Reactions of odd-electron cobaltacycles: Characterization of a persistent 17-electron anionic intermediate in electron-transfer-catalyzed (ETC) substitution reactions

Article abstract of DOI:10.1021/om990246g

The cobaltafluorene complex Cp(PPh₃)CoC₁₂H₈ (1) undergoes an electrochemically irreversible one-electron reduction in THF C_p,c = ca. -2.56 V vs ferrocene) with release of PPh₃ to give a persistent 17-electron Co(II) monoanion, 2 , which was characterized by electrochemistry and by ESR spectroscopy. The unpaired spin density in 2^- is highly delocalized, with the majority being located in the cyclopentadienyl ring rather than the cobaltacyclic fragment. Reoxidation of 2^- in the presence of L (L = phosphines, phosphites) forms Cp(L)CoC₁₂H₈ in high yield. Slow electron-transfer-catalyzed (ETC) substitution processes are found when 1 is electrolyzed in the presence of P(OMe)_3, with the efficiency of the catalysis being dependent on both the relative and absolute concentrations of 1 and P(OMe)_S. The substitution reaction is accounted for by a model in which the anion 2^- is in equilibrium with the 19-electron adducts [Cp(L)CoC₁₂H₈]-, where L = THF, PPh3, P(OMe)₃. Further reduction of 2^- is possible in a reversible one-electron reduction (E_1/2 = -2.80 V) to an 18-electron dianion that is stable in solution. The dianion 2^2- was characterized by ¹H NMR spectroscopy and shown to have C₈ or higher symmetry.

Full text of DOI:10.1021/om990246g

3194
Organometallics 1999, 18, 3194-3200
Rea ction s of Od d -Electr on Coba lta cycles:
Ch a r a cter iza tion of a P er sisten t 17-Electr on An ion ic
In ter m ed ia te in Electr on -Tr a n sfer -Ca ta lyzed (ETC)
Su bstitu tion Rea ction s
Bernadette T. Donovan-Merkert,*,†,‡ Philip H. Rieger,§ and William E. Geiger*,†
Departments of Chemistry, University of Vermont, Burlington, Vermont 05405, and
Brown University, Providence, Rhode Island 02912
Received April 7, 1999
The cobaltafluorene complex Cp(PPh3)CoC12H8 (1) undergoes an electrochemically ir-
reversible one-electron reduction in THF (Ep,c ) ca. -2.56 V vs ferrocene) with release of
PPh3 to give a persistent 17-electron Co(II) monoanion, 2-, which was characterized by
electrochemistry and by ESR spectroscopy. The unpaired spin density in 2- is highly
delocalized, with the majority being located in the cyclopentadienyl ring rather than the
cobaltacyclic fragment. Reoxidation of 2- in the presence of L (L ) phosphines, phosphites)
forms Cp(L)CoC12H8 in high yield. Slow electron-transfer-catalyzed (ETC) substitution
processes are found when 1 is electrolyzed in the presence of P(OMe)3, with the efficiency of
the catalysis being dependent on both the relative and absolute concentrations of 1 and
P(OMe)3. The substitution reaction is accounted for by a model in which the anion 2- is in
equilibrium with the 19-electron adducts [Cp(L)CoC12H8]-, where L ) THF, PPh3, P(OMe)3.
Further reduction of 2- is possible in a reversible one-electron reduction (E1/2 ) -2.80 V) to
1
an 18-electron dianion that is stable in solution. The dianion 22- was characterized by H
NMR spectroscopy and shown to have Cs or higher symmetry.
In tr od u ction
equilibria of eqs 3 and 4 involving conversions between
17-electron and 19-electron systems are in general
difficult to characterize owing to the high inherent
reactivities and lack of NMR properties of the odd-
electron species.
When characterizing an electrode-catalyzed substitu-
tion of PEt3 for PPh3 in cobaltacycle 4 (eq 6), we
postulated the intermediacy of the 17-electron anion 5-.7
Electron-transfer catalysis (ETC) may provide a mild
and therefore preferable route for ligand substitution
processes of organometallic compounds.1-5 Reductively
induced ETC reactions (eq 1) implicitly involve a dis-
-
e
(lig)M-L + L′ cat8 (lig)M-L′ + L
(1)
sociative mechanism (eqs 2-5) in which a key interme-
-
e
Cp(PPh3)CoC4Ph4 + PEt3 cat8
Cp(PEt3)CoC4Ph4 + PPh3 (6)
diate is an electron-deficient complex.6 In the catalytic
(lig)M-L + e- h [(lig)M-L]-
[(lig)M-L]- h [(lig)M]- + L
[(lig)M]- + L′ h [(lig)M-L′]-
(2)
(3)
(4)
Although the observed ETC chemistry argued for
reversibility of the proposed dissociation step of eq 7 to
be reversible, voltammetric measurements were am-
biguous, failing to distinguish between reversible and
irreversible loss of PPh3 from the 19-electron anion 4-.
[(lig)M-L′]- + (lig)M-L h (lig)M-L′ + [(lig)M-L]-
4- h 5- + PPh3
(7)
(5)
cycle given by eqs 2-5, typical of that expected for a
mononuclear complex, this intermediate appears as
[(lig)M]- in one of the propagation steps (eq 3). The
Characterization of the 17 e-/ 19 e- equilibrium of eq 7
was hampered by the instability of 5-, which undergoes
protonation and rearrangement to give the π-complex
CpCo(tetraphenylbutadiene).7
† University of Vermont.
‡ Present address: Department of Chemistry, University of North
Carolina at Charlotte, Charlotte, North Carolina 28223.
§ Brown University.
Given the importance of 17 e-/19 e- equilibria in odd-
electron organometallic reactions,1,8-10 we judged this
(1) Astruc, D. Electron-Transfer and Radical Processes in Transition
Metal Chemistry; VCH: New York, 1995; Chapter 6.
(2) Amatore, C. In Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990; pp 31-34.
(3) Coville, N. J . In Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990; pp 108-141.
(4) Chanon, M., J uliand, M., Poite, J . C., Eds. Paramagnetic Species
in Activation, Selectivity and Catalysis; Kluwer: Dordrecht, The
Netherlands, 1988.
(5) Kochi, J . K. In Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990; Chapter 7.
(6) Leading references to postulated structures of unsaturated
intermediates in ETC reactions may be found in ref 1, pp 434-445.
See also: Poli, R.; Owens, B. E.; Linck, R. G. J . Am. Chem. Soc. 1992,
114, 1303.
(7) Kelly, R. S.; Geiger, W. E. Organometallics 1987, 6, 1432.
(8) Kochi, J . K. In ref 5, pp 201-213.
10.1021/om990246g CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/07/1999

Odd-Electron Cobaltacycles
Organometallics, Vol. 18, No. 16, 1999 3195
proceed for an additional 4 min to ensure completion of the
reaction. Only 0.1 faraday/equiv was required for the elec-
trolysis, which caused the yellow-orange solution to become
lighter yellow. Steady-state voltammograms at a rotating
platinum electrode showed that greater than 95% conversion
to 3 had occurred. Isolation of 3 from the electrolysis solution
was achieved by removing most of the THF under reduced
pressure, adding 20 mL of Et2O, and cooling the mixture to
253 K for 15 min to precipitate most of the supporting
electrolyte. After filtration the filtrate was condensed to
approximately 3 mL and added under N2 to a 2.5 × 15 cm
chromatography column of activity III neutral alumina. After
flushing PPh3 and P(OMe)3 through with hexane, the desired
complex was eluted with Et2O. Removal of ether afforded a
fluffy yellow powder identified as 3. Yield: 48.6 mg (70.5%).
system to be worthy of further study if an intermediate
could be found which was not subject to side reactions
such as the σ/π rearrangement that limited investiga-
tions of 4-/5-. This paper reports results on an analogue
which is indeed impervious to side reactions, at least
over about a 1 h time period at ambient temperatures.
The 17-electron cobaltafluorenyl anion 2- generated by
one-electron reduction of 1 has been characterized by
its electrochemical and ESR spectroscopic properties, by
its conversion to 16- and 18-electron systems 2 and 22-
,
respectively, and by its ability to take part in ETC
substitution reactions. Combination of the ESR results
with those of EHMO calculations shows that there is
surprisingly little delocalization of the odd electron in
2- into the fluorenyl ring. A preliminary account of part
of this work has appeared.11
1
Anal. Calcd: C, 60.00; H, 5.55. Found: C, 60.19; H, 5.54. H
NMR (CDCl3): metallacycle resonances at δ 7.66 (d, 2H), 7.63
(d, 2H), 7.00 (dd, 2H), 6.83 (dd, 2H), 4.97 (s, 5H, Cp), 3.21 [d,
9H, P(OMe)3]. Mass spectrum (CI): parent ion peak at m/e
400, base peak at m/e 277. Cyclic voltammetry (0.5 mM in 0
1M THF/[NBu4][PF6], Pt bead working electrode, SCE refer-
ence electrode): irreversible reduction at Ep,c ) -2.76 V;
partially chemically reversible oxidation at E1/2 ) 0.42 V.
Electr och em ica l Meth od s. Cyclic voltammetry (CV) ex-
periments were performed using a Princeton Applied Research
(PAR) Model 173 potentiostat and a PAR Model 176 current
to voltage converter in conjunction with either a PAR 175
universal programmer or a Hewlett-Packard (HP) Model
3300A function generator equipped with a HP Model 3302A
trigger/phase lock plug-in. For high speed (faster than 200 V/s)
CV experiments, a Wavetec Model 143 20 MHz function
generator and an EI-350 potentiostat built by R. Ensman of
Ensman Instruments, Bloomington, IN, were used. Bulk
electrolysis studies employed the PAR Model 173 together with
a PAR Model 179 digital coulometer. Potentials were moni-
tored using either a Keithley Model 178 or a HP 3435A digital
multimeter.
Exp er im en ta l Section
Electrochemistry experiments employed a conventional
three-electrode arrangement. The working electrode was either
a platinum bead, a disk made of platinum, gold, or glassy
carbon, or a hanging mercury drop electrode. An aqueous SCE
was used as the experimental reference electrode at temper-
atures >273 K; a Ag/AgCl reference electrode was employed
in experiments at lower temperatures. The reference electrode
was always separated from the rest of the cell by a fine frit,
and with the exception of coulometry experiments a Luggin
probe was used in order to minimize iR loss. The electrode
potentials were checked with ferrocene, used as an internal
standard. All potentials reported in this paper are referenced
to the ferrocene potential. Conversion to the SCE scale in THF
requires addition of 0.56 V to the values quoted in this paper.
Electrode pretreatment procedures were as follows: platinum-
bead electrodes were conditioned by holding the tip of the
electrode in the vapors of refluxing nitric acid for 10 min
followed by a cooling period to prevent cracking of the glass
surrounding the electrode tip. The electrode was first rinsed
with distilled water and then soaked in a solution of ferrous
ammonium sulfate in 1 M sulfuric acid for an additional 10
min. It was then rinsed thoroughly with distilled water and
dried with a tissue. The disk electrodes were cleaned by
polishing with diamond pastes (Metadi II, from Beuhler, Ltd.)
of 6, 1, and then 0.25 µm particle size, on an emery cloth. For
ultramicroelectrodes, the polishing substance was Gamma
Micropolish II, 0.05 µm particle size, also from Buehler. After
polishing, the electrode was washed with distilled water and
wiped with a tissue.
P r ep a r a tion s. Gen er a l Con sid er a tion s. Synthetic ma-
nipulations were carried out using standard Schlenk tech-
niques. The dinitrogen gas was purified with RX-11 copper
catalyst (Chemlog), Aquasorb, and activated molecular sieves
(t.h.e. Desiccant, EM Science). Solvents were first dried over
an appropriate drying agent (Et2O, THF, toluene, and hexanes
over potassium; CH2Cl2 over calcium hydride) and then
distilled immediately prior to use.
CpCo(CO)2 and CpCo(PPh3)I2 were prepared by the
literature procedures, except that with the latter a reaction
time of 15 h was used in place of the recommended 3 days.
The preparation of 2,2′-dibromobiphenyl also involved a slight
modification of the published procedure.14 Addition of n-
butyllithium to the solution of o-dibromobenzene was con-
ducted at 183 K rather than at 195 K. At the higher
temperature, an unidentified brown oil was obtained instead
of the desired product. Complex 1 was prepared by the method
of Wakatsuki and co-workers.15
12
13
P r ep a r a tion of 3. The trimethyl phosphite derivative 3
was synthesized by electrochemically reducing a solution of 1
(122.7 mg, 0.228 mmol) in 50 mL of 0.1 M TBAPF6/THF at
-2.00 V in the presence of P(OMe)3 (2.78 g, 22.8 mmol, 100
equiv). Although the i vs t curve indicated completion of the
bulk reduction after 8 min, the electrolysis was allowed to
(9) Trogler, W. C. In Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990; Chapter 9.
(10) Huang, Y.; Neto, C. C.; Pevear, K. A.; Banaszak Holl, M. M.;
Sweigart, D. A.; Chung, Y. K. Inorg. Chim. Acta 1994, 226, 53.
(11) Donovan, B. T.; Geiger, W. E. J . Am. Chem. Soc. 1988, 110,
2335.
(12) Rausch, M. D.; Genetti, R. A. J . Org. Chem. 1970, 35, 3888.
(13) King, R. B. Inorg. Chem. 1966, 5, 82.
(14) Gilman, H.; Gaj, B. J . J . Org. Chem. 1957, 22, 447.
(15) Wakatsuki, Y.; Nomura, O.; Tone, H.; Yamazaki, H. J . Chem.
Soc., Perkin Trans. 2 1980, 1344.
Controlled-potential coulometry experiments were carried
out in a Vacuum Atmospheres drybox under N2. The working
electrode was either a platinum basket or a mercury pool.
When the platinum basket was employed as the working
electrode, a cocylindrical arrangement was used, whereby an

3196 Organometallics, Vol. 18, No. 16, 1999
Donovan-Merkert et al.
Sch em e 1
F igu r e 1. CV scan (v ) 0.2 V s-1) of 5 × 10-4 M 1 in THF/
0.1 M [NBu4][PF6] at 273 K at a Pt electrode.
auxiliary platinum electrode was placed inside a fine fritted
compartment in order to separate the anodic and cathodic
portions of the cell. For low-temperature experiments, the cell
was submerged in a heptane bath which was cooled using a
Flexible Temperature Systems (FTS) Flexi-Cool recirculating
cooler.
Solvents employed in the electrochemistry experiments were
dried and thoroughly degassed on a vacuum line prior to use.
THF (Aldrich, Gold Label) was dried by refluxing for at least
8 h over potassium.
The supporting electrolytes [NBu4][PF6] and [NBu4][CF3-
SO3]16 were recrystallized from 95% ethanol and vacuum-dried
at 373 K.
) -3.34 V). The anodic wave D (which is enhanced
when the negative-going scan is clipped after wave A)
arises from the oxidation of 2-. This scan and the
experiments discussed below show that the reduction
of 1 proceeds cleanly and rapidly through eqs 8 and 9
in an EirrevCrev mechanism.
Oth er Meth od s. 31P NMR spectra were recorded using a
Bruker WM 250 MHz Spectrometer. 1H NMR studies were
carried out with a Bruker 270 MHz spectrometer. In the 1H
NMR experiments, solvent peaks were used to reference the
chemical shifts relative to TMS. In 31P NMR studies, H3PO4
was used as an external standard. ESR spectra were obtained
with a modified Varian E-4 spectrometer using DPPH as an
external standard. Mass spectral analysis was performed on
a Finnigan MAT 4500 series GC/MS system. Microanalysis
was performed by Robertson Laboratory, Inc., Madison, NJ .
Extended Hu¨ckel MO calculations were performed with the
CAChe suite of programs (CAChe Scientific Inc., Beaverton,
OR) using the parameters collected by Alvarez.27 Idealized
bond lengths and bond angles were used for the C5 and C6
rings; the Co-C distances in the CoC4 ring were 1.934 Å, and
the Co-Cp distance was 1.861 Å. The fluorenyl ring system
defined the xz plane, and the Cp ring was parallel to the xy
plane. Earlier calculations on cyclopentadienylcobaltacyclo-
pentadiene, CpCoC4H4,17 showed an energy minimum for the
metallacycle tilted 36° away from the Co-C vector (here taken
as the z axis), 0.16 eV lower than the more symmetric
structure. Accordingly, we extended our EHMO calculations
to tilted conformations. For the cobaltafluorene, the minimum
energy corresponded to the fluorenyl ring in the yz plane,
increasing 0.72 eV for a 12° tilt and 4.47 eV for a 24° tilt. The
additional delocalization afforded by the fluorenyl system is
best exploited in the more symmetrical structure.
1 + e- f 1- Ep,c ) -2.56 V
1- h 2- + PPh3
(8)
(9)
We will also show that 2- may be reduced to the
corresponding 18-electron dianion, 22-, through eq 10
(wave B) or oxidized to the 16-electron metallacycle, 2
(eq 11, wave D). The latter immediately coordinates
available two-electron donors, L (eq 12), to regenerate
a neutral, 18-electron, metallacycle. If L ) PPh3, the
product 2-L of eq 12 is simply the starting material 1.
Other possibilities such as L ) THF or P(OMe)3 will
also be discussed.
2
- + e- h 22- E1/2 ) -2.80 V
(10)
2
- - e- h 2 Ep,a ) -1.26 V
2 + L h 2-L
(11)
(12)
The overall reaction mechanism is indicated in Scheme
1, wherein formation of 2 is assumed to lead first to a
THF adduct owing to the large excess of THF over PPh3.
Replacement of THF by the phosphine ligand would
eventually follow.
Resu lts a n d Discu ssion
F or m a tion of th e 17-Electr on An ion 2-. The
diffusion-controlled18 one-electron19 reduction wave (A)
of 1 has a peak breadth (Ep - Ep/2) of 90 mV, consistent
with an irreversible electron transfer having Rn )
Over view of Red u ctive Beh a vior of 1. Cyclic
voltammetry (CV) provides a simple map of the reduc-
tion of 1 and the consequent reaction products. Wave A
of Figure 1 is the irreversible one-electron reduction of
1 (Ep,c ) -2.56 V at ν ) 0.2 V s-1). Chemically reversible
waves B and C arise from reduction of the two electrode
products, namely the 17-electron metallacycle anion, 2-,
(B, E1/2 ) -2.80 V) and free triphenylphosphine (C, E1/2
(18) A description of the voltammetric diagnostics may be found in:
Geiger, W. E. In Electroanalytical Chemistry; Kissinger, P., Heine-
mann, W., Eds.; Marcel Dekker: New York, 1996; pp 683 ff.
(19) The reduction wave height was approximately equal to the one-
electron height of decamethylferrocene. More definitively, it was about
three-fourths the height of the reversible one-electron oxidation wave
of 2 to 2+, consistent with a one-electron irreversible reduction having
an R value of 0.5, both measurements being made at 253 K. The
oxidation of 2 to 2+ was reversible by cyclic voltammetry, E1/2 ) 0.31
V in THF; bulk oxidation at 218 K produced a stable monocation.
(16) Rousseau, K.; Farrington, G. C.; Dolphin, D. J . Org. Chem.
1972, 37, 3971.
(17) Wakatsui, Y.; Nomura, O.; Kitaura, K.; Morokuma, K.; Yamaza-
ki, H. J . Am. Chem. Soc. 1983, 105, 1970.

Odd-Electron Cobaltacycles
Organometallics, Vol. 18, No. 16, 1999 3197
corresponding to the three principal components of the
g matrix. The spacing of the hyperfine features is as
expected from a second-order perturbation theory of the
simple spin Hamiltonian containing electronic Zeeman
and electron-nuclear hyperfine interaction terms with
coincident principal axes for the g and hyperfine ma-
trixes. The coincidence of the principal axes suggests a
model with C2v or higher symmetry, consistent with the
conclusions based on EHMO calculations.
The spectrum was interpreted by simulations (see
Supporting Information) to give g1 ) 2.330, g2 ) 2.107,
g3 ) 1.996, A1 ) 36.8 × 10-4 cm-1, A2 ) 39.9 × 10-4
cm-1, and A3 < 10 × 10-4 cm-1. If the metal contribution
to the SOMO is a single 3d orbital, we expect the
hyperfine splittings to be given by
A| ) A - (4/7)PFd
A ) A + (2/7)PFd
(13)
(14)
where P ) 282 × 10-4 cm-1 and Fd is the 3d spin density.
21 With A| ) [0((10)] × 10-4 cm-1 and A ) [38.4((1.5)]
× 10-4 cm-1, we have A| - A ) [38.4((1.5)] × 10-4
cm-1, and Fd ) 0.16 ( 0.04. Given the substantial g
value anisotropy, there is almost certainly a sizable
spin-orbit coupling correction which should be applied,
but with a highly delocalized system, such a correction
cannot be made using the g components alone. None-
theless, it is clear that the SOMO is extensively delo-
calized over the carbocyclic fragments with only a
modest contribution from the metal. This conclusion is
entirely consistent with the EHMO calculations, which
predict that the singly occupied molecular orbital (SOMO)
has contributions from Co 3dyz (27.4%) and 4py (4.2%),
from Cp C 2pz (59.6%), and from the fluorenyl C 2pz
(7.6%). The relatively large magnetogyric ratio of cobalt
produces not only large hyperfine splittings but also
relatively large spectral line widths when the anisotro-
pies of those splittings are taken into account. This is
undoubtedly why proton splittings from the Cp ring are
not resolved in spectra of 2-.
F igu r e 2. Steady-state voltammograms at the rotating Pt
electrode recorded (a) before and (b) after bulk reduction
of 5 × 10-4 M 1 in THF/0.1 M [NBu4][PF6] at Eappl ) -2.56
V (scan rate 5 mV s-1, T ) 273 K). The dotted line gives
zero current.
By observing the decay of wave D with time after the
completion of the bulk electrolysis it was determined
that the half-life of the 17-electron metallacycle 2- was
about 1 h at room temperature. Wave D was ultimately
F igu r e 3. EPR spectrum of a frozen solution (T ) 196 K)
of 2- in THF/0.1 M [NBu4][PF6]. The sample was taken
from a 5 × 10-4 M solution of 1 which had been electrolyzed
at Eappl ) -2.56 V.
replaced by a new (irreversible) anodic wave at Ep,a
-1.14 V of unidentified origin.
)
0.52.20 No coupled anodic return feature was observed
even at scan rates of 5000 V/s at a 10 µm diameter Pt
electrode, placing an upper limit on the lifetime of 19-
electron 1- at about 40 µs at 298 K. CV at 213 K (v )
0.2 V s-1) also showed no reversibility for wave A.
Compound 1 was exhaustively electrolyzed at 273 K
with Eappl ) -2.56 V, resulting in a color change from
yellow to orange and a coulomb count of 1.1 faraday/
equiv. A comparison of steady-state voltammograms
(Figure 2) demonstrates that after bulk electrolysis
waves B and D are present as cathodic and anodic
features, respectively, replacing the cathodic wave of 1.
A sample of this electrolyzed solution taken for ESR
analysis gave an intense signal as a frozen solution
(Figure 3) which consisted of two resolved octets (hy-
Con fir m a tion of Liber a tion of P P h 3 in Red u c-
t ion of 1. The postulate that rapid ejection of tri-
phenylphosphine (eq 9) follows formation (eq 8) of the
19-electron metallacycle 1- was confirmed by experi-
ments addressing both the CV and electrolytic time
scales. Regarding the former, the reversible reduction
of the product wave (C) at E1/2 ) - 3.34 V was assigned
to free PPh3 on the basis of its potential matching that
of PPh3 in a control experiment. Addition of a 10-fold
excess of PPh3 to a solution of 1 resulted in a propor-
tional increase of the current for wave C, but no changes
in the other waves were observed. Regarding the
synthetic time scale, a solution of 1 in d8-THF was
sealed under vacuum in an NMR tube which contained
a sidearm with a vacuum-deposited potassium mirror.
The solution was allowed contact with the mirror and
then poured into the tube and monitored by 31P{1H}
7
perfine splitting to 59Co, I ) /2) and a broad singlet
(20) The peak breadth, Ep - Ep/2, is 48 mV/Rn for an electrochemi-
cally irreversible wave, where R is the electrochemical transfer
coefficient.
(21) Morton, J . R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577.

3198 Organometallics, Vol. 18, No. 16, 1999
Donovan-Merkert et al.
spectroscopy. A multiplet present prior to reduction at
63 ppm was replaced by a singlet at -4.8 ppm for free
PPh3.
F or m a tion a n d F a te of 16-Electr on 2. The oxida-
tion of 2- in wave D produces 16-electron 2, which, as
a coordinatively-unsaturated molecule, is expected to
coordinate Lewis bases that might be present. Interest-
ingly, CV scans reversed after transversal through the
oxidation wave D did not reveal the presence of wave A
in the negative-going branch. This means that 2 did not
re-form 1 through eq 12, i.e., by recoordination of PPh3,
at least on the CV time scale. The original compound 1
was re-formed, however, in 65% yield when the oxida-
tion of wave D was accomplished in a bulk electrolysis.
These facts imply that the 16-electron complex 2 reacts
rapidly to give an intermediate which slowly coordinates
PPh3 to re-form 1 (eq 15). Given the makeup of the
F igu r e 4. 1H NMR spectrum of 22- obtained after reduc-
tion of 3 in d8-THF with potassium mirror.
2-THF + PPh3 h 1 + THF
(15)
electrolyte solution, the intermediate is almost certainly
the solvent adduct 2-THF. Despite the much lower
basicity of THF compared to a phosphine, the concen-
tration ratio of about 104 THF/PPh3 undoubtedly makes
the THF adduct the kinetic product in the oxidation of
2-. Experiments in which a 10-fold excess of PPh3 was
present in solution gave the following results: CV, still
no evidence of wave A after scanning through wave D;
bulk re-electrolysis, 75% re-formation of 1. The overall
reduction and reoxidation of 1 is therefore seen to be
chemically reversible; mechanistically speaking, how-
ever, the process occurs through two electrochemically
irreversible processes (eqs 8 and 9 for reduction; eqs 11
and 12 (L ) THF) and 15 for reoxidation) in an EC/
ECC sequence.
Ta ble 1. F or m a l P oten tia ls vs F er r ocen e of
Coba lta cycles in THF /0.1 M [NBu 4][P F 6] a t
Am bien t Tem p er a tu r e
compd
process couple
potential, V
Cp(PPh3)CoC12H8 (1)
redn
oxdn
redn
oxdn
redn
oxdn
oxdn
0/1-
0/1+
-2.56 (irrev, Ep,c
)
1
0.31a
[CpCoC12H8]- (2-)
1-/2- -2.80
2-
1-/0
0/1-
0/1+
0/1+
-1.26 (irrev, Ep,a
)
)
Cp[P(OMe)3]CoC12H8 (3)
-2.76 (irrev, Ep,c
3
0.42b
0.26
Cp(PEt3)CoC12H8
a
Chemically irreversible at 298 K (v ) 0.2 V/s), increasingly
reversible at lower temperatures, and finally fully reversible at
b
253 K (using [NBu4][CF3SO3] as supporting electrolyte. Limited
chemical reversibility at v ) 0.2 V/s.
1
Ta ble 2. Com p a r ison of H Ch em ica l Sh ifts for
F or m a tion of th e 18-Electr on Meta lla cyclic Di-
a n ion 22-. The existence and possible persistence of the
18-electron complex 22- suggested by the reversibility
of wave B was explored. Generated through bulk
electrolysis, the dianion decayed to unknown products.
It was successfully produced, however, through alkali-
metal reductions of either 1 or the trimethyl phosphite
Cp [P (OMe3)]CoC12H8 (3) a n d [Cp CoC12H8]2- (22-) in
d 8-THF (Va lu es in p p m Rela tive to TMS)
1
complex 3. The H NMR spectra generated from reduc-
tion of 3 by a K mirror were more readily interpreted
owing to lack of interference from the reduction products
of PPh3, so we now turn to that data.
proton
δ(3)
δ(22-
)
δ(3) - δ(22-
)
1
2
3
4
7.64
6.71
6.87
7.37
4.94
8.28
6.15
6.27
7.18
4.19
-0.64
0.56
0.60
0.19
0.75
When the P(OMe)3 adduct 3 underwent prolonged
contact with a potassium mirror in d8-THF under a
sealed vacuum, the 1H NMR spectrum of 3 was replaced
by one assigned to 22- (Figure 4). Assignments made
on the basis of decoupling and NOE experiments
(described in detail in Supporting Information) are given
in Table 2 for 3 and 22-. As expected for a complex of
increased negative charge, significant increases in
shielding are seen for almost all the protons in the
fluorenyl ring of 22-. An exception is proton 1, shifted
to 8.28 ppm in 22- from 7.64 ppm in 3. This position is
likely to be dominated by the aniosotropy of the Cp ring,
leading to a net deshielding of this proton. These data
confirm that charge is delocalized into the fluorenyl ring
in 22- with the position farthest from the metal (proton
4) having the smallest charge increase. The largest
increase in shielding, however, is associated with the
Cp ring, for which the chemical shift increases by 0.75
ppm. This is consistent with the MO calculations (see
Cp
above), which indicate a greater involvement of the Cp
ring than the fluorenyl framework in the redox anion-
(s). The dianion retains at least Cs symmetry, a conclu-
sion also consistent with the interpretation of the ESR
spectrum of the 17-electron anion.
Rea ction of 2- w ith P (OMe)3. Complex 1 undergoes
electrocatalytic substitution reactions when it is reduced
in the presence of phosphines or phosphites that are
more basic than PPh3. This implies that the 17 e-/19
e- equilibrium of eq 16 is indeed reversible, even though
2
- + L h [2-L]-
(16)
it may lie far to the left. To complete the electrocatalytic
cycle, the 19 e- adduct [2-L]- must be oxidized to the

Odd-Electron Cobaltacycles
Organometallics, Vol. 18, No. 16, 1999 3199
18-electron complex 2-L by either the electrode or by a
homogeneous oxidant (eq 17). We show below that the
[2-L]- + ox h 2-L + red
(17)
homogeneous reaction route dominates under most
conditions. Preceding that discussion, however, we
briefly describe the electrochemical behavior of the
trimethyl phosphite complex 3, which is formed in the
cathodically initiated ETC substitution reaction of 1 in
the presence of P(OMe)3.
CV scans of 3 obtained under the same conditions as
described in Figure 1 revealed a reductive mechanism
the same as that observed for 1. The reduction of 3 is
more negative (Ep,c ) - 2.80 V) than that of 1, but the
other CV features (waves C and D) are identical. Bulk
reduction of 3 (1 faraday/equiv) gave the expected
solution of 2- resulting from ejection of P(OMe)3 from
3-.
E lect r oca t a lyt ic Su b st it u t ion R ea ct ion s of 1.
When 1 is reduced in the presence of more nucleophilic
phosphines or phosphites (L), substitution products (L
for PPh3) are observed; the substitution reaction is
electrocatalytic with an efficiency that depends on the
concentration of the reactant 1. These experiments give
strong confirmation of the importance of 17 e-/19 e-
equilibria (eq 16) in the substitution reactions.
Complex 1 is unchanged for several hours in the
presence of a 10-fold excess of P(OMe)3. Electrolysis of
the mixture at -2.56 V gives 2- (Figure 5b, anodic wave
at -1.26 V, ca. 75% yield)22 and cathodic features at
ca. -2.80 V identical with those of the trimethyl
phosphite complex 3 (25% yield). The catalytic efficiency
was low, about 1.2, as implied by the coulomb count,
napp ) 0.83. Replicating the experiment at higher
concentrations of P(OMe)3, but keeping the concentra-
tion of 1 at 0.5 mM, did not increase the efficiency of
the electrocatalysis (Table 3). Scheme 2 accounts for this
inefficiency. In it, the 17-electron metallacycle 2- is seen
to be in equilibrium with three possible 19-electron
anions: [2-THF]-, [2-PPh3]-, and [2-P(OMe)3]-. The
oxidation of [2-P(OMe)3]- required to form the neutral
substitution product 3 may occur either at the electrode
(Eappl ) -2.56 V is sufficient to reoxidize [2-P(OMe)3]-
but not [2-PPh3]-) or in homogeneous solution. Any
homogeneous oxidation of [2-PPh3]-, re-forming 1, leads
to no net chemical change through this branch of
Scheme 2 as long as the applied potential remains
sufficiently negative to reduce any 1 that is formed.
If the solution containing 2- and the two competing
Lewis bases is exposed to a Pt-gauze electrode with a
potential sufficient to oxidize 2- (e.g., Eappl ) -1.05 V)
in a “reverse electrolysis”, then the yield of 3 rises
dramatically. Figure 5 shows the voltammograms ob-
tained in such an experiment. The top two scans were
recorded after the cathodic electrolysis of 1 in the
presence of a 20-fold excess of P(OMe)3; the cathodic
wave at E1/2 ca. -2.8 V arises from the reduction of 3,
whereas the anodic wave at -1.2 V is from 2-. No
unreacted 1 is detected. The bottom two scans were
recorded after reverse electrolysis at Eappl ) -0.9 V and
F igu r e 5. (top) Scans after bulk reduction of 5 × 10-4
M
1 at Eappl ) -2.56 V in THF: (c) CV scan (v ) 0.2 V s-1) of
pure 1 with no P(OMe)3 present; (a,b) RPE and CV scans
after electrolysis in the presence of 1 × 10-2 M P(OMe)3.
(bottom) RPE (d) and CV (e) scans after “reverse electroly-
sis”, i.e., back oxidation of 2- at Eappl ) -0.9 V in the
presence of a 20-fold excess of P(OMe)3 showing virtually
exclusive formation of 3.
Ta ble 3. Efficien cies of Electr oca ta lytic
Su bstitu tion of P P h 3 by P (OMe)3 th r ou gh Bu lk
Ca th od ic Red u ction of 1 in th e P r esen ce of
P (OMe)3 in THF a t Am bien t Tem p er a tu r es
concn of P(OMe)3 coulomb count,
chain
concn of 1 (mM)
(mM)
faraday/equiva efficiency
0.5
0.5
4.6
4.6
10
52
4.6
456
0.83
0.91
0.37
0.10
1.25
1.10
2.70
10.0
a
Ratio of coulombs per equivalent of 1 required to achieve >95%
conversion to 3.
Sch em e 2
show that 3 is almost the exclusive product when 2- is
oxidized in the presence of excess P(OMe)3.
That the required oxidation step can also be ac-
complished with a chemical oxidant is implied when
considering that the catalytic efficiency of the substitu-
(22) The use of observed currents to calculate “percent yields” of
the electrochemical reactions assumes similar diffusion coefficients for
reactants and products.

3200 Organometallics, Vol. 18, No. 16, 1999
Donovan-Merkert et al.
tion reaction rose by an order of magnitude when more
concentrated solutions of 1 were employed (Table 3). In
these experiments the reactant 1 acts as the oxidant in
Scheme 2 and in so doing converts another mole of
reactant to the unsaturated intermediate 2- required
for the substitution. The potentials involved suggest
that 1 can reduce 19-electron [2-L]- but not 17-electron
2-. The feasibility of 1 acting as an oxidant was proven
by adding 0.2 equiv of 1 at open circuit to an electrolyzed
solution containing 2- and 10 equiv of P(OMe)3. After
about 20 min the current due to 2- had decreased and
that due to 3 had increased, both by about one-third.
After 0.8 equiv of 1 was added and the mixture was
stirred for 2 h, the yield of 3 doubled and there was no
voltammetric evidence of 1.
The oxidation chemistry can also be performed by an
oxidant unrelated to those appearing in the 17 e-/19 e-
equilibria. This was demonstrated using 9-fluorenone
(FL), which is reduced at E1/2 ) -1.88 V to a radical
anion. Although FL is a very weak oxidizing agent, it
is capable of oxidizing the 19-electron systems [2-L]- of
Scheme 2. FL was added at open circuit to a solution
containing 2- and excess P(OMe)3. After addition of 0.21
equiv of FL the amount of 3 increased at the expense of
2- and the voltammetric wave for FL at E1/2 ) -1.88 V
was completely anodic, indicating that the fluorenone
was present in the reduced form as [FL]-. This trend
continued as more FL was added until all the 2- had
reacted. Further addition of FL gave a cathodic contri-
bution for the fluorenone wave, signaling the end of the
“redox titration” and the absence of any 17 e- cobalta-
cycle.
based ETC reactions may also be rapid and efficient,24
other cases of very slow catalysis are known.25 It seems
clear that the equilibrium constant and kinetics of the
17-electron/19-electron equilibrium are the key factors
in determining which of these scenarios is followed.
Our studies of the cobaltacyclic system allow some
insights to be gained regarding the properties of a 17-
electron mononuclear intermediate and its role in ETC
processes. The property of the intermediate 2- respon-
sible for our ability to monitor it, namely its kinetic and
thermodynamic stability, is also responsible for its
inefficiency as a propagating agent in the electron-
transfer chain. One aspect of this stability is seen in
the apparent positions of the equilibria of Scheme 2,
which must strongly disfavor both the phosphine and
phosphite adducts. In the present system the 17-electron
anion 2- constitutes a “resting state” in the propagation
sequence and a productive exit from this state (i.e., one
which produces 3) depends on the presence of an
oxidant. The stability of 2- may be attributed in part
to its ability to weakly coordinate a THF molecule
(Scheme 2), a property arising from the low (ca. 30%)
metallic character of the SOMO in the formal 17-
electron anion. Our results stand in contrast to another
17 e-/19 e- system involving the ETC substitution of
PPh3 for I- in CpFe(CO)2I.26 In the iron system a set of
competitive equilibria are involved which are analogous
to those of Scheme 2, but the 17 e- complex (the Fp
radical) is unstable, subject to rapid dimerization to
form [Cp2Fe(CO)2]2, precluding the solution character-
ization of the reactive intermediate. The comparatively
greater stability of 2-, while allowing for fuller charac-
terization of the intermediate, is accompanied by a lower
electrophilicity which lowers the efficiency of the ETC
process.
Con clu sion s
Reductively induced ETC reactions with high turn-
over numbers are thought to require an intermediate
which has (i) coordinative unsaturation and (ii) at least
a modest ability to coordinate an incoming nucleophile.
In polynuclear complexes, cleavage of metal-metal or
metal-ligand bonds may generate the required 17-
electron center. In such systems the distribution of
charge over several metal centers apparently favors the
coordination of the nucleophile and these ETC reactions
tend to be quite efficient.23 Although some mononuclear-
Ack n ow led gm en t. We are grateful to the National
Science Foundation for support of this work.
Su p p or tin g In for m a tion Ava ila ble: The NOE spectrum
of 22- and discussion of assignments and simulation of frozen
ESR spectrum of 2-. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM990246G
(24) (a) Neto, C. C.; Kim, S.; Meng, Q.; Sweigart, D. A.; Chung, Y.
K. J . Am. Chem. Soc. 1993, 115, 2077. (b) Huang, Y.; Neto, C. C.;
Pevear, K. A.; Banaszak Holl, M. M.; Sweigart, D. A.; Chung, Y. K.
Inorg. Chim. Acta 1994, 226, 53.
(25) (a) Miholova, D.; Vlcek, A. A. J . Organomet. Chem. 1985, 279,
317. (b) Olbrich-Deussner, B.; Kaim, W. J . Organomet. Chem. 1988,
340, 71.
(26) Liu, Z.; Gipson, S. L. J . Organomet. Chem. 1998, 553, 269.
(27) Alvarez, S. Tables of Parameters for Extended Hu¨ckel Calcula-
tions; University of Barcelona: Barcelona, Spain, 1989.
(23) (a) Hinkelmann, K.; Mahlendorf, F.; Heinze, J .; Schacht, H.-
T.; Field, J . S.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1987,
26, 352. (b) Arewgoda, M.; Rieger, P. H.; Robinson, B. H.; Simpson, J .;
Visco, S. J . J . Am. Chem. Soc. 1982, 104, 5633. (c) Holzmeier, P.; Kisch,
H.; Kochi, J . K. J . Organomet. Chem. 1990, 382, 129. (d) Fassler, T.;
Huttner, G.; Gruenauer, D.; Fiedler, S.; Eber, B. J . Organomet. Chem.
1990, 381, 409.