Gas-phase (ion cyclotron resonance spectrometric) and solid-state (crystallographic) studies of highly substituted pyrazoles (original) (raw)
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Journal of Molecular Structure, 1999
The crystal structure of 3(5)-phenyl-4-bromo-5(3)-methylpyrazole, 3, has been determined by X-ray crystallography. Both tautomers are present simultaneously in the crystals forming cyclic tetramers. The proton disorder, determined by the crystallographic methods, has been established to be dynamic using solid state 13 C CPMAS NMR spectroscopy. This is a new example of an asymmetrically 3,5-disubstituted pyrazole that presents solid state intermolecular proton transfer. The arrangement of the tautomers in the cyclic tetramer is different to the case of 3(5)-phenyl-5(3)-methylpyrazole, 2, previously described. ᭧ 1999 Elsevier Science B.V. All rights reserved.
Journal of Magnetic Resonance, Series A, 1996
The results of various 15 N solid-state NMR experiments perin the molecular principal axis system (PAS) is necessary formed on solid samples of doubly 15 N-labeled 3,5-dimethylpyrafor the analysis of several solid-state NMR experiments, e.g., zole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyrafor distance measurements by dipolar couplings in static zole are reported. In the solid state, these compounds form various powder spectra (4), by rotational resonance experiments (5) hydrogen-bonded complexes. The principal values of the 15 N or multipulse techniques (6), for the evaluation of structures chemical-shift tensors (CST) of amine and imine nitrogen atoms by CST correlation (7), for the detection of molecular moare derived by lineshape analysis of the 15 N NMR spectra of the tions (8), or for the analysis of relaxation phenomena (9). static powders obtained under the conditions of 1 H-15 N cross po-The size of the three principal elements of the CST can larization and 1 H decoupling. The orientations of the 15 N CST in provide additional information, which is empirically interthe molecular principal axis system are obtained by taking into preted and becomes more important with an increasing numaccount the 15 N-15 N dipolar interactions and the 15 N-D dipolar interaction after deuteration of the mobile proton sites. The rela-ber of data available (10). tive orientations of the amine and imine CST in PMP are indepen-In the present paper, we report our investigation of 15 N dently checked by one-dimensional off-magic axis sample spinning CST in 3,5-dimethylpyrazole (DMP), b-5-methyl-3-phemagnetization transfer experiments. The isotropic chemical shifts, nylpyrazole (b-PMP), and 3,5-diphenylpyrazole (DPP) the principal elements, and the orientations of the CST of both (Fig. 1). Our interest in these compounds actually stems nitrogen atoms and the ND distances depend only slightly on the from the study of their solid-state hydrogen bonding and chemical structure and the associated hydrogen-bonded structure. proton tautomerism. The chemical structure variations of The intramolecular structure of pyrazoles, therefore, does not vary these compounds leads to interesting differences in the solidsubstantially when different types of hydrogen-bonded complexes state hydrogen-bonded structure: DMP forms cyclic trimers are formed. ᭧ 1996 Academic Press, Inc. and DPP cyclic tetramers in which degenerate triple and quadruple proton transfers take place as shown by 15 N CPMAS experiments (MAS Å magic-angle sample spin-46
The structure of 3,5-bis(trifluoromethyl)pyrazole in the gas phase and in the solid state
New Journal of Chemistry, 1999
The X-ray molecular structure of the important molecule 3,5-bis(triÑuoromethyl)pyrazole has been determined at 120 K and gave crystals belonging to the triclinic space group. The compound forms tetramers through P1 6 NÈHÉ É ÉN hydrogen bonds and some proton disorder is necessary to explain the geometric features of the monomers. The IR spectra have been recorded in the gas phase (monomers) and in the solid state (tetramers) and analyzed by comparison with the calculated normal frequencies. The use of solid-state NMR spectroscopy combined with ab initio GIAO calculations suggests that a certain amount (about 40^10%) of dynamic disorder involving intramolecular proton transfers could be present in the crystal. We have been interested for several years in the modiÐcation of the properties of azoles produced by triÑuoromethyl substituents. Thus, we have described the e †ect of the replacement of a methyl by a triÑuoromethyl group on the acid-base properties of pyrazoles.1 In that work, we discussed, among other pyrazoles, the case of 3,5-bis(triÑuoromethyl)pyrazole 1 and reported STO-3G calculations on this compound. The same level of calculations was employed to discuss the Ñash vacuum pyrolytic behavior of 1.2 Its gas-phase structure was determined by electron di †raction (ED) and compared with the STO-3G calculated geometry.3 The related case of 3,5-bis-(triÑuoromethyl)-1,2,4-triazole 2 was published subsequently (thermodynamic properties and MP2/6-31G* calculations).4 Finally, a review was devoted to triÑuoromethylazoles, including 1 and 2.5 The recent discovery that Celecoxib, a tri-Ñuoromethylpyrazole derivative, is a powerful antirheumatic and antiarthritic drug, makes these compounds even more interesting.6 Although compound 1 was used as a ligand as early as 1983È 86,7h10 its great development in organometallic and coordination chemistry started in 1995 and was essentially due to Dias et al.11h19 Other authors like Togni,20,21 Venanzi,22 Lalor,23 Zanello,24 and Storr25 and their groups have contributed to this Ðeld. ¤ Supplementary material available : table of calculated and experimental IR frequencies. Conformations of the di †erent monomers. Table of X-ray geometries of di †erent pyrazole derivatives. For direct electronic access see
Crystals
Prior to 2021, 4-bromo-1H-pyrazole (published in 1999) was the only structurally characterized 4-halogenated-1H-pyrazole in the Cambridge Crystallographic Data Center (CCDC). The structures of 4-chloro-1H-pyrazole and 4-fluoro-1H-pyrazole were published in 2021 and 2023, respectively. Herein, we report the crystal structure for 4-iodo-1H-pyrazole, completing the crystallographic data for the series of 4-halogenated-1H-pyrazoles. The bromo and chloro analogs are isostructural, forming trimeric H-bonding motifs, whereas the fluoro and iodo analogs form non-isostructural catemers. We also compare the experimental and theoretical (by DFT calculations) IR and 1H NMR spectroscopic data of the four halogenated 4-X-pzH compounds and unsubstituted pyrazole (pzH). An explanation is offered for some counterintuitive structural, infrared, and 1H-NMR spectroscopic data.
Theoretical study of the structure and tautomerism of N 1-unsubstituted pyrazoles in the solid state
Journal of The Chemical Society-perkin Transactions 2, 1997
Ab initio theoretical calculations have been performed on the pyrazole cyclic dimer, trimer and tetramers as well as on linear oligomers, assuming that there is no relaxation of the geometry during the proton transfer. The ground state and a wide variety of transition states, corresponding to different pathways for intermolecular proton transfer, have been explored and the results compared with experimental data from crystallography and solid state NMR spectroscopy. For the simplest case of the dimer, the reaction path corresponding to a double proton transfer has been explored as well as the effect of relaxing the geometry.
Ab initio study of the effect of N-substituents on properties of pyrazoles
Tetrahedron, 1995
A series of fourteen derivatives of pyrazole have been calculated at the MP2-6-31G** level. The first thirteen are derivatives of the parent pyrazole with different substituents on the nitrogen N(1) and the last one is pyrazole N-oxide. The substituents have been selected to cover a wide range of electronic effects. The theoretical results are discussed in relation with geometries, energies, vibrational spectra, Bader analysis and tautomerism (in the case l-hydroxypyrazole/pyrazole N-oxide).