A R T I C L E S
Piskorski and Jaun
formal Ni(III) valence state if at least one strong donor ligand
occupying an axial position would induce a change of the
compartments between the working electrode and counterelectrode
compartments was used in order to minimize diffusion between the
counterelectrode and working electrode (volume of each compartment
ca. 2.0-2.5 mL, D5-glass fritted disks as diaphragms). The working
electrode for bulk electrolysis and the counterelectrode were semicir-
cular platinum grid electrodes (ca. 8 × 10 mm, 0.5 mm mesh); an
additional small Pt wire-tip electrode (3 mm, ø ) 0.3 mm) was used
for the CV experiments. A silver wire coated with AgCl (anodized in
2
2
2
ground-state configuration from dz to dx -y . Extended X-ray
absorption fine structure (EXAFS) results25 and the generation
24
of MCRox1 by low temperature γ-irradiation from MCRox1-silent,
which has the thiol(ate) sulfur of coenzyme M coordinated in
the axial position, indicate that this axial ligand would have to
be coordinating to the nickel center via sulfur.
0
.1 M HCl) inside a Luggin capillary served as the quasi reference
electrode.
Bulk Electrolysis. TBAPF
Experimental Section
6
(0.774 g) was dissolved in acetonitrile
II
1
. Materials. Ni F430M‚ClO
4
(2) was prepared from coenzyme
or propionitrile (20 mL) freshly filtered over activated alumina. The
electrolyte solution was degassed by three freeze-pump-thaw cycles
and transferred under vacuum into the drybox. Inside the drybox,
F430M (2.0 µmol) was dissolved in the electrolyte to give a 1.0 mM
solution, the electrolysis cell was filled with electrolyte solution and,
in the working electrode compartment, with F430M solution, and the
cyclic voltammogram was recorded with the Pt-wire working electrode
(conditions for CV as given in the caption of Figure 1). Bulk electrolysis
was carried out with vigorous stirring by a magnetic stir bar inside the
working electrode compartment. The potentials were set as given in
Table 1, and electrolysis was continued until the current had dropped
to 1% of its initial value. The net charge transferred was determined
by current integration and was corrected by subtraction of the
background charge transfer as determined by a second experiment with
pure electrolyte solution under otherwise identical conditions (always
F430 that had been extracted from cells of Methanothermobacter
1
marburgensis as described earlier. Traces of halogenated solvents
I
(
which react very rapidly with Ni F430M) and of water were eliminated
from F430M samples by 3-fold precipitation from dry THF with dry
-
3
toluene. The sample was then dried at 10 mbar overnight. All
II
experiments were done with material from a single batch of Ni F430M‚
4
ClO . The concentrations and mole equivalents for 2 are based on
1
photometry at 430 nm in methanol (ꢀ ) 22 000). This molar extinction
coefficient was redetermined with the batch used in our experiments
by determination of the nickel content with ICP-MS.
Tetrabutylammonium hexafluorophosphate (TBAPF
6
, Fluka) was
triply recrystallized from EtOAc/EtOH (2:1), dried at rt for 3 days at
-3
10
mbar, and dried again overnight before use. (Cp*)
2 6
CoPF (Aldrich)
was used without further purification. Methylene blue (Fluka) was dried
-
3
II
at rt and 10 mbar. Its extinction coefficient in DMF was determined
<10% of the charge transferred to Ni F430M). After electrolysis,
as ꢀ ) 73 500 at λmax ) 665 nm. Alumina (alumina N, Super I; ICN
aliquots of the solution in the working electrode compartment were
transferred by a gastight syringe to a UV-vis cell (l ) 0.03 cm) and
two EPR tubes, which were capped with septum caps and taken out of
-3
Biomedicals) was activated for 24 h at 150 °C and 10 mbar and stored
under a nitrogen atmosphere. THF was triply distilled under nitrogen
from potassium and then degassed by three freeze-pump-thaw cycles.
DMF was freshly distilled under vacuum at ca. 10 mbar and 40 °C
over a fractionating column (110 cm) packed with glass beads, using
a reflux ratio of 10:1 and taking the middle 30%, degassed by three
freeze-thaw cycles, and stored in a drybox (Vacuum Atmospheres).
All other chemicals were obtained from Fluka and were used without
further purification.
the drybox where the EPR tubes were immediately frozen in liquid N
and the UV-vis and EPR spectra were recorded. X-Band EPR spectra
were taken on a Bruker EMX with N flow cryostat set to 130 K.
2
2
Conditions: see caption to Figure 4. The g-values were measured with
a NMR gaussmeter and calibrated by co-measurement of solid DPPH
(g ) 2.0037) sealed in a quartz capillary (0.5 mm) that was attached
to the outside of the EPR tube.
II
NaHg Amalgam 0.87% (w/w). Sodium (120 mg) was added to
mercury (1 mL, 13.6 g) placed in a 50 mL flask under nitrogen. The
piece of sodium was dipped below the mercury surface, whereby the
amalgam formed spontaneously.
ZnHg amalgam 15% and 1.5% (w/w). In a two-necked flask a
mixture of mercury (ca. 9 g, 0.67 mL, 45 mmol) and zinc powder (1.3
g, 20 mmol) was covered with 5% acetic acid (ca. 5 mL) and heated
to the boiling point under nitrogen while the flask was shaken manually
until the metal mixture had become homogeneous. After cooling to rt
the amalgam solidified. The aqueous phase was removed and the
amalgam was rinsed several times with water and then stored under
3. Spectroelectrochemistry of the reduction of Ni F430M (2) was
measured in a home-built two-compartment spectroelectrochemistry cell
(Suprasil quartz, l ) 0.03 cm) with a fine Pt mesh electrode (optical
density ) ca. 0.11). The reference cell contained pure electrolyte and
an identical Pt mesh. The counterelectrode compartment was separated
from the working electrode compartment by a D4-fritted glass disk.
The counterelectrode was a Pt wire; the quasi-reference electrode was
Ag/AgCl (0.1 mm, isolated by a Teflon capillary) reaching to the bottom
of the UV-vis cell. The BAS 100W potentiostat was used for stepwise
6
electrolysis. The cell was dried in an oven before use. TBAPF (0.387
-
3
g) was dried at 10 mbar overnight, flushed with nitrogen, and
dissolved in acetonitrile (10 mL) freshly filtered over activated alumina
inside a glovebox. The cell was mounted in the glovebox, closed with
septum caps, flushed with argon, filled with electrolyte and F430M
(2) (ca. 1 µmol) solutions, and degassed with a stream of argon through
a fine needle for 10 min. With the cell in the optical path of the
spectrophotometer, electrolysis was started at a potential of -1.275 V
and then the potential was decreased stepwise to -1.48 V. The reduction
was followed by means of UV-vis spectroscopy.
2
% acetic acid. Liquid 1.5% ZnHg was prepared from 15% ZnHg by
dilution immediately before use. To 10 g of 15% ZnHg covered with
% acetic acid mercury (90 g, 6.7 mL) was added and the mixture was
2
treated as described for 15% ZnHg. The liquid ZnHg was washed
several times with water, then with methanol, and dried in a stream of
nitrogen.
UV-vis spectroscopy was performed on a Perkin-Elmer λ 20
spectrophotometer.
II
II
2
. Cyclic Voltammetry and Bulk Electrolysis of Ni F430M. All
4. Stoichiometry of the Reduction of Ni F430M (2) by Deca-
methylcobaltocene in THF. All reactions were carried out in a drybox
+
potentials are given vs the ferrocenium/ferrocene (Fc /Fc) couple
measured in situ together with the analyte.
3
3
(O
2
2 6
< 8 ppm). Experiment 1: (Cp*) CoPF (1.255 mg, 2.645 µmol)
Apparatus. A BAS 100W (Bioanalytical Systems) electrochemistry
system, with cyclic voltammetry (CV) and bulk electrolysis modes,
was dissolved in dry THF (5.00 mL), NaHg amalgam (0.87%, ca. 0.5
mL) was added and the mixture was stirred for 3 h. Ni F430M (0.425
II
was used. The cell was inside a drybox (O
cables connected via a plug in the wall of the drybox to the external
2
< 4 ppm) with the electrode
mg, 0.386 µmol) was dissolved in dry THF (1.30 mL) and 150 µL
(0.44 pmol) of the solution was placed in every one of three UV-vis
potentiostat. A four-compartment electrolysis cell with two bridging
cells, to which 150 µL of the obtained (Cp*)
solution was added. Experiment 2: (Cp*) CoPF
was dissolved in dry THF (6.50 mL), NaHg amalgam (0.87%, ca. 0.5
2
Co (0.79 pmol, 1.8 equiv)
2
6
(0.710 mg, 1.50 µmol)
(33) Gritzner, G.; K uˆ ta, J. Pure Appl. Chem. 1984, 56, 462-466.
1
3124 J. AM. CHEM. SOC. VOL. 125, NO. 43, 2003
9
Piskorski, Rafal
