Environmental Influence on the Single-Molecule Magnet Behavior of [MnIII6CrIII]3+: Molecular Symmetry versus Solid-State Effects

The structural, spectroscopic, and magnetic properties of a series of [MnIII6CrIII]3+ (= [{(talent‑Bu2)­MnIII3}2{CrIII(CN)6}]3+) compounds have been investigated by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and electronic absorption spectroscopy, elemental analysis, electro spray ionization–mass spectrometry (ESI-MS) and matrix-assisted laser desorption ionization–mass spectrometry (MALDI-MS), cyclic voltammetry, AC and DC magnetic measurements, as well as theoretical analysis. The crystal structures obtained with [CrIII(CN)6]3– as a counterion exhibit (quasi-)­one-dimensional (1D) chains formed by hydrogen-bonded (1) or covalently linked (2) trications and trianions. The rod-shaped anion lactate enforces a rod packing of the [MnIII6CrIII]3+ complexes in the highly symmetric space group R3̅ (3) with a collinear arrangement of the molecular S6 axes. Incorporation of the spherical anion BPh4– leads to less-symmetric crystal structures (4–6) with noncollinear orientations of the [MnIII6CrIII]3+ complexes, as evidenced by the angle between the approximate molecular C3 axes taking no specific values in the range of 2°–69°. AC magnetic measurements on freshly isolated crystals (1a and 3a–6a), air-dried crystals (3b–6b), and vacuum-dried powder samples (3c–6c) indicate single-molecule magnet (SMM) behavior for all samples with Ueff values up to 28 K. The DC magnetic data are analyzed by a full-matrix diagonalization of the appropriate spin-Hamiltonian including isotropic exchange, zero-field splitting, and Zeeman interaction, taking into account the relative orientation of the D-tensors. Simulations for 3a–6a and 3c–6c indicate a weak antiferromagnetic exchange between the MnIII ions in the trinuclear subunits (JMn–Mn = −0.70 to −0.85 cm–1, Ĥex = −2∑ij JijŜi·Ŝj) that is overcome by the stronger antiferromagnetic interaction via the Cr–CN–Mn pathway (JCr–Mn = −3.00 to −5.00 cm–1), leading to an overall ferrimagnetic coupling scheme with an St = 21/2 spin ground state. The differences in Ueff, JMn–Mn, and JCr–Mn for the investigated samples are rationalized in terms of subtle variations in the molecular and crystal structures. In particular, a magnetostructural correlation between the Mn–NCN bond length and the JCr–Mn exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on [MnIII6CrIII]3+ allow the formulation of some key recipes for a rational improvement of the SMM behavior.