Enhanced Spin-Crossover Behavior Mediated by Supramolecular Cooperative Interactions

Three
one-dimensional (1D) hetereobimetallic coordination polymers [FeII(L)2(AgCN)2]·Solv (L = bpt–, 1; L = Mebpt–, Solv
= 1.75EtOH, 2; L = bpzt–, 3) with in situ generated AgCN species were synthesized by solvothermal
reactions of FeII salt, K­[Ag­(CN)2], and the
corresponding ligands [bptH = 3,5-bis­(pyridin-2-yl)-1,2,4-triazole,
MebptH = 3-(3-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, and
bpztH = 3,5-bis­(pyrazin-2-yl)-1,2,4-triazole]. They were further characterized
by X-ray crystallography, magnetic and photomagnetic measurements,
and differential scanning calorimetry. Single-crystal X-ray analyses
show that they are isostructural with 1D zigzag chain structures with
rhombus {Fe2Ag2} units, in which the substituted
bpt– ligand connects the FeII ion and
AgCN species in a cis bridging mode. Then the zigzag chains are packed
into three-dimensional supramolecular structures by π···π
interactions. Most importantly, weak Ag···N interactions
(2.750 Å at 150 K) between the π-stacked neighboring chains
present in complex 3. Magnetic susceptibility measurements
exhibit that complex 1 displays characteristic paramagnetic
behavior in the temperature range investigated. Complex 2 undergoes a gradual spin-crossover (SCO) with critical temperatures T1/2↓ = 232 K and T1/2↑ = 235 K, whereas 3 exhibits an abrupt
SCO with critical temperatures T1/2↓
= 286 K and T1/2↑ = 292 K. The
magnetostructural relationships suggest that the magnetic behaviors
can be modulated from paramagnetic behavior to abrupt and hysteretic
SCO near room temperature through adjustment of the electronic substituent
effect and intermolecular interactions.

通过二价铁盐、K[Ag(CN)2]与对应配体[bptH = 3,5-二(吡啶-2-基)-1,2,4-三唑，MebptH = 3-(3-甲基-2-吡啶基)-5-(2-吡啶基)-1,2,4-三唑，bpztH = 3,5-二(吡嗪-2-基)-1,2,4-三唑]的溶剂热反应（solvothermal reaction），合成了三种原位生成AgCN物种的一维异双金属配位聚合物（hetereobimetallic coordination polymers）[FeII(L)2(AgCN)2]·Solv（其中L = bpt–，记为配合物1；L = Mebpt–，Solv=1.75EtOH，记为配合物2；L = bpzt–，记为配合物3）。后续通过X射线晶体学（X-ray crystallography）、磁学与光磁学表征（magnetic and photomagnetic measurements）以及差示扫描量热法（differential scanning calorimetry）对其进行了全面表征。单晶X射线衍射分析（single-crystal X-ray analyses）结果表明，该系列聚合物均为同构结构，均具有包含菱形{Fe2Ag2}单元的一维锯齿链结构，其中取代后的bpt–配体以顺式桥联模式连接FeII离子与AgCN物种。随后，这些锯齿链通过π…π相互作用（π···π interactions）组装为三维超分子结构。尤为关键的是，在配合物3中，π堆叠的相邻链之间存在弱Ag…N相互作用（Ag···N interactions），150 K下键长为2.750 Å。磁化率（magnetic susceptibility）测试结果显示，配合物1在测试温度范围内表现出典型的顺磁行为。配合物2呈现逐步自旋交叉（spin-crossover, SCO）现象，其临界降温温度T1/2↓=232 K，临界升温温度T1/2↑=235 K；而配合物3则表现出突变型自旋交叉行为，临界降温温度T1/2↓=286 K，临界升温温度T1/2↑=292 K。磁构关系（magnetostructural relationships）研究表明，通过调控电子取代基效应与分子间相互作用，可以将磁行为从顺磁性调控至室温附近的突变型且具有滞后性的自旋交叉行为。