Valence Tautomerism in One-Dimensional Coordination Polymers

The combination of
the divergent bis-pyridyl linking ligands 1,2-bis­(4-pyridyl)­ethane
(1,2-bpe), 4,4′-trans-azopyridine (azpy),
and 1,3-bis­(4-pyridyl)­propane (1,3-bpp) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the
complexes [Co­(3,5-dbdiox)2(1,2-bpe)]∞ (1), [Co­(3,5-dbdiox)2(azpy)]∞ (2), [trans-Co­(3,5-dbdiox)2(1,3-bpp)]∞ (3a), and [cis-Co­(3,5-dbdiox)2(1,3-bpp)]∞ (3b). All species are 1D coordination polymers that crystallize as solvated
forms; the geometric isomers 3a,b cocrystallize.
Complexes 1, 2, and 3a exhibit
around the Co centers a trans disposition of the N-donor atoms from
the pyridyl linkers, while an unusual cis disposition is evident in 3b. Single-crystal X-ray structural analysis at 100 or 130
K of solvated forms of these complexes indicates that all complexes
possess the {CoIII(3,5-dbcat)­(3,5-dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butylsemiquinonate) charge distribution at the temperature of data
collection. Variable-temperature magnetic susceptibility studies reveal
that 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O (3 = 3a·3b)
all exhibit thermally induced valence tautomeric (VT) transitions
above 200 K. Multiple heating and cooling cycles indicate that in
some cases the behavior is strongly dependent on desolvation processes.
Most notably, further desolvation of 1·1.5MeCN·2H2O above 340 K affords χmT values that suggest unusual ferromagnetic coupling in the {hs-CoII(3,5-dbsq)2} valence tautomer.
Compound 3·MeCN·H2O exhibits a two-step
VT transition that may be ascribed to the presence of the cis and
trans geometric isomers. Compounds 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O all also exhibit a single photoinduced VT transition, comparable
to those generally observed for nonpolymeric cobalt–dioxolene
complexes.

将具有不同连接模式的双吡啶类连接配体1,2-二(4-吡啶基)乙烷(1,2-bis(4-pyridyl)ethane, 1,2-bpe)、4,4′-反式偶氮吡啶(4,4′-trans-azopyridine, azpy)与1,3-二(4-吡啶基)丙烷(1,3-bis(4-pyridyl)propane, 1,3-bpp)，分别与钴离子及3,5-二叔丁基二氧杂环戊烯(3,5-di-tert-butyldioxolene, 3,5-dbdiox)配体进行组装，成功得到了系列配合物：[Co(3,5-二叔丁基二氧杂环戊烯)₂(1,2-二(4-吡啶基)乙烷)]ₙ（1）、[Co(3,5-二叔丁基二氧杂环戊烯)₂(4,4′-反式偶氮吡啶)]ₙ（2）、[反式-Co(3,5-二叔丁基二氧杂环戊烯)₂(1,3-二(4-吡啶基)丙烷)]ₙ（3a）以及[顺式-Co(3,5-二叔丁基二氧杂环戊烯)₂(1,3-二(4-吡啶基)丙烷)]ₙ（3b）。

所有目标产物均为一维配位聚合物，均以溶剂化晶态形式存在；其中几何异构体3a与3b会发生共结晶。

配合物1、2与3a的钴中心周围，吡啶连接配体的氮给体原子呈现反式配位排布；而配合物3b中则呈现少见的顺式配位排布。

对上述配合物的溶剂化晶体在100 K或130 K下开展单晶X射线结构分析，结果表明在数据收集温度下，所有配合物均存在{CoIII(3,5-二叔丁基儿茶酚盐)(3,5-二叔丁基半醌盐)}的电荷分布（其中3,5-dbcat = 3,5-二叔丁基儿茶酚catecholate；3,5-dbsq = 3,5-二叔丁基半醌semiquinonate）。

变温磁化率研究显示，配合物1、1·1.5MeCN·2H₂O、2·2EtOH以及3·MeCN·H₂O（3 = 3a·3b）均在200 K以上发生热诱导价态互变（valence tautomeric, VT）转变。

多次升温和降温循环实验表明，在部分体系中，其转变行为强烈依赖于去溶剂化过程。尤为值得关注的是，将1·1.5MeCN·2H₂O在340 K以上进一步脱除溶剂后，测得的χₘT值表明，{hs-CoII(3,5-二叔丁基半醌盐)₂}价态互变异构体中存在罕见的铁磁耦合作用。

配合物3·MeCN·H₂O呈现两步价态互变转变，这可归因于体系中同时存在顺式与反式两种几何异构体。

配合物1、1·1.5MeCN·2H₂O、2·2EtOH以及3·MeCN·H₂O同样均表现出单次光诱导价态互变转变，其转变行为与已报道的非聚合型钴-二氧杂环戊烯配合物的同类转变基本一致。