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)](ClO(4))(3) (tacn = 1,4,7-triazacyclononane) and PbO(2)/methanesulfonic acid produced an isostructural series of complexes [LFeNiFeL](n+) (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](ClO(4))(2). 5CH(3)CN (1*), C(88)H(123)Cl(2)Fe(2)N(11)NiO(8)S(6), and [LFeNiFeL](ClO(4))(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 (1) over bar with a = 13.065(2) Angstrom (13.155(2) Angstrom), b = 13.626(3) Angstrom (13.747(3) Angstrom), c = 14.043(3) Angstrom (16.237(3) Angstrom), 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(mu-SR)(3)Ni(mu-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 Mossbauer 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.