[Fe(bpb)(CN)₂]^- as a versatile building block for the design of novel low-dimensional heterobimetallic systems: Synthesis, crystal structures, and magnetic properties of cyano-bridged Fe^III-Ni ^II complexes [(bpb)^2- = 1,2-bis(pyridine-2-carboxamido) benzenate]

A dicyano-containing [Fe(bpb)(CN)₂]^- building block has been employed for the synthesis of cyano-bridged heterometallic Ni ^II-Fe^III complexes. The presence of steric bpb ^2- ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe₂ trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]²⁺ and [Fe(bpb)(CN)₂]^- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J_NiFe values (based on the spin exchange Hamiltonian H =-2J _NiFeS_Ni(S_Fe(1) + S_Fe(2)): J _NiFe = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm^-1, respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N≡C bond angle, the stronger the Ni⋯Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T_N = 3.5 K. Below T_N (1.82 K), a metamagnetic behavior was observed with the critical field of ∼6 kOe. The present research shows that the [Fe(bpb)(CN)₂]^- building block is a good candidate for the construction of low-dimensional magnetic materials.