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Keywords:

  • Quinones;
  • Ruthenium;
  • Hydrides;
  • Noninnocent ligands;
  • Redox chemistry;
  • Transfer hydrogenation catalysis

Abstract

Reactions of [Ru(PPh3)3(CO)(H)Cl] with the zwitterionic p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6-(n-butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q1), 4-(isopropylamino)-6-(isopropylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q2), and 4-(benzylamino)-6-(benzylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q3) in the presence of a base led to the formation of mononuclear complexes [Ru(PPh3)2(CO)(H)(Q1H)] (1a and 1b), [Ru(PPh3)2(CO)(H)(Q2H)] (2a and 2b), and [Ru(PPh3)2(CO)(H)(Q3H)] (3a and 3b), respectively. The positional isomers (a and b) that were formed in each case were separated by preparative TLC. The structural characterization of 2a and 3a·MeCN helped to identify the isomers, and established the distorted octahedral coordination geometry around the ruthenium center. The bond lengths in the complexes are consistent with localization of the double bonds in Q2H and Q3H in both their monodeprotonated and metal-coordinated forms. The Ru–C–O(carbonyl) bond angle is almost linear. Cyclic voltammetry of the complexes showed one oxidation and one reduction process. These are predominantly centered on the quinonoid ligands, which shows their redox-noninnocent character. Studies of transfer hydrogenation with 2a as a precatalyst showed that, in the presence of KOH, acetophenone could be converted to 1-phenylethanol within 10 h in over 90 % yield.