Electronic structure of linear thiophenolate-bridged heterotrinuclear complexes [LFeMFeL]n(+) (M = Cr, Co, Fe; n = 1-3): Localized vs delocalized models

The reaction of mononuclear [LFe<SUP>III</SUP>] where L represents the tria nionic ligand 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclono nane with CrSO<INF>4</INF>. 5H<INF>2</INF>O, CoCl<INF>2</INF>. 6H<INF>2</IN F>O, or Fe(BF<INF>4</INF>)<INF>2</INF>. 6H<INF>2</INF>O and subsequent oxid ation with ferrocenium hexafluorophosphate or NO(BF4) or reduction with [(t mcn)Mo(CO)<INF>3</INF>] (tmcn = 1,4,7-trimethyl-1,4,7-triazacyclononane) pr oduced an isostructural series of [LFeMFeL]<SUP>n+</SUP> complexes, the fol lowing salts of which were isolated as crystalline solids: (i) [LFeCrFeL](P F<INF>6</INF>)<INF>n</INF> with n = 1 (1a), n = 2 (1b), and n = 3 (1c); (ii ) [LFeCoFeL]X<INF>n</INF> with X = BPh<INF>4</INF> and n = 2 (2b) and X = P F<INF>6</INF> and n = 3 (2c); (iii) [LFeFeFeL](BPh<INF>4</INF>)<INF>n</INF> with n = 2 (3b) and n = 3 (3c). All compounds contain Linear trinuclear ca tions (face-sharing octahedral) with an N<INF>3</INF>Fe(mu-SR)<INF>3</INF>M (mu-SR)<INF>3</INF>FeN<INF>3</INF> core structure. The electron structure o f all complexes has been studied by Fe and M K-edge X-ray absorption near e dge structure (XANES), UV-vis, and EPR spectroscopy, variable-temperature, variable-field susceptibility measurements, and Mossbauer spectroscopy (in zero and applied field). The following electronic structures have been esta blished: (1a) Fe<SUP>II</SUP>(ls)Cr<SUP>III</SUP>Fe<SUP>II</SUP>(ls) (ls = low-spin) with a spin ground state of S<INF>t</INF> = 3/2; (1c) Fe<SUP>III< /SUP>(ls)Cr<SUP>III</SUP>Fe<SUP>III</SUP>(ls) with an S<INF>t</INF> = 1/2 g round state; (2c) Fe<SUP>III</SUP>(ls)Co<SUP>III</SUP>(ls)Fe<SUP>III</SUP>( ls) with an S<INF>t</INF> = 1 ground state; (3c) Fe<SUP>III</SUP>(ls)Fe<SUP >III</SUP>(ls)Fe<SUP>III</SUP>(ls) with an S<INF>t</INF> = 1/2 ground state . For 1b (S<INF>t</INF> = 2) it is found that the two iron ions are spectro scopically equivalent (Fe<SUP>2.5</SUP>) and, therefore, the excess electro n is delocalized (class III): [LFe<SUP>2.5</SUP>Cr<SUP>III</SUP>Fe<SUP>2.5< /SUP>L]<SUP>2+</SUP>. For 2b clearly two different iron sites prevail at lo w temperatures (4.2 K); at higher temperatures (>200 K) they become equival ent on the Mossbauer time scale. Thus, 2b is class II with temperature-depe ndent electron hopping between the Fe<SUP>II</SUP> and Fe<SUP>III</SUP> ion s. 3b is again fully delocalized (class III) with an S<INF>t</INF><SUP></SU P> = 1 ground state; the excess electron is delocalized over all three iron sites. The electronic structure of all complexes is discussed in terms of double exchange and superexchange mechanisms.