Ligand Structure Effects on Molecular Assembly and Magnetic Properties of Copper(II) Complexes with 3‑Pyridyl-Substituted Nitronyl Nitroxide Derivatives

Reaction of Cu­(hfac)2 with
methyl- and bromo-3-pyridyl-substituted
nitronyl nitroxides (LR) leads to assemble
a diverse set of coordination complexes: mononuclear [Cu­(hfac)2L2‑Me], binuclear [{Cu­(hfac)2}2(H2O)­L2‑Me], trinuclear [{Cu­(hfac)2}3(L6‑Br)2], pentanuclear [{Cu­(hfac)2}5(L2‑Me)2], and [{Cu­(hfac)2}5(L2‑Me)4], cocrystals [Cu­(hfac)2(L2‑Br)2]·[Cu­(hfac)2(H2O)2] and [Cu­(hfac)2(L2‑Br)2]·2­[Cu­(hfac)2H2O], one-dimensional polymers [Cu­(hfac)2L2‑Br]n and
[Cu­(hfac)2L6‑Br]n, and cyclic dimers [Cu­(hfac)2L5‑Me]2, [Cu­(hfac)2L5‑Br]2, and [Cu­(hfac)2L6‑Me]2. The molecular
structures of the obtained complexes are strongly affected by the
substituent type and its location in the pyridine heterocycle. Occupation
of the second position of the pyridine ring increases the steric hindrance
of both imine and nitroxide coordination sites of L2‑R, which is favorable for the formation of various conformers
and precipitation of complexes with different molecular structures.
The pentanuclear [{Cu­(hfac)2}5(L2‑Me)2] and [{Cu­(hfac)2}5(L2‑Me)4] complexes do not have
prior analogues and are valuable model objects for investigation of
the mechanism of formation of various coordination polymers. The arrangement
of long Cu–ONO bonds in {CuO6} square
bipyramids due to the weakening nitroxide donor site in complexes,
based on L2‑Me, L2‑Br, and L6‑Br ligands, results
in ferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides. Complexes with substituents that do not considerably
affect the coordination ability of ligands (L5‑Me, L5‑Br, and L6‑Me) exhibit strong antiferromagnetic exchange interactions between
spins of Cu2+ ions and nitroxides.