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Issue 25, 2014
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Switching from antiferromagnetic to ferromagnetic coupling in heptanuclear [Mt6Mc]n+ complexes by going from an achiral to a chiral triplesalen ligand

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Abstract

The chiral triplesalen ligand H6chandRR has been used to synthesize the chiral heptanuclear complexes [{(chandRR)MnIII3}2{FeII(CN)6}](ClO4)2 (RR[MnIII6FeII](ClO4)2) and [{(chandRR)FeIII3}2{FeII(CN)6}](ClO4)2 (RR[FeIII6FeII](ClO4)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 FeIII/FeII couple in RR[MnIII6FeII]2+ and RR[FeIII6FeII]2+ is E1/2 = +0.21 and +0.75 V vs. Fc+/Fc, respectively, which (i) corresponds to a strong stabilization of the reduced FeII species compared to the redox couple of free [FeII/III(CN)6]4−/3− and (ii) indicates a significant difference of the electronic coupling with the {(chandRR)Mt}3+ units (Mt = MnIII, FeIII). 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[MnIII6FeII]2+ and JFe–Fe = +0.235 ± 0.005 cm−1 with DFe = 0 for RR[FeIII6FeII]2+. The comparison of the molecular structures of RR[MnIII6FeII]2+ and RR[FeIII6FeII]2+ with those of the heptanuclear complexes [Mt6Mc]n+ using the achiral triplesalen ligand (talent-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[MnIII6FeII]2+ and RR[FeIII6FeII]2+, which explains the switching from antiferromagnetic coupling in [Mt6Mc]n+ to ferromagnetic coupling in RR[Mt6Mc]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 JMn–Mn.

Graphical abstract: Switching from antiferromagnetic to ferromagnetic coupling in heptanuclear [Mt6Mc]n+ complexes by going from an achiral to a chiral triplesalen ligand

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Publication details

The article was received on 05 Mar 2014, accepted on 17 Apr 2014 and first published on 17 Apr 2014


Article type: Paper
DOI: 10.1039/C4DT00670D
Citation: Dalton Trans., 2014,43, 9690-9703
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    Switching from antiferromagnetic to ferromagnetic coupling in heptanuclear [Mt6Mc]n+ complexes by going from an achiral to a chiral triplesalen ligand

    C. Mukherjee, V. Hoeke, A. Stammler, H. Bögge, J. Schnack and T. Glaser, Dalton Trans., 2014, 43, 9690
    DOI: 10.1039/C4DT00670D

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