Robustness of thioamide dimer synthon, carbon bonding and thioamide-thioamide stacking in ferrocene-based thiosemicarbazones

The role of thioureas in crystal engineering as robust supramolecular synthons is now recognized, but their analogs, namely thiosemicarbazones/N-iminothioureas, have not received the attention they deserve. A series of five structurally related ferrocene-based thiosemicarbazones 1-5 have been designed, synthesized and crystallographically characterized in order to investigate the prevalence of the thioamide dimer synthon and carbon bonding. All of the compounds have shown a general preference for the adoption of the cis, trans conformation about the central thiourea moiety which is ideal for the formation of a dimeric hydrogen-bonded R-2(2) (8) {center dot center dot center dot H-N-C=S}(2) synthon as the building block. Therefore, this dimeric synthon is observed in all of the compounds, with the methyl group particularly set for playing its supportive stabilization role through C-H center dot center dot center dot S and carbon bonding interactions. Carbon bonding has been observed in all of the compounds except compound 2. The centrosymmetrically arranged thioamide protons present in trans conformation through N-H center dot center dot center dot H-C interactions give rise to the formation of tapes of varying topology in all of the compounds except 3, where a staircase arrangement of dimeric molecules is observed. Another notable feature of the crystal packing of 1-5 is the presence of thioamide-thioamide stacking, which has been observed in all five compounds. The prevalence of the thioamide dimer synthon, carbon bonding, and observation of new thioamide-thioamide stacking interactions indicate a much larger role for this class of compounds as a design element in crystal engineering than anticipated so far.