Improved synthesis and crystal structure of the flexible pillared layer porous coordination polymer: Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)4]

Abstract

This paper reports our synthesis of flexible coordination polymer, Ni(L)[Ni(CN)₄], (L = 1,2-bis(4-pyridyl)ethylene (nicknamed bpene)), and its structural characterization using synchrotron single crystal X-ray diffraction. The structure of the purplish crystals has been determined to be monoclinic, space group P2₁/m, a = 13.5941(12) Å, b = 14.3621(12) Å, c = 14.2561(12) Å, β = 96.141(2)°, V = 2767.4(4) ų, Z = 4, D_c = 1.46 g cm⁻¹. Ni(bpene)[Ni(CN)₄] assumes a pillared layer structure with layers defined by Ni[Ni(CN)₄]_n nets and bpene ligands acting as pillars. With the present crystallization technique which involves the use of concentrated ammonium hydroxide solution and dimethyl sulfoxide (DMSO), disordered free bpene ligands and solvents of crystallization (DMSO and water molecules) occupy the pores, resulting in a formula of Ni(bpene)[Ni(CN)₄]·½bpene·DMSO·2H₂O, or Ni₂N₇C₂₄H₂₅SO₃. Without the inclusion of free bpene ligands and solvent molecules, the free volume is approximately 61% of the total volume; this free volume fraction is reduced to 50% with the free ligands present. Pores without the free ligands were found to have a local diameter of 5.7 Å and a main aperture of 3.5 Å. Based on the successful crystal synthesis, we also devised a new bulk synthetic technique which yielded a polycrystalline material with a significantly improved CO₂ uptake as compared to the originally reported powder material. The improved synthetic technique yielded a polycrystalline material with 40% higher CO₂ uptake compared to the previously reported powder material. An estimated 14.4 molecules of CO₂ per unit cell was obtained.