Dinuclear and Trinuclear Nickel Complexes as Effective Catalysts for

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Dinuclear and Trinuclear Nickel Complexes as Effective Catalysts for Alternating Copolymerization on Carbon Dioxide and Cyclohexene Oxide Chen-Yen Tsai,† Fu-Yin Cheng,† Kuan-Yeh Lu,† Jung-Tsu Wu,† Bor-Hunn Huang,† Wei-An Chen,† Chu-Chieh Lin,*,†,‡ and Bao-Tsan Ko*,† †

Department of Chemistry and ‡Research Center for Sustainable Energy and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan S Supporting Information *

ABSTRACT: A series of novel nickel complexes 1−9 supported by NNO-tridentate Schiff-base derivatives have been synthesized and characterized. Treatment of the pro-ligands [L1-H = 2,4-di-tert-butyl-6(((2-(dimethylamino)ethyl)imino)methyl)phenol, L 2-H = 2-(((2(dimethylamino)ethyl)imino)methyl)-4,6-bis(2-phenylpropan-2-yl)phenol, L3-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)phenol] with Ni(OAc)2·4H2O in refluxing ethanol afforded mono- or bimetallic nickel complexes {[(L1)Ni(OAc)] (1); (L2)Ni(OAc)] (2); (L3)2Ni2(OAc)2(H2O)] (3)}. Alcohol-solvated trimetallic nickel acetate complexes {[(L3)2Ni3(OAc)4(MeOH)2] (4); (L3)2Ni3(OAc)4(EtOH)2] (5)} could be generated from the reaction of L3-H and anhydrous nickel(II) acetate with a ratio of 2:3 in refluxing anhydrous MeOH or EtOH. The reaction of nickel acetate tetrahydrate and L4-H to L6-H [L4-H = 2-(((2(dimethylamino)ethyl)imino)methyl)-5-methoxyphenol, L5-H = 2-(((2(dimethylamino)ethyl)imino)methyl)-4-methoxy-phenol, L6-H = 2-(((2-(dimethylamino)ethyl)imino)(phenyl)methyl)phenol] produced, respectively, the alcohol-free trinuclear nickel complexes {[(L4)2Ni3(OAc)4] (7); [(L5)2Ni3(OAc)4] (8); [(L6)2Ni3(OAc)4] (9)} with the same ratio in refluxing EtOH under the atmospheric environment. Interestingly, recrystallization of [(L3)2Ni3(OAc)4(MeOH)] (4) or [(L3)2Ni3(OAc)4(EtOH)] (5) in the mixed solvent of CH2Cl2/hexane gives [(L3)2Ni3(OAc)4] (6), which is isostructural with analogues 7−9. All bi- and trimetallic nickel complexes exhibit efficient activity and good selectivity for copolymerization of CO2 with cyclohexene oxide, resulting in copolymers with a high alternating microstructure possessing ≥99% carbonate-linkage content. This is the first example to apply well-defined trinuclear nickel complexes as efficient catalysts for the production of perfectly alternating poly(cyclohexene carbonate).

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INTRODUCTION

examples, Coates, Darensbourg, Lu, and Lee et al. have synthesized, respectively, a series of Cr(III)3 and Co(III)4 complexes as catalysts executing copolymerization at low CO2 pressures through elevating temperature. Williams et al. have also reported a series of bimetallic zinc(II),5 cobalt(II/III),6 iron(III),7 magnesium(II),8 and zinc(II)−magnesium(II)9 acetate complexes bearing macrocyclic backbones to efficiently catalyze the copolymerization of CO2 with cyclohexene oxide (CHO) at only 1 atm of pressure, which displayed excellent selectivity with >99% carbonate repeated units. Regarding the practical applications, an alternative catalytic system with nickel(II)-based catalysts seems to be a potential candidate because of their air-stable nature and abundant availability.10 However, trinuclear nickel acetate complexes bearing NNOSchiff-base ligands are rarely isolated in the literature.11 In addition, trinuclear complexes as effective catalysts for CHO/

Because of its abundant, inexpensive, nontoxic, and biorenewable character, carbon dioxide (CO2), as one of the sustainable resources, was used to produce functional materials via variable synthetic routes. One of the important chemical reactions is the coupling of epoxide and CO2 catalyzed by suitable and efficient metal complexes to generate cyclic carbonate and aliphatic polycarbonate (Scheme 1).1 Numerous catalytic systems have been developed with cobalt, chromium, iron, magnesium, and zinc complexes chelated by various ancillary ligands.2 For Scheme 1. Coupling of Carbon Dioxide and Cyclohexene Oxide

Received: February 25, 2016

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Synthetic Routes of Nickel Complexes

CO2 copolymerization are also rarely seen.6 Therefore, we envisaged that modulating the synthetic conditions and steric effect of ligand backbones might lead to interesting architectures of complexes with respect to the aforementioned good performance in CO2 copolymerization. Herein, we report a series of novel nickel complexes supported by NNOtridentate Schiff-base derivatives and their application in CHO/CO2 copolymerization. It is worth noting that, for the first time, we successfully designed and synthesized well-defined trimetallic nickel complexes as catalysts to produce poly(cyclohexene carbonate).

afforded monomeric four-coordinate nickel complexes [(L1)Ni(OAc)] (1) and [(L2)Ni(OAc)] (2), whereas treatment of 2(((2-(dimethylamino)ethyl)imino)methyl)phenol) (L3-H) with Ni(OAc)2·4H2O (1.0 mol equiv) under the same synthetic conditions furnished the bimetallic nickel complex [(L3)2Ni2(OAc)2(H2O)] (3) instead. It was noted that L3-H reacted with nickel(II) acetate tetrahydrate by using MeOH/ CHCl3 as the mixing solvent to give a monomeric complex [(L3)Ni(OAc)(H2O)2] containing two coordinated H2O molecules.13 Accordingly, adjustment of different synthetic conditions might give diverse configurations such as monomeric, dimeric, or trimeric complexes. In order to prepare the Ni complex without the coordinated water, the reaction using the anhydrous metal salt and ligand precursor under a dry nitrogen atmosphere was performed. Particularly, two alcoholsolvated trinuclear nickel acetate complexes, [(L3)2Ni3(OAc)4(MeOH)2] (4) and [(L3)2Ni3(OAc)4(EtOH)2] (5), were obtained, respectively, from the L3-H as pro-ligand and Ni(OAc)2 as the metal precursor with a ligand/metal precursor ratio of 1:1 in refluxing anhydrous MeOH or EtOH. Alternatively, complexes 4 and 5 could be easily prepared in high yield from the reaction of L3-H and anhydrous nickel(II) acetate with a molar ratio of 2:3 under conditions of refluxing anhydrous MeOH or EtOH. Interestingly, an alcohol-free

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RESULTS AND DISCUSSION Synthesis and Characterization. Treatment of a series of salicylaldehyde or 2-hydroxybenzophenone derivatives with N,N′-dimethylethane-1,2-diamine yielded varied NNO-tridentate Schiff-base pro-ligands (L1-H to L6-H).12,13 Structurally diverse nickel complexes were synthesized through a direct acetic acid elimination of ligand precursor with nickel(II) acetate precursor as shown in Scheme 2. The reaction of the pro-ligand, 2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)imino)methyl)phenol (L1-H) or 2-(((2-(dimethylamino)ethyl)imino)methyl)-4,6-bis(2-phenylpropan-2-yl)phenol (L2H), with 1.0 mol equiv of Ni(OAc)2·4H2O in refluxing ethanol B

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry trinuclear analogue [(L3)2Ni3(OAc)4] (6) was isolated from the recrystallization of [(L 3 ) 2 Ni 3 (OAc) 4 (MeOH) 2 ] (4) or [(L 3 ) 2 Ni 3 (OAc) 4 (EtOH) 2 ] (5) in a mixed solvent of CH 2 Cl 2 /hexane. Similarly, trinuclear nickel complexes ([(L4)2Ni3(OAc)4] (7), [(L5)2Ni3(OAc)4] (8), and [(L6)2Ni3(OAc)4] (9)) could be synthesized by treatment of Ni(OAc)2· 4H2O with L4-H to L6-H (L4-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-5-methoxyphenol, L 5 -H = 2-(((2(dimethylamino)ethyl)imino)methyl)-4-methoxyphenol, L6-H = 2-(((2-(dimethylamino)ethyl)imino)(phenyl)methyl)phenol) under the stoichiometric ratio in refluxing EtOH without a dry nitrogen atmosphere. All complexes were fully characterized on the basis of ESI mass, IR, and ultraviolet− visible (UV−vis) spectra as well as elemental analysis. IR spectroscopic studies show strong vibration bands of acetate groups, which are typical phenomena for O-monodentate, bridging monodentate, and bridging bidentate bound-acetate anions. The bands in the ranges 220−270 and 270−370 nm might be assigned to the π−π* and n−π* transitions on the absorption spectra (Supporting Information (SI), Figure S1). The solid state architectures of all Ni complexes were further verified by single-crystal X-ray crystallographic determination. Molecular Structures of Nickel Complexes 1−9. Single crystals of complexes 1−9 for X-ray diffraction analysis were obtained from their saturated solutions in ethanol, methanol, or a mixed solution of CH2Cl2/hexane. As shown in Figure S2 and Figure 1, the crystal structures of complexes 1 and 2 are

nickel acetate complexes incoporating the Schiff-base derivative in the literature.10,11,14 As a result, the steric effect of substituents seems to be an important factor for the formation of four-coordinated nickel complexes. The molecular structure of complex 3, as shown in Figure 2, is a dinuclear nickel

Figure 2. ORTEP drawing of complex 3 with probability ellipsoids drawn at 50% level. Hydrogen atoms except for those in O(7) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−N(2) 2.025(2), Ni(1)−O(1) 2.0444(15), Ni(1)−O(3) 2.0490(15), Ni(1)−O(2) 2.0883(17), Ni(1)−O(7) 2.1093(16), Ni(1)−N(1) 2.200(2), Ni(2)−N(4) 2.014(2), Ni(2)−O(2) 2.0454(16), Ni(2)−O(5) 2.0481(16), Ni(2)−O(1) 2.0563(15), Ni(2)−O(4) 2.0793(15), Ni(2)−N(3) 2.215(2), N(2)−Ni(1)− O(2) 169.52(8), O(3)−Ni(1)−O(7) 176.61(7), O(1)−Ni(1)−N(1) 169.71(9), N(4)−Ni(2)−O(1) 171.14(8), O(5)−Ni(2)−O(4) 173.12(7), O(2)−Ni(2)−N(3) 168.31(7).

complex constructed by two less sterically hindered ligand framework species (L3), two different types of acetate groups, and one water molecule, containing two distinct environments for six-coordinated Ni centers. The Ni(1) is chelated by two N atoms and one O atom from L3, μ2-bridged acetate, and a coordinated water. Instead of a water molecule, Ni(2) is bonded by a terminal acetate which exists with intramolecular hydrogen bonding, 1.729(2) Å and 177.5(2)°, to the water molecule coordinated on Ni(1). The geometry around each Ni atom is a distorted octahedron, confirmed by the average bond angles as well as the axial axes, O(3)−Ni(1)−O(7) (176.61(7)°) and O(4)−Ni(2)−O(5) (173.12(7)°). A Ni2O2 core bridging through the phenoxy oxygen atoms and two Ni atoms involving six- and five-membered rings were built around the Ni centers. The dihedral angle between two six-membered mean planes, [Ni(1)/O(1)/C(17)/C(22)/C(23)/N(2)] and [Ni(2)/O(2)/C(5)/C(10)/C(11)/N(4)], is about 37.9°. The Oak Ridge thermal-ellipsoid plot (ORTEP) in Figure S3 shows that trinuclear complex 4 is constituted by three hexacoordinated nickel centers in the presence of two distinct environments, and all Ni atoms adopt a distorted octahedral geometry. As a symmetrical C2 axis (along a-axis) passes through, the Ni(2) atom is chelated by six bridged oxygen atoms from three different types which are the phenoxy group of NNO-backbones, μ2-O of monodentate acetate, and μ2bidentate acetate groups. Each Ni(1) and Ni(1A) is constructed by one O and two N atoms of the NNO-tridentate ligand, three O atoms from two acetate groups, and a coordinated methanol. The crystal structure of nickel complex 5 is isostructural to that

Figure 1. ORTEP drawing of complex 2 with probability ellipsoids drawn at the 50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−O(1) 1.845(2), Ni(1)−N(1) 1.848(3), Ni(1)−O(2) 1.882(3), Ni(1)−N(2) 1.949(3), O(1)−Ni(1)−N(1) 94.32(12), O(1)−Ni(1)−O(2) 87.31(11), N(1)−Ni(1)−O(2) 171.63(13), O(1)−Ni(1)−N(2) 173.61(12), N(1)−Ni(1)−N(2) 85.96(13), O(2)−Ni(1)−N(2) 93.34(12).

isostructural, which differ from the bulky substituents of a phenyl ring with tert-butyl or cumyl groups at ortho- and parapositions. Both compounds exhibited the monomeric and fourcoordinated features with a slightly distorted square planar geometry where both Ni centers slightly deviate from the mean plane about 0.06 and 0.01 Å, respectively. Each Ni atom was bonded by one O and two N atoms of the NNO-tridentate Schiff-base ligand, with one O atom from the terminal acetate anion. The bond lengths between Ni and O(1), O(2), N(1), and N(2) are 1.834(2), 1.885(2), 1.846(3), and 1.942(3) Å for 1 and 1.845(2), 1.882(3), 1.848(3), and 1.949(3) Å for 2, which are all shorter than those reported for six-coordinate C

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of 4 except the coordinated ethanol on Ni(1) and Ni(3) as illustrated in Figure 3. Complex 6 is a heteroleptic species

Figure 3. ORTEP drawing of complex 5 with probability ellipsoids drawn at 50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−N(1) 2.009(5), Ni(1)− O(1) 2.014(5), Ni(1)−O(3) 2.071(5), Ni(1)−O(6) 2.093(4), Ni(1)−O(5) 2.105(5), Ni(1)−N(2) 2.165(6), Ni(2)−N(3) 2.002(5), Ni(2)−O(2) 2.028(5), Ni(2)−O(8) 2.081(5), Ni(2)− O(11) 2.092(4), Ni(2)−O(10) 2.137(5), Ni(2)−N(4) 2.153(6), Ni(3)−O(4) 2.031(5), Ni(3)−O(9) 2.013(5), Ni(3)−O(2) 2.075(4), Ni(3)−O(1) 2.088(4), Ni(3)−O(11) 2.104(5), Ni(3)− O(6) 2.115(4), N(1)−Ni(1)−O(6) 170.7(2), O(3)−Ni(1)−O(5) 177.47(17), O(1)−Ni(1)−N(2) 172.17(19), N(3)−Ni(2)−O(11) 170.4(2), O(8)−Ni(2)−O(10) 176.72(17), O(2)−Ni(2)−N(4) 172.65(19), O(4)−Ni(3)−O(9) 178.3(2), O(2)−Ni(3)−O(1) 178.6(2), O(11)−Ni(3)−O(6) 179.2(2).

Figure 5. ORTEP drawing of complex 7 with probability ellipsoids drawn at 50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−N(1) 1.984(4), Ni(1)− O(1) 2.016(3), Ni(1)−O(5) 2.030(3), Ni(1)−O(3) 2.093(3), Ni(1)−N(2) 2.147(3), Ni(1)−O(4) 2.260(3), Ni(2)−O(6) 2.013(3), Ni(2)−O(1) 2.061(2), Ni(2)−O(3) 2.144(3), N(1)− Ni(1)−O(3) 164.09(12), O(1)−Ni(1)−N(2) 173.68(13), O(5)− Ni(1)−O(4) 155.74(12).

coordinated nickel(II) acetate complexes.10,11,15 It is worth noting that two diverse types of acetate groups exist in complexes 3−9 in the solid state, which is consistent with the observation of IR spectroscopic studies. Copolymerization of Cyclohexene Oxide (CHO) with Carbon Dioxide (CO2). On the basis of the previous studies on copolymerization of epoxides and CO2 utilizing bimetallic nickel acetate complexes as catalysts,10 the NNO-Schiff-based Ni complexes have a predictable potential to act as efficient catalysts for copolymerizing CHO and CO2 without cocatalysts. Copolymerization catalyzed by Ni complexes 1−9 was performed to evaluate their catalytic performances, and representative results were tabulated in Table 1. Experimental results reveal that dinickel complex 3 exhibits a higher catalytic activity than the dicopper complex at the same empirical conditions ([CHO]0/[catalyst]0 = 500/1, pCO2 = 300 psi, 120 °C)12c (entry 1 vs 11), where the CHO conversion of copolymerization catalyzed by 3 was 88% along with excellent copolymerization selectivity (>99% carbonate linkages) for 24 h. Decreasing the concentration of 3 ([CHO]0/[catalyst]0 = 1000/1) slightly reduces the CHO conversion (78%), but TOF increases significantly from 18 to 33 h−1 while the selectivity remains. Under the conditions of [CHO]0/[catalyst]0 = 1000/ 1, 120 °C, and pCO20 = 300 psi, all NNO-tridentate Schiff-base Ni complexes in this work were further conducted to copolymerize CO2 and CHO to compare their catalytic efficiency. As depicted by entries 3 and 4 in Table 1, monomeric Ni complexes 1 and 2 displayed very poor productivity (CHO conversion 99) (>99) (3) (17) (>99) (>99) (>99) (>99) (>99) (>99) (96)

TONc

TOFd/h‑1

Mn (obsd)f (PDI)f

440 780 40 30 810 830 770 840 850 630 440

18 33 2 1 34 35 32 35 35 26 9

10 700 (1.34) 11 600 (1.48)

11 000 (1.51) 11 200 (1.49) 12 500 (1.40) 15 500 (1.43) 15 500 (1.43) 7900 (1.34) 9100 (1.42)

Copolymerization/coupling conditions: 0.1 mmol of catalyst, CHO = 10.0 mL, T = 120 °C, pCO20 = 300 psi, 24 h. bDetermined by comparison of the integrals of signals arising from the methine protons in the 1H NMR spectra, including PCHC carbonate (δ: 4.65 ppm), PCHC ether (δ: 3.3−3.5 ppm), and trans-CHC (δ: 3.9 ppm). cTOF = TON per hour. TON = number of moles of CHO consumed per mole of catalyst. dDetermined by GPC, in THF. eCHO = 5 mL. fReference 12c, 48 h. a

Table 2. Coupling of CHO and CO2 Catalyzed by Tri-Nickel Complex 7a

entry

CO2 (psi)

temp (°C)

time/h

% CHO conversionb

% CHCb

1 2 3 4 5 6 7 8 9 10 11e

300 500 100 300 300 500 500 500 500 500 500

120 120 120 90 150 150 90 120 120 120 120

24 24 24 24 8 8 100 6 12 18 48

84 94 51 17 75 91 39 24 58 85 90

13 (trans) 13 (trans) 36 (trans) 10 (trans) 40 (trans) 31 (trans) 4 (trans) 20 (trans) 14 (trans) 13 (trans) 12 (trans)

% copolymer (% carbonate)b 87 87 64 90 60 69 96 80 86 87 88

(>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99)

TOFc

Mn (obsd)d (PDI)d

35 39 21 7 94 114 4 40 48 47 38

15 500 (1.43) 20 000 (1.29) 5700 (1.49) 5100 (1.17) 5500 (1.49) 9400 (1.51) 7800 (1.19) 4400 (1.18) 10 800 (1.22) 15 200 (1.28) 24 900 (1.26)

a

Copolymerization/coupling conditions: 0.1 mmol of catalyst, CHO = 10.0 mL. bDetermined by comparison of the integrals of signals arising from the methine protons in the 1H NMR spectra, including PCHC carbonate (δ: 4.65 ppm), PCHC ether (δ: 3.3−3.5 ppm), and trans-CHC (δ: 3.9 ppm). cTOF = TON per hour. TON = number of moles of CHO consumed per mole of catalyst. dDetermined by GPC, in THF. eCHO = 20 mL.

increasing the reaction temperature to 150 °C, but it degrades the selectivity of copolymerization to a PCHC/CHC ratio of 60/40. In contrast, the conversion of CHO plunged to 17% at the reaction temperature of 90 °C (Table 2, entries 5 vs 4). Although the lower temperature as well as the higher CO2 pressure (90 °C/500 psi) have induced the higher PCHC selectivity ratio to about 96/4, the conversion is only 39% after 100 h together with the low-molecular-weight copolymer produced. According to the optimal conditions ([CHO]0/ [catalyst]0 = 1000/1, pCO20 = 500 psi, 120 °C, time = 24 h) evaluated from the aforementioned experimental results, tri-Ni complex 7 could achieve a high productivity of copolymerization (CHO conversion =94%, TON = 940) and enable preparation of the narrowly dispersed copolymer with a large

CHO/CO2 coupling (copolymer/CHC = 87/13) and a high constituent of carbonate linkages in the polymeric product (>99%) as shown in entry 8 of Table 1. These comparative results encouraged us to further inspect the detailed catalytic behavior of 7 on studying effects of CO2 pressure and copolymerization temperature as depicted in Table 2. First, adjustment of the initial CO2 pressure was examined to investigate the affection of catalytic efficiency as follows: By raising the pressure to 500 psi, the TOF increased from 35 to 39 h−1, and the selectivity of copolymerization is still maintained. On the contrary, TOF and copolymer selectivity decreased to 21 h−1 and 64/36 (PCHC/CHC), respectively, while the CO2 pressure was reduced to 100 psi (Table 2, entries 2 and 3). Notably, it can sharply promote TOF to 94 h−1 by E

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

catalyst 7. The thermal property of the PCHC sample was studied using differential scanning calorimetry (DSC). Because the produced PCHC is atactic, DSC thermograms revealed no melting endotherm during heating. As illustrated in Figure S9 of SI, the glass transition temperature (Tg) of such a copolymer is approximately 110 °C, which agrees well with the previous observations in the literature.16 In order to understand the concentration effect of the Ni catalyst on the catalytic activity, different catalyst loadings of complex 7 were examined under the optimal conditions (pCO20 = 500 psi and 120 °C). As shown in Table S1 of SI, the TOF values are almost constant from high to low catalyst concentrations ([cat.] = 0.2 to 0.025 mol %) (Table S1, entries S1−S6). These results suggest that the monomeric architecture can be ruled out under the reaction conditions.17 Furthermore, ESI-TOF studies of the copolymerization mixture at the conditions of [CHO]0/[catalyst]0 = 1000/1, time = 3 h, pCO20 = 500 psi, and 120 °C were performed to probe the catalyst structure in solution. On the basis of the ESI-TOF mass spectrum (Figure S10), the peaks corresponding to [K + C 2 H 3 O 2 + (C 7 H 1 0 O 3 ) n + (L4)2Ni3(OAc)3]+ ([39.1 + 59.0 + (142.1)n + 793.1]+, C2H3O2 = OAc, and (C7H10O3)n = (−C6H10−O−C(O)− O−)n) were found after the copolymerization, which implied that a trinuclear configuration exists for the Ni complex in the polymerization solution. On the basis of these results, we believe that the multinucleus structure is kept in solution under the reaction conditions. MALDI-TOF Mass Spectroscopic Studies. The MALDITOF (matrix-assisted laser desorption/ionization time-offlight) spectrum from the purified PCHC sample generated by tri-Ni complex 7 (Table 2, entry 4) has been investigated to understand the microstructure and composition of the PCHC as shown in Figure 8. The mass spectrum displays the isolated copolymers with two series of peaks due to the different alkali metal cations (Na+/K+) desorbed from the matrix. A bunch of peaks divided into a fixed interval with molecular mass about 142 Da, which is peculiar to a repeated unit of PCHC. This result also indicated that the CHO/CO2 copolymerization is alternating and the chain propagation was only initiated by the acetate group. Because of trace quantities of protic species in the Ni catalyst or CHO monomer, these might behave as the chain-transfer agents to result in the lower-molecular-weight polymers compared to the theoretically calculated molecular weight.18

molecular weight (Mn = 20 000 g/mol) as shown in entry 2 of Table 2. It is noteworthy that the linear relationship of Mn versus time as well as low polydispersity indexes (PDIs) exist, which demonstrate a well-controlled character for copolymerization process with tri-Ni 7 (Figure 6 and entries 2 and 8−10

Figure 6. Relationship of Mn (▼) and PDI (●) (determined from GPC analysis) vs time for the copolymerization of cyclohexene oxide and CO2 by using tri-Ni acetate complex 7.

in Table 2). On lowering a catalyst loading to 1:2000, trinuclear Ni complex 7 was still an active catalyst for CO2/CHO coupling without loss of copolymerization selectivity, giving a higher productivity (TON = 1800) and a higher molecular weight PCHC (Mn up to 24 900 g/mol) with a narrow PDI (99%). The 1H NMR spectrum (Figure 7) has confirmed the isolated copolymer with highly alternating microstructure up to >99% carbonate linkages. As can be seen from Figure S7, all the resultant polycarbonates except for production from Ni catalyst 7 with a 0.05 mol % catalyst loading possess a unimodal molecular weight distribution. Nevertheless, the copolymer product was almost atactic, which was verified by the 13C NMR spectrum as shown in Figure S8. In comparison, the catalytic activity for the copolymerization of CHO with CO2 by tri-Ni catalyst 7 is comparable to that of dinuclear nickel catalyst bearing the bis(benzotriazole iminophenolate) ancillary ligand under identical conditions.10b Despite the similar activity of two different kinds of Ni complexes, a notably decreased copolymer selectivity (PCHC/CHC = 87/13) with tri-Ni 7 as the catalyst was observed, implying a backbiting reaction of the growing polymeric chains that occurred during copolymerization by Ni

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CONCLUSION A series of novel nickel acetate complexes containing mono-, di-, and trinuclear architectures that bear NNO-tridentate Schiff-base derivatives have been synthesized and fully characterized by X-ray single-crystal structural analyses. The

Figure 7. 1H NMR spectrum of the purified copolymer produced by using dinickel acetate complex 7 (Table 2, entry 2) in CDCl3. Peak a (δ = 4.65 ppm) is assigned to the methine protons in PCHC, and no significant signal at 3.3−3.5 ppm confirms >99% carbonate linkages in PCHC. Peaks b (4.42 and 3.58 ppm) are assigned to the methine protons on the end group (OCH(CH2)4CHOH). F

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. MALDI-TOF spectrum of the produced PCHC catalyzed by complex 7 (Table 2, entry 4).

(>99%), which is superior to that of the previously reported copper analogue. Complexes 4−9 appear to be the first examples of a structurally well-characterized trinuclear nickel catalyst that is effective for CO2/CHO copolymerization as well as the formation of poly(cyclohexene carbonate)s with controllable molecular weights and a highly alternating microstructure.

structural diversity of these Ni complexes can be classified into three types in the solid state. Complexes 1 and 2 assume fourcoordinate nickel center because of steric demands from bulky substituents at the ortho-position of the phenolate group. Complex 3 exhibits a bimetallic nickel(II) species involving one coordinated water intramolecularly hydrogen bonded to the monodentate acetate group, whereas 4−9 are trimetallic complexes in which four ancillary acetate groups adopt two different types of bonding modes to coordinate metal centers. Di- and trinuclear nickel complexes were demonstrated to effectively catalyze the coupling of CO2 and cyclohexene oxide with good copolymerization selectivity. The best catalytic efficiency of CHO/CO2 copolymerization by using trimetallic nickel(II) acetate complex 7 was attributed to a moderate TOF (48 h−1) together with a high content of carbonate linkages

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EXPERIMENTAL SECTION

General Considerations. Solvents and reagents were dried over calcium hydride (anhydrous methanol (MeOH), anhydrous ethanol (EtOH)). Deuterated solvents were dried over 4 Å molecular sieves. Ni(OAc)2·4H2O, methanol (MeOH, 99%), ethanol (EtOH, 95%), 2,4bis(α,α-dimethylbenzyl)phenol, magnesium chloride, paraformaldehyde, triethylamine, 3,5-di-tert-butyl-2-hydroxybenzaldehyde, salicylaldehyde, 2-hydroxy-4-methoxybenzaldehyde, 2-hydroxy-5-methoxyG

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry benzaldehyde, (2-hydroxyphenyl)(phenyl)methanone, and N,N′-dimethylethane-1,2-diamine were purchased and used without further purification. Cyclohexene oxide (CHO) was purified before use. 1H NMR and 13C NMR spectra were recorded on a Varian Mercury 400 (400 MHz for 1H and 100 MHz for 13C) spectrometer with chemical shifts given in parts per million from the peak of internal TMS. 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 an eluent (flow rate 1.0 mL/min, at 40 °C). The chromatographic column was Phenomenex Phenogel 5μ 103 Å, and the calibration curve used to calculate Mn (GPC) was produced from polystyrene standards. The GPC results were calculated using the Scientific Information Service Corporation (SISC) chromatography data solution 3.1 edition. The Fourier transform IR (FT-IR) spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer in the range 4000−600 cm−1. UV−vis spectra were determined by JASCO U-530 UV−vis spectrometer at 25 °C using CH2Cl2 as solvent. Mass analyses were performed using positive electron spray ionization (ESI+) technique on a Thermo Finnigan TSQ Quantum mass spectrometer for all these complexes upon dissolution in DMSO solvent. MALDI-TOF spectrometry measurements were performed on autoflex speed (Bruker Daltonik GmbH). ESI-TOF spectrometry measurements were performed on autoflex speed (Bruker Daltonic, MicroTOF II) for all these samples upon dissolution in MeOH solvent. Synthesis of NNO-Schiff-Base Ligands. General procedures for ligand precursors preparation were conducted by the reaction of the corresponding 2-hydroxybenzaldehyde or 2-hydroxyacetophenone derivatives (5.0 mmol) with N,N-dimethylethane-1,2-diamine (5.1 mmol) in refluxing ethanol (30.0 mL) for 24 h. The mixture was cooled to room temperature, and the volatile components were removed in vacuo to give the products.12,13 Synthesis of Nickel Complexes 1−9. Synthesis of Complex [(L 1 )Ni(OAc)] (1). A mixture of 2,4-di-tert-butyl-6-(((2(dimethylamino)ethyl)imino)methyl)phenol (1.52 g, 5.0 mmol) and Ni(OAc)2·4H2O (1.25 g, 5.0 mmol) was dissolved in ethanol (30 mL), and the solution was heated under reflux for 24 h. The solution was then cooled to room temperature. Volatile materials were removed to leave about 5 mL under vacuum to yield red solids. The powder was obtained after filtration. Yield: 1.68 g (80%). Anal. Calcd for C21H34NiN2O3: N, 6.65; C, 59.88; H, 8.14%. Found: N, 6.65; C, 59.40; H, 8.44%. Characteristic IR absorptions (cm−1, neat): 1617 (νCN), 1533 (νasymacetate), 1419 (νsymacetate). m/z (ESI-MS, DMSO): 361.1 ([M − OAc]+, 100%, calcd 361.1). Synthesis of Complex [(L2)Ni(OAc)] (2). The synthetic route for complex 2 was the same as that of 1. Yield: 2.18 g (80%). Anal. Calcd for C31H38NiN2O3: N, 5.14; C, 68.24; H, 7.02%. Found: N, 5.15; C, 68.47; H, 6.33%. Characteristic IR absorptions (cm−1, neat): 1612 (νCN), 1538 (νasymacetate), 1403 (νsymacetate). m/z (ESI-MS, DMSO): 485.2 ([M − OAc]+, 100%, calcd 485.2). Synthesis of Complex [(L3)2Ni2(OAc)2(H2O)] (3). The synthetic route for complex 3 was the same as that for 1. Yield: 3.82 g (83%). Anal. Calcd for C26H38Ni2N4O7·H2O: N, 8.57; C, 47.75; H, 6.61%. Found: N, 7.63; C, 47.90; H, 5.70%. Characteristic IR absorptions (cm−1, neat): 3224 (νH2O), 1644 (νCN), 1575 (ν asymbridgingbidentateacetate ), 1540 (ν asymmonodentateacetate ), 1417 (νsym monodentateacetate), 1384 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 557.0 ([M − OAc − H2O]+, 100%, calcd 557.1). Synthesis of Complex [(L3)2Ni3(OAc)4(MeOH)2] (4). A mixture of 2(((2-(dimethylamino)ethyl)imino)methyl)phenol (0.96 g, 5.0 mmol) and Ni(OAc)2 (0.88 g, 5.0 mmol) was dissolved in anhydrous methanol (30 mL), and the solution was heated under reflux for 24 h under a dry nitrogen atmosphere. The solution was then cooled to ambient temperature. Volatile materials were removed to leave about 5 mL under vacuum to yield green solids. The powder was obtained after filtration. Yield: 1.42 g (82%). Anal. Calcd for C32H50Ni3N4O12: N, 6.52; C, 44.75; H, 5.87%. Found: N, 6.44; C, 44.20; H, 6.12%. Characteristic IR absorptions (cm−1, neat): 1644 (νCN), 1583 (ν asymbridgingbidentateacetate ), 1542 (ν asymmonodentateacetate ), 1423

(νsymmonodentateacetate), 1399 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 557.4 ([M − Ni(OAc)3 − 2MeOH]+, calcd 557.1). Synthesis of Complex [(L3)2Ni3(OAc)4(EtOH)2] (5). A mixture of 2(((2-(dimethylamino)ethyl)imino)methyl)phenol (0.96 g, 5.0 mmol) and Ni(OAc)2 (0.88 g, 5.0 mmol) was dissolved in anhydrous ethanol (30 mL), and the solution was heated under reflux for 24 h under dry nitrogen atmosphere. The solution was then cooled to room temperature. Volatile materials were removed to leave about 5 mL under vacuum to yield green solids. The powder was obtained after filtration. Yield: 1.42 g (82%). Anal. Calcd for C34H54Ni3N4O12: N, 6.32; C, 46.04; H, 6.14%. Found: N, 6.86; C, 46.46; H, 5.73%. Characteristic IR absorptions (cm−1, neat): 1644 (νCN), 1579 (ν asymbridgingbidentateacetate ), 1542 (ν asymmonodentateacetate ), 1421 (νsymmonodentateacetate), 1399 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 557.3 ([M − Ni(OAc)3 − 2EtOH]+, calcd 557.1). Synthesis of Complex [(L3)2Ni3(OAc)4] (6). [(L3)2Ni3(OAc)4(MeOH)2] (4) or [(L3)2Ni3(OAc)4(EtOH)2] (5) was dried under vacuum overnight and recrystallized by the mixed solvent of CH2Cl2/ hexane to give [(L 3 ) 2 Ni 3 (OAc) 4 ] (6). Anal. Calcd for C30H43Ni3N4O10·CH2Cl2: N, 6.37; C, 42.33; H, 5.04%. Found: N, 6.63; C, 42.72; H, 5.38%. Characteristic IR absorptions (cm−1, neat): 1646 (νCN), 1583 (νasymbridgingbidentateacetate), 1542 (νasymmonodentateacetate), 1428 (νsymmonodentateacetate), 1386 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 557.3 ([M − Ni(OAc)3]+, calcd. 557.1) Synthesis of Complex [(L4)2Ni3(OAc)4] (7). A mixture of 2-(((2(dimethylamino)ethyl)imino)methyl)-5-methoxyphenol (0.88 g, 4.0 mmol) and Ni(OAc)2·4H2O (1.50 g, 6.0 mmol) was dissolved in ethanol (30 mL), and the solution was heated under reflux for 24 h. The solution was then cooled to room temperature. Volatile materials were removed to leave about 5 mL under vacuum to yield solids. The green powder was obtained after filtration. Yield: 1.37 g (80%). Anal. Calcd for C32H46Ni3N4O12: N, 6.55; C, 44.96; H, 5.42%. Found: N, 6.58; C, 44.70; H, 5.50%. Characteristic IR absorptions (cm−1, neat): 1639 (νCN), 1575 (νasymbridgingbidentateacetate), 1536 (νasymmonodentateacetate), 1428 (νsymmonodentateacetate), 1399 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 617.4 ([M − Ni(OAc)3]+, 100%, calcd 617.1). Synthesis of Complex [(L5)2Ni3(OAc)4] (8). The synthetic route for complex 8 was the same as that for 7. Yield: 1.28 g (75%). Anal. Calcd for C32H46Ni3N4O12: N, 6.55; C, 44.96; H, 5.42%. Found: N, 6.18; C, 44.90; H, 5.79%. Characteristic IR absorptions (cm−1, neat): 1646 (νCN), 1585 (νasymbridgingbidentateacetate), 1540 (νasymmonodentateacetate), 1426 (νsymmonodentateacetate), 1390 (νsymbridgingbidentateacetate). m/z (ESI-MS, DMSO): 619.0 ([M − Ni(OAc)3]+, calcd. 617.1). Synthesis of Complex [(L6)2Ni3(OAc)4] (9). The synthetic route for complex 9 was the same as that for 7. Yield: 1.51 g (80%). Anal. Calcd for C42H50Ni3N4O10: N, 5.92; C, 53.27; H, 5.32%. Found: N, 5.90; C, 52.96; H, 6.09%. Characteristic IR absorptions (cm−1, neat): 1668 (νCN), 1565 (νasymmetric bridging bidentate acetate), 1538 (νasymmetric monodentate acetate), 1413 (νsymmetric monodentate acetate), 1392 (νsymmetric bridging bidentate acetate). m/z (ESI-MS, DMSO): 709.4 ([M− Ni(OAc)3]+, calcd. 709.1). Copolymerization of CO2 and CHO Catalyzed by Complexes 1−9. A representative procedure for the copolymerization of cyclohexene oxide with CO2 (Table 1, entry 8) was exemplified. Ni catalyst (0.1 mmol) was dissolved in 10.0 mL of neat cyclohexene oxide under dry nitrogen atmosphere. A mixing solution was added to the 100 mL autoclave with magnetic stirrer under CO2 atmosphere. CO2 was then charged into the reactor until the pressure of 300 psi was reached, and the stirrer was started. The reaction was performed at 120 °C for 24 h. Then the reactor was placed into ice water, and excess CO2 was released. The CHO conversion was analyzed by 1H NMR spectroscopic studies. Spectral characteristics of cyclohexene carbonate: PCHC carbonate (δ: 4.65 ppm), PCHC ether (δ: 3.3−3.5 ppm), and trans-CHC (δ: 3.9 ppm). The mixture was diluted by CH2Cl2 (50 mL), followed by the addition of 1 N HCl solution (1.0 mL) to quench this reaction, and the final mixture was passed through a short column of neutral alumina to remove the metal salt. After precipitation by adding polymer solution in CH2Cl2 into methanol (50 mL) three times, the off-white polymer was collected by filtration and dried under vacuum overnight. H

DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry X-ray Crystallographic Studies. Single crystals of complexes 1− 9 were obtained from a saturated ethanol, methanol, or CH2Cl2/ hexane mixing solution. Suitable crystals were immersed with FOMBLINY under nitrogen atmosphere and mounted on an Oxford Xcalibur Sapphire-3 CCD Gemini diffractometer employing graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å); intensity data were collected with ω scans. The data collection and reduction were performed with the CrysAlisPro software,19 and the absorptions were corrected by SCALE3 ABSPACK multiscan method.20 The space group determination was based on a check of the Laue symmetry and systematic absences, and was confirmed using the structure solution. The structure was solved and refined with the SHELXTL package.21 All non-H atoms were located from successive Fourier maps, and hydrogen atoms were refined using a riding model. Anisotropic thermal parameters were used for all non-H atoms, and fixed isotropic parameters were used for H atoms. CCDC 1454655−1454663 (for 1− 9) contain the supplementary crystallographic data in this paper.

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(4) (a) Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem., Int. Ed. 2003, 42, 5484. (b) Lu, X.-B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574. (c) S, S.; Min, K. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int. Ed. 2008, 47, 7306. (d) Ren, W.-M.; Zhang, W.-Z.; Lu, X.-B. Sci. China: Chem. 2010, 53, 1646. (5) (a) Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K. Angew. Chem., Int. Ed. 2009, 48, 931. (b) Kember, M. R.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2009, 48, 9535. (c) Thevenon, A.; Garden, J. A.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2015, 54, 11906. (6) Kember, M. R.; White, A. J. P.; Williams, C. K. Macromolecules 2010, 43, 2291. (7) Buchard, A.; Kember, M. R.; Sandeman, K. G.; Williams, C. K. Chem. Commun. 2011, 47, 212. (8) Kember, M. R.; Williams, C. K. J. Am. Chem. Soc. 2012, 134, 15676. (9) (a) Saini, P. K.; Romain, C.; Williams, C. K. Chem. Commun. 2014, 50, 4164. (b) Garden, J. A.; Saini, P. K.; Williams, C. K. J. Am. Chem. Soc. 2015, 137, 15078. (10) (a) Li, C. H.; Chuang, H. J.; Li, C. Y.; Ko, B. T.; Lin, C. C. Polym. Chem. 2014, 5, 4875. (b) Yu, C.-Y.; Chuang, H.-J.; Ko, B. T. Catal. Sci. Technol. 2016, 6, 1779. (11) (a) Mukherjee, P.; Drew, M. G. B.; Estrader, M.; Ghosh, A. Inorg. Chem. 2008, 47, 7784. (b) Mukherjee, P.; Drew, M. G. B.; Gómez-García, C. J.; Ghosh, A. Inorg. Chem. 2009, 48, 5848. (c) Biswas, R.; Diaz, C.; Bauzá, A.; Barceló-Oliver, M.; Frontera, A.; Ghosh, A. Dalton Trans. 2014, 43, 6455. (12) (a) Hung, W.-C.; Huang, Y.; Lin, C.-C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6466. (b) Hung, W.-C.; Lin, C.-C. Inorg. Chem. 2009, 48, 728. (c) Tsai, C.-Y.; Huang, B.-H.; Hsiao, M.-W.; Lin, C.-C.; Ko, B.-T. Inorg. Chem. 2014, 53, 5109. (13) Bhardwaj, V. K.; Hundal, M. S.; Corbella, M.; Gomez, V.; Hundal, G. Polyhedron 2012, 38, 224. (14) Reglinski, J.; Morris, S.; Stevenson, D. E. Polyhedron 2002, 21, 2167. (15) Chattopadhyay, S.; Drew, M. G. B.; Ghosh, A. Eur. J. Inorg. Chem. 2008, 2008, 1693. (16) Koning, C.; Wildeson, J.; Parton, R.; Plum, B.; Steeman, P.; Darensbourg, D. J. Polymer 2001, 42, 3995. (17) Liu, J.; Ren, W.-M.; Lu, X.-B.; Liu, Y. Macromolecules 2013, 46, 1343. (18) (a) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 125, 11911. (b) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed. 2006, 45, 7274. (c) Na, S. J.; S, S.; Cyriac, A.; Kim, B. E.; Yoo, J.; Kang, Y. K.; Han, S. J.; Lee, C.; Lee, B. Y. Inorg. Chem. 2009, 48, 10455. (d) Jutz, F.; Buchard, A.; Kember, M. R.; Fredriksen, S. B.; Williams, C. K. J. Am. Chem. Soc. 2011, 133, 17395. (e) Chatterjee, C.; Chisholm, M. H. Inorg. Chem. 2011, 50, 4481. (19) CrysAlisPro Version 1.171; Agilent Technologies. (20) CrysAlisPro SCALE3 ABSPACK: A Scaling Algorithm for Empirical Absorption Correction Using Spherical Harmonics; Oxford Diffraction, 2005. (21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00478. Absorption spectra of complexes 1−9, 1H NMR spectrum of copolymer selectivity by 7, and GPC traces for the isolated PCHC (PDF) Crystallographic details (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 886-4-22840411-718. Fax: 886-4-22862547. *E-mail: [email protected]. Phone: 886-4-22840411715. Fax: 886-4-22862547. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Ministry of Science and Technology, Taiwan (MOST 1042113-M-005-015-MY3 to C.-C.L. and MOST 105-2119-M-005005 to B.-T.K.).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00478 Inorg. Chem. XXXX, XXX, XXX−XXX