Catalysis and Mechanism of H2 Release from Amine-Boranes by Diiron Complexes

Studies focused on the dehydrogenation of amine-borane by diiron complexes that serve as well-characterized rudimentary models of the diiron subsite in [FeFe]-hydrogenase are reported. Complexes of formulation (mu-SCH2XCH2S)[Fe(CO)(3)](2), with X = CH2, CMe2, CEt2, NMe, NtBu, and NPh, 1-CO through 6-CO, respectively, were determined to be photocatalysts for release of H-2 gas from a solution of H3B <- NHMe2 (B:As), dissolved in THF. The thermal displacement of the tertiary amine-borane, H3B <- NEt3 (B:At) from photochemically generated (mu-SCH2XCH2S)[Fe(CO)(3)][Fe(CO)(2)(mu-H)(BH2-NEt3)], 1-B:At through 6-B:At, by P(OEt)(3) was monitored by time-resolved FTIR spectroscopy. Rates and activation barriers for this substitution reaction were consistent with a dissociative mechanism for the alkylated bridgehead species 2-CO through 6-CO, and associative or interchange for 1-CO. DFT calculations supported an intermediate [I] for the dissociative process featuring a coordinatively unsaturated diiron complex stabilized by an agostic interaction between the metal center and the C-H bond of an alkyl group on the central bridgehead atom of the SRS linker. The rate of H-2 production from the initially formed 1-B:As through 6-B:As complexes was inversely correlated with the lifetime of the analogous 1-B:At through 6-B:At adducts. Possible mechanisms are presented which feature involvement of the pendent nitrogen base as well as a separate mechanism for the all carbon bridgeheads.