Diastereomeric half-sandwich Ru(II) cationic complexes containing amino amide ligands. Synthesis, solution properties, crystal structure and catalytic activity in transfer hydrogenation of acetophenone

Abstract

The cationic complexes [( η 6-arene)Ru(N,O-amino amide)X]Y (arene = p-cymene or indane; N,O-amino amide = ( l)-proline amide or ( l)-phenylalanine amide; X = Cl or I; Y = Cl, I or PF 6) have been synthesised and fully characterized by spectroscopic and analytical methods. In several cases ( 1a, 3a, 4a, 4b, 5) the metal configuration has been definitively established by X-ray analysis on single crystal. The lability of the metal center in solution has been studied by 1H NMR and CD techniques. The highest configurational stability has been found in the complexes of the type [( η 6-indane)Ru(N,O-proline amide)Cl]Y ( 4a,b). The complexes 1b, 2a–b, 3b, 4b and 5 are good precatalysts for the transfer hydrogenation of acetophenone in basic i-PrOH, with ee up to 76% at 30 °C. An ESI(+)-MS study of pre-catalytic solutions has provided useful information on the catalytic mechanism.