Chapter 10
Toward Structural and Mechanistic Understanding of Transition Metal-Catalyzed Atom Transfer Radical Processes Tomislav Pintauer, Blayne McKenzie, and Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, P A 15213
Summary: Structural and mechanistic aspects of transition metal catalyzed atom transfer radical polymerization (ATRP) were discussed. Structures of CuI and CuII active A T R P catalysts were investigated using various spectroscopic techniques and were found to be dependent on the nature of the complexing ligand, solvent and temperature. The overall equilibrium constant for A T R P was correlated with the redox potentials of CuI complexes with nitrogen based ligands and equilibrium constant for halogen dissociation from the corresponding CuII-X complexes. Additionally, various techniques to measure the kinetics of elementary reactions in the A T R P such as activation, deactivation and initiation rate constants were presented.
130
© 2003 American Chemical Society
131
Introduction and Background The synthesis of macromolecules with well-defined compositions, architectures and functionalities represents an ongoing effort in the field of polymer chemistry. Over the past few years, atom transfer radical polymerization (ATRP) has emerged as a very powerful and robust technique to meet these goals (i-3). The basic working mechanism of A T R P (Scheme 1) involves a reversible switching between two oxidation states of a transition metal complex (4,5). It originates from Atom Transfer Radical Addition (ATRA), which is a well known and widely used reaction in organic synthesis (6,7).
Scheme 1. Proposed Mechanism for A T R P . R-X + Mt / Ligand n
-
— ^ — R*
k p
+
n+1
X-Mt /Ligand
H
R-R / R & R
=
Homolytic cleavage of the alkyl halogen bond (R-X) by a transition metal complex in the lower oxidation state generates an alkyl radical and a transition metal complex in the higher oxidation state. The formed radicals can initiate the polymerization by adding across the double bond of a vinyl monomer, propagate, terminate by either coupling or disproportionation, or be reversibly deactivated by the transition metal complex in the higher oxidation state. The thermodynamics and kinetics of the atom transfer step defines the control over final polymer structure, namely molecular weight, polydispersity and end functionality (8). Structural and mechanistic studies are crucial in further understanding of this mechanism and are inherently part of the future developments in the A T R P . The important factors that need to be considered in the structural aspects of the A T R P include the structures of the catalysts in solution and their solvent and temperature dependence, the role of complexing ligand and how does it effect the catalyst properties (e.g. redox potential), participation of the catalyst in side reactions other than atom transfer, and characterization of other active intermediates (e.g. radicals). The mechanistic studies, on the other hand, should aim at determining the rate constants for elementary reactions occurring in the A T R P such as activation, deactivation and initiation, and more importantly correlate them with reaction parameters such as catalyst, alkyl halide and monomer structure, temperature and solvent. Such studies can lead to a
132 development of optimal catalysts and generally improve the overall catalytic process. In this article, we present on overview of the structural and mechanistic aspects of transition metal catalyzed A T R P , with an emphasis on copper catalyzed A T R P .
Structural Aspects of Transition Metal Catalyzed ATRP
General Structural Features of the A T R P Catalysts Structural characterization of the A T R P active transition metal complexes plays an important role in the overall catalytic process and still remains a very challenging task. The catalyst consists of a transition metal center accompanied by a complexing ligand and counterion which can form a covalent or ionic bond with the metal center. The efficient catalyst should be able to expand its coordination sphere and oxidation number upon halogen abstraction from alkyl halide or dormant polymer chains. Additionally, the catalyst should not participate in any side reactions which would lower its activity or change the radical nature of the A T R P process. The concurrent reactions which can occur in the A T R P include: (a) monomer, solvent or radical coordination, (b) oxidation/reduction of radicals to radical cations/anions, respectively, (c) βhydrogen abstraction, (d) disproportionation, etc. So far, a variety of transition metal complexes have been successfully used in the A T R P . They include compounds from Group 6 (Mo (9)), 7 (Re (JO)), 8 (Fe (JJ-J3), Ru (J4% 9 (Rh (75)), 10 (Ni (76), Pd (17)) and 11 (Cu (4,18,19)). The following discussion will concentrate on the copper complexes with nitrogen based ligands commonly used for A T R P in our laboratories.
1
Structural Features of C u Complexes in A T R P 1
A variety of structural techniques have been used to study A T R P active Cu and C u complexes in both solid state and solution. They included solid state X ray crystallography (20,21), extended X-ray absorption fine structure (EXAFS) (22,23), cyclic voltammetry (24), electrospray ionization mass spectrometry (ESI-MS) (25), EPR (26,27), and U V - V i s spectroscopy (21). The catalysts typically consist of a copper(I) halide accompanied by a nitrogen based complexing ligand. Variety of bidentate, tridentate and tetradentate nitrogen based ligands have been utilized in our laboratories and are summarized in M
133 Scheme 2. In addition, ligands such as pyridineimines (18) and phenanthrolines (25) have also been used. Cu complexes with bpy based ligands have been characterized by solid state X-ray crystallography and typically consist of distorted tetrahedral [Cu (bpy) ] cations (Figure 1) accompanied by a non coordinating anions (BF ~, C10 ", Br", Cu Br ") (29-32). The structure of the complex is strongly dependent on the solvent polarity and temperature. For 1
,
+
2
l
4
-4J
4
2
N— \
/
1,4,8,11 -tetraaza-1,4,8,11 -tetramethylcyclotetradecane Me CYCLAM
1,1,4,7,10,10-hexamethyltriethylenetetramine HMTETA
4
l
example, Cu Br complex with 2 eq. of dNbpy in non polar medium such as toluene, methyl acrylate or styrene predominantly exists as [Cu (dNbpy) ][Cu Br ] (22). In polar medium such as M e O H , [Cu'(dNbpy) ][Br] is preferred (Table I) (23). Additional structures have also been proposed in literature and include neutral [Cu (dNbpy)(Br)] complex (33). Tetradentate M e T R E N , H M T E T A and M e C Y C L A M ligands are also expected to form four coordinated complexes with Cu (Figure \).(34,35) The situation with tridentate P M D E T A and tNtpy ligands is different because the 18 electron 1
I
2
2
2
!
2
6
4
1
134
l
Figure 1. Structures of'[Cu (bpy)J and [Cu'(Me TREN)J cations in [Ci/(bpy) ][ClOJ (Ref (31)) and[Cii(Me TREN)J[ClOJ (Ref (35)), respectively.. 6
2
6
f
Table 1. Structural parameters of Cu Br/2dNbpy complex, determined by E X A F S measurements under ambient conditions at the C u K - and B r K edge. a
Solvent Toluene
Sty
MA
MeOH
a
Backscatt. Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu
Ν 3.2(5) 1.8(3) 1.0(2) 2.9(4) 1.4(2) 0.8(1) 2.8(4) 1.3(2) 0.8(1) 3.3(5) 0.0 0.0
r[Â] 2.02(2) 2.25(2) 2.23(2) 2.03(2) 2.25(2) 2.24(2) 2.01(2) 2.26(2) 2.25(2) 2.02(2) / I
Proposed Structure [Cu'(dNbpy) ] [Cu'Br ]' +
2
2
[Cu'idNbpykÏÏCu'BrJ
[Cu'idNbpy^rtCu'Brj]-
+
[Cu'(dNbpy) ] [Br]2
N = coordination number, r = absorber-backscatterer distance.
rule would leave an open coordination sphere which can be occupied by coordinating anions such as Br" or CI" . On the other hand, in the case of noncoordinating anions such as BF ", BPh * or C10 ", the empty coordination sphere can be occupied by the solvent or monomer molecules. The monomer coordination was demonstrated recently in characterization of [Cu (PMDETA)(n-MA)][BPh ] and [Cu'(PMDETA)(ïï-Sty)][BPh ] complexes (36). H N M R studies (29) of some Cu complexes with nitrogen based ligands also indicated a rapid ligand exchange reactions, which in the presence of 4
4
4
,
4
l
4
1
135 coordinating solvent or monomer can give rise to other active species that can coexist in the A T R P medium.
Structural Features of C u " Complexes in A T R P Copper(ll) complexes that are active in the A T R P have been characterized using a variety of spectroscopic techniques (20,21,23). The Cu complexes showed either a trigonal bipyramidal structure in the case of dNbpy ligand ([Cu (dNbpy) Br][Br]), or a distorted square pyramidal coordination in the case of triamines and tetramines ([Cu (PMDETA)Br ], iCu (tNtpy)Br ], [Cu (Me TREN)][Br], Cu (HMTETA)Br] [Br] and [Cu (Me CYCLAM)Br][Br]) (20). Depending on the type of the amine ligand, the complexes were either neutral (triamines) or ionic (bpy and tetramines). The counterions in the case of the ionic complexes were either bromide ( M e C Y C L A M and H M T E T A ) or the linear [Cu Br ]" (dNbpy). No direct correlation was found between the Cu -Br bond length and the deactivation rate constant in the A T R P , which suggested that other parameters such as the entropy for the structural reorganization between the Cu and Cu complexes might play an important role in determining the overall activity of the catalyst in the A T R P . Additionally, E X A F S (23) and U V (37) measurements indicated bromide dissociation in polar medium such as M e O H or H 0 . The E X A F S data at the Cu and Br K-edges for copper(II) complexes are summarized in Table 2. The dissociation of bromide anions from copper(II) centers is evident from a decrease in the Br/Cu coordination numbers. This reaction might be responsible for the fast A T R P in polar medium since the concentration of the deactivator is significantly lower than in nonpolar medium. 11
I!
2
n
n
2
,l
2
n
6
u
4
!
4
2
n
1
11
2
Correlating Redox Potential with Catalyst Activity The coordination of the ligand is inherently related to the ability of metal complexes to be oxidized/reduced by the corresponding R - X (38). The overall equilibrium constant for A T R P (Scheme 3) can be expressed as the product of the equilibrium constants for electron transfer between metal complex (Κ τ), electron affinity of the halogen ( K ) , bond homolysis of the alkyl halide ( K H ) and heterolytic cleavage of the M t - X bond or "halogenophilicity" (K p). Therefore, for a given alkyl halide R - X , the activity of the catalyst in the A T R P depends not only on the redox potential, but also on the halogenophilicity of the transition metal complex. Both parameters are affected by the nature of the transition metal and ligand, including the binding constants, basicity, backΕ
E A
B
n + I
H
OS
4
l
2
+
l
2
Figure 2. Molecular structures ofcopper(II) complexes precipitatedfromATRP solutions. [Cu"(dNbpy)2Brf[Cu Br /- (a), [Cu' (tNtpy)Br J (b), [Cu"(PMDETA)Br,]'(c),[Cu"(HMTETA)Br] [Br]~ (d) and [Cu"(Me CYCLAM)Brf[Br]- (e).
-a
138
Table 2. Room Temperature E X A F S Measurements of C u " B r Complexes with bpy, P M D E T A , and M e T R E N Ligands." 2
6
Complex Cu"Br /2dNbpy
Backscatt. Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu Cu-N Cu-Br Br-Cu
Solvent Solid"
2
MeOH
Cu"Br /PMDETA 2
Solid"
MeOH
H 0 2
Cu"Br /Me TREN 3
6
Solid"
MeOH
H 0 2
Ν 2.02(2) 4.0 2.41(2) 1.0 2.41(2) 1.0 2.03(2) 2.8(4) 2.43(2) 0.6(1) 2.42(2) 0.3(0) 2.09(2) 3.0 2.44(2), 2.64(2) 2.0 2.42(2), 2.63(2) 1.0 2.09(2) 3.1(5) 2.41(2) 1.1(2) 2.39(2) 0.4(1) 2.06(2) 4.0(6) / 0.0 2.09(2) 4.0 2.41(2) 1.0 2.40(2) 1.0 2.13(2) 3.2(5) 2.38(2) 1.4(2) 2.38(2) 0.9(1) 2.06(2) 4.1(6) 2.42(2) 0.3(0) 0.0 /
= coordination number, r = absorber-backscatterer distance. Coordination numbers were fixed to the crystallographically known values for [Cu (bpy) Br]lBr]-, [Cu (PMDETA)Br ] and [Cu (Me TREN)Br] [Br]\ u
H
2
n
2
+
6
139 Scheme 3. Representation of atom transfer equilibrium by redox processes, homolytic dissociation of alkyl halide and heterolytic cleavage of M t - X bond. n + ,
Atom Transfer (Overall Equilibrium) R-X
+ Cu'-Y/Ligand
X-Cu-Y/Ligand
Contributing Reactions Cu'-Y / Ligand ^
Cu -Y/Ligand
^EA
Χ
KBH
W
Θ
R'
R-X
+
K =
X
.11 X-Cu-Y/Ligand
X ° + Cu'-Y/Ligand
KATRP —\Γ~
Θ
1
^
K
EAKBHKHPKET or
ATRP
K K p (1) ET
K
H
K
EA BH
bonding, steric effects, etc. For a complexes that have similar halogenophilicities, the redox potential can be used as a measure of catalyst activity in the A T R P . This was demonstrated in the linear correlation between K and Ei/2 (Figure 2) for a series of Cu complexes with nitrogen based ligands (24,37). However, redox potential might not be sufficient to compare 1
A T R P
-14 Initiator MBrP, CH CN, 25 °C
-16
'
\/—\s
1-18 £ -20 ,n
/
KCU
< HP.A ) V
-22 Ru
«n K(
HPAV
9
