Nanoscale Dodecahedral and Fullerene-Type Organoboroxine and Borazine Cages from Planar Building Units

Boroxine- and borazine-cage analogs to C20, C60, and C70 were calculated and compared in terms of structure, strain indicators, and physical properties relevant to nanoscale applications. The results show C60 and C70 type cages are less strained than the smaller congener, primarily due to minimized bending in the B-arylene-B segments. The smallest cage calculated has a diameter of 2.4 nm, which increases up to 4.9 nm by either variation of the polyhedron (C20 < C60 < C70-type cage) or organic spacer elongation between boron centers. All calculated cages are porous (apertures ranging from 0.6 to 1.9 nm). Molecular electrostatic potential and Hirshfeld population analysis revealed both nucleophilic and electrophilic sites in the interior and exterior cage surfaces. HOMO–LUMO gaps range from 3.98 to 4.89 eV and 5.10–5.18 eV for the boroxine- and borazine-cages, respectively. Our findings provide insights into the design and properties of highly porous boroxine and borazine cages for nanoscience.

−4 These systems are commonly formed through self-assembly driven by reversible noncovalent 5 and/or covalent 6 bond formation.Major inspirations for construction originate from similar processes in Nature that lead to viral capsids and fullerenes.While significant progress has been made in cage design, there remains an ongoing need to identify new abiotic building blocks to generate cage compounds and materials.
In this context, the generation of organic cages through dynamic covalent bond formation can be achieved from building blocks, or tectons, 7,8 with chemical functions in precise geometric arrangements. 9,10−13 From a topological standpoint, a tecton used to generate a cage needs to meet two basic design criteria: (i) multitopic bonding that adheres to symmetry demands to link the components in 3D space (i.e., tetrahedral, cubic, icosahedral) and (ii) sufficient curvature for space to be enclosed.Accordingly, the identification of tectons that provide such chemical information remains a challenge.
Here, we report on tectons based on organoboroxines that while largely planar support the generation of large selfassembled B-based cages.We use computational chemistry and evidence from the literature to support B 3 O 3 -rings bridged by flat linear spacers to generate the spherical cages COBOC-n-BDBA (where: COBOC = Covalent Organic BorOxine Cage; n = 20, 60, or 70 B 3 O 3 -rings; BDBA = 1,4-benzenediboronic acid).The structures of the cages are realized using geometryoptimization by DFT-calculations at the B-P86/def-SV(P) level. 14,15The sizes of the cages range from 2 to 5 nm and, we show, can provide a roadmap to isomorphous assemblies with organoborazine linkers.
While there has been an upsurge to develop COFs based on boronic esters and boroxines, 25−27 there is markedly less attention to develop discrete 3D cages and, more specifically, spherical cages (i.e., cubic symmetry).B-based cages 11,28 have been reported wherein bent (i.e., nonlinear) diboronic acid linkers provide the necessary curvature to generate convex surfaces along the cage structures.The boroxine structure itself exhibits susceptibility to curvature owing to weak π-bonding of the B−O bonds.Boronic acid based 2D COFs on metal surfaces have also been shown to exhibit defects consisting in five-and seven-membered B 3 O 3 -rings that provide curvature (Scheme S2, Supporting Information). 29−36 In a minimalist case, a cage with organoboroxines would utilize the flexible D 3h symmetric B 3 O 3 -ring system to supply curvature that propagates the organic linkers at angles approximating 120°akin to the role of C atoms of C 60 (Scheme S3, Supporting Information).Such a design, however, has not been addressed and can be considered counterintuitive given the planar nature of the B 3 O 3 -ring building blocks.In general, the formation of such cages is predicated on design strategies that adhere to tetrahedral, cubic, and icosahedral symmetries, with Platonic and Archimedean solids serving as a model for spheroid designs. 37,38Given that Platonic and Archimedean solids are built from planar building blocks in the form of polygons, we  hypothesized that the flexibility of the B 3 O 3 -ring system while largely planar could support the formation of 3D cages.The aromatic ring systems of the organic linkers that we study here while linear (i.e., not bent) would also be sufficiently flexible to provide curvature.
Structures of three initial cages COBOC-n-BDBA (where: n = 20, 60, 70) were determined using DFT calculations (Figure 1b-d).For the cages, the B−O bond lengths in the range of 1.389 to 1.391 Å are the same as in solid 1,3,5triphenylboroxine (TPBO), for which a range of (1.384(9)− 1.386(9) Å) was determined by SCXRD analysis. 39The sizes compare favorably to a tetrahedral tetraboroxine cage (3.2 nm). 11COBOC-70-BDBA is approximately spheroidal being defined by transversal (4.18 nm) and longitudinal (4.92 nm) diameters.The windows comprised by the 5-and 6-membered boroxine macrocyclic rings along the surfaces exhibit appreciable sized apertures to the cage interiors (0.66 and 0.89 nm, respectively).The sizes of the macrocycles are in good agreement with reported sites in defects in layers of a 2D boroxine COF derived from BDBA. 29 The B 3 O 3 -ring system in combination with the organic linkers provides a convex surface for each cage.Bending is defined by the angle between straight lines involving calculated centroids of adjacent boroxine units and C 6 H 4 -fragments.We define a system labeled α 55 for fragments located in adjacent pentagonal polygons, α 56 for junctions of pentagonal and hexagonal polygons, and α 66 for junctions of two hexagonal polygons (Figure 2).
Bending in smaller COBOC-20-BDBA (α 55 = 159.6°)is, thus, significantly larger than in COBOC-60-BDBA (α 56 = 165.9°;α 66 = 172.4°)and COBOC-70-BDBA (α 56 = 165.74°−166.35°;α 66 = 171.79°−173.93°).The corresponding bending angles for TPBO molecules in its solid-state structure range from 173.3°to 179.0°. 39Overall, bending can be considered to diminish enthalpy, although the cage formation is entropically favored compared to a 2D boroxine layer structure.We calculate the relative stabilities of the cages as C 70 ≈ C 60 > C 20 , with relative strain energies per minimum formula unit (B 3 O 3 C 9 H 6 ) versus COBOC-70-BDBA being 0.17 kcal mol −1 (COBOC-60-BDBA) and 2.54 kcal mol −1  (COBOC-20-BDBA).For comparison, the enthalpy of formation for the transformation of C 60 per formula unit C atom to C graphite is 9.26 kcal mol −1 . 40rganic cages generally exhibit high symmetries, with spheroids conforming to structures of the Platonic and Archimedean solids. 37,38,41The structures of COBOC-20-BDBA and COBOC-60-BDBA conform to a dodecahedron (Platonic) and truncated icosahedron (Archimedean), respectively.The structure of COBOC-70-BDBA is approximately spheroidal, being elongated similar to C 70 and exhibiting a shape of a rugby ball. 40he Platonic and Archimedean solids are also synthetic roadmaps to cages.Given the structural similarities of boroxine and borazines R 3 B 3 N 3 R 3 ′, isomorphous cages of COBNC-20-BDBA and COBNC-60-BDBA (COBNC = Covalent Organic BoraziNe Cage) were calculated (Figure 3). 42 The inner and outer surfaces of the B-cages were studied using molecular electrostatic potential (MESP) maps (Figure 5).Negative potentials (orange and yellow) are concentrated at the π-clouds, as well as the O and N atoms.The regions are expected to interact with electrophiles, including those for hydrogen bonding.Positive potentials (blue) are concentrated at the H and B atoms, which are expected to act as targets for nucleophilic attack or donors for hydrogen bonding.The windows for access to cage interiors exhibit positive potentials, with the O atom lone pairs shielded by the aromatic C−H hydrogens.The MESP map is confirmed by the distribution of the Hirshfeld atomic charges (Table S1, Supporting Information).We note that the MESP map contrasts C 60 , which is entirely positive in the cage. 44For the B-cages, positive and negative regions are concentrated at the inner and outer surfaces.The B-cages can, thus, be expected to serve as hosts of positively and negatively charged guests, with negative species favored in bypassing positive surfaces of the entrance windows.
In summary, diboronic acids are single-tecton candidates to generate spherical cage assemblies through boroxine and borazine formation.Such cages offer high surface area, a tunable cavity, accessible window sizes, and low weight.We regard the cages accessible from a synthetic standpoint.2D and 3D COFs are of high thermal stabilities (i.e., 500 °C), which are generally expected for the spherical cages. 29,45Boroxinebased materials are robust in dry organic solvents and can be stabilized using substituents (e.g., bulky groups, electron donation). 16,46B−O bonds can also "heal" in assembly reactions (i.e., repairing defects). 18,29We also note that borazines are relatively less stable against hydrolysis.We believe that our findings lay groundwork to prepare the large cages.−49 Diverse synthetic strategies (e.g., solvothermal, microwave-assisted) are also available to transform boronic acids to boroxines.−52 It is likely that the cages can be tailored by facile substitution of the boronic acid linkers, allowing for the diversification of the architectures, work on host−guest chemistry, and surface studies.The insights can pave the way to advance nanoconfinement and explore applications of the versatile structures.−55 ■ ASSOCIATED CONTENT