Synthesis, Structure, and CO2 Reactivity of a Two-Coordinate (Carbene)copper(I) Methyl Complex

Article abstract of DOI:10.1021/om034368r

Two-coordinate copper(I) acetate and copper(I) methyl complexes, bearing an N-heterocyclic carbene (NHC) supporting ligand, have been synthesized and structurally characterized, and the stability of the monodentate acetate has been examined by DFT calculations. The methyl complex readily inserts carbon dioxide at ambient temperature and pressure, regenerating the acetate in near-quantitative yield.

Full text of DOI:10.1021/om034368r

Organometallics 2004, 23, 1191-1193
1191
Syn th esis, Str u ctu r e, a n d CO2 Rea ctivity of a
Tw o-Coor d in a te (Ca r ben e)cop p er (I) Meth yl Com p lex
Neal P. Mankad, Thomas G. Gray, David S. Laitar, and J oseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, 2-214A,
77 Massachusetts Avenue, Cambridge, Massachusetts 02139
Received December 12, 2003
Sch em e 1a
Summary: Two-coordinate copper(I) acetate and
copper(I) methyl complexes, bearing an N-heterocyclic
carbene (NHC) supporting ligand, have been synthesized
and structurally characterized, and the stability of the
monodentate acetate has been examined by DFT calcula-
tions. The methyl complex readily inserts carbon dioxide
at ambient temperature and pressure, regenerating the
acetate in near-quantitative yield.
The catalytic formation of carbon-carbon bonds from
carbon dioxide presents an ongoing challenge in orga-
nometallic chemistry.1 With the goal of developing new
catalytic reactions, we have been interested in the
insertion of CO2 into alkyl complexes of late transition
metals, particularly Cu.2 Many copper(I) alkyls are quite
unstable, but those complexes supported by bulky,
electron-rich phosphines2b-e are notably robust, and
several phosphine-ligated copper(I) alkyls have been
reported to undergo CO2 insertion.2e-h To avoid poten-
tial side reactions involving binuclear reaction path-
ways, we have sought supporting ligands that would
disfavor ligand redistribution3a or bridged-oligomer
formation.3b-f
a
Solvents and conditions: (a) THF, room temperature; (b)
PhCH3, room temperature; (c) Et2O, -45 °C; (d) C6H6, room
temperature.
dentate and bidentate acetates, and the synthesis and
structure of a linear (carbene)copper(I) methyl, which
reacts readily with CO2.
The synthetic routes to (carbene)copper(I) complexes
1-3 are shown in Scheme 1.7 Chloride complex 1 was
synthesized as reported previously;8 its crystal structure
(Figure 1) displays a monomeric, two-coordinate geom-
etry, with a Cu-Cl bond distance of 2.106(2) Å.9 Related
(NHC)copper(I) halides, prepared by N-methylation of
(azolyl)chlorocuprates, display very similar geometries
and bond lengths.6d
Attempts to replace the chloro ligand in 1 with a
methyl group, using a variety of methyl nucleophiles,
did not give clean reactions. (Phosphine)copper(I) meth-
yl complexes are often synthesized by addition of Me2-
AlOR (R ) Et, iPr) to Cu(acac)2 in the presence of free
phosphine;2 however, the analogous reaction using the
free ligand IPr afforded a yellow powder that defla-
Since their discovery by Arduengo and co-workers,4
N-heterocyclic carbenes have become versatile and
prolific ligands in catalysis,5 often supplanting sterically
demanding, electron-rich phosphines. Their use as
ligands for copper complexes, however, remains rela-
tively rare.6 Herein we report the structural character-
ization of a carbene-ligated copper(I) chloride, the
synthesis and structure of a two-coordinate (carbene)-
copper(I) acetate, a computational comparison of mono-
* To whom correspondence should be addressed. E-mail: jsadighi@
mit.edu. Fax: (617) 258-5700.
(5) See for example: (a) Navarro, O.; Kelly, R. A., III; Nolan, S. P.
J . Am. Chem. Soc. 2003, 125, 16194-16195. (b) Trnka, T. M.; Morgan,
J . P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T.-L.; Ding, S.;
Day, M. W.; Grubbs, R. H. J . Am. Chem. Soc. 2003, 125, 2546-2558.
(c) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem., Int.
Ed. 2002, 41, 1745-1747. (d) Herrmann, W. A. Angew. Chem., Int.
Ed. 2002, 41, 1290-1309.
(6) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J . Am. Chem. Soc.
2003, 125, 12237-12245. (b) Arnold, P. L.; Scarisbrick, A. C.; Blake,
A. J .; Wilson, C. Chem. Commun. 2001, 2340-2341. (c) Tulloch, A. A.
D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M. E.; Hursthouse, M. B.;
Eastham, G. Organometallics 2001, 20, 2027-2031. (d) Raubenheimer,
H. G.; Cronje, S.; Olivier, P. J . J . Chem. Soc., Dalton Trans. 1995, 313-
316. (e) Arduengo, A. J ., III; Rasika Dias, H. V.; Calabrese, J . C.;
Davidson, F. Organometallics 1993, 12, 3405-3409.
(7) For complete synthetic, computational, spectroscopic, and crys-
tallographic details, see the Supporting Information.
(8) J urkauskas, V.; Sadighi, J . P.; Buchwald, S. L. Org. Lett. 2003,
5, 2417-2420.
(9) Crystal data for C29H40N2Cl5Cu (1‚2CH2Cl2): monoclinic, space
group P21/m, a ) 9.5298(8) Å, b ) 16.3972(14) Å, c ) 11.1057(10) Å,
â ) 103.348(1)°, V ) 1688.5(3) Å3, Z ) 2, Fcalcd ) 1.293 g/cm3, F(000)
) 684, T ) 194(2) K. Least-squares refinement converged normally
with residuals of R1 (based on F) ) 0.0655, wR2 (based on F2) ) 0.1651,
and GOF ) 1.058 based on I > 2σ(I).
(1) Selected references: (a) Louie, J .; Gibby, J . E.; Farnworth, M.
V.; Tekavec, T. N. J . Am. Chem. Soc. 2002, 124, 15188-15189. (b)
Takimoto, M.; Mori, M. J . Am. Chem. Soc. 2002, 124, 10008-10009.
(c) Yin, X.; Moss, J . R. Coord. Chem. Rev. 1999, 181, 27-59.
(2) (a) Coan, P. S.; Folting, K.; Huffman, J . C.; Caulton, K. G.
Organometallics 1989, 8, 2724-2728. (b) Pasynkiewicz, S.; Pikul, S.;
Poplawska, J . J . Organomet. Chem. 1985, 293, 125-130. (c) Pasynk-
iewicz, S.; Poplawska, J . J . Organomet. Chem. 1985, 282, 427-434.
(d) Miyashita, A.; Yamamoto, A. Bull. Chem. Soc. J pn. 1977, 50, 1102-
1108. (e) Ikariya, T.; Yamamoto, A. J . Organomet. Chem. 1974, 72,
145-151. (f) Marsich, N.; Camus, A.; Nardin, G. J . Organomet. Chem.
1982, 239, 429-437. (g) Miyashita, A.; Yamamoto, A. J . Organomet.
Chem. 1976, 113, 187-199. (h) Miyashita, A.; Yamamoto, A. J .
Organomet. Chem. 1973, 49, C57-C58.
(3) (a) Dempsey, D. F.; Girolami, G. S. Organometallics 1988, 7,
1208-1213. (b) Koenig, T. M.; Daeuble, J . F.; Bretensky, D. M.; Stryker,
J . M. Tetrahedron Lett. 1990, 31, 3237-3240. (c) Goeden, G. V.;
Caulton, K. G. J . Am. Chem. Soc. 1981, 103, 7354-7355. (d) Beguin,
B.; Denise, B.; Sneeden, R. P. A. J . Organomet. Chem. 1981, 208, C18-
C20. (e) Goel, R. G.; Beauchamp, A. L. Inorg. Chem. 1983, 22, 395-
400. (f) Edwards, D. A.; Richards, R. J . Chem. Soc., Dalton Trans. 1975,
637-643.
(4) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . Am. Chem. Soc.
1991, 113, 361-363.
10.1021/om034368r CCC: $27.50 © 2004 American Chemical Society
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1192 Organometallics, Vol. 23, No. 6, 2004
Communications
F igu r e 1. Representation of 1, shown as 50% ellipsoids.
Hydrogen atoms (calculated) and solvent are omitted
for clarity. Selected bond lengths (Å) and angles (deg):
Cu(1)-C(1) ) 1.881(7), Cu(1)-Cl(1) ) 2.106(2); C(1)-
Cu(1)-Cl(1) ) 176.7(2).
F igu r e 2. Representation of 2, shown as 50% ellipsoids.
For clarity, hydrogens and solvent are omitted, and only
one of the two molecules in the asymmetric unit is shown.7
Selected interatomic distances (Å) and angles (deg) [cor-
responding values for the other molecule]: Cu(1)-O(2) )
1.850(3) [1.924(5)], Cu(1)-O(1) ) 2.868(4) [2.673(7)],
Cu(1)-C(1) ) 1.854(4) [1.850(5)], O(1)-C(28) ) 1.166(6)
[1.157(8)], O(2)-C(28) ) 1.280(6) [1.228(8)]; O(2)-Cu(1)-
C(1) ) 177.19(18) [166.0(2)], O(1)-C(28)-O(2) ) 126.2(5)
[120.2(8)].
grated when touched, probably [CuMe]4.10 Prior coor-
dination of the carbene ligand to copper thus appears
necessary.
Reaction of free IPr with copper(I) acetate in benzene
affords (IPr)CuO2CCH3 (2).11 The carbonyl resonance in
the 13C NMR spectrum (128.9 ppm) is well upfield of
other copper(I) acetate carbonyl resonances.12 Complex
2 crystallizes with two molecules in the asymmetric
unit, and slightly different bond distances and angles
are obtained for each. In both molecules, the crystal
structure13 reveals a monodentate acetate ligand.14 This
binding mode is noteworthy, as monodentate copper(I)
carboxylates are relatively rare.15
In the molecule shown in Figure 2, the Cu-O bond
distance (1.850(3) Å) is quite close to that of the linear
bis(acetato)cuprate(I) anion (1.821(8) Å)16 and shorter
than the CuI-O bond distances in three- or four-
coordinate monodentate carboxylates.15 In the other
form of 2 (Figure S5),7 the acetate is somewhat closer
to a κ2-binding mode, with a distortion from linearity
about the copper center and a smaller difference be-
tween the two copper-oxygen distances. However, the
distance from the metal to the proximal oxygen atom
F igu r e 3. Optimized structures calculated for κ1- (A) and
κ2-acetate (B) model complexes. Selected interatomic dis-
tances (Å): for A, Cu(1)-O(2) ) 1.901, Cu(1)-O(1) ) 2.251,
Cu(1)-C(1) ) 1.821, O(1)-H(1) ) 2.435, O(1)-C(2) )
1.251, O(2)-C(2) ) 1.286; for B, Cu(1′)-O(2′) ) 2.014,
Cu(1′)-O(1′) ) 2.039, Cu(1′)-C(1′) ) 1.825, O(1′)-C(2′) )
1.268, O(2′)-C(2′) ) 1.269.
(10) (a) Thiele, K.-H.; Koehler, J . J . Organomet. Chem. 1968, 12,
225-229. See also ref 2c.
(11) Copper(I) acetate (1.06 g, 2.74 mmol) and IPr (0.336 g, 2.74
mmol) were stirred in toluene (25 mL) for 11 h. The resulting solution
was filtered through Celite and then concentrated. The crude product
was triturated with hexanes (3 × 20 mL) and then dried in vacuo,
affording 2 as a white powder; yield 0.777 g (56%).
(1.924(5) Å) is shorter, and that to the distal oxygen
atom (2.673(7) Å) considerably longer, than the corre-
sponding distances in the bidentate acetate (Ph3P)2-
CuO2CCH3 (2.166(3) and 2.228(3) Å).17
(12) Taqui Khan, M. M.; Paul, P.; Venkatasubramanian, K.; Purohit,
S. Inorg. Chim. Acta 1991, 183, 229-237. See also ref 15c.
(13) Two molecules of 2 and one benzene molecule were present in
the crystallographic asymmetric unit. See Figure S5 in the Supporting
Information for the other molecule. Crystal data for C32H42N2O2Cu
(2‚0.5C6H6): monoclinic, space group P21/c, a ) 17.6693(16) Å, b )
23.498(2) Å, c ) 16.4406(15) Å, â ) 115.716(2)°, V ) 6150.0(10) Å3, Z
) 8, Fcalcd ) 1.189 g/cm3, F(000) ) 2344, T ) 193(2). Least-squares
refinement converged normally with residuals of R1 (based on F) )
0.0628, wR2 (based on F2) ) 0.1408, and GOF ) 1.029 based on I >
2σ(I).
(14) We are at present uncertain whether this binding mode is
preferred in solution. A comparison of asymmetric and symmetric
stretching frequencies for the carboxylate moiety in 2 is precluded by
strong vibrations arising from the IPr ligand, in the 1400-1500 cm-1
region, which mask the peaks calculated for the symmetric vibration
in both model complexes A and B. We thank a reviewer for raising
this point.
The relative stabilities of the κ1- and κ2-acetate
binding modes were explored through DFT calculations
on model complexes,7 in which N-methyl groups have
replaced the N-(2,6-diisopropylphenyl) groups of the
carbene ligand in 2. Figure 3 depicts the optimized
geometries of the κ1complex A and κ2 complex B;
bonding distances calculated for A agree favorably with
those determined crystallographically for 2.18 Chelated
isomer B is calculated to be more stable than A by 1.1
kcal/mol, but this difference is smaller than the uncer-
tainty of (5 kcal/mol estimated for the calculation.19
In the κ1 complex A, the calculated distance between
copper and the distal oxygen atom is 2.251 Å, which is
(15) (a) Darensbourg, D. J .; Larkins, D. L.; Reibenspies, J . H. Inorg.
Chem. 1998, 37, 6125-6128. (b) Darensbourg, D. J .; Holtcamp, M. W.;
Khandelwal, B.; Reibenspies, J . H. Inorg. Chem. 1994, 33, 531-537.
(c) Darensbourg, D. J .; Longridge, E. M.; Holtcamp, M. W.; Klausmeyer,
K. K.; Reibenspies, J . H. J . Am. Chem. Soc. 1993, 115, 8839-8840. (d)
Speier, G.; Szabo, L.; Fulop, V. J . Organomet. Chem. 1993, 462, 375-
378.
(17) Darensbourg, D. J .; Holtcamp, M. W.; Longridge, E. M.;
Klausmeyer, K. M.; Reibenspies, J . H. Inorg. Chim. Acta 1994, 227,
223-232.
(18) All calculated vibrational frequencies for A and B are real,
indicating that both forms represent stable minima.
(19) Himo, F.; Siegbahn, P. E. M. Chem. Rev. 2003, 103, 2421-2456.
(16) Darensbourg, D. J .; Longridge, E. M.; Atnip, E. V.; Reibenspies,
J . H. Inorg. Chem. 1992, 31, 3951-3955.

Communications
Organometallics, Vol. 23, No. 6, 2004 1193
shorter than those found crystallographically for 2
(2.868(4) and 2.673(7) Å). An atoms-in-molecules analy-
sis20 finds bond critical points between copper and both
oxygen atoms in A, indicating a bonding interaction
between the metal and the distal oxygen, O(1). The
same analysis also finds a bond critical point connecting
O(1) with a methyl hydrogen 2.435 Å away. No such
interaction within the molecule is observed for 2;
however, the extended crystal structure of 2 (Figure S7)7
shows a 2.238-Å approach (2.347 Å, for the other
molecule in the asymmetric unit) of a distal oxygen atom
to the calculated position of a carbene backbone hydro-
gen in an adjacent molecule.7 The preferential crystal-
lization of the κ1isomer could conceivably result from
this intermolecular stabilization.21 Vibrational anima-
tion finds that a hinging motion of the κ1-acetate
corresponds to a 40.7 cm-1 torsional mode, implying that
a slight energy input elicits a large distortion.
F igu r e 4. Representation of 3, shown as 50% ellipsoids;
hydrogens and solvent are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Cu-C(1) ) 1.913(6),
Cu-C(2) ) 1.887(5); C(2)-Cu-C(1) ) 180.000(1).
of 2 prepared in this manner is identical with that of
authentic 2 prepared from IPr and copper(I) acetate.
In conclusion, an N-heterocyclic carbene ligand sup-
ports monomeric, linear complexes of copper(I), includ-
ing the chloride and the new acetate and methyl
complexes. Calculations suggest that the monodentate
and bidentate binding modes of acetate are of very
similar energies. The neutral, two-coordinate methyl
complex 3 reacts readily and cleanly with CO2 to form
the acetate. The potential of (NHC)copper(I) complexes
to catalyze new carboxylation processes is currently
being explored.
Addition of Me2AlOEt to 2 in diethyl ether led to clean
formation of the two-coordinate methyl complex 3
(Figure 4).22,23 The Cu-Cmethyl distance, 1.913(6) Å, is
similar to that found in dimethylcuprates (ca. 1.94
Å),3a,24 slightly shorter than that of [η5-(C5H4SiMe3)2-
Ti(CCSiMe3)2]CuCH3 (1.966(2) Å),25 and appreciably
shorter than that of (Ph3P)3CuCH3 (2.043(12) Å).2a
Exposure of a benzene solution of 3 to ca. 1 atm of
CO2 at room temperature afforded (IPr)CuO2CCH3 (2)
1
in near-quantitative yield.26,27 The H NMR spectrum
Ack n ow led gm en t. We thank the MIT Department
of Chemistry, the MIT Undergraduate Research Op-
portunities Program, and the Mary and Stewart Rob-
ertson UROP Fund for support. T.G.G. thanks the NIH
for a postdoctoral fellowship. D.S.L. gratefully acknowl-
edges a Lester Wolfe predoctoral fellowship. We thank
the MIT DCIF staff for their assistance and the NSF
(Awards CHE-9808061 and DBI-9729592) for support
of our NMR facilities. Dr. William M. Davis provided
helpful advice in crystallography. Professor Daniel G.
Nocera graciously provided computational time.
(20) Bader, R. F. W. Chem. Rev. 1991, 91, 893-928.
(21) We cannot rule out a steric contribution to the monodentate
binding of the acetate in crystalline 2. However, the close approaches
of the pendant oxygen atoms to C-bonded hydrogen atoms (see Figure
S7 in the Supporting Information), despite being able to rotate away
from them, indicate that the observed interactions are attractive, not
repulsive. Moreover, using a crude model derived from the crystal
structure of 2, we estimate that the intramolecular approaches of the
acetate oxygen atoms to the ligand alkyl groups would be closer in an
alternative κ2-bound form but longer than the intermolecular ap-
proaches in the actual structure. We thank a reviewer for raising this
point.
(22) Dimethylaluminum ethoxide, generated in situ from trimethyl-
aluminum (2.0 mmol) and ethanol (0.117 mL, 2.0 mmol) in toluene/
diethyl ether solution (1:1, 2 mL), was transferred by cannula to a -45
°C suspension of 2 (0.396 g, 0.776 mmol) in diethyl ether (3 mL). The
reaction mixture was warmed gradually to room temperature with
stirring over 4 h and then concentrated to afford a crude yellow solid.
This solid was taken up in hexanes (5 mL), and the precipitate was
collected by filtration and washed repeatedly with hexanes. Yield:
0.256 g (71%).
(23) Crystal data for C33H39N2Cu (3‚C5H12 (pentane hydrogens not
calculated)): monoclinic, space group C2/c, a ) 22.561(3) Å, b )
9.4266(12) Å, c ) 16.890(2) Å, â ) 116.117(2)°, V ) 3225.3(7) Å3, Z )
4, Fcalcd ) 1.086 g/cm3, F(000) ) 1120, T ) 193(2). Least-squares
refinement converged normally with residuals of R1 (based on F) )
0.0869, wR2 (based on F2) ) 0.1758, and GOF ) 1.158 based on I >
2σ(I).
(24) Hope, H.; Olmstead, M. M.; Power, P. P.; Sandell, J .; Xu, X. J .
Am. Chem. Soc. 1985, 107, 4337-4338.
(25) J anssen, M. D.; Kohler, K.; Herres, M.; Dedieu, A.; Smeets, W.
J . J .; Spek, A. L.; Grove, D. M.; Lang, H.; van Koten, G. J . Am. Chem.
Soc. 1996, 118, 4817-4829.
Su p p or tin g In for m a tion Ava ila ble: Text, tables, and
figures giving synthetic and computational details, spectro-
1
scopic data, H and 13C NMR spectra for 3, additional crystal
structures, and structural parameters for 1-3; crystallo-
graphic data are also given as CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM034368R
(26) A solution of 3 (0.0683 g, 0.147 mmol) in benzene (4 mL) was
frozen at -78 °C in the dark. The reaction vessel was evacuated and
back-filled with carbon dioxide (ca. 1 atm). The reaction mixture was
then thawed, warmed to room temperature, and stirred for 18 h.
Concentration in vacuo afforded a tan powder; yield 72.1 mg (96%).
(27) The carboxylation of 3 was also monitored by 1H NMR (in C6D6
solution at ambient temperature, under 1 atm of CO2) and judged to
be 50% complete after 65 min and 95% complete after 2.5 h.