Cyanide-bridged iron complexes as biomimetics of tri-iron arrangements in maturases of the H cluster of the di-iron hydrogenase

Developing from certain catalytic processes required for ancient life forms, the H(2) processing enzymes [NiFe]- and [FeFe]-hydrogenase (H(2)ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H(2)ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent Fe(I)Fe(I) unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.