Co-ligand and Solvent Effects on the Spin-Crossover Behaviors of PtS-type Porous Coordination Polymers

In
our previous work (Chen, X.-Y.; Chem. Commun. 2013, 49, 10977−10979), we have reported
the crystal structure and spin-crossover properties of a compound
[Fe­(NCS)2­(tppm)]·S [1·S, tppm = 4,4′,4″,4‴-tetrakis­(4-pyridyl­ethen-2-yl)­tetraphenylmethane, S = 5CH3OH·​2CH2Cl2]. Here, its analogues [Fe­(X)2­(tppm)]·S [X = NCSe–, NCBH3–, and N­(CN)2– for compounds 2·S, 3·S,
and 4·S, respectively] have been
synthesized and characterized by variable-temperature X-ray diffraction
and magnetic measurements. The crystal structure analyses of 2·S and 3·S reveal that both compounds possess the same topologic
framework (PtS-type) building from the tetrahedral ligand tppm and
planar unit FeX2; the framework is two-fold self-interpenetrated
to achieve one-dimensional open channels occupied by solvent molecules.
Powder X-ray diffraction study indicates the same crystal structure
for 4. The average values of Fe–N distances observed,
respectively, at 100, 155, and 220 K for the Fe1/Fe2 centers are 1.969/2.011,
1.970/2.052, and 2.098/2.136 Å for 2·S, whereas those at 110, 175, and 220 K are 1.972/2.013,
1.974/2.056, and 2.100/2.150 Å for 3·S, indicating the presence of a two-step spin crossover
in both compounds. Temperature-dependent magnetic susceptibilities
(χMT) confirm the two-step spin-crossover
behavior at 124 and 200 K in 2·S, 151 and 225 K in 3·S, and 51
and 126 K in 4·S, respectively.
The frameworks of 2–4 are reproducible
upon solvent exchange and thereafter undergo solvent-dependent spin-crossover
behaviors.

在我们此前的研究工作（Chen, X.-Y.; Chem. Commun. 2013, 49, 10977−10979）中，我们曾报道了配合物[Fe(NCS)₂(tppm)]·S的晶体结构与自旋交叉（spin-crossover）性能[记为1·S，其中tppm = 4,4′,4″,4‴-四(4-吡啶乙烯基-2-基)四苯甲烷，S = 5CH₃OH·2CH₂Cl₂]。本文中，我们合成了该配合物的系列衍生物[Fe(X)₂(tppm)]·S（对应化合物2·S、3·S、4·S的X分别为NCSe⁻、NCBH₃⁻与N(CN)₂⁻），并通过变温X射线衍射与磁性测试对其进行了表征。对2·S与3·S的晶体结构解析结果表明，二者具有相同的拓扑骨架（PtS型），该骨架由四面体配体tppm与平面结构单元FeX₂构筑，并通过二重自穿插形成了可供溶剂分子占据的一维开放孔道。粉末X射线衍射测试结果显示，化合物4·S亦具有相同的晶体结构。针对2·S，在100、155与220 K下测得的Fe1/Fe2中心的Fe-N键平均键长分别为1.969/2.011、1.970/2.052与2.098/2.136 Å；而针对3·S，在110、175与220 K下的对应键长分别为1.972/2.013、1.974/2.056与2.100/2.150 Å，上述结果表明两种化合物均存在两步自旋交叉行为。变温摩尔磁化率（χ_M T）测试结果进一步证实了两步自旋交叉行为：2·S的转变温度分别为124 K与200 K，3·S为151 K与225 K，4·S则为51 K与126 K。化合物2~4的骨架在溶剂交换过程中可保持稳定，且后续表现出溶剂依赖型的自旋交叉行为。