Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [Mn^III₆Os^III]³⁺: Promoting SMM Behavior through 3d–5d Transition Metal Substitution

The reaction of the <i>in situ</i> generated trinuclear triplesalen complex [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with (Ph₄P)₃[Os^III(CN)₆] and NaClO₄·H₂O affords <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>(ClO₄)₃ (= [{(talen^{<i>t</i>‑Bu₂})­Mn^III₃}₂­{Os^III(CN)₆}]­(ClO₄)₃) in the presence of the oxidizing agent [(tacn)₂Ni^III]­(ClO₄)₃ (tacn =1,4,7-triazacyclononane), while the reaction of [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with K₄[Os^II(CN)₆] and NaClO₄·H₂O yields <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>(ClO₄)₂ under an argon atmosphere. The molecular structure of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> as determined by single-crystal X-ray diffraction is closely related to the already published <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^<b>3+</b> complexes (M^c = Cr^III, Fe^III, Co^III, Mn^III). The half-wave potential of the Os^III/Os^II couple is <i>E</i>_1/2 = 0.07 V vs Fc⁺/Fc. The FT-IR and electronic absorption spectra of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> and <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> exhibit distinct features of dicationic and tricationic <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^{<b><i>n</i>+</b>} complexes, respectively. The dc magnetic data (μ_eff vs <i>T</i>, <i>M</i> vs <i>B</i>, and VTVH) of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> 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 <b>D</b>-tensors, and Zeeman interaction, indicating antiferromagnetic Mn^III–Mn^III interactions within the trinuclear triplesalen subunits (<i>J</i>_Mn–Mn⁽¹⁾ = −(0.53 ± 0.01) cm^–1, <i>Ĥ</i>_ex = −2∑_{<i>i</i><<i>j</i>} <i>J</i>_<i>ij</i><b>Ŝ</b>_<i>i</i>·<b>Ŝ</b>_<i>j</i>) as well as across the central Os^II ion (<i>J</i>_Mn–Mn^(2,cis) = −(0.06 ± 0.01) cm^–1, <i>J</i>_Mn–Mn^(2,trans) = −(0.15 ± 0.01) cm^–1), while <i>D</i>_Mn = −(3.9 ± 0.1) cm^–1. The μ_eff vs <i>T</i> data of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> are excellently reproduced assuming an anisotropic Ising-like Os^III–Mn^III superexchange with a nonzero component <i>J</i>_Os–Mn^(aniso) = −(11.0 ± 1.0) cm^–1 along the Os–Mn direction, while <i>J</i>_Mn–Mn = −(0.9 ± 0.1) cm^–1 and <i>D</i>_Mn = −(3.0 ± 1.0) cm^–1. Alternating current measurements indicate a slower relaxation of the magnetization in the SMM <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> compared to the 3d analogue <b>[Mn</b>^<b>III</b>_<b>6</b><b>Fe</b>^<b>III</b><b>]</b>^<b>3+</b> due to the stronger and anisotropic M^c–Mn^III exchange interaction