NEW NICKEL(II) COPPER(II) HETERODINUCLEAR COMPLEXES WITH HEXACOORDINATED AND PENTACOORDINATED NICKEL(II) IONS - MAGNETOSTRUCTURAL CORRELATIONS

Three new heterodinuclear Cu(II)Ni(II) complexes of formula [Ni(cth)Cu(oxpn)](ClO4)2 (1), [Ni(cth)Cu(OHoxpn)](ClO4)2 (2) [Ni(cth)Cu(Me2oxpn)](ClO4)2 (3) in which the nickel(II) ion is octahedrally coordinated, and four new Cu(II)Ni(II) complexes with pentacoordinated nickel(II), [Ni(Me3[12]N3)Cu(oxpn)](ClO4)2 (4), [Ni(Me3[12]N3)Cu(Me2oxpn)](ClO4)2 (5), [Ni(Me4[12]N3)Cu(oxpn)](ClO4)2 (6), and [Ni(Me4[12]N3)Cu(Me2oxpn)](ClO4)2 (7) have been synthesized and characterized. Cth is d,l-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; Me3[12]N3 is 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene, Me4[12]N3 is its 9-methyl derivative; oxpn, OHoxpn, and Me2oxpn are N,N'-bis(3-aminopropyl)oxamido and its 2-hydroxo or 2,2'-dimethyl derivatives. The crystal structures of 1 and 5 have been solved. The complex C24H52N8O2NiCu.2ClO4 (1) crystallizes in the monoclinic system, space group P2(1)/a, with fw = 805.50, a = 24.704 (4) angstrom, b = 9.802 (3) angstrom, c = 16.084 (3) angstrom, beta = 105.44 (3)-degrees, V = 3754 (3) angstrom3, Z = 4, R = 0.068, and R(w) = 0.073. The nickel atom is placed in a distorted octahedral environment. The complex C24H49N7O2NiCu.2ClO4 (5) crystallizes in the monoclinic system, space group P2(1), with fw = 788.50, a = 13.794 (4) angstrom, b = 27.459 (7) angstrom, c = 9.783 (3) angstrom, beta = 95.80 (3)-degrees, V = 3687 (3) angstrom3, Z = 4, R = 0.059, and R(w) = 0.059. The nickel atom is placed in an environment intermediate between a trigonal bipyramid and a square pyramid. The magnetic properties of all these compounds have been investigated. The chi(M)T vs T plots for 1-7 exhibit the typical shapes for this kind of heterodinuclear complex. Using the Hamiltonian H = -JS(Cu)S(Ni), the average J value obtained when nickel(II) is octahedral was found to be ca. -95 cm-1, and when the nickel(II) is pentacoordinated, it is ca. -120 cm-1. Extended Huckel-MO calculations indicate that the doublet-quartet gap should be greater when nickel(II) is pentacoordinated, in agreement with the experimental J values. The EPR spectra (powdered samples or acetonitrile (ACN) solution) of all the complexes show a very intense signal independent of the temperature at approximately g = 2.2 and a broad signal at approximately g = 4, which vanishes when the temperature is lowered. This is interpreted as being due to an axial ZFS in the excited quartet state.