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 (L ^R ) leads to assemble a diverse set of coordination complexes: mononuclear [Cu(hfac)₂L ^2-Me ], binuclear [{Cu(hfac)₂}₂(H₂O)L ^2-Me ], trinuclear [{Cu(hfac)₂}₃(L ^6-Br )₂], pentanuclear [{Cu(hfac)₂}₅(L ^2-Me )₂], and [{Cu(hfac)₂}₅(L ^2-Me )₄], cocrystals [Cu(hfac)₂(L ^2-Br )₂]·[Cu(hfac)₂(H₂O)₂] and [Cu(hfac)₂(L ^2-Br )₂]·2[Cu(hfac)₂H₂O], one-dimensional polymers [Cu(hfac)₂L ^2-Br ] _n and [Cu(hfac)₂L ^6-Br ] _n , and cyclic dimers [Cu(hfac)₂L ^5-Me ]₂, [Cu(hfac)₂L ^5-Br ]₂, and [Cu(hfac)₂L ^6-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 L ^2-R , which is favorable for the formation of various conformers and precipitation of complexes with different molecular structures. The pentanuclear [{Cu(hfac)₂}₅(L ^2-Me )₂] and [{Cu(hfac)₂}₅(L ^2-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-O_NO bonds in {CuO₆} square bipyramids due to the weakening nitroxide donor site in complexes, based on L ^2-Me , L ^2-Br , and L ^6-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 (L ^5-Me , L ^5-Br , and L ^6-Me ) exhibit strong antiferromagnetic exchange interactions between spins of Cu²⁺ ions and nitroxides.