Dinuclear and mononuclear iron(II)-thiolate complexes with mixed CO/CN- ligands: Synthetic advances for iron sites of [Fe]-only hydrogenases

The pentacoordinate, 16-electronFe(11) complex [PPN][Fe(CO)(2)(CN)(S,NH-C6H 5)] (1), stabilized by strong S. N pi-donation of chelating [S,NH-C6H4](2-) ligand, was prepared by the reaction of 2-aminophenyl disulfide and [PPN][ Fe(CO)(4)(CN)]. Protonation of complex 1 by electrophiles (2-mercaptopyrimi dine and 2-aminophenylthiol) yielded hexacoordinate iron(II)-thiolate cyano carbonyl complexes [PPN][Fe(CO)(CN)(S-C4H3N2)(2)] (5) and [PPN][Fe(CO)(2)(C N)(S-C6H4NH2)(S,NH2-C6H4)] (3) respectively. The IR spectrum of complex 5 i n the aprotic solvent CH3CN displayed a weak nu(CN) band at 2090 cm(-1) and a strong nu(CO) band at 1945 cm(-1). Chemical oxidation of complex 5 in CH 3CN at - 20 degrees C with [Cp2Fe][PF6] displayed absorption bands at 2096 and 1962 cm(-1) which were assigned to the nu(CN) and nu(CO) vibrational fr equencies respectively of the thermally unstable neutral Fe-III(CO)(CN)(S-C 4H3N2)(2). Complex 5 was reobtained upon addition of [PPN][BH4] to Fe-III(C O)(CN)(S-C4H3N2)(2) in CH3CN at -20 degrees C. The first dinuclear Fe(II)-t hiolate cyanocarbonyl compound [PPN](2)[(CN)(CO)(2)Fe(mu-S,S-C6H4)](2) (4), the promising structural and functional model compound of the dinuclear ir on active sites of [Fe]-only hydrogenases isolated from D. desulfuricans an d C. pasteurianum, was prepared by reacting 1,2-benzenedithiol with complex 1 in THF at -10 degrees C. The X-ray structural analysis shows that comple x 4 possesses crystallographically imposed centrosymmetry. Two six-coordina te Fe(II) centers are connected via two thiolate bridges, and both CN- liga nds point into the antiparallel direction. The IR spectrum of complex 4 in the aprotic solvent CH2Cl2 revealed a weak absorption band for the CN- liga nds at 2101 cm(-1), and two strong absorption bands fur the CO groups at 20 13 and 1960 cm(-1). When the CH2Cl2 solution of complex 4 was exposed to (C O)-C-13 at 0 degrees C, absorbances at 1968 and 1915 cm(-1) appeared within 10 min. Reappearance of the 2013 and 1960 cm-l bands on the removal of the (CO)-C-13 and replacement with (CO)-C-12 atmosphere demonstrated reversibi lity of the CO ligand lability of complex 4. The vibrational spectroscopies of the Fe(CO)(2)(CN) and Fe(CO)(CN) fragments (nu(CN) ranges from 2094 to 2105 cm(-1), nu(CO) ranges from 1928 to 2013 cm(-1)) found in complexes 1, 3, 4, and 5 may be regarded as spectroscopic references of [Fe] hydrogenase s in the various enzymatic states.