Syntheses, characterizations and theoretical calculations of rhodium(III) 1,2-naphthoquinone-1-oxime complexes

Article abstract of DOI:10.1016/j.ica.2009.12.033

Rhodium(III) complexes of 1,2-naphthoquinone-1-oxime (1-nqo) [Rh(1-nqo)L₂Cl₂] 1-3 [1, L = 4-methylpyridine (mpy); 2, L = 4-phenylpyridine (ppy); 3, L = 4-acetylpyridine (apy)] were prepared. The structure of complex 1 is analyzed by

Full text of DOI:10.1016/j.ica.2009.12.033

Inorganica Chimica Acta 363 (2010) 949–956
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Syntheses, characterizations and theoretical calculations of rhodium(III)
1,2-naphthoquinone-1-oxime complexes
Yi-Nan Liu a, Wan-Zhen Liang b, Xiao-Guang Sang a, Yu-Qiu Huo a, Lap Sze-to c,
a,
Ka-Fu Yung c, , Xiao-Xia Liu
*
*
a Department of Chemistry, Northeastern University, Shenyang 110004, China
b Hefei National Laboratory for Physical Science at Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
c Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2009
Received in revised form 14 October 2009
Accepted 17 December 2009
Available online 24 December 2009
Rhodium(III) complexes of 1,2-naphthoquinone-1-oxime (1-nqo) [Rh(1-nqo)L2Cl2] 1–3 [1, L = 4-methyl-
pyridine (mpy); 2, L = 4-phenylpyridine (ppy); 3, L = 4-acetylpyridine (apy)] were prepared. The structure
of complex 1 is analyzed by single crystal X-ray crystallography. All of the complexes were characterized
by mass spectrometry, 1H–1H COSY NMR and FT-IR. UV–Vis absorption spectroscopy and cyclic voltam-
metry were employed to investigate the electronic transition behaviors of the complexes. The complexes
displayed irreversible metal-localized two-electron reductions from RhIII to RhI on the cyclic voltammo-
gram. While the low-energy absorptions at kmax of 488–490 nm on the UV–Vis spectra of the complexes
Keywords:
Rhodium complex
Crystal structure
DFT calculation
Electronic spectroscopy
Electrochemistry
*
were related to metal to 1-nqo ligand charge transfer [MLCT, dp(Rh) ? p (1-nqo)] and chloride to 1-nqo
ligand charge transfer [LLCT, p
p(Cl) ?
p
*(1-nqo)] based on the theoretical calculations using time-depen-
dent density functional theory (TD-DFT).
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
1H–1H COSY NMR and FT-IR. Theoretical calculations are conducted
on the complexes by DFT at B3LYP level, using the basis sets of
LanL2DZ adding ECP basis set for Rh and 6-31G for other atoms.
The singlet–singlet electronic transitions and the energy level of
the triplet orbital 3p* of the complexes are calculated by time-
dependent density functional theory (TD-DFT) methods [19] using
B3LYP functional with the same basis set as ground state. The elec-
tronic transition and electrochemistry behaviors of the complexes
are discussed based on TD-DFT calculations.
1,2-Naphthoquinone-1-oxime which is abbreviated as 1-nqoH
is the first organic substance that was found to form precipitates
with metal ions [1]. 1-nqo and its isomer, 1,2-naphthoquinone-
2-oxime (2-nqo), are good chelating agents for transition metals
including ruthenium and rhodium [2–13]. The formed complexes
displayed interesting electron transfer properties [4,6,7]. Theoreti-
cal calculations for rhodium complexes can be performed by den-
sity functional theory (DFT) [14], using mPW1PW91 method with
SDD basis set and relativistic effective core potential (ECP) for rho-
dium [15], as well as B3LYP method with LanL2DZ adding ECP basis
set for rhodium [16–18].
2. Experimental
As an extension of our studies on 1,2-naphthoquinone-mono-
oxime (nqo) complexes of transition metals, herein we present the
syntheses of three dichloride-rhodium(III) 1-nqo complexes con-
taining different pyridine type co-ligands, [Rh(1-nqo)(mpy)2Cl2] 1
(mpy = 4-methylpyridine), [Rh(1-nqo)(ppy)2Cl2] 2 (ppy = 4-phenyl-
pyridine) and [Rh(1-nqo)(apy)2Cl2] 3 (apy = 4-acetylpyridine). The
structure of complex 1 is analyzed by single crystal X-ray crystallog-
raphy. All of the complexes are characterized by mass spectrometry,
2.1. Materials and equipments
All chemicals were purchased from commercial sources and used
as received, including hydrated rhodium(III) chloride (RhCl3ꢀ3H2O,
Johnson Matthey, Materials Technology, UK), 1,2-naphthoqui-
none-1-oxime (named also as 1-nitroso-2-naphthol, Lancaster syn-
thesis, England), 4-methylpyridine (Sinopharm Chemical Reagent
Co., Ltd., China), 4-phenylpyridine (Aldrich Chemical Co., Inc.,
USA), 4-acetylpyridine (Alkali Metals Ltd., India), tetrabutylammo-
nium hexafluorophosphate (TBAP, Sigma Chemical Company,
USA). Preparative thin layer chromatographic (TLC) plates were pre-
pared from silica GF254 (Qingdao Haiyang Chemical Co., Ltd.,
China).
* Corresponding authors. Tel.: +86 24 83689510; fax: +86 24 23600159 (X.X. Liu),
tel.: +86 852 28598925; fax: +86 852 25472933 (K.-F. Yung).
E-mail addresses: kfyung@hkucc.hku.hk (K.-F. Yung), xxliu@mail.neu.edu.cn
(X.-X. Liu).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.12.033

950
Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
Table 1
Single crystal X-ray diffraction measurement was conducted on
Crystallographic data and experimental details for [Rh(1-nqo)(mpy)2Cl2] 1.
a Bruker SMART 1000 CCD diffractometer (Bruker, Germany). Fast
atom bombardment mass spectra (FAB MS) were measured on a
MAT 95 mass spectrometer (Finnigan, USA). 1H–1H COSY NMR
spectra were obtained from a DPX-400 NMR spectrometer (Bruker
BioSpin, Swiss). Infrared spectra were recorded on a Spectrum One
FT-IR Spectrometer (Perkin–Elmer, USA) with samples prepared as
KBr pellets. Electronic absorption spectra were recorded on a UV-
2100 spectrophotometer (Beijing RayLeigh Analytical Instrument
Corp, China). Cyclic voltammetry was conducted using a multi-
channel electrochemical analyzer VMP3 (Bio-Logic-Science Instru-
ments, France). All electrochemical data was obtained with a glass
cell, equipped with a glassy carbon working electrode, a platinum
wire reference electrode, and a platinum wire as the auxiliary elec-
trode filled with CH2Cl2 containing 0.1 mol dmꢁ3 tetrabutylammo-
nium hexafluorophosphate (TBAP) as a supporting electrolyte. The
ferrocene/ferrocenium couple was used as an internal standard. All
of the reported potentials were relative potentials (calibrated to
the ferrocene internal standard [Fc0/+1] as zero potential).
Compound
[Rh(1-nqo)(mpy)2Cl2] 1
Empirical formula
Formula weight
Crystal color, habit
Crystal dimensions (mm)
Crystal system
Lattice type
Space group
a (Å)
b (Å)
RhO2N3C22H20Cl2
532.23
red, block
0.10 ꢂ 0.20 ꢂ 0.37
monoclinic
Primitive
P21/n (no. 14)
14.691(2)
9.252(1)
17.469(3)
90
c (Å)
a
(°)
b (°)
111.632(2)
90
2207.1(6)
4
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
1.602
F(0 0 0)
1072
Total number of reflections measured
Number of symmetry-independent reflections
15136
4917
Number of reflections > 2
r
threshold
3784
0.035 and 0.037
1.21 and ꢁ0.36
Ra and Rw [I > 2
r
(I)]
b
2.2. Preparation of rhodium(III) complexes
maximum and minimum peaks (eꢁ Åꢁ3
)
a
R = ||Fo| ꢁ |Fc||/|Fo|.
The reaction mixture of RhCl3ꢀ3H2O (0.053 g, 0.2 mmol) and
1-nqoH (0.069 g, 0.4 mmol) (RhClꢀ3H2O:1-nqoH = 1:2, mol/mol)
in 50 ml ethanol was first refluxed for two hours. The correspond-
ing pyridine type ligand (mpy, ppy or apy) of equal molar amount
with the 1-nqoH was added to the hot mixture and was refluxed
for another 2.5 h. The solvent was then removed under vacuum
and the residue was chromatographed by thin layer chromatogra-
phy (TLC). Orange products were isolated from each reaction mix-
ture respectively as [Rh(1-nqo)(mpy)2Cl2] 1, [Rh(1-nqo)(ppy)2Cl2]
2 and [Rh(1-nqo)(apy)2Cl2] 3 by using solvent mixture of n-hexane/
CH2Cl2 (v/v of 1:7, 1:5 and 1:5, respectively) as eluant.
Rw = [w (|Fo| ꢁ |Fc|)2/wFo2]1/2
.
b
constrains and the convergence accuracy were calculated using
the default procedures. After frequency calculations are performed
on all of the structure optimized complexes, the optimized station-
ary points of complexes 1–3 are confirmed to be local minima as
no imaginary vibrational frequencies appear. The contour plots of
molecular orbitals (MOs) were plotted using the GAUSSION 03 view
program.
Based on the optimized geometries, the vertical excitation ener-
gies of both the singlet and triplet excited states of the complexes
were calculated at TD-B3LYP theoretical level.
2.3. X-ray crystallography
Single crystals of 1 were obtained by slow diffusion of n-hexane
into dichloromethane solution of the complex. A red single crystal
with approximate dimensions of 0.10 ꢂ 0.20 ꢂ 0.37 mm3 was se-
lected and sealed in a glass capillary. All measurements and data
was made and recorded on a Bruker SMART 1000 CCD diffractom-
3. Results and discussion
3.1. Crystal structure of complex 1
In order to establish the molecular structure of the complex,
single crystal X-ray diffraction analysis was carried out for single
crystal of 1. The corresponding molecular structure is shown in
Fig. 1. Selected bond distances and angles are summarized in Table
2. The central Rh(III) adopts a pseudo-octahedral geometry. The
two nitrogen atoms, N(2) and N(3) of the mpy ligands locate in
the equatorial plane with a cis-configuration. The other two posi-
tions of the same equatorial plane are bridged by the oximato N
and quinonal O [N(1) and O(1)] of the 1-nqo ligand. The bidentate
1-nqo is almost co-planar with the metal equatorial plane with the
dihedral angle of 5.48(7)°. The dihedral angle of the metal equato-
rial plane with the two methyl pyridine ligands is 126.8(1)° and
44.5(1)°, respectively. The remaining two vacant sites are occupied
by two chlorides. The two chloride ions are at the apical trans-posi-
tions which are nearly perpendicular to the equatorial plane with
the bond angle of [Cl(2)-Rh(1)-Cl(1) 176.83(4)°]. The bidentate 1-
nqo ligand coordinates to the Rh(III) center through its oximato
N and quinonal O atoms to form a stable five-membered metal-
chelate ring which helps this geometry to gain an extra stability.
This coordination mode is commonly observed in other Rh(III)
complexes containing 1-nqo and its isomer 2-nqo as ligands [6,24].
eter equipped with a graphite monochromated Mo K
a radiation
(k = 0.071073 nm). The data were collected at room temperature
(301 1 K). Crystallographic data and experimental details for
complex 1 were summarized in Table 1. A total of 15136 reflections
were collected, of which 4917 were independent (Rint = 0.028) and
3784 observed reflections with I > 2r(I) were used in the structure
analysis. The structure was solved by direct methods and ex-
panded using Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms could be found from the
difference Fourier map but were placed at geometrical sites with
C–H = 0.95 Å.
2.4. Theoretical calculations
Full geometry optimizations were performed at an Intel Pen-
tium IV 3.0 G computer employing the GAUSSIAN 03 software package
suite of programs, using experimental geometry as input for com-
plex 1 [20]. The complexes 1–3 were treated as an open-shell sys-
tem using the spin unrestricted DFT wavefunction, i.e. the Becke
three-parameters exchange functional in combination with the
LYP correlation functional of Lee, Yang and Parr, B3LYP [21,22],
with 6-31G basis set for C, H, O, N, and Cl atoms, and relativistic
effective core potential basis set of double zeta quality Hay and
Wadt Los Alamos ECP [23] basis set LanL2DZ for Rh atom. All
geometry optimizations were performed without any symmetry
3.2. Synthesis and characterization of rhodium(III) complexes
Three orange complexes, 1–3, were prepared by reflux the cor-
responding pyridine type ligand, mpy, ppy or apy with the reaction

Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
951
Fig. 1. Molecular structure of [Rh(1-nqo)(mpy)2Cl2] 1.
Table 2
mixture of RhCl3ꢀ3H2O and 1-nqoH in ethanol (Scheme 1). The
complexes are characterized by mass spectroscopy, 1H–1H COSY
NMR and FT-IR after TLC purification (Table 3).
Selected bond lengths (Å) and angles (°) for [Rh(1-nqo)(mpy)2Cl2] 1 together with the
calculated values by DFT.
Experimental value
Calculated value
Intense peak for [M–Cl]+ at m/z 496 is observed on the FAB MS
spectrum of complex 1 as the largest daughter ion with no molec-
ular ion peak present. Similar FAB MS spectra are also observed for
complexes 2 and 3. The observed common [M–Cl]+ pattern indi-
cates that there are also one 1-nqo ligand, two pyridine type li-
gands and two chlorides coordinated to the Rh center for 2 and 3
as suggested by the molecular structure of 1 that obtained by X-
ray crystallography. On the basis of 1H–1H COSY NMR measure-
ments, the 1H NMR signals assignments are conducted for all of
the complexes (Table 3). Typical 1H–1H COSY NMR spectrum of
complex 1 is shown in Fig. 2. On all of the spectra, the integral ratio
of 1H signals for 1-nqo and pyridine type ligands is 1:2, which is
consistent with FAB MS results. Complexes 1–3 give very similar
patterns in the 1H NMR spectra for the coordinated 1-nqo and
pyridine type ligands, showing that the chemical environments
Rh(1)–Cl(1)
Rh(1)–Cl(2)
Rh(1)–N(1)
Rh(1)–N(2)
Rh(1)–N(3)
Rh(1)–O(1)
C(1)–O(1)
2.329(1)
2.342(1)
1.979(3)
2.053(3)
2.085(3)
2.025(2)
1.279(5)
1.242(4)
2.4541
2.4561
2.0295
2.0825
2.1108
2.0539
1.3090
1.2733
N(1)–O(2)
Cl(2)–Rh(1)–Cl(1)
O(1)–Rh(1)–Cl(1)
N(1)–Rh(1)–Cl(1)
N(2)–Rh(1)–Cl(1)
N(3)–Rh(1)–Cl(1)
O(1)–Rh(1)–Cl(2)
N(1)–Rh(1)–Cl(2)
N(2)–Rh(1)–Cl(2)
N(3)–Rh(1)–Cl(2)
176.83(4)
88.57(9)
89.8(1)
91.7(1)
90.3(1)
88.55(9)
88.5(1)
91.1(1)
91.1(1)
178.17
90.27
89.11
90.84
90.06
88.32
89.51
90.50
91.14
8
7
RhCl3·3H2O
CH3
mpy
N
7
1, L=
9
+
8'
NO
2
O
1
3
Cl(2)
O(2)
8
L (N2, CL1, O3)
L (N3, CL2, O4)
(1)
N
2, L=
N
Ph
ppy
apy
Rh
5
O
8'
8
7'
(1)
Cl(1)
6
+
7
O
3, L =
N
C
CH3
9
L
7'
8'
Scheme 1. Reaction of RhClꢀ3H2O with 1-nqoH followed by treatment with the pyridine type ligand.

952
Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
Table 3
FAB MS, IR and 1H NMR data of complexes 1–3.
Complex
FAB MS* (m/z)
IR spectra (cmꢁ1
)
1H NMR spectra (1D) (d (ppm), J (Hz))
[Rh(1-nqo)(mpy)2Cl2] 1
496 (532)
dasC–H = 1432
dsC–H = 1361
9.13 (d, 1H, J = 8.3, H1), 8.66 (br.d, 2H, J = 6.5,
0
H7), 8.57 (br.d, 2H, J = 6.4, H7 ), 7.76 (d, 1H,
m
N–O = 1040
J = 9.6, H5), 7.66 (m, 2H, H2,4), 7.48 (m, 1H, H3),
0
7.30 (d, 2H, H8), 7.22 (d, 2H, H8 ), 7.19 (s, 1H,
0
H6), 2.48 (s, 3H, H9 or H9
)
[Rh(1-nqo)(ppy)2Cl2] 2
[Rh(1-nqo)(apy)2Cl2] 3
620 (656)
552 (588)
m
N–O = 1072
9.21 (d, 1H, J = 8.2, H1), 8.98 (br.d, 2H, J = 6.7,
0
dC–H = 765, 730, 693
H7), 8.90 (br.d, 2H, J = 6.8, H7 ), 7.72 (d, 1H,
J = 9.4, H5), 7.68 (m, 2H, H8), 7.72–7.66 (m, 4H,
0
phenyl), 7.65–7.61 (m, 4H, H2,4,8 ), 7.57–7.52
(m, 6H, phenyl), 7.46 (m, 1H, H3), 7.30 (d, 1H,
J = 9.4, H6)
m
m
C@O = 1697
N–O = 1059
9.13 (m, 1H, H1), 9.13 (br.d, 2H, J = 6.7, H7), 9.06
0
(br.d, 2H, J = 6.6, H7 ), 7.91 (dd, 2H, J = 5.2, 1.5,
0
H8), 7.84 (dd, 2H, J = 5.4, 1.4, H8 ), 7.74 (d, 1H,
J = 9.4, H5), 7.67 (m, 1H, H2), 7.64 (m, 1H, H4),
7.48 (m, 1H, H3), 7.25 (m, 1H, H6), 2.71(s, 3H,
0
0
H9 or H9 ), 2.70 (s, 3H, H9 or H9
)
*
Only [MꢁCl]+ is observed.
Fig. 2. 1H–1H COSY NMR spectrum of [Rh(1-nqo)(mpy)2Cl2] 1.
of these ligands are very similar. It shows that the ligand spatial
arrangement of 2 and 3 should be very similar to that of complex 1.
Infrared spectra of 1–3 show strong bands at 1608–1631, 1513–
1515 and 1435–1437 cmꢁ1 respectively which corresponds to the
vibrations of aromatic skeleton [25]. The sharp peaks at 1592–
1593 and 1553–1554 cmꢁ1 are attributed to the pyridine type li-
gands. The in-plane bending vibrations of methyl group in complex
1 are observed at 1432 cmꢁ1 for dasC–H and 1361 cmꢁ1 for dsC–H. For
complex 2, the characteristic vibrations for mono-substituted phe-
nyl ring are found at 765, 730 and 693 cmꢁ1. While the vibration of
carbonyl group on the pyridine type ligand of complex 3 appears at
3.3. Electron transfer characterization
The UV–Vis absorption spectra of the Rh(III) complexes and
1-nqoH in CH2Cl2 are depicted in Fig. 3, while the spectral data
are summarized in Table 4. In UV–Vis absorption spectra, there is
a strong absorption in the UV region (270–290 nm) and three
absorptions (402–410 nm, 488–490 nm and shoulder peaks of
524–528 nm) in the visible region for complexes 1–3. All of absorp-
tion peaks for the complexes have been assigned based on TD-DFT
theoretical calculations (see Section 3.5).
Electrochemical properties of 1–3 are studied by cyclic
1697 cmꢁ1
.
voltammetry in dichloromethane containing 0.1 mol dmꢁ3 TBAP.

Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
953
2.0
1.5
1.0
0.5
0.0
1-nqoH
1
2
3
×10
300
400
500
600
wavelength/nm
Fig. 3. UV–Vis absorption spectra of 1-nqoH (solid line), complexes 1 (dash line), 2 (dot line) and 3 (dash dot line) in CH2Cl2.
Table 4
Calculated excited states, singlet excitation energies (E (ev)), absorption bands, oscillator strengths (f), transition configuration and coefficient by TD-DFT and experimental UV–
Vis absorption bands of complexes 1–3 together with those of 1-nqoH.
Compound
1-nqoH
Excited state
E (ev)
kcalc (nm)
f
Transition configuration
*(CI coefficient)
kexp (nm) (e )
(104 L molꢁ1 cmꢁ1
374 (0.73)
274 (1.81)
[Rh(1-nqo)(mpy)2Cl2] 1
528 sh (0.18)
490 (0.31)
1
2.57
3.1
483
401
285
510
0.003
HOMO-1 ? LUMO
HOMO ? LUMO
HOMO-1 ? LUMO
HOMO ? LUMO
HOMO-4 ? LUMO + 1
HOMO-4 ? LUMO + 2
HOMO-1 ? LUMO + 1
HOMO ? LUMO + 1
HOMO-4 ? LUMO
HOMO-1 ? LUMO
HOMO ? LUMO
ꢁ0.40 (32 %)
0.51 (53 %)
0.39 (31 %)
0.34 (23 %)
ꢁ0.36 (25 %)
0.53 (55 %)
ꢁ0.32 (20 %)
0.41 (34 %)
0.46 (42 %)
ꢁ0.47 (44 %)
0.48 (46 %)
0.44 (38 %)
0.36 (26 %)
0.41 (34 %)
0.41 (34 %)
0.45 (41 %)
0.53 (57 %)
0.43 (37 %)
0.48 (47 %)
0.55 (60 %)
0.35 (25 %)
0.45 (41 %)
0.47 (45 %)
5
0.0349
0.0079
0.0001
402 (1.06)
290 (1.77)
524 sh (0.17)
22
1
4.35
2.43
[Rh(1-nqo)(ppy)2Cl2] 2
2
3
2.45
2.58
505
480
0.0003
0.0039
488 (0.21)
410 (0.63)
6
2.96
419
0.0313
HOMO-1 ? LUMO
HOMO ? LUMO
50
1
4.62
2.44
268
508
0.3197
0.0001
HOMO-8 ? LUMO + 2
HOMO-1 ? LUMO + 3
HOMO ? LUMO + 3
HOMO-4 ? LUMO
HOMO-1 ? LUMO
HOMO ? LUMO
HOMO-2 ? LUMO + 2
HOMO-1 ? LUMO + 2
HOMO-8 ? LUMO + 2
HOMO-7 ? LUMO + 2
270 (2.94)
524 sh (0.22)
[Rh(1-nqo)(apy)2Cl2] 3
2
3
2.48
2.61
499
476
0.0004
0.0038
488 (0.32)
410(0.98)
290(1.95)
12
41
3.11
4.16
398
298
0.0135
0.0662
*
The absolute value of the transition coefficient for transition is above 0.3 that has been listed in front of parentheses.
Representative cyclic voltammogram of complex 3 is shown in
Fig. 4. The electrochemical behaviors of the complexes resemble
that of other dichloride–rhodium(III) complexes containing the iso-
mer of 1-nqo, 2-nqo ligands, such as [Rh(2-nqo)L2Cl2] and [Rh(2-
nqo)2LCl] (L = phenylphosphonate or pyridine type ligands) [6].
The cyclic voltammograms of complexes 1–3 show an irreversible,
metal-localized two-electron redox pair. The reductive and oxidative
peak potentials, Ered and Eox, are presented in Table 5. The reductive
peak potentials Ered are positive-shifted from complex 1 to 2 and
3.4. Ground state geometry optimization
Open-shell system spin unrestricted DFT calculations are car-
ried out for complex 1 using experimental geometry as input by
wave function B3LYP, with relativistic effective core potential
basis set LanL2DZ for Rh atom and 6-31G basis set for C, H, O,
N and Cl atoms. Selected calculated bond lengths and angles
are listed in Table 2 together with the available experimental
data for comparison. Structural optimizations are conducted for
complexes 2 and 3, similarly based on the geometry structure
of complex 1.
further 3, due to the decrease in
p electron-donating ability of the
pyridine type ligands in the complexes from mpy to ppy and apy.

954
Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
5 µA
1.0
0.5
0.0
E / V vs. Fc0/+1
Fig. 4. Cyclic voltammogram of [Rh(1-nqo)(apy)2Cl2] 3 in CH2Cl2 containing 0.1 mol dmꢁ3 TBAP; glassy carbon working electrode, scan rate, 100 mV sꢁ1
-0.5
-1.0
-1.5
.
transitions have a very low absorption coefficient. Singlet–singlet
electronic transitions of complexes 1–3 are calculated by TD-DFT
method based on the optimized molecular structures. Fig. 5 depicts
the plots of the corresponding highest occupied molecular orbital
HOMOs, together with the other two frontier orbitals HOMOs-1
and HOMOs-2 for the three complexes. The lowest-unoccupied
molecular orbital LUMOs is also listed in Fig. 5. The compositions
of the orbitals are in Table 6. The calculated excited states, singlet
excitation energies (E/ev), absorption bands, oscillator strengths
(f), transition configuration and coefficient are in Table 4 together
with the experimental UV–Vis absorption bands of complexes 1–3.
Based on TD-DFT calculations, the intense high-energy absorp-
tions at kmax of 270–290 nm on the UV–Vis spectra can be attributed
Table 5
Electrochemical data for complexes 1–3*.
Ered (V) vs. Fc0/+1
Eox (V) vs. Fc0/+1
[Rh(1-nqo)(mpy)2Cl2] 1
[Rh(1-nqo)(ppy)2Cl2] 2
[Rh(1-nqo)(apy)2Cl2] 3
ꢁ1.458
ꢁ1.290
ꢁ1.215
0.566
0.610
0.704
*
All peak potentials are obtained from cyclic voltammetry at 100 mV sꢁ1 in CH2Cl2/
0.1 mol dmꢁ3 TBAP.
3.5. Excited state calculation
UV–Vis spectra reflect the electronic transition behavior of mol-
ecules, however usually only the singlet–singlet electronic transi-
tions are easy to be observed as the banned singlet–triplet
*
p
*
p (ppy) for
to LLCT [p(1-nqo) ?
(mpy) for complex 1, p
p(Cl) ?
Fig. 5. Contour diagrams of the calculated HOMOs-2, HOMOs-1, HOMOs and LUMOs of complexes 1 (a), 2 (b) and 3 (c).

Y.-N. Liu et al. / Inorganica Chimica Acta 363 (2010) 949–956
955
Table 6
Compositions of the frontier molecular orbitals HOMOs-2, HOMOs-1, HOMOs, and LUMOs for complexes 1–3.
Complex
Composition (%)
Molecular obital
Rh
Cl(1)
Cl(2)
O(1)
O(2)
N(1)
N(2)
N(3)
C1-nqo
CL1
CL2
O(3)
O(4)
[Rh(1-nqo)(mpy)2Cl2] 1
LUMOs
5.30
1.22
7.83
30.14
30.87
1.04
7.64
27.32
27.59
8.30
12.84
2.1
20.55
9.98
3.85
1.02
25.62
2.64
1.17
0.15
0.12
0.08
0.01
0.17
0.20
0.06
0.2
35.21
34.68
6.63
0.56
0.92
0.63
0.97
1.06
HOMOs
22.52
20.42
28.79
0.46
0.58
0.57
HOMOs-1
HOMOs-2
1.44
0.09
7.97
[Rh(1-nqo)(ppy)2Cl2] 2
LUMOs
4.85
15.24
21.29
28.06
1.10
3.51
34.32
30.51
0.89
3.01
31.57
32.10
8.83
16.22
1.02
20.55
13.92
2.85
25.31
3.58
0.72
0.15
0.08
0.07
0.11
0.15
0.16
0.05
0.21
0.22
36.22
42.93
5.66
0.50
0.86
1.03
1.04
1.08
0.40
0.73
1.00
HOMOs
HOMOs-1
HOMOs-2
0.04
0.67
5.71
[Rh(1-nqo)(apy)2Cl2] 3
LUMOs
3.14
12.40
22.33
27.14
1.16
2.39
31.50
27.21
1.00
2.19
32.77
33.46
6.48
16.51
0.38
15.80
14.92
2.54
19.20
3.96
0.60
0.18
0.95
0.07
0.19
0.13
3.75
0.04
0.21
0.30
26.96
46.24
7.11
3.51
0.69
1.19
0.82
12.39
0.33
0.68
1.01
0.91
0.02
0.01
0.02
3.45
0.01
0.01
0.02
HOMOs
HOMOs-1
HOMOs-2
0.17
0.54
8.65
3
Table 7
Energies of LUMOt and electrochemical properties for complexes 1–3.
*
p of the complexes have been
energy level of the triplet orbital
calculated by time-dependent density functional theory (TD-DFT)
method based on this. The low-energy absorptions at kmax of
488–490 nm on the UV–Vis spectra of the complexes can be mainly
assigned to metal to 1-nqo ligand charge transfer [MLCT,
[Rh(1-
[Rh(1-
[Rh(1-
nqo)(mpy)2Cl2] 1
nqo)(ppy)2Cl2] 2
nqo)(apy)2Cl2] 3
EtLUMO (a.u.)
Ered (V)
ꢁ0.04559
ꢁ1.458
ꢁ0.06975
ꢁ1.290
ꢁ0.1022
ꢁ1.215
dp
(Rh) ?
p
*(1-nqo)] together with LLCT from chloride atoms to
*
1-nqo ligand [p
p
(Cl) ? p (1-nqo)]. Based on triplet TD-DFT calcu-
lations, the energy of the lowest-unoccupied molecular orbital
LUMOt decreases along with the decrease of electron-donating
ability of pyridine type ligand, resulting in higher reductive poten-
tial (Ered) of the complexes.
complex 2, and p
402–410 nm are assigned to LLCT [p
[d (Rh) ?
*(1-nqo)] for complexes 1–2, but LLCT [p
p(Cl) ? p
*(apy) for complex 3]. The bands at kmax of
*
p(Cl) ?
p
(1-nqo)] and MLCT
p
p
p(Cl) ?
p
*(apy)]
for complex 3. The low-energy absorptions at kmax of 488-490 nm are
related to metal and chloride to 1-nqo ligand charge transfer exci-
Acknowledgements
*
*
(1-nqo)],
tation [MLCT,
while the shoulder peaks of 524–528 nm can be attributed to
1-nqo to metal and chloride ligand charge transfer [LMCT, (1-
nqo) ? d (1-nqo) ? p
*(Rh); LLCT, *(Cl)].
d
p
(Rh) ?
p
(1-nqo); LLCT,
p
p
(Cl) ?
p
We gratefully acknowledge financial support from National
Natural Science Foundation of China (50973013) and that from
Education Ministry of China. Xiao-Xia Liu and Yi-Nan Liu acknowl-
edge kind help from Prof. Zhongmin Su in Institute of Functional
Material Chemistry, Northeast Normal University, China. L. Sze-to
and K.-F. Yung acknowledge financial support from The University
of Hong Kong.
p
p
p
p
3
*
p is lower
As the energy of the triplet anti-bonding orbital
than that of its singlet counterpart 1p*, electrochemical reduction
3
*
usually occurs as the acceptance of electrons by
p . Triplet TD-
DFT calculations for complexes 1–3 are conducted based on the
optimized structure. The energies of the corresponding lowest-
unoccupied molecular orbitals (LUMOt), ELtUMO are summarized in
Table 7 together with the reductive potentials of the complexes,
Ered. From Table 7, the p-substitutions of the pyridine type ligands
have effect on the energy of the triplet orbital. The energy de-
creases along with the decrease of the electron-donating ability
of the pyridine type ligand, resulting in higher tendency to accept
electrons. This is in agreement with the experimental result of
electrochemistry study (Ered).
Appendix A. Supplementary material
CCDC 735086 contains the supplementary crystallographic data
for complex [Rh(1-nqo)(mpy)2Cl2] 1. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2009.12.033.
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