Reductions by titanium(ii) as catalyzed by titanium(iv)

Abstract

The cobalt(III) complexes, [(NH₃)₅CoBr]²⁺ and [(NH₃)₅CoI]²⁺ are reduced by Ti(II) solutions containing Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent of reaction. Other Co(III)–Ti(II) systems exhibit the usual exponential traces with rates proportional to [Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)–Co(III)Br and Ti(II)–Co(III)I reactions support the reaction sequence:with rates determined mainly by the slow Ti(IV)–Ti(II) ligand exchange (k₁ = 9 × 10⁻³ M⁻¹ s⁻¹ at 22 °C). Computer simulations of the catalyzed Ti(II)–Co(III) reaction in perchlorate–triflate media yield relative rates for reduction by the proposed active [Ti(II)(H₂O)_n−1]²⁺ intermediate; k_Br/k_I = 8.