Reaction of [M(η3-allyl)(η2-amidinato)(CO)2(pyridine)] complexes (M = Mo, W) with bidentate ligands: nitrogen donor vs. phosphorus donor

Abstract

The reactivity of amidinato complexes of molybdenum and tungsten bearing pyridine as a labile ligand, [M(η³-allyl)(η²-amidinato)(CO)₂(pyridine)] (M = Mo; 1-Mo, M = W; 1-W), toward bidentate ligands such as 1,10-phenanthroline (phen) and 1,2-bis(diphenylphosphino)ethane (dppe) was investigated. The reaction of 1 with phen at ambient temperature resulted in the formation of monodentate amidinato complexes, [M(η³-allyl)(η¹-amidinato)(CO)₂(η²-phen)] (M = Mo; 2-Mo, M = W; 2-W), which has pseudo-octahedral geometry with the amidinato ligand coordinated to the metal in an η¹-fashion. The phen ligand was located coplanar with two CO ligands and the η¹-amidinato ligand was positioned trans to the η³-allyl ligand. In solution, both complexes 2-Mo and 2-W showed fluxionality, and complex 2-Mo afforded allylamidine (3) on heating in solution. In the reaction of 1 with dppe at ambient temperature, the simple substitution reaction took place to give dppe-bridged binuclear complexes [{M(η³-allyl)(η²-amidinato)(CO)₂}₂(µ-dppe)] (M = Mo; 5-Mo, M = W; 5-W), whereas mononuclear monocarbonyl complexes [M(η³-allyl)(η²-amidinato)(CO)(η²-dppe)] (M = Mo; 6-Mo, M = W; 6-W) were obtained under acetonitrile- or toluene-refluxing conditions. Mononuclear complex 6 was also obtained by the reaction of binuclear complex 5 with 0.5 equivalents of dppe under refluxing in acetonitrile or in toluene. The X-ray analyses and variable-temperature ³¹P NMR spectroscopy of complex 6 indicated the existence of the rotational isomers of the η³-allyl ligand, i.e., endo and exo forms, with respect to the carbonyl ligand. The different reactivity of complex 1 toward phen and dppe seems to have come from the difference in the π-acceptability of each bidentate ligand.