Synthesis, reactivity, and molecular and crystal structures of some trinuclear diphenylphosphido-bridged carbonyl derivatives of rhodium

Abstract

Reaction of [{Rh(µ-Cl)(CO)₂}₂] with PPh₂H in benzene affords, in the presence of a base and under an atmosphere of CO, a yellow crystalline product, believed to be [Rh₃(µ-PPh₂)₃(CO)₉] and which degrades to an orange-red species, characterised as the heptacarbonyl [Rh₃(µ-PPh₂)₃(CO)₇], on recrystallisation from methanol under CO at 15 °C. The same reaction in ethanol, also under an atmosphere of CO but in the absence of a base, gave an orange-red crystalline derivative established to be [Rh₃(µ-PPh₂)₃(CO)₆(PPh₂H)]. All three complexes are unstable under any atmosphere but CO and are readily and reversibly decarbonylated, with loss of PPh₂H as well in the case of [Rh₃(µ-PPh₂)₃(CO)₆(PPh₂H)], to a green crystalline species characterized as [Rh₃(µ-PPh₂)₃(CO)₅]. The latter condenses slowly in polar solvents to afford the tetranuclear cluster [Rh₄(µ-PPh₂)₄(µ-CO)₂(CO)₄] while it reacts with Cl₂ to give the 50-electron species [Rh₃(µ-PPh₂)₃(µ-Cl)₂(µ-CO)(CO)₃]. Crystal structure determinations on [Rh₃(µ-PPh₂)₃(CO)₅][monoclinic, space group P2₁/n, a= 14.003(5), b= 17.471 (5), c= 16.523(5)Å, β= 103.76(5)°, and Z= 4], [Rh₃(µ-PPh₂)₃(CO)₇][monoclinic, space group P2₁/c, a= 18.16(1), b= 17.935(10), c= 13.119(10)Å, β= 103.5(1)°, and Z= 4], and [Rh₃(µ-PPh₂)₃(CO)₆(PPh₂H)][monoclinic, space group P2₁/c, a= 12.515(5), b= 11.843(5), c= 37.102(5)Å, β= 94.6(1)°, and Z= 4] have revealed that addition of PPh₂H and/or CO to the pentacarbonyl compound is accompanied by an expansion of the Rh₃P₃ ring from an average Rh–Rh distance of 2.766 Å to one of 3.15 Å for [Rh₃(µ-PPh₂)₃(CO)₇] and one of 3.165 Å for [Rh₃(µ-PPh₂)₃(CO)₆(PPh₂H)].