Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies

We synthesized two dinuclear copper complexes, which have ionizable <i>N</i> imidazole and <i>N</i> pyrazole protons in the ligand, respectively, and determined the BDFE of the hypothetical H atom transfer reactions Cu^II(LH_–1) + H^• ↔ Cu^I(L) in MeOH/H₂O (BDFE: bond dissociation Gibbs (free) energy). The ligands have two adjacent <i>N</i>,<i>N</i>′,<i>O</i>-binding pockets, which differ in one <i>N-</i>heterocycle: L^a has an imidazole unit and L^c, a pyrazole unit. The copper­(II) complexes of L^a and L^c have been characterized, and the substitution pattern has only little influence on the structural properties. The BDFEs of the hypothetical PCET reactions have been determined by means of the species distribution and the redox potentials of the involved species in MeOH/H₂O (80/20 by weight). The pyrazole copper complex <b>3</b> exhibits a lower BDFE than the isoelectronic imidazole copper complex <b>1</b> (<b>1</b>, 292(3) kJ mol^–1; <b>3</b>, 279(1) kJ mol^–1). The difference is mainly caused by the higher acidity of the <i>N</i> pyrazole proton of <b>3</b> compared to the <i>N</i> imidazole proton of <b>1</b>. The redox potentials of <b>1</b> and <b>3</b> are very similar

Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies

We synthesized two dinuclear copper complexes, which have ionizable <i>N</i> imidazole and <i>N</i> pyrazole protons in the ligand, respectively, and determined the BDFE of the hypothetical H atom transfer reactions Cu^II(LH_–1) + H^• ↔ Cu^I(L) in MeOH/H₂O (BDFE: bond dissociation Gibbs (free) energy). The ligands have two adjacent <i>N</i>,<i>N</i>′,<i>O</i>-binding pockets, which differ in one <i>N-</i>heterocycle: L^a has an imidazole unit and L^c, a pyrazole unit. The copper­(II) complexes of L^a and L^c have been characterized, and the substitution pattern has only little influence on the structural properties. The BDFEs of the hypothetical PCET reactions have been determined by means of the species distribution and the redox potentials of the involved species in MeOH/H₂O (80/20 by weight). The pyrazole copper complex <b>3</b> exhibits a lower BDFE than the isoelectronic imidazole copper complex <b>1</b> (<b>1</b>, 292(3) kJ mol^–1; <b>3</b>, 279(1) kJ mol^–1). The difference is mainly caused by the higher acidity of the <i>N</i> pyrazole proton of <b>3</b> compared to the <i>N</i> imidazole proton of <b>1</b>. The redox potentials of <b>1</b> and <b>3</b> are very similar

(Electro)chemical N₂ Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]−, 1–, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3– is also accessible by electrochemical reduction of 1– under a N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an “overreduced” but more stable [(L(N2)2Mo0)2μ-N2]2– dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials

Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO₂ Reduction

Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, <b>1</b>, [Re₂(L)­(CO)₆Cl₂], with a proton responsive ligand and its application as a catalyst in the electrochemical CO₂ reduction reaction (L = 4-<i>tert</i>-butyl-2,6-bis­(6-(1<i>H</i>-imidazol-2-yl)-pyridin-2-yl)­phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as α-diimine donors. The complex is an active catalyst for the electrochemical CO₂ reduction reaction. CO is the main product after catalysis, and only small amounts of H₂ were observed, which can be related to the ligand reactivity. The <i>i</i>_c/<i>i</i>_p ratio of 20 in dimethylformamide (DMF) + 10% water for <b>1</b> points to a higher activity with regard to [Re­(bpy)­(CO)₃Cl] in MeCN/H₂O, albeit <b>1</b> requires a slightly larger overpotential (bpy = 2,2′-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of <b>1</b>. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO₂. The first 2e^– reduction is ligand based and leads to homolytic N–H bond cleavage reactions at the imidazole units of <b>1</b>, which is equal to a net double proton removal from <b>1</b> forming [Re₂(LH_–2)­(CO)₆Cl₂]^2–. The second 2e^– reduction process has been identified as an O–H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re₂(LH_–3)­(CO)₆Cl]^2–. In the presence of CO₂, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO₂ instead of O–H bond cleavage

Syntheses and Anion Binding Capabilities of Bis(diarylboryl) Ferrocenes and Related Systems

Isomeric diborylated ferrocenes featuring 1,1′-, 1,2-, and 1,3-substitution patterns have been targeted via a combination of electrophilic aromatic substitution and directed ortho-lithiation protocols. While none of these systems are competent for the Lewis acid chelation of fluoride, related systems featuring a mixed B/Si acceptor set capture 1 equiv of fluoride via a Si–F–B bridging motif

Syntheses and Anion Binding Capabilities of Bis(diarylboryl) Ferrocenes and Related Systems

Isomeric diborylated ferrocenes featuring 1,1′-, 1,2-, and 1,3-substitution patterns have been targeted via a combination of electrophilic aromatic substitution and directed ortho-lithiation protocols. While none of these systems are competent for the Lewis acid chelation of fluoride, related systems featuring a mixed B/Si acceptor set capture 1 equiv of fluoride via a Si–F–B bridging motif

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands