Rhenium(I) di- and tri-carbonyl compounds of polypyridyl-like ligands: Electrochemical reactions of fac-[Re(CO)3(dpk)Cl] (dpk = di-2-pyridyl ketone) with electrophiles and Group I and II metal ions

Article abstract of DOI:10.1039/a608470b

In contrast to the facile formation of fac-[Re(CO)₃(dpk)Cl] 1 from the reaction between [Re(CO)₅Cl] and dpk [(C₅H₄N)₂C(O)], in refluxing toluene, [Re(CO)₅Cl] was recovered unchanged when di-2-pyridylamine, dpa [(C₅H₄N)₂NH], was allowed to react with [Re(CO)₅Cl] under a variety of conditions. However, a mixture of [Re(CO)₅Cl] and PPh₃ in refluxing toluene followed by a CH₂Cl₂ solution of dpa gave cis-[Re(CO)₂(PPh₃)(dpa)Cl]. The intermediate involved appears to be a general synthon for the binding of the Re^I(CO)₂(PPh₃)Cl chromophore to a variety of bi- and mono-dentate nitrogen-donor ligands and has been utilized to isolate cis-[Re(CO)₂(PPh₃)(dpk)Cl]. Nucleophilic addition of water at the carbonylic carbon atom of co-ordinated dpk in 1 and cis-[Re(CO)₂(PPh₃)(dpk)Cl] resulted in the hydration of the keto group and formation of fac-[Re(CO)₃{(C₅H₄N)₂C(O)(OH)}] and cis-[Re(CO)₂(PPh₃){(C₅H₄N) ₂C(O)(OH)}]. The compounds isolated exhibit rich electrochemical and photochemical properties and the potential application of fac-[Re(CO)₃(dpk)Cl] as an electrochemical sensor for electrophiles and Group I and II metal ions is demonstrated.

Full text of DOI:10.1039/a608470b

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Rhenium(I) di- and tri-carbonyl compounds of polypyridyl-like
ligands: electrochemical reactions of fac-[Re(CO)3(dpk)Cl] (dpk ؍
di-2-pyridyl ketone) with electrophiles and Group I and II metal ions
Mohammed Bakir * and Jacinth A. M. McKenzie
Department of Chemistry, The University of the West Indies-Mona campus, Kingston 7, Jamaica,
West Indies
In contrast to the facile formation of fac-[Re(CO)3(dpk)Cl] 1 from the reaction between [Re(CO)5Cl] and dpk
[(C5H4N)2C(O)], in refluxing toluene, [Re(CO)5Cl] was recovered unchanged when di-2-pyridylamine, dpa
[(C5H4N)2NH], was allowed to react with [Re(CO)5Cl] under a variety of conditions. However, a mixture of
[Re(CO)5Cl] and PPh3 in refluxing toluene followed by a CH2Cl2 solution of dpa gave cis-[Re(CO)2(PPh3)(dpa)Cl].
The intermediate involved appears to be a general synthon for the binding of the ReI(CO)2(PPh3)Cl
chromophore to a variety of bi- and mono-dentate nitrogen-donor ligands and has been utilized to isolate
cis-[Re(CO)2(PPh3)(dpk)Cl]. Nucleophilic addition of water at the carbonylic carbon atom of co-ordinated dpk
in 1 and cis-[Re(CO)2(PPh3)(dpk)Cl] resulted in the hydration of the keto group and formation of fac-[Re(CO)3-
{(C5H4N)2C(O)(OH)}] and cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}]. The compounds isolated exhibit rich
electrochemical and photochemical properties and the potential application of fac-[Re(CO)3(dpk)Cl] as an
electrochemical sensor for electrophiles and Group I and II metal ions is demonstrated.
Metal compounds containing polypyridyl-like ligands of the
type (C5H4N)2X (X = π-donor or -acceptor ligand) are of cur-
rent interest for their rich physicochemical properties, diverse
reactivity patterns, and potential applications in many import-
ant chemical processes.1–10 Our interest in the chemistry of
polypyridyl-like compounds such as di-2-pyridyl ketone (dpk)
and di-2-pyridylamine (dpa) was aroused because of their abil-
ity to undergo a variety of interesting reactions upon binding to
transition metals and we were intrigued to explore how vari-
ations in the backbone of these ligands affect their reactivity
and the physicochemical properties of their compounds com-
pared to those containing 2,2Ј-bipyridine (bipy).
Di-2-pyridyl ketone has been reported to form stable com-
pounds with a variety of metal ions in a bidentate N,N- or a
tridentate N,O,N-co-ordination mode.9–18 In the former mode
the back π bonding is enhanced as a result of delocalization of
electrons through the pyridine rings and the oxygen atom of the
keto group. Such delocalization of electron density provides a
span of charged species from a single neutral precursor and a
low-energy M→L charge-transfer pathway which are important
in electro- and photo-catalysis.9 The three-co-ordination mode
results from a nucleophilic attack at the carbonyl moiety and
leads to the loss of π-accepting ability of the ligand and an
increase in electron density around the metal centre.2–5 A variety
of transition-metal compounds containing di-2-pyridylamine
has been reported and the acidity of the amine group has
been utilized to graft some of these compounds into solid
supports.1,19,20
O
C
H
N
N
N
N
N
N
N
dpk
dpa
bipy
containing di-2-pyridyl ketone fac-[Re(CO)3(dpk)Cl].35 These
reactions are solvent dependent, controlled by the rate of diffu-
sion of electroactive species from the electrode surface. Fast
diffusion due to solvent or scan-rate variations inhibits the car-
boxylation of the carbonylic carbon atom of the radical anion
fac-[Re{(C5H4N)2C(O) Ϫ}(CO)3Cl] leading to the generation of
᝽
co-ordinatively unsaturated fac-[Re{(C5H4N)2{C(O) Ϫ}(CO)3]
᝽
᝽
which binds CO2 to form fac-[Re{(C5H4N)2C(O) Ϫ}(CO)3-
(CO2)]. Slow diffusion facilitates the carboxylation of the car-
bonylic carbon atom of fac-[Re{(C5H4N)2C(O)}(CO)3Cl] to
Ϫ
ؒ
form fac-[Re{(C5H4N)2C(O )(CO2 )}(CO)3] which undergoes a
second electron transfer followed by chemical steps leading to
the formation of fac-[Re{(C5H4N)2C(O)(CO2H)}(CO)3]. Here,
we report the isolation and physicochemical properties of the
first series of tri- and di-carbonyl compounds of ReI that con-
tain di-2-pyridyl ketone and di-2-pyridylamine along with the
electrochemical reactions of fac-[Re(CO)3(dpk)Cl] with methyl
chloroformate and Group I and II metal ions.
Experimental
Reagents and reaction procedures
Although rhenium compounds containing polypyridyl lig-
ands have been extensively studied and their potential applic-
ations as catalysts, sensors, sensitizers for energy and electron
transfer and photo probes for DNA have been demonstrated,
rhenium compounds containing polypyridyl-like ligands are
scarce.21–34 Only two reports have appeared on the synthesis of
high-valent rhenium() compounds containing a tridentate
N,O,N-donor ligand (C5H4N)2C(O)(OR) (R = H or alkyl)
which resulted from nucleophilic addition of water or alcohol at
the ketonic carbon atom of di-2-pyridyl ketone during these
reactions.17,18 We recently described the electrochemical reac-
tions of CO2 with the first rhenium()-tricarbonyl compound
Solvents were reagent grade and thoroughly deoxygenated prior
to use. All other reagents were obtained from commercial
sources and used without further purification. The compound
fac-[Re(CO)3(dpk)Cl] 1 was prepared as described previously.35
All reactions were performed under a nitrogen atmosphere
using standard vacuum-line techniques.
Preparations
(a) fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] 2. A quantity of fac-
[Re(CO)3(dpk)Cl] (200 mg, 0.42 mmol) was suspended in etha-
nol (50 cm3), water (0.5 cm3) was added and the mixture was
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578
3571

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refluxed for 24 h. It was then allowed to cool to room tempera-
ture. An off-yellow solid was filtered off, washed with hexane,
diethyl ether and dried; yield 180 mg, 0.38 mmol (90%) (Found:
CO
Ph3P
CO
CO
N
Re CO
Cl
N
Re CO
Cl
C, 35.8; H, 1.9; N, 5.9. C14H9N2O5Re requires C, 35.65; H, 1.95;
N
N
᎐
N, 6.05%). IR (KBr): ν(C᎐O) 2019, 1927, 1866, ν(py) 1608,
᎐
ν(C᎐O) 1240, ν(OH) ≈ 3050 cmϪ1. 1H NMR [(CD3)2SO]: δ 8.82
(d), 8.08 (t), 7.70 (s) and 7.40 (t).
fac-[Re(CO)3(L–L)Cl]
cis-[Re(CO)2(PPh3)(L–L)Cl]
When a quantity of fac-[Re(CO)3(dpk)Cl] was allowed to
reflux in absolute ethanol or ethanol solution containing a few
drops of concentrated hydrochloric acid for an extended period
no colour change was observed and fac-[Re(CO)3(dpk)Cl] was
recovered unchanged. The identity of the product was con-
firmed from its electrochemical and spectroscopic properties.
The IR spectrum (KBr pellet) of the orange-yellow solid
Physical measurements
Electronic absorption spectra were recorded on an HP-8452A
spectrophotometer. Solution H NMR spectra were recorded
1
on a Bruker ACE 200-MHz Fourier-transform spectrometer.
Resonances were referenced internally to the residual protons
in the incompletely deuteriated solvent. Electrochemical meas-
urements were made in solutions of acetonitrile or dimethyl-
formamide with 0.1 mol dmϪ3 tetra-n-butylammonium hexa-
fluorophosphate as the supporting electrolyte. A glassy carbon
disc (0.07 cm2) was used as a working electrode. All potentials
were referenced to the saturated sodium chloride calomel elec-
trode (SSCE) at room temperature and are uncorrected for
junction potentials. The reversible one-electron couple of fer-
rocene was employed as an internal standard to determine the
number of electrons and reversibility of the electrochemical
waves. Voltammetric experiments were performed with the
use of a Princeton Applied Research (PAR) model 173
potentiostat/galvanostat and model 276 interface in conjunc-
tion with a 286 personal computer. Data were acquired with the
use of the EG&G PARC HEADSTART program and manipu-
lated using the Microsoft EXCEL program. A PAR 179 digital
showed the stretching mode of the ketonic group ν(C᎐O) at
᎐
1680 cmϪ1
.
(b) cis-[Re(CO)2(PPh3)(dpk)Cl] 3. A mixture of [Re(CO)5Cl]
(150 mg, 0.42 mmol), PPh3 (250 mg, 0.95 mmol) and toluene
(80 cm3) was refluxed for 24 h. To the hot orange solution a
quantity of dpk (220 mg, 1.21 mmol) dissolved in CH2Cl2 (5
cm3) was added. The reaction mixture changed to brown
instantly and was refluxed for 72 h. It was then reduced in
volume to ≈10 cm3 and pentane added. A red-brown solid was
isolated, chromatographed on an alumina column (CH2Cl2 as
eluent) and recrystallized from CH2Cl2–diethyl ether. A red-
brown solid was filtered off, washed with hexane, diethyl ether
and dried; yield 160 mg, 0.22 mmol (52%) (Found: C, 50.8; H,
3.25; N, 3.75. C31H23ClN2O3PRe requires C, 51.4; H, 3.2; N,
coulometer was used in conjunction with
a PAR 173
᎐
3.85%). IR (KBr): ν(C᎐O) 1931, 1852, ν(C᎐O) 1692, ν(Ph)
᎐
᎐
1
galvanostat/potentiostat for coulometry experiments. Infrared
spectra were recorded as KBr pellets on a Perkin-Elmer 1600
Series FT-IR spectrometer, emission spectra on an SLM
Aminco Booman Series 2, luminescence spectrometer.
Elemental microanalyses were performed by MEDAC Ltd.,
Department of Chemistry, Brunel University, Uxbridge, UK.
1633, ν(py) 1610 cmϪ1. H NMR [(CD3)2SO]: δ 9.18 (d), 8.22
(d), 8.07 (t), 7.50–7.25 (m) and 7.02 (t).
(c) cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}] 4. A quantity
of cis-[Re(CO)2(PPh3)(dpk)Cl] (50 mg, 0.07 mmol) was sus-
pended in ethanol (35 cm3) and water (0.5 cm3) was added. The
purple mixture slowly changed to yellow and was refluxed for
5 h. The resulting bright yellow solution was reduced in volume
to ≈10 cm3 and a microcrystalline yellow solid was filtered off,
washed with hexane, diethyl ether and dried; yield 20 mg, 0.03
Results and Discussion
Preparation of cis-[Re(CO)2(PPh3)(L᎐L)Cl] (L᎐L ؍ dpk or dpa)
mmol (43%) (Found: C, 52.45; H, 3.8; N, 3.65. C31H24N2O4PRe
When dpa was allowed to react with [Re(CO)5Cl] in high-
boiling solvents or in the solid state as a melt it was recovered
unchanged. These reactions are in contrast to the facile reaction
observed between [Re(CO)5Cl] and dpk in refluxing toluene to
form fac-[Re(CO)3(dpk)Cl].35 The failure to incorporate dpa
into rhenium() carbonyl moieties directly from [Re(CO)5Cl]
prompted us to search for alternative precursors. The PPh3 lig-
ands in trans-[Re(CO)3(PPh3)2Cl] are substitution labile and the
compound has been used as a precursor for the synthesis of a
variety of rhenium() carbonyl compounds.36 When a mixture
of [Re(CO)5Cl] and ≈2 equivalents of PPh3 was refluxed in tolu-
ene for 24 h an orange solution developed. This was allowed to
cool to room temperature, changing to yellow, and on refluxing
again it became orange again. The addition of a stoichiometric
amount of dpa dissolved in CH2Cl2 to the hot orange mixture
led to the quantitative formation of bright yellow cis-
[Re(CO)2(PPh3)(dpa)Cl]. Following an identical procedure cis-
[Re(CO)2(PPh3)(dpk)Cl] was isolated in good yield when dpk
was used in place of dpa.
᎐
requires C, 52.75; H, 3.45; N, 3.95%). IR (KBr): ν(C᎐O) 1911,
᎐
1828, ν(Ph) 1637, ν(py) 1619, ν(C᎐O) 1234 cmϪ1
.
(d) cis-[Re(CO)2(PPh3)(dpa)Cl] 5. A mixture of [Re(CO)5Cl]
(200 mg, 0.58 mmol), PPh3 (360 mg, 1.40 mmol) and toluene
(80 cm3) was refluxed for 24 h. To the hot orange reaction mix-
ture was added dpa (225 mg, 1.0 mmol) dissolved in CH2Cl2 (10
cm3) and the mixture was allowed to reflux for 20 h. The result-
ing reaction mixture was cooled to room temperature and the
solution reduced in volume to ≈20 cm3. A bright yellow solid
was filtered off, washed with hexane, diethyl ether and dried;
yield 240 mg (86%) (Found: C, 50.65; H, 3.45; N, 5.9.
C30H24ClN3O3Re requires C, 50.65; H, 3.4; N, 5.9%). IR (KBr):
Ϫ1
᎐
ν(C᎐O) 1900, 1820, ν(Ph) 1625, ν(py) 1580, ν(NH) 3358 cm
.
᎐
1H NMR [(CD3)2SO]: δ 9.9 (s), 8.51 (d), 7.67 (t), 7.12 (m), 7.06
(t) and 6.72 (d).
Electrochemical reactions
However, when 2,2Ј-bipyridine was used in place of dpa a
mixture of fac-[Re(CO)3(bipy)Cl], cis-[Re(CO)2(PPh3)(bipy)-
Cl] and other products was obtained. The identity of this mix-
ture, and the solution chemistry of cis-[Re(CO)2(PPh3)(bipy)-
Cl], are currently under investigation.37 The isolation of the
rhenium() dicarbonyl compounds marks the first instance of a
series of dicarbonylchlororhenium() compounds containing a
bidentate nitrogen ligand and are rare examples of rhenium()
dicarbonyl compounds containing bidentate nitrogen ligands.
The known rhenium() dicarbonyl salts containing polypyridyl
ligands are those reported by Casper et al.38 trans,
(a) Methyl chloroformate with fac-[Re(CO)3(dpk)Cl]. The
electrochemical responses in the presence/absence of methyl
chloroformate or CO2 were measured for 1–10 mmol dmϪ3
MeCN or dimethylformamide (dmf) solutions of fac-[Re(CO)3-
(dpk)Cl] in the presence of 0.1 mol dmϪ3 NBun PF6.
4
(b) Group I and II metal ions with fac-[Re(CO)3(dpk)Cl]. The
electrochemical responses in the absence and presence of stoi-
chiometric amounts of the metal ions were measured for 1–10
mmol dmϪ3 MeCN or dmf solutions of fac-[Re(CO)3(dpk)Cl]
in the presence of 0.1 mol dmϪ3 NBun PF6.
4
3572
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578

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CO
Re
N
X
Cl
CO
X
OC
N
CO
N
Re
ROH/H2O
N
O
O
OH
X = CO or PPh3
Scheme 1 X = CO or PPh3
cis-[Re(CO)2(PMe2Ph)2(bipy)]PF6, trans,cis-[Re(CO)2(PPh3)2-
(bipy)]PF6 and cis-[Re(CO)2(bipy)2]PF6. These salts were pre-
pared from the reaction between fac-[ReI(CO)3(bipy)Z]n
(Z = monodentate ligand; n = 0 or ϩ1) with an excess of Z
in refluxing ethylene glycol or from direct carbonylation of
trans,cis-[Re(PMe2Ph)2(bipy)Cl2]PF6 using formic acid.
The intermediate involved in the synthesis of the cis-
dicarbonylchlororhenium() compounds appears to be a vers-
atile synthon for the binding of the ReI(CO)2(PPh3)Cl moiety to
a variety of bi- and mono-dentate ligands.37 Its identity and
reactions with nucleobases and other nitrogen-donor ligands
are currently under investigation.
Nucleophilic addition of water at the ketonic carbon atom of
co-ordinated di-2-pyridyl ketone in fac-[Re(CO)3(dpk)Cl] and
cis-[Re(CO)2(PPh3)(dpk)Cl]
When a sample of fac-[Re(CO)3(dpk)Cl] or cis-[Re(CO)2-
(PPh3)(dpk)Cl] was suspended in an alcoholic solvent con-
taining a trace amount of water fac-[Re(CO)3{(C5H4N)2-
C(O)(OH)}] or cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}] was
isolated (Scheme 1). In the presence of a trace amount of
concentrated HCl the starting material was recovered
unchanged. This marks the first instance where a pair of rhe-
nium compounds containing non-hydrated N,N-bidentate di-2-
pyridyl ketone and hydrated N,O,N-tridentate hydroxydi-2-
pyridylmethoxide were isolated. The hydration of co-ordinated
di-2-pyridyl ketone has been observed in a variety of molecular
systems and provides an interesting pathway for attaching these
systems to insoluble supports.2–7 High-valent rhenium()–
and technetium()–oxo compounds containing hydrated di-2-
pyridyl ketone have recently been reported and there has been
no evidence for the isolation of such compounds containing
co-ordinated di-2-pyridyl ketone in the bidentate mode.17,18
Fig.
1
Infrared spectra of (a) fac-[Re(CO)3(dpk)Cl], (b) cis-
[Re(CO)3{(C5H4N)2C(O)(OH)}] and (c) cis-[Re(CO)2(PPh3)(dpk)Cl] in
KBr pellets
Table 1 Infrared (KBr) CO stretching bands (cmϪ1
)
᎐
Compound
fac-[Re(CO)3(dpk)Cl]a
ν(C᎐O)
᎐
2020, 1913, 1893
2019, 1927, 1866
1930, 1852
1911, 1828
1900, 1820
fac-[Re(CO)3{(C5H4N)2C(O)(OH)}]b
cis-[Re(CO)2(PPh3)(dpk)Cl]c
cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}]d
cis-[Re(CO)2(PPh3)(dpa)Cl]
fac-[Re(CO)3(bipy)Cl]33
2019, 1917, 1895
1925, 1845
1961, 1890
cis-[Re(CO)2(PPh3)(bipy)Cl]37
38
trans,cis-[Re(bipy)(PMe2Ph)2(CO)2]PF6
trans,cis-[Re(bipy)(PPh3)2(CO)2]PF6
trans,cis-[Re(bipy)2(CO)2]PF6
38
1966, 1894
38
1922, 1853
a
ν(C᎐O) 1698 cmϪ1 b ν(C᎐O) 1235 cmϪ1 c ν(C᎐O) 1692 cmϪ1 d ν(C᎐O)
. . .
᎐
᎐
1234 cmϪ1
.
Spectroscopic properties. The CO stretching frequencies of
the compounds are given in Table 1 and Fig. 1 shows the
infrared spectra of fac-[Re(CO)3(dpk)Cl], fac-[Re(CO)3-
For fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] and cis-[Re(CO)2-
(PPh3){(C5H4N)2C(O)(OH)}] in which the ketone group
underwent nucleophilic addition by water the ketonic ν(CO) in
the range 1690–1660 cmϪ1 disappeared, and the pyridine vibra-
tions appeared at higher energy compared to those of the unhy-
drated precursors. Bands appeared at ≈1235 and ≈3400 cmϪ1
assigned to ν(C᎐O) and ν(OH) stretching modes, respectively.
These results are consistent with those reported for a variety
of transition-metal compounds in which the ketone group suf-
fered hydrolysis.13,15–18 In the spectrum of cis-[Re(CO)2(PPh3)-
{(C5H4N)2C(O)(OH)}] a band appeared at 1637 cmϪ1 assigned
{(C5H4N)2C(O)(OH)}] and cis-[Re(CO)2(PPh3)(dpk)Cl]. Three
᎐
strong bands appeared in the ν(C᎐O) stretching region of the
᎐
tricarbonyl compounds similar to those reported for the car-
bonyl stretching modes of fac-[Re(CO)3(bipy)Cl] and other
fac-tricarbonylhalogenorhenium() compounds containing α-
diimine ligands and confirm the assigned fac geometry.24–26 In
the spectra of the dicarbonyl compounds two strong bands
᎐
appeared in the ν(C᎐O) stretching region due to the sym-
᎐
metrical and antisymmetrical stretching vibrations of the two
mutually cis CO ligands. These bands are similar to those
reported for other cis-dicarbonylrhenium() compounds and
consistent with the assigned cis geometry.36–40 For example,
the IR spectrum of [Re(CO)2{MeC(CH2PPh2)3}]ϩ shows two
to the ν(C᎐C) stretching vibrations of the phenyl group and
confirms the presence of co-ordinated PPh3.
᎐
In the IR spectrum of cis-[Re(CO)2(PPh3)(dpa)Cl] a sharp
strong bands in the ν(C᎐O) stretching region at 1948 and 1887
band appeared at 3330 cmϪ1 assigned to the ν(NH) stretching
᎐
᎐
36
cmϪ1
.
mode consistent with the presence of the NH group and the
absence of hydrogen bonding. The ν(C᎐O) stretching modes
appeared at lower wavenumbers compared to those of cis-
᎐
The N,N-co-ordinated dpk complexes, fac-[Re(CO)3(dpk)Cl]
and cis-[Re(CO) (PPh )(dpk)Cl], have their ketonic ν(C᎐O)
᎐
᎐
2
3
stretching band and the combined ν(C᎐C) and ν(C᎐N) stretch-
[Re(CO)2(PPh3)(dpk)Cl] consistent with the stronger π-donor
᎐
᎐
ing modes of the pyridine rings shifted to higher energy (≈10–
20 cmϪ1) compared to free dpk. This is consistent with the
decrease in electron density in the rings and in accord with
results reported for a variety of transition-metal compounds
containing dpk in a bidentate N,N-co-ordination mode.11,12,16
ability of dpa compared to dpk. The combined ν(C᎐C) and
᎐
ν(C᎐N) pyridine vibrations appeared at higher wavenumbers
᎐
than those observed for free dpa.
In the 1H NMR spectra of these compounds a series of
resonances were observed consistent with the presence of the
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578
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Table 2 Electronic absorption spectral data and electrochemical properties
Compound
Electronic absorption spectra,a λ/nm (ε/dm3 molϪ1 cmϪ1
388 (3082), 284 (13 418), 232 (11 393)
)
CV wave potentials,b E/V vs. SSCE
1 fac-[Re(CO)3(dpk)Cl]
Ϫ0.82, Ϫ1.32, Ϫ1.47
2 fac-[Re(CO)3{(C5H4N)2C(O)(OH)}]
380 (sh) (600), 310 (1900), 266 (3100), 240 (3240), 221 (sh)
ϩ1.3, Ϫ1.98 (Ep,c), Ϫ2.20 (Ep,c)
(5360), 200 (13 100)c
3 cis-[Re(CO)2(PPh3)(dpk)Cl]
4 cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}] 410 (sh) (450), 356 (5980), 270 (9758), 252 (11 650), 230 (sh)
472 (3556), 328 (5418), 270 (13 268), 234 (23 678)
ϩ1.03, Ϫ0.99, Ϫ1.67 (Ep,c
ϩ0.75, Ϫ2.17 (Ep,c
)
)
(24 674)
5 cis-[Re(CO)2(PPh3)(dpa)Cl]
6 fac-[Re(CO)3(bipy)Cl]
502 (4190), 356 (5100), 292 (7590), 232 (10 466)d
386 (4356), 294 (18 598), 242 (19 057)
ϩ1.65 (Ep,a), ϩ0.88, Ϫ2.18 (Ep,c
Ϫ1.35, Ϫ1.7129,e
)
Recorded on MeCN solutions. b Half-wave potentials (unless otherwise stated) recorded in 0.1 mol dmϪ3 NBun4PF6–dmf using a glassy carbon
a
disc electrode (0.07 cm2) at ν = 400 mV sϪ1
platinum-disc electrode at ν = 100 mV sϪ1
.
c Recorded in CH2Cl2 solution. d Incompletely soluble. e In 0.1 mol dmϪ3 NBun4PF6–MeCN using a
.
Table 3 Solvent dependence of the MLCT absorption maxima of
fac-[Re(CO)3(dpk)Cl] 1, cis-[Re(CO)2(PPh3)(dpk)Cl] 3 and cis-[Re-
(CO)2(PPh3){(C5H4N)2C(O)(OH)}] 4 at 298 K
EM LCT/kJ molϪ1
Solvent
1
3
4
(π*)42,43
Triethylamine
Diethyl ether
Tetrahydrofuran
Dimethylformamide
Dimethyl sulfoxide
Carbon tetrachloride
Chloroform
Chlorobenzene
Methylene chloride
Mesitylene
300.61
305.21
311.57
319.90
321.62
291.81
302.13
305.21
308.36
302.13
302.13
302.13
308.36
311.57
251.35
253.48
255.65
261.12
262.37
246.18
250.03
253.48
254.56
252.41
252.41
252.41
254.45
256.75
318.20
323.36
325.12
332.34
334.20
326.00
329.60
330.51
333.27
323.36
323.36
326.90
328.69
330.51
0.14
0.27
0.58
0.88
1.0
0.29
0.58
0.71
0.83
0.41
0.54
0.59
0.73
0.87
Fig.
3
Energies of MLCT absorption (EM LCT) of (᭜) fac-[Re-
Toluene
Benzene
Anisole
Pyridine
(CO)3(dpk)Cl], (᭡) cis-[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}] and (᭿)
cis-[Re(CO)2(PPh3)(dpk)Cl] plotted against Taft’s π* solvent values in
non-chlorinated aliphatic solvents
metal-to-ligand charge transfer (MLCT) transition and intense
high-energy bands originating from ligand-to-metal charge-
transfer (LMCT) transitions appeared in the spectra of these
compounds. These features are similar to those reported for fac-
[Re(CO)3(bipy)Cl] and other rhenium()–diimine complexes of
the type fac-[Re(CO)3(L᎐L)Cl] (L᎐L = α-diimine ligand).24–28
The dicarbonyl compounds have their dπ(Re) → π* (MLCT)
transitions red shifted compared to the tricarbonyl compounds
due to the substitution of a CO ligand with PPh3 which elevates
the energy of the d orbitals and subsequently increases the
π-back-bonding capacity of the co-ordinating diimines. This
effect has been observed previously for a variety of low-valent
metal carbonyl compounds containing α-diimine ligands.28 The
lowest absorption bands in these complexes exhibit a marked
solvatochromic behaviour, a characteristic property of MLCT
transitions.41,42 The solvent dependence of the MLCT absorp-
tion maxima, EM LCT, is given in Table 3 and plots of Taft’s
solvent-polarity scale,42,43 π*, vs. EM LCT for fac-[Re(CO)3-
(dpk)Cl], cis-[Re(CO)2(PPh3)(dpk)Cl] and cis-[Re(CO)2(PPh3)-
{(C5H4N)2C(O)(OH)}] in non-chlorinated aliphatic solvents
are shown in Fig. 3. Straight lines were obtained with slopes of
24.30, 17.30 and 12.70 for fac-[Re(CO)3(dpk)Cl], cis-[Re(CO)2-
(PPh3){(C5H4N)2C(O)(OH)}Cl] and cis-[Re(CO)2(PPh3)(dpk)-
Cl], respectively. The higher the value of the slope the higher is
the sensitivity of the MLCT transition to solvent variation and
points to a large increase in charge-transfer character of the
transition in accord with the increase in energy gap between the
highest occupied molecular orbital (HOMO, dπ orbital) and the
lowest unoccupied molecular orbital (LUMO, π* orbital). The
red shift observed in the spectra of these compounds in going
from fac-[Re(CO)3(dpk)Cl] to cis-[Re(CO)2(PPh3){(C5H4N)2-
C(O)(OH)}] to cis-[Re(CO)2(PPh3)(dpk)Cl] is consistent with
the decrease in the HOMO Ϫ LUMO energy gap and the sensi-
tivity of these transitions to solvent variations. The hydration
of the keto group and the co-ordination of the oxo-anion
increases the electron density at both centres and elevates the
......
Fig. 2 Electronic absorption spectra of fac-[Re(CO)3(dpk)Cl] (
cis-[Re(CO)2(PPh3)(dpk)Cl] (——) and cis-[Re(CO)2(PPh3){(C5H4N)2-
C(O)(OH)}] (------) in MeCN
),
ligands. Four sets of ring resonances appeared in the spectrum
of each molecule pointing to the presence of a mirror plane of
symmetry consistent with the assigned geometry.
The electronic absorption spectral properties of the com-
pounds are summarized in Table 2 and Fig. 2 shows the spectra
of fac-[Re(CO)3(dpk)Cl], cis-[Re(CO)2(PPh3)(dpk)Cl] and cis-
[Re(CO)2(PPh3){(C5H4N)2C(O)(OH)}] measured in dmf. A
low-energy absorption band assigned to a dπ(Re) → π*(N᎐N)
3574
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578

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Fig. 4 Emission spectra of (a) fac-[Re(CO)3(dpk)Cl] in the solid state,
(b) cis-[Re(CO)2(PPh3)(dpa)Cl] in deoxygenated dmf solution at room
temperature and (c) fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] in the solid
state
CO
Re
N
CO
Re
Cl
CO
CO
OC
N
CO
N
Fig. 5 Cyclic voltammograms of (a) fac-[Re(CO)3(dpk)Cl], (b) cis-
dmf, hν
N
[Re(CO)2(PPh3)(dpk)Cl] and (c) fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] in
dmf solutions (0.1 mol dmϪ3 NBun4PF6) at a glassy carbon working
electrode (0.07 cm2) and a scan rate of 400 mV sϪ1 vs. SSCE
O
O
H
Electrochemical properties. The electrochemical properties of
these compounds were investigated using voltammetric (cyclic
and square-wave) techniques and the results are summarized
in Table 2. Fig. 5 shows cyclic voltammograms of fac-
[Re(CO)3(dpk)Cl], fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] and cis-
[Re(CO)2(PPh3)(dpk)Cl]. These results reveal the presence of
Scheme 2
energy of both the π* and dπ orbital. The energy gap in the
hydrated compounds is larger than in the non-hydrated com-
pounds consistent with the blue shift observed in their elec-
tronic absorption spectra and higher solvent sensitivity. Owing
to the low solubility of fac-[Re(CO)3{(C5H4N)2C(O)(OH)}] in a
variety of solvents we were not able to compare the sensitivity
of its MLCT transition to solvent variations.
three waves/peaks assigned to ReII/I, diimine/diimine Ϫ and ReI/0
᝽
consistent with those reported for a variety of tricarbonylhal-
ogenorhenium() compounds containing polypyridyl or α-
diimine ligands.29,33
Excitation of
a deoxygenated sample of cis-[Re(CO)2-
(PPh3)(dpa)Cl] in dmf at room temperature resulted in a yellow
emission at 560 nm [Fig. 4(b)]. Under the same conditions
no emission was observed for fac-[Re(CO)3(dpk)Cl]; however,
excitation of a solid sample of it at room temperature gave a
broad emission at 570 nm [Fig. 4(a)]. The luminescence max-
ima for these compounds occur in the same range as observed
for other fac-[ReI(CO)3(α-diimine)] systems and possibly
The hydrated compounds have their potentials shifted nega-
tively compared to the non-hydrated precursors consistent with
the decrease in electron-withdrawing ability of the hydrated lig-
ands and the increase in electron density around the metal as a
result of substitution of the chloride anion with the strong σ-
donor alkoxy group. The shift is most pronounced for the first
reduction wave which for the hydrated compounds occurs ≈1.20
V to more negative potentials as compared to the non-hydrated
compounds.
3
derived from the MLCT state.44 When a solid sample of fac-
[Re(CO)3{(C5H4N)2C(O)(OH)}] was excited at 350 nm no emis-
sion was observed [Fig. 4(c)]. The quenching of the excited state
of fac-[Re(CO)3(dpk)Cl] in dmf points to a low-lying metal-to-
ketone charge-transfer excited state that reduces the keto group
to hydrol. The photoreduction of dpk to hydrol in non-aqueous
media 45 and the photosensitization of dpk to hydrol by the
excited state of [Ru(bipy)3]2ϩ have been reported 46 and fac-
[Re(CO)3(dpk)Cl] because of its high extent of delocalization
of electrons/charge and the presence of a low-lying MLCT
transition is unexceptional in this regard. This is in accord
with the lack of emission in the hydrated complex, fac-
[Re(CO)3{(C5H4N)2C(O)(OH)}] and consistent with the pho-
toreduction of ketones to alcohols by the excited state of fac-
[Re(CO)3L2X] (X = Cl or I, L = 4-benzoylpyridine or 4-
acetylpyridine).47 The reduction of the keto group changes the
hybridization of the carbonic carbon atom from sp2 to sp3, and
places the oxo-anion at right angles to the N2Re plane in close
proximity to the octahedral co-ordination site. This arrange-
ment leads to a steric congestion or a strain around the metal
and the co-ordination of the oxo-anion (Scheme 2), and is
analogous to the addition of nucleophiles at the ketonic carbon
atom of co-ordinated dpk which changes its co-ordination
mode from bi- to tri-dentate (Scheme 1).
In the voltammograms of cis-[Re(CO)2(PPh3)(L᎐L)Cl]
(L᎐L = dpk or bipy) the oxidation wave assigned to the ReI/II
couple is significantly negatively shifted and the reduction
waves are slightly negatively shifted compared to those of the
tricarbonyl compounds. This is consistent with the increase in
electron density around the metal and the elevation of the
energy of the d orbitals when a CO is substituted with the better
σ donor PPh3 and in accord with the red shift observed in the
electronic absorption spectra of the dicarbonyl compared to the
tricarbonyl compounds.
A comparison of the voltammetric data for cis-[Re(CO)2-
(PPh3)(dpk)Cl], cis-[Re(CO)2(PPh3)(bipy)Cl] and cis-[Re(CO)2-
(PPh3)(dpa)Cl] reveals that a decrease in the electron-with-
drawing ability of the diimine ligands causes the reduction
potentials to shift to more negative values. The shift is pro-
nounced for the first reduction wave where the dpa complex has
potentials 0.92 and 1.19 V more negative than those of the
compounds containing bipy and dpk, respectively. The energy
of the π* orbital increases in the order dpk > bipy ӷ dpa.
Electrochemical reactions of fac-[Re(CO)3(dpk)Cl] with CO2.
The electrochemical reactions of CO2 with fac-[Re(CO)3-
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578
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• –
+ e–
2–
CO
Re
N
X
CO
Re
N
CO
Re
N
CO
Cl
CO
X
Cl
CO
N
Cl
CO
CO
Cl
CO
CO
Cl
CO
CO
+ 2e–, + CO2
– Cl–, + H+
Re
O
Re
N
+ e–
– e–
N
N
N
N
N
– e–
O
CO2H
O
O
O
Scheme 3
I
II
III
– Cl–
CO
CO
CO
Re
N
Cl
CO
CO
CO
O2C
CO
CO
Re
N
Re
N
+ 2e–
– Cl–
+ CO2
– CO2
CO
Re
N
CO
N
N
CO
N
MeO2C
N
CO
CO
CO
Re
N
ClCO2Me
– Cl–
N
CO
O
O
O
Scheme 4
O
O
V
IV
Scheme 5
fac-[Re(CO)3(dpk)Cl] were noted although subtle differences
are apparent. In a reductive scan the first wave maintained its
reversibility and the second wave lost its reversibility signalling
an electrochemical reaction following the second electron trans-
fer. When the electrochemical cell was saturated with CO2 the
first and second reductive waves lost their reversibility [Fig. 6(c)]
signalling electrochemical reaction(s) at both sites. When the
electrochemical cell was purged with nitrogen the resulting solu-
tion [Fig. 6(d)] showed an electrochemical behaviour similar to
that observed before treatment with CO2. These results suggest
that CO2 is more reactive than ClCO2Me and can be confirmed
by calculating the rate constants for the reaction of CO2 and
ClCO2Me with the electrochemically generated species at the
first and second electron transfer. The rate constant for the
reaction of ClCO2Me with the electrochemically generated rad-
ical anion can be estimated from the standard reduction poten-
Ϫ
tials of first reduction wave (EЊ1) and ClCO2Me–ClCO2Me
᝽
(EЊ2) from RT ln K = ϪnF(EЊ1 Ϫ EЊ2) where K = k1/kϪ1 and kϪ1 is
taken as the diffusion limit, kdiff = 5 × 109 dm3 molϪ1 Ϫ1, to
s
estimate an upper value for k1.35 The potential for the
Ϫ
ClCO2Me–ClCO2Me couple was estimated to be <Ϫ2.60 V
᝽
vs. SSCE.48 Thus, ln K < ϪnF/RT (Ϫ0.9 ϩ 2.6) and k1 <
Fig. 6 Cyclic voltammograms of fac-[Re(CO)3(dpk)Cl] (0.01 mol
dmϪ3) in MeCN (0.1 mol dmϪ3 NBun4PF6) at a glassy carbon working
electrode (0.07 cm2) at a scan rate of 400 mV sϪ1 vs. SSCE: (a) under a
nitrogen atmosphere, (b) in the presence of ClCO2Me (0.01 mol dmϪ3),
(c) as in (b) but saturated with CO2 and (d) as in (c) after removal of
CO2 by purging with N2
9.00 × 10Ϫ22 dm3 molϪ1 Ϫ1. The value for the reaction of the
s
same radical anion with CO2 was estimated to have an upper
35
value of 5.00 × 10Ϫ13 dm3 molϪ1 sϪ1
.
These values are in
accord with observed reactivity patterns and point to insigni-
ficant reaction between ClCO2Me and the electrochemically
generated radical anion at the first electron transfer.
(dpk)Cl] were published elsewhere.35 These reactions are solvent
dependent and controlled by the rate of diffusion of the
electroactive species from the electrode surface. In dmf at scan
The quasi-reversibility of the second reduction wave hints at
a slow chemical reaction following the second electron transfer.
As the scan rate decreases the wave shifts to more positive
potentials, signifying a decrease in electron density around the
metal. The rate constants for the reaction of ClCO2Me and CO2
at the second electron transfer are estimated to have upper
rates <800 mV sϪ1 and in MeCN at scan rates <80 mV sϪ1
a
stoichiometric carboxylation of the carbonylic carbon atom
followed by rapid chemical steps leading to displacement of
the chloride anion and attack of the oxo-anion at the metal
(Scheme 3). In MeCN, at scan rates >80 mV sϪ1, and in dmf at
scan rates >800 mV sϪ1 the reaction of CO2 with the electro-
chemically generated radical anion is slow and a second elec-
tron transfer leads to the formation of a co-ordinatively
unsaturated intermediate that binds to CO2 (Scheme 4).
values of 2.54 × 10Ϫ11 and 1.46 × 10Ϫ4 dm3 molϪ1 Ϫ1, respect-
s
ively.35 The low reactivity of ClCO2Me is due to its slow electro-
chemical dissociation to ϩCO2Me and ClϪ as reflected by
appearance of a pre-wave before the first reduction wave. The
results of these voltammograms reveal an electrochemical
pathway for the reaction between ClCO2Me and fac-
[Re(CO)3(dpk)Cl] as shown in Scheme 5. This mechanism is
similar to that we reported for the electrochemical reaction of
fac-[Re(CO)3(dpk)Cl] with CO2 in MeCN,35 and is consistent
with the positive shift noted for the second reduction wave as
the scan rate decreases, absence of any reactivity upon the first
electron transfer and the appearance of a chloride–chlorine
oxidation wave.
Electrochemical reactions of fac-[Re(CO)3(dpk)Cl] with
methyl chloroformate. The reductive carboxylation of unsatur-
ated organic compounds such as ketones, nitro compounds, n-
heterocycles and phthalazine with methyl chloroformate have
been described and the electrophilic character of methyl for-
mate cation was demonstrated.49 Cyclic voltammograms of fac-
[Re(CO)3(dpk)Cl] in the absence/presence of ClCO2Me and
CO2 in MeCN are shown in Fig. 6. In the presence of ClCO2Me
electrochemical signatures [Fig. 6(b)] similar to those we
observed for the electrochemical reactions between CO2 and
The electrochemical responses (cyclic voltammograms) of
fac-[Re(CO)3(dpk)Cl] in the presence of ClCO2Me in dmf are
shown in Fig. 7. In these voltammograms a pre-wave appeared
before the first reduction wave and as the concentration of
3576
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578

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Fig. 7 Cyclic voltammograms of fac-[Re(CO)3(dpk)Cl] (0.01 mol
dmϪ3) in dmf (0.1 mol dmϪ3 NBun4PF6). Other details as in Fig. 6: (a)
under a nitrogen atmosphere; (b) in the presence of ClCO2Me (0.01 mol
Fig. 8 Cyclic voltammograms of fac-[Re(CO)3(dpk)Cl] (0.008 mol
dmϪ3) in dmf solutions (0.1 mol dmϪ3 NBun4PF6) in the presence of
0.01 mol dmϪ3 KPF6 (a), NaBF4 (b) and Mg(ClO4)2 (c) at a glassy
carbon working electrode (0.07 cm2) at a scan rate of 400 mV sϪ1 vs. Ag
dmϪ3) and (c) in the presence of ClCO2Me (0.02 mol dmϪ3
)
• –
•
CO
Re
N
CO
Re
N
CO
Re
N
Cl
CO
CO
Cl
CO
CO
Cl
CO
CO
+CO2Me
+ e–
– e–
N
N
N
O
O
O
MeO2C
I
II
VI
– e– + e–
–
CO
Re
O
CO
OC
N
CO
N
Cl
Re
N
CO
CO
– Cl–
N
O
MeO2C
VIII
MeO2C
VII
Scheme 6
ClCO2Me increases the first wave merges with the pre-wave,
becomes irreversible and the second reduction wave disappears.
An electrochemically generated oxidation wave due to 2ClϪ–Cl2
oxidation appeared only on a reductive scan. With the excep-
tion of the pre-wave, these results are similar to those we
reported for the electrochemical reactions of CO2 with fac-
[Re(CO)3(dpk)Cl] in dmf and point to the formylation of a
carbonylic carbon atom and the attack of the oxo-anion at the
metal as shown in Scheme 6. The pre-wave in these voltam-
mograms is associated with the adsorption and dissociation of
ClCO2Me at the electrode surface.
Fig. 9 Square-wave voltammograms of fac-[Re(CO)3(dpk)Cl] (0.008
mol dmϪ3) in dmf solutions (0.1 mol dmϪ3 NBun4PF6) in the presence
of 0.01 mol dmϪ3 KPF6 (a), NaBF4 (b) and Mg(ClO4)2 (c) at a glassy
carbon working electrode (0.07 cm2), a scan increment of 10 mV and
a frequency of 60 Hz vs. Ag
tion waves and the latter is more sensitive. The first reduction
wave shows a slight increase in reductive current and the second
shifts to more positive potentials consistent with the decrease in
electron density around the metal. The magnitude of the shift
depends on polarizing power of the cation, the cation with larg-
est charge to radius ratio, Mg2ϩ > Naϩ > Kϩ, causing the most
significant perturbation. The potassium cation due to its lower
charge density slightly interacts with the metal-based couple as
noted by the slight positive shift and the reversibility of this
couple. These electrochemical responses are similar to those
observed for a variety of transition-metal and organic redox-
active macrocyclic receptor molecules in the presence of Group
Electrochemical recognition of Group I and II metal ions
by fac-[Re(CO)3(dpk)Cl]. Cyclic voltammograms of fac-
[Re(CO)3(dpk)Cl] in dmf in the presence of NaBF4, KPF6 and
Mg(ClO4)2 are shown in Fig. 8 and Fig. 9 shows square-wave
voltammograms measured for the same solutions. These vol-
tammograms reveal that fac-[Re(CO)3(dpk)Cl] electrochemic-
ally senses Group I and II metal ions in solution. Although
subtle differences in their responses were noted, the general
trend is that metal ions are sensed at the first and second reduc-
J. Chem. Soc., Dalton Trans., 1997, Pages 3571–3578
3577

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I and II metal ions.49–51 Owing to the convenient synthesis of
fac-[Re(CO)3(dpk)Cl] the current system may find a wide range
of applications in a variety of voltammetric and amperometric
molecular sensing devices.
20 M. R. Kratz and D. G. Hendricker, Polymer, 1986, 27, 1641.
21 M. Bakir and B. P. Sullivan, J. Chem. Soc., Dalton Trans., 1995,
1733.
22 M. Bakir, S. Paulson, P. Goodson and B. P. Sullivan, Inorg. Chem.,
1992, 31, 1127.
23 G. A. Neyhart, M. Bakir, J. Boaz, W. J. Vining and B. P. Sullivan,
Coord. Chem. Rev., 1991, 111, 27.
24 G. Tapolsky, R. Duesing and T. J. Meyer, Inorg. Chem., 1990, 29,
2285.
25 R. Lin, Y. Fu, C. P. Brock and T. F. Guarr, Inorg. Chem., 1992, 31,
4346.
26 M. S. Wrighton and D. L. Morse, J. Am. Chem. Soc., 1974, 96, 998.
27 B. D. Rossenaar, D. T. Stufkens and A. Vlcek, Jr., Inorg. Chem.,
1996, 35, 2902.
28 V. D. J. Stufkens, Coord. Chem. Rev., 1990, 104, 39.
29 B. P. Sullivan, C. M. Bolinger, D. Conrad, W. J. Vining and T. J.
Meyer, J. Chem. Soc., Chem. Commun., 1985, 1414.
30 K. Yang, S. G. Bott and M. G. Richmond, Organometallics, 1995,
14, 2387.
31 N. B. Thornton and K. S. Schanze, Inorg. Chem., 1993, 32, 4994.
32 J. C. Vites and M. M. Lynam, Coord. Chem. Rev., 1995, 142, 1.
33 G. J. Stor, F. Hartl, J. W. M. van Outersterp and D. J. Stufkens,
Organometallics, 1995, 14, 1115.
Conclusion
The first series of rhenium() di- and tri-carbonyl compounds
containing polypyridyl-like ligands has been isolated. These
compounds exhibit rich physicochemical properties, and the
potential application of the di-2-pyridyl ketone complexes as
electrochemical sensors for electrophiles and Group I and II
metal ions in solutions has been demonstrated.
Acknowledgements
We acknowledge the Research and Publication Committee and
Board of Postgraduate Studies of University of the West Indies
and the Third World Academy of Sciences for funding and Dr.
Zakir Murtaza for recording the emission spectra.
34 V. W.-W. Yam, K. K.-W. Lo, K.-K. Cheung and R. Y.-C. Kong,
J. Chem. Soc., Chem. Commun., 1995, 1191.
35 M. Bakir and J. A. M. McKenzie, J. Electroanal. Chem., 1997, 425,
621.
36 C. Bianchini, A. Marchi, L. Marvelli, M. Peruzzini, A. Romerosa,
R. Rossi and A. Vacca, Organometallics, 1995, 14, 3203.
37 M. Bakir, unpublished work.
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