A copper (II) supramolecular coordination complex formulated as [Cu-2(mu-ox)(2)(pyz)(3)](n) (1), (pyz = pyrazine and ox = oxalate) has been synthesized under ultrasound irradiation. 1 was characterized using various techniques such as elemental analyses, Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and single crystal X-ray diffraction (SC-XRD). A detailed magnetic characterization of complex 1 has been carried by vibrating sample magnetometer (VSM). The M-H hysteresis loop of 1 revealed a weak ferromagnetic behavior with the highest magnetization of 0.0123 emu/g at room temperature. Compound 1 was used as an inorganic precursor to prepare Cu2O nanoparticles through thermal decomposition at 600 degrees C. The obtained Cu2O has been characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The results of SEM showed octahedron Cu2O nanoparticles with the edge lengths from 5-80 nm. Also, the adsorption ability and the photocatalytic activity of octahedral Cu2O nanoparticles in the removal of rhodamine B (RB) have been investigated. The results showed that the obtained octahedral Cu2O nanoparticles are effective in adsorption and degradation of rhodamine B from contaminated water sources. The maximum adsorption capacity and degradation efficiency of Cu2O nanoparticles were 83.3 mg/g and 91.7%, respectively. It was also found that in comparison with the commercial Cu2O, our fabricated Cu2O nanoparticles exhibit higher catalytic activity.