Stearate Complexes in

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

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Roles of Dinuclear Bridging Bidentate Zinc/Stearate Complexes in Sulfur Cross-Linking of Isoprene Rubber Yuko Ikeda,*,†,‡ Yuta Sakaki,†,§ Yoritaka Yasuda,§ Preeyanuch Junkong,ll,⊥ Takumi Ohashi,†,§ Kosuke Miyaji,†,§ and Hisayoshi Kobayashi*,†,# †

Center for Rubber Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan § Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan ll Department of Chemistry, Faculty of Science, Mahidol University, Ratchthewee, Bangkok 10400, Thailand ⊥ Research Strategy Promotion Center, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan # Professor Emeritus, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan Downloaded by ALBRIGHT COLG at 06:45:29:312 on May 29, 2019 from https://pubs.acs.org/doi/10.1021/acs.organomet.9b00193.

‡

S Supporting Information *

ABSTRACT: The roles of the intermediate [Zn 2 (μO2CC17H35)2]2+·4X (X; a hydroxyl group, water, and/or a rubber segment) in the sulfur cross-linking of isoprene rubber are clarified for the first time using in situ time-resolved zinc K-edge X-ray absorption fine structure spectroscopy and in situ time-resolved infrared spectroscopy along with density functional theory calculations. The combined experimental and computational investigation suggests that N-(1,3benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS) is most easily hydrolyzed on the dinuclear bridging bidentate zinc/ stearate intermediate, when a water molecule coordinates to the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the benzothiazole group in CBS. The newly produced intermediate with coordinated 1,3-benzothiazole2-thiolate and cyclohexylamine (CHA) is also found to most readily induce a sulfur insertion among possible candidates to generate subsequent intermediates, when CHA is removed from the intermediate and a water molecule coordinates to the zinc cation which is coordinated by the nitrogen atom of benzothiazole group. The novel dinuclear bridging bidentate zinc/stearate complexes apparently accelerate the sulfur cross-linking of isoprene rubber. Despite the long history of rubber science and technology, these intermediates have been mysterious. The present work will clarify the vulcanization mechanism and will advance the rubber chemistry for a new paradigm of vulcanization technique in the 21st century.

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INTRODUCTION

metal-based compounds such as ZnO are often used as inorganic activators in vulcanization. The development of techniques for controlling rubber processing is necessary for the production of high-performance rubber materials. Elucidation of the vulcanization mechanism would enable control of the cross-linked structures in vulcanizates. However, the vulcanization mechanism remains unclear because of the complicated chemical reactions that occur during rubber processing.1−10 For example, it is still unclear whether vulcanization occurs via an ionic reaction, a radical reaction, or a mixture of ionic and radical mechanisms. However, the development of analytical techniques based on advances in computer technology has provided new methods for studying vulcanization. For example, Nieuwenhuizen et al. used both experimental and theoretical techniques to show that the reaction between TMTD and ZnO generated an

Rubber products are familiar in everyday life. Rubbers are subjected to cross-linking for preparing various rubber products, the most notable being pneumatic tires for automobiles. Rubbers for uses other than adhesives must be cross-linked to form a three-dimensional network structure to endow elasticity. Among various cross-linking reactions, the most important is vulcanization (i.e., cross-linking with sulfur). Vulcanization was first developed by Goodyear in 1839. Since then, additives such as accelerators, activators, and retarders have been used to improve the processability and mechanical properties of rubber products.1−10 Currently, N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS) and tetramethylthiuram disulfide (TMTD) are commonly used as accelerators in vulcanization. Activators are used together with accelerators to ensure complete vulcanization. Activators are divided into two categories: organic and inorganic. Fatty acids and their derivatives are typical organic activators, and © XXXX American Chemical Society

Received: March 21, 2019

A

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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Organometallics intermediate during vulcanization.11,12 Steudel et al. performed a thermodynamic study on vulcanization using both techniques and reported that 2-mercaptobenzothiazole (MBT) and TMTD both accelerated sulfur vulcanization.13,14 In 2009, our group used small-angle neutron scattering to investigate the effects of the combination and composition of sulfur cross-linking reagents on network formation in isoprene rubber.15 Interestingly, the results showed that combining ZnO with the other reagents was crucial not only for chemical crosslinking of the rubber molecules but also for controlling structural network inhomogeneity in the sulfur cross-linked isoprene rubber. The proposed two-phase inhomogeneous network structure of the sulfur cross-linked isoprene rubber was found to consist of the domains of high network-chain density which were embedded in the rubber network matrix (the mesh network). In addition, the mesh size in the twophase inhomogeneous structure was revealed to be controlled by the amounts of ZnO and stearic acid (StH). In situ timeresolved zinc K-edge X-ray absorption fine structure (XAFS) spectroscopy also indicated the formation of the two-phase network.16 Surprisingly, two different sulfur cross-linking reactions for mesh and network domain formations were found to occur almost simultaneously. These observations are important in rubber science and technology because specific network structures play a key role in preparing high performance rubber materials. However, it remains unclear why a combination of ZnO and StH enables control of the mesh size in CBS-accelerated vulcanization. It is generally thought that StH can react with ZnO to form the activator zinc stearate (ZnSt2). Moreover, zinc 1,3-benzothiazole-2-thiolate with a stearate group as a ligand has been accepted as an active intermediate generated from ZnO, StH, and CBS.17,18 However, the form of the zinc salt of StH that is involved in sulfur cross-linking is not well understood because most studies have focused on materials isolated from vulcanization reactions.1−9 The role of the zinc salt of StH during the vulcanization reaction is therefore unclear. To pursue the challenge for this subject, we used a combination of in situ time-resolved zinc K-edge XAFS spectroscopy, in situ time-resolved Fourier-transform infrared spectroscopy (FT-IR), and density functional theory (DFT) calculations to determine the role of the zinc salt of StH in the sulfur cross-linking reaction. Then, a formation of a new complex generated from ZnO and StH at high temperature was discovered:19 A bridging bidentate zinc/stearate complex was the dominant structure. The zinc/stearate ratio of the complex was surprisingly 2/2. The DFT calculation for identifying the intermediate predominantly suggested its fundamental skeleton to be a dinuclear type bridging bidentate zinc/stearate complex composed of [Zn2(μ-O2CC17H35)2]2+·4X, where X is hydroxyl group, water and/or rubber segment (intermediate I) as shown in Figure 1. Particularly, the calculation showed the existence of two hydroxyl groups with water and/or a rubber segment in the complexes.19 Intermediate I has been unknown despite the long history of rubber science and technology. The newly observed zinc/stearate complex was predicted to accelerate the sulfur cross-linking reaction of rubber like an enzyme.19 The findings are expected to be useful for producing important rubber materials, especially eco-friendly tires, as suggested in the ACS News Service Weekly PressPac in 2015.20 This intermediate I was also cited in a study on vulcanization using ZnO anchored silica nanoparticles.21

Figure 1. Dinuclear type bridging bidentate zinc/stearate complex (labeled as intermediate I), where X indicate water, hydroxyl group, and/or a rubber segment. Reprinted with permission from ref 19, Copyright 2015 American Chemical Society, and ref 10, Copyright 2018 Springer Nature Singapore Pte. Ltd.

On the other hand, the proposed two-phase morphology was confirmed by using the atomic force microscopy nanomechanical mapping with the two-dimensional mapping of reliability index.22 Without StH, the average Young’s modulus of the network in the matrix of the sulfur crosslinked isoprene rubber was found to be very low, possibly because of the lack of intermediate of zinc/stearate complex inducing the mesh network formation. Furthermore, using a linear combination fitting in sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy for the solvent extracted sulfur cross-linked isoprene rubber, the sulfidic linkage of a disulfidic type was found for the first time to be dominant in the CBS-accelerated system when either ZnSt223 or a combination of ZnO and StH24 were used as the activators. Importantly, the presence of the bridging bidentate zinc/stearate complex as an intermediate for the sulfur crosslinking reaction was suggested to induce the generation of abundant disulfidic linkages in the rubber networks. In this paper, we report the first direct experimental and theoretical evidence for the role of this unique zinc/stearate complex in the sulfur cross-linking of rubber, with a particular focus on the generation of active intermediates derived from intermediate I. In addition, several reaction pathways for the generation of new active intermediates and sulfur insertion reaction are introduced, and the most possible pathway and chemical structure of the intermediates are proposed ultimately for the sulfur cross-linking reaction of isoprene rubber. New findings will give us a practical hint not only for controlling the generated active intermediates for the vulcanization reaction but also for producing high-performance rubber materials. The new hidden discovery will be useful in achieving breakthroughs in the traditional, yet indispensable, technique of vulcanization.

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RESULTS AND DISCUSSION Role of Intermediate I in Sulfur Cross-Linking of Isoprene Rubber. Generation of Intermediate II. The role of intermediate I, shown in Figure 1, was investigated using IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5) as a model for the sulfur cross-linking reaction of isoprene rubber because most of the ZnO in the sample was consumed in a reaction with StH to generate intermediate I under the experimental conditions in this study.19 Figures 2a and S1f show the in situ FT-IR spectra of IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5), obtained during vulcanization. Each number and the black arrow in Figure 2a indicate the order and the direction of the band shifts during heating, B

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Organometallics

cm−1 decreased. The peak top of the broad bands further shifted to approximately 1565 cm−1 and widened and weakened as the reaction proceeded. Simultaneously, bands appeared at 1757 and 1713 cm−1 (Figure 2a), corresponding to the ν(CO) bands of the monomer and dimer of StH, respectively, when the band at 1580 cm−1 disappeared. We first focused on these shifts to identify the species generated from I(1), I(2), and/or I(3), because the carboxylate shift is useful for detecting different chemical species.31,32 The in situ FT-IR spectra of IR-ZnO(0.5)-StH(2)-CBS(1), IR-ZnO(0.5)-StH(2)-MBT(0.63), and IR-ZnO(0.5)-StH(2) are shown in Figure 2b,e; Figure 2c,f; and Figure 2g, respectively, for comparison. It is noted that the red spectrum of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) in Figure 2d changes gradually to a light blue spectrum via the green spectrum, which is similar to the spectral change of IR-ZnO(0.5)-StH(2)CBS(1) from the yellow spectrum to the black spectrum, as shown in Figure 2e. This variation is displayed more clearly in Figure 3, where the spectra of IR-ZnO(0.5)-StH(2) at 18.5

Figure 2. In situ FT-IR spectra of (a) IR-ZnO(0.5)-StH(2)-CBS(1)S8(1.5), (b) IR-ZnO(0.5)-StH(2)-CBS(1), and (c) IR-ZnO(0.5)StH(2)-MBT(0.63) in the range of 1800−1500 cm−1. Numbers and black arrows indicate the order and the direction of band shifts during heating, respectively. (d), (e), and (f) show the magnified carboxylate shift of (a), (b), and (c) in the range of 1625−1525 cm−1, respectively. (g) shows in situ FT-IR spectra of IR-ZnO(0.5)-StH(2). The color variation shows 8.4 min (red), 20.9 min (yellow), 21.6 min (light green), 22.4 min (green), 23.1 min (light blue), 23.8 (blue), 24.6 min (purple), 25.3 min (gray), and 26.8 min (black) in (d), 8.2 min (red), 14.1 min (yellow), 19.9 min (light green), 23.6 min (green), 30.9 min (light blue), 38.3 min (blue), 67.6 min (purple), 104.3 min (gray), and 140.9 min (black) in (e), 6.0 min (red), 9.7 min (yellow), 17.0 min (light green), 22.9 min (green), 28.0 min (light blue), 33.1 min (blue), 38.3 min (purple), 43.4 min (gray), and 48.5 min (black) in (f), and 5.3 min (red), 6.0 min (yellow), 6.7 min (light green), 7.5 min (green), 8.2 min (light blue), 23.6 min (blue), 38.3 min (purple), and 52.9 min (black) in (g), respectively.

respectively. At first, the two bands at 1537 and 1398 cm−1, which were identified as antisymmetric and symmetric stretching of COO− (νas(COO−) and νs(COO−)) of ZnSt2, respectively, were clearly observed in Figures 2a and S1f.19,25−30 Upon heating, these bands disappeared, and a band at 1595 cm−1 and one shoulder band at approximately 1560 cm−1 appeared oppositely, as shown in Figures 2a and S1f, which is consistent with the changes observed in IRZnO(0.5)-StH(2) in our previous study.19 These spectral changes occurred gradually as the compound was heated to 144 °C, although CBS and S8 were mixed together with ZnO and StH in isoprene rubber. The bands at 1595 cm−1 and approximately 1560 cm−1 corresponded to the theoretically calculated bands for the ν a s (COO − ) of [Zn 2 (μO2CC17H35)2]2+(OH−)2·(rubber)2), [Zn2(μO2CC17H35)2]2+(OH−)2·(rubber)(water) and/or [Zn2(μO2CC17H35)2]2+(OH−)2·(water)2, as given in Table S1.19 The complexes are abbreviated as intermediates I(1), I(2), and I(3), respectively, herein. These results confirmed that intermediate I was generated in IR-ZnO(0.5)-StH(2)-CBS(1)S8(1.5), even when the CBS and S8 were mixed with ZnO and StH in isoprene rubber. The magnified in situ spectra after the appearance of I(1), I(2), and/or I(3) are shown in the range of 1625−1525 cm−1 (Figure 2d). The bands from approximately 1595 to approximately 1560 cm−1 first broadened, becoming almost trapezoidal, and then the peak top of the broad bands shifted from 1595 to 1580 cm−1, as the intensity of the band at 1595

Figure 3. Variation of FT-IR spectra of IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5) (black), where each number indicates the reaction time in minute during its vulcanization. IR-ZnO(0.5)-StH(2) at 18.5 min (red), IR-ZnO(0.5)-StH(2)-CBS(1) at 41.2 min (blue) and IRZnO(0.5)-StH(2)-CBS(1) at 118.9 min (green), and IR-ZnO(0.5)StH(2)-MBT(0.63) at 41.2 min (yellow) were plotted for comparison.

min, IR-ZnO(0.5)-StH(2)-CBS(1) at 41.2 and 118.9 min, and IR-ZnO(0.5)-StH(2)-MBT(0.63) at 41.2 min are shown for comparison. First, the spectrum of IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5) at 6.2 min is comparable to that of IRZnO(0.5)-StH(2) at 18.5 min, indicating the generation of I(1), I(2), and/or I(3).19 Second, the FT-IR spectra of IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5) at 19.4 and 22.4 min are similar to those of IR-ZnO(0.5)-StH(2)-CBS(1) at 41.2 and 118.9 min (blue and green lines in Figure 3), respectively. The results imply that I(1), I(2), and/or I(3) may have reacted with CBS to generate some species in IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5), that is, intermediate I may work as an activator for accelerator, CBS. This speculation is acceptable because both ZnO and StH are well-known as vulcanization accelerator aids. In rubber technology, CBS is classified as a delayed-action accelerator, which controls an induction period of vulcanC

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Organometallics ization. Conventionally, CBS is produced by protecting the active thiol group of MBT with cyclohexylamine (CHA).33 In this study, therefore, IR-ZnO(0.5)-StH(2)-MBT(0.63) was prepared as a reference sample and subjected to in situ FT-IR to investigate a role of intermediate I. Namely, MBT was reacted with intermediate I instead of CBS. Then, a band at approximately 1580 cm−1 was detected together with a band at about 1595 cm−1, as shown in the spectra of IR-ZnO(0.5)StH(2)-MBT(0.63) (Figures 2c,f and 3 (a yellow line)). This band served as a useful indicator for identifying new intermediates generated from intermediate I because a band was detected at approximately 1580 cm−1 in the FT-IR spectra of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) and IR-ZnO(0.5)StH(2)-CBS(1). The spectra at 23.6 (green) and 30.9 min (light blue) in the case of the former and those at 43.4 (gray) and 48.5 min (black) in the case of the latter are shown in Figure 2d,e, respectively. Similar spectral changes are displayed in Figure 3 as well. This carboxylate shift from 1595 to 1580 cm−1 in both IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) and IRZnO(0.5)-StH(2)-CBS(1) was ascribed to the generation of a new intermediate, probably composed of a deprotected CBS species (resembling MBT) and the dinuclear bridging bidentate zinc/stearate intermediate. The structure of intermediate I is similar to that of a certain type of enzyme,19 which is known to hydrolyze proteins.34 Therefore, intermediate I is expected to hydrolyze CBS during the vulcanization of IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5), that is, CBS may be deprotected by intermediate I. Then, we attempted to identify the structure of the new intermediate by using additional experimental results. First, the chemical structure of the intermediate was investigated by comparing the typical infrared vibration modes of MBT with those of IR-CBS(1), IR-MBT(0.63), IR-ZnO(0.5)-StH(2)CBS(1), and IR-ZnO(0.5)-StH(2)-MBT(0.63). Since the thiol and thione forms of γ(C−H) in the benzene ring of MBT show significant absorptions near 750 cm−1,35 the bands around 750 cm−1 were used to identify the structure of the intermediate. However, the δas(CH2) band in isoprene rubber appeared at 740 cm−1. Therefore, the FT-IR spectra of isoprene rubber were subtracted from those of IR-CBS(1), IRMBT(0.63), IR-ZnO(0.5)-StH(2)-CBS(1), and IR-ZnO(0.5)StH(2)-MBT(0.63). The differential FT-IR spectra of IRMBT(0.63) and IR-ZnO(0.5)-StH(2)-MBT(0.63) in the range of 770−710 cm−1 are shown in Figure 4a,b, respectively. The generation of dinuclear bridging bidentate zinc/stearate intermediate was not observed in IR-MBT(0.63), even at 144 °C, whereas it was observed in IR-ZnO(0.5)-StH(2)MBT(0.63) at 144 °C. The bands corresponding to the thiol (754 cm−1) and the thione (745 cm−1) forms were detected at 144 °C in IR-MBT(0.63), but only the thiol form (752 cm−1) was detected at 144 °C in IR-ZnO(0.5)-StH(2)-MBT(0.63) when the sample contained ZnO and StH. Although the peak shift was very small (from 754 to 752 cm−1), the broad band was found to clearly shift toward a lower wavenumber in IRZnO(0.5)-StH(2)-MBT(0.63) after heating to 144 °C. MBT generally shows a thione−thiol tautomerism.36 Therefore, the tautomerism of IR-MBT(0.63) is reasonable in the isoprene rubber matrix at 144 °C. However, IR-ZnO(0.5)-StH(2)MBT(0.63) did not show any tautomerism, and only the thiol form was observed at 752 cm−1 at 144 °C. This implies that the MBT in IR-ZnO(0.5)-StH(2)-MBT(0.63) was coordinated with the dinuclear bridging bidentate zinc/stearate intermediate in the thiol form.

Figure 4. Differential FT-IR spectra in the range of 770−710 cm−1 of (a) IR-MBT(0.63) (red), (b) IR-ZnO(0.5)-StH(2)-MBT(0.63) (yellow), (c) IR-CBS(1) (green), (d) IR-ZnO(0.5)-StH(2)-CBS(1) (blue) and (e) IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5). The color variation in (e) shows 18.0 min (red), 23.1 min (yellow), 24.6 min (light green), 27.5 min (green), 28.2 min (blue), 31.9 min (black). FT-IR spectra of isoprene rubber was subtracted from each spectrum at the same heating time. Direction of black arrow indicates the progress of heating.

The same analytical technique was applied to the CBS systems. Figure 4c,d show the differential FT-IR spectra of IRCBS(1) and IR-ZnO(0.5)-StH(2)-CBS(1) in the range of 770 to 710 cm−1, respectively. IR-CBS(1) showed only a band at 754 cm−1 at 35 °C, which did not change during heating at 144 °C. However, the band at 754 cm−1 in IR-ZnO(0.5)-StH(2)CBS(1) gradually shifted toward 752 cm−1 to 144 °C. The shift occurred slowly, and the band was broader than that in IR-ZnO(0.5)-StH(2)-MBT(0.63). These results suggest the formation of a new intermediate with a structure similar to that of the intermediate detected in IR-ZnO(0.5)-StH(2)MBT(0.63) at 144 °C. Next, we focus on the differential FT-IR spectra of IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5) (Figure 4e) to investigate the vulcanization mechanism. A band at 752 cm−1 clearly appeared during heating at 144 °C, although a sharp band was initially observed at 754 cm−1 at 35 °C. These changes are consistent with those observed in IR-ZnO(0.5)-StH(2)CBS(1), suggesting the generation of a new intermediate, where a CBS derivative was coordinated with a bridging bidentate structure in the thiol form. Although the in situ FTIR results provide useful information on the second intermediate, it was difficult to definitively identify its chemical structure. A combination of DFT calculations and the FT-IR data was therefore used to identify the intermediate, as discussed in the next section. Mechanism of Generation of Intermediate II. To identify the chemical structure of the second intermediate, we performed DFT calculations for the IR-ZnO(0.5)-StH(2)CBS(1). When CBS was associated with the dinuclear bridging bidentate zinc/stearate intermediate, there were two possibilities, that is, the intermediate coordinated by a nitrogen atom or by a sulfur atom of the heterocyclic ring of the 1,3benzothiazole-2-thiolate anion (BtS−) group in the case of the D

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Scheme 1. Generation Mechanisms of Intermediate II and Intermediate III by a Reaction of Intermediate I with (a) CBS and (b) MBT in Isoprene Rubber, Respectivelya

X and Y are a rubber segment, water, and/or OH−, and W is a rubber segment or water, respectively. The structures in the dotted brackets indicate the expected initial structures before their optimizations.

a

Table 1. Calculated Harmonic Frequencies of νas(COO−) and νs(COO−) in Each Structure for Intermediate II and Intermediate III ligand CBS II(1) II(2) II(3) II(4) II(5) II(6) II(7) II(8) II(9) III(1) III(2) III(3) III(4) III(5) III(6)

a

infrared frequency of COO− (cm−1)

MBTb

X

Y

rubberd H2O OH− rubber H2O OH− rubber H2O OH−

H2O H2O H2O rubber rubber rubber OH− OH− OH−

X

rubber H2O OH− rubber H2O OH−

W

charge

antisymmetric

symmetric

Δνc

cleavage of S−N in CBS

rubber rubber rubber H2O H2O H2O

0 0 −1 0 0 −1 −1 −1 −2 1 1 0 1 1 0

1578 1562 1604 1563 1559 1576 1567 1566 1597 1583 1582 1579 1581 1573 1560

1434 1418 1435 1444 1445 1443 1439 1442 1443 1438 1441 1423 1438 1446 1441

144 144 169 119 114 133 128 124 154 145 141 156 143 127 119

yes yes yes no no no no no no -

N-(1,3-Benzothiazol-2-ylsulfanyl)cyclohexanamine. b2-Mercaptobenzothiazole. cDifferential frequency of COO− by subtracting the symmetric frequency from the antisymmetric frequency. dOne repeating unit of cis-1,4-polyisoprene.

a

Interestingly, the DFT calculation of IR-ZnO(0.5)-StH(2)CBS(1) indicated cleavage of the S−N linkage in CBS only when a water molecule was coordinated to the zinc cation opposite the zinc cation coordinated by the nitrogen atom of the benzothiazole group on the dinuclear bridging bidentate zinc/stearate intermediate, regardless of the ligand (X) in Scheme 1a. It is worth noting that the Brønsted acidity of the water molecule coordinated to the zinc in the zinc/stearate complex was enhanced, similarly to that of zinc enzymes.37,38 A water proton was removed by the nitrogen atom in CHA,

thiol form. First, one complex with CBS, in which two nitrogen atoms of CBS were coordinated to each zinc cation, was proposed (Scheme 1a) because each nitrogen has a higher nucleophilicity than that of sulfur in CBS (Figure S2a). In addition, the DFT geometry optimizations of the structures coordinated by a sulfur atom of the BtS− group showed that the heterocyclic rings of the BtS− group were collapsed or eliminated from the dinuclear bridging bidentate zinc/stearate intermediate (Scheme S1). Only the case in which the nitrogen atom is coordinated to the zinc atom is therefore discussed. E

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Organometallics followed by the generation of BtS− and CHA on the complex, as in Scheme 1a. This is consistent with the predictions made on the basis of FT-IR results in the previous section. For reference, the effects of other ligands on cleavage of the S−N linkage in CBS were confirmed by DFT calculations, as summarized in Table 1. Structures II(1), II(2), and II(3) were concluded to be active intermediates to cleave the S−N linkage in CBS. In fact, the contribution of the water molecule to the vulcanization reaction was confirmed experimentally: A faster curing behavior, that is, a shorter induction period was detected in the conventional curing system that uses ZnO and StH (approximately 12 min) than in the ZnSt2-activated curing system (approximately 14 min) at 144 °C. Usually, water is included in rubber compounds as an impurity because rubber and curing reagents are used as received. Therefore, such water molecules may coordinate with the zinc cation by means of ligand substitution, followed by promotion of the hydrolysis reaction of CBS in both cases. In the rubber compound mixed with ZnO and StH, additionally, water is generated by the reaction of ZnO with StH. Thus, CBS is supposed to be hydrolyzed more easily in the compound when mixed with ZnO and StH than when mixed with ZnSt2. Moreover, we believe that the water molecules help to protonate the hydroxyl group coordinated to the zinc cation of the dinuclear bridging bidentate zinc/stearate intermediates. The Lewis acidic zinc cation center commonly activates the coordinated water toward deprotonation.37,38 This phenomenon is known to occur reversibly in general. The DFT calculations pertaining to the intermediate structures with different X groups indicated the most likely candidate for intermediate II among the structures II(1), II(2), and II(3). It was II(1) because the wavenumber (1578 cm−1) of the νas(COO−) mode in II(1) was similar to the experimental wavenumber (1580 cm−1), as given in Table 1. The band observed experimentally at 1580−1570 cm−1 was broad, and a shoulder band was detected simultaneously at approximately 1560 cm−1 (Figure 2e). Therefore, II(2) may also be a candidate for intermediate II. Namely, intermediate II may be a mixture of II(1) and II(2). This is reasonable because our previous study19 suggested that intermediate I, which is a precursor of intermediate II, was a mixture in the highly viscous liquid state of the isoprene rubber matrix during vulcanization. Note that it was not easy to utilize the experimental νs(COO−) band to determine the most likely intermediate II in this study because the bands in the range 1410−1435 cm−1of IR-ZnO(0.5)-StH(2)-CBS(1) were very broad and weak, even after subtraction of the FT-IR spectrum of isoprene rubber, as shown Figure S3. In contrast, the DFT results for the intermediates generated in the IR-ZnO(0.5)-StH(2)-MBT(0.63) system showed a structure similar to those of intermediate II, in which BtS− was coordinated to a zinc cation (Scheme 1b). This calculation supports the identification of intermediate III, which was formed similarly to intermediate II, as detailed above. Cleavage of the S−H linkage in MBT occurred only when a hydroxyl group was coordinated to the zinc cation opposite the zinc cation coordinated by the nitrogen in MBT on the dinuclear bridging bidentate zinc/stearate intermediate, regardless of the type of ligand (X and W; Scheme 1b). IR-ZnO(0.5)-StH(2)MBT(0.63) showed that the hydrogen atom of the thiol group was removed by the hydroxyl group on the zinc cation, similarly to zinc enzymes.37,38 Based on additional DFT calculations, among the six possible intermediates, namely,

III(1)−III(6), the structures III(1)−III(5) were identified as acceptable intermediates because their νas(COO−) values were consistent with the experimental result (1580 cm−1), as shown in Table 1. The role of intermediate III in the vulcanization reaction will be reported in the near future. Characterization of Intermediate II by XAFS Spectroscopy. Figure 5a,b show the XANES and extended X-ray

Figure 5. (a) XANES and (b) EXAFS spectra of IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5) at 20.1 min (a red solid line) and IRZnO(0.5)-StH(2)-CBS(1) at 41.6 min (a black dotted line).

absorption fine structure (EXAFS) spectra of IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5) at 20.1 min and IR-ZnO(0.5)-StH(2)CBS(1) at 41.6 min after heating, respectively. These XAFS spectra corresponded to the FT-IR spectra at 19.4 min of IRZnO(0.5)-StH(2)-CBS(1)-S8(1.5) and at 41.2 min of IRZnO(0.5)-StH(2)-CBS(1) in Figure 3, respectively. The XANES and EXAFS spectra of the two samples were similar, indicating that the intermediate generated in the IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5) system was similar to that generated in the IR-ZnO(0.5)-StH(2)-CBS(1) system. The intermediate observed in IR-ZnO(0.5)-StH(2)-CBS(1) was therefore proposed as a model for intermediate II in the vulcanization system of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5), on the basis of the FT-IR and DFT results. Furthermore, to confirm the validity of intermediate II, the proposed structures of II(1) and II(2) were subjected to EXAFS analysis using the structural parameters obtained from DFT calculations. Curve fitting of the FT-EXAFS spectra of IR-ZnO(0.5)-StH(2)-CBS(1) was conducted using eq 1 in the Experimental Section, in accordance with our previously reported method.19 The optimized bond lengths and coordination numbers obtained from the DFT calculations were used for fitting. The distances to first-shell N and O atoms were refined in this study to improve the resolution of the EXAFS spectra of the rubber vulcanizates. The FT-EXAFS spectrum of IR-ZnO(0.5)-StH(2)-CBS(1) at 41.6 min was fitted using the structural parameters of II(1) and II(2). The average Zn−O and Zn−N distances (2.02 and 2.09 Å for II(1) and 2.05 and 2.10 Å for II(2), respectively) F

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Organometallics Table 2. Results of EXAFS Fitting for Intermediate II ligand intermediate

Z

II(1)

rubber

II(2)

H2O

e

atom

CNa

Rb (Å)

σ2c (Å2)

ΔE0d (eV)

R-factor

O N O N

2.50 1.00 3.00 1.00

1.95 2.10 2.01 1.90

0.004 0.0006 0.005 0.001

−1.0

2.3 × 10−4

−1.3

4.6 × 10−4

a

Coordination number. bBond length. cDebye−Waller factor. dEnergy shift. eOne repeating unit of cis-1,4-polyisoprene.

determined by optimization of the structures of II(1) and II(2) were used. The average Zn−O and Zn−N coordination numbers were 2.5 and 1.0, respectively, for II(1), and 3.0 and 1.0, respectively, for II(2). The fitting results for the zinc atoms in the first shells of the structures of II(1) and II(2) are summarized in Table 2. Both the experimental FT-EXAFS spectra of IR-ZnO(0.5)-StH(2)-CBS(1) were well fitted by structural parameters of II(1) and II(2), with low R-factors and acceptable parameters such as the Debye−Waller factors.39 For example, the real part of the inverse FT-EXAFS spectrum of the best-fitted result, with an R-factor of 2.3 × 10−4, is shown in Figure 6. The black dotted line in this figure is the fitted result for II(1). Note that intermediate II could be a mixture of II(1) and II(2).

Nucleophilic attack was therefore expected to occur on the sulfur ring, followed by sulfur insertion into intermediate II. In this section, the mechanism of the sulfur insertion reaction is discussed based on the DFT calculation results (optimized structures of local minima and transition states (TSs), and their energies). Seven possible reaction paths, namely, A, B, C, D, E, F, and G, from intermediate II were investigated theoretically, as shown in Scheme 2. Two main factors differentiate the paths. One is the elimination energy of ligand from intermediate II, and the other is the TS activation energy. The elimination energies of paths B, C, F, and G are considerably lower than those of paths D and E. Therefore, paths B, C, F, and G seem more favorable. In three reactions via IV(7), IV(8), and IV(9) along paths F and G, TSs could not be characterized. In detail, approximate structures of the saddle points were located. However, the succeeding intrinsic reaction coordinate (IRC) analysis40,41 only led to the intermediates on the reactant side. This result was interpreted as the saddle point being a type of hump on the reaction coordinate, and the reaction coordinate was essentially a monotonic uphill from the reactant side to the product side. In particular, path B via IV(2) reaching intermediate V(2) was predominant because it had low elimination energy from intermediate II and the lowest activation energy for TS formation among the paths when a water molecule was coordinated to the zinc atom which was coordinated by the nitrogen atom of the benzothiazole group. Each of the paths are discussed below. To confirm the results, experimental shifts of the νas(COO−) bands of intermediates IV and V are compared with their respective theoretically calculated bands in the next section. Path A: The structure of intermediate II was first optimized together with S8 to confirm that intermediate II had a role in the sulfur insertion reaction. The intramolecular S−S bond lengths in S8 were approximately 2.18 and 2.11 Å for II(1) with S8 and II(2) with S8 as shown in Figure 7a,b, respectively. Since the lengths were similar to those of S8, that is, 2.060 Å,42 which indicated that S8 did not coordinate with intermediate II and the S−S bonds in S8 were not broken in either structure. This consideration was supported by the long Zn−S bond lengths of 3.17 and 4.66 Å, respectively, between the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the BtS− and the sulfur atom in S8. These results implied that no sulfur insertion occurred in path A. Although the TSs were searched between S4 (Z = rubber), S5 (Z = H2O) and the predicted structures S6 (Z = rubber), S7 (Z = H2O) shown in a dotted square bracket of Scheme S2, no TSs were detected. There are two possible reasons: (1) steric hindrance of the coordination sphere of the zinc cation by a OH− and CHA, and (2) the lower electrophilicity of the zinc cation coordinated by a OH− and CHA. Optimization for the local minima of structures S6 (Z = rubber) and S7 (Z = H2O) found that OH− cleaved the sulfidic linkages of S8, giving the

Figure 6. Real part of inverse FT-EXAFS spectrum of IR-ZnO(0.5)StH(2)-CBS(1) at 41.6 min. A black dotted line is the fitted result of II(1).

The XAFS analyses clearly supported the generation of intermediate II via intermediate I. It is a key that CBS is hydrolyzed on the dinuclear bridging bidentate zinc/stearate intermediate when a water molecule is coordinated to the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the benzothiazole group in CBS. The combined experimental and computational investigation revealed the role of intermediate I in the sulfur cross-linking of isoprene rubber for the first time. Role of Intermediate II in Sulfur Cross-Linking of Isoprene Rubber. Possible Paths for Sulfur Insertion via Intermediate II. DFT calculations were performed to investigate the role of intermediate II in the sulfur crosslinking of isoprene rubber and to first select a possible reaction mechanism theoretically because the reaction mechanism is complicated. The DFT results of intermediate II indicated that the charges on the sulfur anion in BtS− in II(1) and II(2) were −0.25 and −0.28, respectively. Each BtS− retained its nucleophilic character when coordinated to the zinc atom, although the zinc atom showed little or no back-donation because of the d10 electronic configuration of Zn2+.37,38 G

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Organometallics Scheme 2. Possible Sulfur Insertion Reactions via Intermediate II to Intermediate V in Isoprene Rubbera

a

Z is H2O or a rubber segment.

derivative. In the following models, paths B to E, the energy for elimination of OH− or CHA from the zinc atom was first evaluated by comparing the energies of the reactant intermediate II and the intermediate after ligand elimination. For the latter, the solvation effects were taken into account for the leaving groups. Because isolated OH− was considered to be too unstable, isolated OH− and CHA were optimized in the isoprene trimer medium. The elimination energies of CHA and OH− from intermediate II were calculated to be 27.0 and 116.1 kcal/mol, respectively. The former was much smaller than the latter, and therefore, elimination of CHA from the

structure S8 (Z = rubber) and S9 (Z = H2O) shown in Scheme S2. Furthermore, the calculated harmonic frequencies of νas(COO−) were 1595 and 1607 cm−1 for S8 and S9, respectively, which were out of the range between 1580 and 1565 cm−1. Therefore, path A was excluded from the mechanism of sulfur insertion to intermediate II. Next, we propose a mechanism that involves bipolar attack to the S8 ring by the nucleophilic center of BtS− and the electrophilic center of Zn2+. Then, the OH− or CHA in II(1) and II(2) must be eliminated to reduce the steric hindrance and enable insertion of sulfur into the intermediate II H

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to those for S8.42 This indicated that S8 did not coordinate to the intermediate II derivatives and the S−S bonds in S8 were not cloven in either structure simply by eliminating CHA. This was supported by the long Zn−S bond lengths of 2.74 and 2.75 Å, respectively, between the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of BtS− and the sulfur atom in S8. These results imply that no sulfur insertion occurred in path B. However, the steric hindrances in intermediate IV for path B were apparently small compared with intermediate II. Therefore, TSs were expected to be present. TSs between IV(1) (Z = rubber) and IV(2) (Z = H2O) and the products V(1) and V(2) were investigated. The TSs were characterized by formation of two new bonds between BtS− and S8 and between the zinc cation and S8, and by cleavage of the S−S bond in S8. The IRC analysis led to the products of V(1) and V(2) from the reactants of IV(1) (Z = rubber) and IV(2) (Z = H2O), via TSs labeled as TS[IV(1)−V(1)] and TS[IV(2)−V(2)], respectively. The activation energies were around 3.8 and 3.7 kcal/mol, respectively. These low energies suggested that sulfur insertion proceeded easily in path B, if once a reaction site was provided by elimination of CHA. The changes in the relative energy for the insertion reaction of S8 were plotted in Figure 8. As described later, the relative energy of path B tended to be low when Z was H2O. Path C: We also considered the possibility of sulfur insertion from IV(3) and IV(4), which were formed under low concentrations of curing agents (about a few percent). This alternative sulfur insertion mechanism was investigated by optimizing the structures of IV(3) and IV(4) together with S8. However, the insertion reaction did not proceed spontaneously, which implied that coordination of S8 to the zinc complex involved a reaction barrier. The TSs for this reaction path were also investigated. The TS was characterized by the four bond alternations: Formation of two new bonds between BtS− and S8 and between the zinc cation and S8, and cleavage of two bonds between S−S bond in S8 and Zn−S bond in intermediate IV. The IRC analysis led to the products V(1) and V(2) from the reactants IV(3) and IV(4), via TSs labeled as TS[IV(3)−V(1)] and TS[IV(4)−V(2)], respectively. The activation energies for TS[IV(3)−V(1)] and TS[IV(4)− V(2)] were 34.3 and 39.1 kcal/mol, respectively, which were higher than those for path B. Path B may therefore be favored over path C, and the reaction products, that is, V(1) and V(2), were the same in paths B and C. Furthermore, the relative energies of TS with respect to intermediate II in path C were comparable with that of path B via IV(1) (Z = rubber), but higher than that of path B via IV(2) (Z = H2O). Therefore, path B via IV(2) may proceed more predominantly than path C. Path D: The elimination energy for OH− was 3 to 4 times larger than that for CHA, as described above. However, path D may not be ignored for the reaction at 144 °C. Two reaction paths (paths D and E) of sulfur insertion were therefore considered. For path D, the structures of intermediate II without the hydroxyl group were optimized together with S8. The result is worth noting. If once the OH− was eliminated in an endothermic process, S8 coordinated to the intermediates with S−S bond cleavage, and then the final products V(3) and V(4) were produced directly. As shown in Figure 7e,7f, S8 was attracted by the intermediates, resulting in an increase in the S−S bond length in S8 to 3.07 or 3.15 Å. This suggested that elimination of the OH− reduced the steric hindrance around

Figure 7. Optimized structures under the interactions between intermediate II and S8 in paths A, B, D and F. (a) The structure of II(1) (Z = rubber) with S8 in path A, (b) the structure of II(2) (Z = H2O) with S8 in path A, (c) the structures of II(1) with S8 and without CHA in path B, (d) the structures of II(2) with S8 and without CHA in path B, (e) the structures of II(1) with S8 and without OH− in path D, and (f) the structures of II(2) with S8 and without OH− in path D, (g) the structures of II(1) with S8 and without H2O in path F, and (h) the structures of II(2) with S8 and without H2O in path F. Atoms are represented by colored spheres: H (white), C (gray), N (light blue), O (red), S (yellow), and Zn (dark gray). Arrows in red, blue, and green colors indicate the S−S bond lengths between the sulfur atom of the BtS− and the sulfur atom in S8, S−S bond lengths in S8, and Zn−S bond lengths between the zinc cation opposite the zinc cation coordinated by the nitrogen atom of the BtS− and the sulfur atom in S8, respectively.

zinc cation was much easier than elimination of OH−. When the intermediate II derivative was optimized in the presence of S8, associated intermediates, that is, IV(1) and IV(2), were formed (path B). Otherwise, the reaction immediately gave the final products V(3) and V(4) (path D). When S8 was not located around the intermediate II derivative, optimization gave IV(3) and IV(4) (path C), and IV(5) and IV(6) (path E), with bonding between the zinc cation and sulfur anion in BtS−. However, the ligand elimination energies suggested that the reactions via paths B and C would be favored over those via paths D and E. The details of each path is discussed in the following sections. Path B: The structures of intermediate II derivative without CHA were optimized together with S8 as in the case of path A. The S−S bond lengths in S8 were 2.10 Å when S8 was located near the intermediates as shown in Figures 7c (IV(1) (Z = rubber)) and 7d (IV(2) (Z = H2O)). The lengths were similar I

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Path E: Sulfur insertion via intermediates IV(5) and IV(6) was confirmed to occur by a path similar to path C via the corresponding TSs, leading to the products, that is, V(3) and V(4). The TSs connecting IV(5) to V(3) and IV(6) to V(4) were labeled as TS[IV(5)−V(3)] and TS[IV(6)−V(4)], respectively (Scheme 2). The energy changes along path E from IV(5) to V(3) and IV(6) to V(4) are shown in Figure 8. The activation energies were 48.1 and 45.6 kcal/mol, respectively, that is, higher than those in paths B and C. The reactions in paths B and C therefore proceeded on lower potential energy surfaces than those in paths D and E. In addition, sulfur insertion into intermediate II with a water ligand [Z = H2O in IV(2)] via path B was the most energetically favorable among the five paths discussed above. This result is reasonable because CHA is neutral and is more easily removed than a hydroxyl group from the zinc cation. Note that it seems possible that the hydroxyl group on the zinc cation opposite zinc cation which is coordinated by the nitrogen atom of the benzothiazole group abstracts an adjacent proton from CHA and results in the elimination of water molecule from the zinc cation site. Optimizations brought to IV(9) and IV(10) having the bond between the zinc cation and sulfur anion in BtS−, when S8 did not locate around intermediate II. Thus, the energy for the elimination of H2O from the zinc atom was evaluated similarly to the eliminations of OH− and CHA, where the isolated H2O was optimized in the medium of isoprene trimer to fulfill the environment of H2O in the rubber matrix. As a result, elimination energies of H2O from II(1) and II(2) was calculated to be 40.0 and 30.2 kcal/mol, respectively. Interestingly, the energy was in a same level with that of neutral CHA. The details of these paths (paths F and G) are discussed next. Path F: The structure of intermediate II derivative coordinated by only a cyclohexylamino group was optimized together with S8 to investigate the sulfur insertion reaction. It was found that the intramolecular S−S bond lengths in S8 were around 2.52 Å, which was longer than those of S8 in the literature42 as shown in Figure 7g,h. However, the S−S bond lengths between the sulfur anion of BtS− and the sulfur atom of S8 were quite long (5.61 and 7.54 Å), which suggests no insertion reaction of S8 to the intermediate. Although TSs could not be characterized in path F, the relative energies of reactants (IV(7) and IV(8)) were lower than those of products (V(5) and V(6)). The IRC analysis only led to

Figure 8. Variation of relative energies during sulfur cross-linking reaction from paths B to G (Scheme 2). Note that (b) is the enlarged view of (a). The energies of individual species were represented by the relative energy, which is defined as the difference from the energy of intermediate II. Note that (II−IV) shows the initial relative energies of II after eliminating ligand species (CHA, OH−, or H2O), and IV* shows the relative energies of IV(3), IV(4), IV(5), IV(6), IV(9), and IV(10). Six reaction paths (B to G) were investigated and compared with each other, where the number of included atoms was set to be the same. When ligand species (CHA, OH−, or H2O) were dissociated from zinc complexes, the energies of isolated ligands were evaluated, including the solvation effects by the isoprene trimer medium. The energies of CHA, OH−, or H2O under interaction with isoprene trimer are adopted as the energies of isolated ligands.

the zinc cation and increased the electrophilicity of the zinc cation. This increased the driving force for the sulfur insertion reaction and generated V(3) and V(4) without any TS. This reaction is shown in path D in Scheme 2.

Table 3. Calculated Harmonic Frequencies of νas(COO−) and νs(COO−) in Each Structure for Intermediate IV infrared frequency of COO− (cm−1)

ligand intermediate IV(1) IV(2) IV(3) IV(4) IV(5) IV(6) IV(7) IV(8) IV(9) IV(10)

liganda

Z c

rubber H2O rubber H2O rubber H2O rubber H2O rubber H2O

−

OH OH− OH− OH− CHAd CHA c-C6H11NH−e c-C6H11NH− c-C6H11NH− c-C6H11NH−

charge

antisymmetric

symmetric

Δνb

0 0 0 0 +1 +1 0 0 0 0

1565 1575 1566 1562 1567 1569 1600 1585 1578 1570

1421 1444 1439 1431 1442 1445 1446 1446 1438 1446

144 131 127 131 125 124 154 139 140 124

a A ligand coordinating to zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the BtS−. bDifferential frequency of COO− by subtracting the symmetric frequency from the antisymmetric frequency. cOne repeating unit of cis-1,4-polyisoprene. dCyclohexylamine. e Cyclohexylamino group.

J

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Organometallics Table 4. Calculated Harmonic Frequencies of νas(COO−) and νs(COO−) in Each Structure for Intermediate V infrared frequency of COO− (cm−1)

ligand a

intermediate

Z

ligand

V(1) V(2) V(3) V(4) V(5) V(6)

rubberc H2O rubber H2O rubber H2O

OH− OH− CHAd CHA c-C6H11NH−e c-C6H11NH−

charge

antisymmetric

symmetric

Δνb

0 0 +1 +1 0 0

1581 1565 1587 1591 1586 1581

1404 1447 1438 1439 1432 1440

177 118 149 152 154 141

A ligand coordinating to zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the BtS−. bDifferential frequency of COO− by subtracting the symmetric frequency from the antisymmetric frequency. cOne repeating unit of cis-1,4-polyisoprene. dCyclohexylamine. e Cyclohexylamino group. a

the range 1580−1565 cm−1. The FT-IR spectra indicated that V(3) and V(4) would not be generated preferentially, but they could not be excluded, depending on the vulcanization conditions. This result is consistent with the discussion of reaction paths proposed on the basis of the relative energies of the intermediates and their TSs, as shown in Figure 8: The relative energies along paths D and E for V(3) and V(4) generation, respectively, were considerably higher than those along paths B and C for V(1) and V(2) generation. By contrast, the vibrational modes of the carboxyl groups in IV(7) and IV(8) along path F, IV(9) and IV(10) along path G, and V(5) and V(6) along paths F and G were analyzed by means of DFT calculations; Tables 3 and 4 summarize the results. The calculated harmonic frequencies of νas(COO−) were 1600, 1585, 1578, and 1570 cm−1 for IV(7), IV(8), IV(9), and IV(10), respectively, and 1586 and 1581 cm−1 for V(5) and V(6). A comparison with the experimental spectra, as mentioned above, shows that these results are quite reasonable, except that of IV(7). From a comparison of the relative energy of each path, path B seems the most dominant, and we attempted to determine whether the main intermediate in the vulcanization reaction in this study was V(1) or V(2). Figure 8 suggests that the main intermediate is V(2), based on the calculated activation energy of the sulfur insertion reaction. The differential FT-IR spectra and the planar plots of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) clearly support the predominant generation of V(2), as shown in Figure 9. The νs(COO−) band in IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) did not appear clearly at around 1404 cm−1 and 22 min, but it did appear at around 1440 cm−1 against the νas(COO−) band observed from 1580−1560 cm−1. The experimental symmetric stretching of νs(COO−) of intermediate V appeared in the range 1420−1450 cm−1, and this is important information for comparison with the theoretical data shown in Table 4. Combination of these results with the calculated activation energies shown in Figure 8 suggested that path B was the dominant path for the generation of V(2) among the predicted paths. Note that the DFT calculation predicted the presence of a band at approximately 1565 cm−1 for IV(4), as given in Table 3. The generation of V(2) via path C can therefore not be excluded under the high temperature of the vulcanization reaction. Paths B and C are considered to occur simultaneously during vulcanization. The validity of V(2) was examined by curve fitting of the in situ zinc K-edge XAFS data. The experimental FT-EXAFS spectrum of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) at 22.9 min was used for fitting; the bond lengths and coordination numbers obtained by DFT calculations were used. Figure 10 shows the fitting of V(2). The fitted results (a black dotted

intermediates in the reactant side. This suggested that the potential energy surface was monotonically uphill from the reactant to the product, and the energy differences between IV(7) and V(5) and between IV(8) and V(6) were only 7.6 and 6.0 kcal/mol, respectively. We conclude that path F may be possible under heating at 144 °C because the reactant and product with these small energy differences will coexist in an equilibrium mixture under temperatures of the experimental conditions. Path G: A sulfur insertion reaction was suggested to occur via intermediates IV(9) and IV(10). Only TS connecting IV(10) to V(6), labeled as TS[IV(10)−V(6)], was characterized in the case of H2O ligand, and its activation energy was 48.6 kcal/mol. For IV(9) with a rubber ligand, TS could not be characterized similar to path F. The potential energy surface was monotonically uphill from the reactant (IV(9)) to the product (V(5)) by 15.8 kcal/mol. Although this energy difference is larger than those in path F, path G may be possible under heating at 144 °C as well. Generation of Intermediate V by Sulfur Insertion via Intermediate II. The vibrational modes of the carboxyl groups of intermediates IV and V were analyzed by means of DFT calculations, and the results are summarized in Tables 3 and 4. Considering path B or C, the calculated harmonic frequencies of νas(COO−) were 1565 and 1575 cm−1 for IV(1) and IV(2), respectively, and 1566 and 1562 cm−1, for IV(3) and IV(4). Experimentally, a trapezoidal absorption band of νas(COO−) related to intermediate II was observed at 1580−1560 cm−1 in IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5), as shown in Figure 2d. Intermediates IV(1), IV(2), IV(3), and IV(4) are therefore acceptable. To confirm the proposed structures of intermediate V, we evaluated the harmonic frequencies of νas(COO−) for V(1) and V(2) by means of DFT calculations, and we compared the evaluated values with the experimental frequencies. Table 4 shows the results. The calculated νas(COO−) values of V(1) and V(2) were 1581 and 1565 cm−1, respectively. These theoretical frequencies agree well with the experimental results, which are, 1580 and 1565 cm−1, respectively. This indicated that intermediates V(1) and V(2) were generated predominantly via paths B and C. The vibrational modes of the carboxyl groups in IV(5) and IV(6) along path E and those of V(3) and V(4) along paths D and E were analyzed by DFT calculations as well, and the results are summarized in Tables 3 and 4. The calculated harmonic frequencies of νas(COO−) were 1567 and 1569 cm−1 for IV(5) and IV(6), respectively, and 1587 and 1591 cm−1, for V(3) and V(4). The theoretical frequencies of IV(5) and IV(6) are therefore acceptable, but V(3) and V(4) cannot be the main products because the specific bands should appear in K

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Organometallics

FT-IR similarly to our previous report.19 Figure 11a,b show differential FT-IR spectra in the region 3000−2800 cm−1 for IR-ZnO(0.5)-StH(2)-CBS(1) and IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5), respectively. In both spectra, only the symmetric and antisymmetric methylene stretching showing a gauche disorder were detected at 2853 and 2924 cm−1 as found in the alkane melt,43 and the bands were shifted from 2847 and 2916 cm−1 by heating, respectively. In general, a conformation of gauche chains requires a larger lateral area as compared with an all-trans chain. In addition, the poly(isoprene) segments vibrate vigorously at 144 °C in this study. Therefore, it was clearly suggested that layered multinuclear zinc complexes were not formed in the isoprene rubber matrixes of IR-ZnO(0.5)-StH(2)-CBS(1) and IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5) at 144 °C because of the steric hindrance of the gauche disordered stearate chains. At least, polynuclear typed intermediates of zinc complexes are concluded not to be generated even after the addition of CBS and/or S8 to the compounds of isoprene rubber mixed with ZnO and StH. For the generation of trinuclear and tetranuclear typed intermediates of zinc complexes, a possibility of the generations is considered to be low, because carboxylatebased metal−organic frameworks are known to be especially prone to breakdown under humid conditions.44−53 The characteristic has been attributed to the relatively weak metal−ligand bond between oxygen and the metal.45 In fact, a trend that a four-coordinated zinc−oxygen metal−organic framework is unstable in water has also been observed.54 On the other hand, dinuclear zinc enzymes with bridging carboxylate coordination are well-known in biological systems containing water.38,55,56 Therefore, the dinuclear-type structures for II, IV, and V are most likely for the zinc/stearate complexes during vulcanization under the presence of water in accordance of our previous paper,19 although trinuclear and tetranuclear zinc complexes may not be ignored. Changes of Intermediates during Sulfur Cross-Linking of Isoprene Rubber. We proposed several important intermediates in the sulfur cross-linking of rubber in the presence of ZnO, StH, and an accelerator, CBS, for the first time, by using the results of in situ FT-IR spectroscopy, in situ XAFS spectroscopy, and DFT calculations. The most possible structures of intermediate I, II, and V are illustrated in Figure 12a. Several characteristic bands in the FT-IR spectra indicated the structures of the intermediates, and the accuracy of the suggestions was confirmed by following the in situ changes in the specific bands with time. Figure 12b shows the results of semiquantitative in situ analyses of the 1595, 1580, 1565, 1713, and 1757 cm−1 bands for IR-ZnO(0.5)-StH(2)-CBS(1)S8(1.5) during vulcanization. Each characteristic band was normalized by using the band at 833 cm−1, corresponding to olefinic C−H out-of-plane bending of rubber molecules, as a reference. The validity of the results of our semiquantitative FT-IR analyses was supported by the observation that a band at 1660 cm−1, corresponding to the ν(CC) stretching vibration of isoprene rubber, hardly changed with time during the reaction, as shown in black in the figure. A curing curve for IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) is shown together with the variations in specific bands (Figure 12c); this curing curve was recorded separately, but the temperature control was similar to that used for the in situ FT-IR. The curing behavior clearly showed an increase of torque at approximately 18 min, which suggested a progression of the sulfur cross-linking of

Figure 9. (a) Differential FT-IR spectra and (b) their planar plot of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5). Each number indicates the reaction time in minutes during its vulcanization. The change of color from blue to red in the planar plot indicates the high to low relative transmittance. White color shows the relative transmittance which is out of the range 98.5−100. The blue, light blue, green, yellow, and red dotted lines in (a) indicate the wavenumbers at 1595, 1580, 1565, 1440, and 1404 cm−1, respectively.

Figure 10. Real part of inverse FT-EXAFS spectrum of IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5) at 22.9 min. A black dotted line is the fitted result for V (2).

line) coincided well with the experimental results (a red solid line). The obtained fitting parameters are given in Table 5, along with those for V(1), V(3), V(4), V(5), and V(6). The parameters of all intermediates did not deviate from those (0 < σ2 < 0.02, ΔE0 < 10 eV) of the normal EXAFS fitting.39 Additionally, the R-factors of these intermediates were low. Therefore, the structure V(2) cannot be excluded, at least based on the EXAFS analysis. A combination of the FT-IR and EXAFS data suggested that V(2) was a more probable structure than V(1), V(3), V(4), V(5), and V(6) for intermediate V. Namely, the sulfur insertion reaction occurred readily when the CHA was removed from intermediate II, and water coordinated with the zinc cation, which was coordinated by the nitrogen atom of the benzotiazole group. Here, an impossibility of generations of polynuclear typed intermediates of zinc complexes is discussed using results of L

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 5. Results of EXAFS Fitting for Structure of Intermediate V ligand intermediate

Z

atom

CNa

R (Å)

σ2 (Å2)b

ΔE0 (eV)c

R-factor

V(1)

rubber

3.4 × 10−4

rubber

−1.8

2.5 × 10−4

V(4)

H2O

−3.0

1.2 × 10−4

V(5)

rubber

0.6

2.4 × 10−4

V(6)

H2O

0.004 0.004 0.004 0.007 0.012 0.001 0.003 0.007 0.003 0.005 0.007 0.004 0.007 0.004 0.003 0.005 0.004 0.009

5.2

V(3)

1.97 2.26 1.84 2.05 2.50 2.04 1.98 2.39 2.11 1.96 2.24 1.78 1.94 2.27 2.01 1.97 2.26 1.86

2.3 × 10−4

H2O

2.50 0.50 0.50 3.00 0.50 0.50 2.00 0.50 1.00 2.50 0.50 1.00 2.00 0.50 1.00 2.50 0.50 1.00

0.0

V(2)

O S N O S N O S N O S N O S N O S N

−1.1

1.5 × 10−4

a

Coordination number. bDebye−Waller factor. cEnergy shift.

Figure 11. Differential FT-IR spectra in the region of 3000−2800 cm−1 of symmetric and antisymmetric methylene stretching bands of (a) IR-ZnO(0.5)-StH(2)-CBS(1) and (b) IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5), respectively. The differential spectra were obtained by subtracting infrared spectra of isoprene rubber from each original spectrum at the same elapsed time. The baseline from 3300 to 2600 cm−1 was used for subtraction.

Figure 12. (a) Vulcanization reaction via the most possible active intermediates in IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5). (b) Variations of the specific bands at 1595 cm−1 (blue), 1580 cm−1 (red), 1565 cm−1 (green) 1757 cm−1 (blown), 1713 cm−1 (yellow), and 1660 cm−1 (black) during the sulfur cross-linking reaction of IR-ZnO(0.5)StH(2)-CBS(1)-S8(1.5). The arrows in the inset figure of (b) indicate the time before the relative intensities of each specific band greatly decreases. (c) Curing curve of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) (red). Black lines both in (b) and (c) show the temperature profiles.

rubber. The semiquantitative FT-IR showed that the relative absorbance of the specific bands changed considerably. Consumption of the intermediates at about 24 min apparently stopped further increases in the torque. The detail of changes was explained below. First, an increase in the intensity of the band at 1595 cm−1, corresponding to the generation of I(1), I(2), and/or I(3), was observed during heating to 144 °C, and then the decrease in

the intensity was observed. The intensities of the bands at 1580 and 1565 cm−1 increased and then gradually increased with decreasing intensity of the band at 1595 cm−1, when the temperature became constant at 144 °C. These changes mainly indicate the consumption of I(1), I(2), and/or I(3) to M

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 3. Sulfur Cross-Linking Reaction of Isoprene Rubber via V(2)

dimer of StH, at 1757 and 1713 cm−1, respectively, increased rapidly at approximately 22 min, and the intensities of the bands at 1580 and 1565 cm−1 decreased. This suggests the generation of StH, probably because of decomposition of the intermediates composed of the bridging bidentate zinc/ stearate structure after the sulfur cross-linking reaction, which was activated mainly by V(2). Furthermore, the intensities of the three bands at 1595, 1580, and 1565 cm−1 increased slightly during reversion of the sulfur cross-linking reaction. As we have reported previously, intermediate I was involved in controlling the type of sulfidic linkages formed in the sulfur cross-linking of isoprene rubber.23,24 These novel dinuclear bridging bidentate zinc/stearate complex intermediates may also cause de-cross-linking of sulfidic linkages in the vulcanizate. Details of this aspect will be reported elsewhere in the future.

generate intermediate II. In IR-ZnO(0.5)-StH(2)-CBS(1)S8(1.5), the intensities of the bands at 1580 and 1565 cm−1 further increased during the period 18 to 22 min and then decreased greatly in the order of 1580 and 1565 cm−1, as can be clearly seen in the inset figure of Figure 12b. (This is also supported by Figures 2d and 9a.) Slightly before these intensity reductions, the intensity of the band at 1595 cm−1 decreased abruptly. These changes suggest the rapid conversion of I(1), I(2), and/or I(3) predominantly to V(1) and V(2) via II(1) and II(2), respectively. A combination of the in situ FT-IR results and the curing behavior suggested that the increased intensities of the bands at 1580 and 1565 cm−1 from approximately 18 to 22 min were related to the abrupt increase in the torque during vulcanization. The sulfur crosslinking of isoprene rubber was apparently accelerated by the novel dinuclear bridging bidentate zinc/stearate complexes. Sulfur Cross-Linking of Isoprene Rubber by Dinuclear Bridging Bidentate Zinc/Stearate Intermediates. In this study, for example, one reaction path of sulfur cross-linking via V(2) is proposed in Scheme 3. The attacking species from the vulcanization system is well-known to contain sites for proton acceptance and electron acceptance in a proper steric relationship with the rubber containing allylic hydrogen atoms.6 Therefore, a reaction of sulfur atoms in the left side of S8 fragment in V(2) is postulated, as shown in Scheme 3. Furthermore, the zinc cation has been known to play a role in activating thiolate toward nucleophilic attack.38 Therefore, it is conceivable that the sulfur atom neighboring the zinc in the right side of V(2) extracts the proton at the allyl position of isoprene rubber and resulted in a sulfur cross-linking reaction on the right end of S8 fragment in V(2). Then, activation and reaction energies were estimated. The reaction path from the reactant structure V(2) through the cross-linked structure VII(1) encounters two transition states (TS[V(2)−VI(1)] and TS[VI(1)−VII(1,2)]). The activation and reaction energies were 44.4 and 5.3 kcal/mol from the reactant of V(2) to TS[V(2)−VI(1)] and to intermediate VI(1), respectively. Moreover, the activation and reaction energies were 44.9 and 9.3 kcal/mol from intermediate VI(1) to TS[VI(1)−VII(1,2)] and to the products of “VII(1) + VII(2)”, respectively. These results suggested that this reaction path was acceptable considering the heating at 144 °C under this vulcanization condition. It is worth noting that two different sulfur cross-linking reactions of isoprene rubber could occur on the right and the left sides of S8 fragment in V(2). The details of the DFT calculations for the vulcanization of isoprene rubber via intermediate V will be reported elsewhere, including other reaction paths. Despite the long history of rubber science and technology, the intermediates identified in this study have not been reported previously. Figure 12b shows that the intensities of the specific bands that were identified as the ν(CO) for the monomer and

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CONCLUSIONS We showed, for the first time, the role of [Zn 2 (μO2CC17H35)2]2+·4X (X; a hydroxyl group, water and/or a rubber segment) in the sulfur cross-linking of isoprene rubber. In situ FT-IR and synchrotron in situ XAFS spectroscopic analyses combined with DFT calculations clearly showed that BtS− and CHA were generated on the dinuclear bridging bidentate zinc/stearate intermediate via hydrolysis of CBS by activated water in the zinc/stearate complex, similarly to an enzymatic reaction.57 This reaction occurred on the dinuclear bridging bidentate zinc/stearate intermediate, when a water molecule was coordinated to the zinc cation opposite the zinc cation coordinated by a nitrogen atom of the benzothiazole group. This novel dinuclear bridging bidentate zinc/stearate complex was previously unknown, despite the long history of rubber science and technology. In zinc enzymology, similar dizinc cations bridging by two carboxylate groups are known to play numerous versatile roles in catalysis, in which a water molecule coordinated to a zinc cation plays a critical role in biological control reactions.37,38,55,56,58 The steric hindrance resulting from the bridging bidentate structures of dizinc intermediates may also affect the selectivity (e.g., of an enzymatic reaction).56 This aspect must be related to the hydrolysis of CBS, which affects the vulcanization induction period. Until now, zinc 1,3-benzothiazole-2-thiolate has been widely believed to be the intermediate of vulcanization.1−3,5−9,33,59,60 However, the present work shows that the newly identified II, IV and V (Schemes 1a and 2) are intermediates in the sulfur cross-linking reaction during CBS-accelerated vulcanization with the ZnO and StH system, and the ZnSt2 system. In general, it is difficult to generate BtS− selectively because it is highly active and therefore unstable. However, the bridging bidentate structure of the dizinc intermediate stabilizes BtS− by N

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 6. Recipes for Preparation of Samples sample code IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) IR-ZnSt2(3.88)-CBS(1)-S8(1.5) IR-ZnO(0.5)-StH(2)-CBS(1) IR-ZnO(0.5)-StH(2)-MBT(0.63) IR-CBS(1) IR-MBT(0.63)

units

IRa

ZnO

StHb

g

100

0.5 5.9 × 10−5

2.0 6.7 × 10−5

phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g

100 100 100

0.5 5.9 × 10−5 0.5 6.0 × 10−5

2.0 6.7 × 10−5 2.0 6.8 × 10−5

100

ZnSt2c

CBSd

3.88 5.8 × 10−5

1.0 3.6 × 10−5 1.0 3.6 × 10−5 1.0 3.7 × 10−5

MBTe

S8f 1.5 5.6 × 10−5 1.5 5.5 × 10−5

0.63 3.7 × 10−5 1.0 3.7 × 10−5

100

0.63 3.7 × 10−5

a

Isoprene rubber. bStearic acid. cZinc stearate. dN-(1,3-Benzothiazol-2-ylsulfanyl)cyclohexanamine. e2-Mercaptobenzothiazole. fElemental sulfur. Parts per one hundred rubber by weight.

g

one hundred rubber by weight (phr). The latter is a conventional unit in rubber science and technology. Abbreviations for all samples were assigned on the basis of the combinations of reagents, that is, ZnO, sulfur (S8), ZnSt2, MBT, and CBS. The “IR” and the numbers in parentheses in the sample codes show isoprene rubber and the amount of each reagent mixed with isoprene rubber in phr, respectively. The last number after the hyphen in the code shows the elapsed time in minutes of heating from 35 °C. Fourier-Transform Infrared Spectroscopy. In situ timeresolved FT-IR was performed at 32 scans for 40 s every 44 s across the wavenumber range 4000−400 cm−1 by a single-reflection attenuated total reflectance method on a diamond plate (GradiATR, PIKE Technologies, Wisconsin, USA) in an IR Prestige-21 instrument (Shimadzu Co., Kyoto, Japan) The resolution was 4 cm−1. The temperature was kept at 35 °C for 5 min, was increased from 35 to 144 °C for 10 min, and was maintained at 144 °C for a specific time according to a controlled program. The temperature error was controlled to less than ±1 °C. The sample area was about 2 × 3 mm2 and the thickness was approximately a few millimeters. The peak assignments for the FT-IR spectra of IR-CBS(1), IR-MBT(0.63), IRZnO(0.5)-StH(2)-MBT(0.63), IR-ZnO(0.5)-StH(2)-CBS(1), and IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) are shown in Figure S1. Differential FT-IR spectra were obtained by subtracting the spectrum of isoprene rubber from each original spectrum; the baseline was 1820−700 cm−1. Theoretical Calculations. DFT calculations were performed to investigate the structures, energies, and infrared vibrational frequencies of the zinc dinuclear complexes that were suggested by the experimental results. For the sulfur insertion step, the reaction path was evaluated as follows. First, the TS was characterized, and the TS structures were confirmed by the single imaginary frequency and corresponding eigenvector, that is, the nuclear displacement vectors along the reaction path. IRC analysis40,41 was then performed for the TS in both directions, that is, a reactant and a product. The IRC step width was short, and ordinary optimization was followed at the end point of IRC to connect the TS to the reactant and the product. The activation energy was determined by the energy difference between the reactant and the TS. For the DFT calculations, the Becke threeparameter Lee−Yang−Parr hybrid-type functional62 was used; it was implemented in the Gaussian 09 program.63 For the basis sets of zinc atoms, the Los Alamos ECP64 with the double-ζ valence basis65 was used. The 6-31G(d) and 6-31G basis sets were used for sulfur atoms and the other atoms, respectively. To characterize the predicted structures by the DFT calculations, a repeating monomer unit of cis1,4-polyisoprene was used as a ligand, which is shown as “rubber” to identify the kind of ligand in tables and text. A specific binding site of the rubber molecule attached to the zinc atom was set at a double bond in cis-1,4-polyisoprene. The coordination of repeating monomer unit of cis-1,4-polyisoprene to the zinc cation was investigated as the

complexation, even at the high temperature of vulcanization. This stabilization may come from the steric hindrance created by the bridging bidentate structures of dizinc intermediates, similar to the case of the “reaction bowl” 4-tert-butyl-2,6bis[(2,2″6,6″-tetramethyl-m-terphenyl-2′-yl)methyl]phenyl).61 Furthermore, the derivatives of intermediate II induced sulfur insertion to generate mainly V(2) when CHA was removed from intermediate II (paths B and C in Scheme 2). The sulfur insertion reaction via the elimination of water molecule from intermediate II was also suggested as a possible path under the heating at a high temperature of the vulcanization. The novel intermediate significantly accelerated sulfur cross-linking of rubber molecules at a high temperature, probably via enzymatic-like reactions. The reaction of ZnO and StH generates water, and a small amount of water is generally existed in a rubber matrix. These water molecules affected the sulfur cross-linking reaction in the CBS-accelerated sulfur cross-linking in the ZnO and StH system, and the ZnSt2 system. Water was involved not only in the generation of intermediate II but also in the production of active intermediates IV and V. Therefore, the coordination of water at appropriate positions in the dinuclear bridging bidentate zinc/stearate intermediate is crucial. The results of this study will be useful in the development of rubber science and will facilitate a paradigm shift to the next stage of materials science.10 Further study is necessary to achieve a breakthrough in rubber vulcanization. Thus, elucidation of the overall vulcanization mechanism is in progress, and details will be reported in the near future.

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EXPERIMENTAL SECTION

Sample Preparation. Isoprene rubber (IR2200) was supplied by the JSR Co., Tokyo, Japan. Elemental sulfur (purity: 99.9%, powder, 150 mesh), StH (LUNAC S-25), ZnO (average diameter: 0.29 μm), and CBS (Sanceler CM-G) were rubber-processing commercial grade and used as received. They were supplied by the Hosoi Chemical Industry Co., Ltd., Tokyo, Japan, Kao Co., Tokyo, Japan, Sakai Chemical Industry Co., Ltd., Osaka, Japan, and Sanshin Chemical Industry Co., Ltd., Yamaguchi, Japan, respectively. ZnSt2 (Wako special grade, FUJIFILM Wako Pure Chemical Corporation., Osaka, Japan) and High-grade 2-MBT (purity: 97%, Sigma-Aldrich, Inc., Missouri, USA) were used as received. The rubber compounds were prepared according to the recipes shown in Table 6 by conventional mixing at room temperature on a two-roll mill with a water cooling system (6 × 15 test roll, Kansai Roll Co., Ltd., Osaka, Japan). These amounts of reagents are shown in units of mol/gram and parts per O

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Zn−CC π type coordination. Natural population analysis was used to estimate the charges on atoms for each structure.66 Zinc K-edge XAFS Spectroscopy. In situ synchrotron XAFS spectroscopy was performed every 87 s at the BL-14B2 beamline of SPring-8 in Harima, Japan.67 The rubber compound was sealed in a Teflon tube with Kapton windows and then heated in a laboratorymade reaction cell according to a controlled program, which was set to heat at 35 °C for about 8 min, from 35 to 144 °C for 10 min, and to hold the temperature at 144 °C for a specific time. The temperature control error was less than ±1 °C. The sample was cylindrical; the diameter was 10 mm, and the length was based on the molar concentration of each element in the sample, determined using XAFS_SAMPLE software.68 The X-ray irradiation time was 52 s. The monochromator, which was a Si(311) crystal, was calibrated against the first inflection point of a zinc foil sample. The X-ray absorption spectra of the samples were recorded in transmission mode using ionization chambers. The XAFS spectra were analyzed with the ATHENA and ARTEMIS XAFS analysis package 69 and FEFF6.02L.70,71 R-factor is defined as the sum-of-squares of the fractional misfits as described in IFEFFIT.69 R-factor shows the degree of best fitting. The smaller the R-factor is, the better the fitting accuracy. The EXAFS spectra were analyzed in detail by parameter fitting of the EXAFS data in the range 3−13 Å on the basis of theoretically obtained spectra by using FEFF6.02L. The EXAFS equation is given below.39

χ (k) = S02 ∑ Ni i

fi (k) kDi2

*E-mail for Y.I.: [email protected]. *E-mail for H.K.: [email protected]. ORCID

Yuko Ikeda: 0000-0003-2809-8911 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST ALCA program (Grant Number: JPMJAL1501), and SUMITOMO SCIENCE AND TECHNOLOGY FOUNDATION. The XAFS experiments were performed at the BL-14B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos.2009A1929, 2009B2044, 2010A1778, 2010B1928, 2012A1419, 2012B1891, 2013A1828, 2013B1840, 2014A1574, 2017A1611). The authors thank to Dr. A. Tohsan for her useful supports.

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(1)

2me(E − E0) ℏ

In this equation, k is the wavenumber, S20 is the amplitude reduction factor, Ni is a coordination number, f i(k) is the backscattering factor, Di is the distance from the absorbing atom to the scattering atom, λ(k) is the mean free path of the photoelectron, σi2 is the Debye− Waller factor, δi(k) is the phase shift, me is the mass of the electron, E is the photon energy, and E0 is the edge energy. The EXAFS spectra were analyzed using k3-weighted χ(k). Here, S20 was determined to be 0.9 based on the EXAFS spectrum of a ZnO pellet. Curve fitting was conducted for the Wurtzit structure of ZnO with a coordination number of four. This procedure gave a Zn−O distance of 1.97 Å, which is consistent with data in the literature.72 Curing Behavior. The curing properties of the rubber compound were investigated by using a MR-500 instrument (Rheology Co., Kyoto, Japan) to measure the torque against time. The temperature was set according to a controlled program, which was set at 35 °C for a certain time and heated from 35 to 144 °C for 10 min and then held at 144 °C for a specific time.

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REFERENCES

(1) Bateman, L.; Moore, C. G.; Porter, M.; Saville, B. The Chemistry and Physics of Rubber-like Substances; Bateman, L., Ed.; MacLaren Sons Ltd.: London, 1963. (2) Coleman, M. M.; Shelton, J. R.; Koenig, J. L. Sulfur Vulcanization of Hydrocarbon Diene Elastomers. Ind. Eng. Chem. Prod. Res. Dev. 1974, 13 (3), 154−166. (3) Trivette, C. D.; Morita, E.; Maender, O. W. Prevulcanization Inhibitors. Rubber Chem. Technol. 1977, 50 (3), 570−600. (4) Morita, E. S-N Compounds as Delayed Action Chemicals in Vulcanization. Rubber Chem. Technol. 1980, 53 (3), 393−436. (5) Chapman, A. V.; Porter, M. Natural Rubber Science and Technology; Roberts, A. D., Ed.; Oxford University Press: Oxford, 1988; pp 511−568. (6) Coran, A. Y. The Science and Technology of Rubber, 2nd ed.; Mark, J. E., Erman, B., Eirich, F. R., Eds.; Academic Press: San Diego, 1994; pp 339−385. (7) Coran, A. Y. Chemistry of the Vulcanization and Protection of Elastomers: A Review of the Achievements. J. Appl. Polym. Sci. 2003, 87 (1), 24−30. (8) Ghosh, P.; Katare, S.; Patkar, P.; Caruthers, J. M.; Venkatasubramanian, V.; Walker, K. A. Sulfur Vulcanization of Natural Rubber for Benzothiazole Accelerated Formulations: From Reaction Mechanisms to a Rational Kinetic Model. Rubber Chem. Technol. 2003, 76 (3), 592−693. (9) Heideman, G.; Datta, R. N.; Noordermeer, J. W. M.; van Baarle, B. Activators in Accelerated Sulfur Vulcanization. Rubber Chem. Technol. 2004, 77 (3), 512−541. (10) Ikeda, Y.; Kato, A.; Kohjiya, S.; Nakajima, Y. Rubber Science: A Modern Approach; Springer: Singapore, 2017. (11) Nieuwenhuizen, P. J.; Ehlers, A. W.; Hofstraat, J. W.; Janse, S. R.; Nielen, M. W. F.; Reedijk, J.; Baerends, E. J. The First Theoretical and Experimental Proof of Polythiocarbamatozinc(II) Complexes, Catalysts for Sulfur Vulcanization. Chem. - Eur. J. 1998, 4 (9), 1816− 1821. (12) Nieuwenhuizen, P. J.; Ehlers, A. W.; Haasnoot, J. G.; Janse, S. R.; Reedijk, J.; Baerends, E. J. The Mechanism of Zinc(II)Dithiocarbamate-Accelerated Vulcanization Uncovered; Theoretical and Experimental Evidence. J. Am. Chem. Soc. 1999, 121 (1), 163− 168. (13) Steudel, R.; Steudel, Y. Interaction of Zinc Oxide Clusters with Molecules Related to the Sulfur Vulcanization of Polyolefins (“Rubber. Chem. - Eur. J. 2006, 12 (33), 8589−8602. (14) Steudel, R.; Steudel, Y.; Wong, M. W. Complexation of the Vulcanization Accelerator Tetramethylthiuram Disulfide and Related

where k=

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Corresponding Authors

2 2

e−2Di / λ(k)e−2k σi sin(2kDi + δi(k))

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00193. Optimized Cartesian coordinates together with their single-point energy (a.u.) and the imaginary eigenvalues of transition states (XYZ) Characterization of FT-IR, charges on the atoms estimated by the natural population analysis, differential FT-IR spectra and their planar plot of IR-ZnO(0.5)StH(2)-CBS(1), optimized structures when heterocycle of CBS coordinates to the zinc cation via sulfur, and speculated sulfur insertion reaction via path A (PDF) P

DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.9b00193 Organometallics XXXX, XXX, XXX−XXX