A high-pressure infrared spectral study of the reactions of phosphine-substituted derivatives of tetrairidium dodecacarbonyl with carbon monoxide and hydrogen

Abstract

The reactions of tetranuclear phosphine-substituted iridium carbonyls of the types Ir₄(CO)₉L₃, where L = PPh₃, P(p-Me·C₆H₄)₃, PEt₃, and PPrⁱ ₃, and Ir₄(CO)₈L₄, where L = PEt₃, PPrⁿ ₃, and PBuⁿ ₃, under pressures of carbon monoxide and hydrogen have been investigated by following i.r. spectral changes in a high-pressure spectraphotometric cell. The tetranuclear arylphosphine- and isopropylphosphine-substituted derivatives are cleaved by reaction with carbon monoxide to give the dinuclear species Ir₂(CO)₆L₂, which for L = PPh₃, P(p-Me·C₆H₄)₃, and PPrⁱ ₃, have been isolated and characterised. Further reactions of the dinuclear compounds occur under more forcing conditions with the reversible formation of Ir₂(CO)₇L. Breakdown of the tetranuclear cluster by carbon monoxide occurs less readily with the n-alkylphosphine-substituted derivatives instead initial replacement of phosphine by carbon monoxide occurs yielding Ir₄(CO)₉L₃, Ir₄(CO)₁₀L₂, and finally Ir₂(CO)₇L, where L = PEt₃, PPrⁿ ₃, and PBuⁿ ₃. Reactions of both the aryl- and alkyl-phosphine-substituted iridium carbonyls with carbon monoxide and hydrogen mixed gases follow similar courses, finally leading to cleavage of the tetranuclear and dinuclear compounds with the formation of mononuclear hydridocarbonyls of the type HIr(CO)₃L, where L = PPh₃, P(p-CH₃C₆H₄)₃, PEt₃, PPrⁿ ₃, PPrⁱ ₃, and PBuⁿ ₃. The significance of the formation of the latter species under high pressures and temperatures is discussed in relation to catalytic species involved in the hydroformylation reaction.