A series of transition metal chloro complexes with the tetradentate tripodal tris(2-amino-oxazoline) ligand TAO have been synthesized and characterized. X-ray structural analyses of these compounds demonstrate the formation of the mononuclear complexes [MII(TAO)(Cl)]+, where MII = Cr, Mn, Fe, Co, Ni, Cu and Zn. These complexes exhibit distorted trigonal-bipyramidal geometry, coordinating the metal through an apical tertiary amine, three equatorial imino nitrogen atoms, and an axial chloride anion. All the complexes possess an intramolecular hydrogen-bonding (H-bonding) network within the cavity occupied by the metal-bound chloride ion. The metal-chloride bond distances are atypically long, which is attributed to the effects of the H-bonding network. Nuclear magnetic resonance (NMR) spectroscopy of the Zn complex suggests that the solid-state structures are representative of that observed in solution, and that the H-bonding interactions persist as well. Additionally, density functional theory (DFT) calculations were carried out to probe the electronic structures of the complexes.

The synthesis of a carboxyamidopyridine-pyrazolate dinucleating ligand (H5bppap) is described. Bimetallic iron and cobalt complexes of H5bppap ([MII2H2bppap]+) showed structural differences in both their primary and secondary coordination spheres. The binding of small molecules into the pre-organized ligand cavity is verified by the hydration of [FeII2H2bppap]+ and [CoII2H2bppap]+, leading to the formation of the complexes [{CoII(OH)}CoIIH3bppap]+ and [{FeII(OH)}-FeIIH3bppap]+, in which one of the metal centers has a terminal hydroxo ligand.

The synthesis of MII2 complexes (MII = Co, Mn) with terminal hydroxo ligands has been achieved utilizing a dinucleating ligand containing a bridging pyrazolate unit and appended (neopentyl)aminopyridyl groups. Structurally studies on the complexes revealed that the MII–OH units are positioned in a syn-configuration, placing the hydroxo ligands in close proximity (ca. 3 Å apart), which may be a prerequisite for water oxidation.

No abstract