ACS PRF | ACS
All e-Annual Reports
43389-AC3
Synthesis of Functional and Structural Analogues of the Iron Hydrogenases
The [FeFe]-hydrogenases are the most efficient catalysts known for the reduction of protons to H2.[1] The active site exists in two functional states, Hred, which is S = 0, and Hox, which is S = 1/2. This bimetallic center has spawned considerable research into new diiron-dithiolato complexes. This project entailed research aimed at elucidating the mechanism of the enzymatic catalysis. A specific research goal was the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form.
Themes that were investigated included:
(i) hydrogen production by reduction of diiron dithiolato hydrido complexes. Compounds of the type HFe2(SR)2(CO)4(PR3)2+ have been reinvestigated and shown, when the H is terminal, to produce H2.
(ii) the role of the dithiolate cofactor, often speculated to be SCH2NHCH2S. We prepared a series of compounds of the type Fe2(SCH2)2NH(CO)6-x(PR3)x. These species proved more reactive than the corresponding propanedithiolates which was both a source of problems and inspiration for further investigation. Specifically, these species do not readily sustain 1e oxidation (see below) due to the interfering coordination of the amine.
(iii) the role of mixed valency in the catalytic cycle. This was the area of significant advances. We were finally able to generate the mixed-valence derivative that structurally resembles Hox state of the enzyme. This species binds CO (and NO). 13CO labeling studies showed that the binding is regiospecific. We have not yet unlocked the secret that allows this species to bind H2, but we are closer. We also uncovered the signficant effects of the dithiolate cofactor.
One of the most attractive aspects of this modeling effort was the expanding momentum for the use of organic ligands (e.g., phosphines) in place of the naturally-occurring cyanide and _-SR[4Fe-4S] ligands, which have complicated acid-base behavior that is difficult to control. The specific ligand of greatest value was cis-bis(diphenylphosphino)ethene. Using this species we were able to generate tetraphosphine complexes such as Fe2(SR)2(CO)2(dppv)2, which exhibit a rich acid-base and a spectacular redox chemistries, both of which are well-behaved because of the electronic and steric buffering provided by the dppv.
