Arylazoimidazoleplatinum(II) complexes and their dioxolene derivatives: Single crystal X-ray structure of (catecholato){1-ethyl-2-(p-tolylazo)-imidazole}platinum(II)

Article abstract of DOI:10.1016/S0020-1693(00)00311-X

The reaction of 1-ethyl-2-(arylazo)imidazoles (RaaiEt, 1) [R = H (1a), p-Me (1b), p-Cl (1c)] with K₂PtCl₄ in boiling acetonitrile-water (1:1) produces red-brown Pt(RaaiEt)Cl₂ (2) complexes. Addition of dioxolene in the pre

Full text of DOI:10.1016/S0020-1693(00)00311-X

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 313 (2001) 21–29
Arylazoimidazoleplatinum(II) complexes and their dioxolene
derivatives: single crystal X-ray structure of
(catecholato){1-ethyl-2-(p-tolylazo)-imidazole}platinum(II)
S. Pal a, D. Das a, C. Sinha a,*, C.H.L. Kennard b
a Department of Chemistry, The Uni6ersity of Burdwan, Burdwan 713 104, India
b Department of Chemistry, The Uni6ersity of Queensland, St. Lucia, QLD 4072, Australia
Received 11 May 2000; accepted 6 September 2000
Abstract
The reaction of 1-ethyl-2-(arylazo)imidazoles (RaaiEt, 1) [R=H (1a), p-Me (1b), p-Cl (1c)] with K2PtCl4 in boiling
acetonitrile–water (1:1) produces red–brown Pt(RaaiEt)Cl2 (2) complexes. Addition of dioxolene in the presence of Et3N to
chloroform–methanol solution of Pt(RaaiEt)Cl2 has yielded green-coloured mixed-ligand complexes [Pt(RaaiEt)(O,O)] [O,O=
catecholate (cat) (3), 4-tert-butylcatecholate (tbcat) (4), 3,5-di-tert-butylcatecholate (dtbcat) (5), tetrachlorocatecholate (tccat) (6)].
The structure of [Pt(p-MeaaiEt)(cat)]·1/2H2O (3b) was confirmed by X-ray diffraction. Electronic spectra exhibit ligand-to-ligand
charge transfer transition (LLCT) in the VIS–NIR region; the position and symmetry of the band depend on the substituent type
on the dioxolene and arylazoimidazole frames. This is qualitatively assigned as a HOMO(cat)“LUMO(RaaiEt) transition. Cyclic
voltammograms of the complexes show four successive redox responses: two couples at positive to saturated calomel electrode
(SCE) correspond to catechol to semiquinone and semiquinone to quinone oxidations, and two couples at negative to SCE
correspond to the azo reductions. The difference in the potential E1/2 of the first oxidation and reduction process changes linearly
with the LLCT transition. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Pt(II)-azoimidazole; Catecholato complexes; LLCT; X-ray crystal structures; Electrochemistry
like dioxolene, in ternary complexes of redox-innocent
metal ions shows efficient ligand-to-ligand charge trans-
fer (LLCT) transition [24,29]. This exhibits a negative
solvatochromic effect in palladium(II) complexes [29].
Platinum(II)–diimine complexes are important be-
cause of their charge transfer, luminescence, and elec-
trochemical properties [30–32] and their catalytic [33],
and biological [34–37] activities. The current research
in the field of platinum(II) chemistry with N-donor
ligands is aimed at synthesising compounds analogous
to cisplatin [34–37]. Platinum(II)–catecholates have
anti-cancer activity and are used to label the metal-
loantigens [38]. Platinum(II)–diimine–catecholato com-
plexes exhibit solvent-dependent LLCT transitions [39].
This is a report of the synthesis, spectral characterisa-
tion and electrochemical properties of 1-ethyl-2-(aryl-
azo)imidazole complexes of platinum(II) and their diox-
olene derivatives. The structure of (catecholato){1-
ethyl-2-(phenylazo)imidazole}platinum(II) is supported
by X-ray crystallography.
1. Introduction
This work is part of a systematic investigation on
transition metal complexes of azoimine function in
general [1–28] and arylazoimidazoles [19–27] in partic-
ular. Azoimines are isoelectronic with the diimine sys-
tem [1], and the scope of this function encompasses
from the stabilisation of the low-valent metal redox
state [1–7,19–28], metaloxo complexes [8,9], formation
of organometallic compounds [18] to the metal medi-
ated CꢀH bond activation of the pendant aryl group
following the hydroxylation [13], thiolation [14–16] and
amination [12] reactions. This is part of our continuing
work on platinum metal chemistry [1–15,19–28].
Metallated azoimine fragments exhibit potent M“L
charge transfer transitions [1–7], and a reducing ligand,
* Corresponding author. Fax: +91-342 564452.
E-mail address: c
r sinha@yahoo.com (C. Sinha).
– –
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(00)00311-X

22
S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
2. Experimental
2.1. Materials
plexes were prepared under identical conditions, and
yields varied in the range 45–50%.
2.4. Preparation of catecholato{1-ethyl-2-
(phenylazo)imidazole}platinum(II), [Pt(HaaiEt)(cat)],
(3a)
H2PtCl6·xH2O was purchased from Arrora Matthey,
Calcutta, India. K2PtCl4 was prepared from the re-
ported method [40]. 1-Ethyl-2-(arylazo)imidazoles were
synthesised according to the published procedure [21].
Pyrocatechol (0.098 g, 0.89 mmol) in MeOH (10 cm3)
was added to Pt(HaaiEt)Cl2, 2a, (0.395 g, 0.85 mmol)
which had been dissolved in degassed CHCl3–MeOH
mixture (50% v/v, 30 cm3). Triethylamine (2 mmol) un-
der N2 was added to this solution, stirred for 1 h and
the colour changed from brown–red to green. N2 bub-
bling through the solution evaporated it to a third of its
original volume, producing a dark green precipitate
that was filtered, washed with cold MeOH and dried in
vacuo. The dried mass was dissolved in a minimum
volume of CH2Cl2 and chromatographed over a silica
gel column to give the desired green band, which was
eluted with a 3:2 (v/v) C6H6–MeCN mixture. On evap-
oration a crystalline product was produced with a yield
of 0.208 g (41%). All other complexes were prepared by
similar procedures, with yields varying in the range
40–55%.
Pyrocatechol
(H2cat),
3,5-di-tert-butylcatechol
(H2dtbcat), and tetrachlorocatechol (H2tccat) were ob-
tained from Aldrich. 4-tert-Butylcatechol (H2tbcat) was
purchased from Fluka. Catechols were purified before
use by recrystallisation from benzene. Dichloromethane
and acetonitrile were further purified by distillation
over P4O10. Bu4NClO4 was prepared according to the
reported method [19]. Nitrogen was purified by bub-
bling through alkaline pyrogallol solution and concen-
trated H2SO4. Silica gel (60–120 mesh) for column
chromatography was obtained from SRL. Triethyl-
amine and other chemicals and solvents used for the
preparative work were of reagent grade and were used
as received.
2.2. Physical measurements
The IR spectra (KBr disk, 4000–200 cm−1) were
recorded on FTIR JASCO model 420. UV–VIS–NIR
spectra were collected from Shimadzu UV 160A and
Hitachi U-3501-UV–VIS–NIR spectrophotometers. 1H
NMR spectra were recorded on Brucker AC(F) 200
and 300 MHz FT-NMR spectrometers. Electrochemical
studies were performed on a computer-controlled
EG&G PAR model 270 VERSASTAT electrochemical
instrument with Pt-disk and glassy carbon (GC) elec-
trodes. All measurements were carried out under dini-
trogen environment at 298 K with reference to a
saturated calomel electrode (SCE) in acetonitrile.
[Bu4N][ClO4] was used as the supporting electrolyte.
The reported potentials are uncorrected for junction
potential. Microanalytical data were obtained from a
Perkin Elmer 2400 CHNS/O elemental analyser.
2.5. X-ray crystal structure determination
Crystals suitable for X-ray work were grown by slow
diffusion of benzene into CH2Cl2 solution of the com-
plex [Pt(p-MeaaiEt)(cat)]·1/2H2O (3b). Crystal size is
0.11×0.10×0.15 mm3. Data were collected from an
Enraf–Nonius CAD 4 four-cycle diffractometer em-
ploying graphite monochromatised Mo Ka radiation
,
(u=0.710 69 A) at 293 K. Crystal data and data collec-
tion parameters are listed in Table 1. The data are
corrected for absorption (c scans) using the XTAL
package [41–43]. The compound crystallised in P1. Of
(
the 6660 reflections collected, 6220 with I\2|(I)
(CHECK) were used for structure solution. The struc-
ture was solved using the heavy atom-Patterson method
with SHELXS86 and refined by full-matrix least-squares
analysis with SHELXL97. All non-hydrogen atoms were
refined using anisotropic thermal parameters, and
H-atoms located from difference maps and refined
isotropically.
2.3. Preparation of dicholoro-{1-ethyl-2-
(phenylazo)imidazole}platinum(II), Pt(HaaiEt)Cl2, (2a)
1-Ethyl-2-(phenylazo)imidazole (HaaiEt) (0.42 g,
2.10 mmol) in MeCN–water (15 cm3) was added drop-
wise to an MeCN–water (1:1, v/v; 40 cm3) solution of
K2PtCl4 (0.85 g, 2.05 mmol) and the mixture was
refluxed for 48 h. On slow evaporation, a brown precip-
itate gradually appeared; this was filtered and washed
with cold MeCN–water (1:1, v/v, 3×5 cm3). The dried
mass was dissolved in a minimum volume of CH2Cl2
and chromatographed over a silica gel column. The
desired compound was eluted as a brown–red band by
C6H6–MeCN (2:1, v/v); yield, 0.48 g (50%). Other com-
3. Results and discussion
3.1. Synthesis
Reaction of K2PtCl4 with 1-ethyl-2-(arylazo)-
imidazoles (RaaiEt, 1) in acetonitrile–water mixture
under reflux produced red–brown Pt(RaaiEt)Cl2

S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
23
Table 1
followed by the chromatographic purification gave
green catecholato complexes [Pt(RaaiEt)(O,O)]. The
catechols used are abbreviated as pyrocatechol (H2cat),
4-tert-butyl catechol (H2tbcat), 3,5-di-tert-butyl cate-
chol (H2dtbcat), tetrachlorocatechol (H2tccat) and have
the complexes Pt(RaaiEt)(cat) (3), Pt(RaaiEt)(tbcat)
(4), Pt(RaaiEt)(dtbcat) (5) and Pt(RaaiEt)(tccat) (6).
The complexes are non-conducting and the elemental
analyses (Table 2) support the composition of the
complexes.
Crystallographic data and structure refinement for [Pt(p-
MeaaiEt)(cat)] (3b)
Empirical formula
Formula weight
Temperature (K)
Wavelength (nm)
Crystal system
Space group
C18H18N402.5Pt
525.45
293(2)
0.710 69
Triclinic
(
P1
,
a (A)
8.8479(11)
,
b (A)
11.3939(11)
17.802(2)
89.950(9)
83.669(10)
82.462(9)
1768.1(4)
4
1.974
7.959
1008
0.11×0.10×0.75
452q550
05h510, −135k513,
−215l5 21
6660
6220 (Rint=0.0430)
6220/0/460
,
c (A)
h (°)
i (°)
k (°)
3.2. Single crystal X-ray structure of
[Pt(p-MeaaiEt)(cat)] (3b)
3
,
Volume (A )
Z
(
The complex [Pt(p-MeaaiEt)(cat)] crystallises in P1
zcalc (g cm−3
)
with two molecules in the asymmetric unit (Fig. 1),
which are different as the ethyl group about N41 has a
different conformation. The water molecule O(1W) is
hydrogen bonded to O(5B). No hydrogen atoms were
found for this molecule, although the oxygen has rela-
v (mm−1
F(000)
)
Crystal size (mm3)
2q (°)
Index ranges
,
tively short links at 2.7 A to H(43A) and H(43B). The
Reflections collected
selected bond parameters are listed in Table 3. The
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
,
PtN2O2 is a square plane (deviation B0.04 A). Two
0.981
,
chelate planes Pt, N(1), N(4) (mean deviation 0.02 A)
R1=0.0563, wR2=0.0970
R1=0.0563, wR2=0.1050
2.272 and −1.652
,
and Pt, O(5), O(6) (mean deviation 0.03 A) are planar.
The chelate angles N(1)ꢀPtꢀN(4) and O(5)ꢀPtꢀO(6) are
79.0(3)° (A), 77.8(3)° (B) and 83.4(2)° (A), 82.6(2)° (B)
respectively for molecules A and B (given in the paren-
theses). Two chelate planes are inclined at an angle
5.84° (A), 5.02° (B). The pendant p-tolyl ring is planar
and is inclined at an acute angle 21.10° (A), 23.35° (B)
with the N(1)ꢀPtꢀN(4) chelate ring. This magnetically
distinguishes two ortho CꢀH functions [19] and is seen
in the 1H NMR spectra (vide infra). The average
Largest diff. peak and hole
(e− A−3
)
(2). The addition of Et3N to the solution of 2 and
catechol in a CHCl3–MeOH mixture under N2 changes
the colour immediately from brown–red to green and
the product separates on slow bubbling of N2 for a
period of 1 h or more (Scheme 1). Solvent removal
Scheme 1. (i) MeCN–water (1:1), reflux, 48 h. (ii) Catechols, Et3N in CHCl3–MeOH, stirred, under N2 atmosphere.

Table 2
Microanalytical, UV–VIS and electrochemical data
Compound
Elemental analyses: found (calc.) (%)
UV–VIS umax (nm)
Cyclic voltammetrya
[m (dm3 mol−1 cm−1)]
f
C
H
N
Rcq/Rsq
Rsq/q
azo−/azo
DE1/2 (V)e wCT (eV)
E11/2 (V)
E12/2 (V)
−E13/2 (V)
[DEp (mV)d]
[DEp (mV)d]
[DEp (mV)d]
Pt(HaaiEt)Cl2
Pt(MeaaiEt)Cl2
Pt(ClaaiEt)Cl2
Pt(HaaiEt)(cat)
Pt(HaaiEt)(tbcat) 45.10 (45.08)
Pt(HaaiEt)
(dtbcat)
Pt(HaaiEt)(tccat) 31.75 (31.82)
Pt(MeaaiEt)(cat) 41.85 (41.77)
Pt(MeaaiEt)-
(tbcat)
Pt(MeaaiEt)-
(dtbcat)
Pt(MeaaiEt)-
(tccat)
28.40 (28.32)
30.15 (30.00)
26.40 (26.37)
40.50 (40.55)
2.52 (2.57)
2.86 (2.92)
2.24 (2.20)
3.10 (3.18)
4.30 (4.29)
5.00 (5.20)
12.00 (12.01)
11.58 (11.66)
11.13 (11.18)
11.18 (11.13)
9.85 (10.01)
8.50 (9.10)
545b [165], 482 [457], 412 [14 410]
541c [167], 482 [424], 417 [14 757]
553 [160], 484 [424], 415 [14 240]
807 [7050], 630 [3292], 403 [13 251]
841 [5356], 634 [2639], 405 [9773]
925 [6074], 654 [2230], 468 [3844]c
0.404 [70]
0.470 [75]
0.349 [70]
0.842 [120]
0.871 [120]
0.884 [130]
/
0.414 [105]
0.353 [80]
0.247 [85]
0.850c
0.686 [120]
0.524 [100]
1.256
1.221
1.131
1.538
1.476
1.342
48.70 (48.78)
1.80 (1.87)
3.50 (3.48)
4.50 (4.53)
8.75 (8.73)
10.73 (10.83)
9.80 (9.77)
725 [7950], 600 [4776], 408 [16 348]
790 [7782], 619 [3393], 415 [14 886]
822 [2215], 628 [1887], 420 [8191]
0.780c
0.400 [104]
0.338 [100]
0.743 [90]
0.878 [130]
0.924 [160]
0.810c
0.660 [120]
1.278
1.262
1.571
1.510
46.00 (46.07)
49.56 (49.60)
32.90 (32.97)
5.34 (5.40)
2.10 (2.13)
8.15 (8.90)
8.50 (8.55)
888 [3467], 659 [1739], 417 [9622]
723 [2830], 590 [2040], 420 [9350]
0.214 [90]
0.710c
0.511 [150]
0.895 [140]
0.791 [110]
1.109
1.400
1
(
Pt(ClaaiEt)(cat)
Pt(ClaaiEt)-
(tbcat)
Pt(ClaaiEt)-
(dtbcat)
37.90 (37.95)
42.40 (42.45)
2.75 (2.79)
3.80 (3.87)
10.35 (10.41)
9.45 (9.43)
841 [11 290], 634 [4806], 414 [21 504] 0.452 [94]
883 [5179], 651 [2152], 412 [10 321]
0.780c
0.630 [110]
0.753 [140]
0.764 [109]
1.205
1.154
1.476
1.405
0.381 [100]
0.265 [120]
0.830c
2
46.00 (46.18)
4.69 (4.77)
1.60 (1.62)
8.58 (8.62)
8.00 (8.29)
960 [6553], 667 [1831], 474 [4101],
407 [9533]
741 [625], 590 [476], 412 [1723]
0.570 [100]
0.798 [180]
0.665 [140]
1.063
1.293
Pt(ClaaiEt)(tccat) 30.00 (30.19)
a Solvent is dichloromethane.
b Shoulder.
c Epa (V).
d DEp=Epa−Epc (mV).
e DE1/2=E11/2−E13/2 (V).
f wCT=1241/umax (nm).

S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
25
Fig. 1. Single crystal structure of [Pt(z-MeaaiEt)(cat)]. For clarity, all hydrogen atoms have been eliminated.
PtꢀN(azo) (PtꢀN(1)) and PtꢀN(imidazole) (PtꢀN(4))
3.3. Spectral characterisation
,
distances are approximately 1.980(6) (A), 1.971(6) A (B)
,
The IR spectrum of Pt(RaaiEt)Cl2 (2) exhibits a
single sharp stretch at approximately 1385–1395 cm−1
corresponding to w(NꢁN) compared with free ligand
values (approximately 1410 cm−1). The cis-PtCl2
configuration was established by the appearance of
w(PtꢀCl) at 350 and 335 cm−1 [13,52]. For catecholato
complexes, [Pt(RaaiEt)(O,O)], the NꢁN stretch appears
at approximately 1340 cm−1 and the lowering of the
frequency may be attributed to the extensive d(Pt)“
p*(RaaiEt) back bonding or 3b1(cat)“p*(RaaiEt) in
the catecholato comlexes [29]. The appearance of a
broad band at 3400 cm−1 supports the presence of H2O
in the molecule. On comparing the spectra of
Pt(RaaiEt)Cl2 and [Pt(RaaiEt)(O,O)], additional bands
are observed at 1250–1280 and 1460–1470 cm−1 corre-
sponding to CꢀO (phenolic) vibrations of catechols and
at approximately 555 cm−1 for (PtꢀO).
and 1.980(6) (A), 1.974(6) A (B) respectively. Although
the two catecholato-O are equivalent [44], they are
distinctly different in terms of the PtꢀO and CꢀO bond
distances in this complex. The PtꢀO(6) is elongated by
,
approximately 0.03 A compared with PtꢀO(5). The
,
CꢀO(6) is shorter than CꢀO(5) by 0.04 A, but is far
away from the semiquinone bond length [45]. This
observation is corroborated with the higher trans-influ-
ence provided by N(4) (N(imidazole)) than by N(1)
(N(azo)). From available structural reports on platinum
metal compounds of azoimine, the function MꢀN(azo)
is shorter than the MꢀN(imine) distance (M=Ru [46],
Os [3], Re [4], Pd [47], Pt [48]). The shortening may be
due to the greater p back-bonding, represented as
M(dp)“p*(azo). This also enhances the NꢁN bond
,
length relative to the free ligand value (1.25 A) [3,49].
The X-ray structure determination of dichloro-[1-ethyl-
2-(naphthyl-b-azo)imidazole]platinum(II) suggests that
,
the NꢁN length is 1.280(5) A [50]. The present structure
Table 3
,
has the longest NꢁN bond (1.32 A) in the platinum
Selected bond distances and angles and their estimated standard
deviations for 3b
metal chemistry; this may be due to charge delocalisa-
tion from catechol to the azo function in the coordi-
nated frame involving metal dp-orbitals, which is
supported from solution (VIS–NIR), magnetic (1H
NMR) and redox properties.
The PtꢀN(1) and PtꢀN(4) distances are comparable.
This is quite unusual compared with similar bond
lengths in other platinum metal complexes [3,4,46,47],
and in the case of Pt(II)–naphthylazoimidazole [50] it
,
Distances (A)
Angles (°)
Pt(1A)ꢀN(1A)
Pt(1A)ꢀN(4A)
Pt(1A)ꢀO(5A)
Pt(1A)ꢀO(6A)
Pt(1B)ꢀN(1B)
Pt(1B)ꢀN(4B)
Pt(1B)ꢀO(5B)
Pt(1B)ꢀO(6B)
N(1A)ꢀN(2A)
N(1B)ꢀN(2B)
C(51A)ꢀO(5A)
C(56A)ꢀO(6A)
C(51A)ꢀC(56A)
C(51B)ꢀO(5B)
C(56B)ꢀO(6B)
C(51B)ꢀC(56B)
1.980(6)
1.980(6)
1.956(6)
1.994(5)
1.971(6)
1.974(6)
1.949(5)
1.978(5)
1.320(9)
1.319(9)
1.376(10)
1.327(9)
1.388(11)
1.361(9)
1.333(9)
1.385(11)
N(1A)ꢀPt(1A)ꢀN(4A)
O(5A)ꢀPt(1A)ꢀO(6A)
N(1A)ꢀPt(1A)ꢀO(5A) 176.0(2)
N(4A)ꢀPt(1A)ꢀO(6A) 176.3(2)
N(1A)ꢀPt(1A)ꢀO(6A) 100.0(2)
N(4A)ꢀPt(1A)ꢀO(5A)
N(1B)ꢀPt(1B)ꢀN(4B)
O(5B)ꢀPt(1B)ꢀO(6B)
N(1B)ꢀPt(1B)ꢀO(5B)
N(4B)ꢀPt(1B)ꢀO(6B)
N(1B)ꢀPt(1B)ꢀO(6B)
N(4B)ꢀPt(1B)ꢀO(5B)
79.8(3)
83.4(2)
97.5(3)
77.8(3)
82.6(2)
175.7(2)
177.2(2)
101.4(2)
98.2(2)
,
follows the order PtꢀN(4) (1.983(4) A)BPtꢀN(1)
(2.039 A). This may be due to the preferential binding
,
of platinum(II) to imidazole-N than other N(sp2)
centres present in the same molecule. It is observed
in the 1-methylimidazole-2-yl-pyridine-2-yl ketone
Pt(II) complex that the PtꢀN(imidazole) is shorter than
the PtꢀN(pyridine) distance [51]. Biological mole-
cules and DNA bases also prefer to bind cisplatin via
N(imidazole) (N(7) of guanine, adenine) [37].

26
S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
of the Cl group, and the reverse is true for MeaaiEt
because of the +I effect of the ꢀMe group.
1
The proton numbering scheme for H NMR spectra
is shown for structures 2–6 with data summarised in
Table 4. All aromatic protons of the complexes were
assigned on the basis of spin–spin interactions and
changes on substitution. The NꢀEt signal in
Pt(RaaiEt)Cl2 is supported by the appearance of a
triplet and a quartet signal at approximately 1.6 ppm
and 4.5 ppm respectively. Imidazole 4- and 5-H appear
at 7.6 ppm and 7.4 ppm respectively and are downfield
shifted by \0.4 ppm compared with the free ligand
values. This is in support of strong Pt(II) bonding with
the imidazole group. The aryl protons are also
downfield shifted with reference to the free ligand val-
ues and are moved to higher and lower l values in the
usual manner with the substituents [49]. In catecholato
complexes the RaaiEt protons, in general, suffer signifi-
cant upfield shifting by 0.2–1.0 ppm compared with the
parent chloro complex, with the exception of a doublet
signal of a proton (as evident from integration). The
signal shifting to an upfield position is due to the
charge being delocalised from the HOMO (cat) to the
LUMO (RaaiEt) [29]. The appearance of a downfield
doublet could be due to the asymmetric orientation of
the pendant aryl ring in the ligand. This magnetically
differentiates between the 9- and 13-H, where 9-H is
considered stereochemically nearer to the metal centre.
The X-ray structural data also support this inference;
the p-tolyl plane is inclined at acute angle (21.1°) to the
corresponding chelate plane (vide supra). The coordina-
tion of imidazole-N to Pt(II) is again supported by
coupling of 4-H with 195Pt (I=1/2, abundance 33.7%)
Fig. 2. Electronic absorption spectra of [Pt(HaaiEt)(cat)] (—);
[Pt(HaaiEt)(tbcat)] (- - -); and [Pt(HaaiEt)(dtbcat)] (— · —); and
[Pt(HaaiEt) (tccat)] (· · ·) in CH2Cl2 at 298 K.
The electronic spectra of the complexes were exam-
ined in chloroform. Pt(RaaiEt)Cl2 exhibits three ab-
sorption bands in the visible region at 415, 480 and
545 nm. The transitions below 400 nm are due to an
intraligand charge transfer transition [21] and are not
considered further. The absorption spectra of cate-
cholato complexes [Pt(RaaiEt)(O,O)] are different than
Pt(RaaiEt)Cl2 (Fig. 2, Table 2). A new band appears in
the red to NIR region and is dependent on the nature
of the substituent in dioxolene and azoimidazole frag-
ments [24,29,39]. This band has been assigned as an
LLCT transition involving the HOMO of catechol and
the LUMO of RaaiEt [53] or the redox orbital consti-
tuted by platinum(II) and ligand [39]. The electron-do-
nating substituent (ꢀMe group) in the MeaaiEt site
moves the band position to the shorter wavelength
region and the reverse is true for the electron-withdraw-
ing substituent (ꢀCl group) in ClaaiEt. Thus the wave-
length order follows MeaaiEtBHaaiEtBClaaiEt. The
wavelength trend of the absorption band for a particu-
lar RaaiEt in [Pt(RaaiEt)(O,O)] is tccatBcatBtbcatB
dtbcat. The HOMO of dtbcat is expected to have the
highest energy in the series because of the electron-re-
3
and the coupling constant J(PtꢀH) is 20–25 Hz [54].
The protons of the catecholato ring are on the upfield
side relative to the RaaiEt; this is caused by the elec-
tron-releasing effect of the catecholato oxygen. The
complexes [Pt(RaaiEt)(cat)] give
a
characteristic
AA%BB% pattern, whereas signals in the aliphatic region
(l, 1.2, 1.4 ppm) are due to the tBu group in
Pt(RaaiEt)(tbcat)/Pt(RaaiEt)(dtbcat) complexes. Other
protons in the dioxolene ring are easily identified by
comparing the results reported elsewhere [29].
3.4. Electrochemical studies
The complexes exhibit three/four successive cyclic
redox systems within the potential range +1.5 to
−1.5 V versus SCE. The electrochemical measurements
were carried out with the use of a Pt working electrode
for Pt(RaaiEt)Cl2 (2) and a GC working electrode for
[Pt(RaaiEt(O,O)] (3, 6) in acetonitrile solution under
dinitrogen in the presence of tetrabutylammonium per-
chlorate (Bu4NClO4) as a supporting electrolyte. The
couples appearing at the negative side to the SCE
correspond to azo reduction. The LUMO of azo can
t
leasing effect of two Bu groups. In tccat, the HOMO
has the lowest energy in the series because of the
electron-withdrawing character of the ꢀCl groups. The
LUMO is characterised by an azoimidazole function; in
ClaaiEt the energy of the p*(LUMO) is of lower value
than HaaiEt because of the electron-withdrawing effect

S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
27
accommodate two electrons and, hence, two reductions
are observed (Eq. (1)).
tron extraction from the HOMO (catechol character-
ised). The difference in potential between the first
reduction and first oxidation (DE1/2=E11/2−E13/2) may
be correlated with the LLCT (wCT) transitions that are
observed in the low-energy region (Eq. (3)). This sup-
ports the involvement of the same redox orbitals in
charge transfer transition and redox reactions.
−
−
[ꢀNꢁNꢀ]+Xe [ꢀN···
6
Nꢀ]+“e [ꢀNꢀNꢀ]2−
(1)
wCT=1.174DE1/2+0.052
(3)
(2)
The anodic responses are due to coordinated catechol
oxidation (Eq. (2)) with the compounds undergoing two
successive one-electron redox responses with varying
degrees of reversibility depending upon the substituent
in the catechol ring [29,55].
3.5. Comparison with analogous palladium complexes
Table 5 summarises some spectral and electrochemi-
cal data of the palladium(II) [24,49] and platinum(II)
complexes of analogous 1-ethyl-arylazoimidazoles. It
shows that the LLCT band position is shifted to a
shorter wavelength region, E1/2 of dioxolenes is moved
to less positive values and −E1/2 of azo is also trans-
ferred to more negative values on traversing from palla-
dium(II) to platinum(II). The spectral trend accounts
for the larger energy difference between the participat-
ing HOMO and LUMO in platinum(II) complexes and
is more destabilised relative to palladium(II) complexes.
This may be due to the participation of metal-redox
orbitals with ligand-group of orbitals more efficiently in
platinum than palladium complexes. This is a general
group trend in the family of platinum metals and the
5d-orbitals are relativistically more perturbed by the
external field than 4d-orbitals [57]. This observation is
The complexes [Pt(RaaiEt)(tbcat)] and [Pt(RaaiEt)-
(dtbcat)] give reversible first oxidation couples (DEp=
70–90 mV) followed by a quasi-reversible response
(DEp\120 mV), as evident from peak-to-peak separa-
tion. The data in Table 1 reveal that Epa (anodic peak
potential) is shifted to more positive values due to the
presence of the electron-withdrawing ꢀCl group in
Pt(RaaiEt)(tccat) and the reverse is observed for butyl
groups (Fig. 3). This is because oxidation will be ther-
modynamically more facile when electron density flows
into the catechol ring, leading to the stabilisation of the
positive charge of the cation radical [56]. Thus, reduc-
tion refers to the electron accommodation at the
LUMO (azo characterised) and oxidation is the elec-
Table 4
1H NMR spectral data in CDCl3
Compounda
l (ppm)
4-Hb,e 5-Hb 9-Hb 10,12-H
13-Hb 6-CH2ꢀ 7ꢀCH3 16-H 17-H 18-H 19-H Others
c
Pt(HaaiEt)Cl2
Pt(MeaaiEt)Cl2
Pt(ClaaiEt)Cl2
Pt(HaaiEt)(cat)
7.40 7.24 8.00 7.71c
7.37 7.22 7.95 7.62b
7.49 7.29 8.16 7.85b
7.18 7.04 8.22 7.50c
7.10 6.97 8.16 7.46c
7.03 6.81 8.12 7.44c
7.20 7.07 8.25 7.55c
7.20 7.05 8.25 7.35b
7.15 7.00 8.22 7.33b
7.11 6.94 8.20 7.30b
7.24 7.10 8.28 7.42b
7.16 7.03 8.30 7.44b
7.11 6.93 8.28 7.42b
7.00 6.60 8.25 7.38b
7.25 7.09 8.40 7.68b
8.00
7.95
8.16
7.80
7.74
7.66
7.93
7.85
7.78
7.84
7.94
8.00
7.96
7.90
8.14
4.55
4.52
4.58
4.25
4.24
4.19
4.43
4.28
4.25
4.24
4.33
4.20
4.35
4.30
4.40
1.62
1.60
1.62
1.58
1.54
1.50
1.58
1.55
1.54
1.50
1.55
1.55
1.55
1.22
1.30
2.43g
7.04b 6.51d 6.51d 7.04b
Pt(HaaiEt)(tbcat)
Pt(HaaiEt)(dtbcat)
Pt(HaaiEt)(tccat)
Pt(MeaaiEt)(cat)
Pt(MeaaiEt)(tbcat)
Pt(MeaaiEt)(dtbcat)
Pt(MeaaiEt)(tccat)
Pt(ClaaiEt)(cat)
Pt(ClaaiEt)(tbcat)
Pt(ClaaiEt)(dtbcat)
Pt(ClaaiEt)(tccat)
6.78f
6.58b 6.89b 1.30h
6.40g 1.33h, 1.27h
6.63f
6.52
7.01d 7.01d 6.52
2.41h
6.67f
6.44b 6.69b 2.40h, 1.30h
6.39g 2.40g, 1.33h, 1.26i
2.42g
6.58f
6.53b 6.97d 6.97d 6.53b
6.69f 6.48b 6.82b 1.28h
6.68g 1.27h, 1.41h
6.84f
a With Me4Si as internal standard.
b Doublet (J=7.0–8.0 Hz).
c Triplet (J=10.0–11.0 Hz).
d Multiplet (J=7.0–8.0 Hz).
e 3JPtꢀH=20–25 Hz.
f Singlet.
g p-Me.
h tBu.

28
S. Pal et al. / Inorganica Chimica Acta 313 (2001) 21–29
Fig. 3. Cyclic voltammogram of [Pt(HaaiEt)(cat)] (— · —); [Pt(HaaiEt)(cat)] (- - -); [Pt(HaaiEt)(cat)] (—) and [Pt(HaaiEt)(tccat)] (· · ·) in MeCn.
Table 5
data are available from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ,
Comparison with analogous palladium complexes
UK, on request, quoting the deposition number
147007.
Complex
uLLCT (nm)
E1M/2 (V)
-E1L/2 (V)
Pd(HaaiEt)(tbcat)
Pd(HaaiEt)(dtbcat)
Pd(MeaaiEt)(tbcat)
Pd(MeaaiEt)(dtbcat)
Pt(HaaiEt)(tbcat)
Pt(HaaiEt)(dtbcat)
Pt(MeaaiEt)(tbcat)
Pt(MeaaiEt)(dtbcat)
974
1000
944
982
841
925
822
888
0.45
0.35
0.41
0.32
0.353
0.247
0.338
0.214
0.46
0.50
0.50
0.55
0.868
0.884
0.924
0.895
Acknowledgements
Financial support from the Council of Scientific and
Industrial Research, New Delhi is gratefully acknowl-
edged. Our sincere thanks are due to Professor G.N.
Mukherjee, Department of Chemistry, Calcutta Univer-
sity for recording some NIR spectra and helpful
discussion.
backed up by the unusual enhancement of the NꢁN
bond and shortening of the CꢀO bond in the X-ray
crystal structure and supports the charge shifting the
catecholate part to the azoimine end.
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