Transformation of a ditopic Schiff base nickel(II) nitrate complexinto an unsymmetrical Schiff base complex by partial hydrolyticdegradation: structural and density functional theory studies
Qiang Wang • Yang Liu • Wei Gao •
Zhijun Xu • Yuguang Li • Wei Li • Melanie Pilkington
Received: 29 March 2014 / Accepted: 19 May 2014
� Springer International Publishing Switzerland 2014
Abstract A new Schiff base complex [Ni(H2L1)(NO3)]
(NO3) (1) (H2L1 = 3-[N,N0-bis-2-(5-bromo-3-(morpholi-
nomethyl) salicylideneamino) ethyl amine]) was synthe-
sized from reaction of the ditopic ligand H2L1 with
Ni(NO3)2 in anhydrous MeOH. Complex 1 is stable in the
solid state, but prone to hydrolysis. Recrystallization of 1
from wet MeOH led to the isolation of a novel unsym-
metrical complex [Ni(HL2)(NO3)](NO3) (2) (HL2 = 2-[(2-
(2-aminoethylamino) ethylimino) ethyl)-5-bromo-3-(mor-
pholino methyl) salicylidene amine]). X-ray single-crystal
analysis of complex 2 showed that complex 1 had under-
gone partial decomposition of one imine bond. In contrast,
the Schiff base complex [Ni(HL3)](NO3) (3) (H2L3 =
N,N0-bis(5-methyl-salicylidene) diethylenetriamine) was
stable in wet methanol, and the single-crystal structure of 3
showed that the Ni(II) center was coordinated in an
unsymmetrical square planar geometry. Density functional
theory calculations were performed in order to obtain a
geometry-optimized model of complex 1, in which the
Ni(II) center was coordinated in a similar manner as that
in complex 3. The thermodynamic parameters were
calculated, in order to rationalize the difference in hydro-
lytic reactivity between complexes 1 and 3.
Introduction
Schiff base ligands and their transition metal complexes are
of great interest due to their diverse reactivities, and wide
applications in catalysis, pharmaceuticals, functional
materials, ion recognition and molecular assembly [1–6].
Transition metal complexes of unsymmetrical Schiff base
ligands have attracted much attention in recent years, since
many ligands around metal centers in metalloenzymes are
unsymmetrical [7–9]. However, the monocondensation of
diamines with acetylacetone or salicylaldehyde derivatives
for making ‘‘half units’’ represents a challenging step for
the construction of such unsymmetrical ligands [10].
Although imines are labile and readily hydrolyzed in
aqueous media, they are usually stabilized in metal com-
plexes. Nevertheless, the Schiff base may undergo hydro-
lysis during metal complexation, and it has been reported
that the hydrolysis is dependent on several factors, such as
the nature of the metal, the pH and counter anions [11, 12].
The influence of Lewis acid metals (such as Cu2?, Zn2?
and Ni2?) and/or counter anions (such as NO3-, Cl-,
ClO4-, N3
-, SCN- and NCS-) on the hydrolysis of Schiff
bases during complexation has been well documented [10,
11, 13]. However, their effects on the already formed
Schiff base complexes are relatively less reported. The
stability of Schiff base metal complexes against hydrolysis
is also rarely documented. Schiff base ligands have been
used as candidates for selective extraction and separation
of metal ions as they may contain multidentate mixed aza-
and oxo-cores, and their selectivity can be achieved by
electronic and steric adjustment [6, 14–16]. We report
Q. Wang (&) � Y. Liu � W. Gao � Z. Xu � Y. Li � W. Li (&)
School of Chemistry and Chemical Engineering, Wuhan Textile
University, Wuhan 430073, China
e-mail: [email protected]; [email protected]
W. Li
e-mail: [email protected]
Q. Wang � Y. Li
Engineering Research Center for Cleaner Production of Textile
Printing and Dyeing, Ministry of Education, Wuhan 430073,
China
M. Pilkington
Department of Chemistry, Brock University, St. Catharines,
ON L2S 3A1, Canada
123
Transition Met Chem
DOI 10.1007/s11243-014-9840-y
herein the synthesis, structural characterization and density
functional theory (DFT) calculations for the first example
of partial hydrolysis of an already formed ditopic Schiff
base nickel(II) nitrate complex. The hydrolytic reactivities
of this complex and an analogous complex were also
investigated by calculation and analysis of the thermody-
namic parameters.
Experimental
All chemicals and solvents were obtained from commercial
sources as reagent grade and used as received without
further purification unless mentioned otherwise. Anhydrous
methanol was distilled from magnesium methoxide. Mass
spectrometry measurements were performed on a KRA-
TOS/MSI CONCEPT 1-S Spectrometer. NMR spectra
were recorded in deuterated solvents with a Bruker
AVANCE AV300 spectrometer and measured in ppm
downfield from TMS, unless otherwise stated. FTIR spec-
tra were recorded (4,000–400 cm-1) using an AVATAR
360 spectrometer (Nicolet, USA), and KBr pellets were
used for solid samples. Elemental analyses were carried out
using an Elementar Vario Micro cube analyzer (Elementar
Corporation, Germany). UV–vis spectra were recorded on
a Varian Cary 50 Scan UV–Vis spectrometer (Varian,
USA).
DFT calculations
Becke’s 1988 exchange functional [17, 18] in combination
with the correlation functional of Lee, Yang and Parr
(B3LYP) [19] was employed in the DFT calculations. The
split valence 6–31 ? G (d) basis set was used for ground-
state geometry optimizations and analytical vibration fre-
quency calculations. All calculations were performed with
the Gaussian 03 program [20].
Synthesis of complex 1
5-Bromo-3-(morpholinomethyl) salicylaldehyde (5-BMS)
was prepared via modification of a literature method [21,
22]. To a solution of 5-BMS (0.30 g, 1.0 mmol) in anhy-
drous MeOH (8 ml) was added a solution of diethylene-
triamine (0.05 g, 0.5 mmol) in anhydrous MeOH (3 ml).
The bright yellow solution was refluxed for 2 h under N2
followed by addition of a solution of anhydrous Ni(NO3)2
(0.092 g, 0.50 mmol) in anhydrous MeOH (5 ml). The
mixture was refluxed for 2 h followed by stirring at room
temperature for another 8 h. The pale yellow precipitate
was collected by filtration, washed with MeOH and dried
under vacuum. Yield 0.18 g (65 %). Elemental analysis of
1 (C28H37N7O10Br2Ni): Found (Calcd.), C, 39.5 (39.5); H,
4.4 (4.5); N, 11.5 (11.4). FTIR (KBr, mmax, cm-1): 3,440,
3,276, 2,935, 2,888, 2,844, 1,870, 1,627 (vs, C = N),
1,533, 1,464, 1,384 (vs, NO3-), 1,309, 1,220, 1,128, 1,085,
1,035, 980, 956, 882, 756, 679, 551, 516, 446. FAB MS: m/
z 724 (18 %) for [M–2H–2(NO3)]?; m/z 787 (3 %) for [M–
H–(NO3)]?.
Preparation of complex 2
Cooling of a warm saturated solution of complex 1 in non-
anhydrous MeOH afforded complex 2 as orange crystals
suitable for X-ray diffraction analysis. The crystalline solid
was collected, washed with cooled MeOH and dried under
vacuum. Yield (50 %). Elemental analysis of 2 (C16H25-
N6O8BrNi): Found (Calcd.), C, 33.9 (33.8); H, 4.4 (4.4); N,
14.9 (14.8). FTIR (KBr, mmax, cm-1): 3,445, 3,223 (m,
NH2), 3,062 (m, NH2), 2,971, 2,868, 2,759, 1,624 (vs,
C = N), 1,546, 1,440, 1,383 (vs, NO3-), 1,320, 1,219,
1,149, 1,122, 1,079, 1,048, 956, 886, 867, 833, 789, 665,
577, 531, 445. FAB MS: m/z 506 (12 %) for [M–NO3)]?;
443 (100 %) for [M–NO3–HNO3]?.
Solvent-free synthesis of H2L3
Diethylenetriamine (0.50 g, 5.0 mmol) was added drop-
wise to 5-methyl salicylaldehyde (1.36 g, 10.0 mmol) with
constant grinding using a mortar and pestle, giving a brittle
yellow solid within 2 min of grinding. This solid was
milled to a fine, bright yellow powder to afford H2L3 in
quantitative yield. Elemental analyses of H2L3�2H2O
(C20H25N3O2�2H2O): Found (Calcd.), C, 63.9 (63.6); H,
7.7 (7.3); N, 11.2 (10.9). 1H NMR (300 MHz, CDCl3): dH
13.07 (OH, 2H, br), 8.31 (N = CH, 2H, s), 7.08 (Ph-H, 2H,
d, J = 8.7 Hz), 7.01(Ph-H, 2H, s), 6.83 (2H, Ph-H, 2H,
J = 8.1 Hz), 3.70 (4H, NCH2NH, t, J = 6.0 Hz), 2.99
(4H, NHCH2, t, J = 6.3 Hz), 2.27 (6H, CH3, s). 13C NMR
(75 MHz, CDCl3): dC 166.3 (C = N), 158.7, 133.3, 131.5,
127.8, 118.4, 116.7 (Ar–C), 59.8 (C = NCH2), 50.0
(CH2NH), 20.5 (CH3). FAB MS: m/z: 340 (100 %)
[M ? H]?. FTIR (KBr, mmax, cm-1): 2,918, 2,862, 2,804,
1,638 (vs, C = N), 1,588, 1,493, 1,370, 1,282, 1,229,
1,145, 1,070, 1,038, 996, 938, 869, 824, 782, 673, 588, 570,
459.
Synthesis of complex 3
To a solution of H2L3 (0.17 g, 0.50 mmol) in anhydrous
MeOH (5 ml) was added a solution of Ni(NO3)2�6H2O
(0.15 g, 0.50 mmol) in anhydrous MeOH (3 ml). The
solution was refluxed for 2 h under N2 and then stirred at
room temperature for 5 h. The solution was filtered, and
removal of solvent gave 3 as an orange-yellow solid. Yield
0.20 g (88 %). Elemental analysis of 3 (C20H24N4NiO5):
Transition Met Chem
123
Found (Calcd.), C, 52.3 (52.2); H, 5.3 (5.1); N, 12.2 (12.1).
FTIR (KBr, mmax, cm-1): 3,396, 3,133, 2,942, 2,870, 1,620
(vs, C = N), 1,538, 1,468, 1,384 (vs, NO3-), 1,359, 1,262,
1,223, 1,208, 1,158, 1,117, 1,061, 956, 826, 605, 490. FAB
MS: m/z 396 (100 %) for [M–(NO3)]?. Crystals suitable for
X-ray diffraction analysis were obtained by slow evapo-
ration of a solution of 3 in non-anhydrous MeOH.
Crystal structure determination
X-ray crystallographic data [23, 24] were collected on a
Bruker Smart Apex II CCD diffractometer using graphite
monochromated Mo Ka (k = 0.71073 A) radiation. The
collected data were reduced using the SAINT program, and
empirical absorption corrections were performed using the
SADABS program. The structures were solved by direct
methods and refined against F2 by full-matrix least-squares
methods using SHELXTL version 6.1. All of the non-
hydrogen atoms were refined anisotropically. All other
hydrogen atoms were placed in geometrically ideal posi-
tions and constrained to ride on their parent atoms. The
crystallographic data for complexes 2 and 3�H2O are
summarized in Table 1. Figures of the X-ray crystal
structures and the intermolecular interactions were pre-
pared with the program Diamond [25].
Results and discussion
Ditopic Schiff base ligand H2L1 was synthesized from
condensation of two equivalents of 5-BMS with one
equivalent of triethylenetetramine in refluxing anhydrous
MeOH. When one equivalent of anhydrous Ni(NO3)2 was
added to a solution of H2L1, a yellow precipitate formed
after heating for 2 h. The molecular formulation of the
isolated product was confirmed by elemental analysis,
FTIR spectroscopy and mass spectrometry (Scheme 1).
Complex 1 is stable in the solid state, as attested to by
elemental analysis and IR spectroscopic data. Slow evap-
oration of a solution of 1 in non-anhydrous MeOH or dif-
fusion of Et2O vapor into a solution of 1 in non-anhydrous
MeOH was tried in order to obtain single crystals. How-
ever, both methods afforded single crystals of an unex-
pected novel complex [Ni(HL2)(NO3)](NO3) (2). The
formulation of 2 was confirmed by elemental analysis, IR
spectroscopy and mass spectrometry. The IR spectrum
showed strong absorptions at 1,384 and 1,624 cm-1 cor-
responding to the stretching vibrations of nitrate [10, 11]
and imine (C = N) groups, respectively. The IR spectrum
of 2 also displayed medium–strong intensity bands at 3,223
and 3,062 cm-1, assigned as the asymmetric and sym-
metric stretching vibrations of the reformed NH2 group
[10].
The X-ray single-crystal structure of complex 2 is pre-
sented in Fig. 1a, with selected bond distances and angles
given in Table 2. Figure 1a reveals that one imine bond of
H2L1 in complex 1 has degraded into an NH2 group, and
one salicylaldehyde derivative 5-BMS has been lost, giving
the unsymmetrical complex 2. The results indicate
that complex 1 is unstable and undergoes partial hydrolysis
by trace moisture when exposed to non-anhydrous metha-
nol. X-ray single-crystal diffraction analysis indicates that
the molecular structure unit of 2 consists of one
[Ni(HL2)(NO3)]? cation and one nitrate anion (Fig. 1a).
The transformed asymmetrical ligand HL2 in the mononu-
clear [Ni(HL2)(NO3)]? unit acts as a tetradentate N,N,N,O-
donor, coordinating to the Ni(II) center in a square planar
geometry through three nitrogen atoms (N1, N2, N3) from
diethylenetriamine and one O atom (O1) from phenolate.
One proton has formally transferred from the phenolic
oxygen to the morpholine nitrogen atom to form a positive
cavity. The Ni–N [1.845(2), 1.8894(19) and 1.9206(18) A]
and Ni–O [1.8224(16) A] bond distances and bond angles
around the Ni(II) center are comparable with those reported
for analogous {Ni(II)N3O}? complexes [26–28]. One of the
two nitrates is associated with the positive cavity by a
Table 1 Selected crystallographic data for 2 and 3�H2O
2 3�H2O
Empirical formula C16H25BrN6NiO8 C20H26N4NiO6
Molecular weight 568.04 477.16
Crystal system Monoclinic Orthorhombic
Space group P21/c P212121
Crystal size (mm) 0.23 9 0.22 9 0.20 0.28 9 0.24 9 0.22
a (A) 15.5566 (12) 7.0377 (9)
b (A) 7.4196 (6) 13.3214 (16)
c (A) 23.2161 (14) 24.0035 (19)
a (�) 90.00 90.00
b (�) 125.555 (4) 90.00
c (�) 90.00 90.00
V (A3) 2,180.1 (3) 2,250.4 (4)
Z 4 4
T (K) 150 (2) 293 (2)
Dc (g cm-3) 1.731 1.408
l (mm-1) 2.779 0.905
F (000) 1,160 1,000
h range (�) 2.95–26.00 2.11–23.51
Reflections collected 16,547 23,693
Reflections unique 4,279 4,414
Goodness of fit on F2 1.012 1.010
R [F [ 4r(F)] 0.0273a 0.0583a
RWF2 (all F2) 0.0913b 0.1335b
a R =Pj(Fo - Fc)j/
PFo; b RWF
2 = {P
w(Fo2 - Fc
2)2/P
[w(Fo2)2]}1/
2
Transition Met Chem
123
combination of electrostatic interactions and trifurcated N–
H���O hydrogen bonds to the morpholinium amine protons
(H4A) and the NH2 protons (H3B) (Fig. 1a and Table 3),
while the other acts as a counter anion and bridging group in
the structure lattice to link the [Ni(HL2)]2? units into a left-
handed helical chain by a combination of electrostatic
interactions and N–H���O hydrogen bonds to the diethyl-
enetriamine protons (H2A and H3A) (Fig. 2 and Table 3).
The electronic spectrum of a fresh solution of complex 1
in anhydrous MeOH was recorded and then re-recorded
every hour for five cycles (Fig. 3). After one hour, the UV–
Vis spectrum showed a significant change from the original
spectrum of 1 in anhydrous MeOH. This indicated that the
hydrolysis was relatively fast when exogenous moisture
from the atmosphere entered into the anhydrous MeOH
solution, and hydrolysis was almost complete after 5 h. In
comparison with the UV–Vis spectra of 2 and 5-BMS, the
spectra of the solution from hydrolysis of 1 were
dominated by the absorption bands of 5-BMS. The
absorptions at 312 and 396 nm from complex 2 were
overlapped with the absorption of 5-BMS. Isosbestic points
at approximately 371 and 426 nm indicated that the
hydrolysis of 1 to 2 plus 5-BMS proceeds without the
involvement of any other detectable intermediates.
To further assess the extent to which ligand H2L1 was
prone to hydrolysis, samples were refluxed for 10 h in
either anhydrous MeOH or a mixed solvent of MeOH-H2O
(ratio 10:1 in volume). The UV–Vis spectra showed no
evidence of any hydrolysis, suggesting that the partial
hydrolytic degradation of H2L1 depends on its coordination
to the nickel(II) center [12, 29].
Schiff base H2L3 was synthesized by a solvent-free
method (Scheme 2). The reaction was followed by TLC
and NMR techniques and was found to have reached
completion within 5 min of constant grinding. This short
reaction time, limitation of energy needed for heating and
Scheme 1 Synthetic route and
plausible molecular
conformation and hydrolysis of
1
Fig. 1 a X-ray molecular
structure of 2 (left); b DFT
model of the monocationic part
of 2 (right)
Transition Met Chem
123
the elimination of solvent are all important factors in green
chemistry [30]. Complex 3 was synthesized from the
reaction of one equivalent H2L3 with Ni(NO3)2�6H2O in
methanol (Scheme 2); slow solvent evaporation from a
solution of 3 in non-anhydrous MeOH yielded orange-red
crystals suitable for X-ray diffraction analysis. The X-ray
single-crystal structure of 3�H2O is depicted in Fig. 4a. The
asymmetric unit consists of one [Ni(HL3)] ? cation, one
nitrate anion and one lattice water molecule. The Ni(II)
center is coordinated in a square planar geometry with one
phenolic oxygen and three nitrogen atoms from the Schiff
base, while the other phenolic oxygen of the ligand remains
un-coordinated. The monocationic charge of the {NiN3O}
center is balanced by one nitrate anion, as in complex 2.
The Ni–N [1.857(4), 1.870(4) and 1.892(5) A] and Ni–O
[1.827(4) A] bond distances and bond angles around the
Ni(II) center (Table 2) are comparable with those in
complex 2 and other analogous nickel(II) complexes [26–
28]. Together with the lattice water molecules, the nitrate
anions in 3 link the [Ni(HL3)]? units into a right-handed
helical chain through electrostatic interactions and O–H���Oand N–H���O hydrogen bonds involving the H2 and H2A
protons of [HL3]- (Fig. 5 and Table 3). The unsymmetrical
coordination of {NiN3O} without ligation of the second
phenolic OH group observed for complex 3 offers support
for the proposed structural model of 1 (Scheme 1), which
possesses a similar coordination environment.
As we could not obtain the crystal structure of 1, DFT
calculations at the B3LYP/6-31 ? G(d) level were
Table 2 Selected bond lengths [A] and angles (�) from the X-ray crystal structure for 2 and 3�H2O in comparison with geometry-optimized
models for 2, 3 and 1 calculated using DFT
2 (X-ray) 2 (Calcd.) 3�H2O (X-ray) 3 (Calcd.) 1 (Cacld.)
Ni1–N1 1.8450(2) 1.862 1.857(4) 1.848 1.865
Ni1–N2 1.8894(19) 1.921 1.870(4) 1.899 1.922
Ni1–N3 1.9206(18) 1.946 1.892(5) 1.907 1.939
Ni1–O1 1.8224(16) 1.842 1.827(4) 1.824 1.859
O1–Ni1–N1 96.29(8) 96.01 96.22(18) 95.2 94.9
O1–Ni1–N2 176.79(7) 176.60 177.4(2) 178.2 176.83
O1–Ni1–N3 90.00(7) 90.26 91.24(18) 92.0 93.79
N1–Ni1–N2 86.92(9) 87.03 86.03(19) 86.3 86.22
N1–Ni1–N3 171.59(9) 171.85 172.2(2) 171.6 171.26
N2–Ni1–N3 86.79(8) 86.58 86.56(18) 86.4 85.07
Table 3 Hydrogen bonds arrangement in complexes 2 and 3�H2O (A,
�)
D-H���A d(D - H) d(H���A) d(D���A) \ (DHA)
2
N3 - H3B���O4 0.92 2.07 2.924(3) 153.4(3)
N4 - H4A���O4 0.93 2.23 3.020(3) 141.9(3)
N4 - H4A���O5 0.93 2.11 2.974(4) 153.8(4)
N2 - H2A���O6i 0.93 2.11 3.010(3) 163.8(3)
N3 - H3A���O6ii 0.92 2.24 3.032(4) 143.8(4)
Symmetry codes: (i) 1 - x, -3/2 ? y, 3/2 - z; (ii) -1 ? x, -1 ? y,
z
3�H2O
O2 - H2A���O1 W 0.96 2.05 2.650(6) 119(6)
O1 W - H1X���O3 0.85 2.25 2.715(6) 114(6)
O1 W - H1Y���O3 0.85 2.36 2.715(6) 105(6)
N2 - H2���O4i 0.91 2.10 2.941(8) 153(8)
Symmetry code: (i) 2 - x, -1/2 ? y, 1/2 - z
Fig. 2 Left-handed helical
chain showing one NO3- acts as
a counter-anion and bridging
group of complex 2; Symmetry
code: (i) –x, 0.5 ? y, 1.5 - z
Transition Met Chem
123
performed in order to obtain structural information for 1.
For the geometry-optimized DFT models of complexes 2
(Fig. 1b) and 3 (Fig. 4b), the input structures were acquired
from the CIF data. DFT calculations reproduce the prin-
cipal structure features of the experimental geometries of 2
and 3 very well (see Figs. 1, 4; Table 2 for comparison
between selected bond lengths and angles of the X-ray
crystal structures and DFT models). The differences in
bond lengths and angles between the X-ray crystal struc-
ture of 2 and the calculated model are smaller than 0.044 A
and 0.28 degrees, respectively. The differences in bond
lengths and angles between the X-ray crystal structure of 3
and the calculated model are smaller than 0.029 A and 1�,
respectively. The good agreement between the calculated
and the experimental geometries for both complexes 2 and
3 suggests that this DFT method should also be capable of
producing a reliable geometry for complex 1, for which no
X-ray crystallographic data are available. The DFT model
Scheme 2 Preparation of
Schiff base H2L3 and complex 3
Fig. 3 Electronic spectra of 5-BMS and 2 in MeOH and 1 at different
time intervals of hydrolysis due to moisture entering into the original
anhydrous MeOH solution
Fig. 4 a X-ray molecular
structure of 3�H2O (left); b DFT
model of 3 (right)
Transition Met Chem
123
suggests an unsymmetrical structure for 1 as described in
Scheme 1 and Fig. 6. In this model, the Ni(II) center is
coordinated by one phenolic oxygen and three nitrogen
atoms from the Schiff base, while leaving the other phe-
nolic oxygen uncoordinated, similar to the experimentally
determined coordination geometry of complex 3. It is
worthy of noting that this unsymmetrical coordination
mode was also proposed for a Cd(II)-Schiff base interme-
diate, which gave [Cd(O3N2)](H2O)(NO3) via hydrolysis in
ethanol [12]. Table 2 shows that the bond lengths and
angles around the Ni(II) center in the DFT model of
complex 1 are comparable with those of 2 and 3. It is also
of note that the two six-membered rings of morpholine
exhibit chair conformations in the calculated structure of
complex 1, similar to the chair conformation observed for
complex 2.
In contrast to complex 1, complex 3 was stable in non-
anhydrous methanol and the imine bonds stayed intact
during recrystallization from this solvent. We assume that
the transformed proton and nitrate on one pendant mor-
pholine arm in 1 possibly formed small quantities of
H3O?NO3-, catalyzing nucleophilic attack of water on the
carbon atom of the C = N bond of the unbound salicy-
lidene arm, giving the free amine group [12]. The amine
group so generated will then coordinate to the metal center.
For complex 3, in the absence of such catalytic species of
H3O?NO3-, the remaining unbound salicylidene stayed
intact without hydrolysis. However, other differences in the
steric and electronic configuration of 1 and 3 might also
influence their different hydrolytic reactivities.
DFT-calculated thermodynamic parameters for the relevant
compounds are listed in Table 4. The DFT-calculated
changes of enthalpy (DH), entropy (DS) and Gibbs free energy
(DG) for the hydrolysis of 1 (1 ? H2O ? 2 ? 5-BMS) are
Fig. 5 Right-handed helical
chain of 3.H2O; Symmetry
code: (i) 2 - x, 0.5 ? y, 0.5 - z
Table 4 Calculated thermodynamic parameters
Compound or process H (Hartree) S (Cal/Mol�Kelvin) G (Hartree)
5-BMS -3317.605 130.451 -3,317.667
1 -8,595.179 258.567 -8,595.302
2 -5,353.998 184.523 -5,354.085
3 -2,879.465 199.610 -2,879.560
4 -459.974 90.546 -460.017
5-MS[a] -2,495.890 150.073 -2,495.961
H2O -76.406 45.141 -76.428
State function changes for process of:
1 ? H2O ? 2 ? 5-BMS
-0.018 (-11.30 kcal/mol) =
-47.3 kJ/mol
11.266 = 47.2 J/mol�K -0.022 (-13.8 kcal/mol) =
-57.8 kJ/mol
State function changes for process of:
3 ? H2O ? 4 ? 5-MSa0.007 (4.39 kcal/mol) =
18.4 kJ/mol
-4.132 =
-17.3 J/mol�K0.01 (6.27 kcal/mol) =
26.3 kJ/mol
a 5-MS: 5-methyl salicylaldehyde
Fig. 6 DFT model of the monocationic part of 1
Transition Met Chem
123
-47.3 kJ/mol, 47.2 J/mol�K and -57.8 kJ/mol, respectively.
Since DG\0, this hydrolytic process should be spontaneous,
which is consistent with the experimental results.
DFT calculations were also employed to investigate the
possibility of hydrolysis of 3 (3 ? H2O ? 4 ? 5-MS)
as proposed in Scheme 3. The changes in DH, DS and
DG for the hypothetical hydrolysis of 3 are 18.4 kJ/mol,
-17.3 J/mol�K and 26.3 kJ/mol, respectively. Thus, the
prediction that DG [ 0 confirms that the process is non-
spontaneous, consistent with the experimental results.
Conclusions
We have demonstrated, to the best of our knowledge, the
first example of partial hydrolysis of a complex of
nickel(II) nitrate with a ditopic Schiff base ligand. A
possible mechanism of the partial hydrolysis of the
complex has been identified, and DFT calculations on the
thermodynamic parameters are consistent with the
observed difference in hydrolytic reactivity between
complexes 1 and 3.
Supplementary material
CCDC 885803 and 959117 contain the supplementary
crystallographic data for complexes 2 and 3�H2O. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgments This work was financially supported by the
National Natural Science Foundation of China (Grant No. 21277106);
Scientific Research Foundation for Returned Overseas Chinese
Scholars, State Education Ministry; Natural Science Foundation of
Hubei Province (2008CDB038); Scientific Research Program of the
Educational Department of Hubei Province (D20091703); NSERC
and CRC (M. Pilkington).
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