An Open-Shell IrII/IrIV Redox Couple Outperforms the Conventional IrI/IrIII Pair for the Catalytic Isomerization of Olefins

The chemistry of mononuclear open-shell complexes of precious transition metals is largely underdeveloped, which contrasts with that of first-row transition metals and their catalytic applications. This is particularly true for IrII complexes and its almost inexistent participation in catalysis. Here we report the synthesis and characterization of a small family of mononuclear IrII metalloradicals constructed around a POCOP pincer ligand (POCOP = C6H3-2,6-(OP(tBu)2)2) and explore their performance as catalysts for olefin isomerization, a fundamental process for alkane metathesis. These paramagnetic species are up to 20 times more active than their diamagnetic IrI analogues. Strikingly, the elementary steps involved in the catalytic action of both IrI and IrII are virtually identical according to our computational investigations. However, the proposed unprecedented IrII/IrIV redox cycle presents faster kinetics than the traditional IrI/IrIII route due to much lower barriers for the key oxidative addition and reductive elimination events, a feature that we rationalize by means of the Activation Strain Model-Energy Decomposition Analysis-approach. In essence, we report the first example of a catalytic system based on a mononuclear IrII/IrIV redox couple, a fundamental contribution to the underexplored chemistry of IrII compounds that stresses the necessity to broaden the catalyst search and design guidelines beyond conventional redox cycles, including those based on platinum-group metals.