Synthesis and Characterization of Azidobipyridyl Ruthenium Complexes and Their “Click” Chemistry Derivatives

The ligand 4-azido-2,2′-bipyridyl (1) has been used to prepare 1,2,3-triazole-substituted ligands through copper-catalyzed alkyne/azide cycloaddition (CuAAC or “click” chemistry) with phenylacetylene, ethynylferrocene and 2-ethynylpyridine to yield 4-(4-phenyl-1,2,3-triazol-1-yl)-2,2′-bipyridyl (2a), 4-(4-ferrocenyl-1,2,3-triazol-1-yl)-2,2′-bipyridyl (2b) and 4-[4-(pyridyl-2-yl)-1,2,3-triazol-1-yl]-2,2′-bipyridyl (2c). Complexes of the form [Ru(p-cymene)(Cl)(L)]PF6 (3, L = 1; 4a, L = 2a; 4b, L = 2b) were then prepared and characterized. We also report the synthesis of the complex of the analogous ligand 4,4′-bisazido-2,2′-bipyridyl (1′), [Ru(p-cymene)(Cl)(1′)]PF6 (3′). Complexes 4a and 4b were also prepared by an alternative route, whereby 3 undergoes CuAAC coupling with phenylacetylene and ethynylferrocene respectively. Complexes prepared with ligands that are pre-assembled or “clicked” at the metal show identical 1H NMR spectra. CuAAC coupling of 3 and 2-ethynylpyridine results in the formation of the complex [Ru(p-cymene)(Cl)(2c)]PF6 (4c), which contains a coordinatively vacant pyridyltriazole moiety. The reaction of [Ru(p-cymene)(Cl)2]2 with pre-assembled 2c results in the formation of the dinuclear complex [{Ru(p-cymene)(Cl)}2(2c)]·2PF6 (5), which incorporates ruthenium atoms at both bipyridyl and pyridyltriazole binding domains. The 1H NMR spectrum of the complex shows two signals for the triazole ring proton as well as duplicate signals for other protons of the bridging ligand, which indicates that 5 is produced as a mixture of diastereoisomers. The complex [{Ru(p-cymene)Cl}2{di-4-([1-{2,2′-bipyrid-4-yl}triazol-4-yl]methyl)ether}][PF6]2 (7) was also prepared through the coupling of 3 with dipropargyl ether.