Mononuclear cobalt(II) carboxylate complexes: Synthesis, molecular structure and selective oxygenation study

Article abstract of DOI:10.1016/j.ica.2007.03.026

Some cobalt carboxylate (both mononuclear as well as binuclear) complexes have been prepared by using hindered hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (Tp^iPr2) as supporting ligand. The reaction of [Tp^iPr2Co(NO₃)] (2) with sodium benzoate resulted in the formation of acetonitrile coordinated complex [Tp^iPr2Co(OBz)(CH₃CN)] (3) whereas the reaction of 2 with sodium fluorobenzoate gave coordinately unsaturated five coordinate complex of the type [Tp^iPr2Co(F-OBz)] (4). The oxidation of compound 4 in the presence of 3,5-diisopropylpyrazole 3, 5 - Pr₂ⁱ pzH resulted in the formation of a unique compound [Co (F-OBz) {HB (3 - OCMe₂ - 5 - Prⁱ pz) (3, 5 - Pr₂ⁱ pz)₂} (3, 5 - Pr₂ⁱ pzH)] (5) where only one methine carbon of isopropyl group on pyrazole ring of hydrotris(3,5-diisopropyl-1-pyrazolyl)borate oxidized and coordinated with cobalt center. In compound 5, the binding behavior of fluorobenzoate also changes from bidentate to monodentate and the nonbonded oxygen atom formed intramolecular hydrogen bond with the hydrogen atom of the NH fragment of the coordinated 3, 5 - Pr₂ⁱ pzH. X-ray crystallography and IR studies confirmed the existence of hydrogen bonding in complex 5. The pyrazolato bridged binuclear cobalt(II) complex [(3, 5 - Pr₂ⁱ pzH)₂ Co₂ (μ - 3, 5 - Pr₂ⁱ pz)₂ (NO₂ -OBz)₂] (6) was prepared by the reaction of hydrated cobalt(II) nitrate, 3,5-diisopropylpyrazole and sodium nitrobenzoate where, each cobalt is four coordinate. The X-ray structure of 6 showed that the NH fragment of terminally coordinated 3, 5 - Pr₂ⁱ pzH formed intramolecular hydrogen bonding with nonbonded oxygen atom of monodentately coordinated nitrobenzoate.

Full text of DOI:10.1016/j.ica.2007.03.026

Inorganica Chimica Acta 360 (2007) 3226–3232
www.elsevier.com/locate/ica
Mononuclear cobalt(II) carboxylate complexes: Synthesis,
molecular structure and selective oxygenation study
*
Udai P. Singh , Vaibhave Aggarwal, Asish K. Sharma
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
Received 27 May 2006; received in revised form 23 February 2007; accepted 18 March 2007
Available online 24 March 2007
Abstract
Some cobalt carboxylate (both mononuclear as well as binuclear) complexes have been prepared by using hindered hydrotris(3,5-diiso-
propyl-1-pyrazolyl)borate (TpiPr2) as supporting ligand. The reaction of [TpiPr2Co(NO3)] (2) with sodium benzoate resulted in the forma-
tion of acetonitrile coordinated complex [TpiPr2Co(OBz)(CH3CN)] (3) whereas the reaction of 2 with sodium fluorobenzoate gave
coordinately unsaturated five coordinate complex of the type [TpiPr2Co(F-OBz)] (4). The oxidation of compound 4 in the presence of
3,5-diisopropylpyrazole 3; 5-Pri2pzH resulted in the formation of a unique compound ½CoðF-OBzÞfHBð3-OCMe2-5-PripzÞð3; 5-Pr2i pzÞ2g
ð3; 5-Pri2pzHÞꢀ (5) where only one methine carbon of isopropyl group on pyrazole ring of hydrotris(3,5-diisopropyl-1-pyrazolyl)borate oxi-
dized and coordinated with cobalt center. In compound 5, the binding behavior of fluorobenzoate also changes from bidentate to monod-
entate and the nonbonded oxygen atom formed intramolecular hydrogen bond with the hydrogen atom of the NH fragment of the
coordinated 3; 5-Pri2pzH. X-ray crystallography and IR studies confirmed the existence of hydrogen bonding in complex 5. The pyrazolato
bridged binuclear cobalt(II) complex ½ð3; 5-Pri2pzHÞ Co2ðl-3; 5-Pr2i pzÞ ðNO2-OBzÞ ꢀ (6) was prepared by the reaction of hydrated cobal-
2
2
2
t(II) nitrate, 3,5-diisopropylpyrazole and sodium nitrobenzoate where, each cobalt is four coordinate. The X-ray structure of 6 showed
that the NH fragment of terminally coordinated 3; 5-Pri2pzH formed intramolecular hydrogen bonding with nonbonded oxygen atom
of monodentately coordinated nitrobenzoate.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Mononuclear complexes; Cobalt complexes; Monooxygenation; Intramolecular hydrogen bonding; Crystal structures
1. Introduction
C–H bond functionalization by metal-coordinated
active oxygen species is very interesting subjects from syn-
thetic, catalytic, and bioinorganic point of view [7]. Various
studies on C–H bond functionalization using metal–dioxy-
gen complexes [8–12] revealed that a high valent metal–oxo
(O2ꢁ) species is formed via O–O bond cleavage of metal
The cobalt is an essential element for life although it
does not participate in O2 metabolism. Cobalt is found in
vitamin B12, that catalyzes trans-alkylation and isomeriza-
tion via Co(III)-alkyl intermediate [1]. Besides vitamin B12,
there are several other cobalt containing proteins and
enzymes [2–6] where cobalt is present in the active site or
needed as cofactors for their functions. A common struc-
tural feature found in most of the cobalt-containing pro-
teins and enzymes is the presence of one or more
carboxylate groups derived from aspartate or glutamate
side chains of the protein.
2ꢁ
coordinated O2 species and work as an active intermedi-
ate for C–H bond oxygenation. Many examples of the
ligand oxygenation including TpiPr2 by using metal–oxo/
peroxo complexes [11,12] have been reported in the litera-
ture. In general the aliphatic C–H bond oxygenation is rare
[13–15] compared to aromatic oxygenation [16–18] and in
all reported examples of aliphatic/aromatic C–H bond oxy-
genation, the source of oxygen is generally coordinated
oxo/peroxo to metal center.
*
Coordination chemistry of metal carboxylato species is
an attractive subject from the bioinorganic standpoint
Corresponding author. Tel.: +91 1332 285329; fax: +91 1332 273560.
E-mail address: udaipfcy@iitr.ernet.in (U.P. Singh).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.03.026

U.P. Singh et al. / Inorganica Chimica Acta 360 (2007) 3226–3232
3227
because the carboxylate group of glutamate and aspartate
works as supporting ligand for the metal center in various
metalloproteins and behave as monodentate or bidentate
depending upon the requirement of active site. Accord-
ingly, a variety of mononuclear carboxylate complexes
with coordination number six are available in the literature
[19,20]. But to the best of our knowledge, there are no
examples available for five coordinated mononuclear
cobalt carboxylate complexes. In present paper, we report
the synthesis, structural characterization of mononuclear
cobalt(II) benzoate, fluorobenzoate, binuclear pyrazolato
bridged cobalt(II), and oxidation behavior of fluorobenzo-
ate complex.
Potassium salt of hydrotris(3,5-diisopropyl-1-pyraz-
olyl)borate ½HBð3; 5-Pri2pzÞ3 ¼ TpiPr2ꢀ (1) and [TpiPr2Co-
(NO3)] (2) were synthesized by the literature methods
[25,26].
2.3. [TpiPr2Co(OBz)(CH3CN)] (3)
Compound 2 (0.29 g, 0.46 mmol) and sodium benzoate
(0.07 g, 0.46 mmol) were stirred in a mixture of toluene
and acetonitrile (1:1 ratio) for 12 h. The mixture was fil-
tered over celite and solvent was evaporated to dryness
under vacuum. The purple colored crystals in 73% yield
(0.25 g, 0.36 mmol) were obtained by slow cooling of the
acetonitrile solution at ꢁ20 °C. The elemental analysis
was performed on vacuum dried sample for several hours.
Anal. Calc. for C36H54N7O2BCo: C, 62.98; H, 7.93; N,
14.28. Found: C, 62.36; H, 7.70; N, 14.17%. IR (KBr,
cmꢁ1): m(BH) 2535, m(CN) 2279, mas(COO) 1602, ms(COO)
1473. UV–Vis (toluene, nm, e/Mꢁ1 cmꢁ1): 289 (1969), 574
(161). FD-MS (m/z): 645.
2. Experimental
2.1. Instrumentation
Carbon, hydrogen, and nitrogen were analyzed with a
Vario EL III elemental analyzer after carefully dried sam-
ples under vacuum for several hours. IR spectra were
obtained on a Thermo Nicolet Nexus FT-IR spectrometer
in KBr. The UV–Vis spectra were recorded on Perkin–
Elmer Lambda 35 UV/Vis spectrophotometer. FD-MS
spectra were obtained on a Hitachi M-80 and FAB-MS
spectra were obtained on VG Autospec mass spectrometer.
The room temperature magnetic susceptibility measure-
ments were done on a Princeton applied research vibrating
sample magnetometer Model 155. The single crystal of 3
was coated with FomblinÒ YR-1800, picked up with loop
and mounted on a Siemens Smart-CCD diffractometer
whereas the crystals of 4–6 were treated in same way but
mounted on a Bruker Kappa Apex four circle-CCD diffrac-
tometer. Both systems were equipped with a nitrogen cold
stream operating at 100 ( 2) K. Graphite monochromated
2.4. [TpiPr2Co(F-OBz)] (4)
To the toluene solution (15 ml) of 2 (0.51 g, 0.81 mmol)
was added the acetonitrile solution (10 ml) of sodium p-flu-
orobenzoate (0.13 g, 0.81 mmol) drop-wise and the reac-
tion mixture was stirred for 10 h. The resultant mixture
was filtered over Celite and solvent was evaporated to dry-
ness under vacuum. The compound was dissolved in a mix-
ture of acetonitrile and dichloromethane solvent and the
purple colored crystals were obtained at ꢁ20 °C in 52%
yield (0.30 g, 0.45 mmol). Anal. Calc. for C34H50N6
O2BFCo: C, 61.45; H, 7.58; N, 12.65. Found: C, 61.39;
H, 7.82; N, 12.59%. IR (KBr, cmꢁ1): m(BH) 2534, mas(COO)
1602, ms(COO) 1473. UV–Vis (toluene, nm, e/Mꢁ1 cmꢁ1):
290 (1837), 576 (112). FAB-MS (m/z): 664. leff = 3.89
BM at 295 K.
˚
MoKa radiation (k = 0.71070 A) was used throughout.
Crystal structures were solved by direct methods. Structure
solution, refinement, and data output were carried out with
the SHELXTL program [21,22]. Non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in
geometrically calculated positions by using a riding model.
Images were created with the DIAMOND program [23].
Hydrogen bonding interactions in the crystal lattice were
calculated with SHELXTL and DIAMOND.
2.5. ½CoðF -OBzÞfHBð3-OCMe2-5-Pri pzÞð3; 5-Pr2i pzÞ g
2
ð3; 5-Pri2pzHÞꢀ (5)
To dichloromethane solution (15 ml) of 4 (0.54 g,
0.70 mmol) was added 1 equiv. of 3; 5-Pri2pzH (0.11 g,
0.70 mmol) at room temperature and 30 equiv. of aqueous
30% H2O2 (2.15 ml, 21.0 mmol) drop-wise with continuous
stirring. The reaction mixture was further stirred for one
more hour. The reaction mixture was cooled at ꢁ20 °C
for 10 min and filtered on Celite in cold condition. The
solution was evaporated to dryness and the brown colored
crystals were obtained from the mixture of acetonitrile
and dichloromethane at ꢁ20 °C in 71% yield (0.59 g, 0.70
mmol). The elemental analysis was performed on vacuum
dried sample for several hours. Anal. Calc. for
C43H65N8O3BFCo: C, 62.24; H, 7.80; N, 13.54. Found: C,
61.89; H, 7.54; N, 13.31%. IR (KBr, cmꢁ1): m(BH) 2541,
mas(COO) 1607, ms(COO) 1467. UV–Vis (toluene, nm,
e/Mꢁ1 cmꢁ1): 284 (1837), 569 (112). leff = 4.89 BM at 295 K.
2.2. Materials and methods
All manipulations were carried out under nitrogen
atmosphere by using standard Schlenk tube techniques
unless otherwise stated. The required solvents were care-
fully purified and distilled under nitrogen prior to use by
the literature method [24]. Cobalt(II) nitrate hexahydrate
and sodium benzoate were of the highest grade commer-
cially available and used without further purification. The
p-X-benzoic acids (X = –F, –NO2) were commercially
available and their sodium salts were prepared by reacting
with appropriate amount of sodium hydroxide in water.

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U.P. Singh et al. / Inorganica Chimica Acta 360 (2007) 3226–3232
2.6. ½ð3; 5-Pri2pzHÞ Co2ðl-3; 5-Pr2i pzÞ ðNO2-OBzÞ ꢀ (6)
3. Results and discussion
2
2
2
Co(NO3)2 Æ 6H2O (0.291 g, 1.0 mmol), 3; 5-Pri2pzH
(0.302 g, 2.0 mmol) and sodium nitrobenzoate (0.189 g,
1.0 mmol) were stirred in a mixture of dichloromethane
(20 ml) and acetonitrile (5 ml) for 8 h. The solution was fil-
tered over Celite and the solvent was evaporated to dryness
under vacuum. The violet colored crystals in 75% (0.79 g,
0.75 mmol) yield were obtained by slow cooling of acetoni-
trile solution at ꢁ20 °C. Anal. Calc. for C50H70N10O8Co2:
C, 56.81; H, 6.63; N, 13.25. Found: C, 56.51; H, 6.65; N,
13.10%. IR (KBr, cmꢁ1), mas(COO) 1591, ms(COO) 1471.
The previously prepared [TpiPr2Co(NO3)] (2) was used
as starting material for the preparation of different benzo-
ate complexes reported in the present paper. Complex 3
was prepared by the reaction of 2 with sodium benzoate.
It was crystallized by slow cooling of acetonitrile solution
at ꢁ20 °C and the structure was determined by X-ray crys-
tallography. As shown in Fig. 1, the cobalt center has dis-
torted octahedral geometry with N4O2 donor set having
one coordinated acetonitrile at 6th position. The overall
structure of the present octahedral complex is similar to
one reported by the reaction of binuclear Co(II) hydroxo
complex of TpiPr2 with excess amount of benzoic acid
[26]. The IR spectrum of 3 suggested the bidentate behavior
of coordinated benzoate and was further supported by X-
ray analysis. The Co–O bond lengths (Table 2) are very
leff = 5.78 BM per molecule at 295 K.
Fig. 1. Thermal ellipsoid view of [TpiPr2Co(OBz) Æ CH3CN] (3) drawn at
the 50% probability level. Hydrogen atoms and solvent molecule have
been omitted for clarity.
Fig. 2. Thermal ellipsoid view of [TpiPr2Co(F-OBz)] (4) drawn at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Table 1
Crystal data and data collection details of the complexes [TpiPr2Co(OBz)(CH3CN)] (3), [TpiPr2Co(F-OBz)] (4), ½CoðF-OBzÞfHBð3-OCMe2-
5-PripzÞð3; 5-Pri pzÞ gð3; 5-Pri pzHÞꢀ (5 Æ CH3CN Æ CH2Cl2), ½ð3; 5-Pri pzHÞ Co2ðl-3; 5-Pri pzÞ ðNO2-OBzÞ ꢀ (6)
2
2
2
2
2
2
2
2
Complex
(3)
(4)
(5 Æ CH3CN Æ CH2Cl2)
(6)
Empirical formula
Formula weight
Crystal system
Space group
C
36H54N7O2BCo
C34H50N6O2FBCo
663.54
triclinic
C89H135N17O6F2B2Cl2Co2
1787.52
triclinic
C50H70N10O8Co2
1057.02
triclinic
686.60
monoclinic
P2(1)/c
ꢀ
ꢀ
ꢀ
P1
P1
P1
Unit cell dimensions
˚
a (A)
12.3935(4)
19.6719(8)
16.1545(6)
90.00
105.640(10)
90.00
9.261(6)
15.261(4)
24.851(5)
90.361(3)
95.020(4)
90.911(5)
3498(3)
4
14.026(9)
18.810(10)
21.645(14)
98.232(19)
95.002(18)
103.018(18)
5464(6)
10.3603(8)
11.3682(8)
13.9441(10)
67.8290(10)
70.8480(10)
88.5350(10)
1427.61(18)
1
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
3792.7(2)
4
Z
2
Dcalc (g/cm3)
Data collection
2hmax (°)
1.202
1.260
1.086
1.229
60.0
51.00
13030
6829
51.62
21050
5128
54.8
10775
5272
Number of measured reflections
31027
10972
437
Number of observed reflections
Number of parameters refined
819
1058
324
R
Rw
0.0408
0.0838
0.0461
0.1022
0.0838
0.2192
0.0410
0.1131

U.P. Singh et al. / Inorganica Chimica Acta 360 (2007) 3226–3232
3229
Table 2
similar to the metal–oxygen bond in previously reported
Selected bond distances (A) and bond angles (°) for [TpiPr2Co(OBz)
(CH3CN)] (3), [TpiPr2Co(F-OBz)] (4), ½CoðF-OBzÞfHBð3-OCMe2-5-PripzÞ
ð3; 5-Pri2pzÞ2gð3; 5-Pr2i pzHÞꢀ (5 Æ CH3CN Æ CH2Cl2), ½ð3; 5-Pri pzHÞ Co2-
˚
mononuclear benzoate complexes of cobalt(II) [26] and
manganese(II) [27], but are slightly shorter than the Fe–O
bond distance in a six-coordinate iron(II)–benzoate com-
plex [16] (Fig. 2).
2
2
ðl-3; 5-Pri2pzÞ ðNO2-OBzÞ ꢀ (6)
2
2
[TpiPr2Co(OBz)(CH3CN)] (3)
Attempts were made to prepare the oxygen sensitive
cobalt(II) compound so that the oxidation/oxygenation
studies could be performed. The reaction of complex 2 with
sodium fluorobenzoate in acetonitrile–toluene solution
resulted in the formation of five coordinated [TpiPr2Co
(F-OBz)] (4) whose IR clearly indicated that the fluor-
obenzoate is coordinated in bidentate manner. The suitable
crystal for X-ray data collection was obtained by slow cool-
ing of the acetonitrile–dichloromethane solution of 4 at
ꢁ20 °C. The crystallographic data of 4 are listed in Table
1 and important bond lengths and bond angles are given
in Table 2. As shown in Fig. 3, the metal center of 4 is in
five coordination atmosphere coordinated with three nitro-
gen atom from TpiPr2 ligand and two oxygen atoms from
the fluorobenzoate. The Co–O bond lengths (Table 2) are
similar to that in complex 3, whereas the Co–N bond dis-
tances are slightly smaller than the Co–N bond distances
in 3. On the basis of X-ray data, it may be presumed that
the geometry around metal center may be square pyrami-
dal. As per our effort toward the oxygen activation by tran-
sition metal complexes, a toluene solution of 4 was exposed
to dioxygen at room temperature but no color change was
observed even after cooling the solution at ꢁ50 °C. Since
complex 4 is five coordinate, coordinatively unsaturated,
it was thought that it could be oxidized by suitable oxidiz-
ing agents. To our surprise, during the oxidation of 4 with
H2O2, in the presence of 1 equiv. of free 3,5-diisopropylpy-
razole, we got the monooxygenated intramolecular hydro-
gen bonded cobalt(II) complex 5 (Scheme 1). In complex 5,
one of the six methine groups in the isopropyl substituents
of TpiPr2 distal to the metal center is oxidized in place of
metal and resulted in the six coordinated intramolecular
hydrogen bonded fluorobenzoate complex. As shown in
Fig. 3, the coordination behavior of fluorobenzoate chan-
ged from bidentate to monodentate. The Co1–O1 bond dis-
˚
Bond lengths (A)
Co(1)–N(2)
Co(1)–N(42)
Co(1)–O(60)
2.160(12)
2.088(12)
2.105(10)
Co(1)–N(22)
Co(1)–N(70)
Co(1)–O(61)
2.051(12)
2.211(13)
2.174(11)
Bond angles (°)
N(2)–Co(1)–N(70)
N(22)–Co(1)–O(60)
N(42)–Co(1)–O(60)
O(60)–Co(1)–N(2)
O(61)–Co(1)–N(70)
173.35(5)
170.74(4)
97.98(4)
98.04(4)
82.77(5)
N(22)–Co(1)–N(70)
N(22)–Co(1)–O(61)
N(42)–Co(1)–O(61)
O(60)–Co(1)–N(70)
89.59(5)
108.83(4)
158.69(4)
88.55(4)
[TpiPr2Co(F-OBz)] (4)
˚
Bond lengths (A)
Co(1)–N(7)
Co(1)–N(11)
Co(1)–O(4)
2.039(8)
2.042(9)
2.184(7)
Co(1)–N(9)
Co(1)–O(3)
2.103(9)
2.011(7)
Bond angles (°)
N(7)–Co(1)–N(9)
N(11)–Co(1)–N(9)
N(9)–Co(1)–O(4)
O(3)–Co(1)–N(7)
O(3)–Co(1)–N(9)
91.0(4)
90.0(4)
154.4(4)
114.4(4)
97.7(5)
N(11)–Co(1)–N(7)
N(7)–Co(1)–O(4)
N(11)–Co(1)–O(4)
O(3)–Co(1)–N(11)
O(3)–Co(1)–O(4)
90.6(4)
111.1(4)
102.1(4)
153.5(4)
62.0(3)
½CoðF-OBzÞfHBð3-OCMe2-5-PripzÞð3; 5-Pr2i pzÞ2gð3; 5-Pri2pzHÞꢀ
(5 Æ CH3CN Æ CH2Cl2)
˚
Bond lengths (A)
Co(1)–N(1)
Co(1)–N(5)
Co(1)–O(1)
1.984(9)
1.851(8)
1.951(8)
Co(1)–N(3)
Co(1)–N(7)
2.010(9)
1.985(10)
Bond angles (°)
N(1)–Co(1)–N(3)
N(5)–Co(1)–N(1)
N(5)–Co(1)–N(7)
N(5)–Co(1)–O(1)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(7)
O(3)–Co(1)–N(3)
O(3)–Co(1)–O(1)
87.9(4)
84.0(4)
90.4(4)
171.6(3)
89.4(4)
96.4(4)
176.9(3)
93.7(3)
N(1)–Co(1)–N(7)
N(5)–Co(1)–N(3)
N(7)–Co(1)–N(3)
N(5)–Co(1)–O(3)
O(1)–Co(1)–N(3)
O(3)–Co(1)–N(1)
O(3)–Co(1)–N(7)
173.4(5)
96.2(5)
89.2(4)
81.7(4)
88.7(4)
94.1(4)
88.5(4)
˚
tance of the coordinated oxygen is 1.951(8) A, whereas the
½ð3; 5-Pri2pzHÞ2Co2ðl-3; 5-Pr2i pzÞ2ðNO2-OBzÞ2ꢀ (6)
˚
Bond lengths (A)
˚
Co1–O2 bond distance is 3.474 A. The long distance of
Co1–O2 bond indicated that this oxygen was not involved
in coordination with metal center. The cobalt–nitrogen
bond distances of coordinated TpiPr2 are in the range of
Co(1)–N(2)
Co(1)–N(5)
N(4)–N(5)
O(4)–N(1)
2.004(17)
1.987(16)
1.375(2)
1.220(4)
Co(1)–N(4)
Co(1)–O(1)
O(3)–N(1)
1.986(16)
1.943(15)
1.204(4)
˚
1.851(8)–2.010(9) A, which are shorter than metal–nitrogen
bond distances in 4. The nonbonded oxygen atom of fluor-
obenzoate formed intramolecular hydrogen bond with NH
group of 3; 5-Pri2pzH. The existence of hydrogen bonding
in complex 5 has been established on the basis of the
absence of mN–H band of free pyrazole (which appear at
3180 cmꢁ1 in free 3; 5-Pri2pzH) as well as the location of
hydrogen atom in X-ray structure with bond distance of
Bond angles (°)
N(4)–Co(1)–N(2)
N(5)–Co(1)–N(2)
O(1)–Co(1)–N(4)
107.60(7)
106.84(7)
109.24(7)
N(4)–Co(1)–N(5)
O(1)–Co(1)–N(2)
O(1)–Co(1)–N(5)
111.67(7)
114.68(7)
106.86(7)
reported in the literature for other metal complexes
[27,28]. The formation of monooxygenated cobalt(II) com-
plex is not unusual in the oxidation chemistry of TpiPr2
coordinated cobalt(II) compound as reported by Hikichi
et al. [12]. They have reported the extensive studies on ali-
phatic C–H oxygenation by Co(II)-peroxo species [12] and
˚
˚
N8–Ha, 0.861 A; O2-Ha, 1.921 A. The bond distance
between nitrogen of free pyrazole and the nonbonded oxy-
˚
gen atom of fluorobenzoate is 2.752 A (N8–O2). These dis-
tances are well within the range of hydrogen bonding as

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U.P. Singh et al. / Inorganica Chimica Acta 360 (2007) 3226–3232
Fig. 4. Thermal ellipsoid view of ½ð3; 5-iPri pzHÞ Co2ðl-3; 5-Pri pzÞ
2
2
2
2
Fig. 3. Thermal ellipsoid view of ½CoðF-OBzÞfHBð3-OCMe2-5-PripzÞ
ð3; 5-Pri2pzÞ2gð3; 5-Pri2pzHÞꢀ (5 Æ CH3CN Æ CH2Cl2) drawn at the 30%
probability level. Hydrogen atoms and solvent molecules have been
omitted for clarity.
ðNO2-OBzÞ2ꢀ (6) drawn at the 30% probability level. Hydrogen atoms
have been omitted for clarity.
coordinated pyrazole are slightly longer (Co1–N2,
˚
demonstrated the formation of binuclear mono-
oxygenated, di-oxygenated, and mononuclear fully oxygen-
ated Co(II) complex. In all these complexes, the metal
center is five coordinated and no hydrogen bonding exits.
In present mono-oxygenated Co(II) complex 5, the metal
center is six coordinate and intramolecular hydrogen
bonding exits. To our knowledge this is first example of
monooxygenated Co(II) six coordinate mononuclear
complex with intramolecular hydrogen bonding.
2.004(17) A) than the bridging ones. As shown in X-ray
structure, both nitrobenzoate groups are coordinated
unidentately with cobalt metal ions. The uncoordinated
oxygen atom of both nitrobenzoates form hydrogen bond
with the hydrogen atom (NH fragment) of 3; 5-Pri2pzH.
The nitrogen–oxygen bond distance of 2.771 A (N3–O2),
(N3–Ha, 0.860 A; O2–Ha, 2.002 A) clearly indicates the
˚
˚
˚
presence of hydrogen bonding in complex 6. The Co–Co
separation in this complex is 3.597 A, which is longer than
˚
In other set of experiment, when a mixture of cobalt
(II) nitrate, 3,5-diisopropylpyrazole, and sodium nitro-
benzoate was allowed to react for 6 h, a binuclear
compound of the type ½ð3; 5-Pri2pzHÞ Co2ðl-3; 5-Pr2i pzÞ
the Co–Co separation in binuclear cobalt complexes
having TpiPr2 ligand [12,26,29] but shorter than the
recently reported similar binuclear cobalt(II) complex
˚
with Co–Co distance of 3.629 A [30]. The cobalt atoms
2
2
ðNO2-OBzÞ2ꢀ (6) was isolated. The suitable single crystal
for X-ray data collection was obtained from acetonitrile
solution at ꢁ20 °C. A thermal ellipsoidal view of complex
6, representing a novel di(l-pyrazolato)dicobalt core, is
depicted in Fig. 4. The selected bond lengths and bond
angles are summarized in Table 2. Cobalt–nitrogen bond
distances of both bridging pyrazolato are same (Table 2)
whereas the cobalt–nitrogen bond distance for terminally
are bridged via the N atoms of two bridging pyrazolate
ligands produced through deprotonation of the
3; 5-Pri2pzH molecules. Each cobalt is coordinated by the
one nitrogen atom of the terminal pyrazole molecule
and oxygen atom of the terminal nitrobenzoate anion.
Compound 6 appears to be an unusual binuclear complex
in which each cobalt(II) atom is electron deficient (15
electrons) and is in a distorted tetrahedral environment.
Scheme 1. Stick figure diagram for oxidation reaction.

U.P. Singh et al. / Inorganica Chimica Acta 360 (2007) 3226–3232
[2] M. Kobayashi, S. Shimizu, Eur. J. Biochem. 261 (1999) 1.
3231
4. Conclusion
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Natl. Acad. Sci. USA 95 (1998) 12153.
[TpiPr2Co(OBz)(CH3CN)] complex 3 was obtained by
the reaction of [TpiPr2Co(NO3)] (2) and sodium benzoate.
The X-ray studies demonstrated the presence of six coordi-
nated metal center in complex 3 due to the coordination of
acetonitrile at vacant 6th position of the cobalt center. On
the other hand, the reaction of 2 with sodium fluoroben-
zoate gave five coordinate complex 4 which is coordin-
atively unsaturate with three nitrogen atoms from TpiPr2
ligand and two oxygen atoms from fluorobenzoate. Com-
plex 4 was oxidized with 30% H2O2 in the presence of free
pyrazole. The oxidized product 5 has a unique structure. In
complex 5, the cobalt is still in two oxidation state, one of
the six methine carbon atoms in the isopropyl groups of
TpiPr2 ligand is oxygenated and form bond with cobalt cen-
ter. The coordination behavior of fluorobenzoate is chan-
ged from bidentate to monodentate. The one oxygen
atom of monodentately coordinated fluorobenzote forms
intramolecluar hydrogen bond with NH fragment of the
coordinated 3; 5-Pri2pzH giving six coordination number
to the cobalt center. The formation of pyrazolato bridged
binuclear cobalt(II) complex 6 indicated that the coordi-
nated 3,5-diisopropylpyrazole molecule can behave in both
protonated as well as deprotonated form in same com-
pound. In complex 6, the deprotonated 3; 5-Pri2pzH bridges
between two cobalt centers and protonated 3; 5-Pri2pzH
form intramolecular hydrogen bonds. This may help in
understanding the mechanism of the action of metallo-
enzymes, where the imidazole fragment of the terminal
histidine molecule is present in their active site [31] and
the binuclear metal carboxylate complexes serve as models
[32].
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supplementary crystallographic data for this paper. The
data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge
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mentation Center, Indian Institute of Technology Roorkee
for X-ray measurements.
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