Binding, activation, and oxidative addition of dihydrogen using Ir-pincer systems and implications in hydrogenation catalysis

Abstract

Binding of unreactive small molecules, such as H2, N2, CO2, CH4, etc. on a transition metal center and their subsequent activation are fundamental problems that have been extensively studied in organometallic chemistry for the past four decades. Towards this direction, activation of the H–H bond in H2, has been a topical area of research interest. The ground breaking discovery of Kubas’ complex, [W(η2–H2)(CO)3(PR3)2] in 1984,2 laid the foundation in treating other electronically saturated molecules with strong sigma bonds such as the C–H bond in alkanes in a similar manner for their activation and functionalization. From the discovery of the first dihydrogen complex to a few recently reported sigma methane complexes, the advancement in the binding and activation of either H2 or CH4 using metal complexes is not only restricted to academic research pursuits but also to pave the way for futuristic concepts such as “hydrogen economy” or “methane economy”. In the pursuit of developing a single organometallic system (catalyst) showcasing versatile reactivity towards H2, both in terms of binding and activation, an iridium pincer complex with PPh3 as a co-ligand, [Ir(H)(Cl)(iPr)4(POCOP)(PPh3)] has been synthesized. Two different complexes derived from the same iridium pincer backbone: a weakly coordinated iridium dinitrogen cationic complex, trans-[Ir(H)(N2)(iPr)4(POCOP)(PPh3)]+,3 and a four coordinate, neutral iridium complex, [Ir(iPr)4(POCOP)(PPh3)] ,4 with two vacant sites favoring controlled binding of H2 and the subsequent scission of the H–H bond on the iridium center, has been elucidated. An unusual intramolecular H-atom site exchange occurring between H2 and the hydride ligands trans-disposed to one another, in a trans-[Ir(H)(η2–H2)(iPr)4(POCOP)(DMAP)]+ has been investigated in detail.5 In addition to the binding of H2 to give a σ-H2 complex and its cleavage to form a classical dihydride complex, attempts were made to study the systematic elongation of the H–H bond along the continuum for the oxidative addition of H2 to a metal center. A study of an equilibrium between cis-[Ir(H)(η2–H2)(iPr)4(POCOP)(PPh3)]+ to its tautomeric form [Ir(H)3(iPr)4(POCOP)(PPh3)]+ as a function of pressure of H2, has also been investigated.6 These studies were carried out to design and develop better performance catalysts for hydrogenation. The results of these studies will be presented.