Electron-transfer processes can play an important role in organometallic chemistry. Herein, we demonstrate that even d^0 organometallic complexes exhibit a rich redox chemistry and present electrochemical, kinetics, and stereochemical studies that establish a general outer-sphere oxidation process for d^0 group 4 metallocene complexes. This oxidation chemistry closely parallels the reported LMCT photochemistry of these complexes

Burk, Mark J.

Tumas, William

Ward, Michael D.

Wheeler, David R.

Caltech Authors - Main

Supporting Information for J. Am. Chem. Soc., 1990, 112(16), 6133-6135, DOI: 10.1021/ja00172a042
Terms& Conditions
Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical
Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website
may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any
Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form
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permission system can be found at http://pubs.acs.org/page/copyright/permissions.html.
Copyright © 1990 American Chemical Society
Q.^/35-mt
Supplementary Material 
for
Oxidation Chemistry of d° Organometallic Complexes
Mark J. Burk,* William Tumas,* Michael D. Ward,* David R. Wheelert
Contribution No. 5392
Central Research and Development, E.L du Pont de Nemours and Co., Inc.
Experimental Station, Wilmington, DE 19880.
Experimental Considerations
All manipulations were carried out under inert atmosphere conditions 
using standard drybox and Schlenk line techniques with dry, deoxygenated 
solvents. Methylene chloride and acetonitrile, as well as their deuterated 
counterparts used for NMR studies, were distilled under high vacuum from 
CaH2 after deoxygenation by multiple freeze-pump-thaw cycles. 
Tetrabutylammonium tetraphenylborate (Aldrich) and tetrabutylammonium 
tetrafluoroborate were recrystallized from acetone/water (3/1) and 
acetonitriIe/ethyI acetate, respectively, then dried in vacuo for several days. 2,3- 
Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, Aldrich) was recrystallized and 
dried in vacuo. Ferrocenium salts were prepared from ferrocenium 
tetrachSoroferrate (Alfa). Tetrakis(trifluoromethyl)cyclopentadienone was 
prepared using literature methods1 and purified by vacuum sublimation.
All metallocene dialkyl, diaryl, and metallacycle complexes used in this study 
were prepared according to literature procedures, purified by recrystallization, and 
characterized by nmr spectroscopy. The bis(cyclopentadienyl)titanacyclobutanes 1 
and their stereolabelJed analogs 4 were prepared from the Tebbe reagent and 
appropriate olefin following literature procedures.2 Diphenyltitanocene3 (3) and
(j-was-ma-
diphenylzirconocene4 (2) were obtained from phenyllithium (Aldrich) and the 
respective metallocene dichlorides (Aldrich), while the dibenzyl compounds 5 and 6  
were prepared from CP2 MCI2 and benzyimagnesium chloride (Aldrich) .5 -6 
Dimethylzirconocene4 was purified by vacuum sublimation ( 65 °C, 10-3 Torr), All 
samples used in this study were pure by NMR.
Electrochemical Experiments. Bulk electrolysis, cyclic voltammetry and 
rotating disk experiments were performed in a nitrogen filled drybox (Vac 
Atmospheres) or in airless electrochemical cells using a Princeton Applied 
Research (PAR) 173/175 programmer/potentiostat. Rotating disk experiments 
were perfomed with a Pine Instruments rotator and a Pt disk electrode in a teflon 
sheath. Microelectrode experiments were conducted u$?ng a homemade fast 
scan potentiosat and a Nicolet 3091 digital oscilloscope. Pt electrodes 
(A = 0.029 cm2), microelectrodes (r = 5 fim), and Ag/AgCI reference electrodes 
were obtained from Bioanalytical Systems (BAS). Ag/AgNOs (0.1 M in CH3CN) 
reference electrodes were used for all electrochemical experiments performed 
in the glove box. Microelectrode experiments were performed in an airless 
electrochemical cell with a Ag/AgCI reference electrode (BAS). Ferrocene 
(E° = +0.41 V vs. SCE)7 was added as an internal standard to verify potentials. 
Reported reduction potentials (Table 1) were then corrected to V vs. SCE. The 
electrochemical behavior of most of these complexes was very sensitive to 
adventitious water and/or oxygen; therefore, it was necessary to work under 
rigorously inert conditions. Cyclic voltammetry revealed that passivation of the 
platinum electrode occurred upon potential excursions into the anodic wave of 
the CP2 MR2 complexes. This was especially evident in ultramicroelectrode 
studies, even at scan rates of 1000 mV s_1. Thus, determination of the E1/2 
values at very fast scan rates, where kinetic factors would presumably be 
minimized, were not generally feasible. It was necessary to clean the electrode
y-(t/35-rrB
frequently to obtain consistent cyclic voltammograms. One exception to this 
was titanacyclobutane la : at scan rates of 1000 V s-1, the measured E 1/2 was 
0.86 V vs SCE. The observation that this number was significantly positive of 
the value determined at a conventional electrode is consistent with a very fast 
chemical reaction following the oxidation of 1a.
Electrochemical Mediation Experiments/ Kinetic Analysis. Cyclic 
voltammetric determinations of the reaction rates of the ferrocenium-mediated 
oxidation of Group 4 metallocene complexes were carried out in the glove box with 
either 7 or 15 equivalents of the metal alkyl (relative to the ferrocene). The data were 
analyzed according to an EC scheme:8
Cp2Fe ----- ►  [Cp2Fe]+
i
[Cp2Fe]+ + Cp2MR2 J i!— Cp2Fe + "Product"
by measuring L  values. These limiting currents are related to the rate constant by
L  = nFAC0(Dk*Cz)1̂ 2 
which upon rearrangement affords
I p 1
k *  =  (™ —   -)
nFAC ' DC7
0 z
where k* is the bimolecular rate constant for the reaction between the ferrocenium ion 
and metal alkyl, L  is the limiting current obtained directly from the cyclic
voltammogram, C0  is the ferrocene concentration, D is the ferrocene diffusion 
coefficient and Cz is the metal alkyl concentration.
Rotating Disk Experiments. The heterogeneous electron transfer rate for 
1a was determined by measuring the limiting currents at different applied 
potentials (vs. Ag/AgN03 in BU4 NBF4 /CH3 CN). The rate constant at a given 
potential, ke, was determined using
ke = ie/nFACz
where i6 was estimated from the limiting current at the highest rotation rates (3600 
RPM) or from the intercept of inverse Levich plots (1/i vs. o r1/2). Kinetic measurements 
on metallocene compounds other than 1a were hampered by electrode passivation. 
Product Analyses of Bulk Electrolyses. Bulk electrolyses of several compounds 
for the purpose of product analysis were performed in deuterated solvents using a Li- 
cell with a frit to separate the working and counter electrode solutions. In a typical 
experiment, ca. 100 mg of metallocene complex 1, 2, 3, or 4 was dissolved in 4 ml 
deuterated solvent (CD2CI2 or CD3 CN) along with electrolyte (0.1 M) in the working 
side of the cell (Pt gauze working electrode). The counter electrode was a Pt foil in 0 . 1  
M electrolyte. In order to avoid complications from competing thermal retro 2+2 
cleavage, electrolysis experiments on the titanacyclobutanes 1a and 4 were run at 
0  °C in airless electrochemical cells. The working electrode contents were then 
transferred under Ar to a schlenk tube via cannula. After several freeze-pump-thaw 
cycles, the volatiies were collected by vacuum transfer at 0 °C and analyzed by NMR. 
This procedure prevented the production of alkenes from thermal decomposition2  and 
revealed exclusive production of cyclopropanes. The stereochemistry of the 
deuterated cyclopropanes formed by oxidation of 4 was determined by integration of
- 6 /35-
the methylene resonances (1H NMR).9 For the diphenylmetallocenes 2 and 3, bulk 
electrolyses were performed at room temperature in the drybox and the products 
(biphenyl and/or benzene) were characterized by 1H NMR analysis of the working 
electrode solutions. Electrode passivation was a considerable problem for 2 and 3 
during oxidative electrolysis, preventing high conversions and thereby hampering 
product analysis. Complex 2 was found to be very sensitive to partial hydrolysis 
resulting in the production of (Cp2ZrPh)20 and benzene during electrolysis. We were 
unable to discern whether the benzene produced from electrolysis of 2 was due solely 
to hydrolysis or also resulted from oxidative cleavage as well. Nonetheless the 
electrolytic production of benzene and biphenyl corroborate an electron transfer 
process.
References for Supplementary Material
1. Dickson. R.S.: Wilkinson. G. J. Chem. Soc. 1964. 2699.
2. Lee, J.B.; Ott, K.C.; Grubbs, R.H. J. Am. Chem. Soc. 1982. 104. 749.
Wheeler, D.R., Ph.D. Thesis, California Institute of Technology, 1989.
3. Boekel, C.P.; Teuben, J.H.; De Liefde Meijer, H.J. J. Organomet. Chem. 1974.
SI, 371.
4. Samuel, E.; Rausch, M.D. J. Am. Chem. St. .1973, 6263.
5. Boekel, C.P.; Teuben, J.H.; De Liefde Meijer, H.J. J. Organomet. Chem. 1975,
102. 317.
6. Fachinetti, G.; Floriani, C. J. C. S. Chem. Comm. 1972. 654.
7. Ward, M.D. in "Electroanalytical Chemistry," Bard, A.J.; Ed., Marcei Dekker, Inc.: 
New York, 19891 Vol 16, p. 181.
8. Saveant, J.M.; Vianello, E. Electrochim. Acta 1965 10. 905; Nicholson, R.S.; 
Shain. I. Anal. Chem. 1964 15. 155.
9. Schaeffer, T.; Sebastian, R.; Wasylishen, R.E. Can. J. Chem. 1982. 60, 845.


English

Oxidation chemistry of d^0 organometallic complexes

Supporting Information for J. Am. Chem. Soc., 1990, 112(16), 6133-6135, DOI: 10.1021/ja00172a042
Terms& Conditions
Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical
Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website
may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any
Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form
without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material,
requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink
permission system can be found at http://pubs.acs.org/page/copyright/permissions.html.
Copyright © 1990 American Chemical Society
Q.^/35-mt
Supplementary Material 
for
Oxidation Chemistry of d° Organometallic Complexes
Mark J. Burk,* William Tumas,* Michael D. Ward,* David R. Wheelert
Contribution No. 5392
Central Research and Development, E.L du Pont de Nemours and Co., Inc.
Experimental Station, Wilmington, DE 19880.
Experimental Considerations
All manipulations were carried out under inert atmosphere conditions 
using standard drybox and Schlenk line techniques with dry, deoxygenated 
solvents. Methylene chloride and acetonitrile, as well as their deuterated 
counterparts used for NMR studies, were distilled under high vacuum from 
CaH2 after deoxygenation by multiple freeze-pump-thaw cycles. 
Tetrabutylammonium tetraphenylborate (Aldrich) and tetrabutylammonium 
tetrafluoroborate were recrystallized from acetone/water (3/1) and 
acetonitriIe/ethyI acetate, respectively, then dried in vacuo for several days. 2,3- 
Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, Aldrich) was recrystallized and 
dried in vacuo. Ferrocenium salts were prepared from ferrocenium 
tetrachSoroferrate (Alfa). Tetrakis(trifluoromethyl)cyclopentadienone was 
prepared using literature methods1 and purified by vacuum sublimation.
All metallocene dialkyl, diaryl, and metallacycle complexes used in this study 
were prepared according to literature procedures, purified by recrystallization, and 
characterized by nmr spectroscopy. The bis(cyclopentadienyl)titanacyclobutanes 1 
and their stereolabelJed analogs 4 were prepared from the Tebbe reagent and 
appropriate olefin following literature procedures.2 Diphenyltitanocene3 (3) and
(j-was-ma-
diphenylzirconocene4 (2) were obtained from phenyllithium (Aldrich) and the 
respective metallocene dichlorides (Aldrich), while the dibenzyl compounds 5 and 6  
were prepared from CP2 MCI2 and benzyimagnesium chloride (Aldrich) .5 -6 
Dimethylzirconocene4 was purified by vacuum sublimation ( 65 °C, 10-3 Torr), All 
samples used in this study were pure by NMR.
Electrochemical Experiments. Bulk electrolysis, cyclic voltammetry and 
rotating disk experiments were performed in a nitrogen filled drybox (Vac 
Atmospheres) or in airless electrochemical cells using a Princeton Applied 
Research (PAR) 173/175 programmer/potentiostat. Rotating disk experiments 
were perfomed with a Pine Instruments rotator and a Pt disk electrode in a teflon 
sheath. Microelectrode experiments were conducted u$?ng a homemade fast 
scan potentiosat and a Nicolet 3091 digital oscilloscope. Pt electrodes 
(A = 0.029 cm2), microelectrodes (r = 5 fim), and Ag/AgCI reference electrodes 
were obtained from Bioanalytical Systems (BAS). Ag/AgNOs (0.1 M in CH3CN) 
reference electrodes were used for all electrochemical experiments performed 
in the glove box. Microelectrode experiments were performed in an airless 
electrochemical cell with a Ag/AgCI reference electrode (BAS). Ferrocene 
(E° = +0.41 V vs. SCE)7 was added as an internal standard to verify potentials. 
Reported reduction potentials (Table 1) were then corrected to V vs. SCE. The 
electrochemical behavior of most of these complexes was very sensitive to 
adventitious water and/or oxygen; therefore, it was necessary to work under 
rigorously inert conditions. Cyclic voltammetry revealed that passivation of the 
platinum electrode occurred upon potential excursions into the anodic wave of 
the CP2 MR2 complexes. This was especially evident in ultramicroelectrode 
studies, even at scan rates of 1000 mV s_1. Thus, determination of the E1/2 
values at very fast scan rates, where kinetic factors would presumably be 
minimized, were not generally feasible. It was necessary to clean the electrode
y-(t/35-rrB
frequently to obtain consistent cyclic voltammograms. One exception to this 
was titanacyclobutane la : at scan rates of 1000 V s-1, the measured E 1/2 was 
0.86 V vs SCE. The observation that this number was significantly positive of 
the value determined at a conventional electrode is consistent with a very fast 
chemical reaction following the oxidation of 1a.
Electrochemical Mediation Experiments/ Kinetic Analysis. Cyclic 
voltammetric determinations of the reaction rates of the ferrocenium-mediated 
oxidation of Group 4 metallocene complexes were carried out in the glove box with 
either 7 or 15 equivalents of the metal alkyl (relative to the ferrocene). The data were 
analyzed according to an EC scheme:8
Cp2Fe ----- ►  [Cp2Fe]+
i
[Cp2Fe]+ + Cp2MR2 J i!— Cp2Fe + "Product"
by measuring L  values. These limiting currents are related to the rate constant by
L  = nFAC0(Dk*Cz)1^ 2 
which upon rearrangement affords
I p 1
k *  =  (™ —   -)
nFAC ' DC70 z
where k* is the bimolecular rate constant for the reaction between the ferrocenium ion 
and metal alkyl, L  is the limiting current obtained directly from the cyclic
voltammogram, C0  is the ferrocene concentration, D is the ferrocene diffusion 
coefficient and Cz is the metal alkyl concentration.
Rotating Disk Experiments. The heterogeneous electron transfer rate for 
1a was determined by measuring the limiting currents at different applied 
potentials (vs. Ag/AgN03 in BU4 NBF4 /CH3 CN). The rate constant at a given 
potential, ke, was determined using
ke = ie/nFACz
where i6 was estimated from the limiting current at the highest rotation rates (3600 
RPM) or from the intercept of inverse Levich plots (1/i vs. o r1/2). Kinetic measurements 
on metallocene compounds other than 1a were hampered by electrode passivation. 
Product Analyses of Bulk Electrolyses. Bulk electrolyses of several compounds 
for the purpose of product analysis were performed in deuterated solvents using a Li- 
cell with a frit to separate the working and counter electrode solutions. In a typical 
experiment, ca. 100 mg of metallocene complex 1, 2, 3, or 4 was dissolved in 4 ml 
deuterated solvent (CD2CI2 or CD3 CN) along with electrolyte (0.1 M) in the working 
side of the cell (Pt gauze working electrode). The counter electrode was a Pt foil in 0 . 1  
M electrolyte. In order to avoid complications from competing thermal retro 2+2 
cleavage, electrolysis experiments on the titanacyclobutanes 1a and 4 were run at 
0  °C in airless electrochemical cells. The working electrode contents were then 
transferred under Ar to a schlenk tube via cannula. After several freeze-pump-thaw 
cycles, the volatiies were collected by vacuum transfer at 0 °C and analyzed by NMR. 
This procedure prevented the production of alkenes from thermal decomposition2  and 
revealed exclusive production of cyclopropanes. The stereochemistry of the 
deuterated cyclopropanes formed by oxidation of 4 was determined by integration of
- 6 /35-
the methylene resonances (1H NMR).9 For the diphenylmetallocenes 2 and 3, bulk 
electrolyses were performed at room temperature in the drybox and the products 
(biphenyl and/or benzene) were characterized by 1H NMR analysis of the working 
electrode solutions. Electrode passivation was a considerable problem for 2 and 3 
during oxidative electrolysis, preventing high conversions and thereby hampering 
product analysis. Complex 2 was found to be very sensitive to partial hydrolysis 
resulting in the production of (Cp2ZrPh)20 and benzene during electrolysis. We were 
unable to discern whether the benzene produced from electrolysis of 2 was due solely 
to hydrolysis or also resulted from oxidative cleavage as well. Nonetheless the 
electrolytic production of benzene and biphenyl corroborate an electron transfer 
process.
References for Supplementary Material
1. Dickson. R.S.: Wilkinson. G. J. Chem. Soc. 1964. 2699.
2. Lee, J.B.; Ott, K.C.; Grubbs, R.H. J. Am. Chem. Soc. 1982. 104. 749.
Wheeler, D.R., Ph.D. Thesis, California Institute of Technology, 1989.
3. Boekel, C.P.; Teuben, J.H.; De Liefde Meijer, H.J. J. Organomet. Chem. 1974.
SI, 371.
4. Samuel, E.; Rausch, M.D. J. Am. Chem. St. .1973, 6263.
5. Boekel, C.P.; Teuben, J.H.; De Liefde Meijer, H.J. J. Organomet. Chem. 1975,
102. 317.
6. Fachinetti, G.; Floriani, C. J. C. S. Chem. Comm. 1972. 654.
7. Ward, M.D. in "Electroanalytical Chemistry," Bard, A.J.; Ed., Marcei Dekker, Inc.: 
New York, 19891 Vol 16, p. 181.
8. Saveant, J.M.; Vianello, E. Electrochim. Acta 1965 10. 905; Nicholson, R.S.; 
Shain. I. Anal. Chem. 1964 15. 155.
9. Schaeffer, T.; Sebastian, R.; Wasylishen, R.E. Can. J. Chem. 1982. 60, 845.


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Oxidation chemistry of d^0 organometallic complexes

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