Catalytic activity and fluxional behavior of complexes based on RuHCl(CO)(PPh3)3 and xantphos-type ligands

Article abstract of DOI:10.1021/om5003182

With RuHCl(CO)(PPh₃)₃ as the starting material, the complexes RuHCl(CO)(PPh₃)(L) were prepared for L = Xantphos and closely related ligands. Their catalytic activity in the direct amination of cyclohexanol showed large differences depending on the different backbone structures. In those complexes the Xantphos-type ligand backbones are slightly bent and display fluxionality, studied by VT-NMR. This was assigned to the "flipping" of the backbone via the bridging atoms in the xanthene backbone. Via line shape analysis of the peaks, the Gibbs free energy of activation of the flipping movement was found to be around 56 kJ/mol in all cases. However, the activation enthalpy and entropy differed considerably. Employing RuCl₂(PPh₃)₃ as the precursor resulted in the trans-coordinated complexes RuCl₂(PPh₃)(L) for L = Xantphos, Sixantphos. Fluxionality was no longer observed, due to the fact that in these complexes the O atom in the backbone also coordinates to the Ru.

Full text of DOI:10.1021/om5003182

Article
pubs.acs.org/Organometallics
Catalytic Activity and Fluxional Behavior of Complexes Based on
RuHCl(CO)(PPh3)3 and Xantphos-Type Ligands
Dennis Pingen,† Tomas Lebl,‡ Martin Lutz,§ Gary S. Nichol,† Paul C. J. Kamer,‡ and Dieter Vogt*,†
†School of Chemistry, University of Edinburgh, King’s Buildings, Joseph Black Building, West Mains Road, Edinburgh EH9 3JJ,
Scotland, U.K.
‡School of Chemistry, University of St. Andrews, Fife KY16 9AJ, Scotland, U.K.
§Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands
S
* Supporting Information
ABSTRACT: With RuHCl(CO)(PPh3)3 as the starting material, the complexes RuHCl(CO)(PPh3)(L) were prepared for L =
Xantphos and closely related ligands. Their catalytic activity in the direct amination of cyclohexanol showed large differences
depending on the different backbone structures. In those complexes the Xantphos-type ligand backbones are slightly bent and
display fluxionality, studied by VT-NMR. This was assigned to the “flipping” of the backbone via the bridging atoms in the
xanthene backbone. Via line shape analysis of the peaks, the Gibbs free energy of activation of the flipping movement was found
to be around 56 kJ/mol in all cases. However, the activation enthalpy and entropy differed considerably. Employing
RuCl2(PPh3)3 as the precursor resulted in the trans-coordinated complexes RuCl2(PPh3)(L) for L = Xantphos, Sixantphos.
Fluxionality was no longer observed, due to the fact that in these complexes the O atom in the backbone also coordinates to the
Ru.
synthesis from alcohols, which generally requires activating
groups for the alcohol and protecting groups on the amine to
prevent further nucleophilic attack of the products. Develop-
ment and improvement of the homogeneously catalyzed
conversion of alcohols into primary amines using ammonia is
therefore highly desired. So far, only a few examples of catalysts
have been developed for this reaction.1,5 Only very limited
information on catalyst structure and behavior is available for
those systems, and detailed mechanistic insight is lacking. In
order to develop new catalysts or to improve existing systems,
more mechanistic and structural knowledge is required.
INTRODUCTION
■
The direct synthesis of amines from alcohols via “hydrogen
shuttling”,1 also called “borrowing hydrogen”2 and “hydrogen
autotransfer”,3 has been an intriguing challenge for quite some
time (Scheme 1).4 Amines are important building blocks for
polymers, surfactants, corrosion inhibitors, and a vast range of
fine chemicals. Much effort has been put into their direct
Scheme 1. Hydrogen Shuttling Concept
The activity and selectivity of homogeneous transition-metal
catalysts are largely controlled by the ligands applied. Even
more so, it is often seen that very similar ligands can exhibit
extremely different performance in catalysis.6 In this study a
range of closely related Xantphos-type ligands (Table 1, entries
Received: March 24, 2014
Published: May 28, 2014
© 2014 American Chemical Society
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Supporting Information, Figures S3−S8), on the basis of the
GC analysis of liquid samples (50 μL) withdrawn at regular
intervals during the reaction. The reactions were performed at
150 and 170 °C with a catalyst loading of 1 mol % (Ru:L =
1:1).
Table 1. Bite Angles for the Ligands Employed in This Study
Xantphos (1) gave a highly active catalyst (Figure 1), giving
full conversion after 23 h at 170 °C. At 150 °C the rate was
slightly lower and full conversion was reached after 30 h.
Using Sixantphos (2), the reaction started off at a higher rate
than with Xantphos (1). However, only 70% conversion was
reached within 45 h at 150 °C; the complex seems to be
deactivated. Increasing the temperature to 170 °C resulted in a
higher conversion; after 25 h full conversion was reached
(Figure S3). Employing Thixantphos (3) as the ligand showed
a peculiar behavior. At 150 °C hardly any primary amine was
produced. Instead, in the beginning the intermediate cyclo-
hexanone was the major product but at the end of the reaction
the secondary amine prevailed. Apart from this reaction using
Thixantphos (3) as ligand, we and others never observed the
secondary amine in more than trace quantities in the direct
alcohol amination with ammonia.1a,5b,c Remarkably, increasing
the temperature to 170 °C resulted in a very selective reaction
toward the primary amine.
The flexible DPEphos gave the least active catalyst; only 20%
conversion was reached after 45 h at 150 °C. Even at 170 °C,
the activity remained low; in this case, the conversion went only
up to 50%. Employing Nixantphos (5), which bears an NH
moiety on the bridge instead of an isopropylene group, resulted
in a completely inactive catalyst. Even raising the temperature
did not result in a significant increase of activity. Remarkably,
the N-benzylated ligand BnNixantphos (6) did show activity.
The conversion goes up to 30% at 150 °C and to 80% at 170
°C, both with very high selectivity. However, in both cases full
conversion was not achieved within 50 h. The much more rigid
Benzoxantphos (7) resulted in a very low conversion.
The turnover frequencies of the complexes at both
temperatures can be found in Table 2. An interesting detail
bite angle βn
flexibility range
a
b
ligand
Benzoxantphos (7)
(deg)
(deg)
120.6
114
102−146
99−141
99−139
Nixantphos (5; X = NH, R = H)
BnNixantphos (6; X = N(benzyl), R
114
= H)
Xantphos (1; X = C(CH3)2 R = H)
Thixantphos (3; X = S, R = CH3)
Sixantphos (2; X = Si(CH3)2, R = H)
DPEphos (4; X = none, R = H)
111.7
109.4
108.7
102.2
97−135
94−130
93−132
86−120
a
b
Bite angles are taken from ref 9, all modeled for Rh. Range
accessible within 3 kcal/mol excess strain energy. Flexibility ranges are
taken from ref 10.
1−7) were employed in the Ru-catalyzed direct amination of
cyclohexanol with ammonia. Recently the catalyst derived from
RuHCl(CO)(PPh3)3 and Xantphos was reported to be efficient
and selective for this transformation.5c The wide bite angle
diphosphine Xantphos has been shown to be a versatile ligand
for a large variety of catalytic reactions.6b,7 The Xantphos
backbone is widely regarded to be quite rigid, although there
are well reported examples known, in which the ligand shows
fluxional behavior and where different coordination modes are
observed in seemingly very similar complexes.7a,8
The ligands all have large bite angles, depending on the
bridge in the xanthene backbone. The most flexible ligand,
diphenyl ether based DPEphos, has the smallest bite angle
(Table 1).9
RESULTS AND DISCUSSION
■
The catalytic direct amination of cyclohexanol with NH3 was
performed with complexes generated in situ from RuHCl-
(CO)(PPh3)3 and 1 equiv of the ligand (1−6). Figure 1 shows
the reaction profiles for the reactions employing RuHCl(CO)-
(PPh3)3/Xantphos (other reaction profiles can be found in the
Table 2. Cyclohexanol Amination with RuHCl(CO)(PPh3)3/
a
Xantphos-Type Ligands: TOFs at 150 and 170 °C
b
TOF (h−1) (conversion after 30 h (%))
ligand
150 °C
170 °C
Xantphos (1)
5.6 (96.6)
9.6 (67.3)
3.4 (63.1)
0.4 (17.8)
14.3 (99)
Sixantphos (2)
Thixantphos (3)
DPEphos (4)
35.1 (94.3)
24.3 (94.0)
1.9 (43.4)
c
c
Nixantphos (5)
BnNixantphos (6)
Benzoxantphos (7)
n.d. (1.2)
n.d. (5.0)
0.83 (27.8)
4.40 (68.6)
c
c
n.d. (6.3)
n.d. (14.3)
a
Conditions: 5 mmol of cyclohexanol, 1 mol % of RuHCl(CO)-
(PPh3)3, 1 mol % of ligand, 13.3 mL of tert-amyl alcohol, 2.5 mL of
b
c
NH3 (97.5 mmol). TOFs are calculated at 20% conversion. Not
determined due to very low conversion.
here is that, at 150 °C, Xantphos (1) gives a higher turnover
frequency than Thixantphos (3), with the secondary amine as
major product for the latter. However, increasing the
temperature to 170 °C gives a higher TOF for Thixantphos
(3) with the expected primary amine as product. Even more
interesting is that Nixantphos (5) does not show any
conversion at both temperatures, while the N-benzyl derivative
gives reasonably good activity. The more rigid Benzoxantphos
Figure 1. Reaction profiles for the reactions employing RuHCl(CO)-
(PPh3)3/Xantphos. Conditions: 5 mmol of cyclohexanol, 1 mol % of
RuHCl(CO)(PPh3)3, 1 mol % of Xantphos (1), 13.3 mL of tert-amyl
alcohol, 2.5 mL of NH3 (97.5 mmol). Legend: ( ) 150 °C; ( ) 170
°C; (black) cyclohexanol; (gray) cyclohexylamine.
■
●
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(7) appears to be inactive. In all cases, DPEphos (4) resulted in
the least active complex.11
Intrigued by the observed differences in the TOFs for the
different ligands, we decided to study their coordination
behavior in more detail. It has already been shown that
Xantphos (1) adopted a cis coordination in an octahedral
complex. The structure in the crystal showed the hydride and
PPh3 together with the Xantphos P atoms in the equatorial
plane. The CO and Cl are positioned in axial positions, trans to
each other. However, both ligands appear to be isotropically
disordered.12
In a study of the solution structure of this molecule, it was
shown by NMR that the complex was present in two different
conformations. In solution the complex shows a broad doublet
1
of triplets in the hydride region of the H NMR spectrum at
room temperature (Figure 2), and the 31P NMR spectrum was
Figure 3. Low-temperature H (top) and H{31P} (bottom) NMR
spectra (hydride region) of RuHCl(CO)(PPh3)3/Xantphos after reflux
for 12 h: (top) 1H NMR (toluene-d8, 500 MHz, 223 K, δ ppm) −6.71
(ddd, JHP = 23 Hz, JHP = 31 Hz, JHP = 117 Hz), −7.02 (ddd, JHP =21
Hz, JHP = 31 Hz, JHP = 110 Hz); (bottom) 1H{31P} NMR (toluene-d8,
500 MHz, 223 K, δ ppm) −6.72 (s), −7.04 (s).
1
1
Figure 2. 1H NMR spectrum (hydride region) of RuHCl(CO)-
(PPh3)3/Xantphos after reflux for 12 h (toluene-d8, 500 MHz, 298 K)
δ −6.8 (ddd, JHP = 24.8 Hz, JHP = 27.2 Hz, JHP = 114.4 Hz).
indicative of fluxional processes in the slow exchange regime
(Figure S9, Supporting Information). The broad peaks could be
due to intramolecular exchange or fluxional behavior of the
ligand(s).
Upon cooling to 223 K, the hydride region resolves in a more
complex pattern, attributed to the presence of two complexes in
solution (Figure 3, top).
1
The hydride region of the H NMR spectrum consists of a
splitting pattern with 16 lines. However, the coupling constants
reveal the presence of two doublet of doublets of doublets: one
complex with coupling constants of 23, 31, and 117 Hz and the
other with coupling constants of 21, 31, and 110 Hz. The close
similarity of the coupling constants indicates the presence of
two complexes which are structurally closely related.
Figure 4. Low-temperature 31P{1H} NMR spectrum of RuHCl(CO)-
(PPh3)3/Xantphos after reflux for 12 h (toluene-d8, 202 MHz, 223 K, δ
●
ppm): 39.1 (dd, JPP = 14 Hz, JPP = 307 Hz, ), 35.9 (dd, JPP = 14 Hz,
●
JPP = 301 Hz, ★), 32.9 (dd, JPP = 14 Hz, JPP = 307 Hz, ), 26.7 (dd,
●
JPP = 14 Hz, JPP =301 Hz, ★), 20.6 (t, JPP = 14 Hz, ), 4.2 (t, JPP = 14
▲
Hz, ★), 25.4 (residual RuHCl(CO)(PPh3)3, ).
In order to confirm that there are indeed only two complexes
1
present in solution, a 31P-decoupled H NMR spectrum was
recorded. As expected, only two singlets at 6.72 and 7.04 ppm
in a ratio of 1:1 were now observed (Figure 3, bottom).
The LT 31P NMR spectrum reveals sharp peaks which also
indicate the presence of more than one complex (Figure 4).
The signals located most upfield in the 31P NMR spectrum are
most likely the signals of the phosphine trans to the hydride
(4.2 ppm (★) and 20.6 ppm (●), JPP = 14 Hz), as these lack a
large P−P coupling. The signals at 26.7 and 35.9 ppm form a
doublet of doublets (★) with one large and one smaller
coupling constant (301, 14 Hz), revealing that these are two P’s
trans to each other. The other doublet of doublets (32.9 and
39.1 ppm, 307, 14 Hz) belongs to the second complex (●).
The signal at 25.4 ppm belongs to the precursor RuHCl(CO)-
(PPh3)3.
In order to determine the exact conformations of the
1
complexes in solution, H−31P HMBC NMR spectra were
1
recorded. Because of the different H−31P coupling constants
(around 20, 30, and 100 Hz), three spectra were recorded,
optimized for each coupling constant to give a better coupling
resolution.
The 1H resonance with 100 Hz coupling constants correlates
with the 31P signals around 4 and 20 ppm and thus can be
assigned as the phosphine coordinated trans with respect to the
hydride (Figure 5A). The signal at 4 ppm can be assigned to
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one conformation (conformation 1), whereas the signal at 20
ppm belongs to the other conformation (conformation 2).
Figure 6. 31P−31P EXSY spectrum (T = 223 K) for the exchange in
the complexes.
In order to examine this dynamic behavior in more detail, VT
1H NMR spectra were recorded over a range of temperatures
from 223 to 373 K. Coalescence was observed at about 273 K
(Figure 7).
Figure 5. Correlation spectra of the hydride to the phosphine in
toluene-d8 optimized for (A) 100 Hz coupling at 223 K, (B) 20 Hz
coupling at 223 K, and (C) 30 Hz coupling at 223 K.
The resonance in the 1H NMR spectrum with 20 Hz
coupling correlates with the signals located around 27 ppm
(conformation 1) and 33 ppm (conformation 2) in the 31P
spectrum (Figure 5B). The 1H resonance with a coupling of 30
Hz correlates to the 31P signals located around 36 and 38 ppm
(Figure 5C).
The NMR data confirm that the structure in solution is equal
to the structure in the crystal. However, in solution, the
complex shows an exchange which is most likely due to
intramolecular movement. To exclude other transitions, a 31P
exchange NMR spectrum was recorded (Figure 6).
The signal at −5 ppm resembles free PPh3, as the complex
was prepared in situ, exchanging two PPh3 ligands for
Xantphos. The exchange spectrum shows that there is no
exchange between free PPh3 and the other coordinated
phosphines. The only exchange resolved is between the 31P
signals having the same order of coupling constants. The
phosphines remain on the same position in the complex; thus,
no dissociation occurs. This indicates that the exchange must
be due to the flipping of the Xantphos backbone. The backbone
of free Xantphos is slightly bent with a dihedral angle of 166°.13
However, upon coordination it bends more strongly over the
bridging atoms between the backbone phenyl rings, resulting in
a smaller dihedral angle.
Figure 7. VT 1H NMR (toluene-d8, 500 MHz) spectra of the hydride
region of RuHCl(CO)(PPh3)3/Xantphos, showing the exchange
between two complexes.
The same was observed in the VT 31P NMR (Figure 8). The
signals remain fairly broad though, even at 373 K, indicating
that the fast exchange regime has not yet been reached. This is
also in accordance with some residual broadening of the middle
hydride signals (Figure 8, 328−373 K).
Line-shape analysis based on the simulated spectra (Figures 9
and 10) was performed. The Eyring plot (Supporting
Information) of these data provides the enthalpy, entropy,
and Gibbs free energy of activation.
The activation energy for exchange was determined using
both the hydride region and the 31P (trans to hydride) region.
The resulting Gibbs free energy, enthalpy, and entropy from
the Eyring plot are given in Tables 3 and 4.
The observed dynamic exchange process is most likely
related to the “flipping” motion of the Xantphos backbone.7a,c
The determined entropy of activation between −44 and −48 J
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Table 3. Activation Parameters for Backbone “Flipping”
1
Derived from H NMR Spectra
RuHCl(CO)
(PPh3)
(Sixantphos)
RuHCl(CO)
(PPh3)
(Xantphos)
RuHCl(CO)
(PPh3)
(Thixantphos)
coalescence
283
290
290
temp Tc (K)
ΔH⧧ (kJ
50.2 ( 2.2)
−23 ( 7)
56.7 ( 4.2)
42.7 ( 2.2)
−44 ( 7)
55.2 ( 4.2)
47.0 ( 2.2)
−33 ( 7)
55.6 ( 4.2)
mol−1
)
ΔS⧧ (J mol−1
K−1
)
ΔG⧧ (kJ
mol−1
)
Table 4. Activation Parameters for Backbone “Flipping”
Derived from 31P NMR Spectra
RuHCl(CO)
(PPh3)
(Sixantphos)
RuHCl(CO)
(PPh3)
(Xantphos)
RuHCl(CO)
(PPh3)
(Thixantphos)
Figure 8. VT 31P NMR (toluene-d8, 202 MHz) spectra of
RuHCl(CO)(PPh3)/Xantphos.
coalescence
temp Tc (K)
328
328
Tc = 328
ΔH⧧ (kJ
47.5 ( 2.3)
41.0 ( 2.4)
44.50 ( 2.2)
mol−1
)
ΔS⧧ (J mol−1
−28 ( 7)
−48 ( 7)
−39( 7)
K−1
)
ΔG⧧ (kJ
56.6 ( 4.8)
56.6 ( 4.8)
57.3 ( 4.5)
mol−1
)
Information, Figures S17−S24). Although the barriers for
flipping were comparable, different values for activation
enthalpy and entropy in the flipping process were observed
(Tables 3 and 4).15
No fluxional behavior was observed for the complex of
RuHCl(CO)(PPh3)3 and DPEphos. The NMR spectra
recorded at the different temperatures showed a doublet of
doublets of doublet, which did not change with temperature
(Supporting Information, Figures S25 and S26). At very low
temperature line broadening occurred due to the solution
becoming too viscous.15 From the crystal structure it can be
seen that the diphenyl ether backbone adapts the least
constrained configuration (Figure 11).
Figure 9. Simulated 1H NMR spectra of the hydride region of
RuHCl(CO)(PPh3)(Xantphos) (238−373 K) using gNMR.14
In Tables 3 and 4 the determined activation parameters for
the “flipping” motion of all complexes investigated are given for
comparison. For the complexes with ligands 1−3 ΔG⧧ was
found to be close to 56 kJ/mol, which can be explained by the
similarity of those ligands. The values for ΔH⧧ also vary in a
fairly close range from 41 to 47 kJ/mol. The values for ΔS⧧,
however, show a larger variation. They vary from −28 J mol−1
K−1 for the Sixantphos (2) complex to −48 J mol−1 K−1 for the
Xantphos (1) complex, on the basis of the VT 31P spectra.
However, one should keep in mind that the values for ΔS⧧ are
always afflicted with a larger experimental error.
As Nixantphos (5) appeared to be completely inactive, its
coordination behavior was also studied. Only free ligand and
free metal precursor were now observed in the 31P spectrum,
indicating no coordination via the P of the ligand to ruthenium.
Though unexpected, this might be due to coordination of the
backbone nitrogen (see the Supporting Information, Figures S3
and S4). This is supported by the observations in employing
Figure 10. Simulated 31P NMR spectra of the P trans to the hydride in
RuHCl(CO)(PPh3)(Xantphos) using gNMR.14
1
BnNixantphos (6), which does show activity. The H NMR of
mol−1 K−1 is in line with an intramolecular exchange process.
The value for ΔG⧧ is around 55.8 kJ mol−1, also in good
agreement with previously reported fluxional processes.7a
Several of the other Xantphos-type ligands employed in the
direct amination displayed a similar behavior (Supporting
RuHCl(CO)(PPh3)3 with BnNixantphos does show coordina-
tion by means of a complex splitting pattern in the hydride. In
addition, the 31P NMR now shows coordination. Although the
majority of the peaks in both spectra suggest a complex similar
to that observed for the previously described ligands, additional
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Figure 12. Structure of RuCl2(PPh3)(Sixantphos) in the crystal.19
Displacement ellipsoids are displayed at the 50% probability level.
Selected bond lengths (Å) and angles (deg) with standard
uncertainties given in parentheses: Ru(1)−Cl(3) = 2.4167(15),
Ru(1)−Cl(6) = 2.3903(15), Ru(1)−P(5) = 2.2506(14), Ru(1)−
P(23) = 2.3405(14), Ru(1)−P(24) = 2.3197(13), Ru(1)−O(13) =
2.328(3); P(5)−Ru(1)−P(23) = 101.05(5), P(23)−Ru(1)−P(24) =
157.63(6), P(5)−Ru(1)−P(24) = 98.80(5), P(5)−Ru(1)−O(13) =
175.21(11), Cl(3)−Ru(1)−Cl(6) = 167.70(5).
Figure 11. Molecular structure of RuHCl(CO)(PPh3)(DPEphos) in
the crystal.16 Displacement ellipsoids are displayed at the 50%
probability level. Severely disordered solvent molecules are omitted
for clarity. Selected bond lengths (Å) and angles (deg) with standard
uncertainties given in parentheses: Ru(1)−P(1) = 2.4831(14),
Ru(1)−P(2) = 2.3603(12), Ru(1)−P(3) = 2.3591(12), Ru(1)−
C(55) = 1.824(4), Ru(1)−Cl(1) = 2.4821(10), C(55)−O(2) =
1.149(4); P(1)−Ru(1)−P(2) = 98.18(4), P(2)−Ru(1)−P(3) =
158.23(4), P(1)−Ru(1)−P(3) = 103.11(4).
cyclohexanol, with Sixantphos giving the highest TOF (35.1 h−1
at 170 °C) and the flexible DPEphos giving the lowest TOF
(1.9 h−1 at 170 °C). All Xantphos-type ligands possessing a
fairly rigid backbone showed fluxional behavior in their
complexes RuHCl(CO)(PPh3)(L), as observed by VT-NMR.
Simulating the spectra and line shape analysis with gNMR14
gave a free energy of activation for the flipping motion of about
56 kJ/mol for all of these complexes. The activation entropies
and enthalpies, however, were different. The flexible DPEphos
ligand does not show any restricted dynamics on the NMR
time scale. Complexes of Xantphos-type ligands prepared from
RuCl2(PPh3)3 as the precursor showed trans coordination.
These complexes did not show fluxional behavior, due to
fixation of the ligand via additional coordination of the O atom
in the xanthene backbone.
Obviously, subtle differences in the structure and dynamics
of the ligands investigated in this study give rise to different
behaviors in catalysis. No direct correlation could be found
between the ligand fluxionality in the precursor complexes and
the performance in catalysis. However, obviously the bite angle
plays an important role. The flexible DPEphos with the smallest
bite angle, and BnNixantphos and Benzoxantphos with the
largest bite angles (>114°), show very low activity. The fact that
the two ligands with the largest bite angles show a tendency for
trans coordination might be the reason for their low activity.
Further studies will be necessary to gain a deeper insight into
the reaction mechanism and to develop a structure−perform-
ance relation. For this we foresee that spectroscopic studies
under real reaction conditions, i.e. also in the presence of amine
and alcohol, will be needed.
peaks can be observed. These suggest a complex in which
BnNixantphos is trans coordinated (Supporting Information,
Figures S5−S8).
The more rigid Benzoxantphos (7) resulted in a more
complex mixture of complexes (on the basis of 1H{31P} NMR),
but here the majority of the peaks suggest trans coordination of
the ligand. There are also four conformations possible for this
ligand; the benzo group in the backbone results in an
asymmetric ligand, and thus also the two P’s are inequivalent
(Figures S9−S12 in the Supporting Information).17
It was previously shown by Williams that the coordination
mode of Xantphos can depend on the Ru precursor complex.8d
The use of RuHCl(PPh3)3 resulted in trans coordination of
Xantphos, with the bridging O also coordinated to the metal
center. Due to this extra bond, the backbone was now fixed in a
flat conformation, preventing the flipping motion. The same
was observed when we used RuCl2(PPh3)3 as precursor with
Xantphos as the ligand. The octahedral structure is slightly
distorted but the Xantphos is clearly trans coordinated, the
angle between the Xantphos P atoms being slightly less than
180°. The same holds for the Cl atoms, the angle being slightly
less than 180° due to the bulky PPh3 coordinated in the
plane.12
In addition, Sixantphos (2) as ligand adopts the same
structure with RuCl2(PPh3)3 as precursor. Also here a slightly
distorted octahedron is observed, with both angles, that
between the Sixantphos P atoms and that between the Cl
atoms, being slightly less than 180° (Figure 12). The same has
been reported for DPEphos, also adopting a trans-coordinated
structure.18 Unfortunately, crystals suitable for X-ray diffraction
could not be obtained for the other complexes.
EXPERIMENTAL SECTION
■
Procedures for the Amination of Cyclohexanol. RuHCl(CO)-
(PPh3)3 (0.05 mmol, 1 mol %, 47.6 mg) was weighed into a Schlenk
tube that was purged with argon. To this as added dry degassed tert-
amyl alcohol (15 mL). Subsequently, the ligand (0.05 mmol, 1 mol %)
and cyclohexanol were added (5 mmol, 0.53 mL). The solution was
transferred to a homemade 75 mL stainless steel autoclave, which was
CONCLUSIONS
■
Complexes of RuHCl(CO)(PPh3)3 with Xantphos, Sixantphos,
Thixantphos, DPEphos, BnNixantphos, and Benzoxantphos
have been synthesized. All of these complexes (except for
Benzoxantphos) show activity in the direct amination of
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Article
purged with argon. The autoclave was charged with liquid ammonia
(2.5 mL) and then heated to 150 or 170 °C for at the appropriate time
(see graphs for exact times). Liquid samples (50 μL) were withdrawn
at intervals and analyzed by GC in order to derive reaction profiles.
Synthesis. RuHCl(CO)(PPh3)(Xantphos). RuHCl(CO)(PPh3)3
(0.3 mmol, 285.7 mg) and Xantphos (0.35 mmol, 201.7 mg) were
placed in a Schlenk tube that was purged with Ar. To this was added
dry degassed toluene (10 mL), and the Schlenk tube was closed. The
mixture was stirred under reflux for 3 h. After this time, the reaction
mixture was cooled to room temperature and all solvent was
evaporated in vacuo. The remaining powder was washed with EtOH
(3 × 10 mL), and any remaining EtOH was evaporated. The resulting
solid was then washed with hexanes (3 × 10 mL). All organic solvents
were removed, yielding a yellowish powder (211 mg, 69.5%). Crystals
suitable for X-ray diffraction were grown by dissolving the complex in
benzene and allowing slow diffusion of hexane into the solution.
1H NMR (toluene-d8, 500 MHz, 223 K): δ −6.71 (ddd, JHP = 23
Hz, JHP = 31 Hz, JHP = 117 Hz), −7.02 (ddd, JHP = 21 Hz, JHP = 31 Hz,
JHP = 110 Hz). 31P{1H} NMR (toluene-d8, 202 MHz, 223 K): δ 39.1
(dd, JPP = 14 Hz, JPP =307 Hz), 35.9 (dd, JPP = 14 Hz, JPP =301 Hz),
32.9 (dd, JPP = 14 Hz, JPP = 307 Hz), 26.7 (dd, JPP = 14 Hz, JPP =301
Hz), 20.6 (t, JPP = 14 Hz), 4.2 (t, JPP = 14 Hz). IR (cm−1): 1913 (CO,
hydride overlap). MS (ESI, CH2Cl2): 970.8 (100% [M − Cl]+), 709.1
(83% [M − Cl]+ − PPh3).
31.9 Hz, JHP = 109 Hz). 31P{1H} NMR (toluene-d8, 202 MHz, 223 K):
δ 38.3 (s), 34.8 (d, JPP = 301 Hz), 28.4 (d, JPP = 296 Hz), 25.2 (s),
25.4 (d, JPP = 114 Hz), 8.1 (d, JHP = 109 Hz). IR (cm−1): 1959
(hydride), 1921 (CO). MS (ESI, MeOH): 1029 (100% [M − CO +
H]+ + MeOH), 988.8 (90% [M − H − Cl]+).
RuHCl(CO)(PPh3)(Nixantphos). RuHCl(CO)(PPh3)3 (0.05 mmol,
47.6 mg) and Nixantphos (0.05 mmol, 27.5 mg) were placed in a
Wilmad-Young NMR tube that was purged with Ar. To this was added
dry degassed toluene (0.5 mL), and the tube was closed. The mixture
was heated to reflux for 3 h. After this time, the reaction mixture was
cooled to room temperature and NMR spectra were recorded.
1H NMR (toluene-d8, 500 MHz, 223 K): no hydride signal was
observed. 31P{1H} NMR (toluene-d8, 202 MHz, 223 K): δ 25.7 (PPh3,
RuHCl(CO)(PPh3)3), −5 (free PPh3)3), −18.4 (free Nixantphos).
Benzylnixantphos was prepared following a literature procedure
from Nixantphos and benzyl chloride.10
RuHCl(CO)(PPh3)(BnNixantphos). RuHCl(CO)(PPh3)3 (0.05
mmol, 47.6 mg) and BnNixantphos (0.05 mmol, 32.1 mg) were
placed in a Wilmad-Young NMR tube that was purged with Ar. To this
was added dry degassed toluene-d8 (0.5 mL), and the NMR tube was
closed. The mixture was heated to reflux for 3 h. After this time, the
mixture was cooled to room temperature and NMR spectra were
recorded. Crystals suitable for X-ray diffraction could not be grown. A
small amount of precipitate was observed after several days in the
NMR tube.
RuHCl(CO)(PPh3)(Sixantphos). RuHCl(CO)(PPh3)3 (0.3 mmol,
285.7 mg) and Sixantphos (0.35 mmol, 207.4 mg) were placed in a
Schlenk tube that was purged with Ar. To this was added dry degassed
toluene (10 mL), and the Schlenk tube was closed. The mixture was
stirred under reflux for 4 h. After this time, the reaction mixture was
cooled to room temperature and all solvent was evaporated in vacuo.
The resulting pale yellow powder was washed with EtOH (3 × 10
mL). The remaining powder was dried in vacuo and after that washed
with hexanes (3 × 10 mL). All organic solvents were removed, yielding
an off-white powder. Crystals suitable for X-ray diffraction could not
be grown. Only amorphous precipitation was obtained via diffusion of
hexane into the solution of the complex in benzene (157 mg, 51%).
1H NMR (toluene-d8, 500 MHz, 238 K): δ −5.96 (ddd, JHP = 19
Hz, JHP = 33 Hz, JHP = 116 Hz), −6.29 (ddd, JHP =18 Hz, JHP = 34 Hz,
JHP = 109 Hz). 31P{1H} NMR (toluene-d8, 202 MHz, 238 K): δ 42.5
(d, JPP = 307 Hz), 36.2 (d, JPP = 300 Hz), 34.1 (d, JPP = 307 Hz), 29.3
(d, JPP = 300 Hz), 24.9 (d, JHP =109 Hz), 10.9 (d, JHP = 109 Hz). IR
(cm−1): shoulder at 1921 (hydride), 1921 (CO). MS (ESI, CH3CN):
695.5 (100% [M − H − Cl]2+ − CO − PPh3), 765.5 (79.5% [M − H
− Cl]+ − PPh3 + CH3CN), 986.8 (18.9%, [M − H − Cl]2+).
RuHCl(CO)(PPh3)(DPEphos). RuHCl(CO)(PPh3)3 (0.04 mmol,
38.1 mg) and DPEphos (0.04 mmol, 21.5 mg) were placed in a
Wilmad-Young NMR tube. The powders were degassed and purged
with Ar, degassed, dried toluene-d8 was added, and the mixture was
1H NMR (toluene-d8, 500 MHz, 223 K): δ −6.705 (ddd, JHP = 22,
JHP = 30 Hz, JHP = 110 Hz), −6.435 (dt, JHP = 27 Hz, JHP = 116 Hz),
−6.330 (ddd, JHP = 24 Hz, JHP = 30 H, JHP = 105 Hz). 31P{1H} NMR
(toluene-d8, 202 MHz, 223 K): δ 42.25 (d, JPP = 14 Hz), 40.9 (dd, JPP
= 14 Hz, JPP = 310 Hz), 38.65 (dd, JPP = 14 Hz, JPP = 302 Hz), 30.57
(dd, JPP = 14 Hz, JPP = 310 Hz), 25.65 (dd, JPP = 14 Hz, JPP = 302 Hz),
20.3 (t, JPP = 14 Hz), 14.4 (t, JPP = 14 Hz), 4.8 (t, JPP = 14 Hz). IR
(cm−1): 3053 (CH), 1919 (CO). MS (ESI, CH3CN): 1004.1 (100%,
[M − H − Cl]2+ − CO), 812.4 (76%, [M − H − Cl]2+ − PPh3 +
CH3CN), 1044.0 (48% [M − H − Cl + K]+ − CO).
RuHCl(CO)(PPh3)(Benzoxantphos). RuHCl(CO)(PPh3)3 (0.05
mmol, 47.6 mg) and Benzoxantphos (0.05 mmol, 29.3 mg) were
placed in a Wilmad-Young tube that was purged with Ar. To this was
added dry degassed toluene-d8 (0.5 mL), and the NMR tube was
closed. The mixture was heated to reflux for 3 h. After this time, the
mixture was cooled to room temperature and NMR spectra were
recorded. Crystals suitable for X-ray diffraction could not be grown. A
precipitate was observed after several days in the NMR tube.
1H NMR (toluene-d8, 500 MHz, 223 K): δ −7.42 (ddd, JHP = 24
Hz, 30 Hz, 110 Hz), −7.05 (dt, JHP = 27 Hz, JHP = 112.5 Hz), −6.82
(dt, JHP = 26.5 Hz, JHP = 119 Hz), −6.32 (dt, JHP = 24 Hz, JHP = 105
Hz). 31P{1H} NMR (toluene-d8, 202 MHz, 223 K): δ 40.93 (dd, JPP
=
14 Hz, JPP = 229 Hz), 39.25 (dd, JPP = 14 Hz, JPP = 310 Hz), 36.35
(dd, Jpp = 319 Hz), 34.3 (d, Jpp = 30 Hz), 34.1 (d, JPP = 30 Hz) 26.22
(dd, JPP = 14 Hz, JPP = 319 Hz), 21.75 (dd, JPP = 14 Hz, JPP = 310 Hz),
14.1 (broad s), 12.5 (broad s), 10.2 (broad s). IR (cm−1): 3053 (CH),
1917 (CO). MS (ESI, MeOH): 949.1 (100%, [M − H − Cl]2+ − CO)
978.8 (81%, [M − Cl]+).
1
heated to reflux for 3 h. H and 31P VT-NMR spectra were recorded,
and the solution was transferred under argon to a Schlenk tube. Via
slow diffusion of hexanes into the solution, crystals suitable for X-ray
diffraction were obtained.
1H NMR (toluene-d8, 500 MHz, 298 K): δ −7.09 (ddd, JHP = 18,
JHP = 31, JHP = 111 Hz). 31P{1H} (toluene-d8, 202 MHz, 223 K): δ
45.9 (d, JPP = 298 Hz), 27.4 (d, JPP = 298 Hz) 8.7 (d, JHP = 111 Hz).11
RuHCl(CO)(PPh3)(Thixantphos). RuHCl(CO)(PPh3)3 (0.278
mmol, 255.4 mg) and Thixantphos (0.278 mmol, 160 mg) were
placed in a Schlenk tube purged with Ar. To this was added dry
degassed toluene (10 mL), and the Schlenk tube was closed. The
mixture was stirred under reflux for 6 h. After this time, the reaction
mixture was cooled to room temperature and all solvent was
evaporated in vacuo. The resulting pale yellow powder was washed
with EtOH (3 × 10 mL). The remaining powder was dried in vacuo
and after that washed with hexanes (3 × 10 mL). All organic solvents
were removed, yielding a pale yellow powder. Crystals suitable for X-
ray diffraction could not be grown. Only an amorphous precipitate was
obtained via diffusion of hexane into the solution of the complex in
benzene (178 mg, 63%).
RuCl2(PPh3)(Xantphos). RuCl2(PPh3)4 (0.05 mmol, 47.9 mg) and
Xantphos (0.05 mmol, 28.8 mg) were placed in a Wilmad-Young tube
that was degassed and purged with argon. To this was added toluene-
d8 (0.5 mL), and the mixture was refluxed for 5 h. The solution was
cooled and crystals suitable for X-ray diffraction fell out of solution
(quantitative yield).
31P{1H} NMR (toluene-d8, 400 MHz, 298 K): JPP = 35.1 (d, JPP
=
31 Hz), 57.1 (t, JPP = 31 Hz). IR (cm−1): 3053 (CH), 740, 691 (arom
subst ring). MS (ESI, MeOH): 943.2 (100% [M + H − 2Cl]+), 974.7
(47%, [M − Cl + 2H]+).
RuCl2(PPh3)(Sixantphos). RuCl2(PPh3)3 (0.3 mmol, 285.7 mg) and
Sixantphos (0.35 mmol, 207.4 mg) were placed in a Schlenk tube that
was purged with Ar. To this was added dry degassed toluene (10 mL),
and the Schlenk tube was closed. The mixture was stirred under reflux
for 4 h. After this time, the reaction mixture was cooled to room
temperature and all solvent was evaporated in vacuo. The resulting
pale yellow powder was washed with EtOH (3 × 10 mL). The
1H NMR (toluene-d8, 500 MHz, 223 K): δ −5.88 (ddd, JHP = 26.5
Hz, JHP = 26.5 Hz, JHP = 114 Hz), −6.31 (ddd, JHP = 18.5 Hz, JHP
=
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Article
remaining powder was dried in vacuo and after that washed with
hexanes (3 × 10 mL). All organic solvents were removed, yielding an
off-white powder. Dark red crystals suitable for X-ray diffraction were
grown by slow diffusion of hexanes into a solution of the complex in
benzene (202 mg, 65.5%).
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31P{1H} NMR (toluene-d8, 400 MHz, 298 K): JPP = 36.9 (d, JPP
=
31 Hz), 58.0 (t, JPP = 31 Hz). IR (cm−1): 3051 (CH), 719, 702 (arom
subst ring). MS (ESI, CH3CN): 697.1 (100%, [M − H − Cl]2+ − CO
− PPh3), 773.6 (64%, [M − Cl]+ − PPh3 + CH3CN).
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental methods,
characterization and crystallographic data and analysis methods.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.V.: D.Vogt@ed.ac.uk.
■
Notes
(9) Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans.
1999, 0, 1519−1530.
The authors declare no competing financial interest.
(10) van der Veen, L. A.; Keeven, P. H.; Schoemaker, G. C.; Reek, J.
N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Lutz, M.; Spek, A.
L. Organometallics 2000, 19, 872−883.
ACKNOWLEDGMENTS
■
This research has been funded by The Netherlands Ministry of
Economic Affairs and The Netherlands Ministry of Education,
Culture, and Sciences within the framework of the CatchBio
program. We thank Ton Staring for his technical assistance.
(11) In a recent publication by Deutsch et al. it has been shown that
the employment of DPEPhos can be enhanced by using an isolated
complex and changing the solvent: Baumann, W.; Spannenberg, A.;
Pfeffer, J.; Haas, T.; Kockritz, A.; Martin, A.; Deutsch, J. Chem. Eur. J.
̈
2013, 19, 17702−17706.
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