Synthesis, Characterization, and Dehydrogenation Activity of an Iridium Arsenic Based Pincer Catalyst Daniel F. Brayton, Paul R. Beaumont, Erin Y. Fukushima, Hope T. Sartain, David Morales-Morales, † and Craig M. Jensen* Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States * S Supporting Information ABSTRACT: A new arsenic-based pincer (AsOCOAs) dehydrogenation catalyst has been synthesized, IrHCl{2,6- C 6 H 3 -(O-AsBu t 2 )} (3). Treatment with an equivalent of base (NaO-tert-butoxide) under an atmosphere of hydrogen gas affords the dihydride catalyst IrH 2 {2,6-C 6 H 3 -(O-AsBu t 2 )} (4). The activity of 3 was explored under transfer dehydrogenation conditions with cyclooctane and tert-butyl ethylene, giving a maximum turnover number of 960 at 175 °C in 24 h. Acceptorless dehydrogenations were also explored with pyrrolidine-based molecules, ethylperhydrocarbazole (5), methylperhydroindole (6), and butylpyrrolidine (7), in which all results indicate 3 is roughly half as active as the analogous phosphine-based pincer catalyst 2. Akin to the phosphine pincer catalysts the activity was seen to steadily improve with increasing temperature, peaking at 175 °C, upon which thermal decomposition sets in. ■ INTRODUCTION For decades, hydrogen has been targeted as the utopian fuel of the future on account of its abundance and environmental friendliness. However, a major difficulty in the utilization of hydrogen as a fuel is the problem of high-density storage. Thus, a high-density, high-stability method for storing hydrogen is essential to the implementation of fuel cells in all but a few niche applications. Another major concern about hydrogen is implementing a suitable infrastructure. While this could be done for any form of hydrogen carrier, the barrier to implementing a liquid organic carrier (LOC) of hydrogen would be significantly lower than others as it is a similar type of chemical to the current distribution system. In addition to the easy adaption to existing infrastructures, LOCs have many other practical advantages. They are cheap, abundant LOCs that can reversibly release 7-8 wt % hydrogen. They can be economically manufactured in the massive quantities required to meet the anticipated demand and would eliminate the thermal management problems commonly associated with the systems based on solid-state hydrogen-absorbing materials. We have recently discovered catalysts that could potentially enable this technology. 1-3 In 1997, the Jensen lab discovered that the “pincer” complex IrH(Cl){2,6-C 6 H 3 -CH 2 PBu t 2 } 2 (1) catalyzes the dehydrogen- ation of cycloalkanes to arenes. 1-3 This was the first report of a homogeneous catalyst for this reaction. The unique reactivity of this especially robust and active catalyst can be ascribed to the tridentate “PCP pincer” ligands, which contain two coordinat- ing, neutral phosphorus centers and an anionic, coordinating carbon site. It has been found that the electronic environment of the catalytic metal center of the pincer complex is highly sensitive to minor changes in the PCP pincer ligand. 4-6 It is now well established that dihydro POCOP pincer iridium complexes can selectively dehydrogenate aliphatic groups under much milder conditions than those required for the corresponding heterogeneous catalysts, such as platinum on alumina, without harm to other functional groups of an organic molecule. 5a,b,6 More recent reports have described that related POCOP pincer complex 2 is a highly active precatalyst for the dehydrogenation of alkanes and heterocylic amines, Scheme 1. 4,7 Previous findings have shown that pyrrolidine-based heterocycles are known to have lower ΔH values of dehydrogenation; thus a small family of LOCs was chosen for an initial screening. 8,9 Seeking to improve this system, we made an alteration to the pincer ligand using arsenic in place of phosphorus as the chelating atoms (AsOCOAs). 10 Herein we formally report the synthesis, characterization, and dehydrogen- ation activity of the related arsenic-based iridium pincer complexes IrHCl{2,6-C 6 H 3 -(O-AsBu t 2 ) 2 }(3) and IrH 2 {2,6- C 6 H 3 -(O-AsBu t 2 ) 2 }(4), Scheme 2. To our knowledge only one other arsenic pincer is known of the “ANA” type in which pyridine is the aromatic portion of the pincer, binding through Received: May 13, 2014 Scheme 1. Catalysts 1 and 2 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A dx.doi.org/10.1021/om5005034 | Organometallics XXXX, XXX, XXX-XXX