Crystallography of Representative MOFs Based on Pillared Cyanonickelate (PICNIC) Architecture

Abstract

Abstract:The pillared layer motif is a commonly used route to porous coordination polymers or metal organic frameworks (MOFs). Materials based on the pillared cyano-bridged architecture, [Ni'(L)Ni(CN) 4 ] n (L = pillar organic ligands), also known as PICNICs, have been shown to be especially diverse where pore size and pore functionality can be varied by the choice of pillar organic ligand. In addition, a number of PICNICs form soft porous structures that show reversible structure transitions during the adsorption and desorption of guests. The structural flexibility in these materials can be affected by relatively minor differences in ligand design, and the physical driving force for variations in host-guest behavior in these materials is still not known. One key to understanding this diversity is a detailed investigation of the crystal structures of both rigid and flexible PICNIC derivatives. This article gives a brief review of flexible MOFs. It also reports the crystal structures of five PICNICS from our laboratories including three 3-D porous frameworks (Ni-Bpene, NI-BpyMe, Ni-BpyNH2), one 2-D layer (Ni-Bpy), and one 1-D chain (Ni-Naph) compound. The sorption data of BpyMe for CO 2 , CH 4 and N 2 is described. The important role of NH 3 (from the solvent of crystallization) as blocking ligands which prevent the polymerization of the 1-D chains and 2-D layers to become 3D porous frameworks in the Ni-Bpy and Ni-Naph compounds is also addressed.

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

“…Of those which formed three-dimensional-type structures, all of them had lower symmetry than tetragonal and an in-plane periodic cell that was a supercell of an approximately 7.2 Å × 7.2 Å primitive cell, indicating ligand orientational ordering. Furthermore, various types of CO 2 gas adsorption isotherms are found . The PICNIC-21 structure Ni-ByeMe shows a Type I isotherm, with CO 2 uptake at low pressure, and no hysteresis between CO 2 adsorption and desorption.…”

“…Culp et al have synthesized over 40 distinct PICNIC structures using different ligands . Wong-Ng et al determined the structure of several of these compounds . Of those which formed three-dimensional-type structures, all of them had lower symmetry than tetragonal and an in-plane periodic cell that was a supercell of an approximately 7.2 Å × 7.2 Å primitive cell, indicating ligand orientational ordering.…”

Understanding Material Characteristics through Signature Traits from Helium Pycnometry

“…The geometrical technique chosen is a variation of those previously used by some of the present authors and described in detail elsewhere. [14][15][16] In short, the adsorbent free volume (i.e., the volume of the adsorbent unit cell not eclipsed by the radii of constituent atoms of the framework) is computed using the so-called accessible volume metric described by Frost et al 17 For a given adsorbent structure (see below), each framework atom is assigned an atomic van der Waals (vdW) radius. The unit cell is then gridded into voxels 18 with side length 10 pm or smaller.…”

Structural Basis of CO2 Adsorption in a Flexible Metal-Organic Framework Material

“…In this connection, since their original synthesis by Culp et al, [23], several of the present authors have studied members of the PICNIC (pillared cyanonickelate) class of flexible MOFs, which have a dynamic pillared cyanonickelate architecture [38,39]. This class of flexible MOFs incorporate metal ions bridged through organic linkers into extended coordinate-covalent networks [40]. This and similar architectures offer enormous diversity in the materials design of possible flexible MOFs, arising from an ability to alter the coordination geometry and organic linker properties in numerous combinations, thus creating porous networks of widely varied nanoscale size and chemical nature [41,42].…”

“…NiBpene has been the subject of several studies by some of the present authors [23,39,40,43,44,45]. It has a characteristic CO 2 adsorption/desorption isotherm exhibiting hysteresis between the adsorption and desorption branches and also features on the desorption branches that have been associated with structural changes, as measured by neutron diffraction (ND) and small-angle neutron scattering (SANS) [45].…”