Dinuclear Cp*Co amido and alkoxo complexes: Synthesis, structures, and reactivity

Article abstract of DOI:10.1021/om101065z

New amido- and alkoxo-bridged dinuclear cobalt complexes [Cp*Co(μ-X)]₂ (X = NHTol (2), NHBoc (3), OBn (4); Cp* = η⁵-C₅Me₅; Tol = p-tolyl; Boc = t-BuOCO; Bn = CH₂Ph) have been prepared by the reaction of [Cp*Co(μ-Cl)]₂ (1) with LiX and characterized by X-ray analysis. The amido complexes 2 and 3 are diamagnetic and feature a folded Co₂N₂ butterfly structure with a Co-Co single bond. In contrast, the alkoxo complex 4 is paramagnetic and contains a planar Co ₂O₂ core with a long Co???Co separation. Dynamic stereochemical processes in 2 and 3 were investigated by NMR spectroscopy, and cis-trans isomerization in 2 and Co₂N₂ ring inversion in 2 and 3 were identified. Complexes 2 and 4 reacted with carbon monoxide to give N,N′-di-p-tolylurea and dibenzyl carbonate, respectively, along with [Cp*Co(CO)₂] and [Cp*Co(μ-CO) ]₂. Oxidation of 2 with iodine and ferrocenium triflate afforded the Co(III) complexes [(Cp*Co)₂(μ-I)(μ-NHTol)₂]I (5) and [(Cp*Co)₂(μ-OH₂)(μ-NHTol) ₂][OTf]₂ (6), respectively, the latter being characterized by X-ray analysis.

Full text of DOI:10.1021/om101065z

Organometallics 2011, 30, 1013–1020 1013
DOI: 10.1021/om101065z
Dinuclear Cp*Co Amido and Alkoxo Complexes:
Synthesis, Structures, and Reactivity
Shin Takemoto,* Takashi Honma, and Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1,
Naka-ku, Sakai, Osaka 599-8531, Japan
Received November 12, 2010
New amido- and alkoxo-bridged dinuclear cobalt complexes [Cp*Co(μ-X)]2 (X = NHTol (2),
NHBoc (3), OBn (4); Cp* = η5-C5Me5; Tol = p-tolyl; Boc = t-BuOCO; Bn = CH2Ph) have been
prepared by the reaction of [Cp*Co(μ-Cl)]2 (1) with LiX and characterized by X-ray analysis. The
amido complexes 2 and 3 are diamagnetic and feature a folded Co2N2 butterfly structure with a
Co-Co single bond. In contrast, the alkoxo complex 4 is paramagnetic and contains a planar Co2O2
core with a long Co Co separation. Dynamic stereochemical processes in 2 and 3 were investigated
3 3 3
by NMR spectroscopy, and cis-trans isomerization in 2 and Co2N2 ring inversion in 2 and 3 were
identified. Complexes 2 and 4 reacted with carbon monoxide to give N,N0-di-p-tolylurea and dibenzyl
carbonate, respectively, along with [Cp*Co(CO)2] and [Cp*Co(μ-CO)]2. Oxidation of 2 with iodine
and ferrocenium triflate afforded the Co(III) complexes [(Cp*Co)2(μ-I)(μ-NHTol)2]I (5) and [(Cp*Co)2-
(μ-OH2)(μ-NHTol)2][OTf]2 (6), respectively, the latter being characterized by X-ray analysis.
Introduction
we have synthesized and characterized the dimeric Rh(II) amido
complex [Cp*Rh(μ-NHPh)]2 and the mixed amido-alkoxo
complex [(Cp*Rh)2(μ-NHPh)(μ-OMe)].13f These electron-
rich amido complexes release aniline or methanol upon
heating and can serve as precursors of the transient unsatu-
rated imido species {(Cp*Rh)2(μ-NPh)}, which shows inter-
esting reactivity toward alkynes.13f
With these results in hand, we sought to pursue a com-
parative study with Cp*Co amido and alkoxo complexes. An
earlier work by Koelle et al. provided the synthesis of the dia-
magnetic Co(II) amide [Cp*Co(μ-NH2)]2 and the paramagnetic
Late-transition-metal amido and alkoxo complexes pos-
sess reactive metal-nitrogen and -oxygen bonds and have
been proposed to play important roles in catalytic carbon-
nitrogen and -oxygen bond formation.1-9 Pentamethylcy-
clopentadienyl group 8-10 metal fragments have been used
to produce a number of well-defined late-transition-metal
amido and alkoxo complexes, and extensive studies on the
chemistry of these complexes have provided comprehensive
knowledge of the stability and reactivity of late-transition-
metal amido and alkoxo complexes.10-15
Our interest in the synthesis and reactivity of dinuclear
bridging amido and alkoxo complexes, with group 8 and 9
Cp*M fragments,10c,11m-11q,13f stems from their potential to
provide new reactivity patterns of metal-nitrogen and -oxygen
bonds by a cooperative work of the two metal centers. Recently,
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*E-mail: takemoto@c.s.osakafu-u.ac.jp (S.T.); matuzaka@c.s.osakafu-u.
ac.jp (H.M.). Tel: þ81-72-254-9697. Fax þ81-72-254-9931.
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2011 American Chemical Society
Published on Web 02/08/2011
pubs.acs.org/Organometallics

1014 Organometallics, Vol. 30, No. 5, 2011
Takemoto et al.
alkoxide [Cp*Co(μ-OEt)]2 from [Cp*Co(μ-Cl)]2.12a The
μ-NH2 complex was reported to react with carbon monoxide
togive (CONH)x and [Cp*Co(CO)2], indicating the potential
reactivity of these systems. Bergman and co-workers synthe-
sized [Cp*Co(pin)] (pin = pinacolate).12b,c This compound
has been shown to display unique photochemical reactivity12b
and undergo pinacolate group transfer reactions with carbon
monoxide and heterocumulenes.12c Crystal structures are
available for [(Cp*Co)2(μ-Cl)( μ-NMe2)] and [Cp*Co-
(pin)].12a,b Herein we present the synthesis and X-ray struc-
tures of the new dimeric Cp*Co amido and alkoxo com-
plexes [Cp*Co(μ-X)]2 (X = NHTol (2), NHBoc (3), OBn (4))
along with their solution properties and reactivity, including
amido and alkoxo group transfer to carbon monoxide,
alkoxo-amido ligand exchange, and oxidation to Co(III)
complexes.
Scheme 1
Results and Discussion
Synthesis of [Cp*Co(μ-X)]2 (X = NHTol, NHBoc, OBn).
Treatment of [Cp*Co(μ-Cl)]2 (1)12a with 2 equiv of LiNHTol
in THF at low temperature cleanly produced the green
diamagnetic dimeric amido complex [Cp*Co(μ-NHTol)]2
(2; Scheme 1). Under these reaction conditions, complex 2
was produced as a mixture of cis and trans isomers. The ratio
of these isomers was not fully reproducible. The cis:trans
ratio was typically about 1:3. However, we occasionally
observed 1:2 or even 1:1 ratios. Since both isomers are stable
at room temperature, the cis:trans ratio is likely affected by
subtle differences in reaction conditions under which these
isomers are generated. These isomers showed similar solubi-
lities, and crystallized samples of 2 obtained from THF-
MeCN also contained the cis and trans isomers. The isolated
yield of this isomeric mixture is typically 60%.
[Cp*Co(μ-NHBoc)]2 (3), containing the tert-butoxycarbo-
nylamido ligand (Scheme 1). This complex was produced
exclusively as the trans isomer and isolated in 54% yield as
dark green block-shaped crystals.
Similar treatment of 1 with 2 equiv of LiOBn in THF at
low temperature afforded the alkoxo-bridged dimeric com-
plex [Cp*Co(μ-OBn)]2 (4), isolated in 56% yield as dark
brown needles. In contrast to the amido complexes 2 and 3,
the alkoxo complex 4 is paramagnetic.
The initially produced cis and trans mixture of 2 under-
went thermal isomerization to give a trans-only form, as
shown in eq 1. A solution of 2 in C6D6 containing cis-2 and
trans-2 in a ca. 1:1 ratio was monitored at 60 °C by 1H NMR,
which showed conversion of cis-2 to trans-2 over the course
of 12 h with a good first-order time dependence. A plot of
[cis-2] and [trans-2] vs time is provided in the Experimental
Section (Figure 6). This isomerization reaction could also be
performed in THF on a preparative scale to give trans-2 in ca.
40% isolated yield from CoCl2. The exclusive conversion to
the trans isomer provides a contrast to the previously
reported dinickel complex [Cp*Ni(μ-NHTol)]2, which ther-
mally equilibrates to an isomeric mixture with a trans:cis
ratio of about 2:1.15a
Treatment of 1 with 2 equiv of LiNHBoc in THF at low
temperature afforded the analogous dimeric complex
(13) Rh: (a) Espinet, P.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc.,
Dalton Trans. 1979, 1542. (b) Mashima, K.; Abe, T.; Tani, K. Chem.
Lett. 1998, 1199. (c) Murata, K.; Ikariya, T. J. Org. Chem. 1999, 64,
2186. (d) Matsuzaka, H.; Kamura, T.; Ariga, K.; Watanabe, Y.; Okubo,
T.; Ishii, T.; Yamashita, M.; Kondo, M.; Kitagawa, S. Organometallics
2000, 19, 216. (e) Holland, A. W.; Glueck, D. S.; Bergman, R. G.
Organometallics 2001, 20, 2250. (f) Takemoto, S.; Otsuki, S.; Hashimoto,
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ꢀ
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8904. (g) Tejel, C.; Ciriano, M. A.; Jimenez, S.; Passarelli, V.; Lopez,
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K.; Ikariya, T. J. Org. Chem. 1999, 64, 2186. (h) Holland, A. W.; Glueck,
D. S.; Bergman, R. G. Organometallics 2001, 20, 2250. (i) Rais, D.;
Bergman, R. G. Chem.;Eur. J. 2004, 10, 3970. (j) Ishiwata, K.; Kuwata,
S.; Ikariya, T. Organometallics 2006, 25, 5847. (k) Ishiwata, K.; Kuwata,
S.; Ikariya, T. Dalton Trans. 2007, 3606. (l) Arita, H.; Ishiwata, K.;
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2912. (n) Yuan, J.; Hughes, R. P.; Rheingold, A. L. Organometallics
2009, 28, 4646. (o) Ringenberg, M. R.; Nilges, M. J.; Rauchfuss, T. B.;
Wilson, S. R. Organometallics 2010, 29, 1956.
The characterization of 2-4 was based on elemental
analysis, solution NMR spectroscopy (including variable-
temperature measurements), IR spectroscopy, and single-
crystal X-ray diffraction. Details of these characterization
data are described in the following sections.
X-ray Structures of 2-4. A crystallized sample of the
cis-trans mixture of 2 contained some large crystals suitable
for single-crystal X-ray diffraction. Two of these crystals
were examined for diffraction measurements, both of which
showed the structure of the trans isomer. An ORTEP draw-
ing of trans-2 is shown in Figure 1. The molecule features a
butterfly-shaped Co2N2 central core. The Co-Co distance in
(15) Ni: (a) Holland, P. L.; Andersen, R. A.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 1092. (b) Holland, P. L.; Andersen, R. A.;
Bergman, R. G.; Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1997, 119,
12800.

Article
Organometallics, Vol. 30, No. 5, 2011 1015
Figure 2. ORTEP drawing of 3 with 35% thermal ellipsoids.
Hydrogen atoms, except for that involved in the N-H
hydrogen bond (H1), are omitted for clarity. Selected bond
O
3 3 3
˚
lengths (A) and angles (deg): Co1-Co2 = 2.4301(9); Co1-
Figure 1. ORTEP drawing of trans-2 with 35% thermal ellip-
soids. Hydrogen atoms are omitted for clarity. Selected bond
lengths (A) and angles (deg): Co1-Co2 = 2.4276(8); Co1-
N1 = 1.928(2); Co1-N2 = 1.945(2); Co2-N1 = 1.949(2);
Co2-N2 = 1.936(2); N1-Co1-N2 = 81.29(9); N1-Co2-
N2 = 80.98(9); Co1-N1-Co2 = 77.51(8); Co1-N2-Co2 =
77.45(8).
N1 = 1.9567(18); Co1-N2 = 1.9815(18); Co2-N1 =
1.9537(19); Co2-N2 = 1.9811(17); N1-Co1-N2 = 83.70(7);
N1-Co2-N2
Co1-N2-Co2 = 75.65(6).
˚
= 83.79(7); Co1-N1-Co2 = 76.85(6);
˚
˚
trans-2 is 2.43 A, and the Co-N distances are 1.93-1.95 A.
The short Co-Co distance is consistent with a Co-Co single
bond. The folded Co2N2 core shows dihedral angles of 113°
between the Co2N planes and 111° between the CoN2 planes.
The two p-tolyl substituents on the nitrogen atoms adopt an
axial-equatorial orientation. No appreciable hydrogen bond-
ing was detected for the amido N-H groups. The puckered
structure of trans-2 is similar to those reported for
[(Cp*Co)2(μ-Cl)(μ-NMe2)]12a and [Cp*Rh(μ-NHPh)]2,13f
which also contain metal-metal single bonds. On the other
hand, the diiron and dinickel amido complexes [Cp*Fe(μ-
NMePh)]2 and [Cp*Ni(μ-NHAr)]2 have been reported to
have planar M2N2 cores.10b,15a However, [Cp*Ni(μ-NHBut)]2
has been shown to be puckered due to steric constraints.15a
The structure of the Boc amido complex 3, shown in Figure
2, is very similar to that of trans-2. The Co2N2 core is a folded
butterfly, and the Boc substituents adopt an axial-equatorial
Figure 3. ORTEP drawing of 4 with 35% thermal ellipsoids.
˚
Hydrogen atoms are omitted for clarity. Selected distances (A)
˚
orientation. The Co-Co distance is 2.43 A, and the Co-N
˚
distances are 1.95-1.98A. The Co-N distances in3 are longer
than those in trans-2 by about 0.03 A. An intramolecular
and angles (deg): Co1 Co2 = 3.011; Co1-O1 = 1.924(2);
3 3 3
˚
Co1-O2 = 1.916(2); Co2-O1 = 1.926(2); Co2-O2 =
1.914(2); O1-Co1-O2 = 76.24(9); O1-Co2-O2 = 76.24(9);
Co1-O1-Co2 = 102.90(10); Co1-O2-Co2 = 103.66(10).
N-H O hydrogen bond exists between the amido nitrogen
3 3 3
N1 and the tert-butoxy oxygen O4; the N1-O4 distance is 2.78
A. A similar intramolecular hydrogen bond has been reported
˚
for [Cp*Rh(μ-NHTs)]2 (Ts = p-MeC6H4SO2).13h
signals in the normal chemical shift range. For example, Cp*
protons for cis- and trans-2 were observed at δ 1.50 and 1.62
ppm, respectively. Interestingly, not only cis-2 but also trans-
2 showed equivalent TolNH groups on the NMR time scale.
Most diagnostic were the signals for the methyl groups in the
p-tolyl substituents, which appeared at δ 2.27 (cis) and 2.22
(trans) ppm in the 1H NMR spectrum and at δ 21.1 (cis) and
20.9 (trans) ppm in the 13C NMR spectrum. The NH protons
for cis-2 and trans-2 were observed at δ -2.46 and 0.76 ppm,
respectively, which are close to the NH chemical shifts
reported for [Cp*Rh(μ-NHPh)]2 (δ 1.47)13f and [Cp*Ni(μ-
NHTol)]2 (δ -2.08 (trans isomer) and -1.79 (cis isomer)).15a
The alkoxo complex 4 (Figure 3) has an almost planar
Co2O2 core (interior angle sum 359°) with nearly parallel
Cp* rings. The bridging oxygen atoms adopt a trigonal-
planar geometry (bond angle sum 358°). The long Co Co
3 3 3
˚
separation (3.011 A) and wide Co-O-Co angles (ca. 104°)
indicate the absence of a Co-Co bond. This structure
provides a sharp contrast to that of [(Cp*Co)2(μ-pin)], which
has a folded Co2O2 core, pyramidal oxygen atoms, and a
Co-Co single bond.12b
Solution Structural Characterization. The diamagnetic
nature of 2 was evident from its NMR spectra, showing

1016 Organometallics, Vol. 30, No. 5, 2011
Takemoto et al.
Scheme 2
1
A variable-temperature H NMR measurement was con-
ducted for the isolated sample of trans-2. No significant line
broadening or signal decoalescence was observed over the
temperature range of þ30 to -60 °C. These results indicate
the presence of a rapid inversion of the Co2N2 ring that
makes the two TolNH groups equivalent on the NMR time
scale (Scheme 2). A similar ring inversion process has been
proposed for the thiolate-bridged dimeric complex [Cp*Co-
(μ-SMe)]2.12a
In contrast to 2, complex 3 showed inequivalent BocNH
groups in its 1H and 13C NMR spectra at room temperature.
The 1H NMR spectrum of 3 in C6D6 contained signals for the
NH protons at δ 2.95 and 2.91 ppm and for the t-Bu protons
at δ 1.63 and 1.45 ppm. However, variable-temperature
1H NMR spectra of 3 in chlorobenzene-d5 showed coales-
cence of the t-Bu signals at higher temperature (Figure 4).
The estimated activation energy ΔGq at the coalescence
temperature (85 °C) is 18 kcal/mol. These results indicate
that the inversion of the Co2N2 ring is also present in 3, but
this process is considerably slower than that in 2. The slower
ring inversion in 3 compared to that in 2 is likely due to the
Figure 4. Variable-temperature 1H NMR spectra of 3 in chloro-
benzene-d5.
Scheme 3
existence of an intramolecular N-H O hydrogen bond
3 3 3
between the two BocNH groups, which has been identified
by the X-ray structure of 3 (vide supra).
The 1H NMR spectrum of the alkoxo complex 4 showed
signals at δ 51.0 (2H), 43.1 (30H), 38.8 (4H), 8.1 (2H), 7.9
(4H), and 7.7 (2H) ppm, demonstrating the paramagnetic
nature of 4. The most intense peak at δ 43.1 ppm is assignable
to the Cp* protons. This chemical shift is comparable to
those reported for other paramagnetic Cp*Co(II) complexes
(40-50 ppm);12a,16 for example, the chemical shift for the
Cp* protons in [Cp*Co(μ-OEt)]2 is reported to be 50.5 ppm.
Reactivity. The amido complex trans-2 reacted with car-
bon monoxide in C6H6 or C6D6 at room temperature to give
the urea (TolNH)2CO and [Cp*Co(CO)2]19a,b (Scheme 3).
The urea precipitated as a white solid and was isolated in
60% yield by filtration. The urea was identified by 1H NMR,
IR, and MS spectroscopy by referring to literature data.15a,18
The supernatant solution contained [Cp*Co(CO)2]19a,b in
93% yield, estimated by integration of 1H NMR signals
against an internal standard. These products are the outcome
of amido group transfer from trans-2 to carbon monoxide
and have probably been produced via insertion of carbon
monoxide into metal-amido bonds.17
We also observed the reaction of 4 with carbon monoxide
(Scheme 3). In this case, the reaction was performed in THF
with a low initial temperature to improve the selectivity of
the reaction. The major products were dibenzyl carbonate
(73%), [Cp*Co(CO)2] (70%),19a,b and [Cp*Co(μ-CO)]2
(30%),19a,b identified by 1H NMR and IR spectroscopy.
This reaction has an immediate precedent; the reaction of
[Cp*Co(pin)] with CO gives the cyclic carbonate (pin)CO
and [Cp*Co(CO)2], as reported by Bergman and Holland.12c
However, this type of alkoxo transfer reactivity is still rare.
Although we could not observe any intermediates, the reac-
tion probably proceeded via insertion of CO into a me-
tal-oxygen bond.20
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€
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ꢀ
Yamamoto, A. Organometallics 1988, 7, 2182. (d) Campora, J.; Palma,
P.; del Rıo, D.; Alvarez, E. Organometallics 2004, 23, 1652. (e) Wu, J.;
ꢀ
Sharp, P. R. Organometallics 2008, 27, 4810.

Article
Organometallics, Vol. 30, No. 5, 2011 1017
Scheme 4
Scheme 5
Figure 5. ORTEP drawing of 6 with 35% thermal ellipsoids.
Hydrogen atoms and triflate anions are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Co1 Co1* =
3 3 3
2.7244(12); Co1-N1 = 1.992(3); Co1-N2 = 1.994(3); Co1-
O1 = 2.084(3); Co1-N1-Co1* = 86.31(16); Co1-N2-Co1* =
86.19(16); Co1-O1-Co1* = 81.62(13); N1-Co1-N2 =
80.04(13); N1-Co1-O1 = 82.51(13); N2-Co1-O1 = 75.86(14).
(μ-NHTol)2]I (5) and [(Cp*Co)2(μ-OH2)(μ-NHTol)2][OTf]2
(6), respectively (Scheme 5). Complex 5 is dark green, while
complex 6 is almost black. Both of these products were
isolated in ca. 60% yield after recrystallization. The aquo
ligand in 6 probably came from adventitious water in acetone
used as the reaction solvent. Although a mixture of cis and
trans isomers of 2 was used as the starting material, the two
p-tolyl groups in 5 and 6 were exclusively trans with respect
to the Co2N2 ring. This orientation is maintained in solu-
tion, at least at room temperature, as indicated by inequi-
valent TolNH groups in the 1H and 13C NMR spectra of 5
and 6.
The molecular structure of 6 was determined by single
crystal X-ray diffraction (Figure 5). The molecule has a
crystallographic mirror plane that makes the two Cp*Co
units symmetry-related. All non-hydrogen atoms in the
bridging aquo and TolNH ligands are lying on this mirror
plane, and the two p-tolyl substitutents are trans with respect
to the Co2N2 ring. The distance between the cobalt atoms is
The alkoxo complex 4 underwent an alkoxo-amido ex-
change reaction with 2 equiv of BocNH2 in C6D6 at room
temperature (Scheme 4). This reaction was complete within
30 min and produced the amido complex 3 and benzyl
1
alcohol quantitatively, as identified by H NMR spectros-
copy. The reaction of 4 with TolNH2 was slow at room
temperature but could proceed in toluene at 120 °C in the
presence of a 10-fold excess of TolNH2 to give trans-2 over
the course of 2 days (Scheme 4). Under these conditions, 4
completely disappeared and trans-2 was observed by
1H NMR spectroscopy along with several paramagnetic
byproducts; trans-2 was isolated in 42% yield after removal
of the byproducts and excess TolNH2 by washing with
MeCN.
The ligand exchange reaction between 4 and BocNH2
demonstrates the greater thermodynamic stability of the
amido complex 3 compared to the alkoxo complex 4. The
reaction of 4 with TolNH2 also suggests a similar trend.
These results are consistent with alkoxo-amido exchange
between [Rh(cod)(μ-OMe)]2 and TolNH2 and hydroxo-
amido exchange between [PdPh(PPh3)(μ-OH)]2 and RNH2
(R = t-Bu, i-Bu, sec-Bu, Ph, 2,6-Me2C6H3),21,22 for which
the exchange equilibrium favors the formation of amido
complexes. This trend can be attributed to the greater bridg-
ing capability of amido ligands compared to alkoxo ligands.9
In contrast, alkoxo-amido exchange equilibria observed for
monomeric systems such as [Cp*RuX(PMe3)2],11a [Cp*IrHX-
(PPh3)],14b and [Cp*NiX(PEt3)]15b (X = amido, alkoxo)
tend to less strongly favor amido complexes.
˚
˚
2.72 A, which is much longer than that of trans-2 (2.43 A) and
is consistent with the absence of a Co-Co bond in 6. The
aquo ligand forms hydrogen bonds with both of the two
triflate anions (not shown). Each triflate anion shows the
˚
shortest O O distance of ca. 2.6 A from the aquo oxygen
3 3 3
atom.
Finally, we attempted the reaction of 2 with diphenylace-
tylene, since we previously observed the reaction of the
rhodium analogue [Cp*Rh(μ-NHPh)]2 with diphenylacety-
lene. In this dirhodium system, the reaction proceeded via the
transient imido species {(Cp*Rh)2(μ-NPh)} and produced
the imido-alkyne cycloaddition product [(Cp*Rh)2(μ-
PhNCPhCPh)].13f When the dicobalt amido complex 2 was
heated in C6D6 at 120 °C for 7 days in the presence of
10 equiv of diphenylacetylene, ca. 80% of 2 remained intact.
Although FAB-MS spectrum of the reaction mixture showed
an envelope at m/z 671 that might correspond to a trace
amount of the imido-alkyne cycloaddition product
[(Cp*Co)2(μ-TolNCPhCPh)], we do not have any further
evidence for the formation of such species.
Oxidation of 2 with iodine or ferrocenium triflate afforded
the triply bridged dinuclear Co(III) complexes [(Cp*Co)2(μ-I)-
ꢀ
(21) Tejel, C.; Ciriano, M. A.; Bordonaba, M.; Lopez, J. A.; Lahoz,
F. J.; Oro, L. A. Inorg. Chem. 2002, 41, 2348.
(22) Driver, M. S.; Hartwig, J. F. Organometallics 1997, 16, 5706.

1018 Organometallics, Vol. 30, No. 5, 2011
Takemoto et al.
Conclusions
The new amido- and alkoxo-bridged dinuclear Cp*CoII
complexes 2-4 have been synthesized, and their solid-state
structures and solution properties have been elucidated by
single-crystal X-ray diffraction and NMR spectroscopic
investigations. The data presented here provide a detailed
and clear picture of the structures of the dimeric μ-amido and
μ-alkoxo complexes of the type [Cp*Co(μ-X)]2, helping to
advance our understanding from the earlier work by Koelle
et al.12a The reactions of the amido complex 2 and the alkoxo
complex 4 with carbon monoxide have provided new exam-
ples of stoichiometric C-N and C-O bond-forming reac-
tions of late-transition-metal amido and alkoxo complexes,
yielding the urea and carbonate products, respectively. The
complete transfer of the amido and alkoxo groups from the
metal centers to carbon monoxide has been observed, and
such facile elimination of organic products may be advanta-
geous in developing catalytic processes if viable pathways for
the regeneration of Co-N and Co-O bonds could be
devised. In contrast to the previously reported [Cp*Rh(μ-
NHPh)]2,13f the cobalt amido complex 2 appears to be not
very reactive toward alkynes. However, more work should
be required to elucidate the differences in reactivity between
these two systems, and our study is currently directed to
develop further information about the reactivity of both the
cobalt and rhodium systems in a comparative way.
Figure 6. Plot of normalized concentrations of cis-2 and trans-2
vs time in C6D6 at 60 °C.
(br, 2H, NH). 13C{1H} NMR (C6D6) for cis-2: δ 154.9, 129.5,
128.4, 124.8 (aryl), 82.4 (C5Me5), 21.1 (C6H4Me), 10.1 (C5Me5).
1H NMR (C6D6) for trans-2: δ 7.20 (4H, aryl), 7.00 (4H, aryl),
2.22 (s, 6H, C6H4Me), 1.62 (s, 30H, Cp*), 0.76 (br, 2H, NH).
13C{1H} NMR (C6D6) for trans-2: δ 154.9, 129.4, 129.1, 124.8
(aryl), 82.4 (C5Me5), 20.9 (C6H4Me), 10.3 (C5Me5).
Isolation of trans-[Cp*Co(μ-NHTol)]2 (trans-2). A mixture of
cis and trans isomers of 2 (759 mg, 1.25 mmol, cis:trans = 1:3)
was dissolved in THF (10 mL), and the solution was stirred at
50 °C for 14 h. Then the solution was concentrated to ca. 5 mL
and layered with acetonitrile (15 mL) to give dark green plates of
trans-2. Yield: 458 mg. Anal. Calcd for C34H46N2Co2: C, 67.99;
H, 7.72; N, 4.66. Found: C, 67.74; H, 7.80; N, 4.52. IR (Nujol):
3462 (w), 3285 (w), 3270 (w), 2714 (w), 2490 (w), 2279 (m), 1865
(w), 1767 (w), 1607 (m), 1503 (m), 1247 (m), 1217 (m), 1104 (w),
Experimental Section
1024 (m), 821 (m) cm-1
.
General Remarks. All manipulations were performed under
an atmosphere of nitrogen using standard Schlenk techniques.
[Cp*Co(μ-Cl)]2 (1)12a was prepared in situ from Cp*Li and
anhydrous CoCl2 in THF. The lithium reagents LiNHTol,
LiNHBoc, and LiOBn were prepared by the treatment of
corresponding amines and alcohol with n-BuLi in THF at
-80 °C. Anhydrous solvents were used for all experiments;
these solvents were purchased from Kanto Chemical Co., Inc.
(THF, hexane, and toluene) or Wako Pure Chemical Industries,
Ltd. (acetonitrile and acetone) and degassed before use. Deut-
erated solvents were obtained from Cambridge Isotope Labora-
tories, Inc., degassed, and stored over activated 4A molecular
sieves. Other reagents were purchased from commercial vendors
and used without further purification unless otherwise noted.
1H NMR spectra were recorded on a JEOL ECP500 spectro-
meter at the field strength of 500.16 MHz. 13C{1H} NMR
spectra were obtained on a Varian VNMR400 spectrometer at
the field strength of 100.55 MHz. FT-IR spectra were recorded
on a JASCO FT-IR 4100 spectrometer. FAB-MS analysis was
performed on a JEOL JMS700 spectrometer. Elemental ana-
lyses were performed on a Perkin-Elmer 2400 Series II CHNS/O
analyzer.
[Cp*Co(μ-NHTol)]2 (2, Cis and Trans Mixture). A solution of
[Cp*Co(μ-Cl)]2 (1) in THF (30 mL) was prepared by mixing
CoCl2 (519 mg, 4.00 mmol) and Cp*Li (4.34 mmol), each
suspended in 15 mL of THF. The solution was cooled to
-80 °C and treated with a solution of LiNHTol (4.35 mmol)
in 20 mL of THF. The mixture was warmed slowly to room
temperature and stirred for 14 h. The resulting dark green
solution was evaporated to dryness, and the residue was ex-
tracted with hexane (total 80 mL). The extract was evaporated
to dryness, and the residue was recrystallized from THF-
MeCN to give 2 (cis:trans = 1:3) as dark green crystals. Yield:
759 mg (1.25 mmol, 63% from CoCl2). Anal. Calcd for
C34H46N2Co2: C, 67.99; H, 7.72; N, 4.66. Found: C, 67.70; H,
7.99; N, 4.74. 1H NMR (C6D6) for cis-2: δ 7.53 (4H, aryl), 7.08
(4H, aryl), 2.27 (s, 6H, C6H4Me), 1.50 (s, 30H, Cp*), -2.46
Cis-Trans Isomerization of [Cp*Co(μ-NHTol)]2 (2). A mix-
ture of cis and trans isomers of 2 (19 mg, 0.033 mmol, cis:trans =
1:1) was dissolved in C6D6 (0.75 mL) with 1,3,5-trimethoxyben-
zene (12 mg, 0.073 mmol) as internal standard. The solution was
sealed in an NMR tube and heated at 60 °C in an NMR probe.
The progress of the isomerization reaction was monitored by 1H
NMR spectroscopy. Concentrations of cis-2 and trans-2 were
determined by integration of their Cp* signals against the inter-
nal standard (OMe signal). A plot of normalized concentrations
of cis-2 and trans-2 vs time is shown in Figure 6.
[Cp*Co(μ-NHBoc)]2 (3). A solution of [Cp*Co(μ-Cl)]2 (1) in
THF (30 mL) was prepared by mixing CoCl2 (649 mg, 5.00
mmol) and Cp*Li (5.62 mmol), each suspended in 15 mL of
THF. The solution was cooled to -80 °C and treated with a
solution of LiNHBoc (5.60 mmol) in 15 mL of THF. The
resulting mixture was warmed slowly to room temperature
and stirred for 14 h to give a dark green solution. The solution
was evaporated to dryness, and the residue was extracted with
hexane (total 90 mL). The extract was evaporated to dryness,
and the residue was recrystallized from THF-MeCN to give 3
as dark green block crystals. Yield: 833 mg (1.34 mmol, 54%
from CoCl2). Anal. Calcd for C30H50N2O4Co2: C, 58.06; H,
8.12; N, 4.51. Found: C, 58.07; H, 7.70; N, 4.59. 1H NMR
(C6D6): δ 2.95, 2.91 (br s, 1H each, NH), 1.81 (s, 30H, Cp*), 1.63,
1.45 (s, 9H each, t-Bu). 13C{1H} NMR (C6D6): δ 168.7, 168.3
(CdO), 80.7 (C5Me5), 78.4, 78.0 (OCMe3), 28.9, 28.3 (OCMe3),
10.0 (C5Me5). IR (Nujol): 3322 (s), 3303 (s), 2714 (m), 2391 (w),
2198 (m), 2003 (w), 1716 (s), 1673 (s), 1562 (w), 1069 (m), 1044
(m), 1024 (m), 1044 (m), 1007 (m), 864 (m) cm-1
.
[Cp*Co(μ-OBn)]2 (4). A solution of [Cp*Co(μ-Cl)]2 (1) in
THF (40 mL) was prepared by mixing CoCl2 (649 mg, 5.00 mmol)
and Cp*Li (5.62 mmol), each suspended in 20 mL of THF. The
solution was cooled to -80 °C and treated with a solution of
LiOBn (5.61 mmol) in 15 mL of THF. The resulting mixture was
warmed slowly to room temperature and stirred for 14 h to give
a dark brown solution. The solution was evaporated to dryness,
and the residue was extracted with hexane (total 100 mL).

Article
Organometallics, Vol. 30, No. 5, 2011 1019
Table 1. Crystal Data and Structure Refinement Parameters for 2,3,4, and 6
2
3
4
6 2Me2CO
3
formula
formula wt
T (°C)
C
600.59
23
monoclinic
P21/c (No. 14)
9.358(3)
33.108(9)
10.463(4)
90
108.229(14)
90
3078.9(17)
4
1.296
34H46N2Co2
C30H50N2O4Co2
620.58
23
monoclinic
P21/a (No. 14)
16.742(6)
11.467(3)
18.309(5)
90
102.033(13)
90
3438.0(18)
4
1.199
C34H44O2Co2
602.55
23
C42H60N2O9F6S2Co2
1032.90
23
orthorhombic
Pnma (No. 62)
22.352(5)
13.807(4)
16.094(6)
90
90
90
4967(2)
4
1.381
cryst syst
space group
triclinic
P1 (No. 2)
8.641(5)
11.111(6)
17.681(9)
89.64(2)
83.19(2)
68.685(18)
1568.9(15)
2
1.276
1.084
0.0461
0.1365
1.027
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
Dcalcd (g/cm3)
μ (mm-1
)
1.102
0.997
0.0404
0.1256
1.016
0.825
R1 (I > 2σ(I))a
wR2 (all data)b
GOF on F2
P
0.0422
0.1223
1.047
0.0710
0.2377
1.064
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ (w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
The extract was evaporated to dryness, and the residue was
recrystallized from toluene-MeCN to give 4 as dark brown
needles. Yield: 845 mg (1.40 mmol, 56% from CoCl2). Anal.
Calcd for C34H44O2Co2: C, 67.77; H, 7.36. Found: C, 67.69; H,
Reaction of [Cp*Co(μ-OBn)]2 (4) with TolNH2 To Give
[Cp*Co(μ-NHTol)]2 (2). Complex 4 (151 mg, 0.251 mmol) and
TolNH2 (269 mg, 2.51 mmol) were dissolved in 5 mL of toluene,
and the solution was stirred at 120 °C for 48 h, during which time
the solution changed from dark brown to green. The solution
was evaporated to dryness, and the residue was washed with
MeCN to give trans-2 as a green solid. Yield: 63 mg, 42%. The
purity was checked by 1H NMR spectroscopy.
1
7.60. H NMR (C6D6): δ 50.96 (br s, 2H), 43.08 (br s, 30H),
38.78 (br s, 4H), 8.14 (br s, 2H), 7.86 (br s, 4H), 7.68 (br s, 2H).
IR (Nujol): 2725 (m), 2693 (m), 2385 (w), 2312 (w), 2215 (w),
1858 (w), 1796 (w), 1599 (m), 1583 (m), 1191 (m), 1155 (m), 1096
(m), 1061 (m), 1024 (m), 903 (m), 722 (m) cm-1
.
[(Cp*Co)2(μ-I)(μ-NHTol)2]I (5). To a solution of [Cp*Co(μ-
NHTol)]2 (2) (cis-trans mixture, 70 mg, 0.12 mmol) in 7 mL
of toluene was added a solution of I2 (30 mg, 0.12 mmol) in 3 mL
of toluene at -80 °C. The mixture was warmed slowly to room
temperature and stirred for 12 h to give a dark green suspension.
The dark green solid was collected by filtration, dried under
reduced pressure, and recrystallized from acetone-hexane to
give 5 as a dark green crystalline solid. Yield: 58 mg, 57%. 5
was also obtained in a similar manner using trans-2 as a starting
material. Anal. Calcd for C34H46N2Co2I2: C, 47.80; H, 5.42;
N, 3.28. Found: C, 47.38; H, 5.51; N, 3.08. 1H NMR (acetone-
d6): δ 8.21 (d, 1H, aryl), 8.07 (d, 1H, aryl), 7.82 (d, 1H, aryl),
7.77 (d, 1H, aryl), 7.39 (d, 1H, aryl), 7.25 (d, 1H, aryl), 7.16 (d,
1H, aryl), 7.12 (d, 1H, aryl), 3.67, 3.35 (br s, 1H each, NH),
2.33, 2.25 (s, 3H each, C6H4Me), 1.07 (s, 30H, Cp*). 13C{1H}
NMR (acetone-d6): δ 152.3, 151.8, 133.4, 132.9, 130.7, 130.1,
129.6, 128.9, 128.2, 127.8, 125.3, 122.0 (aryl), 89.6 (C5Me5),
20.9, 20.3 (C6H4Me), 8.6 (C5Me5). IR (Nujol): 3383 (w),
3301 (w), 3190 (m), 2733 (w), 2594 (m), 2227 (m), 1910 (w),
1705 (m), 1602 (m), 1266 (w), 1154 (w), 1031 (m), 920 (m), 719
Reaction of [Cp*Co(μ-NHTol)]2 (2) with CO. Compound
trans-2 (70 mg, 0.12 mmol) and 1,3,5-trimethoxybenzene
(internal standard, 19 mg, 0.11 mmol) were dissolved in 1 mL
of C6D6, and carbon monoxide was introduced into the solu-
tion. The mixture was stirred under a CO atmosphere at room
temperature for 2 h to give a suspension containing a brown
solution and a white precipitate. The 1H NMR spectrum of the
solution showed the formation of [Cp*Co(CO)2] (δ 1.64)19a in
93% yield. Separately, the reaction was repeated in C6H6 (5 mL)
with trans-2 (99 mg, 0.17 mmol). The precipitate was collected
by filtration, washed with hexane (3 mL), and dried in vacuo to
give (TolNH)2CO as a white powder. Yield: 24 mg, 60%.
1H NMR (DMSO-d6): δ 8.81 (br s, 2H, NH), 7.34 (d, 4H, aryl),
7.07 (d, 4H, aryl), 2.23 (s, 6H, C6H4Me). IR (Nujol): 3308 (s),
1644 (m), 1595 (s), 1546 (m), 1514 (m). FAB-MS (m-nitrobenzyl
alcohol): m/z 241 ([M þ H]þ).
Reaction of [Cp*Co(μ-OBn)]2 (4) with CO. Complex 4 (85 mg,
0.14 mmol) and 1,3,5-trimethoxybenzene (internal standard, 23
mg, 0.14 mmol) were dissolved in 5 mL of THF, and the solution
was cooled to -95 °C. Carbon monoxide (1 atm) was introduced
into the solution, and the mixture was stirred under a CO
atmosphere for 2 h, during which period the solution was
warmed to room temperature. An aliquot of the resulting
emerald green solution was evaporated under reduced pressure,
and the residue was analyzed by 1H NMR (C6D6) and IR
(Nujol), which showed the formation of [Cp*Co(CO)2] (70%;
δ 1.60; νCO 2001, 1938 cm-1),19a,b [Cp*Co(μ-CO)]2 (30%; δ
1.42; νCO 1750 cm-1),19c,d and (BnO)2CO (73%; δ 7.05 (m, 10H)
and 4.93 (s, 4H); νCO 1750 (overlapping with μ-CO) and 1260
cm-1). The data for dibenzyl carbonate were identical with those
of an authentic sample.
(m) cm-1
.
[(Cp*Co)2(μ-OH2)(μ-NHTol)2][OTf]2 (6). To a solution of
[Cp*Co(μ-NHTol)]2 (2; cis-trans mixture, 115 mg, 0.191 mmol)
in 5 mL of acetone was added a solution of [Cp2Fe]OTf (129 mg,
0.385 mmol) in 3 mL of acetone at -80 °C. The mixture was
warmed slowly to room temperature and stirred for 1 h to give a
black solution. The solution was evaporated to dryness, and the
residue was washed with diethyl ether (40 mL). The resulting
solid was recrystallized from acetone-hexane to give 6 as black
block-shaped crystals. Yield: 109 mg, 55%. 6 was also obtained
in a similar manner using trans-2 as a starting material. Anal.
Calcd for C36H48F6N2O7S2Co2 2Me2CO: C, 48.83; H, 5.85; N,
3
Reaction of [Cp*Co(μ-OBn)]2 (4) with BocNH2 To Give
[Cp*Co(μ-NHBoc)]2 (3). Complex 4 (61 mg, 0.10 mmol),
BocNH2 (24 mg, 0.203 mmol), and 1,3,5-trimethoxybenzene
(19 mg, 0.11 mmol) as internal standard were dissolved in 1 mL
of C6D6, and the solution was stirred at room temperature for 30
min, during which time the solution changed from dark brown
2.71. Found: C, 48.73; H, 5.79; N, 2.84. The solvating acetone
molecules were also observed by X-ray crystallography.
1H NMR (acetone-d6): δ 8.21 (d, 1H, aryl), 8.15 (d, 1H, aryl),
8.05 (d, 1H, aryl), 7.53 (d, 1H, aryl), 7.51 (d, 1H, aryl), 7.40 (d,
1H, aryl), 7.36 (d, 1H, aryl), 7.27 (d, 1H, aryl), 2.42, 2.38 (s, 3H
each, C6H4Me), 2.69, 1.93 (br s, 1H each, NH), 2.10 (br s, 2H,
OH2), 0.89 (s, 30H, Cp*). 13C{1H} NMR (DMSO-d6): δ 152.3,
150.2, 131.1, 130.8, 129.7, 129.1, 128.2, 127.0, 126.8, 126.4,
1
to green. The H NMR spectrum showed quantitative forma-
tion of 3 and benzyl alcohol.

1020 Organometallics, Vol. 30, No. 5, 2011
Takemoto et al.
122.8, 120.9 (aryl), 120.5 (q, JCF = 320 Hz, CF3SO3), 86.9
(C5Me5), 20.5 (C6H4CH3), 7.8 (C5Me5).
and hydrogen atoms were placed at calculated positions as
riding models. Thermal ellipsoid plots were drawn with the
ORTEP-3 program26 and presented at the 35% probability
level. The crystal data and structure refinement parameters are
summarized in Table 1. The checkcif report for 3 features a level-
A alert, which warns that there is a large variation in U(eq)
values of carbon atoms in the structure. This alert is due to a
minor disorder in a Cp* group and does not affect the results of
the structure determination.
X-ray Crystallography. A single crystal of each sample was
coated with Paratone-N hydrocarbon oil and mounted on a
glass capillary. All measurements were performed on a Rigaku
R-AXIS Rapid imaging plate diffractometer with graphite-
˚
monochromated Mo KR radiation (λ = 0.710 69 A). The frame
data were processed using the Rigaku PROCESS-AUTO
program,23 and the reflection data were corrected for absorption
with an ABSCOR program.24 The structures were solved by a
direct method (SHELXS 97) and refined on F2 by full-matrix
least-squares methods by using SHELX97.25 All non-hydrogen
atoms were refined with anisotropic displacement parameters,
Acknowledgment. This work was supported in part by
a Grant-in-Aid for Scientific Research from the Ministry
of Education, Science, Sports, and Culture of Japan (No.
21550064 and No. 18033046 Research on Priority Area
“Chemistry of Coordination Space”). We also thank
Toyota Motor Corp. for financial support.
(23) PROCESS AUTO, Automatic Data Acquisition and Processing
Package for Imaging Plate Diffractometer; Rigaku Corp., Tokyo, Japan,
1998.
(24) Higashi, T. ABSCOR, Empirical Absorption Correction Based on
Fourier Series Approximation; Rigaku Corp.: Tokyo, Japan, 1995.
(25) Sheldrick, G. M. SHELX97, Program for Crystal Structure
Supporting Information Available: CIF files giving crystal-
lographic data for 2-4 and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
€
€
Determination; University of Gottingen, Gottingen, Germany, 1997.
(26) ORTEP-3 for Windows: Farrugia, L. J. J. Appl. Crystallogr.
1997, 30, 565.