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) {[Zn-2(NH2-bdc)(2)(4-bpdb)]center dot(H2O)(4)}(n) (1) and {[Zn-2(NH2-bdc)(2)(4-bpdb)]center dot(H2O)(4))(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 =N-N= base functionalities but the only difference is the presence of methyl groups in adjacent carbon atoms of the =N-N= 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 x 3.8 angstrom(2) for 1 and 8.0 x 1.6 angstrom(2) 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.