Contributions of inner and outer coordination sphere bonding in determining the strength of substituted phenolic pyrazoles as copper extractants.

Mary R Healy,Thomas Sassi,John Campbell,Philip J Bailey,Jason B Love,Lucy C Emeleus,Adam J Fischmann,Euan D Doidge,Peter A Tasker,Carole A Morrison,James W Roebuck,David J White

doi:10.1039/c5dt04055h

Mary R Healy, Thomas Sassi + Show 10 more

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https://doi.org/10.1039/c5dt04055h

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Abstract

Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are shown to function as Cu-extractants, having similar strength and selectivity over Fe(iii) to 5-nonylsalicylaldoxime which is a component of the commercially used ACORGA® solvent extraction reagents. Substitution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effectiveness of this is very dependent on steric factors. The correlation between the calculated energies of formation of copper complexes in the gas phase and the observed strength of comparably substituted reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests that big differences in strength of extractants arise principally from a combination of the effects of the substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their propensity to buttress inter-ligand hydrogen bonding.

Highlights

Some of the earliest reports of the use of metal complexes of phenolic pyrazoles with the generic structure I (Scheme 1) concern their use in biomimetic investigations.1 They have been shown to have anti-bacterial activity2 and oxorhenium(V) and dioxomolybdenum(VI) complexes have been used as catalysts for epoxidation reactions.3,4 The proximity of the oxygen and the two nitrogen donor atoms in the proligands and their ability to lose protons from both the phenol and pyrazole moieties to become dianionic bridging ligands favours the formation of polynuclear complexes
Scheme 1 Pseudomacrocyclic structures of copper complexes formed by phenolic pyrazoles I and phenolic oximes II
In this paper we focus on the solution chemistry of phenolic pyrazoles and in particular their ability to function as copper solvent extractants in the pH-dependent equilibrium shown in eqn [1]

Summary

Introduction

Some of the earliest reports of the use of metal complexes of phenolic pyrazoles with the generic structure I (Scheme 1) concern their use in biomimetic investigations.1 They have been shown to have anti-bacterial activity2 and oxorhenium(V) and dioxomolybdenum(VI) complexes have been used as catalysts for epoxidation reactions.3,4 The proximity of the oxygen and the two nitrogen donor atoms in the proligands and their ability to lose protons from both the phenol and pyrazole moieties to become dianionic bridging ligands favours the formation of polynuclear complexes. Whilst the introduction of electron withdrawing groups ortho or para to the phenolic OH group will facilitate deprotonation of the extractant to generate the coordinating form and favour complex formation, it will reduce the basicity of the N2O22− donor set, thereby diminishing the strength of the interactions with the copper cation.

Results

Conclusion