Substituent directed selectivity in anion recognition by a new class of simple osmium-pyrazole derived receptors

Indazole-Derived Mono-/Diruthenium and Heterotrinuclear Complexes: Switchable Binding Mode, Electronic Form, and Anion Sensing Events

The article deals with the newer classes of mononuclear: [(acac)₂Ru^III(H-Iz)(Iz^-)] 1, [(acac)₂Ru^III(H-Iz)₂]ClO₄ [1]ClO₄/[1']ClO₄, and [(bpy)₂Ru^II(H-Iz)(Iz^-)]ClO₄ [2]ClO₄, mixed-valent unsymmetric dinuclear: [(acac)₂Ru^III(μ-Iz^-)₂Ru^II(bpy)₂]ClO₄ [3]ClO₄, and heterotrinuclear: [(acac)₂Ru^III(μ-Iz^-)₂M^II(μ-Iz^-)₂Ru^III(acac)₂] (M = Co:4a, Ni:4b, Cu:4c, and Zn:4d) complexes (H-Iz = indazole, Iz^- = indazolate, acac = acetylacetonate, and bpy = 2,2'-bipyridine). Structural characterization of all the aforestated complexes established their molecular identities including varying binding modes (N_a and N_b donors and 1H-indazole versus 2H-indazole) of the heterocyclic H-Iz/Iz^- in the complexes. Unlike [1']ClO₄ containing two NH protons at the backface of H-Iz units, the corresponding [1]ClO₄ was found to be unstable due to the deprotonation of its positively charged quaternary nitrogen center, and this resulted in the eventual formation of the parent complex 1. A combination of experimental and density functional theory calculations indicated the redox noninnocent feature of Iz^- in the complexes along the redox chain. The absence of intervalence charge transfer transition in the near-infrared region of the (Iz^-)₂-bridged unsymmetric mixed-valent Ru^IIIRu^II state in [3]ClO₄ suggested negligible intramolecular electronic coupling corresponding to a class I setup (Robin and Day classification). Heterotrinuclear complexes (4a-4d) exhibited varying spin configurations due to spin-spin interactions between the terminal Ru(III) ions and the central M(II) ion. Though both [3]ClO₄ and 4a-4d displayed ligand (Iz^-/Iz^•)-based oxidation, reductions were preferentially taken place at the bpy and metal (Ru^III/Ru^II) centers, respectively. Unlike 1 or [2]ClO₄ containing one free NH proton at the backface of H-Iz, [1']ClO₄ with two H-Iz units could selectively and effectively recognize F^-, OAc^-, and CN^- among the tested anions: F^-, OAc^-, CN^-, Cl^-, Br^-, I^-, SCN^-, HSO₄^-, and Η₂PΟ₄^- in CH₃CN via intermolecular NH···anion hydrogen bonding interaction. The difference in the sensing feature between [1']ClO₄ and 1/[2]ClO₄ could be rationalized by their pK_a values of 8.4 and 11.3/10.8, respectively.

Electrochemical Anion Sensing: Supramolecular Approaches

Anions play a vital role in a broad range of environmental, technological, and physiological processes, making their detection/quantification valuable. Electroanalytical sensors offer much to the selective, sensitive, cheap, portable, and real-time analysis of anion presence where suitable combinations of selective (noncovalent) recognition and transduction can be integrated. Spurred on by significant developments in anion supramolecular chemistry, electrochemical anion sensing has received considerable attention in the past two decades. In this review, we provide a detailed overview of all electroanalytical techniques that have been used for this purpose, including voltammetric, impedimetric, capacititive, and potentiometric methods. We will confine our discussion to sensors that are based on synthetic anion receptors with a specific focus on reversible, noncovalent interactions, in particular, hydrogen- and halogen-bonding. Apart from their sensory properties, we will also discuss how electrochemical techniques can be used to study anion recognition processes (e.g., binding constant determination) and will furthermore provide a detailed outlook over future efforts and promising new avenues in this field.