Structural influences on the exchange coupling and zero-field splitting in the single-molecule magnet [Mn(III)6Mn(III)]3+.

A comprehensive synthetic, structural, mass spectrometrical, FT-IR and UV/Vis spectroscopic, electrochemical, and magnetic study on Mn(III)(6)Mn(III) (= {(talen(t-Bu(2)))Mn(III)(3)}(2){Mn(III)(CN)(6)}) is presented. The high stability of Mn(III)(6)Mn(III) in solution allows the preparation of different salts and solvates: Mn(III)(6)Mn(III)(3)·3MeOH·3MeCN·3Et(2)O (), Mn(III)(6)Mn(III)(MeOH)(4)(3)·5MeOH (), Mn(III)(6)Mn(III)(MeOH)(6)(3)·9MeOH (), Mn(III)(6)Mn(III)(MeOH)(6)(2)(OAc)·11MeOH (), and Mn(III)(6)Mn(III)(MeOH)(6)(3)·5MeOH·10H(2)O (). The molecular structure of Mn(III)(6)Mn(III) is closely related to the already published Mn(III)(6)M(c) 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)} trication. FT-IR spectra show the characteristic bands of the triplesalen ligand in Mn(III)(6)M(c) and the symmetric ν(C≡N) vibration of the Mn(III)(CN)(6) 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) compounds are rationalized in terms of subtle variations in the molecular structures, especially regarding the distortion of the central Mn(III)(CN)(6) core and the ligand folding. In comparison with the other Mn(III)(6)M(c) compounds, this allows us to establish some general magnetostructural correlations for this class of complexes.