Computational Investigation of the Mechanism for the Activation of CO

Article pubs.acs.org/Organometallics

Computational Investigation of the Mechanism for the Activation of CO by Oxorhenium Complexes Jessica L. Smeltz,† Charles Edwin Webster,‡ and Elon A. Ison*,† †

Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States ‡ Department of Chemistry, 213 Smith Chemistry Building, The University of Memphis, Memphis, Tennessee 38138, United States S Supporting Information *

ABSTRACT: In this paper a computational analysis (B3PW91) of the previously reported reaction of (O)Re(Me)(DAAm) (1; DAAm = N,N-bis(2-arylaminoethyl)methylamine, aryl = C6F5) with CO to produce (CO)Re(OAc)(DAAm) (2) is described. The data suggest that this transformation proceeds by two novel elementary steps that are of fundamental interest to the broader organometallic/ inorganic community: (a) direct insertion of CO into the rhenium−methyl bond in 1 to yield the acyl intermediate (O)Re(Ac)(DAAm) (3) and (b) 1,2-migration, in the presence of CO, of the acyl fragment in 3 to the oxo ligand to yield 2. Evidence is provided for the first example of an insertion reaction where CO inserts directly into a M−R bond without prior formation of a CO adduct. In addition, it was shown that the addition of CO is necessary for the 1,2-migration of the acyl ligand. The data suggest that the addition of CO effectively weakens the Re−Cacyl bond in 3 and enables the facile migration of the acyl ligand.

1. INTRODUCTION The development of catalysts for the efficient oxy-functionalization of hydrocarbons remains a challenge.1 Several transition-metal complexes are capable of activating C−H bonds;2 however, the incorporation of these systems into a catalytic cycle that also involves the insertion of an O atom into the activated C−H bond (oxy-functionalization) remains a rarity. In fact, the electrophilic Pt systems developed by Shilov almost 40 years ago, where CH4 is catalytically converted to CH3OH and CH3Cl, stand as the first examples of a homogeneous catalytic system that is capable of functionalizing hydrocarbons.3 There are some examples of stoichiometric reactions of M−R groups that react with oxygen atom donors to produce M− OR.4 The mechanisms for these reactions are diverse and depend on the oxidation state of the metal as well as the d electron count. For example, in 1996, Brown and Mayer reported the first thermal example of a 1,2-migration of a ligand from a metal center to a terminal oxo ligand.5 In this system, an oxorhenium(V) complex is oxidized to a rhenium(VII) dioxo intermediate. Subsequent 1,2-migration of the phenyl ligand to the terminal oxo group yields an oxorhenium(V) phenoxide complex. The electrophilicity of the rhenium oxo species enables the facile 1,2-migration. This reaction was groundbreaking, because it illustrated a new paradigm for the functionalization of metal hydrocarbyls. In recent years, Periana, Gunnoe, Goddard, and others have reported that organometallic oxorhenium complexes such as Me(O)3Re and Ar(O)3Re (Ar = aryl) undergo oxy-function© 2012 American Chemical Society

alization in the presence of oxygen atom donors such as H2O2 and IO4−.1b,c,6 Computational and experimental data suggest that these reactions proceed by a Baeyer−Villiger type mechanism, where oxy-functionalization occurs by 1,2-migration to a coordinated oxide ligand. The reaction, however, is restricted to transition metals in their highest oxidation state, as the requirement for strong oxygen atom donors inevitably leads to the oxidation of the metal center. These examples illustrate that, despite the importance of reactions that functionalize M− R bonds, examples of these reactions are rare, mechanisms by which they operate are diverse, and in many cases these mechanisms are still not well understood. Consequently, studies aimed at the discovery of new oxy-functionalization reactions and an understanding of their mechanisms is of critical importance. Some of us recently reported a novel reaction for the functionalization of metal carbon bonds where a methyl ligand in (O)Re(Me)(DAAm) (1; DAAm = N,N-bis(2arylaminoethyl)methylamine, aryl = C6F5) in the presence of CO is converted to an acetate ligand in (CO)Re(OAc)(DAAm) (2) (Scheme 1).7 Experimental data suggest that the reaction proceeds by (a) C−C bond formation (migratory insertion) to generate the acyl intermediate (O)Re(Ac)(DAAm) (3) and (b) 1,2-migration of the acyl ligand to the oxo to generate 2. This sequence of reactions represents a novel mechanism in transition-metal chemistry, and we envision that Received: April 24, 2012 Published: May 11, 2012 4055

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paper energies are reported in kcal/mol with gas-phase energies in parentheses and solvation energies without parentheses.

Scheme 1

3. RESULTS AND DISCUSSION DFT calculations were performed to examine the energetics of the reaction depicted in Scheme 1. Geometries were optimized in the gas phase and with the SMD solvation model as implemented in Gaussian 09. The functional and basis sets used are suitable for describing the geometry of complex 1 (Supporting Information); the average deviations in the bond length from the crystallographically (X-ray) determined structure for 117 are 0.99 and 1.02% for solution and gasphase calculated structures, respectively, while the average deviations in bond angles are 2.1 and 1.4% for solution and gasphase structures (Table S1, Supporting Information). As observed experimentally,7 energy changes along the reaction coordinate can be divided into two separate regimes: (a) addition of CO to 1 and (b) the formation of complex 2. Each of these regimes will be discussed in detail below. 3.1. Addition of CO to 1. Complex 1 is five-coordinate; as a result, the approach of the CO ligand can occur along three trajectories, as depicted in Figure 1: (A) cis to the Re−Me bond, (B) cis to the Re−N(amide) bond, and (C) trans to the ReO bond.

these complexes may be incorporated into a catalytic cycle as depicted in Scheme 2, where methane (CH4) is functionalized to acetic acid (CH3COOH) and the 1,2-migration of the acyl ligand is a critical step. Scheme 2

In this current paper we perform a computational study (DFT) of the reaction depicted in Scheme 1. Our goal was to determine the viability of the mechanistic pathway proposed previously and also to gain additional insights with regard to the structure and energetics of key intermediates. An understanding of the energetics of this new reaction may lead to the development of new catalytic systems (as proposed in Scheme 2) for the oxy-functionalization of hydrocarbons.

Figure 1. Trajectories for the approach of the CO ligand to 1.

2. EXPERIMENTAL SECTION

3.1.1. Pathway A, Attack of CO Cis to the Re−Me Bond. The attack of CO cis to the Re−Me bond in a plane containing the oxo ligand, Re, and the methyl ligand is depicted in Schemes 3 and 4.

Computational Methods. Theoretical calculations have been carried out using the Gaussian098 implementation of B3PW91 (the B3 exchange functional9 and PW91 correlation functional10) density functional theory.11 All geometry optimizations were carried out using tight convergence criteria (“opt=tight”) and pruned ultrafine grids (“Int=ultrafine”). All calculations were conducted with the same basis set combination. The basis set for rhenium was the small-core (311111,22111,411) → [6s5p3d] Stuttgart−Dresden basis set and relativistic effective core potential (RECP) combination (SDD)12 with an additional f polarization function.13 The 6-31G(d,p)14 basis sets were used for all other atoms. Cartesian d functions were used throughout: i.e., there are six angular basis functions per d function. All structures were fully optimized, and analytical frequency calculations were performed on all structures to ensure either a zeroth-order saddle point (a local minimum) or a first-order saddle point (transition state, TS) was achieved. The minima associated with each transition state was determined by animation of the imaginary frequency and, if necessary, with intrinsic reaction coordinate (IRC) calculations. Solvation energies were computed using the SMD15 method, with dichloromethane as the solvent, as implemented in Gaussian 09. In this method an IEFPCM16 calculation is performed with radii and electrostatic terms from Truhlar and co-workers’ SMD15 solvation model. For solvent effects, geometry optimizations and frequency calculations were carried out using these models as well. Thermochemical data were calculated using unscaled vibrational frequencies and default parameters at 298.15 K and 1 atm. In this

Scheme 3

The addition of CO to 1 proceeds with a free energy of activation (ΔG⧧) of 32.8 kcal/mol to yield an oxorhenium CO adduct, 4. The intermediate 4 is six-coordinate, with the metal center in a distorted-octahedral environment, and is characterized by the CO ligand cis to the ReO multiple bond (oxo−Re−CO = 87.2°). This geometric arrangement accommodates π back-bonding to CO in the plane incorporating Re and the diamido ligand. However, this geometry orients the methyl ligand 146° from the oxo ligand. This arrangement is unfavorable thermodynamically because of the strong trans 4056

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Scheme 4a

a

The solvent-corrected energy for *TS3 was obtained by running a single-point energy calculation on the structure optimized in the gas phase.

the metal center in a facial orientation. From 6, migratory insertion results in the formation of the acyl complex 7, which is a geometric isomer of the observed acyl complex 3anti. The free energy (ΔG°) for the transformation (1→ 6 → 7) is 38.2 kcal/mol. The high relative energy of 7 suggests that this species is unlikely to be an intermediate and that pathway B is not a viable pathway for the formation of 2. 3.1.3. Pathway C, Attack of CO Trans to the Re−Oxo Bond. As shown in Scheme 6, the approach of CO trans to the

influence of the oxo ligand. As a result, the Re−Me bond in 4 (2.4 Å) is significantly lengthened compared to that in 1 (2.13 Å) and the transformation from 1 to 4 is endergonic overall (ΔG° = 11.9 kcal/mol). From 4, CO can either migrate to the oxo ligand to give the CO2 complex 5 (Scheme 3) or the methyl group can migrate to the CO ligand to give the acyl complex 3 (Scheme 4). Migration of CO to the oxo ligand results in the formation of the CO2 complex 5. It is noteworthy that the formation of 5 does not involve direct attack of the oxo ligand by CO, as no pathway for direct attack at the oxo ligand was found.18 This result is not surprising because CO is a poor nucleophile, and although the oxo ligand may be regarded as electrophilic, the Lewis acidity of the oxo ligand pales in comparison to the metal center. The calculated data suggest that the progression of the reaction through a CO2 adduct (5) is unlikely for this reaction because of the large barrier for the formation of 4 (ΔG⧧298 = 32.8 kcal/mol) and the overall unfavorable thermodynamics (ΔG° = 11.1 kcal/mol). Migration of the methyl ligand in 4 to CO (TS3), however, is facile (ΔG⧧298 = 0.7 kcal/mol) and is overall exergonic for the formation of 3syn (ΔG° = −11.7 kcal/mol). However, pathway A is again unlikely because the reaction must proceed through 4 and, as stated above, the activation barrier for the formation of this intermediate is large (ΔG⧧298 = 32.8 kcal/ mol)and not consistent with experimental observations. 3.1.2. Pathway B, Attack of CO Cis to the Re−N Bond. Another trajectory that was considered for the addition of CO to 1 involves the approach of CO in a plane containing the oxo ligand, Re, and the Re−N(diamide) bond. As shown in Scheme 5, attack of CO along this plane results in adduct 6, which is unstable (ΔG° = 11.3 kcal/mol) relative to 1. However, the free energy of activation for the formation of 6 (ΔG⧧298 = 21.9 kcal/mol) is significantly lower than that of pathway A (ΔG⧧298 = 32.8 kcal/mol). The DAAm ligand in 6 is arranged around

Scheme 6

ReO bond proceeds directly to 3anti with a free energy of activation of 15.7 kcal/mol. For this pathway no CO adduct was calculated as an intermediate. The nonbonding dxy orbital that is responsible for π back-bonding to CO lies in the plane that includes the Re atom and the diamido ligand. The oxo ligand itself is a strong trans-influence ligand; therefore, a geometry where the CO is trans to the oxo would be inherently unstable. As shown in Figure 2, the Re−Me bond (2.41 Å) is significantly lengthened in TS7 compared to that in 1 (2.12 Å). Also, the Me and CO ligands begin to form a bond (Me−CO

Scheme 5a

Figure 2. Optimized structure (B3PW91) for TS7. The structure was optimized using the SMD solvation model with dichloromethane as the solvent. Selected bond lengths (Å) and angles (deg): Re−C25, 1.95; Re−C26, 2.41; Re−O23, 1.74; Re−N2, 1.99; Re−N3, 1.98; Re− N4, 2.29; C25−C26, 1.99; O23−Re−C26, 76; O23−Re−C25, 129; N2−Re−N3, 133.

a The solvent-corrected energy for *TS6 was obtained by running a single-point energy calculation on the structure optimized in the gas phase.

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bond, 1.99 Å). In order to minimize the unfavorable energetics associated with the trans influence of the oxo ligand and to maximize π bonding in TS7, CO begins to move into the plane that contains the dxy orbital: i.e., the plane containing the diamido ligand but orthogonal to the ReO bond. As a result, the oxo−Re−CO angle in TS7 is 129°. As shown in Figure 3, the HOMO of TS7 reflects an interaction of the dxy orbital with the π* orbital of CO.

Scheme 7

(Figure 4). In TS4 the Re−Cacyl bond (2.21 Å) is significantly lengthened and the acyl ligand is oriented perpendicular to the ReO bond (dihedral angle −77.8°). Upon complete rotation to 3syn, the bond length shortens (2.07 Å). To summarize, three trajectories for the addition of CO to 1 were considered. The results of the computations suggest that the formation of the CO2 adduct 5 is both kinetically and thermodynamically disfavored and the acyl complexes 3syn and 3anti are more stable than 5 by 22.8 and 23.7 kcal/mol, respectively. Also, the instability of 5, relative to 1 (ΔG° = 11.1 kcal/mol), and the large free energy of activation (ΔG⧧298 = 32.8 kcal/mol) for the formation of the CO adduct 4 suggest that the progression of the reaction through 4 and consequently 5 is unlikely. The data are consistent with isotope labeling data and the time course for the reaction reported previously,7 which strongly implicates the intermediacy of 3 and suggests that the reaction does not proceed through a CO2 adduct. The large free energy of activation associated with the formation of 4 also suggests that CO does not approach cis to the ReO and Re−Me bonds because these trajectories result in a geometry where the methyl ligand is approximately trans to the ReO and as a result is inherently unstable. Similarly, the formation of 7 by the addition of CO in a plane containing Re, one diamide nitrogen, and the oxo ligand is also unlikely because of the relatively high energy of this intermediate (ΔG° = 38.2 kcal/mol). Furthermore, although the free energy of activation for this pathway (ΔG⧧298 = 21.9 kcal/mol) is lower than that for pathway A (ΔG⧧298 = 32.8 kcal/mol), this barrier is higher than that for pathway C (ΔG⧧298 =15.7 kcal/mol). Therefore, pathway B is not the lowest energy pathway. The computational data suggest that pathway C is the lowest energy pathway for the addition of CO to 1. Importantly, in this pathway, the formation of 3 from 1 proceeds by a mechanism that involves direct insertion of CO into the Re− Me bond, as opposed to the more traditional migration of a methyl group to a coordinated CO ligand. It is also noteworthy that all calculated CO adducts (4 and 6) are unstable relative to 1. In addition, pathways associated with these adducts proceed with large free energies of activation.

Figure 3. HOMO for TS7 showing the interaction of the dxy orbital on Re with the π* orbital of CO.

Pathway C ends with the equilibrium between the two acyl isomers 3anti and 3syn. These isomers differ in the orientation of the acyl oxygen relative to the ReO bond: i.e., in 3anti the acyl oxygen is oriented anti (dihedral angle −180°) to the ReO, while in 3syn the acyl oxygen is oriented syn (dihedral angle 0.33°) to the ReO bond (Figure 4). The free energy of activation, ΔG⧧298, for the conversion of the isomers of 3anti → 3syn (TS4) is 21.1 kcal/mol and the free energy, ΔG°, for the transformation 3anti → 3syn is 0.9 kcal/mol. These results are consistent with experiment, as a 4:1 ratio of the two isomers was observed by 1H NMR spectroscopy. In addition, line broadening of the resonances for the two isomers of 3 was not observed at 75 °C by 1H NMR spectroscopy.7 These results are consistent with the calculated free energy of activation. Similar rotational free energies of activation (ΔG⧧298 = 19−22 kcal/mol) for rotation about a ReCHR bond were calculated by Gladysz and co-workers.19 The calculated free energy of activation for the transformation is a consequence of significant π back-donation from the metal center to the π* orbital of the acyl ligand. As a result of π bonding, the acyl ligand in 3 is best described by the carbene-like resonance structure B (Scheme 7).20 To convert between the two isomers, the ReC double bond in B is broken in TS4. The calculated Re−Cacyl bond length (2.04 Å) for 3anti is comparable to the Re−Cacyl bond length (2.01 Å) for 3 obtained by X-ray crystallography7

Figure 4. Optimized structure (B3PW91) for 3. The oxo−Re−Cacyl−Oacyl dihedral angle (°) are as follows: 3anti (−180); 3syn (0.33); TS4 (−77.8). The Re−Cacyl bond lengths (Å) are as follows: 3anti (2.04); 3syn (2.07); TS4 (2.21). 4058

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The inability of CO to coordinate to and form stable complexes with 1 is likely governed by two factors: (1) the high oxidation state of the metal center (ReV) and (2) the geometry of the complex, which enforces a trans approach of CO to the oxo ligand. While migratory insertion of CO into a M−R bond represents a classic mechanism in organometallic chemistry,21 most of these reactions are thought to proceed via alkyl migrations to coordinated CO ligands. Examples of direct CO insertion into M−R bonds are rare.22 Thus, pathway C is an example of a novel mechanism for CO insertion, where CO directly inserts into a M−R bond without prior formation of a CO adduct. To the best of our knowledge, this report is the first example of such a transformation. The computational data for the addition of CO to 1 are summarized in Figure 5.

Scheme 8

Scheme 9a

Figure 5. Summary of computational data for the addition of CO to 1: (blue) pathway A (migration to oxo); (black) pathway A (migration to methyl); (green) pathway B; (red) pathway C. All energies are computed at the B3PW91 level of theory with basis sets as reported in the text. Reported energies include solvation corrections, which were computed with the SMD solvation model with dichloromethane as the solvent.

The solvent-corrected energy for *8 was obtained by running a single-point energy calculation on the structure optimized in the gas phase.

a

this case, migration of the aryl ligand leads to the formation of the stable ReV complex TpRe(O)(OPh)+. The higher charge and oxidation state of Re in TpRe(O)2Ph+ results in increased electrophilicity for the oxo ligand. This increased electrophilicity enables the migration to be facile. In addition, the complex TpRe(O)(OPh)+ is stabilized by the presence of a spectator terminal oxo group, whereas 1,2-migration of the acyl ligand in 3anti in the absence of CO results in *8, which lacks a terminal oxo group to stabilize the metal center. The effect of spectator metal−oxo bonds has been used to rationalize the increased reactivity of dioxo complexes in comparison to complexes that incorporate a single oxo bond.23 These factors contribute to the instability and high activation energy for the formation of 8 and suggest that pathway D is unlikely for this step.24 The computational data are also consistent with experimental observations, as migration of the acyl ligand is not observed in the absence of CO.7 In light of these results, we pursued two other mechanisms (pathways E and F) that involved initial addition of CO to the two isomers of 3. 3.2.2. Pathway E: Addition of CO to 3syn Followed by Acyl Migration. The interaction of 3syn with CO results in insertion into the Re−Namide bond to generate 9 (Scheme 10). The overall reaction is endergonic (ΔG° = 15.6 kcal/mol) and proceeds with a free energy of activation (ΔG⧧298) of 29.4 kcal/ mol. Importantly, no pathway for the formation of 2 from 9 was

3.2. Formation of 2. Experimental data7 for the addition of CO to 1 (vide supra) strongly implicate the intermediacy of 3; therefore, in investigating the pathway for the formation of 2, three mechanisms were considered that began with 3 as an intermediate. As shown in Scheme 8, the first mechanism considered, pathway D, involves direct 1,2-migration of the acyl ligand to the terminal oxo ligand, followed by addition of CO to generate complex 2. In pathway E, CO is added initially to 3syn. This step is followed by 1,2-migration of the acyl ligand to the oxo to generate 2. Finally, pathway F involves addition of CO to 3anti, followed by 1,2-migration of the acyl ligand to the oxo ligand. Each of these pathways is described in detail below. 3.2.1. Pathway D: Migration of the Acyl Ligand Followed by the Addition of CO. The first mechanism considered involves direct 1,2-acyl migration across the Re−oxo bond in 3anti to generate *8 (Scheme 9) followed by the addition of CO to generate 2 (Supporting Information). Complex *8 is unstable with respect to 3anti (ΔG° = 16.1 kcal/mol). In addition, the activation barrier (ΔG⧧298) for the migration of the acyl fragment is 48.4 kcal/mol. In contrast, the 1,2-phenyl migration across the Re−oxo bond in TpRe(O)2Ph+ (Tp = hydridotris(pyrazoyl)borate), originally reported by Brown and Mayer, proceeds with an activation barrier of 20.9 kcal/mol.5 In 4059

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environment the orbital available for π back-bonding (dxy) occurs in the plane containing the diamido amine and acyl ligand. Therefore, the addition of CO trans to the oxo ligand results in a net partial positive charge on the CO carbon, as the electron density that is removed by σ donation to the metal from the CO ligand is not compensated by π back-bonding. The CO ligand trans to the oxo ligand is stabilized, because in the anti orientation the acyl oxygen is able to form a bond (1.47 Å) with CO. In contrast, no CO adducts were located for the addition of CO trans to the oxo ligand in 1 or 3syn. This result is likely because the interaction of the metal and CO in this case is weak, due to CO being a poor σ donor, and π back-bonding is not possible, as the dxy orbital occurs in a plane that is orthogonal to the potential Re−CO bond. Migration of the acyl fragment to the oxo ligand proceeds from 10 through TS11 (ΔG⧧298 = 3.8 kcal/mol) to the κ1acetate complex 11. As suggested in Figure 6, TS11 is early and features the cleavage of the carbon−oxygen bond and disruption of the Re−acyl multiple bond that was present in 10. Consequently, the acyl fragment twists out of plane (dihedral angle Re−Cacyl−Oacyl−CCO = 22.4° compared to 0.06° in 10) and the Re−Cacyl bond length is significantly lengthened in TS11 (2.16 Å). Inspection of the Mulliken charges in 10 and TS11 reveal the influence of π bonding in both structures. The positive charge at Re is decreased on proceeding from 10 to TS11 (1.27 to 1.23). In TS11, the π back-bonding present in 10 is disrupted; as a result, the electron density at Re is increased. Similarly, the charge at the acyl carbon in 10 (0.056) is lower than that of the corresponding carbon in TS11 (0.084). Again, this result is consistent with π back-bonding in 10, as the transfer of electron density from Re to the π* orbital of the acyl ligand results in a decreased positive charge at the acyl carbon in 10. This data are consistent with the description of the Re− carbon bond in 10 by resonance form B in Scheme 7, while the Re−Cacyl bond in TS11 is best described by resonance form A in Scheme 7. The pathway (Scheme 11) concludes with the formation of the κ2-acetate complex 12 followed by isomerization of the backbone of the diamido amine ligand to produce 2. Importantly, our computational data suggest that the most likely pathway for the formation of 2 involves the addition of CO to 3anti prior to 1,2-acyl migration. The addition of CO effectively weakens the Re−Cacyl bond in TS11 and allows for facile migration. The data presented are consistent with experimental observations, which suggest that the formation of 3 occurs at a significantly faster rate than the formation of 2.7

Scheme 10

found. This result suggests that pathway E is unlikely for this reaction. 3.2.3. Pathway F: Addition of CO to 3anti Followed by Acyl Migration. Addition of CO to 3anti results in the formation of the CO adduct 10, whose structure is best described as a metallolactone (Figure 6). The activation energy, ΔG⧧, for the

Figure 6. Optimized structures (B3PW91) for 10 and TS11.

formation of 10 is 26.0 kcal/mol (Scheme 11). Similar metallolactone-like structures have been proposed by Whited and Grubbs25 and have also been computed by Yates and coworkers26 for reactions of Ir carbenes with CO2. As shown in Figure 6, the Re−Cacyl bond in 10 is short (2.01 Å) and is similar to the Re−Cacyl bond length (2.04 Å) in 3anti, which suggests there is significant back-bonding from the metal dxy orbital to the π* orbital of the acyl ligand. This π-bonding polarizes the C−O bond of the acyl ligand and results in a partial negative charge on the acyl oxygen. In an octahedral

4. CONCLUDING REMARKS The discovery of catalysts for the oxy-functionalization of hydrocarbons would require the development of methods for

Scheme 11

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efficient carbon−oxygen bond formation. In this paper, the activation of CO by complex 1 was investigated by DFT and confirms the novel mechanism for CO activation reported previously.7 The calculated reaction agrees with experimental observations: i.e., the reaction proceeds by a two-step mechanism involving fast CO insertion followed by slow methyl migration. This reaction involves two novel elementary steps that are of fundamental interest to the broader organometallic/inorganic community: (a) direct insertion of CO into the rhenium−methyl bond to yield the acyl intermediate 3 (to the best of our knowledge, this report is the f irst example of an insertion reaction where CO inserts directly into a M−R bond without prior formation of a CO adduct) and (b) 1,2-migration, in the presence of CO, of the acyl fragment in 3 to the oxo ligand to yield 2. 1,2-Migration of the acyl ligand proceeds with prior coordination of CO. The data suggest that that the addition of CO effectively weakens the Re−Cacyl bond and enables the facile migration of the acyl ligand. This report is only the second example of a thermal metal-to-oxo migration and the f irst example of an acyl-to-oxo migration. The reaction described involves two important steps for the selective conversion of CO to C2+ oxygenates: C−C bond formation and C−O bond formation in the form of an acyl migration. Current efforts in our laboratories are aimed at developing these reactions for the design of a homogeneous catalyst for syngas activation.

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ASSOCIATED CONTENT

* Supporting Information S

Tables and text giving Cartesian coordinates for all optimized complexes as well as the full ref 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge North Carolina State University, The University of Memphis High Performance Computing Facility, and the National Science Foundation via a CAREER Award (No. CHE-0955636) for funding.

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REFERENCES

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