Water exchange of the (o-phenylenediamine-N,N,N',N'-tetraacetato)ferrate(III) complex in aqueous solution as studied by variable-temperature, -pressure, and -frequency oxygen-17 NMR techniques

Abstract

The crystal structure of potassium aqua (o-phenylenediamine-N,N,N{prime},N{prime}-tetraacetato)ferrate(III) dihydrate, (K(Fe(OH{sub 2})-(phdta)){center dot}2H{sub 2}O), has been determined by x-ray crystallography. The central iron(III) ion is seven-coordinate, with one water molecule being hydrated. Water-exchange rates of the iron(III) complex with phdta{sup 4{minus}} in aqueous solution have been studied as a function of temperature and pressure by the oxygen-17 NMR line-broadening method. Activation parameters for water exchange have been determined as follows: k(25 C) = (1.2 {plus minus} 0.2) {times} 10{sup 7} s{sup {minus}1}, {Delta}H{sup {double dagger}} = 26 {plus minus} 3 kJ mol{sup {minus}1}, {Delta}S{sup {double dagger}} = -22 {plus minus} 9 J K{sup {minus}1} mol{sup {minus}1}, and {Delta}V{sup {double dagger}} = 4.6 {plus minus} 0.2 cm{sup 3} mol{sup {minus}1}. The positive activation volume indicates that the water exchange proceeds via a dissociative interchange mechanism. 41 refs., 4 figs., 4 tabs.