Synthesis and structures of two cobalt(III) complexes with N4 donor ligands: Isolation of a unique bis-hemiaminal ether ligand as the metal complex

Article abstract of DOI:10.1016/j.poly.2012.11.025

Two cobalt(III) complexes, [CoL¹(N₃) ₂]ClO₄·H₂O (1) and [CoL ²(N₃)₂]NO₃ (2), where L¹ = N,N′-bis-(2-pyridylmethyl)-1,2-ethanediamine, L² = N,N′-bis-(1-methoxy-1-pyridin-2-yl-methyl)-ethane-1,2-diamine and both ligands are tetradentate N₄ donors, have been prepared and characterized by elemental analysis, IR and UV-Vis spectroscopy, cyclic voltammetry and single crystal X-ray diffraction studies. Both compounds assume an asymmetric cis-octahedral (N2/N2) geometry. Complex 1 crystallizes in the orthorhombic space group Pbca with cell dimensions a = 9.7593(2) A?, b = 16.8640(3) A?, c = 25.8206(5) A?, Z = 8, whereas complex 2 crystallizes in the monoclinic space group P2₁/c with cell dimensions a = 14.437(3) A?, b = 15.596(3) A?, c = 19.187(3) A?, β = 93.642(2), Z = 8. In complex 2, a new bis-hemiaminal ether ligand was found to be "trapped". Hemiaminal ethers are known to be very unstable intermediates and no crystal structure of any bis-hemiaminal ether has been reported to date. Complex 2 represents a uniquely stable system containing the trapped bis-hemiaminal ether L².

Full text of DOI:10.1016/j.poly.2012.11.025

Polyhedron 50 (2013) 443–451
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Synthesis and structures of two cobalt(III) complexes with N4 donor ligands:
Isolation of a unique bis-hemiaminal ether ligand as the metal complex
⇑
Mithun Das, Shouvik Chattopadhyay
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Two cobalt(III) complexes, [CoL1(N3)2]ClO4ꢀH2O (1) and [CoL2(N3)2]NO3 (2), where L1 = N,N0-bis-(2-pyri-
Article history:
Received 26 July 2012
Accepted 5 November 2012
Available online 29 November 2012
dylmethyl)-1,2-ethanediamine,
L2 = N,N0-bis-(1-methoxy-1-pyridin-2-yl-methyl)-ethane-1,2-diamine
and both ligands are tetradentate N4 donors, have been prepared and characterized by elemental analy-
sis, IR and UV–Vis spectroscopy, cyclic voltammetry and single crystal X-ray diffraction studies. Both
compounds assume an asymmetric cis-octahedral (N2/N2) geometry. Complex 1 crystallizes in the ortho-
rhombic space group Pbca with cell dimensions a = 9.7593(2) Å, b = 16.8640(3) Å, c = 25.8206(5) Å, Z = 8,
whereas complex 2 crystallizes in the monoclinic space group P21/c with cell dimensions a = 14.437(3) Å,
b = 15.596(3) Å, c = 19.187(3) Å, b = 93.642(2), Z = 8. In complex 2, a new bis-hemiaminal ether ligand
was found to be ‘‘trapped’’. Hemiaminal ethers are known to be very unstable intermediates and no crys-
tal structure of any bis-hemiaminal ether has been reported to date. Complex 2 represents a uniquely sta-
ble system containing the trapped bis-hemiaminal ether L2.
Keywords:
Cobalt(III)
Azide
N4 ligands
Crystal structures
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
by their potential abilities to mimic biological co-factors, such as
cobalamin [20]. It was known that Schiff-bases are formed from
Schiff bases, characterized by an imine or azomethine group
(>C@N–), form a significant class of complexes in medicinal and
pharmaceutical chemistry and are known to have biological appli-
cations due to their antibacterial [1–6], antifungal [3–6] and anti-
an amine and an aldehyde, through an intermediate tetrahedral
hemiaminal [21–23]. The overall reaction is exothermic, but for-
mation of the hemiaminal is endothermic and it is therefore short
lived and typically observed only by spectroscopic methods in low-
temperature matrices. Recently, however, Schiff-base formation in
a molecular capsule was shown to be sufficiently retarded that the
intermediate hemiaminal could be observed using NMR at ambient
temperature [21,22]. An X-ray diffraction study also confirmed
unambiguously the structure of a transient hemiaminal, trapped
in a porous Zn(II) network, which is finally converted to an imine
[23]. In the preparation of such metal complexes, methanol is com-
monly used as the solvent and it is generally assumed that the sol-
vent is innocent in terms of reactivity. However, under certain
circumstances, these labile azomethine linkages of the Schiff bases
can be susceptible to attack by alcohol to generate O-alkyl hemia-
minals or hemiaminal ethers [24]. Interestingly, no crystal struc-
ture of any bis-hemiaminal ether has been reported in the
literature before the present work. We were able for the first time
to trap bis-hemiaminal ether as its cobalt complex. In the present
paper, we report the synthesis, characterization and X-ray crystal
tumor [7,8] activities. Schiff bases also play
a key role as
chelating ligands in main group and transition metal coordination
chemistry, due to their ease of synthesis, stability under a variety
of oxidative and reductive conditions, and their structural versatil-
ity associated with their diverse applications [9–14]. It is also well
known that the formation of complexes with transition metals
leads to an enhancement of their biological activities and decrease
in the cytotoxicity of the Schiff bases [15–17]. The discovery of the
role played by some cobalt(III) Schiff-base complexes as potent
antitumor agents opened up new avenues for the investigation of
cobalt(III) interactions with proteins and nucleic acids [18]. Cobal-
t(III) Schiff base complexes with two amines in axial positions have
also been used as antiviral and antibacterial agents [19].
Encouragement for the syntheses of cobalt(III) complexes of
symmetrical Schiff bases derived from ethylenediamine and for
the study of their physical and chemical properties is provided
structure analysis of two cobalt(III) complexes, one with
a
tetradentate symmetrical bis-hemiaminal ether, N,N0-bis-(1-
methoxy-1-pyridin-2-yl-methyl)-ethane-1,2-diamine, and the
other with the reduced Schiff base, N,N0-bis-(2-pyridylmethyl)-
1,2-ethanediamine.
⇑
Corresponding author.
E-mail address: Shouvik.chem@gmail.com (S. Chattopadhyay).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.025

444
M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
arating funnel. The dichloromethane part was then dried using
2. Experimental
2.1. Materials
anhydrous Na2SO4. It was evaporated to give a yellow liquid of
the reduced ligand (L1). Yield: 1.7 g (70.16%). Anal. Calc. for
C14H18N4 (242.30): C, 69.39; H, 7.49; N, 23.12. Found: C, 70.2; H,
8.1; N, 22.5%. 1H NMR (CDCl3) (ppm) d: 8.53(d, J = 5.6 Hz, 2H, pyr-
idine H1 and H14), 7.64(dd, J = 7.6, 7.6 Hz, 2H, pyridine H3 and
H12), 7.41(dd, J = 6.3, 6.5 Hz, 2H, pyridine H2 and H13), 7.69(d,
J = 6 Hz, 2H, pyridine H4 and H11), 5.8(s, 2H, NH) 4.6(s, 4H, benzyl
CH2), 2.96(m, 4H, alkyl CH2). The ligand has already been used in
the preparation of several transition metal complexes [25].
All starting materials were commercially available, reagent
grade and used as purchased from Sigma–Aldrich without further
purification.
Caution! Since azide complexes of metal ions are potentially
explosive, only small amounts of the materials should be handled
with care.
2.2. Synthesis
2.2.2. [CoL1(N3)2]ClO4ꢀH2O (1)
2.2.1. Ligand (N,N0-bis-(2-pyridylmethyl)-1,2-ethanediamine, (L1))
The reduced Schiff base ligand L1 was synthesized by refluxing
ethane-1,2-diamine (10 mmol, 1.08 g) with pyridine-2-carboxal-
dehyde (20 mmol, 3.04 g) in methanol (20 ml) for ca. 1 h. Then so-
lid sodium borohydride (0.7 g, 20 mmol) was added slowly in
portions to the methanol solution under refluxing conditions. After
completion of the addition, the resulting solution was refluxed fur-
ther for 4 h. The methanol solution was then evaporated using a ro-
tary evaporator and then the rest of the mixture was dissolved in
water. The reduced Schiff base ligand was then extracted from
the aqueous solution by dichloromethane as a solvent using a sep-
A methanol solution (10 ml) of cobalt(II) perchlorate hexahy-
drate (1 mmol, 370 mg) was added to
a methanol solution
(10 ml) of L1 (1 mmol, 240 mg) under refluxing conditions. A solu-
tion of sodium azide (2 mmol, 130 mg) in methanol/water solution
mixture (10 ml in a 4:1 ratio) was then added to it and refluxed for
2 h. The resulting solution was then filtered hot. Crystalline com-
pounds started to separate on standing at room temperature from
the resulting solution and were collected by filtration after ca. 13 h.
Red coloured single crystals suitable for X-ray diffraction were ob-
tained at that time. Yield: 330 mg (65.64%, based on the cobalt
salt). Anal. Calc. for C14H20ClCoN10O5 (502.78): C, 33.45; H, 4.01;
N, 27.86; Co, 11.72. Found: C, 33.5; H, 4.1; N, 27.8; Co, 11.8%. 1H
NMR (DMSO-d6) (ppm) d: 8.82(d, J = 5.4 Hz, 2H, pyridine H1 and
H14), 8.16(dd, J = 7.5, 7.5 Hz, 2H, pyridine H3 and H12), 7.78(dd,
J = 6.3, 6.5 Hz, 2H, pyridine H2 and H13), 7.69(d, J = 6 Hz, 2H, pyr-
idine H4 and H11), 7.05(s, 2H, NH) 4.6(dd, J = 6.9, 10.8 Hz, 2H, ben-
zyl CH2), 4.15(d, J = 17.7 Hz, 2H, benzyl CH2), 2.32(m, 2H, alkyl
CH2), 2.53(m, 2H, alkyl CH2). UV–Vis, kmax (nm), (emax (dm3 molꢁ1
cmꢁ1)) (DMF): 512(327). Magnetic moment: diamagnetic.
Table 1
Crystal data and refinement details of complexes 1 and 2.
Complex
1
2
Formula
C
14H20CoN10O5Cl
C16H22CoN11O5
507.38
Formula weight
Crystal Size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
502.78
0.2 ꢂ 0.2 ꢂ 0.2
293
orthorhombic
Pbca
9.7593(2)
16.8640(3)
25.8206(5)
90
90
90
8
1.572
0.983
2064
62860
0. 2 ꢂ 0.2 ꢂ 0.3
296
monoclinic
P21/c
14.424(3)
15.58(3)
19.169(3)
90
93.644(2)
90
8
1.568
0.853
2096
29143
7443
4264
627
0.060
0.1002, 0.1742
0.0510, 0.1405
Table 3
Selected bond angles around cobalt(III) in complexes 1 and 2.
b (Å)
c (Å)
Complex
1
2
a
(°)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(4)
N(1)–Co(1)–N(5)
N(1)–Co(1)–N(8)
N(2)–Co(1)–N(3)
N(2)–Co(1)–N(4)
N(2)–Co(1)–N(5)
N(2)–Co(1)–N(8)
N(3)–Co(1)–N(4)
N(3)–Co(1)–N(5)
N(3)–Co(1)–N(8)
N(4)–Co(1)–N(5)
N(4)–Co(1)–N(8)
N(5)–Co(1)–N(8)
Co(1)–N(5)–N(6)
Co(1)–N(8)–N(9)
N(12)–Co(2)–N(13)
N(12)–Co(2)–N(14)
N(12)–Co(2)–N(15)
N(12)–Co(2)–N(16)
N(12)–Co(2)–N(19)
N(13)–Co(2)–N(14)
N(13)–Co(2)–N(15)
N(13)–Co(2)–N(16)
N(13)–Co(2)–N(19)
N(14)–Co(2)–N(15)
N(14)–Co(2)–N(16)
N(14)–Co(2)–N(19)
N(15)–Co(2)–N(16)
N(15)–Co(2)–N(19)
N(16)–Co(2)–N(19)
Co(2)–N(16)–N(17)
Co(2)–N(19)–N(20)
84.81(12)
92.21(12)
176.15(12)
89.95(14)
92.12(13)
86.82(12)
92.21(12)
89.67(14)
174.27(13)
85.17(12)
175.70(14)
88.46(13)
92.49(14)
90.63(13)
95.18(15)
117.7(3)
80.95(16)
95.82(16)
177.26(15)
90.32(16)
90.00(16)
85.87(14)
97.82(16)
170.46(18)
90.06(16)
81.63(16)
91.25(14)
172.29(18)
90.74(16)
92.46(16)
93.78(16)
121.3(3)
b (°)
c
(°)
Z
dcalc (g cmꢁ3
)
l
(mmꢁ1
F(000)
)
Total reflections
Unique reflections
Observed data [I > 2
Number of parameters
Rint
R1, wR2 (all data)
R1, wR2 [I > 2r(I)]
4369
3516
283
0.040
0.0655, 0.1657
0.0532, 0.1529
r
(I)]
118.8(3)
122.9(4)
Table 2
Coordination environment around cobalt(III) in complexes 1 and 2.
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
81.83(17)
97.89(17)
178.32(15)
90.98(18)
90.22(17)
86.15(14)
96.64(16)
171.52(18)
90.06(14)
81.29(16)
90.42(16)
170.46(18)
90.49(16)
90.47(16)
94.47(16)
122.0(4)
Complex
1
2
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–N(4)
Co(1)–N(5)
Co(1)–N(8)
1.932(3)
1.966(3)
1.954(3)
1.931(3)
1.942(4)
1.940(4)
1.925(3)
1.984(4)
1.968(3)
1.938(3)
1.943(4)
1.940(4)
Co(2)–N(12)
Co(2)–N(13)
Co(2)–N(14)
Co(2)–N(15)
Co(2)–N(16)
Co(2)–N(19)
–
–
–
–
–
–
1.941(3)
1.976(3)
1.980(4)
1.934(3)
1.940(4)
1.931(4)
122.2(3)

M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
445
2.2.3. [CoL2(N3)2]NO3 (2)
7.5 Hz, 2H, pyridine H2 and H13), 7.82(d, J = 6 Hz, 2H, pyridine
H4 and H11), 7.21(s, 2H, NH), 5.72(d, J = 4.2Hz, 2H, hemiaminal
CH), 3.67(s, 6H, methoxy CH3), 2.84(m, 2H, alkyl CH2), 2.72(m,
2H, alkyl CH2). UV–Vis, kmax (nm),
(DMF): 491(252). Magnetic moment: diamagnetic.
A methanol solution (20 ml) of ethane-1,2-diamine (10 mmol,
1.08 g) and pyridine-2-carboxaldehyde (20 mmol, 3.04 g) was re-
fluxed for ca. 1 h. A methanol solution (10 ml) of cobalt(II) nitrate
hexahydrate (10 mmol, 2.9 g) was then added and refluxed for an
additional 1 h. A solution of sodium azide (20 mmol, 1.30 g) in
methanol–water solution (10 ml in 4:1 ratio) was added to it and
refluxed further for 2 h. The resulting solution was filtered under
hot conditions. The filtrate was left at room temperature. X-ray dif-
fraction quality red coloured single crystals were obtained on
standing overnight by slow evaporation of the filtrate in open
atmosphere. Yield: 290 mg (57.15%, based on cobalt salt). Anal.
Calc. for C16H22CoN11O5 (507.38): C, 37.88; H, 4.37; N, 30.37; Co,
11.62. Found: C, 37.9; H, 4.4; N, 30.3; Mn, 11.6%. 1H NMR
(DMSO-d6) d: 8.97(d, J = 5.4 Hz, 2H, pyridine H1 and H14),
8.28(dd, J = 6.3, 7.5 Hz, 2H, pyridine H3 and H12), 7.94(dd, J = 6.9,
(
emax (dm3 molꢁ1 cmꢁ1))
2.3. Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was per-
formed using a Perkin–Elmer 240C elemental analyzer. IR spectra
in KBr (4500–500 cmꢁ1) were recorded using a Perkin–Elmer RXI
FT-IR spectrophotometer. Electronic spectra in acetonitrile
(1200–350 nm) were recorded in a Hitachi U-3501 spectropho-
tometer. The magnetic susceptibility measurements were done
with an EG & PAR vibrating sample magnetometer, model 155 at
room temperature and diamagnetic corrections were made using
Scheme 1. Preparation of complexes 1 and 2. ꢃ represents a stereocenter.

446
M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
Fig. 1. ORTEP view of complex 1 with 30% ellipsoid probability, including the atom numbering scheme. All the hydrogen atoms and the perchlorate ion are omitted for clarity.
Scheme 2. Coordination geometries of bis(picolyl)ethylenediamine-type ligands.
Fig. 2. H-bonded structure of complex 1. Only the relevant hydrogen atoms are shown for clarity. $ represents symmetry element ꢁ1 + x, y, z.
Pascal’s constants. Electrochemical measurements were performed
in acetonitrile solution under a dry nitrogen atmosphere in con-
ventional three-electrode configurations using a Pt diskworking
electrode, Pt auxiliary electrode and Ag/AgCl reference electrode,
with tetrabutylammonium hexafluorophosphate as the supporting
electrolyte in the potential range from ꢁ2 to 2 V, and were uncor-
rected for any junction contribution. The value for the Fc–Fc+ cou-
ple under our conditions is 0.39 V.
2.4. X-ray crystallography
Single crystals having suitable dimensions for the complexes
were used for data collection using a Bruker SMART APEX II diffrac-
tometer equipped with graphite-monochromated Mo Ka radiation
(k = 0.71073 Å) at 293 K. The molecular structures were solved by
direct methods and refinement by full-matrix least squares on F2
using the SHELX-97 package [26]. Non-hydrogen atoms were refined

M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
447
with anisotropic thermal parameters. Hydrogen atoms were placed
in their geometrically idealized positions and constrained to ride
on their parent atoms. H atoms of water molecules were located
by difference Fourier maps and were kept fixed. Multi-scan absorp-
tion corrections were applied to the data using the program SADABS
[27]. The crystallographic data are summarized in Table 1. Selected
bond lengths and bond angles are shown in Tables 2 and 3,
respectively.
The electronic spectra of the complexes 1 and 2 in DMF solution
exhibit a low-energy absorption at 512 and 491 nm respectively,
attributable to a transition in the visible region of a low-spin cobal-
t(III) in an octahedral geometry, obscuring the transitions of the
divalent metal ions. The high-energy intense band at 326 and
334 nm respectively is associated with a charge-transfer transition
[34].
3.3. Description of the structures
3. Results and discussions
3.3.1. [CoL1(N3)2]ClO4ꢀH2O (1)
The structure of 1 is shown in Fig. 1. The structure shows that
the cobalt atom is in a distorted octahedral environment bonded
to two nitrogen atoms of two azide groups and four nitrogen atoms
of the ligand L1. The bond lengths to the azide are 1.942(4) and
1.940(4) Å, and to the ligand 1.932(3), 1.966(3), 1.954(3) and
3.1. Syntheses
The ligand L1 was prepared by the 1:2 condensation of ethane-
1,2-diamine with pyridine-2-carboxaldehyde, followed by reduc-
tion with sodium borohydride in methanol. L1 was then reacted
with cobalt(II) perchlorate and sodium azide in methanol to pre-
pare complex 1. Complex 2 was synthesized by refluxing ethane-
1,2-diamine with pyridine-2-carboxaldehyde in a 1:2 ratio, fol-
lowed by the addition of cobalt(II) nitrate along with sodium azide
under refluxing conditions. The formation of complexes 1 and 2 is
shown in Scheme 1. The crucial step in the formation of complex 2
is the formation of the hemiaminal ether, which is formed by the
reaction of methanol with these labile azomethine linkages of
the Schiff base. Although no definite reason is available for the for-
mation of this bis-hemiaminal ether, we like to propose the follow-
ing mechanism. After the starting materials are mixed, the first
step may be the formation of a complex with a N4-donor Schiff
base ligand, which is then transformed to a hemiaminal ether li-
gand. The transformation of the initially tetra-coordinated ligand
to a hemiaminal ether maybe attributed to the decrease of elec-
tronic density on the imine nitrogen atom coordinated to the Lewis
acid Co(III), thus weakening the nitrogen–carbon bond and favour-
ing nucleophilic attack by alcohols as the second step in the pro-
cess. We have tried with other alcohols, such as ethanol, propan-
2-ol and butane-1-ol, to prepare similar hemiaminal ether ligands,
but failed.
Table 4
Hydrogen bond distances (Å) and angles (°) for complex 1.
D–Hꢀ ꢀ ꢀA
D–H
Dꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA
\D–Hꢀ ꢀ ꢀA
N(2)–H(2A)ꢀ ꢀ ꢀO(5)
N(3)–H(3A)ꢀ ꢀ ꢀO(1)
O(5)–H(5B)ꢀ ꢀ ꢀO(1)$
0.910(1)
0.910(1)
0.920(16)
2.956(7)
3.009(8)
3.293(12)
2.120(1)
2.100(1)
2.43(16)
156.00(1)
179.00(1)
157.00(14)
Symmetry element $ = ꢁ1 + x, y, z and D, donor; H, hydrogen; A, acceptor.
3.2. IR and electronic spectra
The IR spectrum of complex 1 exhibits a moderately strong,
sharp peak at 3228 cmꢁ1, which is ascribed to the N–H stretching
vibration, therefore confirming that the imine group of the Schiff
base has been reduced. The lowering of the positions of the band
in the complex compared to that in the free ligand (L1) indicates
their coordination to metal centres [28]. The absence of the >C@Nꢁ
moiety is corroborated further by the non-appearance of the
typical strong band due to the imine vibration, which appears in
the region 1620–1650 cmꢁ1 for the corresponding complexes from
the unreduced Schiff bases [29,30]. A very strong band centred at
2032 cmꢁ1 was also observed and is normally assigned to a mono-
dentate azide [29,30]. The sharp, strong, single peak at 1088 cmꢁ1
gives evidence for the presence of ionic perchlorate [31]. A broad
band around 3469 cmꢁ1 in complex 1 is assigned to the OH
stretching vibration of the water molecule, which is involved in
hydrogen bonding [32]. In the IR spectrum of complex 2, appear-
ance of a moderately strong, sharp peak at 3566 cmꢁ1 give evi-
dence for the presence of an N–H bond and the absence of any
band in the region 1620–1650 cmꢁ1 for the >C@Nꢁ moiety of the
Fig. 3. ORTEP view of complex 2 with 30% ellipsoid probability, including the atom
numbering scheme. All hydrogen atoms and the nitrate ion are omitted for clarity.
Table 5
Bond angles around the hemiaminal carbon centers of L2 in complex 2.
O(1)–C(6)–N(2)
O(1)–C(6)–C(5)
O(1)–C(6)–H(6)
N(2)–C(6)–C(5)
N(2)–C(6)–H(6)
C(5)–C(6)–H(6)
O(2)–C(9)–N(3)
O(2)–C(9)–C(10)
O(2)–C(9)–H(9)
N(3)–C(9)–C(10)
N(3)–C(9)–H(9)
C(10)–C(9)–H(9)
114.0(4)
108.0(3)
111(2)
105.8(4)
103(3)
O(6)–C(22)–N(13)
O(6)–C(22)–C(21)
O(6)–C(22)–H(22)
N(13)–C(22)–H(22)
N(13)–C(22)–C(21)
C(21)–C(22)–H(22)
O(7)–C(25)–N(14)
O(7)–C(25)–C(26)
O(7)–C(25)–H(25)
N(14)–C(25)–H(25)
N(14)–C(25)–C(26)
C(26)–C(25)–H(25)
112.9(4)
108.5(4)
116(3)
105(3)
106.6(4)
107(2)
113.8(4)
106.7(3)
112(2)
109(2)
106.2(4)
109(2)
116(2)
Schiff base indicates the formation of
a
hemiaminal ether.
113.8(4)
109.6(4)
115(3)
105.5(4)
107(3)
Compound shows
2
a strong doublet peak at 2019 and
2050 cmꢁ1, suggesting azido-groups with two different environ-
ments, which is evident from the crystal structure determination.
In addition, there is a very strong band centered at 1384 cmꢁ1 cor-
responding to the free nitrate ion [29,33].
105(2)

448
M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
Fig. 4. H-bonded structure of complex 2. Only the relevant hydrogen atoms are shown for clarity. ꢃ represents symmetry element ꢁ1 + x, y, z.
Table 6
In the octahedral complex of a bis(picolyl)ethylenediamine-
Hydrogen bond distances (Å) and angles (°) for complex 2.
type ligand, when it acts as a tetradentate ligand, three different
arrangements of the four donor atoms around the metal atom is
found; one is planar (N4) in which the donors occupy the four
equatorial positions, the second one is asymmetric cis-octahedral
(N3/N) in which one nitrogen is displaced from the equatorial
plane, occupying an axial position of the coordination polyhedron,
and last one is symmetric cis-octahedral (N2/N2) in which two ter-
minal nitrogen atoms are displaced from the equatorial plane,
occupying two axial positions (Scheme 2) [37]. In the present case,
the ligand assumes the third alternative, where the two terminal
binding sites of the ligand, that is two pyridine (ring) N-atoms of
the ligand, are trans to each other.
D–Hꢀ ꢀ ꢀA
D–H
Dꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA
\D–Hꢀ ꢀ ꢀA
N(2)–H(2A)ꢀ ꢀ ꢀO(5)
N(3)–H(3A)ꢀ ꢀ ꢀO(9)
N(13)–H(13A)ꢀ ꢀ ꢀO(8)
N(14)–H(14A)ꢀ ꢀ ꢀO(4)⁄
0.85(4)
0.88(4)
0.92(4)
0.95(4)
3.109(8)
2.855(8)
2.859(6)
2.860(7)
2.30(4)
2.00(4)
1.96(4)
1.95(4)
159(4)
164(4)
164(4)
159(4)
Symmetry element ꢃ = ꢁ1 + x, y, z and D, donor; H, hydrogen; A, acceptor.
1.931(3) Å, which are quite familiar for this type of complex [35].
The conformation of the saturated five-membered ring [Co(1)–
N(2)–C(7)–C(8)–N(3)] is half-chair, twisted on C(7)–C(8), with
There are two H atoms, H(2A) and H(3A), available in the ligand
part for H-bonding. Of these, H(2A) is H-bonded with an oxygen
atom, O(5), of the solvent water molecule and H(3A) is H-bonded
puckering parameters Q = 0.437(4) Å,
Co(1)–N(3) angle is 86.82(12)° and is typical of a five-membered
chelate ring [20].
U = 89.3(4)° [36]. The N(2)–
Table 7
Selected structural parameters for reported metal complexes with hemiaminal ether ligands derived from aromatic aldehydes and primary amines (
a
= N–C–O angle).
Ref.
Complex
d(C–O) (Å)
d(C–N) (Å)
a (°)
[CoL2(N3)2]NO3 (2)
[Cu(bpe,MeOH)Br]ClO4
[NiL](ClO4)2ꢀH2Ob
1.403(6), 1.386(6); 1.393(6), 1.398(6) 1.486(6), 1.500(6); 1.492(6), 1.493(6) 114.0(4), 113.8(4); 112.9(4), 113.8(4) Present work
a
1.387(21)
1.411(5)
1.523(23)
1.479(5)
113.1(1.5)
109.2(3)
[38]
[39]
[40]
[40]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
Cu(L3-MeOH)(CCl3SO3)2
1.455(13)
1.388(8)
1.358(10)
1.475(8)
119.6(7)
117.6(5)
c
c
Cu(L3-EtOH)(CCl3SO3)2
Cu(L3-MeOH)(CCl3SO3)2
1.415(6), 1.405(6)
1.505(15)
1.423(3)
1.448(6), 1.432(7)
1.485(13)
1.473(3)
110.6(3), 119.9(4)
110.7(8)
112.30(18)
113(1)
c
[Cd2L4](ClO4)ꢀCH3OHd
[Ni(L5)(bipy)](ClO4)2
e
[CuL00Br2]ꢀH2Of
1.39(1)
1.47(1)
Trans-[Cl2Ru(CO)2L4b
]
1.392(13)
1.395(6)
1.495(14)
1.471(7)
114.3(8)
113.5(4)
g
[Cu(L6)](ClO4)2
h
[Ni(L2-OMe)2(
l
1,1-N3)(N3)]2
1.402(4), 1.393(4)
1.412(7)
1.468(5), 1.475(4)
1.443(7)
115.1(3), 114.2(3)
114.6(5)
i
[ZnLB°(NCS)2]ꢀH2Oj
PdCl2(Lox
-
j
N,
j
N0)k
1.387(6)
1.480(7)
106.7(5)
[Fe(L1)(Phen)](PF6)2
1.446(6)
1.473(7)
104.9(4)
l
[Fe(L1)(L2°)](PF6)3
1.434(10)
1.416(11)
1.430(9)
1.527(8)
105.5(6)
108.4(6)
104.7(6)
110.2(10)
m
0
Trans-[PtCl2L2 (C2H4)]n
1.477(11)
1.461(10)
1.449(15)
NiL1(SCN)2
l
Hg3(LaH2)2Cl6
1.423(14)
o
a
b
c
bpe = N,N0-bis-(Z-pyridylrnethylene)ethane-l,2-diamine.
L = N-[ethoxy-(1-oxy-pyridin-2-yl)-methyl]-N0-{2-[(1-oxy-pyridin-2-ylmethylene)-amino]-ethyl}-N0-quinolin-8-ylmethyl-ethane-1,2-diamine.
L3 = N,N0-bis-pyridin-2-ylmethylene-cyclohexane-1,2-diamine.
d
e
f
L4 = cryptand: synthesized by the [2+3] condensation of 2-aminoethyl-bi(3-aminopropyl)amine with sodium 2,6-diformyl-4-methylphenolate.
L5 = N-(methoxy-phenyl-pyridin-2-ylmethyl)-N-(phenyl-pyridin-2-yl-methylene)-ethane-1,2-diamine.
L00 = 1-[2-pyridyl(ethoxy)methylamino]-2-(2-pyridylmethyleneamino)ethane.
g
h
i
L4b = (N-(pyrid-2-ylmethoxymethyl)(3-isopropoxypropy1)amine).
L6 = 2,6-(6-(3-aminopropyl)-1-ethoxy-2,6,10-triazaundeca-10-ene-1,11-diyl)pyridine.
L2-OMe = N0-(methoxy-pyridin-2-yl-methyl)-N,N-dimethyl-ethane-1,2-diamine.
LB° = N-[methoxy-(pyridin-2-yl)methyl]-N’-(2-pyridin-2-yltetrahydropyrimidin-1(2H)-yl)ethane-1,2-diamine.
Lox = (4S)-4-isopropyl-2-(pyridin-2-yl)oxazolidine.
j
k
l
L1 = pyridin-2-ylmethylene-(2-pyridin-2-yl-oxazolidin-5-ylmethyl)-amine.
m
n
o
L2° = 1-(3-pyridin-2-yl-imidazo[1,5-a]pyridin-2-yl)-3-[(pyridin-2-ylmethylene)-amino]-propan-2-ol.
L2’ = 4-[1,3]oxazinan-2-yl-benzoic acid.
LaH2 = 6-chloro-2-(quinolin-2-yl)-2,4-dihydro-1H-benzo[d][1,3]-oxazine.

M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
449
Fig. 5. Cyclic voltammogram of [CoL1(N3)2]ClO4ꢀH2O (1) and [CoL2(N3)2]NO3 (2) in DMF at 293 K.
Table 8
Electrochemical data for the oxidation and reduction of the cobalt(III) centre of the complexes 1 and 2.
Complex
Epc(CoIII ? CoII) (V)
CoII/CoI
Epa(CoII ? CoIII) (V)
Epa (V)
Epc (V)
E1/2 (V)
DEp (mV)
1
2
ꢁ0.5
ꢁ0.84
ꢁ0.72
ꢁ1.31
ꢁ1.23
1.08
0.98
470
510
0.92
0.83
ꢁ0.51
E1/2 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials;
D
Ep = (Epa ꢁ Epc).
with the perchlorate oxygen atom, O(1). O(1) is again H-bonded
with H(5B)$ of a symmetry solvent water [$ = ꢁ1 + x, y, z] to form
a one-dimensional H-bonded chain (Fig. 2). The details of the H
bonding distances are given in Table 4.
2 shows bond angels in the range 103(3)–116(2)° around them.
The details of these tetrahedral angels are depicted in Table 5.
There are two H atoms, H(2A) and H(3A), available for H-bond-
ing in the ligand part of one cationic complex present in the unit
cell. Of these two, H(2A) is H-bonded with an oxygen atom, O(5),
of one nitrate and H(3A) is H-bonded with O(9) of another nitrate
anion present in the unit cell. Another oxygen atom, O(8), of the
second nitrate anion is H-bonded with H(13A) of the second cation
present in the unit cell. H(14A) of this cationic moiety is again H-
bonded with O(4)⁄ of a symmetry related nitrate to form a one-
dimensional H-bonded chain [⁄ = ꢁ1 + x, y, z] (Fig. 4). Details of
the H-bonding distances are given in Table 6. The shortest inter-
molecular separation between Co(1)ꢀ ꢀ ꢀCo(2) is 8.870 Å, implying
no metal–metal interaction between the cobalt atoms.
Complex 2 crystallizes in the centrosymmetric space group
P21/c. So the unit cell contains two molecules, one with R,R and
the other with S,S chiralities at the newly formed chiral centres
C(6)/C(22) and C(9)/C(25), i.e. diastereoisomeric pairs of the com-
plex are crystallized together.
Hemiaminal ethers trapped as metal complexes are limited; all
the species reported so far in the footnote 1 along with the relevant
bond lengths and bond angles, including the present work, are sum-
marized in Table 7. It is noteworthy that no bis(hemiaminal ether)
ligands have been trapped as a metal complex before the present
work.
3.3.2. [CoL2(N3)2]NO3 (2)
A view of complex 2, including the atom labelling scheme, is
shown in Fig. 3. The asymmetric unit of the unit cell of 2 consists
of a pair of cationic complexes of cobalt(III) and two nitrate anions.
The coordination geometry around cobalt(III) is close to octahedral,
where four coordination sites are occupied by two hemiaminal N
atoms [N(2) and N(3)] and two pyridyl N atoms [N(1) and N(4)]
of the neutral tetradentate bis-hemiaminal methyl ether ligand
(L2), and the remaining two coordination sites are taken up by
two N-atoms [N(5) and N(8)] of two terminal azides. Interestingly,
the sp2 hybridized pyridyl N-atoms are coordinated in a trans
orientation whereas the sp3 hybridized hemiaminal and azidal N-
atoms are coordinated in a cis-conformation to produce a symmet-
ric cis-octahedral N2/N2 geometry of the ligand (Scheme 2). The
two azidal N-atoms are bent in opposite directions out of the coor-
dination plane to avoid interacting. The extent of bending is differ-
ent for the two cobalt(III) complexes present in the unit cell. The
conformation of the saturated five-membered ring [Co(1)–N(2)–
C(7)–C(8)–N(3)] is half-chair, twisted on C(7)–C(8), with puckering
parameters Q = 0.424(5) Å,
U(Phi) = 90.8(5)° [36]. The N(2)–Co(1)–
N(3) angle is 85.79(2)° and is typical of a five-membered chelate
ring [20]. The two newly generated tetrahedral carbon centers
[C(6)/C(22) and C(9)/C(25)] due to hemiaminal formation in L2 of
1
CSD search file: Supporting Information.

450
M. Das, S. Chattopadhyay / Polyhedron 50 (2013) 443–451
3.4. Cyclic voltammetry
Appendix A. Supplementary data
Electrochemical measurements of 1 and 2 were performed in
DMF solution under a dry nitrogen atmosphere in a conventional
three electrode configuration using a Pt disk working electrode,
Pt auxiliary electrode and Ag/AgCl reference electrode, with TBAP
(tetrabutylammonium perchlorate) as the supporting electrolyte
in the potential range from ꢁ2 to 2 V, and the results were uncor-
rected for any junction contribution. The value for the Fc–Fc+ cou-
ple under our conditions is 0.39 V.
CCDC 860295 and 860296 contain the supplementary crystallo-
graphic data for 1 and 2, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.poly.2012.11.025.
A scan initiated in the negative direction at 0.0 V reveals a first
irreversible reduction wave at ꢁ0.50 and ꢁ0.51 V for complexes 1
and 2 respectively, which is due to Co(III)/Co(II) reduction. A sec-
ond, well-defined quasi-reversible process at ꢁ1.08 and ꢁ0.98 V
for complexes 1 and 2, respectively, is observed, which corre-
sponds to the simple one-electron process of Co(II)/Co(I) reduction.
The criteria of reversibility were checked by observing the con-
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