FULL PAPER DOI: 10.1002/ejic.201101091 Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands – Isomer Separation, Structural Properties, Electrochemistry, and Catalysis Stephan Hohloch, [a] Pierre Braunstein,* [b] and Biprajit Sarkar* [a] Keywords: Quinones / Ruthenium / Hydrides / Noninnocent ligands / Redox chemistry / Transfer hydrogenation catalysis 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-(iso- propylamino)-6-(isopropylimino)-3-oxocyclohexa-1,4-dien-1- olate (Q 2 ), and 4-(benzylamino)-6-(benzylimino)-3-oxocy- clohexa-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 1 –H )] (1a and 1b), [Ru(PPh 3 ) 2 (CO)(H)(Q 2 –H )] (2a and 2b), and [Ru(PPh 3 ) 2 (CO)(H)(Q 3 –H )] (3a and 3b), respectively. The positional isomers (a and b) that were formed in each case were separated by preparative TLC. The structural charac- terization of 2a and 3a·MeCN helped to identify the isomers, Introduction Quinones have fascinated chemists for decades, [1] and their interaction with transition metals has relevance to bio- logical systems. [2–4] The redox noninnocence of such mole- cules imparts many interesting properties to them. [5–12] Thus, metal complexes of quinonoid ligands have been ex- tensively investigated due to their valence ambiguity and captivating electronic structures, [13–18] their engrossing mag- netic properties, [19–22] their use as bridges for molecular and supramolecular systems, [19,23–36] and in homogeneous catal- ysis. [37–40] In recent years, we have developed the chemistry of the potentially antiaromatic zwitterionic quinonoid li- gands Q [41–44] (Scheme 1) and their metal complexes. Metal complexes of these ligands, where the 6π +6π zwitterionic Scheme 1. Zwitterionic quinonoid ligands. [a] Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany Fax: +49-711-68564165 E-mail: sarkar@iac.uni-stuttgart.de [b] Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201101091. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012, 546–553 546 and established the distorted octahedral coordination geom- etry around the ruthenium center. The bond lengths in the complexes are consistent with localization of the double bonds in Q 2 –H and Q 3 –H 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 predomi- nantly centered on the quinonoid ligands, which shows their redox-noninnocent character. Studies of transfer hydrogena- tion 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. form is more stable than the canonical forms, have found use in homogeneous catalysis, [45,46] redox [47–49] and supra- molecular chemistry, [44] and as spacers for “metal–metal coupling”. [50,51] In this work we have extended the chemistry of these zwitterionic ligands to new ruthenium hydride complexes and probed their use ascatalysts for transfer hydrogenation reactions. Hydride complexes of transition metals are inter- mediates in a variety of useful chemical transformations. [52] The syntheses of [Ru(PPh 3 ) 2 (CO)(H)(Q 1 –H )] (1a and 1b), [Ru(PPh 3 ) 2 (CO)(H)(Q 2 –H )] (2a and 2b), and [Ru(PPh 3 ) 2 - (CO)(H)(Q 3 –H )] (3a and 3b), where Q 1 , Q 2 , and Q 3 are 4-(n-butylamino)-6-(n-butylimino)-3-oxocyclohexa-1,4- dien-1-olate, 4-(isopropylamino)-6-(isopropylimino)-3-oxo- cyclohexa-1,4-dien-1-olate, and 4-(benzylamino)-6-(benz- ylimino)-3-oxocyclohexa-1,4-dien-1-olate, respectively, are presented. The separation of the positional isomers (a and b in each case) of these complexes is reported and discussed. Results obtained from elemental analysis, mass spectrome- try, 1 H NMR and IR spectroscopy, and structural analysis have been used to establish the formulation of these metal complexes. Cyclic voltammetry studies have been carried out to elucidate the redox properties of these complexes. Finally, the use of these complexes as precatalysts for trans- fer hydrogenation is presented and explained. Results and Discussion Synthesis, Spectroscopy, and Structures Q 1 –Q 3 were deprotonated by using KOtBu. Reactions of these deprotonated ligands with [Ru(PPh 3 ) 3 (Cl)(CO)(H)]