Exploring oxidation of half-sandwich rhodium complexes: Oxygen atom insertion into the rhodium-carbon bond of κ2-coordinated 2-phenylpyridine

Article abstract of DOI:10.1021/om500660n

The reactions of oxygen atom transfer reagents with Rh(Cp) complexes, each with a bidentate ligand and an accessible coordination site, are described (Cp = η⁵-pentamethylcyclopentadienyl). When [Rh(Cp)(phpy)(NCAr^F)][B(Ar^F)

Full text of DOI:10.1021/om500660n

Article
pubs.acs.org/Organometallics
Exploring Oxidation of Half-Sandwich Rhodium Complexes: Oxygen
2
Atom Insertion into the Rhodium−Carbon Bond of κ ‑Coordinated
2
‑Phenylpyridine
†
‡
†
‡
,‡
Christopher R. Turlington, James Morris, Peter S. White, William W. Brennessel, William D. Jones,*
Maurice Brookhart, and Joseph L. Templeton*
†
,†
†Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
*
S Supporting Information
ABSTRACT: The reactions of oxygen atom transfer reagents with Rh(Cp*) complexes, each with a bidentate ligand and an
5
F
F
accessible coordination site, are described (Cp* = η -pentamethylcyclopentadienyl). When [Rh(Cp*)(phpy)(NCAr )][B(Ar ) ]
4
1
F
F
(
1, phpy = 2-phenylene-κC ′-pyridine-κN, NCAr = 3,5-bis(trifluoromethyl)benzonitrile, B(Ar ) = tetrakis[3,5-bis(trifluoro-
4
methyl)phenyl]borate) was treated with the soluble oxygen atom transfer reagent 2-tert-butylsulfonyliodosylbenzene (sPhIO),
2
oxygen atom insertion into the rhodium−carbon bond of coordinated phpy was observed. This resulted in the formation of a κ
-(2-pyridyl)phenoxide ligand. Following insertion to form a new bidentate ligand, a second equivalent of sPhIO, acting as a
2
neutral, two-electron donor ligand, coordinated to the rhodium center through the iodosyl oxygen. Over time, the sPhIO ligand
dissociates and dimerization occurs to generate a phenoxide-bridged dinuclear species. The 2-(2-pyridyl)phenoxide ligand could
be protonated and cleaved from the mononuclear rhodium-(sPhIO) adduct by treating with a carboxylic acid (pivalic acid) at
room temperature. In addition, when rhodium complex 1 was treated with excess phpy (14 equiv), hydrogen peroxide, and acetic
acid, 5 equiv of 2-(2-pyridyl)phenol formed. Deactivation of the organometallic species, probably due to oxidative degradation of
Cp*, severely limited this catalysis.
INTRODUCTION
and distorted square-planar geometries, respectively, which
minimize repulsive overlap of filled dπ orbitals on the metal and
filled p orbitals of the oxo ligand.
■
Terminal oxo complexes of the late transition metals (groups
9
−11) have been proposed as reactive intermediates in
1 2
In an earlier report, we attempted to synthesize an Ir(V)oxo
biological pathways, industrial processes, and water oxidation
3
complex at low temperatures and characterize it by NMR
catalysis. Despite numerous efforts to synthesize stable late
12
transition metal complexes with terminal oxo ligands, only two
definitive monomeric examples are known: a Pt(IV)oxo
spectroscopy. Our strategy was to treat [Ir(Cp*)(phpy)-
F
F
(NCAr )][B(Ar ) ], which contains a labile, electron-deficient
4
4
,5
compound with a tridentate P−C−N pincer,
and a
mesityl) Ir(V)oxo complex. A dinuclear iridium complex
benzonitrile ligand, with 2-tert-butylsulfonyliodosylbenzene
(sPhIO), a relatively soluble iodosylaryl derivative known to
be an oxygen atom transfer reagent. Dissociation of the nitrile
ligand could open up a vacant coordination site, allowing for
oxygen atom transfer to the metal and oxidation to an Ir(V)oxo
complex. Rather than oxidation at iridium, experimental results
revealed oxygen atom transfer to the nitrile carbon of the
6
,7
(
3
with a bridging oxo ligand that displays an unusually short
iridium−oxygen bond distance (1.858 Å) suggests the presence
8
,9
of an iridium−oxo multiple bond.
The lack of isolated late transition metal oxo compounds
stems from destabilizing, repulsive interactions between filled
dπ orbitals on the metal with filled p orbitals on the oxo
F
1
0
NCAr ligand, followed by insertion into the iridium−carbon
ligand. Orbital overlap between filled metal dπ and filled oxo
p orbitals serves as the foundation for the “oxo wall” concept,
which predicts that an octahedral late transition metal oxo
bond of the chelated phpy ligand, forming a coordinated amide.
1
1
complex would be highly unstable. The (mesityl) Ir(V)oxo
and Pt(IV)oxo complexes described above adopt tetrahedral
Received: June 20, 2014
Published: August 27, 2014
3
©
2014 American Chemical Society
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Article
A second oxygen atom transfer ensued, presumably involving
the metal center, but the complex was not isolated (Scheme 1).
room temperature (Scheme 2), 92% conversion to a new
rhodium complex was observed by H NMR spectroscopy
1
Scheme 2. Two Synthetic Strategies for Preparation of
Scheme 1. Oxygen Atom Transfer to Ir(Cp*) Complexes
F
[
Rh(Cp*)(2-(2-pyridyl)phenoxide)(sPhIO)][B(Ar ) ] (2)
4
determined by using an internal standard). The H and 13C
1
(
F
NMR spectra confirmed that free NCAr was present in the
solution, indicating that NCAr was not oxidized. In addition,
Reactions of Rh(Cp*) complexes 3−15 with oxygen atom
transfer reagents could provide a useful comparison to the
oxidation chemistry of rhodium’s heavier congener, iridium. In
1
F
only 1 equiv of sPhI (the reduced oxidant, 2-tert-butylsulfonyl-
iodobenzene) was observed. The fate of the second equivalent
1
6
addition, Rh(V) complexes are rare, and no terminal
Rh(V)oxo complexes have been reported. Herein, we describe
reactions of Rh(Cp*) complexes with oxygen atom transfer
reagents. It was found that the reactivity of the rhodium
complexes was markedly different from the reactivity of the
reported iridium analogues. A new C−O bond forming reaction
was discovered via oxygen atom insertion into the rhodium−
carbon bond of coordinated phpy. Attempts were made to
cleave the resulting 2-(2-pyridyl)phenoxide ligand from
rhodium. It was also found that sPhIO could coordinate to
rhodium as a neutral, two-electron donor ligand, but no
evidence of oxidation to a Rh(V)oxo complex was observed.
t
of sPhIO was not immediately apparent, but a Bu resonance
t
appeared upfield (1.18 ppm) of the Bu resonances for sPhIO
1
(
1.40 ppm) and sPhI (1.38 ppm) in the H NMR spectrum. In
addition, the Cp* resonance appeared as a singlet at 1.44 ppm,
5
demonstrating that the η -ligand had not been oxidized. The
stability of the new rhodium product at room temperature,
though limited, encouraged attempts to synthesize and isolate
the complex on a preparative scale.
Isolation of the same rhodium product (as confirmed by
NMR spectroscopy) was accomplished by a one-pot reaction
F
containing Rh(Cp*)(phpy)Cl and Na[B(Ar ) ] and 2 equiv of
4
sPhIO (Scheme 2). A crystal of the product suitable for X-ray
analysis was grown by layering pentanes on top of a
concentrated solution of the product in dichloromethane and
storing at −20 °C. The structure (Figure 1) revealed oxygen
atom insertion into the rhodium−carbon bond of Rh(Cp*)-
RESULTS AND DISCUSSION
■
Synthesis. Chloride abstraction from Rh(Cp*)(phpy)Cl in
F
dichloromethane with Na[B(Ar ) ] in the presence of the weak
4
two-electron donor ligand 3,5-bis(trifluoromethyl)benzonitrile
(
[
phpy) and coordination of an sPhIO molecule, forming
F
(
(
NCAr ) allowed facile access to complex 1, [Rh(Cp*)(phpy)-
F
Rh(Cp*)(2-(2-pyridyl)phenoxide)(sPhIO)][B(Ar ) ] (2).
4
F
F
F
NCAr )][B(Ar ) ] (eq 1). Substitution of the NCAr ligand
4
Compound 2 was stable in the solid state but converted to a
dinuclear product in dichloromethane solutions over time. This
dinuclear product will be discussed in more detail later.
One striking feature of product 2 is that an sPhIO molecule
serves as a neutral, two-electron donor ligand to rhodium.
Confirmation of coordination of an iodosylaryl oxidant to a
metal is rare. The sPhIO oxidant is considered to be
nucleophilic because the reported iodine−oxygen bond
distance of 1.848(6) Å suggests extra electron density on the
iodosyl oxygen (as shown by the resonance structure in Scheme
20
by a stronger σ-donor ligand occurred rapidly in solution. In
1
addition, a H NMR spectrum of compound 1 in acetone-d6
21
revealed that exchange was occurring between the acetone
2). In addition, the Lewis acid BF interacts with the iodosyl
3
F
solvent and the NCAr ligand. Other solvent-stabilized
oxygen atom, allowing BF to be used as a solubilizing reagent
3
+
21
[
Rh(Cp*)(phpy)] cations were prepared in situ by adding
for iodosylaryl oxidants. Structural characterization of a
silver salts to solutions of Rh(Cp*)(phpy)Cl in acetonitrile,
dichloromethane, THF, or methanol (Supporting Information).
Oxidation of 1 with 2-Tertbutylsulfonyliodosyl-
benzene (sPhIO). When a solution of the nitrile adduct 1 in
coordinated sPhIO unit in complex 2 further confirms that
the oxidant is nucleophilic at the iodosyl oxygen. Indeed,
coordination of sPhIO in the NMR experiment is favored
relative to coordination of either NCAr or sPhI (via
interaction either with the iodine or with one of the sulfonyl
F
17−19
dichloromethane-d was treated with 2 equiv of sPhIO
at
2
4
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Organometallics
Article
mechanisms have been studied by computational analysis for
an isoelectronic amide insertion into the rhodium−carbon
23
bond of phpy. In this system, phpy amination is accomplished
by treating a Rh(Cp*)(phpy) catalyst with organic azides,
which are two-electron oxidants. DFT analysis suggested that
stepwise oxidation to a Rh(V)nitrenoid, followed by nitrene
insertion into the rhodium−carbon bond, was energetically
more favorable than a concerted mechanism. Our results fail to
support a stepwise mechanism for the isoelectronic oxygen
atom insertion reaction, because coordinated sPhIO in complex
2
does not oxidize the metal to a Rh(V)oxo complex, even
when heated. While the Rh(V) oxidation state cannot be ruled
out in our system, it is clearly difficult to access. On the basis of
the fact that oxygen atom insertion occurs rapidly at room
temperature, oxidation to a rhodium(V)oxo intermediate seems
unlikely in this reaction.
F
+
The oxidation of [Rh(Cp*)(phpy)(NCAr )] (1) with
sPhIO contrasts sharply with the oxidation of the analogous
F
+
iridium complex [Ir(Cp*)(phpy)(NCAr )] . In the iridium
F
case, the NCAr ligand did not dissociate but was instead
1
2
Figure 1. ORTEP drawing of 2, with partial numbering scheme (50%
probability thermal ellipsoids). Hydrogen atoms and the anion are
omitted for clarity. Selected bond lengths (Å), bond angles (deg), and
oxidized via oxygen atom transfer to the nitrile carbon.
Insertion into the iridium−carbon bond of phpy followed,
F
forming an amide (Scheme 1). Dissociation of NCAr from
torsion angles (deg): Rh −O 2.068(4), O −C 1.351(8), Rh −N
rhodium occurred rapidly, however, and NCArF was not
1
4
4
11
1
1
2
1
2
2
3
.115(5), Rh −O 2.093(4), O −I 1.909(4), I −C 2.126(6), I ···O
1
1
1
1
1
1
1
oxidized. This difference in reactivity may be due to a well-
established trend of faster ligand exchange rates for second row
.652(4), O −I −C 92.1(2), O −I −O 165.7(2), C −C −C −N
1
1
1
1
1
2
11
16
17
5(1).
24
transition metals in comparison to third row metals. In
addition, in the iridium case, a second oxygen atom transfer
occurred after formation of the amide, presumably due to
coordination of a second sPhIO oxidant molecule (a second
equivalent of sPhI was observed as a product). That rhodium is
not oxidized after coordination of an sPhIO molecule likely
stems from the trend that second row transition metals are
oxygens of sPhI). The iodosyl oxygen−iodine bond distance
increases from 1.848(6) to 1.909(4) Å upon coordination of
sPhIO. This is consistent with less electron density on the
iodosyl oxygen as a result of electron donation to cationic
rhodium(III), which weakens the bonding interaction with
iodine. The highly charged iodine receives extra electron
density from the sulfonyl oxygen to compensate for the weaker
bond to the iodosyl oxygen, as evidenced by the contraction of
the distance between the iodine and sulfonyl oxygen distance
from 2.707(5) to 2.652(4) Å. The sulfonyl oxygen, iodine, and
iodosyl oxygen also maintain their nearly linear relationship
when bound (∼166°) similar to free sPhIO (∼167°). The
iodosyl oxygen−rhodium bond distance is 2.093(4) Å, which is
within the range of a single bond, and is comparable to the
bond distance between rhodium and the 2-(2-pyridyl)-
phenoxide oxygen (2.068(4) Å). Although sPhIO is a powerful
oxidant, it simply coordinates to Rh(III) here. Coordinated
sPhIO does not oxidize rhodium to a terminal oxo complex
through elimination of sPhI, even when heated.
24
harder to oxidize than third row transition metals.
When a single equivalent of sPhIO was added to compound
1
, roughly half of the compound converted to complex 2 in less
1
than 5 min (as assessed by H NMR spectroscopy). About half
of the original concentration of 1 remained, which suggested
that coordination of sPhIO to the five-coordinate [Rh(Cp*)(2-
(
2-pyridyl)phenoxide)]+ fragment is fast relative to oxygen
atom transfer to the transient, five-coordinate [Rh(Cp*)-
+
(
phpy)] species. After leaving the solution at room temper-
1
ature overnight, H NMR spectroscopy revealed that both
compounds 1 and 2 had cleanly converted to a single, new
rhodium species. All of the sPhIO had also been reduced to
F
sPhI. Neither NCAr nor sPhI was coordinated to the rhodium
center. Instead, the final product was a dinuclear rhodium
species with bridging phenoxide ligands (3, Scheme 3). Dimer
Oxygen atom insertion into the rhodium−carbon bond of
coordinated phpy results in the formation of a coordinated 2-
3
also formed when a dichloromethane solution of the Rh-
(
2-pyridyl)phenoxide ligand. Olefin and carbonyl insertion
sPhIO adduct 2 was left at room temperature for 16 h. Bridging
reactions into the rhodium−carbon bond of Rh(Cp*)(phpy)
complexes have been well-documented.
1
5,22
2
-(2-pyridyl)phenoxide ligands have also been reported for an
These insertions
12
Ir(Cp*) dimer.
occur without any oxidation state changes at the metal. An
oxygen atom insertion, however, could involve redox chemistry
at rhodium. If the first step of phpy oxidation is coordination of
sPhIO to the metal center, then at least two reaction
mechanisms are possible. The first mechanism is a concerted
insertion of oxygen into the rhodium−carbon bond, with
simultaneous release of sPhI. A second possible mechanism
would be a stepwise insertion, initiated by oxygen atom transfer
to rhodium and release of sPhI. Formation of the Rh(V)oxo
intermediate would be followed by insertion, which would
formally reduce the metal back to Rh(III). These two
Compound 3 can be conveniently synthesized in one pot
starting from Rh(Cp*)(phpy)Cl and using hydrogen peroxide
as the oxygen atom donor in place of sPhIO. Addition of a
silver salt to a dichloromethane solution of Rh(Cp*)(phpy)Cl
allowed for the in situ generation of the solvent-stabilized,
+
unsaturated [Rh(Cp*)(phpy)] cation. Addition of hydrogen
peroxide causes formation of dimer 3 as the triflate salt (eq 2).
−
−
Single-crystal X-ray structures of both the OTf and the BF
salts have been obtained, both showing a centrosymmetric
dication (Figure 2).
4
25
4
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Article
Scheme 3. Reaction of Rh Complex 1 with 1 equiv of sPhIO
Cleaving the 2-(2-Pyridyl)phenoxide Ligand from
Rhodium. All attempts to cleave the 2-(2-pyridyl)phenoxide
26
ligand from dinuclear compound 3 were unsuccessful. It was
possible, however, to cleave the bidentate ligand from the
monomeric rhodium compound 2 with acid. Treating 2
(
generated in situ from 1 and 2 equiv of sPhIO) with 10
equiv of 2-phenylpyridine and heating at 80 °C for 3 h yielded
.25 equiv of 2-(2-pyridyl)phenol (relative to rhodium).
Adding pivalic acid to the above reaction facilitated cleavage
0
27
of 2-(2-pyridyl)phenol from rhodium. After 3 h at room
temperature, a solution of 2, with 10 equiv each of 2-
phenylpyridine and pivalic acid, produced 1.4 equiv of 2-(2-
pyridyl)phenol (eq 3). It is possible for more than 1 equiv of 2-
(
2-pyridyl)phenol to be generated in relation to rhodium
because 2 equiv of the sPhIO oxidant are added to form
complex 2. In addition, carboxylates aid C−H activation of 2-
1
3,14,28
phenylpyridine at rhodium.
Because it was possible to
generate more than 1 equiv of 2-(2-pyridyl)phenol from
rhodium, we proceeded to screen conditions for a catalytic
reaction.
Selected catalytic screening reactions are shown in Table 1.
All reactions use precatalyst 1, which is activated in situ by
added oxidant, and 13 mol equiv of 2-phenylpyridine. The
oxidants sPhIO, iodosylbenzene, and hydrogen peroxide were
screened. Reactions with iodosylbenzene and sPhIO were
deemed complete when full conversion of the oxidants to
1
iodobenzene and sPhI was observed by H NMR spectroscopy.
Background reactions in the absence of rhodium precatalyst 1
yielded no 2-(2-pyridyl)phenol (Supporting Information).
All of the turnover numbers were small, ranging from
roughly two to five, with the highest turnovers obtained when
hydrogen peroxide was used. It was found that lower
temperatures and longer reaction times led to higher
conversion of 2-phenylpyridine to 2-(2-pyridyl)phenol (com-
paring entry 1 to 2 and entry 3 to 4). Importantly, oxidative
equivalents that did not lead to productive formation of 2-(2-
pyridyl)phenol had been consumed by the end of these
Figure 2. Thermal ellipsoid drawing of dimer 3 (30% ellipsoids).
Hydrogens and OTf are omitted for clarity. Selected distances (Å):
−
Rh −O , 2.125(5), Rh −O , 2.130(6), Rh −N , 2.199(7).
1
1
1
1A
1
1
a
Table 1. Optimization of Conditions for Catalytic Generation of 2-(2-Pyridyl)phenol
b
entry
oxidant (equiv)
additive (equiv)
temp, time
equiv phenol
c
1
sPhIO (13)
pivalic acid (5)
pivalic acid (5)
pivalic acid (5)
pivalic acid (5)
pivalic acid (19)
pivalic acid (5)
acetic acid (5)
acetic acid (57)
40 °C, 5 h
23 °C, 22 h
80 °C, 3 h
23 °C, 22 h
23 °C, 21 h
23 °C, 21 h
23 °C, 21 h
23 °C, 21 h
1.8
2.3
2.9
4.4
5.1
5.6
5.4
4.2
c
2
sPhIO (13)
3
4
5
6
7
8
iodosylbenzene (14)
iodosylbenzene (14)
iodosylbenzene (14)
H O (56)
2
2
H O (56)
2
2
H O (56)
2
2
a
Reaction conditions: 5.0 mg of 1 (3.0 μmol), 6.5 mg of phpy (42 μmol, 14 equiv), oxidant, carboxylic acid, and CD Cl were stirred in a sealed
2
2
b
1
acetylation vial. Molar equivalents of oxidant, acid, and 2-(2-pyridyl)phenol are in relation to rhodium precatalyst 1. Determined by H NMR
spectroscopy using hexamethyldisiloxane as an internal standard. The reaction was conducted in an NMR tube and was not stirred.
c
4
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Article
reactions. Presumably, catalyst deactivation occurred more
rapidly at higher temperatures. Increasing the pivalic acid
concentration seemed to promote liberation of 2-(2-pyridyl)-
phenol from rhodium (entries 4 and 5). Acetic acid was also
screened as a substitute for pivalic acid, and was found to be
comparable to pivalic acid (entries 6 and 7). Increasing the
acetic acid concentration, however, seemed to inhibit the
hydroxylation reaction (entries 7 and 8). No evidence for more
than one C−H activation/hydroxylation reaction per substrate
molecule was observed.
obtained. The rhodium oxidation product, complex 2,
converted to a phenoxide-bridged dinuclear species after 16
h. The dimer could also be conveniently generated by chloride
abstraction from Rh(Cp*)(phpy)Cl in dichloromethane and
oxidation with hydrogen peroxide. In the presence of pivalic
acid, 2-(2-pyridyl)phenol could be cleaved from mononuclear 2
(generated in situ from oxidation of 1). Complex 1 was also
capable of generating approximately 5 equiv of 2-(2-pyridyl)-
phenol when treated with excess phpy, hydrogen peroxide, and
acetic acid at room temperature. Although these are mild
conditions for hydroxylation of 2-phenylpyridine, deactivation
of the catalyst was facile, presumably due to the oxidative
degradation of the Cp* ligand. Coordination of sPhIO in
complex 2 demonstrates that rhodium is loath to undergo
oxidation to a Rh(V) terminal oxo species.
It was hypothesized that oxidative Cp* ligand degradation
was deactivating the catalyst after a few turnovers. Oxidative
degradation of the Cp* ligand of Ir(Cp*) complexes is
2
9,30
common under harsh oxidizing conditions.
To test if Cp*
ligand degradation might be occurring, compound 1 was mixed
with 6 equiv of PhIO (which is poorly soluble). After 10 min of
1
vigorous shaking at room temperature, H NMR spectroscopy
EXPERIMENTAL SECTION
■
revealed that the original singlet signal for the five Cp* methyl
groups was replaced by at least 12 singlets between 0.6 and 2.2
ppm. Such signals can be attributed to various decomposition
products, such as oxidized ring carbon fragments, hydroxylated
methyl groups, and acetic acid. This is evidence that Cp*
ligand degradation occurs rapidly in the absence of excess phpy.
In the catalytic reactions, the mildest oxidant (hydrogen
peroxide) led to the highest number of turnovers before catalyst
deactivation. Comparison of the two iodosylaryl reagents
Materials and Methods. All reactions were performed under an
atmosphere of dry argon using standard Schlenk and drybox
techniques, unless noted otherwise. Argon was purified by passage
through columns of BASF R3−11 catalyst and 4 Å molecular sieves.
Dichloromethane, hexane, and pentane were purified under an argon
atmosphere and passed through a column packed with activated
alumina. All other chemicals were used as received without further
purification. 1H and C NMR spectra were recorded on Bruker
AVANCEIII400, AVANCEIII500, and AVANCEIII600 spectrometers.
30
13
1
13
1
H NMR and C NMR chemical shifts were referenced to residual H
(
sPhIO and iodosylbenzene) revealed that higher turnovers
13
and C signals of the deuterated solvents. Unless stated otherwise,
yields of NMR reactions were determined using hexamethyldisiloxane
as an internal standard. X-ray diffraction studies were conducted on a
Bruker-AXS SMART APEX-II diffractometer. Crystals were selected
and mounted using a paratone oil on a MiteGen mylar tip. Elemental
analyses were performed by Robertson Microlit Laboratories of
Madison, NJ, or the CENTC Elemental Analysis Facility at the
University of Rochester. Caution! An explosion hazard has been
reported using the initially published procedure for preparation of the
were achieved with PhIO because dissolution of PhIO is slow
relative to sPhIO. For each oxidant, oxidative equivalents are
consumed that do not lead to hydroxylation of 2-phenyl-
pyridine.
In comparison to systems known to catalyze hydroxylation of
arylpyridines, our method is mild, but inefficient. An aerobic
oxidation with PdCl2 and a radical cocatalyst has been
developed by Jiao and co-workers, which achieves greater
than 17 turnovers for phenylpyridine oxidation. The reaction,
however, must be heated to 100 °C under an atmosphere of
oxygen. Ruthenium catalysts have recently been reported as
competent hydroxylation catalysts for aryl groups containing
directing groups, such as esters, amides, ketones, carbamates,
and anilides. While these hydroxylation reactions may be run
at lower temperatures (50−120 °C), they require a chemical
oxidant other than dioxygen. Other strategies for metal-
catalyzed C−H hydroxylation typically involve elevated
temperatures (80−100 °C), or employ stepwise borylation,
followed by oxidation to the phenol. Although the Rh(Cp*)-
phpy) catalyst reported here effects hydroxylation of
31
oxygen atom transfer reagent 2-tert-butylsulfonyliodosylbenzene
17,18
(
sPhIO).
by Lin.
In this work, we have used a recent procedure published
19
F
F
Synthesis of [Rh(Cp*)(phpy)(NCAr )][B(Ar ) ] (1). A 100 mL
4
Schlenk flask was charged with 150 mg of Rh(Cp*)(phpy)Cl (0.35
1
5
F
F
mmol), 350 mg of Na[B(Ar ) ] (0.40 mmol), 107 mg of NCAr
4
3
2
(0.45 mmol), and 20 mL of dichloromethane. The reaction was stirred
for 5 h, at which point the mixture was filtered. The solvent of the
filtrate was reduced to a minimal volume, and the solution was slowly
added to 25 mL of chilled (−78 °C), rapidly stirring hexane. The
mixture was filtered via cannula, and the resulting solid was heated at
0 °C overnight under high vacuum. The product was isolated as a red
powder (320 mg, 61%). Anal. Calcd (found) for C H BF RhN : C,
9.82 (49.55); H, 2.56 (2.75); N, 1.87 (1.80). H NMR (600 MHz,
33
3
34
62
38
30
2
1
(
4
arylpyridines at room temperature, it is easily deactivated in
the presence of oxygen atom transfer reagents. Instability of
precatalyst 1 under oxidative conditions limits its application in
catalytic hydroxylation of arylpyridines.
CD Cl ) δ 8.67 (d, J = 5.4, 1H), 8.12 (s, 1H), 7.92 (d, J = 7.8, 1H),
2
2
7.89 (td, J = 7.2 and 1.2, 1H), 7.86 (s, 2H), 7.80 (d, J = 7.2, 1H), 7.74
overlapping 8H singlet and 1H doublet), 7.56 (s, 4H), 7.38 (td, J =
(
7
1
.2 and 1.2, 1H), 7.30 (td, J = 6.6 and 1.2, 1H), 7.25 (td, J = 7.5 and
.2, 1H), 1.65 (s, 15H). 13C{ H} NMR (125 MHz, CD Cl , −10 °C)
1
2
2
δ 172.7 (d, JC−Rh = 29), 165.5, 161.8 (1:1:1:1 quartet, J
= 50),
C−B
CONCLUSIONS
■
151.3, 144.3, 139.4, 136.4, 134.8, 133.2, 130.0 (q, 2 bond JC−F = 35),
131.6, 128.8 (q, 2 bond JC−F = 33), 128.4, 124.9, 124.7 (q, 1 bond JC−F
= 270), 124.6, 123.7, 121.9 (q, 1 bond JC−F = 270), 120.5, 120.4, 117.5
(septet, 3 bond JC−F = 3.8), 112.2, 99.1 (d, JC−Rh = 6.3), 9.1.
The oxidation of Rh(Cp*) compounds with sPhIO differs from
similar oxidations of Ir(Cp*) complexes. Oxidation of [Rh-
F
F
(
Cp*)(phpy)(NCAr )][B(Ar ) ] (1) resulted in oxygen atom
4
Synthesis of [Rh(Cp*)(2-(2-pyridyl)phenoxide)(sPhIO)][B-
insertion into the rhodium−carbon bond of coordinated phpy
F
F
(Ar )
4
] (2). A 100 mL Schlenk flask was charged with 100 mg of
upon dissociation of the NCAr ligand, and coordination of a
F
Rh(Cp*)(phpy)Cl (0.23 mmol), 220 mg of Na[B(Ar ) ] (0.25
mmol), 170 mg of sPhIO (0.50 mmol), and 15 mL of dichloro-
4
second sPhIO molecule filled the vacant coordination site. This
is a rare example of an iodosylaryl oxidant coordinated to a
metal. Oxidation of phpy to a 2-(2-pyridyl)phenoxide was not
methane. The reaction was stirred for 10 min at 0 °C and cooled to
−
78 °C. The mixture was then filtered via cannula, with the filtrate
being discharged directly from the cannula into 30 mL of chilled (−78
F +
observed in the oxidation of [Ir(Cp*)(phpy)NCAr ] , and
evidence for the coordination of sPhIO to iridium was not
°C), rapidly stirring pentane. The resulting mixture was then filtered
4
446
dx.doi.org/10.1021/om500660n | Organometallics 2014, 33, 4442−4448

Organometallics
Article
and washed three times with 30 mL aliquots of pentane. After heating
the solid at 35 °C for 1 h under high vacuum, 245 mg of an orange
powder was isolated (65%). Compound 2 was also prepared in situ by
dissolving 10 mg of complex 1 (7.0 μmol) and 5.0 mg of sPhIO (15
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
E-mail: jones@chem.rochester.edu (W.D.J.).
E-mail: joetemp@unc.edu (J.L.T.).
■
μmol) in 0.4 mL of dichloromethane-d at room temperature. NMR
2
spectroscopy revealed 92% conversion to compound 2 (using
hexamethyldisiloxane as an internal standard). Crystals suitable for
X-ray diffraction were grown by slow evaporation of a dichloro-
methane solution of 2. Details are given in the Supporting
Information. Anal. Calcd (found) for C H BF INO RhS: C, 46.95
*
*
Notes
6
3
48
24
4
The authors declare no competing financial interest.
1
(
47.10); H, 3.00 (2.78); N, 0.87 (0.87). H NMR (500 MHz, CD Cl ,
2 2
−
10 °C) δ 8.59 (d, J = 5.5, 1H), 8.20 (d, J = 8.5, 1H), 7.91 (td, J = 7.8
and 1.0, 1H), 7.84−7.74 (m, 3H), 7.72 (s, 8H), 7.64 (t, J = 7.5, 1H),
ACKNOWLEDGMENTS
■
C.R.T. acknowledges support from the National Science
Foundation Graduate Research Fellowship Program under
Grant No. DGE-1144081, and J.L.T. acknowledges support
from the National Science Foundation under Grant No. CHE-
1058675. This work was supported, in part, by NSF under the
CCI Center for Enabling New Technologies through Catalysis
(CENTC) Phase II Renewal, CHE-1205189.
7
7
=
.55 (overlapping 4H singlet and 1H doublet or triplet), 7.37 (td, J =
.8 and 1.5, 1H), 7.22 (t, J = 5.5, 1H), 7.11 (d, J = 7.5, 1H), 6.90 (td, J
13
1
7.5 and 1.0, 1H), 1.41 (s, 15H), 1.13 (s, 9H). C{ H} NMR (125
= 50),
MHz, CD Cl , −10 °C) δ 163.3, 161.8 (1:1:1:1 quartet, J
2
2
C−B
1
1
2
1
56.6, 150.1, 139.4, 136.2, 134.8, 133.0, 132.1, 131.3, 130.5, 129.6,
28.8 (q, 2 bond JC−F = 31), 128.3, 127.9, 124.6 (q, 1 bond JC−F
70), 124.1, 123.7, 123.3, 119.2, 117.6 (septet, 3 bond JC−F = 3.8),
15.1, 92.8 (d, JC−Rh = 8.8), 63.5, 22.8, 8.2.
=
Synthesis of [Rh(Cp*)(2-(2-pyridyl)-μ-phenoxide)] [OTf] (3).
2
2
REFERENCES
■
A 10 mg portion of Rh(Cp*)(phpy)Cl (23 μmol) was dissolved in 0.5
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filtered a second time through a second plug of Celite. The solvent was
removed in vacuo, and the solid was dried under high vacuum
overnight, yielding 10.3 mg of dinuclear product 3 (79%). Compound
(
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(
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13
F
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1
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44
46
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4
2
6
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9.0), 7.8.
Catalyst Screening Reactions. For each reaction, 5.0 mg of
2
(
1
=
=
(
precatalyst 1 (3.0 μmol), 6.5 mg of 2-phenylpyridine (42 μmol, 14
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(
4
(
(
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*
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