Solvent Templates Induced Porous Metal–Organic Materials: Conformational Isomerism and Catalytic Activity

Solvent
templates induced Co-based metal–organic materials;
conformational isomers {[Co2(pdpa)­(CH3CN)­(H2O)3]·CH3OH·H2O}n (1) and {[Co2(pdpa)­(CH3CN)­(H2O)3]}n (2) and {[Co5(pdpa)2(μ3-OH)2(H2O)6]·2H2O}n (3) [H4pdpa = 5,5′-(pentane-1,2-diyl)-bis­(oxy)­diisophthalic acid]
were synthesized under the same solvothermal conditions except with
different concentrations of cyclic ethers (1,4-dioxane or tetrahydrofuran)
as structure-directing agents. Structural transformations from a three-dimensional
(3D) framework of 1 containing channels with dimensions
of ∼6 Å × 6 Å to a two-dimensional layer structure
of 2 consisting of large open channels with a size of
∼15 Å × 8 Å and then to a 3D nonporous framework
of 3, resulting from the different concentrations of
cyclic ethers, were observed. The anion−π interactions
between electron-efficient oxygen atoms of cyclic ethers and electron-deficient
dicarboxylic acid aromatic cores in H4pdpa imported into
the synthetic process accounted for the conformational change of the
ligand H4pdpa and the following structural variations.
A systematic investigation was conducted to explore how different
concentrations of structure-directing agents affected the frameworks
of resultant metal–organic frameworks. Furthermore, 1–3 were shown to be available heterogeneous catalysts for the synthesis
of 2-imidazoline and 1,4,5,6-tetrahydropyrimidine derivatives by the
cascade cycloaddition reactions of aromatic nitriles with diamines.
The results showed that the catalytic activity of 2 was
much higher than that of 1 and 3, because
of its unique structural features, including accessible catalytic
sites and suitable channel size and shape. In addition, a plausible
mechanism for these catalytic reactions was proposed, and the reactivity–structure
relationship was further clarified.