Synthesis and characterization of novel pyrazole-based ligands of [5-cyclopentadiene][4-tetraphenylcyclobutadiene]cobalt

Article abstract of DOI:10.1080/15533170701675139

(5-N,N-dimethylaminomethylcyclopentadienyl)(4-tetraphenylcyclobutadiene) cobalt was prepared by the Mannich reaction of the metallocene (5-Cp)Co(4-Ph4C4) with tetramethylmethylenediamine and phosphoric acid. The methyl iodide salt of this compound was rea

Full text of DOI:10.1080/15533170701675139

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Synthesis and Characterization of Novel
Pyrazole‐Based Ligands of [η5‐Cyclopentadiene]
[η4‐Tetraphenylcyclobutadiene]Cobalt
Anil J. Elias a , M. Senthil Kumar a , R. P. Gupta a & N. Behera a
a Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India
Version of record first published: 06 Nov 2007.
To cite this article: Anil J. Elias, M. Senthil Kumar, R. P. Gupta & N. Behera (2007): Synthesis and Characterization of Novel
Pyrazole‐Based Ligands of [η5‐Cyclopentadiene][η4‐Tetraphenylcyclobutadiene]Cobalt, Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry, 37:9, 729-733
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 37:729–733, 2007
Copyright # 2007 Taylor & Francis Group, LLC
ISSN: 1553-3174 print/1553-3182 online
DOI: 10.1080/15533170701675139
Synthesis and Characterization of Novel Pyrazole-Based
Ligands of [h5-Cyclopentadiene]
[h4-Tetraphenylcyclobutadiene]Cobalt
Anil J. Elias, M. Senthil Kumar, R. P. Gupta, and N. Behera
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India
pyrazole ligands has been utilized in the preparation of many
(h5-N,N-dimethylaminomethylcyclopentadienyl)(h4-tetraphe-
nylcyclobutadiene) cobalt was prepared by the Mannich reaction
of the metallocene (h5-Cp)Co(h4-Ph4C4) with tetramethylmethy-
lenediamine and phosphoric acid. The methyl iodide salt of this
compound was reacted with 3,5-dimethylpyrazole and 3-methyl-
5-trifluoromethylpyrazole to prepare the pyrazole derived
metallocenes, [h5-(C5H4CH2)-3,5-(CH3)2(C3HN2)]Co[h4-(Ph4C4)]
(1) and [h5-(C5H4CH2)-3-CH3-5-CF3(C4H2N)]Co[h4-(Ph4C4)] (2).
Compounds 1 and 2 were structurally characterized.
novel metal complexes with interesting electrochemical prop-
erties. In addition, ferrocene-based ligands having pyrazolyl
and substituted pyrazolyl groups having planar chirality and
central chirality has been used in the design of catalysts
useful in asymmetric synthesis.[6]
Among organometallic sandwich compounds, there are only
few compounds whose chemical properties match that of ferro-
cene. Among these, (h5-cyclopentadienyl) (h4-tetraphenylcy-
clobutadiene)cobalt has been very promising in its stability
and reactivity.[7] Recent work carried out on carboxylic acid
derivatives of this metallocene has shown its potential as a sub-
strate whose functional group chemistry can be utilized in the
design of novel chiral organometallic catalysts[8–10] as well as
clusters.[11] Herein we report the synthesis and structural
characterization of the first examples of pyrazolyl derivatives
of this metallocene, which are potential ligands similar to
Keywords dimethylpyrazole, methyltrifluoromethylpyrazole, met-
allocene, cobalt, dimethylaminomethylcyclopentadienyl,
tetraphenylcyclobutadiene
INTRODUCTION
Pyrazole as such or as part of a molecule has been used their ferrocene analogues.
extensively as ligands in coordination chemistry.[1] The stab-
RESULTS AND DISCUSSION
ility and electronic properties of such ligands has helped in
the design of a host of metal complexes and metal clusters,
some of which has shown potential use as light emitting
devices and display devices.[2] Examples of organometallic
compounds known so far having pyrazole moieties has been
mostly centered on ferrocene based molecules (Figure 1).[3–5]
The stability and electroactivity of these ferrocene based
Two examples of pyrazole-derived (h5-cyclopentadienyl)
(h4-tetraphenylcyclobutadiene)cobalt fh5-[3,5-(CH3)2C3HN2]
CH2C5H4g-Co[h4-(Ph4C4)](1)andfh5-[3-(CH3)5-(CF3)C3HN2]
CH2C5H4g-Co[h4-(Ph4C4)] (2) were prepared in this study. We
have attempted to vary the basicity of the pyrazolyl group by
introducing, on the ring, two methyl groups and also by replacing
one of the methyl groups by a trifluoromethyl group. The incor-
poration of one trifluoromethyl group is expected to decrease
the electron-donating property of 2 in comparison to 1 and
increase the ligand-centered ppꢀ energy gap.
Received 2 November 2006; accepted 9 May 2007.
Dedicated to Prof. Dr. Herbert W. Roesky on the occasion of his
70th birthday.
The reaction of Co(PPh3)3Cl with NaCp and diphenyl-
acetylene resulted in the formation of the metallocene (h5-
cyclopentadienyl)(h4-tetraphenylcyclobutadiene) cobalt in
around 50% yield (Scheme 1). The compound is purified
by column chromatography on silica gel. Reaction of (h5-Cp)
Co(h4-Ph4C4) with tetramethylmethylenediamine and phos-
phoric acid as catalyst in acetic acid medium resulted in the
formation of (h5-N,N-dimethylaminomethylcyclopentadienyl)
(h4-tetraphenylcyclobutadiene)cobalt in 70% yield (Scheme 2).
The authors wish to thank Council of Scientific and Industrial
Research (CSIR) India and Department of Science and Technology,
(DST) India for financial assistance in the form of research grants
and IIT Delhi for an equipment grant. We acknowledge DST-FIST
and IITD for funding of the single crystal X-ray diffraction facility
at IIT Delhi. SAIF CDRI, Lucknow is acknowledged for mass
spectral and analytical measurements.
Address correspondence to Anil J. Elias, Department of Chemistry,
Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016,
India. E-mail: eliasanil@gmail.com
729

730
A. J. ELIAS ET AL.
SCH. 3.
compound 1 (5.65 and 4.29 ppm). This may be due to the
presence of a strongly electron-withdrawing CF3 group in
compound 2. Further, the signal of the CH proton is split into a
FIG. 1. Examples of ferrocene-derived pyrazole ligands.
1
multiplet due to long-range coupling with the CF3 group. H-
The synthesis of this compound was reported previously by
Rausch et al.[12] However we have utilized a relatively
inexpensive and carbonyl-free method for the synthesis of
(h5-(Cp)Co(h4-Ph4C4) by slightly modifying the procedure
reported for the preparation of fh5-C5H4[C(O)OMe]gCo
(h4-Ph4C4) by Richards and coworker.[13] The methyliodide
salt of the dimethylaminomethyl metallocene, which was
prepared by reaction of [h5-C5H4CH2N(CH3)2]Co[h4-(Ph4C4)]
cobalt and iodomethane (Scheme 3).
NMR spectra of compound 1 gave two different signals for the
methyl groups (2.05 and 2.12 ppm). This was further confirmed
by the 13C-NMR spectra, which gave two different signals for the
CH3 carbon atoms (11.28 and 13.50 ppm). The 13C-NMR spectra
of compound 2 gave a signal at 29.71 ppm for the -CF3 carbon
1
and at 11.35 ppm for the CH3 carbon atom. H-NMR spectra
of compound 2 gave one signal for the CH3 proton (2.07 ppm)
which was shifted to the upfield region compared to 3-methyl-
5-trifluoromethylpyrazole (2.43 ppm).
This methyliodide salt reacts directly with 3,5-dimethylpyr-
azole and 3-methyl-5-trifluoromethylpyrazole to yield the Structural Studies
pyrazole derived metallocenes 1 and 2 in 45 and 33% yields,
respectively (Scheme 4). Compounds 1 and 2 were purified
by column chromatography over silica gel using a hexane-
ethylacetate mixture as eluent and are quite stable under air
and moisture over extended periods of time.
The crystal structure of compounds 1 and 2 showed many
similarities (Figures 2 and 3). The pyrazole rings of 1 and 2
are oriented at angles of 112.348 and 112.628 respectively
with respect to the cyclopentadienyl rings. The CF3 group
present in compound 2 is oriented away from the cyclopenta-
diene ring. The plane containing the pyrazole ring bisects the
plane containing the cyclopentadienyl ring at an angle of
71.988 in the case of compound 1 and 70.098 in case of
compound 2. The four phenyl rings present in the cyclobuta-
diene ring are oriented at almost equal angles with respect to
the cyclobutadiene ring in both the compounds 1 and 2
(Table 1). This is in contrast to the observation of the (h5-
carboxycyclopentadienyl)(h4-tetraphenylcyclobutadiene)cobalt
where the four phenyl rings of the tetraphenyl cyclobutadiene
ring of 4 are displaced at different angles 37.39, 48.44,
29.25, and 60.498 with respect to the cyclobutadiene ring.
Further the -CF3 group present in compound 2 involved in
Spectral Studies
The 1H-NMR spectra of compounds 1 and 2 differ consider-
ably from each other. Even though there is no difference in the
chemical shift values of cyclopentadienyl protons of com-
pounds 1 and 2, the CH proton of the pyrazole ring and CH2
protons show downfield shifts in the case of compound 2
(6.15–6.40 and 4.36 ppm, respectively) compared to
˚
weak C-F. . .H interaction (2.585(6) A) with -CH3 hydrogen
of the adjacent molecule (Figure 4). The cyclopentadienyl
ring and cyclobutadienyl ring are planar.
SCH. 1.
SCH. 2.
SCH. 4. Where R1, R2 ¼ CH3 (1) R1 ¼ CH3, R2 ¼ CF3 (2).

NOVEL PYRAZOLE-BASED LIGANDS OF COBALT CYCLOPENTADIENE COMPLEXES
731
TABLE 1
Selected bond distances and angles of compounds 1 and 2
Compound
1
Compound
2
˚
Bond lengths (A)
Co(1)-C(7)
Co(1)-C(12)
Co(1)-C(15)
2.058 (3)
1.981 (3)
1.987 (3)
1.473 (4)
1.364 (5)
1.334 (5)
2.020 (3)
1.984 (3)
1.987 (3)
1.469 (4)
1.343 (4)
1.335 (5)
C(12)-C(16)
N(1)- N(2)
N(2)-C(2)
Bond angles (8)
N(1)-C(6)-C(7)
C(12)-C(16)-C(17)
C(13)-C(22)-C(23)
C(14)-C(28)-C(29)
C(15)-C(34)-C(35)
112.3 (3)
119.9 (2)
121.9 (8)
121.6 (0)
120.9 (4)
112.6 (2)
120.3 (0)
122.1 (9)
121.7 (7)
121.6 (5)
FIG. 2. Thermal ellipsoid view of compound 1 with 30% probability factor
(H atoms are omitted for clarity).
In conclusion, we have prepared and structurally character-
ized the first examples of pyrazole-based ligands of [h5-cyclo-
pentadiene][h4-tetraphenylcyclobutadiene]cobalt having different
electronic properties. Further study on the use of these organo-
metallic ligands in synthesis of metal complexes is in progress.
respectively. IR spectra in the range 4000–250 cm21 were
´
recorded on a Nicolet Protege 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 EI mode using a JEOL SX 102/DA-6000
mass spectrometer.
EXPERIMENTAL
All manipulations were carried out using standard Schlenk
techniques under nitrogen atmosphere. THF and toluene were
freshly distilled from sodium benzophenone ketyl under
nitrogen atmosphere and used. Sodium salt of cyclopentadiene,
tris(triphenylphosphine)cobalt chloride,[13] 3,5-dimethylpyra-
zole and 3-methyl-5-trifluoromethylpyrazole were prepared
according to the literature procedures.[14] Diphenylacetylene
X-ray Crystallographic Study
Suitable crystals of compounds 1 and 2 were obtained by
slow evaporation of their saturated solutions in ethyl acetate/
hexane solvent mixture. Single-crystal diffraction studies
were carried out on a Bruker SMART APEX CCD diffract-
1
and triphenylphosphine (Aldrich) were used as such. H and
1
˚
ometer with a MoKa (l ¼ 0.71073 A) sealed tube. All
13Cf Hg spectra were recorded on a Bruker Spectrospin
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.[15] The crystal
structures were solved and refined using the SHELXTL
(version 6.12) package.[16] All hydrogen atoms were included
in idealized positions, and a riding model was used. Non-
hydrogen atoms were refined with anisotropic displacement
parameters. CCDC 626105 and 626104 contains the sup-
plementary crystallographic data for the compounds 1 and 2,
respectively. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/datarequest/cif.
DPX-300 NMR spectrometer at 300 and 75.47 MHz
General Procedure
A stoichiometric mixture of [h5-C5H4CH2N(CH3)3I]Co[h4-
(Ph4C4)], 3,5-dimethylpyrazole (or) 3-methyl-5-trifluoro-
methylpyrazole and potassium carbonate were taken in 20 ml
of dry acetonitrile. The mixture was heated to reflux for
several hours and the progress of the reaction was monitored
FIG. 3. Thermal ellipsoid view of compound 2 with 30% probability factor
(H atoms are omitted for clarity).

732
A. J. ELIAS ET AL.
TABLE 2
Data collection and structure solution parameters of
compounds 1 and 2
Empirical formula
C39H33CoN2 (1)
588.60
C39H30CoF3N2 (2)
642.58
Formula mass
Crystal system
Space group
T (K)
Triclinic
P-1
298 (2)
Monoclinic
P21/c
298 (2)
˚
A (A)
9.841 (5)
11.087 (6)
14.651 (8)
86.746 (9)
75.803 (8)
79.410 (9)
1523.2 (14)
2
11.2529 (18)
19.914 (3)
14.255 (2)
90.00
˚
B (A)
˚
c (A)
a (8)
B (8)
G (8)
95.029 (3)
90.00
3
˚
V (A )
3182.0 (9)
4
1.341
FIG. 4. View of compound 2 with 30% probability factor showing weak
C-F. . .H interaction.
Z
Dcalcd (g/cm3)
M (mm21
1.283
0.593
)
0.587
Tmax/Tmin
u range (8)
Index ranges
0.503/0.206
2.17–25.49
211 ꢁ h ꢁ 11
213 ꢁ k ꢁ 13
217 ꢁ l ꢁ 17
12124
0.939/0.907
2.23–28.26
213 ꢁ k ꢁ 13
224 ꢁ k ꢁ 24
217 ꢁ l ꢁ 17
30841
by thin layer chromatography. After completion of the
reaction, the solvent was evaporated by rotary evaporator and
the compound was separated by a silica gel column using ethyl-
acetate-hexane mixture as the eluent. A small amount of the
compound was recrystallized using the same solvent mixture
to give red-colored, block-like crystals (Table 2).
Refl. collected
Refl. unique
5223
3768
5930
4832
fh5-[3,5-(CH3)2C3HN2]CH2C5H4g-Co[h4-(Ph4C4)] (1)
[h5-C5H4CH2N(CH3)3I]Co[h4-(Ph4C4)] (0.20 g, 0.3 mmol),
3,5-dimethylpyrazole (0.03 g, 0.3 mmol), and potassium
carbonate (0.04 g, 0.3 mmol). Reaction time: 33 hours. Yield:
0.08 g, 45%. MP: 1958C; IR (n, cm21): 3048w, 2922w,
2358w, 1596s, 1545s, 1496vs, 1453s, 1327w, 1064w, 1025s,
772s, 698vs, 572s; H1-NMR: 2.05 (s, 3H, CH3), 2.12
(s, 3H, CH3), 4.29 (s, 2H, CH2), 4.59 (d, 2H, Cp-H), 4.74
(d, 2H, Cp-H), 5.65 (s, 1H, Py-H), 7.23–7.26 (m, 12H,
Ar-H), 7.45–7.49 (m, 8H, Ar-H); 13C-NMR: 11.28 (-CH3),
13.50 (-CH3), 46.07 (Cp-CH2), 75.19 (C4Ph4), 76.59 (C4Ph4),
83.43 (CpC), 83.96 (CpC), 92.68 (ipso-CpC), 105.12
(Py-CH), 126.43 (PhC), 128.12 (PhC), 128.81 (PhC), 136.02
(ipso-PhC), 138.07 (py C-CH3), 147.19 (py N-C); MS (ESþ)
[m/e (species)]: 589 (M þ 1)þ, 493 (M-Py)þ, 415
(Co(C4Ph4)þ. Anal. calcd. for C39H33CoN2: C 79.58, H 5.65;
Found: C 79.43, H 5.51.
Refl. observed
R1(I . 2s (I))
wR2 (all data)
(Dr)max, (Dr)min
0.0534
0.1510
0.0578
0.1341
0.557/–0.671
0.71073
0.520/–0.275
0.71073
˚
L (A)
11.35 (-CH3), 29.71 (-CF3), 47.32 (Cp-CH2), 75.46 (C4Ph4),
76.59 (C4Ph4), 83.42 (CpC), 84.17 (CpC), 91.43 (ipso-CpC),
103.92 (PyC-CH), 126.59 (PhC), 128.19 (PhC), 128.81
(PhC), 135.80 (ipso-PhC), 139.12 (py N-C); MS (ESþ) [m/e
(species)]: 642 (M)þ, 493 (M-Py)þ; Anal. calcd. for
C39H30CoF3N2: C 72.89, H 4.71; Found: C 72.78, H 4.61.
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