Second-sphere coordination of the tris(5,5′-diamino-2,2′- bipyridine)iron complex with arene-carboxylate ligands through N-H⋯O hydrogen bonding

The tris(5,5′-diamino-2,2′-bipyridine)iron(II) complex [Fe(DABP)₃]²⁺ can be engaged in hydrogen-bonding interactions from the amino groups to carboxylate-containing molecules as H-bond acceptors. This so-called second-sphere coordination of the metal complex is investigated here by using benzene-1,4-dicarboxylate (terephthalate, L ¹), benzene-1,3-dicarboxylate (isophthalate, L²), biphenyl-2,2′-dicarboxylate (L³), and benzene-1,3,5- tricarboxylate (trimesate, L⁴) anions for N-H⋯O bonding. The second-sphere coordination of [Fe(DABP)₃]²⁺ is further complemented by water molecules of crystallization in the isolated and structurally elucidated compounds of [Fe(DABP)₃](L ¹)·5H₂O, [Fe(DABP)₃](L ²)·8H₂O, [Fe(DABP)₃](L ³)·12.5H₂O, and [Fe(DABP)₃] ₃(L⁴)₂·20H₂O. The hydrogen-bonding between [Fe(DABP)₃]²⁺ and the dicarboxylate anions (L¹-L³) decreases from a three-dimensional supramolecular network (with L¹) to discrete units (with L² and L³) with the increasing number of crystal water molecules. The tricarboxylate ligand L⁴ is capable of capping the triangular face formed by the three amino groups in [Fe(DABP) ₃]²⁺. The solid-state structures of DABP·2H ₂O and 5.5′-bis(ethoxycarbonylamino)-2,2′-bipyridine (BEBP) are reported.