A new monoclinic polymorph of 1,1′-bis(diphenylthiophosphoryl)ferrocene

In 1,1′-bis(diphenylthiophosphoryl)ferrocene, the S atoms lie to the same side of the molecule. By contrast to this almost syn disposition, in a previous form, the Fe atom lies on a centre of inversion so that the S atoms are strictly anti.


Chemical context
Phosphanegold(I) dithiocarbamates, R 3 PAu(S 2 CNR 0 2 ), attract on-going interest owing to impressive biological activities against both cancer (Jamaludin et al., 2013) and microbes (Sim et al., 2014). It was in the course of these studies that crystals of the title compound, dppfS 2 , an oxidation product of 1,1 0bis(diphenylphosphane)ferrocene (dppf), were isolated as orange needles, being a side-product of a reaction, see Synthesis and crystallization for details. Crystallography shows the title compound to be a new monoclinic polymorph of a previously described C2/c form (Fang et al., 1995). Herein, details of the new polymorph are described along with a comparison with the original polymorph. A discussion of the key structural characteristics of related dppfY 2 , Y = 0, O, S and Se, structures ensues.

Structural commentary
The molecular structure of dppfS 2 is shown in Fig. 1 and comprises two Ph 2 P S units linked via the P atoms through a ISSN 2056-9890 C 5 H 4 FeC 5 H 4 link. The S atoms lie to the same side of the molecule and might be described as having a syn conformation. When viewed down the PÁ Á ÁP axis, the S atoms are gauche with the pseudo S-PÁ Á ÁP-S torsion angle being À53.09 (3) . This represents the major difference between dppfS 2 and its C2/c-dppfS 2 polymorph (Fang et al., 1995). In the latter the Fe atom lies on a crystallographic centre of inversion, implying the S atoms are anti and that the pseudo S-PÁ Á ÁP-S torsion angle is 180 .

Supramolecular features
Globally, the crystal packing features columns of molecules aligned along the a axis. Based on the distance criteria employed in PLATON (Spek, 2009), the most notable intermolecular contact operating in the crystal structure is a Cp-C2-H2Á Á Á(C31-C36) interaction, Table 1, that connects translationally related molecules into a supramolecular chain along the a axis, Fig. 3. Chains pack with no specific directional interactions between them, Fig. 4. In the C2/c-dppfS 2 polymorph, the most prominent directional interaction is a weak C-HÁ Á ÁS contact. The crystal packing efficiencies calculated by PLATON (Spek, 2009) are 69.3 and 67.2%, respectively, indicating the more symmetric structure packs less efficiently.

Database survey
Subsequent to the report of the C2/c form by Fang et al. (1995), a second report appeared (Pilloni et al., 1997). In the latter analysis, the authors suggested that Cc was the correct space group. The assignment of C2/c was later confirmed as being correct (Clemente & Marzotto, 2004).
The structures of several oxidation products of dppf, Ph 2 P( Y)C 5 H 4 FeC 5 H 4 P( Y)Ph 2 , Y = 0, O, S and Se, have been described in the crystallographic literature. The parent compound, i.e. with Y = lone pair, has the Fe atom situated on a centre of inversion (Casellato et al., 1988) The molecular structure of the new P2 1 /c polymorph of dppfS 2 , showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

Figure 2
Overlay diagram of the P2 1 /c (red image) and C2/c (green) polymorphs overlapped so that one Cp ring of each molecule is coincident.
Cg1 is the centroid of the C31-C36 benzene ring.

Figure 3
Supramolecular chain along the a axis sustained by C-HÁ Á Á interactions shown as purple dashed lines. unsolvated form has been reported with the Fe atom again located on a centre of inversion (Pilloni et al., 1993). A monohydrate (Bar et al., 2008;Bolte et al., 1997) as well as a dihydrate (Munyejabo et al., 1994;Fang et al., 1995) have also been described. In the former, the O atoms are approximately syn while the latter is centrosymmetric, i.e. resembling the situation with the Y = S polymorphs. Finally, when Y = Se, centrosymmetric structures are found in the unsolvated form (Arsenyan et al., 2012) as well as in the CH 2 Cl 2 monosolvate (Pilloni et al., 1997). Clearly, there is significant conformational flexibility in the Ph 2 P( Y)C 5 H 4 FeC 5 H 4 P( Y)Ph 2 , Y = 0, O, S and Se, compounds suggesting a low energy barrier for the interchange from one conformation to another. The structural data for Ph 2 P( Y)C 5 H 4 FeC 5 H 4 P( Y)Ph 2 are summarized in Table 2.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3

Figure 4
Unit-cell contents shown in projection down the a axis. The C-HÁ Á Á contacts are shown purple dashed lines. One of the supramolecular chains shown in Fig. 3 has been highlighted in space-filling mode.