Photophysical and electrochemical studies of heteroleptic cyclometallated complexes of rhodium(III) containing 2-benzoylpyridine

Article abstract of DOI:10.1016/j.poly.2006.01.017

Two heteroleptic cyclometallated complexes, cis-[Rh(bzpy)(bipy)Cl₂] (1) and cis-[Rh(bzpy)(phen)Cl₂] (2) (where bzpy = 2-(2-pyridylcarbonyl)phenyl, bipy = 2,2′-bipyridine and phen = 1,10-phenanthroline) have been synthesized and characterized. Based on IR and NMR data, it is evident that the coordinated bzpy ligand is N,C coordinated with Rh(III). Each complex shows high intensity bands in the UV region, assigned as spin-allowed π-π^* transitions. The medium-intensity absorption band profile in the lower energy region can be explained by convolution of carbonyl n-π^* and d-d^* transitions. At low temperature (77 K), each complex shows a broad, symmetric and structureless red emission with a microsecond lifetime and, hence, is assigned as dd^* phosphorescence. Voltammetric data have also been obtained for both complexes. There were two reduction peaks observed for each complex. The first reduction peak involves the transfer of two electrons and the elimination of two chlorides. The redox orbital involved in the second reduction is assigned to be the π^* orbital localized on the carbonyl fragment of bzpy by examination of the influence on the reduction potentials in the presence of phenol.

Full text of DOI:10.1016/j.poly.2006.01.017

Polyhedron 25 (2006) 2160–2166
www.elsevier.com/locate/poly
Photophysical and electrochemical studies of heteroleptic
cyclometallated complexes of rhodium(III) containing
2-benzoylpyridine
*
*
Mei Ching Tseng, Jin Lian Ke, Chang Chi Pai, Shao Pin Wang , Wen Liang Huang
Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan, ROC
Received 25 November 2005; accepted 5 January 2006
Available online 28 February 2006
Abstract
Two heteroleptic cyclometallated complexes, cis-[Rh(bzpy)(bipy)Cl2] (1) and cis-[Rh(bzpy)(phen)Cl2] (2) (where bzpy = 2-(2-pyridyl-
carbonyl)phenyl, bipy = 2,20-bipyridine and phen = 1,10-phenanthroline) have been synthesized and characterized. Based on IR and
NMR data, it is evident that the coordinated bzpy ligand is N,C coordinated with Rh(III). Each complex shows high intensity bands
in the UV region, assigned as spin-allowed p–p* transitions. The medium-intensity absorption band profile in the lower energy region
can be explained by convolution of carbonyl n–p* and d–d* transitions. At low temperature (77 K), each complex shows a broad, sym-
metric and structureless red emission with a microsecond lifetime and, hence, is assigned as dd* phosphorescence. Voltammetric data
have also been obtained for both complexes. There were two reduction peaks observed for each complex. The first reduction peak
involves the transfer of two electrons and the elimination of two chlorides. The redox orbital involved in the second reduction is assigned
to be the p* orbital localized on the carbonyl fragment of bzpy by examination of the influence on the reduction potentials in the presence
of phenol.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Absorption; Phosphorescence; Lifetime; Cyclometallated complexes; CV
1. Introduction
been constantly investigated for years [4–12]. It had been
reported that this versatile ligand could coordinate in a
variety of modes. It can act as a monodentate ligand (with
coordination by either the nitrogen [4–6] or the oxygen [5]
atom), as a neutral bidentate N,O ligand [6,8], and as a
bridging N,O ligand [5] as well as an anionic bidentate
N,C ligand [9–11].
By simply mixing RhCl3 Æ 3H2O with Hbzpy in 2-meth-
oxyethanol for 4 days at room temperature, de Geest and
Steel synthesized Rh(Hbzpy)(bzpy)Cl2 (bzpy = 2-(2-pyridyl-
carbonyl)phenyl) [12]. They reported that the complex had
a five-membered N,O metallacycle involving the chelating
Hbzpy ligand and a six-membered N,C metallacycle
involving the chelating bzpy ligand, on the basis of NMR
chemical-shift analysis. However, this complex was found
to be non-luminescent in our laboratory.
The great interest in photophysical properties of cyclo-
metallated metal complexes is becoming more intensive
[1–3]. Ortho-metallated ligands, such as anionic ppy and
thpy ligands (ppyH = 2-phenylpyridine, thpyH = 2-(2-thie-
nyl)pyridine), exhibit higher ligand field strengths com-
pared with that of 2,20-bipyridine (bipy), due mainly to
the strong-donor capability of carbon [3]. This moves both
the d(t2g) and d*(eg) orbitals to higher energy. Since the lat-
3
ter are destabilized much more than the former, the d–d*
excited state is expected to move to higher energy [3a]. The
coordination chemistry of 2-benzoylpyridine (Hbzpy) has
*
Corresponding authors. Tel.: +886 6 2757575; fax: +886 6 2740552.
Hetero-bischelated Rh(III) complexes coordinated with
anionic bzpy and a polypyridine ligand have not been
E-mail addresses: spwang@mail.ncku.edu.tw (S.P. Wang), wlhuang@
mail.ncku.edu.tw (W.L. Huang).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.01.017

M.C. Tseng et al. / Polyhedron 25 (2006) 2160–2166
2161
1
reported to our best knowledge. In this article, we have
obtained two heteroleptic cyclometallated Rh(III) com-
plexes. Both the photophysical and electrochemical proper-
ties of the two complexes have been thoroughly examined
to probe the bonding information of their coordination
chemistry.
characterized by its H NMR spectrum which was consis-
tent with the literature data [12].
2.3.2. cis-[Rh(bzpy)(bipy)Cl2] Æ 0.5H2O (1)
An intimate mixture of [Rh(Hbzpy)(bzpy)Cl2]
(0.324 g, 0.60 mmol) and bipy (0.562 g, 3.6 mmol) was
placed in a ceramic boat (10 cm and 1.3 cm in length
and width, respectively) and heated in an oven at
160 ꢁC for 15 h. After cooling, the resulting reaction
mixture was stirred for 2 h with 40 ml of methanol
and then filtered. The precipitate was washed repeatedly
with methanol and then dried under vacuum. 0.118 g
(38%) of yellow crystalline product (1) was obtained.
IR (cmꢀ1): m(C@O) 1674; m(Rh–Cl) 343, 324. dH(dmso-
2. Experimental
2.1. Instrumentation
Recording of NMR, IR, mass and absorption spectra
have been detailed elsewhere [13]. Low temperature
(77 K) emission spectra were obtained with a SLM
8000e spectrofluorometer (SLM Instrument, Inc.) com-
pact with photo-multiplier tubes which were used as detec-
tors. An Xe–Hg lamp was utilized as the excitation source.
Emission photoselection measurements of these complexes
under study were obtained with the same instrument as the
emission spectra, augmented with two additional sheets of
polarizer placed in both the paths of the exciting light and
emitting light. The luminescence lifetime was determined
by a LN 3000 nitrogen laser with 337.1 nm pulses triggered
and integrated by a SRS Model SR250 Gated Integrator
(Standford Research Systems, Inc.) and Boxcar Average
Module. Cyclic voltammetry (CV) and normal pulse vol-
tammetry (NPV) experiments were carried out using a
BAS-100B electrochemical analyzer (Bioanalytical Systems,
Inc.) equipped with a three-electrode cell system. All exper-
iments were conducted out under a N2 atmosphere. A Pt
working electrode (BAS Model MF-2103, 1.6 mm dia.), a
Pt wire auxiliary electrode and a saturated calomel refer-
ence electrode (SCE) were used for voltammetric measure-
ments. The ferrocenium/ferrocence redox couple was used
as an internal standard, E1/2 (Fc+/0) = 0.40 V versus SCE.
An H-type cell with a porous glass frit was used for con-
trolled potential electrolysis (CPE) experiments using a Pt
plate for the working and auxiliary electrodes.
d6) 9.19 (1H, d, J = 5.6 Hz, Hb 6) (b0 = bipy), 8.82
(1H, d, J = 8.4 Hz, Hb 3), 8.79–8.69 (3H, m, Hb6
0
0
þ
0
0
0
Hb 6 þ Hb3) (b = bzpy), 8.34 (1H, t, J = 7.6 Hz, Hb 4),
0
0
8.31 (1H, t, Hb 4 ), 8.25 (1H, t, J = 7.6 Hz, Hb5), 8.15
0
0
(1H, d, J = 7.6 Hz, Hb 3 ), 7.95 (1H, d, J = 8.0 Hz,
0
0
0
0
H
b6 ), 7.89–7.85 (2H, m, Hb 5 þ Hb 5 ), 7.68 (1H, t,
0
J = 6.8 Hz, Hb4), 7.66 (1H, d, J = 7.6, 1.2 Hz, Hb3 ),
0
Hb5 ), 7.25 (1H, t,
7.41 (1H, td, J = 7.6, 1.6 Hz,
0
J = 7.2 Hz,
H
b4 ), d(dmso-d6) 187.39 (CO), 163.75,
163.50, 157.37, 157.14, 154.54, 154.31, 153.29, 148.68,
140.77, 140.31, 139.84, 139.53, 129.65, 127.08, 126.93,
126.82, 126.27, 125.09, 123.86, 123.56, 123.47. Anal.
Calc. for C22H17N3O1.5Cl2Rh: C, 50.70; H, 3.29; N,
8.06; Cl, 13.60. Found: C, 50.62; H, 3.38; N, 8.17; Cl,
13.72%.
2.3.3. cis-[Rh(bzpy)(phen)Cl2] Æ 0.5H2O (2)
An intimate mixture of K[Rh(phen)Cl4] (0.464 g,
1.0 mmole) and Hbzpy (0.660 g, 3.6 mmole) was placed
in a ceramic boat (10 and 1.3 cm in length and width,
respectively) and heated in an oven at 210 ꢁC for 15 h.
After cooling, the resulting reaction mixture was stirred
overnight with 50 ml of methanol/H2O (1:1, v/v) solution
and then filtered. The yellow-brown filtrate was evapo-
rated to dryness, followed by washing with dichlorometh-
ane. A deep yellow-green precipitate was obtained and
washed repeatedly with acetone. After drying under vac-
uum, 0.169 g (31%) of yellow-green crystalline product
(2) was obtained. IR (cmꢀ1): m(C@O) 1666; m(Rh–Cl)
342, 325. dH(dmso-d6) 9.97 (1H, d, J = 5.6 Hz, Hb6)
(b = bzpy), 9.55 (1H, d, J = 4.8 Hz, Hp2) (p = phen),
9.00 (1H, d, J = 8.0 Hz, Hp4), 8.92 (1H, d, J = 8.0 Hz,
Hp5), 8.44–8.29 (5H, m, Hb3 + Hb4 + Hp7 + Hp8 + Hp9),
8.16 (1H, dd, J = 8.0, 2.8 Hz, Hp3); 7.99–7.88 (2H, m,
2.2. Reagents
RhCl3 Æ 3H2O (Merck), 2,20-bipyridine, 2-benzoylpyri-
dine and 1,10-phenanthroline (Aldrich) were used as
received. K[Rh(phen)Cl4] was prepared according to the
published procedure [14]. Me2SO (Uvasol, Merck) was
dried for spectroscopy. Et4NClO4 (TEAP), used as a base
electrolyte, was obtained from Showa Chemical Co. and
further purified by three recrystallizations from distilled
H2O. Laboratory grade solvents were used unless otherwise
specified.
0
b5 + Hp6), 7.67 (1H, d, J = 7.6 Hz, Hb6 ), 6.94 (1H, t,
J = 7.2 Hz, Hb5 ), 6.67 (1H, t, J = 7.2 Hz, Hb4 ), 5.86
(1H, d, J = 8.0 Hz, Hb3 ), dC(dmso-d6) 189.55 (CO),
H
0
0
0
156.44, 155.29, 154.99, 152.27, 151.13, 146.55, 140.30,
138.86, 138.76, 138.34, 137.91, 133.66, 130.90, 130.77,
129.21, 128.29, 127.86, 127.69, 126.93, 126.24, 126.13,
125.98, 124.02. Anal. Calc. for C24H17N3O1.5Cl2Rh: C,
52.87; H, 3.14; N, 7.71; Cl, 13.01. Found: C, 52.76; H,
3.21; N, 7.84; Cl, 13.15%.
2.3. Synthesis
2.3.1. [Rh(Hbzpy)(bzpy)Cl2]
[Rh(Hbzpy)(bzpy)Cl2] was prepared according to the
published procedure [12]. The orange-yellow product was

2162
M.C. Tseng et al. / Polyhedron 25 (2006) 2160–2166
3. Results and discussion
1 and 2 exhibit six and eight non-protonated carbon peaks,
respectively. As a further confirmation of the double bond
character of the C@O bond in the N,O ligands, dCO falls
close to 200 ppm [8]. On the basis that dCO of complexes
1 and 2 relative to free Hbzpy (dCO = 193.5 ppm) are
almost the same, the NMR data are inconsistent with
N,O-coordination of 2-benzoylpyridine. Due to the IR
and NMR data, complexes 1 and 2 are consistent with
the N,C-coordination of 2-benzoylpyridine. There are two
possible isomers for [Rh(bzpy)(NN)Cl2] (NN = bipy or
phen) deduced from the hetero-bischelated complexes as
displayed below.
3.1. Synthesis and assessment of structure
First [Rh(Hbzpy)(bzpy)Cl2] was prepared according to
published procedures [12] at this laboratory. Complex 1
was then prepared by heating the intimate mixture of
[Rh(Hbzpy)(bzpy)Cl2] and bipy in an oven at 160 ꢁC for
15 h. Elimination of Hbzpy had been ascribed to the
cleavage of the rather weak Rh–O bond, which led to a
loss of stabilization due to the chelate effect [12]. Replace-
ment of Hbzpy by phen under the same conditions, how-
ever, generated a mixture of at least two isomers, which
we were unable to separate. The synthesis of complex 2
was accomplished by heating an intimate mixture of
K[Rh(phen)Cl4] and Hbzpy in an oven at 210 ꢁC for
15 h. Attempts to get complex 1 by the latter procedure
failed since a mixture of isomers was obtained again.
The synthetic studies of the two different routes in con-
nection with the stereochemistry of complexes 1 and 2
remain unknown to the authors to date.
Cl
Cl
N
N
N
Cl
N
Cl
-
Rh
Rh
N
N
C
-
C
cis-α
cis-β
-
N
C = bzpy, N N = bipy or phen
The base peaks in the mass spectra of complexes 1 and 2
are m/z 512 and 536, corresponding to the molecular mass
of cis-[Rh(bzpy)(bipy)Cl2] and cis-[Rh(bzpy)(phen)Cl2],
respectively. The infrared spectra of the two complexes
exhibit the m(C@O) stretching bands in the vicinity of
1666 cmꢀ1. On the basis that the m(C@O) stretching bands
of complexes 1 and 2, as well as the free Hbzpy ligand (ca.
1668 cmꢀ1), are almost the same, it is evident that the che-
lated bzpy ligand forms a six-member N,C metallacycle,
not a five-member N,O metallacycle. The stereochemistry
of the RhCl2 moiety in these complexes has been deter-
mined by IR data. Both complexes show two m(Rh–Cl)
bands near 342 and 324 cmꢀ1, indicating that the two chlo-
rides are cis to each other [15].
0
In the cis-a conformation, Hb3 of the phenyl moiety of
the bzpy ligand is located above (or below) the pyridine of
the bipy/phen ligand (see Fig. 1). For complex 1, the reso-
0
nance position of Hb3 , d = 7.66, is somewhat deshielded
compared to that of Hb4 , d = 7.25, as presented in Section
0
2. This is therefore inconsistent with the NMR chemical
shift data since this hydrogen would indeed be more
shielded due to the p-electron ring current of pyridine. In
the cis-b isomer, however, Hb6 of the pyridyl moiety of
the bzpy ligand is located below the plane of the pyridine
of the bipy/phen ligand, which exhibit significant upfield
shifts as expected due to the shielding effect of the ring cur-
rent. For complex 1, the resonance position of Hb6,
1
0
The proton peaks in the H NMR spectrum of each
d = 8.79, is more shielded compared to Hb 6 in the plane
complex show sixteen chemically different protons. From
the 13C spectra, complexes 1 and 2 exhibit 22 and 24 car-
bons, respectively. In addition, the 13C DEPT of complexes
of the pyridine of the bipy ligand, d = 9.19, as expected
by the shielding effect of the ring current. Therefore, the
NMR chemical shift data of complex 1 are consistent with
Fig. 1. The proposed structures of cis-[Rh(bzpy)LCl2]: (A) cis b-[Rh(bzpy)(bipy)Cl2] (1); (B) cis a-[Rh(bzpy)(phen)Cl2] (2).

M.C. Tseng et al. / Polyhedron 25 (2006) 2160–2166
2163
the cis-b structure as illustrated in Fig. 1. The conformer of
complex 1 is further evidenced by the NOESY experiment,
having minima (ꢀ10% for complex 1; ꢀ14% for complex 2)
in the polarization spectra. These data also reveal that the
absorption and emission are both linear oscillators [20].
Deviation of the polarization from ꢀ0.33 may be explained
by the existence of other states having similar energies in
the region [21].
available in the supplementary material. Hb 6 (b0 = bipy) is
0
spatially correlated with Hb6 of pyridine in bzpy as demon-
strated in the same figure.
0
For complex 2, the chemical shift of Hb3 , d = 5.86, more
0
upfield than that of Hb6 , d = 7.67, is due to the anisotropic
The slightly structured shape and increased value for the
excitation polarization spectrum in the 3.3–2.5 · 104 cmꢀ1
region suggest that there may be more than one absorption
mode involved in this region, consistent with the carbonyl
n–p* transitions assignment. The polarization spectra
increase sharply to about 0.20 in the lowest energy wave-
length region (below 25 · 104 cmꢀ1) for both complexes.
The magnitude that approaches the limit value 0.5 is
expected for a linear absorber-linear emitter case [20].
Therefore, involvement other transitions is implied and
one could reasonably conclude that the dd* absorption is
buried in this low-energy region. Therefore, the carbonyl
n–p* transition must be involved and in which all spin-
forbidden dd* transitions were likely buried, similar to that
of cis-[Rh(dpk)2Cl2]+ [17].
effect resulting from the inductive ring current of the pyr-
idyl ring. Hb3 is located just above (or below) the pyridine
of the phen ligand, and hence corresponds to the relative
location of protons in space for the cis-a structure as illus-
trated in Fig. 1. The appearance of the chemical shift of
Hb 6 for complex 1, d = 9.19, more upfield than that of
Hp2 for complex 2, d = 9.55, is due to the p-donor ability
of the metallated phenyl ring, which is located trans to
b 6, as demonstrated in the same figure.
Further assignments of the H NMR spectra for com-
0
0
0
H
1
1
plexes 1 and 2 have been done through H COSY NMR
experiments, available in the supplementary material.
3.2. Absorption spectra
The free Hbzpy ligand exhibits one intense
3.3. Luminescence properties
(e ꢁ 104 cmꢀ1 Mꢀ1) absorption at about 3.78 · 104 cmꢀ1
,
arising from the spin-allowed p–p* transition. The weaker
(e ꢁ 102 cmꢀ1 Mꢀ1) absorption band observed at about
2.84 · 104 cmꢀ1 is assigned as the carbonyl n–p* transition
on the basis that the energy of this absorption peak exhibits
a hypsochromic shift when the solvent polarity is increased
[7]. The room temperature absorption spectra for com-
plexes 1 and 2 are presented in Fig. 2A. Both shows an
Hbzpy shows a structural emission with a single
vibrational progression of 1650 cmꢀ1, assigned as an n–p*
transition [22]. The emission spectra recorded in ethanol/
intense (e ꢁ 104 cmꢀ1 Mꢀ1
)
absorption in the 3.1–
3.9 · 104 cmꢀ1 region which most likely arise from a spin-
allowed p–p* absorption of the coordinated ligands
[7,16]. Each complex also exhibits an intermediate
(e ꢁ 103 cmꢀ1 Mꢀ1) absorption in the 3.0–2.7 · 104 cmꢀ1
region. According to the absorption spectra of the related
free ligands [7,16] and cis-[Rh(dpk)2Cl2]+ (dpk = di-2-pyr-
idyl ketone) [17], this absorption can be assigned as a
carbonyl n–p* band [18], which was not observed for cis-
[Rh(bipy)2Cl2]+ and cis-[Rh(phen)2Cl2]+ [19]. It is noted
that the absorption band profile extended to lower energy
(below 2.6 · 104 cmꢀ1), which was not observed for the
related ligands either. This might be the tail of the carbonyl
n–p* band in combination with a rather weak dd*
absorption.
The excitation polarization spectrum of complex 1 is
shown in Fig. 2B. The excitation polarization spectrum
of complex 2 is very similar to that of complex 1. The sym-
metry of each complex is lower than C2V and hence only a
singly symmetric species is considered in the analysis of the
electronic spectrum. In other words, the polarization limits
pertaining to planar oscillators will not be pertinent here.
The calculated limits are 0.5 and ꢀ0.33 [20], resulting from
the combination of a linear absorption and a linear emis-
sion oscillator. The polarizations in the p–p* region of
the two complexes are quite similar, showing little structure
Fig. 2. Absorption and excitation polarization spectra: (A) absorption
spectra (room temperature): cis b-[Rh(bzpy)(bipy)Cl2] (1) (——); cis
a-[Rh(bzpy)(phen)Cl2] (2) (––). (B) excitation polarization spectrum of cis
b-[Rh(bzpy)(bipy)Cl2] (1) (77 K, 689 nm emission).

2164
M.C. Tseng et al. / Polyhedron 25 (2006) 2160–2166
methanol (9:1, v/v) at 77 K of complexes 1 and 2 are pre-
sented in Fig. 3. No emission behavior is observed at room
temperature. Contained in Table 1 are the luminescent data
No emission could be detected from [Rh(Hbzpy)-
(bzpy)Cl2] [12], but emission could be obtained from cis-
[Rh(bzpy)(NN)Cl2] (NN = bipy/phen). On one hand, it
might be possible that the emission is occurring but is
beyond the detection limits of our equipment [24]. On the
other hand, it is possible that the phenyl ring of chelated
Hbzpy can undergo rotation and then facilitate the non-
radiative energy loss in this complex [25]. The bipyridyl-like
ligands are more rigid than the N,O-coordinated Hbzpy
ligand. When the N,O ligand (Hbzpy) is substituted by
bipy/phen (a more rigid ligand), the emission could be
observed. The latter reason at least is therefore supported
by our results.
of these complexes. The red emissions (1.2–1.7 · 104 cmꢀ1
)
recorded for complexes 1 and 2 are broad (with around
2.2 · 103 cmꢀ1 in half-width) and both are symmetric and
structureless, as seen in Fig. 3. The lifetimes measured at
the emission maximum are respectively 58 and 60 ls for
complexes 1 and 2 (see Table 1). The energy of the emission
peak is unaffected by changing the solvent from ethanol/
methanol to DMSO. In comparison with the band shape,
lifetime and peak position studies performed on the cyclo-
metallated Rh(III) complexes [3], the emission types of
complexes 1 and 2 have excluded the possibilities of
The metallated phenyl ring of bzpy is a weaker p-accep-
tor and a stronger-donor compared to bipy or phen. A
strong-donor can move the metal d*(eg) orbitals to a higher
energy, but a strong p-acceptor can lead to the lowering of
the metal d(t2g) orbitals. Therefore, the dd* emission of
complexes 1 and 2 were observed at a higher energy than
that of cis-[Rh(bipy)2Cl2]+ and cis-[Rh(phen)2Cl2]+ (see
Table 1) [16]. It is examined that the degree of raising of
the metal d*(eg) orbitals for complexes 1 and 2 is more than
the degree of lowering of the metal d(t2g) orbitals for cis-
[Rh(bipy)2Cl2]+ and cis-[Rh(phen)2Cl2]+, which results in
3
3
3MLCT, SBLCT or pp* emissions and hence are classi-
fied as a dd* phosphorescence [17,23].
The emission polarization spectra of both complexes,
excited at 313 and 365 nm, are flat with a vanished slope.
This observation indicates that the combination of absor-
ber and emitter(s) gives the same polarization across the
emission band. Therefore, it is reasonable to conclude that
there is no other emission buried in the observed emission
band. In other words, both complexes show a band of sin-
gle emissions rather than multiple ones.
3
the higher energy gap of the d–d* excited state for com-
plexes 1 and 2. In other word, bzpy exhibits a higher ligand
field strength compared with that of bipy or phen.
3.4. Electrochemical properties
Electrochemical studies were conducted in 0.10 M
TEAP/DMSO solutions containing the corresponding
Rh(III) complexes. Cyclic voltammetric data such as the
peak potentials are collected in Table 2. No oxidation wave
was observed when the potential was scanned from 0.0 to
+2.0 V.
3.4.1. Electrochemistry of complexes 1 and 2
The cyclic voltammograms of complexes 1 and 2 have
similar patterns, showing two reduction waves, cI and cII
(see Table 2), and no corresponding oxidation wave
appeared upon scan reversal with respect to cI. The number
Fig. 3. Emission spectra of cis b-[Rh(bzpy)(bipy)Cl2] (1) (——) and cis
a-[Rh(bzpy)(phen)Cl2] (2) (––) (77 K, 313 nm excitation).
Table 2
Cyclic voltammetric reduction potentials for Rh(III) complexesa
Complex
m (V/s)
(Epc I
)
(Epc)II
Table 1
Phosphorescence data and lifetime (s) for Rh(III) complexes at 77 K
1
0.1
2.0
0.1
2.0
0.1
0.1
ꢀ1.19
ꢀ1.24
ꢀ1.16
ꢀ1.22
ꢀ0.84b
ꢀ0.85c
ꢀ1.64
ꢀ1.67
ꢀ1.65
ꢀ1.69
ꢀ1.46b
ꢀ1.46c
Complex
Phosphorescencea
2
mmax (·104 cmꢀ1
)
sp (ls)
1
1.45
58
cis-[Rh(bipy)2Cl2]+
cis-[Rh(phen)2Cl2]+
2
1.49
60
cis-[Rh(bipy)2Cl2]+
cis-[Rh(phen)2Cl2]+
1.42b
1.41b
47b
41b
a
Results for 1.2 mM samples in 0.10 M TEAP/DMSO solutions at a Pt
electrode. Epc in V vs. SCE.
a
b
In EtOH/MeOH (9:1).
Ref. [16].
Ref. [31].
Ref. [32].
b
c

M.C. Tseng et al. / Polyhedron 25 (2006) 2160–2166
2165
of electrons transferred at the wave cI, estimated from con-
trolled-potential electrolysis experiments at ꢀ1.20 and
ꢀ1.15 V (Epc)I, were found to be 1.80 and 1.85 for com-
plexes 1 and 2, respectively, indicating that wave cI for both
complexes involved essentially a two-electron charge trans-
fer process. An oxidation wave due to oxidation of the
chloride ion was also observed at 1.07 V in the voltammo-
grams obtained for the solutions after the electrolysis. For
each complex, the ratio of the peak current of the chloride
ion to that of wave cI is close to one, indicating that two
chlorides had been eliminated from the complex after elec-
trolysis at wave cI. It has also been reported that electro-
chemical reduction of halide-coordinated complexes will
generate products via dissociation of halide(s) [26–30]. As
stated earlier, the emission of the two complexes is con-
firmed to be a dd* transition. The two-electron transfer
reduction (wave cI) results in higher electron density on
the metal and, hence, destabilizes these complexes. This
can rationalize the transfer of two electrons, in which the
chemical reaction involves the loss of two chloride ions,
and lead to the irreversible process at wave cI.
based on IR and NMR data. Through analysis of NMR
chemical shifts, structures of complexes 1 and 2 are consis-
tent with cis-b and cis-a conformers, respectively. Each
complex shows high intensity bands in the UV region,
assigned as spin-allowed p–p* transitions. The medium-
intensity absorption band profile in the lower energy region
can be explained by convolution of carbonyl n–p* and d–d*
transitions. At 77 K, each complex shows a broad, symmet-
ric and structureless red emission with microsecond
lifetime, assigned as dd* phosphorescence. The electro-
chemical data also provide evidence in support of the
assigned dd* emissions of the two complexes. The first
two-electron reduction process occurs at the metal and is
coupled with the elimination of two chlorides. Through
examination of the influence of phenol on the reduction
potentials, the redox orbital involved in the second reduc-
tion is assigned as the p* orbital localized at the carbonyl
fragment of bzpy.
5. Supplementary material
The more negative values of (Epc)I obtained for com-
plexes 1 and 2 compared to those reported for cis-[Rh-
(bipy)2Cl2]+ and cis-[Rh(phen)2Cl2]+, as shown in Table
2, may be ascribed to the higher electron densities on the
metal. This is well understood based on the nature of the
negatively charged bzpy ligand, which is a better r-donor
but a poorer p-acceptor (vide supra). On the whole, reduc-
tions of the neutral species are more demanding than those
for the cationic complexes.
2D NMR for complexes 1 and 2 are available from
the author on request. Copies of the data can be
obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
fax: +44 1223 336033, e-mail: deposit@ccdc.cam.ac.uk
or http:www.ccdc.cam.ac.uk.
Acknowledgement
Normal pulse voltammograms (NPV) were also
recorded for the two complexes. Similar to the cyclic vol-
tammograms, the NPV also exhibited reduction waves cI
and cII for each complex. It was found that the ratio of
the limiting current of waves cI and cII is 2:1. Furthermore,
the ratio of the peak current of wave cI to that of wave cII is
also close to two at relatively high scan rates. Because wave
cI is a two-electron charge transfer process, this result sug-
gests that wave cII is possibly a one-electron charge transfer
process. When phenol was added into the solutions as the
proton donor, no significant changes were observed for
wave cI in the cyclic voltammograms of the complexes,
suggesting that wave cI is due to the reduction at the
metal-localized orbitals. On the other hand, the addition
of phenol resulted in an evident potential shift and peak
current increased for wave cII. Based on the influence of
phenol, the reduction at wave cII can be assigned to be
localized at the carbonyl p* orbital of bzpy. The same result
was observed for the pik-coordinated or dpk-coordinated
complexes of rhodium(III) [13,17].
Financial support from the National Science Council of
the Republic of China is acknowledged.
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