M. Dolores Santa María

Multiple hydrogen bonds and tautomerism in naphthyridine derivatives

The behaviour of three 2,7-disubstituted 1,8-naphthyridines able to exhibit tautomerism has been studied by NMR in solution and in two cases in the solid state. The three derivatives studied are 2,7-dihydroxy- (1), 2-acetamido-7-amino- (3) and 2,7-diacetamido-1,8-naphthyridine (4). To explore the problem of secondary interactions, a series of complexes, with up to four simultaneous hydrogen bonds, where the monomers are generated using pyridine and 4-pyridone as building blocks, have been theoretically studied. The calculated interaction energies have been correlated with the number of hydrogen bonds and with attractive and repulsive secondary interactions. Further analysis of the electron density and orbital interactions shows that the secondary interactions, both attractive and repulsive, have a purely electrostatic origin. The X-ray structure of compounds 3 and 4 have been determined. In the solid state these compounds exist in the “diamino” tautomers with the N–H proton of the amido groups pointing towards the naphthyridine nitrogen. DFT and GIAO calculations have been essential to disentangle the problem of the structure of these compounds.

The Structure of a Non-Symmetric Disordered Tetramer: A Crystallographic and Solid State Multinuclear NMR Study of the Properties of 3(5)-Ethyl-5(3)-Phenyl-1H-Pyrazole

The X-ray structure and the solid-state NMR measurements, mainly 15N CPMAS of the labelled compound, allow to determine the static and dynamic properties of 3(5)-ethyl-5(3)-phenyl-1H-pyrazole. The compound is a tetramer formed by three 5-ethyl-3-phenyl-1H-pyrazole and one 3-ethyl-5-phenyl-1H-pyrazole tautomers in dynamic equilibrium with the complementary situation.

Multinuclear NMR solution studies on complexes of hexakis(pyrazol-1-yl)benzene (hpzb) with Ag(I)

Abstract The reaction of the polydentate N-donor ligand hexakis(pyrazol-1-yl)benzene (hpzb) with AgSbF6/PPh3 or AgClO4PPh3 in a 1:1 ratio leads to the complexes [Ag(PPh3)(hpzb)]SbF6 (1) or [Ag(PPh3)(hpzb)]ClO4 (3). When two or three equivalents of the silver derivatives are used the species [Ag(PPh3)2]X are formed in addition to 1 or 3. In the case of SbF6− the remaining Ag+ competes with AgPPh3+ for coordination to hpzb and the derivative [Ag(hpzb)]SbF6 (2) is also obtained, while in the case of ClO4− the insoluble salt AgClO4 is formed. The same reactions performed with hpzb–15N12 confirmed the presence of the N-donor ligand in 1–3. At room temperature a Ag–P dissociation process is observed for all the derivatives containing Ag–PPh3 fragments. Complexes 1–3 show in solution an intramolecular argentotropic shift that makes the six pyrazolyl rings of the hpzb ligand equivalent on the 1H NMR time scale. When the amount of counteranion present in solution is sufficiently high, an intermediate of lower coordination number (stabilized with the anion) is observed for complexes 1 and 3 in the low temperature 31P NMR spectra in addition to the three-coordinated species. This intermediate is more stable in the case of ClO4− due to the higher coordinating ability of this anion.