Crystal structure of 1-ferrocenyl-2-(4-methylbenzoyl)spiro[11H-pyrrolidizine-3,11′-indeno[1,2-b]quinoxaline]

In the title compound the four-fused-rings system is approximately planar and the pyrrolidine rings of the pyrrolidizine fragment adopt a twist conformation. In the crystal, molecules are linked by C—H⋯O hydrogen bonds and C—H⋯π interactions, forming double-chains parallel to the c axis.


Chemical Context
Spirooxindoles are an important class of naturally occurring substances characterized by highly pronounced biological properties (Sureshbabu & Raghunathan, 2008). Ferrocene derivatives have antimalarial (Biot et al., 2004) and antibacterial (Chohan, 2002) activities. The use of ferrocene in bio-organometallic chemistry has promising applications since ferrocene is a stable non-toxic compound and has good redox properties (Fouda et al., 2007). Ferrocenyloxindoles have also been found to have anticancer (Silva et al., 2010) and antiproliferative activities (Gasser et al., 2011).
The synthesis of novel ferrocenyl-spiro-indanedione-Nmethylpyrrolidines by employing various unusual ferrocene derivatives as efficient 2-components in 1,3-dipolar cycloaddition reactions of azomethine ylides demonstrate that ferrocene-derived dipolarophiles can further be exploited for the synthesis of a variety of complex heterocycles through cycloaddition reactions (Sureshbabu et al., 2009). A wide range of substituted pyrrolizidine scaffolds offers a high level of functional, structural and stereochemical diversity. It has been demonstrated that multicomponent reactions (MCR) could be used for the synthesis of novel ferrocene-grafted dispiropyrrolidine and pyrrolizidine scaffolds through one-pot three-component intermolecular [3 + 2] cycloaddition of azomethine ylides with an unusual ferrocene Baylis-Hillman adduct (Kathiravan & Raghunathan, 2009). The one-pot fourcomponent cycloaddition reaction method was used to synthesize substituted pyrrolizidines containing ferrocene and a spiro-indenoquinoxaline moiety of biological significance (Sureshbabu et al., 2012). In view of the importance of this class of compounds, the synthesis of the title compound was undertaken and its crystal structure is reported herein.

Supramolecular features
In the crystal structure, molecules are linked into double chains running parallel to the c axis by intermolecular nonclassical C-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á Á interactions (Table 1) involving H atoms of the cyclopentadienyl groups as donors (Fig. 2).

Synthesis and crystallization
Ninhydrin (1 mmol) and 1,2-phenylenediamine (1 mmol) were mixed and stirred with methanol (10 ml) for 10 min. To this mixture, proline (1 mmol) and 1-ferrocenyl-3-(4-methylbenzoyl)prop-2-ene dipolarophile (1 mmol) were added and refluxed up to the end of the reaction as observed by thinlayer chromatography. The solvent was removed from the mixture under reduced pressure and the crude product was obtained using column chromatography. The crude extract was purified by petroleum ether and ethyl acetate (4:1 v/v). Single crystals suitable for the X-ray diffraction analysis were obtained by slow evaporation of the solvent at room temperature. The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probability level. H atoms are shown as small spheres of arbitrary radius.

Figure 2
Partial crystal packing of the title compound, showing the formation of a double chain running parallel to the c axis via C-HÁ Á ÁO hydrogen bonds (violet dashed lines) and C-HÁ Á Á interactions (red dashed lines). H atoms not involved in hydrogen-bond interactions have been omitted. Table 1 Hydrogen-bond geometry (Å , ).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in calculated positions, with C-H = 0.93-0.98 Å , and refined using a riding-model approximation, with U iso (H) = 1.5U eq (C) for methyl groups or 1.2U eq (C) otherwise. DELU restraints were applied to atoms C24 and C25.

1-Ferrocenyl-2-(4-methylbenzoyl)spiro[11H-pyrrolidizine-3,11′-indeno[1,2-b]quinoxaline]
Crystal data   (9) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 > σ(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.