Disappearing Polymorphs in Metal–Organic Framework Chemistry: Unexpected Stabilization of a Layered Polymorph over an Interpenetrated Three‐Dimensional Structure in Mercury Imidazolate

Abstract

The “disappearing polymorph” phenomenon is well established in organic solids, and has had a profound effect in pharmaceutical materials science. The first example of this effect in metal‐containing systems in general, and in coordination‐network solids in particular, is here reported. Specifically, attempts to mechanochemically synthesize a known interpenetrated diamondoid (dia) mercury(II) imidazolate metal–organic framework (MOF) yielded a novel, more stable polymorph based on square‐grid (sql) layers. Simultaneously, the dia‐form was found to be highly elusive, observed only as a short‐lived intermediate in monitoring solvent‐free synthesis and not at all from solution. The destabilization of a dense dia‐framework relative to a lower dimensionality one is in contrast to the behavior of other imidazolate MOFs, with periodic density functional theory (DFT) calculations showing that it arises from weak interactions, including structure‐stabilizing agostic C−H⋅⋅⋅Hg contacts. While providing a new link between MOFs and crystal engineering of organic solids, these findings highlight a possible role for agostic interactions in directing topology and stability of MOF polymorphs.

Polymorphism and Supramolecular Isomerism: The Impasse of Coordination Polymers

“…Both products were formed by self-assembly of monomeric units but DOI: http://dx.doi.org/10.5772/intechopen.96930 differed in presenting slipped or pseudoparallel packings. In 2020, a new study from Hanusa and Friščić [99] identified the presence of a disappearing polymorph during the formation of two different Hg(II) imidazolate (Hg(Im) 2 ) phases, synthesizing a new layered structure (sql) with the consequent disappearing of a previously reported interpenetrated dense phase (dia) Hg(Im) 2 [100]. Both forms exhibited an orthorhombic crystal system but having evident structural differences driven by an agostic interaction (C-H••Hg) in the sql form.…”

N-Donor Auxiliary Ligand Influence on the Coordination Mode Variations of V-Shaped Triazole Dicarboxylic Acid Ligand Affording Seven New Luminescent Zn(II) Compounds with Variable Structural Motifs

“…Construction of entangled metal–organic frameworks (EMOFs) as a new class of potential materials has attracted great interest in the fields of synthetic chemistry and materials science. Besides their appealing topological structures, they can also be widely applied in many fields, such as gas storage, gas separation, optical sensors, ion exchange, and drug delivery. − In the classes of entangled phenomena, interpenetration and self-penetration are the most representative patterns and have been regarded as a challenging synthetic target. − At present, several strategies have been suggested for the rational design and synthesis of interpenetrating and self-penetrating structures, in which the selection of rigid/flexible mixed ligands has been proved to be an effective and controllable route. − The long rigid ligands are in favor of constructing highly porous structures, and the resulting framework has a good tendency to the form interpenetration. − The flexible ligands provide advantages in constructing self-penetrating networks because of their flexibility and conformational freedom, which can meet the geometry requirements of metal centers. − …”

Combined Approach of Mechanochemistry and Electron Crystallography for the Discovery of 1D and 2D Coordination Polymers

“…Mechanochemistry is emerging as a promising environmentally friendly strategy for the preparation of new organic and metal–organic materials in the form of molecular crystals, salts, cocrystals, as well as polymeric and framework structures. − This approach drastically reduces the use of solvents and excess reagents in the synthetic process and potentially allows high conversion while minimizing energy consumption and chemical waste . Moreover, mechanochemical synthesis can promote the formation of products, polymorphs, and topologies different from what is obtained with solution methods. , However, the structural characterization of such new materials is typically complicated by the small crystallite sizes and multiple twinning induced by the grinding process and the nucleation in almost solvent-free conditions. Conventional single-crystal X-ray diffraction (SC-XRD) on the crude product is therefore not applicable, and powder X-ray diffraction (PXRD) methods are the characterization of choice.…”

“…7 Moreover, mechanochemical synthesis can promote the formation of products, polymorphs, and topologies different from what is obtained with solution methods. 7,8 However, the structural characterization of such new materials is typically complicated by the small crystallite sizes and multiple twinning induced by the grinding process and the nucleation in almost solvent-free conditions. Conventional single-crystal X-ray diffraction (SC-XRD) on the crude product is therefore not applicable, and powder X-ray diffraction (PXRD) methods are the characterization of choice.…”