Carbon-carbon bond formation on reaction of a copper(I) stannyl complex with carbon dioxide

Article abstract of DOI:10.1021/om8001729

The reaction of (IPr)CuOt-Bu (IPr = 1, 3-bis(2, 6-diisopropylphenyl) imidazol-2-ylidene) with triphenylstannane forms a stannyl complex, (IPr)CuSnPh₃, by deprotonation of the tin-hydrogen bond. This stannyl complex reacts with CO₂ to afford (1Pr)CuO₂CPh as the sole copper-containing species. A tin-carbon bond in (1Pr)CuSnPh₃ also undergoes facile cleavage by mild acids such as 2,4-lutidinium chloride.

Full text of DOI:10.1021/om8001729

2682
Organometallics 2008, 27, 2682–2684
Carbon-Carbon Bond Formation on Reaction of a Copper(I)
Stannyl Complex with Carbon Dioxide
Koyel X. Bhattacharyya,* Jennifer A. Akana, David S. Laitar, Jacob M. Berlin, and
Joseph P. Sadighi†
Department of Chemistry, Massachusetts Institute of Technology, 18-344, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
ReceiVed February 24, 2008
Scheme 1. (IPr)Cu-E Complexes React Differently with
Summary: The reaction of (IPr)CuOt-Bu (IPr ) 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) with triphenylstannane
forms a stannyl complex, (IPr)CuSnPh3, by deprotonation of
the tin-hydrogen bond. This stannyl complex reacts with CO2
to afford (IPr)CuO2CPh as the sole copper-containing species.
A tin-carbon bond in (IPr)CuSnPh3 also undergoes facile
cleaVage by mild acids such as 2,4-lutidinium chloride.
CO2 Depending on Choice of E
Carbon dioxide is a benign and readily available source of
carbon;1 however, thermodynamic and kinetic considerations
limit its practical use in chemical synthesis.2 Metal complexes
that display unique reactivity toward carbon dioxide hold
promise in the development of new catalytic CO2-fixation
reactions.3,4 The reactivity of organocopper complexes with CO2
to form copper carboxylates5,6 encouraged the study of other
complexes containing copper-E bonds (E ) main-group ligand)
(Scheme 1). The N-heterocyclic carbene (NHC) supported
copper-boryl species (IPr)CuB(pin) (IPr ) 1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene, B(pin) ) boryl pinacolate)
reacts with CO2 to form (IPr)CuOB(pin), with concomitant
extrusion of CO.3 DFT calculations support a nucleophilic
Cu-B bond, with initial formation of B-C and Cu-O bonds
upon insertion of CO2.7 The silyl complex (IPr)CuSiPh3 showed
similar net reactivity, reducing CO2 to CO and forming
(IPr)CuOSiPh3.8 To explore whether and how a copper-tin bond
would react with CO2, we synthesized the stannyl complex
(IPr)CuSnPh3 (1). To our surprise, treatment of 1 with CO2
resulted in carbon-carbon bond formation, affording the
benzoate complex (IPr)CuO2CPh (2).13
The copper stannyl complex 1 was initially synthesized by
the addition of Ph3SnH to [(IPr)CuH]2 in benzene solution (eq
1).9 Although this reaction proceeded in good yield, the high
air-sensitivity and thermal instability of the hydride prompted
a search for more synthetically convenient routes to the stannyl
complex. Interestingly, the reaction of Ph3SnH with (IPr)CuMe
instead produces methyltriphenyltin and [(IPr)CuH]2,5 as judged
1
by H NMR spectroscopy. We later found that (IPr)CuOt-Bu
is sufficiently basic to deprotonate the stannane, forming 1 and
1
t-BuOH (eq 2).9 Complex 1 was characterized by H NMR
spectroscopy and single-crystal X-ray diffraction (Figure 1).
* To whom correspondence should be addressed. E-mail: koyel@mit.edu.
† Current address: U.S. Army Infantry School, Fort Benning, GA.
(1) Behr, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 661–678.
(2) Carbon Dioxide Fixation and Reduction in Biological and Model
Systems; Branden, C.-I., Schneider, G., Ed.s; Oxford University Press: New
York, 1994. (b) Baiker, A. Appl. Organomet. Chem. 2000, 14, 751–762.
(3) Laitar, D. S.; Muller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005,
127, 17196–17197.
Single crystals suitable for X-ray analysis were obtained by
slow vapor diffusion of hexanes into a toluene solution of 1 at
-40 °C. Complex 1 possesses a Cu-Sn bond length of 2.469(5)
Å, similar to that of other copper stannyl complexes (2.45-2.50
Å).10
Complex 1 reacts with CO2 (1.04 atm) to form a single NHC-
containing species (eq 3), as determined by 1H NMR spectros-
copy. Single crystals of the product were analyzed by X-ray
diffraction and identified as the benzoate complex 2 (Figure
(4) See, for example: (a) Meshitsuka, S.; Ichikawa, M.; Tamura, K.
Chem. Commun. 1974, 158–159. (b) Beley, M.; Collin, J.-P.; Ruppert, R.;
Sauvage, J.-P. Chem. Commun. 1984, 1315–1316. (c) Beley, M.; Collin,
J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108, 7461–7467.
(d) Hammouche, M.; Lexa, D.; Momenteau, M.; Save´ant, J.-M. J. Am.
Chem. Soc. 1991, 113, 8455–8466. (e) Bhugun, I.; Lexa, D.; Save´ant, J.-
M. J. Am. Chem. Soc. 1996, 118, 1769–1776.
(5) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organo-
metallics 2004, 23, 1191–1193.
(6) (a) Ikariya, T.; Yamamoto, A. J. Organomet. Chem. 1974, 72, 145–
151. (b) Marsich, N.; Camus, A.; Nardin, G. J. Organomet. Chem. 1982,
239, 429–437. (c) Miyashita, A.; Yamamoto, A. J. Organomet. Chem. 1976,
113, 187–199. (d) Miyashita, A.; Yamamoto, A. J. Organomet. Chem. 1973,
49, C57-C58.
(9) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004,
23, 3369–3371.
(7) Zhao, H.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128, 15637–
15643.
(10) (a) Dias, H. V. R.; Wang, X.; Diyabalanage, H. V. K. Inorg. Chem.
2005, 44, 7322–7324. (b) Klett, J.; Klinkhammer, K. W.; Niemeyer, M.
Chem. Eur. J. 1999, 5, 2531–2536.
(8) Laitar, D. S. Synthetic and Catalytic Studies of Group 11 N-
Heterocyclic Carbene Complexes, Ph.D. Dissertation, Massachusetts Institute
of Technology, Cambridge, MA, 2006.
(11) Darensbourg, D. J.; Bauch, C. G.; Reibenspies, J. H.; Rheingold,
A. L. Inorg. Chem. 1988, 27, 4203.
10.1021/om8001729 CCC: $40.75
2008 American Chemical Society
Publication on Web 05/20/2008

Communications
Organometallics, Vol. 27, No. 12, 2008 2683
in this reaction gives rise to a single 119Sn NMR shift of -43.7
ppm. Although this byproduct has eluded definitive identifica-
1
tion, H and 13C NMR suggest that it contains only phenyl
groups (see the Supporting Information). Furthermore, reaction
of 1 with 13CO2 produces (IPr)CuO213CPh as the only obviously
labeled species. No 13CO was observed in a variable-temperature
13C NMR study, and there was no notable enhancement of any
resonances in the 13C NMR spectrum of the isolated tin-
containing byproduct. This extrusion of a main-group element,
likely made possible by the accessibility of both divalent and
tetravalent tin, represents a new mode of reactivity in complexes
of the type (NHC)Cu-E.
2), resulting from a C-C bond-forming pathway. Another metal
triphenylstannyl complex, the anionic [(OC)5W(SnPh3)]- syn-
thesized by Rheingold and co-workers, exhibited no reactivity
toward CO2 even under high pressures.11 A copper stannyl
complex supported by a tridentate phosphine ligand has been
shown by Klein and co-workers to react with CS2, forming the
insertion product L3CuS2CSnMe3 without loss of tin.12
Figure 1. Solid-state structure of 1 shown as 50% ellipsoids. For
clarity, hydrogen atoms, disorder, and solvent have been omitted.
Select bond lengths (Å) and angles (deg): Sn(1)-Cu(1) ) 2.469(5),
Cu(1)-C(1) ) 1.914(2); Sn(1)-Cu(1)-C(1) ) 169.6(8).
The copper-bound triphenylstannyl appears to react as a
source of the phenyl nucleophile, suggesting the net electrophilic
cleavage of a tin-phenyl bond by CO2. Further evidence for
this mode of reactivity is provided in the protonolysis of 1 by
2,4-lutidinium chloride, which formed (IPr)CuCl,15 benzene, and
the tin byproduct (eq 4). The reaction of 1 with tert-butyl
alcohol-d produced (IPr)CuOt-Bu, C6H5D, and the tin byproduct
(eq 5) after heating at 50 °C for 22 h. These electrophilic
cleavage reactions might occur through a concerted cleavage
with extrusion of SnPh2 (Scheme 2, path A) or via an
equilibrium with the copper phenyl species (IPr)CuPh (Scheme
2, path B),16 which is chemically competent in reactions with
CO2 (eq 6) and t-BuOD (eq 7). Such an equilibrium would
strongly favor (IPr)CuSnPh3, the only species visible by NMR.
Similar equilibria have been observed between [M]SnR3 and
[M]R + SnR2 ([M], R ) (R3P)2ClPt, Cl2; Li, Ph).17,18
Scheme 2. Possible Mechanisms for the Conversion of
(IPr)CuSnPh3 to (IPr)CuO2CPh
Figure 2. Solid-state structure of 2 shown as 50% ellipsoids. For
clarity, hydrogen atoms, disorder, and solvent have been omitted.
Select bond lengths (Å) and angles (deg): O(2)-Cu(1) ) 1.8504(11),
Cu(1)-C(1) ) 1.8647(14), O(2)-C(28) ) 1.2803(19), O(1)-C(28)
) 1.231(2); O(2)-Cu(1)-C(1) ) 179.05(6).
The solid-state structure of 2 shows a monodentate carboxy-
late group, as observed for (IPr)CuOAc.5 The reaction stoichi-
ometry implies the formation of a diphenyltin-derived byproduct.
The best-characterized form of diphenyltin(II) is a six-membered
cyclic species, with a 119Sn NMR shift of -208.6 ppm relative
to Me4Sn.14 In contrast, the tin-containing byproduct formed
(14) (a) Neumann, W. P.; Klaus, K. Ann. Chem. 1964, 667, 1–11. (b)
Saito, M.; Okamoto, Y.; Yoshioka, M. Appl. Organomet. Chem. 2005, 19,
894–897.
(15) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5,
2417–2420.
(12) Klein, H. F.; Montag, J.; Zucha, U.; Flo¨rke, U. ; Haupt, H. J. Inorg.
Chim. Acta 1990, 177, 35–42.
(16) Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.;
Pierpont, A. W.; Petersen, J. L.; Boyle, P. D. Inorg. Chem. 2006, 45, 9032–
9045.
(13) Yield calculated with respect to 1,4-dimethoxybenzene internal
standard.

2684 Organometallics, Vol. 27, No. 12, 2008
Communications
In conclusion, the NHC-supported copper stannyl complex
(IPr)CuSnPh3 was synthesized from (IPr)CuO-t-Bu and Ph3SnH.
This complex serves as a source of a nucleophilic phenyl group,
reacting with CO2 to form a benzoate species and with mild
acids to release benzene. Efforts to elucidate the mechanism of
tin-carbon bond cleavage in this complex by DFT calculations
are ongoing.
ment of Chemistry. The MIT UROP office provided generous
support to K.X.B. Colleagues in the research group of Prof.
John E. Bercaw (Caltech) provided valuable assistance in
the preparation of this paper.
Supporting Information Available: Text, tables, and figures
giving synthetic details, spectroscopic data, and structural
parameters and CIF files giving crystallographic data for 1 and
2. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by the NSF
(CAREER award No. CHE-0349204) and the MIT Depart-
(17) [M], R ) (R3P)2ClPt, Cl2: Holt, M. S.; Wilson, W. L.; Nelson,
J. H. Chem. ReV. 1989, 89, 11–49.
(18) [M], R ) Li, Ph: d’Ans, J.; Zimmer, H.; Endrulat, E.; Lu¨bke, K.
Naturwissenschaften 1952, 39, 450.
OM8001729