Cobalt(II)-Based Single-Ion Magnets with Distorted Pseudotetrahedral [N2O2] Coordination: Experimental and Theoretical Investigations

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• 作者：Sven Ziegenbalg ; David Hornig ; Helmar Görls ; Winfried Plass
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：April 18, 2016
• 年：2016
• 卷：55
• 期：8
• 页码：4047-4058
• 全文大小：654K
• 年卷期：0
• ISSN：1520-510X

文摘

The synthesis and magnetic properties of cobalt(II) complexes with sterically demanding Schiff-base ligands are reported. The compounds [Co(L^Br)₂] (1) and [Co(L^Ph)₂]·CH₂Cl₂ (2·CH₂Cl₂) are obtained by the reaction of cobalt(II) acetate with the ligands HL^Br and HL^Ph in a dichloromethane/methanol mixture. 1 and 2 crystallize in the space groups P2₁2₁2 and P1̅, respectively. X-ray diffraction studies revealed mononuclear constitution of both complexes. For 1, relatively short intermolecular Co–Co distances of 569 pm are observed. In compound 2, a hydrogen-bonded dichloromethane molecule is present, leading to a solvent aggregate with remarkable thermal stability for which desolvation is taking place between 150 and 210 °C. Magnetic measurements were performed to determine the zero-field-splitting (ZFS) parameter D for both complexes. Frequency-dependent susceptibility measurements revealed slow magnetic relaxation behavior with spin-reversal barriers of 36 cm^–1 for 1 and 43 cm^–1 for 2 at an applied external field of 400 Oe. This observation is related to an increasing distortion of the pseudotetrahedral coordination geometry for complex 2. These distortions can be decomposed in two major contributions. One is the elongation effect described by the parameter ϵ_T, which is the ratio of the averaged obtuse and acute bond angles. The other effect is related to a twisting distortion of the chelate coordination planes at the cobalt center. A comparison with literature examples reveals that the elongation effect seems to govern the overall magnetic behavior in pseudotetrahedral complexes with two bidentate chelate ligands. Ab initio calculations for complexes 1 and 2 using the CASPT2 method show strong splitting of the excited ⁴T₂ term, which explains the observed strong ZFS. Spin–orbit calculations with the RASSI-SO method confirm the single-molecule-magnet behavior because only small transversal elements are found for the lowest Kramers doublet for both complexes.