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 L2–= 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 CuII(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: CuII⇌*CuII(k1,k–1); Red +*CuII→ products (k2), and Red + CuII→ products (k3). In this sequence CuII and *CuII 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