Optical and Electrochemical Sensing of Co(II), Ni(II) and Cu(II) in DMSO. Pyridyl Thiazole and (Thio)Amide Pincer Ligands versus Di‐2‐pyridyl Ketone Hydrazones

Abstract

The ability of six pincer type molecules (di‐2‐pyridyl ketone acetic acid hydrazone (1), N‐(2,5‐dimethoxyphenyl)‐6‐[(2,5‐dimethoxyphenyl)carbamothioyl]pyridine‐2‐carboxamide (2), 6‐(4,7‐dimethoxy‐2‐benzothiazolyl)‐N‐(2,5‐dimethoxyphenyl)‐2‐pyridinecarboxamide (3), di‐2‐pyridyl ketone thiosemicarbazone (4), bis‐N‐(2,5‐dimethoxyphenyl)pyridine‐2,6‐dicarbothioamide (5), and 6‐(4,7‐dimethoxy‐2‐benzothiazolyl)‐N‐(2,5‐dimethoxyphenyl)‐2‐pyridinecarbothioamide (6)), to detect MBr2 (where M=Co, Ni, and Cu) in DMSO was quantified via formation constants determined from spectrophotometric and voltammetric titrations. The limit of detection (from electronic spectroscopy) of 1–6 ranged from 0.9‐9.5 ppm. The values of β (=log K) for the six species were found to be in the ranges of 4.3–4.9 and 4.0–6.3 for Co(II), 3.9–5.5 and 3.5–4.3 for Ni(II), and 4.0–4.3 and 3.7–5.0 for Cu(II) by UV‐visible and voltammetric methods, respectively. The results suggested that κ3‐NNS (6) and the κ3‐NNO (3) had good affinity for Ni(II) and Cu(II), whereas the κ3‐SNS (5) had greater affinities for Co(II) and Ni(II). The hydrazones (κ3‐NNO, 1 and κ3‐NNS, 4) were generally in the middle of the pack, displaying good affinity for the three metal ions. In a contrasting vein, the κ3‐ONS species (2) was generally the lowest across the pincers. DFT calculations (PBE0/6‐31G(d,p)/SMD) suggested that stable complexes of coordination numbers=4 to 6 were plausible in solution.