Polydentate Analogues of 8-Hydroxyquinoline and Their Complexes with Ruthenium

Selective reduction of 2-nitro-3-methoxybenzaldehyde provides 2-amino-3-methoxybenzaldehyde that undergoes the Friedländer condensation with a variety of acetyl-substituted derivatives of pyridine and 1,10-phenanthroline. After cleavage of the methyl ether, the resulting polydentate analogues of 8-hydroxyquinoline are excellent ligands for ruthenium. The resulting oxidation state of the metal center depends on the anionic character of the ligands. The presence of two electron donating anionic ligands results in a Ru(III) complex as evidenced by paramagnetic NMR behavior. The electronic absorption and redox properties of the complexes were measured and found to be consistent with the anionic character of the 8-HQ moieties. A planar pentadentate ligand provides two Ru–O and two Ru–N bonds in the equatorial plane. An X-ray structure shows that the central pyridine of the ligand is oriented toward the metal but held at a distance of 2.44 Å

Polydentate Analogues of 8-Hydroxyquinoline and Their Complexes with Ruthenium

Selective reduction of 2-nitro-3-methoxybenzaldehyde provides 2-amino-3-methoxybenzaldehyde that undergoes the Friedländer condensation with a variety of acetyl-substituted derivatives of pyridine and 1,10-phenanthroline. After cleavage of the methyl ether, the resulting polydentate analogues of 8-hydroxyquinoline are excellent ligands for ruthenium. The resulting oxidation state of the metal center depends on the anionic character of the ligands. The presence of two electron donating anionic ligands results in a Ru(III) complex as evidenced by paramagnetic NMR behavior. The electronic absorption and redox properties of the complexes were measured and found to be consistent with the anionic character of the 8-HQ moieties. A planar pentadentate ligand provides two Ru–O and two Ru–N bonds in the equatorial plane. An X-ray structure shows that the central pyridine of the ligand is oriented toward the metal but held at a distance of 2.44 Å

Visible Light-Driven Hydrogen Evolution from Water Catalyzed by A Molecular Cobalt Complex

An approximately planar tetradentate polypyridine ligand, 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline (ppq), has been prepared by two sequential Friedländer condensations. The ligand readily accommodates Co­(II) bearing two axial chlorides, and the resulting complex is reasonably soluble in water. In DMF the complex shows three well-behaved redox waves in the window of 0 to −1.4 V (vs SHE). However in pH 7 buffer the third wave is obscured by a catalytic current at −0.95 V, indicating hydrogen production that appears to involve a proton-coupled electron-transfer event. The complex [Co­(ppq)­Cl₂] (<b>6</b>) in pH 4 aqueous solution, together with [Ru­(bpy)₃]­Cl₂ and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λ_max = 469 nm) produces hydrogen with an initial TOF = 586 h^–1

<i>trans</i>-[Ru^II(dpp)Cl₂]: A Convenient Reagent for the Preparation of Heteroleptic Ru(dpp) Complexes, Where dpp Is 2,9-Di(pyrid-2′-yl)-1,10-phenanthroline

The reaction of 2,9-di­(pyrid-2′-yl)-1,10-phenanthroline (dpp) with [RuCl₃·3H₂O] or [Ru­(DMSO)₄Cl₂] provides the reagent <i>trans</i>-[Ru^II(dpp)­Cl₂] in yields of 98 and 89%, respectively. This reagent reacts with monodentate ligands L to replace the two axial chlorides, affording reasonable yields of a ruthenium­(II) complex with dpp bound tetradentate in the equatorial plane. The photophysical and electrochemical properties of the tetradentate complexes are strongly influenced by the axial ligands with electron-donating character to stabilize the ruthenium­(III) state, shifting the metal-to-ligand charge-transfer absorption to lower energy and decreasing the oxidation potential. When the precursor <i>trans</i>-[Ru^II(dpp)­Cl₂] reacts with a bidentate (2,2′-bipyridine), tridentate (2,2′;6,2″-terpyridine), or tetradentate (itself) ligand, a peripheral pyridine on dpp is displaced such that dpp binds as a tridentate. This situation is illustrated by an X-ray analysis of [Ru­(dpp)­(bpy)­Cl]­(PF₆)

Component Analysis of Dyads Designed for Light-Driven Water Oxidation

A series of seven dyad molecules have been prepared utilizing a [Ru­(tpy)­(NN)­I]⁺ type oxidation catalyst (NN = 2,5-di­(pyrid-2′-yl) pyrazine (<b>1</b>), 2,5-di-(1′,8′-dinaphthyrid-2′-yl) pyrazine (<b>2</b>), or 4,6-di-(1′,8′-dinaphthyrid-2′-yl) pyrimidine (<b>3</b>). The other bidentate site of the bridging ligand was coordinated with 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or a substituted derivative. These dinuclear complexes were characterized by their ¹H NMR spectra paying special attention to protons held in the vicinity of the electronegative iodide. In one case, <b>10a</b>, the complex was also analyzed by single crystal X-ray analysis. The electronic absorption spectra of all the complexes were measured and reported as well as emission properties for the sensitizers. Oxidation and reduction potentials were measured and excited state redox properties were calculated from this data. Turnover numbers, initial rates, and induction periods for oxygen production in the presence of a blue LED light and sodium persulfate as a sacrificial oxidant were measured. Similar experiments were run without irradiation. Dyad performance correlated well with the difference between the excited state reduction potential of the photosensitizer and the ground state oxidation potential of the water oxidation dyad. The most active system was one having 5,6-dibromophen as the auxiliary ligand, and the least active system was the one having 4,4′-dimethylbpy as the auxiliary ligand

A Molecular Light-Driven Water Oxidation Catalyst

Two mononuclear Ru­(II) complexes, [Ru­(ttbt)­(pynap)­(I)]­I and [Ru­(tpy)­(Mepy)₂(I)]I (tpy = 2,2′;6,2″-terpyridine; ttbt = 4,4′,4″-tri-<i>tert</i>-butyltpy; pynap = 2-(pyrid-2′-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ_max = 472 nm) LED light source using [Ru­(bpy)₃]­Cl₂ (bpy = 2,2′-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components, light, photosensitizer, sodium persulfate, and catalyst, are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions

New Tridentate Ligand Affords a Long-Lived ³MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and ¹O₂ Production

Two new complexes, [Ru­(tpy)­(qdppz)]­(PF6)2 (1; qdppz = 2-(quinolin-8-yl)­dipyrido­[3,2-a:2′,3′-c]­phenazine, tpy = 2,2′:6′,2″-terpyridine) and [Ru­(qdppz)2]­(PF6)2 (2), were investigated for their potential use as phototherapeutic agents through their ability to photosensitize the production of singlet oxygen, 1O2, upon irradiation with visible light. The complexes exhibit strong Ru­(dπ) → qdppz­(π*) metal-to-ligand charge transfer (MLCT) absorption with maxima at 485 and 495 nm for 1 and 2 in acetone, respectively, red-shifted from the Ru­(dπ) → tpy­(π*) absorption at 470 nm observed for [Ru­(tpy)2]2+ (3) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3MLCT emission is observed for 2 with maximum at 690 nm (λexc = 480 nm) in acetone. The lifetimes of the 3MLCT states of 1 and 2 were measured using transient absorption spectroscopy to be ∼9 and 310 ns in methanol, respectively, at room temperature (λexc = 490 nm). The bite angle of the qdppz ligand is closer to octahedral geometry than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3. Arrhenius treatment of the temperature dependence of the luminescence results in similar activation energies, Ea, from the 3MLCT to the 3LF (ligand-field) state for the two complexes, 2520 cm–1 in 1 and 2400 cm–1 in 2. However, the pre-exponential factors differ by approximately two orders of magnitude, 2.3 × 1013 s–1 for 1 and 1.4 × 1011 s–1 for 2, which, together with differences in the Huang–Rhys factors, lead to markedly different 3MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1O2 production upon irradiation with ΦΔ = 0.69. The present work highlights the profound effect of the ligand bite angle on the electronic structure, providing guidelines for extending the lifetime of 3MLCT Ru­(II) complexes with tridentate ligands, a desired property for a number of applications

A Ru(II) Bis-terpyridine-like Complex that Catalyzes Water Oxidation: The Influence of Steric Strain

The complexation of 2,9-dicarboxy-1,10-phenanthroline (DPA) with [Ru­(tpy)­Cl₃] (tpy = 2,2′;6,2″-terpyridine) provides a six-coordinate species in which one carboxyl group of DPA is not bound to the Ru­(II) center. A more soluble tri-<i>t-</i>butyl tpy analogue is also prepared. Upon oxidation, neither species shows evidence for intramolecular trapping of a seven-coordinate intermediate. The role of the tpy ligand is revealed by the preparation of [Ru­(tpy)­(phenq)]²⁺ (phenq = 2-(quinol-8′-yl)-1,10-phenanthroline) that behaves as an active water oxidation catalyst (TON = 334). This activity is explained by the expanded coordination geometry of the phenq ligand that can form a six-membered chelate ring that better accommodates the linear arrangement of axial ligands required for optimal pentagonal bipyramid geometry. When a 1,8-naphthyidine ring is substituted for each of the two peripheral pyridine rings on tpy, increased crowding in the vicinity of the metal center impedes acquisition of the prerequisite reaction geometry

Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

The tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp² center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl₃, a μ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl₃. Both structures are verified by X-ray analysis. While the Fe^III(dpa) complex shows two reversible one-electron oxidation waves, the Fe^III(ppq) complex shows a clear two-electron oxidation associated with the process H₂O–Fe^IIIFe^III → H₂O–Fe^IVFe^IV → OFe^VFe^III. Subsequent disproportionation to an FeO species is suggested. When the Fe^III(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h^–1. Under the same conditions the mononuclear Fe^III(dpa) complex also evolves oxygen with TOF = 842 h⁻¹

Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands

A series of tetradentate 2,2′:6′,2″:6″,2‴-quaterpyridine-type ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2′-bipyridine and the other two ligands have a 3,3′-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co­(II) complexes are prepared and characterized by ultraviolet–visible light (UV-vis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H₂-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru­(bpy)₃]²⁺ as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co­(ppq)­Cl₂] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h^–1). Cyclic voltammograms showed a significant catalytic current for H₂ production in both aqueous buffer and H₂O/DMF medium. Combined experimental and theoretical study suggest a formal Co­(II)-hydride species as a key intermediate that triggers H₂ generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co­(II) state to the Co­(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands