Syntheses, crystal structures and electronic properties of [Cu(bipym)(C4O4)(H2O)3]·2H2O and [Cu2(bipym)(C4O4)2(H2O)6](bipym = 2,2′-bipyrimidine)

Abstract

Two new mixed-ligand complexes [Cu(bipym)(C₄O₄)(H₂O)₃]·2H₂O 1 and [Cu₂(bipym)(C₄O₄)₂(H₂O)₆]2[bipym = 2,2′-bipyrimidine and C₄O₄^2–= dianion of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione)] have been obtained from aqueous solutions containing Cu(NO₃)₂·3H₂O, bipym and Li₂[C₄O₄] in 1 : 1 : 0.25 and 2 : 1 : 0.5 (Cu²⁺ : bipym: C₄O₄^2–) molar ratio, respectively. The structures of both complexes have been characterized by single-crystal X-ray analysis. Compound 1 consists of mononuclear [Cu(bipym)(C₄O₄)(H₂O)₃] units in which the copper atom exhibits a slightly distroted elongated-octahedral co-ordination with two bipym nitrogens, one squarate oxygen and one water oxygen forming the equatorial plane, and two water molecules in the axial positions. The structure of 2 is built by centrosymmetric bipym-bridged dinuclear [Cu₂(bipym)(C₄O₄)₂(H₂O)₆] units, the geometry of each copper atom being similar to that found in 1. Squarate acts as a monodentate ligand in both compounds whereas bipym exhibits chelating and bis(chelating) co-ordination modes in 1 and 2, respectively. The intramolecular metal–metal separation in 2[5.542(1)Å] is the largest found in bipym-bridged copper(II) complexes. The magnetic behaviour of 2 has been investigated over the temperature range 10–300 K. Fitting of the magnetic susceptibility data for 2 by a simple Bleaney–Bowers expression yields a value for the singlet–triplet energy gap (J) of –139 cm^–1. The magnitude of the observed antiferromagnetic interaction is the smallest found in bipym-bridged copper(II) complexes for which the σ in-plane exchange pathway is operative. Extended-Hückel calculations have been used to analyse how the magnitude of the exchange coupling is influenced by small structural distortions in this family of complexes.