Electronic Structure of Linear Thiophenolate-Bridged Heterotrinuclear Complexes [LFeMFeL]ⁿ⁺ (M = Cr, Co, Fe; n = 1−3):  Localized vs Delocalized Models

Abstract

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 CrSO4·5H2O, 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 Mössbauer spectroscopy (in zero and applied field). The following electronic structures have been established: (1a) FeII(ls)CrIIIFeII(ls) (ls = low-spin) with a spin ground state of S t = 3/2; (1c) FeIII(ls)CrIIIFeIII(ls) with an S t = 1/2 ground state; (2c) FeIII(ls)CoIII(ls)FeIII(ls) with an S t = 1 ground state; (3c) FeIII(ls)FeIII(ls)FeIII(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 (Fe2.5) and, therefore, the excess electron is delocalized (class III): [LFe2.5CrIIIFe2.5L]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 Mössbauer time scale. Thus, 2b is class II with temperature-dependent electron hopping between the FeII and FeIII 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.

X‐ray Absorption Spectra of CuII and CuIII Complexes of N,N′‐1,2‐Phenylenebis(2‐mercapto‐2‐methylpropionamide)

“…This allowed us to conclude that the Cu III compound is likely a www.eurjic.org Table 1. Distances and angles (Å and°), at given copper oxidation number (ON), obtained by geometry optimisation, at B3LYP/6-31G(d,p) level for different spin multiplicity (Spin), by X-ray crystallography [2] and by EXAFS analysis and calculated difference energy values ∆E (eV) [2] Both theoretical and experimental data confirm that a change in the copper oxidation number from ii to iii de-creases the metalϪligand distances of the atoms of the first coordination shell; [1,2,7,8] conversely, an increase in the spin multiplicity, at a given oxidation state of the metal, increases the same metalϪligand distances (see, for example, the calculated CuϪS or CuϪN distances).…”

“…Moreover, the shape and position of the absorption edge help to distinguish different oxidation states of the metal atoms in analogous complexes. [7] In particular, in the rising edge of the XAS spectra of the copper complex, a few weak peaks and shoulders might originate from transitions from 1s to 3d or 4p orbitals, which are typically attributed to bound state forbidden transitions. [6] This information is usually employed to determine the local symmetry of the transition-metal environments.…”

“…[7,8] The presence of three edge peaks was attributed to three electronic transitions involving the 1s, 3d and 4p orbitals of copper. [8] The positions and intensity of such transitions were evaluated by fitting the spectra and rationalised by empirical configuration interaction methods; [8] the parameters were obtained from X-ray photoelectron spectroscopy and XAS studies.…”

Trinuclear Early/Late‐Transition‐Metal Thiolate Complexes

“…[8] In contrast, no examples of heterometallic vanadium/cobalt or vanadium/nickel thiolato complexes were found in the literature. A series of trinuclear metal complexes of the type [LMMЈML] n+ with L = 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane like [LCoNiCoL] n+ (n = 2-4), [LNiCoNiL] n+ (n = 1-3), [9] [LFeNiFeL] n+ (n = 2-4), [10] [LFeMFeL] n+ (M = Cr, Co; n = 1-3) [11] and [LFeMFeL] n+ (M = Ge, Sn; n = 2,3) [12] have been synthesised and characterised with respect to their magnetic exchange properties. It was shown that in these compounds the mechanism of spin coupling cannot be described exclusively in terms of the classical superexchange mechanisms but rather spin-dependent delocalisation by means of double exchange mechanisms [9] had to be invoked.…”

Multiple K-edge XAS for the structural analysis of thiophenolate bridged heterotrinuclear complexes

“…In a forthcoming publication [Glaser et al (1998)] we focus on the preparation of these complexes, whereas in this contribution we discuss the structural changes upon oxidation of the complexes determined by X-ray absorption spectroscopy. …”