Ml. Mader et al., Analogues for the molybdenum center of sulfite oxidase: Oxomolybdenum(V) complexes with three thiolate sulfur donor atoms, INORG CHEM, 39(3), 2000, pp. 525-531
cis,trans- (L-N2S2)(MoO)-O-V(SR) [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercapt
ophenyl)ethylenediamine; R = CH2Ph, CH2CH3, and p-C6H4-Y (Y = CF3, Cl, Br,
F, H, CH3, CH2CH3, and OCH3)] are the first structurally characterized mono
nuclear Mo compounds with three thiolate donors, as occurs at the Mo active
site in sulfite oxidase. X-ray crystal structures of the Cis,trans-(L-N2S2
)(MoO)-O-V(SR) compounds, where R = CH2Ph, CH2CH3, p-C6H4-OCH3, and p-C6H4-
CF3, show a similar coordination geometry about the Mo atom with all three
sulfur thiolate donors in the equatorial plane. This coordination geometry
places two adjacent S p(pi) orbitals parallel to the Mo=O bond, analogous t
o the orientation in the ene-dithiolate ligand in sulfite oxidase; the thir
d S p(pi) orbital lies in the equatorial plane. Charge-transfer transitions
from the S p to the Mo d orbitals occur at approximately 28 000 cm(-1) (ep
silon: 4400-6900 L mol(-1) cm(-1)) and 15 500 cm(-1) (E: 3200-4900 L mol(-1
) cm(-1)).'lhe EPR parameters are nearly identical for all the cis,trans-(L
-N2S2)(MoO)-O-V(SR) compounds (g(1) similar to 2.022, g(2) similar to 1.963
, g(3) similar to 1.956, A(1) similar to 58.4 x 10(-4) cm(-1), A(2) similar
to 23.7 x 10(-4) cm(-1), A(3) similar to 22.3 x 10(-4) cm(-1)) and are typ
ical of an oxo-Mo(V) center coordinated by multiple thiolate donors. The g
and A tensors are related by a 24 degrees rotation about the coincident gz
and Az tensor elements, reflecting the approximate C-s coordination symmetr
y. These EPR parameters more closely mimic those of the low pH form of sulf
ite oxidase and the "very rapid" species of xanthine oxidase than previous
model compounds with two or four thiolate donors. The cis,trans-(L-N2S2)(Mo
O)-O-V(SR) compounds undergo a quasi-reversible, one-electron reduction and
an irreversible oxidation that show a linear dependence upon the Hammett p
arameter, sigma(p), of the Y group. The cis,trans-(L-N2S2)(MoO)-O-V(SR) com
pounds provide a well-defined platform for the systematic investigation of
the electronic structures of the (MoOS3)-O-V centers and their implications
for molybdoenzymes.