Experimental SectionMaterials. The pUC57 plasmid containing optimized nitrobindin (NB) (M75L, M148L) gene was purchased from GeneScript. Oligonucleotides were obtained from Invitrogen, Inc. (Japan). Restriction enzymes are obtained from Takara Bio Inc. (Japan). Nucleotide sequences were determined by Fasmac Co., Ltd (Japan). All reagents of the highest guaranteed grade were purchased and used as received unless otherwise noted. A standard rhodium solution for inductively coupled plasma optical emission spectroscopy (ICP-OES) was purchased from Wako (Japan). Distilled water was demineralized using a Barnstead NANOpure DIamond TM apparatus. 3-carboxy-1,3-propanedithiol S1 and {-(SCH 2 ) 2 CHCO 2 H}[Fe 2 (CO) 6 ] S2 were synthesized as described in a previous report.Instruments. 1 H and 13 C NMR spectra were recorded on a Bruker DPX400 NMR spectrometer. Chemical shifts were reported in ppm relative to the residual solvent resonances. ESI-TOF MS

The effects of redox-inactive metal ions on dioxygen activation were explored using a new FeII complex containing a tripodal ligand with 3 sulfonamido groups. This iron complex exhibited a faster initial rate for the reduction of O2 than its MnII analog. Increases in initial rates were also observed in the presence of group 2 metal ions for both the FeII and MnII complexes, which followed the trend NMe4+ < BaII < CaII = SrII. These studies led to the isolation of heterobimetallic complexes containing FeIII-(μ-OH)-MII cores (MII = Ca, Sr, and Ba) and one with a [SrII(OH)MnIII]+ motif. The analogous [CaII(OH)GaIII]+ complex was also prepared and its solid state molecular structure is nearly identical to that of the [CaII(OH)FeIII]+ system. Nuclear magnetic resonance studies indicated that the diamagnetic [CaII(OH)GaIII]+ complex retained its structure in solution. Electrochemical measurements on the heterobimetallic systems revealed similar one-electron reduction potentials for the [CaII(OH)FeIII]+ and [SrII(OH)FeIII]+ complexes, which were more positive than the potential observed for [BaII(OH)FeIII]+. Similar results were obtained for the heterobimetallic MnII complexes. These findings suggest that Lewis acidity is not the only factor to consider when evaluating the effects of group 2 ions on redox processes, including those within the oxygen-evolving complex of Photosystem II.

The diiron carbonyl cluster is held by a native CXXC motif, which includes Cys14 and Cys17, in the cytochrome c sequence. It is found that the diiron carbonyl complex works well as a catalyst for H(2) evolution. It has a TON of ∼80 over 2 h at pH 4.7 in the presence of a Ru-photosensitizer and ascorbate as a sacrificial reagent in aqueous media.

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Heterobimetallic cores are important unit within the active sites of metalloproteins, but are often difficult to duplicate in synthetic systems. We have developed a synthetic approach for the preparation of a complex with a MnII–(μ-OH)–FeIII core, in which the metal centers have different coordination environments. Structural and physical data support the assignment of this complex as a heterobimetallic system. Comparison with the analogous homobimetallic complexes, those containing MnII–(μ-OH)–MnIII and FeII–(μ-OH)–FeIII cores, further supports this assignment.