Intramolecular hydrogen bond effect on keto-enolization of aroylhydrazone in copper(II) complexes

Three new coordination complexes, [Cu(L)(nic)(NO3)]·CH3OH·0.63H2O (1), 2[Cu(L)(NO3)]·1CH3OH (2) and [Cu(L-NO2)(NO3)] (3) were synthesized from a tridentate N,N,O-donor ligand (HL = 2-acetylpyridine-3-hydroxy-2-naphthoylhydrazone, nic = nicotinamide) and thoroughly characterized by elemental analyses and spectroscopic methods. Structures of the complexes were unequivocally established by single crystal X-ray diffraction technique. The geometry around Cu in complex 1 is a distorted square pyramid and two adjacent copper complexes are linked to a dimer by a pair of hydrogen bonds of the type NH⋯O. These dimers are further stabilized by weak CH⋯π contacts. Metal ions display distorted square planar geometry in 2 and 3. In addition to intramolecular hydrogen bonds in complexes 2 and 3, there are relatively substantial intermolecular π-stackings. Spectroscopic and X-ray structure analyses of complexes 1–3 show the coordination of L− ligand in keto form as NN⊖C(O) (not NNC(O⊖)). In this study conditions for the in situ nitration of naphthoylhydrazone with Cu(NO3)2 during the synthesis of 3 were also established.

Synthesis and structures of three new coordination complexes of Cu(II) with 2-acetylpyridine naphthoylhydrazone are discussed. Spectroscopic and X-ray structure data show the ligand is monoanionic and coordinated in hydrazinide form. Conditions are also reported for the in situ nitration of naphthoylhydrazone with Cu(NO3)2.Figure optionsDownload as PowerPoint slideHighlights
► Three new complexes of copper(II) with aroylhydrazone are discussed.
► X-ray structures of the complexes are presented which show strong pi-stacking.
► In spite of the amide N–H deprotonation, aroylhydrazone is coordinated in the keto form.
► A new method for nitration of naphthoylhydrazone with Cu(NO3)2 is important.
► Reaction conditions for in situ nitration are interesting.