Crystal Structure and Magnetic and EPR Properties of the Heterobinuclear Complex CuNi(fsa)2en(H2O)2·H2O (H4(fsa)2en = N,N′-Bis(2-hydroxy-3-carboxybenzylidene)-1,2-diaminoethane)
Résumé
The goal of this paper is to investigate the exchange interaction in CuNi(fsa)2en(H2O)2·H2O, denoted [CuNi], where (fsa)2en4− is the bichelating ligand derived from the Schiff base N,N′-(1-hydroxy-2-carboxybenzylidene)-1,2-diaminoethane. For a comparison of the structure of the pair states in [CuNi] with those of Cu(II) and Ni(II) single-ion ground states, CuMg(fsa)2en(H2O)2·H2O and Ni2(fsa)2en(H2O)2·H2O, denoted [CuMg] and [NiNi], have also been investigated. The crystal structure of [CuNi] has been solved at −120 °C from 8428 reflections. [CuNi] crystallizes in the trigonal system, space group P31. The lattice constants are a = 12.8071 (4) Å and c = 9.8157 (8) Å with Z = 3. The structure is made of [CuNi] binuclear units, in which the copper atom is in a strictly planar −N2O2 environment and the nickel atom in a pseudooctahedral −O2O2(H2O)2 environment. The crystal structure of [NiNi] has been solved at room temperature from 3262 reflections. The space group is P32, and the structure of the binuclear units [NiNi] is very close to that of the units [CuNi]. The temperature dependence of the magnetic susceptibility of [CuNi], studied in the temperature range 4–300 K, has revealed an energy gap of −3J/2 = 213 cm−1 between the 2A1 ground state and the 4A1 excited state. The average values of the g factors for the two pair states have been compared to those of the single ions, as deduced from the magnetic behavior of [CuMg] and [NiNi]. The EPR powder spectrum of [CuNi] is typical of an axial symmetry. The single-crystal spectra at 4 K exhibit only one signal for any orientation, assigned to the ground-pair doublet state. The g tensor is axial with the unique axis perpendicular to the N2CuO2NiO2 pseudo molecular plane. The principal values are g∥ = 2.22 (5) and g⊥ = 2.30 (0). The signal broadens out against the temperature in an inhomogeneous manner, the broadening being more pronounced on the high-field side. The magnetic and the EPR data are compared. The status of the spin Hamiltonian utilized to interpret these data is discussed. Finally, the mechanism of the exchange interaction is specified. The existence of two exchange pathways, namely, [formula omitted] and [formula omitted], the former being antiferromagnetic and the latter ferromagnetic, is emphasized. © 1982, American Chemical Society. All rights reserved.