Structural transference and regulation of bimodal assembly of charge-assisted hydrogen-bonded networks

The salt [2,2′-(1,4-phenylene)pyrimidine²⁺] [hexacyanocyclopropanediide^2-] ([PBA²⁺][C ₆(CN)₆^2-]) (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 R₂²(15) motif involving a pair of geminal cyano groups (i.e., on the same methylidene carbon atom) of the C₆(CN)₆^2- ion and the other a R₂²(18) motif involving a pair of nongeminal cyano groups (i.e., on different methylidene carbon atoms). The green polymorph (1G) crystallized in the P1 space group and adopted a "staircase" ribbon motif with "risers" comprising two PBA²⁺ ions that bridged C₆(CN)₆^2- ions through N-H⋯NC hydrogen bonds. This form also exhibited two modes of hydrogen bonding, the R ₂²(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)₆ ^2- 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⁺]-[hexacyanocyclopropanediide^2-] ([EBA²⁺][C₆(CN)₆^2-]) (2Y) crystallized in the P2lc space group as yellow bipyramidal plates, forming infinite zigzag hydrogen-bonding ribbons. Unlike 1R and 1G, however, 2Y only exhibits the R₂²(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 R₂²(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.