CHAPTER 1
Synthesis of Transition Metal Dithiolenes
THOMAS B. RAUCHFUSS
School of Chemical Sciences
University of Illinois at Urbana-Champaign
Urbana, IL
CONTENTS
I. INTRODUCTION
2
II. SYNTHESIS FROM PREFORMED ALKENEDITHIOLATES,
1,2-DITHIONES, OR THEIR EQUIVALENT
A.
B.
C.
D.
From Benzenedithiol and Related Derivatives / 4
1. Arene Derivatives / 4
2. Linked Bis(benzenedithiolate) Complexes / 8
3. Heterocyclic and Heteroatomic Dithiolates / 10
From 1,2-Alkenedithiolates / 10
1. Via Reductive Dealkylation / 10
2. By Base Hydrolysis of Dithiocarbonates (Dithiole-2-ones) and
Related Derivatives / 11
From Selected 1,2-Alkenedithiolate Dianions / 15
1. 4,5-Dimercapto-1,3-dithiole-2-thione (dmit2) / 15
2. Inorganic Dithiolates Related to dmit2 / 17
3. Tetrathiafulvalene (TTF)-Derived Dithiolenes / 19
4. From the Thiacarbons [CnSn]2 and Related Derivatives / 20
5. 1,2-Maleonitrile 1,2-dithiolate (mnt2) / 21
Via Thiophosphate Esters (from a-hydroxyketones and a-diketones) / 21
Dithiolene Chemistry: Synthesis, Properties, and Applications,
Progress in Inorganic Chemistry, Vol. 52
Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin
ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc.
1
4
2
THOMAS B. RAUCHFUSS
E.
F.
G.
From 1,2-Dithietes / 22
From 1,2-Dithiones, Including Dithiaoxamides and Esters of Tetrathiaoxalate / 23
Via Intermetallic Dithiolene Transfer / 25
1. Non-Redox Routes / 25
2. Redox Routes / 26
III. TRANSITION METAL PROMOTED ROUTES TO DITHIOLENES
A.
B.
C.
D.
E.
F.
29
Addition of Electrophilic Alkynes to Metal Sulfides / 29
Addition of Unactivated Alkynes to Metal Sulfides / 32
From Metal Sulfides and a-Haloketones and Related Precursors / 37
By Dehydrogenation of Alkanedithiolates / 38
From Dithiocarbonates / 39
Specialized Routes to Dithiolenes / 40
1. S-Dealkylation / 40
2. Insertion into Metal–Alkyne Bonds / 41
3. C
C Coupling Pathways / 42
4. From Alkynes and Thiocarbonyl Derivatives / 43
5. Dithiolene Coupling / 43
6. From Alkynyl Anions / 44
IV. SUMMARY AND OUTLOOK
44
ACKNOWLEDGMENTS
44
ABBREVIATIONS
44
REFERENCES
45
I.
INTRODUCTION
Research on metal dithiolenes has remained continuously active since its
inception in the early 1960s. Initially, the area was driven by the distinctive
redox and structural characteristics of these coordination compounds. After this
discovery phase, dithiolene chemistry was fueled by its connections to materials
science with respect to photonics and electronic conductors. These developments paralleled growth in the area of organic metals; in fact, the preparative
chemistry of dithiolene complexes has greatly benefited from advances in
tetrathiafulvalene chemistry (1). This direction remains active (2, 3). In the
1990s, research on dithiolenes was energized by recognition that virtually all
Mo- and W-containing enzymes feature dithiolene ligands, which are in turn
incorporated into the heterocyclic pyranopterins (4–7). Parallel with the aforementioned developments—discovery, materials-driven studies, and biologically
driven studies—dithiolene complexes continue to appear in many contexts,
often unexpected ones, due to the great stability of the MS2C2R2 ring.
SYNTHESIS OF TRANSITION METAL DITHIOLENES
3
This chapter discusses the synthesis of transition metal dithiolene complexes
and is current to late 2002. Dithiolene chemistry has been reviewed several
times previously, but this is the first review dedicated to synthetic aspects.
Emphasis is placed on more contemporary methods, and the reader should
consult the older reviews, especially those by Mueller-Westerhoff et al. (8) and
McCleverty (9) for discussions of earlier literature. An effort was made to be
comprehensive with respect to methods and the range of complexes examined,
but the chemistry of metal dithiolenes is so vast that it is not practical to be
exhaustive.
In this chapter, the term dithiolene refers to a ligand of the formula R2C2S2,
which depending on one’s formalism could be described as an alkene-1,2dithiolate dianion, a 1,2-dithione, or some oxidation state between these two
extremes (Fig. 1). Benzenedithiolates, their derivatives, and analogues are also
included.
This chapter is divided into two main parts. The first part focuses on reactions
where the dithiolene ligand is generated independently of the metal center. For
the most part, these preparations give alkenedithiolate dianions, which ordinarily are treated with metal electrophiles to form dithiolene complexes. In
the second part, transition metals actively participate in the assembly of the
dithiolenes, usually via the reaction of a metal sulfido species with an alkyne or
hydrocarbon in an equivalent oxidation state.
When considering the synthesis of a dithiolene complex, it is essential to bear
in mind that dithiolenes vary widely in their electronic properties. If one simply
seeks an unsaturated chelating dithiolate, the most convenient options are
benzenedithiolate and the inorganic dithiolenes 1,3-dithiole-2-thione-4,5-dithiolate (dmit2) and 1,2-maleonitrile-1,2-dithiolate (1,2-dicyanoethene-1,2-dithiolate) (mnt2). Large-scale syntheses of these ligands are available. Dithiolenes
Figure 1. Relationships and nomenclature for common dithiolene precursors.
4
THOMAS B. RAUCHFUSS
such as mnt2 and dmit2 that have electronegative substituents behave like
bidentate pseudohalides, and their complexes are usually synthesized via simple
salt metathesis reactions. Akyl-substituted dithiolenes (e.g., [Me2C2S2]2),
are powerful p donors, useful for stabilizing metals in high formal oxidation
states. Syntheses of complexes of such strongly donating dithiolenes often
require redox steps after the initial formation of a metal dithiolene complex.
II.
SYNTHESIS FROM PREFORMED ALKENEDITHIOLATES,
1,2-DITHIONES, AND THEIR EQUIVALENT
A.
From Benzenedithiol and Related Derivatives
1.
Arene Derivatives
Arene-1,2-dithiols are completely stable and are valuable precursors to
dithiolene complexes. Benzenedithiol is the most common member of this class
of ligands, but related derivatives include toluene-3,4-dithiol (10), 3,4,5,6tetrachlorobenzenedithiol (11), 3,4,5,6-tetramethylbenzenedithiol (12), 2,3naphthalenedithiol (13), and quinoxalinedithiol (14) (Fig. 2).
Benzenedithiols are traditionally prepared by reductive dealkylation of 1,2C6R4(SR0 )2, which in turn are obtained by treatment of dibromobenzenes with
alkali metal or cuprous thiolates. The methodology continues to be used, for
example, for crown ether-appended derivatives (15). A newer and more powerful synthesis of 1,2-benzenedithiol and its derivatives has been developed (16).
This method (17) involves reaction of the benzenethiol with 2 equiv of BuLi to
give 2-LiC6H4(SLi), which reacts with elemental sulfur to give the dithiolate
(Eq. 1).
SH
1) BuLi
2) S8
3) H+
SH
ð1Þ
SH
This method has been extended to the synthesis of the bulky benzenedithiol 3(Ph3Si)C6H3-1,2-(SH)2 (18) as well as a series of mixed chalcogenides such as
[1,2-C6H4(S)(Te)]2 (19).
Typically, transition metal benzenedithiolates (and related derivatives) are
prepared by the following methods: salt elimination reactions using a metal
halide and the dithiolate dianion, thiol exchange, and condensation of the free
thiol with oxo, alkoxo, and amido precursors. In one example, the dithiol was
SYNTHESIS OF TRANSITION METAL DITHIOLENES
5
Figure 2. Structures of important arenedithiolate and related ligands.
treated with a metal methyl compound concomitant with the elimination of
methane (Eq. 2) (20).
WMe6 þ 3 C6 H4 ðSHÞ2 ! WðS2 C6 H4 Þ3 þ 6 CH4
ð2Þ
Homoleptic dithiolene complexes, for example, [Ni(S2C6R4)2]z and
[M(S2C6R4)3]z (M ¼ Mo, W), are generally prepared by reaction of the metal
halide and the dithiolate, often followed by oxidation of the initially formed
complexes. An illustrative study is the synthesis of [Ni(S2C6H2(t-Bu)2)2]2,
which can be oxidized to the monoanion and neutral derivatives using air and
iodine, respectively (21). Reactions of MoCl5 and WCl6 with the benzenedithiolate salts give M(S2C6R4)3 (15, 22) or reduced derivatives. Similarly,
treatment of Ti(NMe2)4 with C6H4(SH)2 gives (NMe2H2)2[Ti(S2C6H4)3],
wherein the amido ligand serves not only as a proton acceptor but also generates
the countercation (23). The tris(dithiolenes) are so robust and so easily
formed that they plague syntheses of the oxo-dithiolenes [MO(S2C6R4)2]z
(24). In the case of Mo derivatives, the metathetical reactions can be conducted
6
THOMAS B. RAUCHFUSS
[WO(SPh)4]-
C6H4(SH)2
[WO(S2C6H4)2]BH4-
[WO2(S2C6H4)2]2-
Me3NO
[WO(S2C6H4)2]2-
Figure 3. Illustrative syntheses of oxo-tungsten dithiolenes (25).
in the presence of donor ligands, which inhibit the formation of the tris(dithiolene) complexes. For example, treatment of Na2(S2C6R4) with MoCl4(MeCN)2
in the presence of donor ligands L affords Mo(S2C6R4)2L2 (L ¼ PPh2Me or
MeNC; R ¼ H, Me) (12). These derivatives are closely related to the corresponding dicarbonyls (L ¼ CO; see Section II.G.2).
Compounds of the type [MO(S2C6H4)2]n (M ¼ Mo, W) have received much
attention. A direct approach to [WO(S2C6H4)2] proceeds via the reaction of
WOCl3(thf)2 (where thf ¼ tetrahydrofuran) benzenedithiol, and Et3N (24). Alternatively, thiol exchange routes can be advantageous as a means to minimize
redox processes and formation of [M(S2C6H4)3]n. For example, [WO(SPh)4]
and benzenedithiol give [WVO(S2C6H4)2], which can be subsequently reduced
with NaBH4 to give [WIVO(S2C6H4)2]2 (Fig. 3) (25). Related thiol exchange
reactions involve conversion of [MoO(SC6H4R)4]n (R ¼ Cl, n ¼ 2; R ¼ H,
n ¼ 1) into [MoO(S2C6H3R)2]n (R ¼ H, Me, Ph3Si) (18, 26).
The oxo-bis(dithiolenes) are amenable to further reactions. Treatment of
[MO(S2C6H4)2]2 Me3NO (13) affords [MVIO2(S2C6H4)2]2 (M ¼ Mo, W)
(25, 27). Silylation of the oxo-bis(dithiolene) complexes gives [MVI(OSiR3)(S2C6H4)2] (M ¼ Mo, W), which are versatile precursors to diverse coordination sets on the bis(dithiolene) framework (12, 24). For example, such
[MVI(OSiR3)(S2C6H4)2] derivatives can be oxidized using Me3NO to give
[MVI(O)(OSiR3)(S2C6H4)2] (M ¼ Mo, W) and, in the W case, sulfided using
dibenzyltrisulfide to give [WVI(S)(OSiR3)(S2C6H4)2].
The mildly electrophilic complex MeReO3 condenses with 2 equiv of
benzenedithiol to give MeRe(O)(S2C6H4)2 (28), a rare alkyl metal dithiolene.
Thiol exchange has been employed to probe the strength of metal ligand
bonds as illustrated by the reactions of dicysteinyl peptide-bound derivatives of
[Fe2S2(SR)4]2 with toluene-3,4-dithiol. For strongly chelating dipeptides,
[Fe(SR)2(S2C6H3Me)] derivatives (R ¼ protein) form with H2S elimination,
whereas less strongly chelating dipeptides are displaced by this dithiol to give
[Fe2S2(S2C6H3Me)2]2 (29).
Since several molybdoenzymes feature a single dithiolene ligand active site
(30), the synthesis of monodithiolene complexes has been of interest. Two
general approaches can be envisioned. Stepwise installation of dithiolene
SYNTHESIS OF TRANSITION METAL DITHIOLENES
O
S
7
_
S
Mo
S
S
PhSeCl
_
O
S
Mo
S
Cl
Et3N/Et4NOH
S
S
O
Mo
Mo
O
RS
_
O
2-
O
O
Cl
S
S
Mo
S
S
SR
SR
RS = 2,4,6-(i-Pr)3C6H2S
adamantyl-2-S
Figure 4. Routes to monodithiolene derivatives of Mo (30).
ligands or removal of dithiolenes from bis- and tris(dithiolene) precursors.
One dithiolene can be removed from [MoO(S2C6H4)2] using PhSeCl to
give monodithiolene [MoOCl2(S2C6H4)] together with (PhSeS)2C6H4 (30).
The chloro ligands in [MoOCl2(S2C6H4)] undergo ready substitution to give
diverse mixed-ligand derivatives as shown in Fig. 4. Thiolate–siloxide exchange
was employed to prepare [MoO2(OSiPh3)(S2C6H4)] via the reaction of
MoO2(OSiPh3)2 with Li2S2C6H4.
Benzenedithiolates can also be prepared by H2 elimination when using lowvalent precursor complexes (Eq. 3) (31).
Cp2 V þ C6 H4 ðSHÞ2 ! Cp2 VS2 C6 H4 þ H2
ð3Þ
This H2-elimination route from 1,2-benzenedithiol was also employed in the
synthesis of Fe2(S2C6H4)(CO)6 (32) and the coordinatively unsaturated [Mn(CO)3(S2C6H4)] (33). A series of related coordinatively unsaturated species
[Cr(CO)3(S2C6R4)]2 (R ¼ H, Cl, Me) were prepared by displacement of the
8
THOMAS B. RAUCHFUSS
(OC)2(NC)Fe
-
H
N
S
2Fe2(CO)9
C6H4(SH)2
C6H4(SH)2
S
S
Fe(CO)2(CN)
(CO)2(CN)Fe
Et 4NCN
S
S
S
S
Fe(CO)3
(OC)3Fe
Et4NCN
2(OC)(NC)2Fe
S
2-
-CO
S
(OC)2(NC)2Fe
S
S
+CO
Figure 5. Synthetic routes to Fe(S2C6H4)–CO complexes (38, 39).
solvento ligands from Cr(CO)3(MeCN)3. At higher metal stoichiometry, one
obtains the binuclear [Cr2(CO)6(m-CO)(m-Z2:Z2-S2C6R4)]2 (34). Other coordinatively unsaturated dithiolenes have been prepared by salt-forming methods,
for example, [triphos]Fe(S2C6H4) (35), [Z4-C4Me2(t-Bu)2]Pd(S2C6H4) (36), and
(C5Me5)Ir(S2C6H4) (37). The species Fe2(S2C6H4)(CO)6 undergoes ready degradation upon treatment with Et4NCN, giving rise to both mono- and diiron
derivatives (Fig. 5) (38).
2.
Linked Bis(benzenedithiolate) Complexes
Relatively elaborate benzenedithiol ligands have been prepared via ortho
lithiation of 1,2-benzenedithiol with 3 equiv BuLi, which affords 3-LiC6H3(SLi)2. This trilithiated compound undergoes carbonation to give 2,3-dimercaptobenzoic acid. This carboxy-functionalized dithiolene can be linked via amide
formation to give the bis(benzenedithiol), isolated as its Cp2TiIV derivative (see
also Section II.G.1) (40, 41). Such ligands can be converted to chelating
bis(dithiolene) complexes (Fig. 6). The use of 2,3-dimercaptobenzoic acid
derivatives is inspired by the naturally occurring chelators derived from 2,3dihydroxybenzoate (42).
An improved and very promising methodology to such linked dithiolenes
begins with the ortho lithiation of 1,2-C6H4(S-i-Pr)2, generated on a large scale
SYNTHESIS OF TRANSITION METAL DITHIOLENES
9
CH2OH
SH
SH
S-i- Pr
1) PBr3
2) Mg
3) Na/C10H8
SH
S-i- Pr
SH
(CH2O)n
Li
Cl NaS-i- Pr
S-i- Pr
Cl
S-i-Pr
S-i- Pr
BuLi
S-i- Pr
CO2
O
HS
HS
NH
NH
CO2H
O
SH
SH
1) SOCl2
S-i- Pr
2) C2H4(NH2)2
3) Na/C10H8
S-i- Pr
Figure 6. Hahn’s methodology to bis(benzenedithiolates).
from 1,2-C6H4Cl2, to give the versatile nucleophile 3-LiC6H3-1,2-(S-i-Pr)2 (43).
This revised metalation procedure was employed in the synthesis of the
ethylene-linked dithiolene 1,2-C2H4[3-C6H3-1,2-(SH)2]2, which forms bimetallic complexes with a staircase-like structure (Fig. 7).
4
Figure 7. Structure of Ni2 ðS2 C6 H3 Þ2 C2 H4 2
(40).
10
THOMAS B. RAUCHFUSS
3.
Heterocyclic and Heteroatomic Dithiolates
Heterocyclic analogues of benzenedithiolates are also available. 3,4-Thiophene-dithiolate is generated from the corresponding dibromothiophene (44).
The isoelectronic but inorganic 1,2,5-thiadiazole-3,4-dithiolate, [SN2C2S2]2
(tdas, see Fig. 2) can be prepared by sulfidation of SN2(CCl)2; the dianion forms
bis(chelate) derivatives of Ni(II) and Fe(III) (45).
Complexes of 1,10 -ferrocenedithiolate ([FcS2]2) exhibit properties like
arenedithiolates, one difference being the potential for dative Fe ! M bonding
(46). Ferrocenedithiol and its salts are well known and have been widely
employed as ligands. Some illustrative complexes are Ni(S2Fc)(PMe2Ph) (47)
and TpRe(O)[S2Fc] (48). The olefin polymerization precatalysts FcS2M(NMe2)2
(M ¼ Ti, Zr) were prepared by treatment of M(NMe2)4 with Fc(SH)2,
concomitant with elimination of HNMe2 (49). 1,2,10 ,20 -Ferrocenetetrathiol
(50) could in principle be employed for the synthesis of multimetallic
derivatives.
1,2-Dicarboranedithiolate can be generated by deprotonation of 1,2-dicarborane followed by sulfidation (51–53). The resulting Li2S2C2B10H10 reacts with
metal dihalides to give the corresponding dithiolates.
B.
1.
From 1,2-Alkenedithiolates
Via Reductive Dealkylation
In contrast to arenedithiols, 1,2-alkenedithiols are usually unstable. The
corresponding alkenedithiolate dianions are, however, valuable precursors to
dithiolene complexes, although they vary widely in their ease of manipulation.
Salts of the required cis-[C2R2S2]2 can be generated by the reductive cleavage
of cis-1,2-bis(benzylthio)alkenes using Na/NH3 (few alkali metal salts of
dithiolenes have in fact been characterized in any detail). The reductive dealkylation was discussed above as a route to benzene- and thiophenedithiolates
(15, 43, 44).
Generally speaking, alkenedithiolates derived by this dealkylation route are
strongly reducing and should be handled with complete exclusion of oxidants
and electrophiles (e.g., chlorinated solvents, water). After their isolation as
solids, the disodium dithiolates are generally treated with the metal halide to
give the corresponding dithiolene complexes. In some cases, intermediate
anionic dithiolene complexes are allowed to undergo oxidation by air or solvent
prior to isolation of the final complex. Illustrative is the traditional route to the
M(S2C2H2)n complexes (M ¼ Ni, n ¼ 2; M ¼ Mo, n ¼ 3) (54, 55), the synthesis
of which begins with the Na/NH3 cleavage of cis-1,2-bis(benzylthio)ethene,
SYNTHESIS OF TRANSITION METAL DITHIOLENES
11
which can be made on a large scale from cis-dichloroethene and benzylthiolate
salts (56). Solutions of cis-C2R2S2Na2 are treated with divalent metal salts (e.g.,
Ni, Co, Fe, Cu) to give intermediate anionic species that are subsequently
oxidized to give neutral or monoanionic complexes. Third-row metal centers
resist reduction, thus treatment of (C5Me5)TaCl4 with cis-C2H2S2Na2 gives the
expected (C5Me5)Ta(S2C2H2)2 (57). The compound cis-1,2-C2H2[SC(O)Me]2,
prepared by treatment of cis-1,2-C2H2(SNa)2 with acetyl chloride (54, 58), is a
promising if untested precursor to the dithiolene dianion (55). The reductive
cleavage of the corresponding trans-1,2-bis(benzylthio)ethene gives trans-C2H2(SNa)2, which does not normally form molecular complexes (55) (a complex
derived from a trans-alkenedithiolate is described in Section III.F.3). The
cleavage of benzylthioethers has more recently been used to generate nonplanar
dithiolene ligands shown in Eq. 4 (59).
OMe
PhCH2S
MeO
PhCH2S
S
1) Na/NH3
2) MCl2L2 or
NiCl2, air oxidation or
MoCl5, air oxidation
MeO
M
S
n
MeO
M = Ni(PR3)2, Pd(b py); n = 1
M = Ni; n = 2
M = Mo, n = 3 (2 isomers)
ð4Þ
Because of the nonplanarity of these dithiolenes, the Mo(dithiolene)3 derivative
exists as two isomers, one with C3 symmetry and the other with Cs symmetry.
2.
By Base Hydrolysis of Dithiocarbonates (Dithiole-2-ones)
and Related Derivatives
A powerful route to dithiolene complexes employs alkenedithiolate dianions
generated by the hydrolysis of cyclic unsaturated dithiocarbonates, which are
formally called 1,3-dithiole-2-ones. Representative of the many examples (60),
the base hydrolysis route has been used to prepare the ferrocene-substituted
dithiolene Ni[S2C2H(C5H4)FeCp]2 (61), the sulfur-rich dithiolene [Ni(S2C2S2C2H4)2] (62), the cyano(dithiolenes) trans-{Ni[S2C2H(CN)]2}n (n ¼ 1, 2) (63),
2,3-thiophenedithiolates [Au(S2C4H2S)2] (64), and the tris(styryldithiolate)
12
THOMAS B. RAUCHFUSS
Br
R2
S
R1
O
S
O
S
S
O
Me2CHOCS2R2
R2
OCHMe2
S
R2
R1
AIBN
R2
S
1) OR-
S
MLn
O
S
R1
O
R1
S
2) Ln MX2
R1
S
AIBN = azoisobutyrylnitile
Figure 8. Synthetic routes to alkene dithiocarbonates (dithioles) and their conversion to dithiolene
complexes.
Mo[S2C2H(Ph)]3 (65). As is typical throughout dithiolene chemistry, initially
produced anionic dithiolene complexes are often allowed to undergo airoxidation to give more conveniently isolated derivatives, for example, of the
type [Ni(S2C2R2)2] and Mo(S2C2R2)3.
The required 1,3-dithiol-2-ones can be prepared in several ways. Commonly
used precursors are a-haloketones, which are often commercially available or
can be prepared by halogenation of the ketones (66, 67). The overall procedure
involves a series of efficient steps with well-defined intermediates (Fig. 8). A
key step is the acid-catalyzed cyclization of a a-ketoxanthate ester [RC(O)CH2SC(S)OR] in neat H2SO4 to give the dithiocarbonate (68). The corresponding
reaction using a-keto dithiocarbamate ester [RC(O)CH2SC(S)NR2] generates
the iminium analogues of the cyclic dithiocarbonates (69), although the xanthate
approach still appears preferable.
1,3-Dithiole-2-ones have also been efficiently prepared from alkynes via the
addition of the equivalent of ‘‘CS2O’’, which in turn is derived from diisopropyl
xanthogen disulfide (70) (Fig. 8). The reaction, which is effected using the free
radical initiator AIBN, has been used to prepare 2-thienyl substituted dithiolenes, which can undergo subsequent electropolymerization (71). Xanthate
derivatives of hydroxymethylalkynes (e.g., HOCH2C2R) also convert to 1,3dithioles (72). The use of 1,3-dithiole-2-ones is compatible with functionalized
backbones, for example, the attachment of heterocyclic side groups to the
dithiolene backbone (70, 72).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
13
The 1,3-dithiole-2-ones can be prepared via displacement of ethylene from
ethylenetrithiocarbonate with electrophilic alkynes (73). The modified
trithiocarbonate is then converted to the corresponding dithiocarbonate, base
hydrolysis of which provides [(MeO2C)2C2S2]2. This route was employed in
the synthesis of {Ni[S2C2(CO2Me)2]2} (73), although such complexes are more
routinely generated by the addition of C2(CO2Me)2 to metal sulfido complexes
(see Section III.A). Displacement of ethylene from ethylenetrithiocarbonate
using the electrophile di(2-thienoyl)acetylene (74) gives 2,20 -dithienyldithiolenes
(Eq. 5), which are susceptible to electropolymerization (75, 76).
Z
S
S
C2Z2
Z
S
- C2H4
Z
S
S
S
S
Z
ð5Þ
S
S
[Z = CO2Me, C(O)-2-C4H3S]
The dithiole-2-thione (NC)2C2S2CS, which is derived from mnt2 (see
II.C.5), has been converted to a variety of dithiolene precursors such as the
amide [H2NC(O)]2C2S2CS, the diacid (HO2C)2C2S2CS, and the unsubstituted
derivative H2C2S2CS (77). Butadiene-1,2,3,4-tetrathiolate [S4C4H2]4 has been
prepared from the bis(dithiocarbonate). This tetraanion is a precursor to the
coordination polymer [Ni(S4C4H2)]n (78).
The conversion of the dithiocarbonates into alkenedithiolates involves base
hydrolysis, which is usually effected with sodium alkoxides in alcohol. With the
dianion in hand, the synthesis of complexes follows the usual course, as
described above. Obviously, oxophilic metal centers, for example, Ti(IV) and
Nb(V) (62), are incompatible with the usual alcohol solutions of in situ
generated alkenedithiolates. In such cases, the anhydrous salts Na2S2C2R2 are
employed in nonhydroxylic solvents, although after complex formation protic
solvents are typically employed for cation exchange.
The dithiocarbonate methodology has been used to prepare a number of
molybdenum–dithiolene complexes. In these syntheses, particular attention
must be paid to the molybdenum precursor in order to avoid formation of the
highly stable (and biologically irrelevant) tris(dithiolene) species. For example,
treatment of Ph(H)C2S2Na2 with MoO2(pentane-2,4-dionate)2 gave M[S2C2H(Ph)]3n (M ¼ V, Mo, W) with displacement of the oxo group (79). The use of
[MoO2(CN)4]4 inhibits the formation of the tris complex (80), allowing one to
14
THOMAS B. RAUCHFUSS
obtain the mixed-ligand complex {MoO[S2C2H(Ar)]2}2, which exists as both
cis and trans isomers (Eq. 6).
H
S
S
X
R
2-
O
H
1) OH-
S
H
S
R
Mo
2) [MoO2(CN)4]4-
S
(X = NMe2+, O)
R
S
(and trans isomer)
ð6Þ
O
N
N
MeN
R = Ph; 2-, 3-, 4-C5H4N,
N
H2N
N
N
The use of cyanide ligands to suppress persubstitution by dithiolenes has also
been applied to the synthesis of [Ni(CN)2(dithiolene)]2 (81).
Whereas complexes of ethylenedithiolate [H2C2S2]2 are typically prepared
by the reductive S-dealkylation of cis-H2C2(SCH2Ph)2 (Section II.B), a viable
alternative route involves base hydrolysis of 1,3-dithiol-2-one, H2C2S2(CO).The
parent H2C2S2CO can in turn be prepared on a multigram scale from chloroacetaldehyde (82). This 1,3-dithiol-2-one can be functionalized via deprotonation
followed by C-alkylation (72), thus opening the way to a variety of functional
dithiolenes (Eq. 7).
H
H
S
S
ð7Þ
1) LiNR2
MLn
O
H
S
2) E+
3) OR4) Ln MX2
E
S
A versatile route to RS-substituted dithiolenes entails S-alkylation of the
trithiocarbonate dmit2 (see Section II.C.1), which provides an efficient means
to introduction of diverse functionality to the dithiolene backbone. Subsequent to
CS2C2(SR)2 is converted to the dithiocarbonate
S-alkylation, the resulting S
O CS2C2(SR)2 with Hg(OAc)2 in acetic acid (63, 83, 84). Such dithiocarbonates are more easily hydrolyzed than the trithiocarbonates (72, 85). This approach has been used for the synthesis of Ni[S2C2(S(CH2)nMe)2]2 (n ¼ 2–11)
(86) and related complexes with pendant alkene substituents (Eq. 8) (87).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
S
S
S
S
1) RBr
S
S
SR
S
SR
1) MeO-
O
2) Hg2+
15
S
SR
S
SR
LnM
2) LnM2+
(dmit2-)
ð8Þ
The direct reaction of [Zn(dmit)2]2 with 1,2,-dibromoethylether affords the
ethoxy-substituted trithiocarbonate, which eliminates ethanol to give a sulfurrich dithiolene with extended unsaturation (Eq. 9) (88).
EtO
Br
Br
S
1) [Zn(dmit)2]2H+
2)
(- EtOH)
3) KOH
4) Ni2+
5) air
S
_
Ni
S
S
ð9Þ
2
Recently, it was found that treatment of [Zn(dmit)2]2 with certain alkylating
CS2C2Hagents gives unsymmetrically functionalized derivatives such as S
(SR) (R ¼ 3-CH2C5H4N, C2H4CN). Although mechanism of the C
S scission
remains obscure, these trithiocarbonates are promising precursors to unsymmetrical dithiolenes (89).
C.
From Selected 1,2-Alkenedithiolate Dianions
1.
4,5-Dimercapto-1,3-dithiole-2-thione (dmit2)
The heterocycle dmit2, occasionally referred to as [a-C3S5]2, is one of the
most important dithiolene ligands. The literature on dmit2 is vast, but an
overview of the ligand chemistry including many useful experimental procedures is available (1) as are reviews on specific aspects of the coordination
chemistry (2, 3, 90–92). Most studies on dmit2 are directed toward applications
in materials chemistry, for example, the photonic or electronic properties (90, 93).
The synthesis of dmit2 involves treatment of a dimethylformamide (DMF)
solution of CS2 with Na to give a mixture of dmit2 and CS32. Recent work has
shown that in the presence of CS2, dmit2 actually exists as its deep red
thioxanthate, [dmit CS2]2 (94). The breakthrough discovery that enabled the
proliferation of this ligand was the finding by Hoyer and co-workers (95) that
dmit2 can be conveniently isolated in multigram scale as quaternary ammonium salts of [Zn(dmit)2]2 (Fig. 9).
16
THOMAS B. RAUCHFUSS
Figure 9. Synthetic interrelationships involving dmit2 and other CS2-derived species.
In principle, oxidized derivatives of dmit2 (C3S5)n and [(C3S5)2]2 could be
employed for the synthesis of dmit complexes (83).
Salts of [Zn(dmit)2]2 are air stable in contrast to alkali metal salts of dmit2
itself (94). Diverse organic cations, for example, [CpFe(C5H4CH2NMe3)]þ,
have been used in the isolation of [Zn(dmit)2]2 (96). The basic Hoyer–
Steimecke synthesis of [Zn(dmit)2]2 has been subjected to numerous optimizations, mainly aimed at large-scale syntheses (>50 g) (1, 97–99), although the
original procedure (95) is excellent. The method has been revised so that it
consumes CS2 more efficiently, facilitating its preparation from 13CS2 (94). In
one interesting modification, CS2 and Na are first combined in the reaction flask
and the electron-transfer process is controlled by the addition of DMF (98, 99).
Although the reductive coupling of CS2 has long been assumed to cogenerate
equimolar amounts of dmit2 and [CS3]2, recent reports show that under
appropriate conditions, formation of [CS3]2 can be suppressed (97).
Complexes of dmit2 are commonly generated either via ligand transfer from
[Zn(dmit)2]2 (see Section III.G) or by salt metathesis using Na2dmit. Alkali
metal salts of dmit2 are prepared by hydrolysis of the thioester dmit[C(O)Ph)]2.
This dithioester is prepared by treatment of [Zn(dmit)2]2 with PhC(O)Cl and
SYNTHESIS OF TRANSITION METAL DITHIOLENES
17
isolated as yellow crystals with favorable stability and solubility (1). Ionic
complexes of dmit2 are usually synthesized in a three-step, one-pot procedure:
(a) dmit[C(O)Ph)]2 is hydrolyzed with NaOMe in MeOH; (b) an alcoholic or
aqueous solution of the metal cation, for example, NiCl2(H2O)6, is then added to
give the alkali metal salt of the metal dithiolene complex, for example,
Na2[Ni(dmit)2]; and (c) an aqueous or methanolic solution of a quaternary
salt, for example, R4NCl or Ph4PCl ¼ quatþCl, is added to precipitate
(quat)m[M(dmit)n] salts which are amenable to recrystallization from MeCN
solution. This route is employed to access derivatives of [Ni(dmit)2]2, the subject
of many hundreds of publications (2, 100), which are discussed by Cassoux in
chapter 8 of this volume (100). Starting from dmit[C(O)Ph)]2, one can also
isolate the anhydrous and air-sensitive Csþ and NMe4þ salts of dmit2 (1, 101,
102).
Numerous complexes of dmit2 have been prepared using in situ generated
Na2dmit (103–115), for example, [Mn(dmit)2]24 (113), [Re2(dmit)5]2 (114),
and [Rh(dmit)2] (103). With more inert precursors such as Cp2MoCl2 (116),
alkali metal salts of dmit2 are used, whereas for more reactive metal electrophiles the transfer (see Section II.D) of dmit2 from [Zn(dmit)2]2 is convenient, for example, for the synthesis of Cp2Ti(dmit) from Cp2TiCl2 (117).
Note that the M
Cp linkage is not immune to substitution as shown by the
conversion of Cp2TiCl2 into [CpTi(dmit)2] using Na2dmit (118). A related
strategy for the preparation of dmit2 complexes involves the reaction of preformed [Ni(dmit)2]2 and [Ni(MeCN)6]2þ in a 3:1 ratio to give [Ni2(dmit)3]2,
whose structure is shown in Fig. 10 (94).
Figure 10. Structure of [Ni2(dmit)3]2 (94).
18
THOMAS B. RAUCHFUSS
Whereas mixed-ligand dmit complexes are generally prepared by reaction of
dmit2 sources with substituted metal halides, the displacement of dmit2 from
homoleptic complexes represents an alternative route. For example, treatment of
[Ni(dmit)2]2 with triphos gives [Ni(dmit)(triphos)] (119).
2.
Inorganic Dithiolates Related to dmit2
The coordination chemistry of the oxa derivative of dmit2, [OCS2C2S2]2
(dmid2), has been well developed although the electronic properties of the
resulting complexes have elicited only modest attention. This ligand is generated by the base-degradation of the bis(dithiocarbonate) tetrathiapentalenedione
(TPD) (120). Otherwise, TPD has played an important role in the development
of tetrathiafulvalenes and dithiolenes (121, 122); the reader is referred to
Sections II.C and III.E for related methodology. The corresponding iminecontaining dithiolates [RNCS2C2S2]2 would be interesting ligands.
Isomeric with dmit2 is dmt2, (dmt2 ¼ 4,5-dimercapto-1,2-dithiole-3-thione)
wherein the three carbon atoms are contiguous (see Fig. 10) (2). The synthesis
of dmt2 starts with the dmit2 preparation followed by heating at 120–140 C
to effect the Steimecke rearrangement (123, 124). The complex (NEt4)2[Zn(dmt)2] is isolated as an oil before conversion to the thioester dmt[C(O)Ph)]2, which is usually purified prior to conversion to its metal complexes. One
unusual feature of dmt2 is the reactivity that is latent in the ligand backbone,
illustrated by its reaction with dimethylacetylene dicarboxylate (DMAD) (123).
Tetrathiooxalate, [C2S4]2 (see Fig. 10), is not a true enedithiolate and
strictly speaking falls outside the scope of this chapter. Nonetheless, the
exploration of this ligand is closely tied to dithiolene chemistry. Early researchers mistook dmit2 for [C2S4]2 (125), not realizing that C2 S2
4 reacts with CS2
to give dmit2 (126). Hydrolysis of the bis(dithiocarbonate) C2(S2CO)2 yields
[OCS2C2S2]2 (see above) (127), not [C2S4]4 as has been claimed (128, 129)
(see Section II.C). The alkene C2(SMe)4 has been generated by treatment of
basic solutions of C2(S2CO)2 with methylating agents, but this reaction proceeds
via the intermediacy of OCS2C2(SMe)2. An easy gram-scale route to (Et4N)2C2S4 has been developed (130), based on a simplification of Jeroschewski’s
electrosynthesis (125).
Numerous bi- and polynuclear complexes with M2(C2S4) cores (monometallic derivatives of [C2S4]n are unknown) are assigned as ethylenetetrathiolate
--derivatives as judged by structural criteria, specifically the C
C distance. Such
complexes are often prepared by reductive coupling of CS2 using low-valent
metal complexes such as those of Ni(I), Fe(I), and Ti(II) (131). Tetrathiooxalate
complexes, generated by salt metathesis from (Et4N)2C2S4 (130, 132), undergo
reduction to give ethylenetetrathiolato complexes (Eq. 10) (133).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
+C2S42–
Cl
Cp*Rh
Cl
S
Cl
RhCp*
Cl
S
Cl
+2e–
S
RhCp*
Cp*Rh
+Cl2
Cl
S
S
19
S
RhCp*
Cp*Rh
S
+Cl2
S
ð10Þ
Partial oxidation of C2 S2
4 proceeds with loss of sulfur and coupling to give
the vinylidene dithiolate derivative of dmit2. This planar dianion C4S62 is
isolated as its blue-purple Et4Nþ salt (130). Treatment of this salt with metal
halides affords di- and polymeric complexes, for example, [C4S6][RuCl(arene)]2
and the semiconducting [NiC4S6]n (134) (Eq. 11).
S
S 2-
S
S
I2
S
S
S-
S
S
S-
Ni2+
S
S
S
S
S
S
Ni
n
ð11Þ
3.
Tetrathiafulvalene (TTF)-Derived Dithiolenes
Because of the close structural and preparative connections between the
TTFs and dithiolene chemistry, it is only natural that extended dithiolenes have
been developed with a TTF-like core. These complexes are generally prepared
via the corresponding TTF-based di- or tetrathiolates.
Tetrathiafulvalenetetrathiolate, with the formula [C6S8]4 or [S2C2S2C
CS2C2S2]4, has long been known, but only recently have well-defined molecular complexes been described. Solutions of [C6S8]4 can be generated by
hydrolysis of the corresponding bis(dithiole) OCS2C2S2C
CS2C2S2CO (see
Section II.C) or by lithiation–sulfidation of TTF itself. The former method has
been applied to the synthesis of polymeric complexes (129). The latter method
was employed in the synthesis of C6S8[TiCp2]2 (135), which has been fully
characterized.
In contrast to the binucleating character of tetrathiafulvalenetetrathiolate, a
variety of chelating tetrathiafulvalenedithiolates are also known, and these
species give rise to complexes with especially interesting electrical properties.
The synthetic routes to this family of ligands typically begin with dmit2 as
illustrated in Fig. 11.
The synthesis of this hybrid TTF–dithiolene illustrates the use of the
cyanoethyl group to protect the sulfur atoms of the dithiolene (81, 136, 137).
The trimethylene-capped tetrathiafulvalenetetrathiolate forms a molecular
20
THOMAS B. RAUCHFUSS
S
S
2C3H6Br2
Zn
S
S
S
S
S
S
S
S
1) BrC2H4CN
2) Hg(O2CMe)2/HO2CMe
S
S
CN
S
S
CN
S
S
S
S
S
S
S
S
P(OEt)3
O
CN NC
nS
S
S
S
S
S
1) NMe4OH
2) Ni2+
Ni
S
S
2
Figure 11. Preparation of trimethylenetetrathiafulvalenedithiolate complexes (136).
CS2C2S2(CH2)3]2}2. Electro-oxidization
species of the type {Ni[S2C2S2C
(138) is commonly employed to secure single crystals of dithiolene-based
2
organic metals. When applied to {Ni[S2C2S2C
CS2C2S2(CH2)3]2} , one obtains
crystals of the charge-neutral species [Ni(dithiolene)2], an unusual single
component metalloorganic electrical conductor (139).
4.
From the Thiacarbons [CnSn]2 and Related Derivatives
The coordinating properties of the thiacarbons [CnSn]2 have been of
intermittent interest (140). Beck et al. investigated the coordination chemistry
of tetrathiasquarate, [C4S4]2 (141, 142). This dianion forms an extensive series
of bimetallic complexes [C4S4][MLn]22, where MLn ¼ Rh(PPh3)2þ, Pt(PPh3)2,
PdCl2, Pt(PEt3)2þ (141), and Au(PMePh2)þ (142). Metal carbonyls form similar
complexes as well as derivatives where the squarate is unidentate (143).
Complexes with terminal [C4S4]2 ligands, for example, L2MC4S4 are apparently unknown. Benzenehexathiolato complexes include multimetallic complexes such as C6S6[Au(PPh3)]6 (144), C6S6[Pt(PR3)2]3 (145), and C6S6[TiCp2]3
(146). Such species are prepared by salt-metathesis reactions.
SYNTHESIS OF TRANSITION METAL DITHIOLENES
5.
21
1,2-Maleonitrile 1,2-dithiolate (mnt2)
An easily prepared, versatile, and time-honored dithiolene ligand is maleonitriledithiolate, [(NC)2C2S2]2, or mnt2. The subject of numerous studies, mnt
complexes are well described in earlier reviews (9, 147). The sodium salt of
mnt2 species arises via the reaction of alkali metal cyanide with CS2 followed
by the spontaneous coupling of the intermediate [S2CCN] concomitant with
loss of sulfur (148). Quaternary ammonium salts of mnt2 do not appear to have
been synthesized.
Most complexes of mnt2 are prepared by straightforward salt metathesis
reactions, as expected for this pseudohalide-like dithiolene. Recent studies on
oxo Mo/W derivatives use less obvious methods. Sarkar and co-workers (149)
synthesized [MoO2(mnt)2]2 via the reaction of aqueous [MoO4]2 with
Na2mnt buffered with citrate and phosphate. The phosphate buffer plays a
significant role in this synthesis. The corresponding reaction with HSO3 in
place of the buffer afforded [MoO(mnt)2]2, isolated as its quaternary ammonium salt (Eq. 12).
[MoO4]2
mnt2-
[MoO2(mnt)2]2-
PPh3
[MoO(mnt)2]2-
ð12Þ
Direct access to [MoIVO(mnt)2]2 involves the use of [MoOCl5]2 (150),
analogous to the routes to [TcO(mnt)2], [OsN(mnt)2], and related mixed
-dithiolene complexes (151–153). Treatment of [MoOCl(MeCN)4]þ with a
mixture of [H2C2S2]2 and mnt2 gives [MoO(mnt)(S2C2H2)]2 (12), which,
like [MoO(S2C6H4)2]2 (154), can be oxidized with Me3NO.
The complex [WIVO(mnt)2]2 has attracted attention because, like the
tungsten-containing enzyme acetylene hydratase, it catalyzes the hydration of
alkynes. This complex is synthesized using aqueous [WO4]2, mnt2, and
dithionite (155). The corresponding reaction of [WO4]2, mnt2, and HSO3
gave [WVIO2(mnt)2]2. The use of HSO3 is curious as it is normally
considered a reductant, but the less oxidizing W(VI) center apparently resists
reduction. At low pH, both the [WOn(mnt)2]2 derivatives convert to the
[W(mnt)3]2, especially in the presence of excess mnt2.
A rare example of a unidentate dithiolene is Ru(k1-mnt)(CO)2(terpy) (terpy is
0 0 00
2,2 ,6 ,2 -terpyridine), prepared from the corresponding [RuCl(CO)2(terpy)]þ (156).
The stability of this complex reflects the relatively low nucleophilicity of mnt2.
D. Via Thiophosphate Esters (from a-Hydroxyketones
and a-Diketones)
A historically significant route to dithiolenes starts from a-hydroxyketones
(also called acyloins) (157). This methodology is well suited for the large-scale
22
THOMAS B. RAUCHFUSS
synthesis of homoleptic dithiolene complexes, especially those with aryl and
simple alkyl substituents. Perhaps the most important complexes of this type are
Ni(S2C2R2)2, where R ¼ Me and Ph, which have gained recent attention as
dithiolene-transfer agents for the synthesis of bis(dithiolene) derivatives of Mo
and W (see Section II.G.2) (158).
In the thiophosphate strategy, an 1,2-enedithiol is recognized as a tautomer of
an a-mercaptothione, which in turn is related via S-for-O exchanges to the
corresponding a-hydroxyketone (Eq. 13).
R
R
OH
S
PS2
1) H2O
+ P4S10
- H2S, "P4S8O2"
R
O
R
S
Ni(S2C2R2)2
2) Ni2+
n
ð13Þ
Thus, treatment of a-hydroxyketones with P4S10 gives intermediate species
described as thiophosphate esters (159), although such species have not been
rigorously characterized. Hydrolysis of these thiophosphates followed by treatment with metal sources, for example, [WO4]2(160), NiCl2 (H2O)6 (160), or
Cp2NbCl2 (161) gives dithiolene complexes. Schrauzer and Mayweg (160)
describe a reliable, large-scale (45 g) procedure to Ni(S2C2R2)2, where R ¼ Me
and Ph.
1,2-Diketones (e.g., derivatives of benzil) can be used in place of ahydroxyketones, a modification that broadens the utility of this method (71,
162), despite the fact that the dithione is the incorrect oxidation state to combine
with metal salts. Large numbers of nickel diaryldithiolenes have been prepared
via this sulfiding method (163–165). Representative of the dithiolene complexes
prepared by the P4S10 /diketone route are Ni[S2C2(Ph)(C6H4NMe2)]2,
W[S2C2(C6H4NMe2)2]3, and Ni[S2C2(C6H4OC11H23)2]2, which have interesting
acid–base (162, 166–168) and liquid-crystal properties (65, 169).
E.
From 1,2-Dithietes
1,2-Dithietes (170–172) are four-membered R2C2S2 rings with adjacent
sulfur atoms. Such heterocycles are isomeric with 1,2-dithiones and formally
result from the two-electron oxidation of 1,2-alkenedithiolates (Fig. 1). Among
the few known 1,2-dithietes, bis(trifluoromethyl)dithiete, (CF3)2C2S2, played a
key role in the early stages of dithiolene chemistry. Preparation of this volatile
(and poisonous) liquid dithiete involves the reaction of hexafluoro-2-butyne with
molten sulfur (9). Oxidation of the dithiolene Cp2TiS2C2(CO2Me)2
(Section III.A) gives the dithiete (MeO2C)2C2S2, which has been characterized
crystallographically (173).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
23
Figure 12. Representative complexation reactions involving 1,2-bis(trifluoromethyl)dithiete.
Because of its solubility in nonpolar solvents and its oxidizing character,
(CF3)2C2S2 is well suited for synthesis of dithiolenes starting with nonpolar,
low-valent organometallic precursors, for example, metal carbonyls (8). The
synthesis of dithiolenes from dithietes is illustrated by the reaction of
[CpMo(CO)3]2 and (CF3)2C2S2 to afford [CpMo{S2C2(CF3)2}]2 via a dicarbonyl intermediate (Fig. 12) (174). An unusual method of exploiting the oxidative
character of (CF3)2C2S2 involves its reaction with [MS4]2, which gives
{M[S2C2(CF3)2]3}2 (M ¼ Mo, W) (175). Starting from mixed-oxo-metal
sulfides one obtains oxo-dithiolenes such as {MoO[S2C2C2(CF3)2]2}2
(Fig. 12). The same oxo-molybdenum species can be obtained by reduction of
(CF3)2C2S2 followed by salt metathesis (12).
F.
From 1,2-Dithiones, Including Dithiaoxamides
and Esters of Tetrathiaoxalate
Few dithiolenes are prepared via reactions involving 1,2-dithioketones, a
rare class of compounds prone to oligomerization. The first stable 1,2-dithione,
1,2-bis(4-dimethylaminophenyl)ethane-1,2-dithione, was generated by photolysis
24
THOMAS B. RAUCHFUSS
of the corresponding dithiocarbonate. The resulting dithione exists in
equilibrium with the dithiete (176, 177). The corresponding diphenyl derivative
exists exclusively in the dithiete form, indicating that p-donor substituents
stabilize the dithione form. Cyclohexanedithione (178) (or its dithiete tautomer),
has been trapped in situ with Mo(0) to give the poorly soluble tris(dithiolene)
(Eq. 14).
S
SMe
hν
"C6H8S2"
- MeSCN
N
S
Mo(CO)6
Mo
- 6 CO
S
S
ð14Þ
3
A subset of the 1,2-dithiones are dialkyl tetrathiooxalate esters, C2S2(SR)2.
The parent C2S2(SMe)2 exists in dynamic equilibrium with its dimer (Eq. 15)
(179).
MeS
S
S
SMe
MeS
S
S
SMe
S
SMe
M(CO)x
2
- x CO
S
SMe
S
SMe
M
SMe
S
n
M = Ni, n = 2
M = Mo, n = 3
ð15Þ
The compound C2S2(SMe)2 exhibits oxidizing character; for example, it reacts
photochemically with Ni(CO)4 and Mo(CO)6 to give dithiolene complexes
Ni[S2C2(SMe)2]2 and Mo[S2C2(SMe)2]3 (an alternative synthesis of such complexes is described in Section III.E) (180, 181). The thermal reaction of
Ni(cyclooctadiene)2 and C2S2(SMe)2 in the presence of bidentate ligands
affords the mixed-ligand complexes Ni[S2C2(SMe)2]L2 (L2 ¼ tmeda, bpy or
2,2-bipyridine). The required S2C2(SMe)2 is derived from dmit2 via methylation, conversion to the dithiocarbonate OCS2C2(SMe)2 followed by photodecarbonylation (179). In principle, this methodology could give a range of
SR-substituted dithiolenes.
Related to the dithioesters of dithiooxalic acid are the diamides R2NC(S)C(S)NR2, which are relatively more stable than ordinary dithiones (182, 183).
These 1,2-dithiones are mildly oxidizing as illustrated by their reactions with
Mo(CO)2(PR3)2(MeCN)2 with displacement of MeCN (184). Crystallographic
studies (185) show that Mo(CO)2(PBu3)2[S2C2(NC5H10)2] (NC5H10 ¼ pipepiperidinyl) has some enedithiolate character (r C
C ¼ 1.37 Å). In contrast, the
corresponding tetracarbonyls, Mo(CO)4[S2C2(NR2)2], arising from Mo(CO)4(OPPh3)2, are described as Mo (0) derivatives (184).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
25
Nickel complexes of unstable cyclic dithioamides are generated by sulfurizing the corresponding diamide in the presence of Ni powder using (MeOC6H4)2P2S4, a sulfiding agent akin to P4S10 (Eq. 16).
R
S
R
N
N
R
S
[ArPS2]2, Ni
O
ð16Þ
S
Ni
(R = Et, i-Pr)
O
N
S
N
R
2
Yields are diminished if the thiation is conducted prior to the addition of the
metal, indicative of the thermal instability of these dithioamides. Furthermore,
the yields are lower when Ni(II) salts are used in place of Ni powder, consistent
with the oxidative character of the dithioamide. Nickel dithiolenes of the dithiooxamides exhibit extraordinary extinction coefficients (80,000 dm3 mol1cm1
at 1000 nm) (186, 187).
G.
Via Intermetallic Dithiolene Transfer
Two basic types of dithiolene exchange reaction are practiced, (1) non-redox
reactions, which usually involve use of ZnII and Cp2TiIV based reagents, and (2)
redox reactions, which commonly involve neutral bis(dithiolene) complexes of Ni.
1.
Non-Redox Routes
Dithiolenes of Ti(IV) and Zn(II) (see Section III.A) transfer their alkenedithiolate to softer metals. Chelate-transfer reactivity under mild conditions was
first demonstrated with Cp2TiS2C2R2 (Z ¼ CO2Me, CF3), which reacts with a
variety of metal dichloro complexes, for example, [RhCl2(CO)2], NiCl2(PR3)2,
to give the corresponding late metal dithiolene and titanocene dichloride, which
can be removed by filtration through silica gel with which it reacts (188).
Related zinc complexes, for example, Zn(S2C2R2)(tmeda), where tmeda ¼
tetramethylethylenediamine, also display this chelate-transfer reactivity and
are perhaps still more versatile (see Section III.A) (189). Both the zinc and
titanocene dithiolenes react with main group halides such as CXCl2 reagents
(X ¼ O, S), to give the corresponding XCS2C2R2. The hexametallic dithiolene
[PdS2C2(CO2Me)2]6 (Fig. 13) was prepared by ligand transfer involving PdCl2(cyclooctadiene) and Zn[S2C2(CO2Me)2](tmeda), concomitant with dissociation
of the diene from Pd (190).
Organic salts of [Zn(dmit)2]2 readily undergo dithiolene-transfer reactions
with metal chlorides. For example, this dianion reacts with Cp2TiCl2, NbCl5,
26
THOMAS B. RAUCHFUSS
Figure 13. Structure of [Pd6S2C2(CO2Me)2]6.
VCl3, and AuCl(PPh3) to afford Cp2Ti(dmit) (92, 117), [V(dmit)3]2 (191),
[Nb2S4(dmit)4]2 (115), and dmit[Au(PPh3)]2 (192), respectively. Although
other dmit complexes are rarely employed for dithiolene transfer CpNi(dmit)
was prepared by treatment of [Ni(dmit)2] with Cp2Niþ (193).
2.
Redox Routes
Schrauzer et al. (194) showed that Ni(S2C2R2)2 and metal carbonyls react
upon photolysis resulting in transfer of dithiolene ligands. For example,
Ni(S2C2R2)2 and Fe(CO)5 react to give the binuclear dithiolene complexes
Fe2(S2C2R2)(CO)6 (R ¼ H, Me, Ph); these species can also be prepared by the
photoaddition of Fe2S2(CO)6 to alkynes (195). More recent studies on this kind
of reaction have revealed examples of incomplete transfer of dithiolene ligands,
resulting in the formation of heterometallic complexes, Eq. 17 (Cp0 ¼ C5Me5,
C5H4SiMe3) (196).
0:5 ½FeðS2 C2 Ph2 Þ2 2 þ 0:5 Cp02 Ru2 ðCOÞ4 ! Cp0 RuFeðS2 C2 Ph2 Þ3 þ ð17Þ
Photolysis of M(CO)6 (M ¼ Mo, W) and Ni(S2C2R2)2 gives modest yields of
M(S2C2R2)2(CO)2 (R ¼ Me, Ph) (197). As expected for M(IV) derivatives, the
carbonyl ligands are labile and can be displaced with a variety of donor ligands.
For example, the chelating 1,2-bis(diphenylphosphino)ethane (dppe) reacts with
W(CO)2(S2C2R2)2 to give M(S2C2R2)2(dppe) (Fig. 14). A monodithiolene
derivative, W(S2C2Me2)(CO)4, was also described, although the behavior of
SYNTHESIS OF TRANSITION METAL DITHIOLENES
CO
M(S2C2R2)3
dppe
M(S2C2R2)2(dppe)
X
dppe
M(S2C2R2)2(CO)2
27
1) S2C2R'22Mo(S2C2Ph2)2(S2C2R2)
2) HCl/air
(R = H, CN)
PR'3
M(S2C2R2)2(CO)(PR'3)
(R' = Bu, t-Bu)
SH-, H+
Mo2(S2C2R2)4E2
(R = Ph, C6H4-4-Me)
Figure 14. Selected reactions of M(S2C2R2)2(CO)2 and related derivatives (M ¼ Mo, W) (198–200).
this complex has not been examined [see related work on dithiaoxamide
complexes of Mo (Section II.G)].
After a long hiatus, the dithiolene-transfer reaction involving Ni(S2C2R2)2
was rejuvenated by Holm and co-workers (198) who sought new routes to bis
(dithiolene) complexes of molybdenum and tungsten as models for metalloenzymes. As discussed above, a synthetic challenge in the chemistry of molybdenum and tungsten dithiolenes is often preventing formation of the tris(dithiolene)
complexes, which are substitutionally inert.
Dithiolene transfer from Ni(S2C2R2)2 to Mo/W (0) proceeds more efficiently
when conducted thermally using preformed M(CO)3(MeCN)3. At the stoichiometry of Ni(S2C2R2)2/M(CO)3(MeCN)3 ¼ 2, the yields of M(CO)2(S2C2R2)2
are 30% (Mo) (198) and 70% (W) (165, 199) (Eq. 18).
2 NiðS2 C2 R2 Þ2 þ MðCOÞ3 ðMeCNÞ3 ! 2 ½NiðS2 C2 R2 Þn þ MðCOÞ2 ðS2 C2 R2 Þ2
ð18Þ
The required Ni(S2C2R2)2 complexes can be prepared on a large scale using the
dithiophosphate route (Section II.E). The use of the labile M(CO)3(RCN)3
reagents, which are easily generated thermally (201), facilitates this chelate-transfer
reaction. The resulting M(S2C2R2)2(CO)2 complexes have trigonal-prismatic
structures, as anticipated by the characterization of the related Mo(CO)2(Se2C6H4)2 (202). The series [M(S2C2R2)2(CO)2]n (n ¼ 0, 1, 2) has been
characterized, the dianionic species having been generated by reduction of the
neutral complex with potassium anthracenide, and the monoanion was obtained
by comproportionation (203).
The carbonyl ligands in M(S2C2R2)2(CO)2 are readily substituted by sources
of O2, S2, and Se2 (Fig. 15) (198, 199). Furthermore, phenoxides, arylthiolates, and arylselenoates (198, 204, 205) also displace one or both of the
carbonyl ligands, the determining factor apparently being the steric crowding
around the M(IV) center.
28
THOMAS B. RAUCHFUSS
Figure 15. Representative reactions of M(S2C2R2)2(CO)2 (M ¼ Mo, W) with anions (198, 199,
203–205).
The resulting anionic complexes closely resemble the proposed active site
structures of the molybdopterin-based O-atom transfer enzymes such as
dimethyl sulfoxide reductase (DMSOR) (4–6), which characteristically converts
dimethyl sulfoxide (DMSO) to Me2S. The complexes [M(S2C2Me2)2(OR)], for
example, deoxygenate Me3NO, DMSO, and Ph2SeO, to give the reduced
substrates and the oxo-metallates [MO(S2C2Me2)2(OR)] (M ¼ Mo, W) (158,
200, 205). One dithiolene can be removed from [MoO(S2C2Me2)2] using
PhSeCl to give mono(dithiolenes) [MoOCl2(S2C2Me2)] (30), analogous to
the more extensively developed [MoOCl2(S2C6H4)] (Fig. 4). These monodithiolenes undergo substitution of the chlorides to give diverse alkoxy and
thiolato derivatives, which are structural analogues of the active sites of sulfite
SYNTHESIS OF TRANSITION METAL DITHIOLENES
C(9)
C(4)
C(5)
C(3)
C(10)
C(8)
C(6)
C(2)
29
C(11)
C(7)
C(12)
S(2)
C(1)
S(3)
S(4)
Mo(1)
S(1)
C(17)
C(20)
Co(1)
C(24)
C(13)
C(16)
C(23)
Co(2)
C(21)
O(2)
C(19)
C(15)
C(14)
O(1)
C(22)
C(18)
Figure 16. Structure of Mo(CO)2[CpCo(S2C6H4)]2 (206).
oxidase and assimilatory nitrate reductase (30). As usual, the main challenge in
the preparation of mono(dithiolene) Mo complexes is preventing formation of
the very stable tris(dithiolene) derivatives.
Some insights into the details of dithiolene-transfer reactions are provided
by a study of the reaction of CpCo(S2C6H4) with Mo(CO)3 sources [e.g., Mo(CO)3(py)3/BF3; py ¼ pyridine]. The product is the trimetallic species
Mo(CO)2[CpCo(S2C6H4)]2 (Fig. 16) (206). Structurally elated complexes have
been prepared from [Ni(S2C6H4)2] and sources of Cp*Ruþ (207).
III.
A.
TRANSITION METAL PROMOTED ROUTES
TO DITHIOLENES
Addition of Electrophilic Alkynes to Metal Sulfides
Metal per- and polysulfido complexes react with electrophilic alkynes to give
dithiolenes. The readily available diester DMAD is most commonly employed
30
THOMAS B. RAUCHFUSS
in this reaction (12, 188, 208–222). Other electrophilic alkynes that have been
used in this context are C2(CF3)2 (175, 223, 224), HC2CO2Me (189),
C2[C(O)Ph]2 (225), and C2[C(O)NH2]2 (12, 226). Still more elaborate alkynes
have been employed in the synthesis of pterin-related dithiolenes (217, 227).
Terminal sulfido complexes also are known to add electrophilic alkynes
as illustrated by additions to both Tp*WS2(EPh), where E ¼ O, Se; Tp* ¼
hydrido tris(3,5-dimethylpyrazolyl)borate (228) and WS2(OSiPh3)2(Me4phen)
(229), where Me4phen is tetramethylphenanthroline.
The prototype reaction of DMAD with sulfur-rich metal complexes involves
treatment of Cp2TiS5 with DMAD to give Cp2TiS2C2(CO2Me)2 (Eq. 19) (188).
S
S
Ti
S
S
S
S
C2Z2
S
Ti
Z
Z
ð19Þ
The reaction in Eq. 19 is first order in alkyne and polysulfido complex and is
probably initiated by nucleophilic attack of a coordinated sulfur atom at the
alkyne carbon followed by attack of the incipient carbanion on another part of
the polysulfido chain. In some reactions, vinylpersulfido or sulfur-rich derivatives of dithiolenes are obtained, for example, the ‘‘b isomer’’of Cp2TiS2C2(CO2Me)2 (214) and Cp2MoS3C2R2 (217). These convert to dithiolenes upon
heating or treatment with PR3, respectively. The dithiolenes Cp2TiS2C2R2 are
distinctively green in color and are readily purified. These and related titanocene
complexes are of synthetic value because the dithiolene ligand can be removed
as the free dianion or transferred to a ‘‘softer’’ metal center (Section II.H.1).
DMAD is a highly reactive electrophile, so caution should be exercised in
using this reagent. Illustrative of the complications that one can encounter, the
reaction of Mo(S2)(S2CNEt2)3 with DMAD gives ‘‘melded’’ dithiolene complexes wherein the Et2NC fragment has inserted into the dithiolene M
S bond
(230).
Subsequent to the development of the titanocene dithiolenes, related results
were obtained starting with ZnS6(tmeda), where tmeda ¼ tetramethylethylenediamine and ZnS4(pmdta) (189), where pmdta ¼ pentamethyldiethylenetriamine (ligand). These species react with DMAD and HC2CO2Me to give
dithiolene complexes, and the dithiolene can also be readily removed from the
zinc center. The zinc complexes are more potent dithiolene-transfer agents than
the titanocene complexes. The pentacoordinate complex ZnS4(pmdta) reacts
more rapidly with alkynes than the tmeda derivative (Fig. 17).
The reaction of phenylethynylquinoxaline and [Mo(S)(S4)2]2 gives {Mo[S2C2(Ph)(C8H5N2)]3}2 (227), which probably exists as a mixture of rapidly
SYNTHESIS OF TRANSITION METAL DITHIOLENES
S
31
S
S
S
Me2
N
S
S
Zn
Me2N
pmdta
Me2N
S
S
RC2CO2Me
(R = H, CO2Me)
S
Zn
NMe
S
MeO2C
Me2N
Me2
N
S
R
S
RC2CO2Me
(R = H, CO2Me)
Zn
N
Me2
pmdta
MeO2C
S
Zn
R
XCl2
(X = Cp2Ti, CO)
Me2N
NMe
MeO2C
S
S
X
Me2N
R
S
Figure 17. Preparative relationships involving amine-supported zinc polysulfides and zinc
dithiolenes (189).
interconverting fac and mer isomers (231). These species are unremarkable
except that they undergo oxidation (I2) to produce thiophenic derivatives that are
structurally related to the products resulting from the oxidative degradation of
molybdopterin (5). Although the mechanism of this conversion remains unclear,
the cyclization step is fairly general as illustrated by related oxidations of
families W(IV) dithiolenes (Fig. 18) (228).
The conversion of dithiolenes into thiophenes has been known for many
years (232). A related thiophene-forming reaction has also been observed using
intact dithiolenes (Eq. 20) (233).
ð20Þ
32
THOMAS B. RAUCHFUSS
Me
Me
Ph
S
Ph
N
N
3
BH
W
N
Tp*WS2(EPh)
N
N
EPh
S
N
I2
O
SR
S
N
HN
Ph
H2N
N
H
N
urothione
N
Ph
S
N
S
2
Figure 18. Synthesis of a molybdenum dithiolene (via a [3 þ 2] pathway) and its conversion to a
thiophene derivative. Structure of urothione, the oxidative degradation product of a molybdopterin
(228).
The ethanedithiolate complex ½Mo2 S4 ðS2 C2 H4 Þ2 2 reacts with DMAD to
give the corresponding dithiolene Mo2 S2 ½S2 C2 ðCO2 MeÞ2 2
2 (215), via a process
that involves loss of ethylene. Precedent for this reaction is the reaction
of ethylenetrithiocarbonate with DMAD as discussed in Section II.B.2 (see
Eq. 5).
B.
Addition of Unactivated Alkynes to Metal Sulfides
Alkynes, even those lacking electron-withdrawing substituents, add to
Cp2Mo2S4 and derivatives to give dithiolenes. This discovery marked one of
the seminal developments in the chemistry of metal sulfides because it
foreshadowed the extensive reactivity of sulfido ligands toward diverse small
molecule substrates. Alkynes displace alkenes from the bis(alkanedithiolates)
Cp2Mo2(S2C2H3R)2 to give the bis(dithiolene) derivatives Cp2Mo2(S2C2R0 2)2
(234). For example, treatment of Cp2Mo2(S2C2H4)2 with acetylene gives
ethylene and Cp2Mo2(S2C2H2)2 (Eq. 21). These reactions proceed via the initial
loss of the alkene followed by the binding of the alkyne to the sulfido ligands. In
contrast to the Cp2Mo2S4 system, most metal sulfides react only with electrophilic alkynes (see Section III.A)
SYNTHESIS OF TRANSITION METAL DITHIOLENES
33
S
S
(C5R5)Mo
Mo(C5R5)
S
S
R2C2
R
S
R
S
S
+ R2C2
S
+ R2C2
R
- C2H4
H
S
S
Mo(C5R5)
(C5R5)Mo
S
S
R
- H2
H
ð21Þ
Mo(C5R5)
(C5R5)Mo
S
S
Mo(C5R5)
(C5R5)Mo
S
S
The dithiolene complex can be hydrogenated (2 atm, 60
C) to re-form the
starting ethanedithiolate complex, thereby defining an alkyne–alkene hydrogenation cycle. An alternative entry into these dithiolenes involves reaction of
alkyne with (MeC5H4)2Mo2(m-S)2(m-SH)2 with displacement of H2 (235).
Furthermore, it was found that anti-(MeC5H4)2Mo2(m-S)2(S)2 and species
described as (MeC5H4)2Mo2Sx react with acetylene to give (MeC5H4)2Mo2(S2C2H2)2 (236).
Treatment of Cp2Mo2S2(SH)2 with 1 equiv of PhC2H in the presence of
oxygen gives Cp2Mo2(O)(m-S)2[S2C2H(Ph)], which is also reactive toward H2
(237). Similar oxo-dithiolenes can be obtained in low yields when Cp2Mo2(S)(O)(m-S)2 is treated with acetylene (238). Of the many modifications of the
Cp2Mo2S4 system, perhaps the most widely studied is the methanedithiolate
Cp2Mo2S2(S2CH2), wherein the reactivity is focused on the pair of m-S ligands.
This species forms dithiolenes upon treatment with the alkynes C2H2, C2Ph2,
and C2Et2, but following a general trend, it binds alkenes more weakly (239).
Functionalized Cp ligands have been introduced, for example, C5H4R, where
R ¼ CH2CH2NMe2, CH2CO2Me, CH2CO2 (240); these allow dithiolene
34
THOMAS B. RAUCHFUSS
formation to occur in water. Similarly, the complexes can be water-solubilized
by functionalization of the methylenedithiolate, e.g., Cp2Mo2S2(S2CHCH2CO2).
Alkynes also add to organotungsten and rhenium sulfides, although, in
contrast to the Cp2Mo2S4 chemistry, these reactions are complicated by sulfur
atom transfer processes. For example, Cp2W2S2(m-S)2 reacts with acetylene
over the course of days (room temperature) to give low yields of Cp2W2(S2C2H2)2. Longer reaction times favor the formation of more complex products
such as Cp2W2S3(S2C2H2) and Cp2W2S2(S2C2H2)2; in the latter two examples,
the dithiolene ligands are nonbridging (241). The dithiolene ligands in
Cp2W2S2(S2C2H2)2, but not Cp2W2(S2C2H2)2, are hydrogenated under mild
homogeneous conditions; the reaction forms ethylene and Cp2W2(S)(m-S)2(S2C2H2), that is, with net change in the S/W ratio. As with the analogous
Mo systems, alkynes displace ethylene from Cp2W2(S)(m-S)2(S2C2H4) and
Cp2W2S2(S2C2H4). Acetylene also binds to [(C5Me4Et)2Re2(m-Z2:Z2-S2)2]2þ
(242), but does not displace alkenes from Cp2V2(S2C2H3R)2 (241). In contrast to
Mo analogues, Cp2V2S4 adds only electrophilic alkynes (223).
The inorganic clusters [Mo3(m3-S)(m2-E)(m2-S)2(H2O)9]4þ (E ¼ O, S) add
acetylene at room temperature in aqueous 1 M HCl solution to give the
dithiolene (243). The dithiolene ligand adopts an unusual bonding mode,
wherein each thiolate sulfur atom bridges two metals (Fig. 19). The cluster
O21
O22
O23
Mo2
C2
S3
O33
C1
H2
S1
Mo3
S2
H1
Mo1
O31
O32
O12
O11
O
O13
Figure 19. Structure of the C2H2 adduct of [Mo3(m3-S)(m-O)(m-S)2(H2O)9]4þ (243).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
35
[Mo3(m3-S)(m2-E)(m2-S2C2H2)(H2O)9]4þ is a rare example of a M-dithioleneH2O complex. A related alkyne addition involves the addition of PhC2H to
[Mo(Et2dtc)]2[Rh(PPh3)2]S4Cl, where dtc ¼ dithiocarbamate which gives a
dithiolene-bis(vinylthiolato) derivative, wherein one sulfur of the dithiolene is
triply bridging and the second sulfur is doubly bridging. The reaction is thought
to proceed via initial dissociation of PPh3 (244).
The anion [ReS4], which is isoelectronic with the well-known d0-oxotransfer agents [MnO4] and OsO4, binds a variety of unactivated alkynes
(Fig. 20).
The alkyne þ [ReS4] reaction depends on the reactant ratio; additionally,
the presence of elemental sulfur has a strong influence. A monomeric dithiolene
[ReS2(S2C2R2)] is initially produced, but it subsequently dimerizes (the
corresponding monomeric alkanedithiolate [ReS2(S2C7H9)] is stable) (245,
246). With a deficiency of alkyne, one obtains tetrametallic [Re4S12(S2C2R2)2]4
with an alkyne/Re ratio of 0.5. These derivatives arise via the addition of [ReS4]
to [ReS2(S2C2R2)] followed by dimerization. Treatment of [ReS4] with 2
equiv of alkyne in the presence of elemental sulfur gives [ReS(S2C2R2)2]; such
Figure 20. Synthetic relationships of dithiolenes derived from [ReS4].
36
THOMAS B. RAUCHFUSS
species adopt a square-pyramidal geometry common to d2 complexes (247).
When the alkyne addition is conducted in the presence of RSH (R ¼ H, alkyl,
aryl) one obtains related derivatives [ReS(S2C2R2)(SH)(SR)] (R2 ¼ PhC2,H;
Ph, Ph). Furthermore, by using ethanedithiol it is possible to prepare mixed
dithiolene–dithiolate complexes, for example, {ReS[S2C2(tms)2](S2C2H4)}
(248), where tms ¼ trimethylsilyl.
Related to these results is the finding that [Cp*WS3] adds Ph2C2 to give the
corresponding tungsten(IV) dithiolene (249). In contrast to [ReS4], the dianion
[WS4]2 does not bind Ph2C2. The reactivity of metal sulfido complexes toward
alkynes thus correlates with the charge on the metal sulfide: neutral complexes
(e.g., Cp2M2S4) being more reactive than monoanions, [Cp*WS3] and [ReS4],
which in turn are more reactive than [WS4]2.
The reaction of metal carbonyl complexes with elemental sulfur in the
presence of alkynes has long been known to afford dithiolenes, for example,
of Fe, Mo, and Ni, although usually in low yields (250). This route to dithiolenes
is mechanistically interesting because binary metal sulfides are unreactive
toward alkynes under mild conditions, thus dithiolene formation indicates the
occurrence of reactive MSx intermediates. Insights into this reaction have mostly
come from studies on Cp metal carbonyls. Sulfidation of Cp2 Fe2(CO)4 in the
presence of alkynes gives dithiolenes, for example, Cp2 Fe2(CO)S2C2(CO2Me)2 and the cubanes Cp2 [Ph(R)C2S2]2Fe4S4 (R ¼ Ph, Et, Me) and
Cp3 (Ph2C2S2)Fe4S5 (251–253). These species are representative of other
abiological Fe
S ensembles that feature dithiolenes in place of thiolate terminal
ligands (254, 255).
Insight into the trapping of reactive metal sulfides with alkynes is provided by
studies on the desulfurization of Cp2 Ru2S4 in the presence of alkynes (256).
This reaction gives Cp2 Ru2(m-Z2:Z4-S2C2R2) via the intermediate Cp2 Ru2S2.
Otherwise, Ph2C2 and Cp2 Ru2S4 do not react, and when the desulfurization
S
Cp*Ru
S
RuCp*
S
R
R
-S
S
Cp*Ru
Cl
Cp*Ru
S
S
H
S
S
H
S
R2C2
RuCp*
Cp*Ru
S
RuCp*
-HCl
RuCp*
Cl
Figure 21. Generation of Cp2 Ru2S2 and its trapping with acetylene to give an Z2:Z4-dithiolene
complex (256, 257).
SYNTHESIS OF TRANSITION METAL DITHIOLENES
37
agent (PBu3) is added prior to the addition of the alkyne, only the unreactive
Cp2 Ru4S6 cluster, not dithiolenes, results. The intermediacy of Cp2 Ru2S2 in this
reaction has been independently confirmed because dehydrohalogenation of
Cp2 Ru2Cl2(m-SH)2 in the presence of alkynes gives the same dithiolenes
(Fig. 21) (257).
The CpCo/Rh promoted reaction with sulfur and alkynes is the basis of a
catalytic synthesis of thiophenes (258).
From Metal Sulfides and a-Haloketones and Related Precursors
C.
This method is somewhat related to the thiophosphate method (Section II.E).
Metal complexes of the type LnM(SH)2 react with a-halogenated ketones to give
the corresponding dithiolenes. The bis(thiol) complexes include Cp2Mo(SH)2
and M(SH)2(dppe) (M ¼ Ni, Pd, Pt) (259). The pathway for this dithiolene
synthesis probably begins with the alkylation of one SH ligand, taking
advantage of the nucleophilicity characteristic of such ligands (260, 261).
The halide leaving group can be replaced by phosphate esters (262) (Eq. 22).
The latter may have implications for the biosynthesis of molybdopterin
cofactors, the precursors to which are a-phosphorylated ketones. Note that
molybdopterin cofactor does not contain molybdenum, it is the organosulfur
component that binds Mo and W.
R1
X
SH
+
Ln M
SH
S
R1
S
R2
Ln M
O
R2
- H2O, HX
LnM =
X = Br,
M(Ph2PC2H4PPh2) tosyl, OPO(OEt)2
Cp2Mo
ð22Þ
R1 = H, Me, Ph
N
N
N
N
R2 =
N
This method has found particular use in the preparation of pyridine-substituted
dithiolene complexes, which exhibit pH sensitive luminescence properties
(263). The a-halocarbonyl starting materials could include related precursors
used in the synthesis of unsaturated dithiocarbonates described in Section II.C.
38
THOMAS B. RAUCHFUSS
D. By Dehydrogenation of Alkanedithiolates
Given the considerable stability of dithiolenes, it is not surprising that they
can be generated by dehydrogenation of alkanedithiolato complexes. Indeed,
Pt(S2C2H2Ph2)(bpyR2), where bpyR2 is a substituted 2,2-bipyridine, dehydrogenates upon photolysis of its oxygenated solutions. Photooxidation of the
corresponding ethanedithiolate gives S-oxygenated products instead (264).
Conventional dehydrogenation agents, for example, dichloro-dicyanoquinone,
do not appear to have been applied to the dehydrogenation of alkanedithiolates.
Dehydrogenation has been employed in the synthesis of thiophenedithioles
(Section II.B.2) from the corresponding tetrahydrothiophene derivatives (64).
Electrophilic alkenes react with ZnS4(pmdeta) to give dithiolene complexes
Zn(S2C2R2)(pmdta). The following alkenes were employed in this reaction:
cis- and trans-C2H2(CO2Me)2, C2H3CO2Me, C2H3CN, 1,2-C2H2Me(CN), C2H3CHO, and 1,2-C2H2(CN)(Ph). The reaction proceeds via the reversible formation
of a dipolar intermediate, as indicated by the ability of the polysulfido complexes to catalyze the cis–trans isomerization of C2H2(CO2Me)2 (Eq. 23)
(265). Such dipolar intermediates are proposed to undergo ring closure to
give alkanedithiolato intermediates. Independently prepared alkanedithiolate
Zn[S2C2H2(CO2Me)2](pmdta) reacts with elemental sulfur to give the dithiolene
Zn[S2C2(CO2Me)2](pmdta). The dithiolene ligands can be removed from the Zn
center, for example, with phosgene trimer (COCl2)3 and Cp2TiCl2 to give
OCS2C2H(CN) and Cp2TiS2C2H(CN), respectively (Section II.G.1).
NMe
NMe
Me2N
Me2N
Zn
S
Zn
+ S8, -H2S
Me2N
R
S
S
ð23Þ
Me2N
R
S
Z
Z
An esoteric example of a dehydrogenative route to a dithiolene involves
the thermolysis of Cp2TiS5. In the product, two H atoms on one Cp ring have
migrated to allow the formation of a cyclopentene-1,2-dithiolate derivative
(Eq. 24) (266).
S
S
S
S
S
S
Ti
∆
S
S
Ti
S
S
ð24Þ
SYNTHESIS OF TRANSITION METAL DITHIOLENES
39
The Kajitani–Sugimori group, which has conducted numerous studies on the
ligand-centered reactions of dithiolenes (267), synthesized various CpCo(S2C2RR0 ) derivatives via the reaction of CpCo(CO)2, electrophilic alkenes,
and elemental sulfur (268). The reaction of [TcCl6]2 with ethanedithiol is
claimed to generate small amounts of [Tc2(S2C2H4)2(S2C2H2)2]2 wherein the
dithiolate, not the dithiolene is bridging (269), although the structure assignment
has been disputed (270). The ethanedithiolate Cp*Re(S2C2H4)Cl2 upon thermolysis or treatment with O2 gives the dithiolene Cp*Re(S2C2H2)Cl2 (271).
Similarly, heating Cp*Re(S2C2H3Et)Cl2 and Cp*Re(S2C2H2Me2)Cl2 gives the
corresponding alkyl-substituted dithiolenes without C
S bond scission. The dehydrogenation follows first-order kinetics. Such reactions are relevant to the ability
of metal sulfides to catalyze hydrogen-transfer reactions.
E.
From Dithiocarbonates
As discussed in Section II.C, the hydrolysis of certain unsaturated dithiocarbonates, that is, dithiole-2-ones, gives alkenedithiolate dianions that react
further with metal cations to give dithiolenes. In selected cases, dithiolenes can
be prepared directly from reactions of metal complexes with dithiole-2-ones.
This process has been demonstrated on two occasions using the electrophilic
tetrathiapentalenedione. Treatment of TPD with [MoS4]2 affords [Mo(C3S4O)3]2 and COS. Oxidation of this tris(dithiolene) complex to the
charge-neutral Mo(VI) derivative followed by hydrolysis in the presence of
BuBr gives Mo[S2C2(SBu)2]3 (Eq. 25) (272).
S
S
O
O
S
S
[MoS4]2-
2-
S
Mo
_
COS, S
O
S
S
S
3
(TPD)
1) [Cp2Fe]+
2) NaOMe/RX
S
SR
S
SR 3
Mo
ð25Þ
In a similar reaction, [MoS4]2 reacts with C4S5O, the monothiocarbonyl
derivative of TPD, to give the same products as above, showing the preferential
reactivity of the thiometalate for the dithio- versus the trithiocarbonate. It was
also found that the tetrasulfido complex ZnS4(pmdta) (Section III.A) reacts with
TPD to give Zn(S2C2S2CO)(pmdta) (189).
40
THOMAS B. RAUCHFUSS
Relevant to the quest for molybdopterin-related ligands, heterocyclesubstituted trithiocarbonates were shown to give modest yields of the CpCo
(dithiolene) when treated with CpCo(cod) (cod ¼ 1,5-cyclooctadiene, Eq. 26) (85).
S
S
H
N
S
H
N
S
CpCo(alkene)2
N
O
Me
S
N
H
CoCp
O
Me
H
BnO2C
BnO2C
ð26Þ
The CpCo platform has been employed to display and interrogate the dithiolene
unit with respect to the ligand-based reactivity. The dithiolene ligand cannot,
however, be readily removed from the CpCo site.
The reaction of dmit-derived trithiocarbonates (see Eq. 8) with Cp2Mo2[C2(CO2Me)2](CO)4 followed by treatment with elemental sulfur gives Cp2Mo2(S)(m-S)2(S2C2R2) [R ¼ CO2Me, SMe, SC(O)Ph] (273). The latter is a unique
example of a complex of the tto-derived ligand {S2C2[SC(O)Ph]2}2.
F.
Specialized Routes to Dithiolenes
Dithiolenes have been prepared by many unplanned or unusual methods,
often involving organometallic intermediates (274). Whereas these transformations are presently classified as specialized, future work may show that these
methods enjoy more significance than presently appreciated. The fact that
dithiolenes appear in the products of so many reactions is testament to their
considerable stability.
1.
S-Dealkylation
Complexes of alkyl-linked o-benzenedithiolates eliminate ethylene to give
benzenedithiolato complexes (Eq. 27) (275–277). This reaction is the reverse
of the known addition of alkenes to bis(dithiolene) complexes (278–281).
Photolysis of the S-benzylated dithiolenes M(PhCH2S2C2Ph2)2 (M ¼ Ni, Pd,
Pt) results in S-dealkylation with elimination of benzyl radicals (282). In
general, however, the properties of S-alkylated dithiolenes have not been
SYNTHESIS OF TRANSITION METAL DITHIOLENES
41
thoroughly investigated (199, 283, 284) beyond the work on CpCo derivatives
(267).
S
S
PEt3
PEt3
-C2H4
Os
PEt3
S
S
S
S
S
S
ð27Þ
Os
PEt3
1,4-Dithiin derivatives (dithiabenzenes) have long been known to react with
metal carbonyls to give dithiolenes (285). A recent illustration is provided by the
finding that treatment CpCo(CO)2 with 2-nitro-3,5-diphenyl-1,4-dithiin affords
CpCoS2C2(Ph)H (286).
2.
Insertion into Metal–Alkyne Bonds
Alkyne complexes of the early metals that are nucleophilic at carbon (i.e.,
have metallacyclopropene character) insert electrophiles such as elemental
sulfur. Thus, zirconocene benzyne complexes, which are generated upon
thermolysis of diarylzirconocene complexes, can be trapped in the presence
of sulfur to give benzenedithiolato complexes (287, 288). Some insights into
the mechanism of this process is provided by the finding that the vinylperthiolate
form of Cp2TiS2C2(CO2Me)2 rearranges intramolecularly to give the dithiolene
(214, see also 215) (Eq. 28).
S
S
S
Z
Ti
Z
S
Ti
Z
Z
ð28Þ
Addition of sulfur to the alkyne-bridged complex Cp2Mo2(CO)4(C2R2) efficiently affords dithiolenes Cp2Mo2(S)(m-S)2S2C2R2 [R2 ¼ H2; H, Me; H, Ph;
Et2; (CO2Me)2] in good yields (273). Such complexes had previously been
prepared by alkyne addition to Cp2Mo2Sx derivatives (238). Related dithiolene
complexes have been made by the successive treatment of the same alkyne
complexes with diorganotrithiocarbonates and elemental sulfur (273).
42
THOMAS B. RAUCHFUSS
3.
C
C Coupling Pathways
A remarkable method for the assembly of dithiolenes proceeds via coupling
of the two RCS halves of the ligand mediated by a metal center. Reaction of
[Mn(CO)5] with ArC(S)Cl (Ar ¼ C6H5, 4-ClC6H4, 4-ClC6H4) gives the
unusual example of a trans (or E-) dithiolene complex. This transformation
has been rationalized by invoking a thioacyl Mn[C(S)Ph](CO)5 intermediate,
which due to the carbenoid character of the thioacyl ligand, undergoes C
C
coupling even at very low temperatures (Eq. 29).
Cl
Mn(CO)5
S
NaMn(CO)5
Ar
Ar
Ar
S
25 °C
- 4 CO
S
S
(CO)3Mn
S
Mn(CO)3
Ar
(CO)5Mn
ð29Þ
Decarbonylation of (trans-S2C2Ph2)[Mn(CO)5]2 occurs at 30
C to give
(Z2:Z5-S2C2Ph2)[Mn2(CO)6 (289). Other examples of Z2:Z5-dithiolenes have
the formula Cp2 Ru2S2C2R2 (256, 290).
Reduction of Fe[Z2-SCSMe)(CO)2L2]þ gives the dithiolenes Fe2[S2C2(SMe)2](CO)4L2 [L ¼ PPh3, P(OMe)3] (291). These species degrade in air
to the deeply colored, 16 e monometalic derivatives Fe[S2C2(SMe)2](CO)2(PPh3), which are formally derived from dimethyltetrathiooxalate
(Section II.F). Conceivably, these C
C coupling reactions are related to the
reductive coupling of CS2 by low-valent metal complexes, which leads to
tetrathiooxalate (or ethylenetetrathiolate) complexes, for example, from Cp2TiII
and (triphos)RhI sources (131, 292, 293) (see also Section II.C.2).
The bis(alkylidyne) clusters Cp3Co3(CR)2 react with elemental sulfur to give
CpCoS2C2R2 (R ¼ Bu, Ph, CO2Me, C2SiMe3). Labeling studies show that the
two alkylidyne units from the same cluster are coupled in this reaction, which
probably begins with attack of sulfur on a Co
Co bond (294).
Werner reported the formation of a CpCo(dithiolene) complexes via a
multistep process that begins with C-alkylation of CpCo(PMe2Ph)(CNMe)
followed by reaction with CS2 and further sulfidation with elemental sulfur.
The product is structurally related to CpCo(dmit) (295).
4.
From Alkynes and Thiocarbonyl Derivatives
Dithiocarbamate complexes are known to react with alkynes to give low
yields of dithiolenes and related ligands via pathways that can be difficult to
SYNTHESIS OF TRANSITION METAL DITHIOLENES
43
rationalize (296). For example, the bis(alkyne) complex W(dtc)2(PhC2H)2 reacts
photochemically with phenylacetylene to give WðC5 Ph2 H2 NMe2 ÞðS2 C2 HðPhÞÞðdtcÞ via a proposed dimethyamino carbyne intermediate (297).
The electron-rich complex Fe(CO)2[(P(OMe)3]2(Z2-CS2) adds electrophilic
alkynes via an apparent 1,3-dipolar addition process to give carbene derivatives
that in turn undergo efficient air oxidation to afford the 16 e dithiolene
complexes Fe(CO)[(P(OMe)3]2(S2C2RR0 ) (R, R0 ¼ CO2Me, Ph, CHO, etc.)
(298).
5.
Dithiolene Coupling
Unsubstituted dithiolenes can be coupled via dehydrogenation. Examples of
this behavior come from studies on derivatives of CpCo, a standard platform for
exploring the reactivity of dithiolenes (267). Treatment of CpCoS2C2H(Ph) with
AlCl3 gives small amounts of the 1,2,3,4-tetrathiobutadiene derivative
[CpCo]2(S4C4Ph2) (299). The related benzenehexathiolate complex arises via
dehydrogenative coupling of 3 equiv of CpCoS2C2H2 (Eq. 30) (300).
S
S
CpCo
S
H
CpCo
S
S
CoCp
CpCo
CoCp
S
S
+
S
S
S
H
H
H
S
S
Co
Cp
ð30Þ
Although inefficient and mechanistically mysterious, these transformations
highlight the reactivity inherent in the C
H bond of unsubstituted dithiolenes,
a fertile area for further research (see Eq. 20). Benzenehexathiolate complexes can
also of course be prepared by reactions of metal salts with C6(SH)6 or its salts
(see Section II.A) (144, 146).
Hoffman has shown that mnt2 complexes form phthalocyanines that are
decorated on their exteriors with metal dithiolenes (301, 302). The reaction is
analogous to the conversion of phthalonitriles to phthalocyanines.
6.
From Alkynyl Anions
The complex Fe2(S2)(CO)6 reacts with alkynyllithium reagents to give
dithiolenes after treatment with electrophiles (303). The synthesis proceeds
44
THOMAS B. RAUCHFUSS
via m-alkynylthiolato complexes, which add electrophiles (Hþ, Me3SiCl,
PhCHO) at the carbon adjacent to sulfur.
IV.
SUMMARY AND OUTLOOK
Dithiolene complexes can be prepared by incredibly diverse routes, the range
of which is testament to the stability of the MS2C2R2 ring. These routes are of
interest not only for their synthetic utility but also for the insights that they
provide on the electronic structure of the host–metal complex. Whereas
dithiolenes occur widely as biological cofactors, the chemistry of synthetic
catalysts based on dithiolenes remains minimal—the development of catalytic
chemistry of metal dithiolenes will likely spawn many new synthetic methods.
ACKNOWLEDGMENTS
Our research in this area has been supported by NIH and NSF. I wish to thank the
following for helpful comments on the review: M. Fourmigué (Nantes), J. A. Joule
(Manchester), E. I. Stiefel (Princeton), and C. J. Young (Melbourne). This review is
dedicated to the memory of Dieter Sellmann.
ABBREVIATIONS
AIBN
bpy
cod
Cp
Cp
DMAD
DMF
dmid2
dmit2
DMSO
DMSOR
dmt2
dtc
dppe
Me4phen
mnt2
Azoisobutyronitrile
2,20 -Bipyridine
1,5-Cyclooctadiene
Cyclopentadienyl
Pentamethylcyclopentadienyl
Dimethylacetylene dicarboxylate
Dimethylformamide
4,5-Dimercapto-1,3-dithiole-2-one
4,5-Dimercapto-1,2-dithiole-3-thione-4,5-dithiolate
Dimethyl sulfoxide
Dimethyl sulfoxide reductase
4,5-Dimercapto-1,2-dithiole-3-thione
Dithiocarbamate
Bis(diphenylphosphino)ethane
Tetramethylphenanthroline
Maleonitrile-1,2-dithiolate
(1,2-dicyanoethane-1,2-dithiolate)
SYNTHESIS OF TRANSITION METAL DITHIOLENES
OAc
pmdta
py
quat
tdas
terpy
thf
tmeda
tms
Tp*
TPD
triphos
TTF
tto2
45
Acetate
Pentamethyldiethylenetriamine
Pyridine
Tetraalkylammonium (tetraarylphosphonium)
1,2,5-Thiadiazote-3,4-dithiolate
2,20 ,60 ,200 -Terpyridine
Tetrahydrofuran
Tetramethylethylenediamine
Trimethylsilyl
Hydridotris(3,5-dimethylpyrazoly)borate
Tetrathiapentalenedione
1,1,1-Tris(diphenylphosphinomethyl) ethane
Tetrathiafulvalene
Tetrathiooxalate
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