Polynuclear Nickel(II) Complexes: A Magnetostructural Study of Ni4II, Ni6II, and Ni9II 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(HL1)(2)(mu-OAc)(2)(MeOH)] (1), [Ni-9(L-2)(10)(mu(3)-OH)(2)(mu-OH)(2)(mu-OH2)(2)(OH2)(6)](ClO4)(4) (2), [Ni-6(L-2)(9)((LH)-H-2)(MeOH)(H2O)(2)](ClO4)(3) (3), and [Ni-6(L-3)(3)(mu(3)-O)(2)](ClO4)(2) (4), where H4L1 represents N,N'-dimethyl-N,N'-ethylene-bis(5-bromo-3-formyloxime-2-hydroxybenzylamine); HL2, 1-methylimidazole-2-aldoxime; and H2L3, 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 center dot center dot center dot Ni-II distances of av. 2.935 angstrom 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) over cap = -2 JS(i)center dot 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 similar to 4.71 angstrom. 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 NiO6 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: NiN6, NiO6, NiN3O3(2x), NiN4O2, and NiNO5. 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.