Nickel-Cornered Molecular Rectangles as Polycyclic Aromatic Hydrocarbon Receptors

Two nickel-cornered organometallic metalla-rectangles containing a pyrene-linked-di-N-heterocyclic carbene have been prepared. The dimensions of one side of the rectangle were modulated by either using pyrazine or 4,4-bipyridine. The two molecules were tested as hosts for the recognition of seven small polycyclic aromatic hydrocarbons (PAHs) in [D-6]acetone. By using (HNMR)-H-1 spectroscopy titrations, it could be established that the host-guest stoichiometries of the inclusion complexes formed were 1:1 for the pyrazine-based host, and 1:2 for the host containing bipyridine, in accordance with the larger dimensions of the latter. The molecular structure of the inclusion complex consisting of the bipyridine-containing host and pyrene was determined by X-ray crystallography, which confirms the 1:2 host-guest stoichiometry of this species. The determination of the association constants indicate that the binding strengths are in the order: Perylene<naphthalene<phenanthrene<anthracene<pyrene<triphenylene, ranging from 130-4400m(-1) in the case of the smaller host, thus indicating that larger PAHs show higher binding strengths than smaller ones, except for the case of perylene, which exceeds the size for an optimum dimensional fitting within the cavity. As a proof of concept, the pyrazine host was used as a PAH-scavenger in heptane.