GaCl3-assisted [2 + 3] cycloaddition: A route to tetrazaphospholes

Article abstract of DOI:10.1039/b517459g

The GaCl₃-assisted [2 + 3] cycloaddition of Mes*-N=P-Cl (Mes* = 2,4,6-^tBu₃C₆H₂) with trimethylsilylazide (TMS-N₃) results in the formation of the first tetrazaphosphole, stabilized as a GaCl₃ adduct in high yields (>96%). The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517459g

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GaCl3-assisted [2 + 3] cycloaddition: A route to tetrazaphospholes{
Alexander Villinger, Peter Mayer and Axel Schulz*
Received (in Cambridge, UK) 9th December 2005, Accepted 18th January 2006
First published as an Advance Article on the web 9th February 2006
DOI: 10.1039/b517459g
The GaCl3-assisted [2 + 3] cycloaddition of Mes*–NLP–Cl
(Mes* = 2,4,6-tBu3C6H2) with trimethylsilylazide (TMS–N3)
results in the formation of the first tetrazaphosphole, stabilized
as a GaCl3 adduct in high yields (>96%).
Pentazole and pentaphosphole rings are the full iso(valence)elec-
tronic nitrogen and phosphorus analogues of the cyclopentadie-
nylide anion and the final members of the azaphosphole series
Scheme 1 Synthesis of 3.
(Fig. 1). While pentaphosphole derivatives are unknown, aryl
derivatives of pentazoles have been known since 1957.1 Pentazoles
(e.g. TMS–N3) are used, so that in the [2 + 3] cycloaddition, small
molecules such as TMS–Cl can be eliminated (Scheme 1).
were prepared at low temperatures by coupling diazonium salts
with azide ions following the procedure of Huisgen and Ugi.2
Otherwise, only the formation of a tetrazaphospholium tetra-
chlorogallate would be observed.4,5a
Recently, we reported on the successful preparation of the first
binary member of the azaphosphole series, the 4-bis(trimethyl-
silyl)amino-1,2,4,3,5-triazadiphosphole (1), which was synthesized
in a GaCl3-induced trimethylsilylchloride (TMS–Cl) elimina-
tion in N,N9,N9-tris(trimethylsilyl)]hydrazino(dichloro)phosphane
((TMS)2N(TMS)N–PCl2) followed by a [2 + 3] cycloaddition
reaction.3
Herein, we wish to report (i) a new high-yielding synthetic
procedure and (ii) a full experimental (Raman, 1H, 13C, 31P (MAS)
NMR, MS, X-ray) characterization of the first aryl-tetrazaphosp-
hole (2), stabilized as GaCl3 adduct (3), combined with DFT
calculations.
Adding TMS–N3 to the red solution of Mes*–NLP–Cl5b in
benzene results in a slight brightening of the red colouration. 31P
NMR experiments displayed a new phosphorus resonance (singlet
at d[31P] = 245.1 ppm) after one hour reaction time at ambient
temperature, besides the resonance of the starting material Mes*–
NLP–Cl (singlet at d[31P] = 135.1 ppm; cf. 1: 317.2 and 292.1 ppm,
trizaphosphole (R2N3PC, R = Ph):6 245 ppm). The best yield of
Mes*–N4P is obtained after one hour reaction time (intensity ratio
of 1.00 (Mes*–NLP–Cl) : 0.17 (Mes*–N4P)). Noticeable decom-
position is observed with longer reaction times. Based on the
excellent agreement between experiment and theory (dcalc[31P] =
243.5 ppm), this new resonance, which lies within the typical range
of dicoordinated phosphorus(III) compounds, could be assigned to
1-(2,4,6-tri-tert-butylphenyl)tetrazaphosphole (2). Upon adding
one equiv. of GaCl3 to this reaction mixture, the red colour
immediately vanishes and both 31P NMR resonances disappear,
while only one new singlet resonance at d[31P] = 228.8 ppm is
observed, which could be assigned to the GaCl3 adduct of 2 (cf.
theory: dcalc[31P] = 229.5 ppm). 31P MAS NMR experiments
(which revealed only a single resonance at diso[31P] = 223(15) ppm)
proved the existence of the same molecule in the solid state as that
observed in the solvent NMR study.
To prepare a tetrazaphosphole (aryl–N4P), another member of
the azaphosphole series (Fig. 1), it seemed quite promising to apply
the GaCl3-assisted elimination of TMS–Cl to the reaction of aryl–
NLP–Cl (aryl = Mes* = 2,4,6-tBu3C6H2) with trimethylsilylazide
(TMS–N3) in an 1,3-dipolar cycloaddition reaction (Scheme 1).
t
Reacting an alkyl-substituted azide (R–N3 (R = Bu, CEt3)) with
[Mes*–NLP+][AlCl42] results in the formation of tetrazapho-
spholium salts, first described by Niecke et al.4 However, the idea
of a GaCl3-assisted [2 + 3] cycloaddition to synthesize new
azaphospholes can only work when suitably substituted 1,3-dipoles
Fig. 1 P(III)–N five-membered rings. Bold marked species are known
and fully characterized, the novel tetrazaphosphole is marked by a dashed
rectangle (R = organic or inorganic group).
Removal of the solvent results in a colourless polycrystalline
powder.{ Pure, dry 3 is unstable at ambient temperature, is heat
and shock sensitive and decomposes slowly in the solid state and in
solvents, releasing N2 gas (detected by 14N NMR and MS
experiments). Colourless crystals of 3 rapidly become yellow when
traces of water or oxygen are present. The intrinsic N2 release is
reduced under pressure and at low temperatures. Hence, 3 can be
handled for a short period in the solid state and in common
Department Chemie und Pharmazie, Ludwig-Maximilians-Universita¨t
Mu¨nchen, Butenandtstraße 5–13 (Haus F), 81377 Mu¨nchen, Germany.
E-mail: Axel.Schulz@cup.uni-muenchen.de; Fax: (+49) 89 2180 77492;
Tel: (+49) 89 2180 77772
{ Electronic Supplementary Information (ESI) available: Crystallographic
data for 3 (Tables S1–S3), experimental and computational details
(calculated structural and vibrational data of all described species
(B3LYP)) (Tables S4–S6). See DOI: 10.1039/b517459g.
1236 | Chem. Commun., 2006, 1236–1238
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Fig. 2 ORTEP drawing of the molecular structure of 3 in the crystal.
Thermal ellipsoids with 50% probability at 200 K (hydrogen atoms and
t
the disordered Bu groups omitted for clarity). Selected bond lengths (s)
and angles (u)]: Ga–Cl1 2.146(1), Ga–Cl2 2.136(1), Ga–N1 1.990(4), P–N1
1.631(4), P–N4 1.664(3), N1–N2 1.374(5), N2–N3 1.286(5), N3–N4
1.355(5) and N4–C1 1.460(5); Cl1–Ga–Cl2 114.28(3), Cl1–Ga–N1
101.9(1), Cl1–Ga–Cl2i 114.28(3), Cl2–Ga–N1 105.00(6), Cl2–Ga–Cl2i
114.56(6), N1–P–N4 88.2(2), Ga–N1–P 129.7(2), Ga–N1–N2 116.1(3), P–
N1–N2 114.3(3), N1–N2–N3 111.8(4), N2–N3–N4 111.5(3), P–N4–N3
114.2(3), P–N4–C1 128.0(3) and N3–N4–C1 117.8(3). Symmetry code: i =
x, K 2 y, z.
Fig. 4 View along the a-axis in the crystal of 3.
typical range found for other GaCl3 adducts, cf. 1.978(3) in 1 and
2.003(5) s in Cl3Ga?NMe2SiMe2NMe2.3,9 The N–P–N angle of
88.2(2)u is rather small compared to the P–N–N (114–115u) and
N–N–N angles (111–112u). In the crystal, all molecules are parallel
to each other and the aryl groups are superimposed (Fig. 4). The
short P–N and N–N bond distances, together with the planarity,
indicate the presence of a strongly delocalized 6p-electron system,
which is supported by MO and NBO calculations (NBO = natural
bond orbital analysis).10
organic solvents (e.g. benzene, CH2Cl2, ether, etc.) when cooled
(T , 25 uC) and stored in the dark. 3 is easily prepared in bulk
and in high yields (>96%).
X-Ray quality crystals were obtained from a saturated CH2Cl2
solution of 3 at 25 uC, and the single crystal X-ray study revealed
an intriguing GaCl3-stabilized 1-(2,4,6-tri-tert-butylphenyl)tetraza-
phosphole. 3 crystallizes in colourless needles in the monoclinic
space group P21/m with two units per cell.§ As depicted in Fig. 2,
the PN4 ring is planar, like in the pentazoles and the triazadipho-
sphols.3,7 The PN4 ring is however slightly distorted, with two
larger N–N bond lengths (d(N1–N2) = 1.374(5) and d(N3–N4) =
1.355(5) s) and one very short N–N distance (d(N2–N3) =
1.286(5) s; cf. N–N distances between 1.307–1.338 s in
phenylpentazole and 1.380(5) s in 1). These N–N distances
between 1.28–1.38 s are substantially shorter than the sum of the
covalent radii (dcov(N–N) = 1.48 and dcov(NLN) = 1.20 s),8 which
indicates partial double bond character for all the N–N bonds,
with the N2–N3 bond being close to having a bond order of two.
Hence, the release of molecular nitrogen seems to be predisposed.
However, upon GaCl3 adduct formation, the ‘‘naked’’ PN4 ring
becomes sandwiched between the large aryl and large GaCl3
moieties, leading to an astonishing kinetic stability for 3 (Fig. 3).
A similar bond situation is found for the two P–N bonds
(1.631(4) and 1.664(3) s) of the PN4 ring, which also lie in the
range between a single and double bond (dcov(N–P) = 1.8 and
dcov(NLP) = 1.6 s). The Ga–N bond length of 1.990(4) s is in the
Adducts such as 3 are typical charge transfer complexes, and the
bond between the GaCl3 and the azaphosphole can be regarded as
a donor–acceptor bond.3 The calculated adduct formation ((gas
phase): Mes*–N4P + GaCl3 A 3) is exergonic (DG298 = 22.1 and
DH298 = 212.7 kcal mol21).{ According to NBO analysis, the
charge transfer is about 0.15e in 3 (cf. 0.16e in 1). The P–N s- and
p-bonds are highly polarized and almost ideally covalent between
the adjacent nitrogen atoms of the ring in 2 and 3.
In summary, 3 represents a GaCl3-stabilized tetrazaphosphole,
with an electronic structure that is related to those of aromatic
hydrocarbons that have (4n + 2) p-electrons and therefore formally
obey the Hu¨ckel rule. 3 can formally be regarded as the [2 + 3]
2
cycloaddition product of Mes*–PLN+ and N3 ions.
Caution: Like pentazoles, tetrazaphospholes and their adducts
are explosives and thermally unstable species; appropriate safety
precautions should be taken.
A. S. thanks Prof. Dr. T. M. Klapo¨tke (LMU Mu¨nchen) for his
generous support. We also wish to thank the Leibniz
Rechenzentrum for a generous allocation of CPU time. This
research was supported by the DFG SCHU 1170/4-1.
Notes and references
{ A solution of TMS–N3 (0.115 g, 1.0 mmol) in benzene (10 mL) was
added dropwise to a stirred solution of Mes*NPCl (0.326 g, 1.0 mmol) in
benzene (10 mL) at 5 uC. To the red solution, GaCl3 (0.194 g, 1.1 mmol) in
benzene (20 mL) was quickly added via syringe, resulting in a colourless
solution. Removal of the solvent in vacuo yielded 3 as fine, colourless,
needle-like crystals, yield 0.498 g, 0.98 mmol, 98%; mp 145–150 uC (dec.).
Anal. calc. % (found): C, 42.52 (42.06); H, 5.75 (5.35); N, 11.02 (10.61).
NMR (CD2Cl2): 1H 1.05 (s, 18 H), 1.37 (s, 9 H) and 7.66 (s, 2 H); 13C{1H}
30.9 (s, C9 (CH3)3), 32.9 (d, 5JPC = 3.8 Hz, C5 (CH3)3), 35.4 (s, C9), 37.0 (s,
C5), 124.8 (s, C3), 127.9 (d, 2JPC = 6.9 Hz, C2), 147.1 (d, 3JPC = 3.1 Hz, C1)
and 154.6 (s, C4) (Fig. 2). 31P{1H} 226.7 (s) (in C6D6: 228.8). 31P MAS
NMR (25 uC, vrot = 15 kHz): diso 223 (s). m/z (%): 332 (0.3) [2]+, 287 (0.8) [2
2 NP]+, 261 (4.0) [Mes*NH2]+, 259 (4.6) [Mes* + Me]+, 258 (7.9) [2 2
N3P]+, 246 (11.9) [Mes*]+, 245 (7.3) [Mes* 2 H]+, 244 (37.8) [Mes* 2 2 H]+,
Fig. 3 Space-filling models of 3 (left: N4P ring in plane, right: 90u out of
plane).
t
228 (1.8), 202 (2.1) [Mes* 2 C3H5]+, 188 (4.4) [Mes* 2 Bu]+, 132 (6.4)
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[Mes* 2 2tBu 2 2H]+, 56 (16.4) [tBu]+, 41 (30.8) [C3H5]+, 39 (10.4) [C3H3]+,
36 (4.9) [HCl]+ and 28 (100) [N2]+.
M. Nieger and F. Reichert, Angew. Chem., Int. Ed. Engl., 1988, 27, 12,
1715–1716.
§ A suitable single crystal of [Mes*N4P](GaCl3) (3) was mounted in Kel-F
oil on the end of a glass fiber and transferred to the N2 cold stream of a
Nonius Kappa CCD diffractometer (graphite-monochromated MoKa
5 (a) N. Burford, J. A. C. Clyburne, P. K. Bakshi and T. S. Cameron,
J. Am. Chem. Soc., 1993, 115, 8829–8830; (b) N. Burford, J. A.
C. Clyburne, P. Losier and T. M. Parks, in Synthetic Methods of
Organometallic and Inorganic Chemistry, ed. H. Karsch, Thieme-Verlag,
Stuttgart, 4th edn (Engl.), 1996, vol. 3, pp. 21–28.
6 Y. Charbonnel and J. Barrans, Tetrahedron, 1976, 32, 2039–2043.
7 F. Biesemeier, U. Mu¨ller and W. Massa, Z. Anorg. Allg. Chem., 2002,
628, 1933–1934.
˚
radiation with l = 0.71073 A). Crystal data: N4P1Ga1Cl3C18H29, FW =
3
508.5, monoclinic, V = 1200.47(6) A , a = 6.0103(2) A , b = 14.8760(4) A,
˚
˚
˚
˚
c = 13.4368(4) A, a = 90u, b = 92.2260(13)u, c = 90u, T = 200 K, space
group P21/m (No. 11), Z = 2, m(Mo-Ka) = 1.557 mm21, 22291 reflections
measured, 2857 unique (Rint = 0.119). The final R1 was 0.048 and wR2=
0.124. CCDC 292916. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b517459g
8 Sum of covalent radii: r(P) = 1.1 and r(N) = 0.7 s; A. F. Holleman and
E. Wiberg, Lehrbuch der Anorganischen Chemie, Walter de Gruyter,
1995, pp. 101.
9 W. R. Nutt, J. S. Blanton, A. M. Boccanfusco, L. A. Silks, III,
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J. E. Carpenter, The Structure of Small Molecules and Ions, Plenum
Press, New York, 1988, pp. 227.
1 R. Huisgen and I. Ugi, Chem. Ber., 1957, 90, 2914–2927.
2 (a) I. Ugi, H. Perlinger and L. Behringer, Chem. Ber., 1958, 91,
2324–2330; (b) I. Ugi and R. Huisgen, Chem. Ber., 1958, 91, 531–537.
3 S. Herler, P. Mayer, A. Schulz, A. Villinger and J. J. Weigand, Angew.
Chem., Int. Ed., 2005, 44, 7790–7793.
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W. W. Scho¨ller, Chem. Ber., 1993, 126, 7, 1513–1517; (b) E. Niecke,
1238 | Chem. Commun., 2006, 1236–1238
This journal is ß The Royal Society of Chemistry 2006