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

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 (Tc), quadrupole parameters were also changed discontinuously at Tc. The analysis of spectra observed above Tc 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