Stereoregular Cyclosiloxane-Based Tetracarboxylic Acid as a Flexible Building Block for HOF and CP/MOF Design

Abstract

The stereoregular organosilicon ligand, all-trans-tetra(p-carboxyphenyl)-cyclotetrasiloxane (H4A), was utilized as a novel building block for the construction of the HOF and coordination polymers (CPs/MOFs). Strong hydrogen bonding and dense 3-fold interpenetration in the H4A-HOF determined the high thermal and solvolytic stability of H4A. H4A and M(II) form novel crystalline CPs/MOFs in pyridine-containing media under ambient (in Pyr/H2O, B−F) and solvothermal (in Pyr/DMF, G) conditions. Pyridine plays a crucial role in the formation and stabilization of B−G, facilitating the dissolution of H4A and shaping the coordination environment of M(II). Cd(II)- (B), Zn(II)/Cd(II)- (C), and Co(II)-based (D) products are structurally similar 1D CPs/MOFs of the 2,4C4 topology. Hg(II)- (F) and Cd(II)-based (G) products are structurally similar 2D CPs/MOFs of the sql topology. The influence of solvent inclusion and hydrogen bonding on the CP/MOF aperture size was shown using the example of B−D. The influence of the initial reagent phase distribution and mixing order was demonstrated by the growth of 1D and 2D Co(II)-based CPs/MOFs D and E (hcb topology). If the dissolution of H4A is hindered, the templated formation of E with an inherited large aperture organization (∼52 × 18 Å) occurs. Structural analysis confirms the flexible nature of the ligand and similar organization patterns of the crystals, thus highlighting the usability of H4A for the CP/MOF design and the preparation of functional materials in the future.