Modeling The Active Site Of [NiFe]-Hydrogenase

The hydrogenases are metalloenzymes that act to catalytically interconvert dihydrogen with protons and electrons. Although there are three classes of hydrogenases that are known, the focus of this research will be on the first hydrogenase to be evolved, the [NiFe]-hydrogenase, which contains a heterobimetallic NiFe active site. The [NiFe]- hydrogenase is a heterodimeric enzyme, and it is composed of a large subunit and a small subunit, inside the large subunit is buried the active site deep within the protein, while the small subunit contains a series of [Fe-S] clusters that controls the electron transfer system of the enzyme. The active site is heterobimetallic, and contains an octahedral Fe(CN)2(CO) center. This ferrous center is coordinated to a Ni(S-cys)4 center via two cysteine thiolates. Previous work has shown that Ni(dppe)(pdt) reacts with Fe2(CO)9 in dichloromethane to form (CO3)Fe(pdt)Ni(dppe). The Fe(I)Ni(I) complex, (CO)3Fe(pdt)Ni(dppe), was readily protonated with HBF4 to give the corresponding bridging hydride. This complex served as the first example of a nickel-iron thiolato hydride. Isolation of the first nickel iron-thiolato hydride, as well as a series of nickel-iron- bridging chloride complexes, led to the investigation of different pathways to similar compounds. Attempts to isolate similar [(CO)3Fe(dithiolate)Ni(diphosphine)] complexes by reaction of the mononuclear NiII(diphosphine)(dithiolate) with Fe2(CO)9 were unsuccessful with this original route, except for [(CO)3Fe(pdt)Ni(dppe)]. The reaction of FeI2(CO)4 followed by reduction with cobaltocene with different L type ligands can generate the corresponding substituted compounds, thus this reagent was investigated as a source of the Fe(CO)3 fragment for our (CO)3Fe(dithiolate)Ni(diphosphine) models. The preparation and successful isolation of novel (CO)3Fe(dithiolate)Ni(diphosphine) compounds (that were previously unsuccessful via the Fe2(CO)9 route) and the catalytic activity of their corresponding hydrides was the primary focus of this research. The reactions with the Ni(SR)2(diphosphine) and FeI2(CO)4 appear to proceed through the intermediacy of the cations [(CO)3Fe(SR)2(μ-I)NiL2]+. IR spectra of these intermediates resemble those for the corresponding μ-hydrides but are shifted to higher energies. Also the spectra tend to be more complex, possibly because of decomposition reactions. Addition of cobaltocene to these solutions produced greenish colored intermediates (CO)3Fe(pdt)Ni(diphosphine), which in some cases could be further identified by IR and 31P NMR spectroscopy. The reductive process was optimized for (CO)3Fe(pdt)Ni(dppe), which was obtained in yields competitive with the Fe2(CO)9 + Ni(pdt)(dppe) route. In most cases, such reduced intermediates were converted to the hydrides by protonation with strong acids. The reactions of Ni(pdt)(dppbz) and Ni(pdt)(dcpe) with FeI2(CO)4 followed the expected patterns: stable μ-iodo intermediates were observed, which reduced to the corresponding Fe(I)Ni(I) compound. Protonation of the reduced species with ether-HBF4 gave the targeted hydride [(CO)3Fe(pdt)(μ-H)Ni(dppbz)]BF4 and [(CO)3Fe(pdt)(H)Ni(dcpe)]BF4, and the latter compound was characterized crystallographically