Azapentalenes . XLIV . 1 H and 13 C-NMR study of mesoionic pyrazolo [ 1 , 2-a ] pyrazoles ∗

The H and C chemical shifts as well as the H–H and H–C coupling constants of sixteen pyrazolo[1,2a]pyrazole derivatives have been measured. The most relevant features are discussed using resonance forms and simple additive models. AM1 semi-empirical calculations have been carried out to provide a rationale for the NMR results.

These compounds were discovered simultaneously by Solomons [6][7][8][9] and by Trofimenko [10,11] who described their preparation, chemical reactivity and some spectroscopic properties.Relevant for the present study are two of their conclusions: i) Positive and negative charges are in general delocalized in the aromatic system 3; however, when one of the two pyrazole rings bears substituents able to delocalize the negative charge such as COMe, COPh, CN then the predominant resonance form is best represented by formula 4 ('charge fixed' structure [11]).ii) In 1 H-NMR, for non 'charge fixed' compounds, the 3 J 'ortho' coupling constant has a value of 2.6 Hz [8][9][10][11], larger than in neutral pyrazoles [12] but similar to that found in pyrazolium cations [13].Moreover, there is a 6 J coupling constant between H 2 and H 5 (see formula 5, Fig. 2) of 1.1 Hz [10,11].
Although derivatives of this ring system continue to be studied (mainly for their chemical and biological properties) [14,15], no further NMR studies have appeared since 1966 [9,11].We will report in this paper the 1 H and 13 C-NMR spectroscopy of nine 3a,6a-diazapentalenes 6-14 and two precursors in their syntheses 15 and 16 (Table 1).

Experimental
All the compounds here discussed have already been described [10,11]. 1 H and 13 C-NMR spectra were obtained using a Bruker AC-200 instrument.The chemical shifts are accurate to 0.01 and 0.1 ppm for 1 H and 13 C-NMR respectively.Coupling constants are accurate to 0.2 Hz for 1 H-NMR and 0.5 Hz for 13 C-NMR.A series of 2D-experiments [16] were used to assign the 1 H and 13 C signals of compound 14.

NMR results and discussion
The spectroscopic data are reported in Tables 2 ( 1 H) and 3 ( 13 C).The assignment was straightforward; only in the case of compound 14, were 2D-experiments necessary to confirm the assignment.
The most interesting results of Table 2 are the 3 J and 6 J coupling constants.The greater accuracy of modern measurements does not change the old results: since all compounds 6-13 contain COMe, COPh, CN groups, 'charge-fixed' structures, 3 J = 2.8-2.9Hz coupling constants were measured.In compound 14, 3 J 1,2 = 4.1 Hz (this compound had not been studied previously although it was also classified as 'charge fixed' [11]).
We have collected in Fig. 3 the information available on 3 J in pyrazoles: the value for NHpyrazoles is an average value between 3 J 34 and 3 J 45 due to prototropy; it is clear, nevertheless that a positive charge results in an important increase in the value of the coupling constant.In mesoionic compounds, the value is only 2.6 Hz and this value increases to about 2.85 Hz when the positive charge is localized in one moiety.Compound 14 has a very large 3 J coupling constant which corresponds to a non-aromatic derivative: in ∆ 3 -pyrazolines an equivalent 3 J coupling constant of 4.0 Hz has been measured [17].
Derivatives 15 and 16 are classical 1,2-disubstituted pyrazolium compounds: the 1 H-NMR of compounds in which the pyrazolium ring nitrogens are linked by a trimethylene chain has been described [11,13].The analysis of the multiplets corresponding to the AA'BB'C system of the diazacyclopentene ring, in first-order approximation, yields the following values: a geminal coupling constant J AB = −12.7 to −13.9 Hz and two vicinal coupling constants, one J BC = 4.5-5.6Hz and the other J AC = 0-2.0Hz.Molecular mechanics modelling and a Karplus type relationship indicates an envelope conformation with a pseudo-axial position of the bromine substituent (the J BC coupling corresponds to a dihedral angle of 30 • and the J AC coupling to a dihedral angle of 95 • ).
Since there is no previous report on 13 C-NMR spectroscopy of these compounds, the results of Table 3 deserve some comments.The CH, =C(CN) 2 and (CN) 2 carbon atoms of the [CH=C(CN) 2 ] − group in compound 14 appear at 126.7, 41.8 and 115.8/117.0ppm; in dicyanomethylylides they appear at 148, 46 and 116/117 ppm, respectively [18].The double bond character of the central bond is responsible for the anisochrony of the CN carbon atoms in both cases.
The chemical shifts of compounds 6-13 can be discussed assuming the additivity of substituent effects and considering compound 6 as an internal reference.A multiregression treatment leads to the following conclusions: on carbons C1 and C3, the only important effect is produced by the replacement of 1,3-( Concerning the substituent effects on the same pyrazole ring, they are similar to those observed for neutral pyrazoles, for instance, the ipso-bromine effect of -12.4 ppm on C2 corresponds to the same effect on the C4-pyrazole atom (-12.5 ppm) [19].On the other hand, the substituent chemical shifts (SCS) on the other moiety are characteristic of this family of pyrazolo [1,2-a]pyrazoles showing the sensitivity of the whole mesoionic structure to electron redistribution and, at the same time, verifying the consistency of the assignments (the effects have been calculated by multiregression with r 2 = 0.998).

AM1 semiempirical calculations
To obtain a better description of the systems discussed in this paper we have carried out AM1 calculations [20] on two representative compounds 12 and 14 and on three model compounds 17-19 (Fig. 4).All the structures have been optimized using the original AM1 parametrizations as implemented in the MOPAC6.0package [21].
Since there is no X-ray structure of any member of this family of azapentalenes, we report in Fig. 5 the AM1 optimized geometries (assuming planarity, distances in A ˚, angles in • ) of the parent compound 17 and the dicyano derivative 12.The introduction of two cyano groups alters significantly the geometry of both rings: the C3-N3a (N6a-C1) increases while the N3a-C4 (C6-N6a) bond lenght decreases.
For symmetry reasons there cannot be charge fixation in compounds 17 and 18; on the other hand, compound 19 was selected as a model of pyrazolium ring (the substituents present in compounds 15 and 16 are not necessary for modelling purposes).Comparison of the total charges in compounds 17 and 12 shows the perturbation of the electronic distribution (Fig. 6).
When tetracyano and dicyano derivatives 18 and 12 are compared only the latter has a dipole moment (3.24 D) directed from the positive lower part to the negative upper part along the C5-C2 atoms.If the sum of the total charges for a pyrazole ring (17, -0.5184, 12, -0.3647, 19, -0.0876 electrons) and the3 J( 1 H-1 H) coupling constants reported in Fig. 3   This equation corresponds to the fact that the more positive charge bears the five-membered ring the larger the 3 J( 1 H-1 H) is.δ 13 C and 1 J( 1 H-13 C) are also dependent on the charge distribution.
According to AM1 calculations, compound 14 is planar with an E conformation (the =C(CN) 2 group directed towards H2; the Z conformer lies 4.0 kcal mol −1 higher).In this case, the very large 3 J( 1 H-1 H)= 4.1 Hz is due to the olefinic character of the C1-C2 bond (bond order = 1.57 to compare with 1.41-1.42for compounds 17 and 19).The resonance form that contributes most to the system is represented in Fig. 7, with a double bond between C7 and C8 (1.364A ˚, bond order 1.63) in agreement with the 13 C-NMR results.The large calculated dipole moment, 8.63 D, for compound 14 reflects its 'charge-fixed' structure.
We have calculated, within the same AM1 approximation, compound 20 (which, like cyclooctatetraene is a tub shaped structure) [22].The resonance energies are -222.60 and -213.12eV respectively, that is 219 kcal mol −1 higher for the aromatic structure 17 than for the antiaromatic one 20.

Table 1
Structure of the different compounds studied