Identification and characterization of monoanionic tripodal tetradentate ligand complexes of copper(I) and copper(II) involved in halogen atom transfer reactions

Article abstract of DOI:10.1021/ja045003g

The copper(I) complex of bis-(2-(2-pyridyl)-ethyl)-(2-(N-p-toluenesulfonamido)-ethyl)amine (PETAEA), a monoanionic, tripodal tetradentate ligand, was prepared, characterized, and shown to be an effective catalyst for atom transfer radical polymerization (ATRP). A model atom transfer reaction of Cu(PETAEA) with 1-phenylethyl bromide and TEMPO radical trapping agent was studied. The copper(II) complex formed in this reaction was identified by comparison of its spectroscopic data with that of Cu(PETAEA)Br prepared by an independent synthesis. Kinetic and spectroscopic data indicated that the reaction mechanism involved simple atom transfer from the alkyl halide to the Cu(PETAEA) to form the Cu(PETAEA)Br, and no other intermediates were involved. The solid-state structures of the copper(I) and (II) complexes appeared to be maintained in solution, so this system is an atom transfer reaction in which all of the reactive species are identified and characterized. Copyright

Full text of DOI:10.1021/ja045003g

Published on Web 10/19/2004
Identification and Characterization of Monoanionic Tripodal Tetradentate
Ligand Complexes of Copper(I) and Copper(II) Involved in Halogen Atom
Transfer Reactions
Jocelyn M. Goodwin, Marilyn M. Olmstead, and Timothy E. Patten*
Department of Chemistry, UniVersity of California at DaVis, One Shields AVenue, DaVis, California 95616-5295
Received August 18, 2004; E-mail: patten@chem.ucdavis.edu
Copper(I) complexes with polydentate amine ligands play
important roles as models for metalloproteins1 and as catalysts for
organic transformations2,3 and polymerizations.4,5 A principal mode
of reactivity of these complexes is the promotion of halogen atom
transfer from an organic halide to yield a free radical that can
undergo further addition, coupling, or disproportionation reactions.
Selectivity for addition processes frequently is achieved via the
persistent radical effect, in which initial radical coupling serves to
build up a concentration of copper(II) complex such that the rate
at which the radicals are deactivated in a reverse atom transfer step
becomes faster than the rate at which the radicals can couple or
disproportionate.6,7 Atom transfer radical polymerization (ATRP)
is an example of a polymerization process that functions via such
copper(I/II) atom transfer chemistry.4,5
The solution structures and fundamental reactivities of the copper
Figure 1. X-ray structure of Cu(I)(PETAEA), complex 1 (left), and X-ray
structure of Cu(II)Br(PETAEA), complex 3 (right).
centers that promote atom transfer are just beginning to be
1
the empirical formula, Cu(PETAEA), and H NMR and 13C {1H}
understood, so direct structure-activity relationships have not been
NMR spectra confirmed the presence of the deprotonated ligand
elucidated for these catalyst systems. Deeper insight into the
in the complex. Crystals of complex 1 suitable for X-ray crystal-
lography were grown from saturated solutions of CH3CN, and the
molecular structure of the complex is shown in Figure 1.
Key features of the structure of complex 1 include a distortion
structure, solution dynamics, and chemistry of the copper centers
involved in halogen atom transfer would allow for rational design
of catalysts for selective transformations. Such studies demand a
catalyst system in which the structures and properties of both the
of the coordination sphere away from tetrahedral geometry, as the
copper(I) and copper(II) centers are known. Because the copper(I)
copper center sits closest to the tetrahedron face comprised of the
ion has a d10 electronic configuration, its complexes are labile, and
three pendant donor nitrogen atoms. This distortion can be attributed
their solution structures can be complex, varying with changes in
to the fact that the ethylene bridges from the central nitrogen to
solvent, temperature, and ligand concentration.
the pendant donor groups are not sufficiently long to allow for an
We proposed that a monoanionic, tripodal tetradentate ligand
ideal 109.5° bite angle (the average was 92°). The Cu-N bond
design for copper(I) ions would have structural stabilization
lengths are within the range normally found for copper(I) com-
properties, such as the chelate effect and ligand-metal electrostatic
plexes. The lack of a halide or other counterion indicated that the
attraction, that would lead to simplified solution structures.8,9 Since
sulfonamide group was deprotonated. Variable-temperature 1H
then, advances have been made toward developing ligand systems
NMR spectra recorded up to 90 °C in the presence and absence of
that permit the identification and characterization (in solution and
weakly coordinating functional groups (i.e., ethyl acetate and
the solid state) of the copper(I) and copper(II) complexes involved
pyridine) normally found in ATRP monomers did not show either
in halogen atom transfer reactions.10,11 Here, we report the synthesis
changes in spectral line widths or significant shifts in the positions
and characterization of copper(I) and copper(II) complexes of bis-
of the signals. While these results were negative, they suggested
(2-(2-pyridyl)-ethyl)-(2-(N-p-toluenesulfonamido)-ethyl)amine (PE-
that the dynamic solution structure of complex 1 under the
TAEA) and evidence that these complexes are the primary copper
conditions studied does not involve ligand exchange, ligand
complexes involved in the atom transfer catalytic cycle.
dissociation, or monomer coordination processes.
The ligand, PETAEA, was designed to be tetradentate, mono-
anionic with a soft donor that has a relatively low pKa, and contain
π-accepting pyridyl rings that would help stabilize the +1 oxidation
state of copper. PETAEA was synthesized via the Michael addition
of mono-N-(tert-butoxycarbonyl)-ethylenediamine to 2 equiv of
2-vinylpyridine. Removal of the tert-butoxycarbonyl protecting
group followed by reaction of the primary amine with p-toluene-
sulfonyl chloride produced PETAEA in good yield. Treatment of
PETAEA with sodium hydride followed by addition of CuCl
yielded complex 1, which was obtained as red-orange crystals after
separation from a colorless precipitate. Elemental analysis confirmed
Evaluation of complex 1 as an atom transfer catalyst was
accomplished via ATRPs of styrene and methyl methacrylate
(MMA). Both polymerizations were conducted using 200:1:1 molar
ratios of monomer:initiator (1-phenylethyl bromide (1-PEBr) for
styrene and ethyl 2-bromo-2-methylpropionate for MMA):complex
1, 60% solvent by volume, and temperatures of 110 °C for styrene
and 80 °C for MMA. The polymerization times to reach yields of
72% (styrene) and 77% (MMA) were comparable to those reported
for CuBr/2dNbipy-catalyzed ATRPs (480 min for styrene and 60
min for MMA).12,13 The final molecular weights were 2.37 × 104
for styrene and 2.79 × 104 for MMA, while the molecular weight
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J. AM. CHEM. SOC. 2004, 126, 14352-14353
10.1021/ja045003g CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
to grow crystals of complex 2 from the final reaction mixture, and
X-ray analysis showed that the unit cell of this compound was
identical to that of complex 3. The match between characterization
data for complexes 2 and 3 strongly indicated that the two species
were identical.
Identification of all the species formed in Scheme 1 permitted a
kinetic study of the process. The conversion of 1-PEBr into
1-phenylethyl TEMPO was monitored using LC-MS and under
reaction conditions in which complex 1 was present in a 10-fold
excess relative to 1-PEBr. The kinetics could be analyzed using a
rate law based on a mechanism consisting of a fast equilibrium
step followed by a fast, irreversible trapping step (Scheme 1). A
plot of kobs versus the initial concentration of complex 1 yielded a
linear fit with a slope from which kact could be extracted (1.7 (
0.2 × 10-2 M-1 s-1). The reaction was repeated, but the ratio
[complex 3]o/[[TEMPO]o was varied, and [complex 1]o was kept
constant and in excess of [1-PEBr]o. A plot of 1/kobs versus [complex
3]o/[[TEMPO]o yielded a linear fit with a slope from which kdeact
could be extracted (3 ( 3 × 108 M-1 s-1). These kinetic values
were of a similar order of magnitude as the CuBr/2dNbipy/1-PEBr
system.15 Along with the characterization data, the kinetic studies
supported the conclusion that the mechanism of atom transfer
follows the steps outlined in Scheme 1 with no other intermediates.
We note that complexes 1 and 3 represent a unique ATRP
catalyst system in which the structures of each species involved in
atom transfer is known with a high degree of certainty. Specific
structural parameters such as the addition of electron-withdrawing/
donating groups at various points of the complex, chelate bite
angles, and donor atom types can now be modified systematically
so that direct structure-reactivity relationships can be probed.
distributions were narrow (1.14 and 1.09, respectively). However,
the experimental molecular weights were 50-60% higher than
expected (1.50 × 104 for styrene and 1.56 × 104 for MMA),
possibly indicating inefficient initiation. Nevertheless, the data did
show that complex 1 was an excellent first generation catalyst for
ATRP.
Because complex 1 showed good activity for ATRP and its solid
state and solution structures appeared to be similar, its atom transfer
equilibrium chemistry was investigated further. When 2 equiv of
1-PEBr were added to complex 1 in the presence of 1.1 equiv of
2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) (Scheme 1), the
orange solution turned emerald green.
The formation of the TEMPO adduct of the 1-phenylethyl radical
was confirmed by the presence of its molecular ion signal in the
ESI-MS spectrum of the product solution. The UV-vis absorption
for complex 1 at 370 nm (ꢀ ) 2711 M-1 cm-1) disappeared, and
new absorptions appeared at 400 (ꢀ ) 463 M-1 cm-1), 498 (ꢀ )
102 M-1 cm-1), and 652 (ꢀ ) 110 M-1 cm-1) nm. A clear isosbestic
point was observed in overlay plots of the UV-vis spectra taken
at varying degrees of conversion, indicating that no intermediate
was involved in the conversion of complex 1 into a new copper-
(II) complex, complex 2. This result was also consistent with
literature conclusions that the mechanism of Cu(I) ATRP does not
involve organometallic intermediates or outer-sphere electron
transfer.5
EPR spectra of this solution of complex 2 showed values of g(||)
) 2.256, g( ) ) 2.032, and A(||) ) 159 G, which are consistent
with data found for other trigonal bipyramidal complexes of copper-
(II) with nitrogen-based ligands. Typically, hyperfine coupling to
the copper nucleus would result in a four-line signal for A(||), but
only three were discerned in the spectrum. This observation may
be due to a significant rhombic distortion of coordination geometry
that is seen in the X-ray crystal structure of an independently
prepared complex (vide infra).14
Acknowledgment. We thank the donors of the Petroleum
Research Fund (35150-AC7), administered by the American
Chemical Society, and the Tyco Electronics Foundation Fellowships
in Functional Materials for support of this research. We thank
Gregory Yeagle and Dr. David Britt (UCD) for their assistance
with EPR.
Supporting Information Available: Experimental procedures,
characterization data, and crystallographic data for complexes 1 and 3
and experimental procedures and data for polymerizations, reactions,
and reaction kinetics using complex 1 (PDF, CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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A copper(II) complex of PETAEA was prepared by adding a
solution of the deprotonated ligand to a suspension of anhydrous
CuBr2 in CH3CN. The green solution was filtered from the white
precipitate and cooled, and green crystals formed, complex 3.
Elemental analysis confirmed the empirical formula of the complex,
Cu(PETAEA)Br. Crystals of complex 3 suitable for X-ray crystal-
lography were grown from saturated solutions of CH3CN, and the
molecular structure of the complex is shown in Figure 1. Key
features of this structure include a rhombic distortion of the trigonal
bipyramidal coordination geometry due to the constraining lengths
of the ethylene-bridge arms. The bromide ligand occupies an axial
position. The lack of a second bromide or other counterion indicates
that the sulfonamide group is deprotonated.
The spectroscopic signatures of complex 3 matched those found
for complex 2: UV-vis signals at 400 (ꢀ ) 931 M-1 cm-1), 498
(ꢀ ) 188 M-1 cm-1), and 652 (ꢀ ) 217 M-1 cm-1) nm and an
EPR signal with g(||) ) 2.259, g( ) ) 2.032, and A(||) ) 159 G
(three-line hyperfine coupling pattern). Furthermore, we were able
JA045003G
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