Michael Mastalerz

Shape-Persistent Organic Cage Compounds by Dynamic Covalent Bond Formation

One area of supramolecular chemistry involves the synthesis of discrete three-dimensional molecules or supramolecular aggregates through the coordination of metals. This field also concerns the chemistry of supramolecular cage compounds constructed through the use of such coordination bonds. To date, there exists a broad variety of supramolecular cage compounds; however, analogous organic cage compounds formed with only covalent bonds are relatively rare. Recent progress in this field can be attributed to important advances, not least the application of dynamic covalent chemistry. This concept makes it possible to start from readily available precursors, and in general allows the synthesis of cage compounds in fewer steps and usually higher yields.

Rational Construction of an Extrinsic Porous Molecular Crystal with an Extraordinary High Specific Surface Area

Apart from well-established porous materials, such as zeolites, metal–organic frameworks (MOFs), covalent organic frameworks (COFs), or amorphous polymeric organic materials, porous compounds consisting exclusively of discrete organic molecules can be classified as a relatively new type of material. Such materials can be subdivided mainly into two kinds: intrinsic and extrinsic porous materials. Intrinsic porosity is defined as porosity that is already inherent in the molecular structure, such as shape-persistent voids, clefts, or cavities. Typical compounds that form intrinsic pores in the solid state are calixarenes, cucurbiturils, or shape-persistent organic cage compounds. It was shown that shapepersistent cage compounds seem to be superior within this subclass of materials, with Brunauer–Emmett–Teller (BET) surface areas of up to 2071 m g . In contrast to the high surface areas of the intrinsic porous organic materials, the values of extrinsic porous materials are significantly smaller. One of the first examples is tris(phenylenedioxy)cyclophasphazene (TPP), which was introduced in 1964 by Allcock and co-workers. Sozzani et al. demonstrated first that crystals of TPP can be permanently porous, and later, Hulliger et al. reporting a Langmuir surface area of 240 m g . The prediction of the assembling of molecules in the solid state is still very difficult, therefore it seems to be more promising to search for existing compounds that obviously exhibit pores in the crystalline state, but have not yet been proven to be permanently porous. McKeown and coworkers defined some useful criteria for such a search, and revealed that there are already quite a number of potential compounds, which fulfill those criteria. For 3,3’,4,4’-tetrakis(trimethylsilylethynyl)biphenyl (TTEB), they showed that this approach is successful: TTEB is permanently porous (BET surface area 278 m g ) and adsorbs 0.8 wt % H2 at 77 K and 10 bar. Re-investigations by Chen et al. demonstrated that HOF1a, a compound introduced before, is permanently porous, with a BET surface area of 359 m g 1 and selective adsorption of ethylene over ethane. Other extrinsic porous materials from discrete organic molecules with relatively high surface areas are SOF-1a, (BET surface area: 474 m g ), the triptycene-derived trinuclear nickel salphens of the MacLachlan group, with BET surface areas of up to 499 m g , or the macrocyclic bis(urea) CBDU (BET surface area 341 m g ). The highest BET surface area of an extrinsic organic compound were reported for PUNCs (phthalocyanine unsolvated nanoporous crystals), with values between 850 and 1002 m g . Herein, we describe a rational approach of the construction of a readily accessible molecular precursor, which forms a permanent porous crystal with a very high specific surface area of 3020 m g . For the design of the molecular structure, we have taken into account McKeown s criteria and have also systematically searched the Cambridge Structural Database for compounds that form flat ordered sheets by selfassembling by hydrogen bonding. Interestingly, almost all 4,5disubstituted benzimidazolones 22] compounds formed nearly planar ribbon-like structures by directed H-bonds of the imidazolone units (Scheme 1). Therefore, we found benzimidazolones 22] to be a potential subunit for designing our molecular precursor, exploiting the chance to form

A Permanent Mesoporous Organic Cage with an Exceptionally High Surface Area

Recently, porous organic cage crystals have become a real alternative to extended framework materials with high specific surface areas in the desolvated state. Although major progress in this area has been made, the resulting porous compounds are restricted to the microporous regime, owing to the relatively small molecular sizes of the cages, or the collapse of larger structures upon desolvation. Herein, we present the synthesis of a shape-persistent cage compound by the reversible formation of 24 boronic ester units of 12 triptycene tetraol molecules and 8 triboronic acid molecules. The cage compound bears a cavity of a minimum inner diameter of 2.6 nm and a maximum inner diameter of 3.1 nm, as determined by single-crystal X-ray analysis. The porous molecular crystals could be activated for gas sorption by removing enclathrated solvent molecules, resulting in a mesoporous material with a very high specific surface area of 3758 m(2)  g(-1) and a pore diameter of 2.3 nm, as measured by nitrogen gas sorption.

Porous organic cage compounds as highly potent affinity materials for sensing by quartz crystal microbalances.

Porosity makes powerful affinity materials for quartz crystal microbalances. The shape-persistent organic cages and pores create superior affinity systems to existing ones for direct tracing of aromatic solvent vapors. A shape and size selectivity for the analytes is observed. These organic cages can be processed to thin films with highly reproducible sensing properties.

Periphery‐Substituted [4+6] Salicylbisimine Cage Compounds with Exceptionally High Surface Areas: Influence of the Molecular Structure on Nitrogen Sorption Properties

The synthesis of various periphery-substituted shape-persistent cage compounds by twelve-fold condensation reactions of four triptycene triamines and six salicyldialdehydes is described, where the substituents systematically vary in bulkiness. The resulting cage compounds were studied as permanent porous material by nitrogen sorption measurements. When the material is amorphous, the steric demand of the cages exterior does not strongly influence the gas uptake, resulting in BET surface areas of approximately 700 m(2)  g(-1) for all cage compounds 3 c-e, independently of the substituents bulkiness. In the crystalline state, materials of the same compounds show a strong interconnection between steric demand of the peripheral substituent and the resulting BET surface area. With increasing bulkiness, the overall BET surface area decreases, for example 1291 m(2)  g(-1) (for cage compound 3 c with methyl substituents), 309 m(2)  g(-1) (for cage compound 3 d with 2-(2-ethyl-pentyl) substituents) and 22 m(2)  g(-1) (for cage compound 3 e with trityl substituents). Furthermore, we found that two different crystalline polymorphs of the cage compound 3 a (with tert-butyl substituents) differ also in nitrogen sorption, resulting in a BET surface area of 1377 m(2) g(-1), when synthesized from THF and 2071 m(2) g(-1), when recrystallized from DMSO.

One-pot synthesis of a shape-persistent endo-functionalised nano-sized adamantoid compound

A simple approach by reversible imine condensation to shape-persistent endo-functionalised nanocage compounds is presented.

Post-Modification of the Interior of Porous Shape-Persistent Organic Cage Compounds†

Cage compounds are fascinating molecules for several reasons. They are defined molecular reaction vessels for “uncommon” products, or used to stabilize highly reactive species in their interior. Recently, materials made from purely organic cage compounds showed remarkable permanent porosities, with very high surface areas and good gassorption properties, both, in crystalline as well as amorphous phases. A feature that distinguishes the porous materials derived from cage compounds from those derived from extended network structures (such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) is, that the intrinsically porous building units (the cage molecules) are soluble. This creates several possibilities, which cannot be easily achieved for extended network structures or are even impossible. For instance, Cooper et al. reported on the co-crystallization of organic cage compounds in a binary or even ternary fashion to create porous organic alloys. 11] Very recently, this property was exploited to grow microporous cage crystals in mesoporous silica. Another example of “processable” porosity has been demonstrated by our group: various cage compounds, which are highly porous in the bulk, can be deposited as thin films on quartz crystal microbalances (QMBs) by spray-coating. The modified QMBs showed very good affinities for several aromatic analytes. In 2008, we introduced the one-pot synthesis of the endofunctionalized [4+6] cage compound 3 by reacting four molecules of triamine 1 and six molecules of salicyldialdehyde 2 (Scheme1). What distinguishes this type of cage compounds from others, is that it bears six hydroxy groups pointing to the center of the cage cavity. This structural motif is very rare for organic cage compounds, and the functionalization of the cages interiors through reaction at the hydroxy groups was to date unsuccessful. Herein we present a facile method for the post-synthetic modification of the intrinsic voids in the cage compound, which allows the pore structure of the resulting material to be “fine-tuned” in the solid state. Initially, we attempted to directly synthesize 5 a by the reaction of O-methylated salicyldialdehyde 4a with triamine 1 (Method A in Scheme 1). Unfortunately, only a small amount of cage 5a was detected in the crude product by the MALDI-TOF mass spectroscopy. However, cage 5a could not be isolated by chromatographic methods. Further optimization of the reaction conditions did not offer a decent route to synthesize 5 a in reasonable yield (never exceeding 17 %). H NMR analysis of the crude product (Figure 1b) exemplifies the high complexity of the resulting mixture. Similarly, reactions of triamine 1 with other salicyldialdehyde ethers 4b–4d failed too. These unsuccessful experiments forced us to switch to an indirect approach (see Scheme 1. Synthesis of cavity-modified cage compounds 5a–5e by two different approaches. Method A: direct route by 12-fold imine condensation. Method B: Post-synthetically modification by sixfold Williamson ether formation. For reaction conditions, results, and yields, see Table 1 and Supporting Information.

Exo-Functionalized Shape-Persistent [2+3] Cage Compounds: Influence of Molecular Rigidity on Formation and Permanent Porosity

It was demonstrated recently that shape-persistent organic cage compounds with defined cavities can form porous materials in the solid state. With specific surface areas of up to 2071 m g , these new materials complement existing systems of polymeric porous structures, for example, metal– organic frameworks (MOFs), covalent organic frameworks (COFs), and amorphous porous organic polymers. One method to synthesize organic cage compounds in high yields is the reversible formation of multiple imine bonds. Nevertheless, only some of those cage compounds have been reported to be permanently porous. One of the first examples was introduced by the Cooper group, who synthesized Td-symmetric cage compounds by condensation of four molecules of 1,3,5-triformylbenzene and six molecules of various 1,2-diamines (such cage compounds are usually termed [4+6] cages). The Brunauer– Emmett–Teller (BET) surface areas of these first systems were 624 m g . These materials could selectively adsorb various gases, for example, H2, CO2, or CH4. Very recently, the same group synthesized a porous [4+6] cage compound with a much higher specific BET surface area (1333 m g ) by enlarging the trialdehyde used. We used a complementary approach to synthesize [4+6] cage compounds. Triptycene triamine 1 was reacted with salicyldialdehydes to give endo-functionalized adamantoid cage compounds. These cage compounds show permanent porosity in the crystalline phases, with highly accessible BET surface areas of up to 2071 m g . These cage compounds could also selectively adsorb CO2 (9.4 wt %) over methane (0.98 wt %). The above-mentioned unique systems tend to have shape-persistent cavities due to the intrinsic rigidity of the tetrahedral molecular scaffold. Recently, another type of connection was used to generate porous shape-persistent molecules through an imine condensation of trisamines and dialdehydes or trisaldehydes and diamines, respectively, to give [2+3] cage compounds. However, the reported BET surface areas (10 m g 1 and 99 m g ) of these [2+3] cage compounds are significantly lower than for the [4+6] cage compounds, though a good selectivity for the adsorption of CO2 over N2 was reported. [7,8] The described [2+3] cage compounds contain flexible linking units within the molecules that allow the molecular structure of the cage compounds to twist so that the trigonal planar subunits preferentially interact through p–p stacking and almost close the cavities, which leaves insufficient space inside the cage compound interior for gas sorption. This effect might be the reason for the low measured BET surface areas. To study the influence of the rigidity of the molecular structure of [2+3] cages on their formation and gas sorption properties, we synthesized exo-functionalized [2+3] cage compounds by using two different bissalicylaldehydes as molecular precursors in the reaction with triptycene triamine 1. The more rigid of the two resulting cage compounds, 3 a, was synthesized by a sixfold imine condensation of triptycene triamine 1 and bissalicylaldehyde 2 a under reflux conditions (Scheme 1). The H NMR spectrum of the crude product in [D8]THF suggests that the main product is desired cage compound 3 a, formed in approximately 69 % yield (see Figure 1a). The byproducts could be removed through crystallisation from THF/Et2O to give pure, crystalline cage compound 3 a (Figure 1b). The other, more flexible cage compound, 3 b, was synthesized under similar conditions by using bis-salicylaldehyde 2 b as a reactant instead of 2 a. Note that the H NMR spectrum of the crude product of 3 b shows many more as-yet undefined byproducts than the spectrum of crude 3 a does (Figure 1). By integration of typical regions (around d=9 ppm) at which the imine protons resonate, we estimated that the cage compound was formed in approximately 33 % of the collected precipitate, which corresponds to about 14 % of the whole product library. It is assumed that the higher degrees of rotational freedom of the two salicylaldehydic units connected by an ethylene bridge causes a higher number of reasonable possibilities in the virtual combinatorial library, which results in a higher number of mismatched oligomeric and polymeric byproducts. From this observation, it is concluded that rigid precursors with directed functional groups are beneficial for cage formation. This finding complements suggestions for other organic cage compounds, [a] Dipl.-Chem. M. W. Schneider, Dr. M. Mastalerz Institute of Organic Chemistry II & Advanced Materials Ulm University Albert-Einstein-Allee 11, 89081 Ulm (Germany) Fax: (+49) 731-50-22840 E-mail : michael.mastalerz@uni-ulm.de [b] Prof. Dr. I. M. Oppel Institute of Inorganic Chemistry, RWTH Aachen Landoltweg 1, 52074 Aachen (Germany) Supporting information for this article (including experimental details) is available on the WWW under http://dx.doi.org/10.1002/ chem.201200032.

A shape-persistent exo-functionalized [4 + 6] imine cage compound with a very high specific surface area

The one-pot synthesis of an exo-functionalized [4 + 6] imine cage compound is introduced. The material derived from this compound is highly porous in its amorphous state with a specific surface area of 1037 m(2) g(-1) as determined by nitrogen sorption at 77 K.

A pyrene-fused N-heteroacene with eleven rectilinearly annulated aromatic rings.

A highly soluble pyrene-fused undecacene is realized by end-capping the rectilinear aromatic π-plane with triptycenylene units. Besides the good solubility, the compound shows a high tendency to crystallize. Two polymorphs from dichlorobenzene and chloroform are described. In the polymorph from chloroform, half of the molecules are strongly bent out of the π-plane by 26.4°.