Thermolysis studies on platinacycloalkane complexes

Article abstract of DOI:10.1016/j.jorganchem.2011.03.019

Thermal decomposition studies on platinacycloalkanes of the type Pt(CH ₂)_mL₂ (where m = 6,7,8,10 and L₂ = dppp {1,3-bis(diphenylphosphino)propane}, dppe {1,2-bis(diphenylphosphino) ethane} or L = PPh₃, ^tBu₃P) are described. The results reveal that the organic product distribution depends on various factors such as the nature of ligand, the metal system, the mode of decomposition, the ring size and the temperature. Possible mechanistic pathways for the formation of various products are discussed. These platinacycloalkanes can be used as models for metallacycloalkane intermediates in catalytic reactions.

Full text of DOI:10.1016/j.jorganchem.2011.03.019

Journal of Organometallic Chemistry 696 (2011) 2457e2465
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Thermolysis studies on platinacycloalkane complexes
,
b
,
a
Feng Zheng a, Akella Sivaramakrishna a , John R. Moss
*
a Department of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
b Organic Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermal decomposition studies on platinacycloalkanes of the type Pt(CH2)mL2 (where m ¼ 6,7,8,10 and
L2 ¼ dppp {1,3-bis(diphenylphosphino)propane}, dppe {1,2-bis(diphenylphosphino)ethane} or L ¼ PPh3,
tBu3P) are described. The results reveal that the organic product distribution depends on various factors
such as the nature of ligand, the metal system, the mode of decomposition, the ring size and the
temperature. Possible mechanistic pathways for the formation of various products are discussed. These
platinacycloalkanes can be used as models for metallacycloalkane intermediates in catalytic reactions.
Ó 2011 Elsevier B.V. All rights reserved.
Received 2 February 2011
Received in revised form
9 March 2011
Accepted 17 March 2011
Keywords:
Thermal decomposition
Platinacycloalkane
Reaction mechanism
Product analysis
1. Introduction
stability of the different platinacycloalkanes with different ring
sizes and to discuss the range of organic products produced upon
Metallacycloalkane complexes have been extensively studied
[1] and widely reported [2e21], and are known to be key inter-
mediates in many catalytic reactions. For example, the reactions
such as ethylene trimerisation, tetramerisation and oligomerisation
to higher linear alkenes proceed through the intermediacy of
metallacycloheptanes, metallacyclononanes and other larger size
rings [22e28]. Thermolysis of these metallacycloalkanes is also
important in generating thin films as well as producing alkenes and
other some interesting organic products depending on various
factors such as nature of ligands, temperature, metal and solvent
[29]. In the literature, thermal decomposition studies on metal-
lacycloalkanes have thus far been restricted to small ring size
compounds [3,29]. No kinetic studies have been reported due to the
limitation of the amount and stability of the medium and large
metallacycloalkane complexes available. Recently, we have
discovered a novel route to medium and large, even and odd
membered metallacycloalkanes from their metalealkenyl precur-
sors, using a ring-closing metathesis reaction with Grubbs’ catalysts
(Scheme 1) [30e33]. In light of this, we have carried out thermol-
ysis studies on a variety of metallacycloalkanes with Pt, Pd, Ir and
Rh [34,35]. The purpose of this present article is to assess the
their decomposition.
2. Experimental
General Considerations: All manipulations were carried out
under
a nitrogen atmosphere unless otherwise stated. The
solvents were commercially available and distilled from dark
purple solutions of benzophenone ketyl. 1H, and 31P NMR spectra
were recorded on a Bruker DMX-400 spectrometer and all 1H
chemical shifts are reported relative to the residual proton reso-
nance in the deuterated solvents. GC analyses were carried out
using a Varian 3900 gas chromatograph equipped with an FID and
a 30 m ꢀ 0.32 mm CP-Wax 52 CB column (0.25
mm film thickness).
The carrier gas was helium at 5.0 psi. The oven was programmed to
hold at 32 ꢁC for 4 min and then to ramp to 200 ꢁC at 10 deg/min
and hold 5 min. GCeMS analyses for peak identification were
performed using an Agilent 5973 gas chromatograph equipped
with MSD and a 60 m ꢀ 0.25 mmRtx-1 column (0.5
mm film
thickness). The carrier gas was helium at 0.9 mL/min. The oven was
programmed to hold at 50 ꢁC for 2 min and then ramp to 250 ꢁC at
10 deg/min and hold 8 min. The platinacycloalkane compounds
1e7 were prepared by ring-closing metathesis (RCM) of bis(1-
alkenyl)platinum(II) complexes using Grubbs’ 1st generation
catalyst. The bis(1-alkenyl)platinum(II) complexes were prepared
by the transmetalation reaction of 1-alkenyl Grignard reagents
with corresponding dichloroplatinum(II) complexes, and then
converted into platinacycloalkenes using the RCM reaction. These
* Corresponding author. Organic Chemistry Division, School of Advanced
Sciences, VIT University, Vellore 632 014, Tamil Nadu, India. Tel.: þ91 416 220 2334.
E-mail address: asrkrishna@vit.ac.in (A. Sivaramakrishna).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.019

2458
F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
The thermal decomposition of 1 and 2 was investigated at 170 ꢁC
2 moles
BrMg(CH2)nCH=CH2
(n = 2, 3)
over a period of time. These experiments were carried out at 170 ꢁC
because this temperature resulted in a higher yield of 1-octene.
The amount of 1-octene from the thermal decomposition of 1
increased with decomposition time. A similar result was obtained
in the thermal decomposition of 2. Complex 1 decomposed at
a much slower rate than 2. Formation of 1-octene was complete
after 2.5 h for 1, while for 2, after 1 h no further formation of 1-
octene could be detected. The organic products obtained after
decomposition in both cases consisted of a mixture of alkenes, with
1-octene as the major product, while 2-octenes, 1,7-octadiene and
octane were present as minor products.
L
Cl
Cl
n
L
Pt
Pt
Et2O, -78°C
L
L
n
Grubbs
catalyst
(5 mol%)
CH2Cl2,
reflux
Pd/C, H2
MeOH - CH2Cl2
L
L
L
n
n
(CH2)m
Pt
Pt
All the decomposition experiments were carried out at
170 ꢂ 5 ꢁC for 2 h which we consider as standard decomposition
conditions for the thermolysis of the platinacycloalkanes.
L
1, m = 8, L2 = dppp ;
2, m = 8, L2 = dppe;
3, m = 8, L = But3P;
4, m = 7, L2 = dppp;
5, m = 7, L2 = dppe;
6, m = 6, L = PPh3;
7, m = 6, L2 = dppe;
8, m = 10, L2 = dppp
3.2. Kinetic studies
3.2.1. Kinetics of thermal decomposition of 1 and 2
Kinetic studies were carried out on the thermal decomposition
of the platinacycloalkanes 1 and 2 using a similar procedure to that
reported by Whitesides and co-workers for the smaller ring
compounds [36e38]. The principal reason for choosing these
complexes were because of their stability to air and that they have
a high decomposition temperature.
Scheme 1. Preparation of platinacycloalkanes.
The rates of thermal decomposition of platinacyclononanes L2Pt
(CH2)8 (1, L2 ¼ dppp; 2, L2 ¼ dppe) in cyclohexane were examined
by GC following the appearance of the total hydrocarbon products
from decomposition, which include 1-octene, n-octane, 2-octenes,
1,7-octadiene and cyclooctane. Results were obtained by following
the total concentration of the products relative to that of an added
internal standard (chlorobenzene).
The rate constants for the decompositions were calculated
according to Eq.(1) Where, a0 ¼ [Pt compound]0, x ¼ [total
hydrocarbon products]t.
complexes were then hydrogenated to yield the corresponding
platinacycloalkanes (Scheme 1) [30e33].
3. Results and discussion
Our first objective was to identify the decomposition pathways
for the metallacycloalkanes although it was not always possible to
ascertain all the product-forming reactions, and to assess their
relative importance within various combinations of metal, ligand
and ring size. The second objective was to establish decomposition
rates for an analysis of the comparative thermal stability of
different ring sizes and to understand the effect of additional
neutral ligands on decomposition pathways.
ln½a0=ða0 ꢃ xÞꢄ ¼ kt
(1)
The examination of rate constants for the thermal decomposi-
tion of complexes 1 and 2 was carried out under different condi-
tions and at different concentrations in cyclohexane. The results
obtained have been summarized in Table 1.
3.1. Temperature and time effects on thermolysis of 1 and 2
The decomposition of 1 at 80 ꢁC for 24 h in cyclohexane was
found to be first order with a rate constant (7.0 ꢀ 10ꢃ7
s
ꢃ1). The
The data for the examination of temperature and time effects on
the thermal decomposition of 1 and 2, using cyclohexane as
solvent, were obtained by monitoring the appearance of 1-octene.
The studies of temperature effects were carried out by heating 1
and 2 at various temperatures for the same period (2 h). The
organic products obtained were analysed by injecting the solution
phase, once cooled to room temperature, into a GC.
extent of decomposition [39] of 1 was, however, ca. 6% even after
24 h which indicates complex 1 is quite stable at 80 ꢁC. Compared
with the decomposition for 2 h from which only 1-octene and
octane were formed, after decomposition for 4 h at 80 ꢁC, 2-octene
was also found. This suggests that the formation of 2-octene might
result from isomerisation of 1-octene in a secondary reaction,
which was not only dependent on decomposition temperature but
also on the heating time.
Thermal decomposition of 1 at 80 ꢁC for 2 h only afforded 1-
octene (65%) and n-octane (35%). When the temperature was
increased to above 120 ꢁC, 2-octenes and 1,7-octadiene were also
formed. The amounts of 1-octene as well as the total amount of
hydrocarbon products increased with the temperature when
heating for the same period. In contrast, decomposition of 2 pro-
ceeded much more rapidly, and gave 1-octene (34%), octane (19%),
2-octenes (28%) as well as 1,7-octadiene (16%) and a small amount
of cyclooctane (3%) when the thermolysis was carried out at 80 ꢁC
for 2 h. The product distribution of 2 at different temperatures
varied slightly, but the formation of products remained unchanged.
The amount of 1-octene formed was also found to increase with
temperature. It is interesting to note that a high temperature is
necessary for the thermal decomposition of platinacycles with
stable chelating ligands, i.e. dppe and dppp.
The decomposition of 1 at 170 ꢁC was first order only over
approximately the first 30% of the decomposition for the low
concentration, and over ca.20% for the higher concentration (Fig.1a).
Rate constants in the linear region were independent of the initial
Table 1
Rate constants of the thermal decomposition of 1 and 2 in cyclohexane.
Compound
Concentration (mM)
Rate constant
80 ꢁC (sꢃ1
)
170 ꢁC (k ꢀ 104 sꢃ1
e
)
1
1
1
2
23.2
23.2
2.32
5.74
7.00 ꢀ 10ꢃ7
e
e
e
0.471
0.485
2.01

F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
2459
a 50
b 0.8
45
40
35
30
25
20
15
10
5
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
2
4
6
8
10
0
2
4
6
8
10
-3
Time (s x 10 )
-3
Time (s x 10 )
1 (23.2mM) 1 (2.32mM) 2 (5.74mM)
1 (23.2mM) 1 (2.32mM) 2 (5.74mM)
Fig. 1. Representative speciation (a) and first order (b) plots for the thermal decomposition of 1 (23.2 mM A & 2.32 mM -) and 2 (5.74 mM :) at 170 ꢁC.
concentration of the platinum complex (Fig. 1b). The observed rate
constant for 2 was 4 times greater than that for 1 (Table 1), which
again showed that the thermal stability of platinacyclononane with
dppeligand islowerthanthat withdpppligand. Theproducts formed
from thermal decomposition of 1 at both concentrations were found
to be the same with similar distributions of organic products. Not
surprisingly, the product formation and distribution obtained from
decomposition of 2 was different which could be due to the stability
as a result of the ancillary ligand.
of 2. The same phenomenon was observed in the thermal decom-
position of bis(phosphine)platinum(II) metallacyclopentanes [36].
The decomposition of 2 with added dppe ligand in cyclohexane
gave a clear golden yellow solution. The organic products from the
reaction of 2 with one equivalent of dppp at 170 ꢁC are shown in
Eq.(4) and the metal-containing product was identified as
[Pt0(dppe)2] by 1H and 31P NMR spectroscopy. The major singlet at
d
4.49 in 31P NMR spectrum shifted to
d 32.35 after 2 h, and agreed
with the literature report; no sign of PteH bonds or hydridic
hydrogen were observed [40,41]
3.2.2. Kinetics of thermal decomposition of 1 and 2 with added
ligands
PPh2
Pt0
The rates of thermal decomposition of platinacyclononanes L2Pt
(CH2)8 (1, L2 ¼ dppp; 2, L2 ¼ dppe) in cyclohexane, with added
ligand, were examined by GC following the appearance of the total
hydrocarbon products from decomposition. Using the same ligand
that is already present in the complexes avoids complications of
ligand exchange.
The decomposition of 1 in the presence of added bis(diphenyl-
phosphino)propane (dppp) ligand was carried out in cyclohexane
with 0.1e20 ratios of added dppp to 1. The kinetic behaviour in all
cases was first order only over approximately the first 30% of
decomposition (Fig. 2a). Rate constants in this region were
dependent on the concentration of added dppp and the linear
region increased when the concentration of dppp increased
(Fig. 2c). The overall rate in this region can be expressed by a two-
term rate expression (Eq.(2)) [36], where ka ¼ 4.85 ꢀ 10ꢃ5 sꢃ1 and
kb ¼ 1.69 ꢀ 10ꢃ5 sꢃ1 at 170 ꢁC, [dppp] (mM): (0.224, 22.4).
Ph2P
PPh2
Ph2
P
Pt
Ph2P
+
170oC
+
dppe
(4)
cyclohexane
P
organic prodcuts
Ph2
2
1-octene
40%
+
n-octane
25%
+
2-octenes
16%
+
1,7-octadiene + cyclooctane
2%
17%
These results are in good agreement with the reported reaction
of dppe with [PtPh2(dppe)] (Eq.(5)) [42].
490K
Â
Ã
½PtPh ðdppeÞꢄ þ dppe
PtðdppeÞ þ Ph2
(5)
!
2
2
In contrast, an intense red residue was formed when complex 1
decomposed without added dppp, which was insoluble in cyclo-
hexane and we believe it is to be due to Ptn(L)m clusters [43].
d½1ꢄ
ꢃ
¼ ðka þ kb½dpppꢄÞ
(2)
dt
3.2.3. Kinetics of thermal decomposition of 5 and 8
The thermal decomposition of 2 with added bis(diphenylphos-
Kinetic studies were carried out on the thermal decomposition
of the complexes of the type Pt(CH2)n(dppp) (5, n ¼ 7; 8, n ¼ 10) in
order to examine the effects of different ring sizes on the thermal
stability of platinacycloalkanes. The rates of decomposition of 5 and
8 in toluene-d8 at 90 ꢁC were determined by following the disap-
pearance of 31P peak of the starting platinacycloalkanes using
31P{1H} NMR spectroscopy [44,45].
phino)ethane (dppe) was studied in the same way, and showed the
same general kinetic features (Fig. 2b). Added dppe ligand also
accelerated the reaction; the overall rate in the linear region can be
expressed as shown in Eq.(3) by the similar rate expression as
Eq.(2), where kc ¼ 2.01 ꢀ 10ꢃ4 sꢃ1 and kd ¼ 4.96 ꢀ 10ꢃ6 sꢃ1 at
170 ꢁC, [dppe] (mM): (5.94, 118.8).
Thermal decomposition of 5 at 90 ꢁC was much slower than that
of 8. The decomposition of 8 was complete after heating for 5 h,
which was indicated by the total disappearance of the starting
material peak in the 31P NMR spectrum. However, the decompo-
sition of 5 was complete after 48 h. Whitesides and co-workers
have found that five- and six-membered platinacycloalkanes are
d½2ꢄ
ꢃ
¼ ðkc þ kd½dppeꢄÞ
(3)
dt
The added phosphine ligand influences the formation of not only
the major products (1-octene & n-octane), but also the minor prod-
ucts (2-octenes & 1,7-octadiene), particularly for the decomposition

2460
F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
1.4
1.2
1
a
b
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1-I
2-I
0.8
0.6
0.4
0.2
0
1-II
1-III
1-IV
1-V
2-II
2-III
2-IV
2-V
0
2
4
6
8
10
0
1
2
3
4
5
6
-3
Time (s x 10 )
-3
Time (s x 10 )
c
4.5
4
3.5
3
2.5
2
1
2
1.5
1
0.5
0
0
5
10
15
20
25
[Added ligand] / [Pt]0
Fig. 2. a: The rate of thermal decomposition of 1 depends on the concentration of dppp in solution. Data are for runs in cyclohexane, having [1]0 ¼ 2.32 mM and these values for
[dppp] (mM): 1eI, 0.0; 1eII, 0.224; 1eIII, 2.24; 1eIV, 22.4; 1eV, 44.8. b: The rate of thermal decomposition of 2 depends on the concentration of dppp in solution. Data are for runs
in cyclohexane, having [2]0 ¼ 5.74 mM and these values for [dppp] (mM): 2eI, 0.0; 2eII, 0.594; 2eIII, 5.94; 2eIV, 59.4; 2eV, 118.8. c: Observed rate constants (kobs) for the thermal
decomposition of 1 (A) and 2 (-) as a function of the ratio of added ligand to Pt complex. These curves are derived from the data of a and b.
markedly more stable than the platinacycloheptane [36]. In this
study, we found that the eight-membered platinacycloalkane 5 is
significantly more stable than the larger and conformationally
more mobile eleven-membered ring 8.
comparisons. The first method was to collect the volatile products
using trap-to-trap distillation, and the second one was addition of
appropriate solvent to extract the products. The products obtained
were found to be same using either method, however, the relative
amount of individual product that could be recovered by the former
method was clearly less than the later one.
3.3. Products of decomposition of platinacycloalkanes
Besides this, it was practically difficult to handle a trap-to-trap
distillation when the amount of metallacycle complex was so small
that it could only result in a trace of volatile products. For the
results reported here, we used the method of adding an appropriate
solvent to extract the volatile products when the thermal decom-
position was carried out under solvent free conditions.
The results presented in Table 2 show that the overall decom-
position patterns for the platinacycloalkanes are independent of
the ring size. These complexes can decompose thermally in several
ways to give a range of organic products, including 1-alkenes,
The non-kinetic decomposition studies of platinacycloalkanes
1e6 (Chart 1) were carried out both in solvent and solvent free under
the standard conditions. Heating a ca.10 mg solid sample or 0.5 mL of
0.02 M sample solution in cyclohexane or toluene at 170 ꢁC for 2 h
resulted in formation of a mixture of alkene and n-alkane products
(see Table 2) accompanied by transformation of the solid, or solution,
from its original pale yellow colour to an intense red colour.
Two methods of product extraction were used for the solvent
free thermal decomposition of complex 1 in order to draw
L
L
L
L
L
L
Pt
Pt
Pt
Pt
L
L
1, L2 = dppp;
4, L2 = dppp;
5, L2 = dppe
2, L2 = dppe;
6, L = PPh3;
7, L2 = dppe
8, L2 = dppp
3, L = But3P
Chart 1. Platinacycloalkane complexes 1e8.

F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
2461
Table 2
in cyclohexane and in dichloromethane. Under both solvent free
conditions and in non-halogenated solvent, only linear C8 products
were formed. In contrast, small amounts of cyclooctane were also
obtained when the decomposition of these two complexes was
carried out in dichloromethane. In a solvent capable of oxidative
addition, like dichloromethane, formation of cyclobutane and C5
materials from the decomposition of platinacyclopentanes was
reported by Whitesides and co-workers [38]. We propose that
cyclooctane could be formed in the same way as reported for the
formation of cyclobutane [38]. Platinum(IV) intermediates were
produced by oxidative addition of dichloromethane to platinum(II)
metallacycles, which then generated cyclooctane by reductive
elimination (Eq.(7)). The products with one more carbon than the
metallacyclic group were not observed in this study. In addition, it
was observed that the decomposition residue in dichloromethane
for complex 1 formed white needles with a melting point at
155e160 ꢁC. This residue was identified by 1H and 31P NMR, as the
starting dichloride platinum(II), PtCl2L2.
Products for the thermal decomposition of the platinacycloalkanes 1e7 at 170 ꢁC for
2 h.
Complex Medium
Products observed (%)a
1-alkene 2-alkenes 1,n-diene n-alkane Cycloalkane
1
Solid
Cyclohexane 46
49
21
17
21
21
21
20
24
26
14
22
20
13
27
23
4
4
4
13
12
9
5
6
9
<1
2
26
33
44
23
28
17
20
28
32
35
28
40
20
24
e
e
4
2
2
e
e
8
e
e
e
e
e
e
CH2Cl2
Solid
Cyclohexane 37
Solid 54
Cyclohexane 51
CH2Cl2
Solid
Toluene
Solid
27
41
2
3
32
45
43
50
43
41
44
4
5
Toluene
solid
solid
3
13
9
6
7
a
Products were analysed by GC and GCeMS within the error limits of ꢂ3% for
each determination.
CH2Cl
L
2-alkenes, n-alkanes and dienes (only complex 2 decomposed in
dichloromethane to give cycloalkane).
CH2Cl2
L
(7)
Pt
Pt
Cl
175oC, 2h
L
The formation of 1-alkenes and 2-alkenes is known from the
decomposition of metallacyclopentanes [46], -hexanes and
eheptanes [36], which was in agreement with our observations of
the decomposition of these larger ring-size metallacycloalkanes.
Assuming that the reaction pathway to form 1-alkene takes place
L
A trend of increasing formation of n-alkane was observed when
complexes 1 and 3 decomposed in different media: solvent free < in
solvent (cyclohexane < dichloromethane). The same phenomenon
was found from the decomposition of complexes 2, 4 and 5, whereas
the relative yield of n-alkane is slightly less under solvent free
conditions than in solvent. The chosen solvents for the decompo-
sition studies were cyclohexane, dichloromethane and toluene.
Nevertheless, the fact that n-alkane formed under solvent free
conditions indicates that the hydrogen-atom responsible for
producing the saturated alkane is not only from the solvent but
could be from other sources. The most likely sources are considered
to involve two components: 1) platinum hydride species produced
by releasing diene from platinacycles (Eq.(8)) [45] and 2) hydrogen
transferred from coordinated ligand, e.g. ortho activation of the
phenyl group of PPh2 [51e53].
via the
b-hydride elimination, the hydridoalkenylplatinum(II)
complex should be a key intermediate. Attempts were made to
isolate this intermediate and we carried out a reaction of (dppe)PtCl
(1-pentenyl) with LiAlH4 in THF at room temperature. The reaction
was monitored by 1H NMR, but no hydride signal could be
observed. The reaction solution was then injected to the GC,
n-pentane (24%) and 1-pentene (76%) were identified (Eq.(6)),
which suggests that the alkenyl hydride intermediate is not stable
under these conditions and if formed, undergoes reductive elimi-
nation to give 1-pentene as the major product.
Ph2
P
Ph2
P
THF
Pt
+
Cl
P
Pt
RT
Ph2
(6)
H
P
Ph2
30min
+
LiAlH4
Not isolated
(8)
Attempts to isolate the hydridoalkenylplatinum(II) complex by
reacting a chloride hydridoplatinum(II) complex with Grignard
reagent at ꢃ78 ꢁC also failed as no hydride signal was obtained from
1H NMR after reaction. The products formed were 1-alkene and
n-alkane which were analysed by GCeMS [47].
Due to the absence of any direct evidence, it is suggested that
formation of the alkenyl hydride intermediate might take place
rapidly and perhaps reversibly, or the formation of 1-alkene may
not involve such an intermediate. An alternative pathway might be
via a concerted metal-assisted hydride transfer route [48e50]. The
formation of 2-alkenes could probably be caused by the occurrence
of isomerisation either during or after decomposition. In addition,
the corresponding n-alkane was also obtained as one of the major
products (>20%) in all cases.
The thermal decomposition of platinacycloheptanes 6 and 7 was
first reported by Whitesides and co-workers: decomposition of
platinacycloheptanes in dichloromethane at 60 ꢁC afforded 1-alkene
and 2-alkenes only [36]. In contrast, many more products formed
when these complexes decomposed at higher temperature (170 ꢁC)
in this study. Higher thermolysis temperatures are required for the
stable platinacycle complexes, especially for those with chelating
ligands, which might make available additional reaction pathways.
3.4. The mechanisms of the thermal decomposition of
metallacycloalkanes
The thermolysis products of 1e5 obtained in solvent and solvent
free conditions were compared to determine the influence of
solvent on the decomposition pathways. The product formation
patterns were similar although the relative yields of the products
were different in the different decomposition media. The decom-
position of 1 and 3 were carried out in three media, i.e. solvent free,
The decomposition pathways of metallacycloalkanes docu-
mented so far are not only relevant to those of alkyl complexes, also
relevant to decomposition of metallacycles in catalytic reactions
e.g. ethylene oligomerization [22e28]. We try to draw a detailed
mechanism based on these decomposition pathways to interpret

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F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
L
H
Reductive
elimination
β-H
L
L
+
M
L
M
L
abstraction
M
L
a
L
L
L
Ligand
displacement
Agostic assisted
hydride shift
M
L
L
+
H
M
L
M
b
M-H
L
L
M-H
+
L
M
M
L
H
c
Scheme 2. Possible mechanisms for the formation of 1-octene from a metallacyclononane.
β-H
Isomerisation
L
L
&
M
transfer
a
H
L
L
L
L
Agostic assisted
hydride shift
M
M
L
L
H
M
3-B-2
3-B-1
L
L
L
M
&
&
M
L
b
Scheme 3. Possible mechanisms for the formation of 2-alkenes from a metallacyclononane.

F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
2463
H
L
L
M
octenes
+
L2
+
M
H
a
H
M
H
L
L
L
L
M
L
L
H
M
b
Scheme 4. Possible mechanisms for the formation of n-alkane from a metallacyclononane.
the formation of various products by
a
- and
b
-hydride elimination,
authors pointed out that the appearance of an internal alkene,
ReCH¼CHeR0, could result from the isomerisation of the initially
formed 1-alkene (Scheme 3a) [55]. And some reported that it could
be presumably explained by a metal hydride-catalyzed intra-
molecular isomerisation before the olefin is released into solution
[36]. The proposed mechanism is outlined in Scheme 3b [56]:
ligandehydrogen abstraction, reductive elimination and homolytic
MeC bond cleavage. Some of the discussions on the mechanisms
that follow are based on the literature, and the rest is a consequence
of our recent studies.
3.4.1. The formation of 1-alkene
b-hydride elimination followed by metal-assisted hydrogen shifts
The best documented reaction pathway to produce 1-alkene is
via b-hydride elimination from metalealkyl complexes. This could
also be the possible decomposition mechanism for the formation of
1-alkene from metallacycloalkanes with flexible medium to lager
ring size, even though this pathway is reported to be hindered in the
to form a viable intermediate 3-B-1 which could rise for 1-alkene or
2-alkenes depending upon the regioselectivity of hydrogen
migration through the formation of 3-B-2 [57]. Our experiments
suggest that the metalealkenyl and metalealkyl compounds can
also isomerize 1-alkenes to the corresponding internal 2-alkenes
catalytically in solution [58].
small metallacycloalkanes.
b-Hydride elimination is formally meant
as -hydride abstraction by the metal, followed by reductive elimi-
b
nation of the resultant alkenyl hydride species (Scheme 2a) [54]. The
alternative mechanism for generation of 1-alkene is via a concerted
metal-assisted hydride transfer from C2 to Cn of the metal-
lacycloalkane which was supported by the calculations for Ti and Cr
catalytic trimerisation systems (Scheme 2b) [48e50]. Nowadays,
3.4.3. The formation of n-alkane
For the thermal decomposition of metalealkyl complexes, it has
been suggested that with only a single alkyl ligand, alkanes should
not form unless an intermolecular reaction (Eq.(9)) occurs [52].
MeH þ MeR/2M þ RH
(9)
these two mechanisms are termed as “b-hydride transfer” due to the
difficulty of distinguishing them in various metallacyclic systems.
Furthermore, Miller and Whitesides demonstrated an intermolec-
ular chain reaction pathway for the formation of 1-alkene, in which
a hydridoplatinum intermediate was involved [45] (Scheme 2c).
Similarly, the formation of alkane from the decomposition of
metallacycloalkane complexes should only occur with an extra
hydrogen source other than the metallacyclic moieties such as
solvent, metalehydride species and hydrogen transferred from
coordinated ligand. Therefore, the mechanism of the formation of
n-alkane could be proposed in three ways: intermolecular
hydrogen abstraction (Scheme 4a), intramolecular hydrogen
abstraction (Scheme 4b) and the combination of intermolecular
and intramolecular hydrogen abstraction. Thermal decomposition
studies of 1 in CD2Cl2 gave n-octane as the major product but with
no incorporation of deuterium observed by GCeMS.
3.4.2. The formation of 2-alkenes
While a reaction pathway such as that shown in Scheme 2
accounts well for the formation of 1-alkenes, ReCH¼CH2, some
H
β-H
L
β-H
M
elimination
elimination
L
P
L
M
L
P
L
L
L
P
P
R
R
Reductive
elimination
Pt
Pt
Pt
L
a
D-2
D-1
D-3
Scheme 5. Possible mechanisms for the formation of dienes and cycloalkane from
a metallacyclononane.
Chart 2. Possible intermediate species in the presence of additional neutral ligands.

2464
F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
P
P
P
Reductive
elimination
P
P
P
P
Pt
Pt
P
E-1
β-hydride
elimination
P
P
P
Pt
P
Reductive
elimination
P
P
Pt
H
P
+
P
alkenes
E-2
Scheme 6. Proposed mechanism for the decomposition of platinacyclononanes with added ligand.
3.4.4. The formation of diene and cycloalkane
Smaller amounts of dienes and cycloalkanes were also produced
from certain metallacycles. Dienes might be formed via b-hydride
elimination from an alkenyl hydride species and a reductive elim-
ination pathway could result in CeC bond formation to give
cycloalkane (Scheme 5).
metalehydride species E-2 is presumably the favoured intermediate
due to the fact that no (or no more) cyclooctane was formed with
added ligand, which indicated reductive elimination from the initial
intermediate E-1 to form cyclooctane might be blocked.
3.5.2. Changes of decomposition medium
The effects of decomposition medium on the hydrocarbon
products of the platinacyclononane 1 were observed (Eq.(10)). The
relative amount of the mixture of octenes including 1-octene,
2-octenes and 1,7-octadiene is significantly higher for the solvent
free decomposition than those decomposed in solvent. Decompo-
sition in the halogenated, highly co-ordinating and polar solvent,
dichloromethane, was found to favour formation of n-octane and
cyclooctane. On the other hand, the formation of octane showed
different trends compared to octenes.
3.5. General factors affecting the thermal decomposition of
platinacycloalkanes
Information from the thermal decomposition of small metalla-
cycles have shown that the thermal stability of these complexes is
quite dependent on the nature of metal, size of the ring, solvent and
supporting ligands [29]. These factors also played a significant role
in the current thermal studies on various mediums to large
metallacycles.
mixture of octenes + octane + cyclooctane
solvent free
3.5.1. Additional neutral ligands
74%
26%
The notable effect of the presence of free phosphine ligands (dppp
and dppe) on the thermolytic behaviour of complexes 1 and 2 is the
facilitation of decomposition in every case. This effect is also found in
the decomposition of some bis(phosphine)platinacyclopentanes
with added ligand [36].
cyclohexane
CH2Cl2
(10)
(CH2)
Pt
(dppp)
33%
67%
52%
8
1
4%
44%
There is no clear mechanism for such reactions so far. According
to Whitesides and co-workers [36], this acceleration by added
ligand might be due to either inhibition of the formation of a three-
coordinate intermediate D-1, or to promotion of the formation of
a five-coordinate intermediate D-2 (Chart 2).
Braterman and co-workers also reported that reductive elimi-
nation of biaryl was enhanced by added phosphines in the ther-
molysis reactions of PtL2Ar2 due to their electrodonor capability
[1,40,41,53,59]. Again, a five-coordinate intermediate D-3 was
proposed according to their evidence from the DSC results.
The complexes 1 and 2 with the chelating ligands dppp and dppe,
however, do not involve a three-coordinate intermediate during the
thermal decomposition as they ensure retention of configuration.
The mechanism of the decomposition of 1 (or 2) with additional
ligand is easily rationalized using the hypothesis that a five-coor-
dinated intermediate E-1 could be formed initially followed by
3.5.3. Changes of supporting ligands
The thermal stability of metallacycles is dependent on the ancil-
lary ligand systems. The order of stability for Pt complexes is as
follows: PPh3 < PtBu3 < diphos (diphos ¼ dppe, dppp, dmpe, dcpe).
The role of the supporting ligands seems to hold the balance in the
decomposition pathways of certain metallacycle systems. For
instance, all platinacyclononane complexes (1, L2 ¼ dppp; 2,
L2 ¼ dppe; 3, L ¼ PtBu3) produced 1-octene as major products upon
solvent free decomposition, whereas the relative amount of 1-octene
increased in the order of the ligands: dppe < dppp < PtBu3 (Eq.(11)).
1-octene
41%
+
2-octenes
21%
+
octane
23%
+
1,7-octadiene
13%
L2 = dppe
L2 = dppp
L = PBut3
b
-hydride elimination to form a metalehydride intermediate E-2,
(11)
4%
9%
L2Pt
(CH2)
26%
17%
49%
54%
21%
20%
8
and the reductive elimination of alkenes could be then facilitated by
the added phosphine ligand (Scheme 6). The five-coordinate

F. Zheng et al. / Journal of Organometallic Chemistry 696 (2011) 2457e2465
2465
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D.S. McGuinness, D.H. Morgan, A. Neveling, S. Otto, M. Overett, A.M.Z. Slawin,
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Some useful results pertinent to the thermal decomposition of
metallacycloalkanes emerged from the current study. The rate of
decomposition of platinacycloalkanes in cyclohexane is increased
by adding dppp and dppe ligands respectively. The effect of the
additional ligands could be that of facilitating the reductive elimi-
nation of alkenes from a five-coordinated metalehydride inter-
mediate. The relative amount of 1-octene formed from the thermal
decomposition of platinacyclononanes is dependent on the
decomposition temperature and time. The formation of the total
products including 1-octene, 2-octenes, n-octane, 1,7-octadiene
and cyclooctane is first order for about the first 30% of the
decomposition. These results taken together demonstrate that
changes of decomposition medium, ring size, supporting ligands
and metal centres have significant effects on the decomposition
pathways; moreover, these factors seem to have a cooperative
effect in the decomposition of certain metallacycloalkanes. The
possible decomposition pathways involved in the thermolysis of
these metallacycloalkanes could be:
b-hydride transfer or an
intermolecular chain reaction to form 1-alkene; intermolecular or
intramolecular isomerisation for 2-alkenes; the formation of
n-alkane by an intermolecular hydrogen abstraction, or alterna-
tively an intramolecular hydrogen abstraction from supporting
ligands. The thermolysis data in this study suggest that the medium
to large metallacycloalkanes can be useful models for the inter-
mediates in selective catalytic oligomerisation reactions, particu-
larly ethylene trimerisation and tetramerisation reactions.
[30] A. Sivaramakrishna, H. Su, J.R. Moss, Angew. Chem. Int. Ed. 46 (2007)
3541e3543.
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[35] A. Sivaramakrishna, E. Hager, F. Zheng, B.C.E. Makhubela, J.R. Moss, unpub-
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[36] J.X. McDermott, J.F. White, G.M. Whitesides, J. Am. Chem. Soc. 98 (1976)
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Acknowledgements
[37] G.M. Whitesides, J.F. Gaasch, E.R. Stedronsky, J. Am. Chem. Soc. 94 (1972)
5258e5270.
[38] G.B. Young, G.M. Whitesides, J. Am. Chem. Soc. 100 (1978) 5808e5815.
We thank University of Cape Town, Anglo Platinum, Johnson &
Matthey for the support and Prof. Luigi Nassimbeni for helpful
discussions. Dr. ASRK is highly grateful to VIT University for the
support.
[39]
%
Decomposition
¼
[total hydrocarbon products]/[Pt compound]
0 ꢀ 100% ¼ (total amount of hydrocarbon products)/(theoretical amount of
hydrocarbon products) ꢀ 100%; [100% decomposition means that the theo-
retical concentration of the total decomposition products should be equal to
the initial concentration of Pt compound].
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