The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

Chromium­(II)-based
extended metal atom chains have been the focus of considerable discussion
regarding their symmetric versus unsymmetric structure and magnetism.
We have now investigated four complexes of this class, namely, [Cr3(dpa)4X2] and [Cr5(tpda)4X2] with X = Cl– and SCN– [Hdpa = dipyridin-2-yl-amine; H2tpda = N2,N6-di­(pyridin-2-yl)­pyridine-2,6-diamine].
By dc/ac magnetic techniques and EPR spectroscopy, we found that all
these complexes have easy-axis anisotropies of comparable magnitude
in their S = 2 ground state (|D|
= 1.5–1.8 cm–1) and behave as single-molecule
magnets at low T. Ligand-field and DFT/CASSCF calculations
were used to explain the similar magnetic properties of tri- versus
pentachromium­(II) strings, in spite of their different geometrical
preferences and electronic structure. For both X ligands, the ground
structure is unsymmetric in the pentachromium­(II) species (i.e., with
an alternation of long and short Cr–Cr distances) but is symmetric
in their shorter congeners. Analysis of the electronic structure using
quasi-restricted molecular orbitals (QROs) showed that the four unpaired
electrons in Cr5 species are largely localized in four
3d-like QROs centered on the terminal, “isolated” Cr2+ ion. In Cr3 complexes, they occupy four nonbonding
combinations of 3d-like orbitals centered only on the two terminal
metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level
pattern of the terminal ions, whose coordination environment remains
quite similar irrespective of chain length. We conclude that the extent
of unpaired-electron delocalization has little impact on the magnetic
anisotropy of these wire-like molecular species.

基于二价铬的延展金属原子链（extended metal atom chains）因其对称与非对称结构及磁学特性，长期以来备受学界关注。本研究针对该类别配合物中的四个体系展开探究，分别为X=Cl⁻与SCN⁻的[Cr3(dpa)4X2]及[Cr5(tpda)4X2]配合物，其中Hdpa为二(2-吡啶基)胺（dipyridin-2-yl-amine），H2tpda为N2,N6-二(2-吡啶基)吡啶-2,6-二胺（N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine）。借助直流/交流磁学测试技术与电子顺磁共振（EPR）波谱分析，我们发现所有上述配合物在S=2基态下均具有量级相近的易轴各向异性（|D|=1.5~1.8 cm⁻¹），且在低温条件下表现为单分子磁体（single-molecule magnets）。尽管三核与五核二价铬金属链在几何偏好与电子结构上存在差异，本研究通过配体场计算以及密度泛函理论/完全活化空间自洽场（DFT/CASSCF）计算，阐明了二者磁学性质相似的原因。对于两种X配体而言，五核二价铬物种的基态结构呈非对称特征（即Cr-Cr键长呈现长、短交替分布），而其同系的三核配合物则为对称结构。借助准受限分子轨道（quasi-restricted molecular orbitals, QROs）对电子结构进行分析后发现，五核铬物种中的四个未成对电子主要定域在四个以端基“孤立”Cr²+离子为中心的类3d QRO中；而在三核铬配合物中，未成对电子则占据仅以两个端基金属为中心的四个类3d非成键分子轨道组合。综上，两种体系中的QRO本征值均与端基离子的3d能级分布高度吻合，且无论金属链长度如何，端基离子的配位环境均保持高度相似。本研究最终得出结论：未成对电子的离域程度对这类线状分子物种的磁各向异性影响极小。