Cyclopalladation versus hydroxylation. A case of pH dependence

Article abstract of DOI:10.1016/S0022-328X(01)00787-2

1-Alkyl-2-(naphthyl-α-azo)imidazoles (NaiR, 2) (C₁₀H₇-N=N-C₃H₂N₂1-R; R=Me (a), Et (b), PhCH₂ (c)) have been reacted with Na₂PdCl₄ in MeOH or MeCN solutions of Pd(MeCN)₂Cl₂ to synthesise Pd(NaiR)Cl₂ (3). The reaction of Pd(OAc)₂ in boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex Pd(NaiR-H)Cl (4). The ligand, NaiR, acts as a N,N′-bidentate chelator while NaiR-H acts as a tridentate N,N′,C-cyclometallating ligand. The infrared spectra of 3 exhibit two Pd-Cl stretches correspond to a cis-PdCl₂ geometry, and a single ν(Pd-Cl) band in 4 suggests one Pd-Cl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEt-H)Cl (4b) and the metallation takes place at C(8′)-position. The solution of Pd(NaiR)Cl₂ (3) is also irreversibly transformed into Pd(NaiR-H)Cl (4) when the pH is adjusted to 4.5-6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li₂CO₃ etc.). At higher pH values (8-10) the reaction shows the chelative hydroxylation at the C(2′)-site to synthesise Pd(NaiRO)Cl (5). The structure of the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl (5b). The reaction of Pd(NaiR)Cl₂ in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment with Tollen's reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds have been characterised by elemental analyses, IR, UV-vis and ¹H-NMR data. The solution spectral behaviour has been interpreted by EHMO calculations.

Full text of DOI:10.1016/S0022-328X(01)00787-2

Journal of Organometallic Chemistry 629 (2001) 28–38
www.elsevier.com/locate/jorganchem
Cyclopalladation versus hydroxylation. A case of pH dependence
Joydev Dinda a, Debasis Das a, Prasanta Kumar Santra a, Chittaranjan Sinha a,*,
Larry R. Falvello b
a Department of Chemistry, The Uni6ersity of Burdwan, Burdwan 713104, India
b Departmento de Quimica Inorganica, Uni6ersidad de Zaragoza, 50009 Zaragoza, Spain
Received 8 November 2000; accepted 15 March 2001
Abstract
1-Alkyl-2-(naphthyl-a-azo)imidazoles (NaiR, 2) (C10H7ꢀNꢁNꢀC3H2N21ꢀR; R=Me (a), Et (b), PhCH2 (c)) have been reacted
with Na2PdCl4 in MeOH or MeCN solutions of Pd(MeCN)2Cl2 to synthesise Pd(NaiR)Cl2 (3). The reaction of Pd(OAc)2 in
boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex
Pd(NaiRꢀH)Cl (4). The ligand, NaiR, acts as a N,N%-bidentate chelator while NaiRꢀH acts as a tridentate N,N%,C-cyclometallat-
ing ligand. The infrared spectra of 3 exhibit two PdꢀCl stretches correspond to a cis-PdCl2 geometry, and a single w(PdꢀCl) band
in 4 suggests one PdꢀCl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEtꢀH)Cl
(4b) and the metallation takes place at C(8%)-position. The solution of Pd(NaiR)Cl2 (3) is also irreversibly transformed into
Pd(NaiRꢀH)Cl (4) when the pH is adjusted to 4.5–6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li2CO3 etc.). At higher
pH values (8–10) the reaction shows the chelative hydroxylation at the C(2%)-site to synthesise Pd(NaiRO)Cl (5). The structure of
the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl (5b). The reaction of
Pd(NaiR)Cl2 in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment
with Tollen’s reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds
have been characterised by elemental analyses, IR, UV–vis and 1H-NMR data. The solution spectral behaviour has been
interpreted by EHMO calculations. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: a-Naphthylazoimidazoles; Palladium(II) complex; Cyclopalladated compound; Hydroxylation; X-ray structure; EHMO calculations
strong s-donation capacity [11] of C− compared to N,
the ability to act as a multinucleating bridging ligand
1. Introduction
[12] in supramolecular chemistry and their photochemi-
cal and photophysical properties [13]. The potential
activity of these systems may be due to the p-acidity
(low lying LUMO) of the heterocyclic system. This
property is largely dependent on the number of hetero
atoms, the ring size and the substituents in the hetero-
cyclic ring [14]. In the search for a new cyclometallating
reagent with a heterocyclic system, imidazole is chosen
in the first place because of its biological activity [15]
and appearance as such in proteins, enzymes etc. For
the last few years, we have been engaged in the explo-
ration of the chemistry [16–18] and analytical applica-
tions [19] of arylazoimidazoles. At present, we have
designed naphthylazoimidazoles (1) and 1-alkyl-2-
(naphthyl-a-azo)imidazoles (2) [23]. The purpose be-
hind this choice is to synthesise cyclometallated
compounds of the ligands having an imidazole back-
Metal mediated CꢀH bond activation is a conspicu-
ous challenge in organic synthesis [1–3]. The activation
may occur either by forming the MꢀC bond (inter-
molecular or intramolecular) [1,2] or via the functional-
isation of the CꢀH group nearest to the metal centre
[4–6] of the complex. Over the last three decades,
cyclopalladated compounds have found numerous ap-
plications in organic synthesis [7], in material sciences
[8] as compounds exhibiting liquid crystal properties [9]
and as biologically active compounds [10].
Orthometallating reagents containing mono or
polypyridine binding sites have attracted immense inter-
est because of the higher ligand field strength due to the
* Corresponding author. Tel.: +91-342-57683; fax: +91-342-
564452.
E-mail address: c
r sinha@yahoo.com (C. Sinha).
– –
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00787-2

J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
29
bone. The cyclometallation process involves the elimi-
2.1.2. Pd(NaiMeꢀH)Cl (4a)
nation of H+ from the ligand molecule and requires
addition of a base to maintain the pH of the medium
[2a]. The increase in pH of the medium has another
problem, as evident from the hydroxylation [4] of the
CꢀH function of the pendant aryl ring in Pd(aap)Cl2
(aap=2-(arylazo)pyridine). Although the regiospecific
cyclopalladation of 1-(arylazo)naphthalene is well es-
tablished [2b], there is no report on the reaction of the
heterocyclic-azonaphthalenes. Here we will take into
account the effect of pH on cyclopalladation vis-a-vis
hydroxylation at two different sites of the naphthyl
ring in dichloro-[1-alkyl-2-(naphthyl-a-azo)imidazole]-
palladium(II), and this is supported by X-ray crystal
structure determinations of the products.
The complex was prepared by two different routes as
given below.
2.1.2.1. Method 1. Pd(OAc)2 (0.27 g, 1.20 mM) was
boiled in benzene (25 ml) and filtered hot. To this
solution NaiMe (0.24 g, 1.02 mM) was added and
further refluxed for 1.5 h. The brown-red solution
slowly turned green with a dark precipitate. The solu-
tion was filtered in the hot condition, digested with
LiCl (5 mM) for 0.5 h and reduced by slow evaporation
to half of its original volume. The precipitate so formed
was filtered and dried over P4O10. The dry dark mass
was dissolved in CH2Cl2 and chromatographed over
silica gel column (30×1 ml). A light pink–red band
was eluted first by C6H6 and rejected. The desired green
band was eluted by a MeCN–C6H6 mixture (1:4, v/v)
and a pink mass adhered to the top of the column. The
solution was then evaporated, and the yield was 80%.
Pd(NaiEtꢀH)Cl (4b) and Pd(NaiCH2PhꢀH)Cl (4c)
were prepared by following identical procedures and
the yield was 85%.
2. Experimental
2-(Naphthyl-a-azo)imidazole (NaiH, 1), 1-methyl-
and 1-ethyl-2-(naphthyl-a-azo)imidazole (NaiMe (2a)
and NaiEt (2b)) were prepared by the stated procedure
[20]. PdCl2 was obtained from Arrora Matthey, Cal-
cutta. MeI, EtBr and PhCH2Br were obtained from
Aldrich. Commercially available SRL silica gel (160–
120, 230–400 mesh) was used for column and prepara-
tive chromatography. All other reagents and solvents
were used as received.
2.1.2.2. Method 2. To CH2Cl2–MeOH (1:2, v/v, 20 ml)
a
solution of Pd(NaiMe)Cl2 (0.2 g, 0.48 mM)
CH3COONa (0.5 M) in MeOH was slowly added, and
the pH of the solution was adjusted to 5 (tested by
pH-meter, the working range of pH is 4.5–6). The
solution was stirred and the pH was measured at 10
min intervals and adjusted by adding more AcONa
solution. The pink–red colour of the solution turned
greenish brown slowly and finally green. The stirring
was continued for 5 h. The solution was then evapo-
rated, washed with plentiful water, and then dried over
P4O10. A CH2Cl2 solution of the compound was chro-
matographed over silica gel, and a small pink–red band
was eluted first by CH2Cl2. The green band was eluted
by MeCN–C6H6 (1:4, v/v) and after evaporation, the
yield was 70%.
1
The H-NMR spectra were obtained on a Bruker AC
300F and Varian Gemini 300 MHz FTNMR and refer-
enced to internal SiMe4. Infrared spectra were recorded
as KBr discs on a JASCO-420 FTIR machine. UV–vis
spectra were obtained from a JASCO UV–VIS–NIR
model V-570 spectrophotometer. Elemental analyses
data were collected on a Perkin–Elmer 2400 CHNO/S
instrument. The pH of the solution was measured by
using a Systronics m-361 pH-meter.
The use of NaOH, LiOH, CH3ONa, Li2CO3, K2CO3
or Cs2CO3 as base was tested by following method 2.
The yield of cyclopalladated complexes was not satis-
factory and varied in the 20–40% range.
2.1. Synthesis
2.1.1. Pd(NaiMe)Cl2 (3a)
To the boiling solution of PdCl2 (0.278 g, 1.52 mM)
in MeCN (15 ml), NaiMe (2a) (0.3 g, 1.27 mM) was
added and stirred for 0.5 h. The pink–red precipitate
slowly appeared on slow evaporation in air. It was
filtered and washed with cold MeCN and finally with
Et2O. Then it was dried in vacuo, and the purity tested
by TLC. Yield was 70%.
Pd(NaiEt)Cl2 (3b) and Pd(NaiCH2Ph)Cl2 (3c) were
prepared by identical procedures with 75% yield. The
reaction of a methanolic solution of Na2[PdCl4] and
NaiR also yielded the same product but in a lower
percentage.
2.1.3. Pd(NaiMeO)Cl (5a)
2.1.3.1. By sodium hydroxide. A dilute solution of
NaOH (0.02 M) in 2:3 H2O–MeCN mixture (5 ml) was
added drop by drop for a period of 4 h to a magneti-
cally stirred suspension of 3a (0.2 g, 0.48 mM) in
MeCN (20 ml) in air. Stirring was continued for an-
other 24 h and the pH was checked by the pH-meter
(working range of pH 8–10). The colour of the solution
changed gradually from pink–red to blue. The solution
was then evaporated to dryness in vacuo and the
residue was washed thoroughly with water and was

30
J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
then dried over P4O10. The solid residue was dissolved
in CH2Cl2 and the solution was chromatographed over
a silica gel column prepared in benzene. A MeCN–
C6H6 (1:2) mixture was eluted into the desired blue
compound. The solution was evaporated in vacuo and
gave the pure complex with 70% yield. A pink mass
remained at the top of the column.
The identical procedure was applied for the chelata-
tive hydroxylation of Pd(NaiEt)Cl2 and Pd(NaiCH2-
Ph)Cl2 to synthesise Pd(NaiEtO)Cl (5b) Pd(NaiCH2-
PhO)Cl (5c) with yields of 75 and 82%, respectively.
When excess (say fivefold) NaOH solution was added
all at once, the suspended Pd(NaiR)Cl2 underwent dis-
solution, forming a red–violet solution with subsequent
precipitation of a dark violet product. The hydroxy-
lated product was not formed under these conditions.
A similar reaction was carried out by using NaOMe,
LiOH, Li2CO3, K2CO3 or Cs2CO3, with pH adjusted at
8–10. The product was isolated as above with 50–70%
yield.
The pink–red solution slowly turned green and finally
bluish–green. AgCl was filtered out and the solution
was evaporated. The dark precipitate so formed was
filtered and washed with water and dried over P4O10.
When the product was dissolved in a small volume of
CH2Cl2 and chromatographed over a silica gel column,
a pink–red band was eluted first by C6H6–MeCN (9:1,
v/v), followed by the desired blue band by using a 2:1
C6H6–MeCN mixture. The solution was then evapo-
rated, and the pure compound was isolated with 10%
yield.
2.1.3.3. By Tollen’s reagent. To a H2O–MeCN (1:1, 20
ml) suspension of Pd(NaiMe)Cl2 (0.2 g, 0.4 mM), Tol-
len’s reagent (0.1 g AgNO3 in 5 ml water+1 ml NaOH
(0.2 M) and excess NH3) was added and stirred for 20
min (on further stirring the solution turned brownish-
blue gradually and finally into a brown insoluble pre-
cipitate). The colour of the solution quickly turned
from pink–red to blue. The reaction was monitored by
TLC test at 2 min intervals. The mixture was quickly
filtered (G-4) and evaporated in vacuo. The dark mass
was washed with water, filtered and dried over P4O10.
The dried mass was chromatographed as before, a light
orange band was rejected and a blue band was formed
by C6H6–MeCN (2:1, v/v). The yield was 84%.
2.1.3.2. By sil6er nitrate solution. Pd(NaiMe)Cl2 (0.2 g,
0.48 mM) was suspended in H2O–MeCN (1:1, v/v; 20
ml), and aqueous silver nitrate (0.1 g, 0.59 mM) solu-
tion was added to it. The mixture was refluxed for 30 h.
Table 1
A similar reaction was carried out by using Tl2CO3
and TlNO3+NaOH under suspension, and the hydrox-
ylated product was isolated with a yield \75%.
Summarised crystallographic data for 4b and 5b
Crystal parameters
Empirical formula
Formula weight
Temperature (K)
4b
5b
C15H13ClN4Pd
391.14
297
C
15H13ClN4OPd
421.30
294
2.2. X-ray diffraction studies
,
Wavelength (A)
0.71073
Monoclinic
P21/n
0.71073
Triclinic
P1
Crystal system
Space group
Crystals suitable for X-ray work were grown by slow
diffusion of hexane into CH2Cl2 solution at 298 K. The
crystal data, data collection and refinement parameters
for the two structures are listed in Table 1. The data
were gathered by using an automated cad4 four-circle
diffractometer [21] with graphite-monochromatised
Mo–Ka (u=0.71073) radiation. After the initial reflec-
tion search and indexing, two-dimensional (ꢀ–q) plots
were used to determine the optimum scan parameters
for the collection of intensity data. Through data col-
lections, the intensities of three standard reflections
were monitored at regular intervals, which in no case
showed variations of \5%. The intensities were cor-
rected for Lorentz and polarisation effects and monitor
absorption by using a technique based on azimuthal „
scans. „-Scans were made for 14 and 8 reflections with
bisecting-mode  values in the range 6–70° and 11–86°,
respectively, for Pd(NaiEtꢀH)Cl (4b) and Pd(Nai-
EtO)Cl (5b) for use in absorption corrections [22]. The
structures were solved by direct methods [23a] and
refined by full-matrix least-squares [23b]. Non-hydro-
gen atoms were refined anisotropically. Hydrogen
atoms were included in calculated positions with
Unit cell dimensions
,
a (A)
7.2237(5)
16.3178(13)
12.1354(10)
90
90.490(6)
90
1430.41(19)
4
1.816
7.6324(4)
22.139(2)
27.660(3)
84.325(10)
86.554(6)
81.450(7)
4594.2(7)
12
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
Dcalc (g cm−3
)
1.827
0.977
1222
Transmission coefficienta
Parameters refined
0.7348
234
Crystal size (mm3)
0.35×0.30×0.26 0.51×0.11
×0.08
5.07
10.92
0.961
Rb (%)
2.85
7.10
1.085
c
wR2 (%)
Goodness-of-fit on F2d
a Maximum value normalised to 1.
b R=ꢀFo−Fc/ꢀFo.
c wR2=ꢀw(Fo2−Fc2)/ꢀw(Fo2)2]1/2
;
w=[|2(Fo2)+(g1P)2+g2P]−1
,
P=[max(F2o; 0)+2Fc2]/3; g1=0.0038; g2=1.28.
d The goodness-of-fit is defined as [ꢀw(F2o−Fc2)2/(N0−NP)]1/2
,
where N0 and NP denote the numbers of data and variables, respec-
tively.

J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
31
Table 2
Microanalytical, UV–visa and 1H-NMRb spectral data
Compound Elemental analyses
Found (calc.)%
UV–vis spectral data
l (ppm)
umax (nm) [10−3
m
1-R
4-Hc 5-Hc Naphthyl-H
(M−1 cm−1)]
C
H
N
1d
2a
2b
2c
3a
3b
3c
4a
70.19(70.27)
Gummy
Gummy
4.46(4.51)
25.31(25.23)
395(17.84), 439(10.58)e
396(15.66), 418(13.87)e
399(13.41), 425(12.22)e
400(14.39), 430(12.49)e
10.42f
7.50 7.50 7.60–8.11
6.91 6.80 7.75–7.92
7.11 6.92 7.80–7.95
7.20 7.02 7.77–7.93
7.58 7.05 7.94–8.20
7.64 7.14 7.98–8.25
7.35 7.16 7.20–8.00
7.21 6.95 7.60–8.10
3.72g
4.25h, 1.50i
5.45g
Gummy
40.55(40.64)
42.04(42.12)
49.00(49.04)
44.64(44.57)
2.79(2.90)
3.21(3.28)
3.21(3.27)
2.99(2.92)
13.62(13.55)
13.17(13.10)
11.37(11.44)
14.77(14.86)
405(6.78), 510(5.77), 652(0.48)e 4.08g
400(6.16), 511(4.77), 650(0.67)e 4.43h, 1.56i
395(6.16), 500(4.03), 652(0.89)e 5.52
400(11.52), 453(4.66)e,
640(3.90), 690(2.14)e
400(13.80), 456(5.31)e,
645(4.59), 688(2.80)e
402(10.80), 450(4.840),
630(3.79), 690(2.42)e
3.96g
4b
4c
5a
5b
5c
40.00(40.05)
53.08(52.99)
42.70(42.76)
44.16 (44.24)
51.24(51.18)
3.40(3.33)
3.26(3.31)
2.84(2.80)
3.12 (3.14)
3.26(3.20)
14.40(14.33)
12.43(12.36)
14.14(14.25)
13.70 (13.76)
11.87(11.94)
4.40h,1.55i
5.40g
7.29 7.00 7.70–8.15
7.24 6.91 7.20–7.90
7.11 6.85 7.40–8.50
7.15 6.94 7.35–8.60
7.20 6.94 7.60–8.00
411(6.59), 614(3.08)e, 661(4.46), 3.85g
711(4.36)
423(6.95), 618(4.66)e, 650(6.28), 4.30h,1.59i
706(6.65)
382(11.82), 446(8.88)e,
5.55g
610(5.00), 644(7.40), 705(6.86)
a In CHCl3.
b In CDCl3.
c Doublet (J=7.0–8.0).
d M.p: 1, 20091°C.
e Shoulder.
f Broad.
g Singlet.
h Quartet (J=7.0 8.0).
i Triplet (J=7.0–8.0).
isotropic displacement parameters 1.2 times the
isotropic equivalent for their respective parent atoms.
Atomic coordinates, H-atom coordinates, thermal
parameters with a complete list of bond distances and
angles and a list of structure factors were obtained from
the CCDC, 12, Union Road, Cambridge, CB2 1EZ,
UK on request, the deposition numbers being 154016
(for 4b) and 154017 (for 5b).
3.1. Coordination [(Pd(NaiR)Cl2, 3)] and
cyclopalladated complexes [(Pd(NaiRꢀH)Cl, 4)]
The treatment of Na2[PdCl4] in methanol [4a] or
Pd(MeCN)2Cl2 in acetonitrile [16] with NaiR affords a
pink–red solid 3, which shows the common pattern for
a cis-PdCl2 configuration [4a] with two w(PdꢀCl) at 340
and 318 cm−1. On addition of NaOAc in MeOH to the
reaction mixture or separately to a CH2Cl2–MeOH
solution of Pd(NaiR)Cl2 (3) (the pH of the solution is
checked by the pH-meter and is adjusted at 4.5–6)
under stirring conditions (5 h), the pink–red solution
changes to green with a crystalline precipitate (4) (Eq.
(1) in Scheme 1). The IR spectra of the green solid (4)
show one w(PdꢀCl) (345 cm−1) and the structural study
by X-ray diffraction of 4b (Fig. 1) suggests a cyclopalla-
dated complex. The ligand is then regarded as N,N%,C−
donor type analogue of 2-acetylpyridine phenylhydra-
zone or 6-phenyl-2,2%-bipyridine, and related ligands are
used to prepare cyclometallated complexes [11–13]. The
use of NaOMe, NaOH, LiOH, Li2CO3, K2CO3 or
Cs2CO3 as the base in the MeOH to CH2Cl2 solution of
3. Results and discussion
2-(Naphthyl-a-azo)imidazole (C10H7ꢀNꢁNꢀC3H3N2,
NaiH, 1) is obtained by coupling a-naphthyldiazonium
ion with imidazole at pH 7 and 1-alkyl-2-(naphthyl-a-
azo)imidazole (C10H7ꢀNꢁNꢀC3H2NN(1)ꢀR, NaiR (2),
where R=Me (a), Et (b), PhCH2 (c)) is synthesised
from there by adding alkyl halide in THF in the
presence of NaH [20]. All the complexes presented here
1
have been characterised by IR, UV–vis and H-NMR
spectra (Table 2) and elemental (C, H, N) analyses.

32
J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
Scheme 1. (i) PdCl2 in MeCN/Na2[PdCl] in MeOH, stirr; (ii) CH2Cl2ꢁMeOH, NaOAc, pH 4.5–6.0, stirr; (iii) Pd(OAC)2 in C6H6, reflux; and (iv)
NaOH in MeCN/AgOH, stirr or AgNO3 reflux.
3.2. Hydroxylated complexes (Pd(NaiRO)Cl, 5)
complex 3 also yields the cyclopalladated complex (with
low yield, 10–20%) when the pH of the medium is
adjusted to 4.5–6.0.
Slow addition of dilute solution of sodium hydroxide
in aqueous MeCN to
a
stirred suspension of
Alternatively, complex 4 can also be prepared in high
yield (\80%) by treatment of NaiR with Pd(OAc)2 in
boiling benzene followed by the addition of LiCl [24].
Pd(OAc)2 has been regarded as an ideal electrophile for
CꢀH activation and cyclopalladation reaction [25];
hence it is used for a similar purpose. The treatment of
AgOAc with 3 in CH2Cl2–MeOH also yields 4 with a
comparatively lower yield along with another product,
which is identified as hydroxylated complex 5. A slurry
of the complex and silica gel (230–400 mesh) in a
CH2Cl2–MeOH mixture is kept airtight under thermo-
static conditions (295 K) overnight and the green
product is separated chromatographically with 20%
yield and is also identified as the cyclopalladated com-
pound 4. A pink mass adheres to the top of the column.
The solid surface activation of the CꢀH bond is well
known in literature [26]. In this case the CꢀH activation
leading to cyclopalladation is achieved by the SiO2
surface.
Pd(NaiR)Cl2 (3) in MeCN, under ambient conditions
whilst maintaining the pH at 8–10, leads to a colour
change from pink–red to greenish–blue, and the naph-
Fig. 1. Thermal ellipsoid plot of Pd(NaiEtꢀH)Cl (4b), showing the
atom labelling scheme. Non-hydrogen atoms are represented by their
50% probability ellipsoids.

J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
33
of Cl− (LiCl) (1–5 M equivalent); (ii) the pH of the
medium during hydroxylation must lie at 8–10; (iii) the
presence of air and alkali is mandatory for the reaction
to occur; and (iv) the addition of a Cl− scavenger
(Ag+, Tl+) followed by a base completes the reaction
quickly yielding \80% product.
3.3. X-ray crystallography
The X-ray crystal structures of cyclopalladated
Pd(NaiEtꢀH)Cl (4b) and hydroxylated Pd(NaiEtO)Cl
(5b) have been determined. Views of the molecules are
shown in Figs. 1 and 2, respectively, and selected bond
parameters are listed in Table 3. The crystal lattice of
Pd(NaiEtꢀH)Cl consists of discrete molecules in which
the bonding fashion of ligand is of the N,N%,C− type
(Fig. 1). The entire molecule is essentially planar with
the exception of the ethyl group. The square planar
configuration around the palladium atom involves
PdꢀN(3) (N(imi)), PdꢀN(1) (N(azo)), PdꢀC and PdꢀCl
bonds. N(3)ꢀPdꢀC(9) deviates from linearity (ꢀ20°)
more than N(1)ꢀPdꢀCl(1) (deviation ꢀ5°). This devia-
tion may be due to the acute chelate angle in
N(1)ꢀPdꢀN(3) (76.57°), while that of the other chelate
ring N(1)ꢀPdꢀC(9) is just a slight one from the ideal
square planar angle. The bond distances are almost
comparable to the reported results [4a,27,28]. The
N(1)ꢀN(2) and PdꢀN(3) distances are 1.286(4) and
Fig. 2. Thermal ellipsoid plot of molecule 1, of Pd(NaiEtO)Cl (5b),
showing the atom labelling scheme. Non-hydrogen atoms are repre-
sented by their 50% probability ellipsoids.
tholato complex 5 can be isolated from the reaction
mixture with 70% yield (Eq. (2) in Scheme 1). The IR
spectra of the complexes show one w(PdꢀCl) and the
X-ray structure determination (Fig. 2) suggests hydrox-
ylation of the C(2%)ꢀH bond leading to naphtholato-pal-
ladium(II) (C(2%)ꢀOPd) complex formation.
Activation of the CꢀH bond of the pendant phenyl
ring in a metal complex of 2-(arylazo)-pyridines (aap),
such as Pd(aap)Cl2 [4a], [Co(aap)3](ClO4)2 [4b], Ru-
(aap)2Cl2 [5], leading to an insertion of oxygen (hydrox-
ylation [4], CꢀH“CꢀOH) or sulphur (thiolation [5],
CꢀH“CꢀSH) is known in the literature. Some com-
mon observations in this reaction are: (i) the yield is
heavily suppressed when it is carried out in the presence
,
2.177(3) A, respectively, and they are slightly longer
Table 3
Selected bond distances and angles for (i) 4b and (ii) 5b
(i ) Cyclopalladated complex (C15H13ClN4Pd (4b))
,
Distances (A)
Angles (°)
PdꢀC(9)
PdꢀN(1)
PdꢀN(3)
PdꢀCl(1)
N(1)ꢀN(2)
1.985(4)
1.969(3)
2.177(3)
2.3149(10)
1.286(4)
N(1)ꢀPdꢀC (9)
N(1)ꢀPdꢀN(3)
C(9)ꢀPdꢀCl(1)
N(1)ꢀPdꢀCl(1)
N(3)ꢀPdꢀCl(1)
N(3)ꢀPdꢀC(9)
82.78(15)
76.57(12)
95.72(12)
175.88(9)
104.83(9)
159.34(14)
(ii ) Hydroxylated product (C15H13ClN4OPd, 1/6CH2Cl2 (5b))
,
Distances (A)
Molecule n
1
2
3
4
5
6
Pd(n)ꢀN(n01)
Pd(n)ꢀN(n03)
Pd(n)ꢀO(n)
Pd(n)ꢀCl(n)
N(n01)ꢀN(n02)
1.929(6)
1.996(6)
2.023(5)
2.292(2)
1.317(7)
1.944(6)
1.989(6)
2.030(5)
2.292(2)
1.303(7)
1.949(5)
2.004(6)
2.025(5)
2.295(2)
1.299(7)
1.936(6)
1.990(7)
2.031(5)
2.291(2)
1.300(7)
1.918(6)
1.976(7)
2.028(6)
2.296(2)
1.327(7)
1.937(6)
1.993(6)
2.013(5)
2.289(2)
1.316(7)
Angles (°)
N(n01)ꢀPd(n)ꢀN(n03)
N(n01)ꢀPd(n)ꢀO(n)
N(n03)ꢀPd(n)ꢀCl(n)
O(n)ꢀPd(n)ꢀCl(n)
N(n03)ꢀPd(n)ꢀO(n)
N(n01)ꢀPd(n)ꢀCl(n)
80.2(2)
82.8(2)
99.36(19)
97.67(16)
163.0(2)
179.30(18)
80.3(3)
82.4(2)
99.5(2)
97.73(18)
162.7(2)
179.82(17)
80.2(2)
82.5(2)
100.25(18)
97.09(15)
162.7(2)
179.32(18)
79.4(3)
82.6(2)
99.7(2)
98.46(16)
161.8(2)
176.54(19)
80.4(3)
82.0(2)
99.7(2)
97.89(18)
162.4(3)
178.68(19)
80.5(3)
82.2(2)
99.71(19)
97.65(17)
162.6(2)
179.4(2)

34
J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
than the chloro-[2-(azophenolato)pyridine]Pd(II) (elon-
of the five-membered chelate ring. As there are posi-
tions to form the five-membered chelate ring in this
molecular frame, so the C(2%)ꢀH activation takes place,
followed by the chelation to achieve the product 5.
,
gated by 0.03 A) [27]. This may be due to a more
reduced p-acidity of imidazole than that of pyridine
[14b].
The crystal structure of Pd(NaiEtO)Cl is unusual,
having six molecules of the palladium complex along
with one molecule of CH2Cl2 in the crystallographic
asymmetric unit. These six molecules are organised into
three dimeric units. That is, molecule 1 (around Pd(1))
and molecule 2 (around Pd(2)) are located together,
parallel to each other, with an interplanar distance of
3.4. Effect of pH on the reaction type
CꢀH bond activation followed by cyclopalladation is
observed in acidic pH, 4.5–6.0, while in basic pH,
8.0–10.0, the CꢀH bond is hydroxylated (CꢀH“
CꢀOH) with subsequent metallation. The nature of the
base is immaterial and has the property to maintain the
pH of the medium. The cyclometallation process is
associated with a decrease in pH (Eq. (3)), and there-
fore base addition is not only necessary to increase the
yield of the product, but also mandatory sometimes for
the CꢀM bond formation [2,29].
,
4.04(2) A and a dihedral angle of 1.69(7)°. Other pairs
,
are located at an interplanar spacing (A) and a dihedral
angle (°), respectively, as follows: molecule 3 and 4,
,
,
3.251(9) A, 0.97(7)°; molecule 5 and 6, 3.28(2) A and
1.34(7)°. As an indication of the relative orientations of
the three dimers, we note that the dihedral angle be-
tween molecules 1 and 3 is 53.14(4)° and between
molecules 1 and 5 is 4.21(7)°, and the angle between the
planes of molecules 3 and 5 is 49.65(4)°. The three
dimers are not equivalent to each other. The Pd···Pd
intra-dimer distances are different: Pd(1)···Pd(2)=
(3)
4.7721(9) A;
Pd(3)···Pd(4)=4.3683(9) A
and
In the hydroxylation process, the initial step is the Cl−
substitution and the formation of the PdꢀOH bond. It
is noted that external Cl− suppresses the yield of the
hydroxylated product, while Cl− scavenger Ag+ or Tl+
quickly completes the reaction with a very high yield of
the product, 5. The addition of excess OH− to an
acetonitrile solution of 3 changes the colour instanta-
neously to violet and subsequently precipitates out an
unidentified sticky dark mass, which is on suspension in
a polar solvent and slowly changes to blue, and the
complex 5 is obtained on addition of dilute HCl. The
sticky mass, on acidification with dilute HCl returns to
the parent complex 3. Addition of dilute HClO4 (fol-
lowed by NaClO4) to the violet suspension precipitates
out a pink product, which is also obtained from the
reaction with AgClO4 followed by the addition of
NaClO4. Initial observation (C, H, N; IR and conduc-
tance data) suggests a 1:2 electrolyte of type 6. All these
data support a metal-coordinated hydroxide ion at
higher pH values (8–10) that attacks the CꢀH bond at
the ortho position of the pendant aromatic ring [4a].
This suggests a nucleophilic attack by the metal coordi-
nated hydroxide ion at the ortho position of the pen-
dant aromatic ring, which is subjected to electron
withdrawal by the combined effects of the metal centre,
the azoimine function and the heterocyclic backbone.
This is equivalent to hydroxylation of 1,3,5-trinitroben-
zene by OH− in air [29]. Although there are two CꢀH
bonds (C(2%)ꢀH, C(8%)ꢀH) closest to the metal centre,
hydroxylation occurs only to C(2%)ꢀH, probably on
account of the formation of a five-membered chelate
ring that is more stable than the six-membered chelate
ring. A plausible mechanism of the reaction is given in
Eq. (4) of Scheme 1.
,
,
,
Pd(5)···Pd(6)=5.1419(10) A. The bond distances (Table
3) in the coordination sphere of six molecules differ
slightly and the values are in the range: PdꢀN(1),
1.918–1.949; PdꢀN(3), 1.976–2.004; PdꢀO(1), 2.013–
2.030; PdꢀCl, 2.289–2.296 and N(1)ꢀN(2), 1.299–1.32
,
A. The chelate angles [N(1)ꢀPdꢀN(3), N(1)ꢀPdꢀO] and
the angles around the square plane are also comparable
(differ by 0–3°). The torsion angles vary in a character-
istic manner. All of them represent an unusual and
subtle form of conformational isomerism in a metal
chelate sustained by the crystal lattices. The PdꢀCl and
PdꢀN(1) (N(azo)) distances contract slightly in going
from 4b to 5b. In contrast, the PdꢀN(3) (N(imidazole))
,
distance is contracted by \0.2 A and may be due to
the trans influence [28] given by the PdꢀC bond in 4b,
which is absent in 5b. The N(1)ꢀN(2) distance varies in
,
the range 1.299–1.317 A. The electron releasing effect
of phenolic oxygen in the tightly bound planar triden-
tate structure may play a role for the contraction of
bond distances in the square plane [4a]. This induces
better p-backbonding in 5b and the azo distance is
slightly elongated. The N(imidazole)-PdꢀN(azo) chelate
bite angle is expanded (ꢀ4°) in going from 4b to 5b. In
both the cases all the angles around the metal except
N(1)ꢀPdꢀCl (176–179°) deviate severely from the
square planar value. The structure of 5b agrees well
with those of analogous pyridine derivatives [4a].
In the parent complex 3, two CꢀH bonds remain
closer to the metal centre; the chelatative hydroxylation
at the C(2%)ꢀH will lead to a five-membered chelate
ring, while the C(8%)ꢀH will constitute a six-membered
ring. In a square planar arrangement, the six-membered
chelate ring may need a larger angular strain than that

J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
35
1
The H-NMR spectra of the ligands and the com-
plexes were recorded in CDCl3. 2-(Naphthyl-a-
azo)imidazole shows
a
characteristically broad,
concentration dependent N(1)ꢀH resonance at 10.45
ppm, which disappears upon the addition of D2O. The
alkylation of azoimidazole was supported by the disap-
pearances of l (NꢀH). NaiMe gives a singlet at 3.72
ppm for l Me; NaiEt gives a quartet at 4.25 ppm
(J=8.0 Hz) and a triplet at 1.5 ppm, respectively, for
the ꢀCH2ꢀ group and the ꢀCH3 group. A singlet at 5.45
ppm is referred to the signal of the ꢀCH2ꢀ group of
NaiCH2Ph [20]. Imidazole protons 4- and 5-H in NaiH
appear as an unsplit band at 7.50 ppm, possibly owing
to an intermolecular proton exchange between them. In
alkylated derivatives NaiR, 4- and 5-H appear as dou-
blets, and the upperfield signal corresponds to 5-H
(Table 2). The naphthyl ring protons are affected due to
the electron withdrawing effect of the azo group [20]. In
the coordination complexes, Pd(NaiR)Cl2 (3), all the
protons suffer significant downfield shifting compared
to the ligand values, which may be due to the electron-
withdrawing effect of the coordinated palladium(II). In
the cyclopalladated complexes, Pd(NaiRꢀH)Cl (4), and
the hydroxylated products, Pd(NaiRO)Cl (5), the pro-
tons shift to an upperfield portion compared with the
coordinated complexes (3). This reflects the involve-
ment of metal d-orbitals in the construction of a molec-
ular orbital in these complexes more efficiently
compared with Pd(NaiR)Cl2. The EHMO calculation
also supports the metal orbital contribution to HOMOs
(vide infra).
Fig. 3. UV–vis spectra of Pd(NaiCH2Ph)Cl2 (3c) (----),
Pd(NaiCH2Ph-H)Cl (4c) (- ꢀ-ꢀ -) and Pd(NaiCH2PhO)Cl (5c) (—)
in MeCN at 298 K.
At low pH values (4.5–6), the base acts as a H+
scavenger that is released during the CꢀH activation by
palladium(II) as given in Eq. (3) and is not sufficiently
strong to augment the nucleophilic substitution at the
metal centre.
3.5. Solution spectra
Dichloromethane solutions of the coordination com-
plexes, Pd(NaiR)Cl2 (3), cyclopalladated compounds
Pd(NaiRꢀH)Cl (4) and hydroxylated products
Pd(NaiRO)Cl (5) exhibit distinctly different UV–vis
spectra. The ligands, NaiR, exhibit intra-ligand charge
transfer transitions near 400 nm along with a shoulder
at 420 nm. The pink–red solution of 3 shows transi-
tions near 410 and 500 nm. Green solutions of the
cyclopalladated complexes exhibit three consecutive
transitions near 400, 450, 640 and 690 nm, along with
high-energy transitions corresponding to intra-ligand
charge transfer bands. The spectral differences of 4
from 3 are characteristic of the orthopalladation reac-
tion and the low energy structured absorption in
Pd(NaiRꢀH)Cl (4) may probably represent a dp(Pd)–
p*(L) (L=ligand) charge transfer transition [30,31].
This is also supported by EHMO calculation (vide
infra). On the other hand, naphtholatopalladium(II)
complexes Pd(NaiRO)Cl (5) display a characteristic
absorption pattern in the low energy region of higher
intensities (nearly at 400–650 and 710 nm) than that of
the respective ortho palladated complexes. The diagnos-
tic spectral features are listed in Table 2 and representa-
tive spectra are shown in Fig. 3.
3.6. EHMO calculation and spectral properties
The approximate composition of the frontier orbitals
in the cyclopalladated complexes and the hydroxylated
products has been estimated by extended Hu¨ckel calcu-
lations performed by using the crystallographic
parameters of the respective single crystal data. The
HOMO and LUMO of the complexes are depicted in
Fig. 4. In Pd(NaiEtꢀH)Cl (4b) the HOMO is composed
of 74% metal (64% d-orbital of Pd) orbitals and 24%
ligand orbitals and the LUMO is a ligand dominated
(89%) function. Hence, the intense absorption at 635
nm may be assigned to MLCT transition [32]. In
Pd(NaiEtO)Cl (5b) the HOMO is constituted by a 61%
ligand share and a 36% contribution from metal or-
bitals. The LUMO is made up mainly from a ligand
(91%) function. The imidazolylazonaphthol in 5b has
two interdependent parts bridged by azo group: imida-
zole and naphthol. In HOMO the imidazolyl group
shares 33% and naphthol shares 13% of the function
while the LUMO is constituted by a 70% azonaphthol
group. Thus, the high intense absorption at ca. 650–
700 nm has been characterised as HOMO“LUMO
and may not be considered as a pure MLCT type;

36
J. Dinda et al. / Journal of Organometallic Chemistry 629 (2001) 28–38
Fig. 4. EHMO derived HOMO and LUMO of: (a) Pd(NaiEtꢀH)Cl; and (b) Pd(NaiEtO)Cl.
(c) A.J. Canty, G. van Koten, Acc. Chem. Res. 28 (1995) 406;
(d) R.H. Crabtree, Chem. Rev. 85 (1985) 245;
(e) I.P. Rothwell, Polyhedron 4 (1985) 177;
rather, it may be a transition of intra-valence charge
transfer (IVCT) type.
(f) E.C. Constable, Polyhedron 3 (1984) 1037;
(g) G.W. Parshall, Acc. Chem. Res. 3 (1970) 139;
(h) J. Dehan, M. Pfeffer, Coord. Chem. Rev. 18 (1976) 327;
(i) A.D. Ryabov, Chem. Rev. 90 (1990) 403;
Acknowledgements
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Technology, New Delhi, for financial support. Crystal-
lographic work is funded by Directorate General of
Higher Education, Spain, under grant PB95-0792.
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