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Design and Function of Pre‐organised Outer‐Sphere Amidopyridyl Extractants for Zinc(II) and Cobalt(II) Chlorometallates: The Role of CH Hydrogen Bonds
Author(s) -
Ellis Ross J.,
Chartres Jy,
Henderson David K.,
Cabot Rafel,
Richardson Patricia R.,
White Fraser J.,
Schröder Martin,
Turkington Jennifer R.,
Tasker Peter A.,
Sole Kathryn C.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201103616
Subject(s) - chemistry , coordination sphere , hydrogen bond , protonation , steric effects , pyridinium , outer sphere electron transfer , chloride , amide , inorganic chemistry , medicinal chemistry , ligand (biochemistry) , zinc , cobalt , crystallography , ion , crystal structure , stereochemistry , molecule , organic chemistry , biochemistry , receptor
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 CH and amido NH 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 CH units towards the charge‐diffuse ions [MCl 4 ] 2− , whereas the smaller, “harder” chloride anion prefers to be associated with the amido NH hydrogen‐bond donors.