Unexpected dimerisation of a 2H-azaphosphirene complex

Article abstract of DOI:10.1039/b007107m

Heating a 2H-azaphosphirene complex in the solid state afforded the first head-to-tail dimer, a 2,5-dihydro-2,5-diphosphapyrimidine complex, a η¹-diphosphene complex and another complex having low-coordinated phosphorus centers; whereas the latter was detected only by ³¹P NMR spectroscopy, the dimer was isolated and structurally characterized.

Full text of DOI:10.1039/b007107m

Unexpected dimerisation of a 2H-azaphosphirene complex
Rainer Streubel,* Hendrik Wilkens, Frank Ruthe and Peter G. Jones
Institut für Anorganische und Analytische Chemie der Technischen Universität Braunschweig, Postfach 3329,
D-38023, Braunschweig, Germany. E-mail: r.streubel@tu-bs.de
Received (in Birmingham, UK) 1st September 2000, Accepted 3rd November 2000
First published as an Advance Article on the web
Heating a 2H-azaphosphirene complex in the solid state
afforded the first head-to-tail dimer, a 2,5-dihydro-2,5-di-
phosphapyrimidine complex, a h1-diphosphene complex
and another complex having low-coordinated phosphorus
centers; whereas the latter was detected only by 31P NMR
spectroscopy, the dimer was isolated and structurally
characterized.
In comparison to 2H-azirenes1 I (Scheme 1), which are versatile
building blocks in heterocycle syntheses, the chemistry of
heteroatom-substituted ring systems of type II–IV is much less
Scheme 2 Reagents and conditions: (i) 615 mg of complex 1, 130 °C, 15
explored. For example, transiently2 formed 2H-azasilirenes II
min; column chromatograpy (Al2O3, 250 °C, light petroleum–diethyl ether
readily dimerise in solution in a head-to-head or head-to-tail
90+10); 4: bright yellow solid, yield: 43%, mp 156 °C (decomp.).
manner, the orientation depending mainly on the carbon
substituents.3 Interestingly, even at low temperatures, 1H-
diazirenes III4 preferentially undergo ring enlargement4 or
rearrangement reactions,5 but do not form dimers. Although
2H-azaphosphirenes IV have been claimed as intermediates, no
dimerisation was reported; instead, ring cleavage occurred at
room temperature to yield a short-lived phosphinidene and a
nitrile derivative.6 1H-diazirene complexes V are, to the best of
our knowledge, unknown. Recently, we reported syntheses7 and
reactions of 2H-azaphosphirene complexes VI, which gave
access, e.g. to differently sized unsaturated phosphorus hetero-
cycles either via ring cleavage8 or via selective P–C9 and P–N
bond10 cleavage ring expansion reactions.
1
3
162.1, J(W,P) 264.3 Hz) (d (PM) 160.9 ppm, J(P,P) 162.1,
1J(W,P) 272.4 Hz) revealed the existence of low-coordinated
phosphorus centers,13 thus allowing a tentative structural
assignment with an atom connectivity as shown in Scheme 2
(PA = PNNR,PM. = PNCR2). The unusually great 3J(P,P) value
1
might result from the two-fold h -coordination mode of the two
phosphorus centers.14 The main product, the 2,5-dihydro-
2,5-diphosphapyrimidine tungsten complex 4, was confirmed
by elemental analyses and NMR spectroscopy.† Further
investigations using the analogous 2H-azaphosphirene chro-
mium and molybdenum complexes15 showed that the reaction
course strictly depended on the nature of the metal; in contrast
to the chromium (and tungsten) complex, heating the 2H-
azaphosphirene molybdenum complex afforded the correspond-
ing h -diphosphene molybdenum complex12 as main product.
1
The 31P NMR data of complex 4 are similar to those found for
non-coordinated 2,5-dihydro-2,5-diphosphapyrimidine deriva-
tives, which have been recently synthesized for the first time
using a completely different approach.16 The line broadenings
1
in the H and 13C{1H} NMR spectra of complex 4 at ambient
temperature, which disappear upon cooling to 255 °C are
remarkable; e.g. in the 13C{1H,31P} NMR spectra two distinct
resonances appear for the imino and also for the ipso carbon
atoms of the phenyl rings. So far, we have no explanation for
this phenomenon. Furthermore, the methine carbon atom (d 9.9
ppm) and also the protons of one trimethylsilyl group (d 20.73
ppm) of the three-coordinated phosphorus center resonate at
remarkably high field, which probably indicates a shielding
caused by the ring current of a nearby phenyl group. The
molecular structure of complex 4 (Fig. 1), as established for the
solid state by X-ray crystallography,‡ shows a slightly distorted
boat conformation of the six-membered ring with the P1 and P2
atoms in the bow and stern positions (Fig. 2). The steric
situation of the P2-bonded bis(trimethylsilyl)methyl group is
noteworthy; as shown by the two different distances P2–C20
1.882(9) Å and P2–C13 1.829(10) Å, this group is asymmet-
rically flanked by the two phenyl rings, which act like a pincer,
but only for one of the two trimethylsilyl groups. If this steric
situation is retained in solution, this would convincingly explain
the observed shielding of the proton and carbon atoms of the
respective methine and trimethylsilyl groups.
Scheme 1 2H-azirenes (I), heteroazirenes (II–IV), 1H-diazirene (V) and
2H-azaphosphirene complexes (VI) (I–VI: R, RA = alkyl, aryl; [M] =
metal complex fragment).
Here we report the first dimerisation of a 2H-azaphosphirene
complex leading to a head-to-tail dimer, a 2,5-dihydro-
1
2,5-diphosphapyrimidine complex; a h -diphosphene complex
and another complex having low-coordinated phosphorus
centers were detected as by-products.
Heating the neat 2H-azaphosphirene tungsten complex 111
for a short period afforded three phosphorus-containing pro-
ducts 2, 3 and 4 in a ratio of ca. 1+3+10 (Scheme 2); heating
complex 1 for longer periods, i.e. 1 h, did not significantly affect
1
the ratio. Whereas the h -diphosphene tungsten complex 212
1
([d8]-toluene): d 391.3 ppm (1J(P,P) 513.4 Hz, J(P,W) 230.8
Hz, PNPW(CO)5), 443.1 ppm (1J(P,P) 513.4 Hz) was isolated in
8% yield by column chromatography, the dinuclear complex 3
decomposed during chromatography, but the chemical shifts,
phosphorus–phosphorus and phosphorus–tungsten coupling
constant magnitudes of 3 (AM-type; d (PA) 360.9 ppm, 3J(P,P)
Currently we are investigating the reaction of 2H-azaphos-
phirene complexes with various nickel(II) and palladium(0)
complexes; such complexes have been shown to catalyze the
DOI: 10.1039/b007107m
Chem. Commun., 2000, 2453–2454
This journal is © The Royal Society of Chemistry 2000
2453

(dd, 1J(P,C) 27.0 Hz, 4J(P,C) 1.5 Hz, CH(SiMe3)2), 128.2 (s br, CHaromat.),
128.3 (s br, CHaromat.), 129.7 (s, CHaromat.), 130.1 (s br, CHaromat.), 140.4
(mc, Caromat.), 184.6 (m, PNC), 197.8 (dd, 2J(P,C) 7.4 Hz, 4J(P,C) 2.2 Hz,
cis-CO), 199.5 (d, 2J(P,C) 24.2 Hz, trans-CO). 13C{1H,31P}-NMR (255
°C): d 1.3 (s, SiMe3), 2.4 (s, SiMe3), 3.0 (s, SiMe3), 8.9 (s, CH(SiMe3)2),
28.4 (s, CH(SiMe3)2), 128.1 (s, CHaromat.), 128.7 (s, CHaromat.), 129.6 (s,
CHaromat.), 130.6 (s, CHaromat.), 138.9 (s, Caromat.), 140.3 (s, Caromat.), 183.6
(s, PNC), 185.6 (s, PNC), 197.6 (s, cis-CO), 200.2 (s, trans-CO). 31P{1H}-
NMR: d 20.6 (mc, P2), 79.9 (d, 3J(P,P) 15.2 Hz, 1J(P,W) 266.1 Hz, P1).
‡ Crystal structure determination for fidi1: C33H48N2O5P2Si2W; M =
910.90, monoclinic, space group P2/n, a = 18.440(3), b = 9.831(3), c =
25.145(5) Å, b = 111.491(10)°, U = 4241.3(16) Å3, Z = 4, Dc = 1.426 Mg
m23, m = 2.949 mm21, F(000) = 1840, 7430 independent reflections to
2qmax. 50°, T = 173 K, S = 0.763, R(F > 4s(F)) = 0.0537, Rw(F2) =
0.0731, 171 restraints and 424 parameters, highest peak 1.162 and deepest
hole 20.712 e Å23. The X-ray dataset was collected with monochromated
Mo-Ka radiation (l
= 0.71073 Å) on a Siemens P4 four-circle
diffractometer. CCDC 182/1840. See http://www.rsc.org/suppdata/cc/b0/
b007107m/ for crystallographic files in .cif format.
Fig. 1 Molecular structure of 4 in the crystal (ellipsoids represent 50%
probability levels, hydrogen atoms are omitted for clarity). Selected bond
lengths [Å] and angles [°]: W–C(2) 1.986(10), W–P(1) 2.521(2), P(1)–C(6)
1.796(8), P(1)–N(1) 1.7107(7), P(1)–N(2) 1.698(7), N(1)–C(13) 1.280(9),
N(2)–C(20) 1.267(9), P(2)–C(27) 1.852(9), C(13)–C(14) 1.514(11), C(13)–
C(14) 1.482(12); C(2)–W–P(1) 176.0(3), W–P(1)–C(6) 115.7(3), N(2)–
P(1)–N(1) 103.6(3), N(2)–P(1)–C(6) 107.1(4), N(1)–P(1)–C(6) 102.0(4),
C(13)–P(2)–C(20) 95.5(4).
1 For reviews on 2H-azirenes see: A. Padwa, Acc. Chem. Res., 1976, 9,
371; V. Nair and K. H. Kim, Heterocycles, 1977, 7, 353; H.-J. Hansen
and H. Heimgartner, in 1,3-Dipolar Cycloaddition Chemistry, ed. A.
Padwa, Wiley, New York, 1984, ch. 2, p. 177.
2 A stable 2H-azasilirene derivative has been described, but no reactions
were mentioned: (a) R. Okazaki, H. Suzuki and N. Tokitoh, XIth
International Symposium on Organosilicon Chemistry, Montpellier,
France, 1996, poster abstract PB60; (b) R. Okazaki and R. West, in
Multiply Bonded Main Group Metals and Metalloids, ed. R. West and F.
G. A. Stone, Academic Press, London, 1996, p. 232.
3 M. Weidenbruch and A. Schäfer, J. Organomet. Chem., 1986, 314, 25;
M. Weidenbruch and P. Will, Z. Anorg. Allg. Chem., 1996, 622,
1811.
4 For a review on transiently formed 1H-diazirenes, see: X. Creary, Acc.
Chem. Res., 1992, 25, 31.
5 G. Alcaraz, V. Piquet, A. Baceiredo, F. Dahan, W. W. Schoeller and G.
Bertrand, J. Am. Chem. Soc., 1996, 118, 1060.
6 M. Rahmoune, Y. Y. C. Yeung Lam Ko, F. Tonnard and R. Carrie, PSI-
BLOCS Conference, Palaiseau, France, 1988, poster abstract.
7 R. Streubel, J. Jeske, P. G. Jones and R. Herbst-Irmer, Angew. Chem.,
Int. Ed. Engl., 1994, 33, 80.
Fig. 2 Side-view showing the boat conformation of 4 (reduced molecular
structure).
8 R. Streubel, A. Ostrowski, H. Wilkens, F. Ruthe, J. Jeske and P. G.
Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 378.
9 R. Streubel, H. Wilkens, A. Ostrowski, C. Neumann, F. Ruthe and P. G.
Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 1492.
10 R. Streubel, C. Neumann and P. G. Jones, J. Chem. Soc., Dalton Trans.,
2000, 2495.
11 R. Streubel, A. Ostrowski, S. Priemer, U. Rohde, J. Jeske and P. G.
Jones, Eur. J. Inorg. Chem., 1998, 257.
12 H. Lang, O. Orama and G. Huttner, J. Organomet. Chem., 1985, 291,
293.
dimerisation of 1H-phosphirenes and 1H-phosphirene com-
plexes.17
We are grateful to Priv.-Doz. Dr Dietrich Gudat for low-
temperature NMR measurements and to the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie
for financial support.
13 M. Regitz and O. J. Scherer, Multiple Bonds and Low Coordination in
Phosphorus Chemistry, Thieme, Stuttgart, 1990.
Notes and references
† Satisfactory elemental analysis were obtained for complex 4. NMR data
were recorded in CDCl3 solutions at 50.3 MHz (13C) and 81.0 (31P), using
TMS and 85% H3PO4 as standard references; J/Hz. Selected spectroscopic
data for 4: 1H-NMR (rt): d 20.15 (v br, 18H, SiMe3), 0.34 (s, 18H, SiMe3),
0.98 (d, 2J(P,H) 11.8 Hz, 1H, P2CH), 1.87 (d, 2J(P,H) 4.7 Hz, 1H, P1CH),
14 On the change of the J(P,P) coupling constant value of a 1,3-diphos-
phabuta-1,3-diene derivative upon metal coordination, see: R. Appel, B.
Niemann, W. Schuhn and N. Siabalis, J. Organomet. Chem., 1988, 347,
299.
15 R. Streubel, F. Ruthe and P. G. Jones, Eur. J. Inorg. Chem., 1998,
571.
16 A. Maraval, B. Donnadieu, A. Igau and J. P. Majoral, Organometallics,
1999, 18, 3138.
17 N. H. Tran Huy, L. Ricard and F. Mathey, J. Chem. Soc., Chem.
Commun., 2000, 1137.
1
7.43 (s br, 10H, CHaromat.). H-NMR (255 °C): d 20.73 (s, 9H, SiMe3),
2
0.28 (s, 18H, SiMe3), 0.31 (s, 9H, SiMe3), 0.94 (d, J(P,H) 11.8 Hz, 1H,
P2CH), 1.80 (d, 2J(P,H) 4.7 Hz, 1H, P1CH), 7.30 (s br, 4H, CHaromat.), 7.44
(s br, 6H, CHaromat.). 13C{1H}-NMR (rt): d 2.1 (d br, 3J(P,C) 3.8 Hz, SiMe3),
3.3 (d, 3J(P,C) 1.8 Hz, SiMe3), 9.9 (d, 1J(P,C) 52.6 Hz, CH(SiMe3)2), 29.2
2454
Chem. Commun., 2000, 2453–2454