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 CrSO 4 ·5H 2 O, CoCl 2 ·6H 2 O, or Fe(BF 4 ) 2 ·6H 2 O and subsequent oxidation with ferrocenium hexafluorophosphate or NO(BF 4 ) 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](PF 6 ) n with n = 1 ( 1a ), n = 2 ( 1b ), and n = 3 ( 1c ); (ii) [LFeCoFeL]X n with X = BPh 4 and n = 2 ( 2b ) and X = PF 6 and n = 3 ( 2c ); (iii) [LFeFeFeL](BPh 4 ) n with n = 2 ( 3b ) and n = 3 ( 3c ). All compounds contain linear trinuclear cations (face-sharing octahedral) with an N 3 Fe(μ-SR) 3 M(μ-SR) 3 FeN 3 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 Mössbauer 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 Mössbauer 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.