Diphosphazane-monoxide and Phosphine-sulfonate Palladium

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Diphosphazane-monoxide and Phosphine-sulfonate Palladium Catalyzed Ethylene Copolymerization with Polar Monomers: A Computational Study Jiajie Sun,†,§ Min Chen,‡,§ Gen Luo,† Changle Chen,*,‡ and Yi Luo*,† †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

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‡

S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations have been comparatively carried out to disclose the origin of different catalytic performance of diphosphazanemonoxide and phosphine-sulfonate palladium complexes toward copolymerization of ethylene and vinyl polar monomers. A theoretical comparison of the two catalytic systems indicates that the rigid five-membered backbone and cationic nature of the diphosphazane-monoxide palladium complex are beneficial for its copolymerization activity. Having achieved agreement between theory and experiment, it is found that the favorable 2,1-selective insertion of methyl methacrylate (MMA) into the diphosphazane-monoxide palladium complex originates from less geometrical deformation and the MeO···H interaction between the ancillary ligand and the methyl of MMA. In this catalyst system, it is kinetically difficult for the MMA pre-enchained species to undergo subsequent ethylene insertion due to the steric repulsion between the methyl of MMA and the inserting ethylene moiety, but it relatively favors β-H elimination, yielding the MMA-terminated copolymer observed experimentally. In contrast, comparable insertion energy barriers were observed for phosphine-sulfonate palladium catalyzed MMA 1,2- and 2,1-insertions, which may account for the lower regioselectivity of this catalyst.

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with their palladium counterparts,10 mainly due to the higher oxophilicity of nickel versus palladium. In this field, another breakthrough was achieved using palladium catalysts based on phosphine-sulfonate ligands (Figure 1, II).11 This type of palladium catalysts is capable of copolymerizing ethylene with a series of fundamental polar monomers.12 Even after extensive ligand modifications, the nickel counterparts based on this type of ligands were only capable of copolymerizing ethylene with some special polar monomers.13 Over the past few years, several successful palladium or nickel catalysts have been developed on the basis of bisphosphine monoxides (Figure 1, III),14 phosphine phosphonic amides (Figure 1, IV),15 specially designed Nheterocyclic carbenes (Figure 1, V),16 phosphino-phenolate (Figure 1, VI),17 bisphosphine monoxide with a methylene linker (Figure 1, VII),18 etc. Recently, Chen et al. reported the studies of some palladium and nickel complexes based on diphosphazane-monoxide ligands (Figure 1, VIII).19 This system possesses some unique features compared with the benchmark phosphine-sulfonate based system: (1) phosphine-sulfonate palladium catalysts could not incorporate methyl methacrylate (MMA) comonomer during ethylene polymerization, while such copoly-

INTRODUCTION Polyolefins can be synthesized via various methods. 1 Transition metal catalyzed copolymerization of ethylene with polar comonomers holds great potential in producing polar functionalized polyolefin materials.2 However, this also represents a significant scientific challenge.3 Several issues need to be addressed in order to achieve efficient copolymerizations: (1) the poisoning of the metal center through σ−X coordination instead of CC π-coordination; (2) the formation of stable metal−X chelate after polar monomer insertion, which is difficult to undergo further insertion; (3) X-related side reactions such as elimination. Due to these issues, successful examples of transition metal catalysts are very limited. A major breakthrough was achieved using palladium catalysts based on α-diimine ligands (Figure 1, I) in the 1990s.4 Over the years, this type of palladium catalysts has been demonstrated to mediate olefin copolymerizations with fundamental polar monomers (with a polar group directly attached to the vinyl double bond) such as acrylates,5 vinyl ketones,5 silyl vinyl ethers,6 vinylalkoxysilanes,7 and some other special polar monomers (with a spacer between the polar group and the double bond).8 Recently, Brookhart et al. showed that α-diimine nickel catalysts can efficiently catalyze ethylene copolymerization with some vinylalkoxysilanes.9 However, the α-diimine nickel catalysts generally showed much worse properties in these copolymerizations compared © XXXX American Chemical Society

Received: October 30, 2018

A

DOI: 10.1021/acs.organomet.8b00796 Organometallics XXXX, XXX, XXX−XXX

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and VMD30 softwares. The optimized geometrical structures were plotted by CYLView.31

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RESULTS AND DISCUSSION For a comparative study, both diphosphazane-monoxide and phosphine-sulfonate based catalysts A and B have been chosen as computational models, as shown in Chart 1. In view of the Chart 1. Selected Catalyst Models (Ar = o-OMe-C6H4)

optimized structures of the two kinds of catalysts, the dihedral angle of P−Pd−O−P in A is much smaller than that of P−Pd− O−S in B (30.8 vs 80.8°; for more geometrical parameters, see Figure S1 in the Supporting Information). This reflects more coplanarity and thus in part more rigidity of A with the fivemembered ring backbone.32 1. The Insertion of the First Ethylene Molecule. As previously reported for phosphine-sulfonate based catalysts,33 the ethylene insertion possibly occurs at two sides of A due to the electronic asymmetry feature, viz., trans (monomer trans to the P atom) and cis (monomer cis to the P atom) sites.34 With the data in Table 1, one can compare the energies for the

Figure 1. (a) Transition metal catalyzed coordination−insertion of polar monomers (the polar-monomer problem). (b) Some previously reported ligand frameworks.4,11,14−19

merization catalyzed by diphosphazane-monoxide palladium catalysts gave enolate-terminated polyethylenes;20 (2) experimental studies showed that phosphine-sulfonate palladium catalysts underwent both 1,2- and 2,1-insertions of MMA monomer, while diphosphazane-monoxide palladium catalysts underwent favorable 2,1-insertion of MMA.19 Clearly, these differences originate from the structural differences between these two ligand systems. Encouraged by previous theoretical works on metal catalyzed olefin polymerization,21 we computationally investigated herein the reactivities of diphosphazanemonoxide and phosphine-sulfonate based palladium catalysts toward copolymerization of ethylene with MA or MMA. These mechanistic studies can help to understand the differences in their polymerization properties and shed light on future design of high-performance copolymerization catalysts.

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Table 1. Calculated Relative Free Energies in Solution (kcal/mol) for the Insertion of the First Ethylene Moleculea

catalysts and coordination−insertion manners A A B B

COMPUTATIONAL DETAILS

All of the DFT calculations were performed with the Gaussian 16 program.22 The B3LYP functional23 together with the 6-31G(d) basis set for nonmetal atoms (C, H, O, N, P, and S) and the LANL2DZ24 basis set as well as the associated poseupotential for metal atom (Pd) was used for geometry optimizations. Such basis sets are denoted as BSI. On the basis of the B3LYP/BSI geometries, single-point calculations were further performed at the higher level by using the dispersion-corrected density functional method (B3LYP-D3)25 together with BSII. In BSII, the 6-311G(d,p) was used for the nonmetal atoms and the basis set SDD26 as well as the associated pseudopotential were applied for the Pd atom. In these single-point calculations, the solvation effect of toluene (ε = 2.37) was considered through the SMD model.27 The energy profiles were constructed at the B3LYP-D3/BSII(SMD)//B3LYP/BSI level, including Gibbs free energy corrections taken from frequency calculations in the gas phase (298.15 K, 1 atm). It is noteworthy that considerations of the D3 dispersion correction in geometry optimization could not reproduce well the observed regioselectivity for the phosphine-sulfonate based system (Table S1). It is also noted that the corresponding experiment was carried out at 80 °C (353.15 K)19 and the temperature difference (353.15 vs 298.15 K) has little effect on the calculated energy barriers for ethylene and MMA insertions, with a variation of less than 1 kcal/ mol (Table S2). After carrying out the above calculations, the noncovalent interaction (NCI) analysis28 was conducted for some important transition states (TSs), which were shown by Multiwfn29

(trans) (cis) (trans) (cis)

C1

TS1

P1

ΔG1⧧

−8.8 −4.5 −10.9 −4.7

19.5 9.5 19.9 10.0

−10.7 −13.0 −10.5 −17.8

28.3 14.0 (18.3) 30.8 14.7 (20.9)

a C1, TS1, and P1 denote the coordination complex, insertion TS, and insertion product, respectively. ΔG1⧧ represents the insertion freeenergy barrier, and the data in parentheses is the energy barrier relative to the more stable trans site coordination complex. The energies of the stationary points are relative to the corresponding catalyst and monomer.

insertions at different sites of the two catalysts. In spite of the metal center and ligand backbone, the trans coordination complex is lower in energy than the corresponding cis one. However, insertion at the cis site has a lower free-energy barrier than insertion at the trans site, even if the energy barrier is relative to the more stable trans coordination complex (A, 18.3 vs 28.3 kcal/mol; B, 20.9 vs 30.8 kcal/mol; Table 1). These results suggest that the olefin monomer prefers to coordinate at the trans site and isomerize to the cis site for subsequent insertion. This is in line with the previous results separately demonstrated by Mecking,20,35 Nozaki,33 and Ziegler,36 which showed feasible trans-to-cis isomerization. Actually, in the current system, such isomerizations in ethylene coordination− B

DOI: 10.1021/acs.organomet.8b00796 Organometallics XXXX, XXX, XXX−XXX

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Table 2. Calculated Relative Free Energies in Solution (ΔG, kcal/mol) for Various Insertion Manners of MMA in Both A and B Systemsa

21MMA

21MMA

21MMA

21MMA

Pc2

ΔG2⧧ (21MMAc2)

12MMA

12MMA

12MMA

12MMA

Pc2

ΔG2⧧ (12MMAc2)

Ct2

MMA

catalysts

P1

A

−13.0

−22.0

B

−17.8

−21.9

Ct2′

Ct2

Cc2 Cc2

−19.3 −19.3 −17.6 −16.1

−11.8 −7.2 −11.0 −9.3

TSc2 TSc2

1.6 3.1 2.0 1.9

−22.2 −18.4 −28.0 −33.0

23.6 25.1 23.9 23.8

a

P1 represents the ethylene enchained intermediate (Table 1). MMACt2′ denotes the intermediate with coordinating MMA at the trans site via the carbonyl oxygen atom. 21MMACt2, 12MMACt2, 21MMACc2, and 12MMACc2 represent the coordination of MMA via its vinyl (CC) in 2,1 at the trans site, 1,2 at the trans site, 2,1 at the cis site, and 1,2 at the cis site, respectively. The right subscript “2” means the second monomer insertion event.

Figure 2. Energy decomposition analysis (energy in kcal/mol) and the optimized structures of (a) 21MMATSc2_A and (b) 12MMATSc2_A. Distances are given in Å, and angles are given in deg.

which is higher than that for 2,1-insertion by 1.5 kcal/mol. Moreover, the 2,1-insertion product (21MMAPc2_A) is also lower in energy than the 1,2-insertion one (−22.2 vs −18.4 kcal/mol). These results suggest that the 2,1-insertion manner is more kinetically and thermodynamically favorable than 1,2insertion in the A system. It is noted that the exclusive 2,1insertion product was reported in the previous work.19 The calculated barrier difference of 1.5 kcal/mol could roughly correspond to the 2,1- to 1,2-insertion product ratio of 93:7 (298.15 K). This calculation result suggests that the 2,1insertion is significantly favorable compared with 1,2-insertion, although it does not well reproduce the exclusive 2,1-selectivity reported previously.19

insertion reaction were computationally found to have free energy barriers of 16.1 and 19.7 kcal/mol for the A and B catalyst systems, respectively (Figure S2), which are lower than those for their corresponding cis insertion energy barriers (18.3 and 20.9 kcal/mol). Therefore, only the favorable cis site insertion of the monomers was considered in this study. 2. The Origin of Favorable 2,1-Insertion of MMA in the Cationic Diphosphazane-Monoxide Palladium Catalyst System. After the ethylene enchainment, the insertion of MMA in the cationic A catalyst system was calculated to access the origin of its favorable 2,1-regioselectivity. For a comparison, the case of the neutral B system was also calculated. As shown in Table 2, in the case of A, the 1,2insertion energy barrier ΔG2⧧ (12MMAc2) is 25.1 kcal/mol, C

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Figure 3. Energy profiles for the copolymerization of MA and ethylene mediated by A and B, respectively. The energies are relative to corresponding catalysts and monomers.

ΔEdef(mono). As the energy of the TS, ΔETS, is evaluated with respect to the energy of the two separated fragments, the relation ΔETS = ΔEint + ΔEdef(cat) + ΔEdef(mono) holds.37 As shown in Figure 2, although the interaction between the two fragments in 21MMATSc2_A is weaker than that in 12MMATSc2_A (−62.9 vs −65.9 kcal/mol), the total deformation energy of the former (41.9 + 18.4 = 60.3 kcal/mol) is significantly smaller than that of the latter (43.3 + 22.9 = 66.2 kcal/mol), which compensates for the less negative interaction energy in 21MMA TSc2_A, and eventually makes this TS more stable (−2.6 vs 0.3 kcal/mol). These results suggest that the steric factor (geometrical deformation) accounts for the higher stability of the 2,1-insertion TS of MMA. This finding stimulated us to take a closer look at the structure of the corresponding two TSs, as shown in Figure 2. It is found that, in 21MMATSc2_A, there is a MeO···H interaction between the ancillary ligand and the Me group of the inserting MMA moiety.38 However, the similar interaction is weaker in 12MMATSc2_A (MeO···H contact of 2.55 vs 2.28 Å). Such a stronger attractive hydrogen-bond interaction in 21MMATSc2_A is also suggested by NCI analyses (Figure S3). In addition, there is steric repulsion between the Me group of the inserting monomer unit and the growing chain in 12MMATSc2_A, which is absent in 21MMA TSc2_A. These structure features could also account for the higher stability of 21MMATSc2_A. These differences in MMA insertion regioselectivity may account for the differences in A/B catalyzed copolymerization of ethylene with MMA. 3. Differences in A and B Catalyzed Copolymerization of Ethylene with MA. Experimentally, comparable activities were observed for the A and B systems toward copolymerization of ethylene and MA.39 Figure 3 indicates the computed energy profiles for the insertion of MA into the ethylene pre-enchained active species and the subsequent ethylene insertion. The 2,1-insertion manner was considered for MA monomer, since it is more favorable than the 1,2-

In the case of neutral B, however, the calculated energy barriers are comparable (23.9 vs 23.8 kcal/mol, Table 2). It is noted that the experimentally observed ratio of 1,2- to 2,1insertion product was 62:38.20 No experimental evidence was reported19,20 to show reversible formation of the insertion product, suggesting that such a reaction might not be under thermodynamic control. However, the product ratio of 62:38 could correspond to a small energy barrier difference of ca. 0.3 kcal/mol (298.15 K). In this sense, the comparable energy barriers obtained from the current calculation could be understandable (same ancillary ligand as that in the experimental complex). It is noted, however, that the product ratio of 62:38 was observed for MMA insertion into the Pd− Me bond in CH2Cl2 solution at 80 °C.20 For this reason, additional calculations were carried out for the MMA insertion into the Pd−Me bond, corresponding to the experimental conditions (CH2Cl2, 80 °C). It is found that the calculated energy barriers are also comparable (23.1 kcal/mol for 1,2insertion vs 23.2 kcal/mol for 2,1-insertion, Table S5). It is also noteworthy that the calculated insertion energy barrier (ca. 24 kcal/mol) is in line with the experimental value (22.8− 24.8 kcal/mol),20 suggesting the reliability of the theoretical method used. To further investigate the origin of the A mediated selectivity for 2,1-MMA insertion over 1,2-insertion, the energy decomposition analysis37 has been carried out for the TSs of the two insertion manners. In the energy decomposition analysis, the energies of the monomer moiety and the remaining metal complex (two fragments) in the TS geometries were evaluated via single-point calculations. Such single-point energies of the fragments and the energy of TS were used to estimate the interaction energy ΔEint. These energies, together with the energies of the respective fragments in their optimal geometries, allow for the estimation of the deformation energies of the two fragments, ΔEdef(cat) and D

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Figure 4. Energy decomposition analyses for TSs

21MA

TSc3_A,

21MA

TSc3_B, and

21MMA

TSc3_A.

Figure 5. Topographical steric maps of catalysts A and B.

insertion of ethylene after MA enchainment serves as the ratedetermining step, where the energy barrier is comparable to that in the A system (21.8 vs 20.4 kcal/mol). In this sense, the copolymerization activities of A and B are comparable. It is noteworthy that the energy gap between the carbonyl and vinyl coordination complexes (−12.4 − (−22.1) = 9.7 kcal/mol) in system B is larger than that in system A (4.8 kcal/mol). This suggests that catalyst poisoning is much greater in the B system. It is also noted that, unlike the A case, an insertion product 21MAPc2_B with coordination of the MeO group of newly enchained MA unit was located in the B system, which is relatively more stable compared with 21MAPc2_A without coordination of an oxygen atom (Figure 3). This could also account for the easier poisoning of catalyst B. These poisoning effects and slightly higher insertion energy barriers in the B case could suggest better properties of A toward MA incorporation during ethylene polymerization.

insertion fashion in both catalyst systems (Table S3). As shown in this figure, in the case of A (black line), the carbonyl coordination complex 21MACt2′_A is lower in energy than the cis coordination complex 21MACc2_A by 4.8 kcal/mol. The latter could favorably undergo MA insertion via 21MATSc2_A with an energy barrier of 15.9 kcal/mol relative to the carbonyl coordinating complex. This insertion leads to the intermediate 21MA Pc2′_A featuring carbonyl coordination. Such a favorable trans coordination and cis insertion manners (Table S4) are similar to the phosphine-sulfonate based system.20,33a,34,35 The newly formed 21MAPc2′_A could further undergo ethylene insertion through 21MATSc3_A to yield 21MAPc3_A. This step has an energy barrier of 20.4 kcal/mol and is exergonic by 12.8 kcal/mol relative to 21MAPc2′_A. It is obvious that the subsequent ethylene insertion has a higher energy barrier than the MA insertion (20.4 vs 15.9 kcal/mol) and therefore serves as the rate-determining step at the chain propagation stage. Similarly, in the case of B (red line in Figure 3), the E

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Figure 6. Computed energy profile for the A mediated ethylene insertion and β-H elimination on the basis of MMA pre-enchained species.

subsequent ethylene insertions. In order to understand why the insertion of ethylene into the MMA enchained species has a high energy barrier, the energy decomposition analysis was comparatively performed again for 21MATSc3_A (Figure 3) and 21MMA TSc3_A (Figure 6), both featuring ethylene insertion into polar monomer preinserted species. With a comparison of Figure 4a and c, it is found that the higher energy of 21MMA TSc3_A (ΔETS of 16.7 vs 4.1 kcal/mol) is mainly caused by the deformation of MMA preinserted species (ΔEdef(cat) of 53.8 vs 46.1 kcal/mol). Geometrically, the interatomic distance between the two carbon atoms forming a new C−C bond is longer in 21MMATSc3_A than that in 21MATSc3_A (2.28 vs 2.13 Å, Figure S5), which is due to the repulsion between the additional Me group and the ethylene moiety in the former. This repulsion and the weaker interaction between the ethylene moiety and its counterpart also account for the lower stability of 21MMATSc3_A. These results help to explain the experimental findings that the MA unit is in-chain incorporated but the MMA unit is at the chain-end of the corresponding copolymer. For a comparison, further calculations were also performed for B-mediated MMA polymerization. The results suggest that MMA could not be incorporated into the polyethylene chain and provide a better understanding for the nonoccurrence of copolymerization of MMA and ethylene in the B catalyst system (Figure S6). It is noted that attempts to computationally model the Pd− X (X = P, O) bond dissociation in catalyst A were fruitless (Figure S7), possibly due to the electron deficient nature of the cationic Pd center. Similar is true for the case of Pd−O bond dissociation of B (Figure S8b). Although the Pd−P bond dissociated species was computationally located for the B case (Figure S8a), possibly because of the lesser electronegativity of the P atom compared with that of the O atom, this species is higher in free energy in solution than B by 44.4 kcal/mol. These results could add better understanding to that such catalysts work under the ordinary polymerization conditions.

To further explore the difference between the two catalysts, the energy decomposition analysis was performed again for the rate-determining TSs 21MATSc3_A and 21MATSc3_B. As shown in Figure 4a and b, the lower electronic energy of 21MATSc3_A compared with 21MATSc3_B (4.1 vs 8.2 kcal/mol) is mainly caused by the stronger interaction between the monomer moiety and the metal center (interaction energy of −58.1 vs −55.6 kcal/mol). This is understandable in view of the cationic metal center of 21MATSc3_A, as suggested by the calculated NBO charges on the metal center (0.250 in the A case vs 0.198 in the B case). Therefore, the more positively charged metal center inducing stronger interaction with the monomer unit could contribute in large part to the higher activity of A toward ethylene insertion. On the other hand, the geometrical deformation is slightly smaller in A (46.1 vs 47.9 kcal/mol, Figure 4), which is also suggested by the calculated topographical steric map analyses40 (buried volume %VBur of 45.3 vs 47.8, Figure 5). 4. The Origin of the Formation of the MMA ChainEnd in the A Mediated Copolymerization. It is experimentally found that A was capable of copolymerizing ethylene and MMA, producing copolymers with a MMA unit only at the chain-end,19 suggesting a favorable β-H elimination after MMA insertion. This experimental finding drove us to explore the molecular and electronic factors governing the formation of the MMA chain-end. For this purpose, the ethylene insertion into the MMA preinserted species (21MMAPc2_A, Table 2) and its β-H elimination have been comparatively calculated. As shown in Figure 6, the ethylene insertion (black line) via 21MMATSc3_A has an overall energy barrier of 31.5 kcal/mol, which is difficult to occur under the experimental conditions.19 In contrast, the β-H elimination (blue line) has an energy barrier of only 9.3 kcal/mol relative to the carbonyl coordinating species 21MMAPc2′_A. Although this step is endergonic, the subsequent successive insertions of three molecules of ethylene make it thermodynamically favorable because of the acceptable overall energy barriers (21−28.8 kcal/mol, Figure S4) and exergonic character of the F

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Author Contributions

CONCLUSIONS DFT calculations have been comparably carried out for the copolymerization of ethylene with methyl acrylate (MA) or methyl methacrylate (MMA) catalyzed by diphosphazanemonoxide and phosphine-sulfonate based palladium catalysts, respectively. It has been found that, in the diphosphazanemonoxide based five-membered cationic system, the coordination of monomers favorably occurs at the trans site (monomer trans to the P atom), while their insertions take place at the cis site (monomer cis to the P atom). This situation is similar to the previously reported phosphine-sulfonate based system.33,34 The favorable 2,1-insertion manner in the diphosphazane-monoxide palladium system is mainly ascribed to the geometrical deformation to achieve the insertion event. Besides, the stronger MeO···H interaction between the ancillary ligand and the Me group of the inserting MMA moiety in the 2,1-insertion transition state compared with the 1,2-insertion case also contributes to the stability of the former and therefore to the priority of 2,1-selectivity. In addition, the steric repulsion between the Me group of the inserting MMA and the growing chain could make the 1,2-insertion less favorable. In the copolymerization of MA and ethylene, the rate-determining step is found to be the ethylene insertion into the MA pre-enchained species rather than the MA insertion itself. In comparison with the conventional phosphinesulfonate based system, the diphosphazane-monoxide palladium demonstrates less geometrical deformation of the catalyst during the copolymerization of MA and ethylene, due to the rigidity of the five-membered backbone. The formation of the MMA chain-end in diphosphazane-monoxide palladium mediated copolymerization is mainly due to the steric repulsion between the methyl group of preinserted MMA and the subsequent inserting ethylene moiety, which kinetically hampers the ethylene insertion and therefore a β-H elimination takes place to result in the MMA chain-end.

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§

J.S., M.C.: These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Nos. 21674014, 21704094, and 21690071). The authors thank the Fundamental Research Funds for the Central Universities (DUT2016TB08, DUT18GJ201, DUT18RC(3)002). The Information Center of Dalian University of Technology is acknowledged for part of the computational resources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00796. Figures showing the optimized structures of some important stationary points and the energy profiles for the insertion of the first ethylene molecule, A mediated successive ethylene insertion after β-H elimination, tables giving the relative free energies in solution for polar monomer insertions (PDF) Optimized Cartesian coordinates of all stationary points together with their single-point energy (a.u.) in solution and the imaginary frequencies (cm−1) of transition states (XYZ)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Min Chen: 0000-0003-3992-5759 Gen Luo: 0000-0002-5297-6756 Changle Chen: 0000-0002-4497-4398 Yi Luo: 0000-0001-6390-8639 G

DOI: 10.1021/acs.organomet.8b00796 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00796 Organometallics XXXX, XXX, XXX−XXX