CH•••O hydrogen bond in the crystal structure of a pyrazine-based ligand and determination of the amplitude of the ligand conformational change induced by Cu(II) coordination

The crystallographic structural features of the ditopic ligand pyrazine-2,5-dicarbaldehyde-bis(propionyl-N-methylhydrazone) 1, as well as their comparison with related ligands are herein reported. CH center dot center dot center dot O short contacts (2.46 and 2.55 angstrom) supporting network formation were observed. The amplitude of the conformational switch induced by the formation of the dinuclear rack-like complex [Cu(2)1(terpy)(2)](CF3SO3)(4) of ligand 1 (synthesized by reaction of 2,5-pyrazine-dicarboxaldehyde with two equivalents of N-methyl-propionohydrazide), has been determined on the basis of the X-ray structures and it was found to be of 3 angstrom. Moreover, the correlation between the structural data and the data obtained by 2D NMR spectroscopy, is confirmed by the mean of the structural analysis herein discussed that concerns the protons involved in the NOE effects observed by NOESY spectroscopy. (C) 2009 Elsevier B.V. All rights reserved.