PUB - Publikationen an der Universität Bielefeld

The chiral triplesalen ligand H(6)chand(RR) has been used to synthesize the chiral heptanuclear complexes [{(chand(RR))Mn-3(III)}(2){Fe-II(CN)(6)}](ClO4)(2) ((RR)[(Mn6FeII)-Fe-III](ClO4)(2)) and [{(chand(RR))Fe-3(III)}(2){Fe-II(CN)(6)}](ClO4)(2) (RR[(Fe6FeII)-Fe-III](ClO4)(2)), which have been characterized by single-crystal X-ray diffraction, mass spectrometry, elemental analysis, FT-IR, Mossbauer, and UV-vis spectroscopies, electrochemistry, as well as DC and AC magnetic susceptibility measurements. The half-wave potential of the Fe-III/Fe-II couple in (RR)[(Mn6FeII)-Fe-III](2+) and (RR)[(Fe6FeII)-Fe-III](2+) is E-1/2 = +0.21 and +0.75 V vs. Fc(+)/Fc, respectively, which (i) corresponds to a strong stabilization of the reduced Fe-II species compared to the redox couple of free [Fe-II/(III)(CN)(6)](4-/3-) and (ii) indicates a significant difference of the electronic coupling with the {(chand(RR))M-t}(3+) units (M-t = Mn-III, Fe-III). Analysis of the DC magnetic data (mu(eff) vs. T, VTVH) of both complexes 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 interactions reveals ferromagnetic interactions of J(Mn-Mn) = +0.17 +/- 0.02 cm(-1) with DMn = -3.4 +/- 0.3 cm(-1) for (RR)[(Mn6FeII)-Fe-III](2+) and J(Fe-Fe) = +0.235 +/- 0.005 cm(-1) with D-Fe = 0 for (RR)[(Fe6FeII)-Fe-III](2+). The comparison of the molecular structures of (RR)[(Mn6FeII)-Fe-III](2+) and (RR)[(Fe6FeII)-Fe-III](2+) with those of the heptanuclear complexes [(M6Mc)-M-t](n+) using the achiral triplesalen ligand (talen(t-Bu2))(6-) reveals significant differences in the ligand folding, smaller C-C bond distances in the central phloroglucinol ring and larger HOMA values. This indicates more aromatic character and less heteroradialene contribution in (RR)[(Mn6FeII)-Fe-III](2+) and (RR)[(Fe6FeII)-Fe-III](2+), which explains the switching from antiferromagnetic coupling in [(M6Mc)-M-t](n+) to ferromagnetic coupling in (RR)[(M6Mc)-M-t](n+) by a stronger contribution of the spin-polarization mechanism. This establishes a magnetostructural correlation between the structural parameters describing the aromaticity of the central phloroglucinol unit and the observed exchange couplings J(Mn-Mn).