Mechanism of Isomerization and Methyl Migration in Heterobimetallic Rhenium-Iridium Complexes: Experimental and DFT Study Kothanda Rama Pichaandi, †,⊥ Lara Kabalan, ‡,⊥ Sabre Kais, †,‡ and Mahdi M. Abu-Omar* ,†,§ † Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States ‡ Qatar Environment and Energy Research Institute, Hamd Bin Khalifa University, Qatar Foundation, PO Box 5825, Doha, Qatar § School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering, 480 Stadium Drive, West Lafayette, Indiana 47907, United States * S Supporting Information ABSTRACT: Investigation of the mechanism of conversion of the bimetallic complex [PNP(H)Ir-μ(CH 2 )-μ(O)-Re(O) 2 ][PF 6 ] (1) to its structural isomer [PNP(Me)(CH 3 CN)Ir-ReO 3 ][PF 6 ](2) by detailed kinetics and DFT computational studies is reported. The reaction proceeds by intramolecular rearrangement of 1 to [PNP(Me)Ir-ReO 3 ][PF 6 ](S) via a methyl-bridged [PNP(H)Ir-μ(CH 3 )-Re(O) 3 ]- [PF 6 ](P) intermediate followed by CH 3 CN coordination. The rate-determining step is the transformation of 1 to P displaying ΔG ⧧ of 21.8 kcal/mol. Experimental kinetic results include zero-order dependence on acetonitrile, positive ΔS ⧧ , and deuteration of the bridging methylidene group in the reaction of 1 with CD 3 OD. All of these results support the proposed mechanism. ■ INTRODUCTION Heterobimetallic complexes featuring two metals with vastly different properties and reactivity are of notable interest because of their connection to heterogeneous catalysts that use an early transition metal oxide support and a late noble metal catalyst. 1-3 In the case of homogeneous systems, heterobimetallic complexes of early and late metals are important because they can carry out chemistry that neither metal by itself can do and in enabling multielectron redox processes. Thomas and Lu have independently reported on a number of heterobimetallic complexes and demonstrated their utility in organic transformations and small-molecule activa- tion. 4-9 With increasing R&D in biofuels and biorenewables, 10 our group and others have developed deoxydehydration (DODH) reactions of polyols to alkenes using early transition metal catalysts such as rhenium, molybdenum, and vanadium. 11-18 The prevalence of fracking in the U.S. in the past couple of years has revolutionized the availability of natural gas and light hydrocarbons (LHC). 19 As a result, we have become interested in coupling renewable feedstock such as polyols with LHC, where the biomass-derived molecules act as oxidants of LHC, creating value from both feedstocks. This synergy is illustrated in Scheme 1 for the reaction of pentane with glycerol. The thermodynamics are quite favorable, 51% conversion at 200 °C and 40 bar pressure, 20 conditions that are akin to DODH reaction conditions. Inspired by the work of Goldman and Brookhart on the use of iridium pincer complexes in alkane dehydrogenation, 21-23 we set out to investigate the properties and reactivity of heterobimetallic Re-Ir organometallic systems. These could be viable catalysts for the oxidative coupling of LHC with biomass-derived polyols (Scheme 1). Ir would serve as an alkane dehydrogenation catalyst affording alkene and iridium hydride. Re would promote DODH of polyols. Hydride transfer or spillover from iridium to oxorhenium would result in catalyst regeneration and production of water as a byproduct. Therefore, understanding the fundamental chemistry and behavior of heterobimetallic complexes that feature iridium hydride or alkyl and oxorhenium is paramount in this endeavor. Received: January 8, 2016 Published: February 3, 2016 Scheme 1. Proposed Conversion of Light Hydrocarbons (LHC), Illustrated Here with Pentane (C5), with Biomass- Derived Glycerol Using an Ir-Re Heterobimetallic Catalytic System Article pubs.acs.org/Organometallics © 2016 American Chemical Society 605 DOI: 10.1021/acs.organomet.6b00010 Organometallics 2016, 35, 605-611