A novel coordination polymer based on Co(ii) hexanuclear clusters with azide and carboxylate bridges: structure, magnetism and its application as a Li-ion battery anode.

A novel Co(ii) coordination polymer, [Co(H2O)6][Co6(bpybdc)2(N3)10(H2O)4]·8H2O (bpybdc2- = 1,1'-bis(3,5-dicarboxylatophenyl)-4,4'-bipyridinium), has been synthesized from a rigid zwitterionic tetracarboxylate ligand and azide. In this compound, hexacobalt clusters with mixed μ-1,1-azide, μ3-1,1,1-azide and μ-1,3-carboxylate bridges are linked into chains by μ-1,3-azide bridges, and the chains are interlinked into 2-fold interpenetrated three-dimensional frameworks through the organic ligand and hydrogen bonds mediated by hexaaquacobalt(ii) complex ions. Magnetic analysis suggested that intracluster ferromagnetic and intercluster antiferromagnetic interactions work together to give overall antiferromagnetic ground states for the azide and carboxylate bridged chain. When applied as an anode for lithium-ion batteries, the coordination polymer changes into an amorphous phase and exhibits a relatively high reversible capacity of 510 mA h g-1 with stable cycling behavior and rate performance.