PUB - Publikationen an der Universität Bielefeld

The reaction of the tris(tetradentate) triplesalen ligand H(6)talen(t-Bu2), which provides three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone, with Wit salts under aerobic conditions, affords, in situ, the trinuclear Mn-III triplesalen complex [(talen(t-Bu2)){Mn-III(solv)(n)}(3)](3+). This species then reacts with [(Me(3)tacn)Cr(CN)(3)] to form the tetranuclean complex [(talen)(t-Bu2))Mn-3(III))}{(Me(3)tacn)Cr(CN)(3)}](3+) ([(Mn3CrIII)-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-Bu2))- (Mn-III(MeOH))(3)}{(Me(3)tacn)Cr(CN)(3)(-))](ClO4)(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-Bu2))(MnIII(MeOH))(2)(Mn-III(OAc))}{(Me(3)tacn)Cr(CN)(3)}]-(C lO4)(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 X '' 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-Bu2))Mn-3(III))(2){Cr-III(CN)(6))](3+) ([(Mn6CrIII)-Cr-III](3+)) formed by the reaction of 2 equiv of [(talent(t-Bu2)) {Mn-III(SOlV)(n)}(3)](3+) with 1 equiv of [Cr(CN)(6)](3-) [Glaser, T.; Heidemeier, M.; Weyhermuller, T.; Hoffmann, R.-D.; Rupp, H.; Muller, 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 JMn-Mn exchange, and a significantly longer Mn-N-N=c bond length in [(Mn3CrIII)-Cr-III](3+). Taking into account magneto-structural correlations establishes a supramolecular interaction between the two [(talen(t-Bu2))Mn-3(III)](3+) subunits in [(Mn6CrIII)-Cr-III](3+) responsible for the structural distortion and the short Mn-N-N=C distance which results in a strong J(Mn-Cr) exchange and thus [(Mn6CrIII)-Cr-II](3+) being a single-molecule magnet with a relatively high effective anisotropy barrier of 25.4 K.