Cobaloximes with pyrazine and their dimetallic complexes

Article abstract of DOI:10.1002/ejic.200600280

Mono- and dinuclear cobaloximes with inorganic anions and pyrazine (Pz) as a base/bridging ligand were synthesized and characterized by ¹H, ¹³C, UV/Vis and IR spectroscopy and X-ray structure analysis. Mononuclear complexes XCo-(dioxime)₂Pz were used as precursors for building homo- and heterodimetallic complexes. The X-ray crystal structures of ClCo(dmgH)₂Pz, ClCo(dpgH)₂Pz, [Co(dpgH) ₂(NO₂)₂]Na, and [Co(dmgH)₂(NO ₂)₂]₂[Ag]₂·H₂O are reported. [Co(dmgH)₂(NO₂)₂] ₂[Ag]₂·H₂O shows two Ag^I ions in different environments: one is in tetrahedral, whereas the other is in capped trigonal coordination and the metal-organic framework in this molecule shows helical structure. A cyclic voltammetry study in [XCo(dioxime) ₂]₂-μ-Pz and XCo(dioxime)₂Pz [dioxime = dmgH (dimethylglyoximato), dpgH (diphenylglyoximato)] is reported. The reduction, Co^III to Co^II and Co^II to Co^I, is found to be more difficult in [XCo(dpgH)₂]₂-μ-Pz than in XCo(dpgH)₂Pz. The CV data have been interpreted in terms of cobalt anisotropy. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600280

FULL PAPER
DOI: 10.1002/ejic.200600280
Cobaloximes with Pyrazine and Their Dimetallic Complexes
Debaprasad Mandal[a] and Bhagwan D. Gupta*[a]
Keywords: Cobaloximes / Cyclic voltammetry / X-ray structure / Helical structures
Mono- and dinuclear cobaloximes with inorganic anions and
pyrazine (Pz) as a base/bridging ligand were synthesized
and characterized by 1H, 13C, UV/Vis and IR spectroscopy
and X-ray structure analysis. Mononuclear complexes XCo-
(dioxime)2Pz were used as precursors for building homo- and
heterodimetallic complexes. The X-ray crystal structures of
ClCo(dmgH)2Pz, ClCo(dpgH)2Pz, [Co(dpgH)2(NO2)2]Na, and
[Co(dmgH)2(NO2)2]2[Ag]2·H2O are reported. [Co(dmgH)2-
(NO2)2]2[Ag]2·H2O shows two AgI ions in different environ-
ments: one is in tetrahedral, whereas the other is in capped
trigonal coordination and the metal-organic framework in
this molecule shows helical structure. A cyclic voltammetry
study in [XCo(dioxime)2]2-µ-Pz and XCo(dioxime)2Pz [diox-
ime = dmgH (dimethylglyoximato), dpgH (diphenylglyoxim-
ato)] is reported. The reduction, CoIII to CoII and CoII to CoI,
is found to be more difficult in [XCo(dpgH)2]2-µ-Pz than in
XCo(dpgH)2Pz. The CV data have been interpreted in terms
of cobalt anisotropy.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
the pyrazine-bridged mixed-valence homo- and heterodime-
tallic complexes of Ru, Fe, and Co are known,[9] there are
few in cobaloxime chemistry.[10–12] A mixing of two cobalt
centers with electron delocalization by pyrazine has been
observed in the recently reported pyrazine-bridged organo-
dicobaloximes, [RCo(dioxime)2]2-µ-Pz.[11,12]
Introduction
Organocobaloximes, initially proposed as models of the
B12 coenzyme, have now acquired an independent research
field because of their rich coordination chemistry and po-
tential application in organic synthesis.[1] The variation in
the N–Co–C bond properties, including its strength, has
been described not only in terms of the trans influence of
the axial neutral base but also of the steric and electronic
properties of the alkyl group.[2,3] The influence of the equa-
torial ligands on the axial ligands (cis influence) in dioxim-
ates, Costa’s model, and iminates (cobalt complexes with
tetradentate Schiff bases) has been previously reported.[2]
Such studies are limited in cobaloximes.[4,5] In recent years
the description of spectroscopic data and structure-property
relationships and their correlation to Co–C bonds have
been most emphasized.[6] We have shown that spectral cor-
relations are much better understood in terms of field ef-
fect; the model takes into account the total effect of cobalt
anisotropy of the [Co(dioxime)2]+ metallabicycle (electron-
withdrawing power) and the ring current arising from the
delocalization of electron density throughout the [Co(diox-
ime)2]+ metallabicycle.[5,7]
Among the electrochemical studies on cobaloximes only
few describe a correlation to related cobalt complexes to
show the mutual cis and trans influence.[13] Recent work has
shown that the reduction potential depends upon both the
axial and equatorial ligands.[5,7,14] CV data have been ra-
tionalized in terms of cobalt anisotropy and this in turn is
affected by the axial ligands; the effect is more distinct when
the axial ligand is an inorganic group/atom.[5,7] A single
peak with a very high reduction potential is observed in
[RCo(dioxime)2]2-µ-Pz, which points to electron delocaliza-
tion and also shows that the dicobaloximes are more diffi-
cult to reduce than the corresponding monocobaloximes.[12]
In general, the inorganic cobaloximes XCo(dioxime)2B
(B = neutral base) give a better CV curve than organocoba-
loximes.[5,7] Also, the changes in NMR chemical shifts in
the inorganic complexes are much more marked than in or-
ganocobaloximes and these are more effectively rationalized
with the field effect.[4c,5,7,15]
Ligand-bridged complexes are of interest in view of their
role as reaction intermediates in inner-sphere electron-
transfer processes[8] and pyrazine has been extensively used
as a bridging ligand. Although a number of examples of
It is therefore interesting and useful to study the pyr-
azine-bridged dinuclear inorganic cobaloximes, [XCo(diox-
ime)2]2-µ-Pz. The spectral and CV data in these dinuclear
complexes should complement the reported data in [RCo-
(dioxime)2]2-µ-Pz. It would also be useful to compare the
data with the corresponding mononuclear cobaloximes,
XCo(dioxime)2Pz.
[a] Department of Chemistry, Indian Institute of Technology,
Kanpur 208016, India
Fax: +91-512-2597436
E-mail: bdg@iitk.ac.in
The aim of the present study is (a) to synthesize [XCo-
(dioxime)2]2-µ-Pz, XCo(dioxime)2Pz, and XCo(dioxime)2-
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Eur. J. Inorg. Chem. 2006, 4086–4095

Cobaloximes with Pyrazine and Their Dimetallic Complexes
FULL PAPER
Scheme 1. Cobaloximes.
Pz–M [M = CuI, AgI], (b) to verify the validity of our re-
XCo(dioxime)2Pz (13–18) were prepared by the reaction
cently proposed field effect model in the interpretation of of the corresponding aqua complexes 7–12 with pyrazine in
spectroscopic data in these complexes, and (c) to perform a a 1:1 ratio by slow evaporation of the solvent. These were
CV study (Scheme 1).
the only products formed in dilute solution, but in a con-
centrated solution the precipitation of the dinuclear product
(1 or 4) occurred and no mononuclear product was formed.
Also a rapid evaporation of the solvent resulted in the for-
mation of a dinuclear complex.
Results and Discussion
Synthesis
The synthesis of complexes 1 and 4 was easy; an addition
of pyrazine to the corresponding aqua complex ClCo(diox-
ime)2H2O in a 1:2 molar ratio afforded the corresponding
dicobaloxime.
Two possibilities were considered for the preparation of
complexes 2, 3 and 5, 6:
Spectroscopy: Characterization of the Complexes
1
The H NMR spectra of 1–6 and 13–18 are easily as-
signed based on the chemical shifts. The signals are as-
signed according to their relative intensities and are consis-
tent with the related dioxime compounds previously de-
scribed.[11] Complexes 1–3 are sparingly soluble in most or-
ganic solvents and it took more than 1000 scans to obtain
(a) We have recently synthesized XCo(dioxime)2Py (X =
NO2, N3) complexes by the nucleophilic substitution of the
chlorido ligand in ClCo(dioxime)2Py.[5,7,15,16] An obvious
choice was to follow the same route. However, the reaction
of NaN3 or NaNO2 with [ClCo(dpgH)2]2-µ-Pz (4) did not
give the desired compound, but instead a CoIII complex was
isolated that was sparingly soluble in chloroform and
dichloromethane but soluble in methanol/water and ace-
tone/water. The 1H NMR in [D6]DMSO showed the ab-
sence of a pyrazine peak. It indicated the presence of a salt
(discussed later).
(b) The same procedure as used in the preparation of 1
and 4 was followed. The dicobaloximes were formed in
good yield. However, this procedure required the prepara-
tion of the corresponding aqua complexes, XCo(dioxime)2-
H2O (8, 9, 11, 12).[17] These were prepared from the corre-
sponding aqua(chlorido)cobaloximes (7 or 10) by the nucle-
ophilic substitution of the chlorido ligand. Any change in
concentration or use of excess NaX led to the formation of
a salt along with the desired product. The salt was identical
to the product obtained earlier in the reaction of
[ClCo(dpgH)2]2-µ-Pz (4) with NaNO2 and was identified as
[Co(dpgH)2(NO2)2]– by X-ray crystallography (19). López
et al. have reported a similar type of salt formation in the
reaction of ClCo(dioxime)2B with excess KCN.[18]
1
1
a reasonable H NMR spectrum in CDCl3. The H values
are given in Table 1.
All four protons in the unligated pyrazine are equivalent
and appear as a singlet at δ = 8.59 ppm. The ligated pyr-
azine in 4–6 appears as a singlet which is shifted downfield
as compared to that of free pyrazine. On the other hand,
pyrazine appears as two doublets in 13–18; the one that is
more upfield is assigned to the protons that are in close
proximity to the cobaloxime moiety. The ring current of the
metallabicycle affects these protons.[5,7] Hence, their signals
are more downfield in 16–18 than in 13–15 because of the
1
lower ring current in the dpgH complexes. H NMR spec-
troscopy in the region δ = 7.5–9.0 ppm can easily differen-
tiate between the di- and mononuclear complexes; pyrazine
appears as a singlet in the former and as two doublets in
the latter.
Spectral Studies
NMR
In the earlier NMR study on cobaloximes XCo(di-
oxime)2Py (dioxime = dmgH, dpgH), Pyα and Pyγ were
Eur. J. Inorg. Chem. 2006, 4086–4095
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4087

D. Mandal, B. D. Gupta
Yield (%)
FULL PAPER
1
Table 1. H NMR and yield.
No.
O–H···O
Pz
dmgH
dpgH/others
1
2
–
–
–
8.09
8.16
8.18
8.66
8.74
2.45
2.42
2.45
–
–
–
–
–
–
89
70
75
49
48
58
60
3
4
18.52
18.48
18.36
–
7.82 (d), 7.77 (t), 7.32 (t), 7.18–7.33 (m)
5
7.13–7.29
7.14–7.26 (m)
–
6
8.63
13
8.33 (d)
8.50 (d)
8.35 (d)
8.57 (d)
8.38 (d)
8.55 (d)
8.61 (d)
8.64 (d)
8.66 (d)
8.72 (d)
8.60 (d)
8.62 (d)
–
2.42
14
15
–
2.35
2.42
–
–
58
58
80
70
65
17.93
18.57
18.39
18.41
18.47
–
16[a]
17
7.05–7.28 (m)
7.16–7.34 (m)
7.16–7.32 (m)
7.24–7.44 (m)
–
18
–
19[b,c]
–
[a] 13C NMR: δ = 154.05, 147.31, 145.46, 129.92, 129.80, 129.31, 128.01 ppm. [b] Recorded in [D6]DMSO. [c] [Co(dpgH)2(N3)2]– ([D6]-
1
DMSO): H NMR: δ = 18.59, 7.41 (d, 4 H), 7.39 (d, 8 H), 7.27–7.24 (dd, 8 H) ppm; 13C NMR: δ = 151.5, 130.0, 128.7, 127.5 ppm.
found to be the most affected by a change in X.[5,15] The dpgH complexes (compare 4–6 and 16–18 with Pzα in free
cobaloximes 13–18 are very similar to pyridine complexes pyrazine). This is because of the higher cobalt anisotropy
except that pyridine is replaced by pyrazine (pKa of Pz of dpgH, which completely compensates for the upfield
1.1,[19] Py 5.3[20]). Also, the spectral studies of pyrazine- shift due to the ring current. Similar compensation is also
bridged alkyl- (and benzyl-) dicobaloximes have already observed in the mono(pyrazine) complexes (compare 13–15
been reported from our laboratory.[11,12] Therefore, rather with 16–18) as well as in the pyridine complexes XCo(diox-
than going into the details about the spectroscopic data and ime)2Py, viz. ∆δ of the Pyα signals in the dmgH complexes
cis/trans influence study in the present systems, we want to is around –0.3 ppm, whereas it is close to zero in the dpgH
highlight only the salient points.
complexes.
The coordination shift[21] ∆δ1H of the Pzα signal in 13–
The pyrazine signals in [RCo(dioxime)2]2-µ-Pz were
18 is very close to the coordination shift ∆δ1H of the Pyα found to be shifted downfield in the dpgH complexes and
signal in the corresponding pyridine complexes, indicating upfield in the dmgH complexes as compared to the unlig-
further the similarity in the two systems (Table ST2). As Hα ated pyrazine.[11,12] A similar trend is observed in the inor-
is affected by the dioxime ring current and cobalt aniso- ganic complexes 1–6 and ∆δ of the Pzα signals in [RCo-
tropy, its signal appears more upfield in the dmgH com- (dioxime)2]2-µ-Pz complexes is larger than that in 1–6 be-
plexes than that of the dpgH complexes in both pyridine cause of the higher trans effect of the R group with respect
and pyrazine complexes.[4a,11,12] One of the major differ- to the X.
ences found in the pyridine and pyrazine complexes is the
coordination shift of the β-hydrogen signal. It is almost un-
UV/Vis
altered in the pyrazine complexes, whereas it was shifted
Three bands, a strong band at 263 nm corresponding to
downfield (by 0.2–0.3 ppm) in the corresponding pyridine the CoǞdioxime metal-to-ligand charge transfer (MLCT)
complexes. This is due to the presence of a nitrogen atom and two shoulders around 313 and 330 nm (CoǞPz), are
instead of a carbon atom at the sixth position in pyrazine. observed in 16. A similar spectrum is obtained in 4 except
The cobalt anisotropy is less in the pyrazine complexes be- that the intensity of the band corresponding to CoǞPz is
cause of its lower pKa value and hence it compensates for significantly increased. This is expected because of the addi-
the β-hydrogen signal downfield shift.
tive factor of the two identical chromophoric groups in the
In the [(R/X)Co(dioxime)2]2-µ-Pz complexes, Pzα is affec- dinuclear complexes. A similar result is found in the dmgH
ted by the ring current and the cobalt anisotropy of the two complexes (see Table 2).
metallabicycles (the factors ring current and cobalt anisot-
In general, organo-cobaloximes exist largely as pentaco-
ropy work in opposite directions: the Pzα signal is shifted ordinate species in coordinating solvents like methanol and
upfield by the ring current and downfield by the cobalt an- as hexacoordinate species in chloroform or dichlorometh-
isotropy) as compared to one in (R/X)Co(dioxime)2Pz, ane.[4a,22] Pyrazine-bridged alkyldicobaloximes show similar
therefore ∆δ of the Pzα signals should be much larger or behavior to the alkyl(pyridine)cobaloximes.[11] The base-on/
even doubled in the former. It is found to be –0.2 to base-off behavior of organocobaloximes has generally been
–0.3 ppm in 13–15 and –0.4 to –0.5 ppm in 1–3; however, studied by titration with the base, and the inference is made
the difference is unexpectedly small in the corresponding from the change in shape of the Co–C CT band with the
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Eur. J. Inorg. Chem. 2006, 4086–4095

Cobaloximes with Pyrazine and Their Dimetallic Complexes
Table 2. UV/Vis and IR data.
FULL PAPER
Compound
UV/Vis: λmax [nm]
CoǞPz
IR: ν [cm–1]
C=N
˜
CoǞdioxime
(MLCT)
CoǞPz
(shoulder)
O–H···O
N–O
N–OЈ
(shoulder)
1
4
13
224 (sh), 250 (4.32)
220 (3.71), 258 (4.36)
256 (4.35)
292 (3.77)
292 (4.21)
290 (3.74)
312 (3.76)
325 (4.30)
308 (3.69)
3417.6 (br.)
3433.0 (br.)
3530.0
1563.7
1530.4
1574.9
1242.9
1288.5
1239.9
1091.4
1139.8
1094.1
3446.7
16
263 (4.46)
313 (4.09)
330 (4.04)
3459.8
3421.2
1531.5
1285.6
1136.9
13·CuIPF6
230 (4.91), 259 (4.52)
3443.5, 1566.4, 1239.3, 1092.3, 845
3438.7, 2025.3, 1383.3, 1136.5
[Co(dpgH)2(N3)2]–
–
gradual addition of base to the cobaloxime. It has, however, bridged complex gives an IR spectrum identical to that of
not been possible to obtain a similar information in the dicobaloxime 1 along with a distinct sharp peak at 845 cm–1
–
present inorganic cobaloximes, because the phenomenon for PF6 (Figures S16 and S17).
can be monitored by the CoǞdioxime MLCT band only
and this occurs at the same position as in the pyrazine case.
Cyclic Voltammetry
Titration with CuIBF4 Salt
The cyclic voltammogram (CV) of a cobaloxime shows
The titration of Cu+ with a solution of 13 in dichloro-
three types of redox couples: CoIII/CoII, CoII/CoI, and
methane was monitored by UV/Vis spectroscopy at 233 and
CoIV/CoIII. Organo-cobaloximes, in general, give poor CVs
259 nm. The absorbance of the solution increased until
and most of the time the CoIII/CoII redox couple is not
0.5 equiv. of Cu+ was added (see Figure 1). No spectral
visible. On the other hand, inorganic cobaloximes show rel-
change was observed after the addition of 0.5–1.0 equiv. of
atively better CVs and all the redox couples are prominent.
Cu+. This suggests that there is a 2:1 complexation with
Two common solvents used for the study are dichlorometh-
Cu+. Job’s plot also suggests a 2:1 complexation (Fig-
ane and acetonitrile. We have preferred to use dichloro-
ure S14).
methane because of the higher solubility of the cobalox-
imes. The cyclic voltammogram of 13 (Figure 2A) shows an
irreversible wave in the reductive half at –0.47 V corre-
sponding to CoIII/CoII and a quasireversible wave at
–1.08 V corresponding to CoII/CoI. [Interestingly, a CV of
13 recorded in acetonitrile gives a much better plot in which
all the peaks are prominent and show the reversible nature
(Figure 2B).] In comparison, 16 (Figure 2C) is much more
easily reduced, with values at –0.32 and –0.73 V, respec-
tively. In the oxidation half corresponding to CoIV/CoIII
,
13 and 16 show one reversible wave at +1.25 and 1.38 V,
respectively. The CV data are very similar to those of the
corresponding pyridine analogues except that the reduction
potential values are low and the oxidation halves are high
(see Table 3). This agrees well with the pKa of pyridine and
pyrazine.
The CV data for both the oxidative and reductive halves
can be rationalized on the basis of the cobalt anisotropy.
The higher the cobalt anisotropy, the lower the reduction
Figure 1. Cu+ addition to a solution of [ClCo(dmgH)2Pz] (13). The
Cu+ saturation point is at 0.5 mol-equiv. [ClCo(dmgH)2Pz] =
1×10–5  (1 mL). [Cu+] = 1×10–5  (0.1–1.0 mL).
potential and the higher is the oxidation potential.[5,7]
A
comparison of the data in 13 with those in 16 shows that
the reduction is easier and the oxidation is more difficult in
IR
All IR spectra were recorded as solid samples on KBr 16 because of the higher cobalt anisotropy in dpgH com-
pellets and are tabulated in Table 3. The mononuclear and plexes than in dmgH complexes. We have made a similar
pyrazine-bridged dicobaloximes can be distinguished by IR observation in the gH, dpgH, mestgH, and mixed dioxime
spectroscopy; the O–H···O peak is a sharp doublet in complexes.[5,7,15] In view of this it is quite likely that the
monocobaloximes, whereas it is a broad singlet in the case cobalt anisotropy may be the major factor responsible for
of dicobaloximes. The O–H···O and C=N peaks appear at the reduction/oxidation process.
lower frequency in dpgH complexes as compared to dmgH
We could not record the CV for 1 because of its insolubil-
complexes because of the higher cobalt anisotropy and ity in most organic solvents. The CV graph for 4 has three
lower electron density in the dpgH complexes. The CuIPF6- peaks; CoIV/CoIII is prominent but CoIII/CoII and CoII/CoI
Eur. J. Inorg. Chem. 2006, 4086–4095
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4089

D. Mandal, B. D. Gupta
FULL PAPER
Table 3. CV data in dichloromethane and TBAPF6 at 0.2 Vs–1 at 25 °C.
No.
CoIII/CoII
CoII/CoI
CoIV/CoIII
[b]
[b]
[b]
E1/2 (∆Ep)[a] E1/2
ipc
ipa
ipa/ipc E1/2 (∆Ep)[a] E1/2
ipc
ipa
ipa/ipc E1/2 (∆Ep)[a] E1/2
ipc
ipa
ipa/ipc
[V] ([mV])
[V]
[µA]
[µA]
[V] ([mV])
[V]
[µA]
[µA]
[V] ([mV])
[V]
[µA]
[µA]
4
13
13[d]
16
–0.88[c]
–0.47[c]
–0.22 (130)
–0.32[c]
–0.66[c]
–1.39
–0.98
–0.73
–0.83
–1.17
–0.80
0.1
15
6.1
11.2
–
–
6.3
–
–
–
1.0
–
–1.10[c]
–1.61
–1.59
–1.52
–1.24
–1.63
–1.33
0.4
21
11.5
19.0
–
–
1.33 (220)
1.25 (280)
1.20 (107)
1.38 (120)
1.20 (190)
1.33 (150)
0.82
0.74
0.69
0.87
0.69
0.82
0.2
26.8
9.5
8
40.0
1.2
1.0
1.1
–1.08 (320)
–1.01 (80)
–0.73 (180)
–1.12 (200)
–0.82 (260)
10.7
11.5
12.7
0.5
1.0
0.64
31.2
10.0
23.6
22.3
ClCo(dmgH)2Py
ClCo(dpgH)2Py –0.29 (490)
[a] vs. Ag/AgCl. [b] vs. Fc/Fc+. [c] Values refer to Epc. [d] Value is in CH3CN.
mean equatorial CoN4 plane toward the neutral pyrazine
ligand in 13-I and 13-II, respectively, while the deviation
is –0.016 Å in 16. The positive sign means the deviation is
towards the axial base group.[2]
Figure 2. Cyclic voltammograms of 13 in dichloromethane (DCM)
(A), in CH3CN (B), and 16 in DCM (C) with 0.1  (nBu4N)PF6
as supporting electrolyte at 0.2 Vs–1 at 25 °C.
Figure 3. Molecular structure of ClCo(dmgH)2Pz (13-I and 13-II)
with two molecules in the asymmetric unit. The hydrogen atoms of
the C–H bonds are omitted for clarity.
are not; however, the differential pulse voltammogram
(DPV) clearly shows reduction. The pyrazine-bridged ben-
zyl dpgH complex, [PhCH2Co(dpgH)2]2Pz, also showed a
poor CV in the reduction half.[12] The CoIII/CoII and CoII/
CoI values in 4 are –0.88 and –1.10 V, respectively. The CoII/
CoI reduction potential is much lower than the correspond-
ing value in [PhCH2Co(dpgH)2]2Pz (–1.41 V). This is ex-
pected because of the higher cobalt anisotropy in 4. Simi-
larly, the oxidation potential CoIV/CoIII is higher (+1.33 V)
in 4 than in [PhCH2Co(dpgH)2]2Pz (+1.12 V).
X-ray Crystallographic Studies
Description of Structures 13 and 16
The “diamond” diagrams of molecular structures 13 and
16 along with selected numbering schemes are shown in
Figures 3 and 4, respectively. Selected bond lengths, bond
angles, and structural parameters are given in Table 4. The
Figure 4. Molecular structure of ClCo(dpgH)2Pz (16). The hydro-
gen atoms of the C–H bonds are omitted for clarity.
The Co–Cl [2.2332(9), 2.2262(10), and 2.2185(10) Å] and
crystal structure of 13 contains two molecules in its asym- Co–N5 [1.958(3), 1.967(3), and 1.961(3) Å] bond lengths in
metric unit. As there is a structural variation, they are num- 13-I, 13-II, and 16 are very similar to their those of the
bered as 13-I and 13-II and the crystal data are given corresponding ClCo(dioxime)2Py cobaloximes (dioxime =
separately (Table 4).[23] Cobalt is in a distorted octahedral dmgH or dpgH). This indicates that the bond length does
geometry with four nitrogen atoms of the dioxime in the not change with a slight difference in the cis or trans influ-
equatorial plane and pyrazine and Cl axially coordinated. ence.[2a,5] However, the Co–Cl [2.252(1) Å] and Co–N5
The cobalt atom deviates +0.013 and +0.028 Å from the [1.981(3) Å] bonds are significantly longer in ClCbl.[24]
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Cobaloximes with Pyrazine and Their Dimetallic Complexes
Table 4. Selected bond lengths, bond angles, and structural data.
FULL PAPER
[a]
13
16
16·2DCM
19
[Co(dmgH)2(NO2)2]2Ag2
Co1
Co2
Co–Cl [Å]
Co–N(ax) [Å]
X–Co–N5 [°]
d [Å]
α [°]
τ [°]
2.2332(9)
1.958(3)
179.12
+0.013
4.14
2.2262(10)
1.967(3)
178.11
+0.028
1.04
2.2185(10)
1.961(3)
177.62(9)
–0.016
1.62
86.38
2.2174(10)
1.963(3)
179.15(9)
+0.054
11.38
1.943(3), 1.948(4)
1.937, 1.940; 1.940, 1.940
180
0.0
ca. 0
90, 0
177.0, 180
0
0
81.34
72.63
82.60
ca. 90
[a] Ag1–O8 2.426(6), Ag1–O10 2.428(6), Ag2–O3 2.548 (6), Ag2–O6 2.352(6), Ag2–O11 2.698(5), Ag2–O12 2.383(6), Ag2–O13 2.337(5) Å.
The pyrazine ring is almost parallel to the dioxime C–C 1.948(4) Å]. Of the two units, one has a positional disorder
bonds and the twist angles (τ)[25a] are 81.34, 72.63, and of the NO2 group attached to Co2. The Co–NO2 distances
86.38° in 13-I, 13-II, and 16, respectively. The butterfly are comparable to those of NO2Cbl [1.942(6) Å][24] and
bending angle (α)[25b] in 13-I and 13-II is 4.14 and 1.04°, [Co(dmgH)2(NO2)2]– [1.945(3) Å][26] indicates no cis influ-
respectively, while it is 1.62° in 16.
ence.
When 16 is crystallized with dichloromethane, the single-
crystal analysis shows two dichloromethane molecules in
the unit cell and its molecular structure differs from 16 (Fig-
ure S22). The Co–Cl and Co–N bond lengths are 2.2174(10)
and 1.963(3) Å and comparable to the values in 16; how-
ever, α (11.38°) and d (+0.054 Å) are larger and the twist
angle is smaller (82.60°). These changes are the results of
crystal packing forces due to the extensive H-bonding and
π-interaction.
Description of the Structure of [Co(dmgH)2(NO2)2]2-
[Ag]2·H2O
The molecular structure shows the silver atoms in two
distinct environments: a pseudo-four-coordinate and a
pseudo-five-coordinate arrangement are displayed. A “dia-
mond” picture of the unique portion of this structure is
shown in Figure 6. Two cobaloxime units are repeated in
three dimensions, linked by bridging Ag atoms, to form a
3D coordination polymer. The two silver atoms are in dif-
Description of Structure 19
The characterization of 19 by X-ray analysis shows it to ferent coordination spheres: one is in a tetrahedral environ-
be [Co(dpgH)2(NO2)2]Na (Figure 5). An extensive hydro- ment and the other is in a capped trigonal environment, as
gen-bonding network results in large solvent-accessible demonstrated in Figure 7.
voids of about 211 Å3.
We see two crystallographically inequivalent silver cen-
The crystal structure contains two cobaloxime molecules ters arranged linearly, whose differences arise from the
in its asymmetric unit and has two Na+ as counter cations. binding mode of Ag. Ag1 is in a tetrahedral environment
The two cobaloxime molecules are structural isomers and bridged by four oxygen atoms; two sets of symmetry-equiv-
differ only in their orientation of the NO2 group; one is alent oxygen atoms are present. One set belongs to two ni-
above the dioxime unit [with a Co–Nax distance of 1.943(3)] tro groups and the other set to two dioxime units. The Ag1–
and the other is above the O–H···O unit [Co–Nax distance O bond lengths are 2.426(6) and 2.428(6) Å, respectively.
Figure 5. Molecular structure of [Co(dpgH)2(NO2)2]Na (19). Counter cations and hydrogen atoms of the C–H bonds are omitted for
clarity.
Eur. J. Inorg. Chem. 2006, 4086–4095
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4091

D. Mandal, B. D. Gupta
FULL PAPER
Figure 6. Molecular structure of [Co(dmgH)2(NO2)2]2[Ag]2·H2O.
Figure 7. Ag1 and Ag2 coordination environment in [Co(dmgH)2-
(NO2)2]2[Ag]2·H2O.
Ag2 has a (3+1) capped trigonal coordination sphere
with four oxygen atoms. Three short bonds [Ag2–O6
2.352(6), Ag2–O12 2.383(6), Ag2–O13 2.337(5) Å] with
bridging NO2 groups and a water molecule form the trigo-
nal geometry (slightly deviated from the planar geometry).
The longer bond Ag2–O3 [2.548(6) Å] is with the dioxime
oxygen atom; O11 acts like a chelating atom and has a weak
interaction with Ag2 at a distance of 2.698(6) Å to generate
a five-member silver-containing ring.[27]
Figure 8. (a) Space-filling view of the right-handed helix of
[Co(dmgH)2(NO2)2]2[Ag]2·H2O. (b) 2D helical assembly along the
a-axis. Some of the atoms have been omitted for clarity.
In the solid state, these tetrahedral building blocks and
cobaloxime units are linked together by the oxygen atom of
the coordinated nitro group into a novel 2D helical struc-
ture along the a-axis. A right-handed helix present in the
crystal structure is shown in Figure 8a. The helix is formed
through four Ag–O bonding interactions (O8–Ag1–O8 and
O6–Ag2–O12) along the 21 screw axis (Figure 8b). Only one
Conclusions
Procedures were developed for the synthesis of mono-
of the oxygen atoms of each nitro group takes part in the and dinuclear inorganic cobaloximes with pyrazine. The
formation of the helix. Each coil of the helix contains six changes observed in NMR and CV are much more promi-
cobaloxime residues and six Ag atoms and the distance be- nent than in organocobaloximes and these are more effec-
tween the coils is 42.91(1) Å. Also it shows a 2D metal- tively rationalized with the field effect, a model proposed
organic framework (Figure S23) and an infinite zigzag co- recently by us. Electron delocalization throughout the two
ordination polymer (Figure S24).
metallabicycles through pyrazine is observed in the NMR
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Cobaloximes with Pyrazine and Their Dimetallic Complexes
FULL PAPER
centration of 1 m of each complex. In addition, in a separate
series of experiments, an internal reference system (ferrocene/ferro-
cenium ion) was used. Under the conditions used, the reversible
Fc/Fc+ potential occurred at 0.51 V versus the Ag/AgCl electrode.
and CV studies. [XCo(dioxime)2]2-µ-Pz complexes are more
difficult to reduce than XCo(dioxime)2Pz. Mononuclear
pyrazine complexes behave similarly to the corresponding
pyridine complexes. [Co(dmgH)2(NO2)2]2[Ag]2·H2O shows
two AgI ions in different environments: one is tetrahedral
whereas the other is in a capped trigonal coordination, and
the metal-organic framework in this molecule shows helical
structure.
X-ray Structural Determination and Refinement: Orange crystals
were obtained by slow evaporation of the solvent {acetone/hexane
for 13 and 16; chloroform/methanol/hexane for 19; dichlorometh-
ane/acetone for [Co(dmgH)2(NO2)2]2[Ag]2·H2O}. Single-crystal X-
ray data were collected at 100 K with a Bruker SMART APEX
CCD diffractometer using graphite-monochromated Mo-Kα radia-
tion (λ = 0.71073 Å). The linear absorption coefficients, scattering
factors for the atoms, and the anomalous dispersion corrections
were taken from the International Tables for X-ray Crystallogra-
phy.[30a] The data integration and reduction were processed with
SAINT[31] software. An empirical absorption correction was ap-
plied to the collected reflections with SADABS[32] using XPREP.[33]
All the structures were solved by direct methods using SIR-97[34]
and were refined on F2 by the full-matrix least-squares technique
using the SHELXL-97[30b] program package. All non-hydrogen
atoms were refined anisotropically in all the structures. The hydro-
gen atoms of the OH group of oxime were located on difference
Fourier maps and were constrained to those difference Fourier map
positions. The hydrogen atom positions or thermal parameters were
not refined but were included in the structure factor calculations.
The pertinent crystal data and refinement parameters are compiled
in Table 5. CCDC-299539, -299540, -299541, -299543, and -299544
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Experimental Section
General: Cobalt chloride hexahydrate, dimethylglyoxime (SD Fine
Chemicals, India), and pyrazine (Aldrich Chemical Company) were
used as received. Diphenylglyoxime (Lancaster Chemicals) was
washed with methanol before use. Silica gel (100–200 mesh) and
distilled solvents were used in all chromatographic separations.
ClCo(dmgH)2H2O,[28] ClCo(dpgH)2H2O,[29] ClCo(dmgH)2Pz,[18]
and ClCo(dpgH)2Pz[18] were prepared according to the literature
1
procedure. H and 13C NMR spectra were recorded with a JEOL
JNM LA 400 FT NMR spectrometer (400 MHz for 1H and
100 MHz for 13C) in CDCl3 solution with TMS as internal stan-
dard. NMR spectroscopic data are reported in ppm. UV/Vis spec-
tra were recorded with a JASCO V-570 spectrophotometer in
dichloromethane (dry) and methanol (dry) at 298 K. IR spectra
were recorded with KBr pellets in the range 4000–650 cm–1 using
a Vertex-70 Bruker Spectrophotometer. Elemental analysis was car-
ried out at the Regional Sophisticated Instrumentation Center,
Lucknow (see Table ST1). Cyclic voltammetry measurements were
carried out using a BAS Epsilon Electrochemical workstation with
platinum working electrode, Ag/AgCl reference electrode (3  KCl),
and a platinum wire counter electrode. All the measurements were
performed in 0.1  nBu4N(PF6) in dichloromethane (dry) at a con-
Synthesis
[ClCo(dmgH)2]2-µ-Pz (1): Pyrazine (0.14 mmol, 0.01 g), dissolved
in acetone (1 mL), was added to a clear brown solution of
Table 5. Crystal data and structure refinement details.
13
16
19
[Co(dmgH)2(NO2)2]Ag
Empirical formula
Formula mass
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
(C12H18Cl1Co1N6O4)2
809.41
100(2)
monoclinic
P21/c
10.0217(7)
21.7874(16)
15.5993(11)
90
107.352(10)
90
3251.0(4)
4
1.654
1.251
C32H26Cl1Co1N6O4
652.97
100(2)
monoclinic
P21/c
9.953(7)
24.6908(16)
11.8723(8)
90
102.212(10)
90
2852(2)
2
C29H24Co1N6Na1.5O14
773.96
100(2)
C24H46Ag3Co3N18O26
1503.19
100(2)
monoclinic
C2/c
23.6052(24)
15.5292(17)
14.3357(15)
90
115.993(2)
90
4723.4(8)
4
2.114
2.355
triclinic
¯
P1
9.6382(10)
12.9620(13)
16.5136(17)
73.438(2)
83.668(2)
75.135(2)
1909.7(3)
2
γ [°]
V [Å3]
Z
ρcalcd. [mgm–3]
µ [mm–1]
F (000)
1.521
0.746
1344
1.346
0.535
791
1664
2984
Crystal size [mm]
Index ranges
0.31×0.23×0.21
–8 Յ h Յ 13,
–28 Յ k Յ 28,
–20 Յ l Յ 19
20450
7392
1.046
0.32×0.25×0.21
–12 Յ h Յ 13,
–32 Յ k Յ 31,
–15 Յ l Յ 9
18797
7024
1.101
0.26×0.19×0.16
–12 Յ h Յ 10,
–17 Յ k Յ 15,
–19 Յ l Յ 21
12682
9048
1.056
0.32×0.17×0.12
–23 Յ h Յ 31,
–20 Յ k Յ 14,
–18 Յ l Յ 19
15455
5832
1.116
Reflections collected
Independent reflections
Gof on F2
Final R indices
[I Ͼ 2σ(I)]
R indices (all data)
R1 = 0.0510
wR2 = 0.1137
R1 = 0.0748
wR2 = 0.1273
7392/0/441
R1 = 0.0528
wR2 = 0.1070
R1 = 0.0787
wR2 = 0.1414
7024/0/397
R1 = 0.0866
wR2 = 0.2173
R1 = 0.1119
wR2 = 0.2449
9048/0/488
R1 = 0.0650
wR2 = 0.1188
R1 = 0.1007
wR2 = 0.1522
5832/0/342
Data/restraints/parameters
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4093

D. Mandal, B. D. Gupta
FULL PAPER
ClCo(dmgH)2H2O (7) (0.29 mmol, 0.10 g in 10 mL of MeOH). An
immediate precipitation occurred. The solution was stirred for se-
veral hours to ensure complete precipitation. The yellow precipitate
was filtered, washed with acetone, and dried in vacuo over phos-
phorus pentoxide. Yield 0.091 g (89%) with respect to pyrazine.
N3Co(dmgH)2H2O (0.34 g, 1.00 mmol) in acetone (70 mL). The
solution was stirred for 1 h and kept aside for slow evaporation of
the solvent. The needle-shaped brown crystals, formed after 2–3 d,
were filtered and air-dried. Yield 60%. (a): When acetone was less
than 70 mL, an immediate precipitation occurred and the com-
pound identified was the dinuclear complex 3 (see above). (b): The
use of methanol always led to the formation of a mixture of mono-
and dinuclear complexes.
[ClCo(dpgH)2]2-µ-Pz (4): The same procedure as in 1 was used ex-
cept that ClCo(dpgH)2H2O (10) (0.20 g, 0.34 mmol) was dissolved
in acetone (2 mL) and methanol (1 mL). Yield 0.096 g (49%) with
respect to pyrazine.
XCo(dpgH)2Pz (16–18): These were prepared from the correspond-
ing aqua complexes according to the method outlined for 13–15.
Yield 80%.
[XCo(dioxime)2]2-µ-Pz (2, 3, 5, 6). (a): A solution of NaNO2
(0.03 g, 0.43 mmol) in water (2 mL) was added to a refluxing sus-
pension of 4 (0.53 g, 0.43 mmol) in methanol (30 mL). The reaction
mixture was further refluxed for 0.5 h. The solvent was evaporated
to 2–5 mL. The product isolated was a salt and did not contain the
pyrazine unit. (b): The complexes were prepared from the corre-
sponding aqua complexes according to the method outlined for 1
and 4 (Scheme 2).
Heterodimetallic Complexes
Reaction of CuIX (X = BF4, PF6, ClO4) with ClCo(dmgH)2Pz:
When CuBF4 was added dropwise to ClCo(dmgH)2Pz (13) (both
taken in dry dichloromethane), a dark red precipitate formed,
which was completely insoluble in dichloromethane. (A similar
kind of precipitation occurred with other CuI salts also.) The pre-
cipitate dissolved instantaneously upon the addition of 2–3 drops
of acetonitrile and the color changed to yellow. The precipitate
was totally insoluble in most noncoordinating solvents and hence
it could not be characterized by CV, NMR spectroscopy, or crystal-
lographic studies.
Reaction of AgI(PF6) with XCo(dmgH)2Pz: The addition of AgI to
13 or 16 in dichloromethane/acetone immediately formed a white
precipitate of AgCl, whereas the addition of an acetone solution of
AgPF6 to 14 gave an orange-yellow precipitate. In an effort to grow
a single crystal by the layering process, most of the salt precipitated
from the solution but a few orange crystals appeared on the inside
wall of the tube. X-ray analysis of one of these crystals showed it
to be [Co(dmgH)2(NO2)2][Ag]2H2O. The complex may have formed
during the crystallization process or because of a small impurity
of [Co(dmgH)2(NO2)2]Na in 14. However, this compound can be
reproduced with addition of an AgPF6 solution to [Co(dmgH)2-
(NO2)2]Na salt in acetone.
Attempted Synthesis of Mixed Dicobaloximes ClCo(dmgH)2-Pz-
Co(dpgH)2Cl (20): The synthesis of 20 turned out to be unexpec-
tedly complicated. The procedure used in the synthesis of 1–6 did
not afford the required product, for example the reaction of 10 with
13 (1:1) or the reaction of 7 with 16 (1:1) in chloroform formed 4
instead of the desired complex (20). The same reactions in acetone/
methanol afforded 1 as the exclusive product. It is very difficult to
offer any conclusive evidence for the formation of 4 or 1. It is,
however, possible that 4 is thermodynamically the more stable
product and arises from the intermediate ClCo(dpgH)2-Pz-
Co(dmgH)2Cl, a kinetically controlled product. [We have, however,
not been able to isolate this compound, even at low temperatures
(see Scheme S1).] Similarly, all efforts to synthesize other combina-
tions like ClCo(dmgH)2-Pz-Co(dpgH)2nPr and MeCo(dmgH)2-Pz-
Co(dpgH)2Cl also failed, although many procedures were tried (see
Supporting Information).
Scheme 2.
XCo(dmgH)2H2O (8, 9):[35] These compounds were synthesized by
the nucleophilic substitution of the chlorido ligand by X– in
ClCo(dmgH)2H2O (7). In a typical experiment, a solution of
NaNO2 (0.03 g, 0.43 mmol) in water (2 mL) was added to a re-
fluxing suspension of ClCo(dmgH)2H2O (7) (0.15 g, 0.43 mmol) in
methanol (30 mL). The reaction mixture was further refluxed for
0.5 h. The solvent was evaporated to 2–5 mL and a sufficient
amount of water was added to precipitate the compound; the pre-
cipitate was filtered and dried over phosphorus pentoxide. Yield
(0.12 g, 77%).
Supporting Information (see footnote on the first page of this
article): Representative figures of UV/Vis, IR, NMR spectra, and
X-ray metal-organic framework for the subject compounds.
XCo(dpgH)2H2O (11, 12): The same procedure as outlined for
XCo(dmgH)2H2O was used. Two products, a salt and the desired
compound 11, were formed. The salt was identified as [Co-
(dpgH)2(NO2)2]– by X-ray crystallography (19). However, when a
dilute solution of NaNO2 (0.03 g, in 10 mL) was treated with
ClCo(dpgH)2H2O (10) (0.25 g, 0.43 mmol) in methanol/acetone
(1:1, 30 mL) (11), the desired product was formed exclusively. Yield
(0.145 g, 71%).
Acknowledgments
We thank the CSIR [01(1949)/04/EMR-II] New Delhi, India, for
financial support.
XCo(dmgH)2Pz (13–15): Pyrazine (0.084 g, 1.05 mmol) dissolved in
[1] a) B. Giese, Radicals in Organic Synthesis: Formation of Car-
bon–Carbon Bonds, Pergamon Press, Oxford, UK, 1986; b) R.
acetone (5 mL) was added to
a clear brown solution of
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Cobaloximes with Pyrazine and Their Dimetallic Complexes
FULL PAPER
Scheffold, G. Rytez, L. Walder in Transition Metals in Organic
Synthesis (Ed.: S. Scheffold), Wiley, Chichester, UK, 1983, vol.
3; c) B. P. Branchaud, G. F. Friestad, “Vitamin B12”, in Ency-
clopedia of Reagents for Organic Synthesis (Ed.: L. A. Pa-
quette), John Wiley & Sons, Chichester, West Sussex, 1995, pp.
5511; d) M. E. Welker, Curr. Org. Chem. 2001, 5, 785–807; e)
M. Tada, Rev. Heteroat. Chem. 1999, 20, 97–144; f) Y. L. Nishi-
kubo, B. P. Branchaud, J. Am. Chem. Soc. 1999, 121, 10924–
10927; g) C. J. Tucker, M. E. Welker, C. S. Day, M. W. Wright,
Organometallics 2004, 23, 2257–2262; h) M. W. Wright, M. E.
Welker, J. Org. Chem. 1996, 61, 133–141; i) T. Brown, A.
Dronsfield, A. Jablonski, A. S. Wilkinson, Tetrahedron Lett.
1996, 37, 5413–5416; j) G. B. Gill, G. Pattenden, G. A. Roan,
Tetrahedron Lett. 1996, 37, 9369–9372; k) B. D. Gupta, V. Vi-
jaikanth, J. Organomet. Chem. 2004, 689, 1102–1109 and refer-
ences therein; l) A. A. Gridnev, S. D. Ittel, Chem. Rev. 2001,
101, 3611–3659; m) M. W. Wright, T. L. Smalley, M. E. Welker,
A. L. Rheingold, J. Am. Chem. Soc. 1994, 116, 6777–6791; n)
T. L. Smalley, M. W. Wright, S. A. Garmon, M. E. Welker,
A. L. Rheingold, Organometallics 1993, 12, 998–1000; o) B. P.
Branchaud, Y. L. Choi, Tetrahedron Lett. 1988, 29, 6037–6038;
p) R. R. Pidaparthi, M. E. Welker, C. S. Day, Organometallics
[13] C. Tavagnacco, G. Balducci, G. Costa, K. Täschler, W.
von Philipsborn, Helv. Chim. Acta 1990, 73, 1469–1480.
[14] B. D. Gupta, V. Vijaykanth, V. Singh, Organometallics 2004,
23, 2069–2079.
[15] B. D. Gupta, R. Yamuna, V. Singh, U. Tewari, Organometallics
2003, 22, 226–232.
[16] a) B. D. Gupta, K. Qanungo, R. Yamuna, A. Pandey, U. Te-
wari, V. Singh, V. Vijaikanth, T. Barclay, W. Cordes, J. Or-
ganomet. Chem. 2000, 608, 106–114; b) B. D. Gupta, U. Tewari,
T. Barclay, W. Cordes, J. Organomet. Chem. 2001, 629, 83–92.
[17] It is worth noting that the procedure typically used for the
preparation of RCo(dioxime)2H2O from the corresponding
pyridine complex by Dowex (H+ form) has failed in the inor-
ganic complexes.[11] This might be because pyridine is much
less labile because of the poor trans effect of X as compared
to R.
[18] C. López, S. Alvarez, X. Solans, M. Font-Altaba, Inorg. Chem.
1986, 25, 2962–2969.
[19] D. A. Keyworth, J. Org. Chem. 1959, 24, 1355.
[20] H. C. Brown, D. H. McDaniel, O. Hafliger, in Determination
of Organic Structures by Physical Methods (Eds.: E. A. Braude,
F. C. Nachod), Academic Press, New York, 1955, pp. 567–662.
2006, 25, 974–981; q) X. Hu, B. M. Cossairt, B. S. Brunschwig, [21] Coordination shift ∆δ = [δ(complex) – δ(free base)].
N. S. Lewis, J. C. Peters, Chem. Commun. 2005, 4723–4725; r)
M. Razavet, V. Artero, M. Fontecave, Inorg. Chem. 2005, 44,
4786–4795.
[22] K. L. Brown, D. Lyles, M. Penencovici, R. G. Kallen, J. Am.
Chem. Soc. 1975, 97, 7338–7346.
[23] J. W. Steed, CrystEngComm 2003, 5, 169–179.
[24] G. Garau, S. Geremia, L. G. Marzilli, G. Nardin, L. Randac-
cio, G. Tauzher, Acta Crystallogr., Sect. B: Struct. Sci. 2003,
59, 51–59.
[2] a) L. Randaccio, Comments Inorg. Chem. 1999, 21, 327–376;
b) L. Randaccio, N. Bresciani-Pahor, E. Zangrando, L. G.
Marzilli, Chem. Soc. Rev. 1989, 18, 225–250.
[3] a) M. S. A. Hamza, A. Felluga, L. Randaccio, G. Tauzher, R.
van Eldik, Dalton Trans. 2004, 287–291; b) N. Bresciani-Pahor,
L. Randaccio, E. Zangrando, M. F. Summers Jr, J. H.
Ramsden, P. A. Marzilli, L. G. Marzilli, Organometallics 1985,
4, 2086–2089.
[25] a) The twist angle (τ) is the angle between the pyrazine plane
and one virtual plane, which is formed by considering the mid-
dle point of the C–C bond of two oxime units that pass through
the cobalt atom and the pyrazine nitrogen atom. b) The butter-
fly bending angle (α) is the dihedral angle between two dioxime
planes.
[4] a) B. D. Gupta, K. Qanungo, J. Organomet. Chem. 1997, 543,
125–134; b) B. D. Gupta, V. Singh, R. Yamuna, T. Barclay, W. [26] A. I. Shkurpelo, Yu. A. Simonov, Izv. Akad. Nauk Mold. SSR,
Cordes, Organometallics 2003, 22, 2670–2678; c) B. D. Gupta,
K. Qanungo, T. Barclay, W. Cordes, J. Organomet. Chem. 1998,
560, 155–161.
Ser. Fiz.-Tekh. Mat. Nauk 1979, 46–52.
[27] Y.-B. Dong, X. Zhao, R.-Q. Huang, M. D. Smith, H.-C.
zur Loye, Inorg. Chem. 2004, 43, 5603–5612.
[5] D. Mandal, B. D. Gupta, Organometallics 2005, 24, 1501–1510. [28] N. Yamazaki, Y. Hohokabe, Bull. Chem. Soc. Jpn. 1971, 44,
[6] a) K. L. Brown, S. Satyanarayana, J. Am. Chem. Soc. 1992, 63–69.
114, 5674–5684; b) L. Randaccio, S. Geremia, E. Zangrando, [29] C. López, S. Alvarez, M. Font-Bardia, X. Solans, J. Or-
C. Ebert, Inorg. Chem. 1994, 33, 4641–4650; c) R. S. Drago, J.
ganomet. Chem. 1991, 414, 245–259.
Organomet. Chem. 1996, 512, 61–68.
[30] a) International Tables for X-ray Crystallography, Kynoch Press,
Birmingham, England, 1974, vol. IV; b) G. M. Sheldrick,
SHELXL-97, Program for Crystal Structure Refinement, Uni-
versity of Göttingen, Göttingen, Germany, 1997.
[7] R. Yamuna, B. D. Gupta, D. Mandal, Organometallics 2006,
25, 706–714.
[8] a) V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni,
Chem. Rev. 1996, 96, 759–833, and references cited therein; b)
A. Haim, Acc. Chem. Res. 1975, 8, 264–272.
[9] a) C. Creutz, H. Taube, J. Am. Chem. Soc. 1973, 95, 1086–
1094; b) J. T. Hupp, Y. Dong, Inorg. Chem. 1994, 33, 4421–
4424; c) A. Yeh, A. Haim, J. Am. Chem. Soc. 1985, 107, 369–
[31] SAINT+, 6.02 ed., Bruker AXS, Madison, WI, 1999.
[32] G. M. Sheldrick, SADABS, Empirical Absorption Correction
Program, University of Göttingen, Göttingen, Germany, 1997.
[33] XPREP, 5.1 ed., Siemens Industrial Automation Inc., Madi-
son, WI, 1995.
376; d) K. J. Pfenning, L. Lee, H. D. Wohlers, J. D. Petersen, [34] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
Inorg. Chem. 1982, 21, 2477–2482; e) J. M. Malin, D. A. Ryan,
Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R.
Spagna, J. Appl. Crystallogr. 1999, 32, 115–119.
T. V. O’Halloran, J. Am. Chem. Soc. 1978, 100, 2097–2102.
[10] a) A. W. Herlinger, T. L. Brown, J. Am. Chem. Soc. 1972, 94, [35] a) U. Englert, A. Fischer, A. Gammersbach, Struct. Chem.
388–393; b) A. W. Herlinger, K. Ramakrishna, Polyhedron
1995, 6, 115–119; b) U. Englert, J. Strähle, Gazz. Chim. Ital.
1988, 118, 845–855; c) Z.-Y. Dong, X.-W. Liu, X.-Q. Wang, R.-
J. Wang, G.-Q. Shen, Acta Crystallogr., Sect. E: Struct. Rep.
Online 2003, 59, m260–m262.
1985, 4, 551–561.
[11] B. D. Gupta, K. Qanungo, J. Organomet. Chem. 1998, 557,
243–249.
[12] D. Mandal, B. D. Gupta, J. Organomet. Chem. 2005, 690,
Received: March 30, 2006
Published Online: August 11, 2006
3746–3754.
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