N-Heterocyclic carbene-functionalized ruthenium phosphinidenes: what a difference a twist makes

Abstract

N-Heterocyclic carbenes (NHCs)1 are ubiquitous ligands in transition-metal chemistry and homogeneous catalysis and serve increasingly often as a replacement for tertiary phosphines (R3P). The two ligand classes exert often subtle but crucially different electronic influences on the properties of catalysts.2 Exemplary is the enhanced activity of the second-generation Grubbs metathesis catalyst [(Cy3P)(L)Cl2RudCHPh] [L ) 1,3-dimesityl-4,5-dihydroimidazol- 2-ylidene (H2IMes)] relative to that of the firstgeneration catalyst (L ) Cy3P), which is caused by the differences in σ-donor/π-acceptor ability,3 shape, and symmetry of the ligands.4 Does a similar sensitivity apply to the isolobal phosphinidene5 complexes? We address here the ligand and conformational sensitivities for [(η6-C6H6)(L)RudPMes*] [Mes* ) 2,4,6-tBu3C6H2; L ) IiPr2Me2 (1), L ) Ph3P (2)6] by examining their solutionphase chemistry together with their structure-activity parameters modeled by density functional theory. We simultaneously demonstrate the applicability of phosphinidene complexes to the synthesis of phosphaalkenes (PdC),5a which are unique P-ligands7 and attractive building blocks for P-functionalized polymers.8 The desired novel dark-green crystalline compound 1 (84%) was obtained by a double dehydrohalogenation-ligation sequence9 of the phosphine complex [(η6-C6H6)RuCl2(PH2Mes*)]6 using 3 equiv of IiPr2Me2 in toluene (eq 1):