Environmental Influence on the Single-Molecule Magnet Behavior of [Mn6IIICrIII]3+: Molecular Symmetry versus Solid-State Effects

The structural, spectroscopic, and magnetic properties of a series of [(Mn6CrIII)-Cr-III](3+) (= [{(talen(t-Bu2))-Mn-3(III)}(2){Cr-III(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 [Cr-III(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 [(Mn6CrIII)-Cr-III](3+) complexes in the highly symmetric space group R (3) over bar (3) with a collinear arrangement of the molecular S-6 axes. Incorporation of the spherical anion BPh4- leads to less-symmetric crystal structures (4-6) with noncollinear orientations of the [(Mn6CrIII)-Cr-III](3+) complexes, as evidenced by the angle between the approximate molecular C-3 axes taking no specific values in the range of 2 degrees-69 degrees. 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 U-eff 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 Mn-III ions in the trinuclear subunits (J(Mn-Mn) = -0.70 to -0.85 cm(-1), (H) over cap (ex) = -2 Sigma(i<(S)over cap>(j),(S) over cap (j)) that is overcome by the stronger antiferromagnetic interaction via the Cr-C N-Mn pathway (J(Cr-Mn) = -3.00 to -5.00 cm(-1)), leading to an overall ferrimagnetic coupling scheme with an S-t = 21/2 spin ground state. The differences in U-eff, J(Mn-Mn), and J(Cr-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-N-C N bond length and the J(Cr-Mn) exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on [(Mn6CrIII)-Cr-III](3+) allow the formulation of some key recipes for a rational improvement of the SMM behavior.