Carbonyl compound dependent hydrolysis of mono-condensed Schiff bases: A trinuclear Schiff base complex and a mononuclear mixed-ligand ternary complex of copper(II)

Article abstract of DOI:10.1016/j.ica.2010.04.010

Two Schiff bases, HL¹ and HL² have been prepared by the condensation of N-methyl-l,3-propanediamine (mpn) with salicylaldehyde and 1-benzoylacetone (Hbn) respectively. HL¹ on reaction with Cu(ClO ₄)₂6

Full text of DOI:10.1016/j.ica.2010.04.010

Inorganica Chimica Acta 363 (2010) 2488–2495
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Carbonyl compound dependent hydrolysis of mono-condensed Schiff bases:
A trinuclear Schiff base complex and a mononuclear mixed-ligand
ternary complex of copper(II)
Subrata Naiya a, Biswarup Sarkar b, You Song c, Sandra Ianelli d, Michael G.B. Drew e, Ashutosh Ghosh a,
*
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Bolpur College, Bolpur, Birbhum, West Bengal 731204, India
c State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China
d Dipartimento di Chimica Generale e Inorganica, Chimica Analitica, Chimica Fisica, Università di Parma, Viale delle Scienze 17/A, I-43100 Parma, Italy
e School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Two Schiff bases, HL1 and HL2 have been prepared by the condensation of N-methyl-1,3-propanediamine
Article history:
(mpn) with salicylaldehyde and 1-benzoylacetone (Hbn) respectively. HL1 on reaction with
Received 15 February 2010
Received in revised form 2 April 2010
Accepted 6 April 2010
Cu(ClO4)2ꢀ6H2O in methanol produced a trinuclear CuII complex, [(CuL1)3(
3-OH)](ClO4)2ꢀH2Oꢀ0.5CH2Cl2
l
(1) but HL2 underwent hydrolysis under similar reaction conditions to result in a ternary CuII complex,
[Cu(bn)(mpn)ClO4]. Both complexes have been characterised by single-crystal X-ray analyses, IR and
UV–Vis spectroscopy and electrochemical studies. The partial cubane core [Cu3O4] of 1 consists of a cen-
Available online 13 April 2010
Keywords:
Copper(II)
tral l3-OH and three peripheral phenoxo bridges from the Schiff base. All three copper atoms of the tri-
nuclear unit are five-coordinate with a distorted square–pyramidal geometry. The ternary complex 2 is
mononuclear with the square–pyramidal CuII coordinated by a chelating bidentate diamine (mpn) and
a benzoylacetonate (bn) moiety in the equatorial plane and one of the oxygen atoms of perchlorate in
an axial position. The results show that the Schiff base (HL2) derived from 1-benzoylacetone is more
prone to hydrolysis than that from salicylaldehyde (HL1). Magnetic measurements of 1 have been per-
formed in the 1.8–300 K temperature range. The experimental data clearly indicate antiferromagnetism
Schiff base
Crystal structure
Magnetic properties
Hydrolysis
in the complex. The best-fit parameters for complex 1 are g = 2.18(1) and J = ꢁ15.4(2) cmꢁ1
.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
gands is the hydrolysis of the Schiff base that lead to the formation
of several rearranged products [16–19]. It has been observed that
the hydrolysis is dependent on several factors such as the nature
of the metal ions [16,17], the pH of the reaction medium [18],
the size of the chelate rings formed by the diamine fragment of
the Schiff base [16,17], the coordinating ability of the counter an-
ions [19] etc. The rearrangement or hydrolysis of Schiff base is
found to occur more often with MnII/III than the other metal ions
[20,21]. The acidic pH facilitates the hydrolysis of the ligand in sev-
eral instances [18] whereas the Schiff bases derived from the dia-
mine that forms a five-membered chelate ring with the metal ion
are more stable compared to those that form six-membered che-
late rings [21]. Recently, we have shown that the stability of a
mono-condensed Schiff base is regulated by the anion-directed
cation template effect [19]. In the case of a CuII template, when
one of the four equatorial coordination sites is occupied by a
strongly coordinating anion (e.g. azide or thiocyanate) the triden-
tate Schiff base is stabilized but when the counter anion is a weakly
coordinating group (e.g. perchlorate or nitrate), the Schiff base
undergoes hydrolytic cleavage [19].
Tridentate Schiff bases produced by the mono-condensation of
acetylacetone, salicylaldehyde or their derivatives with various
diamines react readily with metal ions to produce mono-[1–3],
di-[4–7] or trinuclear [8–11] complexes having interesting struc-
ture and magnetic properties [3–11]. These complexes are valuable
to theoretical and experimental chemists, as the steric and elec-
tronic factors of the Schiff base can be tuned systematically by
introducing suitable substituents to bring about subtle structural
variations that are extremely useful for deducing a relationship be-
tween magnetic coupling and structural features [3–11]. The free
amine group of such mono-condensed Schiff bases can also be con-
densed with another carbonyl compound to form unsymmetrical
tetradentate ligands which have been utilised to form complexes
with several metal ions [12–15]. One of the major problems
encountered during the synthesis of the complexes with these li-
* Corresponding author.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.010

S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
2489
In order to investigate the effect of carbonyl compound on the
hydrolytic behaviour of the mono-condensed Schiff base, we have
synthesised two Schiff bases, HL1, [2-{(3-methylamino-propylimi-
no)-methyl}-phenol] and HL2, [7-methylamino-3-methyl-1-phe-
nyl-4-azahept-2-en-1-one] by the condensation of N-methyl-1,3-
propanediamine (mpn) with salicylaldehyde and 1-benzoylacetone
(HBz) respectively. This group of tridentate Schiff bases produced by
the mono-condensation of acetylacetone, 1-benzoylacetone, salicyl-
aldehyde or their derivatives with various diamines are well known
to react readily with CuII ion in slightly alkaline or even in neutral
medium to produce trinuclear CuII complexes with a [Cu3OH] core
[4,8,9,16,17,22–24]. The magnetic properties of such complexes
are of interest as slight variations in the Schiff base ligand can change
the coupling from antiferro- to ferromagnetic. Moreover, these com-
plexes have aroused interest as they can show spin frustration phe-
nomena. We therefore allowed the two Schiff bases HL1 and HL2 to
react separately with CuII ion to produce trinuclear CuII complexes
with a [Cu3OH] core. HL1 produces the desired trinuclear Schiff base
and HL2 (10 mmol) in methanol (10 mL). Triethylamine (15 mmol,
2.1 mL) was then added drop wise to this solution with constant
stirring. In the case of 1, an immediate separation of small amount
of green hydrolysed product occurred during the addition of trieth-
ylamine, which was filtered off. The resulting green filtrate on
keeping in a refrigerator for several days yielded single crystals
as dark green hexagons. In the case of 2, no separation of any solid
product took place after mixing the constituents. The resulting
clear solution was set aside for several days at room temperature
when deep blue single crystals of complex 2, suitable for X-ray
analysis were separated out.
Complex 1: Yield: 5.6 g. 54%. Anal. Calc. for C33.5H49Cl3Cu3N6O13
(1040.76): C, 38.66; H, 4.75; N, 8.07. Found: C, 38.57; H, 4.79; N,
8.11%. kmax/nm
IR(KBr):
(C@N), 1624 cmꢁ1
3523 cmꢁ1
(
e
max/dm3 molꢁ1 cmꢁ1
)
(methnol), 641(113);
m
.
,
m
(N–H), 3272 cmꢁ1
,
m
(O–H),
Complex 2: Yield: 3.9 g. 61%. Anal. Calc. for C14H21ClCuN2O6
(411.32): C, 40.78; H, 5.13; N, 6.79. Found: C, 40.32; H, 4.98; N,
complex [(CuL1)3(
l
3-OH)](ClO4)2ꢀH2Oꢀ0.5CH2Cl2 (1) but HL2 under-
6.91%. kmax/nm (
e
max/dm3 molꢁ1 cmꢁ1) (acetonitrile), 569(81.1);
goes hydrolysis to result in a ternary CuII complex, [Cu(bn)(mpn)-
ClO4]. Herein, we report the syntheses, hydrolytic behaviour,
crystal structures, and the spectral and electrochemical behaviour
of these two complexes. The variable-temperature magnetic study
of complex 1 is also reported.
IR(KBr): , m .
m
(C@N), 1524 cmꢁ1 (N–H), 3267, 3315 cmꢁ1
2.3. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were 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 methanol (1200–
350 nm) were recorded in a Hitachi U-3501 spectrophotometer.
Cyclic voltammetry was carried out using a Sycopel model AEW2
1820F/S instrument. The measurements were performed at 300 K
in acetonitrile solutions containing 0.2 M TEAP and 10ꢁ3 M CuII
complex deoxygenated by bubbling with nitrogen. The working,
counter, and reference electrodes used were a platinum wire, a
platinum coil, and an SCE.
2. Experimental
The diamine, N-methyl-1,3-propanediamine, salicylaldehyde
and 1-benzoylacetone were purchased from Lancaster and were
of reagent grade. They were used without further purification.
2.1. Synthesis of the Schiff base ligands HL1, [2-{(3-methylamino-
propylimino)-methyl}-phenol] and HL2, [7-methylamino-3-methyl-1-
phenyl-4-azahept-2-en-1-one]
The mono-condensed ligands HL1 and HL2 (Scheme 1) have
been synthesised by refluxing solutions of salicylaldehyde
(10 mmol, 1.49 mL, for HL1) and 1-benzoylacetone (10 mmol,
1.620 g, for HL2) respectively in methanol (30 mL) with of N-
methyl-1,3-propanediamine (10 mmol, 1.04 mL) for 1 h. The
resulting dark yellow solutions were then used directly for com-
plex formation.
The magnetisation data were recorded on a Quantum Design
MPMS-XL7 SQUID magnetometer with an external magnetic field
of 2000 Oe in the temperature ranges of 1.8–300 K. The experimen-
tal magnetic susceptibility data are corrected for the diamagnetism
estimated from Pascal’s tables and sample holder calibration.
2.4. Crystallographic studies
2.2. Synthesis of the complexes, [(CuL1)3(l3
-
Data for complex 1 were collected with Mo Ka radiation at
OH)](ClO4)4ꢀH2Oꢀ0.5CH2Cl2 (1) and [Cu(bn)(mpn)ClO4] (2)
150 K using the Oxford Diffraction X-Calibur CCD System. The crys-
tal was positioned at 50 mm from the CCD. 321 frames were mea-
sured with a counting time of 10 s. Data analysis was carried out
with the CRYSALIS program [25]. The structure was solved using
A solution of Cu(ClO4)2ꢀ6H2O (10 mmol, 3.710 g) in methanol
(20 mL) was added to a stirred solution of each the ligands, HL1
C H
6
5
CH
C H
3
6
5
CHO
OH
NHCH
N
O
3
CH
O
O
OH
NHCH
NH
3
NHCH
Reflux
Reflux
3
NH
CH
2
3
2
HL
1
HL
Cu(ClO ) .6H O
Triethylamine
(in methanol)
4 2
2
Cu(ClO ) .6H O
4 2
2
Triethylamine
(in methanol)
2+
ClO
O
3
CH
O
N
H
N
N
N
O
3
2
N
N
O
1
Cu
O
Cu
O
=L
OH
Cu
N
N
Cu
NH
CH
O
C H
6
5
N
3
Scheme 1. Synthesis of the complexes.

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S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
Table 1
anisotropically while the hydrogen atoms, either located from dif-
Crystal data for complexes 1 and 2.
ference electron density maps or placed geometrically, were re-
fined with isotropic thermal parameters. Neutral atom scattering
factors were taken from Cromer and Weber [29] and anomalous
dispersion effects were included in Fcalc [30]. The crystallographic
drawings were made using ORTEP-3 [31]. Significant crystallo-
graphic data are summarised in Table 1.
Complex
1
2
Formula
Formula weight
Crystal system
Space group
a (Å)
C
33.5H49Cl3Cu3N6O13
C14H21ClCuN2O6
412.33
monoclinic
P21/c
8.826(1)
21.441(2)
9.560(1)
108.13(2)
1719.3(4)
4
1.593
1.458
0.035
3960
3087
0.0459, 0.1325
1.032
1040.76
monoclinic
P21/c
19.558(4)
14.3278(9)
14.952(8)
98.45(4)
4144(2)
4
1.668
1.789
0.063
11,997
b (Å)
c (Å)
3. Results and discussion
b (°)
V (Å3)
The tridentate ligand HL1 on treatment with the methanol solu-
tion of copper perchlorate hexahydrate and triethylamine in a
2:2:3 ratio yielded the trinuclear complex 1 (Scheme 1). A small
amount of undefined product which was formed immediately after
mixing the solutions of HL1 and copper perchlorate, was filtered off
and the filtrate on keeping either at room temperature or in a
refrigerator yielded compound 1 as the only product. The quality
of the single crystals, however was better when it was crystallized
at low temperature. On the other hand, the clear solution which
was obtained by the reaction of HL2 and copper perchlorate re-
sulted the ternary complex 2 on keeping either at room tempera-
ture or in the refrigerator showing that HL2 underwent
hydrolysis under the similar conditions.
Z
Dc (g cmꢁ3
)
l
(mmꢁ1
Rint
)
No. of unique data
No. of data with I > 2
R1, wR2
r(I)
5913
0.0533, 0.1140
0.855
Goodness-of-fit (GOF) on F2
direct methods with the SHELXS-97 program [26]. The non-hydrogen
atoms were refined with anisotropic thermal parameters. The
hydrogen atoms bonded to carbon were included in geometric
positions and given thermal parameters equivalent to 1.2 times
those of the atom to which they were attached. Two sets of four
oxygen atoms were obtained around Cl(9) and refined with occu-
pations of x and 1 ꢁ x, x refining to 0.57(1). An absorption correc-
tion was carried out using the ABSPACK program [27]. The structure
was refined on F2 using the SHELXL-97 [26].
3.1. IR and electronic spectra
In compound 1 the presence of a broad band at 3523 cmꢁ1 is as-
signed to m(O–H) of the triply bridging hydroxyl group. The peak
Data for complex 2 were collected at room temperature on Bru-
ker AMX Smart 1000 equipped with an area detector diffractome-
position corroborates the H-bonded hydroxyl group [24]. There is
another broad band at 3384 cmꢁ1 due to the presence of a lattice
water molecule. The sharp peak at 3272 cmꢁ1 indicates the pres-
ence of an N–H group and the strong absorption bands at
ter using graphite monochromated Mo K
a radiation The
absorption correction was performed with the method inserted
in SHELXTL-NT V5.1 [28]. The crystal structure of the complex 2 was
solved by direct method using SHELXS-97 [26] program and refined
by using SHELXL-97 [26]. The non-hydrogen atoms were refined
1624 cmꢁ1 are attributed to
m(C@N) of the Schiff base ligand. A
broad band at 1105 cmꢁ1 indicates the presence of ionic perchlo-
rate ions. The main differences in the IR spectra of 2 from that of
Fig. 1. The structure of the [Cu3L3(OH)]2+ cation in 1 with ellipsoids at 30% probability.

S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
2491
1 are in the regions 3300–3200 and 1650–1600 cmꢁ1. In 2, the
3.2. Description of the crystal structures
presence of two sharp peaks at 3267 and 3315 cmꢁ1, attributable
to mas and ms (NH2) clearly indicates the presence of a free NH2
Complex 1: The structure of 1 consists of a [(CuL1)3(
l
3-OH)]2+
group. On the other hand, the
m
(C@N) band which appears at
cation, shown in Fig. 1, with two perchlorate anions, and one sol-
vent water molecule together with a molecule of dichloromethane
refined with 50% occupancy. Selected bond distances and bond an-
gles for complex 1 are summarised in Table 2. Like the similar
other trinuclear CuII complexes, in this complex, each trinuclear
cluster contains a triangular Cu3 skeleton in which the copper
atoms are held together by two bridging systems: (i) the oxygen
1624 cmꢁ1 for complex 1, is absent in the spectrum of 2. The re-
sults are consistent with the fact that the Schiff base is present in
1 but not in 2 where it has been hydrolysed. In the spectra of 2,
splitting of the m
3(ClO4) (1106 and 1037 cmꢁ1) confirms the coordi-
nation of the perchlorate ion to CuII [16,17].
The electronic spectra in methanol as well as in acetonitrile of
the two complexes display a single absorption band at 641 (for
1) and 569 nm (for 2). This spectrum is typical of d/d transition
bands corroborating the observed square based geometry around
the copper centre [24,32,33].
atom (O1) of the central l3-hydroxo group and (ii) the three bridg-
ing phenoxo oxygen atoms, each of them from a different ligand.
The resulting trinuclear species does not contain any threefold
symmetry axis. The three copper(II) ions and the bridging hydroxo
Table 2
Bond distances (Å) and bond angles (°) of compound 1.
Bond distances
Cu(1)–O(1)
Cu(1)–O(31)
Cu(1)–O(51)
Cu(1)–N(59)
Cu(1)–N(63)
2.157(3)
2.189(3)
1.923(3)
1.997(3)
2.010(3)
Cu(2)–O(1)
Cu(2)–O(11)
Cu(2)–O(31)
Cu(2)–N(39)
Cu(2)–N(43)
1.992(3)
2.333(3)
1.953(3)
1.963(3)
2.052(3)
Cu(3)–O(1)
Cu(3)–O(11)
Cu(3)–O(51)
Cu(3)–N(19)
Cu(3)–N(23)
2.020(3)
1.933(3)
2.216(3)
1.980(3)
2.035(4)
Bond angles
O(51)–Cu(1)–N(59)
O(51)–Cu(1)–N(63)
O(51)–Cu(1)–O(1)
O(51)–Cu(1)–O(31)
N(59)–Cu(1)–O(1)
N(59)–Cu(1)–O(31)
N(59)–Cu(1)–N(63)
N(63)–Cu(1)–O(31)
N(63)–Cu(1)–O(1)
O(1)–Cu(1)–O(31)
89.92(12)
175.65(12)
81.65(10)
89.40(11)
146.47(11)
140.15(12)
94.34(13)
86.76(12)
95.23(12)
72.52(10)
O(31)–Cu(2)–N(39)
O(31)–Cu(2)–O(1)
O(31)–Cu(2)–N(43)
O(31)–Cu(2)–O(11)
N(39)–Cu(2)–O(1)
N(39)–Cu(2)–N(43)
N(39)–Cu(2)–O(11)
N(43)–Cu(2)–O(11)
N(43)–Cu(2)–O(1)
O(1)–Cu(2)–O(11)
90.00(12)
81.32(11)
166.78(12)
92.20(11)
171.30(12)
96.73(13)
106.04(11)
96.83(12)
91.69(12)
74.97(9)
O(11)–Cu(3)–N(19)
O(11)–Cu(3)–O(1)
O(11)–Cu(3)–N(23)
O(11)–Cu(3)–O(51)
N(19)–Cu(3)–O(1)
N(19)–Cu(3)–N(23)
N(19)–Cu(3)–O(51)
N(23)–Cu(3)–O(1)
N(23)–Cu(3)–O(51)
O(1)–Cu(3)–O(51)
93.10(13)
83.99(11)
156.98(14)
103.06(11)
175.40(12)
94.08(14)
99.17(12)
89.95(12)
97.33(13)
78.10(10)
Fig. 2. The hydrogen bonding in complex 1 shown as dotted lines. Only one set of positions for the disordered oxygen atoms around Cl(9) is included.

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S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
group form a flattened trigonal pyramid with the copper atoms
positioned at the corners of an approximate equilateral triangle.
In the pyramidal [Cu3OH] group the average Cu–O–Cu0 angle is
102.1° which is lower than is found in similar trinuclear Cu(II)
complexes with ligands derived from acetylacetone and 1-benzoyl-
Complex 2: The ORTEP-3 view of the complex 2 with atom-num-
bering scheme is shown in Fig. 3. Complex 2 is ternary in the sense
that copper atom is coordinated by two different ligands: a chelat-
ing diamine (mpn) and a benzoylacetonate (bn) moiety. This struc-
ture consists of a mononuclear cation with a weakly coordinated
perchlorate anion.
acetone [8,9,16,17,24]. The oxygen atom (O1) of the central l3-hy-
droxo group is situated above the plane defined by the three
copper atoms at 0.903 Å which is slightly higher than in the corre-
sponding acetylacetone and 1-benzoylacetone analogues
[8,9,16,17,24].
The copper atom is in a five-coordinate square–pyramidal envi-
ronment with a perchlorate oxygen atom, O(3) weakly bound to an
axial site at a distance of 2.587(3) Å (Table 4). The four coordinat-
ing atoms consisting the basal plane are the two carbonyl oxygen
from the 1-benzoylacetonate moiety and two amine nitrogen
atoms. The two Cu–O and two Cu–N bond distances in the basal
plane are slightly different [Cu–O 1.916(2), 1.932(2) Å and Cu–N
1.985(3), 2.023(3) Å] as was also observed for its 1,3-propanedi-
amine analogue [17]. Interestingly, the Cu–O2 (the oxygen atom
closer to the phenyl group) is shorter compared to Cu–O1 but
the reverse trend was observed in the other reported complex
[17]. The deviations of the coordinating atoms N1, N2, O1, O2 from
their least square mean plane are 0.087(3), ꢁ0.088(3), 0.092(2),
ꢁ0.091(1) Å respectively. The deviation of copper(II) from the
same plane is 0.031(1) Å towards the axially coordinated site.
Bond lengths for the metals in the three subunits show some
minor differences. While the bonds to the terdentate ligands are
relatively similar, to oxygen 1.923(3), 1.953(3), 1.933(3) Å, to ami-
no nitrogen 1.997(3), 1.963(3), 1.980(3) Å and to other nitrogen
2.011(3), 2.051(3), 2.035(4) Å, there are significant difference in
the bonds to the l3 bridging oxygen O(1) namely, Cu(1)–O(1)
2.157(2), Cu(2)–O(1) 1.992(3), Cu(3)–O(1) 2.020(2) Å and to the
axial oxygen Cu(1)–O(31) 2.189(3), Cu(2)–O(11) 2.333(3) and
Cu(3)-O(51) 2.216(3) Å. A possible explanation for these differ-
ences may lie in packing effects. One of the perchlorate anions in
complex 1 shows some disorder.
The hydroxide O(1) forms a donor hydrogen bond to the solvent
water molecule O(100) which in turn forms hydrogen bonds to the
two perchlorate ions as shown in Fig. 2. Dimensions of the hydro-
gen bonds are given in Table 3. In addition there are hydrogen
bonds from the saturated nitrogen atoms in the trinuclear unit to
perchlorate oxygen atoms also shown in Fig. 2. N(23)–H(23) forms
a hydrogen bond to a perchlorate oxygen atom O(83), N(43)–H(43)
to the water molecule O(100) and N(63)–H(63) to O(97) or O(94),
two oxygen atoms from alternative tetrahedra in the distorted per-
chlorate anion.
The Addison parameter (s) [34] for the copper atom is 0.06. The
six-membered chelate ring formed by the benzoylacetone moiety
is essentially planar as is observed in other similar systems
[17,24]. The plane of the benzene ring of the benzoylacetonate res-
idue makes an angle of 21.30° with the mean plane of the chelate
ring. The six-membered chelate ring of the starting diamine has a
conformation intermediate between screw-boat and boat [35] hav-
ing the puckering amplitudes Q = 0.547(4) Å, / = 348(2)° and
h = 169.3(3)°.
Both hydrogen atoms of the NH2 group (H1a and H1b) of the
diamine are involved in hydrogen bonding (Table 5 and Fig. 4).
H1b form a strong hydrogen bond with the oxygen atom, O2 of a
benzoylacetone moiety of a neighbouring centrosymmetrically re-
lated unit (2ꢁx, ꢁy, ꢁz) to result in an H-bonded dimeric entity.
H1a forms a weak hydrogen bond with a perchlorate oxygen atom,
O3 of the same centrosymmetrically related unit (2ꢁx, ꢁy, ꢁz). The
Table 3
Hydrogen bonding interactions in complex 1.
D–Hꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA (Å)
D–Hꢀ ꢀ ꢀA (o)
Dꢀ ꢀ ꢀA (Å)
O(1)–H(1)ꢀ ꢀ ꢀO(100)
1.83
1.92
2.13
1.99
2.07
2.64
2.16
2.21
171
176
135
176
157
150
175
150
2.80(1)
2.76(1)
2.78(1)
2.84(1)
2.93(1)
3.46(1)
3.07(1)
3.03(1)
O(100)–H(101)ꢀ ꢀ ꢀO(98)
O(100)–H(101)ꢀ ꢀ ꢀO(93)
O(100)–H(102)ꢀ ꢀ ꢀO(84)
N(23)–H(23)ꢀ ꢀ ꢀO(83)
N(43)–H(43)ꢀ ꢀ ꢀO(100)
N(63)–H(63)ꢀ ꢀ ꢀO(93)
N(63)–H(63)ꢀ ꢀ ꢀO(97)
complex is devoid of any considerable
p
-stacking interaction be-
contact be-
tween the aromatic rings. However, there is a Csp2–H/
p
tween the two hydrogen bonded units (the distance H2cꢀ ꢀ ꢀcentroid
A is 2.75(3) Å, the angle between C2–H91ꢀ ꢀ ꢀCg (ring A) is 144(2)°
2
and the distance C2ꢀ ꢀ ꢀCg (ring A) is 3.596(4) Å; Fig. 4). The Csp
–
Fig. 3. ORTEP plot of 2 with atom-numbering scheme; ellipsoids at 50% probability.

S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
2493
Table 4
1.4
Bond distances and bond angles in compound 2.
0.25
0.20
0.15
0.10
0.05
0.00
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Cu(1)–O(1)
Cu(1)–O(3)
Cu(1)–N(2)
1.9148(19)
2.587(3)
2.019(3)
Cu(1)–O(2)
Cu(1)–N(1)
1.932(2)
1.985(3)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(1)
O(2)–Cu(1)–O(3)
O(2)–Cu(1)–N(2)
O(3)–Cu(1)–N(2)
93.34(8)
176.52(10)
87.18(9)
173.39(11)
99.44(11)
O(1)–Cu(1)–O(3)
O(1)–Cu(1)–N(2)
O(2)–Cu(1)–N(1)
O(3)–Cu(1)–N(1)
N(1)–Cu(1)–N(2)
94.31(9)
86.23(11)
87.22(11)
82.28(10)
93.61(13)
T
χ
χ
Table 5
Hydrogen bonding distances (Å) and angles (°) for the complex 2.
0
50
100
150
/ K
200
250
300
T
D–Hꢀ ꢀ ꢀA
D–H
Dꢀ ꢀ ꢀA
Aꢀ ꢀ ꢀH
\D–Hꢀ ꢀ ꢀA
161(6)
139(2)
0
50
100
150
T
200
250
300
N1–H1aꢀ ꢀ ꢀO30
0.67(4)
1.04(4)
3.195(4)
3.328(3)
2.56(5)
2.47(4)
N1–H11ꢀ ꢀ ꢀO20
/ K
Fig. 5. Temperature dependence of vM (s) and
vMT (h) for complex 1. Solid line
shows the best-fit to the model.
H/p contact distance is typical for this type of interaction [15,36].
The copper–copper distance in the dimeric entity is 5.53 Å.
of complex 1, occupy alternate partial cube corners. Thus, there are
three CuII ion pairs interacting in the complex with two different
3.3. Magnetic properties of 1
exchange pathways for each pair: one involves the
l3-hydroxo
group and the other a bridging phenoxo group of one peripheral
Schiff base. All the three CuII ions are in a distorted square-base
pyramid environment with axial bond lengths larger than 2.1 Å,
significantly longer than equatorial bonds. Since these axial bonds
can also take part in the phenoxo-mediated exchange pathway be-
tween copper ions, we can expect the interactions through the tri-
ply bridging hydroxo ligands to be much stronger than the
phenoxo ones [37]. Thus, we will consider only one magnetic ex-
change constant J for each pair of CuII ions assuming the main con-
tribution to coupling is through the hydroxo ligand as is found in
similar complexes [38]. Moreover, since the three copper atoms
of the [Cu3O4] unit define a quasi-equilateral triangle, we consid-
ered the three metal ions as equivalent, and thus the three
magnetic exchange constants of each [Cu3O4] core will have the
same values. An isotropic Heisenberg–Dirac–van Vleck (HDVV)
The variable-temperature susceptibility for complex 1 was
measured over the temperature range 1.8–300 K with an applied
magnetic field of 2000 Oe. The temperature dependence of
vMT
versus T (vM being the magnetic susceptibility per [Cu3O4] entity)
of complex 1 is shown in Fig. 5. At room temperature, the vMT va-
lue is 1.25 cm3 molꢁ1 K, slightly higher than the spin-only value,
1.13 cm3 molꢁ1 K, based on a CuII unit with S = 1/2 and assuming
3
g = 2, suggesting higher values of g as it is usual for CuII ions. The
v
MT versus T curves show a very smooth decrease with decreasing
temperature until 130 K. On further cooling the
vMT values start
decreasing abruptly to 0.42 cm3 molꢁ1 K at 1.8 K, clearly indicating
the presence of antiferromagnetic exchange interactions in the
[Cu3O4] entity.
To determine the nature of the individual exchange interac-
tions, we noted that the three copper(II) ions of the [Cu3O4] system
Fig. 4. Hydrogen bonding network along with the C–H/p interactions in complex 2.

2494
S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
Hamiltonian formalism for an equilateral triangle [Eq. (1)] was
used to estimate the magnitude of the magnetic exchange constant
J between CuII ions.
Schiff base complexes (class iii), however, shows a classical general
behaviour – larger Cu–O(H)–Cu bond angles and the shorter dis-
tance of the O(H) above the Cu3 plane favour larger antiferromag-
netic contributions [9,38]. The present complex 1, in which the
Schiff base is derived from N-methyl-1,3-propanediamine and sal-
icylaldehyde, shows antiferromagnetic coupling, in accordance
with this classical magnetostructural correlation (Table 6). It is to
be noted that the average Cu–O(H)–Cu angle of this compound is
only slightly greater than that of the two ferromagnetic complexes
of this class. However, one must be cautious of such correlations as
the Cu–O(H)–Cu angle and distance of the O(H) above the Cu3
plane are not the only criteria for the sign and strength of magnetic
coupling, the electronic and structural factors, coming from the dif-
ferent nature of the bridging Schiff base ligands, could also be play-
ing a key role in their magnetic exchange. For example, the
compounds which are derived from the Schiff bases of N,N-dimeth-
ylethylenediamine and 1-benzoylacetone or salicylaldehyde show
relatively strong ferromagnetic coupling [9,24]. The substitution
of hydrogen atoms in the terminal amine group of the Schiff bases
by more voluminous alkyl groups seems to cause a decrease the
overlap of the magnetic orbitals of the CuII ions, giving rise to fer-
romagnetic interactions.
ꢀ
ꢁ
^
^
^
^
^
^
^
H ¼ ꢁ2Jiso S1 S þ S1 S þ S2
S
ð1Þ
2
3
3
The experimental data for complex 1, corrected for diamagnetic
contributions were analysed by Eq. (1) on MT. The magnetic sus-
v
ceptibility deduced from the Hamiltonian is given in Eq. (2), in
which N, g, b, h and k have their usual meanings.
Ng2b2
4kðT ꢁ hÞ
5e3J=kT þ 1
e3J=kT þ 1
vM
¼
ꢂ
ð2Þ
For complex 1, the best-fit parameters obtained are g = 2.18(1),
J = ꢁ15.4(2) cmꢁ1, h = ꢁ0.07(3) K and R = 0.00018, (where, R is
the agreement factor defined as Ri[(
v
MT)iexp ꢁ (
v
MT)icalc]2/
Ri[(v
MT)iexp]2). It is to be noted that in this compound the three
Cu–O(H)–Cu angles are quite different from each other (99.4,
101.9 and 104.9°). We therefore treated also the magnetic data
considering three different J values with the help of the program
ˆ
ˆ ˆ
ˆ ˆ
MAGPACK [39,40], by using the Hamiltonian H = ꢁJ12S1S2 ꢁ J13S1S3 ꢁ
ˆ ˆ
23S2S3. The best-fit (Figs. S1 and S2, Supporting Information)
J
gives a set of parameters J12 = ꢁ21.75, J13 = ꢁ21.75 and J23
=
ꢁ21.74 cmꢁ1, g = 2.18 and R = 0.00025.
3.4. Electrochemical study
For the magnetostructural correlations, the reported
l3-O(H)
copper triangular compounds can be divided into three classes
based on the constituents of the Schiff bases: (i) acetylacetone de-
rived (ii) benzoylacetone derived and (iii) salicylaldehyde derived.
Magnetic studies of these complexes revels that all 11 reported
compounds belonging to class (i) are antiferromagnetically cou-
pled [8,9,11,23], while among the five compounds of class (ii),
three compounds which are derived from N-substituted diamines,
are ferromagnetic [24] and the other two derived from unsubsti-
tuted diamines, are antiferromagnetic [17]. New magnetostructur-
al correlations were proposed for these (class ii) compounds based
on the symmetry of the metal coordination environment [24]. For
such complexes, the calculated J values using density functional
theory methods correlate with the product of the shape measure-
ments for the square–planar coordination for the two CuII cations
involved in the exchange interaction and it was found that larger
distortions from the ideal square–planar coordination result in
stronger ferromagnetic coupling. The salicylaldehyde derived
The redox behaviour of the complex 1 is shown in Fig. 6. The
cyclovoltammogram of the copper complex in acetonitrile solution
display irreversible reductive responses (Epc at ꢁ1.187 V) during
cathodic scan, attributed to the CuII/CuI couple. During the anodic
potential scan, the complexes show an oxidative response at ca.
ꢁ0.145 V with a very narrow width and high peak current. The re-
sponse is typical of the anodic stripping of copper. Therefore, it
may be inferred that the CuII complex undergoes reduction to the
respective CuI complex during the cathodic scan, which subse-
quently undergoes disproportionation to Cu0 and Cu2+. Anodic
stripping is observed, even at a high scan rate of 500 mV sꢁ1, indi-
cating the fast disproportion of the CuI species. Such anodic strip-
ping has also been found in a few other CuII–Schiff-base complexes
[32].
The cyclic voltammogram of complex 2 (Fig. 6) is recorded in
acetonitrile solvent (scan rate is 100 mV sꢁ1). The complex shows
quasi-reversible CuII/CuI reductive response. The reductive response
Table 6
Selected structural parameters for complexes 1–6 related to their magnetic properties.
e
Compoundsa
Ref.
Cu–O(H)–Cu (°)b
Cu–O(H)–Cu (average) (°)c
O(H)–Cu3 plane (Å)d
0.903
J (cmꢁ1
ꢁ15.4
)
[(CuL1)3(
[(CuL3)3(
[(CuL4)3(
[(CuL5)3(
[(CuL6)3(
l
l
l
l
l
3-OH)](ClO4)2ꢀH2Oꢀ0.5CH2Cl2
3-OH)](ClO4)2ꢀ3.75H2O
3-OH)](ClO4)2
This work
101.9
99.4
104.9
107.7/105.7
105.2/104.2
104.0/105.0
105.6
102.7
102.2
101.3/99.4
101.3/99.4
101.3/99.4
102.5
102.1
[38]
[38]
[38]
[9]
105.6/105.0
103.5
0.777/0.793
0.845
ꢁ2.33
ꢁ3.88
+4.02
+7.83
3-OH)](BF4)2ꢀ0.5CH3CN
3-OH)](ClO4)2ꢀ0.5H2O
101.3/99.4
101.4
0.911/0.968
0.932
101.5
100.2
a
(HL1 = 2-[(3-methylamino-propylimino)-methyl]-phenol,
HL3 = 2-[(2-amino-ethylimino)-methyl]-phenol,
HL4 = 2-[(2-methylamino-ethylimino)-methyl]-phenol,
HL5 = 2-[1-(2-dimethylamino-ethylimino)-ethyl]-phenol and HL6 = 2-[(3-amino-propylimino)-methyl]-phenol).
b
Cu–O(H)–Cu is the angle between two CuII ions through the oxygen atom of the hydroxo bridging ligand.
c
Cu–O(H)–Cu (av) is the average value of the three Cu–O(H)–Cu angles of every [Cu3O4] entity.
d
O(H)–Cu3 plane is the out-of-plane shift of the oxygen atom of the central l
3-O(H) bridging ligand with respect to the plane formed by the three CuII ions of the molecule.
e
J (cmꢁ1) is the magnetic exchange constant derived from the fit of the experimental data with Eq. (1).

S. Naiya et al. / Inorganica Chimica Acta 363 (2010) 2488–2495
2495
Appendix A. Supplementary material
CCDC No. 760310 (1) and 668119 (2) contain the supplemen-
tary crystallographic data for compounds 1, and 2. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic 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.2010.04.010.
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Acknowledgements
We thank CSIR, Government of India [Junior Research Fellow-
ship to S.N, Sanction No.09/028 (0702)/2008-EMR-I] and the EPSRC
(UK) and the University of Reading for funds for the X-Calibur Sys-
tem. We also thank ‘‘National Natural Science Foundation of China
(20631030 and 20771057)”.