Quaternary diammonium reagents hosted on polymer nanofibers for hexachloridoiridate(IV) specificity: Experimental and theoretical studies

Abstract

This study is focused on the development of iridium(IV)-specific quaternary diammonium cations containing electron-donating and electron-withdrawing groups. Quaternary diammonium cations used were tetramethylbenzyl-1,10-diammonium decane chloride (QuatDMDAMeBnz), tetrabenzyl-1,10-diammonium decane chloride (QuatDMDABnz), tetratrifluoromethylbenzyl-1,10-diammonium chloride (QuatDMDACF3Bnz), and tetranitrobenzyl-1,10-diammonium chloride (QuatDMDANO2Bnz). Polyvinylbenzylchloride (PVBC) nanofibers were used as a support material, and the functionalised nanofibers were used for the separation of Ir(IV) from Rh(III). The efficiency of the quaternary diammonium cations was investigated in a column study under dynamic flow adsorption conditions. Column sorption of [IrCl6]2- and [RhCl5(H2O)]2- on nanofibers functionalized with diammonium cations showed [RhCl5(H2O)]2- was not adsorbed by the sorbent materials while [IrCl6]2- was loaded onto the column. The loading capacities of [IrCl6]2- on the quaternary diammonium sorbent materials increased in the order of F-QuatDMDAMeBnz (3.08 mg/g; 16.01 mmol/g) < F-QuatDMDABnz (3.39 mg/g; 17.63 mmol/g) < F-QuatDMDACF3Bnz (5.46 mg/g; 28.42 mmol/g) < F-QuatDMDANO2Bnz (7.10 mg/g; 36.96 mmol/g). The loading capacity for Ir(IV) with quaternary diammonium cationic nanofibers increased with an increase in the electron-withdrawing nature of the substituent on the quaternizing group. This trend was further explained by considering the anion binding energies and electrostatic charges of the diammonium cations as well as their ion-pairing interactions with [IrCl6]2- and [RhCl5(H2O)]2- using molecular modeling calculations. The formation of [(L)IrCl6] ion-pairs with the charge diffuse [IrCl6]2- anion was energetically favoured from QuatDMDAMeBnz to QuatDMDANO2Bnz. The positive electrostatic potential (ESP) values of 473.92 kcal/mol for QuatDMDAMeBnz, 495.85 kcal/mol for QuatDMDABnz, 991.12 kcal/mol for QuatDMDACF3Bnz and 1133.05 kcal/mol for QuatDMDANO2Bnz also followed this trend. This trend was also well correlated with binding energies of the respective cations with [IrCl6]2- which were 22.66 kcal/mol for QuatDMDAMeBnz, 27.43 kcal/mol for QuatDMDABnz, 49.19 kcal/mol for QuatDMDACF3Bnz and 51.04 kcal/mol for QuatDMDANO2Bnz.