C O M M U N I C A T I O N S
Acknowledgment. We thank the EPSRC for funding (S.B.) and
Johnson Matthey plc for the loan of hydrated ruthenium trichloride.
Supporting Information Available: Spectroscopic data for com-
pounds 3-6, CIF files giving X-ray crystallographic data for 4 (CCDC
code 616583) and 5 (CCDC code 616582). This material in available
References
(1) N-Heterocyclic Carbenes in Synthesis; Nolan S. P., Ed; Wiley-VCH:
Weinheim, Germany, 2006.
(2) For a comprehensive review of bond activation reactions in NHC
complexes, see: Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004,
248, 2247. For recent reports of C-H activation, see: (a) Scott, N. M.;
Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan, S. P. Angew. Chem.,
Int. Ed. 2005, 44, 2512. (b) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa,
A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516. (c)
Cabeza, J. A.; del R´ıo, I.; Moguel, D.; Sa´nchez-Vega, M. G. Chem.
Commun. 2005, 3956. (d) Corbera´n, R.; Sanau´, M.; Peris, E. J. Am. Chem.
Soc. 2006, 128, 3974. (e) Hanasaka, F.; Tanabe, Y.; Fujita, K.; Yamaguchi,
R. Organometallics 2006, 25, 826. (f) Corbera´n, R.; Sanau´, M.; Peris, E.
Organometallics 2006, 25, 4002.
Figure 1. Molecular structures together with selected bond lengths (Å)
and angles (deg) for (left) 4 (Ru(1)-C(6) 2.1282(18), Ru(1)-C(5)
1.884(2), N(1)-C(6) 1.346(2), N(2)-C(6) 1.366(2), N(1)-C(6)-N(2)
103.91(15)) and (right) 5 (Ru(1)-N(1) 2.1816(18), Ru(1)-C(1) 1.844(3),
N(1)-C(2) 1.323(3), N(2)-C(2) 1.351(3), N(1)-C(2)-N(2) 111.3(2)).
Ellipsoids are shown at 30% probability level.
(3) (a) McGuinness, D. S.; Saendig, N.; Yates, B. F.; Cavell, K. J. J. Am.
Chem. Soc. 2001, 123, 4029. (b) Graham, D. C.; Cavell, K. J.; Yates, B.
F. Dalton Trans. 2006, 1768.
(i) Heating 4 in the presence of 2 equiv of IiPr2Me2 (C6D6 or
THF-d8, 70 °C) affords complex 5 along with traces of Ru(PPh3)3-
(CO)H2. The same transformation occurred upon heating with either
ItBu or IEt2Me2, although the latter reaction was accompanied by
formation of the simple substitution product 6. Simply heating 4
alone in THF did not generate 5.
(ii) Heating a 1:2 mixture of 4 + IiPr2Me2 side-by-side with a
1:2:5 solution of 4 + IiPr2Me2/PPh3 (THF-d8, 70 °C, 5 days) resulted
in a 20% less conversion through to 5 in the latter reaction.
(iii) 5 was not formed upon heating 4 in the presence of 1,3-
diisopropyl-4,5-dimethylimidazolium chloride, DBU, or a proton
sponge.
We have attempted to prove that the N-H in 4 is the source of
the C-H in 5 through N-H/N-D exchange using D2O. However,
these experiments proved inconclusive as not only partial H/D
exchange was observed at nitrogen but also deuterium was
incorporated into the Ru-H bond.
In summary, we have described a rare example of C-N bond
activation of an NHC and the subsequent unprecedented transfor-
mation of the resultant C-2 bound carbene complex 4 to the N-1
bound product 5. The need for free carbene in converting 4 to 5 is
consistent with a base-catalyzed process. Our results provide
experimental support to Crabtree and Eisenstein’s computational
study, which show that while C-binding is favored on moving to
heavier metals, the presence of CO trans to the heterocycle stabilizes
the N-bound form. This is the case with 4 and 5.
(4) (a) Gru¨ndermann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree,
R. H. J. Am. Chem. Soc. 2002, 124, 10473. (b) Lebel, H.; Janes, M. K.;
Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046. (c)
Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E. Organo-
metallics 2004, 23, 166. (d) Viciano, M.; Mas-Marza´, E.; Poyatos, M.;
Sanau´, M.; Crabtree, R. H.; Peris, E. Angew. Chem., Int. Ed. 2005, 44,
444. (e) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L. Angew. Chem.,
Int. Ed. 2005, 44, 5282.
(5) (a) Sini, G.; Eisenstein, O.; Crabtree, R. H. Inorg. Chem. 2002, 41, 602.
(b) Chianese, A. R.; Crabtree, R. H. In ActiVation and Functionalization
of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; ACS Symposium
Series 885: American Chemical Society: Washington, DC, 2004; pp 169-
184.
(6) Sundberg, R. J.; Bryan, R. F.; Taylor, I. F., Jr.; Taube, H. J. Am. Chem.
Soc. 1974, 96, 381.
(7) C- and N-bound tautomers have been postulated in both stoichiometric
and catalytic processes. (a) Mu¨ller, J.; Stock, R. Angew. Chem., Int. Ed.
1983, 22, 993. (b) Tan, K. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem.
Soc. 2003, 124, 3203. (c) Caballero, A.; Jalo´n, F. A.; Manzano, B. R.;
Espino, G.; Pe´rez-Manrique, M.; Mucientes, A.; Poblete, F. J.; Maestro,
M. Organometallics 2004, 23, 5694. (d) Wiedemann, S. H.; Lewis, J. C.;
Ellman, J. A.; Bergman, R. G. J. Am. Chem. Soc. 2006, 128, 2452.
(8) Complex 3 is formed upon heating Ru(PPh3)3(CO)H2 with IiPr2Me2 directly
and is reversible under H2 at 50 °C. Burling, S.; Paine, B. M.; Nama, D.;
Brown, V. S.; Mahon, M. F.; Prior, T. J.; Pregosin, P. S.; Whittlesey, M.
K.; Williams, J. M. J., unpublished results.
(9) 1,3-Diisopropyl-4,5-dimethylimidazolium chloride, presumably formed by
reductive elimination from an initially formed complex such as Ru(NHC)-
(PPh3)2(CO)HCl, precipitated from the reaction mixture at early times
(prolonged heating led to dissolution of the precipitate). The resulting
“Ru(PPh3)2(CO)” fragment would provide a pathway to Ru(PPh3)3(CO)-
H2.
(10) Burling, S.; Mahon, M. F.; Paine, M. F.; Whittlesey, M. K.; Williams, J.
M. J. Organometallics 2004, 23, 4537.
(11) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.; de K. Lewis, A. K. Angew.
Chem., Int. Ed. 2004, 43, 5824.
(12) (a) Galan, B. R.; Gembicky, M.; Dominiak, P. M.; Keister, J. B.; Diver,
S. T. J. Am. Chem. Soc. 2005, 127, 15702. (b) Becker, E.; Stingl, V.;
Dazinger, G.; Puchberger, M.; Mereiter, K.; Kirchner, K. J. Am. Chem.
Soc. 2006, 128, 6572.
This report of C-N cleavage and C-/N-tautomerism, alongside
other recent examples detailing NHC insertion reactions,12 illustrate
new carbene degradation pathways which are of clear relevance to
the catalysis and ionic liquid communities.
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