TRANSITION-METAL COMPLEXES WITH SULFUR LIGANDS .57. STABILIZATION OF HIGH-VALENT FE(IV) CENTERS AND VACANT COORDINATION SITES BY SULFUR PI-DONATION - SYNTHESES, X-RAY STRUCTURES, AND PROPERTIES OF [FE(S2)2(PME3)N] (N = 1, 2) AND (NME4)[FE(S2)2(PME3)2].CH3OH (S2(2-) = 1,2-BENZENEDITHIOLATE(2-))

The iron(IV) complexes [Fe(''S2'')2(PMe3)2] (1) and [Fe(''S2'')2(PMe3)] (2) as well as the related iron(III) complex (NMe4)[Fe(''S2'')2(PMe3)2].CH3OH (4) (''S2''2- = 1,2-benzenedithiolate(2-)) were synthesized and characterized as potential models for FeS oxidoreductases. X-ray structure determination shows that 1 (orthorhombic, P2(1)2(1)2(1), a = 2075.7 (6) pm, b = 1202.4 (2) pm, c = 875.0 (2) pm, Z = 4, R = R(W) = 0.059) contains six-coordinate iron in a strongly distorted octahedral environment; the two phosphine P atoms occupy trans positions, and the P-Fe-P angle is only 159.7 (1)degrees; the FeS4 unit is planar with two long (Fe-S(av) = 229.8 (3) pm) and two short (Fe-S(av) = 219.1 (3) pm) iron-sulfur bonds. In 2 (monoclinic, P2(1)/n, a = 1218.3 (3) pm, b = 730.7 (4) pm, c = 1998.6 (4) pm, beta = 104.20 (2)degrees, Z = 4, R/R(W) = 0.062/0.046) four basal sulfur and one apical phosphorus donors surround five-coordinate iron in a tetragonal pyramid; all four Fe-S bonds are short (d(av) = 218.5 (3) pm). In 4 (triclinic, P1BAR, a = 880.5 (3) pm, b = 931.3 (3) pm, c = 1793.9 (5) pm, alpha = 91.99 (2)degrees, beta = 91.83 (2)degrees, gamma = 94.40 (3)degrees, Z = 2, R/R(W) = 0.078/0.070) two methanol molecules are bound to one of the two independent anions via S...H-O bridges. Both anions are centrosymmetric, the iron centers are pseudooctahedrally surrounded by four sulfur and two trans phosphorus atoms, and the Fe-S distances range from 224.8 (2) to 227.3 (2) pm. Comparison of the structural data reveals that the ''S2'' ligands in all three complexes function as ''innocent'' dianionic dithiolates, and the iron atoms in 1, 2, and 4 have to be regarded as Fe(IV) and Fe(III) centers. This conclusion could be corroborated by Mossbauer, NMR, EPR, UV, and IR investigations as well as magnetic moments. Cyclovoltammetry further revealed two reversible one-electron steps for 1, 2, and 4 each, due to Fe(IV/III) and Fe(III/II) redox couples. The model character of the new complexes for [FeS] centers in redox enzymes is discussed. In the iron-benzenedithiolate systems a linear correlation exists between Fe-S bond lengths and formal electron count. In conclusion, it could be shown that Fe-S bond lengths and the stabilization of vacant coordination sites as well as high oxidation states in Fe/''S2'' complexes depend on sulfur pi-donation.