Crystal structure, sorption properties, and electronic structure of flexible MOF, (Ni-4,4′azopyridine)[Ni(CN)4]

• Published in: Solid state sciences Vol. 118; no. C; p. 106646
• Main Authors: Wong-Ng, W., McCandless, G.T., Culp, J.T., Lawson, M., Chen, Y.S., Siderius, D.W., Chen, Y.P., Li, L.
• Format: Journal Article
• Language: English
• Published: France Elsevier Masson SAS 01-08-2021; Elsevier
• Subjects: Crystal structure; Flexible Ni(CN)4-Based MOF; Multiple twinned crystals; Ni-AzoPyr; Pore size distribution; Sorption hysteresis; Synchrotron X-ray; Sorption hysteresis; Pore size distribution; Ni-AzoPyr; Synchrotron X-ray; Multiple twinned crystals; Flexible Ni(CN)4-Based MOF; Crystal structure
• ISSN: 1293-2558; 1873-3085
• DOI: 10.1016/j.solidstatesciences.2021.106646

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) Å3. 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).
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•Multiple twinned structure of flexible MOF (Ni-4,4′azopyridine)[Ni(CN)4] or (Ni-AzoPyr) was determined.•Electronic structure clarified bonding characteristic of the five-fold Ni coordination (square-pyramid geometry).•Structure and sorption property relationship of Ni-AzoPyr and related flexible MOFs was established.•Hysteresis sorption curves of Ni-AzoPyr show the characteristic threshold pressure associated with the pore opening.