Rational design of a new class of heterobimetallic molecule-based magnets: Synthesis, crystal structures, and magnetic properties of oxamato-bridged [formula omitted] (M′ = Li I and Mn II; M = Ni II and Co II) open-frameworks with a three-dimensional honeycomb architecture

Abstract

Two new series of oxamato-bridged heterobimetallic coordination networks of general formula Li 5[Li 3M 2(mpba) 3(H 2O) 6] · 31H 2O [M = Ni II ( 1a) and Co II ( 1b)] and Li 2[Mn 3M 2(mpba) 3(H 2O) 6] · 22H 2O [M = Ni II ( 2a) and Co II ( 2b)] have been prepared from the metal-mediated self-assembly of the hexakis(bidentate), triple-stranded dinickel(II) 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 Li 3 I M 2 II (M = Ni and Co) hexagonal layers, which are interconnected through three m-phenylenediamidate bridges between the M II ions of opposite propeller chirality ( Δ and Λ) to give dinuclear metallacryptand 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) Å, while the interlayer M–M distance through the triple m-phenylenediamidate bridge is 6.856(3) ( 1a) and 6.851(3) ( 1b) Å. Overall, this leads to an open-framework honeycomb structure with large hexagonal pores of ca. 21.8 ( 1a) and 21.5 Å ( 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 M 2 II (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 ( λ 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 Mn 3 II M 2 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.