Phosphorus-carbon bond cleavage at a di-iron centre: synthesis of μ-phosphidomethyl complexes [Fe 2(CO) 6(μ-CH 2PR 2)(μ-PR 2)] from [Fe 2(CO) 6(μ-R 2PCH 2PR 2)

Abstract

Upon heating in toluene at reflux the di-iron heptacarbonyl complexes [Fe 2(CO) 6(μ-CO)(μ-R 2PCH 2PR 2)] (RPh, Me, Et, iPr, OEt) lose carbon monoxide, resulting in phosphorus-methylene bond cleavage to give the μ- phosphidomethyl complexes [Fe 2(CO) 6(μ-CH 2PR 2)(μ-PR 2)]. The ability of the phenyl group to stabilise the μ- CH 2PR 2 ligand is seen in the thermolyses of the diphosphine complex [Fe 2(CO) 6(μ-CO)(μ-Ph 2PCH 2PMe 2)], which undergoes selective Me 2PCH 2 bond cleavage to yield [Fe 2(CO) 6(μ-CH 2PPh 2)(μ-PMe 2)], and of the bis-diphosphine complexes [Fe 2(CO) 4(μ-CO)(μ-R 2PCH 2PR 2)(μ-Ph 2PCH 2PPh 2)] (RPh, Me), which results only in Ph 2PCH 2 bond cleavage to give [Fe 2(CO) 4(μ-R 2PCH 2PR 2)(μ-CH 2PPh 2)(μ-PPh 2)]. The ubiquity of μ-CH 2PPh 2 is attributed to the existence of a zwitterionic form in which positive charge residing on phosphorus is dispersed into the phenyl rings. The complexes [Fe 2(CO) 4(μ-R 2PCH 2PR 2)(μ-CH 2PPh 2)(μ-PPh 2)] exist as mixtures of geometric isomers a and b, identified by 31P NMR spectroscopy and an X-ray diffraction study on the major isomer a of [Fe 2(CO) 4(μ-Me 2PCH 2PMe 2)(μCH 2PPh 2)(μ-PPh 2)] as its dichloromethane solvate, which contains a cis arrangement of phosphido and phosphidomethyl ligands with the diphosphine lying trans to the latter. Methyl substitution in the diphosphine backbone suppresses phosphorus-methylene bond cleavage and results instead in ortho-metalation and phosphorus-phenyl bond cleavage. Thus, on heating [Fe 2(CO) 6(μ-CO){μ-Ph 2PCH(Me)PPh 2}] carbon monoxide and benzene are lost and [Fe 2(CO) 6{μ-PhPCH(Me)P(Ph)(C 6H 4- o)}] is formed, structurally characterised by X- ray diffraction. Substitution of two methyl groups into the diphosphine backbone favours ortho-metalation more strongly still and UV irradiation of the chelate complex [Fe(CO) 3{η 2-Ph 2PC(Me 2)PPh 2}] in the presence of iron pentacarbonyl yields [Fe 2(CO) 6{μ-PhPC(Me 2)P(Ph)(C 6H 4- o)}] directly. The structure of [Fe 2(CO) 6(μ-CO){μ-Ph 2PCH(Me)PPh 2}] as its hexane solvate was examined by X-ray diffraction, for comparison with that of [Fe 2(CO) 6(μ- CO){μ-Ph 2PCH 2PPh 2}]. The structure analysis was not satisfactory but no significant differences between the molecular structures were observed. The suppression of backbone P-C cleavage by methyl substitution is attributed to the destabilisation of the zwitterionic form of μ-CR 2PPh 2, which has negative charge residing on the carbon.