Voltammetry of monomeric and dimeric oxorhenium(V) complexes of Schiff base ligands N,N′-ethylenebis(acetylacetone)diimine, N,N′-ethylene(salicylidene)diimine and N,N′-phenylenebis(salicylidene)diimine

Article abstract of DOI:10.1016/S0020-1693(96)05384-4

The electrochemical study of six Schiff base complexes of oxorhenium(V) of the general formula Re₂O₃L₂ and ReOClL (L=N,N′-ethylenebis(acetylacetone)diimine (acacen), N,N′-ethylene(salicylidene)diimine (salen) and N,N′-phenylenebis(salicylidene)diimine (salphen)) was undertaken. In non-aqueous media the oxorhenium(V) monomers exhibit an equilibrium in solution (between forms ReOClL and [ReOL]⁺+Cl^-), the position of the equilibrium dependent on L. A one-electron reduction and one-electron oxidation are observed, followed by a fast chemical reaction, resulting in decomposition of the complex. The μ-oxo dimeric complexes underwent a two-electron reduction followed by decomposition to an electroinactive product. Successive one-electron oxidations of each rhenium in Re₂O₃L₂ were observed. Each electron-transfer step was coupled to a chemical reaction; the generation of [Re₂O₃L₂]⁺ was followed by the cleavage of the μ-oxo bond and formation of mono-oxo species. This reaction was much slower than the decomposition which followed generation of [Re₂O₃L₂]²⁺.

Full text of DOI:10.1016/S0020-1693(96)05384-4

Inorganica Chimica Acta 255 (1997) 149–155
Voltammetry of monomeric and dimeric oxorhenium(V) complexes of
Schiff base ligands N,N9-ethylenebis(acetylacetone)diimine,
N,N9-ethylene(salicylidene)diimine and
N,N9-phenylenebis(salicylidene)diimine
U
Lawrence A. Bottomley a, Paul E. Wojciechowski a, Grant N. Holder b,
a School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
b Department of Chemistry, Appalachian State University, Boone, NC 28608, USA
Received 17 October 1995; revised 16 May 1996
Abstract
The electrochemical study of six Schiff base complexes of oxorhenium(V) of the general formula Re2O3L2 and ReOClL (LsN,N9-
ethylenebis(acetylacetone)diimine (acacen), N,N9-ethylene(salicylidene)diimine (salen) and N,N9-phenylenebis(salicylidene)diimine
(salphen)) was undertaken. In non-aqueous media the oxorhenium(V) monomers exhibit an equilibrium in solution (between forms ReOClL
and [ReOL]qqCly), the position of the equilibrium dependent on L. A one-electron reduction and one-electron oxidation are observed,
followed by a fast chemical reaction, resulting in decomposition of the complex. The m-oxo dimeric complexes underwent a two-electron
reduction followed by decomposition to an electroinactive product. Successive one-electron oxidations of each rhenium in Re2O3L2 were
observed. Each electron-transfer step was coupled to a chemical reaction; the generation of [Re2O3L2]q was followed by the cleavage of the
m-oxo bond and formation of mono-oxo species. This reaction was much slower than the decomposition which followed generation of
q
[Re2O3L2]2
.
Keywords: Electrochemistry; Rhenium complexes; Oxo complexes; Schiff base complexes
1. Introduction
Several studies of oxorhenium(V) Schiff base ligands have
appeared [7], undoubtedly prompted by the influence of
ligand identity on the thermodynamic and kinetic stability of
the similar [TcO]3q core toward ligand substitution reac-
tions, an important consideration for an in vivo radioimaging
agent, the action of which depends on its formulation.
Oxorhenium(V) complexes have been known for 4 dec-
ades. Recent interest in these complexes stems from their
utility as intermediates in the synthesis of a large number of
neutral and ionic compounds as well as their catalyticactivity
[1]. Some rhenium complexes show oxo-atom transfercapa-
bility, although rhenium complexes have receivedonlyminor
scrutiny in this regard [2]. Oxorhenium complexes are also
useful in the design of new radiopharmaceuticals. The redox
behavior of the Re and Tc complexes are quite similar, and
the redox behavior is correlated to the bioactivity of 99mTc
radioimaging agents [3]. Thus, many workers evaluate the
reactivity of the rhenium complex prior to evaluation and
testing of the corresponding technetium complex.
2. Experimental
2.1. Materials
Ammonium perrhenate (Aesar Chemicals) was used as
received. The precursor ReOCl3(P(C6H5)3)2 was prepared
and characterized according to the method of Chatt and Rowe
[8]. Ethylenediamine and salicylaldehyde were obtained
from Aldrich and used without further purification. Acetyl-
acetone was obtained from Eastman Chemicals and distilled
under air prior to use. The fraction boiling at 1378C was
collected and redistilled until examination by NMR yielded
Schiff base complexes containing the oxorhenium(V)
moiety are known; these relatively polarizable ligands sta-
bilize the rather acidic [ReO]3q core, as do complexes of
pyridines [4], phosphines [5] and dithiocarbamates [6].
U Corresponding author.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05384-4

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L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
only resonances characteristic of the b-diketone. All Schiff
base ligands were prepared by condensation of either ethyl-
enediamine or o-phenylenediamine and either acetylacetone
or salicylaldehyde, accordingtothemethodofMcCarthy[9].
Recrystallization was from absolute ethanol and characteri-
zation was by NMR and IR spectroscopy. All reactions were
carried out under dry N2 atmosphere.
The electrochemical solvent, Aldrich Gold Label HPLC
grade 1,2-dichloroethane, was used as received. The sup-
porting electrolyte used was [n-Bu4N][ClO4] purchased
from Fisher Chemical, which wassubsequentlyrecrystallized
three times from EtOH, dried in vacuo and kept in an oven
at 1108C prior to use.
Infrared spectra were obtained with a Nicolet 5DXB Fou-
rier transform IR spectrometer. All NMR spectra were
obtained using a Bruker WM-300 Fourier transform spec-
trometer using deuterated solvents obtained from Aldrich.
Cyclic voltammetry/differential pulse voltammetry was per-
formed using an IBM model EC-225VoltammetricAnalyzer.
Output was obtained on a Houston Instruments Omnigraphic
2000 X–Y Recorder (for scan rates F0.500 V sy1)ora
Tektronix model 564 storage oscilloscope equipped with a
type 2A60 amplifier in the X-channel and a type 3A72 dual
trace amplifier in the Y-channel. A model C-12 oscilloscope
camera was used to record data on Polaroid type 667 (ASA
3000) black and white film. Coulometric andconductometric
instruments were designed and built by the electronics shop
in the School of Chemistry. Elemental analyses were
performed by Atlantic Microlab, Atlanta, GA, USA.
no evidence for the formation of a trans-dioxo species (m–s
absorption ;860 cmy1).
2.2.2. Re2O3(salen)2
Two methods of synthesis were employed to obtain this
complex.
Method A: The synthesis of Middleton et al. [10] as mod-
ified above for Re2O3(salphen)2.
Method B: Preparation of ReOCl(salen) followed by m-
oxo dimerization on activated basic alumina in 7:3 (vol./
vol.) CH2Cl2/Me2CO. The dimerization yield was always in
excess of 80% based on the amount of monomer loaded onto
the column and the eluted product required no further puri-
fication. Anal. Calc. for C32H28N4O7Re2: C, 40.34; H, 3.94;
N, 5.87. Found (Method B): C, 40.81; H, 3.98; N, 5.95%.
The IR stretch at 690(s) cmy1 confirmed the presence of the
m-oxo bridge and an absorption at 965(w) cmy1 verified
existence of the (ReO)3q moiety. There were no absorptions
in the 860 cmy1 range corresponding to the formation of a
monomeric trans-dioxo species.
2.2.3. Re2O3(acacen)2
The conditions described for the analogous salphen com-
plex apply except that the reaction is not complete until after
2 h of reflux. Yields decrease as the time of reflux exceeded
this limit; best yield obtained was 40%. Purification was
accomplished by chromatography on basic alumina using 1:1
(vol./vol.) CH2Cl2/MeOH mobile phase. The final, dark
green band was collected. Anal. Calc. for C24H36N4O7Re2:
C, 33.30; H, 4.20; N, 6.50. Found: C, 33.39; H, 4.27; N,
6.19%. IR absorptions at 720(s) and 975(w) cmy1 con-
firmed the presence of the O_Re–O–Re_O structure. IR
absorptions characteristic of monomeric trans-dioxo species
were absent. The preparation of this complex from the mon-
omer was also successfully demonstrated on basic alumina
in CH2Cl2. This is not a useful approach to Re2O3(acacen)2,
however, as synthesis of the pure monomer is difficult.
2.2. Syntheses
2.2.1. Re2O3(salphen)2
Repeated attempts to synthesize this complex according to
literature procedure gave dramatically lower yields than orig-
inally reported [10]. The procedure presented here gave the
highest yields and purities and required the simultaneous
addition of triethylamine and ligand. Into a solution of 1.0 g
(1.2 mmol) ReOCl3(P(C6H5)3)2 in 100 ml of reagent grade
toluene, 0.38 g (1.2 mmol) of H2salphen and 1.0 ml (7.2
mmol) of triethylamine were added simultaneously withvig-
orous stirring. The yellow color of the solution darkened to
brown as the mixture was then brought to reflux for 90 min.
The reaction mixture was cooled to r.t. and filtered. The
brown product was collected and washed with three 10 ml
aliquots of toluene, five 10 ml aliquots of deionized distilled
water and five 10 ml portions of diethyl ether.
The compound was purified on a Sephadex LH-20column.
Elution of Re2O3(salphen)2 with a 1:1 (vol./vol.) CH2Cl2/
Me2CO mobile phase produced a faint yellow band which
was eluted prior to the dark brown band containing the prod-
uct. Yield after chromatography was 62% based on
ReOCl3(P(C6H5)3)2. Anal. Calc. for C40H28N4O7Re2:C,
40.47; H, 2.62; N, 4.44. Found: C, 40.50; H, 2.77; N, 4.42%.
IR stretches at 700(s) and 962(w) cmy1 confirm the pres-
ence of the O_Re–O–Re_O structural backbone. There was
2.2.4. ReOCl(salphen)
This complex proved very elusive due to a tendency for
this complex to form the m-oxo dimer, Re2O3(salphen)2,
even under conditions of rigorous exclusion of atmospheric
moisture and oxygen. The following procedure provided the
highest yield of ReOCl(salphen). In an oven dried flask con-
taining 1.0 g ofReOCl3(P(C6H5)3)2, toluenewhichhadbeen
dried over pellitized Na for36 h wasintroducedwithvigorous
stirring. An amount of 0.38 g (1.2 mmol) of dry, degassed
H2salphen and 0.33 ml (2.4 mmol) of triethylamine were
added simultaneously. No significant color change was
observed until the reaction was brought to reflux. After 60
min a dark brown color appeared and an insoluble material
formed. If allowed to reflux beyond 90 min, a dark oil formed
and yield dropped significantly. Optimum yield was obtained
following 60"5 min of reflux.
Purification was on a Sephadex LH-20 column, as basic
alumina caused the compound to form the m-oxo dimer. A

L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
151
7:3 (vol./vol.) CH2Cl2/Me2CO solution was used as mobile
phase, the system again eluting a faint yellow band before
the dark brown product. Anal. Calc. for C20H14ClN2O3Re: C,
42.11; H, 2.80; N, 4.91; Cl, 6.22. Found: C, 42.30; H, 2.84;
N, 4.90; Cl, 6.24%. IR absorption at 948(m) cmy1 indicated
the presence of the Re_O stretch. There were no IR absorp-
tions characteristic of a m-oxo dimer (;700 cmy1)ora
trans-dioxo monomer (;860 cmy1). Yield of purified
product was 58% after chromatography.
2.2.5. ReOCl(salen)
This complex was successfully prepared in 50.5% yield
using the procedures outlined for ReOCl(salphen) withcare-
fully prepared solvent. Purification was accomplished on
Sephadex LH-20 in 7:3 (vol./vol.) CH2Cl2/Me2CO as pre-
viously outlined. Anal. Calc. for C16H14ClN2O3: C, 38.14; H,
2.78; Cl, 7.05; N, 5.56. Found: C, 37.95; H, 3.05; Cl, 6.80;
N, 5.35%. IR absorption at 958(m) cmy1 indicatestheRe_O
stretch. Bands consistent with either a m-oxo dimer or trans-
dioxo monomer were absent.
Fig. 1. Initial cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) scans of Re2O3(salphen)2 in 0.10 M TBAP/1,2-dichloroethane.
Epsy0.925 V (e4). For the first oxidation (e1), the sepa-
ration of anodic and cathodic peak potentials was invariant
with a scan rate at 0.060 V. The ratio of cathodic and anodic
peak currents (ipc/ipa) increases with scan rate (n) to 0.500
Vs y1, above which it levels off to unity. The current param-
eter ipa/n1/2 increases only slightly over the scan rate
observed (0.050–9.900 V sy1). These criteria correspond to
a reversible single electron-transfer followed byaslowchem-
ical reaction [11]. Repeated scans indicate the product of the
following chemical reaction was electroinactive within the
potential window of the solvent system.
Examination of the diagnostic criteria corresponding tothe
second oxidation (e2) shows a peak potential difference
(DEp) of 0.083 V; the anodic peak potential demonstrated a
0.030 V anodic shift for each ten-fold increase in scan rate
above 0.500 V sy1. Peak current ratio increases with scan
rate while the current parameter increases slightly for scan
rates above 1.000 V sy1. These trends constitute evidence
for an irreversible chemical reaction followingelectrontrans-
fer [11]. Since repetitive scan following the first oxidation
indicate that the product of the chemical reaction following
(e1) is electroinactive, the reduction observed at y0.300 V
(e3) must be an electroactive product formed following the
second oxidation (e2); current parameters indicate the reac-
tion following (e2) is faster than that following (e1). That
the processes (e2) and (e3) are interrelated are evident by
examining the ratio of the two anodic peak currents (ipa,e2
and ipa,e3), which is unity at all scan rates, a relationship that
serves as an indication of two successive and related electron
transfers [11c].
2.2.6. ReOCl(acacen)
This complex was prepared according to the method pre-
viously described for the salphen complex; however, even
meticulously dried solvents and reagents produced a mixture
of monomer and m-oxo dimer under all conditions. Puremon-
omer was obtained by dissolving the washed but unchroma-
tographed product in methylene chloride and then bubbling
dry HCl gas through the solution at a moderate rate for 3 min.
The solution was then degassed with dry N2 for 15 min.
Treatment of the reaction product in this manner quantita-
tively yielded acacen monomer which was then successfully
chromatographed on Sephadex LH-20 in 7:3 (vol./vol.)
solution of CH2Cl2/Me2CO, yielding a single slow moving
dark brown band, which was collected. Anal. Calc. for
C12H18N2O3Re: C, 31.35; H, 3.91; Cl, 7.71; N, 6.09. Found:
C, 31.23; H, 3.85; Cl, 7.55; N, 6.00%. The presence of a
medium strength absorption at 973 cmy1 confirms the pres-
ence of the Re_O moiety. The presence of either the O_Re–
O–Re_O structure or the O_Re_O moiety was discounted
due to the absence of characteristic IR absorption bands.
3. Results and discussion
3.1. Voltammetric behavior of Re2O3L2 complexes
3.1.1. Re2O3(salphen)2
Data that typifiescyclicvoltammetricanddifferentialpulse
voltammetric behavior of the m-oxo dimeric compounds are
shown in Fig. 1. On initial sweep to positive potentials, two
one-electron oxidations were observed at E1/2sq0.610 V
(e1) and q1.058 V (e2) versus SCE. The return scan
revealed a single reduction at Epsy0.300 V (e3). Subse-
quent cathodic scans did not show the (e3) reduction but
revealed a reduction of comparatively greater current at
At scan rates (n) between 0.100 and 0.500 V sy1, both
oxidations (e1 and e2) exhibit evidence of electrochemical
irreversibility, and the reduction at y0.300 V (e3) is
observed. However, when n)0.500 V sy1, the reductive
process (e3) is not observed and the reversibility of the sec-
ond oxidation (e2) (as indicated by current ratios) increases

152
L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
substantially, additional evidence that (e3) is coupled to the
second oxidation.
no electroactivity within the observed potential window,
these electron-transfers are metal-centered.
The most extreme reduction (e4) showed only partial
reversibility over accessible scan rates. The value of the
cathodic peak potential (Epc) was found to shift cathodically
0.016 V per ten-fold increase in scan rate while the anodic
peak potential (Epa) was invariant at 0.0867 V. Peak current
ratio increased with increasing scan rate to a maximum of
0.40 at 9.900 V sy1; ipc/n1/2 (current function) increased
slightly with scan rate. Takentogether, thesecriteriaareindic-
ative of an electron transfer followed by irreversiblechemical
reaction for both processes [11a,c]. Consistent 1:2 relation-
ships between the current magnitude of either oxidation (e1
or e2) and the reduction (e4) over the entire range of scan
rates observed indicate a two-electron process. As H2salphen
is not electroactive withintheaccessibleelectrochemicalwin-
dow, the reduction (e4) must be related to the oxo–rhenium
center and represents either (a) concurrent reduction of both
oxo–metal moieties to Re(IV) or (b) two-electron reduction
of one of the two oxo groups. The scenario (a) is considered
more likely because (i) it is known that Re(V) species are
generally readily reducible [12], and (ii) very similar elec-
trochemical behaviors (successive one-electron oxidations,
two-electron reductions) have been observed for other m-oxo
rhenium dimers carrying anionic ligands [6].
3.2.2. Re2O3(acacen)2
Two one-electron oxidations were observed at Eps
q0.490 V (e9) and Epsq0.925 V (e10), respectively. A
single reduction (e11) was observed at Epsq0.130 V on
reversal of scan direction following (e10). This reduction is
observed only for nF0.500 V sy1. Initial cathodic scan
revealed a reduction at E1/2sy1.302 V (e12).
The reduction at q0.130 V (e11) is observed only for
nF0.500 V sy1.At nG0.500 V sy1, the midpoint potential
was found to be q0.450 V. For n-0.500 V sy1, Epa shifted
0.015 V per ten-fold increase in scan rate. The ratio ipc/ipa
increased to unity with increasing n, and ipa/n1/2 was inde-
pendent of scan rate. These findings indicate a slow and
irreversible chemical reaction following thereversiblecharge
transfer [11,13]. The product of the reaction following the
first oxidation is reduced at q0.130 V. The reaction was
found to be first order with kfs0.2 sy1 using the single scan
method of Nicholson and Shain [11a]. Examination of the
first oxidation by differential pulse voltammetry revealed a
single peak having a width at half height of 0.090 V, corrob-
orating the reversibility of the charge transfer [14].
A corresponding anodic component of the wave observed
at q0.130 V (e11) was not observed at any scan rate. The
value of Epc (e11) was dependent on scan rate, shifting
3.2. Voltammetric behavior of ReOClL complexes
cathodically by 0.028 V over 0.050 V sy1-n-0.500 V sy1
,
3.2.1. Re2O3(salen)2
strongly suggesting the presence of a fast chemical reaction
immediately following the reduction.
An initial anodic scan revealed two successive one-elec-
tron oxidations at Epsq0.521 V (e5) and Epsq1.051 V
(e6). On reversing direction, a single reduction appeared
at q0.272 V (e7). On initial cathodic scan, a wave at
E1/2sy1.060 V (e8) was observed.
For the second oxidative wave (e10), Epa shifted cathod-
ically by 0.030 V between 0.050 V and 0.500 V sy1. Peak
current ratio approached unity only at 9.900 V sy1 while
ipa/n1/2 was independent of scan rate. This diagnostic criteria
is also strongly indicative of an irreversible chemicalreaction
following the charge transfer. As H2acacen is electroinactive
over the potential window observed, these charge transfers
are assigned to the [ReO]3q core, in the manner of the other
m-oxo dimers previously discussed.
The two-electron reduction at E1/2sy1.302 V (e12)
shows some evidence of a following chemical reaction when
n-0.500 V sy1. The value of Epc shifts cathodicallyby0.015
V between 0.050 V and 0.500 V sy1; the peak current ratio
increases rapidly to unity. The current parameter remains
independent of scan rate. The return sweep of a scan with
n-y0.500 V sy1 exhibits an oxidation at y1.050 V of low
current relative to the forward sweep, a process which is not
observed at fast scan rates and is linked to a chemicalreaction
following the reduction (e12).
For the first oxidation (e5), Epa shifts 0.017 V anodically
per ten-fold increase in scan rate. The current ratio is unity
and ipa/n1/2 is independent of n when n)0.040 V sy1. For
the second oxidation (e6), Epa shifts anodically by 0.025 V
per ten-fold increase in n, ipc/ipa rises to unity for nG0.300
Vs y1, and ipa/n1/2 is independent of scan rate. These diag-
nostic criteria strongly indicate irreversible chemicalreaction
following charge transfer for both processes (e5) and (e6).
As was the case with the m-oxo salphen dimer, reversingscan
direction between (e5) and (e6) and examining voltamme-
tric behavior on repetitive scans for both processes indicates
the reduction at Epsq0.272 V (e7) was coupled to the
second oxidation (e6).
The most extreme reduction (e8) also exhibits character-
istics previously encountered in Re2O3(salphen)2. Peak
reductive current is approximately twice that of either oxi-
dative wave (e5 or e6) at identical scan rates and concentra-
tions. The value of Epc shifts 0.036 V per ten-fold increase in
scan rate, while ipa/ipc is relatively constant with n, and the
value of ipc/n1/2 slowly decreases with scan rate. The afore-
mentioned criteria are evidence for a very fast chemical reac-
tion following a two-electron reduction. As H2salen exhibits
3.3. Electron-transfer behavior of Re2O3L2 complexes
In an attempt to discern the chemically coupled Re2O3L2
oxidation products, electrolysis products were characterized
in situ using FT-IR to compute difference spectra. This was
necessary due to interference from the supporting electrolyte.

L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
153
As an example, electrolysis of Re2O3(salphen) was carried
out at q0.750 V, that is, between first and second oxidations.
Comparison of the spectra obtained from this solution with
that of an unelectrolyzed complex acquired in the same man-
ner indicated (i) the m-oxo bridge is cleaved following the
first oxidation, as the characteristic stretch at 706 cmy1 is not
present; (ii) there is no evidence of a trans-dioxo species, as
evidenced by the absence of a strong absorbance at 800"30
cmy1 and (iii) the presence of an absorption at 963 cmy1
that can be assigned to the Re_O stretch [15]. The presence
and/or identity of any trans-ligand could not be established
unambiguously from this data. Electrolyses of salen and aca-
cen dimers gave similar results.
A cyclic voltammogram taken on the product of the elec-
trolysis of Re2O3(salphen) gave a wave-for-wave match of
the corresponding mono-oxo species ReOCl(salphen). An
identical experiment was carried out for the salen and acacen
species with comparable results. On the voltammetric time
scale, voltammetric and coulometric data indicate successive
oxidations of the m-oxo bridged dimer forming a [O_ReV–
O–ReVI_O]q mixed-valent (first oxidation), followed by
the formation of the dicationic complex [O_ReVI –O–
ReVI_O]2q (second oxidation). Reductioninvolvedasingle
two-electron process, which was only observed on an initial
cathodic scan.
Both oxidations are coupled to chemical reactions which
follow charge transfer, with the reaction that followed the
second oxidation being much faster than the reaction follow-
ing the first oxidation (e.g. for Lssalphen, kf2s1.5 and
kf1s0.35 sy1). This chemical coupling impeded characteri-
zation of the initial electron transfer products. Analysis of the
reaction products following electrolysis at potentials suffi-
cient to generate the dication indicated mono-oxo species
formation. Exhaustive electrolysis at cathodic potentials
resulted in an electroinactive solution, regardless of L. The
half-wave potential for the rhenium V/VI couple of the cor-
responding mono-oxo species is anodic of the first andsecond
oxidation potentials of Re2O3L2. Thus, any mono-oxo
Re(VI) species formed would be immediately reduced back
to Re(V). Further, trans-dioxospecieswithdianionicligands
are presently unknown. Satisfaction of the high charge requi-
rements of Re(V) by an oxo group and an anionic ligand
with good p-donorcharacteristicsnormallyresultsinamono-
oxo complex, none of which has ever been converted into a
trans-dioxo species [16]. The presence of the mono-oxo
Re(V) species in the product mixture was confirmed by both
voltammetric and IR results. Our inability to detect a trans-
dioxo Re(VI) species is probably due to proton transferfrom
adventitious water present in the medium to one of the oxo
groups. One resonance form of a possible product of this
oxidation, the [O_ReVI –OA]q species, has the d2 cation
bonded to an oxo radical; this has been shown on numerous
occasions to abstract hydrogen atoms efficiently [17]. The
[O_ReV–OH]q species is not stable in the medium, and
decomposes rapidly to form the mono-oxo species. This is
one possible explanation for the lack of the characteristic
Scheme 1. Electron transfer pathway proposed for Re2O3L2 complexes, long
time scale. Products of reactions coupled to electron-transfers (kf1, kf2) are
postulated.
n[O_ReV_O]q absorption among theelectrolysisproducts.
Nearly identical redox pathways involving these intermedi-
ates are quite well-known for m-oxo rhenium dimers and
extrapolation to the aforementioned Schiff base complexes
seems appropriate in view of their nearly identical electron-
transfer behavior [1c,6,18], although direct evidence for the
existence of a trans-dioxo or oxo–hydroxo species remains
elusive.
On the basis of this information, we postulate that elec-
trooxidation of the dimer to the dication product produces the
species shown in Scheme 1. The Schiff base ligands are omit-
ted for clarity. The second oxidation is well anodic of the V/
VI couple. Thus, during exhaustive electrolysis, the dimeric
cation dissociates with spontaneous reduction of the mono-
oxo Re(VI) species.
3.4. Voltammetric behavior of ReOCl(L) complexes
The monomeric oxorhenium Schiff base complexes were
studied under the same conditions as the m-oxo dimers. For
the acacen complex, two oxidations are observed, one at
q0.950 V and one at q1.185 V. The more extreme wave is
irreversible electrochemically, and is located at a potential
corresponding to the oxidation of free Cly. The q0.950
process corresponds to the oxidation of the [ReO]3q core.
In the salphen (Fig. 2) and salen monomers, the extreme
nature of the overlap of the two oxidations complicated the
analysis of the metal-centered oxidative wave, showing both
Epa and Epc to be independent of scan rate, and ipc/ipa to be
constant with scan rate, a situation which would not normally
be associated with an isolated EC wave.
Cathodic sweeps of the salphen monomer revealed a com-
plex wave with Epsy0.320 and y0.635 V. The more
extreme wave was dependent on scan rate, shifting cathodi-
cally 0.025 V per ten-fold scan rate increase. The peak poten-
tial of the first wave shifted very slightly with n, and issimilar
to the wave observed in the corresponding m-oxo dimer in
that it is only observed following anodic scan. Significantly,
this wave disappeared on the addition of free chloride (as
[n-Pr4N][Cl])tosolutionsofanyoftheReOClLcomplexes,
regardless of oxidation state. Conductivitymeasurementsrel-

154
L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
Table 1
Redox potentials for Re2O3L2 and ReOClL complexes (0.200 V sy1
)
Complex
V/VI
V/IV
E1/2,1 (V)
E1/2,2 (V)
E1/2,1 (V)
E1/2,2 (V)
a
1
a
2
Re2O3(salphen)2
Re2O3(salen)2
Re2O3(acacen)2
ReOCl(salphen)
ReOCl(salen)
q0.610
q0.490
q0.450
q1.197
q1.150
q0.950
y0.920 bc
y1.060 bc
y1.302
y0.320
y0.185
q0.125
c
q0.915
c
y0.635
y0.840
y0.995
c
c
ReOCl(acacen)
a Chloride oxidations: a 1s1.058 V, a 2s0.988 V.
b Peak potentials.
c Not observed.
Scheme 2. Electron transfer pathway proposed for ReOClL complexes.
4. Summary
Fig. 2. Cyclic voltammetric scans of ReOCl(salphen) in 0.10 M TBAP/
1,2-dichloroethane. Upper trace: initial anodic scan; lower trace: initial
cathodic scan. Eoc: point of zero charge.
Electrochemically, the compounds described above are
very similar to those based on N,N-diethyldithiocarbamate
[6]. This is not surprising in view of the fact that both ligand
systems are anionic. Their voltammetric behavior varies rad-
ically from Re2O3Cl4(pyridine)4, which exhibits reversible
oxidations and distinct one-electron reductions, the first of
which is reversible [19]. Unlike the diethyldithiocarbamate
formulations, however, the Schiff bases showed no severe
adsorption of anionic and/or cationic intermediate forms.
The differences between voltammetric behavior of the m-oxo
diethyldithiocarbamate or aforementioned m-oxo Schiff base
complexes and Re2O3Cl4(pyridine)4 cannot be explained by
differences in p-backbonding; this would seem to indicate
that the ligand plays a critical role in the electron-transfer
mechanism. Further, it is intriguing that while the synthesis
of the compound ReOCl(S2CN(C2H5)2)2 is relatively sim-
ple, synthesis of the corresponding Schiff base mono-oxo
complexes is done only with difficulty, and ReOCl3(py)2 is
unknown. Interesting also is the fact that in both diethyldi-
thiocarbamates and these Schiff base complexes, the chloride
in the ReOClL complex is so weakly bound that both are
partially dissociated in solution beforeoxidationorreduction.
On oxidation to Re(VI), the anionic ligands would seem to
be unable to stabilize a rhenium–oxo moiety; the moeity is
so electropositive that it has been shown on numerous occa-
sions to abstract hydrogen atoms [1]. Similarly, Re(IV)
species formed on reduction do not have the high charge
requirement of the Re(VI); the anionic ligand over-satisfies
the requirement, and decomposition after the reduction is the
ative to neat ethylene chloride indicated at least partial dis-
sociation of the coordinated chloride insolutioninthemanner
ReOClL°[ReOL]qqCly. This is not surprising in that
similar trans-oxo/halo technetium and rhenium(V) com-
plexes have displayed analogous dissociative behavior
[6,3d]. The position of the equilibrium appears to depend
somewhat on the identity of L, as salen and salphen com-
plexes have irresolvable metal-centered and chloride oxida-
tions, while those of the acacen complex are readily
distinguishable. This would seem to suggest that the equilib-
rium described lies further toward the dissociated form in
the latter complex and toward the coordinated chloride form
in the former. For the salen/salphen complexes, the
Re(V)™Re(VI) oxidation has an ipa value twice that of ipc,
and a midpoint potential almost identical to that of free Cly.
Thus it appears that both halide and the [ReO]3q core are
oxidized during anodic scan. The acacen complex dis-
plays two separate oxidation steps, with Ep for chloride at
q1.185 V and the complex Ep at q0.950 V. Displacement
of chloride by perchlorate is not anticipated due to the non-
coordinating nature of the latter anion. Redox potentials for
all complexes described in this study are given in Table 1.
The voltammetric, conductometric and coulometric obser-
vations described above are in agreement with the electron-
transfer mechanism proposed in Scheme 2. Once again, the
Schiff base ligands L have been omitted for clarity.

L.A. Bottomley et al. / Inorganica Chimica Acta 255 (1997) 149–155
155
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