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Diprotonated Tetra(2-pyridyl)pyrazine and its Chemical Mimesis due to Different Hydrogen Bridges.

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Diprotonated Tetra(2-pyridy1)pyrazine
and its Chemical Mimesis"""
due to Different Hydrogen Bridges**
By Hans Bock,* Thorsten Vaupel, Christian Nzither,
Klaus Ruppert, and Zdenek Havlas
Dedicated to Professor Gerhard Quinkert
on the occasion of his 65th birthday
Tetra(2-pyridyl)pyrazine, according to cyclovoltammetric
measurements,['l exhibits proton-sponge['. 3l properties.
This is of interest because the title compound contains four
pyridine rings connected to a central pyrazine ring by four
C-C axes; therefore, the altogether 132 degrees of freedom
in the 40 center molecule C,,H,,N, are predominantly reduced to four rotations. Motivated by this dynamic aspect,
we have determined the solid-state structures of the neutral
c0mpound[~1(Fig. 1 top) as well as of its diprotonated salts
with either the "naked" or the "hydrocarbon-surrounded"
counteranions C1- 1 5 ] (Fig. 1 middle) and (C,H,),B(Fig. 1 bottom), respectively.
In the solid-state structure of tetra(2-pyridyl)pyraziner4I
(Fig. 1 top), the two pyridine rings on either side of the
central pyrazine ring are twisted out of its plane by 50" either
up or down. For this inversion-symmetric molecule with
adjacent pyridine rings twisted in opposite directions an
N ... N distance of 324 pm results; this value exceeds the sum
of two N-interference radii of 155 pm[7b1each and, therefore
the repulsive nitrogen lone-pair interactions are reduced.
In the reaction of the title compound with HCI two diagonally opposing pyridine rings are protonated; hydrogen
bridges N +-H . . .CI - are formed to the two electron-rich
Prof. Dr. H. Bock. T. Vaupel, Dipl.-Chem. C. Nither,
Dipl.-Chem. K . Ruppert
lnstitut fur Anorganische Chemie der Universitit
Niederurseler Hang, D-W-6000 Frankfurt am Main 50 (FRG)
Dr. Z. Havlas
Institute of Organic Chemistry and Biochemistry
of the Czechoslovakian Academy of Sciences
Flelningova Nam 2. CS-16610 Prague 6 (Czechoslovakia)
Structures of Charge-Perturbed and Sterically Overcrowded Molecules,
Part 13. The research project has been supported by the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the
State of Hesse. Part 12: H. Bock, N. Borrmann, Z. Havlas, H. Oberhammer. K . Ruppert, A. Simon, Angar. Chem. 1991, 109, 1733; Angew.
Chrfn.I n [ . Ed. Engl. 1991, 30. 1678.
Mimesis = close external resemblance between one animal and another
animal or inanimate object: (Greek: imitation); The Concise O-cfordDfctionirry of'Currenr Eng/ish (Eds.: H. W. Fowler and F. C. Fowler), Clarendon Press, Oxford. 1961.
Angru, Cliein. Int. Ed. Engl. 31 (1992) N o . 3
Fig. 1. Structures of tetra(2-pyridy1)pyrazine (top) and its diprotonated salts
with chloride (middle) and tetraphenylhorate anions (bottom). All six-memhered ring skeletons exhibit inversion centers. Essential bond lengths [pm]
(f1 pm) and angles ["I (k1") are given; for further structural details see [4-61.
chloride anions (rc, = 180 ~ m ) , [ and
~ ~ ]distances between
proton-donor and proton-acceptor centers of 295 pm are
exhibited. Simultaneously, the unprotonated pyridine rings
rotate into planarity with the pyrazine ring (q = 50" +16")
causing the (H)N' ... N distances to shorten to 288 pm, a
distance less than the sum of two N interference radii
(Fig. 1 middle). The N'-H bond lengths of
91 pm as determined by X-ray structural analysis must be
corrected to about 104 pm, based on neutron-diffraction
structural data. Concomitantly and in agreement with literature data,[7b1the distances H + ...CI- must be corrected to
about 185 to 190pm. However, the angles % N'HCI= 171" can presumably be accepted without any correcti~n.[~~]
Exchange of the electron-rich and therefore, preferred
proton-acceptor center chloride ion for the phenyl surrounded one, tetraphenylborate, which under the reaction conditions is not protonated, can be achieved by anion exchange
of tetra(2-pyridine)pyrazine dihydrochloride in acetonitrile
VCH Vrrlugsgesellwhufi m h H , W-6940 Weinheim, fY92
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with Li[B(C6H,),], on addition of which a yellow color is
observed.16] Structure determination of the yellow crystals
proves drastic changes (Fig. 1 bottom). Thus, the torsion
angles of the pyridine rings relative to the pyrazine ring are
reduced to 21" and 26", yielding a flattened molecular skeleton, which allows a rationalization of the color. This is essentially due to the formation of two intramolecular hydrogen
bridges N + - H . . - N between the N centers of adjacent
pyridine rings, which are only 253 pm apart. The uncorrected
distances N+-H = 124 pm and H ... N = 133 pm on both
sides of the bending angle 4 N+-H-N = 160" differ by only
9 pm. These intramolecular N+-H ... N bridges are rather
short even for proton sponges (N+-(H)...N = 265254 pmr3I) as well as almost symmetrical (A(N+-H/
H . . N) = 16- 17 pmr3]) and change, in addition, other
structural parameters of tetra(2-pyridy1)pyrazine (Fig. 1
top). Thus the external C-C-C angles of the pyrazine ring are
expanded to 129"(!) and the differing pyridine @so angles
of 123" and 117" in the dihydrochloride (Fig. 1 middle)
expectedlyr21equalize at 121".
Additional information on the problem of different hydrogen bridges in one compound^"^ '1 is provided by extensive quantum chemical calculations:[81
1) The preferred conformation of tetra(2-pyridy1)pyrazine
in the crystal (Fig. 1 top) is reproduced by a complete AM1
geometry optimization starting from standard structural
parameters.[8a1The alternative arrangement, which would
contain triazachelate pincers and could be constructed by
rotation of all pyridine rings, proves to be energetically less
favorable, presumably due to the resulting N ... N distances
of about 290 pm, which are less then the sum of interference
2) For the diprotonation and the conversion of the two
external H bridges to intramolecular ones, enthalpy of formation contributions of +91 kJmol-' and +443 kJmol-'
are predicted.[8h1However, these gas-phase values must be
corrected for the unknown solvation and lattice energies.
3 ) The AM1 charge distributions for tetra(2-pyridy1)pyrazine M and for its different diprotonation products
[MH: Cl;] and [MH:] calculated from the crystal structure
data, exhibit significant differences: Thus for the H' bridge
and of
centers, residual charges of +0.33 ([MH:CI;])
+0.38 ( [ a : ] ) are estimated as well as -0.82 for the chloride anion. The partial charges at the pyridine N centers
change from -0.12 ( M ) to -0.05 and -0.16 ([MHlCIJ)
and to -0.16 and -0.19 ([MH:]), predicting AqN = 0.11
for the dichloride with one-sided NH' bridges and only
AqN = 0.03 for the bis(tetrapheny1borate) with almost symmetrical intramolecular bridges. As expected, the positive
charges introduced by diprotonation are partially distributed to the peripheral pyridine hydrogens.
4) For a simulation of the H + bridge potentials, which
depend strongly on the N ... N distance,i8c1the structure of
the 4-aminopyridine semiperchlorate provides an appropriate starting point (Scheme
This structure has been determined by neutron diffraction and shows an N " . N distance of 270 pm between the two perpendicular pyridine
rings as well as an unsymmetrical hydrogen bridge (N +-H
117 pm, H ... N 152 pm). Variation of the distance dN between 280 pm and 240 pm and stepwise movement of the
bridge proton along the N-N axis by Ar yield different potential curves after each total geometry optimization of the
pyridine rings (Scheme 1).
Double minima are calculated for structures with N . . N
distances between 280pm and 250pm; the height of the
barrier decreases from 35 to 2 kJmol- ', and the difference in
the N-H bond lengths is reduced from 70 pm to 22 pm. For
VCH Verfa~sgesrfisciiu~l
mhH. W-6940 Weinheim, 1992
280 pm
Ar- Lpml-
Scheme 1 . Potential curves for pyridinium pyridine [Sc] (see text)
the optimum N ... N distance of about 270 pm, an N+H . . . N bond energy of 54 kJmol-' results. On shortening
the N ... N distance from 260 pm to 250 pm, the AM1 calculations predict an increase in energy due to increasing N-N
= 245 pm a single minimum with
repulsion, and for dN...N
equal distances N-H' of 122.5 pm is found. All other calculated properties exhibit only small changes: The pyridine
@so angles vary between 118" ( H . . . N 197pm) and 120"
(N+-H 103pm) and the charges q H + ,between +0.34
( N . . . N 2 8 0 p m ) and +0.36 ( N . . . N 2 5 0 p m ).
5) In the diprotonated tetra(2-pyridy1)pyrazine containing
two intramolecular hydrogen bridges (Fig. 1 bottom), the
proton movement might be coupled to a "flip-flop" molecular dynamics: The adjacent pyridine rings are twisted by 26"
and 21 out of the pyrazine plane. If their differing twisting
angles are not caused by the crystal packing, the transition
from one into the other double minimum should cause a
change of 5". Therefore, in additional AM1 model calculationsrSd1 the proton was shifted in ten steps with
AdNH= 9 pm along the experimentally determined structure
with dN...N
= 253 pm. The following data are obtained for
the barrier of the double minimum: height AAH, =
4 kJmol-', dN-H+
= 128 pm, bonding angle < N(H+)N =
170", pyridine ips0 angles of 121", and a proton charge
qH+= +0.38.
According to the approximate quantum chemical calculations, both tetra(2-pyridy1)pyrazine dihydrochloride and
bis(tetrapheny1borate) also differ in the type of their hydrogen bridge: With the electron-rich chloride ion as the proton
acceptor, due to the rather long N ... N distance of 288 pm,
a simple potential minimum is approximated, in which the
energetically favorably located proton vibrates only around
its equilibrium position. The N+-H ... N bridges in the
"naked" dication, on the other hand, are among the shortest
known13,', 191 with the smallest difference in the bond
lengths N +-H and H . . . N.i3,7 ,
Their formation causes
the N ... N distance to shorten by 35 pm (!) (Fig. 1 middle
and 1 bottom), corresponding to a structure represented by
a shallow double-minimum with a low barrier. The movement of the proton coupled with molecular dynamics, however, seems to be largely frozen in the crystal at 100 KL6'
according to the center of inversion formed.
The mimetic behavior observed for tetra(2-pyridy1)pyrazine upon exchange of the anions of the dichloride in
0570-0833192/0303-0300$3.50+ .25/0
Angew. Chem. In!. Ed. Engl. 31 (1992) N o . 3
acetonitrile by addition of lithium tetraphenylborate presumably occurs in other protonatable nitrogen bases as well
and is potentially of biological importance. In ongoing investigations the preparation of single crystals of metal-cation
chelate complexes'' '1 of tetra(2'-pyridyl)pyrazine will be attempted.
Received: November 14, 1991 [Z 5019 IE]
German Version: Angew. Chem. 1992,104, 348
CAS Registry numbers:
tetra(2-pyridy1)pyrazine dihydrochloride, 138901-51-6; tetra(2-pyridy1)pyrazine hrs(tetraphenylborate), 138901-52-7; tetra(2-pyridyljpyrazine 2500597-4.
[l] H. Bock, D. Jaculi, Z. Nuturjorsch. B 1991, 46, 1091.
[2] See: H. Bock. K. Ruppert, C. Niither, Z. Havlas, H.-F. Herrmann, C.
Arad. I. Gobel, A. John, J. Meuret, S. Nick, A. Rauschenbach, W. Seitz,
T. Vaupel. B. Solouki, Angew. Chem. 1992,104, in press; Angeu. Chem. In!.
Ed. Engl. 1992, 31. in press (May issue) and literature cited therein.
[ 3 ] H. A. Staab, T. Saupe, Angew. Chem. 1988, 100,895; Angeu. Chem. In!.
Ed. EngI. 1988, 27, 865; T. Barth, C. Krieger, F. A. Neugebauer, H. A.
Staah, rhid. 1991, 103, 1006 and 1991,30, 1030 and literature cited therein.
(41 Tetra(2-pyridy1)pyrazine (Alpha): 100 mg were recrystallized from 3 mL
dry HCCI, under an atmosphere of n-hexane. Colorless plates; for the
crystal structure analysis we are grateful to Dr. J. W. Bats (Universitlt
Frankfurt); see T. Vaupel, Diplomurbeil, Universitat Frankfurt, 1992. The
molecule is arranged around a crystallographic inversion center. Angle
sum of all six-membered rings 720". bond lengths C-N 134 pm, C-C (ring)
138-139 pm. C-C 149 pm.
[5] Tetra(2-pyridyljpyrazine dihydrochloride: 520 mg (1.3 mmol) tetra(2pyridy1)pyrazine were stirred in 60 mL aqueous 1 N HCI at room temperature for 2 h. The water was then removed under reduced pressure and the
dried residue recrystallized from 200 mL acetonitrile. Single crystals:
70 mg were dissolved in 100 mL boiling acetonitrile and slowly cooled to
room temperature; after 2 d colorless polyhedra had grown. Crystal structure analysis: C,,H,,CI,N,
(461.23) a = 940.5(2), h =771.8(2), c =
1563.2(6) pm, fl =105.13(3)", V =1095.3 x lo6 pm3 (100 K), Z = 2,
pca,c=1.399 gem-', ~(Mo,,) = 0.32 mm-', monoclinic, space group
P2,jn, Siemens-AED 11 four-circle diffractometer. 6843 measured reflections 3' 5 2 H 5 60", of which 2493 are independent with I >2u(f). Structure resolution by direct methods and difference Fourier techniques
(SHELXTL-PIUS),N = 2943, NP =182, R = 0.033. R, = 0.037, M, = 1/u2
( F ) 0.00008 F2. GOOF = 2.864, Shift/Err 5 0.001, residual electron
density +0.43/-0.30 e 0 k 3 . Empirical absorption correction, extinction
correction. all C, N, and CI atoms were anisotropically refined; H atoms
(from difference Fourier analysis) isotropically refined. The dihydrochloride is arranged around a crystallographic inversion center. Angle sums of
all rings 7 2 0 , bond lengths C-N 134 pm. C-C (ring) 139 pm, C-C 149 pm.
b) Further details of the crystal structure determination are available on
request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur
wissenschaftlich-technische Information mbH, D-W-7514 EggensteinLeopoldshafen 2, on quoting the deposit number CSD-56020, the authors,
and the literature citation.
[6] Tetraphenylborate salt of diprotonated tetra(2-pyridy1)pyrazine: A suspension of 200 mg (0.04 mmol) dihydrochloride in 20 mL acetonitrile was
dissolved in methanol. On addition of 800mg (2.5mmol) lithium tetraphenylborate in 5 mL acetonitrile a yellow color appeared; after 3 h
yellow cubes had grown. Crystal structure analysis: C,,H,,B,N, (1028.7);
u=11.082(6). b=11.160(7), c =12.366(6)pm. a:=80.98(5), /l=
84.87(4). 7 = 63.68(4)", V =1353 x 46 106pm3 (loOK), Z =1,
1.263 g ~ m - jt(MoKJ
= 0.07 mm-'. triclinic, space group Pi, SiemensAED-II four-circle diffractometer, 5461 measured rellections between
3 s 2H I55'. of which 3833 are independent with I z 1 u(0. Structure
resolution by direct methods and difference Fourier technique
(SHELXTL-PIUS),N = 3833, NP = 478, R = 0.054, R , = 0.040, 1%' = 1/
0 2 ( F ) 0.0002 Fz. GOOF = 1.3660, Shift/Err 5 0.001, residual electron
density 0.40:-0.26 e,.&-3. Empirical absorption correction, extinction
correction, B, C, and N atoms were anisotropically refined; H atoms (from
difference Fourier analysis), isotropically refined. The dication is arranged
around a cryStdllOgrdphiC inversion center. Angle sums of all rings 720.
bond lengths C-N 134pm. C-C(ring) 139 pm, C-C 150 pm. For further
details of the crystal structure see [5 b].
[7] Selected reviews on hydrogen-bridge bonds: a) A. F. Wells, Structurul Inorgunic Chcrnrstry, 5th ed., Clarendon Press, Oxford, 1987, pp. 355-376; b)
J. Elmsley. Chem. SOC.Rev. 1980,Y. 91-24;c) R. Taylor, 0. Kennard, Arc.
Chrm. Rex 1984.17,320:d)G. R. Desiraju, Acc. Chem. Res. 1991,24,290.
e) Among the examples for different hydrogen bridges within one compound. the yellow and white polymorphic modifications of 3,6-dichloro25dihydroxyterephthalic acid dimethyl ester which have different intramolecular bridges 0-H ' ' ' 0 and 0-H . - . C I ,are pointed out (Q.-C.
Yang. M. F. Richardson. J. D. Dunitz. Acta Crystallogr. Sect. B, 1988.45,
Angew. Chrm. I n t . Ed. Engl. 3 / (1992J No. 3
312). f ) For mono- and diprotonation see for example R. Schwesinger, M.
Missfeldt, K. Peters, H. G. von Schnering, Angew.. Chem. 1987, 99, 210;
A n g a t . Chem. Int. Ed. Engl. 1987. 26, 1165.
[XI All quantum chemical calculations were performed at the AM1 level
(M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. J. P. Stewart, J. Am. Chem.
Soc. 1985, 107, 3902; program version SCAMP 4.0, T. Clark, Universitlt
Erlangen). a) For the neutral molecule, a complete geometry optimization
starting from standard structural parameters yields AH, = 984 kJmol-I,
C-N 135 pm, C-C (ring) 140 pm, C-C 149 pm. ipso angle in pyridine 117".
twisting angle 54", nonbonding distance N ... N 320 pm; E , = 9.06 eV
(exp. IEY = 8.1 eV; T. Vaupel, Diplomarheit, Universitdt Frankfurt, 1992).
b) The calculations for the diprotonated compounds were based on the
crystal structure data. Dichloride N +-H distance corrected according to
[7c] to 104pm): AHf=1132kJmoI-' ([MH:CI;])
and 2314kJmol-'
(MH:) as well as bis(tetrapheny1borate) AH< = 241 I kJmol-' ([MH;]).
c) For the model calculations of H,C,N+-H ... NC,H,, the starting point
was the structure determined by neutron diffraction [9] with the six-meinbered rings perpendicular to each other and the N . . . N distance varied
between 280 pin and 240 pm. For the individual potential curves AAH,/Ar
(Scheme 1). the bridge proton is moved in 5 pm steps along the N-N axis
and the geometry of thecomplete system including both pyridine rings was
optimized. Additional calculations for coplanar pyridine rings yield similarpotentialcurves, theAAH,valuesofwhichare higher by 2-3 kJmol-'
It has been pointed out that semiempirical calculation procedures often
exaggerate N/N repulsion relative to the H+-bridge attraction due to insufficient N+-H '.. N parametrization. d) For the calculations of diprotonated tetra(2-pyridy1)pyrazine with two intramolecular H + bridges. the
simulation of the "flip-flop" molecular dynamics started with the Cartesian
coordinates of the experimental structure (Fig. 1 bottom), transforming
the latter structure in ten equally weighted steps into that of its mirror
image. In the fifth step the enthalpy of formation increased from
2410 kJmo1-I to 2414 kJmol-'. The structural parameters given correspond to interpolation values.
[9] J. Roziere, J. M. Williams, E. Grech, Z. Malarski, L. Sohzyzk, J. Chem.
P h w . 1980. 72, 61 17; see also P. Tenlon, R. G. Delaplane. I. Olovson, J.
Reziere, Actu Crystallogr. Sect. 1985, C41. 479.
[lo] An extensive literature search, although still incomplete with respect to the
multitude of publications [7c], yields a comparably short N+-H .-.N
bridge of 252.6 pm for 1,6-diazabicyclo[4.4.4]tetradecane hydrochloride
(R. W. Alder, A. G. Orpen, R. B. Sessions, J. Chem. Soc. Chem. Commun.
1983, 999); the position of the H + center is not reported. The shortest
N " . N distance found so Par (247pm) i s found in syn-1,6:8,13-diimino[l4]annulene perchlorate (R. Sestro, T. Pilati, M. Simonetta. E. Vogel,
J. Am. Chem. Soc. 1985, 107, 3185).
(111 See for example: S. Hunig, H. Quast, Neuere Furbige Systeme (Abstr. 11.
Int. Farbensymposium, Elmau. 1964) Verlag Chemie, Weinheim, 1964,
pp. 17-19 and 35-43; T. Vaupel, Diplomurbeit, Universitat Frankfurt
1992; see also H. A. Goodwin, F. Lions, J. Am. Chrm. SOC.1959.8/,6415.
Crystalline Nitrogen Oxides-Crystal Structure
of N,O, and a Remark Concerning the Crystal
Structure of N,O,
By Arndt Simon,* Jorg Horakh, Axel Obermeyer,
and Horsf Borrrnann
Our knowledge of the crystal structures of nitrogen oxides, N,O, N,O,, N,O,, N,O,, and N,O,, is still rather
incomplete. The structure determinations of N,O['] and
N,0,[21 were complicated by the static disorder of the molecules. Raman spectrat3'contradict the linearity of the NO:
ion in NOlNO; as determined in the crystal structure of
N,O, .[41 In the course of a structure determination of N,O,
only a possible unit cell could be proposed.[51
Our results on the structure of the stable cubic and the
metastable monoclinic forms of N,04 have already been
published.[61Here we report on phase transformations in
crystalline N 2 0 3 / N 2 0 4samples, a single-crystal structure
determination of N,O,, and indications of the existence of
[*] Prof. Dr. A. Simon, J. Horakh, A. Obermeyer, H. Borrmann
Max-Planck-Institut fur Festkorperforschung
Heisenbergstrasse 1, D-W-7000 Stuttgart 80 (FRG)
Verlugsgesellsrhajt mbH, W-6940 Wrinheim, 1992
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hydrogen, chemical, diprotonated, pyrazines, bridge, due, mimesis, different, tetra, pyridyl
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