The reaction of [bispyridinotetrakis(t-butyl)phthalocyaninato]iron(II) with dioxygen to give μ-oxo-bis[tetrakis(t-butyl)phthalocyaninato]iron(III)

Abstract

The reaction of [bispyridinotetrakis(t-butyl)phthalocyaninato]iron(II), ( py )2 FePc t, with dioxygen in CH 2 Cl 2, gives almost quantitatively the oxo-bridged μ-oxo-bis[tetrakis(t-butyl)phthalocyaninato]iron(III) as the final, stable product. The reaction order with respect to the starting complex changes within a run. For t = 0, the rate is proportional to the ratio [Formula: see text] (Equation 1) where y0 is the initial absorbance of the solution and a1 and a2 are linear functions of [O2]. For t > 0, the instantaneous rate is instead described by the equation Rate = b [O2][A]/([A]0 - [A]) (Equation 2), where A is ( py )2 FePc t and the coefficient b is again proportional to [ O 2]. A mechanism is proposed implying the presence of trace amounts of ( py )(S) FePc t (S = axially bound CH 2 Cl 2) in equilibrium with ( py )2 FePc t. Both species are able to generate the transient pentacoordinate ( py ) FePc t that may react with py and S, giving back the hexacoordinate precursors, or with dioxygen, giving ultimately the μ-oxo complex. At t = 0, the species ( py )(S) FePc t, expected to be much more labile than ( py )2 FePc t, is at its maximum concentration and is assumed to give the prevaling contribution to the instantaneous rate. As the reaction proceeds, and free pyridine accumulates in the solution, the concentration of ( py )(S) FePc t decreases rapidly and the transient ( py ) FePc t is produced essentially by the bispyridino complex. This fact and the still growing concentration of free pyridine account for the Equation 2. Finally, when the py concentration is buffered by addition of small amounts of base, the reaction attains a simple first order law. Present data further support the iron(III) formulation of the final μ-oxo product and are inconsistent with the iron(II) hypothesis put forward by some authors.