Coupling of arylamines with coordinated Arylazopyrimidines in palladium(II) complexes

Article abstract of DOI:10.1002/1099-0682(200205)2002:5<1124::AID-EJIC1124>3.0.CO;2-K

2-(Arylazo)pyrimidine (1) reacts with palladium chloride and gives Pd(aapm)Cl₂ (2) which undergoes a palladium(II)-mediated novel carbon-nitrogen bond formation reaction of the aromatic amines to the ortho-C-H function of the pendant aryl ring

Full text of DOI:10.1002/1099-0682(200205)2002:5<1124::AID-EJIC1124>3.0.CO;2-K

FULL PAPER
Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II)
Complexes
Prasanta Kumar Santra,[a] Prithwiraj Byabartta,[a] Surajit Chattopadhyay,[b]
Larry R. Falvello,[c] and Chittaranjan Sinha*[a]
Keywords: Nitrogen heterocycles / Palladium / N ligands / Aminations / X-ray structure / EHMO calculation
2-(Arylazo)pyrimidine (1) reacts with palladium chloride and
itions (NIR region) unlike the parent complex, Pd(aapm)Cl2.
gives Pd(aapm)Cl2 (2) which undergoes a palladium(II)-me- The cyclic voltammogram shows an oxidation couple at pos-
diated novel carbon−nitrogen bond formation reaction of the
aromatic amines to the ortho-C−H function of the pendant
itive potential to SCE. The EHMO calculation suggests that
the spectral feature in Pd(aapm)Cl2 (2) is due to the MLCT
aryl ring in the coordinated 2-(arylazo)pyrimidine. The react- [d(Pd)Ǟπ*(azo)] whereas in Pd(aapm-N-C6H4-X-p)Cl (3−6)
ant, Pd(papm)Cl2 (2a) and the product, Pd(papm-N-C6H4-
OCH3-p)Cl (5a) have been characterised by X-ray crystal
structure. The coupled products exhibit low energy trans-
the transition at the NIR region is due to the intravalence
charge transfer (IVCT) transition.
Introduction
gives (azophenolatopyrimidine)palladium(II) complex.[19]
This is believed to be formed by the hydroxylation of the
CϪH bond ortho to the azo group of 2-(arylazo)pyrimidine.
Heterocyclic bases (e.g., pyridine and derivatives R-Py) re-
act with Pd(aapm)Cl2 and substitute the ligand followed by
isomerisation to trans-Pd(R-Py)2Cl2.[26] In continuation of
our work to explore the reactivity of palladiumϪaapm
complexes, we discuss in this article the palladium(II)-medi-
ated CϪN coupling reaction of aromatic amines to the or-
tho-CϪH function of the pendant aryl ring in the coordin-
ated 2-(arylazo)pyrimidine. The reactant and product have
been characterised by X-ray crystallography. The coupled
products exhibit different electronic and redox behaviour
from that of the reactants and have been accounted for by
EHMO calculations.
Arylazoheterocycles are light-sensitive, redox-active, and
efficient colorant materials.[1Ϫ9] Presence of an arylazo
group at the ortho position of the N-heterocycles constitute
an azoimine, ϪNϭNϪCϭNϪ, function.[7Ϫ9] They have
successfully been used to stabilise the low-valent metal
redox state.[7Ϫ17] The coordination of arylazoheterocycles
to a metal centre alter the electrophilic character of the li-
gand and provides a fascinating area in the metal-assisted
organic synthesis. This has been observed in the metal com-
plexes of 2-(arylazo)pyridines.[18Ϫ25] The complexes un-
dergo different organic transformations at the pendant aryl
ring such as hydroxylation,[18Ϫ20] thiolation,[21Ϫ23] and the
CϪN coupling[24,25] reaction with aromatic amines. This
has encouraged us to design 2-(arylazo)pyrimidines [aapm
(1)][13,14,19] that is more π-acidic than 2-(arylazo)pyridine
due to the π-acidity order, pyrimidine Ͼ pyridine. Cop-
per(I)[13] and isomers of ruthenium(II) complexes[14] of 1
have been reported from our group. Palladium(II) forms [2-
Results and Discussion
(arylazo)pyrimidine]dichloropalladium(II)
[Pd(aapm)Cl2
(2)] and its reaction with OHϪ in air or Tollen’s reagent
Synthesis of Parent Complexes
[a]
Department of Chemistry, The University of Burdwan,
The general abbreviation of 2-(arylazo)pyrimidine is
aapm (1). 2-(Phenylazo)pyrimidine [papm (1a)], 2-(p-tolyl-
azo)pyrimidine [tapm (1b)], and 2-(p-chloro-phenylazo)pyri-
midine [Clpapm (1c)] are three ligands used in this work.
They act as N,NЈ-bidentate chelators with an azoimine
function, ϪNϭNϪCϭNϪ, where N refers to N(8)(azo)
and NЈ refers to N(3)(pyrimidine). The reaction of aapm
with Pd(MeCN)2Cl2 in MeCN has yielded orange-red
Pd(aapm)Cl2 (2) in high yield.[19,27]
Burdwan 713 104, India
Fax: (internat.) ϩ 91-0342/564452
E-mail: c
r sinha@yahoo.com
Ϫ Ϫ
[b]
[c]
Department of Chemistry and Chemical Technology,
Vidyasagar University,
Midnapore 721102, India
Departmento de Quimica Inorganica,
Universidad de Zaragoza,
Plaza San Fransisco S/N, 50009 Zaragoza, Spain
Supporting information for this article is available on the
WWW under http://www.eurjic.com or from the author.
1124
WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0505Ϫ1124 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2002, 1124Ϫ1131

Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II) Complexes
FULL PAPER
The C؊N Coupling Reaction
Addition of ArNH2 (in 1:1 stoichiometry) in an MeCN
suspension of Pd(aapm)Cl2 (2) under refluxing conditions,
changes the intense orange-red colour to greenish-brown.
Chromatographic purification of the reaction mixture sep-
arates an orange-brown band. The orange-brown band has
been identified as an amine product coupled to
Pd(aapm)Cl2 and is abbreviated Pd(aapm-N-C6H4-X-p)Cl
[where Xϭ H (3), Me (4), OMe (5), Cl (6)]. The reaction is
shown in Equation (1). The arylamination takes place at the
ortho position of the aryl ring (B ring) of aapm to the azo
function and forms a palladium(II) complex of tridentate
N,NЈ,NЈЈ-chelating ligand [where N is N(8)(azo); NЈ is
N(3)(pyrimidine); NЈЈ is N(15)(arylamine)]. Free ligands do
not couple with arylamines even if forced but they react
smoothly in the palladium(II) complex.
Figure 1. Electronic spectra of Pd(papm-N-OCH3-p)Cl (5a) in
CHCl3
itions (Figure 1). The spectroscopic data are collected in
Table 1. The transitions have been accounted from EHMO
calculation and are assigned to HOMOǞLUMO trans-
itions where the HOMO has a major share from the aryl-
amine group and the LUMO is dominated by the azo group
(vide infra). The observed trend of the band maxima (λmax
)
for the substituent in the aryl ring follows as p-chloroaniline
(6) Ͻ aniline (3) Ͻ p-toluidine (4) Ͻ p-anisidine (5). The
electron-donating substituent in the X-C6H4-N- fragment
increased the energy of the HOMO[27,28] and in p-anisidine
the energy was maximised. In p-choloroaniline the energy
of the HOMO was reduced due to the electron-withdrawing
effect of the Cl group. The substituent (R) in the arylazopy-
rimidine [R-C6H4-NϭN-C4H3N2 (1)] fragment also shifted
the band maxima in the order Pd(Clpapm-N-C6H4-X-p)Cl
(c) Ͼ Pd(papm-N-C6H4-X-p)Cl (a) Ͼ Pd(tapm-N-C6H4-X-
p)Cl (b). This is due to the lowering of the energy of the
LUMOs in c compared to a due to the electron-with-
(1)
Spectral Studies
Infrared spectra of Pd(aapm)Cl2 (2) show two distinct drawing power of Cl. The reverse case is seen in the com-
PdϪCl stretches [ν(PdϪCl) ϭ 340, 360 cmϪ1] in agreement plexes b due to the ϩI effect of the Me group that has a
with cis-PdCl2 configuration.[19,27] A sharp single band at destabilising effect on the LUMOs.
1
1400Ϫ1415 cmϪ1 corresponding to νNϭN in the ligand is
The H NMR spectra of the complexes are compared in
shifted to 1380Ϫ1385 cmϪ1 in Pd(aapm)Cl2. This lowering order to determine the bonding mode and stereochemistry.
of the frequency is due to N-coordination.[18] In the coupled The 1H NMR spectra of the ligands aapm (1) and
products, Pd(aapm-N-C6H4-X-p)Cl (3Ϫ6), νNϭN appears at Pd(aapm)Cl2 (2) were reported from our group.[13,26] The
1290Ϫ1300 cmϪ1; this significant reduction in azo stretch- signals were assigned on the basis of chemical shifts, spin-
ing may be due to the intramolecular charge delocalisation spin interaction and their effect on substitution. In
from the fused N-C6H4-X-p fragment to the azo function Pd(aapm)Cl2 the signals in general are downfield shifted
(vide infra). A single stretch at 340Ϫ350 cmϪ1 in 3Ϫ6 sup- compared to free ligand values.[26] Pd(aapm-N-C6H4-X-
ports the presence of a PdϪCl bond. A chloroform solution p)Cl show complex NMR spectra. The N-aryl (A-ring) and
of Pd(aapm)Cl2 exhibits a strong absorption[19] at ca. azoaryl (B-ring) proton signals appear on the upfield side.
425 nm. On comparison with the free ligand solution spec- The effect of the substituent X (X-C6H4-N) on the signal
tra,[13] the transition has been assigned to the MLCT band movement of the arylamine protons (A-ring protons: 16-H
which is also supported by an EHMO calculation (vide to 20-H) is in accordance with the electronic property of X.
infra). The spectral structure changes abruptly in Pd(aapm- It is observed that the electron-donating substituent (Me/
N-C6H4-X-p)Cl (3Ϫ6) and exhibits an intense band in the OMe) moves the signals upfield while the electron-with-
NIR region (750Ϫ1100 nm) with five/six well-defined trans- drawing group Cl moves them downfield with reference to
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131
1125

P. K. Santra, P. Byabartta, S. Chattopadhyay, L. R. Falvello, C. Sinha
FULL PAPER
Table 1. Solution spectral and cyclic voltammetric data
Compound
Electronic spectroscopic data[a]
Cyclic voltammetric data[b]
E1/2 [V] (∆Ep [mV])
λmax [nm] (ε [dm3·molϪ1·cmϪ1])
Oxidation
Reduction
E11/2
ϪE21/2
ϪE31/2
ϪE41/2
Pd(papm-N-C6H5)Cl (3a)
914 (6289), 819 (7419), 760 (6353), 474 (5975)[c],
410 (10327), 340 (23019)
1.174
(107)
1.275
(105)
1.021
(100)
1.185
(91)
1.042
(95)
1.129
(100)
1.005
(98)
1.190
(105)
1.000
(130)
1.115
(140)
0.957
(160)
0.945
(150)
0.347
(88)
0.364
(90)
0.627
(75)
0.316
(90)
0.382
(100)
0.435
(110)
0.425
(92)
0.389
(94)
0.422
(80)
0.873
(140)
0.834
(160)
0.958
(160)
0.751
(110)
0.943
(120)
0.889
(90)
0.865
(120)
0.783
(100)
0.825
(77)
0.982
(167)
1.103
(100)
1.109
(110)
1.048[d]
Pd(papm-N-C6H4-MeϪp)Cl (4a)
Pd(papm-N-C6H4-OCH3Ϫp)Cl (5a)
Pd(papm-N-C6H4-ClϪp)Cl (6a)
Pd(tapm-N-C6H5)Cl (3b)
921 (6518), 820 (7619), 761 (6925), 479 (6419)[c],
409 (8344), 345 (18295)
926 (8428), 821 (9648), 765 (8963), 473 (9046)[c],
416 (10919), 344 (23127)
901 (6531), 815 (7353), 755 (6689), 470 (6452)[c],
410 (8468), 343 (16846)
878 (6285), 792 (7532), 723 (6375), 470 (6014)[c],
409 (10469), 340 (22310)
1.192
(140)
1.124
(120)
1.114
(110)
1.069[d]
Pd(tapm-N-C6H4-MeϪp)Cl (4b)
Pd(tapm-N-C6H4-OCH3Ϫp)Cl (5b)
Pd(tapm-N-C6H4-ClϪp)Cl (6b)
Pd(Clpapm-N-C6H5)Cl (3c)
882 (6687), 798 (7728), 733 (7081), 466 (7715)[c],
395 (11649), 340 (23337)
894 (9323), 811 (11232), 756 (10426), 475 (11384)[c],
390 (15648), 340 (23254)
865 (8736), 792 (10362), 722 (9840), 474 (10243)[c],
394 (1612 2), 341 (28015)
918 (6715), 825 (7830), 765 (7129), 476 (8117)[c],
411 (10925), 340 (22833)
1.107
(175)
1.120
(180)
1.004
(200)
0.985
(170)
Pd(Cllpapm-N-C6H4-MeϪp)Cl (4c)
Pd(Clpapm-N-C6H4-OCH3Ϫp)Cl (5c)
Pd(Clpapm-N-C6H4-ClϪp)Cl (6c)
922 (8097), 828 (8995), 768 (8143), 481 (9660)[c],
405 (14475), 344 (26010)
0.436
(85)
0.415
(75)
0.365
(83)
0.832
(82)
0.761
(70)
0.745
(70)
930 (1530)[c], 830 (1728), 772 (1590), 407 (3850),
309 (17822)
905 (6792), 818 (7840), 760 (7145), 478 (7824)[c],
409 (11455), 336 (23121)
[a]
[b]
Solvent CHCl3.
Solvent MeCN, supporting electrolyte Bu4NClO4, Pt-disk milli working electrode, Pt-wire auxiliary electrode,
[c]
[d]
reference electrode SCE, at 298 K. Shoulder. Epc.
the unsubstituted system. A similar situation is also ob-
served for the substituent R on the arylazopyrimidine (R-
C6H4-NϭN-C4H3N2) protons of the B-ring (10-H to 13-
H). The cross interactions i.e., the effect of substituent X at
the N-aryl ring on the azoaryl protons or vice versa are
not remarkable. Azoaryl signals (10-H to 13-H) are severely
affected
in
Pd(aapm-N-C6H4-X-p)Cl
relative
to
Pd(aapm)Cl2 and are shifted upfield by 0.2Ϫ1.0 ppm. Pyri-
midine protons (4-H to 6-H of C-ring) in 3Ϫ6 are shifted
upfield by 0.1Ϫ0.5 ppm compared to the parent complex
Pd(aapm)Cl2 and are hardly affected by the substituents in
the aryl rings (X or R). Coupling of arylamine with the aryl
part of aapm (B-ring) has changed the electronic behaviour
of the molecule. The azopyrimidine function is electron-de-
ficient while the N-aryl (ring A) is a σ-donor and may cause
an intramolecular charge-transfer transition. This may be
the reason for the large upfield shifting of the arylazopyrim-
idine proton signals (4-H to 13-H). Solution electronic spec-
tral properties also support this behaviour (vide supra).
Figure 2. Drawing of Pd(papm)Cl2 (2a), showing the atom num-
bering scheme; non-hydrogen atoms are represented by their 50%
probability ellipsoids
a. Geometrical Features
The Pd(papm)Cl2 has a cis-PdCl2 configuration (Fig-
ure 2). The asymmetric unit of the crystal lattice consists of
two molecules. These two molecules have parallel cores with
˚
a Pd···Pd separation of 3.4201(5) A. The molecular packing
diagram viewed down the a axis shows a Pd···Pd separation
between asymmetric units of 3.9881(4) A and the pendant
X-ray Structure Studies
˚
The X-ray structures of Pd(papm)Cl2 (2a) and Pd(papm- phenyl rings are oriented in a parallel manner to ensure
N-C6H4-OCH3-p)Cl (5a) have been determined. Perspective some sort of π-interaction. The packing also exhibits a ben-
molecular views are shown in Figures 2 and 3 and the se- zene molecule at the special position and a weak Pd···Cl
˚
lected bond parameters are listed in Tables 2 and 3.
nonbonding intermolecular interaction (3.3Ϫ3.4 A) where
1126
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131

Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II) Complexes
FULL PAPER
Pd and Cl belong to two different molecules in the asym-
metric unit. The two independent molecules are chemically
similar. The PdN2Cl2 coordination sphere is planar with no
˚
atom deviating more than 0.06 A from the mean plane. The
˚
chelate ring is planar (deviation Ͻ 0.04 A) and the bond
angle lies in the range 77.6Ϯ2°. This deviates from ideal
square-planar angles. The ClϪPdϪCl angle is ca. 90.2°.
The pendant phenyl ring is no longer in plane with the che-
lated fragment and is twisted by ca. 42.5°.
The crystallographic asymmetry unit of Pd(papm-
NC6H4-OCH3-p)Cl contains 25 non-hydrogen atoms, of
which 17 lie in a plane nearly perpendicular to the crystallo-
graphic c axis. These atoms comprise of two chelate rings,
the aromatic groups fused to them, and the Pd and Cl
atoms. The remaining atoms of the asymmetric unit com-
prise the methoxyphenyl group. The Pd atom lies in the
square plane defined by the N3Cl donor centres (deviation
˚
0.02 A). Two chelate planes Pd,N(3),C(2),N(7),N(8) (devi-
Figure 3. Drawing of Pd(papm-N-C6H4-OCH3-p)Cl (5a), showing
the atom numbering scheme; non-hydrogen atoms are represented
by their 50% probability ellipsoids
˚
ation Ͻ 0.01 A) and Pd,N(15),C(14),C(9),N(8) (deviation
Ͻ 0.01 A) are coplanar and the chelate angles are 78.86(14)
˚
and 81.76(14)°, respectively. The pendant methoxyphenyl
ring (A ring) is inclined at an angle of 54.99(15)° with the
principal plane of 17 atoms.
˚
Table 2. Selected bond lengths [A] and angles [°] and their estim-
ated standard deviations for 2a
b. Bond Parameters
Distances
In Pd(papm)Cl2 the PdϪN(azo) bond length is slightly
Pd(1)ϪN(3)
Pd(1)ϪN(8)
Pd(1)ϪCl(1)
Pd(1)ϪCl(2)
N(7)ϪN(8)
C(2)ϪN(3)
2.014(3) Pd(1A)ϪN(3A)
2.041(3) Pd(1A)ϪN(8a)
2.2882(9) Pd(1A)ϪCl(1A)
2.2800(9) Pd(1A)ϪCl(2A)
1.260(3) N(7A)ϪN(8a)
1.344(4) C(2)ϪN(7)
2.009(3)
2.015(3)
2.2845(9)
2.2808(9)
1.260(3)
1.408(4)
1.407(4)
˚
longer (ca. 2.03 A) than that of the PdϪN(pyrimidine) (ca.
˚
2.01 A). This is the reverse in the case of other complexes.
In copper(I) and ruthenium(II) complexes of papm, the
MϪN(azo) distance is significantly shorter than that of the
MϪN(pyrimidine)
distance
[Cu(papm)2(ClO4):[13]
˚
C(2A)ϪN(3A)
1.346(4) C(2A)ϪN(7A)
˚
CuϪN(azo) 1.99 A, CuϪN(pyrimidine) 2.03 A; Ru(-
papm)2Cl2:[14] RuϪN(azo) 2.00 A; RuϪN(pyrimidine) 2.06
˚
Angles
˚
A]. This is due to π-back bonding involving metal t2 and
ligand π*(azo) orbitals.[7,13,14,29] The NϭN bond length is
N(3)ϪPd(1)ϪN(8) 77.74(10) N(3A)ϪPd(1A)ϪN(8A) 77.39(11)
Cl(1)ϪPd(1)ϪCl(2) 89.50(3) Cl(1A)ϪPd(1A)ϪCl(2A) 90.93(4)
N(3)ϪPd(1)ϪCl(1) 93.98(8) N(3A)ϪPd(1A)ϪCl(1A) 94.89(9)
N(3)ϪPd(1)ϪCl(2) 174.76(8) N(3A)ϪPd(1A)ϪCl(2A) 171.90(8)
N(8)ϪPd(1)ϪCl(2) 98.82(8) N(8A)ϪPd(1A)ϪCl(2A) 96.90(7)
N(8)ϪPd(1)ϪCl(1) 171.68(8) N(8A)ϪPd(1A)ϪCl(1A) 172.15(8)
[13]
˚
1.260(3) A and is comparable with that of the copper(I)
complex but shorter than that of the ruthenium(II)[14] com-
plexes. Efficient π-back bonding in (arylazopyrimidine)ru-
thenium(II) complexes enhances the NϭN distance.[14] In
Pd(papm-NC6H4-OMe-p)Cl the PdϪN(azo) [PdϪN(8)],
˚
Table 3. Selected bond lengths [A] and angles [°] and their estim-
ated standard deviations for 5a
Distances
Pd(1)ϪN(3)
Pd(1)ϪN(8)
Pd(1)ϪN (15)
N(7)ϪN(8)
C(2)ϪN(3)
2.029(4)
1.937(3)
2.026(3)
1.315(4)
1.362(5)
C(2)ϪN(7)
Pd(1)ϪCl(1)
C(9)ϪN(8)
C(14)ϪN(15)
1.379(5)
2.302(1)
1.336(5)
1.333(5)
Angles
N(3)ϪPd(1)ϪN(8) 78.86(14) N(8)ϪPd(1)ϪCl(1) 176.29(11)
N(8)ϪPd(1)ϪN(15) 81.76(14) N(1)ϪN(8)ϪN(7)
N(3)ϪPd(1)ϪCl(1) 97.52(11) Pd(1)ϪN(3)ϪC(2)
120.6(3)
110.0(3)
N(15)ϪPd(1)ϪCl(1) 101.88(10) Pd(1)ϪN(15)ϪC(14) 111.5(3)
N(3)ϪPd(1)ϪN(15) 160.59(14) Pd(1)ϪN(8)ϪC(9)
116.4(2)
Figure 4. Cyclic voltammogram of Pd(papm-N-C6H4-OCH3-p)Cl
(5a) in MeCN (0.1  Bu4NClO4) at 298 K
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131
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P. K. Santra, P. Byabartta, S. Chattopadhyay, L. R. Falvello, C. Sinha
FULL PAPER
PdϪN(pyrimidine) [PdϪN(3)], and PdϪN(arylamine) levels in the azoimine function [Equations (2) to (5)]. In the
[PdϪN(15)] distances are 2.028(4), 1.938(3), and 2.028(4) present examples, three negative redox couples may corre-
˚
˚
A, respectively. The NϭN length is 1.314(4) A. The NϭN spond to three reductions [Equations (2) to (4)]. However,
distance is not available for the free ligand; however, the the couple at the positive potential is difficult to assign as
data available in some azo ligands[30Ϫ32] suggest that it is PdII is redox-inactive. We suggest that this couple is due to
˚
ca. 1.25 A. In this case the NϭN distance in 5a is unusually oxidation of the ligand at the easily oxidisable azoarylamine
long compared to the parent complex 2a. This elongation chelated centre (chelate ring 2) [Equation (6)]. The first three
may be due to intramolecular charge delocalisation from redox couples [Equations (2), (3), and (6)] are quasireversible
the arylamine fragment (vide supra) to the azopyrimidine.
(∆EP ϭ 70Ϫ100 mV) and the fourth response [Equation (4)]
is irreversible in nature (∆EP Ͼ 130 mV).
Electrochemistry
The redox behaviour of 3Ϫ6 in acetonitrile solution was
examined by cyclic voltammetry at a Pt-disk working elec-
trode and the potentials are reported with reference to the
SCE. The results are given in Table 1, and a representative
voltammogram is shown in Figure 4. Pd(aapm)Cl2 does not
exhibit any redox response positive to the SCE and on the
negative side a quasireversible redox couple (∆EP Ͼ 130 mV)
followed by an irreversible response at potential Ͼ Ϫ1.0 V
is observed. Pd(aapm-N-C6H4-X-p)Cl (3Ϫ6) exhibited four
successive redox couples: one of them at high positive poten-
tial (1.0Ϫ1.3 V) and three at the negative side of the SCE.
Arylazopyrimidines are reduced at the negative potentials,
Ϫ1.0 to Ϫ1.4 V.[13,14] The reduction is regarded as the elec-
tron accommodation in the LUMO characterised by the azo-
imine function. There are four successive redox-accessible
(2)
(3)
(4)
(5)
(6)
Table 4. Relative percentages of atomic contributions to the MOs of the complexes in i and ii
% Contribution
MO
Energy [eV]
Pd
N (pyrimidine)
NϭN (azo)
azoaryl
arylamine
structure (i)
45a (HOMOϪ1)
46a (HOMO)
47a (LUMO)
Ϫ11.781
Ϫ11.569
Ϫ10.714
Ϫ9.259
93
83
9
2
1
99
8
67
7
48a (LUMOϩ1)
structure (ii)
57a (HOMOϪ2)
58a (HOMOϪ1)
59a (HOMO)
60a (LUMO)
61a (LUMOϩ1)
Ϫ11.623
Ϫ11.591
Ϫ11.457
Ϫ10.724
Ϫ9.260
84
81
37
9
6
4
4
53
9
98
68
13
1128
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131

Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II) Complexes
FULL PAPER
The oxidation of coordinated ortho-phenylenediamine to
semibenzoquinone diimine and benzoquinone diimine are
known in the literature and support this observation.[34,35]
The redox potential varies in the usual manner with the
substituent at the N-aryl (A-ring) and azoaryl (B-ring)
ring.[13,19]
Electronic Structure ؊ EHMO Calculation
In order to have an insight into the difference in elec-
tronic behaviour of Pd(aapm)Cl2 and Pd(aapm-N-C6H4-X-
p)Cl towards their spectral and electrochemical properties,
the MO calculations have been performed in the framework
of extended Hückel formalism on a model complex as
shown in structures i and ii (shown in Table 4). Crystallo-
graphic data were used as the parameters for bond lengths
and bond angles. The relative percentages of the atomic
contributions to the MOs are given in Table 4. In complex
Figure 6. LUMO and HOMO of Pd(papm-N-C6H5)Cl
(60a) (Figure 6, a) is characterised by the azo group (68%),
pyrimidine group (22%), and a small part of the metal or-
bital (dyz, 9%). The HOMO (59a) (Figure 6, b) is computed
from the admixture of the metal dxz orbital (37%) and the
N-aryl (ring A) (53%) and contributes to 90% of the charac-
ter of the function.
Some experimental findings may be rationalised on the
basis of EHMO results. The solution electronic spectra of
complex 2 may exhibit a HOMOǞLUMO (46aǞ47a)
transition corresponding to the MLCT band. The first re-
duction will certainly be localised due to electron accom-
modation at the LUMO (47a) [π*(azo)] and the second elec-
Figure 5. LUMO and HOMO of Pd(papm)Cl2
Pd(aapm)Cl2 the LUMO (47a) (Figure 5, a) is ligand-dom- tron will enter at the LUMOϩ1 (48a), which is purely pyri-
inated with 67% character of azo (NϭN), 5% of pyrimid- midine in character (99%). Thus, the second reduction is
ine-N and a small contribution (9%) from the Pd dyz or- irreversible in nature. The spectral and electrochemical be-
bital. The HOMO (46a) is mainly a metal orbital (83%) haviour of Pd(aapm-NAr)Cl is entirely different from the
(Figure 5, b) and contributed from d orbitals (total 78%, of parent complex. The low-energy spectral band is associated
2
which dz 41% and dxy 37%) and a small part of the 4s with a HOMOǞLUMO (59aǞ60a) charge transfer.[28,35,36]
orbital (5%). In complex Pd(aapm-N-Ar)Cl the LUMO The HOMO is from a major portion (53%) of the NϪAr
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131
1129

P. K. Santra, P. Byabartta, S. Chattopadhyay, L. R. Falvello, C. Sinha
FULL PAPER
brown compound, isolated in 45% yield. The orange-brown prod-
uct is the desired coupled product, Pd(papm-N-C6H5)Cl. Pd(papm-
N-C6H5)Cl (3a): C16H12ClN5Pd (416.2): calcd. C 46.16, H 2.89,
N 16.83; found C 46.25, H 2.95, N 16.70. IR (cmϪ1): νPd-Cl 370,
νNϭN 1296, νCϭN 1585. All the other compounds were prepared
according to the same procedure, the yields varied between 40 and
65% for the orange-brown compound. Pd(papm-N-C6H4-Me-p)Cl
(4a): C17H14ClN5Pd (430.2): calcd. C 47.45, H 3.26, N 16.28; found
C 47.35, H 3.32, N 16.30. IR (cmϪ1): νPd-Cl 368, νNϭN 1295, νCϭN
1580. Pd(papm-N-C6H4-OCH3-p)Cl (5a): C17H14ClN5OPd (446.2):
group (ring A) and the LUMO shares 68% azo and 22%
pyrimidine character. Thus, the transition is characterised
as an intravalence charge-transfer transition (IVCT). Other
spectral transitions may be characterised as HOMOϪ1
(58a)ǞLUMO (60a), HOMOϪ2 (57a)ǞLUMO (60a),
HOMO (59a)ǞLUMOϩ1 (61a), etc. The redox behaviour
at a positive potential to SCE may correspond to electron
extraction from the HOMO (59a), which has a major share
from the NϪAr group. The reductions correspond to se-
quential electron feeding at the LUMO (60a); LUMOϩ1 calcd. C 45.75, H 3.14, N 15.69; found C 45.85, H 3.20, N 15.75.
IR (cmϪ1): νPd-Cl 366, νNϭN 1290, νCϭN 1577. Pd(papm-N-C6H4-
(61a) and LUMOϩ2 (62a).
Cl-p)Cl (6a): C16H11Cl2N5Pd (450.6): calcd. C 42.63, H 2.44, N
15.54; found C 42.50, H 2.51, N 15.63. IR (cmϪ1): νPd-Cl 365, νNϭN
Experimental Section
1298, νCϭN 1580. Pd(tapm-N-C6H5)Cl (3b): C17H14ClN5Pd (430.2):
calcd. C 47.45, H 3.26, N 16.28; found C 47.60, H 3.35, N 16.37.
IR (cmϪ1): νPd-Cl 365, νNϭN 1290, νCϭN 1580. Pd(p-tapm-N-C6H4-
Me-p)Cl (4b): C18H16ClN5Pd (444.2): calcd. C 48.66, H 3.60,
N 15.77; found C 48.84, H 3.68, N 15.85. IR (cmϪ1): νPd-Cl 371,
νNϭN 1288, νCϭN 1585. Pd(tapm-N-C6H4-OCH3-p)Cl (5b):
C18H16ClN5OPd (460.2): calcd. C 46.96, H 3.48, N 15.22; found C
46.89, H 3.56, N 15.30. IR (cmϪ1): νPd-Cl 370, νNϭN 1285, νCϭN
1578. Pd(tapm-N-C6H4-Cl-p)Cl (6b): C17H13Cl2N5Pd (464.6):
calcd. C 43.93, H 2.79, N 15.07; found C 43.75, H 2.85, N 15.15.
IR (cmϪ1): νPd-Cl 367, νNϭN 1280, νCϭN 1585. Pd(Clpapm-N-
C6H5)Cl (3c): C16H11Cl2N5Pd (450.6): calcd. C 42.63, H 2.44,
N 15.54; found C 42.40, H 2.50, N 15.62. IR (cmϪ1): νPd-Cl 365,
νNϭN 1295, νCϭN 1580. Pd(Clpapm-N-C6H4-Me-p)Cl (4c):
C17H13Cl2N5Pd (464.6): calcd. C 43.93, H 2.79, N 15.07; found C
44.13, H 2.82, N 15.00. IR (cmϪ1): νPd-Cl 366, νNϭN 1292, νCϭN
1584. Pd(Clpapm-N-C6H4-OCH3-p)Cl (5c): C18H15Cl2N5OPd
(494.7): calcd. C 44.96, H 3.12, N 14.57; found C 45.15, H 3.18, N
14.50. νPd-Cl 365, νNϭN 1290, νCϭN 1585. Pd(Clpapm-N-C6H4-Cl-
p)Cl (6c): C16H10Cl3N5Pd (485.0): calcd. C 39.59, H 2.06, N 14.44;
found C 39.40, H 2.10, N 14.38. IR (cmϪ1): νPd-Cl 368, νNϭN 1300,
νCϭN 1588.
General Remarks: 2-(Arylazo)pyrimidines were prepared by the re-
ported procedure.[13] PdCl2 was purchased from Arora Matthey.
Aniline, p-toluidine, p-anisidine, p-chloroaniline were received from
the Sisco Research Lab. The purification of acetonitrile and pre-
paration of tetrabutylammonium perchlorate (TBAP) for electro-
chemical work were done as before.[13] Dinitrogen was purified by
bubbling through an alkaline pyrogallol solution. All other chem-
icals and solvents were of reagent grade and were used without
further purification. Commercially available SRL silica gel (60Ϫ120
mesh) was used for column chromatography. Spectroscopic data
were obtained using the following instruments: UV/Vis/NIR spec-
tra: JASCO UV/Vis/NIR model V 570 and Hitachi U-3501. IR
spectra (KBr disk, 4000Ϫ200 cmϪ1): FTIR JASCO model 420. 1H
NMR spectra: Bruker (AC) 300 MHz FTNMR spectrometer. Elec-
trochemical measurements were performed using a computer-con-
trolled PAR model 270 VERSASTAT Electrochemical instruments
with Pt-bead and GC-electrodes. All measurements were carried
out under dinitrogen at 298 K with reference to a saturated calomel
electrode (SCE) in acetonitrile. The reported potentials are uncor-
rected for junction potential.
Preparation of the Complexes: The dichloro[2-(arylazo)pyrimidine]-
palladium(II) [Pd(aapm)Cl2 (2)] complexes were prepared by a lit-
erature method[19] in the yield of 85Ϫ90%. A representative case is
given in detail.
Molecular Orbital Calculations: Extended Hückel Molecular Or-
bital calculations[34,36] were performed using the ICON software
package originally developed by Hoffmann. The orthogonal coord-
inate system chosen for the calculations are defined in Figures 5
and 6. The atomic parameters were taken from the crystallographic
data (Table 5). For drawing figures and diagrams the graphic pack-
age CACAO was used.
a. Dichloro[2-(phenylazo)pyrimidine]palladium(II) [Pd(papm)Cl2
(2a)]: To an MeCN (10 mL) solution of PdCl2 (0.18 g, 1.01 mmol),
2-(phenylazo)pyrimidine [papm (1a)] (0.2 g, 1.09 mmol) was added
dropwise and the mixture was stirred for 1 h. The orange-red pre-
cipitate was filtered and washed with MeOH. The dried mass was
dissolved in a minimum vol. of CH2Cl2 and purified by chromato-
graphy on a silica gel (60Ϫ120 mesh) column and the desired or-
ange-red band was eluted with MeCN/benzene (30% v/v).Yield
85%. NOTE: Benzene is carcinogenic and precaution must be taken
during its use!
X-ray Crystallography: Crystals of Pd(papm)Cl2 suitable for X-ray
work were grown by slow diffusion of benzene into a dichlorome-
thane solution at 283 K. The crystal size was 0.26 ϫ 0.25 ϫ 0.23
mm. The crystals of Pd(papm-N-C6H4-OCH3-p)Cl suitable for X-
ray work were obtained from diffusion of hexane into a dichloro-
methane solution at 298 K. The crystal size was 0.22 ϫ 0.19 ϫ 0.10
mm. Crystal data and collection parameters are listed in Table 5.
b. Chloro[(2-(14-imidophenyl)phenylazo)pyrimidine-N,NЈ,NЈЈ]pallad-
ium(II) [Pd(papm-N-C6H5)Cl (3a)]: To an acetonitrile solution ite monochrochromator (λ ϭ 0.71073 A) at 297(2) K. The crystals
Data were gathered with a CAD4/PC V diffractometer with graph-
˚
(15 mL) of Pd(papm)Cl2 (0.3 g, 0.83 mmol), an aniline (0.1 g,
1.08 mmol) solution in acetonitrile (10 mL) was slowly added. The
were mounted on glass fibres and covered with epoxy. Scan para-
meters were derived from ω-θ scans of 25 reflections. Three monitor
reaction mixture was stirred and refluxed continuously for 4 h and reflections were measured after every 30 min of X-ray exposure as
the colour of the solution changed gradually from orange-red to
greenish-brown. The solution was concentrated in air, and the res-
idue was washed thoroughly first with water (2 ϫ 5 mL) and then
with 50% aqueous ethanol (3 ϫ 5 mL). The residue was dissolved
in dichloromethane (10 mL) and the solution was column-chroma-
tographed on silica gel prepared in benzene. An orange-brown
a check on experimental stability. The orientation of the crystal
was checked after every 500 intensity measurements. Absorption
corrections were based on ψ-scans of 12 reflections with bisecting-
mode Eulerian χ values in the range of 0Ϫ86° for Pd(papm)Cl2
and 2Ϫ83° for Pd(papm-N-C6H4-OCH3-p)Cl. The structure was
solved by direct methods and refined by full-matrix least squares.
band was eluted with an acetonitrile/benzene (1:9) mixture. The In Pd(papm)Cl2 the asymmetric unit consists of two full molecules
eluted solution, on concentration in vacuo, gave a pure orange- and a half molecule of benzene. Systematic absences are consistent
1130
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131

Coupling of Arylamines with Coordinated Arylazopyrimidines in Palladium(II) Complexes
FULL PAPER
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Chakravorty, Polyhedron 1994, 13, 999.
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P. Bandyopadhyay, D. Bandyopadhyay, A. Chakravorty, F. A.
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A. Bharath, B. K. Santra, P. Munshi, G. K. Lahiri, J. Chem.
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B. K. Santra, G. K. Lahiri, J. Chem. Soc., Dalton Trans.
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A. K. Ghosh, P. Majumdar, L. R. Falvello, G. Mostafa, S.
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Table 5. Crystallographic data for 2a·0.25C6H6 and 5a
2a·0.25C6H6 5a
C11.50H9.50Cl2N4Pd C17H14ClN5OPd
[2]
[3]
Empirical formula
Formula mass
Temperature [K]
Crystal system
Space group
381.03
299(2)
446.18
297(2)
[4]
[5]
[6]
[7]
[8]
monoclinic
P21/n
orthorhombic
Pna21
17.1717(19)
14.5001(14)
6.6869(5)
90
1665.0(3)
4
1.780
˚
a [A]
10.6455(7)
22.1264(16)
12.0316(10)
109.216(6)
2676.1(3)
8
1.891
4Ϫ55
0 Յ h Յ 13,
0 Յ k Յ 28,
Ϫ15 Յ l Յ 14
3.06
˚
b [A]
˚
c [A]
[9]
β [°]
3
˚
V [A ]
[10]
[11]
[12]
Z
ρcalcd. [g cmϪ3
2θ range [°]
Index range
]
5Ϫ55
0 Յ h Յ 22,
Ϫ18 Յ k Յ 0,
0 Յ l Յ 8
2.84
6.17
1.052
[13]
[14]
[15]
[16]
[17]
[a]
R1 [I Ͼ 2σ (I)] (%)
[b]
wR2 (%)
6.57
1.018
GOF[c]
Transmission,
max./min.
0.937/0.894
0.974/0.891
Largest diff. peak
0.416 and Ϫ0.429
0.331 and Ϫ0.383
Ϫ3
˚
and hole [eA
]
[a]
[b]
2
2 2
R1 ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo|. wR2 ϭ [Σw(Fo Ϫ Fc ) /Σw(Fo2)1/2].
[c] GOF is defined as [w|Fo Ϫ Fc|/(no Ϫ nv)]1/2, where no and nv
denote the numbers of data and variable, respectively.
[18]
[19]
[20]
with the space group of Pna21 or the centric group Pnma for
Pd(papm-N-C6H4-OCH3-p)Cl. The refinement in the centric group
(Pnma) leads to residuals of R1 ϭ 0.12 and wR2 ϭ 0.44. In space
group Pna21 the values are R1 ϭ 0.0287 and wR2 ϭ 0.0630 and
thus chosen as the correct space group. All data were corrected for
Lorentz polarisation and an empirical absorption correction was
done on the basis of azimuthal scans. Of 6435 reflections in
Pd(papm)Cl2 6117 [I Ͼ 2σ(I)] were used for the structure solution.
In Pd(papm-N-C6H4-OCH3)Cl the unique reflections (2068) were
used for the structure solution. Data were processed with an Alpha
Station 2004/166 (open VMS/Alpha V 6.2) using the program
XCAD4B and the commercial package SHELXTL Rel. 505/VMS.
All non-hydrogen atoms were located from subsequent difference
Fourier maps. CCDC-135187 and -135188 contain the supplement-
ary crystallographic data for [Pd(papm)Cl2] (2a) and [Pd(papm-N-
C6H4-OMe-p)Cl] (5a), respectively. These data can be obtained free
of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
Supporting Information: 1H NMR spectroscopic data, a packing
diagram, and Walsh diagrams are available (see footnote on the
first page of this article).
[32]
[33]
Acknowledgments
[34]
[35]
[36]
C. K. Pal, S. Chattopadhyay, C. Sinha, A. Chakravorty, Inorg.
Chem. 1996, 35, 2442.
M. Haga, E. S. Dodsworth, A. B. P. Lever, Inorg. Chem. 1986,
25, 447.
We are thankful to the University Grants Commission, New Delhi
for financial assistance. The Council of Scientific and Industrial
Research, New Delhi provided a fellowship to P. B. Funding from
the Directorate General of Higher Education and Scientific Re-
search to the Departmento de Quimica Inorganica, Universidad de
Zaragoza, Plaza San-Fransisco, S/NE-50009, Espana, for crystallo-
graphic work (grant PB 98-1593), is gratefully acknowledged.
C. K. Pal, S. Chattopadhyay, C. Sinha, A. Chakravorty, Inorg.
Chem. 1994, 33, 6140.
Received July 27, 2001
[I01280]
Eur. J. Inorg. Chem. 2002, 1124Ϫ1131
1131