Generation of the 15-electron rhodium(II) complex [RhCl(PPh3)3]+ by 1-electron oxidation of wilkinson's catalyst

Article abstract of DOI:10.1021/om010077b

Wilkinson's catalyst, RhCl(PPh₃)₃ (1), undergoes a one-electron oxidation at a sufficiently low potential (E_1/2 = +0.035 V us Fc) to allow a facile electron-transfer reaction with mild oxidants such as the ferrocenium ion. The Rh(II) cation 1⁺ decomposes rapidly to the very reactive intermediate 2, which over longer reaction times gives rise to the three-coordinate complex [Rh(PPh₃)₃]⁺ (4).

Full text of DOI:10.1021/om010077b

Organometallics 2001, 20, 2133-2135
2133
Gen er a tion of th e 15-Electr on Rh od iu m (II) Com p lex
[Rh Cl(P P h 3)3]+ by 1-Electr on Oxid a tion of Wilk in son ’s
Ca ta lyst
Fre´de´ric Barrie`re and William E. Geiger*
Department of Chemistry, University of Vermont, Burlington, Vermont 05405
Received J anuary 31, 2001
Sch em e 1
Summary: Wilkinson’s catalyst, RhCl(PPh3)3 (1), under-
goes a one-electron oxidation at a sufficiently low poten-
tial (E1/ 2 ) +0.035 V vs Fc) to allow a facile electron-
transfer reaction with mild oxidants such as the
ferrocenium ion. The Rh(II) cation 1+ decomposes rap-
idly to the very reactive intermediate 2, which over longer
reaction times gives rise to the three-coordinate complex
[Rh(PPh3)3]+ (4).
Although the very important complex1 RhCl(PPh3)3
(Wilkinson’s catalyst 1) has been the subject of intense
catalytic, mechanistic, and physical studies,2 the pros-
pect of its 1-electron oxidation and the chemical fate of
the putative 15-electron Rh(II) cation 1+ does not seem
to have been addressed. One-electron processes are
certainly expected to affect the dissociative reactions
crucial3 to the activity of Rh catalysts and may be
involved when Rh(I) compounds are combined with
latent oxidants (e.g., silver salts) for catalysis of cy-
cloadditions4 and other organic reactions. There is
precedent for the oxidation of phosphine-substituted Rh-
(I) (and even Rh(III)) complexes being thermodynami-
cally quite facile.5 Given these facts and the known high
reactivity6 of mononuclear Rh(II) complexes, we are
investigating the oxidative redox chemistry of RhCl-
(PPh3)3 and the possible catalytic properties of its
oxidation products. Although the reduction of 1 has been
described in several papers,7 its oxidation by electro-
chemistry or by one-electron-transfer agents does not
appear to have been previously reported.
complex 1+ has a limited lifetime at room temperature,
so that cyclic voltammetry (CV) at scan rates higher
than about 1 V/s is necessary to significantly outrun the
implied follow-up reaction (1+ f 2; Scheme 1). The
stability of 1+ increases at subambient temperatures,
its half-life being about 2 s at 273 K (Figure 1a).9 Full
chemical reversibility at slow scan rates is observed at
temperatures below 240 K (Figure 1b). A variety of
voltammetric and electrolytic methods, including bulk
coulometry, firmly establish a stoichiometry of one
electron for the oxidation. The separation between the
anodic and cathodic peak potentials (∆Ep) was found to
be only qualitatively reproducible on gold or glassy-
carbon electrodes, but occasionally ∆Ep values were
obtained that were quantitatively reproducible and close
to those of the decamethylferrocene/decamethylferroce-
nium couple (Fc*/Fc*+) under the same conditions
(Figure 1b). The observed electrode dependence of the
couple will be fully described at another time, but we
can state that the heterogeneous electron-transfer
process of the couple 1/1+ is reasonably rapid, with a ks
value of ca. 10-2 cm s-1 at a gold electrode (T ) 250
K).10a The voltammetric behavior was independent of
concentration over the range 0.1-3 mM.
RhCl(PPh3)3 undergoes a one-electron oxidation pro-
cess in CH2Cl2-[NBu4][PF6]8 at an E1/2 value of +0.035
V vs ferrocene (Fc) (1 h 1+; Scheme 1). The 15-electron
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(2) (a) J ardine, F. H. Progress in Inorganic Chemistry; Lippard, S.
J ., Ed.; Wiley: New York, 1981; Vol. 28. pp 63 ff. (b) J ardine, F. H.;
Sheridan, P. S. In Comprehensive Coordination Chemistry; Wilkinson,
G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon Press: Oxford,
U.K., 1987; Vol. 4, pp 901ff.
(3) Torrent, M.; Sola`, M.; Frenking, G. Chem. Rev. 2000, 100, 439.
(4) (a) Wender, P. A.; Correa, A. G.; Sato, Y.; Sun, R. J . Am. Chem.
Soc. 2000, 122, 7815. (b) Wender, P. A.; Glorius, L.; Husfeld, C. O.;
Langkopf, E.; Love, J . A. J . Am. Chem. Soc. 1999, 121, 5348.
(5) (a) Valcher, S.; Pilloni, G.; Martelli, M. J . Electroanal. Chem.
Interfacial Electrochem. 1973, 42, App 5-App 6. (b) Pilloni, G.;
Schiavon, G.; Zotti, G.; Zecchin, S. J . Organomet. Chem. 1977, 134,
305. (c) Menglet, D.; Bond, A. M.; Coutinho, K.; Dickson, R. S.; Lazarev,
G. G.; Olsen, S. A.; Pilbrow, J . R. J . Am. Chem. Soc. 1998, 120, 2086.
(d) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 56. (e) Fooladi,
E.; Tilset, M. Inorg. Chem. 1997, 36, 6021.
(7) (a) Olson, D. C.; Keim, W. Inorg. Chem. 1969, 8, 2028. (b) Pilloni,
G.; Valcher, S.; Martelli, M. J . Electroanal. Chem. Interfacial Electro-
chem. 1973, 42, 483. (c) Taqui Khan, M. M.; Samad, S. A.; Siddiqui,
M. R. H.; Bajaj, H. C.; Ramachandraiah, G. Polyhedron 1991, 10, 2729.
(d) Adekola, F. A.; Colin, C.; Bauer, D. Electrochim. Acta 1993, 38,
1331.
(8) Analogous CV behavior was also observed when the supporting
electrolyte employed a poorly coordinating anion: viz., [B(C6F5)4]-.
(9) A first-order reaction rate (kc) for the follow-up reaction 1+ f 2
was measured from the values of ic/ia at different scan rates (Nicholson,
R. S.; Shain, I. Anal. Chem. 1964, 36, 706. Nicholson, R. S. Anal. Chem.
1966, 38, 1406): kc ) 0.4 s-1 at T ) 273 K.
(6) DeWit, D. G. Coord. Chem. Rev. 1996, 147, 209.
10.1021/om010077b CCC: $20.00 © 2001 American Chemical Society
Publication on Web 05/03/2001

2134 Organometallics, Vol. 20, No. 11, 2001
Communications
20% yield, reversible CV) and Epc ≈ -1.5 V (somewhat
broad, irreversible, 40% yield). Raising the temperature
of the solution containing the oxidized forms of 2 and 3
increases the yield of the apparent single wave at about
-1.5 V by an additional 60%. This wave is ascribed to
the major long-term oxidation product of 1, the three-
coordinate cation [Rh(PPh3)3]+ (4) (2 f 4; Scheme
1).11-13 This process was complete in about 1 h at room
temperature. The complexity of the follow-up reactions
of 1+ is consistent with the fact that Rh(II) complexes
are prone to competitive reactions, including dimeriza-
tion, disproportionation, and reductive elimination
processes.5d,e,6
The identity of the 14-electron complex 4 was estab-
lished by NMR spectroscopy and by the close agreement
between literature values and the presently observed
experimental value of its reduction potential.14 To
facilitate the NMR experiments, it was first established
by CV scans that treatment of 1 with an innocent one-
electron oxidant such as the ferrocenium ion results in
the same products and relative yields as those obtained
by bulk electrolysis. Then, in a separate experiment 1
was oxidized by an equimolar amount of [Cp2Fe][PF6]
in cold (ca. 220 K) d2-dichloromethane in an NMR tube.
J ust after mixing, the solution showed low-temperature
spectra with several independent features, consistent
with the complexity of the electrochemical oxidation.
Two 31P{1H} NMR resonances, tentatively assigned to
2 and 3, were detected at 26.7 and 74.2 ppm, respec-
tively. Both of these features were very broad, more
likely owing to fast exchange processes than to the
presence of paramagnetic species, since frozen solutions
of these samples at 140 K are ESR silent. As expected
from bulk electrolysis data, these signals disappear
upon raising the temperature, after which the spectrum
of complex 4 becomes dominant. This approximately
T-shaped complex11-13 shows temperature-dependent
31P{1H} spectra, exhibiting coalescence at 300 K, well-
resolved peaks at reduced temperatures, and detection
of the four-coordinate [Rh(PPh3)4]+ cation at lower
temperatures in the presence of trace PPh3.12b,14 An-
F igu r e 1. CVs recorded under uncompensated solution
resistance conditions in CH2Cl2-[NBu4][PF6] 0.1 M under
N2. Potentials are referenced against the Fc/Fc+ couple. (a)
T ) 273 K, [1] ) 1.1 mM, ν ) 200 mV/s. The dotted line is
the second cycle, showing the reversible reduction of
product 2. The working electrode is a 1 mm diameter
glassy-carbon disk. (b) T ) 230 K, [1] ) 1.0 mM (∆Ep ) 69
mV), [Fc*] ) 0.5 mM (∆Ep ) 64 mV), ν ) 100 mV/s. The
working electrode is a 1 mm diameter gold disk.10a (c) T )
215 K, [1] ) 1.8 mM, ν ) 200 mV/s. The solid line is before
electrolysis at +0.3 V; the dotted line is after electrolysis.
The working electrode is a 1 mm diameter glassy-carbon
disk.
(11) Yared, Y. W.; Miles, S. L.; Bau, R.; Reed, C. A. J . Am. Chem.
Soc. 1977, 99, 7076.
(12) (a) Zotti, G.; Zecchin, S.; Pilloni, G. J . Electroanal. Chem.
Interfacial Electrochem. 1984, 175, 241. (b) Long, J . A.; Marder, T. B.;
Behnken, P. E.; Hawthorne, M. F. J . Am. Chem. Soc. 1984, 106, 2979.
(c) Knobler, C. B.; Marder, T. B.; Mizusawa, E. A.; Teller, R. G.; Long,
J . A.; Behnken, P. E.; Hawthorne, M. F. J . Am. Chem. Soc. 1984, 106,
2990.
The initial product from the decomposition of 1+ is
also electroactive, as shown in Figure 1a. The redox
couple of product 2 is reversible with E1/2 ) -0.335 V.
After this product is formed in about 40% yield10b by
bulk oxidation of 1 at 215 K (Figure 1c), it can be
reduced by electrolysis at -0.6 V. When the solution
containing the reduced form of 2 is warmed to room
temperature, 1 is recovered in ca. 40% yield, as esti-
mated by CV peak current comparisons. Complex 2 has,
to date, resisted identification (vide infra). Low-tem-
perature oxidative electrolysis of 1 also gives other
products having reduction waves at E1/2 ) -0.850 V (3,
(13) Besides the coordination of the metal to three phosphine
ligands, complex 4 appears to also interact with one solvent molecule
in solution or (weakly) to two carbons of one of the phenyl rings in the
solid state.11,12
(14) 31P{1H} NMR data for 4 in CD2Cl2 (in agreement with data
reported in ref 12b) are as follows. (i) At 300 K: coalescence, 36 ppm,
1
broad peak. (ii) Below 250 K: doublet of triplets, 49.3 ppm, J Rh,P
)
2
1
244 Hz, J P,P ) 30.5 Hz, doublet of doublet, 31.0 ppm, J Rh,P ) 133.5
2
Hz, J P,P ) 30.5 Hz, well-resolved peaks. (iii) Below 230 K: additional
1
doublet at 32.1 ppm, J Rh,P ) 133.5 Hz, assigned to the entropically
controlled formation of the four-coordinate [Rh(PPh3)4]+ cation by
reaction of 4 with free phosphine. Unassigned broad peaks (44-48
ppm) were also detected. All chemical shifts are externally referenced
to 80% H3PO4. The hexafluorophosphate heptet was centered at -144.2
and -141.0 ppm in CD2Cl2 and CD3CN, respectively. The irreversible
reduction of [Rh(PPh3)3]+ in CH2Cl2-[NBu4][PF6] (0.1 M) was found
to be dependent on both temperature and free PPh3 concentration (cf.
ref 12a for the electrochemical study of [Rh(PPh3)3(DME)]+ in dimethoxy-
ethane (DME)-[NBu4][ClO4] (0.2 M)). [Rh(PPh3)3]+ was also prepared
as its hexafluorophosphate salt by chloride abstraction from 1 with
TlPF6 in dichloromethane, adapting the procedure described in ref 11.
This allowed independent NMR and electrochemical studies and the
characterization of 4.
(10) (a) On one occasion, examination of the gold electrode with the
naked eye or the optical microscope revealed that the metal surface
had lost its shiny aspect and that it was covered with a fine layer of
black material. That electrode gave reproducible small ∆Ep’s for the
oxidation of 1 and could be reused in several experiments after being
simply rinsed with acetone and let dry. (b) The “% yields” of electrolysis
experiments are based on the assumption that diffusion coefficients
are equal.

Communications
Organometallics, Vol. 20, No. 11, 2001 2135
other short-term product which has been identified is
the chlorotriphenylphosphonium ion, [ClPPh3]+, from its
resonance at 66 ppm. The origin of the chlorine atom
in the phosphonium product is not the chlorinated
solvent, since it is detected as well when the solvent is
d3-acetonitrile.15
oxidizing agents.18 On an electrolytic time scale, very
reactive intermediates are formed which, in part, even-
tually yield the formally 14-electron cation 4. Sum-
marizing the rough stoichiometries of the electrochemi-
cal processes, anodic electrolysis of 1 at low temperature
gives 2 (40%), 3 (20%), and 4 (40%). After intermediate
2 is reduced, warming the solution to room temperature
yields 1 (40%) and 4 (60%). That is, reduced 2 gives back
Wilkinson’s catalyst, whereas 3 yields the three-
coordinate complex 4. The electron-transfer route is not
followed when 1 reacts with Ag(I) triflate. Given the fact
that mononuclear Rh(II) complexes may be active
catalysts or catalyst precursors,19 it seems worthwhile
to explore the possibility that oxidative treatment of
RhCl(PPh3)3 and its analogues may give rise to useful
low-temperature catalytic processes.
Although Ag+ is a strong oxidant in low-polarity
solvents16 (e.g., E1/2 ca. +0.65 V in CH2Cl2),17 treatment
of 1 with Ag[CF3SO2] was shown by NMR monitoring
to rapidly yield 4 by simple chloride abstraction rather
than to follow the complex oxidative path.
The present work shows that Wilkinson’s catalyst is
subject to a 1-electron oxidation, giving the detectable
15-electron Rh(II) monocation 1+. The potential of the
Rh(I)/Rh(II) couple (+0.035 V vs Fc) is sufficiently low
that the possibility of one-electron oxidation must be
considered whenever 1 may interact with even mild
Ack n ow led gm en t. We gratefully acknowledge sup-
port of this research by the National Science Foundation.
(15) (a) Wiley, G. A.; Stine, W. R. Tetrahedron Lett. 1967, 2321. (b)
Dillon, K. B.; Lynch, R. J .; Reeve, R. N.; Waddington, T. C. J . Inorg.
Nucl. Chem. 1974, 36, 815. (c) Independent 31P{1H} NMR measure-
ments were made in our laboratory by dissolving the phosphorane Cl2-
PPh3 (36.5 ppm in CD2Cl2 and 40.5 ppm in CD3CN) in the appropriate
solvent, yielding the ClPPh3+ phosphonium cation by partial ionization.
ClPPh3+ was detected as a singlet at 66 ppm in CD2Cl2 and 69 ppm in
CD3CN.
OM010077B
(18) It should be noted that the oxidation of 1 by ferrocenium is
complete in a short time period, even though its E1/2 potential (+0.035
V) is slightly positive of that of Fc/Fc+. The follow-up reaction 1+ f 2
provides the driving force for completeness of the homogeneous
oxidation process.
(16) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(17) Song, L.; Trogler, W. C. Angew. Chem., Int. Ed. Engl. 1992,
31, 770.
(19) (a) Arzoumanian, H.; Blanc, A. A.; Metzger, J .; Vincent, J . E.
J . Organomet. Chem. 1974, 82, 261. (b) Howe, J . P.; Lung, K.; Nile, T.
A. J . Organomet. Chem. 1981, 208, 401.