Copper complexes of the phenolic oxime family of ligands (3-X-salicylaldoximes) are used extensively as metal solvent extractants. Incorporation of electronegative substituents in the 3-position, ortho to the phenol group, can be used to “buttress” the inter-ligand H-bonding leading to enhancement in extractant strength. However, investigation of the relevant H-bonding in these complexes can be exceedingly difficult. Here we have combined EPR, ENDOR, DFT and X-ray crystallography to study this effect. Analysis of the 1H ENDOR data revealed a variation in the Cu…H16 (oxime proton) distance from 2.92 Å for the unsubstituted complex [Cu(L2)2] compared to 3.65 Å for the X = N(C6H13)2 substituted complex [Cu(L3)2]. DFT calculations showed that this variation is caused by changes to the length and strength of the H-bond between the oximic hydrogen and the phenolate oxygen. Noticeable changes to the Cu…H15 (azomethine proton) distances and the Cu…N bonding parameters were also observed in the two complexes, as revealed through the NA and NQ ENDOR data. Distortions in the structure of the complex and variations in the oximic proton to phenolate oxygen H-bond strength caused by the substituent (X) were confirmed by DFT and X-ray crystallography. DFT directly evidenced the importance of the interaction between H16 and the amine nitrogen of N(C6H13)2 in the buttressed complex, and how the high strength of this interaction may not necessarily lead to enhancement of copper extraction as it can impose an unfavorable geometry in the inner coordination sphere of the complex. Therefore ENDOR, DFT and X-ray structural data all indicate that the aminomethyl substituent (X) ortho to the phenolic oxygen atom provides a particularly strong buttressing of inter-ligand H-bonding in these copper complexes, and these outer sphere interactions can significantly influence structure and stability.
- POLYCRYSTALLINE ENDOR PATTERNS