Generating phosphinidene - N-methylimidazole adducts under mild conditions

Article abstract of DOI:10.1021/om060460m

The nucleophilic attack of N-methylimidazole at the bridge phosphorus of a 7-phosphanorbornadiene pentacarbonylmolybdenum complex induces the collapse of the bridge. According to DFT calculations, the resulting zwitterion displays a very long and weak P - Mo bond. The excess of, N-methylimidazole thus appears to be able to displace the phosphinidene from its molybdenum complex. The final result is a phosphinidene- N-methylimidazole adduct whose structure has also been computed. When applied to the phenyl derivative at 40°C in toluene, this reaction effectively generates the [PhP] adduct, which decomposes to give essentially Ph₄P₄ and Ph₅P₅. At 80°C, PhPH₂ is also produced. In the presence of CCl₄ the phosphinidene adduct inserts into the C-Cl bond to give PhP(Cl)CCl ₃.

Full text of DOI:10.1021/om060460m

Organometallics 2006, 25, 5031-5034
5031
Generating Phosphinidene-N-Methylimidazole Adducts under Mild
Conditions
Irina Kalinina†,‡ and Franc¸ois Mathey*,†
UCR-CNRS Joint Research Chemistry Laboratory, Department of Chemistry, UniVersity of California
RiVerside, RiVerside, California 92521-0403, and A. M. ButleroV Chemical Institute, Kazan State
UniVersity, 18 KremleVskaja Street, 420008 Kazan, Russian Federation
ReceiVed May 25, 2006
The nucleophilic attack of N-methylimidazole at the bridge phosphorus of a 7-phosphanorbornadiene
pentacarbonylmolybdenum complex induces the collapse of the bridge. According to DFT calculations,
the resulting zwitterion displays a very long and weak P-Mo bond. The excess of N-methylimidazole
thus appears to be able to displace the phosphinidene from its molybdenum complex. The final result is
a phosphinidene-N-methylimidazole adduct whose structure has also been computed. When applied to
the phenyl derivative at 40 °C in toluene, this reaction effectively generates the [PhP] adduct, which
decomposes to give essentially Ph4P4 and Ph5P5. At 80 °C, PhPH2 is also produced. In the presence of
CCl4 the phosphinidene adduct inserts into the C-Cl bond to give PhP(Cl)CCl3.
phosphorus ylides, malonate anions, fluoride ions) induces the
collapse of the phosphorus bridge under mild conditions.5 This
observation led us to wonder what would happen if we used as
the nucleophile a base devoid of reactive bonds. In fact, an
example already exists where the nucleophile is tributylphos-
phine.6 The result is a zwitterionic adduct between the phos-
phinidene bridge and tributylphosphine whose chemistry is
similar to that of a classical phosphorus ylide.7 Among the
possible nucleophiles, N-methylimidazole appeared as a logical
choice since it is both a good nucleophile and a good ligand of
group VI metals.8 We thus decided to study the reaction of a
representative 7-phosphanorbornadiene complex such as 1 with
this sp2-N base.
For convenience, we start with the case where water or an
alcohol is present as a co-reagent in the reactive medium. In
this case, not surprisingly, the product of the reaction corre-
sponds to the insertion of the phosphinidene bridge into the
O-H bond.9 The only difference with the case where no
N-methylimidazole is present is that the collapse of the bridge
and the insertion reaction take place at lower temperature and
proceed in higher yield with the base (eq 1).
Introduction
The chemistry of phosphinidenes is presently poorly devel-
oped,1 in part as a result of the lack of versatile methods for
their generation under mild conditions. The two main methods,
i.e., the thermal depolymerization of cyclopolyphosphines at
high temperature and the reduction of dihalophosphines by
electropositive metals, are, indeed, hardly compatible with any
reactive functional substituent and generate a lot of products
other than the desired six-electron monomeric species. In
addition, most of the phosphinidenes have a triplet ground state2
and their chemistry appears to be rather erratic. In fact, presently,
only two phosphinidenes have been generated and convincingly
characterized by spectral methods and trapping reactions, i.e.,
[tBu2P-P]3 and [Mes-P].4 In both cases, their chemistry is
characteristic of a singlet state. In this report, we present a simple
access to singlet N-methylimidazole adducts of these species
that could significantly extend the scope of phosphinidene
chemistry.
Results and Discussion
It has been recently shown that the reaction of 7-phosphan-
orbornadiene complexes with mild nucleophiles (stabilized
* Corresponding author. E-mail: francois.mathey@ucr.edu.
† University of California Riverside.
‡ Kazan State University.
(1) Reviews: Schmidt, U. Angew. Chem., Int. Ed. Engl. 1975, 14, 523.
Mathey F. In Multiple Bonds and Low Coordination in Phosphorus
Chemistry; Regitz, M., Scherer, O. J., Eds.; Thieme: Stuttgart, 1990; pp
33-38. Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon
Copy; Wiley: Chichester, 1998; pp 14-18. For more recent data, see: Shah,
S.; Simpson, M. C.; Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc.
2001, 123, 6925.
(2) Nguyen, M. T.; Van Keer, A.; Vanquickenborne, L. G. J. Org. Chem.
1996, 61, 7077.
(3) Fritz, G.; Vaahs, T.; Fleischer, H.; Matern, E. Angew. Chem., Int.
Ed. Engl. 1989, 28, 315. Fritz, G.; Vaahs, T.; Fleischer, H.; Matern, E. Z.
Anorg. Allg. Chem. 1989, 570, 54.
With water, the reaction goes to completion in ca. 30 h at 40
°C. With ethanol, the reaction is complete in 6 h and the product
(5) Hoffmann, N.; Wismach, C.; Jones, P. G.; Streubel, R.; Tran Huy,
N. H.; Mathey, F. Chem. Commun. 2002, 454. Tran Huy, N. H.; Compain,
C.; Ricard, L.; Mathey, F. J. Organomet. Chem. 2002, 650, 57. Compain,
C.; Tran Huy, N. H.; Mathey, F. Heteroat. Chem. 2004, 15, 258. Compain,
C.; Mathey, F. Z. Anorg. Allg. Chem. 2006, 632, 421.
(6) Le Floch, P.; Marinetti, A.; Ricard, L.; Mathey, F. J. Am. Chem.
Soc. 1990, 112, 2407.
(4) Li, X.; Lei, D.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc.
1992, 114, 8526. Li, X.; Weissman, S. I.; Lin, T.-S.; Gaspar, P. P.; Cowley,
A. H.; Smirnov, A. I. J. Am. Chem. Soc. 1994, 116, 7899. Bucher, G.;
Borst, M. L. G.; Ehlers, A. W.; Lammertsma, K.; Ceola, S.; Huber, M.;
Grote D.; Sander, W. Angew. Chem., Int. Ed. 2005, 44, 3289.
(7) Similar transient adducts are formed with electron-rich nitriles:
Streubel, R. Top. Curr. Chem. 2003, 223, 91.
(8) Ardizzoia, G. A.; Brenna, S.; LaMonica, G.; Maspero, A.; Masciocchi,
N. J. Organomet. Chem. 2002, 649, 173.
(9) Marinetti, A.; Mathey, F. Organometallics 1982, 1, 1488.
10.1021/om060460m CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/02/2006

5032 Organometallics, Vol. 25, No. 21, 2006
Kalinina and Mathey
is isolated in 84% yield. The same conditions are used for the
reaction with glycol. The product is isolated in 49% yield. The
13C NMR spectrum of 4 shows two OCH2 groups at 62.18 (JC-P
) 6.3 Hz) and 73.94 (JC-P ) 14.6 Hz) in CDCl3. For
comparison, the reaction of ethanol with 1 alone goes to
completion in 16 h at 110 °C and 3 is isolated in 27% yield.
The reaction of 1 with N-methylimidazole was also run in
the presence of diphenylacetylene and 2,3-dimethylbutadiene.
The formation of the expected phosphirene or vinylphosphirane
complexes was not observed. Since [PhP-Mo(CO)5] is known
to react with these unsaturated compounds to give three-
membered rings, its intermediacy is ruled out.
In the absence of protic reagent, the reaction of N-methylimi-
dazole proceeds in a different way. It is typically carried out at
80 °C in toluene with 4 equiv of heterocyclic base. The three
main products of the reaction are phenylphosphine, which was
easily identified by 31P NMR (δ31P -125, triplet, 1JH-P ) 195.5
Hz), tetraphenylcyclotetraphosphine, which was recovered in
35% yield by chromatography and identified by 31P NMR (δ31P
-55 in CD2Cl2, singlet)10 and mass spectrometry (molecular
peak at 433), and pentaphenylcyclopentaphosphine, which was
recovered in 10% yield by chromatography and characterized
by 31P NMR (δ31P -5 in CD2Cl2, multiplet)11 and mass
spectrometry (molecular peak at 541). Molybdenum was
recovered as its already described tris-imidazole complex8 that
precipitates from the toluene solution (eq 2).
Figure 1. Computed structure of complex 6. Main bond lengths
(Å) and angles (deg): P15-N20 1.897, P15-C11 1.878, P15-
Mo28 2.642; C11-P15-N20 94.94, C11-P15-Mo28 108.00,
N20-P15-Mo28 106.61.
midal (∑angles at P ) 309.5°). The only unexpected result is
the very long P-Mo bond at 2.642 Å. Such bond lengths
typically lie between 2.37 and 2.53 Å.14 Thus, it becomes highly
likely that N-methylimidazole is able to displace the phosphin-
idene from its labile molybdenum complex. This observation
supports the proposed mechanism that is summarized in eq 3.
When run at 40 °C, the reaction does not produce any primary
phosphine, but exclusively yields the oligomers.
At this point, as noted by several referees, the question
becomes whether the decomplexation reaction initially yields
the free phosphinidene or a zwitterionic adduct with the base.
We thus decided to compute the structure of the [MeP]-N-
methylimidazole adduct 7 at the B3LYP 6-311+G(d,p) level.
We found a genuine minimum (no negative frequency), whose
structure is shown in Figure 2. The most striking characteristic
of this structure is the strict coplanarity of the P-Me bond and
imidazole ring, probably reflecting a stabilizing interaction
between the π lone pair at P and the imidazole π system. The
frontier orbitals of 7 are shown in Figure 3. The HOMO is
essentially localized at P and corresponds to the π lone pair.
By analogy with the case of tributylphosphine,6 it was
tempting to interpret these results as follows. The nitrogen base
would attack the bridge of 1 near room temperature to produce
the zwitterionic adduct 5. In 5, phosphorus is not electrophilic
as in [PhP-Mo(CO)5] but nucleophilic. We thus expect a fast
protonation by water and alcohols but no reaction with alkynes
or conjugated dienes. It must be noted here that a terminal
phosphinidene Fe(CO)4 complex has been stabilized by chelation
with a nitrogen base.12
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
To confirm this hypothesis, we decided to perform DFT
calculations13 on 6 using the B3LYP functional with the 6-31G-
(d) basis sets for all atoms except Mo (lanl2dz). The results
were quite interesting. The computed structure of 6 is shown
in Figure 1. This structure corresponds to a shallow but genuine
local minimum (no negative frequency). Phosphorus is pyra-
(10) Breen, T. L.; Stephan, D. W. Organometallics 1997, 16, 365.
(11) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117, 12645.
(12) Cowley, A. H.; Geerts, R. L.; Nunn, C. M. J. Am. Chem. Soc. 1987,
109, 9, 6523.
(14) Frenking, G.; Wichmann, K.; Fro¨hlich, N.; Grobe, J.; Golla, W.;
Le Van, D.; Krebs, B.; La¨ge, M. Organometallics 2002, 21, 2921.

Phosphinidene-N-Methylimidazole Adducts
Organometallics, Vol. 25, No. 21, 2006 5033
than those used for the reaction of Ph4P4 (reflux in pure CCl4
for 20 h), thus highlighting the enhanced nucleophilicity of our
adducts. The chlorophosphine 8 was detected by its 31P
resonance at 96.4 ppm in toluene and reacted with ethanol and
sulfur for full characterization.
The net effect of the complexation of phosphinidenes by
N-methylimidazole is to stabilize the singlet state and to enhance
the nucleophilicity of phosphorus. Since the proposed access
to these adducts is both simple and versatile, we expect the
development of interesting synthetic applications around these
new P(I) species.
Figure 2. Computed structure of adduct 7. Main bond lengths (Å)
and angles (deg): P15-N20 C1-P5 1.869, P5-N10 1.829, N10-
C6 1.392, C6-C7 1.366, C7-N11 1.372, N11-C12 1.361, C12-
N10 1.342; C1-P5-N10 97.09, P5-N10-C12 124.51, C6-N10-
C12 106.29.
Experimental Section
Reactions were performed under nitrogen using oven-dried
glassware. Dry tetrahydrofuran was obtained by distillation from
Na/benzophenone. Silica gel (70-230 mesh) was used for chro-
matographic separation. Nuclear magnetic resonance spectra were
obtained on a Bruker Avance 300 and Varian Inova spectrometer
operating at 300.13 MHz for 1H, 75.45 MHz for 13C, and 121.496
MHz for 31P. Chemical shifts are expressed in parts per million
downfield from external TMS (1H and 13C) and external 85% H3-
PO4 (31P). Mass spectra were obtained on VG 7070 and Hewlett-
Packard 5989A GC/MS spectrometers. Elemental analyses were
performed by Desert Analytics Laboratory, Tucson, AZ.
(Phenylphosphinous acid)pentacarbonylmolybdenum (2). The
(7-phosphanobornadiene)pentacarbonylmolybdenum complex 1 (0.15
g, 0.27 mmol), 1-methylimidazole (0.09 mL, 1.08 mmol), and water
(0.005 mL, 0.27 mmol) were heated at 40 °C in toluene (1.5 mL)
for 30 h. After evaporation, the residue was purified by column
chromatography on silica gel (2:1 hexane-dichloromethane) to
afford complex 2 (0.087 g, 89% yield): Rf 0.3 (CH2Cl2); 31P NMR
(CDCl3) δ 107.1 (P-H, 1JPH ) 343.7 Hz 3JPH ) 11.9 Hz); 1H NMR
1
(CDCl3) δ 1.69 (OH), 7.47-7.64 (m, 5H, Ph), 7.90 (d, JP-H
)
341.2 Hz); 13C NMR (CDCl3) δ 129.21-138.30 (m, C ortho, meta,
Figure 3. Kohn-Sham orbitals of adduct 7.
2
2
para), 204.80 (d, JC-P ) 10.1 Hz, CO cis), 209.35 (d, JC-P
)
26.4 Hz, CO trans); MS (FAB, 98Mo) m/z 364 (M+, 61%), 336 (M
- CO, 20%), 308 (M - 2CO, 27%), 280 (M - 3CO, 22%), 252
(M - 4CO, 10%), 224 (M - 5CO, 25%). Anal. Calcd for C11H7-
MoO6P: C, 36.49; H, 1.95. Found: C, 36.85; H, 2.33.
The HOMO-1 corresponds to the in-plane σ lone pair at P.
The LUMO is mainly localized on the imidazole ring. These
observations completely fit a zwitterionic formulation with a
nucleophilic phosphorus.
We were eager to find a reaction that would be characteristic
of these imidazole adducts. It is known that tetrachloromethane
reacts with terminal phosphinidene complexes to give the
corresponding dichlorophosphine complexes,15 whereas it reacts
with cyclopolyphosphines to give the corresponding chlorot-
richloromethylphosphines via a stepwise depolymerization reac-
tion at high temperature.16 Our system reacts with CCl4 in
toluene at 80 °C to give chlorotrichloromethylphenylphosphine
(eq 5).
(O-Ethyl phenylphosphinite)pentacarbonylmolybdenum (3).
The (7-phosphanobornadiene)pentacarbonylmolybdenum complex
1 (1 g, 1.77 mmol), 1-methylimidazole (0.57 mL, 7.08 mmol), and
ethanol (0.1 mL, 1.77 mmol) were heated at 40 °C in toluene (10
mL) for 6 h. After vacuum distillation, the residue was chromato-
graphed on silica gel with petroleum ether-ethyl ether (20:1) as
the eluent to yield 0.58 g (84%) of a blue solid: Rf 0.5 (petroleum
1
1
ether); 31P NMR (CDCl3) δ 123.3 (P-H, JPH ) 325.9 Hz); H
3
NMR (CDCl3) δ 1.21 (t, JH-H ) 7.0 Hz, CH3), 3.54-3.77 (m,
1
CH2), 7.46-7.62 (m, 5H, Ph), 7.57 (d, JP-H ) 327.7 Hz); 13C
3
2
NMR (CDCl3) δ 16.25 (d, JC-P ) 5.7 Hz, CH3), 68.96 (d, JC-P
) 14.6 Hz, CH2), 125.50-136.26 (m, C ortho, meta, para), 204.94
2
2
(d, JC-P ) 10.1 Hz, CO cis), 209.91 (d, JC-P ) 27.2 Hz, CO
trans); MS (FAB, 98Mo) m/z 392 (M+, 53%), 364 (M - CO, 38%),
336 (M - 2CO, 21%), 308 (M - 3CO, 36%), 280 (M - 4CO,
6%), 252 (M - 5CO, 22%). Anal. Calcd for C13H11MoO6P: C,
40.02; H, 2.84. Found: C, 40.07; H, 2.92.
(O-2-Hydroxyethyl phenylphosphinite)pentacarbonylmolyb-
denum (4). The (7-phosphanobornadiene)pentacarbonylmolybdenum
complex 1 (1 g, 1.77 mmol), 1-methylimidazole (0.57 mL, 7.08
mmol), and ethylene glycol (0.1 mL, 1.77 mmol) were heated at
40 °C in toluene (10 mL) for 6 h. After evaporation of the solvent,
the residue was purified by chromatography on silica gel. Elution
with petroleum ether-ethyl ether (1:1) gave 0.35 g of complex 4,
yield 49%: Rf 0.4 (1:1 petroleum ether-ethyl ether); 31P NMR
The reaction is complete in less than 1 h, and 8 is the sole
P-containing product according to the monitoring of the reaction
mixture by 31P NMR. These conditions are substantially milder
(15) Khan, A. A.; Wismach, C.; Jones, P. G.; Streubel, R. J. Chem. Soc.,
Dalton Trans. 2003, 2483.
(16) Appel, R.; Milker, R. Z. Anorg. Allg. Chem. 1975, 417, 161.
1
1
(CDCl3) δ 127.2 (P-H, JPH ) 331.8 Hz); H NMR (CDCl3) δ
1.91 (OH), 3.61-3.76 (m, CH2), 7.46-7.64 (m, 5H, Ph), 7.64 (d,

5034 Organometallics, Vol. 25, No. 21, 2006
Kalinina and Mathey
1JP-H ) 331.8 Hz); 13C NMR (CDCl3) δ 62.18 (d, 3JC-P ) 6.3 Hz,
CH2), 73.94 (d, 2JC-P ) 14.6 Hz, CH2), 129.20-135.64 (m, C ortho,
meta, para), 204.79 (d, 2JC-P ) 10.1 Hz, CO cis), 209.49 (d, 2JC-P
) 27.2 Hz, CO trans); MS (FAB, 98Mo) m/z 408 (M+), 380 (M -
CO, 34%), 352 (M - 2CO, 58%), 324 (M - 3CO, 18%), 268 (M
- 5CO, 45%). Anal. Calcd for C13H11MoO7P: C, 38.45; H, 2.73.
Found: C, 38.81; H, 2.82.
O-Ethyl phenyltrichloromethylthiophosphinate (10). The (7-
phosphanobornadiene)pentacarbonylmolybdenum complex 1 (0.51
g, 0.9 mmol), 1-methylimidazole (0.58 mL, 7.2 mmol), and
tetrachloromethane (0.70 mL, 7.2 mmol) were heated at 80 °C in
toluene (5 mL) for 1 h (8, 31P NMR δ 96.4). The mixture was
cooled to room temperature, and ethanol (0.08 mL, 1.8 mmol) was
added. The solution was stirred for a further 10 min and monitored
by 31P NMR (9, 31P NMR δ 123.5). Sulfurization was performed
by addition of sulfur powder (0.06 g, 1.8 mmol) at 40 °C for 1
day. After evaporation of the solvents, the residue was extracted
with ethyl ether and purified by chromatography on silica gel with
hexane as the eluent to yield 0.08 g (29%) of a bright yellow solid,
10: Rf 0.2 (hexane); 31P NMR (CDCl3) δ 82.5; 1H NMR (CDCl3)
3
δ 1.46 (t, JH-H 7.0 Hz, CH3), 4.41-4.61 (m, CH2), 7.44-8.16
(m, 5H, Ph); 13C NMR (CDCl3) δ 16.35 (d, 3JC-P ) 6.2 Hz, CH3),
65.25 (d, 3JC-P ) 6.2 Hz, CH2), 96.83 (d, 1JC-P ) 78.8 Hz, CCl3),
127.96-134.09 (m, C ortho, meta, para); MS (EI) m/z 305 (MH+).
Anal. Calcd for C9H10Cl3OPS: C, 35.61; H, 3.32. Found: C, 34.69;
H, 3.90.
Acknowledgment. The authors thank the University of
California Riverside and the CNRS for the financial support of
this work.
OM060460M