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Further comments on the electrochemistry of [PtCl2(PR3)2] complexes
J. Electroanal. Chem., 179 (1984) 273-276
273
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication FURTHER COMMENTS ON THE ELECTROCHEMISTRY OF [PtCI2(PR3)2] COMPLEXES
J.A. DAVIES * and V. UMA
Department of Chemistry, College of Arts & Sciences, University of Toledo, Toledo, OH 43606 (U.S.A.) 'Received 20th March 1984; in revised form 19th June 1984)
Recently we described how cyclic voltammetry may be employed to study the isomerization reactions of [PtClz(PPh3)2] complexes. Our initial results were described in a letter [1] and later the complete study was reported in a full paper [2]. Our work complemented the early studies of Mazzocchin and co-workers [3], who were among the pioneers of transition metal phosphine electrochemistry, and whose comments precede this paper [4]. By studying the time dependence of cyclic voltammograms of [PtC12(PPh3)2] complexes, and interpreting the observations in terms of a catalytic cis/trans equilibration reaction, certain of the earlier observations [3] could be re-interpreted into one cohesive mechanistic picture [2]. Our comment [1], concerning Mazzocchin's work, that "...investigations...at a platinum electrode are reported to give rise to no meaningful reductive processes" referred to Mazzocchin's statement [3] that " T h e cis-platinum(II) complexes did not exhibit cathodic peaks on platinum electrodes with the exception of cis[PtC12(PPh3)2] which showed a hump in the solvent cathodic discharge". While it is arguable as to whether or not a hump in a solvent discharge is a meaningful reductive process, Mazzocchin [3] further states " T h e trans-platinum(II) complexes underwent reduction at the same potential values as the corresponding cis-complexes", implying that both isomers of [PtClz(PPh3)2] exhibit " a hump in the solvent cathodic discharge" while no other [PtClz(PR 3) 2 ] complexes studied exhibit cathodic peaks on platinum electrodes. Our work [1,2] showed that trans[PtClz(PPh3)2] was reduced at ca. - 2 . 0 V on a platinum electrode and does indeed appear as a hump in the solvent cathodic discharge, as Mazzocchin had correctly stated earlier [3]. However, cis-[PtC12(PPh3)z] exhibits a well-defined peak at - 1 . 5 5 V on a platinum electrode [1,2], well clear of the solvent discharge. The fact that the two isomers equilibrate in solution was postulated by us as a possible reason for such a discrepancy in the observations reported by Mazzocchin [3] and by ourselves [1,2]. As part of our mechanistic picture [1,2], we proposed that the two-electron
* To whom correspondence should be addressed. 0022-0728/84/$03.00
© 1984 Elsevier Sequoia S.A.
274 reduction of [PtCI2(PPh3)2] would produce a labile platinum(0) complex, capable of dissociating triphenylphosphine, and also free chloride ion. A cyclic v o l t a m m o g r a m of NEtnC1, as a chloride source, showed a broad oxidative peak at ca. 0.0 V while a cyclic voltammogram of triphenylphosphine exhibited a number of features including broad peaks at ca. 0.0 V (oxidative) and - 0 . 2 0 V (reductive). Peaks in these positions are apparent in voltammograms of [PtC12(PPh3)2] complexes, figures of which are shown in refs. 1-3. We thus attributed the 0.0 V peak to (i) the presence of C1- a n d / o r (ii) the presence of PPh 3 a n d / o r (iii) possible Pt(0) ~ Pt(II) oxidation process. We were unable to differentiate between these possibilities. The - 0 . 2 V peak we attributed to PPh 3 and observed [1] that this position corresponded to that assigned to Pt(IV) ~ Pt(II) reductions by Mazzochin [3]. Possible Pt(IV) reductions could not explain this peak in our voltammograms as we did not explore potentials capable of generating such species. Mazzocchin, however, was exploring such potential ranges and hence contributions to this peak from such P t ( I V ) ~ Pt(II) reductive processes are entirely possible and indeed seem likely in such cases. Clearly it is of interest, as we stated, to note that two such different processes, one involving triphenylphosphine and the other involving Pt(IV) complexes, may result in peaks in the same potential range. Many workers [5-7] have explored the electrochemistry of triphenylphoshine fully and it is not our intention to repeat or even provide a review of such exhaustive studies. One such study [5], reported in this journal and cited in our earlier reports [1,2], shows a figure of a cyclic voltammogram of 4.0 x 10 -3 M triphenylphosphine in acetonitrile solution containing 0.2 M NaC104 on a platinum electrode. The voltammogram shows two oxidative peaks at + 0.83 V and - 0 . 3 7 V and a single reductive peak at - 0.68 V. All potentials were measured with respect to a Ag/0.1 M A g N O 3 electrode in acetonitrile. Figure 1 shows a voltammogram of 4.0 x 10 -3 M PPh 3 (Aldrich, 99%) in acetonitrile + benzene solution containing 0.1 M BuaNC104 on a platinum electrode (1.77 m m 2) obtained in this laboratory. The profile, over the potential range of interest to us, corresponds well to that previously described, with reasonably similar experimental conditions. The peak potentials we measure are ca. 0.0 V (oxidative) and - 0 . 2 0 V (reductive) using an Ag/AgC1 reference electrode separated from the acetonitrile + benzene medium by a Vycor plug. While we have no data immediately at hand to allow a direct comparison of our potentials with those reported relative to the Ag/0.1 M AgNO 3 electrode in acetonitrile, it is known [8] that an Ag/0.1 M AgC104 electrode in acetonitrile has a measured potential of + 0.495 V vs. SCE. As our reference electrode has a measured potential of - 0.035 V vs. SCE, some rather approximate comparisons of potentials can be made, allowing for differences due to the effects of the nitrate anion compared to the perchlorate anion and due to the differences in solvent systems. With such limitations in mind, the literature values of - 0 . 6 8 V (reductive) and - 0 . 3 7 V (oxidative) transpose to - 0 . 1 5 V (reductive) and +0.16 V (oxidative) on our reference scale. Bearing in mind the limitations of such comparisons, these values are not too far removed from the - 0 . 2 0 V (reductive) and ca. 0.0 V (oxidative) values that we have measured. These peaks may thus be used as indicators that free PPh 3 is present in solution,
275
[5
i/~A 4
0
--4
E/V vs. Ag/AgCI
Fig. 1. Cyclic voltammogram of 4×10 -3 M PPh 3 in CH3CN+30% rate = 200 mV/s).
C 6 H 6 at
a Pt electrode (scan
although we make no attempt to specify their chemical origins. While Bard [7] has indeed stated that triphenylphosphine shows a reduction peak at - 2 . 7 5 V vs. SCE, this is using D M F as the solvent. Bard [7] also states that " t h e results obtained in acetonitrile differ significantly from those in D M F " and cites a reduction peak at - 1 . 6 0 V vs. SCE which appeared irreversible. N o mention of other peaks is made by Bard [7] and other workers [5] have since questioned some aspects of the results for acetonitrile solutions of triphenylphosphine. Clearly the situation for triphenylphosphine is complex and the definitive story of its electrochemistry is still evolving. In conclusion, we would point out that at no time have we criticized the results of Mazzocchin and co-workers, rather we have attempted to supply a coherent interpretation of both their results and our own in terms of a model for cis/trans isomerization. We see no reason to alter any facet of this model at the present time as its validity and usefulness are still clearly apparent. ACKNOWLEDGEMENTS Financial support from the Research Corporation in the form of a Cottrell Research G r a n t and the Petroleum Research Fund, administered by the American Chemical Society is gratefully acknowledged. Thanks are expressed to Johnson Matthey, Inc., and Kigre, Inc. for loans of platinum. REFERENCES 1 J.A. Davies and V. Uma, Inorg. Claim. Acta, 76 (1983) L305. 2 J.A. Davies and V. Uma, J. Electroanal. Chem., 158 (1983) 13.
276 3 4 5 6 7 8
G. Mazzocchin, G. Bontempelli, M. Nicolini and B. Crociani, Inorg. Claim. Acta, 18 (1976) 159. G. Mazzocchin and G. Bontempelli, J. Electroanal. Chem., 179 (1984) 269. G. Schiavon, S. Zecchin, G. Cogoni and G. Bontempelli, J. Electroanal. Chem., 48 (1973) 425. J.M. Sav6ant and S.K. Binh, J. Electroanal. Chem., 88 (1978) 27. K.S.V. Santhanam and A.J. Bard, J. Am. Chem. Soc., 90 (1968) 1118. G. Pilloni, G. Zotti, Q.G. Mulazzani and P.G. Fuochi, J. Electroanal. Chem., 137 (1982) 89.
273
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication FURTHER COMMENTS ON THE ELECTROCHEMISTRY OF [PtCI2(PR3)2] COMPLEXES
J.A. DAVIES * and V. UMA
Department of Chemistry, College of Arts & Sciences, University of Toledo, Toledo, OH 43606 (U.S.A.) 'Received 20th March 1984; in revised form 19th June 1984)
Recently we described how cyclic voltammetry may be employed to study the isomerization reactions of [PtClz(PPh3)2] complexes. Our initial results were described in a letter [1] and later the complete study was reported in a full paper [2]. Our work complemented the early studies of Mazzocchin and co-workers [3], who were among the pioneers of transition metal phosphine electrochemistry, and whose comments precede this paper [4]. By studying the time dependence of cyclic voltammograms of [PtC12(PPh3)2] complexes, and interpreting the observations in terms of a catalytic cis/trans equilibration reaction, certain of the earlier observations [3] could be re-interpreted into one cohesive mechanistic picture [2]. Our comment [1], concerning Mazzocchin's work, that "...investigations...at a platinum electrode are reported to give rise to no meaningful reductive processes" referred to Mazzocchin's statement [3] that " T h e cis-platinum(II) complexes did not exhibit cathodic peaks on platinum electrodes with the exception of cis[PtC12(PPh3)2] which showed a hump in the solvent cathodic discharge". While it is arguable as to whether or not a hump in a solvent discharge is a meaningful reductive process, Mazzocchin [3] further states " T h e trans-platinum(II) complexes underwent reduction at the same potential values as the corresponding cis-complexes", implying that both isomers of [PtClz(PPh3)2] exhibit " a hump in the solvent cathodic discharge" while no other [PtClz(PR 3) 2 ] complexes studied exhibit cathodic peaks on platinum electrodes. Our work [1,2] showed that trans[PtClz(PPh3)2] was reduced at ca. - 2 . 0 V on a platinum electrode and does indeed appear as a hump in the solvent cathodic discharge, as Mazzocchin had correctly stated earlier [3]. However, cis-[PtC12(PPh3)z] exhibits a well-defined peak at - 1 . 5 5 V on a platinum electrode [1,2], well clear of the solvent discharge. The fact that the two isomers equilibrate in solution was postulated by us as a possible reason for such a discrepancy in the observations reported by Mazzocchin [3] and by ourselves [1,2]. As part of our mechanistic picture [1,2], we proposed that the two-electron
* To whom correspondence should be addressed. 0022-0728/84/$03.00
© 1984 Elsevier Sequoia S.A.
274 reduction of [PtCI2(PPh3)2] would produce a labile platinum(0) complex, capable of dissociating triphenylphosphine, and also free chloride ion. A cyclic v o l t a m m o g r a m of NEtnC1, as a chloride source, showed a broad oxidative peak at ca. 0.0 V while a cyclic voltammogram of triphenylphosphine exhibited a number of features including broad peaks at ca. 0.0 V (oxidative) and - 0 . 2 0 V (reductive). Peaks in these positions are apparent in voltammograms of [PtC12(PPh3)2] complexes, figures of which are shown in refs. 1-3. We thus attributed the 0.0 V peak to (i) the presence of C1- a n d / o r (ii) the presence of PPh 3 a n d / o r (iii) possible Pt(0) ~ Pt(II) oxidation process. We were unable to differentiate between these possibilities. The - 0 . 2 V peak we attributed to PPh 3 and observed [1] that this position corresponded to that assigned to Pt(IV) ~ Pt(II) reductions by Mazzochin [3]. Possible Pt(IV) reductions could not explain this peak in our voltammograms as we did not explore potentials capable of generating such species. Mazzocchin, however, was exploring such potential ranges and hence contributions to this peak from such P t ( I V ) ~ Pt(II) reductive processes are entirely possible and indeed seem likely in such cases. Clearly it is of interest, as we stated, to note that two such different processes, one involving triphenylphosphine and the other involving Pt(IV) complexes, may result in peaks in the same potential range. Many workers [5-7] have explored the electrochemistry of triphenylphoshine fully and it is not our intention to repeat or even provide a review of such exhaustive studies. One such study [5], reported in this journal and cited in our earlier reports [1,2], shows a figure of a cyclic voltammogram of 4.0 x 10 -3 M triphenylphosphine in acetonitrile solution containing 0.2 M NaC104 on a platinum electrode. The voltammogram shows two oxidative peaks at + 0.83 V and - 0 . 3 7 V and a single reductive peak at - 0.68 V. All potentials were measured with respect to a Ag/0.1 M A g N O 3 electrode in acetonitrile. Figure 1 shows a voltammogram of 4.0 x 10 -3 M PPh 3 (Aldrich, 99%) in acetonitrile + benzene solution containing 0.1 M BuaNC104 on a platinum electrode (1.77 m m 2) obtained in this laboratory. The profile, over the potential range of interest to us, corresponds well to that previously described, with reasonably similar experimental conditions. The peak potentials we measure are ca. 0.0 V (oxidative) and - 0 . 2 0 V (reductive) using an Ag/AgC1 reference electrode separated from the acetonitrile + benzene medium by a Vycor plug. While we have no data immediately at hand to allow a direct comparison of our potentials with those reported relative to the Ag/0.1 M AgNO 3 electrode in acetonitrile, it is known [8] that an Ag/0.1 M AgC104 electrode in acetonitrile has a measured potential of + 0.495 V vs. SCE. As our reference electrode has a measured potential of - 0.035 V vs. SCE, some rather approximate comparisons of potentials can be made, allowing for differences due to the effects of the nitrate anion compared to the perchlorate anion and due to the differences in solvent systems. With such limitations in mind, the literature values of - 0 . 6 8 V (reductive) and - 0 . 3 7 V (oxidative) transpose to - 0 . 1 5 V (reductive) and +0.16 V (oxidative) on our reference scale. Bearing in mind the limitations of such comparisons, these values are not too far removed from the - 0 . 2 0 V (reductive) and ca. 0.0 V (oxidative) values that we have measured. These peaks may thus be used as indicators that free PPh 3 is present in solution,
275
[5
i/~A 4
0
--4
E/V vs. Ag/AgCI
Fig. 1. Cyclic voltammogram of 4×10 -3 M PPh 3 in CH3CN+30% rate = 200 mV/s).
C 6 H 6 at
a Pt electrode (scan
although we make no attempt to specify their chemical origins. While Bard [7] has indeed stated that triphenylphosphine shows a reduction peak at - 2 . 7 5 V vs. SCE, this is using D M F as the solvent. Bard [7] also states that " t h e results obtained in acetonitrile differ significantly from those in D M F " and cites a reduction peak at - 1 . 6 0 V vs. SCE which appeared irreversible. N o mention of other peaks is made by Bard [7] and other workers [5] have since questioned some aspects of the results for acetonitrile solutions of triphenylphosphine. Clearly the situation for triphenylphosphine is complex and the definitive story of its electrochemistry is still evolving. In conclusion, we would point out that at no time have we criticized the results of Mazzocchin and co-workers, rather we have attempted to supply a coherent interpretation of both their results and our own in terms of a model for cis/trans isomerization. We see no reason to alter any facet of this model at the present time as its validity and usefulness are still clearly apparent. ACKNOWLEDGEMENTS Financial support from the Research Corporation in the form of a Cottrell Research G r a n t and the Petroleum Research Fund, administered by the American Chemical Society is gratefully acknowledged. Thanks are expressed to Johnson Matthey, Inc., and Kigre, Inc. for loans of platinum. REFERENCES 1 J.A. Davies and V. Uma, Inorg. Claim. Acta, 76 (1983) L305. 2 J.A. Davies and V. Uma, J. Electroanal. Chem., 158 (1983) 13.
276 3 4 5 6 7 8
G. Mazzocchin, G. Bontempelli, M. Nicolini and B. Crociani, Inorg. Claim. Acta, 18 (1976) 159. G. Mazzocchin and G. Bontempelli, J. Electroanal. Chem., 179 (1984) 269. G. Schiavon, S. Zecchin, G. Cogoni and G. Bontempelli, J. Electroanal. Chem., 48 (1973) 425. J.M. Sav6ant and S.K. Binh, J. Electroanal. Chem., 88 (1978) 27. K.S.V. Santhanam and A.J. Bard, J. Am. Chem. Soc., 90 (1968) 1118. G. Pilloni, G. Zotti, Q.G. Mulazzani and P.G. Fuochi, J. Electroanal. Chem., 137 (1982) 89.