Oligomerization of terminal phosphinidene complexes

Article abstract of DOI:10.1021/om101144n

The thermolysis of a 7-phenyl-7-phosphanorbornadiene pentacarbonylchromium complex yields a mixture of the cyclic trimer and tetramer of [PhP-Cr(CO) ₅]. In the presence of CuCl and W(CO)₆ at 60 °C, the decomposition of 7-R-7-phosphanorbornadiene pentacarbonyltungsten complexes affords the corresponding diphosphene σ,π complexes [RP=PR][W(CO) ₅]₃ (R = Me, CH₂CH₂CO₂Et, 2-thienyl, 2-N-methylpyrrolyl). The Me derivative has been characterized as its [4+2] cycloadduct with 2,3-dimethylbutadiene. The three others have been characterized by X-ray crystal structure analysis, NMR, and UV, and the electronic structure of the thienyl derivative has been studied by DFT calculations.

Full text of DOI:10.1021/om101144n

NOTE
pubs.acs.org/Organometallics
Oligomerization of Terminal Phosphinidene Complexes
Ngoc Hoa Tran Huy,* Yunpeng Lu, and Franc-ois Mathey*
Nanyang Technological University, Division of Chemistry & Biological Chemistry, 21 Nanyang Link, Singapore 637371
S Supporting Information
b
ABSTRACT: The thermolysis of a 7-phenyl-7-phosphanorborna-
diene pentacarbonylchromium complex yields a mixture of the cyclic
trimer and tetramer of [PhP-Cr(CO)5]. In the presence of CuCl and
W(CO)6 at 60 °C, the decomposition of 7-R-7-phosphanorborna-
diene pentacarbonyltungsten complexes affords the corresponding
diphosphene σ,π complexes [RPdPR][W(CO)5]3 (R = Me,
CH2CH2CO2Et, 2-thienyl, 2-N-methylpyrrolyl). The Me derivative
has been characterized as its [4þ2] cycloadduct with 2,3-dimethyl-
butadiene. The three others have been characterized by X-ray crystal
structure analysis, NMR, and UV, and the electronic structure of the thienyl derivative has been studied by DFT calculations.
’ INTRODUCTION
The reduction of dihalophosphines or the condensation of
dihalophosphines with primary phosphines lead, in most cases,
to a mixture of cyclopolyphosphine oligomers [RP]n, the value of
n decreasing with the steric hindrance of R. When R is extremely
bulky, a diphosphene with a PdP double bond can be obtained
as shown by Yoshifuji in his pioneering work of 1981.1,2 Kinetic
stabilization of the small oligomers (n = 2, 3, 4) can also be
achieved by using bulky complexing groups on the phosphorus
lone pairs. For example, Huttner has shown that PhPdPPh can
be stabilized by two [Cr(CO)5] units.3 On our side, we have
studied the oligomerization of the transient terminal phosphini-
dene complex [PhP-W(CO)5] and obtained a variable mixture
of products with n = 2, 3, or 4 according to the experimental
conditions.4,5 In view of the mildness of these conditions (no
base, no reducing metal), it appeared interesting to check
whether this approach has some generality. This is the subject
of the present report.
The formula of 2 was established by exact mass measurement
and 31P NMR spectroscopy. The NMR spectrum shows a ABX
system with two inequivalent noncomplexed phosphorus nuclei
at -121.8 and -142.5 ppm and the complexed phosphorus at -
42.3 ppm in CDCl3. The formula and the stereochemistry of 3
were established by X-ray crystal structure analysis (Figure 1).
The P4 ring is folded around the P2-P4 axis, with an interplane
angle of 128.5°. The stereochemistry is identical to that of the
tetramer obtained with tungsten under more drastic conditions.5
As can be seen, the less bulky chromium favors the formation of
the tetramer, whereas, logically, the more bulky tungsten favors
the trimer. It is known that the steric bulk of R favors the lower
(RP)n oligomers. For example, the Mg reduction of RPCl2 gives
the pentamer for R = Ph and the trimer for R = tBu.8
At lower temperature with CuCl as a catalyst, the tungsten
complex 1c is known to yield the tricomplexed dimer.4,5 This
reaction cannot be transposed to chromium and molybdenum
since, in these cases, the copper chloride catalysis proved to be
ineffective. We first decided to study what happens when the
phenyl is replaced by an alkyl substituent. In the methyl case, the
expected dimer is produced (we observe a 31P resonance at -
44.8 ppm) but is too reactive for isolation. We allowed it to react
’ RESULTS AND DISCUSSION
It is known from theoretical calculations6 that the P-M bond
strength in R3P-M(CO)5 varies in the order P-W > P-Cr >
P-Mo. This implies that, in the 7-phosphanorbornadiene family,
the stabilization of the bicyclic structure is the highest for
tungsten and the weakest for molybdenum. This phenomenon
has been experimentally demonstrated when devising a precursor
of [FP-Mo(CO)5].7 Thus, we decided to first check the
influence of the metal on the formation of the phosphinidene
oligomers. To our surprise, the thermal decomposition of the
molybdenum complex (1b) takes place slowly at 120 °C, but
yields only a complex mixture of paramagnetic decomposition
products (eq 1). On the contrary, the chromium complex 1a
rapidly decomposes at 120 °C to give a mixture of trimer (2) and
tetramer (3).
Received: December 6, 2010
Published: March 02, 2011
r
2011 American Chemical Society
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Organometallics
NOTE
Figure 1. X-ray crystal structure of tetramer 3. Selected bond lengths
(Å) and angles (deg): P1-P2 2.2454(6), P2-P3 2.2267(6), P3-P4
2.2270(6), P1-P4 2.2370(7), P1-Cr1 2.3921(5); P2-P1-P4
82.40(2), P1-P2-P3 85.52(2), P2-P3-P4 83.05(2), P3-P4-P1
85.72(2).
Figure 2. X-ray crystal structure of dimer 6. Selected bond lengths (Å)
and angles (deg): C1-P1 1.823(11), W1-P1 2.556(2), W3-P1
2.615(2), P1-P2 2.156(4), C11-P2 1.834(11), W2-P2 2.564(3),
W3-P2 2.605(2); C1-P1-P2 104.9(4), C1-P1-W1 117.4(3), C1-
P1-W3 106.9(3), C11-P2-P1 104.7(4), P1-W3-P2 48.79(8).
in situ with 2,3-dimethylbutadiene and obtained the [2þ4]
cycloadduct (4), which was purified and characterized by
NMR spectroscopy and HRMS (eq 2).
Obviously, the coordination of copper by the cyano group will
prevent the formation of this transition state. Also of interest is the
fact that adding acetonitrile to the reaction medium during the
thermal decomposition of 1e does not kill the extrusion catalysis
but suppresses the formation of the dimer. It is clear from the
work of Lammertsma10,11 and our work12,13 that copper chloride
remains coordinated to the extruded phosphinidene. Thus, we
suggest that the formation of the dimer takes place by oxidative
coupling in the coordination sphere of copper (eq 4).
A positive outcome was also observed with the CH2-CH2-
CO2Et substituent. In this case, the dimer 5 is sufficiently stable
and was isolated (eq 3).
We also carried out our dimerization experiments with the
N-methyl-2-pyrrolyl and 2-thienyl substituents. Both dimers
have been characterized by X-ray crystal structure analysis
(Figures 2 and 3). In 6, the C1-P1-P2-C11 unit displays
a quasi-perfect trans arrangement (torsion angle 157.1°), but the
two tungsten atoms coordinated to phosphorus are repelled by
the π-complexed tungsten (W1-P1-P2-W2 138.14°). In 7,
some disorder is observed, but the overall structure is similar.
The most striking phenomenon when comparing the structures
of 6 and 7 is that all of the distances (P-C, P-P, P-W) are
systematically shorter in 7 than in 6. It seems that the thiophene
ring interacts more strongly with the P2W3 unit and strengthens
it more efficiently than the pyrrole ring does. From the previous
data on the phenyl derivative,4 it seems that phenyl more
resembles pyrrole than thiophene from this standpoint. The
comparison of the UV spectra confirms that 7 stands apart
from 6 and the phenyl derivative (Figure 4). This led us to
investigate in more depth the electronic structure of 7 by
DFT calculations. The data are provided in the Supporting
Information. Both the structure and the UV spectrum were
satisfactorily reproduced. The most important findings are that
the P-P bond order is 1 (Wiberg indices) and that both the
On the contrary, with the CH2CH2CN substituent, the CuCl
catalysis does not work and no dimer is formed. It is known that
Ph2PCH2CH2CN coordinates to Ru(II) via its cyano group.9
Besides, on the basis of DFT calculations, Lammertsma has
suggested that the phosphorus extrusion from 7-phosphanorbor-
nadiene complexes proceeds via a transition state where copper is
tricoordinated to W, P, and one carbon at the bridgehead.10
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Organometallics
NOTE
HOMO and the LUMO do not have any significant contribu-
tions from the thiophene substituent. Some contribution from
the localized π orbital in the two thiophene rings occurs only in
the second highest occupied orbital (HOMO-1). Thus, this
P2W3 unit cannot be used as a conjugation transmitter within a
polythiophene chain.
The possibility to synthesize functional diphosphenes such as
5 as stable tungsten complexes offers interesting synthetic
opportunities that we are currently exploring.
’ EXPERIMENTAL SECTION
NMR spectra were obtained on a JEOL ECA400 or JEOL ECA400SL
spectrometer. All spectra were recorded at 298 K unless otherwise
specified. Elemental analyses were performed in the Division of Chem-
istry and Biological Chemistry, Nanyang Technological University.
HRMS spectra were obtained in ESI mode on a Finnigan MAT95XP
HRMS system (Thermo Electron Corp.). X-ray crystallographic ana-
lyses were performed on a Bruker X8 APEX diffractometer. All reactions
were performed under argon. Silica gel (230-400 mesh) was used for
the chromatographic separations. All commercially available reagents
were used as received from the suppliers. The phosphanorbornadiene
precursors were synthesized as described in the literature.14,15
Thermal Decomposition of 1a. A solution of complex 1a (3 g,
5.7 mmol) in toluene (10 mL) was heated at 120 °C for 1 h. The reaction
was complete, as checked by 31P NMR. After evaporation, the residue
was chromatographed on silica gel to give 2 and 3 as yellow powders.
Complex 2: eluent hexane, yield 0.2 g (20%). 31P NMR (CDCl3):
ABX, the two noncomplexed phosphorus atoms resonate at -
121.81(A) and -142.48(B). The complexed phosphorus appears at -
42.30 (X). Exact mass: calcd For C23H15O5P3Cr [M þ H] 516.9616;
found 516.9624.
Complex 3: eluent hexane/CH2Cl2 (9:1), yield 0.36 g (45%). 31P
NMR (CDCl3): the spectrum shows three types of phosphorus atom at
-70.24 (1P), -20.00 (2P), and 43.20 (1P). Exact mass: calcd for
C29H20O5P4Cr [M þ H] 624.9745; found 624.9737. Single crystals
were obtained from CH2Cl2/hexane.
Synthesis of Tetrahydro-1,2-diphosphinine (4). A solution
of 1d (0.6 g, 1.13 mmol), W(CO)6 (0.25 g), and CuCl (0.03 g) in
toluene (5 mL) was heated at 60 °C. Monitoring by 31P NMR showed
that the reaction was complete after 12 h of heating. 2,3-Dimethylbu-
tadiene (2.3 mL, 2 equiv) was added, and the solution was heated for 5 h
at 110 °C. The solvent was removed under high vacuum, and the residue
was chromatographed on silica gel with hexane/CH2Cl2 (9:1) as eluent:
0.115 g of 4 was isolated (28% yield). 31P NMR (CDCl3): δ -23.8 ppm.
1H NMR (CDCl3): δ 1.87 (pseudo t, ∑J(H-P) = 5.8 Hz, 6H, P-Me),
1.90 (s, 6H, Me), 2.68 (d, J(A-B) = 14.6 Hz, 2H, HA CH2P), 2.82 (d
pseudo t, ∑J(B-P) = 8.2 Hz, 2H, HB). 13C NMR (CDCl3): δ 19.36
(pseudo t, ∑J(C-P) = 33.5 Hz, P-Me), 22.13 (s, Me), 38.72 (pseudo t,
Figure 3. X-ray crystal structure of dimer 7. Selected bond lengths (Å)
and angles (deg): C1-P1 1.807(9), W1-P1 2.522(2), W3-P1
2.591(2), P1-P2 2.140(3), C10-P2 1.810(12), W2-P2 2.522(2),
W3-P2 2.596(3); C1-P1-P2 106.0(3), C1-P1-W1 112.1(3), C1-
P1-W3 110.6(3), C10-P2-P1 103.7(12), P1-W3-P2 48.73(7).
Figure 4. UV spectra of [PhPdPPh][W(CO)5]3 (sample 1), 6 (sample 2), and 7 (sample 3) in dichloromethane.
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Organometallics
NOTE
∑J(C-P) = 9.6 Hz, CH2), 126.97 (s, dC), 196.23 (s, ∑J(C-P) = 0 Hz,
cis CO)), 197.77 (pseudo t, ∑J(C-P) = 25.8 Hz trans CO). Exact mass:
calcd for C18H16O10P2W2 819.9282; found 819.9287.
7.17 (m, 2H, Hβ), 7.46 (m, 2H, Hβ), 7.67 (m, 2H, HR). 13C NMR
(CDCl3): δ 128.79, 133.60, 137.70 (s, dCH), 140.36 (pseudo t, dC),
190.22 (s, π WCO, fluxional), 195.99 (s, cis CO), 198.54 (pseudo t, trans
CO). Exact mass: calcd for C18H6O10P2S2W2 (M - W(CO)5 - 2H)
873.7941; found 873.7938. Single crystals were obtained from CH2Cl2.
Synthesis of Diphosphene Complex 5. A solution of 1e (1 g,
1.6 mmol), CuCl (0.08 g), and W(CO)6 (0.7 g) in toluene (15 mL) was
heated at 60 °C for 2 h, under monitoring by 31P NMR. The solvent was
removed under high vacuum, and the compound was chromatographed
at -2 °C on silica gel with hexane/CH2Cl2 (3:7) as eluent: 0.2 g of 5 was
’ ASSOCIATED CONTENT
1
isolated (20% yield). 31P NMR (CDCl3): δ -27.0 ppm. H NMR
(CDCl3): δ 1.25 (t, 3J(H-H) = 7.2 Hz, 6H, Me), 2.07 (m, 2H, CH2),
2.76 (m, 4H, CH2), 2.79 (m, 2H, CH2), 4.15 (q, 4H, OCH2). 13C NMR
(CDCl3): δ 14.16 (s, Me), 36.31 (s, CH2), 36.75 (pseudo-t, ∑J(C-P) =
18.1 Hz, CH2-P), 61.56 (s, OCH2), 170.21 (pseudo-t, ∑J(C-P) = 19.0
Hz, CO2), 195.98 (CO), 197.66 (CO). The presence of only two CO
resonances indicates some fluxionality of the carbonyls.
S
Supporting Information. X-ray crystal structure analyses
b
of compounds 3, 6, and 7. Computational details for 7. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Synthesis of 7-Phosphanorbornadiene (1f). A solution of
1-(1-methyl-2-pyrrolyl)-3,4-dimethylphosphole16 pentacarbonyltung-
Corresponding Author
*E-mail: fmathey@ntu.edu.sg.
1
sten complex (δ31P(CDCl3) -15.3 ppm, J(31P-183W) = 212.4 Hz)
(1.6 g, 3 mmol) and dimethyl acetylenedicarboxylate (1.4 mL, 11 mmol)
in toluene (1.5 mL) was heated at 75 °C for three nights. After
evaporation, the organic residue was chromatographed on silica gel with
hexane/CH2Cl2 (1:3) as the eluent. Yield of 1f: 1.1 g (60%). 31P NMR
(CDCl3): δ 194.2 ppm, 1J(31P-183W) = 242.7 Hz. 1H NMR (CDCl3): δ
2.00, 2.01 (s, 6H, Me), 3.60, 3.70, 3.80 (m, 11H, N-Me, OMe, bridge
’ ACKNOWLEDGMENT
The authors thank the Nanyang Technological University in
Singapore and the CNRS in Paris for the financial support of this
work. Thanks are also due to Dr. Li Yongxin (NTU) for the X-ray
crystal structure analyses, Dr. Lloyd Foong Nien Ah Qune (NTU)
for the UV spectra, and Hon Jing Ti (CBC 491) and Lim Jiahui
(CBC 941) for their help during their HNRS and SRS projects.
CH), 6.14 (m, 2H, CHdCHN, dCH-N), 6.62 (d, 1H, NCCHd). 13
C
NMR (CDCl3): δ 16.20 (s, Me), 35.32 (s, Me-N), 52.46 (s, OMe), 59.07
(d, 1J(C-P) = 25.2 Hz, CH-P), 62.11 (d, 1J(C-P) = 15.2 Hz, CH-P),
108.64 (d, J(C-P) = 7.4 Hz, dH pyr), 115.75 (d, J(C-P) = 9.6 Hz, CH-
1
pyr), 126.63 (s, CH-pyr), 131.85 (d, J(C-P) = 22.9 Hz, P-C-N),
137.11 (d, 2J(C-P) = 17.1 Hz, Me-Cd), 137.85 (d, 2J(C-P) = 18.1 Hz,
Me-Cd), 143.55 (s, dC-CO2Me), 146.49 (s, dC-CO2Me), 164.37 (s,
2
’ REFERENCES
(1) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J.
Am. Chem. Soc. 1981, 103, 4587.
(2) Recent reviews on diphosphenes: Yoshifuji, M. Sci. Synth. 2009,
42, 47. Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877.
(3) Borm, J.; Zsolnai, L.; Huttner, G. Angew. Chem., Int. Ed. Engl.
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1985, 4, 2134.
(5) Tran Huy, N. H.; Inubushi, Y.; Ricard, L.; Mathey, F. Organo-
COMe), 165.72 (s, COMe), 195.81 (d, J(C-P) = 6.6 Hz, cis CO),
198.75 (d, 2J(C-P) = 26.7 Hz, trans CO). The two sides of the
phosphanorbornadiene appear to be inequivalent. This is probably due
to the blocked rotation of the pyrrolyl substituent due to the NMe group.
Synthesis of Diphosphene Complex 6. A solution of 1f (0.65 g,
1 mmol) and CuCl (0.06 g) in toluene (5 mL) was heated at 60 °C for 6
h under monitoring by 31P NMR. The solvent was removed under high
vacuum, and the compound was chromatographed on silica gel with
hexane/CH2Cl2 (2:1) as the eluent: 0.095 g of 6 was isolated as red-
1
brown crystals (20% yield). 31P NMR (CD2Cl2): δ -38.3 ppm. H
metallics 1997, 16, 2506.
NMR (CD2Cl2): δ 4.21 (s, 6H, NMe), 6.13 (m, 2H, Hβ), 6.47 (m, 2H,
Hβ), 6.96 (m, 2H, HR). Exact mass: calcd for C20H11N2O10P2W 2 (M
- W(CO)5 - H) 868.9037; found 868.9039. Single crystals were
obtained from CH2Cl2 at -20 °C.
(6) Frenking, G.; Wichmann, K.; Fr€ohlich, N.; Grobe, J.; Golla, W.;
Le Van, D.; Krebs, B.; L€age, M. Organometallics 2002, 21, 2921.
(7) Compain, C.; Donnadieu, M.; Mathey, F. Organometallics 2006,
25, 540.
Synthesis of 7-Phosphanorbornadiene (1g). A solution of
1-(2-thienyl)-3,4-dimethylphosphole17 pentacarbonyltungsten complex
(δ31P(CDCl3) -5.1 ppm, 1J(31P-183W) = 221.0 Hz) (1.6 g, 3 mmol)
and dimethyl acetylenedicarboxylate (1.6 mL, 12 mmol) in toluene
(2 mL) was heated at 75 °C for three nights. After evaporation, the
organic residue was chromatographed on silica gel with hexane/CH2Cl2
(1:1) as the eluent. Yield of 1g: 1.0 g (50%). 31P NMR (CD2Cl2): δ
(8) Baudler, M.; Gruner, C. Z. Naturforsch. B 1976, 31B, 1311.
(9) Caetano, W.; Alves, J. J. F.; Lima Neto, B. S.; Franco, D. W.
Polyhedron 1995, 14, 1295.
(10) Lammertsma, K.; Ehlers, A. W.; McKee, M. L. J. Am. Chem. Soc.
2003, 125, 14750.
(11) Vlaar, M. J. M.; de Kanter, F. J. J.; Schakel, M.; Lutz, M.; Spek,
A. L.; Lammertsma, K. J. Organomet. Chem. 2001, 617-618, 311.
(12) Deschamps, B.; Mathey, F. J. Chem. Soc., Chem. Commun.
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(14) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc.,
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(15) Espinosa-Ferao, A.; Deschamps, B.; Mathey, F. Bull. Soc. Chim.
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(16) Holand, S.; Maigrot, N.; Charrier, C.; Mathey, F. Organome-
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(17) Bevierre, M. O.; Mercier, F.; Ricard, L.; Mathey, F. Bull. Soc.
188.7 ppm, J(31P-183W) = 247.1 Hz. H NMR (CDCl3): δ 2.25 (s,
6H, Me), 3.68 (d, 2J(H-P) = 2.72 Hz, 2H, CH-P), 3.83 (s, 6H, OMe),
7.02-7.44 (m, 3H, CHd). 13C NMR (CDCl3): δ 15.79 (s, Me), 52.95
(s, OMe), 61.34 (d, 1J(C-P) = 20.0 Hz, CH-P), 127.44 (d, J(C-P) =
7.6 Hz, Th dCH), 129.16 (d, 1J(C-P = 62.3 Hz, Th C-P), 129.95 (s, Th
dCH), 132.25 (d, J(C-P) = 6.7 Hz, Th dCH), 138.28 (d, 2J(C-P) =
18.1 Hz, dC-Me), 145.47 (d, 2J(C-P) = 4.8 Hz, dC-CO2Me), 164.64
(s, COMe), 195.80 (d, 2J(C-P) = 6.7 Hz, cis CO), 198.66 (d, 2J(C-P)
= 26.7 Hz, trans CO).
1
1
Synthesis of Diphosphene Complex 7. Same as for 6. Eluent
hexane/CH2Cl2 (4:1); 0.09 g of 7 was isolated as red-brown crystals
(20% yield). 31P NMR (CD2Cl2): δ -36.1 ppm. 1H NMR (CD2Cl2): δ
Chim. Fr. 1992, 129, 1.
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