Synthesis, characterization and DFT studies of complexes bearing [Re(CO)3]+ core and reactivity towards cyanide ion

Article abstract of DOI:10.1016/j.jorganchem.2019.121098

Mononuclear rhenium(I) complexes having fac-[Re(CO)₃]⁺ core of general formula fac-[Re(CO)₃(L)Cl] have been synthesized in excellent yield by reacting [Re(CO)₅Cl] with L₁ and L₂in a ratio of 1:1 in toluene under inert atmosphere. Here L₁ and L₂ are N-((quinolin-2-yl)methylene)-9H-fluoren-2-amine and N-((anthracen-10-yl)methylene)quinolin-8-amine respectively. Spectroscopic measurements such as NMR, ESI-MS and IR spectroscopy were used to ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)₃(L₁)Cl] and fac-[Re(CO)₃(L₂)Cl] were confirmed by single-crystal X-ray diffraction. The ligands are emissive whereas the metal complexes are weak emitter. A remarkable change in absorption as well as emission behavior was observed in complex 2 upon addition of cyanide ion. Interruption in intraligand charge transfer in complex 2 is the probable reason for the obvious change in spectral response of the same. ¹H NMR titration was performed as the evidence of the previously stated fact. The ground and excited-state geometries, absorption properties of rhenium(I) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and analysis reveal the nature of excitations.

Full text of DOI:10.1016/j.jorganchem.2019.121098

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Synthesis, characterization and DFT studies of complexes bearing [Re(CO) ] core
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and reactivity towards cyanide ion
Tapashi Das, Kajal Krishna Rajak
PII:
S0022-328X(19)30541-8
DOI:
https://doi.org/10.1016/j.jorganchem.2019.121098
JOM 121098
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 August 2019
Revised Date: 17 December 2019
Accepted Date: 22 December 2019
Please cite this article as: T. Das, K.K. Rajak, Synthesis, characterization and DFT studies of complexes
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Journal of Organometallic Chemistry
2019
Synthesis, Characterization and DFT studies of
Complexes Bearing [Re(CO)3]+ Core and Reactivity
Towards Cyanide ion
Tapashi Das, Kajal Krishna Rajak
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata, 700032,
India. Tel.: +91 33 2457 2780; fax: +91 33 2414 6223.
E-mail: kajalrajak@gmail.com /kajalrajak@rediffmail.com
1

Abstract:
Mononuclear rhenium(I) complexes having fac-[Re(CO)3]+ core of general formula fac-
[Re(CO)3(L)Cl] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with L1 and
L2in a ratio of 1:1 in toluene under inert atmosphere. Here L1 and L2 are N-((quinolin-2-
yl)methylene)-9H-fluoren-2-amine
and
N-((anthracen-10-yl)methylene)quinolin-8-amine
respectively. Spectroscopic measurements such as NMR, ESI-MS and IR spectroscopy were
used to ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)3(L1)Cl]
and fac-[Re(CO)3(L2)Cl] were confirmed by single-crystal X-ray diffraction. The ligands are
emissive whereas the metal complexes are weak emitter. A remarkable change in absorption as
well as emission behavior was observed in complex 2 upon addition of cyanide ion. Interruption
in intraligand charge transfer in complex 2 is the probable reason for the obvious change in
spectral response of the same. 1H NMR titration was performed as the evidence of the previously
stated fact. The ground and excited-state geometries, absorption properties of rhenium(I)
complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO)
and analysis reveal the nature of excitations.
Keywords: Schiff base, rhenium(I), spectroscopy, anion sensor, DFT
2

1. Introduction
Rhenium tricarbonylchloro complexes with N^N coordinating Schiff base ligands having d6
electronic configuration is one of the widely studied area in coordination chemistry due to its
interesting photophysical properties [1-2] such as room-temperature luminescence from low-
lying tunable metal to ligand charge transfer (MLCT) excited states [3-9], solar-energy
conversion [10-12], OLED and sensor development [13-18], and biological labeling [19-20].
These imine-based rhenium complexes having [Re(CO)3]+ core show interesting optical behavior
in presence of toxic anions. It is possible to create a highly potential receptor molecule by
varying ligand architecture with the photoactive rhenium(I) metal center. A few groups of
researchers have shown the sensing properties of rhenium(I) complex by hydrogen bonding
(NH···X) followed by deprotonation [21-25]. Anion sensing mechanism by deprotonation of O-
H bond has also been reported [26]. But, anion recognition through the addition of an anion into
the imine bond is rare. Cyanide ion is a very toxic anion which comes into environment through
its use in pharmaceutical industries, insecticides, fertilizers, plastic manufacturing processes,
extraction of metals, electroplating industries etc [27-30]. It is necessary to replace the
conventional methods of testing for cyanide such as titration, distillation, chromatography or
potentiometry as they are expensive, laborious and time consuming. Therefore, there is still a
need to develop more robust and less laborious testing methodologies for cyanide. In this
context, the complexes of rhenium(I) are suitable. It has proven that Re(CO)3]+ core potentially
influence the imine based complex to detect cyanide ion selectively among other anions [31]. So,
the main focus of this paper is the successful synthesis of two rhenium complexes with N^N
coordinating two different Schiff base ligands. We have also described the details of synthetic
procedures of these mononuclear complexes and the characterization is done by various
spectroscopic techniques and single crystal X-Ray diffraction study. The reactivity of these
complexes towards various anions has also been studied. To describe the ground and excited
state behavior of the Re(I)-diimine complexes, the contribution of the fundamental
computational chemistry such as DFT/TDDFT [32] and NTO analysis is indispensable to get
better insight into the geometry, electronic structure and optical properties, TDDFT calculations
of several excited states have been performed.
3

2. Experimental Section
2.1. Materials
Re(CO)5Cl,
2-Aminofluorene,
9-Anthraldehyde,
8-Aminoquinoline,
Quinoline-2-
carboxaldehyde were purchased from Sigma Aldrich. All solvents and chemicals are analytically
pure. All the reactions with Re(CO)5Cl were carried out under argon atmosphere.
2.2. Physical measurements
Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Micromass
QTOF YA 263 mass spectrometer. IR spectra were observed and obtained with a Perkin–Elmer
1
L-0100 spectrophotometer with KBr disk. H NMR spectra were measured on Bruker FT 500
MHz spectrometer in CDCl3and DMSO solvent. By Perkin–Elmer LAMBDA 25
spectrophotometer UV–Vis spectra were investigated. The emission data were collected on a
Horiba FluroMax-4 fluorescence spectrometer. Quantum yields of the complexes were
determined in freeze-pump-thaw-degassed solutions of the complexes using quinine sulfate in
the same solvent as the standard [Φstd= 0.54 (at 298 K) in 0.1M H2SO4 at λex= 350 nm] by usual
method. The quantum yields were calculated using eq 1,
2
Astd
Ar
Ir
ηr
(1)
Φr = Φstd
2
Istd
ηstd
Where, Φr and Φstd are the quantum yields of unknown and standard samples, Ar and Astd (<0.1)
are the solution absorbance’s at the excitation wavelength (λex), Ir and Istd are the integrated
emission intensities, and ηr and ηstd are the refractive indices of the solvent. Time–correlated
single–photon–counting (TCSPC) measurements were carried out for the luminescence decay of
complexes in dichloromethane. On a Hamamatsu MCP photomultiplier (R3809) and were
analyzed by using IBH DAS6 software, the fluorescence decay data were investigated.
2.3. Computational details
The geometrical structures of the ground-state of the selected complexes were optimized by the
DFT method with B3LYP exchange correlation functional approach [33-34]. The geometry of
the complexes was fully optimized in solution phase without any symmetry constraints. There
was a good agreement between the theoretical and experimental structures. On the basis of the
optimized ground state geometry structure, the absorption property in dichloromethane (DCM)
4

and methanol (MeOH) media was calculated by time-dependent density functional theory
(TDDFT) approaches associated with the conductor-like polarisable continuum model (CPCM)
[35]. We computed the lowest 50 singlet – singlet transition and results of the TD calculations
were qualitatively very similar. The TDDFT approach had been demonstrated to be reliable for
calculating spectra properties of many transition metal complexes [36-37]. Due to the presence
of electronic correlation in the TDDFT (B3LYP) method it can yield more accurate electronic
excitation energies. Hence TDDFT had been shown to provide a reasonable spectral feature for
our complex of investigation.
In the calculation, the quasi relativistic pseudo potentials of Re atoms proposed by Hay and Wadt
with 14 valence electrons (outer-core [(5s25p6)] electrons and the (5d6) valence electrons were
employed, and a “double-ξ” quality basis set LANL2DZ was adopted as the basis set for Re
atoms. For H we used 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 geometries. 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 09W software package [38]. Gauss Sum 2.1 program [39] was used to
calculate the molecular orbital contributions from groups or atoms.
2.4. Crystallographic studies
The single crystal suitable for X-ray crystallographic analysis of the complex was obtained by
slow evaporation of acetone solution of the complex. The X-ray intensity data were collected on
Bruker AXS SMART APEX CCD diffractometer (Mo Kα, λ = 0.71073 Ǻ) at 293 K. The
detector was placed at a distance 6.03 cm from the crystal. Total 606 frames were collected with
a scan width of 0.3˚ in different settings of φ. The data were reduced in SAINTPLUS [40] and
empirical absorption correction was applied using the SADABS package [41]. Metal atom was
located by Patterson Method and the rest of the non-hydrogen atoms were emerged from
successive Fourier synthesis. The structures were refined by full matrix least-square procedure
on F2. All non-hydrogen atoms were refined anisotropically. Two large electron density value
near the rhenium metal centre (_refine_diff_density_max 9.428 for Q1) and
(_refine_diff_density_max 7.11 for Q2) was located at the end of the refinement cycle. However,
the connectivity and other broad structural features of the complex were refined to a reasonable
degree with respect to data quality and are undoubtedly correct. All calculations were performed
using the SHELXTL V 6.14 program package [42]. Molecular structure plots were drawn using
5

the Oak Ridge thermal ellipsoid plot (ORTEP) [43]. The CCDC numbers are 1921556 and
1921582 for complexes 1 and 2 respectively. Relevant crystal data is given in Table 1.
Table 1Crystal data and structure refinement parameters for complexes 1 and 2
1
2
Formula
Mr
C26H16ClN2O3Re
626.06
C28H18Cl3N2O3Re
722.99
Crystal system
Space group
a / Å
Triclinic
P -1
Triclinic
P -1
6.9438(7)
9.0741(8)
18.6879(17)
99.723(3)
98.947(3)
100.024(3)
1121.94(18)
2
10.0610(17)
10.9620(19)
13.023(2)
86.860(13)
77.069(13)
69.510(12)
1310.9(4)
2
b / Å
c / Å
α
β
γ
/°
/°
/°
V /Å3
Z
Dcalcd /g cm−3
1.853
1.832
/mm−1
5.566
4.974
µ
2.253-27.482
273 K
1.521-26.855
296
θ
/°
T /K
Reflns collected
5133
5387
R1,a wR2b [I > 0.1052, 0.2766
(I)]
GOF on F2
0.0680, 0.1797
2
σ
1.117
1.032
2
2 2
aR1 = Σ Fo - Fc / Σ Fo . b wR2 = [Σ[w(Fo - Fc2)2] / Σ[w(Fo ) ]]1/2
6

2.5. Syntheses of Ligand
(9H-Fluoren-2-yl)-quinolin-2-ylmethylene-amine (L1): 2-Aminofluorene (500 mg, 2.75 mmol)
was dissolved in 20 ml ethanol followed by addition of 2-quinolinecarboxaldehyde (433.15 mg,
2.75 mmol). The resultant mixture was stirred for 2 hours. A yellow precipitate was obtained and
it was collected under suction filtration and washed with ethanol. Yield: 70 %, 1H NMR (CDCl3,
300 MHz): δ 8.86 (s, 1H); 8.35 (d, 1H, J = 15); 8.17 (dd, 1H, J= 9), 8.67 (d, 1H, J = 9), 8.43 (d,
1H, J = 9), 8.30 (d, 1H, J= 8), 8.17 (t, 1H, J = 12), 7.94 (t, 1H, J = 8), 7.6-7.3 (Ar, 5H), 3.91(9H
of fluorine, 2H).13C NMR {(CDCl3, 500 MHz, δ (ppm)}: 193.05, 150-110; Anal. calcd for
C23H16N2 : C, 86.22, H, 5.03, N, 8.74. Found: C, 86.68, H, 5.44, N, 8.62; ESI-MS (CH2Cl2): m/z
321.159[M+H]+. IR (cm-1): ν (imine C=N): 1614.
3-Ethylidene-2-methylene-2,3-dihydro-naphthalen-1-ylmethylene)-quinolin-8-yl-amine
(L2): 9-Anthraldehyde (500 mg, 2.25 mmol) was dissolved in 20 ml ethanol followed by addition
of 8-Aminoquinoline (396.46 mg, 2.25 mmol). The resultant mixture was refluxed for 2 h with
the addition of 1 drop of acetic acid. A yellow precipitate was obtained and it was collected
1
under suction filtration and washed with ethanol. Yield: 80%, H NMR (CDCl3, 500 MHz): δ
8.88(s, 1H); 8.39 (d, 1H, J=10); 8.23 (q, 2H, J=10), 7.85-7.74 (m, 4H), 7.60 (q, 3H, J=10), 7.38 (t,
1H, J=5). 13C NMR {(CDCl3, 500 MHz, δ (ppm)}: 160-111; Anal. calcd for C24H16N2 : C, 86.72,
H, 4.85, N, 8.43. Found: C, 86.49, H, 4.46, N, 8.28; ESI-MS (CH3CN): m/z 333.190 [M+H]+; IR
(cm-1): ν (imine C=N): 1610.
2.6. Complex Synthesis
fac-[Re(L1)(CO)3Cl],1. Re(CO)5Cl (50mg, 0.147mmol), L1 (47.09 mg,0.147 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. A red coloured solid was obtained.
The product on recrystallization from dichloromethane–hexane afforded red coloured crystals.
Yield: 60%, 1H NMR (DMSO-d6, 500 MHz): δ 9.7 (s, 1H); 9.03 (d, 1H, J=10); 8.71(d, 1H, J=10),
8.4 (d, 1H, J=10), 8.33 (d, 1H, J=10), 8.19 (q, 2H, J=10), 7.99(t, 2H, J=10), 4.09 (s, 2H).13C
NMR {(DMSO-d6, 500 MHz, δ (ppm)}: 196, 206 (CO), 171-118; Anal. calcd for
C26H16N2O3ClRe : C, 49.88, H, 2.58, N, 4.47. Found: C, 49.58, H, 2.17, N, 4.42; ESI-MS
(CH2Cl2): m/z 642.15 [M+H]+. IR (cm-1): ν (CO): 1881, 1920 and 2019.
7

fac-[Re(L2)(CO)3Cl] 2. Re(CO)5Cl (50mg, 0.147mmol), L2 (48.86 mg, 0.147 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. A red colored solid was obtained.
The product on recrystallization from dichloromethane–hexane afforded red colored crystals.
Yield: 68%, 1H NMR (CDCl3, 500 MHz): δ 10.28 (s, 1H); 9.36(d, 1H, J=5), 8.70(s, 1H), 8.60(s,
13
1H), 8.26-7.45(m, 3H); C NMR {(CDCl3, 500 MHz, δ (ppm)}: 196, 198, 207 (CO), 173-120;
Anal. calcd for C27H16N2O3ClRe : C, 50.82, H, 2.53, N, 4.39. Found: C, 50.67, H, 2.08, N, 4.30;
ESI-MS (CH2Cl2): m/z 661.10 [M+Na]+. IR (cm-1): ν (CO): 1879, 1919 and 2013.
3. Result and discussion
3.1. Synthesis
The Schiff base ligands L1 and L2 are synthesized according to the conventional method by the
condensation reaction of an aldehyde and amine (Scheme 1) and are used as neutral bidentate N,
N donor ligands towards Re(I) metal center. These complexes consist of two different
fluorophore segments i.e. 2-Aminofluorene and 9-Anthraldehyde, which are able to show
interesting photophysical properties. It is to be noted that the choice of such ligands helps us to
achieve our goal in the context of synthesis of mononuclear rhenium(I) complexes with
interesting optical properties.
NH2
N
reflux
N
in ethanol
+
N
O
L1
O
N
reflux
+
N
in ethanol
N
NH2
L2
Scheme1. Schematic representation of the synthesis of ligands L1 and L2
8

The stoichiometric reaction of [Re(CO)5Cl] with ligands L1 and L2in 1:1 ratio in boiling toluene
under argon atmosphere afforded red colored complexes of composition [Re(CO)3(L1)Cl]
and[Re(CO)3(L2)Cl] respectively in excellent yields (Scheme 2). The recrystallization of these
complexes from dichloromethane-hexane layer provided red crystals. X-ray crystallographic
study confirms the formation of the complexes 1 and 2.
N
N
Cl
Re(CO)5Cl
N
N
OC
OC
reflux in toluene
CO
1
L1
N
N
CO
CO
CO
Re(CO)5Cl
N
N
Re
Cl
reflux in toluene
L2
2
Scheme 2. Schematic representation of the synthesis of complexes 1 and 2
3.2. Characterization
The ligands L1, L2 and the corresponding complexes [Re(CO)3(L1)Cl] and [Re(CO)3(L2)Cl] were
1
characterized satisfactorily by IR, ESI-MS and H NMR spectroscopy. The IR spectra of the
complexes exhibited the characteristic metal carbonyl stretching frequency in the range of 1880-
2023 cm-1, which is due to the presence of fac-[Re(CO)3]+ core having C3v symmetry. The IR
data of all these compounds are given in the experimental section. The comparison between the
IR stretching frequencies of ligands and corresponding complexes has shown in Fig. (S13 &
S14).
1
The H NMR spectra of the complexes and ligands was recorded in CDCl3, DMSO-d6. The
complexes are diamagnetic and display well resolved NMR spectra in CDCl3 and DMSO-d6
solution and the spectral data are given in the experimental section. The assignment of NMR
peaks is based on the intensity and spin-spin splitting pattern. A singlet due to azomethine
9

hydrogen atom observed at ~ 10 ppm in both the complexes. The active methylene hydrogens (-
CH2) of complex 1is observed at 4.091 and in ligand L1 it is observed at 3.91 ppm which depicts
the downfield shift of methylene proton due to complexation. The 1H NMR spectral features for
the aromatic protons of the ligands and complexes matched well with the composition and
structure.
3.3. Crystal structure description
The molecular structures of the complexes fac-[Re(CO)3(L1)Cl], 1 and fac-[Re(CO)3(L2)Cl], 2
were determined by single-crystal X-ray diffractometer. Both the complexes crystallize in the P -
1space group. The selected bond distances and bond angles for 1 and 2 are listed in Table S3,
and the molecular views are shown in Fig. 1. In these two complexes, ligands behave as N, N
coordinating neutral ligand. Here, the geometry around the Re(I) metal center is distorted
octahedral. It can be characterized by C26-Re-N2 bond angle (167.5) in 1 and C27-Re-N1 bond
angle (174.7) in 2. The Re-Cl bond is relatively longer (~ 0.3 Å) than other Re-N and Re-C
bonds. N1-Re-N2 bond angles are 74.3˚ and 75.5˚ for 1 and 2 respectively, which gives rise to
cyclic five membered ring in both complexes. The carbonyl ligands are arranged in a facial mode
and the remaining equatorial sites are occupied by pyridyl nitrogen and nitrogen coming from
other part of the ligand moiety. The remaining axial site is occupied by a chlorine atom.
Figure 1. ORTEP plot of complex 1 (left) and complex 2 (right) showing essential numbering
and H atoms are omitted for clarity. Solvent (dichloromethane) is omitted in complex 2.
10

3.4. Photophysical study of ligand
The absorption spectra of the ligands L1 and L2 were recorded in acetonitrile at room
temperature (given in Fig 2). L1 and L2 show absorption bands of lowest energy at 361 and 390
nm respectively which are attributed to the n→π* transition. The higher energy absorptions were
observed in UV region at 272 nm and 257 nm for L1 and L2 respectively. The higher energy
absorption of these ligands arises in the UV region at 272 nm and 257 nm respectively. These
bands can be reasonably assigned as an admixture of π→π* transitions and weaker n→π*
transition which are submerged under a stronger π→π*transition.
Figure 2. Absorption spectra of L1 and L2in acetonitrile keeping the concentration (c = 2×10-5
M).
To get better agreement with the experimental photophysical values, TDDFT calculations were
performed for the ligands L1 and L2on the basis of the optimized geometry both in the singlet
ground state (S0) and excited state (S1) in acetronitrile solvent. The absorption energies
associated with their oscillator strengths, the main configurations and their assignments
calculated using the TDDFT method and the S0 geometry for L1 and L2 is discussed here (Fig.
3). The energy difference between HOMO and LUMO of L1 and L2 is 3.3 eV and 2.8 eV
respectively. The calculated absorption bands are in good agreement with the experimental
result. These absorption bands can be assigned to intramolecular charge transfer (ICT) band with
some rearrangement of electron density from fluorene moiety to quinoline moiety in L1 and from
anthracene moiety to quinoline moiety in L2.
11

Figure 3. Frontier molecular orbitals involved in the UV–Vis absorption spectrum of ligands
L1and L2 (for clarity we considered the molecular orbital with the higher value of CI (<0.5) for
the appropriate transition).
The emission property of the ligands is measured in acetonitrile solution. Ligands L1 and L2
show emission band at 405 nm and 470 nm when excited at 360 nm and 390 nm respectively. A
relatively larger stokes shift is observed in case of L2 (80 nm) than L1 (45 nm). The emission
intensity of L2 is also greater than L1. The ligands L1 and L2 were emitters with quantum yields
0.03 and 0.11 respectively. The luminescence parameters for all the ligands are listed in Table 2.
The emission spectra of ligands are shown in Fig. 4.
12

Figure 4. Experimental emission spectra of the ligands L1 (λexcitation = 360 nm) and L2 (λexcitation =
390 nm) having concentration, c = 2 × 10-5 M in acetonitrile at room temperature.
Time-resolved luminescence spectra are proved to be an important tool to understand the decay
process and the emissive nature of the compounds. Thus time-resolved luminescence spectra
were recorded for L1, L2. The emission lifetime (τ), radiative (kr) and nonradiative (knr) decay
rate constants were collected in Table 2. The lifetime of L1 (τ1 = 0.17 ns and τ2 = 1.5 ns) is
relatively higher than that of L2 (τ1 = 0.48 ns and τ2 = 0.15 ns). All these ligands display bi-
exponential decay nature. Two closely lying singlet excited states are mainly responsible for this.
The ligands consist of two organic units linked by C=N bond, in which one part behaves as
donor and another as acceptor. While photoexcitation, electron transfer occurs from donor to
acceptor which creates charge separation resulting in generation of an opposite force that twists
the donor acceptor junction. This intramolecular twisted excited state gives rise to the formation
of TICT (twisted intramolecular charge transfer) state. The bi exponential decay of these two
ligands with very small red shifted emission depicts the existence of TICT state.
13

Table 2 Photophysical parameters of the mononuclear complexes in acetonitrile solution at room
temperature
Compound
Quantum Knr×108 s-1 Kr×107 s-1
λexcitation
λemission
τ1 ns
τ2 ns
Yield (Φ)
(nm)
(nm)
360
390
437
405
470
505
0.03
0.11
0.19
6.2
18
2.5
24
0.17
0.48
1.83
1.54
0.15
6.29
L1
L2
2-CN
1.3
3.05
3.5. Photophysical study of complex
The absorption spectra of the mononuclear complexes (1 and 2) are recorded in acetonitrile at
room temperature (given in Fig 5). As compared to the ligands, the corresponding complexes
show bathochromic shift in the longer wavelength region. This fact illustrates that the
incorporation of rhenium(I) actually responsible for the bathochromic shift. The low lying
absorption bands of these complexes ~ 400 nm are assigned as metal to ligand charge transfer
(MLCT) band [Re(dπ) → ligand π* orbital]. The higher energy absorption band with high molar
absorption value ~ 250 nmcan be assigned as the intraligand charge transfer band (ILCT) and
weaker contribution of MLCT band. Table 3 confirms the usual assignments of all the absorption
bands for complexes 1 and 2.
Figure 5. Absorption spectra of the complexes 1 and 2in acetonitrile keeping the concentration
(c = 2 × 10-5 M).
14

Table 3 Main calculated optical transition for complexes 1, 2 and 3 with vertical excitation
energies and oscillator strength in acetonitrile.
Electronic
transition
Excitation
(eV)
energy Oscillator
Strength (f)
λexp in nm (ε in
Compound
Composition
CI
Assign
M-1cm-1)
H-3→L
H-5→L
2.8284 (438 nm)
3.2150 (385 nm)
0.0515
0.0248
0.6800
0.6866
S0→S4
1MLCT
405 (26722)
327 (14725)
3.8822 (319 nm)
4.6646 (265 nm)
0.2263
0.1538
0.5165 1ILCT
S0→S10
H→L+1
H→L+5
1
S0→S25
0.4619 1MLCT/1LLCT 267 (75764)
2.8336 (437 nm)
2.9175 (424 nm)
0.0134
0.0377
0.6594 1MLCT
0.6358 1MLCT
437(5631)
S0→S2
S0→S3
H-1→L
H-2→L
2.9774 (416 nm)
3.4143 (363nm)
3.6114 (343 nm)
4.8800 (254 nm)
0.1000
0.6607 1ILCT
0.6083 1MLCT
0.5641 1ILCT
0.4158 1ILCT
385 (5429)
369 (6268)
328 (9724)
254 (43477)
S0→S4
S0→S7
S0→S10
S0→S39
H→L+1
2
0.0132
0.1543
0.8658
H-2→ L+1
H-5→L
H-5→L+2
In order to analyze the nature of absorption, we performed an NTO analysis based on the
calculated transition density matrices. This method offers the most 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). Here we refer to the
unoccupied and occupied NTOs as “electron” and “hole” transition orbitals, respectively. Note
that NTOs are not the same as virtual and occupied MO pairs from the ground state calculations.
Table 4 illustrate the natural transition orbitals (NTOs) for mononuclear complexes (1 and 2)
respectively.
Table 4 Natural transition orbitals (NTOs) for the complexes illustrating the nature of singlet
excited states. For each complexes, the respective number of the state, transition energy (eV),
and the oscillator strength (in parentheses) are listed.
15

Complex
1
Transition
Hole
Electron
Assignment
S4
MLCT
W=0.92
2.82 eV
(0.05)
S10
ILCT
W=0.94
3.21eV (0.02)
S25
W=0.42
4.66 eV
(0.15)
LLCT
S2
MLCT
MLCT
ILCT
2
W=0.86
2.83 eV
(0.03)
S3
W=0.79
2.91 eV
(0.03)
S4
W=0.87
2.97 eV (0.1)
S7
MLCT
ILCT
W=0.72
3.414 eV
(0.01)
S10
W=0.62
3.61 eV
(0.15)
S39
ILCT/LLCT
W=0.33
4.88 eV
(0.86)
Based on the TDDFT NTO analysis, it is clear that the absorption bands at longer wavelength
region of these two complexes are purely metal to ligand charge transfer band. In complex 1 the
metal charge density completely shifts over quinoline moiety and in complex 2, metal charge
16

density is delocalized throughout the anthracene and quinoline moiety in their excited states S4
and S2 respectively which in turn makes the metal center devoid of electron density.
The higher energy absorptions of complex 1 comprises of intraligand charge transfer band and
less contributing ligand to ligand charge transfer band where electron density moves from
fluorine moiety to carbonyl ligands. In case of complex 2, higher energy absorption bands are
observed due to ILCT transition with weaker MLCT character as the four excited states (S4, S7,
S10, S39) depicts that the charge density moves in between two ligand fragments and CO ligands.
The emission spectral behavior of two mononuclear rhenium complexes was recorded in
acetonitrile solution at room temperature. The complexes displayed poor luminescence property
due to the excited state quenching in presence of [Re(CO)3]+ core, which practically makes them
non-emitters.
3.6. Geometry optimization and electronic structure
Molecular structures of the complexes 1 and 2 were optimized by DFT at their electronic ground
state (S0) using LANL2DZ basis set. The optimized structures of the complexes are given in
Table S1. The optimized ground state structures of complexes 1and2 at their singlet (S0) state,
possesses distorted octahedral geometry around Re(I) metal center. The fac-[Re(CO)3]+ core is
consisted with each complexes having the angles of ~90˚ between the CO ligands. Theoretical
structural parameters are in excellent agreement with the experimental data for the complexes for
which X-ray data are available. In these complexes all the Re-C bond length occurs near 1.91 Å
and Re-N bond length occurs near 2.25 Å. The Re-Cl bond length of all the complexes is slightly
longer than the Re-C and Re-N bond lengths. All these values deviate from the experimental
parameters faintly, which is dependent on the errors originated from the environmental factors
such as crystal packing and the impact of the medium.
17

Figure 6. Partial molecular orbital diagram of the complexes 1and 2.
The partial frontier molecular orbital compositions in their singlet ground state (S0) are listed in
Table S2. The partial molecular orbital diagram of frontier molecular orbital, which are mainly
involved in the electronic transitions for mononuclear complexes 1 and 2 are shown in Fig. 6.
In the ground state (S0), the energy difference between HOMO (H) and LUMO (L) of all three
mononuclear complexes are very similar to each other, which is 2.91 eV and 2.92 eV for
complexes 1and 2 respectively. Clearly, the energy gap between HOMO and LUMO is is
comparable to each other. Each mononuclear rhenium(I) complex possesses ligand consist of
two moiety joined through imine bond. HOMO of the complexes shows high electron densities
around fluorene [FL (84%)] and anthracene [Anth (90%)] moiety for 1 and 2 respectively but, in
case of LUMO, high electron density is observed in other part of the ligands i.e. quionline
moiety (in the range of 42- 55%). In these two complexes H-1 lies in the range of 0.21-0.63 eV
18

below the respective HOMO levels and H-2 resides below H-1 by ~0.10 eV. This calculation
indicates that H-1 and H-2 levels of these complexes are almost degenerate. Metal orbital
contribution towards molecular orbitals occurs in lower energy levels. Predominant participation
of rhenium d orbitals occurs from H-2 energy level where ~45% or more contribution arises due
to the metal center. Partial involvement of π* orbital of CO contributes ~25% whereas the p
orbital of Cl contributes ~22% in each complex. The HOMO−3 and HOMO−4 are apart from
each other by 0.35 eV in 1, 0.12 eV in 2 and 0.06 eV. LUMO (L) and L+1 of all mononuclear
complexes originates from the ligand π* orbital localized on the C=N bond, quinoline moiety
and aromatic systems attached to the C=N bond contribution.
3.7. Anion sensing properties
The complexes 1, 2 and ligands L1, L2 are insoluble in water. We have checked the compatibility
of these compounds in water through UV-Vis and luminescence spectroscopy by making a stock
solution in DMSO. But, addition of cyanide ion in water medium slowly destroys the system
(Fig S16). So, the anion binding properties of these compounds have been studied by
spectrophotometric and fluorometric titration method in acetonitrile solution (20µM) at room
temperature. Tetrabutylammonium salts of OH-, F-, Cl-, OAc-, Br-, CN-, I- were used in this study.
Among all the compounds, only complex 2 responded positively towards cyanide ion. All the
other anions (OH-, F-, Cl-, OAc-, Br-, I-) remained silent upon addition of 10 equivalents (Fig. 5)
of each anion. The absorption titration curve of 2 with cyanide ion passes through several
isobestic points in UV-Vis region. These isobestic points indicate the formation of new species
1
in the solution medium. In the electronic absorption spectrum, the MLCT (dπ(Re) → L2)
absorption band at 435 nm exhibits a red shift to 495 nm with a sharp isobestic point at 467 nm
(Fig. 7) when cyanide ion concentration slowly increases. It is clearly visible that the peak at 435
nm gradually decreases with the simultaneous increment of the peak at 495 nm. It is assumed
that cyanide ion interacts with the complex by attacking the imine bond and forming a 2-CN
adduct (shown in scheme 3). The binding of cyanide ion is expected to enrich the electron
density of the nitrogen atom of imine bond attached to the anthracene unit and thus render the
1MLCT transition to occur at lower energy. A plot of the absorbance ratio at 495 and 435 nm
(A495/A435) vs concentration of cyanide clearly indicates that A435/A495 increases linearly with R2
= 0.95307.
19

Figure 7. Absorption titration of 2 (c = 2 × 10-5 M) with CN- up to 2.5 equivalent (c = 2 × 10-3
M) in acetonitrile (left); The ratiometric plot of absorbance ratio at 495 and 435 nm vs
concentration of cyanide ion (middle); Selective change in absorption in presence of various
anions (right).
We have investigated the luminescence properties of the non-emissive complexes 1 and 2 in
presence of various anions (OH-, F-, Cl-, OAc-, Br-, CN-, I-) in acetonitrile at room temperature.
Both the complexes were excited with their corresponding lowest energy absorption band and
the spectral change was monitored by varying those anions. An appreciable change was observed
for 2 in presence of cyanide ion. The non-emissive complex 2 displayed a strong emission at 505
nm (Excited at 437 nm) when 15 equivalents of cyanide ion was added to the complex solution
(Fig. 8). This observation suggests that the upcoming cyanide ion somehow can prevent the
conjugation between anthracene and quinoline unit which is likely to be observed in the ground
state picture of ligand unit in 2. Therefore, the interruption of conjugation has occurred by the
addition of cyanide ion into the imine bond attached with anthracene moiety. As a result, the ICT
(intramolecular charge transfer transition) process stopped and the fluorophore moiety becomes
free to emit. We have calculated the detection limit on the basis of 3σ/k, where σ is the standard
deviation of blank measurement. The calculated detection limit is found to be 7.0×10-7 M. LOD
(limit of detection) value indicates sensitivity of the probe towards the analyte. The lower the
value, the better is the sensitivity. Table S4 contains comparative studies on LOD of some
recently published works. Our probe has relatively higher detecting ability over the other works
while a very few are better in comparison to the present work. In order to further investigate the
binding constant of complex 2 with incoming cyanide ion, a plot of 1/∆I (where, ∆ I= I-I0, I =
intensity after cyanide addition, I0= intensity before cyanide addition) vs 1/[CN-] is carried out.
As a result, the fluorescence binding constant is found to be 4.4×10-4 M-1.
20

Figure 8. Spectrofluorometric titration of 2 (c = 2 × 10-5 M) with 10 equivalents of TBACN in
acetonitrile solution at 437 nm (left). The plot of 1/∆ I vs. 1/ [CN-], where ∆I = I – I0 (I =
intensity after cyanide addition, I0 = intensity before cyanide addition) (right).
1
For the better insight of the reaction occurring in presence of CN- ion, H NMR titration was
carried out in CD3CN solution of 2 and the change in spectra was recorded after gradual
1
increment of cyanide ion up to 1.5 equivalents. The H NMR peak of aldimine proton at 10.3
ppm disappeared slowly up to the addition of 1.5 equivalents of cyanide ion. A new peak
appeared at 7.3 ppm simultaneously which is assigned as the cyanomethanamine proton. The
aromatic protons were shifted towards slightly upfield region. This observation supports the fact
that the imine bond of 2 was affected by cyanide ion which induces different electronic
atmosphere around the aromatic region and caused the aromatic protons as well as the
1
cyanomethanamine proton to shift towards upfield region. The relevant H NMR titration is
shown in Fig. 9.
21

Figure 9. The 1H NMR titration of complex 2 in CD3CN solution in presence of 1.5 equivalents
of CN- ion. Generation of cyanomethanamine proton is shown by (*).
CN-
N
N
H
Cl
N
H
CO
CO
CN-
Cl
N
CO
CO
Re
Re
CO
CO
2
N
H
Cl
N
CO
CO
NC
Re
CO
2-CN adduct
Scheme 3 Probable mechanism for the formation of 2-CN adduct.
22

Furthermore, the time resolved luminescence spectra also give evidence to understand the decay
process and the emissive nature of the 2-CN adduct. Life time data are recorded for 2-CN adduct
at room temperature in acetonitrile solution when excited at 437 nm. The emission lifetime (τ),
radiative (kr) and nonradiative (knr) decay rate constant of 2-CN adduct were collected in Table
2. It shows a bi-exponential decay. Upon addition of CN- ion into the solution of 2 enhances the
emission intensity as well as the excited state lifetime due to the formation of 2-CN adduct. This
photoluminescence property mainly originates from the triplet state charge transfer transitions,
which is confirmed by the treatment of triplet oxygen as no quenching of emission intensity
occurs. The short Stokes shift of 2-CN with respect to the longer wavelength absorption of 2
reveals the ligand-centered 3IL nature of the T1 state of 2-CN.
4. Conclusion
In summary, we have synthesized two Schiff base based ligands (L1 and L2) and two monomeric
rhenium(I) complexes of these two ligands. The aim of the present work is to explore their
optical properties and reactivity with various anions. The complexes are characterized by
different spectroscopic techniques and X-ray crystal structure determination. This work includes
investigation of the ground and excited-state geometries of both the ligands and complexes as
well as the study of absorption and luminescence properties of two rhenium(I) complexes by
DFT and TDDFT methods. Furthermore, the complex 2 has shown a very interesting
photophysical property by selective binding with cyanide ion in acetonitrile solution. The
spectral change of complex 2 in presence of high amount of cyanide is prominent in UV-Vis
spectroscopy as well as in fluorescence spectroscopy. Proton NMR spectra confirms that the
change in spectral response is due to the attachment of cyanide ion into the imine bond of
complex 2. So, the studies presented herein provide valuable information about the synthetic
strategy of metal complexes as well the ligands. Further research aimed at the synthesis of
mononuclear/polynuclear rhenium(I) complexes with multiple important ligands is currently in
progress.
23

Acknowledgements
Financial assistance received from Department of Science and Technology, Govt. of West
Bengal, Kolkata, India. We are also thankful to the Department of Science and Technology, New
Delhi, India, for data collection on the NMR facility setup (Jadavpur University) under DST-
FIST program. We also acknowledge the UGC CAS II program, Department of Chemistry,
Jadavpur University, the DST-PURSE program and JU RUSA 2.0 for other facilities.
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