Synthesis and Structure of [2 × 2] Molecular Grid Copper(II) and Nickel(II) Complexes with a New Polydentate Oxime-Containing Schiff Base Ligand

A new polynucleating oxime-containing Schiff base ligand, 2-hydroxyimino-N′-[1-(2-pyridyl)ethylidene]propanohydrazone (Hpop), has been synthesized and fully characterized. pH potentiometric, electrospray ionization mass spectrometric, and spectrophotometric studies of complex formation in H2O/DMSO solution confirmed the preference for polynuclear complexes with 3d metal ions. Single-crystal X-ray diffraction analyses of [Ni4(pop)4(HCOO)4]·7H2O (1), [Cu4(pop-H)4(HCOOH)4]·H2O (2), and [Cu4(pop-H)4(H2O)4]·9H2O (3) indicated the presence of a [2 × 2] molecular grid structure in all three compounds but distinct configurations of the cores: a head-to-tail ligand arrangement with overall S4 symmetry of the grid in the Cu2+ complexes as opposed to a head-to-head ligand arrangement with (noncrystallographic) C2 grid symmetry for the Ni2+ complex. A cryomagnetic study of 3 revealed intramolecular ferromagnetic exchange between copper ions in the grid, while in 1, antiferromagnetic interactions between the metal ions were observed.