Shape‐Persistent Tetrahedral [4+6] Boronic Ester Cages with Different Degrees of Fluoride Substitution

Abstract

In recent years the interest of shape-persistent organic cage compounds synthesized by dynamic covalent chemistry (DCC) has risen, because these cages are potentially interesting for gas sorption or -separation. One such reaction in DCC is the condensation of boronic acids with diols to form boronic esters. Most interestingly, the variety of geometries and sizes for boronic ester cages is much lower than that of, for example, imine-based cages. Here, a small series of shape-persistent [4+6] tetrahedral boronic ester cages is introduced. One cage has a high specific surface area of 511 m g and selectively adsorbs ethane over ethylene and acetylene.

The Chemistry of Porous Organic Molecular Materials

“…[195] There are several possible synthetic routes to POCs; imine condensation, [16,22,24,56,144,[146][147][148][196][197][198][199][200][201][202][203][204][205][206] alkyne methasis, [23,194,[207][208][209][210][211][212][213] and boronic ester condensation [122,195,[214][215][216] are the most commonly used (Figure 7a), although there is scope adopt other bond-forming chemistries. [217][218][219] POC synthesis has been discussed in several recent reviews, [6,8,143,183,[219][220][221][222][223] and choice of bond-formation chemistry is important for defining the bond angle between the precursors in the final POC product. This imine condensation react...…”

“…[6,8,183,219,220] For example, Mastalerz et al demonstrated that the geometry and number of dihydroxy groups on triptycene cores could be varied to direct the synthesis of [4+6] or [8+12] cages, when these triptycene precursors were reacted with 1,4-benzenediboronic acid and 1,3,5-benzenetriboronic acid, respectively (Figure 7, where trp = triptycene). [122,221] While organic cages synthesized using boronic acid condensation reactions are somewhat less common than imine cages, [122,195,[214][215][216][239][240][241][242][243][244] the 3758 m 2 g -1 BET surface area reported for the [8+12] boronic acid cage [122] remains after 5 years the highest reported for a POC material. These POCs were designed by mapping their precursors over different platonic solids (Figure 7), where the Reproduced with permission.…”

“…Porous molecular solids are, so far, less accommodating to postsynthetic modification approaches than framework materials, such as MOFs, at least in the solid-state. Typically, pendant functional Chemistry and precursor geometry can be varied to direct the formation POCs into different topologies; shown for the boronic ester cages, [4+6]-typ-cage [221] and [8+12]-trp-cage, [122] and the imine cages, CC7, [203] CC5. [16] groups amenable to solid-state chemical transformations, such as amines and phenols, [257] are only rarely encountered as freely accessible groups in the pores of molecular solids.…”

A Giant [8+12] Boronic Ester Cage with 48 Terminal Alkene Units in the Periphery for Postsynthetic Alkene Metathesis

“…The broadness of the signals is expected because typically the alkene metathesis is not of high E / Z ‐selectivity [9] and each of the 24 formed internal olefinic bonds is probably present in two isomeric forms resulting a vast set of overall 2 24 (16,777,216) possible stereoisomers! [20] Unfortunately, for cage 10 , no clear reliable mass spectrometric information to underpin the successful metathesis was achieved. However, it is worth mentioning that for some boronic ester cages these were also not detectable by MS, although clearly have been identified by single crystal X‐ray diffraction [4f, 21] …”

Fluorinated Organic Porous Materials

“…Mastalerz and coworkers synthesized three shape-persistent tetrahedral boronic ester cages 15a-c ( Figure 10) by the [4+6] condensation between nonfluorinated or fluorinated diboronic acids 13a-c and a brominated triptycene-based hexaol 14. 74 The fluorinated diboronic acids 13b and 13c have higher Lewis acidity than the nonfluorinated 13a, which reduced the reaction time Cooper and coworkers performed a side-by-side comparison of the fluorinated and nonfluorinated imine-based cages. 75 Using a [4+6] cycloimination of 1,3,5-triformylbenzene (16, Figure 11) with (R,R)-1,2-diphenylethylenediamine (17a) and (R,R)-1,2-bis(4-fluorophenyl)ethane-1,2-diamine (17b), they synthesized two novel imine cages 18a (R ¼ H) and 18b (R ¼ F).…”