Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [(Mn6OsIII)-Os-III](3+): Promoting SMM Behavior through 3d-5d Transition Metal Substitution

Höke V, Stammler A, Bögge H, Schnack J, Glaser T. Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [(Mn6OsIII)-Os-III](3+): Promoting SMM Behavior through 3d-5d Transition Metal Substitution. Inorganic Chemistry. 2014;53(1):257-268.The reaction of the in situ generated trinuclear triplesalen complex [(talen(t-Bu2))Mn-3(III)(solv)(n)](3+) with (Ph4P)(3)[Os-III(CN)(6)] and NaClO4 center dot H2O affords [(Mn6OsIII)-Os-III] (ClO4)(3) (= [{(talen(t-Bu2))Mn-3(III)}(2){Os-III(CN)(6)}](ClO4)(3)) in the presence of the oxidizing agent [(tacn)(2)Ni-III] (ClO4)(3) (tacn =1,4,7-triazacyclononane), while the reaction of [(talen(t-Bu2))-Mn-3(III)(solv)(n)](3+) with K-4[Os-II(CN)(6)] and NaClO4 center dot H2O yields [(Mn6OsII)-Os-III](ClO4)(2) under an argon atmosphere. The molecular structure of [(Mn6OsIII)-Os-III](3+) as determined by single-crystal X-ray diffraction is closely related to the already published [(Mn6Mc)-M-III](3+) complexes (M-c = Cr-III, Fe-III, Co-III, Mn-III). The half-wave potential of the Os-III/Os-II couple is E-1/2 = 0.07 V vs Fc(+)/Fc. The FT-IR and electronic absorption spectra of [(Mn6OsII)-Os-III](2+) and [(Mn6OsIII)-Os-III](3+) exhibit distinct features of dicationic and tricationic [(Mn6Mc)-M-III](n+) complexes, respectively. The dc magnetic data (mu(eff) vs T, M vs B, and VTVH) of [(Mn6OsII)-Os-III](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 Mn-III-Mn-III interactions within the trinuclear triplesalen subunits (J(Mn-Mn)((1)) = -(0.53 +/- 0.01) cm(-1), (H) over cap (ex) = -2 Sigma(i(i)center dot(S) over cap (j)) as well as across the central Os-II ion (J(Mn-Mn)((2,cis)) = -(0.06 +/- 0.01) cm(-1), (J(Mn-Mn)((2,trans)) = -(0.15 +/- 0.01) cm(-1)), while D-Mn = -(3.9 +/- 0.1) cm(-1). The mu(eff) vs T data of [(Mn6OsIII)-Os-III](3+) are excellently reproduced assuming an anisotropic Ising-like Os-III-Mn-III superexchange with a nonzero component J(Os-Mn)((aniso)) = -(11.0 +/- 1.0) cm(-1) along the Os-Mn direction, while J(Mn-Mn) = -(0.9 +/- 0.1) cm(-1) and D-Mn = -(3.0 +/- 1.0) cm(-1). Alternating current measurements indicate a slower relaxation of the magnetization in the SMM [(Mn6OsIII)-Os-III](3+) compared to the 3d analogue [(Mn6FeIII)-Fe-III](3+) due to the stronger and anisotropic M-c-Mn-III exchange interaction