The complex
[Et4N]2[WVIO2(mnt)2]
(1),
[Et4N]2[WIVO(mnt)2]
(2), and
[Et4N]2[WVIO(S2)(mnt)2]
(3) (mnt2-
= 1,2-dicyanoethylenedithiolate) have been synthesized as possible
models for the tungsten cofactor of inactive red
tungsten protein (RTP) and the active aldehyde ferredoxin
oxidoreductase (AOR) of the hyperthermophilic archaeon
Pyrococcus furiosus. The
[Ph4P]+ salt of the complex anion of
1·2H2O crystallizes in space group
Pbcn, with a =
20.526(3) Å, b = 15.791(3) Å, c =
17.641(3) Å, and Z = 4. The
WVIO2S4 core of
[Ph4P]2[WVIO2(mnt)2]·2H2O
has
distorted octahedral geometry with cis dioxo groups.
2 crystallizes in space group
P21212, with a =
14.78(3) Å, b
= 30.08(2) Å, c = 7.37(4) Å, and
Z = 4. The complex anion of 2 has a
distorted square-pyramidal structure with
an axial WO bond. 3 crystallizes in space group
P21/a, with a =
12.238(3) Å, b = 18.873(2) Å, c =
15.026(2)
Å, β = 102.84(2)°, and Z = 4. The anion
of 3 with a terminal oxo group and a dihapto
disulfido ligand in an
adjacent position is the first example of a seven-coordinate W(VI)
species with bis-dithiolene coordination. The
complexes 1−3 have been characterized by IR,
UV−visible, 13C NMR, negative ion FAB mass spectra,
and
electrochemical properties. Complex 1 reacts with
H2S, PhSH, 1,4-dithiothreitol (DTT), or dithionite
(S2O4
2-) to
yield 2 with the oxidation of these reducing agents
suggesting intramolecular electron transfer in the
respective
intermediates across the W(VI)−sulfur bond. Participation of
this type of redox reaction, seemingly unrealistic
from the point of view of real reduction potential values of
1 and of these reductants, is best explained by
the
formation of a precursor complex. This relates to the essential
formation of a Michaelis (enzyme−substrate) complex
wherein the individual chemical identity of the free enzyme and unbound
substrate is lost. Subsequent atom transfer
reaction embodies internal electron transfer between the two redox
partners present in the enzyme−substrate complex.
The terminal oxo group of 2 is readily protonated (pH
< 4) to yield
[WIV(mnt)3]2-.
2 responds to a metal exchange
reaction with MoO4
2- to form
[MoIVO(mnt)2]2- which is
similar to in vitro reconstitution of the molybdenum
cofactor
by MoO4
2- in tungsten formate dehydrogenase
(W-FDH). The model reaction between 2 and
MoO4
2- involves a
stepwise one-electron transfer reaction from W(IV) to Mo(VI) with
the intermediate formation of EPR active W(V)
species. Oxidative addition of elemental sulfur from 2
affords 3, which gives sulfur atom transfer reactions
with
several thiophiles. 3 reacts with Ph3P in a
second-order process (A + 2B type) to yield 2 and
Ph3PS with the
observed rate constant k
2 = 4.3 (± 0.06)
M-1 s-1 at 25 °C
(ΔH
⧧ = 5.14 (± 0.46) kcal/mol,
ΔS
⧧ = −38.35 (± 1.5)
cal/(deg·mol)). A cyclic voltammetric study suggests the
attack of Ph3P across the W−S bond in the WS2
moiety
of 3. 2 catalyzes the reactions
Ph3P + S → Ph3PS and H2
+ S → H2S, demonstrating its sulfur reductase
activity.
No such reaction is observed in the absence of 2.
Fo...