Kinetics and electron paramagnetic resonance evidence of an electron-transfer chain mechanism for PPh3 substitution of [Fe3(CO)12]

Abstract

Rapid PPh₃ substitution of [Fe₃(CO)₁₂] to give [Fe₃(CO)₁₁(PPh₃)] occurs in tetrahydrofuran at ambient temperature. The reaction is independent of the complex concentration. An electron-transfer-catalysed mechanism is operative in this reaction, with [Fe₃(CO)₁₂]˙^– as the catalytically active species. The EPR studies indicate that [Fe₃(CO)₁₂]˙^– is produced rapidly at the beginning of the reaction and its concentration is maintained at a constant value during the reaction. The substitution rate is equal to k[Fe₃(CO)₁₂˙^–] with k=(2.7 ± 0.1)× 10^–3 s^–1. Dissociation of a CO ligand from this radical to yield [Fe₃(CO)₁₁]˙^– is the rate-limiting step for this electron catalysis. It is believed that [Fe₃(CO)₁₂]˙^– is formed from a fast disproportionation of [Fe₃(CO)₁₂] induced by an impurity in PPh₃. The impurity is likely PPh₃O and its concentration is proportional to that of the PPh₃ used.