Ruthenium complexes incorporating azoimine and α-diamine based ligands: Synthesis, crystal structure, electrochemistry and DFT calculation

Article abstract of DOI:10.1016/j.ica.2011.12.050

Ru(II) complexes, trans-[RuCl₂(Azo)L] [where Azo = C ₆H₅NNC(C₆H₅)NC₆H ₅, L = 2,2′-bipyridine (bpy) (C1), 4,4′-dimethyl-2, 2′-bipyridine (dmb) (C2), 1,10-phenanthroline (phen) (C3

Full text of DOI:10.1016/j.ica.2011.12.050

Inorganica Chimica Acta 387 (2012) 45–51
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Ruthenium complexes incorporating azoimine and
a-diamine based ligands:
Synthesis, crystal structure, electrochemistry and DFT calculation
b
Mousa Al-Noaimi a, , Murad A. AlDamen
⇑
a Department of Chemistry, College of Sciences and Arts, Rabigh Campus, King Abdulaziz University, Saudi Arabia
b Department of Chemistry, Faculty of Science, The University of Jordan, Amman 11942, Jordan
a r t i c l e i n f o
a b s t r a c t
Ru(II) complexes, trans-[RuCl2(Azo)L] [where Azo = C6H5N@NAC(C6H5)@NC6H5, L = 2,20-bipyridine (bpy)
(C1), 4,40-dimethyl-2,20-bipyridine (dmb) (C2), 1,10-phenanthroline (phen) (C3), 5-amino-1,10-phenan-
throline (NH2phen) (C4)] were synthesized and characterized by spectroscopy (IR, UV–Vis, and NMR),
cyclic voltammetry and crystallography. The new Azo ligand was isolated as amidrazones, H2Azo {where
H2Azo is C6H5NHAN@C(C6H5)ANHC6H5}, but oxidized to azoimines (Azo) during the formation of the
Ru(II) complexes. A crystallographic analysis of C1 showed that the Ru-center is in a distorted octahedral
coordination sphere in which the donor atoms around the Ru(II) center occupy cis:cis:trans
N,N(Azo):N,N(bpy):Cl,Cl positions. The Ru(II) oxidation state is greatly stabilized by the novel Azo biden-
tate ligand showing Ru(III/II) oxidation couples ranging from 1.10 to 1.15 V. The absorption spectrum of
C1 in acetonitrile was modeled by time-dependent density functional theory.
Article history:
Received 23 October 2011
Received in revised form 21 December 2011
Accepted 26 December 2011
Available online 10 January 2012
Keywords:
Ruthenium
Complexes
Azomethine
X-ray structure
Electrochemistry
DFT calculation
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
addition, the electrochemical parameters EL for the ligands L were
calculated by Lever’s method and the magnitude of EL(L) was found
The chemistry of complexes built from heterocyclic nitrogenous
ligands has developed in different dimensions due to photophysical,
photochemical, catalytic and redox activities [1,2]. The major work
focused on ruthenium(II) complexes containing poly-pyridine
bases. The later bases may have different number of hetero-atoms,
ring size and substituents which resulted in modifying the p-acidity
and regulating the physical and chemical properties of these com-
plexes [3].
to increase as the withdrawing substituent attached to the L moi-
ety is replaced with electron donating groups [5]. As part of our
continuing interest in ruthenium azoimine chemistry, we report
herein the synthesis of
a new family of trans-[RuCl2(Azo)L]
(C1–C4) [Azo is C6H5N@NAC(C6H5)@NC6H5, and L is 2,20-bipyri-
dine (bpy) (C1), 4,40-dimethyl-2,20-bipyridine (dmb) (C2), 1,10-
phenanthroline (phen) (C3) and 5-amino-1,10-phenanthroline
(NH2phen) (C4)]. The effect of the substituents on the ligands L is
Ruthenium(II) complexes based on azoimine ligands (Ph–N@N–
expected to change the p-acceptor properties of these ligands
C(R)@N–Ph), which is isoelectronic with the
C@N–), are being currently examined in our laboratory [4–9]. These
ligands are strong -acceptor and capable of stabilizing metals in
a
-diimines (–N@C–
and thus ‘‘tune’’ the electronic properties of the ruthenium center
and consecutively the energy of MLCT bands. In addition, this work
also presents and discusses the spectroscopic (IR, UV–Vis and, 1H
NMR) and electrochemical (cyclic voltammetry) behavior of C1–
C4, and reports the X-ray structure for C1. The absorption spectrum
of C1 in acetonitrile has also been modeled by time-dependent
density functional.
p
their low oxidation states [4]. The extent of this stabilization de-
pends on the substituents on the phenyl rings (Ph) and the R group
to which the azo and imine groups’ linkage are attached [5].
We have previously reported the syntheses and electrochemis-
try of trans-[Ru(Az)LCl2] (Az is C6H5N@NAC(R)@NC6H5) where R is
an acetyl and acetate group and L are bipyridine- and phenanthro-
line-derivatives [4,5]. The effect of the substituents R on the elec-
tronic properties of the ruthenium complexes was studied [4]. In
2. Experimental
2.1. Materials
Ruthenium trichloride hydrate, lithium chloride, 2,20-bipyridine,
4,40-dimethyl-2,20-bipyridine, 1,10-phenanthroline, 5-amino-1,10-
⇑
Corresponding author. Permanent address: Department of Chemistry, Hash-
emite University, P.O. Box 150459, Zarqa 13115, Jordan. Tel.: +962 5 3903333; fax:
+962 5 3826613.
phenanthroline,
tetrabutylammonium
hexafluorophosphate
E-mail address: manoaimi@hu.edu.jo (M. Al-Noaimi).
(TBAHF) and solvents (reagent grade) were purchased from Aldrich
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.050

46
M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
and were used as received. N-phenylbenzenecarbohydrazonoyl
chloride was synthesized following reported procedures [10].
2.3.4. Trans-[RuCl2(Azo)(NH2phen)] (C4)
Yield = 0.27 g, 40%. I.r. (KBr, cmꢁ1): mN
@N = 1485, mC@N = 1630,
1H-NMR (DMSO) d 8.57 (d, 2H, H7), 8.30 (d, 2H, H1), 7.86 (m, 3H,
H8, H9), 7.58 (m, 7H, H2, H3, H4, (NH2phen)), 7.45 (m, 6H, H5,
H6, (NH2phen)), 7.29 (t, 1H, (NH2phen)), 7.24 (s, 1H, (NH2phen)),
2.2. Preparation of the N,N-diphenylbenzenecarbohydrazonamide
ligand (H2Azo)
6.68 (s, 2H, NH2). UV–Vis in acetonitrile: kmax (nm)
(emax,
A solution of aniline (1.86 g, 20 mmol) and triethylamine
(2.40 g, 24.0 mmol) in 5.0 mL ethanol was added to a 10 mL solu-
tion of N-phenylbenzenecarbohydrazonoyl chloride (4.60 g,
20.0 mmol). The resulted reaction mixture was refluxed for 2 h.
Condensing the solution followed by cooling produced a yellow
solid which was collected by filtration, then washed with water
and recrystallized from 2-propanol. Yield = 2.0 g, 35%. M.p 133–
134 °C. 1H NMR (DMSO) d 10.25 (s, NH), 9.18 (s, NH), 7.96 (d, 2H,
H7), 7.78 (d, 2H, H1), 7.50 (m, 7H, H2, H3, H6, H8, H9), 6.74 (t,
2H, H5), 6.61 (d, 2H, H4). M.p, 173–175 °C. Anal. Calc. for
Mꢁ1 cmꢁ1): 310 (2.95 ꢀ 104), 484 (1.01 ꢀ 104). Anal. Calc. for
RuC31H24N6Cl2: C, 57.06; H, 3.71; N, 12.88. Found: C, 57.15; H,
3.80; N, 12.72%.
2.4. Instrumentation
The 1H NMR spectra were measured on a Bruker-Avance
400 MHz spectrometer using TMS as an internal standard. Micro
analyses (C, H, N) were performed using EURO VECTOR elemental
analyzer model EA3000. IR spectra were obtained by FT-IR JASCO
model 420. Electronic spectra were recorded on a Shimadzu
240-UV–Vis spectrophotometer. Electrochemical measurements
were performed in 99.8% anhydrous acetonitrile (Aldrich, HPLC
grade) using a computer controlled Volta Lab model PGP201 with
a platinum working electrode, a platinum wire auxiliary electrode
and silver wire pseudo-reference electrode. Ferrocene (0.665 V ver-
sus Normal Hydrogen Electrode (NHE)) is used as an internal refer-
ence [11]. To control the temperature, a Haake D8-G refrigerated
bath and circulator was used to maintain the cell temperature at
25.0 0.1 °C. Tetrabutylammonium hexafluorophosphate (TBAHF)
(0.1 M) was twice recrystallized and vacuum dried at 110 °C, and
used as the supporting electrolyte. The experimental solutions were
degased by bubbling with research grade dinitrogen.
C19H17N3: C, 79.41; H, 5.96; N, 14.62. Found: C, 79.55; H, 6.18; N,
14.71%.
2.3. General procedure for the preparation of trans-[RuCl2(Azo)L]
(C1–C4)
A
suspension of ruthenium trichloride trihydrate (0.26 g,
1.0 mmol) and the (H2Azo) (0.29 g, 1.0 mmol) in 100 mL absolute
ethanol were reacted under reflux conditions. After 1 h, 1.0 mmol
of the ligand (L) and excess amount of LiCl (0.25 g, 5.90 mmol)
were added. The mixture was heated for an additional 3 h, after
which the solvent was removed by using a rotary evaporator.
The crude product was dissolved in dichloromethane, filtered and
washed with water to remove the unreacted ruthenium trichloride
and lithium chloride. The filtrate was reduced to 20 mL and puri-
fied by chromatography (50 ꢀ 3 cm) on alumina grade (III). Elution
with acetone gave a yellow band which was discarded followed by
a dark-red band of the product. The products were recrystallized
from slowly evaporating solutions of acetonitrile.
2.5. X-ray crystallography
Single brown plate crystals of C1 were collected. Single crystal
XRD data sets were collected at 100(1) K on a Bruker X8 Kappa
APEXII equipped with graphite monochromator. Crystal data
collection and refinement were performed using the package SMART
[12], SAINT [13], SADABS [14] and SHELXL-97 [15]. Geometric calculations
and molecular graphics were performed with CrystalMaker 6.0
2.3.1. Trans-[RuCl2(Azo)(bpy)] (C1)
Yield = 0.25 g, 52%. I.r. (KBr, cmꢁ1): mN
@N = 1490, mC@N = 1618,
1H-NMR (DMSO) d 8.57 (d, 1H, bpy), 8.48 (d, 1H, bpy), 8.12 (t,
1H, H9), 8.01 (d, 1H, bpy), 7.99 (d, 2H, H7), 7.77 (t, 1H, H3), 7.60
(t, 2H, H8), 7.53 (t, 2H, H2), 7.48 (t, 2H, H5), 7.44 (m, 2H, H1,
H6), 7.38 (m, 3H, H4, bpy), 7.32 (m, 2H, bpy), 7.22 (t, 1H, bpy),
7.13 (t, 1H, bpy), 7.01 (d, 1H, bpy). UV–Vis in acetonitrile: kmax
[16]. The threshold expression of F2 > 2r(F2) was used only for
calculating R-factors(gt) etc. and was not relevant to the choice of
reflections for refinement. Details of the data collection and refine-
ment are given in Table 1. The complex C1 was obtained by recrys-
tallization/synthesis with many solvents such as H2O, CH2Cl2,
acetone, and acetonitrile. We were not able to decide the solvent
multicomponent system that entered in the lattice, and for this
reason we used SQUEEZE. We deleted the correspondance diffraction
pattern that belongs to this solvent. Molecule analysis by program
PLATON [17] using the squeeze routine indicated void space of 264.
Squeezing out this disordered region and continuing the refinement
converged at R1 = 0.0372, S = 1.054 and largest electron density
peak of 0.397 e Åꢁ3 located off Ru1. All nonhydrogen atoms were
refined anisotropically. No decomposition was observed during
data collection. The crystal structure of the complex C1 is depicted
in Fig. 1. Crystal data after void squeeze is given in Table 1.
(nm) (
e
max, Mꢁ1 cmꢁ1): 291 (3.27 ꢀ 104), 513 (1.09 ꢀ 104). Anal.
Calc. for RuC29H23N5Cl2: C, 56.77; H, 3.78; N, 11.42. Found: C,
56.75; H, 3.61; N, 11.71%.
2.3.2. .Trans-[RuCl2(Azo)(dmb)] (C2)
Yield = 0.31 g, 48%. I.r. (KBr, cmꢁ1): mN
@N = 1480, mC@N = 1610,
1H-NMR (DMSO) d 8.41 (s, 1H, dmb), 8.33 (s, 1H, dmb), 7.98 (d,
2H, H7), 7.75 (t, 1H, H9), 7.58 (t, 2H, H8), 7.42 (m, 8H, H1, H4, H5,
H6, dmb), 7.31 (m, 3H, H2, H3), 7.04 (d, 1H, dmb), 6.97 (d, 1H,
dmb), 6.82 (d, 1H, dmb), 2.49 (s, 3H, CH3), 2.43 (s, 3H, CH3). UV–
Vis in acetonitrile: kmax (nm) (
e
max, Mꢁ1 cmꢁ1): 286 (3.14 ꢀ 104),
506 (1.01 ꢀ 104). Anal. Calc. for RuC31H27N5Cl2: C, 58.04; H, 4.24;
N, 10.92. Found: C, 58.25; H, 3.98; N, 10.83%.
2.6. Molecular orbital calculations
2.3.3. Trans-[RuCl2(Azo)(phen)](C3)
The X-ray structure of complex C1 was fully optimized using
LanL2DZ/6-31+G(d,p) level of theory using the GAUSSIAN03 [18] and
was used as starting coordinates to generate trans-[RuCl2(Azo)L]
(C2–C4) geometries. All theoretical calculations were carried out
using Becke’s three-parameter hybrid function with LYP correla-
tion function [19,20] (B3LYP), as implemented in the GAUSSIAN03
program package [18]. The optimized geometric parameters for
complex C1 are gathered in Table 2. Generally, the calculated bond
lengths and angles are in a good agreement with the values based
Yield = 0.29 g, 46%. I.r. (KBr, cmꢁ1): mN
@N = 1495, mC@N = 1606,
1H-NMR (DMSO) d 8.57 (d, 1H, Phen), 8.76 (t, 1H, Phen), 8.20 (t,
2H, H8), 8.04 (d, 2H, H7), 7.82 (d, 1H, Phen), 7.74 (t, 1H, Phen),
7.66 (m, 3H, H2, H9), 7.53 (m, 4H, H5, H4), 6.08 (m, 6H, H6, H1,
H3, Phen), 7.28 (t, 1H, Phen), 7.19 (d, 1H, Phen). UV–Vis in acetoni-
trile: kmax (nm)
(
emax
,
Mꢁ1 cmꢁ1): 296 (3.04 ꢀ 104), 513
(1.10 ꢀ 104). Anal. Calc. for RuC31H23N5Cl2: C, 58.40; H, 3.64; N,
10.99. Found: C, 58.15; H, 3.72; N, 10.83%.

M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
47
Table 1
Table 2
Crystal data and structure refinement for C1.
Bond lengths [Å] and angles [°] for complex C1.
Empirical formula
Formula weight
T (K)
Crystal color and shape
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
RuC29H23Cl2N5
613.49
100(1)
brown, plate
0.71073
monoclinic
C2/c
Bond lengths (Å) (experimental, optimized)
Ru(1)–N(3)
Ru(1)–N(5)
Ru(1)–N(2)
Ru(1)–N(1)
Ru(1)–Cl(1)
Ru(1)–Cl(2)
N(3)–N(4)
N(5)–C(17)
1.940(3), 1.977
2.026(2), 2.080
2.092(3), 2.135
2.137(2), 2.186
2.359(9), 2.440
2.388(9), 2.468
1.329(3), 1.299
1.318(4), 1.316
23.767(5)
b (Å)
c (Å)
8.8496(18)
28.213(6)
90
90.50(3)
90
5934(2)
8
1.376
0.734
2480
Bond angles (°) (experimental, optimized)
N(3)–Ru(1)–N(5)
N(3)–Ru(1)–N(2)
N(5)–Ru(1)–N(2)
N(3)–Ru(1)–N(1)
N(5)–Ru(1)–N(1)
N(2)–Ru(1)–N(1)
N(3)–Ru(1)–Cl(1)
N(5)–Ru(1)–Cl(1)
N(2)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
N(3)–Ru(1)–Cl(2)
N(5)–Ru(1)–Cl(2)
N(2)–Ru(1)–Cl(2)
N(1)–Ru(1)–Cl(2)
Cl(1)–Ru(1)–Cl(2)
76.23(10), 74.98
103.41(10), 103.94
173.04(10), 176.37
177.66(10), 179.35
103.78(10), 105.53
76.87(9), 75.57
89.74(8), 91.89
87.51(8), 88.48
85.53(7), 88.09
92.60(7), 88.51
95.49(8), 96.13
94.80(8), 94.45
92.16(7), 89.10
82.17(7), 83.45
174.65(3), 171.92
a
(°)
b (°)
c
(°)
V (3Å)
Z
Dcalc. (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
)
Crystal size (mm)
Theta range for data collection
Index ranges
0.65 ꢀ 0.16 ꢀ 0.09
1.44–25.68°
ꢁ28 6 h 6 28, 0 6 k 6 10,
0 6 l 6 34
5629
5629 [Rint = 0.0000]
99.9%
Semi-empirical from equivalents
1.0000 and 0.8161
Full-matrix least-squares on F2
5629/0/335
Reflections collected
Independent reflections
Completeness to theta = 25.68°
Absorption correction
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
the electronic absorption spectrum for the complex C1. Orbital con-
tribution was analyzed using GAUSSSUM software [23].
Goodness-of-fit (GOF) on F2
1.054
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0372, wR2 = 0.0762
R1 = 0.0527, wR2 = 0.0816
0.397 and ꢁ0.577
Largest difference in peak and hole
3. Results and discussion
(e Åꢁ3
)
Calc. w = 1/[r2 (Fo2) + (0.0315P)2 + 3.5503P] where P = (Fo
+
2)/3.
c
2
3.1. Synthesis
The N-phenylbenzenecarbohydrazonoyl chloride (1) was pre-
pared by chlorination of N-phenylbenzohydrazide in carbon tetra-
chloride [10], then reacted with aniline to give the amidrazone
ligand (H2Azo) (Scheme 1). The ruthenium(II) complexes of this li-
gand trans-[RuCl2(Azo)L] (C1–C4) were obtained by stepwise equi-
molar addition of RuCl3, H2Azo, substituted bipyridine or
phenanthroline ligands (L) and then an excess of lithium chloride
in refluxing ethanol (Scheme 1). Ethanol presumably acts both as
a solvent and as a reducing agent for Ru(III) [4,5]. The ligand
H2Azo is oxidized to Azo by Ru(III) and/or atmospheric oxygen un-
der the conditions of the synthesis. All the complexes are air stable
as solids or in solution and are soluble in common organic solvents.
Their structures were confirmed by 1H NMR spectra, elemental
analysis and X-ray diffraction for complex C1.
upon the X-ray crystal structure data. LanL2DZ effective core
potential basis set was employed for Ruthenium and 6-31g⁄ for
the rest of the atoms in acetonitrile solution. Time-dependent
density functional theory (TD-DFT) [21,22] and NBO analysis were
performed using B3LYP functional and a mixed basis set, LanL2DZ/
6-31+G(d,p), in acetonitrile as a solvent via polarized continuum
model (PCM). The lowest 20 singlet-to-singlet spin-allowed
excitation states were taken into account for the calculations of
The 1H NMR spectra of complexes C1–C4 show multiplets due
to aromatic protons of the phenyl rings of the the azoimine ligands
and L. In their IR-spectra, all complexes show bands in the ranges
of 1560–1630 and 1480–1495 cmꢁ1 which are assigned to the C@N
and N@N stretching bands of azoimine ligands, respectively.
3.2. Crystal structures
The X-ray structure of C1 was determined and the perspective
molecular view is shown in Fig. 1. The selected bond parameters
are listed in Table 2. This complex crystallizes in the C2/c space
group. The bond length for N(3)–N(4) and N(5)–C(17) are
1.329(3) and 1.318(4) Å, respectively,which is slightly longer than
the bond length for the free azo (N@N) (1.266(3) Å and the free
0
imine C@N (1.29 ÅA) bonds [24]. In addition, both the Ru–N(azo)
and Ru–N(methine) distances of the azoimine ligand (1.940(3)
and 2.026(2) Å) are shorter than Ru–N(bpy) lengths (average,
Fig. 1. Thermal ellipsoid drawing (30%) of C1 complex.
2.115 Å). This shortening may be due to the great p-back donation

48
M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
aniline
NH
NH
NH
Cl
N
N
triethylamine, ethanol
1
H2Azo
3
2
1
Cl
L = N
LiCl
N
N
NH
NH
N
N
N
N
RuCl3
+
Ru
Cl
7
8
9
N
4
5
6
C1-C4
Ligand (L)
Complex
bpy dmb phen aphen
C1 C2 C3 C4
Scheme 1. The chemical structures of ligand H2Azo and complexes (C1–C4).
from Ru-d orbital to the empty p⁄ orbital of the azoimine ligand
[25]. The Ru–N(bpy) distances are longer than those in the corre-
sponding [Ru(bpy)3]2+ complexes [26]. The average Ru–Cl bond
lengths in C1 (2.374(9) Å) is comparable to those reported for sim-
ilar systems [27]. The coordination geometry of the complex is dis-
torted from regular octahedral as indicated from the bond angles
[N(3)–Ru(1)–N(5), N(2)–Ru(1)–N(1) and Cl(1)–Ru(1)–Cl(2) are
76.23(10), 76.87(9) and 174.65(3), respectively].
the half-wave potential of earlier complex was shifted anodically
by ꢂ15 mV compared to those observed with later complexes. This
relationship indicates that the donor ability of the phenanthroline
ligands is slightly weaker than that of the bipyridine ligands [26].
The Ru(III/II) couple is slightly affected by changing the substituent
on the phenanthroline and 2,20-bipyrdine ligands.
Lever has developed an electrochemical parameterization
method to calculate ruthenium(III/II) couples of complexes with
octahedral geometry as shown in Eq. (1) [28]
h
i
X
3.3. Electrochemistry
ERuðIII=IIÞ ¼ 0:97
where
EL þ 0:04 in V versus NHE
ð1Þ
Formal oxidation and reduction potentials of C1–C4 versus NHE
were obtained from cyclic voltammograms recorded at a platinum
electrode in acetonitrile containing 0.1 M tetrabutylammonium
hexaflurophospate (Table 3). Complex C1, as a representative
example (Fig. 2), displays an irreversible reduction wave at ꢁ0.58
which is assigned to azo(0/ꢁ) ligand-centered processes. The
reversible oxidation peak at 1.19 V, is attributed to the metal-cen-
tered Ru(III/II) couple. This couple is slightly affected by changing
the bipyridine ligands by phenanthroline derivatives. However,
R
EL is the sum of electrochemical parameters for each ligand
in the complex. The parameter EL is a measure of the stabilizing
effect a ligand has on the Ru(II) state and so the greater the magni-
tude of positive EL the more positive the Ru(III/II) couple. Based on
the previously found (EL(bpy) = 0.259 [28], EL(Cl) = ꢁ0.24 [28]) and
assuming ligand additivity, the Ru(III/II) couple for C1 was used to
find the (EL(L)) for unreported (Azo) ligand and it was found to be
0.45 V. The large EL value for Azo (EL(Azo) = 0.45) suggests that this
ligand is a stronger p-acid than bpy but it is almost the same as pre-
viously prepared Az ligand (Az = C6H5N@NAC(COCH3)@NC6H5,
EL(Az) = 0.42 [4]).
Table 3
Cyclic voltammetry and electronic spectroscopy data of trans-[RuCl2(Azo)L] (C1–C4).a
(Eo1/2, V)b
Azo(0/ꢁ)c
Electronic spectrakmax (nm)
3.4. Electronic structure
Complex
1
2
3
4
1.19
1.13
1.28
1.23
ꢁ0.58
ꢁ0.57
ꢁ0.50
ꢁ0.55
513
506
513
507
Theoretical calculations were performed on trans-[RuCl2(Azo)L]
(C1–C4); relative percentages of atomic contributions to the lowest
unoccupied and highest occupied molecular orbitals have been
placed in Table 4. Moreover, the isodensity plots for the HOMOs
and LUMOs orbitals for complex C1 are shown in Fig. 3. Results
indicate that the LUMOs are constructed mainly from the p⁄ orbital
of azoimine (70%) and has 20% metal d-orbital character which
a
Solvent MeCN, supporting electrolyte Bu4NPF6 (0.1 M), scan rate 0.1 V s, Pt-disk
working electrode, Pt-wire auxiliary electrode, reference electrode Ag at 25 °C.
Eo M = (E°pa + E°pc)/2.
b
c
1/2
The cathodic peak maximum.

M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
49
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
absorption spectrum of C1 (as representative example) using
TD-DFT method. Excitation energies, oscillator strengths and corre-
sponding transitions compositions for the simulated absorption
bands in acetonitrile solution are listed in Table 5. Both the exper-
imental UV–Vis spectrum of C1 reported in acetonitrile and its sim-
ulated absorption spectrum shown in Fig. 4 are in good agreement
for the band in the visible region whereas a blue shift for simulated
absorption spectrum in the UV region is observed. The observed
electronic absorption spectrum of C1 in acetonitrile is shown in
Fig. 4. For the band in the visible region, TD-DFT calculations show
that the band at kmax = 513 nm which is detected experimentally
(calculated at 510 nm) resulted from HOMO-1 (which has sizable
contributions of Ru(dp) orbitals mixed with chloride and azoimine
orbitals) and HOMO-2 (which has sizable contributions of Ru(d
p
)
orbitals mixed with chloride orbitals) to LUMO which has a signif-
icant contribution from the p⁄ orbital of azomethine. Thus, this
band is assigned to MLMLCT (metal-ligand to metal-ligand charge
transfer). The near-UV region band centered at 439 nm (calculated)
which appear as a shoulder in the experimental spectrum, arises
mainly from the HOMO-3 which is mainly azoimine in character
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Voltage (vs. NHE)
Fig. 2. Cyclic voltammogram for complex C1 in acetonitrile 0.1 M TBAH at 25 °C,
data reported in V vs. NHE with scan rate of 0.1 V/s.
to LUMO, thus this band is assigned to LLCT or
p ?
p⁄. The strong
suggests significant back donation [29]. The lower unoccupied
molecular orbitals (LUMO+1 and LUMO+2) for these complexes
consist mostly of a series of antibonding p⁄ orbital of bipyrdine
and phenanthroline derivatives. The other group of HOMOs
(HOMO to HOMO-2) can be described as t2g Ru orbitals with small
contributions from the azoimine and chloride ligands.
high energy band at 291 nm (calculated at 370 nm) resulted from
HOMO-7, HOMO-8 and HOMO-9 which are mainly azoimine in
character to LUMO. Thus, this band is assigned to LLCT or
too.
p ?
p⁄
The MLCT band energy and Ru(III/II) couple for a Ru(II) mono-
nuclear complex provide information about the relative energies
of the complex’s metal and ligand-based orbitals. The Ru(III/II)
couple shifts to more positive potentials as the metal orbitals are
The lowest 20 singlet-to-singlet spin-allowed excitation states
were taken into account for the calculation of the electronic
Table 4
Relative percentages of atomic contributions to the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO) of trans-[RuCl2(Azo)L] (C1–C4).
MO
Energy (ev)
Ru
L
Cl
Azoimine
C1
LUMO+6
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
ꢁ0.47
ꢁ0.77
ꢁ1.03
ꢁ1.15
ꢁ1.16
ꢁ2.0
21
2
4
59
3
5
19
67
46
69
10
1
9
0
0
0
25
1
0
69
78
22
8
3
4
19
74
7
92
90
9
3
1
3
3
ꢁ3.04
ꢁ5.44
ꢁ5.94
ꢁ6.12
ꢁ6.57
ꢁ6.82
ꢁ6.94
ꢁ6.97
3
70
9
HOMO
21
30
2
8
2
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
22
26
78
44
81
79
53
11
16
0
1
7
4
C2
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2
ꢁ1.14
ꢁ1.91
ꢁ3.01
ꢁ5.41
ꢁ5.92
ꢁ6.09
5
4
20
68
47
70
90
92
7
3
2
1
0
3
20
29
2
4
3
70
9
23
25
3
C3
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2
ꢁ2.01
ꢁ2.34
ꢁ3.06
ꢁ5.45
ꢁ5.94
ꢁ6.09
6
0
19
67
42
58
89
99
9
4
8
0
0
3
21
28
1
5
1
69
9
23
18
24
C4
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2
ꢁ1.93
ꢁ2.22
ꢁ3.02
ꢁ5.43
ꢁ5.9
6
0
19
67
36
43
90
99
8
4
20
43
0
0
3
20
23
4
4
1
70
9
21
10
ꢁ6.02

50
M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
Fig. 3. Isodensity plots of the HOMO and LUMO orbitals of complex C1.
stabilized [29]. For the trans-[RuCl2(Azo)L], the bipyrdine ligands
Table 5
shifts the Ru(III/II) couple approximately ꢂ15 mV negative relative
to phenanthroline analogs. This stabilization, however, does not re-
sult in a noticeable shift in the MLCT band energy due to the equiv-
alent stabilization of the orbitals involved in MLCT events (HOMO’s
and the LUMO azoimine ligand) (Table 4). The difference in the
Computed excitation energies (nm), electronic transition configurations and oscillator
strengths (f) for the optical transitions in the visible region of complex C1 (transitions
with f P 0.02 are listed, H stands for HOMO and L for LUMO).
k
f
510
439
391
375
373
370
358
346
0.1386
0.0796
0.0283
0.028
0.0429
0.0806
0.0289
0.0226
H-2 ? L (28%), H-1 ? L (63%)
H-3 ? L (89%)
energy
D
E = EHOMO ꢁ ELUMO for all complexes is ꢂ2.4 eV.
H-4 ? L (72%), H-1 ? L+1 (11%), H-2 ? L+1 (8%)
H-5 ? L(37%), H-8 ? L (27%), H-2 ? L+1 (12%)
H-7 ? L (35%), H-2 ? L+1 (20%), H-5 ? L (13%)
H-7 ? LUMO (42%), H-8 ? LUMO (16%), H-2 ? L+1 (16%)
H-9 ? L (56%), H-10 ? L (22%), H-5 ? L (7%)
H ? L+4 (46%), H-12 ? L (12%), H ? L+6 (8%)
4. Conclusions
The crystallography and electrochemistry of trans-[RuCl2(Azo)L]
(C1–C4) where Azo = C6H5N@NC(C6H5)@NC6H5, and L is a substi-
tuted bipyridine and phenanthroline ligands show that the new
Fig. 4. Uv-visible spectrum for complex C1 in acetonitrile (black line). Inset shows simulated absorption spectrum (red line) based on TD-DFT calculations, compared to
excitation energies and oscillator strengths.

M. Al-Noaimi, M.A. AlDamen / Inorganica Chimica Acta 387 (2012) 45–51
51
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based on TD-DFT calculations.
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Acknowledgements
This paper was funded by the Deanship of Scientific Research
(DSR), King Abdulaziz University, Jeddah, under Grant No. (2-
662-D1432). The authors, therefore, acknowledge with the thanks
DSR technical and financial support.
Appendix A. Supplementary material
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