Charge-transfer solids using nucleobases: supramolecular architectures composed of cytosine and [Ni(dmit)2] assembled by multiple hydrogen bonds and heteroatomic contacts.

Protonated species of the nucleobase cytosine (C), namely the monoprotonated CH(+) and the hemiprotonated CHC(+), were used to obtain four charge-transfer complexes of [Ni(dmit)2] (dmit: 1,3-dithiole-2-thione-4,5-dithiolate). Diffusion methods afforded two semiconducting Ni(dmit)2 salts; (CH)Ni(dmit)2 (1) and (CHC)[Ni(dmit)2] (2). In salt 1, the Ni(dmit)2 ions with a S = 1/2 spin construct a uniform one-dimensional array along the molecular long axis, and the significant intermolecular interaction along the face-to-face direction results in a spin-singlet ground state. In contrast, salt 2 exhibits the Mott insulating behavior associated with uniform 1D arrays of Ni(dmit)2, which assemble a two-dimensional layer that is sandwiched between the layers of hydrogen-bonded CHC(+) ribbons. Multiple hydrogen bonds between CHC(+) and Ni(dmit)2 seem to result in the absence of structural phase transition down to 0.5 K. Electrooxidation of Ni(dmit)2 afforded the polymorphs of the Ni(dmit)2 salts, (CHC(+))[{Ni(dmit)2}(0.5-)]2 (3 and 4), which are the first mixed-valence salts of nucleobase cations with metal complex anions. Similar to 2, salt 3 contains CHC(+) ribbons that are sandwiched between the 2D Ni(dmit)2 layers. In the layer, the Ni(dmit)2 ions form dimers with a S = 1/2 spin and the narrow electronic bandwidth causes a semiconducting behavior. In salt 4, the CHC(+) units form an unprecedented corrugated 2D sheet, which is sandwiched between the 2D Ni(dmit)2 layers that involve ring-over-atom and spanning overlaps. In contrast to 3, salt 4 exhibits metallic behavior down to 1.8 K, associated with a wide bandwidth and a 2D Fermi surface. The ability of hydrogen-bonded CHC(+) sheets as a template for the anion radical arrangements is demonstrated.