Synthesis and structural characterisation of rhodium hydride complexes bearing N-heterocyclic carbene ligands

Addition of excesses of N-heterocyclic carbenes (NHCs) IEt2Me2, I'Pr2Me2 or ICy (IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene; I'Pr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; ICy = 1,3-dicyclohexylimidazol-2-ylidene) to [HRh(PPh3)(4)] (1) affords an isomeric mixture of [HRh(NHC)(PPh3)(2)] (NHC = IEt2Me2 (cis-/trans-2), I'Pr2Me2 (cis-/trans-3), ICy (cis-/trans-4)) and [HRh(NHC)(2)(PPh3)] (IEt2Me2 (cis-/trans-5), I'Pr2Me2 (cis-/trans-6), ICy (cis-/trans-7)). Thermolysis of 1 with the aryl substituted NHC, 1,3-dimesityl-4,5-dihydroimidazol-2-ylideiie (IMesH(2)), affords the bridging hydrido phosphido dimer, [{(PPh3)(2)Rh}(2)-(mu-H)(mu-PPh2)] (8), which is also the reaction product formed in the absence of carbene. When the rhodium precursor was changed from 1 to [HRh(CO)(PPh3)(3)] (9) and treated with either INIes (=1,3-dimesitylimidazol-2-ylidene) or ICy, the bis-NHC complexes trans-[HRh(CO)(IMeS)(2)] (10) and trans-[HRh(CO)(ICY)(2)] (11) were formed. In contrast, the reaction of 9 with I'Pr2Me2 gave [HRh(CO)(I'Pr2Me2)(2)] (cis-/trans-12) and the unusual unsymmetrical dimer, [(PPh3)(2)Rh(mu-CO)(2)Rh(I'Pr2Me2)(2)] (13). The complexes trans-3, 8, 10 and 13 have been structurally characterised. (c) 2005 Elsevier B.V. All rights reserved.