1-(2,4-Dinitrophenyl)-5-ferrocenyl-3-methyl-1H-pyrazole

In the title compound, [Fe(C5H5)(C15H11N4O4)], the dinitrophenyl and cyclopentadienyl rings make dihedral angles of 53.61 (6) and 23.11 (9)°, respectively, with the pyrazole unit. The two cyclopentadienyl rings are in an eclipsed conformation. The crystal structure is stabilized by intermolecular C—H⋯O interactions, which link molecules into chains parallel to the b axis.

In the title compound, [Fe(C 5 H 5 ) (C 15 H 11 N 4 O 4 )], the dinitrophenyl and cyclopentadienyl rings make dihedral angles of 53.61 (6) and 23.11 (9) , respectively, with the pyrazole unit. The two cyclopentadienyl rings are in an eclipsed conformation. The crystal structure is stabilized by intermolecular C-HÁ Á ÁO interactions, which link molecules into chains parallel to the b axis.

Comment
The design of molecular receptors having the ability to selectively bind and sense cationic, anionic or even neutral guests via a macroscopic physical response is an area of intense activity (Fabbrizzi et al., 1995). Chemical sensors bearing ferrocene nuclei as part of the sensing unit have been widely studied (Basurto et al., 2007). Ferrocene has largely proved to be a simple and remarkably robust building block for the construction of redox-responsive receptors (Beer et al., 1998). To build a sensor molecule with dual response (redox &optical), the title compound has been synthesized and studied to understand the above said properties.
The pyrazole ring has delocalized bonds (Gilchrist,1997;Shi et al., 2005). The substitued cyclopentadienyl rings are parallel to each other. The cyclopentadienyl rings are oriented at angles of 23.11 (9)° and 23.45 (7)° with respect to the planar pyrazole ring. Furthermore, the pyrazole and dinitrophenyl rings make a dihedral angle of 53.61 (6) °. The nitro groups are planar and oriented at angles of 26.61 (12)° and 9.53 (10)° to the phenyl ring. The two cyclopentadienyl rings of the ferrocenyl group are in a nearly eclipsed conformation (Erasmus et al., 1996).
The molecules in crystal are connected via intermolecular C-H···O interaction into a chain extended along the [010] direction.
Experimental 2,4-Dinitrophenylhydrazine (1 g, 5 mmol) was added to a solution of ferrocenoylacetone (1.35 g, 5 mmol) and a catalytic amount of p-toluenesulfonic acid in 50 ml of toluene. Tthe mixture was refluxed with a Dean-Stark apparatus for 14 h.
The solvent was evaporated under vacuum, and the resulting black residue was chromatographed on a silica-gel column using petroleum ether-ethylacetate (9:1) mixture as eluent. The red band was collected which offered dark-red crystal. (62% yield, m.p. 433-435 K).

Refinement
H atoms were geometrically positioned (C-H = 0.93 − 0.98 Å) and treated as riding on their parent atoms, with Ui so (H) = 1.2U eq (C) or 1.5U eq (methyl C). The cyclopentadienyl group C15-C19 was refined as a rigid group.

1-(2,4-Dinitrophenyl)-5-ferrocenyl-3-methyl-1H-pyrazole
Crystal data [Fe(C 5   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.