Structural Characterization of the [CuOR]2+Core

Article abstract of DOI:10.1021/jacs.0c13470

Formal Cu(III) complexes bearing an oxygen-based auxiliary ligand ([CuOR]2+, R = H or CH2CF3) were stabilized by modulating the donor character of supporting ligand LY (LY = 4-Y, N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Y = H or OMe) and/or the basicity of the auxiliary ligand, enabling the first characterization of these typically highly reactive cores by NMR spectroscopy and X-ray crystallography. Enhanced lifetimes in solution and slowed rates of PCET with a phenol substrate were observed. NMR spectra corroborate the S = 0 ground states of the complexes, and X-ray structures reveal shortened Cu-ligand bond distances that match well with theory.

Full text of DOI:10.1021/jacs.0c13470

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Structural Characterization of the [CuOR]2+ Core
V. Mahesh Krishnan,§ Dimitar Y. Shopov,§ Caitlin J. Bouchey, Wilson D. Bailey, Riffat Parveen,
Bess Vlaisavljevich,* and William B. Tolman*
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ABSTRACT: Formal Cu(III) complexes bearing an oxygen-based auxiliary ligand ([CuOR]2+, R = H or CH2CF3) were stabilized
by modulating the donor character of supporting ligand LY (LY = 4-Y, N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Y
= H or OMe) and/or the basicity of the auxiliary ligand, enabling the first characterization of these typically highly reactive cores by
NMR spectroscopy and X-ray crystallography. Enhanced lifetimes in solution and slowed rates of PCET with a phenol substrate
were observed. NMR spectra corroborate the S = 0 ground states of the complexes, and X-ray structures reveal shortened Cu−ligand
bond distances that match well with theory.
nderstanding the molecular structures, spectroscopic
shifts in the LMCT energies (lower), redox potentials
(higher), basicities (lower), and PCET reaction rates (faster)
that may be rationalized by electron withdrawal by the X
groups. Changing the pyridyl group to a piperidine
(LpipCuOH) has the opposite effects, attributed to greater
electron donation by the amine donor. In no case has a
complex with the [CuOH]2+ core been structurally defined by
X-ray crystallography, in large part due to its high reactivity
and poor thermal stability (t1/2 ≈ minutes at −80 °C in THF).
These characteristics and the fact that decomposition yields
paramagnetic Cu(II) species have also inhibited efforts to
obtain NMR spectra that would be useful in confirming the
proposed S = 0 ground state. Significantly slower reactions and
greater stability were observed for the LCuZ derivatives (Z =
halide8,16 or carboxylate17) featuring a less-basic X-type
reactive moiety, of which the halide complexes were
characterized recently by X-ray diffraction and NMR spectros-
copy.16 Inspired by that success, we hypothesized that more
complete characterization of the [CuOH]2+ core might be
attained if its stability could be enhanced. Toward this end, we
targeted two modifications: the placement of an electron-
donating para-methoxy pyridyl group (Y = OMe; LOMe) and
the use of a less-basic alkoxide moiety (CF3CH2O−). On the
basis of previous work,9,11,12 we hypothesized that these
changes would enhance the solution stability, thereby
facilitating handling and characterization. We now report the
confirmation of these hypotheses through the synthesis of the
targeted derivatives and their characterization by X-ray
crystallography and NMR spectroscopy, which with compar-
ison to results from theory provides new insights into the
U
properties, and reactivity of copper−oxygen com-
plexes1−5 is important for gaining insight into the mechanisms
by which copper enzymes and other catalysts function.6,7
Among the various complexes studied, those comprising the
[CuOH]2+ core supported by dicarboxamide ligands (Figure
1)8−14 are notably reactive, attacking C−H and O−H bonds
Figure 1. Complexes studied previously and the compounds that
comprise the focus of this work. R′ = Me or aryl groups.
via proton-coupled electron transfer (PCET) processes and
undergoing electron transfer at high rates (cf. the rate constant
for the reaction of LHCuOH with 1,2-dihydroanthracene 50
M−1 s−1 at −25 °C;9 electron-transfer self-exchange rate
constant ∼104 M−1 s−1 at −88 °C13). The formulations of the
complexes are supported by UV−vis spectroscopy (diagnostic
absorptions with ligand-to-metal charge-transfer (LMCT)
character), EPR silence, resonance Raman spectroscopy
(νCu−O ≈ 630 cm−1),13 EXAFS (avg Cu−N,O ≈ 0.1 Å shorter
than for the [CuIIOH]+ precursor),8 and theory.8,15
Received: January 2, 2021
Published: February 23, 2021
Perturbations of the [CuOH]2+ core have been effected by
the installation of remote substituents on the flanking aryl rings
(LXCuOH, Figure 1).11,12 These perturbations are reflected by
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molecular and electronic structures of complexes with
[CuOH]2+ and [CuOCH2CF3]2+ cores.
infra). A more modest enhancement of reversibility is seen for
[NBu4][LOMeCuOH] relative to the analog supported by LH
(Figure S18).5 The replacement of −OH with −OCH2CF3
shifts the E1/2 by +0.191 V on average, whereas the alteration
of LY had a lesser effect, with an average difference of only
−0.049 V induced by the introduction of the p-OMe
substituent.
The reaction of proligand H2LOMe with Cu(OTf)2 in the
presence of NaOMe and CH3CN led to the isolation of key
precursor LOMeCu(CH3CN) (77%), which upon treatment
with NBu4OH led to [NBu4][LOMeCuOH] (76%). [NBu4]-
[LYCuOCH2CF3] (Y = H or OMe) complexes were obtained
from the respective hydroxide precursors via protonolysis with
HOCH2CF3. The complexes LOMeCu(CH3CN), [NBu4]-
[LOMeCuOH], and [NBu4][LYCuOCH2CF3] (Y = H or
OMe) were characterized by UV−vis and X-band EPR
spectroscopy, CHN analysis, and X-ray crystallography (SI).
The complexes feature a slightly distorted square-planar
geometry (τ4 = 0.15−0.22)18 with Cu−N,O bond distances
typical for such Cu(II) species (Figures S43−S45; Table 1)
and characteristic S = 1/2 rhombic signals with Cu and N
hyperfine patterns in EPR spectra (Figures S7−S9).
The chemical oxidation of [NBu4][LOMeCuOH] or [NBu4]-
[LYCuOCH2CF3] (Y = H or OMe) was performed by adding
1 equiv of FcBArF or AcFcBArF4, respectively, in THF (−80
4
°C) or in 1,2-difluorobenzene (DFB, −25 °C). Immediate
color changes (deep violet or blue) and intense electronic
absorption features (Figure 2) diagnostic of the formation of
Table 1. Bond Distances (Å) Obtained by X-ray
Crystallography for [CuOR]2+ (CuIII) and [CuOR]+ (CuII)
Species and by DFT Geometry Optimization for [CuOR]2+
a
(Theory)
bond
species
CuIII
LOMe, OH LH, OCH2CF3 LOMe, OCH2CF3
Cu−N2
1.841(3)
1.845
1.864(13)
1.875
1.856(3)
1.866
theory
CuII
1.927(2)
−0.086
1.900(3)
1.916
1.925(4)
−0.061
1.950(12)
1.960
1.927(1)
−0.071
1.948(3)
1.960
CuIII−CuII
Cu−N1,3
Cu−O1
mean
CuIII
theory
CuII
Figure 2. (Left) UV−visible absorption spectra of the indicated
compounds in DFB at −25 °C. (Right) Orbitals for
2.027(2)
−0.127
1.799(3)
1.783
2.058(4)
−0.108
1.812(12)
1.818
2.020(1)
−0.072
1.849(5)
1.817
CuIII−CuII
L
OMeCuOCH2CF3 plotted with an isovalue of 0.04 au from the B98
CuIII
functional.
theory
CuII
[CuOH]2+ and [CuOCH2CF3]2+ cores were observed, and
reversible one-electron processes were confirmed through
titrations and the sequential cyclic additions of oxidant and
decamethylferrocene (Figures S22−S27).13 TDDFT UV−vis
transitions exhibit λmax values in agreement with experiment
that are predominantly HOMO to LUMO (Figure 2) and shift
from 546.1 and 537.1 nm in LHCuOH and LOMeCuOH to
578.7 and 573.6 nm in LH CuOCH2 CF3 and
LOMeCuOCH2CF3, respectively, as a result of the stabilization
of the LUMO (Table S4, Figures S50−S58). Also in
agreement with experiment (and precedent17a), additional
transitions with partial HOMO-to-LUMO character contribute
at longer wavelengths to the spectra for the −OCH2CF3
complexes. Resonance Raman spectra of frozen solutions (λex
= 561 nm) of all LYCuOR complexes contain a signal at ∼635
cm−1 that we assign as νCu‑OR by analogy to data acquired for
LHCuOH and theory (Figures S45−S49 and S59, Table
S15).13
1.867(2)
−0.068
−0.102
1.857(4)
−0.045
−0.081
1.838(2)
0.011
−0.051
CuIII−CuII
CuIII−CuII
a
Averaged values are presented for carboxamide Cu−N1 and Cu−N3
bonds. N2 is pyridyl donor. Estimated standard deviations are in
parentheses.
The expected electronic perturbations of the LOMe and
−OCH2CF3 moieties are reflected in cyclic voltammograms
(THF, 0.2 M Bu4NPF6), which contain quasi-reversible waves
associated with [CuOR]2+/[CuOR]+ couples (Table 2). Good
reversibility of the waves for the −OCH2CF3 complexes is
apparent at scan rates as slow as 10 mV/s (Figure S20),
consistent with low reactivity for the oxidized species (vide
Table 2. Reduction Potentials for the [CuOR]2+/+ Couple
and Rate Constants for Reactions with ttbPhOH
a
b
compound
E1/2 (mV)
k2 (M−1 s−1
)
To evaluate how ligand variation influences stability and
reactivity, we compared the reactions of LYCuOH and
LYCuOCH2CF3 (Y = H or OMe) with 2,4,6-tri-tert-
butylphenol (ttbPhOH) to yield the stable phenoxyl
radical.11,19,20 Second-order rate constants were measured
using either 1 equiv (LYCuOH) or 50 equiv (LYCuOCH2CF3)
of ttbPhOH at −25 °C in DFB (Table 2). The data show
significantly higher reactivity for the hydroxide complexes
(≳5000-fold), a difference that may be attributed to the higher
basicity of the −OH vs −OCH2CF3 moieties and/or steric
inhibition for the latter. Modest decreases in the rate constants
c
LHCuOH
−167
15 900
11 800
3.0
LOMeCuOH
−202
+37
LCuOCH2CF3
LOMeCuOCH2CF3
−25
1.5
a
b
Conditions: THF, 0.2 M Bu4NPF6, values vs Fc+/0
.
Conditions:
−25 °C in DFB using either 1 equiv (LYCuOH) or 50 equiv
c
(LYCuOCH2CF3). This value is ∼100 mV lower than reported
previously under the same conditions,11 which we ascribe to the
previous use of external Fc referencing rather than the internal
referencing used herein. (See SI IV for details.)
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for the cases where Y = OMe at parity for −OR may be
rationalized by stabilization of the [CuOR]2+ core by the
electron-donating methoxide substituent. Monitoring the
room-temperature decays of the four [CuOR]2+ species in
the absence of substrate in THF and DFB revealed
complicated kinetic traces, but trends in the overall lifetimes
paralleled the ttbPhOH reactivity trends (SI; cf. t1/2 ≈ 4 h vs <1
h for LOMeCuOCH2CF3 vs LCuOH in DFB).
from orbitals spanning the Cu center and/or its immediate
environment, as reported for less-reactive complexes LCuZ (Z
= F, Cl, Br).16 Furthermore, the lack of significant structural
changes associated with this redox event agrees with the low
reorganization energy of 0.95 eV previously measured for the
[LHCuOH]−/LHCuOH couple.14
While theoretical calculations support a closed-shell S = 0
ground state for the [CuOH]2+ core (Table S5),8 the only
experimental corroboration has come from a dearth of signal in
the X-band EPR spectrum, an observation consistent with
either the S = 0 or S = 1 ground state. Acquiring NMR spectra,
which would distinguish the two spin states, presents
challenges due to the formation of paramagnetic Cu(II)
decay species. We were nonetheless able to observe sharp
The enhanced stability of the new complexes led us to
attempt characterization by X-ray crystallography. We
discovered that the complex prepared by the treatment of
[NBu4][LOMeCuOH] with FcBArF in DFB could be isolated
4
as suitable deep-purple crystals via the layered diffusion of
pentane at −30 °C. Similar attempts with [NBu4]-
[LYCuOCH2CF3] (Y = H or OMe) failed to give suitable
crystals, likely in part due to the formation of highly intractable
1
peaks in the diamagnetic region of H NMR spectra for both
LYCuOH species in 1,2-dichlorobenzene-d4 at −15 °C (Figure
S28−S29), although broadening due to decomposition was
evident for Y = H. The 1H NMR spectrum of the more robust
complex LHCuOCH2CF3 in THF-d8 at −80 °C (Figure S30)
displayed negligible broadening, but some resonances
were obscured by solvent/byproduct signals. Since
LOMeCuOCH2CF3 could be isolated in neat form as a
crystalline solid, NMR spectra of isolated material were
collected (CD2Cl2, −15 °C), and all expected 1H NMR
resonances and J couplings (Figure 4) as well as 13C{1H}
viscous residues containing [NBu4][BArF ]. To circumvent
4
this issue, we employed reactants that would yield insoluble
inorganic salts as byproducts. Thus, we reacted LCu-
(CH3CN)4 and LOMeCu(CH3CN) with NaOCH2CF3 and
then oxidized the resulting crude materials with AcFcSbF6 in
CH2Cl2 or DFB. After the removal of a light-colored
precipitate (presumably NaSbF6), suitable crystals of the
oxidized products were obtained at −30 °C.
Representations of the X-ray structures of LOMeCuOH and
LOMeCuOCH2CF3 (Figure 3) as well as LHCuOCH2CF3
Figure 3. Representations of the X-ray structures of (a) LOMeCuOH
and (b) LOMeCuOCH2CF3, showing all nonhydrogen atoms as 50%
thermal ellipsoids.
1
Figure 4. H NMR spectrum of LOMeCuOCH2CF3 (CD2Cl2, −15
°C).
(Figure S42) show similar square-planar geometries compared
to their [CuOR]+ progenitors (τ4 = 0.11−0.15), but they are
neutral species as expected for one-electron oxidation
products. Comparison of metal−ligand bond distances
between the oxidized and reduced forms (Table 1) indicates
in all but one case shortening upon oxidation, by as much as
0.127 Å. The average Cu−N/O bond contraction in
LOMeCuOH, 0.102 Å, is in excellent agreement with previously
reported EXAFS analyses of LHCuOH (0.1 Å).8 The
trifluoroethoxides show somewhat less contraction and in
LOMeCuOCH2CF3 the Cu−O bond even lengthens slightly, by
0.011 Å, but disorder in the trifluoroethoxide ligand imparts an
inherent inaccuracy to the O atom’s position. Gas-phase
geometry optimizations (SI) for the S = 0 ground states of the
oxidized species are in excellent agreement with the
experimentally determined values (theory in Table 1). Overall,
the bond length differences between the precursor and
oxidation products are consistent with the loss of an electron
NMR peaks (Figure S32) were observed. The sharpness of the
1
observed H and 13C{1H} NMR features in the diamagnetic
chemical shift region confirms an S = 0 ground state, in
agreement with predictions.8
In conclusion, we prepared and characterized a new set of
complexes with [CuOR]+/2+ cores using a modified supporting
ligand and/or core (R = CH2CF3). Both changes attenuate the
PCET reactivity of the oxidized state, the former by lowering
its oxidizing potential and the latter by lowering the basicity of
the proton-accepting site. While each modification has the
opposite effect on the opposite property (e.g., the less-basic
proton acceptor also leads to a more oxidizing species), the
dominant impact is on electronics for the supporting ligand
and basicity for the core, in line with previously observed
reactivity trends and demonstrating how PCET reactivity can
be tuned. These stabilization effects were sufficient to permit,
for the first time, the successful characterization of complexes
with [CuOR]2+ cores by X-ray crystallography and NMR
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spectroscopy. These data are consistent with several
predictions made about LHCuOH, in particular, the con-
servation of geometry with minimal reorganization upon
oxidation,14 the EXAFS-derived contraction of Cu−L bonds
by ∼0.1 Å on oxidation,8 the calculated geometries, and the
diamagnetic S = 0 ground state.8 To the best of our knowledge,
the complexes with R = CH2CF3 are the first alkoxo analogs
bearing the formal Cu(III) oxidation state, and with the
discovery that they share many similarities with their hydroxide
counterparts, including PCET reactivity with phenols, the
potential for steric and electronic tuning that is unavailable
with hydroxide makes this class of compounds promising for
further research into the bond-activation properties of high-
valent copper species.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health (GM 47365) for
financial support. X-ray diffraction data were collected using
diffractometers acquired through NSF-MRI award no. CHE-
1827756. Computations were performed on high performance
computing systems at the University of South Dakota, funded
by NSF award no. OAC-1626516.
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ASSOCIATED CONTENT
* Supporting Information
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DFT structural coordinate file (XYZ)
Experimental details and figures (PDF)
Accession Codes
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AUTHOR INFORMATION
Corresponding Authors
■
Bess Vlaisavljevich − University of South Dakota, Vermillion,
South Dakota 57069, United States; orcid.org/0000-
0001-6065-0732; Email: bess.vlaisavljevich@usd.edu
William B. Tolman − Department of Chemistry, Washington
University in St. Louis, St. Louis, Missouri 63130-4899,
United States; orcid.org/0000-0002-2243-6409;
Email: wbtolman@wustl.edu
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Authors
V. Mahesh Krishnan − Department of Chemistry, Washington
University in St. Louis, St. Louis, Missouri 63130-4899,
United States; orcid.org/0000-0001-9642-5908
Dimitar Y. Shopov − Department of Chemistry, Washington
University in St. Louis, St. Louis, Missouri 63130-4899,
United States; orcid.org/0000-0002-4689-4383
Caitlin J. Bouchey − Department of Chemistry, Washington
University in St. Louis, St. Louis, Missouri 63130-4899,
United States; orcid.org/0000-0003-0729-6839
Wilson D. Bailey − Department of Chemistry, Washington
University in St. Louis, St. Louis, Missouri 63130-4899,
United States; orcid.org/0000-0001-6526-8997
Riffat Parveen − University of South Dakota, Vermillion,
South Dakota 57069, United States; orcid.org/0000-
0003-4941-7512
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Complete contact information is available at:
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Author Contributions
§V.M.K. and D.Y.S. contributed equally.
Notes
The authors declare no competing financial interest.
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