A quantum chemical study of the mechanisms of olefin addition to group 9 transition metal dioxo compounds

The mechanistic aspects of ethylene addition to MO₂(CH₂)(CH₃) (M=Co, Rh, Ir) have been investigated with a Hartree–Fock/DFT hybrid functional at the MO6/LACVP* and B3LYP/LACVP* levels of theory to elucidate the reaction pathways on the singlet, doublet and triplet potential energy surfaces (PES). In the reaction of the IrO₂CH₂CH₃ complex with ethylene, [3 + 2]_C,O addition is the most plausible pathway on the singlet PES, the [3 + 2]_O,O addition is the most favoured pathway on the doublet surface whiles the stepwise [1 + 1] addition involving the oxygen atom of the complex in the first step and the carbon atom of the complex in the second step is the most plausible pathway on the triplet PES. For the reaction of the RhO₂(CH₂)(CH₃) complex, the [2 + 2]_Rh,O addition pathway is the most favoured on the singlet surface, the [2 + 2]_Rh,C is the most plausible pathway on the triplet PES and [3 + 2]_C,O is the most plausible on the doublet surface. For the reactions of the CoO₂(CH₂)(CH₃) complex, the [1 + 2]_O addition is the most plausible on the singlet PES, [3 + 2]_C=Co=O cycloaddition to form the five–membered intermediate is the most preferred pathway on the doublet PES, whiles on the triplet PES the preferred pathway is the [3 + 2] addition across the O=Co=O bond of the metal complex. The reactions of olefins with the Co dioxo complex have lower activation barriers for the preferred [3 + 2] and [2 + 2] addition pathways as well as fewer side reactions than those of the rhodium and iridium systems. This could imply that the cobalt dioxo complexes can efficiently and selectively catalyze specific reactions in oxidation of olefins than Rh and Ir oxo complexes will do and therefore Co oxo complexes may be better catalysts for specific oxidation reactions of olefins than Rh and Ir complexes are. The activation barriers for the formation of the four—or five-membered metallacycle intermediates through [2 + 2] or [3 + 2] cyclo-addition are lower on the triplet PES than on the singlet PES for the formation of similar analogues. There are fewer competitive reaction pathways on the triplet surface than on the singlet PES. Also, cycloadditions that seem impossible on the singlet PES seem possible on the doublet and or triplet PESs, this is the case typically for the Rh and Co complexes, illustrating the importance of multiple spin states in organometallic reactions.Graphical Abstract

Exploring the peri-, chemo-, and regioselectivity of addition of technetium metal oxides of the type TcO3L (L = Cl–, O–, OCH3, CH3) to substituted ketenes: a DFT computational study

The addition of TcO3L (L = Cl, O–, OCH3, CH3) to substituted ketenes along various addition pathways was studied with density functional theory calculations to explore the peri-, chemo-, and regioselectivity of the reactions. In the reactions of TcO3L with dimethyl ketene, the results show that for L = O– and CH3, [1 + 1] addition to form a triplet zwitterionic intermediate is the preferred first step; for L = Cl, the [3 + 2]C=C addition across the O–Tc–Cl bond is the preferred first step and for L = OCH3 the [3 + 2]C=C addition across the O–Tc–OCH3 bond is the preferred first step. In the reactions of TcO3Cl with substituted ketenes, [1 + 1] addition to form a triplet zwitterionic intermediate is the preferred first step for X = Ph, CN, and Cl; the [3 + 2]C=C addition across the O–Tc–O bond of the complex is the preferred first step for X = H, while the [3 + 2]C=C addition across the O–Tc–CH3 bond is the preferred first step. Reactions involving a change in the oxidation state of metal have high activation barriers, while reactions that do not involve a change in oxidation state have low activation barriers. Reactions of ketenes with TcO3L complexes have lower activation barriers for the preferred addition pathways than those of the ReO3L complexes reported in the literature. Thus, the TcO3L complexes may be better catalysts for the activation of the C=C bonds of substituted ketenes than the reported ReO3L complexes.

A computational study of the addition of ReO3L (L = Cl(-), CH3, OCH3 and Cp) to ethenone

The periselectivity and chemoselectivity of the addition of transition metal oxides of the type ReO₃L (L = Cl, CH₃, OCH₃ and Cp) to ethenone have been explored at the MO6 and B3LYP/LACVP* levels of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of ReO₃L (L = Cl⁻, OCH₃, CH₃ and Cp) with ethenone, the concerted [2 + 2] addition of the metal oxide across the C=C and C=O double bond to form either metalla-2-oxetane-3-one or metalla-2,4-dioxolane is the most kinetically favored over the formation of metalla-2,5-dioxolane-3-one from the direct [3 + 2] addition pathway. The trends in activation and reaction energies for the formation of metalla-2-oxetane-3-one and metalla-2,4-dioxolane are Cp < Cl⁻ < OCH₃ < CH₃ and Cp < OCH₃ < CH₃ < Cl⁻ and for the reaction energies are Cp < OCH₃ < Cl⁻ < CH₃ and Cp < CH₃ < OCH₃ < Cl CH₃. The concerted [3 + 2] addition of the metal oxide across the C=C double of the ethenone to form species metalla-2,5-dioxolane-3-one is thermodynamically the most favored for the ligand L = Cp. The direct [2 + 2] addition pathways leading to the formations of metalla-2-oxetane-3-one and metalla-2,4-dioxolane is thermodynamically the most favored for the ligands L = OCH₃ and Cl⁻. The difference between the calculated [2 + 2] activation barriers for the addition of the metal oxide LReO₃ across the C=C and C=O functionalities of ethenone are small except for the case of L = Cl⁻ and OCH₃. The rearrangement of the metalla-2-oxetane-3-one–metalla-2,5-dioxolane-3-one even though feasible, are unfavorable due to high activation energies of their rate-determining steps. For the rearrangement of the metalla-2-oxetane-3-one to metalla-2,5-dioxolane-3-one, the trends in activation barriers is found to follow the order OCH₃ < Cl⁻ < CH₃ < Cp. The trends in the activation energies for the most favorable [2 + 2] addition pathways for the LReO₃–ethenone system is CH₃ > CH₃O⁻ > Cl⁻ > Cp. For the analogous ethylene–LReO₃ system, the trends in activation and reaction energies for the most favorable [3 + 2] addition pathway is CH₃ > CH₃O⁻ > Cl⁻ > Cp [10]. Even though the most favored pathway in the ethylene-LReO₃ system is the [3 + 2] addition pathway and that on the LReO₃–ethenone is the [2 + 2] addition pathway, the trends in the activation energies for both pathways are the same, i.e. CH₃ > CH₃O⁻ > Cl⁻ > Cp. However, the trends in reaction energies are quite different due to different product stabilities. The formation of the acetic acid precursor through the direct addition pathways was unsuccessful for all the ligands studied. The formation of the acetic acid precursor through the cyclization of the metalla-2-oxetane-3-one is only possible for the ligands L = Cl⁻, CH₃ whiles for the cyclization of metalla-2-oxetane-4-one to the acetic acid precursor is only possible for the ligand L = CH₃. Although there are spin-crossover reaction observed for the ligands L = Cl⁻, CH₃ and CH₃O⁻, the reactions occurring on the single surfaces have been found to occur with lower energies than their spin-crossover counterparts.