Computational Analysis of Proton-Coupled Electron Transfer in Hydrotris(triazolyl)borate Mid–Late 3d and 4d Transition Metal Complexes

Design of electrocatalysts for the evolution of H2 and reduction of O2, N2, and CO2, as well as water splitting is essential for the development of alternative energy sources. Typically, the catalytic cycle is controlled by key proton-coupled electron transfer (PCET) processes including sequential or concerted electron transfer (ET) and proton transfer (PT) pathways. Studying the reaction free energies and free energy barriers of PCET processes can thus give insight into the design of more effective electrocatalysts. Herein, the focus is on complexes with the scorpionate ligand hydrotris­(1,2,4-triazole-1-yl)­borate (Ttz), [M­(Ttz)­(CO)3]. From the reaction free energies of the studied “PCET squares” for converting M­(0)– to M­(I)­H+, for Group 6 and 10 complexes, a sequential pathway (PT-ET over ET-PT) is predicted. However, for Group 7–9 metals, a concerted pathway (EPT) is preferred. Analyses of trends in the calculated free energy barriers and reaction free energies of 40 transition-metal complexes suggest that the metal and its electronic structure greatly affect the nature of the PCET processes

Computational Analysis of Proton-Coupled Electron Transfer in Hydrotris(triazolyl)borate Mid–Late 3d and 4d Transition Metal Complexes

Design of electrocatalysts for the evolution of H2 and reduction of O2, N2, and CO2, as well as water splitting is essential for the development of alternative energy sources. Typically, the catalytic cycle is controlled by key proton-coupled electron transfer (PCET) processes including sequential or concerted electron transfer (ET) and proton transfer (PT) pathways. Studying the reaction free energies and free energy barriers of PCET processes can thus give insight into the design of more effective electrocatalysts. Herein, the focus is on complexes with the scorpionate ligand hydrotris­(1,2,4-triazole-1-yl)­borate (Ttz), [M­(Ttz)­(CO)3]. From the reaction free energies of the studied “PCET squares” for converting M­(0)– to M­(I)­H+, for Group 6 and 10 complexes, a sequential pathway (PT-ET over ET-PT) is predicted. However, for Group 7–9 metals, a concerted pathway (EPT) is preferred. Analyses of trends in the calculated free energy barriers and reaction free energies of 40 transition-metal complexes suggest that the metal and its electronic structure greatly affect the nature of the PCET processes

Importance of Nitrogen–Hydrogen Bond p<i>K</i>_a in the Catalytic Coupling of Alkenes and Amines by Amidate Tantalum Complexes: A Computational Study

Density functional theory (DFT) was carried out to study the impact of substituents with different electronic properties upon hydrogen transfer as the rate-determining step in the hydroaminoalkylation catalytic cycle in order to determine the character of the hydrogen atom in the transition state. In the transition state of the rate-determining step, an N-methylaniline substrate ligates to Ta and transfers its hydrogen to the α-carbon of a five-membered tantallacycle and a Ta–C bond is thus broken. Study of the activation energy barriers resulting from the different para- and meta-substituted N-methylanilines and their correlation with computed pKa and bond dissociation free energy (BDFE) values of the N-methylanilines show more obvious correlations between pKa and ΔG‡ values. Assessing the asynchronicity parameter (η) for the studied substituents reveals that pKa is a larger driving force in the rate-determining hydrogen transfer reaction than the BDFE, which suggest a reasonable amount of protic character in the transition state, and possible routes to the design of more active catalysts with greater substrate scope

Effect of Appended S‑Block Metal Ion Crown Ethers on Redox Properties and Catalytic Activity of Mn–Nitride Schiff Base Complexes: Methane Activation

Using density functional theory (DFT), the effects of appended s-block metal ion crown ethers upon the redox properties of the following nitridomanganese­(V) salen complexes were investigated: [(salen)­MnV(N)­(Mn+-crown ether)]n+ (salen = N,N′-bis­(salicydene)­ethylenediamine; M = Na+, K+, Ba2+, and Sr2+ for 1Na, 1K, 1Ba, and 1Sr, respectively; A = complex without Mn+-crown ether and B = without Mn+). NBO analysis of the MnN bond orders, optimized bond lengths, and stretching frequencies changes upon oxidation for all species show that for A, B, and 1Na MnN has more nitridyl character while a nitride form is more significant for 1K, 1Ba, and 1Sr. The results reveal that ΔGrxn(e–) and thus E1/2 are quite sensitive to the point charge (q) of the s-block metal ions (1 for K+/Na+ and 2 for Ba2+/Sr2+). Computations suggest that the degree of delocalization of the HOMO electrons on the supporting ligand is modified by the chelated s-block metal ion. Methane activation by A•+, 1K•+, and 1Ba•+ complexes proceeds via a hydrogen atom transfer (HAT) pathway with reasonable barriers for all complexes with ∼4 kcal/mol difference in energy. The molecular electrostatic potential (MEP) maps indicate a shift in redox potential imposed by the nonredox active cations by altering the electrostatic potential of the complexes. Computations show that the complexes with higher point charge of the incorporated metal ions result in higher N–H bond BDFEs. Changes in predicted properties as a function of continuum solvent dielectric constant suggest that the primary effect of the appended s-block ion is via “through space” interactions

Modeling the Deposition of Metal Atoms on a p-Type Organometallic Conductor: Implications for Stability and Electron Transfer

A computational study of the interaction of metal atoms (M′) with cyclo-[Au(μ-Pz)]3 trimer (T) (Pz = Pyrazolate ligand), a p-type organometallic semiconductor is reported in this article. The metal atoms chosen for the study are both high and low work function electrode metals (M′ = Al, Au, Cu, La, Ni, Pd, Pt, Ru, Ni) used in electronic devices. Metal (M′M) and ligand (M′L) sites of the gold trimer are investigated as the possible sites of deposition for the metal atoms. Examination of metal binding, geometric, and electronic properties indicated that low work function metals La and Ti favor the ligand coordination (M′L); Al, Au, Cu, Ni, Pt, and Ru favor coordination to the metal (i.e., gold) site of the trimer. Pd has equal stability at both the M′L and the M′M sites of the trimer. Changes in geometry of the trimer upon deposition of the metal atom are negligible for M′M−T complexes but more change is seen for M′L−T complexes. All metal atoms except Pd exhibited good orbital hybridization with the gold trimer in M′−T complexes. These combinations of observations suggest that, for these metal-based, p-type conductors will form stable interfaces with good electron transfer with typical source/drain electrode metals

Density Functional Study of Oxygen Insertion into Niobium–Phosphorus Bonds: Novel Mechanism for Liberating P₃^– Synthons

We explore the mechanism of oxygen insertion into niobium–phosphorus bonds to liberate synthetically relevant, phosphorus-containing molecules. Oxygen insertion mechanisms generally proceed through either direct oxygen insertion from an oxo ligand, MO (oxy-insertion), or an insertion of an oxygen atom from an external oxidant, OY (Baeyer–Villiger, BV). Computational methods were employed to elucidate the preferred mechanism for the liberation of the phosphorus moiety from [(η2-P3)­Nb­(ODipp)3] (Dipp = 2,6-iPr2C6H3, P3 = P3-SnPh3) when treated with pyridine-N-oxide as an external oxidant. Careful analysis of conformational isomers and energies clearly suggests that the BV mechanism is the preferred pathway toward phosphorus liberation. Once free, the P3 moiety can react with 1,3-cyclohexadiene to form the Diels–Alder product, which is also modeled in the computational study

A Computational Study of Metal-Mediated Decomposition of Nitrene Transfer Reagents

Metal-mediated decomposition to form nitrene complexes is investigated by using DFT for prototypical organic azides and iodonium imides used in organic synthesis. Each system exhibited exothermic pathways via formation of cyclic intermediates, which decompose to yield LNiNX + Y (L = bis-phosphine, NX = nitrene, Y = N2 or IPh). Also, the typical heterotransfer reagents used in organic synthesis show a greater tendency toward triplet nitrene complexes and hence the potential for metal-free reactivity than aliphatic and aromatic substituted versions

Effect of Ancillary Ligands on Oxidative Addition of CH₄ to Ta(III) Complexes Ta(OC₂H₄)₃A (A = B, Al, CH, SiH, N, P): A Density Functional Theory Study

A DFT study of oxidative addition of methane to Ta­(OC₂H₄)₃A (where A may act as ancillary ligand) was conducted to understand how A may affect the propensity of the complex to undergo oxidative addition. Among the A groups studied, they can be a Lewis acid (B or Al), a saturated, electron-precise moiety (CH or SiH), a σ-donor (N), or a σ-donor/π-acid (P). By varying A, we seek to understand how changing the electronic properties of A can affect the kinetics and thermodynamics of methane C–H activation by these complexes. For every reaction two transition states (H or CH₃ trans to A) leading to two corresponding products were identified. For all A, the TS with H trans to A is favored kinetically; except for SiH and CH, the kinetically favored product is not thermodynamically favored. For the kinetic products, the Δ<i>G</i>^⧧ values for A = B, Al are highest among the 2p and 3p elements, respectively. Upon moving from electron-deficient to electron-rich moieties (P and N) the computed C–H activation barrier for the kinetic product decreases significantly. Thus, changing A greatly influences the barrier for methane C–H oxidative addition by these complexes

Computational Mechanistic Study of Electro-Oxidation of Ammonia to N₂ by Homogenous Ruthenium and Iron Complexes

A comprehensive DFT study of the electrocatalytic oxidation of ammonia to dinitrogen by a ruthenium polypyridyl complex, [(tpy)­(bpy)­RuII(NH3)]2+ (a), and its NMe2-substituted derivative (b) is presented. The thermodynamics and kinetics of electron (ET) and proton transfer (PT) steps and transition states are calculated. NMe2 substitution on bpy reduces the ET steps on average 8 kcal/mol for complex b as compared to a. The calculations indicate that N–N formation occurs by ammonia nucleophilic attack/H-transfer via a nitrene intermediate rather than a nitride intermediate. Comparison of the free energy profiles of Ru-b with its first-row Fe congener reveals that the thermodynamics are less favorable for the Fe-b model, especially for ET steps. The N–H bond dissociation free energies (BDFEs) for NH3 to form N2 show the following trend: Ru-b Ru-a Fe-b, indicating the lowest and most favorable BDFEs for Ru-b complex