Stabilization of acyclic phosphazides using the ortho-closo- dicarbadodecaboranyl residue

Article abstract of DOI:10.1039/c0cc00426j

The reactions of triphenylphosphine with the azido-ortho-closo- dicarbadodecaboranes 1-N₃-2-R-closo-1,2-C₂B ₁₀H₁₀ [R = Me (1a), Ph (1b)] produce the stable and isolatable carboranyl phosphazides 1-(N₃

Full text of DOI:10.1039/c0cc00426j

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Stabilization of acyclic phosphazides using the
ortho-closo-dicarbadodecaboranyl residuew
Robert D. Kennedy*
Received 17th March 2010, Accepted 24th April 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/c0cc00426j
The reactions of triphenylphosphine with the azido-ortho-closo-
dicarbadodecaboranes 1-N3-2-R-closo-1,2-C2B10H10 [R = Me
(1a), Ph (1b)] produce the stable and isolatable carboranyl
phosphazides 1-(N3PPh3)-2-R-closo-1,2-C2B10H10 [R = Me
(2a), Ph (2b)] in good yields. Single crystal X-ray diffraction
analysis revealed that, in the solid state, phosphazide 2a adopts
an unusual s-cis conformation, whereas 2b adopts an s-trans
conformation.
Blanch et al.3 Thence, the addition of a small excess of triphenyl-
phosphine to compounds 1a or 1b in diethyl ether solution at
room temperature quickly resulted in the precipitation of the
phosphazides 1-(N3PPh3)-2-R-closo-1,2-C2B10H10 [R = Me (2a),
Ph (2b)] which could be isolated in 80 and 89% yields, respec-
tively. A single-crystal X-ray diffraction analysis of compound 2a
revealed that the phosphazide {PNNN} unit adopts a planar s-cis
conformation in the crystal (Fig. 1).z The P(1)–N(1) distance
[1.666(2) A], which lies between a formal P–N single bond
(ca. 1.8 A) and a PQN double bond (ca. 1.55 A), together with
the similarity of the N(1)–N(2) and N(2)–N(3) distances,
Phosphazides, which are most commonly encountered as inter-
mediates in the synthetically important Staudinger reaction,1
are typically only transient species and eliminate dinitrogen at
room temperature or below to form the corresponding imino-
phosphorane ylides. It has been suggested that this process
occurs via a four-membered cyclic transition state.2 Various
strategies have been employed to stabilize and isolate phospha-
zides, including their incorporation into cyclic or macrocyclic
frameworks,2b coordination to transition-element2c or main-
group-element centers,2d and the use of substituents which
provide electronic2e and steric2f,g stabilization. Of the ten
acyclic, non-coordinated phosphazides that have been crystallo-
graphically characterized, seven enjoy some degree of electronic
stabilization, either through the incorporation of a bis- or
tris(dialkylamino)phosphine unit, or through an electron-
withdrawing aromatic moiety bound to the terminal nitrogen
atom. In these cases, the phosphazide units adopt s-trans
conformations. In 1996, Molina et al. reported the first acyclic
s-cis phosphazide, Ph3PN3C(CN)Ph2,2f and, in 2009, Fortman
et al. reported the second example of an s-cis phosphazide, viz.
the tricyclohexylphosphine adamantyl phosphazide, Cy3PN3Ad.2g
It is important to isolate other examples of phosphazides in
order to effect delineation and comparison of geometrical
and electronic bonding parameters as the substituents are
varied, and in this report use is made of the binding properties
and steric demands of the ortho-closo-dicarbadodecaboranyl
(o-carboranyl) residue, which differs somewhat from organic
residues such as trityl or adamantyl, to stabilize the phosphazide
unit.
Fig. 1 ORTEP-type representations of the crystallographically derived
molecular structures of 2a (top) and 2b (bottom). The thermal
ellipsoids are drawn at 50% probability. Selected interatomic distances
(A) and angles (1) for 2a: P(1)–N(1) 1.666(2); N(1)–N(2) 1.322(2);
N(2)–N(3) 1.285(2); P(1)ꢀ ꢀ ꢀN(3) 2.703(3); N(3)–C(1) 1.431(2);
C(1)–C(2) 1.693(2); P(1)–N(1)–N(2) 118.5(1); N(1)–N(2)–N(3)
116.2(1); N(2)–N(3)–C(1) 111.4(1); N(2)–N(3)–C(1)–C(2) 88.6(1);
N(3)–N(2)–N(1)–P(1) 3.02(17). For 2b: P(1)–N(1) 1.639(1);
N(1)–N(2) 1.344(1); N(2)–N(3) 1.274(1); N(3)–C(1) 1.432(2);
C(1)–C(2) 1.680(2); P(1)–N(1)–N(2) 110.76(8); N(1)–N(2)–N(3)
111.3(1); N(2)–N(3)–C(1) 110.1(1); N(2)–N(3)–C(1)–C(2) 178.26(9);
N(3)–N(2)–N(1)–P(1) 178.18(8).
The readily accessible azido-o-carborane precursors,
1-N3-2-R-closo-1,2-C2B10H10 [R = Me (1a), Ph (1b)], were
synthesized following a procedure modified from that of
Department of Chemistry and Biochemistry, University of California
at Los Angeles, 607 Charles E. Young Drive (East), Los Angeles,
California, 90095, USA. E-mail: rkennedy@chem.ucla.edu;
Tel: +1 310 825 9505
w Electronic supplementary information (ESI) available: Experimental
procedures and spectra. Computational details. CIF files for compounds
2a, 2b, 3a and 3b. CCDC 770278–770281. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc00426j
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suggest significant delocalization throughout the {PNNN}
unit. The P(1)ꢀ ꢀ ꢀN(3) interatomic distance is significantly
below the sum of the van der Waals radii, and it
has been suggested that the s-cis conformation, although
approaching the transition state for dinitrogen extrusion, is
stabilized via an electrostatic interaction between P(1) and
N(3).4 In support of this, a natural population analysis5 of
compound 2a places a large positive charge (+1.75) on P(1)
and a modest negative charge (ꢂ0.40) on N(3). The negative
charge (ꢂ0.75) on N(1) reflects the partial ylide character of
the P–N bond, and also the delocalization along the {PNNN}
unit. The interatomic distances and angles of the {PNNN}
unit are very similar to those previously reported for Molina
et al.’s2f and for Fortman et al.’s2g s-cis phosphazides, indicating
no special electronic involvement of the o-carboranyl residue.
Although the N(2)–N(3)–C(1)–C(2) torsion angle of 88.6(1)1
indicates some specific orbital interaction between the {PNNN}
unit and the cage, the exo-C–N bond length [1.431(2) A] and the
endo-C–C interatomic distance [1.693(2) A], which are both
sensitive to the degree of exo-p-bonding,6 indicate that the
carboranyl fragment does not behave as a stabilizing p-acceptor
in this case. Furthermore, the calculated bond orders for
N(3)–C(1) and C(1)–C(2) are 1.046 and 0.651, respectively.
Collectively these results suggest that the carboranyl residue
acts mainly as a bulky group in stabilizing the s-cis phosphazide
by sterically preventing it from adopting the transition-state
geometry required for dinitrogen extrusion.
of 2a and 2b despite the differences in their solid-state struc-
tures. 3a and 3b were obtained quantitatively from solutions of
2a and 2b in refluxing benzene. Single crystal X-ray diffraction
analyses were possible in both cases, revealing that both
compounds have typical iminophosphorane-type structures
(Fig. 2).z Compared to the phosphazides, the P(1)–N(1) and
exo-C(1)–N(1) bonds of 3a and 3b are significantly shorter,
whereas the endo-C(1)–C(2) interatomic distances are elongated.
This suggests stronger exo-p-bonding with the carborane cage,
possibly due to the pronounced ylide character of the {PN}
unit. The large positive charge on P(1) (+1.88 for both 3a and 3b),
the large negative charge on N(1) (ꢂ1.11 for 3a, ꢂ1.12 for 3b),
and the calculated bond orders [1.136 and 1.121 for N(1)–C(1),
0.556 and 0.586 for C(1)–C(2)] are in agreement with this
suggestion. Similarly to the phosphazide systems 2a and 2b,
the P(1)–N(1)–C(1)–C(2) torsion angles of 3a and 3b are very
different: in methyl derivative 3a, the {PN} unit approaches a
perpendicular orientation relative to the C(1)–C(2) vector,
whereas in the phenyl derivative 3b the {PN} unit and the
C(1)–C(2) vector are approximately parallel. This may well be
due to the greater steric demand of the phenyl group relative
to the methyl group. It should also be noted that, similarly to
phosphazides 2a and 2b, the endo-C(1)–C(2) distance is shorter
in the phenyl derivative 3b than in the methyl derivative 3a,
In contrast to compound 2a, the {PNNN} unit of the phenyl
derivative 2b adopts an s-trans conformation in the solid state
and the N(2)–N(3)–C(1)–C(2) torsion angle is 178.26(9)1
(Fig. 1).z Despite these gross structural differences, the inter-
atomic distances and angles are remarkably similar. The bond
alternation in the {PNNN} unit is slightly more pronounced in
2b and the bond angles around N(1) and N(2) are smaller than
in 2a. The exo-C(1)–N(3) bond length [1.432(2) A] and the
endo-C(1)–C(2) interatomic distance [1.680(2) A] are similar to
those of compound 2a, indicating that the conformation of the
{PNNN} unit and its orientation relative to the carboranyl
residue have little influence on the degree of exo-p-bonding.
Although it is known that perpendicularly oriented aryl groups
often increase the endo-C(1)–C(2) distance,6,7 the endo-
C(1)–C(2) distance in 2b is 0.13 A shorter than in the methyl
derivative 2a, and it cannot be discounted that the influence of
the phenyl group may mask any changes in geometry resulting
from the different conformation and orientation of the
{PNNN} unit. The similarities between 2a and 2b are also
reflected in the calculated bond orders [1.053 for N(3)–C(1)
and 0.678 for C(1)–N(1)] and the distribution of atomic charge
[+1.78 for P(1), ꢂ0.78 for N(1), ꢂ0.29 for N(3)]. It appears
that the overall conformation of compound 2b may well be a
result of the larger steric demand of the phenyl group com-
pared to the methyl group, which prevents the {PNNN} unit
from adopting the stabilized s-cis conformation.
Fig. 2 ORTEP-type representations of the crystallographically derived
molecular structures of 3a (top) and 3b (bottom). The thermal
ellipsoids are drawn at 50% probability. Selected interatomic distances
(A) and angles (1) for 3a: P(1)–N(1) 1.580(1); N(1)–C(1) 1.368(1);
C(1)–C(2) 1.763(2); P(1)–N(1)–C(1) 141.05(9); P(1)–N(1)–C(1)–C(2)
111.2(1). For 3b: P(1)–N(1) 1.556(1); N(1)–C(1) 1.360(1); C(1)–C(2)
1.784(2); P(1)–N(1)–C(1) 132.16(7); P(1)–N(1)–C(1)–C(2) 173.5 (1);
H(10)ꢀ ꢀ ꢀC(3) 2.862(15).
Compounds 2a and 2b are sufficiently robust that they are
stable in the solid state at room temperature in the dark.
However, they do decompose slowly and cleanly in solution to
form the corresponding iminophosphoranes 1-(NQPPh3)-2-
R-closo-1,2-C2B10H10 [R = Me (3a), Ph (3b)], and there
appear to be no significant differences in the relative stabilities
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suggesting that the influence of the orientation of the {PN}
unit relative to the cage may be masked by the influence of the
perpendicularly oriented phenyl group. An interesting feature
of the structure of 3b is the conspicuous orientation of one
of the phosphine phenyl rings. The ring plane is oriented
approximately parallel to the P(1)–N(1)–C(1)–C(2) plane and
perpendicularly to the plane of the cage-bound phenyl ring.
The short distance between H(10) and the mean plane of the
phenyl ring [2.67(2) A] indicates that a C–Hꢀ ꢀ ꢀp interaction
may be a contributory factor in the stabilization of this
conformation.
reflections collected, 6301 unique (Rint = 0.0165), GooF = 1.029, R1
(Fo) = 0.0353 for 5808 reflections with I > 2s(I), wR2 (Fo2) = 0.0992
for all unique reflections. For compound 3b: colorless, from
CH2Cl2–n-C6H14 at 298 K, 0.40 ꢃ 0.40 ꢃ 0.40 mm, C26H30B10NP,
M = 495.58, monoclinic, space group P21/n (no. 14), a = 14.641(4),
b = 11.920(3), c = 15.492(4) A, a = 90.00, b = 92.397(3), g = 90.001,
V = 2701.2(13) A3, Z = 4, Dc = 1.219 g cmꢂ3, F000 = 1032, Mo-Ka
radiation, l = 0.71073 A, m = 0.120 mmꢂ1 T = 100(2) K, 2ymax
=
,
58.361, 34466 reflections collected, 7235 unique (Rint = 0.0307),
GooF
= 1.037, R1 (Fo) = 0.0375 for 6092 reflections with
I > 2s(I), wR2 (Fo2) = 0.1056 for all unique reflections.
1 Y. G. Gololobov and L. F. Kasukhin, Tetrahedron, 1992, 48, 1353;
Yu. G. Gololobov, I. N. Zhmurova and L. F. Kasukhin, Tetra-
hedron, 1981, 37, 437.
Preliminary investigations show that these interesting new
iminophosphoranes 3a and 3b are relatively resistant to hydro-
lysis and are unreactive substrates for the aza-Wittig reaction.8
Inspection of the molecular structures reveals that the {PN}
unit is deeply embedded and severely hindered in both cases,
perhaps explaining this lack of reactivity. However, the reac-
tion of azide 1a with triethylphosphite at 0 1C results in
immediate effervescence and conversion to the corresponding
triethylphosphorimidate in a high yield. This observation
may have useful synthetic portent, in that less hindered
phosphazines such as these may be reactive substrates, allowing
access to novel N-carboranyl imines and their derivatives via
the aza-Wittig reaction. The simple synthesis and isolation of
two stable phosphazides illustrates how the unique reactivities
and geometries of borane and heteroborane cluster units, here
the ortho-carboranyl unit, may be utilized to stabilize and
study ordinarily reactive and unstable species. More generally,
few attempts have been made to unite the diverse chemistries
of azides9 and deltahedral boron-containing species,10 and
their integration generates exciting prospects.
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I thank Dr Saeed Khan for guidance and assistance with the
crystallographic work and Prof. Yves Rubin for his encourage-
ment and financial support. This work was supported by the
National Science Foundation through research (NSF-CHE-
0617052) and instrumentation grants (NSF-CHE-9974928;
NSF-CHE-9871332).
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Notes and references
z Crystal data for compound 2a: colorless, from CH2Cl2–n-C6H14 at
298 K, 0.30 ꢃ 0.20 ꢃ 0.10 mm, C21H28B10N3P, M = 461.53, triclinic,
ꢀ
space group P1 (no. 2), a = 8.7610(12), b = 11.6685(16), c =
13.3020(18) A, a = 65.7590(10), b = 88.741(2), g = 89.749(2)1,
V = 1239.6(3) A3, Z = 2, Dc = 1.237 g cmꢂ3, F000 = 480, Mo-Ka
=
radiation, l = 0.71073 A, m = 0.128 mmꢂ1, T = 100(2) K, 2ymax
58.421, 15 515 reflections collected, 6433 unique (Rint = 0.0235),
GooF = 1.031, R1 (Fo) = 0.0422 for 5303 reflections with I >
2s(I), wR2 (Fo2) = 0.1171 for all unique reflections. For compound
2b: colorless, from CHCl3–Et2O at 258 K, 0.20 ꢃ 0.15 ꢃ 0.05 mm,
ꢀ
C26H30B10N3P, M = 523.60, triclinic, space group P1 (no. 2), a =
7 Z. G. Lewis and A. J. Welch, Acta Crystallogr., Sect. C: Cryst.
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232, 173.
10.694(5),
b = 11.893(5), c = 12.072(5) A, a = 66.726(5),
b = 89.472(5), g = 82.792(5)1, V = 1397.9(10) A3, Z = 2, Dc
=
1.244 g cmꢂ3, F000 = 544, Mo-Ka radiation, l = 0.71073 A, m =
0.122 mmꢂ1, T = 100(2) K, 2ymax = 58.521, 18 803 reflections
collected, 7371 unique (Rint = 0.0261), GooF = 1.035, R1 (Fo) =
0.0388 for 6141 reflections with I > 2s(I), wR2 (Fo2) = 0.1087 for all
unique reflections. For compound 3a: colorless, from CH2Cl2–n-C6H14
at 298 K, 0.60 ꢃ 0.40 ꢃ 0.30 mm, C21H25B10NP, M = 430.49, triclinic,
ꢀ
space group P1 (no. 2), a = 9.309(2), b = 9.342(2), c = 15.134(5) A,
a = 100.552(4), b = 96.851(4), g = 108.414(2)1, V = 1205.3(6) A3,
Z = 2, Dc = 1.186 g cmꢂ3, F000 = 446, Mo-Ka radiation, l =
0.71073 A, m = 0.125 mmꢂ1, T = 100(2) K, 2ymax = 58.301, 15 952
ꢁc
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4784 | Chem. Commun., 2010, 46, 4782–4784