Pt(IV) derivatives formed by oxidative addition of organic halides to [Pt(CH3)2(N,N-chelate)] substrates: Geometric isomers at equilibrium

Article abstract of DOI:10.1016/S0022-328X(99)00634-8

The geometrical isomerism at equilibrium of Pt(IV) derivatives of general formula [Pt(CH₃)₂(R)X(N-N)] (N-N=2,9-dimethyl-1,10-phenanthroline or 1,10-phenanthroline; R=σ C-bonded ligand; X=halide) has been investigated by variation of R, X, and N-N. The complexes have been obtained mainly through oxidative addition of RX to [Pt(CH₃)₂(N-N)]. Some general trends can be traced in the relative stability of the geometrical isomers of the complexes, and attempts to discriminate sterical and electronic factors have been presented. The first attainment of a compound of the general formula [Pt(CH₃)₂(R)R′(N-N)] containing three different types of σ C-bonded groups is also reported.

Full text of DOI:10.1016/S0022-328X(99)00634-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 593–594 (2000) 445–453
Pt(IV) derivatives formed by oxidative addition of organic halides
to [Pt(CH3)2(N,N-chelate)] substrates: geometric isomers at
equilibrium
Vincenzo De Felice a, Bruno Giovannitti a, Augusto De Renzi b, Diego Tesauro b,
Achille Panunzi c,*
a Uni6ersita` del Molise, Via Mazzini 1, I-86100, Campobasso, Italy
b Dipartimento di Chimica, Uni6ersita` di Napoli ‘Federico II’, Via Mezzocannone 4, I-80134, Napoli, Italy
c Dipartimento di Chimica and Facolta` di Agraria, Uni6ersita` di Napoli ‘Federico II’, Via Mezzocannone 4, I-80134, Napoli, Italy
Received 2 August 1999; accepted 22 October 1999
Dedicated to Professor Fausto Calderazzo on occasion of his 70th birthday.
Abstract
The geometrical isomerism at equilibrium of Pt(IV) derivatives of general formula [Pt(CH3)2(R)X(NꢀN)] (NꢀN=2,9-dimethyl-
1,10-phenanthroline or 1,10-phenanthroline; R=s C-bonded ligand; X=halide) has been investigated by variation of R, X, and
NꢀN. The complexes have been obtained mainly through oxidative addition of RX to [Pt(CH3)2(NꢀN)]. Some general trends can
be traced in the relative stability of the geometrical isomers of the complexes, and attempts to discriminate sterical and electronic
factors have been presented. The first attainment of a compound of the general formula [Pt(CH3)2(R)R%(NꢀN)] containing three
different types of s C-bonded groups is also reported. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Platinum; Octahedral complexes; Equilibrium; Geometrical isomerism
N,N-chelate (NꢀN) which are also of practical interest,
e.g. for the preparation of pendant organometallic
polymers [3] and dendrimers [4], or the development of
biological properties [5], and are the subject of a com-
prehensive recent review by Rendina and Puddephatt
[6].
It is to note, however that an aspect of the stereo-
chemistry of these compounds, i.e. the geometrical iso-
merism at equilibrium, has not been investigated
systematically, while by far more interest was devoted
to the kinetically controlled isomerism, which is rele-
vant mechanistically. The geometry of a few equi-
librium systems is known by previous studies (see
below), but no comprehensive discussion of this aspect
has been made. On the other hand, the polyalkyl
complexes, which can easily display good stability and
moderate lability, appear to be a rare example of
octahedral system, whose isomeric equilibrium compo-
sition can be modulated by an ample variation of the
stereoelectronic properties of the coordination sphere,
1. Introduction
Homogeneous oxidative addition reactions involving
transition metals are pivotal steps of important stoi-
chiometric and catalytic processes and have received
intensive interest over the past four decades [1]. A good
share of the studies concerning these reactions and
possible further transformations of their products have
dealt with Pt(II)/Pt(IV) systems. In these cases, in fact,
it has been possible to gain, in addition to many
synthetic achievements, plenty of results on thermody-
namic, kinetic, mechanistic and stereochemical features,
concerning both the oxidation process and ensuing
reactions, such as reductive elimination or insertion [2].
Attention was mostly devoted to the attainment of
polyalkyl mononuclear Pt(IV) complexes including a
* Corresponding author. Tel.: +39-81-5476560; fax: +29-81-
5527771.
E-mail address: panunzi@chemna.dichi.unina.it (A. Panunzi)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00634-8

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V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
with the sole constraint of NꢀN chelation. This work
aimed to add information on the equilibrium stereo-
chemistry with regard to Pt(IV) dimethyl derivatives,
including a third s C-bonded group R and a N,N-
chelate with in plane sterical hindrance. They can be
obtained as the product of the oxidative addition of
organic halides RX to the good nucleophiles
[Pt(CH3)2(NꢀN)]. The equilibrium stereochemistry of
the complexes [Pt(CH3)2(R)X(NꢀN)] (I) has been inves-
tigated by variation of R, X and NꢀN groups. In
addition, this study has produced the attainment of a
compound of general formula [Pt(CH3)2(R)R%(NꢀN)]
containing three different types of s C-bonded groups.
Also some cationic complexes of type [Pt(CH3)2-
(R)L(NꢀN)]BF4 (II) have been isolated. We observe
that no attempt was made during this work to assess
mechanistic features of the examined oxidative pro-
cesses [5], but involvement of different mechanisms
does not affect our results. Moreover, the attainment
of type I complexes by processes other than oxida-
tive additions is potentially useful for the aim of the
work.
2.2. Halide exchange on I complexes
A 1 M solution of the required sodium halide in
water (0.3 ml) was added to a solution of I (0.05 mol)
in 2 ml of chloroform and the mixture was stirred 30
min at room temperature. The organic phase was sepa-
rated, washed with 2×1 ml of water and dried with
Na2SO4. Addition of diethyl ether caused crystallization
of the product.
2.3. Synthesis of [Pt(CH3)2(R)L(NꢀN)]BF4 complexes
To a solution of complex I (0.05 mmol) in 2 ml of
chloroform, 0.5 ml of a 0.1 M solution of AgBF4 in
CH3CN were added under stirring. After 1 h the silver
halide was filtered off on a small Celite bed, the solu-
tion was concentrated under vacuum and diethyl ether
was added to achieve crystallization of the product.
Alternatively, the reaction was performed in CDCl3 and
1
the filtered solution was transferred to the H-NMR
tube for measurement of the spectrum.
The acetonitrile complex can be used as a starting
material for the other cationic complexes here de-
scribed. This is easily achieved by treatment with a
stoichiometric amount of the neutral ligand L, at room
temperature, of the compound dissolved in a minimum
amount of methylene chloride–nitromethane (10:1). Af-
ter 30 min stirring, the solvents and the released
CH3CN are removed in vacuo affording the nearly pure
ionic complex II. Recrystallization could be made by
methylene chloride–diethyl ether mixtures.
2. Experimental
1H-NMR spectra were recorded on 250-MHz Bruker
model AC-250 spectrometer. CDCl3 and CD3NO2 were
used as solvents, and CHCl3 (l=7.26) and CHD2NO2
(l=4.33) as internal standards. The following abbrevi-
ations are used for describing NMR multiplicities:
s, singlet; d, doublet; m, multiplet. Solvents were
dried before use. All reactions have been carried out
under a nitrogen atmosphere by using Schlenk tech-
niques, and care has been taken in excluding oxygen
and moisture, respectively for the synthesis of iso-
propyl and acyl derivatives. The ligands 1,10-phenan-
throline (phen) and 2,9-dimethyl-1,10-phenanthroline
(dmphen) were available commercially (Aldrich). The
syntheses of [Pt(CH3)2(NꢀN)] is reported elsewhere
[7,8].
2.4. Recon6ersion of [Pt(CH3)2(R)L(NꢀN)]BF4 to I
A 2 M solution of the required sodium halide in
water (0.3 ml) was added to a solution of II (0.05 mol)
in 2 ml of chloroform containing a sufficient amount of
nitromethane to ensure dissolution and the mixture was
stirred for 30 min at room temperature. The organic
phase was separated, washed with 2×1 ml of water
and dried with Na2SO4. Addition of diethyl ether
caused crystallization of the product.
2.5. Synthesis of [Pt(CH3)2(R)R%(dmphen)] complexes
2.1. Oxidati6e addition reaction of RX to
[Pt(CH3)2(NꢀN)]
To 5 ml of a 0.04 M solution of [Pt(CH3)2(R)-
(CH3CN)(dmphen)]BF4, 2 mmol of KR% were added.
Addition of RX (0.1–1 mmol, depending on R) to a
solution of [Pt(CH3)2(NꢀN)] (0.1 mmol, 0.424 g of
dmphen or 0.396 g of phen) in diethylether caused
precipitation of the type I product. In order to attain
high yields, a high yield (80–90%) of precipitate times,
ranging from ca. 1–24 h, according to the nature of
RX, were required. The white–brown yellow precipi-
tate was collected by filtration, washed with diethyl
ether and dried in vacuum.
The
solvent
was
tetrahydrofurane
(R%=CH-
(COOCH3)2) or nitromethane–methanol 4:1 (R%=
CH2NO2). The suspension was stirred for 2 h at room
temperature, filtered on a small Celite bed and the
solvent was removed under vacuum. The residue was
extracted with three portions of methylene chloride (1
ml each) and the extract was concentrated to ca. 1 ml.
Addition of diethyl ether produced crystallization of the
1
white product with ca. 85% yield. Selected H-NMR

V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
447
data (l, in CDCl3, JPtꢀH in Hz): [Pt(CH3)2(C2H5)-
{CH(COOCH3)2}(dmphen)], 3.75 (s, 60, PtCH), 3.25 (s,
NCH3), 2.96 (s, OCH3), 1.25 (s, 81, PtCH3), 0.72 (q, 65,
PtCH2), 0.20 (t, 54, CH2CH3); [Pt(CH3)3{CH-
(COOCH3)2}(dmphen)], 3.67 (s, 60, PtCH), 3.19 (s,
NCH3), 2.96 (s, OCH3), 1.25 (s, 75, PtCH3), −0.05 (s,
60, PtCH3); [Pt(CH3)3(CH2NO2)(dmphen)], 4.65 (s, 50,
PtCH2), 3.14 (s, NCH3), 1.24 (s, 73, PtCH3), −0.08 (s,
53, PtCH3).
ing the chelate, while the other two positions are indi-
cated as axial. This enables easy identification of the
stereochemical features of the Pt(IV) derivatives.
3.2. Type I compounds, equilibrium assessment and
description
The addition of the two fragments R and X deriving
from the halide to the square precursor can produce in
principle four different isomers. However, no trace of
anyone of the two isomers bearing the halide in equato-
rial position was detected in any case by 1H-NMR
spectroscopy. Thus, only two isomers, with Cs and C1
symmetry, respectively were observed, and we will refer
to the former as trans and to the latter as cis (Fig. 1) for
the sake of simple and immediate identification of the
mutual position of R and X.
A preliminary requirement for further discussion is
the discrimination between kinetic and equilibrium con-
trol of the observed cis–trans isomer ratio. In fact, we
wished to discuss particularly the stereochemistry at
equilibrium, in order to trace general trends and to
attempt at least a coarse separation of the influence of
electronic and steric factors on the thermodynamic
control.
3. Results and discussion
3.1. Reactions of organic halides with
[Pt(CH3)2(dmphen)]
All the type I compounds are identified by the R, X,
and NꢀN groups, as listed in Tables 1–3, together with
the 1H-NMR data pertaining to each isomer. The oxida-
tive addition reactions have been carried out in diethyl
ether, obtaining precipitation of fairly pure compounds.
Some runs were also carried out in acetone or benzene.
In agreement with previous findings [9], which show that
stability increases by increasing the rigidity of the biden-
tate ligand, the compounds are resistant to reductive
elimination. Halide exchange reactions were performed
in heterogeneous (chloroform–water) phase. In Table 2
the isomeric composition of the products is reported in
case these were assessed (vide infra) to be at equilibrium.
In the tables, as well as in the text, the notation
equatorial designates ligands within the plane contain-
We recall that the preferred kinetically controlled
stereochemistry for the oxidation reactions, considering
previous results on similar processes [6] appears to be
that affording trans derivatives. However, cis com-
pounds can be preferentially or specifically formed by
other mechanisms [10].
Table 1
1H-NMR data for [Pt(CH3)2(R)X(dmphen)] complexes (R=unsubstituted hydrocarbyl group) a
R
X
PtꢀR
NꢀCH3
PtꢀCH3 eq.
PtꢀCH3 ax.
CH3
CH3
I
3.30(s)
3.30(s)
3.29(s)
3.38(s), 3.28(s)
3.29(s)
3.38(s), 3.28(s)
3.29(s)
3.27(s)
3.27(s)
3.18(s)
3.25(s)
3.28(s)
3.32(s)
3.35(s), 3.30(s)
3.26(s)
1.72(s, 74)
1.60(s, 72)
1.69(s, 74)
1.63 (s, 77)
1.59(s, 74)
1.58(s, 77)
1.71(s, 74)
1.75(s, 74)
1.65(s, 74)
1.86(s, 74)
1.81(s, 73)
1.73(s, 74)
1.78(s, 78)
1.60(s, 74)
1.90(s, 74)
1.65(s, 74)
0.22(s, 70)
0.12(s, 70)
Br
I
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3CH2CH2
CH2ꢁCHCH2
0.96(q, 71, CH2), −0.43(t, 68, CH3)
c, 1.35(t, 54, CH3)
0.96(q, 82, CH2), −0.35(t, 68, CH3)
I b
Br
0.18(s, 67)
0.08(s, 67)
Br b c, 1.20(t, 50, CH3)
I
I
0.96(m, CH2), 0.33(t, CH3), 0.0(m, d, CH2Pt)
1.69(d, 94, CH2Pt), 4.75(m, ꢁCH), 4.00 and 3.73(2dd, ꢁCH2)
1.69(d, 94.5, CH2Pt), 4.8(m, ꢁCH), 4.00 and 3.73(2dd, ꢁCH2)
2.47(s, 95), 6.62(t, 1H); 6.31(t, 2H), 5.81(m, 2H)
6.61(t, 1H), 6.37(t, 2H), 5.88(m, 2H), 2.45(s, 73, 2H),
2.33(m, d, CH), −0.40(d, 2CH3, 60)
CH2ꢁCHCH2 Br
C6H5CH2
C6H5CH2
(CH3)2CH
CH2ꢁCH
CH2ꢁCH
CH2ꢁCH
CH2ꢁCH
I
Br
I
Br
5.76(dd, d, ꢁCH), 5.14 and 3.90(2d, ꢁCH2)
Br b 7.20(dd, d, ꢁCH), 5.32 and 4.98(2d, ꢁCH2)
0.20(s, 72)
0.42(s, 74)
I
5.85(dd, d, ꢁCH), 5.06 and 3.85(2d, ꢁCH2)
7.40(dd, d, ꢁCH), 5.40 and 5.02(2d, ꢁCH2)
I b
3.30(s), 3.22(s)
a The reported figures refer to the trans isomer, unless otherwise stated. The spectra were recorded in CDCl3 solution (reference l 7.26, CHCl3);
the 195Pt coupling constants (Hz) are reported in parentheses; abbreviations s (singlet), d (doublet), dd (double doublet), t (triplet), q (quartet),
m (multiplet). The chemical shifts of the heteroaromatic 2,9-dimethyl-1,10-phenanthroline protons are in the range: l 8.5–8.3 (d, 2H), 7.9–7.75 (d,
2H), 7.8–7.7 (s, 2H).
b Cis isomer.
c CH2 resonance obscured by other signals.
d 195Pt coupling constant not evaluable.

448
V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
Table 2
1H-NMR data for [Pt(CH3)2 (R)X(dmphen)] complexes (R=substituted hydrocarbyl group) a
R
X
PtꢀR
NꢀCH3
PtꢀCH3 eq.
PtꢀCH3 ax.
C6H5CO
C6H5CO
C6H5CO
CH3CO
CH3CO
CH3CO
CH3CO
CH3CH2CO
CH3OCO
Cl b
Br b
I b
8.40(d, 2H), 7.95(m, 3H)
8.30(dd, 2H), 7.65(m, 3H)
8.30(dd, 2H), 7.65(m, 3H)
2.84(s, 8)
1.70(s, 12)
2.94(s, 12)
2.95(s, 12)
3.87 and 3.78(2q, CH2), 1.09(t, CH3)
3.81(s, 8)
3.32(s), 2.98(s)
3.34(s), 2.99(s)
3.30(s), 2.95(s)
3.29(s), 3.00(s)
3.25(s)
3.31(s), 3.05(s)
3.22(s), 2.95(s)
3.28(s), 2.94(s)
3.29(s), 3.08(s)
3.20(s)
3.28(s), 3.05(s)
3.20(s)
3.32(s), 3.34(s)
3.28(s)
3.48(s), 3.31(s)
3.25(s)
3.50(s), 3.32(s)
3.26(s)
3.40(s), 3.28(s)
3.27(s)
3.40(s), 3.28(s)
3.27(s)
3.38(s), 3.27(s)
3.15(s)
3.38(s), 3.27(s)
3.12(s)
3.37(s)
3.29(s)
3.40(s), 3.32(s)
3.31(s)
3.50(s), 3.30(s)
3.32(s)
1.56(s, 75)
1.61(s, 75)
1.63(s, 76)
1.44(s, 75)
1.80(s, 70)
1.50(s, 75)
1.50(s, 75)
1.40(s, 73)
1.90(s, 74)
1.69(s, 72)
1.80(s, 74)
1.71(s, 72)
1.80(s, 72)
1.91(s, 71)
1.82(s, 72)
1.82(s, 72)
1.83 (s, 72)
1.82(s, 72)
1.85(s, 73)
1.68(s, 72)
1.90(s, 73)
1.65(s, 72)
1.50(s, 73)
1.82(s, 73)
1.50(s, 73)
1.98(s, 73)
1.89(s, 72)
1.90(s, 69)
1.89(s, 72)
1.78(s, 72)
1.70(s, 75)
1.80(s, 75)
1.62(s, 72)
1.68(s, 70)
1.82(s, 73)
0.48(s, 73)
0.54(s, 70)
0.59(s, 70)
0.37(s, 73)
Cl b
Cl c
Br b
I b
0.46(s, 75)
0.49(s, 72)
0.39(s, 75)
0.60(s, 72)
Cl b
Cl b
Cl c
I b
CH3OCO
CH3OCO
CH3OCO
3.21(s, 8)
3.80(s, 8)
0.78(s, 72)
0.52(s, 68)
0.21(s, 68)
0.30(s, 66)
0.10(s, 70)
0.20(s, 70)
0.42(s, 68)
0.48(s, 68)
0.60(s, 64)
0.60(s, 64)
0.25(s, 70)
0.14(s, 70)
I c
3.22(s,
)
d
NH2COCH2
NH2COCH2
CH3OCOCH2
CH3OCOCH2
CH3OCOCH2
CH3OCOCH2
CH3COOCH2
CH3COOCH2
CH3COOCH2
CH3COOCH2
C6H5COCH2
C6H5COCH2
C6H5COCH2
C6H5COCH2
NCCH2
NCCH2
NCCH2
NCCH2
HOCH2CH2
HOCH2CH2
HOCH2CH2
HOCH2CH2
C6H5COCH2
I b
4.18(d, 115), 3.08(d, 92); 6.3 and 5.2(2bm, NH2)
2.00(s, 132, CH2); 6.3 and 5.2(2bm, NH2)
3.75(d, 113), 2.98(d, 100), 3.55(s, OCH3)
1.77(s, 100, CH2), 2.85(s, OCH3)
3.98(d, 110), 3.10(d, 100), 3.52(s, OCH3)
1.80(s, 95), 2.80(s, OCH3)
5.35(d, 60), 5.68(d, 70),1.13(s, CH3)
3.95(s, 54), 1.13(s, CH3)
5.40(d, 60), 5.75(d, 70), 1.13(s, CH3)
3.90(s, 54), 1.13(s, CH3)
I c
Br b
Br c
I b
I c
Br b
Br c
I b
I c
Br b
Br c
I b
4.95(d, 114), 3.88(d, 127), 7.0 and 7.25(2m, 5H)
2.66(s, 107), 7.0 and 7.25(2m, 5H)
4.95(d, 114), 3.88(d, 127), 7.0 and 7.25(2m, 5H)
I c
2.72(s, 102), 7.0 and 7.25(2m, 5H)
I b
3.72(d, d), 3.66(d,
1.26(s, 89)
)
d
I c
Br b
Br c
I b
3.50(d, d), 2.50(d, 80)
1.22(s, 89)
3.90(m), 2.65(m, d), 2.40(m,
)
)
d
d
I c
3.90(m), 1.20(m,
)
d
Br b
Br c
Br c
3.80(m), 2.80(m, d), 2.45(m,
3.52(s), 3.29(s)
3.27(s)
3.15(s)
d
3.80(m), 1.50(m,
)
2.66(s, 107), 7.0 and 7.25(2m, 5H)
a The spectra were recorded in CDCl3 solution (reference l 7.26, CHCl3); the 195Pt coupling constants (Hz) are reported in parentheses;
abbreviations s (singlet), d (doublet), dd (double doublet), t (triplet), q (quartet), m (multiplet), bm (broad multiplet). The chemical shifts of the
heteroaromatic 2,9-dimethyl-1,10-phenanthroline protons are in the range: l 8.5–8.3 (d, 2H), 7.9–7.75 (d, 2H), 7.8–7.7 (s, 2H).
b Cis isomer.
c Trans isomer.
d 195Pt coupling constant not evaluable.
Spontaneous isomerization of kinetically controlled
products of type I in order to attain equilibrium isomeric
composition was previously observed, although
metastable kinetically controlled systems are likely con-
ceivable owing to possible inertness of complexes like I.
After the examination of several oxidation reactions we
could identify ca. 25 equilibrium systems. Their isomeric
composition at 25°C in chloroform solution as deter-
mined by integration of the 1H-NMR signals is reported
in Table 4.
Of course, a question concerning the origin of the
stereochemical control arises in case spontaneous isomer-
ization of the early products could not be detected. We
attempted to obtain isomerization in chloroform solution
at room temperature. Lack of observed isomerization
could be due to a very low isomerization rate or to the
achievement of equilibrium before measurement of the
isomer ratio. Moreover, the two conditions could in
principle co-exist. It is also to note that the presence of
a sole isomer in the product is not a favorable situation
for easy and reliable affirmation of a putative equilibrium.
1
According to H-NMR spectroscopy we have obtained
early products with trans or cis stereochemistry or
cis–trans mixtures. In fact, the spectral patterns are
generally sufficient to easily assess the isomeric composi-
tion of the products. The presence of only one signal for
the two CH3ꢀPt groups and a symmetric pattern for the
chelate support a Cs symmetry (trans isomer), on consid-
eration that a rapid intramolecular alkyl exchange is
hardly conceivable. On the other hand two CH3ꢀPt
signals and the non-equivalence of the two halves of the
chelate indicate a C1 symmetry (cis).

V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
449
Table 3
Selected 1H-NMR data for [Pt(CH3)2 (R)X(phen)] complexes a
R
X
PtꢀR
PtꢀCH3 eq.
PtꢀCH3 ax.
0.60(s, 74)
0.80(s, 74)
0.80(s, 70)
0.83(s, 74)
0.93(s, 72)
0.90(s, 75)
d
CH3CH2
CH3CH2
CH3CO
CH3CO
CH3CO
I b
1.48(s, 72)
1.64(s, 72)
1.71(s, 72)
1.75(s, 61)
1.96(s, 72)
1.95(s, 70)
1.68(s, 75)
1.82(s, 75)
1.82(s, 75)
2.02(s, 72)
1.82(s, 72)
1.64(s, 72)
I c
1.49(q, 71, CH2), −0.90(t, 68, CH3)
2.73(s, 12)
2.02(s, 15)
2.80(s, 13)
1.98(s, 14)
8.0(m, 3H), 7.4(d, 2H)
7.1(m, 3H), 6.80(d, 2H)
8.0(m, 3H), 7.4(d, 2H)
8.0(m, 3H), 7.4(d, 2H)
6.4 and 5.3(b, NH2), 3.38(s, 101, CH2)
4.8 and 4.3(b, NH2), 2.16(s, 93, CH2)
Cl b
Cl c
I b
CH3CO
I c
C6H5CO
C6H5CO
C6H5CO
C6H5CO
NH2COCH2
NH2COCH2
Cl b
Cl c
I b
I c
I b
I c
a Spectra recorded in CDCl3 solution (reference l 7.26, CHCl3), the 195Pt (Hz) coupling constants are reported in parentheses; abbreviations s
(singlet), d (doublet), t (triplet), q (quartet), m (multiplet). The chemical shifts of the heteroaromatic 1,10-phenanthroline protons are in the range:
l 9.8–9.2 (d, 2H), 8.6–8.4 (d, 2H), 8.0–7.9 (d, 2H), 8.1–7.9 (s, 2H).
b Cis isomer.
c Trans isomer.
d CH2CH3 resonance obscured by other signals.
In order to clarify the actual situation, two types of
procedures were applied to most of the isolated I
compounds: (a) in case the products have been ob-
tained as isomer mixtures, fractioning by crystallization
or chomatography was attempted and the isomeric
ratios of the starting mixture and of each fraction were
compared. Some attempts to get variations of the initial
ratio was made alternatively by performing the reaction
in a different solvent and/or at a different temperature;
(b) in case the above procedures were uneffective (e.g.
owing to decomposition), the attainment of the com-
pound by an alternative process was attempted. Thus,
the transformation of the neutral product in the related
cationic species [Pt(CH3)2(R)L(NꢀN)]BF4 (L=CH3CN
or other), the restoration of the initial halide complex,
and the comparison of the ratios in the starting and
regenerated product were performed. It is to note that
the involvement of cationic species in the isomerization
was claimed previously [9]. Other attempts to attain
variation of the isomeric ratio were performed by halide
exchange or by substitution of the halide with a fourth
s C-bonded group to give [Pt(CH3)2(R)R%(NꢀN)],
which could possibly be reacted in order to remove
selectively a C-ligand and restore a type I compound
(vide infra).
derivative drastically diminishes the cis content to 35%.
As expected, aging of the solution of this mixture at
Table 4
Percentage of cis form in [Pt(CH3)2(R)X(NꢀN)] complexes at equi-
librium in chloroform solution at 25°C
NꢀN
R
X
%
a
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
dmphen
phen
CD3
CH3CH2
I
50
Cl
Br
I
I
Br
I
40 b,c
36 b,c
28 a,b
22 a,d
86 b
CH3CH2CH2
CH2ꢁCH
94 b
CH2ꢁCHCH2
CH3CO
I
5 e
Cl
Br
I
Br
I
Cl
I
I
Br
I
Br
I
I
Cl
I
100 a
100 c
100 c
15 a,b
5 b,c
68 a,d
79 c
C6H5COCH2
COOCH3
NH2COCH2
CH3OCOCH2
30 a
38 a
20 c
CH3COOCH2
20 a,b
30 c
CH3CH2
CH3CO
10 e
phen
phen
phen
phen
72 a
Some representative details concerning the results
summarized in Table 4 are the following ones.
In case RX is CD3I, CH3COOCH2Br, CH3OO-
CCH2Br, or ICH2CONH2 the isomeric ratio of the
products attains a constant equilibrium ratio in a time
ranging from a few hours to about a week.
The reaction of all acyl halides afforded cis products
specifically and the addition products were investigated
with respect to equilibrium. In particular, lowering of
the temperature to 0°C for a solution of the CH3COCl
84 c
C6H5CO
Cl
I
I
77 a
70 c
phen
NH2COCH2
52 a
a Isomerization spontaneous.
b Isomerization after composition change via cationic complex
formation.
c Isomerization after composition change via halide substitution.
d Isomerization via recrystallization or chromatography.
b
c
e Composition unaffected by the procedures under and
.

450
V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
ble trans influence of halides and aromatic nitrogen
ligands.
As for steric effects it must be considered that due
to the well known in-plane substantial hindrance of
dmphen [12] an axial position should be preferred by
the more sterically demanding alkyl group.
Fig. 1.
It appears reasonable that in case the R groups are
unsubstituted hydrocarbyl groups, whose high donor
ability is, although different, fairly comparable, the
sterical influence is the predominating factor in deter-
mining the equilibrium position. Thus, the observed
preference for trans stereochemistry for R higher than
methyl (Table 4) has a rationale.
The presence of substituents Z on R (R=CH2Z) is
expected to disfavor the cis form owing to higher
sterical demand than for the unsubstituted methyl.
Thus, also the observed comparatively high trans con-
room temperature causes restoration of the pure cis
compound, at equilibrium.
The addition product of C6H5COCH2Br in diethyl
ether is 25% cis while the product from acetone is
100% cis. Both products isomerize to equilibrium (ca.
4 days) attaining a 15/85 cis–trans ratio.
By following a type (a) approach a ratio different
from that occurring in the initial product could be
obtained in the case of BrCH2CN, ICH2CN,
BrCH2CH2OH or ICH2CH2OH derivatives. Although
the work-up (recrystallization or chomatography) af-
fords some decomposition it appeared that the new
ratios were constant in solution for at least 48 h. Thus
the earlier ratios are most probably kinetically
controlled.
For instance, a type (b) approach was effectively
adopted for RXꢁCH3CH2I, CH3COOCH2Br, and
C6H5COCH2Br. The products stemming from these
electrophiles after conversion to cationic species can be
recovered with isomeric ratio substantially different
from that of the starting material. More precisely the
cis percentages are formerly 28, 20, and 15, respec-
tively, and turn to 22, 55 and 82. All the three latter
mixtures change composition in a few hours till to
restore the early isomer ratios, which therefore are
equilibrium values. In case the electrophile is allyl io-
dide, the initial product is a mixture with very little cis
isomer (less than 5%) while no cis form was detected
for the benzyl derivative. Conversion to cationic spe-
cies and restoration of the neutral products reproduces
the starting composition in both cases. Thus, it ap-
pears that also in these two cases the former products
are at equilibrium.
tent
for
R=ꢀCH2COOCH3,
ꢀCH2OCOCH3,
ꢀCH2COC6H5 or ꢀCH2CONH2 with respect to the
methyl analogous, might find an explanation on steri-
cal grounds. On the other hand, most of the Z sub-
stituents are electron withdrawing, and an electronic
influence is expected. We note that a calculation based
on the density functional theory (DFT) [13], shows
nearly the same energy for the cis and trans geometry
in the case NꢀN=dmphen, R=CF3 and X=I, while
in the case of NꢀN, the less hindering phen, the axial
position of CF3 appears to be slightly favored. This
result indicates that the substitution in plane in NꢀN
and the ensuing hindrance are not always the predom-
inating factors and electronic effects can play a sub-
stantial role. In fact, the equilibrium percent content
in cis form of the dmphen–iodide derivatives with
RꢁCH3CH2 or CH2CONH2 is, respectively, 28 and 30
and while the corresponding phen derivatives display
10 and 52 cis percent content. Moreover, the cis con-
tent at equilibrium in [Pt(phen)I(CH3)2(CH2-
CH2COOCH3)] was 72% [14]. It is also to note that in
case the rigidity of the chelate is to some degree re-
lieved, i.e. in [Pt(CH3)(CF3)2I(2,2%-bipyridine)] [11],
both the more encumbering and electron withdrawing
CF3 groups are both found in cis position, most prob-
ably at equilibrium, while in [Pt(CH3)2(CH2Cl)Cl(2,2%-
bipyridine)], the cis percentage at equilibrium is 20
[10].
3.3. Sterical and electronic influences
The stereochemistry of all addition products at equi-
librium can be discussed attempting a broad separa-
tion between steric and electronic influence. The latter
ones should be debated taking first of all in account:
(i) the repulse [11] of two alkyl groups of good donor
ability and high trans influence to stay mutually trans
(on this ground the lack of observation of two iso-
mers, i.e. the two ones bearing X in equatorial posi-
tion, is easily rationalized); (ii) the comparatively
better balance for trans halide/alkyl or nitrogen/alkyl
arrangements, for which however an a priori distinc-
tion seems difficult, on consideration of the compara-
As for the preference, noted above, of substituted
R, it must be allowed that an homogeneous compari-
son should involve only R groups with equally hy-
bridized carbons linked to the metal. Acyl halides
afforded stereospecifically cis products, which at least
in the three investigated cases proved to be at equi-
librium. Table 4 shows that the cis geometry is specifi-
cally adopted also in the other two cases involving sp2
carbon ligands, i.e. CHꢁCH2 and COOCH3. This can
be explained considering that the Csp2 linked groups
can present, in suitable conformation, a lower sterical

V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
451
Table 5
Selected 1H-NMR data for [Pt(CH3)2(R)(dmphen)(L)]BF4 complexes a and isomeric composition
R
L
PtꢀR
L
NꢀCH3
PtꢀCH3 eq. PtꢀCH3 ax.
%
b
b
CH3
CH3
CH3
CH3CN
C6H5CH2CN
C6H5NH2
2.15(s)
4.13(s, CH2)
5.88 and 4.60(2b, 3.03(s)
NH2)
3.15(s)
3.04(s)
1.41(s, 70)
1.38(s, 71)
1.51(s, 69)
0.22(s, 75)
0.28(s, 75)
1.40(s, 75)
CH3
CH2CHCH2NH2
5.60(m, ꢁCH),
4.85(dd, ꢁCH2)
2.90(bm, CH2)
3.12(s)
1.37(s, 70)
0.60(s, 75)
b
CH3
CH3
(CH3)3CCN
(CH3)2C6H5P
1.36(s, 9H)
3.15(s)
3.05(s)
1.36(s,70)
0.37(s,74)
3
1.09(s, JPꢀH
=
1.52(d c, 70) 1.38(d d, 57)
1.60(d c, 70) 1.60(d e, 62)
9.6 Hz)
CH3
CH3
CH3
(C6H5)3P
(CH3)2S
(C6H5)3As
7.3(m), 6.70(t)
2.06(s, 12)
7.40(t), 7.25(t),
6.70(d)
2.40(s)
2.40(s)
5.85 and
4.48(2b, NH2)
5.85 and
4.48(2b, NH2)
1.93(s, 10)
2.04(s, 11)
2.35(s)
3.05(s)
3.15(s)
3.05(s)
1.41(s, 70)
1.66(s, 71)
0.79(s, 68)
1.70(s, 68)
b
b
CH3CH2
CH3CH2
CH3CH2
CH3CN f
CH3CN i
C6H5NH2
g, 0.25(bt, h)
3.10(s)
1.32(s,70)
50
50
80
h
g, 1.20(t,
)
3.18(s), 3.10(s) 1.32(s, 70)
3.09(s) 1.51(s, 70)
0.18(s, 75)
1.32(s, 74)
0.51(s, 69)
f
2.32(q, 68), 0.55(t, 57)
i
CH3CH2
C6H5NH2
g, 0.95(t, 60)
3.18(s), 3.02(s) 1.52(s, 67)
20
CH3CH2
CH3CH2
CH2ꢁCHCH2
(CH3)2S f
(CH3)2S i
CH3CN f
1.62(q, h), 0.12(t, 60)
3.13(s)
1.32(s, 70)
60
40
100
b
b
h
g, 1.05(t,
)
3.22(s), 3.10(s) 1.32(s, 70)
5.1(bm, 1H), 4.9(bm, 1H),
4.1 (bm, 1H), 3.8(bm, 1H),
1.7(bm, 1H)
5.5(m, 1H), 4.74(d, 1H), 4.38
(d, 1H), 2.95(d, 90, 2H)
6.65(m, CH), 5.45(d, 130,
ꢁCH2)
4.58 (d, 81, ꢁCH2)
5.94(m, CH), 4.80(d, 135,
ꢁCH2),
4.18(d, 68, ꢁCH2),
2.50(b, CH2)
3.20(s)
3.08(s)
1.45(s, 74)
1.58(s, 70)
f
CH2ꢁCHCH2
CH2ꢁCH b
C6H5NH2
100
75
CH3CN i
CH3CN f
CH3CN f
2.25(s)
2.20(s)
3.08(s), 3.00(s) 1.40(s, 71)
0.50 (s, 74)
CH2ꢁCH b
3.05(s)
1.49(s, 70)
25
C6H5CH2
C6H5CH2
(CH3)2CH
2.29(s)
5.88 (b,1NH)
2.25(s)
3.11(s)
2.84(s)
3.17(s)
1.62(s, 69)
1.68(s, 73)
1.48(s, 72)
100
100
100
f
C6H5NH2
3.32(s, 88, CH2)
2.25(bs, 1H), −0.32(d, 55,
6H)
CH3CN f
f
(CH3)2CH
C6H5NH2
2.5(bm, 1H), 0.35(d, 54, 3H) 4.75(b, 1NH)
3.05(s)
1.53(s, 71)
100
h
j
C6H5CO
CH3CO
CH3COOCH2
C6H5COCH2
C6H5COCH2
CH3CN
CH3CN
CH3CN i
CH3CN f
CH3CN i
7.53(m, 5H)
2.53(b, 3H)
6.20(d, 71), 5.08(d), 2.11(s)
2.70(s, 109, CH2)
4.18(d, 107, 1H), 3.35(d, 1H) 2.37(s)
2.53(s)
2.53(b)
2.04(s)
2.37(s)
2.96(s)
2.93(s)
1.20(b, )
1.43(s, 77) j
1.02(s, 77)
b
3.08(s), 3.04(s) 1.46(s, 70)
3.03 (s) 1.67(s, 72)
3.08(s), 3.03(s) 1.48(s, 72)
100
65
35
b
b
0.52(s, 67)
a Spectra recorded in CD3NO2 solution (reference l 4.33, CHD2NO2), the 195Pt coupling constants (Hz) are reported in parentheses;
abbreviations s (singlet), d (doublet), dd (double doublet), t (triplet), q (quartet), m (multiplet), b (broad). The chemical shifts of the
heteroaromatic 2,9-dimetyl-1,10-phenanthroline protons are in the range: l 8.5–8.3 (d, 2H), 7.9–7.75 (d, 2H), 7.8–7.7 (s, 2H).
b CDCl3 solution (reference l 7.26, CHCl3).
c 3JPꢀH=9 Hz.
d 3JPꢀH=5 Hz.
e 3JPꢀH=8 Hz.
f Trans isomer.
g CH2 resonance obscured by other signals.
h 195Pt coupling constant not evaluable.
i Cis isomer.
j Fast exchange between axial and equatorial position.

452
V. De Felice et al. / Journal of Organometallic Chemistry 593–594 (2000) 445–453
demand than the Csp3 bonded ligands. It is noted
that the above cited DFT calculations [13], show a
relevant energy difference (ca. 36 kJ mol−1) in the
case of [Pt(CH3)2(COCH3)I(dmphen)] in favor of the
cis form, while in the case of the analogous phen
the stereochemical control of the isolated compounds
but we note the necessary trans arrangement of two
alkyls in these products should be more favored (i) in
case one of the two alkyls bears electron withdrawing
substituents and the other is a good donor. The ob-
served specificity of trans stereochemistry in case
RꢁCH3, R%ꢁCH2NO2 or CH(COOCH3)2 is in agree-
ment with this observation. It is to note the same
preference holds true in case RꢁC2H5 and
R%ꢁCH(COOCH3)2.
compound the difference drops to ca. 19 kJ mol−1
.
We find that in [Pt(CH3)2(COCH3)I(dmphen)] the
only observed isomer is cis, while the cis–trans ratio
in the analogous phen derivative is 84/16.
3.4. Cationic complexes
Cationic Pt(IV) complexes of the type [Pt(CH3)2-
(R)L(NꢀN)]+ are a fairly recent accomplishment of
platinum chemistry. Previous results [9] disclosed their
relationship with five-coordinate species, which are fa-
vored in the case of the anion and L lack of coordi-
nating ability, and their relevance to isomerization
and reductive elimination. The attainment of cations
from the above general formula was attempted during
this work aiming, firstly to obtain by successive halide
treatment, changes of the isomer ratio of the starting
neutral compound. The fair general stability displayed
in solution by most of the cations, in keeping with
the rigidity of the chelate, prompted us to attempt in
some representative examples, the isolation of the
cationic complexes themselves. We could isolate as
crystalline solids the methyl cyanide derivatives with
RꢁCH3, CH2CH3 or COCH3. By 1H-NMR spec-
troscopy (Table 5), it is inferred an equilibrium be-
tween coordinated and free CH3CN for the complexes
at 30°C in chloroform solution, as reported for simi-
lar cations [9]. Thus, the geometrical isomers of these
cations are to be considered at equilibrium, and the
cis–trans ratios are also reported in Table 5. The
stereochemical behavior appears comparable with that
of the neutral complexes.
4. Conclusions
The geometrical isomerism of [Pt(CH3)2(R)X(NꢀN)]
complexes has been investigated on about 25 equi-
librium systems. The influence of the sterical hin-
drance in plane of the chelate appears quite evident,
although the steric pressure relief does not appear to
be the prevailing effect in all cases. Also a clear elec-
tronic influence of the NꢀN substituents and a very
moderate influence of the halide, whose nature in-
duces a fine tuning of the relative stability of the
isomers, can be inferred from the observed isomer
ratios. A potentially useful procedure for the attain-
ment of compounds [Pt(CH3)2(R)(R%)(NꢀN)] contain-
ing three different types of C-bonded groups has been
a synthetic achievement of this work.
Acknowledgements
We thank the Consiglio Nazionale delle Ricerche
and the MURST (Programmi di Ricerca Scientifica di
Rilevante Interesse Nazionale, Cofinanziamento
1998–1999) for financial support, and the Centro In-
terdipartimentale di Metodologie Chimico-fisiche,
Universita` di Napoli ‘Federico II’ for NMR facilities.
We are also indebted with Dr L. Cavallo for helpful
calculations and discussion.
3.5. [Pt(CH3)2(R)R%(NꢀN)] deri6ati6es
During this work N,N-chelate Pt(IV) mononuclear
derivatives containing four s C-bonded ligands were
obtained. But for the previous report of tetramethyl
(R=R%=Me) compounds [15], these [Pt(CH3)2-
(R)R%(NꢀN)] compounds are unprecedented. They
could be prepared by the reaction of the above de-
scribed cationic compounds with potassium salts of
stabilized carbanions (KCH2NO2 or KCH(COO-
CH3)2). The synthesis of the novel compounds was
suggested by the chance that they could be precursors
of new trialkyl systems poorly attainable by another
way, or of known trialkyls with unprecedented isomer
composition. This seemed feasible by halogen treat-
ment but since a quite preliminary result was not
encouraging no further investigation on this point
was attempted. No investigation was also made on
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