Synthesis and spectroscopic characterization of a hydride carbonyl ruthenium(II) complex with 2-methyl-4(5)nitroimidazole as a co-ligand

Article abstract of DOI:10.1016/j.poly.2013.02.077

The reaction between [RuHCl(CO)(PPh₃)₃] and 2-methyl-4(5)-nitroimidazole (L) has been performed and the hydride carbonyl ruthenium(II) complex [RuH(CO)(PPh₃)₂(L)] has been obtained by this reaction, running with deprotonation of the ligand. The molecular structure of the complex has been determined by X-ray crystallography and its spectroscopic characterization has been carried out by IR, ¹H, ³¹P and ¹³C NMR, UV-Vis spectroscopy. Supported by the DFT computational method, the electronic structure is described in order to explain the absorption and emission spectra obtained experimentally. The luminescence property of the complex has been examined and the quantum yield of the fluorescence has been obtained by a comparative method. The observable fluorescence is related to MLLCT excited states. The imidazole derivative ligand plays a role in the creation of the excited states.

Full text of DOI:10.1016/j.poly.2013.02.077

Polyhedron 55 (2013) 18–23
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and spectroscopic characterization of a hydride carbonyl ruthenium(II)
complex with 2-methyl-4(5)nitroimidazole as a co-ligand
⇑
´
Jan Grzegorz Małecki, Anna Maron
Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction between [RuHCl(CO)(PPh3)3] and 2-methyl-4(5)-nitroimidazole (L) has been performed and
the hydride carbonyl ruthenium(II) complex [RuH(CO)(PPh3)2(L)] has been obtained by this reaction, run-
ning with deprotonation of the ligand. The molecular structure of the complex has been determined by X-
ray crystallography and its spectroscopic characterization has been carried out by IR, 1H, 31P and 13C NMR,
UV–Vis spectroscopy. Supported by the DFT computational method, the electronic structure is described
in order to explain the absorption and emission spectra obtained experimentally. The luminescence prop-
erty of the complex has been examined and the quantum yield of the fluorescence has been obtained by a
comparative method. The observable fluorescence is related to MLLCT excited states. The imidazole
derivative ligand plays a role in the creation of the excited states.
Received 2 January 2013
Accepted 20 February 2013
Available online 14 March 2013
Keywords:
Hydride carbonyl ruthenium(II) complexes
Imidazole derivatives
X-ray crystallography
DFT
Absorption and emission electronic spectra
Fluorescence
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
In this paper, the synthesis and study by IR, NMR and UV–Vis
spectroscopy and X-ray crystallography of a hydride carbonyl
Hydride carbonyl complexes of ruthenium(II) containing tri-
phenylphosphine and N-heteroaromatic ligands derived from pyr-
idine, imidazole and benzimidazole are one of the fastest growing
areas in coordination chemistry, mainly due to its potential appli-
cations [1–5]. The wide interest in ruthenium(II) complexes as
luminescence materials is a consequence of their application in
photo-activated molecular systems [6–10]. In this regard, the
attention of scientists is focused on designing spectroscopic prop-
erties by the introduction of specific ligands to the coordination
sphere of the metal center. It is well known that in metal–ligand
probes the heterocyclic ligands play a primary role in the determi-
nation of emissions originating from MLCT excited states. On the
other hand, influences of the coordination environment can change
the character of the excited state from MLCT to MLLCT [11]. Hence,
one can see that not only the ligands directly related to the formed
excited states have a significant effect, but the whole coordination
environment of the central ions is important in this case. The hy-
ruthenium(II) complex with 2-methyl-4(5)-nitroimidazole as a
co-ligand are presented. Based on the crystal structure, computa-
tional research was made in order to determine the electronic
structure of the complex by analysis of its optimized molecular
geometry and its electronic population using the natural bond
orbitals scheme. The latter was used to identify the nature of the
interactions between the ligands and the central ion. The calcu-
lated density of states showed the interactions and influences of
the orbital composition on the frontier electronic structure. Time
dependent density functional theory (TD-DFT) was finally used to
calculate the electronic absorption spectrum. Based on the molec-
ular orbital scheme, the experimental absorption electronic spec-
trum has been interpreted. The luminescent property of the
complex was examined and the quantum yields of the fluorescence
were determined.
2. Experimental
dride ligand is strong
r-donor and through the ‘trans effect’ affects
neighboring ligands. Therefore, study on hydride carbonyl ruthe-
nium(II) complexes seems to be of interest. Earlier we reported
our study on ruthenium hydride carbonyl complexes with imidaz-
ole carboxylic acid derivative ligands [12]. For these complexes,
emissions coming from MLCT excited states was observed and
the nature of N-heterocyclic ligand had a substantial influence on
the observed fluorescent properties.
[RuHCl(CO)(PPh3)3] was synthesized according to a literature
method [13]. All other reagents used for the synthesis of the com-
plex were commercially available and have been used without fur-
ther purification.
2.1. Synthesis of the complex
⇑
The complex [RuH(CO)(PPh3)2(L)] was synthesized by the
Corresponding author. Tel.: +48 011323591886.
E-mail address: ank806@wp.pl (A. Maron´ ).
reaction between [RuHCl(CO)(PPh3)3] (0.2 g, 2.0 ꢀ 10ꢁ4 mol) and
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.02.077

´
19
J.G. Małecki, A. Maron / Polyhedron 55 (2013) 18–23
2-methyl-4(5)-nitroimidazole (0.028 g, 2.2 ꢀ 10ꢁ4 mol). The mix-
ture of compounds was refluxed in methanol (60 mL) for 4 h. After
this time, it was cooled and filtered. Crystals suitable for X-ray
crystal analysis were obtained by slow evaporation of the reaction
mixture.
molecular orbital was calculated using Mulliken population analy-
sis. GAUSSSUM 2.2 [21] was used to calculate group contributions to
the molecular orbitals and to prepare the partial density of states
(DOS) spectrum. The DOS spectrum was created by convoluting
the molecular orbital information with Gaussian curves of unit
height and a FWHM (Full Width at Half Maximum) of 0.3 eV.
IR (KBr, cmꢁ1): 3055
mArH; 2014
m
Ru–H; 1924
mCO; 1501masym NO2;
1479 d(C–CH
plane); 1432
m
Ph(P–Ph); 1365, 1350 msym NO2; 1091
in the
d(C–CH
plane); 743 d(C–C
plane); 695 d(C–C
;
in the plane)
in the
out of the
2.4. Crystal structure determination and refinement
519 d(Ru–(H)CO). UV–Vis (ethanol, nm (loge): 433.6 (1.34), 347.4
(3.97), 274.0 (4.24), 208.2 (4.88). 1H NMR (400 MHz, CDCl3)
A red crystal of [RuH(CO)(PPh3)2(L)] was mounted on a Gemini
A Ultra Oxford Diffraction automatic diffractometer equipped with
a CCD detector, and used for data collection. X-ray intensity data
were collected with graphite monochromated Mo K
(k = 0.71073 Å) at a temperature of 295.0(2) K, with the
d: 7.73–7.65 (m, Im); 7.59–6.87 (m, PPh3), 3.50 (s, CH3), ꢁ15.62
(t, J = 19.8 Hz,
H
(Ru)). 31P NMR (162 MHz, CDCl3) d: 44.28
(s, PPh3). 13C NMR (CDCl3) d: 206.93 (s, C„O); 132.62, 132.10,
a
radiation
scan
131.93, 128.50, 128.34 (Im, PPh3); 77.04 (t, CDCl3); 30.93 (s, CH3).
x
mode. Ewald sphere reflections were collected up to 2h = 50.10°.
The unit cell parameters were determined from least-squares
refinement of the setting angles of the 3853 strongest reflections
for the studied complex. Details concerning crystal data and refine-
ment are gathered in Table 1. Lorentz, polarization and empirical
absorption corrections using spherical harmonics implemented in
the SCALE3 ABSPACK scaling algorithm [22] were applied. The
structure was solved by the Patterson method and subsequently
completed by difference Fourier recycling. All the non-hydrogen
atoms were refined anisotropically using the full-matrix, least-
squares technique. The Ru–H hydrogen atom was found from dif-
ference Fourier synthesis after four cycles of anisotropic refine-
ment and the remaining hydrogen atoms were refined as
‘‘riding’’ on the adjacent carbon atom with individual isotropic
temperature factors equal to 1.2 times the value of the equivalent
temperature factor of the parent atom. Bearing in mind the limita-
tions of Fourier synthesis and the problems in recognizing artifacts
in the immediate neighborhood of heavy atoms, it is doubtful
whether a reliable position for the hydrogen atom bound to the
Ru atom could be found in the difference Fourier map, avoiding
the danger of mistaking the effects of the series termination errors
for a true atomic position. In this complex the Ru–H bond length of
2.2. Physical measurements
The infrared spectrum was recorded on a FT-IR Nicolet iS5 spec-
trophotometer in the spectral range 4000–400 cmꢁ1 using a KBr
pellet. The electronic spectrum was measured on a Lab Alliance
UV–VIS 8500 spectrophotometer in the range 600–180 nm in eth-
anol solution. The 1H, 31P and 13C NMR spectra were obtained at
room temperature in CDCl3 using a Bruker Avance 400 MHz spec-
trometer. Luminescence measurements were made in ethanolic
solutions on a Hitachi F-7000 FL spectrophotometer at room tem-
perature. The quantum yield of fluorescence was calculated using
Gradsg2s
follow equation: Us
¼
Ustd
, where Us is the quantum yield
Gradstdg2
std
of the unknown sample, Ustd is the quantum yield of naphthalene,
as a reference, at 313 nm and is equal to 0.21 [14], Grads and
Gradstd are the gradients from the plots of the integrated fluores-
cence intensity vs the solution absorbencies at the excitation wave-
length, and gs and gstd are the refractive indices of the solvents. The
samples were prepared with absorbencies less than 0.1 at the exci-
tation wavelengths and were gradually diluted in order to mini-
mize re-absorption effects and to avoid inner-filter effects, which
may perturb the quantum yield values.
2.3. DFT calculations
Table 1
Crystal data and structure refinement details of the [RuH(CO)(PPh3)2(L)] complex.
The calculations were carried out using the GAUSSIAN09 [15] pro-
gram. The molecular geometry of the singlet ground state of the
complex was fully optimized in the gas phase at the B3LYP/DZVP
level of theory [16,17]. For the complex a frequency calculation
was carried out, verifying that the obtained optimized molecular
structure corresponds to an energy minimum, thus only positive
frequencies were expected. The DZVP basis set [18] with f func-
tions with exponents 1.94722036 and 0.748930908 was used to
describe the ruthenium atom and the basis set used for the lighter
atoms (C, N, O, P, H) was 6-31G with a set of ‘‘d’’ and ‘‘p’’ polariza-
tion functions. The TD-DFT (time dependent density functional
theory) method [19] was employed to calculate the electronic
absorption spectrum of the complex in the solvent PCM (Polariz-
able Continuum Model) model. In this work 100 singlet excited
states were calculated as vertical transitions for the complex. A
natural bond orbital (NBO) analysis was also made for the complex
using the NBO 5.0 package [20] included in GAUSSIAN09. Natural
bond orbitals are orbitals localized on one or two atomic centers
that describe the molecular bonding in a manner similar to a Lewis
electron pair structure, and they correspond to an orthonormal set
of localized orbitals of maximum occupancy. NBO analysis pro-
1
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C41H35N3O3P2Ru
780.73
295.0(2)
orthorhombic
P212121
12.1794(5)
15.5281(7)
19.3642(7)
90.00
3662.2(3)
4
1.418
0.558
1604
b (Å)
c (Å)
a
= b = c (°)
Volume (Å3)
Z
Calculated density (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
)
Crystal dimensions (mm)
h range for data collection (°)
Index ranges
0.15 ꢀ 0.11 ꢀ 0.07
3.35–25.05
ꢁ14 6 h 6 14,
ꢁ15 6 k 6 18,
ꢁ23 6 l 6 23
12574
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
6220[R(int) = 0.0447]
6220/0/456
1.088
vides the contribution of atomic orbitals (s, p, d) to the NBO
r
and hybrid orbitals for bonded atom pairs. In this scheme, three
p
Final R indices [I > 2
r
(I)]
R1 = 0.0614,
wR2 = 0.0814
R1 = 0.0974,
wR2 = 0.0940
0.737/ꢁ0.573
NBO hybrid orbitals are defined, bonding orbital (BD), lone pair
(LP) and core (CR), which were analyzed on the atoms directly
bonded to or presenting some kind of interaction with the ruthe-
nium atom. The contribution of a group (ligands, central ion) to a
R indices (all data)
Largest diff. peak and hole

´
J.G. Małecki, A. Maron / Polyhedron 55 (2013) 18–23
20
1.67(5) Å does not deviate significantly from the bond length value
located in the CSD 2013 basis. The OLEX2 [23], SHELXS97 and SHELXL97
[24] programs were used for all the calculations. Atomic scattering
factors used were those incorporated in the computer programs.
is shown in Fig. 2). The experimental and calculated geometrical
details as selected bond lengths and angles are listed in Table 2.
The bond lengths and angles are comparable with distances in
other hydride-carbonyl ruthenium(II) complexes with N-heteroar-
omatic ligands. In this complex the Ru–H bond distance of
1.67(5) Å does not differ significantly from the values for other
ruthenium carbonyl hydride complexes found in the Cambridge
Structural Database (CSD; ConQuest v. 1.15; 2013) and that found
in the parent complex (1.7 Å) [25–28].
3. Results and discussion
3.1. Spectroscopic characterization of the complex
In the complex there is a pseudooctahedral coordination envi-
ronment around the central ruthenium(II) ion. The largest devia-
tion from an ideal octahedron comes from the bite angle of the
2-methyl-4(5)-nitroimidazole ligand. The N(1)–Ru(1)–O(2) angle
is equal to 74.49(18)°. The carbonyl group lies in a trans position
to the nitrogen heteroatom of the imidazole ring and the oxygen
donor atom from nitro group is trans to the hydride ligand. The
Ru(1)–C(1) and Ru(1)–H(1) bond lengths are equal to 1.829(7)
and 1.67(5) Å, respectively. Moreover, the C(1)–O(1) bond and
the Ru(1)–C(1)–O(1) angle are equal to 1.158(7) Å and 179.16°.
The Ru–C bond distance is normal for a monomeric Ru(II) carbonyl
complex (the distances generally range from 1.74 to 1.98 Å). In the
complex, the phosphine ligands are in a trans conformation, with
the P(1)–Ru(1)–P(2) angle being equal to 175.92(6)°. Generally,
the presence of a singlet in the 31P NMR spectrum of such a com-
plex shows the equivalence of the triphenylphosphine ligands.
The 1H NMR spectrum of the complex shows a triplet at high
field (ꢁ15.62 ppm), which indicates the presence of the hydride li-
gand coordinated to ruthenium central ion. The shift of the signal is
due to the shielding effect of the metal and to the charge of the hy-
dride ion. The range of the shift is characteristic for complexes with
N,O-donor anionic ligands based on imidazole moieties and reflects
the influence of the coordination environment on the interaction
between the ruthenium central ion and the hydride ligand. The
Ru–H signal is a triplet due to coupling with the two trans phos-
phorus atoms (JHP = 19.8 Hz). Moreover, in the 1H NMR spectrum
of the complex a set of signals corresponding to the PPh3 and
2-methyl-4(5)-nitroimidazole ligands are visible and are given in
Section 2. Additionally, it is shown that there is a lack of signals
coming from NH protons, that correlates with the molecular
structure of [RuH(CO)(PPh3)2(L)]. In the 31P NMR spectrum of
the complex a singlet at 44.28 ppm is visible. It indicates the pres-
ence of magnetically equivalent phosphorus atoms, which proves
the equivalence of the phosphine groups in the structure of the
complex. The 13C NMR spectrum presents signals attributed to
PPh3 and imidazole groups (see Section 2). Moreover, a signal at
206.93 ppm indicates the presence of the carbonyl group. The
carbon from the methyl group of the imidazole derivative ligand
gives a signal at 30.93 ppm.
3.3. Electronic structures
The ground state geometry of the complex was optimized in the
singlet state using the B3LYP functional. The calculation was car-
ried out for a gas phase molecule and in general the predicted bond
lengths and angles are over-estimated by about 0.1 Å and 4°,
respectively. The calculated IR frequencies of the complex show
agreement with the experimental spectrum; the differences can
be explained by the neglect of intermolecular interactions for the
gas phase. From the data collected in Table 2, one may see that
the major difference between the experimental and calculated
geometries is found in the Ru(1)–C(1) distance (0.15 Å).
The IR spectrum of the complex shows a strong C„O band at
1924 cmꢁ1 and a Ru–H stretching band at 2014 cmꢁ1. In the parent
complex, the mCO and mRu–H stretching bands were observed at 1922
and 2020 cmꢁ1, respectively. For hydride carbonyl ruthenium(II)
complexes with N,O-donor heteroaromatic ligands, the shift of
the carbonyl stretching vibration in comparison to this mode for
the parental complex is characteristic. It take place when the
N,O-donor ligand interacts with the ruthenium(II) center in such
way that the delivery of electron density by backbonding to the
carbonyl ligand is possible. In this case, the lack of a visible shift
Based on the optimized geometry of the complex, an NBO
analysis was performed in order to reveal the nature of the coordi-
nation between the ruthenium center and the donor atoms of the
of mCO indicates that 2-methyl-4(5)-nitroimidazole is a rather weak
ligand. The asymmetric and symmetric stretches of the NO2 group
give bands with maxima at 1501 and 1350 cmꢁ1, respectively, but
the first band is partially covered by very strong d(C–CH in the plane)
bands related to triphenylphosphine groups present in the struc-
ture of the compound, and for this reason it is barely visible. The
nitro group stretches in the free ligands are observed at 1505
and 1377 cmꢁ1 and the visible change in the band position of the
symmetric stretches of the NO2 group indicates coordination of
the nitro groups to ruthenium.
3.2. Molecular structure
The reaction between [RuHCl(CO)(PPh3)3] and 2-methyl-4(5)-
nitroimidazole (L) results in red crystals of the title complex
[RuH(CO)(PPh3)2(L)], where L = 2-methyl-4(5)-nitroimidazole. The
2-methyl-4(5)-nitroimidazole ligand is coordinated to the ruthe-
nium(II) central ion as a N,O-donor, with the removal of the imid-
azole NH proton. The deprotonation of the imidazole nitrogen
atom is confirmed by the lack of signals coming from NH protons
in the 1H NMR spectrum. The complex crystallizes in the ortho-
rhombic P212121 space group and Fig. 1 presents the molecular
structure of the complex (a structural drawing of the compound
Fig. 1. Molecular structure of the [RuH(CO)(PPh3)2(L)] complex. Hydrogen atoms
(bonding with carbons) are omitted for clarity.

´
21
J.G. Małecki, A. Maron / Polyhedron 55 (2013) 18–23
analysis were calculated using the GAUSSSUM program, and Fig. 3 pre-
sents the composition of the fragment orbitals contributing to the
molecular orbitals of the complex. In the HOMO orbital of the com-
plex, dRu orbitals and orbitals of the 2-methyl-4(5)nitroimidazole
ligand participate in the same range (43%). The contribution of
the PPh3 ligands is also visible, with a 14% share in this orbital.
The contribution of dRu orbitals in lower occupied orbitals is visible
for the HOMOꢁ1 to HOMOꢁ4 orbitals (23–78%). For these orbitals
the participation of the carbonyl ligand is observed at about 15%. In
the unoccupied (LUMO) orbitals of the complex the triphenylphos-
phine ligands play a significant role, but the LUMO is localized on
the imidazole derivative ligand (95%). The d orbitals of the ruthe-
nium ion share in LUMO+1, LUMO+4 and LUMO+9 orbitals at the
level of 16–19%. On the OPDOS plot the antibonding interaction be-
tween the imidazole ligand and the ruthenium(II) central ion in the
frontier HOMO orbitals is visible. On the other hand, in this energy
range, the influence of the carbonyl ligand on the ruthenium d
orbitals has a positive value, indicating a bonding character of
these interactions.
Fig. 2. Structural drawing of the [RuH(CO)(PPh3)2(L)] complex.
Table 2
Selected bond lengths (Å) and angles (°) for the [RuH(CO)(PPh3)2(L)] complex with the
optimized geometry values.
Exp
Cal
Bond lengths (Å)
Ru(1)–C(1)
Ru(1)–N(1)
Ru(1)–O(2)
Ru(1)–P(2)
Ru(1)–P(1)
Ru(1)–H(1)
O(1)–C(1)
1.829(7)
1.875
2.116(5)
2.254(4)
2.3608(15)
2.3640(15)
1.67(5)
2.164
2.274
2.433
2.433
1.607
1.162
1.158(7)
3.4. Electronic absorption and emission spectra
Angles (°)
C(1)–Ru(1)–N(1)
C(1)–Ru(1)–O(2)
N(1)–Ru(1)–O(2)
C(1)–Ru(1)–P(2)
N(1)–Ru(1)–P(2)
O(2)–Ru(1)–P(2)
C(1)–Ru(1)–P(1)
N(1)–Ru(1)–P(1)
O(2)–Ru(1)–P(1)
P(2)–Ru(1)–P(1)
C(1)–Ru(1)–H(1)
N(1)–Ru(1)–H(1)
O(2)–Ru(1)–H(1)
P(2)–Ru(1)–H(1)
P(1)–Ru(1)–H(1)
O(1)–C(1)–Ru(1)
175.3(2)
173.82
99.92
73.90
89.54
90.88
93.93
89.54
90.87
93.93
172.13
89.44
96.73
170.63
86.09
86.09
178.22
The electronic absorption spectrum was measured on a Lab Alli-
ance UV–VIS 8500 spectrophotometer in the range 500–180 nm in
methanol solution. The experimental spectrum of the complex
100.8(2)
74.49(18)
88.4(2)
91.09(14)
91.06(11)
90.8(2)
90.03(14)
93.02(11)
175.92(6)
89.3(16)
95.4(16)
169.4(16)
86.1(15)
89.9(15)
179.1(6)
ligands. The interactions between the ligands and the central ion
manifest themselves in the charge on the complex metal ion. The
metal ion in the complex is formally in the +2 oxidation state,
but in the studied complex the calculated natural charge is consid-
erably lower than +2, being equal to ꢁ0.786. This indicates that the
donations from the ligands to the central ion have the advantage
over the back donations from the metal to the ligands. The NBO
analysis (Second Order Perturbation Theory Analysis of the Fock Ma-
trix in NBO Basis) also shows that the donation from the 2-methyl-
4(5)-nitroimidazole ligand to the ruthenium central ion, with value
of 347.41 kcal/mol, exceeds the back donation, equal to 33.52 kcal/
mol. In addition to the
r-donor properties, this ligand exhibits
weak -acceptor properties. The analysis shows that the imidazole
p
derivative N,O-donor ligand does not show covalent bonding with
ruthenium; the Coulomb-type interaction between the ruthenium
center and the ligand is clearly visible in the calculated Wiberg
bond indexes, which are considerably lower than one. The Ru–N
and Ru–O bond indices are equal to 0.44 and 0.34, respectively.
The Ru–P bond orders are also smaller than 1 and are close to
0.70. The charge of the hydride ligands is +0.06 and the Wiberg in-
dex of the Ru–H bond is equal to 0.81. The charge on the carbonyl
group, calculated by summing the individual charges on the carbon
and oxygen atoms, is 0.20 for the complex. The Wiberg index of the
CO bond in the complex is reduced by about 0.18 with respect to
free CO (WCO = 2.23).
Analysis of the frontier molecular orbitals is useful for under-
standing spectroscopic properties, such as electronic absorption
and emission spectra. The density of state (DOS) and overlap pop-
ulation density-of-states (OPDOS) in terms of Mulliken population
Fig. 3. The density-of-states and overlap partial density of states diagrams for
interactions between the ruthenium central ion and the ligands in the complex
[RuH(CO)(PPh3)2(L)].

´
J.G. Małecki, A. Maron / Polyhedron 55 (2013) 18–23
22
Table 3
The electronic transitions for the complex calculated with the B3LYP functional by the TD-DFT method.
Wavelength (nm)
f
Transitions
Character
Exp (nm)
433.6
411.55
379.77
373.13
342.95
311.75
298.45
279.81
276.42
270.85
268.01
257.16
247.09
233.69
225.07
0.0627
0.0024
0.0002
0.0265
0.0879
0.1096
0.0187
0.0144
0.0573
0.0252
0.3115
0.0014
0.0121
0.0062
HOMO ? LUMO (96%)
dRu
dRu
dRu
dRu
dRu
dRu
dRu
dRu
dRu
dRu
dRu
dRu
?
?
?
?
p⁄
Hꢁ2 ? LUMO (76%), Hꢁ1 ? LUMO (18%)
p⁄L
p⁄L
Hꢁ3 ? LUMO (99%)
Hꢁ4 ? LUMO (26%), Hꢁ2?LUMO (22%), Hꢁ1 ? LUMO (50%)
Hꢁ4 ? LUMO (64%), Hꢁ1 ? LUMO (24%)
Hꢁ2 ? L+1 (43%), Hꢁ1 ? L+1 (22%)
HOMO ? L+2 (32%), HOMO ? L+3 (64%)
HOMO ? L+4 (71%)
p⁄L
347.4
274.0
L
/
pL
pL
pL
pL
?
?
?
?
p⁄
?
?
?
p⁄
/
/
/
p⁄L
p⁄PPh3
p⁄PPPPhh33/dRu
Hꢁ2 ? L+1 (24%), Hꢁ1 ? L+1 (67%)
?
PPh3
HOMO ? L+5 (85%)
/
/
/
pL
pL
pL
p⁄
Hꢁ4 ? L+1 (51%), Hꢁ2 ? L+1 (11%)
HOMO ? L+9 (84%)
HOMO ? L+13 (61%)
Hꢁ9 ? L+2 (35%), Hꢁ4 ? L+5 (26%)
p⁄PPh3
250.2
208.2
p⁄PPPPhh33/dRu
pL
?
p⁄
PPh3
p⁄
pPPh3
?
PPh3
shows maxima of absorption at 433.6, 347.4, 274.0 and 208.2 nm.
The theoretical absorption spectrum of the complex was obtained
from the calculations of the singlet excited states by the TD-DFT
method. Computation of 100 excited states of the complex allowed
for the interpretation of the experimental spectrum. The selected
excited states and their assignment to the absorption bands are
shown in Table 3. The bands above 300 nm have HOMO/Hꢁ1/
Hꢁ2 ? LUMO character. Taking into account the fact that the d
ruthenium orbitals play a role in these HOMOs, and the LUMO
orbital is localized on the imidazole derivative ligand, the transi-
tions at 433.6 and 347.4 nm have Metal–Ligand Charge Transfer
character. However, given that the triphenylphosphine ligands
mainly participate in the LUMOs and due to the contribution of
in optically diluted solutions by the comparative method, using
naphthalene in ethanol as a standard (UST = 0.21). The solution of
the complex has been excited at a wavelength equal to 274 nm
and the emission maximum was observed at 307 nm. The Stockes
shift is equal to 3923.06 cmꢁ1. The excitation band corresponds to
the transition dRu/pL ?
p⁄PPh3/dRu for the absorption spectrum of
the complex. The assignments are supported by the analysis of
the frontier orbitals of the complex, showing a partial contribution
of ligand nature. As one can see from Table 3, the transitions calcu-
lated in this energy range have MLLCT character and proceed be-
tween the frontier HOMOs and LUMO+1 orbital. On the
formation of excited states, both the metal ion and the imidazole
derivative ligand are involved. In the formed excited states, a wide
share of the triphenylphosphine ligands is visible with an admix-
ture of ruthenium d orbitals and a slight contribution of the imid-
azole derivative ligand. At high optical dilution (O.D. < 0.03), the
excitation band at 318 nm is visible with an emission maximum
at 361 nm. This excitation band has MLCT character and proceeds
between the frontier HOMOs and LUMO orbital. In the formed ex-
cited states a contribution of the imidazole derivative ligand is
both ruthenium d orbitals and
ligand in the frontier HOMO orbital, the bands observed at 274 and
250 nm have been assigned to MLLCT transitions (dRu
p⁄PPh3),
p bonding orbitals of the imidazole
/pL ?
with a small admixture of d–d character. The highest experimental
band, close to 210.0 nm, may result from transitions in the PPh3
ligands.
The fluorescence spectrum of the complex has been prepared in
optically diluted (O.D. < 0.1) ethanolic solution at room tempera-
ture. The measurement of the quantum yield has been examined
dominant. The Stockes shift in this case is equal to 3745.71 cmꢁ1
Moreover, the observed fluorescence properties are also related
.
Fig. 4. Emission spectrum of the [RuH(CO)(PPh3)2(L)] complex in methanolic solution.

´
23
J.G. Małecki, A. Maron / Polyhedron 55 (2013) 18–23
to the fact that in the frontier HOMOs of the complex, the carbonyl
References
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fact that there is no clear participation of one fragment of the
molecular orbital in the excited state, which suggests that more
than one excited state plays a role in the emission band.
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Appendix A. Supplementary data
CCDC 917237 contains the supplementary crystallographic data
for [RuH(CO)(PPh3)2(L)]. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.