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

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 CuII–CuI 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.