Design and function of pre-organised outer-sphere amidopyridyl extractants for zinc(II) and cobalt(II) chlorometallates: the role of C-H hydrogen bonds.
ABSTRACT Four new sterically hindered pyridines, L(1)-L(4)-containing amido substituents at the 2-position act as efficient solvent extractants for [CoCl(4)](2-) or [ZnCl(4)](2-) from acidic chloride solutions through protonation of the pyridino N-centre to form the neutral outer-sphere complexes [(LH)(2)MCl(4)]. These ionophores show very high selectivity for chlorometallate anions over chloride ion and are readily stripped to liberate the free-metal chlorides without the formation of inner-sphere complexes [ML(2)Cl(2)]. Single-crystal X-ray structure determinations of [(L(2)H)(2)CoCl(4)] and [(L(2)H)(2)ZnCl(4)] (L(2) = 2-(4,6-di-tert-butylpyridin-2-yl)-N,N'-dihexylmalonamide) coupled with (1)H NMR spectroscopy and DFT calculations on L(2)H(+) and other complexes of [ZnCl(4)](2-) confirm that the pyridinium NH group does not address the outer co-ordination sphere of the metallanion, but rather forms a hydrogen bond to the pendant amide groups and thus pre-organizes the ligand to present both C-H and amido N-H hydrogen-bond donors to the [MCl(4)](2-) ions. The selectivity for chlorometallates over chloride ions shown by this class of extractants arises from their ability to present several polarized C-H units towards the charge-diffuse ions [MCl(4)](2-), whereas the smaller, "harder" chloride anion prefers to be associated with the amido N-H hydrogen-bond donors.
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ABSTRACT: 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.Inorganic Chemistry 11/2012; · 4.79 Impact Factor
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ABSTRACT: Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.Chemical Communications 01/2013; · 6.38 Impact Factor