Single Site Isomeric Ru WOCs with an Electron-Withdrawing Group: Synthesis, Electrochemical Characterization, and Reactivity

The synthetic intermediate <i>cis­(out),cis</i>-[Ru­(Cl)₂(HL)­(DMSO)₂], <b>1</b> (DMSO = dimethyl sulfoxide), and four new mononuclear ruthenium complexes with general formula <i>out/in</i>-[Ru­(HL)­(trpy)­(X)]^<i>m</i>+ (trpy = 4-<i>tert</i>-butylpyridine; X = Cl^–, <i>m</i> = 1, <b>2a</b>⁺ and <b>2b</b>⁺; X = H₂O, <i>m</i> = 2, <b>3a</b>²⁺ and <b>3b</b>²⁺) based on the ligand 1<i>H</i>-pyrazole-3-carboxylic acid, 5-(2-pyridinil)-, ethyl ester (HL), are synthesized and characterized by analytical, spectroscopic, and electrochemical methods. A linkage isomerism is observed for a DMSO moiety of <b>1</b>, and relevant thermodynamics and kinetics values are obtained through electrochemical experiments and compared to literature. Different synthetic routes are developed to obtain isomeric <b>2a</b>⁺ and <b>2b</b>⁺, with different relative yields. Water oxidation activity of <b>3a</b>²⁺ and <b>3b</b>²⁺ is analyzed by means of electrochemical methods, through foot of the wave analysis, yielding <i>k</i>_obs values of 1.00 and 2.23 s^–1, respectively. Chemically driven water oxidation activity is tested using [(NH₄)₂Ce­(NO₃)₆] as sacrificial electron acceptor, and turnover number (TON) and turnover frequency (TOF) values of TON = 10.8 and TOF_i = 58.2 × 10^–3 s^–1 for <b>3a</b>²⁺ and TON = 4.2 and TOF_i = 15.4 × 10^–3 s^–1 for <b>3b</b>²⁺ are obtained

Synthesis, Structure, and Redox Properties of a <i>trans</i>-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States

In this work we have prepared and characterized two Ru complexes that contain the pentadenatate tda^2– ligand (tda^2– = [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate) that occupies the equatorial positions and two monodentate ligands aqua and/or dmso that occupy the axial positons: [<i>trans</i>-Ru^III(tda-κ-N³O)­(OH₂^ax)₂]⁺, <b>3</b>^<b>III</b>(OH₂)₂⁺, and [Ru^II(tda-κ-N³O)­(dmso)­(OH₂^ax)], <b>4</b>^<b>II</b>. The latter is a useful synthetic intermediate for the preparation of Ru-tda complexes with different axial ligands. The two complexes have been characterized in the solid state by single-crystal XRD and by elemental analysis. In solution, complex <b>4</b>^II has been characterized by NMR spectroscopy as well as the one-electron reduction of complex <b>3</b>^<b>III</b>(OH₂)₂⁺. The electrochemical properties of <b>3</b>^<b>III</b>(OH₂)₂⁺ and <b>4</b>^<b>II</b> have been assessed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex <b>3</b>^<b>III</b>(OH₂)₂⁺ shows the presence of four redox waves that are assigned to the VI/V, V/IV, IV/III, and III/II redox couples. The variation of the redox potentials is analyzed as a function of pH and is graphically presented as a Pourbaix diagram. Finally, the redox potentials displayed by both <b>3</b>^<b>III</b>(OH₂)₂⁺ and <b>4</b>^<b>II</b> are compared to related complexes previously reported in the literature and rationalized on the basis of the electron donating or withdrawing capacity of the auxiliary ligands as well as with regard to their ability to undergo seven-coordination at high oxidation states

New Dinuclear Ruthenium Complexes: Structure and Oxidative Catalysis

The synthesis of new dinuclear complexes of the general formula {[Ru^II(trpy)]₂(μ-pdz-dc)­(μ-(L)}⁺ [pdz-dc is the pyridazine-3,6-dicarboxylate dianion; trpy is 2,2′:6′,2″-terpyridine; L = Cl (<b>1</b>^<b>+</b>) or OH (<b>2⁺</b>)] is described. These complexes are characterized by the usual analytical and spectroscopic techniques and by X-ray diffraction analysis. Their redox properties are characterized by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex <b>2⁺</b> is used as the starting material to prepare the corresponding Ru-aqua complex {[Ru^II(trpy)­(H₂O)]₂(μ-pdz-dc)}²⁺ (<b>3²⁺</b>), whose electrochemistry is also investigated by means of CV and DPV. Complex <b>3²⁺</b> is able to catalytically and electrocatalytically oxidize water to dioxygen with moderate efficiencies. In sharp contrast, <b>3²⁺</b> is a superb catalyst for the epoxidation of alkenes. For the particular case of <i>cis</i>-β-methylstyrene, the catalyst is capable of carrying out 1320 turnovers with a turnover frequency of 11.0 cycles min^–1, generating <i>cis</i>-β-methylstyrene oxide stereospecifically

New Dinuclear Ruthenium Complexes: Structure and Oxidative Catalysis

The synthesis of new dinuclear complexes of the general formula {[Ru^II(trpy)]₂(μ-pdz-dc)­(μ-(L)}⁺ [pdz-dc is the pyridazine-3,6-dicarboxylate dianion; trpy is 2,2′:6′,2″-terpyridine; L = Cl (<b>1</b>^<b>+</b>) or OH (<b>2⁺</b>)] is described. These complexes are characterized by the usual analytical and spectroscopic techniques and by X-ray diffraction analysis. Their redox properties are characterized by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex <b>2⁺</b> is used as the starting material to prepare the corresponding Ru-aqua complex {[Ru^II(trpy)­(H₂O)]₂(μ-pdz-dc)}²⁺ (<b>3²⁺</b>), whose electrochemistry is also investigated by means of CV and DPV. Complex <b>3²⁺</b> is able to catalytically and electrocatalytically oxidize water to dioxygen with moderate efficiencies. In sharp contrast, <b>3²⁺</b> is a superb catalyst for the epoxidation of alkenes. For the particular case of <i>cis</i>-β-methylstyrene, the catalyst is capable of carrying out 1320 turnovers with a turnover frequency of 11.0 cycles min^–1, generating <i>cis</i>-β-methylstyrene oxide stereospecifically

Ru–Zn Heteropolynuclear Complexes Containing a Dinucleating Bridging Ligand: Synthesis, Structure, and Isomerism

Mononuclear complexes <i>in</i>- and <i>out</i>-[Ru­(Cl)­(trpy)­(Hbpp)]⁺ (<i><b>in</b></i><b>-0</b>, <i><b>out</b></i><b>-0</b>; Hbpp is 2,2′-(1<i>H</i>-pyrazole-3,5-diyl)­dipyridine and trpy is 2,2′:6′,2″-terpyridine) are used as starting materials for preparation of Ru–Zn heterodinuclear <i>out</i>-{[Ru­(Cl)­(trpy)]­[ZnCl₂]­(μ-bpp)} (<i><b>out</b></i><b>-2</b>) and heterotrinuclear <i>in,in</i>- and <i>out,out</i>-{[Ru­(Cl)­(trpy)]₂(μ-[Zn­(bpp)₂])}²⁺ (<i><b>in</b></i><b>-3</b>, <i><b>out</b></i><b>-3</b>) constitutional isomers. Further substitution of the Cl ligand from the former complexes leads to Ru–aqua <i>out,out</i>-{[Ru­(trpy)­(H₂O)]₂(μ-[Zn­(bpp)₂])}⁴⁺ (<i><b>out</b></i><b>-4</b>) and the oxo-bridged Ru–O–Ru complex <i>in,in</i>-{[Ru^III(trpy)]₂(μ-[Zn­(bpp)₂(H₂O)]­μ-(O)}⁴⁺ (<i><b>in</b></i><b>-5</b>). All complexes are thoroughly characterized by the usual analytical techniques as well as by spectroscopy by means of UV–vis, MS, and when diamagnetic NMR. CV and DPV are used to extract electrochemical information and monocrystal X-ray diffraction to characterize complexes <i><b>out</b></i><b>-2</b>, <i><b>in</b></i><b>-3</b>, <i><b>out</b></i><b>-3</b>, and <i><b>in</b></i><b>-5</b> in the solid state. Complex <i><b>out</b></i><b>-3</b> photochemically isomerizes toward <i><b>in</b></i><b>-3</b>, as can be observed by NMR spectroscopy and rationalized by density functional theory based calculations

On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl Halides

A computational screening of 42 bidentate phosphines (PP) has yielded promising candidates for Ph–CF₃ reductive elimination from Ni­(II) complexes of the type [(PP)­Ni­(Ph)­(CF₃)]. The computed barriers and synthetic accessibility considerations have identified two PP ligands, dippf and dcypf (Δ<i>G</i>^⧧ = 22.6 and 23.2 kcal/mol, respectively), for experimental studies with 1-Np (1-naphthyl) in place of Ph. Ligand exchange of [(Ph₃P)₂Ni­(1-Np)­Cl] with dippf or dcypf has cleanly produced [(dippf)­Ni­(1-Np)­Cl] and [(dcypf)­Ni­(1-Np)­Cl], the first examples of trans square-planar 1,1′-ferrocenediyl backbone-based diphosphine metal complexes devoid of M···Fe dative interactions. Treatment of these chlorides with CF₃SiMe₃/F^–, AgCF₃/MeCN or [(Ph₃P)₃Cu­(CF₃)] does not furnish isolable or ¹⁹F NMR-detectable [(PP)­Ni­(1-Np)­(CF₃)] (PP = dippf, dcypf). Other transformations have been observed instead, e.g., ligand exchange with the Ag and Cu complexes, the latter leading to [(dcypf)­Cu­(CF₃)], a rare example of well-defined CF₃Cu­(I) species. With CF₃SiMe₃/F^–, indirect evidence has been obtained for intermediacy of [(PP)­Ni­(1-Np)­(CF₃)] (PP = dippf, dcypf) and instantaneous decomposition via pathways other than C–CF₃ reductive elimination. The first Ni­(II) complexes with fluoride trans to a non-electron-deficient aryl, [(Cy₃P)₂Ni­(1-Np)­F] and [(<i>i-</i>PrXantphos)­Ni­(1-Np)­F], have been prepared and fully characterized. Surprisingly, [(Cy₃P)₂Ni­(1-Np)­F] can be produced from [(Cy₃P)₂Ni­(1-Np)­Cl] and CsF rather than AgF that is conventionally used for the synthesis of late transition metal fluorides via X/F exchange. While [(Cy₃P)₂Ni­(1-Np)­F] is unreactive toward CF₃SiMe₃, [(<i>i-</i>PrXantphos)­Ni­(1-Np)­F] is readily trifluoromethylated to produce robust [(<i>i-</i>PrXantphos)­Ni­(1-Np)­(CF₃)], a rare example of complexes of the type [(PP)­Ni­(Ar)­(CF₃)] with PP other than dippe

Redox Non-innocent Ligand Controls Water Oxidation Overpotential in a New Family of Mononuclear Cu-Based Efficient Catalysts

A new family of tetra-anionic tetradentate amidate ligands, <i>N</i>₁,<i>N</i>₁′-(1,2-phenyl­ene)­bis­(<i>N</i>₂-methyl­oxal­amide) (H₄L1), and its derivatives containing electron-donating groups at the aromatic ring have been prepared and characterized, together with their corresponding anionic Cu­(II) complexes, [(L<i>Y</i>)­Cu]^2–. At pH 11.5, the latter undergoes a reversible metal-based III/II oxidation process at 0.56 V and a ligand-based pH-dependent electron-transfer process at 1.25 V, associated with a large electrocatalytic water oxidation wave (overpotential of 700 mV). Foot-of-the-wave analysis gives a catalytic rate constant of 3.6 s^–1 at pH 11.5 and 12 s^–1 at pH 12.5. As the electron-donating capacity at the aromatic ring increases, the overpotential is drastically reduced down to a record low of 170 mV. In addition, DFT calculations allow us to propose a complete catalytic cycle that uncovers an unprecedented pathway in which crucial O–O bond formation occurs in a two-step, one-electron process where the peroxo intermediate generated has no formal M–O bond but is strongly hydrogen bonded to the auxiliary ligand

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

In this work, we describe the synthesis, structural characterization, and redox properties of two new Ru complexes containing the dianionic potentially pentadentate [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate (tda^2–) ligand that coordinates Ru at the equatorial plane and with additional pyridine or dmso acting as monondentate ligand in the axial positions: [Ru^II(tda-κ-N³O)­(py)­(dmso)], <b>1</b>^<b>II</b> and [Ru^III(tda-κ-N³O²)­(py)­(H₂O)^ax]⁺, <b>2</b>^<b>III</b><b>(H</b>_<b>2</b><b>O)</b>⁺. Complex <b>1</b>^<b>II</b> has been characterized by single-crystal XRD in the solid state and in solution by NMR spectroscopy. The redox properties of <b>1</b>^<b>II</b> and <b>2</b>^<b>III</b><b>(H</b>_<b>2</b><b>O)</b>⁺ have been thoroughly investigated by means of cyclic voltammetry and differential pulse voltammetry. Complex <b>2</b>^<b>II</b><b>(H</b>_<b>2</b><b>O)</b> displays poor catalytic activity with regard to the oxidation of water to dioxygen, and its properties have been analyzed on the basis of foot of the wave analysis and catalytic Tafel plots. The activity of <b>2</b>^<b>II</b><b>(H</b>_<b>2</b><b>O)</b> has been compared with related water oxidation catalysts (WOCs) previously described in the literature. Despite its moderate activity, <b>2</b>^<b>II</b><b>(H</b>_<b>2</b><b>O)</b> constitutes the cornerstone that has triggered the rationalization of the different factors that govern overpotentials as well as efficiencies in molecular WOCs. The present work uncovers the interplay between different parameters, namely, coordination number, number of anionic groups bonded to the first-coordination sphere of the metal center, water oxidation catalysis overpotential, p<i>K</i>_a and hydrogen bonding, and the performance of a given WOC. It thus establishes the basic principles for the design of efficient WOCs operating at low overpotentials

Establishing the Family of Diruthenium Water Oxidation Catalysts Based on the Bis(bipyridyl)pyrazolate Ligand System

A bis­(bipyridyl)­pyrazolate (^Mebbp^–) has recently been introduced as a rugged dinucleating, bis­(tridentate) ligand for the formation of efficient diruthenium water oxidation catalysts (<i>J. Am. Chem. Soc.</i> <b>2014</b>, <i>136</i>, 24–27). Now, detailed protocols for the synthesis of a whole family of such dinuclear ruthenium complexes [{Ru­(pyR²)₂}₂­(μ-^R1bbp)­(X,Y)]²⁺ based on the bis­(bipyridyl)­pyrazolate scaffold are reported. The isolation of a synthetic key intermediate allowed the straightforward introduction of different pyridines as axial ligands. Thereby, a set of complexes with different substituents at the pyrazolate backbone (R¹ = Br, H, Me), different pyridines as axial ligand (R² = H, NMe₂, SO₃), and different (non)­bridging units in the <i>in,in</i>-position (X,Y = Cl, H₂O, OAc) has been prepared and thoroughly characterized. Complexes of the type [{Ru­(pyR²)₂}₂­(μ-^R1bbp)­(μ-OAc)]²⁺, with an exogenous acetato bridge, have been used as catalyst precursors in catalytic water oxidation experiments with a sacrificial oxidant. The effect of substitution on the pyrazole core of the ^R1bbp^– ligand as well as on the pyridine ligands on both electrochemistry and catalytic activity has been systematically investigated. The catalyst stability, reflected by the turnover number, is crucially determined by the substituent at the pyrazolate ligand (R¹ = Me > H > Br). In contrast, the axial pyridine ligands modulate the rate of the catalytic process, expressed by the initial turnover frequency (R² = H > NMe₂H⁺)

Nonelectrochemical Synthesis, Crystal Structure, and Physical Properties of the Radical Salt [ET]₂[CuCl₄] (ET = Bis(ethylenedithio)tetrathiafulvalene)

The radical salt [ET]₂[CuCl₄] was obtained by chemical oxidation of bis­(ethylenedithio)­tetrathiafulvalene (ET) with the tetranuclear copper­(II) halide cluster [Cu₄­OCl₁₀]^4–. Although a complex mixture of anions forms in solution during the redox reaction, only this product is obtained as large (>3 mm) single crystals. X-ray diffraction analysis determined that the ET molecules stack in the solid state forming dimerized 1D chains along the <i>a</i> axis, interleaved by [CuCl₄]^2– anions. The ET dimers show very short S···S contacts (<3.41 Å). The physical properties are dominated by these intradimer ET interactions. The magnetic behavior shows antiferromagnetic coupling with a singlet–triplet gap >620 K (430 cm^–1). The Cu²⁺ (<i>S</i> = ¹/₂) centers are magnetically isolated and yield a narrow EPR line in the X-band at <i>g</i> = 2.01. The ET moieties are EPR silent