Multinuclear nuclear magnetic resonance spectroscopic and high-performance liquid chromatographic characterization of silica, grafted with specifically deuterated 4-((propylamino)methyl)benzoic acid

Abstract

Specifically deuterated 4-((propylamino)methyl)benzoic acid-grafted silica (PAMBA-silica) was prepared by benzylation of propylamino-grafted silica (PA-silica) by either in situ reduction by sodium cyanoborodeuteride (NaCNBD3) of the Schiff base, formed by the reaction between PA-silica and 4-formylbenzoic acid, or by NaCNBD3 reduction of the isolated Schiff base. The PAMBA-silicas, which contain amine and carboxylic acid functionalities, were characterized by elemental analysis, 13C, 29Si, and 2H solid state NMR, and HPLC. Solid state 13C NMR revealed that PAMBA-silica prepared by the in situ method consists of di-benzylated, mono-benzylated, and unreacted amino-groups while PAMBA-silica prepared by the two-step synthesis consists of only mono-benzylated and unreacted amino-groups. 29Si solid-state NMR spectra indicated that no significant loss of propylamine groups had occurred during benzylation. Nearly ideal uniaxial rigid-limit 2H NMR spectra of grafted 4-PAMBA ligands indicates that they form a rigid structure, which provides effective electrostatic screening of inner positive charges when the ligands are in zwitterionic form. HPLC columns packed with PAMBA-silica and PA-silica were evaluated for ionic solutes at different pH of the mobile phase. Retention times increased for cations and decreased for anions at increasing pH. These trends show that PAMBA-silicas act as cation and anion exchangers at high and low pH, respectively. The pKa values of grafted carboxylic acid, determined from HPLC of weakly retaining solutes, are close to pKa of the solution PAMBA.