Di(pyridin-2-yl)amino-substituted 1,10-phenanthrolines and their Ru(ii)–Pd(ii) dinuclear complexes: synthesis, characterization and application in Cu-free Sonogashira reaction

Abstract

Dinuclear complexes bearing Ru(II) photoactive centers are of interest for the development of efficient dual catalysts for many photocatalyzed reactions. Ditopic polypyridine ligands, bis(pyridin-2-yl)amino-1,10-phenanthrolines, containing an additional coordination site (bis(pyridin-2-yl)amine, dpa) at positions 3, 4 or 5 of the 1,10-phenanthroline core (Phen-3NPy₂, Phen-4NPy₂ and Phen-5NPy₂) were synthesized. They were used as bridging ligands to obtain dinuclear complexes [(bpy)₂Ru(Phen-NPy₂)PdCl₂](PF₆)₂ (Ru(Phen-NPy₂)Pd) in good yields via stepwise complexation. In these complexes Ru(II) is coordinated to 1,10-phenanthroline, while Pd(II) is bound to the dpa chelating moiety, as established by NMR spectroscopy and X-ray single crystal analysis. The influence of the position of dpa in the phenanthroline ring on the structural, optical and electrochemical properties of Ru(Phen-NPy₂)Pd complexes was studied. The complexes exhibit photoluminescence in argon-saturated MeCN solution with maxima in the range of 615–625 nm, with emission quantum yields ranging from 0.11 to 0.15 for Ru(Phen-NPy₂) complexes and from 0.018 to 0.026 for dinuclear Ru(Phen-NPy₂)Pd complexes. All the complexes absorb visible light in the range of 370–470 nm with high extinction coefficients and can be considered useful as photocatalysts. The Ru^2+/3+ potential in Ru(Phen-NPy₂)Pd complexes showed no significant dependence on the dpa position, while the Pd^2+/0 reduction potential was significantly lower for Ru(Phen-3NPy₂)Pd and Ru(Phen-4NPy₂)Pd, than for Ru(Phen-5NPy₂)Pd (−0.57 V and −0.72 V vs. Ag/AgCl, KCl_(sat.), respectively). The complexes were used as photoactivated precatalysts in Cu-free Sonogashira coupling under blue LEDs (12 W) irradiation. The reaction proceeded roughly three times faster when Ru(Phen-4NPy₂)Pd and Ru(Phen-3NPy₂)Pd were used as catalyst precursors compared to the mixed catalytic system Ru(bpy)₃(PF₆)₂/(RNPy₂)PdCl₂.