Accessing the inaccessible: discrete multinuclear coordination complexes and selective anion binding attainable only by tethering ligands together

Abstract

Novel multimetallic copper pyrazolate complexes, inaccessible using simple pyrazole ligands due to competing, alternative structural motifs, can be obtained by locking pairs of pyrazole ligands together with ethylene tethers. Nanojars based on this tethered pyrazole ligand display unexpected total selectivity for the carbonate over the sulfate ion.

Mapping the Intricate Reactivity of Nanojars toward Molecules of Varying Acidity and Their Conjugate Bases Leading To Exchange of Pyrazolate Ligands

“…Self-assembled metal–organic complexes with potential anion recognition, sensing, and separation applications are an integral part of supramolecular anion binding research efforts and continue to generate sustained interest . Nanojars have recently been introduced as a new class of neutral anion-incarcerating agents with several outstanding features. − Self-assembled from Cu­(II) ions and pyrazole (Hpz) in the presence of a base and the anion to be incarcerated, nanojars of the formula [A⊂{Cu­(μ-OH)­(μ-pz)} n ] 2– (Cu n A; A = anion, n = 22–33) are comprised of three [Cu­(μ-OH)­(μ-pz)] x rings (Cu x ; x = 6–14, except 11). Nanojars are totally selective for doubly charged anions (such as CO 3 2– , SO 4 2– , HPO 4 2– , and HAsO 4 2– ) in the presence of singly charged anions (such as NO 3 – and ClO 4 – ) in large excess. ,,, An extremely efficient binding of the incarcerated anion is demonstrated by the inability of an aqueous Ba 2+ solution to precipitate the highly insoluble barium salt of the anion (e.g., BaSO 4 , K sp = 1.08 × 10 –10 at 25 °C in H 2 O). , Indeed, nanojars completely surround and isolate the incarcerated anion from its surroundings, as revealed by crystallographic analysis. − , Another advantage of nanojars as anion extraction agents is their exceptional stability to highly alkaline conditions.…”

“…Another aim of this work is to test the limits of the analogy between pyrazolate and carboxylate ligands in the coordination chemistry of multinuclear metal complexes, and to attempt to prepare nanojars with all carboxylate ligands. Examples of complexes that have been prepared with both pyrazolate and carboxylate ligands include a trinuclear Co­(III) complex with all pyrazolate ligands, [Co 3 (μ 3 -O)­(μ-4-NO 2 pz) 6 L 3 ] 2– (L = NO 2 – ), all acetate ligands, [Co 3 (μ 3 -O)­(μ-OAc) 6 L 3 ] + (L = py), and also with various mixtures of pyrazolate/acetate ligands, [Co 3 (μ 3 -O)­(μ-pz) 6– x ­(μ-OAc) x L 3 ] + ( x = 2–5; L = Hpz), tetranuclear complexes with a tetrahedral [Co 4 (μ 4 -O)] 6+ core based on pyrazolates or carboxylates (which can also coexist in the same structure), tetranuclear complexes with planar [Cu 4 (μ 4 -L)] 7+ cores (L = OH or Cl) with either pyrazolate, carboxylate or carbonate ligands, hexanuclear copper complexes in which two trinuclear units are bridged by either three pyrazolates or three carboxylates, pyrazolate-based [Ni 8 (OH) 6 (pz) 12 ] vs carboxylate-based [Ni 8 (OH) 4 (H 2 O) 2 ­(Me 3 CCOO) 12 ] cube-like octanuclear complexes, as well as larger multinuclear metallacycles with either pyrazolate , or carboxylate ligands . There would be an obvious advantage to obtaining nanojars with carboxylate instead of pyrazolate ligands, especially when it comes to large scale applications, as variously substituted carboxylate ligands are incomparably more accessible and inexpensive than the corresponding pyrazolate ligands.…”

Pyrene-Functionalized Fluorescent Nanojars: Synthesis, Mass Spectrometric, and Photophysical Studies

“…9 Selective binding of oxyanions has recently been achieved by tethering pairs of pyrazolate ligands together. 10,11 Aiming at detecting NJs using fluorescence and ultimately developing NJ-based sensors for oxyanions, we pursued the synthesis and characterization of fluorescent NJs. Various polar substituents (including nitro, amine, aldehyde, carboxylate) as well as acidic groups (phenol, thiol, carboxylic acid) interfere with NJ formation.…”

“…Consequently, NJs have been developed into highly efficient anion-extracting agents, capable of binding trace amounts of oxyanions and transferring even the most hydrophilic ones from water into long-chain aliphatic hydrocarbon solvents . Selective binding of oxyanions has recently been achieved by tethering pairs of pyrazolate ligands together. , …”

Manganese(II), lead(II) and cadmium(II) coordination complexes containing a tetrazole-based acylamide ligand: synthesis and the influence of the metal ions on the structures

“…Many discrete coordination compounds have been synthesized and characterized in recent years due to the wide range of their potential applications, such as single-molecule magnets, MRI contrast agents and catalysts (Zhang et al, 2006;Murase et al, 2012;Harris et al, 2013;O'Neill et al, 2017). Many factors, for example, the coordination geometry of the metal ions, the ligands, reaction temperature, pH value, metalligand ratio, auxiliary ligands, template, counter-ion and medium, can significantly affect the formation of coordination compounds (Caballero et al, 2011;Jeong et al, 2011;Ahmad et al, 2012;Metherell & Ward, 2014;Noh et al, 2016, Ahmed & Mezei, 2017Spore & Rosi, 2017). Among these factors, the nature of the metal ions and the ligands are especially important in the assembly of the coordination compounds (Zarra et al, 2015;Chen et al, 2017;Feng et al, 2017), as it determines the coordination ability of the ligand, as well as the related connection modes and coordination number and therefore the resulting structures.…”