Triptycene-Based Organic Molecules of Intrinsic Microporosity

Four Organic Molecules of Intrinsic Microporosity (OMIMs) were prepared by fusing triptycene-based components to a biphenyl core. Due to their rigid molecular structures that cannot pack space efficiently, these OMIMs form amorphous materials with significant microporosity as demonstrated by apparent BET surface areas in the range of 515–702 m2 g–1. Bulky cyclic 1′,2′,3′,4′-tetrahydro-1′,1′,4′,4′-tetramethylbenzo units placed on the triptycene termini are especially efficient at enhancing microporosity.

M ost microporous materials are based on crystalline 1 or amorphous 2 network structures with porosity arising from the presence of interconnected voids. In order to allow for solution processability, there has been intense recent interest in the preparation of microporous materials without network structures derived from either discrete small molecules or macromolecules. In most cases microporosity derived from discrete molecules is generated from their self-assembly into ordered porous crystals with cage structures being particularly successful for generating porosity. 3 In contrast, microporosity from non-network macromolecules originates from the inability of rigid contorted polymer chains to pack space efficiently, as demonstrated by the much-studied Polymers of Microporosity (PIMs). 4 The analogous generation of amorphous microporous materials from well-defined discrete molecules has been achieved mainly by using prefabricated pores derived from macrocycles or cage molecules. 5 However, to date, very few discrete molecules have been demonstrated to generate porosity solely from inefficient packing. 6 Recently, we introduced the concept of Organic Molecules of Intrinsic Microporosity (OMIMs) based upon the design of welldefined, rigid molecules with awkward shapes that cannot pack efficiently in the solid state. 7 Molecular packing simulation studies of various OMIMs, including triptycene-based examples OMIM-1 and OMIM-4 (Scheme 1), suggest that the addition of bulky substituents (e.g., t Bu on OMIM-4) increases both the amount and accessibility of microporosity. 7 Here we describe for the first time the synthetic realization of some of the OMIMs used in these packing simulations and investigate the effect of bulky substituents, including a bulky cyclic unit (1′,2′,3′,4′-tetrahydro-1′,1′,4′,4′-tetramethylbenzo), on the amount of intrinsic microporosity generated.
Other examples of naphthobenzobarrelene adducts have been reported, but few have been isolated due to difficulty in separating them from the triptycene coproducts. 12 Diels−Alder reactions are known to be influenced by steric crowding, so it is perhaps surprising that naphthobenzobarrelene 5 is the major product. For example, a recent report describes the exclusive synthesis of a naphthobenzobarrelene adduct by blocking addition across the central ring by placing bulky ferrocenyl groups at the 9,10 positions of anthracene and using the sterically crowded 3-trifluoromethylbenzyne as a coreactant. 12e However, previous studies of benzyne addition to 2,3,5,6tetramethylanthracene also reported naphthobenzobarrelene byproducts, and therefore, in the case of anthracene 3, it appears that the electronic effect of substitution outweighs the potential steric hindrance to addition at the outer rings. The unforeseen formation of 5 facilitated the preparation of OMIM-6 from the demethylated naphthobenzobarrelene 6 and comparison of its properties to those of the isomeric triptycene-based OMIM-5 (Scheme 1).
Data from matrix assisted laser desorption ionization mass spectrometry, solution NMR, elemental analysis, and analytical gel permeation chromatography are consistent with each of the anticipated OMIM structures. OMIM-1 and OMIM-5 demonstrate simple 1 H/ 13 C NMR spectra, whereas those of OMIM-4 and OMIM-6 are complex indicating the presence of regioisomers (also enantioisomers for OMIM-4), as expected due to the less symmetrical nature of the precursors 2 and 6, respectively. OMIM-5 was also characterized by single crystal X-ray diffraction (Figure 1).
Nitrogen adsorption isotherms measured at 77 K from amorphous powdered samples of the OMIMs each demonstrate significant uptake at low relative pressures indicative of microporosity (up to 8 mmol g −1 at p/p°< 0.1; Figure 2). Apparent BET surface areas calculated from these isotherms are 480, 626, 702, and 622 m 2 g −1 for OMIM-1, OMIM-4, OMIM-5, and OMIM-6, repectively. These results validate predictions from molecular packing studies that suggest placing rigid bulky groups on the termini of OMIMs will enhance microporosity. 7 In addition, the rate of nitrogen adsorption is faster for OMIM-4, OMIM-5, and OMIM-6 as compared to OMIM-1. This suggests that the microporosity is also more readily accessible for those OMIMs with bulky terminal substituents due to the more open conformation adopted by the molecules, also as predicted. 7 Although the apparent microporosity is modest as compared to many recently described microporous molecular materials based on crystalline order, 3 the BET surface area of OMIM-5 is comparable to microporous amorphous materials based on cages 5 or many PIMs (e.g., PIM-1 ≈ 760 m 2 g −1 ). 4 In addition, it seems reasonable to conclude from this study that bulky cyclic substituents, such as 1′,2′,3′,4′-tetrahydro-1′,1′,4′,4′-tetramethylbenzo, may be of general utility for the

Notes
The authors declare no competing financial interest.