Metal–Organic Frameworks with Hexakis(4-carboxyphenyl)benzene: Extensions to Reticular Chemistry and Introducing Foldable Nets

Nine
metal–organic frameworks have been prepared with the
hexagon-shaped linker 1,2,3,4,5,6-hexakis­(4-carboxyphenyl)­benzene
(H6cpb) by solvothermal reactions in dimethylformamide
(dmf) or dimethylacetamide (dmac) with acetic acid or formic acid
as modulators: [Bi2(cpb)­(acetato)2(dmf)2]·2dmf CTH-6 forms a rtl-net;
2­(H2NMe2)­[Cu2(cpb)] CTH-7 forms a kgd-net; [Fe4(cpb)­(acetato)2(dmf)4] CTH-8 and [Co4(cpb)­(acetato)2(dmf)4] CTH-9 are isostructural and
form yav-nets; 2­(HNEt3)­[Fe2(cpb)] CTH-10 and the two polymorphs of 2­(H2NMe2)­[Zn2(cpb)]·1.5dmac, Zn-MOF-888 and CTH-11, show kgd-nets; [Cu2(cpb)­(acetato)2(dmf)2]·2dmf, CTH-12, forms a
mixed coordination and hydrogen-bonded sql-net; and 2­(H2NMe2)­[Zn2(cpb)] CTH-13,
a similarly mixed yav-net. Surface area values (Brunauer–Emmett–Teller,
BET) range from 34 m2 g–1 for CTH-12 to 303 m2 g–1 for CTH-9 for samples activated at 120 °C in dynamic vacuum. All compounds
show normal (10-fold higher) molar CO2 versus N2 uptake at 298 K, except the 19-fold CO2 uptake for CTH-12 containing Cu­(II) dinuclear paddle-wheels. We also
show how perfect hexagons and triangles can combine to a new 3D topology laf, a model of which gave us the idea of foldable
network topologies, as the laf-net can fold
into a 2D form while retaining the local geometry around each vertex.
Other foldable nets identified are cds, cds-a, ths, sqc163, clh, jem, and tfc covering the basic polygons
and their combinations. The impact of this concept on “breathing”
MOFs is discussed. I2 sorption, both from gas phase
and from MeOH solution, into CTH-7 were studied by time
of flight secondary ion mass spectrometry (ToF-SIMS) on dried crystals.
I2 was shown to have penetrated the crystals, as layers
were consecutively peeled off by the ion beam. We suggest ToF-SIMS
to be a method for studying sorption depth profiles of MOFs.

本研究以1,2,3,4,5,6-六(4-羧基苯基)苯（1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene, H6cpb）作为六边形连接体（hexagon-shaped linker），在N,N-二甲基甲酰胺（dimethylformamide, DMF）或N,N-二甲基乙酰胺（dimethylacetamide, DMAC）中，以乙酸（acetic acid）或甲酸（formic acid）作为调节剂（modulators），通过溶剂热反应（solvothermal reactions）合成了9种金属有机框架（metal–organic frameworks, MOFs）。各产物及对应拓扑网络如下：[Bi₂(cpb)(乙酸根(acetato))₂(dmf)₂]·2dmf（CTH-6）呈现rtl拓扑网络（rtl-net）；2(H₂NMe₂)[Cu₂(cpb)]（CTH-7）呈现kgd拓扑网络（kgd-net）；[Fe₄(cpb)(乙酸根(acetato))₂(dmf)₄]（CTH-8）与[Co₄(cpb)(乙酸根(acetato))₂(dmf)₄]（CTH-9）为同构结构，均形成yav拓扑网络（yav-net）；2(HNEt₃)[Fe₂(cpb)]（CTH-10）以及2(H₂NMe₂)[Zn₂(cpb)]·1.5dmac的两种多晶型——Zn-MOF-888与CTH-11均呈现kgd拓扑网络；[Cu₂(cpb)(乙酸根(acetato))₂(dmf)₂]·2dmf（CTH-12）形成兼具配位作用与氢键作用的sql拓扑网络（sql-net）；2(H₂NMe₂)[Zn₂(cpb)]（CTH-13）同样属于yav拓扑网络。经120℃动态真空活化的样品，其布鲁诺尔-埃米特-特勒（Brunauer–Emmett–Teller, BET）比表面积介于34 m²·g⁻¹（CTH-12）至303 m²·g⁻¹（CTH-9）之间。在298 K下，除含Cu(II)双核桨轮型结构（dinuclear paddle-wheels）的CTH-12的二氧化碳/氮气吸附比为19倍外，其余化合物的二氧化碳吸附量均为氮气吸附量的10倍左右。本研究还证实，完美六边形与三角形可组合形成新型三维拓扑网络laf（laf-net），基于该拓扑的模型启发了可折叠网络拓扑的研究思路：laf拓扑网络可折叠为二维形式，且保留每个顶点的局部几何构型。已被识别的其他可折叠拓扑网络包括cds、cds-a、ths、sqc163、clh、jem与tfc，涵盖了基本多边形及其组合形式。本研究同时讨论了该概念对“呼吸型”MOFs的研究价值。此外，通过飞行时间二次离子质谱（time of flight secondary ion mass spectrometry, ToF-SIMS）对干燥晶体进行表征，研究了CTH-7从气相与甲醇溶液中吸附碘的过程。结果显示碘已渗透进入晶体内部，离子束可逐层剥离晶体表层。据此，我们提出可采用ToF-SIMS作为研究MOFs吸附深度分布的有效表征方法。