Metal-Controlled Assembly and Selectivity of a Urea-Based Anion Receptor

The terdentate ligand 3 (LH, 2-formylpyridine 4-thiosemicarbazone) forms with FeII and NiII 2:1 complexes of octahedral geometry of formula [MII(LH)2]2+. X-ray diffraction studies have shown that in both complexes the thiourea moieties of the coordinated thiosemicarbazones are exposed to the outside and are prone to establish hydrogen-bonding bifurcate interactions with oxoanions. However, spectrophotometric studies in CHCl3 soln. have shown that only the poorly basic NO3- ion is able to form authentic hydrogen-bond complexes with thiourea subunits, whereas all the other investigated anions (CH3COO-, NO2-, F-) induce deprotonation of the N-H fragment. The extreme enhancement of the thiourea acidity is based on the coordinative interaction of the sulfur atom with the metal, which stabilizes the thiolate form, and it is much higher than that exerted by any other covalently linked electron-withdrawing substituent, for example, -NO2.