Two new Keggin-type polyoxometalate (POM)-based metal–organic complexes (MOCs) H₃[Cu₂(4-dpye)₂(PMo₁₂O₄₀)] (1) and H[Cu₂(4-Hdpye)₂(PMo₁₂O₄₀)(H₂O)₄]·2H₂O (2) were constructed with a new N,N′-bis (4-pyrimidinecarboxamido)-1,2-ethane (4-H₂dpye) ligand by the hydrothermal/solvothermal method. Complex 1 was a 2D layered structure constructed from 1D metal–organic chains [Cu(4-dpye)]_n and Keggin-type [PMo₁₂O₄₀]³⁻ polyoxoanions. Complex 2 displays a 3D supramolecular framework formed by discrete [PMo₁₂O₄₀]³⁻ polyoxoanions and binuclear metal–organic loops [Cu₂(4-Hdpye)₂]. The electrocatalytic behaviors of carbon paste electrodes modified by complexes 1 and 2 (1-CPE and 2-CPE) were investigated. The 1-CPE and 2-CPE were used as electrochemical sensors to detect trace Cr(VI), and the low limits of detection (LOD) are 1.27 × 10⁻⁷ M for 1 and 1.71 × 10⁻⁷ M for 2, which are lower than the maximum allowable concentration of Cr(VI) in drinking water specified by the World Health Organization (WHO). In addition, the performances of complexes 1 and 2 modified carbon cloth electrodes (1-CC and 2-CC) as supercapacitor materials have also been studied. The influence of the structure on electrocatalytic and capacitor performances is discussed.