Comparative studies of the electronic, binding and photophysical properties of a new nona-dentate hemi-cage tripodal HQ pendant trizaza-macrocycle with unfilled, half-filled and completely filled lanthanide ions

Article abstract of DOI:10.1039/C8NJ02217H

The present paper describes the comparative studies of the electronic, photophysical and binding properties of a new C₃-symmetric polydentate ligand, 1,4,7-tris-{(5-methylene-8-hydroxyquinoline)-1,4,7-triazacyclononane} (9N3Me5Ox), and its complexes with 4f⁰, 4f⁷, and 4f¹⁴ metal ions (La³⁺, Gd³⁺, and Lu³⁺) by experimental and theoretical methods using DFT, TDDFT, TDDFTB, ETS-NOCV, NBO and Ligand Field DFT (LFDFT). The ligand and the complexes were synthesized and characterized through elemental analysis, molar conductance, TGA, FT-IR, FT-NMR, ¹H-¹H COSY and ESI-mass spectrometry techniques. The spectral data and structural studies revealed the formation of nine-coordinate compounds with the general formula [Ln(9N3Me5Ox)(H₂O)₃], in which the nonadentate chelator acted as a trinegative hexadentate ligand coordinating to the metal ions through three sets of O,N-donors of 8-hydroxyquinoline groups and three coordinated water molecules. The molecular modeling studies suggest that the metal ion can be easily encapsulated in its central cavity forming hemi-cage complexes without altering the basic metal-ligand coordination sphere. The nature of the bonding between the lanthanide ions and 9N3Me5Ox^3?, elucidated by means of the natural bond orbital (NBO), Morokuma-Ziegler energy decomposition analysis (ETS-NOCV) scheme, suggests that the Ln-L bonds are more electrostatic (～82percent) than covalent (～18percent). The covalent character of the complexes increases in the order Lu > La > Gd. The photoluminescence spectral studies of the metal complexes revealed that the observed luminescence of the compounds in the solid state and solution is of different origin. The vibrational, ¹H and ¹³C NMR spectral data obtained from the DFT optimized structures showed good agreement with the experimental results. The excitation and emission behaviours of the ligand and the complexes were established by molecular orbital analysis of the ground state DFT geometry as well as of the excited state optimized geometry using TD-DFT and TD-DFTB orbital analysis, excitation and emission properties.

Full text of DOI:10.1039/C8NJ02217H

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Comparative studies of electronic, binding and photophysical properties of a new nona-
dentate hemi-cage tripodal HQ pendant trizaza-macrocycle with unfilled, half-filled and
completely filled lanthanide ions
1
2
1*
Rohini , Minati Baral , and B K Kanungo
1
Department of Chemistry, Sant Longowal Institute of Engineering & Technology, Longowal, Punjab−148106, India
2
Department of Chemistry, National Institute of Technology Kurukshetra, Haryana−136119, India
Abstract
The present paper describes the comparative studies of electronic, photophysical and binding
properties of
a
new C ꢀsymmetric polydentate ligand, 1,4,7ꢀtrisꢀ{(5ꢀmethyleneꢀ
3
0
7
8
hydroxyquinoline)ꢀ1,4,7ꢀtriazacyclononane)}, (9N3Me5Ox), and its complexes with 4f , 4f ,
14
3+
3+
3+
4
f metal ions (La , Gd , Lu ) by experimental and theoretical methods using DFT, TDDFT,
TDDFTB, ETSꢀNOCV, NBO and Ligand Field DFT (LFDFT). The ligand and the complexes
were synthesized and characterized through elemental analysis, molar conductance, TGA, FTꢀIR,
1
1
FTꢀNMR, Hꢀ H COSY and ESIꢀmass spectrometry techniques. The spectral data and structural
studies reveal the formation of nine–coordinate compounds with general formula
[
Ln(9N3Me5Ox)(H O) ], in which the nonaꢀdentate chelator acted as a trinegative hexadentate
2 3
ligand coordinating to the metal ions through three sets of O, Nꢀdonors of 8ꢀhydroxyquinoline
groups and three coordinated water molecules. The molecular modeling studies suggest that the
metal ion can be easily encapsulated in its central cavity forming hemiꢀcage complexes without
altering the basic metalꢀligand coordination sphere. The nature of the bonding between the
3
ꢀ
lanthanide ions and the 9N3Me5Ox , elucidated by means of the natural bond orbital (NBO),
Morokuma Ziegler energy decomposition analysis (ETSꢀNOCV) scheme, suggest that the LnꢀL
bonds are more electrostatic (~82%) than covalent (~18%). The covalent character of the
complexes increases in the order Lu > La > Gd. The photoluminescence spectral studies of the
metal complexes revealed that the observed luminescence of the compounds in the solid state
1

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13
and solution are of different origin. The vibrational, H and C NMR spectral data obtained from
the DFT optimized structures showed good agreement with the experimental results. The
excitation and emission behaviour of the ligand and the complexes were established by
molecular orbital analysis of the ground state DFT geometry as well as on the excited state
optimized geometry using TDꢀDFT and TDꢀDFTB orbital analysis, excitation and emission
properties.
1
Introduction
The outstanding photophysical properties of the trivalent lanthanide (Ln) ions have encouraged
1
their use in a wide range of applications, like biomedical, chemical, industrial, analytical as
2
3
fluorescent materials, and probes in studies of biological systems, have lead to the discovery of
several new compounds. In addition, due to the unique electronic structures, optical and
magnetic properties, wellꢀdesigned lanthanide complexes have been widely used in magnetic
resonance imaging (MRI), biological assays, light emitting diodes (LED), telecommunications,
4
optical fibers, light amplifiers, and lasers, etc.. However, when dealing with such types
applications, the greatest attention is focused on the use of these compounds, particular
5
gadolinium complexes, as contrast agents in MRI, because of their effective magnetic moment
7
and electronic relaxation time related with the f configuration. However, in lanthanide ions, due
to of inner core effect, the weak f-f transition parityꢀforbidden, direct excitation is difficult, and
high energy sources are required.
Enhanced luminescence emission with low energy sources can be accomplished by incorporating
lanthanide ions in multidentate linkers of various geometries and denticity which contain organic
chromophores that can absorb energy efficiently and transfer them the metal ion ("antenna
effect"). For this purpose, the essential requirement is high kinetic inertness and possibly
2

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7
thermodynamic stability. Ligands containing βꢀdiketonate,
carboxylate
and 8ꢀ
8
hydroxyquinolinate are mainly used to sensitized lanthanide emission by effectively transfer
light energy to the metal ions, as they show intense absorptions in the ultravioletꢀvis (UV/VIS)
region. 8ꢀHydroxyquinoline (8HQ) and its derivatives are particularly versatile for coordination
9
of the ligands to a wide range of transition metal and lanthanide ions. Majority of studies on the
quinoline chromophores appropriate for the sensitization of lanthanide ions have focused on the
1
0
8
ꢀhydroxyquinoline as a bidentate ligand for directꢀcoordination or podands containing many
1
1
quinoline units for multidentate coordination. Proper design of polydentate chelates utilizing
specific edifice attached to multidentate 8HQ units can also have a coordination control with
predictable properties. It is expected that the complexes with direct coordination of the
lanthanide center, in which the quinoline units are attached with macrocyclic binding sites
enhance the stability of the complexes and achieve a complete coordinative saturation at the
lanthanide center. Also, the incorporation of Ln(III) in carefully designed polydentate ligands
provides the stability of the Ln(III) chelates in physiological media required for biomedical
1
2
applications. Quinolineꢀbased macrocycles have been established as sensors for pH and in
1
3
molecular logic gates. Many such multidentate chelates containing three HQ units attached to a
1
4ꢀ18
macrocycle unit with specific properties have been reported,
and some representative
structures are presented at 1 to 3 in Fig. 1. Some acyclic and cyclic tripodal hexadentate
1
6ꢀ18
chelators,
Fig. 1).
containing three 8HQ units have also been reported from this laboratory (4ꢀ7 of
3

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Fig. 1 Multidentate chelators incorporating 8HQ units: Oxinobactin, Sulfoxinobactin, H thqtcn
3
(
R=H), H thqtcnꢀSO (R=SO H), H tchqtcn, TAME5OX, TMOM5OX, CHTOM5OX, and
3
3
3
3
9
N3Me5Ox.
Incorporation of 8HQ units to the puckered ring structure of the macrocyclic 9N3, and the the
1
9
existence of several rotamers/conformers, has an additional advantage in ligand design that, it
can adjust its geometry to minimum energy on coordination with metal ions giving extra
stability.
Keeping in the above facts in view, in this communication, the detailed synthesis and
structural characterization of the polydentate tripodal ligand, 1, 4, 7ꢀtris(5ꢀmethyleneꢀoxine)ꢀ1, 4,
7
ꢀtriazacyclononane, (9N3Me5Ox), having three 8HQ units linked at 5ꢀposition through
methylene moieties to the backbone of 1, 4, 7ꢀtriazacyclononane (9N3) are reported. It is
important to note that, the ligand shows unusual interesting property: dual fluorescence, at
4

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0
physiological pH due to excimer formation , whereas, there is no such report of dual
+
fluorescence or excimer formation in similar ligands. Since the interaction of H with the ligand
is regarded as ligandꢀproton complex, it was thought worth to investigate if the dual emission
and excimer formation that had occurred in proton coordination, can also occur on metal
coordination. Hence, the complexation of the ligand (9N3Me5Ox) with La(III), Gd(III) and
Lu(III) is also reported in this communication. In addition, the photophysical properties
(electronic and fluorescence), theoretical studies of the ligand and its lanthanide complexes at the
DFT level have been incorporated to confirm the electronic structures and behavior. The nature
of metalꢀligand bonding and photophysical properties of the complexes could be established by
complementary theoretical studies like extended transition state method for energy
decomposition analysis (EDA) combined with the natural orbitals for chemical valence theory
(ETSꢀNOCV), natural bonding orbitals (NBOs), ligand field density functional theory (LFDFT),
timeꢀdependent density functional theory (TDDFT) and timeꢀdependent tightꢀbinding density
functional theory (TDFTB).
2
2
Results and discussion
.1 Design, synthesis and characterization of 9N3Me5Ox
To design a suitable structure, we considered the Cꢀ5 position of the 8ꢀhydroxyquinoline moiety
as a connecting point for the 1, 4, 7ꢀtriazacyclononane basic skeleton for two reasons. First of all,
the coordination sites are naturally located toward the center of the cavity (obtained from
preliminary inꢀsilico study using molecular mechanics), facilitating the formation of a highly
compact cage. Second, due to conformational flexibility of the presence of C3 symmetric cyclic
center on the ligand, which has an additional advantage that it can adjust its geometry on
coordination with metal ions giving extra stability as compared to traditional tripodal, tetra or
5

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1
4ꢀ18
tridentate amines as reported in literature.
This configuration also leaves open the ꢀ2 and ꢀ7
positions, that can be utilized for effective color tuning. The C ꢀsymmetric 9N3Me5Ox [shown
3
in structure 7 Fig. 1] was synthesized by following the general plan (as shown in Scheme 1).
o
Scheme 1. Reagents and conditions: (i) K CO , pꢀTsCl, PTC, 1,2ꢀDBE; (ii)H SO , 90 C, 48hrs;
2
3
2
4
(
iii) HCl gas; (iv) 3KOH; (v) HCHO, 37% HCl, 50°C, 7 hrs, HCl gas; (vi) K CO ,
2 3
acetone:water, reflux, 3 hrs
The ligand was synthesized in multiꢀsteps from the commercially available starting materials 1,
ꢀdibromoethane and diethylenetriamine as in scheme 1. The macrocycle 1, 4, 7ꢀ
2
2
1
triazacyclononane 9 was synthesized by the procedure adopted by Madison et a1.. Since 1, 4,
ꢀtriazacyclononane is an airꢀsensitive malodorous liquid, it was changed into its corresponding
7
salt as a stable white solid by passing the dry HCl gas into it. The derivatization of the oxine is
initiated by 8HQ, a very reasonable precursor, which can be easily converted into a derivative of
5
ꢀchloromethyl 8 by chloromethylation of 8HQ in position 5 (Scheme 1). The chloromethylation
2
2
stage is best performed with formaldehyde (40%) and HCl (37%). To obtain the final product
of ligand 7, the reaction was carried out using a conventional method by refluxing of 9N3 (9)
with three equivalent of 5ꢀchloromethyl 8ꢀhydroxyquinoline hydrochloride (8) in a mixture of
acetone and water in presence of K CO for 3 hours. The compound was isolated as an airꢀstable
2
3
green colour solid; it was recrystallized from chloroform as a pure compound with a yield 65%
6

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1
13
and was characterized by analysis CHN, FTIR, H NMR, COSY and C NMR and mass
spectrometry.
2
.1.1 Infrared spectra
The absorption peak at 3340 cm in the IR spectra of 9N3Me5Ox is due to the OꢀH
stretching of the 8ꢀhydroxyquinoline unit (Fig. S1). The ꢀOH of free oxine is usually observed at
ꢀ1
ꢀ1
3
730ꢀ3520 cm . The observed lower value of ꢀ than the normal, is attributed to the presence
OH
of intramolecular hydrogen bonding between –OH and –N pyridyl group, which can also be
1
attested by the H NMR spectrum of 9N3Me5Ox. The ꢀ(ꢀC=N) of the pyridine ring of quinoline
ꢀ1 23
ꢀ1
is observed at 1627 cm , whereas a peak at ∼1270 cm is due to the CꢀN bond (tertiary
nitrogen carbon bond) stretching vibrations. The stretching vibrations of the aromatic CꢀH
ꢀ1
groups of quinoline units gave very weak bands within the 3105ꢀ3058 cm [ꢀ(CꢀH)asym] and
ꢀ1
ꢀ1
2
987ꢀ2853 cm [ꢀ(CꢀH) ] ranges. The sharp absorption peaks at ∼1231 and ∼1078 cm are
sym
2
4
assigned respectively to [ꢀ(OꢀC)asym] and [ꢀ(OꢀC) ]. In addition, the appearance of a new
sym
ꢀ1
peak at ∼830 cm was observed due to the outꢀofꢀplane deformations of the CꢀH groups of the
ꢀ1
aromatic ring. Absorption peaks around 1578 and 1368 cm are due to ꢀCꢀC of the aromatic ring
2
5
ꢀ1
of the 8HQ nucleus. The obtained weak bands at ∼1960 and ∼1769 cm may be attributed to
the ꢀ and ꢀ of methylene groups (ꢀCH ꢀ). Theoretical calculations were performed by DFT at
as
s
2
GGA(BLYP) level to confirm the assignments of the IR frequencies. The theoretical computed
IR frequencies of the 9N3Me5Ox are reported in section 2.3.2 and the acceptable correlation
2
(
R =0.9955) obtained from theoretical and experimental data as shown in Fig. 7.
2
.1.2 NMR Study
1
The H NMR spectrum of 9N3Me5Ox in DMSO (d ) is shown in Fig. S2. The NMR
6
peaks of podant were broad. The 8HQ peaks were found to be slightly shielded as compared to
7

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6
free 8HQ. The assignment of proton various peaks was confirmed by a correlation experiment.
The COSY spectra of the ligand in the same solvent is presented in Fig. S3. The aromatic ArꢀH
protons of 8ꢀhydroxyquinoline appeared as multiplets. The unit displays signals in range 6.0 to
9
.0 ppm, with double doublet at δ = 4.1 to 4.5 ppm designed to the methylene (ꢀCH ) groups of
2
the pendant arms due to the close proximity of the ring, and four signals at δ = 6.8, 7.3, 8.4 and
.6 ppm for the hydroxyquinoline rings. The ligand showed a broad peak for hydroxyl (ꢀOH)
8
proton of quinoline ring at δ = 9.7 ppm, probably due to the intramolecular Hꢀbonding between
OH proton and N atom of quinoline unit. The result is supported by the ꢀꢁOꢀH observed at 3340
ꢀ1
27ꢀ28
cm of the infrared spectrum. Similar results have also been reported other polypod ligands.
1
3
The C NMR spectrum of 9N3Me5Ox with the proposed assignments scheme is given in Fig.
S4. The ligand showed bands at 31 and 33 ppm for CH −N−CH and NꢀCH ꢀAr carbons,
2
2
2
1
3
respectively. Usually, C NMR band for carbons attach to tertiary nitrogen in 1, 4, 7ꢀ
2
9
triazacyclononane appears at 47.8, but in 9N3Me5Ox, carbons attach to tertiary nitrogen band
are shielded at 31 and 33 ppm. The shielding effects on the cyclic and aliphatic carbons of
9
N3Me5Ox were observed due to, 9N3 cyclic ring attach to 8HQ ring at paraꢀposition. The ring
3
0
showed +R effect, and both the carbons experienced less deshielding effect. Signals at 111.2,
22.3, 127.3, 128.46, 133.5,138.5, 148.4 and 152.4 ppm belong to the aromatic carbon of 8HQ
unit. Peak at 152 is assigned to the –OH group attached to the carbon present in the 8HQ unit.
.1.3 Mass spectra
1
2
+
The highꢀresolution electrospray ionization mass spectrometry MS (ES ) was used to
+
identify the product through evaluation of the molecular ion peak of 9N3Me5Ox. The MS (ES )
+
spectrum of 9N3Me5Ox (Fig. S5) showed a parent peak at m/Z (I%) [M+H] calculated 597.2,
found 599.2, (15%) corresponding to deprotonated [MꢀH] form of ligand.
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2
.2 Synthesis and characterization of metal complexes of 9N3Me5Ox trivalent La, Gd and
Lu metal ions
The lanthanide complexes, [Ln(9N3Me5Ox)(H O) ] (Ln = La, Gd, and Lu), were
2
3
prepared by reacting equimolar amounts of Ln(NO ) .5H O and 9N3Me5Ox in acetonitrile. The
3
3
2
reaction mixture was stirred for 24 hours at room temperature, after which the colour of reaction
mixture changed from straw to dark green with formation of a precipitate, which was filtered,
washed with diethyl ether and dried in vacuum. The compounds were characterized by elemental
1
analysis, ESIꢀmass spectra, FTIR, TGA, H NMR spectra; yield: [La(9N3Me5Ox)(H O) ] 42%,
2
3
[
Gd(9N3Me5Ox)(H O) ] 40% and [Lu(9N3Me5Ox)(H O) ] 48%. Elemental analysis of all
2 3 2 3
complexes is consistent with analytically pure [Ln(9N3Me5Ox)(H O) ]. Analytical data for
2
3
newly synthesized complexes listed in Table 1.
Table 1. Analytical data and some physical properties of ligand (9N3Me5Ox) and their
complexes
o
a
Sr.No
Molecular Formula
Colour
M.P. C
(Λ )
M
Calc. (Found)
C%
H%
N%
1
2
3
4
C H N O
3
Straw
170ꢀ175
274–276
288–290
294–296
9.0
22
20
20
71.98 (71.55)
54.69 (54.12)
53.45 (53.60)
52.30 (52.38
6.04 (6.15)
4.97 (4.84)
4.86 (4.82)
4.75 (4.80))
13.99 (13.25)
10.63 (10.73)
10.39 (10.23)
10.17 (10.04)
36
36
6
[La(C H N O )(H O) ]
Pear green
Phthalo green
Lime green
36
33
6
3
2
3
[Gd(C H N O )(H O) ]
36
33
6
3
2
3
[Lu(C H N O )(H O) ]
36
33
6
3
2
3
a
ꢀ1
2
ꢀ1
Ω cm mol (in DMSO)
Elemental analysis supported by highꢀresolution electrospray ionization time of flight
+
mass spectra TOF MS(ES ) was used to recognize the product by evaluating the molecular ion
peaks of 9N3Me5Ox metal complexes. The mass spectrum of [La(9N3Me5Ox)(H O) ] [Fig. S6
2
3
+
(
a)] showed a peak at m/Z 791.01 attributed to the protonated complex fragments [La(M+H)] .
For [Gd(9N3Me5Ox)(H O) ] complex, the mass spectra evidenced ion peak at m/z 808.3 while
2
3
for [Lu(9N3Me5Ox)(H O) ], the mass ion peak was observed at m/z 826.40. The spectral
2
3
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diagrams are depicted in Fig. S6 (bꢀc). The ionization of the metal complexes was mainly
obtained by the addition of protons.
All the complexes of 9N3Me5Ox are soluble in dimethylsulfoxide (DMSO) and
dimethylformamide (DMF), slightly soluble in acetonitrile, but not soluble in chloroform, ethyl
ꢀ3
acetate, methanol, and ethanol. The molar conductance in 10 M DMSO of the complexes are
31
too low to account for an ionic complex ; therefore, these complex are considered to be neutral.
2
.2.1 Infrared spectra
ꢀ1
The FTIR spectra were of the complexes recorded the region 4000–400 cm is shown in Fig. S7.
ꢀ1
In all the spectra of the complexes, the band at 3339 cm due to ꢀOH of the ligand was
disappeared, indicating the OH group is involved in complex formation. Appearance of a new
ꢀ1
broad band in the 3345ꢀ3400 cm spectral region corresponds to the vibration of the OꢀH
stretching. This can be correlated to the presence of water molecules in the complex.
ꢀ1
Characteristic peaks for coordinated water molecules were observed at around 755 cm due to ꢂ
r
ꢀ1
(
H O) and 643 cm ꢂ (H O). The presence of coordinated water molecules was also further
2
w
2
supported by TGꢀDTG analysis. Other important absorption bands corresponding to vibrations of
ꢀ1
ꢀ1
(
C=N) and (C=C) of pyridine and aromatic groups of the ligand at 1627 cm and 1457 cm were
ꢀ1
ꢀ1
shifted to higher wave number 1657 cm and 1471 cm respectively. The stretching vibration at
ꢀ1
∼
1300 cm was observed for C ꢀO group of the phenolate moiety, while less intensive
ar
vibrations of aromatic CꢀH bonds and HꢀCꢀH group vibrations due to outꢀofꢀplane deformation
ꢀ1
at ∼1066, and ∼755 cm were detected respectively. In low energy region, LnꢀO and LnꢀN
ꢀ1
32
stretching vibrations were identified at ∼460 and ∼411cm ; respectively, as in literature data.
2
.2.2 Thermal gravimetric analysis
1
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Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG)
ꢀ1
studies were carried out in an atmosphere of nitrogen at a heating rate of 20 °C min . The
thermogravimetric curves obtained for all metal complexes were similar [Fig. S8 (a)]. The curve
of one representative complex, [La(9N3Me5Ox)(H O) ], is considered for deliberation. The
2
3
initial loss of mass is due to the loss of lattice water, followed by the coordinated water. The
TGAꢀDTA curves show that between 82 and 130 °C there is 1.90 % loss in the initial mass loss
due to loss of the lattice water molecules. The second loss of mass (2.95 %) between 250 and
2
75 °C, is due to one of the coordinated water molecules. The major loss of mass for water
molecules (6.54%) can be attributed to the two intramolecular bonded coordinated water
molecules that occurred between 315 and 340 °C. Thus, the total loss of mass for water
molecules is 9.49%. This value corresponds to three coordinated water molecules (calcd.
1
0.09%) in the complex. This finding is in good agreement with the observed experimental
elemental analysis data. The decomposition of the complex [La(9N3Me5Ox)(H O) ] begins at
2
3
∼350 °C and continues up to 500 °C, with a significant loss (62.3%) in weight inconsistent with
the destruction of 9N3Me5Ox (calcd. 65.1%). The complex [Ln(9N3Me5Ox)(H O) ] leaves
2
3
1
5.4% residue confirming the presence of the inorganic lanthanide. In [Gd(9N3Me5Ox)(H O) ]
2 3
and [Lu(9N3Me5Ox)(H O) ] complexes, three water molecules were expelled simultaneously at
2
3
1
30 °C to 155 °C. Unlike the [La(9N3Me5Ox)(H O) ] complex, where there was intramolecular
2 3
hydrogen bonding between two water molecules. The results are shown in supplementary data as
Fig. S8 (b) and S8 (c).
1
2
.2.3 H NMR spectra of the [Ln(9N3Me5Ox)(H O) ] complexes
2
3
1
3+
3+
3+
3+
The H NMR spectra of [Ln(9N3Me5Ox)(H O) ] (Ln =La , Gd and, Lu ), with nonꢀ
2
3
equivalent proton peak assignments for the 8HQ moiety and 9N3 central unit are shown in Fig.
1
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S9. The H NMR spectra of [La(9N3Me5Ox)(H O) ] and [Lu(9N3Me5Ox)(H O) ] in DMSOꢀd
6
2
3
2
3
solution shows a single set of 11 signals. This pattern is consistent with a C symmetry of the
s
species in solution, in which the three 8ꢀhydroxy pendant arms are equivalent with C symmetry.
3
The methylene proton of macrocyclic (NꢀCH -CH ꢀN) rings appeared 0ꢀ4 ppm, due to the axial
2
2
and equatorial proton of macrocycle ring. The NꢀCH -CH ꢀN moieties of the macrocycle form a
2
2
complex series of multiplets that were only partially assigned. A double of triplets was observed
at 4.53 ppm as a result of a strong NOE effect between the protons H of the CH group close to
b
2
the H proton of 8HQ. The proton NMR spectra of aromatic proton slightly downfield range 5–9
d
2
8
1
relative to free ligand, due to weak metalꢀligand interaction. The H NMR spectra of complexes
showed the same splitting pattern as in the ligand, except for gadolinium complex [Fig. S9 (iii)].
In particular, the 8HQ ring protons in diamagnetic lanthanum and lutetium complexes are wellꢀ
resolved and slightly downꢀfielded as compared to gadolinium complex. The complex of
gadolinium showed broadband due to its paramagnetic nature caused due to the presence of
seven unpaired electrons. The paramagnetic nature of the metal ion has the faster time of
1
relaxation; as a result, the H NMR signals attached to the ligand have been dramatically
broadened and shifted to upfield. Comparison of the paramagnetic complex
1
[
[
Gd(9N3Me5Ox)(H O) ]
H
NMR spectra, with the diamagnetic complexes
2
3
1
La(9N3Me5Ox)(H O) ] and [Lu(9N3Me5Ox)(H O) ] H NMR spectra, suggests a rough
2
3
2
3
ascription in [Gd(9N3Me5Ox)(H O) ] (metal ion inner sphere) the large widths of the
2
3
coordinated ligand resonances, complex exclude the complete assignment of the protons signals.
The shifts may come either from pseudoꢀcontact, i.e., throughꢀspace dipolar interactions, or from
the contact shift, i.e., direct delocalization of the unpaired electron spinꢀdensity from the metal
due to binding of the ligand to the paramagnetic metal ion.
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2

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3
+
2
.2.4 Electronic spectra of the Ln complexes
The UVꢀvis absorption spectra of the ligand and its lanthanide complexes were recorded
−
5
in dimethyl sulfoxide (DMSO) solution (2×10 M). The results are presented in Fig. 2. The
ligand shows a series of absorption bands located in the UV region 209, 219, 240, 260, and 330
nm. They are characteristically observed for π–π* and n–π* transitions situated on the 9N3
substituted 8HQ ligand with long wavelength absorption trails to 402 nm (Fig. 2). In the metal
complexes, the π–π* bands did not undergo any appreciable change, but slightly redꢀshifted
indicating that the 9N3Me5Ox ligand moiety exists in the complexes. Lanthanum (III) and
lutetium (III) complexes have no significant absorption in the visible region, due to the absence
of 4f orbital electron transition. Upon deprotonation and formation of the complex, a blue shift of
~
30 nm was observed for the bands at 209, 219 and 240 nm. A new broad absorption band
appeared in the nearꢀvisible region with a maximum apparent at 382, 397, or 392 nm for
La(9N3Me5Ox)(H O) ], [Gd(9N3Me5Ox)(H O) ], and [Lu(9N3Me5Ox)(H O) ], respectively
[
2
3
2
3
2
3
and extended to 450 nm. It was found that the molar absorption coefficients of the band at λmax
3
ꢀ1
ꢀ1
2
67 nm were found to be 3950, 1800 and 3550 /dm mol cm for [La(9N3Me5Ox)(H O) ],
2 3
[
Gd(9N3Me5Ox)(H O) ], and [Lu(9N3Me5Ox)(H O) ] indicating that the complexes have
2 3 2 3
strong ability to absorb light in the said region. The spectral shapes indicate that the coordination
of the lanthanide ion with 9N3Me5Ox ligand does not have a significant effect on the energy of
the singlet state of the 9N3Me5Ox.
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3

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Fig. 2 Experimental and theoretical electronic spectra of 9N3Me5Ox and its metal complexes:
(
a) Experimental electronic spectra of [La(9N3Me5Ox)(H O) ], [Gd(9N3Me5Ox)(H O) ] and
2 3 2 3
[
Lu(9N3Me5Ox)(H O) ] in DMSO at 298K (b) Theoretical electronic spectra of 9N3Me5Ox and
2
3
its metal complexes in gas phase.
To ascertain the electronic transitions of 9N3Me5Ox and its complexes obtained experimentally,
the theoretical spectra were calculated using the timeꢀdependent density functional theory (TDꢀ
DFT) at GGA (BLYP) level. The calculated spectra of 9N3Me5Ox and its metal complexes from
the corresponding optimized geometries are discussed in section 2.4.4 and the spectra are shown
in Fig. 2(b).
2
.2.5 Excitation and photoluminescence spectra for metal complexes
The photoluminescence spectra of the studied lanthanide complexes in solid state and in
dimethylsulfoxide solution were measured in the visible spectral region under ultraviolet (UVA)
light excitation (λ = 380 nm). The backgroundꢀcorrected emission photoluminescence spectra
ex
of the complexes are shown in Fig. 3. A broad emission band was observed for all the metal
complexes. It is well known that lanthanideꢀcontaining compounds are luminescent due to f–f
electronic transitions exhibiting sharp emission lines in the UV, Visible and Nearꢀinfrared (NIR)
3
3
3+
spectral regions. Typically, broad emission bands are not observed for Ln ions due to the
absence of molecular/lattice vibrations, which typically cause the homogeneous spectral
broadening. Moreover, the 4fꢀelectrons, which play a role in optical transitions, do not participate
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4

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3
+
much in binding (the covalency of a Ln –ligand bond is at most 5%–7%), so the excitationꢀ
induced rearrangement does not affect the binding pattern in the molecules. The internuclear
distances remain almost unchanged after the excitation, and generates small Stokes shifts. On the
other hand, organic compounds, the electronic transitions are considerably affected by molecular
vibrations, and the excitation often leads to the lengthening of chemical bonds. Thus, the
3
3
emission bands are usually much broader with larger Stokes shifts. In luminescence study in
solution, a broadband at 425–600 nm centered at 478 nm was observed in all the complexes, that
can be assigned to the emissions deriving from the ligand since it incorporates an appropriate
conjugated system. Interestingly, the intensity of the broad bands were more as compared to
3
+
3+
ligand peak. In case of La and Lu complexes, the intensity of broad bands were about 9.1
3
+
times (quantum yield 0.045ꢀ0.043), and 1.2 times for Gd complex (quantum yield 0.006) as
3
+
3+
compared to ligand (quantum yield 0.0049). It is expected that La and Lu cannot significantly
change the emission intensity of 9N3Me5Ox because it has no lowꢀenergy f–f states for energy
3
+
3+
transfer. The observed high enhancement of the fluorescence on complexation of La and Lu
may be attributed due to the fact that photoinduced proton transfer (PPT) and photoinduced
electron transfer (PET) are inhibited in fluorescent complexes. It has been demonstrated that
tertiary amine close to 8HQ derivatives is able to quench the fluorescence quantum yield because
3
4
of PET processes from the amine to 8HQ moiety . The lone pairs of the central Nꢀcontaining
macrocycle can also quench the fluorescence on the free ligand due to the above process. In the
3
+
3+
La and Lu complexes of 9N3Me5Ox, the PPT and PET processes are not active because the
amino group of the macrocyclic ring acts as a base and serves to deprotonate the 8HQ hydroxyl
group. Thus, the PPT processes are precluded because the 8HQ hydroxyl group are deprotonated
and the PET processes are not possible because the proximal amine groups are protonated
1
5

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3
+
(typing up the lone pairs of the electrons responsible for the PET process). In case of Gd
complex, the observation of very weak luminescence spectra is apparent. It may be due to the
3
+
lowest excited state of Gd , is too high to receive energy from the organic ligand, and thus
complex shows the luminescence of the ligand in the absence of charge transfer to the central
atom.
Fig. 3 (a) Excitation, (b) Emission spectra of 9N3Me5Ox and its metal complexes in solid state
(c) Excitation and emission spectra of 9N3Me5Ox and its metal complexes in DMSO.
In solidꢀstate luminescence study, the board bands in the range 400ꢀ650 nm (see Fig. 3b)
were also observed in the powdered samples due to the aromatic ligand.
.5.6. Time Decay study.
2
The luminescence decay curves of the ligand and its La, Gd and Lu complexes in
dimethylsulfoxide solution were recorded at room temperature. The curves were wellꢀfitted by
monoexponential function indicate the presence of a single luminescent site in the complexes.
The luminescence lifetime was determined from the slope of logarithmic plot of decay curve is
shown in Fig 4. To increase intrinsic quantum yield, it is necessary to enlarge the radiative
transition, meanwhile supressing the nonꢀradiative transition. The radiative decay includes the
rate of fluorescence and phosphorescence. The nonꢀradiative decay includes back energy transfer
to the ligand and quenching by presence of high energy oscillators, such as OH in the bound
water molecules, and NꢀH, and CꢀH stretching vibration in ligand.
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6

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Fig. 4. The room temperature luminescence decay curves of the 9N3Me5Ox,
La(9N3Me5Ox(H O) ], [Gd(9N3Me5Ox(H O) ] and [Lu(9N3Me5Ox(H O) ] in
[
2
3
2
3
2
3
dimethylsulfoxide solution (λ = 478 nm).
em
3
+
The radiative (KRAD) and nonradiative (K ) rates of 9N3Me5Ox and its metal complexes (La ,
NR
3
+
3+
Gd , and Lu ) were evaluated using the following equations:
KRADꢁꢃꢁꢄꢅ
(1)
(2)
ꢆꢇꢈꢉꢊ
K ꢃꢁ
NR
ꢋ
ꢍꢎꢏꢐ
ꢌꢁꢁ= ꢍꢑꢎ
(3)
The emission lifetime of the 9N3Me5Ox and complexes are given in Tables 2. The radiative and
9
ꢀ1
9
ꢀ1
nonradiative rates are 0.0053 X 10 s and 1.085 X 10 s for 9N3Me5Ox. The values were
9
ꢀ1
modified after addition of metal of the ligand. The radiative rate changed from 0.0053 X 10 s
9
ꢀ1
9
ꢀ1
to 0.0469 X 10 s (increase) and the nonradiative rate changed from 1.085 X 10 s to 0.768 X
9
ꢀ1
1
0 s (decrease) with changing from ligand to the complexes, [Ln(9N3Me5Ox)(H O) ]. The
2 3
3
+
shortening of decay time in presence of Ln metal ions indicates the change of nonradiative
3
+
relaxation rate of the excited 9N3Me5Ox, i.e., due to energy transfer from 9N3Me5Ox to Ln
metal ions.
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Table 2: Radiative (KRad) and nonꢀradiative (KNR) decay rates and quantum efficiency (η) for
9
N3Me5Ox and its lanthanides metal complexes.
ꢀ1 a
ꢀ1 a
Sr.No
Compound
K
(s )
KNR (s )
τobs(ns)
0.9168
1.0477
0.9150
1.0360
ꢌꢁ(%)
Rad
9
9
9
9
9
1
2
3
4
9N3Me5Ox
0.0053 X 10
0.0429 X 10
0.0065 X 10
0.0469 X 10
1.085 X 10
0.4884
4.7065
0.8463
4.5096
9
[La(9N3Me5Ox)(H O) ]
0.9115 X 10
2
3
9
[Gd(9N3Me5Ox)(H O) ]
0.768 X 10
2
3
9
[Lu(9N3Me5Ox)(H O) ]
1.04 X 10
2
3
2
.3 Theoretical studies
.3.1 Conformational stability and molecular geometry
2
Since we could not isolate any single crystal, a molecular modeling study was carried out using
molecular mechanics, semiꢀempirical as well as by DFT method for the ligand and its metal
complexes to establish the structures. The initial geometry of 9N3Me5Ox that led to the
minimum strain was obtained by the molecular mechanics calculation using the UFF force field
was reoptimized by semiꢀempirical PM6 selfꢀconsistent fields (SCF) method at the Restricted
Hartree–Fock (RHF) level. The resulting PM6 optimized molecular geometry was used for DFT
2
0
level calculations. The results of the previous study on the same ligand showed that the
conformer in which three pendants (8HQ) are in the π− π stacking arrangement has the minimum
energy. It is worth to mention here that, out of three 8HQ rings of 9N3Me5Ox, two were parallel
to each other (parallel πꢀstacking), while the third ring showed perpendicular πꢀstacking with Tꢀ
shaped structure 7 (Fig. 5). The structure can be viewed as a hemiꢀcage bowl (Fig. 5).
3
ꢀ
9
N3Me5Ox is a potential trinegative nonadentate ligand (N O ) that has nine coordination sites:
6 3
three nitrogen atoms from the 9N3 cyclic ring, three nitrogen and three oxygen atoms from the
three 8ꢀhydroxyquinolinate units. The lanthanide ions show coordination numbers ranging from
8
to 12 out of which 9 is more common. It is expected that to satisfy nineꢀcoordination, the nonaꢀ
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dentate ligand, 9N3Me5Ox, can coordinate to the metal ion to saturate its secondary valences to
result in a stable complex. However, molecular mechanics calculations using UFF force field
suggests that the ligand topology is such, that it prevents nine coordination to the same metal ion
by escalating the strain energy as compared to the six coordination. This is due to the wrong
positioning of the three tertiary amine coordination sites of 9N3 that are opposite to the N, Oꢀ
coordination sites of each 8HQ group. Forcing 9ꢀcoordination to a Ln (III) increases the strain
energy tremendously. Anchoring cyclic 9N3 ring can facilitate 6ꢀcoordination with two
coordination positions each in the three oxinate group. It can be seen from the optimized
geometry that the alignments of coordination sites naturally point toward the centre of the cavity
of the ligand facilitating formation of a highly compact cage. This may be due to conformational
flexibility of the central unit 9N3. In order to satisfy the higher coordination number, a number
of water molecules were added to predict a suitable plausible geometry. A coordination scan
(Fig. 6) was carried out with the SYBYL force field (the water strain energy in this force field
3
5
assumes zero ) using molecular mechanics with variation in coordinated water molecule from
three to six, (assigning maximum coordination number 12) to ascertain the number of water
molecules. The scan results suggest that the molecule prefers the 9ꢀcoordination geometry with
stoichiometry [Ln(9N3Me5Ox) (H O) ], that contains three water molecules in the coordination
2
3
sphere. The result is in conformity with the experimental data obtained from infrared spectra and
thermal analysis.
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9

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Fig. 5 DFT optimized ground state geometries of ligand and complexes (7) 9N3Me5Ox, showing
preorganized three 8HQ units as readiness for complex formation (10) [La(9N3Me5Ox)(H O) ],
2
3
(
11) [Gd(9N3Me5Ox)(H O) ], and (12) [Lu(9N3Me5Ox)(H O) ] ], distorted tricapped trigonal
2 3 2 3
3
+
3+
3+
prism structure for La (e), Gd (f), and Lu complexes (g).
Fig. 6 Coordination scan plots for the ligand (9N3Me5Ox) with the varied coordination
3+
number for Ln complexes.
The nine coordinated structures obtained from molecular mechanics and semiꢀempirical
3
6
calculations were further optimized at DFT level using ADF package for further studies using
2
0

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3
7
the generalized gradient approximation (GGA) with the exchangeꢀcorrelation functional
3
8
proposed by Becke correlation functional; Lee, Yang, Parr (BLYP) exchangeꢀcorrelation
3
9
functional using the ZORA and DZ basis sets for metal atoms and lighter elements (the 1s core
4
0
for carbon, nitrogen, and oxygen were kept frozen) in the gas phase. The structures of
optimized molecules are presented in Fig. 5. The structural parameters of the DFT optimized
geometries of the three lanthanide complexes in the ground state as well as in the first excited
state are presented in Table 3. The coordination atmosphere around the lanthanide ion can be
ascribed as a tricapped trigonal prism with three 8HQ pendants of the ligand linked to the metal
ion in the molecules arranged in a helical fashion with small distortion with a pseudo C3
symmetry (Fig. 5). In all the three complexes, triangular faces are formed by the one nitrogen,
one oxygen of 8ꢀhydroxyquinoline and one oxygen of water molecules. The planes of first and
third triangular faces are almost parallel. The second triangular face is slightly distorted due to
elongation of 8ꢀhydroxyquinoline nitrogen. The resulted elongation of LnꢀN bonds is responsible
for the general shape of lanthanide complexes. The values of the LaꢀN bond lengths are 3.427,
3
.698 and 2.757 Å for 8ꢀhydroxyquinoline nitrogen bonds (Fig. 4). Bite angles of 54.3°, 49.4°,
and 63.8° are observed for the N ꢀLaꢀO , N ꢀLaꢀO and N ꢀLaꢀO coordinating units. It has been
1
6
2
5
3
4
observed, in the ground, as well as in the excited state, there is a continuous decrease in the LnꢀN
(
Ox) and LnꢀO (Ox) distances in the series, (i.e., [La(9N3Me5Ox)(H O) ]
>
2
3
[
Gd(9N3Me5Ox)(H O) ] > [Lu (9N3Me5Ox)(H O) ]). In addition, a constant decrease of the
2
3
2
3
dihedral angle of the 8HQ oxygen and nitrogen bridge (N -C-C-O ) was recorded through the
ox
ox
series. These results are in concordance with the trend of the lanthanides, wherever the ionic
radii of the lanthanide metal ion decrease when one moves along the period, generating a bigger
torsion on the ligand. The LnꢀO bond lengths were found to be shorter than the LnꢀN bond
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lengths in complexes by about 1.103 Å. This large bond length difference suggests that oxygen
atoms of the 8HQ have relatively stronger electronꢀdonating capacity toward Ln(III) ion as
compared to the nitrogen atoms of the 8HQ. Minor variations in bond lengths and angles were
exhibited in the excited state geometries for all the [Ln(9N3Me5Ox)(H O) ] systems. From the
2
3
above results, it can be assumed that there is no significant conformational change in the
geometrical parameters in the excitedꢀstates as compared to their ground state. The small
changes in the observed in stoke shifts are due to small structural variations between the ground
and the excited state geometries. It can also be inferred that the ligand, 9N3Me5Ox, is flexible
enough to encapsulate different size lanthanide ions without changing the basic ligand sphere. Its
3
ꢀ
flexibility is analogous to that observed in tpatcn , despite the increased rigidity of the 8HQ
2
8
group than picolinate group.
Table 3. Theoretical calculated geometrical parameter: Bond Length (Åꢊ,ꢁdihedral angles (Deg)
and angles for the ground and first excited state on [Ln (9N3Me5Ox)(H O) ] systems
2
3
Closed-Shell
Open-Shell
[
La(9N3Me5Ox)(H O) ] [Lu(9N3Me5Ox)(H O) ] [Gd(9N3Me5Ox)(H O) ]
2
3
2
3
2
3
Ground
State
Excited
State
Ground
State
Excited
State
Ground
State
Excited
State
ꢒꢓꢔꢈꢕꢇꢖꢗ
ꢒꢓꢔꢈꢕꢘꢖꢗ
ꢒꢓꢔꢈꢕꢙꢖꢗ
ꢒꢓꢔꢈꢚꢇꢛꢜꢝ
ꢒꢓꢔꢈꢚꢘꢛꢜꢝ
ꢒꢓꢔꢈꢚꢙꢛꢜꢝ
ꢒꢓꢔꢈꢚꢞꢝꢗ
3.427
3.501
3.329
3.487
3.359
3.572
3.698
2.757
2.617
2.702
2.565
2.324
3.625
2.609
2.596
2.726
2.336
2.312
3.459
2.514
2.359
2.462
2.302
2.175
3.471
2.436
2.356
2.438
2.113
2.207
3.517
2.620
2.462
2.564
2.400
2.234
3.676
2.772
2.538
2.589
2.408
2.291
ꢒꢓꢔꢈꢚꢟꢝꢗ
ꢒꢓꢔꢈꢚꢠꢝꢗ
∠ꢕꢇꢈꢓꢔꢈꢚꢠ
∠ꢕꢘꢈꢓꢔꢈꢚꢟ
∠ꢕꢙꢈꢓꢔꢈꢚꢞ
∠ꢚꢇꢈꢓꢔꢈꢚꢠ
∠ꢚꢘꢈꢓꢔꢈꢚꢞ
∠ꢚꢙꢈꢓꢔꢈꢚꢟ
2.474
2.390
54.3
49.4
63.8
72.5
136.6
72.0
13.0
ꢀ0.9
2.598
2.655
51.7
50.4
66.4
68.3
127.8
74.6
12.6
ꢀ0.2
2.273
2.273
52.5
53.3
69.9
70.2
136.1
81.8
14.9
ꢀ1.0
2.278
2.561
52.5
53.3
69.9
70.2
136.1
81.8
15.3
ꢀ2.3
2.344
2.346
55.5
52.8
66.7
73.6
137.4
75.1
14.0
ꢀ1.3
2.480
2.542
51.2
50.0
64.7
70.1
134.8
74.2
13.5
ꢀ5.6
∠ꢕꢇꢈꢡꢇꢢꢈꢡꢇꢟꢈꢚꢠ
∠ꢕꢙꢈꢡꢟꢈꢡꢠꢈꢚꢞ
∠ꢕꢘꢈꢡꢘꢙꢈꢡꢘꢞꢈꢚꢟ
ꢀ9.7
ꢀ8.8
ꢀ11.5
ꢀ10.3
ꢀ11.0
ꢀ10.9
1
13
2
.3.2 Theoretical IR spectra, H NMR, and C NMR spectral analyses
2
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The theoretical vibrational spectra and the wavenumbers for all the fundamental modes of
N3Me5Ox computed using GGA (BLYP), along with the experimental values and assignments,
9
are presented in Fig. S10 and Table 5. The ligand molecule consists of 81 atoms and belongs to
C point group, hence the 237 fundamental modes of vibrations are distributed as vib =161 A՛ +
3
7
6 A՛ ՛ (A՛ inꢀplane and A՛ ՛ outꢀofꢀplane vibrations). The assignment of the vibrational
modes of the molecules was done with help of normal coordinate analysis. Theoretically
calculated vibrational spectra of the ligand and the complexes were made accessible to illustrate
the experimental vibrational peaks through the geometrical animation at every peak. The
theoretical calculated vibrational frequencies with respect to experimental values exhibited an
identical trend. Slightly higher values were observed for stretching vibrations in theoretical
results. The slight variation in calculated and experimental data of the spectra may be attributed
due to the fact that, the observed experimental spectral data were obtained in the solid phase,
whereas the theoretical spectra were measured in the gas phase. The trend was persistent with the
experimental results. A linear correlation observed between the experimental and the theoretical
2
2
data provided an acceptable result, (R = 0.9924 for the ligand (Fig. 7a) and (R = 0.9938ꢀ0.9986)
for the complexes Fig. 7(b). This confirms the reliability of the presented band assignments.
It is recognized that precise perditions of molecular geometries are important for calculation of
magnetic properties. To predict theoretically the chemical shifts, it has been shown that a variety
of functional combined with medium to large basis sets to bring results in good concurrence with
1
13
the experimental data. H and C NMR chemical shifts were calculated from the GGA (BLYP)
4
1
optimized molecule, by the GIAO (gaugeꢀincluding atomic orbital) in the gas phase using
tetramethylsilane (TMS) as standard. All relative chemical shifts (δ) are represented as (δ = σref ꢀ
σ), where (σ) is the absolute shielding constant. These were calculated at the same computational
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3

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level. The computed chemical shifts (Table 4) are in good agreement with observed chemical
shift values with an average error of 0.25 [Fig. 7(cꢀd)].
Fig. 7 A linear correlation between the experimental and theoretical IR frequencies of
1
13
9
N3Me5Ox (a) and its metal complexes (b) H NMR (c) and CNMR (d).
1
13
A comparison of experimental and theoretically calculated H and CNMR chemical shifts
along with isotropic magnetic shielding tensors and the infrared frequencies for the studied
systems are listed in Table 4.
1
13
Table 4. Calculated GGA(BLYP) and Experimental (DMSO, 400 MHz) H NMR and C NMR
Chemical Shifts (ppm) and IR Frequencies (KBr) for 9N3Me5Ox. The numbering scheme (Haꢀ
H , C ꢀC ) is shown in Fig. S9.
i
a
k
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NMR Chemical Shifts
IR Frequency
Proton
Ha
Hb
Hc
Hd
He
Hf
Hg
Hh
Hi
Experimental GGA(BLYP) Carbon
Experimental GGA(BLYP) Experimental GGA(BLYP)
3.4
4.35
4.68
6.99
7.09
7.56
8.44
8.739
9.64
3.1
Ca
Cb
Cc
Cd
Ce
Cf
Cg
Ch
Ci
31.29
33.11
31.40
34.12
3609
3332
3058
2911
1627
1576
1371
1158
1074
913
3619
3207
3207
2998
1649
1556
1383
1152
1087
927
4.29
4.48
6.78
7.0
7.22
8.20
8.43
8.99
111.01
122.23
127.09
127.74
127.95
133.33
139.52
148.4
111.44
121.93
126.98
127.34
127.65
138.97
138.34
148.87
153.58
Cj
Ck
152.57
803
846
ꢀ1
Table 5. Comparison of calculated theoretical and experimental IR frequencies (KBr, cm ) for
[
La(9N3Me5Ox)(H O) ], [Gd(9N3Me5Ox)(H O) ] and [Lu(9N3Me5Ox)(H O) ].
2 3 2 3 2 3
[
La(9N3Me5Ox)(H O) ]
[Gd(9N3Me5Ox)(H O) ]
[Lu(9N3Me5Ox)(H O) ]
2 3
2
3
2
3
Experimental GGA(BLYP) Experimental GGA(BLYP) Experimental GGA(BLYP)
3
2
1
390
920
657
3214
2984
1678
3395
2923
2823
3310
2903
2882
2919
2853
1654
2975
2899
1685
1
1
1
1
1
7
6
5
458
387
261
165
029
98
42
08
1484
1312
1210
1189
1072
799
1657
1572
1465
1387
1321
1269
1154
1088
1690
1589
1479
1398
1308
1295
1110
1093
1055
780
1506
1454
1261
1169
1091
1032
795
725
640
577
514
1555
1495
1279
1125
1098
1010
796
740
585
510
485
602
425
1
017
7
7
6
95
40
62
779
625
2
.3.3 Molecular Orbital Analysis
Molecular orbital (MO) compositions for all the three lanthanide complexes were determined
using GGA(BLYP) and an analysis of these compositions was done considering the interaction
3
6
of discrete molecular fragments using the ADF 2017 program. Molecular orbital analysis
displaying the mixing of the atomic orbitals in the wellꢀdefined molecular fragments (FOs) (i.e.,
the Ln(III), 9N3Me5Ox, and the three water molecules) to generate the calculated molecular
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5

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DOI: 10.1039/C8NJ02217H
orbitals (MOs) for a given complex, with the fragment orbitals of interest is presented in Fig. 8.
The partial MO diagram and the interacting FOs to the MOs of interest of on of the
representative complexes, i.e., Lu (III) complex, is shown in the Fig. 9. From the electronic
e
3
+
3+
3+
configurations of La , Gd , and Lu , it is assumed that the compounds of the three analogs of
3
+
Ln should have a similar number of molecular orbitals; however, due to the difference in
occupation in 4f orbitals, the numbering pattern of MOs differs. The MOs analysis of the three
complexes with the numbering of the molecular orbitals is presented in Supporting Table 1. In
the case of lanthanum complex, the 4f orbital is empty, on LCAO of AOs of the metal AOs and
ligand AOs, these orbitals bear high energy.
Fig. 8 Schematic depiction of the fragmentation mode of the complexes [Ln
(
9N3Me5Ox)(H O) ] (Ln= Lu) in the MO analysis.
2 3
From a MOs composition analysis of both closed shell systems, ([La(9N3Me5Ox)(H O) ]) and
2
3
(
[Lu(9N3Me5Ox)(H O) ]), as well as in the open system, ([Gd(9N3Me5Ox)(H O) ]), it was
2
3
2
3
observed that the highest occupied molecular orbitals (HOMOs) are mainly formed by the (p C,
x
p N, p N orbitals) of the ligand, whereas the lowest unoccupied molecular orbitals
x
y
(
LUMOs) are centred in p C, p N, orbitals of ligand and metal d orbitals (Supporting
x x xy
Table 1). Each percentage contribution in the data Supporting Table 1 below corresponds to the
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6

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DOI: 10.1039/C8NJ02217H
indicated SFO. In general, an SFO may be a linear combination of several Fragment Orbitals on
the same, or on symmetryꢀrelated Fragments. Only the first 'member' of such a combination is
specified here. The numbering of the SFOs in this table does not include the Core Orbitals and
starts from one for each symmetry representation. It can be seen that (Supporting Table 1) the
percentage contribution of the AOs does not sum to 100%. It is because only the orbitals with
significant contributions are shown in the table. There is ~28 % contribution of the ligand
FOs in the MOs formed from the 4f orbitals (particularly MOs 178 to184, Fig. 9). The
percentage of individual contributions of the ligand in molecular orbitals are 178 (26.39%), 179
(50.29%), 180 (6.22%), 181 (18.26%), 182 (6.22%), 183 (5.24%) and 184 (31.64%). It can be
seen that the contribution is maximum in MOs 178, 179 and 184, whereas the molecular
orbitals 180ꢀ183 had small ligand contributions. Results of the energy decomposition analysis
(discussed in section 2.4.2), also show 28.82% of covalency in the metalꢀligand bonds. These
results also indicate that nature of metalꢀligand bond (covalent or ionic) depends upon interaction
of ligand orbital with metal orbital.
As expected for rare earth compounds, their role in the complex stability is very small. However,
the low probability that 4f electrons present in the outer shell of the lanthanide atom or ion allow
a small effective overlap between the atomic orbitals of a metal ion with the coordinated ligand
orbitals. As a result, the complexes of lanthanides are considered as to have very less or no
covalent character. These are not affected by orbital geometries. The absence of orbital
interaction also implies that metal variation has a small effect on the complex (apart from size),
particularly in comparison with the transition metals. The lanthanide complexes are held together
through weak electrostatic interaction that is omniꢀdirectional and therefore ligands themselves
dictate the symmetry and coordination of the metal complexes.
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7

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DOI: 10.1039/C8NJ02217H
Fig. 9 Ground state molecular orbital interaction diagram for [Lu(9N3Me5Ox)(H O) ]
2
3
(calculated according to the BLYP level of theory; only the selected orbitals are presented for
clarity).
2
.4 Bonding Nature
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2
.4.1 Atomic population analysis (APO) and NBO analysis
For more information on the binding nature of the lanthanide complexes, atomic population
4
2
analysis (APO), natural orbital analysis (NBO), the net charge, spin density, and charge
transfer analyses were performed in a complex model. The results of the analysis (Table 6),
display that in all cases the systems show a charge transfer from the oxygen atoms of the 8HQ
pendant in 9N3Me5Ox ligand to the lanthanide center. The charge transfer from the 8HQ
nitrogen is slightly larger than that of oxygen of the water molecule. Although the trend of all
4
3ꢀ44
45
population analysis schemes is the same, Mulliken
and Hirchsfield methods underestimate
4
6
the values of the charges providing only a qualitative result; Voronoi and NBO provide the best
description of the charge.
Table 6. Atomic population analysis on the lanthanide center, nitrogen and oxygen atoms
achieved at scalar relativistic level.
Hirschfield
Mulliken
Voronoi
NBO
a
b
NAC
2.002
NEC
[
[
[
La(9N3Me5Ox)(H O) ]
2 3
0
.11 0.17
0.64 0.
La
0.6286
1.6278
2.326
[Core]6s 4f 5d 5f
01
1
.37
4.14
0.01
ꢣꢤꢥꢦ.ꢧꢨ
ꢩꢤꢥꢦ.ꢪꢘꢚ
ꢩꢤꢥꢦ.ꢧꢨ
ꢀ0.0902
ꢀ0.2666
ꢀ0.282
ꢀ0.5589
ꢀ0.8084
ꢀ0.6587
ꢀ0.2353
ꢀ0.3799
ꢀ0.661
ꢀ0.5351
ꢀ1.025
ꢀ0.8132
[Core]2s 2p 3p
1.75
5.26
5.09
[Core]2s 2p
1.71
[Core]2s 2p
Gd(9N3Me5Ox)(H O) ]
2
3
0
.14 7.12
0.65
0.01
Gd
0.6741
1.6770
2.326
2.025(6.9 [Core]6s 4f 5d
c
0.03
0.06
4
)
6p 6d 1
.38
4.13
ꢣꢤꢥꢦ.ꢧꢨ
ꢩꢤꢥꢦ.ꢪꢘꢚ
ꢩꢤꢥꢦ.ꢧꢨ
ꢀ0.0966
ꢀ0.2559
ꢀ0.2775
ꢀ0.5506
ꢀ0.816
ꢀ0.6981
ꢀ0.1875
ꢀ0.3799
ꢀ0.661
ꢀ0.525
ꢀ1.027
ꢀ0.8215
[Core]2s 2p 3p
1
.76
5.27
[Core]2s 2p
1
.71
5.11
[Core]2s 2p
Lu(9N3Me5Ox)(H O) ]
2
3
0
.18 13.98
0.63
0.01
Lu
0.5923
1.8066
1.9289
2.097
[Core]6s 4f 5d
0.03
0.08
6
p
6d 1
.38
4.14
ꢣꢤꢥꢦ.ꢧꢨ
ꢩꢤꢥꢦ.ꢪꢘꢚ
ꢀ0.0878
ꢀ0.2562
ꢀ0.2876
ꢀ0.5719
ꢀ0.8124
ꢀ0.7203
ꢀ0.2076
ꢀ0.3421
ꢀ0.6253
ꢀ0.5311
ꢀ1.038
[Core]2s 2p 3p
1.76
5.27
[Core]2s 2p
1.72
5.14
ꢩ
ꢀ0.8581
[Core]2s 2p
ꢤꢥꢦ.ꢧꢨ
a
b
c
NAC: natural atomic charge, NEC: natural electronic configuration, and spin density value.
Moreover, in open shell molecule, spin density analysis shows that the density of electron is
centered on f orbitals in the gadolinium (6.942) metal ion. These, added to the results found by
2
9