The enzymes containing bimetallic active sites in biology like [NiFe]-, [FeFe]-hydrogenases, and acetyl coA synthase have been the impetus for significant research in bimetallic small molecule models as analogues of base metal catalysts. The active site of the [FeFe]- hydrogenase, in particular, consists of a diiron construct that is held together by an aza-dithiolate linker. The function of this pendant amine base, proximal to the Lewis acidic iron center with an open site, is proven to be critical for efficient H2 production. While the active site of [NiFe]-H2ase is devoid of a pendant amine base, a terminal cysteinyl sulfur appears to assist proton removal; ultra-high resolution crystallographic evidence of a protonated cysteinyl thiolate in the Ni-R1 state by Ogata et al. supports this thesis. Recent pursuits of [MN2S2.Fe(CO)(η5-C5H5)]+, (M = Ni2+, Fe(NO)2+), as electrocatalysts showed hemilability of the bridging cis-dithiolates upon successive reduction and protonation steps facilitating H2 production. Earlier studies using non-innocent, (NO)FeN2S2, bound to the redox-active iron dinitrosyl unit, isolated in two redox states, {Fe(NO)2}9/10, illustrated modest electrocatalysis for proton reduction. To elucidate the role of the iron dinitrosyl unit as catalytically active center, redox innocent NiII instead of Fe(NO)2+ was incorporated to synthesize analogous [NiIIN2S2.Fe(NO)2]0/+ complexes. In this study, we have synthesized such heterobimetallic complexes in two redox levels, {Fe(NO)2}9 and {Fe(NO)2}10, with hemilabile metallodithiolates as pendant bases for H+ relay to produce H2, vide graphic. Computational studies reconcile the required initial thiolate-iron bond cleavage that augments proton relay to the dinitrosyl iron center.

Unleashing a pendant base by reduction-induced hemilability in [NiN2S2.Fe(NO)2]+/0 complexes

253rd National Meeting (2017)

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