Theoretical and computational studies of organometallic reactions: successful or not?

A computational study of cobalt-catalyzed C–H iodination reactions using a bidentate directing group with molecular iodine

A computational methodology was used to collect detailed mechanistic information on the cobalt-catalyzed C–H iodination of aromatic amides with molecular iodine using an N,N′-bidentate directing group.

Activation mechanism of hydrogen peroxide by a divanadium-substituted polyoxometalate [γ-PV2W10O38(μ-OH)2]3-: A computational study

In the present paper, the reaction mechanism corresponding to activation of hydrogen peroxide (H₂O₂) by a divanadium-substituted polyoxometalate (POM) [γ-PV₂W₁₀O₃₈(μ-OH)₂]^3- (I) to form catalytic active species, peroxo complex [γ-PV₂W₁₀O₃₈(μ-η²,η²-O₂)]^3- (III), was studied by using the density functional theory (DFT) calculations method with B3LYP functional. The results indicate that coordination of H₂O₂ to I proceeds via a vanadium-center-assisted proton transfer pathway to remove the first water molecule and form a hydroperoxy intermediate [γ-PV₂W₁₀O₃₈(μ-OH) (μ-OOH)]^3- (II). And intermediate II occurs through three successive water-assisted proton transfer steps to remove the second water molecule and finally forms catalytic active species. The calculated overall energy profiles show that coordination of H₂O₂ to vanadium center requires a proton transfer barrier of about 24 kcal mol^-1. A detailed comparison of molecular geometries and electronic structure shows that the catalytic active species has a very interesting structural feature, where a superoxide radical (O₂^-) was embedded into two vanadium centers, and may be a potential nucleophile. The unique withdrawing electron properties and flexible bonding ability of the γ-Keggin-type POM ligand contribute to the formation of O₂^- radical. The tunable alternate arrangement of W-O bond series in γ-Keggin-type POM ligand contributes to the flexibility of the γ-Keggin-type POM ligand. Meanwhile, our DFT calculations show a good performance of B3LYP-gauge-independent atomic orbital (IGAIM) method for the calculation of ¹H NMR parameters of divanadium-substituted phosphotungstate.

Mechanistic Insights into the Directed Hydrogenation of Hydroxylated Alkene Catalyzed by Bis(phosphine)cobalt Dialkyl Complexes

The mechanism of directed hydrogenation of hydroxylated alkene catalyzed by bis(phosphine)cobalt dialkyl complexes has been studied by DFT calculations. The possible reaction channels of alkene hydrogenation catalyzed by catalytic species (0_T, 0_P, and 0) were investigated. The calculated results indicate that the preferred catalytic activation processes undergo a 1,2 alkene insertion. 0_P and 0 prefer the β hydrogen elimination mechanism with an energy barrier of 9.5 kcal/mol, and 0_T prefers the reductive elimination mechanism with an energy barrier of 11.0 kcal/mol. The second H₂ coordination in the σ bond metathesis mechanism needs to break the agostic H²-βC bond of metal-alkyl intermediates (2_1P and 2_1T), which owns the larger energetic span compared to the reductive elimination. This theoretical study shows that the most favorable reaction pathway of alkene hydrogenation is the β hydrogen elimination pathway catalyzed by the planar (dppe)CoH₂. The hydrogenation activity of Co(II) compounds with redox-innocent phosphine donors involves the Co(0)-Co(II) catalytic mechanism.

A Quantum-Chemical DFT Approach to Elucidation of the Chirality Transfer Mechanism of the Enantioselective Suzuki–Miyaura Cross-Coupling Reaction

The DFT calculations of the simplified model of the asymmetric Suzuki–Miyaura coupling reaction were performed at the M062x/LANL2DZ theory level at first. It was found that enantioselective reactions mediated by the palladium complexes of chiral C,P-ligands follow a four-stage mechanism similar to that proposed previously as one of the most credible mechanisms. It should be underlined that the presence of substituents in the substrates and the chiral ligand at ortho positions determines the energies of possible diastereoisomeric transition states and intermediates in initial reaction steps. This suggests that, in practice, a sharp selection of theoretically possible paths of chirality transfer from the catalyst to the product should have a place and, therefore, the absolute configuration of the formed atropisomeric product is defined and can be predicted.

Heterolytic activation of dihydrogen molecule by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex. Theoretical insights

Heterolytic activation of dihydrogen molecule (H2) by hydroxo-/sulfido-bridged ruthenium-germanium dinuclear complex [Dmp(Dep)Ge(μ-S)(μ-OH)Ru(PPh3)](+) (1) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl) is theoretically investigated with the ONIOM(DFT:MM) method. H2 approaches 1 to afford an intermediate [Dmp(Dep)(HO)Ge(μ-S)Ru(PPh3)](+)-(H2) (2). In 2, the Ru-OH coordinate bond is broken but H2 does not yet coordinate with the Ru center. Then, the H2 further approaches the Ru center through a transition state TS2-3 to afford a dihydrogen σ-complex [Dmp(Dep)(HO)Ge(μ-S)Ru(η(2)-H2)(PPh3)](+) (3). Starting from 3, the H-H σ-bond is cleaved by the Ru and Ge-OH moieties to form [Dmp(Dep)(H2O)Ge(μ-S)Ru(H)(PPh3)](+) (4). In 4, hydride and H2O coordinate with the Ru and Ge centers, respectively. Electron population changes clearly indicate that this H-H σ-bond cleavage occurs in a heterolytic manner like H2 activation by hydrogenase. Finally, the H2O dissociates from the Ge center to afford [Dmp(Dep)Ge(μ-S)Ru(H)(PPh3)](+) (PRD). This step is rate-determining. The activation energy of the backward reaction is moderately smaller than that of the forward reaction, which is consistent with the experimental result that PRD reacts with H2O to form 1 and H2. In the Si analogue [Dmp(Dep)Si(μ-S)(μ-OH)Ru(PPh3)](+) (1Si), the isomerization of 1Si to 2Si easily occurs with a small activation energy, while the dissociation of H2O from the Si center needs a considerably large activation energy. Based on these computational findings, it is emphasized that the reaction of 1 resembles well that of hydrogenase and the use of Ge in 1 is crucial for this heterolytic H-H σ-bond activation.

Cationic intermediates in the Pd-catalyzed negishi coupling. kinetic and density functional theory study of alternative transmetalation pathways in the Me-Me coupling of ZnMe2 and trans-[PdMeCl(PMePh2)2]

The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh(2))(2)] and ZnMe(2) in THF as solvent was analyzed. The results reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh(2))(2)(THF)](+)) opens ionic transmetalation pathways. In contrast, an excess of phosphine retards the reaction because of the formation of a very stable cationic complex with three phosphines ([PdMe(PMePh(2))(3)](+)) that sequesters the catalyst. These ionic intermediates had never been observed or proposed in palladium Negishi systems and warn on the possible detrimental effect of an excess of good ligand (as PMePh(2)) for the process. In contrast, the ionic pathways via cationic complexes with one solvent (or a weak ligand) can be noticeably faster and provide a more rapid reaction than the concerted pathways via neutral intermediates. Theoretical calculations on the real molecules reproduce well the experimental rate trends observed for the different mechanistic pathways.

Noble reaction features of bromoborane in oxidative addition of B-Br σ-bond to [M(PMe3)2] (M=Pt or Pd): theoretical study

Through detailed calculations by density functional theory and second-order Møller-Plesset perturbation theory (MP2) to fourth-order Møller-Plesset perturbation theory including single, double, and quadruple excitations [MP4(SDQ)] methods, we investigated the oxidative addition of the B-Br bond of dibromo(trimethylsiloxy)borane [Br(2)B(OSiMe(3))] to Pt(0) and Pd(0) complexes [M(PMe(3))(2)] (M = Pt or Pd) directly yielding a trans bromoboryl complex trans-[MBr{BBr(OSiMe(3))}(PMe(3))(2)]. Two reaction pathways are found for this reaction: One is a nucleophilic attack pathway which directly leads to the trans product, and the other is a stepwise reaction pathway which occurs through successive cis oxidative addition of the B-Br bond to [M(PMe(3))(2)] and thermal cis-trans isomerization. In the Pt system, the former course occurs with a much smaller energy barrier (E(a) = 5.8 kcal/mol) than the latter one (E(a) = 20.7 kcal/mol), where the DFT-calculated E(a) value is presented hereafter. In the Pd system, only the latter course is found in which the rate-determining steps is the cis-trans isomerization with the E(a) of 15.1 kcal/mol. Interestingly, the thermal cis-trans isomerization occurs on the singlet potential energy surface against our expectation. This unexpected result is understood in terms of the strong donation ability of the boryl group. Detailed analyses of electronic processes in all these reaction steps as well as remarkable characteristic features of [Br(2)B(OSiMe(3))] are also provided.