Synthesis of acylphosphine complexes by controllable migration of acyl groups from molybdenum to phosphido ligands†

Abstract

The acyl complexes [Mo(COR¹)(CO)₂(PPh₂H)(η-C₅H₅)] (R¹ = Me or Et) have been prepared in high yield by the reaction of [MoR¹(CO)₃(η-C₅H₅)] with PPh₂H. Deprotonation of the diphenylphosphine ligand with 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu) at –78 °C produced a phosphorus-centred anion which can be alkylated by treatment with R²I (R² = Me or Et) to give the substituted phosphine acyl complexes [Mo(COR¹)(CO)₂(PPh₂R²)(η-C₅H₅)], or acylated with R²COCl to produce acylphosphine acyl complexes [Mo(COR¹)(CO)₂(PPh₂COR²)(η-C₅H₅)]. If the deprotonation is carried out at room temperature, however, migration of the metal acyl group to the phosphorus atom occurs to give the molybdenum-centred anion [Mo(CO)₂(PPh₂COR¹)(η-C₅H₅)]^–, which can in turn be alkylated with R²I to give the acylphosphine alkyl complexes [MoR²(CO)₂(PPh₂COR¹)(η-C₅H₅)]. The metal-centred anion also undergoes typical reactions with H⁺ and chlorinated solvents to give [MoX(CO)₂(PPh₂COR¹)(η-C₅H₅)] (X = H or Cl). Mechanistic studies showed that (i) on deprotonation of the related complex [MoMe(CO)₂(PPh₂H)(η-C₅H₅)] the methyl group does not undergo migration to phosphorus, and (ii) the reaction is largely intramolecular but with a measurable intermolecular component. A mechanism is therefore proposed involving nucleophilic attack on the acyl carbon atom by the anionic phosphido ligand. Full spectroscopic data for the new complexes are reported and interpreted, and the crystal structure of [Mo(COMe)(CO)₂(PPh₂COMe)(η-C₅H₅)] has been determined.