Studies On The Structures And Properties Of The Complexes Containing Internal Bases Of [FeFe]-Hydrogenase Active Site

[FeFe] Hydrogenases in microorgamisms can reversibly catalyze the proton reduction for hydrogen evolution. Studies on the crystal structures of the enzymes show that the active site of [FeFe] hydrogenases features a square-pyramidal butterfly coordination geometry, which is quite similar to that of the reported organometallic complexes formulated as [Fe₂(μ-SR)₂(CO)_6-nL_n]. The simple structure and the high efficiency of the [FeFe] hydrogenase active site inspire a great interest for chemists. They try to explore the catalytic mechanism for proton reduction and eventually find cheap and efficient catalysts for hydrogen production by mimicking the structure of the active site. In this thesis, the effect of internal N-base on the structures and properties of the diiron dithiolate complexes were studied.Two isomeric diiron dithiolate complexes [(μ-pdt){Fe(CO)₃}{Fe(CO)(κ²-PMP)}] (3) and [(μ-pdt)(μ-PMP)Fe₂(CO)₄] (4) containing the (EtO)₂PN(Me)P(OEt)₂ (PMP) ligand have been prepared. The PMP ligand is in an unsymmetrical chelating mode for 3 and in a symmetrically bridging mode for 4. Complex 3 was converted to 4 in 75% yield after reflux in toluene for a long time. The reactions of 3 with PMe₃ and P(OEt)₃ afforded bis-monodentate P-donor complexes [(μ-pdt){Fe(CO)₂PR₃}{Fe(CO)₂(PMP)}] (R = Me, 5; OEt, 7), respectively, which are formed via the coordination of PR₃ accompanied by an intramolecular CO-migration process from the Fe(CO)₃ to the Fe(CO)(PMP) unit and opening of the Fe-PMP chelate ring. The PMP-monodentate complexes 5 and 7 were converted to a trisubstituted diiron complex (μ-pdt)(μ-PMP)[Fe(CO)PR₃][Fe(CO)₂] (R = Me, 6; OEt, 8) with release of an equiv of CO when they are refluxing in toluene. Variable temperature ³¹P NMR spectra show that trisubstituted diiron complexes each exist as two conformation isomers in solution. All diiron dithiolate complexes obtained were characterized by MS, IR, NMR spectroscopy, elemental analysis, and X-ray diffraction studies.The CO-displacement of [(μ-pdt)Fe₂(CO)₆] with Ph₂PCH₂N(n-Pr)CH₂PPh₂ (PNP) in refluxing toluene gave an unsymmetrical chelating complex[(μ-pdt){Fe(CO)₃}{Fe(CO)(κ²-PNP)}] (9) as a major product, together with a small amount of the symmetrical intramolecular bridging complex [(μ-pdt){μ-PNP}Fe₂(CO)₄] (10) and the intermolecular bridging complex [{(μ-pdt)Fe₂(CO)₅}₂(μ,κ¹,κ¹-PNP)] (11), while the reaction of [(μ-pdt)Fe₂(CO)₆] with Ph₂PCH₂N(Ph)CH₂PPh₂ (PAP) afforded only the intermolecular bridging complex [{(μ-pdt)Fe₂(CO)₅}₂(μ,κ¹,κ¹-PAP)] (12). The molecular structures of 9,11, 12, as well as the doubly protonated complex [9(H_NH_μ)](OTf)₂ were determined by X-ray analyses. The protonation processes of 9 with HBF₄·Et₂O and HOTf were studied. Theμ-hydride of [9(H_μ)]⁺ and [9(H_NH_μ)]²⁺ can take place facile deprotonation in the presence of an equivalent of aniline and rapid H/D exchange with deutons in solution.Three diiron dithiolate complexes [μ-SC(NBn)CH(NHBn)S-μ]Fe₂(CO)₅PR₃ (R = Me, 15; Ph, 16; Pyr, 17) containing a functionalized C₂ bridge with two vicinal basic sites were prepared and characterized. The molecular structures of 16 and its N-protonated form [(16H_N)]OTf were determined by X-ray analyses of single crystals. The complexes 16 and 17 are relatively protophilic. The cyclic voltammograms show that complex 17 can be protonated by the mild acid CF₃COOH. Electrochemical studies show that the first reduction peak of 17 at -1.51 V versus Fc⁺/Fc is 110 mV more positive than that (-1.62 V) found for the analogous diiron azadithiolate complex [{(μ-SCH₂)₂N(CH₂C₆H₅)}Fe₂(CO)₅P(Pyr)₃] (18). Protonation of 17 leads to the anodic shifts of 610-650 mV for the Fe^â… Fe^â… /Fe^â… Fe⁰ reduction potentials. The shifts are apparently larger than that (450 mV) displayed by protonation of 18. The reaction of the all-carbonyl complex [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₆] (14) with two equivalents of bis(diphenylphosphino)methane (dppm) in refluxing toluene forms dppm mono-dentate intermediate [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅(κ¹-dppm)] (19) and a dppmμ-bridging species [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₄(μ-dppm)] (20) depending on reaction time. A desulfurized complex [(μ-S)(μ-dppm)₂Fe₂(CO)₄] (21) was obtained after refluxing in toluene for a long time.