Biomimetic model featuring the NH proton and bridging hydride related to a proposed intermediate in enzymatic H(2) production by Fe-only hydrogenase.
Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe(2)(mu-S(CH(2))(2)N(n)Pr(H)(m)(CH(2))(2)S)(mu-H)(n)(CO)(4)(PMe(3))(2)](2)((2m+2n)+) (1, m = n = 0; [1-2H(N)](2+), m = 1, n = 0; [1-2H(N)2H(Fe)](4+), m = n = 1), are prepared to mimic the active site of Fe-only hydrogenase. X-ray crystallographic analyses of these three complexes reveal that two diiron subunits are linked by two azadiethylenethiolate bridges to construct a dimer-of-dimer structure. (31)P NMR spectroscopy confirms two trimethylphosphine ligands within the diiron moiety are arranged in the apical/basal configuration, which is consistent with the solid-state structural characterization. Deprotonation of the NH proton in [1-2H(N)](2+) and [1-2H(N)2H(Fe)](4+) occurs in the presence of triethanolamine (TEOA), which generates 1 and [1-2H(Fe)](2+), respectively. Deprotonation of the Fe hydride is accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hydride species, [1-2H(Fe)](2+), is a kinetic product which converts to its thermodynamically stable tautomer, [1-2H(N)](2+), in solution, as evidenced by IR and NMR spectroscopy. The pK(a) values of the aza nitrogen and the diiron sites are estimated to be 8.9-15.9 and <8.9, respectively. [1-2H(N)2H(Fe)](4+) has been observed to evolve H(2) electrocatalytically at a mild potential (-1.42 V vs Fc/Fc(+)) in CH(3)CN solution. Catalysis of [1-2H(N)2H(Fe)](4+) is found to be as efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, [1-2H(N)2H(Fe)](4+) undergoes four irreversible reduction processes at -1.26, -1.42, -1.82, and -2.43 V, which are attributed to the reduction events from [1-2H(N)2H(Fe)](4+), [1-2H(Fe)](2+), [1-2H(N)](2+), and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids.
Chiang
Ming-Hsi
MH
Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan. mhchiang@chem.sinica.edu.tw
Liu
Yu-Chiao
YC
Yang
Shu-Ting
ST
Lee
Gene-Hsiang
GH
D001372:Aza Compounds / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D032701:Biomimetics; D001881:Borates / Q000737:chemistry; D002384:Catalysis; D004563:Electrochemistry; D006864:Hydrogenase / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007536:Isomerism; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D011522:Protons
D001372:Aza Compounds; D001881:Borates; D007506:Iron-Sulfur Proteins; D011522:Protons; C118830:iron hydrogenase; D006864:Hydrogenase; C098759:fluoroboric acid; D009584:Nitrogen