The [FeFe]-hydrogenase enzymes catalyze the interconversion of hydrogen with protons and electrons. The active site (H-cluster) is composed of a diiron core in a face-shared bi-octahedral structure. The H-cluster is attached to the protein via coordination of a [Fe4S4(Cys)4 ] cluster to the proximal iron (FeP) center via a cystein thiolate. Each iron is coordinated by a terminal cyanide and terminal carbonyl ligand. The irons are bridged by a carbonyl ligand and a five member light atom bridge, proposed to an azadithiolate bridge (SCH2NHCH 2S). The active state of the enzyme (Hox) is characterized by an open coordination site on the distal iron (FeD). It has been shown that the enzyme is inhibited by CO, resulting in Hox CO. The oxidation state of the irons in Hox and H oxCO is a subject of a large amount of research. Spectroscopy of the enzyme reveals for both states the diiron core is paramagnetic (S= ½). It is proposed that the active state of the enzyme consists of and Fe II-FeI species. Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis. A specific goal is the preparation of mixed-valence complexes that are similar in structure and have spectroscopy analogs to the enzyme with the expectation that function will follow form.
Synthetic model complexes are based on the chelating phosphine cis-1,2-bis(diphenylphosphino)ethylene (dppv). The dppv ligand was chosen because of its preference to chelate to a single iron center, rather than bridge between the two iron centers. The complex Fe2(S2C2H4)(CO)4 (dppv) (1(CO)4) was prepared in high yield by the decarbonylation of Fe2(S2C2H4)(CO) 6 in the presence of dppv. The geometry adopted by the dppv ligand (apical-basal vs. di-basal) is influenced by the nature of the chelating dithiolate. For 1(CO)4, the dppv ligand spans the apical-basal coordination sites, while for the larger 1,3-propanedithioalte (2(CO) 4) the dppv ligand exists in both the apical-basal and di-basal coordination geometries. Complex 1(CO)4 was round to readily react with PMe3 to yield Fe2(S2C2H 4)(CO)3(PMe3)(dppv) (1(CO)3 (PMe3)). The substitution process was found to proceed through a "rotated" transition state, which is favored by the unsymmetrical ligand set.
Oxidation of 1(CO)3(PMe3) in MeCN with two equiv of FcPF6 yielded the diferrous complex [Fe 2(S2C2H4)(μ-CO)(CO)2(PMe 3)(dppv)(NCMe)]2+. When only one equiv of oxidizing agent was used, a 1:1 mixture of unreacted starting material and the diferrous complex was observed, with no evidence for a mixed-valence intermediate. Upon changing the solvent to the non-coordinating CH2Cl2, very different reactivity was observed. The cyclic voltammogram of 1(CO) 3(PMe3) in CH2Cl2 under N2 revealed a reversible one electron oxidation (+20 mV vs. Ag/AgCl). When the cyclic voltammogram was recorded under a CO atmosphere two separate one electron oxidations were observed (-05 mV and +230 mV vs. Ag/AgCl).
On a preparative scale, the addition of one equiv FcBF4 to a CH2Cl2 solution of 1(CO)3(PMe 3) at -45°C, resulted in complete consumption of the starting material and yielded the mixed-valence complex [Fe2(S2 C2H4)(CO)3(PMe3)(dppv)]BF 4 ([1(CO)3(PMe3)]BF4). The structure of [1(CO)3(PMe3)] + was confirmed crystallographically. The "inverted" structure of [1(CO)3(PMe3)]+ approaches that proposed for the Hox state of the active site with a vacant apical coordination site and a bridging carbonyl. The EPR spectrum of [ 1(CO)3(PMe3)]+ is consistent with a S = ½ species, with g values of 2.1384, 2.0280 and 2.0102. Each g value appeared as a triplet, indicating the unpaired spin (FeI) was coupled to two equivalent phosphorus nuclei. From solution EPR data the oxidation state of [1(CO)3(PMe3)]+ can be confidently assigned; the Fe(CO)(dppv) remains FeI while the Fe(CO)2(PMe3) has been oxidized to FeII.
The unsaturated character of [1(CO)3(PMe 3)]+ is indicated by its reactivity with CO, which occurs in seconds. The product; [Fe2(S2C2H 4)(μ-CO)(CO)3(PMe3)(dppv)]+ [1(CO)4(PMe3)]+ is highly unstable and could only be observed under a CO atmosphere. The EPR spectrum for [1(CO)4(PMe3)]+ featured a doublet of triplets, coupled to three phosphorus nuclei with two distinct phosphorus hyperfine couplings (A(31P) = 283, 32 and 34 MHz). Based upon the changes in the EPR, upon binding CO the unpaired spin becomes delocalized over both iron centers resulting in two Fe(1.5) metal centers.
|School:||University of Illinois at Urbana-Champaign|
|School Location:||United States -- Illinois|
|Source:||DAI-B 69/05, Dissertation Abstracts International|
|Keywords:||Enzyme modeling, Hydrogen, Hydrogenases, Iron, Iron-sulfur, Mixed-valence|
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