A chiral 2-D donor - Acceptor array of a bipyrazine N-oxide and tetracyanoethylene

Article abstract of DOI:10.1021/ol991367f

(equation presented) Hexamethylbipyrazine-N,N′,N″,N?-tetraoxide (1) is synthesized in racemic form and cocrystallized with tetracyanoethylene to give a donor-acceptor (DA) networked lattice. The resulting DA₂ cocrystal contains homochiral 2-D DA arrays (layers) cohered by 2.7 ? NO?C=C DA bonds with periplanar O p-orbital/C=C orientations. Layer formation is stereoselective, while interlayer relations yield a racemic lattice.

Full text of DOI:10.1021/ol991367f

ORGANIC
LETTERS
2000
Vol. 2, No. 9
1181-1184
A Chiral 2-D Donor−Acceptor Array of a
Bipyrazine N-Oxide and
Tetracyanoethylene
Byron J. McGee, Lindsay J. Sherwood, Melinda L. Greer,† and
Silas C. Blackstock*
Department of Chemistry, The UniVersity of Alabama, Tuscaloosa, Alabama 35487
blackstock@ua.edu
Received December 16, 1999
ABSTRACT
Hexamethylbipyrazine-N,N′,N′′,N′′′-tetraoxide (1) is synthesized in racemic form and cocrystallized with tetracyanoethylene to give a donor−
acceptor (DA) networked lattice. The resulting DA cocrystal contains homochiral 2-D DA arrays (layers) cohered by 2.7 Å NO‚‚‚CdC DA
2
bonds with periplanar O p-orbital/CdC orientations. Layer formation is stereoselective, while interlayer relations yield a racemic lattice.
The synthesis of orientationally constrained molecular net-
works requires the use of anisotropic molecular association
modes for cohesion. Within a crystal lattice, control of
supramolecular order is known as crystal engineering.1 In
an effort to develop new tools for crystal engineering, we
have been evaluating new, localized organic electron donor-
acceptor2 (DA) associations and their effect on crystal
growth.3 We report here the cocrystallization of bipyrazine
N-oxide (1) and tetracyanoethylene (TCNE).
is significantly shorter than the nonbonded O,C van der
Waals distance5 of 3.22 Å. The previous cocrystal structures
suggested that a bipyrazine tetra-N-oxide (BPTO) such as 1
might serve as a chain crossing linker to afford two-
dimensional DA arrays in a hypothetical BPTO/TCNE
cocrystal.
(1) Some reviews are (a) Desiraju, G. R. Crystal Engineering, The Design
of Organic Solids; Elsevier: Amsterdam, 1989. (b) Gavezzotti, A. Acc.
Chem. Res. 1994, 27, 309. (c) The Crystal as a Supramolecular Entity.
PerspectiVes in Supramolecular Chemistry; Desiraju, G. R., Ed.; Wiley:
New York, 1996. (d) Weber, E. Top. Curr. Chem. 1998, 198, 1.
(2) For reviews of organic donor-acceptor complexation, see: (a) Bent,
H. A. Chem. ReV. 1968, 68, 587. (b) Mulliken, R. S.; Person, W. B.
Molecular Complexes: A Lecture and Reprint Volume; Wiley-Inter-
science: New York, 1969. (c) Foster, R. Organic Charge-Transfer
Complexes; Academic Press: New York, 1969. (d) Herbstein, F. H. Persp.
Struct. Chem. 1971, 4, 166.
(3) (a) Blackstock, S. C.; Poehling, K.; Greer, M. L. J. Am. Chem. Soc.
1995, 117, 6617. (b) Greer, M. L.; Blackstock, S. C. J. Am. Chem. Soc.
1997, 119, 11343. (c) Greer, M. L.; Blackstock, S. C. Mol. Cryst. Liq. Cryst.
1998, 313, 55.
(4) (a) Greer, M. L.; McGee, B. J.; Rogers, R. D.; Blackstock, S. C.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1864. (b) Greer, M. L.; Duncan, J.
R.; Duff, J. L.; Blackstock, S. C. Tetrahedron Lett. 1997, 38, 7665. (c)
Bodige, S. G.; Selby, T. D.; McKay, S. E.; Blackstock, S. C. Trans. Am.
Cryst. Assoc. 1998, 33, 135.
Previous work showed that heterocyclic N-oxides such as
2 bind to TCNE in a manner that supports cocrystallization
with (DA)n or (D2A)n chain formation in the lattice.4 A
hallmark of the DA bonding in such cocrystals is a short
NO‚‚‚CdC intermolecular contact of 2.77-2.91 Å, which
(5) For van der Waals radii in crystals, see: (a) Bondi, A. J. Phys. Chem.
1964, 68, 441. (b) Rowland, R. S.; Taylor, R. J. Phys. Chem. 1996, 100,
7384.
† Current address: Department of Chemistry, University of Dayton.
10.1021/ol991367f CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/08/2000

To avoid CH‚‚‚O hydrogen bonded self-aggregation of the
N-oxide moiety that might compete with DA-mediated
aggregation modes,4c,6 the permethylated BPTO 1 was
desired as a target donor structure. Scheme 1 shows the
Scheme 1
Figure 2. Job plot for 1/TCNE absorption at 496 nm with [1] +
[TCNE] ) 0.020 M in CH2Cl2 at 25 °C.
synthesis of 1. Treatment of trimethylpyrazinone 37,8 with
POCl3 gave 4,8 whose Nickel-mediated coupling using a
procedure derived from Constable et al.9 yielded bipyrazine
5. Oxygenation of 5 with OXONE gave tetra-N-oxide 1.
The mixture of colorless 1 and TCNE in CH2Cl2 yields a
red solution that features a new visible absorption band with
λm ) 496 nm (Figure 1). This absorption is assigned to the
lattice as two-dimensional (2-D) supramolecular layers
(grids) of (DA2)n composition (Figure 3). Each donor binds
Figure 3. (DA2)n layer in (1)(TCNE)2 cocrystal.
Figure 1. UV-vis spectra in CH2Cl2 of (a) 49 mM TCNE, (b)
1.2 mM 1, and (c) 49 mM TCNE + 1.2 mM 1.
to four acceptors, and each acceptor binds to two donors to
cohere the array. In a supramolecular sense, crossed DA
chains form a thatched layer with the BPTO donors serving
as the chain intersection points. The component connectivity
pattern is denoted below.
charge-transfer excitation of the 1/TCNE DA complex.
Benesi-Hildebrand analysis10 of 1/TCNE mixtures in CH2Cl2
provides an estimate of Kf (3.6 ( 0.1 M-1) for complexation
and of ꢀ (4686 cm-1 M-1 at 490 nm) for the complex. A
plot (Figure 2) of absorbance at 496 nm as a function of
continuous variation of the D:A molar ratio (according to
the method of Job) shows a maximum at D:A ) 1:1,
suggesting that an assumed 1:1 stoichiometry of complex-
ation is valid under the solution conditions.
Cooling of concentrated 2:1 TCNE/1 solutions in CH2Cl2
at -20 °C under an ether atmosphere afforded bright red
DA2 cocrystals for which X-ray diffraction analysis at -100
°C has been performed.11 DA aggregation is observed in the
Figure 4 shows the DA connections. Intermolecular D-A
contacts of 2.6-2.7 Å are observed. These contacts are 0.5
1182
Org. Lett., Vol. 2, No. 9, 2000

Figure 4. DA bonding in the (1)(TCNE)2 cocrystal lattice. Dashed
distances are in Å. Top and bottom views are orthogonal perspec-
tives about the horizontal axis.
Figure 5. (above) HOMO of 1 and (below) LUMO of TCNE as
calculated by AM1.
Å shorter than the van der Waals radii sum and clearly
represent DA attractions in the lattice between N-oxide and
TCNE functional groups that we shall refer to as DA bonds.
The DA bond orientations are similar to those previously
found in azodioxide/TCNE13 and pyrazine dioxide/TCNE4a,b
arrays in which the p-orbital axis of the N-oxide orients
parallel to the CdC bond of TCNE. This geometry is
favorable for HOMO/LUMO orbital overlap, which appears
to play a significant structural role, along with dipolar
coulombic attractions, in the N-O‚‚‚TCNE DA bonding.
Figure 5 shows the HOMO of 1 and LUMO of TCNE as
calculated by AM1.
oxygen located to one end of the CdC linkage. In both DA
bond geometries, constructive FMO overlap is apparent. The
presence of two different DA interaction geometries in the
(1)(TCNE)2 cocrystal may be caused by the different
molecular environments of the ortho and meta NO groups
or by other intermolecular close-packing constraints in the
lattice. The “loose” DA bonds will have soft potentials that
allow them to undergo distortion in response to other
energetically demanding features of molecular packing. We
speculate that the “slipped” DA bonds of the (1)(TCNE)2
lattice may result from a collapse or relaxation of the (DA2)n
grid network that would, were it composed of only rigid
bisected DA links, otherwise be strained as a result of porous
structure. Of course, optimum interlayer packing may also
influence the DA2 grid structure and, in turn, the observed
DA bond geometries. Although it seems reasonable to
assume that the bisected NO‚‚‚TCNE bond would be
electronically preferred to the slipped one, there is no
compelling evidence at this time to support this conjecture.
In fact, there is more literature precedence for the slipped
NO‚‚‚TCNE bond geometry.4a,b,13
The ortho and meta NO groups of 1 bind to TCNE with
slightly different geometries. The “bisected” ortho NO‚‚‚
TCNE and “slipped” meta NO‚‚‚TCNE linkages are depicted
below with FMO overlap. Both DA bonds have short 2.7 Å
(6) For preliminary reports of heterocyclic N-oxide CH‚‚‚O hydrogen
bonding in crystal lattices, see: (a) Taylor, R.; Kennard, O. J. Am. Chem.
Soc. 1982, 104, 5063. (b) Bodige, S. G.; Zottola, M. A.; McKay, S. E.;
Blackstock, S. C. Cryst. Eng. 1998, 1, 243.
(7) MacDonald, J. C.; Bishop, G. G.; Mazurek, M. Tetrahedron 1976,
32, 655.
noncovalent NO‚‚‚CdC lengths and parallel oxygen p-
orbital, TCNE CdC bond orientations. For the ortho
NO‚‚‚TCNE bond, a cyclic DA interaction between the oxide
oxygen and the two carbons of the CdC linkage occurs with
a “bisected” topology that could be described, in Dewar-
Zimmerman terms,12 as an ω2a + π2s Mo¨bius aromatic
pericyclic array. For the meta NO‚‚‚TCNE bond a “slipped”
version of the DA interaction is observed, with the oxide
(8) Karmas, G.; Spoerri, P. E. J. Am. Chem. Soc. 1952, 74, 1580.
(9) Constable, E. C.; Morris, D.; Carr, S. New J. Chem. 1998, 287.
(10) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
(11) Monoclinic C2/c lattice: a ) 18.1957(6), b ) 10.0687(1), c )
18.1772(6) Å and R ) γ ) 90°, â ) 118.699(2)°. Data/parameter ) 2522/
227, structure refinement on F2 gave R1 (I > 2σ(I)) ) 0.0514, wR2 ) 0.1074.
See Supporting Information for complete details.
(12) (a) Zimmerman, H. E. Acc. Chem. Res. 1971, 4, 272. (b) Dewar,
M. J. S. Angew. Chem., Int. Ed. Engl. 1971, 10, 761.
Org. Lett., Vol. 2, No. 9, 2000
1183

Since donor 1 is a chiral structure by virtue of its slow
bond rotation about the bipyrazine linkage,14 there exists an
issue of the stereochemical integrity of DA layer assembly.
It is observed that the (DA2)n arrays of the (1)(TCNE)2
cocrystal are homochiral, i.e., they contain only one enan-
tiomer of 1. However, the crystal lattice as a whole is racemic
because alternate layers are of alternate chirality. Presumably,
chiral layer formation is favored because such layers
propagate most readily and/or pack together (in racemic
form) in optimal fashion. Figure 6 displays a packing diagram
of the (1)(TCNE)2 lattice with color-coded layers.
Figure 7. Powder X-ray diffractograms for bulk (1)(TCNE)2 and
for its single-crystal structure.
to form 2-D (DA2)n grids with TCNE in the solid state.
Extremely short NO‚‚‚CdC DA bonds are observed, having
lengths 0.55 Å shorter than van der Waals contact (or 31%
of the way from a nonbonded O‚‚‚C distance toward that of
a single C-O bond). The orientational preference of these
N-oxide/TCNE DA bonds is consistent with HOMO/LUMO
overlap optimization. Finally, the stereoselectivity of the
(DA2)n layer formation in (1)(TCNE)2 is noted.
Figure 6. Packing diagram for (1)(TCNE)2 cocrystal showing
alternating chiral layers.
To test for the presence of polymorphic cocrystals of
1/TCNE, a powder diffractogram of the bulk sample was
obtained and compared to the calculated powder spectrum.
As seen from Figure 7, bulk material has the same powder
peaks as expected for the observed (1)(TCNE)2 crystal and
no additional peaks. This result suggests that little or no
polymorphic cocrystal formation occurred under our crystal
growth conditions.
Acknowledgment. We thank the donors to the Petroleum
Research Fund administered by the American Chemical
Society for funding of this work; the NSF for shared
instrument facilities (CHE-9626144 and DMR-9809423); Dr.
Robin Rogers and Lilian Rogers for assistance with X-ray
data collection; and T. D. Selby, J. Duff, and A. Perkins for
assistance with the synthesis of 1; L.J.S. was supported by
the NSF-REU program at Alabama.
In conclusion, we have shown that a bipyrazine tetra-N-
oxide donor molecule functions as a chain intersection linker
Supporting Information Available: Synthetic proce-
dures, material purifications and spectral data, Benesi-
Hildebrand analysis, Job plot, X-ray tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Greer, M. L.; Blackstock, S. C. J. Org. Chem. 1996, 61, 7895.
(14) Resolution of 1 is being attempted. The report of isolated (S)-3,3′-
dimethyl-2,2′-biquinoline-N,N′-dioxide strongly suggests that 1 also exists
as isolable enantiomeric rotamers. See: Nakajima, M.; Saito, M.; Shiro,
M.; Hashimoto, S. J. Am. Chem. Soc. 1998, 120, 6419.
OL991367F
1184
Org. Lett., Vol. 2, No. 9, 2000