Novel platinum(II) compounds with N-benzylidenebenzylamines: Synthesis, crystal structures and the effect of cis or trans geometry on cycloplatination

Article abstract of DOI:10.1016/j.poly.2008.05.001

Coordination compounds with a trans-stereochemistry were prepared for ligands (4-ClC₆H₄)CH{double bond, long}NCH₂(4′-ClC₆H₄) (L_a), (2,4,6-Me₃C₆H₂)CH{double bond, long}NCH₂(4′-ClC₆H₄) (L_b) and (2,6-Cl₂C₆H₃)CH{double bond, long}NCH₂(4′-ClC₆H₄) (L_c) and were shown to be precursors of the corresponding cyclometallated compounds. The reactions between cis-[PtCl₂(dmso)₂] and ligands ArCH{double bond, long}NCH₂(4′-ClC₆H₄) (Ar = 2-BrC₆H₄ (L_d); 2-ClC₆H₄ (L_e); C₆F₅ (L_f); 2,6-F₂C₆H₃ (L_g)) under previously described conditions for cycloplatination of N-benzylidenebenzylamines gave a cyclometallated compound only for imine L_f; the other imines produced coordination compounds with a cis arrangement from which cyclometallation could not be achieved. Formation of either cis or trans coordination compounds [PtCl₂(L)dmso] (L = N-benzylidenebenzylamine) can be related to steric effects and to the E/Z configuration of the C{double bond, long}N bond. All compounds were fully characterized including structure determinations for trans-[PtCl₂{(4-ClC₆H₄)CH{double bond, long}NCH₂(4′-ClC₆H₄)}SOMe₂] (2a), trans-[PtCl₂{(2,6-Cl₂C₆H₃)CH{double bond, long}NCH₂(4′-ClC₆H₄)}SOMe₂] (2c) and the amine derivative trans-[PtCl₂(4-ClC₆H₄CH₂NH₂)SOMe₂] (2) obtained as a by-product.

Full text of DOI:10.1016/j.poly.2008.05.001

Polyhedron 27 (2008) 2603–2611
Contents lists available at ScienceDirect
Polyhedron
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Novel platinum(II) compounds with N-benzylidenebenzylamines: Synthesis,
crystal structures and the effect of cis or trans geometry on cycloplatination
a
b
b
b,z
Margarita Crespo a, , Raquel Martín , Teresa Calvet , Mercè Font-Bardía , Xavier Solans
*
a Departament de Química Inorgànica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain
b Departament de Cristalꢀlografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Coordination compounds with
a
trans-stereochemistry were prepared for ligands (4-ClC6H4)CH@
NCH2(40-ClC6H4) (La), (2,4,6-Me3C6H2)CH@NCH2(40-ClC6H4) (Lb) and (2,6-Cl2C6H3)CH@NCH2(40-ClC6H4)
(Lc) and were shown to be precursors of the corresponding cyclometallated compounds. The reactions
between cis-[PtCl2(dmso)2] and ligands ArCH@NCH2(40-ClC6H4) (Ar = 2-BrC6H4 (Ld); 2-ClC6H4 (Le); C6F5
(Lf); 2,6-F2C6H3 (Lg)) under previously described conditions for cycloplatination of N-benzylidenebenzyl-
amines gave a cyclometallated compound only for imine Lf; the other imines produced coordination com-
pounds with a cis arrangement from which cyclometallation could not be achieved. Formation of either
cis or trans coordination compounds [PtCl2(L)dmso] (L = N-benzylidenebenzylamine) can be related to
steric effects and to the E/Z configuration of the C@N bond. All compounds were fully characterized
including structure determinations for trans-[PtCl2{(4-ClC6H4)CH@NCH2(40-ClC6H4)}SOMe2] (2a), trans-
[PtCl2{(2,6-Cl2C6H3)CH@NCH2(40-ClC6H4)}SOMe2] (2c) and the amine derivative trans-[PtCl2(4-
ClC6H4CH2NH2)SOMe2] (2) obtained as a by-product.
Received 31 March 2008
Accepted 5 May 2008
Available online 10 June 2008
Dedicated to the memory of Xavier Solans;
he will always be with us
Keywords:
Platinum
N-donor ligands
Cyclometallation
cis–trans Isomers
Crystal structure
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
NCH2(40-ClC6H4) leading to either endo (containing the imine
functionality) or exo cyclometallated platinum compounds [12].
Cyclometallation of N-donor ligands by platinum and palladium
has remained as one of the major topics in organometallic chemis-
try [1]. Recently cis-[PtCl2(dmso)2] has become an useful substrate
for direct cycloplatination, and numerous examples have been re-
ported including tridentate [C,N,X] (X = N [2], S [3] and O[4]) as
well as bidentate [C,N] systems derived from imines [5], oximes
[6], pyridines[7] or even tertiary amines leading to either mono
[8] or dinuclear [9] compounds.
These ligands along with similar ones with ortho substituents of
different bulk were selected for this study.
2. Results and discussion
2.1. Coordination compounds as precursors to cycloplatinated
derivatives
Coordination compounds [PtCl2(L)dmso] (L = nitrogen donor li-
gand) with a trans geometry have been postulated as precursors to
the cyclometallated derivatives [5c,10] and crystal structures of
these compounds confirm the proposed trans arrangement
[5c,8c,10,11]. However, these results do not address specifically
whether only trans isomer of the precursor or also cis isomer
may lead to successful cyclometallation.
In view of these facts, the synthesis of novel coordination com-
pounds [PtCl2(L)dmso] (L = nitrogen donor ligand) was envisaged
in this work in order to ascertain whether their cis or trans arrange-
ment is decisive for a successful cyclometallation process. We have
recently reported the reactions of cis-[PtCl2(dmso)2] with bifunc-
tional N-benzylidenebenzylamines of general formula ArCH-
Initial work was carried out for imines (4-ClC6H4)CH@NCH2(40-
ClC6H4) (La), (2,4,6-Me3C6H2)CH@NCH2(40-ClC6H4) (Lb) and (2,6-
Cl2C6H3)CH@NCH2(40-ClC6H4) (Lc) for which cyclometallated
derivatives 3a, 3b and 3c (see Chart 1) have been previously pre-
pared in a ‘‘one-pot” synthesis using cis-[PtCl2(dmso)2] as metallat-
ing agent [12]. The aim of isolating the corresponding coordination
compounds is (i) to determine the cis or trans arrangement and (ii)
to confirm that these compounds are intermediates in the cyclo-
platination process.
The reaction of the imine 4-ClC6H4CH@NCH2(40-ClC6H4) with
cis-[PtCl2(dmso)2] in refluxing methanol for 4 h produced the
corresponding aldehyde and the amine derivative trans-[PtCl2(4-
ClC6H4CH2NH2)SOMe2] (2) which was characterized crystallo-
graphically. This result suggests hydrolysis of the imine with
adventitious water in the solvent, a process that has been reported
for palladium or platinum compounds under metallation
* Corresponding author. Tel.: +34 934039132; fax: +34 934907725.
E-mail address: margarita.crespo@qi.ub.es (M. Crespo).
z
Deceased.
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.05.001

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M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
Ligands
Cyclometallated compounds
Coordination compounds
5
4
3
2
6
N
1
N
Cl
Cl
Pt
Cl
Cl
Pt Cl
7
SOMe2
Cl
Cl
Me2OS
Cl
N
(2a)
(3a)
6
4,5
3
2
7
1
N
Cl
Cl
Pt SOMe2
8,9
Cl
Cl
Cl
(La)
(2a')
7
Me
Me
Me
Me
N
6
5
Me
Me
Me
2
Me
3
1
Me
4
N
N
Cl
Cl
Pt
Cl
Pt Cl
8
SOMe2
Cl
SOMe2
(3b)
Cl
(2b)
(Lb)
6
Cl
N
5
7
Cl
N
Cl
Cl
3
Cl
2
1
Cl
4
N
Cl
Cl
Pt
Cl
Pt Cl
8
SOMe2
Cl
SOMe2
Cl
(3c)
(2c)
(Lc)
Chart 1.
conditions [2a]. When the reaction was carried out in refluxing dry
toluene, hydrolysis was avoided and a mixture of trans and cis-
[PtCl2{(4-ClC6H4)CH@NCH2(40-ClC6H4)}SOMe2] (2a and 2a0 shown
in Chart 1) was produced. Suitable crystals for structure resolution
of the trans isomer were obtained. The 1H NMR spectra of the cis
and trans isomers display distinct features. For the cis isomer, both
the methylene hydrogen atoms and the methyl groups of the
dimethylsulfoxide ligand appear as non-equivalent which suggest
a low symmetry, and the imine hydrogen displays a large J(H–Pt)
value (117.4 Hz) which is consistent with the presence of a chloro
ligand in a trans position and with an E conformation1 of the imine
as confirmed by a 2D-NOESY experiment. Conversely, for the trans
isomer both the methylene hydrogen atoms and the methyl groups
of the dimethylsulfoxide ligand appear as equivalent and the J(H–Pt)
for the imine hydrogen is reduced (84.5 Hz) compared to the cis iso-
mer which is consistent with the presence of a dimethylsulfoxide li-
1
Z/E conformation has been assigned as for the free imine (not taking into account
the platinum coordination to the nitrogen).

M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
2605
gand in a trans position to the nitrogen atom. The presence of a
cross-peak signal between the imine and the methylene protons in
a 2D-NOESY NMR spectrum supports an E conformation across the
C@N bond.
The reactions of imines Lb and Lc with cis-[PtCl2(dmso)2] were
also studied in refluxing toluene and produced the corresponding
compounds trans-[PtCl2{(2,4,6-Me3C6H2)CHNCH2(40-ClC6H4)}SOMe2]
(2b) and trans-[PtCl2{(2,6-Cl2C6H3)CHNCH2(40-ClC6H4)}SOMe2]
(2c). For these compounds, both the methylene hydrogen atoms
and the methyl groups of the dimethylsulfoxide ligand appear as
equivalent as expected for a trans stereochemistry. The J(H–Pt) val-
ues for the imine hydrogen of 2b and 2c are consistent with a Z
conformation across the C@N bond [13] since it is reduced (37.2
and 42.6 Hz, respectively) compared to that of 2a (84.5 Hz).
In order to confirm that trans-[PtCl2(4-ClC6H4CH@NCH2(40-
ClC6H4))SOMe2] (2a) is a precursor to the cyclometallated com-
pound 3a, the coordination compound was dissolved in toluene
and treated with an equimolar amount of sodium acetate dissolved
in methanol. Refluxing the mixture for 48 h produced quantitative
formation of the cyclometallated compound 3a, thus confirming
that the coordination compound is an intermediate in the cyclo-
metallation process which requires the presence of added base.
Analogous reactions carried out for trans-[PtCl2{(2,4,6-Me3-
C6H2)CHNCH2(40-ClC6H4)}SOMe2] (2b) in refluxing toluene and
for trans-[PtCl2{(2,6-Cl2C6H3)CHNCH2(40-ClC6H4)}SOMe2] (2c) in
refluxing methanol lead to the corresponding exo-metallacycles
3b and 3c.
However, an analogous experiment carried out with a mixture
of trans and cis-[PtCl2(4-ClC6H4CH@NCH2(40-ClC6H4))SOMe2] (2a/
2a0) gave a mixture of the cyclometallated compound 3a and the
cis coordination compound 2a0 which was recovered unchanged.
This result suggests that only the trans isomer may lead to
cyclometallation.
Fig. 2. Molecular structure of 2a.
chloro ligands, a S-bound dimethylsulfoxide and the N-donor li-
gand completing the coordination sphere. The angles between
adjacent atoms in the coordination sphere of platinum lie in the
range 85.9–95.36° (2), 88.40–93.77° (2a) and 87.67–94.16° (2c).
Pt–Cl, Pt–N and Pt–S bond lengths are well within the range of val-
ues obtained for similar complexes of platinum [8c,10,11].
2.2. Crystal structures of trans-[PtCl2(4-ClC6H4CH2NH2)SOMe2] (2),
trans-[PtCl2{(4-ClC6H4)CH@NCH2(40-ClC6H4)}SOMe2] (2a) and trans-
[PtCl2{(2,6-Cl2 C6H3)CHNCH2(40-ClC6H4)}SOMe2] (2c)
For 2a, the configuration at the C@N moiety is E which is the
adequate arrangement for formation of an endo-metallacycle
Crystals of compounds 2, 2a and 2c were grown from dichloro-
methane–methanol. The crystal structures are composed of dis-
crete molecules separated by van der Waals interactions. The
structures of 2 and 2c consist of the packing in the asymmetric unit
of four and two independent molecules, respectively, in both cases
through activation of
a C–H bond at the benzal ring. The
Ptꢀ ꢀ ꢀH(14)–C(14) distance is only 2.651 Å, a smaller value than
the one observed for the ortho hydrogen atoms of the benzylamine
(3.161 Å) which suggest that formation of an endo-cycle from 2a is
more favoured than that of an exo derivative, as experimentally ob-
served [12]. For 2c, the configuration at the C@N moiety is Z, a form
that allows formation of an exo-cycle [12] and the shortest distance
between platinum and the ortho hydrogen atoms is 3.289 Å for
Ptꢀ ꢀ ꢀH(110)–C(110). For 2a and 2c, the distances between platinum
with bond parameters equal within experimental error [3r]. The
structures are shown in Figs. 1–3 and selected molecular dimen-
sions are listed in Table 1. The molecular structures confirm the
geometries predicted from spectroscopic data. The coordination
sphere of platinum is square-planar with two mutually trans
Fig. 1. Molecular structure of 2 (molecule 1).

2606
M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
Fig. 3. Molecular structure of 2c (molecule 1).
Table 1
Selected bond lengths (Å) and angles (°) with estimated standard deviations
Compound 2 (molecule 1)
Compound 2a
Compound 2c (molecule 1)
Pt(1)–N(1)
Pt(1)–S(1)
Pt(1)–Cl(13)
Pt(1)–Cl(12)
N(1)–C(17)
C(16)–C(17)
N(1)–Pt(1)–Cl(13)
S(1)–Pt(1)–Cl(13)
N(1)–Pt(1)–Cl(12)
S(1)–Pt(1)–Cl(12)
C(17)–N(1)–Pt(1)
N(1)–C(17)–C(16)
2.079(10)
2.214(2)
2.280(2)
2.287(3)
1.443(16)
1.523(17)
85.9(3)
89.01(9)
90.0(3)
95.36(10)
118.2(8)
113.7(10)
Pt–N(1)
Pt–S
Pt–Cl(3)
Pt–Cl(2)
N(1)–C(8)
N(1)–C(7)
N(1)–Pt–Cl(3)
S–Pt–Cl(3)
N(1)–Pt–Cl(2)
S–Pt–Cl(2)
C(6)–C(7)–N(1)
C(7)–N(1)–C(8)
N(1)–C(8)–C(9)
2.050(8)
2.222(3)
2.301(4)
2.303(3)
1.304(12)
1.514(13)
88.6(3)
88.40(14)
89.5(3)
93.77(14)
111.1(9)
117.0(8)
129.4(9)
Pt(1)–N(11)
Pt(1)–S(11)
Pt(1)–Cl(11)
Pt(1)–Cl(12)
N(11)–C(17)
N(11)–C(18)
N(11)–Pt(1)–Cl(11)
S(11)–Pt(1)–Cl(11)
N(11)–Pt(1)–Cl(12)
S(11)–Pt(1)–Cl(12)
N(11)–C(18)–C(19)
C(17)–N(11)–C(18)
N(11)–C(17)–C(16)
2.070(6)
2.2202(19)
2.288(3)
2.305(3)
1.276(8)
1.482(10)
89.4(2)
87.67(8)
88.7(2)
94.16(9)
109.5(7)
122.0(6)
124.9(7)
and ortho C–H bonds are in the range reported for an analogous
compound with a coordinated aryl oxime (3.153 Å), described as
an intermediate in the cyclometallation process [10]. The distances
between platinum and ortho hydrogen atoms observed for the 4-
chlorobenzylamine derivative 2 are considerably greater (4.668 Å
is the shortest distance for molecule 1) and, consistently, the cyclo-
metallation of 4-chlorobenzylamine did not occurred so readily as
for the benzylidenebenzylamine La [12].
They were formed as single isomers and the expected E configura-
tion of the C@N bond was confirmed by a cross-peak between the
imine and the methylene protons in the 2D-NOESY NMR spectra
taken for Ld and Lf (see Chart 2).
Ligands Ld and Le might in principle lead to either endo (con-
taining the imine functionality) or exo five-membered metallacy-
cles. Ligands Lf and Lg contain fluoro substituents in the ortho
positions of the benzal ring and, although intramolecular C–F bond
activation for analogous ligands has been reported at electron rich
platinum(II) substrates [14], this process is not expected in princi-
ple for an electrophilic substrate such as cis-[PtCl2(dmso)2]; there-
fore, formation of an exo-metallacycle through C–H activation in
the benzylamine ring is more likely. As shown in Chart 3, both
endo- and exo-cycles can be formed from the E form, while only
exo-cycles are possible for the Z form.
2.3. Reactions of cis-[PtCl2(dmso)2] with ligands ArCH@NCH2(40-
ClC6H4) (Ar = 2-BrC6H4 (Ld); 2-ClC6H4 (Le); C6F5 (Lf); 2,6-F2C6H3 (Lg))
In order to complete this study, the reactions of cis-[PtCl2-
(dmso)2] with ligands Ld–Lg which had not been tested so far
towards cycloplatination, were also studied. The N-benzylideneb-
enzylamine ligands Ld–Lg were prepared by a condensation reac-
tion of 4-chlorobenzylamine with the corresponding aldehyde.
Initially, the reactions of imines Ld-Lg were carried out using cis-
[PtCl2(dmso)2], the imine and CH3CO2Na in a 1:1:2 ratio and heat-

M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
2607
Platinum compounds
Imines
8
Br
6,7
Br
5
5
6
9
7
7
1
N
8
1
2
N
N
N
4
2
Cl
(Ld)
(Le)
(Lf)
(2d')
(2e')
Cl
Pt SOMe2
10,11
Cl
4
3
Cl
3
Cl
8
6,7
Cl
5
5
6
9
1
2
N
8
1
4
Cl
2
4
Cl
Pt SOMe2
10,11
3
Cl
3
Cl
F
1
2
F
1
3
2
5
F
N
F
4
Cl
(3f)
F
Pt
F
Cl
F
3
F
4
Cl
F
SOMe2
6
F
F
F
6
4,5
3
3
4
5
7
(2g')
N
6
1
2
N
2
F
Cl
Cl
(Lg)
Pt
SOMe2
8,9
Cl
F
Cl
Chart 2.
product and no evidence of formation of the corresponding cyclo-
metallated compound was observed.
endo
R
R
With the aim of achieving the syntheses of cyclometallated
derivatives, the reactions of imines Ld and Le were tested under
more drastic conditions. A molar ratio [Pt]:[L]:[acetate] = 1:1:1
was used; sodium acetate was dissolved in a small amount of
methanol and added to a solution of imine and cis-[PtCl2(dmso)2]
in toluene and the mixture was refluxed for 48 h under nitrogen.
Since a higher concentration of the base could favour the cyclomet-
allation process, a molar ratio [Pt]:[L]:[acetate] = 1:1:2 was also
tested for both reactions. In all cases, the coordination compounds
described above were obtained with similar yields to those ob-
tained when methanol was used as a solvent.
H2
Pt
C
C
C
N
H
N
exo
H
H2C
Pt
Cl
exo
Cl
E form
Z form
Chart 3.
Attempts to obtain the corresponding cyclometallated com-
pound were also carried out using the coordination compound
2e0 as starting material. In one experiment, compound 2e0 was dis-
solved in toluene, sodium acetate dissolved in methanol was added
and the mixture was heated under reflux for 48 h; in another
experiment, coordination compound 2e0 in the solid state was
heated at 145 °C in an oven for three days following a literature
procedure for cycloplatination of a benzophenone imine [11]. In
both cases, the coordination compound was recovered and longer
reaction times lead only to partial decomposition with no evidence
of cyclometallation.
ing the obtained mixture in refluxing methanol during 48 h.
According to the NMR spectra of the crude products, only one plat-
inum compound was detected in each reaction, along with forma-
tion of metallic platinum and the corresponding aldehyde. Due to
decomposition processes of the platinum compounds as well as
to hydrolysis of the imines, low yields were obtained. After
work-up of the reaction mixtures, a cyclometallated compound
was obtained in extremely low yield for imine C6F5CHNCH2(4-
C6H4Cl) (Lf) while the other imines produced coordination com-
pounds with a cis arrangement. The proposed structures are shown
in Chart 2. For imine Le, the reaction time was increased up to six
days, however the cis coordination compound was again the only
The obtained compounds were characterized by 1H, 13C (2e0),19F
1
1
(3f and 2g0), COSY H–1H (2d0), NOESY H–1H (2e0, 3f and 2g0) and
gHSQC 1H–13C (2e0, 3f and 2g0) NMR spectra, mass spectrometry

2608
M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
the trans compounds [16]. Further studies indicated that if there is a
high steric hindrance due to ortho substituents in the ligand, direct
formation of the cis isomer takes place [15]. These results are re-
lated to the position of the entering ligand in the five-coordinate
intermediate or transition state: axial for non-bulky ligands and
equatorial for bulky ligands.
and elemental analyses (except for 3f due to its low stability). For
compound 3f, the presence of only three crossing peaks in the aro-
matic region of the {1H–13C}-heterocorrelation spectra can be
taken as evidence of cyclometallation leading to an exo-metallacy-
cle and the presence of a cross-peak signal between the imine and
the methylene protons in a 2D-NOESY NMR spectrum supports an
E conformation across the C@N bond 1. Coordination compounds
2d0, 2e0 and 2g0 display analogous NMR spectral features to those
described in the previous section for compound 2a0. In addition,
the presence of 6 (2e0) or 4 (2g0) crossing peaks in the aromatic re-
gion of the {1H–13C}-heterocorrelation spectra confirms that intra-
molecular C–H activation did not occur in these reactions. A cis
arrangement of these coordination compounds is consistent with
all the spectral data. In addition, the NOESY NMR spectrum of 2e0
indicates that each methyl of the dimethylsulfoxide ligand is close
to one of the aromatic rings of the imine.
The very low yield obtained for cyclometallated compound 3f
was not totally unexpected taking into account the low stability
of exo-metallacycles as previously observed for ligand 2,4,6-
Me3C6H2CHNCH2(40-ClC6H4) [12]. More striking is the failure to ob-
tain cyclometallated derivatives for ligands Ld, Le and Lg which are
analogous to the N-benzylidenebenzylamines previously studied
(La–Lc) for which cyclometallation was successful. This result could
be related to the cis arrangement of the coordination compounds
derived from Ld, Le and Lg versus the trans geometry of La, Lb and
Lc derivatives.
The results here reported can be rationalized on steric grounds
taking into account that upon coordination to metal centres, N-
benzylidenebenzylamines of considerable size might reduce the
steric hindrance through E to Z isomerization around the imine
bond [13,17]. Therefore, three distinct types of coordination com-
pounds [PtCl2(L)dmso] have been obtained: (i) The less bulky li-
gand La produced a trans isomer with an E conformation of the
imine which in the absence of added base isomerizes partially to
the cis isomer 2a0, but in the presence of CH3CO2Na gives the cor-
responding cyclometallated compound 3a; (ii) Ligands Ld, Le and Lg
containing, respectively, one bromo, one chloro or two fluoro sub-
stituents in the ortho positions of the benzal ring are sterically
more demanding than ligand La and direct formation of the cis iso-
mers 2d0, 2e0 and 2g0 takes place; therefore, cyclometallation is
inhibited; (iii) The bulkier ligands Lb and Lc adopt a Z conformation
of the imine to reduce the steric hindrance upon coordination and
this allows formation of trans isomers 2b and 2c which are precur-
sors of cyclometallated compounds 3b and 3c. In conclusion, the
combined effect of steric hindrance due to ortho substituents and
of the E or Z configuration of the C@N bond seems decisive on
the formation of either cis or trans coordination compounds of N-
benzylidenebenzylamines and, consequently, on the success of
the cyclometallation process.
In an attempt to obtain the corresponding trans isomer, the reac-
tion of imine Le with cis-[PtCl2(dmso)2] was studied under the con-
ditions used for imines La, Lb and Lc, which consist in refluxing the
mixture in toluene for 4 h. In contrast to the results obtained for the
latter imines, the reaction of Le produced exclusively the cis isomer
of the coordination compound 2e0 without traces of the trans iso-
mer. In an analogous procedure for imine Lf, no reaction was ob-
served and the starting materials were recovered; this suggests
that the low basicity of the imine bearing an electron-withdrawing
group such as C6F5 inhibits the coordination to platinum.
4. Experimental
4.1. General
Toluene (ACS) and methanol (ACS; max. 0.005 H2O) were used
as received from commercial sources.
Microanalyses were performed at Serveis Cientifico-tècnics
(Universitat de Barcelona) and at Servei de Recursos Científics i
Tècnics (Universitat Rovira i Virgili, Tarragona).
3. Conclusions
Mass spectra were performed at Servei d’Espectrometria de
Masses (Universitat de Barcelona). Electrospray mass spectra were
carried out in a LC/MSD-TOF spectrometer using H2O–CH3CN 1:1
to introduce the sample. CI mass spectra were carried out in a
HP-5988A spectrometer using NH3. NMR spectra were performed
at Unitat de RMN d’Alt Camp (Universitat de Barcelona). 1H, 13C
and 19F spectra were recorded by using Varian Unity 300 (1H,
300 MHz; 19F, 282.2 MHz), Mercury-400 (1H, 400 MHz; 13C,
100.6 MHz; 19F, 376.5 MHz) and Varian Inova DMX-500 (1H,
500 MHz; 1H–1H-COSY;1H–1H-NOESY, 1H–13C-gHSQC) spectrome-
In agreement with previously suggested mechanism for the
cyclometallation process [5c,8c,10], the results here obtained show
that cycloplatination is successful for those ligands leading to a
trans isomer of the coordination compound (La, Lb and Lc), while
the cyclometallation process is inhibited for those ligands leading
to a cis isomer of the coordination compound (Ld, Le and Lg). Crystal
structures for trans complexes 2a and 2c indicate a suitable
arrangement of these compounds as precursors of endo (3a) and
exo (3c) metallacycles, respectively. Unfortunately, suitable crys-
tals for structure determination of the cis coordination compounds
could not be obtained. It is expected, however, that steric effects
are important due to the mutual proximity of dimethylsulfoxide
and imine ligands in an E conformation. In order to minimize the
steric effects, it is likely that both aryl rings of the imine ligand
would be tilted away from the coordination plane in an arrange-
ment which is not suitable for cyclometallation. In addition, for
the cis isomer there is only one Pt–Cl cis to the N-donor ligand
and available for cyclometallation.
ters, and referenced to SiMe4 (1H, 13C), and CFCl3 19F). d values
(
are given in ppm and J values in Hz. Abbreviations used: s = singlet;
d = doublet; t = triplet; m = multiplet; br = broad; NMR labelling as
shown in Charts 1 and 3.
4.2. Preparation of the compounds
4-Chlorobenzylamine was used as received from commercial
sources. cis-[PtCl2(dmso)2] [18] and ligands La, Lb and Lc [12] were
prepared using reported procedures.
It has been reported in the literature that the reaction between
cis-[PtCl2(dmso)2] and nitrogen donor ligands lead to formation of
trans-[PtCl2(L)dmso] [15] in a substitution process that takes place
with stereo-chemical change as shown in Eq. (1).
4.2.1. Preparation of the ligands
Compounds Ld–Lg were prepared from the reaction of 0.507 g
(2.7 mmol) of 2-bromobenzaldehyde (Ld), 0.509 g (3.6 mmol) of
2-chlorobenzaldehyde (Le), 0.503 g (2.6 mmol) of pentafluorobenz-
aldehyde (Lf) or 0.506 g (3.6 mmol) of 2,6-difluorobenzaldehyde
cis-½PtCl2ðdmsoÞ2ꢁ þ L ! trans-½PtCl2ðLÞdmsoꢁ þ dmso
In addition, slow trans to cis isomerization might be observed since
complexes having the cis configuration are usually more stable than
ð1Þ

M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
2609
(Lg) with an equimolar amount (0.396 g (Ld), 0.511 g (Le), 0.371 g
(Lf), 0.511 g (Lg)) of 4-chlorobenzylamine in dichloromethane
(20 mL) with an excess of Na2SO4. The mixture was stirred under
reflux for 2 h and filtered. The solvent was removed in a rotary
evaporator to yield white (Ld, Le) or yellow (Lf, Lg) solids. (2-
BrC6H4)CHNCH2(40-ClC6H4) (Ld): Yield 708 mg (85%). 1H NMR
(400 MHz, CDCl3): d = 8.77 [s, 1H, H5], 8.06 [dd, 3J(H–H) = 7.8,
4J(H–H) = 1.8, 1H, H4], 7.58 [dd, 3J(H–H) = 8.0, 4J(H–H) = 1.2, 1H,
H1], 7.36–7.24 [m, 6H, H2,3,7,8], 4.82 [s, 2H, H6]. CI-MS (NH3) m/z:
308.2 [M]+. (2-ClC6H4)CHNCH2(40-ClC6H4) (Le): Yield 818 mg
(86%). 1H NMR (400 MHz, CDCl3): d = 8.84 [s, 1H, H5], 8.08 [dd,
3J(H–H) = 7.8, 4J(H–H)=1.8, 1H, H4], 7.38–7.26 [m, 6H, H1,2,3,7,8],
4.82 [s, 2H, H6]. (C6F5)CHNCH2(4-ClC6H4) (Lf): Yield 648 mg
(78%). 1H NMR (400 MHz, CDCl3): d = 8.50 [s, 1H, H1], 7.36–7.25
[m, 4H, H3,4], 4.86 [s, 2H, H2]. 19F NMR (376.5 MHz, CDCl3):
d = ꢂ138.67 [dd, 2F, J(F–F) = 24.4; 8.0, Fortho], ꢂ146.68 [t, 1F, J(F–
F) = 21.8, Fpara], ꢂ157.83 [td, 2F, J(F–F) = 21.8; 8.0, Fmeta]. CI-MS
(NH3) m/z: 320.3 [M+H]+. (2,6-F2C6H3)CHNCH2(40-ClC6H4) (Lg):
Yield 746 mg (78%). 1H NMR (400 MHz, CDCl3): d = 8.60 [s, 1H,
H3], 7.39–7.23 [m, 5H], 7.00–6.92 [m, 2H], 4.85 [s, 2H, H4]. 19F
NMR (282.2 MHz, CDCl3): d = -113.67 [t, 3J(F–H) = 7.0, 2F]. CI-MS
(NH3) m/z: 266.7 [M+H]+.
3J(Pt–H4) = 37.2, 1H, H4], {7.33 [d, 3J(H1-H2) = 8.8, 2H], 7.29 [d,
3J(H1–H2) = 8.8, 2H], H1,2}, 6.94 [s, 2H, H6], 4.78 [d, 3J(H3–
H4) = 1.6, 2H, H3], 3.30 [s, 6H, H8], 2.32 [s, 3H, H7]; 2.27 [s, 6H,
H5]. gHSQC (400 MHz): d = 169.7 [C4], 131.9 [C2], 129.3 [C6],
129.0 [C1], 57.9 [C3], 43.8 [C8], 21.1 [C7], 19.2 [C5]. ESI-MS, m/z:
580.06 [MꢂCl]+; 616.04 [M+H]+; 633.06 [M+NH4]+. Anal. Calc. for
C19H24Cl3NOPtS ꢀ H2O: C, 36.0; H, 4.1; N, 2.2; S: 5.1. Found: C,
36.04; H, 3.98; N, 2.25; S, 5.55%.
[PtCl2{(2,6-Cl2C6H3)CHNCH2(40-ClC6H4)}SOMe2] (2c) was ob-
tained from 142 mg (0.34 mmol) of cis-[PtCl2(dmso)2] and
100 mg (0.34 mmol) of the imine Lc which were allowed to react
in refluxing toluene (50 mL) at 100 °C for 4 h. The solvent was re-
moved in a rotary evaporator and the residue was re-crystallized
in dichloromethane–methanol, yielding 2c (yield 140 mg; 64%).
1H NMR (400 MHz, CDCl3): 8.92 [t, 3J(H3–H4) = 1.5, 3J(Pt–
H4) = 42.6, 1H, H4], 7.43 [s, 3H, H5,6,7], {7.36 [d, 3J(H1–H2) = 8.0,
2H], 7.33 [d, 3J(H1–H2) = 8.0, 2H], H1,2}, 4.92 [d, 3J(H3–H4) = 1.5,
3J(Pt–H3) = 27.6, 2H, H3], 3.30 [s, 3J(Pt–H8) = 16.8, 6H, H8]. ESI-MS,
m/z: 640.91 [M+H]+, 604.93 [MꢂCl], 568.96 [Mꢂ2Cl]. Anal. Calc.
for C16H16Cl3NOPtS: C, 29.90; H, 2.51; N, 2.18; S, 4.99. Found: C,
29.8; H, 2.6; N, 2.3; S, 4.9%.
[PtCl2{(2-BrC6H4)CHNCH2(40-ClC6H4)}SOMe2] (2d0) was ob-
tained from 0.207 g (0.49 mmol) of cis-[PtCl2(dmso)2], 0.151 g
(0.489 mmol) of imine Ld and 80 mg (0.975 mmol) which were al-
lowed to react in refluxing methanol (30 mL) for 48 h. The reaction
mixture was filtered, the solvent was removed in a rotary evapora-
tor and the residue was re-crystallized in dichloromethane–meth-
anol, yielding a light yellow solid which was filtered in vacuo. Yield
81.2 mg (25.4%). 1H NMR (400 MHz, CDCl3): d = 9.35 [dd, 3J(H3–
H4) = 7.7, 4J(H2–H4) = 1.6, 1H, H4], 8.87 [s, 3J(Pt–H5) = 115.0,
4J(H5–H6) = 1.9, 1H, H5], 7.69–7.63 [m, 2H, H1,3], 7.67 [d, 3J(H8–
H9) = 8.3, 2H, H8], 7.51 [td, 3J(H2–H1,3) = 7.7, 4J(H2–H4) = 1.7, 1H,
H2], 7.47 [d, 3J(H8–H9) = 8.4, 2H, H9], 5.6 [dd, 2J(H6–H7) = 13.4,
4J(H5–H6) = 1.9, 1H, H6], 4.94 [d, 2J(H6–H7) = 13.4, 1H, H7], 2.63 [s,
br, 3H, H10], 2.48 [s, br, 3H, H11]. Anal. Calc. for C16H17BrCl3NOPtS:
C, 29.44; H, 2.63; N, 2.15; S, 4.91. Found: C, 30.0; H, 2.8; N, 2.3; S,
4.6%.
4.2.2. Preparation of platinum compounds
[PtCl2{(4-ClC6H4)CHNCH2(40-ClC6H4)}SOMe2] (2a/2a0) was ob-
tained from 0.322 g (0.76 mmol) of cis-[PtCl2(dmso)2] and
0.200 g (0.76 mmol) of the imine La which were allowed to react
in dry toluene (50 mL) at 100 °C for 4 h. Unreacted cis-
[PtCl2(dmso)2] was removed by filtration of the reaction mixture,
the solvent was removed in a rotary evaporator and the residue
was re-crystallized in dichloromethane–methanol, yielding a first
crop of orange-yellow crystals of 2a (yield 66 mg; 14.3%) followed
by pale-yellow solid which was a mixture of 2a and 2a0 (yield
115 mg; 24.7%) and a final crop of 2a0 (yield 45 mg; 9.8%). Total
yield 226 mg (48.8%). 2a: 1H NMR (500 MHz, CDCl3): d = 8.62 [d,
3J(H1–H2) = 8.6, 2H, H1], 8.28 [t, 3J(195Pt–H3) = 84.5, 4J(H4–
H3) = 1.5, 1H, H3], 7.51 [d, 3J(H1–H2) = 8.6, 2H, H2], 7.45–7.40 [m,
4H, H5,6], 5.21 [d, 4J(H4–H3) = 1.5, 2H, H4], 3.34 [s, 6H, H7]. gHSQC
(500 MHz): d = 168.2 [C3], 132.1 [C1], 132.0 [C5], 129.1 [C6], 128.9
[C2], 66.6 [C4], 43.4 [C7]. Compound 2a0: 1H NMR (300 MHz,
CDCl3): 8.65 [s, 3J(Pt–H3) = 117.4, 1H, H3], 8.65 [d, 3J(H1–H2) = 8.4,
2H, H1], 7.68 [d, 3J(H6–H7) = 8.5, 2H, H6], 7.58 [d, 3J(H1–H2) = 8.4,
2H, H2], 7.46 [d, 3J(H6–H7) = 8.5, 2H, H7], 5.52 [dd, 2J(H4–
H5) = 13.5, 4J(H3–H4) = 1.9, 1H, H4], 4.94 [d, 2J(H4–H5) = 13.6,
3J(Pt–H5) = 54.4, 1H, H5], 2.67 [s, br, 3H, H8], 2.65 [s, br, 3H, H9].
Anal. Calc. for C16H17Cl4NOPtS: C, 31.59; H, 2.82; N, 2.30; S, 5.27.
Found: C, 31.9; H, 3.0; N, 2.4; S, 5.2%.
[PtCl2{(2-ClC6H4)CHNCH2(40-ClC6H4)}SOMe2] (2e0) was ob-
tained from 0.242 g (0.573 mmol) of cis-[PtCl2(dmso)2], 0.151 g
(0.572 mmol) of imine Le and 93 mg (1.134 mmol) of sodium ace-
tate, using the procedure reported for 2d0. Yield 166.2 mg (47.7%).
1H NMR (500 MHz, CDCl3): d = 9.36 [dd, 3J(H3–H4) = 5.7, 4J(H2–
H4) = 3.6, 1H, H4], 8.95 [s, 3J(Pt–H5) = 120.2, 1H, H5], 7.67 [d,
3J(H8–H9) = 8.4, 2H, H8], 7.60 [d, 3J(H3–H4) = 5.9, 4J(H2–H4) = 3.4,
2H, H2,3], 7.51–7.50 [m, 1H, H1], 7.47 [d, 3J(H8–H9) = 8.4, 2H, H9],
5.58 [dd, 2J(H6–H7) = 13.3, 4J(H5–H6) = 1.9, 1H, H6], 4.95 [d, 2J(H6–
H7) = 13.3, 3J(Pt–H7) = 56.3, 1H, H7], 2.62 [s, 3J(Pt–H10) = 22.8, 3H,
H10], 2.48 [s, 3J(Pt–H11) = 22.8, 3H, H11]. 13C NMR (100.6 MHz,
CDCl3): d = 161.05 [1C, C7], 136.00 [1C, C6], 135.24 [1C, C9],
133.66 [1C, C3], 132.97 [1C, C1], 132.84 [2C, C10], 132.69 [1C, C12],
130.20 [1C, C5], 129.89 [1C, C2], 129.17 [2C, C11], 126.77 [1C, C4],
69.78 [1C, C8], 43.66 [1C, C13], 43.44 [1C, C14]. ESI-MS, m/z:
626.98 [M+NH4]+. Anal. Calc. for C16H17Cl4NOPtS: C, 31.59; H,
2.82; N, 2.30; S, 5.27. Found: C, 31.6; H, 3.2; N, 2.3; S, 6.2%.
[PtCl{C6F5CHNCH2(4-ClC6H3)}SOMe2] (3f) was obtained from
0.203 g (0.480 mmol) of cis-[PtCl2(dmso)2], 0.154 g (0.481 mmol)
of imine Lf and 79 mg (0.963 mmol) of sodium acetate, using the
procedure reported for 2d0. Yield 20 mg (6.6%). 1H NMR
(500 MHz, CDCl3): d = 8.69 [d, 3J(Pt–H1) = 82.9, 4J(H1–H2) = 1.4,
1H, H1], 8.01 [d, 3J(Pt–H5) = 50.9, 4J(H5–H3,4) = 1.7, 1H, H5], 7.08
[t, J(Pt–H3,4) = 20.2, J(H5–H3,4) = 2.0, 2H, H3,4], 5.05 [s, 3J(Pt–
H2) = 25.2, 2H, H2], 3.47 [s, 6H, H6]. 19F NMR (376.5 MHz, CDCl3):
d = ꢂ150.93 [t, J(F–F) = 20.8, 1F, Fpara], ꢂ163.83 [dd, J(F–F) = 20.5;
5.7, 2F, Fortho], ꢂ169.48 [td, J(F–F) = 18.4; 2.1, 2F, Fmeta]. gHSQC
(500 MHz) d = 154.0 [C1], 133.6 [C5], {120.5, 124.5 [C3, C4]}, 68.5
[C2], 44.5 [C6]. ESI-MS, m/z: 663.46 [M+2H20]+.
[PtCl2(4-ClC6H4CH2NH2)SOMe2] (2) was obtained as a by-prod-
uct in the reaction of equimolar amounts of cis-[PtCl2(dmso)2] and
imine La (see above) carried out in refluxing methanol. The solvent
was removed in a rotary evaporator and the residue was re-crystal-
lized in dichloromethane–methanol at low temperature. Orange-
yellow crystals of 4 were obtained. 1H NMR (300 MHz, CDCl3):
d = 7.34 [d, 3J(H1–H2) = 8.6, 2H, H2], 7.28 [d, 3J(H1–H2) = 8.7, 2H,
H1], 4.61 [s, 2H, H4], 4.02 [m, 2H, H3], 3.41 [s, 3J(195Pt–H5) = 23.7,
6H, H5]. ESI-MS, m/z: 502.99 [M+NH4]+; 485.96 [M]+; 449.98
[MꢂCl]+; 414.01[Mꢂ2Cl]+. Anal. Calc. for C9H14Cl3NOPtS: C,
22.25; H, 2.90; N, 2.89; S, 6.60. Found: C, 22.5; H, 3.0; N, 2.9; S,
6.6%.
[PtCl2{(2,4,6-Me3C6H2)CHNCH2(40-ClC6H4)}SOMe2] (2b) was
obtained from 0.079 g (0.187 mmol) of cis-[PtCl2(dmso)2] and
0.051 g (0.184 mmol) of the imine Lb which were allowed to react
in toluene under reflux for 4 h. The solvent was removed in a rotary
evaporator and the residue was dried in vacuo. Upon addition of
diethylether a yellow solid was obtained, filtered and dried in va-
cuo (yield 73.5 mg; 64.9%). 1H NMR (400 MHz, CDCl3): 9.05 [s,

2610
M. Crespo et al. / Polyhedron 27 (2008) 2603–2611
[PtCl2{(2,6-F2C6H3)CHNCH2(40-ClC6H4)}SOMe2] (2g0) was ob-
SHELXL97 computer program using 28622 (2), 9983 (2a) and 9836
P
2
2 2
(2c) reflections. The function minimized was
where w = [
2(I) + (0.0501P)2 + 11.8165P]ꢂ1
(0.0986P)2 + 3.6436P]ꢂ1 (2a) or
wjjFoj ꢂ jFcj j ,
tained from 0.242 g (0.573 mmol) of cis-[PtCl2(dmso)2], 0.151 g
(0.572 mmol) of imine Lg and 93 mg (1.134 mmol) of sodium ace-
tate, using the procedure reported for 2d0. Yield 36.5 mg (10.0%). 1H
NMR (500 MHz, CDCl3): d = 8.80 [s, 3J(Pt–H3) = 120.2, 1H, H3], 7.64
[d, 3J(H6–H7) = 8.4, 2H, H6], 7.61–7.58 [m, 1H, H2], 7.47 [d, 3J(H6–
H7) = 8.4, 2H, H7], 7.11 [t, 3J(H1–H2) = 8.2, 3J(H1–F) = 8.2, 2H, H1],
5.71 [dd, 2J(H4–H5) = 13.5, 4J(H3–H4) = 1.8, 1H, H4], 5.05 [d, 2J(H4–
H5) = 13.5,1H, H5], 2.75 [s, br, 3H, H8], 2.64 [s, br, 3H, H9]. 19F
NMR (376.5 MHz, CDCl3): d = ꢂ104.00 [t, 3J(F–H1) = 7.0]. gHSQC
(500 MHz) d = 162.9 [C3]; 134.0 [C2]; 132.6 [C6]; 128.9 [C7], 112.0
[C1]; 69.8 [C4]; 42.0 [C8]. ESI-MS, m/z: 1237.95 [2M+H20]+;
1182.95 [2MꢂCl]+; 626.99 [M+H20]+; 573.99 [MꢂCl]+; 538.02
[Mꢂ2Cl]+. Anal. Calc. for C16H16Cl3F2NOPtS: C, 31.51; H, 2.64; N,
2.30; S, 5.26. Found: C, 31.9; H, 2.9; N, 2.4; S, 5.3%.
r
(2),
[r
2(I) +
[
r
2(I) + (0.0734P)2 + 4.1280P]ꢂ1
2
2
(2c) and P = (jFoj + 2jFcj )/3. f, f0 and f00 were taken from Interna-
tional Tables of X-ray Crystallography [20]. All hydrogen atoms
were computed and refined using a riding model with an isotropic
temperature factor equal to 1.2 times the equivalent temperature
factor of the atom to which they are linked. Further details are
given in Table 2.
5. Supplementary data
CCDC 68025, 68026 and 68027 contain the supplementary crys-
tallographic data for 2, 2a and 2c. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
4.3. X-ray structure analysis for 2, 2a and 2c
Prismatic crystals were selected and mounted on a MAR 345
diffractometer. Unit cell parameters were determined from 6776
(2), 309 (2a) and 7029 (2c) reflections (3° < h < 31°) and refined
by least-squares methods. Intensities were collected with graphite
Acknowledgement
mono-chromatized Mo K
a radiation. 28622 (2), 9983 (2a) and
This work was supported by the Ministerio de Ciencia y Tec-
nología (Project: CTQ2006-02007/BQU).
9836 (2c) reflections were measured in the range 2.60° < h <
30.00° (2), 3.06° < h < 30.00° (2a) or 2.73° < h < 32.34° (2c). 12272
(2), 5345 (2a) and 7929 (2c) reflections were assumed as observed
References
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tion corrections were made for 2 and 2c.
The structures were solved by direct methods using SHELXS pro-
gram [19], and refined by full-matrix least-squares method with
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Table 2
Crystallographic and refinement data
Compound 2
Compound 2a
Compound 2c
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Empirical formula
Molecular weight
Temperature (K)
Wavelength (Å)
Crystal system, space
group
Unit cell dimensions
a (Å)
b (Å)
C9H14Cl3NOPtS
485.71
293(2)
C16H17Cl4NOPtS
608.26
293(2)
0.71073
monoclinic, P21
C16H16Cl5NOPtS
642.70
293(2)
0.71073
monoclinic, P21/c
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0.71073
triclinic, P1
ꢀ
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13.603(7)
14.699(5)
16.323(6)
67.60(2)
89.04(3)
77.35(2)
2936(2)
8; 2.198
10.225
11.126(6)
7.842(3)
12.127(4)
90
109.93(2)
90
994.7(7)
2; 2.031
7.699
9.890(6)
32.240(15)
13.289(5)
90
97.76(3)
90
4198(4)
8; 2.034
7.425
c (Å)
a
(°)
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b (°)
c
(°)
Volume (Å3)
Z; Dcalc (Mg mꢂ3
)
Absorption coefficient
(mmꢂ1
F(000)
)
1824
580
2448
Crystal size (mm)
h Range for data
collection (°)
0.1 ꢃ 0.1 ꢃ 0.2
0.09 ꢃ 0.05 ꢃ 0.05 0.2 ꢃ 0.1 ꢃ 0.1
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2.60–30
3.06–30
2.73–32.34
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28622/14588
(0.0947)
9983/5596
(0.0714)
98.2 (h = 30.00)
full-matrix least-
9836/9836
(0.0370)
78.1 (h = 25.00)
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Completeness to h
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5596/1/218
9836/8/457
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1.118
1.146
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0.916 and
ꢂ0.763
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[I > 2r(I)]
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ꢂ0.916
(e Åꢂ3
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