Comprehensive DFT Study of the Mechanism of Vanadium-Catalyzed

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Comprehensive DFT Study of the Mechanism of Vanadium-Catalyzed Amination of Benzene with Hydroxylamine Dianyong Tang,*,† Liangfang Zhu,‡ and Changwei Hu*,‡ †

Centre for Functional Molecular Design, Department of Chemistry and Life Science, Leshan Normal College, Leshan, 614000, People's Republic of China ‡ Key Laboratory of Green Chemistry and Technology, MOE, College of Chemistry, Sichuan University, Chengdu, 610064, People's Republic of China

bS Supporting Information ABSTRACT: Reaction pathways and free-energy profiles for the conversion of benzene and hydroxylamine to aniline, catalyzed by NaVO3 and VOSO4, in acetic acid/water, were discussed using density functional theory calculations. Three model catalysts, namely VO2+, VO(H2O)52+, and VO(AcO)(H2O)3+, were investigated and compared. The calculations revealed that the addition elimination pathway was clearly preferred over the C H bond activation pathway with VO2+ as the catalyst. The rate-determining step for all three catalysts is the formation of the amino radical complex. The existence of water and CH3COO effectively reduced the free-energy barriers of the formation of the amino radical complex. Energy decomposition analysis indicated that bonding variations between the solvent (water and CH3COO ) and vanadium played an important role in the amination process. The results obtained using VO(AcO)(H2O)3+ as the catalyst were in good agreement with experimental data.

1. INTRODUCTION One-step production of aniline by the direct amination of benzene is an attractive and challenging method from the viewpoint of both green chemistry and synthetic chemistry, because it involves the functionalization of the relatively stable aromatic ring.1 Numerous attempts have been made to perform this intriguing reaction,2 15 and hydroxylamine has been found to be an effective aminating agent.13 15 In our previous work,15 the direct aerobic amination of benzene to aniline with hydroxylamine chloride catalyzed with NaVO3 was investigated, and a satisfactory aniline yield (64 mol %) with a turnover of 48 was obtained under the optimal reaction conditions. Although the amination process has been investigated by EPR, in situ 51V NMR, and UV vis spectroscopies, the detailed mechanism for the reactions is still unclear, because the amination chemistry involving hydroxylamine and the redox reactions of transition metals are complex. A combination of experimental and computational studies of these reactions can enhance our understanding of the elementary steps occurring under the reaction conditions. Kuzenetsova et al.14 reported a theoretical study on the amination of benzene and toluene by NH2 or NH3+ radicals to obtain the reaction energies in the gas phase and water. However, the generation processes for the amino group or amino vanadium complexes and the key step of the amination are not clear. Therefore, in this paper, theoretical studies of the amination of benzene with hydroxylamine as the aminating agent in acetic acid/water, based on density functional theory (DFT), are r 2011 American Chemical Society

performed. As reported in our previous paper,15,16 V(V) is easily and selectively reduced to V(IV) by hydroxylamine under the experimental conditions; a V(IV) species is used as the initial catalyst in the present work. The present computational study is, to the best of our knowledge, the first comprehensive theoretical investigation of the complete reaction cycle for the amination of benzene with hydroxylamine as the aminating agent. This shows promise for improving the understanding of the amination chemistry of benzene and its derivatives.

2. COMPUTATIONAL MODELS AND METHODS In addition to VO2+, VO(H2O)52+ and VO(AcO)(H2O)3+ were chosen to model the aggregation state of the vanadium catalyst in acetic acid/water media. For these two larger models, only the most possible reaction pathways were investigated. Geometry optimizations as well as frequency calculations for all of the stationary points considered here have been performed at the density functional level of theory, using the hybrid B3LYP functional,17 19 together with the 6-311G(d,p) basis set.20 24 For each optimized stationary point, vibrational analysis was performed to determine its character (minimum or saddle point) and to obtain the zero-point vibrational energy (ZPVE) and thermal corrections at 353.15 K and 1 atm (the experimental conditions). For each transition state, intrinsic reaction coordinate (IRC)25,26 calculations were performed in both Received: June 24, 2011 Published: October 11, 2011 5675

dx.doi.org/10.1021/om2005436 | Organometallics 2011, 30, 5675–5686

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Scheme 1. Proposed Possible Mechanism of Conversion of Benzene to Aniline, Catalyzed by VO2+, in Aqueous Solution

directions to connect the corresponding intermediates at the above level. A step of 0.1 or 0.05 amu1/2 bohr was used in the IRC procedure. Wave function stability calculations were performed to confirm that the calculated wave functions corresponded to the ground state.27 29 The effect of the polarized surroundings of water on the reaction species was evaluated at the UB3LYP/6-311G(d,p) level. Self-consistent reaction field (SCRF) single-point energy calculations on the gas-phaseoptimized structures in a water continuum (water as solvent) were carried out using Tomasi’s polarized continuum model (IEF-PCM)30 with the united atom for Hartree Fock (UAHF) topological model.31 As demonstrated in other systems,32,33 the geometry changes produced by solvation effects were not very large, and the differences in electronic energies between the PCM-optimized structures and the single-point PCM calculations using the gas-phase geometries were usually less than 2 kcal/mol. The solvation free energy is the difference between the free energies in solution and in the gas phase. With regard to the entropy effects, the following discussions were based on the free energies (ΔG) of activation and reaction. Unless otherwise specified, natural charges obtained by natural population analysis (NPA) were used in the following discussions. Natural charges were calculated by the NBO program at the UB3LYP/6-311G(d,p)(IEF-PCM)//UB3LYP/ 6-311G(d,p) level.34,35 To bridge the experimental and theoretical results, the 51V chemical shifts for some stationary points were computed by the gauge-independent atomic orbital (GIAO) method at the UB3LYP/6-311G(d,p)(IEFPCM)//UB3LYP/6-311G(d,p) level.36 39 The 51V chemical shifts were reported relative to VOCl3 and optimized at the same level ( 2176 ppm). Nucleus-independent chemical shifts (NICS)40 were computed by the GIAO method at the optimized UB3LYP/6-311G(d,p) geometries. NICS provides a practical aromaticity index that can be calculated at the ring center (nonweighted mean of the heavy atom coordinates on the ring perimeter). All the calculations reported in the present work were carried out with the Gaussian 03 package.41 The EPR g-tensors (giso) of some intermediates were calculated by the UB3LYP method using the uncontracted all-electron Wachters basis on V(14s11p6d3f),22,42,43 Ahlrichs’ TZVP on C(11s6p7d),44,45 IGLO-III on N(11s7p2d) and O(11s7p2d),46 and EPR-II on H(6s2p) with the

ORCA package47 51 by solution of the coupled perturbed SCF equations combined with the COSMO (conductor-like screening model) solvent model (water as solvent, ε = 80.4, and refractive index = 1.33) and scalar ZORA (zeroth-order regular approximation) relativistic method.52 Molecular orbital (MO) compositions were calculated using the AOMix program53,54 and the Mulliken scheme.55 58 The analysis of the MO compositions in terms of occupied and unoccupied fragment molecular orbitals (OFOs and UFOs, respectively), construction of orbital interaction diagrams, charge decomposition analysis (CDA),59,60 and extended charge decomposition analysis (ECDA) were performed using AOMix-CDA.53,54,59,60

3. RESULTS AND DISCUSSION 3.1. Simple Model Catalyst, VO2+. Based on the in situ NMR

and UV vis characterization of the reactions,15 two possible pathways for the amination of benzene to aniline, catalyzed by VO2+, were proposed; they are shown in Scheme 1. The first stage is the generation of the amino species (•NH2) or amino vanadium complex from hydroxylamine, which is the essential step for the subsequent amination process. Then, the NH2 terminal of the amino complex attacks benzene, which constitutes the addition elimination mechanism; activation of the benzene C H bond by the vanadium atom of the amino vanadium complex constitutes the C H bond activation mechanism. The doublet VO2+ stands below the quartet VO2+ by about 70.61 kcal/mol. Simultaneously, the stationary points on the quartet energy profile are higher than those on the doublet energy profile. Therefore, only the ground-state (doublet) free-energy profiles were investigated in the present paper. In the following sections, generation of the amino vanadium complex is first addressed, and then the addition elimination and C H bond activation mechanisms are discussed. 3.1.1. Generation of the Amino Vanadium Complex. As shown in Scheme 1, the formation of the amino vanadium complex involves the coordination of hydroxylamine to VO2+ 5676

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Figure 1. Free-energy diagrams of the ground state (doublet state) in water at 353 K and 1 atm (units: kcal/mol). The total free energy for the separated reactants is 1382.8858559 hartree at the UB3LYP/6-311G**(IEF-PCM)//UB3LYP/6-311G** level.

Figure 2. Optimized structures involved in the generation step of the NH2 radical complex. Bond lengths and angles are in angstroms and degrees, respectively.

and then cleavage of the N O bond. The relative free energies in water at 353 K and 1 atm are shown in Figure 1, and the optimized structures and selected parameters are shown in Figure 2. As shown in Figure 2, the first step is the coordination of hydroxylamine with VO2+(2Σ) to form an η2-N,O complex, IM1, without a barrier. This step is predicted to be exothermic by about 54.71 kcal/mol and, thus, is thermodynamically feasible. A noticeable positive charge transfer (0.46 au) takes place from the VO2+ fragment to the hydroxylamine fragment. Interestingly, the

N O bond is not obviously activated in this coordination process. The singly occupied molecular orbital (SOMO) indicates that the single electron is mainly localized on the d orbital of the vanadium atom (Figure S1). The spin-density distribution also supports this point (1.2 au on the vanadium atom). Subsequently, a hydrogen transfers from the oxygen of the hydroxylamine fragment to the oxygen of the VO2+ fragment in IM1, leading to the formation of IM2 via TS1/2. As expected, the free-energy barrier is as high as 47.20 kcal/mol under the experimental conditions; the free-energy barrier is about 5677


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Figure 3. Optimized structures involved in the addition elimination mechanism. Bond lengths and angles are in angstroms and degrees, respectively.

47.04 kcal/mol at 353 K. So, this step is not feasible at room temperature. The spin density of IM2 is mainly located on the vanadium atom (1.19 au for vanadium). This hydrogen-transfer step can be considered as the reduction of V(IV) to V(III). The breakage of the N O bond in IM2 then produces IM3 through TS2/3. The calculated activation free energy is 33.00 kcal/mol, which implies that breakage of the N O bond is not feasible. The spin density of IM3 is mainly located on the p orbital of the nitrogen atom (1.04 au for nitrogen). IM3 can be viewed as an amino free-radical complex. Therefore, the single electron transfers from the d orbital of the vanadium atom to the p orbital of the nitrogen atom in the conversion of IM2 to IM3, associated with the oxidation of V(III) to V(IV). In IM2 f TS2/3 f IM3, the breakage of the N O bond gives rise to the p orbitals of the oxygen and nitrogen atoms, together with the p orbital of oxygen overlapping with the d orbital of the vanadium atom because of the adapted symmetry. Thus the d orbital of the vanadium atom plays an essential role in cleavage of the N O bond. In addition, a variation of VV f VIV f VIII f VV is probably involved when using NaVO3 as the initial catalyst. It is also deduced that the vanadium(IV) species can be oxidized to VV when VOSO4 is used as the initial catalyst. This is qualitatively in agreement with our previous experimental study.15 The 51V NMR chemical shift of IM3 is predicted to be about 753 ppm. The predicted EPR g-tensor and A-tensor of IM3 are about 2.0045 and ( )158.3 G,61 respectively.

Comparing the relative free energies of IM1 and IM3 (ΔG = 15.27 kcal/mol, .0), it is found that the formation of the amino vanadium complex is not feasible thermodynamically. At the same time, the calculated activation free energies of the two steps clearly show that the formation of this amino vanadium complex is not feasible kinetically. Nevertheless, the amination of benzene by the simple amino complex IM3 was investigated for comparison. 3.1.2. Addition Elimination Pathway. After generation of the amino complex IM3, the NH2 terminus of IM3 attacks benzene to constitute the addition elimination pathway. The relative free energies in water at 353 K and 1 atm are shown in Figure 1. Optimized structures and selected parameters are shown in Figure 3. Initially, the NH2 terminus of IM3 attacks benzene, resulting in the η1-benzene intermediate IM4 through TS3/4. The freeenergy profile in Figure 1 clearly shows that the amino-addition step (IM3 f TS3/4 f IM4) is exergonic by 31.10 kcal/mol, and a low free-energy barrier of 2.35 kcal/mol needs to be overcome. This step therefore easily occurs both thermodynamically and kinetically. The spin-density surface of TS3/4 shows that the single electron is not located on only one atom (Figure S1). The spin-density distribution in TS3/4 also supports this point (0.36 au on vanadium and 0.76 au on the benzene carbon atoms). The occupied molecular orbitals depicted in Figure S2 demonstrate that the amino-addition step mainly involves interaction of the p orbital of nitrogen with the π orbital of benzene 5678


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Figure 4. Optimized structures involved in the C H bond activation mechanism. Bond lengths and angles are in angstroms and degrees, respectively.

and the p orbital of the amino-connected carbon atom. Interestingly, the single electron in IM4 is mainly located on vanadium; that is, the single electron transfers from the p orbital of the nitrogen atom to the d orbital of the vanadium atom in the course of IM3 f IM4. The C H bond of the amino-connected carbon atom in IM4 is obviously activated (Figure 3), which makes the subsequent hydrogen elimination from the benzene ring occur easily. Afterward, the hydrogen atom on the amino-connected carbon atom transfers to the oxygen atom of V OH or VdO; that is, the hydrogen is eliminated, to form aniline complexes IM5 and IM6 via TS4/5 and TS4/6, respectively. The imaginary frequencies for the two TSs are 853i and 1254i cm 1. These high frequencies are a direct consequence of C H bond cleavage as well as O H bond formation, as the eigenvector coordinates of these imaginary frequencies suggest. The bond distances are reasonable for these transition-state structures, which are responsible for cleavage of the C H bond and formation of the O H bond (Figure 3). These two processes are also exergonic, by 24.41 and 22.87 kcal/mol, respectively, and low free-energy barriers of 1.89 and 8.54 kcal/mol, respectively, need to be overcome. So, hydrogen elimination easily occurs both thermodynamically and kinetically, and the path IM4 f TS4/5 f IM5 is exclusively involved in the hydrogen elimination. In the two TSs, the single electron is mainly localized on the d orbital of the vanadium atom (Figure S1). Finally, the product, aniline, is obtained directly from intermediates IM5 and IM6, and the

catalyst VO2+ is regenerated. These two reactions are very endergonic, by about 30.10 and 61.74 kcal/mol. On the basis of the above discussions, the addition elimination pathway involves two steps: amino addition and hydrogen elimination. The low free-energy barriers of these two steps show that they proceed easily under the experimental conditions. The most feasible pathway for the addition elimination mechanism is IM3 f TS3/4 f IM4 f TS4/5 f IM5. The rate-limiting step is the formation of the amino vanadium complex. 3.1.3. C H Bond Activation Pathway. The coordination of benzene to the vanadium atom in IM3 results in the formation of a benzene complex and then to the C H bond activation pathway. The relative free energies in water at 353 K and 1 atm are shown in Figure 1. Optimized structures and selected parameters are shown in Figure 4. The first step in the C H bond activation pathway is the coordination of benzene to the vanadium atom with the formation of the η1-complex IM7, without a free-energy barrier. This process is predicted to be exergonic by 8.94 kcal/mol. The SOMO of IM7 presented in Figure S1 and the spin densities on the nitrogen atom and the benzene carbon atoms (0.66 and 0.50 au for the nitrogen atom and the benzene carbon atoms, respectively) show that the single electron partially transfers to the benzene ring. Analysis of the occupied molecular orbitals of IM7 shows that the interaction between the d orbital of the vanadium atom and the π orbital of benzene is relevant for the 5679


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Figure 5. Orbital interaction diagrams for TS3/4, IM4, and IM7 (only α-spin orbitals presented). (a) TS3/4 from VO(OH)(NH2)2+ and benzene fragments; (b) IM7 from VO(OH)(NH2)2+ and benzene fragments; (c) IM4 from VO(OH)(NH2)2+ and benzene fragments; and (d) IM4 from VO(OH) 2+ and cyclohexadienamine radical fragments.

benzene-coordination process (Figure S2). Analysis of the molecular orbitals demonstrates that the conjugated π orbital of benzene has been destroyed in IM7. The C H bond of the vanadium-coordinated carbon atom is slightly longer than that of the free benzene, which indicates that the coordination activates the C H bond. However, the activation of the C H bond in IM7 is obviously weaker than that in IM4. Therefore, the subsequent breakage of the C H bond may not be easier than that in the addition elimination pathway; this statement will be confirmed in the following discussion. Second, activation of the C H bond via TS7/8 and TS7/11 gives rise to the phenyl complexes IM8 and IM11, respectively. These two processes are obviously competitive with each other because of their very close free-energy barriers of 42.91 and 43.14 kcal/mol. These two reactions are kinetically less favorable under the experimental conditions than the addition elimination pathway is. In IM8 and IM11, the spin densities are mainly located on the carbon atoms of the phenyl ring. The oxidation state of vanadium in IM8 and IM11 is +5. Third, the intermediates IM8 and IM11 produce the intermediates IM5 and IM6 through carbon nitrogen coupling, together with reduction of VV to VIV. The transition states for these two reactions are TS8/5 and TS11/6, respectively. These two reactions are very exergonic, by 53.70 and 52.15 kcal/mol, with low free-energy barriers of 2.81 and 3.96 kcal/mol. Therefore, both the reactions are irreversible thermodynamically and

feasible kinetically. Additionally, intermediate IM8 can release the coordinated water to form IM9 with an endergonicity of 22.72 kcal/mol. Then, carbon nitrogen coupling in IM9 leads to intermediate IM10 via TS9/10, with a very low free-energy barrier. The coordination of water can effectively enhance this carbon nitrogen coupling process thermodynamically; comparison of path IM9 f TS9/10 f IM10 with path IM8 f TS8/5 f IM5 shows that this enhancement would be the result of stabilization of the corresponding stationary points on IM9 f TS9/10 f IM10 by the coordination of water. Lastly, as in the addition elimination pathway, the product, aniline, is obtained by the decomposition of IM5, IM6, and IM10. These processes are predicted to be very endergonic (Figure 1). As discussed above, the C H bond activation pathway also includes two main steps: C H bond activation and carbon nitrogen coupling. The rate-determining step for the C H bond activation mechanism based on the free-energy barriers is also the amino-complex-generation step, with a free-energy barrier of about 47 kcal/mol. 3.1.4. Comparison of the Two Pathways. Combining the above discussions of the addition elimination and the C H bond activation pathways with the free-energy profiles shown in Figure 1, the species in the addition elimination pathway are obviously lower in free energy than those in the C H bond activation pathway. Therefore, the former pathway is thermodynamically more favorable than the latter one. The highest free-energy barrier 5680


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Figure 6. Free-energy diagrams of the ground state (singlet state) in water for the reduction of [VO2(H2O)4]+ (1A) to [VO(H2O)5]2+ (2A) by hydroxylamine at 353 K and 1 atm (units: kcal/mol). The total free energy for the separated reactants [VO(OH)(H2O)42+ and hydroxylamine] is 1532.448329 hartree at the UB3LYP/6-311G**(IEF-PCM)//UB3LYP/6-311G** level.

(only 2.35 kcal/mol) of the addition elimination pathway is obviously lower than that of the C H activation pathway. So the addition elimination pathway is also kinetically more feasible than the C H activation pathway. Therefore, the addition elimination pathway is exclusively involved in the amination of benzene using hydroxylamine as the aminating agent and catalyzed by NaVO3 or VOSO4. To gain further insight into the difference between the addition elimination and the C H bond activation pathways, the NICS of the benzene ring center (nonweighted mean of the heavy atom coordinates), as suggested by Scheleyer et al.,40 were computed at the UB3LYP/6-311G** level (Table S1). Interestingly, in benzene f TS3/4 f IM4, the aromaticity of the benzene ring is obviously destroyed in TS3/4. The aromaticities of the benzene rings of the other stationary points on the addition elimination pathway are preserved. However, most of the benzene rings in the stationary points on the C H bond activation mechanism show antiaromaticity. So, breakage of the aromaticity of benzene makes the C H bond activation pathway difficult. Therefore, keeping the aromaticity of the benzene ring is essential for the benzene functionalization process under mild conditions. To distinguish the addition elimination and C H activation pathways, as well as the role of vanadium, the orbital interaction diagrams of TS3/4, IM4, and IM7 were constructed. The results are shown in Figure 5. Only α-spin orbitals were presented because the orbital interaction diagram of the β-spin orbitals is similar to that of the α-spin orbitals. The orbital interaction diagram of TS3/4 (Figure 5a) indicates that the attack of the NH2 terminal involves not only the p orbital of the nitrogen atom [HOFO 1 (highest occupied fragment orbital) of the VO(OH)(NH2)2+ fragment] and π orbitals of the benzene fragment (HOFO and HOFO 4 of the benzene fragment) but also the

3dx2 y2 orbital of the vanadium atom [LUFO (lowest unoccupied fragment orbital) of the VO(OH)(NH2)2+ fragment]. These interactions stabilize the π orbitals of benzene and the 3dx2 y2 orbital of the vanadium atom in TS3/4. In intermediate IM4 resulting from TS3/4, the HOMO is mainly constructed from the LUFO of the VO(OH)(NH2)2+ fragment and the LUMO mainly comes from the HOFO and LUFO orbitals of the benzene fragment (Figure 5c). However, in the IM7 originating from the coordination of benzene to VO(OH)(NH2)2+, the d orbital of vanadium is rarely involved (Figure 5b). The π2 orbitals of benzene are almost retained in TS3/4 and IM4 (Figure 5a and b),62 so the π electrons can delocalize on the pentadienyl moieties. However, all three π orbitals are destroyed in IM7 (Figure 5c). Thus, the π electrons in IM7 are localized. Therefore, formation of IM4 is preferred to formation of IM7. Comparing parts a and c of Figure 5, it is found that the single electron, which should be located on the cyclohexadienamine after the attack of the NH2 free radical on benzene,15 is mainly located in the 3dx2 y2 orbital of vanadium. Which factor results in the transformation of this single electron? To answer this question, it is useful to divide IM4 into VO(OH)2+ and cyclohexadienamine radical fragments to construct the orbital interaction diagram. Extended charge decomposition analysis33 indicates that the cyclohexadienamine radical fragment transfers 1.07 au electron to the VO(OH)2+ fragment. As shown in Figure 5d, the HOFO of the cyclohexadienamine radical fragment results in an IM4 LUMO with loss of one electron. The LUFO and LUFO+1 orbitals of the VO(OH)2+ fragment combine into the HOMO of IM4 with receipt of one electron. The orbital energies of the LUFO and LUFO+1 of the VO(OH)2+ fragment are obviously lower than that of the HOFO of the cyclohexadienamine radical fragment. Therefore, the 3d orbital of vanadium plays an import role in this amination process by 5681


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Figure 7. Free-energy diagrams of the ground state (doublet state) in water for the amination of benzene to aniline by [VO(H2O)5]2+(2A) at 353 K and 1 atm (units: kcal/mol). The total free energy for the separated reactants [VO(H2O)52+, hydroxylamine, and benzene] is 1688.878758 hartree at the UB3LYP/6-311G**(IEF-PCM)//UB3LYP/6-311G** level.

accepting a single electron from the cyclohexadienamine radical moiety to stabilize the cyclohexadienamine radical intermediate. 3.2. Water-Solvated Model, VO(H2O)52+. 3.2.1. Formation of the Active Catalyst VO(H2O)52+. In our previous work,16 the oxidation of hydroxylamine by VO2+ in the gas phase was predicted to be achieved by cleavage of the O H bond in hydroxylamine on the energy profile of the ground state. Therefore, only cleavage of the O H bond in hydroxylamine by VO2(H2O)4+ in the ground state is investigated in the present work. The predicted free-energy profiles in water at 353 K and 1 atm are shown in Figure 6. The corresponding optimized structures and selected parameters are depicted in Figure S3. Unlike the situation in the gas phase,16 the active species for the oxidation of hydroxylamine in aqueous solution is VO(OH)(H2O)42+, not VO2+. As shown in Figure 6, the protonation of VO2(H2O)4+ to VO(OH)(H2O)42+ is very exothermic (about 257.18 kcal/mol), which means that VO2(H2O)4+ would be spontaneously protonated to form VO(OH)(H2O)42+ in aqueous solution. Coordination of hydroxylamine to VO(OH)(H2O)42+, with loss of a water ligand, forms η1-NH2OH intermediates IM12 (O-terminus) and IM14 (N-terminus). When the η2-NH2OH intermediate is used as the initial structure, optimization leads to formation of an η1-NH2OH complex. As shown in Figure 6, IM14 is more stable than IM12. Hydrogen then transfers from the NH2OH moiety to the V OH moiety in IM12 and IM14, producing the NH2O radical complexes IM14 and IM15 via TS12/13 and TS14/15, respectively. The freeenergy barriers of the two reaction pathways are predicted be 28.80 and 21.51 kcal/mol, respectively. Obviously, pathway IM14 f IM15 is more favorable than IM12 f IM13, both thermodynamically and kinetically (Figure 6). This favorability may be caused by the small torsion force of the five-membered ring in TS14/15 and the large orbital interaction between the p orbital of the NH2 fragment and the d orbital of vanadium in IM14, IM15, and TS14/15. Additionally, the water-assisted hydrogen transfer (IM12 f IM13 and IM14 f IM15) was investigated. The transition state (TS14/15-w) and product (IM15-w) of the water-assisted IM14 f IM15 could not be located because of the improper torsion angle of H O H. As shown in Figure 6, the free energy of the path IM12 f IM13 is lowered to 3.58 kcal/mol by water. Finally, the decomposition of IM13 and IM15 generates the NH2O radical and the active

species VO(H2O)52+. The two reaction routes are predicted to be exothermic by 20.38 and 12.45 kcal/mol, respectively. Comparing these results with our previous study on oxidation of hydroxylamine by VO2+ in the gas phase,16 it is found that a solvent (water) reduces the free-energy barrier of hydrogen transfer and makes the free-energy profile smooth. 3.2.2. Generation of the Amino Vanadium Complex [VO(OH) (H2O)3(NH2)]2+. The generation of the amino vanadium complex [VO(OH)(H2O)3(NH2)]2+ from VO(H2O)52+ and hydroxylamine is similar to that in the VO2+ model. The calculated free-energy profile is presented in Figure 7, and the corresponding structures are collected in Figure S4. According to Figure 7, the first step, the formation of the encounter complex IM16, is slightly endothermic by about 11 kcal/mol. Next, through the hydrogen-transfer transition state TS16/17, with a free-energy barrier of 33.98 kcal/mol, the NH2O vanadium radical complex IM17 is generated. Furthermore, the free energy of the watercatalyzed IM16 f IM17 process is predicted to be 9.80 kcal/ mol. Finally, the amino vanadium complex IM18 is formed via TS17/18, with a free-energy barrier of 29.10 kcal/mol. The spindensity distribution (1.06 au on nitrogen) of IM18 indicates that IM18 is an amino intermediate. The predicted EPR g-tensor and A-tensor are 2.0055 and ( )103.5 G, respectively. The whole process (IM16 f IM18) is predicted to be endothermic by about 8.20 kcal/mol, indicating that generation of the amino vanadium complex is not spontaneous. Comparing these freeenergy barriers and reaction free energies with those of the VO2+ model, it is found that a solvent (water) facilitates generation of the amino vanadium intermediate. The free-energy profile shown in Figure 7 is obviously smoother than that presented in Figure 1. 3.2.3. Formation of Aniline. Now let us consider the amination of benzene by the amino intermediate IM18 via an addition elimination pathway. The calculated free-energy profile is shown in Figure 7. The corresponding optimized structures are collected in Figure S5 (Supporting Information). The first step is attack of the NH2 terminal of IM18 on benzene to form the cyclohexadienylamino intermediate IM19 via TS18/19, with a freeenergy barrier of 8.61 kcal/mol. The spin-density surface (Figure S8) indicates that a single electron partially transfers to the vanadium and carbon atoms in TS18/19. The selected molecular orbitals of TS18/19 presented in Figure S8 (Supporting Information) make it 5682


Organometallics clear that the amino-addition step mainly involves the p orbital of nitrogen interacting with the π orbital of benzene and the p orbital of the amino-connected carbon atom. Additionally, the single electron is mainly localized on the d orbital of vanadium in IM19. The aniline complex is then derived from IM19 by hydrogen transfer. However, a transition state linking IM19 and the aniline complex cannot be located. Fortunately, we located a bridged-water hydrogen-transfer transition-state, TS20/21. As shown in Figure 7, the addition of a water molecule to IM19 leads to the formation of IM20 through hydrogen bonding. This step is predicted to be endothermic by 3.38 kcal/mol. The bond lengths of the resulting O 3 3 3 H hydrogen bonds in IM20 are 1.814 and 1.699 Å (Figure S5, Supporting Information). Hydrogen transfer then produces IM21, with a small free-energy barrier (Figure 7). The last step, the loss of the product from IM21, is similar to that shown in Figure 1. 3.3. Model Catalyst VO(AcO)(H2O)3+. The optimized structures of VO(AcO)(H2O)3+(2A) are shown in Figure 8. Conformation B ( 1477.11023939 and 1477.21814344 hartree in

Figure 8. Optimized structures of mono-AcO -coordinated vanadium complexes. Bond lengths and angles are in angstroms and degrees, respectively.

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the gas phase and water, respectively) is more stable than conformation A ( 1477.10663709 and 1477.21672171 hartree in the gas phase and water, respectively) because the mutual repulsion of oxygen atoms in conformation B is stronger than that in conformation A. However, the η2-NH2OH complexes resulting from the coordination of hydroxylamine to the vanadium center of conformation B cannot be located. Therefore, the subsequent reaction pathways are derived from conformation A. Scheme 2. General Framework of Energy Decomposition Analysis “Reactant f TS/Product”a and a Sample EDA Framework for IM16 f TS16/17

a L = (H2O)3 for IM16 f TS16/17, IM16 f IM17, IM17 f TS17/18, IM17 f IM18, and IM16 f IM18; L = (AcO)(H2O) for IM22 f TS22/23, IM22 f IM23, IM23 f TS23/24, M23 f IM24, and IM22 f IM24.

Figure 9. Free-energy diagrams of the ground state (doublet state) in water for the amination of benzene to aniline by [VO(AcO)(H2O)3]+(2A) at 353 K and 1 atm (units: kcal/mol). The total free energy for the separated reactants (VO(AcO)(H2O)3+, hydroxylamine, and benzene) is 1688.211128 hartree at the UB3LYP/6-311G**(IEF-PCM)//UB3LYP/6-311G** level. 5683


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Table 1. Energies of Activation and Reaction, and Their Energy Components, in the Generation Process of the Amino Complexes (units: kcal/mol) reaction

ΔEint‑1

ΔEint‑2

ΔEB(M‑L)

ΔEDeform‑L

ΔEDeform‑M

ΔrE/ΔrEq

IM16fTS 16/17

91.27

107.73

16.46

1.11

49.69

34.33

IM16fIM17 IM17fTS 17/18

91.27 109.99

109.99 101.60

18.72 8.39

0.58 1.02

15.30 22.45

2.85 31.86

IM17fIM18

109.99

104.16

5.83

0.01

8.05

13.87

IM16fIM18

91.27

104.16

12.89

0.57

23.35

11.02 28.13

IM22fTS 22/23

107.19

134.48

27.29

3.13

52.39

IM22fIM23

107.19

121.24

14.05

2.73

6.73

4.59

IM23fTS 23/24

121.24

126.24

5.00

1.53

31.53

28.06

IM23fIM24

121.24

126.87

5.63

1.22

12.53

8.12

IM22fIM24 IM1fTS 1/2

107.19

121.24

19.68

3.95

19.27

3.53 50.25 4.91

IM1fIM2

35.13

IM2fTS 2/3 IM2fIM3

13.41

IM1fIM3

18.32

The calculated free-energy profiles are shown in Figure 9, and the corresponding optimized structures are shown in Figures S6 and S7. According to Figure 9, there are two reaction pathways resulting from conformation A. As presented in Figure 9, the first step is formation of the η2NH2OH complexes IM22 and IM27, which are slightly endothermic by 7.45 and 6.84 kcal/mol, respectively Next, through hydrogen-transfer transition states TS22/23 and TS27/28, with free-energy barriers of 26.62 and 31.46 kcal/mol, respectively, intermediates IM23 and IM28, respectively, are generated. In addition, the free-energy barriers of the water-mediated hydrogen transfer (IM22 f IM23 and IM27 f IM28) are predicted to be 1.52 and 2.14 kcal/mol, respectively. Obviously, the water molecule reduces significantly the free-energy barriers of these two hydrogen-transfer processes. Then, breakage of the N O bond in IM23 and IM28 results in the amino intermediates IM24 and IM29 via the transition states TS23/24 and TS28/29, with free-energy barriers of 26.08 and 29.50 kcal/mol, respectively. The spin-density distributions of IM24 and IM29 demonstrate that intermediates IM24 and IM29 can be considered as amino complexes. The predicted EPR g-tensor and A-tensor of IM24 are 2.0049 and ( )116.05 G, which are in agreement with the experimental values (2.0041 ( 0.0005 and 113.7 G).15 The processes IM22 f IM24 and IM27f IM29 are predicted to be endothermic by about 0.64 and 2.17 kcal/mol, respectively, indicating that the generation of the amino vanadium complex is slightly spontaneous. Comparing these free-energy barriers and reaction free energies with those of the VO(H2O)52+ model (Figure 7), it is found that the presence of AcO is favorable for the generation of the amino vanadium intermediate. Thereafter, the amino benzene adducts IM25 and IM30 are generated without barriers. Then, cyclohexadienylamino intermediates IM26 and IM31 are produced through the transition states TS25/26 and TS30/31. Finally, the product intermediates can be easily formed by hydrogen transfer. This step is not calculated, since it is very similar to the step in the two catalyst models above. Comparison of the free-energy profiles in Figure 9 shows that both reaction routes are kinetically facile; thus it is hard to tell which pathway is more favorable.

3.4. Comparison of the Three Model Catalysts. To gain a complete understanding of the mechanistic pathways, we have to compare the free-energy profiles obtained with the three model catalysts; these are shown in Figures 1, 7, and 9. The first conclusion drawn from this comparison is that VO(AcO)(H2O)3+ is the most suitable catalyst model. First, the calculated free-energy barriers for generation of the amino complexes IM3, IM18, and IM24 (47.20, 33.08, and 26.62 kcal/mol for VO2+, VO(H2O)52+, and VO(AcO)(H2O)32+, respectively) demonstrate that the reaction pathway of VO(AcO)(H2O)3+ is feasible under the experimental conditions (373.15 K and 1 atm). The predicted reaction free energies for generation of the amino complexes (IM1 f IM3, IM16 f IM18, and IM22 f IM24) imply that formation of the amino complex IM24 is probably spontaneous. Second, the predicted 51V NMR chemical shifts of IM3, IM18, and IM24 are 753, 802, and 688 ppm, respectively. The EPR g-tensor and A-tensor of IM24 are 2.0049 and ( )116.1 G, respectively, which is in agreement with the experimental values (2.0041 ( 0.0005 and 113.7 G).15 Therefore, the predicted 51V chemical shifts and the EPR g-tensor and A-tensor of the amino complexes IM3, IM18, and IM24 indicate that VO(AcO)(H2O)32+ is the most appropriate model for the amination of benzene to aniline with hydroxylamine as the aminating agent catalyzed by NaVO3. To understand the role of the solvent (water and AcO ) on the amination process, we carried out an energy decomposition analysis63,64 (Scheme 2) on the energies of activation and reaction of the amino generation steps. In Scheme 2, De represents the energy required to dissociate the reactants into two fragments. ΔEdeform refers to the energy needed to deform the two fragments to the geometries they have in the transition states or products. The interaction energies between the two fragments in the reactants and transition states/products are represented by ΔEint‑1 (ΔEint‑1 = De) and ΔEint‑2, respectively. ΔEM‑L represents the change in interaction energy between the ligand and vanadium from reactants to transition states/products. The results of the energy decomposition analysis are presented in Table 1. As shown in Table 1, ΔEDeform‑M is comparable to the energies of activation and reaction of the corresponding steps in the generation process of the amino 5684


Organometallics complex catalyzed by VO2+. ΔEDeform‑L is very small because the change in the solvent geometries is unlikely to be significant. The main contribution to the decrease in the energies of activation and reaction is ΔEM‑L. The binding between oxygen atoms of water (AcO ) ligands and a vanadium atom in TS16/17 (TS22/ 23) and IM17 (IM23) is stronger than that for IM16 (IM22) in the hydrogen-transfer process because the bonding of oxygen and vanadium changes from a secondary bond (O 3 3 3 VdO) to a coordination bond (OfV O) (Figures S4 and S6). Therefore, the presence of the solvents water and AcO lowers the activation energy of the hydrogen-transfer step and changes the thermodynamic tendency. In the N O bond breakage process, the change of the coordination bond (H2OfV O) to a secondary bond (H2O 3 3 3 VdO) results in the endothermicity of IM17(IM23) f IM18(IM24). The water and AcO coordinated pathway (IM22 f IM24) includes transformation of a secondary bond (CH3COO 3 3 3 VdO, IM22) to a coordination bond (CH3COOfVdO, IM23) and transformation of a coordination bond (H2OfV O, IM23) to a secondary bond (H2O 3 3 3 VdO, IM24). The ΔEM‑L of water and AcO assisted IM22 f IM24 is higher, leading to a rapid reduction in the activation and reaction energies. However, the water-assisted reaction process (IM16 f IM18) involves the counteraction of a change of a secondary bond (H2O 3 3 3 VdO) to a coordination bond (H2OfV O) and change of a coordination bond (H2OfV O) to a secondary bond (H2O 3 3 3 VdO); thus the whole process needs to overcome high energy barriers and absorb much more energy. To sum up, the solvent does not essentially change the generation of the amino complex. Moreover, the reduction of activation and reaction energies results mainly from the differences in the bonding energies between the solvent and vanadium.

4. CONCLUSIONS The amination of benzene with hydroxylamine as the aminating agent, catalyzed by vanadium, was investigated in a DFT framework at the UB3LYP/6-311G**(IEF-PCM)//UB3LYP/6-311G** level. Three model catalysts, namely, VO2+, VO(H2O)52+, and VO(AcO)(H2O)3+, were investigated separately and compared. Our calculations reveal that the addition elimination mechanism is clearly preferred over the C H bond activation mechanism, thermodynamically and kinetically, with VO2+ as the catalyst. The rate-determining step is the formation of the amino radical complex. The predicted NICS values for the benzene ring indicate that the benzene ring keeps its aromaticity throughout the addition elimination mechanism. In contrast, the benzene ring exhibits antiaromaticity in the stationary points of the C H bond activation mechanism. The FMO analysis illustrates that the 3d orbital of vanadium plays an important role in this amination process by receiving a single electron from the cyclohexadienamine radical moiety to stabilize the cyclohexadienamine radical intermediate. For pure water as the solvent, the weakly bonded water molecules reduce the free-energy barriers in the rate-determining step through electrostatic interactions, mainly originating from the interaction of the dz2 orbital of vanadium with the p orbital of the oxygen of the axial H2O and the dx2 y2 orbital of vanadium with the p orbital of the oxygen of the equatorial H2O. The predicted free-energy barriers and reaction free energies of the generation of amino complexes imply that the formation of the AcO -coordinated amino intermediate is more favorable than formation of naked amino- and water-coordinated amino intermediates, both

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thermodynamically and kinetically. The consistency between these results and experimental data suggests that VO(AcO)(H2O)3+ may be the main form of the operative catalyst. The EDA results indicate that binding between the solvent (water and AcO ) and vanadium plays an important role in the amination of benzene to aniline using hydroxylamine. To conclude, our calculations have demonstrated that the mechanism for the amination of benzene to aniline using hydroxylamine, catalyzed by vanadium, in a polar solvent is solvent dependent. Since the polar solvent used noticeably affects the chemical equilibrium between VO2+, VO(H2O)52+, and VO(AcO)(H 2 O)3 +, the aniline-production rate is solvent sensitive.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 S8, Cartesian coordinates of optimized structures, and the complete ref 41; these materials are available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(D.T.) Tel/Fax: +86-833-2272106. E-mail: [email protected]. (C.H.) Tel/Fax: +86-28-85411105. E-mail: [email protected] or [email protected].

’ ACKNOWLEDGMENT The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 200720024, No. 20502017) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PRC (2002). ’ REFERENCES (1) Thomas, B. R.; William, T. Science 1999, 284, 1477. (2) Schmerling, L. U.S. Patent 2 948 755, August 9, 1960. (3) Squire, E. N.; Mills, G. U.S. Patent 3 919 155, November 11, 1975; 3 929 889, December 30, 1975. (4) Tan, Y.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 3676. (5) Gao, L.; Zhang, D.; Wang, Y.; Xue, W.; Zhao, X. React. Kinet. Mech. Catal. 2011, 102, 377. (6) Hu, C.; Zhu, L.; Xia, Y. Ind. Eng. Chem. Res. 2007, 46, 3443. (7) Parida, K. M.; Dash, S. S.; Singha, S. Appl. Catal. A: Gen. 2008, 351, 59. (8) Parida, K. M.; Rath, D.; Dash, S. S. J. Mol. Catal. A: Chem. 2010, 318, 85. (9) Matsuda, N.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2011, 13, 2860. (10) Poojary, D.; Borade, R.; Hagemeyer, A.; Dube, C.; Zhou, Z. P.; Nothels, U.; Armbrust, R.; Rasp, C. PCT World Patent Application, WO 00 069 804, November 13, 2000. (11) Hagemeyer, A.; Borade, R.; Desrosiers, P.; Guan, S.; Lowe, D. M.; Poojary, D. M.; Turner, H.; Weinberg, H.; Zhou, X. P.; Armbrust, R.; Fengler, G.; Notheis, U. Appl. Catal., A 2002, 227, 43. (12) Desrosiers, P.; Guan, S. H.; Hagemeyer, A.; Lowe, D. M.; Lugmair, C.; Poojary, D. M.; Turner, H.; Weinberg, H.; Zhou, X. P.; Armbrust, R.; Fengler, G.; Notheis, U. Catal. Today 2003, 81, 319. (13) Kuznetsova, N. I.; Kuznetsova, L. I.; Detusheva, L. G.; Likholobov, V. A.; Pez, G. P.; Cheng, H. J. Mol. Catal. A: Chem. 2000, 161, 1. (14) L€u, Y.; Zhu, L.; Liu, Q.; Guo, B.; Hu, X.; Hu, C. Chin. Chem. Lett. 2009, 20, 238. 5685


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