Platinum complexes of diazo ligands. Studies of regioselective aromatic ring amination, oxidative halogen addition and reductive halogen elimination reactions

Article abstract of DOI:10.1039/b418470j

2-(Arylazo)pyridine ligands, L^1a-1c react with the salt K ₂[PtCl₄] to give the mononuclear complexes [PtCl ₂(L¹)] (1), which readily react with ArNH₂ to yield the monochloro complexes of ty

Full text of DOI:10.1039/b418470j

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F U L L P A P E R
Platinum complexes of diazo ligands. Studies of regioselective
aromatic ring amination, oxidative halogen addition and reductive
halogen elimination reactions†
Manashi Panda,a Srijit Das,a Golam Mostafa,b Alfonso Castin˜eirasc and Sreebrata Goswami*a
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Kolkata, 700 032, India. E-mail: icsg@iacs.res.in; Fax: +91 33 2473 2805
b Department of Physics, Jadavpur University, Kolkata, 700 032, India
c Departamento de Qu´ımica Inorga´nica, Universidade de Santiago de Compostela,
Campus Universitario Sur, E-15782, Santiago de Compostela, Spain
Received 8th December 2004, Accepted 17th February 2005
First published as an Advance Article on the web 4th March 2005
–1c
2-(Arylazo)pyridine ligands, L1a react with the salt K2[PtCl4] to give the mononuclear complexes [PtCl2(L1)] (1),
which readily react with ArNH2 to yield the monochloro complexes of type [PtCl(L2)] (HL2 =
2-[(2-(arylamino)phenyl)azo]pyridine) (2) via regioselective ortho-amine fusion at the pendent aryl ring of
coordinated L1. Oxidative addition of the electrophiles Y2 (Y = Cl, Br, I) to the square-planar platinum(II) complex,
2 has led to syntheses of the corresponding octahedral platinum(IV) complexes, [PtY3(L2)] (3) in high yields.
Ascorbate ion reductions of the platinum(IV) complexes, 3, resulted in reductive halogen elimination to revert to the
platinum(II) complexes 2 almost quantitatively. Isolation of products and X-ray structure determination of the
representative complexes followed all these chemical reactions. In crystal packing, the compound [PtCl2(L1c)] (1c)
˚
forms dimeric units with a Pt · · · Pt distance of 3.699(1) A. In contrast, the crystal packing of 2b revealed that the
molecules are arranged in an antiparallel fashion to form a noncovalent 1D chain to accommodate p(aryl)–p(pyridyl)
and Pt–p(aryl) interactions. Notably, the oxidation of [PtIICl(L2a)] (2a) by I2 produced a mixed halide complex
[PtIVClI2(L2a)] (5), which, in turn, is reduced by ascorbate ion to produce [PtIII(L2a)] with the elimination of ClI. All
the platinum(II) complexes are brown, the platinum(IV) complexes, on the other hand, are green. Low-energy visible
range transitions in the complexes of the extended ligand [L2]− are ascribed to ligand based p–p* transitions. Cyclic
voltammetric behaviour of the complexes is reported.
Our literature survey has revealed that platinum–azo complexes
Introduction
are scarce.7
This work has originated from our interest in the coordination
In this paper we wish to report synthesis and reactivities of the
chemistry1 of 2-(arylazo)pyridine (AAP, L1) ligands involving
transition metal complexes in general, and platinum group
metal complexes, in particular. Rich redox as well as spectral
properties of these systems have been studied over the past two
dichloro platinum complexes [PtCl2(L1)] (1). The pendent aryl
ring of coordinated L1 in 1 undergoes ortho-amination reaction
regioselectively to result a monochloro platinum(II) complex
[PtCl(L2)] (2) of a tridentate ligand (HL2) formed due to amine
decades. During the recent years much attention has focused on
fusion via C–X (X = H, Cl) activation of the coordinated L1.
unusual metal mediated reactions2–4 on the coordinated diazo
Oxidative halogen addition to these monochloro complexes (2)
ligand. The first example of such a reaction, viz. palladium
produce platinum(IV) complexes, [PtY3(L2)] (Y = Cl, Br, I) (3–5).
promoted hydroxylation of the phenyl ring on coordinated L1
The tetravalent complexes, in turn, are reduced to the bivalent
was noted2 in 1982. During the recent past we have been engaged
complexes, 2 by L-ascorbic acid. The chemical isolation as well as
X-ray structure characterization of the products follow all these
in the systematic study of aromatic ring amination reactions
on coordinated L1. The scope of these reactions appears to be
reactions. In the present context we note that oxidative addition
high particularly in the context of design and synthesis5 of new
coupled with reductive elimination in platinum complexes forms
ligands of multiple denticities whose coordination chemistry is
the basis of catalytic cycles8 that are used in catalyzed organic
interesting with many novel features. Furthermore, the recent
reactions. While the examples of oxidative I2 addition to bivalent
discovery of cytotoxic activity6 of the isomeric ruthenium
platinum complexes are not common,9 the reductive elimination
complex, [RuCl2(L1)2], has yielded the possibility of similar
by potential bio-reductants10 such as ascorbic acid has biological
activities in the corresponding or related platinum complexes.
significance.
Results and discussion
A: [PtCl2(L1)] 1
† Electronic supplementary information (ESI) available: Table S1:
Secondary interactions for the complexes 1c, 2b, 4 and 5. Fig. S1: A
supramolecular dimer of [PtCl2(L1c)] 1c formed by p · · · p interaction.
Fig. S2: 1D supramolecular chain of [PtCl(L2b)] 2b constructed by
p · · · p, Pt · · · p interaction running along b axis. Fig. S3: ORTEP
and atom numbering scheme for [PtBr3(L2a)] 4. Fig. S4: A 2D sheet
of [PtBr3(L2a)] 4 constructed from p-stacking and Br · · · Br contacts.
Fig. S5: A dimeric unit of [PtClI2(L2a)] 5 constructed by p · · · p stacking
interaction. Fig. S6: Segmented cyclic voltammograms of [PtCl2(L1a)]
1a, [PtCl(L2a)] 2a and [PtClI2(L2a)] 5. Fig. S7: UV-Vis spectra of 1a, 2a
and 5. See http://www.rsc.org/suppdata/dt/b4/b418470j/
The three AAP ligands, viz. 2-(phenylazo)pyridine (L1a), 2-(m-
tolylazo)pyridine (L1b) and 2-(o-chlorophenylazo)pyridine (L1c)
were used in this work (Scheme 1). The ligands,11 L1a–L1c react
with the salt K2[PtCl4] in boiling aqueous acetonitrile (CH3CN–
H2O, 9 : 1) to produce platinum complexes of general formula
[PtCl2(L1)] (1) in excellent yields (>80%). The complexes are
freely soluble in dimethyl formamide and are only sparingly
soluble in acetonitrile. The single-crystal X-ray structure of a
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5
1 2 4 9
©

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ability1 of the azo-chromophore of L1 over the pyridyl function.
As a consequence, the N–N length12 of the coordinated azo
˚
ligand is elongated (1.271(4) A). In the crystal packing, the
compound 1c forms dimeric units; the two molecules of the
dimer interact weakly with one another by a center of inversion13
(Fig. S1, ESI†). The platinum complexes are diamagnetic. Their
1H NMR spectra are broad (Fig. 2) due to some unidentified
dynamic processes.14
Scheme 1
representative, [PtCl2(L1c)], 1c is solved. Fig. 1 shows the ORTEP
and atom numbering scheme for 1c. Selected bond parameters
are collected in Table 1.
B: Amination of coordinated L1 in [PtCl2(L1)] 1
The bivalent platinum complexes 1 were found to react freely
with primary aromatic amines in neat to yield platinum com-
plexes of the anionic tridentate ligand, [L2]− due to regioselective
C–N bond fusion at ortho carbon of the pendent aryl ring of co-
ordinated L1. The reddish-brown mixture of 1 with neat ArNH2,
upon heating on a steam-bath, produced a brown paste in about
an hour. The crude product, after initial work-up, was purified by
repeated crystallization from dichloromethane–hexane solvent
mixture. The structural analysis of a representative, [PtCl(L2b)],
2b (R = Me) confirms amine fusion at the pendent aryl ring
specifically at a position ortho to the azo-group of L1 (Scheme 2
and Fig. 3).
The extended N,N,N-donor (HL2) binds as a bischelate with
deprotonation of the amine nitrogen (N(14)). In the above
transformation, one of the two coordinated chlorides dissociated
from the starting dichloro complex 1, which, is an essential
prerequisite4c for such an ortho-amine fusion reaction.
The yield of the compound 2 is high (>75%). Formally
this ligand transformation involves nucleophilic aromatic ring
substitution of hydride (as in 1a) or halide (as in 1c). Such
substitution reactions at coordinated ligands generally occur
Fig. 1 ORTEP and atom numbering scheme for [PtCl2(L1c)], 1c.
Hydrogen atoms are omitted for clarity.
The complex 1c is essentially planar with the platinum atom
in a distorted square-planar geometry. The two cis-chlorides lie
almost equidistant from the metal center. Notably, Pt–N(azo)
˚
length, e.g. Pt(1)–N(2) (1.959(3) A) is shorter than that of Pt–
˚
N(py), 1.992(3) A. This may be attributed to greater p-acceptor
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for the complexes 1c, 2b, 4 and 5
1c
Pt(1)–Cl(1)
Pt(1)–N(6)
C(1)–N(6)
2.2888(10)
1.992(3)
1.350(5)
Pt(1)–Cl(2)
N(2)–N(1)
C(11)–N(2)
2.2796(11)
1.271(4)
1.445(5)
Pt(1)–N(2)
N(1)–C(1)
1.959(3)
1.394(5)
Cl(1)–Pt(1)–Cl(2)
90.74(4)
N(2)–Pt(1)–N(6)
78.11(14)
2b
Pt(1)–Cl(1)
Pt(1)–N(11)
C(16)–N(13)
C(15)–N(11)
2.3090(10)
2.018(3)
1.363(4)
1.370(4)
Pt(1)–N(14)
N(14)–C(21)
N(13)–N(12)
N(14)–C(22)
2.031(3)
1.345(4)
1.303(4)
1.402(5)
Pt(1)–N(13)
C(21)–C(16)
N(12)–C(15)
1.928(3)
1.438(5)
1.398(4)
N(14)–Pt(1)–N(13)
N(11)–Pt(1)–Cl(1)
82.06(13)
97.79(9)
N(13)–Pt(1)–N(11)
Cl(1)–Pt(1)–N(14)
79.19(13)
100.95(9)
4
Pt(1)–Br(1)
Pt(1)–N(1)
N(1)–C(5)
N(3)–C(6)
N(4)–C(12)
2.4602(7)
2.051(5)
1.381(8)
1.349(7)
1.427(8)
Pt(1)–Br(2)
Pt(1)–N(3)
C(5)–N(2)
C(6)–C(11)
2.4560(6)
1.972(4)
1.380(8)
1.431(8)
Pt(1)–Br(3)
Pt(1)–N(4)
N(2)–N(3)
C(11)–N(4)
2.4501(7)
2.035(4)
1.297(7)
1.333(7)
N(1)–Pt(1)–N(3)
Br(1)–Pt(1)–Br(3)
5
79.2(2)
177.53(2)
N(3)–Pt(1)–N(4)
81.8(2)
Pt(1)–I(1)
2.6567(4)
2.037(4)
1.346(8)
1.298(6)
1.419(7)
Pt(1)–I(2)
Pt(1)–N(3)
C(11)–C(6)
N(2)–C(5)
2.6551(5)
1.952(5)
1.439(8)
1.384(7)
Pt(1)–Cl(1)
Pt(1)–N(1)
C(6)–N(3)
C(5)–N(1)
2.3515(14)
2.048(4)
1.367(7)
1.373(7)
Pt(1)–N(4)
N(4)–C(11)
N(3)–N(2)
N(4)–C(12)
N(1)–Pt(1)–N(3)
I(1)–Pt(1)–I(2)
78.98(18)
178.20(2)
N(3)–Pt(1)–N(4)
82.20(18)
1 2 5 0
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5

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Scheme 2
chloro substituted ligand L1c, reacts also in an oxygen-free
atmosphere. This is not surprising since hydrogen substitution
in a typical nucleophilic aromatic substitution (SNAr) reaction
needs an oxidizing agent to rearomatize15 the intermediate.
Coordination geometry of the platinum atom is distorted square
planar, deviation from the idealized geometry is primarily due
to geometric constraints imposed by the tridentate ligand.
The Pt–N lengths showed expected variation with that to
the middle nitrogen (N(13)) being shorter than those to the
other two terminal nitrogens viz. N(11) and N(14). The chelate
bite angles are in agreement16 with those of the previously
reported complexes of [L2]− and the Pt–N and Pt–Cl lengths are
similar to related17 platinum complexes of tridentate nitrogenous
ligands. Furthermore, the elongation of the diazo length, N(12)–
˚
N(13), 1.303(4) A in this compound is attributed to extensive
delocalisation4c of charge along the ligand backbone. A packing
view of 2b (Fig. S2, ESI†) reveals the arrangement of the
molecules in an antiparallel fashion to form a noncovalent 1D
chain (Table S1, ESI†) wherein the alternative antiparallel Pt
atoms are displaced up and down from the chain axis. The
complexes 2a and 2b showed highly resolved 1H NMR spectra
in CDCl3 (Experimental section) and a representative spectrum
is displayed in Fig. 2. The pyridyl protons resonated in the low
field region (d 8.0–7.0). Aromatic proton signals are assigned
primarily based on substitution induced change in spin–spin
coupling pattern. The complex 2b of methyl substituted ligand
[L2b]− showed a sharp methyl resonance at d 2.30. Notably the
proton resonances for the complexes4c of the ligand [L2]− appear
high field as compared to that1c of the neutral ligand L1. This
may be attributed to electron delocalization along the ligand
backbone of the coordinated anionic ligand [L2]−.
Fig. 2 1H NMR spectra of (a) 1a in DMSO-d6, (b) 2a in CDCl3 and
(c) 5 in CDCl3 at 300 K.
C: Reversible oxidative addition and reductive elimination
reactions
The bivalent platinum complexes 2 are susceptible to oxidation
and react with halogens, viz. chlorine, bromine and iodine in-
stantaneously to produce the platinum(IV) complexes of type 3–5
almost quantitatively (>95%). In all the cases oxidative halogen
addition occur with concomitant 2e− oxidation of PtII→PtIV
(Scheme 3). It was noted before18 by us that the reaction
of K2[PtCl4] with the preformed HL2 ligand in acetonitrile
produces a tetravalent green complex, [PtCl3(L2)] in a moderate
yield. However, no bivalent platinum complex of type 2 could
be isolated from the above reaction.
The identity of the Cl2 oxidation product, [PtCl3(L2a)] (3) was
confirmed18 by comparing its spectra with those of the reported
sample. However, formulations of its bromo and iodo analogues
Fig. 3 ORTEP and atom numbering scheme for [PtCl(L2b)], 2b.
Hydrogen atoms are omitted for clarity.
at electron deficient aromatic rings induced by the presence of
high valent metal ions. Notably, the reference reaction of 1 with
ArNH2 occur only in neat amine and failed to produce any
isolable products in methanol or 2-methoxyethanol even in the
presence of a very high excess (10 times) of aromatic amines.
Moreover, for the amine fusion reaction of the unsubstituted
ligand L1a the presence of air is necessary, however, the 2-
Scheme 3
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5
1 2 5 1

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were made from their X-ray structure determination (Table 1).
The ORTEP and atom numbering scheme of [PtClI2(L2a)] (5)
are shown in Fig. 4 and that of a closely related compound
[PtBr3(L2a)] (4) are submitted as ESI (Fig. S3†).
D: Solution properties
Cyclic voltammetry of the platinum complexes reported herein
are dominated by two reversible reductive responses, which
are assigned4,24 to ligand reductions (Fig. S6, ESI†). Electronic
spectra of the present platinum complexes are characterized by
low energy structured bands17a appearing in the visible-NIR
range. The transitions near 600 nm in 1 may be ascribed25
as metal to ligand (MLCT) electronic transitions. The lowest
energy transitions in the complexes of the tridentate ligand,
2b and 5 are ascribed26 as intraligand p–p* transfer. Notably,
such low energy transitions in the platinum complexes are
uncommon27 in the literature. Redox and electronic spectral data
of the complexes are collected in the Experimental section. UV-
visible spectra of three representative complexes are submitted
as ESI (Fig. S7†).
E: Conclusion
Successful isolation and characterization of platinum complexes
of 2-(arylazo)pyridine ligand together with some examples of
fascinating chemical reactions viz. platinum promoted aromatic
amine fusion, oxidative halogen addition as well as reductive
dehalogenation using the bio-reductant ascorbic acid, are the
main issues of the present work. The unique versatility of the
chemical transformations that occur in the present complexes
opens up avenues for the synthesis of new platinum–azo systems,
which are scarce in the literature. Our work on the exploration
of oxidative addition of other electrophiles to platinum(II)
complexes for the synthesis of organometallic platinum(IV)
systems is under active search.
Fig. 4 ORTEP and atom numbering scheme for [PtClI2(L2a)], 5.
Hydrogen atoms are omitted for clarity.
The deprotonated extended tridentate N,N,N-donor in each
case binds as a bis-chelating ligand and the coordination geom-
etry around the platinum atom is distorted octahedral. Three
halogen atoms occupy rest three positions. Interestingly in the
mixed-halide complex 5, the two iodine atoms occupy relative
trans-axial positions. Of the three Pt–N bonds, the Pt–N(azo)
is shortest, the N–N lengths are elongated indicating strong
delocalisation of anionic charge along the ligand backbone.
The Pt–Y (Y = Br, I) bond lengths9d,19 are normal. The crystal
packing of 4 reveals intermolecular Br · · · Br contacts linking the
individual monomers into a 2D supramolecular network (Fig.
S4, ESI†). The crystal packing of 5, on the other hand, reveals
a face-to-face p–p stacking (Fig. S5, ESI†) between the pyridine
and phenyl rings.
Experimental
Materials
The starting metal salt K2[PtCl4] was obtained from Arora-
Matthey, Kolkata. Solvents and chemicals used for the syntheses
–
were of analytical grade; 2-(arylazo)pyridine ligands, L1a 1c were
prepared by the published11 procedure.
Physical measurements
A JASCO V-570 Spectrometer was used to record electronic
spectra. The IR spectra were recorded with a Perkin-Elmer 783
spectrophotometer. 1H NMR spectra were measured in CDCl3
and DMSO-d6 with a Bruker Avance DPX 300 spectropho-
tometer with SiMe4 as an internal standard. A Perkin-Elmer
240 C elemental analyzer was used to collect microanalytical
data (C,H,N). Electrochemical measurements were performed
under a dry nitrogen atmosphere at 298 K on a PAR model
370-4 electrochemistry system. The potentials reported herein
are referenced to the saturated Calomel electrode (SCE) and
are uncorrected for junction contribution. The value for the
ferrocenium–ferrocene couple under our experimental condition
is 0.40 V. Tetraethylammonium perchlorate and tetrabutylam-
monium perchlorate were used as the supporting electrolyte. ESI
mass spectra were recorded on a Micromass Quattro II triple
quadrupole mass spectrometer.
Oxidation of [PtCl(L2a)] (2a) by Br2 has resulted in a tri-
bromide complex, [PtBr3(L2a)] (4), while I2 oxidation has, in
contrast, resulted in a mixed-halogen complex, [PtClI2(L2a)] (5).
Since platinum(IV) compounds are generally substitution inert20
it is reasonable that substitution of Cl by Br in 2a occurs before
oxidative addition of Br2. The chloro compound, 2a is indeed
labile and produced [PtBr(L2a)] (2c) and [PtI(L2a)] (2d) when
reacted with aqueous solution of KBr and KI, respectively.
The geometry of the mixed halide product, 5 is interesting.
The compound is trans with respect to two coordinated iodine
atoms. This suggests trans addition of halogen in these oxidation
reactions. This is in line with widely accepted mechanism21 of ox-
idative halogen addition reactions on square planar platinum(II)
complexes. We wish to note here that while the complex 4 is also
obtained upon oxidation of 2c by Br2, the corresponding triiodo
complex, [PtI3(L2a)] could not be isolated from the reaction of
2d and I2 possibly due to steric crowding.
Synthesis of compounds
In order to assess the chemical reversibility of oxidative
halogen addition in our system, the tetravalent complexes, 3–
5 were reacted with a potential bio-reductant,10,22 L-ascorbic
acid. The reactions occurred smoothly and the parent bivalent
platinum complexes, 2 were isolated in almost quantitative yields
(>95%). Of the above noted reactions, the result of reduction
of the mixed-halogen complex, [PtClI2(L2a)] (5) is noteworthy.
In this reaction, Pt(IV)→Pt(II) occurred with the formation of
[PtIII(L2a)] indicating the elimination10a,23 of ClI. This complex
showed a ESI-MS peak at m/z 758, which indeed confirmed its
formation from the above reaction.
Synthesis of [PtCl2(L1a)] 1a. The ligand, 2-(arylazo)pyridine
(L1a) (45 mg, 0.24 mmol) was dissolved in 100 ml acetonitrile.
To this was added an aqueous solution of K2[PtCl4] (100 mg,
0.24 mmol) and the mixture was refluxed on a steam bath for
3 h. During this period the orange colour of the mixture slowly
became reddish-orange. The reaction mixture was then cooled
and filtered. It was left undisturbed for slow evaporation. After
three days dark crystalline products, deposited in the reaction
vessel, were collected by filtration and washed thoroughly with
diethyl ether, and finally air-dried in a vacuum dessicator. Yield:
1 2 5 2
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5

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85%. Anal. Calc. for C11H9N3Cl2Pt: C, 29.4; H, 2.0; N, 9.3.
Oxidative halogen addition reactions
Found: C, 29.6; H, 1.9; N, 9.8%. 1H NMR (DMSO-d6): d: pyridyl
Synthesis of [PtCl3(L2a)] 3. A gentle stream of dichlorine
was bubbled through a deep brown colour solution of complex
[PtCl(L2a)] 2a (105 mg, 0.21 mmol) in dichloromethane (30 ml).
The colour of the solution immediately changed from dark
brown to deep green. A crystalline dark green compound was
obtained by the slow diffusion of hexane into the above green
solution in dichloromethane. Finally the product was crys-
tallized from a dichloromethane–acetonitrile solvent mixture.
Yield: 90%. Anal. Calc. for C17H13N4Cl3Pt: C, 35.5; H, 2.3; N,
protons: 9.53 (d), 8.88 (br), 8.62 (t), 8.24 (br); aromatic protons:
−1
=
=
7.81–7.63 (br). IR (KBr): m 1610 (C N), 1380 (N N) cm . UV-
vis (dimethylformamide), kmax/nm (e//M−1 cm−1): 650sh (2495),
565 (5940), 530 (4820), 475 (4490), 375 (11500), 340sh (9255),
275 (11490). Cyclic voltammetric data (acetonitrile, scan rate
50 mV s−1): reduction, E1/2/V (DEp/mV): −0.22 (70), −1.10
(100).
The compounds [PtCl2(L1b)] 1b and [PtCl2(L1c)] 1c were
synthesized similarly following the above procedure using 2-(m-
tolylazo)pyridine (L1b) and 2-(o-chlorophenylazo)pyridine (L1c),
respectively, in place of 2-(phenylazo)pyridine.
1
9.8. Found: C, 35.3; H, 2.5; N, 9.6%. MS: m/z 575. H NMR
(CDCl3): d pyridyl protons: 8.42 (d), 8.21 (t), 7.93 (d), 7.54 (m);
aromatic protons: 7.44 (m), 7.28 (d), 7.10 (d), 7.04 (t), 6.69 (t),
−1
=
=
6.54 (d). IR (KBr): m 1605 (C N), 1330 (N N) cm . UV-vis
(dichloromethane), kmax/nm (e/M−1 cm−1): 1065sh (1720), 850
(4560), 790 (7170), 720 (6380), 435 (5640), 330sh (10975), 235
(33515). Cyclic voltammetric data (dichloromethane, scan rate
50 mV s−1): reduction, E1/2/V: −0.67, −1.40.
1b. Yield: 80%. Anal. Calc. for C12H11N3Cl2Pt: C, 31.1; H,
2.4; N, 9.1. Found, C, 31.2; H, 2.3; N, 9.1%. 1H NMR (DMSO-
d6): d: pyridyl protons: 9.50 (d), 8.85 (br), 8.59 (t), 8.21 (br);
aromatic protons: 7.75–7.45 (br); 2.4 (aryl CH3). IR (KBr): m
−1
=
=
1600 (C N), 1385 (N N) cm . UV-vis (dimethylformamide),
kmax/nm (e/M−1 cm−1): 630sh (2325), 540 (5985), 470 (4480),
375 (10840), 265 (11265), 220 (7250). Cyclic voltammetric data
(acetonitrile, scan rate 50 mV s−1): reduction, E1/2/V (DEp/mV):
−0.26 (80), −1.17 (80).
Similarly, the compounds [PtBr3(L2a)] 4 and [PtClI2(L2a)] 5
were synthesized by the oxidation of 2a in dichloromethane
(30 ml) with Br2 and I2, respectively. Their yields and analytical
data are as follows.
4. Yield: 95%. Anal. Calc. for C17H13N4Br3Pt: C, 28.9;
H, 1.8; N, 7.9. Found: C, 29.1; H, 1.6; N, 8.3%. MS: m/z
1c. Yield: 85%. Anal. Calc. for C11H8N3Cl3Pt: C, 27.3;
H, 1.7; N, 8.7. Found: C, 27.5; H, 1.5; N, 8.8%. 1H NMR
(DMSO-d6): d: pyridyl protons: 9.55 (d), 9.01 (br), 8.67 (t),
1
708. H NMR (CDCl3): d pyridyl protons: 8.62 (d), 8.15 (m),
7.92 (d); aromatic protons: 7.43 (m), 7.31 (d), 7.00 (t), 6.67
−1
8.38 (br); aromatic protons: 7.78–7.51 (br). IR (KBr): m
=
=
(m), 6.39 (d). IR (KBr): m 1600 (C N), 1325 (N N) cm .
UV-vis (dichloromethane), kmax/nm (e/M−1 cm−1): 1075sh
(1900), 875 (4670), 785 (6890), 715 (5945), 430 (7040), 350sh
(11290), 255 (42215), 230 (37805). Cyclic voltammetric data
(dichloromethane, scan rate 50 mV s−1): reduction, E1/2/V:
−0.71, −1.43.
−1
=
=
1615 (C N), 1395 (N N) cm . UV-vis (dimethylformamide),
kmax/nm (e//M−1 cm−1): 635sh (2490), 570 (5900), 445 (4370),
375 (10975), 275 (11355), 235 (7010). Cyclic voltammetric data
(acetonitrile, scan rate 50 mV s−1): reduction, E1/2/V (DEp/mV):
−0.19 (80), −1.16 (90).
5. Yield: 90%. Anal. Calc. for C17H13N4ClI2Pt: C, 26.9;
H, 1.7; N, 7.4. Found: C, 26.8; H, 1.3; N, 7.2%. MS: m/z
758. H NMR (CDCl3): d pyridyl protons: 8.50 (d), 7.79 (t),
Amination reactions
1
Synthesis of [PtCl(L2a)] 2a. A mixture of [PtCl2(L1a)] 1a
(85 mg 0.19 mmol) and aniline (1 ml) was heated on a steambath
for 1 h. The initial reddish brown colour of the mixture
gradually became dark brown. The mixture was cooled and
washed thoroughly with diethyl ether several times. Further
purification was made by repeated fractional crystallization of
the crude mass from dichloromethane–hexane solvent mixture.
Yield: 80%. Anal. Calc. for C17H13N4ClPt: C, 40.5; H, 2.6; N,
11.1. Found: C, 40.7; H, 2.7; N, 10.7%. MS: m/z 504. 1H NMR
(CDCl3): d: pyridyl protons: 7.92 (d), 7.46 (t), 7.34 (d), 7.32 (t);
7.72 (d), 7.59 (t); aromatic protons: 7.44 (t), 7.32 (m), 7.11
=
(t), 6.77 (t), 6.53 (d), 6.47 (t). IR (KBr): m 1600 (C N), 1325
−1
(N N) cm . UV-vis (dichloromethane), kmax/nm (e/M−1 cm−1):
=
1070sh (1815), 860 (4385), 780 (5435), 715 (4585), 430 (10560),
355sh (13880), 285 (33920), 235 (28650). Cyclic voltammetric
data (dichloromethane, scan rate 50 mV s−1): reduction, E1/2/V:
−0.71, −1.42.
Reductive dehalogenation reaction by L-ascorbic acid
aromatic protons: 7.23 (m), 6.92 (t), 6.84 (d), 6.58 (d), 6.33 (d),
Reduction of all the platinum(IV) complexes was achieved by L-
ascorbic acid following a general procedure. Specific details are
given below for the reduction, [PtCl3(L2a)] 3 → [PtCl(L2a)] 2a.
To a green dichloromethane solution of [PtCl3(L2a)] 3 (110 mg,
0.19 mmol), an aqueous solution of L-ascorbic acid (35 mg,
0.20 mmol) was added and the mixture was stirred at a room
temperature. The colour of the solution changed from green
to brown in about 15 min. The resulting brown solution was
extracted by dichloromethane, washed thoroughly with water
and finally dried over anhydrous Na2SO4. Evaporation of the
dichloromethane solution resulted the crude [PtCl(L2a)] 2a. It
was then recrystallized from a dichloromethane-acetonitrile
solvent mixture. Yield: 95%.
−1
=
=
6.09 (m). IR (KBr): m 1600 (C N), 1310 (N N) cm . UV-vis
(dichloromethane), kmax/nm (e/M−1 cm−1): 1235 (2295), 1065
(4330), 945 (4045), 830sh (2410), 475 (8235), 425 (7650), 360
(18100), 285 (19065), 230 (20195). Cyclic voltammetric data
(dichloromethane, scan rate 50 mV s−1): oxidation, E1/2/V: 1.09;
reduction, E1/2/V (DEp/mV): −0.54 (140), −1.26 (150).
Similarly [PtCl(L2b)] 2b was synthesized following the above
procedure using p-toluidine (1 g) in place of aniline (1 ml).
2b. Yield: 85%. Anal. Calc. for C18H15N4ClPt: C, 41.8; H,
3.1; N, 10.8. Found: C, 41.8; H, 3.3; N, 10.6%. 1H NMR
(CDCl3): d pyridyl protons: 7.89 (d), 7.44 (t), 7.11 (br); aromatic
protons: 6.89 (t), 6.83 (d), 6.55 (d), 6.31 (d), 6.08 (m); 2.3 (aryl
The compounds [PtBr(L2a)] 2c and [PtI(L2a)] 2d were obtained
similarly from the reductions of [PtBr3(L2a)] 4 and [PtClI2(L2a)]
5 with L-ascorbic acid, respectively. The spectral properties of 2a
from the above reduction corresponded exactly to the authentic
sample. Yields and analytical data of 2c and 2d are as follows.
−1
=
=
CH3). IR (KBr): m 1600 (C N), 1315 (N N) cm . UV-vis
(dichloromethane), kmax/nm (e/M−1 cm−1): 1260 (2340), 1075
(4495), 945 (4260), 835sh (2500), 475 (8285), 430 (8390), 360
(18470), 265 (22870), 230 (24230). Cyclic voltammetric data
(dichloromethane, scan rate 50 mV s−1): oxidation, E1/2/V: 1.07;
reduction, E1/2/V (DEp/mV): −0.54 (140), −1.29 (130).
We however, note that the reactions of amines, aniline and
p-toluidine, with [PtCl2(L1c)] 1c were faster, over in 30 min and
produced identical compounds [PtCl(L2a)] 2a and [PtCl(L2b)] 2b,
respectively.
2c. Yield: 95%. Anal. Calc. For C17H13N4BrPt: C, 37.2; H,
2.4; N, 10.2. Found: C, 37.5; H, 2.7; N, 10.4%. MS: m/z 548. 1H
NMR (CDCl3): d pyridyl protons: 8.13 (d), 7.45 (t), 7.36 (m);
aromatic protons: 7.22 (m), 6.89 (t), 6.84 (d), 6.60 (d), 6.34 (t),
−1
=
=
6.20 (d), 6.09 (m). IR (KBr): m 1595 (C N), 1305 (N N) cm
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5
1 2 5 3

View Article Online
Table 2 Crystallographic data
1c
2b
4
5
Empirical formula
Mr
C11H8Cl3N3Pt
483.64
Monoclinic
P21/n
7.3352(3)
19.9225(12)
9.3996(12)
90
96.766(9)
90
1364.0(2)
4
2.355
0.60 × 0.15 × 0.10
3.34–27.98
0.71073
C18H15ClN4Pt
517.88
Monoclinic
P21/n
6.7696(3)
16.4559(8)
14.7526(7)
90
91.1020(10)
90
1643.13(13)
4
2.093
0.39 × 0.09 × 0.08
1.85–28.00
0.71073
C17H13Br3N4Pt
708.13
Monoclinic
P21/c
12.1865(9)
8.5286(6)
18.5423(13)
90
105.1650(10)
90
1860.1(2)
4
2.529
0.13 × 0.14 × 0.17
2.28–28.29
0.71073
C17H13ClI2N4Pt
757.65
Monoclinic
P21/n
9.6500(7)
14.2218(10)
14.8154(10)
90
108.4540(10)
90
1928.7(2)
4
2.609
0.10 × 0.15 × 0.19
2.24–28.29
0.71073
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/Mg m−3
Crystal dimens./mm
h Range for data collect./◦
˚
k/A
Reflns. collected
Unique reflns.
Abs. correction
Dqmax, min/e A
Final R indices [I > 2r(I)]
3866
3289
Psi-scans
0.969, −0.995
R1 = 0.0240
wR2 = 0.0544
10140
3858
SADABS
1.006, −0.761
R1 = 0.0239
wR2 = 0.0355
12039
4579
SADABS
2.22, −2.13
R1 = 0.0364
wR2 = 0.0930
12549
4729
SADABS
3.45, −2.99
R1 = 0.0339
wR2 = 0.0844
−3
˚
UV-vis (dichloromethane), kmax/nm (e/M−1 cm−1): 1270 (2120),
1065 (3210), 940 (3060), 830sh (2340), 480 (6345), 435 (6645),
360 (17260), 280 (20425), 230 (22250). Cyclic voltammetric data
(dichloromethane, scan rate 50 mV s−1): oxidation, E1/2/V: 1.11,
reduction, E1/2/V (DEp/mV): −0.57 (130), −1.29 (140).
2d. Yield: 90%. Anal. Calc. For C17H13N4IPt: C, 34.3; H,
2.2; N, 9.4. Found: C, 34.8; H, 2.1; N, 9.6%. MS: m/z 595.
1H NMR (CDCl3): d: pyridyl protons: 8.47 (d), 7.44 (t), 7.34
using the program SHELXS-9731 and refined by full-matrix
least-squares techniques against F2 using SHELXL-97.32
[PtBr3(L2a)] 4. X-Ray quality crystals (0.13 × 0.14 ×
0.17 mm3) of 4 were obtained by slow diffusion of acetonitrile
into a dichloromethane solution of the compound. The data
were collected at 100(2) K on a Bruker SMART APEX CCD
˚
diffractometer equipped with Mo-Karadiation (k = 0.71073 A).
A total of 12039 reflections were collected out of which 4579
were unique (Rint = 0.0573). The structure was solved by direct
methods using the program SHELXS-9731 and refined by full-
matrix least-squares techniques against F2 using SHELXL-97.32
(m); aromatic protons: 7.17 (m), 6.85 (m), 6.65 (d), 6.35 (t),
−1
=
=
6.07 (m). IR (KBr): m 1595 (C N), 1305 (N N) cm . UV-vis
(dichloromethane), kmax/nm (e/M−1 cm−1): 1235 (2075), 1070
(2730), 865 (3340), 780 (3230), 710 (2455), 430 (8145), 360
(18315), 285 (21865), 235 (22495). Cyclic voltammetric data
(dichloromethane, scan rate 50 mV s−1): oxidation, E1/2/V: 1.05;
reduction, E1/2/V (DEp/mV): −0.56 (120), −1.26 (110).
[PtClI2(L2a)] 5. X-Ray quality crystals (0.10 × 0.15 × 0.19)
of 5 were obtained by slow diffusion of acetonitrile into a
dichloromethane solution of the compound. The data were
collected as noted before for 4. A total of 12549 reflections
were collected out of which 4729 were unique (Rint = 0.0354).
The structure was solved by direct methods using the program
SHELXS-9731 and refined by full-matrix least-squares tech-
niques against F2 using SHELXL-97.32
Crystallography
Crystallographic data for the compounds 1c, 2b, 4 and 5 are
collected in Table 2.
[PtCl2(L1c)] 1c. X-Ray quality crystals (0.60 × 0.15 ×
0.10 mm) of 1 were obtained by slow evaporation of a
concentrated acetonitrile solution of the compound. The data
were collected at 293 K on an Enraf-Nonius CAD4 auto-
matic diffractometer28 equipped with Mo-Ka radiation (k =
CCDC reference numbers 258038–258041.
See http://www.rsc.org/suppdata/dt/b4/b418470j/ for crys-
tallographic data in CIF or other electronic format.
Acknowledgements
˚
0.71073 A), and data were corrected for Lorentz-polarization
effects.29 A total of 3866 reflections were collected, of which
3289 were unique (Rint = 0.0215). A semi-empirical absorption
correction (psi-scans) was made.30 The structure was solved by
Patterson and Fourier methods31 and refined by full-matrix least
squares on F2 using SHELXL-97.32 The Cl(3) atom is disordered
over three positions; the occupancy factor for each was refined,
resulting in a value of 0.57(7) for Cl(3A), 0.10(3) for Cl(3B) and
0.33(8) for Cl(3C).
Financial support received from the Department of Science and
Technology, New Delhi is gratefully acknowledged. M. P. and
S. D. are thankful to the Council of Scientific and Industrial
Research, New Delhi for their fellowship. We are thankful to
Chemistry Department, IIT-Kanpur for X-ray data collections
of two samples.
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data were collected at 173(2) K on a Bruker SMART CCD
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D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5

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D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 9 – 1 2 5 5
1 2 5 5