Synthesis and catalytic activity of [Cp′Co(COD)] complexes bearing pendant N-containing groups

Article abstract of DOI:10.1021/om400330x

The novel Co(I)-complex [Cp^CNCo(COD)] (Cp^CN = η⁵-(C₅H₄CMe₂CH₂CN), COD = 1,5-cyclooctadiene; 3) with a substituted cyclopentadienyl ligand containing a pendant nitrile moiety has been synthesized and characterized by X-ray diffraction. The reactivity of the nitrile group in 3 has been investigated regarding its behavior in cyclization reactions with alkynes, leading to three new complexes containing pendant 2-pyridyl groups. All synthesized complexes have been evaluated as catalysts in the [2 + 2 + 2] cycloaddition reaction of 1,6-heptadiyne and benzonitrile.

Full text of DOI:10.1021/om400330x

Note
pubs.acs.org/Organometallics
Synthesis and Catalytic Activity of [Cp′Co(COD)] Complexes Bearing
Pendant N‑Containing Groups
Indre Thiel,† Martin Lamac,*,‡ Haijun Jiao,† Anke Spannenberg,† and Marko Hapke*,†
̌
†Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
̈
̈
‡J. Heyrovsky Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsk
Czech Republic
̌
ova 2155/3, 182 23 Prague 8,
́
S
* Supporting Information
ABSTRACT: The novel Co(I)-complex [CpCNCo(COD)]
(CpCN = η5-(C5H4CMe2CH2CN), COD = 1,5-cyclooctadiene;
3) with a substituted cyclopentadienyl ligand containing a
pendant nitrile moiety has been synthesized and characterized
by X-ray diffraction. The reactivity of the nitrile group in 3 has
been investigated regarding its behavior in cyclization reactions
with alkynes, leading to three new complexes containing pendant 2-pyridyl groups. All synthesized complexes have been
evaluated as catalysts in the [2 + 2 + 2] cycloaddition reaction of 1,6-heptadiyne and benzonitrile.
which comprised the coordination chemistry of a new nitrile-
substituted Cp ligand (C5H4CMe2CH2CN)− (CpCN) toward
group IV elements,12a we decided to study Co(I)-complexes of
this ligand. We were particularly curious whether the nitrile
moiety, which would be tied in a close proximity of the cobalt
center, would behave only as a potential donor ligand or might
eventually participate as a substrate in the cyclization reaction.
As a result, we report herein the synthesis of a new CpCNCo(I)-
complex with a pendant nitrile group and its subsequent
reactivity under cycloaddition conditions.
INTRODUCTION
■
The coordination chemistry of functionalized cyclopentadienyl
(Cp) or indenyl ligands with the whole range of transition
metals has received profound attention in recent years.1 Various
functional moieties attached to the η5-coordinated Cp ligands
have been used to prepare derivatives of remarkable structural
diversity. Nevertheless, the development of methods leading to
the preparation of some functionalized organometallic com-
pounds and especially of their further synthetic transformations
has remained a challenging area. The latter is true particularly
for inherently sensitive organometallics such as those featuring
highly electrophilic early transition metals.2
RESULTS AND DISCUSSION
■
Among the monocyclopentadienyl (half-sandwich) com-
pounds, there have also been several reports in literature on
CpCo(I)-complexes containing an additional donor group,
which is linked to the Cp ring.3 Prominent examples include
systems with a phosphine ligand connected by variable bridge
The cobalt complex [ClCo(PPh3)3] (1) is a standard precursor
for the generation of Co(I)-complexes with coordinated
(substituted) cyclopentadienyl derivatives.5e,14 The reaction
between the precursor complex 1 and the lithium salt CpCNLi
(2) proceeds very fast, and the bright-green suspension of the
starting complex turns into a clear dark-red solution. The
resulting bisphosphine complex undergoes a subsequent
substitution reaction with 1,5-cyclooctadiene (COD) at
elevated temperatures, resulting in the formation of 3 (Scheme
1). The separation of triphenylphosphine from 3 was achieved
lengths to the Cp ring, as Butenschon et al. have described,4 but
̈
also complexes with chelating olefin moieties.3b,d,5 Related
Fe(II)-complexes with chelating Cp-tethered NHC ligands
have also been reported,6 as well as CpCr(III)-complexes with
coordinated amine donor groups, that have been successfully
applied in olefin polymerization catalysis.7 Moreover, CpRu-
(II)-complexes containing chelating phosphine-,8 amine-,9 or
pyridyl-10 substituted Cp ligands have been described. All these
chelating ligand systems bring about higher stability of the
respective complexes. On the other hand, a rare example of a
Cp-tethered nitrile group in a half-sandwich Ru(II)-complex
has been reported, which did not exhibit the intramolecular
coordination of the pendant moiety.11
Scheme 1. Synthesis of the Nitrile-Tethered CpCNCo(I)-
Complex 3
Our groups have been recently interested in the preparation
of functionalized Cp derivatives and the synthetic modifications
of ligands attached to organometallic moieties,12 as well as in
the development of precatalysts for the [2 + 2 + 2]
cycloaddition reaction.13 On the basis of our previous research,
Received: April 17, 2013
© XXXX American Chemical Society
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by a procedure published by Lipshutz and Blomgren using
Merrifield resin to remove the phosphine as its phosphonium
salt.15
Because CpCo(I)-catalysts prefer the cyclotrimerization of
two alkynes and one nitrile over the generation of benzene
derivatives, we thought it might be possible for 3 to “self-
catalyze” its own pyridine modification by reacting its pendant
nitrile group with two acetylene molecules instead of just
producing benzene, and, indeed, irradiation of a solution of 3 in
THF under an acetylene atmosphere yielded complex 4 with a
pendant pyridine group (Scheme 2). This complex was purified
Scheme 2. Synthesis of Co(I)-Complexes 4−6
Figure 1. ORTEP drawing of the molecular structure of 3. Ellipsoids
are set at 30% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Co1−C1 2.0168(12),
Co1−C2 2.0163(12), Co1−C5 2.0255(13), Co1−C6 2.0185(12),
Co−Cg 1.704, C1−C2 1.411(2), C5−C6 1.407(2), C13−C14
1.5107(15), C14−C15 1.554(2), C15−C16 1.467(2), C16−N1
1.140(2), C13−C14−C15 107.77(9), C14−C15−C16 112.01(10),
C15−C16−N1 178.02(14), C9−C13−C14−C15 −80.62(14), C13−
C14−C15−C16 −62.33(12); Cg is the centroid of the ring C9−C13.
by a short filtration over neutral aluminum oxide with THF
under inert gas atmosphere. Complex 3 was also able to react
with 2-butyne, forming complex 5 with a pendant tetramethyl-
pyridine moiety. Pure compound 5 could be isolated from the
additional cyclization product hexamethylbenzene by crystal-
lization from a pentane solution at −40 °C. A similar result was
obtained when 3 was irradiated in the presence of 1,6-
heptadiyne. In the latter case, the respective tethered bicycle
was formed. Because an excess of 1,6-heptadiyne was used, the
resulting complex 6 could not be separated from the
homocyclization product of three 1,6-heptadiyne molecules.
In addition, we were able to obtain single crystals of 3 and 5
suitable for X-ray structural analysis (Figures 1 and 2). Both
compounds crystallized in a centrosymmetric space group P21/
c with two pairs of inversion-related molecules in the unit cell.
In both structures, the pendant moieties are oriented above the
plane of the respective Cp rings, away from the metal center.
There is obviously no intra- or intermolecular coordination of
the nitrogen atoms of the nitrile or pyridyl functional groups.
Notably, the structure of 5 in the solid state is stabilized by
offset π−π stacking interactions of the pairs of inversion-related
pyridyl rings. The distance of the ring centroids is 3.8506(10)
Å, and the rings are slipped by 1.57 Å. The orientation of the
pendant substituent in 3 with respect to the Cp ring is different
from that observed in group IV metal complexes with the same
ligand.12a The coordination environment of the central metal
Figure 2. ORTEP drawing of the molecular structure of 5. Ellipsoids
are set at 30% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Co1−C1 2.017(2),
Co1−C2 2.017(2), Co1−C5 2.018(2), Co1−C6 2.009(2), Co−Cg
1.712(8), C1−C2 1.408(3), C5−C6 1.402(3), C13−C14 1.515(2),
C14−C15 1.558(2), C15−C16 1.507(3), C16−N1 1.340(2), C13−
C14−C15 108.99(14), C14−C15−C16 115.85(14), C15−C16−N1
114.29(15), C9−C13−C14−C15 −75.4(2), C13−C14−C15−C16
56.0(2), C14−C15−C16−N1 80.8(2); Cg is the centroid of the
ring C9−C13.
atom is, on the other hand, rather unexceptional compared to
other [Cp′Co(COD)] complexes.16
Butenschon et al. already discussed the appropriate length of
̈
the linker between the cyclopentadienyl ring and the phosphine
group to achieve optimal coordination to the cobalt center.4a
While a one-carbon bridge is too short to result in a good
chelation, an ethylene or propylene linker leads to the
respective chelating complexes. In view of these observations,
the coordination of the nitrile moiety to the cobalt center once
the metallacyclopentadiene is formed after oxidative addition of
the two acetylene molecules might be possible. Furthermore,
the particular disposition of a CMe2 group in the ligand might
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principally support its chelating coordination due to the
Thorpe−Ingold effect.17 There have been two reports by
Okuda et al. on cobalt catalysts that are able to perform a [2 + 2
+ 2] cycloaddition reaction on their own ligands.5a,b In contrast
to the results from Okuda et al., a complex with a η4-
coordinated pyridine could not be isolated in our case because
in the presence of free COD the coordination of COD is much
more preferred to that of pyridine.
cyclization reaction between 1,6-heptadiyne and benzonitrile to
form pyridine 7 (Figure 4).13a
To test the possibility of an intramolecular “self-catalyzed” or
intermolecular cycloaddition, we have calculated the coordina-
tion ability of the pendant nitrile group in complex 3 to the
cobalt center. As a model, we were interested in the energetic
and related geometric changes due to the substitution of one
coordinated CC double bond of COD by the nitrile group.
Figure 3 shows the BP86/SVP optimized structures of 3 and
Figure 4. Catalytic evaluation of different [Cp′Co(COD)] complexes:
[CpCo(COD)], 3, and 4.
Figure 3. BP86 optimized structures (bond length in Å and bond
angles in deg) of 3 (left) and 3a (right) (hydrogen atoms are omitted
for clarity).
The results clearly show that while [CpCo(COD)] or 3 give
a full conversion of the diyne in 30 min, complex 4 − having
the most sterically demanding tethered group on the Cp ligand
− needs reaction times as long as 24 h until a nearly full
conversion is achieved. The high activity of 3, even though it
can react with 1,6-heptadiyne to form 6 and therefore create an
even larger pendant group, can be explained by the competition
of the nitrile group of 3 and benzonitrile in the [2 + 2 + 2]
cycloaddition. Because benzonitrile is present in a 100-fold
excess regarding the nitrile of complex 3, the integration of
benzonitrile into the final product is much more likely
compared to the nitrile of another metal complex molecule.
In conclusion, we have described a new [CpCNCo(COD)]
complex containing a pendant nitrile moiety linked to the
cyclopentadienyl ring. This complex is not only able to catalyze
the [2 + 2 + 2] cycloaddition reaction between 1,6-heptadiyne
and benzonitrile, but its own pendant nitrile moiety can be
activated by another complex molecule acting as catalyst to
perform [2 + 2 + 2] cycloaddition reactions employing different
substrates. This reactivity subsequently yields new Co(I)-
complexes with pyridine functionalities attached to the
cyclopentadienyl ligand, which can also catalyze the [2 +
2+2] cycloaddition reaction, however with lowered catalytic
activity.
3a. The BP86/TZVP computed relative energy shows that 3 is
more stable than 3a by 23.58 kcal/mol (22.06 kcal/mol at
BP86/SVP). This huge energy difference shows that the
pendant nitrile group cannot substitute one of the coordinated
CC bonds from COD, and therefore a coordination of the
nitrile group is not possible. The origin for the energy
difference can be clearly seen from the optimized structures, in
particular, the change of the bond angles. For example, the
nearly linear N1−C1−C2 moiety in 3 becomes bent in 3a
(153.09°) and the normal C1−C2−C3 and C2−C3−C4 bond
angles become smaller (114.34 and 108.72° vs 104.65 and
105.36°). In addition, the C2−C3 and C3−C4 bonds in 3 are
longer than in 3a (1.565 and 1.527 Å vs 1.580 and 1.541 Å).
The situation would not change significantly for the
cobaltacyclopentadienyl complex, resulting from the oxidative
coupling of two acetylene moieties to the cobalt center. While
the coordination of a free nitrile to the cobaltacyclopentadienyl
is found barrierless and exergonic by 5.5 kcal/mol and the
nitrile insertion barrier at only 2.2 kcal/mol,18 our theoretical
considerations place the coordination and insertion of the
pendant nitrile group at an energetically unfavorable level to
actually take place. Hence, we can reasonably rule out the
possibility of an intramolecular self-catalyzed cycloaddition and
it is safe to assume that the [2 + 2 + 2] cycloaddition that yields
compounds 4−6 involves the participation of another molecule
of the respective complex, serving as the catalytically active
species in an intermolecular fashion.
EXPERIMENTAL SECTION
■
All experiments were carried out using standard Schlenk technique
under argon atmosphere. Solvents were appropriately dried (THF was
refluxed in the presence of sodium and benzophenone until the
solution was showing a persisting deep-violet color, then distilled;
acetone was dried over P2O5). Merrifield’s resin ((chloromethyl)-
polystyrene, ∼5.5 mmol/g Cl loading) was obtained from Aldrich
(CAS: 55844-94-5).
To investigate the influence of the attached nitrile or pyridyl
group on the catalytic activity of the prepared complexes in the
[2 + 2 + 2] cycloaddition reaction, compounds 3 and 4 as well
as the unsubstituted [CpCo(COD)] were tested in the known
Synthesis of [η5-(C5H4CMe2CH2CN)Co(COD)] (3). A bright-
green suspension of [ClCo(PPh3)3] (1) (1.72 g, 2 mmol) and 3-
cyclopentadienyl-3-methylbutanenitrile lithium salt (CpCNLi, 2) (306
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mg, 2.0 mmol) in THF (10 mL) was stirred at room temperature for
two hours. 1,5-Cyclooctadiene (4 mL, 24 equiv) was added to the
solution, which was stirred at 60 °C for 24 h. The solvent was removed
under reduced pressure and the dark-red oily residue dried under
reduced pressure. Removal of triphenylphosphine was accomplished
by a method published by Lipshutz and Blomgren.15 Merrifield’s resin
(3 g) and sodium iodide (1.78 g) were added and dissolved in an
acetone/THF solution (5 mL/15 mL). After stirring at room
temperature for 24 h, the solution was filtered and the solvent
removed under reduced pressure. The orange residue was taken up in
warm hexane and filtered. Crystallization at −48 °C afforded the
complex 3 as deep-red crystals (isolated yield, 367 mg, 60%; NMR
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1
yield, quantitative); mp 84−85 °C. H NMR (C6D6): δ = 1.35 (s,
CH3, 6H), 1.61 (d, J = 8.3 Hz, CH, 4H), 2.10 (s, CH2, 2H), 2.35 m,
CH2, 4H), 3.16 (t, J = 2.2 Hz, CH, 2H), 3.31 (m, CH2, 4H), 4.62 (t, J
= 2.18 Hz, CH, 2H). 13C NMR (C6D6): δ = 28.7 (s, CH3), 32.3 (s,
COD), 33.8 (s, CH2), 33.9 (s, Cq), 63.9 (s, COD), 80.0 (s, Cp), 84.4
(s, Cp), 110.9 (s, Cq), 118.3 (s, CN). CI (THF) m/z 313 [M+]. CHN
(%) Calcd for: C, 69.00%; H, 7.72%; N, 4.47%. Found: C, 69.16%; H,
7.68%; N, 4.44%.
̈
̈
Exemplary Synthesis of [CppyCo(COD)] (4). A reaction vessel
was charged with 3 (80 mg, 0.26 mmol) and filled with gaseous
acetylene. The complex was dissolved in THF (6 mL), and the
thoroughly stirred, thermostated red solution was irradiated (two 460
W lamps) for 3 h at room temperature. The solution was filtered over
a small amount of neutral Al2O3 (Brockman Type I), which was
washed with THF (10 mL). After removal of the solvent under
reduced pressure, the resulting red−brown oil was dried under vacuum
(yield, 79 mg). NMR spectroscopy showed that a 2:1 mixture of 4 and
3 was obtained. Further irradiation of the complex mixture under the
same conditions as above and filtration over a small amount of neutral
Al2O3 (Brockman Type I), which was washed with THF (10 mL),
afforded 4 as a brown−red oil (yield, 38 mg, 42%). 1H NMR (C6D6):
δ = 1.62 (s, CH3, 6H), 1.67 (d, J = 7.7 Hz, CH, 4H), 2.46 (m, CH2,
4H), 3.13 (s, CH2, 2H), 3.22 (t, J = 2.3 Hz, CH, 2H), 3.43 (m, CH2,
4H), 4.65 (t, J = 2.0 Hz, CH, 2H), 6.52 (d, J = 7.4 Hz, CH, 1H), 6.64
(t, J = 5.7 Hz, CH, 1H), 7.01 (t, J = 7.4 Hz, CH, 1H), 8.52 (d, J = 4.1
Hz, CH, 1H). 13C NMR (C6D6): δ = 29.1 (s, CH3), 32.4 (s, COD),
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160.2 (s, Ar). CI (isobutane) m/z 365 [M+], 257 [M − COD+].
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis procedures, characterization data, and NMR spectra
for the new Cp′Co(I)-complexes 5 and 6, the general
procedure for the catalytic screening, details for the computa-
tional calculations and crystallographic data for complexes 3
and 5 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: marko.hapke@catalysis.de (M.H.); martin.lamac@jh-
(17) Sekabunga, E. J.; Smith, M. L.; Webb, T. R.; Hill, W. E. Inorg.
Chem. 2002, 41, 1205.
(18) Dazinger, G.; Torres-Rodrigues, M.; Kirchner, K.; Calhorda, M.
J.; Costa, P. J. J. Organomet. Chem. 2006, 691, 4434.
inst.cas.cz (M.L.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Uwe Rosenthal for his ongoing support and
helpful discussions and comments. M.H. thanks the DFG
(HA3511/3-1) and M.L. the Czech Science Foundation
(GPP207/10/P200) for financial support as well as the
Fonds der Chemischen Industrie for a Kekule-
I. Thiel.
́
Fellowship for
D
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