Equilibrium studies on mixed ligand complex formation of Co(II), Ni(II), Cu(II) and Zn(II) with N-(2-hydroxybenzyl)-l-histidine (H2hb-l-his) and typical N,N donor ligands: Crystal structure of [Ni(hb-l-his)(bipyridine)] · H2O complex

Article abstract of DOI:10.1016/j.ica.2006.03.031

Equilibrium study on complex formation of Co(II), Ni(II), Cu(II) and Zn(II), hereafter M(II), with the quadridentate (O^-, N, O^-, N) donor ligand, N-(2-hydroxybenzyl)-l-histidine (H₂hb-l-his, hereafter H₂L), in the absence and in the presence of typical (N, N) donor bidentate ligands, 1,10 phenanthroline(phen), 2, 2′-bipyridine(bipy), ethylenediamine(en), hereafter B, in aqueous solution at 25 ± 1 °C was done at a fixed ionic strength, I = 0.1 mol dm^-3(NaNO₃) by combined pH-metric, UV-Vis and EPR measurements provide evidence for the formation of mononuclear and dinuclear binary and mixed ligand complexes of the types: M(L), M (L)₂^{2 -}, M₂(L)²⁺, M₂(H_-1L)⁺, M(L)(B), (B)M(H_-1L)M(B)⁺. The imidazole moiety of the ligand is found to act as a bridging bidentate ligand in the dinuclear M₂(L)²⁺, M₂(H_-1L)⁺ and (B)M(H_-1L)M(B)⁺ complexes, using its N₃ atom and N₁-H deprotonated moiety. Stability constants of the complexes provide evidence of discrimination of Cu(II) from the other M(II) ions by this ligand. Solid complexes: [Ni(L)(H₂O)₂] (1), [Cu(L)(H₂O)] (2), and [Ni(L)(bipy)] · H₂O (3) have been isolated and characterized by various physicochemical studies. Single crystal X-ray diffraction of the ternary complex, 3, shows an octahedral [(O^-,N,N,O^-)(N,N)] geometry with extensive π-π stacking of the aromatic rings and H-bonding with imidazole (N₁-H), secondary amino N-atom, the lattice H₂O molecule, and the carboxylate and phenolate O-atoms.

Full text of DOI:10.1016/j.ica.2006.03.031

Inorganica Chimica Acta 359 (2006) 3243–3251
www.elsevier.com/locate/ica
Equilibrium studies on mixed ligand complex formation of Co(II),
Ni(II), Cu(II) and Zn(II) with N-(2-hydroxybenzyl)-L-histidine
(
H hb-L-his) and typical N,N donor ligands: Crystal structure of
2
[
Ni(hb-L-his)(bipyridine)] Æ H O complex
2
Susmita Bandyopadhyay , G.N. Mukherjee a,*, M.G.B. Drew b
a
a
Department of Chemistry, University of Calcutta, University College of Science, 92, Acharya Prafulla Chandra Road, Kolkata, West Bengal 700 009, India
b
Department of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
Received 14 December 2005; received in revised form 8 March 2006; accepted 24 March 2006
Available online 3 April 2006
Abstract
ꢀ
ꢀ
Equilibrium study on complex formation of Co(II), Ni(II), Cu(II) and Zn(II), hereafter M(II), with the quadridentate (O , N, O , N)
donor ligand, N-(2-hydroxybenzyl)-L-histidine (H hb-L-his, hereafter H L), in the absence and in the presence of typical (N, N) donor
2
2
0
bidentate ligands, 1,10 phenanthroline(phen), 2, 2 -bipyridine(bipy), ethylenediamine(en), hereafter B, in aqueous solution at 25 ± 1 ꢀC
ꢀ
3
was done at a fixed ionic strength, I = 0.1 mol dm (NaNO
3
) by combined pH-metric, UV–Vis and EPR measurements provide evi-
2
ꢀ
2+
dence for the formation of mononuclear and dinuclear binary and mixed ligand complexes of the types: M(L), MðLÞ , M (L)
,
2
2
+
+
M (Hꢀ1L) , M(L)(B), (B)M(Hꢀ1L)M(B) . The imidazole moiety of the ligand is found to act as a bridging bidentate ligand in the dinu-
2
2+ + +
clear M
2
(L) , M
2
(Hꢀ1L) and (B)M(Hꢀ1L)M(B) complexes, using its N
3 1
atom and N –H deprotonated moiety. Stability constants of
the complexes provide evidence of discrimination of Cu(II) from the other M(II) ions by this ligand. Solid complexes: [Ni(L)(H O) ] (1),
2
2
[
2 2
Cu(L)(H O)] (2), and [Ni(L)(bipy)] Æ H O (3) have been isolated and characterized by various physicochemical studies. Single crystal X-
ꢀ
ꢀ
ray diffraction of the ternary complex, 3, shows an octahedral [(O ,N,N,O )(N,N)] geometry with extensive p–p stacking of the aromatic
rings and H-bonding with imidazole (N –H), secondary amino N-atom, the lattice H O molecule, and the carboxylate and phenolate O-
1
2
atoms.
2006 Elsevier B.V. All rights reserved.
ꢁ
Keywords: N-(2-hydroxybenzyl)-L-histidine; Metal(II) complexes; Stability constants; Crystal structure
1
. Introduction
acids and salicylaldehyde, are well documented [2], but
studies on the complex formation equilibria of metal ions
with such ligands are limited [3]. Recently, structural topol-
ogies of some binary and mixed ligand complexes of Cu(II)
with a series of N-(2-hydroxybenzyl) amino acid ligands,
obtained on reduction of the Schiff bases derived from sal-
icylaldehyde and various amino acids, viz., L-alanine
[2d,2e], L-valine [2f] and other amino acids, viz., glycine
[2g], b-alanine [2h], 4-aminobutyric acid [2i], 1-amino-
methanesulfonic acid and 2-aminoethanesulfonic acid [2j],
Schiff bases derived from amino acids and aldehydes
provide useful models for enzymic processes, viz., transam-
ination and racemisation reactions of amino acids [1].
Reduced products of such Schiff bases, because of greater
flexibility arising from reduction of the azo methyne
(
–CH@N–) group, appear to serve as better models in this
respect. Structural studies on the metal complexes of
reduced Schiff base ligands, derived from various amino
1-amino-1-cyclopropanecarboxylic acid [2k], have been
*
extensively investigated by Vittal and coworkers. In this
paper, we report the equilibrium studies on the complex
Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755.
E-mail address: gmchem@rediffmail.com (G.N. Mukherjee).
0
020-1693/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.03.031

3244
S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
H
nol:water (20 ml) was added a solution of Ni(NO ) .6H O
3 2 2
N
(291 mg, 1 mmol) in methanol. The resulting blue reaction
CH2 CH NH CH2
COOH
mixture was stirred for one hour, filtered and allowed to
stand in air at room temperature for slow evaporation. Blu-
ish violet crystals of complex 1 separated within a few days.
The crystals were filtered and washed with water (yield,
208 mg, ca. 70%). Anal. Calc. for C H NiN O : C, 44.1;
N
HO
2 2
H hb-L-his (H L)
1
3
17
3
5
H, 4.8; N, 11.8; Ni, 16.6. Found: C, 43.9; H, 4.6; N, 11.6;
Scheme 1.
ꢀ
1
3
ꢀ1
Ni, 16.4%. kmax/kK (H O) (e/mol dm cm ) 11.1
2
ꢀ
1
(
144.5), 18.1 (100.2), 27.5 (60.7). IR (KBr disk, m/cm ):
formation of Co(II), Ni(II), Cu(II) and Zn(II) with the
ligand N-(2-hydroxybenzyl)-L-histidine (H hb-L-his, here-
after H L) (Scheme 1) in the absence and in the presence
m(OH) 3579; mas (COO) 1579; m(C–N) 1457, m (COO)
s
2
1
391, m(phenolic, C–O) 1250. l /BM (298 K) 2.97.
eff
2
of typical (N,N) donor bidentate ligands, viz., ethylenedia-
mine (en), 2,2 -bipyridine(bipy) and 1,10-phenanthro-
2
.1.3. [Cu(L)(H O)] (2)
2
To a hot slurry of freshly precipitated alkali free
0
line(phen). The ligand (H L) is obtained by reducing the
2
Cu(OH)2 (200 mg, ꢁ2 mmol), H L Æ 0.5H O (540 mg,
2
2
Schiff base derived from L-histidine and salicylaldehyde
with sodium borohydride. Binary complexes of Ni(II)
ꢁ
2 mmol) was added and the mixture was digested on a
steam bath for 10 min. Cu(OH) gradually dissolved giving
2
and Cu(II) with H L and a ternary Ni(II) complex formed
2
a green solution, which was filtered hot and allowed to
stand in air. Green crystals of complex 2 separated out
within a few minutes (yield, 125 mg, ca. ꢁ65%). Anal. Calc.
for C H CuN O : C, 45.8; H, 4.4; N, 12.3; Cu, 18.6.
with it in the presence of bipy as the auxiliary ligand have
been isolated and characterized. The ligand H L appears as
2
an unusual amino acid, containing an imidazole moiety
and a phenolic group in addition to the amino and the car-
1
3
15
3
4
Found: C, 45.4; H, 4.2; N, 12.1; Cu, 18.3%. kmax/kK
2
ꢀ
boxylate functions. The ligand ion,
(
L
,
provides
ꢀ1
3
ꢀ1
(H O) (e/mol dm cm ) 15.1(79.6), 25.7 (346.4). IR
ꢀ
ꢀ
2
O ,N,O ,N) quadridentate chelation, using the carboxyl-
ꢀ1
(KBr disk, m/cm ): m(OH) 3563; mas (COO) 1590; m(C–N)
ate oxygen atom, the secondary amino nitrogen atom, the
phenolate oxygen atom, and the imidazole nitrogen atom,
creating a coordination environment, involving a combina-
tion of hard, border line and soft ligand atoms. The coor-
1482, m (COO) 1385, m(phenolic, C–O) 1252. l /BM
s
e
ff
(298 K) 1.88.
2
.1.4. [Ni(L)(bipy)] Æ H O (3)
3
ꢀ
2
dinated ligand ion, H 1L , provides quinquodentate
(
ꢀ
To a solution of a mixture of H L Æ 0.5H O (270 mg,
ꢀ
ꢀ
ꢀ
2
2
O ,N,O ,N,N ) chelation in dinuclear binary and ter-
1
mmol) and NaHCO (168 mg, 2 mmol) in water (20 ml)
3
nary complexes using the deprotonated N –H moiety of
1
was added a solution of Ni(NO ) Æ 6H O (291 mg,
1
To the refluxing reaction mixture, 2,2 -bipyridine
3
2
2
the imidazole as the fifth coordinating site. The ligand
has a tendency to form H-bonded network due to the pres-
ence of a complementary H-bond donor and acceptor
groups within the same molecule.
mmol) in water. The mixture was refluxed for 30 min.
0
(
156 mg, 1 mmol) in ethanol (15 ml) was added dropwise.
The resulting greenish brown solution was refluxed for
h, filtered hot and the volume of the filtrate was reduced
3
2
. Experimental
by evaporation on a steam bath. Brown needle shaped sin-
gle crystals of complex 3, suitable for X-ray diffraction,
appeared on standing the reaction mixture overnight at
room temperature (yield, 205 mg, ca. 70%). Anal. Calc.
for C H NiN O : C, 56.1; H, 4.6; N, 14.2; Ni, 11.9.
All the reagents other than the ligand (H L) were
2
obtained from commercial sources as A.R. grades and were
used without further purification.
2
3
23
5
4
Found: C, 55.8; H, 4.4; N, 14.1; Ni, 11.7%. kmax/kK
ꢀ
1
3
ꢀ1
2
.1. Synthesis
(H O) (e/mol dm cm ) 8.7 (26.5), 10.4 (30.4), 18.6
2
ꢀ
1
(
24.6). IR (KBr disk, m/cm ): m(OH) 3517; m (COO)
as
2
.1.1. N-(2-hydroxybenzyl)-L-histidine, H hb-L-his 0.5H O
2
1598; m(C–N) 1473, m (COO) 1395, m(phenolic, C–O)
2
s
(
H L Æ 0.5H O)
1207. l /BM (298 K) 2.91.
eff
2
2
The ligand H L Æ 0.5H O was prepared by reducing the
2
2
Schiff base obtained from the condensation reaction of
L-histidine with salicylaldehyde with sodium borohydride
2.2. Analytical and physicochemical measurements
following the reported procedure [2b]. Its purity was
2.2.1. Crystal structure determination of complex 3
Single crystal X-ray diffraction data for complex 3 were
recorded with Mo Ka radiation (k = 0.71073 A) using the
1
checked from its m.p., elemental analysis, IR and
NMR spectral data.
H
˚
MAR research Image Plate System. The crystal was posi-
tioned at 70 mm from the image plate. 95 frames were mea-
sured at 2ꢀ intervals with a counting time of 2 min. Data
analyses were carried out following the procedures [4–6]
2
.1.2. [Ni(L)(H O) ] (1)
2 2
To a solution of a mixture of H L Æ 0.5H O (270 mg,
mmol) and KOH (112 mg, 2 mmol) in 1:1 metha-
2
2
1

S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
3245
ꢀ
3
ꢀ3
ꢀ3
described earlier [7a]. The summary of crystallographic
known amounts (1 · 10
2
to 2 · 10 mol dm ) of
2
+
data and structure refinement parameters is presented in
Table 1, and some selected bond lengths and bond angles
in the structures (Figs. 1 and 2) are shown in Tables 2
and 3.
H B (B = en, bipy, phen) in the form of nitrate salts,
maintaining the metal(II):H L:B molar ratios of 1:1:1 and
2:1:2 at a fixed ionic strength, I = 10 mol dm (NaNO3)
at 25 ± 1 ꢀC (thermostated). Experimental details and
methods of calculation of the equilibrium constants
(Tables 4 and 5) were the same as described earlier [7].
2
ꢀ
1
ꢀ3
2
.2.2. Potentiometric investigations
Equilibrium study for the determination of proton–
ligand and metal(II)–ligand constants involved pH-metric
titration of series of solutions, each of initial volume
2.2.3. Spectral measurements and thermal analysis
Experimental procedure for collecting the data on ele-
mental analysis, TG/DTA, IR, and UV–Vis spectra were
the same as described earlier [7,8]. Room temperature
magnetic susceptibilities of complexes 1, 2 and 3 were
measured in polycrystalline state on a PAR 155 sample
vibrating magnetometer. Solutions of concentrations com-
parable to those used in the potentiometric measurements
were used for EPR and UV–Vis measurements. The EPR
spectra of Cu(II)–H L and Cu(II)–H L–B mixtures at pH
3
ꢀ2
ꢀ3
2
5 cm , containing 1 · 10 mol dm HNO and known
3
ꢀ3
ꢀ3
ꢀ3
amounts (1 · 10
to 5 · 10 mol dm ) of the ligand
+
ꢀ
3
H L Æ 0.5H O in its protonated form (H L ) ðNO Þ in
2
2
3
the absence and in the presence of known amounts
ꢀ
3
ꢀ3
ꢀ3
(
1 · 10 to 2 · 10 mol dm ) of metal(II) nitrates and
Table 1
2
2
Summary of crystal data, data collection and structural refinement
parameters for complex [Ni(L)(bipy)] Æ H O (3)
2
values corresponding to the concentration maxima of the
binary and ternary complex species, Cu(L) and Cu(L)(B),
respectively, as observed in the species distribution curves
Empirical formula
Formula weight
Crystal system
Space group
23 23 5 4
C H N O Ni
492.18
monoclinic
P2
(Figs. 3 and 4) were run on a Varian model E-112 spec-
trometer (X-band) in H O–ethylene glycol at LNT using
2
1
˚
DPPH (g = 2.0036) as the standard in the field range of
1200–5200 G.
a (A)
9.483(10)
7.301(9)
16.466(17)
100.05(1)
1123(2)
2
˚
b (A)
˚
c (A)
b (ꢀ)
3. Results and discussion
˚
3
V (A )
Z
ꢀ3
3.1. Physicochemical studies on solid complexes
dcalc (g cm
)
1.456
Independent, observed reflections (I > 2r(I))
3936, 3460
0.0465, 0.0926
0.205, ꢀ0.309
2
ꢀ1
R
1
, wR(F )
The IR bands in the range 3517–3579 cm
may be
˚
3
Maximum, minimum residual density (e/A )
assigned to m(OH) of lattice water molecules in compounds
C28
C25
C24
C29
C27
C26
C30
C23
C18
C17
C31
C22
C19
N21
N32
O20
C3
C16
C15
N2
Ni1
O11
C14
N4
C5
N12
C9
C6
C13
O1w
C8
O10
C7
Fig. 1. ORTEP view of complex 3 with atom labeling scheme. Thermal ellipsoids are shown at 50% probability.

3246
S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
1
–3 [9]. The presence of lattice water is further supported
by the weight loss percents observed in the temperature
range 40–220ꢀC. The asymmetric m (COO) and the sym-
as
metric m (COO) stretching bands of the complexes fall in
s
ꢀ1
the regions 1579–1598 and 1385–1395 cm , respectively.
The difference between m (COO) and m (COO) stretching
as
s
ꢀ1
frequencies (ꢁ200 ± 5 cm ) suggests only terminal mode
of coordination by the carboxylate groups [10,11]. The
ꢀ
1
strong band around 1273 cm due to the m(C–O) phenolic
in the free ligand shifts to lower frequencies in all these
complexes, suggesting coordination of metal by the pheno-
lic oxygen atom.
The anhydrous products resulting from thermal dehy-
dration of 1–3 between 40 and 200 ꢀC undergo two consec-
utive pyrolytic steps between the temperature ranges 220
and 550 ꢀC, of which the first step (200–360 ꢀC) corre-
sponds to dissociation of the imidazole moiety from the
ligand, and the second step (360–550 ꢀC) corresponds to
pyrolysis of the resulting product to form metallic oxides,
MO.
Electronic spectra of the Ni(II) complexes 1 and 3 show
three well resolved bands (11.1, 18.1, 27.4 kK and 8.7, 10.4
and 18.6 kK, respectively), which on an octahedral geome-
3
3
try correspond to the transitions: m [ A ! T ], m
1
2g
2g
2
3
3
3
3
[
A2g ! T (F)], m [ A ! T (P)], respectively. In both
1g
3
2g
1g
complexes 1 and 3, lowering of Racah parameter (B) of
2
+
Ni ion from 1.1 kK in free ion to 0.8 kK and 0.2 kK
B ) in complexes 1 and 3, respectively, indicates delocaliza-
0
(
tion of metal ion dp-electrons over to the aromatic p-sys-
tems of the coordinated ligands, viz., imidazole moiety in
1
and both imidazole and bipy moieties in 3. Electronic
spectra of the Cu(II) complex, 2, show two peaks. A broad
band centered at 15.1 kK indicates a distorted octahedral
geometry of the complex and a sharp band at 25.7 kK
may be assigned to the charge transfer transition from filled
Fig. 2. Hydrogen bonded packing of 3.
*
2
2
Table 2
rphenolate-O atom to partly filled or vacant r (x ꢀ y )
˚
Some selected bond distances (A) and bond angles (ꢀ) of complex
orbital of Cu(II) ion [12].
[
2
Ni(L)(bipy)] Æ H O (3)
Room temperature magnetic moment (l ) values of
eff
Ni(1)–N(32)
Ni(1)–N(2)
Ni(1)–O(11)
2.073(5)
2.130(4)
2.110(4)
Ni(1)–O(20)
Ni(1)–N(12)
Ni(1)–N(21)
2.078(4)
2.089(4)
2.093(4)
complexes 1–3 are close to the spin only values for Ni(II)
and Cu(II) ions.
O(11)–Ni(1)–O(20)
O(11)–Ni(1)–N(2)
O(11)–Ni(1)–N(12)
O(11)–Ni(1)–N(21)
O(11)–Ni(1)–N(32
O(20)–Ni(1)–N(2)
O(20)–Ni(1)–N(12)
O(20)–Ni(1)–N(21)
94.50(13)
86.49(12)
79.85(12)
174.34(13)
98.94(13)
178.47(13)
92.94(13)
90.77(14)
O(20)–Ni(1)–N(32)
N(2)–Ni(1)–N(12)
N(2)–Ni(1)–N(21)
N(2)–Ni(1)–N(32)
N(12)–Ni(1)–N(21)
N(12)–Ni(1)–N32)
N(21)–Ni(1)–N(32)
88.25(14)
88.38(13)
88.19(14)
90.44(13)
101.93(15)
178.36(13)
79.16(15)
3
.2. Description of crystal structure of complex 3
ORTEP view of complex 3 with the atom numbering
scheme is shown in Fig. 1. Geometry around the Ni(II)
ion in 3 is distorted octahedral with the carboxylate oxygen
atom (O11), the phenolate oxygen atom (O20), the second-
ary amino nitrogen atom (N12) and the pyridine type
Table 3
˚
Hydrogen bond distances (A) and bond angles (ꢀ) of complex [Ni(L)(bipy)] Æ H
2
O (3)
D–H
d(D–H)
d(Hꢂ ꢂ ꢂA)
\DHA
d(Dꢂ ꢂ ꢂA)
A
Symmetry
O1W–H1W
O1W–H2W
N4–H4
0.71(7)
0.94(6)
0.860(6)
0.910(5)
2.29(7)
1.83(6)
1.904(5)
2.138(6)
155(8)
166(6)
159.7(4)
153.0(3)
2.950(6)
2.746(6)
2.726(6)
2.978(7)
O10
O20
O10
O1W
[1 ꢀ x,ꢀ1/2 + y,1 ꢀ z]
[x,ꢀ1 + y,z]
[2 ꢀ x,ꢀ1/2 + y,1 ꢀ z]
N12–H12

S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
3247
Table 4
100
Proton–ligand constants of H
2
L and B ligands
(a)
H L
3
Cu(L)
Constants
Ligands
8
6
4
2
0
0
0
0
0
+
2+
2+
2+
H
3
L
H
2
phen
H
2
bipy
2
H en
logb010–3
logb010–2
logb010–1
logb001–2
logb001ꢀ1
24.43
18.50
10.38
6.86
4.96
5.55
4.23
16.99
9.89
Free Cu(II)
ꢀ ꢀ3
1
[
I = 10 mol dm (NaNO ); 25 ± 1 ꢀC].
3
Table 5
Stability constants of M(II)-L binary and M(II)-L-B(phen,bipy,en)
ternary complexes (M = Co, Ni, Cu and Zn)
3
4
5
6
7
8
9
1
00
II
II
II
II
Constants
B
Co
Ni
Cu
Zn
(b)
Cu(L)2
logb1100
logb1200
logb2100
logb2101
9.36
11.37
14.26
16.03
7.23
15.44
17.80
18.83
11.75
ꢀ9.68
9.14
9.52
8
0
13.61
13.63
4.23
13.55
13.07
3.73
60 Free Cu(II)
logK
d
ꢀ12.50
ꢀ12.76
ꢀ11.44
H L
3
logb1010 [15]
phen
bipy
en
phen
bipy
en
phen
bipy
en
phen
bipy
en
7.83
8.64
6.22
4
2
0
0
0
6.06
5.62
7.13
6.96
7.80
5.15
5.61
10.44
24.44
23.01
25.36
ꢀ0.14
ꢀ0.23
ꢀ0.52
27.46
26.35
26.12
Cu(L)
logb1110
17.80
15.93
15.42
ꢀ0.28
ꢀ0.38
ꢀ0.45
18.90
17.56
16.13
19.80
18.17
17.94
ꢀ0.21
ꢀ0.33
ꢀ0.39
22.06
18.49
18.32
15.41
14.28
14.76
ꢀ0.30
ꢀ0.43
ꢀ0.49
16.34
15.15
14.71
H L
2
DlogK
M
3
4
5
6
7
8
9
1
00
80
logb2121
(c)
Limits of error: ± 0.02–0.05 in log scale.
Temp. 25 ± 1 ꢀC; ionic strength I = 10 mol dm (NaNO
ꢀ1
ꢀ3
3
) in aqueous
6
4
2
0
0
0
0
solution.
Free Cu(II)
nitrogen atom (N2) of the imidazole moiety of the ligand
H L
Cu(L)
3
2
ꢀ
dianion, (L ), coordinating four positions, leaving two
cis-positions vacant for coordination by the two nitrogen
atoms (N21 and N32) of the bipyridine ligand. Four donor
atoms [O20, N12, N2, N32] define a mean equatorial plane,
Cu (H L)
2
-1
Cu (L)
2
Cu(OH)
Cu(OH)2
˚
3
4
5
6
P1 [deviations of ꢀ0.0173 and ꢀ0.0172 A for O20 and N2
˚
pH
to 0.0170 and 0.0175 A for N12 and N32]. The other two
planes formed by [O20, N21, N2, O11] and [N12, N21,
N32, O11] are almost perpendicular to P1 forming dihedral
angles of 79.55ꢀ and 88.91ꢀ, respectively, with P1, and these
two planes are also perpendicular to each other forming a
dihedral angle of 88.58ꢀ. One of the two five-membered
chelate rings, Ni1–O11–C9–C8–N12, has half chair confor-
mation having the puckering parameters [13],
Fig. 3. Species distribution curves of (a) 1:1, (b) 1:2 and (c) 2:1 Cu(II)–
L systems.
H
2
(O10), the phenolate oxygen atom (O20), the H-atoms
attached to the pyrrole type nitrogen atom N(4) of the
imidazole moiety, and the secondary amino nitrogen atom
˚
(N12) and one lattice H O molecule are involved in exten-
q = 0.472(3) A; u = 343.1(5)ꢀ. The other five-membered
2
2
2
sive H-bonding (Fig. 2).
ring constructed by Ni1–N21–C26–C27–N32 is approxi-
mately planar with a N–C–C–N torsion angle of 0.97ꢀ.
The six-membered chelate rings Ni1–N2–C6–C7–C8–N12
and Ni1–O20–C19–C14–C13–N12 have twist boat confor-
mation with puckering parameters q = 0.460(4), q =
3.3. Equilibrium study
2
3
3.3.1. Protonation–deprotonation equilibria of the ligand
H L
˚
ꢀ
0.487(4) A and u = 98.9(5)ꢀ and boat conformation with
2
2
˚
puckering parameters q = 0.814(4), q = 0.008(4) A and
The ligand (H L) is sparingly soluble in water and in
2
3
2
u = 235.4(3)ꢀ, respectively. The carboxylate oxygen atom
2
common organic solvents. It is brought into solution as

3248
S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
pM þ qL þ rB þ sOH ꢀ M L B ðOHÞ
ð1Þ
1
1
1
00
p
q
r
s
Cu(bipy)
(a)
½
M
p
p
L
q
B
r
ðOHÞ ꢃ
Cu(L)(bipy)
s
bpqrs
¼
;
ð1aÞ
q
r
s
80
60
40
20
0
½Mꢃ ½Lꢃ ½Bꢃ ½OHꢃ
where p, q, r and s are stoichiometric numbers. p, q and r
are positive integers or zero. s is a negative integer for a
protonated species, zero for a neutral species and positive
for a hydroxo or a deprotonated species. Charges are omit-
ted for simplicity. bpqrs values have been calculated by ana-
lyzing the pH titration data of the metal–ligand mixtures
using the computer program, SCOGS [14]. The other relevant
constants may be calculated from the bpqrs values using
appropriate relations. Preliminary values of b1010, obtained
from literature [15], have been subjected to computer
refinement using the SCOGS program [14] with the pH titra-
tion data of the 1:1, 1:5 and 1:10 M(II):BH2 mixtures and
the values corresponding to lowest standard deviations
have been accepted. For some of the systems, such as
Co(II)–phen, the preliminary b1010 value has been calcu-
lated from the corresponding value of Ni(phen), assuming
as a first approximation, the difference between the
logb1010 values of Ni(phen) and Co(phen) to be same as
that between Ni(bipy) and Co(bipy), for which literature
H L
3
Free Cu(II)
3
4
5
6
7
8
9
10
00
(
b)
H en
Free Cu(II)
2
Cu(L)(en)
2
þ
80
60
40
20
0
Cu(L)
H L
3
values of b1010 are available [15]. b1100, b1200, b2100, b1010
,
Cu(en)
b1110 and b2121 values are obtained as computer output,
from which the reprotonation constant DlogKM [16] for
mixed-ligand complexes may be calculated using the
relation:
3
4
5
6
7
8
00
(
c)
8
6
4
2
0
0
0
0
0
D log KM ¼ log b1110 ꢀ log b1100 ꢀ log b1010
.
ð2Þ
Cu(bipy)
3
.3.2.1. Binary M(II):H L (1:1, 1:2 and 2:1) equilibria.
2
H L
3
Complex formation equilibria in 1:1 metal(II):H L systems
2
Cu(L)(bipy)
with all the four metal(II) ions (M = Co, Ni, Cu and Zn)
start at pH 4–5. Binary 1:1 complexes, M(L), are the major
metal containing species at pH < 7 (Fig. 3a). These are
formed according to
Cu (H L)(bipy)
2
2
-1
Hbipy
H bipy
Cu(OH)
8
2
Free bipy
6
2
2þ
þ
þ
M
þ H
3
L ꢀ MðLÞ þ 3H
ð3Þ
+
3
4
5
7
2
Consequently, three moles of base per mole of M are
consumed in this pH range, since the buffer regions corre-
sponding to all the three-steps of deprotonation of the li-
gand are lowered in the presence of M(II) ions. 1:1
pH
Fig. 4. Species distribution curves of (a) 1:1:1 Cu(II)–H
Cu(II)–H L-en and (c) 2:1:2 Cu(II)–H L-bipy systems.
2
L-bipy, (b) 1:1:1
2
2
Cu(II)–H L mixture at pH ꢁ 6 shows k
at 25.3 and
2
max
1
4.6 kK and its EPR spectrum gives g = 2.1276, A /
i
i
+
its mono nitrate salt form, H L Æ HNO [i.e., (H L )
G = 48, indicating a octahedral [Cu(II)(O) (N) ] geometry
4 2
2
3
3
ꢀ
+
ðNO Þ]. At pH 6 2 its cation, H L , titrates as a triprotic
[17], in which the carboxylate oxygen atom, the phenolate
oxygen atom, the amino nitrogen atom and the pyridine
type nitrogen atom of the imidazole moiety of the ligand
3
3
acid in the pH range 2–11, due to successive deprotonation
of the –COOH group (pH 2–6) followed by deprotonation
+
of the amino–N H and the –OH protons in the successive
and two H O molecules coordinate the Cu(II) ion.
2
steps.
The speciation curves (Fig. 3b) of 1:2 M(II):H L mix-
2
2ꢀ
tures show the formation of MðLÞ
complexes at
2
3
.3.2. Metal (II)–ligand equilibria
The overall stability constants (bpqrs) of the generalized
pH > 7.5 according to
2ꢀ
þ
MðLÞ þ H
2
L ꢀ MðLÞ þ 2H
ð4Þ
ð5Þ
2
complex species, M L B (OH) , may be defined according
to
p
q
r
s
ꢀ
2ꢀ
þ
MðLÞ þ HL ꢀ MðLÞ þ H
2

S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
3249
H O
2
OH
HN
HC
O
HN
HC
OH
N
2
+
M
M(aq)
N
2 +
2
H
OH
NH
+
2M (aq)
O
NH
2 H +
+
O
C
O
C
H
2 +
H
M (L)
2
2
2
H2L
+
+
H
H O
+
2
H
O
OH
N
2
M
M(aq)
HN
O
N
HC
O
C
H
2
+
M (H L)
1
2
Scheme 2.
2
ꢀ
2+
CuðLÞ2 ion (kmax: 25.1 and 15.2 kK), which is the major
Free M ions are practically non-existent from the very
beginning of complex formation in these systems. The early
stages (pH 2–5) of complex formation, in the ternary 1:1:1
metal containing species in the 1:2 Cu(II):H L system at
pH > 8, is however formed according to
2
+
2+
þ
2ꢀ
2
þ 3Hþ
M(II):H
L :H en systems (Fig. 4b) are dominated by the
3 2
CuðLÞ þ H L ꢀ CuðLÞ
In the 2:1 M(II):H L systems, the imidazole bridged
binuclear complexes, M (L) , appear at pH > 4 (Fig. 3c)
ð6Þ
3
M(L) complexes, which may be formed according to:
+
3
M þ H L ꢀ MðLÞ þ nH ðn ¼ 3; 2Þ
n
ð12Þ
2
+
2
according to
With rise of pH, these binary M(L) complexes take up
the en ligand to form the ternary M(L)(B) complexes
2
þ
þ
ðLÞ2þ þ 3Hþ
2
M
þ H
3
L ꢀ M
2
ð7Þ
(B = en) above pH ꢁ 5 according to equilibrium:
Concentration maximum of the dinuclear complex,
MðLÞ þ BH ꢀ MðLÞðBÞ ðB ¼ enÞ þ nH ðn ¼ 2; 1Þ ð13Þ
n
2
+
Cu (L) (kmax: 32.8 and 14.2 kK), is observed at pH ꢁ 4.
2
Charges are not shown for simplicity. The ternary
M(L)(en) complexes represent the major metal containing
species above pH 8. Stability constants (Table 5) of the
complexes fall in the usual order [18]: Co(II) < Ni(II) <
At higher pH values (pH > 5.5), the dinuclear complex spe-
2
+
cies, M (L) , decompose (equilibrium (8)) to produce the
mononuclear M(L) complexes, and hydroxo species,
M(OH), and it also undergo deprotonation (equilibrium
2
Cu(II) > Zn(II)ꢂDlogK (Table 5) values are found to be
M
(
9)) of the imidazole (N –H) moiety (Scheme 2),
1
less negative than their statistical values [16], indicating fa-
vored formation of the ternary M(L)(B) complexes over the
binary ones.
Kd
2
þ
þ
þ
M ðLÞ2 þ H O ꢀ MðLÞ þ MðOHÞ þ H
M ðLÞ ꢀ M ðH LÞ þ H
ð8Þ
ð9Þ
2
2
þ
þ
þ
2
2
ꢀ1
The decomposition constant (K ) and the deprotonation
d
H
-1
2
constant ðK
Þ may be related to the experimentally
h
g
λmax(nm)
ε(mol cm )
M2ðLÞ
pH
determinable constants according to (8a) and (9a),
respectively
0.6
a
3.00 712
27.5
b
c
d
e
f
g
h
4.00 671
5.00 664
6.00 664
7.00 665
54.7
81.9
89.5
92.7
log K
log K
d
¼ log b110 þ log b101 ꢀ log b210
.
ð8aÞ
ð9aÞ
H
¼ log b2101 ꢀ log b2100
.
8.00 665, 387 98.2, 363.8
.00 666, 397 99.1, 528.1
10.00 663, 397 107.2, 633.9
M2ðLÞ
0
.4
9
3
(
.3.3. Ternary (1:1:1) and (2:1:2) M(II):H L:B equilibria
2
f
B = phen, bipy and en)
Speciation curves (Fig. 4a) of 1:1:1 M(II):H L :H B
+
2+
3
2
0.2
e
2
+
(
B = phen and bipy) ternary systems indicate the M(B)
d
complexes as the major metal containing species at the start
c
2
+
of the reaction (pH ꢁ 2). With rise of pH, these M(B)
b
+
complexes react with H L and H L to form the ternary
a
3
2
0
.0
M(L)(B) complexes according to equilibria:
400
600
800
1000
2
þ
L ꢀMðLÞðBÞ þ 3Hþ
þ
Wavelength (nm)
pH 2–5 MðBÞ2 þ H
3
ð10Þ
þ
þ
Fig. 5. Superimposed electronic spectral curves of 1:1:1 Cu(II):H
mixtures at different pH values; [Cu(II)] 0.001 M in aqueous solution,
I = 0.1 M (NaNO ), l = 1 cm.
2
L:phen
pH > 5 MðBÞ þ H
2
LꢀMðLÞðBÞ ðB ¼ phen;bipyÞ þ 2H
ð11Þ
3

3250
S. Bandyopadhyay et al. / Inorganica Chimica Acta 359 (2006) 3243–3251
N
N
O
HN
HC
N
O
HN
HC
N
N
N
M
M
M
N
N
N
+
O
N
N
O
NH
+
H
M
O
O
+
+
H
C
C
H
H
2
2
2
+
+
M(L)(B)
M(B)
(B)M(H -1 L)M(B)
Scheme 3.
Concentration maxima of Cu(L)(B) complexes occur
around pH 8. Electronic spectral band around 25.1 kK
Fig. 5) of 1:1:1 Cu(II):H L:phen mixture at this pH
4. Conclusion
(
The ligand N-(2-hydroxybenzyl)-L-histidine (H L)
2
2
arises from the charge transfer transition from filled
shows ambidentism and discriminating power in its com-
plexes with 3d M(II) ions. It coordinates as a diprotic
(O , N, O , N) quadridentate ligand in mononuclear bin-
ary M(L) and mixed ligand M(L)(B) complexes (B = bipy,
phen, en) using the carboxylate –O, secondary amino-N,
*
2
2
r(O-phenolate) to vacant r (x ꢀ y ) of Cu(II) ion [12].
The EPR spectra of the mixture at pH give
g = 2.1035 and A /G = 68.6, which are indicative of dis-
ꢀ
ꢀ
8
i
i
torted octahedral [Cu(II)(N) (O) ] geometry [17], in
4
2
which the carboxylate oxygen atom, the phenolate oxy-
gen atom, the secondary amino nitrogen atom and the
pyridine type nitrogen atom of the imidazole moiety of
the ligand and two N-atoms of the (N,N) donor (B)
ligands coordinate the Cu(II) ion. These modes of coor-
phenolate-O and imidazole-(N ) atoms. In dinuclear binary
3
and mixed ligand complexes, it offers quinquodentate
ꢀ
ꢀ
ꢀ
(O ,N,O ,N,N ) coordination. The imidazole–N atom
1
coordinates the second metal ion through deprotonation
of the N –H moiety. Because of a combination of hard
1
2
ꢀ
ꢀ
ꢀ
dination of the L and B ligands are evident from the
(COO , O -phenolate), borderline (–NH) and soft (imid-
azole N , N ) donor groups within the same molecule,
crystal structure of the Ni(II) complex [Ni(L)(bipy)] Æ H O
2
3
1
(
3) (Fig. 1).
the ligand H L is able to discriminate Cu(II) from the other
2
The speciation curves (Fig. 4c) of 2:1:2 M(II):-
3d M(II) ions (Co(II), Ni(II) and Zn(II)) in regard to stabil-
ity of the complexes in solution.
2
+
2+
H L :H B mixtures at pH P 5 indicate the formation
3
2
of imidazolate bridged dinuclear mixed ligand complexes
+
of the types, (B)M(H L)M(B) , according to the
equilibrium:
5. Supplementary data
ꢀ
1
Crystallographic data for the structural analysis of the
complex 3 have been deposited with the Cambridge Crys-
tallographic Data Centre, with CCDC No. 290615. Copies
of this information may be obtained free of charge from
The Director, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.-
ac.uk or www: http://www.ccdc.cam.ac.uk).
MðLÞðBÞ þ MðBÞ ꢀ ðBÞMðHꢀ1LÞMðBÞ þ H
ð14Þ
in which one of the two M(II) ions may be coordinated
by the carboxylate oxygen atom, the phenolate oxygen
atom, the amino nitrogen atom and the pyridine type
nitrogen atom of the imidazole moiety of the ligand
3
ꢀ
H
1L ion and two N-atoms of the (N,N) donor B li-
ꢀ
gands, and the other M(II) ion may be coordinated by
Acknowledgment
the deprotonated (N –H) moiety of the imidazole ring,
1
two N-atoms of the (N,N) donor B ligands and solvent
Author’s grateful thanks are due to the University
Grants Commission (New Delhi) for the award of research
fellowships to S.B.
H O molecules (Scheme 3). Consequently, the 2:1:2
2
Cu(II):H L:phen mixture shows a small blue shift of
2
the absorption maxima with rise of pH from 5 to 8.5,
indicating that a negatively charged (r-basic and p-
acidic) imidazolate moiety provides stronger ligand field
than its neutral form [19].
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