FULL PAPER DOI: 10.1002/ejic.200801169 N-Heterocyclic Carbene Rhodium Complexes and Their Reactions with H 2 and with CO Xiao-Yan Yu, [a] Hongsui Sun, [a] Brian O. Patrick, [a] and Brian R. James* [a] Keywords: Carbene ligands / Carbonyl ligands / Rhodium / Hydrogenation / Hydrides 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-trimeth- ylphenyl)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 di- hydride-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. Reac- tions of the mono-carbenes (1/3, or 2/4) with CO in hexane Introduction Interest in NHC-Rh complexes (NHC = N-heterocyclic carbene) shows no signs of abatement, as can judged by the contents of this special issue of the Eur. J. Inorg. Chem., and other recent publications [1–12] and the references contained therein. Our initial studies in metal-carbene chemistry [13] developed from attempts to replace oxidizable tertiary phosphane ligands in a Ru system that showed catalytic ac- tivity for conversion of N 2 O to N 2 and O 2 . [14] We later re- ported that (carbene-phosphanyl)-Rh I species undergo “standard” oxidative addition of O 2 to give Rh III -peroxide species, [15] while Crudden and co-workers have recently sug- gested that such species are best considered as Rh I with coordinated singlet oxygen. [16] Our studies [13–15] have thus far mainly utilized the simple, unfunctionalized NHCs, N,N-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) (see Scheme 1), in syntheses of the RhCl(diene)(NHC) and [RhCl(COE)(NHC)] 2 complexes, and reactivities of these complexes with O 2 (diene = NBD or COD, and COE = cyclooctene). Because of our interests in catalytic hydrogen- ation/hydroformylation, we then studied reactions of the [RhCl(COE)(NHC)] 2 complexes with H 2 and with CO, with an ultimate goal of comparing catalytic activity of such sys- [a] Department of Chemistry, The University of British Columbia, Vancouver, BC, V6T 1Z1, Canada Fax: +1-604-822-2847 E-mail: brj@chem.ubc.ca © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2009, 1752–1758 1752 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, 1 H and 13 C{ 1 H} 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 hydrogena- tion 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) tems with those of classic Rh-tertiary phosphane sys- tems. [17] There is literature on reactions of H 2 and CO, and related catalysis, within NHC-Rh I systems, [5,9,13,18–21] but not specifically with the COE/IPr/IMes precursors, and our studies have led to identification of new species, as well as some findings on catalytic hydrogenation and isomerization of COE and 1-octene. Our findings on the O 2 and H 2 reac- tions were first presented at a Canadian Chemistry confer- ence. [22] During the writing of this paper, we heard that Crudden’s group had also been studying reactivity of H 2 , CO, and N 2 , toward the same NHC-Rh I species, and their work (with necessarily some overlap) is reported in this journal issue. [23] Scheme 1. Reactions of 1 and 2 with H 2 .