Coordination behaviour and network formation with 4,4′,6,6′-tetracarboxy-2,2′-bipyridine and 4,4′-dicarboxy-2,2′-bipyridineligands with rare and alkaline earth metals

Abstract

Reported herein is the synthesis and structural characterisation of the novel ligand, 4,4′,6,6′-tetracarboxylic acid-2,2′-bipyridine (H₄tcbp), along with an investigation of its coordination chemistry. Single crystal X-ray diffraction studies showed H₄tcbp crystallises as a dihydrate and that intermolecular hydrogen bonding involving adjacent 4,4′-carboxylic acid groups form a R²₂(8) dimer motif whereas the 6,6′-carboxylic acid groups hydrogen bond together via the lattice water molecules [R⁴₄(12)] to form a 2D grid which is 3-fold interpenetrated and yields a 3D hydrogen bonded network. The reaction of H₄tcbp under solvothermal conditions in MeOH with Ln(III) (Ln = Yb, Gd and Sm) lead to the isolation of {[Yb(Me₂tcbp)₂]·H₂O·MeOH·H₃O}, 1, and {[Ln(Me₂tcbp)₂]·2H₂O·H₃O}, where Ln = Gd, 2, or Sm, 3, and when in the presence of Yb(III) and Cu(II) to the mixed-metal species, {[Yb₂(Me₂tcbp)₂]₂Cu(OMe)₂(H₂O)₂]·6MeOH}, 4, which were structurally characterised. Under these synthetic conditions the H₄tcbp ligand di-esterified in situ at the remote 4- and 4′-carboxylic acid groups to form the 4,4′-dimethylcarboxy-6,6′-dicarboxy-2,2′-bipyridine ligand (Me₂tcbp^2–). Although 1 is monomeric, extensive hydrogen bonding between lattice water molecules, hydronium counterions and deprotonated carboxylate oxygen atoms results in 2D bilayer structure. Isostructural 2 and 3 are discrete mononuclear complexes whereas the tri-nuclear 4 forms a 2D sheet through face-to-face π–π interactions between adjacent units. In addition, the structural characterization and thermal analysis of four alkaline earth metal 3D coordination networks are reported, two of which feature the H₄tcbp ligand: [Ba₂(tcbp)(H₂O)₂] 5 and [CuSr(tcbp)(H₂O)₃] 6 (a 5,6-connected net), with the remaining two featuring the related 4,4′-dicarboxy-2,2′-bipyridine (H₂dcbp) ligand: {[Ba(dcbp)(H₂O)_0.4]·0.6H₂O} 7 (a bi-nodal 6-connected net) and {[Sr(dcbp)]·H₂O} 8 (a uni-nodal 6-connected net).