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(III)(6)Mn(III)](3+) (= [{(talen(t-Bu(2)))Mn(III)(3)}(2){Mn(III)(CN)(6)}](3+)) is presented. The high stability of [Mn(III)(6)Mn(III)](3+) in solution allows the preparation of different salts and solvates: [Mn(III)(6)Mn(III)](BPh(4))(3)·3MeOH·3MeCN·3Et(2)O (), [Mn(III)(6)Mn(III)(MeOH)(4)](BPh(4))(3)·5MeOH (), [Mn(III)(6)Mn(III)(MeOH)(6)](BF(4))(3)·9MeOH (), [Mn(III)(6)Mn(III)(MeOH)(6)](PF(6))(2)(OAc)·11MeOH (), and [Mn(III)(6)Mn(III)(MeOH)(6)](lactate)(3)·5MeOH·10H(2)O (). The molecular structure of [Mn(III)(6)Mn(III)](3+) is closely related to the already published [Mn(III)(6)M(c)](3+) complexes (M(c) = Cr(III), Fe(III), Co(III)). ESI mass spectra exhibit the signal of the [{(talen(t-Bu(2)))Mn(III)(3)}(2){Mn(III)(CN)(6)}](3+) trication. FT-IR spectra show the characteristic bands of the triplesalen ligand in [Mn(III)(6)M(c)](3+) and the symmetric ν(C≡N) 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(III)(3) 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(III)(6)Mn(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(III)(6)M(c)](3+) compounds, this allows us to establish some general magnetostructural correlations for this class of complexes.