Thermochromism and Structure of Piperazinium Tetrachlorocuprate(II) Complexes.

The tetrachlorocuprate(II) ion can crystallize in two different structures with the piperazinium dication (pipzH(2)). Both structures contain discrete CuCl(4)(2-) species. A yellow compound (pipzH(2))[CuCl(4)].2H(2)O (1) is monoclinic (C2/c, Z = 4, a = 10.538(3) Å, b = 7.4312(5) Å, c = 17.281(4) Å, beta = 111.900(10) degrees ) and contains the CuCl(4)(2-) ion as a distorted tetrahedron. A green compound (pipzH(2))(2)[CuCl(4)].Cl(2).3H(2)O (2) is triclinic (P&onemacr;, Z = 2, a = 9.264(3) Å, b = 10.447(2) Å, c = 11.366(2) Å, alpha = 68.38 degrees, beta = 82.86(2) degrees, gamma = 83.05(2) degrees ) and contains the CuCl(4)(2-) ion with a square planar geometry. This latter compound shows thermo/photochromism, changing from green to yellow upon heating or laser irradiation.