¹H-NMR studies on thermochromism Ni(II) complexes with N,N-diethylethylenediamine

Abstract

The temperature dependence of the second moment (M₂) of ¹H NMR absorptions was measured in solid thermochromic bis(N,N-diethylethylenediamine)Ni(II) complexes. Perchlorate and tetrafluoroborate having a thermochromic phase transition at T_tr = 383 and 367 K, respectively, showed a discontinuous M₂ change at T_trr indicating that a marked change of the chelate-ring motions takes place at the transition. A small M₂ value of ca. 8 G₂ observed above the respective T_tr could be explained by the excitation of the chelate-ring puckering together with a large-amplitude librations of the whole complex ions. A gradual M₂ decrease upon heating from 100 to 400 K was observed in bromide and iodide exhibiting continuous thermochromism. These M₂ changes and the reported visible electron absorption maxima showed a quite analogous temperature dependence. This suggests that some kind of chelate-ring motion such as the puckering also takes place in the continuous thermochromism complexes.