Photochemically Generated Transients from κ²- and κ³-Triphos Derivatives of Group 6 Metal Carbonyls and Their Reactivity with Olefins

The synthesis and characterization of (κ2-Triphos)­M­(CO)4 derivatives, where M = Mo, W and Triphos = MeC­(CH2PPh2)3, are reported. Photolyses of these metal carbonyls in dichloromethane or CO2-saturated dichloromethane readily afford the (κ3-Triphos)­M­(CO)3 complexes with no evidence of significant solvent or carbon dioxide interactions with the site vacated by CO. However, in the presence of 1-hexene a transient (κ2-Triphos)­M­(CO)3(1-hexene) adduct was observed, which subsequently releases the olefin with formation of the stable κ3-tricarbonyl species. In the case of M = W the kinetic parameters for this process were assessed, with the rate of olefin replacement being inversely proportional to [1-hexene]. A dissociative rate constant of 25.6 ± 1.1 s–1 at 298 K was determined for olefin loss, with the selectivity for 1-hexene vs free phosphine arm addition to the unsaturated intermediate being somewhat surprisingly large at 22. The activation parameters measured were ΔH⧧ = 26.1 ± 0.4 kcal/mol and ΔS⧧ = 36 ± 3 eu, which are consistent with a dissociative substitution reaction. The kinetic parameters for this transformation were unaffected in the presence of excess quantities of CO2. Although no interaction of CO2 with the transient species resulting from CO loss in the κ2 complex was noted on the time scale of 50 ms, an intermediate described as an η2-HSiEt3 complex was observed upon addition of triethylsilane. This latter transient species underwent dissociation with κ3-complex formation about 15 times as fast as its 1-hexene analogue. X-ray structures of the κ2 complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported

Study of Binuclear Silicon Complexes of Diketopiperazine at S_N2 Reaction Profile

A series of binuclear pentacoordinate silicon complexes of diketopiperazine have been synthesized and substituent (leaving group) effects on the Si−O bond coordination have been studied by comparison of the five differently substituted analogous structures (X = F, Cl, OTf, Br, and I). Variable-temperature NMR spectroscopy supported by X-ray crystallography shows, for the first time in binuclear pentacoordinate silicon complexes, a complex equilibrium with both nonionic (O−Si) and ionic (Si−X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution in a binuclear pentacoordinate silicon compound at the silicon atom

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

Study of Binuclear Silicon Complexes of Diketopiperazine at S_N2 Reaction Profile

A series of binuclear pentacoordinate silicon complexes of diketopiperazine have been synthesized and substituent (leaving group) effects on the Si−O bond coordination have been studied by comparison of the five differently substituted analogous structures (X = F, Cl, OTf, Br, and I). Variable-temperature NMR spectroscopy supported by X-ray crystallography shows, for the first time in binuclear pentacoordinate silicon complexes, a complex equilibrium with both nonionic (O−Si) and ionic (Si−X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution in a binuclear pentacoordinate silicon compound at the silicon atom

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

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

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

Catalysis and Mechanism of H₂ Release from Amine-Boranes by Diiron Complexes

Studies focused on the dehydrogenation of amine-borane by diiron complexes that serve as well-characterized rudimentary models of the diiron subsite in [FeFe]-hydrogenase are reported. Complexes of formulation (μ-SCH₂XCH₂S)­[Fe­(CO)₃]₂, with X = CH₂, CMe₂, CEt₂, NMe, NtBu, and NPh, <b>1-</b>CO through <b>6</b>-CO, respectively, were determined to be photocatalysts for release of H₂ gas from a solution of H₃B ← NHMe₂ (B:A^s), dissolved in THF. The thermal displacement of the tertiary amine-borane, H₃B ← NEt₃ (B:A^t) from photochemically generated (μ-SCH₂XCH₂S)­[Fe­(CO)₃]­[Fe­(CO)₂(μ-H)­(BH₂–NEt₃)], <b>1-</b>B:A^t through <b>6</b>-B:A^t, by P­(OEt)₃ was monitored by time-resolved FTIR spectroscopy. Rates and activation barriers for this substitution reaction were consistent with a dissociative mechanism for the alkylated bridgehead species <b>2</b>-CO through <b>6</b>-CO, and associative or interchange for <b>1</b>-CO. DFT calculations supported an intermediate [<b>I</b>] for the dissociative process featuring a coordinatively unsaturated diiron complex stabilized by an agostic interaction between the metal center and the C–H bond of an alkyl group on the central bridgehead atom of the SRS linker. The rate of H₂ production from the initially formed <b>1-</b>B:A^s through <b>6</b>-B:A^s complexes was inversely correlated with the lifetime of the analogous <b>1-</b>B:A^t through <b>6</b>-B:A^t adducts. Possible mechanisms are presented which feature involvement of the pendent nitrogen base as well as a separate mechanism for the all carbon bridgeheads