Crystal Structure, Sorption Properties, and Electronic Structure of Flexible MOF, (Ni-4,4′Azopyridine)[Ni(CN)₄]

The flexible metal organic framework (MOF) compound, Ni(L)[Ni(CN)₄], (L = 4,4′azopyridine (C₁₀N₄H₈), nicknamed AzoPyr) is a 3D porous material that adopts the Hofmann-type structure. This paper reports our synthesis of Ni-AzoPyr and its structural, bonding, and sorption characterization. The red monoclinic crystals (with space group P2/n) were found to be multiple twins with three main components related by twin laws. The lattice parameters are a = 7.102 (3) Å, b = 14.154 (4) Å, c = 25.655 (10) Å, β = 92.575 (12)°, and V = 2577 (2) ų. Ni-AzoPyr adopts a pillared structure with layers defined by the 2-D Ni [Ni(CN)₄]_n nets and AzoPyr ligands as pillars linking between 6-fold coordinated Ni3 sites. An additional AzoPyr ligand was found to cross link between the 6-fold Ni1 sites to the open ends of the four-fold Ni2 sites. This arrangement results in a 5-fold pseudo square-pyramid for Ni2 and a significantly long Ni2–N distance of 2.436 (11) Å. Density functional theory (DFT) calculations show that almost all states in the conduction band minimum (CBM) are occupied by the 6-fold coordinated Ni site, indicating little to no electrons are conducted at the 5-fold coordinated Ni site. Water molecules were found to be entrapped in the cavities of the structure. In addition to the gating adsorption feature of Ni-AzoPyr, using computational approach, we found that in the absence of water molecules, the pores were found to have a local diameter of 5.8 Å with a maximum number of 15.5 CO₂ molecules per unit cell. The inclusion of disordered water solvent molecules gives rise to the formula of Ni(AzoPyr)[Ni(CN)₄]·0.8H₂O, or C₁₉H₁₂N₁₀Ni₂·0.8(H₂O).