Dolores Santa María

Variety in the Coordination Modes of the Ligand Hexakis(pyrazol-1-yl)benzene (Hpzb) to PdII, PtII and CuI Centres

The structure and conformational analysis of eight complexes derived from hexakis(pyrazol-1-yl)benzene (hpzb) have been carried out by means of X-ray crystallography and NMR spectroscopy. The metallic fragments [Pd(C6F5)2, [Pd(η3-2-Me-C3H4)]+, PtCl2 and Cu(PMe3)+] coordinate with two adjacent pyrazole residues yielding complexes with one, two or three metallic centres. The crystal and molecular structures of [{Pd(C6F5)2}2(hpzb)], [{Pd(η3-C4H7)}2(hpzb)](TfO)2, [{Cu(PMe3)}2(hpzb)](BF4)2 and [{Pd(η3-C4H7)}3(hpzb)](TfO)3 have been determined. The structure of each of the complexes depends on the fact that the metal atoms, when two or three are present, prefer to be situated on opposite faces of the benzene ring. Evidence for restricted rotation of the uncoordinated pyrazolyl groups has been found for the free ligand and monometallic derivatives, while free rotation of these groups at room temperature has been observed for the dimetallic complexes. Furthermore, the derivative {[Cu(PMe3)]2(hpzb)}(BF4)2 and probably [{Pd(η3-C4H7)}2(hpzb)](TfO)2 exhibit metallotropy on the NMR time scale. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Imidazole and benzimidazole addition to quinones. Formation of meso and d,l isomers and crystal structure of the d,l isomer of 2,3-Bis(benzimidazol-1'-yl)-1,4-dihydroxybenzene

Abstract The hydroquinones obtained by addition of imidazole, 2-methylimidazole and benzimidazole to 1,4-benzoquinone and 1,4-naphthoquinone have been isolated and identified. In the case of 1,4-benzoquinone they are monosubstituted hydroquinones 1a -1e, o-substituted hydroquinones 2a - 2c, and p-disubstituted derivatives 3a - 3c while in the case of 1,4-naphthoquinone, only disubstituted derivatives 5a and 5c have been isolated. In solution, 2,3-bis(2′-methylimidazol-1′-yl)-1,4-dydroxybenzone (2b), 2,3-bis (benzimidazol-1′-yl)-1,4-dihydroxybenzene (2c) and 2,3-bis(benzimidazol-1′-yl)-1,4-dihydroxynaphthalene (5c) exist as mixtures of meso and d,l isomers. NMR spectroscopy (n.O.e. experiments in 1H NMR and solid-state 13C CPMAS spectra) and AM1 semiempirical calculations have been used to establish the structure of the isomers both in the solid state and in solution as well as their interconversion pathways. Compound 2c-d,l crystallizes with two methanol solvate molecules as guests and it has a crystallographic twofold axis through the middle. The host molecules are linked together by means of the methanol molecules through O-H⋯O/N hydrogen bonds giving rise to chains along the c axis centrosymmetrically related.

The molecular structure of 4-tert-butylpyrazoles in the solid state and in solution: an X-ray, NMR and calorimetric study of the buttressing effect of a 4-tert-butyl substituent

The molecular structures of three 4-tert-butylpyrazoles have been determined at 173 K: 4-tert-butyl-3(5)-isopropylpyrazole 1, 4-tert-butyl-3(5)-neopentylpyrazole 2 and 4-tert-butyl-3(5)-p-tolylpyrazole 3. 1H, 13C and 15N NMR spectroscopies, in solution and in the solid state (CPMAS), have been used to complement the structural information. The major tautomers in solution correspond to the tautomers present in the crystal: 5-isopropyl 1b, 5-neopentyl 2b and 3-p-tolyl 3a. All these compounds crystallise as tetramers, formed by four identical tautomers, through N–H···N hydrogen bonds, the tetramers corresponding to 1b and 2b being folded but that of 3a is the first example of a planar tetramer.

Aromatic propellenes. Synthesis, X-Ray structures and conformational study of polypyrazolylpyridines

Abstract The crystal and molecular structures of three pentapyrazolylpyridines pz5py, dmpz5py and pz4dmpzpy (pz = pyrazol-1′-yl, dmpz = 3,5-dimethylpyrazol-1′-yl) have been determined by X-Ray crystallography. The two pyrazole rings close to the pyridine nitrogen tend to be as coplanar as possible with the pyridine plane. Two different conformations with the nitrogen lone pairs of pyrazole N2 directed towards or opposite to the N1 nitrogen of pyridine, are present in the crystals. Semiempirical AM1 calculations have been performed to evaluate the stability of these conformations which appear to be present also in solution ( 1 H and 13 C NMR and dipole moments).

Aromatic propellenes. Part 10. Conformational study of hexa(imidazol-1-yl)benzene and hexakis(2-methylimidazol-1-yl)benzene by means of NMR and AM1 calculations

Abstract Hexa(imidazol-1-yl)benzene, (imz) 6 bz, and hexakis(2-methylimidazol-1-yl)benzene, (mimz) 6 bz, have been synthesized from hexafluorobenzene and imidazole or 2-methylimidazole anions in tetrahydrofuran. The stabilities of the eight possible conformers of (imz) 6 bz and (mimz) 6 bz have been estimated by semi-empirical calculations at the AM1 level. In both compounds, the absolute minimum corresponds to the conformation in which the β nitrogen lone pair of consecutive imidazole rings is alternately oriented to opposite sides of the benzene plane. While in (imz) 6 bz only the ring signals corresponding to a unique imidazole were observed, the presence of the eight conformers in (mimz) 6 bz has been established by 1 H and 13 C NMR spectroscopy.

The annular tautomerism of 4(5)-phenylimidazole

A new polymorph of 4-phenylimidazole 1a has been characterised by X-ray crystallography. Both polymorphs present a secondary structure of chains, and the observed differences in the topology of their crystal packing are related to the conformational differences in the primary and secondary structural levels. NMR experiments (13C and 15N) reveal that tautomer 1a is the only one observed in the solid state, being also the most abundant in solution (80%) in agreement with high-level theoretical calculations which favour this tautomer over the 5-phenyl one. The results obtained for this compound are relevant for the structure of the related 4(5)-porphyrinylimidazole.

Molecular structure and dynamics of C-1-adamantyl substitutedN-unsubstituted pyrazoles studied by solid state NMRspectroscopy and X-ray crystallography

The influence of the 1-adamantyl group on the structure and the proton transfer dynamics of N-unsubstituted pyrazoles has been determined. Four compounds have been labelled with 15 N and studied by variable temperature 15 N CP MAS NMR spectroscopy: 3(5)-(1-adamantyl)pyrazole 2, 4-(1-adamantyl)pyrazole 3, 3,5-dimethyl-4-(1-adamantyl)pyrazole 4 and 3,5-di(1-adamantyl)pyrazole 5. Compound 2 (a 1∶1 mixture of both tautomers) is a long chain of hydrogen bonded molecules (‘catemer’) and as in most catemers there is no proton transfer since it would imply an ‘infinite’ number of proton jumps. Compound 3, although also a ‘catemer’, is possibly an exception to this rule, in that it seems to show proton transfer. In the solid state, compounds 4 and 5 should be cyclic hydrogen-bonded structures, dimers or trimers, but the activation energies for proton transfer, about 39 kJ mol -1 , are quite low compared with those of 3,5-dimethylpyrazole. It appears that the quasi-spherical shape of the 1-adamantyl substituent and its solid-state plasticity may play a role in lowering these barriers. The crystal structure of 2 has been determined by X-ray analysis. Individual molecules of 2 form chains through N–H · · · N hydrogen bonds (‘catemers’) very similar to those already described for 4-(1-adamantyl)pyrazole and for pyrazole itself; however, the packing of these catemers is different. Tautomers 2a and 2b are present in the crystal in a 1∶1 ratio, forming alternating chains of hydrogen-bonded molecules (2a · · · 2b · · · 2a · · · 2b · · ·); the NH hydrogen atoms are linked to both nitrogen positions (N1 and N2) and show a 1∶1 disorder.