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

The NHC-Rh-I 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 H-2 in hexane to give the dimeric, mono-carbene dihydrido species [Rh(H)(2)Cl(NHC)](2) (NHC = IPr (3), IMes (4)). In the presence of further NHC, the bis-carbene dihydrido species Rh(H)(2)Cl(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)(2)Cl(IPr)(IMes) (5a) was also observed but not isolated. A benzene solution of 5 under D-2 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, H-1 and C-13{H-1} 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)(2)Cl(IMes)(2) for hydrogenation of COE and 1-octene (and isomerization of the latter) are shown to be poor. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)