Crystal Design of Monometallic Single-Molecule Magnets Consisting of Cobalt-Aminoxyl Heterospins

Five N-aryl-N-pyridylaminoxyls, which have no substituent (PhNOpy), one substituent (MeOPhNOpy and tert-BuPhNOpy) at the 4-position, and three substituents (TPPNOpy and TBPNOpy) at the 2,
4, and 6-positions of the phenyl ring, were prepared as new ligands for cobalt-aminoxyl heterospin systems.
The 1:4 complexes, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4] (2), [Co(NCS)2(tertBuPhNOpy)4]
(3), [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4] (5a), and [Co(NCO)2(TBPNOpy)4] (5b), were
obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes
except 4 is a compressed octahedron. In the crystal structure of 1, 2, and 3, the organic spin centers have
various short contacts within 4 Å with the neighboring molecules to form 3D and 2D spin networks. On the
other hand, complexes 5a and 5b have no significant short intermolecular contacts, indicating that they
are magnetically isolated. 1 and 2 behaved as a 3D antiferromagnet with a Neel temperature, TN, of 22 K
and as a weak 3D antiferromagnet with a TN of 2.9 K and a spin-flop field at 1.9 K, Hsp(1.9), of 0.7 kOe,
respectively. 3 was a canted 2D antiferromagnet (a weak ferromagnet) with TN = 4.8 K and showed a
hysteresis loop with a coercive force, Hc, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes
4, 5a, and 5b functioned as single-molecule magnets (SMMs). 5b had an effective activation barrier, Ueff,
value of 28 K in a microcrystalline state and 48 K in a frozen solution.

本研究合成了五种N-芳基-N-吡啶基硝酰基（N-aryl-N-pyridylaminoxyls）化合物作为钴-硝酰基异自旋体系的新型配体，涵盖无取代基的PhNOpy、苯环4位带有单取代基的MeOPhNOpy与叔丁基取代的PhNOpy（tert-BuPhNOpy），以及苯环2、4、6位带有三取代基的TPPNOpy与TBPNOpy。
成功获得了金属钴与配体摩尔比为1:4的系列配合物单晶：[Co(NCS)₂(PhNOpy)₄]（1）、[Co(NCS)₂(MeOPhNOpy)₄]（2）、[Co(NCS)₂(tertBuPhNOpy)₄]（3）、[Co(NCS)₂(TPPNOpy)₄]（4）、[Co(NCS)₂(TBPNOpy)₄]（5a）以及[Co(NCO)₂(TBPNOpy)₄]（5b）。经X射线晶体学表征，除配合物4外，其余所有配合物的分子几何构型均为压缩八面体。
在配合物1、2、3的晶体结构中，有机自旋中心与邻近分子存在多种4 Å以内的短程接触，进而形成三维与二维自旋网络。而配合物5a与5b则未出现显著的分子间短程接触，表明其处于磁孤立状态。
配合物1与2分别表现为奈尔温度（Neel temperature, T_N）为22 K的三维反铁磁体，以及奈尔温度T_N为2.9 K、1.9 K下自旋翻转场（spin-flop field）H_sp(1.9)为0.7 kOe的弱三维反铁磁体。配合物3则为奈尔温度T_N=4.8 K的倾斜型二维反铁磁体（即弱铁磁体），并在1.9 K下呈现出矫顽力（coercive force, H_c）为1.3 kOe的磁滞回线。
另一方面，三取代配合物4、5a与5b均表现为单分子磁体（single-molecule magnets, SMMs）。其中配合物5b的有效活化能垒（effective activation barrier, U_eff）在微晶态下为28 K，在冷冻溶液中则为48 K。