Ring-expanded N-heterocyclic carbene complexes of rhodium with bifluoride, fluoride, and fluoroaryl ligands

Article abstract of DOI:10.1021/om300984v

Thermolysis of Rh(PPh₃)₄H in the presence of the six-membered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6- tetrahydropyrimidin-2-ylidine (6-ⁱPr) gave the monocarbene complex Rh(6-ⁱPr)(PPh₃)₂

Full text of DOI:10.1021/om300984v

Article
pubs.acs.org/Organometallics
Ring-Expanded N‑Heterocyclic Carbene Complexes of Rhodium with
Bifluoride, Fluoride, and Fluoroaryl Ligands
†
†
†
‡
‡
́
Candela Segarra, Elena Mas-Marza, John P. Lowe, Mary F. Mahon, Rebecca C. Poulten,
,
†
and Michael K. Whittlesey*
†
‡
Department of Chemistry, and X-ray Crystallographic Unit, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
S Supporting Information
*
ABSTRACT: Thermolysis of Rh(PPh ) H in the presence of the six-
3
4
membered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6-tetrahydro-
i
i
pyrimidin-2-ylidine (6- Pr) gave the monocarbene complex Rh(6- Pr)-
(
PPh ) H as a 1:2 mixture of the cis- and trans-phosphine isomers 1a and
3 2
1
b. This same isomeric mixture was formed as the ultimate product from
i
treating Rh(PPh ) (CO)H with 6- Pr at room temperature, although
3
3
pathways involving both CO and PPh loss were observed at initial times.
3
i
Treatment of 1a/1b with Et N·3HF generated the bifluoride complex cis-Rh(6- Pr)(PPh ) (FHF) (2a), which upon stirring with
3
3 2
i
anhydrous Me NF was converted to the rhodium fluoride complex cis-Rh(6- Pr)(PPh ) F (3a). Thermolysis of 1a/1b with C F
4
3 2
6 6
i
resulted in C−F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh(6- Pr)(PPh ) (C F )
3
2
6 5
(
5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
In 2005, we reported that substitution of N-alkyl-substituted
five-membered ring N-heterocyclic carbenes (NHCs) into the
rhodium phosphine hydride complex Rh(PPh ) H took place
at room temperature to yield a mixture of cis- and trans-isomers
of the mono- and bis-NHC complexes Rh(NHC)(PPh ) H
Substitution Reactions of Rh(PPh ) H and Rh-
3
4
i
i
(PPh ) (CO)H with 6- Pr. Addition of 3 equiv of 6- Pr (1,3-
3
3
bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine) to a tol-
uene solution of Rh(PPh ) H at room temperature led to the
3
4
3
4
rapid, but incomplete, formation of the mono-NHC complex
3
2
i
and Rh(NHC) (PPh )H (NHC = 1,3-diethyl-4,5-dimethylimi-
Rh(6- Pr)(PPh ) H. Running the reaction at 343 K for 2−3 h
2
3
3 2
dazol-2-ylidene (IEt Me ), 1,3-diisopropyl-4,5-dimethylimida-
led to complete conversion to a 1:2 mixture of the cis- and
trans-phosphine isomers 1a and 1b (Scheme 1), which could be
isolated as an inseparable mixture typically in ca. 70% yield.
2
2
i
zol-2-ylidene (I Pr Me ), 1,3-dicyclohexylimidazol-2-ylidene
ICy)). The formation of such complex mixtures of products
2
2
1
(
i
precluded any subsequent investigations of Rh−H bond
reactivity. Although relatively few NHC-Rh-H complexes have
been described in the intervening time, the examples that have
been reported all exhibit an ability to coordinate and/or activate
Increasing the amounts of ligand used to >3 equiv of 6- Pr had
no effect on the isomer ratio.
Scheme 1
2
,3
inert bonds in small molecules.
With this in mind, we have revisited the substitution
chemistry of Rh(PPh ) H (as well as the related carbonyl
3
4
precursor Rh(PPh ) (CO)H) now toward NHCs with ring
3
3
sizes >5. These so-called ring-expanded carbenes (RE-NHCs)
are known to exhibit much wider N−C−N angles than their
4
five-membered counterparts, a feature we hoped might reduce
the likelihood of generating geometric isomers and/or mono-
and bis-substitution products. We now report the case of the
1
In the H NMR spectrum, there was a hydride resonance
six-membered N-isopropyl-substituted ligand 1,3-bis(2-propyl)-
i
with a doublet of doublets of doublets multiplicity at δ −5.66
3
,4,5,6-tetrahydropyrimidin-2-ylidine (6- Pr), which reacts with
2
2
1
i
( J
= 106.4 Hz, J
= 30.4 Hz, J
= 25.5 Hz) arising
HPtrans
HPcis
HRh
Rh(PPh ) H to give only the monocarbene species Rh(6- Pr)-
3
4
from the minor cis-phosphine isomer 1a, along with a lower
(
PPh ) H (1), albeit still as a mixture of cis (1a) and trans (1b)
3 2
2
frequency doublet of triplets signal at δ −10.35 ( J = 26.2 Hz,
HP
isomers. Thus far, we have focused on the reactivity of 1a/1b
toward fluorine-based reagents because of (i) the interest in
combining hard fluoride ligands with soft metal centers such as
1
J
= 10.2 Hz) for the trans-phosphine species 1b. Both
HRh
isomers showed methine protons at very high frequencies (>δ
5
,6
Rh and (ii) the strong precedence for using metal hydride/
dihydride precursors for the synthesis of novel organometallic
complexes with fluorinated ligands.
Received: October 19, 2012
Published: December 4, 2012
7
©
2012 American Chemical Society
8584
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Organometallics
.5). This feature has been noted previously in other RE-NHC-
Article
i
6
160.869(13)° (cf. 160.39(3)° for the I Pr Me complex). This
2 2
distortion most probably helps to minimize steric interactions
8
,9
containing species and rationalized on the basis of the wide
N−C−N angle (e.g., 116.79(12)° in 1b), which places the
between the phosphines and carbene. The Rh−C distance
NHC
i
i
N- Pr groups proximal to the metal center (see further in
in 1b (2.0928(13) Å) was much longer than in Rh(I Pr Me )-
2
2
3
1
1
discussion of the X-ray data below). The P{ H} NMR
(
PPh ) H (2.068(2) Å), contrary to what might have been
3 2
spectrum displayed two doublets of doublets for 1a, at δ 51.0
expected on the basis of a more strongly σ-donating six-
1
2
1
2
(
JPRh = 146 Hz, J = 24 Hz) and δ 43.1 ( J = 144 Hz, J
PP PRh PP
membered ring NHC being present. As commented on
1
=
24 Hz), and a doublet for 1b at δ 43.1 ( J = 180 Hz). The
9
PRh
elsewhere, it is likely that enhanced steric clashes between
carbenic carbon of only the major isomer 1b could be observed
the metal and N-substituents (resulting from the widened N−
C−N angle in RE-NHCs) result in the carbene ligand being
pushed away from the metal.
13
by C NMR spectroscopy and appeared at a characteristic high
1
0
1
2
frequency (223 ppm) with J and J splittings of 46 and
CRh
CP
9
Hz, respectively.
Slow diffusion of hexane into a concentrated benzene
solution of 1a/1b gave orange crystals of the major isomer
b suitable for X-ray crystallography. The structure (Figure 1)
The carbonyl precursor Rh(PPh ) (CO)H proved to be
3
3
more substitutionally labile than Rh(PPh ) H and reacted
3
4
i
immediately with 6- Pr at room temperature via two separate
1
pathways, one involving CO and PPh loss, the other involving
3
11
dissociation of two of the PPh ligands. Thus, the hydride
3
region of a proton NMR spectrum recorded ca. 15 min after
i
addition of 3 equiv of 6- Pr to Rh(PPh ) (CO)H in C D
3
3
6
6
showed a 1:2.2:6.4:2.9 mixture of 1a and 1b and two new
products, which appeared as a doublet of doublets at δ −2.90 (J
=
111.3, 29.5 Hz) and δ −4.60 (J = 27.5, 19.5 Hz). The
splitting patterns and magnitudes of the coupling constants led
us to assign the new species as the trans-H-Rh-P and trans-H-
Rh-CO isomers, respectively, of the monocarbonyl complex
i
12
Rh(6- Pr)(PPh )(CO)H. Unexpectedly, after 10 days stand-
3
ing at room temperature, the signals for the two isomers of
i
Rh(6- Pr)(PPh )(CO)H had vanished, leaving 1a/1b (in a 1:2
3
ratio) as the only products of the reaction. This transformation
was achievable in only 2 h with heating at 343 K.
Reaction of 1a/1b with Et N·3HF. Addition of NEt ·3HF
3
3
(
TREAT-HF) to the mixture of 1a/1b in toluene solution at
room temperature led to the clean formation of the rhodium
i
bifluoride complex cis-Rh(6- Pr)(PPh ) (FHF) (2a), which was
3
2
Figure 1. Molecular structure of 1b. Ellipsoids are shown at the 30%
level. Hydrogen atoms except for Rh-H and those attached to C(5)
and C(8) are removed for clarity. Selected bond lengths (Å) and
angles (deg): Rh(1)−P(1) 2.2467(3), Rh(1)−P(2) 2.2354(3),
Rh(1)−C(1) 2.0928(13), P(1)−Rh(1)−C(1) 100.30(4), P(1)−
Rh(1)−P(2) 160.869(13).
isolated as a yellow, air-sensitive solid in moderate yield (65%)
upon precipitation from solution with hexane. When THF was
used as the reaction solvent rather than toluene, 2a was still
formed as the major species, although the reaction was not as
3
1
clean, with other uncharacterized products apparent by
P
13
NMR spectroscopy.
The 31P{ H} NMR spectrum of a crystalline sample of the
1
exhibited a very similar distorted square-planar geometry to
that found in the five-membered NHC analogue trans-
compound dissolved in C comprised two highly coupled
D
6 6
i
1
1
2
2
Rh(I Pr Me )(PPh ) H. Thus, the PPh groups were bent in
resonances at δ 60.1 ( J = 219 Hz, J = 180 Hz, J = 38
2
2
3 2
3
PRh PF PP
1
2
2
toward the hydride ligand with a P−Rh−P angle of
Hz) and δ 37.1 ( J = 124 Hz, J = 38 Hz, J = 26 Hz),
PRh PP PF
Scheme 2
8
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Organometallics
Article
indicating that only the cis-phosphine isomer of the complex
was formed (Scheme 2).
The bifluoride ligand was characterized by the appearance of
147.2°, respectively), this value being at the bottom end of the
14
range reported for other M-FHF species.
A preliminary assessment of the thermal stability of 2a
31
1
a very broad, high-frequency doublet (J = 380 Hz in THF-d ) at
revealed there was no change in the P{ H} NMR spectrum of
a sample heated in C D at 343 K for 2 h.
8
1
1
2.1 ppm in the room-temperature H NMR spectrum. Upon
6
6
i
cooling to 204 K, this resonance resolved into a doublet of
doublets centered at δ 12.6, with a large splitting of 370 Hz to
the distal fluorine of the bifluoride ligand (i.e., RhFHF) and a
smaller 42 Hz splitting resulting from coupling to the proximal
fluorine, RhFHF. The room temperature F NMR spectrum
displayed two broadened doublets, one at δ −176 for the distal
fluorine, the other at lower frequency (δ −273) for the
rhodium-bound proximal fluorine. At 204 K, the higher
frequency resonance sharpened to a doublet of doublets with
JFH and JFF couplings of ca. 372 and 117 Hz, respectively. The
lower frequency signal resolved to some extent to become more
multiplet-like in appearance, but not enough to allow any of the
JFH, JFF, JFRh, and JFP splittings to be measured.
Synthesis and Characterization of Rh(6- Pr)(PPh ) F.
3 2
Stirring 2a with an excess of anhydrous Me NF in THF
4
solution resulted in the abstraction of HF and formation of the
cis-phosphine isomer of the rhodium fluoride complex
13b
19
i
Rh(6- Pr)(PPh ) F (3a) (Scheme 2).
3
2
31
1
The P{ H} chemical shift data for this species differed very
little from those for 2a, with a change of only 0.6 ppm between
the two sets of phosphorus signals. The F NMR spectrum was
more distinctive, with a new single, sharp, low-frequency Rh-F
19
14
resonance observed at δ −287.5, with trans and cis J
FP
couplings of 179 and 28 Hz and a J
of 66 Hz. Both the
FRh
chemical shift and coupling constants are very similar to those
15
reported for Rh(PPh ) F.
3 3
The X-ray crystal structure of 2a (Figure 2) confirmed the
cis-phosphine geometry observed in solution. The four-
Bond lengths and angles in the solid-state structure of 3a
(Figure 3) showed only very minor changes from those in 2a.
Figure 2. Molecular structure of 2a. Ellipsoids are shown at the 30%
level. Hydrogen atoms except for Rh-FHF and those attached to C(5)
and C(8) are removed for clarity. Selected bond lengths (Å) and
angles (deg): Rh(1)−P(1) 2.3148(6), Rh(1)−P(2) 2.1778(6),
Rh(1)−C(1) 2.062(2), Rh(1)−F(1) 2.1217(13), P(1)−Rh(1)−C(1)
163.47(6), P(2)−Rh(1)−C(1) 93.02(4), P(1)−Rh(1)−P(2)
102.46(13), P(1)−Rh(1)−F(1) 82.81(4), P(2)−Rh(1)−F(1)
174.34(4), C(1)−Rh(1)−F(1) 81.53(7).
Figure 3. Molecular structure of 3a. Ellipsoids are shown at the 30%
level, and hydrogen atoms except for those attached to C(5) and C(8)
are removed for clarity. Selected bond lengths (Å) and angles (deg):
Rh(1)−P(1) 2.1850(7), Rh(1)−P(2) 2.3162(7), Rh(1)−C(1)
2
.051(3), Rh(1)−F(1) 2.0880(15), P(1)−Rh(1)−C(1) 90.68(7),
P(2)−Rh(1)−C(1) 166.43(8), P(1)−Rh(1)−P(2) 101.90(2), P(1)−
Rh(1)−F(1) 173.99(5), P(2)−Rh(1)−F(1) 83.27(5), C(1)−Rh(1)−
F(1) 83.91(8).
coordinate rhodium center was distorted from a regular square
plane with trans-C-Rh-P and trans-P-Rh-F angles of 163.47(6)°
and 174.34(4)°, respectively. The metrics of the bifluoride
ligand can be compared against those of Rh(PPh ) (PPh F)-
Of most note was the shortening of the Rh−F distance from
2.1217(13) Å to 2.0880(15) Å as a result of the loss of HF.
Upon comparing the structure of 3a with those of 1b and 2a, it
is apparent that in all cases the carbene approximates a 2-fold
rotation symmetry about the Rh(1)−C(1) axis, although there
is a difference of ca. 3° between the N(1)−C(1)−Rh(1) and
N(2)−C(1)−Rh(1) angles for the three compounds (1b:
123.16(10)°, 120.05(10)°; 2a: 118.92(16)°, 121.85(16)°; 3a:
3
2
2
1
5
14g
(
FHF) and Rh(PPh )(COD)(FHF).
The Rh−F bond
3
length (2.1217(13) Å) is significantly longer than in either of
these two compounds (2.082(1) and 2.083(2) Å, respectively),
and the F···F distance is identical (2.325 Å vs 2.329 and 2.331
Å). The Rh−F···F angle (127.44°) is reduced (135.6° and
8
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Organometallics
19.98(19)°, 122.78(19) ). As a result, one of the methine
Article
o
1
hydrogen atoms on either C(5) or C(8) is, on average, 0.11 Å
closer to the metal center than the other.
The thermal stability of 3a was of interest given that Grushin
has reported the degradation of Rh(PPh ) F within 2−3 h at
3
3
3
53 K to give a 1:1 mixture of the F/PPh rearrangement
3
product Rh(PPh ) (PPh F)F and the cyclometalated complex
3
2
2
1
5
Rh(PPh ) (PPh )′. We found no evidence to suggest the
3
2
3
formation of a species analogous to Rh(PPh ) (PPh F)F upon
3
2
2
heating 3a in C D at 343 K, although two products were
6
6
generated (Scheme 2). Heating for 3 h led first to the
appearance of a compound assigned as the trans-phosphine
i
isomer of Rh(6- Pr)(PPh ) F (3b). This exhibited a doublet of
3
2
3
1
1
1
doublets signal in the P{ H} NMR spectrum at δ 30.0 ( J
175 Hz, J = 29 Hz) and a somewhat broad, low-frequency
doublet of triplets resonance in the corresponding F spectrum
at δ −326, with the same F− P splitting and a J coupling
of 80 Hz, comparable to that in 3a. A P{ H} NMR spectrum
recorded after 20 h heating still contained mostly 3a, but now
PRh
2
=
PF
1
9
1
9
31
FRh
3
1
1
also showed in addition to 3b a further product (4) that
1
displayed two doublets of doublets at δ 39.4 ( J = 128 Hz,
PRh
2
1
2
JPP = 28 Hz) and δ −40.7 ( J = 96 Hz, J = 28 Hz). There
PRh
PP
1
9
were no new signals in the F spectrum. We provisionally
assign 4 as a cyclometalated phosphine complex, principally on
the basis of the characteristic very low frequency phosphorus
Figure 4. Molecular structure of 5b. Ellipsoids are shown at the 30%
level, and hydrogen atoms except for those attached to C(5) and C(8)
are removed for clarity. Selected bond lengths (Å) and angles (deg):
Rh(1)−P(1) 2.2981(7), Rh(1)−P(2) 2.3062(7), Rh(1)−C(1)
1
6,17
resonance.
The formation of 3b and 4 as part of a mixture,
along with the incomplete nature of the reaction, has thus far
2
.078(2), Rh(1)−C(29) 2.098(2), P(1)−Rh(1)−C(1) 92.71(7),
prevented their further characterization.
P(2)−Rh(1)−C(1) 97.43(7), P(1)−Rh(1)−P(2) 157.31(2), P(1)−
Rh(1)−C(29) 88.94(7), C(1)−Rh(1)−C(29) 164.38(9).
i
C−F Bond Activation of C F by Rh(6- Pr)(PPh ) H. The
6
6
3 2
1
8
possibility of using C−F bond activation as an alternative
i
route to 3 prompted us to study the reactivity of Rh(6- Pr)-
(
PPh ) H toward hexafluorobenzene. No reaction was
Rh(1)−C(29), 164.38(9)°; P(1)−Rh(1)−P(2), 157.31(2)°)
relative to 1b, 2a, and 3a, presumably because of the ligand
“gymnastics” needed to accommodate the sterically demanding
fluoroaryl ring. The level of this distortion can be readily
evidenced by the maximum deviation of the contributors to the
mean plane containing the four atoms directly coordinated to
rhodium in each of the four structures. These values are 0.05,
0.04, 0.02, and 0.38 Å for 1b, 2a, 3a, and 5b. Moreover, in 5b
there is a substantial tilt of the carbene relative to the Rh(1)−
C(1) axis. The N(1)−C(1)−Rh(1) and N(2)−C(1)−Rh(1)
3
2
observed between 1a/1b and C F (1−5 equiv) in C D over
2
C−F activation was observed, although this gave the
pentafluorophenyl complex trans-Rh(6- Pr)(PPh ) (C F )
6
6
6
6
4 h at room temperature, but upon heating at 363 K for 2 h,
i
3
2
6 5
(5b) (Scheme 3). Only with prolonged heating was 3a formed,
along with the hydrodefluorination products C F H and 1,4-
C F H .
6
5
6
4
2
Scheme 3
o
angles at 115.27(17)° and 128.55(18) , respectively, support
this observation, with the net effect being that the methine
H(5) comes closer to the metal (H(5)−Rh(1), 2.32 Å) than
any of the comparative hydrogen atoms in 1b, 2a, or 3a.
SUMMARY AND CONCLUSIONS
The six-membered-ring N-heterocyclic carbene 6- Pr (1,3-bis(2-
propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine) has been used to
prepare cis- and trans-phosphine isomers of Rh(6- Pr)(PPh ) H
■
i
i
3
2
from either Rh(PPh ) H or Rh(PPh ) (CO)H. Treatment of
3
4
3 3
the hydride complex with Et N·3HF gave the bifluoride
3
i
Complex 5b was isolated as an analytically pure yellow solid
complex Rh(6- Pr)(PPh ) (FHF), which could be converted
3
2
by precipitation from the mixture with hexane. It exhibited a
to the corresponding fluoride analogue upon reaction with a
source of anhydrous fluoride. Attempts to prepare Rh(6- Pr)-
31
1
i
doublet in the P{ H} NMR spectrum, consistent with a trans-
1
9
phosphine stereochemistry. The F NMR spectrum displayed
three multiplets in a 2:2:1 ratio at δ −108, −165, and −167,
which were assigned to the ortho-, meta-, and para-F atoms of
(PPh ) F via C−F activation of hexafluorobenzene instead
3
2
i
allowed isolation of the pentafluorophenyl complex Rh(6- Pr)-
(PPh ) (C F ).
The synthesis and initial reactivity studies of Rh(6- Pr)-
3
2
6 5
19
i
the C F ring by comparison to the literature.
6
5
The X-ray crystal structure of 5b (Figure 4) confirmed the
trans-phosphine arrangement present in solution. The presence
of the C F ligand resulted in the geometry at the rhodium
(PPh ) F (3a) suggest that incorporation of the RE-NHC
brings about noticeable differences to the all-phosphine
3
2
analogues. Unlike Rh(PPh ) F, there is no evidence for
6
5
3 3
center being substantially distorted from square planar (C(1)−
phosphine loss and Rh(μ-F) Rh complex formation from 3a
2
8
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Article
1H, Rh-FHF). 31P{ H} NMR (C
1
D , 298 K): δ 60.1 (ddd, JPRh = 219
1
in solution at room temperature nor F/Ph rearrangement of a
6 6
1
3,15
2
2
1
2
Hz, J = 180 Hz, J = 38 Hz), 37.1 (ddd, J = 124 Hz, J = 38
PPh3 ligand at higher temperature.
We are currently
PF
PP
PRh
PP
2
19
Hz, J = 26 Hz). F NMR (THF-d , 298 K): δ −175.9 (br d, J = 377
pursuing this further in attempts to make other Rh(RE-
NHC)(PR ) F analogues, which will allow the properties of the
PF
8
Hz, 1F, Rh-FHF), −272.6 (br d, J = 188 Hz, 1F, Rh-FHF); 204 K: δ
3
2
1
2
−
176.8 (dd, J = 372 Hz, J = 117 Hz, 1F, Rh-FHF), −274.9 (br
13 1
FH
FF
Rh−F bond to be investigated as a function of NHC and
m, 1F, Rh-FHF). Selected C{ H} NMR (THF-d , 298 K): δ 208.1
8
phosphine. Similarly, if the oxidative addition of H or R SiH
2
1
2
2
3
(
ddd, J
= 110 Hz, J = 48 Hz, J
= 12 Hz, Rh-CNHC).
i
CP(trans)
CRh
CP(cis)
reported for Rh(PMe ) (C F ) takes place with Rh(6- Pr)-
3
3
6
5
Anal. Calcd for C H N F P Rh (834.72): C, 66.19; H, 6.16; N, 3.36.
1
9b,20
46 51 2 2 2
(
PPh ) (C F ),
this would open up the possibility of
3
2
6
5
Found: C, 66.04; H, 6.17; N, 3.45.
cis-Rh(6- Pr)(PPh ) F (3a). Anhydrous Me NF (72 mg, 0.77
mmol) was added in a glovebox to a THF (5 mL) solution of
i
probing the influence of RE-NHCs on the catalytic
functionalization of fluorinated aromatic substrates. In light of
our recent work on catalytic C−F functionalization with Ru-
3
2
4
i
Rh(6- Pr)(PPh
with a J. Young resealable valve. After stirring at room temperature for
h, the solution was reduced to dryness, extracted with benzene (8
) (FHF) (66 mg, 0.079 mmol) in an ampule fitted
3 2
21
NHC complexes, this is an avenue we are actively pursuing.
2
mL), and filtered by cannula. The filtrate was concentrated to ca. 1
mL, and 3a precipitated as a mustard-colored solid upon addition of
hexane upon vigorous stirring. Yield: 20 mg (31%). H NMR (500
EXPERIMENTAL SECTION
■
All manipulations were carried out using standard Schlenk, high-
vacuum, and glovebox techniques using dried and degassed solvents,
unless otherwise stated. NMR spectra were recorded on Bruker
Avance 400 and 500 MHz NMR spectrometers and referenced to
1
3
MHz, C D , 298 K): δ 8.30−7.76 (br, 10H, P(C H )), 7.65 (sept, J
6
6
6
5
HH
=
6.5 Hz, 2H, (CH ) CH), 7.04−6.95 (br, 10H, P(C H )), 6.93−6.78
m, 10H, P(C H )), 2.60 (m, 2H, NCH CH CH N), 2.16 (m, 2H,
3
2
6
5
(
1
13
6 5 2 2 2
residual solvent signals for H and C spectra for C D (δ 7.15, 128.0)
3
6
6
NCH CH CH N), 1.71 (d, J = 6.5 Hz, 6H, N(CH ) CH), 1.31−
1
13
2
2
2
HH
3 2
and THF-d (δ 3.58, 25.4). Unless otherwise quoted, H and
C
3
8
1
.11 (m, 2H, NCH CH CH N), 0.70 (d,
J
= 6.5 Hz, 6H,
1
3
2
2
2
HH
resonances for the PPh ligands and noncarbenic C signals arising
31
1
1
2
3
N(CH ) CH). P{ H} NMR: δ 60.7 (ddd, J = 202 Hz, J = 180
Hz, J = 36 Hz), 37.7 (ddd, J = 121 Hz, J = 36 Hz, J = 28
i
31
1
19
3
2
PRh
PF
from the 6- Pr ligands have been excluded. P{ H} and F spectra
2
1
2
2
PP
19
PRh
PP
PF
were referenced externally to 85% H PO4 (85%) and CFCl3,
2
1
3
Hz). F NMR: δ −287.5 (ddd, J
= 179 Hz, J = 66 Hz,
FP(trans)
FRh
respectively (both δ = 0.0). Elemental analyses were performed by
Elemental Microanalysis Ltd., Okehampton, Devon, UK, or the
Elemental Analysis Service, London Metropolitan University, London,
2
JFP(cis) = 28 Hz, 1F, Rh-F). Anal. Calcd for C H N F P Rh·0.5C H
46
51
2
2
2
6
6
(
853.76): C, 68.93; H, 6.26; N, 3.28. Found: C, 68.93; H, 6.33; N,
3.37.
trans-Rh(6- Pr)(PPh ) (C F ) (5b). Hexafluorobenzene (65 μL,
2
2
22
i
23
UK. Rh(PPh ) H, Rh(PPh ) (CO)H, 6- PrH·BF , and anhy-
3
4
3
3
4
i
2
4
3
2
6 5
drous Me NF were prepared according to the literature.
4
i
0.57 mmol) was added to a solution of 1a/1b (90 mg, 0.11 mmol) in
benzene (5 mL) in an ampule fitted with a J. Young resealable valve,
and the mixture heated at 363 K for 10 h. After cooling, the solvent
was removed, and the residue redissolved in toluene (5 mL) and
cannula filtered. The filtrate was concentrated to ca. 2 mL, and hexane
Rh(6- Pr)(PPh ) H (1a/1b). A Schlenk flask was charged with
3
2
6-iPrH·BF (280 mg, 1.09 mmol) and KN(SiMe ) (217 mg, 1.09
4
3 2
mmol) in toluene (10 mL), and the suspension stirred at room
i
temperature for 30 min. This solution of the free 6- Pr carbene was
added to a J. Young ampule containing Rh(PPh ) H (400 mg, 0.35
mmol) in toluene (20 mL), and the mixture heated at 343 K for 3 h.
After cooling to room temperature, the resulting deep orange-red
solution was filtered by cannula and concentrated to ca. 3 mL, and
hexane was added to afford an orange precipitate. The solid was
isolated by cannula filtration and dried under vacuum to give a mixture
3
4
added with vigorous stirring to bring about precipitation of 5b as a
1
yellow solid. Yield: 43 mg (40%). H NMR (500 MHz, C D , 298 K):
6
6
δ 7.84−7.60 (br, 12H, P(C H )), 7.10−6.98 (br, 20H, P(C H ) + 2 ×
6
5
6
5
N(CH ) CH), 2.54 (m, 4H, NCH CH CH ), 1.54 (m, 2H,
3
2
2
2
2
3
NCH CH CH ), 0.51 (d, JHH = 7.0 Hz, 12H, N(CH ) CH).
P{ H} NMR: δ 29.8 (d, J = 172 Hz). F NMR: δ −107.8 (br
2
2
2
3 2
31
1
1
19
1
of isomers 1a and 1b. Yield: 232 mg (84%). 1a: H NMR (500 MHz,
PRh
3
3
3
C D , 298 K): δ 6.68 (sept, J = 6.6 Hz, 2H, (CH ) CH), 2.62 (m,
d, J = 36.5 Hz, 2F, Rh-o-C
Hz, 2F, Rh-m-C ), −166.7 (t,
Selected C{ H} NMR: δ 214.6 (m, Rh-CNHC). Anal. Calcd for
Rh (962.76): C, 64.89; H, 5.23; N, 2.91. Found: C, 65.0;
6 5
F ), 164.8 (dd, JFF = 31.2 Hz, JFF = 20.4
6
6
HH
3 2
3
2
H, NCH CH CH N), 2.40 (m, 2H, NCH CH CH N), 1.54 (m, 1H,
6
F
5
JFF = 20.4 Hz, 1F, Rh-p-C F ).
6 5
2
2
2
2
2
2
3
13
1
CH CH CH ), 1.47 (m, 1H, CH CH CH ), 1.30 (d, J = 6.6 Hz,
2
2
2
2
2
2
HH
3
6
H, (CH ) CH), 0.58 (d, J = 6.6 Hz, 6H, (CH ) CH), −5.66 (ddd,
HP(trans)
C H N F P
52 50 2 5 2
3
2
HH
3
2
2
2
1
J
= 106.4 Hz, J
= 30.4 Hz, J
= 25.5 Hz, RhH).
H, 5.25; N, 3.03.
HP(cis)
HRh
3
1
1
1
2
P{ H} NMR: δ 51.0 (dd, J = 146 Hz, J = 24 Hz), 43.1 (dd,
X-ray Crystallography. Single crystals of compounds for 1b, 3a,
and 5b were analyzed at 150 K using a Nonius Kappa CCD
diffractometer, while those for 2a (collected at 120 K) were obtained
on an Oxford-Diffraction machine. Data were all collected using Mo
Kα radiation throughout. Details of the data collections, solutions, and
refinements are given in the Supporting Information. The structures
PRh
PP
1J = 144 Hz, J = 24 Hz). 1b: H NMR (500 MHz, C D , 298 K):
2
1
PRh
PP
6
6
3
δ 6.52 (sept, JHH = 6.7 Hz, 2H, (CH ) CH), 2.54 (m, 4H,
3
2
3
NCH CH CH N), 1.53 (m, 2H, CH CH CH ), 0.53 (d, J = 6.7
2
2
2
2
2
2
HH
2
1
Hz, 12H, (CH ) CH), −10.35 (dt, J
= 26.2 Hz, J = 10.2 Hz,
= 180 Hz). Selected C{ H}
3
2
HP(cis)
HRh
RhH). 3 P{ H} NMR: 43.1 (d, J
1
1
1
13
1
PRh
2
5
1
2
were solved using SHELXS-97 and refined using full-matrix least-
NMR: δ 222.7 (dt, J = 46 Hz, J = 9 Hz, Rh-CNHC). Anal. Calcd
CRh
CP
2
5
squares in SHELXL-97. Refinements were generally straightforward,
although notable points were as follows. In 1b, the hydride H(1) was
located and refined at 1.6 Å from Rh1. Additionally, carbons C(2)−
C(4) were each disordered over two sites in a 75:25 ratio. The 75%
occupancy fragment atoms were refined anisotropically. The F-H-F
hydrogen (H(1)) in the structure of 2a was readily located and refined
freely. There was some evidence from the difference electron density
Fourier map that this hydrogen may sometimes move closer to F(2)
than F(1). However, disorder of H(1) was not modeled due to the
uncertainty associated with determining hydrogen positions when
using X-rays.
for C H N P Rh (796.72): C, 69.34; H, 6.45; N, 3.52. Found: C,
46
51
2 2
6
9.12; H, 6.41; N, 3.69.
i
cis-Rh(6- Pr)(PPh ) (FHF) (2a). Et N·3HF (20 μL, 0.123 mmol)
3
2
3
i
was added by syringe to a vigorously stirred solution of Rh(6- Pr)-
(
PPh ) H (100 mg, 0.126 mmol) in toluene (7 mL) in an ampule
3 2
fitted with a J. Young resealable valve. After stirring at room
temperature for 2 h, the solution was reduced to dryness, redissolved
in a minimum amount of benzene, and precipitated under vigorous
1
stirring with addition of hexane. Yield: 68 mg (65%). H NMR (500
MHz, C D , 298 K): δ 13.15 (br d, J = 405 Hz, 1H, Rh-FHF), 8.22−
6
6
3
7
.67 (br, 10H, P(C H )), 7.52 (sept, J = 6.7 Hz, 2H, (CH ) CH),
6 5 HH 3 2
.06−6.93 (br, 10H, P(C H )), 6.91−6.79 (m, 10H, P(C H )), 2.89
The apical carbene atom C(3) in 3a exhibited disorder over two
positions in an 80:20 ratio. Hydrogen atoms attached to C(2) and
C(4) are included at full occupancy based on the 80% occupancy
component of C(3). The motif was also seen to contain half of a
molecule of benzene located proximate to a crystallographic inversion
center.
6
5
6
5
2 2 2 2 2 2
3
2
2
2
2
3
1
.59 (d, J = 6.7 Hz, 6H, N(CH ) CH). Additional selected H
HH 3 2
8
1
1
= 370 Hz, J
= 42 Hz,
HF(distal)
HF(proximal)
8
588
dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics
Article
The asymmetric unit in 5b comprised one molecule of benzene, in
addition to one molecule of the rhodium complex. The solvent and the
carbene backbone atoms C(2), C(3), and C(4) exhibited 75:25
disorder. ADP restraints were applied to the 25% fractional occupancy
carbons in the final least-squares cycles. Distance restraints were also
included in the minor disordered solvent component to assist
convergence.
Crystallographic data for compounds 1b, 2a, 3a, and 5b have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publications CCDC 901475−901478. Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: (+44) 1223 336033, e-mail:
deposit@ccdc.cam.ac.uk].
Am. Chem. Soc. 2001, 123, 10973. (c) Ferrando-Miguel, G.; Gerard,
H.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 2002, 41, 6440.
(d) Jasim, N. A.; Perutz, R. N.; Archibald, S. J. Dalton Trans. 2003,
2184. (e) Clot, E.; Megret, C.; Kraft, B. M.; Eisenstein, O.; Jones, W.
D. J. Am. Chem. Soc. 2004, 126, 5647. (f) Salomon, M. A.; Braun, T.;
Krossing, I. Dalton Trans. 2008, 5197.
(
8) Bazinet, P.; Ong, T. G.; O’Brien, J. S.; Lavoie, N.; Bell, E.; Yap, G.
P. A.; Korobkov, I.; Richeson, D. S. Organometallics 2007, 26, 2885.
9) Armstrong, R.; Ecott, C.; Mas-Marza, E.; Page, M. J.; Mahon, M.
F.; Whittlesey, M. K. Organometallics 2010, 29, 991.
(
́
(
(
10) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385.
11) Note that we found that Rh(PPh ) (CO)H reacted with five-
3
3
membered NHCs via a single pathway involving loss of all three
phosphine ligands to give Rh(NHC) (CO)H complexes. See ref 1.
(12) The NMR data for the proposed trans-H-Rh-CO isomer was
2
ASSOCIATED CONTENT
■
*
S
Supporting Information
very similar to that of the N-mesityl analogue Rh(6-Mes)(PPh )(CO)
3
Synthesis and characterization of Rh(6-Mes)(PPh )(CO)H.
H, which was formed as the only product in the reaction of
Rh(PPh ) (CO)H with 6-Mes. Spectroscopic, analytical, and struc-
3
Crystallographic details and CIF files for 1b, 2a, 3a, and 5b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
3 3
tural characterization of Rh(6-Mes)(PPh )(CO)H are provided as
3
Supporting Information.
(13) Braun and co-workers have reported that addition of TREAT-
HF to the all-phosphine complex Rh(PEt ) H, gave the corresponding
3
4
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
rhodium fluoride complex Rh(PEt ) F rather than the bifluoride
3
3
product Rh(PEt ) (FHF). The latter could be prepared along with
3
3
Rh(PEt ) Cl upon treatment of Rh(PEt ) F with <1 equiv of HCl.
3
3
3 3
(
a) Braun, T.; Noveski, D.; Neumann, B.; Stammler, H.-G. Angew.
Chem., Int. Ed. 2002, 41, 2745. (b) Noveski, D.; Braun, T.;
Kruckemeier, S. J. Fluorine Chem. 2004, 125, 959.
14) (a) Murphy, V. J.; Hascall, T.; Chen, J. Y.; Parkin, G. J. Am.
Chem. Soc. 1996, 118, 7428. (b) Whittlesey, M. K.; Perutz, R. N.;
Greener, B.; Moore, M. H. Chem. Commun. 1997, 187. (c) Roe, D. C.;
Marshall, W. J.; Davidson, F.; Soper, P. D.; Grushin, V. V.
Organometallics 2000, 19, 4575. (d) Jasim, N. A.; Perutz, R. N. J.
Am. Chem. Soc. 2000, 122, 8685. (e) Jasim, N. A.; Perutz, R. N.;
Foxon, S. P.; Walton, P. H. J. Chem. Soc., Dalton Trans. 2001, 1676.
ACKNOWLEDGMENTS
■
̈
We acknowledge financial support from the Erasmus program
(
(
(
C.S.), the Spanish Ministerio de Ciencia e Innovacio
́
E.M.M.), and the University of Bath (R.C.P.). We thank
Johnson Matthey plc for the loan of hydrated RhCl3.
REFERENCES
■
(
1) Douglas, S.; Lowe, J. P.; Mahon, M. F.; Warren, J. E.; Whittlesey,
M. K. J. Organomet. Chem. 2005, 690, 5027.
(f) Kirkham, M. S.; Mahon, M. F.; Whittlesey, M. K. Chem. Commun.
(
2) For an overview of Rh-NHC chemistry, see: Praetorius, J. M.;
Crudden, C. M. Dalton Trans. 2008, 4079.
3) (a) Yu, X. Y.; Patrick, B. O.; James, B. R. Organometallics 2006,
5, 4870. (b) Praetorius, J. M.; Wang, R. Y.; Crudden, C. M. Eur. J.
2
001, 813. (g) Vicente, J.; Gil-Rubio, J.; Bautista, D.; Sironi, A.;
Masciocchi, N. Inorg. Chem. 2004, 43, 5665. (h) Vergote, T.; Nahra,
F.; Welle, A.; Luhmer, M.; Wouters, J.; Mager, N.; Riant, O.; Leyssens,
T. Chem.Eur. J. 2012, 18, 793.
(
2
Inorg. Chem. 2009, 1746. (c) Yu, X. Y.; Sun, H. S.; Patrick, B. O.;
James, B. R. Eur. J. Inorg. Chem. 2009, 1752. (d) Tang, C. Y.;
Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578.
(15) (a) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2004, 126,
3
068. (b) Macgregor, S. A.; Roe, D. C.; Marshall, W. J.; Bloch, K. M.;
Bakhmutov, V. I.; Grushin, V. V. J. Am. Chem. Soc. 2005, 127, 15304.
(
2
e) Tang, C. Y.; Thompson, A. L.; Aldridge, S. Angew. Chem., Int. Ed.
010, 49, 921. (f) Di Giuseppe, A.; Castarlenas, R.; Perez-Torrente, J.
J.; Lahoz, F. J.; Polo, V.; Oro, L. A. Angew. Chem., Int. Ed. 2011, 50,
(c) Grushin, V. V. Acc. Chem. Res. 2010, 43, 160.
(
(
16) Garrou, P. E. Chem. Rev. 1981, 81, 229.
17) The cyclometalated phosphine ligand in Rh(PPh ) (PPh )′ has
3
2
3
3938. (g) Shaw, B. K.; Patrick, B. O.; Fryzuk, M. D. Organometallics
31
a
P chemical shift of ca. δ −54. Kuznetsov, V. F.; Yap, G. P. A.;
2
012, 31, 783. (h) Tang, C. Y.; Phillips, N.; Bates, J. I.; Thompson, A.
Bensimon, C.; Alper, H. Inorg. Chim. Acta 1998, 280, 172.
18) (a) Braun, T.; Peutz, R. N. In Comprehensive Organometallic
L.; Gutmann, M. J.; Aldridge, S. Chem. Commun. 2012, 48, 8096. (i) Di
Giuseppe, A.; Castarlenas, R.; Perez-Torrente, J. J.; Crucianelli, M.;
Polo, V.; Sancho, R.; Lahoz, F. J.; Oro, L. A. J. Am. Chem. Soc. 2012,
(
Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier:
Oxford, 2007; Vol. 1, p 725. (b) Braun, T.; Wehmeier, F. Eur. J. Inorg.
Chem. 2011, 613.
134, 8171.
(
4) (a) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch,
(19) (a) Jones, W. D.; Partridge, M. G.; Perutz, R. N. J. Chem. Soc.,
A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.;
Fallis, I. A. Organometallics 2008, 27, 3279. (b) Binobaid, A.; Iglesias,
M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.; Fallis, I. A.; Cavell, K. J.
Dalton Trans. 2009, 7099. (c) Iglesias, M.; Beetstra, D. J.; Cavell, K. J.;
Dervisi, A.; Fallis, I. A.; Kariuki, B.; Harrington, R. W.; Clegg, W.;
Horton, P. N.; Coles, S. J.; Hursthouse, M. B. Eur. J. Inorg. Chem.
Chem. Commun. 1991, 264. (b) Aizenberg, M.; Milstein, D. Science
1
1
994, 265, 359. (c) Edelbach, B. L.; Jones, W. D. J. Am. Chem. Soc.
997, 119, 7734.
(20) Aizenberg, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 8674.
(21) (a) Reade, S. P.; Mahon, M. F.; Whittlesey, M. K. J. Am. Chem.
Soc. 2009, 131, 1847. (b) Panetier, J. A.; Macgregor, S. A.; Whittlesey,
M. K. Angew. Chem., Int. Ed. 2011, 50, 2783.
2
010, 1604.
(
5) Grushin, V. V. Chem.Eur. J. 2002, 8, 1006.
(22) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg.
(
6) (a) Mezzetti, A.; Becker, C. Helv. Chim. Acta 2002, 85, 2686.
(
b) Becker, C.; Kieltsch, I.; Broggini, D.; Mezzetti, A. Inorg. Chem.
Synth. 1990, 28, 81.
i
2
003, 42, 8417. (c) Yahav, A.; Goldberg, I.; Vigalok, A. J. Am. Chem.
(23) The 6- Pr ligand was prepared using the same methodology as
Soc. 2003, 125, 13634. (d) Laitar, D. S.; Muller, P.; Gray, T. G.;
Sadighi, J. P. Organometallics 2005, 24, 4503. (e) Grushin, V. V.;
Marshall, W. J. J. Am. Chem. Soc. 2009, 131, 918.
for 6-Mes described by the Herrmann group. Herrmann, W. A.;
̈
Schneider, S. K.; Ofele, K.; Sakamoto, M.; Herdtweck, E. J. Organomet.
Chem. 2004, 689, 2441.
(
7) (a) Whittlesey, M. K.; Perutz, R. N.; Moore, M. H. Chem.
(24) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng, J. A.
Commun. 1996, 787. (b) Kraft, B. M.; Lachicotte, R. J.; Jones, W. D. J.
J. Am. Chem. Soc. 1990, 112, 7619.
8
589
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Organometallics
Article
(
25) Sheldrick, G. M. Acta Crystallogr. 1990, 467−473, A46.
Sheldrick, G. M. SHELXL-97, a computer program for crystal structure
refinement; University of Gottingen, 1997.
̈
8
590
dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590