Synthesis and characterization of platinum(II) complexes of 2N1O-donor ligands with a pendent indole ring

Article abstract of DOI:10.1016/j.ica.2008.01.046

The Pt(II) complexes of 2N1O-donor ligands containing a pendent indole, 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-iepp), and 1-methyl-3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-miepp) (H denotes an ionizable hydrogen), were synthesized, and the structure of [Pt(tbu-iepp)Cl] (1) was determined by X-ray analysis. Complex 1 prepared in CH₃CN was revealed to have the C2 atom of the indole ring bound to Pt(II) with the Pt(II)-C2 distance of 1.981(3) ?. On the other hand, [Pt(tbu-miepp)Cl] (2) was concluded to have a phenolate coordination instead of the C2 atom of the indole ring by ¹H NMR spectra. Reaction of 1 with 1 equiv. of Ce(IV) in DMF gave the corresponding one-electron oxidized species, which exhibited an ESR signal at g = 2.004 and an absorption peak at 567 nm, indicating the formation of the Pt(II)-indole-π-cation radical species. The half-life, t_1/2, of the radical species at -60 °C was calculated to be 43 s (k_obs = 1.6 × 10^-2 s^-1).

Full text of DOI:10.1016/j.ica.2008.01.046

Available online at www.sciencedirect.com
Inorganica Chimica Acta 362 (2009) 887–893
www.elsevier.com/locate/ica
Synthesis and characterization of platinum(II) complexes
of 2N1O-donor ligands with a pendent indole ring
a,
b
b
a
*
Yuichi Shimazaki , Tatsuo Yajima , Yasuo Nakabayashi , Yoshinori Naruta ,
Osamu Yamauchi b,*
a Institute for Materials Chemistry and Engineering, Kyushu University, Higashi-ku, Fukuoka 812-8581, Japan
b Unit of Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan
Received 3 December 2007; received in revised form 29 January 2008; accepted 31 January 2008
Available online 9 February 2008
This paper is dedicated to Professor Dr. Bernhard Lippert in celebration of his outstanding contribution
to chemistry and with the very best wishes for his future activities
Abstract
The Pt(II) complexes of 2N1O-donor ligands containing a pendent indole, 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)ben-
zylamino]ethylindole (Htbu-iepp), and 1-methyl-3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-
miepp) (H denotes an ionizable hydrogen), were synthesized, and the structure of [Pt(tbu-iepp)Cl] (1) was determined by X-ray analysis.
Complex 1 prepared in CH3CN was revealed to have the C2 atom of the indole ring bound to Pt(II) with the Pt(II)–C2 distance of
˚
1.981(3) A. On the other hand, [Pt(tbu-miepp)Cl] (2) was concluded to have a phenolate coordination instead of the C2 atom of the
1
indole ring by H NMR spectra. Reaction of 1 with 1 equiv. of Ce(IV) in DMF gave the corresponding one-electron oxidized species,
which exhibited an ESR signal at g = 2.004 and an absorption peak at 567 nm, indicating the formation of the Pt(II)–indole-p-cation
radical species. The half-life, t1/2, of the radical species at ꢀ60 °C was calculated to be 43 s (kobs = 1.6 ꢁ 10ꢀ2 sꢀ1).
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Pt(II) complexes; Indole; Cyclometalation; Indole-p-cation radical; Pt(II)–indole bond
1. Introduction
the catalytic cycle; one of the reaction intermediate, com-
pound I, has been considered to be the ferryl heme and
Trp 191 indole-radical cation [4–6]. Trp coordinates to
metal ions through the amino and carboxylate groups
and may undergo intramolecular stacking interactions [8–
11] as seen in [Cu(L)(Trp)]ClO4 (L = 2,20-bipyridine [12]
or 1,10-phenanthroline [13]), but the indole ring has not
been known to be involved in metal binding in biological
systems, although it can form various bonds with metals
such as palladium in synthetic chemistry [14,15].
We reported earlier that the Cu(I) complexes of tripodal
ligands having an indole moiety in place of one of the coor-
dinating groups were found to have a Cu(I)–(C2@C3) g2
bond in a tetrahedral structure [16]. Pd(II) reacts with
indole-3-acetate (IA) in the 3H-indole form in the presence
of pyridine (py) to give a dimeric complex, [Pd2(IA)2(py)2],
The indole group of the tryptophan (Trp) residue in pro-
teins is known to form a hydrophobic environment for pro-
tein stabilization and specific binding of molecules [1–3]
and to be involved in electron transfer pathways [4–6]. It
is sometimes located close to the active center of metalloen-
zymes with p–p stacking interaction. In the active center of
cytochrome c peroxidase, the indole rings of Trp 51 and
Trp 191 were revealed to be stacked with the heme and
the coordinated imidazole ring of histidine 175, respectively
[7]. The indole ring of Trp 191 plays an important role in
*
Corresponding authors. Tel./fax: +81 72 855 8102 (O. Yamauchi).
E-mail addresses: yshima@ms.ifoc.kyushu-u.ac.jp (Y. Shimazaki),
oyamauchi36bioin@wonder.ocn.ne.jp (O. Yamauchi).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.01.046

888
Y. Shimazaki et al. / Inorganica Chimica Acta 362 (2009) 887–893
where Pd(II) binds with the deprotonated indole C3 atom
and the carboxylate oxygen atom of IA to form a spiro ring
and in addition with the imine nitrogen atom of the other
formation of the indole p-cation radical species upon oxida-
tion with cerium(IV) are reported.
´
Pd(II)-bound IA molecule [17]. Kostic et al. developed
2. Experimental
Pd(II) and Pt(II) complexes which function as unique arti-
ficial peptidases, which recognize the indole moiety and
cleave the adjacent amide or peptide bond specifically
[18–21]. Examples are also known for Pd(II)–indole C2
bonding as a result of a cyclopalladation-like reaction
[22,23]. Recently, we reported some Pd(II) complexes of
2N1O donor ligands with a pendent indole moiety (Scheme
1), which were found to be interconvertible between the
Pd(II)–phenolato(O) and Pd(II)–indole(C2) complexes
with a 2N1O1Cl- and a 2N1C1Cl-donor set, respectively
[24]. One-electron oxidation of the Pd(II)–indole(C2) com-
plex yielded the corresponding Pd(II)–indole p-cation rad-
ical species. Further, the conversion to the Pd(II)–indole
species was concluded to be dependent on the phenolate
O-donor properties, the formation ratio of the indole-bind-
ing complex to the phenolate complex being higher for the
complexes with a higher pKa value of the phenol moiety
[25]. However, the interconversion or the formation of
the Pd(II)–indole r-bonding was limited by factors such
as the distance between the indole ring and the Pd center
and the existence of the phenol moiety and the unsubstitut-
ed indole NH moiety (Scheme 1).
Very recently, we have reported that the properties of the
group 10 metal complexes depend on the central metal ion
[26], and especially their oxidation behavior and properties
of oxidized species are very different. With these points in
mind, we now studied the behavior of the pendent indole
ring in the Pt(II) complexes of the same 2N1O-donor ligands
used for the Pd(II) complexes involving a phenol, a pyridine,
and a tertiary amine as metal binding sites (Fig. 1) [24]. The
structure of a novel indole C2 binding Pt(II) complex and the
2.1. Materials and methods
K2PtCl4 and triethylamine were obtained from Wako,
and other chemicals were obtained from Tokyo Kasei.
All the chemicals used were of the highest grade available
and were further purified whenever necessary [27]. Solvents
were also purified before use by standard methods
[27]. DMSO-d6 was purchased from Cambridge Isotope
Laboratory.
hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-
iepp), 1-methyl-3-[N-2-pyridylmethyl-N-2-hydroxy-3,
Synthesis
of
[N-2-pyridylmethyl-N-2-
5-di(tert-butyl)benzylamino]ethylindole (Htbu-miepp), and
[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzyl-
amino]ethane (Htbu-etpp) have been reported previously
[24,28]. Electronic spectra were measured with a Shimadzu
1
UV-3101PC spectrophotometer. H NMR measurements
were performed with a JEOL JNM-GSX-400 (400 MHz)
NMR spectrometer. Frozen solution ESR spectra were
taken at 77 K in quartz tubes with 4-mm inner diameter
on a JEOL JES-RE1X X-band spectrometer equipped with
a standard low-temperature apparatus. The g values were
calibrated with a Mn(II) marker used as a reference.
2.2. Synthesis of complexes
2.2.1. [Pt(tbu-iepp)Cl] (1)
To a suspension of Htbu-iepp (0.47 g, 1.0 mmol) in
CH3CN (20 mL) was added an aqueous solution of
py5
py6
py4
N
N
Pd(tbu-iepp)Cl
py3
O—
O—
H
N
N
N
in4
in5
in6
HO
in2
N
N
N
Me
N
H in1
N
Pd
in7
N
NH
Pd
Cl
O
Cl
tbu-iepp
tbu-miepp
Pd(tbu-miepp)Cl
N
Me
N
O—
N
HO
N
N
N
Pd
N
N
Me
Pd
Cl
O
Cl
tbu-etpp
Fig. 1. Structures of ligands.
Scheme 1. Interconversion between phenolate and indole-binding Pd(II)
complexes (tert-butyl groups are omitted for clarity).

Y. Shimazaki et al. / Inorganica Chimica Acta 362 (2009) 887–893
889
Table 1
K2PtCl4 (0.42 g, 1.0 mmol), and the mixture was refluxed
Crystallographic data of complex 1
overnight. The solution was filtrated and kept standing at
room temperature to give pale yellow crystals. Anal. Calc.
for C31H38N3OClPt (1): C, 53.25; H, 5.48; N, 6.01. Found:
C, 53.18; H, 5.41; N, 6.02%. 1H NMR (400 MHz, DMSO-
d6): d (vs TMS) 9.60 (s, 1H), 8.69 (d, 1H), 8.47 (s, 1H), 7.74
(d, 1H), 7.73 (t, 1H), 7.35 (q, 1H), 7.25 (q, 1H), 7.20 (m,
2H), 6.85 (m, 3H), 4.71 (d, 1H), 4.32 (d, 1H), 4.17 (d,
1H), 3.98 (q, 1H), 3.57 (m, 1H), 1.23 (s, 9H), 1.15 (s, 9H).
1
Formula
PtC31H38N3OCl
610.51
Formula weight
Crystal color, habit
Crystal dimensions (mm)
Crystal system
orange, platelet
0.10 ꢁ 0.08 ꢁ 0.06
monoclinic
8.529(1)
15.251(3)
21.750(3)
92.054(6)
2827.2(9)
P21/n
˚
a (A)
˚
b (A)
˚
c (A)
b (°)
V (A )
2.2.2. [Pt(tbu-miepp)Cl] (2) and [Pt(tbu-etpp)Cl] (3)
These complexes were prepared in a manner similar to
that described for 1 as pale yellow solids. Anal. Calc. for
C32H40N3OClPt (2): C, 53.89; H, 5.65; N, 5.89. Found:
C, 53.85; H, 5.41; N, 5.88%. 1H NMR (400 MHz,
DMSO-d6): d (vs TMS) 9.00 (d, 1H), 8.18 (t, 1H), 7.77
(d, 1H), 7.47 (t, 1H), 7.25 (d, 1H), 7.12 (d, 1H), 7.08 (d,
1H), 7.02 (t, 1H), 6.88 (s, 1H), 6.75 (m, 2H), 4.96 (d,
1H), 4.72 (d, 1H) 4.64 (d, 1H), 4.32 (d, 1H), 3.59 (s, 3H),
2.90 (m, 3H), 2.61 (d, 1H), 1.36 (s, 9H), 1.20 (s, 9H). Anal.
Calc. for C23H33N2OClPt (3): C, 47.30; H, 5.69; N, 4.80.
3
˚
Space group
Z-value
4
Dcalc (g/cm3)
F(000)
1.643
1392.00
50.86
54.9
46627
6441
6441
334
0.018
l (Mo Ka) (cmꢀ1
2hmax (°)
)
Observed reflections
Independent reflections
Reflections used
Number of variables
R1 [I > 2r(I)]a
Rw (all data)b
0.053
1
Found: C, 47.25; H, 5.71; N, 4.78%. H NMR (400 MHz,
P
P
a
R1 = ||Fo| ꢀ |Fc||/ |Fo| for I > 2r(I) data.
DMSO-d6): d (vs TMS) 9.00 (d, 1H), 8.17 (t, 1H), 7.74
(d, 1H), 7.47 (d, 1H), 7.06 (d, 1H), 6.93 (d, 1H), 4.71 (d,
1H) 4.54 (d, 1H), 4.48 (d, 1H), 4.06 (d, 1H), 2.58 (m,
2H), 1.35 (s, 9H), 1.21 (s, 9H), 0.88 (t, 3H).
P
P
Rw = { x(|Fo| ꢀ |Fc|)2/ xFo2}1/2
;
x = 1/r2(Fo) = {rc2(Fo) + p2/
b
4F2o}ꢀ1
.
3. Results and discussion
3.1. Preparation of Pt(II) complexes
2.3. X-ray structure determination
The X-ray experiments were carried out for a well-shaped
single crystal of complex 1 on a Rigaku RAXIS imaging
plate area detector with graphite monochromated Mo Ka
2N1O-donor tripod-like ligands, Htbu-iepp, where H
denotes an ionizable hydrogen, reacted with K2PtCl4 in
1:1 (v/v) CH3CN/H2O at 80 °C to give [Pt(tbu-iepp)Cl]
(1) as pale yellow crystals. On the other hand, Htbu-miepp,
which is inferred to have the same ligand structure as that
of Htbu-iepp except the N-methylindole moiety, and Htbu-
etpp, which has no indole moiety, reacted with K2PtCl4
under the same conditions to give [Pt(tbu-miepp)Cl] (2)
and [Pt(tbu-etpp)Cl] (3) as a pale yellow powder, respec-
tively. Elemental analysis of complexes 1–3 revealed that
they have a deprotonated 2N1O-donor ligand and one
chloride ion. Interestingly, while the corresponding Pd(II)
complexes were obtained by the addition of a base such
as triethylamine [24], it was not necessary for preparing
these Pt(II) complexes. For the Pd(II) complexes the addi-
tion of a base assisted the conversion of the phenolate com-
plexes to the indole-binding species [25]. However, such a
conversion by base was not observed for any of the Pt(II)
complexes, but instead the Pt(II)–ligand systems showed
a color change to dark brown and gave some decomposi-
tion products, which were not studied any further.
˚
radiation (k = 0.71073 A). The crystal was mounted on a
glass fiber. To determine the cell constants and orientation
matrix, three oscillation photographs were taken for each
frame with the oscillation angle of 3° and the exposure time
of 3 min. Intensity data were collected by taking oscillation
photographs. Reflection data were corrected for both Lor-
entz and polarization effects. The structure was solved by
the direct method and refined anisotropically for non-
hydrogen atoms by full-matrix least-squares calculations.
Each refinement was continued until all shifts were smaller
than one-third of the standard deviations of the parameters
involved. Atomic scattering factors were taken from the lit-
erature [29]. Except for the hydrogen atom of the phenol
OH group, all hydrogen atoms were located at the calcu-
lated positions, assigned a fixed displacement, and con-
˚
strained to ideal geometry with C–H = 0.95 A and N–
˚
H = 0.90 A. The thermal parameters of calculated hydro-
gen atoms were related to those of their parent atoms by
U(H) = 1.2Ueq(C,N). The hydrogen atom of the phenol
OH group was located from the difference Fourier map.
All the calculations were performed by using TEXSAN crystal-
lographic software program package from Molecular Struc-
ture Corporation [30]. Summaries of the fundamental
crystal data and experimental parameters for the structure
determination of complex 1 are given in Table 1.
3.2. Crystal structure of [Pt(tbu-iepp)Cl] (1)
X-ray crystal structure analysis revealed that 1 was a
cyclometalation species having a mononuclear square-pla-
nar geometry formed by the indole C2 carbon, an amine

890
Y. Shimazaki et al. / Inorganica Chimica Acta 362 (2009) 887–893
Table 2
nitrogen, a pyridine nitrogen, and a chloride ion, as shown
in Fig. 2. The Pt–C2 bond length is 1.981(3) A (Table 2),
˚
Selected bond lengths (A) and angles (°) for 1
˚
1
which is within the normal range of the Pd–C2 distances
for the indole-binding Pd(II) complexes of the ligand series
and the Pt–C2 distances for the indole-binding Pt(II) com-
plexes [23,24] but is slightly shorter than the values for the
Pt–C bonds of Pt-phenyl complexes [31–33]. However, the
Pt(II)–C2 distance in 1 is much shorter than the distances
reported for the metal–indole complexes (Pd(II)–
Bond lengths
Pt–N(1)
Pt–N(2)
Pt–C(2)
Pt–Cl
2.082(2)
2.063(2)
1.981(3)
2.3125(7)
Bond angles
N(1)–Pt–N(2)
N(1)–Pt–C(2)
N(1)–Pt–Cl
N(2)–Pt–C(2)
N(2)–Pt–Cl
C(1)–Pt–Cl
82.75(8)
˚
C3 = 2.12–2.15 A [17]; Cu(I)–C2 and Cu(I)–C3 = 2.23–
174.20(8)
93.85(6)
91.55(9)
176.32(6)
91.88(7)
˚
2.27 A) [16]. The indole C2–C3 bond length in 1
˚
(1.388(3) A) is longer than that of the indole-binding Pd(II)
and Cu(I) complexes and uncoordinated indole ring
[16,17]. This may reflect a stronger effect of Pt(II) binding
on the indole ring than that of Pd(II) and Cu(I) binding.
˚
The Pt–N bond lengths of 1 are 2.082(2) and 2.063(2) A
Pd(tbu-miepp)Cl
for Pt–N(pyridine) and Pt–N(amine) (Table 2), respec-
tively, and the order of these bond lengths can be found
for the other Pt complexes [32–34] and is in agreement with
the observations with the Pd(II) complexes [24]. The phe-
nol ring of 1 is located above the coordinated pyridine ring
to be involved in the intramolecular p–p stacking, where
˚
the shortest Cindole–Cpyridine distance is 3.177(4) A and the
angle between the average planes of the two aromatic rings
is 34.3°. It is interesting to note in this connection that the
side chain phenol ring stacks with the coordinated pyridine
ring but not with the indole ring. Such a trend in stacking
was also observed for the indole-binding and phenolate-
binding Pd(II) complexes [24].
Pt(tbu-miepp)Cl
3.3. Characterization of Pt(II) complexes
10
9
8
7
6
The absorption spectra of 1 and 2 in DMF in the range
300–1000 nm showed a similar strong peak at 363
(e = 7500) and 364 nm (e = 6000), respectively. However,
the 1H NMR spectra of 1 and 2 in DMSO-d6 were very dif-
ferent. The chemical shifts for 2 were very similar to those
of [Pd(tbu-miepp)Cl], whose side chain indole ring is not
Chemical Shift / ppm
Fig. 3. 1H NMR spectra of metal-tbu-miepp complexes (metal = Pd(II)
and Pt(II)).
stacked with the coordinated pyridine ring (Fig. 3 and
Table 3) [24,25]. This result suggests that the structure of
complex 2 is a phenolate complex and that the side chain
1
indole ring is not coordinated to Pt(II). Further, the H
NMR chemical shifts for 2 were also very similar to those
of 3 which is without a pendent aromatic ring, indicating
that the side chain indole ring is not stacked with the coor-
dinated pyridine ring in 2 (Scheme 2). On the other hand,
large shift differences were observed for the pyridine and
indole proton signals between 1 and 2 in DMSO (Fig. 4
and Table 3); the signals of pyridine C3 through C6 pro-
tons (py3–py6) in 1 shifted upfield relative to those of 2
with the chemical shift differences, Dd = ꢀ(dcomplex1
ꢀ
d
complex2), of +0.2 to +0.6 ppm. The shifts of the indole
signals (Dd = ꢀ0.5 to +0.1 ppm) observed for the Pt(II)
complexes as well as the corresponding Pd(II) complexes
are in contrast with the rather small differences observed
between the Cu(I)-g2-coordinated indole and free indole
(|Dd| = <0.1 ppm) [16] and are ascribed to a stronger
Fig. 2. ORTEP view of [Pt(tbu-iepp)Cl] (1) drawn with the thermal
ellipsoids at 50% probability level and atomic labeling scheme.

Y. Shimazaki et al. / Inorganica Chimica Acta 362 (2009) 887–893
891
Table 3
Pd(tbu-iepp)Cl
1H NMR chemical shifts (d/ppm) and upfield shifts (Dd)a of pyridine and
indole proton signals for Pd(II) complexes in DMSO-d6
d (Dd) (ppm)
tbu-iepp (1)
tbu-miepp (2)
tbu-etpp (3)
py3
py4
py5
py6
in1
in2
in4
in5
in6
in7
a
7.20 (+0.57)
7.73 (+0.45)
7.20 (+0.27)
8.69 (+0.31)
8.47
7.77
8.18
7.47
9.00
7.74 (+0.03)
8.17 (+0.01)
7.47 (0)
9.00 (0)
Pt(tbu-iepp)Cl
6.88
7.25
6.75
7.02
6.75
7.35 (ꢀ0.10)
6.85 (ꢀ0.10)
6.85 (+0.17)
7.25 (ꢀ0.50)
Dd = ꢀ(d ꢀ dcomplex2), where dcomplex2 refers to the shift for 2.
Pt(tbu-iepp)Cl (1)
N
10
9
8
7
6
Chemical Shift / ppm
Fig. 4. 1H NMR spectra of indole-binding metal-tbu-iepp complexes
(metal = Pd(II) and Pt(II)).
O—
HO
N
N
N
Pt
NH
1000
Cl
N
H
Pt(tbu-miepp)Cl (2)
Me
800
567
N
N
O—
600
400
200
0
N
N
Pt
Cl
N
O
N
Me
Scheme 2. Formation of the Pt complexes with N2O ligands containing
an indole ring (tert-butyl groups in complexes are omitted for clarity).
r-donation to Pt(II) and Pd(II) than to Cu(I) by the indole
ring. These observations support that similar side chain
conformations and coordination structures are maintained
both in the solid state and in solution. The indole-binding
Pt(II) complex 1 is very stable in DMSO at room temper-
400
500
600
700
800
Wavelength / nm
1
Fig. 5. Absorption spectral changes of oxidized 1 with time in DMF at
ꢀ60 °C (5.0 ꢁ 10ꢀ4 M). The spectra were recorded at 10-s intervals.
ature; the H NMR spectrum of 1 in DMSO showed no
spectral changes over one week, whereas the corresponding
Pd(II) complex, [Pd(tbu-iepp)Cl], in DMSO was found to
undergo interconversion between the indole-binding spe-
cies and the phenolate complex [24,25]. The difference in
behavior in solution may be in line with the general prop-
erties of the Pd and Pt complexes in ligand exchange reac-
tions [35].
a new peak at 567 nm in the visible absorption spectrum
(Fig. 5). The reaction was found to be a one-electron oxida-
tion process from the reaction stoichiometry. The ESR spec-
trum of oxidized 1 exhibited a new sharp signal at g = 2.004,
and the amount of unpaired electron was calculated to be
more than 0.85 from the integrated spectrum (Fig. 6). On
the other hand, the absorption spectra of 2 and 3 remained
unchanged upon the addition of Ce(IV), and no peaks char-
acteristic of the phenoxyl radical were observed in our previ-
ous studies [26,36–39]. The indole radicals from tryptophan
3.4. Oxidation with Ce(IV)
Oxidation of 1 by one equivalent of Ce(IV) in DMF at
ꢀ60 °C caused a color change from yellow to green, giving

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Y. Shimazaki et al. / Inorganica Chimica Acta 362 (2009) 887–893
Pt(II) bonding has been established by the molecular struc-
ture of 1, which was also concluded for the solution of 1 in
DMSO-d6 by the downfield shifts of the indole protons due
to effective r-donation by the C2 atom. However, intercon-
version between the Pt–indole and Pt–phenolate complex
was not detected, which is probably due to the inert char-
acter of Pt(II) in ligand exchange reactions. One-electron
chemical oxidation of the Pt(II)–indole complex, 1, yielded
the corresponding Pt(II)–indole-p-cation radical species,
which is supported by the characteristic 567-nm absorption
peak and the ESR signal of the radical species. The stability
of the indole-p-cation radical species was slightly higher
than that from the corresponding Pd(II) complex, which
may also arise from the inertness of Pt(II).
The present observations will add to our knowledge of
the versatile nature of the indole ring in the metal coordi-
nation sphere and give clues to its functions in a variety
of systems.
300
320
B
340
/ mT
Fig. 6. ESR spectrum of one-electron oxidized 1 in DMF at 77 K.
Concentration, 5.0 ꢁ 10ꢀ4 M; microwave power, 1 mW; modulation
amplitude, 0.63 mT.
Acknowledgements
We gratefully acknowledge the support of this work by
the Grants-in-Aid for Scientific Research (No. 17750055 to
Y.S. and No. 16350036 to O.Y.) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan.
and N-methylindole have been reported to show a character-
istic absorption band in the region 510–560 nm [6,40,41],
where the indole-p-cation radical and the neutral indolyl
radical give a band centered at 560 and 510 nm, respectively.
The oxidized Pd(II) complex having the same ligand,
[Pd(tbu-iepp)Cl], showed a similar absorption peak at
ꢂ550 nm, which has been assigned to the Pd(II)–indole-p-
cation radical species. On the basis of these considerations,
we assign the oxidized form of 1 having a peak at 567 nm
to the indole p-cation radical species, although the assign-
ment of the shoulder appearing at 600–650 nm is not clear
at present. The unpaired electron is inferred to be localized
in the indole ring. When the green solution of oxidized 1 in
DMF was left to stand at ꢀ60 °C, it rapidly turned yellow,
and the 567-nm absorption peak disappeared in the first-
order decay. The half-life, t1/2, of oxidized 1 was calculated
to be 43 s (kobs = 1.6 ꢁ 10ꢀ2 sꢀ1). This result shows that
the oxidized form of 1 was slightly more stable than that of
[Pd(tbu-iepp)Cl] (t1/2 = 20 s, kobs = 3.5 ꢁ 10ꢀ2 sꢀ1 ), indi-
cating that the Pt(II) ion has a weak stabilizing effect on
the indole-radical [24]. This characteristic may be due to
the inertness of Pt(II) in ligand exchange. Further stabiliza-
tion of the indole-radical species may be attained by intro-
ducing protecting groups into the indole ring. It should be
mentioned in this connection that, when a methyl group
was introduced into the NH moiety of the indole ring as
the N-alkylindole of tub-miepp in 2, no Pt(II)–indole bond
was formed.
Appendix A. Supplementary material
CCDC 669313 contains the supplementary crystallo-
graphic data for complex 1. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supple-
mentary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2008.01.046.
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