EPR/ENDOR and Computational Study of Outer Sphere Interactions in Copper Complexes of Phenolic Oximes

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 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 1 H ENDOR data revealed a variation in the 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.3

Paramagnetic NMR of Phenolic Oxime Copper Complexes: A Joint Experimental and Density Functional Study

“…It is the hybridisation through admixture of s‐orbitals that gives rise to large hyperfine couplings at some of these nuclei, and, hence, to their large pNMR shifts. Among the H atoms it is the H atom at the imido moiety (H7 in Figure a) that carries the highest spin density, in excellent agreement with recent low‐temperature EPR results . As a consequence, this H atom is the one with the most pronounced downfield shift and can safely be assigned to the resonance observed at δ =272 ppm (Table ).…”

“…The metal centres in the solids are usually too far apart ford irect or indirect exchangec oupling, but it is unclear to what extent spin delocalisationo nto the organic ligandso ccurs, which might in turn modulate any intermoleculari nteraction. Significant spin density on Ha nd Na toms has been detectedt hroughE PR spectroscopy in solution and in the solid state, [5] but in the absence of 13 C-enriched samples, the precise spin distributions over the organic backbonea re unknown. We now present an experimental and computational 1 Ha nd 13 Cs olid-state NMR study of phenolic oxime copper(II) complexesa tn atural abundance in order to fillthat gap.…”

Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic and Spectroscopic Features and Applications for Catalysis of the Henry Reaction

“…This can be related to the difference between the imine and amine nitrogen atoms (difference between the sp 2 and sp 3 hybridization of the copper-ligating nitrogen) and, to a lesser extent, to the higher planarity in the salen case. This is substantiated by the observation that the 14 N spin density of the here-studied Cu II -salan complexes is also 60–70% of that reported for copper complexes of phenolic oximes, of which some have slight tetrahedral distortion. , Similarly, the 14 N isotropic hyperfine values of the pyrrole nitrogen atoms in planar copper porphyrins and copper phthalocyanines are significantly larger than those of the Cu II -salan complexes in this study …”

“…This is substantiated by the observation that the 14 N spin density of the here-studied Cu IIsalan complexes is also 60−70% of that reported for copper complexes of phenolic oximes, of which some have slight tetrahedral distortion. 46,47 Similarly, the 14 N isotropic hyperfine values of the pyrrole nitrogen atoms in planar copper porphyrins and copper phthalocyanines are significantly larger than those of the Cu II -salan complexes in this study. 48 These values are, however, close to those obtained for amino nitrogen (−NH 2 ) couplings of copper(II) dipeptides determined by EDNMR ( N A 0 = 32.0−32.3 MHz).…”

New Insights into the Recovery of Strategic and Critical Metals by Solvent Extraction

“…This is perhaps, surprising, as the bond dissociation energies of metal-ligand bonds are at least one order of magnitude greater than those of hydrogen bonds. Recently however, this was studied further by analysing similar Cu complexes by EPR (Cu 2+ is d 9 with a single unpaired electron) and 1 H ENDOR (electron nuclear double resonance) spectroscopy, DFT calculations, and X-ray crystallography [14]. In this case, the 1 H ENDOR spectra are dominated by coupling of the unpaired electron to the azomethine and oximic protons, which provides information on the Cu…H distances for the various complexes and gives an indication of the strength of the hydrogen bond and Cu-N/O bonds.…”