Article
pubs.acs.org/Organometallics
Diazadiene Complexes of the Heavy Alkaline-Earth Metals Strontium
and Barium: Structures and Reactivity
Volker Lorenz,† Cristian G. Hrib,† Dirk Grote,‡ Liane Hilfert,† Michael Krasnopolski,§
and Frank T. Edelmann*,†
†
Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, Germany
Lehrstuhl für Organische Chemie II and §Lehrstuhl für Anorganische Chemie II, Ruhr-Universität Bochum, Universitätsstraße 150,
D-44801 Bochum, Germany
‡
S Supporting Information
*
ABSTRACT: 1,4-Diaza-1,3-diene (=DAD) complexes of the
heavy alkaline-earth metals strontium and barium have been
synthesized by direct metalation of N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp). The reaction
with Sr metal afforded a mixture of the red enediamide-type
derivative (DADDipp)Sr(DME)2 (2, DME = 1,2-dimethoxyethane) and black (DADDipp)2Sr(DME) (3), which contains two
coordinated DAD radical anions. With barium, only the radical
anion derivative (DADDipp)2Ba(DME) (4) was formed in 82%
yield. For the first time, transfer of a DAD radical anion ligand from an alkaline-earth metal to a rare-earth metal has been
achieved. Reaction of 4 with [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) afforded the novel (DAD)holmium bis(disiloxanediolate)
complex [{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DADDipp) (6). All new complexes (2−4 and 6) have been structurally characterized
by X-ray diffraction. In addition, the radical anion complexes 3, 4, and 6 were characterized by their EPR spectra.
■
INTRODUCTION
In 1975, tom Dieck et al. surprised the coordination chemistry
community with a paper entitled “2,2′-Bipyridyl - a “bad ligand”
for metals in low oxidation states”.1 In this paper the authors
demonstrated that the π-acceptor capacity of certain 1,4-diaza1,3-diene (=DAD) ligands such as tBuNCHCHNtBu is
about twice as high as that of the traditionally used 2,2′bipyridine. This particular contribution by one of the pioneers
in the field had undoubtedly a significant impact on the
development of DAD coordination chemistry. Today, 1,4-diaza1,3-dienes are well established as highly versatile ligands for
nearly every element in the periodic table. The list of stable
DAD complexes encompasses main-group metals,2 early3 and
late4 transition metals, and the lanthanides and actinides.5 A
unique electronic property of the 1,4-diaza-1,3-dienes is that
they are redox-noninnocent and can undergo one- and twoelectron-reduction steps to afford the corresponding radical
anions and the enediamide dianions, respectively, as shown in
Scheme 1.6
This electronic flexibility combined with the possibility of
tuning the steric properties of the diazadiene moiety by
introducing various substituents at both carbon and/or
nitrogen account for the high versatility of DAD ligands in
coordination and organometallic chemistry. Scheme 2 illustrates different coordination modes of DAD ligands which have
been reported in the literature.2−6 The neutral σ2-chelating
coordination mode is most common with late transition metals,
especially in low coordination states. Both the dianionic
enediamide derivatives and the σ2-coordinated radical-anion
© 2013 American Chemical Society
Scheme 1. Stepwise Reduction of 1,4-Diaza-1,3-dienes To
Give the Corresponding Radical Anions and Enediamide
Dianions
complexes are known for various s-, p-, and d-block metals and
the first-row transition metals. In the vast majority of all metal
complexes the diazadienes act as chelating ligands (Scheme 2).
In general, nonchelating coordination modes are also possible,5i
but examples are exceedingly rare.
The chemistry of alkaline-earth-metal DAD complexes has
been fairly well developed, especially for magnesium and
calcium.7,8 A large body of research in this area comprises
group 2 metal complexes of rigid acenaphthene-based DAD
ligands.8 The majority of these complexes have been prepared
by salt metathesis reactions between the metal dihalides and
alkali-metal DAD precursors. Yet another suitable synthetic
route is the direct metalation of diazadienes by alkaline-earth
metals. While all coordination modes depicted in Scheme 2
have already been realized with alkaline-earth metals, it
Received: June 28, 2013
Published: August 13, 2013
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Scheme 3. Direct Synthesis of DADDipp Complexes of Sr and
Ba from the Metals
Scheme 2. Different Coordination Modes of 1,4-Diaza-1,3diene ligands
becomes evident that the radical anion and enediamide type
complexes generally prevail.7 Among the group 2 metals, the
DAD coordination chemistry of the heavier elements strontium
and barium remains the least developed.7b,8c A recent paper by
Mashima et al. reported two synthetic protocols for preparing
complexes of Ca, Sr, and Ba with the bulky DAD ligand N,N′bis(diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp).9
Salt metathesis reactions of the metal diiodides with 1 equiv of
the dipotassium salt of DADDipp gave mono- and dinuclear
group 2 metal complexes containing the DADDipp ligand in its
dianionic enediamide form. The second route involved direct
metalation of DADDipp with the respective alkaline-earth-metal
powders in the presence of iodine. This method yielded again
the enediamide type complexes (DADDipp)M(THF)4 for M =
Ca, Sr, while barium afforded the iodide-bridged binuclear
complex [(DADDipp)Ba(μ-I)(THF)2]2 with the DADDipp ligand
coordinated to barium as a radical anion.9
We report here the synthesis and structural characterization
of three new DADDipp complexes of Sr and Ba made by direct
metalation in the absence of iodine. Moreover, we discovered
that the barium complex (DADDipp)2Ba(DME) can be
successfully employed as a reagent for transferring the DAD
radical anion from the alkaline-earth-metal center to a rareearth metal.
DME ligands in the molecule, which was already indicative for
enediamide dianion coordination of the diazadiene (coordination mode C in Scheme 2). The highest peak in the EI mass
spectrum at m/z 464 (35% relative intensity) could be assigned
to the fragment M+ − 2DME, i.e. [(DADDipp)Sr]+. The 13C
NMR spectrum of 2 showed a corresponding NCH
resonance at δ 114.5, which is in good agreement with the
value reported for (DADDipp)Sr(THF)4 (δ 112.7).9 In the 1H
NMR spectrum, the isopropyl protons of the Dipp substituents
gave rise to a broad signal at δ 3.98 and a broad singlet at δ
1.09. A very broad singlet at δ 5.12 in the 1H NMR spectrum
(THF-d8) could be assigned to the NCH protons of the
diazadiene backbone, a value typical for dianionic enediamide
ligation of DAD’s (cf. δ 5.47 (C6D6) reported for (DADDipp)Sr(THF)4).9 Particularly notable is the broadening of virtually
all signals in the 1H NMR spectrum measured at room
temperature. In particular, the resonance of the olefinic protons
(δ 5.12) was barely distinguishable from the baseline. The same
phenomenon has already been noted by Mashima et al. for the
complexes [K(DAD Dipp )(THF) 2 Ba(μ-I)(THF) 2 ] 2 and
(DADDipp)M(THF)4 (M = Ca, Sr) bearing the DAD ligands
in the dianionic enediamide-type coordination mode.9
A single-crystal X-ray diffraction study confirmed the
presence of the monomeric strontium diazadiene complex
(DADDipp)Sr(DME)2 (2). Figure 1 depicts the molecular
structure of 2. Crystallographic data for 2−4 and 6 are given in
Table 1 in the Supporting Information. The coordination of
chelating DADDipp and two DME molecules leads to a
distorted-octahedral coordination geometry around the central
strontium. The Sr−N distances are Sr−N1 = 2.494(2) Å and
Sr−N2 = 2.419(3) Å. These can be favorably compared to the
Sr−N bond lengths reported for (DADDipp)Sr(THF)4 (Sr−N =
2.475(4) and 2.458(5) Å).9 The Sr−O distances fall in the
narrow range of 2.537(2)−2.637(2) Å and are unexceptional
(cf. Sr−O = 2.448(2)−2.613(4) Å in (DADDipp)Sr(THF)4).9
The most important structural feature of 2 is the long−short−
long sequence in the diazadiene backbone corresponding to a
−NCHCHN− bonding situation which is typical for
dianionic enediamide coordination of DAD ligands (coordination mode C in Scheme 2). Thus with a value of C1−C2 =
1.363(5) Å the central C−C bond is shorter than the two C−N
bonds (N1−C1 = 1.384(5) Å, N2−C2 = 1.407(4) Å). Once
■
RESULTS AND DISCUSSION
Parallel to the investigations by Mashima et al.9 we also studied
the formation of new DAD complexes of the heavy alkalineearth metals Sr and Ba by direct metalation of N,N′bis(diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp).
The main difference was that pure Sr and Ba metals were used
as chips without activation by addition of iodine. The results are
summarized in Scheme 3. When a solution of 1 in DME (=1,2dimethoxyethane) was stirred with an excess of Sr chips, a deep
red color began to develop already after ca. 1 h. In order to
ensure complete reaction, stirring was continued for 1 week.
Two new strontium DAD complexes could be isolated by
fractional crystallization directly from the reaction mixture after
removal of unreacted Sr metal.
A minor component (28% yield) in the product mixture was
a red compound which was shown by elemental analysis,
spectroscopic data (1H and 13C NMR, MS, IR), and X-ray
diffraction to be the enediamide derivative (DADDipp)Sr(DME)2 (2). Separation of 2 from the second reaction product
could be readily achieved, due to its significantly lower
solubility in DME. Both elemental analysis and NMR data
were consistent with the presence of one DADDipp and two
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Figure 1. Molecular structure of (DADDipp)Sr(DME)2 (2). Selected
bond lengths (Å) and angles (deg): Sr−N1 = 2.494(2), Sr−N2 =
2.419(3), N1−C1 = 1.384(5), N2−C2 = 1.407(4), C1−C2 =
1.363(5), Sr−O = 2.537(2)−2.637(2); N1−Sr−N2 = 74.93(9), Sr−
N1−C1 = 104.6(2), Sr−N2−C2 = 106.7(2), N1−C1−C2 = 126.1(3),
N2−C2−C1 = 125.2(4); (N1SrN2)−(N1C1C2N2) = 14.6.
Figure 2. Molecular structure of (DADDipp)2Sr(DME) (3). Selected
bond lengths (Å) and angles (deg): Sr−N1 = 2.613(2), Sr−N2 =
2.600(3), Sr−N3 = 2.626(3), Sr−N4 = 2.613(2), N1−C1 = 1.321(4),
N2−C2 = 1.330(4), N3−C31 = 1.326(4), N4−C32 = 1.325(4), C1−
C2 = 1.396(5), C31−C32 = 1.403(5), Sr−O1 = 2.578(2), Sr−O2 =
2.588(2); N1−Sr−N2 = 67.64(8), N3−Sr−N4 = 66.26(8), Sr−N1−
C1 = 110.4(2), Sr−N2−C2 = 110.2(2), Sr−N3−C31 = 111.6(2), Sr−
N4−C32 = 112.2(2).
again there is excellent agreement with the corresponding
values reported for (DADDipp)M(THF)4 (M = Sr, Ba).9 These
structural details in combination with the spectroscopic results
clearly confirmed the dianionic enediamide coordination of the
DADDipp ligand in 2.
The major product of the reaction of DADDipp with Sr metal
in DME was isolated with 68% yield in the form of dark red
(almost black) crystals by crystallization from the concentrated
mother liquor after separation of 2. The fact that no meaningful
1
H NMR spectrum could be obtained gave an early indication
for the presence of a radical anion DAD complex. The highest
peak in the EI mass spectrum at m/z 929 (30% relative
intensity) corresponded to the molecular ion of the bis(DAD)
complex (DADDipp)2Sr(DME) (3). The peak with 100%
relative intensity at m/z 840 could be easily assigned to the
unsolvated ion [(DADDipp)2Sr]+ formed by loss of the
coordinated DME, while a peak at m/z 464 (82%)
corresponded to [(DADDipp)Sr]+. Other prominent peaks
were assigned to the fragment ions [DADDipp]+ (m/z 378,
18%) and [DADDipp − C3H7]+ (m/z 333, 30%). A single-crystal
X-ray diffraction study confirmed the presence of the strontium
radical anion DAD complex (DADDipp)2Sr(DME) (3) (Figure
2). The coordination geometry around the central Sr atom can
again be best described as distorted octahedral. In contrast to
the long−short−long bonding sequence for the N−C−C−N
backbone of the enediamide type coordinated DADDipp ligand
in 2, these three bonds are equalized in 3 (N1−C1 = 1.321(4)
Å, N2−C2 = 1.330(4) Å, N3−C31 = 1.326(4) Å). This is
typical for the radical anionic coordination of the DAD ligand
(coordination mode B in Scheme 2). In the Ba complex
[(DADDipp)Ba(μ-I)(THF)2]2, which also contains monoanionic DAD ligands, the central C−C bond (1.401(7) Å) is
even longer than the C−N bonds (1.343(7) and 1.314(7) Å).9
The Sr−N distances fall in the narrow range of Sr−N
2.600(3)−2.626(3) Å and are thus significantly longer than
those in 2 (Sr−N1 = 2.494(2) Å and Sr−N2 = 2.419(3) Å) and
(DADDipp)Sr(THF)4 (Sr−N = 2.475(4) and 2.458(5) Å).9 The
Sr−O distances are unexceptional with Sr−O1 = 2.578(2) Å
and Sr−O2 = 2.588(2) Å (cf. Sr−O 2.448(2)−2.613(4) Å in
(DADDipp)Sr(THF)4).9 The predominant formation of radical
anionic 3 in the reaction of DADDipp with Sr metal in DME is
markedly different from the same reaction carried out in the
presence of I2 (1 mol %), which led to exclusive formation of
the enediamide derivative (DADDipp)Sr(THF)4, as reported by
Mashima et al.9
In line with these observations, the analogous reaction of 1
with an excess of barium chips in DME yielded exclusively the
new radical anion complex (DADDipp)2Ba(DME) (4). Stirring
of the reaction mixture for 1 week at room temperature
produced a deep red solution, from which (DADDipp)2Ba(DME) (4) could be isolated in the form of deep red (almost
black) crystals in 92% yield. In this case, the formation of the
enediamide derivative (DADDipp)Ba(DME)2 was not observed.
Due to the radical anion character of the DADDipp ligands in 4,
no meaningful NMR spectra (1H, 13C) could be obtained for
this compound. The highest peak in the EI mass spectrum at
m/z 890 (30% relative intensity) corresponded to the fragment
M+ − DME, i.e. [(DADDipp)2Ba]+. Other prominent peaks
could be readily assigned to the fragment ions [(DADDipp)Ba]+
(m/z 514, 25%), [DADDipp]+ (m/z 378, 40%), and [DADDipp −
C3H7]+ (m/z 333, 100%). The molecular structure of 4 (Figure
3) confirmed the formation of the barium analogue of 3 and the
first mononuclear barium DAD complex. With an average of
2.769 Å the Ba−N distances in 4 are slightly longer than those
in [K(DADDipp)(THF)2Ba(μ-I)(THF)2]2 (Ba−N = 2.720(4)
and 2.706(4) Å). This minor elongation might be traced back
to higher steric congestion in 4 as compared to [K(DADDipp)(THF)2Ba(μ-I)(THF)2]2 in which only one bulky DADDipp
ligand is coordinated to each barium ion. The Ba−O bond
lengths to the coordinated DME in 4 are 2.771(2) and
2.753(2) Å.
The two radical anion complexes 3 and 4 were also
characterized by their EPR spectra. As shown in Figures 4
and 5, the EPR spectra of 3 and 4 in toluene display a sevenline pattern due to hyperfine coupling (hfc) to the 14N and 1H
atoms of the ligand. Simulation of the EPR spectrum of 4
reveals hfc constants of αN = 5.5 G for two 14N centers and αH
= 5.1 G for two 1H atoms. These values are in good agreement
with the hfc constants reported for the barium complex
published by Mashima et al.9 The EPR spectrum of 3 can be
simulated with the same parameters. However, the spectrum
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Figure 3. Molecular structure of (DADDipp)2Ba(DME) (4). Selected
bond lengths (Å) and angles (deg): Ba−N1 = 2.770(2), Ba−N2 =
2.769(2), Ba−N3 = 2.758(2), Ba−N4 = 2.779(2), N1−C1 = 1.322(3),
N2−C2 = 1.327(3), N3−C31 = 1.328(3), N4−C32 = 1.326(3), C1−
C2 = 1.396(3), C31−C32 = 1.406(3), Ba−O1 = 2.771(2), Ba−O2 =
2.753(2); N1−Ba−N2 = 62.56(5), N3−Ba−N4 = 63.03(5), Ba−N1−
C1 = 112.7(2), Ba−N2−C2 = 112.4(2), Ba−N3−C31 = 113.4(2),
Ba−N4−C32 = 112.9(2).
Figure 5. (a) Experimental EPR spectrum of the barium complex 4 in
toluene at room temperature. (b) Simulated EPR spectrum with
(hyperfine coupling) hfc constants αN = 5.5 G for the two 14N centers
and αH = 5.1 G for the 1H atoms (g = 2.0027, ν = 9.770218 GHz).
the 12-membered Si4O6Li2 inorganic ring system but are
significantly displaced, leading to a series of bis(disiloxanediolate) complexes which have been termed
“inorganic lanthanide metallocenes”.11,12 It was shown that
this new class of heterobimetallic lanthanide disiloxanediolates
shares structural similarities with the well-known bent metallocenes containing pentamethylcyclopentadienyl (=C5Me5)
ligands. The latter form a large and well-investigated class of
organolanthanides, with many of them displaing high catalytic
activities in various olefin transformations.13 In both cases two
bulky ligands are coordinated to the central lanthanide ion in a
bent geometry, leaving room for functional groups such as the
chloro ligand in [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5). Thus,
we reasoned that replacement of the chloro functionality by
monoanionic DAD ligands should provide access to an
interesting new class of lanthanide bis(disiloxanediolates) of
the type [{(Ph2SiO)2O}2{Li(THF)2}2]Ln(DAD). These would
complement the well-known lanthanide metallocene derivatives
(C5Me5)2Ln(DAD).5
In fact, treatment of [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5)
with the barium reagent 4 (molar ratio 2:1) in toluene
according to Scheme 4 at reflux temperature led to the
development of a red solution accompanied by formation of a
fine white precipitate of BaCl2. Filtration and subsequent
crystallization directly from the concentrated filtrate afforded
the desired product [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]Ho(DADDipp) (6) as air-sensitive, orange-red prisms in 57%
isolated yield.
The isolation of 6 was facilitated by the virtual insolubility of
the barium chloride byproduct in toluene. Because of the
strongly paramagnetic nature of the Ho3+ ion as well as the
presence of a radical anionic DADDipp ligand, no useful NMR
data (1H, 13C, 29Si) could be gathered for 6. In this case, the EI
mass spectrum was also uninformative, showing only the
fragment ion m/z 333 [DADDipp − C3H7]+ as the highest peak.
Fortunately, X-ray-quality single crystals of 6 could be readily
grown by slow cooling of a saturated solution in toluene to 2
°C. The X-ray diffraction study (Figure 6) clearly established
the successful formation of the target compound
[{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DADDipp) (6). For the first
time, transfer of a DAD radical anion ligand from an alkaline-
Figure 4. (a) Experimental EPR spectrum of the strontium complex 3
in toluene at room temperature. (b) Simulated EPR spectrum with the
same (hyperfine coupling) hfc constants as for the barium complex, αN
= 5.5 G for the two 14N centers and αH = 5.1 G for the 1H atoms (g =
2.0027, ν = 9.769633 GHz).
shows additional lines or splitting which cannot easily be
explained and might be due to impurities.
In the course of this investigation we wondered if the new
alkaline-earth-metal DAD complexes could perhaps be used as
reagents to transfer the DAD ligand to other metals. As a first
example in this direction, we studied the reaction between the
barium DADDipp complex 4 and the chloro-functionalized
holmium bis(disiloxanediolate) derivative [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5).10 Such heterobimetallic lanthanide bis(disiloxanediolates) have been extensively investigated by our
group in recent years.11,12 It was found that the small Sc3+ and
Y3+ ions form heterobimetallic complexes in which the group 3
metal fits into the center of a 12-membered Si4O6Li2 inorganic
ring system formed by two lithium disiloxanediolate units.
Additional chloro functionalities and solvent molecules are
arranged in trans positions. In contrast, medium and large Ln3+
ions such as Pr3+, Sm3+, and Ho3+ do not fit into the center of
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Scheme 4. Synthesis of the Holmium DADDipp Complex 6 via Ligand Transfer from 4
Figure 6. Molecular structure of [{(Ph2SiO)2O}2{Li(THF)}2]Ho(DADDipp) (6). Selected bond lengths (Å) and angles (deg): Ho−
Osilox = 2.243(3), 2.273(2), Ho−N1 = 2.462(3), Ho−N2#1 =
2.462(3), N1−C1 = 1.319(5), C(1)−C(2)#1 = 1.398(9), Li−Osilox
= 1.864(7), 1.889(8), 1.881(8); Osilox−Ho−Osilox = 122.12(14),
85.28(9), 75.86(9), N1−Ho−N2#1 = 69.21(15). Symmetry transformations used to generate equivalent atoms: (#1) −x + 3/2, y, −z +
3
/2.
Figure 7. (a) Experimental EPR spectrum of the Holmium complex 6
in toluene at room temperature. (b) Simulated EPR spectrum with
(hyperfine coupling) hfc constants αN = 6.1 G for the two 14N centers
and αH = 5.1 G for the 1H atoms (g = 2.0033, ν = 9.769995 GHz).
barium complex 4 but has slightly different hfc constants and a
broader line width. Simulation of the experimental spectrum of
6 results in the hfc constants αN = 6.1 G for the two 14N centers
and αH = 5.1 G for the 1H atoms.
earth metal to a rare-earth metal had been achieved with the
synthesis of 6.
As illustrated in Figure 6, the molecular structure of 6 adopts
a highly distorted octahedral coordination geometry and
features a monoanionic DADDipp ligand. Due to the presence
of two very bulky ligands, i.e. the 12-membered
[{(Ph2SiO)2O}2{Li(THF)2}2]2− ring system and the sterically
demanding DADDipp radical anion, the central Ho3+ ion is
sterically saturated without addition of a coordinating solvent.
The bonding situation within the diazadiene backbone
comprises shortened C−N bond lengths (1.319(5) Å) and an
elongated central C−C bond length (1.398(9) Å) in
accordance with the monoanionic (radical anionic) character
of the DADDipp ligand in 6. These values are also in good
agreement with those reported for [(DADDipp)Ba(μ-I)(THF)2]2 (vide supra)9 or the previously reported scandium
DAD radical anion complex (DADDipp)ScCl2(THF)2 (C−N =
1.338(2) and 1.337(2) Å, C−C = 1.399(2) Å).5l The Ho−N
distances in 6 (2.462(3) Å) are in good agreement with those
reported for other organoholmium complexes comprising Ho−
N bonds.14 At 2.243(3) and 2.273(2) Å the Ho−Osilox bond
lengths in 6 are slightly elongated as compared to those in the
chloro precursor 5 (Ho−Osilox = 2.224(4) and 2.205(4) Å) due
to replacement of the chloro ligand by the bulky DADDipp
radical anion. The radical anionic coordination mode of the
diazadiene ligand in 6 was also proven by the EPR spectrum
(Figure 7). The spectrum looks quite similar to that of the
■
CONCLUSIONS
In summarizing the work reported here, we succeeded in the
preparation of three new diazadiene complexes of strontium
and barium by direct metalation. In the case of strontium, both
monoanionic and dianionic coordination of the redox-active
ligand DADDipp was observed, whereas with barium only the
radical anion complex (DADDipp)2Ba(DME) (4) could be
isolated in excellent yield (92%). These results nicely
complement recent findings by Mashima et al.,9 making various
types of diazadiene complexes of the heaviest alkaline earth
metals readily available. Moreover, transfer of a DAD radical
anion ligand from an alkaline-earth metal to a rare-earth metal
has been achieved for the first time. Reaction of 4 with
[{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) afforded the novel
(DAD)holmium bis(disiloxanediolate) complex
[{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DADDipp) (6) as the first
representative of a new class of DAD complexes. All new
complexes (2−4 and 6) have been structurally characterized by
X-ray diffraction. The radical anionic coordination mode of the
DAD ligand in 3, 4, and 6 was also proven by EPR
spectroscopy.
■
EXPERIMENTAL SECTION
General Procedures. All operations were performed with rigorous
exclusion of air and water in oven-dried or flame-dried glassware under
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an inert atmosphere of dry argon, employing standard Schlenk, highvacuum, and glovebox techniques (MBraun MBLab; <1 ppm O2, <1
ppm H2O). THF, DME, and toluene were dried over sodium/
benzophenone and freshly distilled under a nitrogen atmosphere prior
to use. All glassware was oven-dried at 120 °C for at least 24 h,
assembled while hot, and cooled under high vacuum prior to use. The
starting materials N,N′-Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (1; abbreviated DADDipp, Dipp = 2,6-diisopropylphenyl)15 and
[{(Ph2SiO)2O}2{Li(THF)2}2]HoCl(THF)2 (5)11 were prepared
according to published procedures. Strontium and barium metal
chips were purchased from Aldrich and used as received. The NMR
spectra were recorded in C6D6 or THF-d8 solutions on a Bruker DPX
600 (1H, 600.1 MHz; 13C, 150.9 MHz) or a Bruker-AVANCEDMX400 instrument (5 mm BB; 1H, 400.1 MHz; 13C, 100.6 MHz),
1
H and 13C shifts are referenced to internal solvent resonances and
reported in parts per million relative to TMS. IR (KBr) spectra were
measured using a Perkin-Elmer FT-IR 2000 spectrometer. Mass
spectra (EI, 70 eV) were run on a MAT 95 apparatus. X-band EPR
spectra were recorded on a Bruker Elexsys E500 EPR spectrometer
with an ER077R magnet (75 mm pole cap distance) and an ER047
XG-T microwave bridge. The EPR spectra were measured in toluene.
Microanalyses of the compounds were performed using a Leco CHNS
923 apparatus. In the case of compounds 2−4 the carbon values were
found to be off by about 1% (lower). This might be traced back to the
formation of MCO3 (M = Sr, Ba) during the combustion analysis.
Synthesis of (DADDipp)Sr(DME)2 (2) and (DADDipp)2Sr(DME)
(3). A 2.5 g portion (6.6 mmol) of DADDipp (1) was dissolved in 100
mL of DME, and 0.61 g (7.0 mmol, in excess) of strontium chips was
added. The reaction mixture was stirred at room temperature with a
glass-coated stirring bar. After 1 h the reaction mixture turned from
yellow to deep red. The reaction mixture was stirred for 1 week, and
the unconsumed strontium together with a gray precipitate was
removed by filtration. Partial evaporation of 30 mL of DME and
cooling of the deep red solution to 5 °C afforded 1.2 g (28%) of
(DADDipp)Sr(DME)2 (2) as red crystals. The crystalline compound
was removed, the volume of the solution was reduced to 40 mL, and
the solution was stored for 1 week at a temperature of 2 °C to form 2.1
g (68%) of dark red (almost black) crystals of [(DADDipp)2Sr(DME)]
(3). The overall conversion of DADDipp to (DADDipp)Sr(DME)2 and
(DADDipp)2Sr(DME) was virtually quantitative (ca. 96%).
Data for (DADDipp)Sr(DME)2 (2) are as follows. Anal. Calcd for
C34H56N2O4Sr (Mr = 644.4): C, 63.37; H, 8.76; N, 4.35. Found: C,
62.47; H, 8.97; N, 4.43. 1H NMR (400 MHz, d8-THF, 193 K): 1.09
(s(br), 24H, −CH(CH3)2), 3.24 (s, 12H, DME), 3.80 (s(br), 4H,
−CH(CH3)2), 5.12 (s, 2H, CHN), 6.23 (trip, 3JHH = 7.0 Hz, 2H,
para-Ar), 6.66 (d, 3JHH = 7.0 Hz, 4H, meta-Ar) ppm. 13C NMR (100
MHz, d8-THF, 253 K): 25.3−25.7 (−CH(CH3)2), 28.15, 28.19
(−CH(CH3)2), 59.0 (DME), 72.6 (DME), 114.5 (NCH), 120.3
(para-Ar), 122.8 (meta-Ar), 140.9 (ortho-Ar), 158.2 (ipso-Ar) ppm.
Mass spectrum (EI): m/z 464 (35%) [M+ − 2DME], 378 (55%)
[DADDipp], 333 (70%) [DADDipp − C3H9], 203 (35%), 188 (80%),
162 (60%), 178 (38%), 146 (100%). IR (KBr disk): ν 3034 m, 2998
m, 2954 vs, 2865 s, 1662 w, 1628 w, 1581 s, 1546 w, 1458 s, 1417 vs,
1373 vs, 1334 s, 1314 vs, 1265 vs, 1191 m, 1136 m, 1112 vs, 1096 s,
1065 vs, 1025 m, 1005 m, 911 w, 863 s, 851 m, 793 m, 760 m, 728 m,
685 w, 549 w cm−1. Mp: 143 °C dec.
Data for (DADDipp)2Sr(DME) (3) are as follows. Anal. Calcd for
C56H82N4O2Sr (Mr = 930.9): C, 72.25; H, 8.88; N, 6.02. Found: C,
71.37; H, 8.75; N, 6.53%. 1H NMR (600 MHz, C6D6, 298 K): because
of the paramagnetic properties of the radical anionic DAD, it was
impossible to obtain meaningful 1H NMR spectra. 13C NMR (150
MHz, C6D6, 298 K): 23.5, 23.6, 23.7, 24.2, 24.3, 24.4, 28.0, 28.3, 28.5
(−CH(CH3)2), 33.0 (br, −CH(CH3)2), 85.5 (br, DME), 81.8 (br,
DME), 123.4, 123.8, 124.1, (para-Ar), 125.6 (meta-Ar), 136.6 (orthoAr), 148.7 (ipso-Ar), 163.2 (CN) ppm. Mass spectrum (EI): m/z
929 (30%) [M+], 840(100%) [M+ − DME], 464 (82%) [M+ − DME,
DADDipp], 378 (18%) [DADDipp], 333 (30%) [DADDipp − C3H7]. IR
(KBr disk): ν = 3058 m, 2959 s, 2867 m, 1668 w, 1628 m, 1588 w,
1539 w, 1457 s, 1433 s, 1382 m, 1361 m, 1314 m, 1261 s, 1178 w,
1159 w, 1108 w, 1076 m, 1036 w, 926 m, 884 w, 868 w, 837 w, 819 w,
794 m, 756 s, 679 w, 525 w cm−1. Mp: 160 °C dec.
Synthesis of (DADDipp)2Ba(DME) (4). A 2.5 g portion (6.6 mmol)
of DADDipp (1) was dissolved in 100 mL of DME, and 1.0 g (7.3
mmol, in excess) of barium chips was added. The reaction mixture was
stirred at room temperature with a glass-coated stirring bar. After 1 h
the reaction mixture changed from yellow to deep red. The reaction
mixture was stirred for 1 week, and the unconsumed barium together
with a gray precipitate were removed by filtration. Partial evaporation
of 40 mL of DME and cooling of the deep red solution to 2 °C
afforded (DADDipp)2Ba(DME) (4) as deep red (almost black) crystals.
In this case the formation of the enediamide derivative (DADDipp)Ba(DME)2 was not observed. Yield: 3.0 g (92%). Anal. Calcd for
C56H82BaN4O2 (Mr = 980.6): C, 68.59; H, 8.43; N, 5.71. Found: C,
67.72; H, 8.21; N, 6.13. Because of the paramagnetic properties of the
radical anionic DAD, it was impossible to obtain meaningful 1H and
13
C NMR spectra. Mass spectrum (EI): m/z 890 (30%) [M+ − DME],
514 (25%) [M+ − DME, DADDipp], 378 (40%) [DADDipp], 333
(100%) [DADDipp − C3H7]. IR (KBr disk): ν 3052 w, 3011 w, 2961 vs,
2927 sh, 2867 m, 2709 w, 1664 w, 1626 m, 1588 w, 1541 w, 1458 s,
1431 s, 1381 m, 1361 m, 1314 s, 1264 vs, 1196 w, 1180 w, 1158 w,
1107 w, 1070 m, 1054 w, 1037 w, 1026 w, 982 w, 958 w, 924 m, 884
w, 859 w, 835 m, 794 s, 755 s, 525 w cm−1. Mp: 155 °C dec.
Synthesis of [{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DADDipp) (6). A 0.7 g
portion (0.475 mmol) of [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) and
0.25 g (0.25 mmol) of (DADDipp)2Ba(DME) (4) were dissolved in 50
mL of toluene. The reaction mixture was stirred overnight, refluxed for
2 h, and filtered through a P4 glass frit to separate a white precipitate
(BaCl2) from the clear red solution. After the volume of the solution
was reduced under vacuum to 5 mL, the product crystallized in the
form of orange-red prisms. Yield: 0.5 g (57%). Anal. Calcd for
C82H92HoLi2N2O8Si4 (Mr = 1524.8): C, 64.59; H, 6.08; N, 1.84.
Found: C, 64.31; H, 5.89; N, 2.27. Because of the strongly
paramagnetic nature of the Ho3+ ion, it was impossible to obtain
meaningful 1H, 13C, and 29Si NMR data. Mass spectrum (EI): m/z 333
(100) [DADDipp − C3H7], 378 (10) [DADDipp], 439 (25), 517 (7),
559 (1), 594 (1), 637 (8), 715 (3), 792 (0.5). IR (KBr disk): 3067 m,
3048 m, 2999 m, 2962 s, 2869 m, 1959 w, 1892 w, 1825 w, 1774 w,
1625 m, 1590 m, 1568 w, 1460 s, 1428 vs, 1384 m, 1362 m, 1316 m,
1249 s, 1181 m, 1156 m, 1119 vs, 1083 s, 1034 vs, 1016 vs, 994 vs, 821
w, 797 m, 742 s, 701 vs, 683 m, 620 w, 531 vs, 492 s cm−1. Mp: 95 °C
dec.
X-ray Crystallographic Studies. Single crystals of 2−4 were
obtained from saturated solutions in DME at 2−5 °C. In the case of 6,
single crystals were grown by cooling a saturated toluene solution to 2
°C. The intensity data of 2−4 and 6 were collected on a Stoe IPDS 2T
diffractometer with Mo Kα radiation. The data were collected with the
Stoe XAREA16 program using ω scans. The space group was
determined with the XRED3216 program. The structure was solved
by direct methods (SHELXS-97) and refined by full-matrix leastsquares methods on F2 using SHELXL-97.17 Data collection
parameters are given in Table 1 in the Supporting Information.
■
ASSOCIATED CONTENT
S Supporting Information
*
CIF files and tables giving X-ray structural data for 2−4 and 6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*F.T.E.: tel, (+49)-391-6718327; fax, (+49)-391-6712933; email, [email protected].
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/om400622d | Organometallics 2013, 32, 4636−4642
Organometallics
■
Article
Cherkasov, V. K. Chem. Eur. J. 2007, 13, 4981. (g) Walter, M. D.; Berg,
D. J.; Andersen, R. A. Organometallics 2007, 26, 2296. (h) Cui, P.;
Chen, Y.; Wang, G.; Li, G.; Xia, W. Organometallics 2008, 27, 4013.
(i) Makhrova, T. V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A.
Russ. Chem. Bull. 2008, 57, 2285. (j) Panda, T. K.; Pal, K.; Tsurugi, H.;
Mashima, K. Kidorui 2009, 54, 202. (k) Mahrova, T. V.; Fukin, G. K.;
Cherkasov, A. V.; Trifonov, A. A.; Ajellal, N.; Carpentier, J.-F. Inorg.
Chem. 2009, 48, 4258. (l) Panda, T. K.; Kaneko, H.; Pal, K.; Tsurugi,
H.; Mashima, K. Organometallics 2010, 29, 2610 and references cited
therein. (m) Kaneko, H.; Nagae, H.; Tsurugi, H.; Mashima, K. J. Am.
Chem. Soc. 2011, 133, 19626. (n) Trifonov, A. A.; Shestakov, B. G.;
Lyssenko, K. A.; Larionova, J.; Fukin, G. K.; Cherkasov, A. V.
Organometallics 2011, 30, 4882.
(6) (a) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21,
151. (b) Vrieze, K. J. Organomet. Chem. 1986, 300, 307. (c) Mealli, C.;
Ienco, A.; Phillips, A. D.; Galindo, A. Eur. J. Inorg. Chem. 2007, 2556
and references cited therein.
(7) (a) Rieckhoff, M.; Pieper, U.; Stalke, D.; Edelmann, F. T. Angew.
Chem., Int. Ed. 1993, 32, 1079. (b) Lorenz, V.; Neumüller, B.; Thiele,
K.-H. Z. Naturforsch., B: Chem. Sci. 1995, 50, 71. (c) Liu, Y.; Yang, P.;
Yu, J.; Yang, X. J.; Zhang, J. D.; Chen, Z.; Chen, Z.; Schaefer, H. F.;
Wu, B. Organometallics 2008, 27, 5830. (d) Liu, Y.; Zhao, Y.; Yang, X.
J.; Li, S.; Gao, J.; Yang, P.; Xia, Y.; Wu, B. Organometallics 2011, 30,
1599 and references cited therein.
(8) (a) Fedushkin, I. L.; Skatova, A. A.; Chudakova, V. A.; Fukin, G.
K.; Dechert, S.; Schumann, H. Eur. J. Inorg. Chem. 2003, 3336.
(b) Fedushkin, I. L.; Khvoinova, N. M.; Skatova, A. A.; Fukin, G. K.
Angew. Chem., Int. Ed. 2003, 42, 5223. (c) Fedushkin, I. L.; Morozov,
A. G.; Rassadin, O. V.; Fukin, G. K. Chem. Eur. J. 2005, 11, 5749.
(d) Fedushkin, I. L.; Skatova, A. A.; Ketkov, S. Y.; Eremenko, O. V.;
Piskunov, A. V.; Fukin, G. K. Angew. Chem., Int. Ed. 2007, 46, 4302.
(e) Fedushkin, I. L.; Eremenko, O. V.; Skatova, A. A.; Piskunov, A. V.;
Fukin, G. K.; Ketkov, S. Y.; Irran, E.; Schumann, H. Organometallics
2009, 28, 3863 and references cited therein.
(9) Panda, T. K.; Kaneko, H.; Michel, O.; Pal, K.; Tsurugi, H.;
Törnroos, K. W.; Anwander, R.; Mashima, K. Organometallics 2012,
31, 3178.
(10) (a) Scholz, J.; Dlikan, M.; Ströhl, D.; Dietrich, A.; Schumann,
H.; Thiele, K.-H. Chem. Ber. 1990, 123, 2279. (b) Kawaguchi, H.;
Yamamoto, Y.; Asaoka, K.; Tatsumi, K. Organometallics 1998, 17,
4380. (c) Zippel, T.; Arndt, P.; Ohff, A.; Spannenberg, A.; Kempe, R.;
Rosenthal, U. Organometallics 1998, 17, 4429. (d) Galindo, A.; del Rio,
D.; Mealli, C.; Ienco, A.; Bo, C. J. Organomet. Chem. 2004, 689, 2847.
(e) De Waele, P.; Jazdzewski, B. A.; Klosin, J.; Murray, R. E.; Theriault,
C. N.; Vosejpka, P. C.; Petersen, J. L. Organometallics 2007, 26, 3896.
(11) Lorenz, V.; Edelmann, A.; Blaurock, S.; Hrib, C.; Edelmann, F.
T. C. R. Chim. 2010, 13, 577.
(12) (a) Lorenz, V.; Fischer, A.; Brüser, W.; Edelmann, F. T.; Jacob,
K.; Gelbrich, T.; Jones, P. G. Chem. Commun. 1998, 2217. (b) Lorenz,
V.; Fischer, A.; Jacob, K.; Brüser, W.; Edelmann, F. T. Chem. Eur. J.
2001, 7, 848. (c) Giessmann, S.; Blaurock, S.; Lorenz, V.; Edelmann, F.
T. Inorg. Chem. 2007, 46, 10383. (d) Giessmann, S.; Blaurock, S.;
Lorenz, V.; Edelmann, F. T. Inorg. Chem. 2007, 46, 10956.
(13) Review: Edelmann, , F. T. Complexes of Scandium, Yttrium
and Lanthanide Elements.. In Comprehensive Organometallic Chemistry
III; Crabtree, , R. H., Mingos, , D. M. P., Eds.; Elsevier: Oxford, U.K.,
2006,; p 1.
(14) Zhou, X.; Huang, Z.; Cai, R.; Zhang, L.; Zhang, L.
Organometallics 1999, 18, 4128.
(15) Jafarbour, L.; Stevens, E. D.; Nolan, S. P. J. Organomet. Chem.
2000, 606, 49.
(16) XAREA Program for Xray Crystal Data Collection (XRED32
included in XAREA); Stoe, Darmstadt, Germany, 2002.
(17) (a) Sheldrick, G. M. SHELXL-97 Program for Crystal Structure
Refinement; Universität Gö ttingen, Gö ttingen, Germany, 1997.
(b) Sheldrick, G. M. SHELXS-97 Program for Crystal Structure
Solution; Universität Göttingen, Göttingen, Germany, 1997.
ACKNOWLEDGMENTS
Financial support by the Otto-von-Guericke-Universität
Magdeburg is gratefully acknowledged.
■
REFERENCES
(1) tom Dieck, H.; Franz, K.-D.; Hohmann, F. Chem. Ber. 1975, 108,
163.
(2) For selected references, see: (a) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.;
Schaefer, H. F.; Liang, Y.; Wu, B. Chem. Commun. 2007, 2363. (b) Yu,
J.; Yang, X.-J.; Liu, Y.; Pu, Z.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B.
Organometallics 2008, 27, 5800. (c) Liu, Y.; Li, S.; Yang, X. J.; Yang, P.;
Wu, B. J. Am. Chem. Soc. 2009, 131, 4210. (d) Bhadbhade, M.;
Clentsmith, G. K. B.; Field, L. D. Organometallics 2011, 29, 6509.
(e) Cloke, F. G. N.; Dalby, C. I.; Henderson, M. J.; Hitchcock, P. B.;
Kennard, C. H.; Lamb, R. N.; Raston, C. L. J. Chem. Soc., Chem.
Commun. 1990, 1394. (f) Kaim, W.; Matheis, W. J. Chem. Soc., Chem.
Commun. 1991, 597. (g) Gardiner, M. G.; Hanson, G. R.; Henderson,
M. J.; Lee, F. C.; Raston, C. L. Inorg. Chem. 1994, 33, 2456.
(h) Clyburne, J. A. C.; Culp, R. D.; Kamepalli, S.; Cowley, A. H.;
Decken, A. Inorg. Chem. 1996, 35, 6651. (I) Rijnberg, E.; Boersma, J.;
Jastrzebski, J. T. B. H.; Lakin, M. T.; Spek, A. L.; van Koten, G.
Organometallics 1997, 16, 3158. (j) Rijnberg, E.; Richter, B.; Thiele, K.H.; Boersma, J.; Veldman, N.; Spek, A. L.; van Koten, G. Inorg. Chem.
1998, 37, 56. (k) Mair, F. S.; Manning, R.; Pritchard, R. G.; Warren, J.
E. Chem. Commun. 2001, 1136. (l) Baker, R. J.; Davis, A. J.; Jones, C.;
Kloth, M. J. Organomet. Chem. 2002, 656, 203. (m) Baker, R. J.; Farley,
R. D.; Jones, C.; Kloth, M.; Murphy, D. M. Dalton Trans. 2002, 3844.
(n) Baker, R. J.; Jones, C.; Kloth, M.; Mills, D. P. New J. Chem. 2004,
28, 207. (o) Baker, R. J.; Jones, C.; Murphy, D. M. Chem. Commun.
2005, 1339. (p) Baker, R. J.; Jones, C.; Kloth, M. Dalton Trans. 2005,
2106. (q) Jones, C.; Stasch, A.; Woodul, W. D. Chem. Commun. 2009,
113.
(3) For leading recent references, see: (a) Mashima, K.; Matsuo, Y.;
Tani, K. Chem. Lett. 1997, 767. (b) Mashima, K.; Matsuo, Y.; Tani, K.
Organometallics 1999, 18, 1471. (c) Amor, F.; Gomez-Sal, P.; Royo, P.;
Okuda, J. Organometallics 2000, 19, 5168. (d) Galindo, A.; Ienco, A.;
Mealli, C. New J. Chem. 2000, 24, 73. (e) Matsuo, Y.; Mashima, K.;
Tani, K. Angew. Chem., Int. Ed. 2001, 40, 960. (f) Mashima, K.;
Nakamura, A. J. Organomet. Chem. 2001, 621, 224. (g) Matsuo, Y.;
Mashima, K.; Tani, K. Organometallics 2002, 21, 138. (h) Daff, P. J.;
Etienne, M.; Donnadieu, B.; Knottenbelt, S. Z.; McGrady, J. E. J. Am.
Chem. Soc. 2002, 124, 3818. (i) Tsurugi, H.; Ohno, T.; Yamagata, T.;
Mashima, K. Organometallics 2006, 25, 3179. (j) Stanciu, C.; Jones, M.
E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129,
12400. (k) Kreisel, K. A.; Yap, G. P. A; Theopold, K. H. Inorg. Chem.
2008, 47, 5293. (l) Tsurugi, H.; Ohno, T.; Kanayama, T.; ArteagaMü ller, R. A.; Mashima, K. Organometallics 2009, 28, 1950.
(m) Tsurugi, H.; Saito, T.; Tanahashi, H.; Arnold, J.; Mashima, K. J.
Am. Chem. Soc. 2011, 133, 18673.
(4) For leading references, see: (a) Johnson, L. K.; Killian, C. M.;
Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (b) Johnson, L. K.;
Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267.
(c) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121,
1974. (d) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686. (e) Johansson, L.;
Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122,
10846. (f) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739.
(g) Zhong, A. H.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002,
124, 1378.
(5) For leading references, see: (a) Recknagel, A.; Noltemeyer, M.;
Edelmann, F. T. J. Organomet. Chem. 1991, 410, 53−61. (b) Poremba,
P.; Edelmann, F. T. J. Organomet. Chem. 1997, 549, 101−104.
(c) Scholz, J.; Görls, H.; Schumann, H.; Weimann, H. Organometallics
2001, 20, 4394. (d) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.;
Baranov, E. V.; Druzhkov, N. O.; Bochkarev, M. N. Chem. Eur. J. 2006,
12, 2752. (e) Moore, J. A.; Cowley, A. H.; Gordon, J. C.
Organometallics 2006, 25, 5207. (f) Trifonov, A. A.; Borovkov, I. A.;
Fedorova, E. A.; Fukin, G. K.; Larionova, J.; Druzhkov, N. O.;
4642
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