Geometrical configuration of monomethyl-platinum(II) complexes driven by the size of entering nitrogen ligands

Article abstract of DOI:10.1016/S0020-1693(01)00775-7

The reaction of the monoalkyl complex trans-[Pt(DMSO)₂Cl(CH₃)] with a large variety of heterocyclic nitrogen bases L, in chloroform solution, leads to the formation of uncharged complexes of the type [Pt(DMSO)(L)Cl(CH₃)], containing four different groups coordinated to the metal center. Only two out of the three different possible isomers were detected in solution. These two trans(C,N) and cis(C,N) species can be unambiguously identified through ¹H NMR spectroscopy. For the trans(C,N) isomers, average values of ²J_PtH=75±4 Hz and ³J_PtH=36±4 Hz have been observed for the coordinated methyl and DMSO ligands, respectively. In the case of the cis(C,N) isomers, these values increase to ²J_PtH=83±2 Hz, and decrease to ³J_PtH=26±3 Hz due to the mutual exchange of ligands in trans position to CH₃ and DMSO. In the case of bulky asymmetric ligands, such as quinoline, 2-quinolinecarboxaldehyde, 2-methylquinoline, 5-aminoquinoline, 2-phenylpyridine and 2-chloropyridine, slow rotation of the hindered group around the Pt-N bond makes the coordinated DMSO ligand prochiral. NMR experiments have shown that the first reaction product is the trans(C,N) isomer as a consequence of the very fast removal of one DMSO ligand by the nitrogen bases from the starting complex trans-[Pt(DMSO)₂Cl(CH₃)]. This trans kinetic product undergoes a geometrical conversion into the more stable cis(C,N) isomer through the intermediacy of fast exchanging aqua-species. The rate of isomerization and the relative stability of the two isomers depends essentially on the rate of aquation and on the steric congestion imposed by the new L ligand on the metal.

Full text of DOI:10.1016/S0020-1693(01)00775-7

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Inorganica Chimica Acta 330 (2002) 189–196
Geometrical configuration of monomethyl–platinum(II) complexes
driven by the size of entering nitrogen ligands
Luigi Monsu` Scolaro a,b,c,*, Antonino Mazzaglia a, Andrea Romeo a,
Maria Rosaria Plutino b, Mariangela Castriciano a, Raffaello Romeo a,b,
*
a Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Uni6ersita´ di Messina, Salita Sperone 31, 98166 Villagio S. Agata,
Messina, Italy
b Istituto di Chimica dei Prodotti Naturali (ICTPN-CNR), Sezione di Messina, Messina, Italy
c INFM, Sezione di Messina, Messina, Italy
Received 12 October 2001; accepted 13 November 2001
Professor Luigi Venanzi has been a pioneer in the field of inorganic and organometallic chemistry. This paper is a tribute to the scientist and
a friend
Abstract
The reaction of the monoalkyl complex trans-[Pt(DMSO)2Cl(CH3)] with a large variety of heterocyclic nitrogen bases L, in
chloroform solution, leads to the formation of uncharged complexes of the type [Pt(DMSO)(L)Cl(CH3)], containing four different
groups coordinated to the metal center. Only two out of the three different possible isomers were detected in solution. These two
1
trans(C,N) and cis(C,N) species can be unambiguously identified through H NMR spectroscopy. For the trans(C,N) isomers,
2
3
average values of JPtH=7594 Hz and JPtH=3694 Hz have been observed for the coordinated methyl and DMSO ligands,
2
3
respectively. In the case of the cis(C,N) isomers, these values increase to JPtH=8392 Hz, and decrease to JPtH=2693 Hz due
to the mutual exchange of ligands in trans position to CH3 and DMSO. In the case of bulky asymmetric ligands, such as
quinoline, 2-quinolinecarboxaldehyde, 2-methylquinoline, 5-aminoquinoline, 2-phenylpyridine and 2-chloropyridine, slow rotation
of the hindered group around the PtꢀN bond makes the coordinated DMSO ligand prochiral. NMR experiments have shown that
the first reaction product is the trans(C,N) isomer as a consequence of the very fast removal of one DMSO ligand by the nitrogen
bases from the starting complex trans-[Pt(DMSO)2Cl(CH3)]. This trans kinetic product undergoes a geometrical conversion into
the more stable cis(C,N) isomer through the intermediacy of fast exchanging aqua-species. The rate of isomerization and the
relative stability of the two isomers depends essentially on the rate of aquation and on the steric congestion imposed by the new
L ligand on the metal. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Platinum(II) complexes; Nitrogen ligands; Geometrical isomerization; Dimethylsulfoxide
a useful guide to the synthesis of new compounds.
Good examples of this correlation can be found in the
1. Introduction
chemistry of coordination and organometallic platinu-
The use of transition metal complexes containing
m(II) complexes. A number of kinetic studies have been
sulfoxide ligands as precursors in synthetic procedures
devoted to clarify the properties of dimethylsulfoxide
has been known for many years [1–4]. The reactions
(DMSO, hereafter) when acting as coordinated ligand
exploit the fairly high lability of such weakly coordinat-
or reagent in square-planar platinum(II) complexes.
ing molecules, even in solid state thermal synthesis
DMSO shows quite a large trans effect [9–11], ex-
[5–8]. For solution processes the detailed knowledge of
the mechanistic pathways of the substitution represents
ploited in inorganic syntheses [12], and a fairly weak
trans influence [13]. In substitution reactions, it can be
regarded as a poor nucleophilic agent [14–16], closely
resembling ethene. Interestingly, compounds containing
a single molecule of DMSO are quite inert to the
* Corresponding authors. Tel.: +39-090-391 984; fax: +39-090-
393 756.
E-mail address: raf.romeo@chem.unime.it (R. Romeo).
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00775-7

190
L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
substitution [17], while the presence of an additional
molecule increases the substitutional lability [18–20].
Thus, as leaving groups, sulfoxides spanning a mutual
cis position display a relevant labilization effect. A
much easier substitution of both the sulfoxide ligands
by bidentate agents takes place in organometallic com-
plexes of the type cis-[Pt(DMSO)2(R)2] (R=CH3, aryl
group) [21]. These latter reactions can be regarded as
the first example of a dissociative pathway for the
nucleophilic substitution process in square planar plat-
inum(II) complexes [22–24].
together with small amounts of starting material. So far
the factors controlling the geometry of these com-
pounds are unknown. Therefore, in this paper we have
extended the study of this reaction to a large variety of
monodentate nitrogen containing ligands, with the aim
of shedding some light on the factors determining the
geometry of the platinum products.
2. Experimental
Most of the reports in literature refer to the structure
and reactivity of complexes containing two molecules
of sulfoxides spanning mutual cis position. Only few
studies have been focused on complexes containing two
sulfoxide ligands in trans configuration [25–28]. Some
years ago, we reported the first investigation on an
organometallic compound of this type. The complex
trans-bis(dimethylsulfoxide)chloromethyl-platinum(II)
was first synthesized by Eaborn et al. [21] and its trans
geometrical configuration has been proven in the solid
state by X-ray diffraction [29]. Detailed multinuclear
NMR experiments showed that a chloroform solution
of trans-[Pt(DMSO)2Cl(CH3)], depending on the water
concentration in the solvent, contains four different
species, which have been unequivocally assigned as the
equilibrium mixture of the trans complex and its cis
isomer, together with a low concentration of two re-
lated cis and trans-[Pt(CH3)Cl(DMSO)(H2O)] aqua-
species [29]. These latter compounds contribute to the
high reactivity of this system toward a variety of nucle-
ophilic agents. Thus, chloroform solution of the com-
plex trans-[Pt(DMSO)2Cl(CH3)] were widely used as
synthetic precursor of organometallic species containing
the moieties PtꢀCH3 or Pt(DMSO)(CH3) [30]. The reac-
tion with diamines or di-imines, NꢀN, leads to cationic
complexes of the type [Pt(NꢀN)(DMSO)(CH3)]Cl or
neutral complexes of the type [PtCl(NꢀN)(CH3)], con-
taining five- or six-membered chelate rings [31]. When
the NꢀN ligand is a long bridging diamine, dinuclear
compounds of the type [{Pt(DMSO)Cl(CH3)}2(m-NꢀN)]
having potential antitumor properties can be prepared
[32]. Reaction with 2,2%:6%,2%%-terpyridine leads to the
complex [Pt(terpy)(CH3)]Cl, which behaves as an inter-
calator towards double-stranded DNA [33–36].
2.1. Materials
K2PtCl4 (Strem Chemical Co.) was purified from
metallic Pt and K2PtCl6 by dissolving it in water and
filtering. Tetramethyltin was received from Aldrich and
1
its purity was checked through H NMR spectroscopy.
trans-[Pt(DMSO)2Cl(CH3)] was prepared according to
a published method, and was crystallized several times
from dichloromethane/diethyl ether mixtures [30].
Dimethylsulfoxide was purified by liquid chromatogra-
phy on alumina under argon and stored over molecular
sieves. The solvents used were purified and dried by
standard techniques. All the other reagents were of the
highest commercial grade available and were used as
received or were purified by distillation or recrystalliza-
tion when necessary.
The complexes [Pt(L)(DMSO)Cl(CH3)] (L=pyri-
dine, py, (1); 2-methylpyridine, 2-Mepy, (2); 4-N,N-
dimethylaminopyridine, 4-(NMe2)py, (7); 2-phenyl-
pyridine, 2-Phpy (9a–b); 2-chloropyridine, 2-Clpy
(10a–b); 2,6-dimethylpyridine, 2,6-Me2py (13); 2-
methylquinoline, 2-Mequin (16); 5-aminoquinoline, 5-
AQ-N1 (17); acridine orange, AO (19); acridine, Acr
(20a–b)) were prepared according to a previously de-
scribed method [29]. The complexes [Pt(L)(DMSO)-
Cl(CH3)] (L=3-methylpyridine, 3-Mepy, (3); 4-
methylpyridine, 4-Mepy, (4); 4-chloropyridine, 4-Clpy,
(5); 4-cyanopyridine, 4-CNpy, (6); 2-acetylpyridine, 2-
Acpy, (8); 2,4-dimethylpyridine, 2,4-Me2Py, (11a–b);
3,5-dimethylpyridine, 3,5-Me2Py, (12); quinoline, quin
(14a–b);
2-quinolinecarboxaldehyde,
2-CHOquin,
(15a–b); 9-aminoacridine, 9-AA, (18a–b)) were pre-
pared and characterized in situ by reacting the complex
trans-[Pt(DMSO)2Cl(CH3)] (4 mg, 0.01 mmol) with an
equimolar amount of the proper ligand in 0.5 ml of
CDCl3.
The reaction of trans-[Pt(DMSO)2Cl(CH3)] in chlo-
roform with monodentate ligands L, such as amines or
pyridines, yields almost quantitatively uncharged com-
plexes of the type [Pt(DMSO)(L)Cl(CH3)], in which
four different groups are coordinated to the platinu-
m(II) center [29]. Depending on the nature of the
entering ligand, different patterns of behavior have
been recognized: (i) fast build-up of a stable isomer; (ii)
fast formation of an isomer that undergoes a very slow
geometrical isomerization; and (iii) the formation of an
equilibrium mixture containing cis and trans-isomers
2.1.1. cis(C,N)-(3-methylpyridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(3-Mepy)-
(DMSO)Cl(CH3)] (3)
1H NMR (CDCl3) l: 8.56 (m, 3JPtH=44 Hz, H2,
3
1H), 8.52 (m, JPtH=42 Hz, H6, 1H), 7.66 (m, H4, 1H),
4
3
7.33 (m, JPtH=20 Hz, H5, 1H), 3.52 (s, JPtH=26 Hz,

L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
191
2
DMSO, 6H), 2.41 (s, 3-CH3, 3H), 0.89 (s, 2JPtH=83 Hz,
Pt–CH3, 3H).
2.35 (s, 3-CH3, 6H), 0.87 (s, JPtH=82 Hz, Pt–CH3,
3H).
2.1.2. cis(C,N)-(4-methylpyridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(4-Mepy)-
(DMSO)Cl(CH3)] (4)
2.1.9. cis(C,N)-(quinoline)(dimethylsulfoxide)chloro-
methyl–platinum(II), cis(C,N)-[Pt(quin)(DMSO)-
Cl(CH3)] (14a)
1H NMR (CDCl3) l: 8.55 (m, 3JPtH=42 Hz, H2, 2H),
3
1H NMR (CDCl3) l: 9.22 (m, 1H), 9.11 (m, JPtH
=
3
7.25 (m, H3, 2H), 3.51 (s, JPtH=26 Hz, DMSO, 6H),
46 Hz, H2, 1H), 8.39 (m, 1H), 7.95 (m, 1H), 7.92 (m,
2
3
2.44 (s, 4-CH3, 3H), 0.89 (s, JPtH=83 Hz, Pt–CH3,
1H), 7.70 (m, 1H), 7.54 (m, 1H), 3.63 (s, JPtH=28 Hz,
3
3H).
DMSO, 3H), 3.58 (s, JPtH=28 Hz, DMSO, 3H), 0.82
2
(s, JPtH=84 Hz, Pt–CH3, 3H).
2.1.3. cis(C,N)-(4-chloropyridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(4-Clpy)-
(DMSO)Cl(CH3)], (5)
2.1.10. trans(C,N)-(quinoline)(dimethylsulfoxide)chloro-
methyl–platinum(II), trans(C,N)-[Pt(quin)-
(DMSO)Cl(CH3)], (14b)
1H NMR (CDCl3) l: 8.66 (m, 3JPtH=42 Hz, H2, 2H),
3
7.46 (m, H3, 2H), 3.52 (s, JPtH=26 Hz, DMSO, 6H),
1H NMR (CDCl3) l: 9.38 (m, 1H), 9.21 (m, 1H), 8.35
(m, 1H), 7.94 (m, 1H), 7.64 (m, 1H) (a complete
assignment of this region has been prevented by severe
overlap with the signals of the free ligand and of the
predominant cis isomer), 3.24 (s, 3JPtH=35 Hz, DMSO,
3H), 3.23 (s, 3JPtH=35 Hz, DMSO, 3H), 0.89 (s,
2JPtH=76 Hz, Pt–CH3, 3H).
2
0.88 (s, JPtH=82 Hz, Pt–CH3, 3H).
2.1.4. cis(C,N)-(4-cyanopyridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(4-CNpy)-
(DMSO)Cl(CH3)] (6)
1H NMR (CDCl3) l: 8.99 (m, 3JPtH=41 Hz, H2, 2H),
3
7.69 (m, H3, 2H), 3.53 (s, JPtH=28 Hz, DMSO, 6H),
2
0.89 (s, JPtH=81 Hz, Pt–CH3, 3H).
2.1.11. cis(C,N)-(2-quinolinecarboxyaldehyde)-
(dimethylsulfoxide)chloromethyl–platinum(II),
2.1.5. cis(C,N)-(2-acetylpyridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(2-Acpy)-
(DMSO)Cl(CH3)] (8)
cis(C,N)-[Pt(2-CHOquin)(DMSO)Cl(CH3)] (15a)
1H NMR (CDCl3) l: 11.90 (s, CHO, 1H), 9.59 (m,
4JPtH=18 Hz, H8, 1H), 8.59 (m, 1H), 8.18 (m, 1H), 8.02
1H NMR (CDCl3) l: 8.94 (m, 3JPtH=44 Hz, H6, 1H),
3
(m, 1H), 7.85 (m, 1H), 7.82 (m, 1H), 3.66 (s, JPtH=28
4
8.00 (m, H4, 1H), 7.75 (m, JPtH=20 Hz, H3, 1H), 7.56
3
Hz, DMSO, 3H), 3.61 (s, JPtH=28 Hz, DMSO, 3H),
(m, H5, 1H), 3.52 (s, 3JPtH=27 Hz, DMSO, 6H), 3.18 (s,
2
0.70 (s, JPtH=81 Hz, Pt–CH3, 3H).
2
2-CH3CO, 3H), 0.78 (s, JPtH=82 Hz, Pt–CH3, 3H).
2.1.12. trans(C,N)-(2-quinolinecarboxyaldehyde)-
(dimethylsulfoxide)chloromethyl platinum(II),
2.1.6. cis(C,N)-(2,4-dimethylpyridine)(dimethyl-
sulfoxide)chloromethyl–platinum(II),
trans(C,N)-[Pt(2-CHOquin)(DMSO)Cl(CH3)] (15b)
1H NMR (CDCl3) l: 12.14 (s, CHO, 1H), 9.87 (m,
H8, 1H), 8.54 (m, 1H), 8.15 (m, 1H), 8.00 (m, 1H), 7.78
cis(C,N)-[Pt(2,4-Me2py)(DMSO)Cl(CH3)] (11a)
1H NMR (CDCl3) l: 8.46 (m, 3JPtH=43 Hz, H6, 1H),
3
7.17 (m, JPtH=12 Hz, H3, 1H), 7.07 (m, H5, 1H), 3.50
3
(m, 1H), 7.74 (m, 1H), 3.21 (s, JPtH=37 Hz, DMSO,
3
3
(s, JPtH=26 Hz, DMSO, 3H), 3.49 (s, JPtH=26 Hz,
2
6H), 1.00 (s, JPtH=78 Hz, Pt–CH3, 3H).
4
DMSO, 3H), 2.93 (s, JPtH=8 Hz, 2-CH3, 3H), 2.39 (s,
4-CH3, 3H), 0.73 (s, JPtH=83 Hz, Pt–CH3, 3H).
2
2.1.13. cis(C,N)-(9-aminoacridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), cis(C,N)-[Pt(9-AA)-
(DMSO)Cl(CH3)] (18a)
2.1.7. trans(C,N)-(2,4-dimethylpyridine)-
(dimethylsulfoxide)chloromethyl–platinum(II),
trans(C,N)-[Pt(2,4-Me2py)(DMSO)Cl(CH3)] (11b)
1H NMR (CDCl3) l: 8.57 (m, H6, 1H), 7.12 (m, H3,
1H NMR (CDCl3) l: 10.42 (m, 2H), 10.08 (m, NH2,
2H), 7.55 (m, 2H), 7.04 (m, 2H), 6.91 (m, 2H), 3.62 (s,
3JPtH=24 Hz, DMSO, 6H), 0.88 (s, 2JPtH=85 Hz,
Pt–CH3, 3H).
3
1H), 7.06 (m, H5, 1H), 3.26 (s, JPtH=37 Hz, DMSO,
3H), 3.24 (s, 3JPtH=37 Hz, DMSO, 3H), 2.92 (s, 2-CH3,
2
3H), 2.33 (s, 4-CH3, 3H), 0.70 (s, JPtH=74 Hz, Pt–
CH3, 3H).
2.1.14. trans(C,N)-(9-aminoacridine)(dimethylsulfoxide)-
chloromethyl–platinum(II), trans(C,N)-[Pt(9-AA)-
(DMSO)Cl(CH3)] (18b)
2.1.8. cis(C,N)-(3,5-dimethylpyridine)(dimethyl-
sulfoxide)chloromethyl–platinum(II),
1H NMR (CDCl3) l: 10.39 (m, 2H), 9.59 (m, NH2,
2H), 7.80 (m, 2H), (a complete assignment of this
region has been prevented by severe overlap with the
signals of the free ligand and of the predominant cis
cis(C,N)-[Pt(3,5-Me2py)(DMSO)Cl(CH3)] (12)
1H NMR (CDCl3) l: 8.34 (m, 3JPtH=43 Hz, H2, 2H),
3
7.46 (m, H4, 1H), 3.51 (s, JPtH=27 Hz, DMSO, 6H),

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L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
Table 1
Selected 1H NMR data for complexes of the type cis(C,N)- and trans(C,N)-[Pt(DMSO)(L)Cl(CH3)] a
N°
Compound
L
l(Pt–CH3) (2JPtH
)
l(DMSO)(3JPtH
)
l(Harom) b(3JPtH
)
1 c
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
cis(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
trans(C,N)
cis(C,N)
py
0.90 (83)
0.74 (84)
0.89 (83)
0.89 (83)
0.88 (82)
0.89 (81)
0.88 (84)
0.78 (82)
0.29 (84)
0.54 (75)
0.78 (83)
0.77 (78)
0.73 (83)
0.70 (74)
0.87 (82)
0.71 (75)
0.82 (84)
0.89 (76)
0.70 (81)
1.00 (78)
0.88 (76)
0.79 (84)
0.88 (85)
0.94 (71)
1.13 (76)
0.70 (83)
1.07 (72)
3.52 (26)
3.51 (27)
3.52 (26)
3.51 (26)
3.52 (26)
3.53 (28)
3.49 (25)
3.52 (27)
3.47 (27); 3.23 (27)
3.00 (40); 2.19 (36)
3.54 (28); 3.53 (29)
3.33 (34); 3.24 (39)
3.50 (26); 3.49 (26)
3.26 (37); 3.24 (37)
3.51 (27)
8.73(42)
8.64 (42)
8.56 (44); 8.52 (42)
8.55 (42)
8.66 (42)
8.99 (41)
8.18 (41)
8.94 (44)
8.94 (42)
2 c
2-Mepy
3-Mepy
4-Mepy
4-Clpy
4-CNpy
4-(NMe2)py
2-Acpy
2-Phpy
2-Phpy
2-Clpy
3
4
5
6
7 c
8
9a c
9b c
10a c
10b c
11a
11b
12
9.01 (B20)
8.68 (43)
8.75 (15)
2-Clpy
2,4-Me2py
2,4-Me2py
3,5-Me2py
2,6-Me2py
quin
8.46 (43)
8.34 (43)
9.11 (46)
13 c
14a
14b
15a
15b
16 c
17 c
18a
18b
19 c
20a c
20b c
3.24 (37)
3.63 (28); 3.58 (28)
3.24 (35); 3.23 (35)
3.66 (28); 3.61 (28)
3.21 (37)
3.16 (35); 3.22 (35)
3.60 (26); 3.56 (26)
3.62 (24)
3.23 (34)
2.90 (31)
3.68 (27)
3.10 (34)
quin
2-CHOquin
2-CHOquin
2-Mequin
5-AQ-N1
9-AA
9-AA
AO
Acr
Acr
9.02 (46)
cis(C,N)
trans(C,N)
trans(C,N)
cis(C,N)
trans(C,N)
a In CDCl3 at 298 K. Chemical shifts are reported in ppm downfield from TMS, coupling constants are in Hz.
b ortho proton on pyridine ring.
c Data taken from Ref. [29].
3
isomer), 3.23 (s, JPtH=34 Hz, DMSO, 6H), 0.94 (s,
2JPtH=71 Hz, Pt–CH3, 3H).
different groups coordinated to the metal center. The
distribution of the coordinated ligands around the
metal depends on the nature of the reagent, possibly
from its size. The addition of a ligand excess does not
produce any further effect. In this study the series of
reagents has been extended to include other pyridines
and quinolines with different steric requirements. Some
of these complexes were prepared as solids while other
compounds were obtained by direct reaction in situ and
2.2. Instruments
UV–Vis absorption spectra were obtained by using a
Hewlett Packard mod. HP 8453A diode spectrophoto-
meter. 1H NMR spectra were obtained on a Bruker
AMX-R 300 spectrometer equipped with a broadband
probe operating at 300.13 MHz. Sample solutions in
CDCl3 were referenced to the internal residual solvent
peak. Chemical shifts (l) are reported in ppm
downfield from TMS, and coupling constants in Hz.
1
characterized through H NMR spectroscopy. Table 1
collects the most relevant NMR features of these com-
plexes.
1
3.2. H NMR characterization
3. Results and discussion
The presence of four different groups in the coordi-
nation sphere of the metal implies the possible occur-
rence of three well distinct isomers (Scheme 1).
3.1. Reaction with pyridine ligands
We knew from a previous study [29] that the reaction
of a heterocyclic nitrogen ligand L with the stoichio-
metric amount of the complex trans-[Pt(DMSO)2-
Cl(CH3)], in chloroform solution, leads to quite a fast
removal of one sulfoxide ligand affording a neutral
complex [Pt(DMSO)(L)Cl(CH3)], which contains four
Scheme 1.

L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
193
Indeed, only two compounds, A and B, have been
detected in solution and the presence of isomer C can
be safely ruled out. The criterion followed is based
essentially on the inverse correlation observed [37,38]
between the trans influence of a ligand and the value of
the coupling constants with 195Pt (3JPtH) of the protons
of the group trans to it. According to the well known
high trans-influence of the methyl group, the values of
the coupling constants for the protons of dimethylsul-
foxide or pyridine coordinated in trans position to the
methyl are expected to be much lower than those found
when the same ligands are in trans position to a chlo-
ride or an amine [37]. The NMR data in Table 1 show
ampy)(DMSO)(CH3)]+ (2-ampy=2-aminomethylpyri-
dine; pyridyl trans to DMSO isomer, 3JPtH=42 Hz,
and pyridyl trans to methyl group, JPtH=20 Hz) [31].
3
An inspection of Table 1 reveals that all the
trans(C,N) (A) isomers exhibit an average value for the
2JPtH relative to the methyl group of 7594 Hz, and
3JPtH=3694 Hz for the DMSO ligand. This value is
in line with the coupling constant measured for a
DMSO ligand in trans position with respect to a chloro
ligand, as in the case of cis-[Pt(DMSO)2Cl(CH3)]
2
(3JPtH=36 Hz). For the cis(C,N) (B) isomers the JPtH
value for the methyl group increases to 8392 Hz while
3JPtH for the DMSO ligand decreases to 2693 Hz.
These variations reflect the mutual exchange of trans
ligands (Cl for CH3 and L for DMSO). The occurrence
of a CH3ꢀPtꢀCl bond axis is confirmed by the compari-
son with the values of the starting complex trans-[Pt(-
DMSO)2Cl(CH3)] (2JPtH=80 Hz, for the CH3).
3
that the average value for the JPtH coupling constants
of the coordinated DMSO is in the range 24–40 Hz.
This value is larger than those usually observed for a
DMSO ligand in trans position to a methyl group. As
an example, in the case of the complex cis-[Pt(-
3
DMSO)2(CH3)2], the value of JPtH coupling constant is
As previously reported for the derivatives containing
bulky ligands, such as 2-Clpy, 2-Phpy, 2-Mequin, and
of 18 Hz [21], while in cis-[Pt(DMSO)2Cl(CH3)] this
values is much lower (3JPtH=8 Hz) [29]. On this basis,
we can safely rule out the formation of isomer C.
The correct assignment of the two remaining isomers,
the A isomer (nitrogen ligand trans to the carbon donor
atom, trans(C,N)) or the B isomer (nitrogen ligand cis
to the carbon donor atom, cis(C,N)) can be attained
1
5-AQ-N1, the H NMR spectra of the complexes con-
taining 2,4-Me2py, quin and 2-CHOquin exhibit dou-
bled peaks for the coordinated DMSO, each integrating
for a single methyl group. This observation suggests
that, as expected, the two methyl groups of the DMSO
molecule become diastereotopic, due to the asymmetry
and to the slow rotation on the NMR time scale of the
ligand L around the PtꢀN bond. Therefore the torsion
angle between the square coordination plane and the
plane of the aromatic ligand has to be significantly
large (the values of torsion angles for platinum–pyri-
dine complexes are all within the range 45–60°) [40].
3
through the value observed for the JPtH coupling con-
stant of the ortho hydrogen of the pyridine or quinoline
ligand. We successfully exploited this method in a
number of cases to distinguish between two different
pyridine
moieties.
In
the
complex
[Pt(-
DMSO)(py)2(CH3)]+ [31], the H2 protons on the pyri-
dine rings evidence largely different coupling constants
(3JPtHB20 Hz for the py in trans to the methyl group
and 3JPtH=43 Hz for that in trans position to the
3.3. Isomerization
1
DMSO). A relevant feature in the H NMR spectra of
In the case of the most sterically hindered ligands,
such as 2-Mequin, AO and 2,6-Me2py, the trans(C,N)
isomer is quite stable and these compounds can be
[Pt(bipy)(R)Cl] (R=CH3, C6H5) compounds is the sig-
nal of the H(6%) proton of the pyridine ring in trans
position to the carbon atom (at about l 9.6 ppm,
3JPtH=16.2 and 12.9 Hz for R=CH3 and C6H5, re-
spectively) which can be distinguished from that of the
H(6) proton trans to chloride (3JPtH=59.6 and 59.1 Hz,
respectively) on the basis of the magnitude observed for
1
isolated as solids in high yield. Fig. 1 shows the H
NMR spectrum in CDCl3 of the isolated trans(C,N)
isomer containing the bulky acridine orange (3,6-
bis(N,N-dimethylamino)acridine). A methyl group and/
or a condensed aromatic ring in both of the ortho
positions with respect to the donor nitrogen atom
seems to be a prerequisite for the isolation of complexes
in a trans(C,N) configuration. In the case of a less
hindered ligand, such as pyridine or even 2-methyl-
pyridine, the only product detectable in solution soon
after mixing the reagents is the stable cis(C,N) complex.
The same result applies with pyridines containing
strong electron donating or withdrawing groups (4-
(NMe2)py, 4-Clpy, 4-CNpy) in meta or para position
on the pyridine ring. Thus, the geometrical configura-
tion of the reaction product between trans-[Pt-
(DMSO)2Cl(CH3)] and nitrogen bases seems to depend
essentially upon the steric requirements of the entering
3
the JPtH coupling constant [39]. Likewise, in a wide
series of cyclometalated [Pt(NꢀNꢀC)Cl] complexes,
containing a number of terdendate NꢀNꢀC anionic
ligands derived from deprotonated alkyl, phenyl or
benzyl 6-substituted-2,2%-bipyridines, the average value
1
of the H NMR signal of the H(6%) proton is found at
l=9.5290.2 ppm with a coupling to the 195Pt nucleus
3JPtH=14.491 Hz [39]. Large differences in the cou-
pling constant values have also been reported for the
ortho protons in the case of the complexes [Pt-
(phen)(DMSO)(CH3)]+ (phen=1,10-phenanthroline)
3
(trans to the methyl group, JPtH=16.5 Hz, and trans
3
to DMSO, JPtH=44 Hz) and the two isomers [Pt(2-

194
L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
Fig. 2. Selected region of the 1H NMR spectra showing the isomer-
ization process of the complex [Pt(2-Phpy)(DMSO)Cl(CH3)] in
CDCl3 at 298 K, (a) after 10 min, (b) after 12 h.
Fig. 1. 1H NMR spectrum of the complex [Pt(AO)(DMSO)Cl(CH3)]
in CDCl3 at 298 K.
stable cis(C,N) compound (Scheme 2), through the
intermediancy of fast exchanging aqua-species. The
mechanism is reminiscent of that which dictates the
trans to cis isomerization of [Pt(DMSO)2Cl(CH3)] [29].
The aqua-species can be generated by the removal of
the nitrogen ligand through the nucleophilic attack of
water. The diplacement of the DMSO in trans to Cl− is
less likely, due to the weak trans-activating effect of the
chloride ion [29]. The presence of nitrogen ligands
projecting substituents groups above and below the
coordination plane (such as 2,6-Me2py, 2-Phpy, etc.)
markedly decreases the rate of the water attack as well
as that of isomerization. In order to prove the possible
involvement of DMSO or water as catalysts of the
isomerization process, some kinetic measurements have
been performed. Fig. 3 shows typical UV–Vis spectral
changes for the geometrical conversion of the complex
trans(C,N)-[Pt(AO)(DMSO)Cl(CH3)]. This complex
can be easily prepared and isolated as a solid com-
pound and, due to the strong absorption in the UV–
Vis range (for the presence of the highly absorbing AO
chromophore), it is suitable for monitoring the isomer-
ization process in chloroform. The progress of the
isomerization leads to a decrease of the 457 nm band
relative to the starting trans isomer, and to a parallel
and matching increase of a band at 495 nm due to the
final cis isomer. The time trace at 495 nm evidences a
ligand. An analogous pattern of behavior has already
been reported for the reaction of cis-[Pt(DMSO)2Cl2]
with nitrogen-donor ligands (L) leading to cis- or trans-
[Pt(DMSO)(L)Cl2]. Kinetic evidences have suggested
that the isomerization process, leading to the most
stable cis isomer, proceeds through a series of consecu-
tive substitution steps which occur with complete reten-
tion of configuration [28].
Using bulky ligands, we have performed some kinetic
experiments showing that the trans(C,N) isomer is the
first formed kinetic product that subsequently intercon-
verts into the stable cis(C,N) analogue with a rate
dependent on the size of the ligand. Fig. 2 reports a
1
typical time evolution of the H NMR spectra relative
to the complex with 2-phenylpyridine. Soon after mix-
ing the reagents (left side of Fig. 2), the trans(C,N)
isomer is formed with a resonance at l=0.54 (2JPtH
=
75 Hz), attributable to the methyl group directly coor-
dinated to the metal center and two diastereotopic
methyl groups for the DMSO at l 3.00 and 2.19
(3JPtH=40 and 36 Hz), respectively. As time goes by,
the signals relative to the cis(C,N) species appear at l
0.29 Hz (2JPtH=84 Hz) and l 3.47 and 3.23 (3JPtH=27
Hz), respectively (right side of Fig. 2). After 12 h, the
cis isomer is the predominant species in solution.
Considering the complexity of the starting system, i.e.
the chloroform solution of trans-[Pt(DMSO)2Cl(CH3)]
complex, made up by cis and trans sulfoxide and aqua
complexes in mutual fast exchange, it is difficult to
establish a scale of reactivity for these compounds.
However, from previous magnetization transfer experi-
ments we know that both the sulfoxide and the aqua
ligands trans to the methyl group in the complexes
cis-[Pt(DMSO)2Cl(CH3)]
and
cis(C,S)-[Pt(H2O)-
(DMSO)Cl(CH3)], respectively, are very labile and
would be quickly removed by a nitrogen ligand to yield
the trans(C,N) isomer. This kinetically favored com-
pound isomerizes in a subsequent step to the more
Scheme 2.

L. Monsu` Scolaro et al. / Inorganica Chimica Acta 330 (2002) 189–196
195
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1
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The Authors wish to thank the Ministero dell’Uni-
versita`
e della Ricerca Scientifica e Tecnologica
(MURST), Programmi di Ricerca Scientifica di Rile-
vante Interesse Nazionale, Cofinanziamento 2000–2001
and CNR for financial support.

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