Synthesis, structure, and magnetic characterization of a C3-symmetric Mn(III)3Cr(III) assembly: molecular recognition between a trinuclear Mn(III) triplesalen complex and a fac-triscyano Cr(III) complex

Abstract

The reaction of the tris(tetradentate) triplesalen ligand H(6)talen(t-Bu(2)), which provides three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone, with Mn(II) salts under aerobic conditions, affords, in situ, the trinuclear Mn(III) triplesalen complex [(talen(t-Bu(2))){Mn(III)(solv)(n)}(3)](3+). This species then reacts with [(Me(3)tacn)Cr(CN)(3)] to form the tetranuclear complex [{(talen(t-Bu(2)))Mn(III)(3)}{(Me(3)tacn)Cr(CN)(3)}](3+) ([Mn(III)(3)Cr(III)](3+)). The regular ligand folding observed in the trinuclear triplesalen complex preorganizes the three metal ions for the reaction with three facially coordinated nitrogen atoms of [(Me(3)tacn)Cr(CN)(3)]. [{(talen(t-Bu(2)))(Mn(III)(MeOH))(3)}{(Me(3)tacn)Cr(CN)(3)}](ClO(4))(3) (1) was characterized by infrared spectroscopy, elemental analysis, mass spectrometry, electron absorption spectroscopy, and magnetic measurements. The molecular structure was established for the acetate-substituted derivative [{(talen(t-Bu(2)))(Mn(III)(MeOH))(2)(Mn(III)(OAc))}{(Me(3)tacn)Cr(CN)(3)}](ClO(4))(2) (2) by single-crystal X-ray diffraction. Variable-temperature-variable-field and mu(eff) versus T magnetic data have been analyzed in detail by full-matrix diagonalization of the appropriate spin-Hamiltonian, consisting of isotropic exchange, zero-field splitting, and Zeeman interaction components. Satisfactory reproduction of the experimental data has been obtained for the parameters J(Mn-Cr) = -0.12 +/- 0.04 cm(-1), J(Mn-Mn) = -0.70 +/- 0.03 cm(-1), and D(Mn) = -3.0 +/- 0.4 cm(-1). These generate a triply degenerate pseudo S(t) = 7/2 spin manifold, which cannot be appropriately described by a giant spin model and which exhibits a weak easy-axis magnetic anisotropy. This is corroborated by the onset of a frequency-dependent chi'' signal at low temperatures, demonstrating a slow relaxation of the magnetization indicative of 1 being a single-molecule magnet. Comparing the properties to those of the heptanuclear analogue [{(talen(t-Bu(2)))Mn(III)(3)}(2){Cr(III)(CN)(6)}](3+) ([Mn(III)(6)Cr(III)](3+)) formed by the reaction of 2 equiv of [(talen(t-Bu(2))){Mn(III)(solv)(n)}(3)](3+) with 1 equiv of [Cr(CN)(6)](3-) [Glaser, T.; Heidemeier, M.; Weyhermüller, T.; Hoffmann, R.-D.; Rupp, H.; Müller, P. Angew. Chem. Int. Ed., 2006, 45, 6033-6037] demonstrates a lower driving force for formation, a strongly reduced J(Mn-Cr) exchange, a slightly reduced J(Mn-Mn) exchange, and a significantly longer Mn-N(N[triple bond]C) bond length in [Mn(III)(3)Cr(III)](3+). Taking into account magneto-structural correlations establishes a supramolecular interaction between the two [(talen(t-Bu(2)))Mn(III)(3)](3+) subunits in [Mn(III)(6)Cr(III)](3+) responsible for the structural distortion and the short Mn-N(N[triple bond]C) distance which results in a strong J(Mn-Cr) exchange and thus [Mn(III)(6)Cr(III)](3+) being a single-molecule magnet with a relatively high effective anisotropy barrier of 25.4 K.