Homogeneous reduction of carbon monoxide

Abstract

Hydride additions to eighteen electron iron cations containing both cyclopentadienyl and carbonyl Ligands occur regioselectively onto coordinated carbon monoxide to generate formyls which undergo further reduction or disproportionate to iron methyls. Alternatively Ligand loss converts the iron formyls to carbonyl hydrides. The carbonyl hydride [(η 5-C 5H 5)Fe(Ph 2PCH 2CH 2PPh 2)(CO)H] exists in equilibrium with the corresponding formyl complex which in polar solvents disproportionates to the methyl complex [(η 5-C 5H 5)Fe(Ph 2PCH 2CH 2PPh 2)CH 3] but in non polar solvents rearranges to the cyclopentadiene complex [(η 4-C 5H 6)Fe(Ph 2PCH 2CH 2PPh 2)CO]. Deuterium labelling studies have shown this latter migration to be an intramolecular transfer of hydride from the iron to the endo-position of the cyclopentadiene. Hydride reduction of the cation [(η 5-C 5H 5)Fe(PMe 3)(CO) 2] +PF 6 − generates via the formyl [(η 5-C 5H 5)Fe(PMe 3)(CO)CHO] the methyl complex [(η 5-C 5H 5)Fe(PMe 3)(CO)CH 3]. Carbon monoxide insertion into this iron methyl generates the corresponding acetyl complex [(η 5-C 5H 5)Fe(PMe 3)(CO)COCH 3] which is reduced by BH 3.THF to the ethyl complex [(η 5-C 5H 5)Fe(PMe 3)(CO)CH 2CH 3]. Successive clean homologations by treatment with CO and BH 3.THF gives after four carbonylations [(η 5-C 5H 5)Fe(PMe 3)(CO)COCH 2CH 2CH 2CH 3] which on oxidation yields entirely CO-derived pentanoic acid. The carbonyl hydride [(η 5-C 5H 5)Fe(PMe 3)(CO)H] in the presence of carbon monoxide produces the cyclopentadiene complex [(η 4-C 5H 6)Fe(PMe 3)(CO) 2] via an intermolecular hydride transfer from an intermediate formyl which is catalysed by [(η 5-C 5H 5)Fe(PMe 3)(CO) 2] +PF 6 −.