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Abstract Tuning of redox‐active complexes featuring metals with high coordination numbers by incorporation of secondary redox‐inactive cations has received far less attention than it deserves. Here, appending moderate steric bulk to a tripodal ligand framework has been tested for its influence on secondary‐cation‐driven structural and electrochemical tuning of cerium, a lanthanide that tends to adopt high coordination numbers. Aquasi‐C₃‐symmetric cerium(III) complex denoted[Ce]has been prepared that features pendant benzyloxy groups, and this work demonstrates that this species offers a site capable of binding single Na⁺or Ca²⁺ions. Electrochemical and UV‐visible spectroscopic studies reveal equilibrium binding affinity of[Ce]for Na⁺in acetonitrile solvent, contrasting with tight binding of all cations in all other previously studied systems of this type. The modulated cation binding can be attributed to the bulky benzyloxy groups, which impact the thermodynamics of cation binding but do not impede the formation of cerium centers with coordination number 8 upon binding of either Na⁺or Ca²⁺. The Ce(IV/III) reduction potential was found to be tunable under the equilibrium binding conditions, highlighting the potentially significant role that controlled structural changes can play in modulating the solution properties of heterobimetallic complexes. 

Abstract Literature reports have demonstrated that Schiff-base-type ligands can serve as robust platforms for the synthesis of heterobimetallic complexes containing transition metals and the uranyl dication (UO₂²⁺). However, efforts have not advanced to include either synthesis of complexes containing second- or third-row transition metals or measurement of the redox properties of the corresponding heterobimetallic complexes, despite the significance of actinide redox in studies of nuclear fuel reprocessing and separations. Here, metalloligands denoted [Ni], [Pd], and [Pt] that contain the corresponding Group 10 metals have been prepared and a synthetic strategy to access species incorporating the uranyl ion (UO₂²⁺) has been explored, toward the goal of understanding how the secondary metals could tune uranium-centered redox chemistry. The synthesis and redox characterization of the bimetallic complex [Ni,UO₂] was achieved, and factors that appear to govern extension of the chosen synthetic strategy to complexes with Pd and Pt are reported here. Infrared and solid-state structural data from X-ray diffraction analysis of the metalloligands [Pd] and [Pt] show that the metal centers in these complexes adopt the expected square planar geometries, while the structure of the bimetallic [Ni,UO₂] reveals that the uranyl moiety influences the coordination environment of Ni(II), including inducement of a puckering of the ligand backbone of the complex in which the phenyl rings fold around the nickel-containing core in an umbrella-shaped fashion. Cyclic voltammetric data collected on the heterobimetallic complexes of both Ni(II) and Pd(II) provide evidence for uranium-centered redox cycling, as well as for the accessibility of other reductions that could be associated with Ni(II) or the organic ligand backbone. Taken together, these results highlight the unique redox behaviors that can be observed in multimetallic systems and design concepts that could be useful for accessing tunable multimetallic complexes containing the uranyl dication. 

Operandomeasurements of local pH at the nanoscale can significantly improve the understanding of the complex microenvironments that exist in electrochemical systems. However, attempts to easily fabricate a nano-sized pH electrode that can operate under a wide range of pH conditions and have fast temporal responses have been difficult. Here, we show that an anodic-grown Pt/Pt(II) oxide pH sensor manufactured in alkaline conditions (1 M NaOH) shows a near-Nernstian response (−60 mV/pH) from pH 0 to pH 14, is insensitive to dissolved oxygen, cation, and anion identities, and responds correctly in solution with different ionic strengths. This is in contrast to Pt/Pt(II) oxide films grown in acidic media, which do not demonstrate a Nernstian relationship due to cation interference other than H⁺. We observed a response time of 2.25 s, corresponding to 90% of the final measured pH, for an approximately twelve-fold pH step change when growing the Pt(II) oxide layer on a platinum nanoelectrode. Our findings emphasize the influence of solution pH used for anodization synthesis on the anodic Pt(II) oxide pH sensing properties. The direct oxidation approach for fabricating Pt/Pt(II) oxide microelectrode/nanoelectrode pH sensors can simplify the manufacture of real-time pH sensors for complex aqueous environments. 

Incorporation of secondary redox-inactive cations into heterobimetallic complexes is an attractive strategy for modulation of metal-centered redox chemistry, but quantification of the consequences of incorporating strongly Lewis acidic trivalent cations has received little attention. Here, a family of seven heterobimetallic complexes that pair a redox-active nickel center with La3+, Y3+, Lu3+, Sr2+, Ca2+, K+, and Na+ (in the form of their triflate salts) have been prepared on a heteroditopic ligand platform to understand how chemical behavior varies across the comprehensive series. Structural data from X-ray diffraction analysis demonstrate that the positions adopted by the secondary cations in the crown-ether-like site of the ligand relative to nickel are dependent primarily on the secondary cations’ ionic radii and that the triflate counteranions are bound to the cations in all cases. Electrochemical data, in concert with electron paramagnetic resonance studies, show that nickel(II)/nickel(I) redox is modulated by the secondary metals; the heterogeneous electron-transfer rate is diminished for the derivatives incorporating trivalent metals, an effect that is dependent on steric crowding about the nickel metal center and that was quantified here with a topographical free-volume analysis. As related analyses carried out here on previously reported systems bear out similar relationships, we conclude that the placement and identity of both the secondary metal cations and their associated counteranions can afford unique changes in the (electro)chemical behavior of heterobimetallic species. 

Obtaining useful insights from machine learning models trained on experimental datasets collected across different groups to improve the sustainability of chemical processes can be challenging due to the small size and heterogeneity of the dataset. Here we show that shallow learning models such as decision trees and random forest algorithms can be an effective tool for guiding experimental research in the sustainable chemistry field. This study trained four different machine learning algorithms (linear regression, decision tree, random forest, and multilayer perceptron) using different sized datasets containing up to 520 unique reaction conditions for the nitrogen reduction reaction (NRR) on heterogeneous electrocatalysts. Using the catalyst properties and experimental conditions as the features, we determined the ability of each model to regress the ammonia production rate and the faradaic efficiency. We observed that the shallow learning decision tree and random forest models had equal or better predictive power compared to the deep learning multilayer perceptron models and the simple linear regression models. Moreover, decision tree and random forest models enable the extraction of feature importance, which is a powerful tool in guiding experimental research. Analysis of the models showed the complex interaction between the applied potential and catalysts on the effective rate for the NRR. We also suggest some underexplored catalysts–electrolyte combinations to experimental researchers looking to improve both the rate and efficiency of the NRR reaction.