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(III)] where L represents the trianionic ligand 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane with CrSO₄·5H₂O, COCl₂·6H₂O, or Fe(BF₄)₂·6H₂O and subsequent oxidation with ferrocenium hexafluorophosphate or NO(BF₄) or reduction with [(tmcn)Mo(CO)₃] (tmcn = 1,4,7-trimethyl-1,4,7-triazacyclononane) produced an isostructural series of [LFeMFeL](n+) complexes, the following salts of which were isolated as crystalline solids: (i) [LFeCrFeL](PF₆)(n) with n = 1 (1a), n = 2 (1b), and n = 3 (1c); (ii) [LFeCoFeL]X(n) with X = BPh₄ and n = 2 (2b) and X = PF₆ and n = 3 (2c); (iii) [LFeFeFeL](BPh₄)(n) with n = 2 (3b) and n = 3 (3c). All compounds contain linear trinuclear cations (face-sharing octahedral) with an N₃Fe(μ-SR)₃M(μ-SR)₃FeN₃ core structure. The electron structure of all complexes has been studied by Fe and M K-edge X- ray absorption near edge 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 established: (1a) Fe(II)(ls)Cr(III)Fe(II)(ls) (ls = low- spin) with a spin ground state of S(t) = 3/2; (1c) Fe(III)(ls)Cr(III)Fe(III)(ls) with an S(t) = 1/2 ground state; (2c) Fe(III)(ls)Co(III)(ls)Fe(III)(ls) with an S(t) = 1 ground state; (3c) Fe(III)(ls)Fe(III)(ls)Fe(III)(ls) with an S(t) = 1/2 ground state. For 1b (S(t) = 2) it is found that the two iron ions are spectroscopically equivalent (Fe(2.5)) and, therefore, the excess electron is delocalized (class III): [LFe(2.5)Cr(III)Fe(2.5)L]²⁺. For 2b clearly two different iron sites prevail at low temperatures (4.2 K); at higher temperatures (>200 K) they become equivalent on the Mossbauer time scale. Thus, 2b is class II with temperature-dependent electron hopping between the Fe(II) and Fe(III) ions. 3b is again fully delocalized (class III) with an S(t) = 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.