Thermal versus photochemical reductive elimination of aryl chlorides from NHC-gold complexes

Article abstract of DOI:10.1021/om400701f

Two homologous complexes of the type [(NHC)Au^I-Ar], in which the aryl substituent was either phenyl or pentafluorophenyl, were prepared. Treatment of [(IPr)Au^IC₆F₅] with PhICl ₂ leads directly to the expected Au^III oxidation addition product [(IPr)Au^III(Cl)₂C₆F₅]. This complex is thermally stable but undergoes photochemical reductive elimination to deliver [(IPr)Au^ICl] and C₆F₅Cl. In contrast, the reaction of [(IPr)Au^IPh] with PhICl₂ does not deliver an isolable Au^III oxidation addition product but rather leads directly to the formation of [(IPr)Au^ICl] and PhCl, presumably via a [(IPr)Au^III(Cl)₂Ph] intermediate. These related reactivity pathways are rationalized on the basis of the electronic structures of the two [(NHC)Au^I-Ar] complexes.

Full text of DOI:10.1021/om400701f

Communication
pubs.acs.org/Organometallics
Thermal versus Photochemical Reductive Elimination of Aryl
Chlorides from NHC−Gold Complexes
Michael J. Ghidiu,† Allen J. Pistner,† Glenn P. A. Yap,† Daniel A. Lutterman,‡ and Joel Rosenthal*,†
†Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: Two homologous complexes of the type [(NHC)AuI−
Ar], in which the aryl substituent was either phenyl or pentafluor-
ophenyl, were prepared. Treatment of [(IPr)AuIC6F5] with PhICl2 leads
directly to the expected AuIII oxidation addition product [(IPr)-
AuIII(Cl)2C6F5]. This complex is thermally stable but undergoes
photochemical reductive elimination to deliver [(IPr)AuICl] and
C6F5Cl. In contrast, the reaction of [(IPr)AuIPh] with PhICl2 does
not deliver an isolable AuIII oxidation addition product but rather leads
directly to the formation of [(IPr)AuICl] and PhCl, presumably via a [(IPr)AuIII(Cl)2Ph] intermediate. These related reactivity
pathways are rationalized on the basis of the electronic structures of the two [(NHC)AuI−Ar] complexes.
N-heterocyclic carbene (NHC) complexes of gold have found
utility for the catalytic formation of C−C,1−3 C−O, and C−N
bonds.4 Additional emphasis has been placed on use of such
complexes for C−X (X = halogen) bond formation.5−7
Moreover, recent work has shown that photoexcitation of
complexes of the type [(L)AuIII(X)3] (L = phosphine, NHC; X
= Cl, Br) leads to reductive elimination of X2.8,9 Despite the
recent synthetic and catalytic advances involving AuI and AuIII
complexes supported by NHC ligands, the ability of such
systems to mediate the reductive elimination of aryl chlorides
has not been demonstrated. Although previous studies have
shown that other certain Cu10−12 and Pd13 complexes can
promote aryl halide formation, the photochemical and/or
thermal elimination of Ar−X bonds from AuIII complexes of the
type [(NHC)AuIII(X)2Ar] (Ar = aryl; X = halogen) has not
been documented. To this end, we sought to probe the
reactivity of [(NHC)AuI−Ar] complexes with Cl2, particularly
with respect to thermal and photochemical elimination of Ar−
Cl from the Au center.
1).18,19 The corresponding pentafluorophenyl homologue was
prepared by adapting an arene C−H activation strategy that has
Scheme 1. Synthesis of (IPr)AuI−Ar Complexes
been utilized for AuI−phosphine complexes.20 As such, the
reaction of [(IPr)AuICl], pentafluorobenzene, and Ag2O in
DMF at 55 °C delivered [(IPr)AuIC6F5] in 64% yield. Crystals
of this complex were grown from CH2Cl2/pentane solutions via
slow evaporation. The solid-state structure of this compound
(Figure 1a) revealed that the pentafluorophenyl ring is canted
by 75.8° with respect to the imidazole ring that comprises the
IPr framework. This angle is similar to that observed for the
corresponding phenyl derivative [(IPr)AuIPh], in which the
torsional angle is 88.6°.21 Other selected bonding metrics are
given in Table 1. Notably, the AuI−C(IPr) and AuI−C(Ar)
bond lengths of [(IPr)AuIC6F5] are 2.018 and 2.021 Å,
respectively. As shown in Table 1, these metrics are slightly
shorter than the analogous bond lengths for the corresponding
nonfluorinated complex [(IPr)AuIPh].21
In developing suitable platforms to investigate the reactivity
described above, we adopted an NHC ligand sporting ancillary
2,6-diisopropylphenyl groups (IPr = 1,3-bis(2,6-
disopropylphenyl)imidazole-2-ylidene). The bulky aryl groups
appended to this carbene ligand offer enhanced steric
protection to the metal center and help to solubilize the
metal complex.14 Moreover, IPr has been shown to support
many interesting organometallic complexes of gold.15,16 In
surveying the chemistry described in the preceding paragraph,
we targeted two electronically distinct [(IPr)AuI−Ar] com-
plexes in which the aryl group was either phenyl or
pentafluorophenyl.
With both of the aryl-appended NHC−AuI complexes in
hand, we investigated the oxidation of these systems.
Treatment of a CHCl3 solution of [(IPr)AuIC6F5] at 45 °C
with iodobenzene dichloride (PhICl2),22 which is a convenient
source of Cl2,23 leads to the clean formation of a new complex
[(IPr)AuICl]17 was converted to the corresponding phenyl
derivative ([(IPr)AuIPh]) in 66% yield via treatment with
PhB(OH)2 and Cs2CO3 in isopropyl alcohol at 55 °C (Scheme
Received: July 17, 2013
© XXXX American Chemical Society
A
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Communication
Figure 1. Solid-state structures of (a) [(IPr)AuIC6F5] and (b)
[(IPr)AuIII(Cl)2C6F5]. Thermal ellipsoids are shown at the 50%
probability level (F, green; Au, orange; C, black; N, blue; Cl, maroon).
Hydrogen atoms are omitted for clarity.
Figure 2. UV−vis time course showing the changes in the absorption
profile accompanying the photoconversion (λexc ≥ 275 nm) of
[(IPr)AuIII(Cl)2C6F5] (100 μM) to [(IPr)AuICl] in CH2Cl2 at 298 K.
The reactant and product are matched by color to their respective
absorption profiles. Spectra were recorded over the span of 12 h at 30
min intervals.
1
within 3 h. The H NMR spectrum obtained for this new
product is consistent with oxidation of the AuI complex to the
corresponding AuIII dihalide, on the basis of the downfield shift
of the ligand methyne 1H NMR resonances from δ 2.61 to 2.90
ppm. Although only two other NHC−AuIII complexes of the
type [(NHC)AuIII(Cl)2Ar] are known,24 several related
complexes of this type supported by phosphine and arsine
ligands have been reported.25,26
9% upon excitation of [(IPr)AuIII(Cl)2C6F5] with monochro-
matic light of λexc 312 nm. The course of the photoreaction was
1
also followed by H NMR spectroscopy, which confirmed the
formation of [(IPr)AuICl] (Figure S2, Supporting Informa-
tion). Generation of C6F5Cl as the elimination product was
confirmed by a combination of 19F NMR and GCMS.
X-ray diffraction studies confirmed that this material was the
square-planar oxidative addition product [(IPr)AuIII(Cl)2C6F5].
As shown in Figure 1b, the NHC and aryl ligands of
[(IPr)AuIII(Cl)2C6F5] maintain their trans orientation, with
respect to one another. Unlike the case for the AuI precursor,
however, these two ligands are only canted by 14.2° with
respect to one another. Additional bonding metrics for this
complex are provided in Table 1. Interestingly, the AuIII−
C(IPr) and AuIII−C(Ar) bonds of [(IPr)AuIII(Cl)2C6F5] (2.062
and 2.049 Å, respectively) are longer than the corresponding
metrics for [(IPr)AuIC6F5]. A similar trend has been observed
for the homologous mesityl-appended NHC−AuI/III com-
plexes.24
Oxidation of [(IPr)AuIPh] with PhICl2 proceeds in a very
different manner. Addition of PhICl2 to [(IPr)AuIPh] did not
generate a AuIII complex but rather directly delivered
[(IPr)AuICl]. Formation of this product was accompanied by
production of 1 equiv of chlorobenzene, as judged by 1H NMR
and GCMS analyses. Direct formation of [(IPr)AuICl] and
chlorobenzene was also observed to be rapid when this reaction
was carried out in the dark at temperatures as low as −33 °C.
This observation is consistent with initial formation of
[(IPr)AuIII(Cl)2Ph] as an intermediate, which rapidly under-
goes facile reductive elimination to deliver the observed
products (Scheme 2). The intermediacy of this AuIII dichloride
along the reaction pathway is akin to that which has been
invoked for oxidation of homologous phosphine- and arsine-
supported AuI−aryl complexes with X2.25,27 Moreover, this
pathway is bolstered by the isolation of [(IPr)AuIII(Cl)2C6F5]
(vide supra) and related work by Limbach and co-workers,
which demonstrated that a less sterically encumbered NHC−
AuIII complex ([(IMes)AuIII(Cl)2Ar]) could be formed upon
treatment of [(IMes)AuIPh] with PhICl2.24 Our observation
that reductive elimination from the homologus IPr-supported
complex, which is more crowded around the metal center, is
consistent with a reaction pathway in which ligand dissociation
precedes reductive elimination.
[(IPr)AuIII(Cl)2C6F5] is stable for at least several months
when stored at room temperature in the absence of light;
however, we found that samples of this compound exposed to
room light decomposed over the course of weeks to generate
[(IPr)AuICl]. This observation suggested that photoexcitation
of this AuIII dichloride complex induces reductive elimination of
1 equiv of aryl chloride. To further probe this process, solutions
of [(IPr)AuIII(Cl)2C6F5] in CHCl3 were irradiated (λexc ≥ 275
nm) and the course of the photoreaction was monitored by 1H
NMR and UV−vis spectroscopy.
A time course for the changes in the absorption profile
accompanying this photoreaction is displayed in Figure 2. An
isosbestic point is maintained at 265 nm throughout the
photolysis, attesting to a clean and quantitative process. The
final absorbance spectrum is identical with that of [(IPr)AuICl]
and does not change upon continued irradiation. The quantum
yield for this transformation was measured to be approximately
The thermal and photochemical reductive eliminations
observed for the phenyl and pentafluorophenyl NHC−Au
complexes, respectively, may occur through similar pathways.
Although the chloride and C6F5 groups of [(IPr)-
Table 1. Selected Bonding Metrics for [(IPr)AuIPh], [(IPr)AuIC6F5], and [(IPr)AuIII(Cl)2C6F5]
bond length (Å)
bond angle (deg)
Au−C(IPr)
Au−C(Ar)
Au−Cl(1)
Au−Cl(2)
C(IPr)−Au−C(Ar)
Cl(1)−Au−Cl(2)
a
[(IPr)AuIPh]
2.0334(13)
2.018(4)
2.063(7)
2.0396(13)
2.021(5)
2.049(8)
174.44(5)
180
[(IPr)AuIC6F5]
[(IPr)AuIII(Cl)2C6F5]
2.278(2)
2.278(2)
176.3(3)
177.36(8)
a
Data reproduced from ref 21.
B
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approximately 160 mV demonstrates that the gold center of
[(IPr)AuIC6F5] is more electron poor than that of [(IPr)-
AuIPh] and highlights the ability of fluorinated aromatics to
perturb the electronic structure of the gold−NHC complexes.
Electrostatic potential maps constructed for both of the
[(IPr)AuI−Ar] complexes are shown in Figure 3a. These plots
Scheme 2. Proposed Pathway for Reductive Elimination of
Aryl Chloride from Complexes of the Type
[(IPr)AuIII(Cl)2Ar]
AuIII(Cl)2C6F5] are cis to one another, photoexcitation likely
does not induce reductive elimination directly from the four-
coordinate AuIII center. Kochi has demonstrated that reductive
elimination from square-planar AuIII complexes is facilitated by
loss of a ligand to generate an electronically and coordinately
unsaturated AuIII center,28,29 which may either adopt a T- or Y-
shaped geometry.30 It is this three-coordinate intermediate that
ultimately undergoes elimination to generate the corresponding
AuI complex and aryl halide. Related work has shown that
reductive elimination of methyl iodide from [(IPr)AuIII(I)2Me]
occurs via an analogous mechanism.21 On the basis of this
mechanistic precedent, a likely pathway for the observed
photoreaction involves photoinduced loss of a chloride ligand
to generate [(IPr)AuIII(Cl)C6F5]Cl, which then eliminates
C6F5Cl to give the observed AuI product (Scheme 2).
Figure 3. Electrostatic potential maps: (a) [(IPr)AuIPh] and
[(IPr)AuIC6F5]; (b) [(IPr)AuIII(Cl)2Ph] and [(IPr)AuIII(Cl)2C6F5].
Each map was generated using an isovalue of 0.006 e/Å3. Red indicates
areas of more negative electrostatic potential, and blue represents
more positive electrostatic potential.
TD-DFT conducted for [(IPr)AuIII(Cl)2C6F5] supports this
photochemical pathway. These calculations show that the main
absorption (∼290 nm) from which the observed photoreaction
is derived involves an electronic transition resulting in excited-
state population of the LUMO. This molecular orbital is
show that the AuI center of the fluorinated homologue is less
electron rich than the phenyl complex. This result is consistent
with the difference in measured redox potentials (vide supra).
On the basis of the ability of the fluorinated aryl ring to
withdraw electron density from the metal center of [(IPr)-
AuIC6F5], the AuIII center of [(IPr)AuIII(Cl)2C6F5] should also
be more electron poor than the gold atom of an [(IPr)-
AuIII(Cl)2Ph] intermediate. Electron density maps constructed
for these two systems (Figure 3b) also reveal that the metal
center of the fluorinated AuIII complex is more electropositive
than that of the phenyl-appended homologue.
The redox data together with the electrostatic potential maps
provide a framework through which the divergent reactivities of
[(IPr)AuIPh] and [(IPr)AuIC6F5] can be rationalized. The
extent to which the pentafluorophenyl group shifts electron
density away from the metal center of [(IPr)AuIII(Cl)2C6F5]
disfavors dissociation of a hard chloride anion from the
electropositive AuIII center of this complex in comparison to
the analogous process involving the more electron rich AuIII
center of the phenyl complex. As such, a plausible explanation
for the unusual stability of [(IPr)AuIII(Cl)2C6F5] versus
[(IPr)AuIII(Cl)2Ph] stems from the increased electrostatic
interaction between the chloride ligands and the AuIII center
of the pentafluorophenyl complex. This electrostatic stabiliza-
tion raises the barrier for halide dissociation from [(IPr)-
AuIII(Cl)2C6F5], and as a result, thermal reductive elimination
from this complex is not observed. In contrast, the chloride
ligands should be bound much more weakly to the more
electron-rich gold center of the [(IPr)AuIII(Cl)2Ph] inter-
mediate. Facile dissociation of chloride from such an
2
2
comprised of antibonding interactions between dx −y on the
AuIII center and the ligands in the equatorial plane (Figure S3,
Supporting Information). As such, photoexcitation of [(IPr)-
AuIII(Cl)2C6F5] should produce an excited state in which the
AuIII−Cl bonds are significantly weakened, which facilitates
halide dissociation and subsequent reductive elimination.
A related pathway is likely operative for the thermal reductive
elimination observed upon reaction of [(IPr)AuIPh] with
PhICl2. If it is presumed, on the basis of previous work
involving [(IMes)AuIPh], that [(IPr)AuIPh] and PhICl2
initially react to form [(IPr)AuIII(Cl)2Ph], the thermal
dissociation of a chloride from this square-planar complex
would enable reductive elimination of Ph−Cl. The observation
that this process is facile for [(IPr)AuIII(Cl)2Ph] but requires
photoexcitation with a relatively high energy photon for
[(IPr)AuIII(Cl)2C6F5] suggests that the AuIII−Cl bond is
stronger for the pentafluorophenyl homologue. Indeed, thermal
dissociation of chloride from [(IPr)AuIII(Cl)2C6F5] does not
even occur upon treatment of this complex with AgBF4 at 80
°C for several hours. This unusual stability is presumably due to
the extent to which a C6F5 moiety perturbs the electron density
of the Au center. This effect can be perceived by comparing the
AuI/III redox couples of the related AuI−NHC complexes
([(IPr)AuIPh] and [(IPr)AuIC6F5], which were measured to be
1.77 and 1.93 V versus Ag/AgCl, respectively. That the
fluorinated homologue is more difficult to oxidize by
C
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Communication
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In conclusion, we have prepared two homologous complexes
of the type [(NHC)AuI−Ar], in which the aryl substituent was
either phenyl or C6F5. Reaction of these species with PhICl2
proceeds differently, depending on the aryl substituent.
Treatment of [(IPr)AuIC6F5] with PhICl2 leads directly to
isolation of the expected AuIII oxidation addition product
[(IPr)AuIII(Cl)2C6F5]. This complex is thermally stable but
undergoes reductive elimination upon photoexcitation to
deliver [(IPr)AuICl] and C6F5Cl. In contrast, the reaction of
[(IPr)AuIPh] with PhICl2 does not deliver an isolable AuIII
oxidation addition product but rather leads directly to
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ASSOCIATED CONTENT
* Supporting Information
■
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(17) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P.
Text, figures, tables, and CIF files giving experimental methods
with accompanying crystallographic and computational data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.R.: joelr@udel.edu.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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■
N.; Rominger, F.; Jakel, C.; Hashmi, A. S. K.; Limbach, M.
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We thank Gabriel A. Andrade (UD) for assistance with X-ray
crystallography. Research reported in this publication was
supported by the University of Delaware Research Foundation
and the donors of the American Chemical Society’s Petroleum
Research Fund. M.J.G. was supported through a UD Plastino
Summer Fellowship, and D.A.L. was sponsored by the Division
of Chemical Sciences, Geosciences, and Biosciences, Office of
BES, U.S. DOE. Data were acquired using instrumentation
obtained with assistance from the NSF (CHE-0421224, CHE-
0840401, CHE-1048367, and CHE-1229234). D.A.L. also
thanks the Ohio Supercomputing Center.
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