Ruthenated Acetonitrile: Unusual Brønsted Acidity of a Polar “Aprotic” Solvent

Addition of acetonitrile to the complex [Ru(η5-indenyl)(PR2)(PPh3)] (1) gives the unusual metalation product [Ru(η5-indenyl)(CH2CN)(HPR2)(PPh3)] (2), which has been structurally characterized. This reaction clearly demonstrates high Brønsted basicity at the terminal phosphido ligand in 1. 31P{1H} NMR studies show that less acidic N-donor solvents simply disrupt the Ru−P π-bond in 1 to give adducts [Ru(η5-indenyl)(L)(HPR2)(PPh3)] (L = benzonitrile (6) or pyridine (7)), which are in equilibrium with 1 and free L. The analogous acetonitrile adduct (4) was observed by NMR at 240 K during the formation of 2, but is quickly replaced by 2 at higher temperatures. NMR studies of an alternate route to the metalated complex 2, starting from the cationic N-bound acetonitrile adduct [Ru(η5-indenyl)(NCCH3)(HPCy2)(PPh3)][PF6] (3a), along with the demonstrated lability of the benzonitrile and pyridine adducts, suggest that the metalation of acetonitrile by 1 proceeds via an intermolecular C−H addition across the RuP double bond, rather than the intramolecular C−H activation of N-bound acetonitrile. This is confirmed by the observation, by 31P{1H} NMR, of multiple product isotopomers in the reaction of 1 with a 1:1 mixture of d3- and d0-acetonitrile. O-Donor solvents also deprotonated by 1 include water, alcohols, and acetone, which give the complexes [Ru(η5-indenyl)(X)(HPCy2)(PPh3)], where X = OR (8), CH2(O)CH3 (10).