Reversible pyrrole-based proton storage/release in ruthenium(ii) complexes

Metal-Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO2 Hydrogenation under Ambient Conditions

Interconversion between CO₂ + H₂ and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO₂ hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)]ⁿ⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H₂O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H₂O catalyzes FA dehydrogenation at 90 °C with a TOF_max of 45 900 h^-1. Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO₂ hydrogenation in the presence of base to formate under atmospheric pressure (CO₂/H₂ = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h^-1 in aqueous solution and with a TOF value of 29 h^-1 in a methanol/H₂O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO₂ hydrogenation.

Efficient Electrochemical Water Oxidation Mediated by Pyridylpyrrole-Carboxylate Ruthenium Complexes

Spurred by the rapid growth of Ru-based complexes as molecular water oxidation catalysts (WOCs), we propose novel ruthenium(II) complexes bearing pyridylpyrrole-carboxylate (H₂ppc) ligands as members of the WOC family. The structure of these complexes has 4-picoline (pic)/dimethyl sulfoxide (DMSO) in [Ru(ppc)(pic)₂(dmso)] and pic/pic in [Ru(ppc)(pic)₃] as axial ligands. Another ppc^2- ligand and one pic ligand are located at the equatorial positions. [Ru(ppc)(pic)₂(dmso)] behaves as a WOC as determined by electrochemical measurement and has an ultrahigh electrocatalytic current density of 8.17 mA cm^-2 at 1.55 V (vs NHE) with a low onset potential of 0.352 V (vs NHE), a turnover number of 241, a turnover frequency of 203.39 s^-1, and k_cat of 16.34 s^-1 under neutral conditions. The H₂O/pic exchange of the complexes accompanied by oxidation of a ruthenium center is the initial step in the catalytic cycle. The cyclic voltametric measurements of [Ru(ppc)(pic)₂(dmso)] at various scan rates, Pourbaix diagrams (plots of E vs pH), and kinetic studies suggested a water nucleophilic attack mechanism. HPO₄^2- in a phosphate buffer solution is invoked in water oxidation as the proton acceptor.

Neutral Cyclometalated Ir(III) Complexes with Pyridylpyrrole Ligand for Photocatalytic Hydrogen Generation from Water

To explore structure-activity relationships with respect to light-harvesting behavior, a family of neutral iridium complexes [Ir(ppy)₂(L_R)] 1-4 (where ppy = 2-phenylpyridine, and N̂N = 2-(1H-pyrrol-2-yl)pyridine and its functionalized derivatives) were designed and synthesized. The structural modifications in metal complexes are accomplished through the attributions of electron-donating CH₃ in 2, OCH₃ in 3, and electron-withdrawing CF₃ in 4. The structural analysis displays that the pyridylpyrrole acts as one-negative charged bidentated ligand to chelate the iridium center. The electrochemical and photophysical properties of these complexes were systematically studied. The neutral 1-4 as well as the ionic structurally analogous [Ir(ppy)₂(bpy)](PF₆) (5) were utilized as PSs in photocatalytic hydrogen generation from water with [Co(bpy)₃](PF₆)₂ as catalyst and triethanolamine (TEOA) as electron sacrificial agent in the presence of salt LiCl. Complex 1 maintains activity for more than 144 h under irradiation, and the total turnover number is up to 1768. The electrochemical properties and the quenching reaction indicate the H₂ generation by neutral complexes 1-4 is involved exclusively in the oxidative quenching process.

(1,2-Azole)bis(bipyridyl)ruthenium(II) Complexes: Electrochemistry, Luminescent Properties, And Electro- And Photocatalysts for CO2 Reduction

New cis-(1,2-azole)-aquo bis(2,2'-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)₂Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)₂Cl₂] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by ¹H, ¹³C, ¹⁵N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO₂ media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_p(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)₃]Cl₂ as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TON_CO+HCOO^-) as high as 107 for 2c after 24 h of irradiation.

Formation of Iridium(III) Complexes via Selective Activation of the C-H and N-H Bonds of a Dipyridylpyrrole Ligand

Treatment of [Ir(PPh₃)₃Cl] with 2-[5-(pyridin-2-yl)-1H-pyrrol-2-yl]pyridine (Hdpp) in refluxing toluene affords an unexpected pyrrole-metalated iridium(III) hydride complex, [Ir(K²_C,N-dpp)(H)(Cl)(PPh₃)₂] (1), via C_pyrrole-H activation, while the presence of the base KO^tBu as the deprotonation reagent produces a pyridine-metalated iridium(III) hydride complex, [Ir(K³_C,N,N-dpp)(H)(PPh₃)₂] (2), via C_pyridine-H activation. Treatment of [Ir(PPh₃)₃Cl] prepared by a convenient method with Hdpp in the presence of KO^tBu under the refluxing mixture solvent toluene/methanol (2:1, v/v) generates the N,N-chelating complex [Ir(K²_N,N-dpp)(H)(Cl)(PPh₃)₂] (3) together with 1 and the N,N-chelating dihydride complex [Ir(K²_N,N-dpp)(H)₂(PPh₃)₂] (4). Complex 4 is also readily produced by the reaction of [Ir(PPh₃)₃Cl] and Hdpp in the presence of KO^tBu under refluxing methanol or by the reaction of IrCl₃ and PPh₃ in refluxing 2-ethoxyethanol. Complexes 1-4 are fully characterized by NMR, IR, and UV-vis spectroscopy and X-ray diffraction analysis. The dpp^-/dpp^2- ligand shows rich coordination capability, of which pyridine- and pyrrole-cyclometalated coordination modes are first reported. The formation of structural isomers 1 and 3 involved the selective activation of the C-H and N-H bonds of Hdpp is rationalized by theoretical calculations.