Mechanism for the Reaction of CO with Oxorhenium Hydrides

Article pubs.acs.org/Organometallics

Mechanism for the Reaction of CO with Oxorhenium Hydrides: Migratory Insertion of CO into Rhenium Hydride and Formyl Bonds leads to Migration from Rhenium to the Oxo Ligand Nikola S. Lambic, Cassandra P. Lilly,† Roger D. Sommer, and Elon A. Ison* Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: Computational studies (M06) have been performed in synergy with experimental studies to show that the thermodynamics for insertion of CO into an oxorhenium− hydride bond to form a formyl ligand is favorable despite conventional wisdom to the contrary. Further, it is shown that insertion of CO into formyl ligands to form α-dicarbonyl ligands is also a viable pathway and results in hydroxy carbonyl or formate complexes, depending on the nature of the ancillary ligand.

■

INTRODUCTION The dwindling supply of petroleum has led to a search for alternative sources of carbon.1 Syngas (H2 + CO), which can be derived from natural gas, biomass, and coal, is an attractive alternative as it can be used in direct CO hydrogenation to methanol2 or converted to liquid hydrocarbons industrially in the Fischer−Tropsch process.3 However, the mechanism of both heterogeneous and homogeneously catalyzed Fischer− Tropsch and other CO hydrogenation chemistry is still not well understood.2d,3,4 Initially, CO insertion into a metal− hydride bond to afford a formyl complex was frequently proposed as the beginning step for mechanisms for CO reduction.2d,g,5 However, these mechanisms quickly fell out of favor once the unfavorable thermodynamics for the overall process of converting M−H + CO to M−C(O)H was recognized.5b Alternative syntheses of metal formyls revealed that these complexes rapidly led to decarbonylation, in support of the notion that the equilibrium for CO insertion into a metal hydride is unfavorable.6 Metal formyl complexes fall into two types: η1 carbon bonded and η2 carbon−oxygen bonded species. Most metal formyl complexes have been made by the nucleophilic attack at a CO ligand coordinated to an electrophilic metal center.6 An example of an η1 carbon bonded formyl was reported by Wayland and co-workers.7 These complexes were the first to produce observable quantities of metal formyl species under mild conditions. The mechanism for this reaction was not a classic migratory insertion reaction, however, but involved free radicals.7e,g,8 Examples of η2-formyls that result from the insertion of CO into a metal−hydride bond include zirconium hydrides by Bercaw and co-workers and thorium hydrides by Marks and co-workers.9 Coordination of the formyl oxygen to © XXXX American Chemical Society

the highly oxophilic thorium is believed to provide the additional stability necessary for the isolation of this complex. Recently, our group investigated the reaction of oxorhenium methyl complexes with CO.10 The reaction proceeds by the initial insertion of CO into a Re−Me bond, followed by a unique 1,2-acyl migration to the metal oxo to afford a Re(III) acetate complex. The mechanism for the insertion of CO into the rhenium−methyl bond has been extensively studied and proceeds without the formation of a CO adduct prior to methyl migration (direct insertion).10 The electronics of the oxo ligand largely influences the mechanism for insertion, because the strong trans influence of the axial oxo ligand in these squarepyramidal oxorhenium molecules strongly destabilizes complexes with a ligand (in this case CO) trans to the oxo. Further, strong π donation from these d2 metal complexes results in unusually strong rhenium−carbon bonds (see Figure 1). We hypothesized that these propensities could be utilized to induce insertion into the rhenium−hydride bond in an oxorhenium complex. In this paper we show with M0611 calculations and experiments that, with a careful choice of ancillary ligands, the thermodynamics for the insertion of CO into a rhenium− hydride bond to form a formyl ligand is favorable and leads to novel reactions and intermediates. Experimental results consistent with these predictions are presented. Metal formyl complexes have been widely postulated as plausible intermediates in the reduction of CO.5 Data presented here suggest that mechanisms that involve the insertion of CO into a metal− hydride bond should be considered when designing catalysts Received: July 24, 2016

A

DOI: 10.1021/acs.organomet.6b00591 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 2. Alternative Synthesis of 2a

Scheme 3. H/D Exchange of Terminal Hydroxyl Ligand with D 2O

yellow to green, and green crystals of 4 precipitate from solution (Scheme 4).

Figure 1. Trans influence of the oxo ligand in oxorhenium complexes, illustrated by a oxorhenium carbonyl complex. The trans influence of the oxo ligand destabilizes the structure where the oxo and CO are trans. However, the CO ligand is stabilized in the xy plane as a result of π back-bonding.

Scheme 4. Oxidation of 2a to 4

for homogeneous hydrogenation reactions under mild conditions.

■

RESULTS AND DISCUSSION Reactions of 1a with CO. When a benzene solution of 1a was exposed to 40 psi of carbon monoxide in methylene chloride, the hydroxy carbonyl complex 2 was formed (Scheme 1). The reaction was monitored by 1H NMR spectroscopy; the

The FTIR spectrum of 4 suggests a loss of the CO ligand (see the Supporting Information). Two new ReO stretches at 942 and 916 cm−1 were observed by FTIR spectroscopy and are attributed to the slow exchange of the hydroxyl proton between the two oxo sites (Scheme 5). Further, two OH

Scheme 1. Reactivity of 1a with CO

Scheme 5. Proposed Proton Exchange in 4

reaction proceeded cleanly to give the product in approximately 83% yield (see the Supporting Information), and 1a and 2a were the only species detected. Complex 2a was isolated as a yellow powder. By 1H NMR spectroscopy, the Re−OH group is observed at 7.8 ppm. The downfield shift is consistent with several Re−OH species previously synthesized by Mayer and co-workers.12 A single, terminal CO stretch was observed at 1842 cm−1 in the FTIR spectrum for 2a, which is in good agreement with other Re(III) carbonyl species previously isolated by our group.10b,d,13 The O−H stretch was also observed at 3546 cm−1. To confirm the proposed structure for 2a, an alternative synthesis was attempted. Treatment of the complex (MesDAAm)Re(CO)Cl (3) with excess KOH also resulted in 2a (Scheme 2). The 1H NMR and FTIR spectra obtained for this reaction were identical with those for the analogous reaction with 1a and CO (see the Supporting Information). In addition, treatment of a solution of 2a with 5.0 μL of D2O resulted in a loss of the signal for the −OH proton (Scheme 3). Complex 2a progressively oxidizes to the oxo−hydroxy complex (DAAm)Re(O)(OH) (4) upon exposure to air over 16 h at −40 °C. The solution of 2a slowly changes color from

stretches with approximately the same intensity as the two rhenium−oxo stretches were observed at 3523 and 3510 cm−1. Similar behavior was observed by Mayer and co-workers in the complex (acetylene)2Re(O)OH.12 To confirm the proposed structure, 4 was alternatively synthesized according to Scheme 6. The treatment of (MesDAAm)Re(O)Cl (5) with excess NH4OH results in the quantitative formation of 4. The 1H NMR spectra for the products from the reactions depicted in Scheme 5 and 7 were identical. Further, a preliminary crystal structure confirmed the Scheme 6. Alternative Synthesis of 4

B


Article

Organometallics Scheme 7. Reactivity of 1b with CO

Figure 2. Thermal ellipsoid plot of 6 (50% probability ellipsoids). Hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (deg): Re1−C1, 1.846(5); Re−N1, 1.933(4); Re1− O6, 2.203(4); Re2−C3, 1.864(6); Re2−O5, 2.177(4); Re2−N6, 2.193(5); Re1−N3, 1.920(4); Re1−O4, 2.134(4); Re1−N2, 2.238(4); Re2−C2, 1.872(5); Re2−N4, 2.186(4); Re2−N5, 2.223(4); O2−C2, 1.159(7); O1−C1, 1.169(7); O3−C3, 1.172(7); O4−C4, 1.295(6); O5−C4, 1.260(6); O6−C4, 1.296(6); O4−C4−O6, 114.6(4); O5− C4−O6, 123.5(5); O5−C5−O4, 121.8(5); C10−N4−Re2, 109.9(3); C11−N5−Re2, 106.2(3); C13−N6−Re2, 109.9(3).

expected connectivity for 4 (see the Supporting Information). The reaction of 1a with CO is formally a 1,2-hydrogen migration that is induced by CO.12a To the best of our knowledge, this type of reaction has not been reported previously. Reactions of 1b with CO. When a benzene solution of 1b was exposed to 40 psi of carbon monoxide under nitrogen, the carbonate-bridged bimetallic complex 6 was obtained, in addition to the complex [(C6F5-DAAm)Re(CO)3][Re(O)4] (7) (Scheme 7). NMR and FTIR data, as well as an X-ray crystal structure for 6, provided firm evidence to support the two proposed structures. Complex 7 is insoluble in most organic solvents and is easily isolated by filtration.14 The 1H NMR spectrum of 7 features the characteristic four signals, each split into a ddd pattern for the backbone of the DAAm ligand. Two CO stretches were observed by FTIR spectroscopy with a single sharp stretch at 1930 cm−1 for two symmetrical CO ligands in plane and another band of medium intensity at 1899 cm−1, attributed to the apical CO ligand. Complex 6 features a broad singlet at 8.36 ppm for the N−H protons by 1H NMR spectroscopy, as well as six separate signals for the methylene backbone. These data are consistent with the asymmetric bimetallic nature of the complex, which was confirmed by X-ray crystallography (Figure 2). The crystal structure for 6 features two severely distorted Re centers bridged by a carbonate (CO32−) group. The C−O bond lengths within this group are similar and range from 1.260 to 1.296 Å. The geometry around the nitrogen atoms in one of the chelating ligands is best described as distorted tetrahedral, as the angles, for example, for the non-hydrogen atoms around N6 are 114, 114, and 110°. Protonation of the chelating ligand was also confirmed by 1H NMR spectroscopy, as amine protons were observed at 8.36 ppm. In contrast, nitrogen atoms in the chelating ligand on the other half of the molecule are sp2 hybridized, as the sum of the angles around N1 (115, 118, and 127°), for example, is 360°. Further, the Re−N bond lengths to N4 and N6 (2.186(4) and 2.193(5) Å, respectively) are significantly longer than the corresponding bonds to N1 and N3 (1.933(4) and 1.9204(4) Å, respectively). Thus, 6 is formally a Re(III)−Re(I) bimetallic complex. Computational Studies on CO Insertion. Reaction of 1 with CO. M0611 calculations were employed to model the reaction of complexes (DAAm)Re(O)H15 (DAAm = N,Nbis(2-arylaminoethyl)methylamine; aryl = Mes (1a), C6F5 (1b)) with carbon monoxide. By analogy to our prior studies,10b two pathways were initially considered with 1a (Scheme 8).

Scheme 8. Potential Mechanisms for the Reaction of (MesDAAm)Re(O)(H) (1a)

The first pathway is analogous to the previously calculated mechanism for the reaction of CO with (DAAm)Re(O)(CH3).10 Initially, direct insertion of CO into the rhenium− hydride bond affords a rhenium formyl complex. In the presence of CO, this is followed by 1,2-formyl migration to the oxo ligand to afford a rhenium(III) formate complex. The second pathway considered was originally proposed by Wojcicki and co-workers for the reaction of a cationic oxorhenium hydride with CO.14,16 This pathway begins with the nucleophilic attack of CO on the electrophilic oxo ligand, yielding a CO2 adduct. Insertion of CO2 into the rhenium− hydride bond, followed by subsequent CO coordination, also yields a rhenium(III) formate complex. The M06 functional was employed, as it was previously shown to provide accurate calculations for similar molecules.10a All geometry and transition state optimizations utilized a double-ζ 6-31G(d, p)17 basis set on light atoms and SDD18 (added f polarization) on rhenium.19 Energies of intermediates C


Article

Organometallics

insertion into the Re−H bond occurs through TS 2a with a low barrier (ΔG⧧ = 4.8 kcal/mol) to afford the formyl species 9a. The overall reaction for 1a to 9a is slightly exergonic (ΔG° = −0.4 kcal/mol). Addition of CO to 9a results in formyl to oxo migration and the formation of the rhenium(III) formate 10a. Pathway B (black) begins with the initial coordination of CO cis to the ReO to form the CO adduct 11a. The coordination of CO effectively forces the hydride ligand out of the squarepyramidal plane and trans to the oxo ligand. This geometrical arrangement is slightly exergonic (ΔG° = −0.7 kcal/mol). From this adduct, the reaction proceeds through TS 3a (ΔG⧧ = 20.5 kcal/mol) to yield the CO2 adduct 12a, making the reaction 1a to 12a endergonic overall (ΔG° = 13.6 kcal/mol).22 Hydride migration to the CO2 ligand followed by the addition of CO leads to the formation of the ReIII formate 10a. Further pursuit of intermediates along this pathway was not carried out due to the high energies of TS 3a (20.5 kcal/mol) and intermediate 12a (13.6 kcal/mol). Thus, the lowest energy pathway for the activation of CO involves the insertion of CO into a Re−H bond to form a rhenium formyl complex, rather than CO2 adduct formation via the nucleophilic attack of CO on the oxo ligand, as the energies of in both intermediates and transition states were more favorable for the former pathway. Reaction of Rhenium(V) Formyl with CO. Reactions with MesDAAm Ligand. The lowest energy pathway for CO insertion into the rhenium−hydride bond in 1a involves the approach of CO trans to the Re oxo bond to generate the rhenium(V) formyl 9a. Two competing pathways have been identified for the reduction of 9a (Figure 4). The initial reaction of 9a with CO yields the high-energy intermediate 13a (ΔG° = 18.9 kcal/mol). CO inserts into the rhenium−formyl bond in 13a to afford the intermediate 14a (ΔG° = 5.8 kcal/mol), which is a rare example of a transition metal α-dicarbonyl complex that results from CO insertion into a metal−formyl

and transition states were calculated with a triple-ζ (6-311G+ +(d,p))20 basis set on light atoms and SDD (added f polarization) on rhenium and included solvent corrections by utilizing a PCM solvation model with benzene as the solvent.21 The geometries and free energies (ΔG°) of intermediates and transition states (ΔG⧧) for the initial activation of CO by oxorhenium complexes are outlined in Figure 3.

Figure 3. M06 calculated pathways for the reaction of 1a with CO. Pathway A is shown in red, while pathway B is shown in black. Reported energies included are at 298 K in benzene and include solvent corrections by utilizing the PCM solvation model.21 Computational details are provided in the Experimental Section.

Pathway A (red) involves the insertion of CO into the rhenium−hydride bond of 1a. Initial coordination of CO trans to the ReO bond proceeds through TS 1a (ΔG⧧ = 15.5 kcal/ mol) and affords the CO adduct 8a (ΔG° = 2.8 kcal/mol). CO

Figure 4. M06 calculated pathways for the reaction of 1a with CO. Pathway A is shown in red, while pathway B is shown in black. Reported energies included are at 298 K in benzene and include solvent corrections by utilizing the PCM solvation model.21 Computational details are provided in the Experimental Section. D


Article

Organometallics

Figure 5. M06 calculated pathways for the reaction of 1b with CO. Pathway A is shown in red, while pathway B is shown in black. Reported energies included are at 298 K in benzene and include solvent corrections by utilizing the PCM solvation model.21 Computational details are provided in the Experimental Section.

Table 1. Computed (M06) Thermodynamic and Kinetic Parameters for the Insertion of CO into Rhenium Hydride and Methyl Bonds in 1 and 15a complex

ΔG° (kcal/mol)

ΔG⧧ (kcal/mol)

ΔH° (kcal/mol)

1a (Aryl = Mes) 1b (Aryl = C6F5) 15a (Aryl = Mes) 15b (Aryl = C6F5)

−0.4 −1.4 −5.1 −5.7

15.5 13.5 22.9 23.9

−13.1 −12.9 −17.8 −18.7

a

Reported energies included are at 298 K in benzene and include solvent corrections by utilizing the PCM solvation model. Computational details are provided in the Experimental Section.

bond. The first pathway in Figure 4 involves migration of the formyl fragment in 14a to the oxo ligand to produce the rhenium(III) formate 10a. The overall process (9a to 10a) is exergonic (ΔG° = −32.5 kcal/mol) and proceeds with an activation barrier of 19.5 kcal/mol. A second pathway starting from 14a was also identified. The close proximity of the proton of the α-dicarbonyl ligand to the oxo ligand results in proton transfer to produce the rhenium(III) hydroxo complex 2a. The overall reaction (9a to 2a) is also exergonic but less so than the formation of the rhenium(III) formate (ΔG° = −25.2 kcal/mol). However, the activation barrier for this transformation (13.8 kcal/mol) is 5.8 kcal/mol lower than the pathway for the formation of 2a. Therefore, calculations suggest that the lowest energy pathway for the reaction of the rhenium(V) formyl 9 with CO involves initial CO insertion into the rhenium formyl of 9 to produce a rare example of a rhenium α-dicarbonyl complex, 14,23 followed by proton transfer to the oxo ligand in 14 to produce the rhenium(III) hydroxy carbonyl 2.24 Reactions with (C6F5)DAAm Ligand. In order to examine the effects of the electronics of the ancillary diamidoamine ligand, calculations were also explored with 1b, where the aryl ligand was C6F5 instead of mesityl in 1a. As shown in Figure 5, a similar pathway was calculated for 1b. It is noteworthy that the formation of 9b from 1b is more exergonic than the formation of 9a from 1a (ΔG° = −1.4 and −0.4 kcal/mol, respectively). A similar pathway was calculated for the

migratory insertion of CO into the rhenium−formyl bond in 9b. However, the difference in the activation barriers between the pathways for the formation of 2 and 10 was smaller (ΔΔG⧧ = 5.7 and 3.8 kcal/mol for the pathways for 9a,b, respectively). The computational results presented suggest that insertion of CO into the rhenium−hydride bond is favorable both kinetically and thermodynamically for these systems. Insertion of CO into a metal−hydride bond is rare because a metal− formyl bond is thought to be weaker than a metal−hydride bond.5a−e,25 However, we have shown here that with the correct metal, and choice of ancillary ligands, this insertion reaction can be thermoneutral to favorable. As shown in Table 1, in comparison to insertion into the rhenium−methyl bond in (DAAm)Re(O)(CH3) (15), insertion into the rhenium−hydride bond is kinetically favorable. However, insertion into the rhenium−methyl bond is more favorable thermodynamically in comparison to insertion into the rhenium−hydride bond. This is in support of the previously proposed postulate that migratory insertion of CO into a metal−hydrogen bond is governed by thermodynamic rather than kinetic factors.9a The calculations also reveal an unexpected reaction for transition-metal formyl complexes: i.e., migratory insertion of CO into the rhenium− formyl bond to form the α-dicarbonyl species 14.26 The optimized structures for 14, TS 4, and TS 5 are shown in Figures 6 and 7. In order to facilitate proton transfer (Figure 6) from the α-dicarbonyl ligand to the oxo ligand, these groups E


Article

Organometallics

significantly (carbon−carbon = 2.13 and 2.10 Å for TS 10a and TS 5b, respectively). Comparison of Insertion into a Rhenium−Methyl Bond to Insertion into a Rhenium−Hydride Bond. In order to understand the reaction of oxorhenium hydrides with CO, the reactions of 1 and the rhenium methyl complex 15 with CO were compared. We previously reported calculations on the insertion of the oxorhenium methyl complex 15 with CO at the B3PW9127 level of theory.10a,b For this study, calculations with 1a were repeated with the M06 functional. Consistent with our previous results, a similar pathway was calculated for carbonyl insertion into the rhenium−methyl bond in 15 (Figure 8). In addition, calculations revealed that, after initial direct insertion of CO to afford the acyl complex 16 (ΔG° = −5.1 kcal/mol), migratory insertion of CO results in the new α-dicarbonyl intermediate 17. From 17, the migration of the acyl fragment to the oxo ligand leads to the acetate complex 18. The activation barrier for this process is 18.6 kcal/ mol. Unlike the analogous reaction with the oxorhenium hydride 1, no pathway was found for the transfer of the methyl to the oxo ligand. In order to understand this difference in reactivity, the optimized structures of the two α-dicarbonyl complexes 14 and 17 were compared (Figure 9). The carbonyl ligands in 14a are oriented cis, while the dicarbonyls are oriented trans in 17. In addition, as described above, because of the cis orientation in 14a, the formyl proton is in close proximity to the oxo ligand (1.42 Å), which suggests that proton transfer from the α-dicarbonyl ligand to the oxo ligand should be facile. However, this orientation is not present in 17 (dihedral angle 150°);26d as a result, transfer of the methyl group from the α-dicarbonyl ligand to the oxo ligand does not occur and is not observed experimentally. Reactivity of Oxorhenium Hydrides with CO. Computational studies suggest that facile insertion of CO into rhenium− hydride and −formyl bonds should occur in these systems. Reactions of 1a,b with CO appear to be consistent with the computational data. As proposed in Scheme 9, the left-hand portion of 6 (red) results from CO insertion into the rhenium− hydride bond in 1b to produce the rhenium(III) formate 10b, while the right-hand portion of 6 (black) results from CO insertion into the rhenium−hydride bond in 1b to produce the hydroxyl carbonyl complex 2b. The reaction of 10b with 2b results in 6. Comparison with Computational Data. The contrasting reactivities of 1a and 1b with CO are interesting. While the reaction of 1a with CO leads exclusively to the hydroxy carbonyl 2a, the reaction of 1b with CO appears to result in both the hydroxy carbonyl 1b and the Re(III) formate 10b, which combine to form the Re(III)−Re(I) bimetallic 6. This reactivity is consistent with the computational data described above. Recall that it was predicted from M06 calculations that the reaction of 1 with carbon monoxide should result in insertion of CO into the rhenium−hydride bond to generate 9 (Figures 4 and 5). Subsequent insertion of CO into the rhenium bond leads to the α-dicarbonyl complex 14. Complex 14 can then either transfer a proton to the oxo ligand to generate 9 or transfer a formyl group to generate 10. The differences in transition state free energies of activation (ΔΔG⧧) for the formation of 2 and 10 were 5.7 kcal/mol for 2a and 10a and 3.8 kcal/mol for 2b and 10b, respectively. Thus, the calculations accurately predicted the selectivity observed experimentally: i.e.,

Figure 6. Optimized structures (M06) for proton transfer from the αdicarbonyl group in 14 to the oxo ligand: (a) TS 4a, (b) 14a; (c) TS 4b; (d) 14b. Bond lengths are given in angstroms (Å). For clarity, the mesityl and C6F5 groups on the diamido ligand are shown in wireframe format. Computational details are provided in the Experimental Section.

Figure 7. Optimized structures (M06) for formyl group transfer from the α-dicarbonyl group in 13 to the oxo ligand: (a) TS 5a, (b) 14a; (c) TS 5b; (d) 14b. Bond lengths are in angstroms (Å). For clarity, the mesityl and C6F5 groups on the diamido ligand are shown in wireframe format. Computational details are provided in the Experimental Section.

become coplanar in TS 4 (torsion angles −3.20 and 0.00° for TS 4a and TS 4b, respectively). The carbon−carbon bond in TS 4 is significantly lengthened (2.14 Å for TS 4a and TS 4b, respectively); in addition, the carbon−hydrogen bond in the αdicarbonyl group (1.21 and 1.20 Å for TS 4a and TS 4b, respectively) is also lengthened. An O−H bond (1.42 and 1.43 Å, TS 4a and TS 4b, respectively) is present in TS 4. In contrast, in order to facilitate transfer of the HCO group in the α-dicarbonyl ligand to the oxo ligand (Figure 7), the formyl fragment is orthogonal to the rhenium−oxo bond (torsion angles 100 and 96.7° for TS 10a and TS 5b, respectively). TS 5 contains a rhenium−carbon bond to the migrating formyl group (Re−formyl = 2.28 and 2.30 Å for TS 10a and TS 5b, respectively). In addition, a bond has formed between the oxo ligand and the migrating formyl (oxo−formyl = 1.94 and 1.97 Å for TS 10a and TS 5b, respectively) and the carbon−carbon bond in the α-dicarbonyl group is cleaved F


Article

Organometallics

Figure 8. M06 calculated comparison of insertion into a rhenium−methyl bond in 14 to a simplified mechanism for insertion into a rhenium− hydride bond in 1a (see Figure 3 for the detailed mechanism). Reported energies included are at 298 K in benzene and include solvent corrections by utilizing the PCM solvation model.21 Computational details are provided in the Experimental Section.

Scheme 9. Proposed Mechanism for the Formation of 6

Figure 9. Optimized structures (M06) for acyl group transfer from the α-dicarbonyl group in 13 and 16 to the oxo ligand: (a) TS 4a; (b) 14a; (c) TS 7; (d) 17. Bond lengths are in angstroms (Å). For clarity, the mesityl group on the diamido ligand is shown in wireframe format. Computational details are provided in the Experimental Section.

occurs. In the case of C6F5-substituted ligands these adducts are unstable and a direct insertion occurs as in the previously reported DAAm and DAP alkyl complexes.

■

CONCLUSIONS In this manuscript, density functional theory calculations (M06)11 have been performed in synergy with experimental studies to show that the thermodynamics for insertion of CO into a oxorhenium−hydride bond to form a formyl ligand is favorable. The careful choice of ancillary ligand is critical for these insertion reactions, and calculations accurately predict that CO insertion into oxorhenium hydrides with mesitylsubstituted diamidoamine ligands results in a carbonyl hydroxide rhenium(III) complex while oxorhenium hydrides with C6F5-substituted diamidoamine ligands result in both a carbonyl hydroxide rhenium(III) complex and a carbonyl

because of the larger difference in the free energies of activation, 2a should be produced exclusively from 14a while both 9b and 4b should be produced from 14b. These two species then react to produce the bimetallic 6. The mechanism for CO insertion results from the presence of a strong trans influence oxo ligand in the axial position of a square pyramid in these oxorhenium(V) complexes. In carbonylation reactions with metal oxo complexes, the weakening of a ligand trans to the oxo ligand leads to the instability of the resultant carbonyl. Thus, for the mesitylsubstituted DAAm ligand, species such as 8a, 13a, and 17, CO adducts are relatively unstable and facile C−H bond formation G


Article

Organometallics

−CH2−), 2.79 (s, 3H, N−CH3), 2.30 (s, 6H, Mes-CH3), 2.19 (s, 6H, Mes-CH3), 2.18 (s, 6H, Mes-CH3). 13C NMR (CD2Cl2, 300 K, 400 MHz): δ 134.31, 133.36, 130.97, 128.70, 128.65, 60.53, 58.30, 42.51, 20.59, 19.03, 18.95. IR (FTIR, cm−1): ν(CO) 1842 cm−1. Because of the instability of this molecule elemental analysis was not possible. Synthesis of (MesDAAm)Re(O)(OH) (4). In a 25 mL pressure vessel, (MesDAAm)Re(O)H (50.0 mg, 0.09 mmol) was dissolved in 10 mL of CH2Cl2. The reaction mixture was subject to three freeze− pump−thaw cycles, upon which it was pressurized with CO (30 psi). The reaction mixture was stirred at room temperature for 16 h, upon which the solvent was removed in vacuo. The orange residue was then redissolved in a minimal amount of CH2Cl2 and placed in a freezer at −40 °C overnight. After 16 h, a green powder precipitated and was collected on a filter frit and dried in vacuo. 1H NMR (CD2Cl2, 300 K, 300 MHz): δ 9.80 (s, 1H, Re−OH), 6.85 (s, 2H, Mes-aromatic H), 6.84 (s, 2H, Mes-aromatic H), 4.05 (ddd, 3JHH = 12.6 Hz, 3JHH = 8.7 Hz, 2JHH = 4.4 Hz, 2H, ligand −CH2−), 3.48 (td, 3JHH = 8.4 Hz, 3JHH = 5.1 Hz, 2H, ligand −CH2−), 3.38 (s, 3H, N−CH3), 3.22 (td, 3JHH = 8.1 Hz, 3JHH = 4.4 Hz, 2H, ligand −CH2−), 3.01 (td, 3JHH = 8.1 Hz, 3 JHH = 4.4 Hz, 2H, ligand −CH2−), 2.28 (s, 6H, Mes-CH3), 2.22 (s, 6H, Mes-CH3), 1.93 (s, 6H, Mes-CH3). 13C NMR (CD2Cl2, 300 K, 400 MHz): δ 135.77, 135.37, 134.37, 129.10, 128.99, 69.10, 68.67, 20.70, 18.91, 18.63. IR (FTIR, cm−1): 3523, 3510 (−OH, two isomers). 942, 916 (ReO, two isomers) 592, 545 cm−1 (Re−OH, two isomers). Because of the instability of this molecule elemental analysis was not possible. Computational Methods. Theoretical calculations have been carried out using the Gaussian 09 implementation of M06 density functional theory. All geometry optimizations were carried out in the gas phase using tight convergence criteria (“opt = tight”) and pruned ultrafine grids (“Int = ultrafine”). 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)18 with an additional f polarization function.19 The 6-31G(d,p) basis set was used for all other atoms.17 All structures were fully optimized. 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 were determined by animation of the imaginary frequency. Energetics were calculated with the 6-311++G(d,p)20a basis set for C, H, N, O, and F atoms and the SDD basis set with an added f polarization function on Re. Reported energies utilized analytical frequencies and the zero point corrections from the gas-phase optimized geometries and included solvation corrections which were computed using the PCM method,21 with benzene as the solvent, as implemented in Gaussian 09.

rhenium(III) formate that ultimately combine to form a Re(III)−Re(I) bimetallic complex. The most important findings from this study are (1) CO insertions into M−H bonds are indeed thermodynamically favorable despite conventional wisdom to the contrary5a−e and (2) insertion of CO into acyl/formyl ligands to form an αdicarbonyl ligand is also a viable pathway and results in hydroxy carbonyl or formyl complexes depending on the nature of the ancillary ligand. Thus, this work shows that mechanisms that involve CO insertions into M−H or M−acyl/formyl bonds should still be considered and may have important implications for the many catalytic reactions where metal formyls and αdicarbonyls have been proposed as relevant intermediates.

■

EXPERIMENTAL SECTION

General Considerations. Complexes 1a,b, 3, and 17 were prepared as previously reported;10c,13a,28 all other reagents were purchased from commercial resources and used as received. 1H, 13C, and 19F NMR spectra were obtained on 300 or 400 MHz spectrometers at room temperature. Chemical shifts are listed in parts per million (ppm) and are referenced to their residual protons or carbons of the deuterated solvents, respectively. All reactions were run under an inert atmosphere with dry solvents unless otherwise noted. FTIR spectra were obtained in KBr thin films. Elemental analyses were performed by Atlantic Micro Laboratories, Inc. Synthesis of 6. (DAAm)Re(O)H (Ar = C6F5, 1b; 100 mg, 0.154 mmol) was placed in the 100 mL pressure vessel and brought into the glovebox. Anhydrous benzene was then added to dissolve the complex, and the reaction vessel was removed and subjected to three freeze− pump−thaw cycles. The reaction mixture was then pressurized with CO (40 psi) and was stirred overnight. The reaction mixture was then filtered to remove the byproduct [(CO)3Re(DAAm)][ReO4] (7), and the filtrate was concentrated under reduced pressure. Addition of excess hexane resulted in a tan solid, which was collected on the filter frit and dried under vacuum (37.0 mg isolated, 34% yield). 1H NMR (CD2Cl2, 300 K, 300 MHz): δ 8.40 (s, 2H, N−H), 4.16 (overlapping dt, 3JHH = 12.4 Hz, 3JHH = 5.7 Hz, 4H, ligand − CH2) 3.84 (dt, 3JHH = 11.4 Hz, 3JHH = 5.5 Hz, 2H, ligand −CH2−) 3.68 (dt, 3JHH = 13.0 Hz, 3 JHH = 4.4 Hz, 2H, ligand −CH2−) 3.46 (s, 3H, N−CH3) 3.43 (td, 3 JHH = 10.3 Hz, 3JHH = 5.1 Hz, 2H, ligand −CH2−) 3.01 (td, 3JHH = 12.4 Hz, 3JHH = 5.7 Hz, 2H, ligand −CH2−) 2.84 (dd, 3JHH = 13.2 Hz, 2 JHH = 3.4 Hz, 2H, ligand −CH2−) 2.60 (s, 3H, N−CH3). 13C NMR ((CD2Cl2, 300 K, 400 MHz): δ 200.59, 199.83, 188.80, 163.79, 139.17, 137.77, 136.47, 124.92, 63.03, 58.26, 57.96, 48.83. IR (FTIR, cm−1): ν(CO) 1913, 1866, 1816 cm−1. Satisfactory elemental analysis could not be obtained. Isolation of 7. Complex [(CO)3Re(DAAm)][ReO4] (7) was isolated as a byproduct during the preparation of 6 as a white powder (synthesis as described above). Isolated: 14.0 mg (0.018 mmol, 23% yield). 1H NMR (CD3CN, 300 K, 300 MHz): δ 3.92 (dt, 3JHH = 13.5 Hz, 3JHH = 6.6 Hz, 2H, ligand −CH2−), 3.71 (dt, 3JHH = 11.6 Hz, 3JHH = 5.8 Hz, 2H, ligand −CH2−), 2.93 (dt, 3JHH = 11.6 Hz, 3JHH = 5.8 Hz, 2H, ligand −CH2−), 2.81 (dt, 3JHH = 11.8 Hz, 3JHH = 5.8 Hz, 2H, ligand −CH2), 2.37 (s, 3H, N−CH3). IR (FTIR, cm−1): ν(CO) 1930, 1899 cm−1. Synthesis of (MesDAAm)Re(CO)(OH) (2a). In a 25 mL pressure vessel, (MesDAAm)Re(O)H (50.0 mg, 0.09 mmol) was dissolved in 10 mL of CH2Cl2. The reaction mixture was subject to three freeze− pump−thaw cycles, upon which it was pressurized with CO (30 psi). The reaction mixture was stirred at room temperature for 16 h, upon which the solvent was removed in vacuo. The orange residue was then redissolved in a minimal amount of CH2Cl2. Addition of excess pentane precipitated a yellow powder, which was collected on a filter frit and dried in vacuo to afford the product. Isolated: 19.0 mg, 36% yield. 1H NMR (CD2Cl2, 300 K, 300 MHz): δ 7.81 (s, 1H, Re−OH), 6.87 (s, 2H, Mes-aromatic H), 6.86 (s, 2H, Mes-aromatic H), 3.75 (dt overlapping, 3JHH = 13.5 Hz, 3JHH = 6.7 Hz, 4H, ligand −CH2−), 2.97 (tdd, 3JHH = 17.3 Hz, 3JHH = 11.6 Hz, 3JHH = 5.5 Hz, 4H, ligand

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00591. X-ray data, additional experimental and characterization data, computational details, and full Gaussian reference (PDF) Cartesian coordinates for the calculated structures (XYZ) X-ray experimental data for 6 (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for E.A.I.: [email protected]. Present Address †

Department of Chemistry, Physics, and Geoscience, Meredith College, 3800 Hillsborough Street, Raleigh, NC 27607-5298, USA. H


Article

Organometallics Author Contributions

(d) Smeltz, J. L.; Boyle, P. D.; Ison, E. A. J. Am. Chem. Soc. 2011, 133, 13288−13291. (11) Zhao, Y.; Truhlar, D. Theor. Chem. Acc. 2008, 120, 215−241. (12) (a) Tahmassebi, S. K.; Conry, R. R.; Mayer, J. M. J. Am. Chem. Soc. 1993, 115, 7553−4. (b) Spaltenstein, E.; Erikson, T. K. G.; Critchlow, S. C.; Mayer, J. M. J. Am. Chem. Soc. 1989, 111, 617−23. (13) (a) Smeltz, J. L.; Lilly, C. P.; Boyle, P. D.; Ison, E. A. J. Am. Chem. Soc. 2013, 135, 9433−9441. (b) Smeltz, J. L.; Boyle, P. D.; Ison, E. A. Organometallics 2012, 31, 5994−5997. (14) (a) Kim, Y.; Gallucci, J.; Wojcicki, A. J. Am. Chem. Soc. 1990, 112, 8600−2. (b) Kim, Y.; Rende, D. E.; Gallucci, J. C.; Wojcicki, A. J. Organomet. Chem. 2003, 682, 85−101. (15) Feng, Y.; Aponte, J.; Houseworth, P. J.; Boyle, P. D.; Ison, E. A. Inorg. Chem. 2009, 48, 11058−11066. (16) Kim, Y.; Rende, D. E.; Gallucci, J. C.; Wojcicki, A. Inorg. Chim. Acta 2003, 352, 171−178. (17) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (18) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866−872. (19) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111−114. (20) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (b) Miller, W. H.; Handy, N. C.; Adams, J. E. J. Chem. Phys. 1980, 72, 99−112. (21) Tomasi, J.; Mennucci, B.; Cances, E. J. Mol. Struct.: THEOCHEM 1999, 464, 211−226. (22) A pathway was also found for insertion into the Re−H bond in 11a. However, this pathway was found to be higher in energy overall. See the Supporting Information. (23) (a) des Abbayes, H.; Salaun, J.-Y. Dalton Trans. 2003, 1041− 1052. (b) Radu, N. S.; Engeler, M. P.; Gerlach, C. P.; Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1995, 117, 3621−3622. (c) Geoffroy, G.; Sheridan, J. B.; Bassner, S. L.; Kelley, C. Pure Appl. Chem. 1989, 61, 1723−1729. (24) Different pathways for the formation of 9a were also explored. These pathways were all found to be higher in energy. Migration of the H ligand in 8a to the oxo ligand, for example, proceeds with a barrier of 23.4 kcal/mol. However, migration of the formyl H to the oxo ligand proceeds with a barrier of 65.7 kcal/mol. Migration of the H ligand in 1a to the oxo ligand proceeds with a barrier of 48.2 kcal/mol. (25) (a) Blomberg, M. R.; Karlsson, C. A.; Siegbahn, P. E. J. Phys. Chem. 1993, 97, 9341−9350. (b) Pacchioni, G.; Fantucci, P.; Koutecký, J.; Ponec, V. J. Catal. 1988, 112, 34−43. (c) Ziegler, T.; Versluis, L.; Tschinke, V. J. Am. Chem. Soc. 1986, 108, 612−617. (26) (a) Chen, J. T. J. Chin. Chem. Soc. 1992, 39, 603−610. (b) Vetter, W. M.; Sen, A. J. Organomet. Chem. 1989, 378, 485−491. (c) Bassner, S. L.; Sheridan, J. B.; Kelley, C.; Geoffroy, G. L. Organometallics 1989, 8, 2121−2126. (d) Casey, C. P.; Bunnell, C. A.; Calabrese, J. C. J. Am. Chem. Soc. 1976, 98, 1166−1171. (27) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Burke, K.; Perdew, J.; Wang, Y. Electronic Density Function Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998. (28) Lilly, C. P.; Boyle, P. D.; Ison, E. A. Dalton Trans. 2011, 40, 11815−11821.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

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

■

REFERENCES

(1) (a) Olah, G. A.; Goeppert, A.; Prakash, G. S. Beyond Oil and Gas: the Methanol Economy; Wiley: Hoboken, NJ, 2011. (b) Olah, G. A.; Prakash, G. S.; Goeppert, A. J. Am. Chem. Soc. 2011, 133, 12881− 12898. (c) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44, 2636−2639. (d) Olah, G. A. Catal. Lett. 2004, 93, 1−2. (2) (a) Studt, F.; Abild-Pedersen, F.; Varley, J. B.; Nørskov, J. K. Catal. Lett. 2013, 143, 71−73. (b) Studt, F.; Abild-Pedersen, F.; Wu, Q.; Jensen, A. D.; Temel, B.; Grunwaldt, J.-D.; Nørskov, J. K. J. Catal. 2012, 293, 51−60. (c) Grabow, L. C.; Mavrikakis, M. ACS Catal. 2011, 1, 365−384. (d) West, N. M.; Miller, A. J.; Labinger, J. A.; Bercaw, J. E. Coord. Chem. Rev. 2011, 255, 881−898. (e) Bermúdez, J.; Fidalgo, B.; Arenillas, A.; Menéndez, J. Fuel 2010, 89, 2897−2902. (f) Gotti, A.; Prins, R. Catal. Lett. 1996, 37, 143−151. (g) Bradley, J. S. J. Am. Chem. Soc. 1979, 101, 7419−7421. (3) Dry, M. E. Catal. Today 2002, 71, 227−241. (4) (a) Schulz, H. Appl. Catal., A 1999, 186, 3−12. (b) Iglesia, E. Appl. Catal., A 1997, 161, 59−78. (c) Dry, M. E. Appl. Catal., A 1996, 138, 319−344. (d) Jager, B.; Espinoza, R. Catal. Today 1995, 23, 17− 28. (5) (a) Bochmann, M. Organometallics and Catalysis: An Introduction; Oxford University Press: New York, 2015. (b) Hartwig, J. F. Organotransition Metal Chemistry: from Bonding to Catalysis; University Science Books: Mill Valley, CA, 2010. (c) Spessard, G. O.; Miessler, G. L., 2009. (d) Ugo, R.; Psaro, R. J. Mol. Catal. 1983, 20, 53−79. (e) Kuhlmann, E. J.; Alexander, J. J. Coord. Chem. Rev. 1980, 33, 195− 225. (f) Fahey, D. R. J. Am. Chem. Soc. 1981, 103, 136−141. (g) Pruett, R. L. Ann. N. Y. Acad. Sci. 1977, 295, 239−248. (h) Dombek, B. D. J. Am. Chem. Soc. 1980, 102, 6855−6857. (6) Gladysz, J. Adv. Organometal. Chem. 1982, 20 (1). (7) (a) Imler, G. H.; Zdilla, M. J.; Wayland, B. B. J. Am. Chem. Soc. 2014, 136, 5856−5859. (b) Cui, W.; Li, S.; Wayland, B. B. J. Organomet. Chem. 2007, 692, 3198−3206. (c) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2005, 127, 16460−16467. (d) Wayland, B. B.; Coffin, V. L.; Sherry, A. E.; Brennen, W. R. Bonding Energetics in Organometallic Compounds 1990, 428, 148. (e) Wayland, B. B. Polyhedron 1988, 7, 1545−1555. (f) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc. 1986, 108, 3659−3663. (g) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986, 25, 4039− 4042. (h) Wayland, B. B.; Duttaahmed, A.; Woods, B. A. J. Chem. Soc., Chem. Commun. 1983, 142−143. (i) Wayland, B. B.; Woods, B. A.; Pierce, R. J. Am. Chem. Soc. 1982, 104, 302−303. (8) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985, 107, 4333−4335. (9) (a) Fagan, P. J.; Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6959−6962. (b) Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 7051−7064. (c) Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121−127. (d) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100, 2716−2724. (10) (a) Robbins, L. K.; Lilly, C. P.; Smeltz, J. L.; Boyle, P. D.; Ison, E. A. Organometallics 2015, 34, 3152−3158. (b) Smeltz, J. L.; Webster, C. E.; Ison, E. A. Organometallics 2012, 31, 4055−4062. (c) Lilly, C. P.; Boyle, P. D.; Ison, E. A. Organometallics 2012, 31, 4295−4301. I


Mechanism for the Reaction of CO with Oxorhenium Hydrides

Recommend Documents