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.