Stable ionic Rh(I,II,III) complexes ligated by an imidazolium-substituted phosphine with π-acceptor character: Synthesis, characterization, and application to hydroformylation

Article abstract of DOI:10.1021/om400171t

The stable ionic Rh(I,II,III) complexes [Rh^I(acac)(CO)(L)] PF₆ (2), [Rh^II₂(OAc)₄(L) ₂]2PF₆ (3), and [Rh^IIICl₄(L) ₂]PF₆ (4) were synthesized through the complexation of Rh^I(acac)(CO)₂, Rh^II₂(OAc) ₄(H₂O)₂, and Rh^IIICl ₃·3H₂O with the phosphine-functionalized ionic liquid (FIL) 1 ([L]PF₆, L = 1-butyl-2-diphenylphosphino-3- methylimidazolium), respectively. The cation of L in 1 is an imidazolium-substituted phosphine with a positive charge vicinal to the P(III) atom, which acts as an electron-deficient donor with π-acceptor character to afford the stable complexes 2-4 due to the presence of retrodonating π-binding between Rh-P linkage. Due to the weakened reducing ability of L, the redox reaction between L and RhCl₃·3H₂O during the complexation is avoided, leading to the formation of 4, in which the Rh center is in the +3 valence state. Single-crystal X-ray analyses show that 2-4 are all composed of a Rh-centered cation and a PF₆^- counteranion. The cation of 2 possesses structural similarity to Rh ^I(acac)(CO)(PPh₃), the cation of 3 with a D_4h geometry possesses a structural similarity to Rh^II ₂(OAc)₄(PPh₃)₂, and the cation of 4 exhibits an ideal Rh^III-centered octahedral geometry, in which the Rh(III) (d⁶) ion is six-coordinated by four chlorine atoms in the equatorial plane and two L ligands in the axial positions. TG/DTG analyses indicated that the thermal stabilities of 2-4 in air flow were improved dramatically in comparison to the corresponding analogues Rh^I(acac) (CO)(PPh₃), Rh^II₂(OAc)₄(PPh ₃)₂, and Rh^ICl(PPh₃)₃. 2-4 were found to be good to excellent catalysts for homogeneous hydroformylation of 1-octene free of any auxiliary ligand; 3 was the best candidate. The on water effect in rate acceleration was evidently observed over 2 and 4 due to their insensitivity to moisture and oxygen.

Full text of DOI:10.1021/om400171t

Article
pubs.acs.org/Organometallics
Stable Ionic Rh(I,II,III) Complexes Ligated by an Imidazolium-
Substituted Phosphine with π‑Acceptor Character: Synthesis,
Characterization, and Application to Hydroformylation
Hongxing You, Yongyong Wang, Xiaoli Zhao, Shengjie Chen,* and Ye Liu*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, 3663 North Zhongshan Road,
200062 Shanghai, People’s Republic of China
S
* Supporting Information
ABSTRACT: The stable ionic Rh(I,II,III) complexes [RhI(acac)-
(CO)(L)]PF6 (2), [RhII (OAc)4(L)2]2PF6 (3), and [RhIIICl4(L)2]PF6
2
(4) were synthesized through the complexation of RhI(acac)(CO)2,
RhII (OAc)4(H2O)2, and RhIIICl3·3H2O with the phosphine-function-
2
alized ionic liquid (FIL) 1 ([L]PF6, L = 1-butyl-2-diphenylphosphino-
3-methylimidazolium), respectively. The cation of L in 1 is an
imidazolium-substituted phosphine with a positive charge vicinal to the
P(III) atom, which acts as an electron-deficient donor with π-acceptor
character to afford the stable complexes 2−4 due to the presence of
retrodonating π-binding between Rh−P linkage. Due to the weakened
reducing ability of L, the redox reaction between L and RhCl3·3H2O
during the complexation is avoided, leading to the formation of 4, in
which the Rh center is in the +3 valence state. Single-crystal X-ray
analyses show that 2−4 are all composed of a Rh-centered cation and a PF6− counteranion. The cation of 2 possesses structural
similarity to RhI(acac)(CO)(PPh3), the cation of 3 with a D4h geometry possesses a structural similarity to RhII (OAc)4(PPh3)2,
2
and the cation of 4 exhibits an ideal RhIII-centered octahedral geometry, in which the Rh(III) (d6) ion is six-coordinated by four
chlorine atoms in the equatorial plane and two L ligands in the axial positions. TG/DTG analyses indicated that the thermal
stabilities of 2−4 in air flow were improved dramatically in comparison to the corresponding analogues RhI(acac)(CO)(PPh3),
RhII (OAc)4(PPh3)2, and RhICl(PPh3)3. 2−4 were found to be good to excellent catalysts for homogeneous hydroformylation of
2
1-octene free of any auxiliary ligand; 3 was the best candidate. The “on water” effect in rate acceleration was evidently observed
over 2 and 4 due to their insensitivity to moisture and oxygen.
bonds of phosphites are unstable and sensitive to protic
cleavage.16 Although fluoroarylphosphines with inert P−C
linkages can be considered as alternatives, their structural
diversity is limited.22 The recently developed phosphine-FILs
with the positive charge vicinal to the P(III) atom have become
attractive candidates for mimicking the electron-deficient
ligands, due to their advantages of apparent robustness and
the vicinal electrostatic effect derived from the conjugation of a
positive charge with the coordinating P(III) atom.14−19,23 Such
electron-deficient phosphines are of great concern in
coordination chemistry and homogeneous catalysis,24 while
providing an essential difference in coordinating nature as well
as in catalytic performance.
In this paper, through complexation between the phosphine-
FIL of [L]PF6 (1, L = 1-butyl-2-diphenylphosphino-3-
methylimidazolium) and the corresponding Rh(I,II,III) pre-
cursors, three types of ionic Rh complexes (2−4) were
synthesized for the first time, in which the Rh center was in
the oxidative valence of +1, +2, and +3, respectively (Scheme
INTRODUCTION
■
Nonvolatile ionic liquids (ILs) have been recognized as
promising alternative solvents, which can dissolve transition-
metal complexes (homogeneous catalysts) and then prevent
their leaching and deactivation.1−3 On the other hand, ILs can
be functionalized flexibly by varying cations and/or anions that
constitute the IL structures with specific functionalities. Thus, it
is possible to develop abundant functionalized ILs (FILs) dually
possessing the characters of the incorporated functionalities as
well as those of the ILs.4−9 Phosphine-FILs have long been
investigated for the design of ionic organometalates and
application to homogeneous catalysis, in which the positive
charge in the imidazolium ring was either isolated in a position
remote from the P(III) atom10−13 or placed as a neighbor to
the P(III) atom, such as the amidiniophosphine ligands
reported in Canac and Chauvin’s work.14−19
In contrast to the electron-rich phosphine ligands as strong σ
donors,20 electron-deficient ligands remain less explored.
Typical examples of electron-deficient ligands are phosphites
and fluoroarylphosphines. The electron-withdrawing effects of
phosphites result from both σ(−I)-induction and π-retrodona-
tion into the π* orbitals of the P−O linkages.21 However, P−O
Received: March 1, 2013
Published: April 30, 2013
© 2013 American Chemical Society
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Scheme 1. Syntheses of the Ionic Complexes of 2-4 Ligated by the Phosphine-FIL of 1
1). In comparison with the Rh(I,II) analogues, there have been
relatively few reported examples of Rh(III) complexes
containing tertiary phosphines.25 Detailed crystallographic
data of 2−4 are given. The catalytic performance of 2−4
toward hydroformylation of 1-octene was investigated. L as an
electron-deficient ligand with a vicinal electrostatic effect
provides nonclassical coordinating behavior. Consequently,
the geometric configuration, stability, and catalytic performance
of the resultant ionic Rh complexes completely vary from those
of the corresponding PPh3-ligated Rh complexes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The complexation of
RhI(acac)(CO)2 with 1 led to the formation of the ionic
complex 2 in the yield of 67%, following the procedures for the
preparation of Rh(acac)(CO)(PPh3).26 2 was air and moisture
stable both in the solid state and in organic solvents for several
weeks under ambient conditions. Single crystals for X-ray
diffraction analysis were obtained through recrystallization in
CH2Cl2/acetone/n-hexane. The molecular structure depicted in
Figure 1 indicates that 2 is composed of the RhI complex cation
−
and PF6 anion. The structure of the complex cation is similar
to that of Rh(acac)(CO)(PPh3),26 which exhibits a typical
square-planar geometry. The Rh(I) (d8) center, lying at the
center of inversion, is coordinated by L, CO, and the chelated
2,4-pentanedionato ligand. The Rh−P bond distance (2.22 Å)
−
observed in 2 is shorter than those in the PPh3-ligated Rh(I)
complexes Rh(acac)(CO)(PPh3)26 and RhCl(PPh3)327,28 (Rh−
P = 2.23−2.4 Å; Table 1). The P1−Rh1−O1 and C25−Rh1−
O2 bond angles are 179°, and the C25−Rh1−P1 and O2−
Rh1−O1 bond angles are 90°.
Figure 1. Single-crystal structures of 1−4. The PF6 anion and the
solvent molecule are omitted for clarity.
Supporting Information). The 13C NMR spectra of 2 show that
the resonance for P-C in the imidazolium ring shifts to higher
field (138.2 ppm, d, J = 29 Hz) in comparison to that in 1
(142.7 ppm, d, J = 52 Hz), probably due to the π-retrodonated
d electron of the Rh(I) ion into the π* orbital of the P atom
and the resultant P-C conjugation effect. The 31P NMR spectra
of 2 show two types of resonances at 52.8 (doublet) and
−144.0 ppm (quintet), which are attributed to the coordinated
1
The H NMR, 13C NMR, and 31P NMR spectra further
1
support the assigned structure of 2. The H NMR spectra of 2
show that the resonance for o-H in the phenyl ring shifts to
lower field (8.10 ppm) in comparison to that in 1 (7.30 ppm),
due to the electron-withdrawing effect from the Rh+ ion. The
resonances for C(4)-H and C(5)-H in the imidazolium ring are
nearly the same for 1 and 2 (see Supplementary Table 2 in the
L and PF6 , espectively. In comparison to the 31P NMR signal
−
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Table 1. Comparison of the Rh−P Bond Distances and 31P
anion. The complex cation exhibits an ideal octahedral
geometry. The RhIII (d6) center, which is six-coordinated by
four chlorine atoms in the equatorial plane and two
imidazolium-tailed L groups in the axial positions, is situated
exactly in the center of an octahedron. The P1−Rh1−P1a and
Cl−Rh1−Cl1a bond angles are 180°, and the Cl−Rh1−P1 and
Cl1−Rh1−Cl2 bond angles are 89−91° (Table 1). The
identical Rh−P distances (2.38 Å) observed in 4 are in the
classical range, in comparison to those in the PPh3-ligated
Rh(I,III) complexes of Rh(acac)(CO)(PPh3),26 RhCl-
NMR Signals for the Various Rh Complexes
Rh complex
Rh−P bond
31P NMR of
phosphine (ppm)
distance (Å)
Rh(I)
[Rh(acac)(CO)(L)]
PF6 (2)
2.2218(8)
52.8
Rh(acac)(CO)
2.2418(9)
48.6
(PPh3)26
RhCl(PPh3)3 (red)27
2.214(4), 2.322(4)
, 2.334(4)
46.8, 29.828
none
30
(PPh3)3,27 and RhH2Cl(PPh3)3 (Table 1). The cation
a
[RhCl4(L)2]+ is typically a 18-electron configuration which is
thought to be stable. In addition, the highly symmetrical
octahedron further facilitates the stability of [RhCl4(L)2]+ in 4.
The 1H NMR, 13C NMR, and 31P NMR spectra also support
Rh(II)
[Rh2(OAc)4(L)2]
(PF6)2 (3)
2.4368(10), 2.43
29
a
Rh2(OAc)4(PPh3)2
2.4771(5), 2.45
none
[RhCl4(L)2]PF6 (4)
2.3776(15),
2.3776(15)
13.6 (br)
1
30
the assigned structure of 4. The H NMR spectra of 4 show
Rh(III) RhH2Cl(PPh3)3
2.302(1), 2.331(1)
, 2.458(1)
22.0, 40.1
that the resonance for o-H in the phenyl ring (8.70 ppm) shifts
to much lower field in comparison to those in 1 (7.49 ppm).
Similarly, due to the significantly intense electron-withdrawing
effect from Rh(III) ion on the P atom, in the 13C NMR spectra
of 4 all the resonances for the carbons closely related to the P
atom shift to lower field (147.3, s, P-C in imidazolium ring;
133.0 and 129.8, s, in NC(H)C(H)N+; 140.2, s, P-C in phenyl
ring), in comparison to those in 1 (142.7, d, J = 52 Hz, P-C in
imidazolium ring; 127.8 and 125.3, s, in NC(H)C(H)N+; 132.7,
d, J = 20 Hz, P-C in phenyl ring). In comparison to the 31P
NMR signals for the other phosphine-ligated Rh complexes as
given in Table 1, L in 4 shows a high-field chemical shift at 13.6
ppm, indicating the relatively electron rich character of the P
atom, which can be attributed to the π-retrodonation of a d-
electron of the Rh(III) ion and the consolidated electrostatic
interaction Rh3+−Pδ−, as well as the convenient octahedral
configuration. It has been known that, in the preparation of
RhCl(PPh3)3 through the complexation of RhCl3·3H2O with
PPh3, the reduction of Rh(III) to Rh(I) is accomplished by
PPh3 as a reducing reagent due to its strongly electron rich
nature.27,31 In contrast to PPh3, L is an electron-deficient ligand
with weak reductive ability. Consequently, the redox reaction
between L and RhCl3·3H2O in the course of complexation is
avoided, leading to the unchanged valence state for the Rh(III)-
centered complex of 4.
a
Rh−Rh bond distance.
of Rh(acac)(CO)(PPh3) (48.6 ppm) with structural similar-
ity,26 a relatively low-field chemical shift of 2 attributed to L
(52.8 ppm) is observed, which further indicates that L is in an
electron-deficient environment.
The complexation of RhII (OAc)4(H2O)2 with 1 led to the
2
formation of the ionic complex 3. Single crystals for X-ray
analysis were obtained through recrystallization in CH2Cl2/
ethanol/diethyl ether. The molecular structure depicted in
Figure 1 indicates that, in 3, the RhII complex dimer cation is
counteracted by two PF6− anions. The structure of the complex
cation is very similar to that of Rh2(OAc)4(PPh3)2,29 in which
the dirhodium nucleus (RhII, d7) with D4h symmetry is
coordinated by two imidazolium-tailed ligands of L in the
axial position and four acetate groups in the equatorial plane.
Each Rh(II) center is lying at the center of a square plane in a
six-coordinated octahedral geometry. The Rh1A−Rh1−P1 and
O4−Rh1−O1A bond angles are 175 and 89°, respectively. The
bond distances Rh1−P1 (2.44 Å) and Rh1−Rh1A (2.43 Å)
observed in 3 are both shorter than those in
29
Rh2(OAc)4(PPh3)2 (Rh−P, 2.48 Å; Rh−Rh, 2.45 Å; Table
1), indicating the enhanced dative interaction and the improved
stability of 3. The P1−Rh1−O1 and C25−Rh1−O2 bond
angles are 179°, and the C25−Rh1−P1 and O2−Rh1−O1
bond angles are 90°, which are within the standard deviations.
Since 3 is in a low-spin state with an indication of the
presence of one unpaired electron in the Rh(II) nucleus (d7), it
Table 1 summarizes the comparison data of 2−4 with the
PPh3-ligated Rh complexes in terms of the Rh−P distances and
the 31P NMR signals. In contrast to the corresponding Rh
analogues with the same valence state, the Rh−P distances of
the L-ligated Rh complexes 2−4 were universally shorter than
those of the PPh3-ligated complexes (2 vs RhI(acac)(CO)-
1
is endowed with a paramagnetic nature. Consequently, the H
NMR and 31P NMR signals attributed to L in the cationic
complex are broadened to flatness. Undoubtedly, if 3 could be
measured by EPR spectroscopy, the signal for the g factor (ge)
for the free electron would be observed. Unfortunately, the
instrument for EPR spectroscopy is not available in our
university, and the measurement cannot be conducted at the
present stage. However, the 31P NMR spectrum still shows the
signal of the PF6− counteranion at −144.0 ppm, consistent with
its solid-state structure, due to the negligible influence of the
(PPh3) and RhICl(PPh3)3; 3 vs RhII (OAc)4(PPh3)2; 4 vs
2
RhIIIH2Cl(PPh3)3), indicating an enhanced interaction for the
Rh−P linkages due to the π-retrodonation of d-electrons of the
Rh ion to the cationic phosphorus fragment.
In comparison to PPh3 as a typical electron-rich donor, L can
mimic the behavior of the electron-deficient ligand phosphite
with the electron-withdrawing effect, which possesses both the
σ-donor and π-acceptor characters as a result of σ-induction
and π-retrodonation.21 Consequently, the Rh−P linkages in 2−
4 are all strengthened and the stabilities of 2−4 were improved
universally. In addition to π-retrodonation, the vicinal electro-
static effect derived from the conjugation of the positively
charged imidazolium ring with the coordinating P(III) atom
may be another reason to afford the stable complexes 2−4.14
On the other hand, the weak reductive ability of L can avoid the
possible redox reaction between the metal center and L,
−
Rh(II) magnetic moments on the PF6 counteranion.
In comparison with the phosphine-ligated Rh(I,II) analogues,
the Rh(III) complexes containing tertiary phosphines such as
RhH2Cl(PPh3)330 have been rarely reported. 4, in which the Rh
ion is in the +3 valence state, was obtained herein as an orange-
red solid through simple complexation of RhCl3·3H2O with 1.
The molecular structure of 4 (Figure 1) indicates that 4 is
composed of the Rh complex cation [RhCl4(L)2]+ and a PF6
−
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Figure 2. TG/DTG analyses of the phosphine-ligated Rh complexes in an air flow.
a
Table 2. Hydroformylation of 1-Octene Catalyzed by Different Rh Complexes without the Presence of Any Auxiliary Ligand
b
sel (%)
b
c
entry
cat.
additive
conversn (%)
ald. (n + iso)
iso-octenes
L/B
TONald
1
2
2
3
3
4
4
69
98
97
96
45
97
99
81
98
97
87
80
83
97
93
92
86
82
96
83
98
99
85
82
17
3
0.7
0.7
1.0
1.3
2.1
2.2
0.8
0.9
1.1
0.8
0.9
0.8
1140
1900
1800
1770
770
2
H2O
H2O
H2O
H2O
H2O
H2O
3
7
4
8
5
14
16
4
6
1610
1910
1340
1910
1930
1050
1140
7
Rh(acaca)(CO)(PPh3)
Rh(acaca)(CO)(PPh3)
Rh2(OAc)4(PPh3)2
Rh2(OAc)4(PPh3)2
RhCl(PPh3)3
8
17
2
9
10
11
12
1
15
18
RhCl(PPh3)3
a
Conditions: Rh 0.05 mol % (0.01 mmol), H2O 1 mL, 1-octene 20.0 mmol, CO/H2 (1/1) 4.0 MPa, temperature 120 °C, reaction time 2 h.
b
c
Determined by GC. L/B, the ratio of linear nonanals to branched nonanals.
especially when the metal center is in a highly oxidative valence
state such as Rh3+, which facilitates the utilization efficiency of
the ligand (L) and the coordination diversity.
Catalytic Performance of 2−4 for Hydroformylation
of 1-Octene. Supposedly, the differences in the phosphine-
ligated Rh complexes, in terms of the coordinating geometry,
the valence state of the Rh center, and the thermal stability,
could lead to various catalytic behaviors. In this part, the
catalytic performance of 2−4 toward the hydroformylation of
1-octene was investigated in comparison to that of the
corresponding Rh(I,II) analogues. It has been reported that a
substantial rate acceleration could be obtained when the
insoluble reactants were stirred in aqueous suspension, which
was denoted as a remarkable “on water” phenomenon by
Sharpless32 and other researchers.33 A similar “on water” effect
has also been observed in Rh-catalyzed hydroformylations
when water is compatible with the specific phosphine-modified
Rh systems without a negative effect on the catalyst stability
and selectivity.34,35 The insensitivity to moisture and oxygen of
2−4 motivated us to investigate the “on water” effect for
On the other hand, 1 with an electron-deficient nature is
more robust against oxidative degradation due to the intense
delocalization of the lone-pair electrons in the P atom to the
adjacent positive imidazolium ring, leading to insensitivity to
moisture and oxygen under ambient conditions. Consequently,
the L-ligated complexes 2−4 are also moisture and oxygen
insensitive under ambient conditions and possess improved
thermal decomposition temperatures. The TG/DTG analyses
in an air flow (Figure 2) showed that initial thermal
decompositions temperatures of 3 and 4 were up to 180 °C,
which were much higher than those of Rh2(OAc)4(PPh3)2 (150
°C) and RhCl(PPh3)3 (115 °C); the initial thermal
decomposition temperature of 2 (180 °C) was comparable to
that of Rh(acac)(CO)(PPh3) (185 °C).
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hydroformylation over 2−4, with the additional advantage of
easy manipulations in open air.
EXPERIMENTAL SECTION
■
Reagents and Analysis. All synthesis experiments were carried
out under a nitrogen atmosphere using standard Schlenk techniques.
All reagents were purchased from Aladin Reagent Co. in Shanghai,
People’s Republic of China, and used as received. The FT-IR spectra
Free of any auxiliary phosphine and water, when 2−4 were
applied as the catalysts in parallel under the optimal
temperature (120 °C) in Table 2, the best activity with a
TON value of 1800 was observed for 3 but with a poor L/B
ratio of 1.0 (entry 3), which was competitive with those for
Rh(acac)(CO)(PPh3) and Rh2(OAc)4(PPh3)2 (entries 7 and
9). Reasonably, the less stable complex 3 exhibited better
catalytic performance in comparison to 2 and 4 (entry 3 vs
entries 1 and 5). The relatively more stable 2 and 4 resulted in
difficult dissociation of the ligand to accommodate insertion of
1-octene for hydroformylation. Under the same conditions, the
addition of H2O (optimized to be 1 mL) spurred the catalytic
activities of 2 and 4 greatly, resulting in the high TONs of 1900
for 2 and 1610 for 4 without an influence on L/B ratio (entry 2
vs 1; entriy 6 vs 5). In contrast, when the neutral analogues
Rh(acac)(CO)(PPh3) and RhCl(PPh3)3 were used as the
catalysts, the addition of H2O deteriorated the catalytic activity
and the selectivity to nonanals obviously (entry 8 vs 7; entry 12
vs 11). These results suggested that water as an auxiliary
additive in a suitable amount indeed plays a significant role in
promoting the catalytic behaviors of 2 and 4 as the defined “on-
water” effect, while the catalysts were well compatible with
water. Nevertheless, under the applied conditions, the “on-
water” effect was not evidently observed for 3 and
Rh2(OAc)4(PPh3)2, which were both robust against water
and could exhibit remarkable activities even free of any additive.
However, the hydroformylations for 2 and 4 were not efficient
with water as the solvent (>2 mL). Because the Rh catalysts
and 1-octene were poorly soluble in water, the surrounding of
the hydrophobic catalyst and substrate by a large amount of
water limited the accessibility of the Rh catalyst to 1-octene,
resulting in the depressed activation of 1-octene.
1
were recorded on a Nicolet NEXUS 670 spectrometer. The H and
31P NMR spectra were recorded on a Bruker Avance 500
spectrometer. The 31P NMR spectra were referenced to 85% H3PO4
sealed in a capillary tube as an internal standard. Elemental analyses for
CHN were obtained using an Elementar Vario EL III instrument. TG/
DTG analysis was performed using a Mettler TGA/SDTA 851e
instrument and STARe thermal analysis data processing system.
TG/DTG analysis was run in an air flow with a temperature ramp of
10 °C min−1 between 50 and 700 °C. GC analysis was performed on a
Shimadzu-2014 instrument equipped with a Rtx-Wax capillary column
(30 m × 0.25 mm × 0.25 μm).
Synthesis. 1-Butyl-2-diphenylphosphino-3-methylimidazolium
Hexafluorophosphate ([L]PF6, 1). 1 (L = 1-butyl-2-diphenylphosphi-
no-3-methylimidazolium) was prepared according to the reported
methods23 with some modification. Under a nitrogen atmosphere, a
solution of 1-butyl-3-methylimidazolium hexafluorophosphate (15.0 g,
53.0 mmol) in 50 mL of dry CH2Cl2 (refluxed with calcium hydride
and distilled freshly before use) was cooled to −78 °C, and then 27
mL of n-BuLi (2.2 M, in hexane, 59.4 mmol) was added dropwise.
After the mixture was stirred for 1 h, chlorodiphenylphosphine
(PPh2Cl, 11.7 g, 53.0 mmol) was added dropwise. The resultant
mixture was stirred overnight while the reaction mixture was warmed
to room temperature. After quenching excess n-BuLi with deionized
water, the obtained oily mixture was stripped of solvent in vacuo. The
residue was recrystallized from CH2Cl2/EtOH to yield 1 as white
1
solids (18.6 g; yield, 75 wt %). H NMR (δ, ppm, CDCl3): 7.62 (s,
1H, NC(H)C(H)N+), 7.59 (s, 1H, NC(H)C(H)N+), 7.49 (m, 6H,
p-,m-H, PPh2), 7.30 (m, 4H, o-H, PPh2), 4.26 (t, 2H, J = 7.5 Hz,
CH2CH2CH2CH3), 3.48 (s, 3H, NCH3), 1.56 (m, 2H,
CH2CH2CH2CH3), 1.15 (m, 2H, CH2CH2CH2CH3), 0.78 (t, J =
7.5 Hz, 3H, CH2CH2CH2CH3). 31P NMR (δ, ppm, DMSO-d6): −28.6
−
(s, PPh2), −143.5 (quint, PF6 ). 31C NMR (δ, ppm, CDCl3): 142.7 (d,
NCN+, J = 52.0 Hz), 132.7 (d, PC, JCP = 20.0 Hz), 131.0 (s, CHp‑Ar),
130.1 (d, J = 7.0 Hz, CHo‑Ar), 127.8 (s, NC(H)C(H)N+), 127.6 (d, J =
7.0, CHm‑Ar), 125.3 (s, NC(H)C(H)N+), 50.5 (s, CH2CH2CH2CH3),
37.8 (s, CH3), 32.7 (s, CH2 CH2 CH2CH3 ), 19.3 (s,
CH2CH2CH2CH3), 13.3 (s, CH2CH2CH2CH3).
CONCLUSION
■
(1-Butyl-2-diphenylphosphino-3-methylimidazolium)carbonyl-
(2,4-pentanedionato)rhodium Hexafluorophosphate ([Rh(acac)-
(CO)(L)]PF6, 2). Under a nitrogen atmosphere, [Rh(acac)(CO)2]
(commercial, 54 mg, 0.21 mmol) dissolved in dry CH2Cl2 (5 mL) was
added to the ligand 1 (94 mg, 0.2 mmol) and the mixture then
refluxed for 4 h. During refluxing, the reaction solution changed from
green-yellow to yellow rapidly, accompanied by the gas (CO) release.
When the solution was cooled to room temperature, a clear solution
was obtained upon filtration, to which diethyl ether was added to
precipitate yellow solids. The yellow solids were collected after drying
under vacuum in the yield of 67% (98 mg). A sample suitable for X-ray
diffraction analysis was obtained by recrystallization from CH2Cl2/
ethanol/n-hexane. FT-IR (KBr pellet): 3737 (w), 3447 (w), 2960 (s),
1620 (w), 1395 (m), 1260 (m), 841 cm−1 (P−F, s). 1H NMR (δ, ppm,
CDCl3): 8.10 (m, 4H, o-H, PPh2)), 7.61 (m, 6H, p-,m-H, PPh2), 7.54
(s, 1H, NC(H)C(H)N+), 7.49 (s, 1H, NC(H)C(H)N+), 5.53 (m, 1H,
The ionic Rh(I,II,III) complexes 2−4, which were ligated by
the phosphine-FIL 1 as an electron-deficient donor with π-
acceptor character, were synthesized for the first time and fully
characterized. Single-crystal X-ray analyses show that 2 is
composed of the RhI-centered square-planar cation and one
−
PF6 anion, whose complex-cation possesses the structural
similarity to Rh(acac)(CO)(PPh3), that 3 is composed of a
−
dinuclear RhII-centered cation with D4h geometry and two PF6
anions, whose complex cation possesses a structural similarity
to Rh2(OAc)4(PPh3)2, and that 4 is composed of a highly
−
symmetrical RhIII-centered octahedral cation and one PF6
anion. The TG/DTG analyses indicated that the thermal
stabilities of 2−4 in an air flow were improved markedly in
comparison to the corresponding analogues Rh(acac)(CO)-
(PPh3), Rh2(OAc)4(PPh3)2, and RhCl(PPh3)3. When the
moisture- and oxygen-insensitive 2−4 were used as the catalysts
for homogeneous hydroformylation of 1-octene free of any
auxiliary ligand, 3 exhibited the best catalytic performance,
which was competitive with those of Rh(acac)(CO)(PPh3) and
Rh2(OAc)4(PPh3)2. The “on water” effect in rate acceleration
was observed for 2 and 4, obviously due to their compatibility
with H2O. However, the “on-water” effect was negligible in the
case of 3, which exhibited excellent activity regardless of the
presence or absence of water.
OC(CH3)CH(CH3)CO), 3.67 (t, 2H,
J = 8.0 Hz,
CH2CH2CH2CH3), 3.37 (s, 3H, NCH3), 2.17 (s, 3H, trans to P,
OC(CH3)CH(CH3)CO), 1.55 (s, 3H, OC(CH3)CH(CH3)-
CO), 1.50 (m, 2H, CH2CH2CH2CH3), 0.95 (m, 2H,
CH2CH2CH2CH3), 0.72 (t, 3H, J = 7.3 Hz, CH2CH2CH2CH3). 31P
NMR (δ, ppm, CDCl3): 52.8 (d, JP−Rh = 450 Hz, PPh2), −144.0
−
(quint, PF6 ). 31C NMR (δ, ppm, CDCl3): 188.9 (s, COacac), 187.8 (d-
d, JCRh = 73.5 Hz, JCP = 25.0 Hz, CO), 186.9 (s, COacac), 138.2 (d,
NCN+, JCP = 29.0 Hz), 135.1 (d, PC, JCP = 14.0 Hz), 133.2 (d, JCP
=
3.0 Hz, CHp‑Ar), 130.0 (d, J = 13.0 Hz, CHo‑Ar), 127.5 (d, J = 8.0,
CHm‑Ar), 126.9 (s, NC(H)C(H)N+), 125.5 (s, NC(H)C(H)N+), 50.5
(s, CH2CH2CH2CH3), 37.8 (s, CH3), 32.7 (s, CH2CH2CH2CH3), 19.3
2702
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Organometallics
Article
(s, CH2CH2CH2CH3), 13.3 (s, CH2CH2CH2CH3). Anal. Found: C,
44.27; H, 4.45; N, 3.95. Calcd: C, 44.72; H, 4.47; N, 4.01.
ASSOCIATED CONTENT
* Supporting Information
■
S
Bis(1-butyl-2-diphenylphosphino-3-methylimidazolium)-
tetraacetatodirhodium(II) hexafluorophosphate ([Rh2(OAc)4(L)2]-
(PF6)2, 3). A solution of Rh2(OAc)4(H2O)2 (commercial, 96 mg, 0.2
mmol) in 5 mL of ethanol was treated with 1 (234 mg, 0.5 mmol) and
then refluxed for 4 h with vigorous stirring. Upon completion, the
reaction solution changed from green to blood-red while red solids
precipitated at ambient temperature. The red precipitates were
collected after washing with ethanol and drying under vacuum, in
the yield of 80% (220 mg). The sample suitable for X-ray diffraction
analysis was obtained by recrystallization from CH2Cl2/ethanol/
diethyl ether. FT-IR (KBr pellet): 3737 (w), 3447 (w), 2960 (s), 1620
(w), 1395 (m), 1260 (m), 841 cm−1 (P−F, s). Due to the
paramagnetic nature of the Rh(II) center complex cation, the 1H
NMR and 31P NMR signals of 3 attributed to ligand L were broadened
Tables giving crystal data and selected NMR data and CIF files
giving crystallographic data for 1−4. This material is available
free of charge via the Internet at http://pubs.acs.org. The files
CCDC-923401, CCDC-923402, CCDC-923403, and CCDC-
919910 also contain supplementary crystallographic data for 1−
4. These data can be obtained free of charge from the
Cambridge Crystallographic Database.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yliu@chem.ecnu.edu.cn (Y.L.); chenshengjieno1@
sina.com (S.C.). Fax: +86 21 62233424. Tel: +86 21 62232078.
■
Notes
−
to flatness. However the 31P NMR signal attributed to the PF6
The authors declare no competing financial interest.
counteranion was observed clearly at −143.5 ppm (sept, δ, acetone-
d6). Anal. Found: C, 46.28; H, 4.52; N, 5.91. Calcd: C, 46.35; H, 4.67;
N, 5.41.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21273077 and 21076083),
973 Program from Ministry of Science and Technology of
China (2011CB201403), the Ministry of Science and
Technology of China for Croatian-Chinese Scientific and
Technological Cooperation, and the Fundamental Research
Funds for the Central Universities.
Bis(1-butyl-2-diphenylphosphino-3-methylimidazolium)-
tetrachloridorhodium(III) Hexafluorophosphate ([RhCl4(L)2]PF6, 4).
Under a nitrogen atmosphere, the ligand 1 (710 mg, 1.5 mmol)
dissolved in 10 mL of methanol at 45 °C was treated with
RhCl3·3H2O (80 mg, 0.3 mmol) and tetrabutylammonium chloride
(Bu4NCl, 47 mg, 0.17 mmol). The resultant mixture was refluxed for
12 h with vigorous stirring. After the reaction mixture was cooled to
room temperature, orange-red precipitates were collected after
washing with methanol and ethyl ether and drying under vacuum to
give the product 4 (292 mg; yield, 94%). A sample suitable for X-ray
diffraction analysis was obtained by recrystallization from acetone/n-
hexane. FT-IR (KBr pellet): 3435 (w), 3100 (w), 2961 (m), 2930 (w),
2873 (w), 1622 (w), 1570 (w), 1483 (m), 1461 (w), 1435 (s), 1382
(w), 1086 (m), 833 cm−1 (P−F, s), 750 (s), 698 (s), 558 (s). 1H NMR
(δ, ppm, acetone-d6): 8.70 (m, 4 o-H, PPh2), 8.05 (s, 1H,
NC(H)C(H)N+), 7.96 (s, 1H, NC(H)C(H)N+), 7.52 (m, 2 p-H,
PPh2), 7.46 (m, 4 m-H, PPh2), 3.98 (m, br, 2H, CH2CH2CH2CH3),
3.78 (s, 3H), 1.90 (s, br, CH2CH2CH2CH3), 1.05 (s, br,
CH2CH2CH2CH3), 0.71 (t, 3H, J = 7.0 Hz, CH2CH2CH2CH3). 31P
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