Tris(5-methylpyrazolyl)methane: Synthesis and Properties of Its Iron(II) Complex

The new ligand, tris­(5-methylpyrazolyl)­methane (1), has been prepared by the reaction of n-butyl lithium with tris­(pyrazolyl)­methane followed by trimethylation of the tetralithiated species with methyl iodide. The BF4–, ClO4–, and BPh3CN– salts of the Fe­(II) complex of this ligand were also synthesized. The X-ray crystal structure of the BF4– complex (2) at 100 K had Fe–N bond lengths of 1.976 Å, indicative of a low spin Fe­(II) complex, while at room temperature, the structure of this complex had a Fe–N bond distance close to 2.07 Å, indicative of an admixture of approximately 50% low-spin and 50% high-spin. The solid-state structure of the complex with a ClO4– counterion was determined at 5 different temperatures between 173 and 293 K, which allowed the thermodynamic parameters for the spin-crossover to be estimated. Mössbauer spectra of the BF4– complex further support spin-state crossover in the solid state with a transition temperature near 300 K. UV–visible spectroscopy and 1H NMR studies of 2 show that the transition temperature in solution is closer to 400 K. No spin-crossover was observed for [Fe­(1)2]2+·2BPh3CN–. The results allow the separation of effects of groups in the 3-position from those in the 5-position on tpm ligands, and also point toward a small cooperative effect in the spin-crossover for the Fe­(II) complex.