Study on divalent copper, nickel and zinc model complexes for fluoride ion detection

Article abstract of DOI:10.1039/c5ra18559a

A series of new model complexes of 2,4-dihydroxybenzaldoxime (H₃L) with divalent copper and nickel ions having a general formula [M(H₂L)₂]·sol (sol = solvent or nitrogen containing heterocycle) and a new zinc complex [Zn(H₂L)₂(pep)₂] (where pep = 4-(2-(pyridine-4-yl)propyl)pyridine) were synthesized for the detection of fluoride ions. All complexes were structurally characterized by X-ray single crystal diffraction, powder X-ray diffraction and conventional spectroscopic tools. All these complexes show selective interactions with tetrabutylammonium fluoride. The position of the new UV-Vis absorption in each of these complexes in the presence of fluoride ions is guided by the electronic configuration of the metal ion. The complex [Zn(H₂L)₂(pep)₂] has a distinct advantage over the other model complexes, as it can detect fluoride ions through both emission and absorption spectroscopy.

Full text of DOI:10.1039/c5ra18559a

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: J. B. Baruah and
A. Tarai, RSC Adv., 2015, DOI: 10.1039/C5RA18559A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
Study on divalent copper, nickel and zinc model complexes for fluoride ions detection
Arup Tarai, Jubaraj B. Baruah*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati ꢀ781 039, Assam,
India. Fax: +91ꢀ361ꢀ2690762; Ph. +91ꢀ361ꢀ2582311;
email: juba@iitg.ernet.in http://www.iitg.ernet.in/juba
Abstract:
A series of new model complexes of 2,4ꢀdihydroxybenzaldoxime (H L) with divalent copper and
3
nickel ions having a general formula [M(H L) ].sol (sol = solvent or nitrogen containing
2
2
heterocycle) and a new zinc complex [Zn(H L) (pep) ] (where pep = 4ꢀ(2ꢀ(pyridineꢀ4ꢀ
2
2
2
yl)propyl)pyridine) were synthesized for the detection of fluoride ions. All complexes were
structurally characterized by Xꢀray single crystal diffraction, powder Xꢀray diffraction and
conventional spectroscopic tools. All these complexes show selective interactions with
tetrabutylammonium fluoride. The position of the new UVꢀVis absorption in each of these
complexes in the presence of fluoride ions is guided by the electronic configuration of the metal
ion. The complex [Zn(H L) (pep) ] has the distinct advantage over the other model complexes,
2
2
2
as it can detect fluoride ions through both emission and absorption spectroscopy.
Introduction:
1
aꢀb
1c
Due to selective interactions of fluorophore or chromophore containing phenols,
ureas,
1
d
1eꢀk
1lꢀm
1n
thioureas, oximes,
sulphonamides , amides
with fluoride ions, they are used in the
detection of fluoride ions. Fluorescent sensors based on high chemical affinity of the fluoride
2
3
ions towards silicon or boron atoms are reported in literature. The silicon based receptors in
2
aqueous solution, requires several minutes to complete the detection process, which is a
disadvantage to such receptors. In an another approach, the fluoride ions are detected on the
4
a
basis of their specific tendency to coordinate to the hard metal ions such as divalent calcium,
4
b
4cꢀg
trivalent iron or tetravalent zirconium.
Such fluoride sensors based on metal complexes
require pretreatment to avoid interference of other metal ions such trivalent aluminum or iron
4b
ions. Platinum(II) complex possessing triarylboron groups shows distinct phosphorescent
5
6a
response to fluoride ions. Among the other metal complexes, the rutheniumꢀ2,2'ꢀbipyridine,
1

RSC Advances
Page 2 of 18
View Article Online
DOI: 10.1039/C5RA18559A
6
b
the thiourea based iron(III) complexes were used to detect fluorides ions. Fluorescence receptor
having a rhodamine unit senses fluoride ions and was used in imaging of fluoride ions in living
7
cell.
Figure 1: Sites of interaction of fluoride ion in a bisꢀchelated metal complex of hydroxyaromatic
aldoxime.
From the above examples it is clear that chromophoric or fluorophoric compounds capable of
forming hydrogen bond or deprotonation causing signal transduction have been widely used in
fluoride ion detection. Such methodologies have been extended through introduction of
8
biocompatible hydrophilic polymer such as poly(ethylene glycol) to understand their practical
utility in biological in aqueous medium. Besides these compounds containing main group
elements such as boron, silicon, phosphorous having affinity to bind to fluoride ions have been
also used in detection of fluoride by UVꢀVis or fluorescence spectroscopy. On the other hand
chemical affinity of the fluoride ion towards metal ions is conventionally used in detection of
fluoride. Conventionally, the ligand exchange reactions of fluoride ions help to detect of fluoride
ions by spectroscopic tools. In such reactions one or more ligands are released to generate
spectroscopic or electrochemical signals. In the metal containing systems focus is put on the
central metal atom and in many cases preꢀtreatment of the metal complexes are required which
deters them to use in real time detection processes. In recent days modulations of fluorescence of
9
ligands attached to metal ions have shown promise to develop new sensors. Hydroxyaromatic
1
0
oximes are good ligands for metal chelating but their metal complexes have not been explored
for fluoride detection. Use of the polyꢀhydroxyaromaticaldoxime metal complexes such as the
representative structure shown in Figure 1 would have several ways to interact with fluoride
ions. Namely the signal transduction can be due to interaction of fluoride with the lignad or on
2

Page 3 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
the metal site as illustrated in the Figure 1. Further such process may be guided by the central
metal ion. With this background we prepared and studied interactions with fluoride ions of a
series of model complexes of the 2,4ꢀdihydroxybenzaldoxime (H L) with divalent copper,
3
nickel and zinc ions.
Experimental:
Physical measurement: Infrared spectra of the solid samples were recorded on a PerkinꢀElmer
−
1
SpectrumꢀOne FTꢀIR spectrophotometer in the region 4000ꢀ400 cm by making KBr pellets.
UVꢀvisible spectra were recorded on a PerkinꢀElmerꢀLambda 750 UVꢀVis spectrometer at room
temperature. Mass were recorded on a micro mass QꢀTOF (waters) mass spectrometer by using
an acetonitrile / formic acid matrix. Fluorescence emissions were measured in a PerkinꢀElmer
LSꢀ55 spectrofluorimeter by taking definite amounts of solutions of samples and exciting at
required wavelengths. Powder Xꢀray diffraction patterns were collected on a Bruker D2 Phaser
desktop diffractometer with Cu Kα radiation (λ = 1.5418 Å). The single crystal Xꢀray diffraction
data were collected at 296(2) K on a Bruker Nonius SMART CCD diffractometer with MoKα
radiation (λ = 0.71073 Å). SMART software was used for data collection, indexing the
reflections and determination of the unit cell parameters. Structures were solved by direct
methods and refined by fullꢀmatrix leastꢀsquare calculations using SHELXTL software.
Crystallographic parameters are summarized in supporting Table S1.
Experimental procedure for UV–Vis and fluorescence experiments:
ꢀ5
For study of interactions with fluoride ions separate solutions of the complexes 1, 5 and 7 (10
ꢀ3
M solution) as well as the solutions of different tetrabutyl ammonium ions (10 M solution) were
prepared in dimethylsuphoxide or water or DMSOꢀwater mixture (1:1 v/v). To record the UV–
Vis spectra or fluorescence spectra 3 mL solution of the complex was placed in quartz cell of a
10 mm path length at room temperature. Absorption or emission spectra were recorded after each
addition, by adding definite amounts of solutions of anions in aliquot to such a solution by using
micropipette.
Synthesis of ligand H L:
3
To a solution of hydroxylamine hydrochloride (0.138 g, 2 mmol) in ethanol (20 ml) pyridine (1
ml) was drop wise added. Resulting solution was stirred at 25 °C for 15 min. followed by which
2, 4ꢀdihydroxybenzaldehyde (0.276 g, 2 mmol) was added to the reaction mixture and stirred at
3

RSC Advances
Page 4 of 18
View Article Online
DOI: 10.1039/C5RA18559A
for 1 h. Extraction of the reaction mixture by ethylacetate and evaporation of solvent yielded
1
H L as a colorless solid. Isolated yield: 84%. HꢀNMR (400MHz, DMSOꢀd ): 10.98 (s, 1H),
3
6
−
1
1
3
1
8
0.12 (s, 1H), 9.77 (s, 1H), 8.22 (s, 1H), 7.28 (d, J = 8.8 Hz, 1H), 6.32 (m, 2H). IR (KBr, cm ):
363 (br, m), 1643 (m), 1623 (m), 1598 (w), 1524 (s), 1488 (m), 1446 (w), 1365 (w), 1344 (w),
304 (s), 1255 (s), 1208 (s), 1170 (m), 1117 (m), 1003 (s), 973 (s), 957 (m), 933 (w), 857 (w),
30 (s), 801 (m), 733 (w), 712 (w), 631 (w). Mass (ESI) m/z: 154.1049 (m+1); (Calculated exact
mass 153.0426 for C H NO ).
7
7
3
[
Cu(H L) ].2DMF (1): To a wellꢀstirred solution of H L (0.153 g, 1 mmol) in methanol (10
2
2
3
ml), copper(II) nitrate trihydrate (0.120 g, 0.5 mmol) was added. A black precipitate obtained
was dissolved by adding minimum amount of dimethylformamide (DMF). Reaction mixture was
filtered and transparent liquid was kept for crystallization. After 5 days crystals were obtained.
ꢀ1
Isolated yield: ~60 %. IR (KBr, cm ): 3416 (bs), 2925 (m), 1663 (s), 1643 (s), 1621 (s), 1548 (s),
510 (w), 1454 (s), 1384 (w), 1341 (w), 1317 (m), 1253 (m), 1209 (s), 1170 (s), 1126 (s), 1098
m), 1016 (s), 986 (s), 968 (w), 849 (m), 836 (w), 796 (s), 759 (s), 667 (s), 630 (w), 614 (w).
1
(
TGA: 200–250°C (–2DMF; exptl. wt. loss 27.67 %, calcd. 28.40 %). Elemental anal. calcd for
C H CuN O ; C: 46.69 %, H: 5.05 %, N: 10.89 %; found, C: 46.56 %, H: 5.01 %, N: 10.78 %.
20
26
4
8
[
Cu(H L) ].2DMA (2): Complex 2 was prepared by a procedure similar to complex 1 using
2 2
dimethylacetamide (DMA) as solvent for crystallization. After 7days needle type crystals were
ꢀ1
obtained. Isolated yield: 62 %. IR (KBr, cm ): 3400 (bw), 3106 (bs), 1637 (s), 1613 (bs), 1546
s), 1457 (s), 1414 (w), 1397 (m), 1344 (w), 1312 (m), 1257 (s), 1215 (s), 1174 (s), 1137 (s),
012 (s), 985 (s), 970 (w), 858 (m), 842 (s), 814 (m), 761 (s), 796 (s), 661 (w), 620 (m), 630 (m).
(
1
TGA: 200–250°C (–2DMA; exptl. wt. loss 29.56 %, calcd. 32.14 %). Elemental anal. calcd for
C H CuN O ; C: 48.70%, H: 5.53 %, N: 10.33%; found, C: 48.66 %, H: 5.50 %, N: 10.28 %.
22
30
4
8
[
Cu(H L) ].DMSO (3): Complex 3 was prepared in a similar manner to complex 1 using
2 2
dimethylsulfoxide (DMSO) instead of DMF. After 8 days black plate type crystals were
ꢀ1
obtained. Isolated yield: 60 %. IR (KBr, cm ): 3332 (bs), 2922 (w), 1648 (s), 1606 (s), 1481 (s),
453 (s), 1376 (m), 1347 (w), 1328 (w), 1314 (w), 1284 (m), 1247 (m), 1212 (s), 1190 (w), 1129
s), 1014 (s), 987 (s), 953 (w), 850 (m), 834 (s), 795 (w), 786 (m), 761 (m), 698 (m), 651 (m),
30 (s), 610 (m). TGA: 200–250°C (–DMSO; exptl. wt. loss 17.16 %, calcd. 17.52%). Elemental
anal. calcd for C H CuN O S; C: 43.05 %, H: 4.03 %, N: 6.27 %, S: 7.17 %.; found, C: 43.01
1
(
6
1
6
18
2
7
%, H: 4.01 %, N: 6.21 %, S: 7.10 %.
4

Page 5 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
[
(Cu(H L) ].4,4'-bipy (4): Complex 4 was prepared by a procedure similar to synthesis of
2 2
complex 1, but adding 4,4'ꢀbipyiridine (4,4'ꢀbipy). After 15 days black needle shaped crystals
ꢀ1
were obtained. Isolated yield: 58%. IR (KBr, cm ): 3452 (bs), 1620 (s), 1606 (s), 1597 (w), 1549
s), 1516 (w), 1491 (w), 1455 (s), 1408 (s), 1377 (w), 1321 (s), 1256 (s), 1222 (w), 1212 (s),
173 (s), 1134 (m), 1058 (s), 1015 (s), 1001 (w), 982 (s), 964 (w), 846 (m), 832 (s), 799 (s), 760
s), 668 (w), 633 (s), 621 (s), 609 (w). Elemental anal. calcd for C H CuN O ; C: 54.96 %, H:
(
1
(
2
4
20
4
6
3.81 %, N: 10.68 %; found, C: 54.91 %, H: 3.76 %, N: 10.61 %.
[
Ni(H L) ].2DMF (5): To a wellꢀstirred solution of H L (0.153 g, 1 mmol) in methanol (10 ml),
2
2
3
nickel(II) nitrate hexahydrate (0.145 g, 0.5 mmol) was added. A green precipitate was obtained,
which was dissolved in a minimum amount of DMF and filtered, transparent filtrate after 7 days
ꢀ1
gave green block type crystals. Isolated yield: 65%. IR (KBr, cm ): 3380 (bs), 2925 (w), 1659
s), 1643 (s), 1615 (s), 1569 (w), 1555 (s), 1514 (w), 1494 (w), 1451 (s), 1386 (w), 1346 (w),
(
1
310(s), 1261 (w), 1228 (w), 1216 (s), 1170 (s), 1124 (s), 1102 (w), 1020 (s), 987 (s), 955 (w),
61 (m), 845 (m), 831 (s), 794 (s), 769 (m), 668 (s), 647 (s). TGA: 250–300°C (–2DMF; exptl.
8
wt. loss 27.85 %, calcd. 28.71 %). Elemental anal. calcd for C H N NiO ; C: 47.13 %, H: 5.10
2
0
26
4
8
%, N: 10.99 %; found, C: 47.08 %, H: 5.06 %, N: 10.92 %.
[
Ni(H L) ] (6): Complex 6 was obtained from an identical reaction that was used in preparation
2
2
of 5 but was carried out in DMSO. After 6 days green crystals were obtained. Isolated yield:
−
1
6
1
9
7%. IR (KBr, cm ): 3372 (bs), 2923 (w), 1635 (w), 1607 (s), 1574 (w), 1553 (s), 1514 (w),
483 (w), 1449 (s), 1381 (m), 1308 (s), 1261 (m), 1214 (s), 1167 (s), 1122 (s), 1019 (s), 986 (s),
55 (w), 862 (m), 832 (s), 794 (m), 768 (m), 673 (w), 647 (s). Elemental anal. calcd for
C H N NiO ; C: 46.28 %, H: 3.30 %, N: 7.71 %; found, C: 46.22 %, H: 3.25 %, N: 7.66 %.
14
12
2
6
[
Zn(H L) pep) ] (7): To a solution of H L (0.153 g, 1mmol) and 4ꢀ(2ꢀ(pyridineꢀ4ꢀ
2 2( 2 3
yl)propyl)pyridine (pep) (0.198g, 1mmol) in methanol (10 ml), zinc(II) nitrate hexahydrate
0.148g, 0.5mmol) was added and stirred. A white precipitate was obtained. Precipitate was
dissolved in a minimum amount of DMF and filtered. Filtrate on standing, resulted in formation
(
ꢀ1
of crystals after 15 days. Isolated yield: 70%. IR (KBr, cm ): 3039 (w), 2923 (w), 1646 (m),
1
605 (s), 1557 (m), 1500 (s), 1477 (s), 1452 (s), 1419 (s), 1383 (s), 1332 (w), 1274 (s), 1245 (s),
207 (s), 1177 (s), 1119 (s), 1070 (m), 1008 (s), 998 (s), 983 (s), 957 (w), 840 (s), 794 (s), 761
1
(
m), 613 (s). Elemental anal. calcd for C H N O Zn; C: 62.64 %, H: 5.22 %, N: 10.96 %;
40 40 6 6
found, C: 62.60 %, H: 5.16 %, N: 10.88 %.
5

RSC Advances
Page 6 of 18
View Article Online
DOI: 10.1039/C5RA18559A
Results and discussion:
Characterization of the model complexes:
A series of divalent copper, nickel and zinc complexes of H L were prepared by reacting H L
3
3
with respective metal salts in different solvents as illustrated in Scheme 1. The copper complexes
1
-4 have a general composition [Cu(H L) ].sol [when sol = DMF (1), DMA (2), DMSO (3) and
2 2
4,4'ꢀbipyiridine (4)]. The complexes 1-7 were characterized by conventional spectroscopic tools
and single crystal diffraction study. Powder Xꢀray diffraction (PXRD) patterns of the bulk
samples were recorded to ascertain the phase purity and the PXRD patterns of the complexes 1-4
Complex-4
OH
OH
N
O
N
Cu
O
.(4 4' Bipy)
HO
HO
OH
OH
OH
OH
X
N
O
O
OH
Cu(NO
N
O
N
3 2 2
Zn(NO ) .6H O
N
Zn
X
Cu
3
)
2
.3H
2
O
.Solvent
N
O
Pep. HO
OH
Solvent
OH
H L
3
HO
HO
HO
X= 4-(2-(pyridine-4-yl)propyl)pyridine
pep.)
(
Solvent = DMF (Complex-1)
=
=
DMA (Complex-2)
DMSO (Complex-3)
Complex-7
OH
OH
N
O
Ni
O
N
.Solvent
HO
HO
Solvent = DMF (Complex-5)
none (Complex-6)
=
Scheme 1: Synthesis of the metal complexes.
are given in Figure 2 and the rest of the PXRD patterns are included as supporting information.
It is clear from the Figure 2 that the PXRD patterns of the solvates have very close resemblances
6

Page 7 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
with the corresponding simulated pattern, showing that the complexes are obtained in bulk
ꢀ1
comprise of one solvate in each case. IRꢀspectra of the complex 1 has absorptions at 2925 cm
ꢀ1
due to CꢀH stretch, and C=O stretch of the DMF appear at 1663 cm , beside this there are C=N
ꢀ1
ꢀ1
and C=C stretches at 1643 cm and 1621 cm respectively. On the other hand, the complex 2
ꢀ1
has DMA molecules which shows the C=O stretch at 1637 cm ; similarly the complex 3 shows a
ꢀ1
sharp S=O stretching at 1129 cm due to the DMSO. Thermogravimetric analysis has revealed
that these complexes are thermally stable to about 200 °C and they lose respective solvent of
crystallization molecules around 200ꢀ250 °C. All these complexes decompose around 450 °C to
form the respective metal oxide (please refer to thermograms in supplementary materials).
(
a)
Figure 2: (a) Experimental and (b) Simulated powder Xꢀray diffraction patterns of the complexes
-4.
(b)
1
Structurally all the complexes adopt bisꢀchelated structures that are commonly observed in the
11
salicylaldoxime complexes. Complexes 1-3 have similar coordination environment around
copper but they have distinct differences in bond parameters. The metalꢀligand bond lengths are
dependent on the nature of solvent of crystallization molecules present in their lattices. Packing
7

RSC Advances
Page 8 of 18
View Article Online
DOI: 10.1039/C5RA18559A
pattern of such metal complexes are guided the crystallized solvent molecules. As a
representative example, the structure of complex 1 is shown in Figure 3a and the rest are shown
as supporting figures. Bond parameters of complexes 1-3 are compared in Table 1. Complex 1
and 2 are more symmetric, they show two identical CuꢀO and two identical CuꢀN bond distances.
Whereas, the complexes 3 and 4 have show four different metal ligand bond distances.
Differences are attributed to packing requirement of different guest molecules in respective
lattice to form tight packed structure. As a result of packing requirement, two oxygen atoms of
neighboring molecules of complex 4 in crystal lattice are at close vicinities to show a weak
interaction. Thus in solid state, the complex 4 appears to be a hexaꢀcoordinated complex to form
a coordination polymer. In this structure, the axial copperꢀoxygen distance is 2.654 Å, which is
relatively long and high enough to describe as dative bond. On the other hand, the other CuꢀO
bonds of the complex lie between 1.8ꢀ1.9 Å.
(a)
8

Page 9 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
(b)
Figure 3: (a) Selfꢀassembly of the complex 1 and (b) the structure of the zinc complex 7 (30%
thermal ellipsoids).
Table 1: Some of the selective metalꢀligand bond parameters in the complexes 1-4
o
o
Complex No. MꢀL
Length (Å)
< LꢀMꢀL
Angle ( )
< LꢀMꢀL
Angle ( )
1
Cu1ꢀO1
Cu1ꢀN1
1.895(3)
1.945(3)
O1ꢀCu1ꢀO1'
O1ꢀCu1ꢀN1
180.0
87.60(13)
O1’ꢀCu1ꢀN1
N1ꢀCu1ꢀN1'
92.40(13)
180.0
2
3
Cu1ꢀO1
Cu1ꢀN1
1.892(2)
1.937(3)
O1ꢀCu1ꢀO1'
O1ꢀCu1ꢀN1
179.99(1)
92.36(11)
O1’ꢀCu1ꢀN1'
N1ꢀCu1ꢀN1'
87.64(11)
180.00
CuꢀO2
CuꢀO1
CuꢀN2
CuꢀN1
Cu1ꢀO2
Cu1ꢀO1
Cu1ꢀN1
Cu1ꢀN2
1.895(2)
1.907(2)
1.934(3)
1.940(3)
1.898(2)
1.915(2)
1.930(3)
1.941(3)
O2ꢀCuꢀO1
O2ꢀCuꢀN2
O1ꢀCuꢀN2
179.48(12)
92.01(11)
88.05(11)
O1ꢀCuꢀN1
N2ꢀCuꢀN1
92.32(11)
179.37(13)
4
O2ꢀCu1ꢀO2
O2ꢀCu1ꢀN1
O1ꢀCu1ꢀN1
178.52(14)
86.79(11)
92.63(11)
O2ꢀCu1ꢀN2
O1ꢀCu1ꢀN2
N1ꢀCu1ꢀN2
92.00(11)
88.48(11)
175.58(15)
There are many examples of copper complexes in which the 4,4'ꢀbipyridine act as bridging
1
2
ligands by coordinating to axial positions of the copper ions of paddleꢀwheel geometry. In spite
of the fact that 4,4'ꢀbipyridine is a good coordinating ligand, it does not coordinate to the copper
ion in the complex 4. The complexes 1-3 have square planar geometries. In these complexes the
nitrogen atoms of the two oxime ligands are at trans dispositions in the coordination sphere. The
nickel complexes 5 and 6 also have similar coordination environments as that of the copper
9

RSC Advances
Page 10 of 18
View Article Online
DOI: 10.1039/C5RA18559A
complexes 1-3, but differ in the obvious metalꢀligand bond parameters due to the presence nickel
ion in lieu of the copper. The complex 5 is a DMF solvate, whereas the complex 6 is devoid of
solvent of crystallization. Among the copper and nickel complexes we observed an important
difference in their packing patterns. Packing pattern of the nickel complexes have nickel ions
positioned on the top of aromatic rings of a neighboring molecule showing weak stacking
interactions. Shortest distance between a nickel ion and the centriod of an aromatic ring above it
is 3.407 Å for the complex 6 and 3.695 Å for the complex 5. DMF molecules are responsible to
hold the layered packing structure of the complex 5, whereas the layered packing of the complex
6
is held by intermolecular hydrogen bonds between free hydroxyl groups of ligands. The copper
complexes also have layered packing patterns in which copper ions are found to be present on
the top of each other with or without an intervening atom. The zinc complex 7 is a hexaꢀ
coordinate mononuclear complex; which has pep coordinating through the nitrogen atom at one
end of the pep and the other end remains free (Figure 3b). Nitrogen atom of the free end of pep is
hydrogen bonded to a hydroxyl group of a neighboring complex to form selfꢀassembly.
Interactions of fluoride ions:
From the UVꢀVis spectroscopic studies it is found that the complexes 1-7 are selective to detect
fluoride ions in aprotic polar solvents such as DMSO, DMF, DMA and THF etc. UVꢀVis spectra
of these complexes were recorded in water by adding solution of tetrabutylammonium fluoride in
water (Figure S16) and found no noticeable changes to suggest a detection process. We have also
studied similar experiment in DMSOꢀwater mixture and found the solvent system is not suitable
for the detection process (Figure S17). Thus the detection of fluoride by these complexes is
limited to aprotic solvent. Fluorescence emission spectra of complex 7 depend on solvent
polarity, more polar solvent causes red shift of fluorescence emission spectra of complex 7. One
illustrative change in the UVꢀVis spectra on the interaction of the complex 7 with
tetabutylammonium salts such as fluoride, chloride, bromide, phosphate and carbonate is shown
in Figure 4a. It shows that only the terabutylammonium fluoride causes change in the UVꢀVis
spectra of the complexes and effect on the rest of the salts are negligible. The parent ligand as
well as the complexes 1-7 has a strong absorption at 259 nm in DMSO. As illustrated in Figure
4a, when a solution of the complex 7 in DMSO was treated with the respective solution of
10

Page 11 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
tetrabutylammonium salt, no significant changes caused on this absorption by any of these salt
other than tetrabutylammonium fluoride. On interaction with tetrabutylammonium fluoride a
new peak at 343 nm appeared in solution of complex 7. The change caused by the
tetrabutylammonium fluoride ions is shown independently Figure 4b. This figure shows that the
new absorption peak at 343 nm formed on gradual increase of the tetrabutylammonium fluoride
ions passes through an isosbestic point at 307 nm. Similar profiles on the changes in UVꢀVis
absorptions were observed from all these complexes 1-7 on interactions with
tetrabutylammonium fluoride. Positions of the new absorptions observed from such interaction
of the complexes 1-3 are independent of the solvent of crystallization. However, the positions of
the new peaks are dependent on the central metal ions. For example, it occurred at 357 nm for
the copper complexes, whereas at 382 nm for the nickel complexes and 343 nm for the zinc
complex on addition of tetrabutylammonium fluoride ions. From the shifts in absorption peaks of
the nickel, copper and zinc complexes showed a definite trend. The new peak in a solution of
zinc complex appeared at a relatively shorter wavelength, followed by the copper and the nickel
complex. This is attributed to the electronic factors of these metal ions as the occupancies in dꢀ
(a)
(b)
11

RSC Advances
Page 12 of 18
View Article Online
DOI: 10.1039/C5RA18559A
−
5
Figure 4: The changes in the UVꢀVis spectra of the complex 7 (10 M solution in DMSO) on
ꢀ3
addition of (a) different solutions (50µl, 10 M) of tetrabutylammonium salts (fluoride, chloride,
−
3
bromide, phosphate, carbonate) and (b) solution of tetrabutylammonium fluoride (10 M in
DMSO in 10 ꢁl aliquot).
orbitals follow the sequence Ni < Cu < Zn. It was reported earlier that the color changes of metal
1
3
complexes with polyhydroxy ligands can be caused by hydrogen bonds with fluoride ions. The
1
g
ligand H L acts as fluoride sensor showing spectral changes in UVꢀVis region. The UVꢀVis
3
spectra of the copper complex 1 and the nickel complex 5 were recorded at acidic (pH = 6),
neutral and basic (pH = 9) medium in dimethylsulphoxide. It is observed that, in basic medium
the complexes show longer wavelength peak which is similar to the peak that were observed in
presence of fluoride ions (Figure S18). Whereas, the UVꢀVis spectra of the complexes in acidic
medium tally with the peak of the ligand, suggesting that the complexes decompose in acid
medium. These experiments also show that the fluoride detection by these complexes is possible
only at a neutral condition in aprotic medium. Further to these we have examined the
fluorescence emission of the zinc complex (7) in different pH (Figure S19). The zinc complex is
very weakly fluorescent in neutral and in acidic medium. To the solution of this complex at pH =
6
when terabutylammonium fluoride was added no change in fluorescence emission took place.
This suggests that the complex is stable at this pH and does not respond to fluoride ions. But this
complex at pH = 9 showed a strong emission at 415 nm. This showed that a substrate able to
abstract the acidic hydrogen of the ligand should also cause a similar effect. The emission peak
at 415 nm was associated with a shoulder at 375 nm. This shoulder peak however appears as a
prominent peak with comparable intensity to the peak a 415 nm at lower concentration of
fluoride ions. Hence the peak at 375 nm is attributed to arise due to hydrogen bond formation. As
the concentration of fluoride ions was increased proton transfer took place showing the
prominence of the peak at 415 nm from the anionic form of the complex. At low concentration of
fluoride there is equilibrium between hydrogen bonded assembly of metal complex and
deprotonated metal complex caused by fluoride ions. After more amount of fluoride ions the
equilibrium shifts toward deprotonated metal complex which emits at 415 nm. Based on this the
emission spectral changes of the complex at neutral condition by adding different amount of
fluoride ions were recorded. The ligand H L is weakly fluorescent and without complex
3
12

Page 13 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
formation it is not suitable for the detection of fluoride ions by fluorescence technique. The
complex 7 has a fluorescence emission at 346 nm on excitation at 270 nm. However, a new
fluorescence emission at 415 nm (Figure 5a) on addition of fluoride ions to the complex 7 was
observed which is similar to the metal complex at basic pH. The emission was very selective to
the fluoride ions among different ions tested (Figure 5b).
(a)
(b)
−
5
Figure 5: The changes in the fluorescence emission spectra of the complex 7 (10 M solution in
−
3
DMSO) on addition of (a) tetrabutylammonium fluoride (10 ꢁl aliquot of 10 M in DMSO) and
(b) tetrabutylammonium salts (fluoride, chloride, bromide, phosphate, and carbonate) (50 µl of
ꢀ
3
1
0 M).
A series of anions were tested for changes in UVꢀVis with the solution of complex 7 and the
relative sensitivity of the complexes and is shown in Figure S20a by a bar graph diagram. Beside
this the effect of the sensitivity of the emission caused by presence of various anions are
checked and found that the fluoride ions can be easily detected in the presence of other ions such
as acetate, biphosphate, bromide, carbonate, chloride, perchlorate etc (Figure S20b). Hence,
complex 7 is a highly sensitive fluorophore for the detection of fluoride ions.
Comparing the fluorescence emission spectra of the complex 7 in basic medium with the one in
neutral medium in presence fluoride ions showed that deprotonation of hydroxy group of the
ligand was responsible for such an increase in intensity of emission with a shift of fluorescence
1
0
towards longer wavelength. The complexation of divalent metal ions having a d configuration
1
3
through deprotonation generally causes fluorescence enhancement of fluorphoric units. Hence
a highly sensitive fluorophore for the detection of fluoride ions. The detection limit of the
13

RSC Advances
Page 14 of 18
View Article Online
DOI: 10.1039/C5RA18559A
ꢀ6
complex 7 was found to be 4.86 × 10 M which was calculated by using 3σ/k as the detection
limit, where σ is the standard deviation of the complex and k is the slope of intensity vs
concentration plot (Figure S21).
Thus, the basic behavior of the fluoride deprotonate all these complexes to form resonance
10
stabilized anions shown in Scheme 2. Divalent zinc posses a d configuration, generates less
9
8
resonance stabilized anion; whereas, the d and d configuration of the central metal ions have
comparatively higher delocalization, hence the absorption occurs at a longer wavelength than the
corresponding zinc complex. Phenols shows absorption changes on deprotonation by fluoride
1b
14
ions, but such a change may occur without a proton transfer. Specific changes in absorption
by the fluoride ions in the present case but not caused by the respective acetate or hydroxide
tetrabutylammonium salt showed that protonꢀtransfer is not necessarily required for such a
Scheme 2: Resonance stabilized anions generated from the complexes by the interactions of
fluoride ions.
process. In basic medium these complexes are deprotonated, hence fail to detect the fluoride
ions. Based on these observations, Scheme 2 is proposed to explain the absorption changes.
Disadvantage of using these complexes is that the detection of fluoride ions is not possible by
these complexes in pure water. Further support to the mechanism is drawn from EPR spectra for
1
copper complexes and HNMR spectroscopy of zinc and nickel complexes. EPR spectral
features of the copper complexes in DMSO (Figure S22) do not show change in after addition of
tetrabutylammonium fluoride. This suggests that the copper sites are not affected by the presence
of fluoride ions and the effect of fluoride ion is on the external periphery of the complex. In the
1
titration carried out by recording the HNMR of the nickel and zinc complexes (Figure S23ꢀ24)
after addition of different amounts of tetrabutylammonium fluoride did not how shift in the
positions of the signals showing that complex is not decomposed and a fast exchange of proton
by fluoride takes. However the exchange process could not be ascertained as we could not
14

Page 15 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
clearly get the free hydroxy peak in solution. The role of fluoride is further clarified by recording
the UVꢀVis spectra of the complexes in presence of tetrabutylammonium hydroxide (Figure
S25). These complexes behave similarly as the tetrabutylammonium fluoride establishing the
proposed mechanism.
In conclusion, the divalent metal ion other than the conventional hard ions can also be used for
the fluoride detection by putting them as a part of a receptor. Population of dꢀelectron of the
central metal ions in a delocalized system plays a key role in shifting the emission or absorption
spectra with respect to the parent ligand during the detection of fluoride ions. Depending on the
metal ions such as in the complex 7 fluorescence emission can be a tool for the detection of
fluoride ions despite the parent ligand does not have such a capability. Interesting aspect of the
complex 7 is that it has a very weakly fluorescent counterpart, but the complex 7 on the
interactions with the fluoride ions becomes strongly fluorescent to show an emission at longer
wavelength. Incorporation of the basic aromatic heterocyclic compounds as ligands or solvent of
crystallization do not interfere in the fluoride detection.
Supporting information: The CIF of the complexes are deposited to CCDC and they have
CCDC numbers 1402350ꢀ1402356. ORTEP diagram, thermogravimetry, UVꢀVis titrations,
PXRD patterns, and table for crystallographic parameters of the complexes are available as the
supporting information.
References:
1. (a) F. Zheng, C. Yu, X. Hou and S. Wu, Chem. Eur. J., 2013, 19, 936 ꢀ942. (b) V. Bhalla, R.
Tejpal and M. Kumar, Tetrahedron, 2011, 67, 1266ꢀ1271. (c) C. ꢀH. Chen, M. ꢀK. and Leung,
Tetrahedron, 2011, 67, 3924ꢀ3935. (d) D. A. Jose, D. K. Kumar, B. Ganguly and A. Das, Org.
Lett., 2004, 6, 3445ꢀ3448. (e) S. Kondo, M. Nagamine, S. Karasawa, M. Ishihara, M. Unno, Y.
Yano, Tetrahedron, 2011, 67, 943ꢀ955. (f) C. B. Rosen, D. J. Hansen and K. V. Gothelf, Org.
Biomol. Chem., 2013, 11, 7916ꢀ7922. (g) A. Tarai and J. B. Baruah, CrystEngComm, 2015, 17,
2301ꢀ 2309. (h) C. Chen, D. Zhu, Org. Lett., 2005, 7, 4629ꢀ4632. (i) S. Li, C. Zhang, S. Huang,
F. Hu, J. Yin and S. H. Liu, RSC Advances, 2012, 2, 4215ꢀ4219. (j) W. Guan, S. Yamabe, S.
Sakaki, Dalton Trans., 2013, 42, 8717ꢀ8728. (k) D. A. Jose, P. Kar, D. Koley, B. Ganguly, W.
Thiel, H. N. Ghosh and A. Das, Inorg. Chem., 2007, 46, 5557ꢀ5564. (l) S. V. Bhosale, M. B.
15

RSC Advances
Page 16 of 18
View Article Online
DOI: 10.1039/C5RA18559A
Kalyankar and S. J. Langford, Org. Lett., 2009, 11, 5418ꢀ5421. (m) C. Caltagirone, A. Mulas, F.
Isaia, V. Lippolis, P. A. Gale and M. E. Light, Chem. Commun., 2009, 6279ꢀ6281.
2
. (a) T. H. Kim and T. M. Swager, Angew. Chem., Int. Ed., 2003, 42, 4803ꢀ4806. b) S. Y. Kim
and J. I. Hong, Org. Lett., 2007, 9, 3109ꢀ3112. (c) X. F. Yang, Spectrochim. Acta, Part A, 2007,
7, 321ꢀ330. (d) R. Hu, J. Feng, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Li and G. Q. Yang, Angew.
6
Chem., Int. Ed. 2010, 49, 4915ꢀ4918. (e) X. F. Yang, H. Qi, L. Wang, Z. Su and G. Wang,
Talanta, 2009, 80, 92ꢀ97. (f) S. Y. Kim, J. Park, M. Koh, S. B. Park and J. I. Hong, Chem.
Commun., 2009, 4735ꢀ4737.
3. (a) L. Weber, D. Eickhoff, J. Kahlert, L. Bohling, A. Brockhinke, H. Stammler, B. Neumanna
and M. A. Fox, Dalton Trans., 2012, 41, 10328ꢀ10346. (b) S. Arimori, M. G. Davidson, T. M.
Fyles, T. G. Hibbert, T. D. James and G. I. KociokꢀKohn, Chem. Commun., 2004, 1640ꢀ1641.
4. (a) S. Rochat and K. Severin, Chem. Commun., 2011, 4391ꢀ4393. (b) H. Parham, N. Rahbar,
Talanta, 2009, 80, 664ꢀ669. (c) O. A. Zaporozhets and L. Y. Tsyukalo, Anal. Chim. Acta, 2007,
597, 171ꢀ177. d) K. Okazaki, M. Mifune, J. Odo, Y. Tanaka and Y. Saito, Anal. Sci., 1992, 8,
567ꢀ569. (e) P. Sahu, J. D. Panda and B. C. Sinha, Talanta, 1992, 39, 541ꢀ545. (f) L. Kanwar,
K. K. Rao, Sens. Actuators, B 2010, 149, 245ꢀ251. (g) R. F. Devine and G. L. Partington,
Environ. Sci. Technol., 1975, 9, 678ꢀ679.
5. S. B. Zhao, T. McCormick and S. Wang, Inorg. Chem., 2007, 46, 10965ꢀ10967.
6. a) Z. Lin, Y. Zhao, C. Duan, B. Zhang and Z. Bai, Dalton Trans., 2006, 3678ꢀ3684. b) W. Lu,
H. Jiang, F. Hu, L. Jiang and Z. Shen, Tetrahedron, 2011, 67, 7909ꢀ7912.
7
8
1
9
1
. G. Sivaraman and D. Chellappa, J. Mater. Chem. B, 2013, 1, 5768ꢀ5772.
. X. Yong, M. Su, W. Wan, W. You, X. Lu, J. Qu and R. Liu, New J. Chem., 2013, 37, 1591ꢀ
594.
. Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39, 3007ꢀ3030.
0. (a) A. Chakravorty, Cord. Chem. Rev., 1974, 13, 1ꢀ46. (b) P. S. Reddy and K. H. Reddy,
Polyhedron, 2000, 19, 168ꢀ174. (c) D. Mandal and B. D. Gupta, Organometallics, 2005, 24,
1
1
1
1
501ꢀ1510. (d) V. Y. Kukushikin and A. J. L. Pomberio, Coord. Chem. Rev., 1999, 181, 147ꢀ
81.
1. A. G. Smith, P. A. Tasker and D. J. White, Coord. Chem. Rev. 2003, 241, 61ꢀ85.
2. D. L. Reger, A. Debreczeni, B. Reinecke, V. Rassolov, M. D. Smith and R. F. Semeniuc,
Inorg. Chem., 2009, 48, 8911ꢀ8924.
16

Page 17 of 18
RSC Advances
View Article Online
DOI: 10.1039/C5RA18559A
1
2
1
3. J. ꢀS. Wu, J. ꢀH. Zhou, P. ꢀF. Wang, X. ꢀH. Zhang and S. ꢀK. Wu, Org. Lett., 2005, 7, 2133ꢀ
136.
4. (a) M. Cometti and K. Rissanen, Chem. Commun., 2009, 2809ꢀ1829. (b) X. Yong, M. Su,
W. Wang, Y. Yan, J. Qu and R. Liu, Org. Biomol. Chem., 2013, 11, 2254ꢀ2257. (c) J. Wang,
Y. Hou, C. Li, B. Zhang and X. Wang, Sensors and Actuators B, 2011, 157, 586ꢀ593. (d) P. D.
Beer and P. A. Gale, Angew. Chem. Int. Ed., 2001, 40, 486ꢀ516.
17

RSC Advances
Page 18 of 18
View Article Online
DOI: 10.1039/C5RA18559A
Graphical Synopsis:
Study on divalent copper, nickel and zinc model complexes for fluoride ions detection
OH
OH
N
O
O
N
M
HO
HO
M = Cu, Zn, Ni
Site for deprotonation or
hydrogen bond by fluoride ion
A series of mononuclear hydroxyaromatic oxime metal complexes were synthesized for
detection of fluoride ion and shown that the spectral signals of such complexes depend on the
electronic configuration of the metal ion.
Arup Tarai, Jubaraj B. Baruah