Synthesis of highly ethynylated mono and dinuclear Pt(II) tethers bearing the 4,4′-bis(ethynyl)biphenyl (debp) unit as central core

Mono and bisubstituted bis(ethynyl)biphenyl Pt(II) complexes, i.e. trans-[(RCC)Pt(PPh3)2(CCp-C6H4p-C6H4CCH)], R=p-NO2C6H4 (3b), (η5-C5H5)Fe(η5-C5H4) (3c), and trans-[(RCC)Pt(PPh3)2(CCp-C6H4p-C6H4CC)Pt(PPh3)2(CCR)], R=C6H5 (4a),p-NO2C6H4 (4b), (η5-C5H5)Fe(η5-C5H4) (4c), have been synthesized by the dehydrohalogenation reaction from the appropriate Pt monochloro acetylides trans-[(RCC)Pt(PPh3)2Cl], R=C6H5 (1a), R=p-NO2C6H4 (1b), R=(η5-C5H4)Fe(η5-C5H5) (1c) and HCCp-C6H5p-C6H5CCH, (4,4′-bis-ethynylbiphenyl), (DEBP) (2). In order to make possible a direct evaluation of the role of Pt centers on the properties of these highly ethynylated complexes, the corresponding species without Pt were prepared, i.e. C6H5CCCCC6H4C6H4CCCCC6H5, (4,4′-(bis-phenylethynyl)-diethynylbiphenyl) (7), and (η5-C5H5)Fe(η5-C5H4)(CCCCC6H4C6H4CCCC)(η5-C5H4)Fe(η5-C5H5), (4,4′-(bis-ferrocenylethynyl)-diethynylbiphenyl) (8). Preparation of the latter compounds was only achieved by the use of the palladium catalyzed Stille coupling reaction, since the dehydrohalogenation route afforded only homocoupled and polymeric products. The electron-donor or electron-withdrawing ligands coordinated to Pt were chosen for the purpose of tuning the optical properties of the complexes through the different charge distribution along the π-electron conjugation. Characteristic spectroscopic features (UV–vis, FT-IR and NMR) of these complexes are discussed.