The metal–organic framework [Cu(ta)2] (Hta = 1H-1,2,3-triazole), containing Jahn–Teller active Cu(II) ions and 1,2,3-triazolate ligands, is prepared under solvothermal reaction conditions. The compound shows a reversible phase transition from the tetragonal crystal system (α-[Cu(ta)2]: space group I41/amd (no. 141), a = 11.8447(7) Å, c = 18.9782(13) Å, V = 2662.6(3) Å3) to the cubic crystal system (β-[Cu(ta)2]: space group Fdm (no. 227), a = 17.4416(15) Å, V = 5305.9(8) Å3) within the temperature range of 120–160 °C. Both [Cu(ta)2] polymorphs have identical bonding topologies that might be described as fully condensed Kuratowski-type pentanuclear secondary building units of local Td point group symmetry in which four Cu(II) ions occupy the vertices of an imaginary tetrahedron. α-[Cu(ta)2], as opposed to the high-temperature β-phase, shows a strong tetragonal Jahn–Teller distortion of CuN6 coordination octahedra. The compounds are characterized by elemental and thermogravimetric analyses, single crystal and powder X-ray diffraction, FTIR-, UV-vis and fluorescence spectroscopy. Magnetic susceptibility investigations reveal two different Cu(II) sites at a ratio of 1 : 2, in agreement with the solid state structure of [Cu(ta)2]. At low temperatures the formation of antiferromagnetically coupled Cu(II) dimers is observed, leading to a spin frustration of roughly 1/3 of all magnetically active Cu(II) sites.