Synthesis and characterization of a heteroleptic Ru(II) complex of phenanthroline containing oligo-anthracenyl carboxylic acid moieties

Abstract

In an effort to develop new ruthenium(II) complexes, this work describes the design, synthesis and characterization of a ruthenium(II) functionalized phenanthroline complex with extended π-conjugation. The ligand were L(1) (4,7-bis(2,3-dimethylacrylic acid)-1,10-phenanthroline), synthesized by a direct aromatic substitution reaction, and L(2) (4,7-bis(trianthracenyl-2,3-dimethylacrylic acid)-1,10-phenanthroline), which was synthesized by the dehalogenation of halogenated aromatic compounds using a zero-valent palladium cross-catalyzed reaction in the absence of magnesium-diene complexes and/or cyclooctadienyl nickel (0) catalysts to generate a new carbon-carbon bond (C-C bond) polymerized hydrocarbon units. The ruthenium complex [RuL(1)L(2)(NCS)(2)] showed improved photophysical properties (red-shifted metal-to-ligand charge-transfer transition absorptions and enhanced molar extinction coefficients), luminescence and interesting electrochemical properties. Cyclic and square wave voltammetry revealed five major redox processes. The number of electron(s) transferred by the ruthenium complex was determined by chronocoulometry in each case. The results show that processes I, II and III are multi-electron transfer reactions while processes IV and V involved one-electron transfer reaction. The photophysical property of the complex makes it a promising candidate in the design of chemosensors and photosensitizers, while its redox-active nature makes the complex a potential mediator of electron transfer in photochemical processes.

Keywords: conjugation; electrochemistry; oligoathracene; palladium; polypyridyl ligands; ruthenium complex; spectroscopy.

Figure 1

Infra-red spectra of the RuL₁L₂(NCS)₂ complex in KBr pellet.

Figure 2

UV-Vis absorption (a) and emission (b) spectra of the RuL₁L₂(NCS)₂ complex at a concentration of 0.001 g/dm³ in chloroform-methanol (1:1). (c). UV-Vis absorption spectrum of [Ru(L₁)₂(NCS)₂], showing the effect of conjugation at the visible region of the metal-to-ligand charge transfer transition.

Figure 2

UV-Vis absorption (a) and emission (b) spectra of the RuL₁L₂(NCS)₂ complex at a concentration of 0.001 g/dm³ in chloroform-methanol (1:1). (c). UV-Vis absorption spectrum of [Ru(L₁)₂(NCS)₂], showing the effect of conjugation at the visible region of the metal-to-ligand charge transfer transition.

Figure 3

Cyclic voltammetry profiles of 1 × 10⁻³ M of complex L₁, L₂ and [RuL₁L₂(NCS)₂] in freshly distilled DMF containing 0.1 M TBABF₄ supporting electrolyte. Step potential: 5 mV, amplitude: 50 mV vs. Ag|AgCl, frequency: 10 Hz. Scan rate: 100 m Vs⁻¹ vs. Ag|AgCl.

Figure 4

Square wave voltammetry profiles of 1 × 10⁻³ M of complex [RuL₁L₂(NCS)₂] in freshly distilled DMF containing 0.1 M TBABF₄ supporting electrolyte. Step potential: 5 mV, amplitude: 50 mV vs. Ag|AgCl, frequency: 10 Hz. Scan rate: 100 m Vs⁻¹ vs. Ag|AgCl.

Figure 5

Plots of charge vs. time response for processes I, II and III; and IV and V (line a-e, respectively). Scan rate: 200 m Vs⁻¹.

Figure 5

Plots of charge vs. time response for processes I, II and III; and IV and V (line a-e, respectively). Scan rate: 200 m Vs⁻¹.

Figure 6

Anson plot of charge vs. square root of time (s) for processes I, II and III, and V. scan rate: 200 m Vs⁻¹.

Figure 6

Anson plot of charge vs. square root of time (s) for processes I, II and III, and V. scan rate: 200 m Vs⁻¹.

Scheme 1

Synthesis of functionalized oligoanthracenyl and alkenyl phenanthroline ligands L₁ and L₂ and their Ru(II) complex [Ru-L₁L₂(NCS)₂]. {Reaction conditions: (1) MeOH, Et₃N, Pd/C, 24 h; (2–4) DCM/Benzene, or EtOH, Et₃N, Pd/C, 8 h; (5) DMF, 12 h, NaOH/HNO₃, Diethyl ether}.