Synthesis, structure, and ligand exchange reactions of tetramethyleneethane complexes of cobalt

Article abstract of DOI:10.1021/om100611p

The synthesis of (η³:η³-TME)[Co(CO) ₃]₂ (1) was achieved using 2,3-bis(bromomethyl)-1,3- butadiene (TMEBr₂) as the tetramethyleneethane (TME) ligand precursor and Na[Co(CO)₄]. Solution NMR studies suggested an η³:η³-configuration, which has been confirmed in the solid state by single-crystal X-ray diffraction studies. The series of complexes (η³:η³-TME)[Co(CO)₂PR ₃]₂ (R = Me, 2; R = Et, 3; R = n-Bu, 4; R = Ph, 5; R = OPh, 6) were also synthesized by ligand exchange reactions, demonstrating that only one carbonyl may be exchanged for a phosphine group on each metal center. The η³:η³-configuration of the tetramethyleneethane ligand in these complexes was determined by crystallographic studies. The effect of the electron-donating properties of PR₃ was studied by cyclic voltammetry (CV) and infrared spectroscopy. The greatest degree of electron donation was seen when R = Et (3) and lowest when R = Ph (5) or R = OPh (6). Electronic communication between the metal centers was observed by CV. The chemical oxidation of 1 resulted in a highly unstable species that decomposed to {[(CO)₂Co]TME[Co(CO) ₃]}⁺[BF₄]^- (1⁺d), determined by its crystal structure. The synthesis of (η⁴: η⁴-TME)[CoCp*]₂ (7) has been achieved using a dipotassium 2,3-bis(methylene)-1,3-butanediyl (TMEK₂) synthon. NMR studies suggested that 7 adopts an unusual η⁴:η⁴- configuration, which was confirmed with the aid of crystallographic studies. DFT calculations were performed in order to rationalize the bonding for 1, 7, and hypothetical (η⁴:η⁴-TME)[CoCp]₂ (8). The large energy difference between the two coordination isomers 1 and 1a confirmed the η³:η³-configuration. For isomers 7/7a and 8/8a, the energy difference between the two isomers (ca. 15 kJ mol ^-1) is in favor of the η⁴:η⁴- configuration. For complexes 1⁺ and 8⁺, the calculations suggested complete delocalization on the system when one electron was removed.

Full text of DOI:10.1021/om100611p

Organometallics 2010, 29, 5847–5858 5847
DOI: 10.1021/om100611p
Synthesis, Structure, and Ligand Exchange Reactions
of Tetramethyleneethane Complexes of Cobalt
ꢀ
ꢀ
Paulina Aguirre-Etcheverry, Andrew E. Ashley, Gabor Balazs, Jennifer C. Green,
Andrew R. Cowley, Amber L. Thompson, and Dermot O’Hare*
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road,
Oxford, OX1 3TA, U.K.
Received June 24, 2010
The synthesis of (η3:η3-TME)[Co(CO)3]2 (1) was achieved using 2,3-bis(bromomethyl)-1,3-buta-
diene (TMEBr2) as the tetramethyleneethane (TME) ligand precursor and Na[Co(CO)4]. Solution
NMR studies suggested an η3:η3-configuration, which has been confirmed in the solid state by single-
crystal X-ray diffraction studies. The series of complexes (η3:η3-TME)[Co(CO)2PR3]2 (R = Me, 2;
R=Et, 3; R=n-Bu, 4; R=Ph, 5; R=OPh, 6) were also synthesized by ligand exchange reactions,
demonstrating that only one carbonyl may be exchanged for a phosphine group on each metal center.
The η3:η3-configuration of the tetramethyleneethane ligand in these complexes was determined by
crystallographic studies. The effect of the electron-donating properties of PR3 was studied by cyclic
voltammetry (CV) and infrared spectroscopy. The greatest degree of electron donation was seen
when R=Et (3) and lowest when R=Ph (5) or R=OPh (6). Electronic communication between the
metal centers was observed by CV. The chemical oxidation of 1 resulted in a highly unstable species
that decomposed to {[(CO)2Co]TME[Co(CO)3]}þ[BF4]- (1þd), determined by its crystal structure.
The synthesis of (η4:η4-TME)[CoCp*]2 (7) has been achieved using a dipotassium 2,3-bis(methylene)-
1,3-butanediyl (TMEK2) synthon. NMR studies suggested that 7 adopts an unusual η4:η4-configuration,
which was confirmed with the aid of crystallographic studies. DFT calculations were performed in
order to rationalize the bonding for 1, 7, and hypothetical (η4:η4-TME)[CoCp]2 (8). The large energy
difference between the two coordination isomers 1 and 1a confirmed the η3:η3-configuration. For
isomers 7/7a and 8/8a, the energy difference between the two isomers (ca. 15 kJ mol-1) is in favor of
the η4:η4-configuration. For complexes 1þ and 8þ, the calculations suggested complete delocalization
on the system when one electron was removed.
Introduction
the metal orbitals, e.g., (C3H5)4Cr2;4 or π-allyl, where the bond
between the allyl group and the metal atom is delocalized and
multicentric, e.g., (π-C3H5)2Ni.5 In some cases, conversion
from the σ- to the π-bonded type is possible by heating or UV
irradiation.
The C3H5 ligand has been widely studied, and many mono-
metallic organometallic complexes have been synthesized demon-
strating interesting behavior in catalytic reactions, ligand
exchange, and polymerization, and as a reagent in organic
chemistry.6 The allyl-metal complexes can be of the pure π-allyl
Allyl-containing transition metal complexes represent some
of the earliest examples of organometallic complexes.1 These
types of compounds have become of increased interest because
of their importance in preparative organic chemistry and
catalytic reactions; the high catalytic activity is in part due to
the coordination flexibility of allyl-metal bonding.2
On a general basis, organometallic compounds containing
the allyl (C3H5) moiety can be classified as follows: σ-allyl,
where a terminal carbon atom is σ-bonded to the metal atom
with a localized double bond between the two remaining
carbon atoms, e.g., (σ-C3H5)Mn(CO)5;3 μ-allyl, where the
allyl group bridges two metal atoms, being σ-bonded to one
metal atom through a terminal carbon and to the second
metal atom through interaction of the allyl double bond with
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Chem. 1984, 262, 157. (c) Bogdanovi, B.; Borner, P.; Breil, H.; Hardt, P.;
€
*Author for correspondence. E-mail: dermot.ohare@chem.ox.ac.uk.
Tel: þ44 (0) 1865 272686. Fax: þ44 (0) 1865 285131.
(1) Wilke, G.; Bogdanov, B.; Hardt, P.;Heimbach, P.;Keim, W.;Kroner,
M.; Oberkirc, W.; Tanaka, K.; Steinruc, E.; Walter, D.; Zimmermann, H.
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r
2010 American Chemical Society
Published on Web 10/28/2010
pubs.acs.org/Organometallics

5848 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
Scheme 1. Synthesis of Complexes 1-7
Figure 1. (a) (η1:η1-TME)[E(CH3)3]2 (E = Si13, Sn14), (b) (η3:η3-
TME)[NiCp]2,15 (c) (η3:η3-TME)[RuCp*Cl2]2,16 (d) (η3:η3-TME)-
[Mn(CO)4]2,17 (e) (η3:η3-TME)[Fe(CO)3]2.18
type, such as (π-C3H5)2Ni,5 (π-C3H5)3Co,7 or (π-C3H5)4Zr.8
They can also involve other ligands; examples include car-
bonyls (π-C3H5)Co(CO)3,9 triphenyl phosphine (π-C3H5)Ir-
(CO)(PPh3)2,10 and η5-cyclopentadienyl (π-C3H5)Ni(η5-Cp).9b
The allyl fragment can also support substituents (π-C3R5),
be part of a hydrocarbon ring, or be cyclic.1,5,6d
Bimetallic complexes containing two metal centers linked
by a bridging ligand may support metal-metal interactions
depending on the nature of the ligands. The physical and
chemical properties of one of the metal centers can vary
significantly due to the vicinity of the other.11 Applications
for these systems may be found in biological processes, molec-
ular electronics, and theoretical studies of electron-transfer
processes.11g,12
centers π-bonded to each allyl fragment. Structural data for
these known complexes confirm the planarity of the ligand15,16
and agree with calculations that demonstrate the complete
delocalization of the π-electrons across the carbon skeleton,
which should promote effective communication between the
metal centers. However, no studies of such metal-metal inter-
actions have yet been performed.
The most common method for the synthesis of TME com-
plexes is to combine reactive metal fragments, i.e., photo-
lyzed carbonyl M(CO)6, with allene,16-19 but poor yields are
obtained (<20%) and it is not possible to synthesize more
complex systems. Given the limitations of this synthetic
approach, we have investigated the use of the TME precursor
dipotassium 2,3-bis(methylene)-1,3-butanediyl, also referred
to as potassium tetramethyleneethane (TMEK2). TMEK2
can be seen as a six-π-electron donor to one metal center or
a pair of three-π-electron donors in a homobimetallic com-
plex, sharing the π-density equally between the metals. Several
authors have investigated the potential of using TMEK2 in
organic chemistry with various electrophilic reagents, and
this has been met with considerable success.20 Despite this,
no studies describing the direct combination of TMEK2 with
metal precursors has been published.
We were interested in studying the metal-metal inter-
actions in bimetallic systems containing bridging allyl moieties.
An example of a ligand capable of binding two metal centers
is 2,3-dimethyl-1,3-butadiene (TMEH2) since the proximity
of the two allyl fragments should facilitate the communica-
tion of the metal centers; the few known organometallic
complexes with a TME fragment bonded in an η1:η1- and
η3:η3-manner are depicted in Figure 1.
In an η1:η1-complex, each metal center is σ-bonded to each
allyl fragment. η3:η3-TME compounds contain two metal
Since (C3H5)Br has been widely used for the synthesis of
many important organometallic complexes such as (π-C3H5)-
Co(CO)3,21 2,3-bis(bromomethyl)-1,3-butadiene (TMEBr2)
was also chosen as a synthon. In this paper we report the
synthesis and characterization of bimetallic TME complexes
containing the “Co(CO)3” and “CoCp*” (Cp* = η5-C5Me5)
metalfragments. The structural and redox properties of these
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Article
Organometallics, Vol. 29, No. 22, 2010 5849
new complexes are also discussed. We also report several
ligand exchange reactions performed with the TME[Co-
(CO)3]2 (1) system.
Results and Discussion
Synthesis and Characterization. As depicted in Scheme 1,
the dicobalt complex (η3:η3-TME)[Co(CO)3]2 (1) was readily
obtained by reaction of TMEBr2 with 2 equiv of Na[Co-
(CO)4] at -80 °C. The orange-yellow product was purified
by recrystallization from toluene. It is soluble in toluene,
hexane, MeCN, and CH2Cl2. However, 1 appears to react
with MeCN at room temperature when exposed to sunlight
(change of color), presumably due to a ligand exchange reac-
tion occurring between the carbonyl group and MeCN. This
type of reaction has been observed on complexes containing
the Fe(CO)3 fragment, under similar conditions.22 The family of
complexes (η3:η3-TME)[Co(CO)2PR3]2 (R = Me, 2; Et, 3;
n-Bu, 4; Ph, 5; OPh, 6) were obtained by the reaction of 1 with
an excess of the corresponding phosphine precursor, where
only one CO is exchanged. The orange-yellow solids were puri-
fied by recrystallization from hydrocarbon or a mixture of
hydrocarbon/aromatic solvents. All these complexes are slightly
air-sensitive as a solid and in solution, but are stable for months
under an atmosphere of nitrogen.
Figure 2. Molecular structure of 1 from single-crystal diffrac-
tion data. Hydrogen atoms are omitted for clarity, and displace-
ment ellipsoids are drawn at 50% probability. Primed atoms are
generated by -x, -y, -z.
The synthesis of TMEK2 was optimized to high yield (80-
85%) using KOt-Am (Ot-Am=1,1-dimethylpropoxide) instead
of KOt-Bu, which is highly soluble in the hydrocarbon solvent,
used for deprotonation. Furthermore, the byproduct LiOt-
Am can be easily removed from the dipotassium salt by
several washings with hexane.
The metathesis reaction of Cp*Co(acac) with TMEK2 yielded
(η4:η4-TME)[CoCp*]2 (7) through salt elimination. 7 is a
deep purple, air-sensitive solid that can be kept for months
under an atmosphere of nitrogen. It is soluble in toluene,
hexane, CH2Cl2, and MeCN. The η4:η4-configuration was
confirmed by X-ray diffraction (Figure 6). This was unexpec-
ted, as all previously known TME complexes exhibit either
an η1:η1- or an η3:η3-configuration, with the TME ligand
acting as a one-π-electron or three-π-electron donor to each
metal center, respectively. In 7, the TME fragment is acting
as a four-π-electron donor to each metal center when it is
formally only a six-π-electron donor ligand, giving a 34-
electron complex. The electron counting is nontrivial, as the
two Co centers share the electrons of the two central carbon
atoms from the TME unit. The phenomenon of sharing elec-
trons from the same carbon atoms within metal centers is
commonly seen in triple-decker complexes.23 The interesting
η4:η4-configuration was also observed in the triple-decker
complex (μ-η4:η4-arene)[CoCp*]2. The electron count for
this complex is also 34.24
Figure 3. Molecular structure of 2 from single-crystal diffrac-
tion data. Hydrogen atoms are omitted for clarity, and displace-
ment ellipsoids are drawn at 50% probability. Primed atoms are
generated by -x, -y, -z.
In 1976, Hoffmann et al.25 predicted two series of stable
triple-decker complexes (based on CpM-Cp-MCp) with a
30- and 34-electron rule, in analogy to the 18-electron rule for
metallocenes. Even though complexes (μ-η4:η4-C6H5i-Pr)-
[CoCp*]2 and 7 are not “true” triple-decker compounds, as
the metals do not bind to all of the carbon atoms of the
Figure 4. Molecular structure of the decomposition product of
1þ (1þd). Hydrogen atoms and the [BF4]- anion have been omit-
ted for clarity, and displacement ellipsoids are at 50%.
€
(22) Knolker, H. J.; Goesmann, H.; Klauss, R. Angew. Chem., Int.
Ed. 1999, 38, 702.
(23) Beck, V.; O’Hare, D. J. Organomet. Chem. 2004, 689, 3920.
(24) Schneider, J. J.; Denninger, U.; Heinemann, O.; Kruger, C.
Angew. Chem., Int. Ed. Engl. 1995, 34, 592.
(25) Lauher, J. W.; Elian, M.; Summerville, R. H.; Hoffmann, R.
J. Am. Chem. Soc. 1976, 98, 3219.
central structure (arene and TME ligands, respectively), they
do obey the 30- and 34-electron rule of Hoffmann.23
Attempts to oxidize complexes 1, 2, 3, and 7 were perfor-
med using AgBF4 as an oxidizing agent. This reaction was

5850 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
Scheme 2. Formation of 1þ and Decomposition Product 1þd
copy (except 7), cyclic voltammetry (CV), and single-crystal
X-ray diffraction; the results are given below.
1H and 13C NMR Spectroscopy. Table 1 summarizes the
NMR spectroscopic data for all complexes. 1H NMR spectros-
copy of the diamagnetic complexes 1-6 showed resonances
for the TME fragment in a 1:1 ratio, which is consistent with
the metal centers bound in an η3:η3-configuration and is also
consistent with that reported for (η3-C3H5)Co(CO)3. The
latter showed three sets of resonances in a 1:2:2 ratio with a
symmetrical structure. The hydrogen atom of the central
carbon atom appeared at 4.91 ppm as a multiplet., while the
syn- and anti-hydrogens of the terminal carbon atom of the
allyl group appeared as a pair of doublets at 3.06 and 2.15 ppm,
with the splitting due to the hydrogen atom of the central
carbon atom of the allyl group. 13C NMR spectroscopy was
comparable with that reported for (η3-C3H5)Co(CO)3.21 The
spectra of 7 also displayed resonances for the TME fragment
in a 1:1 ratio, consistent with a symmetrical structure, although
the crystal structure showed an η4:η4-configuration.
Figure 5. Compounds 1, 2, and 7 showing the hydrogen displace-
ment from the TME plane.
IR Spectroscopy. IR spectroscopy was used to study the
carbonyl vibrations in the family of complexes 1-6 and to
determine the effect of the different phosphine groups on
these vibrations. (π-C3H5)Co(CO)3 is expected to have two
IR-active vibrations for the carbonyl group (A1 and E) if the
local symmetry is taken into account (C3v) and three IR-
active vibrations (2A0 and A00) if the overall symmetry is
considered (Cs). It has been shown elsewhere that the use
of the former treatment to assign carbonyl frequencies is
adequate.26 Hence, the IR studies of the complexes herein
will only consider local symmetries. The local symmetry of
the Co(CO)2PR3 fragment is Cs; consequently complexes
2-6 should present two IR-active vibrations for the carbonyl
group (A0 and A00). Since the local symmetry of the Co(CO)3
fragment is C3v, 1 should present two IR-active vibrations for
the carbonyl group (A1 and E).26b,27 The IR spectra recorded
(KBr disk) display two bands characteristic of carbonyl
stretchings; these are summarized in Table 2.
As expected, the vibrations observed for the carbonyl
groups for 1 are shifted to lower frequencies when one of
the carbonyl groups is exchanged for a phosphine group on
each metal center. The extent of the shift is influenced by
the substitution on the phosphine group, since this affects its
donor property. The ordering of the PR3 groups according to
their electron-donating properties should be PEt3 > Pn-Bu3 >
PMe3>PPh3>P(OPh)3. The IR frequencies found in this family
of complexes tend to agree with this trend. The values found
for 2, 4, and 5 are close, suggesting that the electron-donating
properties of PMe3, Pn-Bu3, and PPh3 groups are very similar.
The IR spectrum of 6 shows two vibrations for the carbonyl
Figure 6. Molecular structure of 7 from single-crystal diffrac-
tion data. Hydrogen atoms are omitted for clarity, and displace-
ment ellipsoids are drawn at 50% probability.
successful only for 1, where a solid was isolated that was con-
sistent with the formula {TME[Co(CO)3]2}2þ[BF4]- (1þ)
2
given by elemental analysis. This solid appeared to be very
unstable in CH2Cl2 and MeNO2 solutions, decomposing
after approximately one hour. The instability of 1þ can be
seen in the decomposition product obtained from a solution
of 1þ: {[(CO)2Co]TME[Co(CO)3]}þ[BF4]- (1þd) (Scheme 2).
The identity of all the products was established by elemen-
tal analysis, mass spectroscopy, 1H and 13C NMR, IR spectros-
(26) (a) Andrews, D. C.; Davidson, G. J. Chem. Soc., Dalton Trans.
1972, 1381. (b) Paliani, G.; Murgia, S. M.; Cardaci, G.; Cataliotti, R.
J. Organomet. Chem. 1973, 63, 407. (c) Yurchenko, E. N. J. Struct. Chem.
1977, 18, 399.
(27) Paliani, G.; Poletti, A.; Cardaci, G.; Murgia, S. M.; Cataliotti, R.
J. Organomet. Chem. 1973, 60, 157.

Article
Organometallics, Vol. 29, No. 22, 2010 5851
Table 1. Summary of the 1H and 13C NMR Spectroscopic Data for 1-7a
1H resonances
13C resonances
CpC
complex
syn-H
anti-H
Cp*H
CTerm
CCent
CpMe
CO
(η3:η3-TME)[Co(CO)3]2, 1
2.85
2.79
2.83
3.02
2.89
3.16
1.77
1.36
1.54
1.58
1.74
1.44
1.63
-0.44
45.10
39.35
38.81
39.02
43.28
43.66
26.87
98.21
98.14
98.33
98.84
99.15
97.75
69.67
202.75
205.1
205.90
206.13
206.13
205.05
(η3:η3-TME)[Co(CO)2PMe3]2, 2
(η3:η3-TME)[Co(CO)2PEt3]2, 3
(η3:η3-TME)[Co(CO)2Pn-Bu3]2, 4
(η3:η3-TME)[Co(CO)2PPh3]2, 5
(η3:η3-TME)[Co(CO)2P(OPh)3]2, 6
(η4:η4-TME)[CoCp*]2, 7
1.71
89.18
10.15
a Multiplicity of resonances and J values are given in the Experimental Section.
Table 2. Summary of the Carbonyl Stretching Frequencies for 1-6
complex
ν(CO)
ν(CO)
ref
(η3:η3-TME)[Co(CO)3]2, 1a
2053 (A1)
2062 (A1)
1968 (A0)
1957 (A0)
1990 (A0)
1972 (A0)
1973 (A0)
1974(A0)
1985 (A0)
2009 (A0)
2019 (A0)
1985 (E)
1980 (E)
this work
26b
this work
this work
28
this work
29
this work
29
(η3-C3H5)Co(CO)3
a
(η3:η3-TME)[Co(CO)2PMe3]2, 2a
(η3:η3-TME)[Co(CO)2PEt3]2, 3a
1917 (A00)
1920 (A00)
1930 (A00)
1912 (A00)
1912 (A00)
1918 (A00)
1928 (A00)
(η3-C3H5)Co(CO)2PEt3
b
(η3:η3-TME)[Co(CO)2Pn-Bu3]2, 4a
(η3-C3H5)Co(CO)2Pn-Bu3
a
(η3:η3-TME)[Co(CO)2PPh3]2, 5a
(η3-C3H5)Co(CO)2PPh3
c
(η3:η3-TME)[Co(CO)2P(OPh)3]2, 6a
1962 (A00), 1969 (asymmetry)
1974 (A00), 1984 (asymmetry)
this work
30
(η3-C3H5)Co(CO)2P(OPh)3
c
a KBr disk. b Pentane. c Cyclohexane.
Table 3. Summary of the Electrochemical Data for 1-7a
00
groups. ν
A presents an asymmetry on the main band at
(CO)
1969 cm-1. The IR spectrum of (η3-C3H5)Co(CO)2P(OPh)3
displays two vibrations for the carbonyl groups at 2019 and
1974 cm-1. This last vibration also presents an asymmetry of
complex
E1 (V)
E2 (V)
ΔE (mV)
(η3:η3-TME)[Co(CO)3]2, 1
þ0.25b
-0.07b
-0.27b
-0.21b
-0.03b
-0.05b
-0.91d
þ0.50b
þ0.15b
þ0.03b
þ0.07b
þ0.39c
þ0.36c
-0.47c
250
220
300
280
(η3:η3-TME)[Co(CO)2PMe3]2, 2
(η3:η3-TME)[Co(CO)2PEt3]2, 3
(η3:η3-TME)[Co(CO)2Pn-Bu3]2, 4
(η3:η3-TME)[Co(CO)2PPh3]2, 5
(η3:η3-TME)[Co(CO)2P(OPh)3]2, 6
(η4:η4-TME)[CoCp*]2, 7
the main band at 1984 cm-1 30
.
Electrochemical Properties. The electrochemical proper-
ties of 1-7 were measured by cyclic voltammetry in MeCN
(1-6) and CH2Cl2 (7) solutions containing 0.1 M [n-Bu4N]þ-
[BF4]-. The cyclic voltammograms recorded for each complex
exhibited two electrochemical events believed to correspond
to the oxidation of the cobalt centers and suggests that the
oxidation of one metal center affects the oxidation potential
of the second.
Table 3 summarizes the electrochemical data for this series
of complexes. The couples involved are believed to be CoIICoII/
CoIICoI and CoIICoI/CoICoI for complexes 1-6 and CoIIICoIII/
CoIIICoII and CoIIICoII/CoIICoII for 7. Previous electrochem-
ical studies on the mononuclear species (π-C3H5)Co(CO)3
show a two-electron irreversible process at E1/2 = -2.00 V
(referenced to [Cp2Fe]þ/Cp2Fe) that corresponds to the double
reduction of the complex, resulting in decomposition and the
formation of [Co(CO)4]-.29 No oxidation processes have
been reported. Since the processes observed for 1-6 appear
at much more positive potentials compared to (η3-C3H5)Co-
(CO)3, we believe them to be oxidation processes.
a All redox potentials are relative to the couple [Cp2Fe]þ/Cp2Fe.
b Quasi-reversible. c Irreversible oxidation. d Reversible.
of the electronic delocalization mediated by the ligand.31 Com-
plexes 1-4 display significant electronic interaction between
the metal centers, whereas for complexes 5-7, it is not possi-
ble to calculate ΔE, as these systems present irreversible
oxidations.
The electron-donating properties of the PR3 fragments
increase the electron density on the metal center, and as
expected, this produces shifts of E1 and E2 to more cathodic
potentials relative to 1; for this family of complexes, both
oxidations are easier to achieve. In addition PR3 incorpora-
tion stabilizes the oxidized species, as seen by the greater ΔE
values. The greatest electron donation was observed when
PEt3 was used (both couples were shifted to more cathodic
potentials relative to 1, 2, 5, and 6, and the system displayed
significant electronic communication between the metal centers).
The lowest degree of electron donation was observed for 5
and 6, where the oxidized species are less stable than those
of 2-4. The redox couple observed for 6 is shifted to more
cathodic potentials compared to 5. One possible cause of
this is the presence of the O atoms, which increase the
electron density of the P(OPh)3 group relative to PPh3.
Similarly to that observed in the IR spectroscopy studies,
the effect of the electron-donating properties on the redox
potentials followed the trend PEt3>Pn-Bu3>PMe3>PPh3 ≈
P(OPh)3.
The separation of the two electrochemical events, ΔE
(E2 - E1), may be used as a measure of the electronic stabili-
zation imparted on the mixed valence species by the second
metal center in the molecule and thus regarded as a measure
(28) Dickson, R. S.; Yin, P.; Ke, M.; Johnson, J.; Deacon, G. B.
Polyhedron 1996, 15, 2237.
(29) Cardaci, G.; Murgia, S. M.; Paliani, G. J. Organomet. Chem.
1974, 77, 253.
(30) Clarke, H. L.; Fitzpatrick, N. J. Inorg. Nucl. Chem. Lett. 1973,
9, 75.
(31) Barlow, S.; O’Hare, D. Chem. Rev. (Washington, DC, U. S.)
1997, 97, 637.

5852 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
Table 4. Details of Crystal Structure Parameters and Refinements for Complexes 1-7 and 1þd
1
1þd
2
3
4
5
6
7
formula
C
12H8Co2
C11H8BCo2-
F4O5
424.85
C16H26Co2-
O4P2
462.17
C22H38Co2-
O4P2
546.35
C34H62Co2-
O4P2
714.64
C46H38Co2-
O4P2
834.56
C46H38Co2-
O10P2
930.56
C26H38Co2
fw
366.06
468.45
size/mm
cryst class
space group
0.18, 0.22, 0.22 0.15, 0.13, 0.09 0.2, 0.06, 0.04 0.12, 0.12, 0.28 0.06, 0.04, 0.02 0.2, 0.06, 0.04 0.22, 0.18, 0.1 0.06, 0.2, 0.24
monoclinic
P21/c
triclinic
P1
orthorhombic triclinic
monoclinic
P1
triclinic
P1
triclinic
P1
monoclinic
P21/c
P 212121
7.4109(2)
12.9909(3)
15.0675(4)
90
P1
˚
a/A
6.6398(2)
6.7052(2)
8.1460(3)
88.3649(14)
78.8043(14)
76.2844(16)
345.56(2)
1
1.759
182
2.419
5.0-27.5
6.8755(2)
8.7561(3)
8.9500(2)
87.994(2)
75.764(2)
85.0410(10)
520.26(3)
1
0.475
238
1.763
4.26-27.44
3731
6.9927(2)
14.0728(2)
13.3788(2)
90
101.2813(5)
90
1291.13(5)
2
0.405
572
1.432
9.7293(2)
9.7312(2)
10.5514(3)
93.519(1)
102.906(1)
101.6571(1)
947.71(4)
1
1.252
382
0.992
3.99-27.09
7260
9.4902(2)
9.5480(2)
10.3123(2)
92.954(1)
95.132(1)
95.769(1)
924.23(3)
1
1.499
430
1.030
5.15-28.64
15 279
9.4540(1)
18.3056(3)
12.1477(2)
90
95.761(1)
90
2091.68(5)
2
1.478
956
0.929
9.5675(3)
14.2717(5)
9.9283(3)
90
101.8273(12)
90
1131.71(7)
2
1.375
496
1.474
˚
b/A
˚
c/A
R/deg
β/deg
90
90
γ/deg
V/A3
1450.61(6)
4
1.945
836
2.351
Z
Fc/g cm-3
F(000)
μ/mm-1
θ range/deg
5.13-27.43
22 999
5-27.5
13 136
3.98-27.47
9185
5-27.5
10 378
total no. of data 4237
no. of unique
data
no. obsd reflns
GOF
R indices
1574 R(int)
0.023
1377a
1.1439
R1 =0.0242
=
3315 R(int)
0.051
=
2316 R(int)
0.022
=
3055 R(int)
0.022
=
4148 R(int)
0.053
=
4365 R(int)
0.045
=
4767 R(int)
0.024
=
2681 R(int)
0.027
=
3034b
1.0873
R1 =0.0321
2192b
1.050
2564a
1.0828
R1 =0.0229
3193b
1.012
4102b
1.030
4480b
1.019
1911a
1.0238
R1 =0.0294
R1 =0.0367
R1 =0.0818
R1 =0.0579
R1 =0.0541
wR2 =0.0252 wR2 =0.0282 wR2 =0.0650 wR2 =0.0249 wR2 =0.1003 wR2 =0.0833 wR2 =0.0826 wR2 =0.0369
-0.31, 0.23
-3
˚
residuals/e A
-0.33, 0.36
-0.31, 0.38
-0.35, 0.37
-0.25, 0.33
-0.43, 0.53
-0.42, 0.48
-0.45, 0.59
a I > 3σ(I). b I > 2σ(I).
þ
˚
Table 5a. Selected Bond Lengths (A) for Complexes 1-7 and 1 d
1
1þd
2
3
4
5
6
7
Co-C(CO)
1.818(2)
1.780(2)
1.817(3)a
1.839(3)a
1.798(3)b
1.845(3)b
1.803(3)b
2.068(3)a
2.088(3)a
2.088(3)b
2.111(3)b
2.027(2)a
2.020(2)b
1.498(3)
1.770(2)
1.759(2)
1.7790(15)
1.7559(15)
1.773(3)
1.743(3)
1.791(2)
1.739(2)
1.771(2)
1.766(2)
Co-CTerm
Co-CCent
2.092(2)
2.0796(12)
2.061(2)
2.1006(19)
2.0641(14)
2.1045(15)
2.054(3)
2.098(3)
2.035(2)
2.095(2)
2.068(2)
2.0770(19)
2.0109(18)
2.0086(19)
2.0173(17)
2.0257(19)
2.0338(13)
2.023(3)
2.0349(19)
2.0097(17)
2.0124(17)
2.0116(17)
1.546(3)
1.453(2)
1.434(3)
CCent-CCent
CTerm-CCent
1.488(4)
1.414(3)
1.420(3)
1.485(3)
1.413(3)
1.427(3)
1.489(3)
1.429(2)
1.417(2)
1.484(5)
1.430(4)
1.410(4)
1.471(4)
1.358(3)
1.416(3)
1.481(3)
1.413(3)
1.424(3)
1.411(3)a
1.404(3)a
1.422(4)b
1.408(4)b
Co-P
Co-Cp*Ct
mean Co-C(cp*)
2.1692(5)
2.1902(4)
2.1811(9)
2.1318(6)
2.1594(5)
1.6698(2)
2.06(2)
a Corresponds to the Co(CO)2 fragments. b Corresponds to the Co(CO)3 fragments.
Description of the Structures. The solid-state structures of
complexes 1-7 and 1þd were determined by single-crystal
X-ray diffraction. Complexes 3-6 have structures that are
very similar to those of 1 and 2, so are not discussed in great
detail here.
In general, the complexes are located on a crystallographic
center of inversion, which ensures the planes defined by
the two allyl groups are parallel (Figures 2 and 3). For 1,
the local geometry of the Co-TME-Co core closely approxi-
mates to C2h, but the CO ligands are arranged slightly less
symmetrically (due to the π-bonding between the metal and
allyl moiety).2,5,32 The hydrocarbon ligand deviates slightly
from planarity such that the terminal carbon atoms (hereafter
referred to as CTerm) bend slightly toward the cobalt center.
This can be quantified by the deviation of the central carbon
atom (hereafter referred to as CCent) from the plane defined
by the four terminal carbon atoms (0.047(2) A).33 Since the
˚
hydrocarbon moiety consists of two rigorously parallel allyl
groups, it is also possible to calculate the distance between
34
the planes (0.18(1) A). The cobalt atom is displaced by
˚
˚
1.689(1) A along the normal to the plane defined by its co-
ordinated allyl carbon atoms. The structure of 1 has bond
(33) The standard uncertainties for C1 are approximately isotropic,
˚
giving an error of 0.002 A in x, y, and z. Assuming the error for the plane
defined by the four terminal carbon atoms is negligible, this can be
regarded as the error associated with the C1 deviation from the plane. All
six distances (for structures 1-6) are in the range 0.043-0.049, with
errors of 0.0001-0.003, suggesting the deviation is significant. A similar
approach was used to estimate the error for other distances to planes.
(32) Albright, T. A.; Hofmann, P.; Hoffmann, R. J. Am. Chem. Soc.
1977, 99, 7546.

Article
Organometallics, Vol. 29, No. 22, 2010 5853
Table 5b. Selected Bond Angles (deg) for Complexes 1-7 and 1þd
1
1þd
2
3
4
5
6
7
C(CO)-Co-C(CO)
Co-C(CO)-O(CO)
97.48(10)
107.5(9)
105.14(10)
103.30(12)a
98.08(13)b
100.64(13)b
99.80(12)b
178.5(2)a
176.4(3)a
173.3(2)b
176.7(3)b
177.7(2)b
122.6(2)c
122.2(2)c
120.1(2)d
121.2(2)d
116.4(2)a
114.4(2)b
112.79(9)
110.62(7)
110.52(12)
117.96(10)
96.61(9)
176.80(17)
178.68(19)
174.66(18)
177.92(17)
173.53(15)
177.26(14)
173.4(3)
177.9(3)
176.73(19)
177.7(2)
176.16(19)
177.87(18)
CCent-CCent-CTerm
123.2(2)
123.3(2)
123.0(2)
123.3(2)
122.73(16)
123.08(16)
123.5(3)
123.2(3)
123.4(2)
122.4(2)
123.3(2)
123.5(2)
113.54(19)
114.77(19)
CTerm-CCent-CTerm
C(CO)-Co-P
112.96(18)
113.12(17)
113.62(13)
112.8(3)
113.5(3)
112.65(17)
131.69(17)
95.70(6)
97.87(7)
100.33(5)
93.41(5)
94.71(10)
99.07(10)
117.14(9)
112.34(10)
117.22(11)
104.58(13)
99.65(13)
104.10(12)
97.73(7)
90.42(7)
107.58(6)
111.47(7)
Co-P-C(alkyl)
114.50(8)
114.93(8)
117.71(7)
102.83(10)
102.76(10)
102.10(11)
114.41(5)
114.06(5)
117.42(5)
103.54(7)
101.80(7)
103.88(7)
118.29(7)
114.93(7)
111.60(7)
106.47(9)
100.88(9)
102.61(9)
C(alkyl)-P-C(alkyl)
TME Ct-Co-Cp*Ct
CCent-Co-Cp*Ct
CTerm-Co-Cp*Ct
170.781(15)e
144.19(5)/ 143.69(5)e
135.55(6)/ 134.36(6)e
a Corresponds to the Co(CO)2 fragments. b Corresponds to the Co(CO)3 fragments. c Corresponds to the angle between the CCent of the Co(CO)2
fragment and the CCent and CTerm of the Co(CO)3 fragment. d Corresponds to the angle between the CCent of the Co(CO)3 fragment and the CCent and
CTerm of the Co(CO)2 fragment. e TME Ct corresponds to the centroid of the coordinating atoms of the TME ligand, and Cp*Ct corresponds to the
centroid of the central Cp.
lengths and angles that are in agreement with those found in
(η3:η3-C8H12)[Co(CO)3]2 and (η3-C3H5)Co(CO)3.35
the allyl fragment, which has an asymmetric effect on the
carbonyl groups. The bond angle C(CO)-Co-C(CO) for 2
is slightly larger than the equivalent for 1 as a consequence
of the staggered configuration, which allows an increased
separation between the carbonyl groups. The two C(CO)-
Co-P bond angles are reduced compared to the C(CO)-
Co-C(CO) angles in 1 in order to minimize the interaction
between the methyl groups of the PMe3 fragment. It is possi-
ble to see the effect of the hydrogen atoms from the allyl
fragment and from the PMe3 group on the bond angles Co-
P-C(PMe3): the Co1-P1-C8 angle is bigger than the other
two (Figure 3).
The crystal structure of 2 (Figure 3) is very similar to that
of 1, showing the planarity of the hydrocarbon ligand and
the central carbon atom (CCent) barely displaced from the
˚
plane defined by the four terminal carbon atoms (0.048(2) A).
˚
The cobalt atom is displaced by 1.7837(3) A along the normal
to the plane defined by its coordinated allyl carbon atoms.
The carbonyl groups are in a staggered configuration relative
to the PMe3 group. 2 presents a similar asymmetry of the
carbonyl groups to that of 1. However, the Co-CO bond lengths
are shorter due to the electron-donating influence from the
PMe3 group. There is also a general trend that distances
between the metal and the atoms in the allyl-TME ligand are
slightly longer in the compounds with a coordinated phos-
phine ligand. The Co-P bond lengths observed for 1-5
In complexes 2-6, we observe subtle variations in the dis-
tance between the Co and the terminal carbon atoms of the
TME fragment which could be due to the increasing bulk of
the phosphine substituents and the resulting proximity of the
alkyl group to the allyl fragment. The size of the phosphine
substituents has really little influence on the Co-P distance.
The crystal structure of 1þd (Figure 4) shows both Co(CO)x
groups in a syn-configuration and not anti as in 1-6. Only
one [BF4]- group is present, suggesting that only one cobalt
atom has been oxidized, with the loss of CO. The bond
lengths between cobalt atoms and carbonyl carbon atoms are
slightly longer than those found in 1 and more asymmetric
(reflecting the differing oxidation states). The cobalt atoms
˚
range between 2.1318(6)-2.1902(4) A and compare closely
˚
to the value of 2.185(1) A found in the related complex
(η3-C3H5)Co(CO)2PPh3.36
The bond angles found within the TME fragment for 2-6
are in agreement with those found in 1, as expected consider-
ing the PMe3 group mostly affects the metal center and the carbo-
nyl ligands. The difference between the two Co-C(CO)-
O(CO) carbonyl angles is increased for compounds 2-6,
mainly due to the π-bonding between the metal center and
˚
are separated by 3.1079(5) A, suggesting that there is no
direct interaction between the metal centers since the Co-Co
bond length reported for the related complexes of the type
(34) In general, distances between planes cannot be given, because
(except in special cases) the distance varies depending on where the distance
is measured. However, because the hydrocarbon ligand is composed of two
three-atom allyl fragments related by an inversion center, the two ally planes
are rigorously planar and rigorously parallel. The error on this distance is
difficult to evaluate; however, the range of values for the six structures is
0.167-0.193 (mean=0.184; standard deviation=0.009). Thus, although
the distance is small, the consistency and systematic nature of effect support
the premise that this is significant.
37
˚
[Co2(CO)6L2] is 2.6 A (L=CO, P(Oi-Pr)3, Pn-Bu3, PMe3).
(37) (a) Bryan, R. F.; Manning, A. R. Chem. Commun. (Cambridge,
U. K.) 1968, 1316. (b) Ibers, J. A. J. Organomet. Chem. 1968, 14, 423.
(c) Jones, R. A.; Seeberger, M. H.; Stuart, A. L.; Whittlesey, B. R.; Wright,
T. C. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 399.
(d) Farrar, D. H.; Lough, A. J.; Poe, A. J.; Stromnova, T. A. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1995, 51, 2008.
(35) Cann, K.; Riley, P. E.; Davis, R. E.; Pettit, R. Inorg. Chem. 1978,
17, 1421.
(36) Rinze, P. V.; Mueller, U. Chem. Ber. 1979, 112, 1973.

5854 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
Figure 7. Possible isomers of TME[Co(CO)3]2 (1, 1a), TME[CoCp*]2 (7, 7a), and TME[CoCp]2 (8, 8a).
˚
Table 6. Selected Experimental and Calculated Bond Lengths (A) for 1 and 7 and Calculated Bond Lengths for 8
1
7
8
calculated
experimentala
calculated
experimentala
calculated
CCent-CTerm
1.41
1.46
2.07
2.01
2.97
1.414, 1.420
1.487
2.080, 2.091
2.017
2.988
1.43
1.56
1.98
1.99
1.99
1.433. 1.453
1.546
2.009, 2.011
2.011
2.012
1.43
1.54
1.99
2.00
2.00
CCent-CCent
Co-CTerm
Co-CCent
*
Co-CCent
*
a Experimental values for 1 and 7 were obtained from the crystal structure data.
Analogous to 1-6, complex7is located on a crystallographic
center of inversion with approximate (noncrystallographic)
C2h symmetry. The carbon atoms of the TME fragment are
approximately coplanar, but careful examination of the differ-
ence map and refinement of the hydrogen atoms show that
they are significantly out of plane. Despite the challenges
classically associated with finding hydrogen atoms in the
difference map, the data are generally good for all eight
structures reported herein, and, though most obvious for 7,
the trend is visible in all cases (Figure 5). This deviation is
probably due to increased σ-character in the Co-C bonds,
leading to a distortion of the terminal carbon atoms toward
tetrahedral geometry. Distortion of hydrogen atoms from
the plane defining the π-bonded hydrocarbon has been seen
previously.38
longest bond length corresponds to Co1-C4, which lies
above the allyl unit, due to steric hindrance caused by the TME
ligand. In contrast, the bond lengths to C7 and C6 are the
smallest, as they lie furthest from the TME ligand (Figure 6).
Thermal ellipsoid plots of complexes 3-6 are given in the SI
(Figures S1-S4).
Structure and Bonding of TME Complexes. In order to
understand the ability of the TME ligand to adopt an η3:η3-
or η4:η4-configuration depending on the nature of the bound
transition metal fragment, a computational study was under-
taken. The complexes considered were two coordination
isomers for the Co(CO)3 fragment, 1 and 1a, and the two
model isomers for the CoCp* fragment, 8a and 8, where the
Cp* ring was replaced by Cp for computational expedience
(Figure 7). Calculations were also carried out on the mono-
cations 1þ and 8þ to study the electronic delocalization within
these complexes. Fixing the symmetry as C2h enabled geom-
etry optimization in both the η3:η3- and η4:η4-mode for both
complexes.
˚
The cobalt atom lies 1.5432(2) A along the normal to the
plane defined by the four coordinated allyl carbon atoms,
˚
which is considerably shorter than the 1.7837(3) A seen for 2,
reflecting the change from η3 to η4 binding. The Co-Cp* Ct
(Ct = centroid) distance is 1.6698(2) A (Figure 6) and is in
˚
By comparing the relative energies of the isomers, the two
Co(CO)3 fragments prefer the η3:η3-configuration (1), whereas
the two CoCp fragments prefer the η4:η4-configuration (8).
The two fragments Co(CO)3 and CoCp are isolobal yet not
isoelectronic, the former having an electron count of 15 and
the latter 14. Thus the preference of Co(CO)3 for η3-coordination
and CoCp for η4-coordination is not unexpected and is in
agreement with the 18-electron rule in both cases. The energy
difference between 1 and 1a is 136 kJ mol-1 in favor of 1. By
contrast the isomer 8 is only 12 kJ mol-1 favored compared
to 8a. The isomer 8a was found to have a triplet ground state,
indicating electron deficiency. Both 8 and 8a were shown to
be local minima by frequency calculations. Calculations
were also carried out on 7 and 7a (Figure 7). Isomer 7 was
˚
close agreement to 1.70 A, which is the distance observed for
Cp2Co39 and Cp*Co(acac).40
The bond lengths CCent-CTerm for 7(1.434(3) and1.453(2) A)
are slightly longer than those found for 1 (1.414(3) and
˚
˚
1.420(3) A), but very similar to the C-C distances for the arene
4
4
24
˚
group in μ-η :η -(C6H5i-Pr)[Cp*Co]2 (approximately 1.440 A).
˚
In fact, in 7 the cobalt atoms lie 1.5432(2) A away from the
best plane of the four coordinated allyl carbon atoms,
˚
whereas in 1, they lie 1.7862(2) A away from the best plane
of the three coordinated allyl carbon atoms.
The coplanarity of the ligand forces the Cp* ring to tilt in
order to minimize the interaction between the hydrogen
atoms from both fragments. The tilt angle is 10.7° (angle
between the best planes of the TME fragment and the Cp*
ring) and forces the TME Ct-Co-Cp* Ct bond angle to be
less than 180°. This tilt is responsible for the approximately
three differing bond lengths between Co1 and the carbon
found to be favored by 18 kJ mol-1
.
The calculated structures of the complexes are similar to
those obtained experimentally for 1 and 7. Both systems
maintain a relatively planar C6 skeleton, but in the case of 7,
the CH2 groups are twisted to direct the π-orbitals toward
the Co atoms. Table 6 compares the calculated and experi-
mental bond lengths for the complexes 1, 7, and 8. The calcu-
lations reproduce the experimentally determined dimensions
well even when the methyl groups for the Cp* ring were not
considered in the computational model of 8. In essence, 8displays
longer bond distances for the TME fragment CCent-CCent*,
˚
atoms of the Cp* ring (2.098, ca. 2.068, and ca. 2.046 A). The
(38) (a) Cooper, R. I.; Thompson, A. L.; Watkin, D. J. J. Appl.
Crystallogr. 2010, in press. (b) Rees, B.; Coppens, P. Acta Crystallogr.,
Sect. B: Struct. Sci. 1973, B 29, 2516.
(39) Bunder, W.; Weiss, E. J. Organomet. Chem. 1975, 92, 65.
(40) Smith, M. E.; Andersen, R. A. J. Am. Chem. Soc. 1996, 118,
11119.

Article
Organometallics, Vol. 29, No. 22, 2010 5855
“t2g” set of orbitals (again denoted 1σ, 1δ, and 2δ) of the
CoCp fragment differ in that they are virtually nonbonding.
The orbital interaction schemes for 1 and 8 are depicted
in Figure 8. The Cofrag FOs are labeled according to the
fragment axis rotational symmetry. The resultant MOs are
labeled according to their C2h symmetry. The relevant FOs of
the Co(CO)3, CoCp, and TME fragments are illustrated in
the diagram. The isosurfaces of selected MOs of 1 and 8 are
depicted in the SI (Figures S6 and S7).
The CoCp FOs are significantly higher in energy than the
corresponding FOs of Co(CO)3. This leads to greater mixing
with the TME π-orbitals and more back-bonding from the
metal to the TME ligand in 8 than 1. The degree of occupancy
of the various FOs in the complex is shown in Table 7. For 1
the 1σ, 1π, and 2π FOs effectively retain their full complement
of two electrons, indicating that back-bonding to the CO ligands
is not perturbed on complexation. For 8 the FOs 1σ, 1π,
and 2π lose electrons by back-bonding to the TME ligand.
The occupancies of the TME fragment π-orbitals demon-
strate that both donation to the metal from TME and accep-
tance of electrons into the unoccupied π-orbitals (π4 to π6)
are greater on interaction with CoCp to form 8 than Co(CO)3
to give 1; thus the Co-C bond lengths are shorter in 8 than 1.
Also the net result is a greater negative charge on TME in 8
than 1 (Table 7). The direct consequence of this increased
charge is the lengthening of the bonds in the TME fragment.
Delocalization in Monocations 1þ and 8þ. In order to ana-
lyze whether or not the two metal centers are isolated from
each other, spin polarization calculations were performed. In
both cases, the unpaired electron was attributed to one metal
center, and the structure was optimized. Both complexes
were optimized in a doublet spin state. For complexes 1þ and 8þ,
the spin density relaxed during the optimization so that, in
the optimized structure, the spin density on both Co centers
was approximately equal (0.42 and 0.38 electron on each Co
atom in 1þ and 8þ, respectively). The results indicate that
the removal of one electron on each complex leads to the
corresponding cation in which the spin density is delocalized
on both metal centers, consistent with the electrochemical data.
Figure 8. Molecular orbital interaction schemes for 1 and 8. (The
2σ orbital lies at higher energy.)
Conclusion
whereas 1 presents longer Co-CTerm and Co-CCent dis-
tances. The optimized structures of 1 and 8 are depicted in
the SI (Figure S5).
We have synthesized a series of dicobalt complexes con-
taining the TME ligand. Structural and spectroscopic data
indicate the coplanarity of the TME ligand and its ability to
adopt different coordination modes (η3:η3 or η4:η4) accord-
ing to the electron number of the Co centers and the ancillary
ligands. Electrochemical studies suggested electronic com-
munication of Co centers through the ligand in all complexes
studied. DFT calculations show the η3:η3-binding mode to
be significantly more stable for TME[Co(CO)3]2 than η4:η4-
coordination. For TME[CoCp*]2 the η4:η4-mode is energe-
tically favored over η3:η3 by a lesser amount, but the η3:η3-
complex is predicted to be a biradical. TME binds CoCp*
more strongly than Co(CO)3 and carries a larger negative
charge, with two CoCp* ligands accounting for the longer
C-C and shorter Co-C bonds in this complex.
The bonding in complexes 1 and 8 can be described by
considering the interaction of two Co(CO)3 and two CoCp*
fragments (Cofrag), respectively, with the TME ligand. The
fragments retain their individual geometries in the whole
molecule and are in a singlet spin state due to restricted point
calculations. The orbitals of the molecule are then described
as linear combinations of fragment orbitals (FOs). Both
fragments give three frontier orbitals (2σ, 3π, and 4π) for
binding to the TME ligand.
For Co(CO)3, six d-electrons on each Co center are held in
a “t2g” set of orbitals; they are labeled 1σ, 1δ, and 2δ,
indicating their symmetry with respect to the rotational axis
of the Co(CO)3 fragment. These three orbitals back-bond to
the CO groups. A similar effect is seen in the complex
TME[Fe(CO)3]2, where the fragment Fe(CO)3 presents three
lower energy metal-type orbitals (six d-electrons), which are
heavily involved in Fe-CO back-bonding and hardly inter-
act with the TME fragment.41 For CoCp, the electrons in the
Experimental Details
All reactions were carried out under an inert atmosphere of
nitrogen using standard Schlenk line techniques42 or a Braun
(42) Errington, R. J. Advanced Practical Inorganic and Metalorganic
Chemistry; Blackie AþP: London, 1997.
(41) Thorn, D. Inorg. Chem. 1978, 17, 126.

5856 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
Table 7. Occupancy of FOs in 1 and 8 and Charges on the Fragments TME and Cofrag
TME
Cofrag
π1
π2
π3
π4
π5
π6
charge
1σ
1δ
2δ
3π
4π
2σ
charge
1
8
1.79
1.70
1.67
1.44
0.97
1.26
0.92
1.09
0.42
0.71
0.10
0.19
-0.11
-0.64
1.98
1.97
1.98
1.84
1.96
1.91
1.74
1.03
1.08
0.82
0.34
0.4
þ0.06
þ0.32
Unilab glovebox, unless stated otherwise. Where necessary, solvents
were dried by reflux over an appropriate drying agent [sodium-
benzophenone diketyl (THF), sodium (toluene), sodium-potassium
alloy (Et2O, pentane), calcium hydride (CH2Cl2, MeCN)] or
by passage through a column of activated alumina (hexane).
Deuterated solvents for NMR spectroscopy of air-sensitive
materials were dried by reflux over the appropriate drying agent
and purified by trap-to-trap distillation: potassium (C6D6,
toluene-d8).
processed (including interframe scaling and unit cell refinement)
using Denzo-SMN/Scalepack.45 The structures were then
solved using direct methods with either SIR9246 (compounds
1, 1þd, 3, and 7) or ShelXS47 (compounds 2 and 4-6). Subse-
quent full-matrix least-squares refinement was carried out using
either the CRYSTALS program suite48 (compounds 1, 1þd, 3,
and 7) or ShelXL47 (compounds 2 and 4-6). All structures have
been deposited with the Cambridge Crystallographic Data
Center, CCDC. Copies of these data can be obtained free of
charge from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
DFT calculations were carried out using the Amsterdam Den-
sity Functional program (ADF) versions 2004.01 and 2009.01.49
Scalar relativistic corrections were included via the zero-order
regular approximation (ZORA) method,50 together with the
nonlocal exchange correction by Becke51 and nonlocal correla-
tion corrections by Perdew.52 Geometry optimizations were
performed at the LDA level and exchange and correlation
corrections applied post-SCF. The Slater-type orbital basis sets
were of triple-ζ quality augmented with a single polarization
function and two diffuse functions (ADF basis TZP). The core
electrons of Co atoms were frozen up to 3p. Optimizations were
performed without symmetry restraints. Fragment calculations
were performed to enable analysis of the orbital interactions of
the Co(CO)3 and CpCo fragments, respectively, with the TME
fragment. In these calculations the complex is divided into two
fragments, whose molecular and electronic structures are calcu-
lated in a single-point optimization with a restricted singlet
spin state. The geometry of the fragment is preserved from the
optimized structure of the full complex. The molecular orbitals
(MOs) of the complex are formed through a linear combination
of the MOs of the two fragments.
NMR spectra were recorded using a Varian Mercury VX-
Works 300 MHz spectrometer or a Varian Venus 300 MHz
spectrometer. 1H and 13C spectra were referenced to the residual
protio-solvent peak; 31P spectra were referenced externally to
H3PO4. Oxygen- or water-sensitive samples were prepared using
dried solvents under an inert atmosphere in a glovebox. They
were then sealed in tubes fitted with Young’s-type concentric
stopcocks. Fourier transform infrared spectra were recorded on
a Perkin-Elmer Paragon 1000 FT-IR spectrometer (range used
4000-400 cm-1, resolution 1 cm-1) as KBr discs. In the case of
air-sensitive complexes, samples were ground with KBr powder
and loaded into a die in a glovebox, before being quickly pressed
into a disk on the open bench. The spectrum was run immedi-
ately to prevent decomposition. Elemental analyses for air-
sensitive complexes were performed by the Elemental Analysis
Service, London Metropolitan University, London, and mass
spectra by the Mass Spectrometry Service, Chemistry Research
Laboratory, University of Oxford. Electrochemical measure-
ments were performed using an EG&G Princeton Applied
Research model 273 potentiostat/galvanostat and a CH Instru-
ments electrochemical analyzer (the potentiostat was controlled
by a PC running CH Instruments version 2.05 electrochemical
software), at room temperature. All measurements were made
using a standard three-electrode setup: a platinum disk working
electrode, a platinum mesh counter electrode, and a AgCl/Ag
reference electrode. The cell was specially constructed for air-
sensitive samples, having a side arm fitted with a Young’s tap.
Cyclic voltammetry measurements were made under an atmo-
sphere of nitrogen on deoxygenated solutions of ca. 10-3 M with
0.1 M [n-Bu4N]þ[BF4]- as supporting electrolyte added and
using an appropriate dry solvent. Redox potentials were refer-
enced to the [Cp2Fe]þ/Cp2Fe couple at 0 mV. The reversibility of
the redox couple was judged by comparison with the behavior of
the [Cp2Fe]þ/Cp2Fe couple under identical conditions (506 mV
in THF, 406 mV in CH2Cl2, and 400 mV in MeCN, relative to
SCE).43
All reagents were used as received, unless specified otherwise.
PEt3, Pn-Bu3, PPh3, and P(OPh)3 were distilled prior to use (very
toxic). AgBF4 and NaPF6 were supplied by Sigma-Aldrich,
United Kingdom. The following compounds were prepared
according to published procedures: TMEBr2,53 Na[Co(CO)4],54
KOt-Am,55 Cp*Li.56
Preparation of (η3:η3-TME)[Co(CO)3]2, 1. TMEBr2 (10.0 g,
4.17 mmol) was dissolved in hexane (100 mL) and filtered
through dried MgSO4, then added to a solution of Na[Co(CO)4]
(47) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112.
(48) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36 (pt.6), 1487.
(49) (a) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca
Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931. (b) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.;
Baerends, E. J. Theor. Chem. Acc.: Theor., Comput., Model. (Theor.
Chim. Acta) 1998, 99, 391.
(50) (a) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1993, 99, 4597. (b) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem.
Phys. 1994, 101, 9783. (c) van Lenthe, E.; Ehlers, A.; Baerends, E. J.
J. Chem. Phys. 1999, 110, 8943. (d) van Lenthe, E.; Snijders, J. G.; Baerends,
E. J. J. Chem. Phys. 1996, 105, 6505. (e) van Lenthe, E.; van Leeuwen, R.;
Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281.
(51) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D.
J. Chem. Phys. 1988, 88, 1053.
(52) Perdew, J. P.; Yue, W. Phys. Rev. B 1986, 33, 8800.
(53) Gaoni, Y.; Sadeh, S. J. Org. Chem. 1980, 45, 870.
(54) Edgell, W. F.; Lyford, J. Inorg. Chem. 1970, 9, 1932.
(55) Lochmann, L.; Trekoval, J. J. Organomet. Chem. 1987, 326, 1.
(56) (a) Feitler, D.; Whitesides, G. M. Inorg. Chem. 1976, 15, 466.
(b) Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. Inorg. Synth.
1992, 29, 193.
Single-crystal X-ray diffraction experiments were carried out
on crystals that were mounted on a glass fiber or hair using
perfluoropolyether oil and quench cooled to 150 K in a stream of
cold liquid nitrogen using an Oxford Cryosystems CryoStream
unit.44 Data collection was performed using an Enraf-Nonius
FR590 KappaCCD diffractometer using graphite-monochromated
˚
Mo KR X-ray radiation (λ = 0.71073 A). Intensity data were
(43) (a) Barlow, S. Inorg. Chem. 2001, 40, 7047. (b) Connelly, N. G.;
Geiger, W. E. Chem. Rev. (Washington, DC, U. S.) 1996, 96, 877.
(44) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(45) Otwinowski, Z.; Minor, W. Methods in Enzymology: Macro-
molecular Crystallography, Part A; Academic Press: San Diego, CA, 1997;
Vol. 276, p 307.
(46) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.

Article
Organometallics, Vol. 29, No. 22, 2010 5857
(1.620 g, 8.330 mmol) in THF (50 mL) at -80 °C. The mixture
was warmed to room temperature, and a white precipitate
formed gradually. The solution was stirred for 15 h and the
solvent removed under vacuum. The resulting lime green solid
was extracted with toluene (3 ꢀ 50 mL) and filtered through
Celite. This solution was reduced to minimum volume and
stored at -80 °C overnight. The yellow-orange solid formed
was collected and dried under vacuum. Yield: 0.964 g (63.2%)
(2.634 mmol). 1H NMR (C6D6) δ (ppm): 1.36 (s, 4H, C6H8), 2.85
(s, 4H, C6H8). 13C NMR (C6D6) δ (ppm): 45.10 (s, allyl terminal
carbon), 98.21 (s, allyl central carbon), 202.75 (s, CO). IR (KBr)
cm-1: 1985, 2053. Anal. Calcd for C12H8Co2O6: C, 39.37; H,
2.20; Co, 32.97. Found: C, 40.05; H, 1.50; Co, 31.94. MS (EI) m/
z: 365.898 (Mþ), 337.920 (Mþ - (CO)), 309.883 (Mþ - (CO)2),
CH2CH3), 1.74 (s, 4H, C6H8), 3.02 (d, J=4.11 Hz, 4H, C6H8).
13C NMR (C6D6) δ (ppm): 14.32 (s, PCH2CH2CH2CH3), 25.17
(s, PCH2CH2CH2CH3) 29.46 (s, PCH2CH2CH2CH3), 29.78 (s,
PCH2CH2CH2CH3), 39.02 (d, J = 2.02 Hz, allyl terminal
carbon), 98.84 (d, J = 0.86 Hz, allyl central carbon), 206.13 (s,
CO). 31P NMR (C6D6) δ (ppm): 42.17 (s, Pn-Bu3). IR (KBr)
cm-1: 1912, 1972. Anal. Calcd for C34H62O4P2Co2: C, 57.12; H,
8.74; P, 8.67: Co, 16.50. Found: C, 57.24; H, 8.74; P, 8.76; Co,
16.30. MS (EI) m/z: 202.183 (Pn-Bu3). CV (CH2Cl2) V: E1 =
-0.21, E2 = þ0.07 (both quasi-reversible).
Preparation of (η3:η3-TME)[Co(CO)2PPh3]2, 5. A solution of
1 (0.050 g, 0.137 mmol) in THF (40 mL) was added to a solution
of PPh3 (0.108 g, 0.410 mmol) in THF (40 mL). After stirring for
15 min, all volatiles were removed under vacuum, leaving a
yellow-orange, oily product. The residue was washed with
hexane (2 ꢀ 30 mL) to remove unreacted PPh3. The yellow solid
was dissolved in the minimum volume of THF and layered with
hexane. The crystalline solid obtained was collected, washed
with hexane, and dried under vacuum. Yield: 0.050 g (43.8%)
(0.060 mmol). 1H NMR (C6D6) δ (ppm): 1.44 (d, J = 4.40 Hz,
4H, C6H8), 2.89 (d, J=3.52 Hz, 4H, C6H8), 6.87-7.06 (m, two
superimposed peaks, 18H, PPh3), 7.44-7.67 (m, 12H, PPh3).
13C NMR (C6D6) δ (ppm): 43.28 (s, allyl terminal carbon), 99.15
(s, allyl central carbon), 128.75, 128.88, 133.85, 134.00, 136.54,
and 137.05 (all s, PPh3), 130.06 (d, J=2.30 Hz, PPh3), 206.13 (s,
281.889 (Mþ - (CO)3), 253.896 (Mþ - (CO)4), 225.907 (Mþ
-
(CO)5), 197.9048 (Mþ - (CO)6). CV(CH2Cl2) V:E1=þ0.25, E2=
þ0.50 (both quasi-reversible).
Preparation of {TME[Co(CO)3]2}2þ[BF4]-2, 1þ. A solution of
1 (0.100 g, 0.274 mmol) in CH2Cl2 (30 mL) was added to a
solution of AgBF4 (0.106 g, 0.550 mmol) in CH2Cl2 (30 mL).
During the addition a dark solid precipitated and the solution
turned dark green. After stirring for 15 min, all volatiles were
removed under vacuum, leaving a dark green residue. This solid
was washed with hexane (2 ꢀ 15 mL), then dissolved in MeNO2,
and the new solution was filtered into a new Schlenk. The
MeNO2 solution was reduced to minimum volume and the solid
precipitated by the dropwise addition of Et2O. The precipitate
was collected, washed with Et2O (10 mL), and dried under vacuum
to yield an orange solid. Yield: 0.015 g (10.3%) (0.028 mmol).
Anal. Calcd for C12H8Co2O6B2F8: C, 26.71; H, 1.49. Found: C,
26.78; H, 1.50.
CO). 31P NMR (C6D6) δ (ppm): 58.82 (s, PPh3). IR (KBr) cm-1
:
1918, 1974. Anal. Calcd for C46H38O4P2Co2: C, 66.19; H, 4.58;
P, 7.42: Co, 14.12. Found: C, 66.26; H, 4.59; P, 7.34; Co, 14.09.
MS (EI) m/z: 262.071 (PPh3). CV (CH2Cl2) V: E1 = -0.03
(quasi-reversible), E2 =þ0.39 (irreversible oxidation).
Preparation of (η3:η3-TME)[Co(CO)2P(OPh)3]2, 6. A solu-
tion of 1 (0.100 g, 0.273 mmol) in THF (50 mL) was added to a
solution of P(OPh)3 (0.169 g, 0.547 mmol) in THF (50 mL).
After stirring for 15 min, the solution was concentrated to 20 mL
and layered with hexane. The crystalline solid formed was
collected, washed with hexane, and dried under vacuum. Yield:
0.162 g (63.8%) (0.174 mmol). 1H NMR (C6D6) δ (ppm): 1.63
(d, J = 12.21 Hz, 4H, C6H8), 3.16 (d, J = 1.76 Hz, 4H, C6H8),
6.81-6.93 (m, 6H, P(OPh)3), 6.96-7.11 (m, 12H, P(OPh)3), 7.24
(m, 12H, P(OPh)3). 13C NMR (C6D6) δ (ppm): 43.66 (s, allyl
terminal carbon), 97.75 (s, allyl central carbon), 122.20 (d, J =
4.61 Hz, P(OPh), 125.34 (s, P(OPh)3), 130.30 (s, P(OPh)3),
152.47 (d, J = 6.05 Hz, C bound to the O atom in P(OPh)3),
205.05 (s, CO). 31P NMR (C6D6) δ (ppm): 160.51 (s, P(OPh)3.
IR (KBr) cm-1: 1962, 1969, 2009. Anal. Calcd for C46H38O10P2-
Co2: C, 59.35; H, 4.11. Found: C, 59.12; H, 6.03 (slightly impure
probably due to decomposition). MS (EI) m/z: No relevant sig-
nals were observed. CV (CH2Cl2) V: E1=-0.05 (quasi-reversible),
E2 = þ0.36 (irreversible oxidation).
Preparation of (η3:η3-TME)[Co(CO)2PMe3]2, 2. A solution
of 1 (0.050 g, 0.137 mmol) in THF (30 mL) was added to a
solution of PMe3 (0.031 g, 0.41 mmol) in THF (30 mL). A slight
gas evolution was observed with the mixture changing to a
darker orange color. After stirring for 15 min, all volatiles were
removed under vacuum, leaving a yellow solid. The solid was
extracted with hexane (2 ꢀ 15 mL), and the solution was reduced
to a minimum volume and stored at -35 °C overnight. The
orange crystalline solid formed was collected, washed at -35 °C
with hexane, and dried under vacuum. Yield: 0.028 g (43.6%)
1
(0.060 mmol). H NMR (C6D6) δ (ppm): 0.98 (d, J = 8.8 Hz,
18H, PMe3), 1.54 (s, 4H, C6H8), 2.79 (d, J=5.28 Hz, 4H, C6H8).
13C NMR (C6D6) δ (ppm): 20.12 (d, J=27.64 Hz, PMe3), 39.35
(d, J = 2.59 Hz, allyl terminal carbon), 98.14 (s, allyl central
carbon), 205.01 (s, CO). 31P NMR (C6D6) δ (ppm): 15.10 (s,
PMe3). IR (KBr) cm-1: 1917, 1968. Anal. Calcd for C16H26O4-
P2Co2: C, 41.56; H, 5.67. Found: C, 41.73; H, 5.68. MS (EI) m/z:
461.995 (Mþ), 434.01 (Mþ - (CO)), 406.024. CV (CH2Cl2) V: E1
= -0.07, E2 =þ0.15 (both quasi-reversible).
Preparation of TMEK2. The synthesis was adapted from the
literature preparation of TMEK2.57 A solution of n-BuLi (60 mL,
0.150 mol) in hexane (25 mL) was carefully added to a suspen-
sion of KOt-Am (18.240 g, 0.145 mol) in hexane (40 mL). The
mixture was cooled to -80 °C, and a solution of 2,3-dimethyl-
1,3-butadiene (9.18 mL, 0.081 mol) in hexane (30 mL) was added
dropwise. After stirring for 1 h at room temperature, the orange
solid was collected, washed with hexane (3 ꢀ 50 mL), and dried
thoroughly under vacuum. The pyrophoric yellow powder
obtained was used without further purification. Yield: 11.520 g
(90.1%) (0.073 mol).
Preparation of (η3:η3-TME)[Co(CO)2PEt3]2, 3. The same
procedure for the synthesis of 2 was followed using 1 (0.050 g,
0.137 mmol) in THF (40 mL) and PEt3 (0.048 g, 0.410 mmol) in
THF (40 mL). Yield: 0.025 g (31.4%) (0.043 mmol). 1H NMR
(C6D6) δ (ppm): 0.84 (m, 18H, PCH2CH3), 1.24-1.38 (m, 12H,
PCH2CH3), 1.58 (s, 4H, C6H8), 2.83 (d, J=4.40 Hz, 4H, C6H8).
13C NMR (C6D6) δ (ppm): 8.39 (s, PCH2CH3), 21.73 (d, J =
24.76 Hz, PCH2CH3), 38.81 (d, J = 2.30 Hz, C6H8), 98.33
(s, C6H8), 205.90 (s, CO). 31P NMR (C6D6) δ (ppm): 50.28 (s,
PEt3). IR (KBr) cm-1: 1920, 1957. Anal. Calcd for C22H38O4P2-
Co2: C, 48.36; H, 7.01. Found: C, 48.54; H, 7.12. MS (EI) m/z:
118.076 (PEt3). CV (CH2Cl2) V: E1 =-0.27, E2 =þ0.03 (both
quasi-reversible).
Preparation of (η4:η4-TME)[CoCp*]2, 7. A suspension of
Cp*Li (1.775 g, 0.013 mmol) in THF was cooled to -80 °C,
and a solution of Co(acac)2 (3.190 g, 0.013 mmol) in THF
(40 mL) was added dropwise. After stirring for 4.5 h at room
temperature, the mixture was cooled to -80 °C and a suspension
of TMEK2 (1 g, 0.006 mmol) in THF (40 mL) was slowly added.
After stirring for 12 h at room temperature, all volatiles were
removed under vacuum. The residue was extracted with pentane
Preparation of (η3:η3-TME)[Co(CO)2Pn-Bu3]2, 4. The same
procedure for the synthesis of 2 was followed using 1 (0.050 g,
0.137 mmol) in THF (40 mL) and Pn-Bu3 (0.083 g, 0.410 mmol)
in THF (40 mL). Yield: 0.067 g (69.3%) (0.095 mmol). 1H NMR
(C6D6) δ (ppm): 0.86 (t, J = 7.34 Hz, 18H, PCH2CH2CH2-
CH3), 1.11-1.37 (m, 12H, PCH2CH2CH2CH3), 1.38-1.50
(m, 12H PCH2CH2CH2CH3), 1.50-1.62 (m, 12H, PCH2CH2-
(57) Bahl, J. J.; Bates, R. B.; Gordon, B. J. Org. Chem. 1979, 44, 2290.

5858 Organometallics, Vol. 29, No. 22, 2010
Aguirre-Etcheverry et al.
(3 ꢀ 50 mL). This resulting solution was filtered through Celite,
reduced to minimum volume, and stored at -80 °C overnight.
The dark purple microcrystalline solid obtained was collected
and dried under vacuum. Yield: 1.100 g (37.9%) (0.002 mmol).
1H NMR (C6D6) δ (ppm): -0.44 (d, J=2.00 Hz, 4H, C6H8), 1.71
(s, 30H, Cp*), 1.77 (d, J=2.00 Hz, 4H, C6H8). 13C NMR (C6D6)
δ (ppm): 10.15 (s, C5Me5), 26.87 (s, allyl terminal carbon), 69.67
(s, allyl central carbon), 89.18 (s, C5Me5). Anal. Calcd for
C26H38Co2: C, 66.66; H, 8.18. Found: C, 66.74; H, 8.22. MS
(EI) m/z: 468.165 (Mþ), 139,101 (Mþ - (CoCp*2). CV (MeCN)
V: E1 =-0.93 (reversible), E2 =-0.47 (irreversible oxidation).
Supporting Information Available: The molecular structures
of 3-6 obtained from single crystal X-ray structure determina-
tions, the geometry optimised structures of 1 and 7 and calcu-
lated isosurfaces of selected molecular orbitals of 1 and 7. This
material is available free of charge via the Internet at http://
pubs.acs.org.