Control of the stereochemistry of four-co-ordinate copper(II) complexes by pyridinecarboxamide ligands: crystal structure, spectral and redox properties

Abstract

Three copper(II) complexes [CuL¹]·H₂O 1, [CuL²₂]2 and [CuL³₂]3[H₂L¹=N,N'-o-phenylenebis-(pyridine-2-carboxamide); HL²=N-phenylpyridine-2-carboxamide; HL³=N-2,4,5-trichlorophenyl-pyridine-2-carboxamide] have been prepared and their stereochemical properties investigated. Complex 2 has been characterized by X-ray crystallography: space group P2₁/n, a=10.853(4), b=19.015(6), c=10.293(3)Å, β= 105.22(3)°, Z= 4, R= 0.030, R'= 0.034 for 2938 observed reflections. Compared to the known square-pyramidal structure of 1 it is revealed that the deprotonated bidentate ligand L² exerts a measurable degree of geometric control over the co-ordination sphere of 2(distorted towards tetrahedral). Similar effects are also observed from absorption and EPR spectral results. The Cu^II-Cu^I redox potentials in dimethylformamide solution have been determined by cyclic voltammetry. Upon replacement of the tetradentate L¹ ligand by two uninegative bidentate L² or L³ ligands of similar donor set, a marked positive shift (ca. 0.6-0.7 V) was observed, implying the predominance of structural effects.