<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)

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

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

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

Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O–O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond–second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [Ru^II(NPM)­(4-pic)₂(H₂O)]²⁺ (NPM = 4-<i>t</i>-butyl-2,6-di­(1′,8′-naphthyrid-2′-yl)­pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, g_<i>x</i> = 2.30, g_<i>y</i> = 2.18, and g_<i>z</i> = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the Ru^IVO oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [Ru^III(NPM-NO)­(4-pic)₂(H₂O)]³⁺ and [Ru^III(NPM-NO,NO)­(4-pic)₂]³⁺ complexes. A proposed [Ru^V(NPM)­(4-pic)₂O]³⁺ intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O–O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N–O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N–O ligand in promoting water oxidation catalysis

New Water Oxidation Chemistry of a Seven-Coordinate Ruthenium Complex with a Tetradentate Polypyridyl Ligand

The mononuclear ruthenium­(II) complex [<b>Ru</b>]²⁺ (<b>Ru</b> = Ru­(dpp)­(pic)₂, where dpp is the tetradentate 2,9-dipyrid-2′-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel’s group (<i>Inorg. Chem</i>. <b>2008</b>, 47, 1835–1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [^<b>2</b><b>Ru</b>^<b>III</b>]³⁺ and [^<b>2</b><b>Ru</b>^<b>V</b>(O)]³⁺ to [^<b>1</b><b>Ru</b>^<b>IV</b>(OOH)]³⁺ then [^<b>2</b><b>Ru</b>^<b>III</b>(···³O₂)]³⁺ at pH 0, and through [^<b>3</b><b>Ru</b>^<b>IV</b>(O)]²⁺, [^<b>2</b><b>Ru</b>^<b>V</b>(O)]³⁺, and [^<b>1</b><b>Ru</b>^<b>IV</b>(OO)]²⁺ at pH 9 before reaching the same [^<b>2</b><b>Ru</b>^<b>III</b>(···³O₂)]³⁺ species, from which the liberation of the weakly bound O₂ might require an additional oxidation to form [^<b>3</b><b>Ru</b>^<b>IV</b>(O)]²⁺ to initiate further cycles involving all seven-coordinate species

Evidence for Oxidative Decay of a Ru-Bound Ligand during Catalyzed Water Oxidation

In the evaluation of systems designed for catalytic water oxidation, ceric ammonium nitrate (CAN) is often used as a sacrificial electron acceptor. One of the sources of failure for such systems is oxidative decay of the catalyst in the presence of the strong oxidant CAN (<i>E</i>_ox = +1.71 V). Little progress has been made in understanding the circumstances behind this decay. In this study we show that a 2-(2′-hydroxphenyl) derivative (LH) of 1,10-phenanthroline (phen) in the complex [Ru­(L)­(tpy)]⁺ (tpy = 2,2′;6′,2″-terpyridine) can be oxidized by CAN to a 2-carboxy-phen while still bound to the metal. This complex is, in fact, a very active water oxidation catalyst. The incorporation of a methyl substituent on the phenol ring of LH slows down the oxidative decay and consequently slows down the catalytic oxidation. An analogous system based on bpy (2,2′-bipyridine) instead of phen shows much lower activity under the same conditions. Water molecule association to the Ru center of [Ru­(L)­(tpy)]⁺ and carboxylate donor dissociation were proposed to occur at the trivalent state. The resulting [Ru^III–OH₂] was further oxidized to [Ru^IVO] via a PCET process