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

The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) with RhCl3. 3H(2)O has been studied under different experimental conditions. This reaction in ethanol resulted in the formation of [Rh(tptz)Cl-3]. 2H(2)O (1), whereas the bis-chelate complex [Rh(tptz)(2)][ClO4](3). 2H(2)O (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)Cl-3] (5) with tptz in ethanol-water medium. The crystal structures of complexes 1 and 4 have been determined. Crystal data: complex 1, monoclinic, P2(1)/c, a 11.642(2) Angstrom, b = 7.302(2) Angstrom, c = 24.332(3) Angstrom, beta = 96.420(10)degrees, Z = 4, R = 0.040, and wR2 = 0.117; complex 4, triclinic, P (1) over bar, a = 9.581(1) Angstrom, b = 12.933(2) Angstrom, c = 14.493(2) Angstrom, alpha = 82.480(10)degrees, beta = 71.810(10)degrees, gamma = 75.100(10)degrees, 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.