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Density Functional Theory Study of the Structure of the Pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe or PICNIC 21)

Published

Author(s)

Eric J. Cockayne, Winnie Wong-Ng, Yu-Sheng Chen, Jeffrey T. Culp, Andrew J. Allen

Abstract

We use dispersion-corrected density functional theory (DFT) to investigate the structure of the pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (NiBpyMe for short, or PICNIC 21). We model the disorder found in experimental X-ray structure refinement via an ensemble of supercells with ordered ligand orientation configurations. The ensemble-averaged structure agrees very well with experiment, except for the positions of the methyl group hydrogen atoms. While the dihedral angles between the bipyridal rings of each BpyMe ligand of the averaged structure is 90o, the local dihedral angles are about 80o. DFT screening of configurations where the crystallographic a/b ratio is constrained to equal 1 fail to find the configurations that are most stable when a/b is set to its distorted experimental value of a/b = 0.86, demonstrating the difficulty of solving pillared Hofmann structures purely theoretically without experimental input. The waviness of the Ni(CN)2 sheets is explained as a tendency to maximize dispersion interactions between these sheets and the methyl pyridine rings. The qualitative difference between the CO2 adsorption isotherms of Ni-BpyMe and the related compound Ni-Bpene are discussed in terms of the differences in their crystallographic distortions from tetragonal symmetry and residual pore space
Citation
The Journal of Physical Chemistry C
Volume
125
Issue
29

Keywords

Metal-organic frameworks, Hofmann compounds, Microporous crystals, van der Waals interactions

Citation

Cockayne, E. , Wong-Ng, W. , Chen, Y. , Culp, J. and Allen, A. (2021), Density Functional Theory Study of the Structure of the Pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe or PICNIC 21), The Journal of Physical Chemistry C, [online], https://doi.org/10.1021/acs.jpcc.1c01896, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=931481 (Accessed October 16, 2021)
Created July 14, 2021, Updated October 14, 2021