Crystal structure and spectroscopic properties of a polynuclear complex between copper(II), diethylenetriamine, and ferrocyanide

Abstract

The addition of [Fe(CN)₆]^4– to [Cu(dien)]²⁺(dien = diethylenetriamine) ions gives a polynuclear complex of formula [{Cu(dien)}₂·Fe(CN)₆]·6H₂O. The compound has been characterized by i.r., powder-reflectance, and single-crystal polarized spectra, by thermogravimetric analysis, and by its crystal and molecular structure which has been solved by single-crystal X-ray diffractometry. The dark green crystals are monoclinic, with space group C2/c, a= 27.438(3), b= 7.778(1), c= 14.869(2)Å, β= 100.9°, and Z= 8; the asymmetric unit comprises half a molecule and the iron atoms at a centre of symmetry of the unit cell. The structure has been refined by least-squares methods using 1 586 independent reflections, to a conventional R of 0.08. The structure consists of a polymer of [Cu(dien)]²⁺ and [Fe(CN)₆]^4– ions linked together through cyanide bridges. Of the six CN^– groups around each iron, two are not bonded to copper atoms but only interact through hydrogen bonds with the water molecules present in the lattice, two are tightly bonded (Cu–N 1.97 Å) to copper atoms of adjacent [Cu(dien)]²⁺ molecules giving rise to linear Fe–C≡N–Cu linkages and are co-ordinated in the equatorial plane of the square-pyramidal configuration, and the remaining two co-ordinate to copper atoms of adjacent [Cu(dien)]²⁺ molecules in the apical positions of the pyramid (Cu–N 2.21 Å) leading to non-linear Fe–C≡N–Cu chains. The presence of three water molecules per asymmetric unit of the complex gives rise to an extensive hydrogen-bond system among themselves, so leading to a helix which develops one-dimensionally along the b axis, and also with the nitrogens of the free cyano-groups. Infrared and electronic spectral (both powder-reflectance and single-crystal polarized) and thermogravimetric results are discussed and related to the crystal structure.