Dinuclear and Mononuclear Iron(II)−Thiolate Complexes with Mixed CO/CN⁻ Ligands:  Synthetic Advances for Iron Sites of [Fe]-Only Hydrogenases

The pentacoordinate, 16-electron Fe II complex [PPN][Fe(CO) 2 (CN)(S,NH-C 6 H 4 )] ( 1 ), stabilized by strong S, N π-donation of chelating [S,NH-C 6 H 4 ] 2 - ligand, was prepared by the reaction of 2-aminophenyl disulfide and [PPN][Fe(CO) 4 (CN)]. Protonation of complex 1 by electrophiles (2-mercaptopyrimidine and 2-aminophenylthiol) yielded hexacoordinate iron(II)−thiolate cyanocarbonyl complexes [PPN][Fe(CO)(CN)(S-C 4 H 3 N 2 ) 2 ] ( 5 ) and [PPN][Fe(CO) 2 (CN)(S-C 6 H 4 NH 2 )(S,NH 2 -C 6 H 4 )] ( 3 ), respectively. The IR spectrum of complex 5 in the aprotic solvent CH 3 CN displayed a weak ν(CN) band at 2090 cm - 1 and a strong ν(CO) band at 1945 cm - 1 . Chemical oxidation of complex 5 in CH 3 CN at − 20 °C with [Cp 2 Fe][PF 6 ] displayed absorption bands at 2096 and 1962 cm - 1 which were assigned to the ν(CN) and ν(CO) vibrational frequencies respectively of the thermally unstable neutral Fe III (CO)(CN)(S-C 4 H 3 N 2 ) 2 . Complex 5 was reobtained upon addition of [PPN][BH 4 ] to Fe III (CO)(CN)(S-C 4 H 3 N 2 ) 2 in CH 3 CN at −20 °C. The first dinuclear Fe(II)−thiolate cyanocarbonyl compound [PPN] 2 [(CN)(CO) 2 Fe( μ -S,S-C 6 H 4 )] 2 ( 4 ), the promising structural and functional model compound of the dinuclear iron active sites of [Fe]-only hydrogenases isolated from D. desulfuricans and C. pasteurianum, was prepared by reacting 1,2-benzenedithiol with complex 1 in THF at −10 °C. The X-ray structural analysis shows that complex 4 possesses crystallographically imposed centrosymmetry. Two six-coordinate Fe(II) centers are connected via two thiolate bridges, and both CN - ligands point into the antiparallel direction. The IR spectrum of complex 4 in the aprotic solvent CH 2 Cl 2 revealed a weak absorption band for the CN - ligands at 2101 cm - 1, and two strong absorption bands for the CO groups at 2013 and 1960 cm - 1 . When the CH 2 Cl 2 solution of complex 4 was exposed to 13 CO at 0 °C, absorbances at 1968 and 1915 cm - 1 appeared within 10 min. Reappearance of the 2013 and 1960 cm - 1 bands on the removal of the 13 CO and replacement with 12 CO atmosphere demonstrated reversibility of the CO ligand lability of complex 4 . The vibrational spectroscopies of the Fe(CO) 2 (CN) and Fe(CO)(CN) fragments (ν(CN) ranges from 2094 to 2105 cm - 1, ν(CO) ranges from 1928 to 2013 cm - 1 ) found in complexes 1, 3, 4, and 5 may be regarded as spectroscopic references of [Fe] hydrogenases in the various enzymatic states.