Carborane bis-pyridylalcohols as globular and flexible linkers for coordination polymers: from bulk crystals to 2D ultrathin nanosheets

Abstract

Firstly, the synthesis and characterization of a family of carborane-based coordination polymers are described. They are constructed with cobalt or zinc metals source, tritopic or tetratopic carboxylic acids and a bis-pyridylalcohol carborane linker. All new compounds have been fully characterized and their structures are described. One of the structures, CB1MOF-1 possesses large solvent accessible channels and shows a crystal sponge behavior. CB1MOF-1 can incorporate a variety of solvents and their structures have been determined by Single Crystal X-ray Diffraction (SXRD). A higher number of interactions could be observed between aromatic guest molecules and the CB1MOF-1 framework and a correlation has been found between such host-guest intermolecular interactions and the mechanical properties of CB1MOF-1 encapsulating Guest by nanoindentation experiments. Secondly, a reversible single-crystal to single-crystal transformation of CB1MOF-1 is introduced. This 3D porous MOF could be transformed into 2D non-porous structure through replacement of initial solvent DMF by poor hydrogen acceptor molecules following thermal vacuum treatment. In the final 2D structure CB1MOF-1´, the carborane linker coordinate to Co atoms in the same layer instead of in different layers like in CB1MOF-1. Notably, this transformation is reversible via simply heating the 2D phase in DMF solution. By taking advantage of this unprecedented reversible transformation, encapsulation of a large molecule such as fullerene was achieved. Thirdly, a disubstituted 3-pyridylphenylalcohol carborane ligand (CB2) and a B-iodinated bis-pyridylphenylalcohol carborane ligand (I-CB2) were synthesized and characterized. Solvothermal reactions of CB2 with Co(NO3)2 and tri- and di-topic carboxylic acid afford four new carborane-based MOFs CB2MOF-1 to CB2MOF-4. Substitution of CB2 with I-CB2, an iodo-functionalized compound I-CB2MOF-1 was obtained, which is isostructural with CB2MOF-1. The structures of those new MOFs were solved by SXRD and fully characterized. A fast reversible solid to solid transformation was observed for both the materials 2D CB2MOF-1 and I-CB2MOF-1. When the DMF solvent molecules in the channel were exchanged by poor polar solvents or evacuated, a sliding of layers is observed affording CB2MOF-1´ and I-CB2MOF-1´. The initial framework could be recovered simply via immerse the crystals in DMF solution at room temperature for 30 min. CB2MOF-1´ and I-CB2MOF-1´ are non porous to N2 but porous to CO2. Finally, we extend the investigation from the bulk carborane-based MOFs to high aspect ratio metal-organic framework nanosheets. Considering the brittle interaction involved in the 2D carborane based MOFs, they are adopted to exfoliate into ultrathin nanosheets. CB1MON-1 was obtained by exfoliation or delamination of CB1MOF-1´ by fast sonication. Similarly, CB2MOF-1 and I-CB2MOF-1 could also be delaminated into atomic thickness CB2MON-1 and I-CB2MON-1 respectively. The high-resolution images of SEM and TEM give information on their flat and large particle size distribution. The detected height profiles from AFM show a few layers thickness thus identifying its high surface to volume atom ratio. Tyndall light scattering effects demonstrates the existence of nanosheets in solution. More specifically, the GIWAX spectra indicating the nanosheets are highly oriented deposited on the substrate.