Polynuclear nickel(II) complexes: a magnetostructural study of Ni(II)4, Ni(II)6, and Ni(II)9 species with oxime ligands.

Syntheses, crystal structures, and magnetic properties are reported for a tetrametallic (complex 1), a nonametallic (complex 2), and two hexametallic (complexes 3 and 4) nickel(II) clusters, namely, [Ni(4)(HL(1))(2)(mu-OAc)(2)(MeOH)] (1), [Ni(9)(L(2))(10)(mu(3)-OH) (2) (mu-OH) (2) (mu-OH (2))(2)(OH(2))(6)](ClO(4)) (4) (2), [Ni(6)(L(2))(9)(L(2)H)(MeOH)(H(2)O)(2)](ClO(4))(3) (3), and [Ni(6)(L(3))(3)(mu(3)-O)(2)](ClO(4))(2) (4), where H(4)L(1) represents N,N'-dimethyl-N,N'-ethylene-bis(5-bromo-3-formyloxime-2-hydroxybenzylamine); HL(2), 1-methylimidazole-2-aldoxime; and H(2)L(3), N,N'-bis(2,3-butanedionemonoxime-2-ene)-3(aminomethyl)benzylamine. The structure of 1 can be considered as two face-sharing bioctahedral units of [Ni(2)(mu-O(phen))(2)(mu-OAc)] bridged by a two-atom (-N-O-) oximate linker. The Ni(II)...Ni(II) distances of av. 2.935 A preclude metal-metal bonding, although they are remarkably short. Variable-temperature magnetic susceptibility data are fitted to obtain the parameters J (1) = +8.0 cm(-1), J(2) = -16.0 cm (-1), and g = 2.19 (H= -2 JS(i) x S(j)). The ferromagnetic coupling J (1) operates between the nickel(II) centers in the face-sharing bioctahedral units, whereas J(2) represents the antiferromagnetic interactions mediated by a single (-N-O-) bridge separating the two nickel centers at a distance of approximately 4.71 A. A rationale for the disparate nature of interactions based on a comparison with those reported in the literature is forwarded. The structure of 2 consists of two [Ni(4)(L(2))(5)] units linked covalently to a central nickel atom by four oximate and two hydroxy oxygen atoms, resulting in a central octahedral NiO(6) core and thus yielding the nonanuclear nickel(II) cluster. The magnetic data were analyzed by a "two-J" model, yielding pairwise antiferromagnetic exchange interactions, J(1) = -24.0 cm (-1) and J(2) = -5.8 cm (-1), between the nickel centers. The spin ground state of S(t) = 1.0 has been confirmed by magnetization measurements (variable-temperature, variable-field) at different fields. The structure of 3 contains six nickel(II) centers, each of which is six-coordinated but with different coordination environments: NiN(6), NiO(6), NiN(3)O(3)(2x), NiN(4)O(2), and NiNO(5). The ground-state spin has been observed to be S(t) = 1.0 with the axial zero-field splitting parameter D = -7.2 cm(-1). Complex 4 is a rare example of dimeric [Ni(3)(mu(3)-O)](4+) units, in which each of six nickel(II) centers is in square-planar geometry with low-spin d(8) Ni(II) centers, thus rendering diamagnetism to complex 4.