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1974 773Polarographic Behaviour in Acetone of Complexes formed between theBis(7c-cyclopentadienyl)vanadium(lv) Group and some I ,I -Dithio-chelatesBy A. M. Bond, A. T. Casey," and J. R. Thackeray, Inorganic Chemistry Department, University of Melbourne,The polarographic behaviour of complexes formed between the bis(x-cyclopentadienyl)vanadium(lv) group andsome O-alkyl dithiocarbonates and OO'-dialkyl dithiophosphates has been investigated in acetone. Each complexexhibits two well defined one-electron reduction waves, and in most cases further reduction waves. The wavea t most positive potentials is associated with the reversible process [(cp),VIVL]+ -i- e [(C~),V~~IL]O (whereL is an alkyl dithiocarbonate or dialkyl dithiophosphate). For L = methyl dithiocarbonate and dialkyl dithiophos-phate, the neutral vanadium(ll1) complexes are prone to dissociation.Mercury is oxidized in the presence ofreleased ligand providing an example of ECE mechanisms.Parkville, Victoria 3052, Australia++-AN ELECTROCHEMICAL investigation on a series of com-plexes between the bis(x-cyclopentadienyl)vanadium(Iv)group and certain dialkyl dithiocarbamates showedthat they underwent a novel set of processes at amercury electrode which could be described by thetc.rminology EC,EC, [where E+ represents a reduction ofvanadium ( IV) to vanadium(III), C, a subsequent chemicalreaction in which co-ordinated dialkyl dithiocarbamateis released from the metal, an oxidation of mercury(0)to form lYN-dialkyldithiocarbaniatomercury(I) and C, achemical disproportionation equilibrium betweenniercury-( I) and -( 11) dialkyl dithiocarbamates].Herewe report the results of a study on an analogous seriesof complexes with co-ordinated O-alkyl dithiocarbonatesand 00'-dialkyl dit hiophosphates.The physical properties of these complexes 233 stronglysuggest that they are of the forms-> i-chloroethyl dithiophosphate ; dpdtp, 00'-di-isopropyldithiophosphate ; dbdtp, OO'-dibutyl dithiophosphate ;dodtp, 00'-dioctyl dithiophosphate.EXPERIMENTAL1nstrunzentation.-Polarograms were recorded in acetonewith Rfetrohm or PAR instrumentation as described else-where l y 6 y 7 using @lM-tetraethylammonium perchlorate assupporting electrolyte. The reference electrode wasAg/AgCl (0.lM-LiC1; acetone) and the auxiliary electrodewas platinum wire.All test solutions were thoroughly degassed with acetone-saturated argon (unless otherwise stated) and a continuousstream of argon was passed over solutions while measure-ments were being taken.Solutions were tliermostatted to20.0 & 0-1 "C in a water-jacketted cell.Pve9avations.-Bis(x-cyclopentadienyl) (O-alkyl dithiocnv-bonuto)z~anadium(Iv) Tetva$uovoborntes and Bis-(x-cyclo-pentadiexyl) (00'-dialkyl dithiopl~osp2iato)vanadau~~2 ( IV) Tetua-j k o r o - and Tetraphenyl-borates. These complexes wereprepared from (cp) ,VCI, and the appropriate potassiumalkyl dithiocarbonate or 00'-dialkyl clithiophosplioric acidby the methods described elsewliere. 2*where cp represents a s;-bonded cyclopentadienyl group,K an alkyl group, and X the tetraphenyl- or tetrafluoro-borate anion.Under bonding schemes generally usedfor compounds containing (cp),M gr0ups,49~ a site isavailable for the addition of a further electron, therebyenabling the production of neutral vanadium(II1) speciesof the types [ (C~),V~~~(S,P[OR),)]~ and [(cp),V1IC-(S,COR)]O. Any further reduction steps are expected togive rise to complex electrochemistry since the resultingproducts are predicted to be unstable.The iollowing abbreviations have been used in thetext : cp, x-bonded cyclopentadienyl; axan, O-alkyldithiocarbonate (or more commonly ' alkyl xanthate ') ;mxan, O-methyl dithiocarbonate; exan, O-ethyl dithio-carbonate; pxan, O-isopropyl dithiocarbonate; bxan,O-butyl dithiocarbonate ; chxan, O-cyclohexyl dithio-carbonate ; dmdtp, 00'-dimethyl dithiophosphate;dedtp, 00'-diethyl dithiophosphate ; dcedtp, 00'-di-A. M.Bond, A. T. Casey, and J. R. Thackeray, Inorg. Chetn.,1973, la, 887.A. T. Case?* aiid J . I<. Thackeray, AztstrtaZ. J . Chmz., 1972,25, 2085.J . It. Thackeray, 1'h.L). Thesis, University of Melbourne,1973.RESULTS AND DISCUSSIOKPolarographic Behaviour of Potassium O-Alkyl Dithio-carbonates and Ammonium OO'-Dialkyl Ditlziophosphates.-A description of the polarography of the ligands isessential for a complete understanding of the electro-chemistry of the vanadium(1v) complexes. All the alkyldithiocarbonates and dialkyl dithiophosphates studiedexhibited three oxidation waves.Interest in relationto the vanadium(1v) complexes lay in waves 1 and 2which had respective half-wave potentials near -0.35and +O.lO V vs. Ag/AgCl for the alkyl dithiocarbonates,and -0.15 and +0-15 V us. Ag/AgCl for the dialkyldithiophosphates. Because of the nature of electrodeprocesses, Eg-values vary with concentration andThe detailed electrochemistry of waves 1 and 2 is4 C. J. Ballhausen and J. P. Dahl, Acta Chenz. Scawd., 1961,15,6 J. C. Green, M. L. €I. Green, and C . K. Prout, J.C.S. Chem.8 A. M. Bond, A. T. Casey, and J. 13. Thackeray, J . Elcctvo-7 A. M. Bond, A. T. Casey, and J . R. Thackeray, J . Eleclvo-1333.Comm., 1972, 421.ckem. SOC., 1973, 120, 1602.artalyt. Chem., 1973, 48, 7 1 774 J.C.S. Daltondescribed el~ewhere.~~~ The waves are associated with lo-".For all coniplexes the half-wave potentials ofthe processes this wave remained independent of concentration, andthe diffusion currents were directly proportional to 2Hg" +- Gaxan- - concentration over the above range. Plots oflog[(& - i)/i] vs. potential were all linear over thefor wave 1, and range &l log unit and showed the Nernstian slope2Hgl1(axan),- + HgO expected of waves from reversible one-electron reduction2Hg11(axan)3- 4- *e Ldesignatedsteps.In AC polarograms, using conventional or shortcontrolled-drop times, wave 1 had half-widths of 88 + 23Hg1'(axan)2 -k 2e- Ldesignated &)Ifor wave 2. Dialkyl dithiophosphate ligands undergosimilar processes at the d.m.e.mV a t 500 Hz and its peak potential coincided exactlyHalf-wave potentials for alkyl ditliiocarbonate and dialkyl (lithiophospllate colnplescsE+ (wave l ) / V(05. Ag/AgCl)- 0.243- 0.254- - 0 ' 3 7 4-0.288- 0.300- 0.082-0.132- 0.060-0.124 - 0.122- 0.104:E1/4 - EQlQ)V0.0600.0600.0600-0620.0600.0640-0660.0670.0660.0680-068Ei (wave 2) j \-(us. Ag/AgCl)- 1.387- - 1.380- - 1.445- - 1 -436~- 1.43s-- 0.63- 0.70- 0.64-o.(ji'- 0.63- 0.66Further wave(s) at more negative potentials were poorly defined.Polarographic Belaaviour of Bis(x-cyclopentadienyl) (0-alkyl dithiocarbonato) vanadium (IV) Tetrafluor0 berates.-Each of the [ (cP)~V( axan)] [BF4] complexes exhibitedCI c L30zero currcntI I I I I 4 , I I I-0.4 - 0.8 -1.2 -1 -6 -20Volt vs.AgIAgCLFIGURE 1 The DC and AC polarograin (500 Hz, 10 mV p-p,in phase component) of [ (cp) 2V@xan)] [BF,]two one-electron reduction waves and a third wave nearthe cathodic limit (see Table and Figure 1).Wave 1,the wave at most positive potentials, was diffusioncontrolled over the concentration range ca. 10-3 toEh (wave 3)/V(0s. Ag/-%gCl)-- 1.9- 1.9- 1.9- - 1.9___ 1.8with the DC half-wave potential. The heterogeneouscharge-transfer rate constant is therefore very large forthis reduction step.Wave 2 was more drawn out than wave 1 for all thecomplexes. Its associated (El/* - E3,J value wasalways somewhat larger than that expected of a reversibleone-electron process (see Table), but the limitingcurrent of this wave, for any one complex, was almostidentical to the diffusion current of wave 1 for the samecomplex.In the AC sense, wave 2 had a half-widthconsiderably greater than that expected for a reversibleone-electron diffusion-controlled process and its peakpotential was substantially niore negative than the DChalf-wave potential. As expected for a non-diffusioncontrolled AC reduction, wave 2 had a low peak currentrelative to that obtained from the reversible one-electronreduction (wave 1).Wave 3 was also drawn out in the DC sense, butoccurred too close to the cathodic limit for characteriz-ation. In AC polarograms it appeared as a very broadwave having a considerably lower peak current than thereversible wave 1 (see Figure 1).Unless molecular oxygen was removed to an extentbelow that polarographically detectable , a deviation inthe limiting current region of wave 1 (ca.-0.4 V) wasseen. When oxygen was allowed to enter the systemslowly, DC wave 1 was replaced by an extremelycomplex wave whose limiting current was almost exactlydouble the original diffusion current (i.e. equal inmagnitude to the total current height of wave 1 pluswave 2 in the absence of oxygen). Waves 2 and 3 dis-appeared and a wave due to the reduction of oxygenwas observed near -0.7 V.In AC polarograms the current decreased markedly a1974 775the peak potential of wave 1 in the presence of oxygenand a new wave was clearly evident near -0.4 V (seeFigure 2 ) .When test solutions were repurged ofoxygen, polarograms reverted to their normal be-haviour. The addition of free axan anion to a solutionof its corresponding complex also caused the deviationin wave 1 to grow substantially in the DC polarogramand a new wave was seen near +0.1 V. Wave 1 nowhad an anodic current, which was not evident in polaro-grams of the pure complexes even in the presence ofoxygen. A detailed description of the electrode processf3A’ I9 11II I I 1-0.75 -0.55 -0.35 -015 +0.05Volt YS AdAgCI.The AC polarograiii of [(cp),V(mxan)][BF,] in theabsence of oxygen (A) and in its presence (B) (50 Hz, 10 inV,r.m.s., total alternating current)FIGURE 3was gained from AC measurements, where the additionof free axan ligand introduced a new wave near +0.1 Vand a wave whose peak potential and shape coincidedexactly with the new wave obtained near -0.4 V byintroducing oxygen into the system.Furthermore, theaddition of free ligand to a non-deoxygenated solutionhad the effect of increasing the magnitude of the alter-nating current near -0.4 V. Thus the anomalouspolarographic beliaviour of the complexes in the presenceof oxygen may be explained by the oxygen-catalysedrelease of axan anions subsequent to the initial re-duction. The AC peak height of wave 1 decreased inthe presence of oxygen despite the large increase seenin the corresponding limiting current region of the DCpolarogram. This is presumably because the AC pro-* S. Ahrland, J . Chatt, arid N. R.Davies, Quart. RPU., 1853, 12,266.cesses are no longer reversible in the presence of oxygen.The lack of a DC anodic current at potentials morepositive than the first reduction wave proves that freeaxan anions are only generated by the electrode processesand are not present in solution as impurity.The general mechanism for the electrode processesassociated with the first reduction step may now bewrit ten,kr[ (cp) ,VII1 (axan)]’) =+= ‘ [ (cp) 2V111] ’ + axan- (C)kb6axan- + 2H@ 2HgI1(axan),- +- 4e &)where K f and Kb are the forward and backward rateconstants for step (C).In the complete absence of oxygen the process isadequately described solely by step 3 for all but themethyl derivative (see later). The great enhancementof step E, in the presence of oxygen is readily explicablein terms of the chemistry of vanadium.As a typicalclass ‘ a ’ metal it generally forms its most stablecomplexes with ligands which co-ordinate through donoratoms such as oxygen and nitrogen. The isolation andstability of the complexes studied here is presumablydue to the x-bonded cyclopentadienyl groups whichapparently bestow upon the vanadium atom propertiesmore characteristic of latter members of the transition-metal series. Once the metal is reduced to vanadiiim(II1)however, the complexes become most susceptible to thepresence of oxygen (the corresponding [ (~p),TiIII(axan)]~complexes are very oxygen-sensitive) and the resultingformation of oxovanadium-(Iv) and/or -(v) speciescould easily initiate the release of the axan anion.Thelimiting current of the complex wave system between-0-1 and -0.5 V, in the DC polarograms from solutionscontaining oxygen, is close to double the diffusioncurrent from step E in the absence of oxygen. Inaddition to steps E and El, any oxovanadium speciesgenerated in the presence of oxygen may also undergoreduction at (or more positive to) these potentials andthereby add t o the limiting current. Since step E, hasceased by -0.4 V, the height of the wave near -0.5 V isconsistent with all the vanadium(rr1) complexes pro-duced reacting with oxygen to give oxovanadium species,and all these species undergoing a further one-electronreduction step. Waves 2 and 3 are no longer evidentbecause the species causing them are not present insolutions containing oxygen.Slight evidence of the Estep occurring in some polarograms where there was nopolarographically detectable oxygen indicates that the[ (cp),VIII(axan)]O complexes are extremely susceptibleto even minute traces of oxygen. Thus like the corre-sponding dialkyl dithiocarbamate complexes,1 theset34 tf-cR. S. P. Coutts, P. C. Wailes, and J. V. Kingston, Az4stvul. J .Chenz., 1970, 23, 469776 J.C.S. DaltonFree dadtp anions gave two waves in this potentialregion. The first occurred at more positive potentialsthan wave 1 (E,) and the second at slightly morenegative (El). The asymmetry on the cathodic side oft+-complexes can provide an example of a novel oxygeii-catalysed ECE mechanism.For reasons given previously,l the first electron up-take is expected to be associated with the vanadiumatom, and therefore should produce neutral vanadium-(111) complexes.It is difficult to discern whether theelectron of the second reduction step enters an orbitalprimarily ligand in nature or one associated funda-mentally with the vanadium atom. However thespecies resulting from this reduction must be unstableand presumably undergoes subsequent chemical re-actions, since wave 2 is non-reversible. Wave 3 has amuch lower limiting current than waves 1 and 2 andprobably arises from a reduction step involving afragmentation product from previous processes.Polarograpllaic Behaviour of Bis(x-cycloperatadieny1)-(OO'-dialkyl dithioptzosphato)vanadiztm( ~ v ) Salts.-Twomain waves were obtained from polarograms of thesecomplexes and in all cases except for the [(cp),V(dmdtp)]-[BF,] complex, further electrode processes were seen atmore negative potentials (see Table and Figure 3).The half-wave potential of wave 1 was independent ofconcentration, while its limiting current was diffusioncontrolled and linearly dependent on concentration overthe range 10-3-10-4;11.Plots of log[(& - i)/i] us.potential were close to linear, but had gradients slightlygreater than the Nernstian slope expected of waves fromreversible one-electron reduction steps.AC polarograms were very complex in the vicinity ofwave 1. At low frequencies there appeared to be only++c C : L5)0zero current t0 - Odt - 0.8 -1.2 -1-6Volt vs.AglAgClFIGURE 3 The DC polarogram of [(~p)~V(dodtp)][Ph~B]one asymmetric wave near -0-1 V (having a half-heightwidth near 100 mV). As the frequency was increasedthe wave became more asymmetric on the negativepotential side, until at 300 Hz it was obvious that therewere two overlapping waves present (see Figure 4).t-1 I + 0.1 - 0.1 -0.3Volt YS AglAgClFIGURE 4 The -\C polarogram of [(cp) ,V(dedtp)][Ph,B](300 H Z , 10 mV, p-p, in phase component)AC wave 1 is due to the release o f free dadtp anionssubsequent to the reduction of vanadium(1v) tovanadium(m), and the electrode process is described bythe &% mechanism given previously for the alkylditliiocarbonate complexes.The ECE mechanism is clearly evident for the[(cp),V(dadtp)][X] complexes even in the absence ofoxygen.AC peak potentials of wave 1 from the re-duction of vanadium(1v) and wave 1 from free dadtpanions are both frequency dependent and sufficientlywell separated at high frequencies to enable readydetection of both waves. The addition of free dadtpanions to a solution of the corresponding [(cp),V(dadtp)]-[Ph,B] complex caused the wave on the cathodic side ofcomplex wave 1 to grow. When oxygen was allowed toenter test solutions the ECE mechanism was catalysedsubstantially, as evidenced by the nature of the ACpolarograph y .The presence of an ECE mechanism was not readilydiscernible from the shape of the DC wave arising fromthe reduction of vanadiuni(1v) since the half-wavepotential of this wave was too close to that of the freeligand wave 1.However, except for the [(~p)~V(dnidtp)J-[BF,] complex, the height of wave 1 was always sig-+t++--+1974 777nificantly greater than that of wave 2. This is expectedfor the ECE mechanism because the C , dissociation step-> +-[(~p),VIII(dadtp)]~ ‘ [(cp),V] ’ + dadtp-i\-ould lower the concentration of the neutral vanadium-(111) species present at the electrode surface, and it isthis species which is presumably responsible for wave 2.-4 further wave (wave 3) was seen at more negativepotentials for most complexes and is probably associatedwith the dissociation product ‘ [(C~)~VIII] .’ As expectedfor a proposed mechanism of this type, the total heightof waves 2 and 3 is equal to that of wave 1.Further-more, the limiting currents of these waves are bothdrop-time dependent. In AC polarograms, waves 2and 3 are very broad and have low peak currents typicalof non-diffusion controlled processes. It is thereforeconcluded that they arise from kinetically controlledprocesses.In the presence of oxygen DC wave 1 doubles inheight and no processes corresponding to waves 2 and 3are discernible. Clearly a similar mechanism to thatobtained from the alkyl dithiocarbonate complexes inthe presence of oxygen is also operable here.The much greater instability of the [(~p)~VI~I(dadtp)]~complexes as compared to the corresponding dialkyldithiocarbamate and alkyl dithiocarbonate series isconsistent with the behaviour of the original vanadium-(rv) complexes.The [ (cp),VV(dadtp)] [XI complexesare unstable in the atmosphere and revert to brown oilsunless stored under argon, whereas the analogous serieswith the other two sets of ligands are quite stable, evento prolonged exposure to the atmosphere.CycEic b’oZtawznzetry.-The applicability of this tech-nique to ECE-type mechanisms has been well illustratedby Feldberg and Jeftic.lo Samples of voltammogramsobtained at a J-tube mercury electrode are shown inFigure 5. Figure 5A indicates that some dissociation ofinxan ligand from the (CP)~VIII group occurs subsequentto the initial reduction even in the absence of oxygen,since the wave is considerably drawn out on the cathodicsweep and the separation of peak potentials is over100 mV.In contrast, the [(cp),V(bxan)]+ cation yieldsa cyclic voltammogram typical of that expected from areversible system (see Figure 5C). The separationbetween anodic and cathodic peak potentials is 56 mVand there is no evidence of any dissociation. The dis-sociation of the [ ( ~ p ) , V ~ ~ ~ ( m x a n ) ] ~ species was moreevident in the cyclic voltammetry than the DC polaro-graphy. However the shape and nature of the DCwave 1 is also consistent with the cyclic voltammetry,as it, in fact, showed a slight deviation near -0.4 V forthe [(~p),V~~~(mxan)]+ cation, even after stringent pre-cautions had been taken to remove oxygen (see Figure 2).No such effect was seen for the butyl derivative.Theincrease in stability of [ (cp),VIII(axan)]O species in goingfrom mxan to bxan may be attributable to increasedinductive effects. Similar observations were made forthe analogous dithiocarbamato-complexes.1The addition of K(mxan) to a solution of the [(CP)~V-(mxan)]+ complex caused a slight increase in thedifference between cathodic and anodic peak potentials,and produced an anodic current at potentials morepositive than the initial reduction (E) (see Figure 5B).When oxygen was allowed to enter slowly the ECE34tI -0f l f -0I I I I I I+O2 0 -0.2 -0.4 -0.6 -0.8Volt vs Ag/AgCIFIGURE 5 Cyclic voltammograms of [(cp) ,V(axan)] [BFJcomplexes a t a J-tube mercury electrode. Successive sweepsare numbered on the right of each voltammogram and zerocurrent is marked in each case.(A) [(cp),V(mxan)J[BF,]in the absence of oxygen. (B) [ ( C ~ ) ~ V -(mxan)][BF,] + K(mxan). Scan rate 60 mV/s. (C)[(cp),V(bxan)][BF,] in the absence of oxygen. Scan rate200 mV/s. (D) [(cp),V(bxan)][BF,] in the presence ofoxygen.Scan rate 50 mV/s.mechanism became clearly evident (see Figure 5D) ,The reduction wave from step (E) gradually diminishedin height and the reoxidation was barely discernible onanodic sweeps. After the first sweep the oxidationwave from step (E) was of comparable height to thatfrom step (E) on cathodic sweeps.A cyclic voltammogram from [ (cp),V(dpdtp)j [Ph,B] isshown in Figure 6. The separation of the peaks to-gether with their width show that an ECE mechanism isoccurring even in the absence of oxygen.3t3++lo S.W. Feldbergand L. Jeftic, J . Phys. Chem., 1972, 76, 2430778 J.C.S. Daltonanodic peak heights were not equal. The cathodicpeak potential from the [(cp),V(bxan)][BF,] complex,for example, was 0.030 V more negative than the half-wave potential (as expected for a reversible system 11),but was somewhat more negative for the [(cp),V(mxan)]-[BF,] complex. Finally, the lack of a wave correspond-ing to step (E) provides further evidence for the proposedmechanism, since the presence of mercury is requiredfor processes (El) and (E,). Studies at platinumelectrodes are therefore compatible with the proposedf-t f-Linear Sweep and Cyclic Voltammetry at a PlatinumElectrode.-Linear-sweep volt ammograms of the com-plexes could be recorded quite successfully at a platinumelectrode. For the [(cp),V(axan)][BF4] series the firstI I 1 I I 1 I+0.1 0 - 0.1 -0.2 -0.3 -0.4Volt YS AgIAgClFIGURE 6 The cyclic voltammogram of [(~p)~V(dpdtp)] [Ph,B]a t a J-tube mercury electrode. Scan rate = 60 mV/sreduction step was reversible for [ (cp),V(bxan) J [BF,],but not for [(cp),V(mxan)][BF,] or the [(cp),V(dadtp)]-[Ph,B] series.Equivalent observations were made with cyclicvoltammograms. Where systems were exhibiting kineticcomplications, larger separations were obtained betweenanodic and cathodic peak potentials than theoreticallypredicted for Nernstian behaviour and cathodic andmechanisms.CONCLUSIONSThe present w7ork has shown conclusively that, aspredicted from theoretical considerations, a reversibleone-electron charge transfer formally involving thereaction [ (cP)~VIVL]+ + [ ( C ~ ) , V ~ ~ I L ] ~ is a character-istic electrode process of complexes containing the( C ~ ) ~ V I ~ grouping co-ordinated by a dit hio-chelate.Furthermore, it has been shown that the product of thiselectrode process, [ (C~)~VIIIL]~, has varying degrees ofstability depending on the dithio-chelate L, and isalways oxygen sensitive. The observed polarographicbehaviour is entirely consistent with the known chemicaland physical properties of the compounds.C.P.R.A.J. R. ‘1. thanks the Commonwealth of Australia for a[3/623 Received, 26th March, 1973311 P. Delahay, ‘ New Instrumental Methods in Electro-chemistry,’ Interscience, New York, 1054, p. 115

ISSN:1477-9226    

DOI:10.1039/DT9740000773

出版商:RSC    

年代:1974

数据来源: RSC