Structure and Stability of an Azoalkane-Iodine Complex

Full text of DOI:10.1021/jo980265k

5666
J . Org. Chem. 1998, 63, 5666-5667
Str u ctu r e a n d Sta bility of a n
Azoa lk a n e-Iod in e Com p lex
Paul S. Engel,* Shaoming Duan, and
Kenton H. Whitmire
Department of Chemistry, Rice University, P.O. Box 1892,
Houston, Texas 77251
Received February 13, 1998
Halogen complexes of aromatic hydrocarbons,1-3
olefins,4-6 amines,7,8 and other donors9,10 have been
known for many decades, but charge-transfer (CT) com-
plexes of azoalkanes were discovered more recently.11 In
the course of investigating the oxidation of 1,2-dialkyl-
hydrazines by halogens, we found that azoalkanes also
form CT complexes with halogens, which are xσ-type
acceptors.12 While earlier workers11 had mentioned the
color change that occurred on mixing of 2,3-diazabicyclo-
[2.2.2]oct-2-ene (DBO) with iodine, we were able to isolate
a single crystal of this complex and determine its X-ray
structure. Hydrazine also forms complexes with halo-
gens;13 hence, the halogen oxidation of hydrazines to
azoalkanes may be complicated by both hydrazine and
azo CT complexes.
F igu r e 1. UV-vis absorption spectrum in n-heptane of (a)
0.00406 M DBO, (b) 4.11 × 10-4 M I2, offset ) +0.5, (c) 0.00775
M DBO + 4.88 × 10-5 M I2, offset ) +1.2, and (d) 0.0205 M
DBO + 4.57 × 10-4 M I2, offset ) +1.2.
in 100-fold excess and used the Scott equation (7 points,
r ) 0.9998)19 to calculate the extinction coefficient and
the equilibrium constant. The value of ꢀ (18 500) is
similar to that of aliphatic amine-I2 complexes (ꢀ
∼25 000)20 but is smaller than that of pyridine-I2 (ꢀ )
51 700).16 Our ꢀ cannot be compared with that of
previously known DBO CT complexes because no short-
wavelength band was reported.11 The extinction coef-
ficient of our 424 nm band is ∼2000, comparable to that
of DBO-TCNE11 and to the 422 nm band of pyridine-
I2.16 The observed value of K (153 M-1) is far larger than
Mixing a heptane solution of I2 (4.88 × 10-5 M) with
one of DBO (0.00775-0.0493 M) gave immediate rise to
a light yellow color caused by a weak band at 424 nm
and to a stronger band at 266 nm, both attributed to the
CT complex (cf. Figure 1).14,15 The longer wavelength of
the of DBO-I2 CT band (266 nm) than of the analogous
pyridine-I2 band (236 nm)16 is in the direction expected
from the lower IP of DBO (8.19 eV)17 than pyridine (9.26
eV).18 We determined the absorbance of the 266 nm band
at 25 °C as a function of DBO concentration with DBO
that of other known DBO complexes: TCNE (1 M-1),
11
CBr4 (5 M-1), or AgNO3 (1 M-1
)
is smaller than the
highly structure-dependent values for amines (K ) 530-
21
12 100)20 but is very similar to that of R2S-I2
and
pyridine-I2 complexes.16 This last result is in accord
with the sp2 hydridization of nitrogen in both cases but
does not reflect the lower IP of DBO.9
(1) Hubig, S. M.; J ung, W.; Kochi, J . K. J . Org. Chem. 1994, 59,
6233.
(2) Turner, D. E.; O’Malley, R. F.; Sardella, D. J .; Barinelli, L. S.;
Kaul, P. J . Org. Chem. 1994, 59, 7335.
The enthalpy of complexation was calculated from the
266 nm absorbance of a sealed, degassed, and N2 back-
filled heptane solution of 0.0207 M DBO and 4.61 × 10-5
M I2 in a quartz cuvette at four temperatures from 8.8
to 30.6 °C. A plot of ln K vs 1/T yielded ∆H ) -6.2 kcal/
mol (r ) 0.9987), which was combined with our K(25 °C)
to yield ∆S ) -11 eu. These values are similar to those
for I2-ether complexes.15
A single crystal of the DBO-I2 complex was grown by
slow diffusion of ether into a MeCN solution of the
components at -5 °C. Because the crystal was unstable
to X-rays at room temperature, the data were acquired
at -100 °C. The refined structure (R ) 0.0382, cf. Figure
2) reveals that the geometry of the DBO moiety resembles
that of DBO itself.22 DBO is close to C2v symmetry, as
shown by the C3-N2-N1-C6 dihedral angle of 1.5° and
the nearly eclipsed conformation (5.3° deviation) about
the C4-C3 and C7-C8 bonds. Whereas the two C-Nd
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Soc. 1983, 105, 3649.
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Press: London, 1969.
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2484.
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of the 213th ACS National Meeting, San Francisco, CA, Apr 13, 1997;
American Chemical Society: Washington, DC, 1997. paper no. 805.
Liu, R. M.; Margerum, D. W. Inorg. Chem. 1998, 37, 2531.
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N.; Rozeboom, M. D.; Eaton, T. A.; Glass, R. S.; Broeker, J . L.
Tetrahedron 1988, 44, 6811.
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Draxl, K.; Field, F. H. Ionization Potentials, Appearance Potentials,
and Heats of Formation of Gaseous Positive Ions; Nat. Stand. Ref. Data
Ser. Nat. Bur. Stand.: Washington, D.C., 1969; Vol. NSRDS-NBS 26.
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S0022-3263(98)00265-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/17/1998

Notes
J . Org. Chem., Vol. 63, No. 16, 1998 5667
The N-I and I-I distances of 2.431 and 2.807 Å may
be compared with those of 4-picoline-I2 (2.31 and 2.83
Å, respectively),23 Me3N-I2 (2.27 and 2.83 Å, respec-
tively),24 and I2 itself (2.715 Å).25 The longer N-I and
shorter I-I length in DBO-I2 than the amine complexes
is interesting but is probably unrelated to the value of
K.10 The other noteworthy aspects of the structure are
the deviation from linearity of the N-I-I moiety and the
alignment of the I-I bond with one nitrogen lone pair,
indicating a σ complex or hypervalent iodine compound8
rather than a π complex.
In view of the known electron-transfer photochemistry
of azoalkanes,26-29 it is not surprising that the DBO-I2
complex is photolabile. This aspect of DBO-I2, the role
of complexes in the halogen oxidation of hydrazines, and
the chemistry of other azoalkane-halogen complexes are
currently under investigation in this laboratory.30
Ack n ow led gm en t. We thank the National Science
Foundation and the Robert A. Welch Foundation for
financial support.
Su p p or t in g In for m a t ion Ava ila b le: X-ray data for
C6H10N2‚I2 (6 pages). This material is contained in libraries
on microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
F igu r e 2. Structural comparison of DBO with its iodine
complex. Bond lengths are in angstroms and bond angles are
in degrees.
J O980265K
N angles in DBO are equal, those of the complex differ
by 4°. Likewise, the C-N bonds of DBO are equal, but
those of the complex are 1.479 and 1.493 Å. Thus,
complexation with I2 opens the C-NdN angle of the I2-
bound nitrogen, shortens its C-N bond, and also shortens
the NdN bond from 1.260 Å in DBO to 1.241 Å in the
complex. The skeletal distortion of DBO in the complex
is probably due to its relatively high strength and to its
asymmetry. In the earlier DBO complexes,11 both nitro-
gens were associated with an acceptor moiety and the
skeleton was not distorted.
(23) Hassel, O.; Romming, C.; Tufte, T. Acta Chem. Scand. 1961,
15, 967.
(24) Stromme, K. O. Acta Chem. Scand. 1959, 13, 268.
(25) Bolhuis, F. v.; Koster, P. B.; Migchelsen, T. Acta Crystallogr.
1967, 23, 90.
(26) Engel, P. S.; Keys, D. E.; Kitamura, A. J . Am. Chem. Soc. 1985,
107, 4964.
(27) Zona, T. A.; Goodman, J . L. J . Am. Chem. Soc. 1995, 117, 5879.
(28) Adam, W.; Sendelbach, J . J . Org. Chem. 1993, 58, 5316.
(29) Adam, W.; Moorthy, J . N.; Nau, W. M.; Scaiano, J . C. J . Am.
Chem. Soc. 1997, 119, 6749.
(30) We find, for example, that DBO forms an isolable, light yellow
bromine complex whose structure is very similar to that of the iodine
complex.