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 kH2O/kD2O = 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.