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

The reactivity of amidinato complexes of molybdenum and tungsten bearing pyridine as a labile ligand, [M(eta(3)-allyl) (eta(2)-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(eta(3) -allyl) (eta(1)-amidinato) (CO)(2)(eta(2) -phen)] (M = Mo; 2- Mo, M = W; 2- W), which has pseudo-octahedral geometry with the amidinato ligand coordinated to the metal in an eta(1)-fashion. The phen ligand was located coplanar with two CO ligands and the eta(1)-amidinato ligand was positioned trans to the eta(3)-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(eta(3) -allyl) (eta(2)-amidinato) (CO)(2)}(2)(mu-dppe)] (M = Mo; 5-Mo, M = W; 5- W), whereas mononuclear monocarbonyl complexes [M(eta(3)-allyl) (eta(2) -amidinato) (CO) (eta(2)-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 P-31 NMR spectroscopy of complex 6 indicated the existence of the rotational isomers of the eta(3)-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 pi-acceptability of each bidentate ligand.