Next Article in Journal
Merging Metallic Catalysts and Sonication: A Periodic Table Overview
Next Article in Special Issue
Functional and Biochemical Analysis of Glucose-6-Phosphate Dehydrogenase (G6PD) Variants: Elucidating the Molecular Basis of G6PD Deficiency
Previous Article in Journal
Microwave-Assisted Synthesis of Co3(PO4)2 Nanospheres for Electrocatalytic Oxidation of Methanol in Alkaline Media
Previous Article in Special Issue
The Distribution and Strength of Brönsted Acid Sites on the Multi-Aluminum Model of FER Zeolite: A Theoretical Study

Catalysts 2017, 7(4), 120; https://doi.org/10.3390/catal7040120

Article
Quantitative Structure-Thermostability Relationship of Late Transition Metal Catalysts in Ethylene Oligo/Polymerization
by 1,2,*, 1,2, 1,2 and 1,2,*
1
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Academic Editor: José R. B. Gomes
Received: 23 February 2017 / Accepted: 14 April 2017 / Published: 18 April 2017

Abstract

:
Quantitative structure–thermostability relationship was carried out for four series of bis(imino)pyridine iron (cobalt) complexes and α-diimine nickel complexes systems in ethylene oligo/polymerization. Three structural parameters were correlated with thermal stability, including bond order of metal-nitrogen (B), minimum distance (D) between central metal and ortho-carbon atoms on the aryl moiety and dihedral angle (α) of a central five-membered ring. The variation degree of catalytic activities between optimum and room temperatures (AT) was calculated to describe the thermal stability of the complex. By multiple linear regression analysis (MLRA), the thermal stability presents good correlation with three structural parameters with the correlation coefficients (R2) over 0.95. Furthermore, the contributions of each parameter were evaluated. Through this work, it is expected to help the design of a late transition metal complex with thermal stability at the molecular level.
Keywords:
structure–thermostability relationship; late transition metal complex; homogeneous catalysis; ethylene polymerization; molecular modeling

1. Introduction

Since the discovery of nickel and palladium catalysts bearing α-diimine ligands, the growth of late transition metal complex catalysts for ethylene reactivity has been promoted due to their particular features, such as stable structure, low cost and high catalytic activities in the polymerization of ethylene [1,2,3,4,5,6]. In order to get desirable catalytic performance, extensive achievement and progress have been obtained through modifying substituents of used ligands and designing alternative ligands as well as optimizing reaction conditions [7,8,9,10,11,12,13,14,15,16,17,18]. However, in large-scale polymerizations, the exothermic property of the polymerization reaction deactivates the catalysts by the quick decomposition and β-H elimination at elevated bulk reaction temperature [19,20,21,22], greatly hindering the potential development in the field of industrial applications [23].
To improve the thermal stability of a catalyst, tremendous experimental works were conducted and significant progress achieved. Guan et al. early reported a series of novel nickel catalysts bearing the cyclophane ligands (Scheme 1, A) and phosphine imine hybrid ligands (Scheme 1, B), which revealed high activities at high temperatures for ethylene polymerization. The complex A presents a turnover frequency (TOF) of 1.0 × 106 h−1 at 90 °C and complex B shows a TOF of 1.7 × 104 h−1 at 70 °C [24,25,26]. Subsequently, the camphyl-based nickel catalysts (Scheme 1, C) exhibited good thermal stability; meanwhile, the obtained polymer had a significant narrow molecular weight distribution (polydispersity index: PDI < 1.2) [27,28]. The Long group also reported a robust nickel α-diimine catalyst (Scheme 1, D) for ethylene polymerization at a high temperature. The TOF maintains about 1.0 × 105 h−1 at 100 °C. Moreover, the observed molecular weight distribution of the polyethylene is still very narrow (about 1.25) at such a high temperature [29]. In addition, our group reported the nickel complex catalysts containing acenaphthylene ligands (Scheme 1, E), which showed high catalytic activities (106 g·mol−1(Ni)·h−1) and high molecular weight of the produced polyethylene (106 g·mol−1) at temperature of 80 °C [30]. In addition, the iron-based complex catalysts bearing bis(imino)pyridine ligands with bulky substituents (Scheme 1, F) exhibited good thermal stability with catalytic activities of 106 g·mol−1(Fe)·h−1 at 60–80 °C [31,32,33,34,35,36,37].
In the complexity of the whole polymerization process, the underlying reasons for the observed changes of catalytic activities are often unclear, but there is no doubt that catalyst structures play a key role in determination of catalytic activities. As a result, the quantitative structure–property relationship (QSPR) approach has proven to be useful. In our previous studies, the relationship between the structure of a catalyst and its catalytic activity was investigated by molecular modeling, which successfully predicted the catalytic activities of late transition metal complex precatalysts toward ethylene oligo/polymerization, through electronic effects [38,39,40,41] or both from electronic and steric effects [42,43]. Regarding the influence of the catalyst structure on its thermal stability, several possible factors were proposed by previous experimental research [19,24,25,26,29,30,31,32,33,34,35,36]. However, to our best knowledge, there are less detailed works at a molecular level to study the catalyst with thermal stability quantitatively.
Herein, detailed works on this topic at a molecular level were performed to explain the essential mechanism of the catalyst with thermal stability quantitatively. Three structural parameters were defined and calculated to correlate with the thermal stability for four series of typical late transition metal complexes from our previous reports [30,31,32,33,34,35,36,44,45,46,47]. The results showed that a catalyst’s thermal stability has very good relationships with three structural parameters by using multiple linear regression analysis (MLRA). The obtained correlation coefficient values of R2 are over 0.949. By analyzing the contribution of each parameter, the main effect on thermal stability regarding different frameworks of precatalysts was proposed.

2. Results and Discussion

2.1. Definition of the Parameters Related to Thermal Stability

In the present work, three parameters were chosen to characterize the thermal stability of late transition metal complex, including the bond order (B), minimum distance (D) and dihedral angle (α). Regarding bond order (B), it represents the strength of the bond between metal atoms and coordinated heteroatoms within ligands, which are nitrogen atoms for the modeled complexes. The bond strength of metal and nitrogen atoms (M–N) should be strong enough to prevent them from breaking (or decomposition) at high temperatures [26]. Since there are usually two or three M–N bonds, the minimum value of bond order (B) was chosen. According to previous research, the formation of the five- or six-membered metallacycle via intramolecular C–H activation demonstrated in Scheme 2 may deactivate the catalyst, reducing the thermal stability of a catalyst [19]. Therefore, the minimum distance (D) between central metal and carbon atoms highlighted in blue (M–C) was calculated. The parameter α is the dihedral angle of the central five-membered ring shown in the Scheme 2 with the atoms in red. This factor indicates the stability of the complex due to the stress and strain of the central five-membered ring within a catalyst [48]. For an N^N^N three chelated complex, the value of α is the average data for two central five-membered rings.
To measure the thermal stability of different complexes, the variation values of catalytic activities at different reaction temperatures (AT) were calculated by Equation (1):
A T = A opt   A r A r  
where Aopt and Ar are the activities at optimum and room temperatures for each complex, respectively. Most of the model complexes in this work were taken from our previous experimental results, and the detailed values of activities at both optimum and room temperatures were provided in Table S1 of Supplementary Materials.
The calculated parameters (B, D, α) were related to the AT by the MLRA Equation (2), which was resolved by the regression analysis based on the least squares method in Microsoft Excel [49]. After fitting, the values of x0, x1, x2 and x3 were automatically obtained:
A T =   x 0 + x 1 B + x 2 D + x 3
In order to analyze the contribution of each parameter to the thermal stability for each complex system, the values of B, D and α were standardized using the Z-Score method. Then, the contribution was calculated by the Equation (3) based on our previous works:
C o n t r i b u t i o n ( % ) = j = 1 j = n | x i m i j | 1 i = 3 | x i m i j | n × 100 %
where i is the serial number of parameter, j is the order of the complex, mij is the value of each parameter for each complex, xi is the regression coefficient of each parameter, and n is the total number of complexes.

2.2. Thermal Stability of Bis(imino)pyridyliron(cobalt) Complexes

The main framework of the catalyst with thermal stability can be classified into two categories including the bis(imino)pyridyliron(cobalt) and the α-diiminenickel complexes. In this section, the bis(imino)pyridyliron(cobalt) complexes 110 as showed in the Scheme 3 and Scheme 4 from previous reports [31,32,33,34,35,36] were selected to study the thermal stability. For the complexes 15, the small variations of structures lie in the different substituents within the aryl moiety, while, for the complexes 610, the differences are not only the substituents within the aryl moiety, but also the Ar group. Furthermore, the experimental conditions were totally the same for complexes 15, while, for complexes 610, there was little difference. Therefore, the complexes were divided into two systems: complexes 15 and complexes 610.
In this study, the calculated three parameters (B, D, α) are heavily dependent on the geometry structure, so the optimization of the complex structure was performed by the molecular mechanics (MM) method. The comparisons between calculated structure and experimental crystal data for the selected bond lengths and bond angles for complexes 1, 5 and 7 were listed in Table 1. It is clearly seen that the values of standard deviation δ for all three of these complexes are very small, indicating the reasonableness of the optimized structures. This guarantees a reliable relationship between structure parameters (B, D, α) and the thermal stability.
Accordingly, the three structural parameters calculated were obtained and listed in Table 2 for complexes 15. Clearly, the variations of the three parameters as a function of substituents were very small. Regarding the bond order (B), the values are from 0.1028 to 0.1048, meaning that there is little change in bond strength between iron and nitrogen with various substituents. For the minimum distance parameter (D), all of the calculated results were above 2.0 Å, indicating no formation of an inter M–C bond, which improves the thermal stability of complex. As for a dihedral angle (α), it is shown in Table 2 that the obtained values of α were from 7.839° to 8.627°. From previous study, it is indicated that the half-chair configuration (non plane form) of the central five-membered ring is helpful for the stability of the complex [46]. Therefore, the bigger the value of α, the more stable the complex is.
To quantitatively analyze the influence of catalyst structure on thermal stability, the relationship between B, D, α and AT was fitted using the multiple linear regression Equation (2). The regression and correlation coefficient values were obtained and listed in Table 3. Then, the thermal stabilities were calculated by the fitting equation and compared with experimental results for the complexes 15. The result shows that almost all points were in a diagonal line in Figure 1a, suggesting that the calculated and experimental AT values are very close. The obtained correlation coefficient value (R2) is 0.970.
In the same manner, for complexes 610, three parameters were obtained and listed in Table 4. For bond order (B), the calculated values were in the range of 0.0788 to 0.1031, which were smaller compared with the results of complexes 15 (Table 2), especially for complex 10, signifying the potential reduction of their thermal stabilities. For the minimum distance (D), all of the data were over 2.00 Å, indicating that no metallacycle ring formed either. Regarding the dihedral angle (α), the results ranged from 7.335° to 11.413°. From the variation of the structure parameters, the thermal stabilities of complexes 610 should be lower than that of complexes 15. For clarity, the averaged values of the catalytic activities for complexes 15 and 610 were plotted in Figure 2 at room and optimum temperatures. Obviously, the thermal stability of the former is better than the latter, although the catalytic activities at room temperature are almost the same.
Accordingly, from the values of AT in Table 2 and Table 4, the thermal stabilities of complexes 610 are lower than that of complexes 15 correspondingly. This result is in agreement with the decrease of bond order (B), suggesting that this parameter has an outstanding influence on the thermal stability of this series of complex system. By the MLRA method, the regression coefficients were obtained and showed in Table 3. Then, the calculated values of thermal stabilities were obtained and compared with experimental results, which present good correlation with the R2 value of 0.949 as showed in Figure 1b.
In order to quantitatively investigate the influence of each parameter on the thermal stability, the values of three parameters and thermal stabilities were further standardized by the Z-Score method. The obtained values of the B, D, α, AT were collected in Table 2 and Table 4 for complexes 15 and complexes 610, respectively, and the corresponding standard regression coefficients were obtained and listed in Table 3. Then, using Equation (3), the contributions of each parameter were calculated. The obtained contribution values of the B, D and α for the complexes 15 are 48%, 26% and 26%, respectively. Therefore, it indicates that the bond order dominates the thermal stabilities of complexes 15. Similarly, the contributions of the B, D and α for the complexes 610 are calculated and the results are 46%, 33% and 21%, respectively. It suggests that the bond order plays a predominant role in determining the thermal stabilities for complexes 610 as well.

2.3. Thermal Stability of Phenanthrolinyliron Complexes

For the derivatives of bis(imino)pyridyline, the phenanthrolinyliron complexes also showed good thermal stability. Here, we selected a series of complexes 1115 from our previous reports [47] with the structures showed in the Scheme 5. In the same manner, the values of bond order (B), minimum distance (D) and dihedral angle (α) as well as the results of the thermal stabilities (AT), accordingly, were obtained and listed in Table 5.
Compared with complexes 110, the values of bond order (B) and dihedral angle (α) of complexes 1115 decreased obviously. To see the variation of the catalytic activities from experiments, the average values at room temperature and optimum temperature were showed in Figure 2 for complexes 1115. It is clear that the average activities and thermal stabilities of complexes 1115 are obviously lower than that of complexes 15 and 610. These observations are in agreement with the decrease values of the calculated bond order (B) and dihedral angle (α) values within complexes 1115. On another side, the values of thermal stability (AT) for complexes 1115 are large, as showed in Table 5, especially for complexes 11 and 12. This can be explained by the very low catalytic activities at room temperature.
Then, the regression coefficients of fitting equation were calculated, and the values were listed in Table 3. Comparison of calculated and experimental AT was conducted as showed in Figure 3. Obviously, there is a good correlation with the coefficient (R2) value of 0.998, meaning that the thermal stabilities of complexes 1115 were reasonably investigated.
To further analyze the influence of each parameter on thermal stability, the B, D, α and AT values were standardized and the results were collected in Table 5. Based on the standardized fitting equation, the contributions of each parameter were calculated accordingly. The obtained contribution of the dihedral angle (α) is 78%, whereas those of bond order (B) and minimum distance (d) are 14% and 8%, respectively. These results reveal that, different from complexes 15 and 610, the dihedral angle became the major factor to determine the thermal stabilities of complexes 1115.

2.4. Thermal Stability of Acenaphthylnickel Complexes

As discussed in the introduction, besides the framework of bis(imino)pyridyline complex, the system with an acenaphthylene backbone also shows good thermal stability towards ethylene polymerization. In this section, a series of nickel complexes 1620 containing acenaphthylene ligands [30,43,44,45,46] were investigated regarding the property of thermal stability. The structures of model complexes were showed in the Scheme 6.
By the same method, three parameters (B, D, α) were firstly obtained and listed in Table 6 along with the values of thermal stabilities (AT) for each complex. By the regression analysis method, the regression coefficients were obtained and showed in Table 3. Then, the thermal stabilities were calculated and compared with experimental data. Results revealed in Figure 4 that calculation values are in very agreement with experiments with the correlation coefficient (R2) value of 0.998.
Based on standardized values of B, D, α and AT as well as the corresponding standard regression coefficients in Table 3, the contributions of each factor were calculated accordingly, which showed the values of 49%, 22% and 29% for bond order (B), minimum distance (D) and dihedral angle (α), respectively, signifying the dominant role of bond order in the thermal stabilities of complexes 1620.

3. Computational Details

Calculation Method for the Parameters

In order to calculate the bond order (B), the structure of complex was firstly optimized by a molecular mechanism (MM) method based on our previous study, which shows that the optimized structures by MM are closer to experimental crystal results [43]. This was performed by the Forcite program package (6.0, Accelrys Inc., San Diego, CA, USA, 2011) using the Dreiding force field due to its capability of reasonable prediction of the structure [50]. However, there are no parameters for late transition metal elements, such as Fe, Co and Ni, as well as the atom types that connect with metal elements. Therefore, the crystal data of complexes in previous reports [34,45,47], as listed in Table S2 of Supplementary Materials, were added into the file of Dreiding force field. The values of convergence tolerance for the energy and the force were 0.001 and 0.5 kcal·mol−1, respectively. To describe the electrostatic and van der Waals interactions, the atom based Summation and Truncation methods were used, and the cutoff distance of cubic spline was 1.25 nm.
Then, the Mayer bond order was obtained by single point energy calculation by a Gaussian 09 package (C.01, Gaussian, Inc., Wallingford, CT, USA, 2010) using the Natural Bond Orbital (NBO) population [51,52]. Different functional and basis sets may give rise to different bond order results for one structure; therefore, several combinations of parameters were tried as listed in Table 7. From previous research [53], the bond order and bond length should be in accordance with the following Equation (4):
B = exp [ ( R R 0 ) R 0 ] a ,
where B is bond order, R is the experimental bond length, R0 is a constant which stands for the equilibrium bond length, and a is also a constant related to the type of bond. Then, the calculated bond orders of three Fe–N bonds for the complex 12 (Scheme 5) and experimental bond lengths were fitted by this equation. The obtained fitting results (R2) were showed in Table 7. Clearly, the calculation result for bond order using B3LYP/6-31G* [54,55] is better than others. Therefore, this combination of functional and basis set was chosen. As to the dihedral angles (α) for the central five-membered ring, this parameter can be obtained directly from the optimized structure of a complex by MM calculation.
With regard to the minimum distance (D), this was calculated by the molecular dynamics (MD) simulation. All of the MD simulations were performed in the ensemble of the constant of atom number, volume and temperature (NVT) using a Dreiding force field implemented in the Material Studio (6.0, Accelrys Inc., 2011). The nonbonded potential truncation was performed by cubic spline with a cutoff distance of 1.25 nm. The electrostatic and van der Waals interactions were treated by an atom based Summation method with a spline width of 0.1 nm. The temperature kept at 300 K by the Berendsen thermostat with a coupling constant of 0.1 ps. Total simulation time was 4 ns with a time step of 1 fs.
To illustrate the rationalization of three parameters in the model, the results of the linear fitting using one, two and three parameters for the bis(imino)pyridyliron (complexes 15) and acenaphthylnickel (complexes 1620) were compared. It can be seen from the results listed in Table S3 of Supplementary Materials that the fitting results with three parameters for two systems are optimum. Therefore, the model with three parameters is used to investigate the thermal stability for late transition metal complexes.

4. Conclusions

The property of thermal stability for late transition metal complex precatalysts is very important for ethylene polymerization, which greatly influences their potential applications in the field of industry. In this work, four series of complexes with thermal stability containing typical structures were investigated quantitatively by the MLRA method, which was used in our previous studies on the catalytic activities. Based on the proposed mechanism from experimental observation with respect to the thermal stability of a catalyst; herein, three parameters were defined and calculated, including bond order (B), minimum distance (D) and dihedral angle (α). Meanwhile, the parameter (AT) that calculates the variation degree of catalytic activities between optimum and room temperatures was used to represent the thermal stability of each complex. By the MLRA method, the fitting equations were obtained and the calculated values of thermal stabilities were compared with experimental data. The results show very good correlation with the values of coefficient (R2) ranged from 0.949 to 0.998, indicating that the thermal stability of a late transition metal complex towards ethylene polymerization can be quantitatively investigated.
Based on standardized values of three parameters (B, D, α), the contributions of each factor to the thermal stability of complex were evaluated accordingly. For the bis(imino)pypridyliron(cobalt) and acenaphthylnickel complexes, the thermal stability was primarily determined by bond order of metal–nitrogen, while for the phenanthrolinyliron complexes, the dihedral angle plays an important role. To the best of our knowledge, these results are the first reported on thermal stability of a late transition metal complex at the molecular level. It is anticipated that the present results can give guidance for experimental design of a late transition metal complex with high activity at high reaction temperatures.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/7/4/120/s1, Table S1: The catalytic activities for complexes 120 at different temperatures along with the reaction time, Table S2: The bond lengths and bond angles for metal atoms and atoms coordinated with central metal, which were used as parameters of modified Dreiding force field, Table S3: The correlation coefficient (R2) results for the models of Fe and Ni-based complexes that were fitted by one, two and three parameters, respectively.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 2124092 and U1362204) and “One-Three-Five” Strategic Planning of Institute of Chemistry Chinese Academy Sciences (ICCAS). The authors also thank the supercomputer resources and software provided by the University of Missouri-Columbia and the Japan Advanced Institute of Science and Technology.

Author Contributions

Wenhong Yang and Wen-Hua Sun conceived and designed the experiments; Zhifeng Ma performed the experiments; Jun Yi contributed analysis tools; and Wenhong Yang, Zhifeng Ma and Wen-Hua Sun analyzed the data and wrote the paper.

Conflicts of Interest

The authors declare no conflicts of Interest.

References

  1. Johnson, L.K.; Killian, M.C.; Brookhart, M. New Pd(II)- and Ni(II)-based catalysts for polymerization of ethylene and α-olefins. J. Am. Chem. Soc. 1995, 117, 6414–6415. [Google Scholar] [CrossRef]
  2. Johnson, L.K.; Mecking, S.; Brookhart, M. Copolymerization of ethylene and propylene with functionalized vinyl monomers by palladium(II) catalysts. J. Am. Chem. Soc. 1996, 118, 267–268. [Google Scholar] [CrossRef]
  3. Mecking, S.; Johnson, L.K.; Wang, L.; Brookhart, M. Mechanistic studies of the palladium-catalyzed copolymerization of ethylene and α-olefins with methyl acrylate. J. Am. Chem. Soc. 1998, 120, 888–899. [Google Scholar] [CrossRef]
  4. Ittel, S.D.; Johnson, L.K.; Brookhart, M. Late-metal catalysts for ethylene homo- and copolymerization. Chem. Rev. 2000, 100, 1169–1203. [Google Scholar] [CrossRef] [PubMed]
  5. Gibson, V.C.; Spitzmesser, S.K. Advances in non-metallocene olefin polymerization catalysis. Chem. Rev. 2003, 103, 283–315. [Google Scholar] [CrossRef] [PubMed]
  6. Chen, G.; Ma, X.S.; Guan, Z. Synthesis of functional olefin copolymers with controllable topologies using a chain-walking catalyst. J. Am. Chem. Soc. 2003, 125, 6697–6704. [Google Scholar] [CrossRef] [PubMed]
  7. Younkin, T.R.; Connor, E.F.; Henderson, J.I.; Friedrich, S.K.; Grubbs, R.H.; Bansleben, D.A. Neutral, single component nickel (II) polyolefin catalysts that tolerate heteroatoms. Science 2000, 287, 460–462. [Google Scholar] [CrossRef] [PubMed]
  8. Hicks, F.A.; Brookhart, M.A. Highly active anilinotropone based neutral nickel(II) catalyst for ethylene polymerization. Organometallics 2001, 20, 3217–3219. [Google Scholar] [CrossRef]
  9. Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R.I. Palladium catalysed copolymerisation of ethene with alkylacrylates: Polar comonomer built into the linear polymer chain. Chem. Commun. 2002, 7, 744–745. [Google Scholar] [CrossRef]
  10. Gottker Schnetmann, I.; Korthals, B.; Mecking, S. Water-soluble salicylaldiminato Ni(II)-methyl complexes: Enhanced dissociative activation for ethylene polymerization with unprecedented nanoparticle formation. J. Am. Chem. Soc. 2006, 128, 7708–7709. [Google Scholar] [CrossRef] [PubMed]
  11. Luo, S.; Vela, J.; Lief, G.R.; Jordan, R.F. Copolymerization of Ethylene and alkyl vinyl ethers by a (phosphinesulfonate) PdMe catalyst. J. Am. Chem. Soc. 2007, 129, 8946–8947. [Google Scholar] [CrossRef] [PubMed]
  12. Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. Formation of linear copolymers of ethylene and acrylonitrile catalyzed by phosphine sulfonate palladium complexes. J. Am. Chem. Soc. 2007, 129, 8948–8949. [Google Scholar] [CrossRef] [PubMed]
  13. Mu, H.; Pan, L.; Song, D.; Li, Y. Neutral nickel catalysts for olefin homo- and copolymerization: relationships between catalyst structures and catalytic properties. Chem. Rev. 2015, 115, 12091–12137. [Google Scholar] [CrossRef] [PubMed]
  14. Small, B.L.; Brookhart, M.; Bennett, A.M.A. Highly active iron and cobalt catalysts for the polymerization of ethylene. J. Am. Chem. Soc. 1998, 120, 4049–4050. [Google Scholar] [CrossRef]
  15. Small, B.L.; Brookhart, M. Iron-based catalysts with exceptionally high activities and selectivities for oligomerization of ethylene to linear α-olefins. J. Am. Chem. Soc. 1998, 120, 7143–7144. [Google Scholar] [CrossRef]
  16. Britovsek, G.J.P.; Bruce, M.; Gibson, V.C.; Kimberley, B.S.; Maddox, P.J.; Mastroianni, S.; McTavish, S.J.; Redshaw, C.; Solan, G.A.; Strömberg, S.; et al. Polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands: Synthesis, structures, and polymerization studies. J. Am. Chem. Soc. 1999, 121, 8728–8740. [Google Scholar] [CrossRef]
  17. Bianchini, C.; Giambastiani, G.; Rios, I.G.; Mantovani, G.; Meli, A.; Segarra, A.M. Ethylene oligomerization, homopolymerization and copolymerization by iron and cobalt catalysts with 2,6-(bis-organylimino) pyridyl ligands. Coord. Chem. Rev. 2006, 250, 1391–1418. [Google Scholar] [CrossRef]
  18. Flisak, Z.; Sun, W.-H. Progression of diiminopyridines: From single application to catalytic versatility. ACS Catal. 2015, 5, 4713–4724. [Google Scholar] [CrossRef]
  19. Tempel, D.J.; Johnson, L.K.; Huff, R.L.; White, P.S.; Brookhart, M. Mechanistic studies of Pd(II)-α-diimine-catalyzed olefin polymerizations. J. Am. Chem. Soc. 2000, 122, 6686–6700. [Google Scholar] [CrossRef]
  20. Gates, D.P.; Svejda, S.A.; Oñate, E.; Killian, C.M.; Johnson, L.K.; White, P.S.; Brookhart, M. Synthesis of branched polyethylene using (α-diimine) nickel(II) catalysts: Influence of temperature, ethylene pressure, and ligand structure on polymer properties. Macromolecules 2000, 33, 2320–2334. [Google Scholar] [CrossRef]
  21. Berkefeld, A.; Mecking, S. Deactivation pathways of neutral Ni(II) polymerization catalysts. J. Am. Chem. Soc. 2009, 131, 1565–1574. [Google Scholar] [CrossRef] [PubMed]
  22. Guo, L.; Gao, H.; Guan, Q.; Hu, H.; Deng, J.; Liu, J.; Liu, F.; Wu, Q. Substituent effects of the backbone in α-diimine palladium catalysts on homo- and copolymerization of ethylene with methyl acrylate. Organometallics 2012, 31, 6054–6062. [Google Scholar] [CrossRef]
  23. Xie, T.; McAuley, K.B.; Hsu, J.C.C.; Bacon, D.W. Gas phase ethylene polymerization: Production processes, polymer properties and reactor modeling. Ind. Eng. Chem. Res. 1994, 33, 449–479. [Google Scholar] [CrossRef]
  24. Camacho, D.H.; Salo, E.V.; Ziller, J.W.; Guan, Z. Cyclophane-based highly active late-transition-metal catalysts for ethylene polymerization. Angew. Chem. Int. Ed. 2004, 43, 1821–1825. [Google Scholar] [CrossRef] [PubMed]
  25. Camacho, D.H.; Guan, Z. Living polymerization of α-olefins at elevated temperatures catalyzed by a highly active and robust cyclophane-based nickel catalyst. Macromolecules 2005, 38, 2544–2546. [Google Scholar] [CrossRef]
  26. Guan, Z.; Marshall, W.J. Synthesis of new phosphine imine ligands and their effects on the thermal stability of late-transition-metal olefin polymerization catalysts. Organometallics 2002, 21, 3580–3586. [Google Scholar] [CrossRef]
  27. Liu, F.; Gao, H.; Hu, Z.; Hu, H.; Zhu, F.; Wu, Q. Poly(1-hexene) with long methylene sequences and controlled branches obtained by a thermostable α-diimine nickel catalyst with bulky camphyl backbone. J. Polym. Sci. Part A 2012, 50, 3859–3866. [Google Scholar] [CrossRef]
  28. Liu, J.; Chen, D.; Wu, H.; Xiao, Z.; Gao, H.; Zhu, F.; Wu, Q. Polymerization of α-olefins using a camphyl α-diimine nickel catalyst at elevated temperature. Macromolecules 2014, 47, 3325–3331. [Google Scholar] [CrossRef]
  29. Rhinehart, J.L.; Brown, L.A.; Long, B.K. A Robust Ni(II) α-diimine catalyst for high temperature ethylene polymerization. J. Am. Chem. Soc. 2013, 135, 16316–16319. [Google Scholar] [CrossRef] [PubMed]
  30. Du, S.; Kong, S.; Shi, Q.; Mao, J.; Guo, C.; Yi, J.; Liang, T.; Sun, W.-H. Enhancing the activity and thermal stability of nickel complex precatalysts using 1-[2,6-bis(bis(4-fluorophenyl)methyl)-4-methyl phenylimino]-2-aryliminoacenaphthylene derivatives. Organometallics 2015, 34, 582–590. [Google Scholar] [CrossRef]
  31. Yu, J.; Liu, H.; Zhang, W.; Hao, X.; Sun, W.-H. Access to highly active and thermally stable iron procatalysts using bulky 2-[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridine ligands. Chem. Commun. 2011, 47, 3257–3259. [Google Scholar] [CrossRef] [PubMed]
  32. Cao, X.; He, F.; Zhao, W.; Cai, Z.; Hao, X.; Shiono, T.; Redshaw, C.; Sun, W.-H. 2-[1-(2,6-Dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridyliron(II) dichlorides: Synthesis, characterization and ethylene polymerization behaviour. Polymer 2012, 53, 1870–1880. [Google Scholar] [CrossRef]
  33. Zhao, W.; Yu, J.; Song, S.; Yang, W.; Liu, H.; Hao, X.; Redshaw, C.; Sun, W.-H. Controlling the ethylene polymerization parameters in iron pre-catalysts of the type 2-[1-(2,4-dibenzhydryl-6-methyl phenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridyliron dichloride. Polymer 2012, 53, 130–137. [Google Scholar] [CrossRef]
  34. Sun, W.-H.; Zhao, W.; Yu, J.; Zhang, W.; Hao, X.; Redshaw, C. Enhancing the activity and thermal stability of iron precatalysts using 2-(1-{2,6-bis[bis(4-fluorophenyl) methyl]-4-methylphenylimino}ethyl) -6-[1-(arylimino)ethyl]pyridines. Macromol. Chem. Phys. 2012, 213, 1266–1273. [Google Scholar] [CrossRef]
  35. Wang, S.; Li, B.; Liang, T.; Redshaw, C.; Li, Y.; Sun, W.-H. Synthesis, characterization and catalytic behavior toward ethylene of 2-[1-(4,6-dimethyl-2-benzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl] -pyridyl metal (iron or cobalt) chlorides. Dalton Trans. 2013, 42, 9188–9197. [Google Scholar] [CrossRef] [PubMed]
  36. Wang, S.; Zhao, W.; Hao, X.; Li, B.; Redshaw, C.; Li, Y.; Sun, W.-H. 2-(1-{2,6-Bis[bis(4-fluorophenyl)methyl]-4-methylphenylimino}ethyl)-6-[1-(arylimino)ethyl]pyridylcobalt dichlorides: Synthesis, characterization and ethylene polymerization behaviour. J. Organomet. Chem. 2013, 731, 78–84. [Google Scholar] [CrossRef]
  37. Ma, Z.; Yang, W.; Sun, W.-H. Recent progress on transition metal (Fe, Co, Ni, Ti and V) complex catalysts in olefin polymerization with high thermal stability. Chin. J. Chem. 2017. [Google Scholar] [CrossRef]
  38. Yang, W.; Chen, Y.; Sun, W.-H. Assessing catalytic activities through modeling net charges of iron complex precatalysts. Macromol. Chem. Phys. 2014, 215, 1810–1817. [Google Scholar] [CrossRef]
  39. Chen, Y.; Yang, W.; Sha, R.; Fu, R.-D.; Sun, W.-H. Correlating net charges and the activity of bis(imino)pyridylcobalt complexes in ethylene polymerization. Inorg. Chim. Acta 2014, 423, 450–453. [Google Scholar] [CrossRef]
  40. Yang, W.; Yi, J.; Sun, W.-H. Revisiting benzylidenequinolinyl nickel catalysts through the electronic effects on catalytic activity by DFT studies. Macromol. Chem. Phys. 2015, 216, 1125–1133. [Google Scholar] [CrossRef]
  41. Yang, W.; Chen, Y.; Sun, W.-H. Correlating cobalt net charges with catalytic activities of the 2-(benzimidazolyl)-6-(1-aryliminoethyl)pyridylcobalt complexes toward ethylene polymerization. Macromol. React. Eng. 2015, 9, 473–479. [Google Scholar] [CrossRef]
  42. Yi, J.; Yang, W.; Sun, W.-H. Quantitative investigation of the electronic and steric influences on ethylene oligo/polymerization by 2-azacyclyl-6-aryliminopyridylmetal (Fe, Co, and Cr) complexes. Macromol. Chem. Phys. 2016, 217, 757–764. [Google Scholar] [CrossRef]
  43. Yang, W.; Ma, Z.; Sun, W.-H. Modeling study on the catalytic activities of 2-imino-1, 10-phenanthrolinylmetal (Fe, Co, and Ni) precatalysts in ethylene oligomerization. RSC Adv. 2016, 6, 79335–79342. [Google Scholar] [CrossRef]
  44. Rhinehart, J.L.; Mitchell, N.E.; Long, B.K. Enhancing α-diimine catalysts for high-temperature ethylene polymerization. ACS Catal. 2014, 4, 2501–2504. [Google Scholar] [CrossRef]
  45. Fan, L.; Yue, E.; Du, S.; Guo, C.-Y.; Hao, X.; Sun, W.-H. Enhancing thermo-stability to ethylene polymerization: Synthesis, characterization and the catalytic behavior of 1-(2,4-dibenzhydryl-6-chlorophenylimino)-2-aryliminoacenaphthylnickel halides. RSC Adv. 2015, 5, 93274–93282. [Google Scholar] [CrossRef]
  46. Kong, S.; Song, K.; Liang, T.; Guo, C.-Y.; Sun, W.-H.; Redshaw, C. Methylene-bridged bimetallic α-diimino nickel(II) complexes: Synthesis and high efficiency in ethylene polymerization. Dalton Trans. 2013, 25, 9176–9187. [Google Scholar] [CrossRef] [PubMed]
  47. Jie, S.; Zhang, S.; Sun, W.-H. 2-Arylimino-9-phenyl-1, 10-phenanthrolinyl-iron, -cobalt and -nickel complexes: Synthesis, characterization and ethylene oligomerization behavior. Eur. J. Inorg. Chem. 2007, 5584–5598. [Google Scholar] [CrossRef]
  48. Pan, H.; Zhu, L.; Li, J.; Zang, D.; Fu, Z.; Fan, Z. A Thermal stable α-Diimine palladium catalyst for copolymerization of ethylene with functionalized olefins. J. Mol. Catal. A 2014, 390, 76–82. [Google Scholar] [CrossRef]
  49. Wang, G. Excel 2013 of Charting and Data Analysis Skills; China Youth Publishing House: Beijing, China, 2014. [Google Scholar]
  50. Mayo, S.L.; Olafson, B.D.; Goddard, W.A. Dreiding: A generic force field for molecular simulations. J. Phys. Chem. 1990, 94, 8897–8909. [Google Scholar] [CrossRef]
  51. Mayer, I. Bond orders and valences: Role of d-orbitals for hyfervlent sulphur. J. Mol. Struct. 1989, 149, 81–89. [Google Scholar] [CrossRef]
  52. Glendening, E.D.; Landis, C.R.; Weinhold, F. Natural bond orbital methods. WIREs Comput. Mol. Sci. 2012, 2, 1–42. [Google Scholar] [CrossRef]
  53. Lendvay, G. On the correlation of bond order and bond length. J. Mol. Struct. (Theochem) 2000, 501–502, 389–393. [Google Scholar] [CrossRef]
  54. Hehre, W.J.; Ditchfield, R.; Pople, J.A. Self-consistent molecular orbital methods. XII. Further Extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules. J. Chem. Phys. 1972, 56, 2257–2261. [Google Scholar] [CrossRef]
  55. Szekeres, Z.S.; Bogár, F.; Ladik, J. B3LYP, BLYP and PBE DFT band structures of the nucleotide base stacks. J. Quant. Chem. 2005, 102, 422–426. [Google Scholar] [CrossRef]
Scheme 1. The structures of the nickel and iron complexes with thermal stability.
Scheme 1. The structures of the nickel and iron complexes with thermal stability.
Catalysts 07 00120 sch001
Scheme 2. The description of minimum distance (D) and central five-membered ring.
Scheme 2. The description of minimum distance (D) and central five-membered ring.
Catalysts 07 00120 sch002
Scheme 3. The structures of bis(imino)pyridyliron complexes 15.
Scheme 3. The structures of bis(imino)pyridyliron complexes 15.
Catalysts 07 00120 sch003
Scheme 4. The structures of bis(imino)pyridyliron(cobalt) complexes 610.
Scheme 4. The structures of bis(imino)pyridyliron(cobalt) complexes 610.
Catalysts 07 00120 sch004
Figure 1. The plots of calculated value of thermal stability (Cal. AT) versus experimental results (Exp. AT) for complexes 15 (a) and complexes 610 (b).
Figure 1. The plots of calculated value of thermal stability (Cal. AT) versus experimental results (Exp. AT) for complexes 15 (a) and complexes 610 (b).
Catalysts 07 00120 g001
Figure 2. Comparisons of the average catalytic activities for complexes 15, 610 and complexes 1115 at room and optimum temperatures, respectively.
Figure 2. Comparisons of the average catalytic activities for complexes 15, 610 and complexes 1115 at room and optimum temperatures, respectively.
Catalysts 07 00120 g002
Scheme 5. The structures of phenanthrolinyliron complexes 1115.
Scheme 5. The structures of phenanthrolinyliron complexes 1115.
Catalysts 07 00120 sch005
Figure 3. The plots of calculated thermal stabilities (Cal. AT) versus experimental results (Exp. AT) for complexes 1115.
Figure 3. The plots of calculated thermal stabilities (Cal. AT) versus experimental results (Exp. AT) for complexes 1115.
Catalysts 07 00120 g003
Scheme 6. The structures of the acenaphthylnickel complexes 1620.
Scheme 6. The structures of the acenaphthylnickel complexes 1620.
Catalysts 07 00120 sch006
Figure 4. The plot of calculated thermal stability (Cal. AT) versus experimental thermal stability (Exp. AT) for complexes 1620.
Figure 4. The plot of calculated thermal stability (Cal. AT) versus experimental thermal stability (Exp. AT) for complexes 1620.
Catalysts 07 00120 g004
Table 1. The calculated bond lengths and bond angles compared with experimental crystal structures for complexes 1, 5, and 7 along with the values of standard deviation δ. MM, molecular mechanics.
Table 1. The calculated bond lengths and bond angles compared with experimental crystal structures for complexes 1, 5, and 7 along with the values of standard deviation δ. MM, molecular mechanics.
BondBond Lengths (Å)
Complex 1Complex 5Complex 7
X-rayMMX-rayMMX-rayMM
Fe–N12.2282.2012.2012.1952.2782.239
Fe–N22.0612.0722.0572.0612.1112.118
Fe–N32.1782.1952.1572.1962.2682.236
Fe–Cl12.3192.3192.2982.2982.2902.290
Fe–Cl22.2642.2642.2522.2522.2902.289
δ-0.768-0.834-0.925
AngleBond Angles (°)
Complex 1Complex 5Complex 7
X-rayMMX-rayMMX-rayMM
N1–Fe–N272.8073.3373.2173.2972.5273.64
N2–Fe–N374.0273.5474.5173.5173.9273.56
N1–Fe–N3141.12143.32142.21142.56146.42146.21
N1–Fe–Cl1101.7298.40101.3599.0996.8197.77
N2–Fe–Cl191.7292.6792.3392.33121.41121.32
N3–Fe–Cl199.0298.4399.1099.07101.1198.79
N1–Fe–Cl298.72100.4796.68100.1898.51100.62
N2–Fe–Cl2151.10150.26151.60150.52122.61121.12
N3–Fe–Cl2100.62100.45102.45100.1798.8197.02
Cl1–Fe–Cl2117.19117.07115.89117.16155.95154.65
δ-1.45-1.71-1.43
Table 2. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) for complexes 15.
Table 2. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) for complexes 15.
Original Values
ComplexBD (Å)α (°)AT
10.10482.4277.8395.313
20.10482.0738.2455.689
30.10282.2938.2067.710
40.10422.2218.2596.333
50.10482.1478.6274.082
Standardized Values
ComplexB D (Å)α (°)A T
10.59961.4286−1.4173−0.3838
20.5996−1.16750.0347−0.1022
3−1.70670.4459−0.10491.4116
4−0.0923−0.08210.08310.3802
50.2996−0.62481.4044−1.3058
Table 3. The original and standardized regression coefficients (x) values of multiple linear regression equation as well as correlation coefficients (R2) for the complexes. Or., original; St., standardized .
Table 3. The original and standardized regression coefficients (x) values of multiple linear regression equation as well as correlation coefficients (R2) for the complexes. Or., original; St., standardized .
ComplexesTypex0x1x2x3R2
Complexes 1–5Or.199.46−1493.89−4.909−3.2660.970
St.−1.0 × 10−14−0.970−0.501−0.683-
Complexes 6–10Or.−33.32197.025.1490.5360.949
St.−2.3 × 10−161.0810.6160.438-
Complexes 11–15Or.458.54−7450.733.5925.840.998
St.−6.1 × 10−16−0.2440.1101.002-
Complexes 16–20Or.2.344−32.8980.6090.0390.998
St.−1.9 × 10−16−0.9350.3510.166-
Table 4. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) for complexes 610.
Table 4. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) for complexes 610.
Original Values
ComplexBD (Å)α (°)AT
60.10192.3838.1872.719
70.10262.28011.4134.784
80.10312.6078.8855.474
90.09872.2567.3352.054
100.07882.7788.4090.974
Standardized Values
ComplexBD (Å)α (°)AT
60.4725−0.3456−0.4275−0.2560
70.5402−0.80321.66950.8408
80.58870.64950.02541.2072
90.1626−0.9098−0.9803−0.6092
10−1.76411.4092−0.2835−1.1828
Table 5. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) along with thermal stability (AT) for complexes 1115, respectively.
Table 5. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) along with thermal stability (AT) for complexes 1115, respectively.
Original Values
ComplexBD (Å)α (°)AT
110.07703.6771.81555.276
120.07863.6241.30928.386
130.07693.5220.1898.548
140.07743.6890.0936.992
150.07743.6600.1329.585
Standardized Values
ComplexB D (Å)α (°)AT
11−0.67970.63171.38251.622
121.6845−0.15420.75100.321
13−0.8275−1.6666−0.6467−0.639
14−0.08870.8096−0.7677−0.714
15−0.08870.3796−0.7191−0.589
Table 6. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) along with thermal stability (AT) for complexes 1620, respectively.
Table 6. The original and standardized values of bond order (B), minimum distance (D) and dihedral angle (α) along with thermal stability (AT) for complexes 1620, respectively.
Original Values
ComplexBD (Å)α (°)AT
160.11142.6710.2070.337
170.09932.6264.2450.845
180.10172.2511.4120.414
190.08362.7840.1391.294
200.11292.8730.2860.370
Standardized Values
ComplexBD (Å)α (°)AT
160.81830.126−0.601−0.761
17−0.2110−0.0631.7070.466
18−0.0068−1.635−0.088−0.575
19−1.54640.560−0.6391.552
200.94580.973−0.555−0.682
Table 7. The values of bond order (B) by different functional and basis sets and correlation results (R2) with bond lengths (R) from experiments.
Table 7. The values of bond order (B) by different functional and basis sets and correlation results (R2) with bond lengths (R) from experiments.
BondBond Order (B)Bond Length (R)
B3LYP/6–31G*B3LYP/6–311G**BLYP/6–311G**BP86/6–311G**Experiments
Fe–N10.07680.05390.07550.07612.391
Fe–N20.14660.11540.14280.14292.100
Fe–N30.11010.09260.12280.12382.298
R20.9630.5240.8100.800-

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Back to TopTop