Metallocene complexes of iron and cobalt derived from the 4,4′-bis(η5-cyclopentadienyl)octafluorobiphenyl ligand

Article abstract of DOI:10.1021/om990321n

The reaction of decafluorobiphenyl with 10 equiv of sodium cyclopentadienide (NaCp) in THF (65°C, 2 h) afforded 4,4′-bis(cyclopentadienyl)octafluorobiphenyl (1) as a mixture of double-bond isomers in 72percent yield after workup and purification. Subsequent treatment with sodium hydride in THF (25°C, 6 h) afforded disodium 4,4′-bis(cyclopentadienyl)octafluorobiphenyl (2). Reaction of NaCp, ligand 2, and FeBr₂ (21:1:10 mol ratio) in THF afforded mostly ferrocene as well as the diiron complex CpFe(η⁵-C₅H₄-4,4′-C₆F ₄C₆F₄-η⁵-C₅H ₄)FeCp (3), the linear triiron complex Cp[Fe(η⁵-C₅H₄-4,4′-C ₆F₄C₆-F₄-η⁵C ₅H₄)]₂FeCp (4), and the linear tetrairon complex Cp[Fe(η⁵-C₅H₄-4,4′-C ₅F₄C₆F₄-η⁵-C ₅H⁴)]₃FeCp (5), which were isolated by liquid chromatography. Solution voltammetric analysis of 3 showed a single, reversible two-electron oxidation at +200 mV, 4 showed two reversible oxidations (2:1 ratio) at +180 and +372 mV, respectively, and 5 snowed two reversible oxidations (2:2 ratio) at +192 and +382 mV, respectively (all vs Cp₂Fe|Cp₂Fe⁺). A similar reaction of NaCp, ligand 2, and CoBr₂ in THF was worked up with air and dilute aqueous HCl followed by aqueous KPF₆ to afford a mixture of cobaltocenium hexafluorophosphates, from which [Cp₂Co]⁺PF₆^- and the dicobalt complex [CpCo(η⁵-C₅H₄-4,4′-C ₆F₄C₆F₄-η⁵-C ₅H₄)CoCp]²⁺[PF₆^-] ₂ (6) were isolated by liquid chromatography. CV analysis of 6 showed a single, reversible Co^II|Co^III wave at +232 mV relative to Cp₂Co|Cp₂Co⁺. These voltammetric data suggest that no significant electronic communication exists between two metals bridged by this conjugated dibasic ligand (2). A crystal structure of diiron complex 3 demonstrated the conformational preferences of the CpC₆F₄C₆F₄Cp ligand in the solid state with respective torsional angles of 37.5°, 50.1°, and 32.1°.

Full text of DOI:10.1021/om990321n

3610
Organometallics 1999, 18, 3610-3614
Meta llocen e Com p lexes of Ir on a n d Coba lt Der ived fr om
th e 4,4′-Bis(η5-cyclop en ta d ien yl)octa flu or obip h en yl
Liga n d 1
Carl B. Hollandsworth, W. Gary Hollis J r.,*,2 Carla Slebodnick,3 and
Paul A. Deck*,3
Department of Chemistry, Roanoke College, Salem, Virginia 24153, and Department of
Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212
Received May 3, 1999
The reaction of decafluorobiphenyl with 10 equiv of sodium cyclopentadienide (NaCp) in
THF (65 °C, 2 h) afforded 4,4′-bis(cyclopentadienyl)octafluorobiphenyl (1) as a mixture of
double-bond isomers in 72% yield after workup and purification. Subsequent treatment with
sodium hydride in THF (25 °C, 6 h) afforded disodium 4,4′-bis(cyclopentadienyl)octafluo-
robiphenyl (2). Reaction of NaCp, ligand 2, and FeBr2 (21:1:10 mol ratio) in THF afforded
mostly ferrocene as well as the diiron complex CpFe(η5-C5H4-4,4′-C6F4C6F4-η5-C5H4)FeCp
(3), the linear triiron complex Cp[Fe(η5-C5H4-4,4′-C6F4C6F4-η5-C5H4)]2FeCp (4), and the linear
tetrairon complex Cp[Fe(η5-C5H4-4,4′-C6F4C6F4-η5-C5H4)]3FeCp (5), which were isolated by
liquid chromatography. Solution voltammetric analysis of 3 showed a single, reversible two-
electron oxidation at +200 mV, 4 showed two reversible oxidations (2:1 ratio) at +180 and
+372 mV, respectively, and 5 showed two reversible oxidations (2:2 ratio) at +192 and +382
mV, respectively (all vs Cp2Fe|Cp2Fe+). A similar reaction of NaCp, ligand 2, and CoBr2 in
THF was worked up with air and dilute aqueous HCl followed by aqueous KPF6 to afford a
mixture of cobaltocenium hexafluorophosphates, from which [Cp2Co]+PF6- and the dicobalt
complex [CpCo(η5-C5H4-4,4′-C6F4C6F4-η5-C5H4)CoCp]2+[PF6-]2 (6) were isolated by liquid
chromatography. CV analysis of 6 showed a single, reversible CoII|CoIII wave at +232 mV
relative to Cp2Co|Cp2Co+. These voltammetric data suggest that no significant electronic
communication exists between two metals bridged by this conjugated dibasic ligand (2). A
crystal structure of diiron complex 3 demonstrated the conformational preferences of the
CpC6F4C6F4Cp ligand in the solid state with respective torsional angles of 37.5°, 50.1°, and
32.1°.
In tr od u ction
zation of synthetic methods and for the investigation of
intramolecular (intrachain) communication among the
covalently linked ferrocene units.
Steady interest in the synthesis of multimetallic
complexes derived from redox-active transition-metal
complexes is evident in the recent literature. Ferrocene-
containing polymers in which the complexed iron atoms
are integral to the principal chain have received the
most attention.4 The application of the FeII|FeIII couple
as a charge carrier in conducting organic filaments or
films is among the long-range objectives of that re-
search. For this reason, homobimetallic complexes in
which two ferrocene units are linked by simple organic
spacers serve as important benchmarks for the optimi-
Most synthetic approaches to oligoferrocenes are
based on coupling, condensation, or ring-opening reac-
tions of functional substituents attached to the ferrocene
core.5 The step-growth oligomerization of metal halides
such as FeCl2 with “dibasic” ligands (having two teth-
ered cyclopentadienyl groups) is somewhat less common,
partly because molecular weight and end group func-
tionality are difficult to control.6 Yet, the persistent
advantage of the dibasic ligand approach is flexibility
with respect to the tethering group between the ligands
as well as the transition metals and ligated metal
fragments that can be coordinated.
(1) Part of this work was communicated orally: Hollandsworth, C.
B.; Hollis, W. G., J r.; Deck, P. A.; Slebodnick, C. 217th National
Meeting of the American Chemical Society, Anaheim, CA, March 21-
25, 1999. Abstract CHED 146.
(2) Roanoke College. E-mail for WGH: hollis@roanoke.edu.
(3) Virginia Tech. E-mail for PAD: pdeck@vt.edu.
(4) (a) Rulkens, R.; Lough, A. J .; Manners, I.; Lovelace, S. R.; Grant,
C.; Geiger, W. E. J . Am. Chem. Soc. 1996, 118, 12683-12695. (b)
Manners, I. Angew. Chem., Intl. Ed. Engl. 1996, 35, 1603-1621. (c)
Manners, I. Adv. Organomet. Chem. 1995, 37, 131-168. (d) Barlow,
S.; O’Hare, D. Chem. Rev. 1997, 97, 637. (e) Hilbig, H.; Hudeczek, P.;
Ko¨hler, F. H.; Xie, X.; Bergerat, P.; Kahn, O. Inorg. Chem. 1998, 37,
4246. (f) Marder, T.; Fyfe, H.; Mlekuz, M.; Stringer, G.; Taylor, N.
NATO ASI Series E; Laine, R., Ed.; Kluwer: Dodrecht, Netherlands,
1992; Vol. 206, p 331. (g) Patoux, C.; Coudret, C.; Launay, J .-P.;
J oachim, C.; Gourdon, A. Inorg. Chem. 1997, 36, 5037.
(5) (a) Hradsky, A.; Bildstein, B.; Schuler, N.; Schottenberger, H.;
J aitner, P.; Ongania, K.-H.; Wurst, K.; Launay, J .-P. Organometallics
1997, 16, 392-402. (b) Hirao, T.; Kurashina, M.; Aramaki, K.;
Nishihara, H. J . Chem. Soc., Dalton Trans. 1996, 2929-2933. (c) Ribou,
A.-C.; Launay, J .-P.; Sachtleben, M. L.; Li, H.; Spangler, C. W. Inorg.
Chem. 1996, 35, 3735-3740. (d) Stanton, C. E.; Lee, T. R.; Grubbs, R.
H.; Lewis, N. S.; Pudelski, J . K.; Callstrom, M. R.; Erickson, M. S.;
McLaughlin, M. L. Macromolecules 1995, 28, 8713-8721.
(6) (a) Hudeczek, P.; Ko¨hler, F. H. Organometallics 1992, 11, 1773.
(b) Delville, M.-H.; Robert, F.; Gouzerh, P.; Linare`s, J .; Boukheddaden,
K.; Varret, F.; Astruc, D. J . Organomet. Chem. 1993, 451, C10. (c)
Atzkern, H.; Huber, B.; Ko¨hler, F. H.; Mu¨ller, G.; Mu¨ller, R. Organo-
metallics 1991, 10, 238.
10.1021/om990321n CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/13/1999

Metallocene Complexes of Iron and Cobalt
Organometallics, Vol. 18, No. 18, 1999 3611
Ta ble 1. Cr ysta llogr a p h ic Da ta for th e Diir on
Com p lex (3)
We now report the synthesis of a dibasic ligand (2) in
which two cyclopentadienyl anions are tethered by the
rigid-rod-shaped 4,4′-octafluorobiphenylene moiety.
Arylene groups are chemically stable yet fully conju-
gated, improving the possibility of intermetallic elec-
tronic communication.7 Using this “dibasic” ligand in
conjunction with a large excess of the unsubstituted Cp
anion as an added chain-end “capping” reagent, we
obtained homobimetallic complexes of the type CpMCp-
tether-CpMCp, where M ) Co(+) and Fe and the tether
is 4,4′-octafluorobiphenylene. The diiron complex 3 was
analyzed by solution voltammetry and structurally
characterized by single-crystal X-ray diffraction. Triiron
and tetrairon complexes having the general formula
CpFe[Cp-tether-CpFe]nCp (n ) 2, 3) were also isolated.
empirical formula
fw
C32H18F8Fe2
666.16
diffractometer
cryst dimens (mm)
color, habit
cryst system
a (Å)
Siemens P4
0.4 × 0.4 × 0.08
red, rod
monoclinic
7.593(2)
b (Å)
c (Å)
18.420(8)
18.428(3)
101.88(2)
2522.2(9)
P21/c (No. 14)
4
â (deg)
V (Å)3
space group
Z
D
n
calc (Mg m-3
)
1.754
1.232
F000
1336
λ (Mo KR, graphite) (Å)
temp (K)
0.71073
298(2)
Exp er im en ta l Section
θ range for collection (deg)
no. of reflns colld
1.58-25.99
6532
Gen er a l Con sid er a tion s. All reactions were carried out
using routine inert-atmosphere techniques. NMR spectra were
recorded on either a Varian U-400 or a Bruker AM-360
instrument. Carbon NMR spectra were referenced to the
solvent,8 proton NMR spectra were referenced to residual
solvent isotopomers,8 and fluorine NMR spectra were refer-
enced to external C6F6 in CDCl3 (-163.0 ppm). High-resolution
mass spectrometric analyses were performed by the Nebraska
Center for Mass Spectrometry (Lincoln, NE), under fast-atom
bombardment conditions using a 3-nitrobenzoic acid matrix
and analyzing for positive ions. Elemental microanalysis was
performed by Oneida Research Services (Whitesboro, NY) or
Desert Analytics (Tucson, AZ). Solvents were purified by
established methods.9 FeBr2 and KPF6 were used as received
from Aldrich. CoBr2 was prepared from the commercial
hydrate (Fisher) by drying for several hours under vacuum at
150 °C to afford a bright green powder. Decafluorobiphenyl
was used as received from Lancaster.
Cr ysta llogr a p h ic Stu d ies. A red needle of the diiron
complex 3, grown by slow evaporation of a hexanes solution,
was attached to a glass fiber with epoxy and mounted on the
goniometer of a Siemens (Bruker) P4 diffractometer. Unit cell
parameters were determined by least-squares refinement of
40 reflections that had been automatically centered on the
diffractometer.10 Intensity data were collected, processed,10 and
corrected for absorption.11 The structure was solved by direct
methods and refined using the SHELXTL-PC v5.03 program
package.11 The Laue symmetry and systematic absences were
consistent with the monoclinic space group P21/c. Hydrogen
atoms were refined independently using an anisotropic model.
The program package SHELXTL-PC was used for the ensuing
molecular graphics generation.11 Data collection and structure
refinement information for complex 3 is summarized in Table
1.
no. of indep reflns
abs corr method
max, min transm
4971
integration
0.7351, 0.6910
4969/0/451
R1 ) 0.0678, wR2 ) 0.1003
R1 ) 0.0385, wR2 ) 0.0883
1.079
no. of data/restrts/params
R indices (all data)
final R indices [I > 2σ(I)]
GoF on F2
largest diff peak and hole (e Å-3
)
0.406, -0.475
Ta ble 2. Electr och em ica l Da ta for Com p lexes 3-6
complex
MII/MIII oxidation potential, mVa
3
4
5
+200(5)b
+180(5), +372(5) (2:1 ratio)b
+192(5), +382(5) (1:1 ratio)b
+346(5)b,c
(C6F5Cp)2Fe
6
+232(5)d
[(C6F5Cp)2Co]+
+400(5)d,e
Standard errors in parentheses. b Relative to [Cp2Fe]/[Cp2Fe]+
a
in dichloromethane. c Data from ref 12. Relative to [Cp2Co]/
[Cp2Co]+ in acetonitrile. Values relative to [Cp2Fe]/[Cp2Fe]+ may
be obtained by subtracting 1320 mV, as the oxidation potentials
of Cp2Co and Cp2Fe were reported to be -910 and +410 mV vs
SCE, respectively, in acetonitrile (see ref 21a-c). e Data from ref
16.
d
manner, using acetonitrile as the solvent. The apparatus was
a BioAnalytical Systems BAS-100B automated digital poten-
tiostat with a Pt disk working electrode, a Pt wire auxiliary
electrode, and a Ag/AgCl reference electrode. Cyclic voltam-
metric (CV) sweeps for substituted ferrocenes were typically
initialized at +100 mV, scanned to +1100 mV, and reversed
to +100 mV, at a scan rate of 100 mV s-1. CV sweeps for
substituted cobaltocenium hexafluorophosphates were typi-
cally initialized at -100 mV, scanned to -1300 mV, and
reversed to -100 mV, at a scan rate of 100 mV s-1. |Eox - Ered
|
Electr och em ica l Mea su r em en ts. Single-sweep cyclic and
Osteryoung square wave voltammetric data were obtained for
the sparingly soluble substituted ferrocenes 3-5 at nominal
concentrations roughly estimated to be about 2 µM in CH2Cl2
using 0.10 M [n-Bu4N][PF6] as the electrolyte and activated
alumina as an internal desiccant. The analyses of cobaltoce-
nium hexafluorophosphates were carried out in a similar
were less than 70 mV, and Ic/Ia was within 10% of unity, both
indicators of well-behaved, substantially reversible couples.
Shifts in oxidation potential of substituted ferrocenes rela-
tive to internal Cp2Fe were obtained by Osteryoung square
wave voltammetry (OSWV). The cell voltage was typically
swept from -200 to +1100 mV with a step resolution of 4 mV,
a square wave amplitude of 25 mV, and a frequency of 15 Hz
for a total of 256 data points. Shifts in reduction potential of
substituted cobaltocenium hexafluorophosphates relative to
Cp2Co+ were similarly determined by OSWV using a sweep
from -100 to -1300 mV. Internal resistance compensation was
applied to all voltammetric experiments. Electrochemical
potentials obtained by OSWV are collected in Table 2.
(7) Southard, G. E.; Curtis, D. M. Organometallics 1997, 16, 5618.
(8) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds; Wiley: New York, 1991; pp
223-4 and 261.
(9) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(10) XSCANS v2.1; Siemens Analytical X-ray Instruments: Madi-
son, WI, 1994.
(11) Sheldrick, G. M. SHELXTL v5.03 (PC version) An Integrated
System for Solving, Refining, and Displaying Crystal Structures from
Diffraction Data; Siemens Analytical X-ray Instruments: Madison, WI,
1995.
4,4′-Bis(cyclop en ta d ien yl)octa flu or obip h en yl (1). To a
stirred mixture of sodium cyclopentadienide (8.8 g, 0.10 mol),
sodium hydride (1.2 g, 0.050 mol), and THF (200 mL) was
added decafluorobiphenyl (4.0 g, 0.012 mol) in one portion; gas

3612 Organometallics, Vol. 18, No. 18, 1999
Hollandsworth et al.
evolved. The mixture was stirred at ambient temperature for
15 min and then under reflux for 2 h. The resulting dark brown
mixture was cooled, and the solvent was removed under
reduced pressure. The brown residue was taken up in benzene
(300 mL) and hydrolyzed under a strong counterstream of
nitrogen by cautious addition of water (100 mL), during which
gas evolved vigorously. The biphasic mixture was stirred for
0.5 h to ensure complete hydrolysis, suction-filtered through
Celite, and then separated. The aqueous layer was extracted
with 100 mL of hexanes. The organic layers were combined,
washed with water (3 × 50 mL), dried over anhydrous
magnesium sulfate, filtered, and evaporated to afford 5.0 g
(0.012 mol, 98%) of a yellow residue. Trituration with pentane
(2 × 10 mL) afforded 3.7 g (8.7 mmol, 72%) of an off-white
solid. 1H NMR and 19F NMR (CDCl3) confirmed the pure
product as a mixture of cyclopentadiene double-bond isomers.
The ratio of double-bond isomers obtained in individual
experiments varies but is typically 90:10. Major isomer: 1H
NMR δ 3.62 (s, 2 H), 6.73 (s, 2 H), 7.48 (m, 1 H) and 19F NMR
δ -141.26 (m, 4 F), -141.72 (m, 4 F). Minor isomer: 1H NMR
δ 3.38 (s, 2 H), 6.63 (m, 1 H), 6.91 (m, 1 H), 7.01 (m, 1 H) and
19F NMR δ -140.70 (m, 4 F), -142.15 (m, 4 F). Anal. Calcd
for C22H10F8: C, 61.96; H, 2.37. Found: C, 62.19; H, 2.12.
[4,4′-Bis(cyclop en t a d ien yl)oct a flu or ob ip h en yl]d iyl-
d isod iu m (2). A mixture of neutral ligand (1, 3.1 g, 7.3 mmol),
NaH (2.0 g, 83 mmol), and THF (200 mL) was stirred at 25 °C
for 6 h. Unreacted NaH was removed by filtration, and the
filtrate was evaporated. The resulting orange residue was
dried under vacuum (50 °C, 0.05 Torr) for 12 h, washed with
pentane (100 mL), collected on a filter, and dried under
vacuum to afford 3.3 g (7.0 mmol, 96%) of a bright yellow solid.
1H NMR (THF-d8): δ 6.59 (m, 4 H), 5.98 (t, J ) 2.9 Hz, 4 H);
residual THF was observed, for which reason combustion
The red-orange band remaining on the alumina column was
eluted with ethyl acetate and evaporated to afford a red solid
(47 mg) containing mostly 4 as determined by 1H NMR. About
20 mg of the latter substance was subjected to liquid chroma-
tography on silica gel (15 × 2 cm), eluting with 25% dichlo-
romethane in hexanes. After a colorless forerun, eight 25 mL
orange fractions were collected, evaporated, and analyzed by
1H NMR; fractions 1-3 were combined to afford an additional
9.7 mg of 4; fractions 5 and 6 were combined to afford 3.4 mg
of 5. Data for 5: 1H NMR (CDCl3): δ 4.95 (m, 4 H), 4.93 (m,
4 H), 4.83 (m, 4 H), 4.52 (m, 8 H), 4.47 (t, J ) 1.9 Hz, 4 H),
4.19 (s, 10 H). 19F NMR (CDCl3): δ -140.63 (br m, 8 F),
-140.88 (br m, 4 F), -141.05 (br m, 4 F), -141.24 (m, 8 F).
FAB-HRMS C76H34F24[56Fe]4 (M+): m/z 1625.9675, observed
1625.9603.
[CpCo(η5-C5H4-4,4′-C6F4-C6F4-η5-C5H4)CoCp]2+[P F6-]2 (6).
A solution of NaCp (1.76 g, 20 mmol) and dibasic ligand (2,
0.47 g, 1.0 mmol) in THF (100 mL) was added by canulla to a
stirred, blue solution of CoBr2 (2.19 g, 10.0 mmol) in THF (200
mL) at 25 °C. After the addition was complete, the cloudy,
green mixture was stirred for an additional 15 min. The
solvent was evaporated, and the residue was taken up in 100
mL of toluene. Water (100 mL) and then HCl (12 M, 5 mL)
were added, and air was bubbled through the mixture for 10
min. The resulting dark mixture was filtered through Celite.
The biphasic filtrate was separated, and the organic layer was
extracted with water (3 × 50 mL). The combined aqueous
extracts were washed with ether (2 × 100 mL), and then 25
mL of saturated aqueous KPF6 was added with vigorous
stirring. The yellow precipitate was collected on a filter,
washed with water (2 × 10 mL) and ether (2 × 10 mL), and
dried in a vacuum desiccator over anhydrous CaSO4 for 2 days
to afford a yellow solid. The solid was subjected to liquid
chromatography on alumina (20 cm × 5 cm). Fractions
containing cobaltocenium species elute as yellow or sometimes
green bands. The first, yellow band was eluted with acetone,
evaporated, washed with ether to remove traces of diacetone
alcohol, and dried under vacuum to afford 1.05 g of cobalto-
cenium hexafluorophosphate, which was identified by compar-
ing its 1H and 19F NMR spectra with those of a commercial
sample (Aldrich). A second, yellow band was eluted with 1%
methanol in acetone and evaporated. The resulting yellow
residue was rinsed with ether to remove traces of diacetone
alcohol to afford 420 mg of a yellow solid, which was tentatively
identified as a 10:1 mixture of 6/7. Data for 6: 1H NMR
(acetone-d6): δ 6.53 (pentet, J HH ) J HF ) 1.9 Hz, 4 H), 6.25 (t,
J HH ) 1.9 Hz, 4 H) 6.45 (s, 10 H). 19F (acetone-d6): δ -73.8 (d,
J FP ) 715 Hz, 12 F), -138.4 (m, 4 F), -140.3 (m, 4 F). FAB-
HRMS calcd for C32H18F8Co2: (M+ - 2 PF6) 671.9945, observed
671.9927. Data for 7 (tentative): 1H NMR (acetone-d6): δ 6.64
(m, 4 H), 6.51 (m, 4 H), 6.35 (t, J ) 1.9 Hz, 4 H), 6.24 (t, 1.9
Hz, 4 H). 19F (acetone-d6): δ -138.0 (m, 4 F), -139.8 (m, 4 F),
-140.2 (m, 4 F); integration data reveal the presence of a
fourth signal coincident with the resonance at -138.4 assigned
to 6. In a subsequent experiment, identical in all respects
except for the amount of dibasic ligand (2) used (0.050 g
instead of 0.47 g), the second chromatographic band afforded
65 mg of crude 6, which was purified of diacetone alcohol by
triturating with ether (2 × 1 mL) to afford 44 mg of pure 6, as
microanalysis was not attempted. 19F NMR (THF-d8):
δ
-143.2 (m, 4 F), -144.7 (m, 4 F); broad, unassigned resonances
accounted for about 10% of the integrated signal. {1H}13C NMR
(THF-d8): δ 148.6 (d, 1J CF ) 240 Hz, CF), 146.4 (d, 1J CF ) 247
Hz, CF), 126.4 (t, J CF ) 14.4 Hz, C), 112.3 (t, J CF ) 7.5 Hz,
CH), 111.2 (s, CH), 108.4 (s, C), 98.8 (t, J CF ) 15.1 Hz, C).
Cp [F e(η5-C5H4-4,4′-C6F 4-C6F 4-η5-C5H4)]n F eCp [3 (n ) 1),
4 (n ) 2), a n d 5 (n ) 3)]. A solution of NaCp (1.85 g, 21.0
mmol) and dibasic ligand (2, 0.50 g, 1.0 mmol) in THF (100
mL) was added by canulla to a stirred orange solution of FeBr2
(1.98 g, 9.7 mmol) in THF (250 mL) at 25 °C. After the addition
was complete, the cloudy, dark brown mixture was stirred for
an additional 10 min. The solvent was removed under reduced
pressure, and the residue was extracted with CH2Cl2 (200 mL)
and filtered through a 10 cm bed of neutral alumina. The dark
red filtrate was evaporated to afford 1.80 g of a red solid. The
product mixture was purified by liquid chromatography (50
cm × 5 cm) on silica gel. The first band, eluted with hexanes,
was evaporated to afford ferrocene (1.24 g, 6.65 mmol, 69%
based on FeBr2). A second band, eluted with 0.5% ethyl acetate
in hexanes, was evaporated to afford the diiron complex 3 (0.32
g, 0.49 mmol, 49% based on 2 or 10% based on FeBr2). Data
for 3: 1H NMR (CDCl3): δ 4.90 (pentet, J HH ) J HF ) 2.0 Hz,
4 H), 4.50 (triplet, J HH ) 1.9 Hz, 4 H), 4.21 (s, 10 H). 19F NMR
(CDCl3): δ -141.06 (m, 4 F), -141.24 (m, 4 F). {1H}13C NMR
(CDCl3): δ 144.4 (d, J CF ≈ 250 Hz, two unresolved CF), 121.8
(t, C), 103.7 (m, C), 71.4 (s, C), 70.0 (3 partially resolved
singlets, 3 CH). {19F}13C NMR (CDCl3): δ 144.4 (s, CF), 144.2
(s, CF), 121.8 (s, C), 103.7 (s, C), 70.0 (complex multiplet). Anal.
Calcd for C32H18F8Fe2: C, 57.66; H, 2.72. Found: C, 57.80; H,
2.60.
1
determined by H and 19F NMR spectroscopy.
Resu lts a n d Discu ssion
Liga n d Syn th esis. The synthesis of 4,4′-bis(cyclo-
pentadienyl)octafluorobiphenyl (Scheme 1) is based on
the nucleophilic aromatic substitution approach to
perfluoroaryl-substituted cyclopentadienes that we re-
ported earlier.12 The reaction of decafluorobiphenyl with
A third band, eluted with 5% ethyl acetate in hexanes,
similarly afforded 54 mg (0.047 mmol, 9% based on 2 or 1.5%
based on FeBr2) of 4. Data for 4: 1H NMR (CDCl3): δ 4.96 (m,
4 H), 4.85 (br s, 4 H), 4.53 (t, 1.9 Hz, 4 H), 4.48 (t, 1.9 Hz, 4
H), 4.19 (s, 10 H). 19F NMR (CDCl3): δ -140.7 (br m, 4 F),
-140.9 (m, 4 F), -141.2 (br s, 8 F). FAB-HRMS Calcd for
(12) Deck, P. A.; J ackson, W. F.; Fronczek, F. R. Organometallics
1996, 15, 5287-5291.
C
54H26F16[56Fe]3 (M+): m/z 1145.9827, observed 1145.9797.

Metallocene Complexes of Iron and Cobalt
Organometallics, Vol. 18, No. 18, 1999 3613
Sch em e 1
Sch em e 2
sodium cyclopentadienide and sodium hydride followed
by hydrolytic workup results in a nearly quantitative
conversion (98% crude, 72% purified) to the bis(cyclo-
1
pentadiene) (1). H and 19F NMR spectra obtained for
the crude product were consistent with a mixture of
double-bond regioisomers and an overall purity of >95%.
Initial experiments showed that elevated temperature
(65 °C) was needed to ensure alkylation at both the 4
and 4′ positions of the arene. A 10-fold excess of NaCp
was used to minimize diarylated cyclopentadiene prod-
uct. Added sodium hydride was used to trap the product
as its conjugate base to prevent intermolecular Diels-
Alder dimerization during the arylation reaction; dimers
1
in the 19F NMR spectrum coincide, giving an observed
4:4:8 ratio.) The tetrairon complex 5 shows the expected
10:4:4:4:4:4:4 ratio in the 1H NMR spectrum (two signals
were unresolved, giving an observed 10:4:4:4:4:8 ratio)
and the expected 4:4:4:4:4:4 ratio in the 19F NMR (two
pairs of signals were unresolved, giving an observed 8:4:
4:8 ratio). These data suggest that in 4 and 5 all the
Cp-arene bonds also rotate freely.
are readily identified by signals in the H NMR spec-
trum characteristic of the tricyclic dicyclopentadiene
framework.12 Treatment of 1 with sodium hydride
affords the disodium salt 2 in nearly quantitative yield.
The time-averaged D2h symmetry of 2 was confirmed
1
by the presence of only two signals each in the H and
19F NMR spectra.
Meta llocen e Syn th esis. Reactions of the dibasic
ligand 2 with either FeBr2 or CoBr2 resulted in complex
mixtures of step-growth oligometallocenes, which we
found difficult to separate and characterize, partly
because of the variability and instability of the organic
end groups. In subsequent reactions, we therefore used
a 21:1:10 ratio of NaCp:2:MBr2. This reactant ratio
ensured first that Cp was the predominant end group
and second that the product distributions were skewed
toward number-average degrees of oligomerization barely
exceeding 1.0 metal atoms per molecule. Using this
strategy, product mixtures were dominated by Cp2Fe
and [Cp2Co]+[PF6]- (Scheme 2), from which the minor
dimetallic and trimetallic products could be separated.
This strategy worked particularly well for the fer-
rocene complexes, which are stable and sufficiently
soluble in hydrocarbons for separation by gradient liquid
chromatography. The diiron, triiron, and tetrairon
complexes (3, 4, and 5, respectively) were isolated and
characterized by NMR spectroscopy. The diiron complex
was additionally characterized by elemental microanal-
ysis, while the identities of the triiron and tetrairon
complexes, which were isolated only in small amounts,
were confirmed by high-resolution mass spectrometry.
The separation and purification of the dicobalt com-
plex were more challenging. The major product, cobal-
tocenium hexafluorophosphate, was readily separated
by liquid chromatography on alumina, eluting with
acetone. Unfortunately, we were unable to separate the
remaining 10:1 mixture of 6 and 7 by gradient liquid
chromatography. Nevertheless, the 1H and 19F NMR
spectra of 6 and 7 were analogous to the spectra of 3
and 4, respectively, and their unambiguous assignment
was made possible by resolution-enhancement apodiza-
tion of the time-domain spectrum (FID) prior to Fourier
transformation. A small but pure sample of 6 was
obtained in a similar reaction using a 21:0.1:10 ratio of
NaCp:2:CoBr2 to avoid contamination by the tricobalt
species (7).
Electr och em ica l Stu d ies. Solution voltammetry
was used to investigate the possibility of “electronic
communication” between the two metal centers in the
homobimetallic complexes (3 and 6).3f,13 The results of
these analyses are presented in Table 2. Significant
metal-metal interaction would be indicated by two
distinct oxidation waves in either cyclic voltammetry
(CV) or Osteryoung square wave voltammetry (OSWV).
However, when the diiron complex (3) was subjected to
these two measurements, only one oxidation wave was
observed. In fact, the fwhm of the OSWV signal of 3
was the same as that of internal ferrocene. This means
that the two ferrocenyl groups oxidize independently
1
The H NMR spectrum of 3 shows three signals in a
4:4:10 ratio of integrated intensity, whereas the 19F
NMR spectrum shows two equally integrating signals.
These data are consistent with a time-averaged C2h
-
symmetric structure in which Cp-C6F4 and C6F4-C6F4
bonds rotate freely. The homologous triiron complex 4
shows the corresponding 10:4:4:4:4 ratio of intensities
in the 1H NMR spectrum and the 4:4:4:4 ratio of
intensities in the 19F NMR spectrum (two of the signals
(13) (a) Sutton, J . E.; Taube, H. Inorg. Chem. 1981, 20, 3125. (b)
Richardson, D. E.; Taube, H. J . Am. Chem. Soc. 1983, 105, 40. (c)
Chukwu, R.; Hunter. A. D.; Santarsiero, B. D.; Bott, S. G.; Atwood, J .
L.; Chassiagnac, J . Organometallics 1992, 11, 589.

3614 Organometallics, Vol. 18, No. 18, 1999
Hollandsworth et al.
and that no communication between the two Fe centers
was observed. An analogous biphenylene-linked difer-
rocene species of the formula (η5-C5Me5)Fe(η5-C5Me4)-
C6H4-C6H4-(η5-C5Me4)Fe(η5-C5Me5) also showed no vol-
tammetric splitting;14 however one might expect the
conjugation between the cyclopentadienyl and aryl
groups to be disrupted by the presence of the additional
methyl groups on the Cp ligands (which facilitated the
synthesis of the complex).15 Interestingly, the increase
in the FeII|FeIII oxidation potential relative to internal
ferrocene of +200(5) mV observed for 3 is slightly larger
than the effect anticipated for a single pentafluorophe-
nyl substituent of +173(5) mV.12 This demonstrates the
ability of the 4,4′-octafluorobiphenylene unit to render
both coordinated transition-metal fragments highly
electron-deficient simultaneously.
Similar observations were made when the dicobalt
complex (6) was subjected to solution voltammetry. A
single, reversible wave was observed in the cyclic and
Osteryoung square wave voltammograms of 6, suggest-
ing that no significant communication between the two
equivalent CoIII centers exists. A reduction potential of
+232(5) mV relative to internal Cp2Co+ is slightly
higher than that expected for a single C6F5 substituent
of +200(5) mV,16 again showing that the bridging ligand
(2) renders both Co+ centers electron-poor.
F igu r e 1. Thermal ellipsoid plot of 3 (50% probability)
showing atom-labeling scheme. Hydrogen atoms were
located and isotropically refined but are omitted from
this view. Selected bond distances (Å) and angles (deg):
CentC(1)-C(5)-Fe(1), 1.650(4); CentC(6)-C(10)-Fe(1), 1.657(4);
CentC(17)-C(21)-Fe(2), 1.647(4); CentC(22)-C(26)-Fe(2), 1.656;
CentC(1)-C(5)-Fe(1)-CentC(6)-C(10), 178.2(2); CentC(17)-C(21)
-
Fe(2)-CentC(22)-C(26), 178.3(2); CentC(1)-C(5)-C(1)-C(11),
177.2(2); CentC(17)-C(21)-C(17)-C(27), 177.1(2); C(1)-C(11),
1.474(4); C(17)-C(27), 1.476(4); C(14)-C(30), 1.501(4);
C(12)-C(11)-C(16), 114.6(3); C(13)-C(14)-C(15), 115.7-
(3); C(29)-C(30)-C(31), 116.0(3); C(28)-C(27)-C(32), 114.4-
(3).
torsion angles between least-squares planes of the
dibasic ligand are Cp-37.5°-C6F4-50.1°-C6F4-32.1°-Cp.
These conformational preferences had been anticipated
from the torsion angle of decafluorobiphenyl (70° in the
gas phase18 and 64.4° in the solid state19) and from the
Cp-C6F5 torsion angles (25-45°) in other C6F5-substi-
tuted ferrocenes that we have studied.20
CV analysis of 4 showed two reversible oxidations in
an approximately 2:1 current ratio. Analysis by OSWV
showed two peaks at +180(5) and +372(5) mV, respec-
tively, relative to internal ferrocene. The center fer-
rocene unit is substituted by two perfluoroarylene
groups, whereas the terminal ferrocene units are sub-
stituted by only one, leading to the observed oxidation
potentials in accord with the substituent effects de-
scribed for the diiron complex (3). The observation that
no significant additional potential is required to oxidize
the center metallocene unit beyond that predicted on
the basis of substituent effects also demonstrates the
independence of the ferrocene units of 4. The tetrairon
complex (5) showed similar voltammetric behavior: CV
analysis showed two reversible oxidation waves, which
were found by subsequent OSWV analysis to have a 2:2
current ratio at +192(5) and +382(5) mV versus internal
ferrocene.
X-r a y Cr ysta l Str u ctu r e of 3. Table 1 shows the
crystal data for the diiron complex (3). Figure 1 shows
a thermal ellipsoid plot of the solid-state structure of 3,
defines an atom-numbering scheme, and provides se-
lected metric data. All four Cp rings exhibit highly
regular pentahapto coordination with all Fe-C dis-
tances within a narrow range (2.028-2.054 Å). The
unsubstituted Cp ligands are only slightly farther (0.01
Å, measured from the C5 centroids) from the respective
iron atoms than the substituted Cp ligands.17 The
Con clu sion s
Nucleophilic aromatic substitution is an efficient route
to dibasic bis-Cp ligands tethered by the 4,4-octafluo-
robiphenylene spacer. This highly fluorinated spacer
exerts a strong electron-withdrawing influence on both
coordinated CpFe fragments in 3. However, despite the
conjugated nature of the spacer, no electronic “com-
munication” between the two CpFe fragments was
evident from voltammetric analysis. We are continuing
to investigate the synthesis and characterization of
homo- and heterobimetallic cyclopentadienyl complexes
linked by perfluoroarylene spacers.
Ack n ow led gm en t. This work was supported by the
Petroleum Research Fund (30738-G1). administered by
the American Chemical Society. W.G.H. thanks the PRF
for a Summer Faculty Fellowship to pursue this re-
search at Virginia Tech. C.B.H. thanks Roanoke College
for a Bondurant Undergraduate Research Scholarship.
The authors wish to thank Prof. Karen J . Brewer, Dr.
Michael R. J ordan, and J ennifer L. Montgomery for
assistance with the voltammetric analyses.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for the diiron complex 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM990321N
(17) Gassman, P. G.; Winter, C. H. J . Am. Chem. Soc. 1988, 110,
6130-6135.
(18) Almenningen, A.; Hartmann, A. O.; Seip, H. M. Acta Chem.
Scand. 1968, 22, 1013.
(19) Gleason, W. B.; Britton, D. Cryst. Struct. Commun. 1976, 5,
483.
(14) (a) Bunel, E.; Campos, R.; Ruz, J .; Valle, L.; Chadwick, I.; Santa
Ana, M.; Gonzalez, G.; Manriquez, J . M. Organometallics 1988, 7, 474.
(b) Bunel, E. E.; Valle, L.; J ones, N. L.; Carroll, P. J .; Gonzales, M.;
Munoz, N.; Manriquez, J . M. Organometallics 1988, 7, 789.
(15) A rigid biphenylene-bridged bis(zirconocene dichloride) complex
was shown to exhibit improved efficiency in ethylene polymerization
catalysis. Soga, K.; Ban, H. T.; Uozumi, T. J . Mol. Catal. A 1998, 128,
273.
(20) For example, the complex (η5-C6F5Cp)[η5-1-(C6F5)-2-(Me2NCH2)-
Cp]Fe exhibits two Cp-C6F5 torsion angles of 32° and 45°. Deck, P.
A.; Lane, M. J .; Slebodnick, C. Manuscript in preparation.
(21) (a) O’Hare, D.; Green, J . C.; Marder, T.; Collins, S.; Stringer,
G.; Kakkar, A. K.; Kaltsoyannis, N.; Kuhn, A.; Lewis, R.; Mehnert, C.;
Scott, P.; Kurmoo, M.; Pugh, S. Organometallics 1992, 11, 48-55. (b)
Robbins, J . L.; Edelstein, N.; Spencer, B.; Smart, J . C. J . Am. Chem.
Soc. 1982, 104, 1882. (c) Koelle, U.; Khouzami, F. Angew. Chem., Intl.
Ed. Engl. 1980, 19, 640.
(16) The complex (C6F5Cp)2Co+PF6 shows a reversible reduction
-
at +400 mV relative to internal cobaltocenium hexafluorophosphate
under identical conditions. Thornberry, M. P.; Deck, P. A.; Montgomery,
J . L.; Slebodnick, C. Manuscript in preparation.