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Eight new amido functionalized reagents, L 1 −L 8 , have been synthesized containing the sequence of atoms R 2 N−CH 2 −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 [MCl 4 ] 2unit using both N−H and C−H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl 2 ⇌ [(LH) 2 MCl 4 ]. The strengths of L 1 −L 8 as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R 3-n N(CH 2 NR′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 [ZnCl 4 ] 2over [FeCl 4 ] − and Cl − . Hybrid-DFT calculations on the tri-, di-, and monoamides, L 2 , L 3 , and L 4 , help to rationalize these orders of strength and selectivity. The monoamide L 4 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 [ZnCl 4 ] 2over Cl − , represented by the process 2[(LH)Cl] + ZnCl 4 2-⇌ [(LH) 2 ZnCl 4 ] + 2Cl − , is most favorable for L 4 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.

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