A Rhenium–Cyclohexane Complex with Preferential Binding of Axial CH Bonds: A Probe into the Relative Ability of CH, CD, and CC Bonds as Hyperconjugative Electron Donors?

Lawes, Douglas J.; Darwish, Tamim A.; Clark, Timothy; Harper, Jason B.; Ball, Graham E.

doi:10.1002/anie.200600313

Cited by 59 publications

(64 citation statements)

References 48 publications

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“…This finding suggests at least three possibilities for the mode of coordination of the alkane in 3a. (i) The cyclopentane unit is coordinating to the metal center in a -alkane complex fashion with a highly stretched complexed C-H bond, significantly greater than the 1.15-1.17 Å calculated for the complexed C-H distance of Re(Cp)(CO) 2 (cyclohexane) (2c) (12). (ii) The complexed C-H bond of the coordinated cyclopentane has undergone oxidative cleavage to form the cyclopentyl hydride 4a, but the 1 J CH value for the C ␣ -H is unusually large.…”

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confidence: 84%

“…This means that the observed ␦ 1 H and 1 J CH values of the protons on the bound carbon are an average of the values for complexed and uncomplexed hydrogens. Density functional theory calculations suggest some deshielding of the 1 H resonance and an increase in 1 J CH of the uncomplexed hydrogen(s) in Re(Cp)(CO) 2 (cyclohexane) (12) compared with the free alkane. Therefore, the true shielding and reduction of 1 J CH for the complexed hydrogen may be just over twice the observed changes in the averaged 1 H NMR shift and 1 J CH for a CH 2 group compared with the free alkane.…”

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confidence: 99%

“…A quintet grows in at ␦ Ϫ2.29 in the 1 H spectrum that is due to the bound CH 2 unit of the cyclopentane ligand in the previously characterized Re(Cp)(CO) 2 (cyclopentane) (2a) (10), and a singlet at ␦ 4.92 is due to the Cp ligand of 2a. The alkane in complexes 2a-2c is known not to undergo oxidative cleavage to form alkyl hydrides under these conditions (10)(11)(12).…”

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confidence: 99%

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A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy

Ball

Brookes

Cowan

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy

Ball

Brookes

Cowan

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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“…For instance, it is widely accepted that COH activation reactions proceed via alkane -complex intermediates (3) (Fig. 1, compound 3), and such species have recently been detected and studied in lowtemperature NMR experiments (9)(10)(11). Although complexes with intramolecular COH/metal (so-called agostic) interactions have been isolated and crystallographically characterized (3), in systems where strong evidence has been provided for intermolecular metal-alkane coordination in solution (9)(10)(11), isolation and solid state structural characterization have not yet been accomplished.…”

Section: Metal-alkane Binding Energies Have Been Calculated For [Cprementioning

confidence: 99%

Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

Cobar

Khaliullin

Bergman

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Metal-alkane binding energies have been calculated for [CpRe(CO) 2](alkane) and [(CO)2M(C5H4)C'C(C5H4)M(CO)2](alkane), where M ‫؍‬ Re or Mn. Calculated binding energies were found to increase with the number of metal-alkane interaction sites. In all cases examined, the manganese-alkane binding energies were predicted to be significantly lower than those for the analogous rhenium-alkane complexes. The metal (Mn or Re)-alkane interaction was predicted to be primarily one of charge transfer, both from the alkane to the metal complex (70 -80% of total charge transfer) and from the metal complex to the alkane (20 -30% of the total charge transfer).binding energy ͉ COH activation ͉ DFT calculations ͉ manganese C arbon-hydrogen (COH) bond activation reactions are important from a fundamental point of view, as well as in more practical terms to medicine, academia, and industry, as they are being used for the conversion of common, inexpensive alkanes into reactive (and physiologically active) molecules (1, 2). Many transition metal complexes are now known that can be used to activate COH bonds in alkanes (3). With low-valent metal complexes, the first bond-breaking step involves oxidative addition of an alkane to an unsaturated metal center (Fig. 1). The goal of subsequent steps is to convert the alkyl group R into an alcohol ROH or other potentially useful functionalized organic molecule.Many researchers today are focusing on the development of industrially practical organometallic oxidation catalysts (5-8), but there remain fundamental aspects of the COH activation process that are not fully understood. For instance, it is widely accepted that COH activation reactions proceed via alkane -complex intermediates (3) (Fig. 1, compound 3), and such species have recently been detected and studied in lowtemperature NMR experiments (9-11). Although complexes with intramolecular COH/metal (so-called agostic) interactions have been isolated and crystallographically characterized (3), in systems where strong evidence has been provided for intermolecular metal-alkane coordination in solution (9-11), isolation and solid state structural characterization have not yet been accomplished.† † Intermolecular metal-alkane complexes are therefore one of the most important targets in the study of COH activation; an understanding of the mechanisms of COH activation, which will allow researchers to better control and manipulate the outcome of the reactions, is crucial for the advancement of this area of chemistry.Quantum chemical methods have become useful and practical tools in study of TM complexes (14-17). Particularly, advancements in density functional theory (DFT) (16) and the use of effective core potentials (14, 15) have made qualitatively accurate predictions of the structures and chemistry of TM complexes possible at a reasonable computational cost (17). The goals of the present work are twofold: to make computational predictions of the relative stabilities of a selection of alkane -complexes, which will aid synthetic chemists in th...

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“…Their reactivity decreases on going both across and down groups 5,6, and 7 (10)(11)(12), and these observations led to the identification of a very long-lived alkane complex, Re(Cp)(CO) 2 (nheptane) (Cp ϭ 5 OC 5 H 5 ), which has a lifetime of Ϸ25 ms at room temperature (13). The relative stability of Re(Cp)(CO) 2 (alkane) complexes allowed Re(Cp)(CO) 2 (C 5 H 10 ) to be observed at 180 K by NMR spectroscopy (14), and subsequent NMR studies have been carried out to determine the binding modes of a series of related alkanes to the Re(CpЈ)(CO) 2 moiety (15,16).…”

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Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

Cowan

Portius

Kawanami

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W(CO)5(CH4) and M( 5 OC5R5)(CO)2(L) [where M ‫؍‬ Mn or Re, R ‫؍‬ H or CH3 (Re only); and L ‫؍‬ CH4 or C 2 H 6 ]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous nheptane complexes. Re(Cp)(CO)2(CH4) and Re(Cp*)(CO)2(L) [Cp* ‫؍‬ 5 OC5(CH3)5 and L ‫؍‬ CH4, C2H6] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re(Cp*)(CO)3 in scCH4 and demonstrated that the initially generated Re(Cp*)(CO)2(CH4) forms an equilibrium mixture of Re(Cp*) ( T here is considerable interest in sigma-bonded organometallic alkane complexes, particularly since they have been identified as key intermediates in the transition metal-mediated COH activation process (1-3). Although such complexes generally are very short-lived intermediates (4), they have been known for over 30 years. Early experiments involved the photolysis of complexes such as Cr(CO) 6 and Fe(CO) 5 to generate the unstable intermediates Cr(CO) 5 or Fe(CO) 4 in low-temperature matrices, where coordination to cocondensed CH 4 results in the formation of Cr(CO) 5 (CH 4 ) and Fe(CO) 5 (CH 4 ) (5, 6).Flash photolysis experiments have demonstrated that the photolysis of Cr(CO) 6 in cyclohexane solution at room temperature forms Cr(CO) 5 (cyclohexane) (7). Subsequently, several examples of alkane complexes in solution have been reported, and studies on the mechanism of the COH activation process have clearly demonstrated the role of these complexes in oxidative addition reactions (1,8,9). Time-resolved infrared (TRIR) spectroscopy has proved to be a powerful tool for the study of metal carbonyl alkane complexes. Their reactivity decreases on going both across and down groups 5, 6, and 7 (10-12), and these observations led to the identification of a very long-lived alkane complex, Re(Cp)(CO) 2 (nheptane) (Cp ϭ 5 OC 5 H 5 ), which has a lifetime of Ϸ25 ms at room temperature (13). The relative stability of Re(Cp)(CO) 2 (alkane) complexes allowed Re(Cp)(CO) 2 (C 5 H 10 ) to be observed at 180 K by NMR spectroscopy (14), and subsequent NMR studies have been carried out to determine the binding modes of a series of related alkanes to the Re(CpЈ)(CO) 2 moiety (15, 16).The activation of methane is of particular interest because of the potential of using this abundant hydrocarbon as both an energy source and chemical feedstock (17). Organometallic methane complexes have been characterized in low-temperature matrix isolation experiments (5,6,18). In solution, the existence of methane complexes has been inferred by examining product ratios and from the rates of COH activation and reductive elimination reactions in isotopic labeling experiments (19).The lifetime of the...

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A Rhenium–Cyclohexane Complex with Preferential Binding of Axial CH Bonds: A Probe into the Relative Ability of CH, CD, and CC Bonds as Hyperconjugative Electron Donors?

Cited by 59 publications

References 48 publications

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy

Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

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A Rhenium–Cyclohexane Complex with Preferential Binding of Axial CH Bonds: A Probe into the Relative Ability of CH, CD, and CC Bonds as Hyperconjugative Electron Donors?

Cited by 59 publications

References 48 publications

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy

Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

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A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF₃)] studied with NMR and time-resolved IR spectroscopy