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Synthesis of rhenacyclopentadienes and η<sup>2</sup>:η<sup>2</sup>-diyne complexes from a labile dirhenium carbonyl and π-conjugated 1,7-octadiynes: Structural and photophysical characterization

Abstract:

A series of dirhenium carbonyl compounds containing π-conjugated diyne ligands and symmetrical 2,5-bis(aryl)rhenacyclopentadienes has been synthesized by reaction of [Re2(CO)8(CH3CN)2] and 1,8-bis(aryl)-1,7-octadiyne derivatives [aryl = 2-thienyl (a), 2-pyridyl (b), 2-quinolyl (c) and 9-phenanthrenyl (d)]. We found that diyne ligands undergo coordination to the labile rhenium dinuclear in bridging or chelate modes, however, the formation of metallacyclopentadiene complexes was favorably observed. All products were fully characterized by IR, NMR, ESI-MS, UV–Vis and fluorescence spectroscopy. Single-crystal structures of three new 2,5-bis(aryl)rhenacyclopentadienes [Re2(CO)7(μ-η1:η1:η2:η2-C16H14S2)] (1a), [Re2(CO)6(CH3CN)(μ-η1:η1:η2:η2-C16H14S2)] (2a) and [Re2(CO)6(CH3CN)(μ-η1:η1:η2:η2-C26H20N2)] (2c) are described. DFT calculations were used to evaluate the proposed geometries for the two η2:η2-diyne complexes [Re2(CO)6(CH3CN)2(μ-η2:η2-C18H16N2)] (3b), [Re2(CO)6(CH3CN)2(η2:η2-C26H20N2)] (4c), and the rhenacyclopentadiene [Re2(CO)6(CH3CN)(μ-η1:η1:η2:η2-C36H26)] (2d). The photophysical properties of all complexes were assessed, in that they exhibited fluorescence between 408 and 599 nm with quantum yields of Φf = 0.02–0.09. An exceptional fluorescence quantum yield of Φf = 0.64 was found for [Re2(CO)6(CH3CN)2(η2:η2-C26H20N2)] (4c), which has been attributed to its rigid chelate structure. This work represents the second example of fluorescent 2,5-bis(aryl)rhenaclyclopendadienes reported up to date.