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

Authors

W. Wong-Ng, National Institute of Standards and Technology
G. T. McCandless, University of Texas at Dallas
J. T. Culp, Department of Energy
M. Lawson, Boise State University
Y. S. Chen, University of Chicago
D. W. Siderius, National Institute of Standards and Technology
Y. P. Chen, University of Chicago
L. Li, Boise State UniversityFollow

Document Type

Article

Publication Date

8-2021

Abstract

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).

Publication Information

Wong-Ng, W.; McCandless, G. T.; Culp, J. T.; Lawson, M.; Chen, Y. S.; Siderius, D. W.; Chen, Y. P.; and Li, L. (2021). “Crystal Structure, Sorption Properties, and Electronic Structure of Flexible MOF, (Ni-4,4′Azopyridine)[Ni(CN)₄]”. Solid State Sciences, 118, 106646. https://doi.org/10.1016/j.solidstatesciences.2021.106646