Dimeric rhodium-ethylene NHC complexes as reactive intermediates for the preparation of tetra-heteroleptic NHC complexes

Dimeric rhodium complexes with various N-heterocyclic carbene (NHC) ligands have been synthesized and fully characterized. X-ray analysis unambiguously confirms the bimetallic nature of these complexes, and in all cases one molecule of ethylene is coordinated to each metal center in an η²- fashion. The Rh atoms are also coordinated to one NHC ligand and are interconnected by two μ-chlorine bridges. The dimeric nature of the complexes is most likely stabilized due to the significant steric bulk around the metal centers provided by the carbene ligands. Consistent with this, modulating the steric properties and backbone saturation of the ligands was shown to have a significant effect on the stability and geometry of the complexes. Treatment of the carbene dimers with ligands such as PPh₃ results in cleavage of the dimers and a unique synthesis of tetra-heteroleptic complexes of the general formula [ClRh(NHC)(PR₃)(CH₂=CH₂)]. The stabilities of these compounds have been assessed, and although decomposition to Wilkinson's complex is observed upon treatment with an excess of phosphine for prolonged times, the presence of the ethylene ligand provides greatly increased stability compared with the bis-phosphine analogues [ClRh(NHC)(PPh ₃)₂].