Reversible electrochemical generation of a rhodium(II) porphyrin: Thwarting disproportionation with weakly coordinating anions

Article abstract of DOI:10.1021/ic0345830

We report electrochemical generation of a stable Rh(II) porphyrin (Rh^II(F₂₈TPP)) from a four-coordinate Rh(I) precursor [Rh^I(F₂₈TPP)]^- dissolved in weakly coordinating electrolyte solutions. This work provides the first example of an unambiguously reversible oneelectron electrochemical oxidation of a Rh^I(por), and demonstrates that electrochemical oxidation can be performed under conditions that are compatible with alkane activation. These studies begin to classify those media capable of supporting a stable Rh^II(por), and those that induce disproportionation.

Full text of DOI:10.1021/ic0345830

Inorg. Chem. 2003, 42, 4507−4509
Reversible Electrochemical Generation of a Rhodium(II) Porphyrin:
Thwarting Disproportionation with Weakly Coordinating Anions
Haoran Sun, Feng Xue, Andrew P. Nelson, Jody Redepenning, and Stephen G. DiMagno*
Department of Chemistry and Center for Materials Research and Analysis,
UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588-0304
Received May 28, 2003
We report electrochemical generation of a stable Rh(II) porphyrin
but Saveant et al. have described it in terms of a dispropor-
II
I
(Rh (F TPP)) from a four-coordinate Rh(I) precursor [Rh(F TPP)]-
tionation reaction that is coupled to the loss of a ligand from
an electrogenerated Rh(II) intermediate.13,14 Wayland has
suggested that undesirable disproportionation of certain RhII-
(por) to RhI(por) and RhIII(por) is driven by the favorable
thermodynamics associated with binding an axial ligand to
the Rh(III) product.15,16 Finally, Collman and Boulatov have
recently provided some clarification by showing how trieth-
ylphosphine ligands can be used to stabilize the Rh(II) state
of octaethylporphyrin.17
28
28
dissolved in weakly coordinating electrolyte solutions. This work
provides the first example of an unambiguously reversible one-
I
electron electrochemical oxidation of a Rh(por), and demonstrates
that electrochemical oxidation can be performed under conditions
that are compatible with alkane activation. These studies begin to
II
classify those media capable of supporting a stable Rh (por), and
those that induce disproportionation.
We ultimately wished to determine the role of axial
ligation on the stability and reactivity of RhII(F28TPP). Given
this goal, it seemed logical to determine first the electro-
chemical behavior of RhII(F28TPP) in the absence of strong
ligands. This seemed essential if we were to interpret
chemical steps coupled to electron-transfer events in more
complex media. We report here the electrochemical genera-
tion of RhII(F28TPP) from four-coordinate [RhI(F28TPP)]-
dissolved in weakly coordinating electrolyte solutions. Our
results clearly demonstrate that a stable RhII(por) can be
generated electrochemically from a stable RhI(por) precursor.
Furthermore, this approach has enabled us to determine the
formal potential for the RhII(F28TPP)/[RhI(F28TPP)]- couple
where the rhodium is nominally four-coordinate in both
oxidation states. We believe this value will prove to be a
benchmark for interpreting more complicated reaction mech-
anisms featuring rhodium porphyrins.
Selective functionalization of saturated C-H bonds under
mild conditions remains a significant challenge in organo-
metallic chemistry.1,2 Sterically hindered Rh(II) porphyrins
(RhII(por)) are one class of late transition metal complexes
that have been shown to activate light alkanes, particularly
methane, reversibly at room temperature and low (1 atm)
pressure.3-5 Of late, we have shown that electron-deficient
rhodium porphyrins, such as RhII(F28TPP), can so stabilize
the Rh(I) oxidation state that methylrhodium complexes can
act as electrophiles in alkyl-transfer reactions, providing a
route to direct alkane functionalization at low temperature.6
Exploitation of Rh(II) porphyrins in catalytic alkane
functionalization schemes is hindered by the propensity of
these species to undergo rapid irreversible reactions to form
catalytically incompetent products. Kadish et al. have argued
that this instability is due to dimerization of the RhII(por)
metalloradical that occurs upon loss of an axial ligand,7-12
(9) Kadish, K. M.; Araullo, C.; Yao, C. L. Organometallics 1988, 7,
1583-7.
* Author to whom correspondence should be addressed. E-mail:
(10) Kadish, K. M.; Hu, Y.; Tagliatesta, P.; Boschi, T. Inorg. Chem. 1993,
SDIMAGNO1@UNL.EDU.
32, 2996-3002.
(1) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437-50.
(11) Liu, Y. H.; Anderson, J. E.; Kadish, K. M. Inorg. Chem. 1988, 27,
(2) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-932.
2320-5.
(3) Zhang, X.-X.; Wayland, B. B. J. Am. Chem. Soc. 1994, 116, 7897-
(12) Yao, C. L.; Anderson, J. E.; Kadish, K. M. Inorg. Chem. 1987, 26,
8.
2725-7.
(4) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113,
(13) Grass, V.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119,
5305-11.
7526-32.
(5) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259-
(14) Grass, V.; Lexa, D.; Momenteau, M.; Saveant, J.-M. J. Am. Chem.
Soc. 1997, 119, 3536-42.
61.
(6) Nelson, A. P.; DiMagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569-
(15) Ni, Y.; Fitzgerald, J. P.; Carroll, P.; Wayland, B. B. Inorg. Chem.
1994, 33, 2029-35.
70.
(7) Kadish, K. M.; Yao, C. L.; Anderson, J. E.; Cocolios, P. Inorg. Chem.
(16) Wayland, B. B.; Balkus, K. J., Jr.; Farnos, M. D. Organometallics
1989, 8, 950-5.
1985, 24, 4515-20.
(8) Kadish, K. M.; Xu, Q. Y.; Anderson, J. E. ACS Symp. Ser. 1988, 378,
451-65.
(17) Collman, J. P.; Boulatov, R. J. Am. Chem. Soc. 2000, 122, 11812-
21.
10.1021/ic0345830 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/26/2003
Inorganic Chemistry, Vol. 42, No. 15, 2003 4507

COMMUNICATION
gathered for this process also show clean generation of
RhII(F28TPP) from [RhI(F28TPP)]-; three isosbestic points are
observed, and the transition wavelengths for RhII(F28TPP)
are identical to those for photochemically generated
RhII(F28TPP) in the same media.
Electrochemical and redox titration experiments were also
conducted in benzene. Because of the less polar nature of
this solvent, a higher concentration of electrolyte (1.0 M
tetrahexylammonium hexafluorophosphate, THAPF6) was
required to overcome solution resistance problems. Revers-
ible behavior for the RhII(F28TPP)/[RhI(F28TPP)]- couple
(-0.161V) in benzene was observed. Titration of a benzene
solution of 1 (1.0 M THAPF6) with ferrocenium hexafluo-
rophosphate was followed by monitoring the UV-vis
spectrum in the Soret region. A plot of moles of RhII(F28TPP)
produced vs moles of added ferrocenium gave a straight line
with a slope of 1.05, verifying the one-electron nature of
this oxidation. Finally, the absorbance spectrum for the
oxidized product is again indistinguishable from photochemi-
cally generated RhII(F28TPP) dissolved in a benzene solution
containing 1.0 M THAPF6.
In contrast to the reversible electrochemical oxidation of
[RhI(F28TPP)]- seen when TBAPF6 is the supporting elec-
trolyte, TBAClO4 and TBACl electrolyte solutions lead to
more complex behavior. The voltammogram in Figure 1
(middle) (0.1 M TBACl, DFB) shows a single (apparently)
irreversible oxidation at approximately -0.34 V. In addition,
a new reduction peak that is not present in the initial ramp
toward negative potentials appears only after the process or
processes associated with the oxidation at -1.27 V. This
response is consistent with what is expected if the following
equilibrium is coupled to the initial electron-transfer event.
Figure 1. Cyclic voltammograms and thin layer spectroelectrochemistry
of the oxidation of RhI(F28TPP)- in DFB containing 0.1 M TBAPF6 (top)
and in DFB containing 0.1 M TBACl (middle). Cyclic voltammogram in
DFB containing 1.4 M THF and 0.1 M TBAPF6 (bottom). CV experimental
conditions: glassy carbon working electrode, silver wire pseudo reference
electrode, Pt counter electrode, sweep rate ) 100 mV/s.
Cyclic voltammograms of the cobaltocenium salt of
[RhI(F28TPP)]- ([CoIII(Cp)2]+[RhI(F28TPP)]-, 1) dissolved in
1,2-difluorobenzene18 (DFB) are shown in Figure 1. The
synthesis of 1 has been reported previously.6 In the presence
of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)
(Figure 1, top) three reversible electrochemical processes
were observed: RhII(F28TPP)/[RhI(F28TPP)]-, -0.214 V;
[CoIII(Cp)2]+/[CoII(Cp)2], -1.336 V; and [RhI(F28TPP)]-/
[RhI(F28TPP)]-2, -1.629 V. The [CoIII(Cp)2]+/[CoII(Cp)2]
couple serves as a convenient one-electron internal reference
that is present at the same concentration as [RhI(F28TPP)]-.
We measure the formal potential for the [CoIII(Cp)2]+/
[CoII(Cp)2] couple to be -1.336 V vs the [FeIII(Cp)2]+/
[FeII(Cp)2] couple in DFB and -1.325 V in benzene, values
that are in good agreement with that determined in aceto-
nitrile (-1.35 ( 0.01 V) by Stojanovic and Bond.19
The voltammetric wave at -0.214 V in Figure 1 (top) can
be definitively assigned to the RhII(F28TPP)/[RhI(F28TPP)]-
couple. The one-electron response is nearly reversible and
is not coupled to any other reactions on this time scale.
Digital simulation, shown as the dashed line in Figure 1, is
consistent with the assignment of this electrode reaction as
a one-electron process.20,21 Spectroelectrochemical data
2RhII(por) + X- a RhIII(por)X + RhI(por)-
(1)
When the equilibrium in eq 1 lies far to the right,
disproportionation driven by chloride generates Rh(III) and
Rh(I) species, and the latter is rapidly oxidized to give a net
oxidation by two electrons. It is clear that the interaction
between [RhIII(F28TPP)]+ and Cl- in this solvent is signifi-
cant, and for our purposes undesirable, because it is associ-
ated with the rapid depletion of RhII(F28TPP). Verification
that [RhI(F28TPP)]- and [RhIII(F28TPP)]+ are the stable
oxidation states of this rhodium porphyrin in DFB solutions
containing chloride is again obtained by spectroelectrochem-
istry. Oxidation of [RhI(F28TPP)]- generates a single new
species; the UV-vis spectrum is consistent with that of
[RhIII(F28TPP)]+.Likewise,additionofTBACltoaRhII(F28TPP)
solution (DFB, 0.1 M TBAPF6) results in conversion of
RhII(F28TPP) to [RhIII(F28TPP)]+ and [RhI(F28TPP)]-.
The cyclic voltammetry is more interesting (and more
complex) when neutral donor ligands, such as water or THF,
are present in DFB solutions (0.1 M TBAPF6). Cyclic
voltammograms of such solutions still exhibit a reversible
(20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 1980.
(21) DigSim 3.03 (Bioanalytical Systems, Inc.) was used for digital
simulation. Parameters are listed in the Supporting Information.
(18) O’Toole, T. R.; Younathan, J. N.; Sullivan, B. P.; Meyer, T. J. Inorg.
Chem. 1989, 28, 3923-6.
(19) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56-64.
4508 Inorganic Chemistry, Vol. 42, No. 15, 2003

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-
RhII(F28TPP)/[RhI(F28TPP)]- couple at nearly the same
potential as that observed in the absence of the added ligand
(Figure 1, bottom). The chemically or electrochemically
generated RhII(F28TPP) persists for hours under these condi-
tions, indicating a thermodynamic, rather than merely kinetic,
stability of this oxidation state in the presence of these
ligands. In contrast, under the same conditions, pyridine and
benzonitrile induce disproportionation when present at
concentrations equivalent to that of the rhodium complex.
Taken together, these data show that select sets of potential
nucleophiles are compatible with stable RhII(por) species,
and that disproportionation can be driven by either ion-
pairing or coordination events.
weakly coordinating counterion PF6 supports the Rh(II)
-
oxidation state, whereas Cl- and ClO4 do not. Although
no detailed mechanistic information regarding the affinities
of various counterions for RhIII(F28TPP)+ can yet be derived
from this work, use of eq 1 in conjunction with formal
potentials for the four-coordinate species does provide a
thermodynamic framework for our ongoing mechanistic
studies of ligand exchange reactions convolved with
RhII(F28TPP)-based alkane functionalization strategies.
Acknowledgment. We thank the National Science Foun-
dation (CHE-9817247) and the Office of Naval Research
(N00014-00-1-0283) for support of this research.
This work provides the first example of an unambiguously
reversible one-electron electrochemical oxidation of a RhI(por),
and demonstrates that electrochemical oxidation can be
performed under conditions that are compatible with alkane
activation. In addition, these studies begin to classify those
media capable of supporting a stable RhII(por), and those
that induce disproportionation; in DFB and benzene, the
Supporting Information Available: Details regarding (1)
materials and methods, (2) determination of formal redox potentials,
(3) parameters for cyclic voltammetric digital simulation, and (4)
solvent and counterion effects upon voltammetry. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC0345830
Inorganic Chemistry, Vol. 42, No. 15, 2003 4509