Tuning of the copper-thioether bond in tetradentate N3S (thioether) ligands; O-O bond reductive cleavage via a [Cu II2(μ-1,2-peroxo)]2+/[CuIII 2(μ-oxo)2]2+ equilibrium

Article abstract of DOI:10.1021/ja502974c

Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu^I}₂-(O₂) species L = ^DMMESE, ^DMMESP, and ^DMMESDP, which are N ₃S_(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N₃O_(ether) (EOE) ligand. Cu^I and Cu^II complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu^I}₂](B(C ₆F₅)₄)₂, however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu^II(X)]ⁿ⁺ {X = ClO₄^-, n = 1, or X = H₂O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (-130 °C) of {( ^DMMESE)Cu^I}⁺ results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{(^DMMESE)Cu ^II}₂(μ-1,2-O₂^2-)]²⁺ (1^P). Weakening the Cu-S bond via a change to the thioether donor found in ^DMMESP leads to the initial formation of [{( ^DMMESP)Cu^II}₂(μ-1,2-O₂ ^2-)]²⁺ (2^P) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{(^DMMESP)Cu^III} ₂(μ-O^2-)₂]²⁺ (2^O), with 2^P and 2^O in equilibrium (K_eq = [2 ^O]/[2^P] = 2.6 at -130 °C). Formulations for these Cu/O₂ adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu-S bond in ^DMMESDP or substitution with an ether donor in ^DMMEOE leads to only a bis-μ-oxo species (3^O and 4 ^O, respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2^O, 3^O) and not the trans-peroxo isomers (1^P and 2^P) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2^P/2^O equilibrium contrast with previously established ligand-Cu^I/O₂ reactivity and possible implications are discussed.

Full text of DOI:10.1021/ja502974c

Article
pubs.acs.org/JACS
Open Access on 05/22/2015
Tuning of the Copper−Thioether Bond in Tetradentate N3S(thioether)
Ligands; O−O Bond Reductive Cleavage via a [CuII (μ-1,2-peroxo)]2+/
2
[CuIII (μ-oxo)2]2+ Equilibrium
2
Sunghee Kim,† Jake W. Ginsbach,‡ A. Imtiaz Billah,† Maxime A. Siegler,† Cathy D. Moore,†
Edward I. Solomon,‡ and Kenneth D. Karlin*,†
†Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
‡Department of Chemistry, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
ABSTRACT: Current interest in copper/dioxygen reactivity
includes the influence of thioether sulfur ligation, as it
concerns the formation, structures, and properties of derived
copper-dioxygen complexes. Here, we report on the chemistry
of {L-CuI}2-(O2) species L = DMMESE, DMMESP, and
DMMESDP, which are N3S(thioether)-based ligands varied in the
nature of a substituent on the S atom, along with a related
N3O(ether) (EOE) ligand. CuI and CuII complexes have been
synthesized and crystallographically characterized. Copper(I)
complexes are dimeric in the solid state, [{L-CuI}2](B(C6F5)4)2, however are shown by diffusion-ordered NMR spectroscopy to
−
be mononuclear in solution. Copper(II) complexes with a general formulation [L-CuII(X)]n+ {X = ClO4 , n = 1, or X = H2O, n =
2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series.
Oxygenation (−130 °C) of {(DMMESE)CuI}+ results in the formation of a trans-μ-1,2-peroxodicopper(II) species
[{(DMMESE)CuII}2(μ-1,2-O22−)]2+ (1P). Weakening the Cu−S bond via a change to the thioether donor found in DMMESP
leads to the initial formation of [{(DMMESP)CuII}2(μ-1,2-O22−)]2+ (2P) that subsequently isomerizes to a bis-μ-oxodicopper(III)
complex, [{(DMMESP)CuIII}2(μ-O2−)2]2+ (2O), with 2P and 2O in equilibrium (Keq = [2O]/[2P] = 2.6 at −130 °C). Formulations
for these Cu/O2 adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the
addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu−S bond in
DMMESDP or substitution with an ether donor in DMMEOE leads to only a bis-μ-oxo species (3O and 4O, respectively). Reactivity
studies indicate that the bis-μ-oxodicopper(III) species (2O, 3O) and not the trans-peroxo isomers (1P and 2P) are responsible for
the observed ligand sulfoxidation. Our findings concerning the existence of the 2P/2O equilibrium contrast with previously
established ligand-CuI/O2 reactivity and possible implications are discussed.
primarily bi-, tri-, or tetradentate entities.1 However, certain
copper metalloproteins utilize sulfur-containing ligand residues,
such as cysteine and methionine (Met) in “blue” copper
electron-transfer proteins3 and in certain monooxygenases,4
(vide infra), (Figure 1).2
INTRODUCTION
■
Copper ion-mediated dioxygen activation studies have focused
on understanding the kinetics and thermodynamics of reactive
intermediate formation and their subsequent diverse oxidative
reactivity.1 Such work is inspired by the known utility of copper
in oxidative transformation of organic substrates. Additionally,
the probing of ligand-copper(I)-O2 chemistry is motivated by
the presence of copper enzymes which mediate O2-processing.2
The detailed nature of the copper(s) active site environment
(i.e., ligand type, coordination number and geometry, etc.)
dictates the observed chemical reactivity, such as functioning as
an O2-carrier, an oxygenase incorporating O atom(s) into a
substrate, or an oxidase effecting substrate dehydrogenation
chemistry.2 In chemical studies geared toward either practical
oxidations or elucidation of fundamental aspects of copper-
dioxygen chemistry, it is a chelating polydentate ligand which
controls the (L)CuI/O2 reactivity.
Figure 1. Copper enzymes/proteins with S-ligands: (a) Peptidylgly-
cine-α-hydroxylating monooxygenase (PHM), C−H oxygenation, (b)
Azurin, electron transfer.
The majority of the CuI−O2 literature on adduct formation
and reactivity involves systems containing all nitrogen ligands,
Received: March 24, 2014
Published: May 22, 2014
© 2014 American Chemical Society
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Thioether S-ligation is important in the dioxygen activating
monooxygenases peptidylglycine-α-hydroxylating monooxyge-
nase (PHM) (Figure 1a)4 and dopamine β-monooxygenase
(DβM).2a,4a These enzymes contain unique “non-coupled”
binuclear copper centers, separated by ∼11 Å, one copper
center with three histidine ligands (CuH), and the other with
two histidines and one methionine (Met)(CuM). The CuM
center is where O2-activation and substrate hydroxylation
occur, while CuH receives reducing equivalents from ascorbate
and is thought to serve as an electron transfer relay source to
the CuM center as needed for monooxygenase activity.
A number of N3S(thioether) type ligands have also been studied.
In the case of two pyridyl and one amine donors (Chart 1d),
we showed that CuI/O2 reactivity leads to a μ-1,2-trans-peroxo
dicopper(II) species.9 In addition, evidence for a Cu(II)−S
bond in this species was observed in the extended X-ray
absorption fine structure (EXAFS) spectra. For a number of
N3S(thioether) benzimidazole containing ligands (Chart 1e),
Castilla and co-workers tested O2 reactivity with copper(I)
derivatives, but no copper-dioxygen intermediates (as might be
detected by spectroscopic interrogation) were observed during
the complexes’ oxidation to copper(II).10 Depending on the
identity if the Ar and R substituents, superoxide anion could be
detected by a radical trapping agent.10 With guanidine donors
(Chart 1f), Schindler and co-workers11 recently reported the
formation of a side-on μ-η2:η2-peroxodicopper(II) complex that
subsequently isomerized to an equilibrium mixture with a bis-μ-
oxodicopper(III) product.
Undoubtedly, the thioether ligand plays a major role in
setting the electronic structure and coordination required for
CuM O2-binding and activation leading to peptide pro-hormone
(for PHM) oxidative N-dealkylation. However, the precise role
of Met coordination and the actual PHM reaction mechanism
have yet to be fully determined. A crystal structure obtained by
Amzel and co-workers4b reveals dioxygen bound to CuM in an
end-on superoxo fashion as depicted in Figure 1a. Computa-
In the investigations described in this present report, we
explored the chemistry of three N3S(thioether) ligands. Salient
features of these new ligands are that they all contain more
electron-rich 4-methoxy-3,5-dimethylpyridyl (DMM) donors
(relative to ESE, Chart 1d)12 and the ethyl linked thioether
moieties. The thioether donor was tuned across a series of
ligands by using a variety of substituents (either ethyl in
DMMESE, phenyl in DMMESP, or 2,4-dimethylphenyl in
DMMESDP, Chart 1). As will be detailed in this report, addition
of dioxygen to the Cu(I) complex of DMMESE leads to the
formation of a μ-1,2-trans-peroxodicopper(II) complex similar
to the pyridine containing ESE. Weakening of the Cu−S
interaction by substituting a phenyl or dimethylphenyl group
tional or biochemical arguments suggest that this CuII−O2
•−
intermediate initiates the chemistry via substrate hydrogen-
atom abstraction. Other O2-derived species have been
suggested as the H atom abstracting intermediate; these
include a cupric hydroperoxide (CuII−OOH) complex formed
from reduction-protonation of the initially formed CuII−O2
•−
complex or a cupryl (CuIIIO ↔ CuII−O•) entity generated
after even further reduction−protonation.5
Inspired by this biochemistry and with the motivation to
elucidate the role of the Met ligand in PHM and DβM, a variety
of ligand scaffolds with thioether donors have been synthesized,
and their O2-reactivity has been interrogated. Within the
N2S(thioether) family of ligands, copper(I) complexes with anionic
ligands (Chart 1a)6 react to give bis-μ-oxo dicopper(III)
products, however, thioether ligation was ruled out. CuI
complexes with imidazolyl ligands (Chart 1b)7 were oxidized,
but no discrete Cu/O2 intermediates formed. With tertiary
amine donors (Chart 1c) we demonstrated that a μ-η2:η2-
peroxodicopper(II) complex could be generated.8
(
DMMESP and DMMESDP) stabilizes a copper(III) bis-μ-oxo
isomer product as determined from UV−vis and resonance
Raman (rR) spectroscopy. In the case of DMMESP, both Cu2O2
core isomers are present at equilibrium, the first example of
such an equilibrium reaction in the presence of a thioether
donor. The results have also been compared to those observed
for the N3O ligand DMMEOE (Chart 1) that contains an ether
donor, to further assess the effect the thioether donor has on
the activation of dioxygen in these complexes.
Chart 1
EXPERIMENTAL SECTION
■
General. All materials used were commercially available analytical
grade from Sigma-Aldrich chemicals and TCI. Acetone was distilled
under an inert atmosphere over CaSO4 and degassed under argon
prior to use. Diethyl ether was used after being passed through a 60 cm
long column of activated alumina (Innovative Technologies) under
argon. Acetonitrile was stored under N2 and purified via passage
through 2 × 60 cm columns of activated alumina (Innovative
Technologies Inc.). Inhibitor free tetrahydrofuran (THF) and 2-
methyltetrahydrofuran (MeTHF) were purchased from Sigma-Aldrich
and distilled under argon from sodium/benzophenone and degassed
with argon prior to use. Pentane was freshly distilled from calcium
hydride under an inert atmosphere and degassed prior to use.
CuI(CH3CN)4(B(C6F5)4) was synthesized according to literature
protocols,13 and its identity and purity were verified by elemental
analysis and/or 1H NMR. Synthesis and manipulations of copper salts
were performed according to standard Schlenk techniques or in an
MBraun glovebox (with O2 and H2O levels below 1 ppm).
Instrumentation: Bench-top low-temperature UV−vis experiments
were carried out on a Cary Bio-50 spectrophotometer equipped with a
liquid nitrogen chilled Unisoku USP-203-A cryostat using a 1 cm
modified Schlenk cuvette. NMR spectroscopy was performed on
Bruker 300 and 400 MHz instruments with spectra calibrated either to
internal tetramethylsilane (TMS) standard or to a residual protio
solvent. EPR measurements were performed on an X-Band Bruker
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EMX CW EPR controlled with a Bruker ER 041 XG microwave bridge
operating at the X-band (∼9 GHz) in 5 mm quartz EPR tubes. ESI-
Mass spectra were acquired using a Finnigan LCQDeca ion-trap mass
spectrometer equipped with an electrospray ionization source
(Thermo Finnigan, San Jose, CA). Single crystal X-ray diffraction
was performed on suitable crystals of [{(DMMESE)CuI}2](B(C6F5)4)2
(12), [{(DMMESP)CuI}2](B(C6F5)4)2 (22), [{(DMMESDP)CuI}2](B
(C6F5)4)2 (32), [(DMMESE)CuII(ClO4)](ClO4) (1a), [(DMMESP)-
CuII(H2O)](ClO4)2 (2a), and [(DMMESDP)CuII(H2O)](ClO4)2
(3a), which were mounted either on the tip of a glass fiber or on a
loop with a tiny amount of Paratone-N oil and transferred to a N2 cold
stream (100 K: 22, 110 K: 12, 32, 3a, 150 K: 1a, and 240 K: 2a). Data
were collected with either a Xcalibur3 diffractometer [Mo Kα radiation
(λ = 0.71073 Å)] or a SuperNova diffractometer [Cu Kα radiation
(mirror optics, λ = 1.54178 Å)] from Agilent Technologies. Also, see
the Supporting Information for further details (SI). Resonance Raman
(rR) Measurements: A 1.5 mM stock solution of copper(I) complexes
was prepared in MeTHF. A 500 μL aliquot of this copper(I) solution
was added to the 5 mm NMR sample tube, capped with a septum, and
chilled in a pentane/N2(l) bath. Oxygenation of the copper samples
was achieved by slowly bubbling an excess of dioxygen through the
solution using a Hamilton gastight syringe equipped with a three-way
valve and needle outlet. Dioxygen, 16O2 (Airgas OX UHP-300) or 18O2
(Icon 6393), was added to an evacuated Schlenk flask fitted with a
septum for the oxygenation reactions described above. Resonance
Raman spectra were collected on a triple monochromator (Spex 1877
CP with 1200, 1800, and 2400 grooves/mm holographic spectrograph
gratings) with a back-illuminated CCD (Princeton Instruments ST-
135). Samples were placed in a liquid nitrogen finger dewar (Wilmad)
in a ∼135° backscattering configuration, and excitation was provided
by an argon ion laser (Innova Sabre 25/7) and a krypton ion laser
(Coherent I90C−K). Data were collected for 5 min at 20 mW of
power, and samples were hand spun for data collected at 380 nm. Peak
positions were determined from fitting the experimental data with
Gaussian transitions using Peakfit Version 4. For the DMMESE rR
spectra, the CuI spectrum was subtracted from both the 16O2 and 18O2
spectra, and each transition was constrained to have an identical
bandwidth.
filtered and removed by rotary evaporation. The resulting yellow oil
was purified by column chromatography (alumina, 25% ethyl acetate
with hexane, Rf = 0.34) yielding a pale yellow oil (4.71 g, 52% yield).
1H NMR (400 MHz, CDCl3): δ 8.11 (s, 2H), 7.16−7.07 (m, 5H),
3.77 (s, 4H), 3.70 (s, 6H), 2.95−2.91 (t, 2H), 2.81−2.79 (t, 2H), 2.21
(s, 6H), 2.13 (s, 6H). ESI-MS, m/z: 452.42 (M + H+).
DMMESDP. Following a similar methodology as described above, 2,4-
dimethylbenzenethiol 2.20 g (15.92 mmol), 2-chloroethylamine
hydrochloride 2.40 g (20.70 mmol), and potassium carbonate 6.60 g
(60.20 mmol) were stirred in 100 mL methylene chloride in a 250 mL
round-bottom flask at room temperature under Ar. After 5 days, the
mixture solution was washed with water and then dried over Na2SO4.
All solvent was removed by a reduced vacuum resulting in a crude 2-
((2,4-dimethylpheny)thio)ethanamine, and it was used directly for the
subsequent reactions.14 Without further purification, a crude 2-((2,4-
dimethyl-pheny)-thio)ethanamine 1.52 g (8.38 mmol) and 2-
chloromethyl-4-methoxy-3,5-dimethylpyridine hydrochloride (3.73 g,
16.77 mmol), and potassium carbonate (5.80 g, 41.97 mmol) in
CH3CN (100 mL) were placed in a 250 mL round-bottom flask, and
then this mixture was refluxed for 4 days under Ar. After removal of
the solvent, crude yellow oil was dissolved in 100 mL dichloromethane
and washed with water three times. After drying over MgSO4, the
solution was filtered and removed by rotary evaporation. The resulting
yellow oil was purified by column chromatography (alumina, 100%
ethyl acetate, Rf = 0.67) yielding a pale yellow oil (2.36 g, 59% yield).
1H NMR (400 MHz, CDCl3): δ 8.11 (s, 2H), 6.93−6.81(m, 3H), 3.77
(s, 4H), 3.70 (s, 6H), 2.89−2.86 (t, 2H), 2.79−2.75 (t, 2H), 2.25−
2.13 (s, 12H), 2.13 (s, 6H). ESI-MS, m/z: 480.50 (M + H+).
DMMEOE. 2-(Chloromethyl)-4-methoxy-3,5-dimethylpyridine hydro-
chloride (2.26 g, 10.19 mmol), 2-ethoxyl ethanamine (0.41 g, 4.63
mmol), and K2CO3 (3.22 g, 23.30 mmol) in CH3CN (80 mL) were
placed in a 100 mL round flask. The mixture solvent was stirred for 2
days under Ar at room temperature. After removal of the solvent,
crude yellow oil was dissolved in 100 mL dichloromethane and washed
with water. After drying over MgSO4, the solution was filtered and
removed by rotary evaporation. The resulting yellow oil was purified
by column chromatography over alumina (ethyl acetate:hexane = 2:1,
Rf = 0.52) yielding a pale oil (1.52 g, 85% yield). 1H NMR (400 MHz,
CDCl3): δ 8.14 (s, 2H), 3.77 (s, 4H), 3.70 (s, 6H), 3.47−3.44 (t, 2H),
3.38−3.33 (q, 2H), 2.72−2.70 (t, 2H), 2.16 (s, 6H), 2.08 (s, 6H),
1.20−1.10 (t, 3H). ESI-MS, m/z: 388.42 (M + H+).
Synthesis of Ligands. DMMESE. 2-(Chloromethyl)-4-methoxy-3,5-
dimethylpyridine hydrochloride (5.50 g, 24.76 mmol), 2-(ethylthio)
ethylamine (1.24 g, 11.79 mmol), and potassium carbonate (17.10 g,
123.75 mmol) in CH3CN (100 mL) were placed in a 250 mL two-
neck-flask. The mixture solvent was stirred for 4 days under Ar at
room temperature. After removal of the solvent, crude yellow oil was
dissolved in 100 mL dichloromethane and washed with water. After
drying over MgSO4, the solution was filtered and removed by rotary
evaporation. The resulting yellow oil was purified by column
chromatography (silica gel, 100% ethyl acetate, Rf = 0.28) yielding a
Synthesis of Copper(I) Complexes. [(DMMESE)CuI](B(C6F5)4) (1).
In a 100 mL Schlenk flask in the glovebox, 258 mg (0.285 mmol) of
[Cu(CH3CN)4](B(C6F5)4) was dissolved in 7 mL of THF. 115 mg
(0.285 mmol) of DMMESE ligand dissolved in ∼5 mL of THF was
added to the copper solution yielding a pale yellow solution. This
yellow solution was allowed to stir for 10 min at which time ∼100 mL
of degassed pentane was added to the solution. After 1 h, the
supernatant was decanted, and the oil removed from the glovebox and
dried under vacuum for 20 min affording 288 mg of a white powder
(88% yield). Single crystals were obtained by topping of dry pentane
1
pale yellow oil (3.61 g, 76% yield). H NMR (400 MHz, CDCl3): δ
8.14 (s, 2H), 3.75 (s, 4H), 3.70 (s, 6H), 2.74−2.70 (t, 2H), 2.57−2.54
(t, 2H), 2.34−2.29 (q, 2H), 2.21(s, 6H), 2.11(s, 6H), 1.12−1.08 (t,
3H). 1H NMR (300 MHz, THF-d8): δ 8.14 (s, 2H), 3.77 (s, 4H), 3.74
(s, 6H), 2.73−2.70 (t, 2H), 2.64−2.58 (t, 2H), 2.37−2.31 (q, 2H),
2.24(s, 6H), 2.15(s, 6H), 1.16−1.12 (t, 3H). ESI-MS, m/z: 404.39 (M
+ H+).
1
into a solution of the complex in THF. H NMR (300 or 400 MHz,
THF-d8): δ 8.41 (s, 2H), 4.34−4.28 (d, 2H), 4.10−4.04 (d, 2H), 3.81
(s, 6H), 3.03−3.00 (b, 4H), 2.42−2.37 (q, 2H), 2.29(s, 6H), 2.23(s,
6H), 1.27−1.22 (t, 3H). Elemental analysis: (C46H33BCuF20N3O2S)
Calcd: C (48.20), H (2.90), N (3.67); Found: C (48.26), H (2.94), N
(3.42).
DMMESP. In a 250 mL round-bottom flask, 2.21 g (20.060 mmol)
thiophenol, 2-chloroethylamine hydrochloride (3.02 g, 26.04 mmol),
and potassium carbonate (8.32 g, 60.20 mmol) were stirred in 120 mL
methylene chloride at room temperature under Ar. After 48 h, the
mixture solution was washed with water and then dried over Na2SO4.
All solvent was removed by a reduced vacuum resulting in a crude 2-
(phenylthio)ethanamine, and it was used directly for the subsequent
reactions.14 Without further purification, a crude 2-(phenylthio)-
ethanamine (3.073 g, 20.05 mmol) and 2-chloromethyl-4-methoxy-
3,5-dimethyl-pyridine hydrochloride (8.910 g, 40.12 mmol), and
potassium carbonate (11.09 g, 80.21 mmol) in CH3CN (120 mL)
were placed in a 250 mL round-bottom flask, and then this mixture
was stirred for 4 days under Ar at room temperature. After removal of
the solvent, crude yellow oil was dissolved in 100 mL dichloromethane
and washed with water. After drying over MgSO4, the solution was
1
[(DMMESP)CuI](B(C6F5)4) (2). H NMR (300 MHz, THF-d8): δ 8.34
(s, 2H), 7.48−7.31 (m, 5H), 4.25 (d, 2H), 4.09 (d, 2H), 3.73 (s, 6H),
3.50 (s, 2H), 2.99 (s, 2H), 2.24 (s, 6H), 2.17(s, 6H). Elemental
analysis: (C50H33BCuF20N3O2S) Calcd: C (50.29), H (2.79), N
(3.52); Found: C (50.39), H (2.92), N (3.27).
[(DMMESDP)CuI](B(C6F5)4) (3). 1H NMR (300 MHz, THF-d8): δ 8.21
(s, 2H), 7.14−6.90 (m, 5H), 4.22 (d, 2H), 4.01 (d, 2H), 3.73 (s, 6H),
3.42 (s, 2H), 3.02 (s, 2H), 2.41−2.18 (m, 18H). Elemental analysis:
(C52H37BCuF20N3O2S) Calcd: C (51.10), H (3.05), N (3.44); Found:
C (51.06), H (3.08), N (3.34).
1
[(DMMEOE)CuI](B(C6F5)4) (4). H NMR (300 MHz, THF-d8): δ 8.28
(s, 2H), 4.17−4.11 (d, 2H), 3.75 (s, 6H), 3.71−3.67 (d, 2H), 3.12−
3.03 (m, 4H), 2.41−2.35 (d, 2H), 2.25(s, 6H), 2.22(s, 6H), 1.00−0.97
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Figure 2. Representations of the dimeric Cu(I) complexes described in the crystal structures of (a) [{(DMMESE)CuI}2](B(C6F5)4)2 (12), (b)
[{(DMMESP)CuI}2](B(C6F5)4)2 (22), and (c) [{(DMMESDP)CuI}2](B(C6F5)4)2 (32). Selected bond distances (Å) and bond angles (deg) are also
listed. The H atoms and counterions were omitted for clarity.
(t, 3H). Elemental analysis: (C46H33BCuF20N3O3) Calcd: C (48.89),
H (2.94), N (3.72); Found: C (48.87), H (3.11), N (4.21).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Copper(I) Com-
Following a similar methodology as described above, dinuclear
plexes, 1, 2, and 3. Reaction of ligand DMMESE and
copper(I) complexes, [(N5)CuI2](B(C6F5)4)2, and [(XYL-H)CuI2]-
[CuI(CH3CN)4](B(C6F5)4) under Ar in THF gives a white
(B(C6F5)4)2 were synthesized and characterized by NMR analyses (see
SI).
powder, formulated as [(DMMESE)CuI](B(C6F5)4), 1, on the
Synthesis of Copper(II) Complexes. [(DMMESE)CuII(ClO4)](ClO4)
(1a). In a 100 mL Schlenk flask, 305 mg (0.756 mmol) of DMMESE
ligand was dissolved in 20 mL dry acetone. 280 mg (0.756 mmol) of
CuII(ClO4)2·6H2O was dissolved in 20 mL dry acetone, and then
copper solution was added to DMMESE solution. After stirring for 10
min at room temperature, the complex was precipitated as a blue solid
upon addition of diethyl ether (100 mL). The supernatant was
decanted, and the resulting blue solid was washed two times with
diethyl ether and dried under vacuum to afford 411 mg of a copper(II)
complex (82% yield). Single crystals were obtained by vapor diffusion
of diethyl ether into a solution of the complex in acetone. Elemental
analysis: (C22H33Cl2CuN3O10S) Calcd: C (39.67), H (4.99), N (6.31);
Found: C (38.95), H (5.09), N (5.98). EPR spectrum (Figure S18):
X-band (2 mM, ν = 9.186 GHz) spectrometer in acetone at 70 K: g∥ =
2.263, A∥ = 168 G, g⊥ = 2.036.
basis of elemental analysis. Attempts to grow crystals of 1 in
THF/pentane lead to the isolation of a dinuclear complex,
[{(DMMESE)CuI}2](B(C6F5)4)2 (designated as compound 12)
shown in Figure 2. X-ray data and details of the structure
determination for 12 are provided in the SI. Selected bond
lengths and bond angles of this complex are listed in Figure 2.
Complex 12 is found as a centrosymmetric dimer in the solid
state, where each copper ion is coordinated by one ligand via
the three N-donors and by the other ligand via its thioether
atom, thus giving the overall dinuclear structure. Each copper
center is found in a distorted tetrahedral arrangement in which
the three nitrogen atoms and one S atom from the adjacent
copper site are the vertices of a distorted tetrahedron.
Copper(I) complexes containing at least one thioether ligand
reveal Cu−S bond distances between 2.2 and 2.44 Å.7a,8,9b,15
Accordingly, the Cu−S distance of 2.1891(5) Å in the dimer 12
is fairly typical (but suggesting relatively strong binding of the
thioether arm). Following very similar procedures,
[{(DMMESP)CuI}2](B(C6F5)4)2 (22) and [{(DMMESDP)CuI}2]-
(B(C6F5)4)2 (32) were also isolated and crystallized (Figure 2).
Their coordination environments are quite similar to that of
[{(DMMESE)CuI}2](B(C6F5)4)2 (12). In fact, this type of
copper(I) complex dimerization in the solid state has been
observed previously for a number of three- or four-coordinate
complexes, with a thioether donor and even with all nitrogen
donor ligands.6,9b,10,16,17
[(DMMESP)CuII(H2O)](ClO4)2 (2a). Elemental analysis:
(C26H35Cl2CuN3O11S) Calcd: C (42.66), H (4.82), N (5.74);
Found: C (42.69), H (4.98), N (5.69). EPR spectrum (Figure S18):
X-band (2 mM, ν = 9.186 GHz) spectrometer in acetone at 70 K: g∥ =
2.263, A∥ = 168 G, g⊥ = 2.034.
[(DMMESDP)CuII(H2O)](ClO4)2 (3a). Elemental analysis:
(C28H39Cl2CuN3O11S) Calcd: C (44.24), H (5.17), N (5.53);
Found: C (44.27), H (5.48), N (5.54). EPR spectrum (Figure S18):
X-band (2 mM, ν = 9.186 GHz) spectrometer in acetone at 70 K: g∥ =
2.262, A∥ = 168 G, g⊥ = 2.034.
[(DM MEOE)CuII(H2O](ClO4 )2 (4a). Elemental analysis:
(C22H35Cl2CuN3O12) Calcd: C (39.56), H (5.28), N (6.29); Found:
C (40.03), H (5.43), N (6.00). EPR spectrum (Figure S18): X-band (2
mM, ν = 9.186 GHz) spectrometer in acetone at 70 K: g∥ = 2.265, A∥
= 170 G, g⊥ = 2.036.
While the three ligand-copper(I) complexes are dimeric in
the solid state, we postulate that in solution these Cu(I)
complexes are mononuclear species. Support for this
conclusion was obtained from the NMR spectroscopic
technique diffusion ordered spectroscopy (DOSY). This utilizes
pulsed field gradients to measure the translational diffusion of
molecules.18 Our DOSY measurement gives a log D (diffusion
constant) of −9.08 for the [(DMMESE)CuI]+ (1) complex in
The description of experimental procedures for generating and
handling the copper-dioxygen adducts in order to carry out UV−vis,
IR, CV, ESI-MS, and NMR analyses and the reactivity study where
examination of ligand thioether sulfoxidation is probed are further
described in the Supporting Information (SI).
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Chart 2
Figure 3. Representations of the monomeric Cu(II) complexes described in the crystal structures of (a) [(DMMESE)CuII(ClO4)](ClO4) (1a), (b)
[(DMMESP)CuII(H2O)](ClO4)2 (2a), and (c) [(DMMESDP)CuII(H2O)](ClO4)2 (3a). Selected bond distances (Å) and bond angles (deg) are also
listed. The H atoms (except for those of the water ligands found in 2a and 3a) and noncoordinating counterions were omitted for clarity.
THF. From this value, we calculate a hydrodynamic diameter of
1.13 × 10−9 m, which agrees closely (1.08 × 10−9 m) with that
obtained from X-ray structural parameters for the mononuclear
copper(II) complex [(DMMESE)CuII(ClO4)]+ (1a), see SI.19
Further, our diffusion constant for 1 closely matches that
determined via DOSY measurements for the well-known
mononuclear Cu species [(TMPA)CuI(CH3CN)](B-
(C6F5)4),20 however log D varies considerably from that
determined for the discrete dicopper complexes [(N5)(CuI)2]-
(B(C6F5)4)2 and [(XYL-H)CuI2](B(C6F5)4)2, see SI including
Table S1.
Synthesis and Characterization of Copper(II) Com-
plexes. To experimentally probe the effect of the thioether
substituent on the Cu(II)−S interaction, which would also be
expected to occur in copper-dioxygen adducts where the
copper is either CuII or CuIII (vide infra), copper(II)
perchlorate complexes (Chart 2) were synthesized for each
ligand. [(DMMESE)CuII(ClO4)](ClO4) (1a), [(DMMESP)-
CuII(H2O)](ClO4)2 (2a), and [(DMMESDP)CuII(H2O)]-
(ClO4)2 (3a) were generated by simple combination of ligand
and Cu(ClO4)2·6H2O in acetone. The structures of 1a, 2a, and
3a were determined by single crystal X-ray diffraction, which
reveals that all three compounds are mononuclear complexes
with a distorted square pyramidal (SP) coordination (τ = 0.39
for 1a, τ = 0.38 for 2a, and τ = 0.12 for 3a; τ = 0.00 for idealized
SP geometries), see Figure 3.23 The thioether donor groups are
found in an axial position, and the Cu−S bond distances are
2.536 Å in 1a, 2.603 Å in 2a, and 2.691 Å for 3a; similar to the
2.68 Å Cu−S(methionine) bond length observed in an oxidized
form of PHM. The shortest Cu−S bond occurs for complex 1a
with its donating ethyl S-substituent. For 2a and 3a, the aryl
substituent is a poorer donor electronically (then an ethyl
1
Additionally, variable-temperature (VT) H NMR experi-
ments on (1) in THF-d8, acetone-d6, or DMF-d7 (25 °C to −60
°C) show minimal temperature dependence resulting from
chemical dynamics (see Figures S7−S11 for details). This is in
contrast to systems where the dimer is in equilibrium with the
monomer, resulting in significant chemical shift changes with
temperature.6,16 Further, DMF and acetone are reasonably
good ligands for copper(I),21 and even weak solvent
coordination would be expected to break up any dimeric
structure.22
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group), while steric factors are clearly relevant, even with
comparing the Cu−S bond lengths 2a directly with 3a, the
latter with its o-methylaryl substituent. We carried out cyclic
voltammetry experiments on 1a, 2a, and 3a (see the SI). A
slightly more negative E1/2 value observed for 1a compared to
2a and 3a indicates that the ethylthio arm of 1a is at least a
marginally better donor than the thiophenyl-type arms in 2a
and 3a. Overall, however, it is difficult to tease out the
differences in electronic vs steric factors in these and related
complexes.24
The EPR spectra for all of these complexes in solution are of
the axial type (Figure S18) indicating that these mononuclear
Cu(II) complexes are found in a distorted SP environment.
This observation that the S(thioether) atom binds axially in these
copper(II) complexes suggests that such binding can occur in
the copper-dioxygen adducts described below.
Figure 5. UV−vis spectra and time profile of the equilibrium between
2P and 2O in MeTHF at −130 °C.
Oxygenation Reactions of Cu(I) complexes. [(DMMESE)-
CuI]+ (1) + O2(g). Dioxygen reacts with [(DMMESE)CuI]+ (1) in
MeTHF at −130 °C, forming the low-temperature stable
intense indigo blue species (1P)(see Chart 2). The UV−vis
spectrum of 1P is characterized by three transitions at 445
(2150 M−1 cm−1), 521 (8640 M−1 cm−1), and 615 nm (10 850
M−1 cm−1, Figure S23) and is similar to other trans-peroxo
species such as [{(TMPA)CuII}2(μ-1,2-O22−)]2+.25 Yet, the
lower energy charge transfer (CT) transition at 615 nm for 1P
is more intense than the higher energy transition. This intensity
ratio was previously observed in [{(ESE)CuII}2(μ-1,2-O22−)]2+
(Chart 1d, Figure 4b) and was ascribed to a geometric
Figure 6. Equilibrium between (a) a μ-1,2-trans-peroxodicopper(II)
and bis-μ-oxodicopper(III) species; with 1 equiv CH3SO3 , the
−
mixture converts to a pure bis-μ-oxodicopper(III) complex, and (b)
Stack and co-workers’ mixture of side-on μ-η2:η2-peroxodicopper(II)
and bis-μ-oxodicopper(III) species reacts with chelating anions to give
a clean side-on μ-η2:η2-peroxodicopper(II) complex.
that the low-temperature combination of 2P and 2O represents
a dynamic equilibrium mixture can be readily seen from
examination of a series of spectra we recorded for the 2 + O2
reaction at a variety of temperatures, in some cases then also
warming. Those spectra and explanatory comments are given in
Figures S19−S20.
Figure 4. (a) Structure of [{(TMPA)CuII}2(μ-1,2-O22−)]2+ obtained
from X-ray crystallography and (b) low-temperature UV−vis
absorption spectra of [{(TMPA)CuII}2(μ-1,2-O22−)]2+ (purple) and
[{(ESE)-CuII}2(μ-1,2-O22−)]2+ (blue).9b,25
We have previously characterized a similar trans-peroxo/bis-
μ-oxo equilibrium with the all nitrogen tetradentate donor
(BQPA) ligand (BQPA = bis(2-quinolylmethyl)(2-pyridylme-
thy1)-amine). Similarly, [(BQPA)CuI]+ reacts with dioxygen to
distortion from the trigonal bipyramidal coordination environ-
ment in TMPA (Figure 4a) toward a SP geometry in ESE. This
geometric distortion would increase the overlap of the π*ν
orbital with the Cu(II) hole, resulting in a higher intensity CT
transition.9b The similar intensity ratio of the CT transitions in
1P and [{(ESE)CuII}2(μ-1,2-O22−)]2+ indicate it also possesses
a distorted SP geometry, similar to the coordination environ-
ment observed in the X-ray structure of [(DMMESE)-
CuII(ClO4)](ClO4) (1a) (vide supra).
first form a [CuII (μ-1,2-O22−)]2+ species that converts to a
2
[CuIII2(μ-O)2]2+ complex, ktrans→bis‑oxo = 0.16 s−1 (−90 °C); Keq
=3.2 (−90 °C).27 The observation of a trans-peroxo/bis-μ-oxo
equilibrium in these systems differs from what has typically
been presumed, that only a side-on μ-η2:η2-peroxodicopper(II)
could be in (rapid) equilibrium with a bis-μ-oxodicopper(III)
complex (Figure 6b, upper).1
The [CuIII2(μ-O)2]2+/[CuII (μ-1,2-O22−)]2+ equilibrium with
2
DMMESP (Figure 5 and 6a) is also sensitive to the addition of
the coordinating anion methylsulfonate. A spectroscopically
[(DMMESP)CuI]+ (2) + O2(g). Upon oxygenation in MeTHF at
−130 °C, a trans-peroxodicopper(II) species [{(DMMESP)-
CuII}2(μ-1,2-O22−)]2+ (2P) with UV−vis features at 504 and
644 nm was initially formed. Over time (∼30 min) these
features decay, and a new species (2O) with an absorption
feature at 388 nm forms (Figure 5).
pure (UV−vis) bis-μ-oxo species 2O-(CH3SO3 ), λmax = 386
−
nm (13 000) (Figure S22), is obtained with one equiv of
28
(nBu4N+)(CH3SO3 ) (Figure 6a, lower part). Stack and co-
workers29 previously demonstrated that addition of methylsul-
fonate or other chelating anions converts a mixture of side-on
μ-η2:η2-peroxodicopper(II) and bis-μ-oxodicopper(III) species
with bidentate ligands to an anion bridged side-on peroxo
species (Figure 6b). Masuda and co-workers30 also observed
conversion of a [CuIII2(μ-O2−)2]2+ complex to a side-on peroxo
species bridged by a benzoate donor with addition of the latter,
−
However, the absorption features of 2P do not completely
convert to 2O (Figure 5), indicating an equilibrium between
these complexes (Figure 6a, upper), with Keq = [2O]/[2P] = 2.6
at −130 °C.26 The rate constant for the trans-peroxo to bis-μ-
oxo interconversion can be well estimated from the data in
Figure 5, ktrans→bis‑oxo = 9.6 s−1 (−130 °C). Further evidence
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indicating coordinating anions stabilize the side-on peroxo
isomer relative to the bis-μ-oxo.
[(DMMESDP)CuI]+ (3) + O2(g). The reaction of [(DMMESDP)-
CuI](B(C6F5)4) (3) with O2 only gives a bis-μ-oxodicopper-
(III) species (UV−vis criterion), [{(DMMESDP)CuIII}2-
(O2−)2]2+ (3O) (λmax = 388 nm, 13 000 M−1 cm−1), see Figure
S23. This species (3O) is stable for hours at −130 °C with only
minimal decomposition occurring (UV−vis criterion). The
increased size of the dimethylphenyl group weakens the Cu−S
interaction (as evident from the longer Cu(II)−S bond in the
structure of 3a) with the result that DMMESP acts like as a
tridentate ligand, which favors the bis-μ-oxo dicopper(III)
isomer. A steric clash due to the dimethylphenyl substituent
likely also contributes to a destabilization of a trans-μ-1,2-
peroxo dicopper(II) analogue; in the structure of [{(TMPA)-
CuII}2(μ-1,2-O22−)]2+ (Figure 4), the ligand arms are
interdigitated.
The influence of the Cu−S interaction on the oxygenated
products was further probed in comparison to a ligand lacking
the thioether donor atom. Oxygenation of the Cu(I) complex
(4) of DMMEOE, possessing the same three N-donors but
having an ether O atom replacing the sulfur (Charts 1 and 2),
leads to the formation of a new species with prominent optical
absorption at 382 nm (20 000 M−1 cm−1, Chart 2), nearly
identical in λmax and absorptivity to the bis-μ-oxo complexes 2O
and 3O. The further comparison of the UV−vis spectrum of 4O
with those of 2O and 3O and other literature examples15e,31
clearly shows this is a bis-μ-oxodicopper(III) complex,
[{(DMMEOE)CuIII}2(O2−)2}2+ (4O). This suggests that as the
fourth donor is weakened, the bis-μ-oxo isomer is stabilized,
and the ligand behaves like a N3 donor. Since the oxygen atom
of the ether arm in DMMEOE has an extremely weak to
nonexistent interaction with the copper, this also strongly
suggests that the thioether is coordinated to the Cu(II) trans-
peroxo complexes 1P and 2P, similar to [{(ESE)CuII}2(μ-1,2-
O22−)]2+ where a Cu(II)−S bond was characterized by EXAFS
spectroscopic data.9a
Figure 7. Resonance Raman spectra of 2P (a) with 647 nm excitation
and 2O (b) with 380 nm excitation in MeTHF collected at 77 K; see
text.
Scheme 1
Laser excitation of an oxygenated sample of [(DMMESP)CuI]+
(2) that was frozen after ∼90 min and contains mostly 2O
produces one isotope sensitive stretch (16O2: 597 cm−1; 18O2:
570 cm−1, Figure 7b) that is similar to the rR spectra of the
structurally characterized [{Me2tpaCuIII}2(μ-O2−)2]2+ com-
plex34 and the bis-μ-oxo isomer of BQPA.27 This vibration
has been previously assigned as the symmetric Cu2(μ-O2−)2
core-breathing mode. The addition of dioxygen and 1 equiv of
Resonance Raman Spectroscopic Characterization of
1P, 2P, 2O, 2O-(CH3SO3 ), and 3O. Resonance Raman
−
spectroscopy confirms the assignment of 1P and 2P as trans-
peroxo isomers and 2O, 2O-(CH3SO3 ), and 3O as bis-μ-oxo
−
isomers. Laser excitation of oxygenated samples of 2 that were
frozen after ∼1 min contains mostly 2P and yields rR spectra
(Figure 7a) with four isotope sensitive bands that are consistent
in energy and isotope shift with the νO−O (16O2: 830 and 810
cm−1; 18O2: 784 and 772 cm−1) and the symmetric νCu−O
(16O2: 545 and 531 cm−1; 18O2: 518 and 500 cm−1) of
previously characterized end-on peroxo species.1a The presence
of two νO−O is similar to the rR spectra of [{(BQPA)-
CuII}2(O22−)]2+ and results from two end-on peroxo isomers in
both systems.27 These two species likely arise from the
asymmetric coordination environment present in DMMESP,
which yields two different arrangements of the ligand around
−
tetrabutylammonium methanesulfonate (CH3SO3 ) (per cop-
per dimer) to a solution of the CuI salt 2 results in an
absorption spectrum that only contains features arising from a
bis-μ-oxo species (vide infra). The rR spectra of this complex is
indistinguishable (2 cm−1 resolution) from the rR spectrum of
2O, which was synthesized with a B(C6F5)4− counterion (Figure
S25). Additionally, similar rR spectra were observed for 3O
(Figure S26). This further indicates that while 1P and 3O are
pure trans-peroxo and bis-μ-oxo species, respectfully, the
−
oxygenated products of 2 in the presence of B(C6F5)4 (and
the CuII (O22−) core in the interdigitated dimer. In one isomer,
−
not CH3SO3 ) are an equilibrium mixture of these two isomers.
2
the thioether ligands would be anti to each other and in the
other syn (Scheme 1). Also present in the rR spectra is an
isotope insensitive vibration at 415 cm−1. A band at similar
energy and intensity is observed in the rR spectra of
[{(TMPA)CuII}2(O22−)]2+ and has been previously assigned
as the Cu−Namine stretch.25b The rR spectra of 1P are also
characteristic of an end-on peroxo species (Figure S24) and are
consistent with the presence of both syn and anti trans-peroxo
isomers.
The combination of the absorption and rR data indicate that
the coordination environment of 1P and 2P is distorted toward
SP (Scheme 1). Not only is this coordination geometry
observed in the perchlorate crystal structures 1a and 2a but also
the lower energy π*v CT transition in both 1P and 2P is more
intense than the higher energy π*σ CT transition. This is in
contrast to the structure of [{(TMPA)CuII}2(μ-1,2-O22−)]2+
(Figure 4a) where a trigonal bipyramidal coordination
geometry (τ = 0.9) is observed.25a,32 The ligand environment
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is rather flexible since 2P can readily equilibrate to give bis-μ-
oxo species (2O), adopting a CuIII2(μ-O)2 core with a square
planar geometry (Scheme 1) analogous to the crystallo-
graphically characterized complex [{(Me2tpa)CuIII}2(O2−)2]2+
where Me2tpa has two of the three pyridyl donors (of TMPA)
possessing ortho (6-position) methyl groups.33 In this case, the
methylpyridines are axial (Cu···N = 2.5 Å). By analogy, the
apical amine and one pyridyl ligand arm are equatorial in 2O
and 3O, with the other pyridyl and thiophenyl arms interacting
axially with the Cu(III) ion. However, due to the potential
lability of this interaction, it is possible that 2O and 3O do not
possess even a weak, axial thioether−Cu(III) interaction as we
have depicted in Scheme 1.
PPh3 formed. Thus, the ligand (intramolecular) sulfoxidation
was suppressed in favor of a more facile intermolecular
oxidation of PPh3.
SUMMARY AND CONCLUSION
■
The oxygenation of copper(I) salts of a series of N3S DMM
derivatized ligands indicates that the nature of the oxygenated
product can be tuned by modulating the Cu−S interaction.
While a trans-peroxo isomer is favored for DMMESE, an increase
in steric bulk (and/or decrease in donor strength) at the
thioether donor causes a weakening of the Cu−S interaction
resulting in the stabilization of the bis-μ-oxo isomer observed
for DMMESP and DMMESDP with their S-aryl substituents. In the
case of DMMESP, an equilibrium mixture of the two species is
observed, the first example supported by a ligand with a
thioether donor. The addition of the coordinating anion
Ligand Substrate Reactivity of 1P, 2P, and 3O Species.
To seek further insight into Cu-dioxygen bonding and reactivity
in the presence of a thioether donor, the copper(I) complexes
were oxygenated at slightly warmer temperatures to probe for
the possibility of ligand sulfoxidation chemistry. [(DMMESP)-
CuI](B(C6F5)4) (2) was bubbled with dioxygen gas in MeTHF
at −120 °C, forming as described above a mixture of
[{(DMMESP)CuII}2(μ-1,2-O22−)]2+ (2P) and [{(DMMESP)-
CuIII}2(μ-O2−)2]2+ (2O). Further hour-long incubation at
−120 °C led to decomposition of both intermediates resulting
in a color change from dark indigo blue to green. Workup of
this reaction product solution at low temperature (see SI) and
analysis of the organic products, the ligands, and any oxidized
species showed that approximately one out of two DMMESP
thioether-amine (N3S) ligands had undergone sulfoxidation as
determined by TLC, NMR spectroscopy, and ESI-MS
spectrometry, with a ∼ 40% measured yield obtained (Scheme
2).
−
CH3SO3 stabilized the bis-μ-oxo, relative to the trans-peroxo
isomer, in contrast to the effect of coordinating anions on the
side-on peroxo/bis-μ-oxo equilibrium observed in other
systems. Additionally, only ligands with an accessible bis-μ-
oxo isomer oxidize the thioether to the corresponding sulfoxide
at low temperatures indicating this isomer is the oxidant
responsible for this electrophilic reactivity. The observation of
this trans-peroxo/bis-μ-oxo equilibrium in the present N3S and
a previous N4 system27 shifts a previous paradigm which
assumed that a bis-μ-oxo dicopper(III) complex generation
occurs from initially formed side-on μ-η2:η2-peroxodicopper(II)
species. These systems demonstrate a new strategy to reversibly
cleave an O−O bond, which has new potential implications for
the development of a synthetic, copper-based water splitting
catalyst.34
ASSOCIATED CONTENT
* Supporting Information
Scheme 2
■
S
Details concerning X-ray crystallographic analyses including cif
files, variable-temperature NMR, DOSY NMR, UV−vis, IR and
rR spectroscopies, cyclic voltammetry, ESI mass spectrometry,
and the procedures for carrying out the sulfoxidation reactivity.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
karlin@jhu.edu
■
Notes
These results suggest that a monooxygenase-type reaction
occurred with the 2P/2O mixture, as we previously observed in
the room temperature oxygenation of [(ESE)CuI]+.9b Here,
two electrons provided by two copper(I) complexes lead to a
peroxo or bis-μ-oxodicopper product which is capable of only
one oxo-transfer (net) reaction, giving a sulfoxide; the
theoretical yield is a maximum of 50%. By comparison, under
identical conditions, similar reactivity studies were carried out
for the trans-peroxo species, [{(DMMESE)CuII}2(O22−)]2+ (1P)
and bis-μ-oxo species, [{(DMMESDP)CuIII}2(O2−)2}2+ (3O).
The same behavior was observed for 3O leading to ∼43% ligand
sulfoxidation. However, 1P showed no ligand sulfoxidation at
−120 °C (Scheme 2) indicating that the bis-μ-oxodicopper(III)
isomer is responsible for the ligand sulfoxidation, as we had
previously hypothesized with ESE (Chart 1d).9b Interestingly,
when an excess of triphenylphosphine (PPh3) was added to the
2P/2O mixture and allowed to stand for 2−3 h, low-temperature
work-up showed that no sulfoxidation occurred and rather O
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge support of this research from the
National Institutes of Health (GM28962 from the National
Institute of General Medical Sciences to K.D.K. and
R01DK031450 from the National Institute of Diabetes and
Digestive and Kidney Diseases to E.I.S.). The content is solely
the responsibility of the authors and does not necessarily
represent the official views of the National Institutes of Health.
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