Voltammetric, specular reflectance infrared, and X-ray electron probe characterization of redox and isomerization processes associated with the [Mn(CO)2(η3-P2P′)Br]+/0 (P2P′ = {Ph2P(CH2)2}2PPh), [Mn(CO)2(η3-P3P′)Br]+/0

Article abstract of DOI:10.1021/om970468j

Extremely well defined voltammetric responses are obtained for both oxidation of microcrystalline mononuclear trans- and cis,mer-Mn(CO)₂(η³-P₂P′)Br (P₂P′ = {Ph₂P(CH₂)₂}₂-PPh) and cis,mer-Mn(CO)₂(η³-P₃P′)Br (P₃P′ = {Ph₂PCH₂}₃P) and binuclear cis,fac-{Mn(CO)₂-(η²-dpe)Br}₂(μ-dpe) (dpe = Ph₂P(CH₂)₂PPh₂) and reduction of cationic trans-[Mn(CO)₂(η³-P₂P′)Br]BF ₄, trans-[Mn(CO)₂(η³-P₃P′)Br]BF ₄, and trans-[{Mn(CO)₂(η²-dpe)Br} ₂(μ-dpe)](BF₄)₂ when they are attached to a graphite electrode and placed in water containing either 0.1 M NaCl or KCl as the electrolyte. The combination of access to species in different oxidation states and different isomeric forms, as well as mononuclear and binuclear species, enables the rates of isomerization and the extent of electronic communication between the metal centers to be evaluated in the solid state and compared to data in organic solvent systems previously reported. The voltammetric data, combined with specular reflectance IR and X-ray electron probe data, established that the following processes occur at the graphite electrode-microcrystal-water (electrolyte) interface (subscript s denotes solid): trans-Mn(CO)₂(η³-P₂P′)Br_s + Cl^- ? trans-[Mn(CO)₂(η³-P₂P′)Br]Cl _s + e^-; cis,mer-Mn(CO)₂(η³-P₂P′)-Br _s + Cl^- ? cis,mer-[Mn(CO)₂(η³-P₂P′)Br]Cl _s + e^- → trans-[Mn(CO)₂(η³-P₂P′)Br]Cl _s; trans-Mn(CO)₂(η³-P₃P′)Br_s + Cl^- ? trans-[Mn(CO)₂(η³-P₃P′)Br]Cl _s + e^-; cis,mer-Mn(CO)₂(η³-P₃P′)Br]Cl _s + Cl^- ? cis,mer-[Mn(CO)₂(η³-P₃P′)Br]Cl _s + e^- → trans-[Mn(CO)₂(η³-P₃P′)Br]Cl _s; trans-{Mn-(CO)₂(η²-dpe)Br}₂(μ-dpe) _s + 2Cl^- ? trans-[{Mn(CO)₂(η²-dpe)Br} ₂(μ-dpe)]Cl_2s+ 2^e-; cis,fac-{Mn(CO)₂(η²-dpe)Br}₂(μ-dpe) _s + 2Cl^- ? cis,fac-[{Mn(CO)₂(η²-dpe)Br} ₂(μ-dpe)]Cl_2s + 2e^- → irans-[{Mn(CO)₂(η²-dpe)Br} ₂(μ-dpe)]Cl_{2 s}. The reaction pathways in organic solvents are generally analogous. However, the rates of isomerization are slower in the solid state, and shapes of voltammograms and potentials differ significantly. Interestingly, in the binuclear [{Mn(CO)₂(η²-dpe)Br}₂(μ-dpe)] ^2+/0 system, no intermediate [{Mn(CO)₂(η²-dpe)Br}₂(μ-dpe)] ⁺ species are observed in the solid state, implying that the metal centers are oxidized or reduced at the same potentials, unlike the case in the solution phase, where the [{Mn(CO)₂(η²-dpe)-Br}₂(μ-dpe)] ^2+/+ and [{Mn(CO)₂(η²-dpe)Br}₂(μ-dpe)] ^+/0 redox couples are well separated. This result implies that no significant communication occurs between the metal centers in the solid state redox processes.

Full text of DOI:10.1021/om970468j

5006
Organometallics 1997, 16, 5006-5014
Volta m m etr ic, Sp ecu la r Reflecta n ce In fr a r ed , a n d X-r a y
Electr on P r obe Ch a r a cter iza tion of Red ox a n d
Isom er iza tion P r ocesses Associa ted w ith th e
[Mn (CO)2(η3-P 2P ′)Br ]+/0 (P 2P ′ ) {P h 2P (CH2)2}2P P h ),
[Mn (CO)2(η3-P 3P ′)Br ]+/0 (P 3P ′ ) {P h 2P CH2}3P ), a n d
[{Mn (CO)2(η2-d p e)Br }2(µ-d p e)]2+/0 (d p e ) P h 2P (CH2)2P P h 2)
Solid Sta te System s
Alan M. Bond,*,1 Ray Colton,1 Frank Marken,2 and J acky N. Walter3
Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia,
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road,
Oxford OX1 3QZ, U.K., and School of Chemistry, La Trobe University,
Bundoora, Victoria 3083, Australia
Received J une 5, 1997X
Extremely well defined voltammetric responses are obtained for both oxidation of
microcrystalline mononuclear trans- and cis,mer-Mn(CO)2(η3-P2P′)Br (P2P′ ) {Ph2P(CH2)2}2-
PPh) and cis,mer-Mn(CO)2(η3-P3P′)Br (P3P′ ) {Ph2PCH2}3P) and binuclear cis,fac-{Mn(CO)2-
(η2-dpe)Br}2(µ-dpe) (dpe ) Ph2P(CH2)2PPh2) and reduction of cationic trans-[Mn(CO)2(η3-
P2P′)Br]BF4, trans-[Mn(CO)2(η3-P3P′)Br]BF4, and trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2
when they are attached to a graphite electrode and placed in water containing either 0.1 M
NaCl or KCl as the electrolyte. The combination of access to species in different oxidation
states and different isomeric forms, as well as mononuclear and binuclear species, enables
the rates of isomerization and the extent of electronic communication between the metal
centers to be evaluated in the solid state and compared to data in organic solvent systems
previously reported. The voltammetric data, combined with specular reflectance IR and
X-ray electron probe data, established that the following processes occur at the graphite
electrode-microcrystal-water (electrolyte) interface (subscript “s” denotes solid): trans-
Mn(CO)2(η3-P2P′)Brs + Cl- h trans-[Mn(CO)2(η3-P2P′)Br]Cls + e-; cis,mer-Mn(CO)2(η3-P2P′)-
Brs + Cl- h cis,mer-[Mn(CO)2(η3-P2P′)Br]Cls + e- f trans-[Mn(CO)2(η3-P2P′)Br]Cls; trans-
Mn(CO)2(η3-P3P′)Brs + Cl- h trans-[Mn(CO)2(η3-P3P′)Br]Cls + e-; cis,mer-Mn(CO)2(η3-P3P′)Brs
+ Cl- h cis,mer-[Mn(CO)2(η3-P3P′)Br]Cls + e- f trans-[Mn(CO)2(η3-P3P′)Br]Cls; trans-{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe)s + 2Cl- h trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s + 2e-; cis,fac-
{Mn(CO)2(η2-dpe)Br}2(µ-dpe)s + 2Cl- h cis,fac-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s + 2e- f
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s. The reaction pathways in organic solvents are
generally analogous. However, the rates of isomerization are slower in the solid state, and
shapes of voltammograms and potentials differ significantly. Interestingly, in the binuclear
[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+/0 system, no intermediate [{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]+
species are observed in the solid state, implying that the metal centers are oxidized or reduced
at the same potentials, unlike the case in the solution phase, where the [{Mn(CO)2(η2-dpe)-
Br}2(µ-dpe)]2+/+ and [{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]+/0 redox couples are well separated. This
result implies that no significant communication occurs between the metal centers in the
solid state redox processes.
In tr od u ction
they are attached to an electrode as well-spaced micro-
crystals (size < 10 µm). For example, the microcrystals
may be mechanically attached to graphite electrodes
which are then placed in a solvent (electrolyte) medium
in which the compound is insoluble. The redox pro-
cesses can then take place at the electrode-microcrys-
tal-solvent (electrolyte) boundary, with charge neu-
tralization achieved, if necessary, via transport of the
appropriate electrolyte ion present in the solution to the
crystal or from the crystal to the solvent.
Electrochemical studies on redox-active nonconduct-
ing inorganic and organometallic compounds are rare.
However, recently it has been shown4 that voltammetric
studies on nonconducting solids are possible, provided
X Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) Monash University.
(2) Oxford University.
(3) La Trobe University.
(4) (a) Scholz, F.; Lange, B. Trends Anal. Chem. 1992, 11, 359. (b)
Scholz, F.; Meyer, B. Chem. Soc. Rev. 1994, 23, 341. (c) Bond, A. M.;
Colton, R.; Daniels, F.; Fernando, D. R.; Marken, F.; Nagaosa, Y.; Van
Stevenick, R. F. M.; Walter, J . N. J . Am. Chem. Soc. 1993, 115, 9556.
(d) Bond, A. M.; Colton, R.; Marken, F.; Walter, J . N. Organometallics
1994, 13, 5122. (e) Bond, A. M.; Marken, F. J . Electroanal. Chem.
Interfacial Electrochem. 1994, 372, 125.
In the present study, this new technique for studying
the redox properties of organometallic solids has been
applied to oxidation of microcrystals of neutral cis,mer-
and trans-Mn(CO)2(η3-P2P′)Br (P2P′ ) {Ph2P(CH2)2}2-
S0276-7333(97)00468-8 CCC: $14.00 © 1997 American Chemical Society

Redox Properties of Organometallic Solids
Organometallics, Vol. 16, No. 23, 1997 5007
PPh) and the analogous Mn(CO)2(η3-P3P′)Br compounds
(P3P′ ) {Ph2PCH2}3P), together with cis,fac-{Mn(CO)2-
(η2-dpe)Br}2(µ-dpe) (dpe ) Ph2P(CH2)2PPh2), and the
reduction of trans-[Mn(CO)2(η3-P2P′)Br]BF4, trans-[Mn-
(CO)2(η3-P3P′)Br]BF4, and trans-[{Mn(CO)2(η2-dpe)Br}2-
(µ-dpe)](BF4)2. This series of compounds was chosen to
include examples of mononuclear and binuclear systems
where isomerizations in the solid state can occur.
Additionally, since the binuclear complexes have two
metal centers which can be oxidized, the separation of
potential for their oxidation provides information on the
extent of communication between the metal centers in
the solid state.
In order to make the required measurements, the
crystals were mechanically attached to graphite elec-
trodes which were then placed in water containing 0.1
M NaCl or KCl as the electrolyte. The redox reaction
of the water-insoluble compound was then monitored
by voltammetry, specular reflectance IR spectroscopy,
and also X-ray electron probe techniques. Access to both
oxidation states of mononuclear [Mn(CO)2(η3-P2P′)Br]+/0
and [Mn(CO)2(η3-P3P′)Br]+/0 and the binuclear [{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe)]2+/0 systems enables detailed
voltammetric studies to be undertaken in both the
oxidation and reduction directions, while the IR mea-
surements enable stereochemical changes (geometrical
isomerizations) to be detected. The X-ray electron probe
method enables detection of K+ or Cl-, if these ions are
incorporated into the structures as part of the charge
neutralization process.
Solids (1-3 mg) were placed on a coarse grade filter paper,
and the electrode was pressed onto the solid and rubbed over
the material, so some adhered to the electrode surface as an
array of microcrystalline particles of 0.1-10 µm size. For
electrochemical measurements, the electrode was transferred
into the electrochemical cell containing aqueous electrolyte
solution. The electrode surface could be renewed after mea-
surements by dissolving the solid with dichloromethane.
Sp ecu la r Reflecta n ce F T-IR Sp ectr oscop y. The elec-
trode surface with adhered solid exhibits a shiny surface which
is an excellent reflector of light. A specular reflectance system
(angle of incidence 30°) mounted in a Perkin-Elmer FT-IR 1720
spectrometer gave strong and reproducible signals, which
correspond closely with absorption bands observed in solution
spectra. Electrodes were rinsed in water after electrochemical
experiments and dried in air before spectra were acquired.
Typically, 10-20 scans were sufficient when good background
subtraction could be achieved.
X-r ay Electr on P r obe An alysis. Surface elemental analy-
ses by electron probe X-ray measurements were carried out
on a scanning electron microscope (J SM 840) coupled to a TN
5500 X-ray analyzer at an accelerating voltage of 15 kV, with
a probe monitor current ranging from 0.3 to 0.6 nA. A dead
time of 25-30% is present under these conditions for a
spectrum collection time of 100 s. Samples for analysis were
prepared by rubbing a freshly cleaved basal plane carbon
graphite plate (5 mm × 5 mm × 1 mm) onto a small amount
of the carbonyl compound on filter paper. After electrolysis
at the required potential for 120 s in the aqueous (electrolyte)
solution, the plate was transferred to a bath of distilled water
and then dried in air. The carbon plates were attached to the
sample holder with conducting epoxy resin.
Sca n n in g Electr on Micr oscop y. An ETEC Autoscan
system (20 kV accelerator voltage) was used for scanning
electron microscopy measurements. Sample preparation was
similar to that described for the electron probe analyzer. The
surface-modified carbon plate was fixed with double-sided tape
on a stub and gold-plated in a Balzers sputter-coating unit.
Finally, it is of considerable interest to compare
results with solution phase voltammetric studies
which have been conducted in dichloromethane (0.1 M
Bu4NPF6),5-7 since intuitively it might be expected that
isomerization rates and communication between the
metal centers would be significantly different in solid
and solution phases.
Resu lts a n d Discu ssion
tr a n s- an d cis,mer -Mn (CO)2(η3-P 2P ′)Br an d tr a n s-
[Mn (CO)2(η3-P 2P ′)Br ]BF4. (a) Electr och em ical Stu d-
ies. Except for the fact that peak current magnitudes
varied according to the amount of solid on the graphite
electrode surface, voltammograms showed similar char-
acteristics for all experiments. The electrochemical data
are summarized in Table 1, and peak potentials are
referenced against Ag/AgCl (3 M NaCl). The geometries
of the compounds are shown in structures I and II.
Exp er im en ta l Section
Syn th esis. The compounds cis,mer-Mn(CO)2(η3-P2P′)Br
and trans-[Mn(CO)2(η3-P2P′)Br]BF4,5 the analogous η3-P3P′
complexes,6 and the binuclear species cis,fac-{Mn(CO)2(η2-dpe)-
Br}2(µ-dpe)7 were prepared as described previously by interac-
tion of Mn(CO)5Br with the appropriate ligand. trans-
[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2 has previously been char-
acterized in solution,7 and it was isolated by oxidizing cis,fac-
{Mn(CO)2(η2-dpe)Br}2(µ-dpe) with NOBF4 in dichloromethane
solution at room temperature, removing the solvent under
vacuum, and recrystallizing the resulting solid from dichlo-
romethane/hexane.
Rea gen ts a n d Volta m m etr ic In str u m en ta tion . All
reagents were of analytical or electrochemical grade purity.
Water from a Millipore system was used for the preparation
of aqueous electrolyte solution. The reference electrode used
was a Ag/AgCl (3 M NaCl) electrode, and the auxiliary
electrode was made from a platinum sheet. Voltammetric
experiments were performed with a BAS 100 A electrochemical
analyzer (Bioanalytical Systems, West Lafayette, IN). All
solutions were deaerated with high-purity nitrogen for 15 min
prior to making the measurements.
3
3
(II)
cis,mer-Mn(CO) (η -P P')Br
-Mn(CO) (η -P P')Br
trans
(I)
2
2
2
2
A cyclic voltammogram (scan rate, 200 mV s-1) for
the oxidation of trans-Mn(CO)2(η3-P2P′)Br mechanically
attached to a graphite electrode and immersed in
aqueous solution (0.1 M NaCl) at 20 °C (Figure 1a)
shows an oxidation response at 0.46 V. This process
corresponds to that observed in solution,5 except that
Cl- ions are incorporated into the solid to achieve charge
neutrality (see X-ray electron probe data later), and is
The carbon disk electrodes were made from basal plane
pyrolytic graphite (5 mm diameter) fitted into a Teflon holder.
(5) Bond, A. M.; Colton, R.; Walter, J . N. Inorg. Chem. 1997, 36,
1181.
(6) Bond, A. M.; Colton, R.; Walter, J . N. Inorg. Chem., submitted
for publication.
(7) Bond, A. M.; Colton, R.; Walter, J . N. Inorg. Chem., submitted
for publication.

5008 Organometallics, Vol. 16, No. 23, 1997
Bond et al.
Ta ble 1. Volta m m etr ic Da ta (Sca n Ra te,
200 m V s-1) for Solid tr a n s- a n d
cis,m er -Mn (CO)2(η3-P 2P ′)Br a n d
tr a n s-[Mn (CO)2(η3-P 2P ′)Br ]BF 4 Mech a n ica lly
Atta ch ed to Gr a p h ite Electr od es a n d P la ced in
Aqu eou s Solu tion (0.1 M Na Cl)a
potential
(V vs Ag/AgCl)
∆Ep
(mV)
temp
(°C)
ox
red
compound
process
Ep
Ep
Er
1/2
trans0 b
1a,a′
1a,a′
1a,b′
1a,b′
1a,b′
1a,a′
1a,a′
1a,a′
1a,b′
1a,a′
1a,b′
1a,a′
1a,b′
1a,b′
1a,a′
1a,b′
1a,a′
1a,a′
1a,b′
1a,a′
1a,b′
1a,a′
1a,b′
1a,a′
2
0.46
0.42
0.42
0.41
0.36
0.34
0.32
0.32
0.32
0.31
0.31
0.29
0.29
0.29
0.29
0.29
0.29
0.29
0.29
0.30
0.30
0.30
0.30
0.31
0.73
0.81
0.08
0.10
0.16
0.17
0.20
0.06
0.08
0.09
0.16
0.11
0.20
0.10
0.17
0.19
0.11
0.20
0.06
0.06
0.16
0.07
0.18
0.10
0.19
0.11
0.61
0.72
0.27
0.26
0.19
0.29
0.28
0.20
0.20
0.21
0.24
0.21
0.26
0.20
0.23
0.24
0.20
0.25
0.18
0.18
0.23
0.19
0.24
0.20
0.25
0.21
0.67
0.77
380
320
260
240
160
280
240
230
160
200
110
190
120
100
180
90
230
230
130
230
120
200
110
200
120
90
20
30
30
40
50
20
30
40
40
50
50
20
30
40
50
50
20
30
30
40
40
50
50
50
50
50
trans0 c
trans+ b
trans+ c
cis,mer0 d
3
a
Epox, oxidation peak potential; Epred, reduction peak potential;
Er1/2, reversible redox potential; ∆Ep, peak separation. Potentials
b
for first scan. c Potentials after several scans. Potentials are given
d
F igu r e 1. Cyclic voltammograms (first, second, and fourth
scans; scan rate, 200 mV s-1) of solid trans-Mn(CO)2(η3-
P2P′)Br mechanically attached to graphite electrodes and
placed in aqueous solution (0.1 M NaCl) at (a) 20, (b) 30,
and (c) 40 °C.
for second scan as initial oxidation response for process 3 merges
with the background for first scan.
shown in eq 1, where the subscript “s” indicates a solid
compound. The reverse scan reveals a reduction re-
apparent upon repetitive cycling of the potential, while
process 1a′ gradually becomes the dominant reduction
response. A cyclic voltammogram obtained at 40 °C
(Figure 1c) indicates that process 1b′ is much more
prominent than process 1a′ on the first cycle, but
repetitive cycling of the potential shows the gradual
growth of process 1a′. At 50 °C, process 1b′ is the
dominant reduction process, while process 1a′ is barely
observed on the first cycle but becomes more obvious
after several cycles. As the temperature is increased,
the potential difference between the oxidation response
on the first and successive scans decreases, as does the
peak-to-peak separations (∆Ep) for processes 1a,a′ and
1a,b′. Peak splittings, such as that observed for this
system, have been reported for other complexes4b,c which
for the trans-[Mn(CO)2(η3-P2P′)Br]+/0 system are at-
tributed to formation of different phases rather than
new species. IR data given below support this conclu-
sion.
A reductive cyclic voltammogram (scan rate, 200 mV
s-1) of solid trans-[Mn(CO)2(η3-P2P′)Br]BF4 attached to
a graphite electrode and recorded at 20 °C (Figure 2a)
shows on the first cycle only a single reduction process
(process 1a′) and a single oxidation wave (process 1a).
Subsequent cycles show a small shift toward a less
positive potential for the reduction process 1a′, while
the oxidation response appears virtually unaltered. This
contrasts with the voltammograms for the oxidation of
trans-Mn(CO)2(η3-P2P′)Brs + Cl- h
trans-[Mn(CO)2(η3-P2P′)Br]Cls + e- (1)
sponse at 0.08 V, assigned as process 1a′ to differentiate
this response from another reduction wave which is
present at higher temperatures (in this paper, all
reduction responses will be denoted by a prime). The
second cycle contains a sharper oxidation wave (process
1a) which has a greater peak height and occurs at a
significantly less positive potential than the oxidation
response of the first cycle. The reduction response also
shows enhanced peak current on the second cycle but
is observed at almost the same potential as on the first
cycle. Subsequent cycles show little variation from the
second cycle. The difference between the first and
second cycles has been noted previously.4b Presumably,
interaction between the compound and the solvent
during the course of redox recycling produces changes
in the size, shape, and structure of the crystals, which
are most pronounced in the initial stages of the volta-
mmetric experiments.
Cyclic voltammograms were recorded at higher tem-
peratures, after cleaning the graphite electrode and
attaching fresh trans-Mn(CO)2(η3-P2P′)Br for each tem-
perature. At 30 °C (Figure 1b), another reduction
response (process 1b′) is clearly observed when the scan
direction is reversed. Only one oxidation wave is

Redox Properties of Organometallic Solids
Organometallics, Vol. 16, No. 23, 1997 5009
F igu r e 3. Cyclic voltammograms (scan rate, 200 mV s-1
)
at 50 °C of solid cis,mer-Mn(CO)2(η3-P2P′)Br mechanically
attached to a graphite electrode and placed in aqueous
solution (0.1 M NaCl). (a) first and second scans; (b) third
and sixth scans.
1 (the cis,mer+/0, cis,fac+/0, and trans+/0 redox couples,
respectively) which were observed in dichloromethane
and acetone solutions.5 Process 1b′ now also is present.
The current associated with processes 2 and 3 decreases
with successive scans, while processes 1a and 1a′
increase until eventually processes 2 and 3 are no longer
observed. The peak potentials are similar to those
obtained with either trans-Mn(CO)2(η3-P2P′)Br or trans-
[Mn(CO)2(η3-P2P′)Br]BF4 at 50 °C after several scans.
Thus, isomerization from the 17-electron cis,mer+ to the
cis,fac+ and trans+ geometries occurs in both solution
and solid state phases, and the overall mechanism for
the oxidation of solid cis,mer-Mn(CO)2(η3-P2P′)Br is
F igu r e 2. Cyclic voltammograms (first, second, and fourth
scans; scan rate, 200 mV s-1) of solid trans-[Mn(CO)2(η3-
P2P′)Br]BF4 mechanically attached to graphite electrodes
and placed in aqueous solution (0.1 M NaCl) at (a) 20, (b)
30, and (c) 40 °C.
trans-Mn(CO)2(η3-P2P′)Br, where the shape and position
of this process vary considerably between the first and
second cycles. A cyclic voltammogram obtained at 30
°C is shown in Figure 2b. At this temperature, the first
reductive scan indicates that process 1b′ is the dominant
reduction process, but again, repetitive cycling shows
the growth of reduction process 1a′ while process 1b′
diminishes. Thus, regardless of whether the initial
complex is trans-Mn(CO)2(η3-P2P′)Br or trans-[Mn(CO)2-
(η3-P2P′)Br]BF4, process 1a′ is the dominant reduction
process observed after several scans. At 40 °C (Figure
2c) and at 50 °C, the results obtained for trans-[Mn-
(CO)2(η3-P2P′)Br]BF4 are analogous to those observed
for trans-Mn(CO)2(η3-P2P′)Br.
Figure 3a shows a cyclic voltammogram recorded at
50 °C for the oxidation of solid cis,mer-Mn(CO)2(η3-P2P′)-
Br attached to a graphite electrode. On the first cycle,
an oxidation response is barely resolved from the solvent
background, while several reduction waves are observed
on the reverse scan. A second cycle clearly shows the
presence of two oxidation responses (processes 2 and 3)
which are very close together. Reduction responses
(processes 2′ and 3′) corresponding to these oxidation
waves are apparent, together with an additional reduc-
tion response (process 1a′) at a less positive potential.
When the potential range is restricted slightly and
several more cycles are recorded (Figure 3b), three redox
couples are apparent corresponding to couples 2, 3, and
cis,mer-Mn(CO)2(η3-P2P′)Brs + Cl- h
cis,mer-[Mn(CO)2(η3-P2P′)Br]Cls + e- slow8
trans-[Mn(CO)2(η3-P2P′)Br]Cls (2)
In acetone solution at 50 °C, processes 2 and 3 merge
because of fast isomerization between the cis,mer+ and
cis,fac+ geometries,5 but the individual redox couples
are apparent at 20 °C, where the isomerization rate is
slower. The above data suggest that the isomerization
reactions are slower for the surface-confined species.
Similar observations were made for the oxidation of cis-
Cr(CO)2(η2-dpe)2, for which the rate of isomerization
from cis+ to trans+ species appears faster in solution
than in the solid state.4b
(b) IR Sp ectr oscop y. Specular reflectance FT-IR
spectroscopy was used to characterize the redox chem-
istry of the solid materials attached to the graphite
electrode. The IR spectrum of solid trans-Mn(CO)2(η3-
P2P′)Br attached to a graphite electrode (Figure 4a)
shows a single carbonyl stretch at 1901 cm-1, a slightly
higher frequency than that for the corresponding band
observed in dichloromethane solution (1893 cm-1).
When oxidation of the solid is undertaken at 0.400 V
for 10 s, an additional carbonyl IR band is observed at

5010 Organometallics, Vol. 16, No. 23, 1997
Bond et al.
F igu r e 4. Specular reflectance FT-IR spectra (20 °C)
obtained for trans-[Mn(CO)2(η3-P2P′)Br mechanically at-
tached to a graphite electrode (a) before oxidation and (b)
after oxidation at 0.400 V for 10 s in aqueous solution (0.1
M NaCl) at 50 °C.
Ta ble 2. IR Da ta (20 °C) for th e Solid Com p ou n d s
Mech a n ica lly Atta ch ed to Gr a p h ite Electr od es
compound
νCO (cm-1
)
F igu r e 5. X-ray electron probe microanalysis data at 20
°C for solid cis,mer- Mn(CO)2(η3-P2P′)Br mechanically
attached to a graphite electrode (a) before electrolysis,
showing detection of Mn, P, and Br, and (b) after oxidative
electrolysis in aqueous solution (0.1 M KCl), showing
detection of Mn, P, Br, and Cl, but no K.
cis,mer-Mn(CO)2(η3-P2P′)Br
1937, 1862
1901
1967
1940, 1861
1899
1966
trans-Mn(CO)2(η3-P2P′)Br
trans-[Mn(CO)2(η3-P2P′)Br]BF4
cis,mer-Mn(CO)2(η3-P3P′)Br
trans-Mn(CO)2(η3-P3P′)Br
trans-[Mn(CO)2(η3-P3P′)Br]BF4
cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe)
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2
1937, 1865
1965
0.900 V and 50 °C for 1 min using 0.1 M KCl as the
electrolyte. KCl was used rather than NaCl as the
electrolyte in this experiment since X-ray electron probe
microanalysis is not very sensitive for the detection of
light elements such as sodium. However, it is sensitive
to the detection of potassium. Electron probe data
obtained after oxidation of cis,mer-Mn(CO)2(η3-P2P′)Br
are shown in Figure 5b for a section of the surface
containing particles of approximately 1 µm diameter.
The presence of chloride was detected in addition to the
elements mentioned above, but, importantly, no potas-
sium was detected, indicating that the neutral aqueous
KCl on the electrode surface was successfully removed
by the washing process and that neither ion-paired KCl
nor desolvated K+ enters the crystal lattice upon oxida-
tion of cis,mer-Mn(CO)2(η3-P2P′)Br. Analysis of larger
particles often indicated no detectable chloride or only
a very small proportion of chloride, depending on which
section of the particle was analyzed. Thus, the electron
probe data show that oxidation of the solid does not
occur uniformly across the surface of the solid but that
it occurs predominantly at the edges, near the electrode-
crystal-solution interface, as found for other com-
plexes.4b,c Similar results were obtained when trans-
Mn(CO)2(η3-P2P′)Br was oxidized at 0.400 V or when
trans-[Mn(CO)2(η3-P2P′)Br]BF4 was first reduced at
0.000 V and subsequently reoxidized at 0.400 V. Thus,
the reduction step probably involves an initial ion
exchange reaction,
1967 cm-1 (Figure 4b). The relative intensity of this
band increases with the time of oxidation, but after
approximately 1 min this band dominates the carbonyl
region of the spectrum. The position of the additional
band is consistent with the carbonyl stretch obtained
for an isolated sample of trans-[Mn(CO)2(η3-P2P′)Br]BF4.
Thus, IR spectroscopy verifies that solid trans-Mn(CO)2-
(η3-P2P′)Br is converted to trans-[Mn(CO)2(η3-P2P′)Br]+
upon oxidation. The reverse situation also applies,
where trans-[Mn(CO)2(η3-P2P′)Br]+ is converted to trans-
Mn(CO)2(η3-P2P′)Br upon reduction. IR data also con-
firm that processes 1a′,b′ are associated with phase
changes rather than formation of new species.
Solid cis,mer-Mn(CO)2(η3-P2P′)Br attached to a graph-
ite electrode shows two carbonyl IR bands at 1937 and
1862 cm-1 at positions similar to those obtained in
dichloromethane solution (1936, 1866 cm-1), but when
the solid sample is oxidized at 0.900 V for 1 min, a
carbonyl stretch becomes apparent at 1967 cm-1, a
position similar to that of trans-[Mn(CO)2(η3-P2P′)Br]+
in dichloromethane (1970 cm-1), confirming that the
initially generated cis,mer+ isomerizes to trans+ in the
solid state. The IR data are summarized in Table 2.
(c) X-r a y Electr on P r obe An a lysis. An image
produced by scanning electron microscopy of solid cis,-
mer-Mn(CO)2(η3-P2P′)Br mechanically attached to a
graphite plate electrode shows that a wide range of
particle sizes (<1-6 µm) is present, and a significant
proportion of the surface is bare graphite.
X-ray electron probe microanalysis data for a sample
of solid cis,mer-Mn(CO)2(η3-P2P′)Br attached to a graph-
ite electrode are shown in Figure 5. Analysis of the
sample before electrochemical oxidation showed the
presence of Br, P, and Mn (Figure 5a). The fact that
some regions of the surface did not contain these
elements confirmed that much of the surface is bare
graphite. Oxidation of the sample was performed at
trans-[Mn(CO)2(η3-P2P′)Br][BF4]s + Cl- f
-
trans-[Mn(CO)2(η3-P2P′)Br]Cls + BF4 (3)
-
Certainly, BF4 is eliminated from the crystal lattice
upon reduction of trans-[Mn(CO)2(η3-P2P′)Br]BF4, and
Cl- becomes incorporated into the lattice upon reoxi-
dation.
tr a n s- an d cis,mer -Mn (CO)2(η3-P 3P ′)Br an d tr a n s-
[Mn (CO)2(η3-P 3P ′)Br ]BF4. (a) Electr och em ical Stu d-

Redox Properties of Organometallic Solids
Organometallics, Vol. 16, No. 23, 1997 5011
ies. The geometries of the compounds are given in
structures III and IV. The first cycle of a cyclic
3
3
(IV)
cis,mer-Mn(CO) (η -P P')Br
-Mn(CO) (η -P P')Br (III)
trans
2
2
3
3
voltammogram (scan rate, 200 mV s-1) obtained at 20
°C for the oxidation of solid trans-Mn(CO)2(η3-P3P′)Br
attached to a graphite electrode shows an oxidation
response at 0.34 V (process 1), and the corresponding
reduction response (process 1′) is observed at 0.10 V on
the reverse scan. Unlike the case with the trans-[Mn-
(CO)2(η3-P2P′)Br]+/0 system, no splitting of the processes
occurs due to a phase change. Couple 1 is analogous to
that observed in dichloromethane solution6 and is
assigned to the reaction
trans-Mn(CO)2(η3-P3P′)Brs + Cl- h
trans-[Mn(CO)2(η3-P3P′)Br]Cls + e- (4)
with proof for chloride insertion into the solid being
given later. Successive cycles indicate that the oxida-
tion wave shifts to a less positive potential (0.28 V),
although the reduction wave remains virtually unal-
tered. A cyclic voltammogram at 30 °C (Figure 6a) is
similar to that obtained at 20° C, but at 30° C there is
a smaller potential shift of the oxidation response from
the first to subsequent scans. In contrast to the volta-
mmetry at 20 and 30 °C, cyclic voltammograms obtained
at 40 and 50 °C (Figure 6b) show a slight shift to a more
positive potential for the second and subsequent scans
compared with the first scan. At each temperature
studied, the reduction response became constant after
several scans.
F igu r e 6. Cyclic voltammograms (scan rate, 200 mV s-1
)
of solid complexes mechanically attached to graphite
electrodes and placed in aqueous solution (0.1 M NaCl):
(a) trans-Mn(CO)2(η3-P3P′)Br at 30 °C, (b) trans-Mn(CO)2-
(η3-P3P′)Br at 50 °C, and (c) trans-[Mn(CO)2(η3-P3P′)Br]BF4
at 50 °C.
observed, as shown in Figure 7b. At 50 °C, the current
associated with process 1 increases upon successive
scans, while the current associated with the reversible
couple 3 gradually decreases. The peak potentials of
process 1 correlate with those of either solid trans-Mn-
(CO)2(η3-P3P′)Br or trans-[Mn(CO)2(η3-P3P′)Br]BF4 after
several scans at 50 °C. Thus, process 1 is assigned to
the trans+/0 redox couple. At lower temperatures,
process 1 also becomes the dominant redox couple;
however, more scans are required before this is the only
redox couple present (i.e., the isomerization reactions
are faster at higher temperatures).
A cyclic voltammogram (scan rate, 200 mV s-1
)
recorded at 20 °C for the reduction of solid trans-[Mn-
(CO)2(η3-P3P′)Br]BF4 shows a single reduction process
(process 1′), the peak potential of which appears con-
stant over several cycles. A slight shift of the oxidation
wave to a more positive potential is observed on second
and subsequent cycles, and this shift is greater as the
temperature is raised. A cyclic voltammogram obtained
at 50 °C is shown in Figure 6c. The peak separations
between oxidation and reduction waves (∆Ep) become
smaller as the temperature is raised for both trans-[Mn-
(CO)2(η3-P3P′)Br]BF4 and trans-Mn(CO)2(η3-P3P′)Br, and
at 50 °C after several scans, the peak potentials are
identical for the two complexes, as detailed in Table 3.
Figure 7a shows the fifth and tenth cycles of a cyclic
voltammogram obtained at 30 °C for the oxidation of
cis,mer-Mn(CO)2(η3-P3P′)Br. Reversible couples are
observed at 0.80 (couple 3) and 0.19 V (couple 1), and a
small reduction wave is apparent at 0.59 V (process 2′).
An oxidation wave due to process 2 is not detected, but
this response may be obscured by the oxidation wave
of process 3. A similar voltammogram was obtained at
20 °C, but when the electrolyte solution is heated to 50
°C, a reduction wave corresponding to process 2′ is not
The cyclic voltammogram at 50 °C of cis,mer-Mn(CO)2-
(η3-P3P′)Br in the solid state shows similarities to those
for solutions in dichloromethane or acetone at 20 °C.6
In the solution phase reaction, a single redox couple is
observed at 20 °C and is attributed to a combination of
processes 2 and 3, which involves a fast isomerization
between the 17-electron cis,mer+ and cis,fac+ geom-
etries. However, upon cooling of the acetone solution,
the kinetics of the isomerization reactions are slowed
sufficiently to enable the observation of the individual
cis,mer+/0 and cis,fac+/0 redox couples. Thus, analogous
behavior seems likely in the solid state, where at 50 °C
a single redox couple is observed due to processes 2 and
3, whereas at 20 and 30 °C a small reduction wave due
to process 2′ is detected. That is, the slower kinetics in
the solid state relative to studies in solution mean that

5012 Organometallics, Vol. 16, No. 23, 1997
Bond et al.
Ta ble 3. Volta m m etr ic Da ta (Sca n Ra te,
200 m V s-1) for Solid tr a n s- a n d
X-ray electron probe analysis of solid trans- or cis,-
mer-Mn(CO)2(η3-P3P′)Br subsequent to oxidation at an
electrode surface in contact with aqueous solution (0.1
M KCl) showed the presence of chloride (but not
potassium), indicating that the anion enters the crystal
lattice upon oxidation of each complex.
cis,m er -Mn (CO)2(η3-P 3P ′)Br a n d
tr a n s-[Mn (CO)2(η3-P 3P ′)Br ]BF 4 Mech a n ica lly
Atta ch ed to Gr a p h ite Electr od es a n d P la ced in
Aqu eou s Solu tion (0.1 M Na Cl)
potential
tr a n s-[{Mn (CO)2(η2-d p e)Br }2(µ-d p e)](BF 4)2 a n d
cis,fa c-{Mn (CO)2(η2-d p e)Br }2(µ-d p e). (a ) Electr o-
ch em ica l Stu d ies. The geometries of the compounds
are given in structures V and VI. In dichloromethane
(V vs Ag/AgCl)
∆Ep
(mV)
temp
(°C)
compound
couple
Ep
Ep
Er
ox
red
1/2
trans0 a
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
1
2,3
0.34
0.31
0.25
0.23
0.28
0.27
0.27
0.25
0.24
0.24
0.24
0.22
0.25
0.26
0.27
0.25
0.26
d
0.10
0.13
0.15
0.15
0.10
0.13
0.15
0.16
0.08
0.12
0.14
0.15
0.08
0.12
0.14
0.16
0.12
0.59
0.74
0.16
0.72
0.22
0.22
0.20
0.19
0.19
0.20
0.21
0.21
0.16
0.18
0.19
0.19
0.17
0.19
0.21
0.21
0.19
230
180
100
80
180
140
120
90
160
120
100
70
170
140
130
90
20
30
40
50
20
30
40
50
20
30
40
50
20
30
40
50
30
30
30
50
50
trans0 b
trans+ a
trans+ b
cis,mer0 c
2+
2
-[{Mn(CO) (
trans
η
-dpe)Br} (µ-dpe)] (V)
2
2
140
0.85
0.25
0.80
0.80
0.21
0.76
110
90
80
a
b
Potentials for first scan. Potentials after several scans.
c Potentials are given for second scan as initial oxidation response
d
for process 3 merges with the background for first scan. Oxida-
tion response for process 2 is not clearly defined.
2
(
VI
)
η
cis,fac- {Mn(CO) ( -dpe)Br} ( -dpe)
µ
2
2
solution, cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) shows7 two
oxidation responses (designated processes 1 and 2)
which are reversible at -50 °C, but at room temperature
rapid (scan rate, 200 mV s-1) isomerization occurs to
give trans geometry at each manganese atom, so that
processes 1 and 2 are irreversible. Two new reversible
couples (3 and 4) are observed on the reverse and
subsequent scans, due to the redox chemistry of the
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+/+/0 system, which
does not isomerize back to cis,mer on the voltammetric
time scale. These voltammetric reactions are sum-
marized in eqs 5 and 6. Importantly, the successive
cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) h
cis,fac-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]+ + e- h
cis,fac-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+ + e-
(processes 1 and 2 at -50 °C) (5)
trans-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) h
F igu r e 7. Cyclic voltammograms (scan rate, 200 mV s-1
)
of solid cis,mer- Mn(CO)2(η3-P3P′)Br mechanically attached
to a graphite electrode and placed in aqueous solution (0.1
M NaCl) at (a) 30 and (b) 50 °C.
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]+ + e- h
trans-[Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+ + e-
(processes 3 and 4) (6)
voltammograms at 50 °C in the solid state show
characteristics similar to those in acetone solution at
20 °C.
oxidations and reductions occur in solution at consider-
ably different potentials (approximately 120 mV) in both
geometries, indicating that significant electronic com-
munication occurs between the two metal centers in the
one-electron oxidized species.
(b) IR Sp ectr oscop y a n d X-r a y Electr on P r obe
An a lysis. Specular reflectance FT-IR spectroscopy
confirms that trans-[Mn(CO)2(η3-P3P′)Br]+ is generated
by the oxidation of either solid trans- or cis,mer-Mn-
(CO)2(η3-P3P′)Br, and data are given in Table 2.
The first cycle of a reductive cyclic voltammogram at
20 °C (scan rate, 200 mV s-1) of trans-[{Mn(CO)2(η2-

Redox Properties of Organometallic Solids
Organometallics, Vol. 16, No. 23, 1997 5013
Ta ble 4. Volta m m etr ic Da ta (Sca n Ra te,
200 m V s-1) for Solid
tr a n s-[{Mn (CO)2(η2-d p e)Br }2(µ-d p e)][BF 4]2 a n d
cis,fa c-{Mn (CO)2(η2-d p e)Br }2(µ-d p e) Mech a n ica lly
Atta ch ed to Gr a p h ite Electr od es a n d P la ced in
Aqu eou s Solu tion (0.1 M Na Cl)
potential
(V vs Ag/AgCl)
∆Ep
(mV)
temp
(°C)
ox
red
compound
couple
Ep
Ep
Er
1/2
trans+ a
3,4
3,4
3,4
3,4
3,4
3,4
3,4
3,4
1,2
3,4
3,4
0.45
0.46
0.44
0.47
0.44
0.45
0.44
0.47
0.98
0.42
0.47
0.22
0.24
0.28
0.33
0.25
0.26
0.26
0.30
0.71
0.23
0.33
0.34
0.35
0.36
0.40
0.35
0.36
0.35
0.39
0.85
0.32
0.40
230
220
160
140
190
190
180
170
270
190
140
20
30
40
50
20
30
40
50
20
20
50
trans+ b
cis,fac0 c
a
b
Potentials for first scan. Potentials after several scans.
c Potentials are given for second scan both at 20 °C as initial
oxidation response for process 1 merges with the background for
first scan and at 50 °C.
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2 s
+
2Cl- h trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s
+
F igu r e 8. Cyclic voltammograms (scan rate, 200 mV s-1
)
-
2BF4 (7a)
of solid complexes mechanically attached to graphite
electrodes and placed in aqueous solution (0.1 M NaCl):
(a) trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]BF4 at 20 °C, (b)
cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) at 20 °C, and (c) cis,-
fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) at 50 °C.
trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s + 2e- h
trans-{Mn(CO)2(η2-dpe)Br}2(µ-dpe)s + 2Cl- (7b)
has only been generated in solution at low temperature,
with rapid isomerization to cis,fac geometry occurring
at 20 °C.
dpe)Br}2(µ-dpe)](BF4)2 attached to a graphite electrode
(Figure 8a) shows only a single reduction response at
0.22 V, and the corresponding single oxidation response
is observed on the reverse scan at 0.45 V. Successive
cycles show a small shift of the redox couple to a more
positive potential, and the peak separation (∆Ep) de-
creases on the second and subsequent cycles compared
with the first cycle. Voltammograms at higher temper-
atures are similar, but at 40 and 50 °C the separations
between oxidation and reduction peaks is smaller on the
first cycle than on successive cycles, as detailed in Table
4. These data differ when compared to studies in
dichloromethane solution,7 where individual redox
couples (processes 3 and 4) are observed for each metal
center. The consecutive oxidations of the metal centers
in the solution phase imply some electronic communica-
tion between metal centers, since these centers would
be oxidized/reduced at the same potentials if this
electronic interaction were absent. The solid state
results suggest that electronic communication between
the metals is not significant in the solid state and that
the reversible couple observed is an overall two-electron
process which generates trans-{Mn(CO)2(η2-dpe)Br}2(µ-
dpe) on the reduction scan. Since there are two electron
steps, the designation 3,4 and 3′,4′ is used to define the
processes for the binuclear species in the solid state. The
voltammetric data do not reveal the role of the anions
associated with the solid and with the electrolyte.
However, data presented below reveal that an ion
exchange process occurs, so the electrochemical reaction
for trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+ is as given in
eqs 7a and 7b.
A cyclic voltammogram (scan rate, 200 mV s-1
)
recorded at 20 °C for the oxidation of solid cis,fac-{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe) attached to a graphite elec-
trode is shown in Figure 8b. The first scan shows an
oxidation response which merges with the solvent
background, and the corresponding reduction wave is
observed upon reversing the direction of the scan. A
smaller reduction response also is apparent at a less
positive potential, and this process is shown to be
reversible when a second cycle is recorded. The second
scan shows a more defined oxidation response for the
reversible couple at 0.85 V than that observed on the
first scan, and the current associated with the redox
couple at the lower potential increases on further cycling
of the potential. Voltammograms of cis,fac-{Mn(CO)2-
(η2-dpe)Br}2(µ-dpe) in dichloromethane solution at 20
°C show7 two irreversible oxidation responses (processes
1 and 2), which are associated with consecutive oxida-
tions of the metal centers, followed in each case by rapid
isomerization to trans geometry. However, at lower
temperatures these solution phase responses become
reversible, due to a decrease in the isomerization rate
constants. The observation of a reversible response
(Er ) 0.85 V) at 20 °C in the solid state gives further
1/2
evidence for a decrease in the rate constants for isomer-
izations involving surface-attached species. Since pro-
cess 3,4 is observed on repetitive cycling, the reversible
couple at Er ) 0.85 V is assigned to both processes 1
1/2
and 2, since only a single redox couple is apparent for
solid trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2, in con-
trast to electrochemical studies in dichloromethane
solution. The reaction in the solid state is summarized
Electrochemical studies of solid trans-{Mn(CO)2(η2-
dpe)Br}2(µ-dpe) are not possible because this compound

5014 Organometallics, Vol. 16, No. 23, 1997
Bond et al.
by the mechanism
interface leads to the observation of the three redox
couples (cis,mer+/0, cis,fac+/0, and trans+/0), with the 17-
electron trans+ species being the thermodynamically
stable geometry in the oxidized form in the solid state.
Likewise, oxidative studies of solid cis,mer-Mn(CO)2(η3-
P3P′)Br indicated isomerization of the 17-electron cis,-
mer+ to the cis,fac+ and trans+ geometries. The faster
isomerization reaction in solution between the cis,mer+
and cis,fac+ isomers of the P3P′ complexes, compared
with that of the analogous P2P′ complexes (due to steric
effects), also applies in the solid state. As expected, the
rates of isomerization increased upon increasing the
temperature, but a decrease in the rate constants for
the isomerization reactions was noted for the surface-
attached species relative to values in solution. The
slower rate for the studies on solids also was apparent
for oxidative studies of the binuclear complex cis,fac-
{Mn(CO)2(η2-dpe)Br}2(µ-dpe), for which a single revers-
ible two-electron oxidation response was observed at 20
°C, in contrast to the two irreversible oxidation waves
in dichloromethane solution at 20 °C. The presence of
one two-electron responses, rather than two well-
separated one-electron responses, was attributed to a
decrease in communication between the metal centers
in the solid state, presumably related to decreased
flexibility of the complexes in the solid state.
Monitoring of the surface-confined redox processes by
specular reflectance FT-IR spectroscopy verifies that,
in each case, the 18-electron cis or trans complexes are
oxidized to the 17-electron trans+ species. This oxida-
tion process is accompanied by the uptake of chloride
ions from the supporting electrolyte into the solid, as
shown by X-ray electron probe microanalysis. The
incorporation of chloride was noted also for the experi-
ments where the trans+ tetrafluoroborate species at-
tached to the electrode is first reduced at a fixed
potential (and, thus, BF4- is expelled from the solid) and
then reoxidized.
cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe)s + 2Cl-
h cis,fac-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s + 2e-
very slow8 trans-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]Cl2 s
+ 2e- h trans-{Mn(CO)2(η2-dpe)Br}2(µ-dpe)s + 2Cl-
(8)
Figure 8c shows a series of cyclic voltammograms
(scan rate, 200 mV s-1) for the oxidation of solid cis,-
fac-{Mn(CO)2(η2-dpe)Br}2(µ-dpe) at 50 °C. On the first
cycle, an irreversible oxidation response corresponding
to process 1,2 is barely detected above the solvent
background, while the reverse scan shows a very well-
defined single reduction wave due to process 3′,4′. A
second cycle reveals the oxidation response attributed
to process 3,4, but, unlike the case at 20 °C, no oxidative
current is observed for process 1,2 on the second scan.
Thus, the oxidation of cis,fac-{Mn(CO)2(η2-dpe)Br}2(µ-
dpe) generates cis,fac-[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)]2+
,
which isomerizes to trans-[{Mn(CO)2(η2-dpe)Br}2(µ-
dpe)]2+, and, as expected, this isomerization is faster
at 50 than at 20 °C.
(b) IR Sp ectr oscop y a n d X-r a y Electr on P r obe
An a lysis. Specular reflectance FT-IR spectroscopy
shows that the two carbonyl bands of solid cis,fac-{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe) attached to a graphite elec-
trode disappear upon oxidation of this complex and are
replaced by the single carbonyl stretch of trans-[{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe)]2+. This experiment confirms
that the oxidation process is a two-electron process. IR
data are summarized in Table 2.
X-ray electron probe microanalysis indicates the pres-
ence of chloride subsequent to oxidation of cis,fac-{Mn-
(CO)2(η2-dpe)Br}2(µ-dpe) at the solid-electrode-water
(0.1 M KCl) interface. In experiments involving trans-
[{Mn(CO)2(η2-dpe)Br}2(µ-dpe)](BF4)2, which was first
reduced at 0.100 V and then reoxidized at 0.600 V, the
technique indicates that Cl- replaces BF4- in the solid.
Ack n ow led gm en t. We thank the Australian Re-
search Council for financial support. J .N.W. thanks the
Commonwealth Government of Australia for a Post
Graduate Research Award.
Con clu sion s
Electrochemical oxidation of solid cis,mer-Mn(CO)2-
(η3-P2P′)Br at a graphite electrode-water (electrolyte)
OM970468J