NMR observation of trialkylphosphite-palladium(II) and ruthenium(II) complexes in supercritical carbon dioxide

Article abstract of DOI:10.1246/cl.2002.424

A supercritical fluid nuclear magnetic resonance (scNMR) spectroscopic method has been developed using a commercially available NMR spectrometer equipped with a high-pressure zirconia cell, revealing that trialkylphosphite ligands are highly effective for increasing the solubility of dichloropalladium(II) and dichlororuthenium(II) complexes in supercritical carbon dioxide (scCO₂).

Full text of DOI:10.1246/cl.2002.424

424
Chemistry Letters 2002
NMR Observation of Trialkylphosphite-Palladium(II) and Ruthenium(II) Complexes in
Supercritical Carbon Dioxide
Yoshihito Kayaki, Tomoyuki Suzuki, Yushi Noguchi, Satoshi Sakurai,y Mamoru Imanari,y and Takao IkariyaÃ
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology
and CREST, JST, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
yJEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558
(Received January 7, 2002; CL-020009)
A supercritical fluid nuclear magnetic resonance (scNMR)
spectroscopic method has been developed using a commercially
available NMR spectrometer equipped with a high-pressure
zirconia cell, revealing that trialkylphosphite ligands are highly
effective for increasing the solubility of dichloropalladium(II)
and dichlororuthenium(II) complexes in supercritical carbon
dioxide (scCO2).
Supercritical fluids (SCFs) have attracted much scientific and
industrial attention as reaction media because of their unique
properties including pressure tunable density, dielectric constant,
viscosity, and cage effect.1 For homogeneous catalysis in scCO2,
the solubility of molecular catalysts in the CO2 phase strongly
affects the outcome of the reactions, and hence much attention has
been paid to preparing CO2-soluble metal complexes for the
development of efficient catalysts.2 The solubility of the catalysts
as well as the phase behavior of the reaction system under
supercritical conditions is sometimes examined by a visual
inspection of the interior of a sapphire window-equipped reactor
vessel. However, the spectroscopic verification of the homo-
geneity of the reaction mixture is almost never used. This paper
presents the results of an NMR spectroscopic investigation of
trialkylphosphite coordinated Pd(II) and Ru(II) complexes under
scCO2 conditions and catalytic use of the Ru–phosphite system
for vinyl carbamate synthesis from terminal alkyne, diethyl-
amine, and scCO2.
Since the pioneering work on scNMR analysis was reported
by Rathke et al.,3 several research groups have used NMR method
to study transition metal complexes. On the basis of the narrowing
effect on quadrupole nuclei under supercritical conditions, they
investigated the chemical behavior of homogeneous cobalt
hydroformylation catalysts in scCO2 by using a toroid NMR
system4 as a high-pressure probe technique.5 However, to date,
the direct NMR observation of transition metal complexes in
SCFs has been limited because of a lack of more versatile high-
pressure NMR instruments for SCFs. Although capillary NMR
tube-cells6 have been used in commercially available NMR
systems, severe limitation on the detection of the small amount of
compounds or insensitive nuclei has still remained for practical
measurement. It has been reported that high pressure cells made
of sapphire,7 or ceramics8 can be used in commercial probes fitted
within without special modifications of the spectrometer.
In this study, we used a zirconia NMR cell modified for
routine operation under supercritical conditions as depicted in
Figure 1. The high pressure cell assembly consists of a non-
magnetic Ti–Al alloy valve and a ceramic tube having a 9.0mm
o.d., 5.4 mm i.d., and a 152 mm length. The valves can be attached
Figure 1. Schematic diagram of the NMR cell for SCF. (a)
Outside view. (b) Vertical sectional view.
to the top of the ceramic tube and fixed with Kalrez1
perfluoroelastomer O-rings. Pressurized fluids can be introduced
using a syringe pump (ISCO, model 260D) and monitored by a
pressure gauge integrated into a back-pressure regulator (JASCO,
880–81). The temperature is controlled by a variable temperature
(VT) spectrometer system. In order to minimize undesired
convection of the fluids mixture arising from temperature
differences between the top and the bottom of the cell, a sealed
Pyrex tube containing dimethylsulfoxide-d6 was placed into the
cell as shown in Figure 1(b). This deuterated reference enables
one to adjust the shim coil with the guidance of the deuterium lock
signal providing high resolution spectra. The assembly is
operated at pressures up to 20MPa in the NMR spectrometer
(JEOL, LA-300) in the non-spinning mode.9
We examined the solubility of the transition metal complexes
bearing a fluorinated phosphorus ligand, 2,2,2-trifluoro-ethyl-
phosphite, (P(OCH2CF3)3), because the incorporation of fluorous
groups into the ligands and/or counter ions is well known to
increase the solubility of metal complexes in scCO2.2 The
phosphite complexes, trans-[PdCl2{P(OCH2CF3)3g2] (1a) and
trans-[RuCl2{P(OCH2CF3)3g4] (2a) can be easily prepared by
ligand exchange reactions from known compounds.10;11
The 31P NMR spectrum of the Pd(II) complex 1a (16.0mg,
1:9 Â 10À2 mmol) in scCO2 (10.0 MPa, 50 ꢀC) was successfully
obtained as a sharp singlet signal at 96.4 ppm analogously to the
spectra in liquid solvents. Its 19F NMR spectrum displayed a
triplet coupled with the adjacent methylene protons at À78:0 ppm
(3JFH ¼ 7:0 Hz). Similarly, NMR spectra of 2a (20.0 mg,
1:3 Â 10À2 mmol) in scCO2 under the conditions of 15.0MPa
and 50 ꢀC showed a singlet at 129.2 ppm in 31P NMR and a triplet
3
at À77:7 ppm with JFH ¼ 7:7 Hz in 19F NMR. These results
indicated that the phosphite complexes 1a and 2a apparently
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
425
The results obtained here on the phosphite-coordinated
complexes 1 and 2 confirm that the use of phosphite ligands has
proven to be a practical ligand modification for preparing CO2-
soluble transition metal complexes and that scNMR methods are
now widely applicable to measuring the solubility and dynamic
behavior of catalysts in scCO2.
Dedicated to Professor Teruaki Mukaiyama on the occasion
of his 75th birthday.
References and Notes
1
a) P. G. Jessop, T. Ikariya, and R. Noyori, Chem. Rev., 99, 475 (1999). b)
‘‘Chemical Synthesis using Supercritical Fluids,’’ ed. by P. G. Jessop
and W. Leitner, VCH/Wiley, Weinheim (1999). c) R. S. Oakes, A. A.
Clifford, and C. M. Rayner, J. Chem. Soc., Perkin Trans. 1, 2001, 917.
a) D. Koch and W. Leitner, J. Am. Chem. Soc., 120, 13398 (1998). b) S.
Kaintz, A. Brinkmann, W. Leitner, and A. Pfaltz, J. Am. Chem. Soc.,
121, 6421 (1999). c) M. A. Carroll and A. B. Holmes, Chem. Commun.,
1998, 1395. d) C. A. G. Carter, R. T. Baker, S. P. Nolan, and W. Tumas,
Chem. Commun., 2000, 347.
2
Figure 2. 31P NMR spectrum (number of scans ¼ 256) of 2a and
2b in scCO2 (50 ꢀC, 15 MPa). Chemical shifts of the spectra were
refereced to an external 85% H3PO4 (ꢀ 0).
3
4
5
6
7
J. W. Rathke, R. J. Klingler, and T. R. Krause, Organometallics, 11, 585
(1992).
J. W. Rathke, R. J. Klingler, R. E. Gerald, II, K. W. Kramarz, and K.
Woelk, Prog. Nucl. Magn. Reson. Spectrosc., 30, 209 (1997).
B. T. Heaton, L. Strona, J. Jonas, T. Eguchi, and G. A. Hoffman, J.
Chem. Soc., Dalton Trans., 1982, 1159.
dissolved into the CO2 phase and that the complex 2a has two
chlorine atoms in a trans configuration.
The advantage of scNMR spectroscopy was further
confirmed by successfully measuring the 31P NMR spectrum
of
a
fluorine-free triethylphosphite complexes, trans-
[PdCl2{P(OC2H5)3g2] (1b)12 and trans-[RuCl2{P(OC2H5)3g4]
C. R. Yonker, T. S. Zemanian, S. L. Wallen, J. C. Linehan, and J. A.
Franz, J. Magn. Reson., A, 113, 102 (1995).
(2b).13 The Pd complex 1b was proved to be soluble in scCO2
ꢀ
(15.0MPa, 80 C), whereas a triphenylphosphine complex, trans-
[PdCl2{P(C6H5)3g2] gave no signal under similar conditions
a) S. Gaemers, H. Luyten, J. M. Ernsting, and C. J. Elsevier, Magn.
Reson. Chem., 37, 25 (1999). b) S. Bai, C. L. Mayne, R. J. Pugmire, and
D. M. Grant, Rev. Sci. Instrum., 67, 240(1995). c) D. C. Roe, J. Magn.
Reson., 63, 388 (1985).
because of its low solubility in scCO2.
Thanks to the high solubility of the phosphite complexes, we
tested the phosphite-Ru complex 2b on the catalytic reaction of
phenylacetylene with diethylamine and CO2.14 Dixneuf et al. has
reported that the reaction proceeds via a vinylidene–Ru
intermediate, which reacts with N; N-diethylcarbamate anion
(or N; N-diethylcarbamic acid) derived from the amine and CO2
or acetylene to give vinyl carbamates (3) and enynes (4),
respectively. In fact, the reaction with 2b in acetonitrile
proceeded with a poor selectivity as shown in Scheme 1.
However, changing the reaction medium to scCO2 caused a
significant improvement in the reaction rate and the yield of (Z)-
vinyl carbamate 3, possibly due to the increase in the concentra-
tion of the carbamate ion.15 The scNMR investigation on a
dynamic behavior of diethylamine in scCO2 revealed that
formation of N; N-diethylcarbamic acid was reversible and the
concentration of the carbamic acid was highly influenced by the
pressure and temperature.16
8
9
T. Aizawa, O. Sato, Y. Ikushima, K. Hatakeda, and N. Saito, Rev. High
Pressure Sci. Technol., 7, 1426 (1998).
The ceramic cell achieved comparable resolution on protons (about
1.5 Hz of half-line width for measurements on chloroform in acetone-d6
at 300 MHz) to a sapphire tube.
10Treatment of [PdCl 2(CH3CN)2] with 2 mol amount of P(OCH2CF3)3 in
THF at room temperature gave 1a in 76% yield. The Ru complex 2a was
prepared in 84% yield from the reaction of [RuCl2{P(C6H5)3g3] and the
phosphite ligand. Selected analytical data for 1a: 1H NMR (300 MHz,
CDCl3): ꢀ 4.65 (m); 19F NMR (283 MHz, CDCl3): ꢀ À75:8 (t,
3JFH ¼ 7:77 Hz); 13C NMR (75 MHz, CDCl3): ꢀ 65.0(vtq, JCP
¼
2
1
38:5 Hz, JCF ¼ 154:5 Hz, OCH2CF3), 121.8 (q, JCF ¼ 276:1 Hz,
OCH2CF3); 31P NMR (122 MHz, CDCl3): ꢀ 96.9 (s); mp 113–114 ꢀC;
Anal. Calcd for C12H12O6P2Cl2F18Pd: C, 17.29; H, 1.45%. Found: C,
17.26; H, 1.44%. 2a: 1H NMR (300 MHz, CD3OD): ꢀ 4.76 (q,
3
3JHF ¼ 8:3 Hz); 19F NMR (283 MHz, CD3OD): ꢀ À 77:0 (t, JFH
¼
8:1 Hz); 31P NMR (122 MHz, CD3OD): ꢀ 127.5 (s); Anal. Calcd for
24H24O12P4Cl2F36Ru: C, 19.42; H, 1.63%. Found: C, 19.77; H, 1.79%.
11 Y. Kayaki, Y. Noguchi, S. Iwasa, T. Ikariya, and R. Noyori, Chem.
C
Commun., 1999, 1235.
12 The 31P NMR spectrum of 1b (34.2 mg, 6:7 Â 10À2 mmol) showed a
singlet signal at 95.0ppm in scCO 2 (15.0MPa, 80 ꢀC).
13 The 31P NMR spectrum of 2b (12.0mg, 1:4 Â 10À2 mmol) displayed a
sharp singlet at 130.1 ppm in scCO2 (15.0MPa, 50 ꢀC).
´
14 R. Mahe, Y. Sasaki, C. Bruneau, and P. H. Dixneuf, J. Org. Chem., 54,
1518 (1989).
15 During the preparation of this manuscript, a similar result on the vinyl
carbamate formation using [RuCl2(C5H5N)4] and [RuCl2(ꢁ6-
C6H6){P(CH3)3g] as catalysts was reported. M. Rohr, C. Geyer, R.
Wandeler, M. S. Schneider, E. F. Murphy, and A. Baiker, Green Chem.,
3, 123 (2001).
16 Further details of the carbamic acid formation in scCO2 will be reported
elsewhere.
Scheme 1.