N‐Heterocyclic Carbene Rhodium Complexes and Their Reactions with H2 and with CO

Abstract

AbstractThe NHC‐RhI complexes [RhCl(COE)(NHC)]2 1 and 2 [COE = cyclooctene, NHC in 1 = N,N‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) and, in 2, N,N‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes)] react with H2 in hexane to give the dimeric, mono‐carbene dihydrido species [Rh(H)2Cl(NHC)]2 (NHC = IPr (3), IMes (4)). In the presence offurther NHC, the bis‐carbene dihydrido species Rh(H)2Cl(NHC)2 are formed; a crystal structure of the IPr complex 5 is analogous to that of the known IMes analogue. The dihydride‐mixed‐carbene species Rh(H)2Cl(IPr)(IMes) (5a) was also observed but not isolated. A benzene solution of 5 under D2 slowly generates the corresponding dideuteride. Reactions of the mono‐carbenes (1/3, or 2/4) with CO in hexane afford the respective dicarbonyl complexes RhCl(CO)2(NHC) [NHC = IPr (6), IMes (7)], while CO reactions with the bis‐carbene dihydrides give, respectively, the mono‐carbonyl complex RhCl(CO)(IPr)2 (8) and the known IMes analogue. All the complexes are characterized by elemental analysis, 1H and 13C{1H} NMR and IR spectroscopies and, in the case of 5, by X‐ray crystallography. The catalytic activities of 5 and the previously reported Rh(H)2Cl(IMes)2 for hydrogenation of COE and 1‐octene (and isomerization of the latter) are shown to be poor.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)