Regiospecific C(naphthyl)-H bond activation: Isolation and characterization of cyclopalladates

Article abstract of DOI:10.1016/j.ica.2006.10.009

In ethanol medium disodium tetrachloropalladate selectively activates the C8-H bond of naphthyl group present in 1-(2′-hydroxy-5′-methylphenylazo)naphthalene (H₂L¹) at room temperature and produces cyclometallate [PdL¹(PPh₃)] in presence of triphenylphosphine. Under similar reaction condition, the C(naphthyl)-H bond activation of 1-(2′-methoxy-5′-methylphenylazo)naphthalene (HL²) occurs at C2 and cyclopalladate [PdL²Cl] has been isolated as product. The labile nature of the Pd ←:O(R) bond of [PdL²Cl] in solution is established from the electronic and NMR spectra. Cyclopalladate [PdL²Cl] reacts with thallium(I) cyclopentadienide and yields [PdL²(Cp)], where both σ- and π-palladium(II)-carbon bonds exist. All the cyclopalladates have been isolated in pure form and characterized on the basis of their spectral data. The molecular structure of cyclopalladate [PdL¹(PPh₃)] has been determined by single crystal X-ray diffraction method. In [PdL¹(PPh₃)], the metal ion is bonded to C8 (peri-position of 1-azonaphthyl fagment), N1 of diazene function, O1 of phenolic group and P1 of triphenylphosphine. The tetra-coordination around palladium(II) is in a distorted square-planar geometry.

Full text of DOI:10.1016/j.ica.2006.10.009

Inorganica Chimica Acta 360 (2007) 2181–2186
www.elsevier.com/locate/ica
Note
Regiospecific C(naphthyl)–H bond activation: Isolation
and characterization of cyclopalladates
Debatra Narayan Neogi, Achintesh Narayan Biswas, Purak Das,
*
Rupa Bhawmick, Pinaki Bandyopadhyay
Department of Chemistry, University of North Bengal, Siliguri 734013, Darjeeling, West Bengal, India
Received 25 April 2006; received in revised form 2 August 2006; accepted 18 October 2006
Available online 25 October 2006
Abstract
In ethanol medium disodium tetrachloropalladate selectively activates the C8–H bond of naphthyl group present in 1-(20-hydroxy-
50-methylphenylazo)naphthalene(H2L1) at room temperature and produces cyclometallate [PdL1(PPh3)] in presence of triphenylphos-
phine. Under similar reaction condition, the C(naphthyl)–H bond activation of 1-(20-methoxy-50-methylphenylazo)naphthalene (HL2)
occurs at C2 and cyclopalladate [PdL2Cl] has been isolated as product. The labile nature of the Pd :O(R) bond of [PdL2Cl] in solu-
tion is established from the electronic and NMR spectra. Cyclopalladate [PdL2Cl] reacts with thallium(I) cyclopentadienide and yields
[PdL2(Cp)], where both r- and p-palladium(II)-carbon bonds exist. All the cyclopalladates have been isolated in pure form and char-
acterized on the basis of their spectral data. The molecular structure of cyclopalladate [PdL1(PPh3)] has been determined by single
crystal X-ray diffraction method. In [PdL1(PPh3)], the metal ion is bonded to C8 (peri-position of 1-azonaphthyl fagment), N1 of dia-
zene function, O1 of phenolic group and P1 of triphenylphosphine. The tetra-coordination around palladium(II) is in a distorted
square-planar geometry.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Cyclopalladation; Palladium(II)-C(naphthyl) bond; Peri-palladates; Ortho-palladates; Crystal structure
1. Introduction
ions [24]. The naphthalenes with donor bearing substituent
like diazene function at C1 can offer either the ortho- (C2)
or the peri- (C8) position for metallation. Rys et al. have
thoroughly studied [25–27] the cyclopalladation reaction
of 1-(phenylazo)naphthalene and established that the palla-
dium-carbon bond formation occurs at C2 (naphthyl)
exclusively. The peri-position (C8) of 1-(phenylazo)naph-
thalene was palladated by disodium tetracloropalladate,
only when all the ortho-positions of both naphthyl and
phenyl rings were blocked by methyl groups [27].
In continuation of our interest [28–30] in the synthesis
and reactivity of cyclopalladated naphthyl moiety, the
present report deals with the regiospecific C(naphthyl)–H
bond activation of 1-(phenylazo)naphthalene system, mod-
ified by 20-substitution with auxiliary donors (1 and 2), by
disodium tetrachloropalladate at room temperature.
The effect of auxiliary donors on the selectivity of
The regioselective intramolecular C–H bond activation
of aryl ring via cyclometallation and subsequent function-
alization at metallated carbon is of special significance in
organic synthesis [1]. There has been intense interest in
the chemistry of cyclopalladated compounds [2,3], which
have numerous applications in organic synthesis [4–8],
catalysis [9–15], photochemistry [16–20] and metallomeso-
gen chemistry [21–23]. The C(naphthyl)–H bond activation
by palladium(II) following cyclometallation route has
received much less attention in contrast to the C(phenyl)–
H bond activation [2,3], in spite of the fact that the naph-
thalene ring is more reactive towards electrophilic metal
*
Corresponding author. Tel.: +91 35 3269 9425; fax: +91 35 3269 9001.
E-mail address: pbchem@rediffmail.com (P. Bandyopadhyay).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.10.009

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D.N. Neogi et al. / Inorganica Chimica Acta 360 (2007) 2181–2186
C(naphthyl)–H bond activation has been examined The
isolation and characterization of the resulting cyclopalla-
dates have also been described.
rically and held in the riding mode. The last cycles of
refinement included atomic positions for all the atoms,
anisotropic thermal parameters for all non-hydrogen
atoms and isotropic thermal parameters for all the hydro-
gen atoms. Full-matrix-least-squares structure refinement
against jF2j was carried out using SHELXL-97 [36] package
of programs.
4
5
6
7
3
2
2.1.1. Synthesis of [Pd(L1)(PPh3)] (3)
1
A solution of 1-(20-hydroxy-50-methylphenylazo)naph-
thalene (H2L1) (0.06 g, 0.23 mmol) in ethanol (20 mL)
was added drop-wise to an ethanolic solution (20 mL) of
disodium tetrachloropalladate (0.08 g, 0.27 mmol) with
stirring at room temperature.
8
N
N
1'
X
6'
5'
The stirring of the mixture was continued for 2 h and
precipitate started to appear slowly. A solution of triphenyl
phosphine (0.08 g, 0.30 mmol) in benzene (10 mL) was
added drop-wise to the above mixture. The mixture was
further stirred for 0.5 h and the colour of the reaction mix-
ture was changed from red to pink. After the removal of
the solvent, the residue was washed with water and ethanol
respectively and finally dried. The solid mass was dissolved
in dichloromethane and chromatographed on silica gel
(60–120 mesh size). An orange yellow band of unreacted
(1) was eluted by a mixture of petroleum ether and benzene
(1:1, v/v) followed by a pink band of cyclopalladate 3,
which was eluted by pure benzene. The removal of solvent
afforded microcrystals of cyclopalladate [PdL1(PPh3)]
which was finally dried in vaccuo. (Yield: 0.08 g,
0.127 mmol, 55.2%). Anal. Calc. for C35H27N2OPPd: C,
66.83; H, 4.29; N, 4.45. Found: C, 66.78; H, 4.10; N,
4.52%. IR (KBr); m/cmꢀ1): 1433 (N@N), 1099 (Pd bound
2'
3'
H3 C
4'
1 : X = OH; H2L1
2 : X = OMe; HL2
2. Experimental
2.1. Materials and general methods
All chemicals and solvents used for the work were of
reagent grade and used without purification. 1-(20-
hydroxy-50-methylphenylazo) naphthalene (HL1) was
prepared following the reported method [31]. 1-(20-meth-
oxy-50-methylphenylazo) naphthalene (HL2) was prepared
PPh3).
UV/VIS
(UV/VIS
(CH2Cl2),
k/nm
(e,
dm3 molꢀ1 cmꢀ1): 548(9600), 463(7700), 438(6900),
337(7700). 1H NMR in CDCl3 (ppm): 2.27 (s, 3H,
50-Me), 6.32–8.37 (aromatic protons).
by
methylation
of
1-(20-hydroxy-50-methylphenyl-
azo)naphthalene [32]. Thallium (I) cyclopentadieneide
was prepared following reported method [33].
2.1.2. Synthesis of [Pd (L2)Cl] (4)
An orange yellow solution of 1-(20-methoxy-50-meth-
ylphenylazo)naphthalene (HL2) (0.06 g, 0.22 mmol) in
ethanol (20 mL) was added to a brown ethanolic solution
(20 mL) of sodium tetrachloropalladate (0.07 g,
0.24 mmol) with stirring. Green precipitate started to
appear with the stirring, which was continued for 2 h
at room temperature. The green precipitate was collected
by filtration and thoroughly washed with water and eth-
anol respectively. The solid mass was recrystallised from
a mixture of dichloromethane and petroleum ether.
Finally the cyclopalladate [Pd(L2)Cl] (4) was dried in
vacuo (Yield: 0.06 g, 0.144 mmol, 65.4%). Anal. Calc.
for C18H15N2OClPd: C, 51.80; H, 3.60; N, 6.71. Found:
C, 51.42; H, 3.34; N, 6.35%. IR (KBr); m/cmꢀ1): 1452
(N@N). UV/VIS (CH2Cl2), k/nm(e, dm3 molꢀ1cmꢀ1):
604(7820), 574(7620), 430sh(7260), 410(8030), 388(8340),
308(14200). 1H NMR in CDCl3 (ppm): 2.39 (s,3H,
50-Me), 4.18 (s, 3H, 20-OMe) 7.01–8.59 (aromatic
protons).
Elemental microanalyses (C, H and N) were done by
either Perkin–Elmer (Model 240C) or Heraeus Carlo Erba
1108 elemental analyzer. The IR and Electronic spectra
were recorded on Jasco 5300 FT-IR spectrophotometer
and Shimadzu UV-240 spectrophotometer respectively.
NMR spectra were obtained by using Bruker DPX 300
NMR Spectrometer.
A suitable single crystal of [PdL1(PPh3)] (3) was
mounted on a thin glass fiber and transferred to a Sie-
mens Smart-CCD diffractometer equipped with a normal
focus, 2.4 kW sealed tube X-ray source (MoKa radiation,
˚
k = 0.71073 A) operating at 50 kV and 40 mA. The struc-
ture was solved by direct methods using SHELXS-97 [34]
and difference Fourier syntheses. An empirical absorption
correction based on symmetry equivalent reflections was
applied for the compound using SADABS [35] program.
All the hydrogen positions for the compound were ini-
tially located in the difference Fourier map, and for the
final refinement, the hydrogen atoms were placed geomet-

D.N. Neogi et al. / Inorganica Chimica Acta 360 (2007) 2181–2186
2183
2.1.3. Synthesis of [Pd(L2)(Cp)] (5)
3. Results and discussion
Solid thallium cyclopentadienide (TlCp) (0.036 g,
0.134 mmol) was added to a solution of [Pd(L2)Cl] (4)
(0.04 g, 0.096 mmol) in benzene (40 mL) and the mixture
was stirred for 2 h at room temperature. The colour of
the solution became deep reddish violet. The precipitate
of thallium chloride was removed by filtration through
glass sintered (G-4) funnel. The filtrate was chromato-
graphed on cellulose column and a deep reddish violet
band was eluted using benzene as eluant. The removal
of the solvent afforded red microcrystals of [Pd
(L2)(Cp)] (5). (Yield: 0.026 g, 0.058 mmol, 60.4%). Anal.
Calc. for C23H20N2OPd: C, 61.82; H, 4.48; N, 6.27.
Found: C, 61.68; H, 4.24; N, 6.12%. IR (KBr); m/cmꢀ1):
1452 (N@N), 771 (Cp). UV/VIS (CH2Cl2), k/nm (e, dm3
molꢀ1cmꢀ1): 552(3200), 473(3700), 362(3000), 280
(11700). 1H NMR in CDCl3 (ppm): 2.4 (s,3H, 50-Me),
3.9 (s, 3H, 20-OMe), 5.79 (s, 5H, Cp), 6.90–8.81 (aromatic
protons).
3.1. Synthesis
Scheme 1 shows the compounds prepared in this work
and the numbering of the carbon atoms for the following
discussion.
An ethanolic solution of disodium tetrachloropalladate
is treated with an equimolar solution of 1-(20-hydroxy-50-
methylphenylazo) naphthalene (1) at room temperature
and gradually starts to produce solid precipitate, which
readily reacts with PPh3 and yields soluble pink coloured
cyclometallate [PdL1 (PPh3)] (3). 1-(20-methoxy- 50-methyl-
phenylazo)naphthalene (2) also undergoes facile cyclopal-
ladation reaction with disodium tetrachloropalladate in
ethanol and yields green coloured cyclopalladate [PdL2Cl]
(4). Cyclopalladate [PdL2Cl] (4) reacts with thallium(I)
cyclopentadienide and yields red violet [PdL2(Cp)] (5),
where palladium(II) is bound to carbon by both r- and
p-bonds. The proposed compositions of all the cyclopalla-
dates are consistent with the observed elemental analysis
data (Section 2).
Caution: Thallium compounds and benzene are highly
hazardous chemicals. These should be handled with great
care.
4
5
4
5
3
2
3
2
6
7
6
7
1
1
8
8
(ii), (iii)
N
Pd PPh3
O
N
N
1'
N
1'
OH
6'
5'
6'
5'
2'
3'
2'
3'
H3C
H3C
4'
4'
1
3
(i)
4
5
8
4
5
8
5
4
3
3
2
6
3
6
6
2
Cl
7
2
7
7
(v)
(iv)
1
1
1
8
Pd
Pd
N
N
N
OMe
N
N
N
1'
1'
1
2'
OMe
OMe
6'
5'
6'
2'
3'
2'
6'
3'
3'
5'
Me
4'
5'
Me
4
4'
Me
5
4
2
Scheme 1. (i) MeI, K2CO3, 2-butanone, reflux, 3 h; (ii) Na2[PdCl4] (1 equivalent), EtOH, room temperature, 2 h; (iii) PPh3 (1 equivalent), benzene, 0.5 h;
(iv) Na2[PdCl4] (1 equivalent), EtOH, room temperature, 2 h; (v) TlCp, benzene, room temperature.

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D.N. Neogi et al. / Inorganica Chimica Acta 360 (2007) 2181–2186
Fig. 1. Electronic spectra of [Pd(L2)Cl] (4) (6.476 · 10ꢀ5 M) in different solvents at 298 K.
3.2. Spectral properties
Table 1
Electronic spectral data of cyclopalladate (4) in different solvents
The IR spectra of all the cyclopalladates (3–5) show
absorption in the region of 1430–1450 cmꢀ1 which can
be assigned to the N@N group [37]. The IR spectrum
of [PdL1(PPh3)] (3) shows absorption at 1099 cmꢀ1 due
to palladium (II) bound triphenyl phosphine [38]. Cyclo-
palladate [PdL3(Cp)] (5) displays a strong absorption
around 771 cmꢀ1 in its IR spectrum, which is characteris-
tic of p-bonded cyclopentadienyl ring to palladium(II)
[39,40].
Solvent
kmax (e, dm3 molꢀ1 cmꢀ1
)
Dichloromethane
604(7820), 574(7620), 430sh(7260), 410(8030),
388(8340), 308(14200)
Benzene
608(6640), 578(6330), 409sh(8030), 394(8500),
305(11890)
Chloroform
Acetonitrile
607(7410), 574(7260), 412sh(7110), 390(7100),
307(13280)
515(4320), 487(4630), 396(7570), 302(8800)
Dimethylformamide 528(3090), 497(4170), 402(8800), 306(8960)
Pyridine 520(4010), 495(4170), 400(8960), 309(7410)
All the cyclopalladates are soluble in common organic
solvents. The electronic spectral data of the cyclometallates
are given in the experimental section. The electronic spectra
of the cyclopalladates show one or more strong absorp-
tions in the visible region. These transitions are assigned
to metal to ligand charge transfer (MLCT) [d(Pd) ! p
(azo)] bands. The bands observed in the high-energy region
with high molar extinction co-efficient arise from intrali-
gand charge transfer transition [41].
dium(II)–OCH3 bond and coordinate to the metal centre
of [PdL2Cl] in the solution (Table 1).
1
The H NMR spectra of all the cyclopalladates show
that the methyl group attached to the phenyl ring (20-Me)
appear as singlet in the range of 2.27–2.40 ppm. In CDCl3
the signal for methoxy group of [PdL2Cl] (4) appears as a
singlet at 4.2 ppm whereas the signal for the same group
of [PdL2(Cp)] (5) shifts to the higher field at 3.9 ppm, which
demonstrates the opening of palladium(II)-OCH3 bond in
The compound [PdL2Cl] (4) shows solvent dependent
electronic spectra (Fig. 1). In non-coordinating solvents
like dichloromethane, benzene and chloroform, it shows
a strong absorption above 600 nm, which is similar to the
absorption profile of palladium(II) bound terdentate
[C2(phenyl), N(azo), O(phenolato)] phenylazophenol
[42,43]. This observation suggests that Pd(II)–OCH3 bond
remains intact in non-cordinating solvents and monoan-
ionic L2 behaves as a terdentate ligand. In coordinating sol-
vents like acetonitrile, dimethylformamide and pyridine,
the absorption band of [PdL2Cl] shifts to 520–530 nm,
which is characteristic of cyclopalladated [C2, N] naphthy-
lazobenzene [27]. Excess donor solvents like acetonitrile,
dimethylformamide and pyridine open the labile palla-
1
[PdL2(Cp)] (3) due to the presence of Cp group. The H
NMR spectrum of [PdL2(Cp)] (5) also shows a sharp signal
at 5.79 ppm due to the five equivalent protons of cyclopen-
tadienyl ring [40,44]. For all the cyclopalladates, the signals
for aromatic protons appear as complex pattern in the
region 6.3–8.8 ppm with correct integration ratio.
3.3. X-ray structure of [PdL1(PPh3)] (3)
Slow diffusion of the solvent from a solution of
[PdL1(PPh3)] (3) in dichloromethane to hexane layer affor-
ded monoclinic pink crystal of the space group P21/c and

D.N. Neogi et al. / Inorganica Chimica Acta 360 (2007) 2181–2186
2185
Table 3
allowed the establishment of its crystal structure (Fig. 2).
Significant crystal data and data collection parameters
for [PdL1(PPh3)] are listed in Table 2. The selected bond
1
˚
Selected bond lengths [A] and angles [ꢁ] for [PdL (PPh3)] (3)
˚
˚
Bond lengths
(A)
Bond lengths
(A)
Pd(1)–C(8)
Pd(1)–O(1)
P(1)–C(24)
P(1)–C(30)
N(1)–N(2)
N(2)–C(11)
2.015(3)
Pd(1)–N(1)
Pd(1)–P(1)
P(1)–C(18)
O(1)–C(12)
N(1)–C(1)
C(1)–C(2)
2.032(3)
2.056(2)
1.820(3)
1.821(3)
1.275(3)
1.374(4)
2.2816(9)
1.835(3)
1.299(4)
1.427(4)
1.372(5)
Bond angles
(ꢁ)
Bond angles
(ꢁ)
C(8)–Pd(1)–N(1)
N(1)–Pd(1)–O(1)
N(1)–Pd(1)–P(1)
82.92(13)
89.50(10)
178.11(8)
C(8)–Pd(1)–O(1)
C(8)–Pd(1)–P(1)
O(1)–Pd(1)–P(1)
172.40(12)
96.70(10)
90.86(7)
lengths and bond angles for [PdL1(PPh3)] are given in
Table 3.
1-(20-hydroxy-50-methylphenylazo) naphthalene binds
palladium(II) center in terdentate fashion through C8 of
naphthyl fragment, N1 of diazene function and O1 of phe-
nolato group. Terdentate naphthylazophenol and P1 of
phosphine make the tetra coordination environment
around palladium(II) essentially planar but deviates from
the square planar geometry. The maximum deviation is
represented by the ‘bite’ angle C(8)–Pd(1)–N(1) of
82.92(13)ꢁ. This type of deviation is observed for cyclopal-
Fig. 2. Molecular structure of [PdL1(PPh3)] (3).
˚
ladates [45]. The Pd–C8 bond length of 2.015(3) A is in
agreement with reported palladium(II)-C(naphthyl) bond
˚
length of 2.014(8) A [46]. The Pd–N1 bond distance of
˚
Table 2
2.032(3) A is longer than Pd–N(azo) bond lengths of
Crystal data and structure refinement for [PdL1(PPh3)] (3)
˚
1.970(4) A [47]. The lengthening of Pd–N bond is due to
Chemical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
C35H27N2OPPd
629.00
the presence of strong trans influence of triphenylphos-
˚
phine. Here the Pd-P1 bond distance (2.281 A) is within
293(2)
the reported range [48]. The C1–N1 bond length
˚
0.71073
monoclinic
P21/c
˚
˚
1.427(4) A of azapalladacycle is longer than C11–N2
1.374(4) A of diazametallacycle [27]. The shortening of
Unit cell dimensions
C11–N2 distance of diazapalladacycle is expected due to
the presence of enhanced p-electron density than that of
azapalladacycle.
˚
a (A)
15.2140(5)
11.6433(3)
17.7950(5)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
115.223(10)
90
2851.68(14), 4
0.366
4. Conclusions
3
˚
Volume (A ), Z
In this study we have begun to explore how the pheny-
lazonaphthalenes that are modeled by variation of the sub-
stituents at 20-position of the phenyl ring can be employed
to achieve the preferential C–H bond activation of the
naphthyl ring with palladium(II) compounds. The selection
of C(naphthyl)–H bond for activation with disodium tetra-
chloropalladate is controlled by the nature of oxygen
donor (auxiliary donor) present at 20-position of 1-(phenyl-
azo)naphthalene. The regiospecific activation of C8–H
bond of naphthyl group in (1) by palladium(II) can be
explained in terms of the formation of six-membered metal
chelate [N,O] as an initial step, which can then only lead to
the palladation at the peri-position of the naphthyl ring.
The group of o-hydroxyarylazo compounds are well known
to form stable six-membered chelates [O,N] with transition
metal ions [49–51]. On the other hand disodium tetrachlo-
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
0.184
320
Crystal size (mm)
*Maximum and minimum
transmission
h range for data collection (ꢁ)
Limiting indices
0.35 · 0.12 · 0.12
1.000000 and 0.673573
1.48 to 23.25
ꢀ16 6 h 6 16, ꢀ8 6 k 6 12,
ꢀ19 6 l 6 19
Reflections collected
Independent reflections
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r (I)]
R indices (all data)
11541
4084 (Rint = 0.0409)
full-matrix least-squares on F2
4084/0/469
0.994
R1 = 0.0308, wR2 = 0.0605
R1 = 0.0498, wR2 = 0.0667
Largest difference in peak and hole 0.289 and ꢀ0.467
(e Aꢀ3
)

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D.N. Neogi et al. / Inorganica Chimica Acta 360 (2007) 2181–2186
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´
´
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Acknowledgements
The financial support (SR/S1/IC-10/2003) from DST,
Government of India, is gratefully acknowledged. We
thank CSIR (India) and UGC for the award of a senior fel-
lowship (PD) and Special Assistance Programme respec-
tively. We also thank one of the Reviewers for helpful
suggestions.
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graphic data for 1. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
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