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LETTER
Cite this: New J. Chem., 2016,
40, 1923
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Isolation of gravimetrically quantifiable alkali
metal arenides using 18-crown-6†
Maximiliano Castillo, Alejandro J. Metta-Magaña and Skye Fortier*
Received (in Victoria, Australia)
14th October 2015,
Accepted 13th December 2015
DOI: 10.1039/c5nj02841h
www.rsc.org/njc
Small, polyaromatic radical monoanions, e.g. C10H8 , are a valued
class of reagents that are routinely used as potent outer-sphere
reductants. Owing to their high reactivity, these arenides are
typically prepared in solution and used in situ. We have found that
treatment of M[arene ] (M = Li, Na, K; arene = biphenyl, naphthalene, anthracene, perylene) with the 18-crown-6 polyether readily
provides [M(18-c-6)(solvent)n][arene ] as thermally stable, easily
handled, ‘‘bottleable’’ crystalline solids that can be stored indefinitely under inert conditions.
Aromatic hydrocarbons, by virtue of their relatively low-lying p*
LUMO, can be readily reduced by alkali metals to give intensely
coloured, open-shell monoanions. The first well-defined arene
radical, sodium anthracenide, was reported in 1914 by Wilhelm
Schlenk during the formative years of organometallic chemistry.1–3
Over a century later, the chemistry of arene radicals continues to
command attention. From an electronic perspective, these species
display unique aromatic character and have been shown to exhibit
long-range magnetic ordering in the solid-state.4–9 Chemically,
arene radicals find utility in a wide range of applications
including use as initiators in anionic polymerization,10 as
potent bases,3,11 models for graphitic battery materials,12 and
are key intermediates in Birch reductions.13 Arguably, though,
alkali metal arenides are perhaps best known for their use as
potent outer-sphere reducing agents.
While C10H8 , C12H10 , C14H10 , and C20H12 (as their
alkali metal derivatives) are commonly used as powerful reductants,14 the discrete isolation of theses arene radical anions is
seldom performed and hampered by a number of factors. Given
their radical nature, it is well-known that these compounds are
highly sensitive and prone to adventitious oxidation, thus requiring
Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, USA.
E-mail: [email protected]
† Electronic supplementary information (ESI) available: Experimental details,
spectral data, structural figures and crystallographic data for 1–12. CCDC
1431112–1431122. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5nj02841h
preparation and handling under rigorously anaerobic and anhydrous conditions.14 Additionally, these arene radical species are
kinetically unstable, having short storage times, with product
equilibria that are highly solvent dependent.3,15,16 Accordingly,
when employed as reductants, solutions of alkali metal arenides must be freshly prepared in situ immediately prior to use
and necessitate multi-step titrations to accurately determine
concentration.14 Furthermore, attempts to isolate these reductants from solution can result in disproportionation.16 Clearly,
these complicating factors place notable synthetic limitations
which negatively affect the utility of these reagents, especially
the general inability to gravimetrically measure precise stoichiometric amounts.
Impressively, a handful of radical arene reductants, utilizing a hodgepodge of alkali metals and arenes under a variety
of crystallization conditions, have been isolated and characterized in the solid-state.5–9,13,17 Of the relatively few that
have been isolated, Bock and co-workers demonstrated success in synthesizing and crystallographically characterizing
a diverse set of arenides including [Na(triglyme)2][C14H10],7
[M(solvent)n][C20H12] (M = Li, solvent = DME, n = 3; M = Na/K,
solvent = triglyme, n = 2; M = Cs, solvent = tetraglyme, n = 2)18
and others through specific arene, metal and solvent combinations.19,20 More recently, Guijarro et al. were successful
in the solid-state isolation of [Li(tmeda)2][C10H8]8 while Zhou
and co-workers reported the synthesis and structure of
K2(THF)(C10H8).6
These scattershot results, while notable, highlight the lack
of a singular method for the general synthesis and solid-state
isolation of these synthetically valuable reagents, particularly
derivatives of the most popularly used arenides C10H8 ,
C12H10 , C14H10 , and C20H12 . In an attempt to resolve
this issue, we sought to develop a straightforward procedure
for the isolation of common alkali metal arene radical monoanions as well-defined, ‘‘bottleable’’ solids. Herein, we describe
our method and demonstrate proof of principle through the
isolation and structural characterization of twelve alkali metal
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arenides with reduction potentials that range from ca. 2.2 to
3.2 V (vs. Fc0/+).
Initially, we found that storage of concentrated THF
solutions of M[arene ] (M = Li, Na, K; arene = biphenyl,
naphthalene, anthracene) at 25 1C produced large, crystalline blocks of the respective anion within hours. However,
all attempts to isolate these crystals failed as the solids, in
our hands, were observed to rapidly desolvate within seconds
upon removal from solution, often times producing intractable, gummy oils.
We rationalized that desolvation effects could be mitigated
by addition of a chelating base to sequester the alkali metal
cations and protect their coordination sphere from solvent loss.
Accordingly, treatment of M[arene ] in THF with 1 equiv. of
18-crown-6 and subsequent storage at 25 1C affords crystalline solids of [M(18-c-6)(THF)n][arene ] (eqn (1)) in all cases.
Gratifyingly, upon removal from solution and drying under
vacuum, the products retain their shape and form.
While 18-crown-6 has been previously employed for the
successful solid-state isolation of the potassium complexes
[K(18-c-6)(THF)2][C10H8] and [K(18-c-6)(THF)2][C14H10] by Kochi
and Rosokha,5 we have found this common and relatively
inexpensive reagent suitable and convenient for use with both
lithium and sodium metals, thus avoiding the need for specialized and size-specific crown ethers in these reactions.
The number of ancillary solvent molecules in [M(18-c-6)(THF)n][arene ] cannot be readily quantified by NMR spectroscopy due to significant signal broadening – a consequence of
the compounds’ inherent paramagnetism. On the other hand,
the high crystallinity of these compounds makes them amenable to X-ray diffractometry, thus allowing for unambiguous
composition determination (Table 1).
Crystals of [M(18-c-6)(THF)n][arene ] harvested from THF
solutions are typically of satisfactory size and shape for
X-ray crystallographic analyses. In a few instances, most
often with the lithium and sodium salts of naphthalene and
perylene, fine needles too small for crystallographic characterization are produced. However, recrystallization of these
Table 1
compounds from DME solutions does yield X-ray quality
crystals (ESI†).
(1)
arene = C10H8, C12H10, C14H10, C20H12; M = Li, Na, K; n = 0–2.
Examination of the solid-state structures of 1–12 (see Fig. 1
and ESI†) reveals that nearly all crystallize as non-interacting
ion pairs with one notable exception. In 3 (Fig. 1a), the [K(18-c-6)]+
moiety is axially flanked by two bridging [C10H8] anions forming
a close contact network that gives rise to a 1D coordination
polymer. Interestingly, each of the two bridging naphthalenes
exhibits a distinct coordination mode. The first naphthalene
ligates the potassium cations through Z2-binding where the two
K–Carene bond distances (avg. 3.13 Å) and K–Carene dihedral angle
(120.31) are indicative of a typical p–cation interaction. The second
naphthalene engages each potassium through two longer K–Carene
bonds (avg. 3.45 Å) with a notably more obtuse K–Carene
dihedral angle (150.31), parameters that are consistent with
agostic interactions between potassium and the C–H bonds of
the naphthalene.21 In contrast, it should be noted that [K(18-c-6)(THF)2][C10H8] exists as a separated ion pair.5 While the exact
cause of this structural variation is not known, we attribute the
difference to differing crystallization methods and conditions
(ESI†).5
Yields of 1–12 fall within a wide range, from moderate to
excellent (Table 1), with diminished yields most often a result
of solution equilibrium effects or high solubility in THF.3,16
Importantly, and in stark contrast to standing solutions, 1–12
can be stored as solids under nitrogen for extended periods of
time. When kept under strictly anhydrous and anaerobic conditions, we have found solid samples of 1–12 to be unchanged
after almost a year.
In order to demonstrate that the reductive properties of the
twelve arenides isolated in the described fashion were not affected,
the solution redox properties of each complex were examined
Isolated arene radical monoanions and electronic properties
E1/2 a (V)
meff b
(mB)
l (nm)
Compound
% Yield
[Li(18-c-6)][C10H8] (1)
[Na(18-c-6)(DME)][C10H8] (2)
{[K(18-c-6)][m:Z2-C10H8]}N (3)
[Li(k3–18-c-6)(THF)2][C12H10] (4)
[Na(18-c-6)(THF)2][C12H10] (5)
[K(18-c-6)(THF)2][C12H10] (6)
[Li(18-c-6)][C14H10] (7)
[Na(18-c-6)(DME)][C14H10] (8)
37
79
80
42
53
37
30
87
3.09
3.09
3.13
3.18
3.15
3.17
2.53
2.48
[K(18-c-6)(THF)2][C14H10] (9)c
[Li(k3-18-c-6)(DME)][C20H12]0.5C20H12
(10)
[Na(18-c-6)(DME)][C20H12] (11)
[K(18-c-6)(THF)2][C20H12] (12)
31
55
2.49
2.20,
2.87
294,
294,
294,
411,
410,
403,
328,
329,
922
328,
322,
84
38
2.19,
2.28,
2.77
2.80
323, 393, 414, 438, 465, 580, 692, 737, 782, 813, 849, 906, 1010
323, 393, 413, 438, 580, 689, 741, 757, 778, 812, 847, 903, 1008
a
Referenced vs. Fc0/+.
b
318,
327,
326,
643,
451,
649,
348,
342,
327,
373,
374,
829
653,
831
358,
353,
376, 435, 468, 799
442, 469, 798, 875
445, 469, 771, 856
837
369, 407, 550, 598, 641, 659, 696, 735, 759, 813, 921
359, 368, 379, 407, 546, 595, 638, 659, 698, 728, 752, 813,
348, 358, 367, 407, 550, 598, 640, 661, 698, 733, 759, 813, 925
388, 411, 437, 579, 688, 740, 761, 782, 813, 847, 903, 1007
1.53
1.67
2.11
2.08
2.24
2.20
1.67
1.90
1.97
2.20
2.29
2.10
Guoy balance measurement. c Known structure, see ref. 5.
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Fig. 1
Letter
Representative solid-state structures of (a) 3, (b) 4, (c) 8, and (d) 12.
by cyclic voltammetry (CV). In all cases, the compounds exhibit
chemically reversible (ipc/ipa E 1) redox waves with E1/2 values
in full agreement with known reduction potentials (ESI†).3,14
While it has been suggested that the identity of the alkali metal
cation should have a detectable effect on the potential values,14
we observe no systematic effects under our experimental conditions (ESI†). As anticipated, the reducing power of the arene
radical monoanions follows the trend C20H12 o C14H10 o
C10H8 o C12H10 (Table 1). Lastly, while complexes 1–12
each have chemically accessible dianionic forms, we find only
the perylene derivatives 10–12 display a second redox wave in
their CV in THF at room temperature (ESI†).
As each of the four described arene types in 1–12 are
known to exhibit signature electronic absorption features,3,22
the identities of 1–12 were further verified using UV-vis/NIR
absorption spectroscopy (Table 1 and ESI†). Between the complexes within a given arene class (e.g. 1 vs. 2 and 3) the spectra
are qualitatively similar; interestingly, though, the peak definitions and absorbance parameters are found to be cation
dependent (without systematic trend). To the best of our
knowledge, this phenomenon has not been previously reported.
While these observations stand in contrast to the results found
in the respective CV data, the electrochemical experiments are
conducted in the presence of a vast excess of supporting
electrolyte which may impede close M-arene pairing effects.
Finally, the solid-state, room temperature magnetic susceptibilities of the open-shell compounds were measured (Gouy balance).
The effective magnetic moments of 1–12 are unexceptional and
found to range from 1.53 to 2.28mB (Table 1). These values are
comparable to that found for [K2(THF)][C10H8] (1.69mB per anion)
and fall in line with the 1.7mB calculated for an isolated S = 1/2
system.6
In conclusion, we have described for the first time a general
and straightforward procedure for the solid-state isolation of
common arene radical monoanion reductants using 18-crown-6
as a non-cation-specific crystallisation aid. As proof of principle,
we have demonstrated through twelve examples that our methodology can be applied to a wide range of aromatic systems with
varying alkali metal counter cations to give highly crystalline,
well-defined materials. These solids, as compared to their parent
solutions, are remarkably stable, easily stored, readily handled,
gravimetrically quantifiable, and provide good access to potent
outer sphere reducing agents over a wide potential range
( 2.2 to 3.2 V vs. Fc0/+) – attributes which further enhance
the utility of these novel and important radical species.
Experimental†
General synthesis of [M(18-crown-6)(solvent)x]arene
To a stirring solution of the aromatic hydrocarbon in THF was
added freshly cut alkali metal in pieces. The reaction was left to
stir for 4 h during which time the reaction mixture turned a dark,
deep color. At this stage, any unreacted metal was removed and
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18-crown-6 was added to the solution as a solid. After addition of
the crown ether, the reaction mixture was stirred for 15 minutes.
Storage of this solution at 25 1C for 12 hours afforded a crop of
crystals. The mixture was poured over a medium porosity glass
frit and the collected crystals were washed with cold ( 25 1C)
THF (1 5 mL) and cold ( 25 1C) hexanes (1 5 mL). The solid
was dried under vacuum to give dark crystalline material.
Acknowledgements
This research was funded in large part by the University of
Texas at El Paso (UTEP) with added support provided by the
National Science Foundation (NSF) PREM program (DMR-1205302).
M. C. thanks the UTEP NSF-SMARTS program for financial support.
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