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

Abstract

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 BF(4)(-), ClO(4)(-), and BPh(3)CN(-) salts of the Fe(II) complex of this ligand were also synthesized. The X-ray crystal structure of the BF(4)(-) 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 ClO(4)(-) 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 BF(4)(-) complex further support spin-state crossover in the solid state with a transition temperature near 300 K. UV-visible spectroscopy and (1)H 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+)·2BPh(3)CN(-). 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.

Iron complexes of tris(pyrazolyl)ethane ligands methylated in the 3-, 4-, and 5-positions

“…This ligand was prepared by a method previously reported for producing tris(5methylpyrazolyl)methane. 8 A solution of 1 (0.34 g/1.5 mmol) in THF (15 mL) was cooled to between -30 ˚C and -40 ˚C (dry ice/acetonitrile bath) under argon. To the cooled solution was carefully added n-butyllithium (2.4 mL/6 mmol, 2.5 M in hexanes, 4 eq.).…”

“…5b,6 More elaborate functionalization of the tpm carbon has been employed by Reger for the creation of a number of functionalized tpm ligands that form fascinating supramolecular structures when complexed to metal ions. 7 As a result of our interest in the chemistry of tpm complexes that exhibit spin-state transitions near room temperature, 8 we undertook the synthesis and study of a simple series of tpe ligands with methyls on the pyrazole rings ( Figure 2). For simplicity, the tpe ligands are denoted with a superscript indicating where the methyl groups are located on the pyrazole rings (e.g.…”

Accessing spin-crossover behaviour in iron(ii) complexes of N-confused scorpionate ligands

“…22 While the above statement seems hyperbole, Fe[HB(3-cypropylpz)3]2 is the only case of a bulkier Tp R iron(II) complex to show SCO. 7 We sought a ligand design that would allow ready (perhaps "on demand") modification of pyrazolyl groups near the metal centre in order to discover new examples of SCO behaviour in scorpionates with bulkier pyrazolyls and to extend the current capabilities of scorpionates as supporting ligands.…”

The organic chemistry of poly(1H-pyrazol-1-yl)methanes

“…This methodology was used to prepare 350, 351 and 352 from 2 by reaction with chlorodiphenylphosphine and chlorotrimethylsilane, respectively [178][179][180] (for 352 see also ref. [181]), 353 from 1 [182] by reaction with methyl iodide and 354 and 355 from 26 using, in this case, LDA as the base and chlorotrimethylsilane and tert-butylisocyanate as the respective electrophiles [179] (Fig. 13).…”