EPR/ENDOR and computational study of outer sphere interactions in copper complexes of phenolic oximes

Healy, M. R., Carter, E., Fallis, I. A., Forgan, R. S. , Gordon, R. J., Kamenetzky, E., Love, J. B., Morrison, C. A., Murphy, D. M. and Tasker, P. A. (2015) EPR/ENDOR and computational study of outer sphere interactions in copper complexes of phenolic oximes. Inorganic Chemistry, 54(17), pp. 8465-8473. (doi:10.1021/acs.inorgchem.5b01180) (PMID:26288384)

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Abstract

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 interligand H-bonding, leading to an 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] to 3.65 Å for the X = CH2N(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 CH2N(C6H13)2 in the buttressed complex and indicated that the high strength of this interaction may not necessarily lead to an 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 interligand H-bonding in these copper complexes and that these outer sphere interactions can significantly influence structure and stability.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Forgan, Dr Ross
Authors: Healy, M. R., Carter, E., Fallis, I. A., Forgan, R. S., Gordon, R. J., Kamenetzky, E., Love, J. B., Morrison, C. A., Murphy, D. M., and Tasker, P. A.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Inorganic Chemistry
Publisher:American Chemical Society
ISSN:0020-1669
ISSN (Online):1520-510X
Published Online:19 August 2015
Copyright Holders:Copyright © 2015 American Chemical Society
First Published:First published in Inorganic Chemistry 54(17):8465-8473
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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