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

Addition of excesses of N-heterocyclic carbenes (NHCs) IEt2Me2, IiPr2Me2 or ICy (IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene; IiPr2Me2 = 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), IiPr2Me2 (cis-/trans-3), ICy (cis-/trans-4) and [HRh(NHC)2(PPh3)] (IEt2Me2(cis-/trans-5), IiPr2Me2 (cis-/trans-6), ICy (cis-/trans-7)). Thermolysis of 1 with the aryl substituted NHC, 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH2), affords the bridging hydrido phosphido dimer, [{(PPh3)2Rh}2(μ-H)(μ-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 IMes (=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 IiPr2Me2 gave [HRh(CO)(IiPr2Me2)2] (cis-/trans-12) and the unusual unsymmetrical dimer, [(PPh3)2Rh(μ-CO)2Rh(IiPr2Me2)2] (13). The complexes trans-3, 8, 10 and 13 have been structurally characterised.

The rhodium phosphine hydride complexes [HRh(PPh3)4] or [HRh(CO)(PPh3)3] lose between one and three PPh3 ligands upon reaction with aryl and alkyl substituted N-heterocyclic carbenes (NHCs) at mild temperatures. Several mono- and dinuclear products have been structurally characterised, including trans-[HRh(IiPr2Me2) (PPh3)2] (IiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and trans-[HRh(CO)(IMes)2] (IMes = 1,3-dimesitylimidazol-2-ylidene).Figure optionsDownload as PowerPoint slide