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

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 nanoparticles formed by the reduction of the Fe-PNNP precatalysts with base. To probe the dehydrogenation active species we successfully show comparable activity between preformed catalysts, and those generated in situ using commercially available Fe²⁺ sources and substoichiometric amounts of PNNP ligand. This result, when paired with transmission electron microscope images of ∼4 nm iron nanoparticles of reaction solutions provide evidence that the highly active systems studied are heterogeneous in nature. This would be the first report of iron nanoparticles catalyzing H₂ evolution from AB in nonprotic solvents. We also report the evolution of hydrogen from dimethylamine-borane and the resultant product mixtures using the same catalyst series