Manipulating the electrolyte medium to favor either one-electron or two-electron oxidation pathways for (fulvalendiyl)dirhodium complexes

Article abstract of DOI:10.1021/om051101e

The anodic oxidations of three compounds having two Rh-containing moieties linked by a fulvalendiyl (Fv) dianion have been studied in varying electrolyte media. Following the integrated approach to manipulation of solvents and supporting electrolytes (Barriere, F.; Geiger, W. E. J. Am. Chem. Soc., in press), ΔE_1/2 values (Δ_1/2 = E _1/2(2) -E_1/2(1)) of the successive one-electron oxidations of the neutral compounds to the corresponding dications were altered to either favor or disfavor the disproportionation of the mixed-valent intermediate. In the cases of Rh₂Fv(CO)₄ (1) and Rh₂Fv(CO) ₂(μ-dppm) (2) [Fv = C₁₀H₈, dppm = bis(diphenylphosphino)methane], replacing the [PF₆]^- supporting electrolyte anion with [B(C₆F₅)]^- in CH₂Cl₂ changed the voltammograms from those of a single two-electron wave (having inverted E1_1/2 potentials) to those of two distinct normal one-electron waves. In the case of Rh₂Fv-(COD)₂ (3, COD = C₈H₁₂), both the solvent and supporting electrolyte were changed to manipulate ΔE _1/2 between normal and inverted values. Changes of up to 330 mV in ΔE_1/2 were observed, resulting in modifications of over 105 in the disproportionation equilibrium constant, K_disp, of the mixed-valent intermediate 3⁺. The thermodynamic stabilization of 1⁺ and 2⁺ allowed for their IR characterization and confirmed the previously postulated formation of Rh-Rh bonds in the cation radicals. An integrated approach to medium effects is shown to be a powerful method for manipulating the equilibrium concentrations of the various redox states that make up a multiple-electron-transfer process.

Full text of DOI:10.1021/om051101e

1654
Organometallics 2006, 25, 1654-1663
Manipulating the Electrolyte Medium to Favor Either One-Electron
or Two-Electron Oxidation Pathways for (Fulvalendiyl)dirhodium
Complexes
Ayman Nafady,† Teen T. Chin,‡ and William E. Geiger*
Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405
ReceiVed December 29, 2005
The anodic oxidations of three compounds having two Rh-containing moieties linked by a fulvalendiyl
(Fv) dianion have been studied in varying electrolyte media. Following the integrated approach to
manipulation of solvents and supporting electrolytes (Barrie´re, F.; Geiger, W. E. J. Am. Chem. Soc., in
press), ∆E1/2 values (∆E1/2 ) E1/2(2) - E1/2(1)) of the successive one-electron oxidations of the neutral
compounds to the corresponding dications were altered to either favor or disfavor the disproportionation
of the mixed-valent intermediate. In the cases of Rh2Fv(CO)4 (1) and Rh2Fv(CO)2(µ-dppm) (2) [Fv )
C10H8, dppm ) bis(diphenylphosphino)methane], replacing the [PF6]- supporting electrolyte anion with
[B(C6F5)]- in CH2Cl2 changed the voltammograms from those of a single two-electron wave (having
“inverted” E1/2 potentials) to those of two distinct “normal” one-electron waves. In the case of Rh2Fv-
(COD)2 (3, COD ) C8H12), both the solvent and supporting electrolyte were changed to manipulate
∆E1/2 between normal and inverted values. Changes of up to 330 mV in ∆E1/2 were observed, resulting
in modifications of over 105 in the disproportionation equilibrium constant, Kdisp, of the mixed-valent
intermediate 3+. The thermodynamic stabilization of 1+ and 2+ allowed for their IR characterization and
confirmed the previously postulated formation of Rh-Rh bonds in the cation radicals. An integrated
approach to medium effects is shown to be a powerful method for manipulating the equilibrium
concentrations of the various redox states that make up a multiple-electron-transfer process.
reaction of eq 2 is normally favored, B is thermodynamically
stable with respect to the reverse (disproportionation) reaction,
Introduction
Multistep electron-transfer processes are central to many
aspects of redox chemistry.1 In the conceptually simple case of
two one-electron processes (an EE mechanism, written as
successive oxidations of compound A in eq 1) the value of ∆E1/2
determines the equilibrium concentration of the one-electron
intermediate B.
A + C h 2B
Kcomp ) [B]2/[A][C] ) 1/Kdisp (2)
and voltammetric scans show two separate one-electron waves.2d,e
For the much less common case of a negative ∆E1/2, which has
been referred to as involving “inversion” of the normal ordering
of E1/2(1) and E1/2(2),2d a single two-electron wave may be
observed, and the one-electron product B exists only in low
equilibrium concentrations. Owing to differences in the physical
and chemical properties of B compared to A and C, eq 2 may
have a profound influence on the overall redox process,
especially if B is a radical.
A y-e z B y -e- z C
∆E1/2 ) E1/2(2) - E1/2(1) (1)
-
E1/2(1)
E1/2(2)
Since, in the great majority of cases, the ∆E1/2 value for two
successive oxidations is positive,2 the comproportionation
A number of electronic and structural factors are known to
* To whom correspondence should be addressed. E-mail:
William.Geiger@uvm.edu.
favor naturally compressed (i.e., small positive or negative)
† Present address: School of Chemistry, Monash University, Clayton,
Victoria 3800, Australia.
2d,3
values of ∆E1/2
.
In such cases, it may be possible to invert
‡ Present address: ALS Technichem, Malaysia.
the potential ordering through changes in the medium. Solvent
variations have been used to this end, for example, in the
oxidation of tetraaryl-substituted ethylenes4 and in the reduction
of transition metal clusters.5 Manipulation of ∆E1/2 values
(1) For leading references see: (a) Bard, A. J. Pure Appl. Chem. 1971,
25, 379. (b) Lowe, D. J.; Fisher, K.; Thomeley, R. N. F.; Vaughn, S.;
Burgess, B. K. Biochemistry 1989, 28, 8460. (c) Anson, F. C.; Shi, C.;
Steiger, B. Acc. Chem. Res. 1997, 30, 437. (d) Evans, D. H. Acta Chem.
Scand. 1999, 53, 876. (e) Bryce, M. R.; Batsanov, A. S.; Finn, T.; Hansen,
T. K.; Howard, J. A. K.; Kamenjicki, M.; Lednev, I. K.; Asher, S. A. J.
Chem. Soc., Chem. Commun. 2000, 295. (f) Hapiot, P.; Kispert, L. D.;
Konovalov, V. V.; Save´ant, J.-M. J. Am. Chem. Soc. 2001, 123, 6669. (g)
Gileadi, E. J. Electroanal. Chem. 2002, 532, 181. (h) Yandulov, D. V.;
Schrock, R. R. Science 2003, 76, 301.
(2) (a) Polcyn, D. S.; Shain, I. Anal. Chem. 1966, 38, 370. (b) Richardson,
D. E.; Taube, H. Coord. Chem. Rev. 1984, 60, 107. (c) Pierce, D. T.; Geiger,
W. E. J. Am. Chem. Soc. 1992, 114, 6063. (d) Evans, D. H.; Lehmann, M.
W. Acta Chem. Scand 1999, 53, 765. (e) Geiger, W. E. In Laboratory
Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P. T.,
Heineman, W. R., Eds.; Marcel Dekker: New York, 1996; pp 698-700.
(f) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John
Wiley and Sons: New York, 2001; pp 243-247.
(3) (a) Creutz, C. In Progress in Inorganic Chemistry; Lippard, S. J.,
Ed.; John Wiley and Sons: New York, 1983; Vol. 30, p 1. (b) Ward, M.
D. Chem. Soc. ReV. 1995, 121. (c) Astruc, D. Acc. Chem. Res. 1997, 30,
383. (d) Du¨mmling, S.; Speiser, B.; Kuhn, N.; Weyers, G. Acta Chem.
Scand. 1999, 53, 876. (e) Baik, M.-H.; Ziegler, T.; Schauer, C. K. J. Am.
Chem. Soc. 2000, 122, 9143. (f) Lehmann, M. H.; Singh, P.; Evans, D. H.
J. Electroanal. Chem. 2003, 549, 137.
(4) (a) Svanholm, U.; Svensmark, B.; Parker, V. D. J. Chem. Soc., Perkin
Trans. 2 1974, 907. (b) Phelps, J.; Bard, A. J. J. Electroanal. Chem. 1976,
68, 313. (c) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J.
Am. Chem. Soc. 1998, 120, 6931.
(5) Jund, R.; Rimmelin, J.; Gross, M. J. Organomet. Chem. 1990, 381,
239.
10.1021/om051101e CCC: $33.50 © 2006 American Chemical Society
Publication on Web 02/18/2006

Oxidation Pathways for (FulValendiyl)dirhodium
Organometallics, Vol. 25, No. 7, 2006 1655
through ion-pairing effects are more widely known. For
example, with cathodic processes involving product anions, the
one-electron product is thermodynamically destabilized under
stronger ion-pairing conditions typified by replacement of
tetraalkylammonium ions by alkali metal ions.6 An essentially
“mirror image” effect7 is expected for anodic processes involv-
ing product cations if “traditional” anions such as [PF6]-8 are
replaced by strong ion-pairing anions such as halide ions.
However, the greatly increased nucleophilicities of the latter
often lead to unwanted side reactions. A new expedient to this
strategy has been recently introduced. Weakly coordinating
In contrast to complexes 1 and 2, significant structural
changes are not likely to accompany the oxidation of Rh2Fv-
(COD)2, 3, COD ) 1,5-cyclooctadiene. In this case, steric
factors prevent formation of a metal-metal bond and allow
retention of a transoid configuration of the Rh(COD) moieties
in all three oxidation states of 3. This causes the ∆E1/2 value to
be positive, albeit small, even in traditional electrolyte solutions.
As shown below, replacement of [PF6]- by a WCA leads to an
increase in ∆E1/2 (i.e., an increasingly positiVe value), stabilizing
the intermediate 3+ and making possible the recording of its
ESR spectrum. To complete the manipulation of the EE
responses of complexes 1-3, solvation effects were used to
induce a negatiVe change in ∆E1/2 sufficient to achieve potential
inVersion, as manifested in a single two-electron cyclic volta-
mmetry (CV) wave for the process 3/32+. This family of three
dirhodium complexes therefore provides a model example of
how solvent and electrolyte effects can be exploited in an
integrated manner to achieve desired thermodynamic goals in
multielectron anodic reactions.
anions (WCA)9 having highly dispersed negative charges (e.g.,
[B(C6F5)4]-, TFAB,10-12 and [B(C6H3(CF3)2)4]-, BArF24
)
13
provide the desired weak nucleophilicity in addition to being
much more weakly ion-pairing than the traditional anions in
low-polarity solvents.12 These properties make possible the
manipulation of anodic ∆E1/2 values by ion-pairing effects with
little concern about nucleophile-induced follow-up reactions.
A systematic study of media containing halide, traditional, or
weakly coordinating anions has provided an integrated model
for the manipulation of ∆E1/2 values in lower-polarity sol-
vents.7,11 Here we give an example of how this integrated
approach to medium effects makes possible either two separate
one-electron processes or a single (inverted) two-electron
process for the oxidation of compounds having a fulvalene-
type bridge between two Rh atoms.
Two of these complexes, namely, Rh2Fv(CO)4, 1, and Rh2-
Fv(CO)2(µ-dppm), 2 [Fv ) fulvalenediyl, C10H8; dppm ) bis-
(diphenylphosphino)methane], were shown earlier to have single
two-electron oxidations14 in which potential inversion is linked
to formation of strong Rh-Rh bonds in the dications. In neither
case was the radical monocation (1+ or 2+) detected, hindering
judgment on the question of whether the metal-metal bond
forms in the first or second oxidation process. The fact that the
reported values14 of ∆E1/2 were not very large in CH2Cl2/[NBu4]-
[PF6] (-140 mV for 1 and -10 mV for 2) suggested to us that
these compounds might have positiVe ∆E1/2 values in less ion-
pairing media, hopefully allowing direct characterization of the
monocations 1+ and 2+. This expectation was realized by
electrochemistry and spectroscopy, as shown below.
Experimental Section
General Procedures. The dirhodium compounds 1-3 and the
dicobalt compound 4 were prepared by literature methods.14,15 All
procedures were carried out under dinitrogen using rigorous Schlenk
conditions, except for electrochemical experiments, which were
carried out in a Vacuum Atmospheres drybox. Dichloromethane
was twice distilled from CaH2, the second time under static vacuum.
Tetrabutylammonium hexafluorophosphate was prepared by me-
tathesis of [NBu4]I with [NH4][PF6] in acetone/water, recrystallized
three times from ethanol, and dried under vacuum. [NBu4][B(C6F5)4]
was prepared by metathesis of [NBu4]Br with Li[B(C6F5)4]‚nOEt2
(Boulder Scientific Co.) in methanol and recrystallized at least twice
from CH2Cl2/hexane. A detailed description of the metathesis
procedure is available.12 Na[B(C6H3(CF3)2)4] (Boulder Scientific
Co.) was used as received. The tetraethylammonium and tetrabut-
ylammonium salts of [B(C6H3(CF3)2)4]- were prepared by similar
metatheses and recrystallizations. [FeCp2][B(C6F5)4] was prepared
by oxidation of sublimed ferrocene with anhydrous FeCl3, followed
by treatment with Li[B(C6F5)4]‚nOEt2, analogous to the literature
preparation of the [PF6]- analogue.16b,17 A standard three-electrode
configuration was used for voltammetry and coulometry experi-
ments. In the case of bulk electrolyses a three-compartment cell
was used in which the working and auxiliary compartments were
separated by two fine frits to minimize mixing of the solutions.
The experimental reference electrode was a AgCl-coated Ag wire
prepared by anodic electrolysis of the wire in 1 M HCl. All
potentials in this paper are referred, however, to the ferrocene/
ferrocenium reference couple.16 This was accomplished by adding
either ferrocene or decamethylferrocene to the solution as an internal
standard at an appropriate time in the experiment. When deca-
methylferrocene was used as the working internal reference,
(6) Although ion-pairing shifts both the A/B and B/C couples to more
positive potentials, the effect on the latter is stronger owing to the increased
electrostatic effect in the dianion. Early work in this area is nicely
summarized in (a) Swarc, M.; Jagur-Grodzinski, J. In Ions and Ion Pairs
in Organic Reactions; Swarc, M., Ed.; John Wiley and Sons: New York,
1974; Vol. 2, Chapter 1. Other representative studies: (b) Levin, G.; Swarc,
M. J. Am. Chem. Soc. 1976, 98, 4211. (c) Smith, W. H.; Bard, A. J. J.
Electroanal. Chem. 1977, 76, 19 [cyclooctatetraene; see also ref 6f]. (d)
Bo¨hm, A.; Meerholz, K.; Heinze, J.; Mu¨llen, K. J. Am. Chem. Soc. 1992,
114, 688 (polyarenes). (e) Himeno, S.; Takamoto, M.; Ueda, T. J.
Electroanal. Chem. 2000, 485, 49 (keggin-type complexes). (f) Baik,
M.-H.; Schauer, C. K.; Ziegler, T. J. Am. Chem. Soc. 2002, 124, 11167
(density functional theory of ion-pairing interactions). See also ref 2d.
(7) Barrie`re, F.; Geiger, W. E. J. Am. Chem. Soc., in press. See ref 11
for preliminary communication of this work.
(8) We include in this category the anions most commonly used in
nonaqueous solutions such as [BF4]-, [ClO4]-, and [CF3SO3]-.
(9) (a) Beck, W.; Su¨nkel, K. H. Chem. ReV. 1988, 88, 1405. (b) Strauss,
S. H. Chem. ReV. 1993, 93, 927. (b) Reed, C. A. Acc. Chem. Res. 1998,
31, 133. (d) Chen, E. Y.-K.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (e)
Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066.
(10) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 38, 248.
(11) Barrie`re, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U.
T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262.
(12) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76,
6395.
(13) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991, 30,
4687.
(14) Chin, T. T.; Geiger, W. E.; Rheingold, A. L. J. Am. Chem. Soc.
1996, 118, 5002.
(15) Rausch, M. D.; Spink, W. C.; Conway, B. G.; Rogers, R. D.;
Atwood, J. L. J. Organomet. Chem. 1990, 383, 227.
(16) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461. (b)
Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.

1656 Organometallics, Vol. 25, No. 7, 2006
Nafady et al.
Table 1. E1/2 Values (volt vs ferrocene) for Fulvalenyl Dirhodium Complexes in Dichloromethane, 1 ) Rh2Fv(CO)4, 2 )
Rh2Fv(CO)2(µ-dppm) at RT (boldface indicates ∆E1/2 value having potential inversion; TFAB ) [B(C6F5)4]-, BArF24
)
[B(C6H3(CF3)2)4]-)
cmpd
meda
supporting electrolyte
E
1/2(1)
E1/2(2)
∆E1/2 (mV)
measb
1
1
1
1
1
2
2
2
2
(i)
[NBu4][TFAB], 0.05 M
[NBu4][BArF24], 0.05 M
[NEt4][BArF24], 0.05 M
Na[BArF24], 0.02 M
[NBu4][PF6], 0.1 M
[NBu4][TFAB], 0.1 M
[NBu4][BArF24], 0.05 M
Na[BArF24], 0.02 M
[NBu4][PF6], 0.1 M
0.07
0.08
0.08
0.10
0.15
0.17
0.20
0.28
80
90
125
DPVc
DPVc
SWV
CV
CVsim
DPVc
SWV
CV
(ii)
(iii)
(iv)
(v)
(vi)
(ii)
(iv)
(v)
180
d
(single wave, E1/2 ) -0.17)
-140
-0.75
-0.74
-0.76
-0.655
-0.635
-0.575
95
105
185
ca. -10
d
(single wave, E1/2 ) -0.75)
CVsim
a Medium, numbered to help identify matches. b Voltammetric method used to determine ∆E1/2: DPV ) differential pulse voltammetry; SW ) square-
wave voltammetry; CV ) cyclic voltammetry; CVsim ) cyclic voltammetry digital simulations. c Single unresolved peak; ∆E1/2 determined from width of
voltammogram at half-height. d In acetone: see ref 14.
adjustment to the ferrocene reference potential was made by adding
0.61 V if the medium was CH2Cl2/0.1 M [NBu4][B(C6F5)4] or 0.55
V if the medium was CH2Cl2/0.1 M [NBu4][PF6]. The working
electrodes were glassy carbon disks having a diameter of either 1
or 1.5 mm (Bioanalytical systems), polished on a Buehler polishing
cloth with Metadi II diamond paste, rinsed copiously with Nanopure
water, and placed under vacuum for drying. Bulk electrolyses were
performed at Pt basket electrodes. Cyclic voltammetry (CV) scans
were subjected to mechanistic diagnostics employing the procedures
detailed elsewhere.18 Linear scan voltammetry (LSV) at a 2 mV
s-1 scan rate using a 1.5 mm diameter disk achieved quasi-steady-
state conditions. A PARC model 273A potentiostat was employed,
and digital simulations were carried out using Digisim 3.0 (Bio-
analytical Systems). Temperatures were ambient unless otherwise
noted.
The ∆E1/2 values were measured using one of three different
voltammetric methods. Simple visualization of the CV peaks was
satisfactory in many cases. Square-wave voltammetry (SWV,
frequencies of 10 Hz or higher) and differential pulse voltammetry
(DPV, pulse height 10 mV) were employed when the |∆E1/2| values
were less than about 110 mV. If the two one-electron peaks were
Figure 1. Differential-pulse voltammogram of 1 mM 1 in
CH2Cl2/0.05 M [NEt4][B(C6H3(CF3)2)4] at a pulse height of 10 mV,
a scan rate of 2 mV s-1, and a 1.5 mm GC disk electrode.
of the one-electron products precluded their spectroscopic
characterization in earlier work.14
unresolved in DPV scans, the width of the voltammogram at half-
19
height was used to obtain ∆E1/2
.
The precision of ∆E1/2 values
1 (or 2) h 12+ (or 22+) + 2e-
(3)
given in this paper is believed to be 10 mV or better.
In-situ IR spectra were obtained on bulk electrolysis solutions
using the fiber-optic transmission method previously described.20,21
In this method a “dip” probe is connected to an FTIR spectrometer
(Mattson Polaris in this case) through a 1 m fiber-optic cable
(Remspec Corp) and operated in an external reflectance mode at a
gold surface using 4 cm-1 resolution. A spectral window of ca.
1500 to 2200 cm-1 was available for the electrolysis solutions
employed in this study. ESR spectra were obtained with a Bruker
ESP 300E spectrometer.
IA. Oxidation of 1. Replacing the [PF6]- anion by TFAB or
BArF24 increases ∆E1/2 by preferentially decreasing ion-pairing
to the product dications compared to the corresponding mono-
cations. Positive values of ∆E1/2 are observed for 1 in four
different electrolyte media, the highest value being 180 mV in
CH2Cl2/0.02 M Na[BArF24] (electrolyte (iv), Table 1). The two
one-electron waves are visually resolved in this electrolyte and
in CH2Cl2/0.05 M [NEt4][BArF24] (electrolyte (iii), Table 1,
Figure 1). The closeness of the ∆E1/2 values with the two
tetrabutylammonium salts of electrolytes (i) and (ii) are con-
sistent with TFAB and BArF24 having very similar ion-pairing
abilities in dichloromethane. It may at first seem surprising that
still larger ∆E1/2 values were attained when the electrolyte
countercation was changed from [NBu4]+ (∆E1/2 ) 90 mV) to
[NEt4]+ (125 mV) or Na+ (180 mV). This may be qualitatively
understood7,24 as a secondary effect in which a more strongly
ion-pairing electrolyte cation (e.g., Na+) competes with the
analyte cation for the electrolyte anion.
Results
I. Converting 1 and 2 to Separate One-Electron Anodic
Processes. As mentioned in the Introduction, with [PF6]- as
the electrolyte anion, compounds 1 and 2 have negative (i.e.,
inverted) ∆E1/2 values and display single two-electron quasi-
reversible22 anodic waves23 (eq 3).The thermodynamic instability
(17) For the preparation of other ferrocenium salts having fluorine-
containing aryl borate counterions see: Calderazzo, F.; Pampaloni, G.;
Rocchi, L.; Englert, U. Organometallics 1994, 13, 2592. For the structural
characterization of [FeCp2][TFAB] prepared using [N(C6H4Br)3][TFAB]
as oxidant see: O’Connor, A. R.; Nataro, C.; Golen, J. A.; Rheingold, A.
L. J. Organomet. Chem. 2004, 689, 2411.
(18) Geiger, W. E. In Laboratory Techniques in Electroanalytical
Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel
Dekker: New York, 1996; Chapter 23.
(22) “Quasi-reversible” refers to sluggish electron-transfer kinetics. Both
oxidations are chemically reversible on the CV time scale. For a discussion
of quasi-reversibility see ref 2f, pp 191-204.
(23) An added difficulty with oxidation of 1 was the insolubility of its
oxidation products with [PF6]- counterions. This precluded the use of
dichloromethane and related low-donor solvents.
(19) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278.
(20) Shaw, M. J.; Geiger, W. E. Organometallics 1996, 15, 13.
(21) Stoll, M. E.; Geiger, W. E. Organometallics 2004, 23, 5818.
(24) Changes in electrolyte concentrations also influence the changes in
these values. The available concentrations of Na[BArF24] were limited by
its solubility in CH2Cl2.

Oxidation Pathways for (FulValendiyl)dirhodium
Organometallics, Vol. 25, No. 7, 2006 1657
Scheme 1
Figure 2. Cyclic voltammogram of 2 mM 1 in CH2Cl2/0.05 M
[NBu4][TFAB] at a scan rate of 0.2 V s-1, 1 mm GC disk electrode.
pair at 2073 and 2061 cm-1, after which the second of these
diminished along with the band at 2105 cm-1 as two new peaks
at 2153 and 2123 cm-1, known to be those of the dication 12+ 14
,
grew in. As expected, these last two bands became prominent
upon completion of the electrolysis (1.35 F/equiv).25 Three
residual features remain, at 2073, 1881, and 1835 cm-1. Because
the most likely follow-up reaction of 1+ is loss of CO, the bands
at 2073 and 1881 cm-1 are tentatively assigned to singly
bridging [Rh2Fv(CO)2(µ-CO)]+. Although the remaining feature
at 1835 cm-1 falls at the same energy as that reported15 for the
neutral complex Rh2Fv(CO)2(µ-CO), we do not observe a
second band reported at 1979 cm-1. Scheme 1 summarizes our
understanding of the anodic reactions of 1.
The most significant IR assignments are those made to 1+.
The pair at 2105 and 2061 cm-1 are shifted by +67 and +83
cm-1, respectively, from those of 1 and provide a strong
argument for Rh-Rh bond formation in the monocation. Were
this not to occur, one would expect four intense IR bands for
1+, including two close to the energies of the original pair for
1.26,27 Strengthening this reasoning is the fact that the average
shift of 75 cm-1 for ∆νCO (1/1+) is quite similar to that (84
cm-1) observed for the Co analogues [Co2Fv(CO)4]0/1+, in which
Co-Co bond formation has been confirmed by ESR spectros-
copy.28
IB. Oxidation of 2. The oxidation chemistry of the dppm-
bridged compound 2 has the advantage of not being complicated
by homogeneous follow-up reactions, and the crystalline dication
22+ is easily isolable. Analysis of the shape of the single two-
electron CV waves for either 2 or 22+ in [PF6]-containing media
gave a ∆E1/2 value of ca. -10 mV.14 As shown in Table 1,
significant positive increases in ∆E1/2 are achieved with WCA
electrolytes, lowering the disproportionation constant for 2+ by
Figure 3. In-situ IR spectroelectrochemical monitoring of bulk
oxidation of 1 mM 1 in CH2Cl2/0.04 M [NEt4][B(C6H3(CF3)2)4] at
Eappl ) 0.4 V vs Fc/Fc+, 273 K.
Since the ∆E1/2 value of 125 mV with [NEt4][BArF24] was
sufficiently large to promise a spectroscopically detectable
equilibrium concentration of 1+, this electrolyte was used for
one-electron anodic electrolyses of 1 in CH2Cl2. A complication
that must be noted here is that the oxidation of 1 is not
completely chemically reversible,14 and even in WCA electro-
lytes some evidence of chemical follow-up products are seen
in CV scans at room temperature (Figure 2).The fact that the
follow-up reaction is not quenched by substitution of a WCA
for [PF6]- suggests that the kinetic instability of 1+ arises from
factors other than anionic nucleophilic attack, perhaps involving
loss of CO.
(25) The fact that exhaustive electrolysis at Eappl ) 0.4 V required
significantly less than 2 F/equiv is further indication of the kinetic instability
of the monocation 1+.
(26) Four bands are expected for two metal dicarbonyl fragments
electronically coupled in a class II mixed-valent compound of the type
expected if 1+ had a cisoid structure. For examples see ref 27. We also
note that a metal-metal bonded system may show one or more additional
weak bands owing to weak vibrational coupling of the nonadjacent carbonyls
Bulk electrolysis of 1 was carried out (Eappl ) 0.4 V, T )
273 K) and in-situ IR spectra were recorded as the electrolysis
proceeded. As shown in Figure 3, after passage of 0.85 F/equiv,
the original pair of νCO bands at 2038 and 1978 cm-1 for 1 had
largely disappeared, being replaced in the terminal CO region
by two strong features [2105 and ca. 2070 cm-1 (broad)] and
two weak features (2153 and 2123 cm-1). Weak bands also
[ref 27b]. This likely accounts for a third, weak, feature at 2016 cm-1
,
which tracks the pair at 2105 and 2061 cm-1 and is also assigned to 1+
(Table 2).
(27) (a) Pierce, D. T.; Geiger, W. E. Inorg. Chem. 1994, 33, 373. (b)
Atwood, C. G.; Geiger, W. E. J. Am. Chem. Soc. 2000, 122, 5477. (c)
Connelly, N. G.; Kelly, R. L.; Kitchen, M. D.; Mills, R. M.; Stansfield, R.
F. D.; Whiting, S. M.; Woodward, P. J. Chem. Soc., Dalton Trans. 1981,
1317.
develop in the bridging CO region at 1881 and 1835 cm-1
.
(28) Spectra of the radical [Co2Fv(CO)4]+ have hyperfine splittings from
two equivalent metals, Aiso ) 10 G, A| ) 47 G, A ca. -8 G: Nafady, A.,
Geiger, W. E. Unpublished work at University of Vermont.
Continuing the electrolysis to yet higher coulomb counts first
resulted in near resolution of the ca. 2070 cm-1 feature into a

1658 Organometallics, Vol. 25, No. 7, 2006
Nafady et al.
Figure 4. (A) Square-wave voltammogram of 1.6 mM 2 in CH2-
Cl2/0.05M [NBu4][B(C6H3(CF3)2)4], 150 Hz, 1.5 mm GC electrode.
(B) Cyclic voltammogram of 2 under the same conditions, scan
rate 0.1 V s-1
.
Figure 5. In-situ IR spectroelectrochemical monitoring of bulk
oxidation of 0.5 mM 2 in CH2Cl2/0.05 M [NBu4][TFAB] at Eappl
) -0.5 V vs Fc/Fc+: (A) from beginning to passage of ca. 1
F/equiv; (B) continuing to passage of ca. 2 F/equiv.
a factor of about 103 (eq 4). Figure 4 has representative
voltammograms showing the resolved one-electron waves.
(∆E1/2 ≈ 90 mV).29,30 More importantly, information is obtained
about the molecular and electronic structures of 2+. The fact
that the neutral complex 2 has a single νCO absorbance and the
dication 22+ has two carbonyl absorptions is well understood
based on known structures.31 That the monocation also has two
carbonyl bands (νCO ) 2024 cm-1 (s), νCO ) 1992 cm-1 (w))
presents a strong argument in favor of the presence of a Rh-
Rh bond in 2+.
2 2+ h 2 + 22+
Kdisp ) [2+]2/[2][22+]
(4)
Bulk electrolysis (at Eappl ) -0.5 V) and IR spectroelectro-
chemistry were carried out on a light red 0.5 mM solution of 2
in CH2Cl2/0.05 M [NBu4][TFAB] at 273 K. As the electrolysis
proceeded to virtual completion with passage of 0.93 F/equiv,
the solution became a dark yellow-green as the original carbonyl
band at 1942 cm-1 decreased (Figure 5, top) and a new
prominent band grew in at 2024 cm-1, which, along with the
less intense band at 1992 cm-1, is assigned to 2+. The small
absorbance at 2082 cm-1, seen late in this sequence, arises from
the dication 22+. These assignments were confirmed by the
further electrolysis to 22+ (an additional 0.92 F/equiv was
released as the solution became yellow), during which the 2024
and 1992 cm-1 bands were replaced by the pair at 2082 and
It is informative that the energetically less-shifted of the bands
(at 1992 cm-1) for 2+ is still some 50 cm-1 higher in energy
2054 cm-1 (Figure 5B) previously ascribed to 22+ 14
.
(29) The conditions of the voltammetry results quoted in Table 1 (medium
(vi)) and the IR experiment differ slightly in that the latter was performed
at a supporting electrolyte concentration of 0.05 M and the temperature
was 273 K.
The IR results are qualitatively consistent with the voltam-
metry data in that the disproportionation reaction of eq 4 is
shown to be weighted in favor of 2+ under these conditions

Oxidation Pathways for (FulValendiyl)dirhodium
Organometallics, Vol. 25, No. 7, 2006 1659
Figure 6. Fluid solution ESR spectra of 2+ generated by chemical
oxidation of 2 with 1 equiv of [FeCp2][TFAB] in CH2Cl2 at 273
K.
than the 1942 cm-1 band of 2. On the basis of arguments made
earlier on IR shifts in formally mixed-valence complexes,30 the
magnitude of this shift is indicative of strong electronic coupling
between the two metals, strengthening support for metal-metal
bond formation in 2+. It is also worth noting that shifts in νCO
with the oxidation state of 2n+ track the overall change in
molecular charge between n ) 0 and either n ) 1+ (larger
shift, 81 cm-1; average 65 cm-1) or n ) 2+ (larger shift, 140
cm-1, average 125 cm-1).
Finally, a chemical oxidation16b of 2 by 1 equiv of
[FeCp2][TFAB] in CH2Cl2 was performed in order to ap-
proximate the electrochemical conditions and generate 2+ for
ESR purposes. A strong spectrum was obtained at both frozen
(155 K) and fluid (room) temperatures. The former was
approximately axial in shape with no detectable hyperfine
splittings (g ) 2.035, g| ) 1.990, gav ) 2.005). The solution
spectrum (Figure 6, g ) 2.008) displays inhomogeneous line
broadening, indicating the presence of unresolved hyperfine
interactions. Rhodium hyperfine splittings are inherently small
owing to the rather small nuclear g-factor of 103Rh.32
Figure 7. CV scans of 1 mM 3 in (A) CH2Cl2/0.1 M [NBu4][PF6]
and (B) CH2Cl2/0.1 M [NBu4][TFAB], scan rate 0.2 V/s, 1 mm
GC disk electrode.
Table 2. Carbonyl-Region IR Energies for Different
Oxidation States of Rh2Fv(CO)4 (1) and
Rh2Fv(CO)2(µ-dppm) (2) in CH2Cl2
cmpd or
ion
av shifta
largest shifta
ν
CO (cm-1
)
(cm-1
)
(cm-1
)
II. Converting between Separate and Inverted Anodic
Reactions of 3. IIA. Anodic Behavior of RhCp(COD). The
redox-active component of Rh2Fv(COD)2, 3, is the Rh(C5H4R)-
(COD) group. Anodic oxidation of the mononuclear parent
RhCp(COD) has not, to our knowledge, been detailed in the
literature.33 In brief, we find the oxidation of RhCp(COD) to
be a one-electron process of limited chemical reversibility, E1/2
) 0.07 V vs ferrocene in CH2Cl2/0.1 M [NBu4][PF6]. Scan rates
in excess of 10 V s-1 are necessary to see modest chemical
reversibility at room temperature. Bulk electrolyses at 233 K
passed slightly over 1 F/equiv, and the subsequent voltammetry
scans showed an absence not only of [RhCp(COD)]+ but of
other electroactive products as well. CV scans of RhCp(COD)
in CH2Cl2/0.1 M [NBu4][TFAB] are very similar to those seen
in the [PF6]- electrolyte, making it unlikely that the electrolyte
1
2038 (vs), 1978 (vs)
2105 (vs), 2061 (vs), 2016 (w)
2153 (vs), 2123 (vs)
1942 (vs)b
2024 (vs), 1992 (s)
2082 (vs), 2054 (s)
n.a.
75
130
n.a.
65
n.a.
67
115
n.a.
82
1+
12+
2
2+
22+
125
140
a Largest shift ) difference between highest energy νCO band and νCO
of neutral compound; av shift ) average difference between energies of
two νCO bands and that of neutral compound. The weak band is not used
to compute the average shift of 1+. b Relative intensity designations: vs )
very strong, s ) strong, w ) weak.
anion plays a role in the follow-up reaction. Loss of the COD
ligand is likely responsible for the limited longevity of [RhCp-
(COD)]+ and, by inference, of 3+. As shown below, the primary
oxidation products of 3, while still subject to slow decomposi-
tion, are sufficiently long-lived to allow investigation of medium
effects and, ultimately, the characterization of the one-electron
product 3+ by ESR spectroscopy.
IIB. Anodic Oxidation of Rh2Fv(COD)2 in Media Con-
taining Traditional Anions. The oxidation of 3 proceeds
through two unresolved one-electron waves in CH2Cl2/[NBu4]-
[PF6] (Figure 7A) having ∆E1/2 ≈ 60 mV (digital simulations,34
Table 3). The resulting Kdisp of 0.095 means that under these
conditions 3+ makes up roughly 60% of the equilibrium mixture
of the three oxidation states of Rh2Fv(COD)2. Manipulation of
the ∆E1/2 value was approached with the objective of finding
conditions in which there is either (i) a significant increase in
(30) With very precise attention to both the coulometry and absorption
values, along with assumptions concerning the absorptivity of either 2+ or
22+ relative to 2, the value of Kdisp could be determined directly from the
spectroelectrochemical experiment. In the present case, this value is better
determined from the measured ∆E1/2 values. See: Hill, M. G.; Rosenhein,
L. D.; Mann, K. R.; Mu, X. H.; Schultz, F. A. Inorg. Chem. 1992, 31,
4108. Hill, M. G.; Mann, K. R. Inorg. Chem. 1991, 30, 1429.
(31) Even though the two CO ligands in 22+ are chemically equivalent,
the weak vibrational coupling referred to in ref 26 accounts for the
observation of symmetric and asymmetric absorption bands.
(32) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic
Resonance; John Wiley and Sons: New York, 1994; pp 535-536.
(33) Brief mention is made in: Winter, R.; Pierce, D. T.; Geiger, W. E.;
Lynch, T. J. J. Chem. Soc., Chem. Commun. 1993, 1949.

1660 Organometallics, Vol. 25, No. 7, 2006
Nafady et al.
Table 3. E1/2 Values (volt vs ferrocene) for M2Fv(COD)2, M ) Rh (3) and M ) Co (4), TFAB ) [B(C6F5)4]-, BArF24
)
[B(C6H3(CF3)2)4]-
solvent
supporting electrolytea
cmpd
E
1/2(1)b
E
1/2(2)c
∆E1/2 (mV)
log Kdisp
CH2Cl2
CH2Cl2
CH2Cl2
glyme
glyme
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
[NBu4][BArF24]
[NBu4][TFAB]
[NBu4][PF6]
[NBu4][TFAB]
[NBu4][PF6]
3
3
3
3
3
3
-0.25
-0.24
-0.17
-0.14
-0.11
-0.19
-0.07
-0.30
-0.34
0.03
0.02
-0.11
-0.04
-0.16
0.07
n.a.
-0.07
+0.06
280
270
60
100
-50
260
-4.73
-4.57
-1.02
-1.69
0.85
[NBu4][BArF24]
[NBu4][PF6]
-4.40
RhCp (COD)
4
4
[NBu4][PF6]d
[NBu4][TFAB]
230
400
-3.89
-6.76
a 0.10 M. b One-electron oxidation of neutral compound. c One-electron oxidation of monocation. d Remeasurement. See ref 38 for original report.
anion, is obvious from previous discussions. The second
involves using a solvent of greater donor ability, which serves
to thermodynamically stabilize more positively charged prod-
ucts.7 To take full advantage of the change in donor strength,
the dielectric constant of the new solvent must remain low
(ꢀ < 10) in order to retain strong ion-pairing effects.
The solvent effect can be quite large, as shown by comparison
of the ∆E1/2 values for 3 in two solvents of similar dielectric
constants but substantially different donor numbers. When the
electrolyte anion is TFAB, the ∆E1/2 value decreases from 270
mV in CH2Cl2 (ꢀ ) 8.9, donor number ) 0) to 100 mV in
1,2-dimethoxyethane (glyme, ꢀ ) 7.2, donor number ) 20)36
Figure 8. Frozen-solution ESR spectrum of 3+ at 163 K, generated
(Table 3). A additional change of -150 mV for the ∆E1/2 value
by bulk oxidation of 0.6 mM 3 in CH2Cl2/0.05 M [NBu4][TFAB],
Eappl ) -0.1 V, T ) 273 K.
in glyme is achieved by replacement of TFAB by [PF6]-,
resulting in an overall reduction in ∆E1/2 of -320 mV on going
from CH2Cl2/TFAB to glyme/[PF6]-. In Figure 9, these effects
∆E1/2, thereby more strongly favoring 3+, or (ii) sufficient
are demonstrated visually by square-wave voltammetry. Addi-
decrease in ∆E1/2 to achieve potential inversion and a single
tion of 250 equiv of [PF6]- to an original CH2Cl2/[NBu4][TFAB]
two-electron wave for 3/32+. Using the integrated approach to
solution containing 0.6 mM 3 is shown to decrease ∆E1/2 by
medium effects,7 we employed a WCA to accomplish the former
about 140 mV (Figure 9B vs 9A). Stepwise addition of glyme
and a stronger donor solvent to achieve the latter. ∆E1/2 changes
at this point further decreases ∆E1/2 until a symmetric wave is
of up to 330 mV were observed through these manipulations.
seen (Figure 9F), denoting a single two-electron wave in 1:1
No discernible change in either chemical or electrochemical
CH2Cl2/glyme. From the breadth of the wave in Figure 9F (90
reversibility was seen on a voltammetric time scale.
mV at half-height), a ∆E1/2 value of + 35 mV is obtained.37 In
IIC. Increasing ∆E1/2 to Favor 3+. Replacement of [PF6]-
a pure glyme solution containing 0.15 M [NBu4][PF6], the
by either of the more weakly ion-pairing anions TFAB or
experimental square-wave width of 53 mV gives ∆E1/2 ) -50
BArF24 resulted in an increase of over 200 mV in ∆E1/2 and
mV. This value was confirmed by the width of the DPV peak,
separate waves due to 3/3+ and 3+/32+ were observed (Figure
and digital simulations of CV scans over the scan-rate range of
7B). A bulk electrolysis carried out at Eappl ) -0.1 V in CH2-
0.2 to 1 V s-1 (Figure 10 and Figure SM1 in the Supporting
Cl2/0.05 M [NBu4][TFAB] at 273 K was halted after release of
Information) allowed the final assignment of E1/2(1) ) -0.11
approximately 0.95 F/equiv. The solution was probed by
V and E1/2(2) ) -0.16 V. Potential inversion is realized under
voltammetry, after which a sample was removed for ESR
these conditions.
spectroscopy. Some decomposition of oxidized 3 accompanied
We note finally the ∆E1/2 value of 260 mV, observed in
the electrolysis,35 but comparison of LSV peak heights showed
diethyl ether (ꢀ ) 4.2, donor number ) 19.2) containing 0.05
that there was a substantial (ca. 40%) yield of 3+. The frozen-
M [NBu4][ BArF24], which seems surprisingly large considering
glass ESR spectrum of this sample (Figure 8) showed one clean
the strong donor ability of Et2O compared to CH2Cl2, in which
rhombic-shaped signal (g1 ) 2.1116, g2 ) 2.0434, g3 ) 2.0005)
essentially the same value is measured. Proper delineation of
attributed to 3+.
this effect will require a better understanding of anodic processes
IID. Decreasing ∆E1/2 to Disfavor 3+: Toward Inverted
in acyclic ether solvents, measurements that are now feasible
Potentials. Starting from the makeup of the medium CH2Cl2/
owing to the increased solubilities and conductivities of WCA-
[NBu4][TFAB] (low donor solvent/weakly interacting anion),
based electrolytes in these solvents.
there are two options available for decreasing the ∆E1/2 of 270
IIE. Confirmation of 1:2 Ion Pairing for [3]2+:[PF6]-.
mV, one of which, namely, the addition of a stronger ion-pairing
Stepwise addition of [PF6]- to a solution of 3 in CH2Cl2/[NBu4]-
[TFAB] made possible a plot of the change in ∆E1/2 (i.e.,
(34) Specific conditions were as follows: electrolyte concentration 0.075
∆∆E1/2) versus log concentration of [PF6]- (Figure SM2,
Supporting Information). At high concentrations a linear
relationship is apparent having a slope of 107 mV, in reasonable
M; scan rate 200 mV s-1, 1.5 mm diameter glassy carbon electrode;
simulation ks and R values of 0.08 cm s-1 and 0.5, respectively, for both
one-electron couples. Digital simulation of data taken under somewhat
different conditions gave the similar result of ∆E1/2 ) 70 mV. Those
conditions were as follows: electrolyte concentration 0.10 M; scan rate
100 mV s-1; 1.0 mm diameter Pt electrode; ks and R values of 0.1 cm s-1
and 0.5, respectively.
(36) Solvent parameters were taken from Linert, W.; Fukuda, Y.; Camard,
A. Coord. Chem. ReV. 2001, 218, 113.
(37) This value was actually obtained from the width of the DPV wave,
using a working curve constructed from the data of ref 19. The DPV and
SWV peak shapes are very similar for Nernstian sytems.
(35) CV peaks for follow-up products were seen at E1/2 ) -0.28 V (rev)
and Epc ) -1.65 V (irrev), along with a smaller cathodic peak at Epc
-1.12 V (irrev).
)

Oxidation Pathways for (FulValendiyl)dirhodium
Organometallics, Vol. 25, No. 7, 2006 1661
Figure 9. Square-wave voltammograms (frequency 10 Hz, pulse height 25 mV) of 0.6 mM 3 at 1 mm GC disk in (A) CH2Cl2/0.1 M
[NBu4][TFAB], (B) CH2Cl2/0.1 M [NBu4][TFAB] + 250 equiv of [NBu4][PF6], (C) solvent mixture CH2Cl2/glyme (90: 10% v/v) + 0.1
M [NBu4][TFAB] and 250 equiv of [NBu4][PF6], (D) CH2Cl2/glyme (80:20% v/v) + 0.1 M [NBu4][TFAB] and 250 equiv of [NBu4][PF6],
(E) CH2Cl2/glyme (60:40% v/v) + 0.1 M [NBu4][TFAB] and 250 equiv of [NBu4][PF6], (F) CH2Cl2/glyme (50:50% v/v) + 0.1 M [NBu4]-
[TFAB] and 250 equiv of [NBu4][PF6]; y-axis is concentration-normalized current.
agreement with the value of 118 mV expected if the dominant
ion-pairing interaction of 32+ is with two [PF6]- anions, as given
in eq 5:
Measurements of ∆∆E1/2 values accompanying changes in
ion-pairing agents hold, in principle,2d information about the
equilibrium constants of eq 5 as well as those involving the
ion-pairs [3+][PF6] and {[32+][PF6]}+, but the present data are
insufficient to address the complexity of the analysis.
3
2+ + 2 [PF6]- h [32+][PF6]2
(5)

1662 Organometallics, Vol. 25, No. 7, 2006
Nafady et al.
exhibit increased kinetic stabilities when generated in the
presence of WCA anions.9-12,40,41
Discussion
The ∆∆E1/2 values for 3 and 4 in dichloromethane increase
by 220 and 170 mV, respectively, when the supporting
electrolyte anion is changed from [PF6]- to TFAB or BArF24.
These values are in concert with the previously reported ∆∆E1/2
value of +190 mV reported for biferrocene under these
conditions.41 It seems reasonable to conclude that an electrolyte
anion effect (WCA vs traditional anion) of ca. +200 mV in
∆∆E is characteristic of dinuclear fulvalenediyl complexes
having positively charged redox products. This electrolyte
change imparts a decrease of over 103 in Kdisp to the dispro-
portionation of the one-electron product.42 It should be noted
that the estimate of ca. +200 mV pertains to solutions having
a [NBu4]+ cation, and those employing more strongly ion-
pairing electrolyte cations, e.g., [NEt4]+ or Na+, will result in
further increases in ∆∆E1/2. The consequent thermodynamic
stabilization of the one-electron product provides a welcome
option for studies of class II43 mixed-valent systems. In the
present case, this medium effect allowed for the facile spectral
characterization of 3+.
Figure 10. Experimental (solid line) and simulated (circles) CV
scans of 0.3 mM 3 in glyme/0.15 M [NBu4][PF6] at 1 mm GC
disk, scan rate 1 V s-1. Relevant simulation parameters: E1/2(1) )
-0.11 V, ks1 ) 8 × 10-2 cm s-1, R1 ) 0.5; E1/2(2) ) -0.16 V, ks2
) 5.5 × 10-2 cm s-1, R2 ) 0.5; Do(3) ) Do(3+) ) 6.7 × 10-6 cm
s-1; Do(32+) ) 4.7 × 10-6 cm s-1; capacitance ) 5 × 10-8 F cm-2
;
electrode area ) 0.0079 cm2.
The ∆∆E1/2 values for 1 and 2, while qualitatively consistent
with the above model, require a more complex analysis.
Although the change of +220 mV for 1 on going from [PF6]-
to TFAB is consistent with the findings for 3, 4, and biferrocene,
the trans-to-cis reorientation and subsequent formation of a Rh-
Rh bond in the oxidation of 1 has an unknown quantitative effect
on the ∆∆E1/2 value.44 Also, the smaller ∆∆E value of +110
mV for the anion effect on 2 may be influenced by making and
breaking of the Rh-Rh bond in the redox processes. From a
utilitarian viewpoint, the increased ∆E1/2 for 2 made possible
the spectral characterization of 2+. However, further studies will
be required to address the question of whether medium effects
are substantially different for redox processes that include major
structural changes.45
Systematic alterations of both the solvent and the electrolyte
allow even more powerful manipulations of ∆E1/2, as shown
by the 330 mV shift for compound 3 as the medium was
changed from CH2Cl2/[NBu4][BArF24] to glyme/[NBu4][PF6].
It is noteworthy that an inverted ∆E1/2 is made possible for 3/32+
even though this redox pair does not appear to have the capacity
for a structural change of the magnitude ordinarily associated
with potential-inverted two-electron processes.2c,d,3e,46 The large
preferential stabilization of the 3+/32+ couple (compared to 3/3+)
must arise primarily from increases of both ion-pairing and
solvation of the dication 32+ by [PF6]- and glyme, respectively.
In terms of new data on systems previously studied under
traditional electrolyte conditions,14 it is the spectral characteriza-
Figure 11. CV (0.1 V s-1, 1 mm glassy carbon disk) of the dication
42+ formed after bulk electrolysis of 1 mM 4 in CH2Cl2/0.05 M
[NBu4][TFAB] at Eappl ) 0.45 V. The fact that the current is zero
at the rest potential (ca. 0.45 V) is consistent with exhaustive anodic
oxidation of the neutral starting material.
III. Oxidation of 4 under WCA Conditions. For the sake
of comparison with the dirhodium compound 3, the anodic
reactions of the dicobalt analogue Co2Fv(COD)2, 4, were carried
out in CH2Cl2/0.1 M [NBu4][TFAB] (a full paper on the
oxidation of 4 in CH2Cl2/0.1 M [NBu4][PF6] has been pub-
lished38). As expected, the ∆E1/2 value increased (from 230 mV
to 400 mV) as [PF6]- was replaced by TFAB. Furthermore,
the oxidation products were noticeably more persistent. Room-
temperature bulk electrolysis at increasing values of Eappl
produced, first, the dark green monocation 4+ and, then, the
deep blue dication 42+ with no evidence of decomposition (see
Figure 11 for CV of 42+).39 Note that in Figure 11 the current
at the rest potential is essentially zero, showing that the original
neutral compound had been completely converted to the
corresponding dication. LSV scans confirmed this quantitatively.
Cathodic back-electrolysis quantitatively regenerated neutral 4.
The oxidized species 4+ and 42+ join the growing number of
organometallic radical cations and multicharged cations that
(40) Camire, N.; Nafady, A.; Geiger, W. E. J. Am. Chem. Soc. 2002,
124, 7260.
(41) Camire, N.; Mueller-Westerhoff, U. T.; Geiger, W. E. J. Organomet.
Chem. 2001, 637-639, 823.
(42) At 298 K, log Kdisp ) 16.92∆E°, where ∆E° is in volts.
(43) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10,
247.
(44) Note also that the ∆E1/2 value of -140 mV for 1 in [NBu4][PF6] is
only an estimated value, based on digital simulations of a quasi-reversible
electron-transfer system having inverted E1/2 potentials. See ref 14 for details.
(45) Solvation energies, in particular, can be expected to be affected by
some types of structural changes.
(46) For some leading references see: Stoll, M. E.; Belanzoni, P.;
Calhorda, M. J.; Drew, M. G. B.; Fe´lix, V.; Geiger, W. E.; Gamelas, C.
A.; Gonc¸laves, I. S.; Roma˜o, C. C.; Veiros, L. F. J. Am. Chem. Soc. 2001,
123, 10595.
(38) Chin, T. T.; Geiger, W. E. Organometallics 1995, 14, 1316.
(39) Only a 40% yield of 42+ was reported in ref 38 using [PF6]- as the
electrolyte anion, even at the considerably reduced temperature of 243 K.

Oxidation Pathways for (FulValendiyl)dirhodium
Organometallics, Vol. 25, No. 7, 2006 1663
tion of the one-electron intermediates 1+ and 2+ that stands out.
The IR spectra of 1+ and 2+ confirm the presence of the
previously postulated Rh-Rh bonds in these radicals.
Supporting Information Available: Two figures showing
(SM1) simulations of the CV scans of 3 at different scan rates and
(SM2) a plot of ∆∆E1/2 vs log [PF6]- in high concentration range
for compound 3 in CH2Cl2/[NBu4][TFAB] to which [PF6]- is being
added. This material is available free of charge through the Internet
at http://pubs.acs.org.
Acknowledgment. This work was supported at the Univer-
sity of Vermont by the National Science Foundation (CHE
-0092702 and CHE-041703).
OM051101E