Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation

Photolysis of CpRe­(CO)₃ in the presence of dichloromethane results in the initial formation of the CpRe­(CO)₂(ClCH₂Cl) complex followed by insertion of the metal into the C–Cl bond. The activation enthalpy is determined to be 20.4 kcal/mol, and with the assistance of DFT calculations, a radical mechanism is proposed for the oxidative addition reaction. Photolysis of Ni­(CO)₂(PPh₃)₂ with dihalomethanes also results in oxidative addition, but the intermediacy of a halogen-bound adduct has not been established

Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation

Photolysis of CpRe­(CO)₃ in the presence of dichloromethane results in the initial formation of the CpRe­(CO)₂(ClCH₂Cl) complex followed by insertion of the metal into the C–Cl bond. The activation enthalpy is determined to be 20.4 kcal/mol, and with the assistance of DFT calculations, a radical mechanism is proposed for the oxidative addition reaction. Photolysis of Ni­(CO)₂(PPh₃)₂ with dihalomethanes also results in oxidative addition, but the intermediacy of a halogen-bound adduct has not been established

Time Resolved Infrared Spectroscopy: Kinetic Studies of Weakly Binding Ligands in an Iron–Iron Hydrogenase Model Compound

Solution photochemistry of (μ-pdt)­[Fe­(CO)₃]₂ (pdt = μ₂-S­(CH₂)₃S), a precursor model of the 2-Fe subsite of the H-cluster of the hydrogenase enzyme, has been studied using time-resolved infrared spectroscopy. Following the loss of CO, solvation of the Fe center by the weakly binding ligands cyclohexene, 3-hexyne, THF, and 2,3-dihydrofuran (DHF) occurred. Subsequent ligand substitution of these weakly bound ligands by pyridine or cyclooctene to afford a more stable complex was found to take place via a dissociative mechanism on a seconds time scale with activation parameters consistent with such a pathway. That is, the Δ<i>S</i>^⧧ values were positive and the Δ<i>H</i>^⧧ parameters closely agreed with bond dissociation enthalpies (BDEs) obtained from DFT calculations. For example, for cyclohexene replacement by pyridine, experimental Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ values were determined to be 19.7 ± 0.6 kcal/mol (versus a theoretical prediction of 19.8 kcal/mol) and 15 ± 2 eu, respectively. The ambidentate ligand 2,3-DHF was shown to initially bind to the iron center via its oxygen atom followed by an intramolecular rearrangement to the more stable η²-olefin bound species. DFT calculations revealed a transition state structure with the iron atom almost equidistant from the oxygen and one edge of the olefinic bond. The computed Δ<i>H</i>^⧧ of 10.7 kcal/mol for this isomerization process was found to be in excellent agreement with the experimental value of 11.2 ± 0.3 kcal/mol

Acrostichum indet.

The displacement of a CO ligand from an unusually labile rhenium carbonyl complex containing a bidentate carboxyaldehyde pyrrolyl ligand by PPh₃ and pyridine has been investigated. The reaction is found to proceed by an associative, preequilibrium mechanism. Theoretical calculations support the experimental data and provide a complete energetic profile for the reaction. While the Re–CO bond is found to be intrinsically weak in these complexes, it is postulated that the unusual lability of this species is due to the presence of a weak aldehyde Re–O link that can easily dissociate to open a coordination site on the metal center and accommodate an incoming ligand prior to CO loss. The resulting intermediate complex has been identified by IR spectroscopy. The presence of the hemilabile pyrrolyl ligand provides a lower-energy reaction channel for the release of CO and may be of relevance in the design of CO-releasing molecules

Light-Enhanced Displacement of Methyl Acrylate from Iron Carbonyl: Investigation of the Reactive Intermediate via Rapid-Scan Fourier Transform Infrared and Computational Studies

The thermal displacement of methyl acrylate from Fe­(CO)₄(η²-CH₂=CHCOOMe) by phosphine ligands is a relatively slow reaction requiring several hours at elevated temperatures. In the present study, it is observed that photolysis of the tetracarbonyl complex with UV light activates the process such that the reaction is complete within a few seconds. This rate enhancement is due to the formation of an intermediate η⁴ complex where the organic C=O and C=C units of methyl acrylate occupy axial and equatorial coordination sites on the Fe center, respectively, following photochemical CO loss. The displacement of methyl acrylate from this photolytically generated intermediate is facile with a remarkably low barrier of 8.7 kcal/mol. Density functional theory calculations support these experimental observations

Light-Enhanced Displacement of Methyl Acrylate from Iron Carbonyl: Investigation of the Reactive Intermediate via Rapid-Scan Fourier Transform Infrared and Computational Studies

The thermal displacement of methyl acrylate from Fe­(CO)₄(η²-CH₂=CHCOOMe) by phosphine ligands is a relatively slow reaction requiring several hours at elevated temperatures. In the present study, it is observed that photolysis of the tetracarbonyl complex with UV light activates the process such that the reaction is complete within a few seconds. This rate enhancement is due to the formation of an intermediate η⁴ complex where the organic C=O and C=C units of methyl acrylate occupy axial and equatorial coordination sites on the Fe center, respectively, following photochemical CO loss. The displacement of methyl acrylate from this photolytically generated intermediate is facile with a remarkably low barrier of 8.7 kcal/mol. Density functional theory calculations support these experimental observations