Copolymerization of Carbon Dioxide with Epoxides Catalyzed by

May 16, 2017 - *E-mail: [email protected] (B.-T. Ko). Tel. .... Hui-Ju Chuang , Wan-Ling Liu , Chi-Tien Chen , Chih-Kuang Chen , Bao-Tsan Ko...
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Copolymerization of Carbon Dioxide with Epoxides Catalyzed by Structurally Well-Characterized Dinickel Bis(benzotriazole iminophenolate) Complexes: Influence of Carboxylate Ligands on the Catalytic Performance Li-Shin Huang, Chen-Yen Tsai, Hui-Ju Chuang, and Bao-Tsan Ko* Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan S Supporting Information *

ABSTRACT: A series of structurally well-defined dinickel carboxylate complexes based on the RBiIBTP derivatives [RBiIBTP = bis(benzotriazole iminophenolate), where R = 3C for the propylbridged backbone and 5C for the 2,2-dimethyl-1,3-propyl-bridged backbone] were synthesized and developed for copolymerization of CO2 and epoxides. The one-pot reactions of nickel perchlorate with the RBiIBTP-H2 proligands and an appropriate amount of carboxylic acid derivatives (CF3COOH or 4-X-C6H4CO2H; X = H, CF3, OMe) upon the addition of triethylamine in refluxing methanol (MeOH) afforded dinuclear nickel dicarboxylate complexes, which could be formulated as either [(RBiIBTP)Ni2(O2CCF3)2] (1 and 2) or [(RBiIBTP)Ni2(O2CC6H4-4-X)2] (3−7). The dinickel monobenzoate complexes [(RBiIBTP)Ni2(O2CPh)(ClO4)(H2O)] [R = 3C (8) and 5C (9)] were prepared by using a similar synthetic route in tetrahydrofuran under reflux with a ligand precursor to metal salt to benzoic acid ratio of 1:2:1 in the presence of NEt3. Recrystallization of neutral nickel perchlorate complex 8 in a saturated MeOH or ethanol (EtOH) solution gave ionic and alcohol-solvated monobenzoate bimetallic analogues [(3CBiIBTP)Ni2(O2CPh)(S)2]ClO4, where S = MeOH (10) and EtOH (11). Single-crystal X-ray crystallography of dinickel analogues 1−11 indicates that the BiIBTP scaffold performs as a N,O,N,N,O,N-hexadentate ligand to chelate two Ni atoms, and the ancillary carboxylate group adopts a bridging bidentate bonding mode. Catalysis for copolymerization of carbon dioxide (CO2) with cyclohexene oxide (CHO) by complexes 1−9 was systematically investigated, and the influence of carboxylate ligands on the catalytic behavior was also studied. Trifluoroacetateligated dinickel complex 1 efficiently catalyzed CO2 and CHO with a high turnover frequency (>430 h−1) in a controlled fashion, generating perfectly alternating poly(cyclohexenecarbonate) with large molecular weight (Mn > 50000 g/mol). In addition to CO2/CHO copolymerization, bimetallic complex 1 was found to effectively copolymerize CO2 with 4-vinyl-1,2-cyclohexene oxide (VCHO) or cyclopentene oxide, producing the high carbonate contents of poly(VCHC-co-VCHO)s and highly alternating poly(cyclopentene carbonate)s, respectively. This study also enabled us to compare the catalytic efficiency of using cyclic epoxides with different ring strains or functional groups as comonomers by the dinickel catalyst 1.

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INTRODUCTION Of the carbon dioxide (CO2) fixation investigations, metalcatalyzed CO2/epoxide coupling by discrete catalysts provides one of the most promising processes to fix CO2 into high-valueadded fine chemicals such as cyclic carbonates and aliphatic polycarbonates, as illustrated in Scheme 1.1 To effectively activate CO2 and increase their coupling reactivity, homogeneous catalysts have been synthesized based on Al, Co, Cr, Fe, Mg, Zn, and group 4 complexes coordinated by various ancillary ligands.2 Among these studies, the most developed catalysts are salen-type metal complexes by virtue of the easily modifiable electronic and steric effects of such ligands.3 A series of impressive catalytic systems of {[(salcy)CoX]/[ammonium salt]} (X = carboxylate, phenoxide) were therefore reported by Lu et al. and Coates et al., respectively; these binary systems © 2017 American Chemical Society

effectively catalyze propylene oxide and CO2 with a large turnover frequency (TOF; >200 h−1) and good catalytic properties including great copolymerization selectivity (>99% carbonate linkage) and high regioselectivity (head-to-tail linkages >95%).4 Recently, the development of metal catalysts possessing a dinuclear architecture has been paid considerable attention because such bimetallic complexes were shown to realize good catalytic efficiency with satisfactory TOFs.5−7 For instance, dinuclear carboxylate complexes incorporated by macrocyclic ligands of Co(II/III), Fe(III), Zn(II), Zn(II)− Mg(II), and Mg(II) were developed by Williams et al., and these metal complexes were demonstrated to be singleReceived: January 18, 2017 Published: May 16, 2017 6141

DOI: 10.1021/acs.inorgchem.7b00090 Inorg. Chem. 2017, 56, 6141−6151

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Inorganic Chemistry

defined dinickel complexes in catalyzing the copolymerization of CO2 and epoxides were studied.9,10 Because the CO2/ epoxides copolymerization process could be considered to be a series of coordination−insertion steps, the catalytic efficiency of the catalysts was expected to be highly dependent on ancillary ligands coordinated on metal atom(s). As a result, we aim to study metal catalysts containing different carboxylate coligands/ initiators and to compare the influence of such coligands of this kind on catalytic behaviors. In addition to the facilely prepared dinickel acetate complexes, no other carboxylate-ligated nickel complex of BiIBTP ligands has been isolated to date. More importantly, an effective synthetic route for the preparation of bimetallic dicarboxylate complexes denoted as [(L)M2(O2CR)2] (L = multidentate ligand; M = divalent metal; O2CR = carboxylate) has not been reported. In this Article, we present the synthesis, structure, and catalysis for CO2/epoxides copolymerization of novel dinuclear nickel carboxylate complexes supported by BiIBTP derivatives with different diimine backbones. Moreover, we report for the first time the use of well-characterized dinickel complexes as catalysts to generate alternating poly(cyclopentene carbonate)s.

Scheme 1. Production of Cyclic Carbonates (Route I) and Aliphatic Polycarbonates (Route II) from the Coupling of CO2 with Epoxides

component catalysts for the copolymerization of CO2 and cyclohexene oxide (CHO).6 More recently, a series of biphenolate-linked dicobalt complexes using the combination of a trivalent cobalt metal with rigid biphenyl bridging salentype ligands were prepared for the copolymerization of broad meso-epoxides and CO2 with great activity (TOF up to 1409 h−1) and enantioselectivity (>98% ee).7 Considering the development of well-designed dinuclear metal catalysts for the efficient coupling/copolymerization of CO2 with epoxides, nitrogen-heterocycle-based phenoxide or aminebisphenolate derivatives seem to be promising ligand candidates.8 Our group has successfully developed a new family of dicobalt(II) and binickel(II) complexes bearing a bis(benzotriazole iminophenolate) (BiIBTP) or a diaminebis(benzotriazole phenolate) (DiBTP) ligand framework,9 and the latter dinickel acetate complexes were found to effectively catalyze the alternating CO2/CHO copolymerization with a high TOF in a controlled character.9b,c Compared with the earlier reported homogeneous catalysts using the Co(III), Cr(III), and Zn(II) metal centers, only a few structurally well-

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RESULTS AND DISCUSSION Bimetallic nickel carboxylate complexes 1−10 supported by BiIBTP ligands were readily synthesized via a one-pot procedure of nickel perchlorate salt ([Ni(ClO4)2·6H2O]) with the proligands and carboxylic acid derivatives, as outlined in Scheme 2. Ligand precursors 3CBiIBTP-H2 and 5CBiIBTP-H2 were prepared according to the earlier literature procedures.9a,b The treatment of 3CBiIBTP-H2 or 5CBiIBTP-H2 with 2 equiv of [Ni(ClO4)2·6H2O] followed by the addition of trifluoroacetic acid (2 mol equiv) and triethylamine (NEt3, 5.0 equiv) in a refluxing methanol (MeOH) solution gave trifluoroacetatobridged dinickel complexes [(3CBiIBTP)Ni2(O2CCF3)2] (1) and [(5CBiIBTP)Ni2(O2CCF3)2] (2) in moderate yield. The dinickel dicarboxylate analogues [(3CBiIBTP)Ni2(O2CC6H4-4-

Scheme 2. Synthetic Pathways for Dinickel Carboxylate Complexes 1−11

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Inorganic Chemistry Table 1. Selected Bond Lengths and Angles of the Nickel Complexes 1, 3, 8, and 10 1 Ni1−O1 Ni1−O2 Ni1−O3 Ni1−O5 Ni1−O9 Ni1−N7 Ni1−N8 Ni2−O1 Ni2−O2 Ni2−O4 Ni2−O5 Ni2−O6 Ni2−N1 Ni2−N4 O1−Ni1−O2 O3−Ni1−O5 O3−Ni1−O9 N7−Ni1−N8 O1−Ni1−N8 O2−Ni1−N7 O1−Ni2−O2 O2−Ni2−O5 O4−Ni2−O5 O4−Ni2−O6 N1−Ni2−N4 O1−Ni2−N4 O2−Ni2−N1

3

8

2.038(4) 2.042(3) 2.143(4) 2.127(4)

2.0352(16) 2.0595(15) 2.0941(17) 2.1253(18)

2.037(4) 2.041(4) 2.028(4) 2.040(4) 2.070(4)

2.0491(18) 2.050(2) 2.0346(15) 2.0356(16) 2.0580(17)

2.082(4) 2.053(5) 2.082(4)

2.0530(17) 2.0393(19) 2.0712(19)

91.10(14) 161.57(15)

93.32(6) 161.87(6)

95.81(17) 169.69(17) 172.55(16) 91.42(14)

96.26(8) 168.12(7) 172.04(7) 94.06

167.44(15) 100.98(17) 176.15(16) 174.12(16)

169.02(6) 100.50(7) 178.16(7) 175.30(7)

X)2] [X = H (3), CF3 (5), OMe (7)] and [(5CBiIBTP)Ni2(O2CC6H4-4-X)2] [X = H (4) and CF3 (6)] were obtained in 66−76% yield by utilizing [Ni(ClO4)2·6H2O] as the metal precursor under the synthetic route with a ligand precursor to metal salt to benzoic acid derivative (4-X-C6H4CO2H; X = H, CF3, OMe) ratio of 1:2:5 in the presence of NEt3 (5.0 equiv) in a MeOH solution under reflux. It should be noted that the reaction of 3CBiIBTP-H2 with a stoichiometric amount of [Ni(ClO4)2·6H2O] (2 equiv) and benzoic acid (2 equiv) on the basis of the same synthetic method generated mixtures of the dinickel monobenzoate complex [(3CBiIBTP)Ni2(O2CPh)(ClO4)(H2O)] (8) and dibenzoate 3. However, using this kind of synthetic pathway with a ligand precursor/metal salt/ benzoic acid ratio of 1:2:1 in tetrahydrofuran (THF) afforded water-solvated bimetallic nickel perchlorates 8 and [(5CBiIBTP)Ni2(O2CPh)(ClO4)(H2O)] (9) after crystallization from the mixing solution of dichloromethane (CH2Cl2) and hexane. Interestingly, ionic and alcohol-solvated monobenzoate dinuclear analogues [(3CBiIBTP)Ni2(O2CPh)(S)2]ClO4 [S = MeOH (10) and ethanol (EtOH; 11)] could be obtained through recrystallization of the neutral nickel perchlorate complex 8 in a saturated MeOH or EtOH solution. Alternatively, using MeOH as the reaction solvent under a similar synthetic method enabled the preparation of ionic dinickel perchlorate complex 10 in 64% yield. Complexes 1−9 are isolated as air-stable green crystalline solids and fully characterized by elemental analysis and electrospray ionization mass spectrometry (ESI-MS). For example, a daughter peak corresponding to [(3CBiIBTP)Ni2(O2CC6H4-4-CF3)]+ (m/z 1043.6, 100%) was observed in 5 based on the ESI-MS spectrum (Figure S1), which implied that a binuclear

1.998(2) 2.067(2) 2.119(2) 2.137(2) 2.037(3) 1.998(3) 1.994(2) 2.040(2) 2.011(2) 2.246(2) 2.059(3) 2.069(3) 82.82(8) 170.84(10) 97.77(10) 172.76(10) 165.34(9) 83.59(8) 95.80(8) 174.91(9) 105.07(10) 168.56(9) 166.04(9)

10 1.999(2) 2.104(2) 2.154(3) 2.136(3) 2.056(3) 2.009(3) 2.019(2) 2.058(2) 1.999(3) 2.115(3) 2.097(3) 2.031(3) 83.29(9) 83.03(12) 89.52(13) 172.59(12) 100.85(11) 83.98(9)

172.76(10) 106.05(11) 171.24(11) 165.59(10)

configuration for nickel complex 5 exists in solution. The molecular structures of all bimetallic nickel complexes were further characterized by single-crystal X-ray crystallography. Single crystals of nickel complexes 1−11 suitable for X-ray structural analysis were obtained from their saturated acetone, CH2Cl2, toluene, MeOH, or EtOH solutions. X-ray structural determination reveals that complexes 1−7, 8 and 9, and 10 and 11 are isostructural. The molecular structures of 1, 3, 8, and 10 are selectively and detailedly described in the following discussions. The selected bond distances and angles between metals and ligands are tabulated in Table 1. As illustrated in Figure 1, the solid structure of 1 reveals a dinuclear nickel conformation constructed by one ligand scaffold species (3CBiIBTP) and two bridging bidentate trifluoroacetato groups.

Figure 1. ORTEP drawing of complex 1 with probability ellipsoids drawn at the 50% level. H atoms are omitted for clarity. 6143

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Inorganic Chemistry Two Ni atoms are attributed to two different six-coordinated chemical environments: the Ni1 atom is bonded by two N atoms of propyl-linked diimine moieties, whereas two N atoms from varying benzotriazole fragments coordinate to the Ni2 atom. The geometry around each Ni atom in 1 is a distorted octahedron where the average bond distances between the Ni atom and O(phenoxy), N(aldimine), and N(benzotriazole) are 2.037(4), 2.039(4), and 2.065(5) Å, respectively. These sixcoordinated Ni-involved bond lengths are all within the normal ranges compared with the previously reported dinickel complex [(3CBiIBTP)Ni2(OAc)2] (A).9a Additionally, the average bond distance between the Ni atom and O(trifluoroacetate) is 2.106(4) Å, which is close to that [2.091(2) Å] of Ni− O(acetate) in the nickel complex A. The nickel complex 3 also displays a homologous bimetallic carboxylate complex (Figure 2), except that two of the carboxylate moieties are altered by

Figure 3. ORTEP drawing of complex 8 with probability ellipsoids drawn at the 60% level.

2,2-dimethyl-1,3-propyl substituent, as illustrated in Figure S7. The Oak Ridge thermal ellipsoid plot (ORTEP) in Figure 4

Figure 2. ORTEP drawing of complex 3 with probability ellipsoids drawn at the 60% level. H atoms are omitted for clarity.

benzoate groups. As expected, the average bond distances of Ni−O(phenoxy) = 2.0412(16) Å, Ni−N(aldimine) = 2.0496(19) Å, Ni−N(benzotriazole) = 2.0553(19) Å, and Ni−O(benzoate) = 2.0826(17) Å in 3 are comparable to those observed for the analogous nickel complex 1, as listed in Table 1. Single-crystal structures of complexes 2 and 4−7 were further confirmed as bimetallic nickel(II) dicarboxylate species (Figures S2−S6). The Ni-containing bond distances and angles for 2 and 4−7, respectively, fall within the typical range of the dinickel complexes 1 and 3, as depicted in Table S1. The crystal structure of 8, as shown in Figure 3, is a bimetallic nickel monocarboxylate complex composed of a ligand framework of 3C BiIBTP, one benzoate moiety, a perchlorate group, and one water molecule, also including two six-coordinated Ni atoms with different coordination spheres. The Ni2 atom assumes a distorted octahedral geometry with metal atom coordinated by the N2O2 skeleton of a 3CBiIBTP2− ligand, one O atom of bridging bidentate benzoate, and one O atom from a terminal ClO4 group. Instead of a bonded perchlorate, Ni1 is additionally coordinated by one water molecule, which forms an intramolecular hydrogen bonding of O9−H···O8 interaction with the perchlorate molecule bound on Ni2. For comparison, the six-coordinated average bond lengths between the Ni atom and O(phenoxy), N(aldimine), and O(benzoate) are respectively 2.025(2), 2.018(3), and 2.065(2) Å for 8, which are all smaller than those observed for the nickel complex 3, as listed in Table 1. The molecular structure of the nickel complex 9 is akin to that of 8 except for the diimine backbone bridged by the

Figure 4. ORTEP drawing of complex 10 with probability ellipsoids drawn at the 40% level. H atoms are omitted for clarity.

shows that 10 is a discrete dinuclear ionic species and consists of a cationic moiety formulated as [(3CBiIBTP)Ni2(O2CPh)(MeOH)2]+ containing two coordinating MeOH molecules and a counterbalanced ClO4− anion. The two inequivalent Ni atoms also manifest a distorted octahedral environment, with nickel-including bond distances (Table S1) being similar to those found in the monobenzoate complexes 8 and 9 reported in this study. Interestingly, because of coordination of the MeOH coligand, the aldimine N7 atom is trans to the benzoate O3 atom and cis to the phenoxo O2 atom, resulting in the highly twisted N,O-chelating ring (Ni1/O1/C1/C6/C7/N7) away from the Ni2O2 core. Complex 11 is a structurally related species (Figure S8) that presents a configuration similar to that of 10, except coordinating two EtOH molecules on the Ni atoms. It is worth noting that the ancillary BiIBTP ligand in nickel complexes 1−11 adopts a N,O,N,N,O,N-hexadentate coordination mode to bond two metal centers, forming five metal-containing six-membered rings with N,O or N,N chelation. Copolymerization of CO2 with Epoxides. Dinuclear nickel carboxylate complexes 1−9 were performed as singlecomponent catalysts to evaluate catalytic studies of CO2/ epoxides coupling, and representative catalysis results are summarized in Tables 2−4. We first utilized the bimetallic trifluoroacetato complex 1 to copolymerize CO2 and CHO and 6144

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Inorganic Chemistry Table 2. Copolymerization of CHO and CO2 Catalyzed by the Dinickel Complex 1a

entry

temp/°C

time/h

% CHO convnb

% copolymer (% carbonate)b

% CHC (% trans)b

TONc

TOFd

1 2 3 4 5 6 7f 8f 9f 10f 11f 12f 13f 14g

80 100 120 130 140 180 130 130 130 130 130 130 130 130

24 24 24 24 24 24 3 4 6 8 10 16 24 36

23 77 89 91 87 48 34 54 58 69 72 81 86 82

>99 (>99) >99 (>99) 99 (>99) 99 (>99) 96 (>99) 54 (−) >99 (>99) >99 (>99) >99 (>99) 99 (>99) 99 (>99) >99 (>99) >99 (>99) 98 (>99)

99) 4 (90) 46 (81) 99) >99 (>99) >99 (>99) >99 (>99) >99 (>99) 99 (>99) >99 (>99) >99 (>99) 97 (95) 97 (87)

99 (97) >99 (98) 97 (98) 92 (97) 95 (93) 8 (−) 69 (>99) 69 (>99) 71 (>99)

A (OAc) > 7 (O2CC6H4-4-OMe) > 3 (O2CPh). The superior catalytic activity of 1 compared with others highlights that electron-withdrawing carboxylates are beneficial for activating monomers effectively on the Ni atoms. In comparison, a significant decrease of the copolymerization selectivity (copolymer/CHC = 97/3; ≤95% carbonate repeated units) was obtained (entries 9 and 10 of Table 3), while dinickel perchlorate complexes 8 and 9 were utilized as catalysts for catalyzing such coupling. These results indicated that the existence of the perchlorate group was not good for alternating copolymerization. In contrast, a series of carboxylates such as trifluoroacetate, benzoate derivatives, and acetate were shown to be promising initiators/coligands for this kind of copolymerization. In order to broaden the substrate scope in the dinickelcatalyzed CO2/epoxides copolymerization, other cyclic epoxides, 4-vinyl-1,2-cyclohexene oxide (VCHO) and cyclopentene oxide (CPO), served as comonomers to couple CO2 with the best nickel complex 1 in this study. It can be seen from entries 1−5 of Table 4 that the dinickel catalyst 1 was active in catalyzing this kind of copolymerization by employing a VCHO-to-catalyst ratio of 3200 under conditions identical

copolymerization by the bimetallic nickel complex 1. More importantly, a high TOF, more than 430 h−1, was achieved within 4 h. To test the capacity of the higher-molecular-weight PCHC preparation, a lowering of the catalyst loading to 0.015625 mol % was carried out. As depicted by entry 14 of Table 2, the dinickel complex 1 was still an effective catalyst for copolymerizing CO2 and CHO without loss of copolymerization selectivity, affording a narrowly dispersed PCHC with the highest molecular weight (PDI = 1.19; Mn = 52000 g/mol) in nickel-catalyzed CO2/CHO copolymerization to date. Moreover, copolymerization by 1 using such a low catalyst concentration gave TONs exceeding 5200 for 36 h. Similar catalytic characteristics such as superior copolymerization selectivity and satisfactory controllability of PCHC molecular weights by dinickel benzoate 3 were also observed, as given in entries S1−S6 of Table S2 and Figure S10 in the Supporting Information. All of the polycarbonates produced from dinickel catalysts 1 and 3 exhibited bimodal molecular weight distributions, as illustrated in Figure S11.11 The cause of the bimodality by these dinickel catalysts might result from both initiation and chain-transfer reactions (contamination by protic species in the CHO monomer).9b In a comparison with bimetallic nickel complexes containing different ligand frameworks, the catalytic efficiency for copolymerization of CHO with CO2 by the trifluoroacetate-ligated dinickel catalyst 1 is almost the same as that of the best dinuclear nickel catalyst bearing the DiBTP ancillary ligand under identical conditions (0.0625 mol % catalyst, 120 °C, and pCO20 = 300 psi). Despite the similar activity of two different kinds of dinickel complexes, a notably increased molecular weight of the obtained PCHC with the nickel complex 1 as the catalyst was observed. To inspect the influence of ancillary ligands on the catalytic behavior, nickel complexes 1−9 were executed as catalysts to compare their catalytic efficiency under the aforementioned optimized conditions (0.03125 mol % catalyst loading, 130 °C, pCO20 = 300 psi, 4 h). First, the propyl-bridged BiIBTP ligands featuring dinickel complexes 1, 3, and 5 were found to display 6146

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Inorganic Chemistry

capped PCPC chains during copolymerization. Despite the moderate catalytic activity (TOF: 5−7 h−1) and PCPC selectivity of CPO/CO2 copolymerization by the dinickel complex 1, the results presented here provide the first example of using the dinickel dicarboxylate complexes containing BiIBTP derivatives for generating PCPC with high molecular weight (Mn > 13000 g/mol) and excellent carbonate linkages (>99%). In addition, samples including PCHC, PCPC, and PVCHC copolymers obtained from the dinickel catalyst 1 were demonstrated to be atactic microstructures based on the 13C NMR spectrum, as shown in Figures S15−S17.

with those of CO2/CHO copolymerization. Note that the dinickel complex 1 was capable of giving a maximum TOF of 208 h−1 and achieved a good degree of copolymerization selectivity (≥92% copolymer and ≥93% carbonate repeat units), which showed a catalytic performance comparable to that of dinickel catalysts coordinated on DiBTP derivatives.9c Moreover, 1 was found to generate high carbonate contents of poly(VCHC-co-VCHO)s having controllable molecular weights ranging from 36600 to 89000 g/mol. Although highermolecular-weight products could be reached for PVCHCbased copolymers, the overall catalytic performances involving the activity and copolymerization selectivity for VCHO/CO2 coupling were slightly lower than those of CHO/CO2 copolymerization using the same catalyst. The reactivity order of CHO > VCHO on CO2 copolymerization could be ascribed to the less sterically hindered CHO monomer, resulting in favorable CHO monomer coordination or effective polymer chain propagation on the metal center(s). In contrast to copolymerization of CHO/CO2, only low epoxide monomer conversion ( 5 > A > 7 > 3 using the same BiIBTP ligand system under identical conditions. Induction of the electron-withdrawing carboxylates in the dinickel complexes enables enhancement of the catalytic property. The best trifluoroacetate-featuring dinickel catalyst 1 was able to effectively copolymerize CHO and CO2 with a high TOF of 432 h−1 in a controlled manner, giving a narrowly dispersed PCHC possessing high carbonate repeated units (>99%) and high molecular weight (Mn = 52000 g/mol). Not only has an efficient catalysis of the nickel catalyst 1 for CO2/CHO copolymerization been enabled, but also 1 has been further applied to catalyze copolymerization of CO2 with VCHO or CPO to obtain the corresponding polycarbonates. On the basis of CO2/epoxide coupling catalysis data, we concluded that the reactivity of these cyclic epoxides as comonomers by 1 has the order CHO > VCHO > CPO. To the best of our knowledge, well-characterized catalyst 1 appears to be the first example of a dicarboxylate bimetallic nickel complex that is active for CO2/ CPO copolymerization and enables the production of poly(cyclopentene carbonate)s with large molecular weight (Mn > 13000 g/mol) and excellent carbonate linkage contents (>99%).

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EXPERIMENTAL SECTION

General Considerations. 2-(2H-Benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol (C8BTP-H), hexamethylenetetramine, 1,3diaminopropane, 2,2-dimethyl-1,3-propanediamine, nickel(II) perchlo6147

DOI: 10.1021/acs.inorgchem.7b00090 Inorg. Chem. 2017, 56, 6141−6151

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Inorganic Chemistry rate hexahydrate, trifluoroacetic acid, benzoic acid, 4-(trifluoromethyl)benzoic acid, 4-methoxybenzoic acid, triethylamine, ethanol (EtOH), methanol (MeOH), dichloromethane (CH2Cl2), tetrahydrofuran (THF), and carbon dioxide (CO2, 99.95%) were purchased and used without further purification. Cyclohexene oxide (CHO), 4-vinyl1,2-cyclohexene oxide (VCHO), and cyclopentene oxide (CPO) were purified by distillation over calcium hydride (CaH2) prior to use. Deuterated solvents were dried over 4 Å molecular sieves. 3-(2HBenzotriazol-2-yl)-2-hydroxy-5-(2,4,4-trimethylpentan-2-yl)benzaldehyde (C8AldBTP-H) was synthesized according to previous literature procedures.9a 1H and 13C NMR spectra were recorded on a Varian Mercury-400 (400 and 100 MHz) spectrometer with chemical shifts given in parts per million from the peak of internal tetramethylsilane. Microanalyses were performed using a Heraeus CHN-O-RAPID instrument. Gel permeation chromatography (GPC) measurements were performed on a Jasco PU-2080 plus system equipped with a RI-2031 detector using THF (HPLC-grade) as the eluent. The chromatographic column was a Phenomenex Phenogel 5 μ 103 Å, and the calibration curve utilized to calculate Mn(GPC) was produced from polystyrene standards. The calibration curve was constructed by 10 polystyrene standards, in which their molecular weights range from 1580 to 288000. The GPC results were calculated using the Scientific Information Service Corporation (SISC) chromatography data solution 3.1 edition. Mass analyses were performed by employing a positive electron spray ionization (ESI+) technique on a Thermo Finnigan TSQ Quantum mass spectrometer for new synthesized complexes upon dissolution in a dimethyl sulfoxide solvent. Synthesis of Nickel Complexes 1−11. Synthesis of [(3CBiIBTP)Ni2(O2CCF3)2] (1). A solution of 3CBiIBTP-H2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and trifluoroacetic acid (0.37 mL, 2.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h, during which time the formation of a green precipitate was observed. The resulting precipitate was collected by filtration. The residue was washed with hexane, and the resultant solid was collected by filtration and dried under vacuum to give a green solid. Yield: 0.69 g (65%). Anal. Calcd for C49H54F6N8O6Ni2: N, 10.35; C, 54.37; H, 5.03. Found: N, 10.55; C, 54.18; H, 5.10. APCIMS (m/z): 967.4 [100%, (M − O2CCF3)+]. Mp: >250 °C. Synthesis of [(5CBiIBTP)Ni2(O2CCF3)2] (2). A solution of 5CBiIBTPH2 (0.77 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and trifluoroacetic acid (0.37 mL, 2.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h, during which time the formation of a green precipitate was observed. The resulting precipitate was collected by filtration. The residue was washed with hexane, and the final solid was collected by filtration and dried under vacuum to give a green solid. Yield: 0.61 g (55%). Anal. Calcd for C51H58F6N8O6Ni2: N, 10.09; C, 55.16; H, 5.26. Found: N, 9.97; C, 54.62; H, 5.34. APCI-MS (m/z): 995.4 [100%, (M − O2CCF3)+]. Mp: >250 °C. Synthesis of [(3CBiIBTP)Ni2(O2CPh)2] (3). A solution of 3CBiIBTPH2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and benzoic acid (0.61 g, 5.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with ether, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.81 g (74%). Anal. Calcd for C59H64N8O6Ni2: N, 10.20; C, 64.50; H, 5.87. Found: N, 9.93; C, 64.13; H, 6.15. APCI-MS (m/z): 975.5 [100%, (M − O2CPh)+]. Mp: >250 °C. Synthesis of [(5CBiIBTP)Ni2(O2CPh)2] (4). A solution of 5CBiIBTPH2 (0.77 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and benzoic acid (0.61 g, 5.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The

mixture was heated under reflux for 24 h and then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with ether, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.85 g (76%). Anal. Calcd for C61H68N8O6Ni2: N, 9.95; C, 65.03; H, 6.08. Found: N, 9.82; C, 64.44; H, 6.32. APCI-MS (m/z): 1003.5 [100%, (M − O2CPh)+]. Mp: >250 °C. Synthesis of [(3CBiIBTP)Ni2(O2CC6H4-4-CF3)2] (5). A solution of 3C BiIBTP-H 2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and 4-(trifluoromethyl)benzoic acid (0.95 g, 5.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with ether, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.90 g (73%). Anal. Calcd for C61H62F6N8O6Ni2: N, 9.08; C, 59.34; H, 5.06. Found: N, 9.18; C, 59.27; H, 5.08. ESI-MS (m/z): 1043.6 [100%, (M − (O2CC6H4-4-CF3))+]. Mp: >250 °C. Synthesis of [(5CBiIBTP)Ni2(O2CC6H4-4-CF3)2] (6). A solution of 5C BiIBTP-H 2 (0.77 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and 4-(trifluoromethyl)benzoic acid (0.95 g, 5.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with ether, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.97 g (77%). Anal. Calcd for C63H66F6N8O6Ni2: N, 8.87; C, 59.93; H, 5.27. Found: N, 8.66; C, 60.05; H, 5.77. APCI-MS (m/z): 1071.6 [100%, (M − (O2CC6H4-4-CF3))+]. Mp: >250 °C. Synthesis of [(3CBiIBTP)Ni2(O2CC6H4-4-OMe)2] (7). A solution of 3C BiIBTP-H 2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (20 mL), and 4-methoxybenzoic acid (0.76 g, 5.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h, during which time the formation of a green precipitate was observed. The resulting precipitate was collected by filtration. The residue was washed with hexane, and the final solid was collected by filtration and dried under vacuum to give a green solid. Yield: 0.76 g (66%). Anal. Calcd for C61H68N8O8Ni2: N, 9.67; C, 63.23; H, 5.92. Found: N, 9.65; C, 63.12; H, 6.06. ESI-MS (m/z): 1005.6 [100%, (M − (O2CC6H4-4-OMe))+]. Mp: >250 °C. Synthesis of [(3CBiIBTP)Ni2(O2CPh)(ClO4)(H2O)] (8). A solution of 3C BiIBTP-H 2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in THF (30 mL), and benzoic acid (0.12 g, 1.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with hexane, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.65 g (60%). Anal. Calcd for C52H61ClN8O9Ni2: N, 10.23; C, 57.04; H, 5.62. Found: N, 10.10; C, 57.02; H, 5.75. ESI-MS (m/z): 975.5 [100%, (M − O2CPh)+]. Synthesis of [(5CBiIBTP)Ni2(O2CPh)(ClO4)(H2O)] (9). A solution of 5C BiIBTP-H 2 (0.77 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in THF (30 mL), and benzoic acid (0.12 g, 1.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and 6148

DOI: 10.1021/acs.inorgchem.7b00090 Inorg. Chem. 2017, 56, 6141−6151

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Inorganic Chemistry

X-ray Crystallographic Studies. Suitable crystals of dinickel nickel complexes 1−11 were mounted onto a glass fiber using perfluoropolyether oil and cooled rapidly in a stream of cold nitrogen gas to collect diffraction data at 120 or 150 K using an Oxford Gemini S or a Bruker APEX2 diffractometer, and the intensity data were collected with ω scans. Data collection and reduction were performed with the CrysAlisPro software,14 and the absorptions were corrected by a SCALE3 ABSPACK multiscan method.15 The space group determination was based on a check of the Laue symmetry and systematic absences and confirmed using the structure solution. The structure was solved and refined with the SHELXTL package.16 All non-H atoms were located from successive Fourier maps, and H atoms were treated as riding models on their parent C atoms. Anisotropic thermal parameters were used for all non-H atoms, and fixed isotropic parameters were used for H atoms. Drawing of the molecular structure was done by ORTEP.17 Crystallographic data of complexes 1−11 are summarized in Table S3. CCDC 1524863−1524873 (for complexes 1−11) contain the supplementary crystallographic data in this paper.

then cooled to ambient temperature. The volatile components were removed in vacuo. The residue was diluted with CH2Cl2 and extracted with a NaCl-saturated solution. The organic layers were dried over MgSO4 and concentrated by vacuum evaporation. The final residue was washed with hexane, and the resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.82 g (73%). Anal. Calcd for C54H65ClN8O9Ni2: N, 9.98; C, 57.75; H, 5.83. Found: N, 9.55; C, 57.30; H, 5.92. ESI-MS (m/z): 1003.6, [100%, (M − O2CPh)+]. Synthesis of [(3CBiIBTP)Ni2(O2CPh)(MeOH)2]ClO4 (10). A solution of 3CBiIBTP-H2 (0.74 g, 1.0 mmol) and nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol) was dissolved in MeOH (30 mL), and benzoic acid (0.12 g, 1.0 mmol) and triethylamine (0.72 mL, 5.0 mmol) were added. The mixture was heated under reflux for 24 h and then cooled to ambient temperature. The final mixture was concentrated by vacuum evaporation to give a green precipitate in solution. The resulting precipitate was collected by filtration and dried under vacuum to give a green solid. Yield: 0.73 g (64%). Anal. Calcd for C55H72ClN8O11Ni2 (10·MeOH): N, 9.54; C, 56.27; H, 6.18. Found: N, 9.80; C, 56.30; H, 5.99. Complex 10 also could be obtained from crystallization of the neutral nickel perchlorate complex 8 in a MeOH-saturated solution. Synthesis of [(C83CBiIBTP)Ni2(O2CPh)(EtOH)2]ClO4 (11). The nickel complex 11 could be obtained from crystallization of the neutral nickel perchlorate complex 8 in an EtOH-saturated solution. Anal. Calcd for C58H78ClN8O11Ni2 (11·EtOH): N, 9.21; C, 57.28; H, 6.46. Found: N, 9.01; C, 56.99; H, 6.90. Copolymerization of CO2 with CHO-, VCHO-, or CPO Catalyzed by the Dinickel Complex 1. A representative procedure for the copolymerization of CHO with CO2 (Table 2, entry 4) was exemplified by using the dinuclear nickel complex 1 as the catalyst. Complex 1 (34.0 mg, 0.0313 mmol) was dissolved in 50.0 mmol of neat CHO under a dry nitrogen atmosphere. The solution was added to the 100 mL autoclave with a magnetic stirrer under a CO2 atmosphere. CO2 was then charged in the reactor until a pressure of 300 psi was reached, and the stirrer was started. The reaction was performed at 130 °C for 24 h. Then the reactor was placed in an ice water, and excess CO2 was released. The CHO conversion (91%) was analyzed by 1H NMR spectroscopic studies. To calculate the copolymerization selectivity, the product was characterized by methine proton resonances in the 1H NMR spectrum (benzene-d6/CDCl3, 4.5/ 1, v/v), including the copolymer carbonate linkages (broad, δ 4.90− 4.60), copolymer ether linkages (broad, δ 3.70−3.30), and cyclic carbonate (multiples: δ 3.9−3.8 (cis-CHC), 3.30−3.25 (transCHC)).12b After the reaction was dissolved in CH2Cl2 (5 mL), the polymer was precipitated into MeOH. Considering removal of the metal salt, the precipitate was redissolved with CH2Cl2 (20 mL) and washed with aqueous HCl (4 N, 100 mL). The organic layer was extracted and concentrated to 5 mL by vacuum evaporation. The copolymer was then precipitated into MeOH (150 mL) again to give a white solid. A representative procedure for copolymerization of VCHO with CO2 (Table 4, entry 4) was exemplified by using the dinickel complex 1 as the catalyst. Complex 1 (17.0 mg, 0.0156 mmol) was dissolved in 50.0 mmol of neat VCHO under a dry nitrogen atmosphere. Copolymerization of VCHO and CO2 by the nickel complex 1 was performed at 130 °C for 12 h following a procedure similar to that of CHO/CO2 copolymerization. VCHO conversion (58%) was characterized by 1H NMR spectroscopic studies. Spectral characteristics of the copolymer referred according the earlier literature reports.9c,13 A representative procedure for the copolymerization of CPO with CO2 (Table 4, entry 9) was exemplified by using the bimetallic nickel complex 1 as the catalyst. Complex 1 (68.0 mg, 0.0625 mmol) was dissolved in 50.0 mmol of neat CPO under a dry nitrogen atmosphere. Copolymerization of CPO and CO2 by the nickel complex 1 was performed at 100 °C for 72 h according to a procedure similar that of CHO/CO2 copolymerization. The CPO conversion (46%) was determined by 1H NMR spectroscopic studies. Spectral characteristics of the copolymer: PCPC carbonate (δ 4.93) and CPC [δ 5.03 (cis)].

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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.inorgchem.7b00090. Mass spectrum of the nickel complex 5, ORTEP drawing of complexes 2, 4−7, 9, and 11, plots of the molecular weights and PDIs versus CHO conversions for controllable characters catalyzed by complexes 1 and 3, GPC traces for the isolated PCHC, PVCHC copolymer, and PCPC, and polymer characterization by 1H, 13C NMR, and MALDI-TOF spectra as well as crystallographic details (PDF) Accession Codes

CCDC 1524863−1524873 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.-T. Ko). Tel.: 886-422840411-715. Fax: 886-4-22862547. ORCID

Bao-Tsan Ko: 0000-0001-8404-6265 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministry of Science and Technology, Taiwan (MOST 105-2119-M-005005 to B.-T. Ko).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00090 Inorg. Chem. 2017, 56, 6141−6151

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DOI: 10.1021/acs.inorgchem.7b00090 Inorg. Chem. 2017, 56, 6141−6151