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 [LFeIII] where L represents the trianionic ligand 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane with CrSO4. 5H(2)O, CoCl2. 6H(2)O, or Fe(BF4)(2). 6H(2)O 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(mu-SR)(3)M(mu-SR)(3)FeN3 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)(CrFeII)-Fe-III(ls) (ls = low-spin) with a spin ground state of S-t = 3/2; (1c) Fe-III(ls)(CrFeIII)-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): [(LFeCrFeL)-Cr-2.5-Fe-III-L-2.5](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.