- Email: [email protected]
Synthesis and structural characterisation of tetrahedral zinc(II) and trigonal bipyramidal cadmium(II) complexes containing N′-cyclohexyl substituted N,N-bispyrazolyl ligand
Inorganica Chimica Acta 435 (2015) 313–319
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis and structural characterisation of tetrahedral zinc(II) and trigonal bipyramidal cadmium(II) complexes containing N0 -cyclohexyl substituted N,N-bispyrazolyl ligand Sunghye Choi a, Seung Hyun Ahn a, Saira Nayab b,⇑, Hyosun Lee a,⇑ a b
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city 702-701, Republic of Korea Department of Chemistry, Shaheed Benazir Bhutto University, Sheringal, Dir (U), Khyber Pakhtunkhwa, Pakistan
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 6 July 2015 Accepted 8 July 2015 Available online 22 July 2015 Keywords: Zinc(II) Cadmium(II) Molecular structures Methyl methacrylate polymerisation Syndiotacticity
a b s t r a c t The bis(pyrazolyl)-based ligands L1 and L2, where L1 is N,N-bis((1H-pyrazol-1-yl)methyl)cyclohexanamine and L2 is N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)cyclohexanamine, reacted with [ZnCl2] and [CdBr24H2O], respectively, and resulted in mononuclear [LnMX2] (Ln = L1 and L2; M = Zn, X = Cl; M = Cd, X = Br) complexes in a facile yield and high purity. The synthesised complexes were characterised spectroscopically and X-ray crystallographically. The molecular structure of 4-coordinate [L1ZnCl2] was best described as a distorted tetrahedral, whereas 5-coordinate [L1CdBr2] and [L2CdBr2] exhibited distorted trigonal bipyramidal geometry involving the metal centres. [L1ZnCl2] showed the highest catalytic activity for the polymerisation of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) with an activity of 4.40 104 g PMMA/molZn h at 60 °C and resulted in syndiotactic poly(methylmethacrylate) (PMMA). Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction PMMA has received increasing attention as an organic transparent material due to its good electrical insulation and optical applications [1–7]. The improved optical properties of PMMA are mainly dependent on the higher glass transition temperature (Tg) and tacticity. Radical-mediated polymerisation of MMA (generally atactic) cannot achieve a sufficient Tg or syndiotacticity content in the resultant PMMA. Thus, non-radical-mediated MMA polymerisation using various transition metal catalysts that can result in a higher Tg up to 140 °C is an active and promising field of research both in academic laboratories and industry [8–13]. Among the various metals, titanium [14,15], zirconium [16], hafnium [17], cobalt [18–20], chromium [21,22], nickel [23–25], ruthenium [26–28], palladium [29–31], and lanthanide [32]-based complexes with various ligand architectures have mainly been employed for MMA polymerisation with moderate-to-high activities and tacticities. Moreover, despite numerous reports on metal complexes with N-donor ligands [33,34], little is known regarding pyrazolyl-based late transition metal complexes as catalysts for methyl methacrylate polymerisation [35–37] compared with early transition metal ⇑ Corresponding authors. Tel.: +82 53 950 5337; fax: +82 53 950 6330 (H. Lee). E-mail address: [email protected] (H. Lee). http://dx.doi.org/10.1016/j.ica.2015.07.012 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
complexes [38–41] because late transition metal complexes are generally more tolerant towards polar functional monomers (including MMA) due to their less-oxophilic nature. In contrast, the steric and electronic properties of pyrazolyl-based ligands can be fine-tuned by an appropriate choice of substitutions on the 2-N, 3-C, 4-C, and 5-C atoms of the pyrazole moiety [42–45], and have been employed in diverse potential applications [46–50]. In view of these promising features of pyrazolyl-based ligands, we focused on the synthesis, characterisation, and reactivity of various transition metal complexes with pyrazolyl-based ligands and their catalytic activities towards MMA polymerisation [18,51,52]. Specifically, we found that cobalt (II) complexed with N0 -cyclohexyl substituted N,N-bispyrazolyl ligand showed exceptional activity for MMA polymerisation [53]. The aim of this report was to extend our work to a novel N0 -cyclohexyl-substituted N,N-bispyrazolyl ligand and investigate their variable coordination behaviours towards Zn(II) and Cd(II) centres since the development of new highly active catalysts is an extremely attractive yet challenging goal to obtain high-molecular-weight PMMA with improved tacticity. The catalytic activities for MMA polymerisation in toluene at 60 °C were also examined. In addition, we explored the effects of substituents of the pyrazolyl moiety and inexpensive transition metals, zinc(II) and cadmium(II), on the activity and tacticity of the resultant polymer.
314
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
2. Experimental 2.1. Materials Anhydrous [ZnCl2], [CdBr24H2O], cyclohexanamine, 1H-1-pyrazolyl-1-methanol, and methyl methacrylate (MMA) were purchased from Sigma-Aldrich (St. Louis, MO) and anhydrous solvents such as C2H5OH, DMF, diethyl ether (Et2O), and dichloromethane (CH2Cl2) were purchased from Merck (Darmstadt, Germany) and used without further purification. Modified methylaluminoxane (MMAO) was purchased from Tosoh Finechem Corporation (Tokyo, Japan) as 6.9% aluminium (by weight) in a toluene solution and used without further purification. 2.2. Physical measurements 1
H NMR (operating at 400 MHz) and 13C NMR (operating at 100 MHz) spectra were recorded using an Advance Digital 400 NMR spectrometer (Bruker, Billerica, MA); chemical shifts were recorded in ppm units (d) relative to SiMe4 as the internal standard. Infrared (IR) spectra were recorded on a Bruker FT/IR-Alpha (neat) and the data were reported in reciprocal centimetres. Elemental analyses (C, H, N) of the synthesised ligand and complexes were performed using an elemental analyser (EA 1108; Carlo-Erba, Milan, Italy). The molecular weight and molecular weight distribution of the obtained poly (methylmethacrylate) (PMMA) were determined using gel-permeation chromatography (GPC) (CHCl3, Alliance e2695; Waters Corp., Milford, MA). Glass transition temperature (Tg) was determined using a thermal analyser (Q2000; TA Instruments, New Castle, DE). 2.3. Synthesis 2.3.1. Preparation of ligands and corresponding complexes 2.3.1.1. Synthesis of (L1) [53]. Cyclohexanamine (2.29 mL, 20.0 mmol) was treated with CH2Cl2 solution of 1H-1-pyrazolyl1-methanol (3.92 g, 40.0 mmol) at room temperature and was stirred for 36 h. The resultant solution was dried over the MgSO4. The filtrate solvent was removed under reduced pressure and the crude residue was vacuum distiled to give colourless oil (4.44 g, 85.6%). Anal. Cal. for C14H21N5: C, 64.84, H, 8.16, N, 27.00. Found: C, 65.18, H, 8.27, N, 27.52%. 1H NMR (CDCl3, 400 MHz): d 7.53 (d, 2H, J = 1.6 Hz, AN@CHACH@CHANA), 7.51 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.27 (dd, 2H, J = 1.6 Hz, J = 2.0 Hz, AN@CHACH@CHANA), 5.13 (s, 4H, ANACH2ANA), 2.86–2.79 (m, 1H, ipso-C6H11A), 1.76–1.70 (m, 2H, AC6H11A), 1.65–1.55 (m, 3H, AC6H11A), 1.24–1.14 (m, 4H, AC6H11A), 1.08–0.99 (m, 1H, AC6H11A). 13C NMR (CDCl3, 100 MHz): d 140.71 (d, 2C, J = 176 Hz, AN@CHACH@CHANA), 130.39 (d, 2C, J = 185 Hz, AN@CHACH@CHANA), 107.25 (d, 2C, J = 176 Hz, AN@CHACH@CHANA), 66.34 (t, 2C, J = 149 Hz, ANACH2ANA), 60.94 (d, 1C, J = 139 Hz, ipso-C6H11A), 33.40 (t, 2C, J = 130 Hz, o-C6H11A), 31.38 (t, 1C, J = 130 Hz, p-C6H11A), 26.31 (t, 2C, J = 126 Hz, m-C6H11A). IR (liquid neat; cm1): 3111 (w), 2928 (m), 2855 (w), 1510 (w), 1449 (w), 1387 (m), 1258 (m), 1130 (m), 1084 (m), 1042 (s), 966 (m), 882 (w), 745 (s), 653 (w), 614 (m). 2.3.1.2. Synthesis of (L2) [54,55]. N,N-bis((3,5-dimethyl-1H-pyrazol1-yl)methyl)cyclohexanamine (L2) was prepared according to reported method. 2.3.1.3. Synthesis of [L1ZnCl2]. EtOH (10.0 mL) solution of L1 (0.259 g, 1.00 mmol) was treated with EtOH (10.0 mL) solution of [ZnCl2] (0.136 g, 1.00 mmol). Precipitation of white powder occurred while stirring at room temperature for 12 h. The white
powder was filtered and washed with EtOH (50.0 mL 2), followed by washing with Et2O (50.0 mL 2) to give the white solid (0.38 g, 96.0%). Crystals suitable for the X-ray study of [L1ZnCl2] were obtained from Et2O (10.0 mL) diffusion into DMF solution (10.0 mL). Anal. Cal. for C14H21Cl2N5Zn: C, 42.50, H, 5.35, N, 17.70. Found: C, 42.50, H, 5.30, N, 17.68%. 1H NMR (DMSO-d6, 400 MHz): d 7.82 (d, 2H, J = 2.4 Hz, AN@CHACH@CHANA), 7.51 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.29 (t, 2H, J = 2.4 Hz, AN@CHACH@CHANA), 5.22 (s, 4H, ANACH2ANA), 2.75–2.70 (m, 1H, ipso-C6H11A), 1.71–1.57 (m, 4H, AC6H11A), 1.51–1.37 (m, 4H, AC6H11A), 1.26–1.20 (m, 1H, AC6H11A), 0.99–0.94 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 139.67 (d, 2C, J = 184 Hz, AN@CHACH@CHANA), 131.08 (d, 2C, J = 181 Hz, AN@CHACH@CHANA), 106.03 (d, 2C, J = 184 Hz, AN@CHACH@CHANA), 66.68 (t, 2C, J = 149 Hz, ANACH2ANA), 59.17 (d, 1C, J = 136 Hz, ipso-C6H11A), 30.21 (t, 2C, J = 126 Hz, oC6H11A), 25.90 (t, 1C, J = 128 Hz, p-C6H11A), 25.55 (t, 2C, J = 125 Hz, m-C6H11A). IR (solid neat; cm1): 3112 (w), 2928 (w), 2852 (w), 1752 (w), 1702 (w), 1514 (w), 1458 (m), 1408 (s), 1347 (w), 1302 (m), 1242 (m), 1165 (s), 1067 (s), 988 (m), 892 (w), 771 (s), 719 (s), 615 (s). 2.3.1.4. Synthesis of [L2ZnCl2]. [L2ZnCl2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L2 (0.315 g, 1.00 mmol) and [ZnCl2] (0.136 g, 1.00 mmol) to give white solid as final product (0.40 g, 88.7%). Crystals of [L2ZnCl2] suitable for the X-ray study were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C18H29Cl2N5Zn: C, 47.86, H, 6.47, N, 15.50. Found: C, 47.82, H, 6.24, N, 15.73%. 1H NMR (DMSO-d6, 400 MHz): d 6.03 (s, 2H, AN@C(CH3)ACH@C(CH3)ANA), 5.02 (s, 4H, ANACH2ANA), 2.75A2.67 (m, 1H, ipso-NC6H11A), 2.63 (s, 6H, AN@C(CH3)ACH@ C(CH3)ANA), 2.33 (s, 6H, AN@C(CH3)ACH@C(CH3)ANA), 1.80– 1.77 (m, 2H, AC6H11A), 1.65–1.61 (m, 1H, AC6H11A), 1.38–1.33 (m, 2H, AC6H11A), 1.30–1.13 (m, 4H, AC6H11A), 1.09–1.01 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 152.43 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 142.78 (s, 2C, AN@C(CH3)ACH@ C(CH3)ANA), 108.89 (d, 2C, J = 176 Hz, AN@C(CH3)ACH@C(CH3) ANA), 61.83 (t, 2C, J = 65 Hz, ANACH2ANA), 60.36 (d, 1C, J = 144 Hz, ipso-C6H11A), 31.26 (t, 2C, J = 127 Hz, o-C6H11A), 26.59 (t, 1C, J = 126 Hz, p-C6H11A), 25.88 (t, 2C, J = 121 Hz, m-C6H11A), 14.48 (q, 2C, J = 127 Hz, AN@C(CH3)ACH@C(CH3)ANA), 11.79 (q, 2C, J = 129 Hz, AN@C(CH3)ACH@C(CH3)ANA). IR (solid neat; cm1): 3274 (s), 2924 (m), 2854 (w), 1570 (w), 1552 (s), 1462 (s), 1407 (s), 1372 (s), 1344 (m), 1325 (w), 1258 (w), 1224 (m), 1193 (s), 1180 (s), 1047 (w), 1023 (s), 991 (s), 893 (w), 779 (w), 690 (w), 619 (m). 2.3.1.5. Synthesis of [L1CdBr2]. [L1CdBr2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L1 (0.259 g, 1.00 mmol) and [CdBr24H2O] (0.334 g, 1.00 mmol) to give white powder as final product (0.382 g, 72.0%). Crystals suitable for the X-ray study of [L1CdBr2] were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C14H21Br2CdN5: C, 31.63, H, 3.98, N, 13.17. Found: C, 30.90; H, 3.97; N, 12.72%. 1H NMR (DMSO-d6, 400 MHz): d 7.78 (d, 2H, J = 1.6 Hz, AN@CHACH@CHANA), 7.48 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.27 (dd, 2H, J = 1.6 Hz, J = 2.0 Hz, AN@CHACH@CHANA), 5.19 (s, 4H, ANACH2ANA), 2.76–2.70 (m, 1H, ipso-C6H11A), 1.64–1.57 (m, 2H, AC6H11A), 1.50–1.38 (m, 3H, AC6H11A), 1.16–1.07 (m, 4H, AC6H11A), 1.00– 0.92 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 140.05 (d, 2C, J = 190 Hz, AN@CHACH@CHANA), 130.79 (d, 2C, J = 188 Hz, AN@CHACH@CHANA), 106.70 (d, 2C, J = 166 Hz, AN@CHACH@CHANA), 66.18 (t, 2C, J = 150 Hz, ANACH2ANA), 59.74 (d, 1C, J = 110 Hz, ipso-C6H11A), 30.42 (t, 2C, J = 122 Hz, o-C6H11A), 26.89 (t, 1C, J = 123 Hz, p-C6H11A), 25.87 (t, 2C,
315
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
J = 126 Hz, m-C6H11A). IR (solid neat; cm1): 3744 (w), 3609 (w), 3116 (w), 2926 (w), 2850 (w), 1837 (w), 1757 (w), 1647 (w), 1514 (m), 1399 (m), 1278 (m), 1194 (w), 1164 (w), 1101 (m), 1059 (s), 973 (m), 895 (w), 769 (s), 607 (m). 2.3.1.6. Synthesis of [L2CdBr2]. [L2CdBr2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L2 (0.315 g, 1.00 mmol) and [CdBr24H2O] (0.334 g, 1.00 mmol) to yield white powder as final product (0.570 g, 97.0%). Crystals suitable for the X-ray study of [L2CdBr2] were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C18H29Br2CdN5: C, 36.79, H, 4.97, N, 11.92. Found: C, 37.64, H, 5.08, N, 12.11%. 1H NMR (DMSO-d6, 400 MHz): d 5.90 (s, 2H, AN@C(CH3)A CH@C(CH3)ANA), 4.93 (s, 4H, ANACH2ANA), 2.20 (s, 6H, AN@C(CH3)ACH@C(CH3)ANA), 2.18 (s, 6H, AN@C(CH3)ACH@ C(CH3)ANA), 1.66–1.59 (m, 4H, AC6H11A), 1.52–1.50 (m, 1H, ipso-NC6H11A), 1.14–0.97 (m, 6H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 147.87 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 140.51 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 107.47 (d, 2C, J = 177 Hz, AN@C(CH3)ACH@C(CH3)ANA), 62.12 (t, 2C, J = 148 Hz, ANACH2ANA), 58.44 (d, 1C, J = 129 Hz, ipso-NAC6H11A), 28.34 (t, 2C, J = 125 Hz, o-C6H11A), 26.06 (t, 3C, J = 124 Hz, m-, p-C6H11A), 14.40 (q, 2C, J = 127 Hz, AN@C(CH3)ACH@C(CH3)ANA), 11.44 (q, 2C, J = 128 Hz, AN@C(CH3)ACH@C(CH3)ANA). IR (solid neat; cm1): 3845 (m), 3744 (s), 3678 (m), 3616 (m), 2930 (w), 1835 (w), 1743 (m), 1695 (s), 1648 (s), 1544 (s), 1464 (s), 1393 (s), 1291 (m), 1232 (w), 1043 (m), 974 (m), 817 (s), 631 (w). 2.4. X-ray crystallographic studies A summary of selected crystallographic details and structure refinement results for [L1ZnCl2], [L1CdBr2] and [L2CdBr2] are
presented in Table 1, respectively. Data collection were made on a Bruker SMART CCD diffractometer equipped with a graphitemonochromated Mo Ka (k = 0.71073 Å) radiation source under a nitrogen cold stream (200 K). Cell parameter refinements and integration were performed with SMART and SAINT-Plus software packages [56]. Semi-empirical absorption corrections based on equivalent reflections were applied using SADABS [57]. Structures were solved using direct methods and refined using a full-matrix least-squares method on F2 using SHELXTL [58]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added to their geometrically ideal positions. 2.5. Catalytic activity for MMA polymerisation The methyl methacrylate (MMA) was extracted with 10% sodium hydroxide, washed with water, dried over MgSO4, and distiled over calcium hydride under reduced pressure before use. In a Schlenk line, complex (15.0 lmol, 5.93 mg for [L1ZnCl2]; 6.77 mg for [L2ZnCl2]; 7.97 mg for [L1CdBr2]; 8.81 mg for [L2CdBr2]) was dissolved in dried toluene (10.0 mL) followed by the addition of modified MMAO (6.90 wt% in toluene, 3.25 mL, 7.50 mmol, and [MMAO]0/[Pd(II) catalyst]0 = 500) as a co-catalyst. The solution was stirred at 60 °C for 20 min. The MMA (5.00 mL, 47.1 mmol, [MMA]0/[Pd(II) catalyst]0 = 3100) was added to the above reaction mixture and stirred at 60 °C for 2 h to obtain a viscous solution. MeOH (2.00 mL) was added to terminate the polymerisation. The reaction mixture was poured into a large quantity of MeOH (500 mL), and 35% HCl (5.00 mL) was injected to remove the remaining co-catalyst. The resulting polymer was filtered and washed with MeOH (250 mL 2) to yield poly (methyl methacrylate) (PMMA), which was vacuum-dried at 60 °C.
Table 1 Crystal data and structural refinement for [L1ZnCl2], [L1CdBr2], and [L2CdBr2].
Empirical formula Formula weight T (K) Wavelength (nm) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalcu (g/cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h range for data collection (°) Index ranges
[L1ZnCl2]
[L1CdBr2]
[L2CdBr2]
C28H42Cl4N10Zn2 791.25 200(2) 0.71073 monoclinic P2(1)/c
C14H21Br2N5Cd 531.58 200(2) 0.71073 monoclinic P2(1)/c
C18H29Br2N5Cd 587.68 200(2) 0.71073 orthorhombic Pnma
8.5628(5) 13.5754(8) 16.1120(9) 90 90.014(1) 90 1872.9(2) 4 1.885 5.436 1032 0.32 0.25 0.08 1.96–28.31 8 6 h 6 11, 16 6 k 6 18, 21 6 l 6 20 13 008 4520 (0.0209) 96.90% Full-matrix least-squares on F2 4520/0/199 1.218 R1 = 0.0332, wR2 = 0.0743 R1 = 0.0581, wR2 = 0.1435 1.611 and 2.326
12.8520(8) 14.8725(9) 11.5930(7) 90 90 90 2215.9(2) 4 1.762 4.604 1160 0.26 0.18 0.15 2.23–28.29 17 6 h 6 14, 19 6 k 6 19, 15 6 l 6 15 14 846 2807 90.0247) 98.20% Full-matrix least-squares on F2 2807/0/141 1.022 R1 = 0.0315, wR2 = 0.0921 R1 = 0.0447, wR2 = 0.1560 0.967 and 1.312
18.063(4) 13.627(3) 15.861(3) 90 114.68(3) 90 3547 (2) 4 1.481 1.689 1632 0.38 0.22 0.12 1.24–28.56 20 6 h 6 24, 15 6 k 6 18, 21 6 l 6 20 Reflections collected 26 113 Independent reflections (Rint) 8848 (0.1383) Completeness to h = 22.210° 99.90% Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 8848/0/383 Goodness-of-fit (GOF) on F2 0.832 Final R indices [I > 2r(I)] R1 = 0.0639, wR2 = 0.1438 R indices (all data) R1 = 0.1549, wR2 = 0.1886 Largest difference peak and hole (e Å3) 1.683 and 0.974
316
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
3. Results and discussion 3.1. Synthesis and chemical properties Ligands in the current study were obtained in high yields using a single-step synthetic procedure from the reaction of cyclohexylamine and 1H-pyrazolyl-1-methanol or 3,5-dimethyl1H-pyrazolyl-1-methanol in CH2Cl2. Treatment of Ln (Ln = L1 and L2) in a 1:1 ratio with [ZnCl2] and [CdBr24H2O] afforded the corresponding dichloro Zn(II) and dibromo Cd(II) complexes in 72–98% yield. All spectroscopic data were consistent with the proposed Zn(II) and Cd(II) complex formulation. 1H NMR and 13C NMR peaks of the Zn(II) and Cd(II) complexes were slightly shifted compared with ligands due to resonance effects of the N and C atoms of the pyrazole groups. Scheme 1 presents a schematic illustration of the synthetic route for ligands and their corresponding Zn(II) and Cd(II) complexes.
3.2. Description of molecular structures Suitable single crystals for X-ray diffraction studies of Zn(II) and Cd(II) complexes were obtained from diffusion of Et2O into DMF solution. ORTEP drawings for [L1ZnCl2], [L1CdBr2], and [L2CdBr2] are shown in Figs. 1–3, respectively. X-ray crystal data and structural refinement for [L1ZnCl2], [L1CdBr2], and [L2CdBr2] are listed in Table 1. The selected bond lengths and angles of these complexes are listed in Table 2. The central zinc atom adopted distorted tetrahedral geometry via coordination of two N atoms of the pyrazolyl moieties and two chloro ligands. The distortion resulted from the N1–Zn1–N2 angle of 86.7(2)° and enlargement of the Cl1–Zn1–Cl2 angle to 117.32(6)°. The bond angle of Npyrazole–Zn(1)–Cl(1) and Npyrazole– Zn(1)–Cl(2) for [L1ZnCl2] were 113.3(1)° and 104.9(1)°. ZnANpyrazole bond lengths in [L1ZnCl2] were 2.020(4) and 2.033(5) Å. The ZnACl bond lengths were 2.237(2) and 2.226(2) Å for Zn(1)–Cl(1) and Zn(1)–Cl(2), respectively. These values were similar to those reported previously for similar complexes [59–61]. [L1ZnCl2] showed a non-coordinative interaction between the N atom of the amine moiety and Zn(II) centre, as determined based on the Zn1AN5 bond length (3.537 Å). The coordination of the pyrazolyl ligand to the Zn(II) centre resulted in an eight-membered chelate ring, comparable to previously reported
Fig. 1. ORTEP drawing of [L1ZnCl2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
complexes [51]. The cyclohexyl moiety in [L1ZnCl2] appeared to be in a different plane compared to the two pyrazole rings. The Cd(II) complexes, [L1CdBr2] and [L2CdBr2], adopt trigonal bipyramidal geometry via coordination of N atoms of pyrazolyl units and two bromo ligands in addition to the coordinative interaction with N of the N0 -cyclohexyl amine moiety. These complexes were found to be solvent free and monomeric. The CdANamine lengths in [L1CdBr2] and [L2CdBr2] were 2.652(7) Å and 2.718(6) Å, respectively. Similarly, CdANpyrazole bond lengths were 2.255(6) and 2.261(6) Å in [L1CdBr2] and 2.285(4) Å in [L2CdBr2]. The bond lengths of Cd(1)-Br(1) and Cd(1)–Br(2) were 2.559(1) and 2.5666(9) Å in [L1CdBr2] and 2.5920(9) and 2.5656(9) Å in [L2CdBr2], and were slightly affected by the
N
[ZnCl2] EtOH/rt/12 h
R1
N
N
N
Cl
Cl
2 R1
N
NH2
N R1
CH2 Cl2/ rt/ 72 h
[L1ZnCl2 ] (R1 = -H) [L2ZnCl2 ] (R1 = -CH3)
N R1
N
N R1
R1
Zn R 1
R1
OH
N
N R1
N
L1 (R1 = -H) L2 (R1 = -CH3)
R1
N R1 [CdBr2. 4H2 O] EtOH/rt/12 h
N
N
N
R1
N
Cd R 1
Br
Br
[L1CdBr 2] (R1 = -H) [L2CdBr 2] (R1 = -CH3 )
Scheme 1. Synthetic route of ligands (L1 and L2) and corresponding Zn(II) and Cd(II) complexes.
R1
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
317
3.3. Methyl methacrylate (MMA) polymerisation
Fig. 2. ORTEP drawing of [L1CdBr2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
substituent on the pyrazole ring. The Br(2)–Cd–Br(1) bond angles in trigonal bipyramidal [L1CdBr2] and [L2CdBr2] were 108.15(3)° and 110.79(3)°, respectively. Two Namine–Cd–Npyrazole bond angles for [L1CdBr2] were 70.4(2)° and 69.6(2)°, while for [L2CdBr2] it was 69.3(1)°, which was indicative of 5-membered fused ring strain, which had a major effect on [L1CdBr2] and [L2CdBr2]. Additionally, Namine–Cd–Br(1) and Namine–Cd–r(2) angles in [L1CdBr2] were 107.0(2)° and 96.5(2)°, while in [L2CdBr2] these were 102.5(1)° and 101.1(1)°, respectively. A much larger Npyrazole–Cd–Npyrazole angle was observed in [L2CdBr2] compared to [L1CdBr2]. This result can be explained by the steric encumbrance caused by the bulky methyl substituents of the pyrazole rings. The planes of the cyclohexyl and two pyrazole rings were virtually parallel in [L2CdBr2], which has bulky methyl substituents on the two pyrazole rings.
Fig. 3. ORTEP drawing of [L2CdBr2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
The synthesised complexes were assessed for catalytic activity in MMA polymerisation in the presence of MMAO to obtain poly (methyl methacrylate) (PMMA). All complexes yielded PMMA, with Tg ranging from 121 °C to 133 °C. The polymers were isolated as white solids and characterised by GPC in THF using standard polystyrene as a reference. The triad microstructure of PMMA was analysed using 1H NMR spectroscopy [62]. The results of polymerisation including tacticity as isotactic (mm), heterotactic (mr), syndiotactic (rr), and polydispersity index (PDI) as the average degree of polymerisation in terms of the number of structural units and molecules are summarised in Table 3. A reference MMA polymerisation experiment was performed with metal starting material; anhydrous [ZnCl2], anhydrous [cdcl2], or MMAO alone at a specific temperature. The Zn(II) complexes were found to be more active and yielded PPMA with higher molecular weights and narrower pdis compared to their Cd(II) counterparts. The catalytic activities of Cd(II) complexes (3.48 104 g PMMA/molcdh) were similar to the metal starting material; anhydrous [cdcl2] (3.53 104 g PMMA/molcdh). 1 4 However, [L ZnCl2] (4.40 10 g PMMA/molZn h) showed only twofold increases in activity compared to the corresponding anhydrous [ZnCl2] (1.73 104 g PMMA/molZn h). Similarly, the complexes without bulky methyl substituents at pyrazolyl moieties were more active than complexes with methyl substituents. This can be explained by the fact that substituents of the pyrazole ring moiety, which is located near the metal centre, resulted from different electronic and steric environments and thus affected the approach of the monomer to the active site. This is in contrast to our previously reported results, in which more electron-donating substituents resulted in a more active metal centre [51]. [L1ZnCl2] resulted in PMMA with the narrowest PDI and highest molecular weight (11.4 105 g/mol). The PDI range was slightly narrow when the molecular weight of PMMA increased. These results are explained by the narrower PDI range of the highermolecular-weight polymer [63,64]. The synthesized complexes exhibited lower activities compared to our previously reported Co(II) complexes with N,N-bis(1H-pyrazolyl1-methyl)aniline [18]; however they yielded higher molecular weights syndiotactic PMMA with narrower polydispersity indices (PDI) and higher Tg. Similarly Cu(II) complexes with N-(2-furanylmethyl)-N-(1-3,5-dimethyl-1H-pyrazolylmethyl)-N-(phenylmethyl)amine [36] yielded syndiotactic PMMA with rr value up to 0.78 with only 30% conversion. However, all the Zn(II) and Cd(II) complexes in the current study have shown 100% conversion. In addition, Fe(II) complex with pyridylmethylamines [65] and Co(II) complex with phenoxy-imine [66] were reported to show moderate activity and syndiotacticity. As discussed above, Tg is mainly dependent on tacticity of PMMA and resulted in improved optical properties. Thus, MMA polymerisation through coordination complexes is used to achieve PMMA with high syndiotacticity, which resulted in Tg up to 140 °C [66,67,35]. The syndiotacticity of resultant PMMA was around 69%, which was similar to both Zn(II) and Cd(II) complexes. Thus, the resultant syndiotacticity was not sufficient to confer a coordination polymerisation mechanism for complexes in the current study. Moreover, the syndiotacticity was not strongly affected by substituents of the ligand or metal centre. However, Zn(II) complexes yielded PMMA with higher Tg compared to Cd(II) complexes (Table 3; entries 4–6) which is consistent with previous reports. The MMA polymerisation activity of Zn(II) and Cd(II) complexes should be considered as a function of steric encumbrance around the metal centre. Further investigations to improve the catalytic activities in terms of ligand architecture variations and resultant tacticities are currently being performed in our laboratory.
318
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
Table 2 Selected bond lengths (Å) and angles (°) for [L1ZnCl2], [L1CdBr2], and [L2CdBr2]. [L1ZnCl2]
[L1CdBr2]
[L2CdBr2]
Bond lengths Zn(1)–N(1) Zn(1)–N(3) Zn(1)–Cl(2) Zn(1)–Cl(1) N(1)–C(1) N(1)-N(2) N(2)–C(3) N(2)–C(7) C(1)–C(2) C(2)–C(3)
2.023(4) 2.034(5) 2.226(2) 2.237(2) 1.327(7) 1.355(6) 1.331(7) 1.467(7) 1.395(8) 1.356(9)
Cd(1)–N(1) Cd(1)–N(5) Cd(1)–Br(1) Cd(1)–Br(2) Cd(1)–N(3) N(1)-C(1) N(1)–N(2) N(2)–C(3) N(2)–C(4) N(3)–C(4)
2.255(6) 2.261(6) 2.559(1) 2.567(1) 2.652(6) 1.330(1) 1.362(8) 1.346(9) 1.465(9) 1.440(9)
Cd(1)–N(1)#1 Cd(1)–N(1) Cd(1)–Br(2) Cd(1)–Br(1) Cd(1)–N(3) N(1)-C(1) N(1)–N(2) N(2)–C(3) N(2)–C(6) N(3)–C(6)
2.285(4) 2.285(4) 2.567(1) 2.592(1) 2.718(6) 1.322(7) 1.357(6) 1.349(6) 1.450(6) 1.454(5)
Bond angles N(1)–Zn(1)–N(3) N(1)–Zn(1)–Cl(2) N(3)–Zn(1)–Cl(2) N(1)–Zn(1)–Cl(1) N(3)–Zn(1)–Cl(1) Cl(2)–Zn(1)–Cl(1) C(1)–N(1)–N(2) C(1)–N(1)–Zn(1) N(2)–N(1)–Zn(1) C(3)–N(2)–N(1)
106.5(2) 104.9(2) 107.4(2) 113.3(1) 106.8(1) 117.3(1) 106.3(4) 127.3(4) 126.3(4) 110.6(5)
N(1)–Cd(1)–N(5) N(1)–Cd(1)–Br(1) N(5)–Cd(1)–Br(1) N(1)–Cd(1)–Br(2) N(5)–Cd(1)–Br(2) Br(1)–Cd(1)–Br(2) N(1)–Cd(1)–N(3) N(5)–Cd(1)–N(3) Br(1)–Cd(1)–N(3) Br(2)–Cd(1)–N(3)
102.2(2) 136.5(2) 107.0(2) 96.5(2) 101.7(2) 108.2(1) 69.6(2) 70.4(2) 90.6(1) 161.2(1)
N(1)#1–Cd(1)–N(1) N(1)#1–Cd(1)–Br(2) N(1)–Cd(1)–Br(2) N(1)#1–Cd(1)–Br(1) N(1)–Cd(1)–Br(1) Br(2)–Cd(1)–Br(1) C(1)–N(1)–N(2) C(1)–N(1)–Cd(1) N(2)–N(1)–Cd(1) C(3)–N(2)–N(1)
137.7(2) 101.1(1) 101.1(1) 102.5(1) 102.5(1) 110.8(1) 106.0(4) 130.5(4) 118.2(3) 111.3(4)
Table 3 MMA polymerisation by [LnMX2] (Ln = L1 and L2; M = Zn, X = Cl; M = Cd, X = Br) complexes in the presence of MMAO. Entry
1 2 3 4 5 6 7 a b c d e f g h
Catalysta
[ZnCl2]g [CdCl2]g MMAOh [L1ZnCl2] [L2ZnCl2] [L1CdBr2] [L2CdBr2]
Temp.
Yieldb
Activityc 4
Tgd
Mwe
Tacticity
Mw/Mnf 5
(°C)
(g)
10 (g/molCath)
(°C)
%mm
%mr
%rr
10 (g/mol)
60 60 60 60 60 60 60
11.1 22.6 8.97 28.2 20.4 22.2 20.7
1.73 3.53 1.40 4.40 3.18 3.48 3.23
129 123 120 133 133 121 127
9.20 5.60 37.2 7.87 7.81 6.67 6.67
24.2 30.3 10.9 24.4 22.6 24.0 24.0
66.6 64.1 51.9 67.7 69.5 69.3 69.3
1.33 4.72 6.78 11.4 10.8 9.58 9.08
1.58 1.49 2.09 1.54 1.90 1.93 2.09
[M(II) catalyst]0 = 15 lmol, [MMA]0/[MMAO]0/[M(II) catalyst]0 = 3100:500:1. Yield defined the mass of dried polymer recovered/ mass of monomer used. Activity is (g PMMA)/(molCath). Tg is glass transition temperature determined using a thermal analyser. Determined using gel permeation chromatography (GPC) eluted with THF at room temperature by filtration with polystyrene calibration. Mn refers to the number average of molecular weights of PMMA. Blank polymerisation in which anhydrous [ZnCl2] and [CdCl2] were also activated by MMAO. Blank polymerisation by MMAO alone.
4. Conclusion
Acknowledgments
In summary, we demonstrated the synthesis of dichloro Zn(II) and dibromo Cd(II) complexes ligated to N0 -cycloalkyl substituted N,N-bispyrazolyl ligands. The versatility of the ligand is shown to accommodate variable coordination geometries around the metal centres. The molecular structure of [L1ZnCl2] was best described as a distorted tetrahedral geometry, whereas [L1CdBr2] and [L2CdBr2] structures were described as a distorted trigonal bipyramidal geometry involving the metal centre. A coordinative interaction exists between N of N0 -cyclohexyl amine moiety and Cd(II) centre in complexes compared to their Zn(II) counterpart. In addition, the catalytic activities of Zn(II) and Cd(II) complexes for methyl methacrylate (MMA) polymerisation in the presence of modified methylaluminoxane (MMAO) were investigated. The presence of bulky methyl substituents on the pyrazole moieties resulted in decreased catalytic activities, whereas the syndiotacticity of resultant PMMA was not strongly affected by these substituents or by variations in the central metal atom. Finally, the 4-coordinate [L1ZnCl2] showed the highest catalytic activity (4.40 104 g PMMA/molZn h) compared to the remaining complexes in the current study.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of the Science, ICT & Future Planning (Grant No. 2014-R1A1A3049750). Appendix A. Supplementary material CCDC 1063503–1063505 contains the supplementary crystallographic data for [L1ZnCl2], [L1CdBr2] and [L2CdBr2]. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.07.012. References [1] Z.H. Stachurski, Polymer 43 (2002) 7409. [2] F.L. Baines, S. Dionisio, N.C. Billingham, S.P. Armes, Macromolecules 29 (1996) 3096. [3] A.W.M. DeLaat, W.P.T. Derks, Colloids Surf. A 71 (1993) 147. [4] X. Zhang, K. Matyjaszewski, Macromolecules 32 (1999) 1763.
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319 [5] S.M. Aha, S.C. Chang, H.K. Rhee, J. Appl. Polym. Sci. 69 (1998) 59. [6] A.H. Alshehri, M. Jakubowska, M. Sloma, M. Horaczek, D. Rudka, C. Free, J.D. Carey, Appl. Phys. Lett. 99 (2011) 153109. [7] S.Z. Szilasi, R. Huszank, T. Szikra, T. Váczi, I. Rajta, I. Nagy, Mater. Chem. Phys. 130 (2011) 702. [8] M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 101 (2001) 3689. [9] T. Pintauer, K. Matyjaszewski, Coord. Chem. Rev. 249 (2005) 1155. [10] N.V. Tsarevsky, K. Matyjaszewski, Chem. Rev. 107 (2007) 2270. [11] W.A. Braunecker, K. Matyjaszewski, Prog. Polym. Sci. 302 (2007) 93. [12] W. Jakubowski, K. Matyjaszewski, Angew. Chem., Int. Ed. 45 (2006) 4482. [13] T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 30 (1997) 4507. [14] H. Alvès, M. Graullier, A. Riondel, J.M. Paul, M. Birot, H. Deleuze, Green Chem. 11 (2009) 868. [15] Y.J. Kim, K.H. Kim, T.Y. Kwon, Mater. Sci. Eng., C 30 (2010) 219. [16] W. Kaminsky, A. Bark, M. Arndt, Macromol. Symp. 47 (1991) 83. [17] V. Kotzabasakis, S. Mourmouris, M. Pitsikalis, N. Hadjichristidis, D.J. Lohse, Macromolecules 44 (2011) 1952. [18] M. Yang, W.J. Park, K.B. Yoon, J.H. Jeong, H. Lee, Chem. Comm. 14 (2011) 189. [19] Z.H. Li, Y.M. Zhang, M.Z. Xue, L. Zhou, Y.G. Liu, J. Polym. Sci., Part A: Polym. Chem. 43 (2005) 5207. [20] Z.X. Huang, Y.M. Zhang, H. Li, Y.G. Liu, Macromol. Chem. Phys. 209 (2008) 825. [21] U. Peuckert, W. Heitz, Macromol. Rapid Commun. 19 (1998) 159. [22] P.G. Lassahn, V. Lozan, G.A. Timco, P. Christian, C. Janiak, E.P. Winpenny, J. Catal. 222 (2004) 261. [23] D.A. Barnes, G.M. Benedikt, B.L. Goodall, S.S. Huang, H.A. Kalamarides, S. Lenhard, L.H. McIntosh III, K.T. Selvy, R.A. Shick, L.F. Rhodes, Macromolecules 36 (2003) 2623. [24] V. Lozan, P.G. Lassahn, C. Zhang, B. Wu, C. Janiak, G. Rheinwald, H.Z. Lang, Naturforsch. B 58 (2003) 1152. [25] Y.B. Huang, G.R. Tang, G.Y. Jin, G.X. Jin, Organometallics 27 (2008) 259. [26] F. Simal, S. Delfosse, A. Demonceau, A.F. Noels, K. Denk, F.J. Kohl, T. Weskamp, W.A. Herrmann, Chem. Eur. J. 8 (2002) 3047. [27] T. Ando, C. Sawauchi, M. Ouchi, M. Kamigaito, M. Sawamoto, J. Polym. Sci., Part A: Polym. Chem. 41 (2003) 3597. [28] T. Terashima, M. Ouchi, T. Ando, C. Sawauchi, M. Kamigaito, M. Sawamoto, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 4966. [29] C. Mehler, W. Risse, Macromolecules 25 (1992) 4226. [30] P.G. Lassahn, V. Lozan, C. Janiak, Dalton Trans. (2003) 927. [31] B. Berchtold, V. Lozan, P.G. Lassahn, C. Janiak, J. Polym. Sci., Part A: Polym. Chem. 40 (2002) 3604. [32] M. Yousaf, H.B. Ahmad, K.M. Khan, A.F. Zahoor, M.K. Khosa, Y.L. Qian, J. Chem. Soc. Pak. 31 (2009) 467. [33] G.J.P. Britovsek, V.C. Gibson, S. Mastroianni, D.C.H. Oakes, C. Redshaw, G.A. Solan, A.J.P. White, D.J. Williams, Eur. J. Inorg. Chem. (2001) 431. [34] L.G. Furlan, M.P. Gil, O.L. Casagrande Jr., Macromol. Rapid Commun. 21 (2000) 1054. [35] Z. Zhang, D. Cui, A.A. Trifonov, Eur. J. Inorg. Chem. (2010) 2861. [36] C. Lansalot-Matras, F. Bonnette, E. Mignard, O. Lavastre, J. Organomet. Chem. 693 (2008) 393.
319
[37] B. Lian, C.M. Thomas, O.L. Casagrande Jr., C.W. Lehmann, T. Roisnel, J.F. Carpentier, Inorg. Chem. 46 (2007) 328. [38] S. Scheuer, J. Fischer, J. Kress, Organometallics 14 (1995) 2627. [39] D.P. Long, P.A. Bianconi, J. Am. Chem. Soc. 118 (1996) 12453. [40] H. Nakazawa, S. Ikai, K. Imaoka, Y. Kai, T. Yano, J. Mol. Catal. A: Chem. 132 (1998) 33. [41] S. Murtuza, O.L. Casagrande Jr., R.F. Jordan, Organometallics 21 (2002) 1882. [42] I.N. Stepanenko, B. Cebrián-Losantos, V.B. Arion, A.A. Krokhin, A.A. Nazarov, B.K. Keppler, Eur. J. Inorg. Chem. (2007) 400. [43] M. Daoudi, N.B. Larbi, A. Kerbal, B. Bennani, J.P. Launay, J. Bonvoisin, T.B. Hadda, P.H. Dixneuf, Tetrahedron 62 (2006) 3123. [44] J. Pons, A. Chadghan, A. Alvarez-Larena, J.F. Piniella, J. Ros, Inorg. Chim. Acta 324 (2001) 342. [45] R. Touzani, A. Ramdani, T. Ben-Hadda, S.E. Kadiri, O. Maury, H.L. Bozec, P.H. Dixneuf, Synth. Commun. 31 (2001) 1315. [46] S.J. Dougan, M. Melchart, A. Habtemariam, S. Parsons, P.J. Sadler, Inorg. Chem. 45 (2006) 10882. [47] N. Singh, N.K. Sangwan, K.S. Dhindsa, Pest Manag. Sci. 56 (2000) 284. [48] T.B. Hadda, A.T. Kotchevar, M. Daoudi, B. Bennani, N. Ben, Lett. Drug Des. Discovery 2 (2005) 584. [49] I. Bouabdallah, L.A. M’barek, A. Zyad, A. Ramdani, I. Zidane, A. Melhaoui, Nat. Prod. Res. 20 (2006) 1024. [50] J. Hitzbleck, A.Y. O’Brien, G.B. Deacon, K. Ruhlandt-Senge, Inorg. Chem. 45 (2006) 10329. [51] E. Kim, H.Y. Woo, S. Kim, H. Lee, D. Kim, H. Lee, Polyhedron 42 (2012) 135. [52] D. Kim, S. Kim, E. Kim, H.J. Lee, H. Lee, Polyhedron 63 (2013) 139. [53] S. Choi, S. Nayab, J. Jeon, S.H. Park, H. Lee, J. Organomet. Chem. submitted. [54] R. Touzani, A. Ramdani, T. Ben-Hadda, S.E. Kadiri, O. Maury, H. Le Bozec, P.H. Dixneuf, Synth. Commun. 31 (2001) 1315. [55] T. Harit, F. Malek, B.E. Bali, A. Khan, K. Dalvndi, B.P. Marasini, S. Noreen, R. Malik, S. Khan, M.I. Choudhary, Med. Chem. Res. 21 (2012) 2772. [56] SMART and SAINT-Plus v 6.22, Bruker AXS Inc., Madison, Wisconsin, USA, 2000. [57] G.M. Sheldrick, SADABS v 2.03, University of Göttingen, Germany, 2002. [58] SHELXTL v 6.10; Bruker AXS Inc.: Madison, Wisconsin, USA, 2000. [59] M. Parvez, A.P. Sabir, Acta Crystallogr., Sect. C 52 (1996) 1651. [60] P.K. Bharadwaj, H.J. Schugar, J.A. Potenza, Acta Crystallogr., Sect. C 47 (1991) 754. [61] Y. Cui, D. Long, W. Chen, J. Huang, Acta Crystallogr., Sect. C 54 (1998) 1605. [62] T. Kitaura, T. Kitayama, Macromol. Rapid Commun. 28 (2007) 1889. [63] J. Llorens, E. Rude, R.M. Marcos, Polymer 44 (2003) 1741. [64] G.T. Lewis, V. Nguyen, Y. Cohen, J. Polym. Sci. A: Polym. Chem. 45 (2007) 5748. [65] T.V. Laine, U. Piironen, K. Lappalainen, M. Klinga, E. Aitola, M. Leskela, J. Organomet. Chem. 606 (2000) 112. [66] B.K. Bahuleyan, D. Chandran, C.H. Kwak, C.S. Ha, I. Kim, Macromol. Res. 18 (2008) 745. [67] B. Lian, C.M. Thomas, O.L. Casagrande, C.W. Lehmann Jr., T. Roisnel, J.F. Carpentier, Inorg. Chem. 46 (2007) 328.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis and structural characterisation of tetrahedral zinc(II) and trigonal bipyramidal cadmium(II) complexes containing N0 -cyclohexyl substituted N,N-bispyrazolyl ligand Sunghye Choi a, Seung Hyun Ahn a, Saira Nayab b,⇑, Hyosun Lee a,⇑ a b
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city 702-701, Republic of Korea Department of Chemistry, Shaheed Benazir Bhutto University, Sheringal, Dir (U), Khyber Pakhtunkhwa, Pakistan
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 6 July 2015 Accepted 8 July 2015 Available online 22 July 2015 Keywords: Zinc(II) Cadmium(II) Molecular structures Methyl methacrylate polymerisation Syndiotacticity
a b s t r a c t The bis(pyrazolyl)-based ligands L1 and L2, where L1 is N,N-bis((1H-pyrazol-1-yl)methyl)cyclohexanamine and L2 is N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)cyclohexanamine, reacted with [ZnCl2] and [CdBr24H2O], respectively, and resulted in mononuclear [LnMX2] (Ln = L1 and L2; M = Zn, X = Cl; M = Cd, X = Br) complexes in a facile yield and high purity. The synthesised complexes were characterised spectroscopically and X-ray crystallographically. The molecular structure of 4-coordinate [L1ZnCl2] was best described as a distorted tetrahedral, whereas 5-coordinate [L1CdBr2] and [L2CdBr2] exhibited distorted trigonal bipyramidal geometry involving the metal centres. [L1ZnCl2] showed the highest catalytic activity for the polymerisation of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) with an activity of 4.40 104 g PMMA/molZn h at 60 °C and resulted in syndiotactic poly(methylmethacrylate) (PMMA). Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction PMMA has received increasing attention as an organic transparent material due to its good electrical insulation and optical applications [1–7]. The improved optical properties of PMMA are mainly dependent on the higher glass transition temperature (Tg) and tacticity. Radical-mediated polymerisation of MMA (generally atactic) cannot achieve a sufficient Tg or syndiotacticity content in the resultant PMMA. Thus, non-radical-mediated MMA polymerisation using various transition metal catalysts that can result in a higher Tg up to 140 °C is an active and promising field of research both in academic laboratories and industry [8–13]. Among the various metals, titanium [14,15], zirconium [16], hafnium [17], cobalt [18–20], chromium [21,22], nickel [23–25], ruthenium [26–28], palladium [29–31], and lanthanide [32]-based complexes with various ligand architectures have mainly been employed for MMA polymerisation with moderate-to-high activities and tacticities. Moreover, despite numerous reports on metal complexes with N-donor ligands [33,34], little is known regarding pyrazolyl-based late transition metal complexes as catalysts for methyl methacrylate polymerisation [35–37] compared with early transition metal ⇑ Corresponding authors. Tel.: +82 53 950 5337; fax: +82 53 950 6330 (H. Lee). E-mail address: [email protected] (H. Lee). http://dx.doi.org/10.1016/j.ica.2015.07.012 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
complexes [38–41] because late transition metal complexes are generally more tolerant towards polar functional monomers (including MMA) due to their less-oxophilic nature. In contrast, the steric and electronic properties of pyrazolyl-based ligands can be fine-tuned by an appropriate choice of substitutions on the 2-N, 3-C, 4-C, and 5-C atoms of the pyrazole moiety [42–45], and have been employed in diverse potential applications [46–50]. In view of these promising features of pyrazolyl-based ligands, we focused on the synthesis, characterisation, and reactivity of various transition metal complexes with pyrazolyl-based ligands and their catalytic activities towards MMA polymerisation [18,51,52]. Specifically, we found that cobalt (II) complexed with N0 -cyclohexyl substituted N,N-bispyrazolyl ligand showed exceptional activity for MMA polymerisation [53]. The aim of this report was to extend our work to a novel N0 -cyclohexyl-substituted N,N-bispyrazolyl ligand and investigate their variable coordination behaviours towards Zn(II) and Cd(II) centres since the development of new highly active catalysts is an extremely attractive yet challenging goal to obtain high-molecular-weight PMMA with improved tacticity. The catalytic activities for MMA polymerisation in toluene at 60 °C were also examined. In addition, we explored the effects of substituents of the pyrazolyl moiety and inexpensive transition metals, zinc(II) and cadmium(II), on the activity and tacticity of the resultant polymer.
314
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
2. Experimental 2.1. Materials Anhydrous [ZnCl2], [CdBr24H2O], cyclohexanamine, 1H-1-pyrazolyl-1-methanol, and methyl methacrylate (MMA) were purchased from Sigma-Aldrich (St. Louis, MO) and anhydrous solvents such as C2H5OH, DMF, diethyl ether (Et2O), and dichloromethane (CH2Cl2) were purchased from Merck (Darmstadt, Germany) and used without further purification. Modified methylaluminoxane (MMAO) was purchased from Tosoh Finechem Corporation (Tokyo, Japan) as 6.9% aluminium (by weight) in a toluene solution and used without further purification. 2.2. Physical measurements 1
H NMR (operating at 400 MHz) and 13C NMR (operating at 100 MHz) spectra were recorded using an Advance Digital 400 NMR spectrometer (Bruker, Billerica, MA); chemical shifts were recorded in ppm units (d) relative to SiMe4 as the internal standard. Infrared (IR) spectra were recorded on a Bruker FT/IR-Alpha (neat) and the data were reported in reciprocal centimetres. Elemental analyses (C, H, N) of the synthesised ligand and complexes were performed using an elemental analyser (EA 1108; Carlo-Erba, Milan, Italy). The molecular weight and molecular weight distribution of the obtained poly (methylmethacrylate) (PMMA) were determined using gel-permeation chromatography (GPC) (CHCl3, Alliance e2695; Waters Corp., Milford, MA). Glass transition temperature (Tg) was determined using a thermal analyser (Q2000; TA Instruments, New Castle, DE). 2.3. Synthesis 2.3.1. Preparation of ligands and corresponding complexes 2.3.1.1. Synthesis of (L1) [53]. Cyclohexanamine (2.29 mL, 20.0 mmol) was treated with CH2Cl2 solution of 1H-1-pyrazolyl1-methanol (3.92 g, 40.0 mmol) at room temperature and was stirred for 36 h. The resultant solution was dried over the MgSO4. The filtrate solvent was removed under reduced pressure and the crude residue was vacuum distiled to give colourless oil (4.44 g, 85.6%). Anal. Cal. for C14H21N5: C, 64.84, H, 8.16, N, 27.00. Found: C, 65.18, H, 8.27, N, 27.52%. 1H NMR (CDCl3, 400 MHz): d 7.53 (d, 2H, J = 1.6 Hz, AN@CHACH@CHANA), 7.51 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.27 (dd, 2H, J = 1.6 Hz, J = 2.0 Hz, AN@CHACH@CHANA), 5.13 (s, 4H, ANACH2ANA), 2.86–2.79 (m, 1H, ipso-C6H11A), 1.76–1.70 (m, 2H, AC6H11A), 1.65–1.55 (m, 3H, AC6H11A), 1.24–1.14 (m, 4H, AC6H11A), 1.08–0.99 (m, 1H, AC6H11A). 13C NMR (CDCl3, 100 MHz): d 140.71 (d, 2C, J = 176 Hz, AN@CHACH@CHANA), 130.39 (d, 2C, J = 185 Hz, AN@CHACH@CHANA), 107.25 (d, 2C, J = 176 Hz, AN@CHACH@CHANA), 66.34 (t, 2C, J = 149 Hz, ANACH2ANA), 60.94 (d, 1C, J = 139 Hz, ipso-C6H11A), 33.40 (t, 2C, J = 130 Hz, o-C6H11A), 31.38 (t, 1C, J = 130 Hz, p-C6H11A), 26.31 (t, 2C, J = 126 Hz, m-C6H11A). IR (liquid neat; cm1): 3111 (w), 2928 (m), 2855 (w), 1510 (w), 1449 (w), 1387 (m), 1258 (m), 1130 (m), 1084 (m), 1042 (s), 966 (m), 882 (w), 745 (s), 653 (w), 614 (m). 2.3.1.2. Synthesis of (L2) [54,55]. N,N-bis((3,5-dimethyl-1H-pyrazol1-yl)methyl)cyclohexanamine (L2) was prepared according to reported method. 2.3.1.3. Synthesis of [L1ZnCl2]. EtOH (10.0 mL) solution of L1 (0.259 g, 1.00 mmol) was treated with EtOH (10.0 mL) solution of [ZnCl2] (0.136 g, 1.00 mmol). Precipitation of white powder occurred while stirring at room temperature for 12 h. The white
powder was filtered and washed with EtOH (50.0 mL 2), followed by washing with Et2O (50.0 mL 2) to give the white solid (0.38 g, 96.0%). Crystals suitable for the X-ray study of [L1ZnCl2] were obtained from Et2O (10.0 mL) diffusion into DMF solution (10.0 mL). Anal. Cal. for C14H21Cl2N5Zn: C, 42.50, H, 5.35, N, 17.70. Found: C, 42.50, H, 5.30, N, 17.68%. 1H NMR (DMSO-d6, 400 MHz): d 7.82 (d, 2H, J = 2.4 Hz, AN@CHACH@CHANA), 7.51 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.29 (t, 2H, J = 2.4 Hz, AN@CHACH@CHANA), 5.22 (s, 4H, ANACH2ANA), 2.75–2.70 (m, 1H, ipso-C6H11A), 1.71–1.57 (m, 4H, AC6H11A), 1.51–1.37 (m, 4H, AC6H11A), 1.26–1.20 (m, 1H, AC6H11A), 0.99–0.94 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 139.67 (d, 2C, J = 184 Hz, AN@CHACH@CHANA), 131.08 (d, 2C, J = 181 Hz, AN@CHACH@CHANA), 106.03 (d, 2C, J = 184 Hz, AN@CHACH@CHANA), 66.68 (t, 2C, J = 149 Hz, ANACH2ANA), 59.17 (d, 1C, J = 136 Hz, ipso-C6H11A), 30.21 (t, 2C, J = 126 Hz, oC6H11A), 25.90 (t, 1C, J = 128 Hz, p-C6H11A), 25.55 (t, 2C, J = 125 Hz, m-C6H11A). IR (solid neat; cm1): 3112 (w), 2928 (w), 2852 (w), 1752 (w), 1702 (w), 1514 (w), 1458 (m), 1408 (s), 1347 (w), 1302 (m), 1242 (m), 1165 (s), 1067 (s), 988 (m), 892 (w), 771 (s), 719 (s), 615 (s). 2.3.1.4. Synthesis of [L2ZnCl2]. [L2ZnCl2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L2 (0.315 g, 1.00 mmol) and [ZnCl2] (0.136 g, 1.00 mmol) to give white solid as final product (0.40 g, 88.7%). Crystals of [L2ZnCl2] suitable for the X-ray study were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C18H29Cl2N5Zn: C, 47.86, H, 6.47, N, 15.50. Found: C, 47.82, H, 6.24, N, 15.73%. 1H NMR (DMSO-d6, 400 MHz): d 6.03 (s, 2H, AN@C(CH3)ACH@C(CH3)ANA), 5.02 (s, 4H, ANACH2ANA), 2.75A2.67 (m, 1H, ipso-NC6H11A), 2.63 (s, 6H, AN@C(CH3)ACH@ C(CH3)ANA), 2.33 (s, 6H, AN@C(CH3)ACH@C(CH3)ANA), 1.80– 1.77 (m, 2H, AC6H11A), 1.65–1.61 (m, 1H, AC6H11A), 1.38–1.33 (m, 2H, AC6H11A), 1.30–1.13 (m, 4H, AC6H11A), 1.09–1.01 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 152.43 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 142.78 (s, 2C, AN@C(CH3)ACH@ C(CH3)ANA), 108.89 (d, 2C, J = 176 Hz, AN@C(CH3)ACH@C(CH3) ANA), 61.83 (t, 2C, J = 65 Hz, ANACH2ANA), 60.36 (d, 1C, J = 144 Hz, ipso-C6H11A), 31.26 (t, 2C, J = 127 Hz, o-C6H11A), 26.59 (t, 1C, J = 126 Hz, p-C6H11A), 25.88 (t, 2C, J = 121 Hz, m-C6H11A), 14.48 (q, 2C, J = 127 Hz, AN@C(CH3)ACH@C(CH3)ANA), 11.79 (q, 2C, J = 129 Hz, AN@C(CH3)ACH@C(CH3)ANA). IR (solid neat; cm1): 3274 (s), 2924 (m), 2854 (w), 1570 (w), 1552 (s), 1462 (s), 1407 (s), 1372 (s), 1344 (m), 1325 (w), 1258 (w), 1224 (m), 1193 (s), 1180 (s), 1047 (w), 1023 (s), 991 (s), 893 (w), 779 (w), 690 (w), 619 (m). 2.3.1.5. Synthesis of [L1CdBr2]. [L1CdBr2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L1 (0.259 g, 1.00 mmol) and [CdBr24H2O] (0.334 g, 1.00 mmol) to give white powder as final product (0.382 g, 72.0%). Crystals suitable for the X-ray study of [L1CdBr2] were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C14H21Br2CdN5: C, 31.63, H, 3.98, N, 13.17. Found: C, 30.90; H, 3.97; N, 12.72%. 1H NMR (DMSO-d6, 400 MHz): d 7.78 (d, 2H, J = 1.6 Hz, AN@CHACH@CHANA), 7.48 (d, 2H, J = 2.0 Hz, AN@CHACH@CHANA), 6.27 (dd, 2H, J = 1.6 Hz, J = 2.0 Hz, AN@CHACH@CHANA), 5.19 (s, 4H, ANACH2ANA), 2.76–2.70 (m, 1H, ipso-C6H11A), 1.64–1.57 (m, 2H, AC6H11A), 1.50–1.38 (m, 3H, AC6H11A), 1.16–1.07 (m, 4H, AC6H11A), 1.00– 0.92 (m, 1H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 140.05 (d, 2C, J = 190 Hz, AN@CHACH@CHANA), 130.79 (d, 2C, J = 188 Hz, AN@CHACH@CHANA), 106.70 (d, 2C, J = 166 Hz, AN@CHACH@CHANA), 66.18 (t, 2C, J = 150 Hz, ANACH2ANA), 59.74 (d, 1C, J = 110 Hz, ipso-C6H11A), 30.42 (t, 2C, J = 122 Hz, o-C6H11A), 26.89 (t, 1C, J = 123 Hz, p-C6H11A), 25.87 (t, 2C,
315
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
J = 126 Hz, m-C6H11A). IR (solid neat; cm1): 3744 (w), 3609 (w), 3116 (w), 2926 (w), 2850 (w), 1837 (w), 1757 (w), 1647 (w), 1514 (m), 1399 (m), 1278 (m), 1194 (w), 1164 (w), 1101 (m), 1059 (s), 973 (m), 895 (w), 769 (s), 607 (m). 2.3.1.6. Synthesis of [L2CdBr2]. [L2CdBr2] was prepared following the same synthetic procedure as [L1ZnCl2] except utilising L2 (0.315 g, 1.00 mmol) and [CdBr24H2O] (0.334 g, 1.00 mmol) to yield white powder as final product (0.570 g, 97.0%). Crystals suitable for the X-ray study of [L2CdBr2] were obtained from Et2O diffusion into DMF solution. Anal. Cal. for C18H29Br2CdN5: C, 36.79, H, 4.97, N, 11.92. Found: C, 37.64, H, 5.08, N, 12.11%. 1H NMR (DMSO-d6, 400 MHz): d 5.90 (s, 2H, AN@C(CH3)A CH@C(CH3)ANA), 4.93 (s, 4H, ANACH2ANA), 2.20 (s, 6H, AN@C(CH3)ACH@C(CH3)ANA), 2.18 (s, 6H, AN@C(CH3)ACH@ C(CH3)ANA), 1.66–1.59 (m, 4H, AC6H11A), 1.52–1.50 (m, 1H, ipso-NC6H11A), 1.14–0.97 (m, 6H, AC6H11A). 13C NMR (DMSO-d6, 100 MHz): d 147.87 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 140.51 (s, 2C, AN@C(CH3)ACH@C(CH3)ANA), 107.47 (d, 2C, J = 177 Hz, AN@C(CH3)ACH@C(CH3)ANA), 62.12 (t, 2C, J = 148 Hz, ANACH2ANA), 58.44 (d, 1C, J = 129 Hz, ipso-NAC6H11A), 28.34 (t, 2C, J = 125 Hz, o-C6H11A), 26.06 (t, 3C, J = 124 Hz, m-, p-C6H11A), 14.40 (q, 2C, J = 127 Hz, AN@C(CH3)ACH@C(CH3)ANA), 11.44 (q, 2C, J = 128 Hz, AN@C(CH3)ACH@C(CH3)ANA). IR (solid neat; cm1): 3845 (m), 3744 (s), 3678 (m), 3616 (m), 2930 (w), 1835 (w), 1743 (m), 1695 (s), 1648 (s), 1544 (s), 1464 (s), 1393 (s), 1291 (m), 1232 (w), 1043 (m), 974 (m), 817 (s), 631 (w). 2.4. X-ray crystallographic studies A summary of selected crystallographic details and structure refinement results for [L1ZnCl2], [L1CdBr2] and [L2CdBr2] are
presented in Table 1, respectively. Data collection were made on a Bruker SMART CCD diffractometer equipped with a graphitemonochromated Mo Ka (k = 0.71073 Å) radiation source under a nitrogen cold stream (200 K). Cell parameter refinements and integration were performed with SMART and SAINT-Plus software packages [56]. Semi-empirical absorption corrections based on equivalent reflections were applied using SADABS [57]. Structures were solved using direct methods and refined using a full-matrix least-squares method on F2 using SHELXTL [58]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added to their geometrically ideal positions. 2.5. Catalytic activity for MMA polymerisation The methyl methacrylate (MMA) was extracted with 10% sodium hydroxide, washed with water, dried over MgSO4, and distiled over calcium hydride under reduced pressure before use. In a Schlenk line, complex (15.0 lmol, 5.93 mg for [L1ZnCl2]; 6.77 mg for [L2ZnCl2]; 7.97 mg for [L1CdBr2]; 8.81 mg for [L2CdBr2]) was dissolved in dried toluene (10.0 mL) followed by the addition of modified MMAO (6.90 wt% in toluene, 3.25 mL, 7.50 mmol, and [MMAO]0/[Pd(II) catalyst]0 = 500) as a co-catalyst. The solution was stirred at 60 °C for 20 min. The MMA (5.00 mL, 47.1 mmol, [MMA]0/[Pd(II) catalyst]0 = 3100) was added to the above reaction mixture and stirred at 60 °C for 2 h to obtain a viscous solution. MeOH (2.00 mL) was added to terminate the polymerisation. The reaction mixture was poured into a large quantity of MeOH (500 mL), and 35% HCl (5.00 mL) was injected to remove the remaining co-catalyst. The resulting polymer was filtered and washed with MeOH (250 mL 2) to yield poly (methyl methacrylate) (PMMA), which was vacuum-dried at 60 °C.
Table 1 Crystal data and structural refinement for [L1ZnCl2], [L1CdBr2], and [L2CdBr2].
Empirical formula Formula weight T (K) Wavelength (nm) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalcu (g/cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h range for data collection (°) Index ranges
[L1ZnCl2]
[L1CdBr2]
[L2CdBr2]
C28H42Cl4N10Zn2 791.25 200(2) 0.71073 monoclinic P2(1)/c
C14H21Br2N5Cd 531.58 200(2) 0.71073 monoclinic P2(1)/c
C18H29Br2N5Cd 587.68 200(2) 0.71073 orthorhombic Pnma
8.5628(5) 13.5754(8) 16.1120(9) 90 90.014(1) 90 1872.9(2) 4 1.885 5.436 1032 0.32 0.25 0.08 1.96–28.31 8 6 h 6 11, 16 6 k 6 18, 21 6 l 6 20 13 008 4520 (0.0209) 96.90% Full-matrix least-squares on F2 4520/0/199 1.218 R1 = 0.0332, wR2 = 0.0743 R1 = 0.0581, wR2 = 0.1435 1.611 and 2.326
12.8520(8) 14.8725(9) 11.5930(7) 90 90 90 2215.9(2) 4 1.762 4.604 1160 0.26 0.18 0.15 2.23–28.29 17 6 h 6 14, 19 6 k 6 19, 15 6 l 6 15 14 846 2807 90.0247) 98.20% Full-matrix least-squares on F2 2807/0/141 1.022 R1 = 0.0315, wR2 = 0.0921 R1 = 0.0447, wR2 = 0.1560 0.967 and 1.312
18.063(4) 13.627(3) 15.861(3) 90 114.68(3) 90 3547 (2) 4 1.481 1.689 1632 0.38 0.22 0.12 1.24–28.56 20 6 h 6 24, 15 6 k 6 18, 21 6 l 6 20 Reflections collected 26 113 Independent reflections (Rint) 8848 (0.1383) Completeness to h = 22.210° 99.90% Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 8848/0/383 Goodness-of-fit (GOF) on F2 0.832 Final R indices [I > 2r(I)] R1 = 0.0639, wR2 = 0.1438 R indices (all data) R1 = 0.1549, wR2 = 0.1886 Largest difference peak and hole (e Å3) 1.683 and 0.974
316
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
3. Results and discussion 3.1. Synthesis and chemical properties Ligands in the current study were obtained in high yields using a single-step synthetic procedure from the reaction of cyclohexylamine and 1H-pyrazolyl-1-methanol or 3,5-dimethyl1H-pyrazolyl-1-methanol in CH2Cl2. Treatment of Ln (Ln = L1 and L2) in a 1:1 ratio with [ZnCl2] and [CdBr24H2O] afforded the corresponding dichloro Zn(II) and dibromo Cd(II) complexes in 72–98% yield. All spectroscopic data were consistent with the proposed Zn(II) and Cd(II) complex formulation. 1H NMR and 13C NMR peaks of the Zn(II) and Cd(II) complexes were slightly shifted compared with ligands due to resonance effects of the N and C atoms of the pyrazole groups. Scheme 1 presents a schematic illustration of the synthetic route for ligands and their corresponding Zn(II) and Cd(II) complexes.
3.2. Description of molecular structures Suitable single crystals for X-ray diffraction studies of Zn(II) and Cd(II) complexes were obtained from diffusion of Et2O into DMF solution. ORTEP drawings for [L1ZnCl2], [L1CdBr2], and [L2CdBr2] are shown in Figs. 1–3, respectively. X-ray crystal data and structural refinement for [L1ZnCl2], [L1CdBr2], and [L2CdBr2] are listed in Table 1. The selected bond lengths and angles of these complexes are listed in Table 2. The central zinc atom adopted distorted tetrahedral geometry via coordination of two N atoms of the pyrazolyl moieties and two chloro ligands. The distortion resulted from the N1–Zn1–N2 angle of 86.7(2)° and enlargement of the Cl1–Zn1–Cl2 angle to 117.32(6)°. The bond angle of Npyrazole–Zn(1)–Cl(1) and Npyrazole– Zn(1)–Cl(2) for [L1ZnCl2] were 113.3(1)° and 104.9(1)°. ZnANpyrazole bond lengths in [L1ZnCl2] were 2.020(4) and 2.033(5) Å. The ZnACl bond lengths were 2.237(2) and 2.226(2) Å for Zn(1)–Cl(1) and Zn(1)–Cl(2), respectively. These values were similar to those reported previously for similar complexes [59–61]. [L1ZnCl2] showed a non-coordinative interaction between the N atom of the amine moiety and Zn(II) centre, as determined based on the Zn1AN5 bond length (3.537 Å). The coordination of the pyrazolyl ligand to the Zn(II) centre resulted in an eight-membered chelate ring, comparable to previously reported
Fig. 1. ORTEP drawing of [L1ZnCl2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
complexes [51]. The cyclohexyl moiety in [L1ZnCl2] appeared to be in a different plane compared to the two pyrazole rings. The Cd(II) complexes, [L1CdBr2] and [L2CdBr2], adopt trigonal bipyramidal geometry via coordination of N atoms of pyrazolyl units and two bromo ligands in addition to the coordinative interaction with N of the N0 -cyclohexyl amine moiety. These complexes were found to be solvent free and monomeric. The CdANamine lengths in [L1CdBr2] and [L2CdBr2] were 2.652(7) Å and 2.718(6) Å, respectively. Similarly, CdANpyrazole bond lengths were 2.255(6) and 2.261(6) Å in [L1CdBr2] and 2.285(4) Å in [L2CdBr2]. The bond lengths of Cd(1)-Br(1) and Cd(1)–Br(2) were 2.559(1) and 2.5666(9) Å in [L1CdBr2] and 2.5920(9) and 2.5656(9) Å in [L2CdBr2], and were slightly affected by the
N
[ZnCl2] EtOH/rt/12 h
R1
N
N
N
Cl
Cl
2 R1
N
NH2
N R1
CH2 Cl2/ rt/ 72 h
[L1ZnCl2 ] (R1 = -H) [L2ZnCl2 ] (R1 = -CH3)
N R1
N
N R1
R1
Zn R 1
R1
OH
N
N R1
N
L1 (R1 = -H) L2 (R1 = -CH3)
R1
N R1 [CdBr2. 4H2 O] EtOH/rt/12 h
N
N
N
R1
N
Cd R 1
Br
Br
[L1CdBr 2] (R1 = -H) [L2CdBr 2] (R1 = -CH3 )
Scheme 1. Synthetic route of ligands (L1 and L2) and corresponding Zn(II) and Cd(II) complexes.
R1
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
317
3.3. Methyl methacrylate (MMA) polymerisation
Fig. 2. ORTEP drawing of [L1CdBr2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
substituent on the pyrazole ring. The Br(2)–Cd–Br(1) bond angles in trigonal bipyramidal [L1CdBr2] and [L2CdBr2] were 108.15(3)° and 110.79(3)°, respectively. Two Namine–Cd–Npyrazole bond angles for [L1CdBr2] were 70.4(2)° and 69.6(2)°, while for [L2CdBr2] it was 69.3(1)°, which was indicative of 5-membered fused ring strain, which had a major effect on [L1CdBr2] and [L2CdBr2]. Additionally, Namine–Cd–Br(1) and Namine–Cd–r(2) angles in [L1CdBr2] were 107.0(2)° and 96.5(2)°, while in [L2CdBr2] these were 102.5(1)° and 101.1(1)°, respectively. A much larger Npyrazole–Cd–Npyrazole angle was observed in [L2CdBr2] compared to [L1CdBr2]. This result can be explained by the steric encumbrance caused by the bulky methyl substituents of the pyrazole rings. The planes of the cyclohexyl and two pyrazole rings were virtually parallel in [L2CdBr2], which has bulky methyl substituents on the two pyrazole rings.
Fig. 3. ORTEP drawing of [L2CdBr2] with thermal ellipsoids at 50% probability. All hydrogen atoms were omitted for clarity.
The synthesised complexes were assessed for catalytic activity in MMA polymerisation in the presence of MMAO to obtain poly (methyl methacrylate) (PMMA). All complexes yielded PMMA, with Tg ranging from 121 °C to 133 °C. The polymers were isolated as white solids and characterised by GPC in THF using standard polystyrene as a reference. The triad microstructure of PMMA was analysed using 1H NMR spectroscopy [62]. The results of polymerisation including tacticity as isotactic (mm), heterotactic (mr), syndiotactic (rr), and polydispersity index (PDI) as the average degree of polymerisation in terms of the number of structural units and molecules are summarised in Table 3. A reference MMA polymerisation experiment was performed with metal starting material; anhydrous [ZnCl2], anhydrous [cdcl2], or MMAO alone at a specific temperature. The Zn(II) complexes were found to be more active and yielded PPMA with higher molecular weights and narrower pdis compared to their Cd(II) counterparts. The catalytic activities of Cd(II) complexes (3.48 104 g PMMA/molcdh) were similar to the metal starting material; anhydrous [cdcl2] (3.53 104 g PMMA/molcdh). 1 4 However, [L ZnCl2] (4.40 10 g PMMA/molZn h) showed only twofold increases in activity compared to the corresponding anhydrous [ZnCl2] (1.73 104 g PMMA/molZn h). Similarly, the complexes without bulky methyl substituents at pyrazolyl moieties were more active than complexes with methyl substituents. This can be explained by the fact that substituents of the pyrazole ring moiety, which is located near the metal centre, resulted from different electronic and steric environments and thus affected the approach of the monomer to the active site. This is in contrast to our previously reported results, in which more electron-donating substituents resulted in a more active metal centre [51]. [L1ZnCl2] resulted in PMMA with the narrowest PDI and highest molecular weight (11.4 105 g/mol). The PDI range was slightly narrow when the molecular weight of PMMA increased. These results are explained by the narrower PDI range of the highermolecular-weight polymer [63,64]. The synthesized complexes exhibited lower activities compared to our previously reported Co(II) complexes with N,N-bis(1H-pyrazolyl1-methyl)aniline [18]; however they yielded higher molecular weights syndiotactic PMMA with narrower polydispersity indices (PDI) and higher Tg. Similarly Cu(II) complexes with N-(2-furanylmethyl)-N-(1-3,5-dimethyl-1H-pyrazolylmethyl)-N-(phenylmethyl)amine [36] yielded syndiotactic PMMA with rr value up to 0.78 with only 30% conversion. However, all the Zn(II) and Cd(II) complexes in the current study have shown 100% conversion. In addition, Fe(II) complex with pyridylmethylamines [65] and Co(II) complex with phenoxy-imine [66] were reported to show moderate activity and syndiotacticity. As discussed above, Tg is mainly dependent on tacticity of PMMA and resulted in improved optical properties. Thus, MMA polymerisation through coordination complexes is used to achieve PMMA with high syndiotacticity, which resulted in Tg up to 140 °C [66,67,35]. The syndiotacticity of resultant PMMA was around 69%, which was similar to both Zn(II) and Cd(II) complexes. Thus, the resultant syndiotacticity was not sufficient to confer a coordination polymerisation mechanism for complexes in the current study. Moreover, the syndiotacticity was not strongly affected by substituents of the ligand or metal centre. However, Zn(II) complexes yielded PMMA with higher Tg compared to Cd(II) complexes (Table 3; entries 4–6) which is consistent with previous reports. The MMA polymerisation activity of Zn(II) and Cd(II) complexes should be considered as a function of steric encumbrance around the metal centre. Further investigations to improve the catalytic activities in terms of ligand architecture variations and resultant tacticities are currently being performed in our laboratory.
318
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319
Table 2 Selected bond lengths (Å) and angles (°) for [L1ZnCl2], [L1CdBr2], and [L2CdBr2]. [L1ZnCl2]
[L1CdBr2]
[L2CdBr2]
Bond lengths Zn(1)–N(1) Zn(1)–N(3) Zn(1)–Cl(2) Zn(1)–Cl(1) N(1)–C(1) N(1)-N(2) N(2)–C(3) N(2)–C(7) C(1)–C(2) C(2)–C(3)
2.023(4) 2.034(5) 2.226(2) 2.237(2) 1.327(7) 1.355(6) 1.331(7) 1.467(7) 1.395(8) 1.356(9)
Cd(1)–N(1) Cd(1)–N(5) Cd(1)–Br(1) Cd(1)–Br(2) Cd(1)–N(3) N(1)-C(1) N(1)–N(2) N(2)–C(3) N(2)–C(4) N(3)–C(4)
2.255(6) 2.261(6) 2.559(1) 2.567(1) 2.652(6) 1.330(1) 1.362(8) 1.346(9) 1.465(9) 1.440(9)
Cd(1)–N(1)#1 Cd(1)–N(1) Cd(1)–Br(2) Cd(1)–Br(1) Cd(1)–N(3) N(1)-C(1) N(1)–N(2) N(2)–C(3) N(2)–C(6) N(3)–C(6)
2.285(4) 2.285(4) 2.567(1) 2.592(1) 2.718(6) 1.322(7) 1.357(6) 1.349(6) 1.450(6) 1.454(5)
Bond angles N(1)–Zn(1)–N(3) N(1)–Zn(1)–Cl(2) N(3)–Zn(1)–Cl(2) N(1)–Zn(1)–Cl(1) N(3)–Zn(1)–Cl(1) Cl(2)–Zn(1)–Cl(1) C(1)–N(1)–N(2) C(1)–N(1)–Zn(1) N(2)–N(1)–Zn(1) C(3)–N(2)–N(1)
106.5(2) 104.9(2) 107.4(2) 113.3(1) 106.8(1) 117.3(1) 106.3(4) 127.3(4) 126.3(4) 110.6(5)
N(1)–Cd(1)–N(5) N(1)–Cd(1)–Br(1) N(5)–Cd(1)–Br(1) N(1)–Cd(1)–Br(2) N(5)–Cd(1)–Br(2) Br(1)–Cd(1)–Br(2) N(1)–Cd(1)–N(3) N(5)–Cd(1)–N(3) Br(1)–Cd(1)–N(3) Br(2)–Cd(1)–N(3)
102.2(2) 136.5(2) 107.0(2) 96.5(2) 101.7(2) 108.2(1) 69.6(2) 70.4(2) 90.6(1) 161.2(1)
N(1)#1–Cd(1)–N(1) N(1)#1–Cd(1)–Br(2) N(1)–Cd(1)–Br(2) N(1)#1–Cd(1)–Br(1) N(1)–Cd(1)–Br(1) Br(2)–Cd(1)–Br(1) C(1)–N(1)–N(2) C(1)–N(1)–Cd(1) N(2)–N(1)–Cd(1) C(3)–N(2)–N(1)
137.7(2) 101.1(1) 101.1(1) 102.5(1) 102.5(1) 110.8(1) 106.0(4) 130.5(4) 118.2(3) 111.3(4)
Table 3 MMA polymerisation by [LnMX2] (Ln = L1 and L2; M = Zn, X = Cl; M = Cd, X = Br) complexes in the presence of MMAO. Entry
1 2 3 4 5 6 7 a b c d e f g h
Catalysta
[ZnCl2]g [CdCl2]g MMAOh [L1ZnCl2] [L2ZnCl2] [L1CdBr2] [L2CdBr2]
Temp.
Yieldb
Activityc 4
Tgd
Mwe
Tacticity
Mw/Mnf 5
(°C)
(g)
10 (g/molCath)
(°C)
%mm
%mr
%rr
10 (g/mol)
60 60 60 60 60 60 60
11.1 22.6 8.97 28.2 20.4 22.2 20.7
1.73 3.53 1.40 4.40 3.18 3.48 3.23
129 123 120 133 133 121 127
9.20 5.60 37.2 7.87 7.81 6.67 6.67
24.2 30.3 10.9 24.4 22.6 24.0 24.0
66.6 64.1 51.9 67.7 69.5 69.3 69.3
1.33 4.72 6.78 11.4 10.8 9.58 9.08
1.58 1.49 2.09 1.54 1.90 1.93 2.09
[M(II) catalyst]0 = 15 lmol, [MMA]0/[MMAO]0/[M(II) catalyst]0 = 3100:500:1. Yield defined the mass of dried polymer recovered/ mass of monomer used. Activity is (g PMMA)/(molCath). Tg is glass transition temperature determined using a thermal analyser. Determined using gel permeation chromatography (GPC) eluted with THF at room temperature by filtration with polystyrene calibration. Mn refers to the number average of molecular weights of PMMA. Blank polymerisation in which anhydrous [ZnCl2] and [CdCl2] were also activated by MMAO. Blank polymerisation by MMAO alone.
4. Conclusion
Acknowledgments
In summary, we demonstrated the synthesis of dichloro Zn(II) and dibromo Cd(II) complexes ligated to N0 -cycloalkyl substituted N,N-bispyrazolyl ligands. The versatility of the ligand is shown to accommodate variable coordination geometries around the metal centres. The molecular structure of [L1ZnCl2] was best described as a distorted tetrahedral geometry, whereas [L1CdBr2] and [L2CdBr2] structures were described as a distorted trigonal bipyramidal geometry involving the metal centre. A coordinative interaction exists between N of N0 -cyclohexyl amine moiety and Cd(II) centre in complexes compared to their Zn(II) counterpart. In addition, the catalytic activities of Zn(II) and Cd(II) complexes for methyl methacrylate (MMA) polymerisation in the presence of modified methylaluminoxane (MMAO) were investigated. The presence of bulky methyl substituents on the pyrazole moieties resulted in decreased catalytic activities, whereas the syndiotacticity of resultant PMMA was not strongly affected by these substituents or by variations in the central metal atom. Finally, the 4-coordinate [L1ZnCl2] showed the highest catalytic activity (4.40 104 g PMMA/molZn h) compared to the remaining complexes in the current study.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of the Science, ICT & Future Planning (Grant No. 2014-R1A1A3049750). Appendix A. Supplementary material CCDC 1063503–1063505 contains the supplementary crystallographic data for [L1ZnCl2], [L1CdBr2] and [L2CdBr2]. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.07.012. References [1] Z.H. Stachurski, Polymer 43 (2002) 7409. [2] F.L. Baines, S. Dionisio, N.C. Billingham, S.P. Armes, Macromolecules 29 (1996) 3096. [3] A.W.M. DeLaat, W.P.T. Derks, Colloids Surf. A 71 (1993) 147. [4] X. Zhang, K. Matyjaszewski, Macromolecules 32 (1999) 1763.
S. Choi et al. / Inorganica Chimica Acta 435 (2015) 313–319 [5] S.M. Aha, S.C. Chang, H.K. Rhee, J. Appl. Polym. Sci. 69 (1998) 59. [6] A.H. Alshehri, M. Jakubowska, M. Sloma, M. Horaczek, D. Rudka, C. Free, J.D. Carey, Appl. Phys. Lett. 99 (2011) 153109. [7] S.Z. Szilasi, R. Huszank, T. Szikra, T. Váczi, I. Rajta, I. Nagy, Mater. Chem. Phys. 130 (2011) 702. [8] M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 101 (2001) 3689. [9] T. Pintauer, K. Matyjaszewski, Coord. Chem. Rev. 249 (2005) 1155. [10] N.V. Tsarevsky, K. Matyjaszewski, Chem. Rev. 107 (2007) 2270. [11] W.A. Braunecker, K. Matyjaszewski, Prog. Polym. Sci. 302 (2007) 93. [12] W. Jakubowski, K. Matyjaszewski, Angew. Chem., Int. Ed. 45 (2006) 4482. [13] T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 30 (1997) 4507. [14] H. Alvès, M. Graullier, A. Riondel, J.M. Paul, M. Birot, H. Deleuze, Green Chem. 11 (2009) 868. [15] Y.J. Kim, K.H. Kim, T.Y. Kwon, Mater. Sci. Eng., C 30 (2010) 219. [16] W. Kaminsky, A. Bark, M. Arndt, Macromol. Symp. 47 (1991) 83. [17] V. Kotzabasakis, S. Mourmouris, M. Pitsikalis, N. Hadjichristidis, D.J. Lohse, Macromolecules 44 (2011) 1952. [18] M. Yang, W.J. Park, K.B. Yoon, J.H. Jeong, H. Lee, Chem. Comm. 14 (2011) 189. [19] Z.H. Li, Y.M. Zhang, M.Z. Xue, L. Zhou, Y.G. Liu, J. Polym. Sci., Part A: Polym. Chem. 43 (2005) 5207. [20] Z.X. Huang, Y.M. Zhang, H. Li, Y.G. Liu, Macromol. Chem. Phys. 209 (2008) 825. [21] U. Peuckert, W. Heitz, Macromol. Rapid Commun. 19 (1998) 159. [22] P.G. Lassahn, V. Lozan, G.A. Timco, P. Christian, C. Janiak, E.P. Winpenny, J. Catal. 222 (2004) 261. [23] D.A. Barnes, G.M. Benedikt, B.L. Goodall, S.S. Huang, H.A. Kalamarides, S. Lenhard, L.H. McIntosh III, K.T. Selvy, R.A. Shick, L.F. Rhodes, Macromolecules 36 (2003) 2623. [24] V. Lozan, P.G. Lassahn, C. Zhang, B. Wu, C. Janiak, G. Rheinwald, H.Z. Lang, Naturforsch. B 58 (2003) 1152. [25] Y.B. Huang, G.R. Tang, G.Y. Jin, G.X. Jin, Organometallics 27 (2008) 259. [26] F. Simal, S. Delfosse, A. Demonceau, A.F. Noels, K. Denk, F.J. Kohl, T. Weskamp, W.A. Herrmann, Chem. Eur. J. 8 (2002) 3047. [27] T. Ando, C. Sawauchi, M. Ouchi, M. Kamigaito, M. Sawamoto, J. Polym. Sci., Part A: Polym. Chem. 41 (2003) 3597. [28] T. Terashima, M. Ouchi, T. Ando, C. Sawauchi, M. Kamigaito, M. Sawamoto, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 4966. [29] C. Mehler, W. Risse, Macromolecules 25 (1992) 4226. [30] P.G. Lassahn, V. Lozan, C. Janiak, Dalton Trans. (2003) 927. [31] B. Berchtold, V. Lozan, P.G. Lassahn, C. Janiak, J. Polym. Sci., Part A: Polym. Chem. 40 (2002) 3604. [32] M. Yousaf, H.B. Ahmad, K.M. Khan, A.F. Zahoor, M.K. Khosa, Y.L. Qian, J. Chem. Soc. Pak. 31 (2009) 467. [33] G.J.P. Britovsek, V.C. Gibson, S. Mastroianni, D.C.H. Oakes, C. Redshaw, G.A. Solan, A.J.P. White, D.J. Williams, Eur. J. Inorg. Chem. (2001) 431. [34] L.G. Furlan, M.P. Gil, O.L. Casagrande Jr., Macromol. Rapid Commun. 21 (2000) 1054. [35] Z. Zhang, D. Cui, A.A. Trifonov, Eur. J. Inorg. Chem. (2010) 2861. [36] C. Lansalot-Matras, F. Bonnette, E. Mignard, O. Lavastre, J. Organomet. Chem. 693 (2008) 393.
319
[37] B. Lian, C.M. Thomas, O.L. Casagrande Jr., C.W. Lehmann, T. Roisnel, J.F. Carpentier, Inorg. Chem. 46 (2007) 328. [38] S. Scheuer, J. Fischer, J. Kress, Organometallics 14 (1995) 2627. [39] D.P. Long, P.A. Bianconi, J. Am. Chem. Soc. 118 (1996) 12453. [40] H. Nakazawa, S. Ikai, K. Imaoka, Y. Kai, T. Yano, J. Mol. Catal. A: Chem. 132 (1998) 33. [41] S. Murtuza, O.L. Casagrande Jr., R.F. Jordan, Organometallics 21 (2002) 1882. [42] I.N. Stepanenko, B. Cebrián-Losantos, V.B. Arion, A.A. Krokhin, A.A. Nazarov, B.K. Keppler, Eur. J. Inorg. Chem. (2007) 400. [43] M. Daoudi, N.B. Larbi, A. Kerbal, B. Bennani, J.P. Launay, J. Bonvoisin, T.B. Hadda, P.H. Dixneuf, Tetrahedron 62 (2006) 3123. [44] J. Pons, A. Chadghan, A. Alvarez-Larena, J.F. Piniella, J. Ros, Inorg. Chim. Acta 324 (2001) 342. [45] R. Touzani, A. Ramdani, T. Ben-Hadda, S.E. Kadiri, O. Maury, H.L. Bozec, P.H. Dixneuf, Synth. Commun. 31 (2001) 1315. [46] S.J. Dougan, M. Melchart, A. Habtemariam, S. Parsons, P.J. Sadler, Inorg. Chem. 45 (2006) 10882. [47] N. Singh, N.K. Sangwan, K.S. Dhindsa, Pest Manag. Sci. 56 (2000) 284. [48] T.B. Hadda, A.T. Kotchevar, M. Daoudi, B. Bennani, N. Ben, Lett. Drug Des. Discovery 2 (2005) 584. [49] I. Bouabdallah, L.A. M’barek, A. Zyad, A. Ramdani, I. Zidane, A. Melhaoui, Nat. Prod. Res. 20 (2006) 1024. [50] J. Hitzbleck, A.Y. O’Brien, G.B. Deacon, K. Ruhlandt-Senge, Inorg. Chem. 45 (2006) 10329. [51] E. Kim, H.Y. Woo, S. Kim, H. Lee, D. Kim, H. Lee, Polyhedron 42 (2012) 135. [52] D. Kim, S. Kim, E. Kim, H.J. Lee, H. Lee, Polyhedron 63 (2013) 139. [53] S. Choi, S. Nayab, J. Jeon, S.H. Park, H. Lee, J. Organomet. Chem. submitted. [54] R. Touzani, A. Ramdani, T. Ben-Hadda, S.E. Kadiri, O. Maury, H. Le Bozec, P.H. Dixneuf, Synth. Commun. 31 (2001) 1315. [55] T. Harit, F. Malek, B.E. Bali, A. Khan, K. Dalvndi, B.P. Marasini, S. Noreen, R. Malik, S. Khan, M.I. Choudhary, Med. Chem. Res. 21 (2012) 2772. [56] SMART and SAINT-Plus v 6.22, Bruker AXS Inc., Madison, Wisconsin, USA, 2000. [57] G.M. Sheldrick, SADABS v 2.03, University of Göttingen, Germany, 2002. [58] SHELXTL v 6.10; Bruker AXS Inc.: Madison, Wisconsin, USA, 2000. [59] M. Parvez, A.P. Sabir, Acta Crystallogr., Sect. C 52 (1996) 1651. [60] P.K. Bharadwaj, H.J. Schugar, J.A. Potenza, Acta Crystallogr., Sect. C 47 (1991) 754. [61] Y. Cui, D. Long, W. Chen, J. Huang, Acta Crystallogr., Sect. C 54 (1998) 1605. [62] T. Kitaura, T. Kitayama, Macromol. Rapid Commun. 28 (2007) 1889. [63] J. Llorens, E. Rude, R.M. Marcos, Polymer 44 (2003) 1741. [64] G.T. Lewis, V. Nguyen, Y. Cohen, J. Polym. Sci. A: Polym. Chem. 45 (2007) 5748. [65] T.V. Laine, U. Piironen, K. Lappalainen, M. Klinga, E. Aitola, M. Leskela, J. Organomet. Chem. 606 (2000) 112. [66] B.K. Bahuleyan, D. Chandran, C.H. Kwak, C.S. Ha, I. Kim, Macromol. Res. 18 (2008) 745. [67] B. Lian, C.M. Thomas, O.L. Casagrande, C.W. Lehmann Jr., T. Roisnel, J.F. Carpentier, Inorg. Chem. 46 (2007) 328.