Synthesis and Electrochemical Properties of Half-Sandwich Rhodium and Iridium Methyl Complexes

A series of complexes of the form [Cp­(*)­M­(bpy)­(CH₃)]I was accessed by treatment of CpM­(bpy) or Cp*M­(bpy) with methyl iodide (M = Rh, Ir; Cp = cyclopenta­dienyl; Cp* = pentamethyl­cyclopenta­dienyl; bpy = 2,2′-bipyridyl). Solid state structures (X-ray diffraction) reveal the expected distorted octahedral geometry, with Cp or Cp* bound in the η⁵ mode and bpy bound in the typical κ² mode. Electrochemical studies demonstrate that the Cp* complexes undergo a single, quasi-reversible one-electron reduction, whereas the Cp complexes undergo both a quasi-reversible one-electron reduction and a second, more negative, irreversible reduction. Electron paramagnetic resonance studies and comparisons between complexes of different metals suggest that the formulation of the singly reduced species is formally M­(III) complexes with a bound bpy anion radical. The second reduction observed in the Cp complexes, on the other hand, results in cleavage of the M–C bond. Taken together, the results suggest that the compounds have strong metal–methyl interactions, but these can be labilized upon reduction

Molecular Influences on the Quantification of Lewis Acidity with Phosphine Oxide Probes

Gutmann–Beckett-type measurements with phosphine oxide probes can be used to estimate effective Lewis acidity with 31P nuclear magnetic resonance spectroscopy, but the influence of the molecular structure of a given probe on the quantification of Lewis acidity remains poorly documented in experimental work. Here, a quantitative comparison of triethyl (E), trioctyl (O), and triphenyl (P) phosphine oxides as molecular probes of Lewis acidity has been carried out via titration studies in MeCN with a test set of six mono- and divalent metal triflate salts. In comparison to E, the bulkier O displays a similar range of chemical shift values and binding affinities for the various test metal ions. Spectral linewidths and speciation properties vary for individual cation-to-probe ratios, however, confirming probe-specific properties that can impact the data quality. Importantly, P displays a consistently narrower dynamic range than both E and O, illustrating how electronic changes at phosphorus can influence the NMR response. Comparative parametrizations of the effective Lewis acidities of a broader range of metal ions, including the trivalent rare earth ions Y3+, Lu3+, and Sc3+ as well as the uranyl ion (UO22+), can be understood in light of these results, providing insight into the fundamental chemical processes underlying the useful approach of single-point measurements for quantification of effective Lewis acidity. Together with a study of counteranion effects reported here, these data clarify the diverse ensemble of factors that can influence the measurement of Lewis acid/base interactions

Structural and Electrochemical Consequences of [Cp*] Ligand Protonation

There are few examples of the isolation of analogous metal complexes bearing [η⁵-Cp*] and [η⁴-Cp*H] (Cp* = pentamethylcyclopentadienyl) complexes within the same metal/ligand framework, despite the relevance of such structures to catalytic applications. Recently, protonation of Cp*Rh­(bpy) (bpy = 2,2′-bipyridyl) has been shown to yield a complex bearing the uncommon [η⁴-Cp*H] ligand, rather than generating a [Rh^III–H] complex. We now report the purification and isolation of this protonated species, as well as characterization of analogous complexes of 1,10-phenanthroline (phen). Specifically, reaction of Cp*Rh­(bpy) or Cp*Rh­(phen) with 1 equiv of Et₃NH⁺Br^– affords rhodium compounds bearing <i>endo</i>-η⁴-pentamethylcyclopentadiene (η⁴-Cp*H) as a ligand. NMR spectroscopy and single-crystal X-ray diffraction studies confirm protonation of the Cp* ligand, rather than formation of metal hydride complexes. Analysis of new structural data and electronic spectra suggests that phen is significantly reduced in Cp*Rh­(phen), similar to the case of Cp*Rh­(bpy). Backbonding interactions with olefinic motifs are activated by formation of [η⁴-Cp*H]; protonation of [Cp*] stabilizes the low-valent metal center and results in loss of reduced character on the diimine ligands. In accord with these changes in electronic structure, electrochemical studies reveal a distinct manifold of redox processes that are accessible in the [Cp*H] complexes in comparison with their [Cp*] analogues; these processes suggest new applications in catalysis for the complexes bearing <i>endo</i>-η⁴-Cp*H

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

Recent work has shown that Cp*Rh­(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2′- bipyridine] undergoes <i>endo</i> protonation at the [Cp*] ligand in the presence of weak acid (Et₃NH⁺; p<i>K</i>_a = 18.8 in MeCN). Upon exposure to stronger acid (e.g., DMFH⁺; p<i>K</i>_a = 6.1), hydrogen is evolved with unity yield. Here, we study the mechanisms by which this catalyst evolves dihydrogen using density functional theory (M06) with polarizable continuum solvation. The calculations show that the complex can be protonated by weak acid first at the metal center with a barrier of 3.2 kcal/mol; this proton then migrates to the ring to form the detected intermediate, a rhodium­(I) compound bearing <i>endo η</i>⁴-Cp*H. Stronger acid is required to evolve hydrogen, which calculations show happens via a concerted mechanism. The acid approaches and protonates the metal, while the second proton simultaneously migrates from the ring with a barrier of ∼12 kcal/mol. Under strongly acidic conditions, we find that hydrogen evolution can proceed through a traditional metal–hydride species; protonation of the initial hydride to form an H–H bond occurs before migration of the hydride (in the form of a proton) to the [Cp*] ring (i.e., H–H bond formation is faster than hydride–proton tautomerization). This work demonstrates the role of acid strength in accessing different mechanisms of hydrogen evolution. Calculations also predict that modification of the bpy ligand by a variety of functional groups does not affect the preference for [Cp*] protonation, although the driving force for protonation changes. However, we predict that exchange of bpy for a bidentate phosphine ligand will stabilize a rhodium­(III) hydride, reversing the preference for bound [Cp*H] found in all computed bpy derivatives and offering an appealing alternative ligand platform for future experimental and computational mechanistic studies of H₂ evolution

Co₃O₄ Nanoparticle Water-Oxidation Catalysts Made by Pulsed-Laser Ablation in Liquids

Surfactant-free, size- and composition-controlled, unsupported, <5-nm, quantum-confined cobalt oxide nanoparticles with high electrocatalytic oxygen-evolution activity were synthesized by pulsed laser ablation in liquids. These crystalline Co₃O₄ nanoparticles have a turnover frequency per cobalt surface site among the highest ever reported for Co₃O₄ nanoparticle oxygen evolution catalysts in base and overpotentials competitive with the best electrodeposited cobalt oxides, with the advantage that they are suitable for mechanical deposition on photoanode materials and incorporation in integrated solar water-splitting devices

Cp* versus Bis-carbonyl Iridium Precursors as CH Oxidation Precatalysts

We previously reported a dimeric Ir^IV-oxo species as the active water oxidation catalyst formed from a Cp*Ir­(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} precursor, where the Cp* is lost to oxidative degradation during catalyst activation; this system can also oxidize unactivated CH bonds. We now show that the same Cp*Ir­(pyalc)­Cl precursor leads to two distinct active catalysts for CH oxidation. In the presence of external CH substrate, the Cp* remains ligated to the Ir center during catalysis; the active specieslikely a high-valent Cp*Ir­(pyalc) specieswill oxidize the substrate instead of its own Cp*. If there is no external CH substrate in the reaction mixture, the Cp* will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the recently reported Ir­(CO)₂(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation precursor. We compare the reactivity of Ir­(CO)₂(pyalc) and Cp*Ir­(pyalc)­Cl precursors and show that both can lose their placeholder ligands, CO or Cp*, to form substantially similar dimeric Ir^IV-oxo catalyst resting states. The more efficient activation of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp* degradation, and therefore the dicarbonyl is our preferred precatalyst for oxidation catalysis

Catalysis of Proton Reduction by a [BO₄]‑Bridged Dicobalt Glyoxime

We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO₄] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO₄] moiety. Using electrochemical methods, the dicobalt complex was found to be an electrocatalyst for proton reduction in acetonitrile solution. Notably, the overpotential for proton reduction (954 mV) was found to be higher than in the cases of two analogous single-site cobalt glyoximes under virtually identical conditions

Highly Active Mixed-Metal Nanosheet Water Oxidation Catalysts Made by Pulsed-Laser Ablation in Liquids

Surfactant-free mixed-metal hydroxide water oxidation nanocatalysts were synthesized by pulsed-laser ablation in liquids. In a series of [Ni-Fe]-layered double hydroxides with intercalated nitrate and water, [Ni_1–<i>x</i>Fe_<i>x</i>(OH)₂]­(NO₃)_<i>y</i>(OH)_{<i>x</i>−<i>y</i>}·<i>n</i>H₂O, higher activity was observed as the amount of Fe decreased to 22%. Addition of Ti⁴⁺ and La³⁺ ions further enhanced electrocatalysis, with a lowest overpotential of 260 mV at 10 mA cm^–2. Electrocatalytic water oxidation activity increased with the relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets

Electrocatalysis of CO₂ Reduction in Brush Polymer Ion Gels

The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp₂⁺), and Re­(bpy)­(CO)₃Cl (<b>1</b>) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, <b>1</b> dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO₂ reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc^+/0 compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency for CO production. These materials provide a promising and alternative approach to immobilized electrocatalysis, creating numerous opportunities for application in solid-state devices

Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W­(CNXy)₆ and W­(CNIph)₆ (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400–550 nm). MLCT emission (λ_max ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W­(CNXy)₆ and W­(CNIph)₆ with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W­(CNIph)₆ (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bu^<i>n</i>₄N]­[PF₆], to the THF solution promotes formation of electron transfer (ET) quenching products, [W­(CNIph)₆]⁺ and [anth]^•–. ET from *W to benzophenone and cobalticenium also is observed in [Bu^<i>n</i>₄N]­[PF₆]/THF solutions. The estimated reduction potential for the [W­(CNIph)₆]⁺/*W couple is −2.8 V vs Cp₂Fe^+/0, establishing W­(CNIph)₆ as one of the most powerful photoreductants that has been generated with visible light