Binuclear copper(II) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane: synthesis, spectroscopy, and spin coupling across multiple-atom bridges of variable length (3.6–7.6 Å)

Abstract

Thirteen new binuclear copper(II) complexes [LCu(µ-X)CuL]²⁺ with L = 1,4,7-trimethyl-1,4,7-triazacyclononane and X = 2Cl^–, 2Br^–, 2SCN^–, C₂O₄^2–, (HN)₂C₂O₂^2–, Cl₂C₆O₄^2–, H₂C₆O₄^2–, and 2CH₃CO₂^– have been synthesized and characterized based on i.r., electronic, and e.s.r. spectroscopy and variable-temperature (100–300 K) magnetic susceptibility measurements. A varying range of magnetic interactions, no coupling, antiferromagnetic (2J=–460 cm^–1), and ferromagnetic (J=+22 cm^–1) coupling, has been observed between the copper(II) ions in these binuclear systems with variable metal–metal separation estimated to be in the range of 3.6–7.6 Å. The interaction is more effective through a µ-oxamato bridge (2J=–460 cm^–1) than through a µ-oxalato bridge (2J=–300 cm^–1). A moderately strong antiferromagnetic interaction (2J=–60 cm^–1) has been found for the Cl₂C₆O₄²-bridged compound, where the Cu ⋯ Cu separation is expected to be ≈ 7.6 Å. The X-band e.s.r. spectra of the polycrystalline substances at 120 K indicate square-pyramidal geometry for the copper with a (d_{x²– y²})¹ ground state. A mixed bridged compound (µ-1,1 -N₃)(µ-OH), with a ferromagnetic interaction between the copper centres is also described. The difference in magnetic exchange interaction between the different copper(II) systems is discussed.