Iron complexes capped with dendrimer-appended triazacyclononanes as the novel spatially encumbered models of non-heme iron proteins

Poly(benzyl ether) dendrimers with a 1,4,7-triazacyclononane (TACN) focal core (Ln(3)TACN, 2a-4a) and nondendritic L1(3)TACN (1a), upon reaction with FeCl2, followed by NaOAc and NH4PF6, afforded mononuclear iron(II) complexes [Fe-II(eta(2)-OAc)(Ln(3)TACN)](+) (1b-4b), which were oxidized under O-2 to form dinuclear (mu-O) (mu-OAc)(2)diiron(III) complexes (1c-4c) in 54-74% isolated yields. The formation of 1c-4c obeyed second-order kinetics with respect to 1b-4b, respecitvely, where the observed rate constants (k(2)) were clearly dependent on the generation number of the dendritic substituents. Photoirradiation of 1c-4c in the presence of NaOAc gave diiron(II) complexes (1d-4d), which were reoxidized to 1c-4c by O-2, following first-order kinetics with respect to 1d-4d, respectively. The crystal structure of nondendritic 1c showed that the diiron(III) center is surrounded by an aromatic wall of the six 3,5-dimethoxybenzyl substituents, while spectroscopic profiles of dendritic 2c-4c suggested that the geometries of their diiron(III) centers are little different from that of 1c. The diiron(III) center of the largest 4c was highly robust toward alkaline hydrolysis and also insulated electrochemically.