Solvent-templated supramolecular isomerism in 2D coordination polymer constructed by Ni^II₂Co^II nodes and dicyanamido spacers: drastic change in magnetic behaviours

Abstract

Two heterometallic Coordination Polymers (CPs) have been prepared using [Ni^IIL]₂Co^II (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine) as nodes and dicyanamido spacers by varying the solvent for synthesis. Structural characterizations revealed that methanol assisted the formation of a two-dimensional (4,4) connected rhombic grid network of [(NiL)₂Co(NCNCN)₂]_∞ (1a) whereas relatively less polar acetonitrile afforded a different superstructure {[(NiL)₂Co(NCNCN)₂]•CH₃CN}_∞ (1b) with a two-dimensional (4,4) connected square grid network. The presence of acetonitrile molecules in the structure of 1b seems to change the spatial orientation of the terminal metalloligands [NiL] from pseudo-eclipsed in 1a to staggered-like in 1b around the central Co(II). These structural changes in the nodes together with the conformationally flexible dicyanamido spacers, which are cis coordinated to the Co(II) in both trinuclear units, led to the differences in the final 2D network. Variable-temperature magnetic susceptibility measurements revealed that this supramolecular isomerism led to a drastic transition from spin-frustrated antiferromagnetism for 1a to a dominant ferromagnetic behaviour for 1b. The geometrical differences in Ni₂ Co Coordination Clusters (CCs) which are scalene triangular in 1a but nearly linear in 1b, are held responsible for the changes of the magnetic properties. The DFT calculations of exchange interactions between metal centres provide a clear evidence of the role played by the fundamental geometrical factors on the nature and magnitude of the magnetic coupling in these pseudo-polymorphic CPs.