Preparation of Co3O4 Nanoparticles via Thermal Decomposition of Three New Supramolecular Structures of Co(II) and (III) Containing 4′-Hydroxy-2,2′:6′,2′′-Terpyridine: Crystal Structures and Thermal Analysis Studies

Three cobalt(II) and (III) complexes based on the 4′-hydroxy-2,2′:6′,2"-terpyridine (tpyOH) have been synthesized and structurally characterized by X-ray crystallography. The reaction of tpyOH with CoCl 2 ·6H 2 O in a mixture of methanol/CH 2 Cl 2 resulted in the formation of the new complex [Co II Cl 2 (tpyOH)] ( 1 ). On the other hand, the reaction of CoCl 2 ·6H 2 O with tpyOH in a 2:1 or 1:1 mol ratio in methanol under reflux condition affords the new complexes [Co III (tpyOH)(tpyO)][Co II Cl 4 ]·H 2 O ( 2 ) and [Co III Cl 2 (H 2 O)(tpyO)]·H 2 O ( 3 ), respectively. Moreover, the treatment of a methanolic solution of CoCl 2 ·6H 2 O with tpyOH in a branched tube at 60 °C resulted in the formation of three quality crystals of the complexes 1 and 2 as the major products as well as the complex 3 as a minor product. The crystal structure of [CoCl 2 (tpyOH)] ( 1 ) reveals that the cobalt(II) is penta-coordinated by two Cl − and three nitrogen atoms of tpyOH in a distorted square pyramidal geometry. The complex [Co III (tpyOH)(tpyO)][Co II Cl 4 ]·H 2 O ( 2 ) is described as a highly distorted octahedral geometry [CoN 6 ] while the X-ray crystal structure of the complex [Co III Cl 2 (H 2 O)(tpyO)]·H 2 O ( 3 ) shows that cobalt(III) is hexa-coordinated in a slightly distorted octahedral geometry CoCl 2 N 3 O. Several strong noncovalent interactions are present in the crystal structure of 1 – 3 . The hydrogen bonding in 1 involves the OH⋯Cl bridges while there is a hydrogen bonding between tpyO and tpyOH of the next molecule in 2 and hydrogen bridges and π–π interactions for 3 , connecting molecules and ions in the crystalline 1 – 3 to form supramolecular networks. The thermal stabilities of the cobalt complexes reveal that the loss of free terpyridine ligand could not be observed in low temperatures. The hexagonal and spherical Co 3 O 4 nanoparticles (NPCs) were prepared by direct calcination of complexes 1 – 3 at 600 °C in air. The nanostructures of the products were characterized by IR, powder X-ray diffraction, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy which show the purity of the resulting Co 3 O 4 NPCs. The average particle size using Scherrer’s equation is calculated to be about 32–35 nm.