CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF NOVEL [LN(III)CU(II)(4)] (LN=GD, DY, HO) PENTANUCLEAR COMPLEXES - TOPOLOGY AND FERROMAGNETIC INTERACTION IN THE LN(III) CU-II PAIR

The first pentanuclear complexes of formula {Dy[Cu(apox)(2)[Cu(apox)(H 2O)](2)} [ClO4](3) . 7H(2)O (1), {Ho[Cu(apox)][Cu(apox)(H2O)](3)} [PF6 ](3) . 4.5H(2)O (2), {Gd[Cu(apox)](2)[Cu(apox)(H2(O))(2)}[ClO4](3) . 7 H(2)O (3) and {Gd[Cu(apox)][Cu(apox) (H2O)(3)}[PF6](3) . 4.5H(2)O (4) (H(2)apox = N,N'-bis(3-aminopropyl)oxamide) have been synthesized. The crystal structures of complexes 1 and 2 have been determined by X-ray diffraction methods. Complexes 3 and 4 are isostructural with 1 and 2 , respectively. Crystallographic data are as follows: 1 and 3, monocli nic, space group C2/c and Z = 4, with a = 14.646(6) Angstrom, b = 29.4 96(7) Angstrom, c = 16.002(7) Angstrom, and beta = 111.76(2)degrees fo r 1 and a = 14.523(6) Angstrom, b = 29.441(6) Angstrom, c = 15.925(8) Angstrom, and beta = 111.90(4)degrees for 3; 2 and 4, triclinic, P (1) over bar, and Z = 2, with a = 14.346(2) Angstrom, b = 14.454(2) Angst rom, c = 18.107(4) Angstrom, alpha = 90.95(2)degrees, beta = 110.75(2) degrees, and gamma = 106.77(2)degrees for 2 and a = 14.365(6) Angstrom , b = 14.496(5) Angstrom, c = 18.172(7) Angstrom, alpha = 91.27(3)degr ees, beta = 110.74(3)degrees, and gamma = 106.67(3)degrees for 4. A tr ipositive ion is present in these structures, the electroneutrality be ing achieved by three uncoordinated perchlorate (1) or hexafluorophosp hate (2) anions. The lanthanide cations are eight-coordinate with a ps eudo-square-antiprismatic environment formed by carbonyl oxygen atoms from two [Cu(apox)] and two Cu(apox)(H2O)] (1) and one [Cu(apox)] and three [Cu(apox)(H2O)] (2) bidentate ligands. The temperature dependenc e of the magnetic susceptibility of complexes 1-4 was investigated in the range 1.8-300 K. The ligand-field effect, as well as the mixing of the foe-ion states in Dy-III and Ho-III, make extremely difficult the analysis of the overall antiferromagnetic interaction which is observ ed for complexes 1 and 2. The magnetic susceptibility data for complex es 3 and 4 have shown that the ground-state spin for the [(GdCu4II)-Cu -III] unit is S = 11/2, the Gd-III-Cu-II interaction being ferromagnet ic with an interaction parameter J(GdCu) = 0.85 cm(-1) (the interactio n Hamiltonian is of the form H = -JS(A) . S-B). The field dependence o f the magnetization at 2 K of 3 and 4 confirms the nature of the groun d state and of the Gd-III-Cu-II interaction. The influence of the topo logy and of the type of bridging ligand on the nature and magnitude of the magnetic interaction in the Gd-III-Cu-II pair is analyzed and dis cussed in light of available magnetostructural data.