The discovery of cyano-iron carbonyls in the hydrogen processing enzymes, [NiFe]- and [FeFe]-hydrogenase (H2ase) has inspired synthetic efforts by organometallic chemists to reproduce the active sites of these efficient H2-producing or H2-oxidation enzymes. The recent demonstration of the possibility of loading the apo-HydA (the [FeFe]-H2ase enzyme) and apo-HydF (a maturation protein) with synthetic analogues of the active site are convincing that the small molecular models are indeed the essential catalyst.1 The interrogation by advanced EPR spectroscopy on labeled cyanides indicated, as shown in the figure, that the cyanide has flipped from its origin in the synthetic diiron unit and inserts into HydF, placing the cyanide nitrogen next to the 2Fe site; the CN- returns to the terminal position as HydF transfers the diirion unit to the apo-HydA. In order to probe the requirements for such cyanide isomerization to occur we have prepared cyanide-bridged constructs of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As found in classical organometallic chemistry,2 the orientation of the CN bridge is determined by the precursor reagents; no cyanide flipping was observed. DFT computations find an excessively high barrier accounts for the kinetically controlled products. 1. a) Berggren et al., Nature, 2013, 499, 66-70. b) Esselborn et al., Nat. Chem. Bio. 2013, 9, 607-610. 2. Zhu, N.; Vahrenkamp, H. Chem. Ber. Recueil. 1997, 130, 1241-1252.

Biomimetics of tri-iron arrangements involving cyanide as docking agent in maturases of the H cluster of the diiron hydrogenase

251st National Meeting (2016)

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