The syntheses and structures of coordinatively unsaturated aryloxy-hydride complexes of molybdenum, Mo(PMe3)4(OAr)H: reversible CH bond activation and comparison with their tungsten analogues

Mo(PMe3)6 reacts with ArOH (Ar=2,4,6-C6H2Me3, 2,6-C6H3Pri2) to give coordinatively unsaturated aryloxy-hydride derivatives Mo(PMe3)4(OAr)H. The formation of Mo(PMe3)4(OAr)H is in marked contrast to the cyclometalated products of CH bond activation that are obtained for the corresponding tungsten system. Deuterium labeling and magnetization transfer studies, however, demonstrate that the coordinatively unsaturated molybdenum complexes Mo(PMe3)4(OAr)H are in fact kinetically capable of intramolecular oxidative addition of a CH bond of the ortho substituents to yield cyclometalated derivatives that are thermodynamically unstable with respect to the aryloxy-hydride derivatives.