| Abstract
| - A family of distorted heterometallic cubanes, [MnIII3NiII(hmp)3O(N3)3(O2CR)3], have been synthesized and magnetically and structurally characterized. X-ray crystallography indicates structures similar to those of the previously studied S = 9/2 Mn3IIIMnIV family of cubanes. Single-crystal HFEPR studies indicate well-isolated S = 5 spin ground states and negative D values ranging from −0.20 to −0.33 cm−1, and micro-SQUID hysteresis measurements confirm that complexes 1−4 are single-molecule magnets that clearly exhibit symmetry and spin-parity effects.
- A family of distorted heterometallic cubanes, [MnIII3NiII(hmp)3O(N3)3(O2CR)3], where O2CR− is benzoate (1), 3-phenylpropionate (2), 1-adamantanecarboxylate (3), or acetate (4) and hmp− is the anion of 2-pyridinemethanol, was synthesized and structurally as well as magnetically characterized. These complexes have a distorted-cubane core structure similar to that found in the S = 9/2 Mn4 cubane family of complexes. Complexes 1, 3, and 4 crystallize in rhombohedral, hexagonal, and cubic space groups, respectively, and have C3 molecular symmetry, while complex 2 crystallizes in the monoclinic space group Cc with local C1 symmetry. Magnetic susceptibility and magnetization hysteresis measurements and high-frequency electron paramagnetic resonance (HFEPR) spectroscopy established that complexes 1−4 have S = 5 spin ground states with axial zero-field splitting (ZFS) parameters (D) ranging from −0.20 to −0.33 cm−1. Magnetization versus direct-current field sweeps below 1.1 K revealed hysteresis loops with magnetization relaxation, definitely indicating that complexes 1−4 are single-molecule magnets that exhibit quantum tunneling of magnetization (QTM) through an anisotropy barrier. Complex 2 exhibits the smallest coercive field and fastest magnetization tunneling rate, suggesting a significant rhombic ZFS parameter (E), as expected from the low C1 symmetry. This was confirmed by HFEPR spectroscopy studies on single crystals that gave the following parameter values for complex 2: gz = 1.98, gx = gy = 1.95, D = −0.17 cm−1, B40 = −6.68 × 10−5 cm−1, E = 6.68 × 10−3 cm−1, and B42 = −1.00 × 10−4 cm−1. Single-crystal HFEPR data for complex 1 gave gz = 2.02, gx = gy = 1.95, D = −0.23 cm−1, and B40 = −5.68 × 10−5 cm−1, in keeping with the C3 site symmetry of this Mn3Ni complex. The combined results highlight the importance of spin-parity effects and molecular symmetry, which determine the QTM rates.
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