Structural influences on the exchange coupling and zero-field splitting in the single-molecule magnet [MnIII6MnIII]3+

Abstract

A comprehensive synthetic, structural, mass spectrometrical, FT-IR and UV/Vis spectroscopic, electrochemical, and magnetic study on [Mn^III₆Mn^III]³⁺ (= [{(talen^t-Bu₂)Mn^III₃}₂{Mn^III(CN)₆}]³⁺) is presented. The high stability of [Mn^III₆Mn^III]³⁺ in solution allows the preparation of different salts and solvates: [Mn^III₆Mn^III](BPh₄)₃·3MeOH·3MeCN·3Et₂O (1), [Mn^III₆Mn^III(MeOH)₄](BPh₄)₃·5MeOH (2), [Mn^III₆Mn^III(MeOH)₆](BF₄)₃·9MeOH (3), [Mn^III₆Mn^III(MeOH)₆](PF₆)₂(OAc)·11MeOH (4), and [Mn^III₆Mn^III(MeOH)₆](lactate)₃·5MeOH·10H₂O (5). The molecular structure of [Mn^III₆Mn^III]³⁺ is closely related to the already published [Mn^III₆M^c]³⁺ complexes (M^c = Cr^III, Fe^III, Co^III). ESI mass spectra exhibit the signal of the [{(talen^t-Bu₂)Mn^III₃}₂{Mn^III(CN)₆}]³⁺ trication. FT-IR spectra show the characteristic bands of the triplesalen ligand in [Mn^III₆M^c]³⁺ and the symmetric ν(CN) vibration of the [Mn^III(CN)₆]³⁻ unit at 2135 cm⁻¹. UV/Vis spectra are dominated by intense transitions of the trinuclear Mn^III₃ triplesalen subunits above 20 000 cm⁻¹. The electrochemical studies establish the occurrence of ligand-centered oxidations at ≈1.0 V vs. Fc⁺/Fc, an oxidation of the central Mn^III at 0.78 V, and a series of reductions of the terminal Mn^III ions between −0.6 and −1.2 V. AC magnetic measurements indicate single-molecule magnet (SMM) behavior for all compounds. The DC magnetic data are analyzed by a full-matrix diagonalization of the appropriate spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction, taking into account the diamagnetic nature of the central Mn^III at low temperatures as inferred from a previous ab initio study. The spin-Hamiltonian simulations indicate Mn^III–Mn^III interactions in the −0.37 to −0.70 cm⁻¹ range within the trinuclear triplesalen subunits and in the −0.02 to +0.23 cm⁻¹ range across the central Mn^III ion, while D_Mn = −3.1 to −5.0 cm⁻¹. The differences in the exchange parameters and the relaxation behavior of the [Mn^III₆Mn^III]³⁺ compounds are rationalized in terms of subtle variations in the molecular structures, especially regarding the distortion of the central [Mn^III(CN)₆]³⁻ core and the ligand folding. In comparison with the other [Mn^III₆M^c]³⁺ compounds, this allows us to establish some general magnetostructural correlations for this class of complexes.