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

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)ethylenedi-amine, 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 (k(f)) of 8.4 x 10(3) and 9.3 x 10(3) M-1 s(-1), respectively (pH 5.0, 25 degrees 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. k(f) 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.