Cis Influence in trans-Pt(PPh3)2 complexes

Article abstract of DOI:10.1039/b813125b

The cis influence of a series of anionic ligands X and Y has been evaluated through the magnitude of the Pt-P coupling constants for compounds of formula trans-[PtXY(PPh₃)₂]. The order of decreasing cis influence was found to be I > Cl > SePh ～ SPh ～ SEt > NO₃ > AcO ～ NO₂ > H > Me > Ph > mtc (mtc = N,N-dimethylmonothiocarbamato-S); moreover, the cis influences of the various ligands was found to be additive. The X-ray structures of three representative compounds (t-4: X = Cl, Y = NO₃; t-5: X = Cl, Y = AcO and t-12: X = Y = NO₂) have also been determined. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b813125b

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cis Influence in trans-Pt(PPh3)2 complexes†‡
Luca Rigamonti,a Carlo Manassero,b Michele Rusconi,a Mario Manassero*b and Alessandro Pasini*a
Received 29th July 2008, Accepted 6th October 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b813125b
The cis influence of a series of anionic ligands X and Y has been evaluated through the magnitude of
the Pt–P coupling constants for compounds of formula trans-[PtXY(PPh3)2]. The order of decreasing
cis influence was found to be I > Cl > SePh ~ SPh ~ SEt > NO3 > AcO ~ NO2 > H > Me > Ph > mtc
(mtc = N,N-dimethylmonothiocarbamato-S); moreover, the cis influences of the various ligands was
found to be additive. The X-ray structures of three representative compounds (t-4: X = Cl, Y = NO3;
t-5: X = Cl, Y = AcO and t-12: X = Y = NO2) have also been determined.
Table 1 Numbering of cis- and trans-[PtXY(PPh3)2] compounds
Introduction
In the course of our work on Pt(II) phosphine complexes,1
we obtained cis- and trans-[Pt(mtc)2(PPh3)2] (mtc, N,N-
dimethylmonothiocarbamato-S, Me2NC(O)S-). Whilst the plat-
inum phosphorus coupling constants of cis-[PtXY(PPh3)2)] com-
plexes are normally larger than those displayed by the trans isomer,
we found, unexpectedly, that the trans isomer of [Pt(mtc)2(PPh3)2]
showed a Pt–P coupling constant larger than that of the cis
derivative (3676 and 3277 Hz, respectively).1a
One bond coupling constants are dominated by the Fermi
contact interaction of nuclei with s electrons2 and are usually taken
as an estimate of bond strength, provided such bonds involves
hybrid orbitals with some s character. Such strength depends, inter
alia, on the nature of the other ligands. Consequently, the increased
strength of the Pt–P bonds, observed in trans-[Pt(mtc)2(PPh3)2],
must arise from the influence of the mtc ligands cis to the
phosphine groups, in other words from a low cis influence of
the mtc ligands. The cis influence can in fact be defined as the
ability of a ligand A to weaken a bond between a metal and a
ligand B, cis to A. The cis influence is a ground state phenomenon
and it is distinguished from the cis effect, which is the ability
of a ligand A to accelerate the substitution rate of a ligand B,
cis to A. The cis and the trans effects are important tools in
designing synthetic pathways, whereas the cis and trans influences
are important to rationalise bond strengths in complexes, and
therefore the knowledge of a cis influence series is of the outmost
importance.
X
Y
cis
trans
Cl
I
Cl
Cl
Cl
AcO
I
NO3
NO3
Cl
Cl
I
I
c-1
c-2
c-3
c-4
c-5
c-6
c-7
c-8
c-9
c-10
c-11
c-12
t-1
t-2
t-3
NO3
AcO
AcO
AcO
AcO
NO3
NO2
NO2
NO2
t-4
t-5
t-6
t-7
t-8
t-9
t-10
t-11
t-12
I
NO2
trans-[PtXY(PPh3)2], where X and Y are anionic ligands (Table 1).
The choice of exploring only neutral complexes was made in order
to avoid any possible influence of the charge. We also report the
X-ray structures of three compounds, namely t-4 (X = Cl, Y =
NO3), t-5·CHCl3 (X = Cl and Y = AcO) and t-12·2CHCl3 (X =
Y = NO2). The spectra of some cis-[PtX2(PPh3)2] complexes (i.e.
those with X = Y) are also reported and briefly discussed.
Results and discussion
Synthesis and characterisation
There are a number of published procedures for the preparation of
trans-[PtCl2(PPh3)2] (t-1),3 but we have found that a reliable, simple
and selective synthesis is a slight modification of that suggested by
Kukushkin,4 i.e. reaction of trans-[PtCl2(CH3CN)2]5 and PPh3,6
in a 1 : 1.9 ratio (excess of phosphine must be avoided as it
catalyses trans to cis isomerisation3f,7) in nitromethane, in which
t-1 is sparingly soluble. This procedure resulted in yields up to
80%. The compound trans-[PtI2(PPh3)2] (t-2) has been prepared
by reaction of PPh3 with either K2[PtI4] in water–acetone,8 or
cis/trans-[PtI2(PhCN)2] in benzene;9 the former method gave t-2–
c-2 in a 4 : 1 ratio, whereas the latter reaction gave almost pure t-2.
Reaction of t-1 with KI also gives t-2, but in very poor yields.
Stepwise substitutions of the chloride or iodide ligands have
been performed by treatment of t-1 or t-2 with the appropriate
silver salt. In the case of the acetato complexes, we could prepare
only trans-[PtCl(AcO)(PPh3)2] (t-5), treating trans-[PtCl2(PPh3)2]
We then reasoned that in complexes of the type trans-
[PtXY(PPh3)2] the mutual trans influence of the two PPh3 groups
remains constant throughout, consequently any variation of the
properties of the Pt–P bonds in these complexes must arise from
the cis influence of X and Y. We therefore considered these
complexes suitable for an evaluation of such influence and report
herein the 31P NMR data of a number of complexes of formula
aDipartimento di Chimica Inorganica, Metallorganica e Analitica “L. Malat-
esta”, Universita` degli Studi di Milano, via Venezian 21, 20133 Milano, Italy.
E-mail: alessandro.pasini@unimi.it
bDipartimento di Chimica Strutturale e Stereochimica Inorganica, via
Venezian 21, 20133 Milano, Italy. E-mail: mario.manassero@unimi.it
† CCDC reference numbers 695960–695962. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b813125b
‡ Dedicated to Prof. Renato Ugo on the occasion of his 70th birthday
1206 | Dalton Trans., 2009, 1206–1213
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◦
˚
with one mole equivalent of AgAcO, since a second mole of
AgAcO produced isomerisation to cis-[Pt(AcO)2(PPh3)2] (c-6).
The trans diacetato isomer, t-6, has been synthesised by Basato
et al. from platinum acetate.10
Table 2 Selected bond lengths (A) and angles ( ) for t-4, t-5·CHCl3 and
t-12·2CHCl3
t-4
t-5·CHCl3
t-12·2CHCl3
Some compounds of the iodo-Y series, prepared from t-2, could
not be isolated. The Cl–I compound (t-3) was characterised in
the mixture obtained by reaction of t-1 with KI by ESI mass
spectroscopy, which showed a cluster of peaks at m/z 904.0111
(calcd for [M + Na]+, 904.0100). 31P NMR spectroscopy of the
residue of this reaction showed peaks (all with 195Pt satellites) due
to t-1, c-1, two doublets, very likely due to cis-[PtClI(PPh3)2] (c-3),
and a resonance at d 18.8 ppm, which was therefore assigned to
t-3. The I–AcO derivative was not obtained: reaction of t-2 with
AgAcO gave rise to c-6.
Rather interestingly, the reaction of trans-[PtI2(PPh3)2]
with AgNO3 gave, unexpectedly, [PtI(NO2)(PPh3)2] (t-11) and
Ph3PO. Such a redox reaction is likely to be triggered by t-2, since
in a blank experiment (Et4N)NO3 does not oxidise PPh3 under the
same conditions.
Pt–Cl
Pt–P1
Pt–P2
Pt–O1
2.297(1)
2.314(1)
2.302(1)
2.310(1)
2.030(2)
—
88.8(1)
91.5(1)
178.5(1)
179.2(1)
90.1(1)
89.6(1)
—
—
2.311(1)
2.313(1)
2.044(2)
2.337(1)
—
—
2.002(3)
—
—
—
—
—
—
Pt–N
—
Cl–Pt–P1
Cl–Pt–P2
Cl–Pt–O1
P1–Pt–P2
P1–Pt–O1
P2–Pt–O1
P–Pt–P¢
P–Pt–N
P–Pt–N¢
N–Pt–N¢
89.5(1)
88.6(1)
175.1(1)
178.0(1)
89.8(1)
92.1(1)
—
180
—
—
—
—
—
—
89.4(1)
90.6(1)
180
platinum atom displays a square-planar coordination with a slight
tetrahedral distortion, maximum distances from the best plane
The trans structure of the mixed X–Y complexes was con-
firmed by the presence of only one 31P resonance, since for cis-
[PtXY(PPh3)2] two doublets would be expected due to the two non
equivalent P atoms. The geometry of trans-[Pt(NO3)2(PPh3)2] (t-9)
was deduced by the fact that its 31P NMR spectrum is different
from that of the cis isomer.1 Compounds t-4, t-5 and t-12 (and
t-8) were characterised by X-ray diffraction studies (see below).
Moreover, it has been reported that a medium intensity band at
550 5 cm-1 in the IR spectrum, which has been assigned to a
PC3 overtone,11 is typical of the cis configuration of Pt(PPh3)2
derivatives,11 its absence being indicative of the trans geometry.
This feature was found to be in accordance with the NMR or
structural evidence. The nitro linkage isomerism of NO2, observed
in the crystal structure of t-12, is suggested also for t-10 and t-11
by their infrared spectra.12,13
˚
being +0.063(1) and -0.045(1) A for atoms O1 and P1, respectively.
The nitrate ligand is strictly planar, and is almost perpendicular
to the metal best plane (dihedral angle 84.1(1)◦).
The structure of t-5·CHCl3 consists of the packing of trans-
[PtCl(CH3COO)(PPh3)2] and CHCl3 molecules in the molar ratio
1 : 1 with no unusual van der Waals contacts. The complex
is shown in Fig. 2 and selected bond lengths and angles are
reported in Table 2. The platinum atom displays a square-planar
coordination with a very slight square-pyramidal distortion,
maximum distances from the best plane being +0.014(1) and
˚
-0.006(1) A for atoms Pt and O1, respectively. Also, the strictly
planar, acetato ligand is almost perpendicular to the metal best
plane (dihedral angle 88.6(1)◦).
X-Ray structures of t-4, t-5·CHCl3 and t-12·2CHCl3
The structure of t-4 consists of the packing of trans-
[PtCl(NO3)(PPh3)2] molecules with no unusual van der Waals
contacts. An ORTEP14 view of the molecule is shown in Fig. 1,
and selected bond lengths and angles are listed in Table 2. The
Fig. 2 ORTEP view of trans-[PtCl(AcO)(PPh3)2] (t-5) (hydrogen atoms
and CHCl3 molecule not shown for clarity). Ellipsoids are drawn at the
30% probability level.
The structure of t-12·2CHCl3 consists of the packing of trans-
[Pt(NO2)2(PPh3)2] and CHCl3 molecules in the molar ratio 1 : 2
with no unusual van der Waals contacts. An ORTEP view of t-12
is shown in Fig. 3 and selected bond lengths and angles are
reported in Table 2. The platinum atom lies on a crystallographic
inversion centre and therefore adopts a strictly (square)-planar
coordination. The NO2 group is N-coordinated and forms a
dihedral angle of 75.9(1)◦ with the metal coordination plane.
Fig. 1 ORTEP view of trans-[PtCl(NO3)(PPh3)2] (t-4) (hydrogen atoms
not shown for clarity). Ellipsoids are drawn at the 30% probability level.
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Table 3 Pt—P bond lengths of some trans-[PtXY(PPh3)2] complexes
Table 4 31P NMR parameters of trans-[PtXY(PPh3)2] complexesa
˚
X
Y
d/ppm
J
PtP/Hz
Reference
X
Y
Complex
d
Pt–P/A
Reference
b
b
a
Cl
I
Cl
18.8
20.7
24.2 (in C6D6)
16.6
18.3
16.4
28.9
29.6
2555
2630
2745
2760
2853
2873
3019
3147
3157
NO2
Cl
AcO
I
Cl
mtc
Cl
Cl
SePh
Cl
NO2
CF3
AcO
I
t-12
2.337(1)
2.328(2)
2.320b
15
10
16
17
SPh
NO3
AcO
NO2
H
Me
Ph
21
t-6
t-2
t-1
b
2.318(2)
2.3163(11)
2.316(6)
2.312b
b
b
b
Cl
mtc
NO3
AcO
SePh
Me
1
a
t-4
t-5
a
2.306b
22
23
2.300b
18
19
25.4 (in CH2Cl2)
2.296b
Order of decreasing cis influence: I > Cl > SPh > NO3 >
AcO > NO2 > H > Me > Ph
I
a This work. b Mean value.
b
b
I
Cl
F
12.7
18.8
13.6
13.0
2492
2555
2747
2795
24
b
NO2
Order of decreasing cis influence: I > Cl > F > NO2
b
b
AcO
Cl
18.3
13.8
14.6
2853
2990
3080
NO3
AcO
10
Confirms Cl > NO3 > AcO
H
I
28.3
29.9
26.7
28.9
2928
2934
3002
3019
3046
3118
3120
25
25
SCN
NCS
Cl
25
b
N3
CF3
Ph
29.9
29.6 (in CH2Cl2)
31.1 (in CH2Cl2)
25
26
27
Order of decreasing cis influence: I > SCN > NCS > Cl >
N3 > CF3 ~ Ph
Ph
H
31.1 (in CH2Cl2)
29.7 (in CD2Cl2)
23.7 (in CD2Cl2)
3120
3223
3269
27
28
28
Me
F
Order of decreasing cis influence: H > Me > F (confirms H >
Me)
a CDCl3 solutions, unless otherwise stated, ppm from H3PO4. b This work,
or measured in our laboratory.
Fig. 3 ORTEP view of trans-[Pt(NO2)2(PPh3)2] (t-12) (hydrogen atoms
and CHCl3 molecules not shown for clarity). Ellipsoids are drawn at the
30% probability level.
31P NMR spectra
The Pt–P bond distances of t-4 and t-5 are similar and fall into
the range usually found for similar compounds (see Table 3 for
In order to establish a series of cis influence, we have recorded the
31P NMR spectra and measured the 1JPtP values of various series
of trans-[PtXY(PPh3)2] complexes, in which one ligand, X, is kept
constant and Y is varied. These parameters are reported in Table 4,
together with some data obtained from the literature. The orders
of decreasing cis influence of each series, as deduced from the 1JPtP
values, are also included in the Table.
some examples). There is a slight decrease of these bond lengths
17
˚
˚
on passing from t-1 (2.3163(11) A), to t-4 (mean value 2.312 A)
˚
and t-5 (mean value 2.306 A). The Pt–P bond is rather long in
t-12, although not exceptional; for example bonds of this length
are also found in trans-[Pt(NO2)2{(p-MeC6H4)3P}2].20 The Pt–Cl
bond in t-4 (trans to NO3) is shorter than that in t-5 (trans to AcO)
Table 5 reports the data of compounds trans-[PtX2(PPh3)2], i.e.
those with X = Y.
(see Table 2). The Pt–O(acetato) distance is similar to those of
10
˚
trans-[Pt(AcO)2(PPh3)2] (mean value 2.038 A). The Pt–N bonds
˚
in t-12 (2.002(3) A), are shorter than those of the above cited
Table 5 31P NMR parameters of trans-[PtX2(PPh3)2] complexesa
20
˚
p-tolilphosphine complex (Pt–N, 2.030(5) A), while the N–O
˚
X
d/ppm
1JPtP/Hz
Reference
bonds (N–O1 = 1.236(3), N–O2 = 1.240(3) A) are longer
20
˚
(1.213 A).
I
Cl
12.7
20.7
22.4 (in C6D6)
22.4 (in C6D6)
24.1 (in C6D6)
12.6
15.9
14.6
14.9
2496
2630
2831
2860
2867
2900
3060
3080
3676
b, 9
We have also obtained crystals of t-8, but found a disorder
in the NO3 and AcO positions, not enabling an acceptable
refinement. The compound crystallises in the monoclinic space
b
SePh
SPh
SEt
NO3
NO2
AcO
mtc
29
29
˚
29
group P21/n, with a = 9.494(1), b = 19.484(2), c = 9.845(1) A,
b
◦
3
˚
b = 110.54(1) , V = 1705.2(5) A , Z = 2. The platinum atom lies
on a crystallographic inversion centre and the nitrato and acetato
groups are therefore disordered. We checked for a solution in non-
centrosymmetric space group Pn, but with no results. However,
although disordered, this structure confirms the characterisation
of t-8 as trans-[Pt(AcO)(NO3)(PPh3)2].
b
10
1
a CDCl3 solutions, unless otherwise stated, ppm from H3PO4. b This work,
or measured in our laboratory.
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Table 6 Differences of the coupling constants, DJ, for some ligands using t-1, t-2 and t-9 as referencesa
t-1 as reference compound (X = Cl)
t-2 as reference compound (X = I)
t-9 as reference compound (X = NO3)
Y
DJ/Hz
2DJ/Hzb
Y
DJ/Hz
2DJ/Hzb
Y
DJ/Hz
2DJ/Hzb
I
Cl
-67c
0
-134
0
I
Cl
F
0
67
0
134
I
Cl
NO3
AcO
—
-140
-408
-270
0
SePh
SPh
NO3
AcO
NO2
H
Me
Ph
mtc
115c
115
130
223
243
389
517
527
590c
230
230
270
450
410
—
255
0
90
NO2
303c
558
0
1034d
1054e
1180
a Calculated from Tables 3 and 4. b These values refer to the trans-[PtX2(PPh3)2] series (Table 5), unless otherwise stated. c Calculated from the X2 derivative.
d Calculated from the Cl–Me derivative. e Calculated from the Cl–Ph derivative.
Considering all the series of Tables 4 and 5, we obtain the order
of decreasing cis influence: I > Cl > SePh ~ SPh ~ SEt > NO3 >
AcO ~ NO2 > H > Me > Ph > mtc.
of these bond lengths as an indication of decreasing cis influence,
from the values of the X = Y complexes, we obtain the following
order of decreasing cis influence: NO2 > AcO > I > Cl > mtc or,
from the series of the Cl–X derivatives: CF3 > NO3 > Cl > AcO >
Me.
This series is similar to those reported in the literature.26,30 Note:
(i) the high cis influence of I and Cl; (ii) the weak cis influence
of thiocarbamato (mtc), compared to the higher influence of the
other group 16 ligands and (iii) the weak cis influence of H, Ph
and Me.
These orders are different from those obtained from 1JPtP values.
Indeed, a relationship between coupling constants and bond
lengths has been questioned32 (see however, ref. 33 and 34), and in
the present case it seems to hold only for the sequence t-1, t-4, t-5.
However, bond distances reflect factors such as ionic, or covalent
radii, steric hindrance, crystal packing, whereas one-bond Pt–P
coupling constants are mainly sensitive to the effects of X and Y
on the Pt(6 s) orbital30 and, consequently, on bonding interactions,
i.e. on the amount of P→Pt donation.
In the above series, the ligands F, SCN, NCS, N3 and CF3 have
not been included because of the lack of cross comparisons. How-
ever, considering the H–Y series, one can note the rather high cis
influence of the halogenides and pseudohalogenides (SCN being
slightly higher than its linkage isomer) compared to CF3 and Ph.
We can also estimate the contribution to cis influence of the
various ligands calculating the difference, DJ, of the coupling
constants of a given derivative from that of a reference com-
pound. These values, calculated using three different reference
compounds, i.e. t-1, t-2, and t-9, are reported in Table 6. Note that
also these DJ values confirm the trend of cis influence reported
above.
1
Thus, the cis influence series obtained from the JPtP values
is mainly due to electronic (bonding) factors, whereas the series
deduced from the X-ray data are influenced by both steric and
electronic effects, which are difficult to discriminate. An instance
of relationship between bond distances and electronic parameters
of cis ligands has been reported by Kukushkin and Konovalon.35
Additivity of cis influence has been noted previously,21,26,31 and
this property is confirmed by our data. See, for instance, the three
sequences Cl2, Cl–NO3, (NO3)2 (DJ ~ 130 Hz); Cl2, Cl–AcO,
(AcO)2 (DJ ~ 225 Hz) and Cl2, Cl–SPh, SPh2 (DJ ~ 120 Hz).
It is noteworthy that, using additivity as a rule, we can predict
cis-[PtX2(PPh3)] derivatives
Having established a series of the cis influence of some X and Y
ligands, one wonders what is the order of the trans influence of
these ligands. In Table 7, we have collected the 31P NMR data of
some cis-[PtX2(PPh3)2] complexes, and the 1JPtP values increase in
1
the coupling constant of a given compound. For instance JPtP
of t-8 (X = NO3, Y = AcO) can be obtained from the value of
the dichloro complex (t-1), adding 130 Hz (the difference of the
values between t-1 and t-4) and 225 Hz (DJ between the Cl2 and
Cl–AcO derivatives). The result (2985 Hz) is in good agreement
with the experimental value (2990 Hz). In the above calculation,
we have taken the dichloro derivative as a reference compound, but
one can start from any compound, for instance from the dinitrato
derivative t-9, JPtP of the Cl–AcO compound (t-5) can be calculated
from the values of Table 5, as follows: 2900 - 140 + 90 = 2850 Hz,
experimental 2853 Hz.
Table 7 31P NMR parameters of cis-[PtX2(PPh3)2] complexesa
X
d/ppm
1JPtP/Hz
References
b
NO3
AcO
Cl
3.6
5.76
14.9
11.89
4010
3861
3672
3454
3277
2968
2960
1900
10
b
I
9
1
36
21
37
mtc
SePh
SPh
Me
15.7
19.1
19.7 (in C6D6)
27.7 (in CD2Cl2)
Comments on Pt–P bond lengths
a CDCl3 solutions, unless otherwise stated, ppm from H3PO4. b This work,
or measured in our laboratory.
Some Pt–P bond lengths in complexes of the type trans-
[PtXY(PPh3)2] have been collected in Table 3. Taking shortening
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the order: Me < SePh ~ SPh < mtc < I < Cl < AcO < NO3,
which gives a sequence similar to that usually accepted for trans
influence.38 It is neither opposite nor parallel to the above described
series of cis influence, and mtc displays a behaviour similar to that
of the other group 16 donor ligands, as expected. However, it
is important to note that in cis-[PtX2(PPh3)2] both trans and cis
influences are simultaneously operative, at least in principle, even
if the latter are normally believed to be less relevant,32,38 and are
difficult to discriminate. More work should be done to elucidate
this point.
trans-[PtCl2(PPh3)2] (t-1)
t-1 was synthesised using a slight modification of the method
described by Kukushkin.4 PPh3 (650.0 mg, 2.50 mmol) was slowly
added to a heated (50 ◦C) suspension of trans-[PtCl2(CH3CN)2]
(475.0 mg, 1.35 mmol) in nitromethane (10 mL). The mixture
was left at 50 ◦C for 30 min. t-1 was isolated as a yellow solid,
washed three times with hot nitromethane and dried at the air
(720.0 mg, 73%). Found: C 54.70, H 3.77. Calcd for C36H30Cl2P2Pt
(790.57): C 54.69, H 3.83. m/z (ESI) 719 ([Pt(PPh3)2]+, 100%), 754
([PtCl(PPh3)2]+, 30%).
Conclusions
trans-[PtI2(PPh3)2] (t-2)
One-bond coupling constants have often been used to investigate
trans and cis influences in Pt and Rh compounds.39,40 In fact, such
constants can be assumed as an estimate of the strength of a
bond, provided such bond possesses some s character. Coupling
constants can be considered a transfer of information of the
spin state between two nuclei, mediated by the s electrons of
a given bond(s). In Pt(II), back donation can be considered
of little relevance, at least in the first place,41 consequently for
the compounds studied in this paper, we can assume that Pt–P
bonds are formed mainly by donation from an sp3 orbital of
P to an hybrid Pt orbital with some 6 s character. Under this
approximation, Pt–P bonds become weaker as the central platinum
atom becomes less positively charged; in fact the larger cis
influence is displayed by ligands such as halides and pseudohalides,
in particular by Cl or I, which being both s and p donors to Pt(II),42
lower the positive charge on Pt. Also of note is the finding that
strong donors display also a strong (kinetic) cis effect, because
they lower the positive charge on Pt, reducing its electrophilicity.43
Thus, the very low cis influence of mtc can be tentatively ascribed
to a low negative charge of the sulfur donor atom, adjacent to the
PPh3 (137.3 mg, 0.52 mmol) was added to a suspension
of cis/trans-[PtI2(PhCN)2] (180.0 mg, 0.28 mmol) in benzene
(20 mL). The slurry was refluxed for 4 h, cooled to 10 ◦C and
the orange product was obtained by addition of diisopropyl ether
(223.0 mg, 88%). Found: C 44.25, H 3.26. Calcd for C36H30I2P2Pt
(973.46): C 44.42, H 3.11. m/z (ESI) 719 ([M - 2I]+, 35%), 846
([M - I]+, 100), 994 ([M + Na]+, 20).
Alternative preparation. Solid KI (837.0 mg, 5.04 mmol) was
added to a stirred solution of K2PtCl4 (500.0 mg, 1.20 mmol)
in water (10 mL). After 15 min, a solution of PPh3 (639.1 mg,
2.40 mmol) in acetone (10 mL) was added and the mixture was
stirred at room temperature for 2 h, when the orange solid was
filtered, washed with acetone and dried in vacuo (1090.0 mg, 93%).
31P NMR showed that this residue consisted of a 4 : 1 mixture of
t-2–c-2.
Attempted synthesis of trans-[PtClI(PPh3)2] (t-3)
Potassium iodide (12.0 mg, 0.07 mmol) was added to a CHCl3
solution (20 mL) of t-1 (552.7 mg, 0.07 mmol), followed by a
crystal of 18-crown-6. The slurry was refluxed for 8 h, filtered and
the solution was evaporated to dryness in vacuo. 31P NMR of the
yellow residue showed the presence of t-1, c-1 and t-2, two doublets
(JPP 12 Hz) at d 11.5 ppm (JPtP 3685 Hz) and 13.7 (JPtP 3408 Hz),
assigned to c-3 and a singlet with Pt satellites at d 18.8 ppm (JPtP
2556 Hz), assigned to t-3. This compound was approximately 30%
of the mixture. Attempts to isolate it were unsuccessful. Similar
results were obtained using LiI as a source of iodide, or by reaction
of t-2 with KCl in the presence of 18-crown-6.
=
electron withdrawing C O group.
trans-[PtXY(PPh3)2] complexes have proved to be a good model
1
for studying cis influence, by consideration of the JPtP values,
because the mutual trans influence of the phosphine groups is
constant. We have thus obtained a series of cis influence, which
can be rationalised in terms of change of the positive charge on Pt.
Although, in principle, cis-[PtX2(PPh3)2] complexes should be
useful for studying trans influence, such a study is more tricky,
because in these complexes cis influence is also operative and is
difficult to discriminate. We are planning to undertake this study
in this direction.
trans-[PtCl(NO3)(PPh3)2] (t-4)
Experimental
t-1 (113.5 mg, 0.14 mmol) and AgNO3 (24.6 mg, 0.14 mmol) were
suspended in CHCl3 (15 mL) and heated at 50 ◦C for 6 h. The
hot mixture was filtered and the filtrate was evaporated to dryness
yielding t-4 as a yellow solid (100.1 mg, 85%). Found: C 50.48, H
3.54, N 1.68. Calcd for C36H30NClO3P2Pt·1/3CHCl3 (856.91): C
50.92, H 3.57, N 1.63. nmax(KBr)/cm-1 1483 and 1270 (NO3). m/z
(ESI) 840 ([M + Na]+, 40%). Crystals suitable for X-ray diffraction
studies were obtained by slow diffusion of diisopropyl ether into
a CHCl3 solution.
Elemental analyses were performed at the Microanalytical Labo-
1
ratory, the University of Milano. 31P { H} NMR spectra (Tables 4,
5and 7)were recorded ona Bruker AdvanceDRX300at 121 MHz;
d values (ppm) are vs. external H3PO4 (see Tables 4 and 5 for
values). ESI Mass spectra were recorded with a LCQ Advantage
Thermofluxional instrument, or a ICR-FTMS APEX II with a
4.7 T magnet, Bruker Daltonics from CH2Cl2–methanol 1 : 1
solutions. Isotope cluster abundance was checked by computer
simulation.
Chemicals were reagent grade and used as received. cis/trans-
[PtCl2(CH3CN)2],5 trans-[PtCl2(CH3CN)2],5 [PtI2(PhCN)2],9 and
cis-[PtCl2(PPh3)2]44 were synthesised as described in the literature.
All reactions involving silver salts were performed in the dark.
trans-[PtCl(AcO)(PPh3)2] (t-5)
t-1 (110.0 mg, 0.14 mmol) and AgAcO (23.6 mg, 0.14 mmol) were
suspended in CHCl3 (10 mL) and heated at 50 ◦C for 6 h. The
1210 | Dalton Trans., 2009, 1206–1213
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hot mixture was filtered and the filtrate was evaporated to dryness
yielding t-5 as a pale yellow solid (70.0 mg, 61%). Found: C 50.21,
H 3.79. Calcd for C38H33O2ClP2Pt·CHCl3 (933.54): C 50.18, H
3.67. nmax(KBr)/cm-1 1604, 1362 and 1316 (CO). m/z (ESI) 754
([M - AcO]+, 100%), 778 ([M - Cl]+, 10), 814 ([M + 1]+, 5), 836
(M + Na]+, 70). Crystals suitable for X-ray diffraction were grown
by slow diffusion of diisopropyl ether into a CHCl3 solution.
trans-[PtI(NO2)(PPh3)2] (t-11)
Solid AgNO2 (9.2 mg, 0.06 mmol) was added to a solution of
t-2 (60.0 mg, 0.06 mmol) in CHCl3 (10 mL) and the mixture
was refluxed for 6 h. The hot mixture was filtered and the
filtrate was evaporated to dryness yielding t-11 as a yellow solid
(23.1 mg, 43%). Found: C 45.86, H 3.24, N 1.20. Calcd for
C36H30NIO2P2Pt·1/2CHCl3 (952.25): C 46.04, H 3.23, N 1.47.
n
max(KBr)/cm-1: 1408 and 1323 (NO2). m/z (ESI) 719 ([M - NO2 -
I]+, 100%), 765 ([M -I]+, 10%), 845 ([M - NO2]+, 95%), 915 ([M +
Attempted preparation of trans-[Pt(AcO)2(PPh3)2] (t-6)
Na]+, 20%).
Reaction of either t-1 or t-2 with two molar equivalents of AgAcO
gave invariably cis-[Pt(AcO)2(PPh3)2], identified by its 31P NMR
spectrum,10 plus minor amounts of other unidentified compounds.
trans-[Pt(NO2)2(PPh3)2] (t-12)
From t-4. t-4 (80.0 mg, 0.10 mmol) and AgNO2 (100 mg,
0.64 mmol) were suspended in CHCl3 (10 mL) and heated under
reflux for 2 d. The hot mixture was filtered, taken to dryness
yielding pale yellow t-12 (10.0 mg, 12%). Found: C 40.64, H 3.58, N
2.47. Calcd for C36H30N2O4P2Pt·2CHCl3·4H2O (1050.41): C 40.66,
H 3.59, N 2.49. nmax(KBr)/cm-1: 1415 and 1335 (NO2). m/z (ESI)
765 ([M - NO2]+, 60%).
Attempted preparation of trans-[PtI(AcO)(PPh3)2] (t-7)
The reaction of equimolar amounts of t-2 and AgAcO in CHCl3
at 60 ◦C led invariably to cis-[Pt(AcO)2(PPh3)2], c-7, and some
unreacted starting material identified through their 31P NMR
signals.
From t-1. t-1 (156.0 mg, 0.20 mmol) and AgNO2 (75.9 mg,
0.50 mmol) were suspended in CHCl3 and heated under reflux for
10 h. The hot mixture was filtered, taken to dryness yielding t-12 as
a white solid (39.7 mg, 25%). Crystals for X-ray diffraction studies
were grown by diffusion of diisopropyl ether into a chloroform
solution.
trans-[Pt(AcO)(NO3)(PPh3)2] (t-8)
t-4 (100.0 mg, 0.12 mmol) and AgAcO (21.1 mg, 0.12 mmol) were
suspended in CHCl3 (15 mL) and heated at 50 ◦C for 7 h. The
hot mixture was filtered and the filtrate was evaporated to dryness
yielding t-5 as a yellow solid (83.2 mg, 82%). Found: C 53.84, H
4.05, N 1.76. Calcd for C38H33NO5P2Pt (840.71): C 54.29, H 3.96,
N 1.67. nmax(KBr)/cm-1 1647 (AcO), 1483 and 1265 (NO3). m/z
(ESI) 841 ([M + 1]+, 50%). Crystals suitable for X-ray diffraction
studies were obtained by slow diffusion of diisopropyl ether into
a CHCl3 solution.
Attempted preparation of trans-[PtI(NO3)(PPh3)2], reduction of
nitrate to nitrite
trans-[PtI2(PPh3)2] (102.0 mg, 0.10 mmol) and AgNO3 (17.3 mg,
0.10 mmol) were refluxed in CHCl3 (20 mL) for 2 h. The filtered
solution was evaporated to dryness in vacuo leaving a yellow solid
whose 31P NMR showed the presence of t-11 (d 13.0 ppm, JPtP
2795 Hz) and Ph3PO (d 29.8 ppm).
=
cis/trans-[Pt(NO3)2(PPh3)2] (c/t-9)
t-4 (130.0 mg, 0.16 mmol), AgNO3 (100.2 mg, 0.59 mmol) and
18-crown-6 (10 mg, 0.04 mmol) were suspended in CHCl3 (10 mL)
and heated under reflux for 2 d. The hot mixture was filtered,
evaporated to dryness yielding a pale yellow solid, which was
washed with diisopropyl ether (80.5 mg, 60%). The 31P NMR
spectrum of this residue showed only two resonances at d 3.6 ppm
(JPtP 4010 Hz), due to c-9,1 and 12.6 ppm (JPtP 2900 Hz). Elemental
analysis and ESI mass spectra of this residue were consistent with
the stoichiometry [Pt(NO3)2(PPh3)2], consequently the signal at
12.6 ppm was assigned to t-9. Found: C 41.80, H 2.78, N 2.94.
Calcd for C36H30N2O6P2Pt·2CHCl3 (1082.43): C 42.17, H 2.98, N
2.59. nmax(KBr)/cm-1 1481 and 1259 (NO3). m/z (ESI) 866 ([M +
Na]+, 60%).
X-Ray data collection and structure determination†
Crystal data are summarised in Table 8. The diffraction experi-
ments were carried out on a Bruker SMART CCD area-detector
diffractometer, at 223 K for t-4, and at 150 K for t-5·CHCl3 and t-
˚
12·2CHCl3, using MoKa radiation (l = 0.71073 A) with a graphite
crystal monochromator in the incident beam. No crystal decay
was observed, so that no time-decay correction was needed. The
collected frames were processed with the software SAINT,45 and
an empirical absorption correction was applied (SADABS)46 to
the collected reflections. The calculations were performed using
the Personal Structure Determination Package47 and the physical
constants tabulated therein.48 The structures were solved by direct
methods (SHELXS)49 and refined by full-matrix least-squares
2
2
using all reflections and minimising the function Rw(Fo - kFc )2
(refinement on F2). All the non-hydrogen atoms were refined
with anisotropic thermal factors. The hydrogen atoms of the CH3
moiety in t-5·CHCl3 were found in the final Fourier maps and
not refined. All the other hydrogen atoms were placed in their
trans-[PtCl(NO2)(PPh3)2] (t-10)
t-1 (99.0 mg, 0.13 mmol) and AgNO2 (19.4 mg, 0.13 mmol) were
suspended in CHCl3 (10 mL) and heated at 60 ◦C for 6 h. The
mixture was hot filtered and the filtrate was evaporated to dryness
yielding t-10 as a yellow solid (43.2 mg, 44%). Found: C 51.83, H
3.70, N 1.54. Calcd for C36H30NClO2P2Pt·1/4CHCl3 (830.95): C
52.24, H 3.67, N 1.68. nmax(KBr)/cm-1: 1408 and 1331 (NO2). m/z
(ESI) 802 ([M + H]+, 30%), 824 ([M + Na]+, 70%).
˚
ideal positions (C–H = 0.97 A), with the thermal parameter U
1.10 times that of the carbon atom to which they are attached, and
not refined. In spite of the low-temperature data collection, in t-4
there are three carbon atoms with high thermal parameters. In the
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Table 8 Crystallographic data for compounds trans-[PtCl(NO3)(PPh3)2] (t-4), trans-[PtCl(AcO)(PPh3)2]·CHCl3 (t-5·CHCl3) and trans-[Pt(NO2)(PPh3)2]
(t-12·2CHCl3)
t-4
t-4·CHCl3
t-12·2CHCl3
Formula
C36H30ClNO3P2Pt
817.14
Pale yellow
Monoclinic
P21/n
11.901(1)
23.988(2)
12.213(1)
90
111.77(1)
90
3237.9(4)
4
1608
1.676
223
0.28 ¥ 0.32 ¥ 0.49
45.94
0.619–1.000
w
0.30
C39H34Cl4O2P2Pt
933.55
Colourless
Monoclinic
P21/c
12.484(1)
11.084(1)
27.857(2)
90
101.86(1)
90
3772.2(4)
4
1840
1.644
150
0.21 ¥ 0.23 ¥ 0.40
41.60
0.692–1.000
w
0.50
C38H32Cl6N2O4P2Pt
1050.45
Colourless
Triclinic
¯
P1
7.9386(7)
11.158(1)
12.025(1)
91.880(1)
102.303(1)
104.799(1)
1001.8(2)
1
516
1.741
150
0.10 ¥ 0.15 ¥ 0.45
40.61
0.706–1.000
w
0.50
Mr/g mol-1
Colour
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
U/A
Z
3
˚
F(000)
Dc/g cm-3
T/K
Crystal dimensions/mm
m (MoKa)/cm-1
Min. and max. transmission factors
Scan mode
Frame width/◦
Time per frame/sec.
No. of frames
Detector-sample distance/cm
q-range
10
3660
6.00
3–28
Full sphere
61 238, 8584
0.0292
0.028, 0.051
0.020
7338
397
10
1500
6.00
3–28
Full sphere
47 543, 9733
0.0206
0.026, 0.047
0.018
8486
433
30
2160
6.00
3–27
Full sphere
17 082, 4658
0.0437
0.049, 0.053
0.030
4409
241
Reciprocal space explored
No. of reflections (total, independent)
Rint
Final R2 and wR2 indicesa (F2, all reflections)
Conventional RI index [I > 2s(I)]
Reflections with I > 2s(I)
No. of variables
Goodness of fitb
1.003
0.968
0.974
a R2 = [R(|Fo - kFc2|/RFo2], wR2 = [Rw/(Fo - kFc2)2/Rw(Fo2)2]1/2
.
b [Rw(Fo - kFc2)2/(No - Nv)]1/2, where w = 4Fo2/s(Fo2)2, s(Fo2) = [s (Fo2) +
2
2
2
2
(pFo2)2]1/2, No is the number of observations, Nv the number of variables, and p = 0.03 for t-4 and t-5·CHCl3, and p = 0.02 for t-12·2CHCl3.
-3
˚
5 F. P. Fanizzi, F. P. Intini, L. Maresca and G. Natile, J. Chem. Soc.,
Dalton Trans., 1990, 199–202.
final Fourier maps the maximum residuals were 1.98(13) e A
-3
˚
˚
˚
at 0.75 A from Pt, 0.64(8) e A at 0.90 A from Cl4, and
6 Kukushkin used a cis–trans mixture of [PtCl2(CHCN3)2]. We found
that the use of pure trans precursor gave better yields and purity of t-1.
7 G. K. Anderson and R. J. Cross, Chem. Soc. Rev., 1980, 9, 185–215.
8 Note that, although reaction of PPh3 with K2PtCl4 gives almost pure
c-1, the reaction with K2PtI4 gives a rather high yield of t-2, see ref. 7.
9 E. Farkas, L. Kollar, M. Moret and A. Sironi, Organometallics, 1969,
15, 1345–1350.
-3
˚
˚
1.49(22) e A at 0.61 A from Cl2 for t-4, t-5·CHCl3, and t-
12·2CHCl3, respectively.
Acknowledgements
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1206–1213 | 1213
©