Synthesis, characterization, CV, and X-ray structures of aryl cobaloximes

Article abstract of DOI:10.1016/j.jorganchem.2010.06.012

Aryl cobaloximes, ArCo(dioxime)₂Py (1-9) (Ar = phenyl, 1-naphthyl, 2-naphthyl; dioxime = dmgH, dpgH, gH) have been synthesized and characterized primarily by NMR and elemental analyses and molecular structures of four complexes have also been determined crystallographically. The electronic and steric effects of the aryl ring size and the equatorial dioxime affect the NMR chemical shifts and the X-ray structural parameters. The cyclic voltammetry study shows that the aryl cobaloximes are reduced electrochemically more easily as compared to the corresponding alkyl complexes. These complexes have a robust Co-C bond and are very stable in solution and show no sign of decomposition even on irradiation under oxygen atmosphere-an unusual property of any organocobaloxime.

Full text of DOI:10.1016/j.jorganchem.2010.06.012

Journal of Organometallic Chemistry 695 (2010) 2233e2239
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, CV, and X-ray structures of aryl cobaloximes
*
Kamlesh Kumar, B.D. Gupta
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Aryl cobaloximes, ArCo(dioxime)2Py (1e9) (Ar ¼ phenyl, 1-naphthyl, 2-naphthyl; dioxime ¼ dmgH, dpgH,
gH) have been synthesized and characterized primarily by NMR and elemental analyses and molecular
structures of four complexes have also been determined crystallographically. The electronic and steric
effects of the aryl ring size and the equatorial dioxime affect the NMR chemical shifts and the X-ray
structural parameters. The cyclic voltammetry study shows that the aryl cobaloximes are reduced
electrochemically more easily as compared to the corresponding alkyl complexes. These complexes have
a robust CoeC bond and are very stable in solution and show no sign of decomposition even on irra-
diation under oxygen atmosphere e an unusual property of any organocobaloxime.
Received 29 March 2010
Received in revised form
3 June 2010
Accepted 11 June 2010
Available online 19 June 2010
Keywords:
Dioximes
Aryl cobaloximes
Structure elucidation
Cyclic voltammetry
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
structural studies. With an objective to study all these aspects, we
have synthesized and characterized nine aryl cobaloximes, ArCo
Organocobaloximes1 have extensively been used as structural
and functional mimic for vitamin B12 [1e4] ever since these were
first introduced by Schrauzer four decades ago as model of vitamin
B12 [5e7]. These represent a unique class of compounds in organ-
ometallic and bioinorganic chemistry. Although they do not accu-
rately reproduce all chemical properties of B12, the wealth of
available information on their reactivity in comparison with B12
derivatives renders cobaloximes as good starting point for under-
standing the properties of cobalamins [8]. Besides, these have rich
coordination chemistry, application in organic synthesis [9e16]
and catalysis [17,18], and their stability and redox properties are
of particular interest.
(dioxime)2Py (1e9) [Ar ¼ phenyl, 1-naphthyl, 2-naphthyl; dioxi-
me ¼ gH, dmgH, dpgH] (Chart 1). All the complexes are new except
1. The complexes 3, 4, 6 and 9 have also been structurally charac-
terized by X-ray. Since the steric and electronic nature of the axial
and equatorial ligands affect the properties of cobaloximes [37e41],
the dioxime and aryl ring size have been varied to study their effect
on the structure and on the NMR chemical shifts.
2. Experimental section
2.1. Materials and physical measurements
Alkyl cobaloximes with dmgH as equatorial dioxime have been
studied the most and the examples with other dioximes are rather
few and many have recently been reported [19e26]. Surprisingly
the examples of aryl cobaloximes, ArCo(dmgH)2B, are limited and
there is no report with dioxime other than dmgH [6,27e36]. The
literature also lacks information on their general properties like
stability in solution, sensitivity toward oxygen, redox behavior and
Cobalt chloride hexahydrate, bromobenzene, and dimethyl-
glyoxime (SD Fine, India), glyoxime (caution! it is highly flammable
and explosive when dry; from Alfa Aesar, Lancaster), diphenyl-
glyoxime, 1-bromonaphthalene, 2-bromonaphthalene (all from
Aldrich) were used as received without further purifications.
Chlorocobaloximes, ClCo(dioxime)2Py (dioxime ¼ gH, dmgH, dpgH)
were prepared according to the literature procedures [6,21,23]. The
solvents were purified rigorously by standard procedures prior to
their use [42] and THF was dried over sodium/benzophenone.
Cyclic voltammetry measurements were carried out using a BAS
Epsilon electrochemical work station with a glassy carbon working
electrode, Ag/AgCl reference electrode (3 M KCl) and a platinum-
wire counter electrode. All measurements were performed in 0.1 M
nBu4NPF6 in dichloromethane (dry) at a concentration of 1 mM of
each complex. 1H and 13C spectra were recorded on a JEOL ECX-500
* Corresponding author. Present address: Department of Chemistry, IIT Ropar-
140001, India. Tel: þ91 512 2597046; fax: þ91 512 2597436.
E-mail address: bdg@iitk.ac.in (B.D. Gupta).
1
Cobaloximes have the general formula RCo(L)2B, where R is an organic group
s-bonded to cobalt. B is an axial base trans to the organic group, and L is a mono-
anionic dioxime ligand (e.g. glyoxime (gH), dimethylglyoxime (dmgH), 1,2-cyclo-
hexanedione dioxime (chgH), diphenylglyoxime (dpgH), dimesitylglyoxime
(dmestgH), and dithiophenylglyoxime (dSPhgH).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.06.012

2234
K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
d
ppm): 151.82 (C]N, Cq), 149.90 (Pya), 137.75 (Pyg), 125.37 (Pyb),
Ar
134.29, 129.08, 128.29, 125.55, 124.22, 123.92, 123.48, 12.24. Anal.
Calcd for C23H26CoN5O4: C, 55.76; H, 5.29; N, 14.14. Found: C, 55.57;
H, 5.23; N, 14.07.
R '
R '
R '
R '
O
N
H
O
N
C o
N
O
2.2.4. Synthesis of 2-naphthylCo(dmgH)2Py (3)
N
O
This complex was prepared following the above procedure for 1
except that 2-naphthylMgBr was used in place of PhMgBr. Yield:
H
0.371 g (75%). 1H NMR (500 MHz, CDCl3,
d
ppm): Pya ¼ 8.79 (2H, d,
N
J ¼ 4.9 Hz), Pyb ¼ 7.37 (2H, t, J ¼ 6.7 Hz), Pyg ¼ 7.76 (1H, t, J ¼ 7.4 Hz),
7.79 (1H, s), 7.71 (1H, d, J ¼ 7.9 Hz), 7.67 (1H, d, J ¼ 7.9 Hz), 7.62e7.60
(1H, m), 7.41 (1H, d, J ¼ 8.5), 7.32e7.25 (2H, m), dmgH(Me) ¼ 2.00
(12H, s), OeH/O ¼ 18.57 (s). 13C NMR (125 MHz, CDCl3,
d
ppm):
1
6
. 2-naphthylCo(dpgH)2Py
7. PhenylCo(gH)2Py
8. 1-naphthylCo(gH)2Py
9. 2-naphthylCo(gH)2Py
. PhenylCo(dmgH)2Py
2. 1-naphthylCo(dmgH)2Py
3. 2-naphthylCo(dmgH)2Py
4. PhenylCo(dpgH)2Py
150.54 (C]N, Cq), 150.35 (Pya), 137.88 (Pyg), 125.46 (Pyb), 133.39,
133.25, 132.90, 131.44, 127.46, 127.16, 124.83, 124.54, 123.98, 12.33.
Anal. Calcd for C23H26CoN5O4: C, 55.76; H, 5.29; N, 14.14. Found: C,
55.83; H, 5.27; N, 14.21.
5
. 1-naphthylCo(dpgH)2Py
Chart 1. Aryl cobaloximes [ArCo(dioxime)2Py].
2.2.5. Synthesis of phenylCo(dpgH)2Py (4)
This complex was prepared following the general procedure
using PhMgBr and ClCo(dpgH)2Py and the crude product was
purified on a silica gel column using dichloromethane. Yield:
FT NMR Spectrometer (500 MHz for 1H and 125 MHz for 13C) in
CDCl3 with TMS as internal standard and the chemical shift values
are reported in parts per million. Elemental analyses for C, H and N
were performed on CE-440 Elemental Analyzer.
0.556 g (80%). 1H NMR (500 MHz, CDCl3,
d
ppm): Pya ¼ 9.08 (2H, d,
J ¼ 4.8 Hz), Pyb ¼ 7.49 (2H, t, J ¼ 7.0 Hz), Pyg ¼ 7.88 (1H, t, J ¼ 7.6 Hz),
7.65 (2H, d, J ¼ 7.3 Hz), 7.21e7.14 (12H, m), 7.05 (2H, t, J ¼ 7.3 Hz),
6.98 (1H, t, J ¼ 7.1 Hz), 6.93 (8H, d, J ¼ 6.7 Hz), OeH/O ¼ 18.88 (s).
2.2. Syntheses
13C NMR (125 MHz, CDCl3,
d ppm): 151.19(C]N, Cq), 150.42 (Pya),
2.2.1. General procedure for the preparation of ArCo(dioxime)2Py
In a typical procedure ArMgBr, prepared from ArBr (6.0 mmol)
and Mg (6.6 mmol) in 10 mL dry THF, was added drop wise to
a magnetically stirred cold slurry of ClCo(dioxime)2Py (1.0 mmol) in
10 mL dry THF under nitrogen. The bluish-green reaction mixture
was stirred for 2 h and then heated to reflux for ½ h.
The reaction was quenched with 10 mL of cold 10% aqueous HCl
solution. The yellow-orange solid product was filtered. The addition
of petroleum ether to the filtrate afforded some more solid product
which was combined with the initial crude solid product. In some
cases where the solid product did not separate out after the addi-
tion of 10% aqueous HCl solution, the addition of petroleum ether
resulted in the formation of crude yellow-orange solid product. The
crude solid product was washed with ethanol, petroleum ether and
water and dried over P2O5 by applying vacuum. The crude product
was further purified by column chromatography on silica gel.
Yield ¼ 65e80%.
138.35 (Pyg), 125.81 (Pyb), 134.55, 129.85, 129.59, 129.16, 127.89,
127.28, 124.13. Anal. Calcd for C39H32CoN5O4: C, 67.53; H, 4.65; N,
10.10. Found: C, 67.38; H, 4.59; N, 10.21.
2.2.6. Synthesis of 1-naphthylCo(dpgH)2Py (5)
This complex was prepared following the above procedure
adopted for 4 except that 1-naphthylMgBr was used in place of
PhMgBr. Yield: 0.566 g (76%). 1H NMR (500 MHz, CDCl3,
d ppm):
Pya ¼ 9.14 (2H, d, J ¼ 4.9 Hz), Pyb ¼ 7.51 (2H, t, J ¼ 7.0 Hz), Pyg ¼ 7.88
(1H, t, J ¼ 7.6 Hz), 9.04 (1H, d, J ¼ 9.1 Hz), 7.93 (1H, d, J ¼ 7.9 Hz),
7.84e7.83 (1H, m), 7.70 (1H, d, J ¼ 6.7 Hz), 7.54 (1H, t, J ¼ 7.3),
7.48e7.41 (2H, m), 7.19e7.15 (4H, m), 7.12e7.09 (8H, m), 6.82 (8H, d,
J ¼ 7.3), OeH/O ¼ 18.77 (s). 13C NMR (125 MHz, CDCl3,
d ppm):
153.33 (C]N, Cq), 150.07 (Pya), 139.99 (Pyg), 125.90 (Pyb), 138.27,
134.72,134.08,129.84,129.54,129.19,128.82,128.44,128.04, 127.96,
127.86,125.83,124.51,124.05,124.00. Anal. Calcd for C43H34CoN5O4:
C, 69.45; H, 4.61; N, 9.42. Found: C, 69.16; H, 4.54; N, 9.31.
2.2.2. Synthesis of phenylCo(dmgH)2Py (1)
2.2.7. Synthesis of 2-naphthylCo(dpgH)2Py (6)
This complex was prepared following the general procedure
using PhMgBr and ClCo(dmgH)2Py and the crude product was
purified on a silica gel column using 20% ethyl acetate/chloroform.
This complex was prepared following the above procedure
adopted for 4 except that 2-naphthylMgBr was used in place of
PhMgBr. Yield: 0.571 g (77%). 1H NMR (500 MHz, CDCl3,
d ppm):
Yield: 0.351 g (79%). 1H NMR (500 MHz, CDCl3,
d
ppm): Pya ¼ 8.75
Pya ¼ 9.13 (2H, d, J ¼ 4.9 Hz), Pyb ¼ 7.53 (2H, t, J ¼ 7.0 Hz),
Pyg ¼ 7.91 (1H, t, J ¼ 8.5 Hz), 8.09 (1H, s), 7.80e7.75 (2H, m), 7.55
(1H, s), 7.36e7.33 (3H, m), 7.20e7.17 (4H, m), 7.14e7.12 (8H, m),
6.90 (8H, d, J ¼ 6.7 Hz), OeH/O ¼ 19.00 (s). 13C NMR (125 MHz,
(2H, d, J ¼ 4.8 Hz), Pyb ¼ 7.36 (2H, t, J ¼ 7.0 Hz), Pyg ¼ 7.75 (1H, t,
J ¼ 7.6 Hz), 7.38 (2H, d, J ¼ 6.7 Hz), 6.95e6.89 (3H, m), dmgH(Me) ¼
2.03 (12H, s), OeH/O ¼ 18.45 (s). 13C NMR (125 MHz, CDCl3,
d
ppm): 150.34 (C]N, Cq), 150.15 (Pya), 137.74 (Pyg), 125.31 (Pyb),
CDCl3, d ppm): 152.06 (C]N, Cq), 150.46 (Pya), 138.42 (Pyg),
134.85, 126.59, 123.70, 12.24. Anal. Calcd for C19H24CoN5O4: C,
51.24; H, 5.43; N, 15.73. Found: C, 51.42; H, 5.41; N, 15.52.
125.87 (Pyb), 133.52, 132.98, 132.94, 131.79, 129.78, 129.59, 129.19,
128.05, 127.88, 127.72, 127.19, 125.60, 124.54, 124.13. Anal. Calcd
for C43H34CoN5O4: C, 69.45; H, 4.61; N, 9.42. Found: C, 69.36; H,
4.58; N, 9.42.
2.2.3. Synthesis of 1-naphthylCo(dmgH)2Py (2)
This complex was prepared following the above procedure for 1
except that 1-naphthylMgBr was used in place of PhMgBr. Yield:
2.2.8. Synthesis of phenylCo(gH)2Py (7)
0.352 g (71%). 1H NMR (500 MHz, CDCl3,
d
ppm): Pya ¼ 8.81 (2H, d,
This complex was prepared following the above general
procedure using PhMgBr and ClCo(gH)2Py and crude product was
purified on a silica gel column using 30% ethyl acetate/chloroform
J ¼ 4.8 Hz), Pyb ¼ 7.40 (2H, t, J ¼ 9.9 Hz), Pyg ¼ 7.74 (1H, t, J ¼ 7.6 Hz),
8.77 (1H, d, J ¼ 9.1 Hz), 7.78 (1H, d, J ¼ 6.7 Hz), 7.59 (1H, d,
J ¼ 6.7 Hz), 7.38e7.30 (3H, m), 7.08 (1H, t, J ¼ 7.6 Hz), dmgH(Me) ¼
1.99 (12H, s), OeH/O ¼ 18.27 (s). 13C NMR (125 MHz, CDCl3,
mixture. Yield: 0.316 g (71%). 1H NMR (500 MHz, CDCl3,
d
ppm):
Pya ¼ 8.75 (2H, d, J ¼ 4.9 Hz), Pyb ¼ 7.42 (2H, t, J ¼ 7.0 Hz), Pyg ¼ 7.82

K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
2235
(1H, t, J ¼ 7.6 Hz), 7.39 (2H, d, J ¼ 6.7 Hz), 7.33 (4H, s), 7.00e6.96 (3H,
3. Results and discussion
m), OeH/O ¼ 17.85 (s). 13C NMR (125 MHz, CDCl3,
d ppm): 149.95
(Pya), 139.13 (C]N, Cq), 138.37 (Pyg), 125.75 (Pyb), 134.37, 127.29,
124.67. Anal. Calcd for C15H16CoN5O4: C, 46.28; H, 4.14; N, 17.99.
Found: C, 46.35; H, 4.03; N, 17.83.
3.1. Syntheses of the aryl cobaloximes
Organocobaloximes, in general, can be prepared from Co(I)
[6,20,22e26,49e55], Co(II) [56e61] or Co(III) [6,27e30] reagents.
The most common method used is the reaction of Co(I), super
nucleophile, with a variety of substrates via nucleophilic substitu-
tion on alkyl, acyl, vinyl, acetylenic halides. However, this method
works only with the activated aryl halides (arene ring with electron
withdrawing groups) and gives poor yield of aryl cobaloxime
[30,31,36,62]. Since several organocobalt complexes with tetra-
dentate ligands like Salen, BAE have been prepared by the reaction
of cobalt(II) complexes with organic hydrazines in the presence of
molecular oxygen [63], Brown et al., while using this method got
disappointing yields in the synthesis of aryl cobaloximes [30]. The
latter have traditionally been prepared in high yield by the reaction
of aryl magnesium halides with Co(III) complex, ClCo(dmgH)2B
[6,27e30]. We have used this method in the present study. The
method, unlike the alkylation of Co(I), though requires an incon-
venient reverse addition of organomagnesium salt due to the
extremely poor solubility of chlorocobaloxime in ethereal solution,
but gives good yield of the product (65e80%).
2.2.9. Synthesis of 1-naphthylCo(gH)2Py (8)
This complex was prepared following the above procedure
used for 7 except that 1-naphthylMgBr was used in place of
PhMgBr. Yield: 0.362 g (73%). 1H NMR (500 MHz, CDCl3,
d ppm):
Pya ¼ 8.84 (2H, d, J ¼ 5.5 Hz), Pyg ¼ 7.82 (1H, t, J ¼ 7.6 Hz), 8.68
(1H, d, J ¼ 8.5 Hz), 7.75 (1H, d, J ¼ 8.0 Hz), 7.63 (1H, d, J ¼ 8.0 Hz),
7.45 (4H, q, J ¼ 7.3 Hz, 2Hs of Pyb and 2Hs of 1-naphthyl), 7.37
(1H, s), 7.34 (4H, s), 7.11 (1H, t, J ¼ 7.6 Hz), OeH/O ¼ 17.49 (s).
13C NMR (125 MHz, CDCl3,
d ppm): 149.57 (Pya), 140.51 (C]N,
Cq), 138.33 (Pyg), 125.82 (Pyb), 139.61, 134.55, 133.74, 128.61,
127.94, 127.85, 126.43, 124.62, 124.21, 123.72. Anal. Calcd for
C19H18CoN5O4: C, 51.95; H, 4.13; N, 15.94. Found: C, 51.73; H,
4.08; N, 15.67.
2.2.10. Synthesis of 2-naphthylCo(gH)2Py (9)
This complex was prepared following the above procedure
used for 7 except that 2-naphthylMgBr was used in place of
PhMgBr. Yield: 0.323 g (65%). 1H NMR (500 MHz, CDCl3,
d ppm):
Pya ¼ 8.79 (2H, d, J ¼ 4.8 Hz), Pyb ¼ 7.51 (2H, t, J ¼ 7.6 Hz),
Pyg ¼ 7.94 (1H, t, J ¼ 8.5 Hz), 8.16 (1H, s), 7.88 (1H, d, J ¼ 7.9 Hz),
7.00 (2H, t, J ¼ 7.0 Hz), 7.48e7.44 (3H, m), 7.33 (4H, s). 13C NMR
3.1.1. Solubility and stability
The dmgH and dpgH complexes are soluble in common organic
solvents (CH2Cl2, CHCl3) but sparingly soluble in acetone, methanol,
acetonitrile while the gH complexes are sparingly soluble in all
these solvents. Complexes (1e9) are air and moisture-stable solids
and do not show any signs of decomposition in solution over
a period of 24 h under diffused light and in refluxing CHCl3/CH3OH
(1:2) mixture for 5 h in air. Also, these complexes do not show any
affinity toward molecular oxygen even under irradiation in
dichloromethane.
(125 MHz, CDCl3,
d ppm): 150.10 (Pya), 139.39 (C]N, Cq), 138.49
(Pyg), 125.93 (Pyb), 133.82, 132.75, 128.61, 128.31, 127.77, 127.36,
126.45, 126.20, 126.11, 125.82, 125.72, 125.08. Anal. Calcd for
C19H18CoN5O4: C, 51.95; H, 4.13; N, 15.94. Found: C, 52.09; H,
4.03; N, 15.71.
2.3. X-ray crystal structure determination and refinements
3.2. 1H and 13C NMR spectral studies
Single crystals suitable for X-ray crystallographic analyses
were obtained by the slow evaporation of solvent from the
solution of complexes 3, 4, 6 and 9 (CHCl3/CH3OH or CH2Cl2/
1H and 13C NMR data for all the complexes are summarized in
Experimental section. The poor solubility of gH complexes (7e9) in
common organic solvents hampered the spectroscopic study of
these complexes. The 1H NMR spectra of these complexes are easily
assigned on the basis of the chemical shifts. The signals are
assigned according to their relative intensities and are consistent
with the related alkyl and benzyl cobaloximes with dmgH, dpgH
and gH. 13C resonance for Pya, Pyb, Pyg and C]N are observed and
assigned on the basis of chemical shifts. 13C Pya and C]N are very
difficult to assign in certain cases as they have almost identical
chemical shift. In such cases the assignment is confirmed by DEPT
spectra.
CH3OH). Single crystal X-ray data were collected using graphite-
ꢀ
monochromated Mo K
a
radiation (
l
¼ 0.71073 A) on “Bruker
SMART APEX CCD diffractometer” at 293 K for 3 and 100 K for
others. The linear absorption coefficients, scattering factors for
the atoms and the anomalous dispersion corrections were taken
from the International Tables for X-ray Crystallography [43]. The
program SMART [44] was used for collecting frames of data,
indexing reflections, and determining lattice parameters. The
data integration and reduction were processed with SAINT [44]
software. An empirical absorption correction was applied to the
collected reflections with SADABS [45] using XPREP [46]. All the
structures were solved by the direct method using the program
SHELXS-97 [47] except 9 (solved by using the program DIRDIF-
2008) [48] and were refined on F2 by the full-matrix least-
squares technique using the SHELXL-97 [47] program package.
All non-hydrogen atoms were refined with anisotropic
displacement parameters in all the structure. The hydrogen
atom positions or thermal parameters were not refined but
were included in the structure factor calculations. The axial
pyridine and phenyl ligands in complex 4 were found to be
orientationally disordered and the occupancies were fixed at 50/
50 percentage for axial atoms (N5/C29) attached to the cobalt
atom. The naphthalene ring in complex 9 was found to be
disordered and was modeled satisfactorily. The pertinent crystal
data and refinement parameters for compounds 3, 4, 6 and 9 are
compiled in Table 1.
1H NMR spectral characteristics of aryl cobaloximes are quite
different from the reported alkyl and benzyl cobaloximes (Table
S1). The key feature is the Pya resonance which has surprisingly
shifted downfield (0.11e0.27 ppm) in dmgH and gH complexes as
compared to the free Pya resonance. This is unusual since Pya
resonance does not shift on coordination to cobalt in alkyl and
benzyl cobaloximes (Table S2). The downfield shift is even larger in
the dpgH complexes (4e6) in comparison to the alkyl derivatives.
Also, the Pya resonance varies with the aryl ring size and follows
the order 1-naphthyl > 2-naphthyl > phenyl. Since the ring size of
1-naphthyl and 2-naphthyl is similar, the more downfield shift in
1-naphthyl complexes may be due to some electronic reasons. This
is in contrast to the alkyl series where it remains almost constant
for C1eC5 carbon chain (Table S2). This unexpected high
deshielding of Pya protons can occur due to an increased anisotropy
of cobalt on binding with an aryl moiety and/or due to deshielding

2236
K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
Table 1
Crystal data and structure refinement details for compounds 3, 4, 6 and 9.
Parameters
3
4
6
9
Empirical formula
Formula weight
Diffractometer
Temp (K)
C23H26CoN5O4
495.42
Bruker Apex CCD
293(2)
C39H32CoN5O4
693.63
Bruker Apex CCD
100(2)
C43H34CoN5O4
743.69
Bruker Apex CCD
100(2)
C19H18CoN5O4
439.31
Bruker Apex CCD
100(2)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
C2/c
Monoclinic
C2/c
Unit cell dimension
ꢀ
a (A)
9.034(1)
15.782(2)
11.123(1)
21.200(3)
90.000
108.020(2)
90.00
3539.2(8)
4
1.302
22.371(5)
18.677(5)
20.827(5)
90.000(5)
116.051(5)
90.000(5)
7818(3)
8.438(1)
20.028(4)
10.557(1)
90.000
98.023(3)
90.000
1766.8(5)
4
1.652
ꢀ
b (A)
16.246(2)
15.531(2)
90.000
105.219(2)
90.000
2199.6(5)
4
1.496
ꢀ
c (A)
a
b
g
(deg)
(deg)
(deg)
3
ꢀ
V (A )
Z
r
8
(calc), g/cm3
(Mo K
) (mmꢀ1
1.311
0.491
m
a
)
0.822
0.532
1.011
F (000)
1032
1440
3200
904
Crystal size (mm3)
Index ranges
0.32 ꢂ 0.20 ꢂ 0.16
ꢀ12 ꢃ h ꢃ 12
ꢀ21 ꢃ k ꢃ 13
ꢀ19 ꢃ l ꢃ 20
14,210
0.31 ꢂ 0.25 ꢂ 0.21
ꢀ20 ꢃ h ꢃ 19
ꢀ14 ꢃ k ꢃ 11
ꢀ27 ꢃ l ꢃ 28
22,625
0.40 ꢂ 0.35 ꢂ 0.30
ꢀ29 ꢃ h ꢃ 20
ꢀ20 ꢃ k ꢃ 24
ꢀ24 ꢃ l ꢃ 27
25,323
0.42 ꢂ 0.34 ꢂ 0.21
ꢀ10 ꢃ h ꢃ 11
ꢀ26 ꢃ k ꢃ 16
ꢀ14 ꢃ l ꢃ 13
5715
No. of rflns collected
No. of indep rflns
GOOF on F2
5435
1.13
8752
1.142
9707
1.099
2173
1.171
Final R indices (I > 2
s
(I))
R1 ¼ 0.0605
wR2 ¼ 0.1410
R1 ¼ 0.0997
wR2 ¼ 0.2010
5435/0/306
R1 ¼ 0.0577
wR2 ¼ 0.1544
R1 ¼ 0.0832
wR2 ¼ 0.1861
8752/0/443
R1 ¼ 0.0696
wR2 ¼ 0.1726
R1 ¼ 0.1154
wR2 ¼ 0.2154
9707/0/518
R1 ¼ 0.0553
wR2 ¼ 0.1391
R1 ¼ 0.0840
wR2 ¼ 0.1910
2173/0/155
R indices (all data)
Data/restraints/param
by the ring current of dpgH phenyl groups. Pya shifts upfield in
chlorocobaloxime in spite of high cobalt anisotropy. However this
should not to be confused because halocobaloximes work differ-
ently from organocobaloximes as shown recently by 15N NMR
studies [64]. Can the deshielding of Pya resonance be a result of
change in the hybridization as we move from alkyl to aryl? It is
difficult to say but the deshielding of Pya resonance is <0.1 ppm in
CH2]CHCo(dmgH)2Py [65].
bending angle between the two dioxime units (
a
). Positive values of
a
and d indicates bending toward R and displacement toward base
and vice versa. The deviations of the cobalt atoms from mean
equatorial N4 plane are 0.0239(6), ꢀ0.0260(3), 0.0339(6) and
0.0181(6) Å in the compounds 3, 4, 6 and 9 respectively. The
butterfly bending angle in 3 is 4.40ꢁ, while it is ꢀ7.67ꢁ, 5.05ꢁ, and
0.18ꢁ in 4, 6 and 9 respectively.
Only two crystal structures of aryl cobaloximes, 4-pyridylCo
(dmgH)2Bu3P [35] and 4-substituted phenylCo(dmgH)2Py [36], are
known. The CoeC and CoeNpy distances in 3 [1.970(3) and 2.055
The extent of electron density on Co(dioxime)2 can be under-
13
13
stood by comparing Dd
(
C
) [66] values. In general,
d
C
in
C]N
C]N
ꢀ
free dioxime shifts upfield on coordination to cobalt and this
(3) A] are comparable to values in 4-substituted phenylCo
13
upfield shift value [Dd
(
C
)] is much smaller in aryl cobalox-
C]N
(dmgH)2Py.
imes as compared to the corresponding alkyl cobaloximes (Table
S3). Also, the upfield shift in the alkyl complexes is quite similar
The data in Table 2 shows that the expansion of ring size
lengthens the CoeC and CoeNpy bond lengths and also induces
within the same series but this is not so in aryl complexes. The [Dd
change in d and a value from negative to positive in dpgH
13
(
C
)] value is lower in 1-naphthylcobaloxime as compared to
complexes (compare 4 and 6). Keeping the same axial ligand, the
change in equatorial moiety also leads to change in CoeC bond
lengths and it follows the order dpgH > gH > dmgH complex
(compare 3, 6 and 9). In contrast, the CoeC bond length does not
change in the alkyl series, MeCo(dioxime)2Py (Table S4).
C]N
other aryl cobaloximes. This means that the charge density is much
lower in 1-naphthyl complexes. This also supports the finding in
the 1H Pya resonance.
The pyridine ring is almost parallel to the CeC bonds; the dpgH
complexes (4 and 6) have the lowest twist angle (torsion angle, s)
3.3. X-ray crystallographic studies
Molecular structures of the complexes 3, 4, 6 and 9 have been
determined crystallographically. Selected bond lengths and bond
angles are given in Table 2 and the molecular structures with
selected numbering schemes are shown in Figs. 1e4. It can be seen
from Figs. 1e4 that all complexes are similar in structure and their
molecules have distorted octahedral geometry around the central
cobalt atom with four nitrogen atoms of the dioximes (gH, dmgH,
dpgH) in the equatorial plane and, pyridine and phenyl (or
2-naphthyl) are axially coordinated.
Table 2
Selected bond lengths and bond angles.
Parameters
3
4
6
9
ꢀ
CoeC (A)
CoeNpy (A)
CeCoeNpy (deg)
1.970(3)
2.055(3)
178.07(15)
4.40
84.47(25)
0.0239(6)
2.478(4)
2.477(4)
2.023(3)
2.025(3)
179.19(10)
ꢀ7.67
2.057(4)
2.066(3)
179.0(12)
5.05
72.03(20)
0.0339(6)
2.459(3)
2.458(3)
2.014(3)
2.044(5)
176.4(2)
0.18
88.81(9)
0.0181(6)
2.496(4)
ꢀ
a
(deg)
s
(deg)a
78.63(12)
ꢀ0.0260(3)
2.477(2)
2.465(3)
ꢀ
d (A)
ꢀ
O/O (A)
Co(dioxime)2 unit may undergo geometrical deformations,
which is roughly represented by the displacement of the cobalt
atom from mean equatorial N4 plane (d) and by the butterfly
a
MeCo(dmgH)2Py ¼ 89.7ꢁ and for MeCo(dpgH)2Py ¼ 89.8ꢁ.

K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
2237
Fig. 3. Molecular structure of 6. All hydrogen atoms have been omitted for clarity
(hydrogen atoms involved in the intramolecular hydrogen-bonding interactions are
shown).
Fig. 1. Molecular structure of 3. All hydrogen atoms have been omitted for clarity
(hydrogen atoms involved in the intramolecular hydrogen-bonding interactions are
shown).
[67] while the gH complex 9 has the highest. Also the twist angle is
smaller in the aryl cobaloximes as compared to their methyl
derivatives and an increase in ring size decreases the twist angle
(compare 4 and 6). The CeCoeNpy bond angles are, however,
similar to alkyl complexes (Table S4). In all the complexes, the plane
of aryl group is almost perpendicular to the plane of the dioxime
ligand as expected and is nearly coplanar with the pyridine plane.
This co-planarity is due to the steric interaction between axial
groups and the equatorial ligands.
packing diagram of 9 shows 2-D lamellar network involving
ꢀ
intermolecular CeH/O (C9eH9A/O1 ¼2.460(3) A) interactions
as well as
p/p interactions. The CeH/O hydrogen bonds are in
the same plane constituting a hexagon and
p/p stacking occurs
between pyridine and one ring of 2-naphthyl group with a distance
ꢀ
4.003 A between the centroids of stacked rings (Fig. S3).
3.4. Electrochemical study (cyclic voltammetry)
The electrochemical behavior of 1e7 has been studied in order
to know the effect of the aryl groups as well as the dioxime on the
redox potentials of the metal centers. The cyclic voltammograms
3.3.1. Crystal packing
Crystal packing in complexes
CeH/O type intermolecular hydrogen bonding. However, it is
stabilized by CeH/O type intermolecular hydrogen bonding as
3 and 6 is stabilized by
well as by
p
/
p
interactions in 9. The two CeH/O
ꢀ
[C18eH18/O4 ¼ 2.620(3), and C10eH10/O1 ¼ 2.642(3) A] inter-
actions in 3 lead to linear superstructure (Fig. S1). The crystal-
packing diagram of 6 also shows the presence of intermolecular
ꢀ
CeH/O [C41eH41/O2 ¼ 2.433(2) A] hydrogen bonding leading to
the formation of a dimer structural motif (Fig. S2). The crystal-
Fig. 2. Molecular structure of 4. All hydrogen atoms have been omitted for clarity
(hydrogen atoms involved in the intramolecular hydrogen-bonding interactions are
shown).
Fig. 4. Molecular structure of 9. All hydrogen atoms have been omitted for clarity
(hydrogen atoms involved in the intramolecular hydrogen-bonding interactions are
shown).

2238
K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
Table 3
CV data for 1e7 in CH2Cl2 and nBu4NPF6 at 0.1 V/s at 25 ꢁC.
No. Co(III)/Co(II)
Co(IV)/Co(III)
PhCo(dmgH)2Py(1)
a
b
a
b
Epc
(V)
Epc
(V)
ipc
A)
E1/2 (V) E1/2
D
(mV)
Ep
ipc
A)
ipa
A)
ipa/ipc
(
m
(V)
(
m
(
m
1
2
3
4
5
ꢀ1.66 ꢀ2.09 13.89 1.00
0.57
0.56
0.56
0.71
0.75
100
120
185
109
e
13.39 14.13 1.06
14.86 16.56 1.11
33.07 58.49 1.77
16.67 18.15 1.08
ꢀ1.54 ꢀ1.97
ꢀ1.71 ꢀ2.14
8.25 0.99
2.83 0.99
ꢀ1.39 ꢀ1.82 16.47 1.14
ꢀ1.34 ꢀ1.77 41.45 1.18c
irrev
1-naphthylCo(dmgH)2Py(2)
e
12.24
e
6
7
ꢀ1.39 ꢀ1.82 12.69 1.16
0.73
0.81
110
122
5.82 13.02 2.24
12.79 32.68 2.56
ꢀ1.37 ꢀ1.80 11.68 1.24
a
b
c
Vs Ag/AgCl.
Vs Fc/Fcþ Fc/Fcþ (E1/2 ¼ 0.4269 V).
2-naphthylCo(dmgH)2Py(3)
Value refers to Epa
.
indicated by value of ipa/ipc which is greater than one, the oxidized
products (ArCoIV(dioxime)2Py) are stable.
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Complex 5 shows one irreversible oxidative wave corresponding
to Co(IV)/Co(III) at 1.18 V and one irreversible reductive wave cor-
responding to Co(III)/Co(II) at ꢀ1.34 V. The reduction potential in
the aryl cobaloximes, as expected, is less negative as compared to
the alkyl cobaloximes within the same series of dioxime (Table S5).
This shows that the aryl cobaloximes are easily reduced and this
may simply be due to the lower donor power of the aryl group, as
compared to alkyl, which puts less negative charge on the cobalt.
Potential (V)
Fig. 5. Cyclic voltammograms for 1e3 in CH2Cl2 with 0.1 M nBu4NPF6 as supporting
electrolyte at 0.1 V/s at 25 ꢁC.
are shown in Figs. 5 and 6 and the CV data are given in Table 3. In all
cases except 5, one irreversible reductive response corresponding
to Co(III)/Co(II) and one quasi-reversible oxidative response cor-
responding to Co(IV)/Co(III) process is observed. A comparison of
the current heights of two responses strengthens our hypotheses.
In present study the Co(III)/Co(II) redox process is considerably
cathodically shifted and due to this, the Co(II)/Co(I) response is
further cathodically shifted and therefore is not observed even
down to ꢀ1.8 V.
4. Conclusion
In this work we have described synthesis, spectral and structural
characterization of a series of aryl cobaloximes containing different
aryl groups and equatorial dioximes. Aryl ring size and the nature of
dioxime affect the structural parameters and their effect on the Pya
resonance is rather unusual for an organic group of a cobaloxime.
These complexes are reduced electrochemically more easily as
compared to the corresponding alkyl complexes.
The DEp for Co(IV)/Co(III) wave is higher than that of a reversible
one electron process and increases with increasing scan rate
(100e200 mV/s); thus indicating the quasi-reversible nature of the
electron transfer. All complexes undergo oxidation to Co(IV) and as
Acknowledgement
This work has been supported by a grant from CSIR, New Delhi,
India. K.K. thanks UGC, New Delhi, India for Junior/Senior Research
Fellowship.
Appendix. Supplementary material
PhCo(dpgH)2Py(4)
CCDC 727613, 727614, 727615, and 762045 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found in the online version,
at doi:10.1016/j.jorganchem.2010.06.012.
1-naphthylCo(dpgH)2Py(5)
References
2-naphthylCo(dpgH)2Py(6)
[1] R. Banerjee, Chemistry and Biochemistry of B12. Wiley & Sons, Inc., New York,
1999.
[2] K.L. Brown, Chem. Rev. 105 (2005) 2075e2149.
[3] N. Bresciani-Pahor, M. Forcolin, L.G. Marzilli, L. Randaccio, M.F. Summers, P.
J. Toscano, Coord. Chem. Rev. 63 (1985) 1e125 and refs cited therein.
[4] L. Randaccio, N. Bresciani-Pahor, E. Zangrando, L.G. Marzilli, Chem. Soc. Rev.
18 (1989) 225e250.
[5] G.N. Schrauzer, J. Kohnle, Chem. Ber. 97 (1964) 3056e3064.
[6] G.N. Schrauzer, Inorg. Synth. 11 (1968) 61e69.
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Potential (V)
[7] G.N. Schrauzer, Angew. Chem., Int. Ed. Engl. 15 (1976) 417e426.
[8] L. Randaccio, S. Geremia, G. Nardin, J. Wuerges, Cord. Chem. Rev. 250 (2006)
1332e1350 and refs cited therein.
Fig. 6. Cyclic voltammograms for 4e6 in CH2Cl2 with 0.1 M nBu4NPF6 as supporting
electrolyte at 0.1 V/s at 25 ꢁC.

K. Kumar, B.D. Gupta / Journal of Organometallic Chemistry 695 (2010) 2233e2239
2239
[9] B. Giese, Radicals in Organic Synthesis: Formation of CarboneCarbon Bonds.
Pergamon Press, Oxford, U.K, 1986.
[40] B.D. Gupta, K. Qanungo, T. Barcley, W. Cordes, J. Organomet. Chem. 560 (1998)
155e161.
[10] I. Das, S. Chowdhury, K. Ravikumar, S. Roy, B.D. Gupta, J. Organomet. Chem.
532 (1997) 101e107.
[41] B.D. Gupta, V. Singh, R. Yamuna, T. Barcley, W. Cordes, Organometallics 22
(2003) 2670e2678.
[11] A.K. Ghosh, Y. Chen, Tetrahedron Lett. 36 (1995) 505e508.
[12] M.W. Wright, M.E. Welker, J. Org. Chem. 61 (1996) 133e141.
[13] B.D. Gupta, V. Singh, K. Qanungo, V. Vijaikanth, R.S. Sengar, J. Organomet.
Chem. 582 (1999) 279e281 and refs cited therein.
[42] D.D. Perrin, W.L.F. Armarego, Purification of Laboratory Chemicals, third ed.
Pergamon, Oxford, U.K, 1988.
[43] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Bir-
mingham, England, 1974.
[14] T. Brown, A. Dronsfield, A. Jablonski, A.S. Wilkinson, Tetrahedron Lett. 37
(1996) 5413e5416.
[44] Smart & SAINT Software Reference Manuals, Version 6.45. Bruker Analytical
X-ray Systems, Inc., Madison, WI, 2003.
[15] S. Roy, I. Das, K. Bhanuprakash, B.D. Gupta, Tetrahedron 50 (1994) 1847e1858.
[16] J.L. Gage, B.P. Branchaud, Tetrahedron Lett. 38 (1997) 7007e7010.
[17] A. Rockenbaur, M. Eyor, M. Kwielinsci, S. Tyrlik, Inorg. Chim. Acta 58 (1982)
237e242.
[18] S. Nemeth, L. Simandi, J. Mol. Catal. 14 (1982) 87e93.
[19] C. López, S. Alvarez, X. Solans, M. Font-Altaba, Inorg. Chim. Acta 121 (1986)
71e75.
[20] C. López, S. Alvarez, M. Font-Bardia, X. Solans, J. Organomet. Chem. 414 (1991)
245e259.
[21] B.D. Gupta, R. Yamuna, V. Singh, U. Tiwari, T. Barcley, W. Cordes, J. Organomet.
Chem. 627 (2001) 80e92.
[45] G.M. Sheldrick, SADABS, Software for Empirical Absorption Correction, Ver.
2.05. University of Göttingen, Göttingen, Germany, 2002.
[46] XPREP, 5.1 ed. Siemens Industrial Automation Inc., Madison, WI, 1995.
[47] G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement.
University of Göttingen, Göttingen, Germany, 2008.
[48] DIRDIF-2008, Software for Crystal Structure Determination by Patterson and
Direct Methods Applied to Different Structure Factors. University of Nijmegen,
Netherlands, 2008.
[49] D. Dodd, M.D. Johnson, J. Organomet. Chem. 52 (1973) 1e232.
[50] P.J. Toscano, L.G. Marzilli, Prog. Inorg. Chem. 31 (1984) 105e204.
[51] B.D. Gupta, S. Roy, Inorg. Chim. Acta 146 (1988) 209e221.
[52] B.D. Gupta, K. Qanungo, J. Organomet, Chem. 534 (1997) 213e220.
[53] B.D. Gupta, V. Vijaikanth, V. Singh, Organometallics 23 (2004) 2069e2079.
[54] M. Bhuyan, M. Laskar, B.D. Gupta, Organometallics 27 (2008) 594e601.
[55] B.D. Gupta, M. Kumar, S. Roy, Inorg. Chem. 28 (1989) 11e18.
[56] H. Stepowska, S. Tyrlik, M. Hauke, Inorg. Chim. Acta 44 (1980) L69eL71.
[57] H. Blaser, J. Halpern, J. Am. Chem. Soc. 102 (1980) 1684e1689.
[58] G.N. Schrauzer, R.J. Windgassen, J. Am. Chem. Soc. 89 (1967) 1999e2007.
[59] L.I. Simaudi, C.R. Savage, Z.A. Schelly, S. Nemeth, Inorg. Chem. 21 (1982)
2765e2769.
[22] B.D. Gupta, R. Yamuna, D. Mandal, Organometallics 25 (2006) 706e714.
[23] B.D. Gupta, R. Yamuna, V. Singh, U. Tiwari, Organometallics 22 (2003)
226e232.
[24] B.D. Gupta, K. Qanungo, R. Yamuna, A. Pandey, U. Tiwari, V. Vijaikanth,
V. Singh, T. Barcley, W. Cordes, J. Organomet. Chem. 608 (2000) 106e114.
[25] D. Mandal, B.D. Gupta, Organometallics 24 (2005) 1501e1510.
[26] G. Dutta, K. Kumar, B.D. Gupta, Organometallics 28 (2009) 3485e3491.
[27] K.L. Brown, L.-Y. Lu, Inorg. Chem. 20 (1981) 4178e4183.
[28] P. Abley, E.R. Dockal, J.J. Halpern, J. Am. Chem. Soc. 95 (1973) 3166e3170.
[29] K.L. Brown, M. Ngamelue, J. Organomet. Chem. 243 (1983) 339e350.
[30] K.L. Brown, R. LeGates, J. Organomet. Chem. 233 (1982) 259e265.
[31] K.L. Brown, A.W. Awtrey, R.J. LeGates, J. Am. Chem. Soc. 100 (1978) 823e828.
[32] R.C. Stewart, L.G. Marzilli, J. Am. Chem. Soc. 100 (1978) 817e822.
[33] R.C. Stewart, L.G. Marzilli, Inorg. Chem. 16 (1977) 424e427.
[34] K.L. Brown, A.W. Awtrey, J. Organomet. Chem. 195 (1980) 113e122.
[35] W.W. Adams, P.G. Lenhert, Acta Crystallogr. B29 (1993) 2412e2425.
[36] D. Lenoir, H. Dauner, I. Ugi, J. Organomet. Chem. 198 (1980) C39eC42.
[37] L. Randaccio, Comments Inorg. Chem. 21 (1999) 327e376.
[38] L. Randaccio, S. Geremia, E. Zangrando, C. Ebert, Inorg. Chem. 33 (1994)
4641e4650.
[60] R.D. Jones, D.A. Summerville, F. Basolo, Chem. Rev. 79 (1979) 139e179.
[61] T.D. Smith, J.R. Pilbrow, Coord. Chem. Rev. 39 (1981) 295e383.
[62] K.L. Brown, in: D. Dolphin (Ed.), B12, Vol. 1, Wiley-Interscience, New York,
1982, p. 245.
[63] V.L. Goedken, S.M. Peng, Y. Park, J. Am. Chem. Soc. 96 (1974) 284e285.
[64] R.W. Schurko, R.E. Wasylishen, J. Phys. Chem. A 104 (2000) 3410e3420.
[65] K.M. McCauley, S.R. Wilson, W.A. van der Donk, Inorg. Chem. 41 (2002) 393e404.
13
13
[66] Dd
effect of substituent on
[67] The torsion angle (
(
C
) value has been taken instead of
d
(
C
). This is to avoid the direct
) represents the field effect.
C]N
C]N
13
d
(C]N) value. Dd
(
C
C]N
s
) is the angle between two virtual planes that bisects
cobaloxime plane. This shows twist angle of pyridine with respect to the line
[39] B.D. Gupta, K. Qanungo, J. Organomet. Chem. 543 (1997) 125e134.
joining the midpoints of CeC bond of two oxime units.