Adventures in boron chemistry – the prediction of novel ultra-flexible boron oxide frameworks

Abstract

Recent periodic density functional calculations have predicted the existence of ultra-flexible low-energy forms of boron oxides in which rigid boron-oxygen heterocycles are linked by flexible B-O-B bridges. The minima in the energy landscapes of these frameworks are remarkably broad, with widths in excess of those of many hybrid metal-organic frameworks. Enormous changes in cell volume, which can exceed a factor of two, are accompanied by negligible changes in energy. Here we explore the underlying reasons for this behaviour using molecular electronic-structure calculations, periodic density functional theory and template-based geometric simulations. The angular flexibility of the B-O-B bridge depends only upon the geometry of the local B2O5 unit, independent of the configuration of neighbouring bridges. Unique cooperativity between the bending and twisting motions of the bridges leads to considerable anisotropy in framework flexibility. Exceptional flexibility is conferred not only by the intrinsic bending flexibility of the bridges but by topological factors, crucially the relaxation of torsional constraints when B3O3 rings are present. We test these conclusions by showing how the flexibility of the frameworks can be tuned by decoration with isoelectronic rings. The new nanoporous boron oxides presented in this work are predicted to be potential novel guest-host materials because of their flat energy landscapes. Furthermore, such structures can be generated systematically from silicates by the substitution of B2O54- for SiO44-. A borate analogue of β-cristobalite is shown to be isoenergetic with the known B2O3-I polymorph. We raise the possibility of new families of frameworks and zeolite analogues.

Monitoring the melting behavior of boron nanoparticles using a neural network potential

“…We employ the supercell of the crystal with 288 atoms for boron and 360 atoms for B 2 O 3 to avoid the cell size effect. The EOS curves calculated by DFT for boron and B 2 O 3 are both in good agreement with literature values 50,51 (see Fig. S5, ESI †).…”

Energy landscapes of perfect and defective solids: from structure prediction to ion conduction

“…Possible nanoporous low-density forms of B 2 O 3 containing B 3 O 3 rings have been linked to the ease of vitrification of this system [50] and found to be extremely flexible [51,52]. Starting with some known B 2 O 3 structures, and others constructed from two-dimensional sheets, we explored the effect of replacing each three-coordinated B atom in these polymorphs by a B 3 O 3 ring, itself three-coordinate.…”

B–O Oligomers or Ring Species in AlB2: Which is More Selective for Propane Oxidative Dehydrogenation?

“…Note that the spent R-AlB 2 sample maintained its original structure, demonstrating better stability than O-AlB 2 (Figures S7 and S8). This is most likely due to its low oligomer content and rigidity of B–O ring species . To rule out the impact of oxidized Al on the reactivity of ODHP, the performance of Al 2 O 3 in its various crystalline phases was evaluated.…”

“…This is most likely due to its low oligomer content and rigidity of B−O ring species. 25 To rule out the impact of oxidized Al on the reactivity of ODHP, the performance of Al 2 O 3 in its various crystalline phases was evaluated. As shown in Figures S9 and S10, no obvious reactivity for both γ-Al 2 O 3 and θ-Al 2 O 3 appeared in ODHP.…”