Outer-Sphere Coordination Chemistry: Amido-Ammonium Ligands as Highly Selective Tetrachloridozinc(II)ate Extractants

• Published in: Inorganic chemistry Vol. 51; no. 23; pp. 12805 - 12819
• Main Authors: Turkington, Jennifer R, Cocalia, Violina, Kendall, Katrina, Morrison, Carole A, Richardson, Patricia, Sassi, Thomas, Tasker, Peter A, Bailey, Philip J, Sole, Kathryn C
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 03.12.2012
• Subjects: Amides - chemistry; Chlorides - chemistry; Crystallography, X-Ray; Hydrogen Bonding; Ligands; Models, Molecular; Molecular Structure; Quantum Theory; Quaternary Ammonium Compounds - chemistry; Zinc - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic301691d

Eight new amido functionalized reagents, L1–L8, have been synthesized containing the sequence of atoms R2N–CH2–NR′–CO–R″, which upon protonation forms a six-membered chelate with a hydrogen bond between the tertiary ammonium N–H+ group and the amido oxygen atom. The monocationic ligands, LH+, extract tetrachloridometal­(II)­ates from acidic solutions containing high concentrations of chloride ions via a mechanism in which two ligands address the “outer sphere” of the [MCl4]2‑ unit using both N–H and C–H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl2 ⇌ [(LH)2MCl4]. The strengths of L1–L8 as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R3‑n N­(CH2NR′COR″) n molecules, anti-intuitively decreasing with the number of strong hydrogen bond donors present and following the order monoamides > diamides > triamides. Studies of the effects of chloride concentration on extraction have demonstrated that the monoamides in particular show an unusually high selectivity for [ZnCl4]2‑ over [FeCl4]− and Cl–. Hybrid-DFT calculations on the tri-, di-, and monoamides, L2, L3, and L4, help to rationalize these orders of strength and selectivity. The monoamide L4 has the most favorable protonation energy because formation of the LH+ cation generates a “chelated proton” structure as described above without having to sacrifice an existing intramolecular amide–amide hydrogen bond. The selectivity of extraction of [ZnCl4]2‑ over Cl–, represented by the process 2­[(LH)­Cl] + ZnCl4 2‑ ⇌ [(LH)2ZnCl4] + 2Cl–, is most favorable for L4 because it is less effective at binding chloride as it has fewer highly polar N–H hydrogen bond donor groups to interact with this “hard” anion.