Isothiocyanato and azido coordination induced structural diversity in zinc(ii) complexes with Schiff base containing tetrahydrofuran group: Synthesis, characterization, crystal structure and fluorescence study

Article abstract of DOI:10.1039/c4ra11352g

Three new structurally diverse zinc(ii) complexes of formula [H₃O][ZnL₂]ClO₄ (1), [Zn₂(μ-L)₂(NCS)₂] (2) and [Zn₃(μ-L)₂(μ-N₃)₄]_n (3) ha

Full text of DOI:10.1039/c4ra11352g

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PAPER
Isothiocyanato and azido coordination induced
structural diversity in zinc(II) complexes with Schiff
base containing tetrahydrofuran group: synthesis,
characterization, crystal structure and fluorescence
study†
Cite this: RSC Adv., 2014, 4, 65044
*
Haridas Mandal, Sukanta Chakrabartty and Debashis Ray
Three new structurally diverse zinc(II) complexes of formula [H3O][ZnL2]ClO4 (1), [Zn2(m-L)2(NCS)2] (2) and
[Zn3(m-L)2(m-N3)4]n (3) have been synthesized using the same furan-based tridentate ONO-donor Schiff
base ligand HL (2-hydroxybenzyl-2-tetrahydrofurylmethyl)imine and characterized by X-ray structural
analysis. Complex 1 is mononuclear, whereas 2 is a double phenoxido-bridged dinuclear compound. The
novel polymeric compound 3 from azido coordination driven aggregation possesses a very rare 1D
structure in which the dinuclear ligand-bound Zn2L2 fragments are bridged by four ligand-free m1,1-azido
2ꢀ
bridging Zn(N3)4
units resulting in a zigzag arrangement of repeating triangular Zn3 motifs with
structural similarity to the P1 nuclease. Solid state mixing and grinding processes were applied for the
ligand exchange reaction and core conversion. In mechanochemical solvent free synthetic routes 1
reacts with isothiocyanato and azido ions to provide 2 and 3 in pure form. The ligand HL serves as
sensitive fluorescent probe for Zn2+, and complex 1 for SCNꢀ and N3 ions in MeOH medium.
ꢀ
Coordination induced fluorescence enhancement due to intraligand p / p* transition in the presence
ꢀ
of Zn2+ of HL and quenching of emission intensities of 1 with SCNꢀ and N3 anions are accounted for by
Received 27th September 2014
Accepted 21st November 2014
the formation of hitherto unknown complexes [Zn(L)(X)] (where X ¼ ClO4ꢀ, SCNꢀ and N3ꢀ). HL shows
chelation-enhanced fluorescence response from strong metal ion coordination and binding of
isothiocyanato and azido anions with an appreciable lifetime of the fluorophore signals. Excitation at 380
nm of MeOH solutions of all three complexes in air exhibit excited state life-time spanning from 2.5–4.8 ns.
DOI: 10.1039/c4ra11352g
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aggregation of zinc(II) ions has been an active area of research
particularly due to the participation of this metal ion in
neurobiology.6 Coordinating primary chelating ligand and
1. Introduction
Zinc is the second most essential metal ion for the human body
and its biogenic coordination environment plays a number of
important roles in a variety of biological processes.1,2 Like any
other main group metal ZnII ions are best known as a structural
or catalytic cofactor in a large number of metalloproteins and
metalloenzymes. The ligand environment is known to tune the
nucleophilicity of the metal bound active site in several hydro-
lytic enzymes.3 Understanding of the role of zinc ions in a living
system by detecting it in live cells and tissues is an important
area of research.4 To date several Schiff base complexes of zin-
c(II) are known to show interesting photochromic behavior.5 The
search for new ligand systems responsible for coordination and
small ancillary inorganic anions can control the self-assembly
process. Different approaches involving one pot multicompo-
nent synthetic procedure, solvent-based synthesis, hydro-
thermal and many more are introduced. Solvent free
mechanochemical grinding method is an efficient environ-
mentally benecial approach to reduce environmental
contamination.7 The present work reports the use of a new
tetrahydrofuran-based tridentate ligand for zinc coordination
induced uorescence and azido coordination mediated multi-
zinc aggregate formation. Azido coordination induced 1D
network of zinc(II) is extremely uncommon particularly with
Schiff base ligands in comparison to those of manganese(II),
nickel(II) and copper(II).8 Herein a very rare and rather unique
1D zinc(II) coordination polymer is reported using tridentate
furan containing Schiff base ligand having triangular Zn3 motif
of two octahedral followed by one tetrahedral zinc(II). Ligand
specic coordination plasticity of zinc(II) allows it to show
coordination numbers from 3 to 6 and adopts geometries from
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India.
E-mail: dray@chem.iitkgp.ernet.in; Fax: +91 3222 82252; Tel: +91 3222 283324
† Electronic supplementary information (ESI) available: Supplementary
crystallographic data in CIF format for 1, 2 and 3, respectively. CCDC 1006844,
1006845 and 1006846. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ra11352g
65044 | RSC Adv., 2014, 4, 65044–65055
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thiocyanate from S d Fine-Chem Ltd, sodium azide from Merck,
India. Zinc(II) perchlorate hexahydrate was prepared by treating
zinc carbonate with 1 : 1 conc. HClO4–water mixture and crys-
tallized aer concentration on water bath. All other chemicals
and solvents were reagent grade materials and were used as
received without further purication.
2.2. Synthesis
Chart 1 Possible coordination modes of HL.
(2-Hydroxybenzyl-2-tetrahydrofurylmethyl)imine (HL). The
ligand HL was prepared in situ, following a modied literature
procedure,15 by treating a methanolic solution (10 mL) of tet-
rahydrofurfuryl amine (0.515 g, 5 mmol) and salicylaldehyde
(0.52 g, 5 mmol) under stirring condition at room temperature
for 1 h followed by heating at 40 ꢁC for another 45 min (Scheme
S1, ESI†). Room temperature solvent evaporation results a light
yellow gummy and sticky mass. Yield: 0.81 g (79%). Selected FT-
IR bands: (KBr, cmꢀ1; s ¼ strong, vs ¼ very strong, m ¼
medium, br ¼ broad) 3445(br), 2869(m), 1631(vs), 1498(m),
1280(s), 1074(m), 757(s). 1H NMR (CDCl3): d ¼ 1.66–1.72 (m,
1H), 1.74–1.90 (m, 2H), 1.97–2.10 (m, 1H), 3.60–3.68 (m, 2H,
CH2), 3.69–3.71 (m, 1H, CH2), 3.73–3.84 (m, 1H, CH2), 4.10–4.16
(m, 1H, CH), 6.81–6.85 (m, 1H, Ph), 6.92 (d, 1H, J ¼ 8.4 Hz, Ph),
7.21–7.28 (m, 2H, Ph), 8.32 (s, 1H, CH), 13.42 (br, 1H, OH) ppm.
13C NMR (CDCl3): d ¼ 25.8 (CH2), 29.1 (CH2), 63.4 (CH2), 68.3
(CH), 78.0 (CH2), 116.9 (CH-Ph), 118.4 (CH-Ph), 118.8 (Cq, Ph),
131.3 (CH-Ph), 132.1 (CH-Ph), 161.2 (Cq, Ph), 166.3 (CH, imine)
ppm.
planar to octahedral with different ligand systems.9 Known
metalloproteins bearing zinc(II) ions quite oen show distorted
tetrahedral or trigonal bipyramidal geometries.10 In the absence
of any crystal eld stabilization, the ligand backbones with
rigid, semi-rigid and exible coordination arms play the
deciding role for a particular geometry.11 Terminal phenoxido
group bearing tridentate ligands having pyridine and other
terminal ends are known to provide bimetallic and higher order
structures.12 In a particular reaction condition and in support
from small ancillary triatomic bridging groups the chosen
ligand system can lead to metal ion aggregates.8 In this work
formation of mononuclear, dinuclear and a notable 1D coor-
dination chain from the coordination of (2-hydroxybenzyl-2-
tetrahydrofurylmethyl)imine (HL) is achieved with the incor-
poration of six, ve and four coordinated zinc(II) centers by
varying the reactants and the reaction conditions. The versatile
coordination abilities of furan based tridentate ligand HL
(Chart 1) is explored with hydrated zinc perchlorate salt in
absence and in presence of triatomic ancillary anions.
Depending upon the situation. HL can coordinate as bidentate
unit through phenoxido-O and imine-N keeping furfuryl arm as
pendent,13 as tridentate mono-metal centric or can bridge as
edge on fashion through phenoxido-O. Participation of furan
oxygen in coordination is little known and an interesting
observation.14 Normal tridentate coordination of HL to ZnII
provides 1 as mononuclear species. Same reaction in presence
of thiocyanato anion yields double-phenoxido-bridged dinu-
clear complex 2 with pentacoordinated zinc centers. On the
other hand, presence of azido anion leads to double-phenoxido
and azido bridged complex 3 as a polymeric 1D chain bearing
hexacoordinated Zn2 units anked by tetrahedral zinc(II) ions in
a zigzag arrangement. All three complexes have been synthe-
sized in good yield, and characterized by elemental analysis,
FTIR, UV-Vis spectroscopy, and single crystal X-ray diffraction
respectively. Moreover their anion dependent aggregation,
solvent-free mechanochemical solid state core conversion and
photophysical properties were investigated for a systematic
study of Zn(II) coordination chemistry involving THF-arm
bearing Schiff base ligand.
2.3. Complexes
[H3O][ZnL2]ClO4 (1). To the methanolic solution (10 mL)
of in situ generated ligand HL (0.2 g,
1 mmol) solid
Zn(ClO4)2$6H2O (0.19 g 0.5 mmol) was added while stirring at
room temperature over a period of ca. 15 min. The color of the
solution turns yellow upon addition of triethylamine (0.14 mL, 1
mmol) and stirring was continued for further 2 h. Aer slow
evaporation of solvent in air during 5 days, yellow rod like
crystals suitable for X-ray diffraction were separated at the
bottom of the reaction vessel. One of X-ray diffraction quality
crystals was selected and mounted for structure determination.
Yield: 0.35 g (59%). Anal. calc. for C24H31ClN2O9Zn (592.35 g
molꢀ1): C, 48.66; H, 5.27; N, 4.73. Found: C, 48.83; H, 5.48; N,
4.55. Selected FT-IR bands: (KBr, cmꢀ1; s ¼ strong, vs ¼ very
strong, m ¼ medium, br ¼ broad) 3598(br), 1638(vs), 1466(m),
1288(m), 1086(s, br), 758(s), 624(s). Molar conductance, LM:
(MeOH solution) 105 S m2 molꢀ1. UV-Vis spectra [lmax, nm (3, L
molꢀ1 cmꢀ1)]: (MeOH solution) 361 (505), 268 (8270), 236
(17 121).
[Zn2(m-L)2(NCS)2] (2). Method A via direct route. To the
methanolic solution (10 mL) of Zn(ClO4)2$6H2O (0.37 g, 1
mmol) was added ligand HL dropwise (0.2 g, 1 mmol) with
magnetic stirring. The resulting solution gradually turned to
straw yellow upon successive addition of triethylamine (0.14
mL, 1 mmol) and a methanolic solution (5 mL) of ammonium
2. Experimental sections
2.1. Materials
The chemicals used were obtained from the following sources: thiocyanate (0.08 g, 1 mmol). Aer stirring for about 1 h, the
tetrahydrofurfurylamine from Alfa Aesar, salicylaldehyde from reaction mixture was ltered and kept for crystallization. X-ray
Spectrochem Pvt Ltd, triethylamine and ammonium diffraction quality parallelepiped shaped yellow crystals were
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obtained during 5 days. One of these single crystals suitable for
Caution! Although no incidents were recorded in this study,
X-ray diffraction was mounted for crystal structure determina- azido complexes of metal ions involving organic ligands are
tion. Yield: 0.48 g (82%). Anal. Calc. for C26H28N4O4S2Zn2 potentially explosive. Only a small amount of material should
(655.44 g molꢀ1): C, 47.64; H, 4.31; N, 8.55. Found: C, 47.61; H, be prepared, and it should be handled with proper care.
4.30; N, 8.60. Selected FT-IR bands: (KBr, cmꢀ1; s ¼ strong, vs ¼
very strong, m ¼ medium) 2954(m), 2089(vs), 1633(vs), 1472(m),
2.4. Physical measurements
1281(s), 1199(m), 905(m), 759(s), 593(m). Molar conductance,
LM: (MeOH solution) 30 S m2 molꢀ1. UV-Vis spectra [lmax, nm
The elemental analysis (C, H, N) were performed with a Perkin-
Elmer model 240C elemental analyzer. Fourier transform
infrared (FT-IR) spectra were recorded on a Perkin-Elmer RX1
spectrometer. Solution electrical conductivity measurements
(3, L molꢀ1 cmꢀ1)]: (MeOH solution) 363 (695), 266 (1185), 221
(3119).
Method B via mechanochemical conversion of 1. Quantitative
conversion of 1 to 2 was achieved under solvent free condition
were carried out using a Unitech type U131C digital conductivity
meter with a solute concentration of about 10ꢀ3 M. The powder
by thorough mixing and grinding of 1 (0.06 g, 0.1 mmol) with
X-ray diffraction (XRD) measurements were carried out in a
Zn(ClO4)2$6H2O (0.037 g, 0.1 mmol) and ammonium thiocya-
PW1710 diffractometer, a Philips, Holland, instrument. The
absorption and uorescence spectra were collected using a
nate (0.015 g, 0.2 mmol) in an agate mortar for about 45 min.
Addition of water to the grinded paste results a suspension
Shimadzu (model no. UV-2450) spectrophotometer and a Hita-
following dissolution of water soluble ammonium thiocyanate.
chi (model no. F-7000) spectrouorimeter respectively. For
It was ltered followed by careful washing with water–methanol
steady-state measurements, all the samples were excited at 380
nm to collect the emission spectra. For time-resolved
mixture (5 : 1 v/v) to remove un-reacted complex 1 and nally
dried under vacuum over P4O10. Yield: 0.05 g (91%). The yellow
measurements, we used a time correlated single-photon-
powdered product was used for elemental analysis, FT-IR and
counting (TCSPC) instrument from IBH (U.K.). The samples
were excited at 450 nm using a picosecond laser diode (IBH,
powder X-ray diffraction analysis (Fig. S2, ESI†). Anal. calc. for
C
26H28N4O4S2Zn2 (655.44 g molꢀ1): C, 47.64; H, 4.31; N, 8.55.
Nanoled), and the signals were collected at the magic angle
(54.7ꢁ) using a Hamamatsu microchannel plate photomultiplier
tube (3809U). The same setup was used for anisotropy
measurements. The instrument response function of our setup
is ꢂ90 ps. The excellence of the t was judged by the c2 crite-
rion. In multi-exponential decay at least two distinct lifetimes
are present. Average uorescence lifetime (sav) for bi-
exponential decay curve was obtained from the decay time
constants (s) and pre-exponential factors (a) using eqn (1) where
s1 and s2 are the two relaxation time constants with normalized
pre-exponential factors a1 and a2 respectively
Found: C, 47.60; H, 4.33; N, 8.51. Selected FT-IR bands: (KBr,
cmꢀ1; s ¼ strong, vs ¼ very strong, m ¼ medium) 2957(m),
2087(vs), 1634(vs), 1474(m), 1280(s), 1195(m), 909(m), 761(s),
592(m).
[Zn3(m-L)2(m-N3)4]n (3). Method A via direct route. The light
yellow compound 3 was obtained following method A of
complex 2 using NaN3 (0.13 g, 2.0 mmol) instead of NH4SCN.
Flake type of single crystals suitable for X-ray analysis was
obtained from a saturated solution of CH2Cl2–MeOH (1 : 10 v/v)
aer 7 days. One diffraction quality crystal was selected, cleaned
with paraffin oil and used for X-ray structure determination.
Yield: 0.65 g, (84%). Anal. Calc. for C24H28N14O4Zn3 (772.77 g
molꢀ1): C, 37.30; H, 3.65; N, 25.38. Found: C, 37.32; H, 3.63; N,
25.35. Selected FT-IR bands: (KBr, cmꢀ1; s ¼ strong, vs ¼ very
strong, m ¼ medium, br ¼ broad) 2957(br), 2107(s), 2085(vs),
1645(s), 1550(m), 1294(s), 1198(m), 906(m), 756(m). Molar
conductance, LM: (MeOH solution) 18 S m2 molꢀ1. UV-Vis
spectra [lmax, nm (3, L molꢀ1 cmꢀ1)]: (MeOH solution) 360
(695), 266 (1185), 221 (3119).
sav ¼ s1a1 + s2a2
(1)
Fluorescence quantum yields (F) of all the compounds were
determined at room temperature from methanol solution by
comparing the corrected spectrum with that of anthracene (F ¼
0.27) in ethanol16 as the secondary standard using eqn (2).
2
FS
AS ðAbsÞ
nS
R
¼
ꢃ
ꢃ
(2)
2
FR AR ðAbsÞ
nR
Method B via mechanochemical conversion of 1. The complex 3
was also obtained by mechanochemical grinding of 1 (0.06 g,
S
where A is the integrated area under the emission spectral
curve, Abs denotes absorbance at the wavelength of exciting
light, h is the refractive index of the medium, F is the uores-
cence quantum yield, and the subscripts S and R stand in the
recognition of the respective parameters of studied sample and
reference, respectively.
0.1 mmol) following method
B of complex 2. Here
Zn(ClO4)2$6H2O (0.075 g, 0.2 mmol) and NaN3 (0.025 g,
0.4 mmol) were mixed and grinded with 1 (0.06 g, 0.1 mmol). A
solid mass formed during 30 min of grinding. It was suspended
with water then ltered and nally kept under vacuum over
P4O10. The off white ne powder was used for elemental anal-
ysis, FT-IR and powder X-ray diffraction analysis (Fig. S2, ESI†).
Yield: 0.07 g (89%). Anal. calc. for C24H28N14O4Zn3 (772.77 g
2.5. Crystal data collection and renement
molꢀ1): C, 37.30; H, 3.65; N, 25.38. Found: C, 37.33; H, 3.61; N, The X-ray diffraction data of the complex 1, 2 and 3 were
25.35. Selected FT-IR bands: (KBr, cmꢀ1; s ¼ strong, vs ¼ very collected on a Bruker APEX-II CCD X-ray diffractometer using
strong, m ¼ medium, br ¼ broad) 2951(br), 2109(s), 2083(vs), single crystals that uses graphite-monochromated Mo-Ka radi-
˚
1646(s), 1553(m), 1292(s), 1195(m), 908(m), 759(m).
ation (l ¼ 0.71073 A) by u-scan method at 298 K. Information
concerning X-ray data collection and structure renement of the
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Table 1 Crystallographic data and refinement parameters of complexes 1, 2, and 3
Complex
1
2
3
Formula
C
24H28ClN2O9Zn
C26H28N4O4S2Zn2
655.44
P21/c
Monoclinic
6.8404(8)
21.759(2)
9.8058(11)
106.686(3)
1398.9(3)
293
(C24H28N14O4Zn3)n
772.77
P2/c
Monoclinic
11.6671(8)
10.8103(7)
12.2636(9)
103.876(2)
1501.61(18)
293
M/g molꢀ1
Space group
Cryst syst
589.32
C2/c
Monoclinic
23.365(3)
12.7460(18)
17.904(2)
101.513(4)
5224.7(12)
293
˚
a/A
˚
b/A
˚
c/A
b/deg
3
˚
V/A
T/K
Z
8
2
2
Dc/g cmꢀ3
1.498
1.556
1.709
F(000)
2440
672
784
m(Mo-Ka)/cmꢀ1
Cryst dimens (mm3)
No. of rens collected
No. of unique rens
No. of params
R1; wR2 (I > 2s(I))
R(int)
10.97
19.03
24.34
0.31 ꢃ 0.22 ꢃ 0.13
0.29 ꢃ 0.21 ꢃ 0.13
0.33 ꢃ 0.22 ꢃ 0.15
32 409
15 239
14 465
5476
334
0.0584; 0.1555
0.0564
1.037
2628
172
0.0410; 0.0954
0.0184
1.138
2614
204
0.0579; 0.1278
0.0372
1.020
GOF (F2)
CCDC nO.
1006844
1006845
1006846
compound is summarized in Table 1. The SMART soware was
used for data acquisition. Data integration and reduction were
performed with SAINT and XPREP soware.17 Structures were
solved by direct methods using SHELXS-97 18 and rened with
full-matrix least squares on F2 using the SHELXL-97 19 program
package. All non-hydrogen atoms were rened anisotropically.
Multiscan absorption corrections were applied to the data using
the program SADABS.20 The locations of the heaviest atom (Zn)
and the other atoms O, N, S, Cl, and C were subsequently
determined from the difference Fourier maps. The H atoms
were introduced in calculated positions and rened with xed
geometry with respect to their carrier atoms. A summary of the
crystal data and relevant renement parameters are given in
Table 1.
3. Results and discussion
3.1 Synthetic considerations
The ligand HL was prepared by a single step 1 : 1 condensation
of tetrahydrofurfurylamine and salicylaldehyde in methanol
(Scheme S1, ESI†). It was characterized by FT-IR and NMR
spectral measurements. The characteristic vibration of imine
bond appeared at 1631 cmꢀ1. In 1H NMR spectrum of the ligand
in CDCl3, the characteristic imine proton appears at 8.32 ppm,
the broad phenolic OH peak at 13.42 ppm whereas the hydrogen
of the aromatic phenol and the tetrahydrofurfuryl rings are at
6.81–7.28 and 1.66–4.16 ppm, respectively. 13C NMR spectrum
records peak at 166.3 ppm for imine carbon. The potential
metal binding abilities of HL (Chart 1) was next investigated
using hydrated zinc(II) perchlorate salt in presence of triethyl-
amine and exogenous bridging units like azido and thiocyanato
ions as summarized in Scheme 1.
Scheme
complexes 1–3.
1 Summary of the possible synthetic routes for the
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When Zn(ClO4)2$6H2O was treated with HL in 1 : 2 metal–
Compound 3 on the other hand, is formed via azido coor-
ligand ratio in MeOH and NEt3 base, 1 results as yellow crystal dination induced trapping of ZnII ions by ligand bound [Zn2L2]
directly from the reaction mixture in moderate yield. The fragments. A rare 1D chain is obtained both by solvent based
preparation of 1 is summarized in eqn (3).
multicomponent approach and mechanochemical route con-
taining ZnII : L in 3 : 2 ratio in the basic unit. The reaction
involved during mechanochemical transformation is summa-
rized in eqn (7).
2HL + Zn(ClO4)2$6H2O + NEt3 /
[H3O][ZnL2]ClO4 + NHEt3ClO4 + 5H2O (3)
Using a similar approach, two other reactions have been
carried out in the presence of thiocyanato and azido anions. In
both the cases, bis-chelation by two tridenate ligands is pre-
vented due to the simultaneous coordination of these externally
[H3O][ZnL2]ClO4 + 2nZn(ClO4)2$6H2O + 4nNaN3 /
[Zn3(m-L)2(m-N3)4]n + 4nNaClO4 + HClO4 + (12n + 1)H2O (7)
Thus the use of exogenous units not only inhibits the bis-
added anions. Yellow complexes 2 and 3 (Scheme 1) are directly chelation around ZnII but also promotes the ligand to act as
synthesized in ꢂ80% yield from MeOH solution under aerobic bridging unit. Furthermore solvent free mechanochemical
conditions at room temperature by stirring the reaction mixture grinding method not only develops new topologies and metal–
of HL, Zn(ClO4)2$6H2O, NEt3 and NH4SCN or NaN3 in ligand aggregation but also shows the solid state core conver-
1 : 1 : 1 : 1 or 1 : 1.5 : 1 : 2 molar ratio for 1 h respectively. The sion and ligand exchange from octahedral to square pyramidal
synthesis of 2 and 3 are summarized in eqn (4) and (5) in 1 to 2 and octahedral to 1D coordination network containing
respectively
two octahedral units followed by one tetrahedral in 1 to 3. These
transformations were conrmed by comparing the solubility
differences, FTIR spectra (Fig. S1, ESI†) and unit cell parameters
of single crystals grown from powder samples. The powder X-ray
diffraction (PXRD) patterns (Fig. S2, ESI†) of complexes 1–3 are
consistent with those of simulated ones (Fig. S3, ESI†) obtained
based on the structures derived from the single crystal X-ray
measurements, suggesting the phase change during the core
conversions within the complexes. Furthermore it concludes
the bulk materials are consistent with same chemical constit-
uents that resembles with the crystal structures.
2HL + 2Zn(ClO4)2$6H2O + 2NEt3 + 2NH4SCN /
[Zn2(m-L)2(NCS)2] + 2NHEt3ClO4 + 2NH4ClO4 + 12H2O (4)
2nHL + 3nZn(ClO4)2$6H2O + 2nNEt3 + 4nNaN3 /
[Zn3(m-L)2(m-N3)4]n + 2nNHEt3ClO4 + 4nNaClO4 + 18nH2O (5)
The elemental analysis and molar conductivity data conrm
the respective formula for these compounds. Reactions with an
excess of thiocyanato or azido anions did not lead to the
substitution of the terminal tetrahydrofuran oxygen coordina-
tion to the zinc ions in 2 and 3, indicating a reasonably strong
affinity and stability of the tetrahydrofuran binding in these
systems. During these synthesis we have not encountered with
any kind of SCNꢀ or N3ꢀ coordination induced partial extrusion
3.2. IR spectroscopy
The characteristic band of ~nC]N stretching vibration of ligand
HL appears at 1631 cmꢀ1 and broad band around 3435 cmꢀ1 is
due to –OH stretching. The sharp peaks in the spectra of 1–3, at
1633–1645 cmꢀ1 are due to ZnII-bound –C]N stretching
vibrations whereas peaks at 1281–1294 cmꢀ1 are due to ~nC–O
vibrations of the coordinated Lꢀ ligands.22 In complex 1, a sharp
band at 3597 cmꢀ1 is observed due to hydrogen bonded –OH
vibrations from water of crystallization, whereas strong and
sharp peaks at 1086 cmꢀ1 and 624 cmꢀ1 are for IR active
asymmetric stretching (n3) and asymmetric Cl–O bending (n4)
vibrations of intimately hydrogen bonded (O/O distance
2ꢀ
2ꢀ
21
of anionic zinc based species like Cu2(NCS)6 or Cu(N3)4
.
A fundamentally different route of initiating and acceler-
ating these reactions have been explored through use of force in
the form of mechanical grinding in solid state for the trans-
formation of 1 to 2 and 1 to 3. Solid state transformation from
one species to another is a topic of contemporary relevance
especially in solvent free solid-state synthesis. Thorough mixing
and grinding of basic complexing unit 1, Zn(ClO4)2$6H2O and
exogenous bridging units NaN3 and NH4SCN in 1 : 1 : 2 and
1 : 2 : 4 molar ratio proceed through the formation of paste
followed by solidication. Initially it is expected that 1 : 1
reaction between 1 and Zn(ClO4)2$6H2O produces an interme-
diate species, [Zn(L)(ClO4)] where mononegative tridentate
ligand and perchlorido ion simultaneously coordinated to the
ZnII center leaving behind vacant sites around the ZnII ion.
Addition of thiocyanato anion replaces perchlorido anion from
the coordination environment because of its stronger coordi-
nating ability. The reaction involved for the synthesis of 2 via
mechanochemical grinding is summarized in eqn (6).
ꢀ
˚
2.622–2.739 A, vide supra) ClO4 unit present in the crystal
lattice. The characteristic band at 2089 cmꢀ1 of ~nC–N stretching
mode in 2 is due to terminally ZnII-bound SCNꢀ group. The
~nas(N–N–N) asymmetric stretching vibrations of metal bound m1,1
-
N3ꢀ groups at 2085 cmꢀ1 along with a kink around at 2107 cmꢀ1
appear for 3.
3.3. X-ray crystal structural descriptions
Yellow crystals of 1 obtained from a saturated solution of
MeOH, crystallizes in monoclinic C2/c space group. A perspec-
tive view of 1 is presented in Fig. 1 and selected bond distances
[H3O][ZnL2]ClO4 + Zn(ClO4)2$6H2O + 2NH4SCN /
[Zn2(m-L)2(NCS)2] + 2NH4ClO4 + HClO4 + 7H2O (6) and angles around the metal center are listed in Table 2. The
structural analysis reveals that 1 has a bis-chelated mono-
nuclear structure containing a hexacoordinated ZnII ion having
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Table 2 Selected bond distances (A˚) and bond angles (ꢁ) in 1, 2 and 3
˚
Bond distances (A)
Complex 1
Zn1–N2
Zn1–N1
Zn1–O3
2.026(4)
2.035(4)
2.052(3)
Zn1–O1
Zn1–O2
Zn1–O4
2.071(4)
2.239(4)
2.305(4)
Complex 2
Zn1–N2
Zn1–O1
Zn1–N1
1.958(3)
1.989(2)
2.025(3)
Zn1–O2
Zn1/Zn1
Zn1–O1
2.261(3)
3.1011(8)
2.071(2)
Complex 3
Zn1–O1
Zn1–N1
Zn1–N2
Zn1–O1
Zn1–N5
2.049(4)
2.120(6)
2.131(5)
2.098(4)
2.120(5)
Zn1–Zn1*
Zn2–N5
Zn1–O2
Zn2–N2
3.1855(14)
2.037(5)
2.345(5)
1.989(5)
Fig. 1 Pov-Ray view of 1 with partial atom numbering scheme. Color
code. C, teal; H, white, N, blue; O, red; Zn, pink. ClO4ꢀ$and H3O+ ions
are omitted for clarity.
Bond angles (ꢁ)
Complex 1
N2–Zn1–N1
N2–Zn1–O3
N1–Zn1–O3
N2–Zn1–O1
O3–Zn1–O2
O1–Zn1–O2
N2–Zn1–O4
N1–Zn1–O4
160.62(17)
89.96(15)
106.57(16)
101.89(16)
94.40(16)
165.76(14)
77.19(17)
86.68(17)
N1–Zn1–O1
O3–Zn1–O1
N2–Zn1–O2
N1–Zn1–O2
O3–Zn1–O4
O1–Zn1–O4
O2–Zn1–O4
88.55(16)
89.66(13)
91.78(17)
77.21(17)
166.72(14)
89.77(16)
89.33(18)
two tridentate Lꢀ ligands in meridional binding mode. A dis-
torted N2O4 octahedral environment around the ZnII center is
found from almost planar ONOꢀ coordination of two Lꢀ units.
The individual Lꢀ units remain perpendicular (89.2ꢁ) to each
other. The neutral tetrahydrofuran oxygen coordination
˚
provides longer Zn–O bonds in 2.235–2.306 A compared to
Complex 2
N2–Zn1–O1
N2–Zn1–N1
O1–Zn1–N1
N2–Zn1–O1
O1–Zn1–O1
˚
mono-metal centric phenoxido oxygen donors (2.052–2.071 A).
111.67(13)
120.99(14)
127.17(12)
106.51(13)
80.40(10)
N1–Zn1–O1
N2–Zn1–O2
O1–Zn1–O2
N1–Zn1–O2
O1–Zn1–O2
88.53(11)
90.65(12)
98.92(10)
77.41(10)
161.89(10)
The shortest Zn–N bond distances are from imine nitrogen
coordination of 2.026–2.035 A. The six-membered bite (88.5–
˚
90ꢁ) containing the phenoxido donor is greater than the tetra-
hydrofuran bearing ve-membered chelate ring (77.3–77.4ꢁ).
The ve- and six-members chelate rings form inter-planar
angles of 4.9–15.4ꢁ range.
Complex 3
O1–Zn1–O1
O1–Zn1–N5
O1–Zn1–N5
O1–Zn1–N1
O1–Zn1–N1
N5–Zn1–N1
O1–Zn1–N2
O1–Zn1–N2
N5–Zn1–N2
79.65(17)
98.01(18)
85.22(18)
87.5(2)
166.8(2)
93.7(2)
89.81(17)
90.64(18)
170.3(2)
O1–Zn1–O2
N5–Zn1–O2
N1–Zn1–O2
N2–Zn1–O2
N2–Zn2–N2
N2–Zn2–N5
N2–Zn2–N5
N1–Zn1–N2
O1–Zn1–O2
118.56(17)
88.91(18)
74.5(2)
85.40(18)
125.4(3)
103.2(2)
109.5(2)
92.3(2)
In 1 the counter ions ClO4 and H3O+ are stabilized by
ꢀ
intricate hydrogen bonding interactions. The H-atoms could
not be located from the difference Fourier map and Q peaks
aer renement steps. The hydrogen bonding parameters are
listed in Table 3. It is clear from the Fig. 2 that two H3O+ and two
ClO4 ions associate six monomeric ZnL2 units through three
types of hydrogen bonding interactions; C–H/O, Cl–O/H and
O–H/O respectively. There are six C–H/O interactions of
ꢀ
161.16(17)
˚
2.551–2.717 A range with C–H fragments from ligands back-
bone (H5, H7, H10A, H11A, H12A and H19), two with each water
oxygen (O9W) and four with two oxygen atoms (O6 and O8) of
each perchlorido anion. Through two Cl–O/HW and two
O–HW/O interactions both perchlorido oxygen (O7) and H3O+
ion form an eight membered cyclic ring (Fig. S4, ESI†). The
Table 3 Hydrogen bonding parameters for the crystal structure of
complex 1
˚
˚
Interactions
D–H (A)
D/A (A)
H/A (A)
D–H/A (ꢁ)
O/O and C/O distances are 2.622–2.739 A and 3.489–3.606 A
and the C–H/O angles are 132.52–165.43ꢁ respectively.
˚
˚
˚
C–H10A/O9W
C–H10A/O9W
C–H11A/O8
C–H7/O8
C–H5/O6
C–H19/O6
0.970
0.970
0.971
0.929
0.930
0.929
3.489
3.606
3.607
3.330
3.357
3.622
2.557
2.717
2.677
2.631
2.551
2.716
161.39
152.51
160.47
132.52
145.28
165.43
Complex 2 as shown in Fig. 3 crystallizes in monoclinic P21/c
space group directly from the methanol reaction mixture. The
selected bond distances and angles are given in Table 2. It is a
centrosymmetric double phenoxido bridging dinuclear zinc(II),
with the inversion center located at the midpoint of Zn2O2
diamond core. Here two Lꢀ ligands through edge on bridging
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Fig. 2 Hydrogen bonding interactions of lattice ClO4ꢀ and H3O+ ions and C–H/O interactions in complex 1. Color code: C, grey; H, aqua; N,
blue; O, red; Cl, green; Zn, pink. Only participated hydrogen atoms are shown.
planar angle between the ve- and six-members chelate rings is
12.4ꢁ. The degree of distortion around pentacoordinate ZnII
environments (Zn1 and Zn1* respectively) is calculated with
Addison parameter, s5 ¼ 0.59 (s ¼ rb ꢀ ar/60ꢁ where b and a are
the two largest angles around the central atom. s5 ¼ 0 for a
perfect square pyramidal and 1 for a perfect trigonal bipyra-
midal geometry).24 In the present case the distortion is 59% and
hence the geometry of ZnII ion in 2 can be best described as an
intermediate between distorted square pyramid and trigonal-
bipyramid. Both the ZnII centers are elevated from the mean
˚
square plane in opposite direction by 0.601 A. The core struc-
ture consists of two distorted square pyramidal ZnN2O3 frag-
ments connecting through a common O,O-edge from bridging
phenoxido oxygen atoms (Fig. S5, ESI†).
A perspective view of the rare and novel 1D coordination
chain of 3 which crystallizes in monoclinic P2/c space group is
shown in Fig. 4 and selected bond distances and angles are
given in Table 2. The repeating Zn3 asymmetric unit contains
Fig. 3 Pov-Ray view of 2 with partial atom numbering scheme. Color
two partially developed double phenoxido bridged dinuclear
code: C, teal; H, white, N, blue; O, red; S, yellow; Zn, pink. Symmetry
ligand bound Zn2L2 units and azido bound Zn(N3)42ꢀ unit with
code: ꢀx, ꢀy, ꢀz.
chemical formula [Zn3(m-L)2(m-N3)4] (Fig. S6, ESI†). Both these
two units are linterlinked through m1,1-bridging azido N atoms
(N2 and N5*). Among three Zn2+ ions in basic Zn3 motif, two
fashion provide distorted square planar coordination around
ZnII centers (Zn1 and Zn1*) are in octahedral N3O3 coordination
environments whereas the third ZnII ion (Zn2) resides in dis-
torted tetrahedral N4 site. In ZnN4 environment the tetra-
coordinate index s4 ¼ 0.89 can be best described as tetrahedral
geometry [s4 ¼ {360 ꢀ (a + b)}/141ꢁ, where a and b are two
largest angles in the four coordinate geometry].25
each ZnII ion with imine-N, tetrahydrofurfuryl-O and m-phe-
noxido oxygen. To this coordinatively unsaturated system,
through terminal coordination, isothiocyanato anions occupy
apical position to satisfy distorted square pyramidal geometry.
˚
The Zn–O distance (2.261 A) for tetrahydrofurfuryl-O coordina-
tion is signicantly longer as compared to the Zn–O distances
˚
The distorted octahedral environments around Zn1 and
Zn1* can be demonstrated from the deviation of cis angles
(74.5–98ꢁ) and trans angles (161.2–166.8ꢁ) from the ideal values.
Ligand bound Zn2L2 unit with an inversion center forms the
from phenoxido bridges (1.989–2.071 A). The Zn–N distance is
˚
longer for imine coordination (2.025 A) than for isothiocyanato
*
˚
˚
binding (1.958 A). The Zn1/Zn1 separation (3.101 A) is shorter
˚
by 0.1 A compared to the range of previously reported doubly
bridged dizinc complexes.23 The six-membered bite angle
˚
Zn2O2 diamond core with Zn/Zn separation of 3.186 A whereas
(88.5ꢁ) is greater than the ve-membered one (77.4ꢁ). The inter-
˚
all azido bound Zn2 show Zn/Zn separations of 3.484–3.556 A
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Fig. 4 Pov-Ray view of the 1D zigzag chain (3) with partial atom numbering scheme. Color code: C, teal; H, white, N, blue; O, red; Zn, pink.
Symmetry code: ꢀx, ꢀy, ꢀz.
from Lꢀ bound ZnII centers (Zn1 and Zn1*). Two apical N atoms
ꢀ
coordination from m1,1-N3 binding complete the octahedral
arrangement. The third ZnII ion is tetra-coordinated by four
bridging azido groups in a slightly distorted tetrahedral envi-
ronment. The tetrahedral angles are in 103.2–109.5ꢁ around
Zn2. In this trinuclear unit the angular Zn1–Zn2–Zn1* dispo-
sition makes angles of 64.0ꢁ whereas in zigzag arrangement of
repeating Zn3 units make angles of 138.4–161.5ꢁ. The unsym-
˚
metrical azido bridge Zn–N distances are 1.99 and 2.04 A
respectively whereas Zn–N distance from coordinated imine N
˚
falls in between at 2.12 A. The core structure of 3 consists of four
vertex shared octahedral ligand bound ZnN3O3 units around the
central tetrahedral ligand free ZnN4 unit and the assembly is
repeated as 1D chain (Fig. S7, ESI†).
Fig. 5 Experimental UV-Vis absorption spectra of HL and complexes
1–3 (10 mM neat MeOH solution).
3.4. Steady-state absorption and emission spectra
Table 4 Absorption and emission parameters,a and fluorescence
The coordination driven change in ground state electronic
absorption behavior in MeOH of complexes 1–3 are shown in
quantum yieldsb of HL and 1–3
Fig. 5. The zinc complexes are known to record only the charge
lem
transfer transitions as no d–d transitions are expected for d10
Zn2+ ion.26 The ligand HL exhibits four primary absorption
bands in the UV-visible region at 215, 254, 316 and 398 nm
respectively (Table 4). These bands are assigned to the n / p*
and p / p* transitions from the Schiff base ligand (i.e.,
phenolate and the ligand frame-work). The maximum absorp-
tion intensity centered at 398 nm due to intraligand n / p*
trantion is blue shied to about 360 nm in complexes 1–3. The
bands at 220, 255 and 320 nm for 1 are possibly due to the p /
p* transitions centered mainly on the azomethine chromo-
phore (imine p / p*). The 320 nm band is shied to the lower
wavelength region at 270 nm during zinc(II) coordination in 1.
For 2 and 3 these bands appear very close to those obtained
for 1.
System lmax (nm) [3 (Mꢀ1 cmꢀ1)]
(nm) Ff
HL
1
398 (733), 316 (1529), 254 (4336), 215 (8817)
361 (2778), 268 (4541), 236 (9755), 223 (9327)
446
452
0.032
0.078
0.32
0.13
2
360 (2547), 266 (4639), 237 (9460), 221 (11235) 448
3
360 (2652), 268 (4500), 238 (9371), 221 (3119)
447
a
b
Excitation wavelength is 380 nm. The uorescence quantum
efficiency was determined by using anthracene as reference (FR
¼
0.27).16
Emission properties of HL and three metal complexes were
examined in MeOH solution at room temperature (Fig. 6).
The observed emission behavior is solely due to the intraligand
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various amounts of Zn(ClO4)2$6H2O (0–10 mM). The system
shows denite coordination induced enhancement of emission
behavior. The emission is maintained at 452 nm upto addition
of 1 equiv. of Zn2+ leading to the formation of hitherto unknown
complex [Zn(L)(ClO4)] in solution which is different from that of
solid state compound 1. The emission band of the free ligand is
almost at the same wavelength as observed during metal–ligand
titration, indicating emission takes place from the same excited
state. In the absence of metal ions the uorescence of the ligand
is possibly quenched by the occurrence of a photoinduced
electron transfer (PET) process due to the presence of lone pair
of electrons of the donor atoms in the ligand.29 Such a PET
process is prevented by the complexation of HL with metal ions,
thus the uorescence intensity may be greatly enhanced by the
coordination of ZnII. The chelation of the ligand to ZnII
increases the rigidity of the ligand and thus reduces the loss of
energy by thermal vibrational decay.30
Anion dependent uorescence responses were investigated
using 1 as uorescent probe. Spectrouometric titration with
addition of various amount of NH4SCN (Fig. 8, le) and NaN3
(Fig. 8, right) in two separate experiments shows coordination
induced quenching of uorescence intensities up to the addi-
tion of 1 equiv. of exogenous anions. The 1 : 1 composition of 1
and exogenous anions fꢀorms an hitherto unknown complex
[Zn(L)(X)] (where X ¼ N3 and SCNꢀ) which could not be iso-
lated. On excitation at the maximum absorption of 1 (lmax of
360 nm) the emission maximum centered at 450 nm is repro-
duced which is exactly the same as that obtained during metal–
ligand titration. From emission spectra, it is clear that the
quenching of emission intensities as compared to 1 is more
prominent for 2 than that of 3. This observation clearly points to
the fact that uorescent emissions are very much sensitive to
anion replacement and can be diagnosed for the thiocyanato
and azido anions following quenching of emission intensities.
Fig. 6 Fluorescence emission spectra of ligand (10 mM) and metal
complexes (10 mM) in neat MeOH solutions.
p / p* uorescence since ZnII ion with stable d10 congura-
tion is spectroscopically and magnetically inert.27 The HL
molecules form a monomeric ground state with a low quantum
yield (Table 4). The enhancement of uorescence intensities
and hence quantum yields in complexes 1–3 is mainly due to
different metal–ligand aggregation, in absence and presence of
anions, and the nature of the emitting species. The emission
intensities of HL shows specic ZnII-binding induced emission
characteristics. The emission band of HL is very close to the
metal complexes 1–3.1 For 1 the uorescence quantum yield (Ff)
is increased about 2.5 fold in comparison to free HL whereas the
same for 2 and 3 are 4 and 10 fold respectively. This coordina-
tion driven enhancement of uorescence intensity is explain-
able in terms of attainment of more rigidity upon
complexation.28
The emission properties of HL in presence of free Zn2+ ion
were examined in MeOH solution at room temperature (Fig. 7)
to identify the change in uorescence behavior of the ligand
during complexation and aggregation. The emission spectrum
of 10 mM HL records emission maximum at 446 nm when the
solution is excited at 380 nm. The emission intensities of the
tridentate zinc chelator HL was measured in the presence of
3.5. Time-resolved emission studies
Time-resolved uorescence (TRF) spectroscopy is known to be a
valuable tool for imaging cellular components.31 Measurements
of TRF decay proles of HL ligand and its ZnII complexes can
differentiate between different populations of uorescence
emitting uorophores with varying lifetimes, whereas steady-
state uorescence measurements identify unresolved contri-
butions of individual components to the overall uorescence.32
Thus time resolved uorescence measurement is most suitable
to extract the lifetimes of ligand and the complexes in the
excited state. In most cases the uorescence from these were
overlapping but using TRF measurements distinct uorescence
lifetime can be extracted. Lifetime of an excited ligand or
complex molecule is the time taken for an ensemble of such
molecules to decay to 1/e (36.8%) of their initial excited state
population. The uorescence lifetime (sav) thus refers to the
average time a molecule stays in its excited state before emitting
a photon. Time-resolved uorescence emissions of 1–3 along
with the free ligand HL were recorded in MeOH solvent at an
excitation wavelength of 375 nm and spectra were monitored at
the emission wavelength of 450 nm. A comparative decay prole
Fig. 7 Emission spectra of HL (10 mM) in presence of 0, 1, 2, 3, 4, 5, 6, 7,
8, 10 mM of free Zn2+ ions in neat MeOH at room temperature (exci-
tation 380 nm, emission 449 nm).
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Fig. 8 Emission spectra of 1 (10 mM) in presence of SCNꢀ (0, 1, 2, 3, 4, 5, 6, 7, 8, and 10 mM) (left) and N3ꢀ (0, 1, 2, 3, 4, 5, 7, and 10 mM) (right) ions in
neat MeOH at room temperature (excitation 380 nm, emission 449 nm).
with s's of 2.30 and 2.47 ns (38.8% each) and 6.00 and 6.27 ns
(61.2% each) respectively, whereas in complex 3 the sav is very
close to the value for HL at 2.97 ns but the s's are signicantly
different at 2.46 ns (80.2%) and 5.01 ns (19.8%). These values
clearly indicate the different excited state lifetime for all these
species. These results can be compared in terms of the envi-
ronment of the tryptophan residue in the proteins.33 The life-
time of tryptophan residues in proteins range from ꢂ0.1 ns to
ꢂ8 ns. For a bi-exponential t the lifetime distributions of the
two conformations spread in adjacent lifetime intervals. The
longer lived conformation extends in the 6 ns to 9 ns range and
shorter lived conformation extends in the 2 ns to 5 ns range.34
Fig. 9 The comparative decay profile for HL and complexes 1–3.
4. Concluding remarks
is depicted in Fig. 9 and the corresponding lifetime values (s's)
and amplitudes (a's) were obtained using eqn (1) (in Physical We have investigated the coordination and photophysical
Measurements). The uorescence lifetime is not affected by the response of tetrahydrofuran bearing Schiff base HL with ZnII in
concentration of the uorophores, static quenching effects, or absence and presence of thiocyanato and azido ions. Mecha-
the brightness of the light source. In contrast, dynamic nochemical solid state core conversion through ligand
exchange is applied for the synthesis of 2 and 3 using 1 as basic
complexing agent. Here mononuclear octahedral core in 1 is
quenching, resonance energy transfer, and temperature have a
strong impact on the uorescence decay. Thus, the uorescence
lifetime is a preferred parameter in uorescence sensing and converted into edge sharing square pyramidal units in 2 and
imaging. The biexponential tting of the decay processes vertex sharing two octahedral fragments followed by one tetra-
provide the parameters which are given in Table 5.
hedral core in repeated fashion in 3. In solvent based synthesis
The average lifetime (sav) of HL is measured at 2.50 ns with a single Lꢀ bound zinc(II) plays the crucial role as basic template
two relaxation time constants (s's) of 1.78 ns (55%) and 3.37 ns unit for the subsequent formation of three complexes from
(45%) having pre-exponential factors of 0.55 and 0.45. In competitive coordination of the inorganic anions. We have
successfully achieved the synthesis of three different coordi-
nation geometries from the same tridentate ligand with very
uncommon coordination of tetrahydrofuran oxygen. The pho-
tophysical characterizations showed that the chelation induced
emission behaviors of complexes 1–3 and signicant increase in
uorescence quantum yield in comparison to free HL. Spec-
trouometric titrations of HL with ZnII showed coordination
complex 1 and 2 these sav's are higher at 4.56 and 4.80 ns and
Table 5 Photophysical parameters of ligand and metal complexes at
an excitation wavelength of 450 nm
Compound
s1, ns (a1)
s2, ns (a2)
sav, ns
c2
induced uorescence enhancement whereas
1
showed
ꢀ
HL
1
2
1.78 (0.55)
2.30 (0.388)
2.47 (0.388)
2.46 (0.802)
3.37 (0.45)
6.00 (0.612)
6.27 (0.612)
5.01 (0.198)
2.50
4.56
4.80
2.97
1.08
0.96
0.92
0.95
quenching behaviors in presence of externally added N3 and
SCNꢀ anions. In comparison to the ligand itself, the complexes
record different excited state lifetime which are comparable to
that of the tryptophan residue in different protein part.
3
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13 (a) Y. Song, Z. Xu, Q. Sun, B. Su, Q. Gao, H. Liu and J. Zhao, J.
Coord. Chem., 2008, 61, 1212–1220; (b) S. Mandal, A. K. Rout,
A. Ghosh, G. Pilet and D. Bandyopadhyay, Polyhedron, 2009,
28, 3858–3862.
14 X. Li, X. Yang, Y. Li, Y. Gou and Q. Wang, Inorg. Chim. Acta,
2013, 408, 46–52.
Acknowledgements
HM is thankful to the Council of Scientic and Industrial
Research, New Delhi for the nancial support. We are thankful
to Prof. Nilmoni Sarkar for providing the instrumental facility
for TCSPC measurements and DST, New Delhi, for providing
the Single Crystal X-ray Diffractometer facility to the Depart-
ment of Chemistry, IIT Kharagpur under its FIST program.
15 U. A. Bhagwat, V. A. Mukhedkar and A. J. Mukhedkar, J.
Chem. Soc., Dalton Trans., 1980, 2319–2322.
16 (a) A. T. R. Williams, S. A. Wineld and J. N. Miller, Analyst,
1983, 108, 1067–1068; (b) Principles of Fluorescence
Spectroscopy, ed. J. R. Lakowicz, Kluwer Academic/Plenum
Publishers, NY, 2nd edn, 2003.
References
1 A. Sarkar, A. K. Ghosh, V. Bertolasi and D. Ray, Dalton Trans., 17 Saint, Smart and XPREP, Siemens Analytical X-ray
2012, 41, 1889–1896. Instruments Inc., Madison, WI, 1995.
2 N. Zhan, G. Palui, M. Sa, X. Ji and H. Mattoussi, J. Am. Chem. 18 G. M. Sheldrick, SHELXS-97, University of Gottingen,
¨
¨
Gottingen, Germany, 1997.
Soc., 2013, 135, 13786–13795.
3 P. Jiang and Z. Guo, Coord. Chem. Rev., 2004, 248, 205–229. 19 G. M. Sheldrick, SHELXL 97, Program for Crystal Structure
¨
¨
4 W. Maret and Y. Li, Chem. Rev., 2009, 109, 4682–4707.
5 (a) M. Irie, Chem. Rev., 2000, 100, 1683–1684; (b) R. Pandey,
Renement, University of Gottingen, Gottingen, Germany,
1997.
P. Kumar, A. K. Singh, M. Shahid, P. Li, S. K. Singh, Q. Xu, 20 G. M. Sheldrick, SADABS: Soware for Empirical Absorption
¨
Correction, University of Gottingen, Institute fur
A. Misra and D. S. Pandey, Inorg. Chem., 2011, 50, 3189–
3197; (c) S. A. Ingale and F. Seela, J. Org. Chem., 2012, 77,
9352–9356.
¨
Anorganische Chemieder Universitat, Gottingen, Germany,
pp. 1999–2003.
´
6 (a) E. L. Que, D. W. Domaille and C. J. Chang, Chem. Rev., 21 (a) S. Paul, R. Clerac, N. G. R. Hearns and D. Ray, Cryst.
2008, 108, 1517–1549; (b) G. Meloni, T. Polanski, O. Braun Growth Des., 2009, 9, 4032–4040; (b) P. Dhal, M. Nandy,
ˇ´
´
´
and M. Vasak, Biochemistry, 2009, 48, 5700–5707; (c)
D. Sadhukhan, E. Zangrando, G. Pilet, C. J. Gomez-Garcıa
S. W. Suh, J. W. Chen, M. Motamedi, B. Bell, K. Listiak,
and S. Mitra, Dalton Trans., 2013, 42, 14545–14555.
N. F. Pons, G. Danscher and C. J. Frederickson, Brain Res., 22 P. B. Chatterjee, S. M. T. Abtab, K. Bhattacharya, A. Endo,
2000, 852, 268–273.
7 (a) C. R. Hickenboth, J. S. Moore, S. R. White, N. R. Sottos,
E. J. Shotton, S. J. Teat and M. Chaudhury, Inorg. Chem.,
2008, 47, 8830–8838.
J. Baudry and S. R. Wilson, Nature, 2007, 446, 423–427; (b) 23 H. S. Jena and V. Manivannan, Inorg. Chim. Acta, 2013, 394,
A. L. Garay, A. Pichon and S. L. James, Chem. Soc. Rev.,
2007, 36, 846–855.
8 (a) A. Das, G. M. Rosair, M. S. E. Fallah, J. Ribas and S. Mitra,
Inorg. Chem., 2006, 45, 3301–3306; (b) S. Sasmal, S. Sarkar,
210–219.
24 W. A. Addison, T. N. Rao, J. Reedijk, J. V. Rijn and
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349–
1356.
N. Aliaga-Alcalde and S. Mohanta, Inorg. Chem., 2011, 50, 25 L. Yang, D. R. Powell and R. P. Houser, Dalton Trans., 2007,
5687–5695; (c) Y.-Q. Wang, Q. Yue, Y. Qi, K. Wang, Q. Sun
955–964.
and E.-Q. Gao, Inorg. Chem., 2013, 52, 4259–4268; (d) 26 C. L. Dollberg and C. Turro, Inorg. Chem., 2001, 40, 2484–
A. Escuer, J. Esteban, S. P. Perlepes and T. C. Stamatatos, 2485.
Coord. Chem. Rev., 2014, 275, 87–129; (e) S. Naiya, 27 S. Basak, S. Sen, S. Banerjee, S. Mitra, G. Rosair and
´
´
S. Biswas, M. G. B. Drew, C. J. Gomez-Garcıa and A. Ghosh,
M. T. G. Rodriguez, Polyhedron, 2007, 26, 5104–5112.
28 X.-X. Zhou, H.-C. Fang, Y.-Y. Ge, Z.-Y. Zhou, Z.-G. Gu,
X. Gong, G. Zhao, Q.-G. Zhan, R.-H. Zeng and Y.-P. Cai,
Cryst. Growth Des., 2010, 10, 4014–4022.
29 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and
T. E. Rice, Chem. Rev., 1997, 97, 1515–1566.
Inorg. Chem., 2012, 51, 5332–5341.
9 (a) C.-H. Wang, C.-Y. Li, B.-H. Huang, C.-C. Lin and B.-T. Ko,
Dalton Trans., 2013, 42, 10875–10884; (b) A. Erxleben, Coord.
Chem. Rev., 2003, 246, 203–228.
10 (a) A.-X. Zheng, J. Si, X.-Y. Tang, L.-L. Miao, M. Yu, K.-P. Hou,
F. Wang, H.-X. Li and J.-P. Lang, Inorg. Chem., 2012, 51,
10262–10273; (b) K. A. McCall, C.-c. Huang and 30 (a) R. Pandey, P. Kumar, A. K. Singh, M. Shahid, P.-z. Li,
C. A. Fierke, J. Nutr., 2000, 130, 1437S–1446S; (c) M. Dey,
C. P. Rao, P. Saarenketo, K. Rissanen and E. Kolehmainen,
Eur. J. Inorg. Chem., 2002, 2207–2215.
S. K. Singh, Q. Xu, A. Misra and D. S. Pandey, Inorg. Chem.,
2011, 50, 3189–3197; (b) H.-W. Chiang, Y.-T. Su and
J.-Y. Wu, Dalton Trans., 2013, 42, 15169–15182; (c) S. Basak,
S. Sen, C. Marschner, J. Baumgartner, S. R. Batten,
D. R. Turner and S. Mitra, Polyhedron, 2008, 27, 1193–1200;
(d) N. J. Williams, W. Gan, J. H. Reibenspies and
R. D. Hancock, Inorg. Chem., 2009, 48, 1407–1415.
11 Z. He, C. He, Z.-M. Wang, E.-Q. Gao, Y. Liu and C.-H. Yan,
Dalton Trans., 2004, 502–504.
12 (a) C. Biswas, M. G. B. Drew, E. Ruiz, M. Estrader, C. Diaz and
A. Ghosh, Dalton Trans., 2010, 39, 7474–7484; (b) R. Biswas,
S. Mukherjee, P. Kar and A. Ghosh, Inorg. Chem., 2012, 51,
8150–8160.
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¨
31 (a) M. Schaferling, M. Wu and O. S. Woleis, J. Fluoresc., 33 J. R. Lakowicz and G. Weber, Biochemistry, 1973, 12, 4171–
¨
2004, 14, 561–568; (b) Z. Lin, M. Wu, M. Schaferling and
4179.
O. S. Woleis, Angew. Chem., 2004, 43, 1735–1738.
32 F. V. Bright and C. A. Munson, Anal. Chim. Acta, 2003, 500,
71–104.
34 J. R. Alcala, E. Gratton and F. G. Prendergast, Biophys. J.,
1987, 51, 597–604.
This journal is © The Royal Society of Chemistry 2014
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