In this study, a new cobaltous-(hexamethylenetetramine) [ meso -tetra( para -methoxyphenyl)porphyrin complex with the formula [Co$^{II}$(TMPP)(HMTA)] ( I ) was synthesized. The molecular structure was con- firmed in solution by 1 H NMR spectroscopy and mass spectrometry methods, and the single crystal X-ray diffraction structure of ( I ) was determined at both room temperature and low temperature. This species was further characterized by infrared, UV-visible and fluorescence spectroscopies, magnetic susceptibility measurements and cyclic voltammetry. The chemical reactivity behavior was also assessed theoretically through Density Functional Theory (DFT) approach. Magnetic investigation indicates that the Co(II)-HMTA porphyrin ( I ) species at low temperature is a cobaltous low-spin (S = 1/2) species while at high temper- ature complex ( I ) exhibits a spin-crossover low-spin (S = 1/2) ↔ high-spin (S = 3/2). The adsorption kinetic of the “vat yellow 1 dye”was carried out in aqueous solution at pH = 6. The experimental results are better fitted using the pseudo second order model. Furthermore, complex ( I ) was tested as catalyst in the degradation of the vat yellow 1 dye using an aqueous H$_2$O$_2$ solution and by photodegradation under solar light.