Efficient Homogeneous Hydridoirida-β-Diketone-Catalyzed Methanolysis of Ammonia-Borane for Hydrogen Release in Air. Mechanistic Insights

Abstract

The hydridoirida-beta-diketone [(IrH{(PPh2(o-C6H4CO))(2)H})(2)(mu-Cl)][BF4] (2) has been used as a homogeneous catalyst for the methanolysis of ammonia-borane to release up to 3 equivalents of hydrogen in the presence of air. With catalyst loadings as low as 0.2 mol%, ammonia-borane undergoes methanolysis within 6 min at 30 degrees C, with TOF50% of 320 mol(H2) .mol(Ir)(-1).min(-1), or within 80 s at 60 degrees C, with an excellent TOF50% of 1991 mol(H2).mol(Ir)(-1).min(-1), and maintains its catalytic activity in consecutive runs. Triethylamine-borane fails to undergo methanolysis. Kinetic studies indicate first-order dependence on substrate and on catalyst concentration and suggest cleavage of the solvent O-H bond being involved in the rate determining step of the reaction. In methanol solution 2 forms cationic [IrH(MeOH){(PPh2(o-C6H4CO))(2)H}](+) (3) and reacts with Me3N-BH3 to afford a hydridoirida-beta-diketone [IrH(Me3NBH3){(PPh2(o-C6H4CO))(2)H}](+) (4), with the borane adduct eta(1)-coordinated to iridium. Compound [4][BAr4F] shows dynamic behaviour in solution due to exchange of bridging and terminal B-H bonds. A multinuclear NMR study of the catalyzed reaction shows the formation of two ammonia-methoxyborane adduct intermediates, H3N-BH2(OCH3) and H3N-BH(OCH3)(2), and an iridium species proposed of the hydridodiacyl type [IrH(H3NBH3-x(OCH3)(x))(PPh2(o-C6H4CO))(2)] with a coordinated borane adduct. On account of experimental evidence, a simplified catalytic cycle is suggested for the methanolysis of AB to release hydrogen.