PUB - Publikationen an der Universität Bielefeld

The synthesis and structural, mass spectrometric, FT-IR, Mossbauer, and UV/Vis spectroscopic, electrochemical, and DC and AC magnetic properties of three [(Mn6FeIII)-Fe-III](3+) (short for [{(talentBu2)Mn-3(III)}(2){Fe-III(CN)(6)}](3+)) compounds {[(Mn6FeIII)-Fe-III(MeOH)(6)]Cl-3 center dot 11MeOH center dot H2O (1a), [(Mn6FeIII)-Fe-III(MeOH)(6)]Cl-3 center dot 3MeOH center dot H2O (1b), and [(Mn6FeIII)-Fe-III(MeOH)(6)](PF6)(2)(OAc)center dot 11MeOH (3)} and the one-electron reduced congener [(Mn6FeII)-Fe-III(MeOH)(4)](BPh4)(2)center dot 3MeOH center dot toluene (2) are presented. The half-wave potential of the Fe-III/Fe-II couple in [(Mn6FeIII/II)-Fe-III](3+/2+) is E-1/2 = -0.06 V versus Fc/Fc(+). The overall molecular structures of the complexes resemble those of the already published [(Mn6Mc)-M-III](3+) compounds (M-c = Cr-III, Co-III, Mn-III, Fe-III). The [(Mn6FeII)-Fe-III](2+) dication, however, exhibits shorter Mn-N-CN bonds than the tricationic [(Mn6Mc)-M-III](3+) complexes, as well as a higher degree of aromaticity of the central benzene ring of the triplesalen ligand. The electronic absorption spectrum of [(Mn6FeII)-Fe-III](2+) differs considerably from the superimposable spectra of tricationic [(Mn6Mc)-M-III](3+) complexes in exhibiting lower-energy ligand field transitions due to stronger pi-donation of the surrounding ligands and weaker absorption features in the 27000-35000 cm(-1) region, due to a weaker keto-enamine character of the central phloroglucinol unit. AC susceptibility measurements indicate single-molecule magnet (SMM) behavior for 1a and 3. Analysis of the DC magnetic data (mu(eff) vs. T, VTVH) of these compounds by a full-matrix diagonalization of the spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction reveals J(Mn,Mn) = -1.00 to -1.15 cm(-1) ((H) over cap (ex) = -2 Sigma(1(i)(S) over cap (j)), J(Fe,Mn) = +0.50 to +0.80 cm(-1), and D-Mn = -3.5 cm(-1). These values are similar to those previously found for [(Mn6FeIII)-Fe-III][Fe-III(CN)(6)]. For 2, the simulations indicate very weak antiferromagnetic Mn-III-Mn-III interactions within the trinuclear triplesalen subunits (J(Mn,Mn)((1)) = -0.20 cm(-1)) as well as across the central diamagnetic Fe-II ion (J(Mn,Mn)((2,cis)) = -0.06 cm(-1), J(Mn,Mn)((2,trans)) = -0.18 cm(-1)), whereas D-Mn = -3.5 cm(-1). The J(Mn,Mn)((1)) coupling in 2 is much less antiferromagnetic than in the tricationic [(Mn6Mc)-M-III](3+) compounds; this is interpreted in terms of a stronger ferromagnetic contribution to the exchange interaction due to more efficient spin-polarization through the central benzene ring, the aromatic nature of which is more pronounced.