31P NMR and DFT studies on square-planar bis(diphenylphosphinoethyl)phenylphosphine (triphos) complexes of Pt(II) with pyridines and anilines

Article abstract of DOI:10.1016/j.poly.2005.12.011

Cationic complexes of the type [Pt(L)(triphos)](ClO₄)₂, where L is a 4-substituted pyridine {L = 4-R-py = 4-cyanopyridine (4-CN-py), isonicotinic acid (4-COOH-py), methylisonicotinate (4-COOMe-py), 4-acetylpyridine (4-MeCO-py), pyridine (4-H-py), 4-methylpyridine (4-Me-py), 4-aminopyridine (4-NH₂-py)} or a 4-substituted aniline {L = 4-R-an = 4-nitroaniline (4-NO₂-an), 4-cyanoaniline (4-CN-an), 4-chloroaniline (4-Cl-an), aniline (4-H-an), 4-methylaniline (4-Me-an), 4-methoxyaniline (4-MeO-an)}, have been prepared from cis/trans-PtCl₂(SMe₂)₂. The cis and trans ¹J_Pt-P coupling constants measured in CD₃NO₂ were found to be dependent upon the pK_a of L. For the series of the para-substituted pyridines 4-R-py the linear correlations ¹J_Pt-P1 = 2918 - 16.9 pK_a and ¹J_Pt-P2 = 2371 + 5.8 pK_a (P₁ = phosphorus nucleus trans to N, P₂ = phosphorus nuclei cis to N) were found. The complexes with L = para-substituted anilines 4-R-an show a different behaviour. The results are discussed on the basis of DFT calculations.

Full text of DOI:10.1016/j.poly.2005.12.011

Polyhedron 25 (2006) 1979–1984
www.elsevier.com/locate/poly
31P NMR and DFT studies on
square-planar bis(diphenylphosphinoethyl)phenylphosphine
(triphos) complexes of Pt(II) with pyridines and anilines
*
Marco Bortoluzzi , Giuliano Annibale, Giampaolo Marangoni, Gino Paolucci, Bruno Pitteri
`
Dipartimento di Chimica, Universita Ca’ Foscari di Venezia, Calle Larga S. Marta Dorsoduro 2137, 30123 Venezia, Italy
Received 22 November 2005; accepted 16 December 2005
Available online 27 January 2006
Abstract
Cationic complexes of the type [Pt(L)(triphos)](ClO4)2, where L is a 4-substituted pyridine {L = 4-R-py = 4-cyanopyridine
(4-CN-py), isonicotinic acid (4-COOH-py), methylisonicotinate (4-COOMe-py), 4-acetylpyridine (4-MeCO-py), pyridine (4-H-py),
4-methylpyridine (4-Me-py), 4-aminopyridine (4-NH2-py)} or a 4-substituted aniline {L = 4-R-an = 4-nitroaniline (4-NO2-an), 4-cya-
noaniline (4-CN-an), 4-chloroaniline (4-Cl-an), aniline (4-H-an), 4-methylaniline (4-Me-an), 4-methoxyaniline (4-MeO-an)}, have been
1
prepared from cis/trans-PtCl2(SMe2)2. The cis and trans JPt–P coupling constants measured in CD3NO2 were found to be dependent
1
upon the pKa of L. For the series of the para-substituted pyridines 4-R-py the linear correlations JPt–P1 = 2918 ꢀ 16.9 pKa and
1JPt–P2 = 2371 + 5.8 pKa (P1 = phosphorus nucleus trans to N, P2 = phosphorus nuclei cis to N) were found. The complexes with
L = para-substituted anilines 4-R-an show a different behaviour. The results are discussed on the basis of DFT calculations.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Platinum; Triphos; DFT; Heteronuclear NMR; Pyridines; Anilines
1. Introduction
The ligand triphos {triphos = bis(diphenylphosphino-
ethyl)phenylphosphine} is really interesting for hetero-
nuclear NMR studies, because once co-ordinated to
platinum it allows to evaluate at the same time both the
The ability of a transition-metal ion to transmit elec-
tronic effects among ligands has been studied for a long
time in connection with both ground-state and kinetic
properties of co-ordination compounds. Since the initial
development of heteronuclear NMR spectroscopy, a great
number of papers correlating the NMR data and the prop-
erties of co-ordination and organometallic complexes has
appeared in the literature. Mainly because of the favour-
able presence of both platinum and phosphorus NMR-
active isotopes, most of these studies has been concerned
with platinum-phosphine complexes [1] and, since the
1JPt–P coupling constant is an easily determinable data, cor-
relations between this coupling constant and some proper-
ties of the co-ordinate ligands have often been made.
1
trans and the cis JPt–P coupling constants. Here, the prep-
aration of a series of cationic complexes of the type
[Pt(L)(triphos)](ClO4)2 is reported, where the ligands L
are nitrogen-donors, namely 4-substituted pyridines
{4-R-py = 4-cyanopyridine (4-CN-py), isonicotinic acid
(4-COOH-py), methylisonicotinate (4-COOMe-py), 4-acetyl-
pyridine (4-MeCO-py), pyridine (4-H-py), 4-methylpyri-
dine (4-Me-py), 4-aminopyridine (4-NH2-py)} and
4-substituted anilines {4-R-an = 4-nitroaniline (4-NO2-an),
4-cyanoaniline (4-CN-an), 4-chloroaniline (4-Cl-an), ani-
line (4-H-an), 4-methylaniline (4-Me-an), 4-methoxyaniline
(4-MeO-an)}. The results of a 31P NMR study about the
influence of the donor capability of L, as measured by its
pKa, on the trans and cis 1JPt–P coupling constants, are dis-
cussed. DFT calculations were also performed to better
*
Corresponding author. Tel.: +39 0412348558; fax: +39 0412348517.
1
give insight into the observed variations of JPt–P with
E-mail address: markos@unive.it (M. Bortoluzzi).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.12.011

1980
M. Bortoluzzi et al. / Polyhedron 25 (2006) 1979–1984
pKa caused by the changes of the electronic and steric prop-
erties of the co-ordinated ligands L.
separated out of the solution was filtered off, washed with
methanol and diethylether and dried under vacuum (yield
>90%).
2. Experimental
2.4. [Pt(4-R-py)(triphos)](ClO4)2 (1R)
{R = CN, COOH, COOMe, MeCO, H, Me, NH2}
[Pt(4-R-an)(triphos)](ClO4)2 (2R)
All reagents were Aldrich or Fluka products in the high-
est available purity and were used without any further
treatment. Solvents were dried using standard techniques.
{R = NO2, CN, Cl, H, Me, MeO}
2.1. Physical measurements
The complexes [Pt(4-R-py)(triphos)](ClO4)2 (1R) and
[Pt(4-R-an)(triphos)](ClO4)2 (2R) have been prepared by
adding the stoichiometric amount of solid AgClO4 to a
solution of [PtCl(triphos)](ClO4) in nitromethane. In a typ-
ical preparation, 0.021 g (0.1 mmol) of AgClO4 were added
to a 5 mL solution of [PtCl(triphos)](ClO4) (0.086 g,
0.1 mmol) in CH3NO2. After 12 h under stirring the AgCl
formed was filtered off and a slight excess (1.5:1) of the
appropriate pyridine or aniline was added. The resulting
solution was heated to about 50 ꢁC for 2 h and then filtered
if necessary. By slow addition of diethylether the product
which separated out was filtered, washed twice with dieth-
ylether and dried under vacuum (yield >70% in all cases).1
Anal. Calc. for C40H37Cl2N2O8P3Pt (1CN): C, 46.52; H,
3.61; N, 2.71. Found: C, 46.61; H, 3.62; N, 2.73%. Anal.
Calc. for C40H38Cl2NO10P3Pt (1COOH): C, 45.68; H, 3.64;
N, 1.33. Found: C, 45.59; H, 3.65; N, 1.32%. Anal. Calc.
for C41H40Cl2NO10P3Pt (1COOMe): C, 46.21; H, 3.78; N,
1.31. Found: C, 46.09; H, 3.80; N, 1.29%. Anal. Calc. for
C41H40Cl2NO9P3Pt (1MeCO): C, 46.91; H, 3.84; N, 1.33.
Found: C, 46.81; H, 3.82; N, 1.34%. Anal. Calc. for
C39H38Cl2NO8P3Pt (1H): C, 46.49; H, 3.80; N, 1.39.
Found: C, 46.36; H, 3.78; N, 1.40%. Anal. Calc. for
C40H40Cl2NO8P3Pt (1Me): C, 47.02; H, 3.95; N, 1.37.
Found: C, 47.14; H, 3.97; N, 1.39%. Anal. Calc. for
Infrared spectra (4000–400 cmꢀ1, KBr disks) were
recorded on a Perkin–Elmer Spectrum One spectro-
photometer. NMR spectra (1H, 31P{1H}, 31P{1H} with
inverse-gated decoupling) were obtained either on AC
200 or AVANCE 300 Bruker spectrometers in CD3NO2
at temperatures comprised between 298 and 243 K. 1H
NMR spectra are referred to internal tetramethylsilane,
while 31P NMR chemical shifts are reported with respect
to 85% H3PO4. SwaN-MR and MestRe-C software pack-
ages were used to treat the NMR data [2]. The conductivity
of 1 · 10ꢀ3 mol dmꢀ3 solutions of the new complexes in
DMF at 25ꢁ was measured with a Radiometer CDM 83
instrument. Elemental analyses (C, H, N) were performed
by the microanalytical laboratory at the Department of
Pharmaceutical Sciences, University of Padua.
2.2. Computational details
Computational geometry optimisation of the complexes
was performed using the restricted DFT B3PW91 method
with the CEP-121G basis set (CEP-31G on the Pt atom).
Geometry convergence was accelerated using the GDIIS
algorithm and a first refinement of the structures was
obtained using the restricted semi-empirical PM3 method.
Geometry optimisation of the free nitrogen-donor ligands
was obtained with restricted DFT B3LYP/6-311+G** cal-
culations [3]. All calculations were carried out with com-
C39H39Cl2N2O8P3Pt ð1NH Þ: C, 45.80; H, 3.84; N, 2.74.
2
Found: C, 45.93; H, 3.85; N, 2.72%. Anal. Calc. for
C40H39Cl2N2O10P3Pt ð2NO Þ: C, 45.04; H, 3.69; N, 2.63.
2
Found: C, 45.11; H, 3.70; N, 2.65%. Anal. Calc. for
C41H39Cl2N2O8P3Pt (2CN): C, 47.05; H, 3.76; N, 2.68.
Found: C, 47.13; H, 3.74; N, 2.70%. Anal. Calc. for
C40H39Cl3NO8P3Pt (2Cl): C, 45.49; H, 3.72; N, 1.33.
Found: C, 45.60; H, 3.70; N, 1.34%. Anal. Calc. for
C40H40Cl2NO8P3Pt (2H): C, 47.02; H, 3.95; N, 1.37.
Found: C, 46.90; H, 3.93; N, 1.36%. Anal. Calc. for
C41H42Cl2NO8P3Pt (2Me): C, 47.55; H, 4.09; N, 1.35.
Found: C, 47.49; H, 4.11; N, 1.36%. Anal. Calc. for
C41H42Cl2NO9P3Pt (2OMe): C, 46.82; H, 4.03; N, 1.33.
Found: C, 46.90; H, 4.01; N, 1.34.
puters equipped with Intel Pentium
4 Northwood
processors operating at 2.6 GHz frequency or IBM G4
operating at 1.25 GHz. Software used were GAUSSIAN-98
and SPARTAN-02 [4].
2.3. Preparation of the complexes
The [PtCl(triphos)]+ species has already been reported
as the chloride salt [5], but we preferred to isolate it as
the perchlorate salt following the procedure described
below [6].
All the 1R and 2R complexes show the signals due to the
1
To
PtCl2(SMe2)2 [7] (0.390 g, 1.0 mmol) in methanol (25 mL)
stoichiometric amount of solid triphos (0.534 g,
a
warm (ca. 50 ꢁC) solution of cis/trans-
triphos and the pyridine or the aniline ligands in the H
NMR and IR spectra. Some characteristic IR and 1H
NMR data are reported below:
a
1.0 mmol) was added in successive small portions. After
10 min the reaction mixture was allowed to cool to room
temperature and a solution of an excess of LiClO4
(0.202 g, 1.5 mmol) in methanol (10 mL) was slowly added.
The white solid complex [PtCl(triphos)](ClO4) which
1
Safety note: perchlorate salts of metal complexes with organic ligands
are potentially explosive; however, all the prepared compounds, in the
experimental conditions described, appeared to be stable both in solution
and in the solid state.

M. Bortoluzzi et al. / Polyhedron 25 (2006) 1979–1984
1981
Table 1
31P {1H} NMR data for complexes 1–2 and pKa values of the 4-substituted pyridines and anilines
Complex
31P {1H} NMR d (J/Hz)
pKa
1
1
1CN
[Pt(4-CN-py)(triphos)]2+
[Pt(4-COOH-py)(triphos)]2+
[Pt(4-COOMe-py)(triphos)]2+
[Pt(4-MeCO-py)(triphos)]2+
[Pt(4-H-py)(triphos)]2+
82.5 (s, JPt–P1 = 2895, P1); 49.3 (s, JPt–P2 = 2382, P2)
1.90
3.26
3.26
3.51
5.17
6.03
9.11
1.00
1.74
3.98
4.60
5.08
5.34
1
1
1COOH
1COOMe
1MeCO
1H
82.7 (s, JPt–P1 = 2861, P1); 48.8 (s, JPt–P2 = 2390, P2)
1
1
82.8 (s, JPt–P1 = 2861, P1); 48.8 (s, JPt–P2 = 2390, P2)
1
1
82.6 (s, JPt–P1 = 2855, P1); 48.8 (s, JPt–P2 = 2392, P2)
1
1
82.4 (s, JPt–P1 = 2827, P1); 48.6 (s, JPt–P2 = 2404, P2)
1
1
1Me
1NH2
[Pt(4-Me-py)(triphos)]2+
[Pt(4-NH2-py)(triphos)]2+
[Pt(4-NO2-an)(triphos)]2+
[Pt(4-CN-an)(triphos)]2+
[Pt(4-Cl-an)(triphos)]2+
[Pt(4-H-an)(triphos)]2+
82.8 (s, JPt–P1 = 2815, P1); 47.9 (s, JPt–P2 = 2406, P2)
1
1
83.0 (s, JPt–P1 = 2768, P1); 46.7 (s, JPt–P2 = 2424, P2)
1
1
2NO
85.0 (s, JPt–P1 = 3292, P1); 48.4 (s, JPt–P2 = 2364, P2)
2
1
1
2CN
2Cl
85.0 (s, JPt–P1 = 3255, P1); 48.4 (s, JPt–P2 = 2520, P2)
1
1
85.0 (s, JPt–P1 = 2949, P1); 48.1 (s, JPt–P2 = 2461, P2)
1
1
2H
2Me
80.8 (s, JPt–P1 = 2806, P1); 48.2 (s, JPt–P2 = 2447, P2)
1
1
[Pt(4-Me-an)(triphos)]2+
[Pt(4-MeO-an)(triphos)]2+
84.8 (s, JPt–P1 = 2918, P1); 47.8 (s, JPt–P2 = 2469, P2)
1
1
2MeO
80.4 (s, JPt–P1 = 2780, P1); 47.9 (s, JPt–P2 = 2457, P2)
IR (cmꢀ1): 1CN 2237 w (mCN); 1COOH 1733 s (mCO);
ered has been already investigated and discussed in kinetic
studies [10]. It is also known [11] that there is a linear rela-
tionship between the pKa values and the proton affinity, PA
(i.e., the enthalpy of a proton breaking-off in the gaseous
phase) for the series of the considered pyridines and ani-
lines, so the medium contribution to the pKa determination
is almost constant for both the types of nitrogen-donor
ligands.
1COOMe 1730 s (mCO); 1MeCO 1694 s (mCO); 1NH 3466 m,
2
3363 m (mNH); 2CN 2251 s, 2210 sh (mCN).
1H NMR (ppm, 298 K): 1COOMe 3.93 (s, 3H, CH3);
1MeCO 2.56 (s, 3H, CH3); 1Me 2.36 (s, 3H, CH3); 1NH (s,
2
br, 2H, NH2); 2Me 2.19 (s, 3H, CH3); 2OMe 3.93 (s, 3H,
CH3).
1
31P NMR chemical shifts and JPt–P coupling constants
are collected in Table 1. All the KM values measured are
comprised between 140 and 150 Xꢀ1 molꢀ1 cm2, as
expected for 2:1 electrolytes in DMF [8].
3.2. NMR and DFT studies
The 31P {1H} NMR spectra of all the 1R and 2R com-
plexes consist of two sharp peaks, each with 195Pt satellites.
The JP1–P2 phosphorus–phosphorus couplings are unob-
servable for all the prepared complexes, but this is expected
for square-planar triphos derivatives with all the phospho-
rus atoms co-ordinated to the same metal centre [9,12]. The
31P NMR data of the 1R and 2R derivatives are reported in
Table 1, together with the pKa values [13] of the para-
substituted pyridines and anilines.
3. Results and discussion
3.1. Preparation of the complexes
The synthetic routes used for the preparation of
the
complexes
[PtCl(triphos)](ClO4),
[Pt(4-R-py)-
(triphos)](ClO4)2 (1R) and [Pt(4-R-an)(triphos)](ClO4)2
(2R) are outlined in Scheme 1 and the details are given in
Section 2, so here only few comments are made. Although
the isolation of the chloro-complex [PtCl(triphos)]+ as
chloride salt has already been reported [5], we have used
an alternative method starting from the isomeric mixture
of PtCl2(SMe2)2, which we found very convenient for the
synthesis of a wide variety of Pt(II) chloro-complexes with
polydentate ligands [6]. Treatment of [PtCl(triphos)](ClO4)
with silver perchlorate followed by the addition of the
appropriate nitrogen-donor ligand afforded the desired 1R
or 2R complex in pure form and in good yield. All the
characterization data (IR, 1H NMR, 31P NMR, conductiv-
ity, elemental analyses) are in good agreement with the pur-
posed formulations. Spectroscopic data of the 1H
derivative have been also compared with those of the
already known [Pt(pyridine)(triphos)](CF3SO3)2 complex
[9].
There in no apparent regularity in the variation of the
31P chemical shifts, while there are interesting trends for
the coupling constants. For the para-substituted pyridines,
a variation of pKa in the range 1.90–9.11 leads to a linear
1
variation of JPt–P1 (P1 = phosphorus nucleus trans to N)
1
from 2895 to 2768 Hz. The linear relationship JPt–P1
=
2918 4 ꢀ 16.9 0.9 pKa is followed, as shown in
Fig. 1A. There is also a linear rise (42 Hz within the consid-
1
ered pKa range) of the JPt–P2 couplings (P2 = phosphorus
nuclei cis to N) while the pKa value increases (see Fig. 1B);
1
the equation is JPt–P2 = 2371 1 + 5.8 0.2 pKa. It is to
be noted that the derivatives of 4-COOH-py and
4-COOMe-py, both having the same pKa (3.26), give
exactly the same trans and cis JPt–P coupling constants.
1
1
1
The linear relationship between JPt–P1 and JPt–P2 cou-
1
pling constants is given by the equation JPt–P1
=
The pyridines and anilines, all para-substituted, were
chosen to cover a wide range of basicity (1.90 6 pKa pyri-
dines 6 9.11, 1.00 6 pKa anilines 6 5.34). Moreover, the
relationship between the basicity and the nucleophilic
behaviour of some of the nitrogen-donor ligands consid-
9700 300 ꢀ 2.9 0.1 1JPt–P2. The variation of donor
1
capability of the pyridines affects mostly the trans JPt–P
coupling constant, also considering that there are two
cis phosphorus atoms and only one trans phosphorus
atom.

1982
M. Bortoluzzi et al. / Polyhedron 25 (2006) 1979–1984
Scheme 1. Synthesis of [PtCl(triphos)]+, [Pt(4-R-py)(triphos)]2+ (1R) and [Pt(4-R-an)(triphos)]2+ (2R) complexes.
1
1
Fig. 1. Plot of JPt–P1 against pKa (A) and of JPt–P2 against pKa (B) for 1R complexes.
The use of 4-substituted pyridines allows to exclude
1JPt–P is a2Pt so, following relation (1), the decrease of the
trans JPt–P1 coupling constant with the increase of
1
steric effects due to the different R groups, so that the linear
1
relationships between the pKa and the JPt–P values can be
the pKa represents primarily the change in 6s character of
the hybrid orbital of platinum used in the bond with P1.
The increase of donor capability of the co-ordinated pyri-
dine increase the Pt 6s character of the Pt–N bond at the
attributed only to electronic effects. It is generally assumed
1
[1] that the magnitude of the JPt–P coupling constant is
dominated by the Fermi contact term, expressed by the fol-
lowing relation:
1
expense of the trans Pt–P bond. The JPt–P2 trend suggests
ꢀ
ꢀ ꢀ
2
ꢀ
ꢀ
ꢀ
2
that the weakening of the Pt–P1 bond strengthens the
bonds between the metal centre and the other two phos-
phorus atoms in the cis positions.
ꢀ
ꢀ ꢀ
c c a2 a2 w
ð0Þ
DE
w
Pð3sÞ
ð0Þ
ꢀ
ꢀ ꢀ
Pt
P
Ptð6sÞ
Pt
P
1JPt–P
/
.
ð1Þ
1
Mather et al. have reported a correlation of the JPt–P
In the relation, cPt and cP are the gyromagnetic ratios for
the 195Pt and P isotopes, aP2t and a2P represent the s-char-
31
coupling constants with the Pt–P bond lengths in some tri-
alkyl phosphine complexes of Pt(II) and Pt(IV) [14] and
subsequent studies for Pt(0) complexes [15] have shown
similar results, in which the shorter Pt–P distances are asso-
ciated with larger values of 1JPt–P. We tried to correlate the
1JPt–P coupling constants of the 1R complexes with the
acters of the localized hybrid orbitals in a valence bond
description, |wPt(6s)(0)|2 and |wP(3s)(0)|2 are the electron den-
sities of the s orbitals at nucleus, DE is an average singlet–
triplet excitation energy. It is also assumed that the most
important term for the variation of the magnitude of

M. Bortoluzzi et al. / Polyhedron 25 (2006) 1979–1984
1983
bond lengths by optimising with DFT calculations the
geometries of the derivatives of the pyridines with the
greatest pKa difference, the [Pt(4-CN-py)(triphos)]2+ (1CN
)
and [Pt(4-NH2-py)(triphos)]2+ ð1NH Þ cations. The differ-
ence in nucleophilicity between the two considered pyri-
dines is well expressed by both the calculated dipolar
moment l (whose vector is oriented from the R group
towards the pyridine-N atom) and the Mulliken charge
on the pyridine-N atoms for the free species (see Table
2). Also the electrostatic potential surfaces on the pyri-
dine-N atoms shown in Fig. 2 allow to understand the dif-
ferent nucleophilicity of the two pyridines.
2
Fig. 2. Potential electrostatic surfaces on the pyridine-N atom for the
4-CN-py and 4-NH2-py ligands.
The calculated rPt–P1, rPt–P2 and rPt–N bond lengths for
the 1CN and 1NH complexes are reported in Table 2; unfor-
2
tunately, the differences between the corresponding bond
lengths of the two derivatives are too small to be really sig-
nificant, probably because of the relatively small difference
1
of the JPt–P couplings of the two complexes.
Also in the case of the complexes with the para-substi-
tuted anilines 2R a relationship between the pKa values of
1
the anilines and the JPt–P1 coupling constants of the 2R
derivatives is observed (see Fig. 3), but this correlation is
less linear in comparison with the one of the 1R derivatives.
Moreover, for the 2R complexes there is an irregular trend
1
of the JPt–P2 values against pKa.
A possible explanation for the different behaviour of the
2R derivatives compared to 1R complexes is that steric
effects due to the change of the 4-R group of the anilines
cannot be completely excluded, as done in the case of the
4-substituted pyridines. In fact, by rotating the P2–Pt–N–
H
dihedral angle in the optimised models of the
[Pt(4-R-an)(triphos)]2+ cations, a steric interaction between
the R group and the phenyl rings bonded to the P2 atoms
can be observed. If the steric hindrance between the phenyl
rings bonded to the P2 atoms and the aniline blocked the
rotation around the Pt–N bond, two different rotamers at
an energy minimum would be possible, with the phenyl
ring bonded to the P1 atom and the aryl group of the ani-
line in Z (sin rotamer) or E (anti rotamer) position, as
shown in Fig. 4. The calculated energy difference between
the two rotamers is quite small, about 5 kcal/mol.
1
Fig. 3. Plot of JPt–P1 against pKa for 2R complexes.
As, on the contrary, the NMR spectra of the 2R deriva-
tives show the signals of only one species in a temperature
range between 298 and 243 K, there should be free rotation
around the Pt–N bond. This means that the geometry in
solution of the 2R derivatives in not affected only by the
nucleophilicity of the aniline ligand, as observed for the
1R complexes, but also by the different bulk of the 4-R
group. The sum of the electronic and the steric effects on
the geometry of the [Pt(4-R-an)(triphos)]2+ cations
Fig. 4. Rotamers of the [Pt(4-R-an)(triphos)]2+ cations.
Table 2
Calculated rPt–P and rPt–N bond lengths for complexes 1CN and 1NH , dipolar moment and Mulliken charge on the pyridine-N atom for the free ligands
2
˚
˚
˚
Complex
rPt–P1 (A)
rPt–P2 (A)
rPt–N (A)
Pyridine
l (Debye)
N charge
1CN
[Pt(4-CN-py)(triphos)]2+
[Pt(4-NH2-py)(triphos)]2+
2.3258
2.3270
2.4083
2.4055
2.1070
2.1023
4-CN-py
4-NH2-py
2.00
4.08
ꢀ0.060
ꢀ0.112
1NH
2

1984
M. Bortoluzzi et al. / Polyhedron 25 (2006) 1979–1984
[4] (a) M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
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package;
explains the less linear correlation between the pKa and the
1JPt–P1 values of these derivatives and the irregular trend of
the JPt–P2 values.
1
4. Conclusions
In conclusion, the 31P NMR spectroscopy of derivatives
of the Pt(II)–triphos fragment allowed us to observe small
changes of the electronic and steric properties of the co-
1
ordinated ligands by measuring the JPt–P coupling con-
stants. Further investigations will be carried out in order
to ascertain whether this approach could be an effective
way for investigating and comparing other series of mole-
cules with co-ordinating ability.
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Acknowledgement
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