Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 172

catecholate complex from which the extradiol oxygenation
products (pyrones: 39 + 28%) were obtained together with the intradiol oxygenation
product (33%), but not with quinone. 120 These results suggested that the presence of
the vacant site, i.e. formation of the five-coordinate catecholatoiron species as an
intermediate, is the more important for the extradiol cleavage oxygenations rather
than the Fe(II) active center. However, the presence of the vacant site is not enough
for explanation of the selectivity control by Fe(II) and Fe(III) centers. It is known
that the five-coordinate intermediate gives selectively the intradiol cleavage product
in the enzymatic system. 121 Lim et al. reported formation of both the intradiol
(75%) and extradiol (15%) cleavage products from the six-coordinate Fe(III)
complex,

122

Recently, Jo and Que reported the importance of the location of the vacant
site for the extradiol oxygenation. 123 They compared reactivities of a series of
Fe(III) catecholate complexes,
containing tridentate ligands that can coordinate to iron in a facial or meridional
fashion ( 1,4,7-trimethyl-1,4,7-triazacyclononane, TPY = 2,2' : 6',2”-
terpyridine, BnBPA = N -benzyl N , N -bis(2-pyridylmethyl)arnine). 123 The complexes
react with in the presence of AgOTf to remove the Cl ligand. The products
obtained indicated that the facial tridentate ligand favours the extradiol cleavage
rather than the intradiol cleavage. In the proposed mechanism, the facial ligand
allows and substrate to occupy the opposite face and form an intermediate that
leads to the desired extradiol products ( vide infra ).