Switching from antiferromagnetic to ferromagnetic coupling in heptanuclear [M(t)6M(c)](n+) complexes by going from an achiral to a chiral triplesalen ligand.

The chiral triplesalen ligand H6chand(RR) has been used to synthesize the chiral heptanuclear complexes {(chand(RR))Mn(III)3}2{Fe(II)(CN)6}2 ((RR)Mn(III)6Fe(II)2) and {(chand(RR))Fe(III)3}2{Fe(II)(CN)6}2 ((RR)Fe(III)6Fe(II)2), which have been characterized by single-crystal X-ray diffraction, mass spectrometry, elemental analysis, FT-IR, Mössbauer, 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)Mn(III)6Fe(II) and (RR)Fe(III)6Fe(II) is E1/2 = +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 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 (μeffvs. 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 JMn-Mn = +0.17 ± 0.02 cm(-1) with DMn = -3.4 ± 0.3 cm(-1) for (RR)Mn(III)6Fe(II) and JFe-Fe = +0.235 ± 0.005 cm(-1) with DFe = 0 for (RR)Fe(III)6Fe(II). The comparison of the molecular structures of (RR)Mn(III)6Fe(II) and (RR)Fe(III)6Fe(II) with those of the heptanuclear complexes M(t)6M(c) 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)Mn(III)6Fe(II) and (RR)Fe(III)6Fe(II), which explains the switching from antiferromagnetic coupling in M(t)6M(c) to ferromagnetic coupling in (RR)M(t)6M(c) 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 JMn-Mn.