Journal of Electroanalytical Chemistry 447 (1998) 155–171
The electrochemical oxidation of cobalt tris(dithiocarbamates) andtris(diselenocarbamates) in acetonitrile; a combined spectroscopic and
voltammetric study
John A. Alden b, Alan M. Bond a,*, Ray Colton a, Richard G. Compton b, John C. Eklund a,Yvonne A. Mah a, Peter J. Mahon a, Vanda Tedesco a
a Department of Chemistry, Monash Uni6ersity, Clayton, Victoria, 3168, Australiab Physical and Theoretical Chemistry Laboratory, Oxford Uni6ersity, South Parks Road, Oxford, OX1 3QZ, UK
Received 22 September 1997; received in revised form 17 November 1997
Abstract
The electrochemical oxidation of cobalt(III) dithiocarbamates and diselenocarbamates (CoL3) in acetonitrile+0.1 M Bu4NPF6
The oxidation of the cobalt tris(dithiocarbamate) andtris(diselenocarbamate) complexes (CoL3�Co(X2
CNR2)3, X�S, Se) provides a challenging mechanisticproblem that can be interrogated by voltammetric tech-niques. The oxidation process is of considerable interestbecause of the initial generation of an unusual Co(IV)
oxidation state [CoL3]+ species [1], which is consistentwith the ability of the dithiocarbamate ligand to sta-bilise a variety of transition metals in high oxidationstates [2]. Thus, a number of studies have been madeinto the electrochemical oxidation of these complexes[3–5]. In non-coordinating solvents, after the initialformation of [CoL3]+, the major final product of theoxidation process is the binuclear cobalt(III) complex,[Co2L5]+, the overall reaction mechanism being:
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171156
C2 2[CoL3]+� [Co2L5]+ +oxidised ligand (1b)
where the rate-limiting step is the C2 dimerisationprocess.
The structure of the Co(III) binuclear species,[Co2L5]+ (see below)
has been determined using X-ray crystallography. Sincethe bridging S atoms utilise a second lone pair ofelectrons for bridging [6], the structure can be writtenas CoL3+ [CoL2]+ and the binuclear complex is sus-ceptible to nucleophilic attack [5–8]. It is thereforepossible that the electro-oxidation of CoL3 in coordi-nating solvents such as acetonitrile may be more com-plex than that suggested by the reaction scheme givenin Eqs. (1a) and (1b).
In order to obtain quantitative confirmation of acomplex electrode reaction mechanism, it is necessaryto simulate the current-voltage behaviour and to obtainan acceptable fit to the experimental data. In the case ofcyclic voltammetry, current-potential curves can be nu-merically simulated by a host of explicit and implicitnumerical techniques. With this transient technique, themass transport problem merely consists of one-dimen-sional diffusion (for electrodes of macro-dimensions [9]assuming migration may be neglected in the presence ofan excess of supporting electrolyte [10,11]). Recently,the DigiSim [12,13] commercial software package hasbecome available which can simulate a wide range ofelectrode reaction mechanisms. DigiSim takes advan-tage of the fast implicit finite difference (FIFD) al-gorithm developed by Rudolph [14,15]. DigiSim will beused to analyse the cyclic voltammetric behaviour ofthe oxidation of the CoL3 complexes in acetonitrile. Inthe case of the channel electrode, the mass transportproblem has a two-dimensional nature resulting fromthe addition of convection perpendicular to planar dif-fusion. However, the mass transport-limiting currentcan be readily modelled as a function of electrolyte flowrate using the backwards implicit finite difference(BIFD) method [16,17] expanded to include homoge-neous kinetics [18]. Furthermore, the voltammetricwaveshape response for a redox process at a channelelectrode may be modelled using an iterative approach[19], or the more recently developed ‘back-to-back’ gridmethod [20]. However, with any voltammetric tech-
nique, mechanisms involving more than one homoge-neous kinetic rate constant may prove difficult to modelwith a unique set of parameters. This is because theexperimentally generated data cannot distinguish be-tween subtle simulated data differences for a particularmechanism resulting from alterations in the kineticparameters. Any simulated parameters will have anerror range that is clearly limited by the quality of theexperimental data. As a result, more than one set ofrate constants may be used to fit the same electrodereaction mechanism within an acceptable error marginassociated with the experimental data. This is true forthe oxidation of CoL3 in acetonitrile, so that a combi-nation of spectroscopic and voltammetric data are re-quired to elucidate the electrode reaction mechanism.
2. Experimental
2.1. Reagents, compounds and sol6ents
The solvents used were acetonitrile (Mallinckrodt,Biolab Scientific; HPLC grade, 99.9%) anddichloromethane (Mallinckrodt; HPLC grade, 99.9%)which were dried for at least 12 h over molecular sieves.The electrolyte used was tetrabutylammonium perchlo-rate (TBAP; Kodak, Eastman Labs, NY (puris grade)or SACHEM, Austin, US (electrometric grade)), ortetrabutylammonium hexafluorophosphate (Bu4NPF6;as synthesized in Ref. [21]). The CoL3 compounds were:Co(R2NCS2), Co(R2NCSe2)3 (R�Me, Et); Co-(morphCS2)3 and Co(morphCSe2)3 where,
They were prepared using methods based on standardprocedures in the literature [1,4,8]. [Co2(S2CNEt2)5]BF4
and analagous binuclear species were also preparedusing methods from the literature [6].
2.2. Instrumentation and procedures
The channel electrode unit was constructed in PTFEand the associated cover plate was made of opticalquality synthetic silica. The overall channel unit had theusual dimensions [22]. The working electrodes consistedof 4×4 mm platinum blocks sealed flush into thePTFE channel unit. Precise dimensions were deter-mined using a travelling microscope. The electrodeswere polished using a succession of finer alumina slur-ries down to 0.1 mm. A silver wire pseudo-referenceelectrode was located in the quartz cover plate of thechannel unit, the electrode being sealed flush to thecoverplate surface using Araldite™. Alternatively, asaturated calomel electrode upstream of the working
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 157
electrode was utilised as a reference. The potentials ofthese reference electrodes were found to yield consistentresults for the oxidation potential of ferrocene andhence all potentials in this paper are quoted relative tothe reversible oxidation potential for the ferricinium/fer-rocene couple (Fc+/Fc). A platinum gauze counterelectrode was located upstream of the working electrodein order to avoid counter electrode products contami-nating the channel cell. Flow-rates ranging from 10−3
to 10−1 cm3 s−1 were employed in the quantitativeexperiments described below. An ADI InstrumentsMaclab4e/potentiostat system controlled by a Macin-tosh Powerbook microcomputer was used in order toundertake electrochemical measurements at the channelelectrode. Further details of the channel electrode flowsystem used in this work may be found in a comprehen-sive review [22].
Cyclic voltammetric experiments were conducted us-ing in-house fabricated 0.5–1.5 mm radii platinum discelectrodes, 1, 2 or 5 mm radius microdisc electrodes ora Pt channel electrode (under no-flow conditions) inconjunction with an Ag�Ag+ �0.01 M AgNO3+0.1 MBu4NClO4 in acetonitrile or Ag�AgCl�sat LiCl+0.1 MBu4NPF6 in dichloromethane reference electrode and aplatinum wire counter electrode. A Cypress Systems(Model CYSY-1R) potentiostat was used in conjunc-tion with a 386 personal computer for these voltammet-ric measurements. All solutions were thoroughly purgedof oxygen by outgassing with nitrogen that had beenpre-saturated with acetonitrile or dichloromethane. Allexperiments were conducted under ambient temperatureconditions (2091°C).
All channel electrode mechanistic BIFD simulationswere conducted using programs written in FORTRAN77 and run on a 120 MHz Pentium PC or a SiliconGraphics Power Challenge high performance computer.
Bulk electrolyses were carried out using a Bioanalyti-cal Systems 100 electrochemical analyser. The bulkelectrolysis cell contained two Pt baskets which servedas the working and counter electrodes. The workingelectrode was arranged symmetrically inside the counterelectrode and separated by a glass cylinder with aporous glass frit in the base. The reference electrode(Ag�Ag+) was positioned as close as possible to theworking electrode in order to maximise the uniformityof potential over its surface.
Cobalt-59 NMR spectra were acquired on a BrukerAM300 spectrometer operating at 71.67 MHz andchemical shifts were referenced against the secondaryexternal reference, Co(S2CNEt2)3. All data are reportedrelative to a saturated aqueous solution of K3Co(CN)6
using the known [23] chemical shift of Co(S2CNEt2)3.Selenium-77 NMR spectra were recorded on a BrukerDRX500 spectrometer at 95.38 MHz and chemicalshifts were referenced against the external secondaryreference 1 M H2SeO3(aq) (1280 ppm vs. Se(CH3)2 [24]).
Positive ion electrospray mass spectroscopic experi-ments were conducted on a Micromass Platform IIquadrupole spectrometer. The compounds were dis-solved (approximate concentration 1 mM) in acetoni-trile at room temperature. A portion of this solutionwas diluted 1:10 with acetonitrile. The diluted solutionwas injected directly into the spectrometer and thesolution was delivered to the vaporisation nozzle of theelectrospray ion source, via a suitable mobile phase, ata flow rate of 5 ml min−1. Voltages at the first skimmerelectrode were the minimum necessary to maintain astable spray (typically 10–25 V).
3. Results and discussion
3.1. CoL3 cyclic 6oltammetric and microelectrodeexperiments
Fig. 1a and b shows the cyclic voltammetric be-haviour for the first oxidation (process I) ofCo(S2CNEt2)3 and Co(Se2CNEt2)3 (henceforth definedas CoL3: L�X2CNR2 (X�S, Se; R�Et)) in acetonitrile+0.1 M Bu4NPF6 at a scan-rate of 100 mV s−1. As is thecase in dichloromethane [3,4], it can be seen that bothspecies give well defined oxidation processes (EP
OX= +0.49 V (X�S) and +0.18 V (X�Se)) with the potentialfor the thio analogue being ca. 0.3 V more positive thanthat for the seleno species. However, on closer examina-tion and unlike the situation in dichloromethane, it canbe seen that the ratio of the peak currents for theoxidative and coupled reductive (IRED
P /IOXP ) processes
are slightly less than one for both cases (the deviationfrom one is greater for the seleno-analogue) suggestingthat the product of the observed oxidative processes isremoved via a slow chemical process. This initial con-clusion is confirmed by increasing the scan-rate andnoting that IRED
P /IOXP increases and tends towards one
using scan-rates \500 mV s−1 because the chemicalkinetics following the charge transfer process areoutrun.
Accurate formal potentials (E f0) may be determined
from the reversible half-wave potential (E1/2) obtainedfrom voltammetric experiments at microdisc electrodesunder near steady-state conditions, where the effectsassociated with the following chemical kinetics are neg-ligible. In addition, due to the steady-state nature of theexperiment [25] a value for the diffusion coefficient ofCoL3 can be obtained from the relationship:
Ilim=4nFD [A]0r (2)
where Ilim is the limiting current observed for the oxida-tion process (A), n is the number of electrons trans-ferred (for CoL3 this is assumed to be one, in theabsence of kinetic complications [3–5]), F is Faraday’sconstant (96480 C mol−1), D is the diffusion coefficient
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Fig. 1. Cyclic voltammograms obtained for (a) 0.81 mM Co(S2CNEt2)3; and (b) 0.38 mM Co(Se2CNEt2)3 in acetonitrile+0.1 M Bu4NPF6 at aplatinum electrode (Area=0.02 and 0.16 cm2, respectively) using a scan-rate of 100 mV s−1 for the first oxidation process.
of the electroactive species (cm2 s−1), [A]0 is the bulkconcentration of the electroactive species (mol cm−3)and r is the radius of the microelectrode (cm). Thevoltammetric data (formal oxidation potentials and dif-fusion coefficients) obtained from microelectrode exper-iments are summarised in Table 1.
The diffusion coefficient data obtained from steady-state microelectrode voltammetry enable the one-elec-tron peak oxidation current (IOX
P ) for the CoL3 speciesto be calculated as a function of scan rate at a Ptmacro-disc electrode using the Randles–Sevc' ik equa-tion [11], which is valid in the absence of any chemicalreactions associated with the charge transfer step. If theobserved value of IOX
P normalised with respect to the
calculated one-electron current is plotted as a functionof electrode scan-rate (as shown in Fig. 2 for the thiospecies) it can be seen that the effective number ofelectrons transferred, Neff, increases from a value of 1.0as observed at fast scan-rates and expected on the basisof the Randles–Sevc' ik equation, to values nearing 1.4at scan-rates in the 10 mV s−1 range. Analogousbehaviour was observed for the seleno complex. There-fore, it is now postulated that the [CoL3]+ cationinitially formed in the first oxidation process mustgenerate a new electroactive species that is also oxidisedat the platinum electrode, resulting in the overallgreater than one-electron process observed at low scanrates for both the seleno- and thio-species.
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 159
Table 1Voltammetric parameters obtained from oxidation of CoL3 in Ace-tonitrile+0.1 M Bu4NPF6 under near steady-state conditions atmicrodisc electrodesa
Co(S2CNEt2)3 Co(SeCNEt2)3
E f0/Vb (vs. Fc+/Fc) +0.16 (90.05)+0.47 (90.01)
1.24 (90.15)105×D cm−2 s−1 1.26 (90.1)
a Electrode radii=1–5 mm, scan rate=5 mV s−1, temperature=20°C.b This formal potential is independent of electrode radius.
(X�Se) versus Fc+/Fc). This oxidative process is postu-lated to arise from the oxidation of seleno- or thio-car-bamate ligands attached to the [CoL3]+ speciesgenerated via process (I). Importantly, only processes Iand III were observed if the equivalent oxidativevoltammetry is conducted in dichloromethane+0.1 MTBAPF6.
Since it is known that [Co2L5]+ reacts with coordi-nating solvents [6], it is postulated that process I for theoxidation of CoL3 in acetonitrile occurs via the follow-ing reaction mechanism:
E CoL3? [CoL3]+ +e− (3a)
C2 2[CoL3]+�Co2L5]+ +oxidised ligand (3b)
C [Co2L5]+ +2CH3CN�
CoL3+ [CoL2(CH3CN)2]+ (3c)
where the oxidised ligand is the 3,5-bis(N,N-di-ethyliminum)1,2,4 trithiolane dication or the equivalentseleno species [3,4] and the C step is pseudo first-orderin the acetonitrile solvent. Note that half of the originalCoL3 species is generated in the final C step. Thisregenerated material may then be reoxidised at theelectrode surface resulting in an observed currentgreater than that expected for a one-electron process.Furthermore, it is suggested that process II (Eq. (4)) isdue to the irreversible one-electron oxidation of[CoL2(CH3CN)2]+ as this process is observed in ace-tonitrile and not dichloromethane.
[CoL2(CH3CN)2]+� [CoL2(CH3CN)2]2+ +e−
products (4)
Additional details on the overall electrode reactionmechanism can be obtained if cyclic voltammogramsfor the CoL3 species are obtained over a wider potentialrange (see Fig. 3). In both the thio- and seleno-casestwo new voltammetric features (processes II and III)are observed at more positive potentials. The smallanodic feature (process II) is observed at EOX
P = +1.00V (X�S) and EOX
P = +0.63 V (X�Se). This process isbarely visible for the thio-species (Fig. 3a) at 100 mVs−1 but can be clearly seen at 10 mV s−1. Process II ischemically irreversible for the seleno analogue (Fig. 3b)and increases in height relative to the first oxidation,process I, as the scan-rate is decreased. This resultsuggests that the species being oxidised in process II isgenerated via a chemical reaction associated with thereaction of [CoL3]+, [Co2L5]+ or some other cobaltspecies with acetonitrile and involves the oxidation of anewly formed cobalt(III) species. A further irreversiblemulti-electron oxidation process (III) is observed atmore positive potentials for both the seleno- and thio-species (ca. EOX
P = +1.25 V (X�S) and EOXP = +0.85 V
i
Fig. 2. Plot of the observed peak current for oxidation process I of Co(S2CNEt2)3 (1.0 mM in acetonitrile+0.1 M Bu4NPF6) normalised withrespect to the current expected for a reversible one-electron process as a function of the square root of scan-rate.
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Fig. 3. Cyclic voltammograms obtained in acetonitrile+0.1 M Bu4NPF6 at a platinum electrode over a wide potential range for oxidation of (a)1.00 mM Co(S2CNEt2)3 at scan-rates of 10 and 100 mV s−1 (electrode area=0.02 cm2); and (b) 0.30 mM Co(Se2CNEt2)3 at a scan-rate of 100mV s−1 (electrode area=0.16 cm2).
No reductive processes are observed for CoL3 (X�S, Seand R�Et) in acetonitrile in the range from 0 to −1.0V (vs. Fc+/Fc). However, if the potential is scannedthrough process I (corresponding to the oxidation ofCoL3) and the CoL3 species is oxidised, an irreversiblereductive feature is obtained at ERED
P = −0.91 V (X�S)and ERED
P = −0.90 V (X�Se) vs. Fc+/Fc. Noanalogous reduction is observed in dichloromethane ifCoL3 (X�S, R�Et) is initially oxidised (EP
OX= +0.41 Vvs. Fc+/Fc, scan-rate=100 mV s−1) and the potentialis scanned reductively. This process is assigned to theirreversible reduction of [CoL2(CH3CN)2]+, formed inthe mechanism described above, rather than that of the
binuclear species, [Co2L5]+, as this process would beobserved in both dichloromethane and acetonitrile. Forboth the seleno- and thio-cases, two further broadirreversible reduction processes are observed in acetoni-trile, if the potential is scanned to more negative poten-tials at ERED
P = −1.72 V, −2.07 V (X�S, R�Et) andERED
P = −1.78 V, −1.93 V (X�Se, R�Et) vs. Fc+/Fc.These processes are associated with the reduction ofCoL3 [26] and are also observed if the equivalent reduc-tion is conducted in dichloromethane. Following thesetwo further reductions in acetonitrile, a new oxidationprocess is observed at −0.4 V (X�S, R�Et) and −0.65V (X�Se, R�Et). An identical oxidation process was
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 161
Table 2Summary of the redox processes observed in the cyclic voltammetry of Co(X2CNEt2)3 and Co(X2Cmorph)3 (X�S, Se) at a Pt macro-disc electrode(scan-rate=100 mV s−1)
MeCN, acetonitrile; DCM, dichloromethane; n.o., not observed.a V vs. Fc+/Fc.
observed in the voltammetry of the free dithiocarba-mate or diselenocarbamate ligand in acetonitrile,demonstrating that this new process results from theoxidation of free ligand ejected upon reduction of CoL3
[26]. Additional reduction process are observed if thepotential is scanned through oxidation process III (oxi-dation of the ligands in [CoL3]+) and these processesmay be attributed to the reduction of oxidised forms ofthe ligand released on oxidation of [CoL3]+.
Similar behaviour to that described above for thediethyl diseleno- and dithio-carbamate species CoL3
was observed for the equivalent morph species(L�X2CR; X�S, Se; R�morph) in dichloromethane andacetonitrile and the appropriate voltammetric detailsare given in Table 2. It was noted that for the diseleno-carbamate an additional oxidation process was ob-served as a shoulder on the process III (oxidation ofligands in [CoL3]+) wave in both dichloromethane andacetonitrile. It is postulated that this wave results fromthe oxidation of the binuclear species [Co2L5]+. Addi-tionally, it was found that the kinetics of the reaction of
the electro-generated [CoL3]+ cation (R�morph) aremuch slower than for the equivalent reaction of theethyl analogues (X�S, Se) since IRED
P /IOXP for the CoL3
oxidation was close to unity for scan rates B500 mVs−1.
3.2. [Co2L5]+ cyclic 6oltammetric experiments
Cyclic voltammetric experiments on [Co2L5]BF4
(L�X2CNR2; X�S, R�Et) in dichloromethane+0.1 MBu4NPF6 medium, in which these complexes are stable,show three oxidation processes (Table 3). First, a re-versible oxidation process associated with the presenceof CoL3 (from the synthesis of the binuclear species) isobserved (process A), the second process (process B) isassociated with the oxidation of [Co2L5]+ and the thirdresults from oxidation of ligands in [CoL3]+ (processC). If acetonitrile is added to the dichloromethanesolution of [Co2L5]+, the peak current for the oxida-tion of [Co2L5]+ decreases and the equivalent currentassociated with oxidation of CoL3 increases. In addi-tion a new voltammetric feature (process D) is observedat a peak potential of +0.98 V (vs. Fc+/Fc), whichclosely corresponds to that reported for the proposedoxidation of [CoL2(CH3CN)2]+ observed in thevoltammetry of CoL3 (process II, as discussed above).Finally, only processes A, C and D are observed for theoxidation of [Co2L5]+ in 100% acetonitrile solvent.This voltammetric result gives further evidence for thereaction of [Co2L5]+ with acetonitrile to produce CoL3
and [CoL2(CH3CN)2]+.Four processes were observed for the reduction of
[Co2L5]+ in dichloromethane. The two reductions atERED
P = −0.94 V and −1.16 V (vs. Fc+/Fc) are be-lieved to be due to the reduction of [Co2L5]+ and aligand fragment released upon oxidation of [CoL3]+ (atslow scan-rates (B50 mV s−1) a small wave is ob-served at −1.14 V for the reduction of CoL3 if thepotential is scanned through the three oxidation pro-cesses alluded to above). The other two reductions that
Table 3Summary of the redox processes observed in the cyclic voltammetryof Co2(S2CNEt2)5BF4 at a Pt macro-disc electrode (scan-rate=100mV s−1)
DCM, dichloromethane; MeCN, acetonitrile; n.o., not observed.a Vs. Fc+/Fc.
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Fig. 4. Channel electrode hydrodynamic voltammograms in acetonitrile+0.1 M Bu4NPF6 for the oxidation of (a) 1.00 mM Co(S2CNEt2)3
(Vf=1.02×10−2 cm3 s−1); and (b) 0.38 mM Co(Se2CNEt2)3 (Vf=8.0×10−4 cm3 s−1). Electrode area=0.16 cm2, scan-rate=5 mV s−1.
occur at more negative potentials correspond to reduc-tions of CoL3. Similarly, in acetonitrile four reductionprocesses are observed. A reduction at ERED
P = −0.20V is observed only following oxidation process C andis associated with reduction of a ligand fragment re-leased upon oxidation of [CoL3]+. A process at−0.87 V is associated with the one-electron reduc-tion of [CoL2(CH3CN)2]+, formed upon the decom-position of [Co2L5]+ in acetonitrile and occurs atan almost identical potential to that observed inthe voltammetry of CoL3. As in dichloromethane,the two reduction processes occurring at the most nega-tive potentials are associated with the reduction ofCoL3.
3.3. Bulk electrolysis of CoL3
Exhaustive controlled potential electrolysis (potentialof Pt electrode was held at +0.56 V vs. Fc+/Fc) wasapplied to the oxidation of CoL3 (L�X2CNR2; X�S,R�Et) in acetonitrile. Approximately two electrons(1.890.1) were required to achieve this. In contrast theequivalent electrolysis in dichloromethane is a one-elec-tron process [3,4,8]. Furthermore, if the cyclic voltam-metry of the bulk electrolysed CoL3 acetonitrile solutionis examined, no oxidation response for CoL3 was ob-served, indicating that all the CoL3 had been oxidised.However, if dithiocarbamate (S2CNEt2
- ) ligand is addedto the solution, the original CoL3 may be detected via its
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 163
Fig. 5. Comparison of experiment and EC2C theory for the oxidation of CoL3 at a channel electrode. Plots of Ilim vs. V f1/3 for (a) 1.0 mM
Co(S2CNEt2)3 (experiment ("), BI theory (——); k1=6.0×105 mol−1 cm3 s−1, k3=1.10 s−1); and (b) 0.38 mM Co(Se2CNEt2)3 (experiment("), BI theory (——); k1=2.0×106 mol−1 cm3 s−1, k3=0.45 s−1) in acetonitrile+0.1 M Bu4NPF6 (one-electron current calculated using Eq.(5) (— — · — —)).
oxidation peak as observed through cyclic voltammetry.Similarly, if a different dithiocarbamate ligand L% wasadded, CoL2L%was formed. From the mechanism alludedto above in Eqs. (3a), (3b) and (3c), the final cobaltspecies generated through bulk electrolysis must be[Co(SCNEt2)2(CH3CN)2]+ (the other electron consumedis associated with formation of oxidised ligand in the C2
step). It is proposed that when dithiocarbamate ligand(L%) is added to the bulk electrolysis solution, it replacesthe CH3CN ligands in the [Co(SCNEt2)2(CH3CN)2]+
complex to give Co(SCNEt2)2L%.
3.4. Channel electrode experiments
Fig. 4a and b illustrate typical steady-state vol-tammograms for the oxidation of CoL3 (X�S, Seand R�Et) at a platinum channel electrode. Inboth cases, the three oxidative processes (I, II andIII) alluded to above are also observed at similarpotentials. If process I merely consisted of a simpleelectron transfer process, such as oxidation of fer-rocene, the limiting current (Ilim) would be given by[27]:
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171164
Fig. 6. Comparison between experiment and EC2C theory for the oxidation of CoL3 at a channel electrode. Plots of Neff vs. Vf for (a) 1.0 mMCo(S2CNEt2)3 (experiment ("), BI theory (——); k1=6.0×105 mol−1 cm3 s−1, k3=1.10 s−1); and (b) 0.38 mM Co(Se2CNEt2)3 (experiment("), BI theory (——); k1=2.0×106 mol−1 cm3 s−1, k3=0.45 s−1) in acetonitrile+0.1 M Bu4NPF6. Variation of Neff with flow-rate for a 1mM solution of ferrocene (�).
Ilim=0.925nFD2/3[CoL3]0wx2/3e (Vf/h
2d)1/3 (5)
where n is the number of electrons transferred, [CoL3]0is the bulk concentration of the cobalt species (molcm−3), w is the width of the electrode (cm), xe is thelength of the electrode (cm), Vf is the volume flow-rate(cm3 s−1), h is the half-height of the channel unit (cm)and d is the width of the channel unit (cm). Fig. 5shows plots of the limiting current for oxidation pro-cess I as a function of the cube-root of the electrolyteflow-rate, with the one-electron Levich current shownfor comparison. The latter is calculated using values of
D inferred from microdisc experiments. Note that carehad to be taken in the measurement of the limitingcurrents of the seleno complex, as the measured cur-rents were seen to decrease with repetitive measure-ments and reproducible currents for the seleno complexcould be obtained only if the electrode was cleanedvoltammetrically by scanning the potential between +2.0 V and −2.0 V (vs. Ag pseudo reference) after eachvoltammogram was recorded. Fig. 6 illustrates how theeffective number of electrons transferred (Neff) for pro-cess I varies as a function of flow-rate. Data for thereversible one-electron oxidation of ferrocene are also
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 165
Table 4Summary of NMR resonances and their interpretations for solutions of [Co2L5]BF4 (L�X2CNR2) in acetonitrile and dichloromethane
Nucleus Solvent [Co2L5]+species Resonance frequencies/ppm Moiety attributed to resonance frequency
MeCN59Coa X�S 6830 Free CoL3
cis- and trans-[CoL2(CH3CN)2]+6870R�morph6725, 6040 [Co2L5]+
MeCN, acetonitrile; DCM, dichloromethane.a 59Co resonances vs. K3Co(CN)6 reference.b 77Se resonances vs. SeMe2 reference.
included to demonstrate that no experimental artefactsare present. It can be seen that at lower flow-rates,higher Neff values are measured as the solution is overthe electrode for a longer period of time which allowsmore CoL3 to be regenerated in the C step of thereaction scheme and then reoxidised at the electrodesurface. This result is analogous to that observed inFig. 2 using cyclic voltammetry.
The above result supports the hypothesis that processI results from more than an EC2 mechanism, in whichthe overall number of electrons transferred would beexpected to be one. Numerical modelling of the cyclicvoltammetric and channel electrode data in terms of theEC2C mechanism is considered in Sections 3.8 and 3.9.
3.5. 59Co and 77Se NMR in6estigations
The 59Co NMR spectra of CoL3 (L�X2CR; X�S, Seand R�morph) were recorded in both acetonitrile anddichloromethane. In all cases one signal correspondingto the 59Co nucleus in the CoL3 complex was observed
in the range 6800–6900 ppm (vs. a saturated solution ofK3Co(CN)6 in D2O) and the results are summarised inTable 4. The 59Co NMR spectrum of the thio andseleno binuclear complexes [Co2L5]BF4 (X�S, Se andR�morph) in acetonitrile (with minimaldichloromethane (\10% v/v) added to aid solubility)show four and three peaks, respectively (see Table 4).For the thio analogue, it is believed that these fourpeaks correspond to unreacted binuclear species[Co2L5]+ (two peaks 6040 ppm ([CoL2]+ unit) and6725 ppm (CoL3 unit)), free CoL3 (6830 ppm) and amixture of cis- and trans-[CoL2(CH3CN)2]+ (a broadsignal centered at 6870 ppm). For the seleno-species,the three signals correspond to the initial CoL3 species(as confirmed by the 59Co NMR spectrum of CoL3 inacetonitrile) and the cis- and trans-isomers of[CoL2(CH3CN)2]+. For the seleno-analogue, the inter-pretation is confirmed if the 77Se NMR spectrum of thesame [Co2L5]+ species is recorded in acetonitrile solu-tion in which four peaks are observed. The peak at1710 ppm coincides with the signal for the six equiva-lent selenium atoms in free CoL3. We postulate that thepeak at 1780 ppm corresponds to the trans-[CoL2(CH3CN)2]+ species which contains four equiva-lent selenium atoms. Two signals at 1740 and 1655 ppmcorrespond to two inequivalent pairs of Se atoms dueto the different environments in cis-[CoL2(CH3CN)2]+.Integration of the 59Co and 77Se resonance signalsshows that the CoL3, cis-[CoL2(CH3CN)2]+ and trans-[CoL2(CH3CN)2]+ are formed in the ratio 2:1:1, thecis- and trans-isomers of [CoL2(CH3CN)2]+ beingformed in statistically equal amounts. Indichloromethane, the 59Co NMR spectrum of the thio-carbamate shows two signals associated with the binu-
Table 5Summary of ESMS data for [Co2L5]BF4 (L�S2CNR2; R�Me, Et) inacetonitrile
Cationic species m/z values
R�Me R�Et
299 355[CoL2]+
437381[CoL2(CH3CN)2]+
340 396[CoL2(CH3CN)]+
750 858[Co2L5]+
[Co2L4LS]+ 890 890
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171166
Fig. 7. An error surface for oxidation process I based on comparison of the error function (Eq. (6)), determined from the experimental current(1.0 mM solution of Co(S2CNEt2)3 in acetonitrile+0.1 M Bu4NPF6) and the calculated EC2C channel electrode theoretical current at a range ofelectrode flow-rates, with the kinetic parameters k1 and k3.
clear species (one due to the CoL3 moiety and the otherdue to the CoL2
+ unit) and a third signal due to CoL3
impurity associated with the synthesis of the binuclearspecies. Furthermore, the 77Se spectrum of the selenoanalogue shows a number of signals associated with thebinuclear species (see Table 4) in dichloromethane.These results demonstrate that the binuclear species issusceptible to cleavage only in nucleophilic solvents.
The 59Co NMR spectrum of a solution of [Co2L5]BF4
(L�S2CNMe2) in acetonitrile shows only one sharpresonance, which corresponds to the chemical shift ofCoL3 (See Table 4). This is again suggestive of acetoni-trile cleaving the [Co2L5]+ binuclear complex. In con-trast, the equivalent spectrum obtained indichloromethane shows two resonances correspondingto the CoL3 and [CoL2]+ units of the binuclear com-plex (see Table 4). Furthermore, the 59Co NMR spec-trum of [Co2L5]BF4 (L�S2CNEt2) in acetonitrile showsevidence of CoL3, [Co2L5]+ and another cobalt species(presumably [CoL2(CH3CN)2]+) in solution.
In summary (for X�S, Se and general R), the 59Coand 77Se NMR data confirm that [Co2L5]+ is cleaved inacetonitrile to form CoL3 and cis- and trans-[CoL2(CH3CN)2]+.
3.6. Electrospray mass spectrometry (ESMS) studies
Electrospray mass spectrometry allows the transferand subsequent detection of ions in solution to the gasphase with minimal fragmentation and decomposition[28]. Two [Co2L5]BF4 (L�X2CNR2; X�S, R�Me, Et)binuclear complexes dissolved in acetonitrile were in-vestigated using this technique, with pure acetonitrileused as the mobile phase. A variety of cations weredetected and the major species identified are shown inTable 5. For both the methyl and ethyl analogues,strong signals were observed at m/z=299 and 355respectively, corresponding to the [CoL2]+ cations. Sig-nals due to the complex [CoL2(CH3CN)2]+ and the
mono-acetonitrile species [CoL2(CH3CN)]+ are alsoobserved. In addition, a small signal probably corre-sponding to [Co2L5]+ reformed in the gas phase is alsodetected (m/z=750 (R�Me) and 858 (R�Et)). The sul-phur-rich cation [Co2L4LS]+ observed in the ESMSspectrum has been previously reported and is believedto be a product of the reaction of Co2L5
+ and elementalS [29,30] generated from the oxidation of thiuramdisul-phide. The [Co2L4LS]+ cation has been shown not toparticipate in the oxidation process of CoL3 [31,32].The cation [CoL3]+ was observed only when the moreconducting mobile phase of H2O+MeOH+HOAcwas used and when a higher than usual voltage wasapplied to the electrospray nozzle, indicating that thisspecies is formed by tip oxidation. This ESMS dataprovides further evidence that [Co2L5]+ reacts withacetonitrile to form CoL3 and [CoL2(CH3CN)2]+.
3.7. Simulation of 6oltammetric beha6iour: comparisonbetween experimental data and EC2C theory foroxidation of CoL3 (L�X2CNR2; X�S, Se and R�Et)
Full details of the EC2C mechanism and theoreticalapproach used to simulate the cyclic voltammetric andchannel electrode experiments are given in Appendix Afor a generic EC2C mechanism. Investigations demon-strated that using either technique in isolation and onlya single concentration could lead to good fits betweenexperimental and theoretical data utilising a broadrange of kinetic parameters. This peculiarity resultsfrom the fact that each technique is most sensitive tothe rate-limiting kinetic parameter and good fits be-tween experiment and theory can be obtained for awide range of values of the non rate-limiting kineticparameter. Thus, for example, in the case of the chan-nel electrode, it was found that a wide range of coupledvalues of k1 and k2 could be used to model the experi-mental data with the EC2C mechanism using the crite-rion (Eq. (6)) of minimising the variance:
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 167
Table 6Summary of homogeneous chemical rate constants (and error ranges) for the oxidation of CoL3 (L�X2CNR2; X�S, Se and R�Et) in acetonitrilevia an EC2C mechanism determined by a combination of channel electrode and cyclic voltammetry
0.6 (0.2–1.6)1.5 (0.6–2.0)k2/s−1 Binuclear species reaction with acetonitrile rate constant (pseudo first-order)
Error=
D %all Vf
(exptIlim− theoryIlim)2
n %(6)
In Eq. (6), exptIlim is the experimentally observed masstransport limited current for oxidation process I for aparticular electrolyte flow rate, theoryIlim is the calculatedlimiting current for the EC2C mechanism at the equiva-lent flow rate and n % is the number of electrolyte flowrates studied. The difficulty in determining k1 and k2 formeasurements using a single CoL3 concentration isexhibited through a ‘minimum valley’ in the error con-tour plot (Fig. 7, [CoL3]=1 mM), which relates theerror defined by Eq. (6) and the parameters k1 and k2.Note that k1 is probed only up to a value of 107 mol−1
cm3 s−1 in this figure, as values greater than this wereoutside the determined error range. In order to over-come this problem, values of k1 were determined at low[CoL3] and values of k2 were determined at high [CoL3].An analagous procedure was applied to the modellingof the cyclic voltammetry using DigiSim.
The possibility of a reversible C2 step was also exam-ined through simulation of the channel electrode andcyclic voltammetric data. For both techniques it wasfound that good fits between experiment and theorycould be achieved only by effectively fixing the rateconstant for the reverse C2 step (k−1) to zero (explicitlyin BI theory by setting k−1=0 and implicitly inDigiSim by setting k1/k−1 to a large value such thatk−1 tended to zero). This is in line with previous work(see Section 1) in which the products of the C2 step are[Co2L5]+ and oxidised ligand, rather than [Co2L6]2+, noevidence of which has been reported for its existence inthis or other work.
In view of the above results, the EC2C kinetic mech-anism parameters presented in the discussion belowrepresent the combination of parameters (and theirerror ranges) which provide good fits between experi-ment and theory for both cyclic voltammetry and chan-nel electrode voltammetry over the range of CoL3
concentrations examined.
3.8. Mechanistic channel electrode studies of CoL3
(L�X2CNR2: X�S, Se and R�Et)
Fig. 5 illustrates typical fits between experiment and
EC2C theory, for the variation of the limiting currentwith the cube root of flow-rate for oxidation process Iof CoL3. The calculated simple one-electron transfer (E)behaviour is also illustrated. Fig. 6 clearly shows the goodagreement obtained for the variation of the effectivenumber of electrons transferred as a function of theelectrolyte flow rate. Similar results were obtained overa range of CoL3 concentrations. The upper concentrationlimit (1.0 mM (X�S) and 0.4 mM (X�Se)) was determinedby the solubility of the compounds in acetonitrile and thelower limit (0.1 mM (X�S, Se) was governed by the abilityto measure a deviation from one-electron current overthe range of flow rates studied. The average values of thekinetic parameters (and their error ranges) derived for theEC2C mechanism described in Appendix A for both thethio- and seleno-species are given in Table 6. The valuesof the rate constants were determined by minimising theerror function (Eq. (6)) at the relevant CoL3 concentra-tion. The error ranges stated are derived from a combi-nation of channel electrode and cyclic voltammetry andrepresent the range of values obtained using both tech-niques together.
It is also possible to model the voltammetric wave-shape for oxidative process I. Fig. 8 shows a typicalcomparison between experiment and EC2C theory for theoxidation of CoL3 (X�S, R�Et) in acetonitrile for anelectrolyte flow rate of 4.1×10−3 cm3 s−1. The kineticparameters used for the modelling of the voltammetricwaveshape were within the error ranges stated in Table6, thus giving further proof that the mechanism is EC2Cin nature and the kinetic parameters are of the correctmagnitude. Similar results were obtained for the seleno-compound and the parameters in Table 6 were found tobe of the correct order of magnitude in order to obtainsatisfactory fits with the experimental data.
3.9. Mechanistic cyclic 6oltammetric studies of CoL3
(L�X2CNR2; X�S, Se and R�Et)
For the purposes of modelling the cyclic voltam-mograms, the DigiSim parameter inputs of double layercapacitance, uncompensated resistance, formal elec-trode potential, k0 and a (see Table 7) were keptconstant and the effect of the kinetic rate constants k1
and k2 on the fit between experiment and theory wasprobed. A low differential between the experimental
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171168
Fig. 8. Comparison of an experimental voltammogram and BI simulation at a channel electrode for oxidation process I of Co(S2CNEt2)3 (0.32mM) in acetonitrile+0.1 M Bu4NPF6. Scan-rate=5 mV s−1, Vf=0.0041 cm3 s−1, k1=8.5×105 mol−1 cm3 s−1, k3=0.9 s−1.
and theoretical peak currents was essential before the fitwas regarded as acceptable. Fig. 9 contains examples offits obtained between the experimental data for oxida-tion process I and the EC2C theory using kineticparameters similar to those derived from the channelelectrode investigations. Optimised values of k1 and k2
were obtained by using the fitting routine of DigiSim,which minimises the error term defined in Eq. (6) overall the potentials examined in a cyclic voltammogram.The greatest discrepancy between experiment and the-ory is obtained in the region after the peak oxidativecurrent has been reached and the current decreases dueto relaxation of the CoL3 diffusion layer into solution.Equally good fits could be obtained for a range ofelectrode scan-rates (10–500 mV s−1) and CoL3 con-centrations (as for the channel electrode experiments).The average kinetic parameters (and their error ranges)used to fit the theoretical data for oxidation of CoL3
are as shown in Table 6.
4. Conclusions
Three oxidation processes are observed in the cyclicand channel electrode voltammetry of cobalt tris(dithio-carbamate) and cobalt tris(diselenocarbamate) com-plexes (CoL3) in acetonitrile. The first oxidation processis associated with an EC2C process. Numerical mod-elling shows that the dimerisation step is irreversible,which would be expected as the dimer [Co2L6]2+ rapidlydecomposes to [Co2L5]+ and oxidised ligand. It isproposed that the EC2 steps are described by Eqs. (3a)and (3b) and the rate constant k1 represents the secondorder rate constant for the formation of [Co2L5]+.
NMR, ESMS and cyclic voltammetric data all demon-strate that the binuclear product of the C2 step (Co2L5
+)reacts with acetonitrile to regenerate CoL3 and thecation [CoL2(CH3CN)2]+ in the final C step of themechanism shown in Eq. (3c), where k2 represents thepseudo-first order rate constant for the reaction of thebinuclear species with acetonitrile.
The kinetic parameters determined using a combina-tion of cyclic and channel electrode voltammetry forthe EC2C mechanism of CoL3 are summarised in Table6. The value of k1 (X�S, R�Et) is of the same order ofmagnitude as that obtained for the dimerisation rateconstant (k1=2×105 mol−1 cm3 s−1 [4]) indichloromethane, where the binuclear species [Co2L5]+
does not react with solvent and is the final product ofthe one electron oxidation of CoL3. This agreementwould be expected for a C2 step that should be largelyindependent of the nature of the solvent. A previousstudy [5] reported a higher value for k1 for the oxida-tion of Co(S2CNEt2)3 in acetonitrile. However, thekinetic parameter was determined on the assumption ofa purely EC2 mechanism and therefore measured thekinetics of the loss of [CoL3]+ rather than thoseuniquely associated with the dimerisation of the cation.
It was noted that both cyclic voltammetry and chan-nel electrode voltammetry give equivalent results for thequantitative analysis of the EC2C mechanism. How-ever, using either method individually a wide range ofcombinations of kinetic parameters for this mechanismcould be derived from single concentration measure-ments.
In summary, the complex electrochemical oxidationof cobalt(III) dithiocarbamates and diselenocarbamatesin acetonitrile has been shown using a combination of
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 169
Table 7Parameters maintained at constant value in DigiSim simulations of process I for oxidation of CoL3 at 293 K
X�S X�SeParameter Comments
E f0 (vs. Fc/Fc+)/V 0.47 0.16 Values within range determined by microelectrode experiments (see Table 1)
20 20Double layer capacitance Cdl (mF cm−2) Typical value [10]Uncompensated solution resistance (Ru)/V 200 200 Value estimated from peak-to-peak separations of the Fc+/Fc couplea(E step) 0.5 Usual value chosen for this parameter [10]0.5
Set to ensure electrochemical reversibilityk0/cm s−1; (E step) 1.01.0
differing voltammetric and spectroscopic techniques toproceed via an EC2C mechanism.
Acknowledgements
We thank the Sir Robert Menzies Centre for Aus-tralian Studies for awarding a Post-Doctoral Fellow-ship to J.C. Eklund, the ARC for financial support ofthe project, the EPSRC for a studentship and KebleCollege for a senior scholarship for J.A. Alden. We alsoacknowledge Stephen Feldberg for many valuable dis-cussions and J.A. Weigold for assistance in conductingthe 59Co NMR spectroscopy experiments.
Appendix A. Numerical modelling details: the EC2Cmechanism
This general mechanism may be expressed by thesimplified scheme:
E A?B9e− (A1a)
C2 2B�k1
C (A1b)
C C�k2
A+D (A1c)
assuming that: (a) the initial electron transfer is electro-chemically reversible; (b) the C2 and C steps are irre-versible unless otherwise stated; and (c) A isregenerated in the C step and therefore may be re-oxidised.
The interested reader is directed to alternativesources [12,13] for the methodology used in the simula-tion of cyclic voltammograms.
For the channel electrode, the mass transport prob-lem is given by:
( [Z](t
=D(2[Z](y2 −6x
( [Z](x
+kinetic terms (A2)
where [Z] is a general species in the electrode reactionmechanism, the y co-ordinate relates to the directionperpendicular to the electrode surface and the x co-or-dinate describes motion parallel to the electrode surfacein the direction of the electrolyte flow. 6x is the compo-nent of the parabolic flow in the x-direction and isgiven by the expression:
6x=6o [1− (y−h)2/h2] (A3)
with 6o being the centre line velocity of the flowthrough the channel unit and 2h the channel flow-cellheight. In order to simulate the hydrodynamic voltam-metry associated with an EC2C mechanism understeady-state conditions the mass transport problems(described by Eq. (A2)) for the three species A, B andC must be solved. The kinetic terms associated with Eq.(A2) for each species in the mechanism are given inTable 8.
The boundary conditions appropriate to the determi-nation of the limiting current associated with the EC2Cmechanism and the calculation of the associated cur-rent-voltage curve for this process are given by:
x=0 (upstream edge of electrode):
[A]= [A]0, [B]=0, [C]=0 (A4)
y=0 (electrode surface):
[B]/[A]=exp[F/RT(E−E0f )]; (A5)
( [A](y
=−( [B](y
; (A6)
( [C](y
=0; (A7)
y=2h (channel wall opposite electrode):
( [A](y
=( [B](y
=( [C](y
=0 (A8)
where E f0 is the formal redox potential for the A/B
couple, [A]0 is the bulk concentration of A and thediffusion coefficients of the A and B species are as-sumed to be equal.
The coupled mass transport equations (modified tocontain the kinetic expressions given in Table 8) for thethree species in the electrode reaction mechanism maybe solved using the finite difference method mentionedin the introduction [16–20]. Typical grid sizes of J=2000 by K=2000 were utilised, where J is the numberof boxes in the direction perpendicular to the electrodesurface between the electrode and the opposite wall ofthe channel and K is the number of boxes along theelectrode surface [18].
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171170
Fig. 9. Comparison of experimental and simulated cyclic voltammograms for oxidation process I of CoL3 in acetonitrile+0.1 M Bu4NPF6 (a) 1.0mM Co(S2CNEt2)3, scan-rate=100 mV s−1, Pt disc radius=1.5 mm, k1=5.0×105 mol−1 cm3 s−1, k3=1.0 s−1; and (b) 0.38 mMCo(Se2CNEt2)3, scan-rate=500 mV s−1, Pt disc radius=0.5 mm, k1=1.5×106 mol−1 cm3 s−1, k3=0.8 s−1.
The electrode current at each potential is evaluatedfrom the expression:
I=wFD& xe
0
( [A](y
)y 0
dx (A9)
where w is the width of the electrode and xe is thelength of the electrode in the direction of the electrolyteflow and the flux of CoL3 at the electrode surface isderived from the BI simulation outlined above.
Table 8Kinetic terms associated with the mass transport equation (Eq.(A2)) of each species at a channel electrode for an EC2C mecha-nism
Species (Z) Kinetic terms
+k2[C]A−2k1[B]2B+k1[B]2−k2[C]C
J.A. Alden et al. / Journal of Electroanalytical Chemistry 447 (1998) 155–171 171
[12] DigiSim 2.1 for WINDOWS, Bioanalytical Systems, WestLafayette, IN, USA.
[13] M. Rudolph, D. Reddy, S.W. Feldberg, Anal. Chem. 66 (1994)589A.
[14] M. Rudolph, J. Electroanal. Chem. 314 (1991) 13.[15] M. Rudolph, J. Electroanal. Chem. 338 (1991) 85.[16] P. Laasonen, Acta Math. 81 (1949) 30917.[17] J.L. Anderson, S. Moldoveanu, J. Electroanal. Chem. 179 (1984)
107.
[18] R.G. Compton, M.B.G. Pilkington, G.M. Stearn, J. Chem. Soc.Faraday Trans. I 84 (1988) 2155.
[19] R.G. Compton, M.B.G. Pilkington, J. Chem. Soc. FaradayTrans. I 85 (1989) 2255.
[20] M.J. Bidwell, J.A. Alden, R.G. Compton, J. Electroanal. Chem.417 (1996) 119.
[21] A.J. Fry, W.E. Britton, in: P.T. Kissinger, W.R. Heineman(Eds.), Laboratory Techniques in Electroanalytical Chemistry,2nd ed., Marcel Dekker, New York, 1996, pp 481.
Englewood Cliffs, NJ, 1962.[28] M. Yamashita, J.B. Fenn, J. B, J. Phys. Chem. 88 (1984) 4451.[29] L.H. Pignolet, B.M. Mattson, J. Chem. Soc. Chem. Commm.
(1975) 49.[30] A.M. Bond, R. Colton, A. D’Agostino, A.J. Downard, J.C.
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