A new type of phosphaferrocene-Pyrrole-phosphaferrocene P-N-P pincer ligand

Article abstract of DOI:10.1021/om300061d

A 2-ethoxycarbonylphosphaferrocene was reduced to the corresponding 2-hydroxymethyl derivative, which was condensed with pyrrole in a 2:1 ratio in the presence of BF₃ to give a phosphaferrocene-pyrrole- phosphaferrocene pincer ligand. This tridentate ligand, in turn, reacted with [Rh(acac)(CO)₂] to yield a rhodium carbonyl pincer complex. This complex was characterized by X-ray crystal structure analysis and tested in the hydroformylation of internal olefins.

Full text of DOI:10.1021/om300061d

Note
pubs.acs.org/Organometallics
A New Type of Phosphaferrocene−Pyrrole−Phosphaferrocene P-N-P
Pincer Ligand
Rongqiang Tian, Yongxiang Ng, Rakesh Ganguly, and Franco̧ is Mathey*
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: A 2-ethoxycarbonylphosphaferrocene was re-
duced to the corresponding 2-hydroxymethyl derivative, which
was condensed with pyrrole in a 2:1 ratio in the presence of
BF3 to give a phosphaferrocene−pyrrole−phosphaferrocene
pincer ligand. This tridentate ligand, in turn, reacted with
[Rh(acac)(CO)2] to yield a rhodium carbonyl pincer complex. This complex was characterized by X-ray crystal structure analysis
and tested in the hydroformylation of internal olefins.
mixture readily reacts with [Rh(acac)(CO)2] to give the
rhodium carbonyl complex 4 as a rac + meso 1:1 mixture (δ31P
14.7 and 14.6 ppm in CDCl3) (eq 2).
INTRODUCTION
■
Pincer ligands are ubiquitous in recent organometallic
chemistry whenever good thermal stability or good rigidity of
the coordination sphere is required for catalytic applications of
transition metal complexes.1 A variety of complexing atoms
have been used in such structures but, to the best of our
knowledge, only a few examples involve electron-accepting sp2
phosphorus atoms incorporated in either phosphaalkenes,
phosphinines, or phosphaferrocenes.2−6 We have recently
described a simple approach to couple a pyrrole with a
phosphaferrocene that gave us access to a new type of
azacymantrene−phosphaferrocene chelates7 and to some
phosphaferrocene analogues of calixpyrroles.8 We wish to
report here the application of this technique to the preparation
of a new type of P-N-P pincer ligand.
The meso complex was crystallized and characterized by X-
ray crystal structure analysis (Figure 2). The great flexibility of
the pincer ligand prevents any significant distortion of the
square-planar geometry of rhodium with a P−Rh−P bite angle
of 173°. The pyrrole plane is tilted by 33.3° with respect to the
coordination plane of rhodium in order to accommodate the
structural requirements of the metal. The P−Rh bonds appear
to be shorter at 2.25 Å than the typical phosphinine−Rh bonds
in trans-[P2RhCl(CO)] at 2.27 Å.9 The two phenyl substituents
efficiently protect the metal, which lies in the center of a cradle.
This provides a good thermal stability for the complex. No
detectable decomposition of 4 was observed upon heating at 90
°C under 40 bar of H2 + CO. The ν(CO) stretching frequency
was observed at 1990.2 for rac 4 and 1988.6 cm−1 for meso 4 in
dichloromethane. In terms of acceptor properties, this means
that 3 lies between triphenylphosphine and aryl-substituted
phosphinines.9
RESULTS AND DISCUSSION
■
We reinvestigated the condensation of pyrrole with phospha-
ferrocenylmethyl alcohol (2) as described in our preceding
publication7 using a 2/pyrrole ratio of 2:1 (eq 1).
Since one patent describes without details the hydro-
formylation of olefins with rhodium−phosphaferrocene cata-
lysts,10 we decided to perform some tests on 4 (as the isomeric
mixture) with the poorly reactive internal olefins. The
hydroformylation experiments were first carried out on the
difficult case of (−)-α-pinene. They are reported in Table 1
after optimization of the reaction conditions.
The pincer ligand 3 was obtained in 53% overall yield from 1
as a rac + meso 1:1 mixture (δ31P −53.7 and −53.8 ppm in
CDCl3). We were able to crystallize the meso form and to get its
X-ray crystal structure analysis (Figure 1). The rac + meso
Received: January 25, 2012
Published: March 9, 2012
© 2012 American Chemical Society
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Organometallics
Note
Figure 1. X-ray crystal structure of the meso pincer 3. One molecule of hexane in the crystal is not shown. Main bond lengths (Å) and angles (deg):
C19−P1 1.784(4), P1−C14 1.771(4), C14−C13 1.515(5), C13−C12 1.496(5), C12−N1 1.366(4); C19−P1−C14 89.86(18), C12−N1−C12A
110.3(4).
Figure 2. X-ray crystal structure of the meso complex 4. Disordered hydrocarbon molecules in the crystal are not shown. Main bond lengths (Å) and
angles (deg): Rh1−C1 1.848(8), Rh1−N1 2.120(5), Rh1−P1 2.251(2), Rh1−P2 2.248(2), C1−O1 1.137(9); C1−Rh1−N1 177.7(3), C1−Rh1−P2
92.6(2), N1−Rh1−P2 86.19(16), C1−Rh1−P1 94.2(2), N1−Rh1−P1 87.07(16), P1−Rh1−P2 173.17(7).
Table 1. Hydroformylation of α-Pinene:
a
b
pinene (mmol)
L(3)/Rh (mmol/mmol)
P (bar)
T (°C)
time (h)
16
total yield (%)
product 5:6:7
3.15
3
75
120
49
21.8:4.3:1
a
b
1
Isolated yield. Ratio according to H NMR.
Whereas a 2,4,6-triarylphosphinine gives exclusively the
phosphite.11 Several other internal olefins were tested with 3
using the same experimental conditions. In most of the cases,
we got the least substituted aldehyde as the major product, and
the yield was better than with the phosphinine. For example,
2,3-dimethylbutene gives 3,4-dimethylpentanal in 69% yield vs
29% with the triarylphosphinine.9
internal aldehyde 6,9 our ligand 3 gives mostly the terminal
aldehyde 5. This probably reflects the very high steric hindrance
around rhodium and the medium donor properties of 3 that
favor the migration of the double bond from the endo- to the
exo-cyclic position. In terms of yields, 3 is, by far, better than
the phosphinine, but still significantly inferior to a highly bulky
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Organometallics
Note
C), 84.23 (s, Cp* C), 86.26 (t, J = 5.2 Hz, C), 89.89 (d, J = 4.2 Hz, C),
Since many applications of phosphaferrocene complexes in
catalysis have been reported,12 it is clear that the easily
accessible pincer 3 deserves further investigation.
105.91 (s, Py CH), 125.56 (s, Ph CH para), 127.73 (s, Ph CH), 129.72
3
(d, JCP = 3.4 Hz, Ph CH), 135.83 (t, JCP = 5.0 Hz C), 137.36, 37.42,
137.50 (t, C). HRMS: m/z calcd for C51H60Fe2NOP2103Rh (M + 2H)+
979.1904, found 979.1886. IR: ν(CO) = 1990.2 cm−1.
EXPERIMENTAL SECTION
■
ASSOCIATED CONTENT
* Supporting Information
All reactions were performed under nitrogen using solvents purified
and dried by standard methods. NMR spectra were obtained using
JEOL ECA 400, Bruker AV400, or Bruker AV300 spectrometers. MS
spectra were obtained in ESI mode on a Thermo Finnigan LCQ
DECA XP MAX. X-ray crystallographic analyses were performed on a
Bruker X8 APEX diffractometer. Phosphaferrocene 1 was prepared
according to the literature.7 Pyrrole was distilled before use. Other
reagents were commercially available and used without further
purification.
■
S
Complete hydroformylation data; X-ray crystal structure
analysis of compound 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fmathey@ntu.edu.sg.
■
Synthesis of Phosphaferrocenes 3a,b. Lithium aluminum
hydride (35 mg, 0.89 mmol) was added to a 5 mL THF solution of
1 (200 mg, 0.44 mmol) at −15 °C. The reaction mixture was slowly
warmed to room temperature and stirred for 2 h. Excess lithium
aluminum hydride was quenched with a small amount of ethyl acetate
and two drops of deionized water. The solvents were evaporated, and
the resulting precipitate was dissolved in dichloromethane (4 mL).
Pyrrole (15 μL, 0.22 mmol) was added to the solution and stirred at
room temperature for 5 min. BF3·OEt2 (0.1 mL, 0.77 mmol) was
added, and the reaction mixture was stirred for 10 min. Et3N was
added to quench the reaction, and the mixture was filtered via a silica
gel pad quickly and washed with dichloromethane. Purification was
performed via chromatography at −8 °C on silica using 2:1 hexane/
dichloromethane. An orange solid (100 mg, 0.118 mmol) was
obtained, and the yield was 53%. One of the isomers suitable for
NMR analysis was obtained by recrystallization. A crystal of 3 was
grown from a solution of the compound in dichloromethane/
methanol.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Ministry of Education in Singapore for
financial support of this work: grant MOE2009-T2-2-065.
■
REFERENCES
■
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CH2), 5.67 (d, 2H, Py CH), 7.10−7.14 (m, 2H, Ph), 7.19−7.23 (m,
4H, Ph), 7.37−7.39 (m, 4H, Ph), 7.65 (s, 1H, NH). 13C NMR
(CDCl3): δ 10.36 (s, Cp* Me), 11.89 (s, Me), 14.41 (s, Me), 26.89 (d,
2
2JCP = 21.7 Hz, CH2), 82.44 (s, Cp* C), 89.53 (d, JCP = 3.1 Hz, 
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1
CP), 96.94 (d, JCP = 53.8 Hz, CP), 104.84 (s, Py CH), 125.19 (s,
3
Ph CH para), 127.82 (s, Ph CH), 129.49 (d, JCP = 9.6 Hz, Ph CH),
2
130.42 (s, Py C), 140.58 (d, JCP = 17.6 Hz, Ph C ipso). HRMS: m/z
calcd for C50H60Fe2NP2 (M + H)+ 848.2900, found 848.2892.
Synthesis of Rhodium Complexes 4a,b. [Rh(acac)(CO)2] (37
mg) was added to a solution of 3a,b (110 mg) in toluene (5 mL). The
reaction mixture was stirred at room temperature for 2 h, and the
solvent was removed under vacuum. Purification was performed via
chromatography at −8 °C on silica using 15:1 hexane/ethyl acetate.
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fraction) and 4b (second fraction), and the total yield was 67% (86
mg). A crystal of 4a was grown from a solution of the compound in
dichloromethane/hexane.
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1
Complex 4a. 31P NMR (CDCl3): δ 14.63 (1JP−Rh = 173 Hz). H
NMR (CDCl3): δ 1.63 (s, 30H, CH3 Cp*), 2.14 (s, 6H, CH3), 2.27 (s,
6H, CH3), 3.37−3.53 (m, 4H, CH2), 5.70 (s, 2H, Py CH), 7.10−7.14
(t, 2H, Ph), 7.18−7.22 (t, 4H, Ph), 7.46−7.48 (d, 4H, Ph). 13C NMR
(CDCl3): δ 9.81 (s, Cp* Me), 11.08 (s, Me), 13.78 (s, Me), 28.10,
28.18, 28.27 (t, CH2), 80.82 (d, JCP = 4.0 Hz, C), 83.71 (d, JCP = 4.9
Hz, C), 84.30 (s, Cp* C), 86.26 (s, C), 90.08 (s, C), 106.08 (s, Py
CH), 125.52 (s, Ph CH para), 127.69 (s, Ph CH), 129.70 (t, 3JCP = 3.6
Hz, Ph CH), 135.65 (t, JCP = 4.8 Hz C), 137.50 (t, JCP = 6.4 Hz C).
HRMS: m/z calcd for C51H60Fe2NOP2103Rh (M + 2H)+ 979.1904,
found 979.1886. IR: ν(CO) = 1988.6 cm−1.
Klankermayer, J.; Ricard, L.; Seeboth, N.; Stankevic,
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̌
M. Chem.Eur.
1
Complex 4b. 31P NMR (CDCl3): δ 14.72 (1JP−Rh = 173 Hz). H
NMR (CDCl3): δ 1.70 (s, 30H, CH3 Cp*), 2.12 (s, 6H, CH3), 2.23 (s,
6H, CH3), 3.43−3.47 (t, 4H, CH2), 5.70 (s, 2H, Py CH), 7.10−7.14
(t, 2H, Ph), 7.19−7.23 (t, 4H, Ph), 7.42−7.47 (dd, 4H, Ph). 13C NMR
(CDCl3): δ 9.89 (s, Cp* Me), 11.08 (s, Me), 13.74 (s, Me), 28.24,
28.31, 28.40 (t, CH2), 81.07 (t, JCP = 5.2 Hz, C), 83.68 (t, J = 5.0 Hz,
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