Formation and Reactivity of Ir(III) Hydroxycarbonyl Complexes

Kinetic studies show that the reaction of [TpIr(CO)₂] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO₂H)(CO)H] (2) is second order (k = 1.65 × 10^-4 dm³ mol ^-1 s^-1, 25°C, MeCN) with activation parameters ΔH^‡ = 46±2 kJ mol^-1 and ΔS ^‡ = -162±5 J K^-1 mol^-1. A kinetic isotope effect of k_H2O/k_D2O = 1.40 at 20°C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp*Ir(CO)₂] (1*, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp*Ir(CO₂H)(CO)H] (2*). A proposed mechanism consistent with the relative reactivity of 1 and 1* involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2* shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O⋯O) 2.65 Å). Complex 2* is thermally stable but (like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp*Ir(CO)₂H]⁺ (3*) and decarboxylation with OH^- to give [Tp*Ir(CO) H₂] (4*). Complex 2 undergoes thermal decarboxylation above ca. 50 °C to give [TpIr(CO)H₂] (4) in a first-order process with activation parameters ΔH^‡ = 115±4 kJ mol ^-1 and ΔS^‡ = 60±10 J K^-1 mol^-1.