Trinuclear Cu(II) complexes containing peripheral ketonic oxygen bridges and a μ₃-OH core: Steric influence on their structures and existence

Two tridentate Schiff bases, HL¹(6-amino-3-methyl-1-phenyl-4-azahex-2-en-1-one), and HL² (6-amino-3,6-dimethyl-1-phenyl-4-azahex-2-en-1-one) on reaction with Cu(II) perchlorate in the presence of triethyl amine yielded two new trinuclear copper(II) complexes, [(CuL¹)₃(μ₃-OH)](ClO₄) ₂ (1) and [(CuL²)₃(μ₃-OH)](ClO₄) ₂ · 0.75H₂O (2), whereas another tridentate ligand HL³ (7-amino-3-methyl-1-phenyl-4-azahept-2-en-1-one) underwent hydrolysis under the same reaction conditions to result in the formation of a mononuclear complex, [Cu(bn)(pn)ClO₄] (3) [where bn = 1-benzoylacetonate and pn = 1,3-propanediamine]. All three complexes have been characterized by X-ray crystallography. For both 1 and 2 the cationic part is trinuclear with a [Cu₃OH] core held by three carbonyl oxygen bridges between each pair of copper(II) atoms. The structure of 3 is a monomer with a chelating 1,3-propanediamine and a benzoyl acetone moiety. Magnetic measurements of 1 and 2 have been performed in the 2-300 K temperature range. The experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -J(S₁S₂ + S₂S₃ + S₁S₃), yielding as best fit parameters: J = -25.6 cm^-1, g = 2.21 for 1 and J = -11.2 cm^-1, g = 2.10 for 2. The EPR spectra at low temperature could be indicative of spin frustration in complex 1.