Rational design of a new class of heterobimetallic molecule-based magnets:: Synthesis, crystal structures, and magnetic properties of oxamato-bridged M3′M2 (M′ = LiI and MnII; M = NiII and CoII) open-frameworks with a three-dimensional honeycomb architecture

Two new series of oxamato-bridged heterobimetallic coordination networks of general formula Li-5[Li3M2(mpba)(3)(H2O)(6)] center dot 31H(2)O [M= Ni-II (1a) and Co-II (1b)] and Li-2[Mn3M2(mpba)(3)(H2O)(6)] center dot 22H(2)O [M = Ni-II (2a) and Co-II (2b)] have been prepared from the metal- mediated self-assembly of the hexakis(bidentate), triple-stranded dinickel(I) and dicobalt(II) complexes [M-2(mpba)(3)](8) [mpba = meta-phenylenebis(oxamato)] with either monovalent lithium(I) or divalent manganese(II) ions respectively, in water. X-ray structural analyses of 1a and 1b show an anionic three-dimensional network formed by an infinite parallel array of oxamato-bridged (Li3M2II)-M-I (M = Ni and Co) hexagonal layers, which are interconnected through three m-phenylenediamidate bridges between the M-II ions of opposite propeller chirality (Lambda and Lambda) to give dinuclear metallacrypt and cores of the meso-helicate-type. The intralayer Li-M distance through the oxamate bridge is 5.380(3) (1a) and 5.396(5) (1b) angstrom, while the interlayer M-M distance through the triple m-phenylenediamidate bridge is 6.856(3) (1a) and 6.851(3) (1b) angstrom. Overall, this leads to an open-framework honeycomb structure with large hexagonal pores of ca. 21.8 (1a) and 21.5 angstrom (1b) in diameter which are occupied by linear arrays of water-bridged Li-I countercations. Variable-temperature (2.0-300 K) dc magnetic susceptibility and variable-field (0-5.0 T) magnetization measurements on these two series of compounds reveal a distinct magnetic behavior mainly depending on the diamagnetic or paramagnetic nature of the Li-I and Mn-II ions, respectively. Compounds 1a and 1b behave as rather well-isolated MI2II (M = Ni and Co) dimers with a moderate to weak ferromagnetic coupling(J(Ni-Ni) = + 3.17 cm (1) and J(Co-Co) = + 1.03 cm (1)) between the two M-II ions across the triple m-phenylenediamidate bridge, together with either an appreciable to remarkable local ion axial magnetic anisotropy (D-Ni = -3.35 cm (1)) or spin-orbit coupling (lambda(Co) = - 167.3 cm (1)); the next nearest-neighbor magnetic interactions between the M-II ions through the diamagnetic Li-I ions are negligible. Compounds 2a and 2b behave instead as strongly coupled oxamato-bridged (Mn3M2II)-M-II (M = Ni and Co) ferrimagnetic planes which are weakly interacting through the m-phenylenediamidate bridges, leading thus to a long-range 3D ferromagnetic order at T-C = 6.5 K. In addition, the variable-temperature (2.0-12 K) ac magnetic susceptibility measurements on 2a and 2b show an intriguing frequency-dependent magnetic behavior characteristic of a spin-glass dynamics. This glassy behavior is likely attributed to their amorphous character and/or the moderate to strong magnetic anisotropy of the dinickel(II) and dicobalt(II) precursors. (c) 2008 Elsevier B. V. All rights reserved.