Theoretical Mechanistic Studies on Redox-Switchable Polymerization

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Theoretical Mechanistic Studies on Redox-Switchable Polymerization of Trimethylene Carbonate Catalyzed by an Indium Complex Bearing a Ferrocene-Based Ligand Xiaowei Xu,† Gen Luo,† Andleeb Mehmood,† Yanan Zhao,† Guangli Zhou,† Zhaomin Hou,*,‡ and Yi Luo*,† †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, and Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

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‡

S Supporting Information *

ABSTRACT: The redox-switchable mechanism of trimethylene carbonate (TMC) polymerization catalyzed by an indium complex bearing a ferrocene-based alkoxide phosfen chelating ligand has been elucidated by density functional theory (DFT) calculations. Having achieved agreement between computational results and available experimental findings, it is found that the ring-opening of TMC moiety has higher energy barrier than that for migratory insertion and serves as a rate-determining step of the polymerization reaction. In comparison with the reduced state of the indium complex, the experimentally observed higher activity of the oxidized form could attribute to the oxidation-induced elongation and thus weakening of the In−N bond of the complex, which strengthened the interaction between the TMC unit and indium metal center and hence stabilized the corresponding transition states. Such geometry and binding changes upon oxidation are manifested by the analyses of structure, bond indexes, and energy decomposition.

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bis(phenolato) ferrocene-based ligands.3c−f Their results indicated that these catalysts showed diverse redox-switchable performances. Besides, yttrium and indium alkoxide phosfen (phosfen = 1,1′-di(2-tert-butyl-6-diphenyl-phosphin-iminophenoxy) ferrocene) complexes were also found to be effective for redox-switchable homopolymerization of lactide (LA) and trimethylene carbonate (TMC), respectively.4 The yttrium complex showed behavior similar to that of group 4 metal systems where the polymerization activity of both monomers decreases upon oxidation of the ferrocene group. However, the indium complex showed the opposite switchable behavior in TMC polymerization where the activity of oxidized indium complex was higher than its reduced state (Scheme 1). However, the related switchable mechanism needs to be further investigated. Regarding the redox-switchable control mechanism of polymerization processes, a few studies have been reported to date. In 2011, Diaconescu et al. found that the activity of reduced cerium(III) alkoxide complexes bearing Schiff base ligand was higher than their oxidized form(IV) during the ringopening polymerization of L-lactide.2e The related DFT calculations suggested that more negative charge on the oxygen atom of alkoxide in Ce(III) species increased the nucleophilicity of the alkoxide ligand in comparison with the Ce(IV) complexes, accounting for the higher activity of the reduced state. Recently,

INTRODUCTION Redox-switchable catalysis, being capable of serving as an effective polymerization methodology, has attracted considerable attention because it can not only modulate polymerization process and polymer microstructure but provide biodegradable polymers from biomass-derived monomers.1 It is hitherto known that the redox-switchable catalysts possess redox-active centers at either the catalytic metal center2 or the ancillary ligand.3 The mechanism behind the redox-switch is complicated, and the related knowledge is very limited. In the case of redox-active metal center as the site of catalysis, the change of oxidation state of the catalytically active metal may directly lead to different reactivity toward substrates. In the case of redox-active ligand, however, the switchable reactivity of the catalytic center could be achieved through redox control of the ligand. The pioneering work on the redox-switchable polymerization was reported by Long et al.3a In their work, the redoxswitchable polymerization of rac-lactide catalyzed by a titanium bis(iminophenoxide)(salen) complex bearing two ferrocenyl units located on the outer extremities of the ligand scaffold was achieved by altering the oxidation state of Fe center in ferrocenyl units. Some other works found that group 4 metal complexes exhibited unique catalytic performance toward various monomer polymerizations achieved through redox of ligand framework.3b−f For instance, Diaconescu’s group demonstrated redox-switchable polymerization of diverse biomass-derived monomers catalyzed by a series of titanium and zirconium alkoxide complexes bearing the [OEEO]-type (E = N, S) © XXXX American Chemical Society

Received: August 18, 2018

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DOI: 10.1021/acs.organomet.8b00599 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

effective core potential (ECP)8 together with associated basis sets was utilized for In and Fe atoms. Such basis sets are donated as BSI. Single-point energy calculations were performed for the B3LYP/BSI geometries by using the M06 functional9 and a larger basis set BSII. In the BSII, 6-311G** was used for nonmetal atoms, and the basis sets for Fe and In atoms are same as those in geometry optimizations. A d-polarization function (exponent of 0.143)10 was augmented for In atom. In these single-point calculations, benzene solvation effects were considered with the CPCM model.11 The Gibbs free energy correction obtained from frequency calculation was added to the single-point energy to estimate the free energy in solution. It is noted that the energies of stationary points calculated in more polar THF are higher by 1.5−4.3 kcal/mol than the corresponding ones calculated in benzene (Figure S5), but the relative trends are consistent with each other. The energy profiles were described by the relative free energies in solution (ΔG, kcal/mol). The corresponding relative enthalpies (ΔH, kcal/mol) are also provided for estimation of entropy effects. All calculations were carried out by using Gaussian 09 program package.12

Scheme 1. Polymerization of Trimethylene Carbonate Catalyzed by Indium Complexesa

a Oxidized form ox-In and reduced species red-In bearing a ferrocenediyl unit.

the same group investigated the (co)polymerization mechanism of cyclic ester and cyclohexene oxide by redox-switchable catalyst (thiolfan*)Al(OtBu) (thiolfan* = 1,1′-di(2,4-di-tertbutyl-6-thiophenoxy) ferrocene)5 or (salfan)Zr(OtBu)2, (salfan = 1,1′-di(2-tert-butyl-6-N-methylmethylenephenoxy) ferrocene).3f The results indicated that the coordination effect of the carbonyl group of the monomer unit and the comonomer effect played an important role in switchable (co)polymerizations. However, the role of 1,1′-ferrocenediyl (fc) unit in the redoxswitchable behavior has not been fully understood. Stimulated by our previous mechanistic studies on olefin polymerization catalyzed by organometallic complexes,6 we have become interested in the polymerization mechanism of biobased monomers, especially for that mediated by redoxswitchable catalysts. In the present work, DFT method has been applied to investigate the mechanism of TMC polymerization catalyzed by the indium alkoxide phosfen complex (Scheme 1). The redox-switchable control of such a complex has been explored in detail by analyzing the structure and energy aspects during the polymerization. The results indicate that the redox of fc unit has an impact on the coordination ability of the catalytic metal center to the monomer and further on the subsequent reaction steps.

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RESULTS AND DISCUSSION Active Species. On the basis of optimized complexes ox-In (oxidized species) and red-In (reduced species) at various electronic states, the relative electronic energies are compared (Table S1). The result indicates that ox-In and red-In have ground states of singlet and doublet, respectively. Therefore, these two structures (Figure 1) at their lowest states in energy were used for mechanistic calculations, respectively. As shown in Figure 1, the C−N bond lengths (C1−N1 and C2−N2) in ox-In are slightly shorter than those in red-In, while the In−N (In−N1 and In−N2) and NP bond lengths in ox-In are longer. In view of the p−π conjugation between the N atoms and fc unit, such geometrical changes could be the outcome of the redox event occurring at the fc unit. Upon redox, the In−O bond lengths (In−O1, In−O2, and In−O3) in ox-In are slightly shorter than that in red-In. Besides the oxidation induced geometrical changes, it is also noted that the Mulliken charge on Fe atom in ox-In is less negative than red-In by 0.11 (−0.422 vs −0.532). This demonstrates again that the oxidation occurred at Fe center, being in good agreement with the XANES and Mössbauer spectroscopy data showing oxidation of the iron center.4 Chain Initiation Stage. The TMC polymerization catalyzed by metal complexes is considered to follow coordination−

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COMPUTATIONAL DETAILS The B3LYP functional7 was used for geometry optimization and subsequent frequency calculation. The 6-31G* basis set was used for C, H, O, N, and P atoms, and the Stuttgart/Dresden

Figure 1. Optimized geometric structures (distances in Å) of indium complexes (ox-In at doublet and red-In at singlet) bearing a ferrocenyl unit. The aromatic substituents of phosfen ligand are shown as wireframe type for clarity. B


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Organometallics

Figure 2. Computed energy profiles for chain initiation step of the ROP of TMC catalyzed by indium complexes (red-In and ox-In). Free energies are relative to the energy sum of isolated reactants. (In the labeling of stationary points, the “ox” and “red” denote oxidized and reduced forms, respectively. C represents TMC coordinating complex; TS1 stands for the transition states of migratory insertion; TS2 means the transition states of ring-opening; INT1 and INT2 represent the intermediates; P denotes the product of the first monomer insertion).

Figure 3. Isomerization of ox-INT1 to ox-INT2 or ox-INT2’, differing from coordinative oxygen atoms. The energies are relative to the corresponding initial catalyst and TMC.

insertion mechanism featuring migratory insertion of alkoxide and subsequent ring-opening. As shown in Figure 2, oxidized form ox-In initiated reaction starts with the formation of coordination complex ox-C, in which carbonyl oxygen of TMC

coordinates to In atom, and then goes through the migrateinsertion transition state ox-TS1, which leads to ox-INT1. Subsequently, the intermediate ox-INT1 favorably isomerizes to more stable ox-INT2 via rotation of the ring moiety of TMC C


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Figure 4. Structures and energy decomposition analyses of the transition states ox-TS1 and red-TS1. (a) Chemdraw-type structures. (b) Optimized structures (distances in Å). (c) Energy decomposition analyses (kcal/mol).

energy than that for ox-In case aforementioned, suggesting a more favorable ox-In mediated reaction. This is in line with the experimental finding that the oxidized form showed higher activity toward TMC polymerization in comparison with its reduced analogue. To have better understanding of the reactivity discrepancy between the two states, the structures of the two insertion transition states have been compared. It is found that In−N distances in ox-TS1 are significantly longer than that in red-TS1 (2.44 vs 2.31 Å and 2.30 vs 2.24 Å, Figure 4). One can expect that the elongation of In−N bond in ox-TS1 could increase the ability of In center to bind with the remained surrounding ligand such as the TMC unit. To confirm this, the energy decomposition analysis14,15 was performed for the two transition states. In the energy decomposition analysis, the energies of the monomer moiety (fragment B) and the remaining metal complex (fragment A) 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 geometry, allow for the estimation of the deformation energies of the two fragments, ΔEdef (A) and ΔEdef (B). The deformation energy of a fragment is defined as the energy difference between its distorted geometry in TS and its optimized structure. As the energy of the TS, ΔETS, is evaluated with respect to the energy of

around C−O2 bond (see the structures in Figure 3). Instead of the interaction of In···OPh in ox-INT1, a new coordinative interaction between In and O3 atoms exists in ox-INT2. It is noted that such an isomerization could also yield an intermediate ox-INT2′ (Figure 3) having an interaction of In···O4. Although it is more stable than ox-INT2 by 3.0 kcal/ mol, the subsequent ring-opening event of ox-INT2′ suffered from much higher energy barrier compared with ox-INT2 (40.7 vs 19.8 kcal/mol, Figures S1 and 2), which is mainly due to the larger geometrical distortion in the former case (Figure S2). Therefore, ox-INT2′ was not considered further. ox-INT2 could subsequently undergo ring-opening through ox-TS2 with an overall free energy barrier of 19.8 kcal/mol. Such a barrier is higher than that for the insertion step (13.5 kcal/mol for oxTS1, Figure 2). The resulting ring-opening product ox-P has a relative energy of 10.9 kcal/mol. Although the chain initiation step is endergonic due to overestimation of entropy effect and stability of six-membered TMC, ox-P involved chain propagation is less endergonic (vide infra). Such an energy change was also previously observed for ring-opening polymerization of TMC.5,13 Meanwhile, in view of the change in enthalpy, ox-In mediated chain initiation is exothermic by 2.4 kcal/mol. Similarly, in the case of reduced species red-In, the chain initiation occurred through the pathway of red-C → red-TS1 → red-INT1 → red-INT2 → red-TS2 → red-P (Figure 2). However, the energies of all these stationary points are higher in D


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Organometallics the two separated fragments, the relation ΔETS = ΔEint + ΔEdef(A) + ΔEdef(B) holds. As shown in Figure 4, in the case of ox-TS1, the total deformation energy ΔEdef is 55.3 (24.5 + 30.8) kcal/mol, which could be balanced out by its ΔEint (−60.1 kcal/ mol) leading to ΔETS of −4.8 kcal/mol. By contrast, the less negative interaction energy in red-TS1 (−54.9 kcal/mol) could not compensate its larger total deformation energy (ΔEdef = 56.6 kcal/mol), thus producing a higher ΔETS (1.7 kcal/mol). Therefore, the stronger interaction between the monomer moiety and the metal center could account for the higher stability of ox-TS1, which is also in line with the result of geometrical analysis aforementioned. It is noteworthy that it is arbitrary to divide TS2 structure into two fragments for such an energy decomposition analysis. Considering that the energy gap between the oxidized and reduced forms also exists in TS1. Therefore, the energy decomposition analysis on TS1 could also provide useful information for understanding the difference in activity between the oxidized and reduced forms. To obtain more knowledge about the origin of diverse activity of the two states, the rate-determining transition structures (redTS2 and ox-TS2, Figure 2) have been further analyzed. As a result of the oxidation, the electron density of Fe center of the oxidized species should be decreased, as suggested by the Mulliken charge (−0.44 in ox-TS2 vs −0.54 in red-TS2, Table 1). Along with this change, the average charges on two Cps and

stabilized the transition state ox-TS2, accounting for the higher activity of oxidized state. In the first monomer insertion product, there is no chelation effect between the In atom and the CO group, as manifested by the distance of more than 4.5 Å between the In and carbonyl oxygen atoms (Figure S3). This is different from Al-catalyzed TMC polymerization system, where the CO chelation structure was observed.5 In the current system, after the insertion of the first monomer, the discoordination of the CO group allows the coordination of the second monomer. Chain Propagation Stage. On the basis of the ringopening product of the first monomer, the reaction of the second monomer has been calculated to model the chain propagation. By these calculations, one can see whether the higher activity of oxidized state can be reproduced computationally and whether the electron-induced stronger binding between the incoming monomer unit and the In center exists at this stage. As shown in Figure 5, in the case of oxidized state, the coordination− insertion of the incoming monomer into ox-P takes place via ox2TS1 with a free energy barrier of 18.1 kcal/mol. The resulting intermediate, ox-2INT2, subsequently undergoes ring-opening through ox-2TS2 to give product ox-2P. Such a chain propagation step has an overall free energy barrier of 20.9 kcal/mol. It is obvious that the reaction of the second monomer is exergonic by (6.7 − 10.9 =) 4.2 kcal/mol although it is endergonic in the chain initiation step (Figure 2). In view of the lower energy of ox-2P than that of ox-P in the whole process, the chain growth should be eventually an exergonic process because of continuous formation of new chemical bonds. In the case of the reduced state, the reaction is both kinetically and thermodynamically unfavorable compared with the case of oxidized form, as indicated by the reaction pathway of red-P (Figure 5). To further access the origin of lower reactivity of the reduced species, similar energy decomposition analyses have been comparatively carried out for migratory insertion transition states red-2TS1 and ox-2TS1. As shown in Figure 6, both larger total deformation energy (17.6 + 22.5 = 40.1 vs 16.0 + 21.4 = 37.4 kcal/mol) and weaker interaction between the incoming TMC moiety and the remaining metal complex (−43.2 vs −47.9 kcal/mol) made red-2TS1 less stable in comparison with ox2TS1. Also, the interaction between the two fragments (TMC moiety and the remaining part) is dominant in the stability of the two transition states. It is noteworthy that the geometries of the two transition states indicate unexpected O···H hydrogen bonds between the fc unit and the carbonyl group of the incoming TMC unit (Figure S4). These hydrogen bonds could stabilize such transition structures. The hydrogen bond lengths in ox2TS1 are shorter than that in red-2TS1 (2.23/2.46 vs 2.34/2.67 Å, respectively, all shorter than the sum of van der Waals radius of O and H atoms 1.5 + 1.2 = 2.7 Å), which could also account for the stronger interaction between the two fragments of ox2TS1 and resulting higher stability. Considering that the ring-opening is still the rate-determining step in the chain propagation (Figure 5), the structural and electronic characters of ring-opening transition states ox-2TS2 and red-2TS2 have also been analyzed, with the purpose of investigating the factors relevant to higher reactivity of oxidized species. Like the chain initiation step, there is also hydrogen bond interactions between fc and the growing chain in the ringopening transition state. Moreover, such an interaction is stronger in ox-2TS2 than red-2TS2, as manifested by the shorter hydrogen bond lengths (2.19/2.24 vs 2.28/2.26 Å,

Table 1. Parameters of the Ring-Opening Transition States in the Oxidized and Reduced Systemsa ox-TS2 In−N1 In−N2 In−O2 In−O3 N1−C(Cp) N2−C(Cp) In−N1 In−N2 In−O2 In−O3 N1−C(Cp) N2−C(Cp) Fe CpN1 moiety CpN2 moiety

red-TS2

ox-2TS2

Bond Length (Å) 2.23 2.19 2.23 2.49 2.30 2.47 2.57 2.89 2.67 2.05 2.07 2.05 1.41 1.43 1.41 1.37 1.42 1.38 Wiberg Bond Index (WBI) 0.18 0.19 0.18 0.11 0.14 0.11 0.10 0.08 0.09 0.21 0.18 0.21 1.07 1.02 1.07 1.20 1.04 1.18 Mulliken Charge −0.44 −0.54 −0.46 −0.10 −0.40 −0.09 −0.18 −0.41 −0.20

red-2TS2 2.18 2.30 2.94 2.08 1.43 1.42 0.20 0.14 0.08 0.18 1.02 1.04 −0.56 −0.40 −0.40

a

Atom labeling defined in Figures 2 and 5.

neighboring N atoms also become less negative in oxidized state (−0.10 and −0.18 vs −0.40 and −0.41). Such electron effects induced shorter N−C(Cp) distances (1.41 and 1.37 Å vs 1.43 and 1.42 Å) and longer In−N contacts in oxidized species (2.23 and 2.49 Å vs 2.19 and 2.30 Å, Table 1). These changes made In−N interactions weaker in ox-TS2 in comparison with redTS2, as manifested by the Wiberg bond index (WBI, 0.11 vs 0.14, Table 1). Upon this change, the In center binds more tightly to the connecting oxygen atoms in ox-TS2. This is suggested by that the calculated interaction energy between the TMC involved anionic moiety and the remaining cationic metal complex in ox-TS2 is larger than that in red-TS2 by 50.9 kcal/ mol. Such a stronger interaction caused by oxidation event E


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Figure 5. Computed energy profiles for chain propagation step of TMC ROP catalyzed by indium complexes (red-In and ox-In). Free energies are relative to the energy sum of isolated reactants. (The labeling of stationary points is similar to that in Figure 2, whereas the middle number “2” means the second monomer reaction)

monomer unit with the In metal center and therefore stabilized the oxidized species. It is noteworthy that a recent theoretical work3g reported the ε-caprolactone polymerization catalyzed by Fe complexes, where the redox-active bis(imino)pyridine ligand directly binds to catalytic Fe center, unlike the current system having N atoms as a spacer between redox-active fc unit and catalytic In center. However, no mechanism of redox switch was elucidated for the polymerization event in that work.

Figure S4). This could also contribute to the higher stability of ox-2TS2. In contrast, it is observed that the oxidation-induced electron deficiency of Fe center shortened N−C(Cp) distances and therefore elongated In−N contacts (see bond lengths in Table 1). These electronic effects weakened In···N interaction and strengthened In−O bond in ox-2TS2 (see WBI in Table 1). Therefore, the incoming TMC unit bound more tightly to the In center and stabilized the oxidized species. Such a stronger binding is also evidenced by the result that the interaction energy between the growing chain and the remaining metal complex is higher in ox-2TS2 than red-2TS2 by 53.1 kcal/mol. These situations are similar to that in chain initiation step. On the basis of the computational results aforementioned, the higher activity of oxidized species could be ascribed to the oxidation-induced electronic effect that weakened the In···N interaction and strengthened the interaction of incoming

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CONCLUSION The redox-switchable polymerization mechanism of trimethylene carbonate (TMC) catalyzed by an indium alkoxide phosfen complex bearing ferrocene-based ligand has been elucidated by DFT calculations. TMC undergoes the general nucleophilic attack and subsequent ring-opening to achieve its ring-opening polymerization, and the latter is the rate-determining step. The F


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Figure 6. Chemdraw-type (a) and optimized structures (b) of the transition states ox-2TS1 and red-2TS1 (distances in Å) as well as their energy decomposition analyses (c). Energies are given in kcal/mol.

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calculation results indicate that the oxidized state of the indium complex has higher activity than the reduced analogue, being in agreement with experimental observations. The detailed structure and energy analyses revealed the origin of the redoxswitch behavior of the indium complex catalyst. Although the redox occurred at the fc unit, the electronic effect induced longer coordinative In···N (ligand) bonds upon the oxidation and thus increased the coordination ability of the indium center, which makes the monomer unit bind to the indium center stronger and thus stabilized the oxidized species. This could account for the higher reactivity of oxidized indium complex compared with its reduced state. Such knowledge could be helpful for further development of highly efficient redox-switchable polymerization catalysts. There are many other interesting redox-switchable polymerization systems demonstrated experimentally. For example, unlike the current In system, the Y analogue showed different redox-switchable behavior toward TMC polymerization, which might be originated from the combination of multiple factors and the interplay between the metal and its ligand. The theoretical mechanistic studies of the other fascinating redox-switchable polymerization systems will be published in due course.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00599. Relative energies of various spin states of catalysts, energy profiles for ring-opening of Int2′, and optimized structures of key stationary points (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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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.H.). *E-mail: [email protected] (Y.L.). ORCID

Gen Luo: 0000-0002-5297-6756 Yanan Zhao: 0000-0002-3928-3429 Zhaomin Hou: 0000-0003-2841-5120 G


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Yi Luo: 0000-0001-6390-8639 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (No. 21429201, 21674014). Y.L. and G.L. thank the Fundamental Research Funds for the Central Universities (DUT2016TB08, DUT18GJ201, and DUT18RC(3)002). We also thank RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology for part of computational resources.

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Theoretical Mechanistic Studies on Redox-Switchable Polymerization

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