Synthesis, spectral characterization and electrochemical studies of mixed-ligand complexes of platinum(II) with 2-(arylazo)pyridines and catechols. Single-crystal X-ray structure of dichloro{2-(phenylazo) pyridine}platinum(II)

Article abstract of DOI:10.1016/S0277-5387(00)00530-1

The reaction of 2-(arylazo)pyridines, RH₄C₆N=NC₅H₄N (aap, 1 where R = H (pap). p-Me (p-tap). p-Cl (p-Clpap)) with K₂PtCl₄ in 1:2 boiling acetonitrile - water affords orange - red complexes of the type Pt(aap)Cl₂ (2). The IR spectrum shows two r(Pt-Cl) stretches suggesting cis-PtCl₂ configuration. The structural confirmation was carried out by an X-ray diffraction study of Pt(pap)Cl₂ (2a). The addition of catechols to a chloroform-methanol solution of Pt(aap)Cl₂ in the presence of Et₃N yielded green colored mixed-ligand complexes of the formula Pt(aap)(O,O) (O,O = catecholate (cat) (3), 4-tert-butylcatecholate (tbcat), (4), 3,5-di-tert-butylcatecholate (dtbcat) (5) and tetrachlorocatecholate (tccat) (6)). The complexes were characterized by elemental analyses, IR, UV - Vis - NIR and ¹H NMR spectral data. Electronic spectra exhibit a ligand-ligand charge transfer (LLCT) transition at NIR region: the position and symmetry of the band depend on the type of substituent on the catechol frame. This is qualitatively assigned as 3b₁(cat) → π*(aap) transition. Cyclic voltammogram of the complexes show four successive redox responses: two couples positive to SCE are referred to as oxidation of catechol to semiquinone and semiquinone to quinone, respectively and two couples negative to SCE correspond to azo reductions.

Full text of DOI:10.1016/S0277-5387(00)00530-1

www.elsevier.nl/locate/poly
Polyhedron 20 (2001) 363–372
Synthesis, spectral characterization and electrochemical studies of
mixed-ligand complexes of platinum(II) with 2-(arylazo)pyridines
and catechols. Single-crystal X-ray structure of
dichloro{2-(phenylazo) pyridine}platinum(II)
Goutam Kumar Rauth a, Sanjib Pal a, Debasis Das a, Chittaranjan Sinha a,*,
Alexander M.Z. Slawin b, J. Derek Woollins b
a Department of Chemistry, The Uni6ersity of Burdwan, Burdwan 713104, India
b Department of Chemistry, Uni6ersity of St-Andrews, St. Andrews KY16 9ST, UK
Received 23 May 2000; accepted 7 August 2000
Abstract
The reaction of 2-(arylazo)pyridines, RH4C6NꢀNC5H4N (aap, 1 where R=H (pap), p-Me (p-tap), p-Cl (p-Clpap)) with
K2PtCl4 in 1:2 boiling acetonitrile–water affords orange–red complexes of the type Pt(aap)Cl2 (2). The IR spectrum shows two
w(PtꢁCl) stretches suggesting cis-PtCl2 configuration. The structural confirmation was carried out by an X-ray diffraction study
of Pt(pap)Cl2 (2a). The addition of catechols to a chloroform–methanol solution of Pt(aap)Cl2 in the presence of Et3N yielded
green colored mixed-ligand complexes of the formula Pt(aap)(O,O) (O,O=catecholate (cat) (3), 4-tert-butylcatecholate (tbcat),
(4), 3,5-di-tert-butylcatecholate (dtbcat) (5) and tetrachlorocatecholate (tccat) (6)). The complexes were characterized by elemental
analyses, IR, UV–Vis–NIR and 1H NMR spectral data. Electronic spectra exhibit a ligand–ligand charge transfer (LLCT)
transition at NIR region; the position and symmetry of the band depend on the type of substituent on the catechol frame. This
is qualitatively assigned as 3b1(cat)“p*(aap) transition. Cyclic voltammogram of the complexes show four successive redox
responses: two couples positive to SCE are referred to as oxidation of catechol to semiquinone and semiquinone to quinone,
respectively and two couples negative to SCE correspond to azo reductions. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Azopyridine; Catechols; Platinum(II); Structure; Electrochemistry
1. Introduction
analogues of cisplatin [21,22]. The use of N,N-donor
ligands instead of ammonia appeared favorable because
of the thermodynamic stability of the chelate com-
plexes. The p-acidity of the function, ꢁNꢀNꢁCꢀNꢁ, has
manifested itself in [Pd(N,N)(O,O)] (where N,N=2-
(arylazo)pyridines; O,O=catecholates) in a ligand–lig-
and charge transfer transition (LLCT) [8]. The band
position and symmetry depend on the substituents in
the ligand frame and on the polarity of the solvent used
[23–25]. Besides, platinum catecholates have been re-
ported to have anticancer activity and are used as labels
for the preparation of metalloantigens [26]. Transition
metal complexes of catecholate and related ligands are
of wide-ranging interest [27,28]. Herein we account the
synthesis, spectral characterization and electrochemical
properties of 2-(arylazo)-pyridine complexes of platinu-
The p-acidity of the azoimine group, ꢁNꢀNꢁCꢀNꢁ,
and its ability to stabilize low valent metal redox states
has encouraged the study of its chemistry and molecu-
lar architecture design with this group [1–20]. 2-(Ary-
lazo)pyridine (aap, 1) is one such molecule whose
coordination chemistry with transition metals has been
extensively studied [3–18] and maximum effort has
been given to explore the chemical activity of the
platinum group metals [3–14]. However, platinum re-
mains untouched. Much current research on platinu-
m(II) with N-donor ligands is targeted to synthesize
* Corresponding author. Tel.: +91-342-57-683; fax: +91-342-64-
452.
E-mail address: c-r-sinha@yahoo.com (C. Sinha).
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00530-1

364
G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
m(II) and their catecholato derivatives. The structure of
dichloro-(2-(phenylazo)pyridine)platinum(II) is sup-
ported by X-ray crystallography.
solution was evaporated in a steam bath and the solu-
tion was reduced to half of its original volume. The
dark precipitate was filtered and washed with hot water
and finally with cold 1:1 MeCN–H2O (v/v). It was then
dried over CaCl2. The product was then dissolved in a
minimum volume of CH2Cl2 and was chromatographed
on silica gel column (30×1 cm). A deep orange–red
band eluted using 3:1 C6H6–MeCN (v/v). Crystals were
obtained by complete evaporation of the eluted solu-
tion at r.t. The yield was 0.17 g (74%). Pt(p-tap)Cl2 and
Pt(p-Clpap)Cl2 were prepared following an identical
procedure and the yields were 68 and 79%, respectively.
2. Experimental
2.1. Materials
H2PtCl6·xH2O was purchased from Arrora Mathey,
Calcutta, India. K2PtCl4 was prepared from H2PtCl6
following the reported method [29]. 2-(Ary-
lazo)pyridines (aap) were synthesized according to the
published procedure [8]. Pyrocatechol (H2cat), 3,5-di-
2.3.2. Preparation of
catecholato-(2-(phenylazo)pyridine)platinum(II),
[Pt(pap)(cat)] (3a)
tert-butylcatechol
(H2dtbcat),
tetrachlorocatechol
(H2tccat) were obtained from Aldrich. 4-tert-Butylcate-
chol (H2tbcat) was a product of Fluka. Catechols were
Pt(pap)Cl2, (0.325 g, 0.72 mmol) was dissolved in
CHCl3 (15 cm3) and the solution was degassed by
bubbling N2. To this solution pyrocatechol (0.085 g,
0.77 mmol) in MeOH (10 cm3) was slowly added fol-
lowed by triethylamine (2 mmol) under N2. The solu-
tion was stirred for 1 h and the color changed from
orange–red to green. The solution volume was reduced
to one-third of its original volume by N2 bubbling and
the dark green precipitate that formed was filtered,
washed with MeOH and dried over CaCl2. The dried
mass was dissolved in a minimum volume of CH2Cl2,
chromatographed over silica gel and the desired green
band was eluted by 3:2 C6H6–MeCN (v/v). Evapora-
tion of the solvent in vacuo afforded the pure crys-
talline product. The yield was 0.28 g (58%). All other
complexes were prepared by an identical procedure and
the yields varied in the range 45–72%.
purified
by
recrystallization
from
benzene.
Dichloromethane and acetonitrile were further purified
by distillation over P4O10. Tetrabutylammonium per-
chlorate (n-Bu4NClO4) was prepared and recrystallized
by a previously reported method [23]. Nitrogen gas was
purified by successive bubbling through alkaline pyro-
gallol solution and concentrated sulfuric acid. Commer-
cial grade SRL silica gel (60–120 mesh) was used for
column chromatography. Triethylamine, hydrazinesul-
fate and all other chemicals and solvents used in
preparative work were of reagent grade and were used
as received.
2.2. Instrumentation
Microanalytical data (C, H, N) were collected using a
Perkin–Elmer 2400 CHNS/O elemental analyzer. Spec-
troscopic data were obtained using the following instru-
ments: UV–Vis–NIR spectra, Shimadzu UV 160 A
and Hitachi 330; IR spectra (KBr disk, 4000–200
cm−1), FTIR JASCO model 420; 1H NMR from
Brucker AC(F) 300 MHz FT NMR spectrometer. Elec-
trochemical measurements were carried out using com-
puter-controlled PAR model 270 VERSASTAT
electrochemical instruments with a GC milli-electrode.
All measurements were carried out under dinitrogen
environment at 298 K with reference to a saturated
calomel electrode (SCE) in acetonitrile. The reported
potentials are uncorrected for junction potential.
2.4. X-ray crystal structure and analysis
A crystal of Pt(pap)Cl2 suitable for X-ray work was
grown by slow diffusion of hexane into
a
dichloromethane solution at 298 K. The crystal size was
0.1×0.1×0.3 mm3. Data were collected on a Siemens
SMART CCD diffractometer with graphite monochro-
,
matized Mo Ka radiation (u=0.71073 A) at 295 K.
Data for the crystal and the collection parameters are
listed in Table 1. The data were corrected for absorp-
tion effects by an empirical method using azimuthal
scan data. Systematic absences led to the identification
of the space group as monoclinic C2/c. Of the 22 453
reflections collected, 8751 with I\2|(I) were used for
structure solution. The structure was solved by the
conventional heavy atom method and was refined by
the full-matrix least-square method on all Fo2 data using
the SHELXTL 5.03 package on a Silicon Graphics
Indigo-R4000 [20]. Hydrogen atoms were included in
calculated positions and refined with isotropic thermal
parameters.
2.3. Preparation of complexes
2.3.1. Dichloro-{2-(phenylazo)pyridine}platinum(II),
Pt(pap)Cl2 (2a)
To a nitrogen flushed solution of K2PtCl4 (0.22 g,
0.53 mmol) in water (20 ml) under refluxing conditions
was added 2-(phenylazo)pyridine (0.1 g, 0.55 mmol) in
MeCN (10 ml). The reaction was continued for 24 h
and cooled to room temperature (r.t.). The brown–red

G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
365
Table 1
and catechols in CHCl3–MeOH under N2 immediately
changed the color from orange to brown–green and the
products separated on slow bubbling of N2 for a period
of 1 h or more. Removal of the solvents followed by
chromatographic purification afforded the catecholato
complexes [Pt(N,N)(O,O)] (3–6) in high yield. The
complexes are nonconducting and ESR silent. The
compositions of the complexes are supported by micro-
analytical data (Table 2).
Summarized crystallographic data for Pt(pap)Cl2
Empirical formula
Formula weight
Crystal system
Space group
C11H9Cl2N3Pt
449.20
monoclinic
C2/c
56.0852(9)
9.42210(10)
14.3708(2)
99.9230(10)
7480.5(2)
24
295
2.393
11.661
0.368
461
0.0348
0.0687
0.878
,
a (A)
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
3.2. Single-crystal X-ray structure of Pt(pap)Cl2
T (K)
Dcalc (Mg m−3
)
v(Mo Ka) (mm−1
)
An X-ray quality single crystal is obtained by slow
diffusion of dichloromethane solution into hexane. The
asymmetric unit of the crystal lattice constituted of
these molecules and is structurally comparable with
Transmission coefficient a
Parameters refined
b
R
c
wR
Goodness-of-fit d
dichloro{2-(phenylazo)pyridine}palladium(II) [30]. A
view of the triad including the atom numbering scheme
is shown Fig. 1 and selected bond parameters are listed
in Table 3. In terms of bond distances, angles, as well
as gross geometry, the three molecules are closely akin
to one another. Some common features are as follows:
the PtN2Cl2 coordination sphere is a good plane with
a Maximum value normalized to 1.
b R=SꢀFo−Fcꢀ/S Fo.
c wR=[S w(Fo2−Fc2)/S wFo4]1/2
14.3394P], P=(Fo2+2Fc2)/3.
;
w=1/[|2(Fo2)+(0.0473P)2+
d Goodness of fit is defined as [w(Fo−Fc)/(no−nv)]1/2 where no and
nv denote the numbers of data and variables, respectively.
,
no atom deviating \0.045 A from the mean plane. The
3. Results and discussion
dimensions of the chelate ring compare well with those
in other structurally characterized chelates of this lig-
and [3,31]. The average chelate bite angle is 78.1(3)°
and is considerably deviated from the ideal square
planar values. The average ClꢁPtꢁCl angle is approxi-
mately 89.3°. The pendant phenyl ring is planar and
inclined at an acute angle with the chelate ring (lies
between 42–48°).
3.1. Synthesis
The arylazopyridines (aap) and platinum(II) com-
plexes described in this work along with the methods
used for their preparation are shown in Scheme 1. The
arylazopyridines were obtained by the condensation of
nitrosoaromatics with 2-aminopyridine [8]. They re-
acted with K2PtCl4 in acetonitrile–water solution under
refluxing conditions affording orange–red Pt(aap)Cl2
(2) in high yields. Addition of Et3N to solutions of 2
The average PtꢁN(azo) and PtꢁN(Py) distances are
approximately 1.988 and 2.010 A, respectively. The
shortening of the PtꢁN (azo) distance may be due to
significant back bonding between t2(Pt) and p*(pap)
,
Scheme 1. (i) K2PtCl4 in 1:2 MeCN–H2O (v/v) reflux, 24 h; (ii) catechols, Et3N in CHCl3–MeOH stir under N2 atmosphere, 8 h.

366
G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
Table 2
Microanalytical a, IR b and UV–Vis c spectral data
Compound
Elemental analyses (%)
Infrared data (cm−1
)
UV–Vis spectral data umax (nm−1
(m (mol−1 cm−1
)
)
C
H
N
w(NꢀN)
1385
1390
1383
1358
1350
1350
1360
1355
1350
1355
1365
1360
1365
1365
1355
w(PtꢁCl)
w(PtꢁO)
Pt(pap)Cl2
29.31
(29.39)
31.17
(31.09)
27.38
(27.29)
42.05
(41.96)
46.80
(46.48)
50.50
(50.16)
32.60
(32.68)
43.09
(43.19)
47.41
(47.47)
51.04
(50.97)
33.78
(33.85)
39.07
(39.18)
43.66
(43.70)
47.50
(47.42)
31.00
1.92
(2.00)
2.31
(2.37)
1.71
(1.65)
2.73
(2.67)
3.91
(3.87)
4.84
(4.85)
1.50
(1.44)
3.08
(2.99)
4.20
(4.14)
5.00
(5.06)
1.81
(1.72)
2.40
(2.31)
3.54
(3.47)
4.80
(4.43)
1.18
9.44
(9.35)
9.12
(9.06)
8.58
(8.68)
8.57
(8.64)
7.54
(7.75)
7.10
(7.02)
6.79
(6.73)
8.47
(8.40)
7.63
(7.55)
6.95
(6.86)
6.64
(6.58)
8.00
(8.07)
7.35
(7.28)
6.68
(6.64)
6.45
324, 345
318, 340
315, 340
493(2240), 407(7011)
383(8537)
Pt(p-tap)Cl2
489(2968), 412(11,482)
391(10,968)
Pt(p-Clpap)Cl2
Pt(pap)(cat)
493(1940), 385(9743)
367(9090)
870(2300), 646(1800)
384(9090)
570
590
590
580
580
595
600
575
580
585
580
600
Pt(pap)(tbcat)
Pt(pap)(dtbcat)
Pt(pap)(tccat)
937(4619), 652(1776)
386(9000)
1011(5397), 749(2936)
390(7865)
759(4385), 609(3141)
384(12732)
Pt(p-tap)(cat)
857(8191), 632(2065)
401(14647)
Pt(p-tap)(tbcat)
Pt(p-tap)(dtbcat)
Pt(p-tap)(tccat)
Pt(p-Clpap)(cat)
Pt(p-Clpap)(tbcat)
Pt(p-Clpap)(dtbcat)
Pt(p-Clpap)(tccat)
895(5320), 694(5519)
409(15460)
988(9977), 672(1842)
402(12639)
750(5323), 605(2244)
415(11055)
893(3208), 642(2675)
392(17800)
989(4619), 672(1211)
391(7021)
1090(2730), 799(6594)
400(18200)
766(1240), 513(853),
398(6094)
(30.97)
(1.21)
(6.38)
a Calculated values are in parenthesis.
b In KBr disk.
c In CH2Cl2.
orbitals. The NꢀN distance (average approximately 1.273
,
A) is comparable with other reported values [3,31]. A
packing view down to the a-axis suggests two molecular
units (Pt(1) and Pt(3)) in the triad are closely associated
N(7)ꢁPt(1)ꢁN(1)
ꢁC(2)
N(27)ꢁPt(2)
ꢁN(21)ꢁC(22)
N(47)ꢁPt(3)
ꢁN(41)ꢁC(42)
N(1)ꢁPt(1)ꢁN(7) −6.4 Pt(1)ꢁN(7)ꢁN(2)
ꢁN(2)
N(21)ꢁPt(2)
ꢁN(27)ꢁN(22)
N(41)ꢁPt(3)
ꢁN(47)ꢁN(42)
8.4 Pt(I)ꢁN(I)ꢁC(2) −10.0
ꢁN(2)
,
and the PtꢁPt distance is approximately 3.6 A.
−3.7 Pt(2)ꢁN(2I)
ꢁC(22)ꢁN(22)
2.5 Pt(3)ꢁN(4I)
ꢁC(42)ꢁN(42)
3.6
−3.0
2.9
The difference between the three molecules lies primar-
ily in the details of conformation. The pendent phenyl
ring is inclined differently: molecule 1, 42.4°; molecule 2,
48.2°; molecule 3, 45.2°. Pt deviates from the square plane,
,
,
N2Cl2 by 0.036 A (molecule 1), 0.028 A (molecule 2) and
ꢁC(2)
3.9 Pt(2)ꢁN(27)
ꢁN(22)ꢁC(22)
−2.0 Pt(3)ꢁN(47)
ꢁN(42)ꢁC(42)
,
0.020 A (molecule 3). Torsion angles also vary in a
−3.0
0.9
characteristic manner. Examples from those among the
atoms of the chelate ring are given below. The three
molecules represent an unusual and subtle form of
conformational isomerism in a metal chelate sustained by
the crystal lattice.

G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
367
Table 3
Selected bond distances (A) and bond angles (°)
energy compared to the analogous Pd(aap)(O,O)
complexes [8] (approximately 1375 cm−1) which is
corroborated by the general trend in periodic proper-
ties [32] of Pd and Pt. The cis-PtCl2 configuration of
Pt(aap)Cl2 is established by the appearance of two
distinct PtꢁCl stretching bands at 315 and 340 cm−1
which are absent in the catecholato complexes (Table
2). This confirms the nucleophilic displacement of
PtꢁCl and the appearance of a new band around
450–500 cm−1 attributable to a PtꢁO stretch suggests
catecholato binding. On comparing the spectra of
Pt(aap)Cl2 and Pt(aap)(O,O) an additional band at
1250–1280 cm−1 was observed. This corresponds to
the CꢁO stretching vibration of catechols. A strong
band near 1400–1480 cm−1 in Pt(aap)(O,O) is also
observed but this region of the spectrum is compli-
cated by vibrations from azopyridines.
The solution electronic spectra of the complexes
were recorded in dichloromethane. Pt(aap)Cl2 (2) ex-
hibits absorptions at around 490 and 400 nm and the
absorptions below 400 nm are not considered as they
correspond to intraligand charge transfer transitions.
The absorption spectra of catecholato complexes,
Pt(aap)(O,O), are different from their parent com-
plexes (Fig. 2, Table 2). The most significant feature
of the spectrum is the appearance of new band in the
red to near-IR region. The position of the band is
highly dependent on the nature of the substituent(s)
in the catechol and azopyridine fragments [8,23,24].
This band is assigned as a ligand-to-ligand charge
transfer (LLCT) transition involving the HOMO of
catechol and the LUMO of arylazopyridine or the
redox orbitals constituted by platinum(II) and aryla-
zopyridine. The observed trend in the wavelength of
the band maxima for a particular catechol is p-
Clpap\pap\p-tap. The trend of the band maxima
for a particular azopyridine fragment is dtbcat\tb-
cat\cat\tccat. The HOMO of dtbcat is expected to
have the highest energy in the series because of the
electron releasing effect of two But groups. In the
tccat complex, the HOMO has lower energy than the
corresponding cat because of the electron withdraw-
ing character of Cl groups. In p-Clpap the energy of
the p* orbital (LUMO) is of lower value compared to
pap because of the electron withdrawing effect of Cl
groups and the reverse is true of p-tap for the induc-
tive effect of the ꢁMe group. Thus the LLCT band is
qualitatively described [33] as a 3b1(cat)“p*(aap)
charge transfer transition.
,
Distances
Angles
Molecule 1
Pt(1)ꢁN(1)
Pt(1)ꢁN(7)
Pt(1)ꢁCl(1)
Pt(1)ꢁCl(2)
Pt(2)ꢁN(7)
2.012(6)
1.997(6)
2.292(2)
2.303(2)
1.284(8)
N(1)ꢁPt(1)ꢁN(7)
Cl(1)ꢁPt(1)ꢁCl(2)
N(1)ꢁPt(1)ꢁCl(2)
N(7)ꢁPt(1)ꢁCl(1)
N(1)ꢁPt(1)ꢁCl(1)
N(7)ꢁPt(1)ꢁCl(2)
77.8(3)
89.33(8)
173.7(2)
172.8(2)
95.0(2)
97.9(2)
Molecule 2
Pt(2)ꢁN(2)
Pt(2)ꢁN(27)
Pt(2)ꢁCl(3)
Pt(2)ꢁCl(4)
Pt(22)ꢁN(27)
2.006(7)
1.987(7)
2.303(2)
2.297(2)
1.267(9)
N(21)ꢁPt(2)ꢁN(27)
Cl(3)ꢁPt(2)ꢁCl(4)
N(21)ꢁPt(2)ꢁCl(4)
N(27)ꢁPt(2)ꢁCl(3)
N(21)ꢁPt(2)ꢁCl(3)
N(27)ꢁPt(2)ꢁCl(4)
78.2(3)
89.50(9)
176.0(2)
172.3(2)
94.3(2)
97.9(2)
Molecule 3
Pt(3)ꢁN(41)
Pt(3)ꢁN(47)
Pt(3)ꢁCl(5)
Pt(3)ꢁCl(6)
N(42)ꢁN(47)
2.011(6)
1.985(6)
2.293(2)
2.299(2)
1.268(8)
N(41)ꢁPt(3)ꢁN(47)
Cl(5)ꢁPt(3)ꢁCl(6)
N(41)ꢁPt(3)ꢁCl(6)
N(47)ꢁPt(3)ꢁCl(5)
N(41)ꢁPt(3)ꢁCl(5)
N(47)ꢁPt(3)ꢁCl(6)
78.3(3)
89.05(8)
174.4(2)
172.8(2)
94.8(2)
98.0(2)
Fig. 1. Plot of Pt(pap)Cl2 with 50% probability and atom labeling
pattern.
3.3. Spectral studies
The IR spectra of Pt(aap)Cl2 (2) exhibit a sharp
single stretch at approximately 1385 cm−1 correspond
to w(NꢀN) which is lowered by 40 cm−1 from that of
the free ligand value [15] (approximately 1425 cm−1).
In catecholato complexes, Pt(aap)(O,O) (3–6), the
w(NꢀN) appears at (Table 2) approximately 1355
cm−1. The lowering of the frequency may attribute to
the extensive d(Pt)“p8(aap) back bonding or
3b1(cat)“p8(aap) [8,24] in the catecholato complexes.
The w(NꢀN) in the present complexes appear at lower
The stereochemistry of Pt(aap)Cl2 (2) and [Pt(aap-
1
(O,O)] (3–6) is supported by H NMR spectral data.
1
The H NMR spectra of the complexes were recorded
in CDCl3 at 298(2) K. The atom numbering pattern
of complexes is as shown in Scheme 1 and the spec-

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G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
tral data are collected in Table 4. Pyridine protons
(3H–6H) appear in the downfield region relative to
the azoaryl protons (8H–12H). The aryl protons move
in an usual manner [8] with the substituents in the
ring. In catecholato complexes [Pt(aap)(O,O)] (3–6)
these signals experience an upfield shift compared with
the parent compound Pt(aap)Cl2 (2). This may be due
to better charge transfer from the catecholato group to
platinum(II) than the chloride ions or more back-
bonding from the metal d-orbitals to the p*(aap) com-
pared with chloride. The resonance of 6H exhibits
coupling with 195Pt (I=1/2, abundance 33.7%) and the
characteristic AA%BB% pattern, while signals in the
aliphatic region (l; 1.3, 1.6 ppm) for [Pt(aap)(tbcat)]
(4)/[Pt(aap)(dtbcat)] (5) are due to But protons. Com-
plex 4 exhibits a singlet and two doublets in the upfi-
eld region assigned to 15H and 17,18H of tbcat,
respectively. Complexes
4
and
5
exhibit slight
downfield shifting of the catecholato protons (15H and
18H for 4 and 16H and 18H for 5) although the
electron releasing But group should show the reverse
effect. The presence of a But group in the catechol
ring (tbcat, dtbcat) increases the energy of the HOMO
(3b1) and induces increased charge transfer to the
LUMO (p*) of the azopyridine compared to cat in
[Pt(aap)(O,O)]. This may be the reason for the
downfield shift of protons in tbcat and dtbcat relative
to cat as well as the upfield shift of arylazopyridine
protons in the complexes.
3
coupling constant JPtꢁH is 20–30 Hz (Fig. 3). The
protons of the catecholato ring appear upfield relative
to the arylazopyridine signals which is expected in
view of the electron releasing effect of the catecholato
oxygen atoms. The complexes [Pt(aap)(cat)] (3) give a
Fig. 2. Spectra of [Pt(pap)Cl2](—), [Pt(pap)(cat)](- - -), [PT(pap)(tbcat)](···), [Pt(pap)(dtbcat)](—ꢀ—ꢀ—) and [Pt(pap)(tccat)](— - —) in
CH2CL2 at 298 K.

G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
369

370
G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
Fig. 3. Spectrum of [Pt(pap)(tbcat)] in CDCL3 (300 MHz).
4. Redox studies
The redox properties of the complexes according to
cyclic voltammetry are summarized in Table 5. All
measurements were carried out with the use of a GC-
working electrode in acetonitrile solution under a dini-
trogen environment. All potentials are referenced to the
saturated calomel electrode (SCE) in the presence of
tetrabutylammonium perchlorate (n-Bu4NClO4) as sup-
porting electrolyte. Pt(aap)Cl2 exhibits only two re-
sponses at negative potential to SCE and Pt(aap)(O,O)
gives four successive redox processes (Fig. 4) within the
potential range +1.5 to −1.5 V versus SCE. The two
couples at negative potential to SCE are due to azo
reduction. The LUMO of aap can simultaneously ac-
commodate two electrons. Since, this orbital has a large
azo character, the reduction occurs at the azo center
and is not considered further.
The anodic responses are due to the coordinated
catechol oxidation (Eq. (1)) and the compounds un-
dergo two successive one-electron redox processes with
varying degrees of reversibility depending upon the type
of substitutents present in the catecholato ring. The
electron-donating catechols (tbcat, dtbcat) first give a
reversible oxidation couple (Rcq/Rsq) followed by a
second quasireversible couple (Rsq/Rq) while electron-
withdrawing catechol (tccat) only gives a response of
for an irreversible pattern. The redox data in Table 5
suggests that the potential (Epa) is shifted to more
positive values as the substitutent(s) in the bound cate-
chol ring becomes more electron withdrawing.
(1)
This is because the oxidation will be thermodynamically
more facile when electron density flows into the cate-
chol ring leading to stabilization of the positive charge
of the cation radical [8,34]. The potentials of the first
reduction and oxidation can qualitatively be related to
the nature of the orbitals in these redox processes.
These orbitals are involved in the CT transitions as
3b1(cat)“p*(aap) [33].
5. Conclusion
The paper describes the synthesis of dichloro-{2-
(arylazo)pyridine}platinum(II), Pt(aap)Cl2 (2). The X-
ray diffraction study has established the structure of the
complex and suggests that the asymmetric unit of the
crystal lattice consists of three molecules. Nucleophilic
substitution of a cis-PtCl2 motif by catechols (O,O) in
the presence of Et3N resulted in the synthesis of (cate-
cholato){2 - (arylazo) - pyridine}platinum(II), [Pt(aap)-
(O,O)] complexes (3–6). The absorption spectra of
catecholato complexes exhibit significant differences
from the dichloro complex in the red to near-IR region.
The band is assigned to ligand–ligand charge transfer
transition (LLCT). The absorption maxima are red
shifted upon increasing the electron density on the

G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
371
Table 5
Cyclic voltammetric data a
Compound
Catechol centered oxidation
aap-centered reduction
b
b
b
Rcq/Rsq E11/2 (DEp
)
Rsq/Rq E12/2 (DEp
)
aap2−/aap− −E13/2 (DEp
)
aap−/aap −Ep4c
Pt(pap)Cl2
Pt(p-tap)Cl2
0.14 (110)
0.24 (90)
0.14 (100)
0.42 (100)
0.48 (80)
0.55 (80)
0.33 (110)
0.46 (110)
0.54 (70)
0.59 (85)
0.38 (80)
0.39 (100)
0.43 (100)
0.51 (100)
0.31 (90)
1.12
1.28
1.07
1.25
1.38
1.48
1.18
1.28
1.41
1.50
1.21
1.22
1.33
1.43
1.15
Pt(p-Clpap)Cl2
Pt(pap)(cat)
Pt(pap)(tbcat)
Pt(pap)(dtbcat)
Pt(pap)(tccat)
0.68 (90)
0.55 (70)
0.47 (80)
1.04 c
0.62 (100)
0.52 (90)
0.40 (100)
1.00 (120)
0.74 (90)
0.59 (80)
0.51 (80)
1.06 (120)
1.10 (170)
1.02 (140)
0.90 (120)
1.51 c
Pt(p-tap)(cat)
1.05 (175)
0.96 (130)
0.87 (110)
1.43 c
Pt(p-tap)(tbcat)
Pt(p-tap)(dtbcat)
Pt(p-tap)(tccat)
Pt(p-Clpap)(cat)
Pt(p-Clpap)(tbcat)
Pt(p-Clpap)(dtbcat)
Pt(p-Clpap)(tccat)
1.23 c
1.08 (130)
0.95 (130)
1.58 c
a Solvent: 1:1 MeCNꢁCHCl2 (v/v); supporting electrolyte n-Bu4CLO4; electrode GC milli-electrode; solute concentration 10−3 M; scan rate 50
mV s−1; potentials are in volts vs. SCE.
b DEp=(Epa−Epc) in milivolts, where Epa and Epc are anodic and cathodic peak potentials.
c Epa
.
Fig. 4. Voltammogram of [Pt(p-tap)(tbcat)] in MeCN using a GC working electrode (SCE reference) at 298 K.
arylazopyridine ring by appropriate substitutent(s) in
the respective rings. [Pt(aap)(O,O)] exhibit catechol/
semiquinone and semiquinone/quinone oxidations and
azo reductions during cyclic voltammetric studies. The
reversibility of the catechol/semiquinone oxidation cou-
ple is dependent on the electron influx in the catechol
ring.
Data Center, CCDC No. 14472 for compound 2a.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
Acknowledgements
6. Supplementary material
We are thankful to the University Grants Commis-
sion (under the DSA scheme) and the Council of Scien-
tific and Industrial Research, New Delhi for financial
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic

372
G.K. Rauth et al. / Polyhedron 20 (2001) 363–372
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