Vibrational spectra, structure and hydrogen bonding of 5-tert-butylpyrazole and its zinc complexes

New zinc complexes [L2ZnX2] with X ¼ Cl (I), Br (II), I (III) and NO3 (IV), [L3Zn(OClO3)]ClO4 (V) and [L4Zn](ClO4)2 (VI) with 5-tertbutylpyrazole as ligand L were synthesized and characterized by infrared-, Raman-, mass- and NMR-spectroscopy. The assignment of the vibrational frequencies for the complexes in the range of 4000–80 cm21 is proposed. Analysis of the IR spectra in the range of nNH frequencies shows that 5-tert-butylpyrazole forms cyclic associates in the solid using intermolecular NH· · ·N hydrogen bonds. This was proven by a crystal structure determination, which showed that L exists as tetramers in the solid state. In solution there is an equilibrium between tetramers and monomers and, probably, dimers or trimers. In the complexes, the NH-groups of L form intramolecular H-bonds with the X-group. The intramolecular H-bond in VI has interionic character and partially dissociates in solution at high dilution. Ligand vibrations that are sensitive to the coordination and the dependence of their frequencies on the anionic group X has been revealed. q 2004 Elsevier B.V. All rights reserved.