A Comprehensive Study on Triplesalen-Based [Mn~(III)_6Fe~(III)]~(3+) and [Mn~(III)_6Fe~(II)]~(2+) Complexes: Redox- Induced Variation of Molecular Magnetic Properties

摘要

The synthesis and structural, mass spectrometric, FT-IR, M?ssbauer, and UV/Vis spectroscopic, electrochemical, and DC and AC magnetic properties of three [Mn~(III)_6Fe~(III)]~(3+) (short for [{(talen~(tBu2))Mn~(III)_3}_2{Fe~(III)(CN)6}]3+) compounds {[Mn~(III)_(6-) Fe~(III)(MeOH)_6]Cl_3?11MeOH?H_2O (1a), [Mn~(III)_6Fe~(III)(MeOH)_6]- Cl_3?3MeOH?H_2O (1b), and [Mn~(III)_6Fe~(III)(MeOH)_6](PF_6)_2(OAc)? 11MeOH (3)} and the one-electron reduced congener [Mn~(III)_6Fe~(II)(MeOH)_4](BPh_4)_2?3MeOH?toluene (2) are presented. The half-wave potential of the Fe~(III)/Fe~(II) couple in [Mn~(III)_6Fe~(III/II)]~(3+/2+) is E_(1/2) = -0.06 V versus Fc/Fc~+. The overall molecular structures of the complexes resemble those of the already published [Mn~(III)_6Mc]~(3+) compounds (M~c = Cr~(III), Co~(III), Mn~(III), Fe~(III)). The [Mn~(III)_6Fe~(II)]~(2+) dication, however, exhibits shorter Mn-N~(C≡N) bonds than the tricationic [Mn~(III)_6M~c]~(3+) complexes, as well as a higher degree of aromaticity of the central benzene ring of the triplesalen ligand. The electronic absorption spectrum of [Mn~(III)_6Fe~(II)]~(2+) differs considerably from the superimposable spectra of tricationic [Mn~(III)_6M~c]~(3+) complexes in exhibiting lower-energy ligand field transitions due to stronger π-donation of the surrounding ligands and weaker absorption features in the 27000-35000 cm~(-1) region, due to a weaker keto-enamine character of the central phloroglucinol unit. AC susceptibility measurements indicate single- molecule magnet (SMM) behavior for 1a and 3. Analysis of the DC magnetic data (μ_(eff) vs. T, VTVH) of these compounds by a full-matrix diagonalization of the spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction reveals J_(Mn,Mn) = -1.00 to -1.15 cm~(-1) (?_(ex) = -2∑i