Side-on cupric-superoxo triplet complexes as competent agents for H-abstraction relevant to the active site of PHM

Article abstract of DOI:10.1039/c5cc02332g

Copper complexes with N₃S donors mimic the Cu_M site of copper monooxygenases and react with O₂ affording side-on cupric-superoxo complexes capable of H-abstraction from dihydroanthracene and THF. Spectroscopic and DFT data of the Cu-superoxos support a spin triplet ground state for the side-on complexes, as well as a hemilabile thioether.

Full text of DOI:10.1039/c5cc02332g

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Side-on cupric–superoxo triplet complexes as
competent agents for H-abstraction relevant to
the active site of PHM†
Cite this:DOI: 10.1039/c5cc02332g
Received 19th March 2015,
Accepted 28th May 2015
a
b
cd
a
B. N. Sanchez-Eguıa, M. Flores-Alamo, M. Orio and I. Castillo*
´
´
DOI: 10.1039/c5cc02332g
www.rsc.org/chemcomm
Copper complexes with N
3
S donors mimic the Cu
M
site of copper
The formation of a precatalytic end-on cupric superoxo complex
2+
1
ꢀ
À
monooxygenases and react with O
2
affording side-on cupric–superoxo [CuM –Z -O ] has been established by X-ray crystallography
2
3a
2
complexes capable of H-abstraction from dihydroanthracene and THF. (Scheme 1a), although side-on Z -O -bound complexes have also
2
Spectroscopic and DFT data of the Cu–superoxos support a spin triplet been evaluated as viable intermediates responsible for C–H activa-
5
ground state for the side-on complexes, as well as a hemilabile thioether. tion (Scheme 1b). Model complexes have been designed based on
the His
the properties of [Cu–O
Efforts have been focused on tripodal ligands with tetradentate
2
Met active sites of Cu–monooxygenases to gain insight into
] adducts, particularly cupric superoxos.
Dioxygen activation by earth-abundant copper in chemical and
biochemical systems is an intense area of research due to its
relevance in the functionalisation of a wide range of substrates:
2
1
6
N S donor sets, including a recent report on a pyridine-supported,
3
2
+
1
ꢀ
À
6d
2
understanding the intimate details of O activation at copper centres
thioether-containing [Cu –Z -O ] complex. Benzimidazole-
2
may lead to the efficient use of dioxygen in selective oxidative
based N S–copper complexes mimic specific aspects of mono-
3
2+
1
transformations. In copper enzymes, the readily accessible Cu /
oxygenase enzymes: oxygenation of [L Cu]ClO
4
resulted in electro-
+
ꢀ
À
2+
7
Cu redox couple is involved in the selective oxidation of hydro-
catalytic O
2
formation, presumably via a putative Cu –superoxo.
1
d
2
carbons, including the hydroxylation of methane. Intriguingly,
some of the active sites of copper monooxygenases that activate
This reactivity is related to the strong s-donor properties of
benzimidazoles, which place them as closer analogues of histidine
than pyridine and amine derivatives, based on the pK and Kass
a
3
strong aliphatic C–H bonds feature methionine-derived sulfur
donors in addition to the ubiquitous imidazole donors from
histidine residues (His
the Cu sites of peptidylglycine a-hydroxylating monooxygenase
PHM), dopamine b-monooxygenase (DbM), and tyramine b-mono-
2
Met). These thioether donors present in
M
(
4
oxygenase (TbM) appear to modulate the redox potential of the
2+
+
Cu /Cu couple. Nonetheless, a detailed understanding of the
electronic influence of thioethers to selectively hydroxylate unacti-
vated C–H bonds remains elusive. An additional question that
arises in these systems is the inertness of the thioether moiety
towards oxidation during catalytic turnover conditions that could
potentially result in the corresponding sulfoxide or sulfone.
a
Instituto de Qu ´ı mica, Universidad Nacional Aut o´ noma de M ´e xico, Circuito
Exterior, CU, M ´e xico DF, 04510, Mexico. E-mail: joseivan@unam.mx
b
Facultad de Qu ´ı mica, Divisi o´ n de Estudios de Posgrado, Universidad Nacional
Aut o´ noma de M ´e xico, CU, M ´e xico DF, 04510, Mexico
c
Laboratoire de Spectrochimie Infrarouge et Raman, Universit ´e des Sciences et
2
+
1
ꢀ
À
Technologies de Lille, UMR CNRS 8516, 59655 Villeneuve d’Ascq Cedex, France
Scheme 1 (a) Precatalytic end-on Cu–superoxo [Cu –Z -O
2
] in PHM;
analogue; (c) ligands
d
2+
2
ꢀ
À
Institut des Sciences Mol ´e culaires de Marseille, Aix Marseille Universit ´e , CNRS,
(b) the side-on Cu–superoxo [Cu –Z -O ]
2
2
Centrale Marseille, ISM2 UMR 7313, 13397, Marseille, France
employed; (d) mercury diagram of L CuCl at the 50% probability level;
H-atoms omitted for clarity. Selected bond distances (Å): Cu1–N1 2.053(2),
Cu1–N11 2.027(2), Cu1–S26 2.691(1), Cu1–Cl1 2.350(1); angles (1): N1–Cu1–N11
116.35(7), N1–Cu1–S26 123.65(5), N11–Cu1–S26 86.86(5), N1–Cu1–Cl1
112.47(5), N11–Cu1–Cl1 114.36(5), S26–Cu1–Cl1 100.29(2).
†
Electronic supplementary information (ESI) available: Full experimental, crys-
tallographic, and computational details. CCDC 999325, 1036422, 1036423 and
053447. For ESI and crystallographic data in CIF or other electronic format see
1
DOI: 10.1039/c5cc02332g
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2
+ 7a,8
À1
À1 11,12
with Cu .
To further establish the capability of benzimidazole- with extinction coefficients 420 000 M cm
.
Although
based platforms to emulate histidine donors in copper monooxy- incomplete oxygenation could account for the low apparent e values,
À
À
genases, we herein report a detailed study of the reactivity of O
addition of chelating anions such as PhCO2 or CH SO3 that
3
2
1
+
2
+
with [L Cu] and the related [L Cu] , featuring a methylthioether favour the formation of side-on peroxo dimers did not have an effect
group to more closely approach the methionine donor in PHM on O -saturated solutions of [L Cu] (Fig. S15 and S16, ESI†).
n
+
13,14
A
2
1
2
(
L , L : [N-Me-2-CH
ꢀ
À
Comparison of the coordination environment provided by cupric–superoxo species [Cu –Z -O
2
2
(C
7
H
4
N
2
)]
2
NCH
2
CH
2
SR, Scheme 1c).
better fit of the observed optical features corresponds to side-on
2+
2
] that lack thioether bonding,
2
Met donor set in PHM was allowed by the solid- similar to those reported by Tolman and coworkers (see DFT studies
2
L with the His
state structure of L CuCl (and its Cu analogue, Fig. S2 in the ESI†). below). Confirmation of the proposed mononuclear [L Cu –
2
2+
10
n
2+
2
ꢀ
À
2
The complexes were obtained from the reaction of equimolar Z -O
] formulations by resonance Raman spectroscopy was likely
amounts of L and the appropriate copper salt. The Cu centre in precluded by the photosensitivity of frozen solutions (liquid N , l
2
+
2
ex
2
L CuCl is in a distorted tetrahedral environment defined by benz- 335 nm) at À125 or À80 1C in 2-MeTHF or THF, respectively.
1
imidazole N, thioether S, and Cl atoms, with the tertiary amine at a Nonetheless, the reaction of [L Cu]OTf with O in THF-d at
2
8
1
2
non-bonding distance of 2.819(2) Å (Scheme 1d). The average Cu–N À80 1C was monitored by H NMR spectroscopy ([L Cu]OTf is
bond length of 2.04 and the Cu–S distance of 2.691(1) Å are considerably less soluble at this temperature). The spectrum in
comparable to the corresponding Cu–N and Cu–S distances of Fig. S12 (ESI†) reveals a single set of paramagnetically broadened
9
1
2
+
2
.03 and 2.68 Å in PHM. Both L and L give rise to thioether- signals reminiscent of the ESR-silent [Cu(HIPT
3
tren)O
2
] reported
+
15
bridged Cu dimers in the solid state when non-coordinating by the group of Itoh, and assigned as a spin triplet by Karlin,
n
À
À 7a
À
À
16
counterions are employed ([L Cu] X , X = ClO
, CF SO3 (OTf ); Solomon, and coworkers. Signal broadening by thioether-bridged
3
2
2
4
+
10d
CuÁÁÁCu distances 5.407(1) and 5.055(2) Å, Fig. S3 in the ESI†).
dimer formation appears to be restricted to Cu complexes.
To gain insight into the bonding and electronic properties of
n
[
L Cu] OTf are nonetheless monomeric in THF solution, as
2 2
1
n
+
corroborated by H NMR spectroscopy: both complexes give rise to [L CuO
a single set of relatively sharp resonances for the equivalent ware package. Di- and monocopper species, with and without
benzimidazole groups in THF-d S vs. N donor sets), were considered in both
2
] , DFT studies were carried out using the ORCA soft-
1
7
8
, even at À40 1C (Fig. S5, ESI†), S-bonding (N
3
3
n
+
consistent with monomeric [L Cu] formulations. The presence of singlet and triplet states. Experimental data were confronted
aggregates would likely result in more than the two multiplets with theoretical calculations by comparing UV-vis spectra with
1
0d
observed for the NCH CH S linkers between d 3.3 and 3.5 ppm.
TD-DFT results. Geometry optimisation of side-on bridging
2
2
2
+
2
2
Furthermore, DOSY NMR confirmed the monomeric nature of the peroxos [Cu –m-Z :Z -O ] resulted in S-uncoordinated (N )
2
2
3
complexes (Fig. S6, ESI†).
Bubbling O
into THF solutions of [L Cu] at À80 1C resulted different from the experimental ones (Fig. S17 and S18, ESI†),
in ESR-silent species after 15 min, with very similar optical with very intense bands predicted at ca. 410 and 550 nm for L , and
singlet species that gave rise to UV-vis spectra drastically
n
+
2
1
1
2
2
features for both L and L -complexes that are stable for at least 370 and 450 nm for L complexes, respectively (Fig. S19 and S20,
1
À1
À1
2
2
6
h (L : l = 336, 614 nm, e B 1500, 285 M cm ; L : l = 339, ESI†). Optimisation of monomeric thioether-bonded complexes
09 nm, e B 2300, 119 M cm , Fig. 1 and Fig. S12, ESI†). (N S) resulted in triplet end-on cupric–superoxos (Fig. S21 and
3
High energy charge transfer bands have been associated to S22), the singlets being higher in energy by 17 kJ mol
dimeric side-on bridging peroxo complexes [Cu –m-Z :Z -O ] expected. The TD-DFT calculated data for the triplet end-on
À1
À1
À1
as
2
+
2
2
16
2
2
n
+
[
L CuO ] species differ considerably from the experimental UV-vis
spectra, with absorbances at ca. 400 and 550 nm for [L CuO
75 nm for [L CuO ] (Fig. S23, ESI†). Monomeric complexes with
2
1
+
2
] , and
2
+
4
2
uncoordinated thioethers result in side-on structures that were
analysed as singlets and triplets (Fig. 1 and Fig. S24 and S25). A
better match of electronic spectra is obtained for the triplet states
with respect to the experimental ones, with a strong band predicted
around 340 nm, and a d–d band at ca. 740 nm (Fig. 1, red trace, and
Fig. S26, ESI†); in contrast, the calculated UV-vis spectra of the
singlets predict intense transitions around 300 nm (Fig. 1, blue
1
+
À1
trace). For [L CuO
spatial overlap S = 0.568) was obtained from BS-DFT calculations
Fig. S29, ESI†). This value shows that the two unpaired electrons of
2
] an exchange coupling constant J = +250 cm
(
(
the complex are strongly ferromagnetically coupled, consistent with
À1
a triplet ground spin state that is stabilised by 6.0 kJ mol with
2
+
respect to the singlet. Similar results were obtained for [L CuO ]
2
À1
with a calculated exchange coupling constant of +237 cm (spatial
1
+
Fig. 1 Experimental UV-vis spectrum of [L Cu] after bubbling O
2
in THF
overlap S = 0.570) consistent with a strong ferromagnetic interaction.
The triplet is also the favoured ground spin state by 5.7 kJ mol
at À80 1C (intensity in e), and simulated spectra for triplet and singlet
À1
1
2
+
2
S-uncoordinated side-on optimised [L Cu-Z -O ] (intensity in a.u.). Inset:
optimised structure of the triplet.
(Fig. S30 and Table S4, ESI†).
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The dimers are insoluble in THF at À80 1C, with the solid-state
n
+
2
UV-vis spectra distinct from those of [L CuO ] , lacking the intense
absorption at B340 nm (Fig. S42, ESI†). H-abstraction from THF by
2
+
[
1
L CuO ] was confirmed by UV-vis spectroscopy at À40 1C (k =
2
À4 À1
.78 Â 10
s ; t = 3894 s, Fig. S44 and S45, ESI†). The capability
1/2
for C–H activation was further evaluated using 9,10-
dihydroanthracene (DHA) as a H-donor. The intensity of the charge
transfer band at B340 nm diminished over an hour with pseudo
first-order behaviour upon addition of a 10-fold molar excess (3 mM
n
+
solution of DHA in THF) to 0.3 mM solutions of [L CuO
for L , k = 0.002 s ; t1/2 = 315 s, Fig. S45 and S46, ESI†). Dilution of
2
] at À60 1C
2
À1
(
the samples with 3 mL of CH Cl , followed by quenching with 0.5 M
2
2
aqueous Na EDTA, evidenced the presence of anthracene and
2
1
+
anthraquinone as oxidation products. In the case of [L CuO
products were obtained in a 1 : 1 ratio. Significantly, [L CuO
2
] the
2
+
2
+
2
Fig. 2 Molecular orbital diagram of triplet side-on superoxo [L CuO ]
2
] is
with uncoordinated methylthioether.
more selective for H-abstraction without further oxidation, affording
the products in a 7 : 3 ratio (by GC-MS, Fig. 3 and Fig. S48 and S49,
ESI†). Similar H-abstraction and further oxidation of DHA has been
n
+
For both side-on [L CuO
2
] complexes a ferromagnetic inter-
2+
2
2
3+
observed with [Cu2 –m-Z :Z -O ] and [Cu (m-O) ] species, albeit
2
2
2
action is expected from an MO analysis due to the orthogonality
of the two magnetic orbitals depicted in Fig. S31 and S32 (ESI†).
The first one is an almost pure superoxide-centred p orbital
perpendicular to the Cu–OO plane, while the second one
corresponds to the antibonding interaction between the Cu
11b
with longer reaction times, predominant formation of anthraqui-
none, or the requirement for a larger excess of DHA. Among
20
21
Ã
v
cupric–superoxo complexes, singlet side-on species do not react with
10,18
C–H bonds.
Triplet end-on superoxos perform H-abstraction,
2+
6d,22
provided that the Cu centres are in a tetragonal environment,
trigonal bipyramidal (TBP) complexes being less reactive. This
behaviour parallels the reactivity of tetragonal cupric–hydroperoxo
complexes that activate C–H bonds, compared to their relatively
unreactive TBP counterparts. Within this context, triplet side-
Ã
x
d 2
–y
2
and the p orbitals (Fig. 2 and Fig. S33 and S34). Although
23
s
2
+
side-on coordination of the superoxide anion to the Cu
1
6
24
centres has been associated with singlet ground states, it is
n
+
25
2
noteworthy in the case of [L CuO ] that the long, calculated
n
+
Cu–O distances of 1.97 Å imply a relatively weak bond (experi-
mental av. Cu–O distance for side-on cupric–superoxo com-
on complexes [L CuO ] fulfill the requirements for enhanced
2
H-abstraction ability: tetragonal geometry and a triplet ground state.
1
0,18
plexes 1.84 Å).
Consequently, the bonding–antibonding
orbitals and the superoxo
] is weaker than the spin-pairing energy
likely intermediate in magnitude between canonical side-on
interaction between the Cu d
x
Ày
2 2
Ã
n
+
p orbitals in [L CuO
2
s
(
1
6
2
and end-on Cu–O interactions previously described), result-
ing in a triplet ground state. The geometry around the Cu
2
+
centres is square pyramidal, with the O and N-atoms from
benzimidazole donors in the basal positions (Cu–N average
distance 1.96 Å), and the tertiary amine in the apical positions
at a considerably longer distance (2.38 Å), defining N donor
3
n
sets from L (calculated CuÁ Á ÁS distances 5.86 and 5.83 Å,
respectively). The weak Cu–O interactions predicted for
n
+
[
2
L CuO ] may be ascribed to the strongly s-donating benzimi-
dazoles, consistent with the Natural Bond Order (NBO) analy-
sis. The calculated Wiberg index for the Cu–N bonds is indeed
1
9
larger (0.83) than that for the Cu–O bonds (0.73). As a
consequence, the superoxo moieties in the optimised square
pyramidal triplet complexes bear a considerable amount of
spin density, which may predispose them to H-abstraction.
n
In this context, attempts to isolate [L CuO
2
]OTf from reac-
n
+
2
Fig. 3 Proposed reaction of [L CuO
2
]
with DHA affording the final
n
n
2+
1
tions of [L Cu]OTf and O
2
in THF at À78 1C resulted in the products anthracene and [L Cu(m-OH)]
; Mercury diagram of [L Cu-
2+
(m-OH)]
2
at the 50% probability level. Selected bond distances (Å):
commonly observed bridging hydroxo complexes, evidencing
solvent C–H activation by the triplet side-on superoxos. Blue
Cu1–O1 1.913(2), Cu1–O1* 1.917(3), Cu1–N1 2.006(3), Cu1–N3 1.985(2),
Cu1–N5 2.460(3); angles (1): N1–Cu1–N3 96.9(1), N1–Cu1–N5 75.3(1),
N3–Cu1–N5 77.0(1), N1–Cu1–O1 162.3(1), N3–Cu1–O1 93.3(1), N1–
Cu1–O1* 92.3(1), N3–Cu1–O1* 168.5(1); inset: the GC-MS trace of organic
1
crystals of [L Cu(m-OH)] OTf were obtained after a week,
with symmetry-related Cu centres in square pyramidal N O
2
2
2
+
3
2
coordination environments (CuÁ Á ÁS distance 6.01 Å, Fig. 3). products with anthracene at m/z = 178.
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Since charge-transfer bands have been described for Cu –
ChemComm
2
+
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6a,24,25a
hydroperoxo complexes around 360 nm,
rapid H-abstraction from solvent, we ruled out such [L Cu–OOH]
species as the observed oxygenation products of [L Cu] . Cu –
which may form by
n
+
n
+
2+
hydroperoxides were prepared by addition of 5–10 equiv. of 1 : 1
7
n
2+
H
2
O
2
/Et
3
N THF solutions to the appropriate [L Cu] sources. In
addition to being relatively stable at room temperature and ESR-
n
+
active, the electronic spectra are distinct from those of [L CuO
2
]
1
9, 6067; (c) I. Castillo, B. N. S ´a nchez-Egu ´ı a, P. R. Mart ´ı nez-Alanis,
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8 B. U
¨
lk u¨ seven, I. Kizilcikli, A. Tavman and B. Akkurt, Rev. Inorg.
(
Fig. S35–S42, ESI†), with shoulders around 330 and 310 nm
2
+
potentially due to S - Cu charge-transfer bands of thioether-
bound cupric–hydroperoxos.
of [L Cu–OOH] at 20–40 1C resulted in only trace amounts of
6
a,7
Chem., 2001, 21, 369.
Addition of DHA to solutions
9
S. T. Prigge, A. S. Kolhekar, B. A. Eipper, R. E. Mains and
L. M. Amzel, Science, 1997, 278, 1300.
n
+
oxidation products (Fig. S50 and S51, ESI†), excluding them as 10 (a) D. J. E. Spencer, N. W. Aboelella, A. M. Reynolds, P. L. Holland
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efficient H-abstraction agents.
(
b) N. W. Aboelella, E. A. Lewis, A. M. Reynolds, W. W. Brennessel,
3
In summary, benzimidazole-based N S ligands support side-on
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cupric–superoxo complexes as ground state triplets, in contrast to
the prevalent view of side-on superoxos as singlets exclusively. The
long Cu–O bonds opposite to the strong Cu–N s-bonds in square
n
2
+
pyramidal [L Cu–Z -O ] arise from the small overlap between the
2
Ã
Cu dx2–y and the O
2
p
orbitals, giving rise to the triplet ground
2
s
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states. Despite being strong s-donors, benzimidazoles give rise to
copper complexes that are more electrophilic than their pyridine-
+
n
+/2+ 7c
based analogues (E1/2 B À170 mV vs. Fc /Fc for [L Cu]
,
6d
compared to À470 mV for pyridine-based N
3
S systems). This
+
may arise from stabilisation of Cu complexes by a low-lying
7a
LUMO, setting benzimidazoles apart from related nitrogen
donors, and enhancing H-abstraction. The methylthioether in
2
+
1
[
L CuO ] directs H-abstraction from DHA without further oxida-
2
M
tion, placing it as a close analogue of the Cu site of PHM. In this
1
context, the thioethers are weak donors, and may even act as
hemilabile ligands. Their role in triplet side-on superoxo complexes
in related chemical and biochemical systems awaits further
evaluation.
1
1
6 J. W. Ginsbach, R. L. Peterson, R. E. Cowley, K. D. Karlin and
E. I. Solomon, Inorg. Chem., 2013, 52, 12872.
We thank Prof. E. I. Solomon, J. W. Ginsbach for rR; Prof.
J. J. Garc ´ı a for GC-MS; S. Hern ´a ndez-Ortega for crystallography,
17 F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2012, 2, 73.
V. G o´ mez-Vidales for ESR, R. Pati n˜ o for IR, M. Orta for EA, B. 18 (a) K. Fujisawa, M. Tanaka, Y. Moro-oka and N. Kitajima, J. Am. Chem.
Quiroz, R. Gavi n˜ o for DOSY and VT-NMR, L. R ´ı os for ESI-MS.
Funding by Conacyt (151837, 254496), PAPIIT (IN210214), IANAS.
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