Indian Journal of Chemistry Vol. 54A, July 2015, pp. 867-871 Notes Reversible hydration of tetraaquabis(4-aminobenzoato)cobalt(II) Kiran T Dhavskar & Bikshandarkoil R Srinivasan* Department of Chemistry, Goa University, Goa 403206, India Email: srini@unigoa.ac.in Received 22 May 2015; accepted 30 June 2015 The octahedral red complex tetraaquabis(4-amino- benzoato)cobalt(II) (1) exhibits a water induced crystalline- amorphous-crystalline transformation accompanied by chromotropism. The loss of coordinated water from [Co(4-aba) 2 (H 2 O) 4 ] (1) (4-aba=4-aminobenzoate) on heating, leads to the formation of tetrahedral anhydrous [Co(4-aba) 2 ] (2). The amorphous blue compound (2) can be rehydrated to original composition by equilibrating (2) over water vapour. The rehydrated crystalline compound exhibits properties identical with (1). The variation in the structure and properties of (1) due to the coordination and decoordination of water are studied by spectral, thermal, magnetic and conductivity measurements. Keywords: Chromotropism, Crystal-to-amorphous transformations, Dehydration, Rehydration, Reversible hydration, Cobalt, 4-Aminobenzoate Progress and development of science and technology relies mostly on the possibility of obtaining various materials with properties designed to meet practical purposes. This possibility is mainly based on the correlation between structure and properties of materials. Thus careful studies of material properties and correlating the corresponding structural changes can serve as a lead in exploring and devising synthetic methodologies that can help to design materials of potential applications. Several publications have appeared in recent literature, showing the growing research interest in metal-organic materials with flexible and dynamic frameworks especially in those that reversibly change their structures and properties in response to external stimuli as they have many applications 1-5 . These types of changes especially the ones accompanied by loss of coordinated or lattice water have also been reported for zero-dimensional systems for example hydrated organic ammonium salts 6,7 or tetra(aqua)Co(II) complexes 8 . Cobalt is a typical metal ion that shows chromism upon ligand exchange in the solid state. In an earlier work we have shown that tetraaquabis(paranitrobenzoato)cobalt(II) dihydrate can be reversibely hydrated 8 . In this work we have chosen the known zero dimensional cobalt complex, namely tetraaquabis(4-aminobenzoato)cobalt(II) (1), which contains only coordinated but no lattice water molecules 9 for investigating changes in the property on the dehydration and rehydration. The results are described herein. Experimental All the chemicals used in this study were of reagent grade and all the syntheses were carried out using doubly distilled water. Infrared (IR) spectra were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer in the range 4000–400 cm -1 . UV-vis diffuse-reflectance spectra were recorded on a Shimadzu UV-2450 double beam spectrophotometer. BaSO 4 powder was used as reference (100% reflectance). Absorption data were calculated from the reflectance data using the Kubelka-Munk function (a/S=(1-R 2 /2R where a is the absorption coefficient, R the reflectance and S the scattering coefficient). TG-DTA studies were carried out on Netzsch, STA 409 PC (Luxx) analyzer, from RT to 1000 °C in dry air, with a heating rate of 10 K min -1 . Isothermal weight loss studies were performed in an electric furnace fitted with a temperature controller and on a steam bath. The X-ray powder pattern were recorded using Cu-Kα radiations of λ= 1.5418 Ǻ (filtered through Ni) in steps of 0.02 degrees on a RIGAKU Ultima IV diffractometer. The variation of electrical resistivity, as a function of temperature was measured using two probe electrical conductivity setup. The temperature variation of resistance was measured from room temperature to 350 o C using a Keithley electrometer. The magnetic susceptibility in air was determined by Guoy method at room temperature in a field of 9600 gauss using sensitive analytical balance. A Quantum Design PPMS-VSM magnetometer was used for magnetic characterization of the pelletized compounds. The variation of the dc-susceptibility of each sample with temperature was measured from 2 K to 300 K in the ZFC (zero field cooling) and FC (field cooling) modes using a magnetic field of 500 Oe. The magnetization with varying magnetic field of up to 140 kOe was also measured at 2 K and 100 K.