Ligand-based reduction and electron-transfer-induced tub-to-chair isomerization of cyclooctatetraene ligand in Rh(η5-C 5Ph5)(η4-C8H8)

Article abstract of DOI:10.1021/om030661f

The electrochemical reduction of Rh(η⁵-C₅Ph ₅)(η⁴-C₈H₈), 1, has been studied in THF/0.1 M [NBu₄]A, where A = [PF₆]^- or [CF₃SO₃]^-. Cyclic, differential pulse, and square-wave voltammetry establish that 1 exists as two isomers having tub-shaped (1,5-bonded) or chair-shaped (1,3-bonded) cyclooctatetraene (COT) ligands. The former is the major isomer at room temperature. One-electron reduction of 1,5-1 (E_pc ≈ -2.7 V vs ferrocene) results in rapid isomerization to the chair isomer. Electrolysis produces [1,3-1]^- (E_1/2 = -2.37 V), which has ESR spectra consistent with a SOMO that is largely (COT) ligand-based. Equilibrium and rate constants are estimated for a square scheme describing the combined electron-transfer/isomerization sequence. The tub isomer is more highly favored at room temperature for Rh(η⁵-C ⁵Ph₅)(COT) than for the previously studied Co(η⁵-C₅H₅)(COT). Because the redox behavior of 1 closely parallels that of the cobalt analogues, the present result strengthens evidence for the decidedly different electronic structures of nominally isoelectronic Co-group versus Ni-group η⁴-COT compounds. Formally d⁹ complexes of the former are classified as highly delocalized 18 + δ complexes, whereas the latter are more traditional 19-electron systems.

Full text of DOI:10.1021/om030661f

Organometallics 2004, 23, 2205-2208
2205
Liga n d -Ba sed Red u ction a n d Electr on -Tr a n sfer -In d u ced
Tu b-to-Ch a ir Isom er iza tion of Cycloocta tetr a en e Liga n d
in Rh (η5-C5P h 5)(η4-C8H8)
Michael J . Shaw,*,†,§ J ulie Hyde,‡ Colin White,‡ and William E. Geiger*,†
Departments of Chemistry, University of Vermont, Burlington, Vermont 05405, and
University of Sheffield, Brook Hill, Sheffield S3 7HF, England
Received November 17, 2003
Summary: The electrochemical reduction of Rh(η5-
C5Ph5)(η4-C8H8), 1, has been studied in THF/ 0.1 M
[NBu4]A, where A ) [PF6]- or [CF3SO3]-. Cyclic, dif-
ferential pulse, and square-wave voltammetry establish
that 1 exists as two isomers having tub-shaped (1,5-
bonded) or chair-shaped (1,3-bonded) cyclooctatetraene
(COT) ligands. The former is the major isomer at room
temperature. One-electron reduction of 1,5-1 (Epc ≈ -2.7
V vs ferrocene) results in rapid isomerization to the chair
isomer. Electrolysis produces [1,3-1]- (E1/ 2 ) -2.37 V),
which has ESR spectra consistent with a SOMO that is
largely (COT) ligand-based. Equilibrium and rate con-
stants are estimated for a square scheme describing the
combined electron-transfer/ isomerization sequence. The
tub isomer is more highly favored at room temperature
for Rh(η5-C5Ph5)(COT) than for the previously studied
Co(η5-C5H5)(COT). Because the redox behavior of 1
closely parallels that of the cobalt analogues, the present
result strengthens evidence for the decidedly different
electronic structures of nominally isoelectronic Co-group
versus Ni-group η4-COT compounds. Formally d9 com-
plexes of the former are classified as highly delocalized
“18 + δ” complexes, whereas the latter are more tradi-
tional 19-electron systems.
complexes of the type Co(η5-C5R5)(COT), R ) H or
Me.2-7 The 1,5-isomer, which is favored in the 18-
electron neutral compounds, converts rapidly and quan-
titatively to the 1,3-isomer in the radical anion. To date
only cobalt compounds have been shown to produce
such interconversions. By way of contrast, isoelectronic
nickel-group analogues retain their 1,5-COT geometry
in both the 18- and 19-electron forms of the complexes
[M(η5-C5R5)(COT)]z, M ) Ni, R) H, Me; M ) Pd, R )
Ph; z ) +1, 0.2 Extended Hu¨ckel calculations4 and ESR
measurements2,3 showed that the contrasting redox
behavior originates in electronic structure differences
of the Co-group versus Ni-group compounds. The SOMO
of the former is highly delocalized, with at least half of
the orbital residing in the COT ring, whereas that of
the latter is essentially metal-based (mostly dyz).
A point of inquiry is whether Co-group COT com-
pounds of the second or third row exhibit tub/chair
ligand isomerization processes in the 18-electron or 19-
electron systems. With this in mind, we chose to study
the reduction of Rh(η5-C5Ph5)(COT), 1. There is ample
precedence for significantly different reductive electron-
transfer behavior of 18 e- sandwich or half-sandwich
compounds involving either first- or second-row metals
in general,8,9 or Co and Rh in particular.9 In the present
case, however, although there are quantitative differ-
ences in the tub/chair equilibrium constants for the
neutral compounds of the two metals, their overall
behavior is quite similar. As will be shown below, the
favored tub bonding of the cyclooctatetraene ligand in
Rh(η5-C5Ph5)(1,5-COT) is replaced by chair bonding in
the mononanion [Rh(η5-C5Ph5)(1,3-COT)]-. The behavior
of the Co- and Rh-COT anion radicals appears to be
In tr od u ction
The cyclooctatetraene (COT) ligand has been shown
to have a number of possible coordination modes for
π-bonding to a transition metal.1,2 In the two most
common coordination geometries for tetrahapto-bonding
to a single metal, the COT ligand adopts either a tub-
or chairlike shape,2 where the latter (1,3) isomer
involves butadiene-like coordination of adjacent double
bonds. Isomerization between the 1,5- and 1,3-isomers
(3) (a) Moraczewski, J .; Geiger, W. E. J . Am. Chem. Soc. 1979, 101,
3407. (b) Moraczewski, J .; Geiger, W. E. J . Am. Chem. Soc. 1981, 103,
4779.
(4) Albright, T. A.; Geiger, W. E.; Moraczewski, J .; Tulyathan, B. J .
Am. Chem. Soc. 1981, 103, 4787.
(5) Geiger, W. E.; Gennett, T.; Grzeszczuk, M.; Lane, G. A.; Morac-
zewski, J .; Salzer, A.; Smith, D. E. J . Am. Chem. Soc. 1986, 108, 7454.
(6) Grzeszczuk, M.; Smith, D. E.; Geiger, W. E. J . Am. Chem. Soc.
1983, 105, 1772.
(7) Richards, T. C.; Geiger, W. E. J . Am. Chem. Soc. 1994, 116, 2028.
(8) Leading references: (a) Veiros, L. F. Organometallics 2000, 19,
5549. (b) Gusev, O. V.; Ievlev, M. A.; Peterleitner, M. G.; Peregudova,
S. M.; Denisovich, L. I.; Petrovskii, P. V.; Ustynyuk, N. A. J .
Organomet. Chem. 1997, 534, 57. (c) O’Connor, J . M.; Casey, C. P.
Chem. Rev. 1987, 87, 307. (d) Poli, R. Chem. Rev. 1996, 96, 2135. (e)
Siegbahn, P. E. M. J . Am. Chem. Soc. 1996, 118, 1487. (f) Ziegler, T.;
Tschinke, V.; Fan, L.; Becke, A. D. J . Am. Chem. Soc. 1989, 111, 9177.
(9) (a) Zagorevskii, D. V.; Holmes, J . L. Organometallics 1992, 11,
3224, and references therein concerning cobaltocene and rhodocene.
(b) Connelly, N. G.; Geiger, W. E.; Lane, G. A.; Raven, S. J .; Rieger, P.
H. J . Am. Chem. Soc. 1986, 108, 6219. (c) Geiger, W. E. Acc. Chem.
Res. 1995, 28, 351.
is well-known for a number of cobalt-COT compounds
and is greatly facilitated by one-electron reduction of
† University of Vermont.
‡ University of Sheffield.
§ Present address: Southern Illinois University at Edwardsville.
(1) Deganello, G. Transition Metal Complexes of Cyclic Polyolefins;
Academic Press: New York, 1979; Chapter 2.
(2) Geiger, W. E.; Rieger, P. H.; Corbato, C.; Edwin, J .; Fonseca, E.;
Lane, G. A.; Mevs, J . M. J . Am. Chem. Soc. 1993, 115, 2314.
10.1021/om030661f CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/31/2004

2206 Organometallics, Vol. 23, No. 9, 2004
Notes
tion mechanism involving major (A) and minor (C)
isomers, both of which give the anion B when reduced.
These characteristics are strongly reminiscent of the
voltammetric behavior of the analogous cobalt complex
CoCp(COT) (Cp ) η5-C5H5) and support a square-
scheme mechanism (Scheme 1) favoring the 1,5-isomer
in 1 and the 1,3-isomer in 1-.
Bulk electrolysis of 1 in THF at room temperature
(Eappl ) -2.9 V) passed 1.02 F/eq and resulted in [1,3-
1]- as the major product. The ESR signals of samples
from this solution were very broad in fluid solutions
(and undetectable at room temperature) but well-
behaved when frozen. A rhombic g-tensor having g1 )
2.064, g2 ) 2.014, g3 ) 1.892 is observed, with the only
resolved rhodium hyperfine splitting (103Rh, 100%, I )
1/2) being ca. 7 G at g2 (Figure 2). Of the reported 19-
electron metal-(η4-C8H8) complexes, only 1,3-COT iso-
mers have been shown to have g-value components less
than 2.0.2 The low g-value is consistent with mixing of
the ligand-favored SOMO with a close-lying LUMO
(most likely the metal dyz). In contrast, the SOMO of a
1,5-COT isomer would be largely metal dyz and consid-
erably lower in energy than the next highest orbital.
On this basis, the spectrum of 1- is assigned to the chair
isomer [Rh(η5-C5Ph5)(η4-1,3-C8H8)]-.
F igu r e 1. Cyclic voltammogram of 0.9 mM 1 in THF/0.1
M [NBu4][PF6] at ambient temperature. Changing the scan
rate from 0.05 V s-1 (line) to 1.6 V s-1 (dots) increases the
chemical reversibility of the [1,3-1]-/0 couple, here shown
as peak B for the oxidation of [1,3-1]- to neutral 1,3-1 and
peak C for the reduction of 1,3-1 formed in the diffusion
layer. The peak current for the 0.05 V s-1 scan is ap-
proximately 0.3 µA, and the relative currents of the two
scans have been corrected for scan rate.
Sch em e 1
The voltammetry data hold information about the
thermodynamic and kinetic aspects of the interconver-
sions in Scheme 1. The analogous mechanism for the
cobalt congeners has been the subject of a number of
papers using different spectroscopic and voltammetric
approaches.2-7 Included in the earlier studies was a
finding that the homogeneous electron-transfer cross-
reaction of eq 1
dominated by the highly ligand-based nature of the
LUMO in M(η5-C5R5)(COT). The pentaphenyl-cyclopen-
tadienyl derivative was used in order to take advantage
of the generally increased stabilities observed for 19-
electron species compared to their cyclopentadienyl
counterparts.10
[1,3]- + 1,5 h 1,3 + [1,5]-
(1)
Resu lts a n d Discu ssion
played a significant role in the voltammetric behavior,
and detailed simulations at different temperatures,
concentrations, and scan rates were necessary to quan-
tify the equilibrium and rate constants of the square
scheme.7 In the present case we offer a less stringent
analysis, which, nevertheless, results in useful esti-
mates of the rate and equilibrium constants of the
square scheme for 1.
A qualitative analysis of the CV data for 1 suggests
that K(0) . 1, K(-) , 1, and that the tub isomer [1,5-
1]- converts quickly to the chair isomer [1,3-1]- in the
19-electron species (scans up to 20 V s-1 at 228 K failed
to show any reversibility for the couple 1,5-1/[1,5-1]-).
Quantitative voltammetric analysis was carried out by
a sequence of three procedures. First, the approximate
value of kch/tub was obtained from the reversibility of the
B/C couple (Figure 1) using a standard approach.12 Next,
square-wave voltammetry13 was used to obtain both the
NMR spectra of the neutral title compound reveal the
presence of only the tub isomer. In THF/0.1 M [NBu4]-
[PF6],11 1,5-1 undergoes a chemically irreversible one-
electron reduction at ca. -2.7 V vs ferrocene (Epc, ν )
0.2 V s-1, wave A in Figure 1) and gives a product
having anodic wave B. The latter is part of a B/C redox
couple (E1/2 ) -2.37 V), which is chemically reversible
at higher scan rates (dotted line in Figure 1). Careful
inspection of the original negative-going scan (arrow in
Figure 1) discloses a small sigmoidally shaped section
near -2.35 V. This feature, which is better observed by
differential pulse voltammetry (DPV) or square-wave
(SW) voltammetry (vide infra), is suggestive of a reduc-
(10) Connelly, N. G.; Geiger, W. E.; Lane, G. A.; Raven, S. J .; Rieger,
P. H. J . Chem. Soc., Dalton Trans. 1987, 467. The cyclopentadienyl
parent complex RhCp(COT) undergoes reduction at Epc ) -3.05 V vs
ferrocene in THF/0.1 M [NBu4][PF6]. Bulk cathodic reduction of RhCp-
(COT) did not give products that were readily identified (Moraczeski,
J .; Edwin, J ., unpublished data, University of Vermont).
(12) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706.
(13) O’Dea, J . J .; Osteryoung, J .; Osteryoung, R. A. Anal. Chem.
1981, 53, 695.
(11) The supporting electrolyte in subambient temperature experi-
ments was 0.1 M [NBu4][CF3SO3].

Notes
Organometallics, Vol. 23, No. 9, 2004 2207
F igu r e 2. ESR spectrum of [1,3-1]- obtained from a frozen (77 K) sample taken from a cathodic electrolysis solution of
2 mM 1 in THF/0.1 M [NBu4][PF6]. The resonance position of a dpph standard is shown.
F igu r e 4. CV data for 0.72 mM 1 in THF/0.1 M [NBu4]-
[CF3SO3] at 296 K at a 2 mm glassy carbon electrode. Scan
rate is 0.8 V s-1. Experimental data are shown as solid
lines; simulated, as open circles using parameters in
Table 1.
Ta ble 1. Va lu es Used in Sim u la tion s of Cyclic
Volta m m etr y a n d Squ a r e-Wa ve Volta m m etr y
Exp er im en ta l Da ta
parameter
1,3-isomer 1,5-isomer isomerizations
E1/2 (V vs FeCp2)
-2.370
0.05
-2.765
0.02
ks (cm s-1
)
F igu r e 3. Square-wave voltammetry data for 0.72 mM 1
in THF/0.1 M [NBu4][CF3SO3] at 296 K at a 2 mm glassy
carbon electrode. Pulse height is 50 mV; pulse width is 2
mV. Experimental data are shown as solid lines; simulated,
as open circles using parameters in Table 1. Frequency is
10 Hz (A); 100 Hz (B).
R (chg transfer coeff)
K(0) () kch/tub/ktub/ch
K(-) () [1,5-1]-/[1,3-1]-)a
0.5
0.5
)
65
1.2 × 10-5
kch/tub (s-1
)
2.0
0.031
a
k
tub/ch (s-1
)
a
Fixed by other values in the simulation.
for the 1,3-isomer. A value of approximately 2 s-1 is
found for the isomerization rate of 1,3-1 to 1,5-1. The
K(-) value of 1.2 × 10-5 is similar to that reported for
CoCp(COT).7 Only the lower limit of the isomerization
rate for the tub radical anion can be determined from
these data. The lifetime of [1,5-1]- is too short to be
determined by cyclic voltammetry.17
When the CV scan of Rh(η5-C5Ph5)(COT) is taken to
more negative potentials, a second cathodic peak of one-
electron height is observed (Epc ) -3.04 V at ν ) 0.2
V/s), which must arise from the one-electron reduction
of [1,3-1]-. A new product wave is seen at Epa ) -1.62
value of K(0) (from the relative heights of the two peaks)
and the standard heterogeneous electron-transfer rate
(ks) of the 1,5-1 reduction (from its peak width). These
values were then used in both CV simulations14 and
SW simulations,15 which afforded good fits with experi-
ment (Figures 3 and 4). SW voltammetry was particu-
larly valuable here owing to its superiority over cyclic
voltammetry for the measurement of small concentra-
tions,16 of relevance to the determination of 1,3-1. The
final values are given in Table 1.
Notable is the K(0) value of 65, indicating the strong
dominance of the 1,5-isomer in the neutral compounds
and rationalizing the lack of an observable NMR signal
(16) Osteryoung, J .; O’Dea, J . J . In Electroanalytical Chemistry;
Bard, A. J ., Ed.; Marcel Dekker: New York, 1986; Vol. 14, pp 209 ff.
(17) The value used in these simulations for the rate of [1,5-1]- to
[1,3-1]- was 105 s-1. Decreasing this value to 103 s-1 had no discernible
effect.
(14) CV simulations were performed using Digisim 3.1 (Bioanalytical
Systems).
(15) SW simulations were performed using ESP version 2.4 (written
by Carlo Nervi), available at http://lem.ch.unito.it/.

2208 Organometallics, Vol. 23, No. 9, 2004
Notes
in CDCl3: δ 135.3 (4 C, s, uncoord. CHdCH) 133.4 (5C), 131.8
(10C), 127.4 (10C), 126.3 (5C) (all s, C5Ph5), 105.7 (5C, d, J RhC
3.7 Hz, C5Ph5), 79.1 (4C, d, J RhC 12.8 Hz, coord. CHdCH). m/z
(positive FAB): 652 (M+, 100), 548 (M+ - C8H8, 50).
Electr och em istr y a n d Sp ectr oscop y. THF was purified
first by distillation from potassium, then treated with sodium/
benzophenone until the solution was deep purple. The solvent
was then transferred under static vacuum to a flask that was
opened in the drybox. Electrochemical glassware was heated
overnight at 120 °C and transferred quickly to a Vacuum
Atmospheres drybox antechamber. All electrochemical experi-
ments were carried out in the drybox under nitrogen.
The voltammetry scans were carried out using a glassy
carbon electrode of 1 or 2 mm diameter that was polished with
diamond paste on a Buehler microcloth. After the polishing,
the electrode was rinsed with Nanopure water and dried under
vacuum. The working electrode for the bulk electrolysis was
a platinum gauze basket. The functional reference electrode
was a Ag/AgCl electrode prepared by anodizing a silver wire
in HCl solution. It was separated from the working electrode
solution by a fine glass frit. The potentials in this paper are
referred to the ferrocene/ferrocenium couple, as recommended
elsewhere.19 Ferrocene served as an internal reference and was
added to the solution at an appropriate time in the experiment.
The potentials in this paper can be changed to SCE reference
values by addition of 0.56 V. The supporting electrolyte was
0.1 M [NBu4][PF6] unless otherwise noted. Electrochemistry
was performed using a PARC model 273 potentiostat inter-
faced to a personal computer with homemade software. ESR
spectra were obtained with a Bruker spectrometer.
V (Supporting Information Figure SM1). Because these
features are again very similar to those of the CoCp
analogue,3b we ascribe these processes to the formation
of [1,3-1]2- followed by rapid adventitious protonation
to give the cyclooctatriene complex [Rh(η5-C5Ph5)-
(η3-C8H9)]- or another protonation product.
In summary, the one-electron reduction of Rh(η5-
C5Ph5)(η4-C8H8) is structurally and mechanistically very
similar to that of the intensively studied cobalt ana-
logues. The thermodynamically favored form of the 18-
electron compound is the tub-shaped 1,5-COT isomer,
whereas that of the radical anion is the chair-shaped
1,3-COT isomer. The latter is formed rapidly from
electrogenerated [1,5-1]-. The chair form comprises
significantly less of the isomer makeup of the neutral
Rh complex (ca. 2% at room temperature) than found
in the Co analogue CoCp(C8H8) (ca. 20% 1,3-isomer).7
Given the convergent physical properties of the Co and
Rh systems and the highly COT-based radical structure
of the former, the anions [M(η5-C5Ph5)(1,3-COT)]-, M
) Co or Rh, may both be classified as having an “18 +
δ” electronic structure, using the Tyler notation.18
Exp er im en ta l Section
Syn th esis of Rh (η5-C5P h 5)(η4-C8H8). A mixture of [Rh-
(η5-C5Ph5)Br2]2 (0.500 g, 0.353 mmol), sodium carbonate (0.500
g, 4.71 mmol), and cyclooctatetraene (1.80 cm3, 15.9 mmol) in
ethanol (100 cm3) was heated under reflux for 4 h. During this
time the solution changed from deep red to orange and an
orange solid precipitated. The solvent was removed in vacuo,
and the residue was washed with hexane (3 × 5 cm3) and then
extracted into diethyl ether (3 × 50 cm3); removal of the ether
in vacuo left a fine orange solid. This was then chromato-
graphed on neutral alumina; an orange fraction eluted with
hexane/diethyl ether (1:1), and removal of the solvent in vacuo
gave the product as an orange crystalline solid (found C, 78.20;
H, 5.00; RhC43H33 requires C, 79.14; H, 5.10). 1H NMR at
250 MHz in CDCl3: δ 3.85 (4 H, “t”, J HH ) J RhH 1.0 Hz, coord.
CHdCH), 5.80 (4 H, br s, uncoord. CHdCH), 6.98 (10 H, m,
C5Ph5), 7.04 (10 H, m, C5Ph5), 7.10 (5 H, m, C5Ph5). 13C NMR
Ack n ow led gm en t. This work was supported by the
National Science Foundation (CHE 0029702). We are
indebted to Prof. P. H. Rieger for advice in interpreting
the ESR spectra.
Su p p or tin g In for m a tion Ava ila ble: Three figures: (i)
CV data for 1 scanning to -3.2 V, (ii) plot of reversibility
measurables as a function of scan rate (CV) or frequency
(SWV) for EC mechanism, and (iii) table giving SWV values
used to construct (ii). This material is available free of charge
via the Internet at http://pubs.acs.org.
OM030661F
(18) Tyler, D. R. Acc. Chem. Res. 1991, 24, 325
(19) Gritzner, G.; Kuta, J . Pure Appl. Chem. 1984, 56, 461.