Reactivity of a stable copper-dioxygen complex

Article abstract of DOI:10.1039/c7cc05014c

We report the isolation of a room temperature stable dipyrromethene Cu(O₂) complex featuring a side-on O₂ coordination. Reactivity studies highlight the unique ability of the dioxygen adduct for both hydrogen-atom abstraction and acid/base chemistry towards phenols, demonstrating that side-on superoxide species can be reactive entities.

Full text of DOI:10.1039/c7cc05014c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Reactivity of a Stable Copper-Dioxygen Complex
a
a
b
a
a
Diana A. Iovan, Alexandra T. Wrobel, Arthur A. McClelland, Austin B. Scharf, Guy A. Edouard
a
Received 00th January 20xx,
Accepted 00th January 20xx
and Theodore A. Betley *
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report the isolation of
a
room temperature stable suggested that less electron donating ligands could increase
side-on the oxidative potency of such Cu(O ) complexes by inducing a
coordination. Reactivity studies highlight the unique ability of the Cu superoxide and, therefore, electrophilic character.
dioxygen adduct for both hydrogen-atom abstraction and Using weak-field dipyrromethene ligands, our group has
acid/base chemistry towards phenols, demonstrating that side-on demonstrated the stabilization of a unique ferric iminyl
dipyrromethene Cu(O
2
)
complex featuring
a
O
2
2
2
3
2
8, 29
superoxide species can be reactive entities.
species that is competent for C–H amination.
Encouraged
by these results, we set out to probe the potential of this
weakly donating scaffold to similarly stabilize a dipyrrin copper
Activation of dioxygen by copper systems plays an essential
1
-3
4-6
role in both biological and abiological transformations.
Studies on natural enzymes and synthetic complexes have
identified structural motifs of mono-, di-, and tri-copper sites
with varying degrees of dioxygen activation (superoxide,
superoxide complex and explore its reactivity. Herein, we
2
report the isolation of a side-on Cu(κ –O
2
) complex that
persists at room temperature and yet manifests both HAA and
acid/base chemistry towards phenolic O–H bonds.
7
-11
peroxide, oxo), and consequently distinct reactivity profiles.
As such, elucidating the correlation between the initially
generated 1:1 Cu(O ) adduct and the subsequent reactivity of
2
this intermediate towards substrates is essential for rationally
designing synthetic systems competent for selective oxidation
chemistry.
1
2-27
Among the Cu(O
2
) adducts isolated to date,
both end-
on and side-on coordination of dioxygen have been observed
II
to afford a Cu -superoxide unit that features either a triplet
12-21
22-25, 27
(end-on bound)
or a diamagnetic (side-on bound)
ground state. Reactivity studies identified the ability of
terminally bound Cu(O
2
1
) complexes to engage in hydrogen-
2, 14-18
Ar
Ar
atom abstraction (HAA)
from weak C–H and phenol O–H
1
2
Fig. 1 Solid state molecular structures for (a) ( L)Cu (1) and (b) ( L)Cu(O ) (2) with
4, 19
thermal ellipsoids at 50% probability level. Color scheme: Cu, aquamarine; N, blue;
O, red. Hydrogens and solvent molecules are omitted for clarity.
bonds or acid/base chemistry,
whereas no such
2
transformations can be effected at a side-on Cu(κ –O
2
)
2
complex. In line with these observations, DFT investigations
3
Ar
Metalation of ( L)Li with CuCl in tetrahydrofuran affords
Ar
I
Ar
28
(
L)Cu (
diffraction studies on single crystals obtained from
concentrated hexanes solution of reveal a three-coordinate
1) ( L: 5-mesityl-1,9-(2,4,6-Ph -C H )dipyrrin ). X-ray
3 6 2
a
1
2
copper centre featuring an η -interaction with one of the
ortho-phenyl groups of the 2,4,6-Ph -C H substituent (Fig.
3 6 2
1
a). Treatment of
as acetonitrile or ammonia displaces the bound arene to
afford the corresponding solvento adducts (Fig. S7, ESI ).
1 with strongly donating functionalities such
†
Upon exposure to vacuum, however, complete removal of
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these exogenous ligands is observed, demonstrating the coupled to a superoxide radical. However, following a similar
2
35
inherent stability of the η -coordination.
Addition of a stoichiometric amount of dioxygen to a characterized side-on Cu(κ –O
frozen benzene solution of results in immediate colour complete active space self-consistent field (CASSCF) method
protocol
applied for the two crystallographically
2
22,24
2
) complexes,
the use of a
1
change from purple (λmax 575 nm) to dark pink (λmax 545 nm) reveals the complex electronic nature of
2
. A CAS(2,2)
(see ESI†)
new diamagnetic resonances in the H NMR spectrum; identified the two σ(dxz + π*O2) and σ*(dxz – π*O2) active MOs
upon thawing the reaction mixture, along with formation of reference space on a truncated model of
2
1
however, full consumption of
Diffusion of pentane into a saturated benzene solution of
under an atmosphere of O afforded crystals suitable for X-ray with respect to a valence bond description unveiled 87.1%
diffraction, providing structural evidence for a side-on bound contributions from the Cu (O
1
is not observed (Fig. S9, ESI
†
). (Fig. S8a, Table S2, ESI
†). Subsequent localization of these MOs
1
(Fig. S8b, ESI ) and analysis of the resulting wavefunctions
†
2
II
•–
2
) configuration, with 9.4% and
Ar
I
0
III
2–
O
2
adduct ( L)Cu(O
2 2 2
) (2). The solid state molecular structure 3.5% participation of the Cu (O ) and Cu (O ) states,
(
Fig. 1b) displays an elongated O–O bond (1.383(2) Å) respectively (Table S3, ESI†). These results are consistent with
9
, 23, 30-34
2
compared to typical superoxide moieties (1.2–1.3 Å),
those calculated for the analogous β-diketiminato Cu(κ –O )
2
9
, 23, 30-34
35
II
more in line with a doubly reduced O moiety (~1.4 Å).
adduct and suggest a significant Cu -superoxide character
The Cu–O bond lengths of 1.8306(11) and 1.8360(12) Å, for
respectively, are also indicative of strong covalent
interaction. Further, the average Cu–Ndipyrrin bond lengths of
1.878(2) Å) are significantly shorter compared to those which tend to decay above –25 °C,
2
2.
a
Complex
2 exhibits interesting properties and reactivity
2
profile. Unlike the previously characterized Cu(O ) species,
2
2, 9, 14, 17, 19, 22-24
(
2
is
II
observed in several authentic dipyrrinato Cu species surprisingly persistent in solution at room temperature.
Ar
Ar
Ar
( L)CuCl: 1.890(2) Å; ( L)Cu(OEt): 1.919(3) Å; ( L)Cu(OPh): However, despite the relatively strong Cu(O ) interaction, the
(
2
2
1
.904(1) Å, ESI†). However, an anilido imine supported side-on η -arene association is still preferred; exposure of in situ
2
I
Cu(κ –O ) species featuring an O–O bond length of 1.392(3) Å generated
2
to vacuum results in conversion to the starting Cu
(Fig. S9, ESI ), akin to the behaviour reported for a
recently reassigned as a Cu superoxide unit based on Cu K tris(pyrazolyl)borate side-on bound Cu
2
2
7
and similarly contracted Cu–O and Cu–N bond lengths was complex
1
†
II
II
22
superoxide.
pre-edge XANES data as opposed to the originally proposed Additionally, variable temperature H NMR studies identified
1
III
35
Cu peroxide formulation. Finally, resonance Raman on
2
did an equilibrium between
not provide a definitive νO–O as the anticipated region (Fig. generation of at room temperature even in the presence of
S12-13, ESI ). As such, a definitive description of as either a excess dioxygen. O2 binding is favoured at lower
temperatures, with full conversion to observed in situ at –15
Preliminary DFT investigations using crystallographically °C (Fig. S10, ESI ) and oxygen release noted upon warming the
determined parameters identified closed-shell singlet reaction mixture back to room temperature. Such
ground state with the lowest unoccupied molecular orbital temperature dependent reversible O2 binding has been
MO) depicting the σ* interactions between the d orbital and previously described for a tris(tetramethylguanidino)tren end-
1 and 2, justifying the incomplete
8
2
†
2
III
II
Cu peroxide or a Cu superoxide is not possible.
2
36
†
a
(
xz
II
12
both the dipyrrin ligand and the O moiety. Given the weak on Cu superoxide.
2
ligand field of the dipyrrin scaffold and its influence on
Despite its robust nature,
as solution of the in situ generated
well as the multi-configurational character described for such consumption of the dioxygen adduct, giving rise to a H NMR
2
is not inert. Heating a benzene
28, 29
electronic structure evidenced in our previous studies
2
at 60 °C results in full
1
2
2, 24
side-on copper dioxygen species,
we chose to further spectrum with no well-defined features (Scheme 1). Notably,
. As discussed in no ligand hydroxylation was observed. Unfortunately,
literature reports, the broken symmetry (BS) approach fails structural confirmation of the final Cu complex ( ) has been
to provide a satisfactory result for these Cu(O ) systems, and unsuccessful. A frozen toluene EPR spectrum of collected at
examine the electronic structure of
2
3
5
3
3
2
1
II
indeed, we were unable to converge to a BS solution 77 K identifies an S = /2 signal consistent with a Cu
corresponding to cupric centre antiferromagnetically formulation (gCu = 2.24, ACu = 543 MHz, Fig. S14, ESI ). O–O
a
†
Scheme 1 Reactions leading to formation of complex 3.
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bond cleavage and decay of
Ar
possible, likely forming [( L)Cu(OH)]
2
into a copper hydroxide is coordinate, terminal
n
units, as a three-
3
7
copper hydroxide would be unstable.
A
tri-copper bond cleavage and formation of
4
is unclear at the moment,
formulation would be more plausible than a dimeric motif this result is in line with a superoxide character of the bound
given the cupric EPR signature of . Interestingly, the same moiety in
product was detected upon treatment of with excess urea
hydroperoxide at 65 °C (Scheme 1 and Fig. S15, ESI ),
suggesting that a Cu hydroperoxide species may form during
the thermal decomposition of . Such an intermediate would
arise upon a HAA step, transformation which has been
3
O
2
2.
1
†
2
II
12,
previously described for terminal Cu superoxide complexes,
1
4-18
23
albeit not for side-on bound Cu(O
the presence of a weak hydrogen atom source such as 2-
hydroxy-2-azaadamantane rapidly undergoes HAA at room
temperature to afford the corresponding N-oxide organic
radical as confirmed via EPR spectroscopy (Fig. S17, ESI ).
Further, reaction of with 1,4-cyclohexadiene (bond
dissociation energy (BDE) = 77
2
) adducts. Indeed, in
2
Scheme 3 Proposed equilibrium between side-on / end-on O bound in 2 and
subsequent reaction with DMPO.
2
Interestingly, despite the electrophilic character inferred
from the HAA ability of , the Cu/O adduct can also engage in
acid/base chemistry (Scheme 2). Addition of dioxygen to a
2
2
†
2
II
kcal
38
benzene solution of
1
and phenol furnishes two new Cu
). No
/
mol
)
proceeds at 45 °C,
generating a Cu species with an EPR signal not well defined,
but similar to that of (Scheme 1 and Fig. S18, ESI ),
suggesting that HAA from the allylic C–H bond may occur.
Following a similar HAA step, exposure of to 2,4-di-tert-
II
species as indicated by EPR spectroscopy (Fig. S23, ESI
†
1
C–C coupled product is detected by H NMR or GC–MS,
suggesting that a phenoxyl radical is not generated during the
3
†
reaction. Instead, phenol deprotonation and formation of the
2
Ar
corresponding ( L)Cu(OPh) (
7
) was confirmed via independent
).
with more acidic phenols such as 4-nitrophenol
butylphenol at 45 °C furnishes the expected C–C coupled
dimeric product quantitatively.
synthesis of the phenoxide complex (Figs. S4, S23, ESI
Treatment of
†
2
Treatment of
2 with either 2,6-di-tert-butylphenol or 2,4,6-
and 2-amino-4-tert-butylphenol produces a similar outcome,
tri-tert-butylphenol at 45 °C instead allows for isolation of 2,6-
di-tert-butyl-1,4-benzoquinone (identified via GC–MS, major
whereas addition of ammonium chloride furnishes the
Ar
19
respective ( L)CuCl (5) (Figs. S2, S24, ESI†). Both Tolman and
organic product, ESI
species by EPR spectroscopy (Scheme 2 and Fig. S20, ESI
Oxygenation of 2,6- and 2,4,6-substituted phenols to furnish
†
) along with detection of the same
1
4
Itoh have reported formation of copper phenoxides upon
reaction of copper superoxide complexes with para
substituted phenols. In these instances, the hypothesized
mechanism involves deprotonation of the phenol unit by the
superoxide moiety followed by ligand exchange to arrive at a
copper phenoxide and the hydroperoxide radical which
subsequently disproportionates into hydrogen peroxide and
water. Indeed, formation of hydrogen peroxide upon
3
†).
1
,4-benzoquinone derivative products has been demonstrated
1
with several Cu superoxide complexes.
2, 15, 16
Recent
1
6
mechanistic investigations by Karlin et al. have proposed
that the Cu superoxide is involved in both HAA and
subsequent reactivity with the phenoxyl radical to effect the
overall four-electron oxygenation. The reactivity of
2 is
treatment of
S25, ESI ), suggesting that the reaction likely follows an
analogous mechanism as previously described.
The switch in the reactivity of towards phenolic
substrates reflects a unique property of the dipyrrin Cu/O
complex relative to other copper dioxygen species. The similar
pK values (in DMSO) for phenol and 4-methoxyphenol or
2,4,6-tri-tert-butylphenol
deprotonation over HAA is dictated by the BDE of the O–H
bond. As such, undergoes HAA with substrates featuring
2 with phenol was confirmed by iodometry (Fig.
noteworthy considering that the analogous side-on bound
II
antiferromagnetically coupled Cu superoxide complexes
†
1
4, 19
reported display no such chemistry. Complex
potentially rearrange to a terminal O binding mode in situ
which could then undergo HAA and O-atom transfer as
described. Supporting this proposal, reaction of with the
radical trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) affords
2 could
2
2
2
2
a
3
9
II
(~17-19)
suggest
that
a new Cu species (Scheme 3 and Fig. S1, ESI
†
). That was
identified via X-ray crystallography as the ( L)Cu(O–DMPO)
adduct ( ) (Fig. S28, ESI ). Although the mechanism of O–O
Ar
2
kcal
4
†
3
8
BDEs up to 83
/mol and deprotonates those phenols that
necessitate higher oxidative potency for homolytic O–H
cleavage, thus demonstrating both electrophilic and basic
character. This is not
a common feature for other
mononuclear copper superoxide complexes in the literature
which manifest either reactivity profile, but not both. The end-
II
on Cu superoxide reported by Itoh is the only other example
2
to date of a Cu/O adduct engaging in acid/base chemistry
with 4-tert-butyphenol and undergoing HAA activity from
This journal is © The Royal Society of Chemistry 20xx
Scheme 2 Reactivity of ( L)Cu(O ) (2).
2
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Ar
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kcal
38
TEMPO–H (BDE ~ 70
/
mol), albeit access to any stronger O– 14.
T. Tano, Y. Okubo, A. Kunishita, M. Kubo, H. Sugimoto, N.
Fujieda, T. Ogura and S. Itoh, Inorg. Chem., 2013, 52,
kcal
14
H or C–H bonds (BDE > 70
/mol) was not demonstrated.
1
0431.
Complementing the proton and HAA reactivity,
2
also
1
1
5.
6.
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and K. D. Karlin, J. Am. Chem. Soc., 2007, 129, 264.
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promotes two-electron O-atom transfer chemistry to
triphenylphosphine (Scheme 2). Upon addition of dioxygen to
a benzene solution of
1 and substrate, regeneration of 1 was
accompanied by quantitative formation of triphenylphosphine
3
1
oxide as confirmed via both P NMR and GC–MS. No O-atom
transfer was detected when was prepared in the presence of
9
925.
2
1
7.
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dimethylsulfide or alkene sources (i.e., styrene, 1,3-
cyclohexadiene), most likely due to the high binding affinity of
the copper(I) centre for these substrates.
18.
The foregoing results showcase the isolation of a robust
dipyrromethene copper dioxygen adduct. Despite the side-on
1
2
9.
0.
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2
, this complex manifests unique reactivity
compared to previously described mononuclear Cu(O
2
)
species: both HAA and acid/base chemistry towards a range of
phenolic substrates as well as O-atom transfer to
triphenylphosphine. To account for the observed reactivity,
we propose an in situ formation of a more reactive terminal
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TOC Entry
A stable Cu–dioxygen adduct is observed that effects both H–atom abstraction and O–atom transfer
chemistry.