Palladacycles of novel bisoxazoline chelating ligands based on the dimeric cyclobutadiene linked cobalt sandwich compound [(η5-Cp) Co(η4-C4Ph3)]2

Article abstract of DOI:10.1039/c0dt01737j

The reaction of 1,4-diphenylbutadiyne along with diphenylacetylene, when carried out with η⁵-[MeOC(O)Cp]Co(PPh₃)₂ resulted in the cyclobutadiene linked dimeric cobalt sandwich compound {[η⁵-MeOC(O)Cp]Co(η⁴-C₄Ph ₃)}₂ (1) along with the known monomeric compound η⁵-[MeOC(O)Cp]Co(η⁴-C₄Ph₄). Compound 1, on treatment with KOH gave the dicarboxylic acid {η⁵-[HOC(O)Cp]Co(η⁴-C₄Ph ₃)}₂ (2) which on reaction with oxalyl chloride followed by (S)-2-amino-3-methyl-1-butanol, triethylamine and mesylchloride was converted to the bis(cobalt oxazoline) based chiral ligand {[η⁵-(4-iPr-2- Ox)Cp]Co(η⁴-C₄Ph₃)}₂ (3) (Ox = Oxazolinyl). Compound 3 on reaction with palladium acetate gave an NCN bound bis(cobalt-oxazolinyl) palladacycle {[η⁵-(4-iPr-2-Ox)Cp] Co(η⁴-C₄Ph₃)}₂PdOAc (4) having both the oxazolinyl groups bound to the palladium in a chelate mode and one of the Cp rings forming a five membered [C,N] palladacycle. Reaction of 4 with aq. NaCl resulted in the chloro analogue of 4, {[η⁵-(4-iPr-2-Ox)Cp] Co(η⁴-C₄Ph₃)}₂PdCl (5). A nonchiral bidentate bis cobalt oxazoline ligand {[η⁵-(Ox)Cp] Co(η⁴-C₄Ph₃)}₂ (6) was also prepared by replacing (S)-2-amino-3-methyl-1-butanol with 2-amino-1-ethanol in the procedure used for the preparation of 3. A reaction of 1,4- diphenylbutadiyne, diphenylacetylene and (η⁵-Cp)Co(PPh ₃)₂ resulted in the dimer [(η⁵-Cp) Co(η⁴-C₄Ph₃)]₂ (7) and small amounts of a trimer [(η⁵-Cp)Co(η⁴-C ₄Ph₃)]₂[(η⁵-Cp) Co(η⁴-C₄Ph₂)] (8) having no substituents on the Cp rings. All the compounds except 2 were structurally characterized. Structural studies on palladacycles 4 and 5 showed interesting anagostic C-H...Pd interactions between the ortho hydrogen of one of the Cp rings and the palladium centre. Preliminary studies carried out on the palladacycles showed promising catalytic activity in the asymmetric rearrangement of trichloroacetimidates.

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PAPER
Palladacycles of novel bisoxazoline chelating ligands based on the dimeric
cyclobutadiene linked cobalt sandwich compound [(g5-Cp)Co(g4-C4Ph3)]2†
Nem Singh and Anil J. Elias*
Received 10th December 2010, Accepted 18th February 2011
DOI: 10.1039/c0dt01737j
The reaction of 1,4-diphenylbutadiyne along with diphenylacetylene, when carried out with
5
h -[MeOC(O)Cp]Co(PPh3)2 resulted in the cyclobutadiene linked dimeric cobalt sandwich compound
5
4
{[h -MeOC(O)Cp]Co(h -C4Ph3)}2 (1) along with the known monomeric compound
5
4
h -[MeOC(O)Cp]Co(h -C4Ph4). Compound 1, on treatment with KOH gave the dicarboxylic acid
5
4
{h -[HOC(O)Cp]Co(h -C4Ph3)}2 (2) which on reaction with oxalyl chloride followed by
(S)-2-amino-3-methyl-1-butanol, triethylamine and mesylchloride was converted to the bis(cobalt
5
4
oxazoline) based chiral ligand {[h -(4-iPr-2-Ox)Cp]Co(h -C4Ph3)}2 (3) (Ox = Oxazolinyl). Compound 3
on reaction with palladium acetate gave an NCN bound bis(cobalt-oxazolinyl) palladacycle
5
4
{[h -(4-iPr-2-Ox)Cp]Co(h -C4Ph3)}2PdOAc (4) having both the oxazolinyl groups bound to the
palladium in a chelate mode and one of the Cp rings forming a five membered [C,N] palladacycle.
5
4
Reaction of 4 with aq. NaCl resulted in the chloro analogue of 4, {[h -(4-iPr-2-Ox)Cp]Co(h -
5
4
C4Ph3)}2PdCl (5). A nonchiral bidentate bis cobalt oxazoline ligand {[h -(Ox)Cp]Co(h -C4Ph3)}2 (6)
was also prepared by replacing (S)-2-amino-3-methyl-1-butanol with 2-amino-1-ethanol in the
procedure used for the preparation of 3. A reaction of 1,4-diphenylbutadiyne, diphenylacetylene and
5
5
4
(h -Cp)Co(PPh3)2 resulted in the dimer [(h -Cp)Co(h -C4Ph3)]2 (7) and small amounts of a trimer
5
4
5
4
[(h -Cp)Co(h -C4Ph3)]2[(h -Cp)Co(h -C4Ph2)] (8) having no substituents on the Cp rings. All the
compounds except 2 were structurally characterized. Structural studies on palladacycles 4 and 5
showed interesting anagostic C–H ◊ ◊ ◊ Pd interactions between the ortho hydrogen of one of the Cp rings
and the palladium centre. Preliminary studies carried out on the palladacycles showed promising
catalytic activity in the asymmetric rearrangement of trichloroacetimidates.
diene rings.6–10 Unlike biferrocene, chemistry centered around the
dimer of (h -Cp)CoC4Ph4 has not been developed so far. In fact,
Introduction
5
The utility of biferrocene, the cyclopentadienyl linked dimer
of ferrocene as a precursor for making a variety of bidentate
organometallic ligands is well documented.1–5 Chiral and non-
chiral transition metal complexes realized from biferrocene de-
rived ligands with binding sites such as amines, oxazolines and
phosphines have shown unique electrochemical, photophysical
and catalytic activities. Although relatively less reactive, the 18
electron cobalt complex (h -Cp)Co(h -C4Ph4), has been found to
be quite similar to ferrocene, especially in terms of stability and
ease of synthesis and a variety of its derivatives have been prepared
with different substituents on the cyclopentadienyl and cyclobuta-
the only structurally characterized example of a cyclobutadiene
5
4
linked dimer of this kind, {(h -Cp)Co[h -C4Ph2(C4H3S)]}2, was
obtained by Clearfield, Rausch and coworkers as an unexpected
5
product from a reaction of (h -Cp)Co(CO)2 with PhC C(C4H3S)
in an yield of 0.7% in 1977 and its formation was thought to be
from a possible impurity of 1,4-diphenylbutadiyne present along
with the alkyne.11 It is also of interest to note that a reaction of
5
4
5
(h -Cp)Co(PPh3)2 with diynes separated by aryl spacers resulted
in polymers linked through cobaltacyclopentadiene units.12 In
this paper we report the first logical synthesis of cyclobutadiene
5
linked dimers of (h -Cp)CoC4Ph4 having chiral and non-chiral
oxazoline groups bound to the two Cp rings. The presence of the
two oxazoline groups on the free Cp rings makes these ligands
differ from bidentate ligands based on biferrocene where the
binding groups are located mostly on the linked Cp rings. This
provides scope for preparing chelate complexes with variations
in bite angle as well as for realizing coordination polymers. We
also demonstrate the usefulness of the chiral bis(cobalt oxazoline)
ligand by preparing chiral five membered [C,N] palladacycles13
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas,
New Delhi, 110016, India. E-mail: eliasanil@gmail.com; Fax: + 91-11-
26581102; Tel: + 91-11-26591504
† Electronic supplementary information (ESI) available: Selected bond
distances and angles of compounds 1, 3–8; 1H and 13C-NMR spectra
of trichloroacetamidates and their rearranged products and HPLC plots
of compounds 10a–c are provided. CCDC reference numbers 803898–
803904. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt01737j
4882 | Dalton Trans., 2011, 40, 4882–4891
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having sterically bulky basal sites involving as many as six phenyl
rings and with an active site similar to the well known bisoxazoline
based NCN pincer ligands.14
as acetic acid resulting in a novel five membered chiral [C,N]
palladacycle 4 having an acetate unit bound to the palladium
(Scheme 3). A variation of this palladacycle with a chloro group
replacing the acetate unit, 5 was prepared by reacting 4 with
NaCl in acetone-water medium. The palladacycles 4 and 5 were
also found to be highly stable to air and moisture and do not
decompose when kept in the open even for months. They were also
found to have very good solubility in a variety of solvents such
as benzene, toluene, cyclohexane, dichloromethane, chloroform,
tetrahydrofuran and ethylacetate.
A bis oxazoline ligand {[h -(Ox)Cp]Co(h -C4Ph3)}2 6, analo-
gous to 3 but having non-chiral oxazoline units on the Cp rings
was prepared by replacing (S)-2-amino-3-methyl-1-butanol with
2-amino-1-ethanol in the procedure used for the preparation of 3.
Compound 6 was obtained in 75% yield and was characterized
structurally as well as by spectral and analytical techniques
(Scheme 4). Attempts to prepare an analogous palladacycle of
4 using ligand 6 were however found to be unsuccessful resulting
mostly in a black insoluble residue. Identity of a non-chiral and
acetate free palladium containing compound isolated from this
reaction in 8% yield remained inconclusive.
Results and discussion
5
Preliminary studies on three component reactions of [h -
MeOC(O)Cp]Co(PPh3)2 {generated in situ from Na[MeOC(O)Cp]
and CoCl(PPh3)3},9d diphenylbutadiyne and diphenylacetylene
under different reaction conditions were found to give several
products mostly as cobalt sandwich compounds and cobaltamet-
allacycles. However, a reaction involving 2 : 1 : 3 molar ratio of
the same reactants when carried out in refluxing xylene was
found to give selectively the novel cyclobutadiene linked dimeric
5
4
5
4
sandwich compound, {[h -MeOC(O)Cp]Co(h -C4Ph3)}2 (1) in
5
41% yield along with the known monomeric compound [h -
MeOC(O)Cp]Co(h -C4Ph4) (Scheme 1). Both the compounds
4
were readily separated by column chromatography.
The dicarboxylate ester 1, was converted to dicarboxylic acid
2 in 90% yield in refluxing ethanol, using aq. KOH. The highly
stable dicarboxylic acid 2 was however found to be poorly soluble
in solvents such as chloroform, DMSO and acetone, hindering its
crystallization and structural characterization. However, reaction
of 2 with oxalyl chloride and catalytic amounts of DMF followed
by L-valinol [(S)-(+)-2-amino-3-methyl-1-butanol] and triethy-
lamine and finally ring closing carried out with mesyl chloride
yielded the optically active bisoxazoline compound 3 in 80% yield
(Scheme 2). Structural studies indicated that both 1 and 3 have the
two cobalt sandwich units trans to each other.
Quite interestingly, on reacting 3 with palladium acetate in
glacial acetic acid, the RCpCoC4Ph3 units, despite having six
phenyl groups in their vicinity were found to rotate almost 180◦
around the bond linking the two cyclobutadiene units and form
a chelate ring with the palladium centre. In addition, the ortho
hydrogen atom of one of the Cp rings was activated and displaced
We were also interested to ascertain if the same synthetic method
can be utilized for realizing the basic unit of the dimeric ligands de-
5
4
scribed herein namely [(h -Cp)Co(h -C4Ph3)]2 which has not been
reported so far. This was all the more of interest as the monomer
5
4
(h -Cp)Co(h -C4Ph4) has been a cobalt sandwich compound of
both academic and applied interest and many useful derivatives
5
have been directly prepared from it.6c,9a,d,15 A reaction of (h -
Cp)Co(PPh3)2 [generated in situ from NaCp and CoCl(PPh3)3],9d
diphenylbutadiyne and diphenylacetylene in 2 : 1 : 3 molar ratio in
5
4
refluxing xylene was found to yield [(h -Cp)Co(h -C4Ph3)]2 (7)
5
in 39% yield along with a very small quantity of trimer [(h -
4
5
4
Cp)Co(h -C4Ph3)]2[(h -Cp)Co(h -C4Ph2)] (8) (Scheme 5) and the
5
4
monomer (h -Cp)Co(h -C4Ph4). The compounds 7 and 8 were
also structurally characterized.
Scheme 1
Scheme 2
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Scheme 3
Scheme 4
Scheme 5
Spectral data on compounds 1–8
when the ligand 3 was found to form complexes 4 and 5 with
palladium. Similarly, as only one of the oxazoline bound Cp ring
was found to form the palladacycle, the number of ligand signals
in the 1H and 13C-NMR spectra of 4 and 5 were found to be almost
double that of the free ligand 3.
1
In the H-NMR spectra of the dimeric ester 1, the Cp hydrogen
atoms were observed as two signals at d 4.63 and 5.23 ppm while
the corresponding carboxylate ester of the monomer gave peaks
at 4.77 and 5.19 ppm for the same hydrogens.16 On converting
1 to the dicarboxylic acid 2, these peaks were found to shift to
4.59 and 5.17 ppm respectively. The bridging carbon atoms of the
cyclobutadiene rings of 1 showed a shielded peak at 72.75 ppm
in its 13C-NMR while the rest of the ring carbon atoms of
the cyclobutadiene units were resonating at 77.98–79.41 ppm.
Comparison of the 1H-NMR spectra of the chiral and non-chiral
bis oxazoline compounds 3 and 6 showed interesting differences.
While the Cp protons of non-chiral compound 6 showed two peaks
at 4.59 and 5.13 ppm, that of the chiral compound 3 showed four
separate peaks in the range of 4.57 to 5.27 ppm. The OCH2 protons
of the oxazoline rings of 6 were found to give a triplet centered at
3.69 ppm while the same for 3 were observed as a multiplet centered
at 3.45 ppm. The 13C-NMR of 6 showed three peaks in the range of
83.28 to 85.59 ppm while five separate peaks in the range of 82.54 to
85.54 ppm were observed for 3, further supporting its chiral nature.
Noticeable downfield shifts in the 1H-NMR signals of the CH
units of the oxazoline bound isopropyl groups were observed,
While a singlet at d 4.59 ppm was observed in the 1H-NMR for
the Cp of 7, two peaks at d 4.11 and 4.32 ppm in the ratio of 1 : 2
were observed for the trimer 8 clearly indicating deshielding of the
protons on the Cp ring of the cobalt sandwich moiety, present at
the middle of the molecule. For comparison, Cp hydrogen atoms
5
4
of the monomer (h -Cp)Co(h -C4Ph4) were observed as a singlet
at 4.63 ppm. A similar variation in chemical shift was observed in
the 13C-NMR as well (82.75 ppm for 7 and 82.10 and 82.51 ppm
for 8).
X-ray crystal structures of compounds 1 and 3–8. The crystal
structures of the cyclobutadiene bridged dimers 1, 3, 6, 7 and the
trimer 8 are given in Fig. 1, 2, 5, 6 and 7. The bond parameters of
the bridged dimers suggest that the linked cyclobutadiene rings
are partly conjugated as the bridging C–C distances in these
˚
compounds vary from 1.433(6) to 1.482(7) A which are similar
to average C–C bond distances of the cyclobutadiene ring bonds
˚
(1.465 A). The CpCoCb (Cb = cyclobutadiene) units present in
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Fig. 1 ORTEP diagram of the dimeric diester 1 with 30% probability
factor (hydrogen atoms have been omitted for clarity).
Fig. 3 ORTEP diagram of the palladacycle 4 with 30% probability factor
(some hydrogen atoms have been omitted for clarity).
Fig. 2 ORTEP diagram of chiral bisoxazoline compound 3 with 30%
probability factor (hydrogen atoms have been omitted for clarity).
compounds 1, 3, 6 and 7 are oriented anti to each other with
the angle between the mean planes of the two Cb units being 0◦
for 1, 6 and 7 and 1.58(2)◦ for 3. Upon palladium coordination
in complexes 4 and 5, the orientation of the two CpCoCb units
became syn to each other with the angle between the two Cb
units 52.8(2)◦ for 4 and 50.9(1)◦ for 5. This observation also
indicates unhindered rotation around the bridging C–C bonds
of the triphenyl substituted cyclobutadiene rings. Interestingly, the
phenyl groups bound to the adjacent carbon atoms of the bridging
C–C units of 1, 3, 6 and 7 were found to be almost planar to each
other with the angles between their mean planes varying from
3.4(1)◦ to 15.0(1)◦. The cyclopentadienyl rings and the oxazoline
rings of compounds 3 and 6 were found to be almost planar to
each other with a dihedral angle of 4.6(2)◦ for 3 and 0.9(3)◦ for 6.
The structure of the trimer 8 showed two adjacent CpCoCb units
syn to each other with the third in gauche orientation to the middle
unit.
Fig. 4 ORTEP daigram of the palladacycle 5 with 40% probability factor
(some hydrogen atoms have been omitted for clarity).
5
4
(h -Cp)Co(h -C4Ph4) and its dimer 7.7e The bent orientation of
the acetate group in compound 4 (Fig. 3) was found to be quite
similar to that observed in the case of palladium-pybox acetate
complex reported by Vicic and coworkers.17 Table 1 compares the
bond distances and angles centered around the active site of the
catalysts 4 and 5 with the well known (i-Pr-Phebox)Pd–Cl complex
reported by Nishiyama and coworkers.14f As shown, the Pd–N and
Pd–C distances as well as the angles around the square plane of 4,
5 and the (i-Pr-Phebox)Pd–Cl complex are quite comparable.
An interesting observation in the crystal structures of com-
pounds 4 and 5 (Fig. 3 and 4) is the weak anagostic interactions
between the ortho hydrogen of the Cp ring not involved in the
palladacycle formation and the palladium centre. These C–H
activations which have been found to be almost perpendicular to
The crystal structures of the palladacycles 4 and 5 prepared
using the chiral bis(oxazoline) ligand 3 are given in Fig. 3 and
4 respectively. In complexes 4 and 5, the cyclopentadienyl and
cyclobutadiene rings on the cobalt palladacycle units are tilted
at an angle ranging from 8.1(2) to 9.5(2)◦ between each other
while the same tilt is only in the range of 1.3(1) to 3.2(2)◦ for
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Fig. 5 ORTEP diagram of non-chiral bisoxazoline compound 6 with 30%
probability factor (some hydrogen atoms have been omitted for clarity).
Fig. 7 ORTEP diagram of the trimeric compound 8 with 30% probability
factor (hydrogen atoms and solvent molecule have been omitted for
clarity).
of the Cp protons of the palladacycles 4 and 5 is considerably
deshielded (a requirement for the anagostic classification) and is
observed at 8.61 and 7.92 ppm while the rest of the Cp protons
are observed at 4.31 to 5.17 ppm. It is also of interest to note
that in the crystal structure of 4, the O(4)–H(20) distance between
˚
the acetate oxygen and the activated C–H is quite short (2.334 A)
and this also accounts for the higher 1H-NMR chemical shift (d =
8.61 ppm) of this hydrogen compared to the activated hydrogens in
the palladacycle 5. Theoretical calculations by Davies, Macgregor
and coworkers on the general mechanism for a palladacycle
formation by cyclometallation involving Pd(OAc)2 suggests an
agostic interaction-based mechanism stabilized by the acetate
unit.19 The anagostic interaction observed in 5 and the unusual
proximity of the acetate oxygen to the activated C–H supports
their mechanism of palladacycle formation.
Fig. 6 ORTEP diagram of the dimeric compound 7 with 30% probability
factor (hydrogen atoms have been omitted for clarity).
Catalytic studies using palladacycles 4 and 5
the palladium square plane are indicated by C–H ◊ ◊ ◊ Pd distances
˚
ranging from 2.649(1) to 2.660(1) A and C–H ◊ ◊ ◊ Pd angles ranging
The catalytic asymmetric rearrangement of trichloro or triflu-
oroacetimidates to the corresponding chiral allylic trichloro or
trifluoroacetamides is a well known method utilized to evaluate
the enantioselective efficiency of many chiral catalysts.20 To
explore the catalytic activity of the palladacycles 4 and 5 we
from 125.2(3) to 125.9(3)◦. This kind of interaction, clearly
different from the conventional agostic interaction was defined
by Brookhart, Green and Parkin in 2007.18 A closer look at the
1H-NMR also substantiates the anagostic interactions as one
Table 1 Comparison of the selected bond distances and angles of active sites of the catalysts 4–5 from the present study with those of (i-Pr-Phebox)Pd–Cl
◦
(i-Pr-Phebox)Pd–Cl14f
˚
Bond lengths (A) and angles ( )
4
5
Pd–N1
Pd–N2
Pd–C
Pd–L
N1–Pd–L
N1–Pd–C
N2–Pd–L
N2–Pd–C
2.054(3)
2.017(3)
1.977(3)
2.098(4) L
98.8(1) L
80.3 (2)
2.054(5)
2.002(5)
1.987(5)
2.391(1) L Cl
96.9(1) L Cl
80.2(2)
93.0(1) L Cl
90.0(2)
2.056(1)
2.056(1)
1.928(1)
2.391(4) L Cl
100.2(2) L Cl
79.8(2)
100.2(2) L Cl
79.8(2)
O
O
89.6(1) L
91.5(2)
O
4886 | Dalton Trans., 2011, 40, 4882–4891
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Table 2 Aza-Claisen rearrangement of trichloroacetimidates using palladacycles 4 and 5 as catalysts
Entry
Substrate
Catalyst
R
Product
yield (%)a
eeb
1
2
3
4
5
6
9a
9a
9b
9b
9c
9c
4
5
4
5
4
5
Me
Me
n-Pr
n-Pr
PhCH2CH2
PhCH2CH2
10a
10a
10b
10b
10c
10c
80
87
72
78
71
77
96
92
89
90
92
88
a Isolated yield. b determined by chiral HPLC analysis.
have carried out the aza-Claisen rearrangement of three selected
trichloroacetimidates, RCH CHCH2OC(NH)CCl3 (R = Me, n-
Pr and CH2CH2Ph) with these palladacycles. Reactions were
carried out in acetonitrile over a period of 16 h with a catalyst
loading of 2 mol% and the results obtained are given in Table 2.
The yields of the trichloroacetamides after the aza-Claisen
rearrangement are in the range of 71–87% and the ee’s obtained
ranged from 88–96%. Among the catalysts, one having the Pd–
Cl bond, 5 was found to give relatively better yields of the
trichloroacetamides while the catalyst with Pd–OAc bond, 4 has
been found to be better with regard to enantiomeric excess.
Reactions of 9b carried out at room temperature and at 50 ◦C in
acetonitrile with catalyst 5 were not found to proceed. The same
reaction when carried out at 60 ◦C gave only 25% conversion
with marginally higher enantiomeric excess of 92% compared to a
reaction carried out at 70 ◦C. As one of the C–H bonds of a Cp ring
is involved in a weak anagostic interaction it increases the crowding
around the active site and possibly is a reason for the relatively
higher temperature required for the activation of the catalysts 4
and 5. However, the high stability of the palladacycles 4 and 5
to air and moisture and good solubility in a variety of solvents
facilitates easier and rougher handling of these palladacycles as
catalysts.
the Cp ring not involved in the palladacycle formation and the
palladium centre. The palladacycles were found to have high air
and moisture stability and good solubility in a wide range of
organic solvents. The utility of the palladacycles as catalysts in the
aza-Claisen rearrangement of three different trichloroacetimidates
was explored. The reactions, which were found to proceed at
◦
70 C, gave yields in the range of 71–87% and good ee’s ranging
from 88 to 96%. The dimeric chiral and non-chiral bisoxazoline
scaffold provides excellent scope for developing a wide range
of air stable bidentate organometallic chiral catalysts. We are
currently involved in modifying the structural framework of the
palladacycles reported in this paper as well as in making new metal
complexes based on the chiral and non-chiral bis(cobalt oxazoline)
compounds.
Experimental section
General methods
All manipulations of the complexes were carried out using stan-
dard Schlenk techniques under nitrogen atmosphere. Tetrahydro-
furan, xylene and toluene were freshly distilled from sodium ben-
zophenone ketyl under nitrogen atmosphere and used. Isopropyl
alcohol and n-hexane (HPLC grade, Merck) were used for the chi-
5
ral HPLC studies. (h -Cp)Co(PPh3)2,21 1,4-diphenylbutadiyne,22
Conclusion
and trichloroacetimidates18b,23 were prepared according to litera-
ture procedures. Acetic acid, diphenylacetylene, mesyl chloride,
L-valinol and dimethylcarbonate (Aldrich) were used as received.
The first logical synthesis of cyclobutadiene linked dimers and
5
a trimer of the well known cobalt sandwich compound (h -
4
Cp)Co(h -C4Ph4) has been described. Chiral and non-chiral
5
4
bisoxazoline derivatives of the dimer [(h -Cp)Co(h -C4Ph3)]2,
Instrumentation
which can function as bidentate ligands were prepared starting
5
4
1
from the dicarboxylate ester, {[h -MeOC(O)Cp]Co(h -C4Ph3)}2.
1H and 13C { H} NMR spectra were recorded on a Bruker
5
Reaction of the chiral bis(cobalt oxazoline) dimer {[h -(4-iPr-2-
Spectrospin DPX-300 NMR spectrometer at 300 and 75.47 MHz
respectively. IR spectra in the range 4000–250 cm-1 were recorded
on a Nicolet Prote´ge 460 FT-IR spectrometer as KBr pellets.
Elemental analyses were carried out on a Carlo Erba CHNSO
1108 elemental analyzer. Mass spectra were recorded in the
fast atom bombardment mode using a JEOL SX 102/DA-6000
mass spectrometer, in the time-of-flight mass spectrometry mode
(HRMS) using an AB Sciex Spectrometer (model 1011273) and
4
Ox)Cp]Co(h -C4Ph3)}2, with Pd(OAc)2 yielded a novel bis(cobalt
oxazolinyl) [C,N] palladacycle 4, having both the oxazolinyl
groups bound to the palladium centre in a chelating mode
and one of the Cp rings forming the palladacycle. Reaction
of palladacycle 4 with NaCl gave the chloro analogue, 5. The
crystal structures of 4 and 5 showed that there are weak C–
H . . . Pd anagostic interactions between the ortho hydrogen of
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also by Waters Q-TOF Premier Micromass instrument, using
electrospray ionization (ESI) mode. Optical rotations of the
chiral compounds were measured on an Autopol V (Rudolph
Research Flanders, New Jersey) instrument. All the rotations
were measured at 589 nm (sodium ‘D’ line) using chloroform as
solvent and readings were crosschecked by taking measurements at
two different concentrations (200 mg and 400 mg/100 mL). The
enantiomeric excess were determined by using a SHIMADZU
LC6AD HPLC fitted with a Diacel AD-H chiral column. All
HPLC analyses used to determine enantiomeric purity were
calibrated with samples of the racemate.
Reaction of 1,4-diphenylbutadiyne and diphenylacetylene with
[g5-MeOC(O)Cp]Co(PPh3)2
1,4-diphenylbutadiyne (0.606 g, 3.00 mmol) and diphenylacetylene
(0.800 g, 4.50 mmol) in 80 ml of xylene were added to a stirred solu-
5
tion of [h -MeOC(O)Cp]Co(PPh3)2 [prepared in situ by dissolving
5
Na{[h -MeO(O)C]Cp} (0.440 g, 3.00 mmol) and (PPh3)3CoCl
(2.670 g, 3.00 mmol) in 10 ml of THF] and the resultant mixture
was refluxed for 4 h. After cooling the reaction mixture, more
5
of [h -MeOC(O)Cp]Co(PPh3)2 [prepared in situ by dissolving
5
Na{[h -MeO(O)C]Cp} (0.440 g, 3.00 mmol) and (PPh3)3CoCl
(2.670 g, 3.00 mmol) in 10 ml of THF] and diphenylacetylene
(0.800 g, 4.50 mmol) were added so that the overall ratio of
X-ray crystallography
5
PhC CPh, PhC C–C CPh and [h -MeOC(O)]CpCo(PPh3)2
was 3 : 1 : 2. The reaction mixture was refluxed further for 6 h.
Afterwards the mixture was cooled, solvent evaporated off and the
remaining residue chromatographed through a neutral alumina
column and eluted with hexane to remove triphenylphosphine.
Using 4% ethylacetate/hexane mixture the first fraction came out
which upon evaporation gave the monomeric sandwich compound
Suitable crystals of compounds 1, 3–8 were obtained by slow
evaporation of their saturated solutions in ethyl acetate–hexane
or chloroform–hexane solvent mixtures. Single-crystal diffraction
studies were carried out on a Bruker SMART APEX CCD diffrac-
˚
tometer with a Mo-Ka (l = 0.71073 A) sealed tube. All crystal
structures were solved by direct methods. The program SAINT
(version 6.22) was used for integration of the intensity of reflections
and scaling. The program SADABS was used for absorption
correction. The crystal structures were solved and refined using
the SHELXTL (version 6.12) package.24 All hydrogen atoms were
included in idealized positions, and a riding model was used.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. Selected bond distances and angles for compounds
1, 3–8 are given as supplementary data.† Crystallographic data
and data collection parameters for compounds 1, 3–8 are given in
Table 3.
5
4
[h -MeOC(O)Cp]Co(h -C4Ph4)].16 Yield: 1.450 g, 2.70 mmol, 45%.
The second fraction which came out at 6% ethylacetate/hexane
on evaporation gave a red crystalline compound which was
5
4
characterized as the dimer {[h -MeOC(O)Cp]Co(h -C4Ph3)}2 (1).
Yield: 1.135 g, 1.23 mmol, 41%. Mp: 240–242 ◦C. Found: C, 75.33;
H, 4.76. Calc. for C58H44O4Co2: C, 75.49; H, 4.81. IR (n, cm-1):
1
1702 vs. (C O). NMR(CDCl3): H, d 3.26 (6H, s, -COOCH3),
4.63 (4H, s, -CpH), 5.23 (4H, s, -CpH), 7.06–7.25 (22H, m, -PhH),
1
7.43–7.45 (8H, m, -PhH); 13C{ H}, d 51.23 (-OCH3), 72.75, 77.98,
79.41 (CbC), 84.41, 85.10, 87.06 (CpC), 126.53, 127.06, 127.47,
Table 3 X-Ray crystal structure parameters of compounds 1, 3–8
Parameters
1
3
4
5
6
7
8
Formula
MW
cryst syst
C58H44O4Co2 C66H58O2N2Co2 C68H60O4N2Co2Pd C66H57O2N2ClCo2Pd C60 H46O2N2Co2 C54H40Co2
C75H55Co3·CHCl3
1252.35
922.79
Monoclinic
P21/n
13.454(3)
11.491(2)
13.887(3)
90
96.526(4)
90
2133.0(7)
2
1029.00
Monoclinic
P21
10.786(1)
9.789(1)
23.514(2)
90
95.187(1)
90
2472.5(3)
2
1193.44
Monoclinic
P21
14.103(1)
12.723(1)
16.601(1)
90
109.272(1)
90
2811.8(3)
2
1169.84
Monoclinic
P21
14.023(1)
12.149(1)
16.287(1)
90
107.687(2)
90
2643.5(4)
2
944.85
806.72
Monoclinic
I2/a
15.732(2)
11.017(1)
21.238(3)
90
93.824(1)
90
3672.7(7)
4
Triclinic
Triclinic
¯
¯
Space group
P1
P1
˚
a/A
10.876(3)
11.351(4)
11.373(4)
84.505(6)
63.090(5)
62.221(5)
1095.9(6)
1
11.236(4)
13.546(4)
20.574(6)
85.371(6)
74.270(6)
79.674(6)
2963.7(16)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
T/K
100(2)
0.71073
1.437
100(2)
0.71073
1.382
100(2)
0.71073
1.410
100(2)
0.71073
1.470
293(2)
0.71073
1.432
100(2)
0.71073
1.459
293(2)
0.71073
1.403
1.010
1.109
1.53–25.50
29676
10971
˚
l/A
rc (g cm-3)
m (mm-1)
0.829
0.721
0.951
1.056
0.807
0.944
Goodness of fit
q range
1.093
2.00–25.50
10821
1.060
1.74–25.50
24704
1.072
1.30–25.50
28326
0.879
1.31–25.50
25709
1.071
2.03—25.00
10677
1.215
1.92—25.50
17946
Total reflections
Unique reflections
Ovserved data[I >
2s(I)]
3944
3069
9181
8387
10456
9987
9833
8001
3867
3215
3415
3168
8392
Rint
0.0506
0.0387
0.0386
0.0923
0.0706
0.0374
0.0515
R1, wR2 [(I > 2s(I)] 0.0580, 0.1241 0.0425, 0.0944 0.0382, 0.0823
0.0514, 0.0831
0.0653, 0.0873
-0.05(2)
0.0553, 0.1093 0.0540, 0.1286 0.0752, 0.1697
0.0721, 0.1166 0.0588, 0.1317 0.1014, 0.1820
R1, wR2 (all data)
Flack parameter
CCDC number
0.0798, 0.1318 0.0482, 0.0970 0.0405, 0.0833
0.04(1)
803899
0.02(1)
803900
803898
803901
803902
803903
803904
1
2
2
2 2
R1 = R|Fo| - |Fcꢀ/R|Fo|; wR2 = R (|Fo| - |Fc| ) ]} .
4888 | Dalton Trans., 2011, 40, 4882–4891
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127.73, 128.03, 130.30, 134.21, 135.00 (PhC), 166.44 (Cp-C O).
MS(ESI) [m/z (species)]: 924 [M + 1]+.
78.73, 79.18, (-C4Ph3), 82.54 (ipso-CpC), 83.72, 83.94, 84.28, 85.54
(4C-CpC), 126.10, 126.74, 127.52, 127.59,127.62,127.84, 130.35,
130.53 (PhC), 134.51, 134.68, 135.41 (ipso-PhC), 160.76 (C N).
HRMS: Calcd for C66H58Co2N2O2H. Exact mass: 1029.3241
Measured mass: 1029.3236.
Preparation of bis[(g5-(S)-2-(4-methylethyl)oxazolinylcyclopenta-
dienyl)-(g4-triphenyl cyclobutadiene)cobalt] (3)
Potassium hydroxide (0.110 g, 2.00 mmol) dissolved in 2 ml of
water was mixed with 1 (0.190 g, 0.20 mmol) in 50 ml of ethyl
alcohol and the mixture was refluxed for 30 h. Reaction was
quenched with 2 M HCl (5 ml). After extraction with CH2Cl2,
(80 ml) the organic phase was washed with 100 ml of 2 M aqueous
HCl and dried over anhydrous sodium sulfate, filtered, and con-
Synthesis of the palladacycle 4
Palladium acetate (0.024 g, 0.11 mmol) was added to a solution of
3 (0.110 g, 0.11 mmol) in acetic acid (1.0 mL) and the mixture was
◦
stirred at 95 C for 30 min. Afterwards acetic acid was removed
completely under vacuum and the residue was chromatographed
through a silica gel column using 15% ethylacetate–hexane mixture
as the eluent. Evaporation of solvent gave palladacycle 4 as an
orange yellow◦crystalline solid. Yield: 0.122 g, 0.10 mmol, 91%.
Mp: 256–258 C. [a]3D5 = + 1306 (c 0.20 in CHCl3). Found: C,
68.31; H, 5.09; N, 2.29. Calc. for C68H60O4N2Co2Pd: C, 68.43; H,
5.07; N, 2.35. IR (n, cm-1): 1632, 1604 (C N). NMR (CDCl3):
1H, d 0.73–0.86 (m, 12H, -CH3), 1.82 (s, 3H, -COCH3), 2.11 [(1H,
bs, -CH(CH3)2], 2.63 [(1H, bs, -CH(CH3)2], 3.09–3.15 (1H, m,
-CHCH2-), 3.43–3.48 (1H, m, -CHCH2-), 3.82–3.97 (2H, bd, -
CHCH2-), 3.24–3.27 (2H, m, -CHCH2-), 4.48 (1H, s, CpH), 4.50
(1H, s, CpH), 4.75 (1H, s, CpH), 4.86 (1H, s, CpH), 4.92 (1H, s,
CpH), 5.02 (1H, s, CpH), 8.61 (1H, s, CpH), 6.76–6.80 (2H, m,
PhH), 6.89–6.96 (3H, m, PhH), 7.01–7.21 (9H, m, PhH), 7.24–7.34
(6H, m, PhH); 7.40–7.55 (8H, m, PhH); 8.11–8.13 (2H, m, PhH).
5
centrated under reduced pressure to give an orange solid bis[(h -
4
carboxycyclopentadienyl)(h -triphenylcyclobutadiene)cobalt] (2).
Yield: 0.160 g, 0.18 mmol, 90% (which was used directly in the
next transformation). Mp: 320–322 ◦C (dec.). Found: C, 75.22;
H, 4.48. Calc. for C56H40O4Co2: C, 75.17; H, 4.51. IR (n, cm-1):
1
1676vs (C O). NMR(CDCl3): H, d 4.59 (4H, s, -CpH), d 5.17
(4H, s, -CpH), d 7.13–7.20 (10H, m, -PhH), d 7.25–7.27 (4H, m,
-PhH); d 7.42–7.44 (6H, m, -PhH).
An oven dried, 100 mL, round-bottomed flask was fitted with a
stirring bar and an argon inlet. The flask was flushed with argon
and then charged with crude 2 (0.160 g, 0.17 mmol) and CH2Cl2
(10 mL). Oxalyl chloride (0.150 g, 1.17 mmol) and DMF (1 drop)
were added sequentially. Upon addition of the latter, gas evolution
was observed. The resulting solution was maintained at room
temperature. After 30 min, the solution was concentrated using
a rotary evaporator. Byproducts and excess oxalyl chloride were
removed by repeated extraction of residue (3¥ 15 mL) with CH2Cl2
to yield the acid chloride as a red-brown residue, which was used
directly in the next step.
1
13C{ H} d 14.48, 14.65 [-CH(CH3)2], 18.59, 19.11 [-CH(CH3)2],
24.60 (COCH3) 29.28 [-CH(CH3)2], 30.94 [-CH(CH3)2], 66.96
(-CHCH2-), 67.39 (-CHCH2-), 71.95 (-CHCH2-), 72.93
(-CHCH2-), 70.10, 73.83, 74.03, 74.93, 77.20, 78.06, 78.18, 79.74
(-C4Ph3), 80.35, 82.64, 83.24, 83.57, 86.44, 86.86, 87.12, 88.01,
90.91, 96.72 (-CpC), 125.83, 126.26, 126.36, 126.51, 126.63,
126.70, 126.86, 127.57, 127.80, 127.86, 128.15, 128.29, 128.60,
129.12, 129.46, 130.25, 130.36 (PhC), 133.75, 134.01, 134.24,
135.06, 135.53, 136.04 (ipso-PhC), 165.55, 172.57 (C N), 176.62
(COCH3). HRMS: Calcd for C66H58Co2N2O2PdH. Exact mass:
1135.2275 Measured mass: 1135.2266.
An oven dried, 100 mL, round-bottomed flask was fitted with
a stirring bar and an argon inlet. The flask was flushed with
argon and then charged with (S)-2-amino-3-methyl-1-butanol [L-
valinol] (0.040 g, 0.38 mmol), triethylamine (2 mL) and CH2Cl2
(8 mL) using syringes. A solution of the crude acid chloride in
10 mL of CH2Cl2 was also transferred to the flask. The resulting
solution was maintained at room temperature, and after 2 h, the
solution was cooled to 0 ◦C using an ice bath. Mesyl chloride
(0.080 g, 0.70 mmol) was then added and the resulting solution
was allowed to warm to room temperature and stirred. After 16
h, the solution was transferred to a separating funnel and washed
with 20 mL of saturated aqueous sodium bicarbonate and 20 mL
of brine. The organic layer was then dried over anhydrous sodium
sulfate, filtered, and concentrated using a rotary evaporator.
The residue was purified through a silica gel column using 9 : 1
hexane–ethyl acetate mixture as eluent. Evaporation of the solvent
Synthesis of the palladacycle 5
The palladacycle 4 (0.108 g, 0.09 mmol) was dissolved in 1 mL
acetone and to this solution, aqueous sodium chloride (2 M,
0.5 mL) was added and the mixture stirred at room temperature
overnight. The resulting yellow precipitate was dissolved in
dichloromethane (40 mL), transferred to a separating funnel and
washed with aqueous sodium bicarbonate (100 mL) and brine
(100 mL). The organic layer was then dried over anhydrous sodium
sulfate, filtered and concentrated using a rotary evaporator. The
residue was purified through a column of neutral alumina using
12% ethyl acetate - hexane mixture as eluent. Evaporation of
solvent gave a red crystalline powder which was characterized
as palladacycle 5. Yield: 0.099 g, 0.085 mmol, 94%). Mp: 302–
5
4
gave bis{[h -(S)-2-(4-methylethyl)oxazolinylcyclopentadienyl](h -
tetraphenylcyclobutadiene)cobalt} (3) as an yellow crystalline
◦
solid. Yield: 0.140 g, 0.13 mmol, 80%. Mp: 235–237 C. [a]D35
=
-62.5 (c 0.20 in CHCl3). Found: C, 76.88; H, 5.69; N, 2.76.
Calc. for C66H58O2N2Co2: C, 77.03; H, 5.68; N, 2.72. IR (n,
◦
1
cm-1): 1649 vs. (C N); NMR(CDCl3): H, d 0.72–0.74 (6H, d,
305 C (dec.). [a]3D5 = + 1346 (c 0.20, CHCl3). Found: C, 67.82;
-CH3), 0.97–1.00 (6H, d, -CH3), 1.43–1.51 [(2H, m, -CH(CH3)2],
3.41–3.52 (4H, m, -CHCH2-), 3.62–3.69 (2H, m, -CHCH2-), 4.58
(2H, s, CpH), 4.62 (2H, s,CpH), 5.12 (2H, s, CpH), 5.27 (2H,
s, CpH), 7. (02–7.23 (22 H, m, PhH), 7.41–7.43 (4H, m, PhH)
H, 4.85; N, 2.34. Calc. for C66H57O2N2ClCo2Pd: C, 67.76; H,
4.91; N, 2.39. IR (n, cm-1) 1633, 1603 (C N); NMR(CDCl3):
1H, d 0.73–0.81 (6H, m, -CH3), 0.87–0.94 (6H, m, -CH3), 2.52
[(1H, bs, -CH(CH3)2], 2.85 [(1H, bs, -CH(CH3)2], 3.20–3.26 (1H,
m, -CHCH2-), 3.77–3.85 (3H, m, -CHCH2-), 3.98–4.16 (1H, m,
-CHCH2-), 4.34–4.38 (1H, m, -CHCH2-), 4.43 (1H, s, CpH), 4.68
1
7.52–7.54 4H, m, PhH). 13C{ H} d 18.27 (-CH3), 19.79 (-CH3),
32.98 [-CH(CH3)2], 69.47 (-CHCH2-), 72.51 (-CHCH2-), 77.79,
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(1H, s, CpH), 4.81 (1H, s, CpH), 4.89 (1H, s, CpH), 5.03 (1H, s,
CpH), 5.18 (1H, s, CpH), 7.92 (1H, s, CpH), 6.84–7.13 (9H, m,
PhH), 7.18–7.28 (10H, m, PhH), 7.41–7.43 (6H, m, PhH); 7.53–
126.32, 127.52, 127.96, 128.18, 130.13, 135.70 (PhC). MS(ESI)
[m/z (species)]: 807 [M + 1]+.
On further eluting the column at the same polarity, a final frac-
tion was obtained which upon evaporation yielded an yellow crys-
1
7.55 (2H, m, PhH); 7.96–7.99 (2H, m, PhH). 13C{ H} d 13.97,
5
4
5
15.56 [-CH(CH3)2], 18.76, 29.68 [-CH(CH3)2], 28.46 [-CH(CH3)2],
31.06 [-CH(CH3)2], 66.31 (-CHCH2-), 67.58 (-CHCH2-), 69.98
(-CHCH2-), 71.59 (-CHCH2-), 75.70, 76.36, 77.20, 78.68, 79.08,
79.15 (-C4Ph3), 80.48, 81.30, 83.49, 83.80, 86.23, 86.42, 86.68,
87.84, 90.27, 99.52, (-CpC), 126.13, 126.42, 126.68, 126.83, 127.04,
127.81, 127.93, 127.99, 128.19, 128.78, 129.31, 129.44, 129.67,
130.36 (PhC), 133.59, 134.08, 134.59, 134.75, 135.28, 135.48 (ipso-
PhC), 165.20, 172.05 (C N).
talline solid which was identified as [(h -Cp)Co(h -C4Ph3)]2[(h -
4
Cp)Co(h -C4Ph2)] (8) Yield 0.045 g, 0.04 mmol, 2%. Mp: 212–
◦
214 C. Found: C, 72.76; H, 4.47. Calc. for C75H55Co3·(CHCl3):
C, 72.88; H, 4.51.IR (n, cm-1): 1598 m, 1495 m (C C). NMR
(CDCl3): 1H, d 4.11 (5H, s, -CpH), d 4.4.27 (10H, s, -CpH), 6.91–
7.56 (40H, m, -PhH); 13C{ H}, d 75.43, 77.20, 78.56, 78.81 (CbC),
1
82.10, 82.51 (CpC), 125.73, 126.10, 126.53, 127.07, 127.44, 127.62,
127.96, 130.38, 130.73 (PhC), 135.85, 138.90, 137.59 (ipso PhC).
MS(ESI) [m/z (species)]: 1134 [M + 1]+.
Preparation of bis[(g5-oxazolinylcyclopentadienyl)-(g4-triphenyl
cyclobutadiene)cobalt]2 (6)
General procedure for rearrangement of allylic
trichloroacetimidates catalyzed by palladacycle 4 and 5
Compound 6 was prepared by a similar procedure which gave 3
using 2-amino-1-ethanol (0.037 g, 0.38 mmol) instead of L-valinol.
Yield: 0.115 g, 0.12 mmol, 75%. Mp: 262–265 ◦C. Found: C, 75.90;
H, 4.81; N, 3.04. Calc. for C60H46O2N2Co2: C, 76.07; H, 4.89; N,
2.96. IR (n, cm-1): 1653 vs. (C N); NMR(CDCl3): 1H, d 3.49–3.55
[(4H, t, J = 9.3 Hz, -NCH2CH2O], d 3.67–3.73 [(4H, t, J = 9.0 Hz,-
NCH2CH2O], 4.59 (4H, s, -CpH-), 5.13 (4H, s, -CpH-), 7.04–
A solution of catalysts 4 or 5 (0.02 mmol) and CH3CN (0.5 mL)
was added to trichloroacetimidates 9a–c (1 mmol) in an oven-
dried, 2 mL reaction vial. The vial was capped and then maintained
at 70 ◦C. After 16 h, the solution was concentrated. Purifica-
tion by silica gel column chromatography using 2% EtOAc–
hexane providedallylic trichloroacetamides10a–c (Table 2). Chiral
HPLC analysis (Shimazdu, Diacel AD-H column, 10–20% IPA/n-
hexane, 1.0 mL min-1) was used to determine the enantiomeric
excess of 10a–c.
1
7.23 (22 H, m, -PhH), 7.44–7.47 (8H, d, -PhH). 13C{ H} d 54.40
(-NCH2CH2O), 66.57 (-NCH2CH2O), 72.74, 77.20, 77.25
(-C4Ph3), 83.28, 83.84 (-CpC), 85.59 (ipso-CpC), 126.09, 126.72,
127.57, 127.79, 130.33 (PhC), 134.52, 135.20 (ipso-PhC),
161.25(C N). MS(ESI) [m/z (species)]: 947 [M + 2]+.
Acknowledgements
The authors thank Department of Science and Technology (DST),
India for financial assistance in the form of research grants
(SR/SI/IC-31/2007) and (SR/SI/IC-43/2010) to A. J. E. Nem
Singh thanks UGC India for a junior/senior research fellowship.
The authors thank Dr N. G. Ramesh, Department of chemistry,
IIT Delhi for his unstinted help in carrying out chiral HPLC
measurements and also for valuable suggestions and discussions.
Thanks are due to Prof. A. Ramanan, IIT Delhi for help in X- ray
structural analysis. We thank DST-FIST and IITD for funding
the single crystal X-ray diffraction facility at IIT Delhi.
Reaction of 1,4-diphenylbutadiyne and diphenylacetylene with
(g5-Cp)Co(PPh3)2
An analogous reaction to the one which yielded compound 1 was
carried out by taking 1,4-diphenylbutadiyne (0.606 g, 3.00 mmol)
and diphenylacetylene (0.800 g, 4.50 mmol) in 80 ml of xylene
5
which were added in a stirred solution of (h -Cp)Co(PPh3)2
[prepared in situ by dissolving NaCp (0.265 g, 3.00 mmol)
and (PPh3)3CoCl (2.670 g, 3.00 mmol) in 10 ml of THF] and
the resultant mixture was refluxed for 4 h. After cooling the
5
reaction mixture, more of (h -Cp)Co(PPh3)2 [prepared in situ by
dissolving NaCp (0.265 g, 3.00 mmol) and (PPh3)3CoCl (2.670 g,
3.00 mmol) in 10 ml of THF] and diphenylacetylene (0.800 g,
4.50 mmol) were added so that the overall ratio of PhC CPh,
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PhC C–C CPh and (h -Cp)Co(PPh3)2 was 3 : 1 : 2. The resulted
mixture was refluxed further for 6 h. After completion, the
reaction mixture was cooled, solvent was evaporated off and the
remaining residue chromatographed through a neutral alumina
column and eluted with hexane to remove triphenyl phosphine.
Using 2% ethylacetate/hexane mixture the first fraction came out
which upon evaporation gave the monomeric sandwich compound
5
4
[(h -Cp)Co(h -C4Ph4)]6c Yield: 1.210 g, 1.52 mmol, 42%. On
increasing polarity to 3%, the second fraction came out which
upon evaporation gave a red crystalline compound characterized
5
4
as [(h -Cp)Co(h -C4Ph3)]2 (7). Yield: 0.943 g, 1.17 mmol 39%.
Mp: 261–263 ◦C. Found: C, 80.31; H, 5.06. Calcd for C54H40Co2:
C, 80.39; H, 5.00. IR (n, cm-1): 1596 m, 1495 m (C C), NMR
1
(CDCl3): H, d 4.58 (10H, s, -CpH), 7.06–7.14 (18H, m, -PhH),
1
7.38–7.41 (4H, m, -PhH), 7.44–7.47 (8H, m, -PhH); 13C{ H},
d 71.41 (bridge CbC), 77.2, 77.94 (CbC), 82.75 (CpC), 126.02,
4890 | Dalton Trans., 2011, 40, 4882–4891
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