Synthesis and comparative studies of cyclopalladated complexes with ortho C-H activation of aromatic rings bearing electron donating and electron withdrawing groups

Article abstract of DOI:10.1080/15533174.2013.862641

The present study was undertaken to throw more light on the cyclopalladation mechanism. The reactivity of Schiff base ligands {N - (4-methoxy benzylidene) -4-methoxyaniline (1) and N - (4-nitro benzylidene) -4-methoxyaniline (2)} by interaction with palla

Full text of DOI:10.1080/15533174.2013.862641

This article was downloaded by: [New York University]
On: 05 June 2015, At: 00:17
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsrt20
Synthesis and Comparative Studies of Cyclopalladated
Complexes With Ortho C‒H Activation of Aromatic
Rings Bearing Electron Donating and Electron
Withdrawing Groups
A. M. Nassara, A. M. Hassana, N. M. Ibraheemb & B. H. Hekala
a Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt
b Organometallic Chemistry Department, National Research Center, Dokki, Giza, Egypt
Accepted author version posted online: 20 Nov 2014.Published online: 06 Jan 2015.
Click for updates
To cite this article: A. M. Nassar, A. M. Hassan, N. M. Ibraheem & B. H. Hekal (2015) Synthesis and Comparative Studies
of Cyclopalladated Complexes With Ortho C‒H Activation of Aromatic Rings Bearing Electron Donating and Electron
Withdrawing Groups, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 45:6, 813-820, DOI:
10.1080/15533174.2013.862641
To link to this article: http://dx.doi.org/10.1080/15533174.2013.862641
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2015) 45, 813–820
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.862641
Synthesis and Comparative Studies of Cyclopalladated
Complexes With Ortho CꢀꢀH Activation of Aromatic Rings
Bearing Electron Donating and Electron Withdrawing Groups
A. M. NASSAR1, A. M. HASSAN1, N. M. IBRAHEEM2, and B. H. HEKAL1
1Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt
2Organometallic Chemistry Department, National Research Center, Dokki, Giza, Egypt
Received 27 May 2013; accepted 2 November 2013
The present study was undertaken to throw more light on the cyclopalladation mechanism. The reactivity of Schiff base ligands {N -
(4-methoxy benzylidene) -4-methoxyaniline (1) and N - (4-nitro benzylidene) -4-methoxyaniline (2)} by interaction with palladium
(II) acetate is allowed to the isolation of different complexes (1a–f) and (2a–f). All the newly synthesized compounds are
characterized using elemental analysis, IR, UV-VIS, 1HNMR and mass spectral tools as well as TGA.
Keywords: electrophilic mechanism, synthesis, cyclopalladated complexes, spectroscopy
Introduction
From this point of view, the aim of this work is to explore the
effect of Para substituted donating and withdrawing groups
Cyclopalladation reactions have been widely studied, and N- on the C-H activation process.
donor ligands capable of giving palladacycles have been thor-
oughly explored during the last decades and more recently
Experimental
with regard to the synthesis, reactivity, structural and mecha-
nistic aspects.[1–7] Cyclometallated complexes present numer-
ous applications in organic and organometallic synthesis,[8] in
Materials and Reagents
insertion reactions,[9] in optical resolution,[10] in the determi-
P-anisaldhyde, p-anisidine and p-nitro benzaldhyde and Pd
nation of the enantiomeric excess,[11] in the synthesis of new
metal misogynic compounds,[12] in biologically active com-
pounds,[13,14] in catalytic materials,[15] in the building of new
self assembly supramolecular species,[16] and to promote
unusual coordination environments.[17] As an agent for the
intermolecular activation of C-H bonds, palladium(II) was
historically classified as a typical electrophile.[18] Bruce
et al.[19] demonstrated that the palladium atom acts as an
electrophile since the favored site of attack in asymmetrically
substituted aromatic ring with an electron-releasing group. It
is commonly assumed that the first step in the cyclopallada-
tion reaction is coordinative bonding of the nitrogen to the
palladium atom.[20,21] Takahashi and Tsuji[22] reported that
cyclopalladation of asymmetrically substituted azobenzene
leads to a palladium–carbon s bond formed preferentially
with the benzene ring having and electron-donating group.
Facile C-H activation is also well known and electron rich.
(OAC)2 are Merck or Aldrich and organic solvents used
(methanol, ethanol, dichloromethane, n-hexane diethyl ether,
and acetone) were HPLC or extra pure grades and were used
without further purification.
Instruments
Percentages of C, H and N were determined in the Micro
analytical Laboratory, Cairo University, Giza, Egypt. IR
spectra were recorded using KBr pellets on a Perkin-Elmer
1430 Spectrometer in the region (4000–200 cm¡1) at Tanta
University, Gharbia, Egypt. Electronic spectra were mea-
sured in UV/Vis range (195–1100 nm) using a Perkin Elmer
lambda 35 UV/Vis at Al-Azhar University, Cairo, Egypt.
1
The HNMR spectra were recorded on DEITAZ NMR 500
MHZ Spectrometer at the National Research Centre, Dokki,
Giza, Egypt. The mass spectra were recorded on GC-MSA-
QP 5050A Shimadzu at the National Research Centre,
Dokki, Giza, Egypt. Magnetic susceptibility measurements
were carried out at room temperature on a Sherwood Scien-
tific Magnetic Balance at Mansoura University, Egypt. Ther-
mogravimetric analysis (TGA) was carried out with
Shimadzu thermal analyzer model 60H at Cairo University,
Egypt.
Address correspondence to A. M. Nassar, Chemistry Depart-
ment, Faculty of Science, Al-Azhar University, Nasr City,
Cairo, Egypt. E-mail: nassar_tanta@yahoo.com
Color versions of one or more of the figures in the article can be
found online at http://www.tandfonline.com/lsrt.

814
Nassar et al.
Synthesis of Bromo Bridged Complexes (1c) and (2c)
Synthesis of Schiff Base Ligands (1) and (2)
The Schiff base ligands used under study were prepared by The complexes were prepared by dropwise addition of metha-
the same general procedure. P-anisidine 5.0 g (0.041 mol) dis- nolic solutions of NaBr to a solution of complex (1a) or (2a)
solved in absolute ethanol (30 mL) was mixed with the in dichloromethane. The precipitates immediately formed for
appropriate aldehydes, p-anisaldehyde or p-nitrobenzalde- complexes (1c) and (2c), respectively. The precipitates washed
hyde (0.041 mol), in 20 mL absolute ethanol. The reaction with hot water, acetone and diethyl ether and air dried. Com-
mixture was refluxed for 1 h then cooled at room temperature plex (1c) yellow solid, M.p. 276ꢁC; M. Wt. 852.21; Anal.
and allowed to form crystals. The crystals of the desired Calcd. for C30H29Br2N2O4Pd2: C, 41.79; H, 3.39; N, 3.25;
ligand were collected by filtration through a Buchner funnel, Pd, 24.69%, Found: C, 41.75; H, 3.35; N, 3.21; Pd, 24.60%;
recrystallized from petroleum ether (60–80), and finally dried main IR Peaks (KBr, cm¡1): ⱱ(CHN)1583. nasPd-Br
at room temperature. Ligand 1, white crystals, M.p. 154ꢁC; 320 cm¡1, nS Pd-Br 307 cm¡1, nPd-C 590 cm¡1
M. Wt. 241.29; Anal. Calcd. for C15H15NO2: C, 74.76; H,
.
Complex (2c) Browen solid, M.p. 190ꢁC; M. Wt. 883.21;
6.27; N, 5.81; Found: C, 74.64; H, 6.20; N, 5.92; main IR Anal. Calcd. for C28H22Br2N4O6Pd2: C, 38.08; H, 2.51; N,
Peaks (KBr, cm¡1): ⱱ(CHN)1606. Ligand (2) Orange crystals 6.34; Pd, 24.10%, Found: C, 38.05; H, 2.29; N, 6.37; Pd,
M.p. 128ꢁC; M. Wt. 256; Anal. Calcd. for C14H12N2O3: C, 24.15%; main IR Peaks (KBr, cm¡1): ⱱ(CHN) 1594. nasPd-Br
65.62; H, 4.72; N, 10.93, Found: C, 65.31; H, 4.20; N, 3.50; 323 cm¡1, nS Pd-Br 309 cm¡1, nPd-C 600 cm¡1
main IR Peaks (KBr, cm¡1): ⱱ(CHN)1610.
.
Synthesis of Iodo Bridged Complexes (1d) and (2d)
The complexes were prepared by dropwise addition of a
methanolic solutions of NaI, to a solution of complex (1a) or
(2a) in dichloromethane. The precipitates immediately
formed for complexes (1d) and (2d) respectively. The precipi-
tates washed with hot water, acetone and diethyl ether and
air dried. Complex (1d) yellow solid, M.p. 245ꢁC; M. Wt.
947; Anal. Calcd. for C30H32I2N2O6Pd2: C, 35.99; H, 3.40;
N, 2.80; Pd, 21.26%, Found: C, 35.95; H, 3.42; N, 2.80; Pd,
21.29%; main IR Peaks (KBr, cm¡1): ⱱ(CHN)1581. nasPd-I
313 cm¡1, nS Pd-I 295 cm¡1,nPd-C 555 cm¡1. Complex (2d)
yellow solid, M.p. 220ꢁC; M. Wt. 977; Anal. Calcd. for
C28H22 I2N4O6Pd2: C, 34.42; H, 2.27; N, 5.37; Pd, 21.78%,
Found: C, 34.50; H, 2.29; N, 5.76; Pd, 21.75%; main IR
Peaks (KBr, cm¡1): ⱱ(CHN) 1596. nasPd-I 314 cm¡1, nS Pd-I
Synthesis of Acetato Bridged Complexes (1a) and (2a)
A solution of Pd(OAC)2 (0.45gm,0.002 mol) in methylene
chloride (30 mL) was treated with solution of the imine 1 or 2
(0.002 mol) and refluxed for 15 min for complex 1a and 3 h
for complex 2a then filtered. The filtrate was concentrated to
10 mL. Addition of n-hexane to the solution allow to precipi-
tate the complexes, which were washed with diethyl ether and
air-dried. Complex (1a) yellow solid, M.p. 190ꢁC; M. Wt.
811.48; Anal. Calcd. for C34H34N2O8Pd2: C, 50.32; H, 4.22;
N, 3.45; Pd, 26.23%, Found: C, 50.31; H, 4.20; N, 3.46;
Pd,26.20%; main IR Peaks (KBr, cm¡1): ⱱ(CHN)
1579.nas(OAC) 1573 cm¡1
, , nPd-C
nS (OAC)1414 cm¡1
570 cm¡1. Complex (1b) orange solid, M.p. 175ꢁC; M. Wt.
841.43; Anal. Calcd. for C32H28N4O10Pd2: C, 45.68; H, 3.35;
N, 6.66; Pd, 25.30%, Found: C, 45.68; H, 3.20; N, 6.69; Pd,
25.35%; main IR Peaks (KBr, cm¡1): ⱱ(CHN)1594.
297 cm¡1,nPd-C 530 cm¡1
.
nas(OAC) 1580 cm¡1, nS (OAC)1419 cm¡1, nPd-C 593 cm¡1
.
Synthesis of Monomeric Complexes (1e) and (2e)
A mixture of pyridine (0.079 g, 0.001 mol) and complex (1b)
(0.4 g, 0.0005 mol) or complex (2b) (0.397 g, 0.0005 mol)
was refluxed for 2 h in dichloromethane, and the solid formed
of complexes (1e) or (2e) was filtered, washed with acetone
and diethyl ether, and then air dried. Complex (1e) Browen
solid, M.p. 185ꢁC; M. Wt. 461; Anal. Calcd. for
C20H21ClN2O3Pd: C,50.12; H,4.24; N 5.85; Pd 22.20%,
Found: C, 50.15; H, 4.40; N 5.80; Pd,22.20%; main IR Peaks
Synthesis of Chloro Bridged Complexes (1b) and (2b)
The complexes were prepared by drop wise addition of a
methanolic solutions of NaCl, to a solution of complex (1a)
or (2a) in dichloromethane. The precipitates immediately
formed for complexes (1b) and (2b), respectively. The precipi-
tates washed with hot water, acetone and diethyl ether and
air dried. These complexes can also prepared by the refluxing
of equal amounts of PdCl2 and ligands (a) or (b) in methanol
for 1 h for complex (1b) and 4 h for complex (2b). Complex
(1b) yellow solid, M.p. 280ꢁC; M. Wt. 763.33; Anal. Calcd.
for C30H32Cl2N2O6Pd2: C, 45.02; H, 4.03; N, 3.50; Pd,
26.59%, Found: C, 45.05; H, 4.09; N, 3.50; Pd, 26.55%; main
IR Peaks (KBr, cm¡1): ⱱ(CHN)1581. nasPd-Cl 320cm¡1, nS
Pd-Cl 303 cm¡1, nPd-C 576 cm¡1. Complex (2b) light green
solid, M.p. 249ꢁC; M. Wt. 794.24; Anal. Calcd. for
(KBr, cm¡1): ⱱ(CHN)1581. nPd-Cl 325 cm¡1, nPd-C 555 cm¡1
.
Complex (2e) yellow crystals, M.p. 250ꢁC; M. Wt. 476.22;
Anal. Calcd. for C19H16ClN3O3Pd: C, 47.92; H, 3.39; N
8.82; Pd, 22.35%, Found: C, 47.90; H, 3.35; N, 8.80; Pd,
22.30%; main IR Peaks (KBr, cm¡1): ⱱ(CHN) 1602. nPd-Cl
333 cm¡1, nPd-C 588 cm¡1
.
Synthesis of Monomeric Complexes (1f) and (2f)
C28H22Cl2N4O6Pd2: C, 42.43; H, 2.79; N, 7.05; Pd, 26.80%, A mixture of triphenyl phosphine (0.262 g, 0.001 mol) and
Found: C, 42.30; H, 2.73; N, 7.02; Pd, 26.85%; main IR complex (1b) (0.4 g, 0.0005 mol) or complex (2b) (0.397 g,
Peaks (KBr, cm¡1): ⱱ(CHN)1590. nasPd-Cl 335 cm¡1, nS Pd- 0.0005 mol) was refluxed for 2 h in dichloromethane, and the
Cl 310 cm¡1, nPd-C 603 cm¡1
.
solid formed of complexes (1f) or (2f) was filtered, washed with

Ortho CꢀꢀH Activation of Aromatic Rings
815
Sch. 1. General mechanism of cyclometallation reaction.
acetone and diethyl ether, and then air dried. Complex (1f) yel- 4.19; Pd, 16.10%; main IR Peaks (KBr, cm¡1): ⱱ(CHN)1572.
low crystals, M.p. 280ꢁC; M. Wt. 608.44; Anal. Calcd. for nPd-Cl 330 cm¡1, nPd-C 602 cm¡1, nPd-PPh3 1095 cm¡1
C33H29ClNO2PPd: C, 61.50; H, 4.54; N, 2.17; Pd, 16.15%,
.
Found: C, 61.38; H, 4.40; N 2.15; Pd, 16.50%; main IR Peaks
(KBr, cm¡1): ⱱ(CHN)1596. nPd-Cl 331 cm¡1, nPd-C 596 cm¡1
,
Results and Discussion
nPd-PPh3 1098 cm¡1. Complex (2f) yellow solid, M.p. 275ꢁC;
M. Wt. 623.41; Anal. Calcd. for C32H26ClN2O3PPd: C, 58.29;
H, 3.97; N, 4.25; Pd, 16.14%, Found: C, 58.25; H, 3.90; N
Cyclopalladation of two Para-substituted imines N-
(4-methoxy benzylidene)-4-methoxyaniline 1 and N-(4-nitro
Reagents and conditions: (i) Pd(OAc)2/CH2Cl2; (ii) (NaCl/MeOH), (iii) (NaBr/MeOH),
(iv) (NaI/MeOH), (v) Pyridine/MeOH, (vi) PPh3/CHCl3
Sch. 2. Cyclopalladation reactions of ligand (1).

816
Nassar et al.
Reagents and conditions: (i) Pd(OAc)2/CH2Cl2; (ii) (NaCl/MeOH), (iii) (NaBr/MeOH),
(iv) (NaI/MeOH), (v) Pyridine/MeOH, (vi) PPh3/CHCl3
Sch. 3. Cyclopalladation reactions of ligand (2).
benzylidene)-4-methoxyaniline 2 was carried out under the electrophilic C-H activation mechanism.[23,24] Treatment of
similar reaction conditions. For the Schiff base derivative the acetato bridged complexes 1a and 2a with NaCl, NaBr,
contain Para substituted releasing group in the metallated and NaI gave the corresponding halido bridged complexes
ring 1, 90% yield of cyclopalladated complex [Pd {p- which have the general formula [Pd{pꢀꢀYꢀꢀC6H4ꢀꢀCH H
OCH3C6H4ꢀꢀCH H NꢀꢀC6H4 ꢀꢀpꢀꢀOCH3] {m-OAC}2 NꢀꢀC6H4ꢀꢀpꢀꢀOCH3]2{m-X}2 where X D Cl (1b, Y D
2
(1a) after 15 min stirring with a bidentate [C (sp2 phenyl), OCH3; 2b, Y D NO2), X D Br (1c, Y D OCH3; 2c, Y D
N] ligand and a central Pd (m -OAc)2Pd Unit was obtained. NO2), and X D I (1d, Y D OCH3; 2d, Y D NO2). Treatment
On the other hand, the Schiff base derivative contain Para of complexes 1b and 2b with pyridine and PPh3 resulted in
withdrawing group in the metallated ring 2, only 25% yield the formation of mononuclear cyclopalladated complexes
of the cyclopalladated complex [Pd {pꢀꢀNO2ꢀꢀC6H4ꢀꢀCH [PdꢀꢀR{pꢀꢀYꢀꢀC6H4ꢀꢀCH
D
NꢀꢀC6H4ꢀꢀpꢀꢀOCH3],
H NꢀꢀC6H4ꢀꢀpꢀꢀOCH3] {m-OAC}2 (2a) after 3h stirring where R D pyridine and Y D OCH3 (1e), R D pyridine and
2
under reflux with a bidentate [C(sp2,phenyl),N] ligand and a Y D NO2(2e),R D PPh3 and Y D OCH3(1f) and R D PPh3
central Pd(m -OAc)2Pd unit was obtained. From this result and Y D NO2 (2f), where the nitrogen and phosphorous
we can conclude that electron donating functional groups atoms are trans to the metallated carbon atom. The cyclo-
favor CꢀꢀH activation, while electron withdrawing func- metallation reaction (Scheme 1) would probably proceed by
tional groups reduce CꢀꢀH activation, which confirmed an initial rapid coordination of the nitrogen to the metal ion,

Ortho CꢀꢀH Activation of Aromatic Rings
817
Fig. 1. Mass spectrum of ligand (2).
followed by attack of the metal at an ortho position of the groups. The mass spectra of ligands 1 and 2 shows the molec-
ring to form five-membered ring.
ular ion peak at m/e D 241 for ligand 1 and m/e D 256 for
The acetato bridged complexes 1a and 2a have folded ligand 2 (Figure 1), which is in agreement with their molecu-
open book shape structure. Metathetical reaction of com- lar weight.
plexes 1a and 2a in dichloromethane with methanolic solu-
tions from NaCl, NaBr, and NaI gave the corresponding
chloro bridged 1b and 2b, bromo bridged 1c and 2c and iodo
bridged 1d and 2d complexes, respectively with unfolded pla-
nar structure. Complexes 1b and 2b can also obtained by
Cyclopalladated Complexes
Infrared spectra
reaction of ligands 1 and 2 with PdCl2 in methanol. Mono-
merization of complexes 1b and 2b by pyridine gave the com-
plexes 1e and 2e, respectively and by PPh3 gave the
complexes 1f and 2f, respectively.
All the complexes display weak multiple bands in the range
3000–2840 cm¡1 due to aromatic and aliphatic C¡H
stretches. All the orthopalladate species display strong band
in the range (1592–1581 cm¡1) cm¡1. This band is due to the
azomethine fragment (¡C D N) of the Schiff base. The shift
of the C D N stretch to lower wave number as compared to
that of the free ligands is expected due to N coordination of
the azomethine in all complexes.[26]
The Ligands
The infrared spectra of complexes 1a and 2a exhibit the
typical asymmetric and symmetric stretching modes of the
acetato groups with strong absorptions at 1579–1580 cm¡1
and 1414–1419 cm¡1. Dn of acetato bands indicate the pres-
ence of bridging mode.[27] The infrared spectra of complexes
1b and 2b show two asymmetric n Pd¡Cl stretching absorp-
tions at 320–335 cm¡1 and 303–310 cm¡1, which give good
evidence to the presence of chloro bridges. The n Pd¡Br
(bridging) in complexes 1c and 2c shows stretching absorp-
tions at 320–323 cm¡1 and 307–309 cm¡1 and n Pd¡I (bridg-
ing) in complexes 1d and 2d shows absorption at 313–
314 cm¡1 and 295–297 cm¡1. The far infrared region of all
The infrared spectra of ligands 1 and 2 exhibited a strong
bands at 1606 cm¡1 and 1610 cm¡1 attributed to C D N
stretching vibration. IR spectrum of ligand 2 shows strong
bands at 1511, 1338 cm¡1 assigned to asymmetric and sym-
1
metric nitro group.[25] The HNMR spectra of the ligands 1
and 2 in DMSO show a singlet at d D 8.53 ppm and d D
8.83 ppm, respectively, attributed to the resonance of imine
proton. The spectra show signals at d D 3.81 ppm due to
methoxy group protons. Spectrum of 1shows another signal
observe at d D 3.82 ppm for the second methoxy group pro-
tons. The spectra show the AA0BB0 system at d D 6.93–
6.98 ppm due to (H2, H2’) and d D 7.83–7.87 ppm due to
(H3, H30) for 1 and at d D 6.99–7.04 ppm due to (H2, H20)
and 8.14–8.36 ppm (H3, H30) for 2, which is characteristic of
meta and ortho aromatic protons of para disubstituted aryl
Fig. 2. Geometrical isomers of complexes (1e) and (2e).
Fig. 3. Geometrical isomers of complexes (1a) and (2a).

818
Nassar et al.
Fig. 4. Mass spectrum of complex (1b).
complexes show a medium intensity band in the range (507– gen atom, Crociani et al.[30] reported that the higher fre-
515) cm¡1. This band is attributed to n M¡N, This band quency band was attributed to n Pd¡Cl trans to $-bonded
observed as broad in the spectrum of complex 1e and 2e due carbon. For the configuration of the monomeric complexes
to the vibrational coupling of two M¡N bonds in this there are two possible isomers as shown in (Figures 2A and
complexes.[28,29]
B) For the infrared spectra of complexes 1e, 1f and 2e, 2f the
All the complexes show strong band in the range 520– band observed in range 325–336 cm¡1 indicates that the com-
600 cm¡1 due to n Pd¡C. The reaction of pyridine and PPh3 plexes are in trans arrangement to Pd-C bond (Figure 2A).
with complexes 1b and 2b undergoes scission of the Pd2X2
1HNMR spectra
The H1NMR spectra of complexes 1a and 2a show the pres-
unite to give mononuclear cyclopalladate complexes 1e and
2e and 1f and 2f respectively, on the basis of higher trans-
influence of a $-bonded carbon compared with that of nitro-
ence of a broadened unresolved pattern in the aromatic
Fig. 5. Electronic spectrum of complex (1d).

Ortho CꢀꢀH Activation of Aromatic Rings
819
region which can explain the folded open-book shape struc- Absorption below 400 nm are due to d¡d spin allowed tran-
ture of this complexes, the folded structure of the acetate sition from the three lower lying ‘d’ levels to the empty
bridged complexes caused the aromatic ring protons signals dx2¡y2 orbital. The bands are attributed to A1g ! A2g,
1
1
1
1
1
to be broadened, probably due to the anisotropic effect of the 1A1g ! B1g, and A1g ! Eg, transitions.[36] The electronic
ring currents.[31]
spectra as well as the diamagnetic behavior of the cyclopalla-
There are two possible structures for the arrangement of dated complexes suggest the square planar arrangement of
the ligands in the acetates,[32] ab-hg type which the methyl the ligands around the palladium atoms.
protons are magnetically equivalent (Figure 3A), and non-
equivalent methyl protons in an ab-gh type (Figure 3B).
In case of ab-hg type, the magnetically equivalent methyl
protons appear as sharp singlet in the range 1.8¡2.2 ppm,
but in case of ab-gh type, the nonequivalent methyl hydro-
gens appear as two resonances at 1.12¡2.2 ppm.
Thermal analysis
TGA of complex 1c
1
The HNMR spectra of complexes 1a and 2a show trans
arrangement of the ligand in the acetates, inferred from the
acetate bridged methyl protons appearing as singlet signal at
d D 1.81ppm, ascribable to ab-hg structural isomer; on the
1
other hand, the HNMR spectra of the halo bridged and
monomeric complexes show the absence of signals in the
TGA of complex 1d
range 1.1–2.2 ppm, indicating the absence of acetates.
1
The HNMR spectra of complexes 1f and 2f show cou-
pling of imine proton to phosphorous, also irradiation of the
31P resonance caused the (CHHN) doublet resonances to col-
lapse to singlets.[33]
Orthometallation of Schiff base ligands after deprotona-
tion of ortho proton (H2) clears from comparing the
1HNMR spectra of the ligands 1 and 2 and their complexes.
The 1HNMR spectra of the ligands show the AA’BB’ system,
1
on the other hand, the HNMR spectra of the complexes
show change in AA’BB’ system into a set of three different
signals (XYZ) pattern corresponding to the remaining pro-
tons (H2¡, H3, and H3¡) of the orthometallated ring. This
result shows that one aromatic ring proton (H2) was substi-
TGA of complex 1f
TGA of complex 2f
1
tuted by Pd, also the HNMR spectra of the cyclopalladate
complexes show upfield shift of the azomethine proton is con-
sistent with the weakening of the CHN bond due to the N-
coordination and the trans disposition of the Pd and the pro-
ton around the CHN.[34] All the spectra showing a sharp sig-
nal in the range (3.30–3.74 ppm) refer to the equivalent
methoxy group protons.
Mass spectra
The mass spectrum of the complex 1b (Figure 4) shows a
molecular ion peak at m/e D 800.33 which is corresponding
to the cyclopalladate moiety (C30H32Cl2N2O6Pd2) and the
base peak was found in the spectrum at m/e D 240.95 corre-
sponding to the Schiff base moiety after ortho deprotonation.
Mass spectrum of complex 1f shows a molecular ion peak at
m/e D 644.44 corresponding to the cyclopalladate moiety
(C33H29ClNO2PPd). The base peak was found in the spec-
trum at m/z D 262 (100%) corresponding to the PPh3.
Conclusion
From two Schiff base ligands {N - (4-methoxy benzylidene)
4-methoxyaniline (1) and
-4-methoxyaniline (2)}, two new series of cyclopalladated
complexes (1a–f) and (2a–f) were synthesized and character-
ized. The effect of Para substituted electron donating and
N
-
(4-nitro benzylidene)
Electronic spectra and magnetic measurements
The electronic spectra of the cyclopalladated complexes were withdrawing groups on the C-H activation process was
recorded at 800–200 nm in DMF. The broad band in the visi- studied. The palladium ion acts as an electrophile as the
ble region of 420–480 nm may be assigned to the charge favored site of attack in substituted aromatic ring with an
transfer from dp (Pd) ! metallated carbon (Figure 5).[35] electron-releasing group, we can conclude that, electron

820
Nassar et al.
donating functional group favors C-H activation, while
electron withdrawing functional group reduces C-H activa-
tion, which confirms an electrophilic C-H activation
mechanism.
pentacoordination in palladium(II). Organometallics 1999, 18,
5484–5487.
18. Rayabov, A. D. Thermodynamics, kinetics, and mechanism of
exchange of cyclopalladated ligands. Inorg. Chem. 1987, 26, 1252–
1262.
19. Bruce, M. I.; Goodall, B. L.; Stone, G. A. Cyclometallation reac-
tions. Part 17. Comparative studies of the manganation and palla-
dation of some substituted azobenzenes J. Chem. Soc. Dalton
Trans. 1978, 687–694.
References
20. Viatcheslav, I. S. Asymmetric cyclopalladation as a tool of enantio-
selective synthesis. Pure Appl. Chem. 1983, 55,1837–1844.
21. Parshall, G. W. Intramolecular aromatic substitution in transition
metal chemistry. Acc. Chem. Res. 1970, 3, 139–144.
22. Takahashi, H.; Tsuji, J. Organic syntheses by means of noble metal
compounds. XXXIII. Carbonylation of azobenzene-palladium
chloride complexes. J. Organomet. Chem. 1967, 10, 511–517.
23. Jung, M. J.; Youn, S. W. Pd-catalyzed ortho-methylation of aceta-
nilides via directed C-H activation. Bull. Korean Chem. Soc. 2011,
32, 2865–2866.
1. Omae, I. Organometallic intramolecular-coordination compounds
containing a nitrogen donor ligand. Chem. Rev. 1979, 79, 287–321.
2. Dunina, V. V.; Zalevskaya, O. A.; Potapov, V. M. General princi-
ples and characteristics of cyclopalladation reactions. Russ. Chem.
Rev. 1988, 57, 250–269.
3. Newkome, G. R.; Puckett, W. E.; Gupta, W. K.; Keifer, G. E.
Cyclometalation of the platinum metals with nitrogen and alkyl,
alkenyl, and benzyl carbon donors. Chem. Rev. 1986, 86, 451–489.
4. Omae, I. Organometallic Intramolecular-Coordination Compounds;
Elsevier, Amsterdam, 1986.
24. William, L.; Brennessel, W.; Jones, W. C-H Activation of phenyl
imines and 2-phenylpyridines with [Cp*MCl2]2 (M) Ir, Rh): regiose-
lectivity, kinetics, and mechanism. Organometallics 2009 28, 3492–
3500.
5. Omae, I. Recent studies on organometallic intramolecular-coordi-
nation compounds. Coord. Chem. Rev. 1988, 83,137–167.
6. Koten, G. V. Tuning the reactivity of metals held in a rigid ligand
environment. Pure Appl. Chem. 1989, 61, 1681–1694.
25. Stuart, B. H. Infrared Spectroscopy: Fundamentals and Applications;
Wiley, London, 2004.
26. Belmonte, M.; Escudero, A. E.; Martin, E.; Kleij, A. Isolation and
characterization of unusual multinuclear Schiff base complexes:
rearrangements reactions and octanuclear cluster formation. Dalton
Trans. 2012, 41, 5193–5200.
7. Raybov, A. D. Mechanisms of intramolecular activation of C-H
bonds in transition metal complexes. Chem. Rev. 1990, 90, 403–424.
8. Pfeffer, M.; Sutter, J.; Rottevel, M. A.; Decian, A.; Fischer, J. Fur-
ther insight into the mechanism of the palladium induced carbocyc-
lisation of aryl rings. Tetrahedron 1992, 48, 2427–2440.
9. Ryabov, A. D.; Van Eldik, R.; Lebonge, G.; Pfeffer, M. Insertion
of alkynes into the palladium-carbon bond of palladacycles, mecha-
nistic information from high-pressure kinetic and x-ray structural
data. Organometallics 1993, 12, 1386–1393.
10. Wild, S. B. Resolutions and reactions of chiral phosphorus and
arsenic ligands. Coord. Chem. Rev. 1997, 166, 291–311.
11. Chooi, S. Y. M.; Leung, P. H.; Lim, C. C.; Mok, K. F.; Quek, G.
H.; Sim, K. Y.; Tan, M. K. Versatile chiral palladium(II) complexes
for enantiomeric purities of 1,2-diamines. Tetrahedron Asymm.
1992, 3, 529–532.
27. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compound; Wiley, New York, 1986.
28. Hassan, A. M.; Nassar, A. M.; Hussien, Y. Z.; Elkmash, A. N. Syn-
thesis, Characterization and biological evaluation of Fe (III), Co
(II), Ni (II), Cu (II), and Zn (II) complexes with tetradentate schiff
base ligand derived from protocatechualdehyde with 2-aminophe-
nol. Appl. Biochem. Biotechnol. 2012, 167, 581–594.
29. Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd
edn.; Methuen, London, 1958.
30. Crociani, B.; Boschi, I.; Pietropolo, R.; Belluco, V. Far-infrared
study of palladium (II) halogen complexes with chelating ligands
containing nitrogen and s carbon as donor atoms. J. Chem. Soc. A
1970, 92, 531–535.
12. Espinet, P.; Esteruelas, M. A.; Oro, L. A.; Serrano, J. L.; Sola, E.
Transition metal liquid crystals: Advanced materials within the
reach of the coordination chemist. Coord. Chem. Rev. 1992, 117,
215–274.
31. Hiraki, K.; Fuchita, Y.; Nakaya, H.; Takakara, S. Prepara-
tions and characterization of cyclopalladated complexes of
1-ethyl-2-phenylimidazole. Bull. Chem. Soc. Jpn. 1979, 52,
2531–2534.
32. Villa, J. M.; Gayoso, M.; Pereira, M. T.; Lopez, M.; Alonso, G.;
Fernandez, J. J. Cyclometallated complexes of PdII and MnI with
N,N-terephthalylidenebis (cyclohexylamine). J. Organomet. Chem.
1993, 445, 287–294.
33. Vicente, J.; Abad, J.; Frankland, A.; Ramiz, M.; Arelleino, D. Syn-
thesis and reactivity of 2-aminophenylpalladium(II) complexes:
insertion reactions of oxygen and carbon monoxide into carbon-
¡palladium bonds—new examples of “transphobia”. Chem. Eur. J.
1999, 5, 3066–3075.
34. Nassar, A. M.; Ibrahem, N. M.; Makky, E. A. Cyclopalladated
complexes of 4-methoxy-N-{(E)-(piperidin-1-yl)phenyl) methylene}
aniline: synthesis, spectral characterization and biological evalua-
tion. Egy. J. Chem. 2011, 54, 469–480.
13. Navarro, C.; Lopez, I.; Rodriguez, J.; Garcia, J.; Raithby, P.;
Masagur, J.; Alonsa, C. Analysis of two cycloplatinated com-
pounds derived from N-(4-methoxyphenyl)-alphabenzoylbenzyli-
denamine. Comparison of the activity of these compounds with
other isostructural cyclopalladated compounds. J. Med. Chem.
1993, 36, 3795–3801.
14. Navarro, R. C.; Lopez, S. I.; Gonzalez, V. M.; Perez, J. M.;
Alvarez, V. A.; Martin, A.; Raithby, P. R.; Masaguer, J. R.;
Alonso, C. Cyclometalated complexes of platinum and palladium
with N-(4-chlorophenyl)-a-benzoylbenzylideneamine. In vitro cyto-
static activity, dna modification, and interstrand cross-link studies.
Inorg. Chem. 1996, 35, 5181–5187.
15. Bose, A.; Saha, C. Orthometalated palladium (II) complex-cata-
lysed reduction of nitroalkanes and nitriles. J. Mol. Catal. 1989, 49,
271–283.
ꢀ
16. Lopez, T. M.; Fernandez, A.; Fernandez, J. J.; Castro, J. S.; Suarez,
A.; Vila, J. M.; Pereira, M. T. Cyclometalated palladium(II) frag-
ments as building blocks in the construction of new heteronuclear
metalomacrocycles. Organometallics 2001, 20, 1350–1353.
35. Kamaraj, K.; Bandyopadhyay, D. Mechanism of palla-
dium¡carbon bond oxidation: dramatic solvent effect. Organome-
tallics 1999, 18, 438–446.
17. Vila, J. M.; Pereira, M. T.; Ortigueira, J. M.; Fernandez, J. J.;
36. Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd edn.; Elsev-
ier, Amsterdam, 1984.
ꢀ
ꢀ
Fernandez, A.; Lopez, T. M.; Adams, H. Cyclometalated com-
plexes with triphosphine ligands: a novel route for promoting