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: wangqche@hotmail.com; qiang_wang@wtu.edu.cn

W. Li

e-mail: liwei_wuse@qq.com

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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