16-electron through 19-electron complexes of the 1,5-COD and 1,3-COD isomers of (η5-C5Ph5)Rh(η4-C 8H12): Electrochemical evidence for an oxidatively induced agostic interaction

Article abstract of DOI:10.1021/om980788i

The redox properties of Cp?Rh (1,5-COD) (1) and Cp?Rh (1,3-COD) (2) (Cp? = η⁵-C₅Ph_5, COD = η⁴-cyclooctadiene) have been investigated by cyclic voltammetry and bulk coulometry. Both compounds display a four-membered electron-transfer series involving complexes of overall 2+, 1+, 0, and 1- charges. The 19-electron complexes [Cp?Rh(COD)]^- are shortlived, rapidly releasing [C₅Ph₅]^-. The persistent 17-electron complexes [Cp?RhCCOD)]⁺ have been characterized by ESR and optical spectroscopies. Complex 1⁺ displays an axially symmetric g-tensor, demonstrating that the 17-electon system retains the approximate Cgu symmetry of its 18-electron precursor. The oxidized forms of 2ⁿ⁺ (n = 1 or 2) undergo slow isomerization to those of lⁿ⁺; that is, isomerization of the diolefin ligand from the 1,3- to the 1,5-isomer occurs in the higher oxidation states of [Cp?Rh(COD)]ⁿ⁺. The 1,3-COD ligand imparts an unexpected thermodynamic stabilization of the higher Rh oxidation states. Formation of the 17-electron and 16-electron complexes occurs at potentials ca 0.25 and 0.62 V, respectively, lower than expected. The increasing stabilization of the Rh(II) and Rh(III) complexes is ascribed to progressive formation of an agostic interaction between the metal atom and the hydrogen on carbon 5 of the 1,3-cyclooctadiene ring.

Full text of DOI:10.1021/om980788i

5486
Organometallics 1998, 17, 5486-5491
16-Electr on th r ou gh 19-Electr on Com p lexes of th e
1,5-COD a n d 1,3-COD Isom er s of (η5-C5P h 5)Rh (η4-C8H12):
Electr och em ica l Evid en ce for a n Oxid a tively In d u ced
Agostic In ter a ction †
Michael J . Shaw,‡ William E. Geiger,*,‡ J ulie Hyde,§,| and Colin White*,§
Departments of Chemistry, University of Vermont, Burlington, Vermont 05405, and
University of Sheffield, Brook Hill, Sheffield S3 7HF, England
Received September 21, 1998
The redox properties of Cp‡Rh (1,5-COD) (1) and Cp‡Rh (1,3-COD) (2) (Cp‡ ) η5-C5Ph5,
COD ) η4-cyclooctadiene) have been investigated by cyclic voltammetry and bulk coulometry.
Both compounds display a four-membered electron-transfer series involving complexes of
overall 2+, 1+, 0, and 1- charges. The 19-electron complexes [Cp‡Rh(COD)]- are short-
lived, rapidly releasing [C5Ph5]-. The persistent 17-electron complexes [Cp‡Rh(COD)]+ have
been characterized by ESR and optical spectroscopies. Complex 1+ displays an axially
symmetric g-tensor, demonstrating that the 17-electon system retains the approximate C2v
symmetry of its 18-electron precursor. The oxidized forms of 2n+ (n ) 1 or 2) undergo slow
isomerization to those of 1n+; that is, isomerization of the diolefin ligand from the 1,3- to the
1,5-isomer occurs in the higher oxidation states of [Cp‡Rh(COD)]n+. The 1,3-COD ligand
imparts an unexpected thermodynamic stabilization of the higher Rh oxidation states.
Formation of the 17-electron and 16-electron complexes occurs at potentials ca 0.25 and
0.62 V, respectively, lower than expected. The increasing stabilization of the Rh(II) and
Rh(III) complexes is ascribed to progressive formation of an agostic interaction between the
metal atom and the hydrogen on carbon 5 of the 1,3-cyclooctadiene ring.
In tr od u ction
electron anion [CpCo(1,5-COD)]-, which has been re-
ported to isomerize to the 1,3-isomer.5 In a recent
review7 one of us emphasized that the evidence for
formation of the 1,3-COD isomer in the Co complex was
indirect, the strongest corroboration being identification
of free 1,3-C8H12 after degradative oxidation of the 19-
electron metal complex. There are, thus far, no reports
of comparative redox potentials for metal complexes
containing 1,5-bonded vs 1,3-bonded COD ligands, mak-
ing E1/2 values unreliable in identifying isomeric com-
plexes that resist isolation.
The recent report8 of 1,5-COD and 1,3-COD complexes
of (C5Ph5)Rh has finally made it possible to compare the
reduction and oxidation potentials of the two relevant
metal-cyclooctadiene isomers. We therefore report our
studies of the redox properties of [Cp‡Rh(1,5-COD)], 1,
and Cp‡Rh(1,3-COD), 2 (Cp‡ ) η5-C5Ph5). These com-
plexes are found to exist in a string of four oxidation
states [Rh(III)/(II)/(I)/(0)] possessing from 16 to 19
valence electrons. We have been able to identify an
inefficient isomerization process of the 1,3-isomer to the
1,5-isomer which takes place in higher oxidation states
and have found oxidation potential shifts that may be
ascribed to agostic interactions involving an aliphatic
hydrogen of the 1,3-cyclooctadiene ligand. Reduction of
either isomer to the 19-electron Rh(0) complex results
in rapid loss of the pentaphenylcyclopentadienyl anion
without evidence for prior isomerization.
A number of metals are known to promote the
isomerization of cyclic and acyclic olefins and polyole-
fins.1 The fact that various types of organometallic
reactions are accelerated in odd-electron complexes2
suggests the possibility that metal-olefin isomerization
processes may be profitably enhanced by oxidation or
reduction of 18-electron complexes. Electron-transfer-
induced isomerizations have been studied in depth for
cyclooctatetraene (COT) complexes of Co3,4 and, to a
lesser extent, Ni and Pd.4 Cyclooctadiene (COD) com-
plexes of the later transition metals have also received
some attention,5,6 but the metal-1,5-COD bond has
generally been found to be unisomerized in the electron-
transfer products. A possible exception is the 19-
† Structural Consequences of Electron-Transfer Reactions, Part 35.
Part 34: DiMaio, A.-J .; Rheingold, A. L.; Chin, T. T.; Pierce, D. T.;
Geiger, W. E. Organometallics 1998, 17, 1169.
‡ University of Vermont.
§ University of Sheffield.
| Formerly J ulie Baghdadi.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; J ohn Wiley and Sons: New York, 1994; p 210.
(2) Trogler, W. C. Organometallic Radical Processes; Elsevier:
Amsterdam, 1990.
(3) 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,
and references therein.
(4) 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.
(5) Moraczewski, J .; Geiger, W. E. J . Am. Chem. Soc. 1981, 103,
4779.
(6) (a) Ko¨lle, U.; Werner, H. J . Organomet. Chem. 1981, 221, 367.
(b) Lane, G.; Geiger, W. E. Organometallics 1982, 1, 401. (c) Ko¨lle, U.;
Ting-Zhen, D.; Keller, H.; Ramakrishna, B. L.; Raabe, E.; Kru¨ger, C.;
Raabe, G.; Fleischhauer, J . Chem. Ber. 1990, 123, 227.
(7) Geiger, W. E. Acc. Chem. Res. 1995, 28, 351.
(8) Baghdadi, J .; Bailey, N. A.; Dowding, A. S.; White, C. J . Chem.
Soc., Chem. Commun. 1992, 170.
10.1021/om980788i CCC: $15.00 © 1998 American Chemical Society
Publication on Web 11/04/1998

Isomers of (η5-C5Ph5)Rh(η4-C8H12
)
Organometallics, Vol. 17, No. 25, 1998 5487
Ta ble 1. E1/2 P oten tia ls of Meta l-Cycloocta d ien e
Com p lexes (Volt vs F er r ocen e) in CH2Cl2/0.1 M
[NBu 4][P F 6]
complex
2+/1+ 1+/0
1-/0
remarks
Cp‡Rh(1,5-COD), 1
1 in THF
0.72
0.25
0.09
-0.01
0.05
stable 1+, 2+ ions
-3.17 ECE mechanism,
loss of [C5Ph5]-
Cp‡Rh(1,3-COD), 2
cation(s) eventually
form 1
Exp er im en ta l Section
2 in THF
-3.08a anion loses [C5Ph5]-
irrev if ν < 2 V/sb
CpRh(1,5-COD)
(η5-C9H7)Rh(1,5-COD)
in THF
CpCo(1,5-COD)
CpCo(1,5-COD)
in THF
-2.19 anion loses [C9H7]- c
Ch em ica ls a n d Sp ectr a l Mea su r em en ts. Compounds 1
and 2 have been previously reported,8 but full synthetic details
are included here for the first time.
0.90d -0.22
refs 18 and e
-3.01 ref 5
(C5P h 5)Rh (1,5-COD). C5Ph5H (0.910 g, 2.00 mmol) was
heated in 50 mL of xylene to 120 °C. A 1.4 mL sample of a
1.4 M solution of MeLi in hexanes (2.0 mmol) was added over
5 min, during which time the solution changed from clear
yellow to cloudy yellow-orange. After heating the mixture for
2 h at 120 °C, [RhCl(COD)]2 (0.500 g, 1.01 mmol) was added
and the resulting solution heated for an additional 1 h. After
cooling, the solution was filtered through “Hiflo” and the
solvent removed under vacuum to leave a deep red residue.
Column chromatography on neutral alumina with 4:1 diethyl
ether/petroleum ether gave the product as a brown-yellow solid
(0.110 g, 8.5% yield). Recrystallization from dichloromethane/
petroleum ether (bp 60-80 °C) gave orange-yellow cubic
a
Chemically irreversible, peak potential at ν ) 0.05 V/s given.
b Chin, T. T. Unpublished data, University of Vermont. c Reference
26. T ) 233 K. e Moraczewski, J .; Geiger, W. E. Organometallics
d
1982, 1, 1385.
voltammetric characterization. 1H NMR spectra were run on
a 250 MHz Bruker instrument, and UV-vis spectra were
recorded using a Perkin-Elmer Lambda 6 spectrometer. Chemi-
cal oxidation of 2 was carried out as follows. Ferrocenium
hexafluorophosphate (25 mg, 76 µmol) was added to a solution
of Cp‡Rh(1,3-COD) (50 mg, 76 µmol) in 10 mL of CH2Cl2 at
233 K. The solution was stirred for 30 min and then
evaporated under reduced pressure. The residue was dissolved
in CDCl3 for 1H NMR analysis: (δ values) free 1,3-COD, 1.5,
2.2, 5.6, 5.8; 1, 1.95, 2.37, 3.66 plus phenyl region. No free
1,5-COD was observed.
Electr och em istr y. Except for experiments with the opti-
cally transparent thin layer (OTTLE) cell, all electrochemical
operations were conducted inside a Vacuum Atmospheres
drybox under N2. Details of the electrochemical methodologies
have been publshed.11 The supporting electrolyte {[NBu4][PF6]
in all cases} concentration was 0.1 M except for OTTLE cell
experiments (1.0 M). Rotating Pt electrode (RPE) scans were
obtained with a synchronous rotator operating at 1800 rpm.
Bulk electrolyses were carried out at a Pt gauze cylinder in a
cell in which the anodic and cathodic compartments were
separated by a fine glass frit. In this paper all potentials are
reported vs the ferrocene/ferrocenium couple (Fc), as recom-
mended by IUPAC.12 Conversion to the aqueous SCE scale
requires addition of 0.46 V for solutions of the chlorinated
hydrocarbons or 0.56 V for THF. Evaluation of voltammetric
diagnostics for diffusion control and chemical and electro-
chemical reversibility followed procedures summarized else-
where.13
crystals of (C5Ph5)Rh(1,5-COD) (found C, 78.9; H, 5.80; C43H37
-
Rh requires C, 78.7; H, 5.70). 1H at 250 MHz in CDCl3: δ
2.00 (4 H, q, J HH 6.0 Hz, CH2), 2.40 (4 H, q, J HH 4.0 Hz, CH2),
3.70 (4 H, s, CHdCH), 6.91, 7.03, 7.08 (25 H, m, C5Ph5). 13C
in CD2Cl2: δ 134.8 (5 C), 132.6 (10 C), 127.7 (10 C), 126.4 (5
C) (all s, C5Ph5), 105.8 (5 C, d, J RhC 3.7 Hz C5Ph5), 78.4 (4 C,
d, J RhC 14.4 Hz, CHdCH), 32.5 (4 C, s, CH2). m/z (positive
FAB): 656 (M+, 100), 548 (M+ - C8H12, 70), 446 (C5Ph5+, 60).
(C5P h 5)Rh (1,3-COD). [RhBr2 (C5Ph5)]2 (0.800 g, 0.560
mmol), sodium carbonate (0.800 g, 7.55 mmol), and 1,5-
cyclooctadiene (8 mL, 65.3 mmol) in 150 mL of ethanol were
stirred at room temperature for 24 h, during which time the
color changed from red to green and a green solid formed. The
latter was filtered off and extracted with diethyl ether until
the washings were colorless (5 × 100 mL). Removal of the
solvent in vacuo gave a bright yellow solid, which, after
vacuum-drying, was found to be a mixture (85:15) of the 1,3-
and 1,5-COD isomeric complexes, respectively (0.530 g, 71%).
Recrystallization was carried out by slow diffusion of hexane
into dichloromethane at -20 °C to give pure (C5Ph5)Rh(1,3-
COD) as orange crystals (found C, 77.9; H, 5.70; RhC43H37
requires C, 78.7; H, 5.70). 1H in CDCl3: δ 1.20 (2 H, m, CH2),
1.35 (2 H, m, CH2), 1.80 (2 H, m, CH2), 2.00 (2 H, m, CH2),
3.55 (2 H, s, CHdCH), 4.65 (2 H, d, J HH 5.0 Hz, CHdCH), 6.90,
7.01, 7.06 (25 H, m, C5Ph5). 13C in CD2Cl2: δ 134.9 (5 C), 132.5
(10 C), 127.6 (10 C), 126.4 (5 C) (all s, C5Ph5), 104.3 (5 C, d,
J RhC 5.1 Hz, C5Ph5), 84.5 (2 C, d, J RhC 5.8 Hz, CHdCH), 61.7
(2 C, d, J RhC 17.8 Hz, CHdCH), 28.7 (2 C, s, CH2), 25.4 (2 C,
Resu lts a n d Discu ssion
Oxid a tion of 1 a n d 2: P r od u ction of 17e a n d 16e
Com p lexes. Complex 1 undergoes two reversible one-
electron oxidations with E1/2 ) 0.09 V, E1/2 ) 0.72 V
vs Fc (Table 1), supporting the existence of the three-
membered electron-transfer series:
1
2
m, CH2). m/z (positive FAB): 656 (M+, 100), 548 (M+ - C8H12
,
50), 446 (C5Ph5+, 70). This complex was also prepared using
the above method, but substituting 1,3-COD for 1,5-COD.
The solvents used for electrochemistry were reagent grade
chemicals distilled in vacuo from drying agents: CH2Cl2 and
1,2-C2H4Cl2 from CaH2 and THF from potassium. [NBu4][PF6]
was recrystallized from 95% ethanol and dried in vacuo at 373
K. [PPN]Cl {PPN ) bis(triphenylphosphine)imminium chlo-
ride} was purchased from Strem Chemicals. Ferrocenium
hexafluorophosphate and acetylferrocenium hexafluorophos-
phate were prepared by oxidation of the corresponding fer-
rocenes with Ag[PF6].9 The complex [(COD)Rh(THF)x]+ was
1 h 1+ h 12+
Cyclic voltammetric (CV) measurements (Figure 1)
with 0.05 V/s < ν < 0.3 V/s established that both couples
were diffusion controlled and reversible at 278 K in CH2-
(9) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(10) The general method is taken from: Schrock, R. R.; Osborne, J .
A. J . Am. Chem. Soc. 1971, 93, 3089.
(11) Richards, T. C.; Geiger, W. E. J . Am. Chem. Soc. 1994, 116,
2028.
(12) Gritzner, G.; Kuta, J . Pure Appl. Chem. 1984, 56, 461.
(13) Geiger, W. E. In Laboratory Techniques in Electroanalytical
Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel
Dekker, Inc.: New York, 1996; Chapter 23.
generated in situ:10 [(COD)RhCl]2 (7.9 mg, 1.6 µmol) was
8a
dissolved in 10 mL of THF and treated with Ag[PF6] (8.3 mg,
3.3 µmol). The pale yellow solution was stirred for 10 min
and then filtered through
a plug of glass wool into an
electrochemical cell containing 0.50 g of [NBu4][PF6] for

5488 Organometallics, Vol. 17, No. 25, 1998
Shaw et al.
Ta ble 2. ESR Resu lts for 17-Electr on
Meta l-Cycloocta d ien e Com p lexes in CH2Cl2/
CH4Cl2 Med ia
radical
[Cp‡Rh(1,5-COD)]+ 77
ambient
[Cp‡Rh(1,3-COD)]+ 77
ambient
[CpCo(1,5-COD)]+ 77
temp (K)
g1
g2
g3
g
2.181 2.181 1.997 (g )
2.114 2.053 1.989
2.329 2.120 2.003
2.064
2.055
(ref 18)
evidence) even at 233 K over 10 min. Exhaustive re-
reduction at Eappl ) -0.4 V produced only about 10% of
the starting material, 1.
The 1,3-isomer 2 likewise undergoes two one-electron
1
2
oxidations (Figure 1); in this case E1/2 ) -0.01 V, E1/2
) +0.25 V (Table 1):
2 h 2+ h 22+
F igu r e 1. Cyclic voltammograms at 233 K of the oxida-
tions of 1.2 mM 1 (top) and 0.95 mM 2 (bottom) in CH2-
Cl2/0.1 M [NBu4][PF6], 0.25 mm Pt disk, ν ) 0.20 V/s.
The first of these processes is Nernstian and uncom-
plicated. The reversibility of the second varied, in
apparent response to solution conditions (especially, we
believe, dryness). In most experiments the second
oxidation was also Nernstian, but in some replicates the
reversibility of this process (both chemical and electro-
chemical) was reduced. Decreased chemical reversibil-
ity was accompanied by the appearance of small waves
at potentials appropriate for 1. This observation is
supported by bulk electrolysis experiments (vide infra)
which give evidence of the 1,5-isomer when the 1,3-
isomer is oxidized to the dication.
Electrolysis of 1.6 mM 2 at the first oxidation wave
(Eappl ) 0.15 V) at 218 K gave napp ) 1.0e as the solution
changed from yellow to green. RPE scans of the
electrolyzed solution showed that 75% of the complex
was in the oxidized form 2+. Re-reduction (Eappl ) -0.5
V) of 2+ gave back the original amount of 2.
Cl2. CV peak separations were essentially equal to
those of Cp2Fe, suggesting that the redox reactions are
approximately Nernstian. Plots of E vs log(ilim - i)/i
taken from rotating Pt electrode (RPE) scans confirmed
this conclusion, slopes of 55 and 62 mV being observed
for the first and second waves, respectively, close to the
theoretical value of 56 mV at 278 K.
The oxidation product 1+ was sensitive to nucleo-
philes. After addition of equimolar chloride as [PPN]-
Cl the first oxidation wave of 1 was no longer revers-
ible.14 Full chemical reversibility of this anodic process
and, even more so, that of the 1,3-COD isomer 2
required pure electrolyte solutions of minimal water
content.
The robustness of 1+ was established by bulk elec-
trolysis. When a 1 mM solution of 1 was oxidized (Eappl
The dication 22+ formed when bulk electrolysis was
carried out at the second wave (Eappl ) 0.5 V) decom-
posed slowly at 218 K, as evidenced by voltammetric
currents (CV and RPE of 22+) that decreased with time.
Exhaustive re-reduction at -0.5 V resulted in waves for
both 1 and 2. Anodic electrolysis at a slightly higher
temperature (240 K) gave a brown solution, which
turned orange when raised to ambient temperature and
showed waves for 1 but not 2. The decomposition
product(s) of 22+ include 1 in one or more of its oxidation
states, but there is little that can be said about the
mechanism of the process.
Evidence for 1 was also obtained when 2 was chemi-
cally oxidized by equimolar [Cp2Fe][PF6] in CH2Cl2.
After 10 min at 298 K, NMR spectra of the evaporated
and redissolved (in CDCl3) residue showed that isomer
2 had completely reacted. The only signals observed
(see Experimental Section) were those of free 1,3-COD
(80% yield) and the 1,5-isomer 1 (20% yield). No free
1,5-COD was detected.
) 0.3 V) at a Pt basket at room temperature (napp
)
0.9e), the solution changed from yellow to purple and
RPE scans indicated complete conversion to the 17e
complex 1+. Samples of this solution removed for ESR
analysis retained their purple color for several hours
at room temperature. The neutral complex 1 was
quantitatively regenerated by exhaustive re-reduction
(Eappl ) -0.4 V). The optical spectrum of 1+ was
obtained by spectro-electrochemistry using a gold mini-
grid electrode sandwiched between two quartz windows
(an OTTLE cell).15 A 5 mM solution of 1 (λmax ) 485
nm, ꢀ ) 380) was electrolyzed at the potential of the
first oxidation, giving a spectrum attributed to 1+ (λmax
) 554 nm, ꢀ ) 2300 assuming complete conversion). Re-
reduction of the cation yielded the original spectrum of
1. The intensity of the transition for 1+ suggests an
assignment as a charge-transfer band,16 most likely L(π)
f M.
Solutions of the 16e dication 12+ were produced by
bulk electrolysis at Eappl ) 0.9 V. CV and RPE scans of
ESR Sp ectr a of 17-Electr on Com p lexes 1+ a n d
2+. ESR-active samples of 1+ were generated either by
bulk electrolysis (vide ante) or by oxidation with equimo-
lar acetylferrocenium ion. Room-temperature solutions
of 1+ have a symmetric single peak at g ) 2.064 (Table
2). At 77 K in 1:1 CH2Cl2/C2H4Cl2 a strong spectrum
was observed which apparently has axial symmetry (g|
) 2.181 and g ) 1.997 (Figure 2). An extra nuance
was noted near g ) 2.0, which makes the low-field side
of the g peak appear to be split. Simulations show that
the resulting green solution indicated formation of 12+
,
but significant decomposition occurred (voltammetric
(14) A product wave appears at E1/2 ) -0.85 V. A likely product of
the reaction of 1+ with Cl- is the chloro-bridged dinuclear complex
[Cp‡Rh(Cl)]2.
(15) (a) Murray, R. W.; Heineman, W. R.; O’Dom, G. W. Anal. Chem.
1979, 39, 1666. (b) Hawkridge, F. M.; Kissinger, P. T. In ref 13, Chapter
9.
(16) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry;
Academic Press: New York, 1979.

Isomers of (η5-C5Ph5)Rh(η4-C8H12
)
Organometallics, Vol. 17, No. 25, 1998 5489
F igu r e 2. ESR spectra of frozen solutions (T ) 77 K) of
17-electron radicals produced by chemical oxidation of
neutral complexes. Top: 1+ in CH2Cl2/C2H4Cl2 (oxidant:
acetylferrocenium tetrafluoroborate). Bottom: 2+ in CH2-
Cl2 (oxidant: ferrocenium tetrafluoroborate).
F igu r e 3. Cyclic voltammograms at ambient temperatures
of the reductions of 1 (top) and 2 (bottom) in THF/0.1 M
[NBu4][PF6], 0.8 mm Pt disk, ν ) 0.80 V/s, concentrations
) 1.2 mM. Inset gives result (unscaled) at ν ) 100 V/s for
1.
this shape cannot arise from a small deviation from
axial symmetry nor from a Rh hyperfine interaction.
There may be a small amount of the pentaphenylcyclo-
pentadienyl radical present, or there may be differences
in radical environments in the frozen solution. Frozen
solutions of 1+ in pure CH2Cl2, a nonglassy matrix, have
the signals in Figure 2 in addition to a broad line at g
≈ 2.06. The latter evidently arises from crystallites of
1+ rather than the randomly oriented radicals favored
in CH2Cl2/C2H4Cl2, which forms a better glass.17 Lack-
ing hyperfine splittings, the spectra of 1+ cannot delin-
eate the precise orbital makeup of the SOMO (semioc-
cupied MO), but they do indicate that the radical retains
the approximate C2v symmetry of 1, since lower sym-
metry would lead to a splitting of the g components
into a rhombic g-tensor (as seen below for 2+).
certainly operative, given the increased metal-ligand
covalency normally associated with Rh π-complexes
compared to their Co congeners.20,21
Kin et ic St a biliza t ion Im p a r t ed by t h e (C5P h 5)
Liga n d . Kinetic stabilization of the 17-electron species
by the pentaphenylcyclopentadienyl ligand is apparent
in the contrasting lifetimes of 1+ and [CpRh(1,5-COD)]+.
The latter has only brief existence under the same
conditions that give long-lived 1+. Unpublished work
by T. T. Chin at the University of Vermont showed that
the oxidation of CpRh(1,5-COD) (E1/2 ) 0.05 V vs Fc) is
chemically reversible only at CV scan rates over 20 V/s
in CH2Cl2. In contrast, even the 16-electron dications
of 1 and 2 are detectable on the voltammetric time scale.
The increased lifetimes of the C5Ph5 complexes most
likely arise from steric hindrance of reactions at the
metal center or the five-membered ring that are com-
monly encountered with 17-electron complexes of this
general type.22
Red u ction of 1 a n d 2 in THF . The reductions of
both isomers give unstable anions. At a scan rate of ν
) 0.1 V/s, 1 displays a cathodic wave (Epc ) -3.23 V)
that is 2.6 times the height of the one-electron oxidation
wave (1/1+) previously detailed. As the scan rate is
increased to 0.8 V/s, the ratio of current functions falls
to 2.0 and a coupled anodic wave appears, suggesting
the onset of chemical reversibility for the 1/1- couple
(Figure 3), E1/2 ) -3.17 V. At scan rates above about
10 V/s the wave is chemically reversible (Figure 3,
inset: CV at 100 V/s). The evidence is consistent with
Spectra of 2+ were similarly generated, in this case a
rhombic g-tensor being observed: g1 ) 2.114, g2 ) 2.053,
g3 ) 1.989 (Table 2). The average of the three values,
2.052, is in good agreement with the observed isotropic
spectrum, g ) 2.055, measured for fluid solutions of
2+ at 233 K.
Both isomers display much smaller g-value anisotro-
pies than their cobalt analogue [CpCo(1,5-COD)]+ (Table
2),18 mostly notably because of the value of g1, which is
2.329 in the Co complex, 2.18 in 1+, and 2.11 in 2+. Since
the spin-orbit coupling constant of Rh is considerably
larger than that of Co, one would expect the opposite
trend in g-values, all other bonding factors remaining
equal. The fact that g1 (almost surely gy, using the
coordinate system of ref 18) is so much closer to the free
spin value in the Rh analogues must arise from either
(i) less metal character in the SOMOs of the Rh
complexes and/or (ii) greater separation of the 3dxz-
containing ground state from the first excited state
(20) Green, J . C.; Powell, P.; van Tilborg, J . E. Organometallics 1984,
3, 211.
(21) (a) Green, J . C. Struct. Bonding (Berlin) 1981, 43, 37. (b)
Lichtenberger, D. L.; Calabro, D. C.; Kellogg, G. E. Organometallics
1984, 3, 1623. (c) Lichtenberger, D. L.; Gruhn, N. E.; Rempe, M. E.;
Geiger, W. E.; Chin, T. T. Inorg. Chim. Acta 1995, 240, 623.
(22) Leading references may be found in: (a) Harlow, R. L.;
McKinney, R. J .; Whitney, J . F. Organometallics 1983, 2, 1839. (b)
Connelly, N. G.; Freeman, M. J .; Manners, I.; Orpen, A. G. J . Chem.
Soc., Dalton Trans. 1984, 2703. For a discussion of the metal vs ligand
character in the HOMOs of CpM(CO)2, M ) Co, Rh, see: Lichtenberger,
D. L.; Calabro, D. C.; Kellogg, G. E. Organometallics 1984, 3, 1623.
2
containing a 3dz contribution. The former is almost
(17) This effect has been previously noted: Quarmby, I. C.; Geiger,
W. E. Organometallics 1992, 11, 436.
(18) Chin, T. T.; Sharp, L. I.; Geiger, W. E.; Rieger, P. H. Organo-
metallics 1982, 1, 1385.
(19) Albright, T. A.; Moraczewski, J .; Tulyathan, B.; Geiger, W. E.
J . Am. Chem. Soc. 1981, 103, 4787.

5490 Organometallics, Vol. 17, No. 25, 1998
Shaw et al.
Sch em e 1. Rep r esen ta tion of a gostic sta biliza tion
en er ies of th e oxid ized for m s of 2. Th e sh ift of A is
0.25 V; th a t of B is 0.62 V. Th e d otted lin es
an ECE mechanism for the reduction of 1, with the 19-
electron complex 1- (apparent half-life 0.1 s) undergoing
decomposition to electroactive product(s) Z:
r ep r esen t th e exp ected E1/2 va lu es in th e a bsen ce
of a gostic in ter a ction s, a ssu m in g a r a isin g of th e
HOMO by 0.15 eV in th e n eu tr a l 1,3-isom er .
1 + e- a 1- f Z (+ ne-) f Zn-
One of these products is the pentaphenylcyclopentadi-
enyl anion, detected on reverse sweeps as a reversible
couple with E1/2 ) -0.84 V.23,24
The reduction of 2 occurs at a potential 150 mV
positive of that of 1 (Epc ) -3.08 V at ν ) 0.095 V/s). In
this case the wave is of one-electron height and is
chemically irreversible, independent of ν. As with the
1,5-isomer, the reduction of 2 releases [C5Ph5]- (CV
product wave at E1/2 ) -0.84 V). A point of difference
in the reductions of 1 and 2 is that a second cathodic
wave, also irreversible, is seen when 2 is scanned to
more negative potentials (Figure 3).
Since release of [C5Ph5]- leaves the formal fragment
(COD)Rh, which would undoubtedly be solvated if
formed, we generated the complex [(1,5-COD)Rh-
(THF)x]+ for comparison of its redox potentials to those
of the product waves of 2. Two irreversible waves were
observed for [(1,5-COD)Rh(THF)x]+ at Epc ) -2.09 and
-2.54 V, neither of which coincides with the product
wave at -3.3 V for the reduction product of 2.
considers that, contrary to loss of the diolefin, loss of
the C5-hydrocarbon provides a mechanism for carrying
off the negative charge.
Com p a r ison of th e P oten tia ls of th e 1,5-COD a n d
1,3-COD Isom er s. The one-electron oxidation of 1 (E1/2
) +0.09 V) occurs 100 mV positive of the corresponding
oxidation of 2. The converse is true for the one-electron
reductions of the two isomers, the potential of 1 being
about the same amount negative of that of 2. That is,
the 1,3-isomer is both easier to oxidize and easier to
reduce than its 1,5-analogue. The more facile reduction
is explicable in the simple notion of increased metal-
ligand delocalization in the LUMO of the 1,3-isomer.
The more facile oxidation of 2 requires more consid-
eration, since this contrasts with expectations based on
reported ionization potentials of CpRh (η4-diolefin)
complexes. Green and co-workers20 showed that the
first ionization potentials of complexes of two conjugated
diolefins (viz., butadiene and 1,3-cyclohexadiene) were
higher (by an average of 0.15 eV) than those of com-
plexes of either 1,5-COD or bis-ethylene. On this basis
one would predict an ordering of the first oxidation
potentials for 1 and 2, namely, E1/2(2) > E1/2(1), opposite
of that observed. Taking into account that the mea-
sured value of E1/2(2) is 0.1 V less than E1/2(1), there is
an unaccounted-for stabilization of about 0.25 V ()0.15
V + 0.10 V) in the formation of 2+.
An even larger negative shift of the oxidation poten-
tial of the 1,3-isomer is seen for the second oxidation of
2 compared to 1 (0.47 V). The separation of the two
E1/2 values for the stepwise oxidation of 2 is therefore
quite small, 0.26 V [cf. 0.63 for 1 and 1.12 V for CpCo-
(1,5-COD)]. These shifts are depicted graphically in
Scheme 1, in which the dotted lines are the relative
oxidation potentials expected from the photoelectron
spectroscopy data of Green.20
Because solvation energies should be very similar for
the various redox states of 1 compared to 2, the
progressive stabilization of higher oxidation states of 2
must have its origin in electronic and molecular struc-
tural differences between the two compounds. ESR
spectra of the monocations do not suggests major
differences in the metal-ligand delocalization (vide
ante), and there is no evidence for isomeric intercon-
versions on the voltammetric time scale. The E1/2 shifts
None of the electrochemical data suggest that either
of the 19-electron anions isomerizes to the other form.
19-Electr on Sp ecies: F a te of [Cp ‡Rh (COD)]-.
Both 19-electron species generated in this study, namely,
1- and 2-, are subject to rapid loss of the C5Ph5 ring,
most likely as the anion [C5Ph5]-, the longer-lived
species 1- having a half-life of ≈0.1 s at room temper-
ature. Cleavage of [C5R5]- from analogous 19e rhodium
complexes has been observed previously. The parent
system CpRh(1,5-COD) does not show a reduction prior
to electrolyte breakdown,25 but Cp‡Rh(CO)2 is reduced
with loss of [C5Ph5]-.23 More direct precedent comes
from the studies of Amatore et al.26 on the reduction of
(indenyl)Rh(1,5-COD) (indenyl ) η5-C9H7). In spite of
the general ability of the indenyl ligand to stabilize
hypervalent metals (most likely through hapticity low-
ering27), the anion [(indenyl)Rh(COD)]- decomposes
with a half-life of about 1 s at room temperature.26 The
increased kinetic and thermodynamic stabilities of 19-
electron species containing the C5Ph5 ring as compared
to the C5H5 ring have been previously noted, as has been
the propensity of such systems to decompose by loss of
[C5Ph5]-.23,24
The ring-cleavage reactions of 1-, 2-, and perhaps
[(indenyl)Rh(COD)]- 26 are most simply explicable in
terms of dissociation of the 19-electron radical without
slip-fold distortion of the five-membered ring. The
SOMO orbitals of the Rh complexes 1- and 2- are
expected to be predominantly Rh 4dyz, having significant
antibonding character between the metal and the five-
membered ring.21,22 It is therefore not surprising that
the 19e system loses [C5Ph5]-, especially when one
(23) Connelly, N. G.; Geiger, W. E.; Lane, G. A.; Raven, S. J .; Rieger,
P. H. J . Am. Chem. Soc. 1986, 108, 6219.
(24) Lane, G. A.; Geiger, W. E.; Connelly, N. G. J . Am. Chem. Soc.
1987, 109, 402.
(25) Geiger, W. E. Unpublished data (in THF).
(26) Amatore, C.; Ceccon, A.; Santi, S.; Verpaux, J .-N. Chem.-Eur.
J . 1997, 3, 279.
(27) O’Connor, J .; Casey, C. Chem. Rev. 1987, 87, 307.

Isomers of (η5-C5Ph5)Rh(η4-C8H12
)
Organometallics, Vol. 17, No. 25, 1998 5491
may be accounted for, however, in terms of increasingly
strong interactions between an aliphatic hydrogen of the
cyclooctadiene ring and the metal atom as the oxidation
state of the complex increases.
The geometry of the 1,3-COD ligand in 2 places a C-5
hydrogen (below, left) about 2.53 Å from the rhodium
atom (in contrast, that of the C-8 carbon is 3.03 Å
away).8,28 The former already places the ring hydrogen
within the M-H distance suitable for an agostic inter-
action.29 The empty metal orbital necessary to fully
The 18e anion was not structurally characterized, but
the isoelectronic cobalt complex has no significant M-H
interactions. In proceeding from the 18e anion to the
“16e” cation, the C8 ring becomes more asymmetric to
allow the interacting C-H group to swing in closer to
the metal. A similar set of geometric changes is very
likely operative in the sequence 2 f 2+ f 22+
.
Mech a n ism of F or m a tion of 2. The fact that Cp‡-
Rh(1,3-COD) is the only known complex in which the
cyclooctadiene ligand has been confirmed as being
bonded in a 1,3-fashion to rhodium raises the question
of the reason(s) for its formation and stability. It had
been previously8 suggested that the isomerization of
cyclooctadiene occurs through a 19e intermediate formed
under the reducing conditions (namely, base and alco-
hol) used in the preparation of 2. The present results
indicate that this is not the case.
A more reasonable alternative is the formation of a
cyclooctenyl intermediate, Cp‡Rh(1,3-C8H13)Br, in the
reaction of [Cp‡RhBr2]2 with 1,5-COD. An analogous
complex, (C5Me5)Rh(C8H13)Cl, has been isolated from
the reaction of [(C5Me5)RhCl2]2 with 1,5-COD under
similar conditions (Na2CO3 in ethanol), which ultimately
lead to (C5Me5)Rh(1,5-COD).31 Whereas the stronger
donor C5Me5 stabilizes the Rh(III) intermediate, the C5-
Ph5 ligand does not, leading to ready elimination of HBr.
This elimination would be promoted by the steric effect
of the pentaphenylcyclopentadienyl ligand32 and by the
undoubted stability of complex 2.
“turn on” the agostic interaction is achieved when the
metal center is doubly oxidized (above, right), creating
a three-center, two-electron situation. The 17-electron
monocation is expected to have a smaller agostic inter-
action owing to its three-center, three-electron local
electronic structure. This model predicts a progressively
larger negative shift of the oxidation potentials arising
from the increasingly greater agostic stabilization of the
higher oxidation states. The flexibility of the aliphatic
backbone of the 1,3-COD ligand allows for a shortening
of the Rh-H distance without introducing undue strain
into the C8 ring. The same cannot be said for the 1,5-
COD ligand, which has no aliphatic hydrogens at
suitable distances in 1 and whose relatively rigid
structure will not allow for such interactions in higher
oxidations states.
Su m m a r y
(1) Both the 1,5- and 1,3-isomers of Cp‡Rh(COD)
oxidize to persistent 17-electron monocations, the life-
time of the 1,5-isomer being enormously increased over
that (≈1 s) of the unsubstituted cyclopentadienyl ana-
logue [CpRh(1,5-COD)]+. The 16-electron dications are
detected voltammetrically for both the 1,5- and 1,3-
isomers.
(2) ESR spectra of both 17-electron complexes, 1+ and
2+, are explicable in terms of a SOMO having a smaller
metal dxz contribution than seen previously for the
cobalt analogue [CpCo(1,5-COD)]+. The axially sym-
metric g-tensor of 1+ suggests that this radical retains
approximate C2v symmetry.
Spectroscopic (NMR) confirmation of the proposed
agostic interaction has thus far failed owing to the
paramagnetic nature of 2+ and the short lifetime of 22+
.
Diagnosis of agostic interactions in odd-electron species
is, indeed, rare. Precedent for this model exists, how-
ever, in the impressive structural studies of Ittel et al.,
on cyclooctenyl iron complexes.30 These workers iso-
lated the complexes {[P(OMe)3]3Fe(η3-C8H13)}m in the
oxidation states m ) -1, 0, +1. The orientation of the
C8 ring (shown below) in the formally 16-electron
monocation allows the coordination of an aliphatic-H
with an Fe-H distance of 1.95 Å. The neutral (17e)
(3) The E1/2 values of 2 demonstrate that the higher
oxidation states of the 1,3-isomer are significantly
stabilized (cf. 1,5-isomer) and that the energy of stabi-
lization increases with increasing oxidation state. A
progressively stronger agostic interaction between the
Rh atom and the C5 aliphatic hydrogen of the cyclooc-
tadiene ring is proposed to account for this thermody-
namic stabilization, estimated as 0.62 eV (14 kcal/mol)
complex has a weak agostic interaction, dFeH ) 2.77 Å.
in the dication 22+
.
(28) H atoms were not located in the structure of 2 in ref 8.
H-positions were calculated assuming a C-H bond length of 0.96 Å.
(29) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250,
395.
(30) (a) Ittel, S. D.; Krusic, P. J .; Meakin, P. J . Am. Chem. Soc. 1978,
100, 3264. (b) Williams, J . M.; Brown, R. K.; Schultz, A. J .; Stucky, G.
D.; Ittel, S. D. J . Am. Chem. Soc. 1978, 100, 7407. (c) Harlow, R. L.;
McKinney, R. J .; Ittel, S. D. J . Am. Chem. Soc. 1979, 101, 7496.
(31) Moseley, K.; Kang, J . W.; Maitlis, P. M. Chem. Commun. 1969,
155.
(4) The reduction potential of the 1,3-isomer is ca 0.1
V positive of that of the 1,5-isomer. Both 19e complexes,
1- and 2-, expel [C5Ph5]- quite rapidly.
Ack n ow led gm en t . Research at the University of
Vermont was supported by the National Science Foun-
dation (CHE94-16611).
(32) Adams, H.; Bailey, N. A.; Browning, A. F.; Ramsden, J . A.;
White, C. J . Organomet. Chem. 1990, 387, 305.
OM980788I