An investigation of the barriers to trimethylenemethane (TMM) rotation in compounds of the type η 4-TMMFe(CO 2 L (L  tBuNC, AsPh 3, tertiary phosphines); examples of coordinated tertiary phosphine rotational isomerism

Abstract

Variable temperature 1H NMR studies of the compounds η 4-TMMFe(CO) 2L (L  PMe 2Ph, PMePh 2, PPh 3, PBz 3, tBuNC, AsPh 3, P( p-MeOC 6H 4) 3) have been carried out in order to determine the barriers to TMM rotation. It is found that the values of Δ G ‡ are lowered by good σ, donor ligands L, the variation in Δ S ‡ being a more important factor than the variation in Δ H ‡. The compounds containing the ligands PMe 2Ph, PMePh 2, PCy 2Ph and PCyPh 2, with two different substituents, also exhibit conformational isomerism, as indicated by doubling of the carbonyl stretching bands in the IR spectra; MMX calculations confirm the identity of the conformations as the two possible staggered rotamers of C 1 and C s symmetry.