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

Lunsford, AM, Blank, JH, Moncho, S, Haas, SC, Sohail, M ORCID: https://orcid.org/0000-0002-8221-818X, Brothers, EN, Darensbourg, MY and Bengali, AA (2016) Catalysis and Mechanism of H2 Release from Amine-Boranes by Diiron Complexes. Inorganic Chemistry, 55 (2). pp. 964-973. ISSN 0020-1669

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Abstract

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 (μ-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 H2 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 (μ-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(μ-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 H2 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.