Supramolecular 2D/3D isomerism in a compound containing heterometallic Cu^II₂Co^II nodes and dicyanamide bridges

Abstract

Three new heterometallic copper(II)–cobalt(II) complexes [(CuL²)₂Co{dca}₂]•H₂O (1), [(CuL¹)₂Co{dca}₂]_n (2a) and [(CuL¹)₂Co{dca}₂]_n (2b) [dca^– = dicyanamide = N(CN) ₂^–] have been synthesized by reacting the “metallo-ligand” [CuL¹] or [CuL²] with cobalt(II) perchlorate and sodium dicyanamide in methanol–water medium (where H₂L¹ = N,N′-bis(salicylidene)-1,3-propanediamine and H₂L² = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine). The three complexes have been structurally and magnetically characterized. Complex 1 is a discrete trinuclear species in which two metallo-ligands coordinate to a cobalt(II) ion through the phenoxido oxygen atoms along with two terminally coordinated dicyanamide ions. On the other hand, complexes 2a and 2b are one of the very scarce examples of supramolecular isomers since they present the same [(CuL¹)₂Co{dca}₂] trinuclear units (very similar to the trinuclear core in 1) and differ only in their superstructures. Thus, although each Cu₂Co trimer in 2a and 2b is connected to four other Cu₂Co trimers through four μ^1,5-dca^– bridges, 2a presents a square two-dimensional structure (each Cu₂Co trimer is connected to four in-plane Cu₂Co trimers); whereas, 2b shows a triangular three-dimensional lattice (each Cu₂Co trimer is connected to three in-plane and one out-of-plane trimers). Variable-temperature magnetic susceptibility measurements show the presence of moderate antiferromagnetic exchange interactions (ferrimagnetic) in all the cases mediated through the double phenoxido bridges that have been fitted with an anisotropic model including spin–orbit coupling in the central Co(II) ion.