Molecular Recognition by Chalcogen Bond: Selective Charge‐Transfer Crystal Formation of Dimethylnaphthalene with Selenadiazolotetracyanonaphthoquinodimethane

Abstract

The title nonplanar electron acceptor (1) fused with a selenadiazole ring selectively forms a crystalline charge-transfer complex (CT crystal) with 2,6-dimethylnaphthalene (2,6-DMN). On the other hand, the sulfur analogue (2) has less recognition ability and forms CT crystals with both 2,6-and 2,7-DMN. X-ray analyses of 1, 2, and their CT crystals revealed that the Se ••• N chalcogen bond (ChB) in 1 is strong enough to determine the crystal packing with the formation of a cavity suitable for 2,6-DMN. On the contrary, ChB through S ••• N contact in 2 competes with other weak interactions such as a C-H ••• N hydrogen bond. The stronger ChB involving Se is the key for 1 to separate 2,6-DMN (>97 wt%) from a complex isomer mixture containing ca. 10 wt% each of 2,6and 2,7-DMN by a simple, efficient and straightforward mixingfiltration-heating process.

Chalcogen Bond versus Weak Hydrogen Bond: Changing Contributions in Determining the Crystal Packing of [1,2,5]-Chalcogenadiazole-Fused Tetracyanonaphthoquinodimethanes

“…Since the sheets are repeated along the a-axis without significant interaction, the most characteristic feature is the sheet-like structure composed of dyad-ribbon networks. This packing is quite similar to that of 1A (Figure S6) without methyl substitution, 12 showing that attachment of two methyl groups does not alter the packing motif because strong ChB through CN•••Se-N is the dominant factor in determining the crystal structure in 1A and 1B. Selenadiazolo-TCNNQ with further annulation of a benzene ring (1C) crystallizes in triclinic P-1 (Z ¼ 2).…”

“…Density functional theory (DFT) calculations were conducted for 1B, 1C, 2B, and 2C [M06-2X/6-31G(d,p)] (Figure S1) and the results were compared to those of 1A and 2A calculated using the same function and basis set. 12 The 6-31G(d,p) basis set with a reasonable calculation cost would be satisfactory since the results were compared only among 1A-1C and 2A-2C, which have similar geometrical and electronic structures.…”

“…Such a coplanar arrangement of molecules resembles the packing of 2A in its molecular complex with an electron-donating guest. 12 Each molecule is further connected along the screw axis extending to the crystallographic c-axis. Three kinds of catemer structures are formed not only by the CN•••S-N contact (ii) but also by CN•••C-H contacts of (v) and (vi) (Figure S4).…”

“…ChB through CN•••E-N contacts is a useful supramolecular synthon in crystal engineering, and thus chalcogenadiazole-fused electron acceptors can recognize the regioisomers of electron-donating guests in their crystalline molecular complexes. 11d By following the general trend with stronger interaction for a Se-involving ChB than for a S-involving ChB, thiadiazole-fused acceptors can only provide the less reliable synthon of CN•••S-N. 12 However, by proper design to suppress competing interactions such as WHB, S-involving ChB can become the decisive factor of crystal packing. In this way, this work has demonstrated that the contribution of ChB in crystal engineering can be altered.…”

“…The resulting three-dimensional cavity endows 1A with a remarkable ability to recognize the regioisomers of the guest, thus proving the importance of ChB through the CN•••Se-N contact in supramolecular chemisty. 12 In contrast, despite the similar molecular geometry and electronic structure, the sulfur analogue (2A) does not show similar recognition properties because WHB through CN•••H-C contacts is a predominant factor in determining its crystal packing due to the less-effective ChB through CN•••S-N contacts.…”