Communication pubs.acs.org/Organometallics
C−H Activation of Methyltrioxorhenium by Pincer Iridium Hydride To Give Agile Ir−Re Bimetallic Compounds Kothanda Rama Pichaandi,† Phillip E. Fanwick,† and Mahdi M. Abu-Omar*,†,‡ †
Brown Laboratory, Negishi Brown Institute for Catalysis (NBIC), Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States ‡ School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering, 480 Stadium Drive, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: C−H activation of the CH3 group on methyltrioxorhenium (MTO) by [PNPIrH2]+ (1) affords the bimetallic complex [PNP(H)Ir-μ(CH2)-μ(O)-Re(O)2]+ (2). Investigation of the mechanism revealed that C−H activation is the ratedetermining step, and it is preceded by MTO coordination to 1 followed by the release of H2. Reversible isomerization of 2 to [PNP(Me)(CH3CN)Ir-ReO3][PF6] (3) is observed when the solvent is exchanged from noncoordinating to coordinating and vice versa. While the Ir−Re bond in 2 is supported by two bridging ligands, oxo and alkylidene (CH2), the Ir−Re bond in 3 is not supported by any bridging ligands. Hence, the reversibility between these two structural isomers defines the agile nature of the Ir−Re metal−metal bond.
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PNPIr. On the contrary, we discovered the unanticipated formation of a bimetallic heteronuclear Ir−Re oxo alkylidene complex (2) from the CH activation of the methyl (CH3) group on MTO by [PNPIr(H)2]+ (1). Furthermore, reversible structural isomerization of 2 to 3, possessing an unsupported iridium−rhenium bond, was also observed. Complex 2 was synthesized in 89% isolated yield by the reaction of 17 with MTO in dichloromethane. We propose the mechanism in Scheme 1. MTO coordinates to 1 in a reversible manner, forming complex A, which reductively eliminated H2 to form B. Intermediate B undergoes an intramolecular C−H activation, eventually forming 2. The following experimental results support this mechanism: (i) Addition of MTO to 1 at 280 K instantaneously resulted in broadening of the Ir-H signal in the 1H NMR with the disappearance of 31P{1H} and 13C{1H} NMR signals of 1. This indicates the rapid exchange of MTO coordination to 1 and vice versa. At temperature lower than 280 K no coordination of MTO with 1 was observed. After ∼10 min the 1H NMR signals of 1/A disappeared including loss of H2 with the appearance of a new set of signals, which we assign to intermediate B (Figure S2), in addition to a small amount of product 2. As the reaction progressed, the intensity of signals corresponding to B decreased with an increase of 2. While the structural assignments of intermediate B cannot be certain as the ReCH3 is not distinguishable from that for MTO, they are
onversion of natural gas into methanol or higher alkanes (paraffins) is of paramount importance because of its abundance in nature and the prevailing production of shale gas in the U.S.1 Utilization of natural gas via syn gas (CO and H2) routes requires high temperature and pressure, making it capital intensive and cost ineffective.2 Therefore, direct partial oxidation of methane (and other light alkanes) at low temperature (
