Ferromagnetic and antiferromagnetic copper(ii) complexes: Counterplay between zero-field effects of the quartet ground state and intermolecular interactions

Abstract

The linear trinuclear Cu(II) complexes [Cu₃(L₁)₄(H₂tea)₂] (1), [Cu₃(L₂)₄(H₂tea)₂]·2CH₃CN (2), [Cu₃(L₂)₄(H₂tea)₂] (3), [Cu₃(L₁)₂(H₂tea)₂(NO₃)₂] (4) and the dinuclear complex [Cu₂(L₁)₂(H₂tea)₂] (5), where L₁ = 2-thiophene carboxylate, L₂ = 2-(thiophen-2-yl)-acetate and H₂tea = the single deprotonated form of triethanolamine have been prepared and characterised while the crystal structures of 1–4 have been determined. The variable-temperature magnetic susceptibilities of complexes 1–5 have been measured in the range 2–300 K under various external fields in the range 0.02–1.0 T. X-band EPR spectra of 1–5 compounds were recorded at 4–100 K. Complexes 1, 2 and 3 found to have the same J = 33 cm⁻¹ and g values 2.16(1), 2.20(1) and 2.16(1) respectively while for 5J = 15 cm⁻¹ and g = 2.06(1) revealing a clear ferromagnetic exchange between Cu(II) ions. Complex 4 was found to be antiferromagnetic with J = −28 cm⁻¹ and g = 2.21(1). The polycrystalline powder X-band EPR spectrum of complexes 1, 2, and 3 at 4 K are dominated by a transition at 1600 G (g = 4.3) which unambiguously identifies the spin of the ground multiplet (S = 3/2) while the antiferromagnetic complex 4 has a derivative centered at g = 2.1 indicative of a ground doublet (S = 1/2). Concerning complex 5 a spectrum of a dominant derivative centered at g = 2.06(1) is observed with a very weak half field transition (ca. 1500 G) indicative of the ferromagnetic nature of the system. Furthermore, for complexes 2 and 3 a strong temperature dependence of this spectroscopic g-factor is revealed and change of the g_eff from the liquid helium temperature to the room temperature is almost 2 units.