A model structure for the resting state of cyanocuprate reagents R2Cu(CN)Li2. The X-ray crystal structure of [Ar2Cu(CN)Li2(THF)4](∞)1 [5]

Full text of DOI:10.1021/ja9802448

9688
J. Am. Chem. Soc. 1998, 120, 9688-9689
point to a preference for structure c10,26 or d.5,8 Molecular weight
determinations by cryoscopy (THF) strongly indicated that this
type of cyanocuprate exists as a discrete monomeric species:
[R2Cu(CN)Li2].6b At this point, it should be emphasized that the
composition of organocopper (and aryllithium) aggregates are
strongly solvent dependent (vide infra).6,11 Our earlier investiga-
tions have shown that the use of the C,N-chelating aminoaryl
anion [C6H4CH2NMe2-2]-,12 instead of the simple parent phenyl
anions has a stabilizing effect on a variety of copper complexes
including organocopper(I) species.13,14 This has allowed the
isolation of well-defined compounds that have been unequivocally
characterized both in the solid state and in solution.
A Model Structure for the Resting State of
Cyanocuprate Reagents R2Cu(CN)Li2. The X-ray
1
Crystal Structure of [Ar2Cu(CN)Li2(THF)4]∞
Claudia M. P. Kronenburg,† Johann T. B. H. Jastrzebski,†
Anthony L. Spek,‡ and Gerard van Koten*,†
Department of Metal-Mediated Synthesis, Debye Institute
Laboratory of Crystal and Structural Chemistry, BijVoet
Center for Biomolecular Research, Utrecht UniVersity
Padualaan 8, 3584 CH Utrecht, The Netherlands
ReceiVed January 21, 1998
We now report on the synthesis, isolation, and structural
characterization (X-ray) of [Ar2Cu(CN)Li2(THF)4]∞ (1, Ar )
[C6H4CH2NMe2-2]-), which represents the first fully elucidated
example of a cyanocuprate of the general stoichiometry R2Cu-
(CN)Li2. Compound 1 is obtained as the only reaction product
when 2 equiv of ArLi is treated with 1 equiv of CuCN in THF at
low temperature (-78 °C; eq 1). Crystals suitable for single-
crystal X-ray diffraction were obtained from a solution of 1 in
THF at -30 °C.
Among organocopper reagents, cyanocuprates are members of
a versatile class of reagents that find useful application in or-
ganic synthesis.2,3 Depending on the synthetic route, two different
types can be distinguished: (i) 1:1 cyanocuprates, formulated as
RCu(CN)Li, obtained from the reaction of equimolar amounts
of an organolithium with CuCN, and (ii) 2:1 cyanocuprates with
R2Cu(CN)Li2 stoichiometry, a 2:1 molar mixture of an organo-
lithium with CuCN. Initially, it has been proposed that 2:1 cyano-
cupratesexistinsolutionasCu(I)dianionicspecies,[R2Cu(CN)]2-2[Li]+
and 1:1 species as monoanions, [RCu(CN)]-.4 In both of these
cyanocuprates, the CN- anion is thought to act simply as a non-
transferable ligand during reactivity. Despite their widespread
use in organic synthesis, there has been little success in obtaining
reliable structural (solid-state or solution) data about cyanocu-
prates. This has led to considerable debate among academic re-
searchers concerning the proposed structural motifs of these
compounds.
The molecular structure in the solid state of 1 reveals a linear
(zigzag) polymeric chain, consisting of alternating [Ar2Cu]-
(10) Huang, H.; Alvarez, K.; Cui, Q.; Barnhart, T. M.; Snyder, J. P.; Penner-
Hahn, J. E. J. Am. Chem. Soc. 1996, 118, 8808.
Four different models have been put forward to describe the
structure of 2:1 cyanocuprates: (a) a dianionic species with a
three-coordinate Cu(I) center,4 (b) a monoanionic copper species
(11) (a) Wehman, E.; Jastrzebski, J. T. B. H.; Ernsting, J.-M.; Grove, D.
M.; van Koten, G. J. Organomet. Chem. 1988, 353, 133. (b) Jastrzebski, J. T.
B. H.; van Koten, G.; Konijn, M.; Stam, C. H. J. Am. Chem. Soc. 1982, 104,
5490. (c) Reich, H. J.; Gudmundsson, B. O. J. Am. Chem. Soc. 1996, 118,
6074. (d) Hope, H.; Power, P. P. J. Am. Chem. Soc. 1983, 105, 5320. (e)
Lorenzen, N. P.; Weiss, E. Angew. Chem., Int. Ed. Engl. 1990, 29, 300.
(12) The CH2NMe2 substituent in [C6H4CH2NMe2-2]- (abbreviated as Ar-)
can be considered as a well-positioned intramolecular “solvent” molecule,
11d
viz. the similar structural features of Ph4Li4(OEt2)4 and Ar4Li4 as well as
11e
of Ph4Cu2Li2(OEt2)2 and Ar4Cu2Li2.
(13) van Koten, G.; Leusink, A. J.; Noltes, J. G. J. Chem. Soc., Chem.
Commun. 1970, 1107.
with a lithiumcyanide cationic counterion,5,25 (c) a neutral trinu-
clear structure with a diagonally coordinated Cu(I) center and
LiCN incorporated in the overall structure,6 and (d) a mixture of
the neutral cuprate and LiCN.7
(14) (a) van Koten, G.; Noltes, J. G. J. Chem. Soc., Chem. Commun. 1972,
940. (b) van Koten, G.; Jastrzebski, J. T. B. H.; Muller, F.; Stam, C. H. J.
Am. Chem. Soc. 1985, 107, 697.
(15) X-ray data for 1 are available as Supporting Information.
(16) Leoni, P.; Pasquali, M.; Ghilardi, C. A. J. Chem. Soc., Chem. Commun.
1983, 240.
(17) Hope, H.; Olmstead, M. M.; Power, P. P.; Sandell, J.; Xu, X. J. Am.
Chem. Soc. 1985, 107, 4337.
(18) Cyanocuprate 1 contains a two center-two electron (2c-2e) Cu-C bond
whereas in Ar4Cu2Li2 each aryl group is 3c-2e bonded to a CuLi atomic pair.
(19) Bertz, S. H. J. Am. Chem. Soc. 1991, 113, 5470.
(20) (a) Boche, G.; Bosold, F.; Marsch, M.; Harms, K. Angew. Chem. 1998,
110, 1779. (b) Hwang, C.-S.; Power, P. P. J. Am. Chem. Soc. 1998, 120, 6409.
(21) Penner-Hahn et al. reported an ν(CtN) of 2115 cm-1 for Me2Cu(CN)-
Li2 in solution which would be in agreement with ab initio calculations of a
seven-membered cyclic cyanocuprate.10 The ν(CtN) value (2115 cm-1) of [Ar2-
Cu(CN)Li2], 1, in THF is, however, very similar to that previously reported
for the 2:1 cyanocuprates.10
(22) It should be noted that the applied methodology (measurements of
freezing point depression) presents a value indicative for the amount of
particles present in solution and not the molecular weight of the individual
species. If 1 completely dissociates in THF solution into cationic and anionic
species which simultaneously associate to higher aggregates (Li2CNf
(Li2CN)nn+; n g 3), a virtual molecular weight close to the value of a neutral
monomeric species will be calculated. Such processes a priori may not be
excluded since it has been well established that a combination of Li+, CN-,
and additional solvent molecules might give rise to highly aggregated species
as is exemplified in the solid-state structure of [LiCN(pyridine)2]∞.23
(23) Purdy, A. P.; Houser, E.; George C. F. Polyhedron 1997, 16, 3671.
(24) (a) Nakamura, E.; Mori, S.; Nakamura, M.; Morakuma, K. J. Am.
Chem. Soc. 1997, 119, 4887. (b) Nakamura, E.; Mori, S.; Nakamura, K. J.
Am. Chem. Soc. 1997, 119, 4900.
Extensive spectroscopic studies have been performed (EXAFS,8
NMR5,9,26 and IR10) to elucidate the structure of dialkylcyanocu-
prates. One of these studies prefers structure a9 but the others
* Authortowhomcorrespondenceshouldbeaddressed.E-mail: g.vankoten@chem.uu.nl.
† Debye Institute.
‡ Bijvoet Center for Biomolecular Research.
(1) Ar ) [C6H4CH2NMe2-2]-: the structure of the title compound in the
solid state is represented by (µ2Li2-C,N-CN)Li2‚(THF)4(µ2Cu,Li-CCu,NLi-2-
Me2NCH2C6H4)2Cu.
(2) Lipshutz, B. H.; Wilhelm, R. S. J. Am. Chem. Soc. 1981, 103, 7672.
(3) (a) Posner, G. H., An Introduction to Synthesis Using Organocopper
Reagents: John Wiley & Sons: New York, 1980. (b) Lipshutz, B. H.; Sengupta,
S. Org. React. 1992, 41, 139. (c) Lipshutz, B. H.; Wilhelm, R. S.; Kozlowski,
J. A. Tetrahedron 1984, 40, 5005. (d) Bertz, S. H.; Fairchild, E. H.
Encyclopedia of Reagents for Organic Synthesis: Wiley: New York, 1995;
pp 1312-1315, 1341-1343, 1346-1349. (e) Krause, N.; Gerold, A. Angew.
Chem., Int. Ed. Engl. 1997, 36, 186.
(4) Lipshutz, B. H. Synthesis 1987, 325.
(5) Bertz, S. H. J. Am. Chem. Soc. 1990, 112, 4031.
(6) (a) Snyder, J. P.; Spangler, D. P.; Behling, J. R. J. Org. Chem. 1994,
59, 2665. (b) Gerold, A.; Jastrzebski, J. T. B. H.; Kronenburg, C. M. P.; Krause,
N.; van Koten, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 755.
(7) Bertz, S. H.; Miao, G.; Eriksson, M. Chem. Commun. 1996, 815.
(8) (a) Stemmler, T. L.; Barnhart, T. M.; Penner-Hahn, J. E.; Tucker, C.
E.; Knochel, P.; Bo¨hme, M.; Frenking, G. J. Am. Chem. Soc. 1995, 117, 12489.
(b) Barnhart, T. M.; Huang, H.; Penner-Hahn, J. E. J. Org. Chem. 1995, 60,
4310. (c) Stemmler, T.; Penner-Hahn, J. E.; Knochel, P. J. Am. Chem. Soc.
1993, 115, 348.
(25) Mobley, T. A.; Mu¨ller, F.; Berger, S. J. Am. Chem. Soc. 1998, 120,
1333.
(26) Bertz, S. H.; Nilsson, K.; Davidsson, O.; Snyder, J. P. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 314.
(9) (a) Lipshutz, B. H.; Sharma, S.; Elsworth, E. L. J. Am. Chem. Soc.
1990, 112, 4032. (b) Lipshutz, B. H.; James, B. J. Org. Chem. 1994, 59, 7585.
S0002-7863(98)00244-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/09/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9689
Trip2}]2,20b in contrast to the 2:1 cyanocuprates where a direct
CN-Cu bond is absent.21
Most likely the polymeric solid-state structure of 1 is broken
down in THF solution as a result of the consecutive dissociation
of the N-Li interactions, which links the anionic and cationic
units (e.g., by solvation). In the limiting situation, this would
give rise to completely dissociated [Ar2Cu]- anionic and [(THF)3Li-
(CN)Li(THF)3]+ cationic species. This conclusion seems to be
corroborated by the observation that the 1H NMR spectrum of 1
in THF-d8 (RT) shows only one singlet resonance for both the
NMe2 protons (δ ) 2.20 ppm) and for the benzylic protons (δ )
1
3.73 ppm). Moreover, the fact that the H NMR spectra of 1
(THF-d8) are temperature independent (RT to -78 °C) suggests
that no complex equilibria take place on the NMR time scale
Molecular weight determinations (cryoscopy) of 1 (22.5 mM)
in THF solution points to the existence of 1 as a nondissociated
monomeric, i.e., neutral, species22 with Ar2Cu(CN)Li2 stoichi-
ometry, for which a proposed structure c is schematically shown
(vide supra). To compare the solution structure to the known
ionic solid-state structure of 1 its conductivity in THF was
measured. The observed molar conductivity of 57.1 Ω-1 cm-1
for THF solutions of 1 (41.5 mM) is about 500 times larger than
that measured for the parent organolithium compound [Ar4Li4].11a
This latter species is known to exist in THF solution as a discrete
neutral dimeric complex [Ar2Li2(THF)4].11b-c This (somewhat
surprising) result strongly suggests that ionic species are present
in THF solutions of 1, which would contradict the earlier proposed
neutral trinuclear CuLi2 structure, i.e., c.
In conclusion, the solid-state structure of the 2:1 cyanocuprate
[Ar2Cu(CN)Li2(THF)4] is comprised of an ionic structure with
[Ar2Cu]- and [LiCNLi]+ units, whereas according to recent solid-
state studies, the 1:1 cyanocuprate [t-BuCu(CN)Li(Et2O)2]∞
contains a t-BuCu-CN-Li structural motif.20 Since the lithium-
bound carbon atom of the organolithium reagent is a stronger
Lewis base than the carbon atom of the cyanide in a 1:1 cyan-
ocuprate, it is not unexpected that the addition of an equimolar
amount of RLi to RCu(CN)Li results in the conversion of the
cyanide-bridged species into a compound with more stable
[R2Cu]- and [Li2CN]+ structural units. The stability of these ions
makes the 2:1 cyanocuprate an unique combination. The observed
structure (Figure 1) for 1 in the solid state does not conform to
any of the previously proposed models a, c, and d6,8,10 but does
correspond to model b.5,25 Especially noteworthy is the unprec-
edented role of the CtN ligand as a bridge between the two
lithium atoms via coordination of both carbon and nitrogen. The
basic role of the CtN anion in the 2:1 cyanocuprate appears to
be as a [Li2CN]+ counterion for the [R2Cu]- anion and this can
be a general function of nontransferable anions in cuprate chem-
istry. Unfortunately, the solid-state structure does not provide
any insight into the mechanism of C-C bond formations that
are promoted by such reagents, since it is highly unlikely that
the observed “resting state” will remain after substrate addition.24,27
Figure 1. ORTEP drawing (50% probability level) of [Ar2Cu(CN)Li2-
(THF)4]∞, 1. representation of [Ar2Cu(CN)Li2(THF)4]∞, 1. Note that the
CN orientation is arbitrarily chosen.15
anionic and [Li2(CN)(THF)4]+ cationic units (see Figure 1). Each
[Li2(CN)(THF)4]+ unit contains a CtN anion (1.156 Å) which
bridges the two lithium atoms (Li(1)-C(1) and Li(1B)-N(10)
2.045 Å; Li(1)-C(10) 175.6(3)°),15 while to each lithium atom
two additional THF molecules are coordinated. As the cyanide
anion is located on a crystallographic inversion center, the cyanide
carbon and nitrogen atoms are indistinguishable. The formal
[Ar2Cu]- and [Li2(CN)(THF)4]+ units in the polymeric chain are
linked together via coordination of the nitrogen atom of the 2-(di-
methylamino)methyl substituent of the Ar anion to the lithium
atom of an adjacent [Li2(CN)(THF)4]+ cation. The latter N-Li
(2.090 Å) interaction renders each lithium atom four coordinate.
The linear C(1a)-Cu-C(1) (180°) arrangement found in the
[Ar2Cu]- anionic units is in agreement with earlier reported
structures that contain free [R2Cu]- structural fragments, e.g., [Cu-
(dppe)2][Cu(Mes)2]16 and [Li(12-crown-4)2][CuPh2].17 The ob-
served Cu-C(1) distance (1.917(2) Å) in 1 is slightly shorter (0.03
Å) than that reported for the neutral cuprate Ar4Cu2Li2 pointing
to a slight increase in the Cu-C bond order in 1.14b,18
An important question is whether the ionic polymeric structure
of 1 is retained in solution. Attempts to dissolve 1 in benzene or
toluene results in an immediate disproportionation reaction giving
rise to the formation of a solution of the known neutral cuprate
14
Ar4Cu2Li2 and a precipitate which is assumed to be LiCN.
However, cyanocuprate 1 dissolves readily in THF. The 13C-
{1H} NMR spectrum of 1 (THF), contains a single carbon
resonance at 158.1 ppm that has been assigned to the cyanide
carbon atom. This chemical shift value is comparable to that
found in [Ph2Cu(CN)Li2] (158.7 ppm), [Et2Cu(CN)Li2] (158.8
ppm), and [Me2Cu(CN)Li2] (158.9 ppm).5 The presence of CN-,
in the form of LiCN, is not observed (δ ) 162 ppm) for 1.9
Consequently, it must be concluded that the various anions such
as Ph-, Et-, or Me- and in our case Ar-, which have different
σ-donor capabilities, do not significantly influence the cyanide
carbon chemical shift.5,25 This is in agreement with the proposal
that the CN anion is exclusively part of the LiCNLi cation. In
contrast, it is noteworthy that the chemical shift values for the
CtN carbon for the monoorganocyanocuprates, [RCu(CN)Li],
do vary with the nature of the R group,19 thus indicating that in
the 1:1 cyanocuprates a Cu-CN bond is present indeed. This
has also recently been observed in the X-ray structure determi-
nation of [t-BuCu(CN)Li]20a and of [Li(THF)2{Cu(CN)C6H3-2,6-
Acknowledgment. We thank Dr. R. A. Gossage (Utrecht University)
for careful reading of the manuscript and helpful suggestions and Dr. E.
T. G. Lutz for the IR measurements. Prof. dr G. Boche (University of
Marburg) is thanked for communicating the unpublished results of the
monocyanocuprate.20a
Supporting Information Available: X-ray structural data for 1, and
tables of fractional coordinates, thermal parameters, bond distances and
angles, and hydrogen coordinates, and 1H and 13C NMR spectra of 1 (11
pages, print/PDF). An X-ray crystallographic file, in CIF format, is
available through the Web only. See any current masthead page for
ordering information and Web access instructions.
JA9802448
(27) The reactivity has been tested in a substitution reaction of 1 with a
primary halide. Reaction of 1 equiv of 1-octylbromide with 2 equiv of 1 (THF,
-78 °C, 2 h) yields 75% of the substituted product, 2-(octyl)dimethylben-
zylamine. Comparing this result with earlier reported data of substitution
reactions with species of the form R2Cu(CN)Li2,3c one can say that 1 reacts
as a “higher order” or better 2:1 bis(aryl)cyanocuprate.