Benzonitrile Adducts of Terminal Diarylphosphido Complexes: Preparative Sources of “RuPR2”

Dehydrohalogenation of secondary diarylphosphine ruthenium complexes in the presence of benzonitrile yields stable, isolable nitrile adducts of the formula [Ru­(η5-indenyl)­(PAr2)­(NCPh)­(PPh3)], in which the terminal phosphido ligand is pyramidal at P and contains a stereochemically active lone pair. Unlike the analogous carbonyl adducts [Ru­(η5-indenyl)­(PAr2)­(CO)­(PPh3)], these benzonitrile complexes behave as masked sources of the highly reactive planar phosphido complexes [Ru­(η5-indenyl)­(PAr2)­(PPh3)], which contain a RuPAr2 π bond. This is illustrated by the addition (or cycloaddition) reactions of the benzonitrile adducts with dihydrogen, methyl iodide, and 1-hexene, as well as their thermal decomposition via orthometalation of the PPh3 ligand. Enthalpies of CO vs NCPh dissociation from the [Ru­(η5-indenyl)­(PR2)­(PPh3)] fragments (R = alkyl, aryl) have been calculated, as has the trajectory of addition of H2 to the model planar phosphido complex [Ru­(η5-indenyl)­(PMe2)­(PMe3)]. The latter study shows the intermediacy of an η2-H2 adduct, [Ru­(η5-indenyl)­(PAr2)­(η2-H2)­(PPh3)], in the formation of [RuH­(η5-indenyl)­(HPMe2)­(PMe3)], a further indication of the importance of the variable binding modes of the terminal phosphido ligand in this system.