Structural Transference and Regulation of Bimodal Assembly of Charge-Assisted Hydrogen-Bonded Networks

The salt [2,2‘-(1,4-phenylene)pyrimidine2+][hexacyanocyclopropanediide2-] ([PBA2+][C6(CN)62-]) (1)
crystallized from organic solvents as two polymorphic forms through charge-assisted N−H···NC hydrogen bonding
between the oppositely charged ions. The red polymorph (1R) crystallized in the Fdd2 space group, forming ribbons
in which the pair of N−H groups on the same side of the PBA2+ ion entered two modes of hydrogen bonding, one
that can be described as a

(15) motif involving a pair of geminal cyano groups (i.e., on the same methylidene
carbon atom) of the C6(CN)62- ion and the other a

(18) motif involving a pair of nongeminal cyano groups (i.e., on
different methylidene carbon atoms). The green polymorph (1G) crystallized in the P1̄ space group and adopted a
“staircase” ribbon motif with “risers” comprising two PBA2+ ions that bridged C6(CN)62- ions through N−H···NC
hydrogen bonds. This form also exhibited two modes of hydrogen bonding, the

(15) motif and a more complex one
involving multiple molecules. The different colors of the polymorphs were assignable to π−π charge-transfer
interactions between the weak C6(CN)62- electron donor and the weak PBA2+ electron acceptor in the solid state, a
feature suggested by single-crystal X-ray diffraction and confirmed by infrared spectroscopy. Heating 1G above 275
°C produced a crystalline white polymorph (1W). The 1R form, however, exhibited thermal stability up to at least
300 °C. The related salt [2,2‘-(1,2-ethanediyl)pyrimidine2+]-[hexacyanocyclopropanediide2-] ([EBA2+][C6(CN)62-]) (2Y)
crystallized in the P2/c space group as yellow bipyramidal plates, forming infinite zigzag hydrogen-bonding ribbons.
Unlike 1R and 1G, however, 2Y only exhibits the

(18) motif because the shorter span of the EBA2+ molecule, as
defined by the intramolecular separation between N−H groups, did not allow formation of the

(15) motif. These
structures reveal that the hydrogen-bonding modes of a bimodal system can be regulated through adjustment of
key molecular structure parameters, which in the case of EBA2+ produces a single hydrogen-bonding mode that is
required for a hypothetical cyclic motif. Furthermore, the structural parameters of the ribbon motifs provide guidance
for the choice of guest molecules capable of promoting formation of a cyclic motif through templating.