Spectroscopic structural investigation of N-benzyl piperazinium tetraCl-cuprates(II) 1 hemihydrate 1 anhydrous unequally flattened [CuCl4]2-tetrahedra

Two compounds [bzpipzn] [CuCI,J*0.5H20 (yellow) and [bzpipzn] [CuCI,] (green) ( [bzpipznI2+ = N-benzylpiperazinium dication) have been prepared and investigated by means of electronic and vibrational spectra, magnetic moments, d.s.c., and crystal-structure measurements. The crystal structures were determined by threedimensional X-ray diffraction. The two compounds crystallize in the space groups P2,/a (yellow) and P2, (green) with unit-cell dimensions a = 17.015(1), b = 16.977(2), c = 11.377(2) A, p = 97.156(4)", Z = 8 and a = 12.075 7(3), b = 28.479 8(10), c = 9.925 8(9) A, p = 109.54(1)*, Z = 8, for the yellow and green compounds respectively. The structures have been refined to R 0.036 and 0.067, respectively. The yellow complex consists of two crystallographically independent [CUCI,]~- anions with moderately flattened tetrahedral geometry, two [bzpipznI2+ cations, and a water molecule of crystallization. The green complex consists of four independent discrete [CuCI,] 2- anions, unequally flattened, and four independent [bzpipzn] 2+ cations, which bridge the anions through hydrogen bonds involving their N-bonded H atoms. The role of hydrogen bonding in the flattening of the tetrahedral [CuCI,12- ions is discussed. The spectroscopic and magnetic properties of the complexes are explained in the light of their known crystal structures. The transformation of the yellow form to the green form by heating (quasi-irreversible thermochromism) is also discussed.