Mechanism of Vanadium-Catalyzed Deoxydehydration of Vicinal Diols

Article pubs.acs.org/Organometallics

Mechanism of Vanadium-Catalyzed Deoxydehydration of Vicinal Diols: Spin-Crossover-Involved Processes Yuan-Ye Jiang,†,‡ Ju-Long Jiang,† and Yao Fu*,† †

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China S Supporting Information *

ABSTRACT: Vanadium-catalyzed deoxydehydration (DODH) reactions provide a cost-effective approach for the conversion of vicinal diols to olefin and polycyclic aromatic hydrocarbons. In this paper, density functional theory (DFT) calculations employing M06 and M06-L methods were conducted to clarify the mechanism of Vcatalyzed DODH. Three types of mechanisms generally proposed for transition-metal-catalyzed DODH, associated with the previously omitted spin crossover processes, were considered herein. As a result, a different catalytic cycle including a new olefin-formation mechanism was located, which is in contrast to the findings of a recent study. We found that the favorable mechanism involves the condensation of diols to form vanadium(V) diolate, reduction of the vanadium(V) diolate by PPh3, and spincrossover steps to form a triplet vanadium(III) diolate. Thereafter, single C−O bond cleavage occurs followed by another spin crossover to form a singlet alkylvanadium(V) intermediate. The final concerted V−O/C−O bond cleavage generates an olefin and finishes the catalytic cycle. The reduction of vanadium(V) diolate by PPh3 and the extrusion of olefin have close overall free energy barriers of 34.3 and 33.7 kcal/mol, respectively. These results suggest that both steps influence the reaction rate. On the other hand, the two mechanisms starting by the reduction of the oxovanadium(V) catalyst with either PPh3 or a secondary alcohol were excluded due to their higher energy demands in the reduction and the olefin-formation stages. The good consistency between the experimental observations and the calculation results verified the proposed mechanism and also enabled us to clarify the reason for the efficiency of different reductants.

1. INTRODUCTION The depletion of fossil fuels is driving widespread interest in biomass as a renewable supply chain for the chemical industry.1 Different from the traditional hydrocarbon feedstock derived from fossil fuels, biomass-derived small molecules, especially polyols (e.g., sugar, glycerinum), are usually oxygen-rich and hyperfunctionalized. Therefore, efficient methods for removing oxygen-containing functionalization groups are required for the utilization of biomass.2 In this respect, various deoxygenation methods, including dehydration,3 hydrodeoxygenation,4 hydrogenolysis,5 decarbonylation,6 decarboxylation,7 and direct deoxygenation,8 have been developed. In addition to these advances, transition-metal-catalyzed deoxydehydration (DODH), which typically transforms a vicinal diol into an olefin directly, has also gradually grown over the last two decades.9 The pioneering work was achieved by Andrews et al. in 1996. They realized the transition-metal-catalyzed DODH of vicinal diols with (C5Me5)ReO3 as the catalyst and with PPh3 as the reductant.10 Thereafter, the DODH reactions based on rhenium catalysts has been largely expanded.11 For instance, the Abu-Omar group12 and Nicholas group13 reported MeReO3-catalyzed DODH with reductants such as hydrogen, © XXXX American Chemical Society

Na2SO3, alcohols, metals, and hydroaromatics. Meanwhile, the catalytic activities of cyclopentadiene-based trioxorhenium carbon- and ceria-supported catalysts in DODH were also investigated.14 On the other hand, Ellman, Bergman, and coworkers reported DODH with low-valent [Re2(CO)10] and BrRe(CO)5.15 Furthermore, Toste, Abu-Omar, Zhang, and coworkers expanded the scope of substrates in MeReO3-catalyzed DODH to polyols and even 2-ene-1,4-diols and 2,4-diene-1,6diols.16 In addition to rhenium catalysts, the DODH with noble ruthenium catalysts was reported, but the efficiency is not satisfactory.17 Despite the achievement of rhenium-catalyzed DODH reactions, increasing attention has been paid to cost-effective catalysts based on the cheaper and earth-abundant molybdenum and vanadium. Fristrup, Montilla, and Galindo have made contributions to Mo-catalyzed DODH reactions.18 Nevertheless, the present Mo-catalyzed DODH reactions require elevated temperatures (195−250 °C) and the olefin yields hardly exceed 80% due to the competitive dehydration of diols and transfer hydrogenation of the hereby-formed aldehyde and Received: July 27, 2016

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DOI: 10.1021/acs.organomet.6b00602 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

paths A and B are both plausible for V-catalyzed DODH and characterization of (dipic)VV-glycolates,22 supporting the intervention of a reduced glycolate. Very recently, the two pathways proposed by Nicholas et al. were evaluated by Galindo with the aid of B3LYP methods.23 By comparing the energy barriers of each elementary steps, Galindo proposed that the preferred mechanism involves the reduction of VV catalyst by PPh3 followed by the condensation of diols, and the olefin formation occurs via the widely reported concerted C−O cleavage. Our recent mechanistic study on V-catalyzed C−O and C−C bond cleavages of lignin model compounds shows that the twostate reactivity24 of vanadium catalysis can lead to unexpected mechanistic profiles.25 Inspired by this, a new mechanism was discovered for the olefin formation stage in our investigation on the previously unaddressed spin crossover processes in Vcatalyzed DODH. The new mechanism involves the single C− O cleavage on triplet vanadium(III) diolate, a spin-crossover to form an alkylvanadium(V) intermediate, and a final concerted V−C/C−O cleavage. Meanwhile, our study supports a sequence of condensation/reduction/olefin formation for Vcatalyzed DODH. In the presence of the new olefin-formation mechanism, the reduction of vanadium(V) diolate by PPh3 was found to be slightly slower than the extrusion of olefin from vanadium(III) diolate. This pathway is different from that proposed by Galindo, yet it has a relatively more reasonable overall free energy barrier, and our calculation results are also consistent with the experimental observation about the efficiency of different reductants.

ketone. In contrast, the Nicholas group employed [n-Bu4N](dipic)VO2 (dipic = 2,6-pyridinedicarboxylate) as the catalyst and realized DODH reactions of glycols at lower temperatures (150−170 °C) with increased yields of 85−97% (Scheme 1).19 Scheme 1. V-Catalyzed Deoxydehydration of Vicinal Diol

Later on, the Krische group successfully applied this vanadiumcatalyzed DODH reaction to synthesize fluoranthenes and acenes, the potential building blocks of organic electronic devices and organic semiconductors.20 With the rapid development of experimental methodologies, many efforts have also been devoted to clarifying the mechanisms of DODH reactions. Nonetheless, the reported studies are mainly limited to MeReO3-catalyzed systems.9,21 It was generally proposed that olefin is generated via the concerted cleavage of two C−O bonds of metal diolate intermediates, whereas three different pathways have been proposed for the formation of the metal diolates. Taking MeReO3 as an example, one possible mechanism starts with the condensation of diols on A to form B, which is further reduced to give C (path A, Scheme 2).16c Finally, C produces olefin and

2. COMPUTATIONAL METHODS DFT calculations were performed by using the Gaussian09 program.26 B3LYP was proposed to systematically overestimate energy barriers27 as shown in a previous study on V-catalyzed DODH,23 while M06 gave better performance and was recommended for both main-group and transition-metal chemistry.27 Therefore, geometry optimization was performed at the level of the M06 method27 in the solution phase with the SMD solvation model28 (solvent = benzene). The ultrafine integration grid was used in the DFT calculations.29 6-31G(d) was used for main-group atoms, while LANL2DZ and the associated basis set30 with a polarization function (ζ( f) = 1.751)31 were used for vanadium in the geometry optimizations. At the same level of theory, frequency analyses were performed to verify the optimized structures as intermediates or transition states and also to obtain the thermodynamic corrections. Meanwhile, an intrinsic reactant coordinate (IRC) analysis32 was performed to check whether or not transition states connect the correct stationary points. The M06-L method27 was recommended for transition-metal thermodynamics, which is important for calculating the relative stability of transitionmetal intermediates and was employed to recalculate the single-point energies in the solution phase on the basis of the optimized structures. For single-point energy calculations, a larger basis set denoting 6311+G(d,p) for main groups and SDD33 for V were used. Possible triplet states of vanadium complexes were investigated with unrestricted DFT calculations. The program developed by Harvey et al. was used to located the minimum energy crossing points (MECP) between the different spin states.34 Because MECPs are not stationary points in the full dimensions of potential energy surfaces, the approximate relative free energies of MECPs are estimated by adding the electronic energy gaps between the MECPs and the corresponding precursors to the relative free energies of the precursors.35 A value of 1.9 kcal/mol was added to the energies of each species to account for the change in standard states from the gas phase to aqueous solution.36 The solution-phase single-point energies corrected by solution-phase Gibbs free energy corrections referring to 298.15 K and 1 mol/L were used for discussions.

Scheme 2. Different Mechanistic Proposals for MeReO3Catalyzed DODH Reactions

regenerates A to finish the catalytic cycle. In contrast, reduction of the ReVII catalyst to generate D followed by condensation of diols was also proposed (path B, Scheme 2).16a,21b,e The DFT studies from Wang, Zhang, Su, and co-workers also support the sequence of reduction/condensation, but MeReVO(OH)2 (E), rather than D, was pointed out to be the key intermediate (path C, Scheme 2).16d,21a Recently, an alternative ReIII−ReV cycle has also been proposed for the MeReO3-catalyzed DODH reactions.21d As to the mechanism of earth-abundant-metalbased catalysts, the Fristrup group performed a DFT study on Mo-catalyzed DODH and proposed a mechanism similar to path A.18c On the other hand, Nicholas et al. pointed out that B


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3. RESULTS AND DISCUSSION As shown in Scheme 3, the DODH of 1,2-propanediol (sub) by the anionic vanadium species 1 and PPh3 was used as the model

the spin states of the vanadium complexes in the following discussions, the symbols -s and -t are used to denote the singlet and triplet states, respectively. According to the literature, three possible mechanisms differing in the sequence of elementary steps (reduction and condensation) as well as the involved intermediates were considered (Scheme 2). Meanwhile, the associated spin crossover between the vanadium complexes in different valence states, which was omitted in previous studies, was considered herein to explore whether or not a more favored pathway can be located accordingly. 3.1. Path A for V-Catalyzed DODH. Path A starts through the condensation of vicinal diols to form a vanadium diolate. In this stage, hydrogen transfer between the primary hydroxyl group of 1,2-propanediol and the oxo ligand of 1 and that between the secondary hydroxyl group of 1,2-propanediol and the oxo ligand of 1 are both possible as the first step. We found that the two pathways lead to close overall free energy barriers and similar mechanistic details; thus, we mainly discuss the more favored path and the results of the other path are given in Scheme S1 in the Supporting Information. As shown in Scheme 4, the singlet state of pentavalent vanadium catalyst 1 is remarkably more stable than its triplet state: i.e., 1-s is stable than 1-t by about 70 kcal/mol. This result is understandable for the d0 vanadium(V) complexes. From 1-s, the primary hydroxyl group of 1,2-propanediol approaches the oxo ligand and forms the intermediate 2-s with a slight energy increase of 0.2 kcal/mol. Then hydrogen transfer occurs via TS1-s (Figure 1) and forms 3-s with an energy barrier of 13.7 kcal/mol. 3-s isomerizes to 4-s to bring the secondary hydroxyl group close to the hydroxide ligand. Thereafter, hydrogen transfer proceeds via TS2-s and generates the H2O-containing vanadium diolate 5-s. This step is more

Scheme 3. Model Reaction and Mechanisms Considered for DFT Studies on V-Catalyzed DODH Reactions

reaction for the DFT calculations. Note that the formal charge −1 of the anionic vanadium complexes has been omitted for clarity in the following graphics and schemes. To distinguish

Scheme 4. Calculated Free Energy Profile of Condensation and Reduction by PPh3 in V-Catalyzed DODH Reactions (in kcal/ mol)

C


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corresponding transition states TS4-s, TS5-s, and TS7-s have relative free energies of 37.3, 39.0, and 36.5 kcal/mol, respectively (Scheme 5). Interestingly, the geometry optimization of the triplet transition state for the extrusion of olefin from the vanadium(III) diolate leads to the cleavage of a single C−O bond to form a carbon-radical-containing vanadium(IV) intermediate. Specifically, this pathway starts from 7-t via TS4-t to generates 9-t, from which OPPh3 is released to form 10-t. In addition, the cleavage of the C−O bond at a planar position via TS4′-t was considered but was less feasible than TS4-t by 3.7 kcal/mol (Scheme 4b). In 10-t, the spin density mainly spreads on the vanadium center and the carbon radical (the values are 1.080 and 1.016, respectively), supporting the notion that 10-t is a vanadium(IV) complex. We then considered the direct cleavage of the remaining C−O bond on 10-t to form propene. However, no transition state was located for this process due to the increase of energy with an increase in the C−O bond length (Figure S1 in the Supporting Information). It is understandable that this triplet pathway generates not only propene but also the highly instable 1-t. Therefore, spin crossover was considered again to transform the triplet vanadium intermediate to a singlet vanadium intermediate. This spin crossover proceeds via MECP2 with an electronic energy barrier of 10.8 kcal/mol (referring to 10-t). After MECP2, the carbon radical combines with the vanadium(IV) center to form an electron pair. From 10-s, concerted V−C/C−O cleavage occurs via TS6-s and generates propene and the active 1-s to finish the catalytic cycle. It is noted that we also examined H2Oparticipating stepwise pathways via TS7-t and TS7′-t and the stepwise pathways in the absence of OPPh3 and H2O via TS5-t and TS5′-t. As shown in Scheme 4b, all of these paths are less favored than the stepwise pathway shown in Scheme 4a by different degrees (see Scheme S2 in the Supporting Information for details). The relative free energies of these transition states indicate that the coordination of OPPh3 or H2O to vanadium provides extra stabilization energy to the triplet C−O bond cleavage process (e.g., TS4-t and TS7-t vs TS5-t and TS5′-t). This is possibly because the formal oxidation state of vanadium changes from +3 to +4 during this process, meaning that the vanadium center is more electron deficient. The relative free energy of TS6-s is 33.7 kcal/mol, lower than that of TS3-s by only 0.7 kcal/mol, suggesting that the reduction of vanadium(V) diolate is the main ratedetermining step while the extrusion of olefin also plays a significant role. On the other hand, the pathway starting by the hydrogen transfer between the secondary hydroxyl group of 1,2propanediol and the oxo ligand of 1-s has a total free energy barrier of 34.6 kcal/mol (Scheme S1 in the Supporting Information), and thus it is also competitive with the pathways shown in Schemes 4 and 5. 3.2. Path B for V-Catalyzed DODH. As shown in Scheme 6, path B starts by the reduction of the vanadium(V) catalyst. In this pathway, 1-s combines with PPh3 to form the intermediate 11-s. This step is endergonic by 8.2 kcal/mol and is less favored than the combination of 1-s and 1,2-propanediol (Scheme 4) by 8.0 kcal/mol. From 11-s, reduction could occur but the corresponding transition state was not located, though great efforts were devoted to it. To estimate the energy demand of this step, a relaxed energy surface scan of the distance between the oxo ligand and the phosphorus atom of PPh3 was conducted. The electronic energy of the highest energetic point during the approach of PPh3 toward the oxo ligand is

Figure 1. Optimized structures of key transition states and MECP2 in path A. Bond lengths (in blue) are given in angstroms.

difficult than the first hydrogen transfer, with an energy barrier of 28.6 kcal/mol (referring to 1-s and 1,2-propanediol). The extrusion of H2O from 5-s forms 6-s with an energy decrease of 1.4 kcal/mol. Similarly to the reduction of oxorhenium complexes by NaSO3− reported by Liu and Nicholas,21e here the P atom of PPh3 attacks the oxo ligand of 6-s via TS3-s and reduces 6-s to the VIII complex 7-s. This step is relatively difficult with an energy barrier of 34.3 kcal/mol. We also tested the nucleophilic coordination of PPh3 to the vanadium center after one carboxylate ligand dissociates, but the departure of PPh3 from vanadium along with the recoordination of the carboxylate ligand always occurs during the geometry optimization. This result indicates that the reduction of vanadium(V) diolate by PPh3 prefers the outer-sphere mechanism via TS3-s. It is known that some VIII complexes have triplet ground states.37 Indeed, we found 7-s can feasibly transform to 7-t via MECP1 (the electronic energy of MECP1 is higher than that of 7-s by 2.5 kcal/mol) with an energy decrease of 10.9 kcal/mol. In the stages of the condensation of 1,2-propanediol and the reduction by PPh3, the triplet pathway except for 7 is disfavored, as indicated by the high energies of the corresponding intermediates (1-t−6-t). In the next step, extrusion of propene proceeds to finish the catalytic cycle. The previous mechanistic studies uniformly located a concerted mechanism in which two C−O bonds are simultaneously cleaved on vanadium diolates to generate olefins.21,23 We examined this mechanism and found that the D


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Scheme 5. Calculated Free Energy Profile of the Extrusion of Olefins in Path A (in kcal/mol): (a) Dominant Pathway; (b) Key Transition States in the Less Favored Pathways

Scheme 6. Calculated Free Energy Profile of the Stage of Reduction by PPh3 and the Stage of Condensation in the V-Catalyzed DODH Reaction via Path B (in kcal/mol)

is exergonic without transition states existing therein (thus 11-t is unstable and cannot be located). Nevertheless, this pathway starts from the highly unstable energetic point 1-t; thus, we excluded this pathway. We also considered the spin-crossoverinvolved reduction via MECP4.25 MECP4 is structurally different from MECP3: e.g., the distance of the removed oxo ligand and the phosphorus atom of PPh3 is 1.748 Å in MECP4 but is much shorter in MECP3 (1.515 Å). The longer O−P

higher than that of 11-s by 32.2 kcal/mol (Figure S2 in the Supporting Information). When the relative free energy of 11-s is taken into account, the overall energy demand for the reduction of 1-s by PPh3 is estimated to be about 40 kcal/mol. After the reduction, the singlet vanadium(III) intermediate 12-s is formed while it can transform to the triplet complex 12-t readily via MECP3. On the other hand, we found that the approach of PPh3 to the oxo ligand in the triplet energy profile E


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Organometallics distance in MECP4 suggests that MECP4 appears at an earlier stage of triplet-state reduction in comparison with MECP3. MECP4 is less stable than 11-s by 35.9 kcal/mol in electronic energy, meaning that the related pathway is even more unfavorable than the singlet reduction from 11-s to 12-s. The next stage is the condensation of vicinal diols to form vanadium diolates. Akin to the discussions for path A, we only present the more favored condensation pathway here while the details of the less favored paths are given in Scheme S4 in the Supporting Information. The ligand exchange of 12-t with 1,2propanediol generates 13-t, in which the primary hydroxyl group coordinates to the vanadium center. Then hydrogen transfer occurs via TS8-t and forms 14-t. The energy barrier of this elementary step is not very high: i.e., 20.1 kcal/mol from 13-t to TS8-t. However, the overall energy barrier of this step is 35.6 kcal/mol due to the endergonic reduction of 1-s by PPh3. 14-t isomerizes to 15-t and undergoes the next hydrogen transfer via TS9-t. Given that there is a vacant coordination site on vanadium in TS9-t, the coordination of H2O or PPh3 to vanadium in the transition states was considered as well. However, the related transition states TS9-iso1-t and TS9iso2-t are less stable than TS9-t by 4.1 and 4.6 kcal/mol, respectively. After TS9-t, the H2O-coordinated VIII complex 16t is formed. The ligand exchange of 16-t with OPPh3 generates 7-t, from which the extrusion of propene can be realized via the pathways shown in Scheme 4. On the other hand, the singletstate condensation pathway is found to be relatively disfavorable (from 13-s to 16-s in Scheme 6) and is excluded accordingly. 3.3. Path C for V-Catalyzed DODH. Path C starts by the reduction of 1, usually with the aid of a secondary alcohol, to generate [(dipic)V(OH)2]− and a ketone.16d,21a Here, the oxidation of the secondary hydroxyl group of 1,2-propanediol was considered for this mechanism. As shown in Scheme 7a, 1-s combines with 1,2-propanediol to form 17-s and undergoes hydrogen transfer via TS10-s. Different from the case for TS1s, the reacted hydroxyl group is located at the planar position of TS10-s to make the secondary carbon accessible for the oxo ligand in the following steps. Meanwhile, the hydrogen transfer via TS10-s is much more difficult than that via TS1-s by 16.7 kcal/mol, probably due to the strong trans effect of the two oxo ligands in TS10-s. After TS10-s, 18-s is formed and undergoes hydrogen transfer via the five-membered transition state TS11s. This step has a high overall energy barrier of 47.5 kcal/mol. The similar step in the Re-catalyzed DODH reactions with iPrOH as the reactant was also found to be the ratedetermining step.21a TS11-s leads to 19-s, and extrusion of hydroxyacetone from 19-s generates the vanadium hydroxide 20-s. 20-s can further transform to the more stable 20-t readily via MECP5. By comparison, the triplet-state reduction process is much less favored (17-t, TS10-t, 18-t, and TS11-t) until the VIII intermediates 19-t forms. From 20-t, the condensation of 1,2-propanediol can occur to generate vanadium diolates (Scheme 7b). The coordination of the secondary hydroxyl group of 1,2-propanediol to the vanadium center of 20-t generates 21-t, in which a hydrogen bond between the hydroxide ligand and the primary hydroxyl group of 1,2-propanediol forms. Thereafter the metathesis of the V−OH bond and O−H bond occurs via TS13-t and generates 22-t. 22-t undergoes another metathesis via TS14-t and generates 23-t to complete the condensation. In contrast, the corresponding singlet condensation is also less possible, mainly because this stage proceeds on vanadium(III)

Scheme 7. Calculated Free Energy Profiles of V-Catalyzed DODH Reactions (in kcal/mol): (a) Reduction of VV Catalysts by a Secondary Alcohol; (b) Condensation of Vicinal Diols and Extrusion of Olefins

complexes. In addition, the condensation starting from the primary hydroxyl group of 1,2-propanediol was also examined and this pathway is less kinetically favored by 0.4 kcal/mol (Scheme S5 in the Supporting Information). 23-t generates 16t by releasing H2O. The following extrusion of propene from 16-t is realized via TS7-t, as discussed for Scheme 5. 3.4. Favorable Mechanism for V-Catalyzed DODH. According to the above results, V-catalyzed DODH prefers the mechanism in a condensation/reduction/extrusion of olefin sequence (path A). This mechanism includes the condensation of diols on oxovanadium(V) catalysts, the reduction of vanadium(V) diolates followed by spin crossover to form triplet vanadium(III) diolates, the single C−O bond cleavage on triplet vanadium(III) diolates with the following spin crossover to form alkylvanadium(V) intermediates, and the final concerted V−C/C−O bond cleavage to produce olefin and regenerate the active oxovanadium(V) catalysts (Scheme 8). The reduction of vanadium(V) diolates by PPh3 is the main rate-determining step with an energy barrier of 34.3 kcal/mol, yet the olefin formation stage also partially controls the reaction rate with a non-negligible energy demand. Path B is excluded due to the high energy demand of the reduction of the vanadium(V) oxo complex by PPh3 as well as the later condensation stage (estimated to be 40.4 and 35.6 kcal/mol, respectively). Path C is also less possible due to the higher overall energy barriers of the reduction by secondary alcohols and the extrusion of olefins (47.5 and 43.4 kcal/mol, respectively). Our proposed mechanism (path A) is different from that proposed by Galindo (path B). Interestingly, we noticed that the free energy gaps of some steps can differ by over 20 kcal/ F


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4. CONCLUSION Transition-metal-catalyzed deoxydehydration provides a promising tool for the production of olefins from biomass-derived diols and polyols. A DFT study was performed to elucidate the mechanism of V-catalyzed DODH of vicinal diols. The dominant mechanism includes condensation of vicinal diols to form vanadium(V) diolate, reduction of the vanadium(V) diolate by PPh3 to form the vanadium(III) diolate, single C−O bond cleavage followed by spin crossover to form the alkylvanadium(V) intermediate, and concerted V−O/C−O bond cleavage to generate the olefin. The rate-determining step in this catalytic cycle is the reduction of the vanadium(V) diolate by PPh3, while the extrusion of olefin also partially controls the reaction rate. Meanwhile, the spin crossover existing in this mechanism facilitates the transformations between VV and VIII intermediates as well as the extrusion of olefin. The discovery of the stepwise olefin-formation mechanism resulting from the two-state reactivity of vanadium catalysts provides new mechanistic insights into the transitionmetal-catalyzed DODH. Capture of the carbon-radical-containing intermediates to validate our proposed mechanism and catalyst design balancing the energy demands of both reduction of vanadium and extrusion of olefin to explore more efficient Vcatalyzed DODH are ongoing in our laboratory.

Scheme 8. Proposed Mechanism for V-Catalyzed DODH

mol according to the choices of DFT methods. For example, 14-t is calculated to be the most stable intermediate and the free energy gap between 14-t and the singlet concerted C−O cleavage transition state TS5-s is calculated to be 48.0 kcal/mol on the basis of the B3LYP method in Galindo’s study. This value is too high to be accepted for the generation of olefins under the reported experimental conditions (150−170 °C, 24− 96 h). In contrast, on the basis of the recommended methods for transition-metal chemistry (M06 and M06-L) by Truhlar, the energy gap between 14-t and TS5-s is calculated to be only 28.0 kcal/mol, and the overall free energy barrier for TS5-s is 39.0 kcal/mol (referenced to 1-s). More importantly, after considering the spin crossover in the olefin formation stage, we located a new and more kinetically feasible pathway: 7-t → TS4-t →MECP2 → TS6-s. Through this pathway, the overall free energy barrier of olefin formation is lowered to 33.7 kcal/ mol, which is much more reasonably consistent with the reported experimental conditions. Although no specific studies were available to answer which DFT methods were better for vanadium-catalyzed reactions to the best to our knowledge, the extremely high overall energy barriers based on B3LYP still make the related conclusions in the case study questionable. As evidence for the success of M06 and M06-L, a recent review from the Schoenebeck group shows that the popularity of M06 and M06-L has already exceeded or approaches that of B3LYP in energy calculations for studies on homogeneous organometallic catalysis involving Ni, Pd, Ir, and Rh in 2013−2014 (the ratios of the three methods are 38%, 14%, and 22%, respectively).38 On the other hand, it was experimentally observed that secondary alcohols (e.g., 2,4-dimethyl-3-pentanol) are less efficient reductants in V-catalyzed DODH. Secondary alcohols are known to promote MeReO3-catalyzed DODH by reducing MeReO3 to MeReO(OH2) first. In contrast, reduction of 1-s to [LVIII(OH)2]− is highly kinetically disfavored probably due to the instability of the resulting singlet VIII intermediate 19-s. Meanwhile, the reduction of VV complex 1-s followed by the condensation of diol to generate VIII complex 16-t is not thermodynamically beneficial: i.e., it is endergonic by 13.9 kcal/ mol. Therefore, the overall free energy barrier of the transformations after 16-t is also raised to be above 40 kcal/ mol. According to these two reasons, our calculation results are qualitatively consistent with the low efficiency of secondary alcohols and supports the fact that extra reductants (otherwise diols act as the reductant) are required to promote V-catayzed DODH reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00602. Detailed results of less favored pathways and , potential energy scans of 10-t and 11-s (PDF) Cartesian coordinates and calculated energies (in hartrees) of all structures presented herein (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.F.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB215305), NSFC (21325208, 21402181, 21572212), IPDFHCPST (2014FXCX006), CAS (KFJ-EW-STS-051, YZ201563), FRFCU, and PCSIRT. We thank the supercomputer center of USTC for computational support.

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REFERENCES

(1) (a) Morais, A. R. C.; da Costa Lopes, A. M.; Bogel-Łukasik, R. Chem. Rev. 2015, 115, 3−27. (b) Besson, M.; Gallezot, P.; Pinel, C. Chem. Rev. 2014, 114, 1827−1829. (c) Straathof, A. J. J. Chem. Rev. 2014, 114, 1871−1908. (d) Mäki-Arvela, P.; Simakova, I. L.; Salmi, T.; Murzin, D. Y. Chem. Rev. 2014, 114, 1909−1971. (e) Lanzafame, P.; Centi, G.; Perathoner, S. Chem. Soc. Rev. 2014, 43, 7562−7580. (2) (a) Sousa, S. C. A.; Fernandes, A. C. Coord. Chem. Rev. 2015, 284, 67−92. (b) Herrmann, J. M.; König, B. Eur. J. Org. Chem. 2013, 2013, 7017−7027. (c) Gollakota, A. R. K.; Reddy, M.; Subramanyam, M. D.; Kishore, N. Renewable Sustainable Energy Rev. 2016, 58, 1543−1568. (d) Arceo, E.; Marsden, P.; Bergman, R. G.; Ellman, J. A. Chem. Commun. 2009, 3357−3359. (3) (a) Wang, H.; Kong, Q.; Wang, Y.; Deng, T.; Chen, C.; Hou, X.; Zhu, Y. ChemCatChem 2014, 6, 728−732. (b) Pholjaroen, B.; Li, N.; G


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Organometallics Wang, Z.; Wang, A.; Zhang, T. J. Energy Chem. 2013, 22, 826−832. (c) Li, X.-L.; Pan, T.; Deng, J.; Fu, Y.; Xu, H.-J. RSC Adv. 2015, 5, 70139−70146. (d) Dai, J.; Fu, X.; Zhu, L.; Tang, J.; Guo, X.; Hu, C. ChemCatChem 2016, 8, 1379−1385. (e) Kobayashi, H.; Yokoyama, H.; Feng, B.; Fukuoka, A. Green Chem. 2015, 17, 2732−2735. (4) (a) Buntara, T.; Noel, S.; Phua, P. H.; Melian-Cabrera, I.; de Vries, J. G.; Heeres, H. J. Top. Catal. 2012, 55, 612−619. (b) ten Dam, J.; Hanefeld, U. ChemSusChem 2011, 4, 1017−1034. (c) Xia, Q.; Chen, Z.; Shao, Y.; Gong, X.; Wang, H.; Liu, X.; Parker, S. F.; Han, X.; Yang, S.; Wang, Y. Nat. Commun. 2016, 7, 11162. (d) Parsell, T. H.; Owen, B. C.; Klein, I.; Jarrell, T. M.; Marcum, C. L.; Haupert, L. J.; Amundson, L. M.; Kenttämaa, H. I.; Ribeiro, F.; Miller, J. T.; AbuOmar, M. M. Chem. Sci. 2013, 4, 806−813. (e) Stanowski, S.; Nicholas, K. M.; Srivastava, R. S. Organometallics 2012, 31, 515−518. (f) Sutton, A. D.; Waldie, F. D.; Wu, R.; Schlaf, M.; Silks, L. A.; Gordon, J. C. Nat. Chem. 2013, 5, 428−432. (5) (a) Sergeev, A. G.; Hartwig, J. F. Science 2011, 332, 439−443. (b) Ruppert, A. M.; Weinberg, K.; Palkovits, R. Angew. Chem., Int. Ed. 2012, 51, 2564−2601. (c) Li, Z.; Assary, R. S.; Atesin, A. C.; Curtiss, L. A.; Marks, T. J. J. Am. Chem. Soc. 2014, 136, 104−107. (d) Molinari, V.; Giordano, C.; Antonietti, M.; Esposito, D. J. Am. Chem. Soc. 2014, 136, 1758−1761. (e) Jin, X.; Shen, J.; Yan, W.; Zhao, M.; Thapa, P. S.; Subramaniam, B.; Chaudhari, R. V. ACS Catal. 2015, 5, 6545−6558. (f) Zaheer, M.; Kempe, R. ACS Catal. 2015, 5, 1675−1684. (g) Kusumoto, S.; Nozaki, K. Nat. Commun. 2015, 6, 6296. (6) (a) Dawes, G. J. S.; Scott, E. L.; Le Nôtre, J.; Sanders, J. P. M.; Bitter, J. H. Green Chem. 2015, 17, 3231−3250. (b) Wang, S.; Vorotnikov, V.; Vlachos, D. G. ACS Catal. 2015, 5, 104−112. (c) John, A.; Hogan, L. T.; Hillmyer, M. A.; Tolman, W. B. Chem. Commun. 2015, 51, 2731−2733. (d) Huang, Y. B.; Yang, Z.; Chen, M. Y.; Dai, J. J.; Guo, Q. X.; Fu, Y. ChemSusChem 2013, 6, 1348−1351. (e) Miranda, M. O.; Pietrangelo, A.; Hillmyer, M. A.; Tolman, W. B. Green Chem. 2012, 14, 490−494. (7) (a) Kellicutt, A. B.; Salary, R.; Abdelrahman, O. A.; Bond, J. Q. Catal. Sci. Technol. 2014, 4, 2267−2279. (b) Deng, J.; Zhang, Q.-G.; Pan, T.; Xu, Q.; Guo, Q.-X.; Fu, Y. RSC Adv. 2014, 4, 27541−27544. (c) Fu, J.; Lu, X.; Savage, P. E. ChemSusChem 2011, 4, 481−486. (d) Han, J.; Sun, H.; Ding, Y.; Lou, H.; Zheng, X. Green Chem. 2010, 12, 463−467. (e) Maetani, S.; Fukuyama, T.; Suzuki, N.; Ishihara, D.; Ryu, I. Chem. Commun. 2012, 48, 2552−2554. (8) (a) Hansen, C. A.; Frost, D. A. J. Am. Chem. Soc. 2002, 124, 5926−5927. (b) Fernandes, T. A.; Bernardo, J. R.; Fernandes, A. C. ChemCatChem 2015, 7, 1177−1183. (9) Raju, S.; Moret, M.-E.; Klein Gebbink, R. J. M. ACS Catal. 2015, 5, 281−300. (10) Cook, G. K.; Andrews, M. A. J. Am. Chem. Soc. 1996, 118, 9448−9449. (11) (a) Harms, R. G.; Herrmann, W. A.; Kühn, F. E. Coord. Chem. Rev. 2015, 296, 1−23. (12) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 9998−10000. (13) (a) Vkuturi, S.; Chapman, G.; Ahmad, I.; Nicholas, K. M. Inorg. Chem. 2010, 49, 4744−4746. (b) Ahmad, I.; Chapman, G.; Nicholas, K. M. Organometallics 2011, 30, 2810−2818. (c) McClain, J. M., II; Nicholas, K. M. ACS Catal. 2014, 4, 2109−2112. (d) Boucher-Jacobs, C.; Nicholas, K. M. ChemSusChem 2013, 6, 597−599. (e) BoucherJacobs, C.; Nicholas, K. M. Organometallics 2015, 34, 1985−1990. (14) (a) Denning, A. L.; Dang, H.; Liu, Z.; Nicholas, K. M.; Jentoft, F. C. ChemCatChem 2013, 5, 3567−3570. (b) Raju, S.; Jastrzebski, J. T. B. H.; Lutz, M.; Klein Gebbink, R. J. M. ChemSusChem 2013, 6, 1673−1680. (c) Ota, N.; Tamura, M.; Nakagawa, Y.; Okumura, K.; Tomishige, K. ACS Catal. 2016, 6, 3213−3226. (15) Arceo, E.; Ellman, J. A.; Bergman, R. G. J. Am. Chem. Soc. 2010, 132, 11408−11409. (16) (a) Shiramizu, M.; Toste, F. D. Angew. Chem., Int. Ed. 2012, 51, 8082−8086. (b) Shiramizu, M.; Toste, F. D. Angew. Chem., Int. Ed. 2013, 52, 12905−12909. (c) Yi, J.; Liu, S.; Abu-Omar, M. M. ChemSusChem 2012, 5, 1401−1404. (d) Li, X.; Wu, D.; Lu, T.; Yi, G.; Su, H.; Zhang, Y. Angew. Chem., Int. Ed. 2014, 53, 4200−4204.

(17) Stanowski, S.; Nicholas, K. M.; Srivastava, R. S. Organometallics 2012, 31, 515−518. (18) (a) Dethlefsen, J. R.; Lupp, D.; Oh, B.-C.; Fristrup, P. ChemSusChem 2014, 7, 425−428. (b) Hills, L.; Moyano, R.; Montilla, F.; Pastor, A.; Galindo, A.; Á lvarez, E.; Marchetti, F.; Pettinari, C. Eur. J. Inorg. Chem. 2013, 2013, 3352−3361. (c) Dethlefsen, J. R.; Lupp, D.; Teshome, A.; Nielsen, L. B.; Fristrup, P. ACS Catal. 2015, 5, 3638−3647. (19) Chapman, G.; Nicholas, K. M. Chem. Commun. 2013, 49, 8199− 8201. (20) Geary, L. M.; Chen, T.-Y.; Montgomery, T. P.; Krische, M. J. J. Am. Chem. Soc. 2014, 136, 5920−5922. (21) (a) Qu, S.; Dang, Y.; Wen, M.; Wang, Z. X. Chem. - Eur. J. 2013, 19, 3827−3832. (b) Dethlefsen, J. R.; Fristrup, P. ChemCatChem 2015, 7, 1184−1196. (c) Korstanje, T. J.; Jastrzebski, J. T. B. H.; Klein Gebbink, R. J. M. Chem. - Eur. J. 2013, 19, 13224−13234. (d) Bi, S.; Wang, J.; Liu, L.; Li, P.; Lin, Z. Organometallics 2012, 31, 6139−6147. (e) Liu, P.; Nicholas, K. M. Organometallics 2013, 32, 1821−1831. (f) Liu, S.; Senocak, A.; Smeltz, J. L.; Yang, L.; Wegenhart, B.; Yi, J.; Kenttämaa, H. I.; Ison, E. A.; Abu-Omar, M. M. Organometallics 2013, 32, 3210−3219. (22) (a) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Sutton, A. D.; Thorn, D. L. J. Am. Chem. Soc. 2009, 131, 428−429. (b) Crans, D. C.; Felty, R. A.; Chen, H.; Eckert, H.; Dad, N. Inorg. Chem. 1994, 33, 2427−2438. (23) Galindo, A. Inorg. Chem. 2016, 55, 2284−2289. (24) (a) Schröder, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139−145. (b) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393−1407. (25) Jiang, Y.-Y.; Yan, L.; Yu, H.-Z.; Zhang, Q.; Fu, Y. ACS Catal. 2016, 6, 4399−4410. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2013. (27) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (28) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378−6396. (29) (a) Grafenstein, J.; Izotov, D.; Cremer, D. J. Chem. Phys. 2007, 127, 214103. (b) Johnson, E. R.; Becke, A. D.; Sherrill, C. D.; DiLabio, G. A. J. Chem. Phys. 2009, 131, 034111. (c) Wheeler, S. E.; Houk, K. N. J. Chem. Theory Comput. 2010, 6, 395−404. (30) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (31) 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. (32) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161−4163. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (33) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866−872. (34) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95−99. (35) For selected examples of using electronic gaps to estimate the feasibility of spin crossover, see: (a) Dhuri, S. N.; Cho, K.-B.; Lee, Y.M.; Shin, S. Y.; Kim, J. H.; Mandal, D.; Shaik, S.; Nam, W. J. Am. H


Article

Organometallics Chem. Soc. 2015, 137, 8623−8632. (b) Popp, B. V.; Stahl, S. S. J. Am. Chem. Soc. 2007, 129, 4410−4422. (36) (a) Li, H.; Hall, M. B. J. Am. Chem. Soc. 2015, 137, 12330− 12342. (b) Plata, R. E.; Singleton, D. A. J. Am. Chem. Soc. 2015, 137, 3811−3826. (37) (a) Krzystek, J.; Ozarowski, A.; Telser, J.; Crans, D. C. Coord. Chem. Rev. 2015, 301−302, 123−133. (b) Ye, S.; Neese, F.; Ozarowski, A.; Smirnov, D.; Krzystek, J.; Telser, J.; Liao, J.-H.; Hung, C.-H.; Chu, W.-C.; Tsai, Y.-F.; Wang, R.-C.; Chen, K.-Y.; Hsu, H.-F. Inorg. Chem. 2010, 49, 977−988. (c) Hessen, B.; Lemmen, T. H.; Luttikhedde, H. J. G.; Teuben, J. H.; Petersen, J. L.; Huffman, J. C.; Jagner, S.; Caulton, K. G. Organometallics 1987, 6, 2354−2362. (38) Sperger, T.; Sanhueza, I. A.; Kalvet, I.; Schoenebeck, F. Chem. Rev. 2015, 115, 9532−9586.

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