Methanol Dehydrogenation by Iridium N‑Heterocyclic Carbene Complexes Jesú s Campos, † Liam S. Sharninghausen, † Michael G. Manas, and Robert H. Crabtree* Chemistry Department, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States * S Supporting Information ABSTRACT: A series of homogeneous iridium bis(N- heterocyclic carbene) catalysts are active for three trans- formations involving dehydrogenative methanol activation: acceptorless dehydrogenation, transfer hydrogenation, and amine monoalkylation. The acceptorless dehydrogenation reaction requires base, yielding formate and carbonate, as well as 2-3 equivalents of H 2 . Of the few homogeneous systems known for this reaction, our catalysts tolerate air and employ simple ligands. Transfer hydrogenation of ketones and imines from methanol is also possible. Finally, N-mono- methylation of anilines occurs through a “borrowing hydro- gen” reaction. Notably, this reaction is highly selective for the monomethylated product. ■ INTRODUCTION Methanol, produced from natural gas, coal, or biomass, 1 is an abundant and cheap resource with many applications. 2-4 It has even been suggested as a future energy carrier and synthetic feedstock in the “methanol economy”. 5 Methanol can be activated by catalyzed conversion to the more reactive H 2 CO, with formation of metal hydrides or the release of H 2 (Scheme 1). 6,7 The carbonyl group can then undergo nucleophilic attack with or without metal mediation. With OH - as a nucleophile, formate and a second molecule of H 2 can be produced, and {formate + H + } can be subsequently dehydrogenated to CO 2 (path A in Scheme 1). 8 If the nucleophile is an amine, an imine intermediate can lead to methylation of the amine in a “borrowing hydrogen” (BH) reaction (path B in Scheme 1). 6,7,9,10 Alternatively, the hydrogen borrowed from methanol could be transferred to a second molecule containing a CX (X = C, N, O) bond by hydrogen transfer (path C in Scheme 1). 6,11 However, no single catalyst has so far been proven to be capable of accomplishing all of these reactions under relatively mild conditions. 6,8 In this paper, we apply our new family of homogeneous iridium bis(N- heterocyclic carbene) (bis-NHC) catalysts to these reactions. Methanol can serve as an excellent storage reservoir for hydrogen, 1,8,12 which can, in turn, generate electricity via a fuel cell. 13 Liquid organic hydrogen carriers such as methanol have garnered much recent attention, 1,14,15 in part because such fuels would be compatible with the current gasoline-based infra- structure. 16 Methanol is especially attractive because of its low cost, high energy density (up to 3 H 2 per MeOH, or 12.6 wt % H), and the fact that no C-C bond cleavage is required for hydrogen release. 1 A major current challenge lies in developing catalysts that completely and selectively release all H 2 from methanol under mild conditions. 1,8 Traditional heterogeneous methanol dehydrogenation cata- lysts convert methanol to CO 2 and H 2 but often require high temperature (>200 °C) and pressure, produce unwanted CO (a poison for most fuel-cell catalysts), and can be pyrophoric. 1,17 Early homogeneous catalysts based on ruthenium did produce H 2 without CO contamination but suffered from low activity [turnover frequency (TOF) < 4 h -1 ] and limited reactivity, releasing only a single H 2 per MeOH. 18 Some new homogeneous acceptorless dehydrogenation catalysts involve ruthenium complexes with cooperative chelating ligands (Figure 1, 1-5). 19-23 Beller and co-workers Special Issue: Small Molecule Activation: From Biological Principles to Energy Applications Received: October 15, 2014 Published: January 23, 2015 Scheme 1. Methanol Activation by Initial Dehydrogenation Forum Article pubs.acs.org/IC © 2015 American Chemical Society 5079 DOI: 10.1021/ic502521c Inorg. Chem. 2015, 54, 5079-5084