Synthesis and Characterization of the Heptanuclear [Mn6III coIII]3+ Triplesalen Complex: Evidence for Exchange Pathways Involving Low-spin CoIII

The reaction of the tert-butyl-substituted triplesalen ligand H(6)talen(t-Bu2) with Mn-II(OAc)(2)center dot 4H(2)O and K-3[Co-III(CN)(6)] results in the formation of the heptanuclear complex [{(talen(t-Bu2))(Mn-III (MeOH))(3)}(2) {Co-III (CN)(6)}](PF6)(2)(OAc)center dot IIMeOH ([(Mn6CoIII)-Co-III](PF6)(2)(OAc)center dot IIMeOH, 1 center dot IIMeOH), which has been characterized by FT-IR spectroscopy, elemental analysis, ESI-MS, single-crystal X-ray diffraction, and magnetic measurements. The molecular structure of the [(Mn6CoIII)-Co-III](3+) complex is closely related to the already published analogs [(Mn6CrIII)-Cr-III](3+) and [(Mn6FeIII)-Fe-III](3+). Variable-temperature variable-field and mu(eff) vs. 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, taking into account the relative orientation of the D tensors. This allowed a careful inspection of the Mn-III-Mn-III exchange interaction involving a diamagnetic central metal ion. A satisfactory reproduction of the magnetic data required the incorporation not only of an exchange interaction between the Mn-III ions belonging to one triplesalen half unit, but also of an exchange coupling between Will ions belonging to different triplesalen subunits. Satisfactory reproduction of the experimental data has been obtained for the parameter set J(1) = -(0.50 +/- 0.04) cm(-1),J(2) = +(0.05 +/- 0.02) cm(-1), and D= -(2.5 +/- 0.5) cm(-1). A detailed analysis of the J(1) coupling taking into account the molecular structures of the three available heptanuclear complexes [Mn-6(III)](3+) (M = Cr, Fe, Co) indicates that the exchange interaction between the Mn-III ions belonging to the same triplesalen subunit involves not only an exchange pathway through the central phloroglucinol unit but also an exchange pathway through the central metal ion.