Chapter 15
The Copper Catalyst in Atom Transfer Radical Polymerizations: Structural Observations 1
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2
Guido Kickelbick , Ulrich Reinöhl , Teja S. Ertel , Helmut Bertagnolli , and Krzysztof Matyjaszewski 2
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1
Institut für Anorganische Chemie, Technische Universität Wien, Getreidemarkt 9/153,1060 Wien, Austria Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 2
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X-ray structure determination and E X A F S analysis were used to determine the structure of copper complexes with 2,2'-bipyridine (bpy) and linear triamine N,N,N',N",N"-pentamethyldiethylenetriamine (pmdeta) as ligands. In the case of the bpy system the Cu complex was identified as a cationic tetrahedral species where the copper is surrounded by two bpy ligands. The structure of the anion depends on the particular catalyst and reaction medium but in some systems can be represented by a linear [Br Cu] . The Cu species generated in a model reaction was a cationic trigonal bipyramidal complex where the metal is surrounded by two bpy ligands and one bromine. In the case of the linear triamine (pmdeta) the Cu atom is surrounded by one pmdeta ligand and a bromine. From crystal structure analysis it was concluded that the Cu complex shows a distorted square pyramidal structure with two very different Cu-Br distances. (I)
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(II)
2
(I)
(II)
Introduction Controlled radical polymerization techniques are based on the quantitative initiation and regulation of the concentration of radicals during the polymerization reaction. L o w concentration of radicals disfavors the termination reactions by combination or disproportionation. Two methods belong to the most promising in this field: (i) the nitroxide mediated systems rely on the cleavage of a C - 0 bond by the reversible formation a stable nitroxyl radical and a growing radical (I) and (ii) atom transfer radical polymerization (ATRP) where the radicals are formed by the reversible cleavage of a C - X bond under transition metal catalyzed conditions (2,3).
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© 2000 American Chemical Society
212 Both methods reveal excellent control over molecular weights and polydispersities combined with the ability to prepare well-defined copolymers (4). However, A T R P can be applied to a larger variety of monomers and use more diversified initiator molecules. Although A T R P and the atom transfer radical addition (ATRA) method have been known for a relatively long time, the mechanism of the process including structure of all intermediates is not yet fully understood. The catalytic cycle in transition metal mediated A T R P involves the switching between two oxidation states of a transition metal compound. The lower oxidation state complex adds a halogen atom by homolytic cleavage of a C - X bond forming a radical and promoting the transition metal compound into a higher oxidation state (Scheme 1). The equilibrium is strongly shifted to the side of the so-called dormant species. The coordination compound affects the equilibrium constants and therefore the control in the reaction is strongly dependent on the metal and the ligands forming the catalytic active species. Metals that are most commonly known for usage in A T R P reactions are Cu (2,3), Ru (5), Fe (6-8), N i (9, 10), Pd (11) and Rh (12) in combination with a variety of different ligands tuned for every metal and the polymerization process. Whether the catalyst is suitable for the polymerization reaction depends strongly on the right combination of metal and ligand. For the future development of new catalyst as well as the proper understanding of the mechanism it is important to know structural details of the different types of catalytically active species in the reaction. This paper concentrates on the copper/amine mediated reactions because those are among the most commonly used A T R P catalytic systems. Two ligands used in this study are 2,2'-bipyridine (bpy) derivatives and linear triamines. In the case of the bpy system the ratio between C u halide and ligand that have been used varies from 1:1 to 1:3. (13) However, there is no clear indication whether the different ratios have any influence on the mechanism of the reaction. (I)
Scheme 1 M
Pn-X
X: Mt: L: M:
+
m
Mt L
Halogen Metal Ligand Monomer
XMt
m + 1
L
Termination reactions
Ή-Ρ =Pn η
P-Pq
+
2 XMt
m + 1
L
In this study X-ray crystallography and extended X-ray absorption fine structure (EXAFS) analysis were used to clarify the nature of the catalyst in the solid state as well as in solution. X-ray crystallography provided the first suggestions for structures of possible intermediates in A T R P . Also, we were able to crystallize compounds from
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reaction solutions and analyze their structures. To verify whether the results from the solid state can be transferred to solution reactions, E X A F S analysis was used.
Results and Discussion Cu/bpy based systems Possible structures involved in the mechanism of Cu/bpy mediated A T R P can be concluded from X-ray structures of related C u and C u complexes. C u surrounded by two bidentate ligands like bpy forms typically a [(bpy) Cu] cation with a tetrahedral structure in a mixture of 1:2 between the copper halide and bpy. This structure type is found in literature accompanied by different counterions like [C10 ]" (14) ox [Cl Cu]~ (15). A bridged structure for the C u species is also possible where the halogen atoms are in bridging positions and only one bpy is bound to every copper center leading to a neutral dimer which is known to occur for bromine and iodine but not for chlorine atoms (15). The C u species being part of the A T R P equilibrium should form a trigonal bipyramidal cation where the copper center is surrounded by two bpy ligands and one halogen atom. This structure type has many literature examples for chlorine, bromine and iodine as halogen atoms accompanied by a collection of different counterions. (16) Due to this large number of examples it appears to be a very stable class of compounds. A s a typical representative of this class of complexes we selected [(bpy) CuBr][BF ] (17) which was used in our studies as a model compound. The proposed mechanism of a Cu/bpy mediated A T R P is shown in Scheme 2 based on the above-mentioned structures. (I)
(II)
(I)
+
2
(I)
4
2
(II)
2
4
Scheme 2
R
R
- /=R
Some information on the structure of amine coordinated copper complexes in A T R P was available from U V model studies (18). However, the structure of counterions and their role in the equilibrium remained obscure. Originally it was assumed that the counterion is a pure halide anion, which is released from the copper halide used in the reaction. This explanation is also supported by the stoichiometry of
214 the reaction where in the typical A T R P case one eq. of C u X was mixed with two eq. of bpy. However, no example for [(bpy) Cu] and only a few examples for [(bpy^CuX]* cations in combination with a simple halide as counterion are found in the literature (19-21). A first hint of the kind of anions involved in A T R P was a crystal structure from crystals precipitated after an A T R P reaction from the solution which showed a trigonal bipyramidal [dNbpy CuBr] cation with a linear [Br Cu]" anion (Figure 1). +
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+
2
2
+
Figure 1. Crystal structure of the complex [dNbpy CuBr] [Br Cu]" prepared from crystals which precipitated from an A T R P reaction solution of methyl acrylate. 2
2
However, all the data above is based on crystal structure observations and therefore only typical for the solid state. To extend these observations to solution an additional technique was needed which is capable to give structural information for the catalytic active systems in solution without interacting with the observed species. For this purpose E X A F S analysis was used. This technique provides information about the distances and the number of the surrounding atoms through the analysis of the fine structure at the absorption edge of an atom.(22) Its big advantage is the applicability in solid state as well as in solution. Especially strong X-ray absorbers like copper and bromine, both typical atoms for the catalytic active A T R P species, can be examined with a high reliability. Therefore, this technique seemed to be ideal for the investigation of A T R P systems. The first step in the analysis was to compare experimental E X A F S data for bond lengths with data from single crystal X-ray diffraction. Therefore model compounds were crystallized and their crystal data proved by single crystal X-ray structure determination. A s model compounds [(bpy) Cu][C10 ] (14) and [(bpy) CuBr][BF ] 2
4
2
4
215 (17) were used. The experimental Cu-N and Cu-Br distances show a good agreement with literature values (Table I).
Table I. Comparison of experimental bonding distances [pm] from EXAFS measurements and single crystal X-ray structures for [(bpy) Cu] and [(bpy) CuBr] . +
2
+
2
[(bpy) Cu][C10 ] [(bpy) CuBr][BF ] 2
4
2
4
Cu-N EXAFS 198.0 201.0
Cu-N crystal (14)
202.0, 202.1 199.5, 199.6, 206.8,211.4
Cu-Br EXAFS
Cu-Br crystal (17)
241.0
241.9
The experiments in solution were carried out following standard A T R P procedures. Styrene and methyl acrylate were used as solvents to verify possible interactions between the coordination compound and the monomer, which are able to influence the structure of the catalyst. Substituted 4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy) was used as ligand for a better solubility of the complex in the nonpolar monomers. The results of the measurements are shown in Table II.
Table Π. Results from EXAFS measurements from different ligand to copper halide mixtures, number of backscattering atoms and their distances [pm]. Mixture
CuBr + dNbpy CuBr + 2 dNbpy CuBr + dNbpy CuBr + 2 dNbpy CuBr + 2 dNbpy 2
St St MA MA MA
Cu-Br
Cu-N
Monomer Nr. of Ν atoms 4 4 4 4 4
Distance
Nr. of Br atoms
Distance
201.0 203.0 200.0 203.0 208.0
2 2 2 2 3or4 1 2
225.0 227.0 226.0 229.0 234.0 243.0 224.0
201.0 CuBr + 2 dNbpy 4 St polymerization @ 95°C 225.0 2 202.0 CuBr + 2 dNbpy MA 4 polymerization @ 80°C The first question was whether different ratios between CuBr and dNbpy affect the structure of the complex in solution. For both ratios 1:1 and 1:2 between CuBr and dNbpy two different types of C u species were observed. One C u atom
216 surrounded by 4 nitrogen atoms in a Cu-N distance from 200.0 pm to 203.0 pm and one C u atom surrounded by two bromine atoms in a distance from 225.0 pm to 229.0 pm. This result fits well with the data from a [(dNbpy) Cu] cation and a [Br Cu]~ anion. The most important conclusion from this data is that the predominant structure of complexes is the same whether one equivalent of CuBr is mixed with one or two equivalents of bpy. Therefore in the case of a 1:2 mixture usually used in A T R P it is possible that free bpy is present in solution. Figure 2 shows a comparison between 3 Fourier-filtered experimental and calculated kx(k) function and the corresponding Fourier functions for a mixture of one eq. CuBr and two eq. dNbpy in styrene at room temperature. The curve for the simulated data for a mixture of the model compounds [(dNbpy) Cu] and [Br Cu]~ fits very well with the experimental plot which demonstrates the reliability of the model. +
2
2
+
2
2
1
kfÂ- ]
r [ A ] 3
Figure 2. Fourier-filtered experimental (dotted line) and calculated (solid line) kx(k) function and the corresponding Fourier transformations Mod(r) for a mixture of one eq. CuBr, two eq. dNbpy in styrene at RT, measured at the Cu K-edge; k = 3.8-11.8 A- . 1
In Figure 3 the simulated line is divided in the functions of the anionic species and the cationic species alone. The superposition of these functions results in the simulated curve of the mixture. The Fourier-transformed functions (b parts of the figures) display directly the distances of all backscattering atoms, which is in this case a mixture of different species. The comparison of E X A F S data in styrene and methyl acrylate, which has a potentially larger interaction with the transition metal (oxygen lone pairs), reveals that there is no influence of the monomer on the complex.
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3
Figure 3. Simulation of the k %(k) function and the corresponding Fourier transformations Mod(r) [(dNbpy) Cu] cation (doted line) [Br Cu]" anion (dashed line) and the superposition of both (solid line). +
2
2
The analysis of the mixture of CuBr with 2 equivalents of dNbpy shows 4 Cu-N distances of 208.0 pm and 3 or 4 Cu-Br distances of 234.0 pm and one Cu-Br distance of 243.0 pm. In this case it was not possible to determine the exact coordination number of the C u atom. The data fits, as expected, with the trigonal bipyramidal [dNbpy CuBr] cation. The remaining Cu-Br distances belong to the corresponding anions which matches well with the literature known distorted tetrahedral [Br Cu] ~ or planar [Br Cu]~ anions (23). Unfortunately, E X A F S can not provide a quantitative information on the proportion of Br" and [Br Cu]", [Br Cu]~, [Br Cu] " anions. Figure 4 shows the good fit of Fourier-filtered experimental (dotted line) and calculated (solid line) k %(k) function as well as the corresponding Fourier transformations Mod(r) for a mixture of one eq. Cu Br and two eq. dNbpy in methyl acrylate at room temperature. Additionally the corresponding composition of the simulated curve of the mixture from the cationic [(dNbpy) CuBr] and anionic fraction [Br Cu] " is shown in Figure 5. Polymerization reactions of styrene and methyl acrylate also were investigated. No differences between the 1:1 and 1:2 mixtures of CuBr and dNbpy were observed by E X A F S . There is a clear indication that the A T R P equilibrium is strongly shifted to the side of the C u species. Although a C u species is definitely formed during the process, as directly observed by E P R (24), we were not able to detect this species by E X A F S analysis due to its low concentration (
