Heteroleptic Dipyrrinato Complexes Containing 5‑Ferrocenyldipyrromethene and Dithiocarbamates as Coligands: Selective Chromogenic and Redox Probes

Six heteroleptic dipyrrinato complexes [Ni­(fcdpm)­(dedtc)] (1), [Ni­(fcdpm)­(dipdtc)] (2), [Ni­(fcdpm)­(dbdtc)] (3), [Pd­(fcdpm)­(dedtc)] (4), [Pd­(fcdpm)­(dipdtc)] (5), and [Pd­(fcdpm)­(dbdtc)] (6) (fcdpm = 5-ferrocenyldipyrromethene; dedtc = diethyldithiocarbamate; dipdtc = diisopropyldithiocarbamate; dbdtc = dibutyldithiocarbamate) have been synthesized and characterized by elemental analyses and spectral (ESI-MS, IR, 1H, 13C NMR, UV–vis) and electrochemical studies. Crystal structures of 1, 2, 4, and 5 have been authenticated by X-ray single-crystal analyses. Nickel-based complexes 1–3 display selective chromogenic and redox sensing for Hg2+ and Pb2+ ions, while palladium complexes 4–6 display selective chromogenic and redox sensing only for Hg2+. Electronic absorption, ESI-MS, and electrochemical studies indicated that sensing arises from interaction between 1–3 and Hg2+/Pb2+ through sulfur of the coordinated dithiocarbamates, while it arises from the pyrrolic nitrogen of fcdpm and dithiocarbamate sulfur from 4–6 and Hg2+. Different modes of binding between Ni and Pd complexes have further been supported by theoretical studies. The receptor–cation binding constants (Ka) and stoichiometry between probes and Hg2+/Pb2+ have been estimated by the Benesi–Hildebrand method and Job’s plot analysis. Detection limits for 1–3 toward Hg2+/Pb2+ and 4–6 for Hg2+ have been found to be reasonably high.