Synthesis and reactivity of copper(i) complexes with an ethylene-bridged bis(imidazolin-2-imine) ligand

Article abstract of DOI:10.1039/b715590e

A series of copper(i) complexes with a sterically hindered, bidentate ligand, BL^iPr, derived from an N-heterocyclic carbene precursor have been isolated, characterized and their reactivity studied. The ethylene-bridged bis(imidazolin-2-imine) ligand (BL^iPr) provides strongly donating N-donor atoms for the stabilization of a copper(i) metal center, priming it for reactivity. The complexes [(BL^iPr)Cu(XyNC)]PF₆ (4) and [(BL^iPr)CuCl] (5) were characterized by X-ray crystallography and exhibit trigonal coordination at the copper centers. The reactivity of [(BL ^iPr)Cu]SbF₆ toward dioxygen was studied at low temperature, indicating formation of a thermally sensitive intermediate with intense UV/Vis features and an isotope-sensitive vibration at 625 cm ^-1 (599 cm^-1 with ¹⁸O₂). The intermediate is assigned as containing the bis(μ-oxo)dicopper(iii) core, [2](PF₆)₂, and the related, stable hydroxo form was crystallized as [{(BL^iPr)Cu}₂(μ-OH)₂] (PF₆)₂, [3](PF₆)₂. The reactivity of 5 as a catalyst for the ATR polymerization of styrene was assessed in terms of reaction kinetics and polymer properties, with low PDI values achieved for polymers with molecular weights up to 30 000 g mol^-1. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715590e

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www.rsc.org/dalton | Dalton Transactions
Synthesis and reactivity of copper(I) complexes with an ethylene-bridged
bis(imidazolin-2-imine) ligand†
Dejan Petrovic,a Lyndal M. R. Hill,b Peter G. Jones,a William B. Tolmanb and Matthias Tamm*a
Received 10th October 2007, Accepted 6th November 2007
First published as an Advance Article on the web 29th November 2007
DOI: 10.1039/b715590e
A series of copper(I) complexes with a sterically hindered, bidentate ligand, BLiPr, derived from an
N-heterocyclic carbene precursor have been isolated, characterized and their reactivity studied. The
ethylene-bridged bis(imidazolin-2-imine) ligand (BLiPr) provides strongly donating N-donor atoms for
the stabilization of a copper(I) metal center, priming it for reactivity. The complexes
[(BLiPr)Cu(XyNC)]PF6 (4) and [(BLiPr)CuCl] (5) were characterized by X-ray crystallography and
exhibit trigonal coordination at the copper centers. The reactivity of [(BLiPr)Cu]SbF6 toward dioxygen
was studied at low temperature, indicating formation of a thermally sensitive intermediate with intense
UV/Vis features and an isotope-sensitive vibration at 625 cm−1 (599 cm−1 with 18O2). The intermediate
is assigned as containing the bis(l-oxo)dicopper(III) core, [2](PF6)2, and the related, stable hydroxo form
was crystallized as [{(BLiPr)Cu}2(l-OH)2](PF6)2, [3](PF6)2. The reactivity of 5 as a catalyst for the ATR
polymerization of styrene was assessed in terms of reaction kinetics and polymer properties, with low
PDI values achieved for polymers with molecular weights up to 30 000 g mol−1.
Introduction
Within the last few years poly(guanidine) ligands have attracted
much attention because they have found remarkable applications
as superbasic proton sponges1 and as ligands for transition metal
complexation,2 with particular emphasis on copper coordination
and dioxygen activation chemistry.3,4 For instance, Henkel and
co-workers reported a number of bis(guanidine) ligands and their
2
2
use in dioxygen activation, leading to the formation of l-g :g -
peroxodicopper(II) and/or bis(l-oxo)dicopper(III) complexes.3
The groups of Holthausen, Schindler and Sundermeyer reported
the first crystallographically characterized end-on 1 : 1 Cu/O2
adduct stabilized by the sterically congested, superbasic tris(tetra-
methylguanidino)tren (TMG3tren) ligand.4a These poly(guani-
dine) ligands attain their unique properties from the ability to
delocalize a positive charge over the guanidine moiety, producing
compounds with considerably enhanced basicity and N-nucleo-
philicity. In related poly(imidazolin-2-imine) ligands (Scheme 1),
these characteristics become even more pronounced, since the
imidazolium ring is particularly effective in stabilizing a positive
charge. This results in a considerable transfer of negative charge
to the exocyclic nitrogen at the 2-position of the N-heterocycle,5
which can be described by the two limiting resonance structures
A and B (Scheme 1), with the contribution of the mesomeric form
B becoming even more pronounced upon metal complexation.
The first example of an ethylene-bridged bis(imidazolin-2-
imine) system of this type was introduced by Kuhn et al.;6 however,
their synthetic protocol is confined to the use of the ligand
Scheme 1 Two limiting mesomeric structures of poly(imidazolin-2-imine)
ligands BLR and TLR (A and B).
precursor 2-imino-1,3-dimethylimidazoline, obtained by a multi-
step protocol from 2-aminoimidazole.7 Recently, we introduced a
novel and versatile method for the preparation of imidazolin-2-
imine ligands employing imidazolin-2-ylidenes as readily available
starting materials.5b These imines are valuable ligands in their own
right,8 and are also useful building blocks for the design and
preparation of novel multidentate poly(imidazolin-2-imine) lig-
ands. For instance, we showed that copper(I) complexes of the 2,6-
bis(imidazolin-2-imino)pyridine pincer ligand TLtBu (Scheme 1)
are highly reactive and exhibit a pronounced tendency to form sta-
ble, square-planar copper(II) complexes, allowing effective aerobic
CO2 fixation, C–Cl bond activation and Cu(I) disproportionation.9
Another confirmation of the pronounced electron-donor proper-
ties of the imidazoline-2-imine ligands comes from 16-electron
ruthenium and molybdenum half-sandwich complexes supported
aInstitut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t
Carolo-Wilhelmina zu Braunschweig, Hagenring 30, D-38106, Braun-
schweig, Germany. E-mail: m.tamm@tu-bs.de; Fax: +49-531-391-5387
bDepartment of Chemistry and Center for Metals in Biocatalysis, University
of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455, USA
† CCDC reference numbers 663318–663320. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b715590e
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by ethylene-bridged bis(imidazolin-2-imine) ligands (BLR).10 The
unusual stability of these coordinatively unsaturated complexes
can be ascribed to the strongly p-basic nature of the novel bidentate
ligands, leading to a weak propensity of the metal centers to
coordinate other p-basic ligands such as chloride. In contrast,
strong binding of r-donor/p-acceptor ligands such as CO or
isocyanides was observed.10
N-Functionalized ligands have been widely used to mimic
active sites of copper-containing enzymes and to study dioxygen
activation, leading inter alia to the stabilization of Cu2O2 moieties
2
2
as bis(l-oxo)dicopper(III) (O-core) or l-g :g -peroxodicopper(II)
cores (P-core), depending on the type of substituents and solvents
used.11 Employment of poly(imidazolin-2-imine) ligands with
enhanced electron donor abilities should help to stabilize copper in
higher oxidation states, which might predominantly lead to the for-
mation of the O-core. To address this question, we present herein
the isolation of copper(I) complexes of the ligand 1,2-bis(1,3-
diisopropyl-4,5-dimethylimidazolin-2-imino)ethane, BLiPr, along
with an initial investigation of their reactivity towards dioxygen.
Since copper(I) halide complexes with related chelating N-donor
ligands have been used as efficient catalysts in atom transfer radical
polymerizations (ATRP),12,13 the application of BLiPr copper(I)
systems in the ATRP of styrene is also reported.
Results and discussion
Scheme 2 Synthesis of Cu(I) diimine complexes for reaction with
dioxygen.
Synthesis and cyclic voltammetry of [(BLiPr)Cu]PF6, [1]PF6
The synthesis of 1,2-bis(1,3-diisopropyl-4,5-dimethylimidazolin-
2-imino)ethane, BLiPr, was achieved starting from 1,3-diisopropyl-
4,5-dimethylimidazolin-2-imine5b as described elsewhere.10a The
reaction of BLiPr with [Cu(CH3CN)4]PF6 in THF followed by
precipitation with diethyl ether affords an off-white, extremely
air-sensitive solid. The 1H and 13C NMR spectra of this material
in acetone-d6 exhibit one set of resonances for the imidazolin-2-
imine moieties, indicating the formation of a symmetric species
1
in solution. Upon coordination of the copper ion, the H NMR
resonances of the bridging ethylene hydrogen atoms are shifted
to lower field, whereas upfield shifts are observed for all other
resonances. Elemental analysis of the solid suggests the empirical
formula [(BLiPr)Cu]PF6, [1]PF6, without solvent incorporation
(Scheme 2).
Fig. 1 Cyclic voltammetry trace for [1]PF6 (0.1 mM in acetonitrile with
0.1 M (TBA)PF6), T = 25 ◦C, against a Ag/AgCl reference electrode (3 M
Numerous efforts to obtain single crystals suitable for X-ray
diffraction analysis led to samples for which only the atom connec-
tivity could be established.14 The results indicated the formation
of a dimeric species, [(BLiPr)2Cu2](PF6)2, with both copper atoms
acquiring an almost linear geometry by coordination of two nitro-
gen atoms from the two different ligands. Several related structures
have been previously reported for analogous propylene-bridged
guanidine copper(I) complexes.3a Fast interconversion between
dimeric and monomeric species in solution is very likely and
probably strongly solvent dependent. Considering the structure of
the isocyanide complex [(BLiPr)Cu(XyNC)]PF6, [4]PF6, described
below, the formation of a tricoordinate [(BLiPr)Cu(NCCH3)]+
complex seems probable in acetonitrile solution. Very low redox
potentials were observed in cyclic voltammograms of [1]PF6 in
acetonitrile at room temperature, consistent with the powerful
electron donating capability of the supporting ligand (Fig. 1).3a,4a
Two quasireversible one-electron oxidations were observed, with
KCl); the ferrocene/ferrocenium couple displays a redox potential E◦
+476 mV, and the values are referenced to E◦ (Fc/Fc+) = 0 mV.
=
the process at E1/2 = −1.239 V assigned to the Cu(I)/Cu(II) couple
and the process at E1/2 = +0.128 V to the Cu(II)/Cu(III) couple,
both with respect to the ferrocene/ferrocenium couple. Cyclic
voltammetry at different scan rates (20 mV s−1 < m < 200 mV s−1)
evidences processes that are reversible in acetonitrile (Fig. 1).
Dioxygen reactivity
Solutions of [(BLiPr)Cu]PF6 are exceedingly sensitive to air. To
achieve higher solution concentrations in the study of dioxygen
reactivity, the more soluble Cu(I) complex [(BLiPr)Cu]SbF6 was
prepared in an analogous manner to [1]PF6. Exposure of THF◦or
acetone solutions of [(BLiPr)Cu]SbF6 to dry dioxygen at −80 C
results in a color change from pale yellow to intense purple
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(Scheme 2). The reaction progress was monitored by UV/Vis
absorption spectroscopy (Fig. 2). Oxygenation is irreversible in
acetone, as evidenced by retention of the UV/Vis profile after
extensive purging of the solution with argon. The solution changes
color to red-brown upon warming, with loss of the UV/Vis fea-
tures, indicating that the purple oxygenated product is a thermally
sensitive reaction intermediate. The Cu/O2 intermediate, formu-
lated as a bis(l-oxo)dicopper(III) complex [2](SbF6)2 (vide infra),
exhibits a UV/Vis profile with strong absorptions at 400 nm (e ∼
15 400 M−1 cm−1 per Cu2 complex) and 585 nm (e ∼ 16 900 M−1
cm−1 per Cu2 complex), with a weaker band at ∼ 760 nm (e ∼
1700 M−1 cm−1 per Cu2 complex). The high intensities of the
dominant absorptions are reminiscent of those observed for com-
plexes with 2 : 1 Cu/O2 cores, i.e. l-1,2-peroxodicopper(II), l-g :g -
peroxodicopper(II) and bis(l-oxo)dicopper(III) complexes.15,16 The
presence of two intense absorption features is characteristic of the
latter (kmax ∼ 300–330 nm, e ∼ 10 000 to 20 000 M−1 cm−1 and kmax
∼ 400–450 nm, e ∼ 10 000 to 30 000 M−1 cm−1),15 while transitions
for peroxo species are typically at more remote energies relative
to the UV/Vis profile here. However, the energies of the intense
absorptions are offset from the typical ranges for bis(l-oxo) com-
plexes, with the low energy feature (585 nm) not conforming with
reported observations for this core (400–450 nm). As such, we are
limited to proposing that the 400 and 585 nm absorptions are due
to the oxo → Cu(III) ligand-to-metal charge transitions (LMCT)
of a [Cu2(l-O)2]2+ core on the basis of the dual signature of the
intense bands and the appearance of the higher energy feature in
the characteristic range reported for the lower energy feature of
bis(l-oxo) complexes supported by neutral N-donor ligands.16c
shifts to 599 cm−1 upon 18O-substitution (Fig 3). The frequency
and isotopic shift (26 cm−1) are consistent with the assignment as
an Ag symmetry [Cu2(l-O)2]2+ core vibration of [{(BLiPr)Cu}2(l-
O)2](SbF6)2 on the basis of analogy with reported examples of
bis(l-oxo)dicopper complexes.17 No O-isotope sensitive vibrations
were observed in the ∼ 700–850 cm−1 range, precluding assignment
of the reaction intermediate as a peroxo–dicopper(II) species
and indicating little or no formation of the isomeric l-g :g -
peroxodicopper(II) complex. A second peak of much reduced
intensity and with the same isotopic shift (26 cm−1) was observed at
581 cm−1. This phenomenon has been observed for a mixed-metal
bis(oxo) complex,18 and could arise from coupling of the Cu2O2
core mode with other molecular vibrations or from the presence
of a second bis(l-oxo) species. The latter could conceivably arise
from the formation of a dimer of dimers in which each ligand is
coordinated to two copper centers involved in neighboring [Cu2(l-
O)2]2+ cores.
2
2
2
2
Fig. 3 Resonance Raman spectra of the product of oxygenation of
[(BLiPr)Cu]SbF6 (∼20 mM) using 16O2 (dashed line) and 18O2 (solid line)
in acetone, 514.5 nm excitation. Apart from the solvent-derived peak at
535 cm−1, all peaks are due to the copper complexes.
Allowing the oxygenated acetone solution of [1]PF6 to warm to
room temperature resulted in a change of color and conversion to
a magnetically coupled dicopper(II,II) species (Scheme 2). Brown
crystals of the bis(l-hydroxo)dicopper(II,II) complex [3](PF6)2 were
isolated from the decomposed solution and were structurally
characterized by X-ray crystallography. The inversion-symmetric
cation of [3](PF6)2 is depicted in Fig. 4, and is structurally
very similar to the previously reported examples of bis(l-
hydroxo)dicopper(II,II) complexes19 with a typical square-planar
geometry around the copper atoms. Short copper–nitrogen bond
distances are observed (Cu–N1 = 1.916(2) and Cu–N2 = 1.950(2)
◦
Fig. 2 UV/Vis spectra (−80 C) of [(BLiPr)Cu]SbF6 (dashed line) and
its oxygenated product [2](SbF6)2 (solid line) in acetone (absorptivity
presented per Cu).
To confirm the identity of the 2 : 1 Cu/O2 core and elicit whether
2
2
any of the isomeric l-g :g -peroxodicopper(II) form exists in
equilibrium with the postulated bis(l-oxo) complex, we collected
resonance Raman spectra for frozen acetone solutions of the
oxygenated intermediate with an excitation energy of 514.5 nm
using natural abundance and 18O-enriched dioxygen. A series of
peaks in the ∼ 540–650 cm−1 range were observed for oxygenation
of a ∼ 5 mM sample of [(BLiPr)Cu]SbF6. The O-isotope sensitive
peaks were distinguished from ligand vibrations by increasing the
Cu(I) concentration to ∼ 20 mM, with the resulting spectrum
exhibiting enhanced intensity of a strong peak at 625 cm−1 that
˚
A), whereas the copper–oxygen bond lengths fall in the expected
˚
range (Cu–O = 1.932(2) and Cu–OA = 1.930(2) A).
Syntheses and structures of Cu(I)–isocyanide and –chloride
complexes
The study of isocyanide binding to metal centers is a use-
ful approach to explore bonding and structure, and has also
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C2v symmetry in solution. The IR spectrum of [4]PF6 exhibits a
strong C≡N stretching vibration at 2125 cm−1, slightly shifted to
higher wavenumbers with respect to the free isocyanide.20 This
observation is in agreement with negligible p-back-donation from
copper(I) to the isocyanide ligand.21
Yellow crystals suitable for X-ray diffraction analysis were
grown by slow diffusion of n-hexane into a THF solution of
[4]PF6. The molecular structure of the cation in [4]PF6 (Fig. 5)
shows a planar coordination geometry at the copper atom, which
is very similar to the one observed for b-diketiminato Cu(I)
isocyanide adducts.22 The coordination geometry is distorted
from trigonal (D = 12.7◦, where D = (N1–Cu–C25) − (N2–Cu–
C25)), which is not uncommon for Cu(I) complexes and might be
attributed to packing effects. The Cu(I) ion lies slightly outside
˚
the N2C coordination plane (by 0.04 A), and the Cu–N distances
Fig.
4 Representation of the structure of the cation portion of
˚
(1.977(3) and 1.957(3) A) fall in the expected range compared
to other (N,N)Cu(I)–isocyanide complexes.23 A remarkably short
[3](PF6)2, with non-hydrogen atoms as 50% thermal ellipsoids. Selected
◦
˚
bond lengths [A] and angles [ ]: Cu–N1 1.916(2), Cu–N2 1.950(2),
Cu–O 1.932(2), Cu–OA 1.930(2), Cu · · · CuA 3.0487(6), N1–C3 1.342(3),
N2–C14 1.340(3); N1–Cu–N2 84.52(8), O–Cu–OA 75.72(8), Cu–O–CuA
104.28(8), N1–Cu–O 100.78(8), N2–Cu–OA 99.32(8). Atoms labeled “A”
are symmetry-generated (operator 1 − x, 1 − y, 1 − z).
˚
Cu–C(isocyanide) bond is observed (1.814(4) A), which is identical
with the distance observed for a neutral b-diketiminato Cu(I)
complex containing coordinated XyNC.22b
been exploited to stabilize reactive copper(I) complexes for X-
ray crystallographic analysis. For this purpose, the isocyanide
adduct [(BLiPr)Cu(XyNC)]PF6, [4]PF6, was synthesized by react-
ing 2,6-dimethylphenyl isocyanide (XyNC) and [1]PF6 in THF
(Scheme 3). In the 1H and 13C NMR spectra, coordination of the
isocyanide is clearly indicated by the observation of the expected
isocyanide resonances together with upfield shifted resonances for
the BLiPr ligand in comparison to [1]PF6. The complex displays
Fig. 5 Representation of the structure of the cation in [4]PF6, with
non-hydrogen atoms as 50% thermal ellipsoids. Selected bond lengths
◦
˚
[A] and angles [ ]: Cu–N1 1.977(3), Cu–N2 1.957(3), Cu–C25 1.814(4),
N1–C1 1.319(4), N4–C14 1.328(4), C25–N7 1.146(4); N1–Cu–C25
130.6(1), N2–Cu–C25 143.3(1), N1–Cu–N4 86.0(1), Cu–C25–N7 175.8(3),
C25–N7–C26 167.1(4).
Although tricoordinate copper(I) complexes are well
established,24 only a limited number of such low-coordinate
copper(I) chlorides with chelating N-donor ligands have been
structurally characterized.25,26 BLiPr reacts readily with CuCl in
THF to give the highly air sensitive yellow compound [(BLiPr)-
CuCl] (5) (Scheme 3). In the 1H NMR spectrum, a downfield shift
is observed for the resonances of the bridging ethylene hydrogen
atoms, whereas all other resonances are shifted to higher field in
comparison to the free ligand. The coordinated chloride ligand
f
f
can be easily removed by reaction of 5 with NaBAr4 [BAr4
=
B{3,5-C6H3(CF3)2}4] in acetone at room temperature, resulting in
f
the formation of NaCl and the cationic complex [(BLiPr)Cu]BAr4 .
The H and 13C NMR spectra of this species are consistent with
1
the spectra observed for the cationic complex [1]PF6 obtained
by reaction of BLiPr with [Cu(CH3CN)4]PF6, suggesting only
dissimilarity in the counterion.
Scheme 3 Synthesis of Cu(I)–isocyanide and –chloride complexes.
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Diffusion of n-hexane into a THF solution of 5 yielded the
product as yellow crystals, which were characterized by X-ray
diffraction analysis. The molecular structure of 5 (Fig. 6) displays
a trigonal coordination geometry at the copper center (Cu situated
˚
0.08 A out of the N2Cl coordination plane), with typical Cu–N
and Cu–Cl bond lengths.26 The coordination geometry is distorted
from trigonal planar (D = 16.3◦), which could be associated with
the short contact observed between the copper and the hydrogen
atom from one of the isopropyl groups. The C22–H22 · · · Cu
angle of 150◦ and the Cu · · · H22 distance of 2.32 A (∼ 0.3 A
˚
˚
˚
shorter than sum of the van der Waals radii, rvdW(H) = 1.20 A,
r
vdW(Cu) = 1.40 A)27 indicate that the contact can be considered as
˚
an intramolecular hydrogen bond of the X–H · · · M type.28,29 For
comparison, the shortest C–H · · · Cu contacts in [3](PF6)2 and
˚
[4]PF6 are 2.88 and 2.60 A, respectively.
Fig. 6 Representation of the structure of 5, with non-hydrogen atoms as
◦
˚
50% thermal ellipsoids. Selected bond lengths [A] and angles [ ]: Cu–N1
2.058(1), Cu–N2 2.002(1), Cu–Cl 2.1590(4), N1–C3 1.324(2), N2–C14
1.313(2); N1–Cu–Cl 128.98(3), N2–Cu–Cl 145.24(3), N1–Cu–N2 85.22(5).
Fig. 7 Kinetics of the styrene polymerization (top) and evolution of
molecular weight with monomer conversion at 110 ◦C (bottom): toluene
5 mL, [styrene]0 = 1.45 M, [1-PECl]0 = [(BLiPr)CuCl]0 = 0.0145 M.
Atom transfer radical polymerization of styrene
Conclusions
Over the past few years, copper(I) halide complexes with nitrogen-
based ligands have attracted much attention because of their
catalytic use in atom transfer radical polymerization (ATRP).12,13
Therefore a preliminary study of [(BLiPr)CuCl] activity as a styrene
ATRP catalyst has been performed. 1-Phenylethyl chloride (1-
PECl, one molar equivalent relative to the catalyst) was used
as an initiator, the catalyst to styrene ratio was 1 : 100, toluene
was used as a solvent, and the reaction mixture was heated
at 110 ◦C. Compound 5 was active under these conditions,
and the monomer conversions were periodically determined by
gravimetry. Fig. 7 shows the kinetics of the styrene polymerization,
and the linear logarithmic kinetic plot of ln([M]0/[M]) versus
polymerization time indicates that the concentration of growing
radicals is constant. The experimental molecular weight Mn
increases with monomer conversion (Fig. 7); the polymer samples
have Mn values smaller than 30 000 g mol−1 compared to the
PS standard and relatively low polydispersities (Mw/Mn = 1.14–
1.59). The 1H NMR spectra of the polystyrene samples in CDCl3
exhibit the expected phenyl group signals in the aromatic area
and CH and CH2 chain signals in the range 1.2–2.0 ppm. The
absence of a proton signal around 4.5 ppm, which is usually
attributed to the methine proton geminal to halogen,30 implies
that no chloride chain end group was detected in the polymer. The
Mayo mechanism for the initiation of the thermal polymerization
of styrene31 could also be excluded since the peak at 3.9 ppm,
which is always assigned to the protons in oligomers produced by
self-initiation, could not be observed.
The copper(I) complexes of the bis(imidazolin-2-imine) ligand
BLiPr presented in this paper provide further evidence for the
strong electron releasing ability of this new class of multidentate
guanidine-type ligands. This behaviour is reflected by the obser-
vation of a bis(l-oxo)dicopper(III) core upon oxygenation at low
temperature, as indicated by resonance Raman spectroscopy. The
strong offset of the UV/Vis bands from the typical ranges for bis(l-
oxo) complexes, which is likely to be a result of the unusual ligand
properties, warrants further investigation. The ease of oxidation of
these copper(I) imine complexes also makes these systems useful
catalysts for atom transfer radical polymerizations (ATRP).
Experimental
General considerations
All operations were performed in an atmosphere of dry argon us-
ing Schlenk and vacuum techniques. All solvents were purified by
standard methods and distilled prior to use. Acetone was vacuum
˚
transferred from CaSO4 before drying over activated 3 A molecular
sieves (3 cycles) immediately prior to use. All Cu(I) complexes
were prepared and stored in a glove-box under a dry argon or
nitrogen atmosphere or were manipulated using standard inert
atmosphere vacuum and Schlenk techniques. Copper(I) chloride
was obtained from Acros and [Cu(CH3CN)4]PF6 was obtained
from Strem chemicals and used as received. 1,3-Diisopropyl-
4,5-dimethylimidazolin-2-imine,10a was prepared according to a
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literature procedure. Styrene was dried over CaSO4 and freshly
distilled before use under reduced pressure at 25 ◦C. Labeled
dioxygen (18O2) was purchased from Icon Isotopes, Inc.
and 1.51 (24 H, d, CHCH3); dC (50.32 MHz; acetone-d6) 156.1
(NCN), 121.9 (NCCH3), 61.3 (CH2), 50.3 (NCH), 22.6 (CHCH3)
and 10.3 (CCH3); Found: C, 45.82; H, 6.77; N, 13.22. Calc. for
C24H44N6CuPF6: C, 46.11; H, 7.09; N, 13.44%.
Physical methods
[(BLiPr)Cu]SbF6, [1]SbF6
1H and 13C NMR spectra were recorded on Bruker DPX 200,
Bruker DPX 400 and Bruker AV 600 MHz devices. The chemical
shifts are given in ppm relative to TMS. The spin coupling patterns
are indicated as s (singlet), d (doublet), m (multiplet), sept (septet)
and br (broad, for unresolved signals). Elemental analysis (C, H,
N) was performed on an Elementar Vario EL III CHNS elemental
analyzer; IR-spectra were collected on a Bruker Vertex 70. Mass
spectrometry was performed with a Finnigan MAT 90 device.
UV/Vis spectra were recorded on an HP8453 (190–1100 nm) diode
array spectrophotometer. Low-temperature spectra were acquired
using a Unisoku cryostat-controlled UV/Vis cell holder. Analyte
solutions were placed in anaerobic cuvettes sealed with rubber
septa and secured with parafilm. Oxygenations were carried out
by bubbling dry O2 gas through the analyte solution at ambient
pressure. Excess O2 was removed by purging the resulting solution
with dry Ar gas. Samples for resonance Raman spectroscopy
were formed by creating a headspace of 16O2 over a solution of
the Cu(I) complex in a Schlenk flask at −80 ◦C or by adding
∼10 mL of 18O2 to a frozen solution of the Cu(I) complex
(−196 ◦C) and reacting the mixture at −80 ◦C. Solutions were
prepared in acetone with Cu(I) complex concentrations of ∼ 5 mM
and ∼ 20 mM. Resonance Raman spectra were collected on an
Acton AM-506 spectrometer using a Princeton Instruments liquid
N2 cooled (LN1100-PB) CCD detector and ST-1385 controller
interfaced with Winspec software. A Spectra-Physics BeamLok
2065–7S Ar laser was used to provide excitation at 514.5 nm.
The spectra were obtained at −196 ◦C using a backscattering
geometry; samples were frozen in a copper cup attached to a
liquid nitrogen cooled coldfinger. The spectral window was cali-
brated with liquid indene and spectra were internally referenced
to solvent. Baseline corrections (polynomial fits) were carried
out using Grams32/Spectral Notebase Versions 4.04 (Galactic).
Cyclic voltammetry was measured on a Metrohm lAutolab Type
III device against a Ag/AgCl reference electrode (3 M KCl),
complex concentration was 0.1 mM in acetonitrile, (TBA)PF6
Compound [1]SbF6 was synthesized using the same procedure
as described for [1]PF6 and formation was confirmed by H and
1
13C NMR spectroscopy. dH (200 MHz; acetone-d6) 5.13 (4 H,
sept, NCH), 3.42 (4 H, s, CH2), 2.27 (12 H, s, CCH3) and 1.52
(24 H, d, CHCH3); dC (50.32 MHz; acetone-d6) 156.1 (NCN),
121.9 (NCCH3), 61.3 (CH2), 50.3 (NCH), 22.6 (CHCH3) and 10.3
(CCH3).
[{(BLiPr)Cu}2(l-O)2](SbF6)2, [2](SbF6)2
A 1 mM solution of [1]SbF6 in acetone was oxygenated at −80 ◦C
by bubbling dry O2 through the solution for 5 min. The color
changed from yellow to purple, indicating formation of [2](SbF6)2.
Excess O2 was removed by bubbling dry Ar through the solution
◦
and the UV/Vis spectra were collected at −80 C. kmax(acetone,
◦
−80 C)/nm 400 (e/M−1 cm−1 15 400 per Cu2 complex), 585
(16 900 per Cu2 complex) and 760 (1700 per Cu2 complex). The
same UV/Vis profile was observed upon oxygenation of [1]PF6 in
acetone at −80 ◦C; kmax(acetone, −80 ◦C)/nm 397 and 581.
[{(BLiPr)Cu}2(l-OH)2](PF6)2, [3](PF6)2
A purple solution of [2](PF6)2 was warmed to room temper-
ature causing development of a brown color. Brown crystals
of [3](PF6)2 were formed at room temperature and isolated
from the acetone solution. Found: C, 45.82; H, 7.38; N, 13.06.
Calc. for C48H90N12Cu2O2P2F12: C, 44.89; H, 7.06; N, 13.09%;
kmax(acetone)/nm 370 (e/M−1 cm−1 1820, sh, per Cu2 complex)
and 629 (125, sh, per Cu2 complex); mmax(ATR)/cm−1 3500–3200
(br, O–H), 2984, 1634, 1583, 1378, 1197, 836, 686.
[(BLiPr)Cu(XyNC)]PF6, [4]PF6
2,6-Dimethylphenyl isocyanide (13.1 mg, 0.1 mmol) and [1]PF6
(62.5 mg, 0.1 mmol) were mixed and dissolved in THF (1 mL).
Diffusion of n-hexane into THF solution over several days yielded
pale-yellow crystals, which were isolated, washed with additional
n-hexane (2 × 5 mL) and dried in vacuo. Yield: 58 mg (77%). dH
(200 MHz; acetone-d6) 7.20–7.37 (3 H, mult, Xy-H), 5.07 (4 H,
sept, NCH), 3.40 (4 H, s, CH2), 2.22 (12 H, s, CCH3) and 1.48 (24
H, d, CHCH3); dC (50.32 MHz; acetone-d6) 157.5 (NCN), 136.6
(o-C) 131.4 (p-C), 130.1 (m-C), 120.5 (NCCH3), 57.5 (CH2), 49.5
(NCH), 22.8 (CHCH3), 19.8 (o-CH3) and 11.5 (CCH3); Found: C,
52.02; H, 6.41; N, 12.24. Calc. for C33H53N7CuPF6: C, 52.41; H,
7.06; N, 12.96%; mmax(ATR)/cm−1 2125 (C≡N).
◦
(0.1 M), T = 25 C. The Fc+/Fc couple displays a reversible
cyclic voltammetric trace with a redox potential E◦ = +476 mV
under these conditions. Molecular weight determinations were
carried out with the following set-up: THF-GPC device; columns-
PL-Gel-Mixed-C (5 lm); detectors-RI, UV and light scattering
calibration with polystyrene.
[(BLiPr)Cu]PF6, [1]PF6
A solution of BLiPr (140 mg, 0.336 mmol) in THF (5 mL) was
added dropwise to a solution of [Cu(CH3CN)4]PF6 (122 mg,
0.326 mmol) in THF (5 mL) at room temperature. Upon addition
of the ligand, a white precipitate formed; the suspension was
stirred for 3 h. The product was fully precipitated upon addition
of Et2O (20 mL) and n-hexane (30 mL). Filtration, washing with
n-hexane (2 × 10 mL) and drying in vacuo afforded the product as
an off-white solid. Yield: 198 mg (97%). dH (200 MHz; acetone-d6)
5.12 (4 H, sept, NCH), 3.41 (4 H, s, CH2), 2.27 (12 H, s, CCH3)
[(BLiPr)CuCl], 5
A solution of BLiPr (175 mg, 0.42 mmol) in THF (10 mL) was
added dropwise to a suspension of CuCl (39 mg, 0.394 mmol) in
THF (5 mL) at room temperature. During addition of the ligand
the CuCl dissolved. The yellow solution was stirred for 1 h followed
by reduction of THF volume to 5 mL. Diffusion of n-hexane over
several days yielded the product in the form of yellow crystals,
892 | Dalton Trans., 2008, 887–894
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Table 1 Summary of crystallographic data for [3](PF6)2, [4]PF6 and 5
Complex
[3](PF6)2
[4]PF6
5
Formula
M
C48H90Cu2F12O2P2
1284.34
0.20 × 0.15 × 0.02
Monoclinic
P21/n
8.7395(6)
25.0543(14)
13.6929(8)
98.799(4)
2962.9(3)
2
C33H53CuF6N7P
756.33
0.22 × 0.20 × 0.06
Monoclinic
P21/n
13.0257(14)
16.6650(18)
17.4258(19)
96.982(2)
3754.6(7)
4
C24H44ClCuN6
515.64
0.33 × 0.28 × 0.15
Monoclinic
P21/n
16.7423(12)
9.2407(6)
18.3128(14)
107.664(3)
2699.6
Crystal size/mm
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
V/A
3
˚
Z
4
D
calc/g cm−3
1.440
0.86
1348
100
1.338
0.69
1592
133
1.269
0.93
1104
133
l/mm−1
F(000)
T/K
2hmax
◦
/
55.8
52.7
61
Refl. measured
50894
7050
1.01
0.0444
0.041
0.107
32833
7680
1.07
0.1113
0.065
0.145
56246
8246
1.10
0.0316
0.032
0.088
Refl. indep.
GOF
Rint
R1
wR2
which were isolated, washed with additional n-hexane (2 × 5 mL)
and dried in vacuo. Yield: 151 mg (74%). dH (200 MHz; acetone-d6)
5.09 (4 H, sept, NCH), 3.36 (4 H, s, CH2), 2.12 (12 H, s, CCH3)
and 1.36 (24 H, d, CHCH3); dC (50.32 MHz, acetone-d6) 156.6
(NCN), 119.4 (NCCH3), 57.7 (CH2), 48.7 (NCH), 22.9 (CHCH3)
and 11.8 (CCH3); Found: C, 55.82; H, 8.68; N, 15.75. Calc. for
C24H44N6CuCl: C, 55.90; H, 8.60; N, 16.30%; m/z (EI) 514 (M+,
2%), 416 (BLiPr+, 5), 208 (BLiPr/2+, 100).
[4]PF6 the isocyanide ligand displays somewhat high and markedly
anisotropic displacement parameters, corresponding to a slewing
motion. The methyl H atom positions of this ligand, which are in
any case difficult to assign, should therefore be interpreted with
caution.
Acknowledgements
We thank the National Institutes of Health (GM 47365) and
the Deutsche Forschungsgemeinschaft (Ta 189/6-1) for financial
support. We thank Professor Lawrence Que, Jr. at the University
of Minnesota for access to resonance Raman spectroscopy instru-
mentation. We are indebted to Dipl.-Chem. Steffen Harling and
Professor Dr Henning Menzel for GPC measurements. The X-ray
crystal structure determinations performed by Dr Cristian Hrib,
Dr So¨ren Randoll and Dipl.-Chem. Constantin Daniliuc are also
gratefully acknowledged.
Procedure for styrene ATR polymerization
[(BLiPr)CuCl] (45 mg, 87.2 lmol, 1 mol%) was dissolved in toluene
(5 mL) followed by addition of styrene (1 mL, 8.72 mmol)
and 1-phenylethylchloride (11.6 lL, 87.2 lmol). The reaction
mixture was submerged in a preheated oil bath and refluxed
for 20 h. Samples were taken periodically and quenched by
adding to methanol (50 mL). Polystyrene was isolated, dried
and twice purified by dissolution in THF (5 mL), filtration and
precipitation with MeOH (50 mL). Isolated polystyrene samples
were characterized by gel permeation chromatography (GPC), 1H
NMR and elemental analysis. The monomer conversions were
determined gravimetrically based on isolated polystyrene.
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894 | Dalton Trans., 2008, 887–894
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The Royal Society of Chemistry 2008
©