The deeply buried active sites of the hydrogenase enzymes are exemplars of nature's strategy for protection from adventitious oxygen. Still such deleterious events occur, and additional defenses of the [NiFe], and [FeFe]-H2ases active sites are called on to protect the integrity of these molecular catalysts. The affinity of oxygen for sulfur, in [NiFeS]-H2ase, and selenium, in [NiFeSe]-H2ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. We have explored the difference in O2 reactivity between small [NiFe] models containing SPh- and SePh- in bridging positions. The particular synthetic analogue chosen resulted in O-atom uptake at the chalcogens, leading to the conversion of the 4-membered Ni(µ-EPh)(µ-SN2’)Fe (E = S or Se) to a 5-membered Ni-O-E-Fe-S arrangement, where the O atom is inserted between E and Ni. Analysis of all the products of such reactions found multiple O-atom uptake, similar to the O-damaged [NiFe]-H2ase active sites, indicating that the models might yield further information. The dimer splitting reaction that yielded the [N2SNiSPh] precursor to the [NiFe] bimetallics offers opportunity to modify the electronic character of S according to substituent effects on the phenyl ring. This new series of (N2S)NiSPh’ and the NiFe bimetallics shown in Figure 2 offer a wider range of possibilities for isolating the various oxygenates. In the X=NMe2 case, the 2-O atom uptake complex was isolated and identified as the SPh binding the second oxygen. The 1-O atom uptake can be removed chemically and electrochemically while 2-O uptake is inert.

Oxygen uptake in complexes related to [NiFeS] and [NiFeSe] hydrogenase active sites

2019 Fall National Meeting

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