In- and Out-of-Cavity Interactions by Modulating the Size of Ruthenium Metallarectangles

Cationic (arene)ruthenium-based tetranuclear complexes of the general formula [Ru₄(<i>η</i>⁶-<i>p</i>-cymene)₄(<i>μ</i>-<i>N</i>∩<i>N</i>)₂(<i>μ</i>-<i>OO</i>∩<i>OO</i>)₂]⁴⁺ were obtained from the dinuclear (arene)ruthenium complexes [Ru₂ (<i>η</i>⁶-<i>p</i>-cymene)₂(<i>μ</i>-<i>OO</i>∩<i>OO</i>)₂Cl₂] (<i>p</i>-cymene=1-methyl-4-(1-methylethyl)benzene, <i>OO</i>∩<i>OO</i>=5,8-dihydroxy-1,4-naphthoquinonato(2−), 9,10-dihydroxy-1,4-anthraquinonato(2−), or 6,11-dihydroxynaphthacene-5,12-dionato(2−)) by reaction with pyrazine or bipyridine linkers (<i>N</i>∩<i>N</i>=pyrazine, 4,4′-bipyridine, 4,4′-[(1<i>E</i>)-ethene-1,2-diyl]bis[pyridine]) in the presence of silver trifluoromethanesulfonate (CF₃SO₃Ag) (<i>Scheme</i>). All complexes <b>4–12</b> were isolated in good yield as CF₃SO₃^- salts, and characterized by NMR and IR spectroscopy. The host–guest properties of the metallarectangles incorporating 4,4′-bipyridine and (4,4′-[(1<i>E</i>)-ethene-1,2-diyl]bis[pyridine] linkers were studied in solution by means of multiple NMR experiments (1D, ROESY, and DOSY). The largest metallarectangles <b>10–12</b> incorporating (4,4′-[(1<i>E</i>)-ethene-1,2-diyl]bis[pyridine] linkers are able to host an anthracene, pyrene, perylene, or coronene molecule in their cavity, while the medium-size metallarectangles <b>7–9</b> incorporating 4,4′-bipyridine linkers are only able to encapsulate anthracene. However, out-of-cavity interactions are observed between these 4,4′-bipyridine-containing rectangles and pyrene, perylene, or coronene. In contrast, the small pyrazine-containing metallarectangles <b>4–6</b> show no interaction in solution with this series of planar aromatic molecules.