Physical Parameters and Electron-Transfer Kinetics of the Copper(II/I) Complex with the Macrocyclic Sexadentate Ligand [18]aneS₆

Abstract

The electron-transfer kinetics of the complex formed by copper(II/I) with the sexadentate macrocyclic ligand 1,4,7,10,13,16-hexathiacyclooctadecane ([18]aneS6) have been measured in acetonitrile with a series of three oxidizing agents and three reducing agents. These studies have been supplemented by determinations of the redox potential and the stability constants of the Cu(I)- and Cu(II)([18]aneS6) complexes in both acetonitrile and aqueous solution. The Marcus cross relationship has been applied to the cross-reaction rate constants for the six reactions studied to resolve the electron self-exchange rate constant for the Cu(II/I)([18]aneS6) complex. An average value of k11 = 3 x 10(3) M(-1) s(-1) was obtained at 25 degrees C, mu = 0.10 M in acetonitrile. This value is approximately 2 orders of magnitude smaller than the values reported previously for the corresponding Cu(II/I) complexes with the quadridentate and quinquedentate homoleptic homologues having all ethylene bridges, namely, 1,4,7,10-tetrathiacyclododecane ([12]aneS4) and 1,4,7,10,13-pentathiacyclopentadecane ([15]aneS5). This significant difference in reactivity is attributed to the greater rearrangement in the geometry of the inner-coordination sphere that accompanies electron transfer in the Cu(II/I)([18]aneS6) system, wherein two Cu-S bonds are ruptured upon reduction. In contrast to other Cu(II/I) complexes with macrocyclic polythiaethers that have self-exchange rate constants within the same range, no evidence for conformationally gated electron transfer was observed, even in the case of the most rapid oxidation reaction studied.

Distortional isomerism with copper(i) complexes of 3,7-diazabicyclo[3.3.1]nonane derivatives

“…Indeed, the experimentally determined stability constants of a number of bispidine-copper(I) complexes are basically constant and similar to other copper(I) complexes. [23][24][25][26] The isomerism shown in Fig. 1 is an interesting example of distortional isomerism (bond stretch isomerism), 27-29 and it efficiently illustrates the general observation that the coordination geometry of transition metal complexes of the very rigid bispidine ligands are very elastic, i.e.…”

Synergistic effect of perchlorate ions and acetonitrile medium explored for extension in copper redoximetry

“…The strong π acceptor nature of aceto nitrile ligand (solvent) stabilizes copper(I) more than copper(II) giving an increase in redox potential. The calculated copper(II)−copper(I) perchlorate redox potential in acetonitrile as per IUPAC recommenda tions [43] against ferrocence reference electrode is 0.619 V [44] equivalent to 1.20 V (standard hydrogen electrode) [45] corresponding to a free energy of 115.81 kJ/mol for the electron transfer process which is sufficient to break the bond between the coordinated thiourea and cuprous ion and thus affect the oxidation of the thiourea in copper(I)−thiourea complexes.…”

Investigating the copper coordination, electrochemistry, and Cu(ii) reduction kinetics of biologically relevant selone and thione compounds

“…32 This Cu + complex is likely formed upon acetonitrile coordination after copper reduction and dissociation of the oxidized dmise or dmit ligands due to the high concentration of acetonitrile compared to dmise or dmit ligand and the high affinity of Cu + for acetonitrile. 62 Kinetic studies were also performed to determine the rates of Cu 2+ reduction by dmise and dmit. For these experiments, equimolar amounts of dmise or dmit were combined with Cu(OTf ) 2 in acetonitrile and the increasing absorbance of the reduced [Cu (NCCH 3 ) 4 ] + complex (λ max = 207 nm) was monitored.…”