(Catecholato)iron(III) complexes with tetradentate tripodal ligands containing substituted phenol and pyridine units as structural and functional model complexes for the catechol-bound intermediate of intradiol-cleaving catechol dioxygenases

This paper reports the synthesis and structures of (catecholato)iron(III) complexes with tetradentate tripodal ligands (LR′,R″={2-hydroxy-3-R′-5-R″-phenyl-bis(2-pyridylmethyl)amine}) containing substituted phenol and pyridine units: [FeIII(LR′,R″)(DBC)] (1a: R′,R″=H,H; 1b: R′,R″=Me,Me; and 1c: R′,R″=H,Cl, DBC=3,5-di-tert-butylcatecholato). X-ray structure analysis has revealed that the coordination arrangement around the iron atom of 1a is similar to that proposed for the active site of the catechol-bound intermediate of protocatechuate 3,4-dioxygenase (3,4-PCD). The series of complex 1 derivatives has been synthesized by two different methods: (i) reaction of [FeIII(LR′,R″)(acac)]+ (2a: R′,R″=H,H; 2b: R′,R″=Me,Me; and 2c: R′,R″=H,Cl, acac=acetylacetonato) with 1 equiv. of DBC and 1 equiv. of Et3N in N,N-dimethylformamide (abbreviated as DMF), and (ii) reaction of [FeIII(LR′,R″)Cl2] (3a: R′,R″=H,H; 3b: R′,R″=Me,Me; and 3c: R′,R″=H,Cl) with 2 equiv. of AgOTf (OTf=O3SCF3−), 1 equiv. of the catechols, and 2 equiv. of Et3N in DMF. The exogenous acac ligand of 2 acts as a Lewis base like the Tyr447 ligand in the active site of 3,4-PCD in the formation of the catechol-bound intermediate. Complexes 1, 2, and 3 have been characterized by X-ray analysis, visible and EPR spectroscopies, and cyclic voltammetry. Oxygenation of the bound DBC on 1 in the presence of O2 has also been investigated and is discussed based on the Lewis basicity of the tripodal ligand containing the substituted phenolato group which is introduced to mimic the Tyr408 ligand of 3,4-PCD.