The Role of Alkoxide Initiator, Spin State, and Oxidation State in Ring

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

The Role of Alkoxide Initiator, Spin State, and Oxidation State in Ring-Opening Polymerization of ε‑Caprolactone Catalyzed by Iron Bis(imino)pyridine Complexes Manuel A. Ortuño,† Büsra Dereli,† Kayla R. Delle Chiaie,‡ Ashley B. Biernesser,‡ Miao Qi,‡ Jeffery A. Byers,‡ and Christopher J. Cramer*,† †

Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ‡ Eugene F. Merkert Chemistry Center, Department of Chemistry, Boston College, 2609 Beacon Street, Chestnut Hill, Massachusetts 02467, United States S Supporting Information *

ABSTRACT: Density functional theory (DFT) is employed to characterize in detail the mechanism for the ring-opening polymerization (ROP) of ε-caprolactone catalyzed by iron alkoxide complexes bearing redox-active bis(imino)pyridine ligands. The combination of iron with the non-innocent bis(imino)pyridine ligand permits comparison of catalytic activity as a function of oxidation state (and overall spin state). The reactivities of aryl oxide versus alkoxide initiators for the ROP of ε-caprolactone are also examined. An experimental test of a computational prediction reveals an Fe(III) bis(imino)pyridine bis-neopentoxide complex to be competent for ROP of ε-caprolactone.

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INTRODUCTION One means to improve the sustainability associated with the manufacture of a given material is to engineer the material to be biodegradable.1 Polyesters are generally amenable to biodegradation, and a popular approach to the construction of such polymers is the controlled ring-opening polymerization (ROP) of cyclic esters (i.e., lactones) derived from various sources.2,3 Ring-opening polymerization is commonly catalyzed by coordination complexes.4 As catalysts, iron-based complexes are appealing as iron is earth-abundant and exhibits versatile chemical reactivity.5 However, the few iron catalysts reported for lactone polymerizations thus far6−13 have exhibited low activities and/or yielded broad molecular weight distributions (polymerizations of lactide and cyclic carbonates have also been reported6,7,14). Nevertheless, iron-based systems remain of special interest because they offer unique opportunities for tuning of the catalyst spin state, overall charge, and oxidation stateeither of the metal or the ligandover a broad range of accessible redox potentials. Controlled polymerizations have been reported that have exploited the redox tunability of iron, although the metal itself was not the active site for catalysis,15,16 and some of us17−20 have also recently reported iron-alkoxide catalysts based on the bis(imino)pyridine framework21,22 that efficiently polymerize a wide variety of oxygenated monomers. In these latter cases, the combination of an iron center with the redox non-innocence of the bis(imino)pyridine ligand, which has been previously exploited for a number of other purposes,23−25 enables fine control over the electronic properties of the various complexes and their related catalytic © XXXX American Chemical Society

activities (Scheme 1). In particular, neutral monoalkoxide bis(imino)pyridine “Fe(I)” complex 1a is very active for the Scheme 1. Iron-Catalyzed Ring-Opening Polymerization of ε-Caprolactone

polymerization of lactones.20 For example, ε-caprolactone polymerizes at room temperature in 10 min with 99% conversion at 0.05 mol % catalyst loading. The corresponding aryloxide derivative 1b, by contrast, takes 24 h to reach 80% conversion at 2.0 mol % catalyst loading, which suggests that polymer initiation (in addition to propagation) is a considerable factor that dictates the overall reactivity of the catalyst. In addition to this factor, neutral bis-alkoxide Fe(II) complexes 1c also function for polymerization, but they are much less active as a catalyst, reaching only 8% conversion after 10 min. These results combined highlight the importance of the Received: November 21, 2017

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DOI: 10.1021/acs.inorgchem.7b02964 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

hexafluorophosphate (0.6 mg, 1.81 μmol) was then added to the vial. The reaction mixture was stirred for 10 min at room temperature, and the solution turned blue. Additional ε-caprolactone (99.0 mg, 867 μmol) was then added to the vial and stirred at room temperature. Complete conversion was seen after 60 min to yield a polymer with Mn = 48.9 kg mol−1 (Mn(theor) = 56.4 kg mol−1) and Mw/Mn = 1.2.

nature of the alkoxide initiator and the electronic structure of the iron center, but a detailed understanding of these influences remains to be established. We here employ computational modeling to develop mechanistic understanding at the atomic level of detail for the polymerization of ε-caprolactone26 catalyzed by bis(imino)pyridine iron complexes (Scheme 1), paying special attention to the particular roles of alkoxide initiators, spin state, and oxidation state, the tunabilities of which render these catalysts so versatile. We predict the Fe(III) oxidation state to be competent for catalysis and report experiments confirming that prediction. We note that computational studies of iron catalysts for ring-opening polymerization of lactones have not previously been reported,27,28 although substantial work has appeared for Al-based catalysts,29−31 other metals such as Sc, Zn, Y, Sn, La, U,32 and enzymes.33

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RESULTS AND DISCUSSION Many lactide polymerization reactions catalyzed by metals proceed through a coordination−insertion mechanism,29 which is presumed to be the mechanism for ε-caprolactone polymerization catalyzed by bis(imino)pyridine iron alkoxide complexes.51 A prototypical coordination−insertion mechanism is shown in Scheme 2a and entails (i) coordination of monomer Scheme 2. (a) General Coordination−Insertion Mechanism for the Ring-Opening Polymerization of ε-Caprolactone and (b) Iron-Based Catalysts Investigated in This Study

METHODS

Computational Details. All calculations were performed at the density functional theory (DFT) level34 using the M06-L local density functional35 as implemented in Gaussian 09.36 The M06-L density functional accounts for medium-range electron correlation effects,37 has proven robust for the modeling of redox non-innocent ligands38 and iron spin-state splitting energies,20,39 and partially captures the multireference character of bis(imino)pyridine ligands.40 Numerical integrations were performed with an ultrafine grid. An automatic density-fitting set generated by the Gaussian program was employed to reduce the computational cost. Geometry optimizations were performed in the gas phase with the Def2-TZVP basis set for iron, and Def2-SVP basis sets were used for the rest of the atoms.41 The natures of all intermediate and transition-state (TS) structures were confirmed by analytic computation of their vibrational frequencies at 298.15 K; all minima were characterized by zero imaginary frequencies while all transition-state structures had exactly one. Transition-state structures were verified to connect with the corresponding reactants and products by following normal modes associated with their characteristic imaginary frequencies. All frequencies below 50 cm−1 were replaced by 50 cm−1 when computing vibrational partition functions.42 Selected geometry optimizations and single-point calculations were performed using different density functional approaches: TPSSh-D3BJ,43−45 MN15,46 PW6B95,47 and ω-B97XD.48 Solvent corrections in toluene (ε = 2.374), the solvent used experimentally,20 were added to gas-phase geometries using the SMD continuum model.49 For highest accuracy, single-point calculations in solution were performed using the Def2-TZVP basis set for iron and Def2-TZVPD basis sets for the rest of the atoms.41,50 For all species a factor of RT × ln(24.46) was added to account for the 1 atm to 1 M standard-state change at 298.15 K. Experimental Details. General Considerations. Unless stated otherwise, all reactions were carried out in oven-dried glassware in a nitrogen-filled glovebox. Solvents were used after passage through a solvent purification system under a blanket of argon and then degassed briefly by exposure to vacuum. Nuclear magnetic resonance (NMR) spectra were recorded at ambient temperature on spectrometers operating at 400−600 MHz for 1 H NMR. Gel permeation chromatography (GPC) was performed on an Agilent GPC220 in THF at 40 °C with three PL gel columns (10 μm) in series. Molecular weights and molecular weight distributions were determined from the signal response of the RI detector relative to polystyrene standards. εCaprolactone was dried over CaH2 and distilled prior to polymerization. The doubly alkoxide substituted analogue of 1d was synthesized as described previously.18,20 General Procedure for the Polymerization of ε-Caprolactone with a Doubly Alkoxide Substituted Analogue of 1d. To a 7 mL vial with stir bar was added 2,6-dimethylphenyl bis(imino)pyridine iron bis(neopentoxide) (1.05 mg, 1.75 μmol) in toluene (2.00 mL). εCaprolactone (1 mg, 8.76 μmol) was then added to the vial and stirred for 10 min to allow the complex to initiate. Ferrocenium

to the catalyst 1 to generate coordination complex 2, (ii) insertion of the alkoxide into the activated carbonyl group leading to an orthoalkoxide intermediate 3, (iii) reorganization of the ether oxygen atoms about iron to deliver complex 4, and (iv) a ring-opening step that elongates the polymer chain by one unit and provides new alkoxide 5. Polymer chain growth then continues by further incorporation of monomer in this cycle. Scheme 2b collects the iron catalysts considered in this mechanistic study: “Fe(I)”-ethoxide 1a, “Fe(I)”-4-methoxyphenoxide 1b, Fe(II)-bis-ethoxide 1c, and cationic “Fe(III)”bis-ethoxide 1d. Ethoxide is a simplified model for neopentoxide, which was used experimentally,20 and Fe(I) and Fe(III) are in quotation marks to emphasize that these are formal assignments of oxidation state, as described further below. Electronic Structure of Iron-Based Catalysts. Previously reported experiments and theory support a quartet spin state as the ground state electronic structure for the formally Fe(I) alkoxide species.20 Figure 1 shows spin density surfaces for quartet 1a and 1b, where roughly 4 unpaired electrons are located on the iron center and 1 unpaired electron with opposite spin is found on the bis(imino)pyridine ligand. Thus, these formally Fe(I) complexes are best described as high-spin Fe(II) centers antiferromagnetically coupled to a singly reduced B


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Inorganic Chemistry

step of ε-caprolactone polymerization in the quartet and doublet surfaces of 1a. Gibbs free energy reaction coordinates for the initiation step of ε-caprolactone polymerization are shown in Figure 2, where

Figure 2. Reaction coordinates (G298, kcal mol−1) for the initiation step of ε-caprolactone polymerization with 1a. Quartet surface in blue, doublet surface in red.

Figure 1. Mulliken spin densities (au, surfaces with 0.006 isovalue) for catalysts 1a−1d and 1d-PF6. L = bis(imino)pyridine, Ar = 4-methoxyphenoxide.

infinitely separated quartet 1a and ε-caprolactone are taken as the zero of relative energy. On the quartet spin-state surface (blue line), coordination of monomer to 1a is slightly endergonic to deliver 2a at 1.0 kcal mol−1. The weak endergonicity derives from loss of entropy, recalling that these are standard-state free energies (i.e., assuming 1 M concentration for all substrates); at very high ε-caprolactone:catalyst ratios, however, the equilibrium would be expected to favor bound complex. Insertion of ethoxide into the activated carbonyl group via TS2-3a proceeds with an activation free energy of 13.5 kcal mol−1 relative to separated reactants and leads to reactive intermediate 3a at 12.6 kcal mol−1. Substrate 3a undergoes reorganization of the oxygen coordination environment about the metal center31 via TS3-4a found at 19.7 kcal mol−1. The resulting intermediate 4a then undergoes ring opening via TS4-5a, which constitutes the ratedetermining (or turnover-limiting) step, having a relative activation free energy of 20.0 kcal mol−1 above 2a. Subsequent product 5a is poised to repeat addition of ε-caprolactone monomers to grow the polymer chain. The energy of intermediate 5a is predicted to be 5.5 kcal mol−1 above 2a, indicating that initiation is weakly endergonic (although subsequent decoordination of the terminal carbonyl group in order to bind a new monomer is predicted to be exergonic and moreover leads to an increase in entropy associated with overall ring opening, as further elaborated below in the context of propagation). A similar mechanistic scenario is found for the doublet spinstate surface (red line), although an orthoalkoxide intermediate having a κ1-coordination mode (3a′) is found that has no analogue on the quartet surface. The major difference between spin states is thermodynamics for binding ε-caprolactone, which is considerably less favorable in the doublet surface (2a at 15.0 kcal mol−1). Ring-opening via TS4-5a remains the ratedetermining step on the doublet surface, with an activation free energy of 31.7 kcal mol−1 relative to separated reactants on the same doublet surface. The two spin surfaces are predicted never to cross at the M06-L level, so there appear to be no issues of two-state reactivity associated with initiation/polymerization. This continues to be the case even for single-point calculations at the TPSSh-D3BJ level (Figure S2), which otherwise predict a doublet ground state for 1a by 0.9 kcal mol−1. Nevertheless, the

bis(imino)pyridine radical anion.52,53 As for the formally Fe(II) bisalkoxide complex 1c, theory predicts a quintet spin state as the ground state electronic structure in agreement with experimental data for an aryloxide derivative.18 Figure 1 displays the spin density surface for 1c, where again approximately 4 unpaired electrons are located on the iron center. There is nevertheless substantial charge transfer with associated spin polarization involving the ligands: the ethoxide groups are predicted to be slightly oxidized by about 0.5 electron with the charge balance localized on the bis(imino)pyridine ligand. With regard to the cationic, formally Fe(III) bis-alkoxide complex 1d, theory predicts a high-spin sextet state as the ground state electronic structure. From the spin density surface of 1d (Figure 1) we can infer that the bis(imino)pyridine ligand is mostly redox-innocent and that the ethoxide groups are oxidized by about 1 electron.54 These results hold for other population analyses and density functionals (Table S1). Since 1d has been experimentally isolated with PF6−,18 we also computed the ion pair 1d-PF6, but similar results as obtained with 1d were found (Figure 1). Compounds 1e and 1f, which are the aryloxide derivatives of Fe(II) 1c and formally Fe(III) 1d, respectively, exhibited the exact same features as 1c and 1d (Figure S1). Indeed, the computed reduction potential55 of −0.92 V (vs Fc/Fc+) for the 1e/1f couple (Table S2) is very similar to the experimental value of −0.84 V for 1f (Figure S4). Although this result does not directly prove the oxidation state of iron, the good agreement between theory and experiments further supports the ligand-based oxidation predicted by DFT. Therefore, we suggest that formally Fe(III) complexes are best described as high-spin Fe(II) centers with roughly 1 unpaired electron residing on the alkoxide groups. Spin State Influence on Initiation. As opposed to many metals used for ring-opening polymerization, the use of iron compounds as polymerization catalysts introduces the consideration that different spin states may exist along the reaction coordinate, particularly in the presence of possibly noninnocent ligands. Therefore, the contribution that spin may have in these systems was assessed by computing the initiation C


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Inorganic Chemistry

modeled the initiation step for the 4-methoxy-phenoxide catalyst 1b and compare it to the previously discussed ethoxide analogue 1a. The same mechanism is assumed to apply for the polymerization via 1b as observed with 1a. Consistent with the reaction coordinate for 1a (Figure 2), we computed the initial catalyst 1b, the adduct 2b, the ring-opening transition state TS4-5b, and the product 5b in the quartet spin state. The free energies of 2b, TS4-5b, and 5b relative to separated reactants are predicted to be −2.8, 27.7, and 15.1 kcal mol−1, respectively. This leads to a predicted activation free energy (from 2b to TS4-5b) of 30.5 kcal mol−1 (Figure 3). The

reduced Lewis acidity of the doublet state compared to the quartet leads to all intermediates along the reaction coordinate to continue to prefer the high spin state, with little variation in predicted relative energetics along that reaction path. Such agreement between a local and a hybrid functional increases confidence in the general conclusions with respect to the influence of spin state. Put succinctly, the low-spin (doublet) is not Lewis acidic enough to bind species sufficiently to promote polymerization. Instead, the initial ring opening of εcaprolactone catalyzed by 1a takes place on the high-spin quartet surface and the rate-determining step for initiation is the ring-opening process via TS4-5a which is 20.0 kcal mol−1 higher in energy than adduct 2a. Propagation. Considering the propagation of the polymer chain for quartet 1a (Scheme 3), the ester oxygen in 5a (6.5 Scheme 3. Coordination of a Second Monomer (6a) and Subsequent Incorporation into the Polymer Chain via Ring Opening (TS6a)a

a

Figure 3. Reaction coordinates (G298, kcal mol−1) for the ring-opening step of ε-caprolactone polymerization with 1a (ethoxide) and 1b (4methoxy-phenoxide).

Relative free energy in kcal mol−1.

kcal mol−1) is replaced by a second ε-caprolactone monomer to form 6a (2.8 kcal mol−1). Thus, the release of the carbonyl of the growing polymer chain and its replacement by the carbonyl of a new monomer is predicted to be exergonic by 3.7 kcal mol−1. The ring-opening TS structure for propagation, TS6a, is found at 20.1 kcal mol−1 above 6a, in essentially quantitative agreement with the value for the initiation step (20.0 kcal mol−1), as might be expected given the common nature of the linear alkoxide nucleophiles. We note that the net free energy change from 2a to 6a, which corresponds to addition of one monomer unit to the alkoxide chain of the lactone-coordinating catalyst, is predicted to have a free energy change of +1.8 kcal mol−1 when the monomer is at 1 M (standard state) concentration. We note, however, that theoretical predictions of ring-opening polymerization free energies usually employ only a single configuration for the growing polymer chainbecause of the substantial computational expense of sampling its significant configurational phase spaceand this approach inevitably underestimates the favorable entropy associated with ring opening.56 A rough estimate for the exergonicity associated with this entropic gain, at least for relatively small monomer rings, is nRT ln 3, where n is the number of single bonds in the ring. For n = 6, this is about 4 kcal mol−1, leading to an overall standard-state exergonicity of about −2.2 kcal mol−1, which is consistent with equilibrium polymerization proceeding to about 98% conversion. While this is an approximate approach for dealing with configurational entropy, its quite reasonable quantitative agreement with observed experimental conversions supports the applicability of the computational model. Alkoxide Influence on Initiation. Experimental results showed a dramatic influence of the initiating alkoxide group on polymerization reaction rates, where neopentoxide and 4methoxy-phenoxide complexes yielded 99% conversion in 10 min and 24 h, respectively.20 To explore this observation we

considerably larger activation energy predicted for initiation via 1b (30.5 kcal mol−1) compared to 1a (20.0 kcal mol−1) is consistent with the experimental observation of a more sluggish initiation with aryloxides than with alkoxides.20 It is worth noting that the ring opening of additional ε-caprolactone monomers during propagation is carried out by alkoxide groups demanding ca. 20 kcal mol−1 (Scheme 3), thus the large activation energy associated with aryloxides will only affect the initial stage of the polymerization by dictating an induction period. Figure 4 displays the transition-state structures for the ringopening step that occurs from the ethoxide (TS4-5a) and 4-

Figure 4. Optimized TS structures for TS4-5a and TS4-5b and relevant metric data.

methoxy-phenoxide (TS4-5b) initiators. Both geometries present very similar structural parameters. The only noteworthy differences we observe are that the breaking C−O1 bond distance appears to be slightly shorter for TS4-5a, and the N− Fe−O2 angle is larger for TS4-5b. The later character of the D


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Inorganic Chemistry

TS2-3c at 21.9 kcal mol−1 followed by rate-determining ring opening via TS4-5c at 26.5 kcal mol−1.57 Despite the different coordination number between “Fe(I)” and Fe(II) catalysts, the breaking C−O bond distance in TS4-5c (Figure 6 left) is quite

latter TS structure is consistent with the greater endothermicity associated with it for this elementary step. Based on computational analysis, the more electron-donating nature of the aliphatic alkoxide compared to the aromatic one plays two roles. First, increased electron density on the metal makes it less Lewis acidic so that ε-caprolactone is bound to iron 3.8 kcal mol−1 less strongly in 2a than in 2b; there is thus less of a thermodynamic sink associated with the initial complex in the aliphatic alkoxide system. In addition, there is less driving force promoting the ring-opening reaction via TS4-5b (27.7 kcal mol−1) to generate an aryl ester than via TS4-5a (21.0 kcal mol−1) to generate an alkyl ester. The same trend is observed for the products 5b (15.1 kcal mol−1) and 5a (6.5 kcal mol−1). Indeed, the free energies computed at the same level of theory for the reaction of free ε-caprolactone with ethanol and 4methoxyphenol to form the corresponding linear esters are −0.2 and 7.5 kcal mol−1, respectively (Figure S3). The ΔΔG of 7.7 kcal mol−1 favoring the aliphatic alkoxide ester for the free substrates is in line with the relative ΔΔG of 6.7 kcal mol−1 between TS4-5b and TS4-5a in the catalyzed system. A similar value of 8.6 kcal mol−1 is also found between 5b and 5a. These results reflect the relative stability of the esters that is directly related to the lower nucleophilicity of the aryloxide. Oxidation State Influence on Initiation. We now address the influence of the oxidation state of the iron-based catalysts by comparing the monoalkoxide “Fe(I)” complex 1a with the bis-alkoxide Fe(II) complex 1c and the cationic bis-alkoxide “Fe(III)” complex 1d. Both 1a and 1c are active catalysts toward ε-caprolactone polymerization, but 1a is considerably more active, reaching 99% conversion in 10 min while 1c only yielded 8% conversion in the same amount of time.20 Experiments for 1d are reported in the next section to validate the theoretical prediction. Figure 5 compares the Gibbs free energy reaction coordinates for the initiation step of ε-caprolactone polymer-

Figure 6. Optimized TS structures for TS4-5c and TS4-5d and relevant metric data.

similar to those for TS4-5a and TS4-5b (Figure 4). Based on literature precedent,58 we also evaluated a possible catalyst side reaction involving the liberation of an imine arm of the bis(imino)pyridine ligand in 2c, but the resulting structure (2cde, Figure S4) was found to be significantly higher in energy than 2c (20.5 kcal mol−1) and was not explored further. The larger activation free energy for 1c (26.5 kcal mol−1) agrees with the slower polymerizations experimentally observed when compared to 1a (20.0 kcal mol−1). However, it is not as large as that for the “Fe(I)” aryl alkoxide complex 1b (30.5 kcal mol−1), which explains why large induction periods were not detected for reactions catalyzed by 1c. Finally, Figure 5 also shows the Gibbs free energy reaction coordinates for the initiation step of ε-caprolactone polymerization via sextet “Fe(III)” 1d (purple line). The reaction coordinate of 1d has a higher activation free energy than that for 1a. However, due to the formal positive charge of the system, the carbonyl group now binds more strongly to iron in 2d (4.4 kcal mol−1) and 5d (6.8 kcal mol−1) compared to the neutral analogues 2c (11.5 kcal mol−1) and 5c (20.6 kcal mol−1). The rate-determining step is the ring-opening polymerization via TS4-5d at 24.0 kcal mol−1 (Figure 6, right). We also included the counterion PF6− in the initial catalyst 1d-PF6, the adduct 2d-PF6, and the ring-opening transition state TS4-5dPF6 (Figure 5, dashed purple line). The presence of PF6− “quenches” the positive charge of the complex as seen by the higher free energies of 2d-PF6 (9.1 kcal mol−1) and TS4-5-PF6 (26.3 kcal mol−1). In sum, calculations predict that 1d and 1d-PF6 should be similarly active to or more active than 1c but less active than 1a for the ring-opening polymerization of ε-caprolactone. This prediction is surprising insofar as cationic formally Fe(III) complexes akin to 1d are completely inactive for lactide polymerization at room temperature.17,18 Nevertheless, we elected to test this prediction, and found that when εcaprolactone was exposed to the formally Fe(III) doubly alkoxide substituted analogue of 1d, it did indeed catalyze the ring-opening polymerization with activity similar to that of neutral Fe(II) bis-alkoxide complexes analogous to 1c, precisely as predicted by theory: complete conversion was observed after 60 min at room temperature. At our current stage of understanding, it is unclear why lactide and caprolactone behave so differently with such Fe(III) catalysts, but our results

Figure 5. Reaction coordinates (G298, kcal mol−1) for the initiation step of ε-caprolactone polymerization with quartet 1a (blue), quintet 1c (green), sextet 1d (purple), and sextet 1d-PF6 (dashed purple).

ization via quartet “Fe(I)” 1a (blue line, recapitulated from above) and quintet Fe(II) 1c (green line). The reaction through 1c follows the same mechanism as previously described, but now all intermediates and transition states are higher in energy than those for 1a. Due to the stereoelectronic effect resulting from two alkoxide groups bound to iron in 1c as opposed to one alkoxide in 1a, the coordination of εcaprolactone to form 2c (with loosely bound monomer) is significantly uphill (11.5 kcal mol−1). Insertion takes place via E


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Inorganic Chemistry Notes

suggest that a combination of factors may be at play, including the propensity of the monomer to bind to the catalyst and the energetics associated with ring opening of the bound tetrahedral intermediate. Overall, the computed (and experimentally verified) reactivities follow the trend 1a > 1d ≈ 1c. To gain further insight, it is instructive to consider the free energies of the ratedetermining ring-opening transition-state structure relative to the catalyst−substrate complexes (i.e., the free energy differences between TS4-5 and 2; see Figure 5). For the formally Fe(I) species 1a, this free energy difference is 20.0 kcal mol−1, and smaller differences are computed for Fe(II) (1c, 15.0 kcal mol−1) and formally Fe(III) (1d, 19.6 kcal mol−1, 1d-PF6, 17.2 kcal mol−1) (Figure 5). The trend in this free energy difference is inverse to the trend in free energies of activation (which are computed as the free energies of TS4-5 relative to separated reactants), so it is clear that overall reactivity depends both on the fundamental ability of the metal to catalyze the bondmaking and bond-breaking steps along the reaction coordinate and also on its ability to bind substrate and reactive intermediates. Thus, 1d binds ε-caprolactone more efficiently than 1c, which might be expected given that the former is a cation while the latter is uncharged. As for 1a, the redox noninnocence of the bis(imino)pyridine ligand allows access to a masked low-coordinate (monoalkoxide) Fe(II) center, which facilitates binding considerably relative to 1c.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.J.C. acknowledges the U.S. National Science Foundation (CHE-1361595) and J.A.B. the US Army Research Office (66672-CH W911NF-15-1-0454) for financial support and the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported within this paper.

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CONCLUSIONS We have studied the electronic structures of a series of ironbased catalysts bearing bis(imino)pyridine ligands and their associated reaction mechanism for the ring-opening polymerization of ε-caprolactone. The computational results are supported by experiments and provide the following conclusions: (i) all catalysts studied are best described as Fe(II) centers containing redox-active bis(imino)pyridine or alkoxides ligands; (ii) high-spin species are more Lewis acidic than lowspin intermediates; (iii) the sluggish catalysis observed for aryloxide initiators is associated with long induction periods attributable to the formation of less thermodynamically favored aryl esters in the initiation step; (iv) the coordination of the ester monomer is a key factor to explain the reactivity trends, where the bis(iminio)pyridine ligand plays a major role in modulating the coordination sphere of iron (mono- vs bisalkoxide). Overall, this study provides significant mechanistic insight that can be used to understand the ring-opening polymerization processes, which will pave the way for rational design of more efficient, selective, and switchable catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02964. Additional figures, redox potentials, energies, and Cartesian coordinates for all optimized species (PDF)

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REFERENCES

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Jeffery A. Byers: 0000-0002-8109-674X Christopher J. Cramer: 0000-0001-5048-1859 F


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Inorganic Chemistry

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The Role of Alkoxide Initiator, Spin State, and Oxidation State in Ring

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