P. Mialane et al., Aminopyridine iron catecholate complexes as models for intradiol catechol dioxygenases. Synthesis, structure, reactivity, and spectroscopic studies, INORG CHEM, 39(12), 2000, pp. 2440-2444
Four new Fe(III) catecholate complexes, [(bispicMe(2)en)Fe-III(DBC)](+), [(
bispicCl(2)Me(2)en)Fe-III(DBC)](+), [(trispic-Meen)Fe-III(DBC)](+), and [(B
QPA)Fe-III(DBC)](+), which all contain aminopyridine ligands, were synthesi
zed. The structure of [(bispicMe(2)en)Fe-III(DBC)](+) was determined by X-r
ay diffraction. It crystallizes in the triclinic space group P(1)over bar w
ith a = 10.666(3) Angstrom, b = 13.367(5) Angstrom, c = 17.685(2) Angstrom,
alpha = 93.46(2)degrees, beta = 93.68(2)degrees, gamma = 109.0-(3)degrees,
V = 2387.4 Angstrom(3), and Z = 2. All of these complexes were found to be
active toward oxidation of catechol by O-2 in DMF at 20 degrees C to affor
d intradiol cleavage products. The catechol was quantitatively oxidized, ma
inly (90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction ra
tes were measured, and for the first three (topologically similar) complexe
s, a correlation of the second-order kinetic constants k with the optical p
arameters of the two LMCT O(DBC) --> Fe(III) bands was found. In particular
, k increases with the epsilon(max) of the charge-transfer bands. The k val
ue of the complex [(BQPA)Fe-III(DBC)](+), containing a tripodal ligand, is
smaller than expected on the basis of these correlations. This discrepancy
could be related to steric hindrance induced by the BQPA ligand. However, t
he much lower activity of the bispicen-Fe(III)-type complexes compared to t
hat of the [(TPA)Fe-III(DBC)](+) complex synthesized by Jang et al. (J. Am.
Chern. Sec. 1991, 113, 9200-9204), despite similar epsilon(max) values, sh
ows that a knowledge of optical and NMR parameters values is not sufficient
to explain the dioxygenase activity rate, in their study of protocatechuat
e 3,4-dioxygenase, Orville et al. (Biochemistry 1997, 36, 10052-10066) sugg
ested that asymmetric chelation of the catecholate to Fe(III) is of great i
mportance in the efficiency of the intradiol dioxygenase reaction. Indeed,
a comparison of the X-ray structures of [(TPA)Fe-III-(DBC)](+) and [(bispic
Me(2)en)Fe-III(DBC)](+) shows that the Fe(III)-O bonds differ by 0.019 Angs
trom in the former and are identical in the latter. Asymmetry could also pl
ay a role in the model complexes. An alternative explanation is the possibl
e existence of a low-spin state for [(TPA)Fe-III(DBC)](+), as recently iden
tified in [(TPA)Fe-III(cat)](+) by Simaan et al.