Assembly of Co(ii)/Cu(ii)–azido polynuclear polymers: structural diversity and magnetic behavior

Abstract

Hydrothermal reaction of cyano-substituted viologen derivatives and NaN₃ with corresponding metal salts in the presence/absence of H₂PTA (H₂PTA = p-phthalic acid) yielded three azide-based polynuclear coordination polymers: [Co₄(N₃)₄(PTA)₃(4-CV)₂]_n·2nH₂O 3D (1), [Co₄(N₃)₄(PTA)₃(3-CV)₂]_n·nH₂O 3D (2), and [Cu₆(N₃)₁₄(4-CV)₂]_n 2D (3) (3/4-CV⁺ = 4,4′-bipyridinium-N-methyl-3/4-benzonitrile). The azido bridges are employed to construct two types of polynuclear cluster units: tetranuclear [Co₄(μ_1,1-N₃)₄] and hexanuclear [Cu₆(μ_1,1-N₃)₆(μ_1,1,1-N₃)₄]. Polymers 1 and 2 are isostructural, and exhibit similar 3D frameworks constructed by unprecedented linear tetranuclear Co^II₄ units with a four-connected 6⁵·8 topology. Polymer 3 features a 2D (4,4) topological network built up by hexanuclear Cu^II₆ segments. The azide anion exhibits diverse coordination modes: μ_1,1-N₃ in 1 and 2, and μ_1,1-, μ_1,3-, μ_1,1,1- and μ_1,1,3-N₃ in 3. Variable-temperature magnetic susceptibility data denote that 1 and 2 display a weak antiferromagnetic interaction in Co^II₄ units and 3 shows a dominant ferromagnetic exchange in Cu^II₆ units.