Cobaloximes with bis(thiophenyl)glyoxime: Synthesis and structure - Property relationship study

Article abstract of DOI:10.1021/om900065k

Alkyl and non-alkyl cobaloximes with bis(thiophenyl)glyoxime, X/RCo(dSPhgH)₂Py (X) Cl, R) Me, Et, Pr, Bu, Bn) have been synthesized and characterized for the first time. The X-ray structures of the complexes ClCo(dSPhgH)₂Py, EtCo(dSP

Full text of DOI:10.1021/om900065k

Organometallics 2009, 28, 3485–3491
3485
Cobaloximes with Bis(thiophenyl)glyoxime: Synthesis and
Structure-Property Relationship Study
Gargi Dutta, Kamlesh Kumar, and B. D. Gupta*
Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, India
ReceiVed January 28, 2009
Alkyl and non-alkyl cobaloximes with bis(thiophenyl)glyoxime, X/RCo(dSPhgH)2Py (X ) Cl, R )
Me, Et, Pr, Bu, Bn) have been synthesized and characterized for the first time. The X-ray structures of
the complexes ClCo(dSPhgH)2Py, EtCo(dSPhgH)2Py, and BuCo(dSPhgH)2Py are reported. The orientation
of SPh groups with respect to the dioxime plane varies with the steric bulk of the axial ligand and affects
1
the NMR chemical shifts. The cis and trans influence has been studied by H NMR, 13C NMR, and
X-ray diffraction. The steric cis influence of the equatorial thiodioxime affects the Co-C bond reactivity
in their cobaloxime complexes. The molecular oxygen insertion into the Co-C bond and steric cis influence
are related to each, and both follow the same order, dmestgH . dpgH > chgH > dSPhgH g dmgH >
gH. A cyclic voltammetry study shows that the reductions, Co(III)/Co(II) and Co(II)/Co(I), are easier in
ClCo(dSPhgH)2Py as compared to other chlorocobaloximes (gH, dmgH, dpgH, mestgH).
complexes with different chelates has been systematically
investigated.1,6-8 The influence of the equatorial ligands on the
axial ligands (cis influence) in Costa’s model and iminates
(cobalt complexes with a tetradentate Schiff base) has also been
reported.1b,c However the similar investigation in dioximates is
little studied since the majority of the reported complexes have
dmgH as the equatorial ligand. Our work on cobaloximes with
different dioximes has shown that the effect of dioxime on the
Co-C bond (cis influence) far exceeds the trans influence of
the axial base.9 In recent years, the description, spectroscopic
data, structure-property relationship, and their correlation to
Co-C bonds have been most emphasized.9a,10 The overall
evidence from literature strongly suggests that many of the
chemical properties related to the axial fragment such as
geometry, kinetics, and spectroscopic behavior are significantly
affected by a change in the equatorial ligand (cis effect and cis
influence).1e,f,6,8,9b,11 The dimesitylglyoxime (dmestgH) com-
plexes have the maximum cobalt anisotropy and the highest
steric cis influence among the commonly studied dioximes. The
overall cis influence follows the order dmestgH > dpgH > dmgH
> chgH > gH.9a Since the cis influence affects the Co-C bond
Introduction
Cobaloximes have been extensively studied and reviewed
over the past four decades.1 [Cobaloximes have the general
formula RCo(L)2B, where R is an organic group σ-bonded to
cobalt. B is an axial base trans to the organic group, and L is
a monoanionic dioxime ligand (e.g., glyoxime (gH), dimeth-
ylglyoxime (dmgH), 1,2-cyclohexanedione dioxime (chgH),
diphenylglyoxime (dpgH), dimesitylglyoxime (dmestgH), and
dithiophenylglyoxime (dSPhgH).] More than 1500 complexes
and >150 crystal structures have been reported. Since the Co-C
bond cleavage is the key step involved in B12-dependent
enzymatic or cobaloxime-mediated reactions,2-5 the strength
of the Co-C bond as a function of steric and electronic factors
with a wide range of axial ligands in cobaloximes and related
* Corresponding author. Tel: +91-512-2597046. Fax: +91-512-2597436.
E-mail: bdg@iitk.ac.in.
(1) (a) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio,
L.; Summers, M. F.; Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1, and
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Marzilli, L. G. Chem. Soc. ReV. 1989, 18, 225. (c) Randaccio, L. Comments
Inorg. Chem. 1999, 21, 327. (d) Gupta, B. D.; Yamuna, R.; Singh, V.;
Tiwari, U. Organometallics 2003, 22, 226. (e) Gupta, B. D.; Qanungo, K.;
Barcley, T.; Cordes, W. J. Organomet. Chem. 1998, 560, 155. (f) Gupta,
B. D.; Qanungo, K.; Yamuna, R.; Pandey, A.; Tiwari, U.; Vijaikanth, V.;
Singh, V.; Barcley, T.; Cordes, W. J. Organomet. Chem. 2000, 608, 106.
(g) Gupta, B. D.; Roy, S. Inorg. Chim. Acta 1988, 146, 209. (h) Gupta,
B. D.; Mandal, D. Organometallics 2006, 25, 3305. (i) Gupta, B. D.; Roy,
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49. (b) Gupta, B. D.; Kumar, M.; Roy, S. Inorg. Chem. 1989, 28, 11. (c)
Gupta, B. D.; Roy, S. Tetrahedron Lett. 1984, 3255. (d) Roy, S.; Das, I.;
Bhanuprakash, K.; Gupta, B. D. Tetrahedron 1994, 50, 1847. (e) Gupta,
B. D.; Kumar, M. Inorg. Chim. Acta 1988, 149, 223. (f) Gupta, B. D.; Das,
I.; Dixit, V. J. Chem. Res. 1992, 306. (g) Roy, M.; Kumar, M; Gupta, B. D.
Inorg. Chim. Acta 1986, 114, 87. (h) Gupta, B. D.; Vijaikanth, V. J.
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M. B.; Marzilli, L. G. Inorg. Chem. 1996, 35, 5646. (c) Polson, S. M.;
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Nardin, G.; Slouf, M.; Srnova, I. Inorg. Chem. 1999, 38, 4087.
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1997, 543, 125. (c) Gupta, B. D.; Vijaikanth, V.; Singh, V. J. Organomet.
Chem. 1998, 570, 1. (d) Gupta, B. D.; Roy, M.; Das, I. J. Organomet. Chem.
1990, 397, 219. (e) Gupta, B. D.; Tiwari, U.; Barcley, T.; Cordes, W. J.
Organomet. Chem. 2001, 629, 83. (f) Gupta, B. D.; Singh, V.; Yamuna, R.
Organometallics 2003, 22, 2670.
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10.1021/om900065k CCC: $40.75
2009 American Chemical Society
Publication on Web 05/05/2009

3486 Organometallics, Vol. 28, No. 12, 2009
Dutta et al.
X-ray Crystal Structure Determination and Refinements. A
slow evaporation of solvent from the solution of complexes 1, 3,
and 5 (CH2Cl2/CH3CN/MeOH for 1 and CH2Cl2/MeOH for 3, 5)
in the refrigerator resulted in the formation of orange crystals.
Single-crystal X-ray data were collected using graphite-monochro-
mated Mo KR radiation (λ ) 0.71073 Å) on a Bruker SMART
APEX CCD diffractometer at 100 K. The linear absorption
coefficients, scattering factors for the atoms, and the anomalous
dispersion corrections were taken from the International Tables
for X-ray Crystallography.13a The data integration and reduction
were processed with SAINT14 software. An empirical absorption
correction was applied to the collected reflections with SADABS15
using XPREP.16 All the structures were solved by the direct method
using SIR-9717 and were refined on F2 by the full-matrix least-
squares technique using the SHELXL-9713b program package. All
non-hydrogen atoms were refined anisotropically in all the structure.
The hydrogen atoms of the OH group of oxime were located on
difference Fourier maps and were constrained to those difference
Fourier map positions. The hydrogen atom positions or thermal
parameters were not refined but were included in the structure factor
calculations. The pertinent crystal data and refinement parameters
of 1, 3, and 5 are compiled in Table 1. The CIF files for the
compounds have been deposited with the Cambridge Crystal-
lographic Data Centre (CCDC numbers for 1, 3, 5 are 715421-
715423). Copies of the data can be obtained free of charge from
the Director, CCDC, 12 Union Road, Cambridge CB2 1EX, UK
(fax +44-1223-336033; e-mail deposit@ccdc.cam.ac.uk, or www:
http://www.ccdc.cam.ac.uk/).
strength and the insertion of oxygen involves Co-C bond
cleavage in these complexes, both processes are related to each
other and follow the same order.9c,d,12
Hence there has been a sustained interest in the synthesis of
organocobaloximes with new or modified equatorial ligands.
Bis(thiophenyl)glyoxime has been used as the equatorial ligand
in the present study. These are particularly selected because their
cobaloximes become the ideal systems for studying the
structure-property relationship. All the previously studied
cobaloximes had dioximes with carbon substituents. The goal
is to see how the SPh substituent on the dioxime affects the
cobalt anisotropy and the ring current in the metallabicycle and
in turn affects the Co-C bond strength. It is a priori difficult
to predict whether the SPh group is electron releasing due to
the lone pair on S or electron withdrawing due to the higher
electronegativity of S as compared to its carbon analogue. The
study will also allow us to make direct comparison with the
already reported work on gH, dmgH, chgH, dpgH, and dmestgH
complexes. Since the steric bulk of the dioxime affects the
Co-C bond, it would be of interest to study the conformation
of the SPh group on the dioxime, which, in principle, can adopt
the up-up, down-down, up-down, and down-up orientation
with respect to the dioxime plane, depending upon the steric
bulk of the axial ligands. We have, therefore, undertaken the
present study on RCo(dSPhgH)2Py (R ) Cl, Me, Et, Pr, Bu,
benzyl, 1-6). All the complexes are new and have been
synthesized for the first time. The complexes 1, 3, and 5 have
also been characterized by X-ray diffraction.
Dichloroglyoxime. We always got much lower yield than the
reported when we prepared dichloroglyoxime using the literature
procedure.18 The following modified procedure works well and
afforded a better yield.
Glyoxime (5.0 g) in 100 mL of water produced a suspension.
Concentrated HCl (25 mL) was added with stirring, which resulted
in a clear solution after 5-7 min. The solution can be warmed on
a heating mantle for 5 min if the clear solution is not obtained.
The solution was transferred to the specially designed glass vessel
that has an outer jacket and was cooled to 0 °C by Julabo
refrigerator circulator. First a slow stream of chlorine gas was
bubbled into the glyoxime solution for exactly 15 min. This was
followed by a medium stream (the yield drops if chlorine gas is
bubbled at a fast pace) of chlorine gas for exactly 2 h. Although
the product started to appear after 20-25 min, the white powdery
precipitate formed after 2 h was kept aside at room temperature
for 10 h or overnight for the precipitate to settle. It was filtered
off, washed with water (three times), and air-dried. Yield ) 4.23 g
(48%; average yield based on 10 experiments).
Dithiophenylglyoxime (dSPhgH). Dichloroglyoxime (0.10 g,
0.636 mmol) was dissolved in 5-8 mL of methanol with stirring.
Thiophenol (0.13 mL, 1.334 mmol) was added dropwise, and the
stirring was continued for 10 min. The temperature of the solution
was brought down to 0 °C by using an ice bath. Potassium carbonate
(0.09 g, 0.636 mmol) was added, the ice bath was removed, and
the solution was stirred for 10-12 min to dissolve potassium
carbonate. Two pellets of potassium hydroxide were added, and
the stirring was continued for an additional 20-25 min. The solution
was filtered to remove the precipitate if any. The filtrate was left
in a conical flask overnight for slow evaporation of the solvent.
Experimental Section
Cobalt chloride hexahydrate (SD fine, India), glyoxime (Caution!
it is highly flammable and explosive when dry) (Alfa Aesar,
Lancaster), iodomethane, and ethyl and benzyl bromide (Aldrich
Chemical Company) were purchased and used as received. Silica
gel (100-200 mesh) and distilled solvents were used in all reactions
and chromatographic separations. A Julabo UC-20 low-temperature
refrigerated circulator was used to maintain the desired temperature.
Cyclic voltammetry measurements were carried out using a BAS
Epsilon electrochemical workstation with a platinum working
electrode, a 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 JNM Lambda 400
FT NMR spectrometer (400 MHz for 1H and 100 MHz for 13C) in
CDCl3 solution with TMS as internal standard. NMR data are
reported in ppm. Elemental analysis was carried out at IIT Kanpur.
(14) SAINT+, 6.02 ed.; Bruker AXS: Madison, WI, 1999.
(15) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(16) XPREP, 5.1 ed.; Siemens Industrial Automation Inc.: Madison, WI,
1995.
(17) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(12) (a) Bhuyan, M.; Laskar, M.; Mandal, D.; Gupta, B. D. Organo-
metallics 2007, 26, 3559. (b) Dutta, G.; Laskar, M.; Gupta, B. D.
Organometallics 2008, 27, 3338.
(13) (a) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol. IV. (b) Sheldrick, G. M. SHELXL-97:
Program for Crystal Structure Refinement; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
(18) (a) Ungnade, H. E.; Kissinger, L. W. Tetrahedron 1963, 19, 143.
(b) Ramprasad, D.; Busch, D. H. U.S. Patent 4,680,037, July 14, 1987.

Cobaloximes with Bis(thiophenyl)glyoxime
Organometallics, Vol. 28, No. 12, 2009 3487
Table 1. Crystal Data and Structure Refinement Details for 1, 3, and 5
[ClCo(dSPhgH)2Py] · CH2Cl2
EtCo(dSPhgH)2Py
BuCo(dSPhgH)2Py
empirical formula
fw
C67H56Cl4Co2N10O8S8
1645.44
C35H32CoN5O4S4
773.83
C37H36CoN5O4S4
801.92
temp (K)
cryst syst
space group
unit cell dimens
a (Å)
100(2)
100(2)
orthorhombic
Pca21
298(2)
monoclinic
Cc
triclinic
j
P1
11.074(5)
12.615(5)
25.496(5)
99.706(5)
99.115(5)
90.263(5)
3465(2)
23.069(5)
8.306(5)
25.458(5)
10.961(5)
14.499(5)
90.000(5)
118.331(5)
90.000(5)
3561(2)
b (Å)
c (Å)
18.064(5)
90.000(5)
90.000(5)
90.000(5)
3461(2)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
2
4
4
F(calc) (mg/m3)
1.577
0.938
1684
0.35 × 0.30 × 0.25
-10 e h e 13,
-15 e k e 15,
-30 e l e 26
18 725
1.485
0.785
1600
0.30 × 0.25 × 0.20
-30 e h e 24,
-11 e k e 10,
-24 e l e 21
21 612
1.496
0.765
1664
0.30 × 0.25 × 0.20
-26 e h e 33,
-14 e k e 14,
-19 e l e 14
11 388
µ(Mo KR) (mm-1
F(000)
)
cryst size (mm3)
index ranges
no. of reflns collected
no. of indep reflns
GOOF on F2
12 678
1.034
8163
1.195
5634
1.047
final R indices (I > 2σ(I))
R1 ) 0.0735
wR2 ) 0.1944
R1 ) 0.1033
wR2 ) 0.2278
12 678/0/892
R1 ) 0.0623
wR2 ) 0.1520
R1 ) 0.0810
wR2 ) 0.1835
8163/1/442
R1 ) 0.0802
wR2 ) 0.1967
R1 ) 0.0917
wR2 ) 0.2295
5634/2/446
R indices (all data)
data/restraints/params
Table 2. Elemental Analysis Data for 1-6
Table 5. Selected Bond Lengths and Angles and Structural Data for
1, 3, and 5
% found (calculated)
H
1
3
5
no.
formula
C
N
Co-C/Co-Cl
Co-N
C-Co-N/Cl-Co-N
d (Å)
R (deg)
τ (deg)
2.231 (17)
1.976 (5)
178.53 (16)
-0.020
2.024 (6)
2.078 (5)
174.1 (2)
-0.015
4.81
2.011 (9)
2.123 (7)
174.1 (4)
+0.015
3.18
1
2
3
4
5
6
C33H27ClCoN5O4S4
C34H30CoN5O4S4
C35H32CoN5O4S4
C36H34CoN5O4S4
C37H36CoN5O4S4
C40H34CoN5O4S4
50.58(50.81)
54.03(53.76)
53.99 (54.34)
55.02(54.90)
55.09(55.43)
57.89(57.49)
3.48(3.46)
3.92(3.94)
4.10(4.13)
4.34(4.31)
4.46(4.49)
4.04(4.06)
9.02(8.97)
9.16(9.21)
9.01(9.04)
8.93(8.88)
8.68(8.73)
8.35(8.37)
4.12
72.710
75.368
65.038
Table 3. 1H NMR Data (ppm) for 1-6
turned the solution green. Air was passed through the solution for
about half an hour to get the desired product. The solid product
was filtered, washed with water and dried over P2O5 overnight. Then
it was purified on a silica gel column using dichloromethane. Yield
) 0.238 g (73%).
Py
compd
no.
aromatic
other
proton
R
ꢀ
γ
O-H · · · O proton Co-CH2
1
2
3
4
5
6
7.95
8.25
8.21
8.22
8.23
8.13
7.80
7.83
7.82
7.82
7.81
7.81
17.87
18.15
18.05
18.07
18.10
18.11
7.09-7.24
6.88-7.28
6.87-7.26
6.88-7.26
6.89-7.26
6.76-7.22
1.17
2.01
1.84
1.87
3.13
RCo(dSPhgH)2Py (2-6): General Procedure. A solution of 1
(0.100 g, 0.128 mmol) in about 25 mL of ethanol was purged with
N2 for 10 min and was cooled to 0 °C with stirring. The solution
turned blue after the addition of a few drops of aqueous NaOH
followed by sodium borohydride (0.012 g, 0.320 mmol in 0.5 mL
of water). The color of the solution immediately turned orange on
the addition of alkyl halide (0.192 mmol in 1 mL of ethanol). The
stirring was continued at 0 °C for 1 h. The volume of the reaction
mixture was reduced to 5 mL by evaporation on a Rotovap and
then poured into 20 mL of water containing a few drops of pyridine.
The resulting orange precipitate was filtered, washed with water,
and dried. The product was purified on the silica gel column using
dichloromethane. Yield ) 53-80%.
0.357
0.942, 0.721
1.16, 0.904, 0.755
Table 4. 13C NMR Data (ppm) for 1-6
no.
CdN
PyR
others
1
145.20
150.95
139.32, 131.11, 130.21, 129.19,
127.82, 126.23
2
142.28
149.98
138.20, 132.10, 130.31, 129.07,
127.58, 125.80, 29.79
3
5
142.02
142.07
149.95
149.88
132.05, 130.54, 128.88, 127.49
132.00, 130.55, 128.90, 127.52,
125.65, 32.80, 23.57, 13.76
132.17, 130.44, 128.78, 128.25,
127.35, 125.53
CHN analysis of 1-6 is given in Table 2.
6
142.30
150.20
Results and Discussion
The solid crystalline compound appeared as colorless needles, which
were washed with dichloromethane. Yield ) 0.132 g (69%). The
same yield was obtained when the reaction was scaled up five times.
ClCo(dSPhgH)2Py (1). In a typical reaction CoCl2 · 6H2O (0.100
g, 0.420 mmol) was added to a stirred solution of dithiophenyl-
glyoxime (0.255gm, 0.840 mmol) in ethanol (20 mL). The stirring
was continued for 10-15 min, during which the solution turned
bluish-green. It was heated for 8-10 min with a hot air gun, and
0.0631 mL (0.783 mmol) of pyridine was added dropwise, which
Synthesis. Dithiophenylglyoxime was prepared from dichlo-
roglyoxime and thiophenol in the presence of K2CO3 following
the literature procedure.19 We have synthesized six new
complexes, X/RCo(dSPhgH)2Py (1-6), by a general procedure
used for the preparation of dmgH complexes reported earlier.
(19) Nicolaides, D. N.; Kouimtzis, A. Chim. Chronika 1974, 3, 63.

3488 Organometallics, Vol. 28, No. 12, 2009
Dutta et al.
Table 6. CV Data for ClCo(dioxime)2Py in CH2Cl2 and TBAPF6 at 0.2 V/s at 25 °C
CoIII/CoII CoII/CoI
CoIV/CoIII
compound
Epc (V)a
Epc (V)b
E1/2 (V)a
E1/2 (V)b
E1/2 (V)a
E1/2 (V)b
ClCo(gH)2Py
-0.39(irrev)
-0.66(irrev)
-0.50(irrev)
-0.68(irrev)
-0.25(irrev)
-0.81
-1.08
-0.92
-1.10
-0.67
-0.66(113)
-1.12(200)
-0.85(105)
-1.01(120)
-0.47(69)
-1.08
-1.54
-1.27
-1.43
-0.90
ClCo(dmgH)2Py
ClCo(dpgH)2Py
ClCo(dmestgH)2Py
ClCo(dSPhgH)2Py
1.20(190)
1.22(200)
1.33(120)
1.40(83)
0.77
0.79
0.904
0.97
b
a vs Ag/AgCl. vs Fc/Fc+. Fc/Fc+ (E1/2 ) 0.4269).
Table 7. CV Data for 1, 3, and 6 in CH2Cl2 and TBAPF6 at 0.2 V/s at 25 °C
CoIII/CoII
CoII/CoI
CoIV/CoIII
no.
Epc (V)a
Epc (V)b
ipc (µA)
E1/2 (V)a
E1/2(V)b
ipc (µA)
E1/2 (V)a
E1/2 (V)b
ipc
ipa
1
3
6
-0.253
-1.269
-1.166
-0.679
-1.695
-1.592
1.07
8.36
2.74
-0.475(69)
-0.901
2.50
1.401(83)
1.155(97)
1.150(75)
0.975
0.729
0.724
1.03
6.17
1.37
4.53
7.87
2.24
b
a vs Ag/AgCl. vs Fc/Fc+. Fc/Fc+ (E1/2 ) 0.4269).
Figure 1. Molecular structure of 1.
Figure 3. Molecular structure of 5.
Figure 2. Molecular structure of 3.
Table 8. Pseudo-First-Order Rate Constant (kobs) for Oxygen
Insertion in PhCH2Co(dioxime)2Py
complex
kobs (s-1
)
PhCH2Co(gH)2Py
7.1 × 10-4
1.2 × 10-3
1.29 × 10-3
4.8 × 10-3
5.0 × 10-2
Figure 4. Cyclic voltammogram of 1 in CH2Cl2 with 0.1 M
(nBu4NPF6) as supporting electrolyte at 0.2 V s-1 at 25 °C.
PhCH2Co(dmgH)2Py
PhCH2Co(dSPhgH)2Py
PhCH2Co(dpgH)2Py
PhCH2Co(dmestgH)2Py
and in addition 1, 3, and 5 have also been characterized by
X-ray. The NMR assignments are consistent with the related
cobaloxime complexes with dmgH, gH, and dpgH. PyR and
CdN occur close to each other, and the assignment is confirmed
by DEPT. The free dithiophenylglyoxime is not completely
soluble in CDCl3, and a drop of DMSO-d6 is necessary to
dissolve it.
The synthesis of RCo(dSPhgH)2Py involves the oxidative
alkylation of CoI with the corresponding halide. Ethanol is a
better solvent than methanol, and a large excess of solvent is
essential during the generation of cobaloxime(I); otherwise the
precipitation occurs after the addition of NaBH4.
Spectroscopy. The free bis(thiophenyl)glyoxime20 and its
The cis and trans influence study includes the investigation
of all possible steric and electronic effects of the dioxime on
complexes 1-6 have been characterized by H and 13C NMR,
1

Cobaloximes with Bis(thiophenyl)glyoxime
Organometallics, Vol. 28, No. 12, 2009 3489
NMR of MeCo(dSEtgH)2Py.21 Here SEt lacks ring current and,
as expected, no shielding of the PyR proton occurs and δ1HPyR
is found to be almost identical with MeCo(dmgH)2Py.
Since 13C operates through-bond, the through-space interac-
tion is minimized in the 13C chemical shift. Hence the δ13CPyR
carbon should be similar to the other cobaloxime complexes
where such through-space interaction with PyR does not occur.
This is indeed so, and the chemical shift value is almost identical
in RCo(dmgH)2Py and RCo(dSPhgH)2Py (Supporting Informa-
tion, Table S2). It is clear that the observed high upfield shift
[∆δ1H(PyR)] in 2-6 occurs due to the through-space interaction
and not because of the high ring current of the Co(dioxime)2.
The extent of electron density (i.e., total effect of cobalt
anisotropy and ring current) on Co(dioxime)2 for different
dioximes (keeping R/X constant) can be understood by compar-
22
ing ∆δ(13CCdN
)
in R/XCo(dioxime)2Py. In general, the
δ13CCdN in free dioxime shifts upfield on coordination to cobalt.
This upfield shift [∆δ(13CCdN)] is much smaller in 1-6 as
compared to the corresponding gH, dmgH, and dpgH analogues
but is comparable to the mesitylglyoxime complexes (Supporting
Information, Table S3). The order based on the upfield shift
value follows gH > dmgH > dpgH > chgH > dSPhgH >
dmestgH. So the charge density on CdN in the thiodioxime
complexes is much lower than the other dioxime complexes.
This may further mean that the cobalt anisotropy of the dSPhgH
complex is close to dmestgH complexes, and this is possible
only if the overall effect of SPh is taken as an electron-
withdrawing group. This effect shows up in the CV study also.
The variation in the R/X group also affects δ13CCdN (compare
1 and 2) and is similar to our earlier observations; for example
the upfield shift value in 1 is much smaller, as expected, than
2 because of the higher cobalt anisotropy in the chloro complex.
Figure 5. Cyclic voltammograms of 3 and 6 in CH2Cl2 with 0.1 M
(nBu4NPF6) as supporting electrolyte at 0.2 V s-1 at 25 °C.
Table 9. Pseudo-First-Order Rate Constant (kobs) for Oxygen
Insertion in RCo(dioxime)2Py
complex
kobs (s-1
)
EtCo(dmgH)2Py
EtCo(dpgH)2Py
EtCo(dSPhgH)2Py
MeCo(dmestgH)2Py
nBuCo(dmestgH)2Py
6.45 × 10-4
6.95 × 10-4
5.95 × 10-4
2.9 × 10-4
4.5 × 10-4
the axial ligands and vice versa. Generally the chemical shift
of PyR and the R-carbon bonded to Co is studied since these
are affected most by the dioxime. Three factors simultaneously
acting on a particular atom to determine the NMR chemical
shift of the coordinated ligand from the free ligand are (a) cobalt
anisotropy (cobalt anisotropy is the total field effect of the
CoC4N4 system; the field effect is the combination of the
inductive effect of cobalt and the effect of donation through
Cofdioxime and Cofaxial ligand and back-donation) (leads
to deshielding of all the ligand atoms through the σ backbone
and it decreases with the distance from the metal), (b) long-
range effect of magnetic anisotropy such as the ring current
arising from either the metallabicycle, the axial ligand pyridine,
or the SPh group, and (c) metal to ligand back-bonding, which
may alter the ring current and/or the cobalt anisotropy.
In tune with our earlier work, the trans influence is monitored
through the coordination shift (∆δ ) δ
- δ
base) of
complex
free
the γ-proton only. The chemical shift of the Pyγ proton is a net
result of the interplay of cobalt anisotropy and the trans effect
of the R/X group. The ∆δ Pyγ value of 0.4 ppm in 1-6 is larger
than the corresponding dmgH and chgH analogues but resembles
the dpgH complexes (Supporting Information, Table S4). This
means that the cobalt anisotropy in 1-6 is high like in dpgH,
which leads to more electron withdrawal from pyridine and
causes deshielding of the γ-proton. The same observation was
made on the basis δ13CCdN data also.
The cis influence of the dioxime on the R-carbon bonded to
cobalt is measured by its chemical shift; for example Co-CH2R
in 2-6 is downfield shifted from the corresponding dmgH
complex but is highly upfield shifted with respect to the dpgH
or dmestgH complexes (Supporting Information, Table S5); that
is, the chemical shift of Co-CH2 in 2-6 lies between the
corresponding dmgH and dpgH complexes. The same pattern
shows up in the molecular oxygen insertion rate data into these
complexes (see later).
Correlations. The coordination shift of the CdN group of
the dioximato moiety is much lower in 1-6 as compared to
the corresponding value in the gH, dmgH, dpgH, and dmestgH
complexes with identical R and B groups. The correlation of
the ∆δ(13C, CdNdSPhgH) in 1-6 is fairly linear with the other
dioximes.
A comparison of ∆δ1H(PyR) (∆δ1H(PyR) ) δ1Hcomplex
-
δ1H free Py) in RCo(dioxime)2Py [dioxime ) gH, dmgH, chgH,
dpgH, dmestgH, dSPhgH] shows an upfield shift of 0.30-0.44
ppm in 2-6 compared to the shift in other dioxime complexes
(Supporting Information, Table S1). This is unexpected and
unusually quite high. One should not jump to the conclusion
and attribute this to the high ring current of the metallabicycle
in 2-6 since the upfield shift can also occur due to through-
space interaction of the phenyl ring of the SPh group with the
PyR proton. Its contribution, if any, can be confirmed from X-ray
and 13C NMR study.
The PyR proton is quite close (3.732 and 3.436 Å in 3 and 5)
to the dioxime SPh group (see X-ray details later) and is highly
shielded by its ring current.1b This is further confirmed by H
1
(21) MeCo(dSEtgH)2Py: PyR ) 8.60 ppm, Pyꢀ ) 7.37 ppm, Pyγ ) 7.77
ppm, SEt group(-CH2) ) 3.26, 3.10 ppm, SEt group(Me) ) 1.10 ppm,
H-1 ) 1.25 ppm, O-H · · · O ) 18.50 ppm.
(20) dSPhgH, 1H NMR: O-H · · · O ) 11.38 ppm, aromatic proton )
7.14-7.28 ppm. 13C NMR: CdN ) 145.35, other carbons ) 134.47, 128.51,
128.37, 128.10.
(22) Instead of δ (13C, CdN), the ∆δ (13C, CdN) value has been taken.
This is to avoid the direct effect of the substituent on the δ(CdN) value.
∆δ(13C, CdN) represents the field effect.

3490 Organometallics, Vol. 28, No. 12, 2009
Dutta et al.
∆δ(13C, CdNdSPhgH) ) 1.92(14) + 1.12(4)[∆δ(13C, CdNdpgH)]
(r2 ) 0.99, esd ) 0.08)
ligands. First, the different electronic nature of the chelate
macrocycle could change the metal-orbital mix involved in the
axial ligand bonding (electronic cis influence), and second, the
more flexible macrocycle may bend toward the axial ligand,
thus increasing the axial bond length.1b
Interaction between the Axial Ligands and the Equatorial
Dioxime Ligand Side Groups. The present results are reminis-
cent of the earlier work by Randaccio et al., who have studied,
in detail, the interactions of axial ligands with the equatorial
ligand bridging the side groups in diphenylborylated co-
baloximes by NMR and X-ray.25-27 They observed that the
phenyl groups on BPh2 assumed different fast interconverting
conformations in solution, depending on the interaction between
the BPh, phenyls, and the axial ligands. Their result suggested
that the steric effects (steric trans influence) played an important
role in this process.
∆δ(13C, CdNdSPhgH) ) 4.49(71) + 1.46(15)[∆δ(13C, CdNdpgH)]
(r2 ) 0.96, esd ) 0.27)
∆δ(13C, CdNdSPhgH) ) 5.63(35) + 1.16(5)[∆δ(13C, CdNdpgH)]
(r2 ) 0.99, esd ) 0.11)
Structural Studies. The geometry of the central cobalt atom
is distorted octahedral with four nitrogen atoms of the dioxime
in the equatorial plane and pyridine and Cl (or Et, Bu) are axially
coordinated. The deviation of the central cobalt atom (d) from
the mean equatorial N4 plane is -0.020, -0.015, and +0.015
Å, respectively, for 1, 3, and 5.
Previous structural studies on cobaloximes have shown that
changing the axial R group in RCo(dioxime)2B causes only a
small distortion on the Co(dioxime)2 moiety.1b The emphasis
of the structural study has mainly been focused on four
parameters: (i) Co-C and Co-Npy bond lengths, (ii) the
puckering of the dioxime ligand, i.e, butterfly bending angle
(R),23 (iii) deviation of the cobalt atom from the mean equatorial
N4 plane (d), and (iv) the torsion angle24 between the axial base
pyridine and the dioxime ligand. R and d are the guiding factors
in defining the geometrical deformation in the Co(dioxime)2
moiety. A large amount of the reported structural data in
RCo(dmgH)2Py complexes further reveals that R seems to be
more influenced by the bulk of the R group than the d, and the
variations obtained in d and R fall in a fairly wide range [R
from -12.3° [R ) CH2C(Me)(COOEt)2] to +6.3° (R )
CHdCH2) and d from -0.03 Å [R ) CH2C(Me)(COOEt)2 to
+0.04 Å (R ) Me)]. Therefore, the bending of the two dioxime
units is the most significant distortion in these compounds.
Unfortunately, there is a lack of enough data in the literature to
see if a change in the dioxime moiety, keeping the same axial
ligands, has any influence on these parameters. The structural
data on the thiodioxime complexes are, therefore, significant.
A comparison of R and d value in MeCo(dioxime)2Py shows a
range of +2° to +7.3° and +0.03 to -0.017, respectively, as
we move from gH to dmestgH (Supporting Information, Table
S6). The distortion imposed by thiodioxime is similar to the
dpgH ligand. When the equatorial ligand (chel) is a dianion of
a tetradentate Schiff base such as salen, acacen, and salophen
in RCo(chel)Py, it imposes a much larger deviation in the R
value as compared to the dioxime complexes (Supporting
Information, Table S7).
Although our systems differ from Randaccio because the
substituent SPh is on the dioxime imine carbon rather than on
the bridging OHO, as was the case in Randaccio’s work,
nevertheless the orientation of SPh group with respect to
the dioxime plane does affect the structure and the NMR. In
the present work the conformation of SPh with respect to the
dioxime plane varies with the steric bulk of the axial ligand;
for example the up-up, down-down conformation in 3 changes
to an up-down, up-down conformation in 5. The conformation
changes further in 1 as shown below.
One of the SPh phenyl groups having a down orientation is
close to the pyridine R-proton and shows the C-H(PyR) · · · π
interaction with distances of 4.32, 3.732, and 3.436 Å in 1, 3,
and 5, respectively. This interaction has led to the upfield shift
1
of the PyR proton in the H NMR.
CV Study. Very little work has been reported on the CV
studies in cobaloximes, and hence there is a lack of sufficient
data to correlate and generalize the electrochemical behavior
of these complexes. One of the major problems associated is
the change in coordination number of cobalt during the
reduction/oxidation process.28 Most of the reported CV work
has been done in (X/R)Co(dmgH)2B to understand the effect
of base B. The effect of dioxime on the CV value has virtually
not been studied. This may be due to the lack of cobaloximes
with the dioximes other than dmgH. In general, the inorganic
cobaloximes give a better cyclic voltammogram than the
organocobaloximes.
The Co-Cl or Co-C bond distances [2.231, 2.024, 2.011
Å] and Co-NPy bond distances [1.976, 2.078, 2.123 Å] in 1, 3,
and 5 do not differ significantly from the reported values for
the corresponding (R/X)Co(dioxime)2Py complexes. The elec-
tronic nature of R influences the Co-NPy bond length, and more
electron-donating R increases the length.1a This is what is
observed here also (compare the data between 3 and 5 in Table
5). A further comparison with the RCo(chel)Py complexes
shows that while the Co-C distances are almost same, the
Co-NPy distance is much larger in the Schiff base complexes.
This can be due to two principal aspects of the equatorial
Three types of redox couples, CoIII/CoII, CoII/CoI, and CoIV/
CoIII, in the cyclic voltammogram of (X/R)Co(dioxime)2B are
(25) Asaro, F.; Naradin, G.; Pellizer, G.; Perissini, S.; Randaccio, L.;
Siega, P.; Tauzher, G.; Tavagnacco, C. J. Organomet. Chem. 2000, 601,
114.
(26) (a) Dreos, R.; Geremia, S.; Nardin, G.; Randaccio, L.; Tauzher,
G.; Vuano, S. Inorg. Chim. Acta 1998, 272, 74. (b) Dreos, R.; Tauzher, G.;
Vuano, S.; Asaro, F.; Pellizer, G.; Nardin, G.; Randaccio, L.; Geremia, S.
J. Organomet. Chem. 1995, 505, 135.
(27) Asaro, F.; Dreos, R.; Geremia, S.; Nardin, G.; Pellizer, G.;
Randaccio, L.; Tauzher, G.; Vuano, S. J. Organomet. Chem. 1997, 548,
211.
(28) (a) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am.
Chem. Soc. 1981, 103, 5558. (b) Alexander, V.; Ramanujum, V. V. Inorg.
Chim. Acta 1989, 156, 125.
(23) (a) The dihedral angle (butterfly bending angle, R) is the angle
between two dioxime planes of each cobaloxime unit. (b) The positive sign
of R and d indicates bending toward R and displacement toward the base
and vice versa.
(24) The torsion angle (τ) is the angle between two virtual planes that
bisects the cobaloxime plane. Each plane is formed considering the middle
point of the C-C bond of two oxime units that passes through cobalt and
pyridine nitrogen.

Cobaloximes with Bis(thiophenyl)glyoxime
Organometallics, Vol. 28, No. 12, 2009 3491
expected, and the CV of both oxidative and reductive halves
can be rationalized on the basis of cobalt anisotropy; the higher
the cobalt anisotropy, the lower the reduction potential and the
higher the oxidation potential.9a,29
cobaloximes, then the oxygen insertion data in the present
thiodioximes complexes should fit in well and strengthen the
concept further.
The rate studies are carried out following the procedures
outlined in our earlier paper.12 The reactions follow a pseudo-
first-order kinetics, and the rate data in 3 and 6 support our
previous observations; for example the benzyl complexes have
a higher rate of insertion, as expected, than the alkyl complexes.
The rate lies between the dmgH and dpgH complexes in the
benzyl complexes (Table 8). This is as per our expectation,
arrived on the basis of NMR. The overall rate follows the order
dmestgH . dpgH > dSPhgH g dmgH > gH. The generality of
this correlation, however, is difficult to test in the alkyl
cobaloximes since the pseudo-first-order kinetics of oxygen
insertion is slow and does not vary much with the alkyl group
or with the dioxime. Nevertheless it is useful information (Table
9).
The cyclic voltammogram of 1 shows one irreversible peak
at -0.253 V (irrev) and one reversible peak (-0.475 V) in the
reductive half corresponding to CoIII/CoII and CoII/CoI, respec-
tively. In the oxidation half only one reversible wave corre-
sponding to CoIV/CoIII (1.40 V) is observed. The CoII/CoI moiety
should be affected only by the dioxime moiety since there is
no axial ligation in CoI. A comparison with the other chloro-
cobaloximes (gH, dmgH, dpgH, dmestgH) (Table 6) shows that
1 is the most easily reduced among these. It points to high cobalt
anisotropy in 1, which is possible only if SPh acts as an electron-
withdrawing group. The NMR data also gave the same
information.
The cyclic voltammogram of 3 and 6, on the other hand,
shows only one irreversible peak at -1.26 and -1.16 V,
respectively, in the reduction side, corresponding to CoIII/CoII,
and one reversible peak at 1.15 V in the oxidation side,
corresponding to CoIV/CoIII; 6 is easier to reduce compared to
3 because benzyl is a poorer electron donor than Et. Due to
enhanced σ donation by the R group, the CoIII state in 3 and 6
is substantially stabilized. In other words, the CoIII/CoII redox
process in 3 and 6 is considerably cathodically shifted. Due to
this, the CoII/CoI response is further cathodically shifted and
therefore is not observed even down to -1.6 V.
Conclusion
The steric cis influence affects the Co-C bond stability/
reactivity, and a good correlation between the rate of oxygen
insertion and the steric cis influence has been found in the alkyl
and benzyl cobaloximes with dithiophenyloxime as the equato-
rial ligand. Both follow the same order: dmestgH . dpgH >
dSPhgH g dmgH > gH. This supports our earlier results. As
the substituent on S controls the orientation of the SR group
and may alter the cobalt anisotropy and/or the ring current of
the metallabicycle, which, in turn, will affect the Co-C bond
stability, more systems should be studied to answer this question.
Rate Studies. The insertion of molecular oxygen into the
Co-C bond has been used to test the reactivity of the Co-C
bond in cobaloximes. Since the cleavage of the Co-C bond is
the primary step in the oxygen insertion and the effect of
dioxime (cis influence) is felt most on the Co-C bond, both
are related to each other. In the earlier work we found a good
correlation between these two factors; both followed the same
order in the benzyl cobaloximes: dmestgH . dpgH > chgH >
dmgH > gH.9c,d,12 If this correlation is a general feature of all
Acknowledgment. This work has been supported by a
grant from DST, New Delhi, India.
Supporting Information Available: Molecular structure com-
1
parison data, H and 13C NMR table, comparison table of NMR,
and CIF files for X-ray crystal structures of 1, 3, and 5. This material
is available free of charge via the Internet at http://pubs.acs.org.
(29) Yamuna, R.; Gupta, B. D.; Mandal, D. Organometallics 2006, 25,
706.
OM900065K