Synthesis and Characterization of Rhodium Complexes Containing 2,4,6-Tris(2-pyridyl)-1,3,5-triazine and Its Metal-Promoted Hydrolytic Products: Potential Uses of the New Complexes in Electrocatalytic Reduction of Carbon Dioxide

Abstract

The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) with RhCl3·3H2O has been studied under different experimental conditions. This reaction in ethanol resulted in the formation of [Rh(tptz)Cl3]·2H2O (1), whereas the bis-chelate complex [Rh(tptz)2][ClO4]3·2H2O (2) was obtained in a two-step reaction in acetone; the chlorides from RhCl3 were removed in the first step using AgClO4, and the ligand tptz was added in the second step. Complexes 1 and 2, when refluxed in ethanol−water (1:1), resulted in metal-promoted hydrolysis of tptz to bis(2-pyridylcarbonyl)amide anion (bpca) and 2-picolinamide (pa), yielding the complexes [Rh(bpca)(pa)Cl][PF6]·H2O (3) and [Rh(bpca)2][ClO4] (6), respectively. A mixed-ligand complex, [Rh(bpca)(tpy)][PF6]2·CH3CN (4), was obtained by the reaction of either 1 with tpy or [Rh(tpy)Cl3] (5) with tptz in ethanol−water medium. The crystal structures of complexes 1 and 4 have been determined. Crystal data: complex 1, monoclinic, P21/c, a = 11.642(2) Å, b = 7.302(2) Å, c = 24.332(3) Å, β = 96.420(10)°, Z = 4, R = 0.040, and wR2 = 0.117; complex 4, triclinic, P1̄, a = 9.581(1) Å, b = 12.933(2) Å, c = 14.493(2) Å, α = 82.480(10)°, β = 71.810(10)°, γ = 75.100(10)°, Z = 2, R = 0.030, and wR2 = 0.082. The two water molecules of complex 1 make short contacts with the carbon atoms adjacent to the metal-bound nitrogen atom of the triazine ring; this observation provides some insight about the "intermediate" of the hydrolysis. X-ray and NMR data suggest that the electron-withdrawing effect of the metal ion is the major responsible factor for the hydrolysis of tptz. The cyclic voltammograms of the complexes exhibit a metal-based 2e reduction (Rh(III) → Rh(I)) at the potential range −0.42 to −0.98 V vs SCE, followed by ligand-based redox couple(s). These novel complexes show effective catalytic properties for the electrocatalytic reduction of carbon dioxide in the potential range −1.26 to −1.44 V.