Methane C–H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study

This paper reports a density functional theory study of 3d transition-metal methoxide complexes with potentially redox-noninnocent pincer supporting ligands for methane C–H bond activation to form methanol (L_<i>n</i>M-OMe + CH₄ → L_<i>n</i>M–Me + CH₃OH). The three types of tridentate pincer ligands [terpyridine (NNN), bis­(2-pyridyl)­phenyl-<i>C</i>,<i>N</i>,<i>N</i>′ (NCN), and 2,6-bis­(2-phenyl)­pyridine-<i>N</i>,<i>C</i>,<i>C</i>′ (CNC)] and different first-row transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the reaction mechanism as well as the effect of the metal identity on the thermodynamics and kinetics of a methane activation reaction. Spin-density analysis indicates that some of these systems, the NNN and NCN ligands, have redox-noninnocent character. A four-centered, kite-shaped transition state, σ-bond metathesis, or oxidative hydrogen migration has been found for methane activation for the complexes studied. Calculations suggest that the d electron count is a more significant factor than the metal formal charge in controlling the thermodynamics and kinetics of C–H activation and late 3d metal methoxides, with high d counts preferred. Notably, early-to-middle metals tend toward oxidative hydrogen migration and late metals undergo a pathway that is more akin to σ-bond metathesis, suggesting that metal methoxide complexes that favor σ-bond metathesis pathways for methane activation will yield lower barriers for C–H activation

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

Effect of Ancillary Ligands (A) on Oxidative Addition of CH₄ to Rhenium(III) Complexes: A = B, Al, CH, SiH, N, and P Using MP2, CCSD(T), and MCSCF Methods

A computational study of oxidative addition (OA) of methane to Re­(OC₂H₄)₃A (A = ancillary ligand, which thus may interact with the metal) was carried out. The choice of ancillary ligands has been made based on their electronic properties: A = B or Al (Lewis acid), CH or SiH (electron precise), N (σ-donor), and P (σ-donor/π-acid). The main objective of this study was to understand how variation in A affects the structural and electronic properties of the reactant d⁴-Re­(III) complex, which can ultimately tune the kinetics and thermodynamics of OA. Results obtained from MP2 calculations revealed that, for OA of CH₄ to Re­(OC₂H₄)₃A, the order of Δ<i>G</i>^‡ for a choice of ancillary ligand is B > Al > SiH > CH > N > P. Single point calculations for Δ<i>G</i>^‡ obtained with CCSD­(T) showed excellent agreement with those computed with MP2 methods. MCSCF calculations indicated that oxidative addition transition states are well described by a single electronic configuration, giving further confidence in the MP2 approach used for geometry optimization and Δ<i>G</i>^‡ determination, and that the transition states are more electronically similar to the d⁴-Re­(III) reactant than the d²-Re­(V) product

Control of C–H Bond Activation by Mo-Oxo Complexes: p<i>K</i>_a or Bond Dissociation Free Energy (BDFE)?

A density functional theory (DFT) study (BMK/6-31+G­(d)) was initiated to investigate the activation of benzylic carbon–hydrogen bonds by a molybdenum-oxo complex with a potentially redox noninnocent supporting liganda simple mimic of the active species of the enzyme ethylbenzene dehydrogenase (EBDH)through deprotonation (C–H bond heterolysis) or hydrogen atom abstraction (C–H bond homolysis) routes. Activation free-energy barriers for neutral and anionic Mo-oxo complexes were high, but lower for anionic complexes than neutral complexes. Interesting trends as a function of substituents were observed that indicated significant H^δ+ character in the transition states (TS), which was further supported by the preference for [2 + 2] addition over HAA for most complexes. Hence, it was hypothesized that C–H activation by these EBDH mimics is controlled more by the p<i>K</i>_a than by the bond dissociation free energy of the C–H bond being activated. Therefore, the results suggest promising pathways for designing more efficient and selective catalysts for hydrocarbon oxidation based on EBDH active-site mimics

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

Methane C–H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study

This paper reports a density functional theory study of 3d transition-metal methoxide complexes with potentially redox-noninnocent pincer supporting ligands for methane C–H bond activation to form methanol (L_<i>n</i>M-OMe + CH₄ → L_<i>n</i>M–Me + CH₃OH). The three types of tridentate pincer ligands [terpyridine (NNN), bis­(2-pyridyl)­phenyl-<i>C</i>,<i>N</i>,<i>N</i>′ (NCN), and 2,6-bis­(2-phenyl)­pyridine-<i>N</i>,<i>C</i>,<i>C</i>′ (CNC)] and different first-row transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the reaction mechanism as well as the effect of the metal identity on the thermodynamics and kinetics of a methane activation reaction. Spin-density analysis indicates that some of these systems, the NNN and NCN ligands, have redox-noninnocent character. A four-centered, kite-shaped transition state, σ-bond metathesis, or oxidative hydrogen migration has been found for methane activation for the complexes studied. Calculations suggest that the d electron count is a more significant factor than the metal formal charge in controlling the thermodynamics and kinetics of C–H activation and late 3d metal methoxides, with high d counts preferred. Notably, early-to-middle metals tend toward oxidative hydrogen migration and late metals undergo a pathway that is more akin to σ-bond metathesis, suggesting that metal methoxide complexes that favor σ-bond metathesis pathways for methane activation will yield lower barriers for C–H activation

Mapping the Basicity of Selected 3d and 4d Metal Nitrides: A DFT Study

Nitride complexes have been invoked as catalysts and intermediates in a wide variety of transformations and are noted for their tunable acid/base properties. A density functional theory study is reported herein that maps the basicity of 3d and 4d transition metals that routinely form nitride complexes: V, Cr, Mn, Nb, Mo, Tc, and Ru. Complexes were gathered from the Cambridge Structural Database, and from the free energy of protonation, the pKb(N) of the nitride group was calculated to quantify the impact of metal identity, oxidation state, coordination number, and supporting ligand type upon metal-nitride basicity. In general, the basicity of transition metal nitrides decreases from left to right across the 3d and 4d rows and increases from 3d metals to their 4d congeners. Metal identity and oxidation state primarily determine basicity trends; however, supporting ligand types have a substantial impact on the basicity range for a given metal. Synergism of these factors in determining the overall pKb(N) values is discussed, as are the implications for the catalytic reactivity of metal nitrides

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-<i>ⁱ</i>Pr₂C₆H₃, P₃ = P₃-SnPh₃) when treated with pyridine-<i>N</i>-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

Computational Study of Methane C–H Activation by Earth-Abundant Metal Amide/Aminyl Complexes

Density functional theory, augmented by multiconfiguration SCF (MCSCF) simulations, was used to understand the factors that control methane C–H activation by Earth-abundant, 3d metal (Cr - Ni) [(κ³-CNC)­M­(NH₂)] complexes via hydrogen atom abstraction (HAA) and [2 + 2] pathways. Calculations suggest a significant amide/aminyl, i.e., [(κ³-CNC)^2–M³⁺(NH₂)⁻] ⇔ [(κ³-CNC)^2–M²⁺(NH₂)^•], admixture in the electronic ground states of these complexes and thus significant unpaired electron density (radical character) on the NH₂ ligand. The spin coupling between the aminyl radical and spin density on the central metal ion is interesting, particularly for the cobalt aminyl complex, in which both ferromagnetic and antiferromagnetic triplet states are found to be close in energy via both DFT and MCSCF methods. Modeled complexes are computed to have reasonable barriers to methane activation, with Δ<i>G</i>^⧧ values being in approximately the upper 20s to mid 30s kcal/mol, generally decreasing toward the right in the 3d series, which loosely tracks with spin density (radical character) on the aminyl nitrogen, a switch from [2 + 2] to HAA activation pathways, and more favorable thermodynamics for C–H scission

C–H Bond Activation of Methane by Pt^II–N-Heterocyclic Carbene Complexes. The Importance of Having the Ligands in the Right Place at the Right Time

A DFT study of methane C–H activation barriers for neutral NHC–Pt^II–methoxy complexes yielded 22.8 and 26.1 kcal/mol for oxidative addition (OA) and oxidative hydrogen migration (OHM), respectively. Interestingly, this is unlike the case for cationic NHC–Pt^II–methoxy complexes, whereby OHM entails a calculated barrier of 26.9 kcal/mol but the OA barrier is only 14.4 kcal/mol. Comparing transition state (TS) and ground state (GS) geometries implies an ∼10 kcal/mol “penalty” to the barriers arising from positioning the NHC and OMe ligands into a relative orientation that is preferred in the GS to the orientation that is favored in the TS. The results thus imply an intrinsic barrier arising from C–H scission of ∼15 ± 2 kcal/mol for NHC–Pt^II–methoxy complexes. Calculations show the importance of designing C–H activation catalysts where the GS active species is already structurally “prepared” and which either does not need to undergo any geometric perturbations to access the methane C–H activation TS or is not energetically prohibited from such perturbations