Multi-Technique Experimental Benchmarking of the Local Magnetic Anisotropy of a Cobalt(II) Single-Ion Magnet

A comprehensive understanding of the ligand field and its influence on the degeneracy and population of d-orbitals in a specific coordination environment are crucial for the rational design and enhancement of magnetic anisotropy of single-ion magnets (SIMs). Herein, we report the synthesis and comprehensive magnetic characterization of a highly anisotropic CoII SIM, [L2Co]­(TBA)2 (L is an N,N′-chelating oxanilido ligand), that is stable under ambient conditions. Dynamic magnetization measurements show that this SIM exhibits a large energy barrier to spin reversal Ueff > 300 K and magnetic blocking up to 3.5 K, and the property is retained in a frozen solution. Low-temperature single-crystal synchrotron X-ray diffraction used to determine the experimental electron density gave access to Co d-orbital populations and a derived Ueff, 261 cm–1, when the coupling between the dx2 – y2 and dxy orbitals is taken into account, in very good agreement with ab initio calculations and superconducting quantum interference device results. Powder and single-crystal polarized neutron diffraction (PNPD, PND) have been used to quantify the magnetic anisotropy via the atomic susceptibility tensor, revealing that the easy axis of magnetization is pointing along the N–Co–N′ bisectors of the N,N′-chelating ligands (3.4° offset), close to the molecular axis, in good agreement with complete active space self-consistent field/N-electron valence perturbation theory to second order ab initio calculations. This study provides benchmarking for two methods, PNPD and single-crystal PND, on the same 3d SIM, and key benchmarking for current theoretical methods to determine local magnetic anisotropy parameters.深入理解配体场及其在特定配位环境中对d轨道简并度与占据数的影响，对于合理设计并提升单离子磁体(single-ion magnets, SIMs)的磁各向异性至关重要。本文报道了一种高各向异性二价钴单离子磁体[L₂Co](TBA)₂（其中L为N,N′-螯合草酰胺配体）的合成与全面磁表征，该配合物在常温常压条件下稳定存在。动态磁化强度测量结果表明，该单离子磁体展现出大于300 K的自旋反转有效能垒（effective energy barrier, Ueff）以及最高可达3.5 K的磁阻塞温度，且该性能在冷冻溶液中得以保留。通过低温单晶体同步辐射X射线衍射（synchrotron X-ray diffraction）测定实验电子密度，我们得以获取二价钴的d轨道占据数，并在考虑dx²-y²与dxy轨道间耦合作用后推导得到Ueff为261 cm⁻¹，该结果与从头算（ab initio calculations）及超导量子干涉器件(superconducting quantum interference device, SQUID)的测试结果吻合极佳。本研究采用粉末与单晶体极化中子衍射(powder and single-crystal polarized neutron diffraction, PNPD、PND)，通过原子磁化率张量对磁各向异性进行定量分析，结果显示易磁化轴沿N,N′-螯合配体的N–Co–N′平分线方向（偏移3.4°），接近分子轴，该结果与完全活性空间自洽场/二阶N电子价态微扰理论（complete active space self-consistent field/N-electron valence perturbation theory to second order）的从头算结果吻合良好。本研究针对同一3d族单离子磁体，为PNPD与单晶体PND两种方法提供了基准测试数据，同时也为当前用于测定局域磁各向异性参数的理论方法提供了关键基准。