Terminal Phosphido Complexes of the Ru(η5-Cp*) Fragment

In situ generation of the five-coordinate complex Ru(eta(5)-Cp*)(PR2)(PPh3) (2), via dehydrohalogenation of Ru(eta(5)-Cp*)Cl(PR2H)(PPh3), has allowed its reactivity toward a range of small molecules to be compared with that of its well-studied analogue Ru(eta(5)-indenyl)(PR2)(PPh3) (1), in a study designed to assess the likelihood of variable hapticity in the chemistry of complex 1. Reactions of 2 with hydrogen, carbon monoxide, phenylacetylene, ethylene, acrylonitrile, and 1-hexene demonstrate enhanced nucleophilicity/basicity of the terminal phosphido ligand in 2 relative to that in complex 1. Complex 2 also exhibits greater lability of the PPh3 ligand, leading to substitutional product mixtures that were not observed for 1. Both of these features are consistent with the more electron-rich and sterically imposing nature of the Cp* ligand in 2 relative to the indenyl ligand in 1. Nevertheless, the fundamental transformations of the phosphido ligand are comparable for the two complexes. This suggests that variable hapticity does not play a role in reactions of indenyl complex 1, since eta(5)-eta(3) shifts are unlikely to occur for Cp* complex 2. The implications of these reactivity studies for the design of highly active, yet stable, ruthenium half-sandwich catalysts for hydrophosphination are discussed.