Biphenyl-bridged dicobaloximes: Synthesis, NMR, CV, and X-ray study

Article abstract of DOI:10.1021/om7009045

4,4'-Disubstituted biphenyl-bridged dicobaloximes, Py(L) ₂Co-CH₂-Ar-CH₂-Co(L)₂Py [L = dmgH, dpgH], have been synthesized and characterized. The cobalt-bound CH₂ is diastereotopic and the dmgH (Me) sho

Full text of DOI:10.1021/om7009045

594
Organometallics 2008, 27, 594–601
Biphenyl-Bridged Dicobaloximes:
Synthesis, NMR, CV, and X-Ray Study†
Mouchumi Bhuyan, Moitree Laskar, and B. D. Gupta*
Department of Chemistry, Indian Institute of Technology Kanpur, India 208016
ReceiVed September 11, 2007
4,4′-Disubstituted biphenyl-bridged dicobaloximes, Py(L)2Co-CH2-Ar-CH2-Co(L)2Py [L ) dmgH,
dpgH], have been synthesized and characterized. The cobalt-bound CH2 is diastereotopic and the dmgH
1
(Me) shows nonequivalence in the H NMR. This may occur due to atropisomerism and/or restricted
rotation of the C-Ph/Co-C bond. Two X-ray structures of the biphenyl-bridged dicobaloximes, reported
for the first time, show no direct or indirect interaction between the two cobaloxime units [Co(dioxime)2Py].
The NMR spectra and X-ray structural details complement each other. Three redox processes, CoIII/CoII,
CoII/CoI, and CoIV/CoIII, are observed in the CV study. Two cobalt centers are oxidized at different
potentials as two discrete units even though these are symmetrically located with respect to each other
with no interaction between them.
solution; the cobalt-bound CH2 became diasteretopic and the
dioxime protons showed nonequivalence at subzero tem-
pearture.5 This was attributed to the restricted rotation of
the Co-C and/or C-Ph bond. Many inherent problems are
associated with the synthesis of such compounds since these
are much more unstable in solution as compared to the cor-
responding monocobaloximes. Because of the presence of two
inherently weak Co-C bonds, it was hoped that these might
find better use as precursors in homolytic displacement
reactions, more than the monocobaloximes derivatives.6 Also
there is very little information on the structural and electro-
chemical behavior of the organo-bridged dicobaloxime
complexes, in general. The idea is to play with the steric
factor of the bridging ligand and to use the steric bulk as a
handle to cause restricted rotation in the organo-bridged
dicobaloximes; the biphenyl-bridged analogues become suit-
able systems for such a study.
Introduction
Although the cobaloximes [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),
dimethylglyoxime (dmgH), diphenylglyoxime (dpgH))] have
been extensively studied over the past four decades,1 the
dinuclear complexes containing bridging ligands are of
continuing interest since the presence of the bridging ligand
has been shown to affect the geometry of the complex and
the redox properties of the metal.2a The study of these
complexes helps us in understanding the electronic interaction
in the supramolecules that have promising applications.2b
Though numerous ligand bridged bimetallic complexes have
been reported,3a little is known about the neutral organo-
bridged bimetallic complexes.3b Organo-bridged dico-
baloximes of the type B(L)2Co-R-Co(L)2B (L ) dioxime)
were virtually unknown until recently.4 In our recent studies
we saw that the ortho and meta xylylene-bridged dico-
baloximes exhibited very interesting NMR behavior in
Since the appropriately substituted biphenyl derivatives
give rise to axial chirality-atropisomerism7 the study of
biphenyl-bridged dicobaloximes having the ortho positions
substituted by bulky groups like -CH2Co(dmgH)2Py is
undertaken. Baddeley8 and Harris et al.9 have reported that
while the major part of the origin of the energy barrier to
inversion in the biphenyl derivatives is the steric strain at
the transition state due to nonbonded interactions between
atoms (or groups) at the orthopositions, it is also considerably
affected by the para substituents. In view of this we have also
studied the effect of substitution at 4,4′-positions in the biphenyl-
bridged dicobaloximes.
† Dedicated to Prof. Karen Eichstadt, Department of Chemistry and
Biochemistry, Ohio University.
* 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
references therein. (b) Randaccio, L.; Bresciani-Pahor, N.; Zangrando, E.;
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.;
Tewari, U. Organometallics 2003, 22, 226. (e) Gupta, B. D.; Qanungo, K.;
Barclay, T.; Cordes, W. J. Organomet. Chem. 1998, 560, 155. (f) Gupta,
B. D.; Qanungo, K.; Yamuna, R.; Pandey, A.; Tewari, U.; Vijaikanth, V.;
Singh, V.; Barclay, T.; Cordes, W. J. Organomet. Chem. 2000, 608, 106.
(2) (a) Xue, W. M.; Goswami, N.; Eichhorn, D. M.; Orizondo, P. L.;
Rillema, D. P. Inorg. Chem. 2001, 39, 4460. (b) Lehn, J. M. Angew. Chem.,
Int. Ed. 1988, 29, 1304.
(5) (a) Gupta, B. D.; Vijaikanth, V.; Singh, V. Organometallics 2004,
23, 2069. (b) Vijaikanth, V.; Gupta, B. D.; Mandal, D.; Shekhar, S.
Organometallics 2005, 24, 4454.
(6) (a) Gupta, B. D.; Singh, V.; Qanungo, K.; Vijai Kanth, V.; Sengar,
R. S. J. Organomet. Chem. 1999, 582, 279. (b) Gupta, B. D.; Roy, S.
Tetrahedron Lett. 1986, 27, 4905. (c) Gupta, B. D.; Dixit, V.; Das, I. J.
Organomet. Chem. 1999, 572, 49. (d) Das, I.; Chowdhury, S.; Ravikumar,
K.; Roy, S.; Gupta, B. D. J. Organomet. Chem. 1997, 532, 101.
(7) Nasipuri, D. Stereochemistry of Organic Molecules; Wiley Eastern
Limited: New Delhi, 1993.
(3) (a) Mandal, D.; Gupta, B. D. Eur. J. Inorg. Chem. 2006, 20, 4086.
(b) Kheradmadan, S.; Heinze, K; Schmalle, H. W.; Berke, H. Angew. Chem.,
Int. Ed. 1999, 38, 2270.
(4) (a) Zhang, X.; Li, Y.; Mei, Y.; Chen, H. J. Organomet. Chem. 2006,
691, 659. (b) Mavunkal, I. J.; Hearshaw, M. A.; Moss, J. R.; Bacsa, J.
Inorg. Chim. Acta 2004, 357, 2748. (c) Finch, Karol P.; Moss, J. R. J.
Organomet. Chem. 1988, 346 (2), 253. (d) Gupta, B. D; Qanungo, K. J.
Organomet. Chem. 1997, 534, 213.
(8) Baddeley, Nature 1946, 157, 694.
(9) Harris, M. M.; Ling, C. K. J. Chem. Soc. 1964, 1825.
10.1021/om7009045 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/01/2008

Biphenyl-Bridged Dicobaloximes
Organometallics, Vol. 27, No. 4, 2008 595
Table 1. Crystal Data and Structure Refinement Details for 3a and 7a
3a
7a · 2C6H6 · C2H5OH
empirical formula
fw
C40H52Cl2N10O10Co2
1021.68
C
108H100Cl2N10O12Co2
1918.74
temp (K)
100(2)
100(2)
diffraction measurement
device/scan method
cryst syst
space group
unit cell dimens
a (Å)
Smart CCD area detector/ꢀ-ω
Smart CCD area detector/ꢀ-ω
tetragonal
P42 21 2
monoclinic
C2/c
14.763(5)
21.412(6)
b (Å)
14.763(5)
19.172(2)
c (Å)
21.544(5)
24.722(10)
R (deg)
90
90
90
4695(2)
90
ꢁ (deg)
113.537(3)
90
9304(11)
γ (deg)
V (Å3)
Z
8
4
F(calcd) (Mg/m3)
1.394
0.879
2040
1.370
0.484
4008
µ (mm-1
F(000)
)
cryst size (mm3)
index ranges
0.35 × 0.25 × 0.20
-19 e h e 13, -19 e k e 16,
-28 e l e 28
31 223
5845
0.30 × 0.25 × 0.22
-15 e h e 28, -25 e k e 24,
-33e l e 32
30 106
11 531
no. of reflns collected
no. of indep reflns
GOOF on F2
1.077
0.969
final R indices (I < 2σ(I))
R indices (all data)
data/restraints/param
R1 ) 0.0652, wR2 ) 0.1594
R1 ) 0.0910, wR2 ) 0.1730
5845/0/289
R1 ) 0.0859, wR2 ) 0.1998
R1 ) 0.1841, wR2 ) 0.2449
11531/0/612
Fc+) couple occurs at E1/2 ) +0.43 (71) V versus the Ag/AgCl
electrode under the same experimental conditions.
We have chosen two different dioximes, dmgH and dpgH,
to see the effect of steric bulk in the dioxime moiety in
-CH2Co(dioxime)2Py. We also report the first crystal structures
of biphenyl-bridged dicobaloximes. The electrochemical studies
have also been done for the first time.
X-ray Crystal Structure Determination and Refinements. We
could obtain single crystals of 3a from slow diffusion of pentane
in a dichloromethane solution. The crystals of 7a are found by slow
evaporation of a mixture of benzene and ethanol in dichloromethane.
The structure 3a was solved in noncentric chiral space group P42212.
The space group for the structure 7a is C2/c. Single-crystal X-ray
data were collected at 100 K on a Bruker SMART APEX CCD
diffractometer using graphite-monochromated Mo KR radiation (λ
) 0.71073 Å). The linear absorption coefficients, scattering factors
for the atoms, and the anomalous dispersion corrections were taken
from the International Tables for X-ray Crystallography.14 Cell
parameters were retrieved using SMART software and refined using
SAINT on all observed reflections. The data integration and
reduction were processed with SAINT15 software. An empirical
absorption correction was applied to the collected reflections with
SADABS16 using XPREP.17 The structures were solved by the
direct methods using SHELXTL18 and were refined on F2 by the
full-matrix least-squares technique using the SHELXL-9719 program
package. All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms of the OH group of oxime were located on
difference maps and were constrained to those difference map
positions. The hydrogen atom positional or thermal parameters were
not refined but were included in the structure factor calculations.
The pertinent crystal data and refinement parameters are compiled
in Table 1.
Experimental Section
General Methods. 2,2′-Bis(bromomethyl) biphenyl was pur-
chasedfromAldrichandwasusedasreceived.4,4′-Diflouro,104,4′-dichloro,11
4,4′-dibromo-[2,2′-bis(bromomethyl)]biphenyl,12
ClCo(dmgH)2Py, and ClCo(dpgH)2Py13a,3b were synthesized ac-
cording to the literature procedure. Silica gel (100–200 mesh) and
distilled solvents were used in all chromatographic separations. The
synthetic work was carried out in subdued light and under a blanket
of argon or nitrogen.
1H and 13C NMR spectra were recorded on a JEOL JNM
LAMBDA 400 FT NMR spectrometer (400 MHz for 1H and
100 MHz for 13C) in CDCl3 with TMS as an internal standard.
NMR data are reported in ppm. Elemental analysis was carried
out using a Thermoquest CE Instruments CHNS-O elemental
analyzer. A Julabo UC-20 low-temperature refrigerated circulator
was used to maintain the desired temperature. Cyclic voltam-
metry measurements were carried out using a BAS Epsilon
electrochemical workstation in dichloromethane (dry) at a
concentration of 1 mM of each complex with 0.1 M tetra-n-
butylammonium hexafluorophosphate (nBu4NPF6) as the sup-
porting electrolyte. All the measurements were performed with
a BASi platinum disk or glass carbon working electrode, a Ag/
AgCl reference electrode (3 M NaCl), and a platinum-wire
counter electrode. The reversible ferrocene/ferrocenium ion (Fc/
A CIF file is deposited with the Cambridge Crystallographic Data
Center (CCDC number for 3a is 660482 and for 7a is 660483).
(14) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol. IV.
(15) SAINT+, version 6.02; Bruker AXS: Madison, WI, 1999.
(16) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Go¨ ttingen: Go¨ttingen, Germany, 1997.
(17) XPREP, version 5.1; Siemens Industrial Automation Inc.,: Mad-
ison,WI, 1995.
(18) Sheldrick, G. M. SHELXTL Reference Manual: Version 5.1; Bruker
AXS: Madison, WI, 1997.
(10) Oki, M.; Iwamura, H.; Yamamoto, G. Bull. Chem. Soc. Jpn. 1971,
44, 262.
(11) Le Fevre, V. J. Chem. Soc. 1938, 967.
(12) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992,
114, 6227.
(13) (a) Schrauzer, G. N. Inorg. Synth. 1968, 11, 61. (b) Gupta, B. D.;
Yamuna, R.; Singh, V.; Tiwari, U.; Barclay, T.; Cordes, W. J. Organomet.
Chem. 2001, 627, 80.
(19) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of Go¨ ttingen: Go¨ttingen, Germany, 1997.

596 Organometallics, Vol. 27, No. 4, 2008
Bhuyan et al.
Copies of the data can be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2 1EX, U.K. (fax +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk/).
chromatography is the best way to separate the dicobaloxime from
monocobaloxime, in certain cases the compounds decomposed on
the silica gel column. In such cases these were purified by repeated
fractional crystallization under argon atmosphere. In fact, one
should first try to separate the products by partial crystallization
(dicobaloximes are much less soluble than the monocobaloximes
in common organic solvents), and if that fails, only then should
column chromatography be tried.
Synthesis of Biphenyl-Bridged Dicobaloximes: General Pro-
cedure. One milliliter of aqueous sodium hydroxide (1 pellet in 2
mL of water) was added to a suspension of ClCo(dioxime)2Py (1
mmol in 15 mL of methanol). The reaction mixture was purged
with argon for 20 min while cooling it to 0 °C. A deaerated aqueous
solution of sodium borohydride (0.04 g, 1 mmol in 0.5 mL of water)
was added dropwise. The solution turned blue in color and showed
the presence of cobaloximes(I). An argon-purged solution of
dihalide (0.5 mmol in ca. 1 mL of diethyl ether) was added to it
dropwise. The stirring was continued in the dark for 2–3 h, during
which the solution became yellow-orange. The reaction mixture
was poured into100 mL of ice-cold water containing a few drops
of pyridine. The orange-yellow precipitate was filtered on a sintered
funnel, washed with water until the filtrate was pale yellow in color,
and dried over P2O5 in the dark, and the crude product was subjected
to column chromatography.
1
The H NMR of the mono- and dicobaloximes 1–8 is given
in Table 2, and the elemental analysis, 13C NMR, and ratio of
mono- and dicobaloximes under different conditions are given
as Supporting Information as Tables S1, S2, and S3.
Spectroscopy. All the complexes have primarily been
1
1
characterized with H and 13C NMR. In the H NMR spectra,
Co-CH2, dioxime, O-H · · · O, and pyridine resonances and in
the 13C NMR CdN, Co-CH2, and pyridine are clearly
distinguished and are easily assigned on the basis of their
chemical shifts. The assignments are consistent with the related
organo-bridged complexes reported from our group.4d,5
The CH2 protons in the dicobaloximes 1a-8a appear as
double-doublets, suggesting that these protons are diastereotopic.
The J value corresponds to the geminal coupling.
Chromatographic Separation. Since dicobaloximes decompose
readily in solution and in the presence of visible light, the entire
process of column chromatography including the evaporation of
solvent and crystallization were carried out in diffused light. It is
recommended that the rate of elution must be kept fast during the
column chromatography.
Ha is located close (3.268 Å) to one of the phenyl rings of
the biphenyl and is affected by its ring current; it is upfield
shifted as compared to Hb. Therefore, the substituent in the 4,4′
position affects the chemical shift of Ha. On the other hand the
chemical shift of Hb remains almost constant (see 2a, 3a, and
4a). A similar pattern is observed in the dpgH complexes
(5a-8a) also; the proximity of Ha to the phenyl ring is almost
the same (3.292 Å) as in the corresponding dmgH complex.
The strong downfield shift of CH2 in the dpgH complexes is as
expected and is due to the higher cobalt anisotropy in these
complexes. The same has been observed earlier in the benzyl
cobaloximes, BnCo(dioxime)2Py (dioxime ) dmgH, dpgH). The
separation between the two doublets (∆δ) is much smaller in
the dpgH complexes (40–136 Hz) as compared to the dmgH
complexes (145–180 Hz). It seems to be a general feature since
the same was observed earlier in the xylylene-bridged dico-
baloximes also. This might have occurred due to the change in
the dihedral angle between the two phenyl groups of the
biphenyl moiety, resulting due to the bulkiness of the [Co-
(dioxime)2] moiety. This has been verified from the crystal
structure details.
dmgH Complexes 1–4. The crude product containing a mixture
of monocobaloxime and biphenyl-bridged dicobaloxime was dis-
solved in a minimum amount of dichloromethane and was loaded
on a silica gel column, pre-eluted with dichloromethane. The
polarity of the solvent was carefully and slowly increased with an
ethyl acetate/dichloromethane mixture (10–40%) until an orange-
red band corresponding to monocobaloxime was distinctly visible.
This band was completely eluted out with ethyl acetate/dichlo-
romethane (80:20). The dicobaloxime was eluted out with 100%
ethyl acetate and an ethyl acetate/methanol (90:10) mixture.
dpgH Complexes 5–8. The polarity of the solvent was carefully
increased with an ethyl acetate/dichloromethane (2–4%) mixture
when the orange band corresponding to monocobaloximes was
eluted out. The dicobaloxime was eluted out with an ethyl acetate/
dichloromethane (10:90) mixture.
Results and Discussion
Synthesis. Biphenyl-bridged dicobaloximes (1a-8a) were
synthesized following a procedure published recently by us on
the xylylene-bridged dicobaloximes.5b In addition to the required
product a small amount of methyl-substituted monocobaloxime
(1b-8b) was also formed as a side product. The combined yield
of mono- and dicobaloxime is around 70–80%. However their
ratio depends upon many factors such as reaction time, the
dihalide:CoI ratio, and the nature of the dihalide used. For
example (a) for Y ) H the yield of dicobaloxime (1a or 5a) is
around 55% with reaction time 5 min to 1 h; however it falls to
15–20% when the reaction time is longer (>5 h). (b) For Y )
H or F the yield is always higher than when Y ) Cl or Br,
where it is 30–40%. (c) when the ratio of biphenyl dibromide:
CoI is 3:1, monocobaloxime is the major product (∼70%);
however with 1:3 ratio, no improvement in yield of dico-
baloxime is observed.
The overall observations suggest that the best conditions for
preparing the biphenyl-bridged dicobaloximes are 0 °C, dihalide:
CoI ratio 1:2, and the reaction time 2–3 h. The dicobaloximes are
orange-red colored solids and are stable under nitrogen atmosphere.
However, decomposition occurs in solution over a period of time,
even under N2 atmosphere. We have even noticed the decomposi-
tion of the dicobaloximes in the NMR tube. Since column
The dmgH(Me) appears as two singlets with a separation of
about 0.13 ppm in all cases, and the substitution in the 4,4′
position does not alter this value; however it is downfield shifted
as compared to the parent dicobaloxime (Y ) H).
Moving from dicobaloximes to the monocobaloximes
(1b-8b), the Co-CH2 remains diastereotopic and dmgH(Me)
appears as two singlets. Interestingly, in the biphenyl complexes
with only one ortho substituent such similar nonequivalence
occurs at subzero temperature; for example the nonequivalence
is observed at -15 °C (CH2) and at -25 °C (dmgH) (Me) in
2-biphenyl CH2Co(dmgH)2Py.20 This means that the barrier to
rotation between the two phenyl rings in this molecule is small,
but substitution by Me at the 2′ position increases this barrier
and facilitates the phenomenon of nonequivalence. This is
expected and can be easily explained. The coplanar conforma-
tion for any biphenyl with a single ortho substituent has
maximum destabilization from the steric interaction of the
substituent with the nearer ortho-hydrogen on the second phenyl
ring and maximum stabilization from the π-π interaction of
(20) (a) Mandal, B. D.; Gupta, B. D. Organometallics 2006, 25, 3305.
(b) Mandal, B. D.; Gupta, B. D. Organometallics 2007, 26, 658.

Biphenyl-Bridged Dicobaloximes
Organometallics, Vol. 27, No. 4, 2008 597
a
Table 2. 1H NMR Data for 1a-8a and 1b-8b at Room Temperature in CDCl3
pyridine
no.
CH2-Co
dmgH/dpgH
R (d)
ꢁ (t)
γ(t)
O-H · · · O
aromatic
CH3
2.10
1a 2.75(d) (6.4) 1.78, 1.91
3.02(d) (6.4)
1b 2.66(d) (6.8)
3.04(d) (7.2)
2a 2.52(d) (6.4)
2.89(d) (6.8)
2b 2.41(d) (7.0)
2.91(d) (6.8)
3a 2.43(d) (6.6)
2.87(d) (6.8)
3b 2.39(d)(5.9)
2.89(d) (6.0)
4a 2.43(d) (6.8)
2.88(d) (7.2)
4b 2.40(b)
8.37
(4.8)
8.43
(5.2)
8.36
(5.2)
8.41
(4.8)
8.35
(4.8)
8.40
(5.2)
8.35
*
*
*
7.57
(7.6)
7.62
(7.6)
17.76
6.49(d) (7.2), 6.89(d) (6.8), 7.13–7.31(m)
1.87, 1.92
1.83, 1.96
1.92, 1.98
1.84, 1.99
1.93, 2.00
1.84, 1.99
1.91, 2.00
*
17.94
17.75
17.99
17.77
17.98
17.75
17.88
18.40
18.38
18.38
18.52
18.27
18.32
18.33
18.38
6.72(d) (7.6), 6.90–6.96(m), 7.16–7.22(m)
7.18* 7.61
(6.8) (7.6)
7.22* 7.65
(6.8) (7.6)
7.18* 7.61
(6.8) (7.6)
7.22
(6.8) (7.6)
*
[6.18(d) (2.8), 6.20(d) (2.4) J ) 8.0], 6.98–7.03(m), 7.12–7.14(m)
[6.37(d) (2.8), 6.40(d) (2.8) J ) 12.0], 6.80–6.95 (m), 7.27–7.31(m)
6.43(d) (2.0), 7.14–7.16 (d) (8.0), 7.30 (d) (2.0)
6.61(d) (2.4), 6.82(d) (8.4), 7.15–7.17(m)
2.06
2.07
2.07
2.04
7.65
7.60
(7.6)
7.64
(7.6)
6.43 (d) 7.08–7.44
8.40
6.61(d)6.81–7.17
2.89(b)
5a 3.44 (6.0)
3.74 (5.6)
5b 3.74(6.4)
3.45(6.0)
8.74
(5.2)
8.74
(6.0)
8.72(b)
7.73* 7.32
(7.2) (6.8)
7.75
(6.0)
*
6.69(d) (7.2), 6.91(d) (6.8), 7.05–7.19(m), 7.44–7.46(m), 7.55(d) (7.2)
6.69(d), 6.91(d), 7.05–7.33(m), 7.43–7.55(m), 7.72(d)
6.73(b), 6.91(b), 7.13–7.26(m), 7.33, 7.47
*
6a 3.24(b)
*
*
*
*
*
*
7.76(b)
3.58(b)
6b 3.03(d) (6.0)
3.55(d) (6.4)
7a 3.29(d) (6.8)
3.56(d) (6.8)
7b 3.23(d) (6.4)
3.50(d) (6.4)
8a 3.31(d) (5.6)
3.56(d) (5.8)
8.74
(4.8)
8.59
(5.2)
8.63
(4.8)
8.69
7.37* 7.80
(6.8) (8.0)
6.77(d) (7.2), 6.84(d) (6.4), 6.79–7.02(m), 7.11–7.25(m), 7.49–7.53(m) 2.13
6.68–7.28
*
*
*
*
7.39
(6.8)
7.68
(7.6)
7.75
6.65–7.30
6.73–7.34
6.72–7.36
1.97
8b 3.31(d)(5.2)
3.44(d) (5.4)
8.71
7.82
2.01
a * ) Merge with aromatic protons, b ) broad.
the two rings. In the orthogonal conformation, these opposing
contributions to the molecular energy are both at a minimum,
so the overall lowest energy conformation for the biphenyl will
be somewhere between these extremes, more or less near
orthogonal, depending on the size of the substituent. This is an
interesting observation since many groups have been trying to
design organic biphenyl systems that show chirality at room
temperature.
Scheme 1. Biphenyl-Bridged Cobaloximes
The separation between the two doublets (∆δ) is much
larger in the monocobaloximes [200–240 Hz in the dmgH
complexes (1b-4b) and 108–208 Hz in the dpgH complexes
(5b-8b)] than the corresponding dicobaloxime complexes
(1a-4a and 5a-8a). This suggests that the dicobaloximes
are more rigid than the monocobaloximes and the orientation
of two [Co] centers in the dicobaloximes is fixed in such a
way as to eliminate or partly reduce the steric interactions
between the two cobaloxime units (the dihedral angle between
the two phenyl rings around the C-C bond defines the
torsion) and thus results in a smaller separation in the peaks.
Similarly, the large ∆δ value in the monocobaloximes
indicates a smaller dihedral angle and hence greater interac-
tion between the two geminal hydrogens.21 The 1H NMR
data in the corresponding gH complex (Scheme 1; L ) gH;
A comparison of 1H NMR in the di- and monocobaloximes
(1a-4a and 1b-4b) shows that the dmgH (Me) appears as
two singlets in both cases. However, one singlet has an almost
constant chemical shift, whereas the other one is downfield
shifted by 0.1 ppm in the monocobaloximes as compared to
the dicobaloximes. This is in contrast to the CH2 protons,
where one set of doublets was upfield shifted in the
monocobaloximes.
The diastereotopic nature of CH2 can occur due to atropi-
somerism and/or restricted rotation of the C-Ph/Co-C bond,
as observed recently in the xylylene-bridged dicobaloximes5b
and in the ortho-substituted benzyl cobaloximes.20 It is very
difficult to distinguish between the two processes at this stage
(Figure 1).
X-ray Crystal Structures. The diamond diagram of the
molecular structures for 3a and 7a along with the selected
numbering scheme is shown in Figures 2 and 3. The selected
bond lengths and bond angles are given in Table 3. The
geometry around each cobalt atom in 3a and 7a is distorted
octahedral with four nitrogen atoms of the dioxime in the
equatorial plane, with pyridine and biphenyl moieties in the axial
positions. The crystal structure of 7a shows one disorder:
1
Y ) H) [the H NMR data of gH complex (δ in ppm) 2.94,
3.36(CH2Co), 8.40 (4.8) (Pya), 7.65 (7.2)(Pyg), 6.99– 7.13
(m); the ligand protons and Pyb are merged with the aromatic
protons] support these arguments. Here the less bulky dioxime
exerts fewer steric interactions and should have a small
dihedral angle. Thus the separation of the peaks should be
large. This is what is observed also (∆δ ) 168 Hz).
(21) Mazzanti, A.; Lunazzi, L.; Minzoni, M.; Anderson, J. E. J. Org.
Chem. 2006, 71, 5474.

598 Organometallics, Vol. 27, No. 4, 2008
Bhuyan et al.
Figure 1. Biphenyl-Bridged Dicobaloximes: Possible C-Ph and
C-C Bond Rotation.
Figure 3. Molecular structure of 7a.
in each structure. Each cobaloxime unit has many structural
similarities and can be looked at as an independent benzyl
cobaloxime; for example, in structure 3a, the Co-C bond length,
the butterfly bending angle (R) between the two dioxime units,
the CH · · · π and π · · · π interactions, and the orientation of the
benzyl group over the dmgH(Me) are very similar to those found
earlier in the benzyl and ortho-substituted benzyl cobaloximes.20
A similar trend follows in the dpgH complex, 7a, except that
the R is much higher (6.37°) as compared to the benzylCo-
(dpgH)2Py complex (1.75°). The high R value can be explained
by the C-H · · · π interaction [(3.307(11) and 3.693(8) Å)]
between one of the aromatic rings of biphenyl and the H atom
of the equatorial aromatic ring.
The number of crystal structures of the organo-bridged
dicobaloximes is limited; a few structures on the alkyl-bridged4
and only one structure on the xylylene-bridged dicobaloximes5b
have been reported so far. Some important differences are
observed from the reported m-xylylene-bridged dicobaloxime
(A); the dicobaloxime units are symmetrical and the distance
between the cobalt atoms [7.148(1) and 7.106(21) Å for 3a for
7a] is shorter than the corresponding distance in A (7.871 Å);
R is identical (1.63°) for both Co1 and Co2 in 3a, whereas these
are different (2.99° and 8.81°) in structure A. The displacement
(d) from the mean equatorial N4 plane for both the cobalt atoms
in 3a is the same (+0.014) and is toward pyridine, whereas it
is +0.029(9) for Co1 and -0.0063 (8) for Co2 in A. [For
cobaloximes, RCo(dioxime)2Py, the positive sign of R and d
indicates bending toward R and displacement toward B and Vice
Versa.] These differences point to many more observations; for
example, no direct or indirect interaction between the two
cobaloxime units [Co(dioxime)2Py] in 3a and 7a is observed
since the structural parameters in the two units are identical.
This is in contrast to the m-xylylene complex (A), where the
sign of d and different value of R suggest that one dioxime
unit on Co1 pushes the other dioxime unit toward Co2, with
the result that one cobalt atom is displaced toward pyridine and
the other toward the benzyl group. This interaction cannot occur
directly between the two dioxime units since these are too far
away. This occurs via the aromatic ring.
Figure 2. Molecular structure of 3a.
the O (OW1) atom of one H2O molecule is attached to one
of the Co-CH2 protons and the H · · · O distance is found to
be 1.09 Å.
In view of the recent structural studies on cobaloximes we
have focused mainly on (i) the axial Co-N and the Co-C bond
lengths, (ii) the puckering of the equatorial dioxime ligand, i.e.,
butterfly bending angle (R),22 (iii) the torsion angle (τ)23 between
the axial base pyridine and equatorial ligand, (iv) the deviation
of the cobalt atom from the mean equatorial N4 plane (d), and
(v) the dihedral angle between the two phenyl rings of the
biphenyl. These points assist in defining the cis or trans influence
of axial as well as equatorial ligands.
Two distinct cobaloxime units, Co(dioxime)2Py, are axially
bridged by the biphenyl group to form the dicobaloxime moiety
(22) The dihedral angle (butterfly bending angle, R) is the angle between
two dioxime planes of each cobaloxime unit.
(23) 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
the pyridine nitrogen.
The pyridine ring is practically planar and parallel to the
dioxime C-C bonds, its conformation being defined by a twist
angle of 85.045(8)° in 3a and 89.501(1)° in 7a. The bridging
biphenyl in 3a is sandwiched between the two dmgH wings,

Biphenyl-Bridged Dicobaloximes
Organometallics, Vol. 27, No. 4, 2008 599
Table 3. Selected Bond Lengths (Å), Bond Angles (deg), Torsion Angles (deg), and Structural Data for 3a and 7a (X1 and X2 for comparison)a
3a
7a
X1
X2
Co-Co (Å)
7.148(1)
57.85
7.106(21)
64.35
biph twist angle (deg)
Co-C (Å)
2.065(4)
2.068(3)
175.01(19)
117.1(3)
130.68
-20.12
-83.61
+0.014
1.63
2.046(5)
2.041(5)
174.81(18)
120.1(4)
155.27
-6.99
-78.66
0.0
2.064(15)
2.056(14)
2.056(5)
2.061(4)
Co-Nax (Å)
C-Co-Nax (deg)
Co-C-C(Ph) (deg)
N5-Co-C-C
Neq-Co-C-Cb
Co-C-C-C
d (Å)
177.1(1)
176.8(2)
116.6(3)
158.8
117.9(3)
155.1
-20.1
-12.7
-93.94
-0.0371
1.86
-91.65
-0.0275
1.75
R (deg) (dihedral angle)
6.80
89.501(1)
τ (deg)
85.045(8)
86.454
88.96
π · · · π (Å)
3.564(19)
3.183(22)
3.748(3)
2.880(4), 3.189
C-H · · · π (Å)
3.421(0), 3.861(0)
3.307(11), 3.693(8)
a X1 ) C6H5CH2Co(dmgH)2Py;31 X2 ) C6H5CH2Co(dpgH)2Py.20b
and the directly linked aromatic ring of the biphenyl unit forms
an angle of 26.47° with the cobaloxime plane.24 However, this
angle is 36.94° in 7a. The values point to how crowded or
flattened the cobaloximes are; 3a is much more flattened as
compared to 7a. The equatorial planes (the plane of four
nitrogens of dioxime) of the two cobaloxime units are staggered
with respect to each other in 3a, whereas they are partially
staggered in 7a.
Both the dioxime planes in the cobaloxime units in each
structure are bent toward the biphenyl moiety, and R is the same
for both the dioxime units (1.63°, 1.63° in 3a and 6.80°, 6.80°
in 7a). Both the cobalt atoms (Co1 and Co2) are displaced (d)
equally by 0.014 Å from the equatorial N4 plane of the two
dioximes in 3a, and the displacement is toward pyridine.
However in 7a each Co is coplanar with the N4 plane of the
dioxime unit and the d value is zero.
A critical value for biphenyl derivatives lies in the dihedral
angle around the central C-C bond, and this angle between
the two phenyl rings is 57.85° and 64.35° in 3a and 7a,
respectively. That means that the phenyl units are in a cisoid
conformation (the two [Co] units are located on the same side
of the biphenyl scaffold). This conformation is comparable to
the reported 2,2′-dimethoxy-substituted biphenyl derivatives.25
As a consequence, the two biphenyl moieties are approximately
arranged orthogonally in the crystal.
these could be reversibly reduced in DMSO solution. Two
common solvents used for the study are dichloromethane and
acetonitrile. We have preferred to use dichloromethane because
of the higher solubility of the cobaloximes. We have studied
the electrochemical behavior of 1a, 2a, 3a, 5a, 8a, 2b, and 5b
in order to find out the effect of the bridging biphenyl group as
well as the effect of the dioxime on the redox potentials of the
metal centers. The ligand oxidation is also observed, but this
occurs at higher potential (>1.6 V). Since the X-ray structural
parameters have indicated that the two cobalt centers are almost
identical and each unit resembles the isolated benzyl cobaloxime,
it would be interesting to see if the electrochemical behavior
also gives the same information or not. The cyclic voltammo-
grams are given in Figure 4, and the redox potential data are
given Tables 4 and 5.
In the reductive half, only one irreversible peak at -0.92,
-1.23, and -1.43 V corresponding to one-electron reduction
of CoII/CoI is observed in 1a, 2a, and 8a. However in 3a and
5a both processes, CoIII/CoII and CoII/CoI, are observed. The
reduction of the dpgH complexes, as expected, is easier than
the dmgH complexes because of the higher cobalt anisotropy
in the former.29
Two irreversible waves in the oxidation half are observed in
the dmgH complexes 1a and 2a, whereas the peaks are
reversible in 3a. The oxidation becomes more difficult, as
expected, with the substitution at the 4,4′- position in the
biphenyl by the electron-withdrawing groups (2a and 3a). The
same phenomenon is observed in the dpgH complexes also.
Electrochemical Study (Cyclic Voltammetry). The cyclic
voltammogram (CV) of a cobaloxime shows three types of redox
couples: CoIII/CoII, CoII/CoI, and CoIV/CoIII. Organo-co-
baloximes, in general, give poor CV, and most of the time the
CoIII/CoII redox couple is not observed.26 None of the studies
have reported the values for all three redox systems. On the
other hand, inorganic cobaloximes show relatively better CV
and all the redox couples are prominent. The CV study of
organocobaloximes becomes complicated due to the change in
coordination number (cleavage of axial base and/or Co-C bond)
during the redox process and therefore has not been studied
that well. Finke and co-workers27 found that the reduction of
alkylcobaloximes was irreversible under all conditions of
solvents and scan rate, whereas Le Hoang et al.28 found that
The presence of two oxidation potentials in the dmgH as well
as in the dpgH complexes indicates that two cobalt centers [Co]
are oxidized at different potentials as two discrete units. A
similar observation was made earlier in the xylylene-bridged
dicobaloximes.5b This, however, is surprising since the present
systems, unlike the xylylene-bridged dicobaloximes, have two
cobalt centers symmetrically located with respect to each other
with no direct or indirect interaction between the two cobaloxime
units as per the X-ray details. Therefore, only one oxidation
potential was expected. In the m-xylylene-bridged dicobaloximes
the two cobalt centers were unsymmetrically oriented with
respect to each other. The present systems resemble the
symmetrical pyrazine-bridged dicobaloximes, [RCo(dioxime)2]2-
µ-Pz, where a single oxidation potential was observed due to
the high electron delocalization between the two [Co] centers
(24) This plane is defined as the plane passing through N1 C8 C9 N2
and N3 C12 C13 N4 of the structure 3a.
(25) Delogu, G.; Fabbri, D.; Dettori, M. A.; Salle, M.; Le Derf, F.; Blesa,
M.-J.; Allain, M. J. Org. Chem. 2006, 71, 9096.
(26) Mandal, D.; Gupta, B. D. Organometallics 2005, 24, 1501.
(27) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am.
Chem. Soc. 1981, 103, 5558.
(29) 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.
(28) Le Hoang, M. D.; Robin, Y.; Devynck, J.; Bied-Charreton, C.;
Gaudemer, A. J. Organomet. Chem. 1981, 222, 311.

600 Organometallics, Vol. 27, No. 4, 2008
Bhuyan et al.
Figure 4. Cyclic voltammograms of (a) monocobaloximes and (b) biphenyl-bridged dicobaloximes in DCM with 0.1 M (nBu4NPF6) as
supporting electrolyte at 0.2 V s-1 at 25 °C.
Table 4. CV Data of Cobaloximes in DCM and TBAPF6 at 0.2V/s at 25 °C
CoIII/CoII
CoII/CoI
CoIV/CoIII
no. Epc (V) ipc (µA) Epc (V) ipc (µA) Epc1 (V) ipc (µA) Epa1 (V) ipa (µA) E1/2a(V) Epc2 (V) ipc (µA) Epa2 (V) ipa (µA) E1/2 (V)
1a
2a
3a
5a
8a
2b
-0.92
-1.23
-1.08
-1.49
-1.43
-1.12
4.89
0.74
0.80
0.91
0.96
1.05
1.02
0.87
1.14
1.05
1.39
1.4
3.49
3.21
3.39
16.1
3.65
2.11
2.75
11.5
5.17
3.01
-0.56
-0.99
0.83
0.89
2.55
0.87
0.95
0.98
3.31
1.02
16.4
9.34
a E1/2 Fc/Fc+ ) 0.4269 V.
Table 5. Comparative CV Data Taken in DCM and TBAPF6 at 0.2 V/s at 25 °C
CoIII/CoII
CoII/CoI
CoIV/CoIII
compound
Epc (V)
Epc (V)
Epc (V)
Epa(1) (V)
E1/2 (V)
Epc (V)
Epa(2) (V)
ref
PhCH2Co(dmgH)2Py
PhCH2Co(dpgH)2Py
[PhCH2Co(dmgH)2]2 µ-Pz
[PhCH2Co(dpgH)2]2-µ-Pz
[Co(dmgH)2Py]2-µ-m xylylene
-1.04
-1.3
-1.28
-1.27
-1.3
0.842
1.09
0.917
1.01
0.879
1.05
1.023
1.203
this work
this work
30
30
5b
-0.88
-0.887
0.98
1.34
through pyrazine.30 The presence of two oxidative potentials
in the present systems suggests that one cobaloxime unit affects
the other cobaloxime unit in terms of its electrochemical
behavior, and this occurs via the bridging biphenyl rings.
Since the X-ray structural parameters in 3a and 7a show that
each cobaloxime unit behaves as an independent benzyl
cobaloxime, the electrochemical data in the dicobaloximes
should resemble the corresponding benzyl cobaloximes. How-
ever a comparison of the data with the benzyl cobaloximes
shows that the dicobaloximes are more difficult to oxidize, and
unlike the BnCo(dioxime)2Py, they do not show any reversible
wave in the oxidation half. This difference further suggests the
transmission of the effect of one cobaloxime unit on the other.
Again the monocobaloxime 5b shows an irreversible oxidation
wave, whereas 2b shows a single reversible oxidation wave just
like in the simple benzyl cobaloximes. However, the oxidation
is more difficult compared to the BnCo(dioxime)2Py.
Conclusion
4,4′-Disubstituted biphenyl-bridged dicobaloximes have been
synthesized. Two crystal structures, reported for the first time, show
two distinct cobaloxime units, Co(dioxime)2Py, bridged by biphe-
nyl, that can be looked at as two independent benzyl cobaloximes.
The two cobaloxime units are symmetrical and have no direct or
indirect interaction between them. The NMR and electrochemical
study, however, show it otherwise. The two cobalt centers are
oxidized at different potentials as two discrete units. Also the
oxidation of the cobalt centers becomes more difficult by the
substitution of electron-withdrawing groups at the 4,4′-position in
the biphenyl. The cobalt-bound CH2 is diastereotopic and the J
1
value in the H NMR corresponds to the geminal coupling. The
dioxime protons, dmgH(Me), also show nonequivalence in a 1:1
ratio. The chemical shift of one of the CH2 protons is affected
by the electron-withdrawing groups at the 4,4′-position in biphenyl.
The NMR data suggest that the dicobaloximes are more rigid than
the methyl-substituted biphenyl monocobaloxime and the orienta-
tion of two [Co] centers in the dicobaloximes is fixed in such a
(30) Mandal, D.; Gupta, B. D. J. Organomet. Chem. 2005, 690, 3746.
(31) Bresciani, P.; Nevenka; Randaccio, L.; Zangrando, E.; Antolini,
L. Acta Crystallogr. Part C 1988, 44, 2052.

Biphenyl-Bridged Dicobaloximes
Organometallics, Vol. 27, No. 4, 2008 601
Supporting Information Available: Table of CHN analysis
data, 13C NMR, yields of mono- and dicobaloximes under different
conditions, representative figures, and the CIF files for X-ray crystal
structures of 3a and 7a. This material is available free of charge
via the Internet at http://pubs.acs.org.
way as to eliminate or partly reduce the steric interactions between
the two cobaloxime units. The nonequivalence may occur due to
atropisomerism and/or restricted rotation of the C-Ph/Co-C bond.
Acknowledgment. We thank DST (SR/S1/IC-12/2004)
New Delhi, India, for financial support. We thank Dr. D.
Mandal for helpful discussions.
OM7009045