In search of complexes exhibiting functional and structural characteristics of the active centers of nitrogenases, the Fe" complexes [Fe(L)('bi~Sl)] with the pentadentate amine-thioether-thiolate ligand ' N H S~)~-(L = N2H4, CH3-OH, THF, pyridine, NH3, NHCH3NH2, P(OMe)3, P(OPh)3; ' N~s 4 '~-= dianion of 2,2'-bis[(2-mercaptophenyl)-thioldiethylamine), [Fe('N&)],, and the free ligand ['N&'-H2].HCl were synthesized and characterized. The molecular structures of the complexes with L = N2H4, NH3, and CH3OH have been elucidated by X-ray structure analyses. The crystal data for [ F~( N~H~) ( " H S~' ) ] (la) are orthorhombic space group Pbca, a = 1102.1 (7), b = 1567.2 (8), c = 2206.6 (14) pm, 2 = 8, and R / R , = 0.046/0.046. The crystal data for [Fe(CH30H)('NHSl)] (2) are orthorhombic space group Pbca, a = 1152.8 (3), b = 1471.5 (9, c = 2237.0 (5) pm, 2 = 8, and R / R , = 0.069/0.054. The crystal data for [Fe(NH3)('NHS,')] (5) are orthorhombic space group Pbca, a = 1107.0 (9, b = 1547.1 (8), c = 2171.9 (9) pm, 2 = 8, and R / R , = 0.040/0.035. u donor ligands (N2H4 or NHj) cause high-spin states; u donor, T acceptor ligands (CO, NO, or N z H~) , however, cause low-spin states of the iron centers. The high-spin 18e-complexes (L = N&, NH3, CH3OH) exhibit considerably larger F e S and Fe-N distances than the low-spin 18e-compounds (L = CO, '/2(N2H2)). The 19e-complex [F~(NO)("HS~')] with low-spin FelI takes an intermediate position.The correlation between coligands L, spin states, and bond lengths is interpreted in terms of molecular orbital schemes, which also explain why the 18e-low-spin complexes are highly substitution inert and the 18e-high-spin complexes are very labile. With regard to the active centers of "Fe-only" nitrogenases, [Fe-('NHS~)] complexes provide a model. Interrelation of iron spin states, coordination of either u or u -~ ligands, and substitution lability or inertness may not only support the reduction steps leading from N2 via N2Hz over N2H4 to NH3, but also enable the final step in a catalytic cycle, the substitution of NH3 by N2.
