A Highly Reactive Functional Model for the Catechol Dioxygenases. Structure and Properties of [Fe(TPA)DBC]BPh₄

[Fe^III(TPA)DBC]BPh₄, a new functional model for the catechol dioxygenases, has been synthesized, where TPA is tris(2-pyridylmethyl)amine and DBC is 3,5-di-tert-butylcatecholate dianion. The TPA complex reacts with 0₂ within minutes to afford intradiol cleavage, in 98% yield, which is the highest conversion observed of all [Fe(L)DBC] complexes studied. More interestingly, the TPA complex is the fastest reacting of all the [Fe(L)DBC] complexes studied. Kinetic studies of the reaction of the complex with 1 atm of O₂ in DMF under pseudo-first-order conditions show that the TPA complex reacts approximately three orders of magnitude faster than the corresponding NTA complex, where NTA is N,N-bis(carboxymethyl)glycine. Both the high specificity and the fast kinetics can be associated with the high Lewis acidity of the ferric center in the TPA complex. To investigate the factors determining reactivity, we have solved the crystal structure of [Fe(TPA)DBC]BPh₄ (space group P1, a = 12.464 (5) Å, b = 13.480 (6) Å, c = 15.980 (8) Å, α = 85.11 (4)°, β = 83.96 (4)°, γ = 70.76 (4)°, V= 2517 (4) ų, Z = 2, R = 0.054 and R_w = 0.063). Compared with other complexes in the [Fe(L)DBC] series, the iron-catecholate interaction in the TPA complex is significantly stronger, resulting in the enhanced covalency of the metal-catecholate bonds and low-energy catecholate LMCT bands. The enhanced covalency is reflected by the isotropic shifts exhibited by the DBC protons, which indicate increased semiquinone character. The greater semiquinone character in the TPA complex correlates well with its high reactivity toward O₂. These trends provide substantial evidence for the substrate activation mechanism proposed for the oxidative cleavage of catechols.