Three-Coordinate Copper(II) Aryls: Key Intermediates in C-O Bond Formation

Article abstract of DOI:10.1021/jacs.7b04046

Copper(II) aryl species are proposed key intermediates in Cu-catalyzed cross-coupling reactions. Novel three-coordinate copper(II) aryls [Cu^II]-C₆F₅ supported by ancillary β-diketiminate ligands form in reactions between c

Full text of DOI:10.1021/jacs.7b04046

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Communication
Three Coordinate Copper(II) Aryls: Key Intermediates in C-O Bond Formation
Subrata Kundu, Christine Greene, Kamille D. Williams, Tolani K
Salvador, Jeffery A. Bertke, Thomas R. Cundari, and Timothy H. Warren
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017
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Three Coordinate Copper(II) Aryls: Key Intermediates in C-O
Bond Formation
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Subrata Kundu,a Christine Greene,a Kamille D. Williams,a Tolani K. Salvador,a,b Jeffery A. Bertke,a
Thomas R. Cundari,*,c and Timothy H. Warren*,a
aDepartment of Chemistry, Georgetown University, Box 571227-1227, Washington, D. C. 20057 United States
bDepartment of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, United States
cDepartment of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas,
Denton, TX 76203, United States
Supporting Information Placeholder
Scheme 1. Representative examples and proposed
mechanism6 of copper catalyzed diaryl ether synthesis.
ABSTRACT: Copper(II) aryl species have been proposed as
key intermediates in copper-catalyzed cross coupling reac-
tions. Novel three coordinate copper(II) aryls [CuII]-C6F5 sup-
ported by ancillary β-diketiminate ligands form in reactions
between copper(II) alkoxides [CuII]-OtBu and B(C6F5)3. Crys-
tallographic, spectroscopic, and DFT studies reveal the geo-
metric and electronic structures of these unusual copper(II)
organometallic complexes. Reaction of [CuII]-C6F5 with the
free radical NO(g) results in C-N bond formation to give
[Cu](η2-ONC6F5). Most remarkably, addition of the phenolate
anion PhO- to [CuII]-C6F5 directly affords the diaryl ether PhO-
C6F5 with concomitant generation of the copper(I) species
[CuI](solvent) and {[CuI]-C6F5}-. Experimental and computa-
tional analysis supports a redox disproportionation between
[CuII]-C6F5 and {[CuII](C6F5)(OPh)}- to give {[CuI]-C6F5}- and
[CuIII](C6F5)(OPh) unstable towards reductive elimination to
[CuI](solvent) and PhO-C6F5.
mediates,6 the ability of first-row transition metals to participate
in single electron transformations raises the possibility of
copper(II) aryls as intermediates (Scheme 1).6c Examples of well-
characterized copper(II) aryl and copper(III) aryl complexes,
however, are rare.7,8,9 Notably, most form via copper-mediated C-
H bond activation of ancillary macrocyclic ligands that kinetically
stabilize the Cu-aryl linkage.7,8 Recently, Tilley and coworkers
Copper-catalyzed cross coupling reactions offer significant
cost and environmental benefits as compared to more tradi-
tionally employed Pd variants.1 Over a hundred years ago,
Ullmann et al. reported Cu-mediated homo- and hetero-
coupling reactions to afford biaryls, diaryl amines, and diaryl
ethers.2 Methodologies for copper catalyzed C-, N-, O-, and S-
aryl bond formation have since experienced remarkable im-
provements, offering broad synthetic scope under ambient
reaction conditions (Scheme 1).1 In particular, revolutionary
developments by Chan, Lam, and Evans led to a complemen-
tary strategy that employs aryl boronic acids (ArB(OH)2) with
amine (RNH2) or alcohol (ROH) based nucleophiles for the
formation of Ar-NHR and Ar-OR bonds (Scheme 1).3
-
demonstrated the activation of an arylborato anion BAr4 by a
dicopper(I) complex to provide a {[CuI]2(µ-Ar)}- species that
undergoes reversible one-electron oxidation to afford the corre-
sponding mixed-valent [CuII,I]2(µ-Ar) complex.9 Inspired by the
Chan-Lam-Evans coupling in which a copper-aryl intermediate is
thought to form between Cu(OAc)2 and ArB(OH)2,10 we targeted
discrete, mononuclear copper(II) aryls [CuII]-Ar.
Scheme 2. Transarylation of [R2NNF6]CuII-OtBu (2R).
The success of copper-catalyzed coupling reactions motivates
an understanding of the active species to further improve and
expand these synthetic methodologies. Based on oxidative
addition/reductive
elimination
steps
involving
Pd-aryl
intermediates in Pd cross-coupling catalysis,4 Cu-aryl
intermediates have been envisioned in Cu catalyzed protocols.5
While a d8/d10 redox couple would correspond to CuIII/CuI inter-
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with the colorless borinic acid anhydride (C6F5)2B-O-B(C6F5)2
(Scheme 3). [iPr2NNF6]Cu-C6F5 (3iPr) possesses a similar, planar
T-shaped structure as 3Me with a slightly longer Cu-Caryl distance
of 1.955(3) Å (Figure S38). The borinic acid anhydride occurs
from rapid dehydration of the borinic acid (C6F5)2B-OH produced
upon transmetallation. [iPr2NNF6]Cu-C6F5 (3iPr) may also be
prepared in the reaction of [iPr2NNF6]Cu-OtBu (2iPr) and B(C6F5)3
(Scheme 2a). The greater thermal sensitivity of [iPr2NNF6]Cu-
C6F5 (3iPr) relative to 3Me has hindered the isolation of
analytically pure 3iPr. Thus we performed further characterization
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and reactivity studies with 3Me
.
9
The cyclic voltamogram of [Me2NNF6]Cu-C6F5 (3Me) in
fluorobenzene at room temperature exhibits a quasi-reversible
reduction wave centered at -420 mV vs Cp2Fe+/Cp2Fe (Figure
S3). Cobaltocene reduction of [Me2NNF6]Cu-C6F5 (3Me) in
fluorobenzene allows for isolation of the corresponding copper(I)
arylate [Cp2Co]+{[Me2NNF6]Cu-C6F5}- ([Cp2Co]6) in 61% yield.
X-ray analysis reveals only modestly altered bond distances and
angles in the copper(I) arylate 6 as in the copper(II) analogue 3Me
(Figure 1b). The Cu-Caryl distance (1.928(3) Å) in this T-shaped
copper(I) aryl is essentially identical to that in the copper(II) aryl
(Cu-Caryl = 1.9289(14) Å), while the Cu-Nβ-dik distances increase
to 2.056(2) and 1.928(2) Å, which reflect the anionic charge and
lower Cu oxidation state in 6. The Nβ-dik-Cu-C22 angles in anion 6
of 110.05(9) and 153.11(9)°, respectively, are almost unchanged
from those in neutral 3Me. DFT optimized geometries predict a
similar trend in the variation of Cu-Nβ-dik distances in 6 and 3Me
(Figures S44 and S46). NMR spectra in THF-d8 exhibit sets of 1H
and 19F resonances that fully support the diamagnetic nature of 6
including sharp, distinct 19F resonances arising from backbone
CF3 and terminal C6F5 groups (Figures S10 and S11).
Figure 1. X-ray structures of (a) copper(II) aryl (3Me) and
(b) copper(I) arylate (6) with cobaltocenium counterion.
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In analogy to the reaction of many copper(I) β-diketiminates
t
[CuI] with BuOOtBu that form three coordinate copper(II) tert-
butoxide
complexes
[CuII]-OtBu,11
reaction
of
[Me2NNF6]Cu(NCMe)12 (1Me-MeCN) with BuOOtBu provides
[Me2NNF6]CuII-OtBu (2Me). Since related [CuII]-OtBu species
undergo transesterification with acyl-protected phenols AcOAr to
give copper(II) aryloxides [CuII]-OAr,11d we anticipated a related
transarylation reaction with the highly Lewis acidic
tris(pentafluorophenyl)borane B(C6F5)3. Addition of B(C6F5)3 to
[Me2NNF6]CuII-OtBu (2Me) in pentane at room temperature results
in a rapid color change from brownish-purple to dark violet
(Scheme 2a). Crystallization from pentane provides dark violet
[Me2NNF6]Cu-C6F5 (3Me) in 63% yield. In contrast, addition of
BPh3 to 2Me results in rapid formation of the biaryl Ph-Ph with
reduction to copper(I) (Scheme 2b).
t
X-ray diffraction reveals a short Cu-Caryl distance of 1.9290(14)
Å (Figure 1a). The sum of angles around the copper center in 3Me
is 359.77(6)°, indicating trigonal planar coordination at copper. A
To provide insight into the electronic structure of these novel
copper(II) aryl complexes 3, we carried out EPR, UV-vis, and
DFT studies. The isotropic X-band EPR spectrum of 3Me in
heptane at 293 K exhibits a four-line signal characteristic of a S =
½ copper(II) site (Figure S28). Simulation of the isotropic EPR
spectrum provides giso = 2.093 with Aiso(Cu) = 175 MHz. The
frozen glass EPR spectrum of 3Me in heptane at 80 K provides a
rhombic signal and its simulation gives g1 = 2.115, g2 = 2.099, g3
= 2.062 with A1(Cu) = 95, A2(Cu) = 290, A3(Cu) = 110 MHz
(Figure S29). Notably, EPR spectra for these CuII-aryls are not
axially biased (g1 ≈ 2.20, g2,3 ≈ 2.05) as found in related three
coordinate [CuII]-X species (X = amide, alkoxide, thiolate, hal-
ide).11,16 The UV-vis spectrum of [Me2NNF6]CuII-C6F5 (3Me) in
heptane at 25 °C shows a strong optical band at λmax = 565 nm
(4150 M-1cm-1) with a less intense, very broad feature at 910 nm
(405 M-1cm-1) (Figure S2). The UV-vis spectrum of 3iPr is similar
to that of 3Me (Figures S2 and S7).
DFT studies suggest that the SOMO for [CuII]-aryl species 3
results from a π-interaction between the Cu d orbital destabilized
via σ-donation by the β-diketiminate N-donors and the π-system
of the C6F5 ring. This interaction shifts e- spin density onto the
C6F5 ring (0.12 e- total) away from the formally d9 CuII center
(0.34 e-) (Figures 2 and S68, Table S6). The precise spin
distribution depends on the energy levels of the [Cu] and Ar
fragments. For instance, raising the energy of the aryl-based
orbitals places these MOs closer in energy to the Cu-C π* SOMO
and enhances the aryl group contribution. Thus, we predict and
compute considerably higher electron spin on the aryl group in the
putative [Me2NNF6]Cu-C6H5 (0.23 e- total) (Figure S69) relative
to its perfluorophenyl congener. Interestingly, enhanced spin
density on the Cu-bound aryl correlates with decreasing Cu-aryl
bond strength. While DFT predicts the Cu-aryl bond dissociation
free energy (BDFE) in [CuII]-C6F5 (3Me) to be 56.2 kcal/mol, the
predicted BDFE for [Me2NNF6]Cu-C6H5 is much lower at 36.0
kcal/mol (Table S7). This difference likely contributes to the
T-shaped distortion is observed, however, with two distinct Nβ-dik
-
Cu-C22 angles of 110.39(6)° and 153.56(6)°. This results in a
corresponding disparity in Cu-Nβ-dik distances of 1.9241(12) and
1.8611(12) Å, respectively, with the shorter Cu-N distance essen-
tially trans to the aryl group. The T-shaped geometry in the solid
state structure of 3Me could gain stabilization from π-stacking
between the electron-rich N-aryl ring and electron deficient
pentafluorophenyl ring.13 An unrestricted DFT geometry optimi-
zation
at
the
BP86/6-31+G(d)/gas//BP86+GD3BJ/6-
311++G(d,p)/SMD-benzene level of theory on 3Me, however,
leads to a Y-shaped trigonal planar geometry (Nβ-dik-Cu-C22 =
132.1°) with a Cu-C22 distance of 1.951 Å. Nonetheless, these
DFT calculations predict a very low free energy difference
between the Y- and T-shaped geometries (0.3 kcal/mol; Figures
S44-S45).
To more closely model transarylation via air-stable [CuII]-OH
intermediates14 in the Chan-Lam-Evans coupling (Scheme 1, step
a), we examined the reaction between {[Me2NNF6]Cu}2(µ-OH)2
(4Me 12
and B(C6F5)3. Unfortunately, no reaction occurred,
)
perhaps due to very tight interactions between two [CuII]-OH
moeties (Cu•••Cu= 3.0195(6) Å).12 Employing a more sterically
demanding ligand that possesses o-iPr N-aryl β-diketiminate
substituents, {[iPr2NNF6]Cu}2(µ-OH)2 (4iPr) (Cu•••Cu= 3.112(1)
Å) can be synthesized by exposing a pentane solution of
[iPr2NNF6]Cu(η2-benzene)15 (1iPr-C6H6) to air at room
temperature. 4iPr undergoes smooth transmetallation with
B(C6F5)3 in pentane to give violet [iPr2NNF6]Cu-C6F5 (3iPr) along
Scheme 3. Transmetallation of {[R2NNF6]CuII}2(OH)2 (4R,
R = Me, iPr).
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Scheme 5. Analysis of reaction stoichiometry for the reac-
tion of [CuII]-C6F5 (3Me) with phenolate anion.
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5
6
7
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9
this redox disproportionation is predicted to be favorable by 4.7
kcal/mol. Facile reductive elimination from [CuIII](C6F5)(OPh) (9)
to the diaryl ether-bound copper(I) complex [CuI](PhOC6F5) (10)
(ΔG = -13.5 kcal/mol) proceeds through a low energy transition
state with ΔG≠ = 16.5 kcal/mol (Scheme 6c and 70; Figure S55).
Displacement of the sterically hindered diaryl ether product from
the copper(I) center by benzene gives the observed [CuI](η2-
benzene) (1Me-C6H6) (ΔG = -2.1 kcal/mol; Scheme 6d). The
overall transformation that delivers the diaryl ether product along
with two reduced copper species is highly favorable from an
enthalpy (ΔHtotal = -48.4 kcal/mol) and free energy (ΔGtotal = -35.0
kcal/mol) perspective.
Figure 2. (a) Qualitative frontier orbital interactions in
trigonal copper(II)-C6F5/C6H5 species. (b) Spin density plot
for 3Me
markedly lower thermal stability of [Me2NNF6]Cu-Ph whose
attempted synthesis results in the biaryl Ph-Ph (Scheme 2b).
Nonetheless, these BDFEs for dissociation of an aryl radical Ar•
from [CuII]-C6F5 or [CuII]-Ph are too high to allow for such a
dissociation to readily proceed around room temperature; we in-
stead consider a redox disproportionation (Scheme 6b).
Illustrating the reactivity of [Me2NNF6]Cu-C6F5 (3Me) with
radicals, addition of 1 equiv. NO(g) to 3Me in pentane results in C-
N coupling to provide the diamagnetic C-nitroso complex
[Me2NNF6]Cu(η2-ONC6F5) (7) in 77% isolated yield (Scheme 4).
X-ray crystallographic analysis reveals η2-O,N coordination of the
C6F5NO ligand to the β-diketiminato Cu center (Figure S40)
similar to the previously reported [Me2NNF6]Cu(η2-ONPh).17
Unfortunately, attempts at C-O bond formation with sterically
hindered, O-based free radicals such as TEMPO or 2,4,6-
tBu3C6H2O• failed and led to formation of the biaryl F5C6-C6F5 in
21% and 38% yields, respectively.
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.
Scheme 6. DFT calculated thermodynamics of each indi-
vidual mechanistic step in C-O coupling mediated by
[Me2NNF6]CuII-C6F5 (3Me) in benzene.
Scheme 4. Reductive nitrosylation of [Me2NNF6]CuII-C6F5
(3Me).
Remarkably, addition of the phenoxide anion in [Bu4N]OPh to
[Me2NNF6]Cu-C6F5 (3Me) in benzene at room temperature results
in immediate formation of PhO-C6F5 (Scheme 5). A plot of the
yield of PhO-C6F5 vs. different equivalents of phenolate anion
exhibits a maximum at 0.5 equiv. phenolate anion / Cu (Scheme
5, Table S2). This suggested 2:1 reaction stoichiometry between
3Me and phenolate is also confirmed by a Job plot (Figure S18).
In the addition of 1 equiv. PhO- to 2 equiv. [CuII]-C6F5 (3Me) in
In the absence of a covalently bound coupling partner,
[Me2NNF6]Cu-C6F5 (3Me) is stable at room temperature in
nonpolar solvents such as pentane, benzene, and toluene for
several days. At elevated temperature (90 °C) in benzene, howev-
1
benzene-d6, H and 19F NMR analyses indicate the formation of
PhO-C6F5 (45% yield based on 1 equiv. 3Me) along with
[Me2NNF6]Cu(η2-benzene)
(1Me-C6D6)
and
anionic
er, 3Me decays over
a
period of 16 hours to give
{[Me2NNF6]Cu-C6F5}- (6) in 49% and 38% yields, respectively
(Figures S19 and S20). A small amount of the biaryl F5C6-C6F5
(5-15%) also forms in these experiments (Figure S20).
decafluorobiphenyl (F5C6-C6F5) in 73% yield. 19F NMR analysis
reveals the formation of the copper(I) complex [CuI](η2-benzene)
(1Me-C6H6) in 78% yield, indicating that [CuI] forms along with
C-C coupling of the copper-bound aryl group (Scheme 7). Most
interestingly, coordinating solvents such as acetonitrile dramati-
cally accelerate the decay of [CuII]-C6F5 (3Me) to cleanly provide
[CuI]-NCMe (1Me-NCMe) and F5C6-C6F5 in quantitative yield
(NMR and GC-MS, Figures S23-S25). Acceleration by addition
of a Lewis base would seem to rule out the intermediacy of simple
bimolecular intermediates [CuII]2(µ-C6F5)2, since such dimeriza-
tion should be even less favored upon binding of an additional
Lewis base to the [CuII] center. Moreover, spontaneous loss of
•C6F5 radicals is also unlikely due to the high BDFE of [CuII]-
C6F5 (56.2 kcal/mol) along with the absence of any C-O coupled
Based on the reaction stoichiometry which requires two [CuII]-
C6F5 for each PhO- along with the formation of the copper(I)
arylate anion {[CuI]-C6F5}- (6), we propose that PhO- coordinates
to [CuII]-C6F5 followed by a redox disproportionation of resulting
anion {[CuII](C6F5)(OPh)}- (8) and [CuII]-C6F5 (3Me) (Scheme 6
and
S70).
Examined
computationally
at
BP86/6-
31+G(d)/gas//BP86+GD3BJ/6-311++G(d,p)/SMD-benzene level
of theory, coordination of PhO- to [CuII]-C6F5 (3Me) results in the
copper(II) aryl phenolate complex {[CuII](C6F5)(OPh)}- (8)
(Scheme 6a). Reductive elimination of C6F5-OPh from 8 would
give
the
monoanionic,
zero-valent
copper
complex
{[Me2NN]Cu}- which is uphill by 38.5 kcal/mol in free energy
(Table S7). Instead, DFT studies reveal this more electron-rich,
anionic species 8 to be susceptible towards electron transfer to the
more electron-poor [CuII]-C6F5 (3Me) to give the copper(III) aryl
phenolate [CuIII](C6F5)(OPh) (9) and the copper(I) arylate anion
{[CuI]-C6F5}- (6) (Scheme 6b). Driven by phenolate coordination,
t
product in the addition of TEMPO or Bu3C6H2O• to [CuII]-C6F5.
We currently are considering a related redox disproportionation
mechanism that generates {[CuIII](Ar)(L)}+ / {[CuI]-Ar}- that may
promote the formation of transient [CuIII](C6F5)2. Computational
studies reveal that [CuIII](C6F5)2 is exceedingly unstable towards
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Scheme 7. [Me2NNF6]CuII-C6F5 (3Me) mediated homocou-
pling reactions.
(3) Chan, D. M. T.; Monaco, K. L.; Wang, R. P.; Winters, M. P.
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and S66, Table S7) with a reductive elimination free energy barri-
er of ~ 2 kcal/mol. The [CuIII](C6F5)2 intermediate could not be
optimized without constraint of either the Cu-Caryl or Caryl-Caryl
bond lengths, consistent with facile ligand-induced C-C coupling
chemistry experimentally observed.
Transmetallation of copper(II) alkoxides or hydroxides [CuII]-
OR (R = alkyl, H) with the electron-deficient borane B(C6F5)3
provides entry into an unprecedented family of three coordinate
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copper(II) aryls [CuII]-C6F5. Facile reduction of [CuII]-C6F5 (3Me
)
to the copper(I) arylate complex {[CuI]-C6F5 (6) (E1/2 = -420 mV
vs. Fc+/Fc) enables redox disproportionation upon coordination of
the phenolate nucleophile to provide [CuIII](C6F5)(OPh) that
reductively eliminates PhO-C6F5, coupling the incoming
phenolate nucleophile with the bound Cu-aryl. These mechanistic
findings provide insight into the mechanism of C-O coupling in
the Chan-Lam-Evans coupling and support
a
redox
disproportionation pathway that involves CuI, CuII, and CuIII
organometallic intermediates.6 Ongoing studies probe the
reactivity of other nucleophiles Nu (e.g. amides, thiolates) with
these novel copper(II) aryls [CuII]-Ar for Nu-Ar coupling.
ASSOCIATED CONTENT
Supporting Information. Experimental, characterization, and compu-
tational details (PDF) and X-ray crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
thw@georgetown.edu (T.H.W.); t@unt.edu (T.R.C)
ACKNOWLEDGMENT
T.H.W. acknowledges the National Science Foundation (NSF) for
support via grants CHE-1300774 and CHE-1337975 (X-ray).
T.R.C is grateful to the NSF for partial support of this research via
grant CHE-1464943; some of the calculations employed the UNT
Chemistry high-performance computing facility, supported by a
grant from the NSF (CHE-1531468). S.K. and T.H.W. thank the
Georgetown Environment Initiative for additional support. We are
grateful to Boulder Scientific Co. for a generous gift of B(C6F5)3.
(12) Hong, S.; Hill, L. M. R.; Gupta, A. K.; Naab, B. D.; Gilroy, J.
B.; Hicks, R. G.; Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2009,
48, 4514-4523.
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(15) Sakhaei, Z.; Kundu, S.; Donnelly, J. M.; Bertke, J. A.; Kim,
W. Y.; Warren, T. H. Chem. Commun. 2017, 53, 549-552.
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121, 7270-7271. (b) Spencer, D. J. E.; Reynolds, A. M.; Holland, P.
L.; Jazdzewski, B. A.; Duboc-Toia, C.; Le Pape, L.; Yokota, S.;
Tachi, Y.; Itoh, S.; Tolman, W. B. Inorg. Chem. 2002, 41, 6307–
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T. R.; Warren, T. H. J. Am. Chem. Soc. 2014, 136, 10930–10940.
(17) Williams, K. D.; Cardenas, A. J. P.; Oliva, J. D.; Warren, T.
H. Eur. J. Inorg. Chem. 2013, 3812–3816.
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