Tuning the Regiospecificity of Cleavage in FeIII Catecholate Complexes: Tridentate Facial versus Meridional Ligands

Abstract

A facial tridentate ligand is a key feature of iron catecholate complexes that elicit extradiol cleavage of catechols (see picture). The facial geometry allows O2 and the catecholate dianion to form a tridentate intermediate on the opposite face. Accordingly, complexes of a meridional tridentate ligand do not elicit any extradiol cleavage but instead yield quinone or result in intradiol cleavage.

Mononuclear non-heme iron(III) complexes of linear and tripodal tridentate ligands as functional models for catechol dioxygenases: Effect of N-alkyl substitution on regioselectivity and reaction rate

“…Also upon introducing methyl groups at amine N-atom of TACN ligand to obtain Me 3 TACN, a quantitative amount of extradiol product has been observed with any base. 41 This illustrates that the increase in Lewis acidity of the iron(III) center enhances the O 2 attack and that the steric bulk of the NMe 2 group does not discourage it. The second order reaction rate constants (table 5) of complexes [Fe(L)(DBC)(Sol)] + in DCM solvent is 19 > 18 > 17; 14 > 16 ∼ 12 > 13, which is in accordance with the decrease in Lewis acidity of the adducts in the non-coordinating solvent (cf.…”

“…Solomon et al have very recently found that the resting extradiol-cleaving enzymes fail to react with O 2 but become reactive upon substrate binding. 35 Recent growing interest has been shifted towards the extradiol-cleaving catechol dioxygenase and their model complexes and several small molecule analogues [36][37][38][39][40][41][42][43][44] have been isolated and studied to get insight into the reaction mechanism of catechol cleavage. Earlier Funabiki et al 37 reported the first model by using FeCl 2 /FeCl 3 with pyridine/bipyridine mixture.…”

“…Que et al and Bugg et al have studied the reactivity of [Fe(TACN)(DBC)Cl] with molecular oxygen and suggested that facial coordination of tridentate ligand and both mono-and dianionic catecholate binding lead to extradiol cleavage and that the iron-catecholate adduct must contain a vacant coordination site for oxygen binding. [39][40][41] Que et al 41 and Palaniandavar et al 45 have recently reported that tridentate ligands with meridional coordination yields quinone and intradiol cleavage products as the major products and confirmed that facial coordination of tridentate ligands is essential to elicit extradiol cleavage (scheme 3 47 All these studies reveal that the steric and electronic properties of ligands coordinated to iron(III) lead to a dramatic difference in the cleavage preferences between the structurally similar catalysts.…”

“…This observation is very much in contrast to the major amount of the oxidative extradiol cleavage products observed for the iron(III)-DBC 2− adducts of the facially coordinating cyclic tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 TACN). 41 This fascinating result implies that the ancillary tridentate ligand allows the both reactants (catecholate dianion and molecular oxygen) to occupy the opposite face and form a tridentate peroxo intermediate that leads to extradiol cleavage. 41 Since all the present ligands are meridionally coordinated to iron(III) center, only a meridional plane is available for both the reactants and hence the two reactants are unable to form a tridentate peroxo intermediate with the available meridional sites.…”

“…41 This fascinating result implies that the ancillary tridentate ligand allows the both reactants (catecholate dianion and molecular oxygen) to occupy the opposite face and form a tridentate peroxo intermediate that leads to extradiol cleavage. 41 Since all the present ligands are meridionally coordinated to iron(III) center, only a meridional plane is available for both the reactants and hence the two reactants are unable to form a tridentate peroxo intermediate with the available meridional sites. From these observations, it is evident that the tridentate peroxo intermediate is essential to elicit extradiol cleavage such an intermediate is formed when one of faces of the octahedron is available for reactants.…”