Intramolecular C−H Bond Activation and Redox Isomerization across Two-Electron Mixed Valence Diiridium Cores

Abstract

Metal−metal cooperativity enables the reaction of carbon-based substrates at diiridium two-electron mixed valence centers. Arylation of Ir20,II(tfepma)3Cl2 (1) (tfepma = bis[(bistrifluoroethoxy)phosphino]methylamine) with RMgBr (R = C6H5 and C6D5) is followed by C−H bond activation to furnish the bridging benzyne complex Ir2II,II(tfepma)3(μ-C6H4)(C6H5)H (2), as the kinetic product. At ambient temperature, 2 isomerizes to Ir2I,III(tfepma)3(μ-C6H4)(C6H5)H (3) (kobs = 9.57 ± 0.10 × 10−5 s−1 at 31.8 °C, ΔH⧧ = 21.7 ± 0.3 kcal/mol, ΔS⧧ = −7.4 ± 0.9 eu), in which the benzyne moiety is conserved and the IrIII center is ligated by terminal hydride and phenyl groups. The same reaction course is observed for arylation of 1 with C6D5MgBr to produce 2-d10 and 3-d10 accompanied by an inverse isotope effect, kh/kd= 0.44 (kobs = 2.17 ± 0.10 × 10−4 s−1 in C6D6 solution at 31.8 °C, ΔH⧧ = 24.9 ± 0.7 kcal/mol, ΔS⧧ = −6.4 ± 2.4 eu). 2 reacts swiftly with hydrogen to provide Ir2II,II(tfepma)3H4 as both the syn and anti isomers (4-syn and 4-anti, respectively). The hydrides of 4-syn were directly located by neutron diffraction analysis. X-ray crystallographic examination of 2, 2-d10, 3, and 4-syn indicates that cooperative reactivity at the bimetallic diiridium core is facilitated by the ability of the two-electron mixed valence framework to accommodate the oxidation state changes and ligand rearrangements attendant to the reaction of the substrate.