A DFT study on the mechanism of the gas phase reaction of niobium with acetaldehyde

Transcription

1 Indian Journal of Chemistry Vol. 51A, November 2012, pp A DFT study on the mechanism of the gas phase reaction of niobium with acetaldehyde Yong Wang a, b & Gui-hua Chen a, * a School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai , PR China b College of Materials Science and Engineering, Nanjing University of Technology, Nanjing , PR China chenguihua518@hotmail.com Received 5 July 2012; revised and accepted 10 October 2012 The reaction between niobium atom and acetaldehyde has been investigated with a DFT approach. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction starts with the formation of a η 2 -CH 3 CHO-metal complex followed by C C, aldehyde C H, methyl C H and C O bond activation. These reactions can lead to several different products, viz., NbCO+CH 4, NbCOCH 2 +H 2, NbCOCH 3 +H, NbCHO CH 2 +H, NbH+CH 3 CO, NbO+CH 3 CH and NbO+C 2 H 4. The spin-forbidden reaction, 6 Nb+CH 3 CHO 2 IM9 is found to be energetically most favorable. According to the identified reaction mechanisms, the sextet quartet and quartet doublet surface crossings are suggested and the crossing region is approximately determined. The present results may be helpful in understanding the mechanism of the title reaction and in experimental investigations of the reaction. Keywords: Theoretical chemistry, Density functional calculations, Potential energy surfaces, Niobium, Acetaldehyde The chemistry of interaction of gas phase transition metal atoms and cations with hydrocarbon molecules has been an active area of research, both theoretical and experimental, due to its significance in catalytic and materials science 1-6. To better understand the fundamental aspects of this reaction, several studies have been reported on the investigation of the relevant mechanism As one of the simple carbonylcontaining organic molecules, acetaldehyde is used in many applied reactions such as organic synthesis, catalysis, etc. Since catalytic hydrogenation of carbon monoxide as well as C H activation of aldehydes is usually promoted by metal-bearing catalysts, it is interesting and instructive to explore the reactions of gas phase transition metal ions and atoms with aldehyde, which may lead to a better understanding of fundamental aspects of elementary transition metal reactions initiated by C H insertion. Earlier studies have focused mainly on the reactions between acetaldehyde and first row transition metal (from Sc to Zn) cations such as Co +, Fe +, Cr +, Ti, + Ni, + etc Some experimental techniques such as ion beam mass spectrometry and crossed beam spectroscopy have revealed that decarbonylation of acetaldehyde to yield CH 4 +MCO + is a dominant process at low energy condition 6. Based on these experiments, Guo et al. 7-9 have theoretically investigated the gas phase reaction of Co +, Fe +, Ti +, Cr + and Ni + with CH 3 CHO. Their results show that for Co +, Fe + and Cr +, decarbonylation proceeds through C C activation rather than aldehyde C H activation, whereas both C C and aldehyde C H activation may result in the decarbonylation of CH 3 CHO by Ni + cation. Compared with the first row cations, heavier metal atoms and cations have received relatively less attention, and only a few theoretical and experimental studies have been reported Bayse et al. 14 have investigated the reaction between yttrium atom and acetaldehyde in detail. From these previous experimental and theoretical studies on the decarbonylation of acetaldehyde by first row cations and the larger transition-metal Y, it is clear that the transition metal change from first row to metal Y leads to the reaction change in mechanism, both in early and late reaction stages. In addition, these earlier studies mainly focused on the reaction between first row cations and aldehyde while investigations on the reaction between aldehyde and neutral atoms are still scarce. Since the charge may affect the mechanistic details, it is necessary to study the reaction between neutral atoms and aldehyde. While the first row elements have been studied, of the second row transition metals (from Y to Cd) only yttrium has been studied for this reaction system. It is

2 1554 INDIAN J CHEM, SEC A, NOVEMBER 2012 of interest to investigate if niobium, a typical second row early transition metal, follows a similar reaction mechanism as Y in the reaction with CH 3 CHO. If not, what is the difference between niobium and yttrium? Prompted by these questions, we investigated the reactions of Nb atom with CH 3 CHO by using DFT methods. Since niobium is a typical representative of the second row transition metal (yttrium may be categorized as a lanthanum analogue because of its chemical properties), the calculated results are expected to predict and give an insight into reactions that can not be reached experimentally under the considered conditions. Methodology The potential energy surface for the title reaction has been considered in detail. All molecular geometries (reactants, intermediates, transition states and products) were optimized by employing the UB3LYP density functional theory method 23. This method has chosen since the previous calculations on the reaction between niobium and carbon dioxide showed that this hybrid functional can provide accurate results 24. In all calculations, the effective core potentials (ECP) of Stuttgart 25 basis set was used for niobium; the 5s and 4d in niobium were treated explicitly by a (8s7p6d) Gaussian basis set contracted to [6s5p3d]. For non-metal atoms, a standardized G** basis set was used. For all the species involved in the reaction, the enthalpy at 0 K is discussed in the study. The harmonic vibration analyses were performed at the same level of theory for all optimized stationary points to determine their properties (minimum or first-order saddle point) and to evaluate the zero point vibrational energies (ZPEs). To verify whether the located transition states connected the expected minima, intrinsic reaction coordinate (IRC) calculations were carried out for each transition state at the same level 26. All calculations in the present study were performed using the Gaussian 03 program 27. Results and Discussion In order to evaluate the computational accuracy, before the discussion on the mechanisms of the title reaction, we first calculated the bond dissociation energies (BDE) for several species involved in the reaction. As shown in Table 1, where some available experimental values are also listed for comparison, these theoretical values are in agreement with experimental findings, indicating that the theoretical Table 1 Theoretical and experimental excitation energies and bond dissociation energies for Nb + and Nb Excitation energy (ev) Bond dissociation energy (ev) Species Calc. Expt. Species Calc. Expt. 3 Nb a Nb + O ± 0.11 b 1 Nb a Nb + CO ± 0.05 b 4 Nb a Nb O c 2 Nb a a Ref. 28; b Ref. 29; c Ref. 30. level chosen in this work is reliable for describing the features of the PESs of the title reaction. The optimized geometries of the stationary points over the PESs for the title reaction are depicted in Fig. 1. The profiles of the PES are shown in Fig. 2. We also inspected the values of <S 2 > for all species involved in the reaction and found that the deviation in <S 2 > is less than 5 %. This shows that spin contamination is small in all the calculations. As shown in Fig. 2, the title reaction starts with the formation of a η 2 -CH 3 CHO-metal encounter complex IM1. From Fig. 1, one can see that in IM1, the niobium atom bonds with the carbon and oxygen simultaneously. As a result of oxygen polarizing charge towards niobium, the C O bond is weakened, and is lengthened by 0.077, and Å in 6 IM1, 4 IM1 and 2 IM1 respectively. Energetically, 6 IM1 lies 27.7 kcal/mol lower than the reactants of 6 Nb + CH 3 CHO. For 4 IM1 and 2 IM1, they are computed to be 17.9 kcal/mol more stable and 4.9 kcal/mol higher than the corresponding sextet initial complex 6 IM1 respectively. Clearly, the ground state of η 2 -CH 3 CHO-metal encounter complex IM1 is quartet. As the excited reactants 4 Nb + CH 3 CHO are 15.5 kcal/mol higher in energy than the ground analogues, it is obvious that inter-system crossing occurs in the course of binding pathway. It may be noted that although several trials were undertaken to search for possible transition states that connect the reactants and IM1, no such transition states were obtained. Obviously, the formation of IM1 is a barrier-free exothermic reaction. Once the encounter complex IM1 is formed, four possible reaction pathways can be followed by C C, aldehyde C H, methyl C H and C O activation, which will be discussed over three different PESs respectively. 4 NbCO+CH 4 channel First, we will discuss the product P1 formation channel on quartet PES. As shown in Fig. 2, this reaction channel starts with the formation of an

3 WANG & CHEN: DFT STUDY ON GAS PHASE REACTION OF NIOBIUM WITH ACETALDEHYDE 1555 Fig. 1 Optimized geometries for some stationary points located on the Nb +CH 3 CHO [distances in Å].

4 1556 INDIAN J CHEM, SEC A, NOVEMBER 2012 Fig. 2 Potential energy surface profiles for the reaction of (a) 6 Nb+CH 3 CHO, (b) 4 Nb+CH 3 CHO, and, (c) 2 Nb+CH 3 CHO. [Relative energies include ZPE correction]. encounter complex 4 IM1. Along this reaction pathway, the Nb atom can insert into the C C bond via a transition state 4 TS 12. From Fig. 1, one can see the distance between two carbon atoms in 4 TS 12 is lengthened from Å to Å. This indicates that the C C bond is activated. The unique imaginary frequency is 390.3i cm -1, and the corresponding normal mode corresponds to the rupture of the C C bond with the Nb atom inserted into it. For the inserted intermediate 4 IM2, the ground state is 4 A' with C 1 symmetry, which is 49.6 kcal/mol more stable in energy than the ground state reactants 6 Nb+CH 3 CHO. As shown in Fig. 1, the C C distance in 4 IM2 is lengthened to Å, which means that this bond has ruptured thoroughly. The two calculated Nb C bonds are and Å respectively, NBO analysis of this complex shows that the Nb atom interacts with these two atoms. Along this reaction

5 WANG & CHEN: DFT STUDY ON GAS PHASE REACTION OF NIOBIUM WITH ACETALDEHYDE 1557 coordinate, the C C bond activation is followed by a H-shift to form 4 IM6, with an energy barrier of 6.9 kcal/mol. From Fig. 2(b), we can see that via the transition state 4 TS 26, there is an aldehyde H migration to the carbon centre of methyl group. The next step is the non-reactive dissociation of 4 IM6 to generate products. Calculated results showed that the C C bond insertion species 4 IM6 can dissociate directly without exit barrier to the product P1( 4 NbCO+CH 4 ) through Nb C bond rupture. This step is endothermic by 10.1 kcal/mol. In earlier theoretical studies on the gas phase reactions of acetaldehyde with first row transition metal cations (such as Cr +, Fe +, Co +, Ni + ), Guo et al. 7-9 reported that the decarbonylation process leading to M + (CO)+CH 4 is highly exothermic. Bayse et al. 14 have examined the reaction between yttrium atom and CH 3 CHO, both experimentally and theoretically. In their study, no Y(CO)+CH 4 species was observed, and no feasible Y(CO)+CH 4 formation channel was determined. Though no corresponding experimental studies have been reported, the discussion above shows that all the species along this decarbonylation channel are computed to be below the ground reactants ( 6 Nb + CH 3 CHO). We propose that even at low energies, these decarbonylation products can be observed in the reaction of Nb atom with acetaldehyde. Hence, compared with the Y atom, niobium may decarbonylate aldehyde more effectively. 4 NbCOCH 2 +H 2 channel With respect to the P2 ( 4 NbCOCH 2 +H 2 ) formation channel, one possible channel which originates from aldehyde C H bond activation has been confirmed. In this mechanism, the 4 IM1 can interconvert to 4 IM3 via 4 TS 13, over a barrier height of 10.4 kcal/mol. In 4 IM3, the Nb atom is inserted into the aldehyde C H bond with the relative energy of kcal/mol. After 4 IM3 formation, the carbon atom in carbonyl group captures one methyl H atom to form 4 IM4 via 4 TS 34, with an energy barrier of 42.3 kcal/mol. The next step corresponds to the isomerization between 4 IM4 and 4 IM7 via 4 TS 47 directly; the relative energy of this transition state is kcal/mol. Through this step, one hydrogen atom is transferred towards the Nb atom and binds with the H atom that connects the metal centre to form a H 2 molecule subsequently. From Fig. 2(b), one can see that the second step, i.e., the process of 4 IM4 formation is rate determining along this path. In 4 IM7, the hydrogen moiety is coordinated to the Nb atom. The binding energy between H 2 and 4 NbCOCH 2 is calculated to be 8.4 kcal/mol. In a previous study on the reaction of Y( 2 D)+CH 3 CHO, Bayse et al. 14 have reported the product channel for H 2 formation in the reaction between yttrium and CH 3 CHO. Their calculations show that this channel is of no importance, as the relative energy of the rate determining transition state is 6.6 kcal/mol, which is above that of the reactants Y+CH 3 CHO. However, in the present study the energies of hydrogen formation channel are below 0.0 kcal/mol. Therefore, this H 2 elimination pathway is spontaneous. From the above discussion we can see that as representative of early second row transition metals, te niobium is more effective in dehydrogenation of acetaldehyde than yttrium. 3 NbCOCH 3 +H formation channel Another exit of 4 IM3 is the direct rupture of the H NbCOCH 3 bond to generate the products P3 ( 3 NbCOCH 3 +H). This dissociation process is computed to be endothermic by 75.0 kcal/mol. We also investigated the channel of direct rupture of HNb COCH 3 bond to form P6 ( 3 NbH+CH 3 CO), which is endothermic by 89.3 kcal/mol. Compared with the P2 ( 4 NbCOCH 2 +H 2 ) formation channel mentioned above, as the dissociation energy is higher than the methyl H-shift energy barriers (75.0 and 89.3 versus 42.3 kcal/mol), the possibility of P3 ( 3 NbCOCH 3 +H) and P6 ( 3 NbH+CH 3 CO) formation originating from 4 IM3 is negligible. 3 NbCHOCH H formation channel Now we will discuss the alternative C H activation channel, which may lead to the product P4 ( 3 NbCHOCH H) formation. It is very clear from Fig. 2(b) that this route passes through the metalmediated methyl H migration followed by the nonreactive-dissociation. Initial complexes 4 IM1 and 4 IM8 are connected through a direct, one-step methyl H shift occuring via the saddle point 4 TS 18. Geometrically, the activation C H bond is elongated to Å and simultaneously the distance between. niobium and hydrogen atoms is shortened to Å. The imaginary frequency of 4 TS 18 is 812.6i cm -1, and the normal mode corresponds to the rupture of methyl C H bond resulting in niobium binding with the H atom. One exit of 4 IM8 is direct rupture of the H NbCH 2 CHO bond to account for the products 3 NbCH 2 CHO+ 2 H, but this is a highly endothermic process with the dissociation energy of 69.4 kcal/mol

6 1558 INDIAN J CHEM, SEC A, NOVEMBER 2012 Aldehyde C O channel Previous studies of M (or M + ) +CH 3 CHO mainly focused on the C C or C H activation reaction, which may lead to formation of HMCH 3 +CO or MCH 2 CO+H 2. In the present study, we have also explored the possibility of the aldehyde C O bond activation mechanism. Our results predict that C 2 H 4 elimination through aldehyde C O bond activation is also a possible reaction channel. Originating from 4 IM1, the C O bond activation species, 4 IM5, can be formed by the insertion of Nb atom into the aldehyde C O bond. Energetically, the transition state ( 4 TS 15 ) is calculated to be kcal/mol below the energies of the ground reactants. Subsequently, the direct, onestep H shift occurring via saddle point 4 TS 59 may yield an intermediate 4 IM9. This step can release a large amount of energy (23.0 kcal/mol). Obviously, 4 IM9 is the lowest point along the quartet PES of the title reaction. When compared with the C C, C H and C O bond activation, one can see that the first step of this mechanism, i.e., 4 IM1 4 IM5 isomerization process, requires a high activation energy of 26.0 kcal/mol. With the excess energy gained in the formation of 4 IM1, this C O activation process can be completed favorably, and hence, this channel is also feasible in energy. From Fig. 1, one can see that in 4 IM9, there exist weak interactions between ethene and 4 NbO, with the binding energy of 18.3 kcal/mol. In the previous study on the reaction between yttrium atom and aldehyde 14, no C 2 H 4 and YO species were detected, and no feasible C 2 H 4 +YO formation channel was observed theoretically. Obviously, in the reaction between Nb and CH 3 CHO, the aldehyde C O activation is also a spontaneous pathway. It should be noted here that one exit of 4 IM5 is the direct rupture of the ONb CHCH 3 bond to account for the products P7 ( 3 NbO+CH 3 CH), but this is a highly endothermic process with the dissociation energy of 76.1 kcal/mol. Potential energy surfaces When compared with the PESs of the C C and C H activation routes as discussed above, we found that the aldehyde C O activation is the most different due to the relatively high energy barrier of 26.0 kcal/mol ( 4 TS 15 ). From these routes for the formation of the C C, C H and C O activation intermediates, 4 IM2, 4 IM3, 4 IM8 and 4 IM5, Nb centre is inserted into the methyl C H via 4 TS 18, with an energy barrier of 7.4 kcal/mol, which is lower by 15.9, 3.0 and 18.6 kcal/mol than that of the C C ( 4 TS 12 ), aldehyde C H ( 4 TS 13 ) and C O ( 4 TS 15 ) activation steps respectively. According to the exponential law of reaction rate [k = Aexp(-E a /RT)], the reaction rate of 4 TS 18 is approximately , , and times as fast as that of 4 TS 12, 4 TS 13 and 4 TS 15 at room temperature respectively. Clearly, starting from the η 2 -CH 3 CHO-metal complex 4 IM1, the channel which leads to 4 IM8 formation through methyl C H activation is most feasible. In addition, as 4 TS 18 is 38.2 kcal/mol below the separated ground reactants ( 6 Nb + CH 3 CHO), the formation of 4 IM8 is spontaneous in energy. Similar to that on the quartet PES, we also located possible activation channels on the sextet and doublet PESs. As shown in Fig. 2, the reaction mechanism on these PESs are in general similar to that of the quartet one. The reaction starts with a η 2 -CH 3 CHO-metal complex followed by several possible pathways: C C, aldehyde C H and methyl C H activation. From Fig. 2, one can see that after IM1 formation, most of the intermediates, transition states and products involved in the reaction on the quartet PES, lie below the analogues on the doublet and sextet PESs, (except IM5, TS 59 and IM9, the ground state is doublet). Obviously, the reaction path involves the spin crossover in different reaction steps. As the ground state 6 Nb is calculated to be 15.5 kcal/mol more stable than the excited state 4 Nb (see Fig. 2), and the initial complex 4 IM1 is 17.9 kcal/mol more stable than 6 IM1, we speculate that the first inter-system sextet-quartet crossing occurs during the process of 6 Nb+CH 3 CHO 4 IM1. The region where the spin crossover occurs, and the structure and energy of crossing point between the two different potential energy surfaces were investigated. We chose an approach suggested by Yoshizawa et al. 31 for approximately locating the crossing points of two PESs of different multiplicities. Then we defined the distance between Nb and O as a function, which is depicted in Fig. 3. For a given Nb O bond length, all other geometrical degrees of freedom were optimized for each spin. Along the energy curve, we find a crossing point CP1, which is at the length of Nb O bond length Å with energy of Hartree. Therefore, the reaction may jump from the sextet PES to the quartet one near the crossing point CP1. As can be seen from Fig. 3, after passing point CP1, the quartet PES can provide a low-energy reaction pathway towards the initial η 2 -CH 3 CHO-metal complex 4 IM1. From Fig. 2, one

7 WANG & CHEN: DFT STUDY ON GAS PHASE REACTION OF NIOBIUM WITH ACETALDEHYDE 1559 potential energy surfaces, denoted as two state reactivity (TSR) 32-35, which has been confirmed by experimental studies. Conclusions In the present study, the reaction mechanisms between Nb atom and CH 3 CHO have been investigated on three different PESs. The reaction is expected to occur spontaneously over the quartet PES after initial complex IM1 formation. Originating from the intermediate complex 4 IM1, initial C C bond insertion may lead to a decarbonylation product 4 NbCO+CH 4. Our calculations confirm two channels for C H bond activation, initial aldehyde C H insertion or direct methyl H shift to the Nb centre. The former occurs through Nb insertion into aldehyde C H, followed by a methyl H-shift yielding NbCOCH 2 +H 2. The latter C H activation is more feasible due to the relatively low activation energy. However, the direct collapse of 4 IM8 to form P3 is highly endothermic and hence 4 IM8 is stable and an abundant species in the title reaction. Also, the initial aldehyde C O activation pathway can lead to formation of C 2 H 4 +NbO. For the title reaction between Nb atom and CH 3 CHO, we found that the reaction system is likely to change its spin multiplicity in going from the entrance channel to the exit channel. Fig. 3 (a) Potential energies from R( 6 Nb+CH 3 CHO) to 4 IM1 along the distance between niobium and oxygen atoms [1, quartet; 2, sextet]; (b) Potential energies from 4 TS 15 to 2 IM5 along the quartet IRC. [1, quartet; 2, doublet]. can see the doublet transition state 2 TS 15 is 6.7 kcal/mol above quartet PES, while 2 IM5 is 22.4 kcal/mol more stable than 4 IM5. Hence, we propose that the other inter-system crossing occurs during the 4 TS 15 2 IM5 process. Along the IRC we find another crossing point CP2, which is after 4 TS 15 with energy of Hartree. Thus, the reaction may jump from the quartet PES to the doublet one near the crossing point CP2. To conclude, the minimum energy pathway of C O activation may proceed as 6 Nb+CH 3 CHO CP1 4 IM1 4 TS 15 CP2 2 IM5 2 TS 59 2 IM9, which is calculated to be exothermic by 90.4 kcal/mol. Obviously, 2 IM9 is the lowest point over the PES of title reaction. The reactions catalyzed by metallic systems may often involve a change in the spin states and proceed via a non-adiabatic way on two or more Acknowledgement This work was supported by the Zhejiang Provincial Natural Science Foundation of China, PR China, under grant No. Y and No. Y References 1 Zhang Q & Bowers M T, J Phys Chem A, 108 (2004) Halle L F, Crowe W E, Armentrout P B & Beauchamp J L, Organometallics, 3 (1984) Sonnenfroh D M & Farrar J M, J Am Chem Soc, 108 (1986) Sievers M R, Jarvis L M & Armentrout P B, J Am Chem Soc, 120 (1998) Haynes C L, Fisher E R & Armentrout P B, J Am Chem Soc, 118 (1996) Tolbert M A & Beauchamp J L, J Phys Chem, 90 (1986) Zhao L M, Zhang R R, Guo W Y, Wu S J & Lu X Q, Chem Phys Lett, 414 (2005) Zhao L M, Guo W Y, Zhang R R, Wu S J & Lu X Q, ChemPhysChem, 7 (2006) Chen X F, Guo W Y, Zhao L M, Fu Q T & Ma Y, J Phys Chem A, 111 (2007) Ma Y, Guo W Y, Zhao L M, Hu S Q, Zhang J, Fu Q T & Chen X F, J Phys Chem A, 111 (2007) 6208.

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