Trigonal-Bipyramidal Geometry Induced by an External Water Ligand in a Sterically Hindered Iron Salen Complex, Related to the Active Site of Protocatechuate 3,4-Dioxygenase

A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces
the endogenous His2Tyr2 donor set with water as an external ligand. X-ray crystal structures of a series of iron
model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted
trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural
change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies
with the external ligand that is bound in the order Cl ≪ EtO < H2O, which is consistent with the spectrochemical
series. The distortion in the model system is not due to steric repulsions but electronic interactions between the
external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model
complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density
functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal
geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique
distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically
induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by
the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.