Oxamato-bridged trinuclear Ni^IICu^IINi^II complexes with irregular spin state structures and a binuclear Ni^IICu^II complex with an unusual supramolecular structure: Crystal structure and magnetic properties

Four oxamato-bridged heterotrinuclear Ni^IICu^IINi^II complexes of formula {[Ni(bispictn)]₂Cu(pba)}(ClO₄) ₂·2.5H₂O (1), {[Ni(bispictn)]₂Cu(pbaOH)}(ClO₄)₂ ·H₂O (2), {[Ni(cth)]₂Cu(pba)}(ClO₄)₂ (3), and {[Ni(cth)]₂Cu(opba)}(ClO₄)₂· H₂O (4) and a binuclear Ni^IICu^II complex of formula [Cu(opba)Ni(cth)]·CH₃OH (5) have been synthesized and characterized by means of elemental analysis, IR, ESR, and electronic spectra, where pba = 1,3-propylenebis-(oxamato), pbaOH = 2-hydroxyl-1,3-propylenebis(oxamato), opba = o-phenylenebis(oxamato), bispictn = N,N′-bis(2-pyridylmethyl)-1,3-propanediamine, and cth = rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The crystal structures of 1, 3, and 5 have been determined. The structures of complexes 1 and 3 consist of trinuclear cations and perchlorate anions, and that of 5 consists of neutral binuclear molecules which are connected by hydrogen bonds and π-π interactions to produce a unique supramolecular "double" sheet. In the three complexes, the copper atom in a square-planar or axially elongated octahedral environment and the nickel atom in a distorted octahedral environment are bridged by the oxamato groups, with Cu···Ni separations between 5.29 and 5.33 Å. The magnetic properties of all five complexes have been investigated. The χ_MT versus T plots for 1-4 exhibit the minimum characteristic of antiferromagnetically coupled NiCuNi species with an irregular spin state structure and a spin-quartet ground state. The χ_MT versus T plot for 5 is typical of an antiferromagnetically coupled NiCu pair with a spin-doublet ground state. The Ni^II-Cu^II isotropic interaction parameters for the five complexes were evaluated and are between 102 and 108 cm^-1 (Ĥ = -JŜ_Cu·Ŝ_Ni).