Fac-Tricarbonyl rhenium(I) complexes of 2-(alkylthio)-N-((pyridine-2-yl) methylene)benzenamine: Synthesis, spectroscopic characterization, X-ray structure and DFT calculation

Article abstract of DOI:10.1016/j.ica.2013.01.013

The paper presents a combined experimental and computational study of tricarbonyl complexes fac-[Re(CO)₃(L¹/L²)Cl] (1a/1b) (L¹ = 2-(methylthio)-N-((pyridine-2-yl)methylene)benzenamine and L² = 2-(ethy

Full text of DOI:10.1016/j.ica.2013.01.013

Inorganica Chimica Acta 399 (2013) 138–145
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fac-Tricarbonyl rhenium(I) complexes of 2-(alkylthio)-N-((pyridine-2-yl)methy-
lene)benzenamine: Synthesis, spectroscopic characterization, X-ray structure and
DFT calculation
⇑
Mahendra Sekhar Jana, Ajoy Kumar Pramanik, Subhankar Kundu, Deblina Sarkar, Tapan Kumar Mondal
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 March 2012
Received in revised form 3 January 2013
Accepted 4 January 2013
Available online 26 January 2013
The paper presents a combined experimental and computational study of tricarbonyl complexes
fac-[Re(CO)3(L1/L2)Cl] (1a/1b) (L1 = 2-(methylthio)-N-((pyridine-2-yl)methylene)benzenamine and
L2 = 2-(ethylthio)-N-((pyridine-2-yl)methylene)benzenamine). The Schiff base ligands L1 and L2 have
been synthesized and characterized by elemental and spectroscopic analysis. Along with spectral charac-
terizations, the structural confirmation by single crystal X-ray study has been done for complex 1a. The
rhenium atom adopts a distorted octahedral geometry and is coordinated by three carbonyl ligands in a
fac arrangement. The electronic structure, redox properties, absorption and emission properties of ligands
and complexes have been explained based on DFT and TDDFT calculations. The complexes show MLCT
bands at 443–448 nm and emitted at 405–410 nm upon excitation at 320–326 nm, characterized as ILCT
transitions.
Keywords:
Re(I) carbonyl complex
X-ray structure
Electrochemistry
Photophysical property
DFT calculation
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
model complexes to mimicking the spectroscopic and structural
properties of the active sites of metallo proteins [34–37]. In this
The transition metal complexes having
p
-acidic diimine
work we have synthesized the fac-[Re(CO)3(L1/L2)Cl] (1a/1b) com-
plexes with thioether containing Schiff base ligands L1 and L2
(where L1 = 2-(methylthio)-N-((pyridine-2-yl)methylene)benzen-
amine and L2 = 2-(ethylthio)-N-((pyridine-2-yl)methylene)ben-
zenamine). The characterization of fac-[Re(CO)3(L1/L2)Cl] (1a/1b)
were carried out by spectral and elemental analysis, and the struc-
ture has been confirmed by single crystal X-ray structure for com-
plex 1a. The electronic transitions, assignment of spectral bands
and redox properties have been explained based on DFT and TDDFT
calculations.
(–N@C–C@N–) functional polypyridyl ligands display exciting pho-
tochemical and photophysical properties, and have application in
many technological fields [1–4]. The excited states of these com-
pounds are often sufficiently long-lived to become engaged in en-
ergy transfer reactions. Their electroluminescent properties have
been applied in solar energy converters [5], as electroluminescent
materials in OLED-type devices [4,6–11], and, particularly as lumi-
nescent probes for long-range electron-transfer studies in proteins
and other biomolecular systems [12–14], and light-emitting elec-
tronic devices [15,16]. The photochemistry and photochemical
properties of rhenium tricarbonyl complexes with this type of li-
gands were extensively studied by D.J. Stufkens in the 90’s [17–
19]. Interest in this type of photoluminescent compounds remains
strong as evidenced by a number of recent reviews [6,20,21]. Study
of the redox properties of these compounds can yield insight into
their photophysical and photochemical properties [22–25].
2. Experimental
2.1. Materials
2-(Alkylthio)thiobenzenamine was prepared following the pub-
lished procedure [38]. Pyridine-2-carboxaldehyde, 2-aminothio-
phenol and Re(CO)5Cl were purchased from Aldrich, and used as
received. All other chemicals and solvents were of reagent grade
and were used without further purification.
Chelating ligands containing N,N,S donor atoms have unique
interest in the coordination chemistry because of the stability,
chemical and electrochemical activities and diversity in binding
to metal ions [26–33]. In recent years, several groups have used
polydentate ligands with thioether moieties for the synthesis of
2.2. Physical measurements
⇑
Microanalyses (C, H, N) were performed using a Perkin-Elmer
CHN-2400 elemental analyzer. The electronic spectra were mea-
Corresponding author. Fax: +91 033 24146584.
E-mail address: tkmondal@chemistry.jdvu.ac.in (T.K. Mondal).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.01.013

M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
139
sured on Lambda 25 Perkin Elmer spectrophotometer in acetoni-
trile solution. The IR spectra were recorded on RX-1 Perkin Elmer
spectrophotometer in the spectral range 4000–400 cmꢀ1 with the
samples in the form of KBr pellets. Luminescence property was
measured using LS-55 Perkin Elmer fluorescence spectrophotome-
ter at room temperature (298 K) in acetonitrile solution by 1 cm
path length quartz cell. Fluorescence lifetimes were measured
using a time-resolved spectrofluorometer from IBH, UK. The instru-
ment uses a picoseconds diode laser (NanoLed-03, 370 nm) as the
excitation source and works on the principle of time-correlated
single photon counting [39]. The goodness of fit was evaluated
by v2 criterion and visual inspection of the residuals of the fitted
function to the data. 1H NMR spectra were recorded in CDCl3 on
Bruker 300 MHz FT-NMR spectrometers in presence of TMS as
internal standard. Cyclic voltammetric measurements were carried
out using a CH1 Electrochemical workstation. A platinum wire
working electrode, a platinum wire auxiliary electrode and Ag/AgCl
reference electrode were used in a standard three-electrode config-
uration. [nBu4N][ClO4] was used as the supporting electrolyte and
the scan rate used was 50 mV sꢀ1 in acetonitrile under dinitrogen
atmosphere. The reported potentials are uncorrected for junction
potential.
ture was allowed to cool to room temperature. After removal of
the solvent in reduced pressure the crude product was purified
by column chromatography on neutral alumina using toluene
and acetonitrile mixture (v:v, 3:1) as eluent. The solvent mixture
was removed in reduced pressure to obtain brown solid of 1a. Yield
was 107 mg, 74%.
Complex 1b was synthesized following the same procedure as
of 1a. Yield was 69%.
Anal. Calc. for C16H12ClN2O3ReS (1a): C, 35.99; H, 2.27; N, 5.25.
Found: C, 35.85; H, 2.26; N, 5.23%. IR data (KBr, cmꢀ1): 2018, 1913
m
(CO); 1589 m
(C@N). 1H NMR data (CDCl3, ppm): 8.87 (1H, d,
J = 4.0 Hz), 8.52 (1H, d, J = 8.0), 7.97 (2H, m), 7.69 (1H, t,
J = 8.1 Hz), 7.21 (2H, m), 6.83 (2H, t, J = 8.0 Hz), 2.83 (3H, s). Epa
(ReI/ReII): 1.15 V; Epc: ꢀ1.12 V.
Anal. Calc. for C17H14ClN2O3ReS (1b): C, 37.26; H, 2.57; N, 5.11.
Found: C, 37.16; H, 2.57; N, 5.09%. IR data (KBr, cmꢀ1): 2020, 1916
m
(CO); 1595 m
(C@N). 1H NMR data (CDCl3, ppm): 8.92 (1H, d,
J = 3.8 Hz), 8.53 (1H, d, J = 8.0), 7.93 (2H, m), 7.66 (1H, t,
J = 8.0 Hz), 7.19 (2H, m), 6.80 (2H, t, J = 7.9 Hz), 3.13 (2H, q,
J = 7.6 Hz), 1.41 (3H, t, J = 7.5 Hz). Epa (ReI/ReII): 1.12 V; Epc
ꢀ1.15 V.
:
The luminescence quantum yield was determined using
carbazole as reference with a known /R of 0.42 in MeCN. The
complex and the reference dye were excited at the same
wavelength, maintaining nearly equal absorbance (ꢁ0.1), and
the emission spectra were recorded. The area of the emission
spectrum was integrated using the software available in the
instrument and the quantum yield is calculated according to the
following equation:
2.4. Computational details
Full geometry optimization was carried out using the density
functional theory method at the B3LYP level for the representative
complex 1a [40,41]. All elements except rhenium were assigned
the 6-31G(d) basis set. The SDD basis set with effective core poten-
tial was employed for the rhenium atom [42,43]. The vibrational
frequency calculation was performed to ensure that the optimized
geometry represents the local minima and there are only positive
eigenvalues. All calculations were performed with GAUSSIAN03
program package [44] with the aid of the GaussView visualization
program. Natural bond orbital analyses were performed using
the NBO 3.1 module of GAUSSIAN03. Vertical electronic excitations
based on B3LYP optimized geometry were computed using the
time-dependent density functional theory (TDDFT) formalism
[45–47] in acetonitrile using conductor-like polarizable continuum
model (CPCM) [48–50]. GaussSum [51] was used to calculate the
fractional contributions of various groups to each molecular
orbital.
/S=/R ¼ ½AS=ARꢂ ꢃ ½ðAbsÞR=ðAbsÞSꢂ ꢃ ½gS2
=
gR2
ꢂ
Here, /S and /R are the luminescence quantum yield of the sam-
ple and reference, respectively. AS and AR are the area under the
emission spectra of the sample and the reference respectively,
(Abs)S and (Abs)R are the respective optical densities of the sample
and the reference solution at the wavelength of excitation, and gS
and gR are the values of refractive index for the respective solvent
used for the sample and reference.
2.3. Synthesis of complexes
2.3.1. Preparation of ligands (L1/L2)
2-(Methylthio)benzenamine (210 mg, 0.66 mmol) and pyri-
dine-2-carboxaldehyde (72 mg, 0.67 mmol) were refluxed for
10 h in dry ethanol. The reaction mixture was cooled and solvent
was removed under reduced pressure. The gummy mass of L1 so
obtained was thoroughly washed with n-hexane. Yield was
123 mg, 82%.
2.5. Crystal structure determination and refinement
Details of crystal analysis, data collection and structure refine-
ment data for 1a is given in Table 1. Crystal mounting was done
on glass fibers with epoxy cement. Single crystal data collections
were performed with an automated Bruker SMART APEX CCD diffrac-
Ligand L2 was synthesized following the same procedure of L1.
Yield was 80%.
tometer using graphite monochromatized Mo
(k = 0.71073 Å). Reflection data were recorded using the
technique. Unit cell parameters were determined from least-
squares refinement of setting angles with in the range
1.84 6 h 6 25.00°. Out of 11918 collected data 2956 with I > 2 (I)
K
a
radiation
x
scan
Anal. Calc. for C13H12N2S (L1): C, 68.39; H, 5.30; N, 12.27. Found:
C, 68.46; H, 5.31; N, 12.30%. IR data (KBr, cmꢀ1): 1622 (C@N). 1H
m
h
NMR data (CDCl3, ppm): 8.76 (1H, d, J = 4.1 Hz), 8.40 (1H, d, J = 8.0),
7.82 (2H, m), 7.55 (1H, t, J = 8.2 Hz), 7.18 (2H, m), 6.78 (2H, t,
J = 8.0 Hz), 2.67 (3H, s).
r
were used for structure solution within hkl parameters
ꢀ9 6 h 6 9, ꢀ11 6 k 6 11, ꢀ13 6 l 6 12. The structures were
solved and refined by full-matrix least-squares techniques on F2
using the SHELXS-97 program [52]. The absorption corrections were
done by the multi-scan technique. All data were corrected for Lor-
entz and polarization effects, and the non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were generated using
SHELXL-97 [53] and their positions calculated based on the riding
mode with thermal parameters equal to 1.2 times that of the asso-
ciated C atoms, and participated in the calculation of the final R-
indices.
Anal. Calc. for C14H14N2S (L2): C, 69.39; H, 5.82; N, 11.56. Found:
C, 69.44; H, 5.81; N, 11.58%. IR data (KBr, cmꢀ1): 1625 (C@N). 1H
m
NMR data (CDCl3, ppm): 8.70 (1H, d, J = 3.8 Hz), 8.38 (1H, d, J = 7.9),
7.85 (2H, m), 7.51 (1H, t, J = 8.1 Hz), 7.16 (2H, m), 6.71 (2H, t,
J = 8.2 Hz), 2.98 (2H, q, J = 7.4 Hz), 1.37 (3H, t, J = 7.4 Hz).
2.3.2. Preparation of [Re(CO)3(L1/L2)Cl] (1a/1b)
Re(CO)5Cl (100 mg, 0.27 mmol) and L1 (63 mg, 0.28 mmol) in
toluene (50 ml) were refluxed for 4 h. The resulting reaction mix-

140
M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
Table 3
Some selected bond distances (Å) and angles (°) of 1a.
CO
Bond distance (Å)
X-ray
Calc.
N
N
N
N
CO
CO
Re(1)–Cl(1)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–C(14)
Re(1)–C(15)
Re(1)–C(16)
C(14)–O(1)
C(15)–O(2)
2.457(4)
2.149(10)
2.164(12)
1.904(17)
1.925(16)
1.901(19)
1.110(18)
1.120(18)
1.13(2)
2.499
2.196
2.210
1.913
1.932
1.926
1.158
1.149
1.152
Re
Cl
S
S
R
R
C(16)–O(3)
Angles (°)
N(1)–Re(1)–Cl(1)
N(1)–Re(1)–N(2)
N(1)–Re(1)–C(14)
N(1)–Re(1)–C(15)
N(1)–Re(1)–C(16)
N(2)–Re(1)–Cl(1)
N(2)–Re(1)–C(14)
N(2)–Re(1)–C(15)
N(2)–Re(1)–C(16)
C(14)–Re(1)–Cl(1)
C(14)–Re(1)–C(15)
C(14)–Re(1)–C(16)
C(15)–Re(1)–Cl(1)
C(15)–Re(1)–C(16)
Re(1)–C(14)–O(1)
Re(1)–C(15)–O(2)
Re(1)–C(16)–O(3)
84.1(3)
73.1(4)
94.6(5)
171.2(5)
100.5(6)
81.3(3)
96.8(5)
98.6(6)
171.9(6)
178.0(4)
89.4(6)
88.5(6)
91.7(4)
82.79
74.41
94.55
171.1
98.05
83.54
94.29
98.49
170.9
177.3
91.17
90.06
91.43
89.06
179.6
179.0
177.8
2-(Alkylthio)-N-((pyridin-2-yl)
methylene)benzenamine
Where, R = Me (L1) and Et (L2)
fac-[Re(CO)3(L1/L2)Cl] (1a/1b)
Scheme 1. Structure of ligands (L1/L2) and fac-[Re(CO)3(L1/L2)Cl] (1a/1b)
complexes.
Table 1
87.5(7)
Crystallographic data and refinement parameters for 1a.
177.1(14)
179.5(16)
177.3(14)
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C16H12ClN2O3ReS
533.99
triclinic
ꢀ
P1
8.1877(2)
9.9841(3)
11.1123(3)
84.575(2)
86.105(2)
70.027(2)
849.32(4)
2
b (Å)
c (Å)
3. Results and discussion
a
(°)
3.1. Synthesis and formulation
b (°)
c
(°)
V (Å3)
The fac-[Re(CO)3(L1/L2)Cl] (1a/1b) complexes were prepared by
the reaction of Re(CO)5Cl with Schiff base ligands L1 and L2 (where,
L1 = 2-(methylthio)-N-((pyridine-2-yl)methylene)benzenamine
and L2 = 2-(ethylthio)-N-((pyridine-2-yl)methylene)benzenamine)
in toluene under refluxing condition in 1:1 ratio. The structures
of the ligands and the complexes having general formula fac-
[Re(CO)3(L1/L2)Cl] (1a/1b) are shown in Scheme 1. In the course
of reaction two CO groups were replaced by pyridine-N and
imine-N donor centers of the ligand. Characterizations of ligands
and complexes were carried out by IR, NMR, UV–Vis and elemental
analysis. In the IR spectra, two typical bands around 2018–2020
and 1913–1916 cmꢀ1 are observed (Calculated (scaling factor
0.97), 2024, 1952, 1918 cmꢀ1 for 1a), which correspond to the
Z
qcalcd (g cmꢀ3
)
2.088
7.450
293(2)
l
(mmꢀ1
)
T (K)
hkl range
F(000)
h range (°)
Reflections collected
ꢀ9–9, ꢀ11–11, ꢀ13–12
508
1.84 to 25.00
11918
2956 [0.0352]
2558
2956/0/205
0.0644, 0.1664
0.0746, 0.1739
1.040
Unique reflections (Rint
)
Observed data (I > 2
r(I))
Data/restraints/parameters
b
R1a, wR2 (I > 2
r(I))
R1, wR2 (all data)
Goodness-of-fit (GOF) on F2c
Largest difference in peak and hole (e Åꢀ3
)
2.101 and ꢀ1.434
fac-Re(CO)3 fragment in the complexes. The
the complexes are observed at around 1589–1595 cmꢀ1. The
(C@N) for free ligands are observed at 1622–1625 cmꢀ1, which
m(C@N) stretching in
P
P
a
R1
=
|(|Fo| ꢀ |Fc|)|/ |Fo|.
P
P
b
2
wR2 = { [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]}1/2, w = 1/[
r
2(Fo2) + (0.1123 P)2 + 8.6320 P],
where P = (Fo2 + 2 Fc2)/3.
m
P
GOF = { [w(Fo ꢀ Fc2)2]/(n–p)}1/2
, where n = number of measured data and
c
2
is significantly shifted to lower frequency region in the complexes
supporting coordination of imine-N (Table 2). The 1H NMR spectra
p = number of parameters.
Table 2
FT-IR, UV–Vis, emission and life time data of ligands (L1 and L2) and complexes (1a and 1b).
a
b
Compd.
m
(CO) (cmꢀ1
)
kmax (Mꢀ1 cmꢀ1
)
Emissionb
Life timeb
kex (nm)
kem (nm)
/
v2
sf (ns)c
L1
–
–
379 (4804), 321 (7027), 362(31497)
382 (5124), 323 (8142), 365(29435)
443 (5070), 326 (11728), 261 (26795)
448 (5642), 320 (10765), 266 (22756)
321
323
326
320
397
401
405
410
0.024
0.022
0.013
0.014
1.05
1.03
0.98
1.05
3.03
2.97
4.49
4.21
L2
1a
1b
2018, 1913
2020, 1916
a
b
c
KBr disk.
Acetonitrile as solvent.
Mean fluorescence lifetime,
s
f = a1
s
1 + a2
s
2, where a1 and a2 are relative amplitude of decay process;
s
1 = 1.039 ns,
s
2 = 4.532 ns (a1 = 43% and a2 = 57%) for L1;
2 = 6.296 ns (a1 = 48% and a2 = 52%) for
s
1b.
1 = 1.084 ns,
s
2 = 4.482 ns (a1 = 45% and a2 = 55%) for L2;
s
1 = 1.211 ns, 2 = 6.349 ns (a1 = 46% and a2 = 54%) for 1a and
s
s1 = 1.946 ns, s

M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
141
of ligands and complexes are recorded in CDCl3 solution and the
signals appeared as expected. The important observation is the
down field shifting of pyridine protons by 0.3–0.6 ppm in the com-
plexes compare to free ligand values supporting the coordination
of pyridine-N to metal center (see Section 2).
3.2. Molecular structure
Single crystal suitable for structure determination was obtained
by slow diffusion of n-hexane into dichloromethane solution of 1a.
The crystallographic data collection and refinement parameters are
given in Table 1; selected bond lengths and angles are given in Ta-
ble 3. ORTEP plot with atomic numbering scheme is shown in
Fig. 1. The rhenium atom adopts a distorted octahedral geometry
and is coordinated by three carbonyl ligands in a fac arrangement,
two nitrogen atoms (pyridine-N and imine-N) of L1 ligand and
chlorine atom. The deviation of the Rhenium coordination sphere
from the ideal octahedron is because of the small bite angle of
the five membered chelate ring (Re1–N1–C5–C6–N2) [73.1(4)°].
The Re–N(pyridyl) bond, Re(1)–N(1), 2.149(10) Å is shorter than
the Re–N(imine), Re(1)–N(2), 2.164(12) Å. The Re–C(CO) (Re(1)–
C(14), 1.904(17); Re(1)–C(15), 1.925(16) and Re(1)–C(16,
1.901(19) Å) and C–O (C(14)–O(1), 1.110(18); C(15)–O(2),
1.120(18) and C(16)–O(3), 1.13(2) Å) bond distances are found as
expected for similar rhenium carbonyl complexes [54–56].
Fig. 1. Ortep structure of 1a with 35% ellipsoidal probability.
3.3. DFT calculation and electronic structure
Table 4
Energy and compositions of some selected molecular orbitals of 1a.
The geometry of L1 and fac-[Re(CO)3(L2)Cl] (1a) were optimized
in singlet state by the DFT method with the B3LYP hybrid func-
tional. The optimized geometric parameters for 1a are given in Ta-
ble 3. The calculated bond parameters for 1a are well correlated
with the X-ray crystal structure data.
The energy and compositions of some selected molecular orbi-
tals for 1a are given in Table 4. Contour plots of some selected
molecular orbitals of L1 and 1a are presented in Fig. 2 and Fig. 3
respectively. The LUMO of L1 have p⁄ character while the HOMO
MO
Energy (eV)
% Of composition
Re
CO
L
Cl
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢀ0.62
ꢀ0.74
ꢀ0.91
ꢀ1.10
ꢀ1.92
ꢀ3.16
ꢀ5.95
ꢀ6.02
ꢀ6.49
ꢀ6.58
ꢀ7.11
ꢀ7.29
ꢀ7.40
ꢀ7.51
ꢀ7.93
ꢀ8.46
ꢀ9.03
28
25
24
09
02
04
44
41
12
57
08
24
08
18
11
02
0
48
39
25
12
01
03
21
18
05
23
04
08
03
05
12
01
0
24
35
51
79
97
92
02
05
81 (S, 61)
19
74
12
76
48
05
95
0
01
0
0
0
01
33
36
02
01
14
56
13
29
72
02
0
HOMO
HOMOꢀ1
HOMOꢀ2
HOMOꢀ3
HOMOꢀ4
HOMOꢀ5
HOMOꢀ6
HOMOꢀ7
HOMOꢀ8
HOMOꢀ9
HOMOꢀ10
to HOMOꢀ2 are
p character with reduced contribution of pp(S)
orbitals. The low lying virtual orbitals, LUMO to LUMO+2 corre-
spond to p⁄ orbital of L1, while LUMO+3 to LUMO+5 show greater
mixing of dp
(Re) and p⁄(CO) orbitals along with the contribution of
L1 for complex 1a. The HOMO and HOMOꢀ1 orbitals have the
mixed contribution of dp(Re), pp(Cl) and p-orbitals of CO. The
HOMOꢀ2 concentrated on S atom (64% p
contribution from d
(Re) orbitals. HOMOꢀ3 has 57% Re(d
reduced contribution of CO (23%).
p
(S)) along with reduced
100 (S, 45)
p
p) and
HOMO (E, -5.90 eV)
HOMO-1 (E, -6.45 eV)
HOMO-2 (E, -7.12 eV)
LUMO (E, -2.10 eV)
LUMO+1 (E, -1.11 eV)
LUMO+2 (E, -0.63 eV)
Fig. 2. Contour plot of some selected molecular orbitals of L1.

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M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
HOMO
LUMO
HOMO-1
LUMO+1
HOMO-2
LUMO+2
HOMO-3
LUMO+3
Fig. 3. Contour plot of some selected molecular orbitals of 1a.
Table 5
Natural bond orbitals (NBOs) analysis result of 1a.
Bonds
Occupancy
Bond orders
1.3449
Contribution/hybridization
Natural charges (a.u.)
Atoms
Charge
Re(1)–C(14)
1.9484(0.1539)
1.7684(0.4327)
1.7205(0.4461)
1.9145(0.0874)
1.9608(0.0984)
1.9912(0.0304)
1.9970(0.0226)
1.9962(0.2525)
1.9951(0.2206)
1.9972(0.0187)
1.9955(0.2164)
1.9949(0.1872)
(37.46%)Re(sd2.22) + (62.54%)C(sp0.51
(83.72%)Re(d) + (16.28%)C(p)
)
Re(1)
Cl(1)
O(1)
O(2)
O(3)
N(1)
N(2)
C(14)
C(15)
C(16)
ꢀ0.863
ꢀ0.409
ꢀ0.470
ꢀ0.447
ꢀ0.452
ꢀ0.379
ꢀ0.368
0.670
(84.40%)Re(d) + (15.60%)C(p)
Re(1)–C(15)
Re(1)–C(16)
C(14)–O(1)
C(15)–O(2)
1.2643
1.2859
2.0319
2.0965
(34.40%)Re(sd2.81) + (65.60%)C(sp0.53
)
)
(36.91%)Re(sd2.14) + (63.09%)C(sp0.52
(30.68%)C(sp1.92) + (69.32%)O(sp1.33
(31.26%)C(sp1.96) + (68.74%)O(sp1.29
(24.36%)C(p) + (75.64%)O(p)
)
)
(24.86%)C(p) + (75.14%)O(p)
0.735
0.727
C(16)–O(3)
2.0818
(31.09%)C(sp1.89) + (68.91%)O(sp1.25
(24.67%)C(p) + (75.33%)O(p)
)
(24.86%)C(p) + (75.14%)O(p)
To understand the nature of Re–CO bonding the natural bond
orbitals (NBOs) calculation has been performed on the optimized
structure of 1a. The occupancies and hybridizations of the calcu-
lated Re–C and C–O natural bond orbitals (NBOs) are summarized
in Table 5. For each carbonyl group three natural bond orbitals
should be detected for the C–O bond, and one orbital for the Re–
C bond [57]. But in our case the NBO analysis is found as expected
Table 6
Some selected singlet–singlet vertical electronic transitions of L1.
Key excitations
Character
k
E (eV)
Osc. strength
(f)
(nm)
(95%)HOMO ? LUMO
p
p
(S) ? L(p⁄
)
391.0
331.5
270.9
263.9
3.1709 0.0573
3.7407 0.0839
4.5765 0.1416
4.6987 0.2082
(94%)HOMOꢀ1 ? LUMO L(
(67%)HOMOꢀ2 ? LUMO L(
(89%)HOMOꢀ3 ? LUMO L(
p
p
p
) ? L(p⁄
) ? L(p⁄
) ? L(p⁄
)
)
)
Table 7
Some selected singlet–singlet vertical electronic transitions of 1a.
Key excitations
Character
k
E (eV)
Osc.
(nm)
strength (f)
(88%)HOMO ? LUMO
d
p
(Re)/
451.6 2.7457 0.0698
Cl ? L(p⁄
)
(82%)HOMOꢀ3 ? LUMO
(91%)HOMOꢀ4 ? LUMO
dp
(Re) ? L(p⁄
)
406.9 3.0470 0.0251
354.9 3.4934 0.2200
266.2 4.6573 0.1613
L(
p
p
) ? L(p⁄
) ? L(p⁄
)
)
(75%)HOMOꢀ4 ? LUMO+1 L(
for Re–CO bonds trans to pyridine-N and imine-N, while three nat-
ural bond orbitals for Re–C bond and one for carbonyl group trans
to Cl are detected (Table 5). Among the three NBOs of Re–C bond
trans to Cl, one has 37% Re and 63% C character while other two
have ꢁ84% d
p(Re) contribution along with reduced contribution
from C atom of carbonyl group.
The bonding between CO ligands and Re atom is supported by
Fig. 4. UV–Vis of L1 (–) and 1a (
) in acetonitrile.
the resonance structure
from the calculated atomic

M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
143
performed in implicit acetonitrile. The bands at 379–382 nm for
the ligands has been assigned as p
(S) ? L(p⁄) transitions, while
the bands at 321–323 nm and 262–265 nm have
p⁄ character
p
p
?
(Table 6). The calculated vertical electronic transitions for 1a show
two weak transitions at 452 nm and 407 nm corresponding to
HOMO ? LUMO and HOMOꢀ3 ? LUMO transitions having
d
p
p
p
(Re) ? p⁄(L) character along with reduced contribution of
(Cl) ? p⁄(L) transitions (Table 7, Fig. 5). The high energy bands
at 320–326 nm and 261–266 nm are
p
(L) ? p⁄(L), intra-ligand
charge transfer (ILCT) transitions.
The ligands (L1/L2) exhibit intense emission at 397–401 nm
upon excitation at 321–323 nm. For complexes (1a/1b) the emis-
sion occurs at higher energy (405–410 nm) than the lowest absorp-
tion band (443–448 nm). It would indicate the presence of a
ligand-localized excited state completely decoupled from the rest
of the complex. We do not observe emission upon excitation in
MLCT band at 443–448 nm for the complexes. So, the p
(L) ? p⁄(L)
transitions are taken to investigate the emission properties (Ta-
ble 2, Fig. 6). The emission quantum yields (/) of the compounds
are in the range 0.013–0.024.
Lifetime data of the compounds are taken at 298 K in acetoni-
trile solution when excited at 370 nm. The fluorescence decay
curve was deconvoluted with respect to the lamp profile. The ob-
served florescence decay fits with bi-exponential nature for both
the ligands and complexes (Table 2, Fig. 7). We have used mean
Fig. 5. Energy level diagram of MOs involved in singlet–singlet vertical excitations.
fluorescence lifetime (
s
f = a1
s
1 + a2
s
2, where a1 and a2 are relative
[A]
1000
100
10
1
0
20
40
60
80
100
120
Fig. 6. Excitation and emission spectra of L1 (–) and 1a (
) in acetonitrile.
Time (ns)
charges (Table 5). The calculated natural charges on the carbon
atoms of the carbonyl ligands are positive, whereas the oxygen
atoms are negatively charged. The least positively charged carbon
of CO groups is in the trans position to the halogen atom as ex-
pected. The calculated charge on the rhenium atom (ꢀ0.863 a.u.)
is considerably lower than the formal charge +1. The population
[B]
1000
100
10
2
2
2
of the 5dxy, 5dxz, 5dyz, 5dx
and 5dz of the rhenium atom is
ꢀy
1.1695, 1.4150, 1.2881, 1.4299 and 1.3146.
3.4. TD DFT calculation and electronic spectra
The L1 and L2 exhibit moderately intense absorption bands at
379–382 nm and 321–323 nm along with a shoulder at 262–
265 nm. The complexes (1a/1b) show one moderately intense
broad band at 443–448 nm along with an intense peak at 320–
326 nm and a shoulder at 261–266 nm in acetonitrile (Table 2,
Fig. 4).
1
0
20
40
60
80
100
120
Time (ns)
Fig. 7. Exponential decay profile of (A) ligand, L1 and (B) fac-[Re(CO)3(L1)Cl] (1a) in
To assign the nature of electronic transitions in the compounds
TDDFT calculations on the optimized geometry of L1 and 1a were
acetonitrile.

144
M.S. Jana et al. / Inorganica Chimica Acta 399 (2013) 138–145
Appendix A. Supplementary material
CCDC 867934 contains the supplementary crystallographic data
for [Re(CO)3(L2)Cl] (1a). These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2013.01.013.
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4. Conclusion
New pyridyl Schiff base ligands (L1/L2) containing thioether
group have been synthesized and characterized. The rhenium car-
bonyl complexes containing the {Re(CO)3}+ fragment and Schiff
base ligands (L1/L2) with general formula fac-[Re(CO)3(L1/L2)Cl]
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Financial support received from the Department of Science and
Technology, New Delhi, India (No. SR/FT/CS-73/2010) is gratefully
acknowledged. Special thanks to Prof. C. Sinha, Department of
Chemistry, Jadavpur University, Kolkata, India for his constant
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to CSIR, New Delhi, India for fellowship.

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