Formation and Reactivity of Ir(III) Hydroxycarbonyl Complexes

• Published in: Inorganic chemistry 2006-08, Vol.45 (16), p.6269-6275
• Main Authors: Elliott, Paul I. P, Haslam, Claire E, Spey, Sharon E, Haynes, Anthony
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0601844

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.