Structural, spectroscopic, and magnetic properties of mono- and bi-nuclear copper(II) complexes of N-(2-pyridinyl)ketoacetamides

Abstract

The synthesis and properties of two N-(2-pyridinyl)ketoacetamide ligands and their mono- and binuclear copper(II) complexes are described. A crystal structure determination for one of the ligands, benzoyl-N-(2-pyridinyl)acetamide, HL²(as its hydrochloride hydrate), shows that it belongs to the orthorhombic space group P2₁2₁2₁, with a= 16.224(9), b= 12.045(6), and c= 7.41 5(4)Å. The β-diketone moiety exists in the enol tautomeric form with unequal C–O bond lengths [1.24(1) and 1.37(1)Å] and other geometric features similar to those in related β-diketones. In solution the ligands display keto-enol tautomerism. Mononuclear copper(II) complexes of type [CuL₂] display spectral and magnetic properties typical of square-planar copper(II)β-diketone chelates. In contrast, binuclear species of type [Cu₂L₂(ClO₄)₂] display very strong antiferromagnetic coupling. The crystal structure of one example involving the N-(2-pyridinyl)acetoacetamide ligand, viz. [Cu₂L¹₂(ClO₄)2]·2H₂O, shows that it belongs to the triclinic space group P, with a= 10.522(6), b= 9.132(5), c= 8346(5)Å, α= 102.38(12), β= 121.86(14), and γ= 93.75(10)°. The binucleating ligands, L¹, are disposed trans to each other in a planar arrangement about the two Cu atoms. The pyridine N and keto O atoms are co-ordinated to each Cu with the amide O acting as a bridging atom. The Cu–Cu distance [3.022(2)Å] and Cu–O–Cu angle [102.0(2)°] can be correlated with the size of the exchange constant, 2J=–588 cm^–1, in a similar manner to the values for related µ-alkoxo-copper(II) complexes. The perchlorate groups are weakly co-ordinated to each Cu atom in a bridging bidentate manner.