A combined experimental and computational study of a ruthenium(II) polypyridyl complex: Synthesis, characterization, electronic structures and spectral properties

Article abstract of DOI:10.1016/j.poly.2018.06.011

A novel ruthenium(II) complex of [Ru(bpy)₂(ipipH)](PF₆)₂ (in which bpy = 2,2-bipyridine, ipipH = 2-(4-imidazol-1-yl)phenyl-imidazol[4,5-f][1,10]phenanthroline) was synthesized, and characterized using FT IR, ¹H NMR, UV–Vis, and ESI mass spectroscopic measurements. The equilibrium geometries, spectral properties, and metal–ligand interaction of the [Ru(bpy)₂(ipipH)]²⁺ complex were investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Natural bonding orbital (NBO) analysis indicates that the electrostatic interaction dominates the metal–ligand interaction.

Full text of DOI:10.1016/j.poly.2018.06.011

Accepted Manuscript
A combined experimental and computational study of a ruthenium(II) polypyr-
idyl complex: Synthesis, characterization, electronic structures and spectral
properties
Yong Xia, Aokui Sun, Zhihui Miao, Tian Zhao, Meifang Cui, Yuejun Liu
PII:
S0277-5387(18)30328-0
https://doi.org/10.1016/j.poly.2018.06.011
POLY 13219
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
20 March 2018
5 June 2018
Please cite this article as: Y. Xia, A. Sun, Z. Miao, T. Zhao, M. Cui, Y. Liu, A combined experimental and
computational study of a ruthenium(II) polypyridyl complex: Synthesis, characterization, electronic structures and
spectral properties, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.06.011
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A combined experimental and computational study of a ruthenium(II)
polypyridyl complex: Synthesis, characterization, electronic structures
and spectral properties
Yong Xiaa,*, Aokui Suna, Zhihui Miaob, Tian Zhaoa, Meifang Cuic, Yuejun Liua,*
a
Key Laboratory of Advanced Packaging Materials and Technology of Hunan Province, Hunan University of
Technology, Zhuzhou, 412007, China
b
Center for Catalysis, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida
32611, United States
c College of Chemistry and Chemical engineering, Southeast University, Nanjing, Jiangsu 211189, PR China
* Corresponding author: E-mail: xiayong@hut.edu.cn; yjliu_2005@126.com
Abstract: A novel Ruthenium(II) complex of [Ru(bpy)2(ipipH)](PF6)2 (in which bpy=2,2-bipyridine, ipipH=2-(4-
1
imidazol-1-yl)phenyl-imidazol[4,5-f][1,10]phenanthroline) was synthesized, and characterized using FT IR, H
NMR, UV-Vis, and ESI mass spectroscopic measurements. The equilibrium geometries, spectral properties, and
metal-ligand interaction of the [Ru(bpy)2(ipipH)]2+ complex were investigated by density functional theory (DFT)
and time-dependent density functional theory (TD-DFT). Natural bonding orbital (NBO) analysis indicates that
the electrostatic interaction dominates the metal-ligand interaction.
Keywords: Ruthenium(II) complex; DFT; TD-DFT; Metal-ligand interaction
1. Introduction
Ruthenium (II) polypyridyl complexes have received considerable attention due to their rich photochemical
properities, long luminescence lifetime, large Stokes shift, and high photochemical stability[1-3]. Therefore they
are most widely used in the field of photochemistry, photophysics and biochemistry[4-7]. The unique
1
photophysical properties of [Ru(bpy)3]2+(bpy=2,2’-bipyridine) are characterized by its strong MLCT visible
3
absorption, metastable MLCT excited state, and reversible redox chemistry[8-10]. These unique properties of
ruthenium (II) polypyridyl systems are resulted from the localized ruthenium dπ and polypyridyl π* orbitals, each
of which present independent redox active behavior[11, 12]. The behavior of such complex has attracted attention
to exploit these materials for solar cells[13-15], light emitting devices[16-18], photocatalysis[19-21], and
biomedical diagnostics[22, 23]. Recently there has been an expanding interest in the DNA binding properties of
[Ru(bpy)3]2+ derivatives which may be tailored by design[24-30]. Because the imidazole moiety is an important
ingredient of the purine bases in DNA, a series of imidazole and substituted imidazole-containing polypyridine
1

ligands and their ruthenium (II) complexes have thoroughly studied. These studies addressed several important
issues related to their mode of binding, site-specificity, and light-switching behaviors toward DNA[31-33].
With the aforementioned in mind, we designed and synthesized a novel Ru(II) complex with
1
imidazole-containing polypyridine ligand ([Ru(bpy)2(ipipH)](PF6)2). The complex is characterized by IR, H
NMR, electrospray mass spectra, and UV-vis spectroscopies. The photophysical properties of the complex are also
investigated. To get better insight into the geometry and the electronic structure, geometry optimizations of the
ligand and complex are carried out by means of DFT and TD-DFT calculations.
2. Experimental section
2.1 Materials
The starting complex Cis-[Ru(bpy)2Cl2]·2H2O[34], 1,10-phenanthroline-5,6-dione[35] and 4-(imidazol-1-yl)
benzaldehyde[36] were synthesized as previously described in the literature. All the chemicals and solvents were
analytically pure and used without further purification.
2.2 Synthesis of 2-(4-imidazol-1-yl)phenyl-imidazol[4,5-f][1,10] phenanthroline (ipipH).
The ligand was synthesized following procedures as previously described in the literature[37]. A mixture of
1,10-phenanthroline-5,6-dione (0.206 g, 1 mmol), 4-(imidazol-1-yl)benzaldehyde (0.172 g, 1 mmol) and
ammonium acetate (2 g, 26 mmol) were dissolved in glacial acetic acid (40 mL) and refluxed for 6 h under
nitrogen. The solution was cooled to room temperature and diluted with water (40 mL), then neutralized with
concentrated aqueous ammonia. A yellow precipitate was formed and washed with water, the crude product was
purified by recrystallization from a mixture of ethanol and water to obtain light yellow crystals. Yield: 0.18g
(50%). IR (KBr, cm-1): 3361, 3043, 1617, 1520, 1487, 1420, 1388, 799, 734. 1H NMR (ppm, DMSO): δ=9.04 (q,
J=2.6Hz, 2H); 8.95 (d, J=8.0 Hz, 2H); 8.44 (s, 1H); 8.43 (d, J=5.2 Hz, 2H); 7.93 (d, J=8.4 Hz, 2H); 7.91 (s, 1H);
7.84 (q, J=8.4 Hz, 2H); 7.18 (s, 1H). MS [CH3OH, m/z]: 363.11[M+H]+
2.3 Synthesis of Ruthenium(II) complex
Cis-[Ru(bpy)2Cl2]·2H2O (0.052 g, 0.1 mmol) and ipipH (0.036 g, 0.1 mmol) were added to ethanol (16 mL)
and H2O (4 mL), heated at 100℃under nitrogen for 8h, then cooled to room temperature. After filtration, saturated
K(PF6) was added dropwise. A red-brown precipitate was obtained. The product was purified by column
chromatography on alumina with acetonitrile-water (1:1, v/v) as eluent. The solvent was removed under reduced
pressure and red-brown precipitate was obtained. Yield: 0.041g (40%). IR (KBr, cm-1): 3402, 1633, 1543, 1440,
1
1325, 1184, 837, 761. H NMR (ppm, DMSO): δ=9.08 (d, J=6.9 Hz, 2H); 8.85 (t, J=8.7 Hz, 4H); 8.70 (d, J=6.0
Hz, 1H); 8.44 (t, J=7.6 Hz, 2H); 8.23 (t, J=7.8 Hz, 2H); 8.08 (m, 4H); 8.00 (d, J=3 Hz, 2H); 7.93 (d, J=7.1 Hz, 2H);
2

7.80 (s, 2H); 7,59 (t, J=6.0 Hz, 4H); 7.46 (t, J=6.6 Hz, 1H); 7.34 (m, 2H); 7.14 (d, J=8.1 Hz, 1H).
2.4 Physical Measurements
1
IR spectra were performed with TENSOR 27, H NMR spectroscopy were recorded on a 300MHz NMR
instrument using (CD3)2SO as solvent at room temperature and TMS as the internal standard. UV-Vis absorption
spectra were determined with a SHIMADZU UV-Vis spectrophotometer, Emission spectra were measured with
Horiba Jobin Yvon Inc.FluoroLog 3-TCSPC. Electrospray mass spectra (ES-MS) were carried out with Agilent
5975E.
2.5 Computational details
The geometrical structures of the ligand and complex were optimized by the DFT method with B3LYP[38,
39] exchange correlation functional approach, and a basis set LanL2DZ was adopted as the basis set for Ru atom,
6-31G(d) basis set for the other atoms. The vibrational frequency calculation was also performed on them to
ensure that the optimized geometries represent the local minima and there are only positive eigen values. Default
options were employed for all optimizations. No symmetry constraints were imposed during the optimizations.
Spectral properties in N,N-Dimethylformamide (DMF) media were calculated by time-dependent density
functional theory (TD-DFT) approach associated with the polarizable continuum model (PCM)[40]. For TD-DFT
calculating, the 80 lowest spin-allowed singlet-singlet transitions were taken into account. All the calculations
were performed with the Gaussian 03W software package[41]. Natural transition orbital (NTO) analysis was
performed with the Multiwfn software package[42].
3. Results and discussion
3.1 Synthesis of Ligand and Complex
Two bidentate ligands bpy and ipipH used in this work are shown in Figure S1. The synthesis and properties
of ligand ipipH was described elsewhere[37]. The two ligands were used as a bidentate N, N donor ligand for the
preparation of the Ru(II) complex (Scheme 1). The stoichiometric reaction of with ipipH in a ratio of 1:1 in
boiling ethanol/H2O (4/1) afforded the red-brown complex [Ru(bpy)2(ipipH)]2+ in a good yield (Scheme 1). The
1
IR, H NMR, and electrospray mass spectroscopic measurements confirm the formation of [Ru(bpy)2(ipipH)]2+
(see Experimental section).
3.2 Structural Analysis
The optimized geometries of ligand and complex are illustrated in Figure 1. The corresponding geometrical
parameter from the current calculation is listed in Table 1. The average length of the Ru-N bonds and the average
ligand bite angle of the current complex are 2.118 Å and 77.9˚, respectively, which are in agreement with the
3

reported complexes Ru(bpy)3 (2.056 Å, 78.7˚)[43] and Ru(phen)3 (2.063 Å, 79.8˚)[44]. Although there are some
differences in size and shape among phen, bpy and ipiph, the length of the Ru-N bond and the ligand bite angle of
the complex are no significantly different. Compared to the ligands, the distances between nitrogen atoms
(N3-N4=2.69, N5-N6=2.65, N7-N8=2.65) in the complex are shorter, which indicate strong interaction between
Ru and ligands. The dihedral angles of N4-N3-Ru-N5, N5-N6-Ru-N7, and N7-N8-Ru-N4 are 97.5˚, 98.1˚, and
97.1˚, respectively. These results indicate that the complex contains a six-coordinated ruthenium atom chelated by
one ipipH and two bpy ligands in the expected distorted octahedral fashion. Furthermore, the dihedral angle
C1-N1-C2-C3, which reflects the planarity of the ligand, decreases approximate 9 degree in complex. It may be
attributed to the ruthenium(II)-induced intramolecular charge transfer (ICT). Especially, the dihedral angle
N5-C7-C8-N6 decreases approximate 33 degree, which may be resulted from the electrostatic effect of the N…
Ru2+ interactions.
3.3 NBO analysis
Natural bond orbital analysis was carried out to understand the metal-ligand interaction. The Wiberg bond
indices (WBIs) of Ru-N and natural atomic charges are shown in Table 2. In terms of natural atomic charges
analysis, positive charges are located on the ruthenium atom, and negative charges are on the nitrogen of the
ligands. These results confirm the strong donor-acceptor interactions between the ruthenium and the ligands,
which is supported by the reference on the other systems[45]. Moreover, the WBIs for all Ru-N bonds are
approximately 0.4. Thus, the metal-ligand bonds are possibly partly ionic and partly covalent.
In the NBO analysis, the NBO second-order interaction energies provide an in-depth interpretation to the
stabilizing effect of the metal ions, which are calculated by the following equation:
F2 (i, j)
E2  Eij  qi
 j i
where qi is the donor orbital occupancy,

i and j are diagonal elements (orbital energies) of the Fock

matrix, and F(i, j)is the off-diagonal NBO Fock matrix element. The strength of the donor-acceptor interaction
could be reflected by the stabilization energy E2. The main second-order perturbation stabilization energies E2
obtained by NBO analysis are summarized in Table 3. The calculated results show that the interaction energies E2
of the [Ru(bpy)2(ipipH)]2+ complex are mainly attributed to the lone pair electrons of nitrogen atoms and the LP*
(1-center valence antibond lone pair) the metal cation Ru2+. The minimum E2 value (LP N7 → LP*(4) Ru) is
17.61 kcal/mol, while the maximum E2 value (LP N4 → LP*(4) Ru) is up to 99.03 kcal/mol. These results
indicate that the driving forces are the electrostatic interactions between the metal ion and the electron-donating
4

nitrogen atoms in the [Ru(bpy)2(ipipH)]2+ complex.
3.4 Spectral Properties
The absorption spectra of the ligand ipipH were recorded in DMF solution at room temperature. There were
two well resolved peaks (282 and 330 nm) in the absorption spectra. Ligand ipipH exhibit an absorption at 282 nm
is due to the various π-π* electronic transition, while a weaker n-π* transition was submerged under a stronger
π-π* transition. The absorption at 330 nm corresponds to the ICT transition of the ligand. Ru(bpy)2Cl2 shows three
absorption peaks at 299 nm, 384 nm, 561 nm. The absorption bands at 299 nm, 384 nm are attributed to the
intraligand (IL) π-π* transition while the absorption band at 561 nm is from MLCT transition. The complex also
exhibits three absorption peaks at 294 nm, 345 nm and 458 nm, respectively. The high-energy absorption bands at
294 nm and 345 nm are attributed to the intraligand (IL) π-π* transition, while the low energy bands
approximately at 458 nm is due to the metal-ligand charge transfer (MLCT) transition. These assignments were
supported by theoretical calculations [from frontier molecular orbital (FMO) and natural transition orbital (NTO)
analysis]. The experimental absorption spectra for ipipH, Ru(bpy)2Cl2, and [Ru(bpy)2(ipipH)]2+ are shown in
Figure S2. The calculated absorption energies associated with their oscillator strengths, the main configurations as
well as the experimental result are given in Table 4. The isodensity plot of frontier of ipipH ligand and
[Ru(bpy)2(ipipH)]2+ complex is given at Figure 2.
For ipipH ligand, in the ground state (S0) the electron density of HOMO is completely delocalized
throughout the whole molecule whereas the electron density in HOMO-2 is mainly resides on the imidazole
moiety and 1H-imidazo[4,5-f][1,10]phenanthroline moiety. On the other hand, the electron density of LUMO is
residing at the whole molecule whereas the electron density in HUMO+1 is mainly resides on phenanthroline
moiety. Hence the peak appears at 330 nm and 282 nm are mainly due to intramolecular charge transfer (ICT),
which is associated with the HOMO → LUMO+1 transition and HOMO-2 → LUMO transition, respectively.
Thus, the two absorption bands, assigned to the S0 → S2 and S0 → S7 transitions, have ICT character, involving
rearrangement of electron density.
For [Ru(bpy)2(ipipH)]2+ complex, the electron density of HOMO (S0) is mainly resides on the imidazole
moiety of ipipH ligand. The electron density of HOMO-4 is completely delocalized on the ruthenium(II), while
the electron density of LUMO is residing at the two bidentate ligands bpy. Hence the peak appears at 458 nm is
due to the MLCT transition. The electron density of HOMO-7 is delocalized throughout the ipipH ligand and the
ruthenium(II) whereas the electron density in HOMO-8 is mainly resides on 1H-imidazo[4,5-f][1,10]
phenanthroline moiety of the ipipH ligand. On the other hand, the electron density of LUMO+2 is residing at the
phenanthroline moiety of the ipipH ligand and the ruthenium(II). Hence the peak appears at 294 nm and 345 nm
5

are mainly due to ILCT and MLCT transition, which is associated with the HOMO-7 → LUMO+2 transition and
HOMO-8 → LUMO+2 transition, respectively.
NTO analysis presented a clearer representation of the transition density. Here the unoccupied and occupied
NTOs were defined as “electron” and “hole” transition orbitals, respextively. It should be note that NTOs are not
the same as virtual and occupied MO pairs from the ground state calculations. The natural transition orbitals of the
complex are shown in Figure S3. Based on the NTOs analysis the complex could be characterized as an admixture
of MLCT and ILCT states.
4. Conclusions
In summary, a combined experimental and computational study of [Ru(bpy)2(ipipH)]2+ complex was carried
out. Structural analysis indicates the complex contains a six-coordinated ruthenium atom chelated by one ipipH
and two bpy ligands in the expected distorted octahedral fashion. The calculated structure parameters are
comparable with the experimental values. From the current calculation results, we have characterized the
low-lying electronic states as admixture of ILCT and MLCT in character. The investigation provides a deeper
understanding for the interaction of the metal-ligand and the spectral properties.
Acknowledgement
The author wishes to acknowledge the financial supports from the National Natural Science Foundation of
China (No. 51704108; No.11372108) and the Natural Science Foundation of Hunan Province (No.14JJ5021).
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8

Scheme 1. Schematic representation for the synthesis of the complex
9

Figure 1. The obtained optimized geometry of ligand and complex
10

Figure 2 The isodensity plot of frontier of ipipH ligand and [Ru(bpy)2(ipipH)]2+ complex
11

Table 1. Selected structural parameters (distance in Å and angles in degrees) for ligand and complex.
Parameter
Ru-N3
Ligand (bpy/ipipH)
Complex
2.121
2.126
2.115
2.115
2.117
2.116
2.69
/
Ru-N4
/
Ru-N5
/
Ru-N6
/
Ru-N7
/
Ru-N8
/
N3-N4
2.74
N5-N6
2.81
2.65
N7-N8
2.81
2.65
N3-Ru-N4
N5-Ru-N6
N7-Ru-N8
N4-N3-Ru-N5
N5-N6-Ru-N7
N7-N8-Ru-N4
C1-N1-C2-C3
C4-C5-C6-N2
N5-C7-C8-N6
/
78.4
/
77.7
/
77.7
/
/
97.5
98.1
/
97.1
36.2
3.6
35.1
27.7
7.8
1.7
12

Table 2 The WBIs of Ru-N for complex and natural charge at B3LYP/6-31G(d) level.
Wiberg bond indices
(WBIs)
Natural charge on
ruthenium
0.593
Natural charge on
nitrogen
-0.421
Ru-N3
Ru-N4
Ru-N5
Ru-N6
Ru-N7
Ru-N8
0.4030
0.4079
0.593
-0.425
0.4062
0.593
-0.428
0.4086
0.593
-0.429
0.4092
0.593
-0.427
0.4103
0.593
-0.427
13

Table 3 Selected NBO second-order interaction energies (E2, kcal/mol) of the complex.a
Donor NBO(i) → acceptor NBO(j)
LP N3 → LP*(4) Ru
LP N3 → LP*(5) Ru
LP N3 → LP*(6) Ru
LP N4 → LP*(4) Ru
LP N4 → LP*(6) Ru
LP N5 → LP*(4) Ru
LP N5 → LP*(6) Ru
LP N6 → LP*(4) Ru
LP N6 → LP*(5) Ru
LP N6 → LP*(6) Ru
LP N7 → LP*(4) Ru
LP N7 → LP*(5) Ru
LP N7 → LP*(6) Ru
LP N8 → LP*(4) Ru
LP N8 → LP*(5) Ru
LP N8 → LP*(6) Ru
[Ru(bpy)2(ipipH)]2+
31.53
65.34
22.25
99.03
22.39
96.53
22.46
19.09
78.58
22.69
17.61
80.18
22.73
30.05
67.77
22.56
a LP: 1-center valence lone pair, LP*: 1-center valence antibond lone pair.
14

Table 4 The selected calculated optical transition for ipipH and [Ru(bpy)2(ipipH)]2+ with composition in terms of
molecular orbital contribution of the transition, vertical excitation energies, and oscillator strength in DMF.
Electronic
transitions
Oscillator
strength
λexp
Composition
Excitation energy
CI
(nm)
S0 → S2
HOMO-2 → LUMO
HOMO → LUMO
3.7014 eV(335 nm)
0.2487
0.58345
-0.30595
-0.12013
0.59236
-0.23468
0.10833
-0.14445
0.12374
-0.10520
0.64984
-0.13866
0.60154
0.11975
0.24549
0.12189
0.48981
0.20337
-0.20362
-0.23140
331
282
HOMO → LUMO+1
HOMO-2 → LUMO
HOMO-2 → LUMO+1
HOMO → LUMO+1
HOMO → LUMO+2
HOMO → LUMO+3
HOMO → LUMO+4
S0 → S7
4.3313 eV(286 nm)
0.1723
ipipH
S0 → S14 HOMO-4 → LUMO
HOMO-1 → LUMO+3
2.7216 eV(456 nm)
3.5255 eV(352 nm)
0.0890
0.5094
458
345
S0 → S38 HOMO-7 → LUMO+2
HOMO-5 → LUMO+2
HOMO-4 → LUMO+3
[Ru(bpy)2(ipipH)]2+
HOMO-1 → LUMO+8
S0 → S71 HOMO-8 → LUMO+2
HOMO-4 → LUMO+8
4.1762 eV(297 nm)
0.1095
294
HOMO-3 → LUMO+3
HOMO → LUMO+14
15

A combined experimental and computational study of a ruthenium(II) polypyridyl complex.
16