Synthesis and characterization of cobalt(II)–salicylate complexes derived from N4-donor ligands: Stabilization of a hexameric water cluster in the lattice host of a cobalt(III)–salicylate complex

The syntheses and structural characterization of four cobalt(II)–salicylate complexes, [(TPA)CoII(HSA)](ClO4) (1), [(isoBPMEN)CoII(HSA)](BPh4) (2), [(TPzA)CoII(HSA)](ClO4) (3) and [(6Me3TPA)CoII(HSA)](BPh4) (4) [TPA = tris(2-pyridylmethyl)amine, isoBPMEN = N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, TPzA = tris((3,5-dimethyl-1H-pyrazole-1-yl)methyl)amine and 6Me3TPA = tris(6-methyl-2-pyridylmethyl)amine] are described. While 2, 3 and 4 are unreactive towards dioxygen, 1 reacts slowly with molecular oxygen to a cobalt(III)–salicylate complex, [(TPA)CoIII(SA)](ClO4) (1a). Two different crystalline forms, 1a and 1a·4H2O were isolated depending upon the condition of oxidation and crystallization. The solid-state structures of cobalt(III)–salicylate unit in both 1a and 1a·4H2O show a six-coordinate distorted octahedral coordination geometry at the cobalt(III) center ligated by the tetradentate ligand (TPA) where the dianionic salicylate (SA) binds in a bidentate fashion through one carboxylate and one phenolate oxygen. The hydrated form 1a·4H2O reveals a hexameric water cluster formation in the inorganic lattice host. The complex cation and the perchlorate counterion are involved in stabilizing the (H2O)6 cluster in a rare ‘pentamer planar+1’ conformation. A one-dimensional water tape consisting of edge-shared water hexamers is observed. The water tape represents a subunit of ice structure.

Graphical abstractA ‘pentamer planar+1’ type hexameric water cluster is stabilized in the crystal lattice of a cobalt(III)–salicylate complex and forms a one-dimensional water tape.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Syntheses and characterization of four mononuclear cobalt(II)–salicylate complexes. ► Oxidation of a cobalt(II)–salicylate complex with dioxygen. ► Isolation of two different crystalline forms of the cobalt(III)–salicylate complex. ► Stabilization of a hexameric water cluster in the hydrated cobalt(III) complex. ► A rare ‘pentamer planar+1’ type water cluster with a one-dimensional water tape.