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Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

Porous materials are important in a wide range of applications including molecular separations and catalysis. We demonstrate that covalently bonded organic cages can assemble into crystalline microporous materials. The porosity is prefabricated and intrinsic to the molecular cage structure, as opposed to being formed by non-covalent self-assembly of non-porous sub-units. The three-dimensional connectivity between the cage windows is controlled by varying the chemical functionality such that either non-porous or permanently porous assemblies can be produced. Surface areas and gas uptakes for the latter exceed comparable molecular solids. One of the cages can be converted by recrystallization to produce either porous or non-porous polymorphs with apparent Brunauer–Emmett–Teller surface areas of 550 and 23 m2 g-1, respectively. These results suggest design principles for responsive porous organic solids and for the modular construction of extended materials from prefabricated molecular pores.

Porous organic cages

The past decade has seen a reawakening of solid-state approaches to chemical synthesis, driven by the search for new, cleaner synthetic methodologies. Mechanochemistry, i.e., chemical transformations initiated or sustained by mechanical force, has been advancing particularly rapidly, from a laboratory curiosity to a widely applicable technique that not only enables a cleaner route to chemical transformations but offers completely new opportunities in making and screening for molecules and materials. This Outlook provides a brief overview of the recent achievements and opportunities created by mechanochemistry, including access to materials, molecular targets, and synthetic strategies that are hard or even impossible to access by conventional means.

http://pubs.acs.org/doi/pdf/10.1021/acscentsci.6b00277

Mechanochemistry: A Force of Synthesis

Three thermally and chemically stable isoreticular covalent organic frameworks (COFs) were synthesized via room-temperature solvent-free mechanochemical grinding. These COFs were successfully compared with their solvothermally synthesized counterparts in all aspects. These solvent-free mechanochemically synthesized COFs have moderate crystallinity with remarkable stability in boiling water, acid (9 N HCl), and base [TpBD (MC) in 3 N NaOH and TpPa-2 (MC) in 9 N NaOH]. Exfoliation of COF layers was simultaneously observed with COF formation during mechanochemical synthesis. The structures thus obtained seemed to have a graphene-like layered morphology (exfoliated layers), unlike the parent COFs synthesized solvothermally.

Mechanochemical synthesis of chemically stable isoreticular covalent organic frameworks.

The condensation of multiple building blocks in a ball mill allows molecular cages with a size up to 3.1 nm to be built.

Synthesis of molecular nanostructures by multicomponent condensation reactions in a ball mill.

The multicomponent reaction of diboronic acids with a catechol and a tripyridyl linker results in the formation trigonal prismatic cages. The cages feature six dative boron–nitrogen bonds as structure-directing elements. The size of the cages can be varied by changing the diboronic acid building block. The cages are able to encapsulate polyaromatic molecules such as triphenylene or coronene.

Dative boron–nitrogen bonds in structural supramolecular chemistry: multicomponent assembly of prismatic organic cages

Multicomponent reactions between 1,4-benzenediboronic acid, catechol, and different pyridyl ligands are reported. The condensation of 1,4-benzenediboronic acid with catechol gives 1,4-bis(benzodioxaborole)benzene. Upon crystallization, the ester aggregates with the N-donor ligands through dative BN bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles or polymeric structures are obtained. 1D polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a 2D network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. These results highlight the utility of dative BN bonds in structural supramolecular chemistry and crystal engineering.

Burcak Icli

Papers

Synthesis of molecular nanostructures by multicomponent condensation reactions in a ball mill.

Dative boron–nitrogen bonds in structural supramolecular chemistry: multicomponent assembly of prismatic organic cages

Multicomponent assembly of macrocycles and polymers by coordination of pyridyl ligands to 1,4-bis(benzodioxaborole)benzene.