Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound*

A Molecular Cage Accessed by Threefold Click Reaction of a C3v-Symmetric Triazido-Functionalized Tribenzotriquinacene

The hitherto unknown hexakis(halomethyl)-functionalized tribenzotriquinacenes (TBTQs) 9 and 10 were synthesized from the key 4b,8b,12b-tribromo-TBTQ derivative 6 by an improved route in 67% overall yield. Extension of the bowl-shaped framework of 9 or 10 by threefold condensation with propargylamine or 2-azidoethylamine afforded the corresponding TBTQ-trialkyne 11 and TBTQ-triazide 12, respectively. While attempts to construct bis-TBTQ cages, including homodimerization of 11 and heterocoupling of 11 with 12, were unsuccessful, triazide 12 was found to undergo threefold [3 + 2]-cycloaddition with 3-ethynylaniline and phloroglucinol tripropargyl ether under click chemistry conditions. The latter reaction enabled facile capping of the TBTQ bowl to give the novel cage compound 5 in 22% yield.

Size Determination of Organic Cages by Diffusion NMR Spectroscopy

Reliable structure elucidation of covalent organic cage compounds remains challenging as routine analysis might leave ambiguities. Diffusion-ordered NMR spectroscopy (DOSY) allows insight into the molecular size and mass of the species present in solution, but a systematic evaluation of the diffusion behavior for cage assemblies is rarely considered. Here we report the synthesis of four series of covalent organic cages based on tribenzotriquinacenes and diboronic acids with varying geometry and exohedral substituents. We provide a guideline for the consistent measurement of diffusion coefficients from 1 H-DOSY NMR spectroscopy, which was utilized to study the diffusion behavior for the whole set of cages and selected examples from the literature. For structurally similar cages, a linear correlation between the solvodynamic volume and the molecular mass allows precise size determination. For more complex systems, multiple parameters, such as window size or rigid exohedral functionalization. further modulate cage diffusion in solution.

The sharp structural switch of covalent cages mediated by subtle variation of directing groups

It is considered a more formidable task to precisely control the self-assembled products containing purely covalent components, due to a lack of intrinsic templates such as transition metals to suppress entropy loss during self-assembly. Here, we attempt to tackle this challenge by using directing groups. That is, the self-assembly products of condensing a 1:2 mixture of a tetraformyl and a biamine can be precisely controlled by slightly changing the substituent groups in the aldehyde precursor. This is because different directing groups provide hydrogen bonds with different modes to the adjacent imine units, so that the building blocks are endowed with totally different conformations. Each conformation favors the formation of a specific product that is thus produced selectively, including chiral and achiral cages. These results of using a specific directing group to favor a target product pave the way for accomplishing atom economy in synthesizing purely covalent molecules without relying on toxic transition metal templates.

Design of a D3h-symmetry prismatic tris-(ferrocene-1,1ʹ-diyl) molecular cage bearing boronate ester linkages

This paper presents a simple, highly selective, and efficient (isolated yield 68%) synthesis of a novel D3h-symmetry prismatic tris-(ferrocene-1,1ʹ-diyl) organic cage (FcB-cage) by incorporating the boronate ester as a linkage...

Isoreticular Crystallization of Highly Porous Cubic Covalent Organic Cage Compounds*

Modular frameworks featuring well-defined pore structures in microscale domains establish tailor-made porous materials. For open molecular solids however, maintaining long-range order after desolvation is inherently challenging, since packing is usually governed by only a few supramolecular interactions. Here we report on two series of nanocubes obtained by co-condensation of two different hexahydroxy tribenzotriquinacenes (TBTQs) and benzene-1,4-diboronic acids (BDBAs) with varying linear alkyl chains in 2,5-position. n-Butyl groups at the apical position of the TBTQ vertices yielded soluble model compounds, which were analyzed by mass spectrometry and NMR spectroscopy. In contrast, methyl-substituted cages spontaneously crystallized as isostructural and highly porous solids with BET surface areas and pore volumes of up to 3426 m2  g-1 and 1.84 cm3  g-1 . Single crystal X-ray diffraction and sorption measurements revealed an intricate cubic arrangement of alternating micro- and mesopores in the range of 0.97-2.2 nm that are fine-tuned by the alkyl substituents at the BDBA linker.

Metal-Stabilized Boronate Ester Cages

Molecular cages with arylboronate ester caps at the vertices are described. The cages were obtained by metal-templated polycondensation reactions of a tris(2-formylpyridine oxime) ligand with arylboronic acids. Suited templates are triflate or triflimide salts of Zn^II, Fe^II, Co^II, or Mn^II. In the products, the metal ions are coordinated internally to the pyridyl and oximato N atoms adjacent to the boronate ester, resulting in an improved hydrolytic stability of the latter. It is possible to decorate the cages with cyano or aldehyde groups using functionalized arylboronic acids. The aldehyde groups allow for a postsynthetic modification of the cages via an imine bond formation.

Cucurbitimines - imine cages with concave walls

The variety of shape-persistent organic cages by imine bond formation has tremendously enlarged in recent years by using different building blocks (aldehydes and amines) in the condensation reactions. Here, we describe the use of a kinked tetraldehyde to generate pumpkin-shaped cages with concave walls, similar to cucurbiturils.

Kinked tetraaldehyde building blocks lead in condensation reactions with diamines to pumpkin shaped cages – the cucurbitimines.

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages

One of the most applied reaction types to synthesize shape‐persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n‐hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.

Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound*

A highly strained covalent organic cage compound was synthesized from hexahydroxy tribenzotriquinacene (TBTQ) and a meta-terphenyl-based diboronic acid with an additional benzoic acid substituent in 2'-position. Usually, a 120° bite angle in the unsubstituted ditopic linker favors the formation of a [4+6] cage assembly. Here, the introduction of the benzoic acid group is shown to lead to a perfectly preorganized circular hydrogen-bonding array in the cavity of a trigonal-bipyramidal [2+3] cage, which energetically overcompensates the additional strain energy caused by the larger mismatch in bite angles for the smaller assembly. The strained cage compound was analyzed by mass spectrometry and 1 H, 13 C and DOSY NMR spectroscopy. DFT calculations revealed the energetic contribution of the hydrogen-bonding template to the cage stability. Furthermore, molecular dynamics simulations on early intermediates indicate an additional kinetic effect, as hydrogen bonding also preorganizes and rigidifies small oligomers to facilitate the exclusive formation of smaller and more strained macrocycles and cages.