Enzyme Models DOI: 10.1002/ange.200803939 Synthetic Support of De Novo Design: Sterically Bulky [FeFe]- Hydrogenase Models** Michael L. Singleton, Nattamai Bhuvanesh, Joseph H. Reibenspies, and Marcetta Y. Darensbourg* Small-molecule models of the active site of [FeFe] hydro- genase (H 2 ase) [1] are synthetic targets for the possibility that they might be adsorbed onto solid supports, such as graphite, and used in the construction of electrodes to be used in H + reduction and H 2 oxidation, as a replacement for platinum electrodes. [2] A distinct structural disparity between the most obvious precursor to models of the two iron subsites of the H cluster, the classic organoiron complex, [(m-S(CH 2 ) 3 S-)- {Fe I (CO) 3 } 2 ], [3, 4] (Figure 1, B), and the enzyme active site (EAS) structure (Figure 1, A), is the orientation of the edge- sharing square-pyramidal arrangements, that define the coordination sphere about each iron. The EAS, exhibiting a “rotated” edge-bridged square-pyramid/inverted square-pyr- amid geometry around the iron centers, has been structurally characterized in a mixed-valent, Fe I Fe II oxidation state. [1] However, the structure is apparently retained throughout the redox states of the catalytic cycle including the reduced Fe I Fe I form. [7] That the reduced form does not rearrange to the more symmetrical conformer, as found in B, is seemingly enforced by the secondary coordination sphere (hydrogen bonding of the cyanide groups to nearby lysine or serine residues) and steric effects from the surrounding protein matrix (the proximity of a phenylalanine and several other hydrophobic residues to the “open” or rotated site). [8] The significance of the rotated structure apparently lies in the requirement of an open site on the catalytically active iron for the binding of H 2 or H  in a terminal position. [9] Since the elucidation of the [FeFe] H 2 ase EAS structure, several hundred small-molecule models based on the simple complex B ([(m-SRS){Fe(CO) 3 } 2 ], R = bridging group) have been synthesized, in attempts to mimic both the structure and function of the enzyme. With an inestimable number of permutations involving different CO-ligand substitutions, isomeric forms, and bridging “R” substituents, [10] the use of theoretical studies to explore the unique features of the EAS as well as to limit the optimal synthetic targets has proven to be a valuable strategy for our work. Density functional (DFT) calculations by Tye et al. helped to define the criteria for reproducing the geometry found in the two-iron subsites of [FeFe] H 2 ase. [11] The rotated geometry (a transition state) of [(m-SRS){Fe I (CO) 3 } 2 ] was found to be stabilized by substitu- tion of CO by more electron-donating ligands, such as cyanide or phosphine, particularly when in the apical position of the unrotated Fe(CO) 2 L moiety and trans to the semibridging CO arising from full rotation (see Figure 1). Both one-electron reduction and oxidation decrease the Fe I  Fe I bond order to 1 = 2 , and the transition state that represents the rotated form of Fe I Fe 0 is some 4 kcal mol 1 lower in energy than that arising from the Fe I Fe I complex. Unexpectedly the ground state of the Fe I Fe II complex is, in some cases, predicted to be stable in the rotated form. [11, 12] Such computations presaged the syn- thesis of the Fe I Fe II complexes C (complex 1 ox , bearing a bulky N-heterocyclic carbene (NHC) ligand, 1,3-bis(2,4,6- trimethylphenyl)imidazol-2-ylidene, IMes) and D (Figure 1). These complexes are thermally sensitive but sufficiently stable to permit characterization by X-ray crystallography as the first rotated structures in small-molecule models. [11, 12] Notably, both 1 ox and D have ligand-based arene groups, positioned to flank the open site on iron in the rotated unit, thus indicating the need for a steric effect to stabilize the rotated structure. Spin density computations suggest that the inverted square-pyramidal moiety incorporates d 7 Fe I , whereas the unrotated group incorporates Fe II . [13] Figure 1. A) The enzyme active site of [FeFe]-hydrogenase, showing the “rotated” geometry. [1] B) Parent model complex [(m-pdt){Fe(CO) 3 } 2 ]. [4] C, D) Recently reported model complexes with similar geometries to the [FeFe]-hydrogenase active site. [5, 6] [*] M. L. Singleton, Dr. N. Bhuvanesh, Dr. J. H. Reibenspies, Prof. M. Y. Darensbourg Department of Chemistry, Texas A&M University College Station, TX 77843 USA Fax: (+ 1) 979-845-0158 E-mail: marcetta@mail.chem.tamu.edu [**] We acknowledge financial support from the National Science Foundation (CHE-0616695 and CHE-0541587) and the R. A. Welch Foundation (A-0924). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200803939. Zuschriften 9634  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2008, 120, 9634 –9637