Ruthenium hydride complexes with zwitterionic quinonoid ligands-isomer separation, structural properties, electrochemistry, and catalysis

摘要

Reactions of [Ru(PPh _3) _3(CO)(H)Cl] with the zwitterionic p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6-(n- butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q ~1), 4-(isopropylamino)-6-(isopropylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q ~2), and 4-(benzylamino)-6-(benzylimino)-3-oxocyclohexa-1,4-dien-1- olate (Q ~3) in the presence of a base led to the formation of mononuclear complexes [Ru(PPh _3) _2(CO)(H)(Q _(-H) ~1)] (1a and 1b), [Ru(PPh _3) _2(CO)(H)(Q _(-H) ~2)] (2a and 2b), and [Ru(PPh _3) _2(CO)(H)(Q _(-H) ~3)] (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 Q _(-H) ~2 and Q _(-H) ~3 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.