Co-operativity effects between adjacent metal sites—kinetics of methyl iodide addition to pyrazolyl-bridged iridium dimers: argument for a concerted mechanism in the reversible two-fragment, two-centre oxidative addition to the cyclo-octa-1,5-diene di-iridium(I) complex [Ir(COD)(µ-pz)]2

Abstract

Addition of Mel occurs irreversibly to [Ir(CO)₂(µ-pz)]₂[(1), pzH = pyrazole] in two kinetically distinguishable steps (ΔH^‡, ΔS^‡ respectively 32 ± 2, 40 ± 5 kJ mol^–1, –152 ± 8, –157 ± 16 J mol^–1 K^–1); by contrast Mel and [Ir(COD)(µ-pz)]₂[(2), COD = cyclo-octa-1,5-diene]) enter into equilibrium, ΔG⁰₂₉₈=–5 ± 2 kJ mol^–1, with ΔH^‡= 89 ± 4 kJ mol^–1, ΔSΔ= 40 ± 15 J mol^–1, K^–1 for the forward reaction, in striking contrast with activation parameters for related reactions including that of (1) and interpretable together with lack of any kinetic salt or solvent dependence as evidence for a concerted mechanism.