Reactivity of the indole ring in palladium(II) complexes of 2N1O-donor ligands: Cyclopalladation and π-cation radical formation

Article abstract of DOI:10.1021/ja031587v

The Pd(II) complexes of new 2N10-donor ligands containing a pendent indole, 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-iepp), 1-methyl-3-[N-2-pyridyl-methyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino] ethylindole (Htbu

Full text of DOI:10.1021/ja031587v

Published on Web 05/18/2004
Reactivity of the Indole Ring in Palladium(II) Complexes of
2N1O-Donor Ligands: Cyclopalladation and π-Cation Radical
Formation
Takeshi Motoyama,† Yuichi Shimazaki,*,‡ Tatsuo Yajima,† Yasuo Nakabayashi,†
Yoshinori Naruta,‡ and Osamu Yamauchi*,†
Contribution from the Unit of Chemistry, Faculty of Engineering, Kansai UniVersity, Suita,
Osaka 564-8680, Japan, and Institute for Materials Chemistry and Engineering,
Kyushu UniVersity, Higashi-ku, Fukuoka 812-8581, Japan
Received December 9, 2003; E-mail: yshima@ms.ifoc.kyushu-u.ac.jp; osamuy@ipcku.kansai-u.ac.jp
Abstract: The Pd(II) complexes of new 2N1O-donor ligands containing a pendent indole, 3-[N-2-
pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-iepp), 1-methyl-3-[N-2-pyridyl-
methyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-miepp), 3-[N-2-pyridylmethyl-N-2-hy-
droxy-3,5-di(tert-butyl)benzylamino]methylindole (Htbu-impp), and 3-(N-2-pyridylmethyl-N-4-hydroxyben-
zylamino)ethylindole (Hp-iepp) (H denotes a dissociable proton), were synthesized, and the structures of
[Pd(tbu-iepp)Cl] (1a), [Pd(tbu-iepp-c)Cl] (1b), [Pd(tbu-miepp)Cl] (3), and [Pd(p-iepp-c)Cl] (4) (tbu-iepp-c
and p-iepp-c denote tbu-iepp and p-iepp bound to Pd(II) through a carbon atom, respectively) were
determined by X-ray analysis. Complexes 1a prepared in CH2Cl2/CH3CN and 3 prepared in CH3CN have
a pyridine nitrogen, an amine nitrogen, a phenolate oxygen, and a chloride ion in the coordination plane.
Complex 1b prepared in CH3CN has the same composition as 1a and was revealed to have the C2 atom
of the indole ring bound to Pd(II) with the Pd(II)-C2 distance of 1.973(2) Å. The same Pd(II)-indole C2
bonding was revealed for 4. Interconversion between 1a and 1b was observed for their solutions, the
equilibrium being dependent on the solvent used. Reaction of 1b and 4 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 ∼550 nm, indicating the formation of the Pd(II)-indole π-cation radical species. The
half-life, t1/2, of the indole radical species at room temperature was calculated to be 20 s (kobs ) 3.5 × 10-2
s-1) for 1b. The cyclic voltammogram for 1b in DMF gave two irreversible oxidation peaks at Epa ) 0.68
and 0.80 V (vs Ag/AgCl), which were ascribed to the oxidation processes of the coordinated indole and
phenolate moieties, respectively.
Introduction
through the amino and carboxylate groups and may undergo
intramolecular stacking interactions9-11 as typically revealed for
Aromatic amino acid residues play vital roles in the stabiliza-
tion and functions of proteins.1-3 The tyrosyl residue serves as
an important metal binding site and forms the metal-phenoxyl
radical species as has been established for Cu-containing
galactose oxidase.4,5 On the other hand, the indole ring of the
tryptophyl residue with the highest hydrophobicity among
R-amino acids6 is known to form a hydrophobic environment
for specific binding of molecules and to be involved in electron-
transfer pathways.7,8 Tryptophan (Trp) coordinates to metal ions
[Cu(L)(Trp)]ClO4 (L ) 2,2′-bipyridine12 or 1,10-phenanthro-
line13). The indole ring is not known to be involved in metal
binding in biological systems, but it can form various metal-
indole bonds in chemical systems. We reported earlier that
alkylindoles coordinate to Pd(II) through the imine nitrogen
atom of the tautomeric 3H-indole ring where the NH hydrogen
atom is bound at the C3 atom.14 Pd(II)-C3 bond formation was
observed for the complex of indole-3-acetate (IA) in the 3H-
indole form, [Pd2(IA)2(py)2] (py ) pyridine), where Pd(II) binds
with IA to form a spiro ring through the carboxylate oxygen
† Kansai University.
‡ Kyushu University.
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K.; Petsko, G. A. AdV. Protein Chem. 1988, 39, 125-189. (c) Burley, S.
K.; Petsko, G. A. J. Am. Chem. Soc. 1986, 108, 7995-8001. (d) Serrano,
L.; Bycroft, M.; Fersht, A. R. J. Mol. Biol. 1991, 218, 465-475. (e) Hunter,
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9
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J. AM. CHEM. SOC. 2004, 126, 7378-7385
10.1021/ja031587v CCC: $27.50 © 2004 American Chemical Society

Indole Ring in Pd(II) Complexes of 2N1O-Donor Ligands
A R T I C L E S
and C3 atoms with deprotonation.15 Kostic´ et al. developed
unique artificial peptidases using Pd(II) and Pt(II) which
recognize the indole moiety and cleave the adjacent amide or
peptide bond specifically.16 Other examples are also known for
Pd(II)-indole C2 bonding as a result of a cyclopalladation-
like reaction.17 The pendent indole ring in metal complexes of
tripodal ligands, where one of the coordinating groups was
replaced with an indole ring, was found to bind with Cu(I) by
η2-type coordination of the C2dC3 bond to form a tetrahedral
structure.18
On the other hand, formation of the indolyl radical from a
tryptophyl residue in proteins has been established for the
intermediate, compound I, formed in the catalytic reaction of
cytochrome c peroxidase (CcP).4,7,8,19,20 Indolyl radical formation
is possible with other biological systems,4 and more recently
Gray et al. reported the formation of the photogenerated
tryptophan radical in modified azurins.21 Indole radicals in
chemical systems have been generated by pulse radiolysis,22
but their characterization has been difficult because of the
instability. We observed previously that the Cu(I) complex of
the N,N-bis(3-indolyl)methyl derivative of 2-aminomethylpyr-
idine in CH2Cl2 reacted with dioxygen to form a CuIII2(µ-O)2
intermediate, giving a compound with a bis(indolyl) moiety with
a spiro ring structure as a decomposition product.23 This strongly
suggested that the bis(indolyl) structure resulted from the
coupling of two vicinal indolyl radicals, although we could not
detect radical species in the course of the reaction.
With these points in mind, we now investigated the behavior
of the pendent indole ring in the Pd(II) complexes of 2N1O-
donor ligands involving a phenol, a pyridine, and a tertiary
amine as metal binding sites (Figure 1). The indole ring
exhibited a stacking interaction with the coordinated pyridine
ring and replaced the phenolate group under suitable conditions
to form complexes with a direct Pd(II)-C2 bond, which were
shown to give the indole π-cation radical species upon oxidation
with cerium(IV).
Figure 1. Structures of ligands.
3-[N-2-Pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]-
ethylindole (Htbu-iepp). To a solution of 3,5-di(tert-butyl)salicylal-
dehyde25 (2.34 g, 10 mmol) and 3-(N-2-pyridylmethylamino)ethylin-
dole23,26 (2.6 g, 10 mmol) in methanol (50 mL) was carefully added
sodium cyanoborohydride (0.63 g, 10 mmol). The resulting solution
was stirred for 6 h to give a white powder, which was recrystallized
from CH3COOC2H5. Yield: 2.14 g (45%). 1H NMR (400 MHz,
CDCl3): δ (vs TMS) 10.83 (br, 1H), 8.52 (d, 1H), 7.92 (s, 1H), 7.54
(t, 1H), 7.20 (m, 4H), 7.11 (m, 2H), 7.00 (t, 1H), 6.98 (s, 1H), 6.95 (s,
1H), 3.93 (s, 2H), 3.89 (s, 2H), 2.97 (m, 4H), 1.44 (s, 9H), 1.28 (s,
9H).
Experimental Section
Materials. Indole and sodium cyanoborohydride were obtained from
Tokyo Kasei, triethylamine was from Wako, and PdCl2 was from
Aldrich. All of the chemicals used were of the highest grade available
and were further purified whenever necessary.24 Solvents were also
purified before use by standard methods.24 DMSO-d6 was purchased
from the Cambridge Isotope Laboratory.
3-[N-2-Pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]-
methylindole (Htbu-impp). Indole (0.73 g, 6.2 mmol) and N-2-
pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamine25 (2.03 g,
6.22 mmol) were dissolved in methanol (50 mL), to which an aqueous
solution of formaldehyde (0.50 g, 6.2 mmol) and a small amount of
acetic acid were added. The resulting solution was stirred for 6 h to
give a white powder, which was recrystallized from CH3COOC2H5.
(15) Takani, M.; Masuda, H.; Yamauchi, O. Inorg. Chim. Acta 1995, 235, 367-
374.
1
Yield: 2.0 g (72%). H NMR (400 MHz, CDCl3): δ (vs TMS) 11.04
(16) (a) Kaminskaia, N. V.; Johnson, T. W.; Kostic´, N. M. J. Am. Chem. Soc.
1999, 121, 8663-8664. (b) Kaminskaia, N. V.; Ullmann, G. M.; Fulton,
D. B.; Kostic´, N. M. Inorg. Chem. 2000, 39, 5004-5013. (c) Kaminskaia,
N. V.; Kostic´, N. M. Inorg. Chem. 2001, 40, 2368-2377. (d) Milovic´, N.
M.; Kostic´, N. M. Met. Ions Biol. Syst. 2001, 38, 145-186.
(17) Robson, R. Inorg. Chim. Acta 1982, 57, 71-77.
(br, 1H), 8.56 (d, 1H), 8.16 (s, 1H), 7.65 (m, 2H), 7.36 (d, 2H), 7.27
(d, 2H), 7.14 (m, 3H), 6.85 (d, 1H), 3.93 (s, 2H), 3.84 (s, 2H), 3.83 (s,
2H), 1.46 (s, 9H), 1.26 (s, 9H).
1-Methyl-3-[N-2-Pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)-
benzylamino]ethylindole (Htbu-miepp). Htbu-miepp was prepared in
a manner similar to that described for Htbu-iepp from 3,5-di(tert-butyl)-
salicylaldehyde25 (2.3 g, 10 mmol) and 1-methyl-3-(N-2-pyridylmethyl-
amino)ethylindole (2.7 g, 10 mmol).26,27 Yield: 3.6 g (75%). 1H NMR
(400 MHz, CDCl3): δ (vs TMS) 8.46 (d, 1H), 7.65 (s, 1H), 7.52 (t,
(18) Shimazaki, Y.; Yokoyama, H.; Yamauchi, O. Angew. Chem., Int. Ed. 1999,
38, 2401-2403.
(19) Sivaraja, M.; Goodin, D. B.; Smith, M.; Hoffman, B. M. Science 1989,
245, 738-740.
(20) Poulos, T. L.; Fenna, R. E. Met. Ions Biol. Syst. 1994, 30, 25-75.
(21) Di Bilio, A. J.; Crane, B. R.; Wehbi, W. A.; Kiser, C. N.; Abu-Omar, M.
M.; Carlos, R. M.; Richards, J. H.; Winkler, J. R.; Gray, H. B. J. Am.
Chem. Soc. 2001, 123, 3181-3182.
(22) (a) DeFelippis, M. R.; Murthy, C. P.; Broitman, F.; Weinraub, D.; Faraggi,
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Getoff, N.; Surdhar, P. S.; Armstrong, D. A.; Singh, A. J. Phys. Chem.
1991, 95, 3639-3643.
(25) (a) Shimazaki, Y.; Huth, S.; Hirota, S.; Yamauchi, O. Bull. Chem. Soc.
Jpn. 2000, 73, 1187-1195. (b) Shimazaki, Y.; Huth, S.; Odani, A.;
Yamauchi, O. Angew. Chem., Int. Ed. 2000, 39, 1666-1669.
(26) Yajima, T.; Shimazaki, Y.; Ishigami, N.; Odani, A.; Yamauchi, O. Inorg.
Chim. Acta 2002, 337, 193-202.
(27) Kuehne, M. E.; Huebner, J. A.; Matsko, T. H. J. Org. Chem. 1979, 44,
2477-2480.
(23) Shimazaki, Y.; Nogami, T.; Tani, F.; Odani, A.; Yamauchi, O. Angew.
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(24) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals; Pergamon Press: Elmsford, NY, 1966.
9
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A R T I C L E S
Motoyama et al.
1H), 7.25 (m, 2H), 7.15 (t, 1H), 7.11 (q, 1H), 6.96 (t, 1H), 6.87 (t,
2H), 6.79 (s, 1H), 3.91 (s, 2H), 3.85 (s, 2H), 3.69 (s, 3H), 2.95 (m,
4H), 1.43 (s, 9H), 1.41 (s, 9H).
(d, 1H), 4.31 (d, 1H), 4.20 (d, 1H), 3.68 (d, 1H), 3.59 (td, 2H), 3.22
(d, 1H), 3.14 (d, 1H). Anal. Calcd for 5 (C21H29N2OClPd): C, 53.97;
1
H, 6.25; N, 5.99. Found: C, 53.54; H, 6.23; N, 5.84. H NMR (400
MHz, DMSO-d6): δ (vs TMS) 8.67 (d, 1H), 8.02 (t, 1H), 7.63 (d,
1H), 7.48 (t, 1H), 7.01 (d, 2H), 6.96 (br, 1H), 6.89 (d, 1H), 4.66 (q,
1H), 4.06 (m, 2H), 3.45 (t, 1H), 1.35 (s, 9H), 1.21 (s, 9H).
3-(N-2-Pyridylmethyl-N-4-hydroxybenzylamino)ethylindole
(Hp-iepp). Hp-iepp was prepared in a manner similar to that described
for Htbu-iepp from 4-hydroxybenzaldehyde (1.2 g, 10 mmol) and 3-(N-
2-pyridylmethylamino)ethylindole (2.6 g, 10 mmol). Yield: 2.02 g
X-ray Structure Determination. The X-ray experiments were
carried out for the well-shaped single crystal of complex 1b on a Rigaku
RAXIS imaging plate area detector with graphite monochromated Mo
KR radiation (λ ) 0.71073 Å). 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 an oscillation
angle of 3° and an exposure time of 3 min. The X-ray experiments for
1a, 3, and 4 were carried out on a Rigaku MSC Mercury CCD
diffractometer with graphite monochromated Mo KR radiation (λ )
0.71073 Å). For the determination of the cell constants and orientation
matrix, six oscillation photographs were taken for each frame with an
oscillation angle of 0.3° and an exposure time of 10 s. Intensity data
were collected by taking oscillation photographs. Refraction data were
corrected for both Lorentz and polarization effects. The structures were
solved by the direct method and refined anisotropically for non-
hydrogen atoms by full-matrix least-squares calculations except for the
disordered terminal carbons of one of the tert-butyl groups in complex
3. The treatment of the disordered carbons in 3 was made in such a
way that the six apparent carbons were placed on the tert-butyl group,
and the occupancy of each carbon was obtained by calculation. All of
the disordered carbons were refined isotropically. 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 literature.28 Except for the hydrogen atoms of the phenol
OH group and the disordered carbons in 3, all hydrogen atoms were
located at the calculated positions, assigned a fixed displacement, and
constrained to ideal geometry with C-H ) 0.95 Å and N-H ) 0.90
Å. The thermal parameters of calculated hydrogen atoms were related
to those of their parent atoms by U(H) ) 1.2Ueq(C,N). The hydrogen
atoms of the phenol OH groups were located from the difference Fourier
maps, while no hydrogen atoms were assigned to the disordered carbons
in 3. All of the calculations were performed by using the TEXSAN
crystallographic software program package from the Molecular Structure
Corp.29 Summaries of the fundamental crystal data and experimental
parameters for the structure determination of complexes 1a, 1b, 3, and
4 are given in Table 1.
1
(57%). H NMR (400 MHz, DMSO-d6): δ (vs TMS) 10.72 (br, 1H),
9.29 (s, 1H), 8.45 (d, 1H), 7.70 (t, 2H), 7.46 (d, 2H), 7.20 (m, 2H),
7.01 (t, 2H), 6.87 (t, 2H), 6.70 (d, 2H), 3.16 (s, 2H), 2.70 (t, 2H), 2.67
(t, 2H), 2.50 (s, 2H).
N-2-Pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamine
(Htbu-pep). To a solution of 2-pyridylmethylamine (1.08 g, 10 mmol)
in methanol (100 mL) was added 3,5-di(tert-butyl)salicylaldehyde (2.34
g, 10 mmol), and to the resulting solution was carefully added sodium
tetrahydroborate (0.30 g, 8 mmol) with stirring. The reaction mixture
was then stirred for 12 h at room temperature, acidified by addition of
concentrated HCl, and evaporated almost to dryness under a reduced
pressure. The residue was dissolved in saturated aqueous Na2CO3 (50
mL) and extracted with three 100-mL portions of CHCl3. The combined
extracts were dried over Na2SO4 and evaporated almost to dryness under
a reduced pressure to give a white powder, which was recrystallized
from diethyl ether. Yield: 2.22 g (68%). 1H NMR (400 MHz, DMSO-
d6): δ (vs TMS) 8.53 (d, 1H), 7.78 (t, 1H), 7.37 (d, 1H), 7.28 (t, 1H),
7.08 (d, 1H), 6.85 (d, 1H), 3.87 (s, 2H), 3.84 (s, 2H), 1.35 (s, 9H),
1.22 (s, 9H).
[Pd(tbu-iepp)Cl] (1a). To a solution of Htbu-iepp (0.47 g, 1.0 mmol)
in 1:1 (v/v) CH2Cl2/CH3CN (20 mL) was added PdCl2 (0.18 g, 1.0
mmol). A few drops of triethylamine were added to the resulting
solution, which was stirred overnight at room temperature to give orange
crystals. Anal. Calcd for 1a (C31H38N3OClPd): C, 60.99; H, 6.27; N,
6.88. Found: C, 60.88; H, 6.31; N, 6.89. 1H NMR (400 MHz, DMSO-
d6): δ (vs TMS) 10.74 (s, 1H), 8.69 (d, 1H), 8.11 (t, 1H), 7.77 (d,
1H), 7.52 (t, 1H), 7.23 (d, 1H), 7.10 (s, 1H), 7.09 (s, 1H), 6.96 (t, 1H),
6.88 (d, 1H), 6.71 (m, 2H), 5.47 (d, 1H), 4.64 (d, 1H), 4.48 (d, 1H),
3.75 (d, 1H), 2.99 (m, 2H), 2.85 (m, 1H), 2.70 (m, 1H), 1.36 (s, 9H),
1.22 (s, 9H).
[Pd(tbu-iepp-c)Cl] (1b). To a suspension of Htbu-iepp (0.47 g, 1.0
mmol) in CH3CN (20 mL) was added PdCl2 (0.18 g, 1.0 mmol). A
few drops of triethylamine were added to the resulting solution, which
was refluxed overnight to give yellow crystals. Anal. Calcd for 1b
(C31H38N3OClPd): C, 60.99; H, 6.27; N, 6.88. Found: C, 60.85; H,
1
Spectroscopies. Electronic spectra were measured with a Shimadzu
UV-3101PC spectrophotometer. 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 a 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.
6.41; N, 6.89. H NMR (400 MHz, DMSO-d6): δ (vs TMS) 9.52 (s,
1H), 8.44 (d, 1H), 8.34 (s, 1H), 8.09 (d, 1H), 7.65 (t, 1H), 7.36 (q,
1H), 7.29 (q, 1H), 7.15 (m, 2H), 6.84 (m, 3H), 4.74 (d, 1H), 4.43 (d,
1H), 4.07 (q, 2H), 3.77 (m, 1H), 3.41 (m, 1H), 1.23 (s, 9H), 1.20 (s,
9H).
[Pd(tbu-impp)Cl] (2), [Pd(tbu-miepp)Cl] (3), [Pd(p-iepp-c)Cl]‚
CH3CN (4), and [Pd(tbu-pp)Cl] (5). These complexes were prepared
in a manner similar to that described for 1b as orange crystals (2, 3,
and 5) and yellow crystals (4), respectively. Anal. Calcd for 2 (C30H36N3-
OClPd): C, 60.41; H, 6.08; N, 7.04. Found: C, 60.38; H, 6.08; N,
1
Collection of NMR Data for the van’t Hoff Plot. The H NMR
data were measured for each of five 1-mL aliquots of 1 mM 1b in
DMSO-d6 after keeping them in a bath thermostated at 20, 30, 40, 50,
and 60 °C, respectively. The average values of concentrations of 1a,
which were calculated from the data collected from five independent
measurements, were used for the van’t Hoff plot.
Electrochemistry. Redox potentials of 1a and 1b (1.0 mM) in dried
DMF containing 0.1 M tetra-n-butylammonium perchlorate (TBAP)
as supporting electrolyte were determined at room temperature under
deaerated conditions by cyclic voltammetry using a BAS 100B
electrochemical analyzer with a three-electrode system. A glassy-carbon
and a platinum wire were used as the working electrode and the counter
electrode, respectively. The reversibility of the electrochemical processes
1
7.05. H NMR (400 MHz, DMSO-d6): δ (vs TMS) 10.97 (br, 1H),
8.22 (d, 1H), 8.04 (d, 1H), 7.59 (t, 1H), 7.28 (d, 1H), 7.10 (m, 3H),
7.01 (m, 3H), 6.91 (t, 1H), 4.94 (d, 1H), 4.80 (d, 1H), 4.17 (d, 1H),
4.00 (d, 1H), 3.60 (d, 1H), 3.47 (d, 1H), 1.44 (s, 9H), 1.26 (s, 9H).
Anal. Calcd for 3 (C32H40N3OClPd): C, 61.54; H, 6.46; N, 6.73.
1
Found: C, 61.48; H, 6.48; N, 6.75. H NMR (400 MHz, DMSO-d6):
δ (vs TMS) 8.65 (d, 1H), 8.07 (t, 1H), 7.76 (d, 1H), 7.47 (t, 1H), 7.22
(d, 1H), 7.11 (d, 1H), 7.04 (d, 1H), 7.00 (t, 1H), 6.86 (s, 1H), 6.72 (t,
1H), 6.66 (d, 1H), 5.00 (d, 1H), 4.65 (d, 1H), 4.48 (d, 1H), 3.72 (d,
1H), 3.61 (s, 3H), 3.37 (d, 1H), 2.97 (m, 3H), 2.67 (m, 1H), 1.37 (s,
9H), 1.23 (s, 9H). Anal. Calcd for 4 (C24H23N3OClPd‚CH3CN): C,
55.57; H, 4.85; N, 10.37. Found: C, 55.54; H, 4.83; N, 10.44. 1H NMR
(400 MHz, DMSO-d6): δ (vs TMS) 9.45 (br, 1H), 9.45 (br, 1H), 8.53
(d, 1H), 7.83 (t, 1H), 7.30 (m, 6H), 6.84 (m, 2H), 6.47 (d, 2H), 4.75
(28) Ibers, J. A.; Hamilton, W. C. International Tables for X-ray Crystallography;
Kynoch: Birmingham, 1974; Vol. IV.
(29) Crystal Structure Analysis Package, Molecular Structure Corp., 1985 and
1999.
9
7380 J. AM. CHEM. SOC. VOL. 126, NO. 23, 2004

Indole Ring in Pd(II) Complexes of 2N1O-Donor Ligands
A R T I C L E S
Table 1. Crystallographic Data
1a
1b
3
4
Pd2C52H53N9O2Cl2
formula
PdC31H38N3OCl
610.51
orange, platelet
0.45 × 0.16 × 0.02
orthorhombic
17.276(5)
PdC31H38N3OCl
610.51
yellow, prism
0.12 × 0.23 × 0.21
monoclinic
8.5354(6)
PdC32H40N3OCl
624.54
orange, needle
0.18 × 0.03 × 0.05
monoclinic
11.777(2)
formula weight
crystal color, habit
crystal dimensions (mm)
crystal system
a (Å)
1119.76
yellow, prism
0.26 × 0.12 × 0.10
monoclinic
12.252(2)
14.313(2)
15.292(2)
76.069(9)
69.391(8)
86.72(1)
2435.0(6)
P-1
b (Å)
c (Å)
69.1(1)
9.519(2)
15.3198(9)
21.615(2)
8.450(2)
30.106(6)
R (deg)
â (deg)
87.961(3)
96.9551(9)
γ (deg)
V (Å3)
11 364(17)
Fdd2
16
2824.6(4)
P21/n
4
2974(1)
P21/n
4
space group
Z value
2
D
F(000)
calc (g/cm3)
1.427
5056.00
7.76
55.0
21 911
6015
6015
335
0.038
0.101
1.436
1264.00
7.81
1.395
1296.00
7.43
1.527
1140.00
9.00
55.0
21 652
10 870
10 870
604
0.047
µ (Mo KR)/cm-1
2θmax/deg
55.0
55.0
observed reflns
independent reflns
reflns used
26 676
6438
6438
334
0.030
0.077
23 270
6566
6566
343
0.040
0.108
no. of variables
R1 [I > 2σ(I)]a
R
w (all data)b
0.114
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo| for I > 2σ(I) data. b Rw ) {∑ω(|Fo| - |Fc|)2/∑ωFo }1/2; ω ) 1/σ2(Fo) ) {σc (Fo) + p2/4‚Fo }-1
.
Figure 2. ORTEP view of [Pd(tbu-iepp)Cl] (1a) drawn with the thermal
ellipsoids at the 50% probability level and the atomic labeling scheme.
Figure 3. ORTEP view of [Pd(tbu-miepp)Cl] (3) drawn with the thermal
ellipsoids at the 50% probability level and the atomic labeling scheme.
was evaluated by standard procedures, and all potentials were recorded
against the Ag/AgCl reference electrode which was calibrated with the
ferrocene/ferrocenium redox couple.
coordinated and is without any interactions within the complex
molecule.
The reactions of Htbu-iepp and Hp-iepp with PdCl2 in
CH3CN with refluxing gave [Pd(tbu-iepp-c)Cl] (1b) and [Pd-
(p-iepp-c)Cl]‚CH3CN (4), respectively, as yellow crystals.
Complex 1b was also obtained by refluxing 1a in CH3CN for
12 h and was found to have the same formula as 1a. No such
conversion was observed for 2, where a methylene group instead
of an ethylene group bridges the tertiary nitrogen atom and the
indole ring. Complexes 1b and 4 were disclosed to have the
structures shown in Figures 4 and 5, respectively, where the
Pd(II) ion binds with the C2 atom of the indole ring in addition
to the amine and pyridine nitrogens in a square-planar geometry.
The unit cell of 4 consists of two crystallographically indepen-
dent Pd(II) complexes and three CH3CN molecules. The Pd-
N, Pd-C, and Pd-Cl bond lengths of 1b and 4 (Pd-N ) 2.07-
2.11; Pd(1)-C ) 1.97-1.98; Pd-Cl ) 2.31-2.32 Å) (Table
2) are within the ranges reported for Pd(II) complexes.26,31 As
compared with 1a, 1b and 4 have a longer Pd-N(1) distance,
Results and Discussion
Preparation and Structures of Pd(II) Complexes. 2N1O-
donor tripod-like ligands, Htbu-iepp and Htbu-impp where H
denotes a dissociable proton, reacted with PdCl2 and triethyl-
amine in 1:1 (v/v) CH2Cl2/C2H5OH at room temperature to give
[Pd(tbu-iepp)Cl] (1a) and [Pd(tbu-impp)Cl] (2), respectively,
as orange crystals. A complex containing an N-methyltryptamine
moiety, [Pd(tbu-miepp)Cl] (3), was also obtained as an orange
powder by the reaction in CH3CN at 80 °C. X-ray crystal
structure analysis revealed that 1a and 3 have a mononuclear
square-planar geometry formed by a phenolate oxygen, an amine
nitrogen, a pyridine nitrogen, and a chloride ion, as shown in
Figures 2 and 3, respectively. Complex 2 having a ligand very
similar to that of 1a and 3 is inferred to have the same
coordination structure. The Pd-N and Pd-O bond lengths of
these complexes (Pd-N ) 2.00-2.08; Pd-O ) 1.97-2.00 Å)
(Table 2) correspond well with those reported for Pd(II)
complexes.26,30 The side-chain indole ring of 1a and 3 is not
(30) Barnard, C. T. J.; Russel, M. J. H. In ComprehensiVe Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon: Oxford, 1987; Vol. 5, Chapter 53.
9
J. AM. CHEM. SOC. VOL. 126, NO. 23, 2004 7381

A R T I C L E S
Motoyama et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1, 3, and 4
4
1a
1b
3
molecule 1
molecule 2
Bond Lengths
Pd-N(1)
Pd-N(2)
Pd-O(1)
Pd-C(1)
Pd-Cl
2.011(4)
2.044(4)
1.985(3)
2.092(2)
2.078(2)
2.008(3)
2.039(3)
1.975(2)
2.103(3)
2.074(3)
2.113(3)
2.072(3)
1.973(2)
2.3174(6)
1.980(3)
2.3198(9)
1.983(3)
2.3111(9)
2.317(1)
2.3255(8)
Bond Angles
N(1)-Pd-N(2)
N(1)-Pd-O(1)
N(1)-Pd-C(1)
N(1)-Pd-Cl
N(2)-Pd-O(1)
N(2)-Pd-C(1)
N(2)-Pd-Cl
O(1)-Pd-Cl
C(1)-Pd-Cl
83.1(2)
175.1(1)
82.95(7)
83.5(1)
176.8(1)
81.7(1)
82.2(1)
173.56(8)
94.37(5)
172.5(1)
93.82(9)
173.3(1)
94.44(9)
95.2(1)
94.1(1)
94.66(8)
93.8(1)
90.77(8)
176.64(5)
91.9(1)
175.41(8)
91.3(1)
176.56(8)
176.2(1)
87.8(1)
177.71(8)
88.12(7)
91.96(6)
92.6(1)
92.1(1)
Table 3. Absorption and Cyclic Voltammetric Data for Pd(II)
Complexes in DMF
wavelength/nm (ꢀ/M-1 cm-1
)
E/V vs Ag/AgCl
tbu-iepp (1a)
tbu-iepp (1b)
tbu-impp (2)
tbu-miepp (3)
p-iepp (4)
450 (940, sh)
340 (4100, sh)
290 (14000)
340 (3900, sh)
340 (3900, sh)
295 (9200)
0.97
0.68
0.89
1.08
0.80
1.01
450 (920, sh)
450 (900, sh)
0.70
0.83
in the intramolecular π-π stacking. The shortest Cindole-Cpyridine
distance is 3.172(3) Å for 1b and 3.060(5) and 3.133(5) Å for
4, and the angle between the average planes of the two aromatic
rings is 33.1° for 1b and 37.5° and 58.4° for 4. It is interesting
to note in this connection that the side-chain phenol and indole
rings stack only with the coordinated pyridine ring and do not
stack with each other. This finding corresponds well with our
previous observation on analogous complexes.26
Figure 4. ORTEP view of [Pd(tbu-iepp-c)Cl] (1b) drawn with the thermal
ellipsoids at the 50% probability level and the atomic labeling scheme.
Characterization of the Complexes. (a) Spectral Proper-
ties. The absorption spectra of 1b and 4 in DMF in the range
250-1000 nm exhibited a strong peak at 290 nm (ꢀ ) 14 000)
and 295 nm (ꢀ ) 9200), respectively, while 1a, 2, and 3 showed
peaks at 340 and 450 nm (Table 3), supporting that 2 has the
same coordination structure as that of 1a and 3. Because the
450-nm peak is assigned to the phenolate-to-Pd(II) charge-
transfer band (LMCT),26 lack of this peak in 1b and 4 indicates
that the phenolate oxygen is not coordinated in these complexes
in DMF, supporting that the Pd(II)-indole C2 bond is main-
Figure 5. ORTEP view of [Pd(p-iepp-c)Cl] (4) drawn with the thermal
ellipsoids at the 50% probability level and the atomic labeling scheme.
which may be ascribed to the difference in the trans effects of
Pd-O and Pd-C bonds. The indole C2-C3 bond length in 1b
(1.379(3) Å) and 4 (1.375(5) and 1.374(6) Å) is only slightly
longer than that of uncoordinated indole ring,18,23,26 which is
reminiscent of the corresponding bond length of an η2-
coordinated indole (1.379(7) Å).18 The Pd(II)-C distance in
1b and 4 is much shorter than that of the reported metal-indole
complexes (Pd(II)-C3 ) 2.12-2.15;15 Cu(I)-C2 and Cu(I)-
C3 ) 2.23-2.27 Å18) but is within the normal range for other
cyclopalladated complexes.31-33 The phenol ring of 1b and 4
is located above the coordinated pyridine ring to be involved
1
tained in solution as well as in the solid state. The H NMR
chemical shifts for 1a and 5 which is without a pendent aromatic
ring are very similar, and this supports that the side-chain indole
ring is not stacked with the coordinated pyridine ring in 1a. On
the other hand, large shift differences were observed between
the indole protons of 1a and 1b in DMSO; the signals of indole
C4, C5, and C7 protons in 1b shifted downfield relative to those
of 1a with the chemical shift difference, ∆δ ) -(δ - δ1a), of
-0.1 to -0.6 ppm. These shifts which were also observed for
4 reflect σ-donation to Pd(II) by the indole ring and are in
contrast with the rather small differences observed between the
Cu(I)-η2-coordinated indole and free indole.18 A large upfield
shift was detected for the indole NH proton (∆δ ) 2.40 ppm).
(31) (a) Newkome, G. R.; Puckett, W. E.; Gupta, V. K.; Kiefer, G. E. Chem.
ReV. 1986, 86, 451-489. (b) Yoneda, A.; Hakushi, T.; Newkome, G. R.;
Morimoto, Y.; Yasuoka, N. Chem. Lett. 1994, 175-176. (c) Orpen, A. G.;
Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem.
Soc., Dalton Trans. 1989, S1-S83. (d) Newkome, G. R.; Puckett, W. E.;
Kiefer, G. E.; Gupta, V. K.; Fronczek, F. R.; Pantaleo, D. C.; McClure, G.
L.; Simpson, J. B.; Deutsch, W. A. Inorg. Chem. 1985, 24, 811-826.
(32) Omae, I. Organometallic Intramolecular-Coordination Compounds; Elsevier
Science Publishers: Amsterdam, NY, 1986.
(33) (a) Crabtree, R. H. Chem. ReV. 1985, 85, 245-269. (b) Alsters, P. L.; Engel,
P. F.; Hogerheide, M. P.; Copijn, M.; Speck, A. L.; van Koten, G.
Organometallics 1993, 12, 1831-1844.
9
7382 J. AM. CHEM. SOC. VOL. 126, NO. 23, 2004

Indole Ring in Pd(II) Complexes of 2N1O-Donor Ligands
A R T I C L E S
Table 4. 1H NMR Chemical Shifts (δ/ppm) and Upfield Shifts
(∆δ)a of Pyridine and Indole Proton Signals for Pd(II) Complexes
in DMSO-d6
substitution mechanism.34 The rate-determining step in such
processes is the C-H bond cleavage, which is normally regarded
as an irreversible process.34,36 In reversible cyclopalladation
reactions of aromatic ligands, on the other hand, kinetically
controlled isomers have been reported to be different from
thermodynamically controlled isomers,35 which explains the
present structural conversion where the phenolate complex 1a
is kinetically favored and the cyclopalladated complex 1b is
thermodynamically favored in CH3CN. Our results indicate that
the C-H bond scission is a reversible step in the present
interconversion between 1a and 1b. The energy gap between
1a and 1b, ∆E, was estimated from the van’t Hoff plot using
δ (∆δ)/ppm
tbu-iepp (1a) tbu-iepp (1b)
tbu-impp (2)
tbu-miepp (3)
tbu-pp (5)
py3
py4
py5
py6
in1
in2
in4
in5
in6
in7
7.77 7.16 (+0.61) 7.01 (+0.76) 7.76 (+0.01) 7.63 (+0.14)
8.11 7.65 (+0.46) 7.59 (+0.52) 8.07 (+0.04) 8.02 (+0.09)
7.52 7.15 (+0.37) 6.91 (+0.61) 7.47 (+0.05) 7.48 (+0.04)
8.69 8.44 (+0.25) 8.22 (+0.47) 8.65 (+0.04) 8.67 (+0.02)
10.74 8.34 (+2.40) 10.97 (-0.23)
6.88
7.10 (-0.22) 6.86 (+0.02)
7.22 7.36 (-0.14) 8.04 (-0.82) 7.22 (0.0)
6.72 6.83 (-0.11) 6.91 (-0.19) 6.72 (0.0)
6.96 6.83 (+0.13) 7.10 (-0.14) 7.00 (-0.04)
6.72 7.29 (-0.57) 7.10 (-0.38) 6.66 (+0.06)
1
the H NMR data in DMSO (see Experimental Section) to be
15 kJ mol-1, which is relatively large when compared with the
rather small enthalpy of formation (60-120 kJ mol-1) of a
Pd-C bond by cyclopalladation.34,36
a ∆δ ) -(δ - ∆δ1a), where δ1a refers to the shift for 1a.
Complexes 1b and 2 exhibited upfield shifts of the coordinated
pyridine proton signals relative to those for 1a, and the ∆δ
values (0.25-0.76 ppm) (Table 4) clearly show that the pyridine
ring is stacked with the aromatic ring in solution. The NMR
spectral behavior of 2 is very similar to that of the Pd(II)
complexes of 2N1O-donor ligands with a pendent indole ring
stacked with the pyridine ring.26 These observations substantiate
that the side-chain conformations as well as the coordination
structures are maintained both in the solid state and in solution.
Formation of the Pd(II)-indole σ-bond observed for 1b and
4 may be related with the favorable six-membered chelate ring
formed by the indole C2 and amine nitrogen atoms. We infer
from the structures of 1b and 2 that the observed structural
difference is a consequence of the steric requirements for the
side-chain indole ring to approach the Pd center.18 Considering
that complex 3 containing an N-methylindole moiety instead
of the NH-indole in 1a did not undergo the conversion to Pd-
indole species under a similar condition of 1b formation, the
conversion may require dissociation of the indole-NH proton,
which is considered to be analogous to the tautomeric proton
migration giving the 3H-indole ring in some Pd complexes with
Pd(II)-C3 and -N1 bindings.14,15,17
Redox Properties. Oxidation of 1b by 1 equiv of Ce(IV) in
DMF at -60 °C caused a color change from yellow to green,
giving a new peak at 550 nm in the visible absorption spectrum
(Figure 6). The reaction was found to be a one-electron oxidation
process from the reaction stoichiometry. Complex 4 was also
oxidized to exhibit a similar absorption peak at 551 nm. The
ESR spectra of oxidized 1b and 4 exhibited a new sharp signal
at g ) 2.004, and the amount of unpaired electron was calculated
to be more than 0.90 from the integrated spectrum. On the other
hand, the absorption spectra of 1a and 3 remained unchanged
upon addition of Ce(IV), and no peaks characteristic of the
phenoxyl radical25,37,38 were observed. The oxidized species of
(b) Structural Changes. As expected from the conditions
for isolation of 1a and 1b, conversion of 1a to 1b occurred in
solution with the structural changes shown in Scheme 1. When
the solution of 1a in CH3CN/CH2Cl2 was kept at room
temperature for a few days, two kinds of crystals, orange crystals
of 1a and yellow crystals of 1b, separated. The solution of 1b
in DMSO, on the other hand, exhibited a color change from
yellow to orange with the appearance of a 450-nm peak of 1a
upon standing for 1 day at room temperature, indicating the
conversion from 1b to 1a. This change was not observed in
CH3CN and CH2Cl2. Dependence of the interconversion on
solvents may be due to the solubility of the complexes; 1b is
much less soluble in CH3CN/CH2Cl2 than in DMSO and is not
converted to 1a in this solvent mixture, whereas conversion from
1a to 1b proceeds because 1b crystallizes out of the solution.
1
The H NMR spectra indicated that the ratio of 1a to 1b was
1:0.3 in DMSO after 24 h at room temperature and 1:1 at 60
°C. Because of decomposition of 1b, complete conversion of
1b to 1a was not possible at 100 °C and higher temperatures.
To understand the mechanism of the conversion, we carried out
an isotope-labeling experiment; deuterated 1b, 1b-dphenol with
the deuterated phenol moiety, was converted in DMSO-d6 at
70 °C to 1a-dindole whose indole C2 position was over 85%
deuterated, but nondeuterated 1b did not give any indole-
deuterated complex. The reaction of 1a in 2:1 (v/v) CD3CN/
D2O gave 1b with the nondeuterated phenol moiety. No such
conversion was detected for complex 4, where coordination of
the intramolecular phenolate oxygen is not possible. These
results suggest that the phenol OH group takes part in the
process of the structural conversion between 1a and 1b, where
the chelate effect is inferred to be important and the phenolate
oxygen abstracts the indole C2 proton to form 1b. Complex 1b
separated from the solution as soon as it was formed, and this
may explain why the phenol OH group was not deuterated in
the above experiment. It is generally agreed that cyclopalladation
reactions of aryl compounds proceed by an electrophilic
(34) (a) Parshall, G. W Acc. Chem. Res. 1970, 3, 139-144. (b) Ryabov, A. D.
Chem. ReV. 1990, 90, 403-424. (c) Beller, M.; Riermeier, T. H.; Haber,
S.; Kleiner, H.-J.; Herrmann, W. A. Chem. Ber. 1996, 129, 1259-1264.
(35) (a) O’Keefe, B. J.; Steel, P. J. Inorg. Chem. Commun. 1999, 2, 10-13. (b)
O’Keefe, B. J.; Steel, P. J. Organometallics 2003, 22, 1281-1292.
(36) Go´mez, M.; Granell, J.; Martinez, M. Eur. J. Inorg. Chem. 2000, 217-
224.
(37) (a) Halfen, J. A.; Young, V. G., Jr.; Tolman, W. B. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1687-1690. (b) Halfen, J. A.; Jazdzewski, B. A.;
Mahapatra, S.; Berreau, L. M.; Wilkinson, E. C.; Que, L., Jr.; Tolman, W.
B. J. Am. Chem. Soc. 1997, 119, 8217-8227. (c) Sokolowski, A.;
Leutbecher, H.; Weyhermu¨ller, T.; Schnepf, R.; Bothe, E.; Bill, E.;
Hildebrandt, P.; Wieghardt, K. J. Biol. Inorg. Chem. 1997, 2, 444-453.
(d) Zurita, D.; Gautier-Luneau, I.; Me´nage, S.; Pierre, J.-L.; Saint-Aman,
E. J. Biol. Inorg. Chem. 1997, 2, 46-55. (e) Jazdzewski, B. A.; Young, V.
G., Jr.; Tolman, W. B. Chem. Commun. 1998, 2521-2522. (f) Benisvy,
L.; Blake, A. J.; Collison, D.; Davis, E. S.; Garner, C. D.; McInnes, E. J.
L.; McMaster, J.; Whittaker, G.; Wilson, C. Chem. Commun. 2001, 1824-
1826. (g) Thomas, F.; Gellon, G.; Gautier-Luneau, I.; Saint-Aman, E.;
Pierre, J.-L. Angew. Chem., Int. Ed. 2002, 41, 3047-3050. (h) Shimazaki,
Y.; Huth, S.; Hirota, S.; Yamauchi, O. Inorg. Chim. Acta 2002, 331, 168-
177.
(38) (a) Sokolowski, A.; Mu¨ller, J.; Weyhermu¨ller, T.; Schnepf, R.; Hildebrandt,
P.; Hildebrandt, K.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 1997,
119, 8889-8900. (b) Sokolowski, A.; Adam, B.; Weyhermu¨ller, T.;
Kikuchi, A.; Hildebrandt, K.; Schnepf, R.; Hildebrandt, P.; Bill, E.;
Wieghardt, K. Inorg. Chem. 1997, 36, 3702-3710. (c) Shimazaki, Y.; Tani,
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9
J. AM. CHEM. SOC. VOL. 126, NO. 23, 2004 7383

A R T I C L E S
Motoyama et al.
Scheme 1. Interconversion between [Pd(tbu-iepp)Cl] Isomers 1a and 1b and Indole Ring Deuteration (tert-Butyl Groups Are Omitted for
Clarity in the Complexes)
1b and 4 are considered to assume one of the three possible
forms, an indole radical, a phenoxyl radical, or a Pd(III)
complex. Mononuclear Pd(III) complexes are relatively uncom-
mon; the two well-characterized examples are [Pd([9]-aneS3)2]3+
([9]-aneS3 ) 1,4,7-trithiacyclononane)39 and [Pd([9]-aneN3)2]3+
([9]-aneN3 ) 1,4,7-triazacyclononane),40 both of which have
been structurally characterized by X-ray analysis. Several others
have been generated electrochemically or otherwise studied in
situ.41,42 It has been reported that a Pd(III) complex, [Pd([9]-
aneN3)2]3+, exhibits the absorption peaks at 383 nm (ꢀ ) 590
M-1 cm-1) and 314 nm (ꢀ ) 1240 M-1 cm-1) and that [Pd-
([9]-aneS3)2]3+ exhibits a peak at 475 nm (ꢀ ) 3000 M-1
cm-1).39,40 The ESR spectra of these Pd(III) complexes showed
Figure 6. Absorption spectral changes of oxidized 1b with time in DMF
a broad isotropic or anisotropic signal based on the 2A1g ground
at -60 °C (5.0 × 10-4 M). The spectra were recorded at 10-s intervals.
spin state in the range g ) 2.005-2.123.39-41 The spectral
properties of oxidized 1b and 4 are very different from those
of the Pd(III) complexes, and the sharp signal at g ) 2.004
exhibited by 1b and 4 indicates the formation of an organic
radical species of the Pd(II) complexes (Figure 7), although the
g value is slightly larger than the value of 2.001 expected for
organic radicals and no nitrogen superhyperfine structures were
observed. While the di(tert-butyl)phenoxyl radical species has
been reported to show a characteristic intense absorption band
at about 400 nm due to the π-π* transition of the phenoxyl
radical,25,37,38 oxidized 1b and 4 did not show this band. When
the green solution of oxidized 1b in DMF was left to stand at
Figure 7. ESR spectrum of one-electron-oxidized 1b in DMF at 77 K.
-60 °C, it rapidly turned yellow, and the 550-nm absorption
Concentration: 5.0 × 10-4 M; microwave power, 1 mW; modulation
peak disappeared in the first-order decay (Figure 6). The half-
life, t1/2, of oxidized 1b and 4 was calculated to be 20 (kobs
amplitude, 0.63 mT.
)
3.5 × 10-2 s-1) and 18 s (kobs ) 3.9 × 10-2 s-1), respectively.
This shows that despite the difference in the phenol ring
substituents oxidized 1b and 4 exhibit similar stability, which
is in contrast with the dependence of the stability of the phenoxyl
radical-metal complexes on phenol ring substituents.25 These
results exclude the possibility that the radical is assigned to the
phenoxyl radical. On the other hand, the indole radicals from
tryptophan and N-methylindole have been reported to show a
characteristic absorption band in the region 510-560 nm,4,22
where the indole-π-cation radical and the neutral indolyl radical
give a band centered at 560 and 510 nm, respectively. On the
basis of these considerations, we assign the oxidized forms of
1b and 4 having a peak at ∼550 nm to the indole π-cation
radical species, whose unpaired electron is inferred to be
(39) (a) Blake, A. J.; Holder, A. J.; Hyde, T. I.; Schro¨der, M. J. Chem. Soc.,
Chem. Commun. 1987, 987-988. (b) Blake, A. J.; Holder, A. J.; Hyde, T.
I.; Roverts, Y. V.; Lavery, A. J.; Schro¨der, M. J. Organomet. Chem. 1987,
323, 261-270.
(40) (a) Blake, A. J.; Gordon, L. M.; Holder, A. J.; Hyde, T. I.; Reid, G.;
Schro¨der, M. J. Chem. Soc., Chem. Commun. 1988, 1452-1454. (b)
McAuley, A.; Whitcombe, T. W. Inorg. Chem. 1988, 27, 3090-3099.
(41) (a) Kirmse, R.; Stach, J.; Dietzsch, W.; Steimecke, G.; Hoyer, E. Inorg.
Chem. 1980, 19, 2679-2685. (b) Chandrasekhar, S.; McAuley, A. Inorg.
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7384 J. AM. CHEM. SOC. VOL. 126, NO. 23, 2004

Indole Ring in Pd(II) Complexes of 2N1O-Donor Ligands
A R T I C L E S
localized in the indole ring. The relatively short half-life of these
radical species indicates that they are less stable than the metal-
phenoxyl and other metal-organic radical species.25,37,38
Table 3 shows the redox potentials of 1a, 1b, 2, and 4
measured in DMF under anaerobic conditions at a scan rate of
100 mV/s in the range 0-1.5 V where no Pd redox wave was
observed. Complexes 1a, 1b, and 4 exhibited two irreversible
oxidation peaks (1a, 0.97 and 1.08 V; 1b, 0.68 and 0.80 V; 4,
0.70 and 0.83 V (vs Ag/AgCl)). Considering that the redox
potential of a Cu(II)-coordinated phenolate oxygen is lower than
that of free phenol and that the potential of the indole ring is
higher than that of the phenol ring,4,22 the first oxidation peak
of 1a is assigned to the oxidation of the coordinated phenolate
moiety and the second one is assigned to that of the indole ring.
On the other hand, the first and second oxidation peaks of 1b
and 4 are assigned to the oxidation of the coordinated indole
and free phenol rings, respectively, because the oxidation
potential of the second peak agrees well with that of the phenol
moiety weakly coordinated to Cu(II).37 The redox potential of
the indole ring of 1b and 4 was found to be lower than that of
1a, which shows that the Pd(II)-bound indole moiety is more
easily oxidized than the unbound one.
The heme site of CcP has two tryptophyl residues, and one
of them, Trp 191, is known to give an indolyl radical species
in the course of the reaction.4,8,20 The active-site structure of
the enzyme43 shows that both coordinated histidine (His 175)
and Trp 191 are hydrogen-bonded to a proximal aspartyl residue
(Asp 235), which should affect the electron density of the indole
ring.8 Pd(II)-indole C2 bonding with deprotonation facilitates
the indole radical formation as seen from the lower redox
potential (Table 3) and may be compared with the Cu(II)-
phenolate oxygen bonding in galactose oxidase. There has been
no information of direct metal-indole interactions in biological
systems, but our results suggest that the indole NH-Asp 235
hydrogen bond may make the indole ring negatively charged
and thus favor the radical formation.
The present findings demonstrate the versatile nature of the
indole ring in the Pd(II) coordination sphere and will provide
further information on its functions in chemical and biological
systems.
Acknowledgment. We gratefully acknowledge the support
of this work by a Grant-in-Aid for Scientific Research (No.
13440202 to O.Y.) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. This work was
supported in part by the Kansai University Grant-in-Aid for
Faculty Joint Research Program, 2002, and the “Nanotechnology
Support Project” of the Ministry of Education, Culture, Sports,
Science, and Technology of Japan, for which we express our
sincere thanks.
Conclusion
We prepared and characterized the Pd(II) complexes of a
series of new tripod-like 2N1O-donor ligands with a phenol
ring and a pendent indole moiety. The direct indole carbon-
Pd(II) bonding has been established by the molecular structure
of 1b and 4, which was also concluded for the solution of 1b
in DMSO-d6 by the downfield shifts of the indole protons due
to effective σ-donation by the C2 atom. Complexes 1a and 1b
were interconvertible in CH3CN and DMSO as shown in
Scheme 1. One-electron chemical and electrochemical oxidations
of the Pd(II)-indole complexes, 1b and 4, yielded the corre-
sponding Pd(II)-indole π-cation radical species, which is
supported by the characteristic 550-nm absorption peak and the
ESR signal of the radical species. To the best of our knowledge,
this is the first observation of the indole π-cation radical species
in metal complexes.
Supporting Information Available: X-ray crystallographic
data (CIF) for complexes 1a, 1b, 3, and 4, and 1H NMR spectra
showing the interconversion. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA031587V
(43) Finzel, B. C.; Poulos, T. L.; Kraut, J. J. Biol. Chem. 1984, 259, 13027-
13036.
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