1. Zeynep Çimşir, Zeliha Durmaz, Abdurrahman Şengül and Hülya Arslan*
2,6-BIS(NH-BENZIMIDAZOL-2-YL)PYRIDINE LIGAND AS NEW CATALYST COMPLEX IN COPPER BASED ATRP
Zeynep Çimşir, Zeliha Durmaz, Abdurrahman Şengül and Hülya Arslan*
Bülent Ecevit University, Chemistry Department Zonguldak, TURKEY
Introduction
Result and Discussion
Atom transfer radical polymerization (ATRP) has emerged as one of the most powerful, rapidly developing areas of chemistry and one of the widely used synthetic techniques in polymer science which is based on the use of an alkyl halide as a initiator and a metal in the lower oxidation state as catalyst and co-catalyst as ligand.
2,6-bis(NH-benzimidazol-2-yl)pyridine (L1) was synthesized by iminization reaction of pyridine-2,6-dicarboxylic acid with o-phenylenediamine. Linear first-order kinetic plots were observed for ATRP of methyl methacrylate using L1 as a ligand, indicating that the number of active species is constant during the polymerization and that the contribution of termination reactions is limited (Figure 1).
ATRP has attracted great attention in particular thanks to the synthesis of polymers with predetermined molecular weight, narrow molecular weight distribution as well as desired composition and molecular architecture. A variety of transition-metal complexes have been used as catalysts for ATRP including Cu(I), Ru(II), Fe(II), Ni(II), Pd(II), Rh(III) and Re(II). Nitrogen-based ligands generally work best for copper-mediated ATRP. Many studies have focused on developing new ligands and new metals that increase the activity and selectivity of the catalyst. The choice of ligand is one of the key factors influencing the reactivity of the catalyst and it is very important. Due to continue searching the new ligand for Cu-based ATRP [1-2], in this presentation 2,6-bis(NH-benzimidazol-2-yl)pyridine was synthesized and it’s ligand effect in Cu-based ATRP was investigated.
Figure 1. Kinetic plots for ATRP of MMA
The Synthesis and Characterization of Ligand
Pyridine-2,6-dicarboxylic acid (3.35 g) was stirred with o-phenylenediamine (4.7 g) in syrupy phosphoric acid (40 ml)at ca. 230 C for 4 h. The colored molten mixture was poured into and vigorously stirred in cold water. When cooled the bulky blue-green precipitate was collected by filtration and then stirred in hot, 10% aqueous sodium carbonate solution (300 ml). The resulting solid was filtered off and recrystallized from methanol to give long, white prisms (3.3 g, 53%).
Table 2. ATRP of MMA using L1
a) MMA in acetonitrile at 90 oC. [MMA]o/[EBİB]o/[CuCl]o/[BPBD]o = 200/1/1/x.
b) Last point of kinetic datas. Molecular weights were measured by GPC using polymethyl methacrylate standarts.
c) fini.eff.=Mn,th / Mn,GPC
When different amount of ligand were used in ATRP of methyl methacrylate, apparent rate constant vs [ligand]/[CuX] ratio showed a maximum at the [ligand]/[catalyst] ratio of two. These means that the most rapid and efficient reaction occurs at 2:1 ligand/CuX ratio and two ligands molecule are binding one molecule of catalyst that can be seen from Figure 2 and Scheme 2.
Figure 2. Apparent rate constant vs ligand/CuX ratio for ATRP of MMA using L1 as a ligand.
Scheme 2. ATRP mechanism using L1 as ligand and schematic presentation of Ligand/catalyst complex.
Scheme 1. The synthesis of 2,6-bis(NH-benzimidazol-2-yl)pyridine (L1)
m.p.: 350C. IR (ATR, /cm-1): 3140 (CHar), 1574 (C=N), 1458, 1437 (C=C). 1H NMR (dmso-d6, ppm): 13 (2H, s), 8.35 (2H, d, J = 7.73 Hz), 8.17 (1H, m), 7.77 (4H, m), 7.33 (4H, m). ESI-MS (m/z): [M+] (C19H13N5, Mw: 311.3)
ATRP of Methyl Methacrylate
Monomer Methyl Methacrylate (MMA)
Initator Ethyl 2-bromoisobutyrate (EBIB)
Catalyst CuCl
Ligand L1
Temperature oC
Solvent Acetonitrile /MMA :1/10 (v/v)
[M]o:[I]o:[CuCl]o:[L]o =200:1:1:x
ATRP of MMA was carried out using L1 as a ligand following experimental procedure: Given amount of CuCl, ligand and initiator (Ethyl 2-bromoisobutyrate) were placed in a 25-mL a round-bottom flask sealed with a plastic cap. The flask was deoxygenated by vacuum-nitrogen cycles. Given amount of monomer (MMA) and acetonitrile were added to the flask via a syringe and the flask was placed in a silicon oil bath at 90 oC. All liquid components were bubbled with nitrogen prior to transferring them into flask. Polymerizations were performed at various molar ratios of monomer/initiator/CuCl/ligand, such as 200/1/1/3, 200/1/1/2 200/1/1/1.5, 200/1/1/1, 200/1/1/0.5, and 200/1/1/0.33, the adequate was taken periodically via a syringe to follow the kinetics of the polymerization process.
Conclusion
2,6-bis(NH-benzimidazol-2-yl)pyridine was synthesized by useful and simple reaction pathway.
Linear first-order kinetic plots were observed for ATRP of methyl methacrylate using L1 as a ligand .
Cyclic voltammetry confirmed that CuCl/L1 complex in acetonitrile gave reversible redox couples. When anodic-cathodic peak potential separation (ΔE) is about 60 mV theoretically, it is mentioned about reversibility for a one-electron redox process. The anodic-cathodic peak potential separation (ΔE) of CuCl/L1 complex is 98 mV for run no 2 in Table 1.
Cyclic Voltammetric Measurements
Cyclic voltammetry measurement was performed at room temperature with a Potentioscan Weking POS 88 instrument. Electrochemical experiments were performed in a three-electrode cell system under argon atmosphere using a platinium wires as the counter electrode and working electrode and a silver wire as referans electrode is dipped in a 0,05 M tetrabuthylamoniumtetrafluoroborate (Bu4NBF4) acetonitrile solution (Table 1).
References
(1) Hülya ARSLAN, Yasemin KUCUK, Ayfer MENTES and Metin H. ACAR “A kinetic study of atom transfer radical polymerization of styrene with bis(2- pyridyl)ethylenedimethanimine derivative ligands” Turk.J. Chem. 37, (2013). DOI: /kim
(2) Hülya ARSLAN, M.Gürkan KAPTAN, Orçun Zırtıl, M.Emre HANHAN, Şadi ŞEN “The Synthesis of (N1E,N4E)-N1,N4-Bis(pyridine-2-yl) ethylene)benzene-1,4-diamine and Investigation of its Efficiency as New Binuclear Catalyst Complex in Copper Based ATRP” Polymer Bulletin 71(5), (2014). DOI: /s