Pillared-bilayer porous coordination polymers of Zn(II): enhanced hydrophobicity of pore surface by changing the pillar functionality

Abstract

Two new isostructural porous coordination polymers of Zn(II) {[Zn₂(NH₂- bdc)₂(4-bpdb)]·(H₂O)₄}_n (1) and {[Zn₂(NH₂-bdc)₂(4-bpdh)]·(H₂O)₄}_n (2) [4-bpdb = 1,4-bis-(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene and NH₂-bdc = 5-amino-1,3-benzenedicarboxylate] have been synthesized using a mixed ligand system by solvent diffusion and structurally characterized through single crystal X-ray diffraction, variable temperature powder X-ray diffraction and thermogravimetric analysis. Both the coordination polymers are constructed using linear Schiff base linkers of similar length having [double bond, length as m-dash]N–N[double bond, length as m-dash] base functionalities but the only difference is the presence of methyl groups in adjacent carbon atoms of the [double bond, length as m-dash]N–N[double bond, length as m-dash] group in the 4-bpdh ligand. Single-crystal structure analysis revealed that both compounds 1 and 2 have two-dimensional (2D) pillared-bilayer framework structures containing 1D channels (8.3 × 3.8Ų for 1 and 8.0 × 1.6 Ų for 2) filled with lattice water molecules. Channel dimensions in 2 decrease due to the presence of methyl groups. The desolvated frameworks of 1 and 2 are rigid which is evidenced by variable temperature PXRD. Both the compounds show type-I CO₂ uptake profiles and the differences in CO₂ adsorption uptakes have been corroborated to their void space (27.1% for 1 and 17.1% for 2). Desolvated forms of compound 1 exhibit remarkably high water adsorption capacity even at low vapor pressure whereas desolvated forms of compound 2 show very low water vapor uptake, which could be ascribed to the hydrophobic nature of the pore surface of 2.