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Praseodymium in the +5 oxidation state is a long-sought connection between lanthanide, early-transition and actinide metal redox chemistries. Unique among the lanthanide series, evidence for molecular pentavalent praseodymium species has been observed in the gas phase and noble gas matrix isolation conditions. Here we report the low-temperature synthesis and characterization of a molecular praseodymium complex in the formal +5 oxidation state, [Pr5+(NPtBu3)4][X−] (where tBu = tert-butyl and X− = tetrakis(pentafluorophenyl)borate or hexafluorophosphate). Single-crystal X-ray diffraction, solution-state spectroscopic, solution magnetometric, density functional theory and multireference wavefunction-based methods indicate a highly multiconfigurational singlet ground state. An inverted ligand field drives this unique electronic structure, which establishes a critical link in understanding the bonding of high-valent metal complexes across the periodic table. 

A mechanism for the concerted pathway of coupled electron- and phase-transfer reactions (CEPhT) is proposed. CEPhT at three-phase interfaces formed by a solid electrode, an insulating organic solvent, and an aqueous electrolyte is driven by electric double layer (EDL) spillover, with significant electrostatic potential gradients extending a few nanometers into the insulating phase. This EDL spillover phenomenon is studied using scanning electrochemical cell microscopy to interrogate the oxidation of ferrocene in toluene to ferrocenium in water, (Fc)tol → (Fc+)aq + e–. Finite element method simulations of the electrostatic potential distribution and species concentration profiles enable the calculation of complete i–E curves that incorporate mass transport, electron transfer, phase transfer, and the EDL structure. Simulated and experimental i–E traces show good agreement in the current magnitude and the effect of the supporting electrolyte, identifying an unexpected dependence of overall reaction kinetics on the concentration of the supporting electrolyte in the aqueous phase due to EDL spillover. An interfacial toluene/water mixing region generates a unique electrochemical microenvironment where concerted electron transfer and solvent shell replacement facilitate CEPhT. Kinetic expressions for concerted and sequential CEPhT mechanisms highlight the role of this interfacial environment in controlling the rate of CEPhT. These combined experimental and simulated results are the first to support a concerted mechanism for CEPhT where (Fc)tol is transported to the interfacial mixing region at the three-phase interface, where it undergoes oxidation and phase transfer. EDL spillover can be leveraged for engineering sample geometries and electrostatic microenvironments to drive electrochemical reactivity in classically forbidden regions, e.g., insulating solvents and gases. 

Abstract The imidophosphorane ligand, [NP^tBu₃]⁻(^tBu=tert‐butyl), enables isolation of a pseudo‐tetrahedral, tetravalent praseodymium complex, [Pr⁴⁺(NP^tBu₃)₄] (1‐Pr), which is characterized by a suite of physical characterization methods including single‐crystal X‐ray diffraction, electron paramagnetic resonance, and L₃‐edge X‐ray near‐edge spectroscopies. Variable‐temperature direct‐current magnetic susceptibility data, supported by multiconfigurational quantum chemical calculations, demonstrate that the electronic structure diverges from the isoelectronic Ce³⁺analogue, driven by increased crystal field. The four‐coordinate environment around Pr⁴⁺in1‐Pr, which is unparalleled in reported extended solid systems, provides a unique opportunity to study the interplay between crystal field splitting and spin‐orbit coupling in a molecular tetravalent lanthanide within a pseudo‐tetrahedral coordination geometry.