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 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)](2-) unit 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)](2-) over [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)](2-) over 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.