C-CN bond activation of benzonitrile with [Rh -I(dippe)] -

Article abstract of DOI:10.1021/om200342f

The complex [Rh(dippe) (μ-Cl)]₂ (1) was reduced with potassiummetal to produce the highly reactive Rh^-I species [Rh(dippe) (μ-K 3 THF)]₂ (2). 2 was characterized by NMR spectroscopy ( ₃₁P, ₁H) and IR spectroscopy after forming the derivative K[Rh(dippe) (CO)₂] (2a). The C-CN bond of benzonitrile was cleaved by oxidative addition when it was reacted stoichiometrically with 2 to form the anionic Rh(I) complex K[Rh(dippe) (CN) (Ph)] (3). 3 has been characterized by NMR spectroscopy (₃₁P, ₁H, ₁₃C), and by the use of ₁₃C-labeled benzonitrile. C-CN bond cleavage was also attempted by reacting benzonitrile with the zwitterionic complex [Rh(dippe) (μ6-Ph-BPh3)] (4). A product 5 was formed from reaction with 2 equiv of benzonitrile. Reaction with ₁₃C-labeled benzonitrile has ruled out the possibility of C-CN cleavage in this species, and a C-Hbond activation pathway is operative instead.

Full text of DOI:10.1021/om200342f

ARTICLE
pubs.acs.org/Organometallics
CÀCN Bond Activation of Benzonitrile with [RhÀI(dippe)]À
Matthew R. Grochowski, James Morris, William W. Brennessel, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S Supporting Information
b
ABSTRACT: The complex [Rh(dippe)(μ-Cl)]2 (1) was reduced with potas-
sium metal to produce the highly reactive RhÀI species [Rh(dippe)(μ-K THF)]2
3
(2). 2 was characterized by NMR spectroscopy (31P, 1H) and IR spectroscopy
after forming the derivative K[Rh(dippe)(CO)2] (2a). The CÀCN bond of
benzonitrile was cleaved by oxidative addition when it was reacted stoichiomet-
rically with 2 to form the anionic Rh(I) complex K[Rh(dippe)(CN)(Ph)] (3). 3
has been characterized by NMR spectroscopy (31P, 1H, 13C), and by the use of
13C-labeled benzonitrile. CÀCN bond cleavage was also attempted by reacting benzonitrile with the zwitterionic complex
[Rh(dippe)(η6-Ph-BPh3)] (4). A product 5 was formed from reaction with 2 equiv of benzonitrile. Reaction with 13C-labeled
benzonitrile has ruled out the possibility of CÀCN cleavage in this species, and a CÀH bond activation pathway is operative instead.
’ INTRODUCTION
Reaction of benzonitrile with [(dippe)Rh(μ-H)]2 at room
temperature has been examined by Fryzuk.7 The reactivity of this
group 9 analogue was found to be dramatically different than the
group 10 dippe-hydrides. Addition of the metal hydride to the
nitrile carbon occurred to form the corresponding imide that
bridges both Rh centers to give the complex [Rh(dippe)]2(μ-H)-
(μ-NdCHPh) (A) (eq 3). The reaction is general for nitriles and
gives analogous products [Rh(dippe)]2(μ-H)(μ-NdCHR) (R =
CH3, o-tolyl).7 This reactivity is not unexpected since the loss of
H2 would give a pair of odd electron [Rh0(dippe)] fragments,
unlike the group 10 analogues that give saturated d10 fragments.
Alternative methods were, therefore, sought to generate a reactive
d10 monomeric [Rh(dippe)]À fragment that could lead to CÀ
CN bond cleavage, as seen with [Ni(dippe)]. The two different
methods that were examined are described herein.
The activation of CÀCN bonds is a formidable challenge given
their high bond strengths. For example, the CÀCN bond strength of
benzonitrile is 132.3(2.0) kcal/mol.1 Despite their strength, activa-
tion of CÀCN bonds has been accomplished previously with [Ni-
(dippe)H]2.2 When 2 equiv of benzonitrile was added to [Ni(dippe)-
H]2 in THF, loss of H2 occurred to give the η2-nitrile complex
Ni(dippe)(η2-NCPh) (eq 1). The η2-nitrile complex was long-
lived and was isolated and characterized by X-ray diffraction and
NMR and IR spectroscopies. At room temperature, slow conversion
to the CÀCN activated product Ni(dippe)(CN)(Ph) occurred and
the Ni(II) square-planar product was fully characterized. The reaction
is versatile and has been used to activate CÀCN bonds in cyano-
quinolines,2 aromatic nitriles,3 and 2-methyl-3-butenenitrile.4 Similar
reactivity has been seen with a bis-N-heterocyclic carbene analogue.5
Reaction of benzonitrile with the group 10 analogue
[Pt(dippe)H]2 has also been examined.6 The reactivity was more
complicated than the nickel case, and a series of CÀH and CÀCN
bond activated complexes were isolated and characterized. The
reaction required more strenuous conditions with a reaction
temperature of 140 °C in neat benzonitrile being required (eq 2).
’ RESULTS AND DISCUSSION
Generation of Reactive [Rh(dippe)(μ-K THF)]2. The first
3
method attempted involved reducing the dinuclear RhI complex
[Rh(dippe)(μ-Cl)]2 (1) with potassium metal with the intended
goal of generating a monomeric reactive [RhÀI(dippe)]À d10
fragment. 1 was initially reacted with 2 equiv of potassium in
THF, and the reaction was monitored periodically by 31P NMR
spectroscopy. After several hours of stirring, a new doublet was
observed at δ 102.21 (1JRhÀP = 190 Hz) in addition to the
doublet resonance for 1 at δ 102.53 (1JRhÀP = 201 Hz). After 2
days of stirring, all of 1 was converted to the new product, which
Received: April 21, 2011
Published: October 13, 2011
r
2011 American Chemical Society
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Figure 2. νco stretching frequencies of literature examples related to
K[Rh(dippe)(CO)2]: (a)16 1927 (m), 1874 (s) cmÀ1, (b)17 1860 (m),
1805 (s) cmÀ1, (c)18 1929 (s), 1858 (vs) cmÀ1
.
Figure 1. IR spectrum of 2a taken in THF.
is believed to be a formally Rh0 complex. The observance of an
NMR signal for this species suggests it is dimeric [Rh(dippe)]2.
Addition of 2 more equiv of potassium resulted in the exclusive
formation of a new broadened doublet centered at δ 104.70
(1JRhÀP = 171 Hz), and this resonance was assigned to the RhÀI
complex [Rh(dippe)(μ-K THF)]2 (2) (eq 4). The proposed
3
structure with bridging potassium ions coordinated by THF is
similar to that seen in other anionic metal dimers (Li4Cr2Me8-
(THF)4,8 Na2Ni2Ph4(C2H4)(THF)5,9 Li4Pt2Me8(ROR)4,10
Li3Fe2Ph6H3(THF)5).11 In addition, DFT calculations show 2
to be a stable, bound structure (see the Supporting Information).
The reaction solution changed from yellow-orange (1) to a dark
red (2) upon reduction. Eisenberg has shown that the reduction
of [Rh(diphos)2]+ (diphos = Ph2PCH2CH2PPh2) occurred in
two discrete single-electron steps, with initial formation of
[Rh(diphos)2], followed by reduction to [Rh(diphos)2]À with
sodium naphthalenide.12
Figure 3. 31P NMR of 2a after evaporation and redissolving in THF-d8.
NMR spectrum by the multiplet hydride resonance at δ À5.08
and the 31P{1H} NMR spectrum by a doublet of multiplets at δ
104.6 (eq 5). Formation of [Rh(dippe)(μ-H)]2 is proposed to
form from the protonation of 2, most likely from residual water in
the THF solvent or from the glassware. Typically, 20 mg of 1
(0.025 mmol) was used for a reaction, and given the small
amount of 1 used and the highly reactive nature of 2, it is not
surprising that some [Rh(dippe)(μ-H)]2 formed.
Bogdanovic was able to reduce Rh(diphos)2Cl with activated
magnesium to obtain [Rh(diphos)2]MgCl.13 He called this
product a “Grignard analogous” rhodium phosphine complex
since the RhÀI center was nucleophilic and reacted with electro-
philes, such as methyl iodide, to give Rh(diphos)2(CH3). The
[Rh(diphos)2]MgCl complex shares many similar characteristics
to 2. Rh(diphos)2Cl was yellow in solution, but the solution
changed to dark red upon reduction, the same as observed for
conversion of 1 to 2. The 31P NMR spectrum of Rh(diphos)2Cl
displays a doublet at δ 58.2 (1JRhÀP = 133 Hz), which became a
broadened doublet centered at δ 62.0 (1JRhÀP = 197 Hz) when
reduced to [Rh(diphos)2]MgCl. 2, like [Rh(diphos)2]MgCl, is
highly air-sensitive in the solid state and decomposed to a white
powder after a few seconds of exposure to air.
An established method for characterizing low valent metal
complexes is to derivatize them with CO to form a low valent
metal carbonyl complex.14 This derivative can then be character-
ized by IR spectroscopy as a means of determining the degree of
backbonding to the carbonyl ligands, as low valent species cause a
substantial red shift of the CO vibrations. When one atm of CO
was introduced to a THF solution of 2, a rapid reaction ensued as
the solution color changed from dark red to clear orange. An IR
spectrum of the reaction solution was taken, and carbonyl
stretches were observed at νco = 1833 (m), 1777 (m), and
1708 (w) cmÀ1 (Figure 1). These stretches are consistent with
the formation of K[Rh(dippe)(CO)2], 2a, as having been formed.
A comparison can be made to similar complexes that are known
in the literature (Figure 2). 2a appears to be in the RhÀI range
based on these examples; however, the weak νco at 1708 cmÀ1
might be explained by potassium ion binding to a carbon
monoxide ligand (ion paring), which is known to give rise to
1
The H NMR spectrum of 2 is unexceptional with broad
resonances ranging from δ 2.35 to 1.10 (THF-d8), as would be
expected for a dippeÀmetal complex. Reduction of 1 to give 2
could also be accomplished with activated magnesium using
Bogdanovic’s procedure, or with KC8. The use of potassium
naphthalenide brought about rapid reduction in a matter of minutes;
however, this sample of 2 contained additional imurities from this
method. All of the reduction methods led to the formation of
1
[Rh(dippe)(μ-H)]2 as a side product, as detected in the H
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Figure 5. 1H NMR spectrum of K[(dippe)Rh(13CN)(Ph)] (3) in
THF-d8.
Figure 4. 31P NMR spectrum of K[(dippe)Rh(13CN)(Ph)] (3) in
THF-d8.
bands at higher and lower energy.15 A 31P NMR spectrum was
taken of 2a, showing a doublet at δ 95.20 (1JRhÀP = 150 Hz)
(Figure 3). Over the period of a day, however, the compound
decomposed and only a broad resonance (ν1/2 = 1800 Hz) was
also observed centered at δ 74.
Activation of the CÀCN Bond in Benzonitrile. A THF
solution of 2 was added to a stoichiometric amount of 13CN-
labeled benzonitrile (2 equiv) and stirred for 2 days at room
temperature. During this time, the reaction solution changed
from dark red to cherry red, as conversion to the CÀCN bond
cleavage product K[(dippe)Rh(CN)(Ph)] (3) occurred (eq 6).
Figure 6. 13C NMR spectrum of K[(dippe)Rh(13CN)(Ph)](3) in
THF-d8. 13CN resonance detail (inset).
The 31P NMR spectrum of 3 showed two nonequivalent
phosphorus resonances that both have a doublet of doublets of
doublets splitting pattern when 13CN-labeled benzonitrile was
used (Figure 4). A significant trans effect was observed for the
RhÀP coupling constants, with PA trans to the cyano group
showing 1JRhÀP = 154 Hz (δ 82.84), and PB trans to the phenyl
group showing 1JRhÀP = 115 Hz (δ 91.08). Splitting by the 13CN
gave a 2JPÀC = 95 Hz for PA, which is trans to the labeled cyano
group. PB is cis to the cyano group and shows a smaller coupling
of 2JPÀC = 15 Hz.
were found at δ 142.43, 125.68, and 119.79 and were correlated
to their respective proton peaks at δ 7.64, 6.78, and 6.57 by a
13
1HÀ C HSQC experiment. The ipso carbon of the phenyl
group was difficult to observe since this weak resonance is
predicted to be split into a doublet of doublets of doublets.
The formation of 3 was typically accompanied by the forma-
tion of a side product that has been identified as Fryzuk’s
[(dippe)Rh]2(μ-H)(μ-NdCHPh) (A),7 which formed when
benzonitrile reacted with [(dippe)Rh(μ-H)]2 that formed dur-
ing the reduction of 1. Elimination of A can be achieved by
eliminating water from the solvent and reaction glassware,
although this is not easy with the reactivity of 1. A ratio of 3/A
of 4:1 was observed under typical conditions (see the Experi-
mental Section). Separation of 3 and A can be achieved by
washing the crude residue with hexanes to remove A.
1
The H NMR spectrum of 3 (Figure 5) also supports the
assignment of the structure, with dippe methyl and ethylene
resonances located between δ 0.88À1.39 (28H). The methine
resonances show their characteristic multiplet splitting patterns
at δ 2.05 and 1.94 (2H each), and the phenyl resonances were
located at δ 7.64 (1H), 6.78 (2H), and 6.57 (2H).
A
13C NMR spectrum of 3 showed a very large doublet of
doublets of doublets resonance for the 13CN group centered at δ
2
160.58 with the aforementioned cis and trans JPÀC couplings,
This method is effective in isolating 3 but is also problematic.
Coordination of THF to 3 appears to be necessary for the
stability of 3. Washing 3 with hexanes and then drying by vacuum
caused the red residue to turn into a cinnamon-colored powder.
Elemental analysis showed that significant decomposition had
and a rhodium coupling of 1JRhÀC = 47 Hz (Figure 6). The value
1
found for JRhÀC is very similar to that found for Milstein’s
(PNP)Rh(CN) (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)-
pyridine) complex with a 1JRhÀC = 52 Hz.14 The aryl resonances
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Figure 8. 31P NMR of 5 (THF-d8).
Figure 7. ORTEP drawing of [(dippe)Rh(η6-Ph-BPh3)] (4). Ellip-
soids are shown at the 50% probability level. All hydrogen atoms are
omitted for clarity.
(integration normalized to the largest resonance): δ 190 (0.66),
177 (0.10), 165 (0.16), 163 (1.00), and 155 (0.15). The only
coupling observed was a doublet for the resonance at δ 190 (J =
7.5 Hz); all of the other 13CN resonances were singlets. A
resonance with the doublet of doublets of doublets pattern
indicative of a (PAPB)RhÀCN group was not observed (see
the Supporting Information for these spectra).
occurred, as the %C was low by 26%! In a separate trial, the crude
mixture was purified by washing with hexanes and then redissol-
ving in THF. The solution was evaporated to dryness and gave a
red residue; elemental analysis of this sample revealed that
decomposition occurred, but not as significantly as before (8%
low on %C).
The major resonance at δ 163 is proposed to be an imine
1
13
carbon on the basis of DEPT-135 and HÀ C HSQC NMR
experiments. A positive signal was observed for this resonance
when a DEPT-135 NMR experiment was performed, indicative
Attempted CÀCN Activation with [(dippe)Rh(η6-Ph-BPh3)].
A second approach to CÀCN bond activation of benzonitrile
attempted to generate a reactive [(dippe)Rh]+ fragment that
would employ a Rh(I)/(III) oxidative addition pathway. Initially,
AgBF4 was reacted with 1 in an attempt to abstract chlorides
from 1 to generate [(dippe)Rh]+. However, this method was
problematic and resulted in multiple decomposition products.
Instead, the sodium salt NaBPh4 was reacted with 1 in THF
solution heated at 60 °C for 8 h (eq 7). This resulted in the
precipitation of NaCl and the appearance of a new doublet at δ
104.22 (1JRhÀP = 204 Hz) in the 31P NMR spectrum. X-ray
diffraction of the orange solid (Figure 7) revealed the zwitterionic
η6-arene product [(dippe)Rh(η6-Ph-BPh3)](4). 4 is known in the
literature, having been previously used as a hydroboration
catalyst.19 When 4 was reacted with 2 equiv of benzonitrile in
THF at 120 °C, a new product (5) was formed.
1
of a carbon bonded to an odd number of hydrogens, and a H
13
correlation for δ 163 was found at δ 7.4 in a 1HÀ C HSQC ex-
periment. An IR stretch at 1967 cmÀ1 was observed for a bound
benzonitrile, along with a broad imine absorption at 1455 cmÀ1
for the Lewis acid coordinated imine. A similar boron adduct of a
Lewis acid coordinated metalloimine, (bipy)(Et)Ni-N(BEt3)d
CHPh, has been reported in the literature with “no IR bands
above 1500 cmÀ1”.20
The other four 13CN resonances were related to one another
since addition of their integrations adds up to 1.0 and accounts
for the second equivalent of benzonitrile added. The resonance at
δ 190 is proposed to be a Rh-coordinated η1-benzonitrile due to
its small coupling constant (vide supra). The other three reso-
nances appear to be side products that form from the coordinated
benzonitrile. The resonance at δ 177 was identified as benzo-
phenone imine (by comparison with an authentic sample), while
the resonance at δ 155 was confirmed by independent synthesis to
be the allene analogue Ph2CdNdBPh2.21 The minor resonance
at δ 165 was not identified.
An integration of approximately 25H for aryl proton reso-
nances was observed in the 1H NMR spectrum, corresponding to
the large number of resonances observed between δ 130 and 140
in the 13C NMR spectrum. Approximately 32H were found in
the alkyl region, and it is instructive that the pattern observed is
indicative of an approximate mirror plane symmetry. Each of the
groups in this region appear in two nonequivalent, but equal,
resonances: 12H each for two nonequivalent sets of methyl
groups, 2H each for two nonequivalent sets of ethylene protons,
and 2H each for two nonequivalent sets of methine protons
The 31P NMR spectrum (Figure 8) of 5 is similar to that of 3,
with two nonequivalent phosphorus nuclei at δ 91.79 and 87.49
that were each split into a doublet of doublets. The JRhÀP
coupling constants are 186 and 119 Hz, which are suggestive of
CÀCN bond cleavage; however, this possibility was eliminated
from consideration when the reaction was repeated with 13CN-
labeled benzonitrile. Only a small coupling of 2JPÀC = 5.5 Hz was
observed for the resonance at δ 87.49. Five different 13CN-
labeled resonances were observed in the 13C NMR spectrum
1
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2a, which was characterized by IR spectroscopy as K[Rh(dippe)-
(CO)2] and has IR data similar to analogous complexes in the
literature. Reaction of 2 with stoichiometric benzonitrile pro-
duces the CÀCN cleavage product Rh(dippe)(Ph)(CN) (3).
This formulation for 3 has been confirmed by use of 13CN-
labeled benzonitrile. Synthesis of 3 was hindered by formation of
the side product A. The isolation of pure 3 has been difficult to
achieve, as it appears that the stability of 3 is dependent upon the
presence of THF. Product 5 was synthesized from reaction of the
zwitterionic complex [(dippe)Rh(η6-Ph-BPh3)] (4) with 2 equiv
of benzonitrile. The use of 13CN benzonitrile in the reaction
helped to rule out the possibility of CÀCN bond cleavage in 5.
The formation of an imine carbon seemed to suggest that the
product may arise from CÀH bond activation of benzonitrile,
followed by nitrile insertion. The reaction is highly dependent
upon stoichiometry, with 1 equiv of benzonitrile resulting in
decomposition and an excess resulting in the formation of
multiple products.
Figure 9. 1H NMR and proposed structure for 5.
’ EXPERIMENTAL SECTION
(Figure 9). On the basis of the experimental data acquired, the
proposed structure for 5 is shown in eq 8.
All operations were performed under a nitrogen atmosphere unless
otherwise stated. THF was distilled from dark purple solutions of
sodium benzophenone ketyl or from NaK. Carbon monoxide was
purchased from Air Products Co. 1,2-Bis(diisopropylphosphino)ethane
was synthesized according to a literature method.23 [Rh(dippe)-
24
21
(μ-Cl)]2 (1) and Ph2CdNdBPh2 were synthesized as previously
reported. Sodium tetraphenylborate was purchased from Sigma-Aldrich,
dried under vacuum, and stored under nitrogen. 13C-carbonyl-labeled
acetic acid was purchased from Cambridge Isotopes Laboratories and
was reacted with benzenesulfonamide (purchased from Sigma-Aldrich)
to produce Ph13CN using the method of Short.25 Celite was heated to
200 °C under vacuum overnight and stored under nitrogen. A Siemens-
SMART 3-Circle CCD diffractometer was used for X-ray crystal struc-
ture determinations. Elemental analyses were obtained at the CENTC
Elemental Analysis Facility at the University of Rochester, funded by
NSF CHE-0650456. All 1H, 31P, and 13C NMR spectra were recorded
on a Bruker Avance 400 NMR spectrometer. The 1H chemical shifts are
reported relative to the residual proton resonance in the deuterated
solvent. 31P chemical shifts are reported relative to the signal of external
85% H3PO4. Mass spectra were recorded using a Shimadzu LCMS-
2010. DFT calculations were made using Gaussian 09.26
The cyclometalated product is proposed to form by activation
of the ortho CÀH bond of benzonitrile with concomitant nitrile
insertion. This is proposed to be the rate-determining step since
only 4 and 5 were observed by 31P NMR spectroscopy. Addition
of a phenyl group from the BPh4 anion is also proposed to occur.
A report in the literature showed that the analogous complex
[(diphos)Rh(η6-Ph-BPh3)] transfers a phenyl to Rh to form
[(diphos)RhPh] upon heating in acetone at 70 °C.22 A phenyl
group was not transferred when 4 was heated at 120 °C by itself in
THF. 4 has a resonance at δ À7.21 in the 11B NMR spectrum,
whereas no signal was observed for 5. This absence of a resonance
suggests that the boron atom is coupled to a quadrupolar atom, such
as nitrogen, which would account for no resonance being observed.
The formation of 5 was found to be highly dependent on the
amount of benzonitrile employed. When 1 equiv of benzonitrile
was used in the reaction, decomposition of 5 occurred as 4 was
Synthesis of [(dippe)Rh(μ-K THF)]2 (2). In a nitrogen-filled
3
glovebox, [Rh(dippe)(μ-Cl)]2 (1) (17.8 mg, 0.0222 mmol) was dis-
solved in 2 mL of THF and added to excess potassium metal (21 mg,
0.537 mmol). The reaction solution was stirred for 5 h, during which
time the solution changed from a clear yellow-orange to a cloudy dark
red. Filtration through a fine fritted funnel was used to remove pre-
cipitated KCl. 2 was isolated by evaporating THF to obtain a dark red
residue. 1H NMR (400 MHz, THF-d8, 25 °C): δ 3.62 (s, 8H), 2.05 (m,
2H), 1.85 (m, 2H), 1.78 (m, 8H), 1.30À0.85 (br m, 28 H). 31P{1H}
NMR (162 MHz, THF-d8, 25 °C): δ 104.70 (d, JRhÀP = 171 Hz).
Synthesis of K[(dippe)Rh(CO)2] (2a). In a nitrogen-filled glove-
box, [Rh(dippe)(μ-Cl)]2 (1) (9.7 mg, 0.0121 mmol) was dissolved in
2 mL of THF and added to excess potassium metal (5.7 mg, 0.145 mmol).
The reaction solution was stirred for 3 h, during which time the solution
changed from a clear yellow-orange to a cloudy dark red. The unfiltered
solution was placed in a flame-dried ampule. Nitrogen was removed by
the freezeÀpumpÀthaw degassing (3Â) method. One atmosphere of
CO was introduced to the reaction vessel. The solution color changed
from dark red to a clear orange. An aliquot of this solution was used to
obtain an IR spectrum: 1833 (m), 1777 (m), 1708 (w) cmÀ1. The
reaction solution was later evaporated to dryness and dried for 2 h. It was
redissolved in THF-d8, and 31P and 1H NMR spectra were taken.
1
consumed. Benzene, as detected by H NMR, was generated
from the decomposition. Use of excess benzonitrile (10 equiv)
resulted in the formation of two additional products by 31P NMR
spectroscopy, which could not be identified. Under these condi-
tions, benzophenone imine became the major 13CN-labeled
product. It should be noted that 5 is an extremely air-sensitive
dark green product that decomposes rapidly in air, but is stable
under nitrogen in the solid and solution states for extended
periods of time.
’ CONCLUSION
The formation of the reactive RhÀI complex [(dippe)Rh-
(μ-K THF)]2 (2) has been achieved by reduction of [(dippe)-
3
Rh(μ-Cl)]2 (1) by potassium metal. Reaction with CO produced
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1
31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 95.20 (d, JRhÀP
=
been deposited at the Cambridge Crystallographic Data Centre
and allocated the deposition number CCDC 818643.
150 Hz). 1H NMR (400 MHz, THF-d8, 25 °C): δ 2.35 (br s, 4H), 1.67
(br m, 4H), 1.10 (br m, 24H). Anal. Calcd (found) for C16H32KO2P2Rh:
%C, 41.74 (38.06); %H, 7.01 (6.53); %N, 0.00 (À0.02).
’ AUTHOR INFORMATION
Synthesis of K[(dippe)Rh(13CN)(Ph)] (3). In a nitrogen-filled
glovebox, unfiltered 2 from the procedure above was added portion-wise
by pipet to a Schlenk flask containing 13CN-labeled benzonitrile (4.62 μL,
0.0444 mmol). The reaction solution was stirred for 44 h, during which
time the solution changed from dark red to cherry red. The reaction was
complete after 1 day of stirring. An aliquot of the reaction solution was
taken and analyzed by 31P NMR spectroscopy, which showed that 80%
of 3 and 20% of A had formed (by integration). The reaction solution
was filtered through Celite to remove KCl. A was removed by concentrat-
ing the reaction solution to dryness and washing the residue with hexanes
(3 mL) to remove A. The solid was redissolved in THF, evaporated to
dryness, and dried for 1 day by vacuum. Yield: 9.8 mg (43%). Past trials
had shown that failure to redissolve in THF leads to significant de-
composition typified by the formation of a cinnamon brown powder and
poor analytical data (Anal. Calcd (found): %C, 49.70 (23.42); %H, 7.35
(2.94); %N, 2.76 (1.52)). 1H NMR (400 MHz, THF-d8, 25 °C): δ 7.64
(br s, 2H), 6.78 (t, J = 6.8 Hz, 2H), 6.57 (t, J = 6.8 Hz, 1H), 2.05 (m, 2H),
1.94 (m, 2H), 1.39À1.20 (m, 10H), 1.10À1.00 (m, 12H), 0.90 (dd, J =
Corresponding Author
*E-mail: jones@chem.rochester.edu.
’ ACKNOWLEDGMENT
Acknowledgement is made to the U.S. Department of Energy,
Office of Basic Sciences, for their support of this work (Grant
DOE86-ER-13569), and to the CENTC Elemental Analysis
Facility at the University of Rochester, funded by NSF CHE-
0650456.
’ REFERENCES
(1) Luo, Y.-R. Handbook of Bond Dissociation Energies In Organic
Compounds; CRC Press: New York, 2007.
(2) Garcia, J. J.; Brunkan, N. M.; Jones, W. D. J. Am. Chem. Soc. 2002,
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(3) Li, T.; Brennessel, W. W.; Jones, W. D. Organometallics 2010,
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(4) Swartz, B. D.; Reinartz, N. M.; Brennessel, W. W.; Jones, W. D.
J. Am. Chem. Soc. 2008, 130, 8548.
7, Hz, 6H). 13C NMR (100 MHz, THF-d8, 25 °C): δ 160.59 (ddd, JRhÀP
=
47.3, JPÀC = 94.5, JPÀC = 15.5 Hz), 142.43 (s), 125.68 (s), 119.79 (s),
27.42 (s), 27.26 (s), 21.00 (d, J = 6.8 Hz), 20.31 (d, J = 6.8 Hz), 19.55 (s),
19.07, ipso carbon not observed, alkyl resonances may be obscured by
overlap with THF. 31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 91.08
(5) Schaub, T.; D€oring, C.; Radius, U. Dalton Trans. 2007, 1993.
(6) Swartz, B. D.; Brennessel, W. W.; Jones, W. D. Organometallics
2011, 30, 1523.
(ddd, 1JRhÀP = 115, 2JPÀC = 15.5, 2JPÀP = 22 Hz), 82.84 (ddd, 1JRhÀP
=
2
1
154, JPÀC = 94.5, JPÀP = 22 Hz). Anal. Calcd (found) for C21H37-
RhP2NK: %C, 49.70 (41.06); %H, 7.35 (5.58); %N, 2.76 (2.44).
Synthesis of [(dippe)Rh(η6-Ph-BPh3)] (4). 1 (19.2 mg, 0.0240
mmol) and NaBPh4 (16.0 mg, 0.0467 mmol) were dissolved in 1 mL of
THF and placed in a J-Young NMR tube. The tube was placed in an oil
bath and heated for 6.5 h, during which time NaCl precipitated from
solution. The solution was filtered through Celite and evaporated to
dryness to obtain an orange crystalline solid (32.5 mg, 99%). 1H NMR
(400 MHz, THF-d8, 25 °C): δ 7.18 (d, J = 6.8 Hz, 6H), 6.94 (t, J =
8.0 Hz, 6H), 6.84 (t, J = 6.8 Hz, 3H), 6.68 (d, J = 6.0 Hz, 2H), 6.56 (t, J =
6.0 Hz, 1H), 6.13 (t, J = 6.0 Hz, 2H), 1.68 (m, 4H), 1.18 (m, 4H), 1.02
(m, 24H). 13C NMR (100 MHz, THF-d8, 25 °C): δ 137.23 (s), 126.38
(s), 123.41 (s), 104.59 (s), 100.66 (s), 95.71 (s), 30.81À18.82 (dippe).
31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 91.79 (dd, 1JRhÀP = 186,
2JPÀP = 23 Hz), 87.49 (dd, 1JRhÀP = 119 Hz, 2JPÀP = 23 Hz). 11B{1H}
NMR (160 MHz, THF-d8, 25 °C): δ À7.21 (s). This compound was
reported previously in ref 19.
(7) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J. Can. J. Chem. 1992,
70, 2381.
(8) Krausse, J.; Marx, G.; Schodl J. Organomet. Chem. 1970, 21, 159.
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Int. Ed. 1976, 15, 48.
(10) Wyrwa, R.; Goris, H. Z. Anorg. Allg. Chem. 1999, 625, 1904.
(11) Bazhenova, T. A.; Kachapina, L. M.; Shilov, A. E.; Antipin,
M. Y.; Struchkov, U. T. J. Organomet. Chem. 1992, 428, 107.
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24, 1852.
(13) Bogdanovic, B.; Leitner, W.; Six, C.; Wilczok, U.; Wittmann, K.
Angew. Chem., Int. Ed. Engl. 1997, 36, 502.
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(15) See, for example: Kinney, R. J.; Jones, W. D.; Bergman, R. G.
J. Am. Chem. Soc. 1978, 100, 7902.
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13, 213.
Synthesis of Metalloimine 5. In a nitrogen-filled glovebox, 4 (9.6
mg, 0.014 mmol) was dissolved in 1 mL of THF and placed in a J-Young
NMR tube. Benzonitrile (2.9 μL, 0.028 mmol) was added by syringe to
the sample. Nitrogen was removed by the freezeÀpumpÀthaw degassing
(2Â) method. The tube was then heated at 120 °C for 18 h, during which
time the solution colored changed from a clear orange to a opaque dark
green. When the solution or solid of 5 turns red, it has decomposed. 1H
NMR (400 MHz, THF-d8, 25 °C): δ 7.8À7.0 (br m, 25H), 6.86 (t, J =
7.6 Hz, 2H), 6.58 (t, J = Hz, 1H), 2.25 (m, 2H), 2.14 (m, 2H), 1.51 (m,
2H), 1.33 (m, 2H), 1.21 (m, 12H), 1.10 (m, 12H). 13C NMR (100 MHz,
THF-d8, 25 °C): δ 189.8 (d, J = 7.5 Hz), 162.8 (s), 149.59 (s), 143À121
(aryl), 27.52À19.48 (dippe).
(18) Chan, A. S. C.; Shieh, H.-S.; Hill, J. R. J. Organomet. Chem. 1985,
279, 171.
(19) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J. Am.
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’ ASSOCIATED CONTENT
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Supporting Information. 31P and 13C NMR spectra for
S
b
13
5À C, tables of crystallographic data for 4, and DFT calcula-
tions of structure 2. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystal structure data for 4 has
5609
dx.doi.org/10.1021/om200342f |Organometallics 2011, 30, 5604–5610

Organometallics
ARTICLE
Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K.
N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N,;
Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,
S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT,
2009.
5610
dx.doi.org/10.1021/om200342f |Organometallics 2011, 30, 5604–5610