Diphenylphosphido-bridged dirhodium complexes containing combinations of 1,5-cyclooctadiene, chelating diphosphines, and monodentate phosphine ligands

Abstract

The syntheses, characterization, and phosphorus-3 1 NMR spectra of 17 new diphenylphosphido-bridged dirhodium complexes of bidentate and monodentate phosphines are presented. Treatment of [RhCI(COD)], and [IrCl(COD)], with lithium diphenylphosphide in THF at room temperature produces the corresponding diphenylphosphido-bridged complexes [ M(p-PPh2)(COD)], in excellent yields. The 1,5-cyclooctadiene (COD) ligands of [Rh(p(-PPh,)(COD)], are replaced stepwise by chelating diphosphines, LL (e.g., Ph2P(CH2),,PPhz, n = 1-4) to produce complexes of the types [(LL)Rh(p-PPh2)2Rh(COD)] and [Rh(p-PPh,)(LL)],. In contrast, reactions with the monodentate phosphines PPh,, PMePh,, PEt,, and PCy, yield only mixed phosphine-COD complexes of the types (R3P)2Rh(pPPh2)2Rh(COD) and (Cy,P)Rh(p-PPh,),Rh(COD). These unusual complexes possess a rhodium-rhodium bond, two dissymmetrically bridging diphenylphosphido groups, and two different stereochemistries around the rhodium atoms. One rhodium is tetrahedral and surrounded by four phosphorus atoms, and the other rhodium (bonded to COD) is nearly planar. In the (R3P)2Rh(p-PPh2)2Rh(COD) complexes, the phosphorus resonance of the p-PPh, group is shifted downfield approximately 300 ppm compared to the position for analogous complexes of the chelating diphosphines; the large shift is attributed to the formation of the rhodium-rhodium bond.