Experimental and theoretical investigation of a metalloreceptor bearing a [Re(CO)3]+ core incorporating a multifunctional ligand: Selective reactivity towards Zn2+ and CN- ions

Article abstract of DOI:10.1039/c9dt00901a

The multifunctional ligand HL [(E)-1-(((di(pyridin-yl)methylene)hydrazono)methyl)naphthalen-2-ol] has been used in the present work. The mononuclear complex 1, ([Re(HL)(CO)₃Cl]), has been synthesised in excellent yield by reacting rhenium(i) pentacarbonyl chloride [Re(CO)₅Cl] with HL in a 1:1 ratio in dry toluene under an argon atmosphere. The complex 2, ([Re(L)(CO)₃]), has also been prepared in a similar way in the presence of triethylamine. X-ray crystallographic study reveals that in complex 1, the ligand binds as an N^N coordinating bidentate ligand while in 2 it behaves as a monoanionic N^N^O coordinating tridentate ligand. Complexes 1 and 2 were characterised by different physicochemical techniques and the observed properties have been interpreted with the help of DFT (density functional theory) and TDDFT (time dependent density functional theory) calculations. Interestingly, complex 1 interacts with Zn(OAC)₂ in methanol to yield amido binding highly reactive α-amino ether containing rhenium(i) complex 3, ([Re(HL-OMe)(CO)₃]). The X-ray structure and spectral properties of the generated complex 3 have also been studied. Complex 1 selectively reacts with a cyanide ion and a high detection limit (5.0 × 10^-8 M) was observed. The properties of intermediates and the final product upon addition of cyanide were also matched with the DFT, TDDFT and NTO (natural transition orbital) analysis.

Full text of DOI:10.1039/c9dt00901a

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Experimental and theoretical investigation of a
+
metalloreceptor bearing a [Re(CO) ] core incor-
3
Cite this: DOI: 10.1039/c9dt00901a
porating a multifunctional ligand: selective reactiv-
2
+
−
ity towards Zn and CN ions†
Tapashi Das and Kajal Krishna Rajak
*
The multifunctional ligand HL [(E)-1-(((di(pyridin-yl)methylene)hydrazono)methyl)naphthalen-2-ol] has
been used in the present work. The mononuclear complex 1, ([Re(HL)(CO) Cl]), has been synthesised in
excellent yield by reacting rhenium(I) pentacarbonyl chloride [Re(CO) Cl] with HL in a 1 : 1 ratio in dry
toluene under an argon atmosphere. The complex 2, ([Re(L)(CO) ]), has also been prepared in a similar
3
5
3
way in the presence of triethylamine. X-ray crystallographic study reveals that in complex 1, the ligand
binds as an N^N coordinating bidentate ligand while in 2 it behaves as a monoanionic N^N^O coordinat-
ing tridentate ligand. Complexes 1 and 2 were characterised by different physicochemical techniques and
the observed properties have been interpreted with the help of DFT (density functional theory) and
TDDFT (time dependent density functional theory) calculations. Interestingly, complex 1 interacts with
Zn(OAC)
2
in methanol to yield amido binding highly reactive α-amino ether containing rhenium(I)
Received 28th February 2019,
Accepted 12th April 2019
complex 3, ([Re(HL-OMe)(CO) ]). The X-ray structure and spectral properties of the generated complex 3
3
have also been studied. Complex 1 selectively reacts with a cyanide ion and a high detection limit (5.0 ×
DOI: 10.1039/c9dt00901a
0−8 M) was observed. The properties of intermediates and the final product upon addition of cyanide
1
rsc.li/dalton
were also matched with the DFT, TDDFT and NTO (natural transition orbital) analysis.
ligand may bind to the metal ions in several ways as shown in
Scheme 1. We have been able to isolate the compounds with
Studies on rhenium(I) α-diimine complexes have received a structural motifs I (complex 1) and II (complex 2).
great deal of attention in recent years. The interest mainly
Introduction
1
arises because these classes of compounds exhibit interesting
2
light harvesting properties, as well as activate the energy or
3
electron transfer processes in protein molecules. The emis-
sion properties of rhenium(I) complexes with N^N coordinat-
ing pyridine-imine based ligands are associated with the low
1
a,b,4
lying metal-to-ligand charge-transfer (MLCT) states.
The
close environment around the metal center regulates the
excited state properties of the molecule and the small change
in the ligand architecture can strongly influence its pro-
5
perties. This work concerns about the design of an unsymme-
trical bi-functional ligand and the coordination pattern of the
+
same with a [Re(CO)
3
]
core. In this paper, we have used a
mixed Schiff base ligand, HL, derived from di-pyridylketone
and 2-hydroxynaphthaldehyde linked via hydrazine. The
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata
700032, India. E-mail: kajalrajak@hotmail.com, kkrajak@chemistry.jdvu.ac.in
†
Electronic supplementary information (ESI) available: Fig. S1–S17 and Tables
S1–S4. CCDC 1836766–1836768. For ESI and crystallographic data in CIF or
Scheme 1 Structure of the ligand and all probable rhenium(I)
other electronic format see DOI: 10.1039/c9dt00901a
complexes.
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The complexes were characterized by single crystal X-ray complex. The most probable pathways for the observed spec-
structure determination along with the help of various spectro- tral behaviour upon addition of the cyanide ion are reported.
scopic methods. Their electrochemical behaviour in aceto-
nitrile solution is also examined.
The properties of both ground and excited states for
medium-sized metal complexes can be calculated at the first-
In complex 1, the azomethine nitrogen and phenolic –OH principles level using density functional theory (DFT) and
of the naphthol moiety are suitably oriented to interact with especially the improvement of time-dependent density func-
1
7
incoming cationic species. Thus, it is expected that the use of tional theory (TDDFT), with good accuracy. Here we also
this ligand will provide a promising approach for the synthesis report a detailed theoretical calculation with the aim to gain
of homo or heteronuclear metal organic hybrid luminescent better insight into its geometry, electronic structure and com-
materials having interesting optical and chemical properties. plete assignment of the experimentally observed spectral pro-
1
0
By taking advantage of these facts, we have used a d metal perties of the complexes using density functional theory (DFT),
ion particularly Zn(II) with the aim to isolate a new series of time-dependent density functional theory (TDDFT) and
6
luminescent
mixed-metal
rhenium(I)
complexes.
natural transition orbital (NTO) analysis.
Unfortunately, we were unable to isolate any mixed-metal
rhenium(I) complexes bearing Zn(II) when complex 1 is
allowed to react with the Zn(II) ion in methanol. Instead of that
Results and discussion
Syntheses and characterization
+
we have isolated a rhenium(I) complex having a fac-[Re(CO)3]
core containing a reactive α-amino ether group. Only a limited
7
number of examples are reported in the literature where the
The synthesis of the multifunctional ligand (HL) is accom-
plished by two steps as specified in Scheme 2. The stoichio-
metric reaction of hydrazine hydrate and di-2-pyridylketone
α-amino ether is stabilized via metal ion coordination.
However, to the best of our knowledge, the authentic isolation
of rhenium(I) complexes containing a reactive α-amino ether
group is not known. In addition to the feasibility of cationic
binding, the presence of acidic OH protons in the suitably
oriented vacant pocket of the coordinated rhenium(I) species,
fabricates the yellow coloured oily compound SB·NH
2
. The
compound SB·NH is further allowed to react with 2-hydroxy-
2
naphthaldehyde, which effectively produces a new ligand (HL)
as a yellow solid. The synthesized ligand was characterized by
1
1
, gives open access to interact with the anionic species. It is
H NMR and IR spectroscopy and the data are listed in the
believed that the incoming anion interacts with the receptor
via hydrogen bonding interaction or proton transfer reactions.
Experimental section.
8
5
The stoichiometric reaction of [Re(CO) Cl] with HL in
This has motivated us to check the change in the properties of
boiling toluene for 8 hours under an argon atmosphere
−
−
−
−
−
and CN−
1
with various anions such as F , Cl , Br , I , NO
3
afforded an orange coloured complex, [Re(CO) Cl(HL)], 1, in
3
ions. In practice only the cyanide ion responds.
good yield. Interestingly when the same reaction is carried out
The use of cyanide as an important raw material in the
pharmaceutical industries, insecticides and fertilizers as well
as an important reagent in plastic manufacturing, extraction
of metals, and the electroplating industries is mainly respon-
for 24 hours, a new complex, [Re(CO)
addition to complex 1. However, in the presence of triethyl-
amine, [Re(CO) (L)], 2, is produced in good yield. The high
yield of 2 is associated with the deprotonation of the ligand in
the presence of triethylamine which facilitates the formation
of imine nitrogen. Complex 1 is coordinated with rhenium(I)
through two pyridyl nitrogen atoms, it contains a free –OH
group and an uncoordinated pyridine nitrogen atom and it is
3
(L)], 2, is obtained in
3
9
sible for cyanide ion toxicity in the environment. Thus the
detection of cyanide ions is an important task for the scientific
community as the cyanide ion is highly toxic to the living body
and it also has detrimental effects on the environment and
1
0
human health. The fluorometric determination of cyanide in
various samples is an emerging field of research due its low
1
1
cost and easy operation. It has been listed in the literature
that a CN− ion is able to adjust the ICT state which also trig-
gers its emission properties by interacting with fluorescent
1
2
dyes. In this context, detection of cyanide ions by a metal ion
1
3
coordinated receptor has become an influential subject to
the researchers to overcome the difficulties such as slow reac-
1
4
tion rate and poor selectivity in the presence of other anions.
1
5
However, a limited number of metalloreceptors bearing a
+
[
Re(CO) ] core have been used as an optical sensor to detect
3
different anions based on the abstraction of dissociable
1
6
protons present at the ligand frame. Moreover, the paucity of
well-characterized rhenium(I) complexes which can detect
exclusively the cyanide ion has prompted us to search for new
systems. Herein we also describe the spectroscopic detection
Scheme 2 Schematic representation of the synthesis of the multifunc-
(UV-Vis and PL method) of cyanide using the synthesized tional ligand, HL.
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Scheme 3 Schematic representation of the synthesis of complex 3.
expected to accommodate foreign ions into the vacant site. 1
was allowed to react with one equivalent of Zn(OAc) , Cd(OAc)
2
2
Fig. 2 ORTEP plot of 2 showing essential numbering and H atoms are
omitted for clarity.
2
and Hg(OAc) in methanol, in order to synthesize hetero-
6
10
dinuclear species having d –d configurations. However, only
in the presence of Zn(OAc)2 we have isolated α-amino ether
containing rhenium(I) species (vide infra), [Re(CO)
3
(L-OMe)], 3
(Scheme 3).
IR spectra
The IR spectra of all the complexes were recorded in a KBr
+
disk. The presence of a fac-[Re(CO)
3
] core having pseudo-C3v
symmetry can be attributed to the three metal carbonyl stretch-
1
8
−1
ing frequencies observed in the range 1900–2014 cm . The
remaining characteristic IR data (experimental and calculated)
are given in the Experimental section.
Crystal structure
The molecular structures of fac-[Re(HL)(CO)
3
Cl], 1, fac-[Re(L)
Fig. 3 ORTEP plot of 3 showing essential numbering and H atoms are
omitted for clarity.
(
CO) ], 2 and fac-[Re(HL-OMe)(CO) ], 3, are determined by
3
3
using a single crystal X-ray diffractometer. The molecular struc-
tures of 1, 2 and 3 are shown in Fig. 1–3 respectively.
The selected bond lengths and bond angles are listed in
Table 1. In 1, the ligand (HL) binds with the metal ion as an
N^N coordinating neutral bidentate ligand, leaving the pyridyl
nitrogen (N4) and phenolic –OH group uncoordinated. But 2
and 3 bind with HL as the N^N^O and N^N^N coordinating
mode, where one pyridyl nitrogen is free in 2 and a phenolic
–
OH group is free in 3. In all the three complexes, the carbonyl
ligands are arranged in facial mode and the equatorial sites are
occupied by pyridyl nitrogen and ketimine nitrogen (for 1), phe-
nolic oxygen (for 2) and another pyridyl nitrogen (N4, for 3).
The remaining axial positions are occupied by a chlorine
atom (for 1) and imine nitrogen (for 2 and 3). Thus, the geo-
metry around the rhenium(I) centre in all the complexes
acquires a distorted octahedral geometry. The coordinated
pyridyl moiety and uncoordinated hydroxy-naphthyl moiety in
1
lie on the same plane (dihedral angle 2.97°) whereas, the co-
ordinated and uncoordinated pyridyl groups are out of plane
dihedral angle 85.44°). The distance between imine nitrogen
(
and phenolic oxygen (O⋯N = 2.565 Å) in 1 recommends the
existence of strong hydrogen bonds. An interesting structural
feature of 3 is revealed from the bond angles around C1
(
∼
Fig. 3). All the bond angles centring C1 in complex 3 are
107°, which confirms the sp hybridization of the C1 atom.
3
NMR spectra
All the complexes are diamagnetic and display well resolved
3
NMR spectra in CDCl solution and the spectral data are given
Fig. 1 ORTEP plot of 1 showing essential numbering and H atoms are
omitted for clarity.
in the Experimental section. The assignment of NMR peaks is
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Table 1 Bond angles and bond lengths of complexes 1, 2 and 3
1
2
3
Bond Length (Å)
C1–N1
Re1–C24
Re1–C25
Re1–C23
Re1–N3
1.299(7)
1.918(7)
1.925(8)
2.028(9)
2.181(5)
2.198(5)
2.459(2)
C1–N1
1.294(5)
2.121(3)
1.892(4)
1.913(5)
1.916(5)
2.126(3)
2.194(4)
C1–N1
1.479(7)
1.915(7)
1.909(7)
1.918(7)
2.140(4)
2.153(5)
2.207(5)
Re1–O1
Re1–C23
Re1–C25
Re1–C24
Re1–N2
Re1–N4
Re1–C24
Re1–C23
Re1–C25
Re1–N1
Re1–N3
Re1–N4
Re1–N1
Re1–Cl1
Bond angles (°)
C24–Re1–C25
C24–Re1–C23
C25–Re1–C23
C24–Re1–N3
C25–Re1–N3
C23–Re1–N3
C24–Re1–N1
C25–Re1–N1
C23–Re1–N1
N3–Re1–N1
C24–Re1–Cl1
C25–Re1–Cl1
C23–Re1–Cl1
N3–Re1–Cl1
N1–Re1–Cl1
C2–C1–C7
86.0(3)
92.2(3)
90.6(3)
96.3(3)
175.3(2)
93.4(3)
169.8(3)
104.2(2)
87.5(3)
73.59(18)
93.6(2)
91.0(2)
C23–Re1–C25
C23–Re1–C24
C25–Re1–C24
C23–Re1–O1
C25–Re1–O1
C23–Re1–N2
C24–Re1–O1
C25–Re1–N2
C24–Re1–N2
O1–Re1–N2
C23–Re1–N4
C25–Re1–N4
C24–Re1–N4
O1–Re1–N4
N2–Re1–N4
C2–C1–C7
89.47(19)
89.3(2)
89.6(2)
C24–Re1–C23
C24–Re1–C25
C23–Re1–C25
C24–Re1–N1
C23–Re1–N1
C25–Re1–N1
C24–Re1–N3
C23–Re1–N3
C25–Re1–N3
N1–Re1–N3
C24–Re1–N4
C23–Re1–N4
C25–Re1–N4
N1–Re1–N4
N3–Re1–N4
C2–C1–C7
89.2(3)
89.3(3)
86.8(3)
170.0(3)
98.2(2)
97.8(2)
98.1(3)
94.4(2)
172.5(2)
74.70(18)
97.1(3)
173.3(2)
95.4(2)
75.34(17)
82.61(18)
107.1(5)
176.14(16)
92.34(16)
96.93(15)
94.13(17)
95.06(17)
172.24(18)
79.52(12)
92.38(16)
173.39(17)
96.75(18)
85.45(12)
78.41(13)
117.9(4)
174.0(2)
84.77(14)
86.54(14)
121.0(5)
based on the intensity and spin–spin splitting pattern. All
three complexes show a doublet near 9 ppm, and the proton
adjacent to the pyridyl nitrogen atom appears as a doublet at
∼8.9 ppm. A singlet due to an azomethine hydrogen atom was
observed at ∼10 ppm in all cases. In complex 1, the peak of
phenolic OH is observed at 11.20 ppm which disappears upon
addition of D O.
2
Geometrical studies
Rhenium(I) complexes (1, 2 and 3) are diamagnetic at room
temperature. Optimizations for these complexes are accom-
plished in the presence of solvent. The main geometrical opti-
mized parameters (bond length and bond angle) of complexes
1
, 2 and 3 are given in the ESI (Table S1†). The modelled geo-
Fig. 4 Partial molecular orbital diagram of complexes 1, 2 and 3.
metries possess a distorted octahedral arrangement around
the rhenium(I) metal centre. The optimized structures of these
complexes are very close to the experimentally observed struc-
tures. A slight variation in structural parameters may arise due LUMO. The HOMO–LUMO energy gap in all the mononuclear
to the crystal lattice distortion in the real molecules. The Re–N complexes is close to each other and the corresponding values
and Re–O bond lengths are in the range of 2.13–2.29 Å in both are 3.06 eV for 1, 2.88 eV for 2, and 3.01 eV for 3. The corres-
the theoretical calculation and experimental observation. The ponding orbital contribution for the three complexes is given
good agreement between the calculated and experimental data in Table S3.† The electron density in the HOMO of all the com-
highlights the importance of relativistic effects for an appropri- plexes mainly resides on the naphthalene moiety (in the range
ate description of the geometrical structures of the present of 40–78%) and the imine bonds (in the range of 25–40%). In
rhenium(I) complexes. The isodensity plot of some selected the case of 2, the electron density also resides on the
frontier molecular orbitals in their singlet ground state (S
0
) is rhenium(I) centre (11%) which is not observed in the other
listed in Table S2.† A partial molecular orbital diagram with two complexes. In the case of the LUMO, the electron density
the HOMO and LUMO for all the three complexes is shown in mainly resides on the pyridyl moiety (in the range 40–95%)
Fig. 4. In the ground state (S
), the HOMO of 2 is of a much and the imine bond (in the range of 35–47%) whereas 1 bears
0
lower energy than 1 and 3, which is the same in the case of the a very low density around the imine bond.
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UV-Vis spectra and DFT studies
The absorption spectral behaviour of the ligand and complexes
1
and 2 is recorded in acetonitrile while that of 3 in methanol
at room temperature. The UV-Vis spectral parameters with the
experimental molar extinction coefficient (ε) of HL and 1–3 are
listed in Table 2. The relevant electronic spectra are given in
Fig. 5. HL shows four characteristic peaks in the range of
2
29–392 nm which can be assigned as n–π* and π–π* intra-
molecular charge transfer transitions. The absence of a peak
above 400 nm ensures the predominance of an enol over a
keto tautomer in HL. All the three complexes display a strong
peak varying between 336 and 500 nm having an extinction
−
1
−1
coefficient in the range of 6000–10 200 M cm . The bands
in the longer wavelength region of these complexes can be
attributed to the metal to ligand charge transfer [Re (dπ) →
HL] transition. The intense absorption bands (ε
3
=
1 000–53 000 M− cm ) of the complexes at 220–260 nm are
1
−1
Fig. 5 Absorption spectra of 1–2 in acetonitrile and 3 in methanol at
room temperature.
attributed to the ligand π–π* transition. In order to gain clear
insight into the vertical excitation and the nature of absorp-
tion, NTO analysis is performed based on the calculated tran-
action of 1 with Zn(OAc)2 is responsible for the above-men-
5
,17,19
sition density matrices.
This method describes the most
tioned transformation. At first, the interaction of Zn(OAc)
with uncoordinated pyridine nitrogen (N4), imine nitrogen
N2) and the phenolic oxygen of 1 enhances the electrophilicity
2
compact representation of the transition density between the
ground and excited states in terms of an expansion into single-
particle transitions (hole and electron states for each given
excitation). The unoccupied and occupied NTOs are referred to
as “electron” and “hole” transition orbitals, respectively.
Table S4† illustrates the natural transition orbitals (NTOs) for
complexes 1, 2 and 3. Based on our TDDFT NTO analysis the
band at 260 nm of 1 can be characterized as an admixture of
(
of ketimine carbon (C1). In the next step, solvent methanol
attacks C1 and simultaneously the ketimine bond gets reduced
and N4 binds with the rhenium(I) center followed by the
removal of the chloride ion and Zn(OAc) . The proposed
2
mechanism of the reaction and the NMR spectra of the inter-
action of 1 with Zn(OAc) are given in the ESI (Fig. S14 & S15†).
1
1
2
MLCT and ILCT states. Bands in the region 300–400 nm orig-
Time dependent UV-spectral studies were performed in
methanol–dichloromethane (1 : 1) to gain better insight into
1
1
inate from the mixed MLCT and ILCT state whereas that of
in the region 400–500 nm mainly arises from ILCT along with
very little MLCT character.
1
the rate of the reaction in the presence of Zn(OAc) . The
1
2
absorption band at 426 nm of 1 diminishes instantly and two
new peaks appeared at 387 and 523 nm respectively after the
Insights into the reactivity of 1 with Zn(OAc)2
stoichiometric addition of Zn(OAc) to the solution of 1. This
2
Complex 1 is prone to react with Zn(OAc)
the reaction is carried out in dichloromethane–methanol (1 : 1) species formed by interacting Zn(OAc)
solvent due to the low solubility of 1 in methanol. Addition of dependent spectral plot of decolourization of the dark red
Zn(OAc)
into 1 changes the reddish yellow solution to dark solution to yellow is given in Fig. 6.
red rapidly. Again, it decolorizes to yellow within two hours
It is clear from the plot that the newly generated peak at
in solution. Here spectral change can be attributed to the generation of new
2
with 1. The time
2
2
and pure complex 3 was obtained from the reaction mixture by 523 nm decreases and that of 387 nm increases with time and
slow evaporation. The mechanism behind the formation of 3 this spectral change passes through an isosbestic point at
is not very much clear to us but it is believed that a weak inter- 413 nm which is associated with the transformation of this
Table 2 Main calculated optical transition for complexes 1, 2 and 3 with vertical excitation energies and oscillator strength in acetonitrile
Electronic
transition
Excitation
energy (eV)
Oscillator
strength ( f )
λexp in nm
−1
Compound
Composition
CI
Assign
(ε in M cm−1
)
1
1
S
S
S
S
S
S
S
S
0
0
0
0
0
0
0
0
→ S
→ S
→ S68
→ S
→ S
→ S
3
8
H−2 → L
H−1 → L+1
H−3 → L+5
H → L
H → L+1
H−3 → L
H−3 → L+5
H → L+3
2.7780 (446 nm)
3.7070 (334 nm)
5.4515 (227 nm)
2.2722 (545 nm)
3.1538 (393 nm)
3.2471 (381 nm)
4.7941 (258 nm)
3.1209 (397 nm)
0.0585
0.0085
0.0652
0.1470
0.0264
0.0969
0.0132
0.3936
0.6965
0.6240
0.4371
0.6981
0.6787
0.6806
0.4522
0.61660
MLCT
426 (10 068)
336 (6042)
225 (53 000)
507 (5000)
372 (7100)
1
MLCT
1
1
MLCT/ ILCT
1
1
1
1
1
1
2
3
1
4
5
MLCT/ ILCT
1
MLCT/ ILCT
ILCT
1
→ S35
→ S
MLCT/ ILCT
258 (31 000)
386 (9158)
1
4
ILCT/ MLCT
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2 ∞
Fig. 6 Time evolution of the UV-Vis spectra of 1 with Zn(OAc) at room temperature. Inset: A ratio-metric plot of A387/A523 (left); plot of ln[(A − A )/
(A − A )] vs. time (monitored at 387 nm) (right).
0 ∞
species into complex 3, with advancement of time. The plot of charge transfer (ICT) band. A plot of the absorbance ratio at
2
A387/A523 vs. time (s) gives a straight line having a R value of 606 and 426 nm (A606/A426
0
) vs. the concentration of
2
.99 (see Fig. 6) which corroborates with the gradual decay of cyanide clearly indicates that A606/A426 increases linearly R =
the intermediate with respect to the time. The reaction is 0.95. The blue colour is stable in the case of F , OAc
found to be a pseudo first order reaction. Hence, the calcu- and OH .
−
−
−
−
3
−1
But in the case of CN− ions, the blue solution gradually
changes to greenish yellow with time. With advancement of
time the absorption maximum at 606 nm decreased gradually,
on the other hand an enhancement of intensity at 390 nm
lated rate constant of the formation of 3 is 1.1 × 10
s .
Selective response towards cyanide
The selective sensing of anions is highly significant because
anions are widely used and play important roles in both
environmental and life sciences. In this connection, the anion
sensing properties of the ligand as well as the complexes were
studied with various anions. In the present study we have
found that complex 1 exhibits sensing activity for the cyanide
ion only while that of complexes 2, 3 and the ligand, HL exhi-
bits no sensing activity (Fig. S16†). The sensing activity was
monitored with the help of UV-Vis and luminescence spectral
methods.
(
Fig. 7) was observed. The absorption band at 390 nm is pre-
sumably attributed to the MLCT character. This unique feature
can be attributed to the involvement of the classical “Strecker”
2
0
reaction, where an aldimine (HCvN) carbon of 1 is attacked
by a cyanide ion. It is assumed that 1.5 equivalents of cyanide
ion first abstract the –OH proton and form HCN in the reac-
tion medium which on increase in time, undergoes the
Strecker reaction to produce a [1-CN] adduct. To check the
selectivity of the anions we have performed the reaction with
−
−
−
1
.5 equivalents of CN ion and 1.5 equivalents of F , OAc or
−
OH ions. It is to be noted that here we observed the same
spectral changes i.e. the blue solution gradually changes to
yellow with time (Fig. S17†). Thus, we can say that the bi-func-
UV-Vis spectroscopic method
The UV-Vis spectra of 1 in CH
at wavelengths of 426 nm and 336 nm at room temperature
Table 2). Investigation of the spectral behaviour of 1 (20 μM)
is carried out in CH CN solution in the presence of various
3
CN show two absorption peaks
tional ligand possesses superior selectivity to detect the
cyanide ion over the reported system bearing [Re(CO) ] .
3
(
+ 16a,f
3
So, the choice of such ligands helps us to achieve our goal to
generate metalloreceptors containing rhenium(I) which can
detect exclusively the cyanide ion.
−
−
−
−
−
−
−
anions such as Cl , Br , I , NO
3
, F , OAc , and OH . The
orange colour solution of 1 changes to blue upon addition of
F , OAc , OH and CN in which a new band appears at
−
−
−
−
6
4
06 nm and a blue shift of 426 nm band is observed at
08 nm. However, no such changes are observed in the case of
Emission spectroscopy
−
−
−
−
−
Cl , Br , I , and NO
3
ions. Upon gradual addition of 1.5 Upon gradual addition of CN ions to the acetonitrile solution
equivalents of X− (F , OAc , OH or CN ) ions in the aceto- of a sensor (c = 2 × 10 M), the emission intensity increases
nitrile solution of complex 1 (c = 20 μM), the intensity of the by 272 fold at 470 nm. The Stokes shift of emission maxima is
band at 606 nm increases while that of at 408 nm decreases 90 nm. Initially, 1 shows very low phosphorescence upon exci-
−
−
−
−
−5
−
with a generation of an isosbestic point at 492 nm (Fig. 7). The tation at 380 nm. Upon increasing the concentration of CN
new band at 606 nm can be attributed to the intramolecular (0–200 μM) to a solution of 1 (20 μM), the intensity of emission
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Fig. 7 Absorption titration of 1 (c = 2 × 10− M) with CN− up to 1.5 equivalents (c = 2 × 10−3 M) in acetonitrile and the inset shows the plot of A606
A
5
/
426 vs. the concentration of CN− (left); time evolution absorption spectra of the solution containing 1 and 1.5 equivalents of CN in acetonitrile at
−
room temperature (right).
increases periodically at 470 nm (Fig. 8). Further addition of 0.050 μL i.e. 5.0 × 10− M, which is higher than the reported
8
−
16a,f
+
CN does not influence the emission intensity.
complexes
3
bearing a [Re(CO) ] core.
A competitive experiment was also carried out in a aceto-
The weak luminescence of rhenium(I) complexes is con-
nected with the internal charge transfer (ICT) processes in the
−
nitrile solution of 1 in the presence of 10 equivalents of CN
−
−
−
−
and 10 equivalents of various other anions F , OAc , OH , Cl , excited state. In 1, the π-electron clouds of the ligand moiety
Br , I and NO3− to further explore the utility of complex 1 as are linked through the ketimine and aldimine bonds (vide
−
−
−
an ion-selective chemosensor for CN (Fig. 9). The emission supra). This conjugation facilitates the internal charge transfer
−
spectrum of 1 with CN was not influenced by the subsequent (ICT) process in the pure complex as well as in its deproto-
addition of competing anions (Fig. 9). This result indicates nated form of a free –OH group. The ICT process in the excited
that 1 can selectively detect the cyanide ion.
state favours the non-radiative decay process in the system
The detection limit was estimated from the luminescence which corroborates with the weak luminescence properties of
titration. The plot of emission intensity vs. the CN− concen- the rhenium(I) complex in the presence or absence of F ,
−
−
−
−
−
−
−
tration monitored at 471 nm shows a good linear relation- OAc , OH , Cl , Br , I and NO
3
ions. However, the addition
2
1
ship (Fig. 9). We have calculated the detection limit on the of cyanide ions into the acetonitrile solution of 1 probably
basis of 3σ/k, where σ is the standard deviation of the blank stops the ICT process through the breaking of the aldimine
measurement. The calculated detection limit is found to be bond by the cyanide ion and hence enhances the emission
intensity (Scheme 4).
2
2
At room temperature in the solution state, the complexes
are very weak emitters when complex 1 was excited at 426 nm
and complex 2 was excited at 507 nm. The quantum yield of 1,
2
and 1-CN was found to be 0.006, 0.005 and 0.658 respect-
ively. To gain insight into the relaxation dynamics of 1-CN, the
excited state lifetime was measured in acetonitrile solution at
room temperature at 471 nm. It displays bi-exponential decay
(
see Fig. 10) having lifetimes τ
Quenching of the emission intensity of 1-CN in the pres-
ence of molecular oxygen in acetonitrile solution suggests the
1 2
= 2.6 ns and τ = 7 ns.
2
g
triplet nature of the emissive state. It is presumably assumed
that the origin of the bi-exponential decay with a short lifetime
is attributed to the existence of two closely lying triplet excited
states. A very small Stokes shift with respect to the lowest
3
energy absorption band is probably due to the IL (intraligand,
π* → π) character of the emissive state. The high luminescence
Fig. 8 Spectrofluorometric titrations of 1 (c = 2 × 10− M) with 10
5
intensity along with a strong radiative decay constant (k
r
3
=
equivalents of TBACN in acetonitrile solution. Inset: The plot of 1/ΔI vs.
7
−1
−
5.92 × 10 s ) is associated with the confinement of the IL
transition between the two fragments of the ligand moiety
1
/[CN ], where ΔI = I − I
0
(I = intensity after cyanide addition, I
0
= inten-
sity before cyanide addition).
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−
Fig. 9 Anion sensitivity of complex 1 by a spectrofluorometric method. Inset: The plot of emission intensity vs. concentration of CN (left); black
−5
bars represent the emission sensitivity of 1 (c = 2 × 10 ) towards various anions. Red bars represent the emission response measured after the
−
−
addition of CN (10 equivalents) in 1 in the presence of anions (A ) (right).
which arises after the addition of cyanide into the imine
CHvN) bond seizing the conjugation with 2-hydroxynaphthyl
moiety. This result is in good agreement with the non-radiative
(
7
−1
decay rate (knr = 3 × 10 s ) of excited 1-CN.
−
1
Confirmation of the interaction of complex 1 with CN by H
NMR, IR spectroscopy and ESI-MS studies
To evaluate the nature of the interaction of CN− with 1, ESI
1
mass, IR and H NMR spectroscopy methods were performed.
The mass spectrum of the isolated compound (1-CN adduct)
+
shows a molecular mass of 687.0701 ([M ], see Fig. S12†),
which corresponds to the formula of the [1-CN] adduct (calcu-
lated m/z 687.120). The formation of the [1-CN] adduct is
further confirmed by IR spectroscopy.
Scheme 4 Plausible mechanism for the formation of the [1-CN]
adduct.
An appearance of a new peak at 2206 cm− in IR spec-
1
troscopy clearly indicates the addition of cyanide to the ald-
imine bond, which is absent in the mother compound
1
(
Fig. S13†). H NMR titration of 1 upon gradual addition of
−
3
CN was carried out in CDCl solution to gain better insight
into the interaction between 1 and cyanide ions. It is clear
1
from the H NMR (see Fig. 11) titration plot that there is a
periodical disappearance of –OH at δ 11.2 ppm and the ald-
imine proton (H*) at δ 10.28 ppm with increasing concen-
tration of cyanide ions. On the other hand there is an appear-
2
3
ance of a new proton at δ 7.8 ppm. The disappearance of the
NMR signal at 11.2 ppm is due to the abstraction of the pheno-
lic proton. Whereas the signal at δ 10.28 ppm diminishes and
the appearance of a new peak at δ 7.8 ppm can be assigned to
the attack at the aldimine carbon atom resulting in the trans-
formation of an aldimine proton to cyanomethanamine,
[
–CH(CN)N] proton (H
a
).
The chemical shifts, IR stretching frequency and ESI-MS
Fig. 10 Changes in the time-resolved photoluminescence decay of
complexes 1-CN in CH
3
CN at room temperature.
strongly support the formation of cyanomethanamine. The
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strategy in the field of generation of metalloreceptors of
rhenium(I). In addition, these results are proved to be
helpful in the identification of the behaviour of metallore-
ceptors incorporating multifunctional ligands. Further
research aimed at the synthesis of mononuclear/polynuclear
rhenium(I) complexes using different polydentate ligands is
currently in progress.
Experimental section
Materials
Re(CO) Cl and 2,2′-dipyridyl ketone were purchased from
5
Sigma Aldrich. All solvents and chemicals are analytically
5
pure. All the reactions with Re(CO) Cl are carried out under an
argon atmosphere.
Caution! Cyanide salts are highly toxic and should be handled
with care and in very small amounts.
1
Fig. 11 H NMR titration of 1 in CDCl
3
solution in the presence of 1.25
−
equivalents of CN ions. H
a
and H
b
represent the –OH proton and ald- Physical measurements
imine proton respectively. Generation of a cyanomethanamine proton is
shown by *.
UV–Vis spectra were recorded on a Perkin–Elmer LAMBDA 25
spectrophotometer. IR spectra were obtained with a Perkin–
Elmer L-0100 spectrophotometer. H NMR spectra were
1
recorded on a Bruker FT 400 MHz spectrometer, where tetra-
methylsilane (TMS) was used as an internal reference. The H
plausible mechanism for the formation of the cyanide adducts
is proposed in Scheme 4.
1
NMR spectra of all the complexes were recorded in CDCl
solvent. The atom-numbering scheme used for H NMR was
3
1
the same as that used in the crystallography. Electrospray
ionization mass spectrometry (ESI-MS) spectra were obtained
on a Micromass Qtof YA 263 mass spectrometer. Elemental
analyses (C, H, N) were performed on a Perkin–Elmer 2400
series II elemental analyzer. The precursor salts of all the
anions used in this work such as tetra-n-butylammonium
cyanide, fluoride, chloride, bromide, iodide, acetate, nitrate
and hydroxide were purchased from Sigma-Aldrich. The
quantum yields of the complexes were determined in freeze–
pump–thaw-degassed solutions of the complexes by a relative
Conclusions
In summary, we have successfully developed a new multi-
functional ligand to generate rhenium(I) complexes of our
choice. We have synthesized N^N and N^N^O binding mono-
nuclear rhenium(I) complexes 1 and 2 respectively. The com-
pounds are characterized by single crystal X-ray crystallogra-
phy and other analytical techniques. We have performed
detailed DFT and TDDFT calculations in order to gain better
insights into their properties. The main aim of the present
work is to generate metalloreceptors by tuning the reaction
2
+
method using [Ru(bipy)3] in the same solvent as the stan-
dard. The quantum yields were calculated using eqn (1).
1
0
conditions and hence to find out their reactivity toward d
cations and various anions. The N^N coordinated species
show tremendous reactivity in the presence of specific
Astd Ir ηr2
Φr ¼ Φstd
ð1Þ
Ar Istd ηstd2
cations and anions. Complex 1 reacts with Zn(OAC)
have isolated and characterized the highly reactive α-amino
ether containing rhenium(I) complex. We have also studied unknown and standard samples (Φstd = 0.08951 (at 298 K) in
the transformation by the UV-Vis spectroscopic method. CH CN at λex = 450 nm), A and Astd (<0.1) are the solution
Complex 1 selectively reacts with cyanide ions which helps absorbances at the excitation wavelength (λ ), I and I are
2
and we
In this equation, Φ and Φ are the quantum yields of
r
std
3
r
ex
r
std
r
us to detect the cyanide ion. The cyanide ion terminates the the integrated emission intensities, and η and ηstd are the
ICT process of the metalloreceptor which enhances the refractive indices of the solvent. Experimental errors in the
luminescence properties of the system. The reasonably high reported luminescence quantum yields were about 10%. Time-
−
8
−
detection limit (5.0 × 10 M) for the CN ion proves the sen- correlated single photon counting (TCSPC) measurements
sitivity for cyanide detection. We have also performed were carried out for the luminescence decay of complexes in
detailed DFT and TDDFT calculations to characterize the acetonitrile. For TCSPC measurements, the photoexcitation
generated species upon addition of cyanide ions. The calcu- was made at 370 nm using a picosecond diode laser (DD-370)
lation strongly supports our assumptions. The studies pre- in a Delta flex modular fluorescence lifetime system apparatus.
sented herein provide valuable insights into the synthetic The emission decay data were collected on a Horiba PPD-900.
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Computational details
Table 3 Crystal data and structure refinement parameters for com-
plexes 1, 2 and 3
The geometrical structures of the ground-state of the selected
complexes were optimized by the DFT method with the
B3LYP exchange correlation functional approach. The geo-
metry of the complexes was fully optimized in the solution
2
4
1
2
3
2
5
Formula
C
25
H
16
4
N O
4
ClRe
C
25
H
15
N
4 4
O Re
C
26 20 4 5
H N O Re
M
r
658.08
621.06
654.67
phase without any symmetry constraints. There was good Crystal system
Monoclinic
C2/c
Tetragonal
I41/a
Monoclinic
P21/n
Space group
agreement between the theoretical and experimental struc-
tures. On the basis of the optimized ground state geometry
a/Å
b/Å
31.198 (8)
9.814(2)
17.652(4)
90
22.5847(5)
22.5847(5)
17.1723(5)
90
8.7123(2)
11.2312(3)
24.3530(6)
90
structure, the absorption properties in dichloromethane c/Å
α/°
(DCM) and methanol (MeOH) media were determined by the
2
6
β/°
γ/°
117.801(12)
90
4781(2)
8
1.829
5.235
90
90
8759.1(5)
16
1.886
5.591
1.490–27.498
296 K
90.308(2)
90
2382.90(10)
4
1.825
5.145
time-dependent density functional theory (TDDFT) approach
3
associated with the conductor-like polarizable continuum V/Å
2
7
Z
D
model (CPCM).
We computed the lowest 50 singlet–
calcd/g cm−
3
singlet transition and the results of the TD calculations were
−1
μ/mm
qualitatively very similar. The TDDFT approach had θ/°
1.476–27.498
296
1.672–25.527
296
T/K
been demonstrated to be reliable for calculating the spectra
properties of many transition metal complexes. Due to
2
8
Reflns collected
, wR
the presence of electronic correlation in the TDDFT GOF on F
B3LYP) method it can yield more accurate electronic
5474
5043
4418
R
1
2
[I > 2σ(I)] 0.0421, 0.1619
0.0255, 0.1011 0.0343–0.0834
0.789 1.042
2
1.301
(
excitation energies. Hence TDDFT had been shown to
provide a reasonable spectral feature for our complex of
investigation.
Syntheses of the ligand
HL: 2,2′-Dipyridyl ketone (1 g, 5.428 mmol), 2 ml hydrazine
hydrate and 1 drop of HCl were dissolved in 20 ml ethanol and
refluxed for 2 h. The resulting yellow coloured solution was
refluxed for 8 h. Then the solvent was evaporated under
reduced pressure. The remaining oily product was washed
with water and separated into ethyl acetate to eliminate
unreacted reactants. Then the organic part was collected over
dry Na SO and evaporated to obtain an oily product SB·NH
In the calculation, the quasi relativistic pseudo potentials
2
9
of Re atoms proposed by Hay and Wadt with 14 valence elec-
2
6
6
trons (outer-core [(5s 5p )] electrons and the (5d ) valence elec-
trons were employed, and a “double-ξ” quality basis set
LANL2DZ was adopted as the basis set for Re atoms. For H we
used the 6-31(g) basis set and the 6-31+G(d) basis set for C, N,
O, and Cl atoms for the optimization of the ground state geo-
metries. Figures showing MOs and the difference density
plots were prepared by using the Gauss View 5.1 software. All
the calculations were performed with the Gaussian 09 W soft-
2
4
2
1
(
(
7
see Scheme 1). Yield: 75%, H NMR (CDCl
d, 1H, J = 16 Hz); 8.41 (s, 1H); 7.71 (d, 1H, J = 8), 7.60 (q, 1H),
.28 (d, 1H, J = 8), 7.16–7.06 (m, 3H), 4.24 (s, 1H). The product
3
, 400 MHz): δ 8.58
3
0
31
ware package. The Gauss Sum 2.1 program was used to cal-
culate the molecular orbital contributions from groups or
atoms.
obtained from the above procedure (500 mg, 2.52 mmol) was
dissolved in 20 ml ethanol followed by the addition of
2-hydroxy naphthaldehyde (433 mg, 2.52 mmol). The solution
was then refluxed for another 6 h. It was cooled to room temp-
erature and a yellow colour precipitate was obtained. It was col-
Crystallographic studies
lected under suction filtration and washed with ethanol. Yield:
The single crystal suitable for X-ray crystallographic analysis of
the complex was obtained by slow evaporation of acetone solu-
tion of the complex. The X-ray intensity data were collected on
1
80%, H NMR (CDCl
3
, 400 MHz): δ 12.71 (s, 1H); 9.79 (s, 1H);
8.84 (d, 1H, J = 4), 8.7 (s, 1H), 8.16 (t, 2H, J = 4), 7.91 (d, 1H,
J = 4), 7.78–7.82 (m, 3H), 7.53 (t, 2H, J = 8), 7.45 (s, 1H), 7.26
d, 1H, J = 8), 7.07 (s, 1H). Anal. calcd for C22 O: C, 48.00;
H, 2.15; N, 5.60. Found: C, 48.10; H, 2.30; N, 5.73. IR (cm ):
a Bruker AXS SMART APEX CCD diffractometer (Mo K
0
6
α
, λ =
.71073 Å) at 293 K. The detector was placed at a distance of
.03 cm from the crystal. Total 606 frames were collected with
(
16 4
H N
−
1
ν (O–H): 3043; ν (imine CvN): 1622, 1603.
a scan width of 0.3° in different settings of φ. The data were
3
2
reduced in SAINTPLUS and empirical absorption correction
3
2
Complex synthesis
was applied using the SADABS package. The metal atom was
located by the Patterson method and the rest of the non-hydro-
3
fac-[Re(HL)(CO) Cl], 1. HL (97 mg, 0.276 mmol) and
gen atoms emerged from successive Fourier synthesis. The Re(CO)
5
Cl (100 mg, 0.276 mmol) were refluxed in 30 ml of
structures were refined by the full matrix least-squares pro- toluene in an oil bath for 8 h. After cooling to room tempera-
2
cedure on F . All non-hydrogen atoms were refined anisotropi- ture, the solvent was removed under reduced pressure. The
cally. All calculations were performed using the SHELXTL crude mass was dissolved in a minimum volume of benzene
3
3
V 6.14 program package. Molecular structure plots were and subjected to column chromatography on a silica gel
3
4
drawn using the Oak Ridge thermal ellipsoid plot (ORTEP).
column (60–120 mesh). A faint red band was observed and
The CCDC numbers are 1836767, 1836766 and 1836768 for 1, eluted using 20% ethyl acetate : hexane solution. A red product
2
and 3† respectively (Table 3).
was obtained with a very low yield. After that, an orange band
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was eluted using 45% ethyl acetate : hexane solution. An Science and Technology, New Delhi, India, for data collection
orange coloured solid was obtained after the removal of on the NMR facility setup (Jadavpur University) under the
solvent under reduced pressure. The product was recrystallized DST-FIST program. We also acknowledge the UGC CAS II
in dichloromethane–hexane. Slow diffusion of dichloro- program, Department of Chemistry, Jadavpur University, and
methane into hexane afforded beautiful red colored crystals. the DST-PURSE program for other facilities.
1
Yield: 60%, H NMR (CDCl
3
, 400 MHz): δ 11.206 (s, 1H); 10.28
(s, 1H), 9.16 (d, 1H, J = 8), 8.88 (d, 1H), 8.27 (d, 1H, J = 12),
7
1
.42 (t, 1H, J = 12), 7.76 (d, 1H, J = 8), 7.89 (d, 1, J = 12), 7.97 (s,
References
+
2 2
H). ESI-MS (CH Cl ): m/z 657 [M] . Anal. calcd for
C
25
H
16
N
4
O
4
Re: C, 45.42; H, 2.90; N, 8.47. Found: C, 45.50; H,
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−
1
2
.93; N, 8.60. IR (cm ): ν (CO): 2017, 1886.25, 1917.02;
−
1
ν (imine CvN): 1621.96; IRtheo (cm ): ν (CO): 2009, 1898,
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II
1
935; Epa (Re /Re couple): 1.30 V (irr).
fac-[Re(HL)(CO) ], 3. To a mixture of dichloromethane and
3
methanol, complex 1 (5 mg, 0.0076 mmol) was added followed
by Zn(II) acetate (2.78 mg, 0.0076 mmol) at 28 °C. Due to the
low solubility of 1 in methanol, dichloromethane was used. It
readily changed the colour from yellowish red to dark red. The
solution was stirred for a few minutes and then it was kept for
slow evaporation. After a few days yellow coloured crystals of
complex 3 appeared. Yield: 70%; ESI-MS (CH
2
2
Cl ): m/z 655.259
+
[M + H] ; anal. calcd for C26H N O Re: C, 47.55; H, 3.38; N,
19 4 5
−
1
8
3
.53. Found: C, 47.58; H, 3.40; N, 8.59; IR (cm ): ν (O–H):
375.69; ν (CO): 2005.46 (2 py), 1878.99; ν (methoxy): 1190;
−
1
IRtheo (cm ): ν (CO): 2015, 1906.
fac-[Re(L)(CO) ], 2. Re(CO) Cl (100 mg, 0.276 mmol), HL
3
5
(70 mg, 0.276 mmol) and triethylamine (0.039 mL, 0.28 mmol)
were taken in 30 ml toluene and then the resulting mixture
was refluxed for 10 h. After cooling to room temperature, the
solvent was removed under reduced pressure. The crude mass
was dissolved in a minimum volume of benzene and subjected
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to column chromatography on
a
silica gel column
(60–120 mesh). A red band was eluted using 45% ethyl acetate :
hexane solution. A red coloured solid was obtained after the
removal of the solvent under reduced pressure. The product
on recrystallization from dichloromethane–hexane afforded
red coloured crystals. Now the red product was obtained with
1
high yield. Yield: 60%, H NMR (CDCl , 300 MHz): δ 9.18 (t,
3
1
7
6
2
H, J = 3), 8.70 (t, 1H, J = 3), 8.61 (s, 1H), 8.26 (d, 1H, J = 6),
.9–7.45 (m, 9H), 6.95 (d, 1H, J = 9). ESI-MS (CH Cl ): m/z
23.007 [M + H] . Anal. calcd for C H N O Re: C, 48.00; H,
2
2
+
2
5
15 4 4
−
1
.15; N, 5.60. Found: C, 48.10; H, 2.30; N, 5.73. IR (cm ):
−
1
ν (CO): 2011, 1906, 1882; ν (CvN): 1620, IRtheo (cm ): ν (CO):
I
II
2
030, 1917, 1892; Epa (Re /Re couple): 1.48 V (irr).
Conflicts of interest
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Acknowledgements
Financial assistance received from the Department of Science
and Technology, Govt. of West Bengal, Kolkata, India is
acknowledged. We are also thankful to the Department of
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