Nitro group and K⁺-based secondary building units for the self-assembly of 3D coordination polymers built on dinuclear dianionic helicate connectors

Two 3D coordination polymers based on dinuclear dianionic helicates, K-2[M-2(II)(L)(3)] M = Mn-II (1) and Ni-II (2), were obtained by slow liquid-liquid diffusion of solutions of 2-hydroxy-5-nitrobenzaldehyde, 4,4 '-methylenedianiline, KOtBu and the metal salt at room temperature. The resulting dianionic triply stranded helicates self-assemble into novel 3D coordination polymers, in which the helicate units act as connectors. The assembly is promoted by the phenolate bridging ability and the coordination of the pre-organized NO2 groups to K-(O-N-O)(4)-K nodes, resulting in robust paramagnetic materials. SC-XRD shows that the metal centers are located 11.257(2) and 11.264(1) & Aring; apart. The Mn-L bond lengths correspond to the high-spin (HS, S = 5/2) state for 1. The magnetic behavior for both complexes shows weak magnetic exchange, as expected due to the spin carrier separation, and the confirmation of the HS-sate for the Mn-II centers. For complex 1, an antiferromagnetic exchange is detected, whereas for complex 2, a mixture of intramolecular antiferromagnetic and intermolecular ferromagnetic exchanges are detected. Thus, we demonstrated that nitro groups can be used for the stabilization of anionic complexes because coordination to K+ results in stable intricate architectures.