[FeIII(TPA)DBC]BPh4, a new functional model for the catechol dioxygenases, has been synthesized, where is tris(2-pyridylmethyl)amine and DBC is 3,-di-tert-butylcatecholate dianion. The TPA with complex reacts with O2 within minutes to afford intradiol cleavage, in 98% yield, which is the highest convesion obsesrved 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 react appproximately 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 athe 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 P1̄ a = 12.464 (5) ̊, b = 13.480 (6) ̊, c = 15.980 (8) ̊, α = 85.11 (4)°, β =83.96 (4)°, γ ?? 70.76 (4)° V = 2517 (4) ̊3, Z = 2, R = 0.054 and Rw = 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 exibited by the DBC protons, which indicate increased semiquinone character. The greater semiquinone character in the TPA complex correlates well its high reactivity toaward O2. These trends provide substantial evidence for the substrate activitation mechanism proposed for the oxidative cleavage of catechols.
|Number of pages||5|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1991 Dec 1|
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