Syntheses, structures and properties of a new Cu(II) coordination polymer based on 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) ligand

Ligated Solvent Influence on Interpenetration and Carbon Dioxide and Water Sorption Hysteresis in a System of 2D Isoreticular MOFs

Four two-dimensional (2D), fluorinated metal-organic frameworks (MOFs), [Cu(hfipbb)(DMF)]_n·0.5(DMF)_n (1), [Cu(hfipbb)(DEF)]_n (2), [Cu₃(hfipbb)₃(DMA)₃]_n·6(DMA)_n·2(H₂O)_n (3), and [Cu₂(hfipbb)₂(DEA)₂]_n·2(DEA)_n·2(H₂O)_n (4), have been synthesized where hfipbb = 4,4'-(hexafluoroisopropylidene)bis(benzoate), DMF = N,N'-dimethylformamide, DEF = N,N'-diethylformamide, DMA = N,N'-dimethylacetamide, and DEA = N,N'-diethylacetamide. The choice of either a formamide or acetamide solvent ligand leads to a 2D, doubly interpenetrated (1 and 2) or noninterpenetrated (3 and 4) MOF structure. Despite their lower potential void spaces, the doubly interpenetrated structures have superior carbon dioxide and hydrogen sorption properties. Their 195 K CO₂ sorption isotherms display inflection points, followed by ∼3-fold increases in their sorption capacities and very large extents of hysteretic behavior. This shows that small changes in the identity of the ligated solvent ligand can affect whether the resulting MOF is interpenetrated or noninterpenetrated and so drastically affect the sorption properties. In addition, the activated phase of a fifth MOF, synthesized through DMF ligand exchange with water in 1 (1W), does not display an inflection point and subsequent increased CO₂ sorption at 195 K, despite having the same degree of interpenetration, showing that even more subtle differences in the desolvated phases can lead to marked differences in their sorption behavior.

Solvent Templating and Structural Dynamics of Fluorinated 2D Cu-Carboxylate MOFs Derived from the Diffusion-Controlled Process

The layered 2D MOFs, owing to their enhanced flexibility and tunability, have recently emerged as a promising alternative to the 3D microporous MOFs in the quest for novel responsive functional materials. However, maintaining the simultaneous control over self-assembly of molecular building blocks as well as ordered stacking of MOF layers poses a significant synthetic challenge. We report on the controlled 2D MOF formation based on a case study of solvent-templated growth of a series of 2D Cu(II)–carboxylate MOFs varying in stacking modes and distances using a diffusion-controlled MOF deposition approach in various solvent mixtures. Moreover, we demonstrate the structural dynamics of the developed 2D MOFs involving both in-plane and out-of-plane movements of the individual 2D layers triggered by solvent exchange, which allowed for selective postsynthetic transformations between the developed 2D MOFs. We also investigated the gas adsorption properties of the developed MOFs, which demonstrates a remarkable crystal size effect on the N₂ adsorption capacity using a model 2D MOF system.

A room temperature diffusion-based approach has been used to prepare a new family of 2D MOFs based on Cu²⁺ paddlewheel clusters and bifunctional fluorinated carboxylate linkers. The stacking distance and geometry of the MOF layers was depending on the solvent mixture used. Remarkably, the developed 2D materials could be postsynthetically transformed into one another by solvent exchange, which was selectively controlled by the type of guest molecules involved.