Diorganoplatinum(II) complexes with chelating PN ligand 2-(diphenylphosphinoamino)pyridine; synthesis and kinetics of the reaction with MeI

Article abstract of DOI:10.1039/b9nj00523d

New organoplatinum(II) complexes [PtR₂(PN)] (PN = 2-(diphenylphosphinoamino)pyridine, R = Me, 1a, or p-MeC₆H ₄, 1b) were synthesized by the reaction of [Pt(p-MeC ₆H₄)₂(SMe₂)₂] or [Me ₂Pt(μ-SMe₂)₂PtMe₂] with 1 and 2 equiv. of PN, respectively. The reaction of Pt(II) complexes 1 with MeI gave the Pt(IV) complexes [PtR₂(PN)MeI] (R = Me; 2a, and p-MeC ₆H₄; 2b). All the complexes were fully characterized using multinuclear (¹H, ³¹P, ¹³C, and ¹⁹⁵Pt) NMR spectroscopy. Density functional theory calculations have been performed to find approximate structures for all described complexes. The platinum(II) complexes have a 5d_π(Pt)-π*(PN) metal-to-ligand charge-transfer band, which was used to easily follow the kinetics of their reactions with MeI. The classical S_N2 mechanism was suggested. The rates of the reactions at different temperatures were measured and were consistent with the proposed mechanism, large negative ΔS? values were found in each reaction. The PN chelating complexes [PtR ₂(PN)], 1, reacted almost 100 or 300 times slower with MeI as compared to that of the NN chelating complex [PtR₂(bpy)] (bpy = 2,2′-bipyridine) in acetone or benzene, respectively. This was attributed to the π-acceptance through the P ligating atom of PN ligand, which decreases the electron density of Pt(II) in PN chelating complexes. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010.

Full text of DOI:10.1039/b9nj00523d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Diorganoplatinum(II) complexes with chelating PN ligand
2-(diphenylphosphinoamino)pyridine; synthesis and kinetics
of the reaction with MeIw
S. Masoud Nabavizadeh,*a Elham S. Tabei,a Fatemeh Niroomand Hosseini,b
Niloofar Keshavarz,a Sirous Jamalic and Mehdi Rashidi*a
Received (in Victoria, Australia) 30th September 2009, Accepted 17th November 2009
First published as an Advance Article on the web 15th January 2010
DOI: 10.1039/b9nj00523d
New organoplatinum(II) complexes [PtR2(PN)] (PN = 2-(diphenylphosphinoamino)pyridine,
R = Me, 1a, or p-MeC6H4, 1b) were synthesized by the reaction of [Pt(p-MeC6H4)2(SMe2)2] or
[Me2Pt(m-SMe2)2PtMe2] with 1 and 2 equiv. of PN, respectively. The reaction of Pt(II) complexes
1 with MeI gave the Pt(IV) complexes [PtR2(PN)MeI] (R = Me; 2a, and p-MeC6H4; 2b). All the
complexes were fully characterized using multinuclear (1H, 31P, 13C, and 195Pt) NMR
spectroscopy. Density functional theory calculations have been performed to find approximate
structures for all described complexes. The platinum(II) complexes have a 5dp(Pt)-p*(PN)
metal-to-ligand charge-transfer band, which was used to easily follow the kinetics of their
reactions with MeI. The classical SN2 mechanism was suggested. The rates of the reactions at
different temperatures were measured and were consistent with the proposed mechanism, large
negative DSz values were found in each reaction. The PN chelating complexes [PtR2(PN)], 1,
reacted almost 100 or 300 times slower with MeI as compared to that of the NN chelating
complex [PtR2(bpy)] (bpy = 2,20-bipyridine) in acetone or benzene, respectively. This was
attributed to the p-acceptance through the P ligating atom of PN ligand, which decreases
the electron density of Pt(II) in PN chelating complexes.
analog [Pt{(CH2)4}(NN)]. A secondary a-deuterium KIE
Introduction
study involving the reaction of MeI/CD3I with some organo-
platinum(II) complexes has also been reported to confirm the
operation of the SN2 mechanism in the oxidative addition of
MeI to some organoplatinum(II) complexes.6 So it is well
established that these reactions proceed by an SN2 mechanism;
however, sometimes the concerted three-center mechanism is
also a possibility, but although it has been proposed for some
reactions, it has never been demonstrated by experimental
evidence.3 The concerted three-center mechanism is usually
proposed in the oxidative addition of C–H and C–C bonds,3b,c
and also we have proposed it in the oxidative addition of
the O–O bonds in H2O2 and dibenzoyl peroxide to some
diarylplatinum(II) complexes [PtAr2(NN)] (in which Ar is Ph
or some simple substituted Ph groups).5b The kinetics of
reaction of MeI with some binuclear organoplatinum(II)
complexes, including a bridging biphosphine ligand, have
recently been studied and a step-wise oxidative addition
of MeI to the platinum(II) centers of each complex were
described.7
The area of homogeneous, transition metal catalyzed alkane
functionalization reactions has significantly progressed over
the last several decades.1 Among the metal complexes,
platinum species have been extensively applied as catalyst
for many reactions in the petrochemical industries.2 This has
led to an increased level of research in the area of oxidative
addition reactions by platinum complexes, and the number of
platinum complexes that have been shown to activate C–X
(X = halide) bonds to form stable alkyl and aryl complexes
has developed considerably in the last several years. The
dimethylplatinum(II) complexes [PtMe2(NN)], in which NN
are various diimine ligands such as 2,20-bipyridine (bpy) and
1,10-phenanthroline (phen), have been extensively investigated
by Puddephatt et al.3 and we have recently studied the
oxidative addition of some alkyl halides,4 e.g. EtI, EtBr, nPrI,
nPrBr, nBuI and an epoxide5a to [PtMe2(NN)] or metallacycle
a Department of Chemistry, College of Sciences, Shiraz University,
71454, Shiraz, Iran. E-mail: nabavi@chem.susc.ac.ir,
rashidi@chem.susc.ac.ir; Fax: +98 711 228 6008;
Tel: +98 711 228 4822
On the other hand, bidentate ligands with a nitrogen
and a phosphorus donor atom (P,N ligands) have attracted
considerable attention in the field of transition metal
catalysis.8 The development and application of P,N ligands
is a significant field of homogeneous catalysis, and this ligand
type has proven its value in many reactions.8 The chemistry of
this class of ligand has been reviewed.8c–e
b Department of Chemistry, Islamic Azad University, Shiraz Branch,
Shiraz, 71993-37635, Iran
c Department of Chemistry, Institute for Advanced Studies in Basic
Sciences, 45195-1159, Gava Zang, Zanjan, Iran
w Electronic supplementary information (ESI) available: Table S1,
DFT calculated bond angles (deg) of compounds 1–2; Fig. S1,
absorbance-time curves; Fig. S2, plots of first-order rate constants
versus concentration of MeI; and Fig. S3, Eyring plots. See DOI:
10.1039/b9nj00523d
In spite of the comparative ease of phosphorus–
nitrogen bond forming reactions compared to those in which
ꢀc
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phosphorus–carbon bonds are formed, relatively few examples
of transition metal complexes with pyridine–phosphine (PN),
in which the donor atoms (i.e. P and N) are separated by
amino groups, are known.9
pressure. The off- white solid product was washed with cold
dry ether. Yield: 84%; Anal. Calcd. for C20H24IN2PPt: C,
37.3; H, 3.8, N, 4.3. Found: C, 38.0; H, 3.9, N, 4.3%. NMR in
2
3
CDCl3: d (1H) 0.65 [d, J(PtH) = 70.0 Hz, J(PH) = 7.5 Hz,
In continuation of our interest in the oxidative addition
reactions of alkyl halides with organoplatinum(II) complexes,
in the present work, new organoplatinum complexes
3H, Me ligand trans to I], 1.49 [d, 2J(PtH) = 70.0 Hz,
2
3J(PH) = 7.5 Hz, 3H, Me ligand cis to P], 1.60 [d, J(PtH) =
60.0 Hz, 3J(PH) = 7.5 Hz, 3H, Me ligand trans to P],
6.15 [bd, 2J(PH) not observed, 1H attached to N], 8.42 [d,
[PtR2(PN)] (PN
= 2-(diphenylphosphinoamino)pyridine,
3
R = Me, 1a, or p-MeC6H4, 1b) were synthesized and the
oxidative addition of MeI to these complexes was studied and
the results obtained here were compared with complexes
containing NN ligands. The kinetics of oxidative addition to
platinum complexes including PN ligands has not been
studied, despite the fact that the PN ligands, especially in
which an amine acts as spacer between the aromatic ring and
the phosphine, are of great interest because of their possible
role in catalysis and coordination chemistry of transition
metals.8
3J(HH) = 7.5 Hz, J(PtH) = not resolved, 1H, pyridyl C[6]
proton of coordinated pyridine]; d (31P) 45.4 [s, 1J(PtP) =
1275 Hz].
[Pt(p-MeC6H4)2(PN)], 1b
To solution of cis-[Pt(p-MeC6H4)2(SMe2)2] (320 mg,
0.68 mmol) in acetone (30 ml) was added a solution of PN
(189 mg, 0.68 mmol) in the same solvent (10 ml) and the
solution was stirred at room temperature for 1.5 h. The solvent
was removed under reduced pressure. The off-white solid
product was washed with n-pentane. Yield: 91%; Anal. Calcd.
for C31H29N2PPt: C, 56.7; H, 4.5, N, 4.3. Found: C, 55.9; H,
4.9, N, 4.0%. NMR in CDCl3: d (1H) 2.08 and 2.17 [s, 6 H,
2 Me groups on the p-tolyl ligands], 5.90 [bd, 2J(PH) = 4.2 Hz,
1H attached to N], 8.20 [d, 3J(HH) = 5 Hz, 3J(PtH) = 20 Hz,
1H, pyridyl C[6] proton of coordinated pyridine]; d (13C) 13.1
and 14.9 [s, 2 Me groups on the p-tolyl ligands], PN carbons
Experimental
The 1H NMR spectra were recorded on a Bruker Avance DPX
250 MHz spectrometer. 31P, 13C and 195Pt NMR spectra were
recorded on a Bruker Avance DRX 500 MHz. References
were TMS (1H, 13C), H3PO4 (31P), and aqueous K2PtCl4
(
195Pt). CDCl3 was used as solvent. All the chemical shifts
1
160-110; d (31P) 69.5 [s, J(PtP) = 2016 Hz].
and coupling constants are in ppm and Hz, respectively.
Kinetic studies were carried out by using a Perkin-Elmer
Lambda 25 spectrophotometer with temperature control
using an EYELA NCB-3100 constant-temperature bath.
The microanalysis was performed using a Thermofinnigan
Eager 300 CHN–O elemental analyzer. Complex
cis-[Pt(p-MeC6H4)2(SMe2)2],10 the dimeric precursor cis,cis-
[Me2Pt(m-SMe2)2PtMe2]11 and 2-(diphenylphosphinoamino)-
pyridine (PN)8l,12 were prepared by the literature methods.
[PtMeI(p-MeC6H4)2(PN)], 2b
Excess of MeI (100 ml) was added to solution of
[Pt(p-MeC6H4)2(PN)] (40.8 mg, 0.062 mmol) in acetone and
stirred at room temperature for 6 h. The solvent was removed
under reduced pressure. The yellow oily product was washed
with cold dry ether to yield light yellow powder. Yield: 84%;
Anal. Calcd. for C32H32IN2PPt: C, 48.2; H, 4.0, N, 3.5.
Found: C, 47.8; H, 4.1, N, 3.4%. NMR in CDCl3: d (1H)
[PtMe2(PN)], 1a
2
1.45 [d, J(PtH) = 68.43 Hz, 3H, Me ligand trans to I], 2.20
and 2.30 [s, 6 H, 2 Me groups on the p-tolyl ligands], 6.15
To a solution of cis,cis-[Me2Pt(m-SMe2)2PtMe2] (125.5 mg,
0.22 mmol) in acetone (30 ml) was added a solution of PN
(121.5 mg, 0.44 mmol) in acetone (10 ml) and stirred at room
temperature for 1 h. The solvent was removed under reduced
pressure to yield the sponge-like product. The product was
washed with cold dry ether to yield a solid white powder.
Yield: 88%; Anal. Calcd. for C19H21N2PPt: C, 45.3; H, 4.2, N,
5.6. Found: C, 44.6; H, 4.3, N, 5.2%. NMR in CDCl3: d (1H)
0.78 [d, 2J(PtH) = 65.0 Hz, 3J(PH) = 7.5 Hz, 3H, Me ligand
trans to P], 0.91 [d, 2J(PtH) = 87.5 Hz, 3J(PH) = 7.5 Hz, 3H,
Me ligand cis to P], 5.84 [bd, 2J(PH) = 5.0 Hz, 1H attached to
N], 8.81 [d, 3J(HH) = 5 Hz, 3J(PtH) = 20 Hz, 1H,
pyridyl C[6] proton of coordinated pyridine]; d (13C) ꢁ23.8
2
[bd, J(PH) = 4.2 Hz, 1H attached to N] 8.42 [d, J(HH) =
3
7.5 Hz, 1H, pyridyl C[6] proton of coordinated pyridine];
1
d (31P) 48.2 [s, J(PtP) = 1247 Hz].
Kinetic studies of the reaction of [PtMe2(PN)] and
[Pt(p-MeC6H4)2(PN)] with MeI
A solution of [PtMe2(PN)], 1a, in benzene (3 ml, 2.6 ꢂ 10ꢁ4 M)
and [Pt(p-MeC6H4)2(PN)], 1b, in acetone (3 ml, 5.0 ꢂ 10ꢁ4 M)
in a cuvette was thermostatted at 25 1C and a known excess of
MeI was added using a microsyringe. After rapid stirring, the
absorbance at l = 350 and 340 nm for 1a and 1b, respectively,
was collected with time (Fig. 3 and S1w). The pseudo-first-
order rate constants (kobs) were evaluated by nonlinear least-
squares fitting of the absorbance-time profiles to a first order
equation (eqn (1)):
1
2
[d, J(PtC) = 730 Hz, J(CPcis) = 4 Hz, Me ligand cis to P],
1
2
14.0 [d, J(PtC) = 678 Hz, J(CPtrans) = 114 Hz, Me ligand
trans to P]; d(31P) 73.2 [s, 1J(PtP) = 2126 Hz]; d (195Pt) ꢁ2333
1
[d, J(PtP) = 2129 Hz].
[PtMe3I(PN)], 2a
Abst = AbsN + (Abs0 ꢁ AbsN) exp(ꢁkobst)
(1)
Excess of MeI (100 ml) was added to solution of [PtMe2(PN)]
(68 mg, 0.14 mmol) in acetone (30 ml) and stirred at room
temperature for 1 h. The solvent was removed under reduced
A plot of kobs versus [MeI] was linear and the slope gave the
second-order rate constant. The same method was used at
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other temperatures (Fig. S2w) and activation parameters were
obtained from the Eyring equation (eqn (2) and Fig. S3w).
ꢀ
ꢁ
ꢀ
ꢁ
z
z
DH
RT
k2
T
kB
h
DS
R
ln
¼ ln
þ
ꢁ
ð2Þ
Similar methods were used to study the other reactions at the
corresponding lmax and the data are collected in Table 2.
Fig. 1 1H (left) and 13C (right) NMR spectra of complex 1a in the Me
Computational details
region. Assignments are given on the spectra.
Density functional calculations were performed with the
program suite Gaussian98.13 The LANL2DZ basis set was
used with B3LYP method. All geometries were optimized. To
evaluate and ensure the optimized structures of the molecules,
frequency calculations were carried out using analytical
second derivatives. In all cases only real frequencies were
obtained for the optimized structures.
1
The H NMR spectrum of 1a in CDCl3 at room tempera-
ture contains one signal as a broad doublet for NH proton at
d = 5.84 with 2J(PH) = 5.0 Hz as well as four for the
inequivalent pyridyl protons on the ligand, the furthest down-
field of which at d = 8.81 has 195Pt satellites, 3J(PtH) = 20 Hz.
The presence of two inequivalent Pt–Me groups (see Fig. 1) is
evident from the Pt–CH3 signals of equal intensity (d = 0.91
and 0.78 ppm), both with coupling to one 31P nucleus as well
to 195Pt (2J(PtH) = 87.5 Hz and 3J(PH) = 7.5 Hz; cis to P and
2J(PtH) = 65.0 Hz and 3J(PH) = 7.5 Hz; trans to P,
respectively). The 31P{1H} NMR spectrum of 1a shows one
signal with 195Pt satellites characteristic of a phosphorus trans
Results and discussion
Synthesis and characterization of the complexes
Very recently, Goldberg et al. reported8m the synthesis of
the platinum(II) complex [PtMe2P0N0], where P0N0
=
to a methyl on Pt(II) (d = 73.2, 1J(PtP) = 2126 Hz). In the 13
C
2-((ditert-butylphosphino)methyl)pyridine, by reaction of the
bidentate ligand 2-((ditert-butylphosphino)methyl)pyridine
with the Pt(II) dimethyl complex [(m-SEt2)PtMe2]2 in toluene
at high temperature (100 1C) for 90 min. Also, it has been
shown that attempts to synthesize and isolate the complex 1a,
through the slow addition of PN ligand to a solution of
[PtMe2(cod)] in CH2Cl2, from the reaction mixture has not
been successful due to its extremely high solubility in all
common solvents.8l But we have synthesized the complex 1a
very easily by reaction of cis,cis-[Me2Pt(m-SMe2)2PtMe2]
with 2-(diphenylphosphinoamino)pyridine ligand at room
temperature in acetone and isolated the complex as pure
and stable product. The analogous arylplatinum complex
[Pt(p-MeC6H4)2(PN)], 1b, was synthesized similarly by the
reaction of [Pt(p-MeC6H4)2(SMe2)2] with 1 equimolar of PN.
The preparative methods are described in the Scheme 1. The
platinum(II) complexes, 1a and 1b, reacted cleanly with MeI to
give the corresponding organoplatinum(IV) complexes, 2. The
complexes were fully characterized using 1H, 13C{1H},
31P{1H} and 195Pt{1H} NMR spectroscopy and elemental
analysis. Full data are collected in the experimental section.
NMR spectrum of 1a, two methyl ligands (see Fig. 1) appear
as two different set of doublets at d = ꢁ23.8 ppm, accom-
panied by platinum satellites with 1J(PtC) = 730 Hz,
2J(CPcis) = 4 Hz for Me group cis to P, and d = 14.0 ppm
1
accompanied by platinum satellites with J(PtC) = 678 Hz,
2J(CPtrans) = 114 Hz for Me group trans to P. Its 195Pt NMR
spectrum shows a doublet at d = ꢁ2333 with 1J(PtP) =
2126 Hz. Similar spectral data are obtained for complex
[Pt(p-MeC6H4)2(PN)], 1b, except instead of Me–Pt, two
singlets were observed at d = 2.08 and 2.17 for the Me
substituents on the Ar ligands.
1
In the H NMR spectrum of complex [PtMe3I(PN)], 2a, a
2
3
doublet at d = 0.65 with J(PtH) = 70.0 Hz and J(PH) =
7.5 Hz was assigned to the Me ligand trans to I. A doublet at
2
3
d = 1.49 with J(PtH) = 70.0 Hz and J(PH) = 7.5 Hz was
assigned to the Me ligand cis to P and a doublet at d = 1.60
with 2J(PtH) = 60.0 Hz and 3J(PH) = 7.5 Hz were attributed
to the Me ligand trans to P. The hydrogen attached to N
appeared as a broad doublet at d = 6.15. The pyridyl C[6]
proton of coordinated pyridine appeared at d = 8.42 with
3J(HH)
=
7.5 Hz. The 31P{1H} NMR spectrum of
[PtMe3I(PN)], 2a, is representative of the product. The value
1
of J(PtP) in this platinum(IV) complex is 1275 Hz, which is
considerably lower than the corresponding value of 2126 Hz
for the starting platinum(II) complex [PtMe2(PN)], 1a.
DFT-computed geometries for complexes 1 and 2
The possible geometries for complexes 1 and 2 were calculated
theoretically. The calculated bond distances and bond angles from
DFT-optimized structures (B3LYP/LANL2DZ)13 for compounds
1 and 2 are included in the Table 1 and 1S. The DFT-optimized
structures for these complexes are shown in Fig. 2.
As can be seen from Table 1, for complex 1a, the Pt–C bond
lengths for the two methyl groups are within the expected
Scheme 1
ꢀc
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Fig. 3 The changes in the UV-vis spectrum during the reaction of
[PtMe2(PN)] (2.64 ꢂ 10ꢁ4 M) and MeI (0.27 M) in benzene at T =
25 1C: (a) initial spectrum (before adding MeI); (b) spectrum at t = 0;
successive spectra recorded at intervals of 30 s, (c) final spectrum.
Fig. 2 Optimized structures of complexes 1 and 2.
determined from measurement at different temperatures
(Fig. S3w) and the data are given in Table 2. These reactions
followed good second-order kinetics, first order in both
platinum(II) and MeI reagent, with remarkable reproducibility
(ꢃ5%). These observations suggest an SN2 mechanism of
oxidative addition of methyl iodide to the platinum(II) complex.3
Also, the large negative values of DSz are typical of oxidative
addition by a common SN2 mechanism which involves nucleo-
philic attack of the metallic center to methyl group of MeI and
formation of a cationic intermediate [PtR2(PN)Me]+Iꢁ.
Table 1 DFT calculated bond lengths (A) of complexes 1–2 based on
the optimized structures
1a
1b
2a
2b
Pt1–P1
2.402
2.210
2.069
2.079
—
2.422
2.204
2.033
2.041
—
2.548
2.268
2.080
2.088
2.100
2.912
2.539
2.269
2.050
2.071
2.109
2.937
Pt1–N1
Pt1–C18
Pt1–C19
Pt1–C20
Pt1–I1
—
—
As shown in Table 2, in reactions of the Pt(II)–NN
complexes or the PN analog with MeI at different tempera-
tures and different solvents, the NN complexes reacted more
than 100–300 times faster than the corresponding PN analog.
For example, MeI in acetone at 25 1C reacted nearly 100 times
range for a Pt(II)-Me moiety.14 The Pt–C bond trans to the
phosphorus is longer (2.079 A) than that trans to nitrogen
(2.069 A), as would be expected based on trans influence
2
arguments and is in agreement with J(PtH) values reported
faster with [Pt(p-MeC6H4)2(bpy)]6 (k2 = 6.2 ꢂ 10ꢁ1 L molꢁ1 sꢁ1
)
for complex [PtMe2(PN)], 1a. Also, as expected, the bond
lengths of platinum(II) complexes, 1, is shorter than the
corresponding bonds for the product platinum(IV) complexes, 2.
For example the Pt–P bond in complex 1a is shorter (2.402 A)
than that in 2a (2.548 A). This confirms the larger 1J(PtP)
value of the complex 1a compared to that of the complex 2a.
Also, the Pt–P bond lengths for complexes 1a and 1b are 2.402
and 2.422 A, respectively, which is in agreement with the
1J(PtP) values found for these complexes in 31P NMR spectra
(2126 or 2016 Hz for complex 1a or 1b, respectively).
than with [Pt(p-MeC6H4)2(PN)] (k2 = 6.3 ꢂ 10ꢁ3 L molꢁ1 sꢁ1).
The same behavior has been seen in the reaction of dimethyl
analogues with MeI in benzene. This trend could be explained
by the influence of donating/accepting character of P and N
atoms. The p-acceptor character of the phosphorous ligands
stabilizes a metal center in a low oxidation state,8d so ligands
with P ligating atom, such as PN, decrease electron density on
platinum(II) core and make it reluctant to oxidative/addition
reactions based on SN2 mechanism. On the other hand
nitrogen s-donor ability in NN ligand, such as 2,20-bipyridine
makes the metal core more susceptible to oxidative addition
reactions8d that help to stabilize intermediate oxidation states,
so complexes with NN ligands undergo oxidative addition
reactions readily.
The kinetic study
The kinetics of oxidative addition of MeI to [PtR2(PN)]
(R = Me, 1a, in benzene and R = p-MeC6H4, 1b, in acetone)
were studied by using UV-vis spectroscopy. In each case, a
known excess of MeI reagent was used and the disappearance
of the MLCT band for 1a at l = 350 nm and for 1b at l = 340 nm
was used to monitor the reaction. The reactions followed good
first-order kinetics (Fig. 3 and S1w). Graphs of these first-order
rate constants against the concentration of the MeI gave
good straight line plots passing through origin, showing
a first-order dependence of the rate on the concentration of
MeI (Fig. S2w). Thus, the overall second-order rate constants
were determined. The activation parameters were also
Conclusions
New organoplatinum complexes [PtR2(PN)] (R = Me, 1a, and
p-MeC6H4, 1b, 2-(diphenylphosphinoamino)pyridine (= PN))
were synthesized by the reaction of [Pt(p-MeC6H4)2(SMe2)2]
or [Me2Pt(m-SMe2)2PtMe2] with 1 and 2, respectively, equiv. of
PN and the kinetic of reaction of MeI with [PtR2(PN)]
complexes was studied.
ꢀc
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Table 2 Second-order rate constantsa and activation parameters for reaction of MeI with [Pt(p-MeC6H4)2(PN)] or [Pt(p-MeC6H4)2(bpy)] in
acetone and [PtMe2(PN)] or [PtMe2(bpy)] in benzene
102 k2/Lmolꢁ1 sꢁ1 at different T/1C
DHz/kJ molꢁ1
DSz/J Kꢁ1 molꢁ1
10
20
25
30
40
Complex
Solvent
[Pt(p-MeC6H4)2(PN)]
[Pt(p-MeC6H4)2(bpy)]b
[PtMe2(PN)]
Acetone
Acetone
Benzene
Benzene
0.25
28
1.48
636
0.47
48
2.48
888
0.63
62
3.24
1093
0.87
83
4.13
1200
1.50
133
6.22
1699
41.8 ꢃ 0.6
34.9 ꢃ 0.4
33.4 ꢃ 0.7
21.6 ꢃ 1.4
ꢁ147 ꢃ 5
ꢁ132 ꢃ 2
ꢁ161 ꢃ 7
ꢁ153 ꢃ 5
[PtMe2(bpy)]c
a
b
c
Estimated errors in k2 values are ꢃ5%. bpy = 2,20-bipyridine, from ref. 6. From ref. 15.
According to the kinetic results, the oxidative addition
reaction of MeI with [PtR2(PN)] follows a good second order
kinetic, first order with respect to both reactants. The entropy
of activation, DSz, has a large negative value in each reaction
consistent with an SN2-type mechanism. The operative
mechanism involves nucleophilic attack of platinum(II) on
the carbon atom of MeI to give the transient cationic
platinum(IV) intermediate, [PtR2(PN)Me]+Iꢁ, which rapidly
rearranges to [PtR2(PN)MeI].
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9 (a) W. Seidel and H. Z. Scholer, Z. Chem., 1967, 11, 431;
It is interesting to note that the observed rate constants for
the reaction of PN complexes, [PtR2(PN)], with MeI are lower
than the related rate constants reported typically for NN
complexes e.g. [PtMe2(bpy)]3,15 and [Pt(p-MeC6H4)2(bpy)].6
This was attributed to the p-acceptance through the P atom
of PN ligand, which decreases the electron density of Pt(II) in
the complex with PN containing ligand.
Acknowledgements
We thank the Iran National Science Foundation (INSF),
Shiraz University Research Council, and the Islamic Azad
University, Shiraz Branch for financial support.
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ꢀc
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