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

Abstract

Addition of acetonitrile to the complex [Ru(η 5 -indenyl)(PR 2 )(PPh 3 )] ( 1 ) gives the unusual metalation product [Ru(η 5 -indenyl)(CH 2 CN)(HPR 2 )(PPh 3 )] ( 2 ), which has been structurally characterized. This reaction clearly demonstrates high Brønsted basicity at the terminal phosphido ligand in 1 . 31 P{ 1 H} NMR studies show that less acidic N-donor solvents simply disrupt the Ru−P π-bond in 1 to give adducts [Ru(η 5 -indenyl)(L)(HPR 2 )(PPh 3 )] (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(η 5 -indenyl)(NCCH 3 )(HPCy 2 )(PPh 3 )][PF 6 ] ( 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 31 P{ 1 H} NMR, of multiple product isotopomers in the reaction of 1 with a 1:1 mixture of d 3 - and d 0 -acetonitrile. O-Donor solvents also deprotonated by 1 include water, alcohols, and acetone, which give the complexes [Ru(η 5 -indenyl)(X)(HPCy 2 )(PPh 3 )], where X = OR ( 8 ), CH 2 (═O)CH 3 ( 10 ).