How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

Density Functional Theory Study of the Reaction Mechanism for Competitive Carbon−Hydrogen and Carbon−Halogen Bond Activations Catalyzed by Transition Metal Complexes

Carbon−hydrogen and carbon−halogen bond activations between halobenzenes and metal centers were studied by density functional theory with the nonempirical meta-GGA Tao−Perdew−Staroverov−Scuseria functional and an all-electron correlation-consistent polarized valence double-ζ basis set. Our calculations demonstrate that the hydrogen on the metal center and halogen in halobenzene could exchange directly through a kite-shaped transition state. Transition states with this structure were previously predicted to have high energy barriers (J. Am. Chem. Soc. 2005, 127, 279), and this prediction misled others in proposing a mechanism for their recent experimental study (J. Am. Chem. Soc. 2006, 128, 3303). Furthermore, other halo−carbon activation pathways were found in the detailed mechanism for the competitive reactions between cationic titanium hydride complex [Cp*(tBu3PN)TiH]+ and chlorobenzene under different pressure of H2. These pathways include the ortho-C−H and Ti−H bond activations for the formation and release of H2 and the indirect C−Cl bond activation via β-halogen elimination for the movement of the C6H4 ring and the formation of a C−N bond in the observed final product. A new stable isomer of the observed product with a similar total energy and an unexpected bridging between the Cp* ring and the metal center by a phenyl ring is also predicted

Monoiron Hydrogenase Catalysis: Hydrogen Activation with the Formation of a Dihydrogen, Fe−H^δ−···H^δ+−O, Bond and Methenyl-H₄MPT⁺ Triggered Hydride Transfer

A fully optimized resting state model with a strong Fe−Hδ−···Hδ+−O dihydrogen bond for the active site of the third type of hydrogenase, [Fe]-hydrogenase, is proposed from density functional theory (DFT) calculations on the reformulated active site from the recent X-ray crystal structure study of C176A (Cys176 was mutated to an alanine) mutated [Fe]-hydrogenase in the presence of dithiothreitol. The computed vibrational frequencies for this new active site model possess an average error of only ±4.5 cm−1 with respect to the wild-type [Fe]-hydrogenase. Based on this resting state model, a new mechanism with the following unusual aspects for hydrogen activation catalyzed by [Fe]-hydrogenase is also proposed from DFT calculations. (1) Unexpected dual pathways for H2 cleavage with proton transfer to Cys176−sulfur or 2-pyridinol’s oxygen for the formation and regeneration of the resting state with an Fe−Hδ−···Hδ+−O dihydrogen bond before the appearance of methenyl-H4MPT+ (MPT+). (2) The strong dihydrogen bond in this resting state structure prevents D2/H2O exchange. (3) Only upon the arrival of MPT+ with its strong hydride affinity can D2/H2O exchange take place as the arrival of MPT+ triggers the breaking of the strong Fe−Hδ−···Hδ+−O dihydrogen bond by taking a hydride from the iron center and initiating the next H2 (D2) cleavage. This new mechanism is completely different than that previously proposed (J. Am. Chem. Soc. 2008, 130, 14036) which was based on an active site model related to an earlier crystal structure. Here, Fe’s role is H2 capture and hydride formation without MPT+ while the pyridone’s special role involves the protection of the hydride by the dihydrogen bond

How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

How Electron Flow Controls the Thermochemistry of the Addition of Olefins to Nickel Dithiolenes: Predictions by Density Functional Theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored

Kinetic C−H Oxidative Addition vs Thermodynamic C−X Oxidative Addition of Chlorobenzene by a Neutral Rh(I) System. A Density Functional Theory Study

Density functional theory (DFT) is used to explore competitive C−H and C−Cl oxidative additions (OA) of chlorobenzene by the neutral Rh(I) complex: (PNP)RhI [PNP = bis(Z-2-(dimethylphosphino)vinyl)amino]. Consistent with experimental results, our calculation shows that C−Cl OA (ΔG‡ = 16.0 kcal·mol−1) is kinetically competitive with C−H OA (ΔG‡ = 16.7 kcal·mol−1) and that the C−Cl OA is thermodynamically preferred by 28.3 kcal·mol−1 over the most stable C−H OA product. Hence, the only experimentally observed product was from C−Cl OA