H. Zheng et al., Complexes with Fe-2(III)(mu-O)(mu-OH), Fe-2(III)(mu-O)(2), and [Fe-3(III)(mu(2)-O)(3)] cores: Structures, spectroscopy, and core interconversions, J AM CHEM S, 121(10), 1999, pp. 2226-2235
We have synthesized the first complexes with bis(mu-oxo)diiron(III) and (mu
-oxo)(mu-hydroxo)diiron-(III) cores (1 and 2, L = TPA (a), 5-Et-3-TPA (b),
6-Me-3-TPA (c), 4,6-Me-6-TPA (d), BQPA (e), BPEEN (f), and BPMEN (g)) and f
ound them to have novel structural properties. In particular, the presence
of two single-atom bridges in these complexes constrains the Fe-Fe distance
s to 2.7 - 3.0 Angstrom and the Fe-mu-O-Fe angles to 100 degrees dr smaller
. The significantly acute Fe-O-Fe angles (e.g, 92.5(2)degrees for 1c and 10
0.2(2)degrees for 2f) enforced by the Fe2O2(H) core endow these complexes w
ith UV-vis, Raman, and magnetic properties quite distinct from those of oth
er (mu-oxo)diiron(III) complexes. Complex Ic exhibits visible absorption ba
nds at 470 (epsilon = 560 M-1 cm(-1)) and 760 nm (epsilon approximate to 80
M-1 cm(-1)), while complexes 2 show features at ca. 550 (epsilon approxima
te to 800 M-1 cm(-1)) and ca. 800 nm (epsilon approximate to 70 M-1 cm(-1))
, all of which are red shifted compared to those of other (mu-oxo)-diiron(I
II) complexes. These complexes also exhibit distinct nu(Fe-O-Fe) vibrations
at ca. 600 and ca. 670 cm(-1) assigned to the nu(sym) and the nu(asym) of
the Fe-O-Fe units, respectively. The relative intensities of the nu(sym) an
d nu(asym) bands are affected by the symmetry of the Fe-O-Fe units; an unsy
mmetric core enhances the intensity of the nu(asym). Complexes 2 exhibit an
other band at ca. 500 cm(-1), which is assigned to the Fe-(OH)-Fe stretchin
g mode due to its sensitivity to both (H2O)-O-18 and (H2O)-H-2. Magnetic su
sceptibility studies reveal J = 54 cm(-1) for Ic and ca. 110 cm(-1) for 2 (
H = JS(1).S-2), values smaller than those for the antiferromagnetic interac
tions found in (mu-oxo)diiron(III) complexes. This weakening arises from th
e longer Fe-mu-O bonds and the smaller Fe-mu-O-Fe angles in the Fe2O2(H) di
amond core structure. These spectroscopic signatures can thus serve as usef
ul tools to ascertain the presence of such core structures in metalloenzyme
active sites. These two core structures, Fe-2(mu-O)(2) (1) and Fe-2(mu-O)(
mu-OH) (2), can also be interconverted by protonation equilibria with pK(a)
's of 16-18 in CH3CN. Furthermore, the Fe-2(mu-O)(2) core (1) isomerizes to
the Fe-3(mu(2)-O)(3) core (7), while the Fe-2(mu-O)(mu-OH) core (2) exhibi
ts aquation equilibria to the Fe-2(mu-O)(mu-H3O2) core (5), except for L =
6-Me3-TPA and 4,6-Me-6-TPA. It is clear from these studies that electronic
and steric properties of the ligands significantly affect the various equil
ibria, demonstrating a rich chemistry involving water-derived ligands alone
.