Structural versatility in cobalt(ii) complexes with 1,2,4,5-benzenetetracarboxylic acid (H4bta) and 4,4′-bipyridine-N,N′-dioxide (dpo)

Abstract

Four new high-spin cobalt(II) complexes of formula [Co(H₂O)₆](H₂bta)·dpo·4H₂O (1), [{Co(H₂O)₄(dpo)}₂(bta)]·4H₂O·(2), [Co(H₂O)₂)(dpo)₂(H₂bta)]_n (3) and [Co(H₂O)₃(dpo)(bta)_1/2]_n (4) (H₄bta = 1,2,4,5-benzenetetracarboxylic acid and dpo = 4,4′-bipyridine-N,N′-dioxide) have been synthesized and their structures solved by single crystal X-ray diffraction methods. Compound 1 is an ionic salt whose structure is made up of [Co(H₂O)₆]²⁺ cations, H₂bta²⁻ anions, uncoordinated dpo groups and crystallization water molecules, which are linked by extensive hydrogen bonds to afford a three-dimensional network. The structure of 2 consists of bta-bridged dinuclear cobalt(II) complexes where four coordinated water molecules and a terminally bound dpo ligand complete the six coordination around each cobalt atom. The bta ligand in 2 adopts the bis-monodentate bridging mode through two trans carboxylate-oxygen atoms, the intramolecular cobalt–cobalt distance being 11.46(2) Å. The structure of 3 is constituted by uniform chains of cobalt(II) ions bridged by the dideprotonated H₂bta²⁻ species through two trans carboxylate-oxygen atoms. Two coordinated water molecules and two terminally bound dpo ligands achieve the six coordination around each cobalt atom. The intrachain cobalt–cobalt separation is 11.387(1) Å. The structure of 4 consists of corrugated layers of cobalt(II) ions bridged by bis-monodentate dpo and bta⁴⁻ ligands, (through two trans carboxylate-oxygen atoms), three coordinated water molecules in a fac arrangement achieving the six-coordination around each cobalt atom. The investigation of the magnetic behaviour of 2–4 in the temperature range 1.9–300 K shows the occurrence of very weak antiferromagnetic interactions between the high-spin cobalt(II) ions, the strong decrease of χ_MT upon cooling which is observed for this family of complexes ligands being mainly due to the depopulation of the higher energy Kramers doublets of the high-spin octahedral cobalt(II) centers.