Theoretical Investigation On The Direct Amination And Hydroxylation Of Benzene Catalyzed By Vanadium-Based Catalysts

The direct amination and hydroxylation of benzene, which involves the activation of C-H bond, preserves a challenging topic in chemistry and attracts much attention recently. In the present paper, density functional theory B3LYP hybrid functional was used to investigate the mechanism of the direct amination and hydroxylation of benzene to aniline and phenol, respectively, catalyzed by vanadium-based catalysts.The oxidation of hydroxylamine by VO2+ to the primary product HNO in gas phase were investigated with density functional theory (DFT) at the B3LYP/6-311++G(d, p) level. Calculations including geometry optimization and vibrational analysis for the stationary points on the ground and the first excited states were performed. Two possible mechanisms were investigated. The fist one is the one-electron mechanism through firstly the formation of NH2O·and·NHOH radical complexes ([VO(OH)(NH2O)]+ and [VO(OH)(NHOH)]+) from NH2OH and VO2+, and then the oxidation of the stable intermediate NH2O·by VO2+ to produce the products HNO and VO(OH)+. The one-electron mechanism is predicted to be spin-conserved and the rate-limiting step is the cleavage of the O-H bond with 27.92 kcal/mol energy barrier. The other one is the two-electron mechanism in which the first half is also the formation of NH2O·and·NHOH radical complexes and the second half is the intra-molecular hydrogen transfer of NH2O·and·NHOH radical complexes to the product HNO together with the reduction of VIV to VIII. The crossing point between singlet and triplet potential energy surfaces (PESs) results in the stable product triplet V(OH)2+ in the two-electron mechanism. The two-electron mechanism may be kinetically competitive with the one-electron mechanism only if the spin inversion between singlet and triplet PESs occurs easily, while, the one-electron mechanism is energetically more favorable than the two-electron mechanism. Therefore, the one-electron mechanism is predominant. The topological description, based on the gradient field analysis of the electron localization functional, on all of the key minima and transition states along the feasible reaction channels suggests that all of the hydrogen transfer processes involve firstly the cleavage of O-H or N-H bond followed by the formation of a covalent O-H bond.The amination of benzene with NH2OH as amination agent catalyzed by vanadium was investigated in the framework of density functional theory at UB3LYP/6-311G**(IEF-PCM)// UB3LYP/6-311G** level. Three model catalysts, VO2+, VO(H2O)52+, and VO(Ac)(H2O)3+, had been investigated separately, and the results were 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 nuclear independent chemical shift (NICS) values of benzene ring indicate that the benzene ring keeps its aromaticity throughout the addition-elimination mechanism. In contrast, the benzene ring exhibits anti-aromaticity in the stationary points of the C-H bond activation mechanism. The fragment molecular orbital (FMO) analysis illustrates that 3d orbitals of vanadium play an import role in this amination process through receipt of single electron from the cyclohexadienamine radical moiety to stabilize the cyclohexadienamine radical intermediate. For pure water solvent, the weakly bonded water molecules lower the free energy barriers of rate-determining step. The predicted free energy barriers and reaction free energies of the generation of amino complexes imply that the formation of the Ac- coordinated amino intermediate is more favorable than that of naked amino and water coordinated amino intermediates thermodynamically and kinetically. The consistency between these results and experimental facts suggests that VO(Ac)(H2O)3+ may be the main form of the operative catalyst. The energy decomposition analysis indicates that the variation of binding energy between solvent and vanadium plays an important role in the amination process of benzene to aniline with hydroxylamine. To conclude, our calculation have demonstrated that the mechanism of amination of benzene to aniline with hydroxylamine as amination agent catalyzed by vanadium in polar solvent is solvent-dependent. Since the polar solvent used noticeably affects the chemical equilibrium between VO2+, VO(H2O)52+, and VO(Ac)(H2O)3+, the productive rate of aniline is solvent-sensitive.The mechanism of benzene to phenol with H2O2 as oxidant catalyzed by vanadium in CH3CN sovent was invistgated at the B3LYP/6-311G(2d,2p)(IEF-PCM) //B3LYP/6-311G(2d,2p) level. Three possible species, VO(O2)(CH3CN)4+, (O2)2V(μ-O)2V(O2)(CH3CN), and (O2)2V(μ-O)2VO(CH3CN), were employed as the active catalyst to elucidate the reaction mechanism. When VO(O2)(CH3CN)4+ is serverd as the catalyst, the C-H bond activation step is the rate-liminting step with about 38 kcal/mol of free energy of activation at 298.15 K and 1atm. This results indicates that VO(O2)(CH3CN)4+ is not the active catalyst at room tempreture. The solvated solvent molecules, CH3CN, interact with the catalytic center, vanadium, through electrostatic interaction. For the binuclear vanadium species, (O2)2V(μ-O)2V(O2)(CH3CN), there are two reaction pathways. The pathway through phenyl intermediate (HOO)(O2)V(μ-O)2V(O2)Ph (the double-center pathway) is preferred than that of phenyl intermediate (O2)2V(μ-O)2V(OOH)Ph (the single-center pathway). Because the double-center pathway inlvolves d orbitals of two vanadiums interacted with p orbital of carbon and p orbitals of -OOH in transition states of the C-H bond activation and hydroxyl transfer steps. The double-center pathway is predicted to be feasible at room tempreture with a free energy barrier of about 25 kcal/mol. While the free energy barrier of the single-center pathway is about 40 kcal/mol, which indicates that this pathway is not feasible at room tempreture. For the other binuclear vanadium species, (O2)2V(μ-O)2VO(CH3CN), there is only one reaction pathway. In the transition states of this reaction pathway, the orbital interaction only involves d orbitals of vanadium of (O2)2V fragment interacted with the orbitals of reaction centers. For all of these pathways, the predicted NICSs of the phenyl rings in these intermediates indicate that the phenyl rings in these intermediates maintain aromaticity. Therefore, (O2)2V(μ-O)2V(O2)(CH3CN) is the most suitable catalyst for the benzene to phenol with H2O2 as oxidant catalysed by vanadium in CH3CN solvent. In order to obtain the best results in experiments, we must make (O2)2V(μ-O)2V(O2)(solvent) as the main species of vanadium in the reaction system.A theoretical study was carried out at the B3LYP level of theory for the (CO)4Cr(μ-PH2)2RhH(CO)(PH3)-catalyzed hydroformylation of phosphinobutene. Four mechanisms are possible. The first one is the chelate associative mechanism in which the main species adopts chelate coordination mode with the rhodium center. The second one is also the associative mechanism in which the key species adopts monodentate coordination mode with rhodium center. The third one is the monodentate dissociative mechanism, which is similar to the popularly accepted mechanism of hydroformylation of alkenes catalyzed by the monometallic Rh catalysts. And the last one is chelate dissociative mechanism in which the main species also equip with chelate coordination mode. The comparison of the four possible mechanisms indicates that the hydroformylation of phosphinobutene catalyzed by the Rh-Cr bimetallic catalyst is obviously different from the previously characterized mechanism of the monometallic rhodium catalyst. The hydroformylation of phosphinobutene catalyzed by the Rh-Cr bimetallic catalyst involves firstly the formation of the chelate acyl species through the chelate associative mechanism including the olefin addition, olefin insertion, carbonyl insertion steps, then the CO addition to the chelate acyl species leading to the formation of the monodentate acyl species, and finally the conversion of the monodentate acyl species to the product aldehyde through the H2 coordination, H2 oxidative addition and aldehyde elimination. The carbonyl insertion is the rate-limiting step for the catalytic cycle. Therefore, the introduction of cooperative metallic chromium remodels the mechanism. In addition, some other new points are obtained: (1) The CO concentration or partial pressure is helpful for the transform of the chelate acyl species to the product aldehyde. (2) The bimetallic Rh-Cr-catalyzed hydroformylation favors the branched product with a percentage ratio of nearly 100% both kinetically and thermodynamically in benzene solution. (3) The breakage of the orbital interaction between Rh and Cr atoms in olefin addition step indicates that this associative mechanism can be viewed as both associative and dissociative. The chromium serves as an orbital reservoir in olefin addition and insertion steps via the variation of the orbital interaction between Rh and Cr atoms. (4) The calculated free energy barriers imply that the catalytic activity of the Rh(I)-Cr bimetallic complex is higher than that of the monometallic Rh catalysts. (5) The adaptability of the Rh-μ-P-Cr-μ-P four-membered ring in the reaction process effectively demonstrates the cooperativity of chromium with the catalytic center rhodium. These results are in good agreement with the experimental studies.