Synthesis, crystal, molecular and electronic structures of ruthenium complexes with a benzoxazole derivative ligand

Article abstract of DOI:10.1016/j.poly.2011.09.045

[RuCl₂(HBO)(PPh₃)₂] and [RuCl(CO)(HBO)(PPh₃)₂] complexes with the 2-(2-hydroxyphenyl)benzoxazole (C₁₃H₉NO₂) ligand were synthesized and characterized by infra red, proton and phosphorus nuclear magnetic resonances, electronic absorption and emission spectroscopies and X-ray crystallography. The experimental studies were completed by theoretical calculations. The calculations show that the donor properties of the carbonyl group predominates the π-acceptor ability in the ruthenium(II) complex. The small transfer of electron density to the acceptor π carbonyl orbitals is compensated by the presence of the chloride acceptor ligand. The electronic structures of these complexes, presented in particular by density of states diagrams, have been correlated with their ability to fluorescence and have been used to analyze the UV-Vis spectra.

Full text of DOI:10.1016/j.poly.2011.09.045

Polyhedron 31 (2012) 159–166
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal, molecular and electronic structures of ruthenium complexes
with a benzoxazole derivative ligand
J.G. Małecki
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:
Received 28 July 2011
Accepted 7 September 2011
Available online 12 October 2011
[RuCl2(HBO)(PPh3)2] and [RuCl(CO)(HBO)(PPh3)2] complexes with the 2-(2-hydroxyphenyl)benzoxazole
(C13H9NO2) ligand were synthesized and characterized by infra red, proton and phosphorus nuclear mag-
netic resonances, electronic absorption and emission spectroscopies and X-ray crystallography. The
experimental studies were completed by theoretical calculations. The calculations show that the donor
properties of the carbonyl group predominates the
p-acceptor ability in the ruthenium(II) complex.
The small transfer of electron density to the acceptor p⁄ carbonyl orbitals is compensated by the presence
of the chloride acceptor ligand. The electronic structures of these complexes, presented in particular by
density of states diagrams, have been correlated with their ability to fluorescence and have been used to
analyze the UV–Vis spectra.
Keywords:
Ruthenium complexes
2-(2-Hydroxyphenyl)benzoxazole
X-ray structure
UV–Vis
Luminescence
DFT
Ó 2011 Elsevier Ltd. All rights reserved.
TD-DFT
1. Introduction
Here is reported an experimental and quantum chemical
study of ruthenium(III) and ruthenium(II) complexes with the
Benzoxazole and their derivatives have been studied in recent
years because of their high quantum yields [1], high non-linear
optical effectiveness, biological activities [2,3], photo-stable prop-
erties and capabilities to ligate transition metals [4,5]. In addition,
benzoxazole compounds and their metallic complexes have been
widely applied for use in chemistry and medicine [6–8]. However
the substituent in the 2-position of the benzoxazole ring may be
quite easily changed to modify the photophysical properties of
the compound. Luminescent and redox-active transition–metal
complexes are an important class of molecules. For this reason,
2-(2-hydroxyphenyl)benzoxazole ligand. The quantum chemical
study includes characterization of the molecular and electronic
structures of the complexes by analysis of optimized molecular
geometries and electronic populations by using the natural bond
orbital 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 in the frontier electronic structure. The
time dependent density functional theory (TD-DFT) was finally
used to calculate the electronic absorption spectra. Based on a
molecular orbital scheme, these results allowed the interpretation
of the UV–Vis spectra obtained at the experimental level. The com-
plexes reported in this paper combine the interest in ruthenium
coordination compounds and complexes containing N,O-heterocy-
clic ligands [21–32].
complexes of ruthenium containing
p-acceptor nitrogen-bearing
ligands, especially N,O-heteroaromatics, have been widely studied
as systems for the conversion of energy. Studies on the synthesis
and characterization of ruthenium complexes containing
N-heteroaromatic ligands have received considerable recent atten-
tion, owing to their interesting photophysical and photochemical
properties. The N-donor ligands have energetically low lying
2. Experimental
p-antibonding orbitals, which can accept electrons from filled
metal d orbitals. As a consequence, they can exhibit charge transfer
bands with interesting spectroscopic properties in the visible
region [9]. Ligands containing the oxazole ring are widely studied
All reagents used for the synthesis of the complexes are
commercially available and have been used without further
purification.
and their p-donor properties are interesting. Its combination with
other donor atoms should in principle afford complexes with
tunable spectroscopic properties [10–20].
2.1. Synthesis of [RuCl2(HBO)(PPh3)2] (1) and [RuCl(CO)(HBO)(PPh3)2]
(2)
The complexes were synthesized in the reactions between
E-mail address: gmalecki@us.edu.pl
[RuCl2(PPh3)3] (1), and [RuHCl(CO)(PPh3)3] (2) (1 Â 10À4 mol) and
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.045

160
J.G. Małecki / Polyhedron 31 (2012) 159–166
2-(2-hydroxyphenyl)benzoxazole (HBO) in methanol solutions
(100 cmÀ3). The mixture of the compounds was refluxed in meth-
anol for 3 h. After this time, the reaction mixture was cooled and
filtered. Crystals suitable for X-ray crystal analysis were obtained
by slow evaporation of the reaction mixture.
measurements were made in methanolic solutions on an F-2500
FL spectrophotometer at room temperature.
2.3. Computational methods
1: Yield 57%. IR (KBr, cmÀ1): 3056
1605 (C@C), 1469 d(C–CH in the plane), 1434
1092 d(C–CH in the plane), 742 d(C–C out of the plane), 693 d(C–C in the plane)
m
ArH, 2925
m
C@H, 1632 m(C@N)
,
,
,
The calculations were carried out using the GAUSSIAN09 [33] pro-
gram. Molecular geometries of the singlet ground state of com-
plexes 1 and 2 were fully optimized in the gas phase at the
B3LYP/DZVP level of theory [34,35]. For each compound a fre-
quency calculation was carried out, verifying that the optimized
molecular structure obtained corresponds to an energy minimum,
thus only positive frequencies were expected. The DZVP basis set
with f functions 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, Cl) was 6-31G with a set of
‘‘d’’ and ‘‘p’’ polarization functions. The TD-DFT (time dependent
density functional theory) method [36] was employed to calculate
the electronic absorption spectra of the complexes in the solvent
PCM model. In this work 90 singlet excited states were calculated
as vertical transitions for the complexes. A natural bond orbital
(NBO) analysis was also made for all the complexes using the
NBO 5.0 package [37] included in GAUSSIAN09. Natural bond orbitals
are orbitals localized on one or two atomic centers that describe
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 provides the contri-
m
m
Ph(P–Ph), 1231 m(C–O/C–O–C)
520 m(P–Ph + P–Ru). UV–Vis (methanol, nm, loge): 786.8 (1.17), 550.6
(1.03), 434.2 (1.10), 317.8 (1.97), 298.6 (1.99), 211.6 (5.02). 1H
NMR (d, CDCl3): 8.05 (dd, J = 7.9, 1.5 Hz), 7.14 (d, J = 8.3 Hz, 2H),
7.03 (t, J = 7.6 Hz), 7.81–7.21 (m, PPh3). 31P NMR (d, CDCl3):
29.410 (s, PPh3).
2: Yield 62%. IR (KBr, cmÀ1): 3055
m
ArH, 2884
m
CH, 1939 m(CO)
,
,
,
1640
1250
m
(C@N), 1610
m(C@C), 1470 d(C–CH
plane), 1433 mPh(P–Ph)
in the
m(C–O/C–O–C), 1091 d(C–CH
plane), 742 d(C–C
out of the plane)
in the
694 d(C–C in the plane), 516 m(P–Ph + P–Ru). UV–Vis (methanol, nm, loge):
441.8 (1.26), 404.6 (1.38), 301.6 (2.56), 276.2 (sh), 210.6 (4.92). 1H
NMR (d, CDCl3): 8.11 (m), 7.71 (dd, J = 23.8, 12.2 Hz, 7H), 7.68–7.11
(m, PPh3), 7.06 (dd, J = 19.9, 11.4 Hz), 6.87 (t, J = 6.8 Hz), 6.70 (d,
J = 8.5 Hz), 6.40 (t, J = 7.5 Hz). 31P NMR (d, CDCl3): 37.049 (d,
PPh3, J = 28.74 Hz), 33.788 (d, PPh3, J = 28.69 Hz).
2.2. Physical measurements
Infrared spectra were recorded on a Nicolet Magna 560 spectro-
photometer in the spectral range 4000–400 cmÀ1 using KBr pellets.
Electronic spectra were measured on a Lab Alliance UV–Vis 8500
spectrophotometer in the range of 900–180 nm in methanol solu-
tion. The 1H and 31P NMR spectra were obtained at room temper-
ature in CDCl3 using a Bruker 400 spectrometer. Luminescence
bution of atomic orbitals (s, p, d) to the NBO
r and p hybrid orbi-
tals for bonded atom pairs. In this scheme, three NBO hybrid
orbitals are defined, bonding orbitals (BD), lone pair (LP) and core
(CR), which were analyzed on the atoms directly bonded to or pre-
senting some kind of interaction with the ruthenium atom. The
contribution of a group (ligands, central ion) to a molecular orbital
was calculated using Mulliken population analysis. GAUSSSUM 2.2
[38] was used to calculate group contributions to the molecular
orbitals and to prepare the partial density of states (DOS) spectra.
The DOS spectra were 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.
Table 1
Crystal data and structure refinement details of [RuCl2(PPh3)2(HBO)] (1) and
[RuCl(CO)(HBO)(PPh3)2] (2).
1
2
Empirical formula
Formula weight
T (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
49H38Cl2NO2P2Ru C50H38ClNO3P2Ru
906.71
899.27
2.4. Crystal structure determination and refinements
295.0(2)
monoclinic
P21/n
295.0(2)
monoclinic
P21/c
A brown crystal of [RuCl2(HBO)(PPh3)2] (1) and a yellow plate of
[RuCl(CO)(HBO)(PPh3)2] (2) were mounted in turn on an Xcalibur,
Atlas, Gemini Ultra Oxford Diffraction automatic diffractometer
equipped with a CCD detector, and used for data collection. X-ray
14.1131(5)
18.0940(7)
16.2073(6)
90
9.9682(5)
21.7460(14)
19.1375(12)
90
b (Å)
c (Å)
a
(°)
b (°)
102.235(4)
90
4044.7(3)
4
1.489
0.642
91.322(5)
90
4147.3(4)
4
1.440
0.565
c
(°)
V (Å3)
Z
Dcalc (mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
1852
1840
Crystal dimensions (mm)
h range for data collection (°)
Index ranges
0.19 Â 0.14 Â 0.07 0.27 Â 0.13 Â 0.09
3.42–25.05
À16 6 h 6 16
À21 6 k 6 21
À19 6 l 6 17
22947
3.47–25.05
À11 6 h 6 11
À23 6 k 6 25
À22 6 l 6 22
18039
Reflections collected
Independent reflections
7147
7271
[Rint = 0.0456]
7147/0/514
1.038
[Rint = 0.0570]
7271/0/523
1.037
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R1 = 0.0395
wR2 = 0.0825
R1 = 0.0735
wR2 = 0.0921
0.588 and À0.475
R1 = 0.0439
wR2 = 0.0878
R1 = 0.0556
wR2 = 0.0120
1.962 and À1.022
R indices (all data)
Largest difference Peak and hole
(e ÅÀ3
)
Fig. 1. The experimental IR spectrum of 2 with calculated infra red frequencies.

J.G. Małecki / Polyhedron 31 (2012) 159–166
161
intensity data were collected with graphite monochromated MoK
radiation (k = 0.71073 Å) at a temperature of 295(2) K, with the
a
x
algorithm (CrysAlis RED, Oxford Diffraction Ltd., Version 1.171.
29.2) were applied. The structures were solved by the Patterson
method and subsequently completed by difference Fourier recy-
cling. All the non-hydrogen atoms were refined anisotropically
using the full-matrix, least-squares technique. All the hydrogen
atoms were found from difference Fourier synthesis after four cy-
cles of anisotropic refinement, and refined as ‘‘riding’’ on the adja-
cent carbon atom with an individual isotropic temperature factor
equal to 1.2 times the value of equivalent temperature factor of
scan mode. Ewald sphere reflections were collected up to
2h = 50.10°. The unit cell parameters were determined from the
least-squares refinement of the setting angles of 7842 and 10920
strongest reflections for complexes 1 and 2, respectively. Details
concerning crystal data and refinement are gathered in Table 1.
Lorentz, polarization and empirical absorption corrections using
spherical harmonics implemented in SCALE3 ABSPACK scaling
Fig. 2. The molecular structures of [RuCl2(PPh3)2(HBO)] (1) and [RuCl(CO)(HBO)(PPh3)2] (2) with 50% probability displacement ellipsoids. Hydrogen atoms are omitted for
clarity.

162
J.G. Małecki / Polyhedron 31 (2012) 159–166
Table 2
Selected bond lengths (Å) and angles (°) for [RuCl2(PPh3)2(HBO)] (1) and [RuCl(CO)(HBO)(PPh3)2] (2).
1
2
Exp
Calc
Exp
Calc
Bond lengths (Å)
Ru(1)–O(2)
Ru(1)–N(1)
Ru(1)–Cl(1)
Ru(1)–Cl(2)
Ru(1)–P(1)
Ru(1)–P(2)
1.997(2)
2.120(2)
2.334(9)
2.348(8)
2.4418(9)
2.4401(9)
2.031
2.176
2.390
2.402
2.518
2.499
Ru(1)–C(50)
Ru(1)–O(2)
Ru(1)–N(1)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–Cl(1)
C(50)–O(1)
1.824(9)
2.076(5)
2.140(7)
2.369(2)
2.362(2)
2.427(2)
1.174(10)
1.8600
2.1191
2.1721
2.4592
2.4642
2.4564
Angles (°)
N(1)–Ru(1)–O(2)
Cl(1)–Ru(1)–O(2)
N(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(2)
Cl(2)–Ru(1)–O(2)
Cl(1)–Ru(1)–Cl(2)
P(1)–Ru(1)–O(2)
P(2)–Ru(1)–O(2)
Cl(1)–Ru(1)–P(1)
Cl(2)–Ru(1)–P(1)
Cl(1)–Ru(1)–P(2)
Cl(2)–Ru(1)–P(2)
N(1)–Ru(1)–P(1)
N(1)–Ru(1)–P(2)
P(1)–Ru(1)–P(2)
87.98(9)
177.32(7)
94.19(7)
174.62(7)
86.72(6)
91.14(3)
90.66(8)
89.59(8)
87.67(3)
88.54(3)
91.95(3)
87.87(3)
92.42(7)
91.19(7)
176.38(3)
85.16
176.86
92.66
174.12
89.07
93.14
95.01
85.51
87.27
88.80
92.40
86.48
94.21
90.56
175.24
P(1)–Ru(1)–P(2)
C(50)–Ru(1)–Cl(1)
P(1)–Ru(1)–N(1)
P(1)–Ru(1)–C(50)
N(1)–Ru(1)–Cl(1)
P(2)–Ru(1)–N(1)
P(2)–Ru(1)–C(50)
P(2)–Ru(1)–Cl(1)
P(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–C(50)
P(2)–Ru(1)–O(2)
Cl(1)–Ru(1)–O(2)
N(1)–Ru(1)–O(2)
C(50)–Ru(1)–O(2)
P(1)–Ru(1)–O(2)
Ru(1)–C(50)–O(1)
99.75(8)
94.80(3)
89.24(19)
95.90(3)
82.30(2)
170.70(19)
86.00(3)
88.39(8)
166.95(8)
95.50(3)
92.20(18)
86.14(17)
86.40(3)
177.9(3)
83.39(17)
179.20(8)
101.49
93.62
89.64
96.54
82.18
168.18
86.92
86.18
167.56
95.85
92.20
86.92
85.14
178.93
83.04
178.54
the parent atom. The OLEX2 [39], SHELXS97 and SHELXL97 [40] pro-
grams were used for all the calculations. Atomic scattering factors
were those incorporated in the computer programs.
and carbonyl ligands in complex 2. The distortions from octahedral
are not only due to the smaller bite angle for the chelating interac-
tion of the HBO ligand but also because of the cis disposition of the
PPh3 ligands in complex 2. Selected bond distances and angles of
the complexes are listed in Table 2. The Ru–N and Ru–O lengths
in the Ru(III) complex 1 are slightly shorter than in complex 2,
indicating the stronger interaction of the HBO ligand. Similarly,
the lengths of the Ru–PPh3 bonds in the complexes are confirmed
by the stronger donor–acceptor properties of PPh3 ligands in cis
positions [41]. In the same manner, based on the Ru–Cl bond
lengths in the complexes, it can be inferred that in complex 2 the
3. Results and discussion
3.1. Spectroscopic characterization of the complexes
The 31P NMR spectra of the complexes show a singlet at
29.410 ppm for complex 1 and two doublets at 37.049 (J =
28.74 Hz) and 33.788 (J = 28.69 Hz) ppm in the case of complex 2.
The two doublet signals indicated mutually cis disposed triphenyl-
phosphine ligands in the complex. The 31P signal in complex 1 is
shifted to a higher field compared with complex 2, which corre-
sponds to an interaction with the central ruthenium ion in the for-
mal oxidation state +3. The 1H NMR spectra of the complexes
show sets of signals corresponding to the PPh3 and HBO ligands.
The IR spectra of the complexes present ring C@N and C@C
stretching modes of benzoxazole at 1632, 1640 and 1605,
1610 cmÀ1, which are close to the stretches of the free ligand, ob-
served at 1633 and 1607 cmÀ1. The vibration of the ether part of
the benzoxazole ring gave bands with maxima at 1231 and
1250 cmÀ1 in complexes 1 and 2, respectively. The experimental
and calculated IR spectra of complex 2 are presented in Fig. 1.
The strong band with a maximum at 1939 cmÀ1 is attributed to
the stretching mode of the C„O group in the [RuCl(CO)-
(HBO)(PPh3)2] complex.
p-acceptor properties of chloride ligands are predominant over
the donor properties in ruthenium(II) complexes. The ruthenium
chloride bond length is longer in complex 2 by about 0.09 Å more
than the average Ru–Cl bond value in [RuCl2(HBO)(PPh3)2]. It is
interesting that the Ru(1)–Cl(1) bond length at the opposite vertex
to the phenolate O atom is longer than the chloride trans to the
N-benzoxazole donor. This indicates a stronger repulsion felt by
the N atom opposite the phenolate O atom in the meridional
isomer. The carbonyl group in complex 2 is in a trans position to
the oxygen atom of the phenolate part of the HBO ligand and the
Table 3
Hydrogen bonds for [RuCl2(PPh3)2(HBO)] (1) and [RuCl(CO)(HBO)(PPh3)2] (2)
(Å and o).
D–HÁ Á ÁA
d(D–H)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
<(DHA)
1
C(3)–H(3)Á Á ÁCl(1)
C(13)–H(13)Á Á ÁO(1)
C(21)–H(21)Á Á ÁCl(1)
C(31)–H(31)Á Á ÁO(2)
C(37)–H(37)Á Á ÁO(2)
0.93
0.93
0.93
0.93
0.93
2.46
2.35
2.62
2.47
2.41
3.214(4)
2.693(4)
3.463(3)
2.911(5)
3.233(5)
138.0
101.0
150.0
109.0
148.0
3.2. Structures of the complexes
Single-crystal structures of complexes 1 and 2 are represented
with OTEP diagrams in Fig. 2. All the crystals were obtained from
the reaction solutions. Both complexes belong to monoclinic space
groups, P21/n and P21/c, respectively. The complexes adopt a dis-
torted-octahedral geometry around ruthenium, consisting of one
2-(2-hydroxyphenyl)benzoxazole ligand with two triphenylphos-
phine and one chloride ligand in complex 1, and with chloride
2
C(7)–H(7)Á Á ÁO(1)
C(13)–H(13)Á Á ÁO(3)
C(27)–H(27)Á Á ÁO(2)
C(43)–H(43)Á Á ÁCl(1)
C(49)–H(49)Á Á ÁO(2)
0.93
0.93
0.93
0.93
0.93
2.50
2.41
2.27
2.80
2.50
3.327(14)
2.744(14)
3.078(11)
3.252(10)
2.937(12)
148.8
100.9
145.6
110.9
109.3

J.G. Małecki / Polyhedron 31 (2012) 159–166
163
strong donation from the benzoxazole nitrogen forces the cis
arrangement of the triphenylphosphine ligands. Different supra-
frequencies of complex 2, shown in Fig. 1, confirm the calculated
structures with experimental one, and the differences in the calcu-
lated and experimental spectra mainly result from the negligence
of intermolecular interactions for the gas phase.
molecular interactions, such as hydrogen bonding and
p–p
stacking interactions play a role in the molecules of the complexes.
The hydrogen bonds are collected in Table 3. Additionally, there are
offset heat-to-tail intermolecular
p–p stacking interactions, with
inter-plane distances of about 3.71 and 3.64 Å and angles of
26.9° and 21.6° in complexes 1 and 2, respectively, as is shown
in Fig. 3.
3.3. The ground state geometries and electronic structures
The optimized geometries of the complexes are given in Table 2.
The optimized bond distances and angles agree well with the
experimental values, the theoretical results showing the same
trends as the experimental observations The calculated IR
Fig. 4. The overlap partial density of states diagram for complex 2.
Fig. 3. View of the
p
–p
stacking interactions in complexes 1 and 2.
Fig. 5. The overlap partial density of states a and b diagrams for complex 1.

164
J.G. Małecki / Polyhedron 31 (2012) 159–166
The NBO analyses were performed for the complexes, which
bonding with ruthenium. Coulomb-type interactions between
the ruthenium central ions and HBO ligands are clearly visible
in the calculated Wiberg bond indices, whose values are consider-
ably lower than unity. In the complexes the WRu–N values are
close to 0.45 and WRu–O is 0.55 in complex 1 and 0.43 for 2.
The bond indices for the ruthenium chloride and phosphine li-
gands are also smaller than 1 (ꢀ0.7). For the carbonyl group of
complex 2, three natural bond orbitals were detected for the C–
O bond, and one for the Ru–C bond. The Ru–C bond orbitals are
polarized towards the carbon atom, and the C–O bond orbitals
are polarized towards the oxygen end. The oxygen atom of the
carbonyl ligand has one lone pair (LP) orbital. The charge of the
CO ligand can be easily calculated by summing the individual
charges on the carbon and oxygen atoms, and has a value of
0.20. This value shows that there is some charge transfer between
CO and the Ru fragment. The Wiberg indices of the CO bond in
the complex are reduced (by about 0.21) with respect to free
CO (WCO = 2.23). The weak value is in agreement with elongation
of the C–O bond in the complex and charge distribution in the
terminal bonding carbonyl group to the ruthenium central ion.
The bonding can be described as the result of two electron trans-
allowed knowing the nature of the coordination between ruthe-
nium and the atoms of the ligands directly interacting with it.
This methodology also gave a better understanding of the opti-
mized molecular structures. In the analysis was found that the
2-(2-hydroxyphenyl)benzoxazole ligand does not show covalent
fers. First this binding mode involves a
r-donation from the CO
lone pair orbital to the LUMO orbital of complex. On the other
hand, the highest occupied orbitals of the metallic fragment,
mainly 4dRu orbitals, are involved in a
p back-bonding from the
metal to the CO p⁄ orbitals. This is consistent with the crystallo-
graphic data from Table 2, where the C„O bond length is
1.174(10) Å. Fig. 4 shows the interaction between the ruthenium
atom and the ligands in complex 2. As one can see, the p-acceptor
interaction between the carbonyl group towards the ruthenium
central ion is very small compared with the Ru–Cl interaction.
However, a small transfer of electron density to the acceptor p⁄
carbonyl orbitals is compensated by the presence of acceptor
chloride and PPh3 ligands. The interaction of the HBO ligand with
the Ru(IIII) central ion in complex 1 and Ru(II) in 2 are similar, as
presented in Figs. 4 and 5. The second order perturbation theory
analysis allows an estimation of the flow of charges between the
central atom and the ligands. The stabilization energies associated
with the charge transfer from the central ruthenium atoms to the
HBO ligand are 23.88 and 39.62 kcal/mol for complexes 1 and 2,
respectively. The donations from the HBO ligand to the ruthe-
nium central ions are 131.89 and 161.98 kcal/mol in 1 and 2.
The data suggest that the donation from ligands to dRu orbitals
plays a role in the electronic structure of the complexes, which
Fig. 6. The density of states (DOS) diagrams for complexes 1 (
a
and b spin) and 2.
Fig. 7. The experimental and calculated UV–Vis spectra of complex 1.

J.G. Małecki / Polyhedron 31 (2012) 159–166
165
Table 4
The electronic transitions for complexes 1 and 2 calculated with B3LYP functional by the TDDFT method.
k (nm) (f)
Contributions
Exp. k (nm)
1
768.1 (0.02)
561.4 (0.01)
430.1 (0.01)
HOMOÀ2(b) ? LUMO(b) (78%); HOMO(b) ? LUMO(b) (16%)
HOMOÀ3(b) ? LUMO(b) (77%)
786.8
550.6
434.2
HOMO(a) ? LUMO+1(a) (25%); HOMO(a) ? LUMO+2(a) (15%); HOMO(b) ? LUMO+2(b) (16%)
428.5 (0.00(3)) HOMOÀ7(b) ? LUMO(b) (42%); HOMOÀ5(b) ? LUMO(b) (14%)
418.7 (0.01)
392.4 (0.01)
387.1 (0.02)
345.3 (0.02)
HOMO(a) ? LUMO(a) (13%), HOMOÀ18(b) ? LUMO(b) (28%), HOMO(b) ? LUMO+1(b) (10%)
HOMOÀ12(b) ? LUMO(b) (29%), HOMOÀ1(b) ? LUMO+1(b) (13%)
HOMOÀ14(b) ? LUMO(b) (51%)
HOMOÀ1(
a) ? LUMO+2(a) (17%), HOMOÀ22(b) ? LUMO(b) (12%), HOMOÀ1(b) ? LUMO+1(b) (14%), HOMOÀ1(b) ? LUMO+3(b)
(13%)
327.5 (0.03)
317.5 (0.02)
311.6 (0.26)
285.8 (0.02)
278.7 (0.11)
276.8 (0.15)
HOMOÀ2(
HOMOÀ1(
HOMOÀ3(
HOMOÀ4(
HOMOÀ5(
HOMOÀ4(
a
a
a
a
a
a
) ? LUMO(
a
) (44%), HOMOÀ2(B) ? LUMO+1(B) (43%)
) ? LUMO(
a
) (24%)
317.8
298.6
) ? LUMO+1(
) ? LUMO+1(
a
) (10%), HOMOÀ2(
a) ? LUMO+1(a) (12%), HOMOÀ2(b) ? LUMO+2(b) (36%)
a) (11%), HOMO(b) ? LUMO+7(b) (55%)
) ? LUMO(
a
) (34%), HOMOÀ4(b) ? LUMO+1(b) (35%)
) ? LUMO+1(
a) (15%), HOMOÀ3(b) ? LUMO+2(b) (13%), HOMO(b) ? LUMO+10(b) (10%)
2
446.0 (0.00(4)) HOMOÀ1 ? LUMO (14%), HOMOÀ1 ? LUMO+1 (13%), HOMO ? LUMO (33%), HOMO ? LUMO+1 (23%)
441.8
404.6
393.2 (0.13)
322.4 (0.03)
HOMO ? LUMO (35%), HOMO ? LUMO+1 (59%)
HOMOÀ1 ? LUMO (28%), HOMOÀ1 ? LUMO+1 (31%)
305.4 (0.00(2)) HOMOÀ2 ? LUMO (17%), HOMOÀ1 ? LUMO+3 (13%)
301.0 (0.01)
299.6 (0.03)
276.9 (0.03)
271.3 (0.10)
HOMOÀ2 ? LUMO (12%), HOMO ? LUMO+6 (60%)
HOMOÀ3 ? LUMO+1 (16%), HOMOÀ2 ? LUMO (21%), HOMOÀ1 ? LUMO+3 (10%)
HOMO ? LUMO+8 (45%)
HOMOÀ7 ? LUMO (18%), HOMOÀ6 ? LUMO (28%)
301.6
276.2
f-denote the oscillators strength.
can be seen in the charges of the ruthenium central ions. The nat-
ural atomic charges on the ruthenium central ion in the com-
plexes are À0.08 in 1 and À0.49 in 2. The close to zero charge
of the central ruthenium atom in complex 1 explains the absence
of a paramagnetic shift in the 1H NMR spectrum and the larger
charge gathered on the central ion in complex 2 indicates the pre-
ground state to the t42ge1 doublet state configuration and two
g
transitions from the ground state to the t42ge1 quartet states. In
g
the presents complex, three observed bands, typical of octahedral
ruthenium(III) complexes, originate from the ground term 2T2,
and these can be assigned to 2T2 ? 4T1 (12710 cmÀ1), 2T2 ? 4T2
(18162 cmÀ1) and 2T2 ? 2A1, 2T1 (23031 cmÀ1) transitions in an
increasing order of energy. Taking into account the d–d transitions,
the ligand field parameter 10Dq can be estimated as 27267 cmÀ1
for complex 1, Racah’s parameters are B = 546 cmÀ1, C = 2605
cmÀ1 and the nephelauxetic parameter has value b55 = 0.84.
The bands observed in the vicinity of 300 nm have been attrib-
dominant influence of the
r-donor carbonyl ligand.
Analysis of the frontier molecular orbitals is useful for under-
standing spectroscopic properties such as electronic absorption
and emission spectra of organometallic complexes. The densities
of states (DOS) in terms of Mulliken population analysis were cal-
culated using the GAUSSSUM program and Fig. 6 presents the compo-
sition of the fragment orbitals contributing to the molecular
orbitals for complexes 1 and 2. The HOMO of the [RuCl2(PPh3)2(H-
BO)] complex is localized on the ruthenium atom with a significant
contribution from the HBO ligand. Similarly the LUMO is located on
uted to intra- and interligand (pbC6H6 ? 3dphosphorus and
transitions with an admixture of Metal–Ligand Charge Transfer
transitions (dRu and dRu
p⁄Ph). The highest experi-
mental bands, close to 210.0 nm, may result from transitions in
the PPh3 ligands and from
p⁄ excitations in the N-heteroaro-
p
?
p⁄
)
C@C
?
p⁄
?
N–ligand
p
?
the ruthenium (73% b) with an admixture of the HBO ligand (90%
spin). The d orbitals of the ruthenium central ion contribute to
HOMOÀ1 to HOMOÀ3 (33–26%) and LUMO+1( )/+2(b) and LU-
MO+2( )/+3(b) in complex 1. In the ruthenium(II) complex 2, the
a
matic ligands. Fig. 7 presents the experimental and calculated
UV–Vis spectra of the [RuCl2(PPh3)2(HBO)] complex (1).
a
Complex (2) has a low-spin d6 configuration and in an octahe-
dral geometry d–d transitions are assigned to 1A1 ? 1T1 and
1A1 ? 1T2. In a lower symmetry field, three bands are expected, as-
signed to 1A1 ? 1A2, 1A1 ? 1B1 and 1A1 ? 1E transitions. The 10Dq,
a
HOMO orbital is localized on the HBO (69%) and PPh3 (19%) ligands
and the LUMO on the ruthenium (27%), HBO (22%) and PPh3 (45%).
The ruthenium dz orbital is visible in LUMO+3 with a significant
Racah’s and nephelauxetic parameters are 10Dq = 26229 cmÀ1
,
2
admixture of the triphenylphosphine ligand. The ruthenium orbi-
tals play a role in HOMOÀ1, HOMOÀ2, HOMOÀ4 and HOMOÀ5,
with a contribution of the chloride and HBO orbitals.
B = 528 cmÀ1, C = 2427 cmÀ1 and b55 = 0.73.
The theoretical absorption spectra of the complexes were ob-
tained from the calculation of the singlet excited states with the
TD-DFT at the B3LYP/DZVP/6-31++G(d) level in methanol solvent
phase. Computation of 90 excited states of the complexes allowed
the interpretation of the experimental spectra in the 260–240 nm
range. Selected excited states, assigned to the absorption bands,
are shown in Table 4. Generally, in the calculated energy range
for complex 1 the transitions between H-2(b), H-3(b) and the
HOMO(b) to LUMO(b) at 786 and 550 nm are visible. The band with
3.4. Experimental and theoretical electronic spectra
Octahedral low-spin d5 complexes such as [RuCl2(PPh3)2(HBO)]
2
have the T2g ground state, corresponding to the electronic state
t52g. As expected for this, the electronic spectrum is complex and
2
many d ? d transitions may be expected from the T2g ground
a maximum at 434 nm is attributed to the HOMO(
to LUMO( ) transitions. These transitions have d ? d character
with an admixture of HBO orbitals, as is clearly seen from the dia-
a
), HOMOÀ1(
a)
state. The high intensity band in the electronic spectra of the com-
plexes is assignable to the allowed charge-transfer transition from
the ligand to the/metal, from the p-level of donor atoms to the
a
p
(
p⁄) level. The strong field electrostatic matri-
grams in Fig. 6. There are similar characteristics for the bands with
maxima around 300 nm. In the case of the ruthenium(II) complex
incomplete metal t2g
ces of Tanabe and Sugano predict eight transitions from the t5
2g

166
J.G. Małecki / Polyhedron 31 (2012) 159–166
Appendix A. Supplementary data
CCDC 807943 and 806891 contain the supplementary crystallo-
graphic data for [RuCl2(HBO)(PPh3)2] and [RuCl(CO)(HBO)(PPh3)2]
complexes. 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, Cambridge CB2 1EZ,
UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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Acknowledgments
The GAUSSIAN09 calculations were carried out in the Wrocław
Centre for Networking and Supercomputing, WCSS, Wrocław,
Poland (http://www.wcss.wroc.pl) under Grant No. 18.