Synthesis, characterization, and variable-temperature 1H NMR behavior of organo-bridged dicobaloximes

Article abstract of DOI:10.1021/om034273p

Organo-bridged dicobaloximes with four different dioximes Py(L) ₂CoCH₂-R-CH₂Co(L)₂Py (L = dmgH, dpgH, chgH, and gH) have been synthesized and characterized by ¹H and ¹³C NMR and FAB mass sp

Full text of DOI:10.1021/om034273p

Organometallics 2004, 23, 2069-2079
2069
Syn th esis, Ch a r a cter iza tion , a n d Va r ia ble-Tem p er a tu r e
1H NMR Beh a vior of Or ga n o-Br id ged Dicoba loxim es†
B. D. Gupta,* V. Vijaikanth, and Veena Singh
Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, India
Received October 31, 2003
Organo-bridged dicobaloximes with four different dioximes Py(L)2CoCH2-R-CH2Co(L)2Py
1
(L ) dmgH, dpgH, chgH, and gH) have been synthesized and characterized by H and 13C
NMR and FAB mass spectroscopy. The cis influencing order observed in dicobaloximes is
similar to the previously observed order in monocobaloximes. The cyclic voltammetric results
show that an irreversible single-step two-electron reduction of CoIII to CoI takes place. The
Co-C bond in 4a cleaves during crystallization and results in the formation of o-vinylbenzyl
cobaloxime. The variable-temperature 1H NMR study suggests that the Co-C bond rotation
is restricted and its magnitude depends on both the nature of the bridging ligand and the
dioxime.
In tr od u ction
appeared on the polymethylene-bridged dicobaloximes,7,8
organo-bridged rhodoximes,9 and ligand-bridged dicobal-
oximes10,11 and cobaloximes leading to self-assembly.12
Organo-bridged dicobaloximes with dioximes other
than dmgH are virtually unknown.8 Here we report the
synthesis and characterization of biphenyl- and xy-
lylene-bridged dicobaloximes with four different di-
oximes (dmgH, dpgH, chgH, and gH). [dmgH ) dimethyl
glyoxime; dpgH ) diphenyl glyoxime; chgH ) 1,2-
cyclohexanedione dioxime (nioxime); gH ) glyoxime.]
The synthesis and organometallic chemistry of orga-
nocobaloximes have been investigated quite extensively
ever since Schrauzer first highlighted their importance
as models for coenzyme B12.1 In view of the inherent
weak Co-C bond, organocobaloximes are known to
catalyze a variety of chemical processes and provide
excellent handle in the form of a reactive Co-C bond.2
They have been utilized in organic synthesis2c,d,3 and
in polymer chemistry.4
Although numerous examples of novel mononuclear
organocobaloximes have been described,5 there are only
a few reports on organo-bridged dicobaloximes. The
earliest examples of such complexes were reported by
Schrauzer6a and J ohnson,6b but no spectral character-
ization was described. Recently, a few reports have
1
The variable-temperature H NMR behavior of organo-
bridged dicobaloximes is also reported.
The character of the Co-C bond has been the center
of much speculation and study since the discovery that
coenzyme B12 contains such a linkage. The weakening
of the Co-C bond in cobaloximes RCo(L)2B has been
interpreted as a function of steric and electronic proper-
ties of R, L, and B ligands.1e,g,h The Co-C bond is
weakened by bulky R, flexible L, and bulky and electron-
donating B ligands. Apart from structural properties,
trends in the spectroscopic, thermodynamic, and kinetic
properties as a function of steric and electronic proper-
ties of the axial ligands R and B have been qualitatively
interpreted.13 Among the various spectroscopic tech-
niques, 1H NMR spectroscopy has been found to be
* Corresponding author. Tel: +91-512-2597046. Fax: +91-512-
2597436. E-mail: bdg@iitk.ac.in.
† Dedicated to Dr. Animesh Chakravorty, Indian Association of
Science, Kolkata, India.
(1) (a) Schrauzer, G. N.; Kohnle, J . Chem. Ber. 1964, 97, 3056. (b)
Schrauzer, G. N. Angew. Chem., Int. Ed. Engl. 1976, 15, 417. (c) Dodd,
D.; J ohnson, M. D. J . Organomet. Chem. 1973, 52, 1. (d) Toscano, P.
J .; Marzilli, L. G. Prog. Inorg. Chem. 1985, 31, 105. (e) Bresciani-Pahor,
N.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.; Summers, M. F.;
Toscano, P. J . Coord. Chem. Rev. 1985, 63, 1. (f) Gupta, B. D.; Roy, S.
Inorg. Chim. Acta 1988, 146, 209. (g) Randaccio, L.; Bresciani-Pahor,
N.; Zangrando, E.; Marzilli, L. G. Chem. Soc. Rev. 1989, 18, 225. (h)
Randaccio, L. Comments Inorg. Chem. 1999, 21, 327.
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1999, 20, 97. (d) Welker, M. E. Curr. Org. Chem. 2001, 5, 785.
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(c) Uetake, T.; Nishikawa, M.; Tada, M. J . Chem. Soc., Perkin Trans.
1 1997, 3591. (d) Stokes, H. L.; Welker, M. E. Organometallics 1996,
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Roberts, G. E.; Heuts, J . P. A.; Davis, T. P. Macromolecules 2000, 33,
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molecules 1996, 29, 481.
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Organometallics 2003, 22, 2670. (b) Gupta, B. D.; Yamuna, R.; Singh,
V.; Tiwari, U. Organometallics 2003, 22, 226. (c) Coogan, M. P.;
Stanton, L. S.; Walther, T. J . Organomet. Chem. 2003, 677, 125. (d)
Llop, J .; Vinas, C.; Teixidor, F.; Victori, L. J . Chem. Soc., Dalton Trans.
2002, 1559. (e) Gupta, B. D.; Singh, V.; Qanungo, K.; Vijaikanth, V.;
Yamuna, R.; Barclay, T.; Cordes, W. J . Organomet. Chem. 2000, 602,
1.
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Soc., Perkin Trans. 2 1972, 311.
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Chem. 2003, 42, 612.
10.1021/om034273p CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/24/2004

2070 Organometallics, Vol. 23, No. 9, 2004
Gupta et al.
against 4622 data to give Rw(F2) ) 0.1182 and S ) 1.188 for
weights of w ) 1/[σ2(Fo2) + (0.1505P)2 + 4.734P] where P )
useful for the study of cis and trans influence in
cobaloximes.1d,e We wish to find out (a) if the trends in
the spectroscopic properties, found previously in mono-
cobaloximes, can be extended to dicobaloximes as well
as (b) if the bridging organic group has any effect on
the redox potential of the metal centers and (c) whether
the variable-temperature 1H NMR study gives any
information on the Co-C bond rotation.
2
[Fo2 + 2Fc ]/3. The final R(F) was 0.0909 for the 3581 observed,
[I > 2σ(I)]. The largest shift/su was 0.368 in the final
refinement circle. The final difference map had maxima and
minima of 0.939 and -0.720 e Å-3
.
Syn th esis of Or ga n o-Br id ged Dicoba loxim es. Dico-
baloximes have been synthesized by two methods (methods A
and B) by a slight modification of the literature methods.
Meth od A. One milliliter of aqueous sodium hydroxide (1
pellet in 2 mL of water) was added to a suspension of ClCo-
(L)2Py (1 mmol) in methanol (30 mL). The reaction mixture
was purged with argon for 0.5 h, and a deaerated aqueous
solution of sodium borohydride (0.04 g, 1 mmol in 0.5 mL of
water) was added. The solution was cooled to 0 °C, and 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 5 h, during which the solution
became yellow-orange. The reaction mixture was poured into
100 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.
Meth od B. CoCl2‚6H2O (0.95 g, 4 mmol) and dimethylgly-
oxime (0.94 g, 8 mmol) were stirred in methanol (20 mL), and
dry nitrogen was passed through the mixture for 0.5 h. An
aqueous solution of sodium hydroxide (ca. 0.32 g, 2 mL, 8
mmol) was added to the mixture, followed by pyridine (0.32 g,
3.2 mL, 4 mmol). The mixture was cooled to 0 °C, and aqueous
sodium hydroxide (ca. 0.40 g, 2 mL, 10 mmol) was added. A
deep blue solution of cobaloxime(I) was formed. After 10 min
the appropriate organic dihalide (1 mmol in 2 mL of diethyl
ether) was added dropwise to the reaction mixture. The color
changed from blue to red immediately. The reaction mixture
was stirred for 5 h and poured into 100 mL of ice-cold water
containing a few drops of pyridine. The orange-yellow precipi-
tate 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.
Sep a r a tion of P r od u cts. (Since dicobaloximes decompose
in solution and in the presence of visible light within a few
hours, the rate of elution was kept faster. The column
chromatography, evaporation of solvent, and crystallization
were carried out in diffused light.) dmgh/chgH/gH complexes:
The crude product containing a mixture of monocobaloxime
and organo-bridged dicobaloxime was dissolved in a minimum
amount of chloroform and was loaded on a silica gel column,
pre-eluted with chloroform. The polarity of the solvent was
carefully increased with an ethyl acetate/chloroform mixture
(10-40%) until an orange-red band corresponding to mono-
cobaloxime was distinctly visible. This band was completely
eluted out with ethyl acetate/chloroform (80:20). The dico-
baloxime was eluted out with 100% ethyl acetate and an ethyl
acetate/methanol (90:10) mixture.
Exp er im en ta l Section
Cobalt chloride hexahydrate, dimethylglyoxime (Merck
India), diphenylglyoxime, 1,2-cyclohexanedione dioxime, gly-
oxime, 2,2′-bis(bromomethyl) biphenyl, o-, m-, and p-xylylene
dibromide, thiophene, and 2,5-furan dimethanol (Fluka or
Aldrich) were used without further purification. 2,5-Bis-
(chloromethyl)thiophene,14 2,5-bis(chloromethyl)furan,15 and
ClCo(L)2Py16-18 were synthesized according to the literature
reports. 1H NMR and 13C NMR spectra were recorded on a
J EOL-J NM LAMBDA 400 model spectrometer in CDCl3 using
TMS as internal reference. FAB mass spectra were recorded
on a J EOLSX 102/DA-6000 data system. Elemental analysis
was carried out using a Thermoquest CE Instruments CHNS-O
elemental analyzer. Cyclic voltammetric studies were per-
formed on a PAR model 273A polarographic analyzer in CH2-
Cl2 containing 0.1 M tetrabutylammonium hexafluorophos-
phate as the supporting electrolyte.
X-r a y Str u ctu r a l Deter m in a tion a n d Refin em en t. The
details regarding the data collection for the single-crystal X-ray
elucidation of compound 20, the cleaved product during the
crystallization of 4a , are given below. A reddish orange crystal
with dimensions 0.50 × 0.30 × 0.12 mm3 was selected for the
structural analysis with the aid of the microscope. Intensity
data were collected using a Mercury CCD-AFC8 instrument
at 293 K. Graphite-monochromated Mo KR radiation (λ )
0.7107 Å) was used. Coverage of unique data was 80.1%
complete to 28.57° in θ. A total of 4623 data were measured
in the range 1.90° < θ < 28.57°. The data were collected for
absorption by the empirical method, giving minimum and
maximum transmission factors of 0.68 and 1.00. The data were
merged to form a set of 4622 independent data with R(int) )
0.038. The monoclinic space group P21/n was determined by
statistical tests and verified by subsequent refinement. The
structure was solved by direct methods using the SHELXS-
97 program19a and refined by full-matrix least-squares methods
on F2 using SHELXL-97.19b Hydrogen atom positions were
initially determined by geometry and refined by a riding model.
Non-hydrogen atoms were refined with anisotropic displace-
ment parameters. A total of 293 parameters were refined
(13) (a) Brown, K. L.; Lyles, D.; Pencovici, M.; Kallen, R. G. J . Am.
Chem. Soc. 1975, 97, 7338. (b) Marzilli, L. G.; Bayo, F.; Summers, M.
F.; Thomas, L. B.; Zangrando, E.; Bresciani-Pahor, N.; Mari, M.;
Randaccio, L. J . Am. Chem. Soc. 1987, 109, 6045. (c) Brown, K. L.;
Satyanarayana, S. J . Am. Chem. Soc. 1992, 114, 5674. (d) Datta, D.;
Sharma, G. T. J . Chem. Soc., Dalton Trans. 1989, 115. (e) Pahor, N.
B.; Geremia, S.; Lopez, C.; Randaccio, L.; Zangrando, E. Inorg. Chem.
1990, 29, 1043. (f) Randaccio, L.; Geremia, S.; Zangrando, E.; Ebert,
E. Inorg. Chem. 1994, 33, 4641. (g) Drago, R. S. J . Organomet. Chem.
1996, 512, 61.
(14) Griffing, J . M.; Salisbury, L. F. J . Am. Chem. Soc. 1948,
70, 3416.
(15) Timko, J . M.; Moore, S. S.; Walba, D. M.; Hiberty, P. C.; Cram,
D. J . J . Am. Chem. Soc. 1977, 99, 4207.
dpgH Complexes: The polarity of the solvent was carefully
increased with an ethyl acetate/chloroform (2-4%) mixture
when the orange band corresponding to monocobaloximes was
eluted out. The dicobaloxime was eluted out with an ethyl
acetate/chloroform (10:90) mixture.
In certain cases, the dicobaloximes were purified by crystal-
lization from a chloroform/hexane mixture. The dicobaloximes
(16) Schrauzer, G. N. Inorg. Synth. 1968, 11, 61.
(17) 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.
(18) (a) Lo´pez, C.; Alvarez, S.; Aguilo, M.; Solans, X.; Font-Altaba,
M. Inorg. Chim. Acta 1987, 127, 153. (b) Toscano, P. J .; Swider, T. F.;
Marzilli, L. G.; Bresciani-Pahor, N.; Randaccio, L. Inorg. Chem. 1983,
22, 3416.
(19) (a) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Analysis (release 97-2); University of Gottingen: Gottingen, Germany,
1997. (b) Sheldrick, G. M. SHELXTL-97, Version 5.03; Siemens
Analytical X-ray Division: Madison, WI, 1994.
1
(Figure 1) were characterized by the elemental analyses, H
and 13C NMR, and FAB mass spectroscopy (Tables 2-5). The
monocobaloximes were characterized by 1H NMR spectroscopy
(Table 6). The yields of cobaloximes are given in Table 1.
Resu lts a n d Discu ssion
Syn th esis. Although organocobaloximes have been
synthesized by several methods, the oxidative alkylation

Synthesis of Organo-Bridged Dicobaloximes
Organometallics, Vol. 23, No. 9, 2004 2071
F igu r e 1. Structures of di- and monocobaloximes.
Ta ble 1. Yield of Di- a n d Mon ocoba loxim es (by
m eth od A)
Ta ble 2. Elem en ta l An a lysis Da ta for
Dicoba loxim es 1a -15a
L ) dmgH
1a (40), 1b (12) 5a (40)a
2a (55), 2b (26) 6a (69), 6b (10) 10a (19)
3a (30), 3b (30) 7a (31)
4a (30), 4b (30) 8a (31)
L ) dpgH
L ) chgH
L ) gH
13a (15)
14a (13), 14b (2)
11a (80)b 15a (31)b
12a (19)
C found
(calcd)
H found
(calcd)
N found
(calcd)
no.
formula
9a (35)
1a
2a
3a
4a
5a
6a
7a
8a
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
40H50Co2N10O8 52.43 (52.41) 5.44 (5.49) 15.24 (15.28)
34H46Co2N10O8 48.56 (48.58) 5.55 (5.52) 16.66 (16.66)
34H46Co2N10O8 48.57 (48.58) 5.55 (5.52) 16.63 (16.66)
34H46Co2N10O8 48.54 (48.58) 5.56 (5.52) 16.63 (16.66)
16 (36)
18 (74)
17 (12)
19 (18)
80H66Co2N10O8 67.95 (67.99) 4.74 (4.71)
9.94 (9.91)
74H62Co2N10O8 66.44 (66.47) 4.64 (4.67) 10.43 (10.47)
74H62Co2N10O8 66.43 (66.47) 4.64 (4.67) 10.44 (10.47)
74H62Co2N10O8 66.46 (66.47) 4.65 (4.67) 10.49 (10.47)
48H58Co2N10O8 56.50 (56.47) 5.78 (5.73) 13.69 (13.72)
42H54Co2N10O8 53.37 (53.39) 5.75 (5.76) 14.85 (14.82)
42H54Co2N10O8 53.35 (53.39) 5.74 (5.76) 14.83 (14.82)
42H54Co2N10O8 53.35 (53.39) 5.74 (5.76) 14.83 (14.82)
32H34Co2N10O8 47.75 (47.77) 4.20 (4.26) 17.42 (17.41)
26H30Co2N10O8 42.83 (42.87) 4.18 (4.15) 19.25 (19.23)
26H30Co2N10O8 42.81 (42.87) 4.12 (4.15) 19.27 (19.23)
a
Mixture of monocobaloximes are obtained as the side products.
Purified by fractional recrystallization.
b
9a
of cobaloxime(I) (method A, Scheme 1) and Schrauzer’s
disproportionation method (method B) find a much
wider use in the synthesis. The major product is the re-
quired organo-bridged dicobaloxime (1a -15a ) [dmgH
(1a -4a ), dpgH (5a -8a ), chgH (9a -12a ), and gH (13a -
15a ) (Figure 1)], and monocobaloxime is obtained as the
side product in a few cases (Table 1). The yield of dico-
baloxime is always higher in method A than in method
10a
11a
12a
13a
14a
15a
B. In method A, methyl-substituted benzyl cobaloxime
and/or halomethylbenzyl cobaloxime is the side product,

2072 Organometallics, Vol. 23, No. 9, 2004
Ta ble 3. 1H NMR Sp ectr oscop ic Da ta for Dicoba loxim es in CDCl3
Gupta et al.
pyridine
no.
CH2Co
L
OH‚‚‚O
R (d)
â (t)
γ (t)
others
1a
2.75 (d) (6.4),
3.02 (d) (6.4)
2.78
1.78, 1.91
17.76
8.37 (4.8)
a
7.57 (3.8)
6.49 (d) (7.2), 6.89 (d) (6.8),
7.13-7.31 (m)
2a
3a
4a
1.92
1.90
1.91, 2.01
18.16
18.23
8.37 (5.2)
8.52 (5.2)
8.51 (4.8)
7.26
7.27
7.23 (6.4)
7.66 (7.6)
7.65 (6.8)
7.63(7.2)
6.32 (s), 6.70 (t) (7.6), 6.86 (d) (7.2)
6.62
2.75
2.60 (d) (6.0),
3.03 (d) (6.4)
3.44 (d) (6.0),
3.74 (d) (5.6)
6.65 (d) (4.0), 6.78 (d) (4.0)
5a
6.69 (d) (7.2),
6.91 (d) (6.8),
7.08-7.18 (m)
6.96 (d) (6.4)
6.95 (d) (6.8),
7.15-7.25 (m)
6.91 (d) (6.8),
6.97 (d) (6.8)
1.37-1.56 (m),
2.23-2.34 (m),
2.50
18.40
8.74 (5.2)
7.32 (6.8)
7.73 (7.2)
7.05, 7.45 (t) (4.4), 7.55 (7.2)
6a
7a
3.45
3.40
18.74
18.70
8.87 (5.2)
8.84 (5.2)
7.39 (6.8)
7.39 (6.8)
7.79 (8.0)
7.79 (7.2)
6.83 (t) (7.6), 7.45a
a
8a
9a
3.23 (d) (6.8),
4.01 (d) (6.4)
2.69, 3.12
18.79
8.82 (4.8)
8.38
7.35 (6.8)
7.74 (6.8)
7.60 (8.0)
7.19-7.29 (m)
7.28-7.31 (m)
6.57 (d) (8.0), 6.91-6.95 (m), 7.18
10a
2.86
1.40-1.50 (m),
17.75
17.78
8.54 (6.4)
7.27 (6.0)
7.68 (8.0)
6.57, 6.72 (t) (7.6), 6.89 (d) (6.4)
2.45 (s)
11a
12a
2.79
1.43, 2.45
1.44-1.60 (m),
2.43
8.53 (5.2)
8.52 (4.8)
7.27 (6.8)
7.26 (6.4)
7.68 (8.0)
7.66 (7.2)
6.70
2.69 (d) (6.8),
3.10 (d) (6.4)
2.94, 3.36
2.96
6.74 (d) (3.6), 6.83 (d) (3.6)
13a
14a
15a
a
8.40 (4.8)
8.53 (4.8)
8.52
a
7.65 (7.2)
7.75 (7.2)
7.74
6.99-7.13 (m)
6.74-6.79 (m), 6.98 (d) (7.6)
6.77
7.27
7.40
17.52
17.60
7.34 (7.2)
a
2.88
a
Merged with aromatic protons.
Ta ble 4. 13C NMR Sp ectr oscop ic Da ta for Dicoba loxim es in CDCl3
no.
CH2Co
CdN
CPyR
CPyγ
aromatic
others
1a
2a
3a
4a
5a
29.7
31.1
32.0
30.5
30.6
149.1, 148.6
149.2
149.2
149.6, 149.2
151.1, 150.5
150.0
150.3
150.2
150.2
149.9
137.1
137.3
137.2
137.1
137.7
124.6, 124.9, 125.3, 128.0, 133.4, 141.5, 143.6
125.1, 125.6, 127.0, 128.0, 145.9
125.0, 128.0, 143.4
11.4, 12.1
11.9
11.8
123.6, 125.0, 130.2, 145.2
11.7, 12.0
125.2, 125.5, 127.0, 127.5, 128.6, 129.3, 129.6,
129.8, 130.2, 133.5, 141.8, 144.0,
125.5, 126.1, 127.6, 128.7, 129.8, 130.0, 146.8
125.4, 127.6, 128.8, 129.2, 129.7, 130.0, 144.4
125.2, 125.3, 127.4, 127.5, 127.8, 128.4, 129.8,
130.0, 130.2, 131.5, 145.3, 149.9
124.4, 124.8, 125.1, 127.6, 133.4, 141.4, 144.0
125.1, 125.6, 127.0, 128.8, 146.3
125.0, 128.3, 143.6
6a
7a
8a
34.9
36.0
33.0
150.9
151.0
151.3, 150.9
150.2
150.1
150.1
137.8
137.8
137.6
9a
30.0
30.6
32.0
30.4
32.8
149.9
150.3
150.2
150.4
138.4
149.6
150.1
150.1
150.1
149.9
137.0
137.3
137.3
137.1
137.9
21.4, 24.6, 25.2
21.3, 25.2
21.3, 25.1
21.4, 24.9, 25.1
-
10a
11a
12a
15a
123.8, 124.9, 130.3, 145.4
125.5, 128.2, 143.3
as reaction time, solvent, dihalide:CoI ratio, and the
nature of the dihalide. For example, (a) 2a is obtained
in 47% yield in method A when the reaction time is 5
min; however it falls to 13% if the reaction time is longer
(>5 h). (b) When the ratio of m-xylylene dibromide:CoI
is 3:1, monocobaloxime 2b is the major product (∼85%).
However with a 1:3 ratio, no improvement in the yield
of dicobaloxime is observed and at the same time the
separation of dicobaloxime from the other cobalt-
containing products is difficult. (c) When acetonitrile is
used as the solvent instead of methanol, precipitation
of dicobaloxime does not occur on pouring the reaction
mixture in water, which makes the separation of dico-
baloxime difficult. The overall observations suggest that
the best conditions for preparing the organo-bridged
dicobaloximes are by method A in methanol at 0 °C with
a dihalide:CoI ratio of 1:2 with the reaction time of 5 h.
Ta ble 5. Ma ss Sp ectr oscop ic Da ta for
Dicoba loxim es 1a -4a
compound no.
principal peaks (relative intensity)
1a
M 917 (15), 838 (38), 758 (67), 469 (38),
289 (63), 154 (100)
2a
3a
4a
M 841 (37), 762 (71), 645 (86), 566 (100),
463 (25), 393 (26), 289 (77)
M 841 (30), 762 (25), 658 (73), 645 (100),
566 (81), 463 (65), 289 (60)
M 841 (14), 762 (20), 658 (27), 463 (46),
393 (100), 289 (56)
whereas halomethyl benzyl cobaloxime is formed in
method B. Heteroaromatic bridged dicobaloximes
(X ) S, O) are not formed in either of the methods; only
methyl-substituted monocobaloximes are obtained (16-
19). All our efforts failed to prepare o-xylylene-bridged
dicobaloxime with glyoxime.
While optimizing the reaction conditions, we find that
the yield of dicobaloxime depends on many factors such
The yield of dicobaloxime with various dioximes
follows the order dpgH ≈ dmgH > chgH > gH (Table
1). The overall yield of dicobaloximes is found to be
better when these are purified by fractional crystalliza-
tion rather than by column chromatography. Dico-
baloximes are orange-red in color. The solids are stable

Synthesis of Organo-Bridged Dicobaloximes
Organometallics, Vol. 23, No. 9, 2004 2073
Ta ble 6. 1H NMR Sp ectr oscop ic Da ta for Mon ocoba loxim es in CDCl3
pyridine
no.
CH2Co
L
OH‚‚‚O
R (d)
â (t)
γ (t)
others
1b
2.66 (d) (6.8) 3.04 (d) (7.2) 1.87, 1.92
17.94
8.43 (5.2)
a
7.62 (7.6) 2.10, 6.72 (d) (7.6), 6.90-6.96 (m),
7.16-7.22 (m)
3b
2b
4b
2.85
2.83
2.95
1.94
1.94
1.96
18.25
18.23
18.35
8.54 (6.0)
8.54
a
a
7.68 (7.2) 2.08, 6.81-6.89 (m)
7.68 (6.4) 2.29, 6.78-6.93 (m)
8.55 (4.8) 7.28 (6.8) 7.68 (7.2) 2.11, 6.82 (d) (3.6), 6.89 (d) (7.2),
7.02-7.06 (m)
6b
3.37
6.93 (d) (8.0),
7.16-7.24 (m)
7.24
18.71
8.55 (4.8) 7.42
7.82 (7.2) 4.43, 7.12 (d) (7.6)a
14b
16
17
2.99
2.97
3.37
17.64
18.25
18.64
8.54 (6.0) 7.34 (6.8) 7.75 (7.6) 2.30, 6.92-7.09 (m)
8.54 (6.4) 7.28 (6.0) 7.69 (7.6) 2.17, 6.38, 6.53
8.83 (5.2) 7.36 (7.2) 7.83 (8.0) 2.21, 6.73, 6.92
2.04
7.03 (d) (8.0),
7.17-7.31 (m)
2.06
7.06 (d) (8.0),
7.18-7.28 (m)
18
19
2.71
3.31
18.23
18.70
8.55 (5.2) 7.28 (7.2) 7.69 (7.2) 2.01, 5.73, 5.93
8.86 (5.2) 7.40 (6.4) 7.79 (7.2) 2.01, 5.86, 6.25
a
Merged with aromatic protons.
Sch em e 1
under nitrogen atmosphere. However, decomposition
occurs in solution when exposed to air.
(c) O-H‚‚‚O. In general, the downfield shift of the
O-H‚‚‚O resonance follows the order p-xylylene > m-
xylylene > biphenyl. It is known that the O-H‚‚‚O reso-
nance appears downfield as the electron-donating power
of the substituent in the axial ligand increases. This can
be explained on the basis of electron density in the
Ch a r a cter iza tion . 1H NMR Sp ectr a . In the 1H
NMR spectra of dicobaloximes, Co-CH2, dioxime,
O-H‚‚‚O, pyridine, and the bridging phenyl ring hy-
drogens are clearly distinguished.
+
Co(L)2 metallobicycle.20 Electron-donating groups in-
(a ) Co-CH2. The CH2 protons appear as a singlet in
m- and p-xylylene-bridged dicobaloximes, and it occurs
slightly downfield in the meta isomer as compared to
the para isomer. However, it appears as a double
doublet in biphenyl- and o-xylylene-bridged dicobaloximes
(Table 3), suggesting that both the hydrogens in each
CH2 group are in different environments. The non-
equivalence is caused by the steric crowding in these
molecules. The coupling constants correspond to the
geminal coupling. It is also noted that out of the two
hydrogens only the hydrogen appearing downfield is
affected by the restricted rotation.
The chemical shift difference between the geminal
protons is larger in o-xylylene-bridged dicobaloximes
(172 Hz for 4a , 312 Hz for 8a , 164 Hz for 12a ) when
compared with biphenyl-bridged dicobaloximes (108 Hz
for 1a , 120 Hz for 5a , 172 Hz for 9a ). The large
difference in 8a is due to two bulkier CH2Co(dpgH)2Py
groups present ortho to each other. This makes the
geminal protons of Co-CH2 appear far from each other.
The chemical shifts of CH2 protons follow the order
dpgH > gH > chgH > dmgH (Table 3).
crease the electron density in the metallobicycle, and the
concomitant increase in the electron density on the oxy-
gen increases the strength of the hydrogen bond, there-
by causing the resonance to occur at lower field. The
downfield shift of the O-H‚‚‚O resonance follows the
order dpgH > dmgH > chgH > gH. In general, the
O-H‚‚‚O peak in biphenyl-bridged cobaloximes appears
0.4 ppm upfield compared to the other dicobaloximes of
the same series.
(d ) P yr id in e. Pyridine protons are clearly distin-
guished except in a few cases where Pyâ protons merge
with the bridging phenyl ring. The chemical shifts of
Pyâ and Pyγ protons follow the order dpgH > gH > chgH
> dmgH. The same order was observed for Co-CH2
protons. This shows that the equatorial dioximes affect
the properties of both axial ligands (R and B) in the
same order. No regular trend is observed for PyR
protons. The coordination shifts ∆δ (δcomplex - δPy) in
pyridine have been used to measure the cis-influence
of the equatorial ligand.17 The coordination shift in PyR
(∆1HPyR) (Supporting Information) in dpgH complexes
is large and opposite in sign when compared with other
dioximes. The coordination shifts ∆1HPyâ and ∆1HPyγ
follow the order dpgH > gH > chgH g dmgH. A similar
order has been observed earlier in monocobaloxime.17
(b) Dioxim e. DmgH methyl groups appear as a sing-
let in 2a and 3a , whereas two singlets are observed in
biphenyl- (1a ) and o-xylylene (4a )-bridged dicobaloxime.
The chemical shift difference between the two peaks is
slightly higher in 1a (52 Hz) as compared to 4a (40 Hz).
The dmgH methyls in xylylene-bridged dicobaloximes
appear upfield when compared to polymethylene-
bridged dicobaloximes.7,8
(20) Gilaberte, J . M.; Lo´pez, C.; Alvarez, S.; Font-Bardia, M.; Solans,
X. New J . Chem. 1993, 17, 193.

2074 Organometallics, Vol. 23, No. 9, 2004
Gupta et al.
1
13C NMR Sp ectr a . In comparison to H NMR, only
a few studies on the 13C NMR spectra of cobaloximes
have been reported.1e,7 The 13C resonances of dioxime
[dmgH (Me), chgH (C1 and C2), dpgH (phenyl ring
carbons)], Co-CH2, Pyâ, and Pyγ are assigned on the
basis of chemical shifts, and these assignments are
consistent with those of the related and previously
described monocobaloximes.1e,17 Dioxime CdN and PyR
carbons appear very close to each other at around 150.0
ppm. The unambiguous assignment has been made
using DEPT spectra (Supporting Information).
(a ) Co-CH2. CH2 bound to Co appears as a low-
intensity peak because of the coupling of C with 59Co.
The latter has a spin of 7/2 and a natural abundance of
100%.7 The chemical shift of CH2 depends both on the
dioxime and on the bridging aromatic ring, and it follows
the order dpgH > gH > chgH ≈ dmgH (Table 4);
p-xylylene > m-xylylene > o-xylylene > biphenyl bridged
dicobaloxime.
(b) Dioxim e. Two peaks are observed for the oximinic
carbon in the biphenyl (1a , 5a , and 9a ) and o-xylylene
(4a , 8a and 12a ) bridged dicobaloximes. The downfield
chemical shift of CdN(oximinic) follows the order dpgH >
chgH > dmgH . gH. The higher downfield shift in dpgH
complexes in comparison to other dioximes can be
explained on the basis of electron-donating ability of the
equatorial group.20 The CdN(oximinic) carbon appears
downfield and the DmgH methyl appears upfield in
xylylene-bridged dicobaloximes when compared with
polymethylene-bridged dicobaloximes.7
(c) P yr id in e. The resonance of the γ-C of pyridine
seems to be more affected by the nature of the dioxime.
The coordination shift for all the dioximes follows the
order ∆13CPyγ > ∆13CPyR (Supporting Information). The
coordination shift follows the order dpgH ≈ gH > chgH
≈ dmgH. PyR appears downfield in xylylene-bridged
dicobaloximes when compared with polymethylene-
bridged dicobaloximes.7
Ma ss Sp ectr a . The FAB mass spectra have been
recorded for dicobaloximes 1a -4a . The spectra show a
very prominent molecular ion peak and a regular
fragmentation pattern (Table 5).
Electr och em ica l Stu d ies. The cyclic voltammogram
of the alkylcobalt complexes becomes complicated due
to axial base and Co-C bond cleavage upon reduction1d
and hence have received little attention. Finke and co-
workers found that the reduction of alkylcobaloximes
was irreversible under all conditions of solvents and
scan rate,21 whereas Le Hoang et al. found that these
could be reversibly reduced in DMSO solution.22
In general, the cyclic voltametric studies on co-
baloximes describing the entire redox process are few.23,24
We have carried out CV studies on 1a -4a to find out
the effect of the bridging xylylene group on the redox
potential of the metal centers. The cyclic voltammo-
grams are given in Figure 2, and the redox potential
data are given in Table 7. In the reductive half, only
F igu r e 2. Cyclic voltammograms of dicobaloximes 1a -
4a in CH2Cl2 with 0.1 M (NBun4)PF6 as supporting elec-
trolyte at 0.1 V s-1 at 25 °C.
Ta ble 7. Cyclic Volta m m etr ic Da ta for 1a -4a in
CH2Cl2 a t 0.1 V s-1 a t 25 °C
E
(V)
pc(I)
E
pc(II) ∆Epc(II)
(V) (mV)
Epc(III) ∆Epc(III)
compond no.
(V)
(mV)
1a
2a
3a
4a
-1.384 irrev 0.806 irrev
-1.308 irrev 0.952 94
-1.412 irrev 0.916 96
-1.304 irrev 0.756 irrev
1.398
104
1.172
1.374
108
152
one irreversible peak, in the range of -1.30 to -1.41 V,
corresponding to a single-step two-electron reduction of
CoIII to CoI is observed in all cases. This is similar to
the results by Finke21 and Costa.25 These results show
that both the metal centers undergo reduction at the
same potential. The shift of 0.1 V toward more positive
value on going from para to meta or ortho isomer
indicates that there is an increased electron density on
the Co atoms in 3a than in 2a or 4a . This means that
the xylyl group is more electron donating in the para
position than in the ortho or meta position. This sup-
ports the NMR data in these complexes. It is also
observed that the ipc value corresponding to CoIII-CoI
reduction is higher in 3a than in 2a or 4a .
(21) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J .
Am. Chem. Soc. 1981, 103, 5558.
(22) Le Hoang, M. D.; Robin, Y.; Devynck, J .; Bied-Charreton, C.;
Gaudemer, A. J . Organomet. Chem. 1981, 222, 311.
(23) Asaro, F.; Dreos, R.; Nardin, G.; Pellizer, G.; Peressini, S.;
Randaccio, L.; Siega, P.; Tauzher, G.; Tavagnacco, C. J . Organomet.
Chem. 2000, 601, 114.
(24) Ngameni, E.; Ngoune, J .; Nassi, A.; Belombe, M. M.; Roux, R.
Electrochim. Acta 1996, 41, 2571.
(25) Costa, G.; Puxeddu, A.; Tavagnacco, C. J . Organomet. Chem.
1985, 296, 161.

Synthesis of Organo-Bridged Dicobaloximes
Organometallics, Vol. 23, No. 9, 2004 2075
Ta ble 8. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for 20
o-vinylbenzylcobaloxime
empirical formula
fw
C21H28CoN5O4
473.41
temperature
wavelength
cryst syst
293(2) K
0.7107 Å
monoclinic
space group
unit cell dimens
P21/n
a ) 9.1495(10) Å
b ) 21.373(3) Å
c ) 11.4878(11) Å
R ) 90°
â ) 93.195(6)°
γ ) 90°
volume
2243.0(5) Å3
Z
4
density(calcd)
abs coeff
1.402 Mg/m3
0.802 mm-1
F(000)
cryst size
θ range for data collection
index ranges
992
0.50 × 0.30 × 0.12 mm3
1.90-28.57°
-11 e h e 11, -28 e k e 27,
-14 e l e 14
4623
F igu r e 3. ORTEP drawing of 20 with 50% displacement
ellipsoid probability. Hydrogen atoms and the disorder in
the vinyl carbon C22 have been omitted for clarity.
no. of reflns collected
no. of indep reflns
completeness to θ ) 28.57°
abs corr
4622 [R(int) ) 0.038]
80.1%
empirical, R. A. J acobson,
MS Corp, 1998
1.00 and 0.68
full-matrix least-squares on F2
4622/0/293
coplanar within (0.03 Å, and the displacement of the
Co atom out of their mean plane is 0.013 Å, whereas it
is 0.037 Å in PhCH2Co(dmgH)2py (21).28 It is possible
that the vinyl group in 20 may have affected the
geometry of the cobaloxime. We have, therefore, com-
pared the structural parameters in 20 with the known
structure 21 (Table 9).
max. and min. transmn
refinement method
no. of data/restraints/params
goodness-of-fit on F2
1.188
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole
R1 ) 0.0909, wR2 ) 0.2621
R1 ) 0.1182, wR2 ) 0.2921
0.939 and -0.720 e‚Å-3
The butterfly-bending angle in 20 is 1.5°, whereas it
is 4.9° in 21. The average Neq-Co-Nax and Neq-Co-
Cax bond angles in 20 are 90.5° and 89.6°, whereas the
same angles in 21 are 91.1° and 88.9°, respectively.
These parameters in 20 suggest that the Co atom is
coplanar with four N atoms of the dioxime. The Co-C
bond length in 20 is 2.11 Å and is longer by 0.05 units
as compared to 21. No difference is observed in the Co-
Npy bond distance. The difference in the structural
parameters in 20 and 21 can be explained by the steric
cis influence caused by the o-vinylbenzyl group in 20.
Since the vinyl group is in the ortho position, bending
of the equatorial plane toward the R group is disfavored,
and the steric interaction between the vinyl group and
the dioxime makes the oxime ligands move away from
the R group. This causes the lengthening of the Co-C
bond (Figure 4). The crystal structure of 20 becomes the
first example of a o-substituted benzyl cobaloxime. A
similar effect is seen when the crystal structures of
i-prCo(dmgH)2Py and i-prCo(dmgH)2(o-NH2Py) are com-
pared.29
Only a few reports have appeared on the oxidation of
cobaloximes, mainly on the aquacobaloximes.23,24,26 In
the present case the oxidation potential value of CoIII
(Epc(II)) is found to be similar to the value in aqua
cobaloximes.26 The oxidation is reversible in 2a and 3a
and is irreversible in 1a and 4a . Ligand oxidation
(Epc(III)) is also observed in 1a , 3a , and 4a at higher
potential. It is also seen that sterically crowded co-
baloximes 1a and 4a behave similarly.
Sin gle-Cr ystal X-r ay Stu dies. All efforts to grow the
single crystals for the dicobaloximes have not been suc-
cessful since these complexes are unstable in solution.
Many solvent systems and methods were tried. We could
get good quality crystal for 4a in a CH2Cl2/hexane mix-
ture. To our surprise, the crystal structure showed the
structure of monocobaloxime 20 having a vinylbenzyl
group. The crystallographic data and important struc-
tural details are given in Table 8. The ORTEP diagram
of 20 is shown in Figure 3. We have made no attempt
to characterize the decomposed organic products.
It is difficult to comment on the origin of the vinyl
group. One possibility is the cleavage of the Co-C bond
followed by the reaction of the resultant radical
The structure shows that the cobaloxime units propa-
gate as a two-dimensional layer through intermolecular
hydrogen bonding. The importance and the nature of
the nonclassical intermolecular hydrogen bonding have
been described by Desiraju.30,31 Three nonclassical
hydrogen bonds are found in the structure. C5, C9, and
C12 act as donors and the dioxime oxygens O2, O3, and
Py(L)2CoIIICH2-C6H4-CH2 with the solvent dichloro-
•
methane to yield Py(L)2CoIIICH2-C6H4-CH2-CH2Cl.
This eliminates HCl to form the observed product.
The crystal structure shows that the cobalt atom
exhibits a distorted octahedral stereochemistry with
four oxime N-donors in the equatorial positions. Pyri-
dine has an orientation similar to that found in RCo-
(dmgH)2Py compounds.27 The four oxime N-donors are
(27) Hirota, S.; Polson, S. M.; Puskett, J . M., J r.; Moore, S. J .;
Mitchell, M. B.; Marzilli, L. G. Inorg. Chem. 1996, 35, 5646.
(28) Pahor, N. B.; Randaccio, L.; Zangrando, E. Acta Crystallogr.
1988, C44, 2052.
(26) Halpern, J .; Chan, M. S.; Hanson, J .; Roche T. S.; Topich, J . A.
J . Am. Chem. Soc. 1975, 97, 1606.
(29) Summers, M. F.; Toscano, P. J .; Bresciani-Pahor, N.; Nardin,
G.; Randaccio, L.; Marzilli, L. G. J . Am. Chem. Soc. 1983, 105, 6259.

2076 Organometallics, Vol. 23, No. 9, 2004
Gupta et al.
F igu r e 4. Schematic representation of the steric cis
influence caused by the interaction of the o-vinylbenzyl
group with the equatorial dioxime moiety in 20.
Ta ble 9. Selected Bon d Len gth s a n d Bon d An gles
for o-Vin ylben zyl Coba loxim e (20) a n d
Ben zylcoba loxim e (21)28
parameter
20
21
Co(1)-C(14)
2.112(5)
1.896(5)
1.881(5)
1.888(5)
1.895(5)
2.058(5)
1.266(8)
1.317(8)
1.296(8)
1.297(8)
1.495(9)
1.541(12)
1.335(19)
1.356(15)
113.9(4)
80.7(2)
98.4(2)
81.3(2)
99.6(2)
178.8(2)
177.6(2)
175.5(2)
88.7(2)
93.0(2)
92.1(2)
2.065(4)
1.875(3)
1.881(3)
1.876(3)
1.879(3)
2.056(3)
1.294(5)
1.290(4)
1.299(5)
1.301(5)
1.474(5)
F igu r e 5. View of a single layer of 20 showing the C-H‚
‚‚O interaction. The o-vinylbenzyl and dioxime carbons C6,
C7, and C8 are not shown for clarity.
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-N(4)
Co(1)-N(5)
N(1)-C(1)
N(2)-C(2)
N(3)-C(3)
N(4)-C(4)
C(14)-C(15)
C(20)-C(21)
C(21)-C(22A)
C(21)-C(22)
F igu r e 6. Repeating structural motif of the supramolecu-
lar two-dimensional network of 20 showing the tubular
arrays A (14-membered ring) and B (12-membered ring).
Co(1)-C(14)-C(15)
N(1)-Co(1)-N(2)
N(2)-Co(1)-N(3)
N(3)-Co(1)-N(4)
N(4)-Co(1)-N(1)
N(1)-Co(1)-N(3)
N(2)-Co(1)-N(4)
N(5)-Co(1)-C(14)
N(1)-Co(1)-C(14)
N(2)-Co(1)-C(14)
N(3)-Co(1)-C(14)
N(4)-Co(1)-C(14)
N(1)-Co(1)-N(5)
N(2)-Co(1)-N(5)
N(3)-Co(1)-N(5)
N(4)-Co(1)-N(5)
C(20)-C(21)-C(22)
C(20)-C(21)-C(22A)
da
116.7(2)
81.4(1)
98.9(1)
81.5(1)
98.2(1)
177.9(2)
177.5(1)
177.1(1)
92.0(1)
88.4(1)
86.0(1)
89.1(1)
90.9(1)
91.9(1)
91.1(1)
90.6(1)
Ta ble 10. C-H‚‚O In ter a ction s in o-Vin ylben zyl
Coba loxim e (20)
C-H
(Å)
H‚‚‚O
(Å)
C‚‚‚ ‚O
(Å)
C-H‚‚‚O
(deg)
symmetry
C5-H5-O3
C9-H9-O2
C12-H12-O4
0.9609
0.9295
0.9297
2.5440
2.4373
2.4088
3.4285
3.1642
3.3013
153.09
135.07
160.88
1+x, y, z
-x, -y, 1-z
1-x, -y, 2-z
84.6(2)
90.0(2)
91.1(2)
89.3(2)
mixed dioxime ligands also a similar interaction has
been observed recently.5a
1
Va r ia ble-Tem p er a tu r e H NMR Stu d ies. 1H NMR
91.4(2)
spectra have been recorded for dicobaloximes 2a , 3a ,
4a , 6a , and 14a at different temperatures. The spectra
have been recorded in the temperature range +25 to
-60 °C. The following peaks are considered for study:
(a) O-H‚‚‚O resonance, (b) CH2 resonance, (c) oxime Me,
Ph, H resonance (for dmgH, dpgH, and gH), (d) pyridine
resonance, and (e) bridging phenyl ring resonance.
The O-H‚‚‚O peak appears as a singlet, PyR hydro-
gens appear as a doublet, and Pyâ and Pyγ hydrogens
appear as triplets at all temperatures. The bridging
phenyl group shows a singlet in 3a , two signals in 4a ,
and a doublet and a triplet in 2a at all temperatures.
The cobalt-bound CH2 and the oxime hydrogen signals
show a marked variation in splitting pattern depending
on the bridging xylyl group and the dioxime. For
example, both CH2 and dmgH methyl protons appear
as singlets in 3a at all temperatures, whereas the CH2
appears as a double doublet and dmgH methyls appear
117.6(9)
114.2(11)
0.013 Å
+1.5°
0.037 Å
+4.9°
Rb
a
Displacement of the cobalt atom from mean N4 plane toward
b
the axial N. Butterfly-bending angle
O4 act as acceptors in the C-H-O interactions found
in the molecule (Figures 5 and 6). The hydrogen-bonding
parameters are given in Table 10. Hydrogen bonding
involving the pyridine R-hydrogen (C9-H9-O2) leads
to a 12-membered-ring formation. Similarly a 14-
membered ring is formed by C12-H12-O4 interaction
(involving the pyridine â-hydrogen). The third interac-
tion is caused by the dioxime methyl hydrogen, which
propagates in a linear fashion. In cobaloximes with
(30) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565.
(31) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 270.

Synthesis of Organo-Bridged Dicobaloximes
Organometallics, Vol. 23, No. 9, 2004 2077
F igu r e 7. Variable-temperature 1H NMR spectra of the
CH2Co signal for m-xylylene-bridged dicobaloximes con-
taining different dioximes: (a) 2a , (b) 6a , and (c) 14a .
F igu r e 8. Variable-temperature 1H NMR spectra of di-
oxime hydrogens of m-xylylene-bridged dicobaloximes: (a)
dmgH methyl of 2a , (b) dpgH phenyl of 6a , and (c) gH
hydrogen of 14a in CDCl3.
as two singlets in 4a at all temperatures. But in 2a ,
both CH2 and dmgH methyls appear as singlets at room
temperature, but these peaks broaden at 0 °C and
sharpen below this temperature, and finally at -60 °C
CH2 protons appear at 2.65 (J ) 5.2 Hz) and 2.82 (J )
5.6 Hz) ppm with the chemical shift difference (∆δ) of
68 Hz (Figure 7a) and dmgH methyl signals appear at
1.85 and 2.06 ppm (∆δ ) 84 Ηz) (Figure 8a).
The CH2 group in other m-xylylene-bridged dico-
baloximes, 6a and 14a , shows a pattern similar to that
in dmgH complex 2a except that the coalescence tem-
perature is higher in dpgH complex 6a and is lower in
gH complex 14a as compared to 2a (Figure 7b,c). A
similar pattern is observed for dioxime signals also
1
(Figure 8b,c). The details of the H NMR values for the
characteristic peaks at different temperatures are given
in Tables 11 and 12.
In general, a difference in chemical shift is observed
on varying the temperature from +25 to -60 °C. For
example, O-H‚‚‚O and Pyγ signals shift downfield by
0.3 and 0.1 ppm, respectively (Table 11). The phenyl
hydrogen (H2), ortho to both CH2 groups (Figure 9),
shifts upfield by 0.17 ppm in 2a and 0.13 ppm in 14a .

2078 Organometallics, Vol. 23, No. 9, 2004
Ta ble 11. 1H NMR Sp ectr a l Da ta of 2a a t Differ en t Tem p er a tu r es
Gupta et al.
pyridine
xylyl ring
temp (°C)
Co-CH2
CH3
1.92
O-H‚‚‚O
R (d)
â
γ (t)
24
0
2.77
2.76
18.17
18.26
18.29
18.36
18.41
18.45
18.49
8.52 (4.8)
8.52 (5.2)
8.52 (5.2)
8.52 (5.2)
8.51 (5.2)
8.51 (4.8)
8.51 (4.0)
7.27
7.28
7.28
7.29
7.32
7.31
7.31
7.66 (7.6)
7.68 (8.0)
7.70 (7.6)
7.71 (8.0)
7.72 (7.6)
7.73 (7.6)
7.75 (6.8)
6.32
6.29
6.26
6.23
6.20
6.18
6.15
6.86 (d) (7.2)
6.70 (t) (7.2)
6.73 (t) (7.6)
6.74 (t) (7.6)
6.76 (t) (7.6)
6.79 (t) (8.0)
6.80 (t) (7.6)
6.82 (t) (7.2)
1.92
6.87 (d) (7.6)
6.88 (d) (7.6)
6.89 (d) (7.2)
6.90 (d) (7.6)
6.91 (d) (7.2)
6.92 (d) (7.2)
-15
-30
-40
-50
-60
2.70, 2.82
2.68, 2.82
2.67 (6.0), 2.83 (6.0)
2.66 (5.2) 2.82 (6.0)
2.65 (5.2) 2.82 (5.6)
1.87, 2.00
1.86, 2.02
1.85, 2.03
1.85, 2.05
1.85, 2.06
Ta ble 12. 1H NMR Sp ectr a l Da ta for CH2 a n d Dioxim e Hyd r ogen s of m -Xylylen e-Br id ged Dicoba loxim es
(2a , 6a , a n d 14a ) a t Differ en t Tem p er a tu r es
CH2
dioxime hydrogen
temp (°C)
2a
6a
14a
2.96
2a methyl
6a phenyl
14a NdC-H
24
10
5
2.77
2.76
3.45
3.45
1.92
6.96 (d) (6.4), 7.13-7.20 (m)
6.96 (d) (6.4), 7.14-7.19 (m)
6.96 (d) (6.0), 7.16-7.19 (m)
6.96, 7.17
6.95-6.99, 7.16-7.22 (m)
6.96 (6.8), 6.99 (7.2), 7.17-7.23 (m)
6.96 (6.4), 7.00 (7.6), 7.15-7.24 (m)
7.27
3.44, 3.46
3.44, 3.46
3.43 (6.0), 3.47 (5.6)
2.95
2.94
7.28
7.29
0
1.92
-10
- 15
-30
-35
-40
-50
-60
2.70, 2.82
2.68, 2.82
1.87, 2.00
1.86, 2.02
3.44 (6.4), 3.48 (6.4)
2.92
7.30
7.30
7.30
7.31, 7.33
2.67 (6.0), 2.83 (6.0) 3.44 (6.0) 3.49 (6.0)
2.66 (5.2), 2.82 (6.0) 3.45 (5.2) 3.49 (5.2)
2.65 (5.2), 2.82 (5.6) 3.46, 3.49
2.97, 2.90
2.88, 2.99
2.88, 2.99
1.85, 2.03
1.85, 2.05
1.85, 2.06
6.96 (7.2), 7.01 (7.2), 7.19-7.23 (m)
6.96 (6.8), 7.03 (6.8), 7.20-7.26 (m)
6.97 (6.4), 7.04 (6.8), 7.20-7.28 (m)
(a) four methyl groups in each cobaloxime unit, (b) one
methyl group in each dioxime unit, or (c) two methyl
groups of the same dioxime in each cobaloxime unit. The
structure details in benzyl cobaloximes28,32 and the
NMR studies in o-bromobenzyl cobaloximes33 have been
1
helpful in rationalizing the H NMR data. Two singlets
of equal intensity for the dmgH methyl are observed in
(o-Br)C6H4CH2Co(dmgH)2Py at low temperature, indi-
cating the nonequivalence of dmgH protons. DmgH
methyl groups appear as a singlet in the mixed dioxime
complex (o-Br)C6H4CH2Co(dmgH)(dpgH)Py even at -50
°C. This means that both the methyl groups are identi-
cal and hence rules out the second possibility. A
combination of these two inferences supports the third
possibility. The third possibility gains indirect support
from the crystal structures of the benzyl cobaloximes.
For example, the crystal structures of PhCH2Co-
(dmgH)2Py,28 PhCH2Co(dmgH)(dpgH)Py,32 PhCH2Co-
(chgH)(dpgH)Py,32 and (o-vinyl)C6H5CH2Co(dmgH)2Py
show that the orientation of the phenyl group is the
same in all four cobaloximes and the plane of the
aromatic ring of the benzyl group is above the dioxime
moiety. The ring current of the bridging aromatic ring,
therefore, affects one of the dioxime units in each
cobaloxime moiety. We have proposed a structure for
the m-xylylene-bridged dicobaloxime based on these
results (Figure 9).
F igu r e 9. Proposed orientation of dioximes in m-xylylene-
bridged dicobaloximes
The overall results suggest that rotational freedom
exists in organo-bridged dicobaloximes and the degree
of freedom depends on both the steric and electronic
properties of the bridging ligand and the dioxime.
For example, both the cobaloxime units are far away
from each other in 3a ; each one behaves as an indepen-
dent benzyl cobaloxime unit such that the rotational
freedom is not affected in any way. Hence, no change
in splitting pattern is observed in CH2 and in the dmgH
methyl even at low temperature. Also, when similar
studies are carried out in C6H5CH2Co(L)2Py (L ) dmgH,
dpgH), no splitting in CH2 and dioxime hydrogens is
observed even at -50 °C. This supports the above
viewpoint.
The results in 2a and 6a suggest that free rotation of
the Co-C bond is possible at room temperature, but it
becomes restricted at lower temperature and this rota-
tion is further affected substantially by the substituents
present on the dioxime moiety, with the result that the
coalescence temperature for the CH2 signal follows the
order 6a > 2a > 14a .
Con clu sion s
Organo-bridged dicobaloximes containing four differ-
ent dioximes have been synthesized. The cyclic volta-
mmetric results show that irreversible two-electron
reduction of CoIII-CoI takes place. The Co-C bond in
4a is cleaved during the crystallization process and
results in the formation of o-vinylbenzyl cobaloxime. The
crystal structure of the latter shows that it propagates
1
The variable-temperature H NMR study has proved
to be quite useful to know the orientation of the dioxime
with respect to the Co-C bond. There are in all eight
methyl groups in the dmgH complexes 1a -4a , and the
appearance of two peaks of equal intensity suggests that
four methyl groups are magnetically nonequivalent to
the remaining four. Four methyl groups in each set may
be comprised of any one of the following possibilities:
(32) Gupta, B. D.; Yamuna, R.; Singh, V.; Tewari, U.; Barclay, T.;
Cordes, W. J . Organomet. Chem. 2001, 627, 80.
(33) Gupta, B. D.; et al. Unpublished results.

Synthesis of Organo-Bridged Dicobaloximes
Organometallics, Vol. 23, No. 9, 2004 2079
as a two-dimensional layer by intermolecular C-H‚‚‚O
We thank Dr. W. Cordes, Department of Chemistry and
Biochemistry, University of Arkansas, Fayetteville, AR,
for the crystallographic help.
1
interaction. The variable-temperature H NMR of xy-
lylene-bridged dicobaloximes suggests that the Co-C
bond rotation is restricted and its magnitude depends
on the nature of the bridging ligand and the dioxime.
On the basis of the studies, a structure of dicobaloxime
has been proposed.
Su p p or tin g In for m a tion Ava ila ble: Table containing
coordination shift values for pyridine in dicobaloximes (1H and
13C NMR); tables of atomic coordinates and equivalent isotropic
displacement parameters, bond lengths and angles, and aniso-
tropic displacement parameters for 20. This material is
available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. We thank the Department of
Science and Technology (DST), New Delhi, India, for
funding the project (Project No. SP/S1/F20/99). V.V.
thanks CSIR, New Delhi, India, for a SRF fellowship.
OM034273P