Methyl and methane elimination in the gas phase reaction of zirconium atom with 2-butyne: A DFT stud

Available online at www.sciencedirect.comCHINESEScienceDirectC HEMICALL .ETTERSELSEVIERChinese Chemical Letters 21 (2010) 1501-1504www.elsevier.com/locate/ccletMethyl and methane elimination in the gas phase reaction ofzirconium atom with 2-butyne: A DFT studyTao Hong Lia, Chuan Ming Wang b, Xiao Guang Xie C,*a Department of Chemistry, Soulhwest Forestry University, Kunming 650224, ChinaDepartment of Biology, Honghe University, Menzi 661 100, China"Department of Chemistry, Yinnan University, Kunming 650091, ChinaReceived 23 February 2010AbstractThe mechanisms for CH3- and CH4-elimination in the gas phase reaction of ground-state Zr with 2-butyne has been investigatedin detail using B3LYP method. For the elimination of CH3, two mechanisms which are similar to those previously found forthe reactions of Y and Zr with propyne are identifed. The mechanism for the elimination of CH4 was revealedas: Zr + CH3C=CCH3→π-complex→TS (H-migration) →HZr- (H2CCC)- CH3→TS(C- C insertion) →(H2CCC)- HZr-CH3→TS (H-migration)→CH4 + ZrC3H2.◎2010 Xiao Guang Xie. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Mechanism; Zirconium; 2-Butyne; B3LYPIn a recent experiment, Davis et al. investigated the reactions of Y (D), Zr (F), Nb (°D) and Mo('s, SS) withpropyne and 2-butyne using crossed molecular beams [1]. In this study, H2-elimination products were observed for allthe metals at low collision energies (Econ). For the reactions with propyne, the C- C bond cleavage products,MCCH + CH3, were only observed for Y as a competitive channel at Ecoll≥63 kJ/mol. Very recently, we performed atheoretical study in which the C-C bond cleavage mechanisms were identified for both ground-state Y and Zr withpropyne [2]. The calculated high energy barriers rationalized the absence of the product, ZrCCH + CH3. For thereactions with 2-butyne, besides the H2-elimination products, the CH4-elimination products, MC;H2 + CH4 were alsoobserved for Y, Zr and Nb. The authors claimed that this was the first observation of methane elimination in neutraltransition metal-hydrocarbon reactions [1]. As they did not propose any mechanisms for this product channel, in thepresent study, we extended our calculations to the reaction of ground-state Zr (4d-5s",“F) with 2-butyne. Consideringthat the mechanisms for H2-elimination have been extensively studied [2-7] while the C-C bond cleavage reactionsare relatively unexplored, our calculations only focus on the mechanisms for the CH3- and CH4-elimination.* Corresponding author.E-mail address: xgxie @ynu.edu.cn (X.G. Xie).中国煤化工1001-8417/$ - see front matter◎2010 Xiao Guang Xie. Published by Elsevier B.V. on.MHCNMHGAll rights reserved.doi: 10.1016j.cclet.2010.07.0251502T.H. Li et al./Chinese Chemical Letters 21 (2010) 1501-15041. Computational methodAll calculations have been carried out at density functional theory level (DFT) employing the hybrid functionalB3LYP [8,9] as implemented in GAUSSIAN98 program package [10]. The all electron basis set TZVP [11] was usedfor C and H. For Zr the Stuttgart/Dresden relativistic effective core potential (RECP) basis set SDD [12] was used, withits matching valence basis set [8s7p6d/6s5p3d] for 4s4p4d5s electrons. The combination of B3LYP with above basissets is denoted as B3LYP/BS1. The geometries of all the stationary points were optimized at this level of theory.Harmonic vibrational analysis was carried out at the same level to characterize the nature of the stationary points.Zero-point energies (ZPE) have been used to correct all the relative energies. All connectivities of minima andtransition states were verified by intrinsic reaction coordinate (IRC) calculations. Additional single point calculationswere performed employing larger basis sets, in which standard basis set 6-311+G** [13,14] was used for C and H andthe Peterson's small-core RECP augCc-tzvp-pp [15] basis set for Zr [11s10p9d3f2g/6s6p5d3f2g]. This theoreticallevel is denoted as B3LYP/BS2.2. Result and discussionFor the reactions of Y, Zr + propyne, we found that the larger basis set (6-311+G** + aug-cc-dzvp-pp)underestimated the energy barriers [2], while the barrier obtained using smaller basis set TZVP + SDD agree betterwith the experimental results. In the present study, similar situation is encountered, the combination of 6-311+G* witha larger triple-ξ basis set augCc-tzVp-PP lowers the energies of TZVP + SDD by 14 kJ/mol on average (Table 1).Therefore, in this work, unless otherwise indicated, the relative energies discussed below also refer to those ofTZVP + SDD (BS1).As shown in Fig. 1, two pathways (a and b) were identified for the elimination of CH3 and they are basically similarto those we previously identified for the reaction of Zr with propyne [2]. The corresponding potential energy profilesare shown in Fig. 2. This reaction starts from the formation of a π-complex, IMI, which is stabilized by 207.8 kJ/molrelative to the reactants. This binding energy is slightly smaller than that (219.0 kJ/mol) of the Zr-propyne complex.Pathway a demonstrates the mechanism of direct C- C bond insertion. From IM1, Zr inserts into one ofthe sp- -sp' C-Cbonds, forming the intermediate IM2. This step proceeds through the transition state, TS1. IRC calculationdemonstrated that with the formation of Zr- C1 bond, the Zr- C3 bond became longer and the structure finally evolvedto IM2, in which Zr, C2, C3 and C4 are basically in a line. This intermediate is different from the cyclic intermediateformed in the Zr + propyne reaction [2]. Fig. 2 shows that TS1 has a barrier of 49.3 kJ/mol relative to the reactants andthis barrier is slightly higher than the corresponding barrier (43.0 kJ/mol) of Zr + propyne reaction [2].Table 1The total energies (E) (hartree), ZPE-corrected relative energies (Erer) (kJ/mol) and the imaginary frequencies (IMG) (cm- ).SpeciesIMGZPEB3LYP/BS1B3LYP/BS2EretErel3Zr+ CaH6220.5-202.9437280.0- 202.801 8369IM1221.1-203.023184-207.8-202.885778-219.6IM2204.4-203.019347-214.5.202.8829208-228.8IM3207.2- 203.020293-214.2202.8825801-225.2IMI 90.3- 202.982289- 131.3-202.8452496-144.086.5- -202.997454-.175.0- 202.860119186.9649i208.9-202.991224- 136.2- 202.8552685-151.8 .TS2606i209.2- 202.92068649.3- 202.785880930.7TS3312i192.6-202.9330870.2-202.7986524-19.4TS4724i191.0- 202.963423-81.1- 202.8269268-95.3TS5129i192..3- 202.930201中国煤化工275-15.7TS61214i186.5-202.930008二992-11.72Zr- -CCCH3 + CH3198.- 202.908699MHCN MH G44262.93ZrCCCH2 + CH4202.4-202.96461712.8-202.82607-80.3T.H. Li et al./Chinese Chemical Letters 21 (2010) 1501- -15041503.41.8942.091454“引后2197272224TS2,C;Tss, CIMs.CIN1,C2v。1oo2117132.5DoTSI,cII3.CTS6,G18.0 180.0 。2.10922zCCCH5, Cn129➊24IN2C,TS3.C1108313231.272199 “72vcCCH2.Cx .“130".2.199080]i 120.0n,10891.7arTS.CNIN4,C,CHIFig. 1. Optimized geometries of the stationary points at the B3LYP/BS1 level (distances in A and angles in degrees).69.69.3Zzr-QCCH5+ CH,' TS13Zr+ C,H。0.2, 7.2.10.0 ;TS3 ↓TS6-72.8;'Zr-CCH2+CH,-136.231.3 ,"IM4TS2、_175ir;IMs. -214.2/IMIM3IM2Fig. 2. Potential energy diagram for pathways a, b and c at B3LYP/BS1 level.Pathway b is another mechanism corresponding to the elimination of CH3. In this pathway, after IM1, one of methylH atoms can migrate to Zr, forming the intermediate IM3. This step proceeds via the transition state TS2 with a barrierof 71.6 kJ/mol relative to IM1. The C-H activation intermediate IM3 locates below the reactants by 214.5 kJ/mol andit is the most stable species among all the intermediates. After IM3, Zr inserts into the C1-C2 bond, forming theintermediate IM4. This step proceeds via TS3 with a barrier of 214.7 kJ/mol relative to IM3. In the next step, the Zr-bound H atom migrates back to C1 forming an intermediate which is equivalent to the direct C- C insertionintermediate IM2. This step proceeds via TS4 by overcoming a barrier of 50.2 kJ/mol relative to IM4. Among the threetransition states along this pathway, TS3 has the highest energy, but it (中国煤化工nts by 0.2 kJ/molwhich is much smaller than the barrier of TS1 (49.1 kJ/mol). This resultHCNMHGinsertion after theH-migration intermediate (IM3) is energetically more favorable than tGnitial π-complex(IM1). However, the highest barrier for this reaction is not resulted from the transition states, but from the high1504TH. Li et al./ Chinese Chemical Letters 21 (2010) 1501-1504endothermicity that caused by loss of an unstable radical CH3. Fig. 2 shows that the formation of the products,CH3 + ZrCCCH3, is endothermic by 69.6 kJ/mol. This barrier is close to the collision energy of 71.0 kJ/mol used inexperiment [1]. Suppose that our calculation underestimated this barrier to some extent, the true barrier would behigher than 71.0 kJ/mol. Thus we can understand the absence of CH3-elimination products in experiment. Once thecollision energy exceeds this thermodynamic barrier, pathways a and b can be competitive processes.For the elimination of CH4, two mechanisms are possible. One is the direct insertion of Zr into a sp- sp' C-C bond,followed by H-migration from one methyl group to another. However this mechanism is unlikely to be feasiblebecause in the insertion intermediate, IM2, the C4 methyl group is far away from the Zr-bound methyl group in space.As a result, the H-migration from C4 to C1 would be very circuitous. In contrast, a mechanism similar to pathway bshould be more favorable. Fortunately, we have identified such mechanism which is shown in Fig. 1 as pathway c.In pathway b, the Zr atom inserts into the C1-C2 bond of IM3, forming the intermediate IM4. In pathway C, the Zralternatively undergoes insertion into the C3- C4 bond, forming another intermediate IM5. This step proceeds throughTS5 with a barrier of 222.2 kJ/mol relative to IM3. From IM5, the Zr-bound H atom migrates to C4, leading to theproduct ZrCCCH2 + CH4. Note that the transition state for this step, TS6, locates 83.3 kJ/mol above TS4. This isunderstandable, because TS6 corresponds to the formation of the final products, involving formation of one C-H bond,but cleavage of two bonds, Zr- C and Zr- H bond. In contrast, TS4 does not involve the cleavage of Zr- _C bond, butcorresponds to formation of a stable intermediate, involving cleavage of a C-H bond and compensated by theformation of a Zr- -H bond.As it can be seen in Fig. 2, the rate-determining transition state TS5 has a small barrier of 7.7 kJ/mol above thereactants. Further, the formation of the products, ZrC;H2 + CH4, is exothermic by 72.8 kJ/mol. These results suggestthat this reaction can take place efficiently even under low-energy condition. However, according to our previouscalculations on the reactions of Y and Zr with propyne [2], the present theoretical level may underestimate the barrierby around 15 kJ/mol. Thus, the true barrier for TS5 is likely to be higher than 20 kJ/mol. In contrast, as we found in thereaction of Zr with propyne [2], the elimination of H2 is barrierless and does not allow the C- C bond cleavagepathways to compete efficiently. Therefore, in the experiment of Davis et al., the CH4-elimination products wereobserved to account only 12% of the total products at Ecol1= 71 kJ/mol.AcknowledgmentsThis work is supported by the General Programs of the Applied Basic Research of Science and Technology,Department of Yunnan Province (Nos. 2008ZC095 and 2007B066M).References[1] R.Z. Hinichs, JJ. Schroden, HF Davis, J. Plhys. Chem. A 112 (2008) 3010.[2] TH. Li, C.M. Wang, S.W. Yu, et al. Chem. Phys. Lett. 475 (2009) 34.[3] P.A. Willis, H.U. Stauffer, R.Z. Hinrichs, et al. J. Phys. Chem. A 103 (1999) 3706.[4] M. Porembski, J.C. Weishaar, J. Phys. Chem. A 105 (2001) 4851.[5] M. Porembski, J.C. Weishaar, J. Phys. Chem. A 105 (2001) 6655.[6] R.Z. Hinrichs, JJ. Schroden, HF. Davis, J. Am. Chem. Soc. 125 (2003) 860.[7] E.D. Glendening, J. Phys. Chem. A 108 (2004) 10165.[8] A.D. Becke, J. Chem. Phys. 98 (1993) 1372.[9] C. Lee, w. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[10] MJ. 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