Laha Biswajit, Khullar Sadhika, Gogia Alisha, Mandal Sanjay K
Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali, Punjab 140306, India.
Dalton Trans. 2020 Sep 15;49(35):12298-12310. doi: 10.1039/d0dt02153a.
In a one-pot self-assembly reaction of Co(OAc)2·4H2O, thiophene-2,5-dicarboxylic acid (H2tdc) and four different bis(tridentate) polypyridyl spanning ligands under ambient conditions, a series of structurally diverse metal-organic frameworks has been synthesised and characterized by single crystal X-ray diffraction: {[Co2(tdc)2(tpbn)(H2O)2]·solvent}n (solvent = 2H2O, 1; solvent = 2CH3OH, 2H2O, 1a), {[Co2(tdc)2(tphn)]·solvent}n (solvent = H2O, 2; solvent = CH3OH, 2.5H2O, 2a), {[Co2(tdc)2(tpchn)(H2O)2]·solvent}n (solvent = 5H2O, 3; solvent = C2H5OH, 2H2O, 3a), and {[Co2(tdc)2(tpxn)]·solvent}n (solvent = 6H2O, 4; when no solvent, 4a), where tpbn (N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,4-diaminobutane), tphn = N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, tpchn = N,N'-(cyclohexane-1,4-diylbis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) and tpxn = N,N'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine). There is a profound effect of the nature of spacer between the alkyl nitrogens in the spanning ligands (flexible vs. semiflexible) on the molecular structures of 1a-4a. The notable differences are (a) the binding mode of the tridentate part of polypyridyl ligands to the Co(ii) center is facial in 1a, 3a and 4a but meridional in 2a, (b) the Co(ii) centers in 1a-3a are hexacoordinated (with a coordinated water in 1a and 3a) but are pentacoordinated in 4a, and (c) the binding mode of tdc linker is bis(monodentate) in 1a, 3a and 4a but chelated in one end and monodentate in the other end in 2a. Thus, the overall framework structure of 1a, 2a, 3a and 4a is cis-decalin type 2D polymer, ladder-shaped 1D polymer, hexagonal 2D net and cis-decalin type 2D polymer, respectively. Their thermal stabilities have been established by thermogravimetric analysis (TGA). The presence of an unsaturated metal center in 4 has provided us an opportunity for its use as an efficient Lewis acid catalyst for the Knoevenagel condensation reaction of malononitrile with various aldehydes (100% conversion in 60 minutes with 2 mol% catalyst in methanol).
在Co(OAc)₂·4H₂O、噻吩 - 2,5 - 二羧酸（H₂tdc）与四种不同的双（三齿）多吡啶桥连配体于环境条件下进行的一锅法自组装反应中，合成了一系列结构多样的金属有机框架，并通过单晶X射线衍射进行了表征：{[Co₂(tdc)₂(tpbn)(H₂O)₂]·溶剂}ₙ（溶剂 = 2H₂O，1；溶剂 = 2CH₃OH，2H₂O，1a），{[Co₂(tdc)₂(tphn)]·溶剂}ₙ（溶剂 = H₂O，2；溶剂 = CH₃OH，2.5H₂O，2a），{[Co₂(tdc)₂(tpchn)(H₂O)₂]·溶剂}ₙ（溶剂 = 5H₂O，3；溶剂 = C₂H₅OH，2H₂O，3a），以及{[Co₂(tdc)₂(tpxn)]·溶剂}ₙ（溶剂 = 6H₂O，4；无溶剂时，4a），其中tpbn（N,N',N'',N'''-四（2 - 吡啶甲基）-1,4 - 二氨基丁烷），tphn = N,N',N'',N'''-四（2 - 吡啶甲基）-1,6 - 二氨基己烷，tpchn = N,N'-（环己烷 - 1,4 - 二亚甲基）双（1 - （吡啶 - 2 - 基）-N - （吡啶 - 2 - 基甲基）甲胺）且tpxn = N,N'-（1,4 - 亚苯基双（亚甲基））双（1 - （吡啶 - 2 - 基）-N - （吡啶 - 2 - 基甲基）甲胺）。桥连配体中烷基氮之间间隔基团的性质（柔性与半柔性）对1a - 4a的分子结构有深远影响。显著差异在于：（a）多吡啶配体三齿部分与Co(ii)中心的结合模式在1a、3a和4a中为面式，但在2a中为经式；（b）1a - 3a中的Co(ii)中心为六配位（1a和3a中有一个配位水），但在4a中为五配位；（c）tdc连接体的结合模式在1a、3a和4a中为双（单齿），但在2a中一端螯合而另一端单齿。因此，1a、2a、3a和4a的整体框架结构分别为顺式十氢化萘型二维聚合物、梯型一维聚合物、六方二维网络和顺式十氢化萘型二维聚合物。通过热重分析（TGA）确定了它们的热稳定性。4中不饱和金属中心的存在为我们提供了将其用作丙二腈与各种醛进行Knoevenagel缩合反应的高效路易斯酸催化剂的机会（在甲醇中使用2 mol%催化剂，60分钟内转化率达100%）。
