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Crystal Structure, Sorption Property and Electronic Structure of the Flexible MOF, (Ni-4,4'azopyridine)[Ni(CN)4]



Winnie Wong-Ng, Gregory T. McCandless, Jeffrey T. Culp, Matthew Lawson, Yu-Sheng Chen, Daniel Siderius, Lan Li


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 an unusual 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, indicating little to no electrons are conducted at the 5-fold coordinated Ni. 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 5.8Å 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).
Solid State Sciences


Ni-AzoPyr, Multiple twinned crystals, Flexible Ni(CN)4-Based MOF, Synchrotron X-ray, Pore size distribution, Sorption hysteresis, Crystal structure


Wong-Ng, W. , McCandless, G. , Culp, J. , Lawson, M. , Chen, Y. , Siderius, D. and Li, L. (2021), Crystal Structure, Sorption Property and Electronic Structure of the Flexible MOF, (Ni-4,4'azopyridine)[Ni(CN)4], Solid State Sciences, [online],, (Accessed May 26, 2024)


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Created August 16, 2021, Updated October 14, 2021