Kinetics of the reaction of nitric oxide with polypyridylamine iron(II) complexes

Abstract

The reactions of three polypyridylamine ferrous complexes, [Fe(TPEN)]2+, [Fe(TPPN)]2+, and [Fe(TPTN)]2+, with nitric oxide (NO) (where TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, TPPN = N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-propylenediamine, and TPTN = N,N,N′,N′-tetrakis(2-pyridylmethyl)trimethylenediamine) were investigated. The first two complexes, which are spin-crossover systems, presented second-order rate constants for complex formation reactions (kf) of 8.4 × 103 and 9.3 × 103 M−1 s−1, respectively (pH 5.0, 25 °C, I = 0.1 M). In contrast, the [Fe(TPTN)]2+ complex, which is in low-spin ground state, did not show any detectable reaction with NO. kf values are lower than those of high-spin Fe(II) complexes, such as [Fe(EDTA)]2− (EDTA = ethylenediaminetetraacetate) and [Fe(H2O)]2+, but higher than low-spin Fe(II) complexes, such as [Fe(CN)5(H2O)]3− and [Fe(bipyridine)3]2+. The release of NO from the [Fe(TPEN)NO]2+ and [Fe(TPPN)NO]2+ complexes were also studied, showing the values 15.6 and 17.7 s−1, respectively, comparable to the high-spin aminocarboxylate analogs. A mechanism is proposed based on the spin-crossover behavior and the geometry of these complexes and is discussed in the context of previous publications.