Supramolecular structures containing transition metal ions with potential inclusion and host-guest applications have been extensively explored in recent years. [1][2][3][4] Stang and co-workers contributed to the formation of a combinatorial library of cyclic molecular polygons via the systematic combination of building blocks with predetermined angles. 5 The initial focus was directed toward molecular squares, i.e., macrocycles fabricated by ciscoordinated transition metal corners and rigid or semirigid bifunctional ligand edges. 6-8 Subsequent efforts were devoted to the synthesis of molecular rectangles to improve selectivity and sensitivity in molecular recognition and separation. 9-12 Hupp et al. were the first to report the preparation of rhenium thiolatebased rectangles that exhibited interesting electrochemistry. 9,10 Subsequent work by the same group led to the synthesis, characterization, and preliminary binding properties of a new class of tetracationic rectangular molecules with triflate as counterion. 10 To tune the cavity size, Sullivan et al. recently also reported the preparation of a series of molecular rectangles based on fac-Re-(CO) 3 corners containing 4,4′-bipyridine as one side and two η 2 -alkoxy or hydroxy bridges as the other. 12 We report herein the synthesis and characterization of a new class of molecular rectangles 3 (Scheme 1) that exhibit luminescence in solution at room temperature. Unlike earlier "rectangles", compounds 3 are neutral, possess larger cavities, and have no counterions inside the channels.Activation of Re(CO) 5 Br (1) with Me 3 NO at 0°C followed by addition of 0.5 equiv of 4,4′-bipyridine in toluene gave a paleyellow, shiny product {Re(CO) 4 Br} 2 (µ-bpy) (2a) in 58% yield. Further treatment of 2a at 5°C in CH 2 Cl 2 with 2 equiv of Me 3 -NO and a required amount of acetonitrile yielded intermediate-{(Re(CO) 3 (NCMe)Br} 2 (µ-bpy), which produced the molecular rectangle 3a (14% yield) after subsequent titration with pyrazine. Following a similar strategy, the bimetallic edges 2b and 2c were synthesized from trans-1,2-bis(4-pyridyl)ethylene and pyrazine, respectively. Compounds 2a and 2b were then converted into 3b and 3c by reacting with trans-1,2-bis-(4-pyridyl)ethylene and pyrazine, respectively.Rectangles 3 and their corresponding edges 2 were characterized spectroscopically. The FAB-MS analyses of 3a and 2a exhibit signals corresponding to the molecular ions at m/z ) 1880 and m/z ) 916, respectively, with the experimental isotope pattern matching the calculated values. The 1 H NMR spectrum of 3a shows the presence of two predominant isomeric forms in solution at room temperature, most likely due to the orientation of the CO/Br trans-ligand pairs with respect to the plane containing the Re atoms.Yellow single crystals of 3a were grown from acetone, and X-ray diffraction studies were carried out. 13 An ORTEP diagram of 3a is shown in Figure 1. The structure consists of a molecular rectangle in which two Br(CO) 4 Re-bpy-Re(CO) 4 Br edges are Crystal data for 3a...

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