When 3+1 is Less Than 2+2: Surprisingly Moderate Hydricities in Heteroleptic Rhodium Complexes Containing Triphosphine and Monophosphine Ligands

Abstract

Rhodium(I) complexes with the formula [Rh(PP2)(PR3)]+ (PP2 = PhP(CH2CH2PPh2)2; R = Ph, Et, or Me) were synthesized and characterized. Their reactions with H2 in the presence of strong bases, forming rhodium(I) hydrides with the formula HRh(PP2)(PR3), were studied, and the structure of HRh(PP2)(PPh3) was determined by X-ray crystallography. These are the first structurally characterized rhodium complexes with this combination of triphosphine and monophosphine ligands. Their thermodynamic hydricities were determined from the equilibrium constants of their heterolytic H2 activation equilibria. The values of ΔG°H– range from 37.3 kcal/mol for HRh(PP2)(PMe3) to 42.4 kcal/mol for HRh(PP2)(PPh3) in MeCN. These hydricities are surprisingly modest in comparison to their close analogues with the HRh(diphosphine)2 ligand framework, which are among the most powerful known hydride donors. Due to this decreased reactivity, the hydricities are in a nearly optimal range for CO2 hydrogenation, balancing the favorability of hydride transfer with the free energy for H2 activation. The structural factors underlying the thermodynamic trends, including the relatively small variation in hydricity imparted by the monophosphine and the higher overall ΔG°H– values compared to electronically similar bis(diphosphine) complexes, are largely attributable to the small bite angles within the chelate rings of the triphosphine ligand in these complexes.