Gongfang Hu

We report that the molecular bismuth–palladium pincer complex (BiPdCl) catalyzes the aldol-type coupling of aldehydes with methyl isocyanoacetate with unusually rapid turnover at room temperature. Kinetic and stoichiometric studies reveal saturation behavior toward both substrates, a reversible aldehyde-dimerization side process, and a palladium–isocyanoacetate adduct as the resting state under catalytic conditions. Counterion and metal comparisons show that chloride is uniquely enabling and that the nickel analog is markedly less active. DFT calculations, supported by microkinetic modeling, rule out isonitrile-induced chloride migration from palladium to bismuth, and instead identify a cooperative pathway in which chloride-assisted enolate formation, concerted C–C and C–O bond formation, and subsequent release of an N,O-carbene collectively determine reactivity, with carbene tautomerization to the oxazoline product constituting the overall rate-limiting event. This mechanistic proposal highlights how the strongly donating bismuthinide ligand compresses the catalytic energy landscape and prevents thermodynamic trapping, thereby enabling efficient turnover unmatched by related palladium systems.

This communication reports the synthesis of a new neutral and a cationic Ni(II) complex featuring a PBiP pincer ligand (BiNiCl and BiNiAcN) through oxidative addition of a Ni(0) precursor. Crystal structures revealed that the sigma-donating bismuthane exerts insignificant trans influence on the nickel. Cyclic voltammetry and orbital analysis highlight the redox-noninnocence of the PBiP ligand and potential redox-induced structural decomposition of the complexes.

Main-group metals as supporting ligands for transition metals offer potential for bimetallic synergistic effects. We investigated a bismuth-palladium system utilizing a PBiP pincer ligand (BiPdCl) as precatalysts for electrocatalytic proton reduction with thiophenol. Rinse tests revealed the formation of a surface-bound active species, likely comprised of Pd-based heterogenous materials. NMR and UV-visible spectroscopic studies unveiled that in diluted solutions of polar coordinating solvents, the chloride trans to the bismuthane readily dissociates, affording the solvent-coordinated species (BiPdsolvent) as the predominant solution species. Following thiol– solvent ligand exchange, an equilibrium mixture of the solvent-bound species and the thiolate complex (BiPdS) forms. The crystal structures of BiPdS and an acetonitrile-coordinated model complex (BiPdAcN) shows the thermodynamic trans influence of the bismuthane ligand. Structural analysis, corroborated by computational investigations, suggests that the reduction of BiPdsolvent is primarily ligand-based than Pd-centered, hinting at a Bi(III)–Pd(0) oxidation states rather than the Bi(I)–Pd(II) model suggested by the Lewis structure of BiPdsolvent. The electron-rich Pd and electron-poor Bi centers explain its electrochemical decomposition similar to other Pd(0) species. Our findings shed light on the electronic properties and reactivities of the studied Bi–Pd compounds, offering an example of ambiguous oxidation states within this system.

Electrocatalytic ammonia oxidation (AO) at room temperature and pressure allows energy-economical and environmentally-friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]⁺, a homogenous, earth-abundant metal electrocatalyst that realizes the title reaction in water. The complex selectively catalyzes the ammonia oxidation electrochemically at 94% Faradaic efficiency without generating the less-valuable N₂ product commonly seen in molecular AO catalysis. Moreover, our complex is designed to not mediate water oxidation (WO), which is beneficial for high selectivity in AO in aqueous media. Initial mechanistic studies by electrochemistry propose monometallic catalysis and water or ammonia-nucleophilic attack for N–O bond formation. Figure 1

A multielectrode-based electrochemical cell allows the structural characterization of an amorphous thin-film water oxidation catalyst under various electrochemical potentials using high-energy X-ray scattering and atomic pair distribution function (PDF) techniques. A multielectrode with five electrodes provides a sufficiently low background signal to enable high-energy X-ray scattering (HEXS) measurements and amplifies the extremely low HEXS signals from samples for high-resolution PDF analysis of in situ data from thin-film catalysts. Glassy carbon (GC) creates a relatively low intensity HEXS pattern and is used as a working electrode. Instead of a three-dimensional (3D) porous electrode architecture, the flat geometry of the electrode enables various deposition techniques to be used for the preparation of a highly conductive metal oxide layer. PDF analysis demonstrates high spatial resolution for a 230 nm thick amorphous iridium oxide film deposited on two roughened 60 µm thick GC electrodes. The PDF analysis resolves the domain size and distinguishes changes in fine structure which are directly correlated with the structure and function of the catalysts. The results bring the opportunity to analyze the structure of nanometre-scale amorphous thin-film catalysts in an electrolyte-compatible and compact 3D-printed electrochemical cell in a three-electrode configuration.

Electrocatalytic ammonia oxidation at room temperature and pressure allows energy-economical and environmentally friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]⁺, a homogeneous electrocatalyst that realizes the title reaction in water at 94% Faradaic efficiency. The catalyst exhibits high selectivity against water oxidation in aqueous media, as [Cu(bipyalk)]⁺ is not competent for water oxidation.

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir₂(pyalc)₂(H₂O)₄-(μ-O)]²⁺ (Ir_Molecular, pyalc = 2-(2'pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrO_x) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for Ir_Molecular contrasting with potential-dependent kinetics for IrO_x. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of Ir_Molecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

One ongoing challenge in the field of iridium-based water oxidation catalysts is to develop a molecular precatalyst affording well-defined homogeneous active species for catalysis. Our previous work by using organometallic precatalysts Cp*Ir(pyalk)OH and Ir(pyalk)(CO)₂ (pyalk = (2-pyridyl)-2-propanolate) suggested a μ-oxo-bridged Ir dimer as the probable resting state, although the structure of the active species remained elusive. During the activation, the ligands Cp* and CO were found to oxidatively degrade into acetic acid or other products, which coordinate to Ir centers and affect the catalytic reaction. Two related dimers bearing two pyalk ligands on each iridium were crystallized for structural analysis. However, preliminary results indicated that these crystallographically characterized dimers are not active catalysts. In this work, we accessed a mixture of dinuclear iridium species from a coordination precursor, Na[Ir(pyalk)Cl₄], and assayed their catalytic activity for oxygen evolution by using NaIO₄ as the oxidant. This catalyst showed comparable oxygen-evolution activity to the ones previously reported from organometallic precursors without demanding oxidative activation to remove sacrificial ligands. Future research along this direction is expected to provide insights and design principles toward a well-defined active species.