TRANSITION-METAL COMPLEXES WITH SULFUR LIGANDS .88. DEPENDENCE OF SPIN STATE, STRUCTURE, AND REACTIVITY OF [FE(II)(L)('NHS4')] COMPLEXES ON THE COLIGAND-L (L = CO, N2H2, N2H4, NH3, PYRIDINE, NHCH3NH2, CH3OH, THF, P(OCH3)3, P(OPH)3) - MODEL COMPLEXES FOR IRON NITROGENASES ('NHS4'(2-) = DIANION OF 2,2'-BIS[(2-MERCAPTOPHENYL)THIO]DIETHYLAMINE)

In search of complexes exhibiting functional and structural characteristics of the active centers of nitrogenases, the Fe(II) complexes [Fe(L)('NHS4')] with the pentadentate amine-thioether-thiolate ligand 'NHS4'2- (L = N2H4, CH3OH, THF, pyridine, NH3, NHCH3NH2, P(OMe)3, P(OPh)3; 'NHS4'2- = dianion of 2,2'-bis[(2-mercaptophenyl)-thio]diethylamine, [Fe('NHS4')]x, and the free ligand ['NHS4'-H-2].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 [Fe(N2H4)('NHS4')] (1a) are orthorhombic space group Pbca, a = 1102.1 (7), b = 1567.2 (8), c = 2206.6 (14) pm, Z = 8, and R/R(w) = 0.046/0.046. The crystal data for [Fe(CH3OH)(NHS4')] (2) are orthorhombic space group Pbca, a = 1152.8 (3), b = 1471.5 (5), c = 2237.0 (5) pm, Z = 8, and R/R(w) = 0.069/0.054. The crystal data for [Fe(NH3)('NHS4')] (5) are orthorhombic space group Pbca, a = 1107.0 (5), b = 1547.1 (8), c = 2171.9 (9) pm, Z = 8, and R/R(w) = 0.040/0.035. Sigma-donor ligands (N2H4 or NH3) cause high-spin states; sigma-donor, pi-acceptor ligands (CO, NO, or N2H2), however, cause low-spin states of the iron centers. The high-spin 18e- complexes (L = N2H4, NH3, CH3OH) exhibit considerably larger Fe-S and Fe-N distances than the low-spin 18e- compounds (L = CO, 1/2(N2H2)). The 19e- complex [Fe(NO)('NHS4')] with low-spin Fe(II) 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('NHS4')] complexes provide a model. Interrelation of iron spin states, coordination of either sigma or sigma-pi ligands,and substitution lability or inertness may not only support the reduction steps leading from N2 via N2H2 over N2H4 to NH3, but also enable the final step in a catalytic cycle, the substitution of NH3 by N2.