Coordination chemistry and reactivity of a cupric hydroperoxide species featuring a proximal h-bonding substituent

Article abstract of DOI:10.1021/ic302071e

At -90 °C in acetone, a stable hydroperoxo complex [(BA)Cu ^IIOOH]⁺ (2) (BA, a tetradentate N₄ ligand possessing a pendant -N(H)CH₂C₆H₅ group) is generated by reacting [(BA)Cu^II(CH₃COCH₃)] ²⁺ with only 1 equiv of H₂O₂/Et₃N. The exceptional stability of 2 is ascribed to internal H-bonding. Species 2 is also generated in a manner not previously known in copper chemistry, by adding 1.5 equiv of H₂O₂ (no base) to the cuprous complex [(BA)Cu^I]⁺. The broad implications for this finding are discussed. Species 2 slowly converts to a μ-1,2-peroxodicopper(II) analogue (3) characterized by UV-vis and resonance Raman spectroscopies. Unlike a close analogue not possessing internal H-bonding, 2 affords no oxidative reactivity with internal or external substrates. However, 2 can be protonated to release H₂O₂, but only with HClO₄, while 1 equiv Et₃N restores 2.

Full text of DOI:10.1021/ic302071e

Communication
pubs.acs.org/IC
Coordination Chemistry and Reactivity of a Cupric Hydroperoxide
Species Featuring a Proximal H‑Bonding Substituent
Sunghee Kim,† Claudio Saracini,† Maxime A. Siegler,† Natalia Drichko,‡ and Kenneth D. Karlin*,†
‡
†Department of Chemistry and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218,
United States
S
* Supporting Information
ABSTRACT: At −90 °C in acetone, a stable hydroperoxo
complex [(BA)CuIIOOH]+ (2) (BA, a tetradentate N4
ligand possessing a pendant −N(H)CH2C6H5 group) is
generated by reacting [(BA)CuII(CH3COCH3)]2+ with
only 1 equiv of H2O2/Et3N. The exceptional stability of 2
is ascribed to internal H-bonding. Species 2 is also
generated in a manner not previously known in copper
chemistry, by adding 1.5 equiv of H2O2 (no base) to the
cuprous complex [(BA)CuI]+. The broad implications for
this finding are discussed. Species 2 slowly converts to a μ-
1,2-peroxodicopper(II) analogue (3) characterized by
Figure 1. (a) trans-μ-1,2-peroxodicopper(II) complex with the ligand
UV−vis and resonance Raman spectroscopies. Unlike a
close analogue not possessing internal H-bonding, 2
affords no oxidative reactivity with internal or external
substrates. However, 2 can be protonated to release H2O2,
but only with HClO4, while 1 equiv Et3N restores 2.
TMPA, (b) Masuda’s [(bppa)CuIIOOH]+ complex stabilized by ligand-
derived H-bonding,4 (c) CuIIOOH species which undergo ligand-based
oxidative N-dealkylation chemistry,6 and (d) the new ligand, BA, the
subject of the current study. See the text.
scaffold by inputting H-bonding moieties on one or more of the
6-pyridyl positions, i.e., using a pivalamide group py-NHC(O)-t-
Bu (Figure 1b). In ongoing studies where we systematically alter
the pendant substrate group to aid in the elucidation of reaction
mechanism(s), we designed and synthesized a new ligand (BA),
possessing a potential H-bonding group (Figure 1d). Herein, we
report the synthesis, characterization and reactivity of the
complex [(BA)CuIIOOH](ClO4), which exhibits different
behavior than many other CuIIOOH containing complexes,
providing new insights and new questions for the future.
As a starting point, we synthesized the copper(II) complex
[(BA)CuII(CH3COCH3)](ClO4)2 (1) by simple combination of
the ligand and Cu(ClO4)2·6H2O in acetone.8 Single crystal X-ray
diffraction analysis determined its structure, revealing a slightly
distorted trigonal-bipyramidal (TBP) coordination (τ = 0.83; τ =
1.00 for idealized TBP geometries; Figure 2). EPR measure-
ments reveal a signature “reverse” axial (g∥ = 2.07, g⊥= 2.22)
spectrum known for TBP, and the two readily apparent d−d
envelopes seen in a UV−vis spectrum also have the relative
intensities known to be associated with solution TBP
coordination geometries.9
n this Communication, we describe the generation, character-
Iization, and reactivity of a copper(II) hydroperoxide complex
bearing a new tripodal tetradentate chelating ligand, which
confers unusual thermal stability and where its synthesis and
reactivity are distinctive. In the biochemistry of copper enzymes
which process molecular oxygen, mononuclear copper species
derived from copper(I) and dioxygen, such as cupric superoxide,
cupric hydroperoxide, and even high-valent copper oxo species,1
have all been considered as possible important intermediates in
certain copper monooxygenases (inserting one or two atoms
from O2 into a C−H substrate) and oxidases (effecting substrate
dehydrogenations where O2 is reduced to either H2O2 or water).
Examples are peptidyl-glycine-α-hydroxylating monooxygenase
and copper amine oxidases.2
It is critical to elucidate fundamental aspects of the formation
and structural/spectroscopic and reactivity characteristics of all
these intermediates. A part of our research program includes such
investigations, especially for CuIIO2•− and CuIIOOH complexes.
Although a number of well-characterized3 and even structurally
defined4 CuIIOOH complexes have been described, there is still a
considerable deficit in our understanding of the fundamental
properties of copper(II) hydroperoxide species, such as the
means for their formation from CuIIO2•− precursors5 and their
intrinsic scope of reactivity and mechanism(s).
The new mononuclear copper(II) hydroperoxide species,
formulated as [(BA)CuIIOOH]+ (2), could be generated at −90
°C in acetone by reacting complex 1 with 1 equiv of H2O2 (50%
aq) in the presence of Et3N (1 equiv) under Ar (Figure 2).
Complex 2 is bright green and exhibits a LMCT band at 393 nm
(ε = 1600 M−1 cm−1) which is characteristic of known ligand−
Tris(2-pyridylmethyl)amine (TMPA) and copper(I)/O2
chemistry lead to the now well-studied trans-μ-1,2-
peroxodicopper(II) complex [{(tmpa)CuII}2(O22−)]2+ (Figure
1a).7 Earlier, Masuda and Yamaguchi3a elaborated on the TMPA
Received: September 25, 2012
© XXXX American Chemical Society
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Inorganic Chemistry
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With the finding of H-bonding in 3, it follows that ligand−NH
to hydroperoxo H-bonding also occurs in 2, as supported by the
following: (i) The crystal structure of 1 reveals the existence of
intramolecular H-bonding between the ligand side arm N−H
and O atom of acetone (Figure 2 and SI8), and the acetone O
atom is in the same position with respect to copper and the
ligand, as is the proximal O atom in the CuIIOOH moiety. (ii)
DFT calculations8 confirm this assumption. In fact, the angle
∠O−H−N relaxes from ∼162° in 18 to 167° and 168° in the
optimized structure of 2 (calculated with the B3LYP or BP86
DFT functionals, respectively). The DFT-calculated values are
closer to the H-bonding ideal value of 180°.11 In addition, the
O···N distance is shortened in 2 compared to in 1, indicating that
H-bonding is stronger in 2; (Figure S4 and Table S2).8
Moreover, both the ∠O−H−N angle and the O···N distance
in 2 compare closely to those in Masuda’s [(bppa)Cu(OOH−)]+
(see Figure 1b) complex (see Table S2).
Figure 2. Displacement ellipsoid plot (50% probability) and
ChemDraw representation of [(BA)CuII(CH3COCH3)]2+ (1) and
reaction with 1 equiv H2O2/Et3N to generate 2 in ∼7 min. Also shown
are UV−vis spectra illustrating the formation of 2 (0.6 mM) at 393 (ε =
1600 M−1 cm−1), 672 (ε = 260 M−1 cm−1), and 823 (ε = 310 M−1 cm−1)
nm from 1 and an EPR spectrum of 2 (2 mM; X-band, ν = 9.186 GHz;
acetone at 70 K): g∥ = 2.00, A∥ = 90 G, g⊥ = 2.21, A⊥ = 93 G.
Interestingly, 2 can be generated from the reaction of the
copper(I) complex [(BA)CuI]B(C6F5)48 with 1.5 equiv of H2O2
in the absence of Et3N at −90 °C in acetone (eq 1).
CuIIOOH complexes. Again, solution TBP coordination is
indicated for 2, with the two d−d bands at 672 and 823 nm and
the observed EPR spectral characteristics (Figure 2). Ligand−
CuIIOOH complexes with TBP coordination are in fact
common.3a A highly notable characteristic of 2 is that its
formation requires only the addition of 1 equiv of H2O2; in all
other examples reported, excesses of hydrogen peroxide and base
(typically Et3N) are required for full formation.3a,d This indicates
exceptional stability for 2, which we propose to be due to H-
bonding taking place (see also below).
[(BA)CuI]+ + 3/2H2O2 → [(BA)CuII−OOH]+ + H2O
(1)
The yield, based on comparison of the absorptivity (at 393
nm) of 2, compared to that made via the 1/H2O2/Et3N
syntheses, is ≥85% for 1.5 equiv of H2O2 and essentially
quantitative with 4 equiv of H2O2. In fact, this is a new approach
in copper chemistry, to generate a CuIIOOH species by hydrogen
peroxide oxidation of the reduced metal complex. However, in
nonheme iron (nh-Fe) chemistry, such reactions, i.e., ligand−FeII
+ H2O2 giving FeIIIOOH species (which may go on to FeIVO
complexes), are relatively common.12 By analogy, a set of
reactions, eqs 2−4, can be proposed for our copper case. The
sequence of reactions required necessarily includes a high-valent
copper oxo product (best described as a CuIIO• species)13
generated by a “Fenton” type reaction (but formal peroxide
heterolytic cleavage)14 in the first step (eq 2).
Complex 2 is very stable at −90 °C in acetone but converts to a
new trans-μ-1,2-peroxodicopper complex [{(BA)-
CuII}2(O22−)]2+ (3) at −50 °C (Figure 3a). This could also be
CuI + H2O2 → CuIIO• + H2O
(2)
CuIIO• + CuI → CuIIOCuII
(3)
CuIIOCuII + 2H2O2 → 2CuIIOOH + H2O
(4)
CuI + 3/2H2O2 → CuIIOOH + H2O
(5)
The analogous nh-Fe product would be the now well-known
FeIVO complex; in one case from Que and co-workers,15 it
does only require 1 equiv of H2O2 plus a catalytic “base”. This
high-valent CuIIO• species would immediately react with the
starting copper(I) complex to give a highly basic CuIIOCuII
compound (eq 3), which further combines with H2O2 to give the
observed final products in the correct stoichiometry (eq 5). In
the Supporting Information, we show an alternative sequence of
reactions (a mechanism) where the more classic first reaction
produced is copper(II) + hydroxyl radical (•OH). However, this
may15a or may not occur in nh-Fe chemistry.
The importance of this finding, eq 1, is that unusual (e.g.,
CuIIOCuII) and unknown (e.g., CuIIO•) species are likely formed
in the reaction sequence (eqs 2−4). The characterization of such
entities is critically important to a full understanding of CuI/O2
(bio)chemistry. Future high priority investigations include
trapping and characterization of such intermediates accompanied
by detailed mechanistic inquiries.
Figure 3. (a) Transformation of 2 (0.6 mM) to a trans-μ-1,2-
peroxodicopper complex 3 or by reacting a [(BA)CuI]+ complex with
O2. (b) UV−vis spectra at −50 °C in acetone, where 2 (green)
transforms to 3 (purple), 2 → 3 + H2O2. (c) rR spectrum of 3 (0.6 mM)
measured in THF at −80 °C (λexcitation = 515 nm).8
directly generated by reacting [(BA)CuI]+ with O2 in acetone/
THF at −50 °C. Complex 3 was characterized by UV−vis, EPR
(“silent”), and resonance Raman (rR) spectroscopies (Figure
3b,c). The latter reveals an (O−O) stretch at 843 cm−1 (Δ18O2 =
−46 cm−1) and ν(Cu−O) = 540 cm−1 (Δ18O2 = −27 cm−1). On
the basis of prior work of Masuda et al.3c,10 and by comparison to
what is observed for [{(tmpa)CuII}2(O22−)]2+, lacking any H-
bonding,7 the UV−vis blue shift (525 to 498 nm) and rR shift of
ν(O−O) to higher frequency (831 to 843 cm−1) indicate that H-
bonding to the peroxo O atoms occurs in 3 (see Figure S88).
B
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Inorganic Chemistry
Communication
The enhanced relative stability of 2 compared to the close
analogues shown in Figure 1c, ascribed to H-bonding within 2,
and also reflected by the need for only 1 equiv of H2O2 for
formation (vide supra), in fact does inhibit oxidative N-
dealkylation chemistry within 2. In other words, prolonged
standing of 2 at −90 °C or warming gives no detectable
benzaldehyde. Furthermore, no spectral change occurs upon the
reaction of 2 with exogenous substrates such as a 2,6-di-tert-
butyl-4-methoxyphenol, 1-benzyl-1,4-dihydronicotinamide,1c
10-methyl-9,10-dihydroacridine,1c reducing agents (decamethyl-
ferrocene and cobaltocene), and 1-hydroxy-2,2,6,6-tetramethyl-
piperidine (TEMPO-H).5a
AUTHOR INFORMATION
Corresponding Author
*E-mail: karlin@jhu.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors acknowledge support of this research from the
National Institutes of Health, R01 GM28962.
■
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addition of HClO4 (1 equiv) to a solution of 2 in acetone at
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ASSOCIATED CONTENT
* Supporting Information
Synthetic and analytical details, UV−vis and EPR spectra, X-ray
structural details, and a cif file. This material is available free of
charge via the Internet at http://pubs.acs.org.
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