Proton Transfer to CpRuH(CO)(PCy3) Studied by Low-Temperature IR and NMR Spectroscopy

Abstract

The interaction of the ruthenium hydride complex RuH ≡ CpRuH(CO)(PCy3) (1) with various proton donors AH ≡ CF3CH2OH (2a), (CF3)2CHOH (2b), (CF3)3COH (2c), CF3COOH (2d), and HBF4 (2e) has been studied by variable-temperature IR spectroscopy using hexane and CH2Cl2 as solvents of different polarity. A low-temperature NMR study of the interaction of 1 with 2c was performed using [D8]methylcyclohexane, CD2Cl2, and a liquefied mixture of CDF2Cl/CDF3 (2:1). The first stage of the proton transfer process was found to be the formation of hydrogen-bonded complexes of the type RuH···HA. The hydrogen bonds in these complexes are of medium strength (−ΔH° = 5.3−7.6 kcal mol−1). The second stage is the slow conversion of the H-complex to a dihydrogen complex to which a hydrogen-bonded ion-pair structure [Ru(η2-H2)]+···A− was assigned. The kinetics of this unusually slow proton transfer reaction was monitored in the case of 2c at 200 K in CH2Cl2. Fast protonation of 1 by 2d leads additionally to a species assigned as the free cationic complex [Ru(η2-H2)]+, whose formation is driven by the formation of the homoconjugated anionic complex [AHA]−. At temperatures above 220 K both the hydrogen-bonded ion pair and the free cationic complex easily release dihydrogen, producing RuA.