Mono and bimetallic CoIII Schiff base complexes: Coordination induced ligand imidazolidine ring cleavage and stabilization

Article abstract of DOI:10.1016/j.ica.2011.03.058

2-(2′-Hydroxyphenyl) imidazoline ring grafted dinucleating diimine-diamine-tris-phenol ligand (H₃aeas) has been obtained from a two-step reaction of 2-hydroxy acetophenone, N,N′-bis-(2-aminoethyl) ethylenediamine and 2-hydroxy benzaldehyde. Rea

Full text of DOI:10.1016/j.ica.2011.03.058

Inorganica Chimica Acta 372 (2011) 160–167
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Mono and bimetallic CoIII Schiff base complexes: Coordination induced ligand
imidazolidine ring cleavage and stabilization
Supriti Paul a, Wing-Tak Wong b, Debashis Ray a,
⇑
a Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
b Department of Chemistry, The University of Hong Kong, Pokfulam Road, Pokfulam, Hong Kong SAR, PR China
a r t i c l e i n f o
a b s t r a c t
2-(20-Hydroxyphenyl) imidazoline ring grafted dinucleating diimine-diamine-tris-phenol ligand (H3aeas)
has been obtained from a two-step reaction of 2-hydroxy acetophenone, N,N0-bis-(2-aminoethyl)ethy-
lenediamine and 2-hydroxy benzaldehyde. Reaction of the ligand with Co(ClO4)2ꢀ6H2O and NEt3 in
MeOH–DCM solvent mixture yielded the monometallic complex [Co(aea)]ClO4ꢀH2O (1) of imidazolidine
ring hydrolyzed hexadentate proligand H2aea. Any solvent derived MeOꢁ bridged Co2 complex could
not be obtained due to facile cobalt coordination assisted hydrolytic cleavage of substituted imidazoli-
dine ring. When the reaction is carried out with Co(NO3)2ꢀ4H2O and CoCl2ꢀ6H2O in presence of NH4NCS
Article history:
Available online 1 April 2011
Dedicated to Prof. S.S. Krishnamurthy.
Keywords:
N3O4 ligand
Imidazoline ring
Hydrolysis
Cobalt
and NaN3 in MeOH–DCM and MeOH under aerobic conditions, preassembly of bimetallic [Co2(
and [Co2(
-N3)]5+ cores takes place on the solvent derived methoxido and azido clips through non-
hydrolytic pathways in [(SCN)2Co2( -OMe)( -OMe)(
-aeas)]ꢀDMF (2), and cocrystals of [(N3)2Co2(
aeas)] (3a) and [(N3)2Co2( -N3)( -aeas)] (3b), respectively.
Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
l
-OMe)]5+
l
l
l
l
l-
Thiocyanate
Azide
l
l
1. Introduction
and bridging of CoIII ions. This hydrolytic ring cleavage reaction
counters the formation of Co2 complex. Molecular structural char-
acterization unequivocally establishes the mononuclear CoIII com-
plex 1 as the end product of the reaction. On the contrary the
presence of in-situ generated and externally added MeOꢁ and
The use of binucleating N,O donor ligands for the synthesis and
characterization of Co2 complexes has been the subject of recent
interest because of their controlled preparations, solution and solid
state stability, molecular structure and biological significance [1–
5]. These Co2 complexes may function as mimics of the active bio-
sites such as in methionine aminopeptidase [6] and can show DNA
cleavage activity [7]. Cobalt ions in distorted octahedral and trigo-
nal bipyramidal geometries have been identified in the metallohy-
drolase responsible for the removal of the N-terminal methionine
from the protein chain [8]. Also this type of metallohydrolase is
known to require two or more metal ions for hydrolysis of phos-
phate ester and peptide bonds [9]. We thus have undertaken the
studies of reactivity of cobalt ions with the hydrolysable imidazoli-
dine ring bearing binucleating ligands. The formation and cleavage
of imidazolidine rings are important in various fields [10]. We
examined the complexation behavior of imidazolidine ring fixed
tris-phenolate ligand H3aeas (Scheme 1) toward different cobalt
salts and solvent media in absence and presence of solvent derived
and externally added molecular clips. The study made with the
intention to identify the stability and suitability of the ligand het-
erocyclic backbone toward hydrolysis yielding H2aea (Scheme 1)
ꢁ
N3 anions function as potential inhibitors for the hydrolysis of
the imidazolidine ring of the ligand. The main objectives of the
present investigation are (i) to identify the ligand hydrolysis during
metal ion coordination, (ii) to scrutinize the role of solvent derived
and externally added ancillary ligands having bridging and termi-
nal binding potential, (iii) inhibition of the coordination-induced
hydrolysis of the imidazolidine ring of the tris-phenolate ligand,
and (iv) to provide Co2 complexes on pre-assembled [Co2(
l-
OMe)]5+ and [Co2( -N3)]5+ fragments. We have successfully
l
achieved our goal by the use of NH4SCN and NaN3 to adjust the
pH of the reaction medium and to provide coligands. While explor-
ing the metal ion coordination processes toward aeas3ꢁ, we found
that the reactions of a particular cobalt(II) salt control reactivity
pattern and the production of the different types of complexes in
the presence and absence of solvent-derived or externally added
template bridges. Herein along with complex [Co(aea)]ClO4ꢀH2O
(1) of hydrolyzed ligand, two other complexes [(SCN)2Co2(
OMe)( -aeas)] (2), and cocrystals of [(N3)2Co2( -OMe)( -aeas)]
(3a) and [(N3)2Co2( -N3)( -aeas)] (3b) derived from H3aeas
l-
l
l
l
l
l
following two different (solvent-derived or external) template
binding routes are reported. All the three complexes were spectro-
scopically examined and crystallographically characterized.
⇑
Corresponding author.
E-mail address: dray@chem.iitkgp.ernet.in (D. Ray).
0020-1693/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.058

S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
161
(C4) 116.91; (C6) 132.44; (C7) 163.52; (C8) 48.92; (C9) 51.20; (C10
52.70; (C11) 14.49; (C12) 89.62; (C13) 127.97; (C16) 130.90; (C17
)
)
119.25; (C18) 130.23. UV–Vis spectra (MeCN solution), [kmax, nm
(e
, L molꢁ1 cmꢁ1)]: 318 (53), 282 (37), 252 (140), 215 (423).
2.3.3. [Co(aea)]ClO4ꢀH2O (1) from H2aea
To a pale yellow solution of H2aea (0.32 g, 1.02 mmol) in meth-
anol (30 ml), a MeOH solution of NEt3 (0.43 ml, 3 mmol) was
slowly added. Aqueous pink solution of Co(ClO4)2ꢀ6H2O (0.75 g,
2.05 mmol) was added drop wise with magnetic stirring during
30 min. The reaction mixture was then stirred for another
30 min. A pink solid was separated on solvent evaporation in air.
The product was collected by filtration through glass frit and
washed thoroughly with hexane. The compound was finally dried
in vacuo over P4O10. The solid obtained was dissolved in MeCN
and after 9 days brown crystals were obtained. Yield: 0.62 g
(55%). Anal. Calc. for C22H28N4O7ClCo: C, 47.62; H, 5.08; N, 10.09.
Found: C, 46.76; H, 4.98; N, 9.88%. FT-IR bands (KBr, cmꢁ1): 3402
(br), 3240 (m), 1598 (vs), 1540 (s), 1458 (m), 1436 (vs), 1329
(vs), 1237 (s), 1143 (s), 1090 (s), 1113 (vs), 1014 (m), 857 (m),
757 (s), 625 (s), 585 (m), 529 (m). Molar conductance, KM: (MeOH
Scheme 1.
2. Experimental
2.1. Materials
The chemicals used were obtained from the following sources:
cobalt(II) carbonate and triethyl amine (Merck), triethylenetetra-
mine (Sisco), cobalt(II) nitrate tetrahydrate, cobalt(II) chloride
hexahydrate (Lancaster) and 2-hydroxy benzaldehyde, 2-hydroxy
acetophenone, sodium azide, ammonium thiocyanate (S.D. Fine
Chem.). All other chemicals and solvents were reagent-grade mate-
rials and were used as received without further purification.
solution) 92
X ,
ꢁ1 cm2 molꢁ1. UV–Vis spectra (MeCN solution) [kmax
nm (
e
, L molꢁ1 cmꢁ1)]: 630 (212), 555 (259), 386 (5182), 438
(2327).
2.2. Physical measurements
2.3.4. [Co(aea)]ClO4ꢀH2O (1) from H3aeas following hydrolysis
Pink aqueous solution of Co(ClO4)2ꢀ6H2O (0.75 g, 2.05 mmol)
was added drop wise with magnetic stirring during 30 min to a
yellow 30 ml MeOH solution of H3aea (0.5 g, 1.02 mmol) and
NEt3 (0.43 ml, 3 mmol) and stirred for another 1 h. Following sol-
vent evaporation in air a pink solid was separated, which was col-
lected by filtration through G4 glass frit and washed thoroughly
with ice-cold MeOH and hexane to remove the cleaved 2-hydroxy
benzaldehyde. The compound was finally dried in vacuo over
P4O10. Yield: 0.45 g (55%). Single crystals for comparison were ob-
tained from MeCN solution.
The elemental analyses (C, H, N) were performed with a Perkin-
Elmer model 240C elemental analyzer. FT-IR spectra were recorded
with a Perkin-Elmer RX1 spectrometer. The solution electrical con-
ductivity and electronic spectra were obtained using a Unitech
type U131C digital conductivity meter with a solute concentration
of about 10ꢁ3 and a Shimadzu UV 3100 UV–Vis–NIR spectropho-
tometer respectively. The room-temperature magnetic susceptibil-
ities in the solid state were measured using a home-built Gouy
balance fitted with a Polytronic d.c. power supply.
2.3. Synthesis of ligand and complexes
2.3.5. Co2(-OMe)(-aeas)(NCS)2ꢀDMF (2)
2.3.1. 1,3-Bis-[30-aza-40-(3-methyl-20-hydroxyphenyl)-prop-4-en-10-
yl]-ethane-1,2-diamine (H2aea)
A 50 ml solution of N,N0-bis-(2-aminoethyl)ethylenediamine
(2.0 g, 11.4 mmol) in MeOH was added to a solution of 2-hydroxy
acetophenone (3.1 g, 22.9 mmol) in 20 ml MeOH. The resulting
pale yellow solution was stirred for ca. 1 h to give a dark yellow
solution. The ligand was isolated as yellow oil. Yield 4.0 g (86%).
A MeOH (10 ml) solution of Co(NO3)2ꢀ4H2O (0.59 g, 2.05 mmol)
was added to a stirred aqueous solution (2 ml) of NH4NCS (0.16 g,
2.05 mmol). After being stirred for 20 min a MeOH–DCM (1:1) yel-
low solution of (10 ml) H3aeas (0.5 g, 1.02 mmol) and NEt3
(0.43 ml, 3 mmol) was added to the former solution and stirring
was continued for another 30 min. After solvent evaporation a
brown solid was obtained. The solid was isolated, washed with
cold MeOH, and dried under vacuo over P4O10.. Brown crystals of
2 were obtained from the saturated DMF solution after 2 weeks.
Yield: 1.35 g (80%). Anal. Calc. for C35H41N7O5S2Co2: C, 51.15; H,
5.02; N, 11.93. Found: C, 49.76; H, 4.88; N, 11.69. Selected FT-IR
bands (KBr, cmꢁ1): 2025 (vs), 1629 (vs), 1531 (m), 1451 (s), 1322
(m), 1264 (m), 1146 (m), 1093 (m), 949 (m), 755 (m). Molar con-
2.3.2. 2-(20-Hydroxyphenyl)-1,3-bis[4-methyl-4-(2-hydroxyphenyl)-
3-azabut-3-enyl]-1,3-imidazolidine) (H3aeas)
A 10 ml MeOH solution of N,N0-bis-(2-aminoethyl)ethylenedia-
mine (1.0 g, 6.84 mmol) was added drop wise to another 10 ml
MeOH solution of 2-hydroxy acetophenone (1.8 g, 13.6 mmol) with
stirring at room temperature to get the proligand H2aea as yellow
solution [11]. After half an hour stirring, a 10 ml MeOH solution of
2-hydroxy benzaldehyde (0.717 g, 6.8 mmol) was added. Stirring
was continued for another 30 min and evaporation of solvent pro-
vided yellow solid of H3aeas. The product was isolated by filtration,
washed with water, and finally dried in vacuo over fused CaCl2.
Yield 1.75 g (53%), mp 139 °C. Anal. Calc. for C29H34N4O3: C,
71.62; H, 6.99; N, 11.52. Found: C, 71.51; H, 6.90; N, 11.51%. FT-
ductance, KM (CH3CN solution): 6
(DMF solution) [kmax, nm (
X
ꢁ1 cm2 molꢁ1. UV–Vis spectra
e
, L molꢁ1 cmꢁ1)]: 639 (470), 349
(4600), 275 (7600) nm.
2.3.6. [Co2(-N3)0.5(-OMe)0.5(-aeas)(N3)2] (3)
A 10 ml MeOH solution of CoCl2ꢀ6H2O (0.56 g, 2.04 mmol) was
added to another 10 ml MeOH–DCM (1:1) yellow solution of
H3aeas (0.5 g, 1.02 mmol) and NEt3 (0.43 ml, 3 mmol) and stirred
at room temperature. After 30 min, aqueous solution of NaN3
(0.27 g, 4.15 mmol) was poured rapidly to former solution and
the reaction mixture was stirred for another 1 h. The brown solid
was isolated following solvent evaporation, washed with cold
MeOH, and dried under vacuo over P4O10.. Brown crystals of 2 were
obtained from the saturated DMF solution after 2 weeks. Yield
1.67 g (80%). Anal. Calc. for Co2C29.5H32.5N11.5O3.5: C, 49.07; H,
IR (cmꢁ1, KBr disk):
(phenolic C–O) 1449 (s);
1H NMR (CDCl3, ppm): d(terminal phenolic OH) 16.02 (s, 2H);
m
(phenolic OH) 3435 (b);
m(C@N) 1615 (s);
m
m
(CH2) 883(m); (aromatic CH) 759 (s).
m
d(pendent phenolic OH) 10.73 (s, 1H); (H8) 3.49 (m, 4H); (H2–5
)
)
7.18–7.49 (m, 8H); (H9) 1.75 (t, 4H); (H10) 2.78 (m, 4H); (H11
2.22 (s, 6H); (H12) 3.88 (s, 1H); (H15–18) 6.71–6.99 (m, 4H); 13C
NMR (CDCl3, ppm): d(C1,14) 158.21; (C2–15) 118.73; (C3–5) 120.85;

162
S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
4.54; N, 22.31. Found: C, 48.76; H, 4.38; N, 22.01%. Selected FT-IR
bands (KBr, cmꢁ1): 3396 (b), 1625 (s), 1597 (s), 1528 (s), 1443
(s), 1397 (m), 1341 (m), 1230 (m), 1185 (m), 1146 (m), 1034 (m),
988 (m), 756 (m) cmꢁ1. Molar conductance, KM (CH3CN solution):
protocol a semi-rigid 2-hydroxy phenyl ring substituted five-mem-
bered imidazolidine ring spacer has been inserted within the back-
bone of H2aea and has quite easy synthetic accessibility (Scheme
2). No imidazolidine rings are formed at the two ends of the poly-
amine chain. Recently, similar tris-phenolate ligand and its coordi-
nation chemistry were reported [15]. In MeOH–DCM medium
reactions of Co(ClO4)2ꢀ6H2O with either H2aea or H3aeas in pres-
ence of NEt3 exclusively lead to the formation of pink [Co(aea)]ClO4
(1) (Scheme 2). The mononuclear complex formation with N4O2
surrounding has been structurally characterized to establish the
coordination driven imidazolidine ring hydrolysis. Both synthesis
of H3aeas and formation of monometallic complex 1 clearly point
to the facile attachment and removal of 2-hydroxy benzaldehyde
molecule during central imidazolidine ring formation and cleav-
age. Terminal imine bonds are not hydrolyzed during the metalla-
tion reactions with two ligands. Complete removal of hydroxy
phenyl substituted imidazolidine ring takes place instead of any
reductive-cleavage to any 2-hydroxy-N-benzyl derivative of the
parent hexadentate ligand [16]. This sort of ligand hydrolysis
behavior in presence of cobalt ions may be remotely considered
close to the reactivity of metallohydrolases. In MeOH–DCM, the
reaction of Co(NO3)2ꢀ4H2O and NH4NCS followed by addition of a
methanolic solution of H3aeas in a 2:2:1 molar ratio yields brown
6
X e,
ꢁ1cm2 molꢁ1. UV–Vis spectra (MeCN solution) [kmax, nm (
L molꢁ1 cmꢁ1)]: 562 (310), 375 (2400), 271 (12 500) nm.
Caution! Metal azide complexes are potential explosives. Only a
small amount of material should be prepared and handled with
caution.
2.4. X-ray data collection, structure determination and refinement
The intensity data of complexes 1 and 2 were collected 301(2) K
on Bruker-Nonius Mach3 CAD4 and Bruker AXS SMART APEX CCD
diffractometers and complex 3 at 293(2) K on Bruker SMART 1000
CCD area detector using single-crystal graphite-monochromated
Mo Ka radiation (k = 0.71073 Å) by the x-scan method. Informa-
tion concerning X-ray data collection and structure refinement of
the compound is summarized in Table 1. For complex 1, a total
of 1981 reflections were recorded with Miller indices hmin = 0,
hmax = 5, kmin = 0, kmax = 18, lmin = 0, and lmax = 27. For complex 2,
a total of 7612 reflections were recorded with Miller indices
hmin = ꢁ20, hmax = 20, kmin = ꢁ17, kmax = 16, lmin = ꢁ21, and
lmax = 20. For complex 3, a total of 7132 reflections were recorded
with Miller indices hmin = ꢁ13, hmax = 13, kmin = ꢁ21, kmax = 17,
lmin = ꢁ15, and lmax = 24. The structure was solved by direct meth-
od using the SHELXS-97 [12] and refined by full-matrix least squares
methods using SHELXL-97 [13] implemented in WINGX system of
programs [14]. Direct phase determinations yielded Co, and most
of the C, N, O atoms. In the final cycles of full-matrix least squares
on F2 all non-hydrogen atoms were assigned anisotropic thermal
parameters. For complexes 1 and 2 absorption corrections were
not applied on the collected reflections, as a result the maximum
and minimum in the final difference map seems to be bigger.
[Co2(
l
-OMe)(
l
-aeas)(NCS)2]ꢀDMF (2) in 80% yield. The reaction of
CoCl2ꢀ6H2O with H3aeas and NEt3 in MeOH–DCM followed by
quick addition of aqueous solution of NaN3 under room tempera-
ture stirring in a 2:1:2 molar ratio yields brown complex 3 in
80% overall yield. Crystallization from DMF provides [Co2(l-
OMe)(l-aeas)(N3)2] (3a) and [Co2(l-N3)(l-aeas)(N3)2] (3b). Thus
in presence of auxiliary ligands NCSꢁ and N3 the solution stability
of hydrolysis prone aeas3ꢁ has been enhanced and these groups are
trapped as part of new [Co2] complexes. The formulation of all
complexes is consistent with elemental analysis and electrical con-
ductivity data.
ꢁ
3.2. Role of template anions in the stabilization of the ligand
imidazolidine ring
3. Results and discussion
3.1. Syntheses of ligands and their complexation with CoIII
In the presence of solvent-derived MeOꢁ and externally added
N3 templates, and terminal N3 and NCSꢁ coordinations, the
hydrolysis of the imidazolidine ring of aeas3ꢁ was prevented with
concomitant formation of new trivalent Co2 complexes. These syn-
ꢁ
ꢁ
The hexadentate proligand H2aea and the binucleating H3aeas
belong to the family of Schiff base ligands. In a two-step reaction
Table 1
Crystallographic data for 1–3.
1
2
3
Molecular formula
Molecular weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C
22H26ClCoN4O7
C35H38Co2N7O5S2
818.70
monoclinic
P21/a
C29.5H32.5Co2N11.5O3.50
722.03
monoclinic
P21/n
552.85
orthorhombic
Pbca
14.000
15.637
22.872
16.3742(19)
13.9850(2)
17.1584(19)
111.5220(10)
3655.2(6)
1.488
10.035(2)
16.259(4)
19.082(5)
92.531(4)
3110.3(13)
1.542
b (Å)
c (Å)
b (°)
U (Å3)
5007.1
1.467
8
Dc (g cmꢁ3
Z
)
4
4
F(0 0 0)
2288
1692
1488
Crystal size/mm
(mmꢁ1
h (°)
R1, wR2 [I > 2
Goodness-of-fit (GOF) on F2
max, min (e Åꢁ3
0.38 ꢂ 0.27 ꢂ 0.12
0.34 ꢂ 0.22 ꢂ 0.13
0.42 ꢂ 0.27 ꢂ 0.08
l
)
0.841
1.074
1.121
2.15–24.96
0.0976, 0.2902
1.119
1.170, ꢁ0.631
1.28–26.71
0.1075, 0.3280
1.072
2.877, ꢁ1.117
2.14–27.53
0.0598, 0.1316
1.049
0.006, ꢁ0.000
r
(I)]
)
2
Crystal 1: w ¼ 1=½r2ðFo2Þ þ ð0:1837PÞ þ 9:4034Pꢃ where P ¼ ðFo2 þ 2F2c Þ=3.
2
Crystal 2: w ¼ 1=½r2ðFo2Þ þ ð0:1111PÞ þ 43:9053Pꢃ where P ¼ ðFo2 þ 2F2c Þ=3.
2
Crystal 3: w ¼ 1=½r2ðFo2Þ þ ð0:0342PÞ ꢃ where P ¼ ðFo2 þ 2F2c Þ=3.

S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
163
Scheme 2.
thetic routes also establish that solvent derived MeOꢁ units are
against chelating coordination of the central ethylenediamine part
of any polydentate ligand [11].
produced only in the presence of N3 and NCSꢁ anions. Also termi-
ꢁ
ꢁ
nal N3 coordinations from two ends did not provide any tetranu-
ꢁ
cleation through cis-1; 3-N3 bridges. The nature of the final
reaction product is greatly influenced by the ligands used in differ-
ent steps, the solvent system, and the sequence of addition of reac-
tants. These in-situ generated and externally added molecular clips
3.4. Infrared spectroscopy
The sharp peaks in the FT-IR spectra of complexes 1, 2 and 3
(Fig. S1 in Supporting Informations) at 1598, 1601 and
1602 cmꢁ1, respectively, are characteristic of the C@N functionality
of the ligand aeas3ꢁ. Broad medium bands at 3402 and 1545 cmꢁ1
function as inhibitors for the hydrolysis of the ligand aeas3ꢁ
.
3.3. Competitive binding of diimine chain and imidazolidine ring with
CoIII in absence and presence of ancillary molecular clips
are observed corresponding to
m(OH) stretching and bending
vibrations from lattice water molecule in 1. In addition, complex
ꢁ
Air oxidations of the cobalt centers are responsible for the imi-
dazolidine ring hydrolysis for hardness matching and the transfor-
mation is dependent on the metal ion and terminally bound acidic
1
shows
a
strong sharp band for uncoordinated ClO4 at
ꢄ1113 cmꢁ1 and metal ion bound phenolic C–O stretching
frequency at 1243 cmꢁ1 [15]. Complex 2 reveals C–N and C–S
stretching modes (mCN and m
CS) at 2111 and 748 cmꢁ1 for terminal
3þ
water molecules (pK1 for CoðH2OÞ6 is 1.75) [17]. The coordinated
water has a pKa substantially lower than free water and can release
thiocyanate coordinations [18]. The CꢁN stretching frequency is in
higher side and characteristic for MꢁNCS coordinations. The com-
plexes 3a and 3b show sharp strong bands at 2089 and 2088 cmꢁ1
for the masðN3ꢁÞ stretching mode [19] and the msðN3ꢁÞ modes
appear at 1339, 1443 cmꢁ1 [20]. The positions of these bands are
consistent with the bridging and terminal azide coordinations, as
observed in other CoIII complexes [21].
a proton easily. Protonation of first imidazolidine nitrogen atom
þ
followed by nucleophilic attack of one sided ½Co—OHꢃ2 to the
2-imidazolidinyl carbon leads to the observed hydrolysis and
transforms the ethylenediamine (part of the imidazolidine ring)
bridging coordination to
a favored chelating coordination.
Recently, we have identified polynuclear bridging coordination

164
S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
3.5. Electronic spectroscopy
are shown in Figs. 1–3. Selected interatomic distances and angles
are collected in Tables 2 and 3.
In MeCN solution, the compound 1 shows the d–d bands at
The monometallic species in 1 is composed of an approximate
= 309 L molꢁ1 cmꢁ1) with charge-transfer bands at 383
octahedral [CoIIIL1]+ cation with nearby ClO4 anions. (Fig. 1)
ꢁ
581 nm (
e
and 249 nm. In DMF solution, the compound 2 shows a band at
Molecular structure detects the meridional binding of two ON2
halves in preference to the facial modes. The imine nitrogen atoms
bind trans to each other in 1. The central ethylene bridge in com-
plex 1 has the gauche configuration. The Co–N(imine), Co–
N(amine) and Co–O(phenolic) bond distances are not exceptional
639 nm
(e ) and is characteristic of the
= 271 L molꢁ1 cmꢁ1
1A1g–1T1g transitions for octahedral CoIII centers [22]. The li-
gand ? CoIII and intra-ligand charge transfer transitions occur at
377 nm (
e
= 980 L molꢁ1 cmꢁ1) and 366 nm (939 L molꢁ1 cmꢁ1
)
)
275 nm (1730 L molꢁ1 cmꢁ1
)
and 303 nm (1430 L molꢁ1 cmꢁ1
[23]. The meridional ONN halves provide N–C–C–N torsion angles
1
with an overlapping contribution of A1g–1T2g in the former. In
of 47.93° of the central ethylenediamine part of chelated aea2ꢁ
.
ꢁ
DMF solution, 3 provides crystal field transitions at 640 nm
Intramolecular hydrogen bonding is observed between the ClO4
(e
= 221 L molꢁ1 cmꢁ1) and ligand–metal charge transfer transi-
anions and single amine nitrogen (2.984 Å), and terminal phenox-
ide ion and uncoordinated water molecule (2.951 Å). The near
most oxygen atom of perchlorate anion is 4.47 Å from the metal
ion center. The perchlorate anions show distorted tetrahedral angle
in the range of 104.73–115.94°, with the average Cl–O bond angle
of 1.42 Å. Along all the three axes the crystal lattice packing dia-
grams show regular patters (Fig. S3 (1) in Supporting
Informations).
tions are observed at 377, 365, 321 and 305 nm with molar extinc-
tion coefficients of 1527, 1505, 1728 and 2103 L molꢁ1 cmꢁ1 [20].
3.6. Description of structures
3.6.1. [Co(aea)]ClO4ꢀH2O (1)
Single crystals of 1, 2 and 3 suitable for X-ray structure determi-
nations were obtained by slow evaporation of solutions of 1, 2 and
3 in MeCN, DMF and DMF, respectively after 7–10 days. The atom
labeling scheme and molecular views (Diamond) of the complexes
3.6.2. ½CoIII2ð
l-OMeÞðl-aeasÞðNCSÞ2ꢃ ꢀ DMF (2)
Complex 2 (Fig. 2) is a neutral aggregate of two CoIII ions linked
by the template action of solvent-derived MeOꢁ and capping coor-
dination of the aeas3ꢁ ligand. The unit cell consists of [Co2(
l-
OMe)(l-aeas)(NCS)2] [25] neutral molecules, without any solvent
of crystallization. Each tetradentate N2O2 pocket binds CoIII ions
with central bridging phenoxide anion and meridional coordination
of terminal phenoxide O, imine N and imidazolidine N atoms (NCN
group). The MeOꢁ group, in preference to the NCSꢁ group, contrib-
utes to the exogenous bridging within the complex and functions
as a template in presence of NH4SCN, acting as a base. Each Co
atom in distorted N3O3 octahedral environment sharing the edge
defined by the bridging phenoxido and methoxido O atoms. The
N2O2 planes intersecting at this edge form an angle of 159.85°, as
a result of the folding imposed by the imidazolidine ring [N2ꢀꢀꢀN3
separation of 2.29 Å]. The geometric restrictions resulting from
aeas3ꢁ also cause the Co–O–Co angles to be different at 96.29°
and 97.04° for endogenous phenolate and exogenous methoxide
oxygen atoms, respectively. The sum of the angles around the oxy-
gen atoms of the central phenolate and methoxide bridges are
329.03° and 339.56° and demonstrate that the pyramidal distor-
Fig. 1. ORTEP [24] representation of [Co(aea)]ClO4ꢀH2O (1) at the 40% probability
level. Hydrogen atoms are omitted for clarity.
Fig. 2. ORTEP [24] representation of ½CoIII2ð
l-OMeÞðl-aeasÞðNCSÞ2ꢃ ꢀ DMF (2) at the 40% probability level. Hydrogen atoms are omitted for clarity.

S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
165
Fig. 3. ORTEP [24] representation of Co2(l-OMe)(l-aeas)(N3)2] (3a) [Co2(l-N3)(l-aeas)(N3)2] (3b) at the 30% probability level. Hydrogen atoms are omitted for clarity.
Table 2
Selected bond lengths [Å] in 1 (a), 2 (b) and 3 (c).
1 (a)
2 (b)
2 (c)
Co(1)–O(2)
Co(1)–N(1)
Co(1)–N(4)
N(2)–Co(1)
O(1)–Co(1)
Cl(1)–O(4)
Cl(1)–O(5)
Cl(1)–O(6)
Cl(1)–O(3)
1.881(9)
1.891(10)
1.929(10)
1.938(12)
1.889(11)
1.386(11)
1.41(2)
O(4)–Co(2)
O(4)–Co(1)
Co(1)–O(1)
Co(1)–N(5)
Co(1)–N(1)
Co(1)–O(2)
Co(1)–N(2)
Co(1)–Co(2)
Co(2)–O(3)
Co(2)–N(6)
Co(2)–N(4)
Co(2)–O(2)
Co(2)–N(3)
S(1)–C(30)
S(2)–C(31)
1.911(7)
1.919(7)
1.866(7)
1.892(9)
1.899(9)
1.924(6)
1.994(9)
2.8699(18)
1.855(7)
1.882(9)
1.899(9)
1.929(6)
2.008(8)
1.621(11)
1.600(10)
Co(1)–O(1)
Co(1)–O(4)
Co(1)–N(2)
Co(1)–N(11)
Co(2)–O(3)
Co(2)–N(3)
Co(2)–N(8)
Co(1)–O(2)
Co(1)–N(1)
Co(1)–N(5)
Co(2)–O(2)
Co(2)–O(4)
Co(2)–N(4)
Co(2)–N(11)
N(6)–N(7)
1.850(3)
1.956(15)
2.010(4)
1.84(3)
1.849(3)
2.002(4)
1.908(5)
1.956(3)
1.889(4)
1.945(4)
1.949(3)
1.97(2)
1.905(4)
1.86(3)
1.145(7)
1.199(7)
1.28(3)
1.441(15)
1.449(9)
N(9)–N(10)
N(12)–N(13)
O(4)–C(30)
1.42(2)
tion is more pronounced in the former case. The intramolecular
CoꢀꢀꢀCo distance is 2.87 Å which is less than dicobalt(II) complexes
[19]. The O–Co–O angles [81.62° and 81.51°] within the metallacy-
solution as well as in the crystal lattice of the neutral compound.
Complex 3 is a neutral aggregate of CoIII centers bridged by
methoxido and azido units and chelated by aeas3ꢁ in the same
manner as in complex 2. In a similar fashion the imidazolidine unit
(N2ꢀꢀꢀN3 distance of 2.29 Å) received stabilization and results a
folding with respect to the cobalt bearing two N2O2 molecular
planes. The intramolecular CoꢀꢀꢀCo distance of 2.89 Å indicate no
influence of the terminal azido binding groups. The terminal Co–
N(azide) distances (1.834 and 1.859 Å) are shorter than the Co–N
distances from thiocyanate in 2. The linearity of the azide terminal
ligands are judged from the N–N–N angles in the 173–177° range.
clic Co2(l-O)2 diamond core are smaller. The bridging Co–O(phen-
oxide) distances (av. 1.926 Å) are longer than the terminal ones (av.
1.860 Å). The Co–N (imidazolidine) (2.008 and 1.994 Å) and Co–
N(imine) (av. 1.899 and 1.899 Å) distances are clearly different
and the terminal Co–N(thiocyanate) distances are 1.882 and
1.892 Å. The NCS group remains almost collinear (171.45° and
170.30°) with the Co–N axis. Along all the three axes the crystal
lattice packing diagrams show regular patters (Fig. S3 (2) in Sup-
porting Informations).
ꢁ
Compared to thiocyanate binding in 2, the angular binding of N3
is reflected in the Co–N–N angles of 117.08° and 122.25° in 3.
Along the three crystallographic axes the crystal lattice packing
diagrams show regular patterns with weak intra- and inter-molec-
3.6.3. [Co2(
l-OMe/N3)(l-aeas)(N3)2] (3)
The unit cell of 3 consists of a 0.5:0.5 mixture of two species
[Co2(
l
-OMe)(
l
-aeas)(N3)2] (3a) [Co2(
l-N3)(l
-aeas)(N3)2] (3b)
ular C–Hꢀꢀꢀ
p interactions (Fig. S3 (3) and S4 in Supporting Informa-
0
(Fig. 3). The CoIII ions have strong preference for terminal azido
groups but both methoxido and azido bridged species coexist in
tions). There are several solvent accessible voids of volume ꢄ10 ÅA3
each in the lattice, which are too small for any solvate molecule.

166
S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
Table 3
Selected bond angles [°] in 1 (a), 2 (b) and 3 (c).
1 (a)
2 (b)
3 (c)
O(4)–Cl(1)–O(5)
O(4)–Cl(1)–O(6)
O(5)–Cl(1)–O(6)
O(4)–Cl(1)–O(3)
O(5)–Cl(1)–O(3)
O(6)–Cl(1)–O(3)
O(2)–Co(1)–O(1)
O(2)–Co(1)–N(1)
O(1)–Co(1)–N(1)
O(2)–Co(1)–N(4)
O(1)–Co(1)–N(4)
N(1)–Co(1)–N(4)
O(2)–Co(1)–N(3)
O(1)–Co(1)–N(3)
N(1)–Co(1)–N(3)
N(4)–Co(1)–N(3)
O(2)–Co(1)–N(2)
O(1)–Co(1)–N(2)
N(1)–Co(1)–N(2)
N(4)–Co(1)–N(2)
N(3)–Co(1)–N(2)
103.6(10)
114.3(12)
110.6(10)
104.7(7)
115.9(15)
107.7(8)
88.5(4)
87.1(5)
92.3(5)
92.2(5)
86.9(4)
179.0(7)
175.5(4)
92.0(5)
97.3(5)
83.4(5)
92.6(5)
176.4(4)
84.3(5)
96.5(4)
87.2(6)
C(32)–O(4)–Co(2)
C(32)–O(4)–Co(1)
Co(2)–O(4)–Co(1)
O(1)–Co(1)–N(5)
O(1)–Co(1)–N(1)
N(5)–Co(1)–N(1)
O(1)–Co(1)–O(4)
N(5)–Co(1)–O(4)
N(1)–Co(1)–O(4)
O(1)–Co(1)–O(2)
N(5)–Co(1)–O(2)
N(1)–Co(1)–O(2)
O(4)–Co(1)–O(2)
O(1)–Co(1)–N(2)
N(5)–Co(1)–N(2)
N(1)–Co(1)–N(2)
O(4)–Co(1)–N(2)
O(2)–Co(1)–N(2)
O(1)–Co(1)–Co(2)
N(5)–Co(1)–Co(2)
N(1)–Co(1)–Co(2)
O(4)–Co(1)–Co(2)
O(2)–Co(1)–Co(2)
N(2)–Co(1)–Co(2)
O(3)–Co(2)–N(6)
O(3)–Co(2)–N(4)
N(6)–Co(2)–N(4)
O(3)–Co(2)–O(4)
N(6)–Co(2)–O(4)
N(4)–Co(2)––O(4)
O(3)–Co(2)–O(2)
(6)–Co(2)–O(2)
N(4)–Co(2)–O(2)
O(4)–Co(2)–O(2)
O(3)–Co(2)–N(3)
N(6)–Co(2)–N(3)
N(4)–Co(2)–N(3)
O(4)–Co(2)–N(3)
O(2)–Co(2)–N(3)
O(3)–Co(2)–Co(1)
N(6)–Co(2)–Co(1)
N(4)–Co(2)–Co(1)
O(4)–Co(2)–Co(1)
O(2)–Co(2)–Co(1)
N(3)–Co(2)–Co(1)
C(25)–O(2)–Co(1)
C(25)–O(2)–Co(2)
Co(1)–O(2)–Co(2)
C(30)–N(5)–Co(1)
C(31)–N(6)–Co(2)
121.5(9)
121.0(9)
97.0(3)
88.5(4)
93.0(4)
92.9(4)
91.8(3)
93.8(4)
171.9(3)
88.5(3)
174.3(4)
92.1(3)
81.5(3)
178.5(4)
90.3(4)
86.1(4)
89.3(3)
92.8(3)
99.5(2)
134.1(3)
131.2(2)
41.4(2)
41.91(17)
82.0(3)
88.8(4)
93.9(3)
93.1(4)
91.3(3)
93.5(4)
171.7(3)
88.3(3)
174.3(4)
92.0(3)
81.6(3)
179.1(3)
90.2(4)
86.4(4)
88.6(3)
92.6(3)
99.1(2)
134.1(3)
130.8(2)
41.6(2)
41.80(17)
81.4(3)
116.8(5)
116.0(5)
96.3(3)
170.3(10)
171.5(10)
O(1)–Co(1)–O(2)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(5)
O(2)–Co(1)–O(4)
O(2)–Co(1)–N(2)
O(2)–Co(1)–N(11)
O(4)–Co(1)–N(2)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(5)
N(2)–Co(1)–N(5)
N(5)–Co(1)–N(11)
O(2)–Co(2)–O(4)
O(2)–Co(2)–N(4)
O(2)–Co(2)–N(11)
O(3)–Co(2)–N(3)
O(3)–Co(2)–N(8)
O(4)–Co(2)–N(3)
O(4)–Co(2)–N(8)
N(3)–Co(2)–N(8)
N(4)––Co(2)–N(8)
N(8)–Co(2)–N(11)
Co(1)–O(2)–Co(2)
Co(2)–O(2)–C(18)
Co(1)–O(4)–Co(2)
Co(2)–O(4)–C(30)
O(1)–Co(1)–O(4)
O(1)–Co(1)–N(2)
O(1)–Co(1)–N(11)
O(2)–Co(1)–N(1)
O(4)–Co(1)–N(1)
O(2)–Co(1)–N(5)
O(4)–Co(1)–N(5)
N(11)–N(12)–N(13)
N(1)–Co(1)–N(11)
N(2)–Co(1)–N(11)
O(2)–Co(2)–O(3)
O(2)–Co(2)–N(3)
O(2)–Co(2)–N(8)
O(3)–Co(2)–O(4)
O(3)–Co(2)–N(4)
O(3)–Co(2)–N(11)
O(4)–Co(2)–N(4)
N(3)–Co(2)–N(4)
N(3)–Co(2)–N(11)
N(4)–Co(2)–N(11)
Co(1)–N(5)–N(6)
Co(2)–N(8)–N(9)
Co(1)–N(11)–Co(2)
Co(2)–N(11)–N(12)
88.10(13)
94.33(17)
91.19(18)
82.9(6)
92.14(14)
78.5(11)
88.5(6)
87.08(18)
92.30(19)
88.42(19)
95.4(11)
82.7(5)
92.55(14)
78.2(10)
178.90(16)
90.7(2)
88.5(6)
93.0(5)
88.3(2)
91.85(19)
97.5(10)
95.87(12)
117.5(3)
95.1(8)
124.4(16)
90.1(6)
178.54(17)
89.4(12)
93.81(14)
174.4(5)
173.88(18)
91.0(6)
65(3)
171.4(10)
89.3(12)
88.42(14)
92.59(15)
175.56(16)
91.3(6)
94.26(16)
90.5(12)
172.6(4)
86.11(17)
89.3(12)
169.4(9)
117.0(4)
122.2(6)
103.4(14)
114(2)
Intramolecular C–Hꢀꢀꢀ
p
interactions are found in C20–H20Aꢀꢀꢀcen-
interactions are
hydrolysable coligands and other external bridges in this reaction
system in order to induce the formation of novel heterometallic
and high nuclearity complexes.
troid of C13–C18 ring at 3.11 Å distance; C8–H8Bꢀꢀꢀcentroid of
C13–C18 ring at 2.91 Å. Intermolecular C–Hꢀꢀꢀ
p
seen in C20i–H20Aiꢀꢀꢀcentroid of C1–C6 ring: 3.11 Å (i: x ꢁ ½,
½ ꢁ y, z ꢁ ½); C11ii–H11Aiiꢀꢀꢀcentroid of C22–C27 ring: 2.74 Å (ii:
½ ꢁ x, y ꢁ ½, ½ ꢁ z).
Acknowledgments
We are thankful to DST, New Delhi, for providing the Single
Crystal X-ray Diffractometer facility in the Department of Chemis-
try, IIT Kharagpur under its FIST program. S.P. acknowledges the
Research Fellowship from IIT Kharagpur. W.-T. Wong acknowl-
edges the funding from Hong Kong Government.
4. Concluding remarks
In absence of externally added pseudohalides SCNꢁ or N3ꢁ, 20-
hydroxy-2-phenyl imidazolidine grafted binucleating tris-phenol
ligand showed CoII coordination (from cobalt(II) perchlorate) in-
duced hydrolysis of the in-built heterocyclic ring, following air oxi-
dation of the metal center and transformation of the N,N0-
imidazolidine bridging coordination motif to N,N0-chelating motif
Appendix A. Supplementary material
CCDC 772883, 712202 and 713537 contains the supplementary
crystallographic data for this paper for 1–3. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2011.03.058.
in perchlorate salt of [CoIIIaea]+. In presence of MeOꢁ and N3 col-
ꢁ
igands, showing preassembly of Co2 species, the reactions in air are
incapable of splitting the imidazolidine ring and favor imidazoli-
dine nitrogen coordinations in high yield formations of the assem-
blies 2 and 3. We would like extending this work by using

S. Paul et al. / Inorganica Chimica Acta 372 (2011) 160–167
167
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