Synthesis, Isomerization, and Catalytic Transfer Hydrogenation Activity of Rhodium(III) Complexes Containing Both Chelating Dicarbenes and Diphosphine Ligands

Article abstract of DOI:10.1021/acs.organomet.5b00809

Different rhodium(III) complexes [Rh(C,C)(P,P)X₂]⁺ bearing both a cis-chelating dicarbene and a diphosphine ligand were synthesized (C,C = methylene(4,4′-diimidazolylidene); P,P = 1,2-bis(diphenylphosphino)ethane (dppe), (R)-(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R-BINAP); X = halide, carbanion, NCMe). Solution analysis by NMR spectroscopy indicate a dynamic behavior of the complexes and cis/trans isomerization processes, likely through dissociation of the nonchelating ligands X (X = halide, NCMe), and eventually also involving the diphosphine ligand, identified by the formation of phosphine oxides. The presence of a diphosphine ligand in addition to the dicarbene substantially enhances the catalytic activity of the rhodium center in the transfer hydrogenation of ketones in iPrOH/KOH, reaching over 4000 turnover numbers and turnover frequencies around 1000 h^-1 vs 330 h^-1 for the phosphine-free analogue. Optimization of the catalytic conditions allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %. The chiral R-BINAP ligand enhances catalytic activity, though no enantioselectivity was induced in the transfer hydrogenation of fluoroacetophenone as prochiral substrate.

Full text of DOI:10.1021/acs.organomet.5b00809

Article
pubs.acs.org/Organometallics
Synthesis, Isomerization, and Catalytic Transfer Hydrogenation
Activity of Rhodium(III) Complexes Containing Both Chelating
Dicarbenes and Diphosphine Ligands
Kevin Farrell,† Helge Muller-Bunz,† and Martin Albrecht*,†,‡
̈
†School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
‡Departement fur Chemie und Biochemie, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
̈
S
* Supporting Information
ABSTRACT: Different rhodium(III) complexes [Rh(C,C)(P,P)X2]+ bearing both a cis-chelating dicarbene and a diphosphine
ligand were synthesized (C,C = methylene(4,4′-diimidazolylidene); P,P = 1,2-bis(diphenylphosphino)ethane (dppe), (R)-
(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R-BINAP); X = halide, carbanion, NCMe). Solution analysis by NMR
spectroscopy indicate a dynamic behavior of the complexes and cis/trans isomerization processes, likely through dissociation of
the nonchelating ligands X (X = halide, NCMe), and eventually also involving the diphosphine ligand, identified by the formation
of phosphine oxides. The presence of a diphosphine ligand in addition to the dicarbene substantially enhances the catalytic
activity of the rhodium center in the transfer hydrogenation of ketones in iPrOH/KOH, reaching over 4000 turnover numbers
and turnover frequencies around 1000 h−1 vs 330 h−1 for the phosphine-free analogue. Optimization of the catalytic conditions
allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %. The chiral R-BINAP ligand
enhances catalytic activity, though no enantioselectivity was induced in the transfer hydrogenation of fluoroacetophenone as
prochiral substrate.
when considering the development of Grubbs’ second
generation olefin metathesis catalyst,3 which was undoubtedly
one of the hallmark achievements10 that spurred the application
of NHCs as spectator ligands in transition metal catalysis.3−5 In
these olefin metathesis catalysts, incorporation of a NHC ligand
only led to an improved catalytic activity with a mixed
phosphine/NHC ligand pattern,11 while the bis(NHC)
complex was much less active.12 The lability of the phosphine
ligand and the more covalent CNHC−Ru bond work
complementary to each other to enhance the activity of the
catalyst.13 Similar enhancement of catalytic performance has
been observed in palladium complexes that were used for olefin
and alkyne hydrogenation, and alcohol dehydrogenation.14
Here we report our work on applying this concept on
rhodium(III) systems by installing both phosphine ligands
and abnormal carbene ligands.15 Abnormal carbenes exhibit
exceptionally strong donor characteristics for formally neutral
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) and phosphines are ubiq-
uitous ligands in transition metal catalysis and promote a broad
array of catalytic transformations including hydrogenations,1
C−H activations,2 olefin metathesis3 and cross coupling
reactions.4 In recent years, NHCs have emerged as robust
and powerful ligands for stabilizing high oxidation state metal
centers due to their generally stronger donor properties than
phosphine ligands.5 NHCs have been incorporated into many
traditionally phosphine-ligated catalysts, sometimes leading to
enhanced activity.6 While at early stages, this strategy has
prompted the view of NHCs as “superphosphine” ligands,7 a
direct comparison between NHCs and phosphines is obviously
prevented by their vastly different steric effects and diverging
bonding properties.8
Even though replacement of phosphine ligands with NHCs
has been highly prolific and well-investigated, much less work
has been directed toward complexes containing NHCs and
phosphines as complementary ligands at the same metal
center.9 This underdevelopment is particularly remarkable
Received: September 22, 2015
Published: December 16, 2015
© 2015 American Chemical Society
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Scheme 1. Synthesis of Dicarbene dppe Rhodium(III) Complexes 3 and 4
ligands, and impart in many cases advantageous catalytic
properties.16 The utilization of chelating dicarbenes and
diphosphine ligands provides substantial benefits for the
rhodium center to catalyze transfer hydrogenation reactions.
The isomerization of trans-3a to cis-3a is gradual in CDCl3
with an initial 6:1 mixture that converts to an approximate 2:3
ratio after 5 days. The isomerization in CD2Cl2 is more than 1
order of magnitude slower and after 3 weeks the cis and trans
isomers were present in about equal ratios. This rate
dependence suggests that solvent polarity may be relevant,
presumably for stabilizing a transient dicationic intermediate
generated through reversible I− dissociation. In addition to the
isomerization, traces of dppe oxide were observed over this
time frame, suggesting slow phosphine dissociation from the
rhodium coordination sphere when stored in CD2Cl2 for
prolonged time.19
RESULTS AND DISCUSSION
■
Synthesis of Dicarbene dppe Rhodium(III) Com-
plexes. The known17 dimeric rhodium(III) complexes 1 and
2 were suitable precursors for phosphine coordination, each of
these complexes containing a C4-bound diimidazolylidene
chelate (Scheme 1). Reaction of complexes 1 and 2 with 1,2-
bis(diphenylphosphino)ethane (dppe) afforded the monome-
tallic complexes 3 and 4, respectively. Cleavage of the dimeric
structure was indicated by the well-resolved 1H NMR signals in
CD2Cl2. The single set of imidazolylidene resonances for trans-
3a suggests two magnetically equivalent, symmetry-related
carbene ligands. The methylene group linking the two
heterocycles appears as a sharp singlet (δH = 7.2), pointing
to a fast inversion of the boat-like six-membered metallacycle.
In the 13C NMR spectrum, the carbenic carbon of trans-3a
resonates as a doublet of doublet of doublets centered at δC =
134.9 due to coupling to 103Rh and two different 31P nuclei
(1JCRh = 32.8 Hz, 2JCPcis = 9.7 Hz, 2JCPtrans = 147 Hz). This signal
The reaction of the C,C,C-tridentate carbene rhodium dimer
2 with dppe gave a more complicated 31P NMR spectrum with
signals for four different species. Two of these were comprised
of magentically identical phosphines, while the other two gave
rise to AB patterns and revealed a dissymmetric dppe ligand.
The two major isomers were assigned to Cs-symmetric trans-4
and C1-symmetric cis-4 (where trans and cis refer to the relative
−
position of iodide to the carbanion ligand CHR2 ; cf. Scheme
1). Both species feature similar Rh−P coupling constants (1JPRh
= 83.7 Hz for trans-4; 1JPRh = 78.3 and 82.0 Hz for cis-4). These
coupling constants are in the same range as those in the
bidentate dicarbene dppe complexes 3. Accordingly, the trans
influence of the carbene and the anionic alkyl carbon are
similar, underpinning the exceptionally strong donor properties
of these mesoionic carbenes and their strong relationship with
carbanionic ligands.
1
pattern combined with the data from H NMR spectroscopy
and the presence of a single doublet in the 31P NMR spectrum
at δP = 20.3 (1JPRh = 76.2 Hz) indicates a Cs-symmetric complex
trans-3a in solution with enantiotopic phosphorus atoms and
with the iodide ligands in mutual trans position. However, upon
standing for extended periods of time, gradual isomerization of
The two minor isomers both featured substantially
deshielded 31P resonances. For example, the higher symmetry
species has δP = 57.4 vs δP = 28.9 in trans-4. Moreover, the Rh−
P coupling constants are markedly increased with 1JPRh = 131.3.
The minor unsymmetric species showed one similarly large
coupling constant of 137.6 Hz (δP = 50.0) and a smaller one
80.0 Hz (δP = 29.8 ppm). These larger coupling constant
suggest either a change of geometry or a significant electronic
change at the rhodium center. Electronic changes might ensue
from iodide dissociation and the formation of a dicationic
rhodium(III) solvento complex, though solvent coordination
seems less probable in weakly coordinating solvents such as
those used for NMR spectroscopic analyses (CDCl3, CD2Cl2).
Geometrical changes may include the formation of a
pentacoordinate system, or a fac- to mer- change of the
tridentate dicarbene ligand, or a combination of these
processes.17,20 The low fraction of these minor components
trans-3a was observed in solution. Two emerging doublets of
2
doublets at δ = 48.3 (1JPRh = 118.4 Hz, JPP = 7.7 Hz) and
P
19.6 (1JPRh = 81.6 Hz) ppm indicated formation of the C1-
symmetric isomer cis-3a containing the iodide ligands in mutual
cis position, one trans to a carbene, and the other trans to a
phosphine ligand. The larger Rh−P coupling constant was
attributed to the phosphine trans to iodide, while the smaller
constant is similar to that observed in trans-3a (1JPRh 81.6 Hz vs
76.2 Hz), thus supporting a phosphine trans to an abnormal
carbene ligand. This isomerization suggests that the all-cis
isomer is kinetically less preferred, but thermodynamically the
most favored configuration, corroborated also by a solid state
analysis of cis-3b.17a Indeed, analogous isomerization of trans-
3b to cis-3b was observed on standing in solution, with 31P
NMR chemical shifts and coupling constants almost identical to
those of trans- and cis-3a, respectively.18
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and their similar properties to complex 4 prevented a more in-
Ligand Exchange Reactions. Reflux of complex 3a in 1,2-
dichloroethane induced halide exchange and installed two
chloro ligands at the rhodium center to yield complex 5
quantitatively (Scheme 2). The complete exchange of all three
iodides was confirmed by elemental analysis and mass
spectrometry, and likely involves a Finkelstein reaction, i.e.,
substitution of chloride in the solvent by metal-decoordinated
iodide.24 NMR spectroscopy indicated the presence of cis- and
trans-5 in a 3:1 ratio, corroborating the cis-isomer to be
thermodynamically more stable. The 31P NMR signals of cis-5
were shifted downfield compared to those of cis-3a and
appeared at δP = 53.9 (1JPRh = 123.7 Hz) and 28.7 (1JPRh = 81.6
Hz; Table 1). The larger Rh−P coupling constant of the low-
field resonance is in agreement with a smaller trans influence of
Cl− vs I−.25 In the chloro complex, the carbenic carbon
resonances were well resolved. One carbon resonance appeared
as a doublet of doublet of doublets due to coupling with
rhodium, a trans phosphorus and a cis phosphorus (δC = 141.0
depth analysis.
1
The H NMR signals for the aromatic heterocyclic protons
for trans-4 appear as a low field singlet (δH = 7.22), while those
of cis-4 are magnetically inequivalent and appear as two singlets
at δH = 6.59 and 4.36 ppm. The remarkable upfield shift of the
latter resonance suggests strong shielding, presumably imposed
by a phenyl group of the dppe ligand. While the 13C NMR
signals for the carbenic nucleus were not resolved for any
isomer of 4, the rhodium bound alkyl carbon resonance was
detected for cis-4 as a doublet of doublet of doublets at δC =
36.7 due to coupling with rhodium (1JCR2h = 20.9 Hz) and both
phosphorus atoms (2JCPtrans = 93.0 Hz, JCPcis = 5.4 Hz).
Similar to complexes 3a,b, the isomeric mixture of 4 was also
unstable in solution at room temperature over extended periods
of time and revealed the slow formation of dppe oxide in
CD2Cl2.21 The formation of dppe oxide indicates that in both
complexes the diphosphine ligand has some lability, perhaps
imparted by the strong trans effect of the dicarbene ligand.
Phosphine dissociation may be fast and reversible, unless the
phosphine is oxidized, thus leading eventually to the observed
decomposition products. When complexes 3 and 4 were stored
as CD2Cl2 solutions at 4 °C instead of room temperature, they
were completely stable over several months and no dppe oxide
or other decomposition products were detected.
1
2
2
ppm; JCRh = 33.1 Hz, JCPcis = 13.0 Hz, JCPtrans = 149.5 Hz),
while the other carbene carbon appeared as a doublet of triplets
due to coupling to rhodium and two cis-positioned phosphorus
1
2
atoms (δC = 135.4 ppm; JCRh = 40.3 Hz, JCP = 9.0 Hz).
Complexes 3 and 4 reacted smoothly with AgBF4 in MeCN
and gave the corresponding solvento complexes 6 and 7,
respectively (Scheme 2). The 31P NMR signals shifted
significantly downfield upon halide abstraction, e.g., cis-6a
featured two doublets of doublets at δP = 59.4 and 44.3 (Table
1). The coupling constants for the tricationic complexes 6 were
similar to those of the chloro complexes 5, and thus in
agreement with a similar trans influence of Cl− and MeCN.
According to 31P NMR spectroscopy, isomer trans-6 was
initially formed as the major isomer, though over 3 h, the
mixture equilibrated at 1:1 cis/trans ratio for both complexes 6a
and 6b. This isomerization is faster than in complex 3,
supporting the fact that dissociation of the monodentate ligand
is indeed critical for the trans−cis isomerization.
A crystal structure determination of trans-4 confirmed the
arrangement of the ligands deduced from solution analysis
(Figure 1).22 The rhodium center is in a distorted octahedral
The reaction of complex 4 with AgBF4 gave four species. The
two major components showed the same chemical shift and
coupling constants as the 31P NMR spectrum of 4 in CD3CN,
suggesting facile iodo for MeCN exchange in this complex. The
two minor species were more deshielded and showed large
Rh−P coupling constants (>130 Hz). The formation of four
new species upon halide abstraction suggests that the
unidentified minor isomers of 4 arise as a result of a
geometrical change of the complexes trans- and cis-4 rather
than halide dissociation and formation of a pentacoordinate
systems, as such a pentacoordinate complex would likely not be
observed in strongly coordinating solvents such as MeCN.
Crystal structures of complexes cis-6a and trans-7 were
obtained and revealed both a distorted octahedral geometry
around the rhodium center (Figure 2). While metal−ligand
bond lengths are expected to alter according to the generally
observed trans influence carbanion ∼ mesoinic carbene >
phosphine > MeCN, complex cis-6a features the longer Rh−
NMeCN bond trans to the phosphine (2.125(3) Å) rather than
trans to the carbene (2.098(3) Å; Table 2). Statistically, the
difference is larger than 3σ and thus significant, however the
absolute difference of 0.03 Å may be goverened by other than
electronic effects such as crystal packing constrains. The Rh−
CNHC bond trans to phosphorus is 0.05 Å longer than the Rh−
CNHC bond trans to MeCN, which is conformal with the
expected trans influence difference. Complex 7 shows
considerably less distortion from octahedral geometry than
Figure 1. ORTEP representation of trans-4 (50% probability
ellipsoids, hydrogen atoms, cocrystallized solvent molecules, and
iodide counterion omitted for clarity); selected bond lengths and
angles: Rh−C1 2.024(2) Å, Rh−C7 2.025(2) Å, Rh−C5 2.158(2) Å,
Rh−P1 2.3401(6) Å, Rh−P2 2.3516(6) Å, Rh−I 2.7490(2) Å, C7−
Rh−C1 88.31(8)°, C5−Rh−I 165.24(6)°.
geometry, with trans-positioned alkyl and iodide ligands, C5−
Rh−I 165.24(6)°. The dicarbene bite angle is 88.3° and thus
some 5° larger than in a related complex containing two MeCN
ligands instead of the chelating diphosphine,17b suggesting
some flexibility in the tridentate bonding. The Rh−CNHC bonds
are relatively long, 2.025(2) Å when trans to the dppe ligand,23
which is expected when considering the stronger trans influence
of phosphine compared to NCMe (Rh−CNHC 1.96 Å in the
MeCN analogue). The Rh−Calkyl bond is 2.158(2) Å and thus
considerably longer than the Rh−CNHC bonds.
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Scheme 2. Ligand Exchange Reactions Yielding Complexes 5, 6, and 7
Table 1. 31P NMR Data for cis and trans Bidentate Dicarbene
Rh(III) Complexes
Table 2. Selected Bond Lengths (Å) and Angles (deg) for cis-
6a and trans-7
a
3a
4
5a
5a (CD3CN)
cis-6a
trans-7
trans
cis
20.5 (75.2)
48.3 (116.6)
20.8 (80.7)
27.0 (75.1)
53.9 (123.7)
28.7 (81.6)
n.d.
28.6 (72.8)
55.9 (120.6)
40.8 (77.7)
Rh−C1
Rh−C9
Rh−P1
Rh−P2
Rh−N5
2.072(4)
2.021(3)
2.3618(9)
2.2685(9)
2.125(3)
2.098(3)
87.04(14)
−
2.027(2)
2.037(2)
2.3475(5)
2.3443(6)
2.085(2)
2.113(3)
87.41(8)
171.03(7)
176.13(7)
59.4 (122.4)
44.3 (78.7)
a
δP in ppm (JPRh in Hz in parentheses) in CD2Cl2 unless specified; n.d.
= not determined.
a
Rh−X
C1−Rh−C9
C5−Rh−N5
C1−Rh−P1
complex 4, which can be quantified with the distortion index Σ
= 26.6° for 7 and much higher for 4 (Σ = 56.5°).26 In addition,
the Rh−C5 bond is 0.045 Å shorter in 7 than in 4, perhaps as
consequence of both, the weaker trans influence of MeCN as
compared to iodide and because of the reduced steric
implications of MeCN relative to the larger iodide ligand.
The variable bond lengths and angles of the tridentate C,C,C-
177.68(10)
a
The sixth ligand X in complex cis-6a is N6, in complex trans-7 X is C5
(see Figure 2).
Figure 2. ORTEP representation of the cationic portion of (a) cis-6a and (b) trans-7 (50% probability ellipsoids; hydrogen atoms, counterions, and
cocrystallized solvent molecules omitted for clarity).
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dicarbene ligand in complexes 4 and 7 suggests some flexibility
of this ligand scaffold.
Conversions remained high when lowering the loadings of
3a,b to 0.05 mol % (entries 9,10). Complex 3b, with C2-phenyl
and N-butyl substituents was the most robust system with
turnover numbers (TON) up to 1880. A further decrease of the
catalyst loading to 0.01 mol % resulted in a sharp decline in
activity (entries 11,12). Even though the TONs remained in
the low thousands conversion was prohibitively low. Increasing
the substrate rather than lowering the catalyst loading to keep
the 10,000:1 substrate/catalyst ratio improved the spectro-
scopic yield of the alcohol, and complex 3b achieved TONs of
almost 5000 (entry 13). This improvement constitutes one of
the highest performances for rhodium(III) complexes in
transfer hydrogenation.30 The better performance at higher
substrate loading may point to an adverse effect of minor
impurities in the solvent when the concentration of the
complex is too low.
Catalytic Transfer Hydrogenation. The new complexes
3−6 were used as precatalysts for the transfer hydrogenation of
ketones.27 Initially, benzophenone was used as a model
substrate and activities were compared to complex 1, which
previously revealed substantial activity. The background base-
induced reduction is poor giving only 8% of diphenylmethanol
after 24 h (Table 3, entry 1). Complex 1a is effective at low
Table 3. Catalytic Transfer Hydrogenation of Benzophenone
a
with Rhodium(III) Dicarbene dppe Complexes 1−6
yield/%
Optimizations of the reaction conditions employing
complexes 3a and 3b on focused on the use of the base as
additive (Table 4). Standard conditions involved KOH as a
entry
[Rh]
mol % [Rh]
0.5 h
2 h
24 h
TON
1
−
−
0
1
8
−
2
1a
3a
3b
4
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.05
0.05
0.01
0.01
26
77
97
10
68
10
15
39
36
4
50
100
100
23
96
25
23
55
76
7
82
n.d.
n.d
71
100
59
26
78
94
22
18
46
550
670
670
470
670
390
170
1560
1880
2200
1800
4600
3
Table 4. Optimization Studies with Rhodium(III) Dicarbene
a
4
dppe Complexes 3a and 3b
5
6
5a
6a
6b
3a
3b
3a
3b
3b
7
8
yield/%
9
entry
[Rh]
mol % base
0.5 h
2 h
24 h
10
11
12
13
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3b
3b
3b
3b
10
5
77
65
6
100
93
n.d.
96
0
3
b
0.01
21
33
1
9
9
b
a
1
30
97
84
21
31
64
80
General conditions: benzophenone (1.0 mmol), KOH (50 μL, 2 M in
10
5
100
100
37
n.d.
n.d.
39
H2O, 0.1 mmol), iPrOH (5 mL), reflux temperature; yields
determined by 1H NMR spectroscopy, n.d. = not determined.
b
Benzophenone (10 mmol).
1
b
1
60
67
a
b
General conditions as in Table 3, 0.15 mol % [Rh]. Water (45 μL)
catalyst loadings (0.15 mol % [Rh]) although it does not reach
full conversion even after 24 h (entry 2). The bidentate
dicarbene diphosphine complexes 3a,b afforded the most active
catalysts of the series and gave complete conversion within 2 h
at 0.15 mol % loading (entries 3, 4). For example, the turnover
frequency at 50% conversion raised from TOF50 = 330 h−1 for
1a up to TOF50 > 1300 h−1 upon introducing a phosphine
ligand as in complex 3b (TOF50 > 1000 h−1 for complex 3a).28
The chloride complex 5a showed similar activity to the iodide
analogue 3a, in agreement with facile activation of the halide
species with the in situ formed KOiPr (entry 6). While previous
work revealed higher performance for the chloro analogue of
1a,17a the introduction of a diphosphine ligand evidently
reduces the influence of the halide on the catalytic activity.
Sampling at early stages of a 3a-catalyzed transfer hydro-
genation indicates a fast catalyst activation and no significant
induction period (Figure S1).29
The tridentate C,C,C-dicarbene complex 4 was considerably
less active than 3 presumably due to the availability of only one
site for hydride formation (entry 5). Even after 24 h, full
conversion was not reached and formation of a black precipitate
pointed to the decomposition of the complex. The solvento
complexes 6a,b were unstable under the catalytic conditions
and a dark precipitate formed within less than 30 min. These
observations may rationalize the comparably poor conversions
(entries 7,8), and they suggest that the heterogenized system is
not efficient.
added.
base, which was added in 10 mol % with respect to the
substrate as a 2 M aqueous solution. No reaction was observed
in the absence of base or upon using NaOAc in the same molar
ratio (0.1 mmol, 10 mol %) as a less corrosive base.31 Reducing
the amount of KOH from 10 to 5 mol % lowered the catalytic
rate slightly, though full conversion was readily achieved (entry
2). However, at 1 mol % base, activity was only marginal (entry
3). Since lowering the amount of KOH was also accompanied
by a reduction of the aqueous fraction, a run was carried out
using 1 mol % base (5 μL of a 2 M solution) together with an
additional amount of water (45 μL) to achieve the same 100:1
iPrOH/H2O ratio as in the runs performed with 10 mol % base.
Under these conditions, the conversions improved substantially
and reached up to 80% after 24 h (entry 4). While this
beneficial effect of added water may be attributed to a bulk
solvent effect, other factors may contribute, such as an
enhanced polarity of the reaction medium to facilitate iodide
dissociation or iodide stabilization to induce catalytic turnover.
Such milder conditions may become attractive when corrosion
or base-sensitivity of the substrate is an issue. Similar trends
were observed for complex 3b, and conversions with 1 mol %
base were acceptable provided water was added to the reaction
mixture (entries 5−8).
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The substrate scope for transfer hydrogenation with 3b was
expanded to include other ketones and imines (Table 5).
benzylimine (PhCNCH2Ph) was not converted. Likewise
carbonyl groups of esters as in methyl benzoate were not
hydrogenated, and instead base-induced saponification was
observed in the presence of 10 mol % base. Classical substrates
such as (substituted) acetophenone was transfer hydrogenated
in good conversion (entries 3−6). The activity was moderately
dependent on the electronic nature of the substituent, and in
agreement with previous studies, electron-withdrawing sub-
stituents increase the electrophilicity of the carbonyl group and
hence facilitate reduction.27 Phenolic substrates such as 4-
hydroxy-acetophenone are not converted (entry 7), presumably
because of the higher acidity of phenoles compared to iPrOH,
thus preventing formation of the alkoxide that is presumed to
be essential for ensuing β-hydrogen elimination and effective
hydrogen transfer from iPrOH. Aldehydes are converted
unselectively and produce the primary alcohol as well as
benzylidene acetone (entry 8). Finally, enones with an α,β-
unsaturated ketone functionality such as trans-chalcone were
fully transfer hydrogenated with complex 3b, and yielded the
saturated alcohol due to transfer hydrogenation of the ketone
and the olefin (24 h; entry 9). Samples taken after 30 min, i.e.,
at early conversion stages, revealed formation of the saturated
ketone as the major product, suggesting a preference for the
reduction of the olefinic CC bond prior to ketone reduction.
These results are in agreement with the selectivity observed for
a NHC ruthenium(II) catalyst.32
Table 5. Catalytic Transfer Hydrogenation of Various
a
Substrates with Complex 3b
Synthesis of Dicarbene BINAP Rhodium(III) Com-
plexes and Catalytic Activity. The facile coordination of
dppe and the enhanced catalytic performance of the rhodium
center when ligated by both dicarbene and diphosphine ligands
provides a straightforward methodology for catalyst modifica-
tion through variation of the phosphine. We have probed this
approach by using the axially chiral BINAP ligand33 which may
afford a precatalyst with potential for asymmetric transfer
hydrogenation reactions. Thus, heating the dinuclear rhodium
complexes 1 and 2 in CH2Cl2 with (R)-BINAP yielded the
rhodium complexes 8 and 9, respectively, in moderate to good
yields (52−74%; Scheme 3).
a
b
c
General conditions as in Table 3. 5 h. Yield shown as yield of I/II.
Yield shown as III/IV.
d
Cyclohexanone was fully reduced to cyclohexanol within 2 h at
0.05 mol % catalyst loading (2000 TON; entry 1), achieving a
turnover frequency at 50% conversion, TOF50 around 3000 h−1.
Hydrogenation of 2-acetylpyridine was slower than related aryl
ketones but full conversion was reached within 24 h (entry 2).
This substrate is usually challenging as both the substrate and
the product alcohol are potentially chelating ligands that tend
to poison the catalyst. In 3b, the presence of chelating
dicarbene and diphosphine ligands and their variable
coordination modes (cf. isomerization, Scheme 1) presumably
prevents such chelation and thus inhibits catalyst poisoning.
Since the pyridyl unit was not deactivating the catalyst, imines
were investigated as substrates. However, benzylidene
The pertinent NMR unambiguously evidenced coordination
of BINAP to the rhodium center and revealed the specific
ligand arrangement. Thus, the cis isomer of complex 8 was
identified as the initial product by the two doublets of doublets
2
centered at δP = 7.66 (1JPRh = 119.7 Hz, JPP = 20.3 Hz) and
Scheme 3. Synthesis of Dicarbene Binap Rhodium(III) Complexes 8 and 9
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Article
1.04 (1JPRh = 79.3 Hz; CD2Cl2 solution). The Rh−P coupling
constants were diagnostic and identified a weakly donating
iodide trans to the low-field phosphine, while the smaller Rh−P
coupling constant of the higher field resonance suggests a
carbene in trans position.34 Notably the imidazolylidene proton
resonances are significantly separated in the 1H NMR spectrum
(δH = 7.33 and 4.66) due to shielding of one of the protons by
the naphthyl group of BINAP, thus further supporting an all-cis
coordination.
and much less by trans influences. Interestingly, the
imidazolylidene proton adjacent to C9 points toward a
naphthyl ring of the BINAP ligand (H10−CgNaphth 2.477 Å).
With this short distance, ring current effects become relevant
and presumably account for the substantial shielding of this
proton in the H NMR spectrum (vide supra, δH = 4.66),
suggesting that the crystallographically determined structure is
also preserved in solution.
1
Complex 9 containing a BINAP ligand and a C,C,C-
tridentate dicarbene ligand was stable and does not undergo
any isomerization in solution. It is characterized by two well
resolved doublets of doublets in the 31P NMR spectrum at δP =
Complex 8 isomerizes rapidly in CDCl3 solution, and after 4
h, two new well resolved doublets of doublets emerged slightly
downfield, at δP = 5.0 and 9.0 for a new species, 8′ (3:1 ratio of
8:8′ after 4 h from an initial 25:1 ratio). The coupling constants
for 8′ are very similar to those of the initially formed species 8,
thus indicating that one phosphorus remains trans to a carbene
while the other phosphorus is trans to iodide.35 In contrast to 8,
the NMR signals for 8′ are well resolved at room temperature.
These data may indicate that the isomerization here arises due
to an intraligand transformation, for example inversion of the
metallacycle containing the NHC ligands. Again, the isomer-
ization is slower in CD2Cl2 than in CDCl3 and only traces of 8′
were observed after 6 h in solution, and about 30% after 3
weeks. No BINAP oxide was detected, indicating the complex is
stable in solution at room temperature for extended periods.
While numerous attempts failed to crystallize complex 8,
single crystals suitable for X-ray diffraction analyses were
obtained from the corresponding solvento complex. This
complex was prepared by exchange of the iodide ligands with
2
13.37 (1JPRh = 81.3 Hz, JPP = 25.2 Hz) and 0.56 (1JPRh = 79.9
Hz). While 31P NMR analysis does not give conclusive
evidence of the ligand arrangement due to the similar coupling
constants imparted by the trans carbene carbon and the anionic
alkyl carbon (cf. cis- and trans-4), the 13C NMR signals provide
strong support for a trans conformation of 9. The carbene
signals were well resolved and appeared as two distinct doublet
of doublet of doublets centered at 153.0 and 150.0 ppm, each
with a large C−Ptrans coupling (>120 Hz) and a small cis
coupling (∼10 Hz) in addition to the coupling to the 103Rh
nucleus. Moreover, the rhodium-bound alkyl carbon appears as
a doublet of triplets due to coupling with two cis-positioned
2
phosphorus atoms (1JCRh = 28.4 Hz, JCP = 4 Hz). Unlike
complex 8, the imidazolylidene protons in complex 9 appear at
a similar field (δH = 6.98 and 6.92).
Complexes 8 and 9 were tested as transfer hydrogenation
catalysts under standard conditions and compared to the dppe
analogue, complex 4 (Table 6). Complex 4 showed moderate
BF4 using AgBF4 in MeCN.36 The molecular structure
−
confirmed the all cis ligand arrangement (Figure 3). The two
Table 6. Transfer Hydrogenation of 4-Fluoroacetophenone
a
with Complexes 4, 8 and 9
yield/% (% ee)
entry
[Rh]
0.5 h
2 h
24 h
1
2
3
4
8
9
24 (7)
100 (4)
85 (4)
55 (2)
n.d.
70 (9)
n.d.
97 (6)
n.d.
a
General conditions: 4-fluoroacetophenone (1 mmol), KOH (2 M, 50
μL, 0.1 mmol), iPrOH (5 mL), 0.15 mol % [Rh], reflux; n.d. = not
determined; enantiomeric excess as % ee determined by chiral HPLC
in parentheses.
Figure 3. ORTEP representation of the solvento complex of 8 (50%
probability ellipsoids; hydrogen atoms, BF4 counterions and
−
cocrystallized solvent molecules omitted for clarity). Selected bond
lengths (Å) and angles (deg): Rh−C1, 2.028(3), Rh−C9, 2.026(3),
Rh−P1, 2.3091(7), Rh−P2, 2.4498(9), Rh−N5, 2.096(2); Rh−N6,
2.106(3); C1−Rh−C9, 86.97(14), C1−Rh−P2, 174.70(9).
activity toward 4-fluoroacetophenone, yielding 70% of the
corresponding alcohol after 24 h (entry 1). Both 8 and 9 were
substantially more active and gave complete conversion after
0.5 and 2 h, respectively (entries 2,3). Hence, BINAP
accelerates the hydrogen transfer reaction compared to dppe.
We speculatively attribute the increased catalytic activity to the
enhanced rigidity of the BINAP ligand, which may facilitate
substrate coordination in a putative five-coordinate active
species. The enantiomeric excess (% ee) of the alcohol product
was poor for each run, even when determined at low
conversion (5 min). Hence, under the applied conditions no
enantioselectivity was imparted by the BINAP ligand, likely
because of the high reaction temperature.30d,37
Rh−CNHC bond lengths are identical even though one carbene
is trans to phosphorus and the other is trans to MeCN (cf. X-ray
data for trans-6a). In contrast, the Rh−P bond trans to the
carbene is a substantial 0.14 Å longer than the Rh−P bond trans
to MeCN (2.4498(9) vs 2.3091(7) Å). Finally, the two Rh−
N
MeCN bonds are essentially equally long (2.106(3) vs 2.096(2)
Å). This latter comparison suggests that the trans influence of
the phosphine and the carbene is not hugely different, and that
the Rh−P and Rh−CNHC bond lengths are governed by sterical
constraints imposed by chelation and phosphorus substituents,
5729
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Organometallics
Article
mol−1) × CH2Cl2: C, 41.13; H, 3.84; N, 4.21. Found: C, 41.10; H,
4.11; N, 4.56.
CONCLUSIONS
■
A series of mixed dicarbene diphosphine rhodium(III)
complexes with different spectator ligands were synthesized
and tested in transfer hydrogenation catalysis. The complexes
display dynamic behavior in solution as indicated by the cis/
trans isomerization. The formation of dppe oxide indicates
some lability of dppe coordination in solution, which is
presumably facilitated by the strong trans effect of the carbene
donors. BINAP is more rigid in its coordination to rhodium
and no BINAP oxide was formed in the same time frame.
Incorporation of the diphosphine ligands increases the catalytic
activity of the rhodium center compared to the precursor
dicarbene rhodium(III) complexes. The process is robust and
tolerates additional water and lowering of the base loading, and
even difficult substrates, such as 2-acetylpyridine are efficiently
converted to the hydrogenated product. When the halide
Complex 4. Complex 2 (203 mg, 0.158 mmol) and 1,2-
bis(diphenylphosphino)ethane (126 mg, 0.316 mmol) were dissolved
in dry CH2Cl2 (5 mL) and stirred at room temperature for 19 h. The
reaction mixture was filtered through Celite and eluted with CH2Cl2
(100 mL). After solvent evaporation, the residue was purified by
gradient column chromatography (SiO2; CH2Cl2 then CH2Cl2/
acetone, 1:2) yielding the title complex as a mixture of four isomers
(270 mg, 82%, 2:1:0.4:0.15). Crystals of 4 were obtained by slow
diffusion of Et2O onto a CH2Cl2 solution of the complex.
1
Spectroscopic data of cis-4: H NMR (CD2Cl2, 400 MHz): δ 8.01
(t, 2H, 3JHH = 8.5 Hz, Hphenyl), 7.87−7.79 (m, 2H, Hphenyl), 7.71 (t, 2H,
3JHH = 8.2 Hz, Hphenyl), 7.52−7.19 (m, 8H, Hphenyl), 7.10−7.02 (m, 4H,
3
H
phenyl), 6.86 (t, 2H, JHH = 8.3 Hz, Hphenyl), 6.59 (s, 1H, Himid), 4.76
2
3
(dd, 1H, JHH = 12.2 Hz, JHH = 7.0 Hz, NCH2), 4.36 (s, 1H, Himid),
4.32−4.23 (m, 2H, NCH2 and NCHMe2), 4.15−3.90 (m, 2H, NCH2
and NCHMe2), 3.79 (dd, 1H, 2JHH = 12.9 Hz, 3JHH = 8.8 Hz, NCH2),
3.08−2.63 (m, 5H, PC2H4P and RhCH), 2.30 (s, 3H, Cimid−CH3),
2.25 (s, 3H, Cimid−CH3), 1.48, 1.33, 0.98, 0.92 (4 × d, 3H, 3JHH = 6.6
−
anions were replaced for BF4 , the complexes were no longer
stable under catalytically relevant conditions. Under the
conditions tested BINAP does not impart any selectivity
under the applied conditions but leads to higher catalytic
activity of the C,C,C-dicarbene rhodium(III) catalyst than dppe
does.
Hz, NCH(CH3)2). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 152 (Cimid
−
Rh),38 150 (Cimid−Rh),38 137.8 (NCimidN), 137.0 (NCimidN, 4JCP = 1.2
Hz), 136.4−128.0 (Cphenyl), 118.8, 115.6 (2 × Cimid−H), 62.0 (NCH2,
3JCP = 6.0 Hz), 58.7 (NCH2, JCP = 5.6 Hz), 49.7, 48.9 (2 ×
3
1
2
NCH(Me)2), 36.7 (Calkyl−Rh, JCRh = 20.9 Hz, JCPtrans = 93.0 Hz,
2JCPcis = 5.5 Hz), 28.9, 26.8 (2 × PCH2), 23.9, 23.3, 23.1, 22.6 (4 ×
NCH(CH3)2), 12.3, 12.2 (2 × Cimid−CH3). 31P{1H} NMR (CD2Cl2,
162 MHz): δ 35.9 (dd, 1JPRh = 78.3 Hz, 2JPP = 3.5 Hz), 31.27 (dd, 1JPRh
EXPERIMENTAL SECTION
■
General Comments. Complexes 1a, 1b, 3b, and 2, were
synthesized as reported in the literature.17 All other reagents were
used as received from commercial suppliers. Unless specified, NMR
spectra were recorded at 25 °C on Varian spectrometers operating at
400 or 500 MHz (1H NMR), 100 MHz (13C NMR) and 162 MHz
(31P NMR) respectively. Chemical shifts (δ in ppm, coupling
constants J in Hz) were referenced to residual solvent signals (1H,
13C) or external H3PO4 (31P). Assignments are based on homo- and
heteronuclear shift correlation spectroscopy. Elemental analyses were
performed at UCD Microanalytic Laboratory using an Exeter
Analytical CE-440 elemental analyzer, residual solvent was confirmed
by NMR spectroscopy and also by X-ray structure determinations. Gas
chromatography was carried out on a Shimadzu GC-174 gas
chromatograph equipped with an Agilent DB5 column (J&W, 30
m) and a diode array detector at 120 °C.
Complex 3a. Complex 1a (200 mg, 0.134 mmol) and 1,2-
bis(diphenylphosphino)ethane (122 mg, 0.306 mmol) were dissolved
in dry CH2Cl2 (5 mL) and stirred at room temperature for 19 h. The
reaction mixture was filtered through Celite and eluted with CH2Cl2
(100 mL). After solvent evaporation, the residue was purified by
gradient column chromatography (SiO2; CH2Cl2 then CH2Cl2/
acetone, 1:1) affording complex 3a as a mixture of two isomers
2
1
= 82.0 Hz, JPP = 3.5 Hz). Spectroscopic data for trans-5: H NMR
(CD2Cl2, 400 MHz): δ 7.22 (s, 2H, Himid), 2.62 (s, 6H, Cimid−CH3),
1.50−1.40 (m, 12H, NCH(CH3)2), other signals overlapping with cis-
4. 31P{1H} NMR (CD2Cl2, 162 MHz): δ 28.9 (d, JPRh = 83.7 Hz).
1
ESI-MS (m/z): 915.1644, calcd for [(C,C,C-dicarb) (dppe)(I)Rh]+
915.1689. Anal. calcd for C43H51I2N4P2Rh (1042.55 g mol−1): C,
49.54; H, 4.93; N, 5.37. Found: C, 49.41; H, 4.99; N, 4.98.
Complex 5a. Complex 3a (185 mg, 0.162 mmol) was dissolved in
1,2-dichloroethane (10 mL) and refluxed for 24 h. The product was
precipitated with Et2O (50 mL) and isolated by centrifugation. After
solvent evaporation complex 5a was isolated as a mixture of two
isomers (137 mg, 97%) in a 1:0.3 ratio. Spectroscopic data for cis-5a:
1H NMR (CD2Cl2, 400 MHz): δ 8.22−8.17 (m, 2H, Hphenyl), 7.99−
7.95 (m, 2H, Hphenyl), 7.43−7.24 (m, 14H, Hphenyl), 7.08 (d, 1H, 2JHH
=
12.1 Hz, lowfield AB part of NCH2N), 6.64−6.60 (m, 2H, Hphenyl),
2
6.55 (d, 1H, JHH = 12.1 Hz, highfield AB part of NCH2N), 6.08 (s,
1H, Himid), 4.60 (s, 1H, Himid), 4.48−4.34 (m, 1H, NCHMe2), 3.89
3
(septet, 1H, JHH = 6.5 Hz, NCHMe2), 3.47−3.29 (m, 2H, PCH2),
3.08−2.93 (m, 5H, CH2P and Cimid−CH3), 2.85 (s, 3H, Cimid−CH3),
3
1.23 (m, 6H, NC(CH3)2), 0.58, 0.42 (2 × d, 3H, JHH = 6.5 Hz,
NC(CH3)2). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 141.3 (NCimidN,
1
2
4JCP = 5.3 Hz), 141.0 (Cimid−Rh, JCRh = 33.1 Hz, JCPcis = 13.0 Hz,
1
(215 mg, 70%). Spectroscopic data of trans-3a: H NMR (CD2Cl2,
2JCPtrans = 149.5 Hz), 140.0 (NCimidN), 135.4 (Cimid−Rh, JCRh = 40.3
1
400 MHz): δ 7.65−7.60 (m, 8H, Hphenyl), 7.40−7.33 (m, 12H, Hphenyl),
7.02 (s, 2H, NCH2N), 6.62 (s, 2H, Himid), 4.41 (septet, 2H, 3JHH = 6.6
Hz, NCHMe2), 3.19−3.13 (m, 4H, PC2H4P) 3.03 (s, 6H, Cimid−CH3),
2
3
Hz, JCP = 9.0 Hz), 134.7−127.8 (Cphenyl), 122.1 (Cimid−H, JCP = 8.9
3
Hz, JCP = 4.5 Hz), 121.3 (Cimid−H, J = 8.3 Hz, J = 3.4 Hz), 59.3
1
3
1.31 (d, 12H, JHH = 6.6 Hz, NCH(CH3)2). 13C{1H} NMR (CD2Cl2,
(NCH2N), 50.3, 49.6 (2 × NCHMe2), 28.4 (PCH2, JCP = 17.3 Hz),
28.0 (CH2P, 1JCP = 17.5 Hz), 23.2, 22.4, 22.37, 22.0 (4 × NC(CH3)2),
12.8, 12.4 (2 × Cimid−CH3). 31P{1H} NMR (CD2Cl2, 162 MHz): δ
53.9 (dd, 1JPRh = 123.7 Hz, 2JPP = 10.3 Hz), 28.7 (dd, 1JPRh = 81.6 Hz,
1
100 MHz): δ 140.9 (m, NCimidN), 134.9 (C−Rh, JCRh = 32.8 Hz,
2JCPcis = 9.7 Hz, 2JCPtrans = 147 Hz), 134.3 (Cipso + Cortho), 130.6 (Cpara),
3
128.1 (Cmeta), 125.9 (Cimid−H, JCP = 8.5 Hz), 62.1 (NCH2N), 50.4
2JPP = 10.3 Hz). Spectroscopic data for trans-5a: H NMR (CD2Cl2,
1
(NCH(Me)2), 28.1 (PC2H4P), 23.0 (NCH(CH3)2), 12.8 (Cimid
−
CH3). 31P{1H} NMR (CD2Cl2, 162 MHz): δ 20.5 (d, 1JPRh = 75.2 Hz).
400 MHz): δ 7.61−7.54 (m, 8H, Hphenyl), 7.43−7.24 (m, 12H, Hphenyl),
6.47 (s, 2H, Himid), 4.48−4.34 (m, 2H, NCHMe2), 3.0−2.8 (PC2H4P),
1
Spectroscopic data of cis-3a: H NMR (CD2Cl2, 400 MHz): δ 8.14−
3
2.87 (s, 6H, Cimid−CH3), 1.36 (d, 6H, JHH = 6.5 Hz). Reminding
8.10 (m, 2H, Hphenyl), 8.02−7.97 (m, 2H, Hphenyl), 7.52−7.47 (m, 4H,
Hphenyl), 7.28−7.19 (m, 8H, Hphenyl), 6.56 (s, 1H, Himid), 4.63 (s, 1H,
signals overlapping with the cis isomer. 13C{1H} NMR (CD2Cl2, 100
MHz): δ 140.8 (m, NCimidN), 138.7 (Cimid−Rh, 1JCRh = 32.6 Hz, 2JCPcis
= 9.9 Hz, 2JCPtrans = 150.0 Hz), 134.7−127.8 (Cphenyl), 124.7 (Cimid−H,
3JCP = 8.6 Hz), 59.1 (NCH2N), 50.2 (NCHMe2), 27.1 (m, PC2H4P),
22.6 (NCH(CH3)2), 11.8 (Cimid−CH3). 31P{1H} NMR (CD2Cl2, 160
MHz): δ 27.0 (d, 1JPRh = 75.1 Hz). ESI-MS (m/z): 831.1770, calcd for
[(dicarb) (dppe) (Cl)2Rh]+ 831.1786. Anal. calcd for
3
Himid), 3.85 (septet, 1H, JHH = 6.7 Hz, NCH(Me)2), 3.57 (m, 2H,
NCH(Me)2 and PC2H4P), 2.54 (s, Cimid-CH3), 0.62, 0.34 (2 × d, 3JHH
= 6.7 Hz, NC(CH3)2). 31P{1H} NMR (CD2Cl2, 162 MHz): δ 48.3
(dd, 1JPRh = 116.6 Hz, 2JPP = 7.3 Hz), 20.8 (dd, 1JPRh = 80.7 Hz, 2JPP
=
7.3 Hz). ESI-MS (m/z): 1015.0544, calcd for [(dicarb) (dppe)-
(I)2Rh]+ 1015.0499. Anal. calcd for C41H48I3N4P2Rh (1142.41 g
5730
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C41H48Cl3N4P2Rh (868.06 g mol−1) × 4 CH2Cl2: C, 44.75; H, 4.67; N,
4.64. Found: C, 44.79; H, 4.36; N, 5.02.
diffusion of Et2O onto a CH2Cl2 solution of the isomeric mixture.
1
Spectroscopic data of cis-7: H NMR (CD3CN, 400 MHz): δ 7.92−
Complex 6a. Complex 3a (73 mg, 0.064 mmol) and AgBF4 (50 mg,
0.257 mmol) were dissolved in MeCN (10 mL) and stirred in the
absence of light at room temperature for 18 h. A yellow suspension
formed and was filtered through Celite. After removing all volatiles
under reduced pressure, complex 6a was obtained as a mixture of two
isomers (71 mg, quantitative). Crystals suitable for X-ray diffraction
studies were obtained by slow diffusion of Et2O onto a CH2Cl2/
MeCN (1:1) solution of the complex. Spectroscopic data39 for trans-
7.81 (m, 2H, Hphenyl), 7.74−7.68 (m, 2H, Hphenyl), 7.66−7.23 (m, 12H,
3
Hphenyl), 7.21−7.15 (m, 1H, Hphenyl), 7.06 (t, 2H, JHH = 8.6 Hz,
Hphenyl), 6.99−6.92 (m, 1H, Hphenyl), 6.80 (s, 1H, Himid), 5.09 (s, 1H,
Himid), 4.48−4.36 (m, 1H, NCHMe2), 4.21−4.12 (m, 1H, NCH2),
3
3.99 (septet, 1H, JHH = 6.6 Hz, NCHMe2), 3.56−3.46 (m, 1H,
NCH2), 3.45−3.13 (m, 4H, PC2H4P), 3.12−3.05 (m, 1H, NCH2),
2.55 (s, 3H, Cimid−CH3), 2.16−2.11 (m, 1H, NCH2), 1.95 (s, 3H,
3
Cimid−CH3), 1.41, 1.35, 1.04, 0.92 (4 × d, 3H, JHH = 6.6 Hz,
1
6a: H NMR (CD3CN, 400 MHz): δ 7.78−7.36 (m, 20H, Hphenyl),
NCH(CH3)2), CH−Rh not detected. 13C{1H} NMR (CD3CN, 100
MHz): δ 140.4 (NCimidN), 138.8 (NCimidN), 134.3−129.3 (Cphenyl),
120.7 (Cimid−H, 3JCP = 13.0 Hz, 3JCP = 3.6 Hz), 117.7 (Cimid−H), 59.7,
59.6 (2 × NCH2), 50.2, 49.5 (2 × NCH(CH3)2), 36.8 (CH−Rh), 25.3
(PC2H4P), 22.8, 22.7, 22.4, 22.2 (4 × NCH(CH3)2), 11.6, 10.5 (2 ×
Cimid−CH3), Cimid−Rh not resolved. 31P{1H} NMR (CD3CN, 162
2
6.26 (s, 2H, Himid), 6.27 (d, 1H, JHH = 13.9 Hz, lowfield AB part of
NCH2N), 5.90 (d, 1H, 2JHH = 13.9 Hz, highfield AB part of NCH2N),
3
4.49 (septet, 2H, JHH = 6.4 Hz, NCHMe2), 3.87−2.88 (m, 4H,
3
PC2H4P), 2.74 (s, 6H, Cimid−CH3), 1.34, 1.14 (2 × d, 6H, JHH = 6.4
Hz, NCH(CH3)2). 13C{1H} NMR (CD3CN, 100 MHz): δ 143.8
(NCimidN, JCP = 4.6 Hz), 134.7−128.1 (Cphenyl), 134 (Cimid−Rh),38
MHz): δ 38.9 (dd, JPRh = 76.5 Hz, JPP = 5.1 Hz), 35.6 (dd, JPRh
=
4
1
2
1
2
124.6 (m, Cimid−H), 58.9 (NCH2N), 50.7 (NCH(Me)2), 27.4 (m,
PC2H4P), 22.9, 22.3 (NCH(CH3)2), 10.6 (Cimid−CH3). 31P{1H}
NMR (CD3CN, 162 MHz): δ 28.6 (d, 1JPRh = 72.8 Hz). Spectroscopic
data for cis-6a:1H NMR (CD3CN, 400 MHz): δ 7.78−7.36 (m, 14H,
Hphenyl), 7.34−7.28 (m, 2H, Hphenyl), 7.25−7.17 (m, 2H, Hphenyl),
82.6 Hz, JPP = 5.1 Hz). Spectroscopic data of trans-7: 1H NMR
(CD3CN, 400 MHz): δ 7.65−7.40 (20H, Hphenyl), 7.11 (s, 2H, Himid),
4.48−4.36 (m, 2H, NCHMe2), 2.09 (s, 6H, Cimid−CH3), 1.54, 1.46 (2
× d, 6H, JHH = 6.6 Hz, NCH(CH3)2). 13C{1H} NMR (CD3CN, 100
3
MHz): δ 139.7 (br, NCimidN), 134.3−129.3 (Cphenyl), 117.1 (Cimid−H,
2
3JCP = 7.9 Hz, JCP = 2.7 Hz), 50.4 (NCH(CH3)2), 23.6, 22.2
3
6.75−6.67 (m, 2H, Hphenyl), 6.36 (d, 1H, JHH = 14.0 Hz, lowfield AB
2
(NCH(CH3)2) 10.9 (Cimid−CH3). 31P{1H} NMR (CD3CN, 162
MHz): δ 32.6 (d, 1JPRh = 83.6 Hz), other signals overlapping with cis-7
and not resolved. Minor species: 31P{1H} NMR (CD3CN, 121 MHz): δ
57.3 (d, 1JPRh = 133.0 Hz); 51.7 (dd, 1JPRh = 133.8 Hz, 2JPP = 7.2 Hz),
part of NCH2N), 5.93 (d, 1H, JHH = 14.0 Hz, highfield AB part of
NCH2N), 5.56 (s, 1H, Himid), 5.06 (s, 1H, Himid), 4.55−4.47 (m, 1H,
NCHMe2), 4.06 (septet, 1H, 3JHH = 6.6 Hz, NCHMe2), 3.87−2.88 (m,
4H, PC2H4P), 2.83, 2.66 (2 × s, 3H, Cimid−CH3), 1.25, 0.87, 0.58, 0.46
3
(4 × d, 3H, JHH = 6.6 Hz, NCH(CH3)2). 13C{1H} NMR (CD3CN,
1
2
35.7 (dd, JPRh = 74.6 Hz, JPP = 7.2 Hz). Anal. calcd for
C45H54B2F8N5P2Rh (1003.41 g mol−1) × 1.25 CH2Cl2: C, 50.06; H,
5.13; N, 6.31. Found: C, 50.04; H, 4.87; N, 5.95.
100 MHz): δ 145.3 (NCimidN), 143.3 (NCimidN), 134.7−128.1
(Cphenyl), 133 (Cimid−Rh),38 126 (Cimid−Rh),38 124.6 (Cimid−H),
122.5 (Cimid−H), 59.6 (NCH2N), 51.1, 50.7 (2 × NCH(Me)2), 27.4
Complex 8. Complex 1a (100 mg, 0.067 mmol) and (R)-(+)-2,2′-
bis(diphenylphosphino)-1,1′-binaphthalene (55 mg, 0.087 mmol)
were dissolved in dry CH2Cl2 (5 mL) and stirred at 50 °C for 18 h
under a nitrogen atmosphere. After evaporation of all volatiles, the
residue was purified by gradient column chromatography (SiO2;
CH2Cl2 then CH2Cl2/acetone, 1:2), which gave orange complex 8 as a
mixture of two isomers (95 mg, 52%, 97:3). Spectroscopic data of cis-
(PC2H4P), 22.5, 22.1, 21.8, 21.7 (4 × NCH(CH3)2), 11.1, 10.6 (2 ×
1
Cimid−CH3). 31P{1H} NMR (CD3CN, 162 MHz): δ 55.9 (dd, JPRh
=
120.6 Hz, 2JPP = 9.7), 40.8 (dd, 1JPRh = 77.7 Hz, 2JPP = 9.7 Hz). ESI-MS
(m/z): 380.6226, calcd for [(dicarb) (dppe)Rh]2+ 380.6205.
Complex 6b. Complex 3b (0.060 g, 0.046 mmol) and AgBF4 (0.050
g, 0.185 mmol) were dissolved in MeCN (10 mL) and stirred in the
dark at room temperature for 2 h. The yellow suspension was filtered
through Celite and all volatiles were removed under reduced pressure,
yielding complex 6b as a mixture of two isomers (0.58 g, quantitative).
1
8: H NMR (CD2Cl2, 400 MHz): δ 8.55−8.33 (br, 2H, Haryl), 7.91−
7.11 (m, 21H, Haryl), 7.33 (s, 1H, Himid), 6.99 (d, 1H, 2JHH = 12.9 Hz,
lowfield AB part of NCH2N), 6.88−6.40 (m, 7H, Haryl), 6.29 (d, 1H,
2JHH = 12.9 Hz, highfield AB part of NCH2N), 5.76−5.70 (m, 2H,
Haryl) 4.66 (s, 1H, Himid), 4.34 (septet, 1H, 3JHH = 6.7 Hz, NCHMe2),
Spectroscopic data39 for trans-6b: H NMR (CD3CN, 400 MHz): δ
1
7.86−7.36 (m, 26H, Hphenyl), 7.22−7.09 (m, 2H, Hp3henyl), 6.77−6.70
3
(m, 2H, Hphenyl), 6.05 (s, 1H, Himid), 5.96 (d, 1H, JHH = 14.0 Hz,
4.02 (septet, 1H, JHH = 6.7 Hz, NCHMe2), 3.22, 2.66 (2 × s, 3H,
3
3
lowfield AB part of NCH2N), 5.66 (d, 1H, JHH = 14.0 Hz, highfield
Cimid−CH3), 1.33, 1.20, 1.16, 0.73 (4 × d, 3H, JHH = 6.7 Hz,
NCH(CH3)2). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 142.6
AB part of NCH2N), 5.43 (s, 1H, Himid), 3.91−3.70 (m, 2H, NCH2),
3.85−3.75, 3.71−3.49, 3.39−3.26 (3 × m, 1H, PCH2), 3.23−3.09 (m,
1H, NCH2), 3.17−3.06 (m, 1H, PCH2), 2.99−2.90 (m, 1H, NCH2),
1.51−1.40, 1.13−0.99 (2 × m, 2H, NCH2CH2), 0.85−0.76, 0.75−0.72
(2 × m, 2H, CH2CH3), 0.72−0.70, 0.57−0.52 (2 × m, 3H, CH2CH3).
1
1
(NCimidN), 141.7 (Caryl, JCP = 3.1 Hz), 141.6 (Caryl, JCP = 3.0 Hz),
139.8−125.4 (Car3yl), 139.8 (NCim3 idN, 4JCP = 5.5 Hz), 129 Cimid−H),38
125.5 (Cimid−H, JCP = 6.9 Hz, JCP = 3.3 Hz), 62.4 (NCH2N), 51.1,
50.6 (2 × NCHMe2), 23.4, 22.8, 21.8, 21.0 (4 × NCH(CH3)2), 14.5,
12.8 (2 × Cipso−CH3); Cimid−Rh not resolved. 31P{1H} NMR
4
13C{1H} NMR (CD3CN, 100 MHz): δ 146.3 (NCimidN, JCP = 4.2
Hz), 144.5 (NCimidN), 135 (Cimid−Rh),38 134.9−121.35 (Cphenyl),
129.8 (Cimid−H), 128 (Cimid−Rh),38 127.6 (Cimid−H), 60.0 (NCH2N),
48.8, 48.2 (2 × NCH2), 32.3, 32.2 (2 × NCH2CH2), 29.2 (PCH2),
22.0 (CH2P), 19.9, 19.4 (2 × CH2CH3), 13.4, 13.3 (2 × CH2CH3).
31P{1H} NMR (CD3CN, 162 MHz): δ 55.8 (dd, 1JPRh = 119.5 Hz, 2JPP
1
2
(CD2Cl2, 162 MHz): δ 7.7 (dd, JPRh = 119.7 Hz, JPP = 20.3 Hz),
1
2
1.0 (dd, JPRh = 79.3 Hz, JPP = 20.3 Hz). Selected spectroscopic data
of cis-8: 31P{1H} NMR (CD2Cl2, 162 MHz): δ 8.5 (dd, JPRh = 120.0
1
Hz, 2JPP = 20.8 Hz), 5.0 (dd, 1JPRh = 76.8 Hz, 2JPP = 20.8 Hz). ESI-MS
(m/z): 1239.1154, calcd for [(dicarb) (dppe)(I)2Rh]+ 1239.1125.
Anal. calcd for C59H56I3N4P2Rh (1366.0 g mol−1) × 2.5 CH2Cl2: C,
46.78; H, 3.89; N, 3.55. Found: C, 47.13; H, 3.55; N, 3.71.
1
2
= 9.0 Hz), 44.2 (dd, JPRh = 79.0 Hz, JPP = 9.0 Hz). Selected
1
spectroscopic data for trans-6b: H NMR (CD3CN, 400 MHz): δ
7.72−7.35 (m, 30H, Hphenyl), 6.46 (s, 2H, Himid), 5.91 (d, 1H, JHH
3
=
Complex 9. Complex 2 (117 mg, 0.091 mmol) and (R)-(+)-2,2′-
bis(diphenylphosphino)-1,1′-binaphthalene (77 mg, 0.124 mmol)
were dissolved in dry, degassed CH2Cl2 (5 mL) and stirred at 50
°C for 16 h. All volatiles were evaporated and the residue was purified
by gradient column chromatography (SiO2; CH2Cl2 then CH2Cl2/
acetone, 1:2), yielding complex 9 as a dark red solid (170 mg, 74%).
1H NMR (CDCl3, 400 MHz): δ 8.28 (t, 1H, 3JHH = 7.9 Hz, Haryl), 8.21
14.0 Hz, NCH2N), 5.83 (br, 1H, NCH2N), 3.80−3.75 (m, 4H,
NCH2), 3.63−3.46 (br, 2H, PCH2), 3.23−3.06 (br, 2H, CH2P), 1.53−
1.41 (m, 4H, NCH2CH2), 1.15−1.03 (m, 4H, CH2CH3), 0.73 (t, 6H,
3JHH = 7.0 Hz, CH2CH3). 31P{1H} NMR (CD3CN, 162 MHz): δ 30
1
(br d, JPRh = 71 Hz). ESI-MS (m/z): 456.6608, calcd for [(dicarb)
(dppe)Rh]2+ 456.6517.
3
3
3
Complex 7. Complex 4 (101 mg, 0.097 mmol) and AgBF4 (57 mg,
0.291 mmol) were dissolved in MeCN (5 mL) and stirred at room
temperature for 2 h under exclusion of light. The reaction mixture was
filtered through Celite and eluted with MeCN (20 mL) yielding the
title complex as a mixture of four isomers (97 mg, quantitative).
Crystals suitable for X-ray diffraction studies were obtained by slow
(dd, 1H, JHH = 8.7 Hz, JHH = 6.4 Hz, Haryl), 8.06 (t, 2H, JHH = 8.7
3
3
Hz, Haryl), 7.86 (d, 1H, JHH = 8.6 Hz, Haryl), 7.72 (d, 1H, JHH = 8.8
Hz, Haryl), 7.70−7.62 (m, 3H, Haryl), 7.51 (d, 1H, 3JHH = 8.1 Hz, Haryl),
7.32−7.04 (m, 12H, Haryl), 6.98 (br, 1H, Himid), 6.95−6.89 (m, 1H,
3
Haryl), 6.92 (s, 1H, Himid), 6.88−6.82 (m, 2H, Haryl), 6.64 (t, 1H, JHH
= 8.4 Hz, Haryl), 6.61−6.56 (m, 3H, Haryl), 6.49 (t, 1H, 3JHH = 8.2 Hz,
5731
DOI: 10.1021/acs.organomet.5b00809
Organometallics 2015, 34, 5723−5733

Organometallics
Article
3
3
Haryl), 5.82 (d, 1H, JHH = 8.2 Hz, Haryl), 5.59 (d, 1H, JHH = 8.4 Hz,
Haryl), 4.70 (br, 1H, CH−Rh), 4.13−4.06 (m, 1H, NCH2), 3.94
AUTHOR INFORMATION
■
3
3
Corresponding Author
*Phone: +41316314644. E-mail: martin.albrecht@dcb.unibe.ch.
(septet, 1H, JHH = 6.6 Hz, NCHMe2), 3.84 (septet, 1H, JHH = 6.6
Hz, NCHMe2), 3.58−3.50 (m, 2H, NCH2), 3.29−3.21 (m, 1H,
NCH2), 2.09, 2.02 (2 × s, 3H, Cimid−CH3), 1.30 (d, 3H, JHH = 6.6
3
Notes
Hz, NCH(CH3)2), 1.26−1.23 (m, 6H, NCH(CH3)2), 1.21 (d, 3H,
3JHH = 6.6 Hz, NCH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz): δ
153.0 (Cimid−Rh, 1JCRh = 37.5 Hz, 2JCPtrans = 130 Hz, 2JCPcis = 9.1 Hz),
151.0 (Cimid−Rh, 1JCRh = 37.5 Hz, 2JCPtrans = 125 Hz, 2JCPcis = 12.4 Hz),
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
1
143.5 (Caryl, JCP = 3.8 Hz), 143.3 (Caryl, JCP = 3.8 Hz), 137.6−125.2
We thank the European Research Council (StG 208651 and
CoG 615653), Science Foundation Ireland (10/RFP/
CHS2844) and the NUI Travel Scheme (scholarship to K.F.)
for financial support.
3
(Caryl), 136.6, 136.3 (2 × NCimidN), 121.1 (Cimid−H, JCP = 9.3 Hz),
3
3
120.3 (Cimid−H, JCP = 10.8 Hz), 59.4 (NCH2, JCP = 4.6 Hz), 56.1
3
(NCH2, JCP = 3.8 Hz), 50.0, 48.8 (2 × NCHMe2), 35.0 (CH−Rh,
2
1JCRh = 28.4 Hz, JCP = 4.0 Hz), 23.4, 23.0, 22.7, 22.5 (4 ×
NCH(CH3)2), 12.2, 12.1 (2 × Cimid−CH3). 31P{1H} NMR (CDCl3,
162 MHz): δ 13.4 (dd, 1JPRh = 81.3 Hz, 2JPP = 25.2 Hz), 0.6 (dd, 1JPRh
REFERENCES
■
2
= 79.9 Hz, JPP = 25.2 Hz). ESI-MS (m/z): 1139.2308, calcd for
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
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met.5b00809.
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Representative time−conversion profile for catalytic
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̈
Muller-Bunz, H.; Albrecht, M. Dalton Trans. 2013, 42, 4197.
̈
(17) (a) Yang, L.; Kruger, A.; Neels, A.; Albrecht, M. Organometallics
̈
2008, 27, 3161. (b) Kruger, A.; Neels, A.; Albrecht, M. Chem.
Commun. 2010, 46, 315.
(18) The 31P NMR resonance frequencies of 3b do not agree with
the literature data (see ref 17a). The newly determined chemical shift
values are, for trans-3b: 31P{1H} NMR (CD2Cl2, 162 MHz): δ 21.1
(1JPRh = 76.3 Hz); for cis-3b: 31P{1H} NMR (CD2Cl2, 162 MHz): δ
(32) Horn, S.; Gandolfi, C.; Albrecht, M. Eur. J. Inorg. Chem. 2011,
2011, 2863.
48.3 (1JPRh = 115.1 Hz, JPP = 6.7 Hz), 24.6 (1JPRh = 82.6 Hz).
2
(33) (a) Noyori, R. Angew. Chem., Int. Ed. 2013, 52, 79. (a1) Noyori,
R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40. (b) Noyori, R.;
Okhuma, T.; Kitamura, M.; Takaya, H.; Sayo, N.; Kumobayashi, H.;
Akuragawa, S. J. Am. Chem. Soc. 1987, 109, 5856. (c) Takaya, H.;
Akutagawa, S.; Noyori, R. Org. Synth. 1989, 67, 20.
(19) The ratio of phosphine oxide (8% of total phosphine) is similar
to the quantity of the ratio of a single doublet, suggesting formation of
a monodentate phosphine complex with one phosphine residue
oxidized and unavailable for chelation, or the formation of free dppe
oxide and a species with enantiotopic phosphine residues. In addition,
another isomer with an AB doublet in the 31P NMR spectrum was
detected in some 15%, together with trans-3a (35%) and cis-3a (34%)
as the major species.
(34) The less shielded doublet of doublets at δP = 7.66 is broad at
room temperature and well resolved at −20 °C, which may point to
dynamic coordination of the corresponding phopshine site.
1
(35) The more deshielded resonance at δP = 9.0 shows a large JPRh
(20) Kruger, A.; Haller, L. J. L.; Muller-Bunz, H.; Serada, O.; Neels,
̈
̈
̈
coupling of 121.2 Hz, consistent with a trans MeCN ligand, while the
A.; Macgregor, S. A.; Albrecht, M. Dalton Trans. 2011, 40, 9911.
(21) After 3 weeks in CD2Cl2, trans-4 (33%), cis-4 (28%) were the
major species in solution. Moreover, dppe oxide (8%) was present and
two Cs-symmetric and two C1-symmetric species in 17%, 7%, 6%, and
1%, with the following respective 31P{1H} NMR data (162 MHz):
17% species: δ 37.5 (dd, 1JPRh = 78.3 Hz, 2JPP = 5.1 Hz), 34.8 (dd, 1JPRh
= 83.2 Hz, 2JPP = 5.1 Hz); 7% species δ 31.9 (d, 1JPRh = 84.4 Hz). 6%
1
more shielded signal at dP = 5.0 ppm shows a smaller JPRh coupling
constant of 78.7 Hz (2JPP = 21.1 Hz) indicating a trans carbene.
(36) Complex 8 (32 mg, 23 μmol) and AgBF4 (18 mg, 94 μmol)
were stirred for 16 h at room temperature in MeCN (5 mL). The
suspension was filtered through Celite and eluted with MeCN (10
mL). Evaporation of the filtrate to dryness gave the solvent complex as
a pale yellow solid (31 mg, quantitative). Crystals suitable for X-ray
diffraction studies were obtained by slow diffusion of Et2O into a
MeCN solution of the complex. 31P{1H} NMR (CD3CN, 162 MHz):
species: δ 50.0 (dd, 1JPRh = 138.0 Hz, 2JPP = 7.6 Hz), 29.8 (dd, 1JPRh
80.2 Hz, JPP = 7.6 Hz); 1% species: δ 57.4 (d, JPRh = 131.3 Hz).
=
2
1
(22) Dissolving crystals in gave spectra that were in full agreement
with trans-4. In addition, minor quantities of a solvento complex were
detected due to the presence of cocrystallized H2O.
(23) See, for example: (a) Poyatos, M.; Sanau, M.; Peris, E. Inorg.
Chem. 2003, 42, 2572. (b) Mata, J. A.; Chianese, A. R.; Miecznikowski,
J. R.; Poyatos, M.; Peris, E.; Faller, J. W.; Crabtree, R. H.
Organometallics 2004, 23, 1253. (c) Leung, C. H.; Incarvito, C. D.;
1
2
1
δ 30.6 (dd, JPRh = 127.6 Hz, JPP = 27.5 Hz), 9.62 (dd, JPRh = 72.4
2
Hz, JPP = 27.5 Hz).
(37) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc. 1996, 118, 2521.
(38) Resonance frequency deduced indirectly by HSQC or HMBC
experiments.
(39) This complex was not available in sufficient quantities after
recrystalliation for elemental analysis.
(40) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112.
(41) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
Crabtree, R. H. Organometallics 2006, 25, 6099. (d) Turkmen, H.;
̈
Pape, T.; Hahn, F. E.; Cetinkaya, B. Eur. J. Inorg. Chem. 2008, 2008,
5418.
(24) Kruger, A.; Michaud, G.; Dowling, A.; Muller-Bunz, H.;
̈
̈
Albrecht, M. Z. Anorg. Allg. Chem. 2015, 641, 2250.
5733
DOI: 10.1021/acs.organomet.5b00809
Organometallics 2015, 34, 5723−5733