| Popis: |
N, N, N′, N′-Tetramethyl-p-phenylenediamine (TMPD or Wurster's blue) is a well-known electrondonor[1]. It can form charge-transfer complexes with electronegative quinones[2]. Such complexes are stabilized by the electrostatic attraction between electron-rich donors and electron-deficient acceptors [3]. In two such co- crystals studied previously, π-stacks of alternating quinone and TMPD molecules are present [2]. Here we report a study on π-stacking and multicentric bonding in charge-transfer complexes with partially charged TMPD radical cation. We have prepared a series of co-crystals of TMPD and differently substituted quinones ; 2, 3-dicyano-5, 6- dichloroquinone (1), tetrafluorquinone (2), tetrachlorquinone (3), tetrabromquinone (4), 7, 7, 8, 8-tetracyanoquinodimethane (5) and 2, 3, 5, 6- tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (6). These systems were studied by means of variable-temperature crystallography (80 - 400 K) and X-ray charge density [4]. Complexes of TMPD with 3 and 5 have been observed previously [2]. There are three types of stacks found in this series of compounds (Figure 1.). First, in cocrystals with 1, 3, 4 and 5 moieties form equidistant stacks of alternating TMPD and quinones. The second type is pancake-bonded dimers of TMPD and quinone in complex with 2. And in co- crystal with 6 moieties form 2D arrays of alternating TMPD and quinone molecules. For crystals of TMPD with 3 and 4, polymorphic phase transitions at lower temperatures have been observed when orientations of the moieties change from parallel (at room temperature) to inclined (at low temperature). Besides that, we studied structure and behavior of TMPD radical cation in its novel chloride salt co-crystal with 2, 5- dichlorohydroquinone. In that co-crystal, TMPD radical cations stack in pancake-bonded dimers, so this is the first pancake-bonding between radical cations studied by Xray charge density. |
