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

Eight new amido functionalized reagents, L-1-L-8, 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 reversible arrow [(LH)(2)MCl4]. 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 R3-nN(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, 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 [ZnCl4](2-) over Cl-, represented by the process 2[(LH)Cl] + ZnCl42- reversible arrow [(LH)(2)ZnCl4] + 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.