Squarato-metal(II) complexes. 1: Structural and magnetic characterization of squarato-bridged dinuclear nickel(II) and copper(II) complexes

A new series of dinuclear squarato-bridged nickel(II) and copper(II) complexes [Ni2(2,3,2-tet)2(μ1,3-C4O4)(H2O)2](ClO4)2 (1), [Ni2(aepn)2(μ1,3-C4O4)(H2O)2](ClO4)2 (2), [Cu2(pmedien)2(μ1,3-C4O4)(H2O)2](ClO4)2.4H2O (3) and [Cu2(DPA)2(μ1,2-C4O4)(H2O)2](ClO4)2 (4) where C 4 O 4 2 - is the dianion of 3,4-dihydroxycyclobut-3-en-1,2-dione (squaric acid), 2,3,2-tet = 1,4,8,11-tetraazaundecane, aepn = N-(2-aminoethyl)-1,3-propanediamine, pmedien = N,N,N′,N″,N″-pentamethyldiethylenetriamine and DPA = di(2-pyridylmethyl)amine were synthesized and structurally characterized by X-ray crystallography. The spectral and structural characterization as well as the magnetic behaviour of these complexes is reported. In this series, structures consist of the ClO 4 - groups as counter ions and the C 4 O 4 2 - bridging the two M(II) centers in a μ-1,3- (1–3) and in a μ-1,2-bis(monodentate) (4) bonding fashions. The coordination geometry around the Ni(II) ions in 1 and 2 is six-coordinate with distorted octahedral environment achieved by N atoms of the amines and by one or two oxygen atoms from coordinated water molecules, respectively. In the Cu(II) complexes 3 and 4, a distorted square pyramidal geometry is achieved by the three N-atoms of the aepn or DPA and by an oxygen atom from a coordinated water molecule. The electronic spectra of the complexes in aqueous solutions are in complete agreement with the assigned X-ray geometry around the M(II) centers. The complexes show weak antiferromagnetic coupling with ∣J∣ = 1.8–4.2 cm−1 in the μ-1,3- bridged squarato compounds 1–3, and J = −16.1 cm−1 in the corresponding μ-1,2- bridged squarato complex 4. The magnetic properties are discussed in relation to the structural data.