The mononuclear cobalt(II) complexes [Co(L^N8)](BPh₄)₂·C₄H₁₀O (1-BPh₄) and [Co(L^N8)](NO₃)₂·CH₃CN (2-NO₃) (L^N8 = 1,4,7,10-tetrakis(2-pyridinemethyl)-1,4,7,10-tetraaza-cyclododecane) have been synthesized and fully characterized. They differ well beyond a formal replacement of the counter anions, with their Co(II) centers in significantly different coordination geometries. In 1-BPh₄, the Co(II) ion is not only coordinated by six N atoms from the L^N8 ligand but also associated with the two remaining N atoms of L^N8 via weak interactions. In contrast, the Co(II) ion in 2-NO₃ is only six-coordinated in a distorted trigonal prismatic geometry. Magnetic anisotropy and slow magnetic dynamics are drastically affected by these different environments around Co(II). The combination of dc magnetic data, high-frequency and -field electron paramagnetic resonance (HFEPR) and theoretical calculations unambiguously reveals large negative zero-field split (ZFS) parameters D for both complexes and a large difference between the D values. Both 1-BPh₄ and 2-NO₃ show slow magnetic relaxation at zero field. Magnetic dilution experiments reveal effective energy barriers of 54(4) cm⁻¹ for 1-BPh₄@Zn and 95(5) cm⁻¹ for 2-NO₃@Zn and confirm that the slow magnetic relaxation for both complexes originates from single molecule behaviour. Ab initio computational studies explain their electronic structures and the origin of the large negative magnetic anisotropy; they support the corresponding experimental observations. Further magneto-structural analyses reveal that different distortions from the ideal trigonal prismatic geometry exert drastic effects on D values and slow relaxation, and that the additional weak intramolecular interactions between Co and N reduce the axial anisotropy.

中文翻译：

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弱分子内相互作用和三棱柱配位畸变对零场Co(II)单离子磁体磁性能的影响

单核钴( II )配合物[Co(L ^N8 )](BPh ₄ ) ₂ ·C ₄ H ₁₀ O ( 1-BPh ₄ )和[Co(L ^N8 )](NO ₃ ) ₂ ·CH ₃ CN ( 2 -NO ₃ ) (L ^N8 = 1,4,7,10-四(2-吡啶甲基)-1,4,7,10-四氮杂-环十二烷)已被合成并充分表征。它们的不同之处远远超出了抗衡阴离子的正式替代，其 Co( II ) 中心具有显着不同的配位几何形状。在1-BPh ₄中，Co( II )离子不仅与L ^N8配体的六个N原子配位，而且还通过弱相互作用与L ^N8 的剩余两个N原子缔合。相比之下，2-NO ₃中的 Co( II ) 离子仅在扭曲的三棱柱几何结构中是六配位的。磁各向异性和慢磁动力学受到 Co( II ) 周围这些不同环境的极大影响。直流磁数据、高频场电子顺磁共振 (HFEPR) 和理论计算的结合明确地揭示了两种配合物的大负零场分裂 (ZFS) 参数D以及D值之间的巨大差异。1-BPh ₄和2-NO ₃均在零场下表现出缓慢的磁弛豫。磁稀释实验揭示了1-BPh ₄ @Zn的有效能垒为 54(4) cm -1 ^， 2-NO ₃ @Zn的有效能垒为95(5) cm ^-1，并证实这两种配合物的慢磁弛豫均源于单一分子行为。从头算计算研究解释了它们的电子结构和大负磁各向异性的起源；他们支持相应的实验观察。进一步的磁结构分析表明，理想三棱柱几何形状的不同变形对D值和缓慢弛豫产生巨大影响，并且 Co 和 N 之间额外的弱分子内相互作用降低了轴向各向异性。