A new P,N-chelating ligand combining phosphaferrocene and azacymantrene units

Article abstract of DOI:10.1021/om101160w

A 2-(ethoxycarbonyl)ferrocene is transformed into a 2-(pyrrolylmethyl) phosphaferrocene, whose reaction with Mn₂(CO)₁₀ yields a 2-(azacymantrenylmethyl)phosphaferrocene. This new ligand gives a chelate with Mo(CO)₄ whose structure shows that molybdenum is displaced from the P-lone pair axis by 16.5°, indicating that the lone pair is less directional than the N lone pair. It appears that P behaves as a stronger donor than does N in this case.

Full text of DOI:10.1021/om101160w

NOTE
pubs.acs.org/Organometallics
A New P,N-Chelating Ligand Combining Phosphaferrocene and
Azacymantrene Units
Rongqiang Tian, Aholibama Escobar, and Franc-ois Mathey*
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S Supporting Information
b
ABSTRACT: A 2-(ethoxycarbonyl)ferrocene is transformed
into a 2-(pyrrolylmethyl)phosphaferrocene, whose reaction
with Mn2(CO)10 yields a 2-(azacymantrenylmethyl)phos-
phaferrocene. This new ligand gives a chelate with Mo(CO)4
whose structure shows that molybdenum is displaced from
the P-lone pair axis by 16.5°, indicating that the lone pair is less directional than the N lone pair. It appears that P behaves as a
stronger donor than does N in this case.
All our attempts at synthesizing η5-pyrrolyl complexes
,N-chelating ligands derived from phosphaferrocenes have
proven their worth in asymmetric catalysis since the work of
P
through the pyrrolide ion failed, probably because the phospha-
ferrocene unit is poorly stable in nucleophilic media. We were
finally successful when using the direct reaction of 3 with
manganese carbonyl (eq 2).
Fu on phosphaferrocene-oxazolines.1,2 Other nitrogen hetero-
cycles such as pyridines, pyrazoles, and imidazoles3-6 have also
been combined with phosphaferrocenes in such chelating sys-
tems. In this report, we wish to describe and structurally
characterize another member of this family based on phospha-
ferrocenes and azacymantrenes. The use of azacymantrenes
offers additional possibilities in asymmetric catalysis resulting
from the combination of their own planar chirality with that of
phosphaferrocenes. These possibilities will be investigated later.
’ RESULTS AND DISCUSSION
We started from the functional phosphaferrocene 1, synthesized
as shown in Scheme 1. The phenyl substitution is needed for
improving the stability of the system. The phospholide ions resulted
from the successive [1,5]-shifts of phenyl and ethoxycarbonyl
substituents as described in the literature.7 Phosphaferrocene 1
was then converted into its pyrrolylmethyl derivative 3, as shown in
eq 1. The yield of the conversion of 1 into 3 was satisfactory (61%).
The pyrrole complexation mainly affords a major product
(δ(31P) -54.2 ppm) but a minor isomer (δ(31P) -54.7 ppm) is
also formed (major/minor ratio 94/6). This is, no doubt, a con-
sequence of the steric repulsion between the two bulky FeCp*
and Mn(CO)3 complexing groups. Azacymantrenylmethyl-
phosphaferrocene 4 easily gives the molybdenum chelate 5,
which was characterized by X-ray crystal structure analysis
(Figure 1). This structure gave us some information on the
ligating properties of 4. There is some strain in the chelate ring, as
indicated by the reduction of the tetrahedral angle at the CH2
bridge down to 106.5° and the decrease of the P-Mo-N angle
from 90° to 80.5°. This strain is accommodated by the deviation
of the P-Mo bond from the P-lone-pair axis by 16.5° and the
N-Mo bond from the N-lone-pair axis by only 3.5°. This result
Received: December 10, 2010
Published: February 16, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om101160w Organometallics 2011, 30, 1738–1740
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Organometallics
NOTE
Scheme 1. Synthesis of the Starting Phosphaferrocene
’ EXPERIMENTAL SECTION
All reactions were performed under nitrogen using solvents purified
and dried by standard standards. Nuclear magnetic resonance spectra
were obtained using a JEOL ECA 400 or Bruker AV400 spectrometer
operating at 400 MHz for 1H, 100.56 MHz for 13C, and 161.89 MHz for
31P. Chemical shifts are expressed in parts per million downfield from
internal TMS (1H and 13C) and external 85% H3PO4 (31P). All coupling
constants (J values) are reported in hertz (Hz). MS spectra were obtained
in the ESI mode on a Thermo Finnigan LCQ DECA XP MAX. X-ray
crystallographic analyses were performed on a Bruker X8 APEX diffract-
ometer. 3,4-Dimethyl-1-phenylphosphole was prepared according to the
literature.12 Pyrrole was distilled before use. Other reagents were com-
mercially available and were used without further purification.
Synthesis of Phosphaferrocene 1. A mixture of 3,4-dimethyl-
1-phenylphosphole (2 g, 10.6 mmol) and t-BuOK (1.44 g, 12.7 mmol) in
THF (10 mL) were stirred in a pressure tube for 14 h at 140 °C (31P
NMR: δ þ70.1 ppm). The brown solution was transferred to a two-neck
round-bottom flask, and ethyl chloroformate (1.4 g, 12.7 mmol.) was
added slowly to the solution at -10 °C. The reaction mixture was stirred
at room temperature for 20 min and then heated to 60 °C for 1 h. t-
BuOK (1.49 g, 13.3 mmol) was added to the mixture at 0 °C, and the
mixture was stirred at room temperature for 3 h (31P NMR: δ þ106.9
ppm). ZnCl2 (1.81 g, 13.3 mmol) was added to the phospholide solution
at 0 °C. The mixture was stirred at 0 °C for 5 min and then at room
temperature for 30 min. In a separate flask, Cp*Li was prepared by
adding n-BuLi (5.2 mL, 8.2 mmol.) to 1,2,3,4,5-pentamethylcyclopen-
tadiene (1.25 mL, 8 mmol) in dry THF at -15 °C and stirred at -15 °C
for 5 min and then at room temperature for 30 min. The suspension
solution of Cp*Li was then added quickly to the FeCl2 solution in THF
at -15 °C. The mixture was stirred at room temperature for 30 min to
obtain [Cp*FeCl]n. The phospholide/ZnCl2 solution was then added
dropwise to the dark green [Cp*FeCl]n solution at -78 °C, and the
resulting mixture was slowly warmed to room temperature and stirred at
room temperature for 20 h. Purification was performed via cold column
chromatography on silica using 2/1 hexane/dichloromethane. The
orange-red band collected gives a red solid (2.44 g, 5.42 mmol), and
the yield was 51% from 3,4-dimethyl-1-phenylphosphole.
Figure 1. X-ray crystal structure of chelate 5. Selected bond lengths (Å)
and angles (deg): P1-Mo1 = 2.5143(5), N1-Mo1 = 2.3210(18),
Mo1-C32 = 1.976(2), C32-O5 = 1.158(3), Mo1-C33 = 1.964(2),
C33-O6 = 1.156(3); P1-Mo1-N1 = 80.48(4), C17-P1-C20 =
91.49(9), C24-N1-C27 = 105.85(17), C20-C23-C24 = 106.51(16).
clearly shows that the P-lone-pair axis is much less directional
than the N-lone-pair axis. A similar finding has been very
recently reported in the literature for chelates derived from a
2-(2-pyridyl)phosphinine.8 Also noteworthy is the fact that N is
P
planar ( (angles at N) = 358.1°), whereas P is slightly pyra-
P
midal ( (angles at P) = 352.2°). As a general rule, η5-phospholyl
complexes are weaker donors and better acceptors than the
corresponding η5-pyrrolyl complexes. However, it is known that
the ancillary ligands play a major role in the nucleophilicity of
these complexes. For example, azacymantrene is protonated at
manganese,9 whereas, upon replacement of one CO by one
triphenylphosphine, the protonation takes place at nitrogen.10 In
our case, using the lengthening of the Mo-trans-CO bond as a
criterion, it seems clear that P and N have comparable donating
properties (1.976(2) vs 1.964(2) Å) and this is probably due to
the presence of the Cp* ligand.
1
31P NMR (CDCl3): δ -39.5. H NMR (CDCl3): δ 1.27 (t, 3H,
3JHH = 7.2 Hz, CH2CH3), 1.57 (s, 15H, CH3), 2.10 (s, 3H, CH3), 2.30 (s,
3H, CH3), 4.04-4.18 (m, 2H, OCH2), 7.08-7.35 (m, 5H, Ph). 13C
NMR (CDCl3): δ 9.64 (s, Cp* Me), 12.74 (s, Me), 13.55 (s, Me), 14.43
(s, Me), 59.86 (s, OCH2), 79.04 (d, 1JCP = 55.5 Hz, dCP), 83.51 (s, Cp*
C), 93.39 (d, 2JCP = 5.8 Hz, dCMe), 95.81 (d, 2JCP = 5.0 Hz, dCMe),
102.56 (d, 1JCP = 53.5 Hz, dCP), 125.73 (s, Ph CH para), 127.75 (s, Ph
It is likely that a ligand such as 4 can find some use in catalysis,
but this is not the only possibility. The recent discovery that
phosphaferrocene complexes can have interesting nonlinear
optical properties11 adds another possible application.
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Organometallics
NOTE
CH), 129.54 (d, 3JC2P = 9.5 Hz, Ph CH), 139.05 (d, 2JCP = 17.8 Hz, Ph C
ipso), 172.97 (d, JCP = 17.9 Hz, CdO). HRMS: m/z calcd for
C25H32O2PFe (M þ H)þ 451.1498, found 451.1489.
(s, 1H, Py), 5.14 (s, 1H, Py), 6.43 (s, 1H, NCHdC), 7.15-7.43 (m, 5H,
Ph). 13C NMR (CDCl3): δ 10.21 (s, Cp* Me), 11.84 (s, Me), 14.56 (s,
Me), 26.16 (d, 2JCP = 14.5 Hz, -CH2-), 82.53 (s, Py CH), 82.90 (d,
Synthesis of Phosphaferrocene 3. Lithium aluminum hydride
(80 mg, 2 mmol.) was added to 2-phenyl-5-(ethoxycarbonyl)phospha-
ferrocene 1 (450 mg, 1 mmol) in dry THF (10 mL) at -15 °C. The
reaction mixture was slowly warmed to room temperature and stirred for
2 h. Excess LiAlH4 was quenched with a small amount of ethyl acetate
and 2 drops of deionized water. The solvents were evaporated, and the
resulting precipitate was dissolved in freshly distilled pyrrole (10 mL).
JCP = 10.4 Hz), 84.04 (s, Cp* C), 86.84 (s, Py CH), 88.02 (d, JCP = 5.2
Hz), 88.22 (d, JCP = 2.1 Hz), 91.83 (s, Py C) 112.13 (d, JCP = 4.2 Hz, Py
CH), 124.02 (d, 1JCP = 56.0 Hz), 125.95 (s, Ph CH para), 128.16 (s, Ph
CH), 129.45 (d, 3JCP = 4.5 Hz, Ph CH), 136.84 (d, 2JCP = 14.5 Hz, Ph C
ipso), 203.18 (d, JCP = 12.5 Hz, Mo CO), 203.18 (d, JCP = 12.5 Hz, Mo
CO), 208.81 (d, JCP = 10.4 Hz, Mo CO), 214.90 (d, JCP = 41.5 Hz, Mo
CO), 219.55 (d, JCP = 8.3 Hz, Mo CO), 220.60 (s, Mn CO). HRMS:
m/z calcd for C35H35NO7P55MnFe98Mo (M þ H)þ 820.9935, found
820.9938.
BF3 OEt2 (0.13 mL, 1 mmol) was added to the pyrrole solution, the
3
mixture was stirred for 15 min, and a small amount of triethylamine was
then added. Purification was performed via cold column chromatogra-
phy on silica using 20/1 hexane/ethyl acetate. The dark orange band
collected gives a red oil (279 mg, 0.61 mmol), and the yield was 61%.
Complex 2. 31P NMR (CDCl3): δ -52.6. 1H NMR (CDCl3): δ 1.68
(s, 15H, Me Cp*), 2.09 (s, 3H, Me), 2.19 (s, 3H, Me), 4.09-4.30 (m,
2H, CH2O), 7.12-7.16 (m, 1H Ph), 7.22-7.26 (m, 2H, Ph), 7.41-7.43
(m, 2H, Ph). 13C NMR (CDCl3): δ 10.46 (s, Cp* Me), 11.60 (s, Me),
14.18 (s, Me), 60.83 (d, 2JCP = 23.1 Hz, CH2O), 82.81 (s, Cp* C), 90.70
(d, 2JCP = 3.8 Hz, dCMe), 93.76 (d, 2JCP = 4.8 Hz, dCMe), 95.13 (d,
1JCP = 55.8 Hz, dCP), 98.52 (d, 1JCP = 53.6 Hz, dCP), 125.48 (s, Ph
CH para), 127.89 (s, Ph CH), 129.57 (d, 3JCP = 9.6 Hz, Ph CH), 140.04
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving NMR data for
b
all the new compounds and a CIF file giving the X-ray crystal
structure analysis of compound 6. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*To whom correspondence should be addressed. E-mail: fmathey@
ntu.edu.sg.
2
(d, JCP = 17.4 Hz, Ph C ipso). HRMS: m/z calcd for C27H30FeOP
(M þ H)þ 409.1384, found 409.1379.
Complex 3. 31P NMR (CDCl3): δ -53.4. 1H NMR (CDCl3): δ 1.70
(s, 15H, Me Cp*), 1.95 (s, 3H, Me), 2.17 (s, 3H, Me), 3.31-3.47 (m,
2H, CH2), 5.89-5.90 (t, 1H, CH), 6.04-6.06 (m, 1H, CH), 6.55-6.57
(m, 1H, N-CH), 7.10-7.14 (m, 1H, Ph), 7.20-7.24 (m, 2H, Ph),
7.40-7.43 (t, 2H, Ph), 7.91 (s, 1H, NH). 13C NMR (CDCl3): δ 10.20
’ ACKNOWLEDGMENT
We thank the Nanyang Technological University in Singapore
for financial support of this work. Thanks are also due to Dr. Li
Yongxin (NTU) for the X-ray crystal structure analysis and to
Ms. Ng Yong Xiang and Hui Yen Li Eunice for experimental help.
2
(s, Cp* Me), 11.62 (s, Me), 14.27 (s, Me), 26.59 (d, JCP = 22.1 Hz,
CH2), 82.41 (s, Cp* C), 89.60 (d, 2JCP = 4.0 Hz, dCMe), 93.47 (d, 2JCP
= 5.0 Hz, dCMe), 94.92 (d, 1JCP = 54.3 Hz, dCP), 97.01 (d, 1JCP = 53.3
Hz, dCP), 104.99 (s, Py CH), 108.02 (s, Py CH), 116.31 (s, Py CH),
125.17 (s, Ph CH para), 127.72 (s, Ph CH), 129.28 (d, 3JCP = 10.1 Hz,
Ph CH), 131.41 (s, Py C), 140.25 (d, 2JCP = 17.1 Hz, Ph C ipso). HRMS:
m/z calcd for C27H32NPFe (M þ H)þ 458.1698, found 458.1700.
Synthesis of Phosphaferrocene 4. Phosphaferrocene 3 (338
mg, 0.74 mmol) and Mn2(CO)10 (293 mg, 0.75 mmol) were dissolved
in dry toluene in a sealed tube. The mixture was heated to 145 °C for 24
h. Purification was performed via cold column chromatography on silica
using hexane and then 15/1 hexane/ethyl acetate. The pale orange band
collected gives a brown solid (130 mg, 0.21 mmol), and the yield was
29%.
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1
31P NMR (CDCl3): δ -54.2. H NMR (CDCl3): δ 1.70 (s, 15H,
Cp*), 2.04 (s, 3H, Me), 2.18 (s, 3H, Me), 3.27 (m, 2H, CH2), 5.00 (s, 1H
Py), 5.02 (s, 1H, Py), 5.91 (1H, s, NCHdC), 7.11-7.42 (m, 5H, Ph).
13C NMR (CDCl3): δ 10.14 (s, Cp* Me), 11.91 (s, Me), 14.23 (s, Me),
(8) Campos-Carrasco, A.; Broecks, L. E. E.; Weemers, J. J. M.; Pidko,
E. A.; Lutz, M.; Masdeu-Bultꢀo, A. M.; Vogt, D.; M€uller, C. Chem. Eur. J.,
in press (DOI: 10.1002/chem.2010022586).
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(12) Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983.
29.13 (d, 2JCP = 21.7 Hz, -CH2-), 82.59 (s, Cp* C), 89.47 (d, 2JCP
=
3.9 Hz, dCMe), 93.43 (d, 2JCP = 4.6 Hz, dCMe), 94.37 (d, 1JCP = 54.9
Hz, dCP), 97.69 (d, 1JCP = 53.5 Hz, dCP), 104.88 (s, Py CH), 108.40
(s, Py CH), 116.40 (s, NCHdC), 125.26 (s, Ph CH para), 127.71 (s, Ph
CH), 129.26 (d, 3JCP = 9.4 Hz, Ph CH), 130.90 (s, NCRdCH), 139.90
(d, 2JCP = 17.6 Hz, Ph C ipso), 223.17 (s, CO). HRMS: m/z calcd for
C31H35NO3P55MnFe (M þ H)þ 611.1084, found 611.1099.
Synthesis of Chelate Complex 5. A solution of phosphaferro-
cene 4 (178 mg, 029 mmol) and Mo(CO)6 (80 mg, 0.3 mmol) in
toluene was stirred at 100 °C for 3 h. Purification was performed via cold
column chromatography on silica using hexane/ethyl acetate (10/1 to
4/1). The pale orange band collected gives a brown solid (111 mg, 0.135
mmol), and the yield was 47%. Crystals of 5 were grown from a solution
of the compound in dichloromethane/methanol.
1
31P NMR (CDCl3): δ -5.6. H NMR (CDCl3): δ 1.70 (s, 15H,
Cp*), 2.17 (s, 3H, Me), 2.28 (s, 3H, Me), 3.09-3.23 (m, 2H, CH2), 4.97
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