Structural, Magnetic and Spectral Properties of Iron(II) and Nickel(II) Complexes of N¹-(Pyridin-2-yl)-3,5-dimethylpyrazole

Abstract

Iron(II) and nickel(II) complexes of N¹-(pyridin-2-yl)-3,5-dimethylpyrazole (L) are described. Salts of [FeL₃]²⁺ undergo a reversible temperature-induced singlet (¹A₁) ↔ quintet (⁵T₂) transition, both in the solid state and in solution. The singlet- quintet change in acetone solution is characterized by ΔH = 25 ± 2 kJ mol^-1 and ΔS = 95 ± 5 J K^-1 mol^-1. Mossbauer spectral data for solid [FeL₃] [BF₄]₂ confirm a change in ground state for the metal atom with change in temperature and indicate two low-spin species at low temperatures and two high-spin species at elevated temperatures. The average M-N bond length changes accompanying the spin change have been estimated as 0.19 Å from data obtained by determination of the structures of [FeL₃][ClO₄]₂ at 135 K and 294 K and of [NiL₃][BF₄]₂ at 294 K by single-crystal X-ray diffractometry. The crystals were all refined as disordered structures in space group P3c1 with Z = 2. Crystal data: [FeL₃] [ClO₄]₂ at 294 K, a = b 10.489(2), 17.521(5) Å ;[FeL₃][ClO₄]₂ at 135 K, a = b 10.355(3), c 17.464(5) Å ; [NiL₃] [BF₄]₂ at 294 K, a = b 10.404(3), c 17.618(8) Å .