Systematic studies of 17-electron rhenium(II) carbonyl phosphine complexes

Article abstract of DOI:10.1021/om970897h

Oxidative electrochemical properties in dichloromethane have been examined for compounds of the types cis- or trans-[Re(CO)₂(P-P)₂]⁺ and trans-Re(CO)(P-P)₂X (P-P = diphosphine ligand, X = Cl, Br) which exhibit a range of charge and ligand types. In all cases 17e Re(II) species are formed, although the oxidation potentials vary over a reasonably wide range. The Re(II) compounds have been characterized by spectroelectrochemical (IR) and electrospray mass spectrometry (ESMS) studies. The stabilities of the Re(II) species are dependent upon charge and ligand types. The trans configuration is strongly preferred in the Re(II) state, so when the starting material is cis, there is rapid isomerization following electron transfer. The major form of reactivity detected is facile reduction of the trans-[Re-(CO)₂(P-P)₂]²⁺ species. The particularly high stability of trans-[Re(CO)(dPe)₂X]⁺ (dpe = Ph₂P(CH₂)₂PPh₂) allows solid-state studies to be undertaken for the trans-[Re(CO)(dPe)₂X]^+/0 couple when the compounds are mechanically attached to a graphite electrode immersed in water (electrolyte).

Full text of DOI:10.1021/om970897h

Organometallics 1998, 17, 2977-2985
2977
System a tic Stu d ies of 17-Electr on Rh en iu m (II) Ca r bon yl
P h osp h in e Com p lexes
Alan M. Bond,*,1 Ray Colton,1 David G. Humphrey,1 Peter J . Mahon,1
Graeme A. Snook,1 Vanda Tedesco,1 and J acky N. Walter2
Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia, and School of
Chemistry, La Trobe University, Bundoora, Victoria 3083, Australia
Received October 15, 1997
Oxidative electrochemical properties in dichloromethane have been examined for com-
pounds of the types cis- or trans-[Re(CO)2(P-P)2]+ and trans-Re(CO)(P-P)2X (P-P )
diphosphine ligand, X ) Cl, Br) which exhibit a range of charge and ligand types. In all
cases 17e Re(II) species are formed, although the oxidation potentials vary over a reasonably
wide range. The Re(II) compounds have been characterized by spectroelectrochemical (IR)
and electrospray mass spectrometry (ESMS) studies. The stabilities of the Re(II) species
are dependent upon charge and ligand types. The trans configuration is strongly preferred
in the Re(II) state, so when the starting material is cis, there is rapid isomerization following
electron transfer. The major form of reactivity detected is facile reduction of the trans-[Re-
(CO)2(P-P)2]2+ species. The particularly high stability of trans-[Re(CO)(dpe)2X]+ (dpe )
Ph2P(CH2)2PPh2) allows solid-state studies to be undertaken for the trans-[Re(CO)(dpe)2X]+/0
couple when the compounds are mechanically attached to a graphite electrode immersed in
water (electrolyte).
In tr od u ction
systematic variation of ligands and charge. These
compounds exhibit a wide range of potential for oxida-
tion, and the stabilities of the Re(II) oxidation products
are examined as a function of charge, ligand type, and
oxidation potential.
Studies on the oxidation of 18e manganese carbonyl
halide complexes with phosphine ligand derivatives are
extensive.3a Although isomerization may follow the
initial electron transfer step, the final product is usually
a stable 17e Mn(II) derivative.4 While there is a large
number of Re(I) carbonyl halide derivatives known,3b
there has been much less study of the oxidation of these
compounds than for those of manganese. The major
differences are that the rhenium compounds are more
difficult to oxidize than their manganese analogues,5
and consequently, the Re(II) products of electron trans-
fer are likely to be more reactive in a thermodynamic
sense. Furthermore, there may be the possibility of
overall disproportionation reactions giving stable seven
coordinate Re(III) derivatives,3c a reaction which is not
generally available in manganese chemistry. In the
photooxidation of Re(I) carbonyl compounds containing
2,2′-bipyridine and related ligands, it appears that Re-
(II) is short-lived and reverts to Re(I) by a variety of
mechanisms.6
Exp er im en ta l Section
Ma ter ia ls. Re(CO)5X complexes were prepared by interac-
tion of Re2(CO)10 and halogen.7 The diphosphines (dpbz )
o-(Ph2P)2(C6H4); dpm ) Ph2PCH2PPh2; dpe ) Ph2P(CH2)2PPh2),
NOBF4, and “magic blue”, (p-BrC6H4)3NSbCl6, were used as
purchased (Strem). All solvents were analytical reagent,
electrochemical, or HPLC grade.
Syn th eses a n d Ch a r a cter iza tion of P r od u cts. All of the
compounds are either known compounds or simply new
examples of established types. The compounds were charac-
terized by IR and 31P NMR spectra, and their molecular
weights of the cationic species were determined by electrospray
mass spectrometry (ESMS). In all cases the diphosphines,
when present, are chelated. Absence of impurities such as free
halide and phosphine ligand was confirmed by voltammetry.
All reflux reactions were conducted under a nitrogen atmo-
sphere, although all the 18e products are air stable.
cis- a n d tr a n s-[Re(CO)2(d p bz)2]Br a n d cis- a n d tr a n s-
[Re(CO)2(d p bz)2]BF 4. A 0.125 g (0.31 mmol) amount of
Re(CO)5Br was treated with 0.303 g (0.66 mmol) of dpbz in
refluxing mesitylene (60 mL) for 168 h. Upon cooling, the
resulting white solid was washed with hexane (yield 0.30 g,
88%). Anal. Calcd for [Re(CO)2(dpbz)2]Br: C, 61.29; H, 3.98.
Found: C, 61.15; H, 3.92. The IR spectrum showed two
bands in the carbonyl region, and the 31P NMR spectrum
(Table 1) showed a singlet and two close triplets of equal
intensities. The ES mass spectrum for a solution of the
compound in dichloromethane showed only a single peak at
m/z 1136, which corresponds to the intact ion [Re(CO)2-
In this paper we describe the electrochemical oxida-
tion of a series of Re(I) carbonyl complexes having
(1) Monash University.
(2) La Trobe University.
(3) Comprehensive Organometallic Chemistry; Wilkinson, G., Stone,
F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Vol
4: (a) p 57, (b) p 173, (c) p 180.
(4) (a) Bond, A. M.; Colton, R.; McCormick, M. J . Inorg. Chem. 1977,
16, 155. (b) Bond, A. M.; Colton, R.; McDonald, M. E. Inorg. Chem.
1978, 17, 2842.
(5) (a) Bond, A. M.; Colton, R.; Panagiotidou, P. Organometallics
1988, 7, 1767. (b) Bond, A. M.; Colton, R.; Gable, R. W.; Mackay, M.
F.; Walter, J . N. Inorg, Chem. 1997, 36, 1181.
(6) (a) Schoonover, J . R.; Gordon, K. C.; Argazzi, R.; Woodruff, W.
H.; Peterson, K. A.; Bignozzi, C. A.; Dyer, R. B.; Meyer, T. J . J . Am.
Chem. Soc. 1993, 115, 10996. (b) Schoonover, J . R.; Strouse, G. F.; Dyer,
R. B.; Bates, W. D.; Chen, P.; Meyer, T. J . Inorg. Chem. 1996, 35, 273.
(7) Colton, R.; McCormick, M. J . Aust. J . Chem. 1976, 29, 1657.
S0276-7333(97)00897-2 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 06/11/1998

2978 Organometallics, Vol. 17, No. 14, 1998
Bond et al.
Ta ble 1. In fr a r ed a n d 31P NMR Sp ectr oscop ic
Da ta
Re(CO)(dpe)2Br: C, 58.31; H, 4.44; P, 11.37. Found: C, 58.39;
H, 4.40; P, 11.38.
tr a n s-Re(CO)(d p e)(d p m )Br . Re(CO)5Br (0.169 g, 0.39
mmol) was reacted with dpe (0.157 g, 0.39 mmol) in refluxing
mesitylene (60 mL) for 1.2 h to yield the known compound fac-
Re(CO)3(dpe)Br in solution (IR: 2031, 1955, 1909 cm-1). dpm
(0.158 g, 0.40 mmol) was added and the mixture refluxed for
a further 160 h. The solution was cooled, and addition of
hexane yielded a white precipitate (yield 0.31 g, 72%). The
IR and 31P NMR data (Table 1) are consistent only with the
formulation trans-Re(CO)(dpe)(dpm)Br.
ν(CO),
J P-P
Hz
,
compd
cm-1
δ(31P), ppma
cis-[Re(CO)2(dpbz)2]+
1980,
1923
1924
1980
1970,
1912
1928
1989
35.1 t, 29.0 t
<5
trans-[Re(CO)2(dpbz)2]+
trans-[Re(CO)2(dpbz)2]2+
cis-[Re(CO)2(dpm)2]+
31.8
-33.3 t, -45.7 t
-34.1
16
trans-[Re(CO)2(dpm)2]+
trans-[Re(CO)2(dpm)2]2+
tr a n s-Re(CO)(d p m )2Br . Re(CO)5Br (0.13 g, 0.32 mmol)
and dpm (0.28 g, 0.73 mmol) were reacted in refluxing
mesitylene (60 mL) for 184 h. The solution was filtered hot,
and upon cooling, a pale yellow solid precipitated which was
washed with hexane and dried (yield 0.05 g, 15%). IR and
NMR data (Table 1) are consistent only with the formulation
trans-Re(CO)(dpm)2Br.
trans-Re(CO)(dpe)2Br
trans-[Re(CO)(dpe)2Br]+
trans-Re(CO)(dpe)2Cl
trans-Re(CO)(dpm)2Br
trans-Re(CO)(dpm)(dpe)Br 1837
trans-Re(CO)(dpm)(dpbz)Br 1830
trans-Re(CO)(dpm)(dpbz)Cl 1829
1818
1892
1818
1818
25.0
29.3
-30.6
35.4 dd, -35.3 dd 185, 24
41.9 dd, -33.4 dd 184, 24
43.8d, -31.2 d
185b
tr a n s-Re(CO)(d p m )(d p bz)Br . Re(CO)5Br (0.115 g, 0.28
mmol) and dpbz (0.126 g, 0.28 mmol) were reacted in refluxing
mesitylene (60 mL) for 1.3 h to yield fac-Re(CO)3(dpbz)Br in
solution (IR: 2034, 1962, 1914 cm-1). dpm (0.122 g, 0.29
mmol) was added and the mixture refluxed for a further 110
h. Upon cooling, hexane, was added to yield a solid which was
filtered off, washed with hexane and dried (yield 0.25 g, 78%).
The analogous chloride compound was prepared by a similar
procedure from [Re(CO)5Cl].
a
b
d ) doublet, dd ) doublet of doublets, and t ) triplet. Not
further resolved.
Ta ble 2. ESMS Da ta
compd
cis- and trans-
additivea
ions (m/z)
[Re(CO)2(dpbz)2]+ (1136)
[Re(CO)2(dpbz)2]PF6
cis-[Re(CO)2(dpm)2]BF4
trans-Re(CO)(dpe)2Cl
trans-Re(CO)(dpe)2Br
[Re(CO)2(dpm)2]+ (1011)
magic blue [Re(CO)(dpe)2Cl]+ (1046)
magic blue [Re(CO)(dpe)2Br]+ (1090)
tr a n s-[Re(CO)(d p e)2Br ]BF 4. This cation was first identi-
fied electrochemically, as described in the text, but it was also
prepared in solution by bulk electrolysis of trans-Re(CO)-
(dpe)2Br and isolated by NOBF4 oxidation as follows. trans-
Re(CO)(dpe)2Br (0.050 g) was dissolved in dichloromethane (10
mL) and placed in an electrochemical cell. Solid NOBF4 was
added and the reaction followed by microelectrode voltamme-
try in the absence of electrolyte. As NOBF4 is insoluble, the
reaction is slow and the solution was degassed to remove NO.
When the reaction was complete, after about 1 h, the solution
was filtered to remove excess NOBF4 and evaporated to yield
a sticky solid. Hexane was added to the solid and allowed to
evaporate to yield a green solid which voltammetry showed
to be trans-[Re(CO)(dpe)2Br]BF4. As the compound is para-
magnetic, NMR spectroscopy cannot be used, but ESMS
confirmed the molecular formulas of the cation (Table 2).
Other cations of the type trans-[Re(CO)(P-P)2X]+ were pre-
pared in solution by oxidation of trans-Re(CO)(P-P)2X with
“magic blue” and characterized by ESMS.
trans-Re(CO)(dpm)(dpbz)Cl magic blue [Re(CO)(dpm)(dpbz)Cl]+
(1080)
trans-Re(CO)(dpm)(dpbz)Br magic blue [Re(CO)(dpm)(dpbz)Br]+
(1124)
a
Magic blue ) (p-BrC6H4)3NSbCl6.
(dpbz)2]+. All these data are consistent with the product being
a mixture of cis- and trans-[Re(CO)2(dpbz)2]Br. The propor-
tions of isomers were the same for a reflux time of 68 h. Since
the bromide ion is electrochemically active within the potential
range of interest for these compounds, the material was
dissolved in dichloromethane, and 20 mM AgBF4 in toluene
solution was added until bromide ion was no longer detectable
by voltammetry. The solid AgBr was filtered off and voltam-
metry carried out on the filtered solution. The spectroscopic
and mass spectral properties of the [Re(CO)2(dpbz)2]BF4 salt
were identical to those of the bromide analog.
cis-[Re(CO)2(d p m )2]BF 4. The known8 compound cis,mer-
Re(CO)2(η1-dpm)(η2-dpm)Br was prepared by reacting Re(CO)5-
Br (0.5 g, 1.23 mmol) and dpm (0.96 g, 2.5 mmol) in refluxing
mesitylene (60 mL) for 9 h and isolated as an insoluble product
upon cooling (yield 0.95 g, 73%). A 0.5 g (0.46 mmol) amount
of this product was dissolved in acetone (50 mL), 0.089 g (0.46
mmol) of AgBF4 in water/acetone was added slowly, and the
solution was stirred for about 1 h. The solvent was removed
under vacuum and the resultant solid extracted with dichlo-
romethane. White cis-[Re(CO)2(dpm)2]BF4 was obtained after
removal of the solvent (yield 0.26 g, 52%). The complete
removal of bromide ion was confirmed by voltammetry in CH2-
Cl2, the 31P NMR and IR spectra showed clean spectra with
only the bands reported in Table 1, and the ES mass spectrum
showed only a single peak due to the intact ion [Re(CO)2-
(dpm)2]+ (Table 2).
Electr och em ica l Meth od s. Voltammetric measurements
were typically obtained with 1.0 mM solutions of compound
in dichloromethane, acetone, or acetonitrile with 0.1 M Bu4-
NBF4, Bu4NPF6, or Bu4NClO4 as the electrolyte using a
Cypress Systems (Lawrence, KA) model CYSY-1 computer-
controlled electrochemical system or a BAS 100A electrochemi-
cal analyzer (Bioanalytical Systems, West Lafayette, IN). For
conventional cyclic voltammetric experiments the working
electrode was either a glassy carbon disk (0.5 mm radius) or
a platinum disk (0.8 mm radius), the auxiliary electrode was
a platinum wire, and the reference electrode was Ag/AgCl
(saturated LiCl in dichloromethane (0.1 M Bu4NPF6)) sepa-
rated from the test solution by a salt bridge. Near steady-
state voltammograms (scan rate 10 mV s-1) were recorded
using 5 or 12.5 µm radius platinum microdisk electrodes. The
reversible voltammetry for oxidation of an approximately 0.5
mM ferrocene (Fc) solution in the same solvent was used as a
reference redox couple, and all potentials are quoted relative
to Fc+/Fc. Solutions were purged with solvent-saturated
nitrogen before voltammetric measurements and then main-
tained under an atmosphere of nitrogen during measurements.
tr a n s-Re(CO)(d p e)2X (X ) Cl, Br ). These compounds
were prepared by the method previously described8 for the
chloride compound by reacting Re(CO)5X (0.5 g, 1.23 mmol
(Br)) with dpe (1.0 g, 2.5 mmol) in refluxing mesitylene (60
mL) for about 100 h. The products were washed with hexane
and dried (yield >95%). IR and 31P NMR spectra are consis-
tent with the trans geometry (Table 1). Anal. Calcd for
AC cyclic voltammograms were obtained using the fast
Fourier transform instrumentation described elsewhere.10 The
admittance form of readout was used.
(8) Carr, S. W.; Shaw, B. L.; Thornton-Pett, M. J . Chem. Soc., Dalton
Trans. 1987, 1763.

17-Electron Rhenium(II) Carbonyl Complexes
Organometallics, Vol. 17, No. 14, 1998 2979
Bulk electrolysis experiments were undertaken with either
a PAR model 273 potentiostat or the BAS 100 A electrochemi-
cal analyzer using a large glassy carbon working electrode, a
platinum gauze auxiliary electrode separated from the test
solution by a salt bridge, and the same reference electrode as
used in the voltammetric studies.
Solid-state voltammetric studies were made using carbon
disk electrodes (radius 2.5 mm) made from basal plane
pyrolytic graphite. Solids (1-3 mg) were placed on a coarse
grade filter paper and the solid was rubbed onto the electrode
using a cotton bud, so some adhered to the electrode surface
as an array of microcrystalline particles.11 For electrochemical
measurements, the electrode was transferred into the electro-
chemical cell containing aqueous electrolyte solution. The
electrode surface could be cleaned after measurements by
dissolving the solid in acetone.
Sp ectr oscop ic Meth od s. NMR spectra were recorded on
Bruker AM 300 spectrometers, 31P at 121.496 MHz in dichlo-
romethane or acetone and 13C at 75.469 MHz in CDCl3
solution. The high-frequency positive convention is used for
chemical shifts with external 85% H3PO4 and internal TMS
references, respectively. Infrared spectra were recorded on a
Perkin-Elmer FT-IR 1720X or a Perkin-Elmer 1430 IR spec-
trometer. ESR spectra were recorded on a Bruker ESP 300
spectrometer.
Infrared spectroelectrochemical experiments were carried
out using a modified IR reflection-absorption (IRRAS) cell,12,13
mounted on a specular reflectance accessory located in the
sample compartment of a Bruker IFS 55 FTIR spectrometer.
The electrode arrangement consisted of a polished platinum
disk working electrode (radius 2.5 mm), a platinum gauze
auxiliary electrode, and a silver wire pseudo-reference elec-
trode. Electrolyses were carried out by stepping the potential
of the working electrode from a rest potential to a potential
sufficient to cause electrolysis (Eappl), typically 200 mV past
the peak potential, and single scan IR spectra (resolution )
1.0 cm-1) were collected as a function of time. Electrolyses
were performed with a Princeton Applied Research Corp.
(PAR) (Princeton, NJ ) model 174A polarographic analyzer. The
Bu4NPF6 electrolyte solution was prepared as for the volta-
mmetric experiments, and solutions were deoxygenated before
syringing into the IRRAS cell. The cell itself was thoroughly
flushed with nitrogen prior to addition of the sample and
maintained under a nitrogen atmosphere throughout the
experiments.
Electr osp r a y Ma ss Sp ectr om etr y. Electrospray mass
spectra of cationic complexes were obtained with a VG Bio-Q
triple quadrupole mass spectrometer using a water/methanol/
acetic acid (50:50:1) mobile phase. Solutions of the compounds
(2.0 mM in dichloromethane) were mixed, if necessary, with
oxidant as described in the text. The mixed solution was then
diluted 1:10 with methanol and immediately injected directly
into the spectrometer via a Rheodyne injector fitted with a 10
µL loop. A Pheonix 20 micro LC syringe pump delivered the
solution to the vaporization nozzle of the electrospray source
at a flow rate of 5 µL min-1. Nitrogen was used as the drying
gas and for nebulization with flow rates of approximately 3 L
min-1 and 100 mL min-1, respectively. The voltage on the first
skimmer (B1) was usually 40 V. Peaks are identified by the
most abundant mass in the isotopic mass distribution. In all
cases the agreement between experimental and theoretical
isotopic mass patterns was excellent.
F igu r e 1. (a) Cyclic voltammograms at 20 °C for the
oxidation of 1 mM cis,trans-[Re(CO)2(dpbz)2]BF4 in dichlo-
romethane (0.1 M Bu4NPF6) at a Pt disk electrode (radius
0.8 mm) at a scan rate of 500 mV s-1. (b) Cyclic voltam-
mogram at 20 °C for a solution of trans-[Re(CO)2(dpbz)2]-
BF4 in dichloromethane (0.1 M Bu4NPF6), produced by
electrolysis, at a Pt disk electrode (radius 0.8 mm) at a scan
rate of 100 mV s-1
.
voltammograms at 20 °C at a platinum macrodisk
electrode for the synthesized mixture of cis- and trans-
[Re(CO)2(dpbz)2]BF4 in dichloromethane (0.1 M Bu4-
NPF6) at a scan rate of 100 mV s-1. On the first forward
scan two oxidation responses are observed, processes 1
and 2, with the latter being very close to the solvent
limit. Examination of the reverse scan shows that
process 2 is seen to be chemically irreversible. In
contrast, the appearance of process 1′ (all reduction
processes are signified by a prime) shows that redox
couple 1 (comprising processes 1 and 1′) has consider-
able chemical reversibility. A cyclic voltammogram in
which the potential is switched just after process 1
shows that process 1′ is enhanced if the potential is
switched after process 2. On the basis of comparison
with comparable metal carbonyl systems,4,5b,14 and
Resu lts a n d Discu ssion
(a ) Electr och em ica l a n d Sp ectr oscop ic Stu d ies
on th e Oxid a tion of cis- a n d tr a n s-[Re(CO)2(d p bz)2]-
BF 4 in Solu t ion . Figure 1a shows oxidative cyclic
(9) Carr, S. W.; Fontaine, X. L. R.; Shaw, B. L. J . Chem. Soc., Dalton
Trans. 1987, 3067.
(10) Hazi, J .; Elton, D. M.; Czerwinski, W. A.; Schiewe, J .; Vicente-
Beckett, V. A.; Bond, A. M. J . Electroanal. Chem. 1997, 437, 1.

2980 Organometallics, Vol. 17, No. 14, 1998
Bond et al.
Ta ble 3. Volta m m etr ic Da ta (Sca n Ra te 100 m V
s-1) in Dich lor om eth a n e (0.1 M Bu 4NP F 6) a t a
P la tin u m Electr od e a t 20 °C
decreases in absorbance by half, leaving a less intense
band at 1923 cm-1, while the band at 1980 cm-1
increases in absorbance. These changes indicate that
ν(CO) for trans-[Re(CO)2(dpbz)2]+ and trans-[Re(CO)2-
(dpbz)2]2+ occur at 1924 and 1980 cm-1, respectively,
coincidently overlapping with the bands from cis-[Re-
(CO)2(dpbz)2]+ (νCO ∼ 1923 and 1980 cm-1). Following
oxidation at Eappl ) +1.2 V, if the potential of the
working electrode is stepped to +1.5 V then complete
disappearance of the band at 1923 cm-1 occurs and is
accompanied by a slight increase in absorbance at 1980
cm-1. This second oxidative step, i.e. oxidation of cis-
[Re(CO)2(dpbz)2]+, is accompanied by the rapid isomer-
ization of cis-[Re(CO)2(dpbz)2]2+ to trans-[Re(CO)2-
(dpbz)2]2+ (eq 2). The small absorbance change at 1980
cm-1 is due to the coincidence of the trans-[Re(CO)2-
(dpbz)2]2+ and one of the cis-[Re(CO)2(dpbz)2]+ carbonyl
bands, the former growing upon oxidation while the
latter collapses. No bands are observed that can be
attributed to transient cis-[Re(CO)2(dpbz)2]2+, even at
low temperature, implying that the isomerization (eq
2) is still fast at temperatures down to -60 °C.
process
∆Ep
Er
(mV)
ox a
red
a
a
starting compd
cis- and trans-
or couple Ep
Ep
1/2
1
1.085
1.005
1.045 80
[Re(CO)2(dpbz)2]BF4
cis-[Re(CO)2(dpm)2]BF4
trans-Re(CO)(dpe)2Br
2
1
2
1
3
1
3
1
3
1
3
1.350
0.810
1.340
0.115
0.860
0.730
0.045
0.770 80
0.080 70
0.020 70
trans-Re(CO)(dpe)(dpm)Br
trans-Re(CO)(dpm)2Br
trans-Re(CO)(dpbz)(dpm)Br
0.055 -0.015
0.725
-0.005 -0.100 -0.050 95
0.590
0.080
0.720
0.005
0.040 75
a
V vs Fc+/Fc.
theoretical considerations,15 it seemed probable that
redox couple 1 is due to the reaction
trans-[Re(CO)2(dpbz)2]+ h
trans-[Re(CO)2(dpbz)2]2+ + e- (1)
Controlled potential bulk oxidative electrolysis with
coulometric monitoring of a solution of cis- and trans-
[Re(CO)2(dpbz)2]+ in dichloromethane (0.1 M Bu4NClO4
or Bu4NBF4) at the potential of process 2 leads to the
passage of considerably more than one electron per
molecule, and the current never decays to zero. Micro-
electrode voltammetry under near steady-state condi-
tions was used for in-situ monitoring of the bulk
electrolysis experiment with Bu4NBF4 as the electrolyte.
As the electrolysis proceeds, the response due to process
2 gradually decreases and finally disappears. The
microelectrode voltammetry also shows that the sign of
the current due to process 1, initially fully oxidative,
changes to show approximately equal reductive and
oxidative components which indicates that a mixture
of trans-[Re(CO)2(dpbz)2]2+ and trans-[Re(CO)2(dpbz)2]+
exists in solution. The proportions of the complexes in
solution does not change upon further prolonged elec-
trolysis. However, when the oxidizing potential is
removed, 31P NMR and IR spectra of a sample of the
solution show only the presence of the 18-electron trans-
[Re(CO)2(dpbz)2]+. These results suggest that trans-[Re-
(CO)2(dpbz)2]2+ is unstable at room temperature on time
scales significantly longer than that of the voltammetric
and IR spectroelectrochemical experiments.
and process 2 is due to the reaction sequence
where the isomerization is believed to proceed via an
internal twist mechanism.4 Throughout this paper the
identifiers 1 and 2 are associated with Re(I)/Re(II)
couples relating to trans isomers (1) and cis isomers (2).
Cyclic voltammograms for couple 1 were studied as a
function of scan rate over the range 20-500 mV s-1. At
low scan rates the peak to peak separation, ∆Ep, is 65
mV and close to the value expected for a fully reversible
redox couple. At a scan rate of 500 mV s-1, the value
of ∆Ep is 120 mV. Since the known reversible one-
electron oxidation of Fc under equivalent conditions
gives the same variation of ∆Ep with scan rate, it is
concluded that process 1 is fully reversible. The differ-
ence from the theoretical value is thought to be due to
uncompensated resistance effects. This was confirmed
by showing that ∆Ep increased with concentration for
a given scan rate. Table 3 lists cyclic voltammetric data
for this and other compounds at representative scan
rates.
A reductive bulk electrolysis of the previously oxidized
solution was carried out at 0.500 V vs Fc+/Fc (less
positive than redox couple 1). A near steady-state
voltammogram at a microdisk electrode of the reduced
solution shows only process 1 with the same limiting
current as the combined processes 1 and 2 for the
original solution. The sign of the current confirms that
the species is the reduced form, trans-[Re(CO)2(dpbz)2]+,
rather than trans-[Re(CO)2(dpbz)2]2+. Figure 1b shows
a cyclic voltammogram at a Pt macrodisk electrode for
the solution containing only trans-[Re(CO)2(dpbz)2]+ in
dichloromethane (0.1 M Bu4NBF4). Couple 1 is now
fully reversible with ipox ) ipred. Under these conditions,
in the absence of any cis isomer, it is possible to fully
the characterize process 1, and it can readily be shown
that it is both chemically and electrochemically revers-
ible.
IR spectroelectrochemical studies, using an IRRAS
cell,12,13 have been undertaken to examine the products
formed upon oxidation of the Re(I) complexes. The IR
spectrum of the cis/trans mixture contains only two ν-
(CO) bands, at 1924 and 1980 cm-1. Upon oxidation at
Eappl ) +1.2 V (i.e. at a potential sufficient to oxidize
only trans-[Re(CO)2(dpbz)2]+) the band at 1924 cm-1
(11) Bond, A. M.; Colton, R.; Daniels, F.; Fernando, D. R.; Nagaosa,
Y.; Van Stevenick, R. F. M.; Walter, J . N. J . Am. Chem. Soc. 1993,
115, 9556.
(12) Best, S. P.; Clark, R. J . H.; Cooney, R. P.; McQueen, R. C. S.
Rev. Sci. Instrum. 1987, 58, 2071.
(13) Best, S. P.; Ciniawsky, C. A.; Humphrey, D. G. J . Chem. Soc.,
Dalton Trans. 1996, 2945.
(14) Bond, A. M.; Colton, R.; Cooper, J . B.; Traeger, J . C.; Walter,
J . N.; Way, D. M. Organometallics 1994, 13, 3434.
(15) Mingos, D. M. P. J . Organomet. Chem. 1979, 179, C29.

17-Electron Rhenium(II) Carbonyl Complexes
Organometallics, Vol. 17, No. 14, 1998 2981
The IR spectrum of the solution shows a single
carbonyl absorption at 1924 cm-1, and its 31P NMR
spectrum shows a single resonance at δ 31.8 ppm, which
are the same values as those assigned to trans-[Re(CO)2-
(dpbz)2]+ in the mixture of cis and trans isomers
prepared synthetically (Table 1). The ES mass spec-
trum of the solution showed a single peak at m/z 1136,
which corresponds to the molecular weight of [Re(CO)2-
(dpbz)2]+.
The only mechanism which accords with all these
data is that both trans- and cis-[Re(CO)2(dpbz)2]+ are
oxidized (processes 1 and 2, respectively) and that
although cis-[Re(CO)2(dpbz)2]2+ isomerizes to trans-[Re-
(CO)2(dpbz)2]2+ on the voltammetric time scale, this
species is partially reduced by the solvent or adventious
water back to trans-[Re(CO)2(dpbz)2]+ on the time scale
of bulk electrolysis, which explains the passage of more
than one electron per molecule in the oxidative elec-
trolysis. Since trans-[Re(CO)2(dpbz)2]2+ and cis-[Re-
(CO)2(dpbz)2]2+ are generated at such positive potentials
(Er1/2(1) ) 1.045 V; Epox(2) ) 1.350 V vs Fc+/Fc), it is
not surprising that they are so easily reduced back to
the stable trans-[Re(CO)2(dpbz)2]+ form. The fact that
the product after reductive electrolysis is entirely trans-
[Re(CO)2(dpbz)2]+ shows that isomerization in the 18e
state to cis-[Re(CO)2(dpbz)2]+ is extremely slow. Since,
after the electrolytic oxidation and reduction cycles, the
conversion of the original mixture of cis- and trans-[Re-
(CO)2(dpbz)2]+ to trans-[Re(CO)2(dpbz)2]+ is quantita-
tive, there are no detectable side reactions, such as
disproportionation.
F igu r e 2. (a) Cyclic voltammograms at 20 °C for a 4 mM
solution of cis-[Re(CO)2(dpm)2]BF4 in dichloromethane (0.1
M Bu4NPF6) which has been subjected to partial oxidative
bulk electrolysis. (b) Near steady-state voltammogram at
a Pt microdisk electrode (radius 5 µm) for the same
solution, scan rate 100 mV s-1
.
(b) Electr och em ica l a n d Sp ectr oscop ic Stu d ies
on t h e Oxid a t ion of cis-[R e(CO)2(d p m )2]BF 4 in
Solu t ion . A fundamental difference between this
system and the [Re(CO)2(dpbz)2]+ system is that only
the cis isomer of [Re(CO)2(dpm)2]+ is present in the
sample. Oxidative cyclic voltammograms at 20 °C (scan
rate range 20-500 mV s-1) of cis-[Re(CO)2(dpm)2]BF4
in dichloromethane (0.1 M Bu4NPF6) at a platinum disk
electrode (radius 0.8 mm) give rise to an irreversible
oxidation response (process 2) near the solvent limit on
the initial oxidation scan, and on the reverse scan a
reduction response (process 1′) is apparent. On the
second scan and subsequent cycles the corresponding
oxidation, process 1, is now apparent together with
process 2. The overall electrochemical reactions are
similar to those of the analogous [Re(CO)2(dpbz)2]2+/+
system and consistent with the scheme shown in eq 3.
Proof of the validity of this scheme is given in the
following sections.
F igu r e 3. Changes in carbonyl region of the IR spectrum
upon oxidation of cis-[Re(CO)2(dpm)2]BF4 in an IRRAS cell
in dichloromethane (0.1 M Bu4NPF6) at -60 °C.
1912 cm-1) decrease in intensity, and a new strong band
grows at 1989 cm-1 (Figure 3). These changes in the
IR spectrum are consistent with the rapid isomerization
of cis-[Re(CO)2(dpm)2]2+ to trans-[Re(CO)2(dpm)2]2+, as
postulated from voltammetric experiments. No bands
are observed which can be assigned to cis-[Re(CO)2-
(dpm)2]2+, so the isomerization (eq 3) is still fast at low
temperature. Attempted rereduction at Eappl ) +1.0 V
does not cause any change in the IR spectrum, nor does
any current flow through the cell, thus confirming that
oxidation of cis-[Re(CO)2(dpm)2]+ results in the forma-
tion of a species with different redox characteristics, i.e.
trans-[Re(CO)2(dpm)2]2+. At Eappl ) +0.5 V, reduction
is achieved and the ν(CO) band at 1989 cm-1 gives way
a band of similar intensity at 1928 cm-1 which can be
attributed to the trans-[Re(CO)2(dpm)2]+ cation. While
there is no evidence of isomerization of trans-[Re(CO)2-
(dpm)2]+ to cis-[Re(CO)2(dpm)2]+ at -50 °C, at room
temperature some of the latter does re-form. Accom-
panying the formation of trans-[Re(CO)2(dpm)2]2+ are
Epox(2) values (process 2) for cis-[Re(CO)2(dpbz)2]2+/+
and cis-[Re(CO)2(dpm)2]2+/+ are very similar. However,
the Er values (couple 1) for the corresponding trans
1/2
isomers differ by over 250 mV (Table 3).
The oxidation of cis-[Re(CO)2(dpm)2]BF4 was carried
out in dichloromethane (0.1 M Bu4NPF6) in the IRRAS
cell at low temperature. As the electrolysis proceeds,
the ν(CO) bands for the parent cis complex (at 1970 and

2982 Organometallics, Vol. 17, No. 14, 1998
Bond et al.
two unidentified carbonyl containing complexes with ν-
(CO) bands at 2027 and 1965 cm-1, respectively. While
these bands are quite prominent at 20 °C, their intensity
is greatly diminished when the oxidation is carried out
at low temperature. These bands do not arise from the
same species since that at 2027 cm-1 is observed to
collapse upon reduction while that 1965 cm-1 remains.
Bulk electrolysis of a 4 mM solution of cis-[Re(CO)2-
(dpm)2]BF4 in dichloromethane (0.1 M Bu4NPF6) at 1.39
V caused little color change in the solution. The close
proximity of process 2 to the solvent limit means that
exhaustive oxidative electrolysis is difficult to achieve.
A cyclic voltammogram of the resulting solution after
partial electrolysis is shown in Figure 2a, which shows
the reversible couple 1 and the irreversible process 2.
The sign of the current in a near steady-state voltam-
mogram at a Pt microelectrode (Figure 2b) shows that
the majority of the species responsible for couple 1 is in
the reduced form. After exhaustive electrolysis of cis-
[Re(CO)2(dpm)2]+ only trans-[Re(CO)2(dpm)2]+ was ob-
served in solution. The infrared spectrum of the solu-
tion shows a new strong absorption at 1928 cm-1, and
the combination of electrochemical and IR evidence
suggests that trans-[Re(CO)2(dpm)2]+ is the major spe-
cies produced in solution after bulk oxidative electrolysis
of cis-[Re(CO)2(dpm)2]+. However, a small concentration
of trans-[Re(CO)2(dpm)2]2+ which rapidly converts to
trans-[Re(CO)2(dpm)2]+ is detected as an intermediate,
as shown by voltammetry at a microelectrode.
The bulk electrolysis was repeated using Bu4NBF4 as
the supporting electrolyte so that 31P NMR spectra could
be acquired without spectral interference from the
concentrated supporting electrolyte. The NMR spec-
trum showed two weak triplets (J (P-P) ) 16 Hz), due
to unoxidized cis-[Re(CO)2(dpm)2]+, and a strong singlet
at δ -34.1 ppm, assigned to trans-[Re(CO)2(dpm)2]+.
All of these data confirm that trans-[Re(CO)2(dpm)2]2+
is unstable on the longer time scale of bulk electrolysis.
Thus, despite the lower Er1/2 value for process 1 relative
to the dpbz compound, the cation trans-[Re(CO)2-
(dpm)2]2+ is still reduced by adventious water, or light,
to its 18e analogue.
trans-[Re(CO)2(dpe)2]BF4 has been prepared before by
the interaction of trans-Re(N2)(dpe)2Cl and TlBF4 in
refluxing tetrahydrofuran for 12 days, with CO being
passed through the solution continuously.16 The com-
pound exhibited a reversible one-electron oxidation
process on the voltammetric time scale in acetonitrile
at a potential of 1.42 V vs SCE which, after correction
for the different reference electrodes, is qualitatively in
the same potential range as couple 1 for other trans-
[Re(CO)2(P-P)2]+ compounds described in this work.
F igu r e 4. (a) Cyclic voltammogram at 20 °C for the
oxidation of 1 mM trans-Re(CO)(dpe)2Br in dichloromethane
(0.1 M Bu4NPF6) at a Pt disk electrode (radius 0.8 mm) at
a scan rate of 100 mV s-1. (b) Near steady-state voltam-
mogram (scan rate 100 mV s-1) recorded at a Pt microdisk
electrode (radius 5 µm) for a 1 mM solution of trans-Re-
(CO)(dpe)2Br in dichloromethane (0.1 M Bu4NPF6). (c) Near
steady-state voltammograms (scan rate 100 mV s-1) re-
corded at a Pt microdisk electrode (radius 5 µm) during
the oxidative bulk electrolysis of trans-Re(CO)(dpe)2Br in
dichloromethane (0.1 M Bu4NPF6).
observed. For cyclic voltammograms in which the
potential is switched between processes 1 and 3, redox
couple 1 is completely chemically reversible over a wide
range of scan rates (10-500 mV s-1) and the peak to
peak separation at a scan rate of 10 mV s-1 is 68 mV,
which is typical for a reversible one-electron process in
a solvent like dichloromethane when noncompensated
resistance is present. Voltammetric data at a glassy
carbon macrodisk electrode are very similar to that
obtained at the platinum macrodisk electrode. Fourier
transform AC (admittance) cyclic voltammograms (Fig-
ure 5) of the first process almost overlap in the positive
and negative potential sweep directions, confirming the
(c) Electr och em ica l a n d Sp ectr oscop ic Stu d ies
on th e Oxid a tion of tr a n s-Re(CO)(d p e)2Br in Solu -
tion . Figure 4a shows a cyclic voltammogram for
oxidation of a 1 mM solution of trans-Re(CO)(dpe)2Br
in dichloromethane (0.1 M Bu4NPF6) at 20 °C using a
scan rate of 100 mV s-1 at a platinum disk electrode
(radius 0.8 mm). The voltammogram shows an oxida-
tion response (process 1) and an irreversible response
ox
(process 3) with Ep ) 0.860 V vs Fc+/Fc. On the
reverse scan
a reduction response (process 1′) is
(16) Pombeiro, A. J . L. Inorg. Chim. Acta 1985, 103, 95.

17-Electron Rhenium(II) Carbonyl Complexes
Organometallics, Vol. 17, No. 14, 1998 2983
F igu r e 5. Fourier transform AC (admittance) cyclic vol-
tammogram for oxidation of a 1 mM solution of trans-Re-
(CO)(dpe)2Br in dichloromethane (0.1 M Bu4NPF6) at a Pt
disk electrode (radius 0.8 mm). Frequency ) 20 Hz, and
total AC amplitude ) 5 mV.
F igu r e 6. Changes in the carbonyl region of the IR
difference spectrum upon oxidation of trans-[Re(CO)(dpe)2Br]
in an IRRAS cell in dichloromethane (0.1 M Bu4NPF6) at
20 °C.
chemical reversibility and also confirming that the rate
of electron transfer is very fast. Thus, the data suggest
that redox couple 1 in dichloromethane is due to the
process
very fast
trans-Re(CO)(dpe)2Br y
z
trans-[Re(CO)(dpe)2Br]+ + e- (4)
Evidence presented below confirms the assignment of
trans-[Re(CO)(dpe)2Br]+ as the product.
Process 3 remains irreversible at high scan rates (up
to 10 V s-1) and low temperatures (-50 °C). A near
steady-state voltammogram at a platinum microdisk
electrode (radius 5 µm) shows the background limiting
currents for oxidation processes 1 and 3 to be equal
(Figure 3b), although the plateau region between the
two processes has a definite slope for reasons which are
unknown. Since the first process is shown to be a
reversible one-electron process under these near steady-
state conditions (Er1/2 ) 0.080 V vs Fc+/Fc and E versus
ln (id - i)/i is linear with slope RT/F), process 3 can be
assigned to the reaction
F igu r e 7. Near steady-state voltammograms at 20 °C at
a Pt microdisk electrode (radius 5 µm) before and after
oxidation of trans-Re(CO)(dpe)2Br with NOBF4 in dichlo-
romethane (no electrolyte).
3c shows the near steady-state voltammograms recorded
during the electrolysis. The half-wave potential of the
response (Er1/2) remained constant, but the sign of the
current reversed from oxidation to reduction as the
electrolysis proceeded, and the solution changed from
colorless to dark green. Furthermore, during the course
of the electrolysis the limiting current region assumes
the ideal potential independent shape. Controlled-
potential reductive electrolysis enables quantitative
regeneration of trans-Re(CO)(dpe)2Br to be achieved.
Coulometric measurement showed that 0.95 ( 0.05
electrons per molecule was passed through the solution
during both exhaustive oxidation and reduction elec-
trolysis experiments. A cyclic voltammogram at a
conventional macroelectrode electrode after oxidative
electrolysis was qualitatively similar to that of the
starting material, but the IR spectrum of the oxidized
Since the emphasis of this paper is on the generation
of 17-electron Re(II) species, no further studies on the
Re(III) products were undertaken.
The oxidation of trans-[Re(CO)(dpe)2Br] was carried
out in dichloromethane (0.1 M Bu4NPF6) in an IRRAS
cell at 20 °C. The changes in the carbonyl region of the
IR spectrum upon oxidation are shown in Figure 6. The
ν(CO) band for trans-[Re(CO)(dpe)2Br] at 1818 cm-1
gives way to a single band at 1892 cm-1, which is
assigned to the Re(II) cation, trans-[Re(CO)(dpe)2Br]+.
These spectral changes are fully reversible in that
rereduction regenerates, in its entirety, the original
spectrum of trans-[Re(CO)(dpe)2Br].
solution gave a single carbonyl stretch at 1892 cm-1
,
compared with 1818 cm-1 for trans-Re(CO)(dpe)2Br.
trans-[Re(CO)(dpe)2Br]BF4 was prepared in dichlo-
romethane by oxidizing trans-Re(CO)(dpe)2Br with
NOBF4, and the oxidation was monitored by microelec-
trode voltammetry in the absence of electrolyte. Figure
7 shows the voltammograms before and after chemical
oxidation. Not only does the sign of the current change,
as expected, but after oxidation the magnitude of the
current is only 48% the initial value. This decrease in
current after oxidation is due to the migration of the
charged product to the auxiliary electrode. Theoreti-
Bulk oxidative electrolysis of a 4 mM solution of trans-
Re(CO)(dpe)2Br in dichloromethane (0.1 M Bu4NPF6) at
a potential slightly more positive than process 1 was
monitored at a platinum microdisk electrode. Figure

2984 Organometallics, Vol. 17, No. 14, 1998
Bond et al.
cally, under steady-state conditions at a microdisk
electrode in the absence of electrolyte, one-electron
reduction of a monovalent cation should exhibit half the
current observed for a one-electron oxidation of a
neutral species.17 Thus, the microelectrode voltamme-
try is in complete agreement with theory and the data
confirm the chemical reversibilty, as well as the charge
on the species associated with the trans-[Re(CO)-
(dpe)2Br]+/0 couple.
In principle, the product of oxidation could have been
either the cis or trans isomer, since no direct spectro-
scopic evidence to identify the isomeric form could be
obtained. However, the voltammetry (single processes
for both oxidized and reduced forms with very fast
electron transfer) and IR evidence (single νCO band
shifted by ≈74 cm-1) are in complete accord with
retention of the trans configuration. Additionally, even
though trans-[Re(CO)(dpe)2Br]BF4 is paramagnetic and
has no observable 31P NMR spectrum, its effect upon
the 31P NMR spectrum of trans-Re(CO)(dpe)2Br can be
observed upon its addition to solutions of the diamag-
netic 18e compound. Addition of about 0.2-0.4% of the
paramagnetic species causes the single 31P resonance
of trans-Re(CO)(dpe)2Br at δ 25.0 ppm to broaden
considerably and move to lower frequency, and addition
of about 1% of the paramagnetic caused loss of the
signal. However, addition of the strong reductant
cobaltocene regenerated the sharp signal of trans-Re-
(CO)(dpe)2Br at δ 25.0 ppm. These observations are
consistent with a fast self-exchange (electron exchange)
reaction between isostructural 18e and 17e species.14
F igu r e 8. Cyclic voltammogram (scan rate 10 mV s-1) at
20 °C for solid trans-Re(CO)(dpe)2Br attached to a graphite
electrode immersed in water (0.1 M NaClO4).
Ta ble 4. Solid -Sta te Volta m m etr ic Da ta for
tr a n s-Re(CO)(d p e)2Br
concn scan rate
ox a
red a
electrolyte (M)
(mV s-1
)
Ep
Ep
∆E (mV) E1/2 (V)
NaClO4
0.01
0.1
1.0
0.01
0.1
1.0
0.05
0.05
0.05
0.05
0.05
0.05
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
10
10
10
100
100
100
10
100
10
100
10
100
10
10
10
0.80
0.77
0.73
0.83
0.80
0.75
0.74
0.79
0.77
0.78
0.75
0.79
1.08
0.59
0.82
0.84
1.03
0.60
0.86
0.87
0.52
0.46
0.46
0.50
0.44
0.42
0.47
0.43
0.50
0.43
0.46
0.43
0.66
0.49
0.59
0.67
0.61
0.42
0.54
0.66
280
340
270
330
360
330
270
360
270
350
290
360
420
100
230
170
420
180
320
210
0.66
0.61
0.59
0.66
0.62
0.59
0.60
0.61
0.63
0.61
0.61
0.61
0.82
0.54
0.70
0.76
0.82
0.51
0.70
0.77
NaClO4
CsClO4
Et4NClO4
KClb
KPF6
KBF4
KNO3
KClb
KPF6
KBF4
KNO3
trans-Re*(CO)(dpe)2Br + trans-[Re(CO)(dpe)2Br]+ h
trans-[Re*(CO)(dpe)2Br]+ + trans-Re(CO)(dpe)2Br
10
100
100
100
100
(6)
In contrast, electron transfer reactions involving a
structural change are likely to be slow on the NMR time
scale.
The ESR spectrum (77 K) of a frozen dichloromethane
solution of isolated trans-[Re(CO)(dpe)2Br]BF4 gives a
broad signal at g ) 2.3.
a
V vs Ag/AgCl. All potentials quoted vs Ag/AgCl (3 M KCl);
E1/2 of [Fe(CN)6]3-/4- in 1 M KCl ) 0.27 V. Poorly defined
oxidation peak.
b
state voltammograms (scan rate 10 mV s-1) for trans-
Re(CO)(dpe)2Br mechanically attached to a graphite
electrode which is then placed in water (0.1 M NaClO4),
in which both the oxidized and reduced forms of the
compound are insoluble, are shown in Figure 8. The
response observed on the first cycle is complex, whereas
on second and subsequent cycles a well-defined chemi-
cally reversible redox response, couple 1, is observed.
The dissimilarity of the initial and subsequent scans,
where a constant response is seen, has been observed
previously in solid-state voltammetry.18 Solid-state
voltammetry of trans-[Re(CO)(dpe)2Br]BF4 is essentially
indistinguishable from that of trans-Re(CO)(dpe)2Br
except that even on the first cycle the only response is
a well-defined process 1.
Voltammetry in acetone and acetonitrile using satu-
rated solutions of sparingly soluble trans-Re(CO)-
(dpe)2Br also gave two well-defined oxidation responses.
The reversible potentials for process 1 are 0.15 and 0.12
V, and the peak potentials for process 3 are 0.85 and
0.78 V vs Fc+/Fc in acetone and acetonitrile, respec-
tively. No evidence of solvent substitution reactions was
found on the voltammetric time scale in either acetone
or acetonitrile.
Voltammetry in dichloromethane solution of other
complexes trans-Re(CO)(P-P)2Br (including those with
different diphosphines) is generally similar, and data
are summarized in Table 3. The corresponding cations
trans-[Re(CO)(P-P)2Br]+ were produced in solution by
oxidation of trans-Re(CO)(P-P)2Br with magic blue and
identified by ESMS (Table 2). Monitoring by both
voltammetry and ESMS showed the cations to be long-
lived in solution.
The potentials of the solid-state voltammograms were
found to be dependent upon both electrolyte concentra-
tion and scan rate, as shown in Table 4. A higher
concentration of electrolyte tended to shift both the
oxidation and reduction peak potentials to less positive
values. The potentials are essentially independent of
(d ) Solid -Sta te Volta m m etr y of tr a n s-Re(CO)-
(d p e)2Br a n d tr a n s-[Re(CO)(d p e)2Br ]BF 4. Solid-
(17) Cooper, J . B.; Bond, A. M.; Oldham, K. B. J . Electroanal. Chem.
1992, 331, 877.
(18) (a) Bond, A. M.; Colton, R.; Marken, F.; Walter, J . N. Organo-
metallics 1994, 13, 5122.

17-Electron Rhenium(II) Carbonyl Complexes
Organometallics, Vol. 17, No. 14, 1998 2985
the identity of the electrolyte cation but are dependent
on the anion (Table 4). This shows that the mechanism
of oxidation in the solid state involves the incorporation
of the anion into the crystal structure and its expulsion
upon subsequent reduction. The reaction associated
with couple 1 is
predicted theoretically15 and generally observed in
earlier work, so that cis Re(II) species generated by
oxidation isomerize rapidly to the trans isomers.
There is no evidence for rapid disproportionation or
equivalent reactions which are a feature of the chem-
istry of some isoelectronic carbonyl compounds.19 Simi-
larly, no disproportionation reactions were observed in
photochemical studies on Re(I) carbonyl compounds
containing 2,2′-bipyridine and related ligands, where the
photochemically generated Re(II) species are short-lived
and quickly revert to Re(I) by a variety of mechanisms.6
Thus, the conclusion is that Re(II) carbonyl com-
pounds described in this study are surprisingly stable
in solution, although it should be noted that the related
compounds [Re(CO)2(P2P′)X]+ (P2P′ ) (Ph2PCH2CH2)2-
PPh) do show some tendency to undergo a slow overall
disproportionation reaction to form the Re(III) cation
[Re(CO)2(P2P′)X2]+.5b The same Re(III) cation, rather
than the Re(II) species, is also produced by oxidation of
[Re(CO)2(P2P′)X]+ by bromine.5b
trans-Re(CO)(dpe)2Br + X- h
trans-[Re(CO)(dpe)2Br]X + e- (7)
After attachment to the electrode of trans-[Re(CO)-
(dpe)2Br]BF4, ion exchange with the electrolyte anion
occurs so consequently the voltammetry is that of trans-
[Re(CO)(dpe)2Br]X without complications on the first
scan.
Con clu sion s a n d Gen er a l Discu ssion
The 18e Re(I) carbonyl starting materials considered
in this paper are either neutral or monocationic, and
the other ligands vary from just diphosphines to a
mixture of diphosphines and halides. This variation in
charge and ligand types is reflected in a substantial
potential range (E° or Epox) for oxidation (Table 3).
However, despite this wide potential range, Re(II) is
always thermodynamically stable in solution with these
ligand combinations, although the strongest oxidants
among the Re(II) species are reactive to adventious
impurities, or light, and revert back to their 18e
analogues. The trans form is dominant in Re(II), as
Ack n ow led gm en t. The authors acknowledge the
financial support of the Australian Research Council
and thank Dr. Georgii Lazarev and Dr. Richard Webster
for recording the ESR spectrum. D.G.H. thanks the
Australian Research Council for the provision of an
Endeavour Fellowship, and J .N.W. thanks the Com-
monwealth of Australia for a Post Graduate Research
Award.
OM970897H
(19) Bond, A. M.; Colton, R. Coord. Chem. Rev. 1997, 166, 161.