A N3S(thioether)-ligated CuII-superoxo with enhanced reactivity

Article abstract of DOI:10.1021/ja511504n

Previous efforts to synthesize a cupric superoxide complex possessing a thioether donor have resulted in the formation of an end-on trans-peroxo-dicopper(II) species, [{(Ligand)Cu^II}₂(μ-1,2-O₂^2-)]²⁺. Redesign/modification of previous N₃S tetradentate ligands has now allowed for the stabilization of the monomeric, superoxide product possessing a S_(thioether) ligation, [(^DMAN₃S)Cu^II(O₂^?-)]⁺ (2^S), as characterized by UV-vis and resonance Raman spectroscopies. This complex mimics the putative Cu^II(O₂^?-) active species of the copper monooxygenase PHM and exhibits enhanced reactivity toward both O-H and C-H substrates in comparison to close analogues [(L)Cu^II(O₂^?-)]⁺, where L contains only nitrogen donor atoms. Also, comparisons of [(L)Cu^II/I]ⁿ⁺ compound reduction potentials (L = various N₄ vs ^DMAN₃S ligands) provide evidence that ^DMAN₃S is a weaker donor to copper ion than is found for any N₄ ligand-complex.

Full text of DOI:10.1021/ja511504n

Communication
pubs.acs.org/JACS
II
A N3S
-Ligated Cu -Superoxo with Enhanced Reactivity
(thioether)
†
†
‡
‡
†
Sunghee Kim, Jung Yoon Lee, Ryan E. Cowley, Jake W. Ginsbach, Maxime A. Siegler,
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
Scheme 1
ABSTRACT: Previous efforts to synthesize a cupric
superoxide complex possessing a thioether donor have
resulted in the formation of an end-on trans-peroxo-
II
2− 2+
dicopper(II) species, [{(Ligand)Cu } (μ-1,2-O )] .
2
2
Redesign/modification of previous N S tetradentate
3
ligands has now allowed for the stabilization of the
monomeric, superoxide product possessing a S
(
thioether)
DMA
II
•−
+
S
ligation, [( N S)Cu (O )] (2 ), as characterized by
3
2
UV−vis and resonance Raman spectroscopies. This
II
•−
complex mimics the putative Cu (O ) active species of
2
the copper monooxygenase PHM and exhibits enhanced
reactivity toward both O−H and C−H substrates in
II
•−
+
Within the subfield of copper-dioxygen synthetic bioinor-
comparison to close analogues [(L)Cu (O )] , where L
2
ganic chemistry, one long-standing goal has been to produce
contains only nitrogen donor atoms. Also, comparisons of
II
•−
II/I]n+
Cu (O2 ) species that can be studied in detail, and to this end
several cupric superoxo complexes have been reported with
[
(L)Cu
4
compound reduction potentials (L = various
DMA
DMA
N vs
N S ligands) provide evidence that
3
N S is a
3
4
ligands containing either three or four N-atoms. In an attempt
weaker donor to copper ion than is found for any N4
ligand-complex.
to mimic the active site donors of PHM and DβM, considerable
effort has been devoted toward generating thioether containing
I
tridentate or tetradentate ligands to study their Cu /O
2
5
reactivity. In our own efforts we have been able to generate
binuclear peroxo-dicopper complexes with S(thioether) ligation,
but there has been no report of a mononuclear Cu (O
he copper monooxygenases peptidylglycine-α-hydroxylat-
ing monooxygenase (PHM) and dopamine-β-monoox-
ygenase (DβM) possess a dicopper active site, but it is
noncoupled”; the two copper ions are about 11 Å apart.
6
T
II
•−
)
2
species coordinated by a S(thioether) donor. Herein, for the first
1
“
time we present the spectroscopic evidence and reactivity of the
Extensive biochemical and biophysical research has shown that
one copper ion (designated Cu or Cu ) receives and passes
DMA
new mononuclear cupric superoxo complex, [( N S)-
3
II
•−
+
S
H
A
Cu (O2 )] (2 ) (Scheme 1b) possessing thioether S-ligation
that exhibits enhanced reactivity toward both O−H and C−H
substrates.
electron reducing equivalents to the His Met N S
2
2 (thioether)
ligated Cu (or Cu ) center, where O and substrate binding
M
B
2
2
DMA
7
occur. Recent computational analyses lead to the hypothesis
that an initial O -adduct, a Cu centered cupric superoxide
This new ligand,
N S, differs from our previously
3
2
M
reported N S ligands in two important ways. First, it possesses
3
II
•−
8
{
Cu (O2 )} moiety, forms from oxygenation of the fully
highly electron-rich dimethylamino (DMA) groups at the para
I
I
reduced (Cu ···Cu ) enzyme. This species performs the H-atom
abstraction of the peptide prohormone substrate (in PHM)
leading to C−H hydroxylation and formation of the product
hormone. Reaction mechanisms suggesting other possible
reactive intermediates being responsible for substrate C−H
position of the two pyridyl donors; this ligand design approach
is the same previously employed to stabilize a mononuclear
II
•−
c,h,9
Cu (O ) complex containing a N4 tripodal tetradentate
2
4
ligand.
Second, the thioether moiety is capped with a
bulkier o-methyl benzyl substituent to slow dimerization
3
6d
DMA
attack have been proposed. In support of the importance of
the Cu (O2 ) reaction intermediate’s involvement, a crystal
relative to previous designs. Treatment of
N S with
3
II
•−
I
[Cu (CH
CN)
]B(C
F
)
in THF, followed by pentane
3
4
6
5
4
structure obtained by Amzel and co-workers in the presence of
addition leads to the isolation of bright yellow powders with
DMA
I
7
formula [( N S)Cu ]B(C F ) (1). Single crystals could be
a substrate inhibitor reveals dioxygen bound to Cu in an end-
3
6 5 4
M
1
b
grown from 2-methyltetrahydrofuran (MeTHF)/pentane at
room temperature under Ar. As shown in Figure 1a, the
cuprous complex is a four-coordinate monomer ligated by two
on fashion, as depicted in Scheme 1a.
Undoubtedly, the thioether ligand plays a critical role in
determining the electronic structure and functions of Cu site
M
2
leading to C−H bond activation. However, the precise role of
Met coordination and the actual PHM reaction mechanism
have yet to be fully elucidated.
Received: November 8, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/ja511504n
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
−
1
resonance Raman (rR) spectroscopy (ν
= 821 cm ,
O−O
−1
7
νCu−O = 547 cm ). Although the initially formed species has a
short lifetime, the characteristic λmax value of 425 nm and the
II
transformation to a more stable (μ-1,2-peroxo)Cu strongly
2
DMA
suggest it is the S-ligated cupric superoxo species [( N S)-
Cu (O2 )] (2 ).
3
II
•−
+
S
Interestingly, when polar or potentially hydrogen-bonding
solvents such as acetone, ethanol, methanol, or trifluoroethanol
DMA
II
•−
+
S
(
TFE) are mixed with MeTHF, [( N S)Cu (O )] (2 ) is
3 2
much more persistent than is found in MeTHF only (Figure
a; λmax = 418, 605, 743 nm). There appears to be a shift of the
3
DMA
Figure 1. Displacement ellipsoid plots of the cations: (a) [( N S)-
3
I
+
DMA
II
+
Cu ] (1) and (b) [( N S)Cu (H O)(ClO )] (3). Hydrogen
3
2
4
atoms were removed for clarity.
pyridyls, one tertiary amine, and one thioether atom in a
distorted pyramidal geometry providing an open site for
potential dioxygen binding. The Cu−S bond length in 1
I
(
S(
2.2327(5) Å) falls in the usual range (2.2−2.44 Å) for Cu −
6
complexes. Many previously studied N S
−
thioether)
3 (thioether)
I
6d
Cu complexes form dimers in the solid state, and the
incorporation of the larger o-methylbenzyl group in this study
may disfavor their formation.
DMA
II
Furthermore, the copper(II) complex [(
N S)Cu
3
(
H O)(ClO )](ClO ) (3) (Figure 1b) was generated and
2
4
4
7
structurally characterized. A slightly distorted octahedral
coordination is observed. The thioether donor group is found
in an axial position with Cu−S bond distance of 2.6974(6) Å.
Electron paramagnetic resonance (EPR) measurement on 3
7
reveals a standard axial spectrum of copper(II) complex.
Bubbling O into a MeTHF solution of 1 at −135 °C led to
2
the immediate formation of a new species (λ = 425 nm) that
max
rapidly (∼50 s) converted to the thermodynamically more
DMA
stable end-on trans-peroxo-dicopper(II) species [{( N S)-
3
II
2− 2+
P
Cu } (μ-1,2-O )] (2 ) (Figure 2, top) exhibiting strong
2
2
−1
−1
UV−vis absorptions at λ = 534 (6500 M cm ) and 604
max
1
−1
(
6600 M− cm ) nm (Figure 2, bottom) consistent with the
Figure 3. (a) Low-temperature (−135 °C) UV−vis absorption
S P
12
spectrum of 2 (containing ∼11% 2 , λ = 526 nm, ε = 6500)
feature of previously synthesized S -ligated (μ-1,2-
max
(
thioether)
II
6
P
as recorded ∼40 s after bubbling O into a MeTHF:TFE (4:1)
peroxo)Cu . The formulation of 2 is confirmed by
2
2
S
solution of 1 (0.098 mM). (b) rR spectra of frozen 2 (0.62 mM, λ =
ex
7
4
13 nm, 77 K) in MeTHF:TFE (4:1).
equilibrium constant to the mononuclear superoxide over the
binuclear trans-peroxo complex. This result could be due to the
formation of a hydrogen bond between an O-atom of the
superoxide ligand and the TFE solvent, as observed in a
1
0
“
classical” cobalt(III)-superoxo complex with TFE.
S
The rR spectra of 2 (λ = 413 nm) reveal two dioxygen
ex
isotope sensitive vibrations (Figure 3b). An O−O stretch is
−
1
−1
18
observed at 1117 cm which shifts to 1056 cm upon
O
2
18
−1
substitution (Δ O = −61 cm ) and a Cu−O stretch at 460
2
−1
18
−1
cm (Δ O = −20 cm ). The energy and isotope shifts of
2
S
these vibrations confirm the assignment of 2 as an end-on
superoxide species. In fact, these parameters closely match
those for previously described cupric-superoxo complexes with
DMA
DMM
tripodal tetradentate N4 ligands ( tmpa and
(
tmpa)
−
1
4c,h
S
Table 1), both having νO−O = 1121 cm . The species 2 is
assigned as having an S = 1 (triplet) ground state based on
7
,11
literature precedent.
In MeTHF:TFE (4:1 v:v), an approximately 4:1 (mol/mol)
Figure 2. Low-temperature UV−vis absorption spectra of the reaction
S
P
12
equilibrium mixture of (2 )/(2 ) forms within ∼150 s
of 1 with O at −135 °C in MeTHF (0.2 mM). The superoxo product
2
S
DMA
I +
2
is observed immediately upon O addition (red) and converted to
following oxygenation of [( N S)Cu ] (1) at −135 °C. This
3
2
P
the peroxo 2 (t = 50 s, blue).
mixture shows minimal decomposition over 1 h; thus, we could
B
DOI: 10.1021/ja511504n
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
II
I
I
13
Table 1. Comparison of LCu /LCu Redox Potentials and
Scheme 2. Oxygenated Products of Ligand-Cu Complexes
II
•−
+
Reactivity of Derived [(L)Cu (O )] Complexes
2
a
II
•−
b
+
Reaction comparisons of [(L)Cu (O2 )] complexes (MeTHF:TFE
(
4:1) at −135 °C) toward substrates. Reactions do occur, but only
c
+ II
above −100 °C.
E
(vs Fc /Fc) for [(L)Cu (solvent)](ClO )
/2 4 2
7
1
II
complexes as determined by cyclic voltammetry.
MeTHF (Scheme 2c), both Cu -superoxo and (μ-1,2-peroxo)-
II
7
DMA
Cu complexes are generated, as in the case of
N S. This
2
3
S
DMA
carry out reactivity studies of 2 with the O−H and C−H
substrates, 2,6-di-tert-butyl-4-methoxyphenol (p-OMe-DTBP)
strongly suggests that the S(thioether)-atom donor of
ligand is coordinated in both Cu -superoxo/μ-1,2-peroxo
N S
3
II
S
P
III
2
and N-methyl-9,10-dihydroacridine (AcrH ). Pseudo-first-order
complexes, 2 and 2 . If it were not, a bis(μ-oxo)Cu
complex
N S)Cu (O
•−)]+ (2 ) is the first
3 2
2
17
kinetic behavior was observed upon addition of p-OMe-DTBP
would prominently form.
In summary, [(
DMA
II
S
(
with t1/2 = 3 min) or AcrH (t = 2 min) as monitored by the
2
1/2
S
disappearance of the 418 nm UV−vis band of 2 . Following
workup at room temperature the products were analyzed as 2,6-
di-tert-butyl-1,4-benzoquinone and 10-methyl-9-acridone, re-
spectively. Independent observations demonstrate that the
complex 2 is unreactive toward these substrates. Warming up
the 2 solution does not lead to sulfur oxygenation which can
occur in bis(μ-oxo)Cu species.
reported example of a cupric superoxo species supported by a
(thioether) donor. This advance allowed us to determine that a
superoxide species from an N S donor was more reactive
toward O−H and C−H bonds than the corresponding N
complex. These results indicate that one role of the Met ligand
in PHM and DβM is to activate the superoxide species for
reaction by increasing its electrophilicity. The synthesis of this
species will also allow us to further consider the role of
S
3
7
4
P
S
III
2
6d
S
To assess the effect of the thioether donor in 2 , we
II
•−
compared the reactivity of (L)Cu (O ) complexes (L =
thioether ligation on critical downstream O -reduced (and
2
2
4c,h
II
DMA
DMM
protonated) derivatives, such as Cu -hydroperoxo and to
perform detailed kinetic analysis of the preliminary reactivity
study presented here.
tmpa or
tmpa) toward the same substrates. Notably,
both cupric superoxide complexes showed no reactivity under
identical conditions (Table 1), although they do react above
S
−
100 °C. Thus, 2 with S(thioether)-ligation is more reactive than
ASSOCIATED CONTENT
Supporting Information
the closely related N superoxide compounds. The difference in
■
4
*
S
reactivity is rationalized by the different donor strengths of the
corresponding ligands. The substitution of a S(thioether) for a
N(pyridine) donor decreases the ligand field strength, consistent
with the Cu redox potentials for the separately isolated Cu
complexes in which thioether coordination raises the reduction
potential by 230 mV (Table 1). As a result, we hypothesize
that 2 is more electrophilic and hence better able to accept an
H-atom from either an O−H or C−H substrate, as compared
to the N complexes.
Details concerning X-ray crystallography analyses, including
CIF files, NMR, UV−vis, EPR, and rR spectroscopies, cyclic
voltammetry, ESI mass spectrometry, and procedures for
carrying out the reactivity studies. This material is available
free of charge via the Internet at http://pubs.acs.org.
II/I
II
7
S
AUTHOR INFORMATION
■
Corresponding Authors
*edward.solomon@stanford.edu
*karlin@jhu.edu
4
The influence of the Cu−S interaction on the oxygenated
products was further probed by comparing Cu/O species with
different donor atoms binding Cu. Three structurally analogous
ligands were synthesized replacing the sulfur site with carbon,
oxygen, and nitrogen (Scheme 2). The isolated Cu complex
2
Notes
The authors declare no competing financial interest.
7
I
DMA
with tridentate
N ligand exhibits a distinctive 382 nm
ACKNOWLEDGMENTS
3
7
■
absorption band upon O addition at −135 °C in MeTHF.
This absorption spectrum is consistent with a bis(μ-oxo)Cu
complex, which is known for many other products from low-
temperature oxygenation of Cu −N precursors.
identical reaction conditions, oxygenation of the Cu ( N O),
2
The authors acknowledge support of this research from the
National Institutes of Health (GM28962 to K.D.K., DK31450
to E.I.S., and Ruth L. Kirschstein National Research Service
Award F32-GM105288 to R.E.C.).
III
2
I
6d,14
Under
3
3
I DMA
possessing three N-donors and an ether O-atom, leads to the
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II
■
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4
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DOI: 10.1021/ja511504n
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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(
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(
9) We believe the increased stability comes about as a result of a
II
•−
larger rate (constant) for formation of a Cu (O ) complex, due to
2
the nature of the ligand donor group; the rate of reaction of
II
•−
I
Cu (O2 ) with a second ligand-Cu species is not as enhanced a
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(
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1
(
S
(
P
peroxo complex 2 was determined by first finding and calculating the
molar absorptivity (ε) for a solution of pure 2 in MeTHF at −135 °C.
Note: as described, initial oxygenation of a solution of a known
P
S
P
concentration of 1 gives the 2 /2 mixture, but waiting 5 min leads to
P
complete conversion to 2 .
(
13) See the Supporting Information for cyclic voltammograms and
II
I
redox potential values for the Cu /Cu complex couples for the
DMA
DMA
DMA
N ,
3
N O, and
3
N ligands.
4
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D
DOI: 10.1021/ja511504n
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX