Dioxygen reactivity of new bispidine-copper complexes

Article abstract of DOI:10.1021/ic2019296

The reactivity of copper complexes of three different second-generation bispidine-based ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane; mono- and bis-tetradentate; exclusively tertiary amine donors) with dioxygen [(reversible) binding of dioxygen by copper(I)] is reported. The UV-vis, electrospray ionization mass spectrometry, electron paramagnetic resonance, and vibrational spectra (resonance Raman) of the dioxygen adducts indicate that, depending on the ligand and reaction conditions, several different species (mono- and dinuclear, superoxo, peroxo, and hydroperoxo), partially in equilibrium with each other, are formed. Minor changes in the ligand structure and/or experimental conditions (solvent, temperature, relative concentrations) allow switching between the different forms. With one of the ligands, an end-on peroxodicopper(II) complex and a mononuclear hydroperoxocopper(II) complex could be characterized. With another ligand, reversible dioxygen binding was observed, leading to a metastable superoxocopper(II) complex. The amount of dioxygen involved in the reversible binding to Cu^I was determined quantitatively. The mechanism of dioxygen binding as well as the preference of each of the three ligands for a particular dioxygen adduct is discussed on the basis of a computational (density functional theory) analysis.

Full text of DOI:10.1021/ic2019296

Article
pubs.acs.org/IC
Dioxygen Reactivity of New Bispidine-Copper Complexes
Peter Comba,*,† Christina Haaf,† Stefan Helmle,† Kenneth D. Karlin,*,‡ Shanthi Pandian,†
and Arkadius Waleska†
†Anorganisch-Chemisches Institut, Universitat Heidelberg, INF 270, D-69120 Heidelberg, Germany
̈
‡Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: The reactivity of copper complexes of three different
second-generation bispidine-based ligands (bispidine = 3,7-
diazabicyclo[3.3.1]nonane; mono- and bis-tetradentate; exclusively
tertiary amine donors) with dioxygen [(reversible) binding of dioxygen
by copper(I)] is reported. The UV−vis, electrospray ionization mass
spectrometry, electron paramagnetic resonance, and vibrational spectra
(resonance Raman) of the dioxygen adducts indicate that, depending on
the ligand and reaction conditions, several different species (mono- and
dinuclear, superoxo, peroxo, and hydroperoxo), partially in equilibrium
with each other, are formed. Minor changes in the ligand structure and/
or experimental conditions (solvent, temperature, relative concen-
trations) allow switching between the different forms. With one of the
ligands, an end-on peroxodicopper(II) complex and a mononuclear
hydroperoxocopper(II) complex could be characterized. With another ligand, reversible dioxygen binding was observed, leading
to a metastable superoxocopper(II) complex. The amount of dioxygen involved in the reversible binding to CuI was determined
quantitatively. The mechanism of dioxygen binding as well as the preference of each of the three ligands for a particular dioxygen
adduct is discussed on the basis of a computational (density functional theory) analysis.
(tacn), hydrotripyrazolylborate, tris(ethylamino)ethane (tren),
β-diketiminate ligands, and their derivatives. In particular also
the “superbasic” tetramethylguanidino-substituted amine li-
gands, has led to exciting discoveries, and this has been
reviewed extensively.9,11,12 The transition-metal coordination
chemistry of a large range of tetra-, penta-, and hexadentate
bispidine (3,7-diazabicyclo[3.3.1]nonane) ligands started to
attract the attention of coordination chemists less than a decade
ago, although the first bispidine derivatives were described by
Mannich.13−16 The first-generation bispidine ligands have
mixed aliphatic/aromatic nitrogen-donor sets and enforce
distorted cis-octahedral coordination geometries; the corre-
sponding copper complexes have been shown to provide
interesting enzyme models and efficient catalysts.17−22 Of
particular interest for the copper−dioxygen chemistry is the fact
that the enforced square-pyramidal geometry with an axial
amine and in-plane coordination of the substrate (i.e., the
dioxygen-derived ligand) leads to an unusual stability of the
peroxo complexes,17,18,20,21 specific reactivities19,23 and inter-
esting spectroscopic properties.21,24
INTRODUCTION
■
A broad range of aerobic oxidation reactions are catalyzed by
copper enzymes and low-molecular-weight model complexes. A
number of catalytically relevant mono- and dinuclear [CuO2]n+
and [Cu2O2]2+ species have been identified and thoroughly
studied spectroscopically, structurally, with computational
methods, and in terms of their reactivity. Apart from the
various types of oxygen adducts (dioxygen, superoxo, peroxo,
and oxo) and copper in different oxidation states (CuI, CuII,
and CuIII), it is particularly the [Cu2O2]2+ core that has
attracted much attention, specifically in terms of the various
possible isomers [bis(μ-oxo), μ-η2:η2-peroxo, and trans-μ-1,2-
peroxo].1−9 The suitability of copper for the activation of
dioxygen derives from the favorable redox potentials, tunable
over a broad range by the coordination geometry and donor
sets. A large variety of ligand systems are known and serve as a
valuable basis for biomimetic copper chemistry;4−8,10,11 the
reactivity and stability ranges depend on the geometry enforced
and the donor set provided by the ligand. Structure−activity
correlations have been established and are used to modulate the
properties of the copper center in order to establish mimics for
specific natural systems for electron transfer, dioxygen trans-
port, oxidation, or oxygenation reactivity.
In the recently introduced second-generation bispidine
ligands with pure aliphatic donor sets,16,25 very different, i.e.,
distorted trigonal, structures (trigonal bipyramidal or trigonal
prismatic) are enforced, and this has important consequences
The copper−dioxygen chemistry of a range of bi-, tri-, and
tetradentate amine-, imine-, and pyridine-containing ligands are,
in the area of copper-based oxygen activation, well-established
tris(methylpyridine)amine (tmpa), 1,4,7-triazacyclononane
Received: September 2, 2011
Published: February 14, 2012
© 2012 American Chemical Society
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Scheme 1. Syntheses and Structures of the Ligands L1, L2, and L3
for the electronic properties and therefore also for the complex
reactivity and stability. Three of these new types of bispidine
ligands (L1, L2, and L3 in Scheme 1) and their copper−
dioxygen chemistry are discussed in the present report.
during the synthesis, a significantly more stable CO-substituted
complex is obtained, and this was used for characterization by
1H NMR and IR spectroscopies. Copper(II) complexes of L2
and L3 were obtained in good yield (60−70%) by the reaction
of stoichiometric amounts of the ligands and CuII salts in
MeCN (see the Experimental Section), the corresponding L1-
based copper(II) complex has been fully characterized, and the
X-ray structure has been reported.16,25
RESULTS AND DISCUSSION
■
1. Synthesis of the Ligands and Complexes. The
tetradentate ligand L1 has been described before.16,25 L1 as well
as the tetradentate derivative L2 and the dinucleating bis-
tetradentate ligand L3 are derived from the known piperidine
precursor P1 (Scheme 1);25 formaldehyde and 4-methoxyphe-
nylethaneamine for L2 and m-xylenediamine for L3 are the
“locking groups”, which, after basic extraction with diethyl
ether, produce ligands as pure solids in reasonably good yields.
The copper(I) complexes of L1, L2, and L3 were obtained
from [CuI(CH3CN)4][B(C6F5)4]26 and the ligands in
dioxygen-free tetrahydrofuran (THF), containing several
drops of acetonitrile (MeCN) to stabilize the resulting CuI
cation; n-pentane was used to precipitate the complexes. It was
not possible to isolate and fully characterize the CuIL1 complex
as a solid, even with CO used as an electron-withdrawing
coligand, which might confer extra stability. This is probably
due to the aliphatic nitrogen-donor set and the specific
coordination geometry, which stabilize the oxidized form [the
stability of the corresponding copper(II) complex has been
determined and found to be relatively high].16 In contrast, the
yellow solid of the dinuclear complex [CuI2(L3)][B(C6F5)4)]2
was stable for several days, and it was also characterized in
solution by 1H NMR spectroscopy. A powder of the
mononuclear complex [CuI(L2)][B(C6F5)4], obtained by the
addition of n-pentane to the reaction mixture, shows the first
signs of decomposition after a few hours. In a CO atmosphere
2. Solution Properties of the Copper Compounds. A
1
slight broadening of the signals in the H NMR spectra of
[CuI(L2)(CO)][B(C6F5)4] and [CuI2(L3)][B(C6F5)4]2 prob-
ably results from slow oxidation of the CuI complexes in
solution. As expected from the rigid ligand cavity and earlier
structural analyses,16,25 no isomerism in the ligand binding
mode is apparent.21 The vibrational frequency of the carbonyl
group in the IR spectrum of [CuI(L2)(CO)][B(C6F5)4] is at
2100 cm−1, i.e., in the expected range for end-on coordination
of CO to CuI.27,28
The structures of first-row transition-metal complexes with
tetra- and pentadentate second-generation bispidine complexes
are best described as distorted trigonal bipyramidal (tbp).16,25
Because of the relatively small N−Cu−N angle, enforced by the
2
2
diazaheptane cycle, the copper(II) complexes have a “dx −y
2
2
ground state” (i.e., the unpaired electron is in dx −y ; in axial
2
tbp symmetry, the unpaired electron instead is in dz ), and this
is reflected in their electron paramagnetic resonance (EPR)
spectra.29 That of [CuII(L2)(OH2)]2+ is very similar to the
spectrum of the L1-based bispidine-copper(II) complex (see
Table 1). The electronic spectra of [CuII(L2)(OH2)]2+ and
[CuII (L3)(solvent)2]4+ in MeCN have the expected dd
2
transitions at approximately 600 nm; however, the low-energy
transition, due to splitting of the eg set of orbitals (in Oh) in an
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Table 1. EPR Parameters of the Copper(II) Complexes [in
MeCN/Toluene or MeOH, 90 K; X-Band Frequencies
a
(Approximately 9 GHz)]
A∥ [10−4
A⊥ [10−4
complex
g∥
g⊥
cm−1
]
cm−1
]
16
[CuII(L1)(NCCH3)]2+
[CuII(L2)(OH)]+
2.21
2.22
2.08
2.05
170
170
15
15
a
The spin Hamiltonian parameters are determined by spectral
simulation with XSophe (the spectra and simulations are given as
the Supporting Information).
axial field, was not resolved for the L2- and L3-based complexes
(Table 2).
Table 2. UV−Vis−Near-IR Spectral Data of the Copper(II)
Figure 1. Oxygenation of [CuI(L1)][B(C6F5)4] (acetone, T = −80 °C,
2.5 × 10−4 M), recorded within 105 min. Color code: blue (for
comparison), [CuI(L1)]+; black, oxygenated complex. The values for
extinction coefficients are based on 100% conversion.
Complexes at Ambient Temperature in MeCN
λ1 [nm]/ε [l/
(mol·cm)]
λ2 [nm]/ε [l/
(mol·cm)]
complex
16
[CuII(L1)(NCCH3)]2+
[CuII(L2)(OH2)]2+
[CuII2(L3)]4+
903/341
627/684
633/136
599/248
system are different from those usually observed for end-on
peroxodicopper(II) complexes with local CuII tbp coordina-
tion,30,31 and this presumably is due to the unusual
coordination geometry enforced by the second-generation
bispidine ligands (see the description of the structures above).
In fact, such a pattern of charge-transfer (CT) bands with
“reversed intensity ratios” was shown to occur in end-on
peroxodicopper(II) complexes with ligands that favor local axial
Electrochemical measurements in MeCN indicate for
[CuII(L2)(OH2)]2+ an irreversible reduction at −270 mV (vs
SCE, Table 3). This potential is significantly more positive than
CuII symmetry, i.e., with a dx −y ground state.34,35 The end-on
peroxo complex [(CuII(L1))2(O2)]2+, with 618, 520, and 450
nm absorptions, appears to slowly isomerize to another similar
species, suggested to be a conformer (see below). The
isosbestic point at 420 nm indicates a clean monophasic
transformation.
Table 3. Cyclic Voltammetry (CV) Data of the Second-
Generation Bispidinecopper Compounds in MeCN, 0.1 M
(Bu4N)(PF6), E1/2 vs SCE
2
2
complex
E1/2 [mV]
[CuII(L1)(NCMe)]2+
[CuII(L2)(OH2)]2+
[Cu2II(L3)(solvent)2]4+
−377
−270
An intensely green solution is obtained at −120 °C in 2-
methyltetrahydrofuran (MeTHF; see the Supporting Informa-
tion for the spectra). In addition to the two bands at 613 and
515 nm, assigned to trans-[(CuII(L1))2(O2)]2+, there is a strong
additional transition in this solvent at 452 nm, which we assign
to [CuII(L1)(O2)]+, i.e., a mononuclear η1-superoxocopper(II)
complex (a similar spectrum is obtained in THF; see the
Supporting Information). Moreover, there is another new
transition at 340 nm, which is due to a third species, probably a
hydroperoxo complex ([CuII(L1)(OOH)]+; a species with a
similar spectrum is observed in diethyl ether; see the
Supporting Information). This putative hydroperoxo complex
forms relatively slowly (see below for further characterization of
this species).
Computational Analysis. Density functional theory (DFT)
calculations were performed to confirm the existence of the
proposed mono- and dinuclear complexes as oxygenation
products of [CuI(L1)]+. There is a relatively large body of
published computational work, especially on the theoretically
challenging dinuclear systems with Cu2O2 “diamond”
cores,36−46 and much of this has recently been reviewed.9
Apart from the general problem of choosing the appropriate
functional,47 the electron distribution in the “diamond” core as
a function of the ligand-enforced structure clearly varies with
the theoretical method used.9,43,44 All spin states were
considered, and the structural parameters as well as the relative
stabilities are based on the widely tested and used setup
involving the B3LYP functional and a triple-ζ basis set (for
details, see the Experimental Section; the energies reported
−396 (red), −697 (red), −555(ox)
that of [CuII(L1)(NCMe)]2+ (−377 mV) and suggests a higher
stability of the copper(I) complex of L2 compared to that of
L1.29 For the dinuclear complex [CuII (L3)(solvent)2]4+, two
2
reduction peaks and one oxidation peak are observed (see the
Supporting Information for the electrochemical traces). This
suggests an interaction between the two CuI cations in the
dinuclear complex, followed by ready decomposition of the
dicopper(I) complex.
a. Oxygenation Experiments. Oxygenation of the
Copper(I) Complex with L1. Time-Resolved UV−Vis Spec-
troscopy. The tetradentate ligand L1 forms copper(I)
complexes with rich oxygenation reactivity. For the oxygenation
experiments, [CuI(L1)]+ was synthesized in situ30 by the
addition of a [CuI(MeCN)4][B(C6F5)4] solution in a strictly
oxygen-free atmosphere to a solution of L1 in the desired
solvent and with the required concentrations. At low
temperature (−80 °C) in acetone, a deep-blue species is
generated by bubbling molecular oxygen through the in situ
generated complex (see Figure 1); identical spectra are
observed in the concentration range from 2.5 × 10−4 to 2 ×
10−3 M. The spectrum of the oxygenated species (see Figure 1)
is dominated by a band at 618 nm with higher energy shoulders
at about 520 and 450 nm. These features are in the region
where UV−vis transitions of trans-μ-1,2-peroxodicopper(II)
compounds are observed, such as in the well-characterized
30,31
32,33
[(CuII(Me6tren))2(O2)]2+
and [(CuII(tmpa))2(O2)]2+
systems. However, the intensity ratios in the L1-based bispidine
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Table 4. Key Structural Parameters and Spin Densities for Various Oxygenated Copper Complexes Involving the Bispidine
Ligand L1 (Lowest-Energy Conformations and Spin States, Where Appropriate; See the Text)
interatomic distances [Å]
spin densities
O
complex
Cu−O
O−O
Cu−Navg
Cu
O
[(CuII(L1A))2(O2)]2+
[CuII(L1)(O2)]+
[CuII(L1)(O2H)]+
1.938, 1.939
1.970
1.516
1.360
1.530
2.193
2.147
2.403
0.49, −0.49
0.45
0.18
0.17
0.22
−0.18
−0.55
−0.01
1.911
0.50
Figure 2. Orbitals involved in the major transitions of the mononuclear complexes.
include corrections for zero-point energies and solvation). The
electronic transitions for the various complexes were calculated
with ab initio methods and time-dependent DFT (TDDFT),
except for the dinuclear complex, for which, because of the
larger size of the model, only a TDDFT analysis was
performed. Two models were used for the computation of
this complex; (i) the terminal methyl groups of the three
nitrogen donors (see Scheme 1) were replaced by hydrogen
atoms in [(CuII(L1A))2(O2)]2+; (ii) the methyl groups were
modeled to allow possible interactions of the methyl hydrogen
atoms with the O−O bridge of [(CuII(L1))2(O2)]2+. The
optimized structural parameters and spin densities for the
lowest-energy structure of the trans-[(CuII(L1A))2(O2)]2+
complex are listed in Table 4, together with the corresponding
data for the mononuclear superoxo and the hydroperoxo
complexes. All possible spin states and different orientations of
the peroxo and superoxo groups (end-on or side-on) were
considered, and the relative energies and spin densities are
given in the Supporting Information.
differing by 1.8 kJ/mol. TDDFT calculations of the dinuclear
trans-[(CuII(L1A))2(O2)]2+ complex yield peaks of higher
intensity at 580, 530, and 490 nm and a peak with very low
intensity around 670 nm; this is in acceptable agreement with
the experimental absorptions at 620, 520, and 460 nm (see, e.g.,
Figure 1).48 These peaks are characteristic for CT transitions.
According to the DFT calculations, the absorptions at 580 and
530 nm correspond to the usual π*-to-Cu CT transitions
involving the peroxo group and the two CuII centers, and the
lower intensity peak at 490 nm is assigned to be due to ligand-
to-metal CT (LMCT) transitions (see the Supporting
Information).
As mentioned, compared to most other known trans-μ-
peroxodicopper complexes, the experimentally observed
absorption spectrum of trans-[(CuII(L1))2(O2)]2+ is signifi-
cantly different with respect to the intensities, and the TDDFT-
predicted transition energies are not as accurate as one might
have hoped. Therefore, TDDFT calculations were also
performed for the known dinuclear trans-μ-peroxo-Cu-tmpa
complex. That absorption spectrum has a more intense peak at
530 nm and a less intense transition at 604 nm.32,33 The
calculated spectrum of the dinuclear peroxo-Cu-tmpa complex
has the highest intensity peak at 567 nm, in reasonable
agreement with the experimental value, within the limits of the
method used. Further low-intensity signals were found at 550,
530, and 507 nm.
The dinuclear trans-[(CuII(L1A))2(O2)]2+ complex was
modeled with three different configurations of the O−O
bridge, i.e., end-on trans-peroxo, side-on (μ-η2-η2) peroxo, and
bis(μ-oxo). Only the trans-μ-1,2-peroxo (end-on) structure was
found to be stable; the other two collapsed upon optimization
to the stable isomer. Possible spin states for the trans-
[(CuII(L1A))2(O2)]2+ complex were then considered. The
open-shell singlet and triplet states are relatively close in
energy (7.5 kJ/mol in favor of the singlet state). This indicates
that the dicopper(II) complex has a diamagnetic ground state,
and antiferromagnetic coupling of the two CuII centers is also
evident from their spin densities (see Table 4). Single-point
calculations with a larger basis and inclusion of the solvent (B2)
predict the two spin states to be even closer in energy, i.e., just
The direct reaction of O2 with [Cu(L1)]+ results in a
mononuclear superoxocopper(II) complex, but a mononuclear
peroxocopper(II) complex may also emerge from further
reactions (see also the experimental observations above); the
peroxide and superoxide moieties can be bound in side-on or
end-on orientations. The calculations predict that the superoxo
complex is favored by 8.0 kJ/mol over the peroxo complex.
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Figure 3. rR spectra of the product of oxygenation of [CuI(L1)]+ in MeTHF (left) and acetone (right), −80 °C, c = 22 × 10−3 M, λlaser = 623 nm: full
line, 18O2; dotted line, 16O2.
This is mainly due to the change in the electronic configuration
at the CuII center.49 The side-on configuration of the superoxo
complex of [CuII(L1)]2+ was found to be sterically hindered and
reverted to the end-on complex upon optimization. The relative
energies of the triplet and open-shell singlet states of the end-
on superoxo complex [CuII(L1)(O2)]+ were found to favor the
triplet state by 7.4 kJ/mol (basis set B2). The open-shell singlet
state was calculated using the broken-symmetry approach that
takes nondynamic correlation effects into account with DFT.
TDDFT calculations of the singlet state afforded a peak at 426
nm with the maximum intensity, and this agrees well with the
experimentally found range of ∼410−450 nm (dependent on
the solvent used; see the experimental spectra). This supports
the mononuclear superoxo species as a product of the oxidation
of [CuI(L1)]+, as suggested by the low-temperature time-
dependent UV−vis spectra. In fact, all of these results are in line
with relatively recent findings on structurally related
tetradentate tripodal ligands and corresponding η1-
superoxocopper(II) species formed from their (ligand)CuI/O2
reactions;45,52−54 for example, a triplet S = 1 ground state
emerges from spectroscopic and computational studies45,53 on
the X-ray structurally characterized complex [(tmg3tren)-
CuIIO2]+ (where tmg3tren = 1,1,1-tris[2-[N2-(1,1,3,3-
tetramethylguanidino)]ethyl]amine).52 Also, the prominent
UV−vis spectroscopic features observed for [CuII(L1)(O2)]+
are very much like those known for other established
superoxocopper(II)(ligand) species.8,45,53−55
Because of the relatively small size of the molecules, ab initio
calculations were also carried out to calculate the absorption
bands of the two mononuclear complexes. The absorption
spectrum of the superoxo complex [CuII(L1)(O2)]+ was
calculated with a reference space of (16,9), i.e., with two half-
filled orbitals. The SORCI-predicted spectrum has absorption
maxima at 457 and 470 nm, in good agreement with the
experimentally observed transitions. The absorption at 457 nm
is a dd transition (see Figure 2). The TDDFT-calculated value
of the transition for the mononuclear end-on hydroperoxo
complex [CuII(L1)(OOH)]+ is in the range of ∼360 nm, which
is in good agreement with the experimental spectral range of
340−370 nm. SORCI calculations for this species were
performed with a reference space of (19,10). The ab initio
calculated spectrum shows maximum-intensity peaks at 336 and
415 nm and therefore supports the analysis discussed above.
The major transition involved is a hydroperoxo-to-CuII LMCT
transition; see Figure 2.
Resonance Raman (rR) Spectra of the Oxygenated
Copper(I) Complexes with L1. For further identification of
the oxygenated [CuI(L1)]+ complexes, rR spectra were
recorded (excitation wavelength of 623 nm, MeTHF or
Table 5. Experimental and Calculated rR Transitions of the
Oxygenation Product of [CuI(L1)]+ at −80 °C (Computed
Values Not Scaled; See the Text)
O−O, Cu−O, O−O, Cu−O,
Δ(16,18) Δ(16,18)
16O2
16O2
18O2
18O2
O−O
Cu−O
solvent
[cm−1
]
[cm−1
]
[cm−1
]
[cm−1
]
[cm−1
]
[cm−1
25
]
MeTHF
811,
801
547
766
522
45,
35
acetone
816,
804
550
768
530
48,
36
20
calculated 808,
798
495,
495
acetone, −80 °C; see Figure 3 and Table 5). In MeTHF with
16O2, two bands for the O−O stretching mode are seen at 811
and 801 cm−1, and these shift to one transition at 766 cm−1
with 18O2. In acetone, the corresponding bands are at 816 and
804 cm−1 with 16O2 and at 768 cm−1 with 18O2. The Cu−O
vibration in MeTHF is at 547 cm−1 (16O2) and 522 cm−1
(18O2) and the corresponding transition energies in acetone are
at 550 cm−1 (16O2) and 530 cm−1 (18O2). These energies are as
expected for the O−O and Cu−O modes of trans-μ-1,2-
peroxocopper(II) adducts4 and indicate that the blue species
generated in acetone at −80 °C (see Figure 1) indeed is a trans-
[(CuII(L1))2O2]2+ complex. The presence of two signals for the
O−O bridge in natural abundance dioxygen might be the result
of torsional isomers. DFT calculations of different torsional
isomers support this assumption, and the relative energies are
found to vary in a range of approximately 20 kJ/mol. The
lowest-energy isomer is shown in Figure 4 and has a dihedral
angle of 180°, involving the N7a−Cua−Cub−N7b centers. This
torsional angle was varied to 0°, 30°, 60°, and 90° to find the
relative energies of the various conformations. The Cu−O and
O−O stretching frequencies, calculated for the two lowest-
energy structures (180° and 120°), are presented along with
the experimental values in Table 5. The O−O stretching
frequencies are in reasonable agreement with the experimental
frequencies, but there is some discrepancy with the Cu−O
band. Another possible interpretation of the double peaks with
the 16O2 and one peak with the 18O2 peroxo complex,
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oxygen activation chemistry. At least three oxygenated species
are present, and the reaction pathway depends on the solvent,
the relative concentrations, and the temperature (see Scheme
2).
From the spectroscopic data (UV−vis, EPR, and rR) and in
combination with the computational characterization, we
conclude that at very low temperature and in low
concentrations, first an end-on superoxo complex [CuII(L1)-
(O2)]+ is formed. In acetone, diethyl ether, or MeTHF at −80
°C, this reacts in a very fast process to trans-
[(CuII(L1))2(O2)]2+, and this is a common pathway.4 In
THF, the superoxo complex [CuII(L1)(O2)]+ is longer-lived,
and it is also stabilized in MeTHF at −120 °C. The dinuclear
trans-peroxo complex [(CuII(L1))2(O2)]2+ (see Figure 1;
acetone solution) decays slowly. In diethyl ether and
MeTHF, the trans-[(CuII(L1))2(O2)] species is converted to
a mononuclear hydroperoxo complex, [CuII(L1)(OOH)]+,
which also can be prepared from [CuII(L1)]2+ and H2O2.
Reaction mixtures studied were warmed to ambient temper-
ature, and from those solutions, electrospray ionization mass
spectrometry (ESI-MS) spectra were recorded, which reveal
[CuII(L1)]2+ as the main decomposition fragment, and only
traces of species with an oxygenated ligand backbone.
Figure 4. Lowest-energy isomer of the dinuclear trans-
[CuII(L1))2(O2)]2+ complex (hydrogen atoms omitted for clarity;
Cu−O = 1.94 Å; O−O = 1.52 Å; see the Supporting Information for
details).
independent of the solvent, is the occurrence of a Fermi
resonance.45,56−58
EPR Spectra of the Oxygenated Copper(I) Complexes with
L1. The dinuclear trans-[(CuII(L1))2(O2)]2+ complex is
antiferromagnetically coupled (see also the DFT analysis
above), and the mononuclear superoxo species [CuII(L1)(O2)]+
has a triplet ground state. This also emerges from the calculated
spin densities of these complexes (see Table 4; there is an
energy difference of 11.4 kJ/mol in favor of the triplet relative
to the open-shell singlet state for the superoxo complex
[CuII(L1)(O2)]+). Therefore, these complexes are not expected
to show EPR transitions at liquid-nitrogen temperatures,53 and
indeed frozen solutions of the oxygenated complex in acetone
and THF are EPR-silent. However, a frozen solution (−80 °C)
of the oxygenated [CuI(L1)]+ complex in MeTHF, where the
UV−vis spectra suggested the presence of a mononuclear
hydroperoxo complex (see above), shows an EPR signal, with
parameters as expected for a mononuclear hydroperoxo
complex, [CuII(L1)(OOH)]+ (Figure 5 and Table 6; for a
dinuclear complex such as trans-[(CuII(L1))2(O2)]2+, a half-
field signal would also be expected, and this was not detected
here). An additional small signal of a mononuclear copper(II)
species, also visible in Figure 5, is assigned to a minor impurity
of a copper(II) decay product. As expected for a hydroperoxo
complex, the spectrum is well resolved,59 and upon warming,
the solution exhibits the known pattern for [CuII(L1)]2+.16 The
quantum-chemically computed g and A tensor parameters (see
above and the Experimental Section) are in good agreement
with the experimental values for both the hydroperoxo and
MeCN species, which were recorded and computed for
comparison (see Table 6).
b. Oxygenation of the Copper(I) Complex with L2. L2 has a
ligand cavity that is identical with that of L1, but the ligand has a
methoxy-substituted phenyl ring (see Scheme 1) in order to
potentially mimic an enzymatic substrate found in close
proximity to the CuI/O2-derived site. In spite of the seemingly
small change in the ligand structure compared to L1, the
copper(I)−dioxygen chemistry is very different. In view of the
observed redox potentials, this is not entirely unexpected (see
above and Table 3). As the oxygenation product, only one
intensely green species with maxima in the electronic spectra at
ca. 400 and 650 nm was found, independent of the solvent
(acetone, THF, and MeTHF), temperature (−80 to −120 °C),
and concentration (c = 2 × 10−4−2.5 × 10−3 M; see Figure 6).
The UV−vis spectrum is as expected for a mononuclear η1-
superoxo-[CuII(L2)(O2)]+ complex;4,30 see also above. The
clean isosbestic point suggests that this is the only oxygenation
product. This is also supported by the fact that the relative
intensities of the two bands at 664 and 402 nm do not change
[2.1−2.4 (402 nm) to 1 (664 nm)] if the concentration is
varied over the range c = 2 × 10−4−2.5 × 10−3 M.
The superoxocopper(II) oxygenation product is stable at
−80 °C for about 24 h. If warmed to room temperature, the
color changes from intensely green to very light green. Upon
cooling again to −80 °C, the dark-green color is reproduced;
i.e., binding of dioxygen and formation of the mononuclear
Reaction Pathways for Oxygenation of the Copper(I)
Complex with L1. The L1-based copper(I) complex has a rich
Figure 5. EPR spectrum of [CuII(L1)O2H]2+ (left) and [CuII(L1)(NCCH3)]2+ (right); frozen solutions (90 K, MeCN/toluene): continuous line,
experiment; dashed line, X-Sophe64 simulation.
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Table 6. Spin Hamiltonian Parameters of the Experimental EPR Spectra (X-Band Frequencies, X-Sophe64 Simulation; See
Figure 6) (and Calculated Parameters; DFT, See the Text) of [CuII(L1)(O2H)]2+ and [CuII(L1)(NCCH3)]2+
complex
gx
gy
gz
Ax [10−4 cm−1
]
Ay [10−4 cm−1
]
Az [10−4 cm−1
]
[CuII(L1)(O2H)]+
[CuII(L1)(NCCH3)]2+
2.034 (2.026)
2.08 (2.06)
2.074 (2.065)
2.232 (2.137)
2.21 (2.27)
31 (27)
15 (30)
33 (33)
162 (168)
170 (172)
gy = gx
Ay = Ax
Scheme 2. Reaction Pathways for Oxygenation of [CuI(L1)]+
Figure 7. Cycles of [CuII(L2)(O2)]+ (acetone, saturated with O2; −80
°C; c = 5.0 × 10−4 M) with its decay product [CuI(L2)]+, as a function
of the temperature (closed vessel, absorption at λ = 402 nm (see the
text). The cycles involve cooling to −80 °C (maximum absorption at
402 nm) and warming to ambient temperature within approximately
30 min (minimum absorption at 402 nm).
Figure 6. Decay of [CuI(L2)(O2)]+ (acetone, saturated with O2; −80
°C; c = 5.0 × 10−4 M), recorded over 10 min while warming up to
ambient temperature. Color code: blue (for comparison), [CuI(L2)]+;
black, oxygenated complex. The values for the extinction coefficients
are based on 100% conversion of [CuI(L2)]+ + O2 ⇌ [CuI(L2)(O2)]+.
Figure 8. Half-life (t1/2) ln(At/A0) vs t [s], where A0 = absorption at t
= 0 s and At = absorption at t, of [CuII(L2)(O2)]+ at −35 °C; THF, c =
5 × 10−4 M; λ = 408 nm.
superoxo complex (temperature-dependent equilibrium) is to a
large extent reversible (see Figure 7), and this behavior appears
to be the first such example thus far reported. The amount of
dioxygen liberated upon warming of the reaction mixture was
determined quantitatively with pyrogallol as the indicator (see
the Experimental Section):60 in a solution of 1.0 × 10−3 mol/L
of [CuI(L2)]+ (THF, −80 °C), the reversibly bound dioxygen is
determined to be 75% of the expected amount (see Supporting
Information). The decomposition of the L2-based superoxo
complex [CuII(L2)(O2)]+ was also followed spectrophotometri-
cally (THF, −35 °C), and the resulting half-life time is t1/2 = 30
min. Figure 8 shows a first-order decay kinetic trace, and this
emphasizes the existence of only one oxygenation product (i.e.,
the superoxo complex), which decomposes to [CuII(L2)]2+.
This is further supported by ESI-MS spectrometry, where
[CuII(L2)]2+ was detected as the main decomposition product.
DFT calculations on the superoxo complex [CuII(L2)(O2)]+
were performed, and it was found that the triplet state with an
end-on orientation is 11.9 kJ/mol lower in energy than the
side-on-oriented singlet state.
c. Comparison of the Two Systems Based on Ligands L1
and L2. Although the structural difference between the ligands
L1 and L2 is small, the copper−dioxygen chemistry is very
different. Therefore, the O2 binding energies between the two
systems and the steric hindrance induced by the aromatic
moiety upon formation of the dinuclear L2-based complex were
studied by DFT calculations. The optimized structures of the
mononuclear superoxo complexes [CuII(L1)(O2)]+ and
[CuII(L2)(O2)]+ and of the corresponding dinuclear trans-
peroxo complexes, together with relevant structural parameters,
are shown in Figure 9. Similar to the mononuclear L1-based
superoxo complex, the triplet and open-shell singlet species are
relatively close in energy for the L2-based system. The strain
accompanying the formation of the dinuclear trans-peroxo-
[(CuII(L1))2(O2)]2+ complex was calculated by modification of
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preferred because of the steric strain induced by a potential
trans-peroxo bridge. After warming to −80 °C, the three bands
decrease in intensity, and a new band at 398 nm is formed. This
is very similar to the spectrum observed in acetone. A solution
of the oxygenated complex (−120 °C) was allowed to warm to
ambient temperature and was then analyzed by ESI-MS
spectrometry. Interestingly, a partially oxidized ligand was
characterized (see the Supporting Information). The main
fragment is a bispidine-derived aldehyde, which is proposed to
be formed by attack at the CH benzylic position near to the m-
xylene group (see Scheme 3 and also the Supporting
Scheme 3. Oxidative Decomposition of [CuII (L3)(O2)2]2+
2
Figure 9. DFT-optimized geometries and key structural parameters of
the mono- and dinuclear copper(II)−dioxygen complexes with L1 and
L2 (hydrogen atoms are omitted for clarity).
the optimized [(CuII(L2))2(O2)] complex to yield the
corresponding L1-based dicopper(II) species, followed by a
single-point calculation. The resulting approximate strain
energy for formation of the dinuclear peroxo complex
[(CuII(L2))2(O2)]2+, induced by the N7-based substituent in
L2, is 79.8 kJ/mol. A considerable strain also emerges from the
optimized geometry of the dinuclear L2- in comparison to the
L1-based complex shown in Figure 9: the slightly elongated
Cu−O distances (0.02−0.04 Å) for the L2-based complex
compared to that of the L1-based system reveals the steric
hindrance that may prevent the ready formation of the
dinuclear complex. In addition, the binding of superoxide and
peroxide to mono- and dinuclear CuII(L1) and CuII(L2)
complexes were compared: in the mononuclear case, formation
of the end-on superoxo-[CuII(L1)(O2)]+ complex is favored
over the [CuII(L2)(O2)]+ analogue by 11.1 kJ/mol. Similarly,
the peroxo binding energy for the dinuclear species is found to
be in favor of the L1-based complex by 20.3 kJ/mol. Both
findings are largely due to the sterics of the methoxypheny-
lethyl group and are in agreement with the experimental
observations.
Information). This is not an unexpected reaction, and similar
pathways have been described before.55,61−63 However, at this
time we cannot be sure about what species is effecting the
oxidative N-dealkylation reaction, a superoxo, (CuII)2-peroxo,
or another [CuI2(L3)]2+/O2-derived species.
CONCLUSIONS
■
The copper(I) complexes of three second-generation bispidine
ligands (one of them dinucleating) were oxygenated, and the
oxygenation products as well as their formation and decay
pathways were studied. Because of the high reactivity of the
copper(I) precursors and the intermediates, the character-
ization of some of the species involved is not unambiguous if
taken alone. However, the thorough spectroscopic analysis of
some key species as well as their computational analysis leads to
a self-consistent overall picture of the systems.
d. Oxygenation of the Dicopper(I) Complex with L3. The
ligand L3 with its m-xylene bridge provides the possibility to
form both trans-peroxo complexes (Cu:O2 ratio of 2:1) and
complexes that are oxygenated at both copper centers (Cu:O2
ratio of 2:2). The electronic spectrum generated at −80 °C in
acetone (c = 1.3 × 10−3 M) has two bands at 334 and 406 nm,
as well as a weak band at 637 nm (see the Supporting
Information). The oxygenation product is not stable; i.e., the
band at 406 nm decays within 60 min at −80 °C. The
assignment of the spectra to specific oxygenated complexes is
not unambiguous.
The μ-peroxodicopper(II) complex of the L1-based ligand is
thoroughly characterized by its time-dependent UV−vis spectra
and the resonance Raman transitions with 16O2- and 18O2-
labeled peroxo bridges. The computational analysis in this case
leads to a better understanding of a few details but primarily
serves to validate the theoretical model used. Also well
characterized is the mononuclear hydroperoxocopper(II)
complex of L1, and this is primarily based on the EPR
spectrum, which is compared to other CuIIL1 spectra and,
importantly, is in good agreement with the computed spectrum.
The time-dependent UV−vis spectra support these assignments
and show the various pathways for interconversion between the
various species. The third species that can be assigned without
much speculation is the mononuclear superoxocopper(II)
complex of L2. The assignment primarily is based on the
clean formation equilibrium that only involves two copper-
The UV−vis spectrum of oxygenated [CuI2(L3)]2+ at −120
°C in MeTHF has three bands at 412, 563, and 678 nm (see
the Supporting Information). These electronic transitions are
typical for an end-on superoxo complex.55 Although the
structure of [CuI2(L3)]2+ allows for both trans-peroxo and
end-on superoxo, the end-on superoxo complex seems to be
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based species: the copper(I) complex of L2 and the
corresponding superoxocopper(II) complex. This is a clean
1:1 (CuI:O2) reaction, and we have shown that it is reversible
over many cycles (with a minor amount of decay products
formed, as one would expect), and the superoxocopper(II)
complex has the expected electronic properties; specifically, its
UV−vis spectrum has the expected transitions. On the basis of
this assignment of the L2-based superoxocopper(II) complex,
we are able to also assign the much more reactive L1-based
superoxocopper(II) complex by its time-dependent UV−vis
spectrum; the two structures are very similar to each other (as
expected, and supported by the DFT-optimized structures),
and the assignment of the L1-based superoxo complex is also
strongly supported by the computed UV−vis spectra. We
therefore believe that all important species in the L1- and L2-
based complexes are part of a self-consistent interpretation with
well-characterized key species. The compounds involved in the
L3-based copper−dioxygen chemistry are not well character-
ized, and this system is only presented here to show possible
pathways and reactivities of these systems. Of specific interest is
the very different stabilities/reactivities of the L1- and L2-based
superoxocopper(II) complexes, and it is quite clear that this is a
result of efficient shielding of the active site with the L2 ligand
substituent. Small geometric differences, as observed in L3, that
lead to amine dealkylation support this interpretation and point
toward future studies of the reactivities of these superoxo
complexes with external substrates.
Elemental analyses were obtained from the analytical laboratories of
the chemical institutes at the University of Heidelberg on a Vario EL
(Elementary) instrument.
Computational Details. The presence of various oxygenated
species upon reaction with each of the three bispidine ligands was
supported by theoretical calculations. Geometry optimizations and
frequency calculations were carried out using Jaguar,65 employing the
hybrid density functional, B3LYP,66,67 and the effective core
pseudopotential, LACVP (basis B1).68 The effect of the solvent and
a larger basis set, LACV3P**++ (designated as B2), were used for
single-point calculations on the LACVP-optimized geometries. MeCN
was used as the solvent with an ϵ of 37.5 and a probe radius of 2.183,
as implemented in Jaguar. Initially, the nonhybrid functional BP86 was
used to calculate the relative energies of various orientations of the
copper−dioxygen complexes. However, side-on orientation of the
mononuclear [CuI(L1)(O2)]2+ complex failed to optimize at the BP86
method. With B3LYP, there were no such problems, and this
functional is widely used for the study of the reaction mechanisms of
copper−dioxygen complexes.69 Spectroscopic calculations were carried
out using the program ORCA.70,71 TDDFT calculations were
performed using the B3LYP functional and a triple-ζ basis set,
TZVP,72 on copper and all heavy atoms, with the split-valence basis set
for the rest of the molecules [SV(P)]. Because of computational
expense, the effect of the solvent was not considered in the TDDFT
calculations. EPR calculations involved the BHLYP73 functional and
the CP(PPP) basis set74 on the metal, the IGLO-III75 basis set on the
atoms directly bound to copper and the SV(P), SV/J, basis set for the
rest of the atoms. Multireference configuration interaction (MRCI)
calculations for the prediction of absorption spectra were carried out
using the spectroscopy-oriented configuration interaction (SORCI)
method.70 Appropriate reference spaces were chosen for the
complexes.44 The initial orbitals for these calculations were chosen
from BP8676,77 calculations, which produced quasi-restricted orbitals
that were rotated to form an adequate active space. The thresholds
Tsel, Tpre, and Tnat for these MRCI calculations were set to 10−6, 10−5,
and 10−5, respectively, and were shown to enhance the computational
efficiency with a minimal loss of accuracy.70 Resonance Raman spectra
were calculated on BP86/TZVP-optimized geometries and checked
for zero imaginary frequencies with the same method. The vibrational
frequencies were not scaled for the complexes discussed here. If not
mentioned otherwise, the relative energies reported in the paper
include zero-point corrections and solvent effects.
Low-Temperature Oxygenation Experiments. The in situ
generated copper(I) complexes were prepared in the appropriate
solvent using CuI(CH3CN)4(B(C6F5)4,26 to which was added a
solution of an equimolar amount of the appropriate ligand. After
standing for 5−10 min, the reaction solution was cooled (about 15
min) and molecular oxygen was bubbled through the solution for 5−
15 s. THF, MeTHF, and diethyl ether (without stabilizers) were used
for −80 °C measurements (cold bath = acetone, dry ice, controlled by
a thermometer). For the measurements at −120 °C (cold bath = liquid
nitrogen, n-pentane, temperature controlled by a thermometer),
MeTHF was used as the solvent.
Quantitative Measurement of Molecular Oxygen Concen-
trations. Five reaction flasks were each charged with 0.4 g of
pyrogallol and dissolved in 10 mL of deoxygenated NaOH (33% in
H2O) in a glovebox. After removal from the glovebox, 0.2, 0.4, 0.6, 0.8,
and 1.0 mL of dioxygen were added via a gastight syringe and the
flasks were reintroduced into the glovebox. After 18 h, 0.2 mL of the
total volume was transferred to a crown-capped quartz cuvette and
diluted with 1.8 mL of deoxygenated NaOH (33% in H2O). From the
calibration and using linear regression results, y = 1.5885x − 0.0558, R2
= 0.9952 with NaOH (33% in H2O) as the baseline. The samples were
then treated similarly. The flask with the pyrogallol solution was
connected to the warmed-up reaction solution and was closed using a
piece of plastic tubing, before the sample flask was opened to the flask
with the pyrogallol solution. The reaction solution was left stirring for
18 h, until 0.2 mL of the flask with pyrogallol had been introduced in a
capped cuvette diluted with 1.8 mL of deoxygenated NaOH (33% in
H2O) and analyzed.
It is of interest to compare the systems presented here with
those based on other ligand systems. The main difference
between the first- and second-generation bispidine ligands is in
terms of the structures they enforce to the metal ions; clearly,
the difference in the donor sets also is of importance,
specifically with respect to the redox potentials, which
obviously are of importance in terms of oxygen activation:
while the first-generation bispidines enforce square-pyramidal
geometries with very stable μ-peroxodicopper(II) (in-plane-
coordinated peroxo group), the second-generation bispidines
lead to distorted tbp complexes with an apical peroxo group;
note that the ligand-enforced distortion from trigonal symmetry
2
2
leads to a dx −y ground state and to a reactivity that strongly
differs not only from that of the first-generation bispidine-based
systems but also from those of other ligand-copper complexes
described in the literature.
EXPERIMENTAL SECTION
■
Materials and Measurements. Chemicals (Aldrich and Fluka)
were used without further purification if not otherwise stated. L1 and
[CuII(L1)(NCCH3)](BF4)2 were described before.25 NMR spectra
were recorded at 200.13 MHz (1H) and 50.33 MHz (13C) on a Bruker
AS-200 or a Bruker DRX-200 instrument with the solvent signals used
as references. IR spectra were recorded with a Perkin-Elmer Spectrum
100 FT-IR spectrometer from KBr pellets. Mass spectra were obtained
with a JEOL JMS-700 or Finnigan TSQ 700/Bruker ApexQe hybrid
9.4 FT-ICR instrument. Electronic spectra were measured with a Tidas
II J&M or a Jasco V-570 UV−vis−near-IR spectrophotometer. EPR
measurements were performed on a Bruker ELEXSYS-E-500 instru-
ment at 125 K; spin Hamiltonian parameters were obtained by
simulation of the spectra with XSophe.64 For electrochemical
measurements, a BAS-100B Workstation was used, with a three-
electrode setup, consisting of a glassy carbon working electrode, a
platinum-wire auxiliary electrode, and, for MeCN solutions, an Ag/
AgNO3 reference electrode [0.01 M AgNO3, 0.1 M (Bu4N)(PF6),
degassed CH3CN] and solutions of the complexes in MeCN/0.1 M
(Bu4N)(PF6); the potential of the Fc+/Fc− couple for the MeCN
setup had a value of +91 mV (MeCN, scan rate of 100 mV/s).
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[CuII(L1)(OOH)]2+. [CuII(L1)(NCCH3)](BF4)2 (4 mg) was dis-
solved in 10 mL of MeOH to give a 5.5 × 10−4 mol/L solution. This
was cooled to −80 °C, and 0.1 mL of NEt3 and 0.2 mL of H2O2 (30 wt
% in water) were slowly added. The initially blue reaction solution
instantly turned to violet. This color faded away when the solution was
left to warm to room temperature. At −80 °C, the violet species was
stable for at least 0.5 h, enabling physical measurements to be applied.
Syntheses. Caution! Although no difficulties were found using the
perchlorate salts described, these are potentially explosive and need to be
handled with care. Heating, especially when dry, must be avoided.
(HCOO)]+, 629.30 [CuII(L2)]+. Elem anal. ([CuII(L2)]-
(ClO4)2·3.5H2O). Calcd: C, 48.51; H, 5.88; N, 6.29. Found: C,
48.42; H, 5.81; N, 6.46. An X-ray structure of this complex was
obtained, and it has the expected coordination geometry;16,25 however,
the quality of the structure (R ∼ 9%) precludes its publication.
II
[Cu2 (L3)](BF4)4. A total of 0.80 mmol of CuII(BF4)2(H2O)6 was
dissolved in 3 mL of CH3CN and added to solution of 0.41 mmol of
L3 in 3 mL of CH3CN. After stirring at room temperature overnight,
the resulting green-blue solution was treated with a diethyl ether
diffusion, resulting in a blue solid, 73% yield (0.30 mmol).
Precipitation of the product could also be carried using MeOH as
the solvent along with a diethyl ether diffusion. IR (KBr-pellet): 3619,
3555, 3030, 2953, 2876, 1743, 1693, 1603, 1500, 1448, 1367, 1320,
1283, 1059, 764, 699 cm−1. E1/2 (CH3CN, 100 mV/s): −396ox mV,
3-(4-Methoxyphenethyl)-1,5-diphenyl-7-(1,4,6-trimethyl-1,4-dia-
zepan-6-yl)-3,7-diazabicyclo[3.3.1]nonan-9-one (L2). A total of 1.53
mmol of (4-methoxyphenyl)ethanamine, 3.06 mmol of a form-
aldehyde solution, and 0.35 mL of glacial acetic acid were mixed at
0 °C in 4 mL of MeOH. The ice bath was removed, and 1.53 mmol of
1-(1,4,6-trimethyl-1,4-diazacycloheptan-6-yl)-3,5-diphenylpiperidin-4-
one in 2 mL of MeOH was added. The reaction mixture was stirred for
8 h at 65 °C. The solvent was removed in vacuo. The resulting oily
solid was dissolved in dichloromethane, the pH was adjusted using
KOH to about ∼13, and this solution was extracted three times with
30 mL of dichloromethane. The combined organic phases were dried
with Na2SO4. The solvent was removed to yield a white solid (1.00
mmol, 67%.).1H NMR (CDCl3, 200.13 MHz): δ 1.12 (s, 3H, CCH3),
2.21 (s, 6H, NCH3), 2.23 (d, 2J = 13.8 Hz, 2H, CCH2ax), 2.45 (m, 4H,
CH2CH2), 2.75 (d, 2J = 13.8 Hz, 2H, CCH2eq), 2.78 (m, 4H,
−697ox mV, −555red mV. ESI+-MS (MeCN): m/z 592.18 [CuII (L3)]-
2
II
(OH)(F)(H2O)22+. Elem anal. ([Cu2 (L3)](BF4)2(F)2·2MeOH).
Calcd: C, 56.40; H, 6.41; N, 8.10. Found: C, 56.52; H, 6.04; N, 7.86.
[CuI(L3)](B(C6F5)4).
A
total of 0.21 mmol of CuI(B-
(C6F5)4(CH3CN)4 and 0.10 mmol of L3 were stirred in 5 mL of
rigorously deoxygenated THF with 3 drops of CH3CN under an
oxygen-free atmosphere. After 30 min of stirring, 50 mL of
deoxygenated n-pentane was added to precipitate the product. The
solid was collected and the solvent removed in vacuo to yield 73%
yield (0.07 mmol) as a yellow powder.
2
NCH2CH2PhOMe), 3.07 (d, J = 10.6 Hz, 2H, CH2axNCH2CH2),
ASSOCIATED CONTENT
2
2
■
3.25 (d, J = 11.0 Hz, 2H, NCH2axC), 3.51 (d, J = 10.6 Hz, 2H,
CH2eqNCH2CH2), 3.72 (d, 2J = 11.0 Hz, 2H, NCH2eqC), 3.75 (s, 3H,
OCH3), 7.14 (d, 3J = 8.8 Hz, 2H, CHarCOCH3), 7.26 (m, 12H,
CHPh). 13C NMR (CDCl3, 50.27 MHz): δ 25.10 (1C, CCH3), 32.95
(1C, CH2PhOMe), 48.80 (2C, NCH3), 54.60 (1C, CCH3), 55.23 (1C,
OCH3), 58.67 (1C, CH2CH2PhOMe), 59.41 (2C, CH2CH2), 60.12
(2C, CCPh), 62.11 (2C, NCH2CCPh), 64.60 (2C, NCH2CCH3), 66.16
(2C, CH2NCH2CH2), 113.80 (1C, CMeO/o), 126.43 (2C, CPh/p),
126.85 (4C, CPh/m), 127.83 (4C, CPh/o), 129.53 (2C, CMeO/o), 132.20
(1C, CH2CMeO), 143.62 (2C, OCMeO), 157.92 (1C, CPh), 211.70 (1C,
CO). IR (KBr-pellet): 3026, 2935, 2802, 1731, 1611, 1512, 1462,
1447, 1246, 758, 715, 698 cm−1. ESI-MS (MeOH): m/z 599.21
[L2H(CH3OH)]+, 567.21 [L2H]+. Elem anal. (L2·0.5H2O). Calcd: C,
75.10; H, 8.23; N, 9.73. Found: C, 75.18; H, 8.34; N, 9.50.
S
* Supporting Information
Additional time-resolved electronic spectra, an experiment
showing O2 recovery from [CuII(L2)(O2)]+, EPR spectra of
[CuII(L1)(NCMe)]2+ and [CuII(L2)(OH)]+, CV diagrams of
[CuII(L1)(NCMe)]2+, [CuII(L2)(OH)]+, and [CuII (L3)-
2
(NCMe)2]4+, an ESI-MS experiment showing ligand oxidation
of [Cu2 (L3)(solvent)]n+ upon oxygenation, and details of the
I
DFT calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
[7,7′-(1,3-Phenylenebis(methylene)]bis[1,5-diphenyl-3-(1,4,6-tri-
methyl-1,4-diazepan-6-yl)-3,7-diazabicyclo[3.3.1]nonan-9-one)]
(L3). A total of 0.18 mL (1.36 mmol) of m-xylylendiamine, 1.1 g of P1
(2.8 mmol), and 0.46 mL (6.2 mmol) of a H2CO solution (37%) were
dissolved in 10 mL of THF, 10 mL of DME, and 6 mL of HOAc. The
reaction mixture was stirred at 80 °C for 18 h, whereupon the solvent
was removed in vacuo. The resulting oily solid was suspended in 2 M
HCl, and this aqueous phase was extracted once using diethyl ether.
Using KOH, the pH was adjusted to ∼13, and the resulting solution
extracted three times with 30 mL of dichloromethane. The combined
organic phases were dried with Na2SO4, and the solvent was removed
*E-mail: peter.comba@aci.uni-heidelberg.de (P.C.), kkarlin1@
jhu.edu (K.D.K.). Fax: +49-6226-546617.
ACKNOWLEDGMENTS
■
Generous financial support by the German Science Foundation
(DFG) and the University of Heidelberg (LGFG, “Molecular
probes”) is gratefully acknowledged. K.D.K. acknowledges
financial support from the U.S. National Institutes of Health.
1
to yield 800 mg (0.83 mmol) of product, 61%. H NMR (CDCl3,
REFERENCES
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