Crystal structure and electronic properties of dibromo- and dichloro-tetrakis[µ3-bis(2-pyridyl)amido]tricopper(II) hydrate

Abstract

The crystal structures of [Cu₃(bipyam–H)₄Cl₂]·H₂O (1) and [Cu₃(bipyam–H)₄Br₂]·H₂O (2) where bipyam–H = bis(2-pyridyl)amide, have been determined by X-ray analysis, in the orthorhrombic space group Pnn2: (1), a= 14.092(3), b= 12.895(3), (c)= 11.190(2)Å, Z= 2, and R= 0.032 for 2 453 observed and 2 029 unique reflections; (2), a= 14.186(3), b= 13.040(3), c= 11.313(2)Å, Z= 2, and R= 0.043 for 1 574 observed and 1 465 unique reflections. The two structures are isomorphous with near isostructural [Cu₃(bipyam–H)₄X₂] units in special positions of two-fold symmetry and a non-co-ordinated water molecule. The Cu₃N₁₂X₂ chromophores involve nearly linear Cu₃ units, Cu–Cu–Cu 178.4°(mean), terminated by the two halide anions. The four separate bipyam–H ligands act as tridentate ligands, involving co-ordination to the three separate copper(II) ions, with Cu–Cu distances of 2.471(1) and 2.468(1)Å, for (1) and (2), respectively. If the Cu–Cu separations are ignored, the central Cu atom in both structures involves a four-co-ordinate rhombic coplanar CuN₄ chromophore generated by the central amido nitrogens of the four bipyam–H ligands. The two terminal Cu atoms involve a square-based pyramidal CuN₄X chromophore, generated by the terminal pyridine nitrogens of the four bipyam–H ligands and an axial halide anion. An average dihedral angle of 48° is involved between the planes of the pyridine rings of the individual bipyam–H ligands, which results in a spiral configuration of the [Cu₃(bipyam–H)₄X₂] units. The spin-only magnetic moment of complex (1) is ca. 1.40 B.M. per Cu atom, consistent with antiferromagnetic coupling between the copper(II) atoms of the trimer. Both complexes are e.s.r. silent, again consistent with strong antiferromagnetic coupling. The electronic spectra of (1) and (2) have a band maximum at 15 500 cm^–1, with a high-energy shoulder at 19 230 cm^–1, consistent with the two different stereochemistries present.