Rapid C-H bond activation by a monocopper(III)-hydroxide complex

Article abstract of DOI:10.1021/ja207882h

One-electron oxidation of the tetragonal Cu(II) complex [Bu ₄N][LCuOH] at -80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH₂). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M^-1 s^-1 at -80 °C, ΔH^? = 5.4(2) kcal mol^-1, ΔS^? = -30(2) eu) and with very large kinetic isotope effects (cf. k_H/k_D = 44 at -70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.

Full text of DOI:10.1021/ja207882h

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Rapid CꢀH Bond Activation by a Monocopper(III)ꢀHydroxide
Complex
Patrick J. Donoghue,† Jacqui Tehranchi,† Christopher J. Cramer,† Ritimukta Sarangi,‡ Edward I. Solomon,‡,§
and William B. Tolman*,†
†Department of Chemistry, Center for Metals in Biocatalysis, and Supercomputing Institute, University of Minnesota,
207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
‡Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
§Department of Chemistry, Stanford University, Stanford, California 94305, United States
S Supporting Information
b
Chart 1. Copper Oxygen Intermediate Cores
ABSTRACT: One-electron oxidation of the tetragonal
Cu(II) complex [Bu4N][LCuOH] at ꢀ80 °C generated
the reactive intermediate LCuOH, which was shown to be
a Cu(III) complex on the basis of spectroscopy and theory
(L = N,N0-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarbox-
amide). The complex LCuOH reacts with dihydroanthra-
cene to yield anthracene and the Cu(II) complex LCu-
(OH2). Kinetic studies showed that the reaction occurs
via H-atom abstraction via a second-order rate law at high
rates (cf. k = 1.1(1) Mꢀ1 sꢀ1 at ꢀ80 °C, ΔH‡ = 5.4(2) kcal
molꢀ1, ΔS‡ = ꢀ30(2) eu) and with very large kinetic iso-
tope effects (cf. kH/kD = 44 at ꢀ70 °C). The findings
As starting material we used the tetragonal copper(II) com-
plex LCu(CH3CN) (1, L = N,N0-bis(2,6-diisopropylphenyl)-
suggest that a Cu(III)ꢀOH moiety is a viable reactant in
oxidation catalysis.
2,6-pyridinedicarboxamide, Figure 1), for which we report a new
X-ray crystal structure (Figure S1). Treatment of 1 dissolved in
THF or suspended in Et2O with a solution of Bu4NX (X = Cl
or OH) in MeOH yielded deep green or blue [Bu4N][LCuX]
([Bu4N][2] or [Bu4N][3]), respectively. Both products were
isolated as solids and characterized by UVꢀvis, FTIR, and EPR
spectroscopy, and CHN analysis.12 Notable data include a sharp
ν(OH) at 3628 cmꢀ1 in the FTIR spectrum (nujol) of [Bu4N]-
[3] and essentially axial EPR signals for both complexes con-
sistent with tetragonal Cu(II) species (Figures S2ꢀS3 and Table
S1 in the Supporting Information). Crystals of both complexes
suitable for characterization by X-ray diffraction were obtained by
metathesis with PPNCl (Figures 1 and S4, Table 1). While the
structure of [PPN][2] is straightforward, in [PPN][3] the longer
than expected CuꢀO distance of 1.947(2) Å13 led us to suspect
that the crystals were a compositionally disordered14 mixture of
the chloride and hydroxide complexes, with the chloride ligand
having been derived from displacement of hydroxide by the
added PPNCl. Consistent with this notion, the UVꢀvis spectral
features for [Bu4N][3] in THF cleanly converted to those for
[Bu4N][2] upon addition of Bu4NCl (1 equiv from titration
experiments; Figure S5). In addition, comparison of UVꢀvis
spectra of crystals of [PPN][3] (3 batches) to those of pure
[Bu4N][2] and [Bu4N][3] indicated chloride contamination
levels of 22ꢀ28%. Finally, DFT (mPW) calculations were
performed for 2ꢀ and 3ꢀ that yielded minimum energy
opperꢀoxygen species are implicated as intermediates in a
C
wide range of oxidation reactions promoted by metallo-
enzymes,1 synthetic complexes,2,3 and materials.4 Understanding
the structures, properties, and reactivities of such species is
critical for obtaining mechanistic insights into oxidation catalysis
and for developing new, selective catalytic reagents and pro-
cesses. Efforts to isolate and characterize reactive copperꢀ
oxygen intermediates through studies of copper-containing en-
zymes and synthetic model complexes have led to the identi-
fication of several core motifs,5 exemplified by those shown
in Chart 1. Complexes comprising cores AꢀE have been well-
defined and, as a result, have often been considered as viable
oxidants in various catalytic reactions.6 The monocopper-oxo/
oxyl core (F) has also been discussed7 as a candidate capable of
attacking hydrocarbon substrates (cf. by peptidylglycine α-
hydroxylating monooxygenase8), but examples of such species
have only been characterized in the gas phase9 and through
theoretical calculations.8,10 We report herein the preparation and
characterization by spectroscopy and theory of a new type of
copperꢀoxygen intermediate containing a hydroxo-copper(III)
core (G), which formally may be considered as a protonated
version of F.11 The complex comprising G rapidly abstracts
hydrogen atoms from the CꢀH bonds of dihydroanthracene,
thus providing a key precedent for the possible involvement of G
in oxidation catalysis.
Received: August 19, 2011
Published: October 17, 2011
r
2011 American Chemical Society
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|

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Figure 2. UVꢀvis spectra of [Bu4N][3] (blue dashed line, right cuvette
in photograph) and 4 (solid black line, left cuvette in photograph) in
acetone at ꢀ80 °C.
Figure 1. Complexes and reactions studied in this work, with a
representation of the anionic portion of the X-ray crystal structure of
[PPN][2] shown in the box as 50% thermal ellipsoids (H-atoms omitted
for clarity).
Table 2. Selected Structural Parameters for 4 from mPW
Calculationsa
singlet
triplet
CuꢀN1
1.872
1.952
1.961
1.810
1.90
1.964
2.106
2.106
1.844
2.01
Table 1. Selected Interatomic Distances for 2ꢀ and 3ꢀ from
CuꢀN2
X-ray Crystallography and mPW Calculationsa
CuꢀN3
2ꢀ
3ꢀ
CuꢀO
CuꢀN/O (avg)
a All distances in Å.
X-ray
mPW
X-ray
mPW
CuꢀN1
CuꢀN2
CuꢀN3
CuꢀXb
CuꢀN/O
(avg)
1.926(2)
1.992(2)
1.993(2)
2.1842(8)
1.97
1.961
2.073
2.073
2.216
2.04
1.920(2)
1.996(2)
2.010(2)
1.9465(19)
1.97
1.947
2.078
2.080
1.863
1.99
absorption features decayed within minutes upon warming to
ꢀ60 °C. Titration experiments monitored by UVꢀvis spectros-
copy (Figure S12) showed that the oxidation reaction required 1
equiv of oxidant to reach completion. Reduction of this species
with 1 equiv of decamethylferrocene (Fc*) added to the chilled
solution yielded UVꢀvis features consistent with 3 and Fc*+
(Figures S13). These data are consistent with a reversible one-
electron process that generates LCuOH (4, Figure 1).
a Distances in Å; X-ray data for PPN+ salts. b For 2ꢀ, X = Cl; for 3ꢀ,
X = OH.
Lacking crystals suitable for X-ray crystallography, we turned
to DFT calculations and X-ray absorption spectroscopy to
further evaluate the nature of 4. The one-electron oxidation of
3ꢀ could conceivably result in (a) a metal-based oxidation to
yield a Cu(III) species or (b) a ligand-based oxidation to yield a
Cu(II)-ligand radical. A Cu(III) species should be a singlet
whereas a Cu(II)-ligand radical could be either a triplet or an
open-shell singlet. Starting from optimized mPW structures of
3ꢀ, a closed-shell singlet structure for 4 was located easily. This
oxidized singlet maintained the square-planar geometry and
symmetry elements identified in its Cu(II) precursor (Cs). Con-
sistent with the singlet Cu(III) formulation, and in agreement
with EXAFS results (see below), upon oxidation of 3ꢀ to 4 the
copperꢀligand bonds contract; the average CuꢀN bond short-
ens by 0.08 Å, and the CuꢀN/O bond average shortens by
0.09 Å (Table 2). Finally, the results of gas phase time dependent
DFT (TD B98) calculations for the singlet of 4 match well
with the experimentally observed UVꢀvis features (Figure S14):
the singlet wave function for 4 was calculated to have a single
transition at 589 nm (2.10 eV), within 0.2 eV of the observed
transition at 540 nm (2.30 eV). The dominant one-electron
excitation in this transition, with a weight of about 67%, is from
an orbital with largely ligand (L) π-character to one largely
comprised of the Cu dx2ꢀy2 (Figure S15). In sum, calculations
support a singlet Cu(III) formulation for 4.
structures with CuꢀN bond distances somewhat longer than
those measured by X-ray crystallography (Table 1), as is com-
mon for mPW calculations with bulky ligands.15 However, a
significantly shorter CuꢀOH distance was calculated for 3ꢀ
(1.863 Å) than was observed in the X-ray crystal structure
(1.9465(19) Å). EXAFS data collected for [Bu4N][3] (powder,
not exposed to PPNCl) were best fit with 4N/O donors16 and
average metalꢀligand distances of 1.95 Å (Figures S6 and S7),
slightly shorter than those determined by X-ray crystallography and
mPW (Table 1). The EXAFS data collected for [Bu4N][2] were
best fit with 3 N/O donors averaging 1.98 Å and one Cl donor at
2.21 Å, which is also in good agreement with the X-ray crystal
structure and mPW calculations. Taken together, the data support
the hypothesis of a compositionally disordered X-ray crystal struc-
ture for [PPN][3] with a CuꢀO distance that is overestimated by
∼0.1 Å.
The tetragonal Cu(II) complex [Bu4N][3] exhibits a pseu-
doreversible oxidation in acetone solution (0.1 M Bu4NPF6)
at room temperature with E1/2(3ꢀ) = ꢀ0.076 V vs Fc+/Fc
(Figure S11). Chemical oxidation of [Bu4N][3] in acetone was
performed using Fc+PF6ꢀ at ꢀ80 °C,17 which provided a deeply
colored purple solution characterized by an intense low energy
electronic absorption feature with λmax(ε) = 540 nm (∼12 400
Mꢀ1 cmꢀ1) (Figure 2). The solutions were EPR silent. The
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Figure 3. Normalized Cu K-edge XAS spectra of [Bu4N][3] (black)
and 4 (red). The inset shows the second derivative spectra of the pre-
edge region showing the shift on going from [Bu4N][3] to 4.
Figure 4. Plots of ln(k/T) vs 1/T (where k = second-order rate con-
stant, T = temperature in K) for the reaction of 4 with DHA (black
diamonds) or DHA-d4 (blue triangles). The shown linear fits were used
to calculate the activation parameters listed in the text via the Eyring
equation (R = 0.997 and 0.985, respectively).
Consistent with this conclusion, structures corresponding to a
ligand-based oxidation were more difficult to locate by mPW.
Thus, attempts to identify an open-shell singlet via a broken-
symmetry formalism for 4 converged to closed-shell singlet
solutions. A triplet structure can be located but is energetically
disfavored relative to the closed-shell singlet by 19.0 kcal/mol.
Metalꢀligand bond distances calculated for the triplet also are
relatively unchanged relative to those calculated for the precursor
3ꢀ, in contrast to the EXAFS results (see below). Thus, the
triplet 4 actually has a slightly longer average CuꢀN/O bond
length than the starting complex 3ꢀ (2.01 vs 1.99 Å). Finally, TD
B98 results for the triplet included several transitions in the near-
IR range that were not observed experimentally.
Figure 5. Comparison of the second-order rate constants (k in units of
Mꢀ1
s
ꢀ1, on a logarithmic scale) for oxidation of DHA at ꢀ80 °C by
Cu K-edge XAS data provide direct experimental evidence for
the Cu(III) formulation of 4. As shown in Figure 3, the Cu K pre-
edge energy position of 4 is shifted ∼1.7 eV to higher energy
relative to that of the Cu(II) complex [Bu4N][3], indicating one-
electron metal-based oxidation with an associated increase in
ligand field strength of the Cu center. A shift in the rising edge by
∼1.1 eV is also observed on going from [Bu4N][3] to 4, reflecting
the increase in the effective nuclear charge on Cu in 4.18 The
EXAFS data (Figures S6 and S8, Table S3) provide further
confirmation of the Cu(III) nature of 4. Fits to the data show an
∼0.1 Å shortening of the first shell CuꢀO/N bond distances in 4
(4 CuꢀN/O at 1.86 Å) relative to [Bu4N][3] (4 CuꢀN/O at
1.95 Å), typical for oxidation of a Cu(II) to a Cu(III) complex.
Inspired by reports of CꢀH bond activation by Mn(III),
Mn(IV), and Fe(III) hydroxide complexes,19 we examined the
reactivity of 4 with dihydroanthracene (DHA), a commonly
studied substrate useful for comparison purposes. The UVꢀvis
features of 4 smoothly decayed upon introduction of 0.5 equiv of
DHA to acetone solutions at low temperature, concomitant with
the growth of bands due to the formation of anthracene (85% yield
by GC/MS). The final inorganic product was determined to be
the Cu(II) complex LCu(OH2) (5, Figure 1) on the basis of
comparison of the UVꢀvis spectrum to that of independently
synthesized material, which was fully characterized, including by
X-ray crystallography (Figure S16). The UVꢀvis features of 4
decayed according to pseudo-first-order kinetics when the reac-
tion was performed with excess DHA (10ꢀ40 equiv), and a plot
of kobs vs [DHA] was linear with an intercept that corresponded
to the self-decay rate (Figures S17 and S18). These data indicate
an overall second-order rate law ꢀd[4]/dt = k[4][DHA]. A very
large kinetic isotope effect (kH/kD) of 44 at ꢀ70 °C was observed
when the reaction was performed with DHA-d4, and the
selected copper, iron, and manganese complexes with that of LCuOH
(4). For (g) and (h), reported rate constants were used; for (a)ꢀ(f), the
rate constants were calculated using the Eyring equation from the
reported activation parameters. (a) [CuIII(Pre)]+ (ref 20); (b) [FeIII-
(Hbim)(H2bim)2]2+ (ref 21); (c) [FeIII(PY5)OH]2+ (ref 22); (d)
[MnIII(H3buea)O]ꢀ, [MnO4]ꢀ (refs 23, 24); (e) [MnIII(PY5)OH]2+
(ref 25); (f) [MnIV(H3buea)O]ꢀ (ref 23); (g) [L0(CH3CN)FeIVO]2+
(ref 26); and (h) [L0FeIII(OH)(μ-O)(L0)FeIVO]2+ (ref 26). Abbre-
viations: Pre = 3,9-dimethyl-4,8-diazaundecane-2,10-dionedioximate,
H2bim = 2,20-bi-imidazoline, PY5 = 2,6-bis(bis(2-pyridyl)methoxy-
methane)pyridine, H3buea3ꢀ = triply deprotonated form of tris[(N0-
tert-butylureayl)-N-ethylene]amine, L0 = tris((4-methoxy-3,5-dimethyl-
pyridin-2-yl)methyl)amine.
temperature dependence of this effect was evaluated by an Eyring
analysis (Figure 4, Table S4); apparent ΔH‡H = 5.4(2) kcal molꢀ1
,
ΔS‡H = ꢀ30(2) eu, ΔH‡D = 6.2(3) kcal molꢀ1, ΔS‡D = ꢀ34(3)
eu. Based on these parameters, the KIE was calculated to be 29 at
25 °C, well beyond the semiclassical limit. Finally, we note that
4 reacted essentially instantaneously at ꢀ80 °C with 2,4,6-tri-
tert-butylphenol to yield the phenoxyl radical (UVꢀvis, EPR).
Together, the reactivity and kinetic data support a mechanism for
oxidation of DHA to anthracene that involves rate-determining
H-atom abstraction by the Cu(III)ꢀOH moiety, with a tunneling
contribution being a reasonable rationale for the larger than
semiclassically predicted kH/kD values.
The second-order rate constants for reactions of selected rele-
vant Cu(III) or metal-oxo or ꢀhydroxo complexes with DHA at
ꢀ80 °C are compared to that of 4 in Figure 5.20ꢀ26 The rate of
H-atom abstraction from DHA by LCuOH is significantly faster
than most other nonheme iron (b,c,g), manganese (dꢀf), and
copper (a) reagents and is comparable to a recently reported
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high-spin Fe(III)Fe(IV)-oxo complex (h). While not as intrinsi-
cally reactive as the (P+)Fe(IV)dO intermediate (compound I)
in cytochrome P450,27 the high rate for DHA oxidation by 4 is
nonetheless extraordinary in the context of known oxidizing
capabilities of copperꢀoxygen species.5,7,28 While further work is
necessary to understand the basis for this reactivity, we speculate
that the high basicity of the hydroxide group is a key factor in view
of the fact that the potential for the 3ꢀ/4 couple is modest
(ꢀ0.076 V vs Fc+/Fc).29 Similar arguments have been advanced
to rationalize observed rates of H-atom transfer by Mn-oxo.30,31
and Fe(IV)-imido32 complexes, as well as cuprous oxide sur-
faces.33 The relatively minor structural differences (overall geo-
metry, metalꢀligand bond distances) between 4 and 5 may also
result in a small reorganization energy that could contribute to a
high reaction rate.34
In summary, we have prepared a new type of copperꢀoxygen
intermediate and shown through theory and experiment that it is
best described as a singlet Cu(III)ꢀOH complex. High rates of
H-atom abstraction from phenols and DHA by this complex
were observed, indicating that such a species should be consid-
ered as a viable intermediate in catalytic oxidation reactions.
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’ ASSOCIATED CONTENT
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(17) XAS samples were prepared using (p-tolyl)3N+PF6
.
ꢀ
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’ ACKNOWLEDGMENT
This research was supported by the NIH (Grants R37-
GM47365 to W.B.T.; DK-31450 to E.I.S.) and the NSF
(CHE-0952054 to C.J.C.). SSRL operations are funded by the
Department of Energy, Office of Basic Energy Sciences. The
SSRL Structural Molecular Biology program is supported by the
National Institutes of Health, National Center for Research
Resources, Biomedical Technology Program, and the Department
of Energy, Office of Biological and Environmental Research. This
publication was made possible by Award P41 RR001209 from the
National Center for Research Resources (NCRR), a component
of the National Institutes of Health (NIH).
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’ NOTE ADDED AFTER ASAP PUBLICATION
The values in Table 2 were corrected, to agree with the data
discussed in the text, on November 2, 2011.
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