Metal Chelation Aptitudes of Bis(o-azaheteroaryl)methanes As Tuned by Heterocycle Charge Demands¹

Abstract

We describe the synthesis of a number of 1,3-azol-2-yl-, 1,3-benzazol-2-yl-, and azinyl-based bis(o-azaheteroaryl)methanes (LH, L(-) = Het(2)CH(-)) and their coordinating properties toward divalent transition metals (Zn, Cu, Co, Ni, Hg, Pd). This extended investigation includes both symmetrical and unsymmetrical ligands based on several substituted and/or unsubstituted thiazole, benzothiazole, benzoxazole, benzimidazole, pyridine, and quinoline derivatives. Depending on the structure and electron properties of the ligand, a vast set of neutral chelates ML(2) were obtained, where the ligand is present in its carbanionic form L(-). Additionally, we have prepared salt complexes [M(LH)(n)]X(m), where the ligand is present as a neutral system. Neutral chelates were typically obtained by the reaction of the ligand with metal acetates in alcoholic solution; salt complexes were formed by reaction with other metal salts such as chlorides. By exploring the coordinating properties of several bisheteroarylmethane ligands based on heteroaromatics of variable pi-electron structure and substitution pattern, we demonstrate that the formation of neutral chelates is strictly dependent on the electron-withdrawing capacity (charge demand) of the heteroaromatic moiety. The latter primarily dictates the efficiency by which the negative charge of the anionic ligand L(-) is stabilized by delocalization in ML(2) and, therefore, the stability of the chelate itself. On the basis of the large number and the variable nature of the nitrogen ligands used, we confirm the general validity of the charge-demand-dependent formation of chelates. This key factor can therefore be used for the efficient design of new pi-deficient heteroaromatic nitrogen ligands in chelates of great potential in many synthetic, catalytic, and technological fields.

Pyridazine‐Based Ligands and Their Coordinating Ability towards First‐Row Transition Metals

“…Syntheses of the complexes were inspired by literature procedures. [37,38] Solutions of 3 tol or 3 tBu in ethanol were refluxed, which was followed by the addition of M(NO 3 ) 2 (M = Co, Ni, Cu, Zn) as a 0.1  solution in ethanol. Coordination was immediate, as a change of color was apparent.…”

Five‐Membered Heterocycles: 1,3‐Azoles

“…A range of acid derivatives can substitute for carboxylic acids (Scheme 10.139): esters [397], orthoesters [398], nitriles [399], imidates [400] phosgene) [401] and anhydrides [402]. Concerning the use of orthoester the reaction conditions may vary considerably depending on the reactivity of the 2-substituted aniline.…”

“…A procedure strictly related to the previous method is the reaction of N-acyl derivatives of 244-246, which undergo thermal dehydration to afford the corresponding benzoazoles (Scheme 10.140) [407][408][409][410].…”

Silica Gel Chemically Modified with Ethyl-2-benzothiazolylacetate for Selective Extraction of Pb(II) and Cu(II) from Real Water Samples

“…27 SiAPTS (20 g) was suspended in 150 mL of dried toluene. Ethyl-2-benzothiazolylacetate (1 g) was added into the mixture.…”