Preorganized tridentate analogues of mixed hydroxyoxime/carboxylate nickel extractants

A series of 22 tridentate unsatd. mono-anionic ligands having the atom-sequence Y-C=C-N=CH-C=C-Z-1, with Y = N, O, or S and Z = O or S, was studied to establish whether this backbone could be used to develop strong solvent extractants for nickel(II) which will preferably also show a high selectivity over iron(III) in the pH-dependent process: 2LHorg + NiSO4 = [(L)2Ni]org + H2SO4. All are capable of forming octahedral [(L)2Ni] complexes with a mer-arrangement of the YNZ-1 donor set. X-ray crystal structures of three salicylaldimine proligands derived from 3-bromo-5-t-butyl-2-hydroxybenzaldehyde show these to have pre-organized donor sets in which the three donors are held in an approx. orthogonal arrangement by intramol. hydrogen bonds. The tautomers obsd. are dependent on the nature of the Y atom and the extent to which it is favorable for this to form a bonding interaction with the acidic hydrogen atom on the salicylaldimine unit. X-ray crystal structure detns. of seven of the [(L)2Ni] complexes show these to have significantly distorted octahedral coordination geometries which partly account for the proligands proving to be fairly weak Ni-extractants. DFT calcns. show that extractant strength is dependent on a combination of the binding strength of the YNZ-1 donor set to the nickel ion and on the ease of deprotonation of the extractant. On this basis 3-nitro-4-t-octyl-6-(quinolin-8-imino)phenol is predicted, and is found, to be the strongest Ni-extractant. The extractants have low hydrolytic stability, reverting to their aldehyde precursors when solns. in water-immiscible solvents are contacted with aq. acid, making them poor candidates for development as reagents for nickel recovery based on pH-swing processes of the type shown above. [on SciFinder(R)]