Crystal structures and spectroscopic properties of polycyano-polycadmate host clathrates including a CT complex guest of methylviologen dication and aromatic donor

A series of polycyano-polycadmate (PCPC) host clathrates including a CT complex of methylviologen dication (MV2+) and an aromatic donor as a guest were synthesized, and their crystal structures and spectroscopic properties were investigated. The PCPC host has a framework structure built with Cd2+ ions as coordination centres and cyanides as bridging ligands. This framework host has negative charge and includes a cationic guest together with an ordinary neutral guest. MV2+, which is a strong acceptor, was included as a cationic guest and an aromatic compound, which works as a donor, was included as a neutral guest. Crystal structures of seven clathrates, whose neutral guests were o-cresol, m-cresol, p-cresol, 1-methylnaphthalene, 1,2,4-trimethoxybenzene, pyrrole and aniline, were determined by single crystal X-ray diffraction methods. In all cases MV2+ and the neutral guest formed a CT complex with a face to face stacking structure and were included as a CT complex guest. However, depending on each clathrate the ratio of aromatic donor to MV2+ was different and several variations were found in their PCPC host structures. The clathrates had their own colour depending on their neutral guest. The plot of the CT transition energies estimated from optical CT bands against the ionization potentials of the neutral guests satisfied a linear relationship predicted by Mulliken theory. However, the CT transition energies observed in the clathrates showed a shift to lower energy by ca. 0.6 eV compared with those observed in corresponding acetonitrile solutions.