Spin-Dependent Delocalization in Three Isostructural Complexes [LFeNiFeL](2+/3+/4+) (L = 1,4,7-(4-tert-Butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane).

The reaction of mononuclear [LFe(III)] where L represents the trianionic ligand 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane with NiCl(2).6H(2)O and subsequent oxidations with Ni(III)(tacn)(2)(3) (tacn = 1,4,7-triazacyclononane) and PbO(2)/methanesulfonic acid produced an isostructural series of complexes LFeNiFeL()(+) (n = 2 (1), n = 3 (2), n = 4 (3)), which were isolated as PF(6)(-) (1, 3) or ClO(4)(-) salts (2). The molecular structures were established by X-ray crystallography for LFeNiFeL(2).5CH(3)CN (1), C(88)H(123)Cl(2)Fe(2)N(11)NiO(8)S(6), and LFeNiFeL(3).8acetone (2), C(102)H(156)Cl(3)Fe(2)N(6)NiO(20)S(6). Both compounds crystallize in the triclinic space group P&onemacr; with a = 13.065(2) Å (13.155(2) Å), b = 13.626(3) Å (13.747(3) Å), c = 14.043(3) Å (16.237(3) Å), alpha = 114.47(3) degrees (114.20(2) degrees ), beta = 97.67(3) degrees (96.57(2) degrees ), gamma = 90.34(3) degrees (98.86(2) degrees ), Z = 1(1) (values in parentheses refer to 2). The cations in 1, 2, and 3 have been determined to be isostructural by Fe and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. All compounds contain linear trinuclear cations (face-sharing octahedral) with an N(3)Fe(&mgr;-SR)(3)Ni(&mgr;-SR)(3)FeN(3) core structure. The electronic structures of 1, 2, and 3 have been studied by Fe and Ni K-edge X-ray absorption near edge (XANES), UV-vis, EPR, and Mössbauer spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Complexes 1 and 3 possess an S(t)() = 0 whereas 2 has an S(t)() = (1)/(2) ground state. It is shown that the electronic structures cannot be described by using localized valences (oxidation states). Delocalized models invoking the double-exchange mechanism are appropriate; i.e., spin-dependent delocalization via a double-exchange mechanism yields the correct ground state in each case. 1, 2, and 3 represent the first examples where double exchange stabilizes a ground state of minimum spin multiplicity.