Co-C bond reactivity and cis influence relationship in benzylcobaloximes with glyoxime and dimesitylglyoxime

Article abstract of DOI:10.1021/om7003578

Benzyl cobaloximes, ArCH₂Co(dioxime)₂Py, with two different dioximes, glyoxime (gH) and dimesitylglyoxime (dmestgH), have been synthesized and characterized by ¹H and ¹³C NMR and UV-vis spectroscopy. The dioxy a

Full text of DOI:10.1021/om7003578

Organometallics 2007, 26, 3559-3567
3559
Co-C Bond Reactivity and Cis Influence Relationship in
Benzylcobaloximes with Glyoxime and Dimesitylglyoxime
Mouchumi Bhuyan, Moitree Laskar, Debaprasad Mandal, and B. D. Gupta*
Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, India
ReceiVed April 12, 2007
Benzyl cobaloximes, ArCH2Co(dioxime)2Py, with two different dioximes, glyoxime (gH) and
1
dimesitylglyoxime (dmestgH), have been synthesized and characterized by H and 13C NMR and UV-
vis spectroscopy. The dioxy adducts, ArCH2(O2)Co(dioxime)2Py, were prepared by the insertion of
molecular oxygen into the Co-C under photochemical conditions. The rate of insertion depends upon
the nature of the dioxime and follows the order dmestgH . dpgH > chgH > dmgH > gH. The steric
cis influence of the dioximes on the Co-C bond also follows the same order. The study also suggests
that the interactions between the axial benzyl group and the dioximes have significant influence on the
Co-C bond reactivity. Such interactions are also seen in the molecular structure of [C6H4CH2Co(gH)2-
(4-tBuPy)] and in the dioxy complexes 4-CN-C6H4CH2(O2)Co(dmestgH)2Py and 2-naphthylCH2-
(O2)Co(dmestgH)2Py. On the basis of the results we have supported the cage mechanism for the oxygen
insertion into the Co-C bond.
based on spectral and structural studies, have shown that, in
addition to the trans effect of the axial base, the cis influence
of the equatorial dioxime on the axial ligands has significant
effect toward the stability of the Co-C bond.5-7 The inherently
weak Co-C bond in the organocobaloximes shows homolytic
cleavage with visible light similar to the activation of vitamin
Introduction
One of the unique and intriguing properties of the coenzyme
B12 is different catalytic activity of two different coenzymes.
How the Co-C bond is activated toward homolysis or het-
erolysis is an enduring subject of research.1,2 The recent
crystallographic data on cobalamins suggest that the structural
effects of changes in “R” are similar to those found in the
cobaloximes, RCo(dmgH)2B, and sometimes can be related to
their chemical reactivity.2 [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), and dimesitylglyoxime (dmestgH)).]
Since the Co-C bond cleavage is the key step involved in B12-
dependent enzymatic or cobaloxime-mediated reactions, 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
in the related complexes with different chelates has been
systematically investigated.2 The Co-C bond length indeed
responds to the steric rather than electronic effect in the
organocobaloximes.2 The bond lengths in structurally character-
ized complexes vary over a remarkably broad range of 0.2 Å
from methyl to adamantyl cobaloximes.3 Spectroscopic evidence
has been presented that even longer bonds occur in more
sterically hindered systems, which have thus far proved to be
too unstable for X-ray structural characterization.4 The results,
B
12 by apoenzyme.8 The insertion of molecular oxygen into the
Co-C bond has extensively been used to test the reactivity of
these compounds,7 and its rate in the benzyl cobaloximes was
found to follow the order dpgH > chgH > dmgH. The cis
influence also has the same order.7 Since the Co-C bond
cleavage is the key step in these reactions and the effect of the
cis influence is felt most on the Co-CH2 bond, the question is
if these two factors, cis influence and rate of insertion, are related
to each other in some way.
Since the cobaloximes with dimesitylglyoxime (dmestgH)
show the highest steric cis influence among the known dioximes
[dmestgH > dpgH > chgH > dmgH > gH],5 we have
synthesized the benzyl cobaloximes with glyoxime and
dimesitylglyoximesthe two extreme casessand studied their
insertion reaction with molecular oxygen. The benzyl system
is particularly chosen because of the inherently weak Co-C
bond and the weak interaction between the benzyl group and
the dioxime may influence the Co-C bond reactivity especially
because these have been shown to slow down the Co-C and
C-Ph bond rotation.9 In order to see the influence of these
interactions on the Co-C bond stability/reactivity, we have
measured and compared the rate data with the alkyl analogues
where such interactions are lacking. We have also studied the
* Corresponding author. E-mail: bdg@iitk.ac.in. Tel: +91-512-2597046.
Fax: +91-512-2597436.
(1) Brown, K. L. Dalton Trans. 2006, 1123.
(5) Mandal, D.; Gupta, B. D. Organometallics 2005, 24, 1501, and
references therein.
(2) Randaccio, L. Comments Inorg. Chem. 1999, 21, 327.
(6) Gupta, B. D.; Qanungo, K. J. Organomet. Chem. 1997, 543, 125.
(7) (a) Gupta, B. D.; Vijaykanth, V.; Singh, V. J. Organomet. Chem.
1998, 570, 1. (b) Gupta, B. D.; Roy, M.; Das, I. J. Organomet. Chem. 1990,
397, 219.
(8) (a) Pratt, J. M.; Whitera, B. R. D. J. Chem. Soc. A 1971, 252. (b)
Ramakrishna, D. N.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 1
1984, 423. (c) Jensen, F.; Madan, V.; Buchnan, D. H. J. Am. Chem. Soc.
1971, 93, 5285.
(3) (a) Randaccio, L. Inorg. Chem. 1983, 22, 3416, and references therein.
(b) Bresciani-Pahor, N.; Marzilli, L. G.; Randaccio, L.; Summers, M. F.;
Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1. (c) Randaccio, L.; Bresciani-
Pahor, N.; Zangrando, E.; Marzilli, L. G. Chem. Soc. ReV. 1989, 18, 225.
(4) (a) Gupta, B. D.; Vijaikanth V.; Singh, V. Organometallics 2004,
23, 2069. (b) Randaccio, L.; Bresciani-Pahor, N.; Toscano, P. J. Marzilli,
L. G. J. Am. Chem. Soc. 1981, 103, 6347. (c) Trommel, J. S.; Warncke,
K.; Marzilli, L. G. J. Am. Chem. Soc. 2001, 123, 3358. (d) Siega, P.;
Randaccio, L.; Marzilli, P. A.; Marzilli, L. G. Inorg. Chem. 2006, 45, 3359.
(9) (a) Mandal, D.; Gupta, B. D. Organometallics 2006, 25, 3305. (b)
Mandal, D.; Gupta, B. D. Organometallics 2007, 26, 658.
10.1021/om7003578 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/12/2007

3560 Organometallics, Vol. 26, No. 14, 2007
Bhuyan et al.
Scheme 1. Benzyl Cobaloximes and Oxygen Insertion into
Spectroscopy: Characterization of these Complexes. The
cis and trans influence in alkyl cobaloximes, RCo(dioxime)2Py
(dioxime ) gH, dmgH, chgH, dpgH, dmestgH), have already
been discussed in detail in our previous publications.5,6,12 We
therefore will highlight only the salient features in the present
complexes.
the Co-C Bond
The complexes were preliminary characterized by 1H and 13
C
NMR spectroscopy, and the spectral data for 1-10 and 1a-
1
10a have been tabulated in Tables 1-4. The H NMR spectra
are easily assigned on the basis of their chemical shifts. The
signals are assigned according to their relative intensities and
are consistent with the literature values on the corresponding
alkyl cobaloximes. Since no 13C NMR has been reported for
any oxygen-inserted cobaloxime, we have assigned the values
keeping in view the chemical shifts in the parent complexes.
We have recently observed that the interaction between the
axial and equatorial dioxime ligand affects the structure, Co-C
bond reactivity, and NMR chemical shifts in benzyl co-
baloximes; namely, such interactions cause restriction of Co-C
and/or C-Ph rotation at subzero temperature and are partly
responsible for the nonequivalence of dioxime and CH2 protons
in 2-substituted benzyl cobaloximes, 2-XC6H4CH2Co(dioxime)2Py
[dioxime ) dmgH, dpgH, and gH].9 The crystal structures of
benzyl cobaloximes support this; the benzyl group always lies
over one of the dioxime wings and has π-interaction with the
dioxime ring current. Such interactions have considerably
affected the structure of pyrazine-bridged dicobaloximes; the
alkyl complex attains the staggered conformation, whereas the
benzyl analogue acquires the eclipsed conformation.13 The same
type of π-interaction between the axial and equatorial ligand
has been reported by Randaccio et al.14 in RCo(DBPh2)2B and
Stynes et al.15 in LFeII(DBPh2)L′, where this interaction defines
the ligand’s orientation.
structural features of these oxygen-inserted complexes since
there is no report of a crystal structure with Co(O2) bound to a
primary carbon atom.10
Results and Discussion
Synthesis. Two series of complexes, ArCH2Co(dmestgH)2Py
(1-5) and ArCH2Co(gH)2Py (6-10), have been synthesized
(Scheme 1). All these complexes are new, and their elemental
analysis data are given in Supporting Information Table S1.
ClCo(dmestgH)2Py was synthesized according to the proce-
dure by Busch et al.11 The preparation required the addition of
Et3N. However, when we tried to prepare it using the conven-
tional procedure reported for ClCo(dioxime)2Py [dioxime ) gH,
dmgH, dpgH], we could get a maximum yield of 10%. The
side product was EPR active and looked like a [Co-(O2)•]
radical. No attempt was made to analyze this.
The synthesis of ArCH2Co(dmestgH)2Py was accomplished
according to the procedure previously established for the alkyl
cobaloximes.5 Ethanol is a better solvent than methanol, and a
large excess of ethanol and 10-fold excess of NaBH4 were
essential; otherwise the yield was poor. The workup must be
carried out rapidly under an argon atmosphere since the Co-C
bond is highly sensitive toward oxygen; otherwise the product
is contaminated with the oxygen-inserted product. For example,
a solution of 3 and 5 kept for crystallization in air gave the
crystal of the oxygen-inserted products 3a and 5a, respectively.
The gH complexes (6-10) were synthesized by a general
procedure detailed earlier for RCo(gH)2Py. The workup pro-
cedure was similar to that described in our recent papers; the
addition of acetic acid during workup is essential to compensate
for the loss of the acidic gH proton in the basic medium.10b
Oxygen Insertion: General Comments. The insertion of
oxygen is free radical nonchain in nature and shows general
characteristics as found in earlier studies; for example, (a) the
reaction does not proceed in the dark at 0 °C, (b) the reaction
stops as soon as the irradiation is stopped, and (c) the reaction
is inhibited by the free radical trap galvinoxyl. The best
temperature for the reaction is 0 °C, though it proceeds at
ambient temperature also. The reaction follows a first-order
kinetics. The overall order of reaction is 2, i.e., first order with
respect to both the complex and oxygen.7a
A reactivity difference toward oxygen in the benzyl and alkyl
cobaloximes, RCo(dmestgH)2Py, has also been observed; the
benzyl complex gives the oxygen-inserted product, whereas the
alkyl analogues give air-stable Co(II).16
Such interactions play a key role and affect the chemical shifts
of the mesityl group in 1-5 in comparison to the alkyl
derivatives.5,17 The ortho-methyl groups in free dmestgH2 are
equivalent and appear at δ 2.14 ppm, whereas these are
nonequivalent and the methyl at the 2-position is highly shielded
by the ring current of the axial pyridine in alkyl or benzyl
complexes and appears at around 1.45 ppm. The chemical shift
of 6-Me depends on the nature of the axial R group; it is
significantly shifted upfield and appears at 1.98 ppm in the
benzyl (1) as compared to 2.19 ppm in the methyl analogue.
This upfield shift is even larger in 2-naphthylcobaloxime (5).
This is due to the interaction of the ring current of the benzyl
or 2-naphthyl group with the dioxime. The 4-Me group is not
(12) Gupta, B. D.; Yamuna, R.; Singh, V.; Tewari, U. Orgnometallics
2003, 22, 226.
(13) Mandal, D.; Gupta, B. D. J. Organomet. Chem. 2005, 690,
3746.
(14) (a) Dreos, R.; Tauzher, G.; Vuano, S.; Asaro, F.; Pellizer, G.; Nardin,
G.; Randaccio, L.; Geremia, S. J. Organomet. Chem. 1995, 505, 135. (b)
Asaro, F.; Dreos, R.; Geremia, S.; Nardin, G.; Pellizer, G.; Randaccio, L.;
Tauzher, G.; Vuano, S. J. Organomet. Chem. 1997, 548, 211. (c) Dreos,
R.; Geremia, S.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano, S. Inorg.
Chim. Acta 1998, 272, 74.
(15) (a) Stynes, D. V.; Leznof, D. B.; de Silva, D. G. A. H. Inorg. Chem.
1993, 32, 3989. (b) Stynes, D. V. Inorg. Chem. 1994, 33, 5022. (c) Vernik,
I.; Stynes, D. V. Inorg. Chem. 1996, 35, 6210, and references therein.
(16) Mandal, D.; Bhuyan, M.; Laskar, M.; Gupta, B. D. Organometallics
2007, 26, 2795-2798.
(10) (a) Giannotti, C.; Fontaine, C.; Chiaroni, A.; Riche, C. J. Organomet.
Chem. 1976, 113, 57. (b) Chiaroni, A.; Pascard-Billy, C. Bull. Soc. Chim.
Fr. 1973, 781. (c) Alcock, N. W.; Golding, B. T.; Mwesigye-Kibende, S.
J. Chem. Soc., Dalton Trans. 1985, 1997.
(11) Lance, K. A.; Goldsby, K. A.; Busch, D. H. Inorg. Chem. 1990,
29, 4537.
(17) Mandal, D.; Chadha, P.; Laskar, M.; Bhuyan, M.; Gupta, B. D.
Tetrahedron Lett. 2007, 48, 2377.

Co-C Bond ReactiVity in Benzylcobaloximes
Organometallics, Vol. 26, No. 14, 2007 3561
Table 1. 1H NMR Data for 1-5 and 1a-5a at Room Temperature in CDCl3
ligand (dmestgH)
pyridine
Me (2)
Me (4)
Me (6)
aromatic
CH2
R
â
γ
O-H‚‚‚O
aromatic/others
1
1a
2
2a
3
3a
4
4a
5
5a
1ba
1cb
1.45
1.52
1.45
1.52
1.44
1.54
1.45
1.52
1.45
1.53
1.52
1.51
2.15
2.17
2.15
2.17
2.16
2.18
2.14
2.18
2.13
2.16
2.18
2.19
1.98
2.04
1.98
2.04
1.98
2.01
2.00
2.05
1.83
2.01
2.48
2.24
6.58, 6.68
6.62, 6.70
6.58, 6.70
6.62, 6.72
6.60, 6.71
6.63, 6.72
6.58, 6.69
6.61, 6.70
6.57, 6.63
6.65, 6.62
6.62, 6.80
6.63, 6.77
3.55
4.71
3.45
4.66
3.41
4.74
3.53
4.69
3.72
4.88
4.62
3.50
8.97
8.75
8.94
8.73
8.91
8.73
8.96
8.75
8.98
8.76
8.67
8.72
7.36
a
7.84
7.84
7.84
7.85
7.86
7.85
7.84
7.83
c
18.83
19.10
18.80
19.08
18.77
19.08
18.79
19.10
18.91
19.15
18.68
7.06(t), 7.16(t), 7.56(d)
7.16-7.36(m)
7.36
7.26
7.37
7.34
7.36
7.33
c
7.34
c
7.33
7.02(d), 7.48(d)
7.22(d), 7.11(d)
7.33(d), 7.61(d)
7.48(d), 7.42(d)
6.96(t), 7.15(d), 3.74 (OMe)
6.83-7.07(m), 3.72 (OMe)
7.95-7.26(m)
7.37-7.76(m)
7.25-7.29(m)
7.85
7.70
7.85
a 1b ) PhCH2(SO2)Co(dmestgH)2Py. b 1c ) Me(O)2Co(dmestgH)2Py. c Merge with aromatic.
Table 2. 1H NMR Data for 6-10 and 6a-10a at Room Temperature in CDCl3
pyridine
CH2 (s)
gH
R (d)
â (t)
γ (t)
aromatic/others
O-H‚‚‚O
6
6a
7
7a
8
8a
9
9a
10
10a
3.01
4.42
2.92
4.36
2.87
4.47
3.00
4.40
3.18
4.59
7.24
7.56
7.26
7.57
7.28
7.59
7.25
7.57
7.21
7.53
8.54
8.34
8.51
8.32
8.47
8.32
8.54
8.34
8.55
8.33
7.35
7.31
7.35
7.30
7.35
7.31
7.35
7.31
7.34
7.35
7.75
7.75
7.75
7.75
7.77
7.77
7.77
7.75
7.75
7.72
7.11-7.05(m)
17.67
17.72
17.65
6.93(d), 7.24(d)
7.00(d), 7.12(d)
7.19-7.22(m)
6.99(d), 7.32, 7.29, 7.11(d)
7.37(d), 7.55(d)
17.37
17.66
17.63
17.74
17.74
17.68
6.66-6.80(m), 6.93-7.00(m), 3.79 (OMe)
6.79(d), 6.85(d), 7.17(t), 3.76 (OMe)
7.40-7.69(m)
7.41-7.44(m) 7.76-7.81(m)
Table 3. 13C NMR Data for 1-5 and 1a-5a at Room Temperature in CDCl3
pyridine
CdN (dmestgH)
R
â
γ
aromatic/others
Me (dmestgH)
CH2
1
153.27
151.32
124.87
138.39
138.67, 138.15, 137.06, 130.10, 128.56,
128.44, 128.29, 126.32, 124.96
138.64, 136.67, 131.60, 128.99, 128.69,
128.15, 126.14
146.54, 139.07, 138.83, 137.99, 137.04,
131.23, 128.63, 128.48, 128.36, 126.19
138.84, 136.64, 130.64, 128.83, 128.64,
128.33, 128.07, 127.83, 126.22
154.97, 139.03, 137.80, 137.04, 132.06,
130.44, 129.23, 128.68, 128.47, 126.01,
107.66, 120.19 (CN)
20.94, 20.53, 20.37
32.79
76.89
31.38
77.44
30.37
1a
2
154.90
153.42
154.80
153.80
152.04
151.55
152.02
151.33
125.02
124.94
125.01
125.04
139.18
138.50
139.06
138.68
21.06, 20.17, 19.95
20.97, 20.47, 20.36
21.04, 20.26, 19.90
21.00, 20.45, 19.93
2a
3
3a
4
154.89
153.35
152.02
151.35
125.02
124.90
139.17
138.40
138.63, 136.66, 131.60, 128.99, 128.68,
128.14, 126.13,
21.05, 20.16, 19.95
20.97, 20.55, 20.37
149.46, 138.69, 138.17, 137.04, 128.56,
128.29, 126.35, 159.71, 138.98, 129.24,
122.66, 114.67, 111.78, 55.22 (OMe)
125.31, 126.29, 127.14, 127.44, 128.05,
128.65 132.80, 133.18, 134.69, 136.63,
55.03 (OMe)
32.71
78.49
4a
154.78
152.05
125.00
138.94
21.04, 20.27, 19.92
5
154.08
154.78
156.44
154.59
152.05
152.05
151.55
151.98
125.25
125.00
124.87
125.00
c
139.24, 138.97, 136.63, 133.18, 129.11,
128.87, 128.06, 127.50, 126.75, 126.47
136.63, 134.69, 133.18, 132.80, 128.65,
128.05, 127.44, 127.14, 126.29, 125.31
148.49, 139.36, 136.77, 135.71, 131.43,
129.18, 128.22, 128.13, 127.56, 125.86
138.84, 136.71, 128.72, 128.10, 126.27
21.06, 20.26, 19.92
21.04, 20.27, 19.92
21.01, 20.84, 20.33
19.88, 20.27, 21.10
38.07
78.49
65.18
65.79
5a
1ba
1cb
138.94
139.90
139.08
a 1b ) PhCH2(SO2)Co(dmestgH)2Py. b 1c ) Me(O)2Co(dmestgH)2Py. c Merge with aromatic.
affected at all and appears at the same chemical shift in the
alkyl and benzyl complexes (Supporting Information Figures
S1 and S2).
The crystal structure of MeCo(dmestgH)2Py shows that both
the axial positions are very much crowded and laterally
compressed by the four methyl groups of the equatorial dmestgH
ligand.5 Due to this steric crowding, pyridine is puckered
(strained) and the axial methyl has an interaction with the 6-Me
group (∼2.47 Å) (Supporting Information Figure S3). Any
substitution on the axial methyl will further increase the steric
crowding and lead to a higher bending angle (R), which in turn
pushes 2-Me closer to pyridine. This is due to the cis influence
of the dioxime on the Co-C bond. In the absence of the crystal
structure we cannot confirm the increase in the bending angle;
1
however its effect is clearly visible in the H NMR chemical
shifts of 2-Me. For example, on increasing the alkyl chain length
from R ) Me to Et, Pr, or benzyl, the 2-Me is upfield shifted
from 1.51 to 1.45 ppm.
The effect of the π-interaction is also seen in the gH
complexes; gH protons in 6-10 are upfield shifted (δ 7.24 ppm)

3562 Organometallics, Vol. 26, No. 14, 2007
Table 4. 13C NMR Data for 6-10 and 6a-10a at Room Temperature in CDCl3
Bhuyan et al.
pyridine
CdN (gH)
R
â
γ
aromatic
CH2
6
6a
7
7a
8
8a
9
138.00
140.09
138.10
140.14
138.31
140.15
138.11
149.88
150.75
149.89
150.79
149.81
150.73
149.87
125.57
125.68
125.62
125.75
125.75
125.76
125.58
a
138.31, 128.67, 127.71, 124.98
136.35, 129.54, 128.76, 128.03, 127.73
144.26, 130.31, 129.79, 127.84
134.99, 133.59, 130.86, 128.24
152.37, 132.41, 131.41, 107.59, 120.19 (CN)
133.89, 129.53, 128.56, 107.01, 120.08 (CN)
159.09, 147.07, 128.50, 121.38, 113.23,
111.47, 55.13 (OMe)
138.82
138.05
138.91
a
138.97
a
78.34
77.48
32.29
34.47
78.27
9a
140.10
138.08
140.15
150.76
149.95
150.71
125.69
125.62
125.70
138.83
138.03
138.84
137.86, 128.99, 121.74, 114.48, 113.88,
55.17 (OMe)
137.95, 128.34, 127.25, 126.86, 125.92,
125.87, 124.62
133.96, 133.16, 128.35, 127.98, 127.51,
127.19, 126.58, 126.18, 125.52
10
10a
78.36
a Merge with CN(gH).
as compared to the reported alkyl analogues (δ 7.42 ppm). Thus,
the benzyl or 2-naphthyl ring must attain a proper orientation
to show a π-interaction with the dioxime ring current. To
perceive how important this requirement is, we have compared
the chemical shifts of 1-5 with the PhCH2(O2)Co(dmestgH)2-
Py and Bn(SO2)Co(dmestgH)2Py complexes since the orienta-
tion of the benzyl group varies significantly in these complexes
(see later).18
Important differences are noticed when the chemical shifts
of dmestgH complexes (1-5) are compared with the gH
complexes (6-10); for example, CH2 is highly downfield shifted
(0.5-0.6 ppm) in 1-5 as compared to the value in 6-10. A
similar trend is observed for PyR and O-H‚‚‚O also. This is
due to the higher cobalt anisotropy of dmestgH complexes. ∆δ-
(13C, CdN) is a good measure of the cobalt anisotropy as
observed in many cobaloximes; the higher the value, the higher
the cobalt anisotropy.5 [Instead of δ13C(CdN), the ∆δ(13C, Cd
N) 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 (combined effect of cobalt anisotropy and ring
the reverse is true in the dmestgH complexes. The data show
that ∆δ(1H, PyR) in 1a-10a is upfield shifted as compared to
1-10, and it is 0.4 ppm downfield shifted in 1-5 as compared
with 6-10. A similar shift is observed when 1a-5a is compared
with 6a-10a. The upfield shift of PyR in 1a-10a means the
ring current contribution is more, and the downfield shift in
1-5 or 1a-5a means the cobalt anisotropy contribution is more.
The ring current effect is more in 1a-10a because PyR is closer
to the dioxime ring (Co-NPy bond length is slightly shorter
than in organocobaloxime). The behavior is similar to the
inorganic cobaloximes.
The insertion of O2 affects the dioxime protons also. The gH
protons shift downfield by 0.3 ppm in the dioxy complexes as
compared to the parent cobaloximes due to higher cobalt
anisotropy in the former. This is much larger compared to the
shift in the dmgH (0.1 to ∼0.2 ppm)7 and dmestgH complexes
(0.02-0.1 ppm).
The effect of cobalt anisotropy is much more pronounced in
the gH complexes since “H” is directly bonded to the dioxime,
and it is one bond away in the dmgH complexes and is too far
away in the dmestgH complexes.
current) and is equal to δcomplex - δfree
(see ref 5 for
ligand
details).] The present results also support this; ∆δ(13C, CdN)
is 8-9 ppm higher in 1-5 as compared to complexes 6-10. A
comparison with the other benzylCo(dioxime)2Py complexes
gives the cis influence order of the dioximes as dmestgH >
dpgH > chgH ≈ dmgH > gH (Supporting Information Table
S2). The same order was also observed in alkylCo(dioxime)2Py
complexes.5,6,12
Interestingly, the chemical shift of 2-Me is identical in MeO2-
Co(dmestgH)2Py, C6H5CH2O2Co(dmestgH)2Py, and C6H5CH2-
SO2Co(dmestgH)2Py and is justified in view of the above
discussion.20 The crystal structures of the dioxy complexes show
that the strain is released and the bending angle decreases (∼3°)
with the result that 2-Me moves away from the pyridine ring
(3.1 Å compared to 2.8 Å, see Figure 2).
Comparison of 1-10 with 1a-10a. Incorporation of di-
oxygen into the Co-C bond affects the chemical shift of RCH2,
Comparison with the SO2-Inserted Complexes. A further
change in conformation of the benzyl group occurs in C6H5-
CH2SO2Co(dioxime)2Py.18 Here the benzyl group lies vertically
up and perpendicular to the dioxime plane and is too far away
to have any interaction with the dioxime protons. The 1H NMR
spectrum of C6H5CH2SO2Co(dmestgH)2Py17,20 should, therefore,
be similar to that of XCo(dmestgH)2Py. However, 6-Me is
highly deshielded and the signal appears at δ 2.48 ppm. This is
due to its interaction with the SO2 group that lies close to it
(Supporting Information Figure S2). A similar observation was
made earlier in the corresponding gH and dmgH complexes;
1
PyR, and the dioxime in the H and 13C NMR spectra of 1a-
10a. RCH2 is directly attached to O2 and hence shifts highly
downfield by 1.1-1.4 ppm (and about 74 ppm in 13C NMR).
A higher magnitude of ∆δ(13C, CdN) [1.5-2.0 ppm] in 1a-
10a as compared to 1-10 points to the higher cobalt anisotropy
in the dioxy complexes. The pyridine resonance is affected in
a similar way. A comparison of ∆δ(1H, PyR) gives useful
information.
The coordination shift of PyR is affected both by cobalt
anisotropy and by the ring current of the dioxime, and both
operate in the opposite directions.5,19 The net value depends
upon the interplay of these two factors; in the gH complexes
the ring current effect is more than the cobalt anisotropy, and
(20) Me(O2)Co(dmestgH)2Py (1H NMR; 400 MHz, CDCl3): δ Py: R-H
8.72 (d, 2H, J ) 5.2 Hz), γ-H 7.85 (t, 1H, J ) 7.4 Hz), â-H 7.33 (t, 2H,
J ) 6.8 Hz); dmestgH(CH3): 1.51 (s, 12H), 2.19 (s, 12H), 2.24 (s, 12H);
dmestgH(H): 6.63 (s, 1H), 6.67 (s, 1H); Co(O2)CH3: 3.50 (s, 3H). Bn-
(SO2)Co(dmestgH)2Py (1H NMR; 400 MHz, CDCl3): δ O-H‚‚‚O: 18.68;
Py: R-H 8.67 (d, 2H, J ) 6.0 Hz), γ-H 7.70 (t, 1H, J ) 8.0 Hz), â-H
merge with aromatic; dmestgH(CH3): 1.52 (s, 12H), 2.18 (s, 12H), 2.48
(s, 12H); dmestgH(H): 6.62 (s, 1H), 6.80 (s, 1H); Co(O2)CH2: 4.62 (s,
2H); aromatic: 7.25-7.29 (m).
(18) Chadha, P.; Mahata, K.; Gupta, B. D. Organometallics 2006, 25,
92.
(19) Gupta, B. D.; Yamuna, R.; Mandal, D. Orgnometallics 2006, 25,
706.

Co-C Bond ReactiVity in Benzylcobaloximes
Organometallics, Vol. 26, No. 14, 2007 3563
Figure 1. Molecular structure of 11.
Figure 3. Molecular structure of 5a.
the twist angle of the pyridine plane (τ) is 79.62°. [d is the
deviation of the cobalt atom from the mean equatorial N4 plane;
the butterfly bending angle (R) is the dihedral angle between
two dioxime planes, and τ is the torsion angle between two
planes, axial base pyridine and the plane that bisects the dioxime
C-C bonds through the cobalt atom. The positive sign of R
and d indicates bending toward R and displacement toward base
and Vice Versa (see ref 5 for details).] Interestingly, the Co-C
(2.061(4) Å) and Co-Nax (2.054(3) Å) distances, d (+0.02 Å),
R (2.68°), and τ (74.36°) in 2-Br-C6H4CH2Co(gH)2Py are very
similar to the corresponding values in 11.
Description of the Molecular Structures of 3a and 5a. A
slow evaporation of solvent from the solution of 3a and 5a
(dichloromethane, acetonitrile, benzene, and hexane) results in
the formation of orange crystals. The X-ray data analysis of
these crystals shows the composition as 4CN-C6H4CH2-
(O2)Co(dmestgH)2Py‚2CH3CN‚C6H6 (3a) and 2-naphthylCH2-
(O2)Co(dmestgH)2Py‚2CH2Cl2 (5a). Diamond diagrams of the
molecular structures for 3a and 5a along with selected number-
ing schemes are shown in Figures 2 and 3, respectively. Selected
bond lengths and bond angles are given in Table 5. The O-O
distances of 1.435/1.399 Å are similar to those in the reported
peroxo cobaloxime complexes.
Figure 2. Molecular structure of 3a.
for example, gH (or dmgH) protons appear at δ 7.24 (2.00)
ppm in C6H5CH2Co(gH)2Py and C6H5CH2Co(dmgH)2Py and
at δ 7.55 (2.30) ppm in C6H5CH2SO2Co(gH)2Py and C6H5CH2-
SO2Co(dmgH)2Py.12,18 However, unlike dmestgH, the downfield
shift here is not due to the close proximity of the SO2 group
with the gH or dmgH(Me) protons, but it results mainly from
the change in cobalt anisotropy. The gH or dmgH(Me) protons
are close to the [Co(dioxime)2]+ moiety and are affected much
more than the dmestgH(Me) protons.
These are the first crystal structures of the oxygen-inserted
cobaloximes with Co(O2) bound to a primary carbon atom. All
three previously reported crystal structures have either a
secondary or tertiary carbon bound to Co(O2) with dmgH as
the equatorial ligand.10 The geometry about the central cobalt
is distorted octahedral with four nitrogen atoms of the dioxime
in the equatorial plane and pyridine and the oxygen atom axially
coordinated. A comparison of the molecular structure of 3a with
the reported cumyl(O2)Co(dmgH)2Py (X1)10a shows that the
Co-NPy and Co-O distances, the C-H···π interaction, and the
orientation of the Bn-O-O group are almost identical. Interest-
ingly, the benzyl group in ArCH2Co(dmgH)2Py9 shows a similar
orientation as well as a C-H···π interaction to that in 3a and
5a.
The R-O-O-R′ dihedral angle is a significant parameter
for all the peroxides. The Co-O-O-C dihedral angles in 3a
and 5a are strikingly similar to the reported alkylperoxyco-
baloximes. Evidently, the same minimization of lone pair-lone
pair (LP-LP) interactions that governs the structure of hydrogen
peroxide (dihedral angle 112°) is the dominant factor in 3a and
5a. The Co-O-O-C dihedral angle gives useful information
X-ray Crystallographic Studies. Description of the Struc-
ture of C6H5CH2Co(gH)2(4-tBuPy) (11). The Diamond dia-
gram of the molecular structure along with the numbering
scheme is shown in Figure 1, and selected bond lengths and
bond angles are given in Table 5. The geometry around the
cobalt atom is distorted octahedral, with four nitrogen atoms
of the dioxime (gH) in the equatorial plane. Benzyl and 4-t-
BuPy are axially coordinated. 4-tBuPy is coordinated to the
cobalt atom with bending (C6-N1-Co1 172.65°), as observed
in many crystal structures,18,21 and the bending is due to the
crystal-packing force. The benzyl group is located over the
dioxime wing and shows some interaction with the dioxime
moiety. Co-C/Co-Nax distances (2.049(8)/2.051(6) Å) and
Co-C-C/N-Co-C angles (119.7(5)/178.6(3)°) are almost
identical with the other benzyl cobaloximes with dmgH as the
equatorial ligand. The deviation of the cobalt atom from the
mean equatorial N4 plane (d) is +0.021 Å, the butterfly bending
angle between two equatorial dioximes halves (R) is 2.54°, and
(21) (a) Galinkina, J.; Rusanov, E.; Wagner, C.; Schmidt, H.; Stro¨hl,
D.; Tobisch S.; Steinborn, D. Organometallics 2003, 22, 4873.

3564 Organometallics, Vol. 26, No. 14, 2007
Bhuyan et al.
Figure 4. Orientation of the ArCH2 group over the cavity in the molecular structure of 3a and 5a.
Table 5. Selected Bond Lengths (Å), Bond Angles (deg), and Structural Data for 3a, 5a, and 11 and Comparison with Other
Dioxy Complexes (X1 ) C6H5(CH3)2C(O2)Co(dmgH)2Py; X2 ) 4-Me-C6H4(CH3)CH(O2)Co(dmgH)2Py; X3 )
C2H5OOC(CH2)2(CH3)CH(O2)Co(dmgH)2Py)
3a
5a
X1
X2
X3
11
Co-Oax/Cax
Co-Nax
1.896(2)
1.995(3)
1.435(3)
1.421(4)
116.43(17)
107.5(2)
106.2(2)
115.4(3)
60.8(4)
173.80(11)
+0.011
3.17
79.81
3.079(1)
1.875(4)
1.978(4)
1.399(5)
1.446(7)
1.897
1.994
1.454
1.898
2.013
1.456
1.923(4)
2.007(5)
1.415(7)
1.428(8)
2.049(8)
2.051(6)
O-O
O-C
1.461
1.449
Co-O-O/ Co-C-C
O-O-C
116.5(3)
112.41
111.15
-131.57
100.77
172.34
175.25
0.027
112.84
107.15
-113.94
104.92
164.36
176.88
0.029
115.3(3)
119.7(5)
108.1(4)
-114.0(4)
105.3(5)
175.3(5)
171.8(2)
+0.021
3.14
108.2(5)
100.0(4)
112.21
63.66
177.97
0.011
Co-O-O-C
O-C-C
O-O-C-C
Npy-Co-Oax/Cax
d (Å)
178.6(3)
+0.023
2.54
79.62
3.999(5)
R (deg)
6.91
88.16
3.432
3.17
79.83
τ (deg)
88.24
3.110(2)
C-H···π
about the bond pair-lone pair (BP-LP) and LP-LP repulsions
around the O-O bond in such complexes; for example cumyl-
(O2)Co(dmgH)2Py (X1), having the tertiary carbon bonded to
Co(O2), has the highest value of -131.6°, whereas it is 100.0-
(4)° in the reported sec-alkyl(O2) complex (X3).10c The higher
the angle, the greater the BP-LP interaction. The C-O-O
angle also gives similar information; the cumyl(O2) complex
(X1) has the largest angle (111.1°) among the reported structures
and has the maximum BP-LP interaction. Although Co(O2) is
bound to a primary carbon in 3a and 5a, the dihedral angle
suggests that the steric crowding by the dmestgH(Me) on the
phenyl (or naphthyl) ring is more like the steric crowding
between the tertiary carbon and the dioxime in X1. Can this be
the reason that the dmestgH complexes are more unstable than
the corresponding dmgH complexes since the tertiary carbon
bound to cobalt is much more unstable than the primary (in
general, the Co-C bond stability follows the order 1° > 2° >
3°)? The steric interaction of the [Co(dioxime)2] moiety with
the axial ligand can also be inferred from the Co-O-O angle;
it is much higher in the dmestgH complexes (3a and 5a) than
the dmgH complex (116° vs 112°). [We have not considered
the comparison with the value of 115° in X3 since it lacks the
interaction between the axial and the equatorial ligand.] This
also suggests a higher steric cis effect of the dmestgH ligand
than the dmgH.
5a, whereas the benzyl ring is somewhat tilted in 3a. This is
why the O-CH2-C angles are so different in 3a and 5a [115.4°
and 105.3°]. However, the Co-O-O angle is almost the same
[116.4° and 116.5°], indicating identical steric crowding due to
the [Co(dmestgH)2] moiety. The most striking difference lies
in the O-O-C-C dihedral angle, which defines the orientation
of the C-C bond of the benzyl or naphthyl group; for example,
it drastically changes from 60.8° in 3a to 175.3° in 5a. This is
explained as follows.
The 4-CN-C6H4CH2- group in 3a partly occupies the cavity
created by four methyl groups of two dmestgH ligands. The
phenyl ring is above the cavity and has interactions with the
equatorial ligand (Figure 4). However, the bulkier 2-naphthyl
group cannot fit into this cavity and undergoes inherent
interaction with the dioxime from the sideways direction in such
a way that it attains an orientation with a very large dihedral
angle [the O-O and the C-C(Ph) plane are almost parallel
(175°) to each other] (Figure 4). The cumyl(O2) complex (X1)
also shows a similar orientation of the bulkier CH(Me)Ph with
a large dihedral angle. Although the complex X3 bears a
secondary carbon and lacks interaction with the dioxime, the
axial group orients with a very small dihedral angle of 63.66°.
This suggests that both the steric cis influence and the
interactions between axial and equatorial ligands are important
and contribute to the Co-C bond stability.
There are a few important differences between the molecular
structures of 3a and 5a; the displacement of cobalt atom (d)
from the N4 plane is toward pyridine in both 5a and 3a. The
naphthyl ring is almost perpendicular to the dioxime plane in
To summarize, the dmestgH ligand has the highest steric cis
influence among all the reported complexes with other dioximes
and Co(O2) though bound to a primary carbon in 3a and 5a,

Co-C Bond ReactiVity in Benzylcobaloximes
Organometallics, Vol. 26, No. 14, 2007 3565
Since the Co-C bond cleavage is the key step in the insertion
reaction and the effect of the cis influence of the dioxime is
felt most on the Co-CH2 bond,7a the very high rate of insertion
in the dmestgH complex is quite justified keeping in view the
steric cis influence observed in the crystal structures. However,
it is worth noting that the electronic cis influence for dpgH and
dmestgH complexes is quite similar on the basis of the 1H NMR
data (δ(Co-CH2) 3.55 ppm in both).
A reactivity difference in the oxygen insertion between alkyl
and benzyl cobaloximes has been observed. In general, the
benzyl complexes are more reactive than the alkyl complexes,
although the Co-C bond length is similar in both. The steric
cis influence of the methyl and benzyl group on the Co-C bond
in the dmgH complexes, RCo(dmgH)2Py, is found to be similar
(R and d values are similar). However this is not the case in
the dmestgH complexes; the steric cis influence is higher in
the benzyl than the methyl [higher R value in the benzyl]. This
is due to the steric crowding of the axial organic group by the
Figure 5. Comparison of the plot of ln(At - A∞) versus time (s)
for PhCH2Co(gH)2Py (6), PhCH2Co(dmgH)2Py, PhCH2Co-
(dmestgH)2Py (1), and CH3Co(dmestgH)2Py.
1
6-Me of dmestgH as observed in the H NMR. Interestingly,
on moving from methyl to any higher alkyl chain (C2-C4, ethyl
to n-butyl), the steric cis influence of the dmestgH on the CH2
group becomes similar to the benzyl cobaloxime.
Table 6. Pseudo-First-Order Rate Constants (kobs) for
Oxygen Insertion into the Co-C Bond in RCo(dioxime)2Py
In the recent study we have found that the Co-C bond
homolysis in alkyl and benzyl cobaloximes, RCo(dmestgH)2Py,
leads to different product formation; the benzyl complex gives
the oxygen-inserted product, whereas the alkyl derivative forms
air-stable CoII(dmestgH)2(Py)2.16 The formation of CoII as the
end product in the alkyl cobaloxime points to its stabilization
by the macrocyclic ligand. The oxygen-inserted product is
formed in the benzyl case due to the stabilization of CoII by
the macrocyclic ligand and the formation of stable benzyl
radical, which remains intact inside the cage by the interaction
with the macrocyclic ligand. This leads to a buildup of the
persistent radicals in solution and steers the reaction in a highly
selective manner. The very fact that the dioxy complex is formed
indicates that the benzyl group is in the vicinity of the reaction
center [CoII(O2)]. This can be seen as a cage effect. However,
there is a possibility that the difference in reactivity may partly
arise due to the difference in the stability of the benzyl and
alkyl radicals. This reactivity difference is similar to that in the
AdoCbl and MeCbl.
RCo(dioxime)2Py
kobs (s-1
)
PhCH2Co(gH)2Py
7.1 × 10-4
1.2 × 10-3
4.8 × 10-3
5.0 × 10-2
4.5 × 10-4
2.9 × 10-4
PhCH2Co(dmgH)2Py
PhCH2Co(dpgH)2Py
PhCH2Co(dmestgH)2Py
n-BuCo(dmestgH)2Py
MeCo(dmestgH)2Py
and the steric crowding by the dmestgH methyl on the phenyl
(or naphthyl) ring is more like the tertiary carbon in the dmgH
complex.
Rate Studies. Kinetic Runs. The insertion reaction is usually
carried out at 0 °C to avoid any side reactions due to thermal
conditions. However it has been reported that the rate of
insertion does not vary much between 0 and 25 °C.7,22 We have,
therefore, studied the rate of oxygen insertion in 1 and 6 at
ambient temperature because of convenience only. In order to
compare the data with the other dioximes, we have also carried
out the rate studies in the corresponding dmgH and dpgH
complexes under similar conditions. We have also confirmed
that the rates (kobs) do not vary much between 0 and 25 °C in
these complexes. The Co-C CT band varies with the dioxime
and occurs between 350 and 420 nm. So before starting the
kinetic runs we have done a preliminary experiment to obtain
the point where the largest change in absorbance occurs, and
these are at 385, 363, and 390 nm for dmestgH, dmgH, and gH
complexes, respectively. Ocean Optics software in the UV-
vis machine allows continuous scan for the change in absorbance
So a comparison of the rate data in these complexes would
be of interest. The kobs is 2.9 × 10-4, 4.5 × 10-4, and 5.0 ×
10-2 s-1 for Me, n-Bu, and BnCo(dmestgH)2Py; the rate is much
higher in the benzyl than in the alkyl complexes. Also these
rates are much higher than in the corresponding dmgH
complexes (see Table 6). The difference in the rates between
the benzyl and alkyl must be due to the additional interaction
between the axial and the equatorial ligand. The kobs in the gH
complex 6 is the smallest among all the dioxime complexes
and fits well into the cis influence order given above.
at a particular wavelength with time. The rate constants (kobs
)
The different product formations and the difference in rate
in the n-butyl and benzyl complexes suggest that not only the
Co-C bond reactivity is different but the recombination step
also has an influence on the oxygen insertion rate and the
formation of Bn(O2) is facilitated by the persistent radical effect
(cage efficiency) of the benzyl radical.16 This is similar to the
difference in the Ado and MeCbl. Interestingly, the interaction
between the axial and equatorial ligands (plausible cage
mechanism Scheme 2) has also had a significant influence on
the product yield in the insertion reaction; for example it is much
lower (40%) in the butyl than in the benzyl analogue (80%).
were calculated from the slope of the linear plots of ln(At -
A∞), where At is the absorbance at time t and A∞ is the final
absorbance versus time (s), and are given in Figure 5 and Table
6. The rate data show that the insertion depends upon the
equatorial ligand and follow the order dmestgH . dpgH >
dmgH > gH. The steric cis influence on the Co-C bond also
follows the same order. We had observed the dependence of
the rate of insertion on the cis influence in our earlier studies,7a
but a conclusion could not be drawn from the data based only
on the dmgH and dpgH complexes.
The X-ray structure details also give useful information. In
general, d is positive, R is low, and τ is ∼90° in MeCo-
(22) Giannotti, C.; Fontaine, C.; Septe, B. J. Organomet. Chem. 1974,
71, 107.

3566 Organometallics, Vol. 26, No. 14, 2007
Bhuyan et al.
Scheme 2. Plausible Cage Effect Mechanism of Oxygen Insertion Reaction
NMR spectra were recorded on a JEOL JNM Lambda 400 FT NMR
(dioxime)2B (gH, dmgH, dpgH) (Supporting Information, Table
S3). In contrast, d is negative, R is high, and τ is highly deviated
from 90° in the dmestgH complex, MeCo(dmestgH)2Py.5 Its
crystal structure also shows that the Py ring is puckered and
has steric interaction with the 2-Me groups of dmestgH. The
corresponding benzyl complexes are highly unstable in solution
1
instrument (at 400 MHz for H and at 100 MHz for 13C NMR
spectra) in CDCl3 solution with TMS as internal reference.
Elemental analysis was carried out using a thermoquest CE
instruments CHNS-O elemental analyzer. UV-vis spectra were
recorded on an Ocean Optics USB4000 (BASi, USA) UV-vis
spectrometer in dry chloroform.
1
and suggest more strain in these molecules. The H NMR has
X-ray Structural Determination and Refinement. Orange
crystals were obtained by slow evaporation of the solvent (dichlo-
romethane/acetonitrile/benzene/hexane) for 3a and 5a (in a methanol/
dichloromethane mixture for 11 [C6H4CH2Co(gH)2(4-tBuPy)]).
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 for 3a and 5a and the data
collected at 293 K on a CAD4 for 11. The linear absorption
coefficients, scattering factors for the atoms, and anomalous
dispersion corrections were taken from International Tables for
X-ray Crystallography.24a The data integration and reduction were
processed with SAINT25 software. An empirical absorption cor-
rection was applied to the collected reflections with SADABS26
using XPREP.27 The structures were solved by the direct method
using SIR-9728 and were refined on F2 by the full-matrix least-
squares technique using the SHELXL-9724b 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 positions 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 7.
The cif files have been deposited with the Cambridge Crystal-
lographic Data Centre (CCDC numbers for 11, 3a, and 5a are
265406, 607226, and 634745). 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/).
Synthesis of ArCH2Co(dioxime)2Py (1-10). The compounds
1-5 were synthesized by the general procedure detailed earlier for
the synthesis of RCo(dmestgH)2Py (where R is an alkyl group)
and involved the reaction of CoI with organic halide in ethanol
(yield ) 85-90%).5 The compounds 6-10 were synthesized and
purified by the procedure used for the synthesis of benzylCo-
(gH)2Py; the addition of 2 or 3 drops of acetic acid was necessary
during workup (yield 45-65%).8b
already shown that the 2-Me protons are more upfield shifted
in the benzyl complex as compared to the methyl analogue due
to its higher interaction with the pyridine ring current. This is
possible if the bending angle of the dioxime is high, which will
push the 2-Me groups closer to pyridine. The insertion of oxygen
releases this strain and leads to lesser puckering of the dioxime.
This is why a much smaller upfield shift of 2-Me in the oxygen-
inserted product is observed. The low value of R (3.17° and
3.14°) and d (+0.011/+0.021 Å) in 3a and 5a supports this
view. The release of strain might be the driving force for the
higher rate constant for the O2 insertion reaction in the dmestgH
complexes, so much so that the otherwise less reactive alkyl
analogues undergo facile oxygen insertion; for example the
MeCo(dmestgH)2Py complex goes to half completion and then
attains an equilibrium state after 2 h, and the n-butyl complex
goes to completion in 2 h.
Conclusion
The steric cis influence affects the Co-C bond stability/
reactivity, and a good relationship has been found between the
rate of oxygen insertion and the steric cis influence in benzyl
cobaloximes. Both follow the same order dmestgH > dpgH >
chgH > dmgH > gH. In addition to the steric cis influence of
the dioximes, the interactions between the axial and equatorial
ligands also play a significant role in the Co-C bond stability/
reactivity and in the product formation. Such interactions are
also present in the molecular structures of the dioxy complexes.
The results support the cage mechanism for the insertion of
oxygen in these complexes. Since the steric cis influence of
the mixed dioxime complexes fall between the parent co-
baloximes, it would be interesting to study the rate of oxygen
insertion in the mixed dioxime complexes. However, complica-
tions will arise because these complexes equilibrate in solution
to a mixture of products.
(23) (a) Tcheou, F.-K.; Shih, Y.-T.; Lee, K.-L. J. Chin. Chem. Soc. 1950,
17, 150. (b) Grice, R.; Owens, L. N. J. Chem. Soc. 1963, 1947.
(24) (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.
(25) SAINT+, 6.02 ed.; Bruker AXS: Madison, WI, 1999.
(26) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(27) XPREP, 5.1 ed.; Siemens Industrial Automation Inc.: Madison, WI,
1995.
Experimental Section
General Comments. Cobalt chloride hexahydrate (SD Fine,
India), glyoxime (Alfa Aesar, Lancaster), benzyl chloride, 4-chlo-
robenzyl chloride, 3-methoxybenzyl chloride, and 2-chlorometh-
ylnaphthalene were purchased from either Aldrich or Fluka and
were used as such. Dichloroglyoxime,11 dimesitylglyoxime,11 ClCo-
(dioxime)2Py (dioxime ) glyoxime12 and dimesitylglyoxime5), and
4-cyanobenzyl bromide23 were prepared according to literature
procedures. Silica gel (100-200 mesh) and distilled solvents were
(28) 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.
used in all reactions and chromatographic separations. 1H and 13
C

Co-C Bond ReactiVity in Benzylcobaloximes
Organometallics, Vol. 26, No. 14, 2007 3567
Table 7. Crystal Data and Structure Refinement Details for 3a, 5a, and 11
3a‚2CH3CN‚C6H6
C63H69N8O6Co1
5a‚2CH2Cl2
11
empirical formula
fw
C58H64Cl4N5O6Co1
1127.87
C20H26Co1N5O4
459.39
1093.19
temp (K)
diffrn measurement
device/scan method
cryst syst
space group
unit cell dimens
a (Å)
100(2)
100(2)
293(2)
Smart CCD area detector/æ-ω
Smart CCD area detector/æ-ω
Enraf-Nonius CAD4 Mach2/θ-2θ
triclinic
P1h
monoclinic
P21
monoclinic
P21/n
14.2170(11)
8.6212(6)
10.177(4)
b (Å)
14.3381(11)
26.734(2)
10.854(4)
c (Å)
16.6229(13)
13.5996(10)
19.864(7)
R (deg)
112.352(2)
95.690(2)
111.594(2)
2799.0(4)
90
90
101.0(2)
90
â (deg)
102.9950(10)
90
3054.2(4)
γ (deg)
V (Å3)
2153.9(14)
Z
2
2
4
F(calcd) (Mg/m3)
1.297
0.367
1156
1.226
0.506
1180
1.417
0.833
960
µ(mm-1
F(000)
)
cryst size (mm3)
index ranges
0.28 × 0.22 × 0.20
-18 e h e 18, -14 e k e 19,
-21 e l e 22
18 867
13 342
0.24 × 0.22 × 0.15
-11 e h e 9, -35 e k e 34,
-11 e l e 17
14 960
10 776
0.928
0.21 × 0.16 × 0.14
0 e h e 12, 0 e k e 12,
-23 e l e 23
4014
3786
no. of reflns collected
no. of indep reflns
GOF on F2
1.043
1.005
final R indices (I > 2σ(I))
R1 ) 0.0748, wR2 ) 0.1987
R1 ) 0.1041, wR2 ) 0.2195
13 342/0/717
R1 ) 0.0667, wR2 ) 0.1441
R1 ) 0.851, wR2 ) 0.1515
10 776/1/677
R1 ) 0.0801, wR2 ) 0.1430
R1 ) 0.1879, wR2 ) 0.1836
3786/0/271
R indices (all data)
no. of data/restraints/params
Molecular Oxygen Insertion in RCH2Co(dioxime)2Py (1-
10): General Procedure.7 A solution of ArCH2Co(dioxime)2Py
(0.07 g) in 15 mL of dichloromethane was irradiated at 0 °C with
2 × 200 W tungsten lamps at approximately 10 cm distance, and
oxygen was bubbled through the solution. The progress of the
reaction was monitored with TLC on silica gel using a 20%
ethylacetate/CH2Cl2 mixture for 1-5 and 100% ethyl acetate for
6-10. The reaction was complete in 1 h. There was a distinct color
change from yellow-orange to dark brown at this stage. At the end
of reaction, the solvent was evaporated and the crude product was
purified on a silica gel column using ethyl acetate and CH2Cl2 as
the eluent. Yield: 80-83% for 1a-5a and 70-80% for 6a-10a.
Kinetic Study. The kinetics study of the oxygen insertion in
ArCH2Co(dioxime)2Py under photochemical conditions was fol-
lowed spectrophotometrically at 0 °C as well as at ambient
temperature under pseudo-first-order conditions. Oxygen was
bubbled through the solution and hence used in large excess. The
progress of the reaction was monitored following the changes
(decreases) in absorbance (the absorbance due to the Co-C CT
band) at regular intervals. The kobs rate constants were obtained
from the slope of the linear plots of ln(At - A∞), versus time, where
At is the absorbance at time t and A∞ is the final absorbance. Kinetic
data were analyzed with ORIGIN 6.1.
Acknowledgment. We thank DST (SR/S1/IC-12/2004) and
CSIR [01(1949)/04/EMR II], New Delhi, India, for financial
support.
Note Added after ASAP Publication: References 16 and
17 have been corrected. The paper was published on the Web
on June 12, 2007. The corrected version was reposted on June
15, 2007.
Supporting Information Available: Table of CHN analysis
data and molecular structure comparison data, representative figures
of 1H and 13C NMR and UV-vis spectra, and CIF files for X-ray
crystal structures of 11, 3a, and 5a. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM7003578