Geometric control in mononuclear complexes has come to the forefront in the field of molecular magnets due to its profound effects on relaxation pathways and blocking temperature in single molecule magnets (SMMs). Herein we report the synthesis and magnetic characterization of six trigonally symmetric, divalent Fe, Co, and Ni molecules, with the rigid geometry enforced via the use of the tris-anionic, tetradentate ligand MST (N,N′,N′′-[2,2′,2′′-nitrilotris-(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamide)). A systematic study on the effect of converting between trigonal monopyramidal complexes, (Me₄N)[M(MST)], and trigonal bipyramidal complexes, (Me₄N)[M(MST)(OH₂)] was conducted experimentally and computationally. It was found that (Me₄N)[Ni(MST)] exhibits a very large, near record zero-field splitting parameter (D) value of −434 cm⁻¹, owing to an extremely low lying first excited state. The trigonal monopyramidal cobalt and iron complexes exhibit slow magnetic relaxation under applied fields, resulting in barriers of 45 K and 63.9 K respectively. Coordination of a single water molecule in the open axial site of the trigonal monopyramidal complexes exerts drastic dampening effects on the D value as well as slow relaxation. Computations reveal that coordination of water rotates the D_zz axis away from the C₃ axis of symmetry resulting in a smaller D value. The aquo species (Me₄N)[Co(MST)(OH₂)] also exhibits magnetic relaxation under an applied field, but the barrier is reduced to 9.9 K. Water coordination totally quenches the magnetic behavior in the iron complex, and reduces the D value for nickel to −185 cm⁻¹. These results showcase the drastic effect that a small change in the coordination environment can have on magnetic behavior, as well as that trigonal monopyramidal geometry can lead to near record D values.

中文翻译：

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配位球对三角对称分子中异常大的零场分裂和慢磁弛豫的影响†

单核配合物的几何控制因其对单分子磁体（SMM）的弛豫路径和阻断温度的深远影响而成为分子磁体领域的前沿。在此，我们报告了六个三角对称二价 Fe、Co 和 Ni 分子的合成和磁性表征，通过使用三阴离子四齿配体 MST ( N , N ', N ''-[ 2,2',2''-次氮基三-(乙烷-2,1-二基)]三(2,4,6-三甲基苯磺酰胺))。通过实验和计算对三角单锥体配合物(Me ₄ N)[M(MST)]和三角双锥体配合物(Me ₄ N)[M(MST)(OH ₂ )]之间的转化效果进行了系统研究。研究发现，由于第一激发态极低， (Me ₄ N)[Ni(MST)]表现出非常大的、接近记录的零场分裂参数( D )值-434 cm ^{-1 。}三角单锥体钴和铁络合物在外加磁场下表现出缓慢的磁弛豫，分别产生 45 K 和 63.9 K 的势垒。三角单锥体复合物的开放轴位点中单个水分子的配位对D值以及缓慢松弛产生剧烈的抑制作用。计算表明，水的配位使D _zz轴旋转远离C ₃对称轴，导致D值更小。水合物种 (Me ₄ N)[Co(MST)(OH ₂ )] 在外加磁场下也表现出磁弛豫，但势垒降低至 9.9 K。水配位完全淬灭了铁配合物中的磁性行为，并降低了镍的D值为-185 cm ^-1。这些结果展示了配位环境的微小变化可能对磁性行为产生的巨大影响，以及三角单锥体几何形状可能导致接近记录的D值。