Electrochemical Reduction of Carbon Dioxide to Methanol in the Presence of Benzannulated Dihydropyridine Additives

Dihydropyridines (DHPs) have been postulated as active intermediates in the pyridine-mediated electrochemical conversion of CO₂ to methanol; however, the ability of isolated DHPs to facilitate methanol production in a fashion similar to that of their parent aromatic <i>N</i>-heterocycles (ANHs) has not been tested. Here, we use bulk electrolysis to show that 1,2- and 1,4-DHPs (1,2-dihydro­phenanthridine and 9,10-dihydro­acridine) can mediate the substoichiometric electrochemical reduction of CO₂ to methanol and formate with Faradaic efficiencies similar to those of the corresponding ANHs at Pt electrodes. 1,2-Dihydro­phenanthridine furthermore exhibits improved CO₂ reduction activity compared to its parent ANH (phenanthridine) at glassy carbon electrodes. These results provide the first experimental evidence for the participation of DHPs as additives in electrochemical CO₂ reduction

Aryl Ether Cleavage by Group 9 and 10 Transition Metals: Stoichiometric Studies of Selectivity and Mechanism

The reactivity of terphenyl diphosphines bearing aryl–methyl ether or aryl–aryl ether moieties with M⁰ (M = Ni, Pd, Pt), M′^I (M′ = Co, Rh, Ir), or M^II centers was investigated to gain mechanistic insight into intramolecular aryl–ether bond cleavage in structurally related metal complexes. Rh^I converts the aryl–methyl ether moiety to an aryl C–H bond. This is similar to reactivity previously observed at Ni⁰ that involves C–O oxidative addition, β-H elimination liberating CH₂O, reductive elimination of an aryl C–H bond, and decarbonylation of CH₂O. Ir^I leads to unselective aryl and alkyl C–O bond activation. In the presence of excess CO, Rh^I and Ir^I display a shift in selectivity and reactivity and cleave the alkyl C–O bond. Co^I does not perform C–O cleavage. Alkyl C–O bond activation was observed with M^II–halide complexes with loss of MeCl via a Lewis acid–base mechanism. Pd⁰ and Pt⁰ cleave selectively the O–Me bond via oxidative addition. With a diaryl ether moiety, Pd⁰ and Pt⁰ are found to be capable of performing aryl C–O bond activation. Various levels of interactions between the central arene and the metal center were observed, and these were correlated with trends in bond activation. Overall, selective cleavage of the stronger aryl ether C–O bond was observed only with Ni⁰ and Rh^I. Pd⁰ and Pt⁰ can perform aryl ether C–O cleavage, but if available, they will cleave the weaker O–Me bond. This study provides insight into the relative reactivity of group 9 and 10 metal centers with aryl ether bonds and suggests future directions for designing systems for metal-catalyzed cleavage of ether C–O bonds in synthetic methodology as well as lignin deoxygenation

Covalent Attachment of Ferrocene to Silicon Microwire Arrays

A fully integrated, freestanding device for photoelectrochemical fuel generation will likely require covalent attachment of catalysts to the surface of the photoelectrodes. Ferrocene has been utilized in the past as a model system for molecular catalyst integration on planar silicon; however, the surface structure of high-aspect ratio silicon microwires envisioned for a potential device presents potential challenges with respect to stability, characterization, and mass transport. Attachment of vinylferrocene to Cl-terminated surfaces of silicon microwires was performed thermally. By varying the reaction time, solutions of vinylferrocene in di-<i>n-</i>butyl ether were employed to control the extent of functionalization. X-ray photoelectron spectroscopy (XPS) and electrochemistry were used to estimate the density and surface coverage of the silicon microwire arrays with ferrocenyl groups, which could be controllably varied from 1.23 × 10^–11 to 4.60 × 10^–10 mol cm^–2 or 1 to 30% of a monolayer. Subsequent backfill of the remaining Si–Cl sites with methyl groups produced ferrocenyl-terminated surfaces that showed unchanged cyclic volammograms following two months in air, under ambient conditions, and repeated electrochemical cycling

Phenanthridine-Containing Pincer-like Amido Complexes of Nickel, Palladium, and Platinum

Proligands based on bis­(8-quinolinyl)­amine (<b>L1</b>) were prepared containing one (<b>L2</b>) and two (<b>L3</b>) benzo-fused N-heterocyclic phenanthridinyl (3,4-benzoquinolinyl) units. Taken as a series, <b>L1</b>–<b>L3</b> provides a ligand template for exploring systematic π-extension in the context of tridentate pincer-like amido complexes of group 10 metals (<b>1-M</b>, <b>2-M</b>, and <b>3-M</b>; <b>M</b> = Ni, Pd, Pt). Inclusion of phenanthridinyl units was enabled by development of a cross-coupling/condensation route to 6-unsubstituted, 4-substituted phenanthridines (<b>4-Br</b>, <b>4-NO</b>_<b>2</b>, <b>4-NH</b>_<b>2</b>) suitable for elaboration into the target ligand frameworks. Complexes <b>1-M</b>, <b>2-M</b>, and <b>3-M</b> are redox-active; electrochemistry and UV–vis absorption spectroscopy were used to investigate the impact of π-extension on the electronic properties of the metal complexes. Unlike what is typically observed for benzannulated ligand–metal complexes, extending the π-system in metal complexes <b>1-M</b> to <b>2-M</b> to <b>3-M</b> led to only a moderate red shift in the relative highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap as estimated by electrochemistry and similarly subtle changes to the onset of the lowest-energy absorption observed by UV–vis spectroscopy. Time-dependent density functional theory calculations revealed that benzannulation significantly impacts the atomic contributions to the LUMO and LUMO+1 orbitals, altering the orbital contributions to the lowest-energy transition but leaving the energy of this transition essentially unchanged

Dipalladium(I) Terphenyl Diphosphine Complexes as Models for Two-Site Adsorption and Activation of Organic Molecules

A <i>para</i>-terphenyl diphosphine was employed to support a dipalladium­(I) moiety. Unlike previously reported dipalladium­(I) species, the present system provides a single molecular hemisphere for binding of ligands across two metal centers, enabling the characterization and comparison of the binding of a wide variety of saturated and unsaturated organic molecules. The dipalladium­(I) terphenyl diphosphine toluene-capped complex was synthesized from a dipalladium­(I) hexaacetonitrile precursor in the presence of toluene. The palladium centers display interactions with the π-systems of the central ring of the terphenyl unit and that of the toluene. Exchange of toluene for anisole, 1,3-butadiene, 1,3-cyclohexadiene, thiophenes, pyrroles, or furans resulted in well-defined π-bound complexes which were studied by crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory. Structural characterization shows that the interactions of the dipalladium unit with the central arene of the diphosphine does not vary significantly in this series allowing for a systematic comparison of the binding of the incoming ligands to the dipalladium moiety. Several of the complexes exhibit rare μ–η²:η² or μ–η²:η¹(O or S) bridging motifs. Hydrogenation of the thiophene and benzothiophene adducts was demonstrated to proceed at room temperature. The relative binding strength of the neutral ligands was determined by competition experiments monitored by NMR spectroscopy. The relative equilibrium constants for ligand substitution span over 13 orders of magnitude. This represents the most comprehensive analysis to date of the relative binding of heterocycles and unsaturated ligands to bimetallic sites. Binding interactions were computationally studied with electrostatic potentials and molecular orbital analysis. Anionic ligands were also demonstrated to form π-bound complexes

Dipalladium(I) Terphenyl Diphosphine Complexes as Models for Two-Site Adsorption and Activation of Organic Molecules

A <i>para</i>-terphenyl diphosphine was employed to support a dipalladium­(I) moiety. Unlike previously reported dipalladium­(I) species, the present system provides a single molecular hemisphere for binding of ligands across two metal centers, enabling the characterization and comparison of the binding of a wide variety of saturated and unsaturated organic molecules. The dipalladium­(I) terphenyl diphosphine toluene-capped complex was synthesized from a dipalladium­(I) hexaacetonitrile precursor in the presence of toluene. The palladium centers display interactions with the π-systems of the central ring of the terphenyl unit and that of the toluene. Exchange of toluene for anisole, 1,3-butadiene, 1,3-cyclohexadiene, thiophenes, pyrroles, or furans resulted in well-defined π-bound complexes which were studied by crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory. Structural characterization shows that the interactions of the dipalladium unit with the central arene of the diphosphine does not vary significantly in this series allowing for a systematic comparison of the binding of the incoming ligands to the dipalladium moiety. Several of the complexes exhibit rare μ–η²:η² or μ–η²:η¹(O or S) bridging motifs. Hydrogenation of the thiophene and benzothiophene adducts was demonstrated to proceed at room temperature. The relative binding strength of the neutral ligands was determined by competition experiments monitored by NMR spectroscopy. The relative equilibrium constants for ligand substitution span over 13 orders of magnitude. This represents the most comprehensive analysis to date of the relative binding of heterocycles and unsaturated ligands to bimetallic sites. Binding interactions were computationally studied with electrostatic potentials and molecular orbital analysis. Anionic ligands were also demonstrated to form π-bound complexes

Boryl/Borane Interconversion and Diversity of Binding Modes of Oxygenous Ligands in PBP Pincer Complexes of Rhodium

A series of Rh complexes derived from a PBP-type pincer ligand have been synthesized and characterized. It was previously reported that reaction of [(COD)­RhCl]₂ with 2,2′-bis­(diisopropylphino)­triphenylborane (<b>1</b>) resulted in a mixture of complexes containing a <i>Z</i>-type borane interaction (<b>2-Cl</b>), a boryl pincer (<b>3a-Cl</b>), and a η² binding of the B–Ph bond to Rh (<b>4-Cl</b>). In this work, we demonstrate that analogous complexes are accessible by replacement of chloride with potentially bidentate acetylacetonate, carboxylate, and trifluoromethanesulfonate ligands. In addition, a new type of isomer was observed in complexes with acetate and pivalate, where the carboxylate bridges between Rh and B (<b>3b-OAc</b>, <b>3b-OPiv</b>). All of these types of complexes are isomeric, and the preference for particular isomers for different anionic ligands varies. These isomers differ and are related by a change in the coordination mode of the oxygenous ligands and the migration of the Ph group between B and Rh

Site-Selective Benzannulation of <i>N</i>‑Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [(<i>P^N</i>)Cu]₂(μ-X)₂ Dimers

Benzannulated bidentate pyridine/phosphine (<i>P^N</i>) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu­(I) complexes of the form [(<i>P^N</i>)­Cu]₂(μ-X)₂. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu₂X₂ cores. Comparing the photophysical properties of complexes of (quinolinyl)­phosphine ligands with those of π-extended (phenanthridinyl)­phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure

A Titration Method for Standardization of Aqueous Sodium Chlorite Solutions Using Thiourea Dioxide

Accurate and cost-effective methods for the analysis of oxychlorine compounds in water are critical to modern chlorine-based water treatment. With alternatives to elemental chlorine and hypochlorite bleaches growing in popularity, simple quantification methods for the disinfectant chlorine dioxide (ClO2) in water, as well as chlorite (ClO2–) and chlorate (ClO3–), which are commonly used precursors in ClO2 generation, are required. However, currently, regulated standard methods require specialized equipment and do not effectively discriminate between molecular and ionic species. In this contribution, we present a simple titration-based method for chlorite determination in water using commercially available and easy-to-handle reagents. Specifically, chlorite is reduced with a slight excess of thioureadioxide (TUD). The remaining reductant is then back-titrated against a known amount of potassium permanganate, affording calculatable chlorite concentrations through measured consumption of a reductant and a clear visual endpoint upon accumulation of excess KMnO4. Straightforward methods for chlorite standardization with reasonable error and accuracy for field and/or lab application have the potential to greatly enhance quality assurance and therefore assist in resource deployment in water treatment

A Titration Method for Standardization of Aqueous Sodium Chlorite Solutions Using Thiourea Dioxide

Accurate and cost-effective methods for the analysis of oxychlorine compounds in water are critical to modern chlorine-based water treatment. With alternatives to elemental chlorine and hypochlorite bleaches growing in popularity, simple quantification methods for the disinfectant chlorine dioxide (ClO2) in water, as well as chlorite (ClO2–) and chlorate (ClO3–), which are commonly used precursors in ClO2 generation, are required. However, currently, regulated standard methods require specialized equipment and do not effectively discriminate between molecular and ionic species. In this contribution, we present a simple titration-based method for chlorite determination in water using commercially available and easy-to-handle reagents. Specifically, chlorite is reduced with a slight excess of thioureadioxide (TUD). The remaining reductant is then back-titrated against a known amount of potassium permanganate, affording calculatable chlorite concentrations through measured consumption of a reductant and a clear visual endpoint upon accumulation of excess KMnO4. Straightforward methods for chlorite standardization with reasonable error and accuracy for field and/or lab application have the potential to greatly enhance quality assurance and therefore assist in resource deployment in water treatment