Luminescent Zn(ii) and Cd(ii) complexes with chiral 2,2′-bipyridine ligands bearing natural monoterpene groups: synthesis, speciation in solution and photophysics

Article abstract of DOI:10.1039/d0dt01438a

Mononuclear zinc(ii) and cadmium(ii) complexes, ZnLCl₂(1), CdLCl₂(2), ZnL¹Cl₂·2H₂O (3), and CdL¹Cl₂·2H₂O (4), with chiral ligands containing a 2,2′-bipyridine moiety and natural terpene (+)-limonene (L) or (+)-3-carene (L¹) moieties were synthesized. In these complexes theLandL¹ligands are shown to coordinate Zn²⁺and Cd²⁺ions through the 2,2′-bipyridine moiety. The acetamide group of the ligands interacts with M²⁺ions by forming N?M²⁺and C-O?M²⁺contacts and N-H?Cl hydrogen bonds with coordinated Cl^?ions. In solutions the complexes have several conformers differing by the degree of the turn of the acetamide moiety relative to the ligand core and the type of its interaction with the coordination core. The ligands and complexes exhibit luminescence with the quantum yield increasing in the order: ligand < cadmium(ii) complex < zinc(ii) complex. The complexes3and4demostrate excitation wavelength independent single-channel fluorescence. As opposed to3and4, the complexes1and2demonstrate excitation wavelength dependent emission with nanosecond and microsecond lifetimes of the excited states. According to our TD-DFT calculations, an interplay of ligand centered and halide to ligand transitions facilitates two deactivation channels in1and2: S₁-S₀and T₁-S₀

Full text of DOI:10.1039/d0dt01438a

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Sheludyakova, A. V. Tkachev and M. B. Bushuev, Dalton Trans., 2020, DOI: 10.1039/D0DT01438A.
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DOI: 10.1039/D0DT01438A
ARTICLE
Luminescent Zn(II) and Cd(II) complexes with chiral 2,2’-bipyridine
ligands bearing natural monoterpene groups: synthesis,
speciation in solution and photophysics
Tatyana E. Kokina,*a,b Marianna I. Rakhmanova,a Nikita A. Shekhovtsov,*a,b Ludmila A. Glinskaya,a
Vladislav Y. Komarov,a,b Alexander M. Agafontsev,b Andrey Y. Baranov,a Pavel E. Plyusnin,a,b Liliya A.
Sheludyakova,a Alexey V. Tkachev *b and Mark B. Bushuev *a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mononuclear zinc(II) and cadmium(II) complexes, ZnLCl2 (1), CdLCl2 (2), ZnL1Cl2·2H2O (3), and CdL1Cl2·2H2O (4), with chiral
ligands containing a 2,2’-bipyridine moiety and natural terpene (+)-limonene (L) or (+)-3-carene (L1) moieties were
synthesized. In these complexes the L and L1 ligands are shown to coordinate Zn2+ and Cd2+ ions through the 2,2’-bipyridine
moiety. The acetamide group of the ligands interacts with M2+ ions by forming N···M2+ and C=O···M2+ contacts and N–H···Cl
hydrogen bonds with coordinated Cl– ions. In solutions the complexes have several conformers differing by the degree of
the turn of the acetamide moiety relative to the ligand core and the type of its interaction with the coordination core. The
ligands and complexes exhibit luminescence with the quantum yield increasing in the order: ligand < cadmium(II) complex
< zinc(II) complex. The complexes 3 and 4 demostrate excitation wavelength independent single-channel fluorescence. As
opposed to 3 and 4, the complexes 1 and 2 demonstrate excitation wavelength dependent emission with nanosecond and
microsecond lifetimes of the excited states. According to our TD-DFT calculations, an interplay of ligand centered and halide
to ligand transitions facilitates two deactivation channels in 1 and 2: S1–S0 and T1–S0.
Luminescent zinc(II) and cadmium(II) complexes are one of
the most important classes of fluorescent materials and
chemosensors.38-55 It worth noting that zinc(II) is one of the
Introduction
Synthesis of chiral metal complexes have attracted much
attention in recent years.1-8 Chiral metal complexes can
demonstrate non-trivial photophysical properties like circularly
polarized luminescence or be used as chiral luminescent
probes.9-19 Importantly, these compounds demonstrate great
potential for the development of coordination chemistry,
photophysics and such interdisciplinary fields as chemical
biology, pharmacology and medicine.20-27 The use of optically
pure organic compounds, including natural products or their
derivatives, as ligands is a promising way for stereoselective
synthesis of chiral metal complexes.28,29 Synthetic nitrogen-
containing derivatives of natural terpenes are appealing ligands
for binding metal ions,30-32 especially in the case of combining a
terpene moiety and various azaheterocycles in a single
molecule to form hybrid multidentate chiral ligands.33-37
most important metals of life playing crucial roles in many
physiological processes.56 Cadmium(II) is a toxic analogue of
zinc(II), nevertheless, its complexation with biomolecules and
their analogs allows researchers to reveal their role in various
biochemical processes.57 Although non-emissive due to filled
d10 configuration, zinc(II) and cadmium(II) often enhance
emission of organic ligands by reducing probability of
luminescence quenching by photoinduced electron transfer
(PET). This enhancement of emission as a result of complexation
is referred to as chelation enhanced fluorescence (CHEF).
Complexation of chiral ligands with zinc(II) and cadmium(II)
opens up prospects for creating new classes of chiral metal
complexes. Systematic studies in this direction has led to
stereoselective formation of luminescent zinc(II) complexes
with pinene-substituted 2,2’-bipyridine ligands.58 Chiroptical
properties of these complexes were studied by circular
dichroism spectroscopy and circularly polarized luminescence
spectroscopy.58 Recently we have reported zinc(II) and
cadmium(II) chloride complexes with chiral nitrogen-containing
fused terpene-based heterocycles.59-61 The free ligands and the
complexes display bright blue luminescence. In the solid state
the complexes with the ligand containing the (–)-α-pinene
moiety fused to N,N-chelating dihydrophenantroline core show
lower quantum yield in comparison with the free ligand,60
however, their quantum yield is about an order of magnitude
higher than the quantum yields of the Zn(II) and Cd(II)
a. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of
Sciences, 3, Acad. Lavrentiev Ave., Novosibirsk, 630090, Russia. E-mail addresses:
kokina@niic.nsc.ru (Tatyana E. Kokina), shna1998@mail.ru (Nikita A.
Shekhovtsov), bushuev@niic.nsc.ru (Mark B. Bushuev). Fax: +7 383 330 94 89; Tel:
+7 383 316 51 43.
b. Novosibirsk State University, 2, Pirogova str., Novosibirsk, 630090, Russia.
c. N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of
Russian Academy of Sciences, 9, Acad. Lavrentiev Ave., Novosibirsk, 630090,
Russia. E-mail address: atkachev@nioch.nsc.ru (Alexey V. Tkachev)
† Electronic Supplementary Information (ESI) available: TGA data, Crystallographic
data, NMR data, IR spectra, X-ray powder diffraction data, photoluminescence
spectra and DFT calculation data. CCDC 1997852-1997853. See
DOI: 10.1039/x0xx00000x
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complexes with a pyridophenazine ligand derived from chiral Schemes 2 and 3).30 α-Amino oximes B and B' were prepared by
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pynopyridine.59 Zinc(II) and cadmium(II) complexes with a treatment of the corresponding nitrDoOsoI:c1h0.l1o0r3i9d/eDs0DoT0f14(3+8)A-
derivative of 2,2’-bipyridine containing (–)-α-pinene moiety and limonene and (+)-3-carene with a water-alcohol solution of
one five-membered carbocyclic manifested the CHEF effect.61
ammonia.64 (E)-3-Phenyl-1-(pyridin-2-yl)prop-2-en-1-one was
Among heteroaromatic ligands, 2,2’-bipyridine is one of the prepared by the condensation of benzaldehyde with 2-
most popular and versatile.62 It coordinates metal ions acetylpyridine in the presence of NaOH in water at room
preferably in a N,N-chelating fashion and is widely used in such temperature as described previously.65 The complexes were
fields as coordination and organometallic chemistry, catalysis, synthesized using ZnCl2 and CdCl2·2.5H2O (pure for analysis),
electrochemistry, spin crossover, photophysics, etc. In this EtOH (rectified), CH2Cl2 and i-PrOH (chemically pure).
context, as a part of our on-going research at the crossroads of
Synthesis
such areas as coordination chemistry, organic chemistry of
natural terpenes and photophysics, we endeavored to study Synthesis of N-((1S,4R,Z)-2-(acetoxyimino)-1-methyl-4-(prop-
complexation and photophysics of zinc(II) and cadmium(II) 1-en-2-yl)cyclohexyl)acetamide (C, see Scheme 2). A solution of
complexes with chiral ligands containing the 2,2’-bipyridine acetyl chloride (10.39 g, 0.1324 mol) in CHCl3 (20 mL) was added
moiety as a pre-requisite for the N,N-chelating coordination and dropwise with stirring to a solution of amino oxime B (see
a chiral moiety of the natural terpene (+)-limonene or (+)-3- Scheme 2) (10.06 g, 0.0552 mol) and NEt3 (13.37 g, 0.1321 mol)
carene (Scheme 1). Here we report (i) synthesis and in CHCl3 (150 mL) under cooling in an ice bath. The reaction
characterization of zinc(II) and cadmium(II) complexes with the mixture was stirred with cooling for more 15 min, kept for 10 h
chiral ligands N-((5S,8S)-8-methyl-4-phenyl-5-(prop-1-en-2-yl)- at room temperature, and shaken with 1 M aqueous
2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinolin-8-yl)acetamide (L) hydrochloric acid (20 mL). The organic phase was separated,
and N-[(1aS,3S,7bR)-1,1,3-trimethyl-6-(pyridin-2-yl)-7-phenyl- washed with water until the wash water was neutral, dried with
1a,2,3,7b-tetrahydro-1H-cyclopropa
[f]-quinolin-3- anhydrous Na2SO4, and concentrated in vacuo. The resulting
yl]acetamide (L1), (ii) X-ray single crystal structures of zinc(II) N,O-diacetyl derivative was used in the synthesis of L.
and cadmium(II) complexes with the ligand L, (iii) photophysical Synthesis of N-((5S,8S)-8-methyl-4-phenyl-5-(prop-1-en-2-yl)-
properties of the organic ligands and the complexes.
2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinolin-8-yl)acetamide (L).
A mixture of the N,O-diacetyl derivative C (see Scheme 2) (0.525
g, 2.0 mmol), diisopropylamine (0.404 g, 4.0 mmol), CuI (0.080
g, 0.4 mmol), and (E)-3-phenyl-1-(pyridin-2-yl)prop-2-en-1-one
(0.46 g, 2.2 mmol) in DMSO (10 mL) was kept under an Ar
atmosphere for 48 h at 80˚C, cooled to room temperature, and
diluted with an aqueous ammonia solution (50 mL). Then
sodium tartrate (0.1 g) was added, and the reaction products
were extracted with EtOAc (3×25 mL). The combined organic
extracts were dried with anhydrous Na2SO4 and concentrated in
vacuo. The resulting dark oily product was chromatographed
using the petroleum ether–EtOAc elution system (10 : 1 → 1 : 1,
v/v). Ligand L was obtained as a fine white powder. Yield: 0.115
g (15% based on the N,O-di-acetyl derivative C). High-resolution
EI-MS (70 eV). Found: m/z 397.2152 [M]+. C26H27O1N3.
Calculated: [M]+ = 397.2149. EI-MS (70 eV), m/z (Irel (%)): 397
[M]+ (100), 382 (41), 340 (33), 337 (16311 (20), 300 (19), 299
(22), 298 (65), 297 (84), 283 (18), 119 (50), 47 (15), 43 (16). IR
(КBr), ν/cm–1: 3294 (N—H), 3055 (Ar—H), 1656 (C=O).
Synthesis of [ZnLCl2] (1), [CdLCl2] (2), ZnL1Cl2·2H2O (3) and
CdL1Cl2·2H2O (4). A solution of a metal salt (ZnCl2, 0.020 g, 0.15
mmol for 1; CdCl2·2.5H2O, 0.034 g, 0.15 mmol for 2; ZnCl2, 0.020
g, 0.15 mmol for 3; CdCl2·2.5H2O, 0.017 g, 0.075 mmol for 4) in
EtOH (2 – 3 mL) was added to a solution of a ligand (L, 0.040 g,
0.10 mmol for 1; L, 0.040 g, 0.10 mmol for 2; L1, 0.040 g, 0.10
mmol for 3; L1, 0.020 g, 0.050 mmol for 4) 7 in EtOH (2 – 3 mL).
The resulting solution was allowed to evaporate slowly at room
temperature. Small white crystals formed when the volume of
the solution became ca. 1 – 2 mL. The crystals were filtered off,
washed with cold i-PrOH, and dried in vacuo at room
temperature. Yield of 1: 0.024 g (45 %). Anal. Calc. for
C26H27N3OCl2Zn (533.8): C, 58.5; H, 5.1; N, 7.9 %. Found: С, 58.2;
H, 5.1; N, 7.7 %. Yield of 2: 0.040 g (72 %). Anal. Calc. for
O
O
N
NH
N
NH
N
N
L1
L
Scheme 1. Structural formulae of L and L1.
Experimental part
Reagents
The ligand L1 was prepared by a procedure reported in Ref. 63.
The ligand L was synthesized using freshly distilled organic
solvents (petroleum ether with b.p. 40—70 °C, ethyl acetate,
chloroform, and ethanol (rectified 95%)), sodium nitrite
(analytical grade), 35% hydrochloric acid (reagent grade); 25%
aqueous ammonia (analytical grade), acetyl chloride (high-
purity grade), benzaldehyde (Reakhim, 99%), 2-acetylpyridine
(Acros, 98%), NaOH (analytical grade), CuI (Reakhim, analytical
grade), diisopropylamine (Acros, 99%), dimethyl sulfoxide
(distilled in vacuo and stored over 4A molecular sieves), high-
purity argon, sodium tartrate dihydrate (Reakhim, analytical
grade), and anhydrous sodium sulfate (analytical grade). The
ligand L was purified by preparative adsorption column
chromatography using silica gel with a mesh size of 50—160 μm
(IMID); the separation was controlled by thin-layer
chromatography on commercial Sorbfil plates (silica gel on
polyamide film). (+)-Limonen and (+)-3-carene nitroso chlorides
were prepared by passing gaseous nitrosyl chloride over a
solution of the terpenes A or A' in dichloromethane (see
2 | J. Name., 2012, 00, 1-3
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C26H27N3OCl2Cd (580.8): С, 53.8; H, 4.7; N, 7.0 %. Found: С, 53.4; coupling constants (J = 135 Hz). Spin-spin constaVnietwJAsrtiicglne Ownlianes
DOI: 10.1039/D0DT01438A
H, 4.9; N, 7.0 %. Yield of 3: 0.029 g (51 %). Anal. Calc. for not determined.
C26H31N3O3Cl2Zn (569.8): C, 54.8; H, 5.5; N, 7.4 %. Found: С, 54.4;
H, 5.6; N, 7.8 %. Yield of 4: 0.040 g (68 %). Anal. Calc. for
Photophysics
C26H31N3O3Cl2Cd (616.9): С, 50.6; H, 5.0; N, 6.4 %. Found: С, The luminescence and luminescence excitation spectra were
50.6; H, 5.1; N, 6.8 %.
measured on
a
Fluorolog-3 (Horiba Jobin Yvon)
spectrofluorimeter with a 450W xenon lamp and a double
monochromator for absorption and emission. The excitation
Characterization of the compounds
Elemental analysis was performed on a Euro EA 3000 analyzer. spectra were corrected for the spectral range of the lamp
IR absorption spectra were recorded on SCIMITAR FTS 2000 and intensity. The emission spectra were corrected for the response
VERTEX-80 spectrophotometers in the range 4000 – 100 cm–1. of monochromators and the detector using the correction
Thermogravimetric analysis (ESI, Fig. S1-S5) was carried out spectra provided by the manufacturer. The first and second
under He flow at a heating rate of 10 °C min−1 using an NETZSCH harmonics of the excitation source were blocked by filters.
STA 449 F1 Jupiter STA up to 600 °C in Al2O3 pans.
DFT and TD-DFT calculations
X-ray crystallography
Conformational analysis of the ligands L and L1 and
The unit cell parameters of 1 and 2 and intensities of reflections corresponding Zn and Cd complexes was was carried out using
were measured at 150 K by means of a Bruker X8 Apex CCD ORCA program system69 (ver. 4.1.0). Solvation effects of CH2Cl2
diffractometer equipped with a two-coordinate detector by the were taken into account within the conductor-like polarizable
standard technique (Mo Kα radiation, λ = 0.71073 Å, graphite continuum model. Geometry optimization was performed using
monochromator).66
Crystallographic
characteristics, the hybrid exchange-correlation functional PBE0 with the
experimental data, and structure refinements are listed in Table valence triple-zeta basis set def2-TZVP with new polarization
S1 (ESI). The structures of 1 and 2 (Fig. 1, ESI, Fig. S6, S7) were functions70 and the auxiliary basis def2/J71 utilizing the atom-
solved by SHELXT67a and refined by SHELXL67b supported with pairwise dispersion correction with the Becke-Johnson damping
Olex2 GUI.68 The H atoms on the C and N atoms were located scheme (D3BJ).72,73 For the Cd atoms, effective core potentials
by means of a difference Fourier map and were refined via a were used74 as a good alternative to scalar relativistic all-
riding model. Selected distances and bond angles in the electron calculations. To speed up the calculations, the RIJCOSX
structures of 1 and 2 are listed in Table S2 and Table S3 (ESI). approximation was applied.75 Geometries of conformers in
Crystallographic data have been deposited with the Cambridge cartesian coordinates are given in ESI (Table S8-S23).
1
Crystallographic Data Centre, CCDC Nos 1997852 (1), 1997853 Parameters of Н and 13C NMR spectra of the ligands and
(2)
and
can
be
obtained
via complexes are listed in ESI (Table S24, S25).To analyze the
electronic structures of the complexes 1 and 2, quantum
chemical calculations were performed using a density
functional theory (DFT) approach with the three-parameter
https://www.ccdc.cam.ac.uk/structures.
X-ray powder diffraction
X-ray powder diffraction analysis of the complexes 1 – 4 was hybrid functional of Becke based on the correlation functional
carried out on a Shimadzu XRD-7000 diffractometer (Cu Kα of Lee, Yang, and Parr (B3LYP)76,77 for geometry optimizations
radiation, Ni filter, 5 – 60° 2θ range, 0.03° 2θ step). and time dependent DFT calculations (TD-DFT) (ESI, Tables S26-
Polycrystalline samples were slightly ground with heptane in an S43). The LANL2DZ basis set78-80 was applied for the calculations
agate mortar, the resulting suspension was deposited on the of 1 and 2. All calculations were performed in Gaussian 0981 and
polished side of a standard glass holder, and smooth thin layers Multiwfn82 programs. Experimental X-ray single crystal
formed after drying (thickness ∼ 100 μm). X-ray powder geometries were used for the ground state geometry (S0)
diffraction patterns of the complexes are given in ESI (Fig. S8 – optimizations of 1 and 2. The ground state molecular
S11). Simulated X-ray powder diffraction patterns of 1 and 2 are geometries of 1 and 2 are found to be in a good agreement with
in agreement with the experimental patterns (ESI, Fig. S9, S10). the geometries obtained from the X-ray diffraction analysis.
Resulting ground state optimized geometries were used as
NMR spectroscopy
initial guesses for the excited state geometry optimizations of 1
NMR spectra were recorded on a Bruker DRX500 instrument and 2. All frequencies in harmonic approximation for the
1
(500.13 MHz for H, 125.75 MHz for 13C) for solutions at a calculated minimum energy geometries are positive. Atomic
concentration of 10-20 mg mL–1 at 27-30 °C using the following coordinates are given in ESI (Tables S40, S41).
signals of the solvent as the internal standard: δH 7.24 and δC
76.90 for CDCl3. The signal assignments were made based on 13C
NMR spectra with J modulation (proton noise decoupling, the
Results and discussion
opposite direction for signals of atoms with an even number of
Synthesis and characterization
attached protons tuned to the constant J = 135 Hz), as well as
Synthesis of the ligands L and L1 and the complexes 1 – 4 is
shown in Schemes 2 and 3. Natural terpenes (+)-limonene (A,
Scheme 2) and (+)-3-carene (A’, Scheme 3) were used as starting
1
on two-dimensional homonuclear H-1H correlation spectra,
heteronuclear 13C-1H correlation spectra recorded for direct
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Scheme 3. Synthesis of the compounds L1, 3 and 4.
compounds for the synthesis of L and L1. These compounds can
be transformed into α-amino oximes B and B’ by reacting
terpenes A and A’ with nitrosyl chloride and, after that, with
aqueous ammonia. The α-amino oximes B and B’ are key
intermediates in the synthesis of the ligands. They react with
acetyl chloride to form N,O-diacetyl derivatives C and C’. These
compounds, C and C’, are condensed with (E)-3-phenyl-1-
(pyridin-2-yl)prop-2-en-1-one in the presence of cuprous iodide
and diisopropylamine which serve as synergetic catalysts for
this reaction. The structure and purity of the ligands L and L1 are
confirmed by NMR spectroscopy. The complexes 1 – 4 were
synthesized by reacting zinc(II) and cadmium(II) chlorides with L
and L1 in EtOH. For the synthesis of the complexes, the 1.5 : 1
metal-to-ligand molar ratio was applied. The yields of the target
products are ca. 45 – 70 %. The complexes are soluble in MeCN,
Me2CO, CH2Cl2 and CHCl3. According to NMR and DFT data for 1
– 4 and X-ray single crystal analysis for 1 and 2 in the complexes
the L and L1 ligands coordinate Zn2+ and Cd2+ ions through the
2,2’-bipyridine moiety (N,N-chelating coordination). The
acetamide group of the ligands interact with M2+ ions by
forming N···M2+ and C=O···M2+ contacts and N–H···Cl hydrogen
bonds with coordinated Cl– ions (see below).
View Article Online
DOI: 10.1039/D0DT01438A
The compounds 1 and 2 show high thermal stability (ESI, Fig.
S1,S2). The decomposition of 1 in the temperature range 300 –
600 °C proceeds in a few poorly separated steps. For the
complex 2, the first decomposition step is observed in the
temperature range 160 – 300 °C and is accompanied by a
gradual loss of mass (~ 6 %). Further decomposition proceeds in
several stages. The second and third stages in the range 300 –
420 °C lead to the loss of another 30% of the mass. The
intermediate product formed in the third stage decomposes
above 500 °C. The total residues of 1 and 2 at 600 °C are 39 and
25 % respectively.
The complexes 3 and 4 contain water molecules. The first
step of the weight loss, ca. 6 % for both complexes, is observed
in the range 50 – 100 °С for 3 and 100 – 140 С for 4 (ESI, Fig.
S3, S4). It corresponds to the complete removal of water of
crystallization (the calculated water content in the complexes 3
and 4 is 6.3 and 5.8 %). The dehydrated forms of these
complexes are stable up to 250 °C. Further decomposition of 3
proceeds in two stages in the temperature range 250 – 600 °C.
The total residue of the complex 3 (ca. 26 %) is in agreement
with the calculated value for ZnCl2 (23.9 %). Thermolysis of 4
proceeds in two steps in the temperature range of 250 – 400 °C
and leads to a loss of another 62 % of the mass, which
corresponds to complete removal of the organic ligand. The
residue (ca. 32 %) approximately corresponds to the calculated
value for CdCl2 (29.7 %). Further thermal destruction of the
complex 4 in the temperature range from 470 to 590 °C is
accompanied by endo-effects at 570 °C. This corresponds to the
melting and evaporating of CdCl2 (ESI, Fig. S5). The residue of 4
at 600 °C is 4 %.
O
NH2
NH
AcCl
NOH
NOAc
NOCl
NH3
NEt3
CHCl3
A
B
C
CuI, NH(i-Pr)2
N
Ph
O
Cl
Cl
DMSO, 80°C, Ar
In the IR spectra of the ligands L and L1 a band of stretching
vibration of the N–H group, (N–H), is observed at 3404 cm–1
for L and at 3289 cm–1 and 3196 cm–1 L1 (ESI, Table S44). The
position of this band shifts to the low-frequency region in the
spectra of complexes 1 and 2, respectively. In the spectra of the
complexes 3 and 4 the wide bands with the maxima at 3447 and
3389 cm–1 are related to overlapping vibrations (NH) and
(H2O). The positions of the stretching–deformation vibrations
of the heterocyclic rings including vibrations of the phenyl group
in the IR spectra of the complexes 1 – 4 shift towards high-
frequency region as compared to the spectra of L and L1 (ESI,
Table S44). This shift of the bands is informative of the
coordination of the N atoms of the ligands L and L1 to Zn2+ and
Cd2+ ions. Change in the position and splitting of the (C=O)
band in the spectra of the complexes 1 – 4 as compared to the
spectra of L and L1 (ESI, Table S44) indicates the participation of
this functional group in intermolecular interactions. The far-IR
spectra of the complexes exhibit the bands of the stretching
vibrations of the M–N and M–Cl bonds (ESI, Fig. S16, S17, Table
S42, S43, S44).
O
M
O
HN
NH
N
N
MCl2
N
N
M = Zn, Cd
[ZnLCl ]
2
1
)
2
)
(
(
L
[CdLCl ]
2
Scheme 2. Synthesis of the compounds L, 1 and 2.
O
NH2
NH
AcCl
NEt3
NOH
NOCl
NOAc
NH3
CH2Cl2
A'
B'
C'
CuI, NH(i-Pr)2
N
Ph
O
Cl
N
Cl
DMSO, 80°C, Ar
M
O
O
NH
N
NH
N
N
.2H2O
MCl2
X-ray single crystal structure of 1 and 2
M = Zn, Cd
According to X-ray single crystal diffraction data, the complexes
1 and 2 crystallize in the space group P212121 and have similar
unit cell parameters (ESI, Table S1). Both crystal structures
[ZnL1Cl ].2H O
3
)
(
2
2
L1
[CdL1Cl ].2H O
4
)
(
2
2
4 | J. Name., 2012, 00, 1-3
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consist of isolated mononuclear [ZnLCl2] and [CdLCl2] 0.084(2) Å for the compounds 1 and 2 respectively). The Cl(1)
View Article Online
DOI: 10.1039/D0DT01438A
molecules. The molecular structure of the complex 1 is shown and Cl(2) atoms deviate from this plane by 1.998(2), 1.747(2)
in Fig 1. Selected bond distances and angles in the structures of Å for 1 and 1.953(2), 2.051(2) Å for 2. The second chelate cycle,
1 and 2 are listed in ESI (Tables S2, S3). The metal atoms M(1)N(1)C(2)C(1)N(3) significantly deviates from planarity. In
coordinate two N atoms of the bipyridine moiety of L, one N the structure of the coordinated L molecule the angle between
atom of the NH group of L and two Cl atoms in the cis-positions. the phenyl ring and the plane of the pyridine ring
The geometry around the metal atoms is a trigonal bipyramid N(1)C(2)C(3)C(15)C(14)C(13) along the C(15)–C(16) bond is
N3Cl2 with the N(1), Cl(1) and Cl(2) atoms occupying the 70.5(1) (for 1) and 76.4(1) (for 2). The six-membered terpene
equatorial positions of the bipyramid and the N(2) and N(3) carbocycles in the structures of both complexes adopt a sofa
atoms occupying the axial ones.
conformation. The С(3) and С(6) atoms deviate from the
In the structure of 1, the Zn–N(amino group) bond length С(1)С(2)С(4)С(5) plane by –0.180(3), 0.663(3) Å in the structure
(Zn(1)–N(3) 2.713(2) Å) is longer than the Zn–N(bipyridine 1 and by –0.173(4), 0.662(4) Å in the structure of 2.
moiety) distances (Zn(1)–N(1) 2.095(2) Å, Zn(1)–N(2) 2.108(2) Å)
Hirshfeld analysis of the structures of 1 and 2 shows
(ESI, Table S2). In the structure of 2, the difference between the intermolecular interactions involving the oxygen atom of the
Cd–N(amino group) bond length (Cd(1)–N(3) 2.637(2)) and Cd– carbonyl group of one complex molecule and the phenyl
N(bipyridine moiety) ones (Cd(1)–N(1) 2.308(2) Å, Cd(1)–N(2) hydrogen atoms of another molecule: О···Н–С(18) = 3.180 and
2.338(2) Å) is smaller (ESI, Table S2). Such a difference between 3.207 Å for 1 and 2 respectively. The intermolecular distances
the M–N bond lengths can be related to the difference in the are within the range typical for van der Waals interactions (the
ionic radii of Zn2+ and Cd2+.
sum of the van der Waals radii of the C and O atoms is 3.2 Å).
This leads to the formation of a 1D helical chain running along
the a axis (ESI, Fig. S6). Weak C–H···Cl intermolecular
interactions assemble the helical chains in the 3D
supramolecular structure (ESI, Fig. S7). The parameters of these
supramolecular interactions for 1 and 2 are given in Table S3
(ESI). The formation of π–π stacking interactions is blocked due
to the turn of the phenyl rings relative to the bipyridine moiety
and due to the non-planar geometry of the terpene moiety. As
a result, the crystal structures of 1 and 2 are rather loose.
X-ray powder diffraction patterns of the compounds 1 and 2
are in agreement with the patterns simulated using the X-ray
single crystal data (ESI, Fig. S9, S10).
Speciation studies
According to DFT calculations, there are two conformations of L
(La and Lb) and L1 (L1a and L1b) in solution which differ in energy
by 4.6 and 3.5 kcal/mol for L and L1, respectively (Table 1). The
population distribution of the lowest energy conformers for
both compounds is over 99%, suggesting that both L and L1 exist
in the solution predominantly as single conformers, which is
well in agreement with the NMR data.
Figure 1. Molecular structure of [ZnLCl2] (1).
As a result of the coordination of the ligand L molecule, two five-
membered chelate cycles MN2C2 (M = Zn, Cd) form. The
M(1)N(1)C(13)C(22)N(2) (M = Zn, Cd) chelate cycle has a
virtually planar structure. An average deviation from the root-
mean-square atomic plane of the moiety is 0.1 Å (0.070(2) Å and
Table 1. Conformations of L and L1. Energies of the conformers relative to La and L1a (kcal/mol) and conformational population distribution (%, at 293K).
La
Lb
L1a
L1b
0.00 (99.96 %)
4.57 (0.04 %)
0.0 (99.8 %)
3.5 (0.2 %)
1
In the H NMR spectra of the complexes 1 – 4, the most coordination of bipyridine fragment to the metal (ESI, Tables
significant changes in chemical shifts compared to free ligands S24, S25). The changes in chemical shifts are also observed for
L and L1 occur for bipyridine protons, which is caused by the protons of the terpene skeleton located next to the
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acetamide group (H(6) and H(10) for L; Н(2) and Н(10) for L1) exchange between the two most stable conformeVrisewinArsticoleluOtniloinne
(ESI, Tables S24, S25). Upon complexation, the mutual for both compounds. There are identical sDeOtsI:o1f0.m10u3l9t/iDp0leDtTs0i1n4t3h8Ae
arrangement of these protons and the acetamide fragment NMR spectra of the complexes 1 and 2. The difference in
varies greatly compared to the free ligands (ESI, Tables S4–S23). chemical shifts can be explained by the fact that in both cases
DFT calculations reveal that the coordination compound 1 fast exchange spectra are observed, but the population
has two energetically stable conformers in solution (Scheme 4). distribution of the forms involved in the exchange turns out to
The conformer 1a is consistent with the X-ray single crystal be different for 1 and 2.
geometry, whereas in the conformer 1b the СH3CONH fragment
is rotated 180 degrees relative to the C(1)–N bond (Table 2). The
O
Cl
M
Cl
M
Cl
M
O
O
H
N
H
Cl
N
Cl
N
Cl
N
population distribution is 69% for 1a and 30% for 1b. Like the
complex 1, the complex 2 adopts several forms, two of which
are the most stable (the conformational population distribution
is 92% for 2a and 8% for 2b) (Table 3). The conformer 2a agrees
with the geometry obtained from the X-ray single crystal
diffraction analysis. Compared to 2a, in the form 2b the
acetamide fragment is rotated 180 degrees relative to the C(1)–
N bond (Scheme 4). Since the peaks in the NMR spectra of the
complexes 1 and 2 are not broadened, there is probably a fast
N
H
N
N
N
N
1a
2a
, M = Zn (69 %)
, M = Cd (92 %)
1b
2b
, M = Zn (30 %)
, M = Cd (ca. 8 %)
Scheme 4. Speciation in the solutions of 1 and 2 (population is shown in brackets, %).
Table 2. Conformations of the complex 1, energies of the conformers relative to 1a (kcal/mol) and conformational population distribution (%, at 293K).
1a
1b
1c
1d
1e
0.00 (69 %)
0.48 (30 %)
2.65 (1 %)
29.04 (0 %)
8.0 (0 %)
Table 3. Conformations of the complex 2, energies of the conformers relative to 2a (kcal/mol) and conformational population distribution (%, at 293K).
2a
2b
2c
2d
2e
0.00 (92 %)
1.44 (7.8 %)
3.73 (0.2 %)
11.6 (0 %)
5.6 (0 %)
Compared to the complexes with L, it is not possible to form Among these three forms, 3Ia is the most energetically stable.
coordination compounds with L1 with the simultaneous Since the peaks in the NMR spectrum of 3 are not broadened,
coordination of three nitrogen atoms to the metal atom (Zn, Cd) there is a fast exchange between these three forms in solution.
due to the geometry of the L1 ligand. According to our
Like the complex 3, there are three possible forms for the
computations, the formation of two types of the complex 3 is complex 4 in solution: 4Ia, 4II and 4Ib (Table 5). However, in
possible, 3I and 3II (Table 4). In both types, the bipyridine comparison with the complex 3, the most energetically stable
fragment is coordinated to Zn atom. In 3I, there is a hydrogen form is 4II. In 4II, the oxygen atom of the acetamide fragment is
bond between the proton of the acetamide fragment and the Cl additionally coordinated to the Cd atom. The NMR spectrum of
atom, the coordination number of Zn atom is 4; in 3II, the the complex 4 has strongly broadened peaks that can be caused
oxygen atom of the acetamide fragment is additionally by exchange processes involving forms with additional
coordinated to the zinc atom, increasing its coordination coordination of traces of water to Cd atom in the CDCl3 solvent,
number to 5. 3I has two energetically different conformations, as shown in Scheme 5. In case of the complex 2, such
3Ia and 3Ib. The conformational population distribution is 78, equilibrium is not realized because Cd atom is bound to the
17 and 5% for 3Ia, 3II and 3Ib, respectively (Scheme 5, Table 4).
6 | J. Name., 2012, 00, 1-3
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nitrogen atom of the acetamide fragment and this bond is λmax = 428 nm for 2) can be selectively excited by increasing the
View Article Online
DOI: 10.1039/D0DT01438A
stronger than the bond to the oxygen atom in the complex 4.
excitation wavelength to 360 nm. The complexes demonstrate
both fluorescence and phosphorescence as the lifetimes of the
excited states of high- and low-energy emission bands for 1 and
2 are in nanosecond and microsecond domain, respectively
(Table 6). The resultant emission color at λex = 300 – 360 nm, as
displayed on CIE 1931 chromaticity diagrams (ESI, Fig. S18, S19),
is blue for both complexes.
Cl Cl
Cl Cl
Cl Cl
O
H
N
O
O
H
M
N
M
N
M
N
N
NH
N
N
N
Wavelength, nm
250
300
350
400
450
500
3Ia
4Ia
3Ib
4Ib
, M = Zn (78 %)
, M = Cd (0 %)
, M = Zn (5 %)
, M = Cd (0 %)
3II
4II
, M = Zn (17 %)
, M = Cd (100 %)

ex = 340 nm
ex = 320 nm
ex = 300 nm
L
1,0
0,8
0,6
0,4
0,2
0,0
M = Cd
+ H2O

- H2O

H2O
H
Cl
O
Cl
N

em = 380 nm
Cd
N
N
40000
36000
32000
28000
24000
20000
Scheme 5. Speciation in the solutions of 3 and 4 (population is shown in brackets, %).
-1
Wavenumber, cm
Fig. 2. Normalized emission (λex = 300, 320 and 340 nm) and excitation (λem = 380 nm)
spectra of the ligand L in the solid state at 300 K.
Table 4. Conformations of the complex 3, energies of the conformers relative to 3Ia
(kcal/mol) and conformational population distribution (%, at 293K).
3Ia
3II
3Ib
Wavelength, nm
250
300
350
400
450
500
1,0
0,8
0,6
0,4
0,2
0,0
1
em = 360 nm


ex = 300 nm
ex = 320 nm

ex = 360 nm

ex = 380 nm
x5
0.0 (78 %)
0.88 (17 %)
1.62 (5 %)

em = 530 nm
Table 5. Conformations of the complex 4, energies of the conformers relative to 4II
40000
36000
32000
28000
24000
20000
(kcal/mol) and conformational population distribution (%, at 293K).
-1
Wavenumber, cm
4II
4Ia
4Ib
Fig. 3. Normalized emission (λex = 300, 320, 360 and 380 nm) and excitation (λem = 360
and 530 nm) spectra of the complex 1 in the solid state at 300 K.
Wavelength, nm
250
300
350
400
450
500
1,0
0,8
0,6
0,4
0,2
0,0


ex = 300 nm
ex = 320 nm
2

em = 350 nm
0.0 (100 %)
4.5 (0 %)
5.1 (0 %)
Luminescence


ex = 360 nm
ex = 380 nm
In the solid state, upon excitation at λex = 300 – 360 nm the
ligand L shows emission with the maximum in the near UV
range. The emission band of L is centered at 375 nm (Fig. 2). In
the solid state the complexes 1 and 2 demonstrate excitation
wavelength dependent emission (Fig. 3, 4). The excitation of the
complexes with 300 – 340 nm light results in the appearance of
a high-energy emission band centered at 357 nm for 1 and 353
nm for 2, with a shoulder stretching up to 500 nm for both
complexes. The low-energy emission band (λmax = 426 nm for 1;
x3

em = 430 nm
40000
36000
32000
28000
24000
20000
Wavenumber, cm-1
Fig. 4. Normalized emission (λex = 300, 320, 360 and 380 nm) and excitation (λem = 350
and 430 nm) spectra of the complex 2 in the solid state at 300 K.
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the spectrum of L1 in the solid state at room temperature are in
View Article Online
Wavelength, nm
1
DOI: 10.1039/D0DT01438A
nanosecond domain ‡ (Table 6). In contrast to L , both
complexes with this ligand, 3 and 4, show excitation wavelength
independent emission with an emission band centered at 440
and 430 nm, respectively (Fig. 6, 7). Excited state lifetimes for
this emission band lying in the nanosecond domain point to
fluorescence of both complexes.
250
300
350
400
450 500 550 600
L1
1,0
0,8
0,6
0,4
0,2
0,0

ex = 340 nm

ex = 320 nm

ex = 360 nm
Тable 6. The quantum yields (PLQY), excitation wavelengths (ex, nm), the wavelengths
of the recording of the luminescence kinetic (reg, nm), emission decay times () for L and
L1 and the complexes 1 – 4 recorded for the solid state at 300 K.

em = 400 nm

em = 500 nm
Compound
PLQY / ex, nm
λex, nm
320
λreg, nm
380
τ,
40000
36000
32000
28000
24000
20000
16000
L
<0.1 / 300
3 ns; 1 ns
3 ns; 21 ns
15 ns; 3 ns
-1
Wavenumber, cm
350
530
Fig. 5. Normalized emission (λex = 320, 340 and 360 nm) and excitation (λem = 400 and
500 nm) spectra of the ligand L1 in the solid state at 300 K.
1
10 / 360
320
380
300
320
300
380
380
380
380
430
380
450
400
530
435
435
67 μs
52 ns; 12 ns
274 μs
Wavelength, nm
2
3 / 300
250
300
350
400
450 500 550 600
ex
=
1,0
0,8
0,6
0,4
0,2
0,0
3
380 nm
L1
<0.1 / 300
9 ns
29 ns, 5 ns
4 ns, 14 ns
18 ns, 5 ns
ex
=
3
4
26 / 380
5 / 360
340 nm
ex
=
280 nm
The emission quantum yield for both series of compounds
increases in the following order: ligand < cadmium(II) complex
< zinc(II) complex. It reaches 10 % for 1 and 26 % for 3.

em = 435 nm
DFT calculations and double channel emission of 1 and 2
40000
36000
32000
28000
24000
20000
16000
-1
To gain an insight into the phenomenon of double-channel
emission depending on excitation energy, we calculated the
absorption spectra for the complexes 1 and 2 with the aim of
identifying the lowest and most intensive transitions. First 20
vertical absorptions from the ground state to the excited singlet
states and 10 vertical absorptions from the ground state to the
excited triplet states have been calculated for the complexes
(ESI, Tables S26, S28, S30). According to the TD-DFT results, the
lowest singlet-singlet absorption (S0–S1) of the complex 1 falls
in the violet region at 424 nm (f = 0.0015) and corresponds to
an almost purely HOMO – LUMO transition (99.6%). The HOMO
orbital is localized at Cl– ions with a small admixture of a d-
function on the Zn atom, whereas the LUMO orbital is
contributed by L ligand. Thus, the absorption S0–S1 has halide to
ligand charge transfer character, XLCT (X = Cl–). The higher
energy absorptions S0–Sn (n = 2, 7, 8, 9, 11, 14) also feature pure
XLCT character. The intensities of these absorptions are
primarily very weak (oscillator strengths are of the order of 10–
Wavenumber, cm
Fig. 6. Normalized emission (λex = 280, 340 and 380 nm) and excitation (λem = 435 nm)
spectra of the complex 3 in the solid state at 300 K.
Wavelength, nm
250
300
350
400
450 500 550 600
1,0
0,8
0,6
0,4
0,2
0,0
4
ex
=
360 nm
ex
=
340 nm
ex
=
280 nm

em = 435 nm
40000
36000
32000
28000
24000
-1
20000
16000
Wavenumber, cm
4
and 10–3) because of the strong space separation of the
Fig. 7. Normalized emission (λex = 280, 340 and 360 nm) and excitation (λem = 435 nm)
spectra of the complex 4 in the solid state at 300 K.
occupied and unoccupied molecular orbitals involved in these
transitions. Singlet-singlet absorptions in the region 282−377
nm have mixed ligand-centered charge transfer (LC) and XLCT
character because the filled molecular orbitals involved in the
dominant excitations have a significant ligand L percentage
contribution. The most intensive electronic transitions S0–S17
(290 nm, f = 0.2149) and S0–S18 (287 nm, f = 0.1694) have pure
LC character. The lowest spin-forbidden singlet–triplet
The emission spectrum of L1 in the solid state at room
temperature is excitation wavelength dependent (Fig. 5). The
excitation of L1 with 320–340 nm light results in the appearance
of a high-energy band centered at 380 nm with a shoulder
expanding to 650 nm. The low-energy band is centered at ca.
530 nm; it can be excited by increasing the excitation
wavelength to 360 nm. The lifetimes of both emission bands in
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absorptions in the range 331–432 nm have either pure XLCT or
mixed XLCT and LC character.
View Article Online
DOI: 10.1039/D0DT01438A
It is worth noting that among all HOMO orbitals of the
complex 1, only the orbitals HOMO-8, HOMO-9 and HOMO-10
have a significant contribution of pyridine-pyridine-phenyl part
of the ligand L (hereafter designated as PyPyPh) (ESI, Tables S26,
S30). Other HOMO orbitals are mainly localized on Zn and Cl
atoms and on atoms of L not involved in the PyPyPh part. LUMO,
LUMO+1 and LUMO+2 orbitals are completely contributed by
PyPyPh fragment of L. Compared to other calculated singlet-
singlet absorptions, transitions from the ground state to S12, S17
and S18 are fully ligand-centered, have high oscillator strengths
and involve only HOMO-10, HOMO-9, HOMO-8 and LUMO
orbitals. Therefore, we can designate S12, S17 and S18 orbitals as
S1 , S2 and S3 , respectively, meaning that the transitions S0–Sx*
(x = 1–3) are entirely intraligand and localized in the PyPyPh part
of L. Since the S0–Sx* transitions are very different from the S0–
Sn (n ≠ 12, 17, 18) transitions in oscillator strength, in transition
character and involved MOs, there is, most likely, a low degree
of overlap between the vibrational wavefunctions of Sx* and Sn
orbitals. From this point of view, the internal conversion from
the state S1* to lower-lying states S1, S2, S3, etc. is much slower
than the S1*–S0 fluorescence and these states do not participate
in the emission.
*
*
*
Fig. 8. Photophysical properties of the complex 1 in the solid state summarized in a
simplified Jablonski diagram
Table 7. Interconnection between emission types, their mechanisms and involved
molecular orbitals for the complex 1.
Transition
Mechanism
S1* – S0
Involved molecular orbitals
1LC + 1XLCT
According to the TD-DFT computations, the double-channel
emission mechanism of the complex 1 can be described in the
following steps: (1) the S0–S2* and S0–S3* excitation, (2) the
internal conversion S2*–S1*, S3*–S1*, (3a) S1*–S0 fluorescence
(high-energy emission band), (3b) S1*–T1 intersystem crossing
and T1–S0 phosphorescence (low-energy emission band) (Fig. 8).
The most intensive absorptions S0–S2* at 290 nm and S0–S3* at
287 nm are in excellent agreement with the experimental
excitation spectrum maximum of the complex 1 (λmax = 288 nm,
Fig. 3). The geometry of the complex 1 can be optimized both at
the S1* and S2* states, however, the oscillator strength of S2*–
S0 transition (f = 0.0029, on the S2* optimized geometry) is much
lower than the oscillator strength of S1*–S0 transition (f =
0.0148, on the S1* optimized geometry), thus, radiative
relaxation occurs from the lowest excited singlet state in the
PyPyPh part of L, in accordance with the Kasha rule. The
calculated S1*–S0 transition (on the S1* optimized geometry) is
a superposition of HOMO-8 – LUMO (87.3 %) and HOMO-7 –
T1 – S0
3LC
The photophysical properties of the complex 2 are similar to
those of the complex 1. The lowest energy singlet-singlet
absorptions in the region 301–416 nm are mostly not intensive
and have either pure XLCT or mixed XLCT and LC character (ESI,
Tables S27, S29, S31). Upon increase of energy, the LC nature of
some S0–Sn transitions becomes more pronounced. S0–S13, S0–
S17 and S0–S19 are lowest energy single-singlet transitions which
are localized primarily in PyPyPh part of L, thus, we can
designate these transitions as S0–S1*, S0–S2* and S0–S3*,
respectively. The radiative relaxation occurs from the orbitals
S1* and T1. The TD-DFT geometry optimization of the complex 2
in the S1* state reveals that the calculated S1*–S0 emission
energy is equivalent to 367 nm, which is 14 nm higher than the
maximum of the high-energy emission band (353 nm). This
transition is 95.0 % HOMO-8 – LUMO with 1LC character and a
1
LUMO (7.7 %) transitions with LC character and an admixture
of 1XLCT (ESI, Tables 7, S32, S33); the calculated emission energy
is 3.3975 eV (365 nm), which is only 8 nm higher compared to
the experimental maximum of the high-energy emission band
(357 nm). Among all triplet states of the complex 1, only the
geometry of T1 state can be optimized. The T1–S0
phosphorescence (on the T1 optimized geometry) is mainly
contributed by HOMO-6 – LUMO (40.1 %) and HOMO-2 – LUMO
(38.5 %) transitions with 3LC character (Table 7, ESI, Tables S36,
S37).
1
little admixture of XLCT (ESI, Tables S34, S35). The T1–S0
emission (on the T1 optimized geometry) is a superposition of
HOMO-7 – LUMO (35.6 %), HOMO-3 – LUMO (27.9 %), HOMO-
4 – LUMO (13.6 %) and HOMO-5 – LUMO (7.5 %) transitions with
3LC character (ESI, Tables S38, S39).
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Conclusions
Acknowledgements
View Article Online
DOI: 10.1039/D0DT01438A
Combining chiral terpene moieties with azaheterocycles is an The authors thank the Multi-Access Chemical Research Centre
appealing strategy for the design of new classes of chiral organic SB RAS for spectral and analytical measurements and the
ligands and metal complexes with these ligands. Here we Information Technology Centre of the Novosibirsk State
demonstrated that the method which includes the following University for providing access to the cluster computational
sequence of transformations: natural terpene → α-amino resources to carry out quantum-chemical calculations. The
oxime → N,O-diacetyl derivative → 2,2’-bipyridine bearing the research was supported by the Ministry of Science and
terpene group can be applied not only to (+)-3-carene but also Education of the Russian Federation. This paper is dedicated to
to (+)-limonene. The importance of this method of the synthesis memory of Professor Stanislav V. Larionov (1936 - 2018), our
of 2,2-bipyridine-based chiral ligands bearing terpene groups late colleague whose work constitutes a milestone in
becomes evident since 2,2-bipyridine is one of the classical and coordination chemistry.
ubiquitous ligands in the field of coordination chemistry. The
reactions of MCl2 (M = Zn, Cd) with the 2,2’-bipyridine-based
chiral ligands having the (+)-limonene and (+)-3-carene
moieties, L and L1, allowed us to study their complexation with
Notes and references
‡ The low-energy emission band can be observed in the emission
spectra of both ligands only in the solid state. It disappears in the
solutions of the ligands in MeCN (ESI, Fig. S20, S21). This fact let
us conclude that the low-energy emission band is related to
supramolecular association in the solid state.
MCl2 (M = Zn, Cd) and to synthesize new luminescent zinc(II) and
cadmium(II) complexes, ZnLCl2 (1), CdLCl2 (2), ZnL1Cl2·2H2O (3),
and CdL1Cl2·2H2O (4). In the complexes the L and L1 molecules
were found to coordinate Zn2+ and Cd2+ ions through the 2,2’-
bipyridine moiety. Interactions between the acetamide group
of the L and L1 molecules and the coordination cores of 1 – 4
(N···M2+, C=O···M2+, N–H···Cl) were found in solutions of the
complexes. In solutions the complexes exist in the form of
several conformers differing by the degree of the turn of the
acetamide moiety relative to the ligand core and the type of its
interaction with the coordination core.
Photophysical properties of the ligands L and L1 and the
complexes 1 – 4 have been investigated both experimentally
and theoretically. The complexes 1 and 2 demonstrate
excitation wavelength dependent luminescence in the solid
state. The lifetimes of the excited states in 1 and 2 are in
nanosecond and microsecond domain, indicating that there are
two deactivation channels: fluorescence and phosphorescence.
According to our TD-DFT calculations, the high-energy
fluorescence bands at 357 and 353 nm in 1 and 2 are due to
intraligand transitions in the PyPyPh part of L with a slight
admixture of halide to ligand charge transfer; the low-energy
phosphorescence bands at 426 and 428 nm are caused by
intraligand transitions involving all atoms of L. TD-DFT
calculated emission and excitation maxima for 1 and 2 are well
in agreement with the experimental data. As opposed to 1 and
2, the complexes 3 and 4 demostrate excitation wavelength
independent single-channel fluorescence with nanosecond
lifetimes of the excited states.
1
2
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Conflicts of interest
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84 A. Gusev, E. Braga, E. Zamnius, M. Kiskin, M. Kryukova, A.
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DOI: 10.1039/D0DT01438A
Coordination of chiral ligands containing a 2,2’-bipyridine moiety and natural terpene (+)-
limonene or (+)-3-carene groups to zinc(II) and cadmium(II) leads to excitation
wavelength dependent emission.