Studies on ²H NMR and single crystal X-ray diffraction of thermochromic bis (N,N-diethyl-1,2-ethanediamine) nickel (II) complexes with and without structural phase transitions

Abstract

The thermochromic complex bis(N,N-diethyl-1,2-ethanediamine) ((C₂H₅)₂NC₂H₄NH₂) nickel (II) perchlorate([Ni(dieten) ₂](ClO₄)₂) was studied by single crystal X-ray diffraction and found to form a triclinic lattice with space group P1̄, a = 8.108(1)Å, b = 8.835(1)Å, c = 9.736(1)Å, α = 94.24(1)°, β = 114.28(1)°, γ = 116.49(1)° and Z = 1 being isomorphous with that of [Cu(dieten) ₂](ClO₄)₂ which has been reported to have a thermochromic phase transition. The temperature dependences of ²H NMR spectra in [Ni(dieten-d₂)₂]X₂ (dieten-d₂: (C₂H₅)₂NC₂H₄ND₂; X: ClO₄, BF₄, Br, NO₃, I) were observed and quadrupole coupling constants e²Qq and asymmetry parameters η were evaluated. Below room temperature, an almost rigid structure of the ND₂ group was derived by analyzing the spectra. For perchlorate and tetrafluoroborate with colour changes at respective phase transition temperatures (T_c), quadrupole parameters were also changed discontinuously at T_c. The analysis of spectra observed above T_c afforded two-site jumps of the N-D bond by angles of 52–55° supporting the ring-puckering model of the five-membered chelate ring. However, iodide having no phase transition showed continuous changes of both e²Qq and η values with increasing temperature up to 450 K. These results which cannot be attributed to the puckering motion were explained by whole complex reorientation. These molecular motions are discussed in connection with the colour change observed in these complexes.