Thermochromic nitro–nitrito interconversion mediated by weak-linked amide in nickel (II) diaminodiamide complexes in the solid state

The red-violet nickel (II) complex [Ni (C8H18N4O2) (NO2)2]·H2O (3) was synthesized from an aqueous solution containing nickel chloride, sodium nitrite and 4,7-diazadecanediamide (1). The NO2− ions in compound 3 are bound to the metal center through the nitrogen atoms. On heating the solid sample of compound 3, the nitro anion rotates to give nitrito coordination and changes the compound color from red-violet to blue. Significantly, the structure transformation along with the color change is a reversible process. For purposes of comparison, the green nickel (II) complex [Ni (C10H22N4O2) (H2O)2]·2 (NO2)·2H2O (4) was synthesized from an aqueous solution containing nickel chloride, sodium nitrite and 4,7-dimethyl-4,7-diazadecanediamide (2). The NO2− ions are not coordinated to the nickel metal center. On heating the solid complex 4, neither color change nor spectral modifications were observed.