Polymorphism in pentaerythritol-derived ferrocenyl dithiophosphonates with intramolecular S-S coupling: a structural and computational study

Four new polymorphs (1–4) of pentaerythritol-derived ferrocenyl S-S coupled dithiophosphonates were obtained. The four polymorphs all crystallized in the monoclinic system but different space groups. Notable differences between polymorphs 3 and 4 lie in their respective unit cell parameters; a fifth polymorph 5 of the same compound crystallized in the orthorhombic system and was previously reported by us. Structural deviations between polymorphs were performed by molecular overlays and the statistical values were expressed as root mean square deviation (RMSD). The highest structural deviations were between polymorphs 1 & 4 and 2 & 4 with an RMSD of about 1.0 while polymorphs 3 & 4 had a smaller difference with an RMSD of about 0.25. The solid-state molecular organization in the five polymorphs was also studied and discussed in terms of molecular conformation, crystal packing and hydrogen-bonded networks. Polymorphs 1–5 contain a variety of intermolecular and intramolecular non-classical hydrogen bonding interactions which form hydrogen-bonded supramolecular architectures. In all polymorphs, these hydrogen bond interactions were between the hydrogen atoms of the substituted and unsubstituted ferrocenyl moiety and the adjoining sulfur atoms of the dithiophosphonate group. A DFT geometry optimization found that 2 had the lowest energy and verified the molecular overlay. The reactivity indices further proved 2 to be the most reactive.