Cyclophanes and [2]catenanes as ligands for transition metal complexes: Synthesis, structure, absorption spectra, and excited state and electrochemical properties

Abstract

Two novel cyclophanes (L14+ and Ln24+), derived from the previously investigated cyclobis(paraquat-p-phenylene) tetracation by the replacement of one and two p-phenylene spacers by 2,2'-bipyridine units suitable as binding sites for the coordination of transition metals, have been synthesized, as have the [2]catenanes L34+ and L44+ incorporating L14+ and the macrocyclic polyethers bis-p-phenylene-34-crown-10 and 1,5-di-naphtho-38-crown-10. L14+, L24+, L34+, and L44+ were then used to synthesize some novel mono- and binuclear ruthenium(II), rhenium(I), silver(I), and copper(I) complexes, which were characterized by mass spectrometry, NMR spectroscopy, and, where possible, X-ray crystallography. The absorption spectra, luminescence properties, and electrochemical behavior of the L14+, L24+, and L44+ ligands and the complexes [Re(CO)3L1Cl]4+, [Re(CO)3L4Cl]4+, [{Re(CO)3Cl}2L2]4+, [Ru(bpy)2L1]6+, [Ru(bpy)2L4]6+, and [{Ru(bpy)2}2L2]8+ were investigated. Besides the ligand-centered bands, the Re(I) and Ru(II) complexes display metal-to-ligand charge-transfer (MLCT) bands in the visible region similar to those of model compounds [Re(CO)3(Cl)(bpy)] and [Ru(bpy)3]2+. None of the complexes studied emits at room temperature, because the potentially luminescent MLCT excited state undergoes electron-transfer quenching by the paraquat-type units in the ligands. In a rigid matrix at 77 K, where electron transfer cannot occur, emission is observed from the complexes containing the cyclophane ligands L14+ and L24+, but not from those containing the catenane ligand L44+, in which quenching can still take place by energy transfer to a low-energy CT excited state of the catenane moiety. In the potential window examined (-2.2/+2.1 V), L14+ and L24+ can accept reversibly five and six electrons, respectively, with processes localized on their paraquat- or bpy-type units. The catenane ligand L44+, besides the reduction processes associated with the L14+ cyclophane, undergoes two oxidation processes involving the dioxynaphthalene moieties of the crown ether. The complexes exhibit several redox processes (up to a total of nine exchanged electrons in the case of [Ru(bpy)2L4]6+ and [{Ru(bpy)2}2L2]8+) that can be assigned to i) reduction of the paraquat- and bpy-type moieties of the ligands, ii) reduction of the bpy ligands (in the Ru complexes), iii) oxidation of the metals, and iv) oxidation of the dioxynaphthalene units of the crown ether (in complexes containing the catenane ligand).