Mechanistic Studies on the Carboxylation of Hafnocene and ansa

Nov 19, 2013 - Homolytic or Heterolytic Dihydrogen Splitting with Ditantalum/Dizirconium Dinitrogen Complexes? A Computational Study. Xuelu Ma , Ming ...
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Mechanistic Studies on the Carboxylation of Hafnocene and ansaZirconocene Dinitrogen Complexes with CO2 Xuelu Ma,† Yanhui Tang,‡ and Ming Lei*,† †

State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ College of Material Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: A DFT study on the carboxylation of hafnocene and ansa-zirconocene dinitrogen complexes with CO2 indicates that the most favorable initial CO2 insertion into M−N (M = Hf, Zr) proceeds by a stepwise path rather than a concerted [2 + 2] path. The calculated results explain the regioselectivity of the N−C formation in experiments. In addition, a comparative analysis of ring tension and charge distribution unveils the different activities of N−N bond cleavage in the CO and CO2 direct N−C bond formation reactions.

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INTRODUCTION Dinitrogen activation and its chemical transformations are some of the most challenging subjects in the chemical industry.1 Because of the high energy demands associated with the Haber−Bosch process, the direct conversion of typically inert atmospheric N2 into more value-added organic molecules is an attractive strategy for the evolution of ammonia-independent synthetic pathways.2 It is meaningful and useful to develop an efficient method for the assembly of nitrogen−carbon (N−C) bonds from two potential feedstocks: dinitrogen and carbon dioxide.3 Using the 1,2-addition method to form N−C bonds would be a complementary synthetic method to the Chatt-type functionalization procedure that relies on electrophilic addition or acylation of coordinated N2.4 In 2004, Fryzuk and co-workers reported the cycloaddition of terminal alkynes with ([PhP(CH2SiMe2NSiMe2CH2)2PPh]Zr)2 (μ2,η2,η2-N2) to generate alkenyl hydrazido species with bridging acetylide ligands, which made a breakthrough in the framework of N−C bond formation.5 In order to extend the scope of 1,2-addition to N−C bond formation reactions, Chirik et al. treated [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2) (1-N2 in Scheme 1) with internal alkynes and heterocumulenes.6 However, more rapid side-on, end-on isomerization reduces the reactivity of N−C bond formation.7 In 2007, Chirik et al. reported N−C bond formation from N2 and CO2 promoted by a hafnocene © 2013 American Chemical Society

Scheme 1. Metallocene Zirconium and Hafnium Dinitrogen Compounds with Side-on Coordination

dinitrogen complex ((η5-C5Me4H)2Hf]2(μ2,η2,η2-N2; see 2-N2 or 1Hf in Scheme 1). Addition of 1 equiv of CO2 to 1Hf produced predominately AHf (1-N2C2O4 in ref 8) in 30% yield, where the same nitrogen atom was carboxylated twice, and another product, BHf, with 15% yield was predicted (see Scheme 2a).8 In 2008, by modification of hafnocene using [Me2Si]-bridged ansa-zirconocene (3-N2 or 1Zr in Scheme 1), BZr (2-(NCO2)2 in ref 9), possessing a core structure similar to that of BHf, with 50% yield could be obtained from the reaction of 1Zr with CO2 (see Scheme 2b) .9 Received: August 7, 2013 Published: November 19, 2013 7077

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Scheme 2. Reaction of N−C Bond Formation by Addition of CO2 to (a) 1Hf and (b) 1Zr

basis set was used for all other atoms.16 This basis set combination will be referred to as BSI. All of the transition states (TSs) were further confirmed by vibrational analysis and characterized by one and only one imaginary frequency. Intrinsic reaction coordinate (IRC) calculations along the whole pathway were performed in order to verify more details around transition states and corresponding intermediates. The atomic polar tensor (APT) charge was utilized for the charge population because experimental evaluation of density functional charge schemes found that the commonly used Mulliken charge analysis was generally deficient.17 Considering the solvent effect in this reaction, all the single-point solvent energies were also calculated using the polarizable continuum model (PCM) and toluene as solvent in the gas-phase optimized structures. 18 Charge decomposition analysis (CDA) and natural population analysis (NPA) were also applied.19 All potential energies were corrected for zero-point energy (ZPE) contributions, and Gibbs free energies were calculated at 298.15 K. For each minimum, singlet and triplet spin states have been considered and the singlet spin states are always the most stable by 20−30 kcal/mol. Thus, the reaction takes place on the singlet potential energy surface.

Interestingly, hafnocene and ansa-zirconocene dinitrogen complexes (1 Hf and 1 Zr ) have similar configurational stabilities,10 but their major carboxylation products with CO2 are different. It is important to unveil the nature of N−C formation and the coordination mode of CO2 in the system.11,12 Because of the lack of any mechanistic or computational studies, herein we performed density functional theory (DFT) calculations to explore plausible pathways for proposed reactions in the carboxylation of hafnocene and ansazirconocene dinitrogen complexes with CO2 (see Scheme 2). In this paper, we focused on the following critical issues: (i) the coordination mode of CO2 and the mechanism of N−C bond formation, (ii) the preference of major products obtained with different metallocene complexes, and (iii) the factors related to N−N bond cleavage.

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COMPUTATIONAL METHODS

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All of calculations were studied with the Gaussian 09 program package.13 In this study, two fundamental key steps are involved: CO2 coordination and N−C bond formation. On the basis of our previous relevant work about CO-assisted N2 functionalization activated by a binuclear hafnium complex,14 full models were also used in this study to be the same as the real system reported by experiments.8,9 Simplified models (all the tert-butyl and methyl groups on the ring of cyclopentadienyl ligand were replaced by hydrogen atoms) were also used for a comparison with the full models (see Figure S1 in the Supporting Information). We fully optimized all the structures reported in this paper at the B3LYP level. The hybrid density functional method is reliable and was used in our previous papers.14,15 The effective core potential of Hay and Wadt with a double-ζ valence basis set (LANL2DZ) was chosen to describe Hf and Zr. The 6-31G*

RESULTS AND DISCUSSION 1. Coordination Mode of CO 2 and N−C Bond Formation. In the mechanism of N−C bond formation, the first step is the coordination of the first CO2 to the hafnocene dinitrogen complex 2-N2 (1Hf) and ansa-zirconocene dinitrogen complex 3-N2 (1Zr) to give 2Hf and 2Zr, respectively. As shown in Figure 1, two different pathways of N1−C1 bond formation were discussed: a stepwise pathway (path 1) and a concerted pathway (path 2). In path 1, CO2 interacts with 1Hf and 1Zr to give 3Hf and 3Zr with C2 symmetry, respectively, and then 2Hf and 2Zr could be obtained by an oxygen atom (O1) 7078

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Figure 1. Potential energy profiles of the coordination of the first CO2 to 1Hf and 1Zr.

Figure 2. Contour plots of important molecular orbitals of 3Hf and 2Hf.

swing with M1−N1 bond cleavage. In path 2, the first CO2 interacts with 1Hf and 1Zr by [2 + 2] cycloaddition to form the N1−C1 bond directly, giving 2Hf and 2Zr. The intermediate 3 has an interesting structure: the carbon atom (C1) of CO2 bonds with the nitrogen atom (N1) of 1 and a C2-symmetric structure is formed in spite of the ligand. On the other hand, TS1-2 and TS1-3 are very similar to each other but totally different in nature. The N−C bond of the fourmembered-ring transition state is much shorter in the latter than in the former (the N−C distances in TS1-2Hf, TS1-3Hf, TS1-2Zr, and TS1-3Zr are 2.448, 1.955, 2.271, and 2.054 Å, respectively). The M−N bond of the four-membered-ring transition state TS1-3 is longer (the M−N distances in TS12Hf, TS1-3Hf, TS1-2Zr, and TS1-3Zr are 2.138, 2.267, 2.194, and 2.254 Å, respectively), and a greater weakening of the M−N bond enhances electron transfer to the metal atom.20 It is explicit in the ansa-zirconocene system that the first CO2 coordinates with Zr2 along the direction of the tert-butyl group of the cyclopentadienyl ligand, which is favorable for the other side to coordinate with Zr1. The phenomenon is the same as the orientation of CO insertion to the ansa-hafnocene complex.14 The tert-butyl groups on the cyclopentadienyl ligand make the M−N bond length much longer, which is conductive to CO/CO2 coordination. Meanwhile, the stepwise pathway of the first CO2 coordination involves the change of coordinated metal from M2 to M1 in Figure 1. There are two coordination modes for CO2 with metallocene dinitrogen complexes in the carboxylation of hafnocene and ansa-zirconocene dinitrogen complexes 2 and 3. In comparison to the N1−N2 bond length in the initial side-on structure 1 (1.421 Å in 1Hf and 1.385 Å in 1Zr), the N1−N2 bond lengths in 2Hf and 2Zr slightly increase by 0.055 and 0.077 Å, respectively. In addition, the N−N bond lengths in 3Hf and 3Zr are elongated to 1.544 and 1.504 Å, respectively. In the different CO2 coordination modes, the N1− C1 distances are 1.406 Å (1.409 Å) in 2Hf (2Zr) and 1.395 Å (1.406 Å) in 3Hf (3Zr), respectively. In the formation of 2 and 3, CO2 coordinates with 1, which involves the transfer of electrons from the metal center to CO2 (see Figure 2). From the CDA results19,21 (see the Supporting Information) in the hafnocene system, 2Hf (3Hf), the net number of electrons obtained by the CO2 fragment is 0.628 (0.464). The orbital ⟨152a⟩ (⟨152a⟩) donates 0.052 (0.052) electron from the hafnocene dinitrogen complex to CO2, which is the primary source of the donor−acceptor bonding.

However, orbitals ⟨152a⟩ in 2Hf/3Hf remove 0.551/0.177 electron from the overlap region between CO2 and the hafnocene dinitrogen complex, which stabilizes 2Hf/3Hf by diminishing electron repulsion. Meanwhile, the orbital ⟨153a⟩ (⟨151a⟩) back-donates 0.036 (0.036) electron from CO2 to the hafnocene dinitrogen complex in 2Hf (3Hf). This causes −0.025 and 0.122 electron to be removed from the overlap region between CO2 and the hafnocene dinitrogen complex for 2Hf and 3Hf, respectively. The former shows the evident accumulation of electrons from the respective occupied fragment orbitals in the overlap region and is beneficial to bonding between CO2 and the hafnocene dinitrogen complex of 2Hf. However, the latter still stabilizes 3Hf by diminishing electron repulsion. Meanwhile, NPA results indicate that the natural charges on the O1, C1, and O2 atoms in 3Hf/2Hf are −0.681/−0.781, 0.967/0.952 and −0.646/−0.656, respectively (see Table 1). In addition, the charge on the N1 atom changes from −0.682 in 3Hf to −0.606 in 2Hf, while the charge on the Hf1 atom changes from 1.581 in 3Hf to 1.640 in 2Hf. The stepwise pathway performs the transfer of electrons from the metal center to the oxygen atom of CO2 gradually. The potential energy profiles along the two pathways of N1− 1 C bond formation in the first CO2 insertion into 1 are also shown in Figure 1. In path 1, the barriers of the C1−N1 bond formation step (from 1 to 3) and M−N bond breakage step (from 3 to 2) are 5.6/13.5 and 1.5/9.3 kcal/mol, respectively (5.6/13.5 denotes 5.6 kcal/mol for the hafnocene system and 13.5 kcal/mol for the ansa-zirconocene system, respectively). However, the energy barrier of the concerted [2 + 2] cycloaddition step in path 2 is 10.6/20.2 kcal/mol. It is obvious that path 1 is a favorable CO2 insertion pathway in comparison with path 2. Also, the rate-determining step of path 1 is the N−C bond formation step. In other words, in the first CO2 insertion, the stepwise pathway is more favored than the concerted pathway. 2. Carboxylation Mechanism and Its Difference in Major Products. Meanwhile, the difference in the major product formations is also a critical issue for the hafnocene and ansa-zirconocene systems. As shown in Figure 3, with the insertion of the second CO2 into the M−N bond of 2Hf and 2Zr, the final carboxylated products of metallocene dinitrogen complexes could be obtained. If the second CO2 coordinates along the cis direction, AHf and AZr are generated with a C2 symmetry core. The CO2 cis coordination pathways in the 7079

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Table 1. Natural Population Analysis (NPA) for the Coordination of the First CO2 to 1Hf Hf

1 TS1Hf-2Hf 2Hf TS1Hf-3Hf 3Hf TS3Hf-2Hf

Hf1

N1

Hf2

N2

C1

O1

O2

1.449 1.482 1.640 1.421 1.581 1.589

−0.688 −0.770 −0.606 −0.509 −0.682 −0.673

1.450 1.428 1.642 1.571 1.600 1.634

−0.699 −0.770 −0.869 −0.595 −0.839 −0.854

NA 1.097 0.952 0.766 0.967 0.939

NA −0.536 −0.781 −0.596 −0.681 −0.722

NA −0.475 −0.656 −0.733 −0.646 −0.614

Figure 3. Different insertion directions of the second CO2: (blue solid line) path AHf; (purple dashed line) path AZr; (blue dashed line) path BHf; (purple solid line) path BZr.

hafnocene and ansa-zirconocene systems are named path AHf and path AZr, respectively. If CO2 coordinates along the trans direction, BHf and BZr are produced and the corresponding pathways are named path BHf and path BZr, respectively. As mentioned before, the stepwise pathway of CO2 coordination involves a change of the coordinated metal. In addition, the metal coordination mode in 2Hf and 2Zr is restricted. The insertion of the second CO2 into the M−N bond proceeds via a concerted pathway to give the final product. After the coordination of the second CO2 molecule with 2Hf and 2Zr, 2HfA, 2HfB, 2ZrA, and 2ZrB are produced. Figure 4 shows the energy profiles along the reaction pathway from 2Hf and 2Zr to products with the second CO2. The second CO2 coordination step from 2 to 2A is exothermic by 0.3/11.1 kcal/mol, but that from 2 to 2B is endothermic by 7.4/5.4 kcal/mol. In the hafnocene system, the second CO2 insertion into the M−N bond from 2HfA to AHf and 2HfB to BHf

proceeds with potential energy barriers of 6.8 and 8.2 kcal/mol, respectively. In the ansa-zirconocene system, the barriers of the second CO2 insertion step from 2ZrA to AZr and 2ZrB to BZr are 15.5 and 0.9 kcal/mol, respectively. In the second carboxylation of two different metallocene systems, the major product configuration is the exact opposite of the other. In the hafnocene system, the energy barrier of AHf formation is 1.4 kcal/mol lower than that of BHf formation. Also, these processes are exothermic at 24.9 and 70.1 kcal/mol, respectively, which are almost irreversible. In the ansazirconocene system, BZr formation is strongly exothermic by 55.6 kcal/mol and AZr formation is exothermic by 14.7 kcal/ mol. The isomer 2ZrA, with lower energy relative to 2ZrB, does not lead to the major product BZr.9 In addition, the less stable intermediate 2ZrB is significantly more reactive. The energy barrier of BZr formation is 14.6 kcal/mol lower than that of AZr formation, which is also largely influenced by 7080

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complete scission in the N−C bond formation of this reaction but could in the reaction of CO-assisted N2 activation.22 As shown in Figure 5, in the CO-induced N−N cleavage activated

Figure 4. Potential energy profiles of product formation in the carboxylation of 1Hf and 1Zr with the second CO2.

steric effects of the ligand environment. As mentioned before, in the ansa-zirconocene system the CO2 coordinates with Zr along the direction of the tert-butyl group of the cyclopentadienyl ligand, which is more favorable than that along the other direction (trans direction). The tert-butyl groups on the cyclopentadienyl ligand in 2Zr form an open pocket, and the M−N distance is much longer (2.156 Å in 2ZrA and 2.065 Å in 2HfA), which are conductive to CO2 coordination to form 2ZrA. Interestingly, the C2−O2 bond is formed in 2ZrA with the second CO2 coordinating with 2Zr. That is, the C atom of the second CO2 forms a bond with the O2 atom of the first inserted CO2 during the second CO2 insertion with 2Zr in path AZr. Then the C2−O2 bond is cleaved with the formation of a N−C bond from 2ZrA to AZr via TS2Zr-AZr (see Figure 3). The difference between path AZr and path AHf has been verified by IRC analysis (see the Supporting Information). This makes 2ZrA more stable than 2Zr due to the C2−O2 bond formation. Therefore, it costs much more energy to break the C2−O2 bond and form the C2−N1 bond, which makes the barrier much higher. In addition, the HOMO−LUMO gaps of 2HfA, 2HfB, 2ZrA, and 2ZrB also explain the different energy barriers for the second CO2 insertion step from the viewpoint of frontier molecular orbital theory. As shown in Table 2, the HOMO−LUMO gaps

Figure 5. Comparison of energies of the CO and CO2 N−N bond activation reactions.

by a binuclear hafnium complex (see 4-N2 in Scheme 1), the N−N bond is completely broken with a barrier of 4.3 kcal/mol, which is a strongly irreversible step with a large exothermicity.14 In this study of the carbonylation of N2 with CO2, with an increase of the N−N distance from 1.476 Å in 2 to 2.876 Å in the final structure 2′, the relative energy would increase to 42.9 kcal/mol (see Figure 5). This implies that N−N cleavage would be very difficult in this system. Although the intermediate 5 of the CO reaction and 2 of the CO2 reaction have similar structural features, they are different in activities of N−N bond cleavage. According to the ∠HfCN1 angle of 86.9° in the fourmembered ring of 5 in the CO reaction and ∠HfCN1 angle of 113.5° in the five-membered ring of 2 in the CO2 reaction, the more strained ring tension provides the possibility for N−N bond cleavage. As shown in Table 3, the charges of the atoms of the ring in the dinitrogen activation are represented in both CO and CO2 reactions. First, the absence of an additional oxygen atom with electronegativity in the CO2 reaction causes the higher total charge of the ring. The total charges of the four atoms of the ring in the CO reaction are 0.256 in 5 and 0.211 in Ic, respectively, while the total charges of the five atoms of the ring in the CO2 reaction are 0.275 in 2 and 0.684 in 2′. Obviously, dinitrogen activation with the ring charge dispersion is favorable, which leads to the dinitrogen cleavage in the CO reaction. On the other hand, the charge of the dinitrogen moiety in 5 in the CO-assisted pathway is −2.130, while that in 2 in the CO2-addition pathway is −1.830. In a theoretical study of N−N splitting of a functionalized μ,η1,η2 coordinated N2 ligand,23 it was suggested that the N−N bond cleavage step could be described as an electron transfer from the dz2-like orbital of the transition metal into the unoccupied σ* orbital of the dinitrogen moiety. A more negative charge of N−N may lead to N−N cleavage with Hf−N bond formation in the COassisted reaction. However, the lower negative charge of the dinitrogen moiety in the CO2 addition reaction weakens the interaction with the transition metal, which has an adverse effect on the N−N bond cleavage.

Table 2. Energies (in au) of Frontier Molecular Orbitals of 2HfA, 2HfB, 2ZrA, and 2ZrB ELUMO EHOMO Egap

2HfA

2HfB

2ZrA

2ZrB

−0.0610 −0.1653 0.1043

−0.0403 −0.1539 0.1136

−0.0540 −0.1868 0.1328

−0.0590 −0.1575 0.0985

are 0.1043 and 0.1136 au in 2HfA and 2HfB, respectively, and are 0.1328 and 0.0985 au in 2ZrA and 2ZrB, respectively. The narrow HOMO−LUMO gap of 2HfA requires less energy to give the major product AHf, while the larger HOMO−LUMO gap of 2ZrA requires more energy to form AZr. Similarly, the major production of BZr costs less energy than the minor production of AZr. 3. N−N Bond Cleavage. In this carboxylation of Hf and ansa-Zr dinitrogen complexes with CO2, the N−C bond could be formed in the CO2 insertion into the M−N bond of metallocene. Interestingly, the N−N bond could not achieve a 7081

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Table 3. Atomic Charges in the Ring in Dinitrogen Activation N1

C

N2

Hf

O

−0.819 −0.036 N1

1.321 0.417

−1.011 −0.292

1.652 0.953

−0.868 −0.358

CO2 reaction 2 2′ CO reaction 5 Ic

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−1.026 −0.957

C

N2

Hf

0.275 0.684 sum of the ring

1.052 1.020

−1.104 −1.507

1.334 1.655

0.256 0.211

ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving CDA results, NPA results, IRC analysis, imaginary frequencies of transition states, optimized geometries and energies of all stationary points along the reaction pathways, and a comparison of the energy profiles in the full model and simplified model. This material is available free of charge via the Internet at http://pubs.acs.org.

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dinitrogen moiety −1.830 −0.328 dinitrogen moiety −2.130 −2.464

(8) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Angew. Chem., Int. Ed. 2007, 46, 2858−2861. (9) Knobloch, D. J.; Toomey, H. E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 4248−4249. (10) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132 (30), 10553−10564. (11) Fan, T.; Chen, X.; Lin, Z. Chem. Commun. 2012, 48 (88), 10808−10828. (12) Ballmann, J.; Pick, F.; Castro, L.; Fryzuk, M. D.; Maron, L. Inorg. Chem. 2013, 52, 1685−1687. (13) Frisch, M. J., et al. Gaussian 09, Revision B.01; Gaussian, Inc., Wallingford, CT, 2010. (14) Ma, X.; Zhang, X.; Zhang, W.; Lei, M. Phys. Chem. Chem. Phys. 2013, 15 (3), 901−910. (15) Zhang, W.; Tang, Y.; Lei, M.; Morokuma, K.; Musaev, D. G. Inorg. Chem. 2011, 50, 9481−9490. (16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Hay, P. J.; Willard, R. W. J. Chem. Phys. 1985, 82, 299−310. (17) Cioslowski, J. J. Am. Chem. Soc. 1989, 111, 8333−8336. (18) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995− 2001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681. (19) Lu, T.; Chen, F. J. Comput. Chem. 2012, 33, 580−592. (20) Liu, C.; Guan, X.; Su, Z. Sci. China Chem. 2012, 55 (09), 1910− 1915. (21) (a) Chen, Y.; Hartmann, M.; Frenking, G. Eur. J. Inorg. Chem. 2001, 2001, 1441−1448. (b) Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. J. Am. Chem. Soc. 2005, 128, 278−290. (22) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. Nat. Chem. 2010, 2, 30−35. (23) Studt, F.; MacKay, B. A.; Fryzuk, M. D.; Tuczek, F. Dalton Trans. 2006, No. 9, 1137−1140.

CONCLUSION In summary, this work studied the carboxylation of hafnocene and ansa-zirconocene dinitrogen complexes with CO2 using DFT methods. Two coordination modes of CO2 with metallocene were presented. The favorable first CO2 insertion pathway is a stepwise pathway rather than a concerted cycloaddition. In addition, the calculated results explain well the regioselectivity of the N−C bond formation, which is largely due to the steric effects of the ligand and C2−O2 bond formation in 2ZrA. A comparative analysis of ring tension and charge in both CO and CO2 reactions show that the more strained ring tension and the ring charge dispersion could lead to possible N−N bond cleavage, but the lower negative charge of the dinitrogen moiety in the CO2 addition reaction weakens the interaction with the transition metal, which has an adverse effect on the cleavage of the N−N bond.

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sum of the ring

AUTHOR INFORMATION

Corresponding Author

*M.L.: tel, 86-10-6444-6598; fax, 86-10-6444-6598; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (Grant No. 21373023 and 21072018).

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REFERENCES

(1) (a) Chen, Y.; Zhou, Y.; Chen, P.; Tao, Y.; Li, Y.; Qu, J. J. Am. Chem. Soc. 2008, 130, 15250−15251. (b) Zhou, M.; Jin, X.; Gong, Y.; Li, J. Angew. Chem., Int. Ed. 2007, 46, 2911−2914. (2) Li, Y.; Li, Y.; Wang, B.; Luo, Y.; Yang, D.; Tong, P.; Zhao, J.; Luo, L.; Zhou, Y.; Chen, S.; Cheng, F.; Qu, J. Nat. Chem. 2013, 5, 320−326. (3) (a) Chen, Y.; Liu, L.; Peng, Y.; Chen, P.; Luo, Y.; Qu, J. J. Am. Chem. Soc. 2011, 133, 1147−1149. (b) Ma, X.; Lei, M. Prog. Chem. 2013, 25 (8), 1325−1333. (4) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95 (4), 1115−1133. (5) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem. Soc. 2004, 126 (31), 9480−9481. (6) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2005, 127 (21), 7901−7911. (7) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2005, 127 (40), 14051−14061. 7082

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