Reversible CO2 Hydrogenation, Neutron Crystallography, and Hydride Reactivity of a Triiridium Heptahydride Complex: Article No. e202501943

The authors report the structure, reactivity, and catalytic utility of a triiridium complex, [Ir3H6(..mu..3-H)(PN)3]2+ (2-H, PN = (2-pyridyl)CH2PBut2). Despite its unusual stability to unsaturated organics, electrophiles, and even CF3SO3D, they find that complex 2-H catalyzes hydrogenation of CO2 to formate (TONIr = 9600) and reverse formic acid dehydrogenation (TONIr = 54 400). The hydrogenation operates via a reactive intermediate [Ir3H4(..mu..-H)4(PN)3]+ (5). Neutron crystallography and DFT-supported neutron vibrational spectroscopy of 2-H reveal Ir-H bond lengths and elucidate the vibration modes within the Ir3H7 core. Stoichiometric oxidation of 2-H produces four classes of iridium complexes of varied nuclearity and hydride structure: tetra- and pentanuclear clusters [Ir3H6(..mu..3-AuPPh3)(PN)3]2+ (2-Au) and [Ag{Ir2H4(..mu..-OAc)(PN)2}2]3+ (6) are generated using AuPPh3+ and AgOAc, respectively. Further oxidation to class [Ir2H3(..mu..-X)2(PN)2]+ is possible with AgOAc, Hg(OAc)2, or I2. Finally, a TEMPO/HCl system completely oxidizes the hydrides and gives [Ir2Cl4(..mu..-Cl)2(PN)2] (11).