The catechol dioxygenase reactivity of iron(III) complexes using tripodal ligands was investigated. Increasing, as well as decreasing, chelate ring sizes in the highly active complex [Fe(tmpa)(dbc)]B(C6H5)(4) (tmpa = tris[(2-pyridyl)methyl]amine; dbc = 3,5-di-tert-butylcatecholate dianion), using related ligands, only resulted in decreased reactivity of the investigated compounds. A detailed low-temperature stopped-flow investigation of the reaction of dioxygen with [Fe(tmpa)(dbc)]B(C6H5)(4) was performed, and activation parameters of Delta H-double dagger = 23 +/- 1 U mol(-1) and Delta S-double dagger = -199 +/- 4 J mol(-1) K-1 were obtained. Crystal structures of bromo-(tetrachlorocatecholato-O,O')(bis((2-pyridyl)methyl)-2-pyridylamin e-N,N',N'')-iron(III), (mu-oxo)-bis(bromo)(bis((2-pyridyl)methyl)-2-pyridylamine-N,N',N'',N'")-diiron(111), dichloro-((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N',N'',N'")-iron(111) and (tetrachlorocatecholato-O,O')((2(2-pyridyl) ethyl) bis((2-pyridyl) methyl)amine-N,N', N'', N'") -iron (I 11) are reported.