Coordination Effects on Electron Distributions for Rhodium Complexes of the Redox-Active Bis(3,5-di-<i>tert</i>-butyl-2-phenolate)amide Ligand

New rhodium complexes of bis­(3,5-di-tert-butyl-2-phenol)­amine ([ONOcat]­H3) were synthesized, and their electronic properties were investigated. These compounds were prepared by combining [ONOq]K and [(cod)­Rh­(μ-Cl)]2 in the presence of an auxiliary donor ligand to yield complexes of the type [ONO]­RhLn (n = 3, L = py (1); n = 2, L = PMe3 (2a), L = PMe2Ph (2b), PMePh2 (2c), PPh3 (2d)). Single-crystal X-ray diffraction studies on [ONO]­Rh­(py)3 (1) revealed a six-coordinate, octahedral rhodium complex. In the case of [ONO]­Rh­(PMe3)2 (2a), X-ray diffraction showed a five-coordinate, distorted square-pyramidal coordination environment around the rhodium center. While 1 is static on the NMR time scale, complexes 2a–d are fluxional, displaying both rapid isomerization of the square-pyramidal structure and exchange of coordinated and free phosphine ligands. UV–vis spectroscopy shows stark electronic differences between 1 and 2a–d. Whereas 1 displays a strong absorbance at 380 nm with a much weaker band at 585 nm in the absorption spectrum, complexes 2a–d display an intense (ε > 104 M–1 cm–1), low-energy absorption band in the region 580–640 nm; however, in the cases of 2a and 2b, the addition of excess phosphine resulted in changes to the UV–vis spectrum indicating the formation of six-coordinate adducts [ONO]­Rh­(PMe3)3 (3a) and [ONO]­Rh­(PMe2Ph)3 (3b), respectively. The experimental and DFT computational data for the six-coordinate complexes 1, 3a, and 3b are consistent with their formulation as classical, d6, pseudo-octahedral, coordination complexes. In the five-coordinate complexes 2a–2d, π-bonding between the rhodium center and the [ONO] ligand leads to a high degree of covalency and metal–ligand electron distributions that are not accurately described by formal oxidation state assignments

Coordination Effects on Electron Distributions for Rhodium Complexes of the Redox-Active Bis(3,5-di-<i>tert</i>-butyl-2-phenolate)amide Ligand

New rhodium complexes of bis­(3,5-di-<i>tert</i>-butyl-2-phenol)­amine ([ONO^cat]­H₃) were synthesized, and their electronic properties were investigated. These compounds were prepared by combining [ONO^q]K and [(cod)­Rh­(μ-Cl)]₂ in the presence of an auxiliary donor ligand to yield complexes of the type [ONO]­RhL_<i>n</i> (<i>n</i> = 3, L = py (<b>1)</b>; <i>n</i> = 2, L = PMe₃ (<b>2a</b>), L = PMe₂Ph (<b>2b</b>), PMePh₂ (<b>2c</b>), PPh₃ (<b>2d</b>)). Single-crystal X-ray diffraction studies on [ONO]­Rh­(py)₃ (<b>1</b>) revealed a six-coordinate, octahedral rhodium complex. In the case of [ONO]­Rh­(PMe₃)₂ (<b>2a</b>), X-ray diffraction showed a five-coordinate, distorted square-pyramidal coordination environment around the rhodium center. While <b>1</b> is static on the NMR time scale, complexes <b>2a</b>–<b>d</b> are fluxional, displaying both rapid isomerization of the square-pyramidal structure and exchange of coordinated and free phosphine ligands. UV–vis spectroscopy shows stark electronic differences between <b>1</b> and <b>2a</b>–<b>d</b>. Whereas <b>1</b> displays a strong absorbance at 380 nm with a much weaker band at 585 nm in the absorption spectrum, complexes <b>2a</b>–<b>d</b> display an intense (ε > 10⁴ M^–1 cm^–1), low-energy absorption band in the region 580–640 nm; however, in the cases of <b>2a</b> and <b>2b</b>, the addition of excess phosphine resulted in changes to the UV–vis spectrum indicating the formation of six-coordinate adducts [ONO]­Rh­(PMe₃)₃ (<b>3a</b>) and [ONO]­Rh­(PMe₂Ph)₃ (<b>3b</b>), respectively. The experimental and DFT computational data for the six-coordinate complexes <b>1</b>, <b>3a</b>, and <b>3b</b> are consistent with their formulation as classical, d⁶, pseudo-octahedral, coordination complexes. In the five-coordinate complexes <b>2a</b>–<b>2d</b>, π-bonding between the rhodium center and the [ONO] ligand leads to a high degree of covalency and metal–ligand electron distributions that are not accurately described by formal oxidation state assignments

O–O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis

A deeper mechanistic understanding of the key O–O bond formation step of water oxidation by the [Ru­(bda)­(L)₂] (bdaH₂ = 2,2′-bipyridine-6,6′-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electron-donating substituents in [Ru­(bda)­(4-X-py)₂] and [Ru­(bda)­(6-X-isq)₂] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O–O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π–π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest Δ<i>G</i>^⧧ for the O–O coupling step to be [Ru­(bda)­(4-I-py)₂] and [Ru­(bda)­(6,7-(OMe)₂-isq)₂] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru­(bda)­(4-I-py)₂] (330 s^–1) is a 10-fold enhancement with respect to that of [Ru­(bda)­(py)₂], and the turnover frequency for [Ru­(bda)­(6-OMe-isq)₂] reaches 1270 s^–1, two times faster than [Ru­(bda)­(isq)₂]

Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

In order to gain a deeper mechanistic understanding of water oxidation by [(bda)­Ru­(L)2] catalysts (bdaH2 = [2,2′-bipyridine]-6,6′-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda2– ligand was synthesized and studied using stopped-flow kinetics. The additional −CF3 groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium­(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10–4 M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium­(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10–6 M), the RDS was a bimolecular step with kH/kD ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of −CF3 groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant

Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

In order to gain a deeper mechanistic understanding of water oxidation by [(bda)­Ru­(L)₂] catalysts (bdaH₂ = [2,2′-bipyridine]-6,6′-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda^2– ligand was synthesized and studied using stopped-flow kinetics. The additional −CF₃ groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium­(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10^–4 M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium­(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10^–6 M), the RDS was a bimolecular step with <i>k</i>_H/<i>k</i>_D ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of −CF₃ groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant

Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

In order to gain a deeper mechanistic understanding of water oxidation by [(bda)­Ru­(L)₂] catalysts (bdaH₂ = [2,2′-bipyridine]-6,6′-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda^2– ligand was synthesized and studied using stopped-flow kinetics. The additional −CF₃ groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium­(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10^–4 M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium­(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10^–6 M), the RDS was a bimolecular step with <i>k</i>_H/<i>k</i>_D ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of −CF₃ groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant

Redox Behavior of Rhodium 9,10-Phenanthrenediimine Complexes

New square-planar rhodium complexes of the redox-active ligand 9,10-phenenthrenediimine (phdi) have been prepared and studied. The complexes [dpp-nacnacCH3]Rh(phdi) (2a; [dpp-nacnacCH3]− = CH[C(Me)(N-iPr2C6H3)]2−) and [dpp-nacnacCF3]Rh(phdi) (2b; [dpp-nacnacCF3]− = CH[C(CF3)(N-iPr2C6H3)]2−) have been prepared from the corresponding [nacnac]Rh(CO)2 synthons by treatment with Me3NO in the presence of the phdi ligand. Complexes 2a and 2b are diamagnetic, and their absorption spectra are dominated by intense charge-transfer transitions throughout the visible region. Electrochemical studies indicate that both the phdi ligand and the rhodium metal center are redox-active, with the [nacnac]− ligands serving to modulate the one-electron-oxidation and -reduction redox potentials. In the case of 2a, chemical oxidation and reduction reactions provided access to the one-electron-oxidized cation, [2a]+, and one-electron-reduced anion, [2a]−, the latter of which has been characterized in the solid state by single-crystal X-ray diffraction. Solution electron paramagnetic resonance spectra of [2a]+ and [2a]− are consistent with S = 1/2 spin systems, but surprisingly the low-temperature spectrum of [2a]− shows a high degree of rhombicity, suggestive of rhodium(II) character in the reduced anion

Redox Behavior of Rhodium 9,10-Phenanthrenediimine Complexes

New square-planar rhodium complexes of the redox-active ligand 9,10-phenenthrenediimine (phdi) have been prepared and studied. The complexes [dpp-nacnacCH3]Rh(phdi) (2a; [dpp-nacnacCH3]− = CH[C(Me)(N-iPr2C6H3)]2−) and [dpp-nacnacCF3]Rh(phdi) (2b; [dpp-nacnacCF3]− = CH[C(CF3)(N-iPr2C6H3)]2−) have been prepared from the corresponding [nacnac]Rh(CO)2 synthons by treatment with Me3NO in the presence of the phdi ligand. Complexes 2a and 2b are diamagnetic, and their absorption spectra are dominated by intense charge-transfer transitions throughout the visible region. Electrochemical studies indicate that both the phdi ligand and the rhodium metal center are redox-active, with the [nacnac]− ligands serving to modulate the one-electron-oxidation and -reduction redox potentials. In the case of 2a, chemical oxidation and reduction reactions provided access to the one-electron-oxidized cation, [2a]+, and one-electron-reduced anion, [2a]−, the latter of which has been characterized in the solid state by single-crystal X-ray diffraction. Solution electron paramagnetic resonance spectra of [2a]+ and [2a]− are consistent with S = 1/2 spin systems, but surprisingly the low-temperature spectrum of [2a]− shows a high degree of rhombicity, suggestive of rhodium(II) character in the reduced anion

Light-Driven Water Splitting by a Covalently Linked Ruthenium-Based Chromophore–Catalyst Assembly

The preparation and characterization of new Ru­(II) polypyridyl-based chromophore–catalyst assemblies, [(4,4′-PO₃H₂-bpy)₂Ru­(4-Mebpy-4′-epic)­Ru­(bda)­(pic)]²⁺ (<b>1</b>, bpy = 2,2′-bipyridine; 4-Mebpy-4′-epic = 4-(4-methylbipyridin-4′-yl-ethyl)-pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline), and [(bpy)₂Ru­(4-Mebpy-4′-epic)­Ru­(bda)­(pic)]²⁺ (<b>1</b>′) are described, as is the application of <b>1</b> in a dye-sensitized photoelectrosynthesis cell (DSPEC) for solar water splitting. On SnO₂/TiO₂ core–shell electrodes in a DSPEC configuration with a Pt cathode, the chromophore–catalyst assembly undergoes light-driven water oxidation at pH 5.7 in a 0.1 M acetate buffer, 0.5 M in NaClO₄. With illumination by a 100 mW cm^–2 white light source, photocurrents of ∼0.85 mA cm^–2 were observed after 30 s under a 0.1 V vs Ag/AgCl applied bias with a faradaic efficiency for O₂ production of 74% measured over a 5 min illumination period

Reactivity of a Series of Isostructural Cobalt Pincer Complexes with CO₂, CO, and H⁺

The preparation and characterization of a series of isostructural cobalt complexes [Co­(<i>t</i>-Bu)₂P^EPy^EP­(<i>t</i>-Bu)₂­(CH₃CN)₂]­[BF₄]₂ (Py = pyridine, E = CH₂, NH, O, and X = BF₄ (<b>1a</b>–<b>c</b>)) and the corresponding one-electron reduced analogues [Co­(<i>t</i>-Bu)₂P^EPy^EP­(<i>t</i>-Bu)₂­(CH₃CN)₂]­[BF₄]₂ (<b>2a</b>–<b>c</b>) are reported. The reactivity of the reduced cobalt complexes with CO₂, CO, and H⁺ to generate intermediates in a CO₂ to CO and H₂O reduction cycle are described. The reduction of <b>1a</b>–<b>c</b> and subsequent reactivity with CO₂ was investigated by cyclic voltammetry, and for <b>1a</b> also by infrared spectroelectrochemistry. The corresponding CO complexes of (<b>2a</b>–<b>c</b>) were prepared, and the Co–CO bond strengths were characterized by IR spectroscopy. Quantum mechanical methods (B3LYP-d3 with solvation) were used to characterize the competitive reactivity of the reduced cobalt centers with H⁺ versus CO₂. By investigating a series of isostructural complexes, correlations in reactivity with ligand electron withdrawing effects are made