The preparation and reactivity of bidentate phosphato complexes of cobalt(III). Cis-[Co(cyclen)PO 4], cis-[Co(en) 2PO 4] and the p-(nitrophenylphosphato)bisethylenediaminecobalt(III) cation

Abstract

The preparation of cis-[Co(cyclen)PO 4] and the p-nitrophenylphosphatobisethylenediaminecobalt(III) cation are described (cyclen = 1,4,7,9-tetra-azacyclododecane). Cis-[Co(cyclen)PO 4] undergoes rapid ring opening in acidic or basic solution to give the monodentate phosphato species. Loss of monodentate phosphate in acidic solution follows a rate expression of the form k obs = k O + k H[H +] where k O = 5 × 10 −4 s −1 and k H = 4.05 × 10 −2 M −1 s −1 at 25 °C and I = 0.49 M(NaClO 4). The acid-catalysed reaction displays a solvent deuterium isotope effect k D 2O /k H 2O of ca. 1.4 consistent with a mechanism involving a rapid pre-equilibrium protonation followed by rate determining loss of monodentate phosphate. For [Co(en) 2PO 4] in acidic solution, both ring opening of the phosphato ring and loss of monodentate phosphate show a first order dependence on the hydrogen ion concentration and solvolytic pathways are of negligible importance. For ring opening k H = 2 M −1 s −1 and for loss of monodentate phosphate k H = 1.1 M −1 s −1 at 25 °C and I = 0.19 M (NaClO 4). Loss of monodentate phosphate from the complex [Co(cyclen)OH(OPO 3)] − also takes place in basic solution and the reaction shows a first order dependence on the hydroxide ion concentration with k OH = 2.7 × 10 −2 M −1 s −1 at 25 °C and I = 0.49 M (NaClO 4). For loss of monodentate phosphate from [Co(en) 2OH(OPO 3)] − k OH = 1.4 × 10 −2 M −1 s −1, while for ring opening of bidentate phosphate in [Co(en) 2PO 4], k OH is ca. 1.75 M −1 s −1.