STRUCTURE AND MAGNETIC AND SPECTROSCOPIC PROPERTIES OF A NIIICUIINIII TRINUCLEAR SPECIES

The two compounds of formula {[Ni(bapa)(H2O)]2Cu(pba)}(ClO4)2 and [Ni(bapa)(H2O)Cu(pba)]-2H2O, hereafter abbreviated as [NiCuNi] and [NiCu], respectively, have been synthesized. bapa is bis(3-aminopropyl), and pba is 1, 3-propylenebis(oxamato). The crystal structure of [NiCuNi] has been solved. It crystallizes in the orthorhombic system, space group Pna21, with a = 9.826 (2) Å, b = 12.793 (1) Å, c = 27.266 (3) Å, and Z = 4. The structure consists of NiIICuIINiII trinuclear cations and noncoordinated perchlorate anions. The nickel atoms are in a distorted octahedral environment, and the copper atom is in an environment markedly distorted from square planar to tetrahedral. The central copper atom is bridged to the terminal nickel atoms by oxamato groups with Cu⋯Ni separations of 5.305 (2) and 5.326 (2) Å. The magnetic properties of both compounds have been investigated. The χMT versus T plot (χM is the molar magnetic susceptibility and T the temperature) for [NiCuNi] exhibits the minimum characteristic for this kind of polymetallic species with an irregular spin state structure. The Ni(II)-Cu(II) isotropic interaction parameter was found to be J = −90.3 cm−1 (H = −JSNi•SCu). The powder EPR spectrum at 10 K is poorly resolved. It shows a broad signal corresponding to the envelope of the two Kramers doublets arising from the spin-quartet ground state. The χMT versus T plot for [NiCu] is typical of an antiferromagnetically coupled pair with a spin-doublet ground state and a doublet-quartet energy gap equal to 3J/2 = −141.9 cm−1. The electronic spectra of [NiCuNi] has also been investigated. It shows a triplet → singlet spin-forbidden transition associated with the nickel(II) ion and activated by an exchange mechanism. © 1990, American Chemical Society. All rights reserved.