Evidence for Iron Nanoparticles Catalyzing the Rapid Dehydrogenation of Ammonia-Borane

Abstract

A series of precatalysts of the general formula [Fe(NCMe)(L)(PPh2C6H4CH═NCHR-)2][BF4]2 (where L = CO or NCMe, and R = Ph or H) were tested for the dehydrogenation of amine-boranes. They have already been used in our lab for the transfer hydrogenation or direct hydrogenation of ketones and the oxidative kinetic resolution of alcohols. We compared a series of sterically- (R = H or Ph) and electronically- (L = NCMe or CO) varied precatalysts in both protic and aprotic solvents for the release of hydrogen from ammonia-borane (AB) and studied the products by NMR. At room temperature in tetrahydrofuran (THF) we optimized our systems, and achieved maximum turnover frequencies (TOF) of up to 3.66 H2/sec and 1.8 total H2 equivalents, and in isopropanol we were able to release a maximum of 2.9 equiv of H2 and reuse some of our catalytic systems. In previous mechanistic studies we provided strong evidence that the active species during transfer hydrogenation (TH) and oxidation catalysis are zerovalent iron nanoparti...