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Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [MnIII6OsIII]3+: Promoting SMM Behavior through 3d–5d Transition Metal Substitution

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posted on 06.01.2014 by Veronika Hoeke, Anja Stammler, Hartmut Bögge, Jürgen Schnack, Thorsten Glaser
The reaction of the in situ generated trinuclear triplesalen complex [(talent‑Bu2)­MnIII3(solv)n]3+ with (Ph4P)3[OsIII(CN)6] and NaClO4·H2O affords [MnIII6OsIII](ClO4)3 (= [{(talent‑Bu2)­MnIII3}2­{OsIII(CN)6}]­(ClO4)3) in the presence of the oxidizing agent [(tacn)2NiIII]­(ClO4)3 (tacn =1,4,7-triazacyclononane), while the reaction of [(talent‑Bu2)­MnIII3(solv)n]3+ with K4[OsII(CN)6] and NaClO4·H2O yields [MnIII6OsII](ClO4)2 under an argon atmosphere. The molecular structure of [MnIII6OsIII]3+ as determined by single-crystal X-ray diffraction is closely related to the already published [MnIII6Mc]3+ complexes (Mc = CrIII, FeIII, CoIII, MnIII). The half-wave potential of the OsIII/OsII couple is E1/2 = 0.07 V vs Fc+/Fc. The FT-IR and electronic absorption spectra of [MnIII6OsII]2+ and [MnIII6OsIII]3+ exhibit distinct features of dicationic and tricationic [MnIII6Mc]n+ complexes, respectively. The dc magnetic data (μeff vs T, M vs B, and VTVH) of [MnIII6OsII]2+ are successfully simulated by a full-matrix diagonalization of a spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the D-tensors, and Zeeman interaction, indicating antiferromagnetic MnIII–MnIII interactions within the trinuclear triplesalen subunits (JMn–Mn(1) = −(0.53 ± 0.01) cm–1, ex = −2∑i<j Jiji·j) as well as across the central OsII ion (JMn–Mn(2,cis) = −(0.06 ± 0.01) cm–1, JMn–Mn(2,trans) = −(0.15 ± 0.01) cm–1), while DMn = −(3.9 ± 0.1) cm–1. The μeff vs T data of [MnIII6OsIII]3+ are excellently reproduced assuming an anisotropic Ising-like OsIII–MnIII superexchange with a nonzero component JOs–Mn(aniso) = −(11.0 ± 1.0) cm–1 along the Os–Mn direction, while JMn–Mn = −(0.9 ± 0.1) cm–1 and DMn = −(3.0 ± 1.0) cm–1. Alternating current measurements indicate a slower relaxation of the magnetization in the SMM [MnIII6OsIII]3+ compared to the 3d analogue [MnIII6FeIII]3+ due to the stronger and anisotropic Mc–MnIII exchange interaction.