Influence of co-ordination number on copper(I)–copper(II) redox interconversions. Part 3. Reduction of a sterically constrained bis-(substituted phenanthroline) complex of copper(II) by iron(II) and ruthenium(II) complexes

Abstract

Further kinetic studies are reported for reduction of the copper(II) complex, [CuL₂]^2–, containing the ligand L^2–= 2,9-dimethyl-4,7-bis[(sulphonyloxy)phenyl]-1, 10-phenanthroline. The methyl substituents in the 2,9 positions create a steric hindrance which limits the extent of co-ordination to Cu^II(bis and not tris complexes are formed) and five- and four-(tetrahedral) co-ordinate complexes are believed to be present. With [Fe(CN)₆]^4–, [Fe(edta)]^2–(edta = ethylenediaminetetra-acetate), [Fe(CN)₅(PPh₃)]^3– and [Ru(NH₃)₅(pyz)]²⁺(pyz = pyrazine) as reductants (abbreviation Red) limiting kinetics are observed, and a self-consistent interpretation is possible in terms of the sequence: Cu^II⇌*Cu^II(k₁,k_–1); Red +*Cu^II→ products (k₂), and Red + Cu^II→ products (k₃). In this sequence Cu^II and *Cu^II are the five- and four-co-ordinate forms respectively. The step k₃ is additional to the sequence previously proposed for [Fe(CN)₆]^4–, the results for which have been modified accordingly. Contributions from k₃ are not apparent with [Fe(CN)₅(PPh₃)]^3– and [Ru(NH₃)₅(pyz)]²⁺, whereas with [Ru(NH₃)₅(py)]²⁺(py = pyridine) reaction via k₃ appears to be dominant. Possible explanations of the balance between k₂ and k₃ in terms of different E^⊖ values and ability to react inner-sphere are considered