Synthesis, molecular structure and spectral analysis: DFT-TDDFT computational study of ruthenium complex of tetradentate N,N′- bis(benzimidazole-2yl-ethyl)-ethylenediamine

Article abstract of DOI:10.1016/j.molstruc.2011.01.005

Ruthenium complex of N,N′-bis(benzimidazol-2-yl-ethyl)ethylenediamine (L¹) was prepared and characterized by analytical methods. Structural and spectral properties of N,N′-bis(benzimidazol-2-yl-ethyl) ethylenediamine (L¹), its dianio

Full text of DOI:10.1016/j.molstruc.2011.01.005

Journal of Molecular Structure 989 (2011) 70–79
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, molecular structure and spectral analysis: DFT–TDDFT computational
study of ruthenium complex of tetradentate N,N0-bis(benzimidazole-2yl-ethyl)-
ethylenediamine
Hernández J. Guadalupe a, Jayanthi Narayanan b, Thangarasu Pandiyan c,
⇑
a Centro Tecnológico, Facultad de Estudios Superiores (FES-Aragón), Universidad Nacional Autónoma de México (UNAM), Estado de México, CP 57130, Mexico
b División de Ingenería en Informatica, Universidad Politecnica del Valle de México, Av. Mexiquense, Tultitlan, Estado de México, CP 54910, Mexico
c Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Coyoacán 04510, México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Ruthenium complex of N,N0-bis(benzimidazol-2-yl-ethyl)ethylenediamine (L1) was prepared and charac-
terized by analytical methods. Structural and spectral properties of N,N0-bis(benzimidazol-2-yl-ethyl)eth-
ylenediamine (L1), its dianionic structure (L2), and their complexes such as [RuL1Cl(PPh3)]+ and
[RuL2Cl(PPh3)]+ were studied by DFT. In the structures, the ruthenium ion is positioned in an equatorial
plane formed by amine, benzimidazole nitrogens in a distorted octahedral geometry, and chloride and
triphenylphospine are axially coordinated. Furthermore, the molecular orbital of [Ru(L)Cl(PPh3)]Cl
(L = L1 or L2) proves that the HOMOs are localized over the benzimidazole and amine moieties, favoring
a strong bond with the metal. DFT-TDDFT was used to analyze the molecular orbitals contribution to
MLCT bands that were observed in the visible region; interestingly, the calculated spectrum of
[RuL1Cl(PPh3)]+ qualitatively agrees only with high energy bands (465 nm and 350 nm) of the experimen-
tal spectrum, and other visible bands (ꢀ580 and ꢀ790 nm) observed in the experimental spectrum coin-
cide with the TD-DFT of [RuL2(PPh3)Cl]+. However, electrochemical studies show existence of only
[RuL1Cl(PPh3)]+ in the solution.
Article history:
Received 22 September 2010
Received in revised form 11 January 2011
Accepted 14 January 2011
Available online 23 January 2011
Keywords:
DFT
Molecular orbitals
TDDFT
Ruthenium (II)
Benzimidazole
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
In the aspect of solar cells, cis-dithiocyanatobis-(2,20-bipyridyl-
4,40-dicarboxylic acid)ruthenium(II) is one of the most efficient
Ruthenium(II) complexes are generally stable, diamagnetic, and
kinetically inert, and have received great attention for their unpa-
ralled photophysical properties because they exhibit interesting
chemical and physical behavior; for example, several ruthenium
compounds have been proposed as potential anticancer substances
[1–4], chemosensors [5,6], and in dye-sensitized solar cells [7,8]. It
has been well documented that metal complexes can bind to DNA
covalently as well as noncovalently [9–13]. For instance, the com-
plex ImH[trans-RuCl4(DMSO)(Im)] (NAMI-A, Im = imidazole) is the
first ruthenium complex to enter clinical trials for cancer treatment
[1,14], where it has been observed that it has high selectivity for
solid tumor metastases [15] (prevents the spread of cancer) and
low host toxicity. Furthermore, organometallic ruthenium(II)-
heterogeneous charge transfer sensitizers and is widely used in
the TiO2-based dye-sensitized solar cell [6,18–26]. The ruthenium
polypyridine octahedral complexes are widely used as dyes in
dye-sensitized solar cells; the use of ruthenium metal is particu-
larly attractive because in its octahedral coordination geometry,
one can introduce specific ligands in a controlled manner to tune
photophysical, photochemical, and electrochemical properties
[27]. The main feature of the ruthenium compounds is that they
can be used as chromophores to produce solar energy because they
absorb light in the visible region through metal-to-ligand charge-
transfer (MLCT). Therefore, for the mechanistic details of the
photo-catalyst function and for the design of efficient molecular
devices related to solar energy conversion, understanding the elec-
tronic excited states in ruthenium complexes is crucially impor-
tant. Thus, the development of ruthenium complexes that exhibit
the MLCT bands at extended absorption (low energy region) is a
challenging research topic.
arene complexes [(g
6-arene)Ru(II)(en)Cl]+ (arene = benzene or
benzene derivatives, en = ethylenediamine) have recently been re-
ported as anticancer compounds both in vitro and in vivo studies
[16,17].
Although extensive studies have been made on ruthenium com-
pounds containing polypyridyl ligands [6,18–26], the reports on
ruthenium compounds containing benzimidazole is limited even
though benzimidazole and amine can stabilize ruthenium(II)
⇑
Corresponding author. Tel.: +52 55 56223499.
E-mail address: pandiyan@servidor.unam.mx (T. Pandiyan).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.01.005

H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
71
because of their moderate
p
-acceptor amine and
p
-donor benz-
relation was treated at the B3LYP [35,36] and the choice of this
method was based on the results obtained from 6-31G⁄⁄ used for
C, N, Cl, P, and H atoms, and for Ru2+, DGDZVP full electron basis
set was used [37]. The structures of L1 and L2 was fully optimized
and the data were then used as the input for the optimization of
[RuL1 Cl(PPh3)]+ and [RuL2Cl(PPh3)]+. For the analysis of the molec-
ular orbital contribution to the electronic absorption bands of the
complexes, DFT–TDDFT technique was employed by using the
Gaussian-09 program with the B3LYP exchange–correlation func-
tional, and DGDZVP basis set to calculate the electronic transitions
in gaseous state and also in solvent medium (in methanol as well
as in CH2Cl2) to see the solvents effect [38].
imidazole nitrogens. Additionally, the complexes of benzimidazole
derived ligands with many transition metal ions have been studied
in detail [28–33], but ruthenium compounds using with N,N0-bis
(benzimidazole-2yl-ethyl)ethylendiamine (L1) do not appear in
the literature. Therefore, the present study deals with the geomet-
rical and spectral analyses of the new compound [Ru(L1)Cl(PPh3)]Cl.
To determine the structural and spectral parameters, DFT–TDDFT
method is employed to interpret how the chloride and triphenyl-
phosphine are stabilized in the coordination sphere of the complex
along with the ligand.
2. Experimental/computational details
2.4. Preparations of N,N0bis (benzimidazole-2yl-ethyl)ethylenediamine
(L1)
2.1. Materials
The ligand was synthesized by using the procedure reported
elsewhere [39]. N,N0-bis(b-carbamolethyl)ethylenediamine (10.1 g,
0.05 M), which was prepared as reported elsewhere [40], was
hydrolyzed by refluxing 3.0 h with NaOH (4 g, 0.1 M) in water
(40 mL). After the neutralization of excess NaOH with HCl, then
1,2-diaminobenzene (10.8 g, 0.1 M) was added to the solution,
and refluxed for 35 h in HCl solution (260 mL, 4 N). On cooling
the solution mixture, the hydrochloride of the compound was ob-
tained and then neutralized with aqueous ammonia. The crude
product obtained was then re-crystallized from aqueous ethanol
and dried over P2O5 vacuum. The spectral results of the compound
agreed with the reported one: Elemental analysis, MS and NMR
spectra for L1 (0.80 g, 4.65%): Calc. for C20H24N6.5H2O: C, 54.77%,
H, 7.81 y N, 19.16%. Found: C, 53.62%, H, 7.99%, N, 18.44%. MS,
(ID, m/z (%)): MS: m/z = 349 (M + 1), 144 (100) [C9H8N2]+; 1H
NMR (CD3OD): d (ppm) = 2.80–3.31 (s, 4H, ANACH2ACH2ANA),
4.91 (t, 8H, 2ꢃBzim ACH2ACH2A), 7.18–7.48 (m, 8H, 2ꢃBzim-
ring). 13C NMR (CD3OD): 13C NMR (CD3OD): d (ppm) = 154.68
(HNACH@N), 123.39–116.41 (benzimidazole ring), 49.99–48–29
(ACH2ANHACH2), 29.98 (ACH2A).
All commercially available reagents employed were: ethylene-
diamine (99%), acrylamide, 1,2-phenylenediamine (99.5%), tri-
phenylphosphine (99%), ruthenium chloride hydrate (RuCl3 xH2O)
(Aldrich); tetrabutylammonium hexaflorophosphate (Smith) was
re-crystallized twice from aqueous ethanol.
ꢁ
2.2. Physical measurements
On a Fisons (Model EA 1108 CHNSO), elemental analyses for the
compounds were carried out. 1H, 13C and 31P NMR spectra were re-
corded for the compounds on Varian Gemini (300 MHz) by using
TMS as an internal standard and GC–MS was recorded with a Joel
JMS-Axsosha instrument. Electronic spectra were measured for
the ruthenium complex in methanol as well as in dichloromethane
by a Perkin–Elmer Lambda-900 double beam UV/Vis/NIR spectro-
photometer to see the solvents effect.
2.3. Computational procedure
By employing the Gaussian-09 program [34], the DFT calcula-
tions were performed for N,N0-bis (benzimidazole-2yl-ethyl)ethy-
lenediamine) (L1), and its dianionic structure (L2) i.e., after
removing hydrogen atom from secondary amine nitrogens. Fur-
thermore, the calculations were carried out with spin unrestricted
orbitals for [RuL1Cl(PPh3)]+ and [RuL2Cl(PPh3)]ꢂ. The exchange cor-
2.5. Ruthenium(II) complexes
2.5.1. [RuL1Cl(PPh3)]+
To a solution of L1 (0.438 g, 1.0 mmol) dissolved in methanol
(20.0 mL), a solution of [RuCl2(PPh3)3] (0.958 g, 1.0 mmol) dis-
Table 1
The optimized geometrical parameters (bond length (Å) and bond angles (°)) for the [Ru(II) L1Cl(PPh3)]Cl, [Ru(IV)L2Cl(PPh3)]Cl gaseous state (L1 = N,N0-bis(benzimidazol-2yl-
ethyl)ethylendiamine; L2 = dianionic N,N0-bis(benzimidazol-2yl-ethyl)ethylendiamine).
B3LYP/DGDZVP
Bond length (Å)
B3LYP/DGDZVP
Bond length (Å)
L1
L2
[RuL1Cl(PPh3)]+
[RuL2Cl(PPh3)]+
N(1)AC(2)
N(3)AC(2)
1.315
1.386
1.498
1.541
1.466
1.471
1.528
1.463
1.465
1.532
1.501
1.314
1.386
1.315
1.384
1.502
1.532
1.469
1.498
1.537
1.497
1.467
1.537
1.499
1.315
1.385
N(1)ARu(1)
2.184
2.221
2.196
2.217
2.504
2.434
2.231
2.005
1.991
2.245
2.445
2.556
N(12)ARu(1)
C(2)AC(10)
C(11)AC(10)
N(12)AC(11)
N(12)AC(13)
C(13)AC(130)
N(120)AC(130)
N(120)AC(110)
C(110)AC(100)
C(100)AC(20)
N(10)AC(20)
N(30)AC(20)
N(120)ARu(1)
N(10)ARu(1)
Cl(1)ARu(1)
P(1)ARu(1)
Bond Angle (o)
N(1)ARu(1)AN(120)
N(10)ARu(1)AN(12)
Cl(1)ARu(1)AP(1)
N(1)ARu(1)AN(12)
N(1)ARu(1)AN(10)
N(12)ARu(1)AN(120)
N(10)ARu(1)AN(120)
N(1)ARu(1)AP(1)
N(12)ARu(1)AP(1)
N(120)ARu(1)AP(1)
N(10)ARu(1)AP(1)
N(1)ARu(1)ACl(1)
N(12)ARu(1)ACl(1)
N(120)ARu(1)ACl(1)
167.54
164.23
178.54
87.77
101.01
80.36
89.74
91.39
82.89
92.96
99.90
88.54
88.56
87.409
171.43
164.51
177.67
88.87
99.84
82.59
88.33
88.97
92.44
92.11
100.44
88.83
86.73
89.95

72
H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
Fig. 1. Optimized structures of L1 and L2 in the gaseous state.
solved in methanol:CH2Cl2 (1:1, 30.0 mL) was added and the
resulting solution was refluxed for 24 h. The solution was cooled
to room temperature, and then concentrated by removing excess
methanol via rotary evaporation. The residue obtained was washed
with diethyl ether and then re-crystallized from methanol (20 mL).
The greenish yellow solid was obtained (Yield: 0.81 g, 84%). Ele-
mental analysis for RuC38H39N6Cl2P (782): calcd. C 58.31, H 4.98,
N 10.74; Found C 57.47, H 5.46, N 9.46. MS (FAB, m/z (%)): 747
(M+, C38H39N6ClPRu), 748 (M+1, RuC36H40N6ClP), 262 (C9H11N3Ru),
449 (C20H24N6Ru), 485 (C20H24N6ClRu). 1H NMR (CD3OD): d =
2.15–3.34 (s, 4H, ANACH2ACH2AN–), 4.88 (m, 8H, 2ꢃBzim
ACH2ACH2AN), 7.54–7.64 (m, 2ꢃBzim-ring and of PPh3). 13C{1H}
NMR (CD3OD): d = 166.36 (NHACH@N), 133.80–124.71 (Bzim-aro-
matic ring), 49.85 (ACH2ACH2 A). 31P{1H} NMR (CD3OD):
d = 33.33 ppm (s, P (Ph)3). IR selected signals (cmꢂ1): 3393 (OH),
3233 (NAH), 1535 (C@N).
on benzimidazole and amine was analyzed (Table 2). The results
indicate that there is present an excess electron density over nitro-
gens (two NBzim and two Namine) that can facilitate bond formation
with the metal. Besides, the hardness of the ligands, determined by
(LUMO–HOMO)/2), [43,44] shows that the ligands can coordinate
efficiently with Ru2+ or Ru4+. In the molecular orbital analysis, since
the energy difference between HOMO and HOMO-1 (ꢂ0.14 eV for
L1; ꢂ0.24 eV for L2) is very small, both orbitals can be involved
cooperatively in the bond formation with the metal ions (Fig. 2).
Thus, the orbitals HOMO and HOMO-1 are localized over
N1AC2@N3 of benzimidazole ring that favor the formation of
bonds with ruthenium ion.
It is known that Ru(II) has a 4d valence shell, which is more
spatially extended than metals with a 3d valence shell, and forms
low-spin complexes. In [RuL1(PPh3)Cl]+, the Ru atom presents a
Table 2
2.5.2. [RuL2Cl(PPh3)]+
Mulliken charges for [Ru(II) L1Cl(PPh3)]Cl, [Ru(IV)L2Cl(PPh3)]Cl gaseous state
(L1 = N,N0-bis(benzimidazol-2yl-ethyl)ethylendiamine; L2 = dianionic N,N0-bis(ben-
zimidazol-2yl-ethyl)ethylendiamine).
The molecular structure (Fig. 3b) of the complex was computa-
tionally constructed and then optimized to see dianionic effect in
the spectral and spectral parameters.
Atoms
B3LYP/DGDZVP
B3LYP/ DGDZVP
[RuL1Cl(PPh3)]+
L1
L2
[RuL2Cl(PPh3)]+
3. Results and discussion
N(1)
ꢂ0.273
ꢂ0.493
ꢂ0.470
ꢂ0.279
ꢂ0.483
ꢂ0.488
0.233
ꢂ0.503
ꢂ0.278
ꢂ0.307
ꢂ0.291
ꢂ0.304
ꢂ0.488
0.262
ꢂ0.286
ꢂ0.192
ꢂ0.184
ꢂ0.272
ꢂ0.492
ꢂ0.480
0.294
ꢂ0.529
ꢂ0.281
ꢂ0.344
ꢂ0.343
ꢂ0.277
ꢂ0.514
0.232
ꢂ0.350
ꢂ0.622
ꢂ0.605
ꢂ0.354
ꢂ0.490
ꢂ0.489
0.340
ꢂ0.541
ꢂ0.342
ꢂ0.339
ꢂ0.336
ꢂ0.306
ꢂ0.566
0.337
ꢂ0.383
ꢂ0.358
ꢂ0.345
ꢂ0.374
ꢂ0.481
ꢂ0.400
0.363
ꢂ0.587
ꢂ0.388
ꢂ0.358
ꢂ0.340
ꢂ0.347
ꢂ0.559
0.362
N(12)
N(120)
N(10)
N(3)
N(30)
C(2)
C(10)
C(11)
C(13)
C(130)
C(110)
C(100)
C(20)
Ru(1)
Cl(1)
P(1)
3.1. Geometrical analysis of L1 & L2 and their complexes
[RuL1Cl(PPh3)]+ and [RuL2Cl(PPh3)]+
The ground state geometrical data for N,N0-bis (benzimidazole-
2yl-ethyl)ethylendiamine (L1) and its dianionic structure (L2) are
presented (Table 1 and Fig. 1). The results show that there is pres-
ent an intra-molecular hydrogen bond N12AHꢁ ꢁ ꢁN120 (2.48 Å) in
the molecular structure of L1 and the observed H-bond length
(Fig. 1) is within the reported values 2.3–2.5 Å [41,42] and the dis-
tance is smaller than the sum of their van der Waals radii (1.5 Å for
N and 1.1–1.2 Å for H). However, the H-bond disappeared when
the ligand structure was de-protonated to give the L2 structure.
Furthermore, the charge distribution over the ligands, particularly
0.519
0.677
ꢂ0.438
ꢂ0.351
0.533
0.565

H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
73
.
Fig. 2. Frontier molecular orbitals (HOMOs and LUMOs) analysis: (a) L1; (b) L2
Fig. 3. Optimized structure of (a) [RuL1(PPh3)Cl]+; (b) [RuL2(PPh3)Cl]+ at gaseous state and in methanol.
+2 oxidation state with a low-spin of 4d6 5s0 electronic configura-
P1
2.434 Å
tion, which is relatively more stable than that of Ru(IV) having
4d4 5s0 configuration. The structures of [RuL1Cl(PPh3)]+ and
[RuL2Cl(PPh3)]+ are optimized and the results show (Fig. 3) that
ligand L1 is coordinated with ruthenium (II) ion in such a way that
the metal ion exhibits a distorted octahedral geometry. In the
structure, Ru(II) is bonded with two amines, and two benzimid-
azole nitrogens, forming an equatorial plane consisting of N(1),
N(12), N(120) and N(10) atoms and their distances with the metal
ion are: 2.184, 2.221, 2.196 and 2.217 Å respectively (Table 1);
the axial bond lengths 2.434 Å for Ru(II)-P and 2.506 Å for Ru(II)-
Cl were calculated from axially coordinated triphenyl phosphine
P(1) and Cl(1) with the metal ion, agreeing with the published
bond lengths [45]. According to Pearson’s Principle of Hard and
Soft Acids and Bases (HSAB) [46,47]: Hard Acids prefer to bond with
Hard Bases, and Soft Acids prefer to bond with Soft Bases. The hard
Ru(IV) cation reacts preferentially with a hard base such as
P1
2.556
N12
N12
N12'
N12'
Ru(II)
Ru(IV)
N1
N1
N1'
N1'
2.504 Å
Cl1
2.434 Å
Cl1
Scheme 1. Axial bond distance changes during Ru(II) oxidize to Ru(IV).
N
amine, or axial chloride ion, while soft Ru(II) bonds with soft base
as Nbzim (bzim = benzimidazole) or axial triphenyl phosphine.
Hence there results shorter Ru(IV)ANamine or Ru(II)ANbzim distance
compared to that of Ru(IV)ANbzim or Ru(II)ANamine. Similarly, the

74
H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
Fig. 4. Frontier molecular orbitals (HOMOs and LUMOs): (a) [RuL1(PPh3)Cl]+; (b) [RuL2(PPh3)Cl]+
.
hard nature of Clꢂ causes it to coordinate with Ru(IV), while the
soft nature of triphenylphosphine makes it interact with Ru(II),
resulting in smaller distances for Ru(IV)ACl or Ru(II)AP bonds than
for Ru(II)ACl or Ru(IV)-P (see data and Scheme 1 given below):

H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
75
Fig. 4 (continued)

76
H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
(b)
(a)
(π)
(dz )
-5.3
-5.4
LUMO+1
LUMO
d(z2)
2
LUMO+9
-2.58
(dx2-y2, π)
-2.69 LUMO+8
(dx2-y2, π)
(dz2, π)
(π)
-2.75
-3.28
LUMO+7
LUMO+4
ΔΕ= 3.11 eV
-3.31 LUMO+3
-3.36
LUMO+2
(π)
(dz2, π)
(dz2,
-3.54 LUMO+1
-3.72
LUMO
π)
-8.5
-8.6
(dxy
(dyz
,
,
σ)
σ)
HOMO
HOMO-1
(π)
-8.7 HOMO-2
-8.8
(π)
HOMO-3
ΔE= 4.02 eV
-8.9
-9.0
-9.1
-9.2
-9.3
HOMO-4
HOMO-5
(dzy
,
,
σ−π
σ−π
)
)
(dxy
HOMO-6
HOMO-7
(
σ−π
)
HOMO
-7.74
-7.78 HOMO-1
-8.04
(dxy, σ)
(dyz, σ)
(dxz, σ)
(dxz
,
σ−π
)
-9.4
-9.5
HOMO-2
(dyz, σ)
HOMO-8
Energy level [eV]
Energy level [eV]
Fig. 5. Frontier molecular orbital energy level diagram of (a) [RuL1(PPh3)Cl]+; (b) [RuL2(PPh3)Cl]+
.
Fig. 6. Electronic spectra of [RuL1(PPh3)Cl]+: in CH2Cl2 (. . ..) and in methanol (-).
the ruthenium (II) complexes containing benzimidazoles [48,49].
The resulting bond angles for trans bonded atoms around the
metal center are: 167.54° for N(1)ARu(1)AN(120), 164.23° for
N(10)ARu(1)AN(12), and 178.54° for Cl(1)ARu(1)AP(1) and other
bond angles are in the range of 80.36° to 101.01°, showing that
the metal presents a distorted octahedral structure. This is consis-
tent with the published crystal structures of the other metals
Cu(II), Ni(II) with the present ligand L1 that favors the formation
of distorted octahedral geometry. For the case of [RuL2Cl(PPh3)]+,
a strong interaction between N12 or N120 with Ru4+ is expected;
thus, a smaller bond distance for Ru(IV)ANA(12) (2.005 Å) or
Ru(II)ANbzim (2.184 Å) <
Soft–soft
Ru(II)AP (2.434 Å) <
Soft–soft
Ru(IV)ANbzim (2.231 Å) >
Hard–soft
Ru(II)ACl (2.504 Å) >
Soft–hard
Ru(II)ANamine (2.221 Å)
Soft–hard
Ru(IV)AP(2.556 Å)
Hard–soft
Ru(IV)ANamine(2.005 Å)
Hard–hard
Ru(IV)ACl (2.445 Å)
Hard–hard
The obtained bond distances (2.184–2.221 Å for Ru(II)AN) of
the complex fall approximately in the range of those reported for

H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
77
Table 3
TDDFT spectral data of electronic transitions [Ru(II) L1Cl(PPh3)]Cl, [Ru(IV)L2Cl(PPh3)]Cl gaseous state (L1 = N,N0-bis(benzimidazol-2yl-ethyl)ethylendiamine; L2 = dianionic N,N0-
bis(benzimidazol-2yl-ethyl)ethylendiamine) with oscillator strength f > 0.0011.
W(nm)
Osc. Strength (f)
Composition
Character
Theory (nm)
[RuL1Cl(PPh3)]+
473.3
0.0022
HOMO ? LUMO (29%)
MLCT
471.43
HOMO-1 ? LUMO + 1 (4%)
HOMO-1 ? LUMO + 7 (3%)
HOMO-1 ? LUMO + 9 (2%)
HOMO-2 ? LUMO + 7(31%)
HOMO-2 ? LUMO + 8 (9%)
HOMO ? LUMO + 7 (9%)
HOMO ? LUMO + 8 (3%)
HOMO-2 ? LUMO + 3 (3%)
HOMO ? LUMO + 1 (49%)
HOMO ? LUMO (33%)
HOMO ? LUMO + 7 (6%)
HOMO ? LUMO + 3 (3%)
HOMO ? LUMO + 4 (2%)
HOMO-1 ? LUMO + 1(46%)
HOMO-1 ? LUMO (41%)
HOMO-1 ? LUMO + 7 (4%)
HOMO-1 ? LUMO + 3 (2%)
HOMO ? LUMO + 2 (43%)
HOMO ? LUMO + 1 (14%)
HOMO-1 ? LUMO + 2 (3%)
HOMO ? LUMO + 2 (35%)
HOMO ? LUMO + 4 (20%)
HOMO ? LUMO + 3 (13%)
HOMO ? LUMO + 9 (4%)
437.94
351.2
0.0011
0.0021
0.0039
MLCT
348.21
335.13
328.77
0.0025
0.0159
MLCT
335.71
[RuL2Cl(PPh3)]+
654.67
0.0143
HOMO-1 ? LUMO (80%)
HOMO-6 ? LUMO (6%)
HOMO-3 ? LUMO (5%)
HOMO ? LUMO+1 (78%)
HOMO-3 ? LUMO (4%)
HOMO-3 ? LUMO (51%)
HOMO-6 ? LUMO (8%)
HOMO-4 ? LUMO (42%)
HOMO-3 ? LUMO (26%)
HOMO-8 ? LUMO (4%)
HOMO-6 ? LUMO (42%)
HOMO-4 ? LUMO (20%)
HOMO-7 ? LUMO (2%)
HOMO-5 ? LUMO (56%)
HOMO-7 ? LUMO(30%)
HOMO-6 ? LUMO (7%)
HOMO-8 ? LUMO (4%)
HOMO-7 ? LUMO (45%)
HOMO-3 ? LUMO (3%)
MLCT
625.00
609.08
596.72
572.19
0.0025
0.0057
0.0016
MLCT
MLCT
MLCT
538.19
523.17
0.0123
0.0043
MLCT
MLCT
525.00
504.95
0.024
MLCT
Note: For [RuL1Cl(PPh3)]+, experimental electronic bands observed were: 791 nm (555), 581 nm (870), 465 nm (823);
e
ꢃ 10ꢂ4, Mꢂ1 cmꢂ1 is presented in the parenthesis.
Ru(IV)ANA(120) (1.991 Å) is obtained than that for Ru(IV)AN(1)
(2.231 Å) or Ru(IV)AN(10) (2.245 Å). However, the dihedral angles
obtained for the complex are almost similar to the angles resulted
for [RuL1Cl(PPh3)]+.
In addition, molecular orbital (MO) analysis establishes the
formation of bonds through the energy stabilization of the orbitals
of rutheniumr(II) with those of ligands; all low-lying HOMOs
result from the overlap of the metal and ligand orbitals. For
instance, for [RuL2Cl(PPh3)]+, the HOMOs orbitals were obtained
through the combinations of d [Ru(IV)] with the p orbitals of L2;
the orbitals (HOMO to HOMO-x; x = 1–8) (Figs. 4b and 5b)
Furthermore, having analyzed the electronic charge density of
the ligands and the ruthenium(II) complexes calculated by the Mul-
likan method, it was found there is a considerable change in the
charge density of the donor atoms of the ligands after the formation
of the complexes (Table 2). For example, for the L1, the charge
densities were: N1 (ꢂ0.273), N12 (ꢂ0.493), N120 (ꢂ0.470), and N10
(ꢂ0.279); however, after its complex formation with Ru(II), the val-
ues were changed to ꢂ0.350, ꢂ0.622, ꢂ0.605, and ꢂ0.354. It appears
that there is a steep change in the values for the coordinated nitro-
gens because of the charge transfer from ligand to metal during
the complex formation. A similar charge transfer was also observed
during the complex formation of dianionic L2 with Ru(IV), i.e., the
charges of N1 (ꢂ0.286), N12 (ꢂ0.192), N120 (ꢂ0.184), and N10
(ꢂ0.272) were changed to ꢂ0.383, ꢂ0.358, ꢂ0.345, and ꢂ0.374 after
the complex formation with Ru(IV). Also, it was observed that the
charges were moved from 0.519 for Ru(II) to 0.677 for Ru(IV).
resulted from the mixing of the d orbital (metal) with
p/p of the
ligand, where L1 contributes considerably to the
p/p character of
Ru(II), the LUMO being derived through the combination of the
p
⁄ type [N(1), N(12), N(120) N(10)] of ligands with d (Ru ion).
The LUMO is being generated because of the d2z (Ru) mixture with
the orbitals of atoms [N(1), N(12), N(120), N(10), P(1) and Cl(1)],
and thus confirms the existence of a RuAP bond in the geometry.
Moreover, the p-character of the benzimidazole moiety presented
in the complex was corroborated: the HOMO, and other low-lying
orbitals (HOMO-x (x = 1–8) for RuL22+ were observed. Similarly,
the HOMOs orbitals were obtained through the combinations
of d [Ru(II)] with the p orbitals of L1 for [RuL1Cl(PPh3)]+ (Figs. 4a
and 5a).

78
H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
Fig. 7. a) Experimental absorption spectra in methanol; b) TD-DFT spectra: (i) [RuL1(PPh3)Cl]+; (ii) [RuL2(PPh3)Cl]+.
3.2. Electronic absorption spectra
those methods. However, there is a significant variation for the
protonated and de-protonated Ru complexes in the calculated elec-
tronic spectra [50]. In addition to the de-protonated effect, the sol-
vent effect is also associated with the absorption bands. Thus, the
MLCT band observed at 790.6 nm in methanol is shifted to
654.7 nm in the TD-DFT spectrum of [RuL2(PPh3)Cl]+. In a previous
report [38], the contribution of Ru d orbital with the ligand orbital
was analyzed, for instance, in Ru(II) complexes [Ru(H3-tctpy)-
(NCS)3]ꢂ1 and [Ru(H3-tctpy)(SCN)3]ꢂ1 (H3-tctpy = 4,40,400-tricar-
boxy-2,20:6,200-terpyridine), the contribution of d orbital mixed
with ligand orbital in solvent was explained; for [Ru(H3-tctpy)-
(NCS)3]ꢂ1, where both axial position are occupied by NCS, a lower
contribution of d orbital in solvent (94% in ethanol, 98% in gaseous
phase) was reported; while in [Ru(H3-tctpy))(SCN)3]ꢂ1, where SCN
are coordinated in both axial positions, a higher contribution of
d orbital in solvent (88% in ethanol than 83% in gas phase) was ob-
served. Therefore, the main absorption features of the experimental
spectrum are reproduced by theory despite having a red shift in the
lowest-energy band in the visible region. In addition, it was seen
that there is present a solvent effect in the DFT–TD electronic spec-
tra, i.e., the spectral bands of [RuL1Cl(PPh3)]Cl (473, 437, 351 and
348 nm) at gaseous state are slightly moved to 477, 436, 360 and
349 nm in methanol included calculated spectra (see Supplemen-
tary materials). It appears that the transitions originate from the
HOMO-x (x = 0, 1, 3, 4, 6 and 8) (RuANbzim) couple to the LUMO
or LUMO + 1) at 654.7, 609.1, 596.7 and 572.2 nm. We therefore
see that the lowest-energy band in the visible region is derived from
an excited state of mixed Ru(IV)AN, i.e., HOMOs localized on the
ligand, especially on the benzimidazole and amine moieties consid-
erably overlap with the LUMO of metal ion through MLCT. At short-
er wavelengths (538.2 and 505.03 nm) the positions are in near
agreement with the experimental values of 581.2 nm and
465.3 nm with a difference of 40 to 60 nm; the bands (538.2,
523.2 nm and 505.0 nm) are due to three electronic transitions of
mixed MLCT character from HOMO-x (x = 3, 4, 5, 6, 7 and 8) to the
higher-lying LUMO orbitals (d2z ). Similarly, other mixed MLCT tran-
sitions are from the orbitals HOMO-x (x = 3 and 7) to dxy character of
LUMO, and appear at 465 nm in methanol and at 505 nm in the
The absorption spectra of [RuL1(PPh3)Cl]+ recorded in different
solvents (Fig. 6) show that there is present a solvent effect in the
absorption spectra; for example, the absorption spectrum mea-
sured for the complex in CH2Cl2 is red shifted when compared to
that observed in methanol. For [RuL1Cl(PPh3)]Cl, the spectrum in
the visible region is dominated by the metal- to- ligand charge-
transfer (MLCT, d ? p⁄) bands: 790.6 nm (Ru(II) d ? benzimid-
azole), 581.3 nm (Ru(II) d ? P(Ph3) and, 465.4 (amine N ? d Ru(II))
(Table 3). Furthermore, in the complex, both processes i.e., an elec-
tron donation from the
r orbital of the ligand toward an empty
d orbital of the metal and a simultaneous donation from a
filled d orbital to a p⁄ anti-bonding orbital of the ligand, promote
each other for the visible bands. Since the ligand L1 has
r
donor
orbitals localized on N and
p
-donor and p⁄-acceptor orbitals delo-
calized on benzimidzole rings, the back-donation between the
ligand and the Ru orbitals is significant. The strongest absorption
in the UV region can be assigned as an intraligand p–
p⁄ transitions.
3.3. TD–DFT absorption spectrum
Since the visible bands of [RuL1(PPh3)Cl]+ in methanol were
broad, TD–DFT was then used to analyze the orbital contributions
to the MLCT spectral bands. The calculated absorption spectrum
of the complex is shown in Fig. 7 and the most representative cal-
culated optical transitions being presented Table 3. The calculated
spectrum qualitatively agrees only with the high energy bands of
the experimental spectra (i.e., peaks ꢀ465 nm and ꢀ350 nm), how-
ever, other visible bands (580 and 790 nm), that were observed in
the experimental spectra, did not appear in the calculated spectra.
The TD-DFT of [RuL2(PPh3)Cl]+, where the ligand is presented in
dianionic form and Ru ion is at +4 oxidation state, is red-shifted
compared to the calculated spectrum of [RuL1(PPh3)Cl]+ and
almost matches the visible experimental bands. Although the pres-
ence of Ru-complex of ligand L1 is confirmed in the solution by
spectral electrochemical methods, the existence of its de-proton-
ated L2 with Ru(IV) in the solution is unable to be confirmed by

H.J. Guadalupe et al. / Journal of Molecular Structure 989 (2011) 70–79
79
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TDDFT with a sizable intensity. It shows that in the frontier orbitals
for the ground state of the complex, the HOMOs have a significant
contribution from Ru t2g d-orbital with a
p contribution from the
benzimidazole. The HOMO of the complex consists mainly of an
anti-bonding combination of a t2g orbital on Ru ion and a p orbital
on Clꢂ.
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4. Conclusion
DFT and TD–DFT studies show that the complex [Ru(L1)Cl-
(PPh3)]Cl is present in distorted octahedral geometry, where since
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the absorption spectra. This is the reason why in the spectra, three
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(ꢀ580 and 790 nm) in the experimental spectrum approximately
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Acknowledgements
The authors acknowledge the Direccíon General de Asuntos del
Personal Académico (Project PAPIIT No. IN226310 for economic
support. We also thank DGSCA-UNAM for the computation
facilities.
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P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople Gaussian,
Revision D 01; Gaussian Inc., Wallingford, CT, 2004.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.01.005.
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