PUB - Publikationen an der Universität Bielefeld

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]Xn 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)3M(mu-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^IIIFe^II(ls) (ls = low-spin) with a spin ground state of St = 3/2; (1c) Fe^III(ls)Cr^IIIFe^III(ls) with an St = 1/2 ground state; (2c) Fe^III(ls)Co^III(ls)Fe^III(ls) with an St = 1 ground state; (3c) Fe^III(ls)Fe^III(ls)Fe^III(ls) with an St = 1/2 ground state. For 1b (St = 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^2.5Cr^IIIFe^2.5L]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 St = 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.