Reactivity of Nickel(II) and Copper(II) Complexes of a β-Aminohydrazone Ligand with Pyridine-2-aldehyde: Macrocyclization vs Unprecedented Pyrazole Ring Synthesis via C–C Bond-Forming Reaction

The synthesis and characterization of a mononuclear nickel(II) complex [Ni(L2)](ClO4)2 (1) and an analogous mononuclear copper(II) complex [Cu(L2)](ClO4)2 (2) of a 15-membered azamacrocycle (L2 = 3-(2-pyridyl)-6,8,8,13,13,15-hexamethyl-1,2,4,5,9,12-hexaazacyclopentadeca-5,15-diene) are reported. The macrocyclic ligand is formed during the reaction of 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11-dione dihydrazone (L1) with pyridine-2-aldehyde (PyCHO) templated by metal ions. The X-ray crystal structure of 1 exhibits a distorted square-pyramidal coordination geometry, where the metal ion sits in the macrocyclic cavity and the pendant pyridine group of L2 occupies the axial position. While 1 is stable in the presence of an excess of PyCHO, 2 reacts further with copper(II) salt and PyCHO to form a mononuclear copper(I) complex, [Cu(H2L3)](ClO4)3 (3). The structure of the complex cation of 3 reveals a distorted tetrahedral coordination geometry at the copper center with a pseudo 2-fold screw axis. A two-dimensional (2D) polymeric copper(II) complex, {[Cu2(L4)2](ClO4)2}n (4) is obtained by reacting complex 2 (or [Ni(L1)](ClO4)2) with copper(II) perchlorate and pyridine-2-aldehyde in a methanol–water solvent mixture. Complex 4 is also obtained by treating 3 with copper(II) perchlorate and pyridine-2-aldehyde in the presence of a base. The X-ray structural analysis of 4 confirms the formation of a pyrazolate bridged dimeric copper(II) complex. The extended structure in the solid state of 4 revealed the formation of a 2D coordination polymer with the dimeric core as the repeating unit. The ligand (HL4) in 4 is a 3,4,5-trisubstituted pyrazole ring formed in situ via C–C bond formation and represents an unprecedented transformation reaction.