Reaction of Amines with Aldehydes and Ketones Revisited: Access To a Class of Non-Scorpionate Tris(pyrazolyl)methane and Related Ligands

The reaction of amines with aldehydes and ketones has been exploited for over 150 years to produce Schiff bases, one of the most popular classes of compounds in both organic and coordination chemistry. In certain cases, however, compounds other than Schiff bases have been reported to result from such reactions. After conducting a representative reaction under various different conditions and identifying several reaction intermediates by NMR spectroscopy, mass spectrometry and X-ray crystallography, we now report a unified picture that explains the scattered and often inconsistent results obtained with 3(5)-aminopyrazole derivatives and other related molecules. Acid catalysis, which is often employed in Schiff base synthesis, radically changes the course of reaction and leads to bis(pyrazol-4-yl)methane derivatives instead of the expected Schiff base products. The stoichiometry of the reaction is also found to be crucial for obtaining quantitative conversions. A total of 31 new compounds have been isolated and characterized as a result of this study, including a reaction intermediate, 2 Schiff bases, and 28 bis- or tris(pyrazol-3(4)-yl)methane ligands. The latter defines a new class of "non-scorpionate" ligands, with three nonchelating pyrazole moieties, as opposed to the well-known "scorpionate" tris(pyrazol-1-yl)-borate and -methane ligands.