Molecular structure and dynamics of C-1-adamantyl substituted N-unsubstituted pyrazoles studied by solid state NMR spectroscopy and X-ray crystallography

Abstract

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.