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 RhCl 3 ‚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 ][ClO 4 ] 3 ‚2H 2 O (2) was obtained in a two-step reaction in acetone; the chlorides from RhCl 3 were removed in the first step using AgClO 4 , 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][PF 6 ]‚ H 2 O (3) and [Rh(bpca) 2 ][ClO 4 ] (6), respectively. A mixed-ligand complex, [Rh(bpca)(tpy)][PF 6 ] 2 ‚CH 3 CN (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) 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, P1 h, a ) 9.581(1) Å, b ) 12.933(2) Å, c ) 14.493(2) Å, R ) 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) f 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.

Electrochemical Hydrogenation of [Ru(bpy)2(napy‐κN)(CO)]2+: Inhibition of Reductive RuCO Bond Cleavage by a Ruthenacycle

“…The ESI mass spectra of a CH 3 CN solution of 3(PF 6 ) prepared in H 2 O and D 2 O exhibit signals at m/z 573 and m/z 574, respectively, for the monocationic parent complexes thus indicating that 3 + is produced by a proton-coupled oneelectron reduction of 2 + . The 1 H NMR spectrum of 3(PF 6 ) prepared in H 2 O shows a new multiplet signal at d = 3.66 ppm with an intensity of two protons.…”

“…A number of metal complexes have been proven to work as catalyst precursors in photo- [1] and electrochemical [2] reduction of CO 2 , although the reduction products are limited to CO and HCOOH as twoelectron-reduced products. The generation of CO occurs by reductive cleavage of metalÀCO bonds derived from M-h 1 -CO 2 and M-h 1 -CO(O)H complexes in protic media.…”

“…A number of metal complexes have been proven to work as catalyst precursors in photo- [1] and electrochemical [2] reduction of CO 2 , although the reduction products are limited to CO and HCOOH as twoelectron-reduced products. The generation of CO occurs by reductive cleavage of metalÀCO bonds derived from M-h 1 -CO 2 and M-h 1 -CO(O)H complexes in protic media. The requisites for multielectron reduction of CO 2 and CO catalyzed by metal complexes, therefore, are electrochemical hydrogenation of CO in aqueous media without accompanying reductive cleavage of the M À CO bonds.…”

“…The generation of CO occurs by reductive cleavage of metalÀCO bonds derived from M-h 1 -CO 2 and M-h 1 -CO(O)H complexes in protic media. The requisites for multielectron reduction of CO 2 and CO catalyzed by metal complexes, therefore, are electrochemical hydrogenation of CO in aqueous media without accompanying reductive cleavage of the M À CO bonds. We have been studying electro-and photochemical reduction of CO 2 using polypyridyl ruthenium carbonyl complexes, [3] in which ligandbased redox reactions supply the electrons required for the reduction of CO 2 .…”

Fabrication of Nanostructured Palladium Within Tridentate Schiff-Base-Ligand Coordination Architecture: Enhancement of Electrocatalytic Activity Toward CO2 Electroreduction

“…Historically, macromolecular complexes of certain transition metals were demonstrated to exhibit appreciable activity toward electroreduction of carbon dioxide. Representative examples include complexes of iron, cobalt, and nickel with terdentate ligands [23], mono(bipyridine)carbonylruthenium complexes (investigated both in solution or as polymeric thin films) [24], rhodium complexes containing 2,4,6-Tris(2-pyridyl)-1,3,5-triazine [25], bimetallic palladium complexes [26], or iron(0) porphyrins [27,28]. In addition to the catalytic activity of a metallic site, the activating role of a ligand should also be appreciated.…”