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 � 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) � ;[FeL3][ClO4]2 at 135 K, a = b 10.355(3), c 17.464(5) � ; [NiL3] [BF4]2 at 294 K, a = b 10.404(3), c 17.618(8) � .