C(sp2)-F Oxidative Addition of Fluorinated Aryl Ketones by iPrPCPIr

Article abstract of DOI:10.1021/acs.organomet.7b00466

The reaction of ^iPrPCPIrH₄ (^iPrPCP = κ³-2,6-C₆H₃(CH₂P(ⁱPr)₂)₂) with ≥2 equiv of 2,3,4,5,6-pentafluoroacetophenone (AP-F5) in aromatic solvents at

Full text of DOI:10.1021/acs.organomet.7b00466

Article
pubs.acs.org/Organometallics
2
iPr
C(sp )−F Oxidative Addition of Fluorinated Aryl Ketones by PCPIr
Miles Wilklow-Marnell, William W. Brennessel, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
*
S Supporting Information
ABSTRACT: The reaction of iPrPCPIrH ( PCP = κ -2,6-
iPr
3
4
i
C H (CH P( Pr) ) ) with ≥2 equiv of 2,3,4,5,6-pentafluor-
6
3
2
2 2
oacetophenone (AP-F5) in aromatic solvents at room
temperature leads to the hydrogenation of AP-F5 and
formation of the corresponding 5-coordinate alkoxy hydride
species (I) within several minutes. Heating at ≥120 °C quickly
iPr
provides the cyclometalated trans C-H product PCPIr(κ-O,C-OC H F )H (III), which slowly isomerizes to the cis C-H
8
3 4
iPr
product PCPIr(κ-C,O-OC H F )H (V). The fate of the fluoride during formation of III and V is not entirely clear, though the
production of HF is implicated by significant glass etching and several crystal structures. When PCPIrH is allowed to react
overnight with ≥2 equiv of AP-F5 at room temperature, formation of C−F oxidative addition product PCPIr(κ-O,C-OC H F )
F (II) is observed by P and F NMR spectroscopy. When PCPIrH activated with tert-butylethylene is employed, formation
8
3 4
iPr
4
iPr
8
3 4
3
1
19
iPr
4
of II occurs within 10 min. Analogous reactivity is observed with several other fluorinated aryl ketones, and the crystal structure
of the C−F oxidative addition product of 2,6-difluoroacetophenone (IIAF2) has been determined. This represents the first well-
2
defined examples of stable iridium fluorides formed by C(sp )−F oxidative addition.
INTRODUCTION
proposed ex post facto based on reaction products which no
longer contain one, or both, of the corresponding C−Ir−F
bonds. Perhaps the earliest example of potential C−F oxidative
■
Organofluorides have become pervasive in everyday life and
chemical industry/research due to their use in common
materials such as Gore-Tex fabrics and “non-stick” Teflon
cookware coatings, a range of pharmaceuticals such as
fluconazole and fluoxetine, as well as ligands and counterions
for organometallic/coordination compounds such as triflate or
14
addition to iridium was demonstrated by Doyle in 1982. The
reaction of Ir(CO)(PR ) Cl (R = Ph, Me) in benzene with
3
2
formyl fluoride at room temperature was found to provide
IrH(F)(CO)(PR ) Cl in good yields with concomitant release
3
2
1
,2
of CO (Figure 1). Initial C−F oxidative addition to form the
iridium formyl fluoride, followed by decarbonylation to the
iridiumfluorohydridocarbonyl with subsequent release of CO,
was proposed due to initial C−H activation being deemed
BARF anions. However, the selective fluorination of organic
molecules remains challenging, and it is often easier to highly or
completely fluorinate a given substrate, especially by methods
3
−5
amenable to large scale production.
The challenges of
“
unlikely” under ambient conditions. Unfortunately, character-
organofluorine chemistry arise from the unique properties
fluorine imparts on C−F bonds, which are highly polarized and
among the strongest of all chemical bonds. For example, simple
changes in reagents or conditions can dictate whether electro/
nucleophilic, radical, or other reaction mechanisms proceed, in
part due to strong electrostatic interactions between fluorine
and other atoms (e.g., hydrogen bonding). Given these
considerations, C−F bond formation and activation by
transition metal complexes has been studied widely in recent
decades as a means toward selective fluorination of organic
molecules, or selective defluorination of highly fluorinated
substrates to form useful synthetic intermediates or compounds
useful in their own right.
Quite an extensive body of research on C−F activation by
organometallic species and the reactivity of organometallic
fluoride complexes now exists,
exception. However, while a number of systems have shown
well-defined examples of C(sp )−F oxidative addition to metal
centers, mainly group 6 and 10 metal species,
ization of the metal-fluoride products was limited to elemental
analysis and IR spectroscopy. No mechanistic study was
attempted, and the veracity of C−F oxidative addition in this
case remains somewhat dubious, though certainly possible.
Roughly a decade later, an example reported by Milstein and
15
3
,6
co-workers involved the, then unknown, activation of C−F
bonds by a phosphine assisted pathway. The complex
MeIr(PEt ) was found to form Ir(PEt ) (PEt F)(C F ) upon
3
3
3 2
2
6 5
thermolysis at 60 °C in C F (Figure 1) with loss of methane
6
6
and ethylene. At the time, a mechanism was proposed involving
cyclometalation of a phosphine ethyl group via C−H activation,
followed by electron transfer to C F elimination of methane
6
6,
•+
•−
and ethylene to provide [Ir(PEt ) (PEt )] [C F ] , and
oxidative addition of [C F ] to give the iridium(III) aryl-
3
2
2
6 6
•
−
6
6
7
−10
fluoride (unobserved), followed by reductive elimination, to
with iridium being no
16
yield the final product. Calculations conducted much later
2
indicate that 1,2 addition of the C−F bond across the Ir−P
bond to form a metallophosphorane intermediate is more
7
,11−13
there are
few with iridium. C−F activation by complexes of iridium has
rarely been reported to proceed through oxidative addition, and
when direct C−F oxidative addition is invoked, it is often
Received: June 19, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00466
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Article
provided spectrum contains more resonances than expected,
the peak intensities are inconsistent with the proposed
products, and one of the proposed products is an iridium
hydrido-fluoride that would almost certainly display H−F
coupling which could be indicative of this formulation, but is
not mentioned in any way. As well, a large excess of sodium
methoxide is used. Many bases are known to react with aryl-
9
,11,21
fluorides on their own,
and in particular, sodium
methoxide has been demonstrated to efficiently undergo S Ar
N
substitution with 4-fluorobromobenzene to form 4-bromoani-
22
sole in methanol at 102 °C with added polar aprotic solvent.
These conditions are quite similar to those used by Ding, and it
is surely possible that C(aryl)−O activation by iridium
subsequent to defluoromethoxylation with sodium methoxide
could lead to the observed products, among other possible
mechanisms.
It seems there is a clear lack of well-defined examples of
2
C(sp )−F oxidative addition by organometallic complexes of
iridium. Here, we report the oxidative addition of ortho C−F
iPr
iPr
3
bonds in fluorinated aryl ketones by PCPIr ( PCP = κ -2,6-
i
C H (CH P( Pr) ) ) to ultimately form cyclometalated iridium
6
3
2
2 2
2
Figure 1. Notable examples of proposed C(sp )−F oxidative addition
hydride species. The initial C−F activation products are
2
by iridium complexes.
surprisingly stable, in contrast to previous reports of C(sp )−
F oxidative addition to iridium, and clear coupling between the
PCP ligand phosphines and iridium bound fluoride is observed.
None of the earlier reactions described above provide examples
where simple C−F oxidative addition has occurred to give an
Ir(C)(F) product. Here, the first crystal structure of an iridium
likely. Nevertheless, this discovery has led to the successful
incorporation of phosphine assisted C−F activation into
reactions such as the iridium catalyzed synthesis of thioethers
by C−S cross-coupling of aryl fluorides and disulfides in the
17
2
presence of stoichiometric phosphine additives.
fluoride resulting from C(sp )−F oxidative addition is reported,
iPr
Shortly after Milstein’s report of phosphine assisted C−F
activation, Perutz and co-workers demonstrated apparent C−F
oxidative addition by CpIr(PMe )H via photolysis in neat
and variations in reactivity between PCPIrH and direct
4
iPr
PCPIr precursors, as well as the origin of the hydride ligand,
are explored.
3
2
1
8
2
C F . Both CpIr(PMe )(η -C F ) and CpIr(PMe )(C F )H
6
6
3
6
6
3
6 5
were formed concomitantly in a 6:5 ratio, with the C−F
activation product forming presumably by loss of HF after C−F
addition to Ir (Figure 1). It was proposed that these products
form by independent mechanisms, in contrast to the rhodium
analogue which was determined to participate in C−F oxidative
RESULTS AND DISCUSSION
■
While investigating molecular hydrogen acceptors for dehydro-
iPr
genations catalyzed by PCPIr, we considered fluorinated aryl
ketones in the hope that the strong C−F bonds ortho to the
ketone moiety would prevent the formation of stable
cyclometalated species as we have previously observed with
2
addition subsequent to η -coordination. Again, however, a
discrete C−F activation product was not observed or isolated.
Experiments with deuterium labeling showed that the hydride
of the pentafluorophenyl adduct comes from the starting
23
related non-fluorinated substrates. In this effort, an excess (3
equiv) of the substrates 2,3,4,5,6-pentafluoroacetophenone
(
AP-F ), 2,6-difluoroacetophenone (AP-F ), and perfluoroben-
complex making H dissociation unlikely, and ring-slip or H-
5
2
2
iPr
zophenone (BP-F ) was reacted with PCPIrH in toluene at
shift mechanisms were proposed to account for the open
coordination sites necessary for C−F addition.
10
4
room temperature. Indeed, the disappearance of the hydride
and phosphine signals of the starting complex was observed by
In a remarkable example related to the present work,
Goldman and co-workers discovered the unprecedented
NMR spectroscopy with AP-F within 5−10 min, whereas AP-
5
3
tBu
F took somewhat longer (ca. 45 min). Dehydrogenation of
activation of C(sp )−F bonds by
PCPIr, with several
2
19
iPr
PCPIrH was markedly slower with BP-F , requiring greater
examples of isolated Ir−F complexes. Although outwardly
proceeding by C−F oxidative addition, combined experimental
and computational studies revealed that C−H addition occurs
first, followed by α-fluorine migration or β-fluoride elimination
depending on substrate.
4
10
than 24 h for full disappearance of signals associated with the
tetrahydride. In all instances, a number of resonances were seen
by 31P NMR spectroscopy, and a single product was not
obtained over the course of 24−48 h at room temperature. The
species observed were found to vary with reaction time,
temperature, and substrate as described in more detail below.
Somewhat more recently, Ding and co-workers proposed
2
C(sp )−F oxidative addition during the reaction of (dfppy) Ir-
2
(
μ-Cl) Ir-(dfppy) with sodium methoxide to form a
In the case of AP-F reacting at room temperature, the initial
5
2
2
3
1
heteroleptic mononuclear complex (dfppy = 2-(4,6-
major product presented a singlet at δ 56.6 in the P NMR
spectrum. This peak correlates with a H NMR resonance
20
1
difluorophenyl)pyridyl) (Figure 1). The intermediacy of
iridium fluorides formed via oxidative addition in this process
was proposed based on the observation of “characteristic” peaks
in the 19F NMR spectrum of a band obtained through TLC
analysis of the reaction mixture without any reference to prior
works establishing this characteristic nature. Furthermore, the
manifesting as a triplet at δ −32.1 (J = 13 Hz). The chemical
PH
shift of the observed hydride is consistent with a 5-coordinate
23,24
iPr
RO−H activation product.
The reaction of PCPIr
iPr
(generated from PCPIrH activated with tert-butylethylene
4
[TBE]) and 2,3,4,5,6-pentafluorophenylethanol gave rise to the
B
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same major product, confirming the designation of this species
iPr
as PCPIr(κ-O-OC H F )H (I). Thus, in the presence of
8
4 5
iPr
excess AP-F5, PCPIrH will reduce 2 equiv of the ketone,
4
with the second remaining attached as an alkoxy-hydride
species (I), and any additional equivalents remaining unaffected
(Scheme 1).
Scheme 1. Reaction of iPrPCPIrH with 2,3,4,5,6-
4
Pentafluoroacetophenone (AP-F ) To Form I
5
Figure 2. Molecular structure of iPrPCPIr(κ-C,O-OC H F )H (V)
determined by X-ray crystallography.
8
3 4
In contrast, no clear evidence of a similar iridium O-H adduct
was found with AP-F . When an excess (3 equiv) of AP-F is
2
2
iPr
reacted with PCPIrH at room temperature in toluene, a
doublets of doublets at δ −105.2 has a value of 8.7 Hz, in good
accord with the observed hydride.
4
31
major product represented by a singlet at δ 59.2 in the
P
NMR spectrum is observed, similarly to AP-F However, no
Intrigued, we set out to determine the origins of this species
5
.
2
associated hydride signal is seen, which may indicate a fluxional
species, either by reversible C−H or by O−H activation. The
and whether more traditional oxidative addition of the C(sp )−
F bond, followed by some exchange process, led to V or if one
of the more unique pathways common to fluorocarbons, but
otherwise rare, had been followed. First, an excess (3 equiv) of
tBu
related complex PCPIr has been reported to undergo rapid
intermolecular arene exchange via reversible C−H activation
25
iPr
faster than the NMR time-scale, and accordingly, no hydride
AP-F5 was added to PCPIrH4 at room temperature in
tBu
31
resonance is observed for PCPIr(Ph)H and related arene C-
toluene, and then heated at 150 °C while monitoring by P,
1
19
H adducts at room temperature, though a defined singlet is
H, and F NMR spectroscopy. Reaction times, product
found in the 31P NMR spectrum.
concentrations, and the number/proportions of “side” products
were found to vary somewhat depending on the particular
batch of solvent or metal complex used, as well as the newness
of the NMR tube, and, though qualitatively informative,
comparisons of reaction rates or “cleanliness” must be taken
with some caution. Nonetheless, shortly after combining and
When BP-F10 was employed, ≥95% iPrPCPIrH4 was
consumed after 24 h at room temperature, and among other
peaks, a very broad resonance (ca. 431 Hz) was observed in the
3
1
P NMR spectrum centered at δ 56.8, which may possibly be
attributed to a highly fluxional O-H adduct. Again, at this point,
1
31
the H NMR spectrum did not display a hydride resonance
mixing reagents, P NMR spectroscopy showed that, besides I,
which could clearly be assigned as an O−H activation product.
and other species, a small signal attributed to V was already
present in solution (∼2.5% of total products) as well as a
These room temperature product mixtures were then heated to
3
1
1
1
50 °C to force additional reaction.
somewhat unusual doublet at δ 38.4 (∼5.8%; most P{ H}
iPr
After heating at 150 °C for 24 h, none of the three substrates
NMR spectra of the PCPIr complexes appear as singlets. A
tested provided a single product, and they were deemed as poor
candidates for hydrogen acceptors in dehydrogenation
doublet could indicate a P−F coupling due to the presence of
an Ir-F moiety). After heating at 150 °C for 1 h, though the
concentration of V had increased (∼19%), by far the main
product (∼66%) was represented by a singlet at δ 45.3, which
slowly converted to V over the course of approximately 6 days
at 150 °C (V ca. 97% of the combined concentration of III and
V). This species (III) displays a broad hydride resonance at δ
reactions. We likely would have moved on at this point and
iPr
ceased investigating the reactivity of
PCPIrH4 with
fluorinated ketones were it not for the successful crystallization
iPr
of an iridium complex from the reaction of PCPIrH and AP-
4
F (pentane, −17 °C). The molecular structure was determined
5
19
by single crystal X-ray diffraction, which showed that, contrary
to our expectations, cyclometalation via C−F activation ortho to
the ketone moiety had indeed occurred but that, curiously, the
anticipated fluoride was replaced by a hydride ligand to give
complex V (Figure 2). Characterization of this species by NMR
−11.5, as well as 4 peaks in the F NMR spectrum which are
very similar to those of V, though the peak attributed to the
fluorine ortho to the C−Ir bond appears as more of a somewhat
overlapped doublet of quartets, and is shifted upfield to δ
−114.3. The chemical shift of this hydride, and slow conversion
of III to V, is consistent with the assignment of III as the trans
spectroscopy showed it to be the major product observed after
4 h at 150 °C, displaying a singlet at δ 46.2 in the 31P{ H}
1
C−H isomer of V (Scheme 2). As shown in previous work,
23,26
2
R
NMR spectrum (toluene). The hydride, found at δ −25.4,
exhibits the usual phosphorus coupling as well as coupling to a
single fluorine atom, most likely that ortho to the C−Ir bond
C−H oxidative addition of aryl C−H bonds to PCPIr is not
directing group assisted, and instead a cyclometalated trans C−
H species is formed first, followed by slow isomerization to the
cis C−H complex.
(
F1 in Figure 2), as evidenced by the resolution of this peak as
a triplet of doublets (JPH = 15.4 Hz, JFH = 8.8 Hz in C D ). The
Analogous reactivity is followed using AP-F or BP-F to
6
6
2
10
1
9
expected four resonances are clearly defined in the F NMR
spectrum between δ −104 and −166. Notably, the weakest
coupling observed in an apparent overlapping doublet of
form products VAF2 and VBF10, respectively. However, while BP-
F required roughly the same reaction time as AP-F at 150 °C,
10
5
near complete conversion to VAF2 occurred within only ca. 24
C
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Article
Scheme 2. Isomerization of trans C−H Adduct III to cis C−
H Isomer V, and Related Products Formed with 2,6-
Difluoroacetophenone (VAF2) and Perfluorobenzophenone
for an iridium-fluoride. The concentration of II only decreased
to 44% of total products after 21 days at room temperature in
toluene.
Using AP-F under otherwise identical conditions, 31P NMR
(VBF10)
2
spectroscopy revealed a doublet at δ 37.7 (JFP = 26.2 Hz) as the
major product (46%), and an associated virtual triplet was
19
found at δ −358.5 in the F NMR spectrum. Interestingly, over
4
days of mixing at room temperature, BP-F10 did not lead to
any identifiable iridium fluoride complexes under these
iPr
conditions. Instead, as PCPIrH was slowly consumed, a
4
31
signal at δ 50.3 in the P NMR spectrum, attributable to VBF10,
was seen to grow in as the major species, among others. This
result, and the observation of small amounts of V immediately
iPr
after mixing PCPIrH and 3 equiv of AP-F (vide supra),
4
5
suggests that a path independent of III leads to V and its
analogues at room temperature which is supplanted by another
mechanism leading quickly to III at high temperatures,
followed by slow isomerization to V.
h. The hydride resonances of all three cis C−H final products
are shown in Figure 3. A simple triplet is seen for the hydride
resonance of VAF2, presumably due to the lack of a fluorine
atom ortho to the PCPIr−C bond.
Multiple attempts were made to crystallize complexes II and
III from reaction mixtures in which they were the main
product. Though neither was isolated, several crystals were
obtained of complexes which demonstrated that HF was likely
being generated during these reactions, and that fluoride loss
from II does not necessarily lead directly to III or V. One of the
structures determined is an analogue of II and III wherein an
aquo ligand is located in place of the fluoride or hydride,
respectively, denoted as IIH2O (Figure 4a). While this result
points to the potential lability/reactivity of the iridium bound
fluoride and possible role of water in this process, the likely
formation of HF is also evident. Water being a neutral ligand,
IIH2O requires a counterion to satisfy the positive charge of the
−
Ir(III) center which, unexpectedly, was found to be BF ,
4
almost certainly resulting from the action of HF on the
31
borosilicate glass of the NMR tube. Although this material
was completely insoluble in C D , it readily dissolved in THF-
1
Figure 3. H NMR spectra showing hydride resonances of V (a), V
(
6
6
AF2
31
b), and VBF10 (c) in toluene.
d , displaying a singlet at δ 36.1 in the P NMR spectrum and
8
19
the anticipated 4 aryl-F signals in the F NMR spectrum as
−
In light of the fact that a species readily identifiable as an
well as a large broad resonance at δ −151 for BF . Also
obtained were several crystals of the minor product [ PCPIr-
(H O) H]F (VI), ostensibly the product of the reaction
between PCPIr, HF, and adventitious water (Figure 4b). The
free fluoride counterion is stabilized by hydrogen bonding to
four different aquo ligands in the extended lattice (Figure 4c),
similarly to reported copper(II) fluoride complexes [Cu-
4
iPr
iridium-fluoride was not observed during the formation of
products V, VAF2, and VBF10 at 150 °C, we considered the
possibility that C−F cleavage by iridium was not involved in
this process, or that the iridium fluoride formed was too short-
lived to be detected by NMR spectroscopy. However, the
feasibility of C−F addition to iridium was readily confirmed at
lower temperature. When a toluene solution of PCPIrH and
equiv of AP-F5 was instead allowed to mix at room
temperature overnight, the small doublet observed shortly after
mixing (vide supra) had grown to approximately 45% of total
products ( P NMR spectroscopy), representing the major
species in solution. Analysis of the F NMR spectrum showed
resonances of equal area in the normal aryl-fluoride region.
The chemical shifts of these peaks are consistent with a
tetrafluorobenzene bound cis to the PCP ligand Ir−C bond,
and the carbonyl oxygen coordinated trans, as compared to III.
A fifth resonance, a triplet, was also observed at δ −347.1 (vide
infra) which seemed to indicate an iridium-bound fluoride
coupled to two equivalent phosphorus atoms, and this species
2
2
iPr
iPr
(en)
(cyclam)
(H
O)
(H
]F
·4H
O (en = ethylenediamine) and [Cu-
·4H O (cyclam = 1,4,8,11-tetra-azacyclote-
4
2
2
2
2
2
3
2
2
O)
2
]F
2
2
3
2
tradecane). The elucidation of these structures called into
question the extent to which water facilitates the dissociation of
fluoride from II. Bergman has reported that, in the presence of
L-type ligands (including water), complexes of the family
31
1
9
4
Cp′Ir(PMe
3
)(aryl)F (Cp′ = C
5 5 5 4
Me or C Me Et) exist in an
equilibrium with [Cp′Ir(PMe
)(aryl)L]F which strongly favors
3
33
Cp′Ir(PMe )(aryl)F in “dry” solvents. Addition of water
shifts this equilibrium with hydrogen bonding stabilizing the
3
fluoride counterion, favoring [Cp′Ir(PMe )(aryl)L]F·(H O)
3
2
n
at water concentrations > 1.8M, illustrating the contribution of
water in fluoride dissociation from this group of iridium
complexes.
iPr
was deemed to be PCPIr(κ-O,C-OC H F )F (II). In support
8
3 4
of this assignment, the measured coupling constants of the
When Cp′Ir(PMe )(C H )F was monitored by NMR
3
6
5
31
19
doublet ( P NMR) and triplet ( F NMR) are both 25 Hz, and
the far upfield 19F NMR chemical shift of the fluoro ligand is
similar to a number of reported organometallic iridium
spectroscopy in THF-d with 54.2 equiv of added water,
8
equilibrium with [Cp′Ir(PMe )(C H )OH ]F·(H O) was
3
6
5
2
2
n
established within <2 h at −31 °C. In contrast, when a
2
7−30
fluorides.
This species was found to be remarkably stable
mesitylene solution (∼5 ppm of H O by Karl Fischer titration)
2
D
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Article
Bergman due to the extremely poor solubility of water in
mesitylene. However, during this period, repeated sonication
was conducted to form a water−mesitylene emulsion and the
sample was mixed by spinning in between sonications to
facilitate contact of the reagents with water. Given the >50 °C
higher temperature and 10-fold greater excess of water, it would
seem that II is much less susceptible to water assisted fluoride
dissociation than the complexes studied by Bergman. That said,
3
1
when heated at 150 °C, peaks at δ 49.3 and 47.6 in the
P
NMR spectrum, seen earlier as minor impurities, quickly (20
min) became the major species in solution. These new products
each displayed hydride signals as triplets at δ −26.7 and −27.2
1
in the H NMR spectrum, indicating possible O−H addition
products with water. Formation of III was apparently inhibited
under these conditions. After 5.5 h at 150 °C, III only
represented about 19% of total products, and II was still readily
detectable, whereas II was absent after only 1 h at 150 °C in
“
dry” toluene, though that sample was not held for a prolonged
period at room temperature before heating. Full conversion to
V (III not detectable) required approximately 9 days as the
water generated products at δ 49.3 and 47.6 were slowly
31
consumed as monitored by P NMR spectroscopy.
iPr
When PCPIrH is reacted with AP-F in C H saturated
4
5
6
6
with H O at room temperature, no evidence of II is observed
2
31
19
by P and F NMR spectroscopy, even after 10 days. Instead,
the same products seen after H O addition in the mesitylene
2
3
1
experiments above at δ 49.3 and 47.7 are observed by P NMR
spectroscopy, representing ca. 70% of total species. Heating at
1
20 °C effects formation of III and subsequently V, albeit
slower than at 150 °C in mesitylene with excess water (V ca.
3% of combined concentration III and V after 7 days for H O
4
2
saturated C H ).
6
6
When C H saturated with D O was used, the formation of
6
6
2
III and V was even slower. After 9 days, V represented only
4
5% of the combined concentration of III and V. This positive
isotope effect indicates that water is activated by O−H/D
cleavage when present in excess, and that conversion of these
species to III is rate determining under the specific conditions.
The hydride signal of V was readily found in the reaction
conducted in D O saturated C H , though somewhat poorly
2
6
6
resolved, demonstrating that this hydride does not necessarily
originate from traces of water, but also does not rule out that
water may act as a hydride source when present. It is worth
noting that, when excess water is used, extensive etching of the
NMR tube is observed, suggesting increased formation/activity
of HF. Jensen and co-worker have reported the formation of
tBu
tBu
PCPIrH(OH) by addition of water to PCPIrH activated
4
iPr
34
with excess TBE. Replicating this procedure with PCPIrH4
generated an apparently highly fluxional species with a broad
resonance in the 31P NMR spectrum (ca. 324 Hz) at δ 49.1,
and an associated broad hydride signal at δ −28.4 by H NMR
spectroscopy. Though not as well-defined as PCPIrH(OH),
1
tBu
iPr
this seemed consistent with formation of PCPIrH(OH).
Figure 4. Molecular structures determined by X-ray crystallography of
iPr
iPr
Addition of AP-F (1.5 equiv) to this species in toluene gave
(
a) [ PCPIr(κ-O,C-OC H F )H O]BF (IIH2O), (b) [ PCPIr-
5
8
3
4
2
4
(
H O) H]F (VI), and (c) extended lattice of VI showing hydrogen
rise to the same products at δ 49.3 and 47.7 (SI section 1.1)
seen in the 31P NMR spectra of the benzene and mesitylene
experiments above. The only other species present was a singlet
at δ 46.6 (∼20%).
2
2
bonding of free fluoride anions to iridium aquo ligands.
of iPrPCPIrH and 3 equiv of AP-F which had progressed to
3
no effect on the concentration of II was observed by P NMR
spectroscopy after 24 h at room temperature. Admittedly,
though used in greater excess, the effective concentration of
water in this experiment is likely much lower than that used by
4
5
We next employed iPrPCPIr(TBE)35 as a precursor to
4% II was treated with 593 equiv of degassed deionized water,
3
1
iPr
PCPIr in reactions with fluorinated aryl ketones, which
presumably avoids the formation of free alcohols and iridium
O−H addition products. We hypothesized that, under these
conditions, exclusive formation of II might occur via direct C−
E
DOI: 10.1021/acs.organomet.7b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
F oxidative addition to the 14 electron iPrPCPIr fragment.
Though quantitative formation was not realized, within 10 min
iPr
after combining PCPIr(TBE) and AP-F5 in toluene, II
3
1
represented 54% of products as determined by P NMR
iPr
spectroscopy ( PCPIrH + AP-F gave only 45% II after ≥14
4
5
h). After 3 h mixing at room temperature, the relative
concentration of II had increased to 63%. Similarly, under
analogous conditions using BP-F , a doublet was observed at δ
1
0
3
1
19
4
0.4 (∼46%) in the P NMR spectrum, with an associated F
NMR resonance, resolved as a triplet, at δ −359.6. Both peaks
had coupling constants of 29.7 Hz, and this species was
designated as IIBF10
.
The reactivity of AP-F with iPrPCPIr(TBE) was somewhat
2
divergent. After mixing reagents, the main product (>76%)
31
seen by P NMR spectroscopy was a singlet at δ 59.2, the same
iPr
as that observed after addition of AP-F to PCPIrH , though a
2
4
small doublet at δ 37.7 (∼5%) was noted. Again, no hydride
resonance was detected by NMR spectroscopy which appeared
to account for this product, nor was an Ir−F resonance
1
observed. However, low temperature H NMR experiments
(
flame-sealed NMR tube, 500 MHz, other experiments 400
MHz) revealed that a partially resolved quartet observed at δ
12.3 at room temperature devolved into an undefined broad
−
signal at 0 °C. At −20 °C, this signal decoalesced into two
separate resonances at δ −12.1 and −12.7, which sharpened
upon further cooling to −40 °C, though no distinct multiplicity
was found. This behavior is consistent with the formation of a
Figure 5. 31P NMR resonances of iridium fluoride products (a) II, (b)
IIAF2, and (c) IIBF10 as well as associated 19F NMR Ir−F resonances
(d−f).
C−H activation product ortho to the C−F bond of AP-F which
allowing this solution to slowly evaporate at −10 °C. X-ray
diffraction revealed that the expected product of C−F addition
2
resolves as two rotamers at low temperature (Scheme 3, Figure
(
II ) was in fact formed (Figure 6), substantiating the
AF2
Scheme 3. Proposed Formation of C−H Activation
i
Pr
Rotamers by the Reaction of PCPIr(TBE) and AP-F in
2
Toluene
S-15), and may present as a quartet due to coincidentally equal
3
1
19
P and long-range F coupling constants. Warming to room
temperature gave an identical spectrum to that before cooling.
Thermodynamic selectivity for oxidative addition of C−H
bonds ortho to C−F bonds in fluoroarenes with cyclo-
pentadienyl Rh complexes has been known for quite some
3
6
time, and has been found to be generally applicable to a range
1
2,37−39
of transition metal-ligand systems.
iPr
After mixing the reaction of PCPIr(TBE) and AP-F for 18
h at room temperature, the P NMR doublet at δ 37.7 had
grown to 58% of total products and the singlet at δ 59.2 was
Figure 6. Molecular structure of C−F oxidative addition product
2
3
1
iPr
PCPIr(κ-O,C-OC H F)F (II ) determined by X-ray crystallog-
8
6
AF2
raphy. The fluoro ligand (F1) is disordered by ∼22% chloride
substitution.
1
9
nearly undetectable. Inspection of the F NMR spectrum
revealed a virtual triplet at δ −358.5, consistent with products
II and II
as well as the results from the overnight room
assignment of products II and IIBF10 as well. An unusual
amount of electron density was found around the fluoro ligand
of IIAF2 which modeled well as 22% substitution by chloride.
Redissolution of these crystals in toluene revealed the same
doublet at δ 37.7 in the 31P NMR spectrum as seen in the
reaction solution, as well as a smaller singlet at δ 29.9 in the
same ratio as determined between F and Cl in the crystal
structure of IIAF2. Treatment of a toluene solution concentrated
in IIAF2 with excess LiCl and stirring overnight led to the
complete disappearance of IIAF2 with an accompanying increase
of the singlet at δ 29.9 (IIAF2Cl), strongly supporting the X-ray
BF10
iPr
temperature reaction of PCPIrH and AP-F . Thus, this
4
2
species was assigned as iridium fluoride IIAF2. The characteristic
3
1
19
P and F NMR resonances of products II, IIAF2, and IIBF10
are shown in Figure 5.
With more concentrated solutions of C−F oxidative addition
products in hand, we again attempted crystallization. Though II
and IIBF10 did not provide material suitable for X-ray diffraction,
a small amount of crystals were obtained from the reaction
iPr
solution of PCPIr(TBE) and AP-F by removing volatiles in
2
vacuo, extracting the residue with pentane, filtering, and
F
DOI: 10.1021/acs.organomet.7b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
diffraction results. In conjunction, when a toluene solution of
Scheme 5. Intramolecular C−H vs C−F Oxidative Addition
iPr
>50% II was treated in the same manner, consumption of II
Experiment between PCPIr(TBE) and BP-F Showing
5
was observed with concomitant increase of a singlet at δ 30.2
Kinetically Preferred C−H Activation at Room Temperature
31
(
II ) in the P NMR spectrum, which had been seen as an
with a Thermodynamic Preference for C−F Activation
Cl
impurity in nearly all other reactions with AP-F . The source of
When Heated at 150 °C To Form II
5
BF5
chloride in these reactions likely stems from the use of LiEt BH
3
iPr
iPr
40
in the preparation of PCPIrH from PCPIrHCl, which
4
could be expected to leave behind some amount of LiCl. In
further support of the assignment of these iridium fluorides,
iPr
when 11.5 mg of PCPIr(TBE) was reacted with 1 equiv of
AP-F in toluene, generating II and subsequently treated with 1
5
equiv of Et O·BF , complete disappearance of II and formation
2
3
3
1
of a product at δ 41.8 (s) in the P NMR spectrum occurred in
0% yield after 10 min, which increased to 77% after overnight
7
mixing at room temperature. Filtration of this solution to
remove a small amount of colorless solid and then storing at
room temperature overnight in a sealed vial not dried by flame
or oven led to the formation of ≤2.5 mg of pale orange crystals.
∼16% of total products. Prolonged mixing at room temperature
(
5 days) did not affect product ratios further. However, if the
3
1
sample was instead heated at 150 °C for 15 min, P NMR
spectroscopy showed the doublet at δ 39.2 to be the major
species (∼48%) and an associated virtual triplet was located in
These crystals were found to be spectroscopically (in THF-d )
8
and crystallographically identical to IIH2O. On the basis of these
results, it is apparent that the first definitively observed, and in
19
2
the F NMR spectrum at δ −363 as well as 4 equal peaks with
one case isolated, examples of C(sp )−F oxidative addition
chemical shifts very similar to II. Thus, these resonances were
products of iridium have been demonstrated (Scheme 4).
iPr
attributed to the C−F activation product PCPIr(κ-O,C-
OC(C H )(C F ))F (II , Scheme 5, SI Table S-1). These
Scheme 4. Formation of C−F Oxidative Addition Products
6
5
6
4
BF5
41,42
iPr
37
findings are in line with calculations
that indicate C−H
II, II , and II
from the Reaction of PCPIr(TBE)
AF2
BF10
addition to Ir(I) is kinetically preferred over C−F addition,
though the products of C−F addition are significantly more
downhill thermodynamically, mainly due to repulsive inter-
actions in the transition state for C−F addition and
directionality of the fluorine orbitals involved in bond
formation in comparison to the simple spherical s-type orbitals
of hydrogen. Prolonged heating at 150 °C led to an iridium
hydride species analogous to the other substrates employed
and the Corresponding Fluorinated Aryl Ketones.
Subsequent Reactivity of II and IIAF2 with Fluoride
Abstraction Reagents Et O·BF and LiCl Also Shown
2
3
(
VBF5, SI section 1.1).
While some difference in initial reactivity is noted between
substrates, all seem to progress to a trans C−F oxidative
iPr
addition product when reacted with PCPIr(TBE), ultimately
forming cis C−H iridium hydrides given sufficient thermal
impetus. The origin of the hydride in these products is the
subject of much curiosity. As shown above, although water may
play some role in this process, it is certainly not the exclusive
source of hydrogen atoms as evidenced by the formation of III
and V in D O saturated benzene. The hydride does not
2
exclusively originate from the substrate either, as evidenced by
the formation of VBF10. Attempts to remove potential sources of
hydrogen atoms and prevent the formation of III or V were all
unsuccessful. For example, using an oven-dried Teflon NMR
tube liner only prolonged the overall reaction time slightly, and
Given the results with AP-F , we were also interested to
2
determine the kinetic/thermodynamic preference for C−H
iPr
versus C−F oxidative addition to PCPIr. In this effort, an
intramolecular competition experiment was conducted by the
iPr
reaction of
PCPIr(TBE) with 3 equiv of 2,3,4,5,6-
did not prevent formation of III or V during the reaction of
pentafluorobenzophenone (BP-F ) in toluene. Within 10−20
5
iPr
3
1
PCPIrH and AP-F in toluene at 150 °C. HF was not
min after combining reactants, P NMR spectroscopy showed
4 5
1
1
observed in the downfield region of the H NMR spectrum.
the major product to be a singlet at δ 46.4 (>50%), and the H
Likewise, conducting this reaction in C D , or employing
NMR spectrum exhibited a hydride triplet at δ −9.25. These
6
6
iPr
iPr
43
PCPIrD4, also did not prevent formation of III or V. When
PCPIr(TBE), or PCPIr(CH CH ), is used as a precursor
chemical shifts are very similar to our previous results with
2
3
iPr
44
benzophenone, suggesting the product is the trans C−H
2 2
iPr
iPr
activation product
PCPIr(κ-O,C-OC(C H )(C F ))H
to PCPIr, the overall reaction time is lengthened (by 1−3
31
6
4
6 5
(
Scheme 5). In further corroboration, 3 new signals were
days depending on conditions) and, importantly, the P NMR
resonance of II is observed to slowly dissipate over the course
of formation of III and isomerization to V, whereas heating a
toluene solution of mainly I at 150 °C rapidly generates III
without observation of II. In conjunction, when a solution
1
9
observed by F NMR spectroscopy consistent with an
unaffected C F moiety, though the para-F was largely obscured
6
5
by excess BP-F . A small doublet (∼12% total products) was
5
3
1
observed as well at δ 39.2 in the P NMR spectrum. Mixing for
4 h at room temperature resulted in only a slight difference in
iPr
2
concentrated in II, formed from PCPIrH and AP-F at room
4
5
3
1
product ratios, with the P NMR doublet only increasing to
temperature, is heated at 150 °C for 1 h, complete
G
DOI: 10.1021/acs.organomet.7b00466
Organometallics XXXX, XXX, XXX−XXX

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Article
disappearance of II is seen with III becoming the major
product (∼64%) and V representing the majority of other
species in solution (∼26%). These results indicate that
formation of III is accelerated by 1-(pentafluorophenyl)-
ethanol, though clearly it is not necessary. As a final effort,
rigorous measures were taken to ensure that all sources of O−
H and easily activated C−H bonds were as low as possible.
Scheme 6. Proposed Mechanism for the Formation of
iPr
Products II, III, and V Starting from Either PCPIrH or
4
iPr
PCPIr(TBE) and AP-F Showing the Variation of
5
Mechanism Based on the Presence or Absence of Initial
Heating
iPr
PCPIr(CH CH ) was used as Ir(I) precursor, and AP-F was
2
2
5
stored for 3 days over 3 Å molecular sieves activated by heating
for 12 h at 230 °C under high vacuum. The solvent used was
para-xylene-d10 distilled from a purple solution containing Na
and benzophenone, and passed through a column of freshly
activated alumina before use. These materials were combined in
an oven-dried Teflon NMR-tube liner within a flame-dried J-
Young NMR tube. These efforts did not prevent the formation
31
of V, but after 11 days at 150 °C, II was still detectable by
P
NMR spectroscopy in a 1-to-10 ratio with V. Under these
conditions, III was never observed to be the major species in
solution, and after 80 h of reaction, III was no longer
detectable. For the remainder of the reaction, the concentration
of II slowly decreased as V increased. This may be explained by
the rate of isomerization of III to V outpacing the formation of
III from II as the hydride source is depleted. Evidently, II is
able to scavenge even the slightest traces of hydrogen
containing compounds in order to form the, apparently,
thermodynamically preferred products III and V. Though the
nature of the F−H exchange process is not entirely clear, based
on these observations, we can propose a mechanism for the
iPr
iPr
transformation of PCPIrH or PCPIr into products II, III,
4
and V when reacted with AP-F (Scheme 6).
5
As can be seen in Scheme 6, and described earlier, addition of
iPr
≥
2 equiv of AP-F to PCPIrH leads to formation of I and an
5 4
equivalent of 1-(pentafluorophenyl)ethanol. When left to react
at room temperature overnight, II becomes the major species in
solution, possibly due to I existing in equilibrium with free
iPr
PCPIr and AP-F . If instead this mixture is heated at 150 °C
5
shortly after mixing reactants, the rapid formation of III as a
major product is observed. This is believed to proceed through
(bis)alkoxide intermediate IV via the concerted metalation
deprotonation reaction shown. Similar reactivity has been
reported by Hughes and Rheingold in the defluoroprotonation
or methylation of CF R moieties (where R = perfluoroalkyl)
2
F
F
bound to Cp*Ir(PMe )H or Cp*Ir(PMe )CH , respec-
3
3
3
4
5,46
tively.
Once formed, IV can undergo double β-hydride
elimination, and release of H , to give free PCPIr, AP-F , and
iPr
2
5
2
,3,4,5-tetrafluoroacetophenone. Preferential C−H oxidative
addition of 2,3,4,5-tetrafluoroacetophenone would then lead to
III. This route would help to explain the fact that the 1-
in formation of III from I or II. The even longer reaction times
noted under extremely dry conditions in Teflon lined NMR
tubes indicates that other O-H sources such as water or Si-OH
functionalities of glass may play a role as well. In any case, once
III is formed, both routes follow the same course of
isomerization to V, consistent with our previous findings with
(
pentafluorophenyl)ethanol initially generated upon reaction of
iPr
PCPIrH and AP-F is seen to dissipate over the course of the
4
5
reaction, leaving only AP-F , iridium based products, and a
small amount of unknown side products at the end of the
reaction as observed by F NMR spectroscopy. When II,
5
1
9
23
benzophenone.
iPr
formed from PCPIrH and ≥2 equiv of AP-F at room
4
5
CONCLUSIONS
temperature (i.e., in the presence of 1-(pentafluorophenyl)-
ethanol), is heated at 150 °C, the rapid disappearance of II and
formation of III is also noted, presumably by a similar pathway.
■
The reactions of iPrPCPIrH4, iPrPCPIr(TBE), and iPrPCPIr-
(CH CH ) with fluorinated aryl-ketones to form cyclo-
2
2
iPr
However, when II is generated by the reaction of PCPIr
precursors PCPIr(TBE) or PCPIr(CH CH ) and AP-F ,
longer reaction times are noted as well as the persistence of II
for most, or all, of the reaction progress in forming V. This
supports the assistance of excess 1-(pentafluorophenyl)ethanol
metalated cis C−H addition products by monodefluorination
have been demonstrated. Depending on the conditions, the
reaction proceeds via ortho C−F oxidative addition, followed by
H/F exchange and isomerization, or by a proposed concerted
metalation deprotonation mechanism, which ultimately leads to
iPr
iPr
2
2
5
H
DOI: 10.1021/acs.organomet.7b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the same products by selective C−H activation. The C−F
oxidative addition products have been characterized by NMR
spectroscopy, and the first crystal structure of an iridium-
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00466.
■
*
S
2
fluoride formed by C(sp )−F oxidative addition is reported.
Fluoride loss from C−F oxidative addition products is believed
to be assisted by the presence of O-H groups from, possibly,
Supplementary data including selected H, 19F, and 31
NMR spectra (PDF)
1
P
water or hydrated glass, but in particular fluorinated aryl
iPr
alcohols formed by the initial reaction of PCPIrH with
4
fluorinated aryl ketones. The extension of this reactivity to
catalytic defluorofunctionalization is currently under inves-
tigation in our lab.
Accession Codes
CCDC 1556151−1556154 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
EXPERIMENTAL SECTION
■
General. All manipulations were carried out under an argon
atmosphere either in a Vacuum Atmospheres glovebox or by modified
Schlenk techniques. All NMR spectra were collected on a Bruker AMX
AUTHOR INFORMATION
Corresponding Author
*E-mail: jones@chem.rochester.edu.
4
00 MHz spectrometer, except for low temperature experiments which
■
3
3
1
1
were conducted on a Bruker AMX 500 MHz spectrometer. All
NMR spectra were referenced to external H PO . For quantitative
P
P
3
4
NMR spectra, an inverse-gated decoupling sequence was used in
conjunction with a relaxation delay of 4 s. Proton NMR spectra were
ORCID
William W. Brennessel: 0000-0001-5461-1825
William D. Jones: 0000-0003-1932-0963
referenced to residual deuterated solvent signal. External trifluoroacetic
acid was used as reference for 19F NMR spectra. All aromatic solvents
were dried over sodium/benzophenone, distilled from the resultant
purple solution prior to use, and stored over 3 Å molecular sieves. All
other reagents were used as received from commercial sources without
further purification unless noted. X-ray structure collection was
conducted on a Bruker SMART APEX II CCD platform
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
iPr
iPr
This work was supported by the NSF under the CCI Center for
Enabling New Technology through Catalysis (CENTC), CHE-
1205189.
diffractometer. The PCPH ligand and PCPIrH complex were
4
42,47
synthesized by literature methods,
or slight variations thereof.
iPr
General Procedure for Reactions of PCPIrH and Fluori-
4
nated Aryl Ketones. To a J-Young NMR tube, 5−15 mg (9.4−28.1
iPr
μmol) of PCPIrH was added, followed by 0.5 mL of toluene or
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4
■
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(
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19
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1
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(
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(
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2
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∼
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(
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4
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991, 0, 258−259.
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48
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I
DOI: 10.1021/acs.organomet.7b00466
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Organometallics
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(
(
(
8
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1
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(
(
(
(
(
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(
3
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−
(
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(
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(
1
(
990, 220, 1−12.
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35) Generated from iPrPCPIrH4 and excess tert-butylethylene,
(
followed by removal of volatiles under high vacuum. Exists as a
mixture of iPrPCPIr(CHCHCMe3)H and solvent C−H addition
products in solution.
(
36) Selmeczy, A. D.; Jones, W. D.; Partridge, M. G.; Perutz, R. N.
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(
(
́
(
2
(
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Vol. 2, p 616.
(
1
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2
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869.
(
43) Generated by addition of 1 atm D to a toluene solution of
2
iPr
PCPIrH , mixing, degassing, and repeating D addition several times.
4
2
(
44) Synthesized by adding 1 atm ethylene to a degassed toluene
iPr
solution of PCPIrH and heating at 60 °C for 3−5 min, followed by
removal of volatiles under high vacuum.
4
(
45) Hughes, R. P.; Zhang, D.; Zakharov, L. N.; Rheingold, A. L.
Organometallics 2002, 21, 4902−4904.
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(
(
1
J
DOI: 10.1021/acs.organomet.7b00466
Organometallics XXXX, XXX, XXX−XXX