Organoplatinum(IV) tris-chelate complexes, each having a cyclic metallacarbonate ring: Synthesis, characterization and kinetic studies of the formation

Article abstract of DOI:10.1039/b314611a

The complexes [Pt{(CH₂)₄}(NN)], 1a (NN = 2,2′-bipyridine) and 1b (NN = 1,10-phenanthroline) react with 2,3-epoxypropylphenyl ether in the presence of CO₂ to give tris-chelate platina(IV)cyclopentane complexes characterized by ¹H and ¹³C NMR spectroscopy as [Pt{(CH₂)₄}(CH ₂CHCH₂OPhOCO₂)(NN)], 2. The reactions proceed by the S_N2 mechanism and the rates were independent of concentration of CO₂. It is demonstrated that for 1a, the reaction proceeds 2.32 times faster than the similar reaction in which the dimethyl analog, [PtMe ₂ (2,2′-bipyridine)], is used. The analog tris-chelate complex [Pt{(CH₂)₄}(CH₂CHPhOCO₂)(phen)], 3a, was similarly synthesized.

Full text of DOI:10.1039/b314611a

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Organoplatinum(IV) tris-chelate complexes, each having a cyclic
metallacarbonate ring: synthesis, characterization and kinetic
studies of the formation
Mehdi Rashidi,* Nahid Shahabadi† and S. Masoud Nabavizadeh
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran.
E-mail: rashidi@chem.susc.ac.ir; Fax: ϩ98-711-228-0926; Tel: ϩ98-711-228-4822
Received 12th November 2003, Accepted 8th January 2004
First published as an Advance Article on the web 22nd January 2004
The complexes [Pt{(CH2)4}(NN)], 1a (NN = 2,2Ј-bipyridine) and 1b (NN = 1,10-phenanthroline) react with
2,3-epoxypropylphenyl ether in the presence of CO2 to give tris-chelate platina()cyclopentane complexes
characterized by 1H and 13C NMR spectroscopy as [Pt{(CH2)4}(CH2CHCH2OPhOCO2)(NN)], 2. The reactions
proceed by the SN2 mechanism and the rates were independent of concentration of CO2. It is demonstrated that
for 1a, the reaction proceeds 2.32 times faster than the similar reaction in which the dimethyl analog, [PtMe2
(2,2Ј-bipyridine)], is used. The analog tris-chelate complex [Pt{(CH2)4}(CH2CHPhOCO2)(phen)], 3a, was similarly
synthesized.
Introduction
There has been a great interest in the use of carbon dioxide as a
cheap feedstock for the production of organic materials. In this
regards, it is known that oxirans and carbon dioxide are cyclo-
added to give cyclic carbonates and a variety of transition metal
catalysts has been successfully used in the process.1–3 It is pro-
posed that the reactions are proceeded via cyclic metallacarb-
onate complexes. By the reaction of [PtMe2(NN)] (NN = 2,2Ј-
bipyridine, bpy, or NN = 1,10-phenanthroline, phen), which
bears an electron rich platinum() centre, with oxiranes and
carbon dioxide, it has been possible to prepare a series of cyclic
platinacarbonate complexes.4,5
In this study, this cycloaddition reaction has been performed
using [Pt{(CH2)4}(NN)], 1, to prepare tris-chelate organo-
Scheme 1
platinum() complexes, each containing a cyclic metallacarb-
onate ring. These are important complexes, as the metallacycles
are of considerable interest because of their possible role in
catalysis,6,7 and also organometallic tris-chelate complexes are
not common. In fact, to the best of our knowledge no tris-
chelate organoplatinum() complex has already been reported.
Also, kinetics of formation of the tris-chelate organo-
platinum() products are investigated and the results are com-
pared with results of similar studies of the dimethyl analog.4
scopy and H/C HETCOR experiments were performed to
ascertain the assignments.
In the 13C NMR spectrum, for the major isomer 2a, two
singlets with platinum satellites at δ = 16.6 (1J(PtC) = 748 Hz)
and δ = 18.6 (1J(PtC) = 748 Hz) were assigned to the two differ-
ent Cα atoms of platinacycle ring. These 1J(PtC) values are
rather larger than corresponding values for the dimethyl analog
[PtMe2(CH2CHCH2OPhOCO2)(bpy)] (1J(PtMe) = 719 Hz and
719 Hz),4 confirming the higher donor ability of CH2 groups of
the platinacycle ring compared with that of Me ligands.8–10 This
may be related to the kinetics of formation of these two analogs
as described in the next section. The two different Cα atoms of
platinacycle ring for the minor isomer 2aЈ appeared at δ = 16.7
(1J(PtC) = 748 Hz) and 17.6 (1J(PtC) = 748 Hz). The Cβ atoms
of platinacycle rings were also different and appeared at δ =
31.5 (2J(PtC) = 18 Hz) and 32.7 (2J(PtC) = 18 Hz) for isomer 2a
and at δ = 31.0 (2J(PtC) = 18 Hz) and 33.2 (2J(PtC) = 17 Hz) for
isomer 2aЈ. All these signals are appeared in the expected
regions.8 The carbon atom of CH2 group of metallacarbonate
ring resonated in Cα region4,8 with a large value of 1J(PtC) at δ =
27.2 (1J(PtC) = 726 Hz) and 25.9 (1J(PtC) = 726 Hz) for isomers
2a and 2aЈ, respectively. The Cβ atom of metallacarbonate ring
is connected to oxygen and therefore appeared at considerably
lower field at δ = 69.7 (2J(PtC) = 64 Hz) and 69.4 (2J(PtC) =
62 Hz) for isomers 2a and 2aЈ, respectively. Likewise, C atom of
CH2OPh substituent resonated at low field at δ = 74.1 (3J(PtC) =
40 Hz) and 73.8 (3J(PtC) = 40 Hz) for isomers 2a and 2aЈ,
respectively, and the data clearly indicate that it is situated in
the β-position to the platinum. Also see the experimental
Results and discussion
Synthesis and characterization of the platinum(IV) tris-chelate
complexes
The deep red complexes [Pt{(CH2)4}(NN)], 1a (NN = bpy) or
1b (NN = phen), reacted cleanly with excess 2,3-epoxy-
propylphenyl ether at 0 ЊC in acetone and in the presence of
CO2 gas to form, after 2 days, yellow solutions, from which
solid products having general formula [Pt{(CH2)4}(CH2CH-
CH2OPhOCO2)(NN)], 2 (Scheme 1), were obtained.
The platinum () product of each reaction was shown to
exist as an approximately 2 : 1 mixture of isomers, 2a and
2aЈ (NN = bpy) and 2b and 2bЈ (NN = phen), which could not
be separated. 2a and 2b are arbitrarily assigned as the major
isomers and 2aЈ and 2bЈ as the minor isomers, although the
opposite assignment is possible. The complexes 2 were fully
1
characterized by H NMR and specially 13C NMR spectro-
section for the data for C᎐O and bpy carbons.
᎐
† Present address: Chemistry Department, College of Sciences, Razi
University, Kermanshah, Iran.
The 1H NMR data are collected in the experimental section.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 6 1 9 – 6 2 2
619

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Particularly important is that CH2OPh protons and β-CH pro-
ton of metallacarbonate ring which are close to oxygen, reson-
ated at low field around 4.5 ppm. This and the H/C HETCOR
experiments were useful in assigning the structures. The two
isomers of the phen analog 2b and 2bЈ were similarly character-
ized by their data which are collected in the experimental
section.
In a similar manner, the tris-chelate analog [Pt{(CH2)4}-
(CH2CHPhOCO2)(phen)], 3a, was prepared as a mixture of
isomers by the reaction of [Pt{(CH2)4}(phen)] and styrene oxide
in the presence of CO2 and characterized by 1H and 13C NMR
spectroscopy (see the experimental section).
The kinetic study
Fig. 3 Plots of first-order rate constants (kobs/sϪ1) for the reaction of
The kinetics of addition of the 2,3-epoxypropylphenyl ether to
[Pt{(CH2)4}(NN)] in acetone was studied by using UV-vis
spectroscopy. In each case, excess epoxide was used and the
disappearance of the MLCT band at λ = 480 nm (1a) or 475 nm
(1b) was used to monitor the reaction. The reactions followed
good first-order kinetics (Figs. 1 and 2). Graphs of these first-
order rate constants against the concentration of epoxide gave
good straight line plots passing through origin, showing a first-
order dependence of the rate on the concentration of epoxide
(Fig. 3). Thus, the overall second-order rate constants were
determined. The activation parameters were also determined
from measurement at different temperatures (Fig. 4) and the
data are given in Table 1. These reactions followed good second-
order kinetics, first order in both platinum() and epoxide
reagent, with remarkable reproducibility and in the presence of
CO2, the results were unchanged. The rates of reactions in
benzene were too slow to be measured and therefore they were
[Pt{CH2}4(NN)] with 2,3-epoxypropylphenyl ether (᭿, T =15 ЊC,
NN=phen; ᭹, T =25 ЊC, NN =bpy; ꢀ, T =35 ЊC, NN =phen; ᮀ,
T =45 ЊC, NN =bpy) in acetone at different temperatures versus
concentration of 2,3-epoxypropylphenyl ether.
Fig. 4 Eyring plots for the reaction of [Pt{CH2}4(NN)] (᭿, NN=bpy;
᭹, NN=phen) with 2,3-epoxypropylphenyl ether in acetone.
measured in the more polar acetone solvent. Thus, all these
observations strongly suggest an SN2 mechanism of oxidative
addition of the epoxide to the platina()cycle11 and as the rates
are independent of concentration of CO2, a dipolar intermedi-
ate [{(CH2)4}(NN)Ptϩ–CH2CH(CH2OPh)OϪ] is formed by
nucleophilic attack of electron rich platinum() centre on the
least substituted carbon atom of the epoxide. Therefore, steric
effects determine the observed selectivity on formation of com-
plexes 2 in which the CH2OPh substitution in each complex is
located at the β-position to the platinum atom in the metalla-
carbonate ring. The large negative values of ∆S‡ are typical
of oxidative addition by a common SN2 mechanism which
involves a polar transition state.
Fig. 1 Absorbance–time curves for the reaction of [Pt{CH2}4(phen)]
with 2,3-epoxypropylphenyl ether (1.93–3.38 M, [ether] increases
reading downward) in acetone at 45 ЊC.
The ∆H‡/∆S‡ compensation plot of reactions of [Pt{CH2}4-
(NN)] and [PtMe2(bpy)] complexes with 2,3-epoxypropylphenyl
ether is shown in Fig. 5 which gives a good straight line. This
may be taken as evidence for operation of a common mechan-
ism (SN2) in this series of reactions.12,13
For the reactions of complexes [Pt{(CH2)4}(bpy)], 1a, and
[PtMe2(bpy)]4 with 2,3-epoxypropylphenyl ether at 25 ЊC, the
metallacycle complex 1a reacted 2.32 times faster than the di-
methyl analog complex [PtMe2(bpy)], which confirms the
stronger donor ability of the (CH2)4 group compared to that of
the CH3 groups, which indicates that the metallacycle complex
1a is more electron rich than the dimethyl complex [PtMe2-
1
(bpy)].8,9 This observation may also be related to the J(PtC)
values of Pt–C bonds in the platinum() products. Thus, the
rate of formation of the platina()cycle complex 2a (in which
the Pt–Cα bond is stronger with J(PtC) = 748 Hz) is greater
1
Fig. 2 First-order plots for the reaction of [Pt{CH2}4(NN)] with
2,3-epoxypropylphenyl ether at different temperatures in acetone:
(a) [ether] (3.38 M), T =15 ЊC, NN =bpy; (b) [ether] (2.41 M),
T =35 ЊC, NN =bpy; (c) [ether] (1.93 M), T =45 ЊC, NN =phen;
(d) [ether] (3.38 M), T =25 ЊC, NN=phen.
than that of the dimethyl platinum() complex [PtMe2(CH2-
CHCH2OPhOCO2)(bpy)], for which the Pt–C bond is weaker
with 1J(PtC) = 719 Hz. It may also be interesting to note that for
[Pt{(CH2)4}(NN)] complexes at different temperatures, when
D a l t o n T r a n s . , 2 0 0 4 , 6 1 9 – 6 2 2
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Table 1 Second-order rate constants and activation parametersa for oxidative addition of 2,3-epoxypropylphenyl ether to [Pt{CH2}4(NN)] in
acetone
Complex
T /ЊC
104 k2/dm3 molϪ1 sϪ1
∆H‡ b/kJ molϪ1
∆S‡ b/J molϪ1 KϪ1
[Pt{CH2}4(bpy)], 1a
15
25
35
45
15
25
35
45
25
1.13 0.07
2.13 0.03
3.90 0.06
5.95 0.05
1.49 0.04
2.53 0.06
4.54 0.08
6.82 0.09
0.92
40.1 1.9
Ϫ181
6
[Pt{CH2}4(phen)], 1b
36.7 1.4
Ϫ190
5
[PtMe2(bpy)]c, d
48
2
Ϫ160 15
a Values given based on 95% confidence limits from least square regression analysis. b Obtained from the Eyring equation. c Taken from ref. 4.
d The ratio k2 for [Pt{CH2}4(bpy)]/k2 for [PtMe2(bpy)] at 25 ЊC = 2.32.
H, 4.2; N, 4.4. C24H26N2O4Pt requires C, 47.9; H, 4.3; N, 4.7%.
NMR data: 13C, isomer 2a: δ = 16.6 [s, 1J(PtC) = 748 Hz, one Cα
1
of platinacycle ring], 18.6 [s, J(PtC) = 748 Hz, other Cα of
2
platinacycle ring], 31.5 [s, J(PtC) = 18 Hz, one Cβ of platina-
2
cycle ring], 32.7 [s, J(PtC) = 18 Hz, other Cβ of platinacycle
ring], 27.2 [s, 1J(PtC) = 726 Hz, Cα of metallacabonate], 69.7 [s,
2J(PtC) = 64 Hz, Cβ of metallacarbonate], 74.1 [s, 3J(PtC) = 40
Hz, CH OPh carbon], 157.4 [s, C᎐O], bpy carbons; 154.1 [C ],
᎐
2
2
122.1 [C3], 140.9 [C4], 126.1 [C5], 147.1 [C6]; isomer 2aЈ: δ = 16.7
[s, 1J(PtC) = 748 Hz, one Cα of platinacycle ring], 17.6 [s,
1J(PtC) = 748 Hz, other Cα of platinacycle ring], 31.0 [s, 2J(PtC)
2
= 18 Hz, one Cβ of platinacycle ring], 33.1 [s, J(PtC) = 17 Hz,
1
other Cβ of platinacycle ring], 25.9 [s, J(PtC) = 726 Hz, Cα of
metallacabonate], 69.4 [s, J(PtC) = 62 Hz, Cβ of metallacarb-
2
onate], 73.8 [s, 3J(PtC) = 40 Hz, CH OPh carbon], 158.4 [s, C᎐
Fig. 5 The ∆H‡/∆S‡ compensation plot of reaction of [Pt{CH2}4-
(NN)] (NN=bpy and phen), 1, and [PtMe2(bpy)] complexes with 2,3-
epoxypropylphenyl ether in acetone.
᎐
2
O], bpy carbons; 156.8 [C2], 122.3 [C3], 141.1[C4], 126.5 [C5],
147.4[C6]; 1H, δ = 0.5–1.6 and 2–2.5 [α-CH2 protons of platina-
cycle rings, for both isomers], 0.4–1.5 [β-CH2 protons of
platinacycle rings, for both isomers], 1.9 (major isomer) and
1.75 (minor isomer) [α-CH2 protons of metallacarbonate], 4.1
[β-CH protons of metallacarbonate, for both isomers], 4.4
[CH2OPh protons of metallacarbonate, for both isomers]; bpy
protons; isomer 2a: 8.32 [3J(H3H4) = 8.0 Hz, H3], 7.74 [3J(H4H5)
= 7.5 Hz, H4], 7.42 [3J(H5H6) = 6.2 Hz, H5], 8.65 [H6]; isomer
2aЈ: 8.45 [3J(H3H4) = 8.0 Hz, H3], 7.86 [3J(H4H5) = 7.5 Hz, H4],
7.66 [3J(H5H6) = 6.2 Hz, H5], 8.65 [H6] νmax/cmϪ1: 1608 and 1648
NN = phen, the reaction proceeds, as expected, slightly faster
than when NN = bpy (by a factor of 1.1–1.3), which further
confirms the operation of the SN2 mechanism. The reaction of
[Pt{CH2}4(phen)] and styrene oxide was too slow for any proper
kinetic studies.
It has been suggested that the coupling of epoxide with CO2
in the presence of electron rich catalysts which gives a cyclic
carbonate proceeds via the oxidative addition of epoxide to the
metallic centre followed by coupling of CO2 to give a cyclic
metallacarbonate.1,14 Thus the above results provide a strong
support for this mechanism.
(C᎐O).
᎐
Synthesis of [Pt{(CH2)4}(CH2CHCH2OPhOCO2)(phen)],
2b؉2bЈ
This was prepared similarly using [Pt{(CH2)4}(phen)]. Yield:
41%; mp 110 ЊC (decomp.). Found: C, 49.5; H, 4.2; N, 4.4.
C26H26N2O4Pt requires C, 49.9; H, 4.2; N, 4.5%. NMR data:
Experimental
1
The 13C and H NMR spectra were recorded as CDCl3 solu-
tions on a Bruker Avance DPX 250 MHz spectrometer and
TMS (0.00) was used as external reference. All the chemical
shifts and coupling constants are in ppm and Hz, respectively.
Kinetic studies were carried out by using a Philips PU 8675
spectrometer, with temperature control using a poly-science 900
constant temperature bath. Melting points were recorded on a
Buchi 530 apparatus and are uncorrected. The complexes
[Pt{(CH2)4}(bpy)]9 and [Pt{(CH2)4}(phen)]8 were prepared as
described previously.
1
13C, isomer 2b: δ = 20.5 [s, J(PtC) = 732 Hz, two Cα atoms of
platinacycle ring], 34–35 [Cβ atoms of platinacycle ring], 29.4 [s,
2
1J(PtC) = 713 Hz, Cα of metallacabonate], 72.0 [s, J(PtC) =
66 Hz, Cβ of metallacarbonate], 73.6 [s, 3J(PtC) = 41 Hz,
CH OPh carbon], 160.2 [s, C᎐O], phen carbons; 148.1 [C ],
᎐
2
2
126.3 [C3], 137.4 [C4], 128.6 [C5], 146.1 [C11], 131.5 [C12]; isomer
2bЈ: δ = 18.5 [s, 1J(PtC) = 732 Hz, two Cα atoms of platinacycle
ring], 34–35 [Cβ of platinacycle ring], 29.0 [s, 1J(PtC) = 713 Hz,
Cα of metallacabonate], 72.1 [2J(PtC) = 66 Hz, Cβ of metalla-
3
carbonate], 76.0 [s, J(PtC) = 40 Hz, CH2OPh carbon], 160.6 [s,
Synthesis of [Pt{(CH2)4}(CH2CHCH2OPhOCO2)(bpy)],
2a؉2aЈ
C᎐O], phen carbons; 147.6 [C ], 126.5 [C ], 137.5 [C ], 128.9 [C ],
᎐
2
3
4
5
1
145.8 [C11], 130.7 [C12]; H, δ = 0.5–1.5 and 2.5–3.3 [α-CH2 pro-
tons of platinacycle rings, for both isomers], 0.5–1.6 [β-CH2 pro-
tons of platinacycle rings, for both isomers], 2.10 (major isomer)
and 2.15 (minor isomer) [α-CH2 protons of metallacarbonate],
4.25 [β-CH protons of metallacarbonate, for both isomers],
4.5 [CH2OPh protons of metallacarbonate, for both isomers],
phen protons; isomer 2b: 9.27 [3J(H2H3) = 5 Hz, H2], 7.89
[3J(H3H4) = 8 Hz, H3], 8.57 [H4], 7.92 [H5]; isomer 2bЈ: 9.18 [H2],
To a solution of [Pt{(CH2)4}(bpy)] (0.1 g) in acetone (50 ml) a
large excess of 2,3-epoxypropylphenyl ether (3 ml) was added.
A stream of CO2 was bubbled through the resulting mixture at
0 ЊC while stirring for 2 h. The solution was then allowed to
stand at 0 ЊC for 2 days while stirring, after which a yellow color
had developed. This was filtered and the solvent was removed
under vacuum. The oily residue was purified from CH2Cl2/
ether. Yield: 34%; mp 125–127 ЊC (decomp.). Found: C, 47.4;
7.78 [H3], 8.51 [H4], 7.92 [H5] νmax/cmϪ1: 1608 and 1648 (C᎐O).
᎐
D a l t o n T r a n s . , 2 0 0 4 , 6 1 9 – 6 2 2
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Synthesis of [Pt{(CH2)4}(CH2CHPhOCO2)(phen)], 3a؉3aЈ
A plot of kobs versus [epoxide] was linear (Fig. 3), and the
slope gave the second-order rate constant. The same method
was used at other temperatures (15 ЊC, 35 ЊC and 45 ЊC), and
activation parameters were obtained from the Eyring equation
(eqn. 2 and Fig. 4). Similar procedures were performed in the
presence of excess CO2.
This was prepared similarly using [Pt{(CH2)4}(phen) ] and the
corresponding oxirane, styrene oxide. Yield: 28%; mp 115–
120 ЊC (decomp.). NMR data: 13C, isomer 3a: δ = 17.8 [s,
1J(PtC) = 745 Hz, one Cα of platinacycle ring], 27.1[s, 1J(PtC) =
745 Hz, other Cα of platinacycle ring], 33.4 [s, 2J(PtC) = 23 Hz,
2
one Cβ of platinacycle ring], 34.2 [s, J(PtC) = 21 Hz, other Cβ
1
of platinacycle ring] 28.7 [s, J(PtC) = 719 Hz, Cα of metalla-
(2)
2
cabonate], 79.1 [s, J(PtC) = 63 Hz, Cβ of metallacarbonate],
160.0 [s, C᎐O], phen carbons; 147.8 [C ], 126.9 [C ], 139.2 [C ],
᎐
2
3
4
In a similar way the rate constants and activation parameters
were determined for the reaction of [Pt{(CH2)4}(phen)] (at λ =
475 nm) with the epoxide and the results are collected in
Table 1.
128.1 [C5], 146.6 [C11], 131.5 [C12]; isomer 3aЈ: δ = 14.7 [s, one
Cα of platinacycle ring], 24.2 [s, other Cα of platinacycle ring],
33 [s, one Cβ of platinacycle ring], 34.6 [s, other Cβ of platina-
1
cycle ring], 30.0 [s, J(PtC) = 719 Hz, Cα of metallacabonate],
75.2 [s, Cβ of metallacarbonate], 159.1 [s, C᎐O], phen carbons;
᎐
148.1 [C2], 126.5 [C3], 139.5 [C4], 128.5 [C5], 146.4 [C11],
130.7 [C12]; 1H, δ = 0.9–1.6 and 2.4–2.8 [α-CH2 protons of
platinacycle rings, for both isomers], 0.6–1.1 [β-CH2 protons of
platinacycle rings, for both isomers], 2.04 (major isomer) and
1.83 (minor isomer) [α-CH2 protons of metallacarbonate], 5.12
and 5.02 [β-CH protons of metallacarbonate, for both isomers],
phen protons; isomer 3a: 9.26 [3J(H2H3 = 5 Hz, H2], 7.85
[3J(H3H4) = 8.5 Hz, H3], 8.45 [H4], 7.94 [H5]; isomer 3aЈ: 9.21
[3J(H2H3 = 5.1 Hz,H2], 7.82 [3J(H3H4) = 8.3 Hz H3], 8.55 [H4],
7.91 [H5].
Acknowledgements
We thank the Shiraz University Research Council, Iran (Grant
No. 78-SC-1189-657) for financial support.
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1988, 110, 3704.
Abst = Abs∞ ϩ (Abs0 Ϫ Abs∞) exp (Ϫkobst)
(1)
D a l t o n T r a n s . , 2 0 0 4 , 6 1 9 – 6 2 2
622