Synthesis and β-hydrogen elimination of water-soluble dialkylplatinum(II) complexes in water

Article abstract of DOI:10.1246/bcsj.76.183

Water-soluble cis-dialkylplatinum(II) complexes cis-[PtR₂L₂] (L₂ = 2 TPPTS, 2 THMP, DHMPE; R = Me, Et, Ph) have been prepared by ligand displacement reactions of [PtR₂(1,5-COD)] with L₂. They are stable in water; the diethylplatinum(H) complexes cis-[PtEt₂(TPPTS)₂] undergo preferential disproportionation of the ethyl groups in water, giving ethylene and ethane in 1: 1 ratio via β-hydrogen elimination in preference to protonolysis at 80 °C. A retardation effect of added free TPPTS on the reaction rate and a deuterium labelling study suggest a mechanism involving prior dissociation of TPPTS for reversible β-hydrogen elimination.

Full text of DOI:10.1246/bcsj.76.183

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 183–188 (2003)
183
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β
Synthesis and -Hydrogen Elimination of Water-Soluble
Dialkylplatinum(Ⅱ) Complexes in Water
Water-Soluble Dialkylplatinum(Ⅱ) Complexes
S. Komiya et al.
Sanshiro Komiya,* Miho Ikuine, Nobuyuki Komine, and Masafumi Hirano
03
3
8
Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588
SJA8
09-2673
230
(Received August 16, 2002)
Water-soluble cis-dialkylplatinum(Ⅱ) complexes cis-[PtR2L2] (L2 = 2 TPPTS, 2 THMP, DHMPE; R = Me, Et, Ph)
have been prepared by ligand displacement reactions of [PtR2(1,5-COD)] with L2. They are stable in water; the diethyl-
platinum(Ⅱ) complexes cis-[PtEt2(TPPTS)2] undergo preferential disproportionation of the ethyl groups in water, giving
ethylene and ethane in 1:1 ratio via β-hydrogen elimination in preference to protonolysis at 80 °C. A retardation effect
of added free TPPTS on the reaction rate and a deuterium labelling study suggest a mechanism involving prior dissocia-
tion of TPPTS for reversible β-hydrogen elimination.
02
02
.2
Many transition metal-mediated reactions and catalyses try
to avoid moisture, because putative organometallic intermedi-
ates are believed to be easily hydrolyzed under the reaction
conditions. Despite this consideration, organometallic reac-
tions in water are becoming one of the promising areas of envi-
ronmentally benign reaction systems,1 since water is regarded
as one of the most safe solvents and water/organic solvent bi-
phasic catalysis can provide easy separation of hydrophobic
organic products in industrial applications. In addition, many
useful and well-established transition metal-mediated chemi-
cal transformations, which had been extensively developed in
recent years, can be extended to the reactions of hydrophobic
compounds in aqueous media. Thus, fundamental understand-
ing of aqueous organometallic reactions has intrinsic impor-
tance in order to develop new transition metal-promoted chem-
ical processes in water. To date, however, very limited water-
soluble complexes having metal–carbon σ-bonds are known.
Although methyliridium(Ⅰ) complex trans-[IrMe(CO)(TPP-
MS)2] (TPPMS = diphenylphosphinobenzene-3-sulfonic acid
sodium salt) is known as one of the water-soluble organome-
tallic compounds (Chart 1), its Ir–C bond was immediately
hydrolyzed in water to form the corresponding hydroxoiridi-
um(Ⅰ) complex.2 Pringle and co-workers3 also reported cis-
[PtMe2(THMP)2] and cis- and trans-[PtMeX(THMP)2]
[THMP = tris(hydroxymethyl)phosphine] with insufficient
characterization due to facile decomposition during the isola-
tion processes. We now describe the synthesis and thermal sta-
bility of water-soluble diorganoplatinum(Ⅱ) complexes cis-
[PtR2L2] with TPPTS [TPPTS = 3,3ꢀ,3ꢁ-phosphinidynetris-
(benzenesulfonic acid) trisodium salt], THMP, or DHMPE
{1,2-bis[di(hydroxymethyl)phosphino]ethane} ligands by
ligand substitution reactions of cis-[PtR2(1,5-COD)] (COD =
cyclooctadiene). Among them, diethylplatinum(Ⅱ) complexes
undergo smooth and selective disproportionation of the ethyl
groups via β-hydrogen elimination even in water. A part of the
results has been published in a preliminary form.4
Experimental
All manipulations were carried out under an atmosphere of ni-
trogen or argon using standard Schlenk techniques. Water was pu-
rified by simple distillation, and other solvents were dried over
and distilled from appropriate drying agents under N2 and were
stored under nitrogen before use. [PtR2(1,5-COD)] were prepared
according to the literature methods.5 [Pt(CH2CD3)2(1,5-COD)] (D
content at the methyl group was estimated as 89%) was prepared
by the reaction of [PtI2(1,5-COD)] with (CD3CH2)MgBr. TPPTS
was purchased from Aldrich and used as received. THMP and
DHPME were prepared according to the literature methods.6 IR
spectra were measured on a JASCO FT/IR-410 spectrometer.
NMR spectra were obtained on a JEOL LA-300 (1H 300.4 MHz)
spectrometer. Chemical shifts were relative to DSS (1H) and 85%
H3PO4 (31P). Elemental analyses were performed with a Perkin-
Elmer 2400 series Ⅱ CHN analyzer. Gases were quantitatively an-
alyzed by gas chromatography (Shimadzu GC-8A) using the inter-
nal standard method.
Synthesis of Water-Soluble cis-Dialkylplatinum(Ⅱ) Com-
plexes. A typical procedure for [PtMe2(TPPTS)2] (1a) is given.
To an ethanol solution (2 mL) of [PtMe2(1,5-COD)] (22.8 mg,
0.0685 mmol) was added TPPTS (74.0 mg, 0.130 mmol) in water
(1 mL). Stirring at room temperature for several minutes gave a
colorless solution. Addition of acetone to the concentrated solu-
Chart 1.

184 Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Water-Soluble Dialkylplatinum(ꢀ) Complexes
43.9 (s, 1JP-Pt = 1600 Hz).
tion of the reaction mixture gave white powder, which was washed
with hexane and dried under vacuum. Recrystallization from wa-
ter with acetone gave white powder of 1a•4H2O (80.1 mg, 80%).
Anal. Found: C, 31.89; H, 2.92%. Calcd for C38H38Na6O22P2PtS6:
C, 31.83; H, 2.62%. 1,5-COD liberated (95%). IR (KBr): 3331
(νOH), 1194 (νasSwO), 1039 cm−1 (νsSwO). 1H NMR (D2O) δ
0.42 (m, 2JH-Pt = 69 Hz, 6H, Pt-CH3), 7.39 (t, 3JH-H = 8 Hz, 6H, 5-
CH of aryl), 7.53 (t, 3JH-H = 3JH-P = 8 Hz, 6H, 6-CH of aryl), 7.76
(d, 3JH-H = 8 Hz, 6H, 4-CH of aryl), 7.79 (d, 3JH-P = 8 Hz, 6H, 2-
Thermolysis of Water-Soluble cis-Dialkylplatinum(Ⅱ) Com-
plexes. Thermolysis reactions were carried out in glass bottles
(10 mL) under reduced pressure. Typically, 2b (21.0 mg, 0.0144
mmol) was placed in a glass bottle with a serum cap and the sys-
tem was evacuated. Water (2.0 mL) was then added through the
serum cap by using a hypodermic syringe. Methane (1.19 mL)
was added by using a calibrated hypodermic syringe as an internal
standard. The solution was heated to 80 °C and the aliquots (20
µL) of the gas phase were periodically taken with a gas-tight
microsyringe for quantitative GC analysis (Porapak Q column).
Protonolysis of Water-Soluble cis-Dialkylplatinum(Ⅱ) Com-
plexes. Protonolysis of 1a is given as a typical example. 1a
(14.3 mg, 0.0194 mmol) was placed in a glass bottle (10 mL) with
a serum cap and the system was degassed. A large excess of 12 M
hydrochloric acid was added by using a hypodermic syringe, lib-
erating only methane. Ethane (1.19 mL) was added as an internal
standard and methane (184%/Pt) was detected by GC method.
For 1b (17.1 mg, 0.0123 mmol), the yield of ethane liberated was
166%/Pt. For 3a (10.8 mg, 0.0246 mmol), the yield of methane
liberated was 201%/Pt. Protonolysis of other cis-dialkylplati-
num(Ⅱ) complexes with THMP or DHMPE ligands (2a–b, 3b)
were performed by using in-situ prepared samples in DMSO. A
typical procedure for 2a is given. The DMSO solution (200 µL) of
2a was prepared in situ from [PtMe2(1,5-COD)] (5.0 mg, 0.015
mmol) and THMP (3.9 mg, 0.032 mmol) in a glass bottle (10 mL)
with a serum cap and the system was degassed. Dry ether solution
of hydrogen chloride (0.38 M, 1 mL) was added by using a hypo-
dermic syringe at room temperature. Ethane (0.113 mL) was
added as an internal standard. Methane (203%/Pt) was detected
by GC. For 2b and 3b, 208%/Pt and 154%/Pt of ethane were
detected.
CH of aryl). 31P{1H} NMR (D2O) δ 29.3 (s, JP-Pt = 1855 Hz).
Mp 186–190 °C (dec.).
1
[PtEt2(TPPTS)2] (1b)•4H2O: Colorless powder from water/ace-
tone. Yield 95.3 mg (72%). Anal. Found: C, 32.82; H, 3.19%.
Calcd for C40H42Na6O22P2PtS6: C, 32.86; H, 2.90%. 1,5-COD lib-
erated (101%). IR (KBr): 3442 (νOH), 1191 (νasSwO), 1038 cm−1
3
(νsSwO). 1H NMR (D2O): δ 0.77 (m, JH-Pt = 74 Hz, 6H, Pt-
CH2CH3), 1.13 (m, 2JH-Pt = 70 Hz, 4H, Pt-CH2CH3), 7.39 (t, 3JH-H
3
= 8 Hz, 6H, 5-CH of aryl), 7.52 (t, 3JH-H = JH-P = 8 Hz, 6H, 6-
CH of aryl), 7.71 (d, 3JH-H = 8 Hz, 6H, 4-CH of aryl), 7.85 (d, 3JH-P
= 8 Hz, 6H, 2-CH of aryl). 31P{1H} NMR (D2O) δ 29.0 (s, 1JP-Pt
= 1677 Hz). Mp 158–164 °C (dec.).
[PtMe2(THMP)2] (2a): The compound was not isolated in a
pure form and the yield was estimated by NMR in DMSO-d6 as
82%. 1,5-COD liberated (93%). 1H NMR (DMSO-d6/D2O = 1/5
2
(v/v)): δ 0.49 (brs, JH-Pt = 66 Hz, 6H, Pt-CH3), 4.29 (br, 12H,
PCH2OH). 31P{1H} NMR (DMSO-d6/D2O = 1/5 (v/v)) δ 13.2
(s, 1JP-Pt = 1700 Hz).
[PtEt2(THMP)2] (2b): The compound was not isolated in a pure
form and the yield estimated by NMR in DMSO-d6 was 86% .
1,5-COD liberated (96%). 1H NMR (DMSO-d6/D2O = 1/5 (v/v)):
δ 1.14 (m, 6H, Pt-CH2CH3), 1.2 (q, 3JH-H = 8 Hz, 2JH-Pt = 129 Hz,
4H, Pt-CH2CH3), 4.33 (brs, 12H, PCH2OH). 31P{1H} NMR
(DMSO-d6/D2O = 1/5 (v/v)) δ 8.9 (s, 1JP-Pt = 1470 Hz).
[PtPh2(THMP)2] (2c): Colorless powder from ethanol/hexane.
Yield 78.7 mg (58%). Anal. Found: C, 36.46; H, 4.50%. Calcd
for C18H28O6P2Pt: C, 36.19; H, 4.72%. 1,5-COD liberated (92%).
Deuterium Labeling Study. [Pt(CH2CD3)2(TPPTS)2] was
prepared by the reaction of [Pt(CH2CD3)2(1,5-COD)] (24.1 mg,
0.0668 mmol) with two equivalents of TPPTS (72.0 mg, 0.127
mmol). Yield 90.3 mg (88%). The deuterium NMR spectrum in-
dicates no incorporation of D in the methylene group and the D
2
1H NMR (D2O) δ 4.06 (s, JH-Pt = 11 Hz, 12H, PCH2OH), 6.79
(t, 3JH-H = 7 Hz, 2H, p-Ph), 7.01 (t, 3JH-H = 7 Hz, 4H, m-Ph), 7.41
(d, 3JH-H = 7 Hz, 3JH-Pt = 57 Hz, 4H, o-Ph). 31P{1H} NMR (D2O)
δ 4.5 (s, 1JP-Pt = 1660 Hz).
content in the methyl group was estimated as 89% by H NMR.
1
[Pt(CH2CD3)2(TPPTS)2] (69.1 mg, 0.0471 mmol) in water (2.0
mL) was heated to 80 °C for 2 h. The evolved gases were collect-
ed by using a Toepler pump through the cold trap (−40 °C) and
the IR spectrum of the gas was measured using a gas-cell with
KRS-5 windows. The following bands (cm−1) were observed:
1387 (CH2wCD2), 1339 (cis-CHDwCHD), 1301 (trans-CHD-
wCHD), 987 (trans-CHDwCHD), 943 (CH2wCD2), 918
(CHDwCD2), 842 (cis-CHDwCHD), 764 (CHDwCD2), 751
(CH2wCD2), 725 (CHDwCD2, trans-CHDwCHD). No character-
istic bands for C2H4, C2H3D, and C2D4 were observed.7
[PtMe2(DHMPE)] (3a): Colorless powder from ethanol/hex-
ane. Yield 39.9 mg (63%). Anal. Found: C, 21.05; H, 4.66%.
Calcd for C8H22O4P2Pt: C, 21.87; H, 5.05%. 1,5-COD liberated
(100%). 1H NMR (D2O) δ 0.50 (t, 3JH-P = 7 Hz, 2JH-Pt = 69 Hz,
6H, Pt-Me), 1.96 (m, 4H, PC2H4), 4.14 (dd, 2JH-H = 14 Hz, 2JH-P
=
2.1 Hz, 4H, PCH2OH), 4.25 (d, 2JH-H = 14 Hz, 3JH-Pt = 12 Hz, 4H,
PCH2OH). 31P{1H} NMR (D2O) δ 49.3 (s, 1JP-Pt = 1640 Hz).
[PtEt2(DHMPE)] (3b): The compound was not isolated in a
pure form and the yield estimated by NMR in DMSO-d6 was 91%.
1,5-COD liberated (94%). 1H NMR (DMSO-d6/D2O = 1/5 (v/v))
δ 1.25 (q, 3JH-H = 8 Hz, 2JH-Pt = 71 Hz, 4H, Pt-CH2CH3), 1.26 (m,
Results and Discussion
Synthesis of Water-Soluble Diorganoplatinum(Ⅱ) Com-
2
6H, Pt-CH2CH3), 1.88 (m, 4H, PC2H4), 4.09 (dd, JH-H = 14 Hz,
plexes.
When (1,5-cyclooctadiene)dimethylplatinum(Ⅱ),
2
2
2JH-P = 3 Hz, 4H, PCH2OH), 4.22 (d, JH-H = 14 Hz, JH-Pt = 11
Hz, 4H, PCH2OH). 31P{1H} NMR (DMSO-d6/D2O = 1/5 (v/v)) δ
48.3 (s, 1JP-Pt = 1460 Hz).
[PtMe2(1,5-COD)] was treated with two equivalents of TPPTS
in ethanol/H2O (ca. 2:1) at room temperature, immediate
ligand displacement took place to give cis-dimethylbis-
(TPPTS)platinum(Ⅱ), cis-[PtMe2(TPPTS)2] (1a) in 80% yield
with liberation of 1,5-COD (95%) (Scheme 1). Addition of ex-
cess acetone to the concentrated aqueous solution gave analyt-
ically pure colorless powder of 1a. The diethylplatinum(Ⅱ) an-
alog 1b was also prepared in a similar manner in 72% yield.
[PtPh2(DHMPE)] (3c): Colorless powder from ethanol/hex-
ane. Yield 54.2 mg (64%). Anal. Found: C, 38.38; H, 4.81%.
Calcd for C18H26O4P2Pt: C, 38.37; H, 4.65%. 1H NMR (D2O) δ
3
2.10 (m, 4H, PC2H4), 4.11 (m, 8H, PCH2OH), 6.85 (t, J
= 7
H-H
Hz, 2H, p-Ph), 7.08 (t, 3J H-H = 7 Hz, 4H, m-Ph), 7.46 (t, 3JH-H
=
3
4JH-P = 7 Hz, JH-Pt = 52 Hz, 4H, o-Ph). 31P{1H} NMR (D2O) δ

S. Komiya et al.
Bull. Chem. Soc. Jpn., 76, No. 1 (2003) 185
Scheme 1.
They are soluble in water and thermally stable at room temper-
ature, but started to darken above 150 °C without melting in
the solid state.
ed in a pure form, but the formation was unambiguously deter-
mined by the 1H and 31P{1H} NMR. Signals due to methylene
and methyl protons of the ethyl groups in 2b and 3b are very
close to each other giving a complicated multiplet at δ 1.1–1.2
with distinct 195Pt satellites of only the methylene quartet sig-
nal. The phosphorus ligands in 2a–c are also considered to oc-
cupy positions trans to the ethyl ligand, similar to other dialkyl-
platinum(Ⅱ) complexes, because the JP-Pt values are relatively
small (1700 Hz for 2a, 1470 Hz for 2b, 1660 Hz for 2c).
While dimethylplatinum(Ⅱ) complex 1a is also stable in wa-
ter for more than two weeks at 50 °C, diethylplatinum(Ⅱ) com-
plex 1b slowly decomposed under similar conditions to give a
mixture of ethylene and ethane in a day at room temperature.
However, when 1a or 1b was treated with concentrated hydro-
chloric acid, immediate protonolysis took place to evolve
methane (184%/1a) or ethane (166%/1b). Protonolysis of 1a
with weaker acids such as acetic acid and formic acid proceed-
ed much more slowly in water and dimethyl malonate did not
react with 1a at all at room temperature. On the other hand,
dimethylplatinum(Ⅱ) complexes 2a or 3a having THMP or
DHMPE ligand are relatively unstable in water at room tem-
perature in comparison with 1a, slowly liberating methane.
The fact suggests additional protonolysis pathways probably
enhanced by THMP and DHMPE ligands, though the detailed
mechanism is not clear at present. Protonolyses of 2a and 3a
with strong acid such as HCl also immediately liberated quan-
titative amounts of methane (203%/2a, 160%/3a). Diphenyl-
platinum(Ⅱ) complexes 2c and 3c are very stable even in water
at room temperature and the aqueous solution stood without
decomposition for a few days at room temperature. They are
thermally more stable than dimethyl- or diethylplatinum(Ⅱ)
complexes in water. While 2a completely decomposed at 50
°C in 7 h, 2c slowly decomposed during seven days to give
benzene (163%) under the same conditions.
31P{1H} NMR spectrum of 1a in D2O shows a singlet at δ
1
29.3 with 195Pt satellites. The JPt-P value of 1855 Hz is rela-
tively small, suggesting that two phosphorus nuclei are placed
trans to the methyl ligands with strong trans influence, as
would be consistent with the square planar cis configuration of
1a.8 The 1H NMR displays a characteristic second order
A3XXꢀAꢀ3 multiplet centered at δ 0.42 due to the Pt-Me groups
of cis-[PtMe2(PR3)2] with 195Pt satellites (2JH-Pt = 69 Hz).9 1b
also shows two multiplets having 195Pt satellites centered at δ
0.77 (3JH-Pt = 74 Hz) and 1.13 (2JH-Pt = 70 Hz) assignable to
methyl and methylene protons of the Et groups. The 31P{1H}
NMR spectrum of 1b in D2O shows a singlet at δ 29.0 with
195Pt satellites (1JP-Pt = 1677 Hz). Addition of free TPPTS to a
D2O solution of 1b did not cause any apparent line broadening
or chemical shift change of the signals of the coordinated and
uncoordinated TPPTS in the 31P{1H} NMR, implying that fast
phosphine ligand exchange process does not take place on the
NMR time scale.
Dimethyl-, diethyl-, and diphenylplatinum(Ⅱ) derivatives
with THMP or DHMPE ligands were similarly prepared
[PtR2L2: L = THMP, R = Me (2a), Et (2b), Ph (2c); L2 =
1
DHMPE, R = Me (3a), Et (3b), Ph (3c)]. The H NMR of
dimethylplatinum(Ⅱ) complexes 2a and 3a displays a triplet at
δ 0.49 and 0.50 with 195Pt satellites assignable to the Pt-Me.
For 3a a characteristic second-order multiplet at δ 1.96
(AX2Xꢀ2Aꢀ) assignable to the bridging methylene protons of
the DHMPE ligand was observed. Consistently, diastereotopic
methylene protons of CH2OH groups in the coordinated
DHMPE ligand in 3a appear as an AB quartet at δ 4.14 and
4.25: the latter involves Pt satellites, suggesting chelation of
the DHMPE ligand. Diphenyl derivatives 2c and 3c with
THMP or DHMPE ligands were also isolated and show the
ortho-H protons of the phenyl groups with 195Pt satellites. Di-
ethylplatinum(Ⅱ) analogues cis-[PtEt2L2] with these ligands
(L2 = 2 THMP (2b), DHMPE (3b)) were difficult to be isolat-
Thermolysis of cis-Diethylplatinum(Ⅱ) Complexes in
Water. Of particular interest is the thermolysis of cis-dieth-
ylplatinum(Ⅱ) complex 1b, since it may undergo competing β-
hydrogen elimination reaction and/or hydrolysis. Heating of

186 Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Water-Soluble Dialkylplatinum(ꢀ) Complexes
1b in water at 80 °C liberated ethylene and ethane in an ap-
proximately 1:1 ratio in 2 h, indicating that disproportionation
of the ethyl groups preferentially took place over protonolysis
even in water [Eq. 1]. Thermolyses of other diethylplati-
num(Ⅱ) complexes 2b and 3b in D2O at 80 °C for 30 min also
liberated a mixture of ethylene and ethane, however, the ratios
of ethylene to ethane were approximately 0.35 and 0.5 for 2b
and 3b, respectively, indicating that concomitant protonolysis
of the ethyl groups was also taking place simultaneously in ad-
dition to disproportionation of the ethyl groups. This protonol-
ysis may also be enhanced by the hydroxy group in THMP or
DHMPE ligands.
d
−
[PtEt2(TPPTS)2 ] = kobsd[PtEt2(TPPTS)2 ]
(2)
dt
Estimated rates constants kobsd are 5.6 × 10−4 s−1 and 2.7 ×
10−5 s−1 in the absence and presence of free TPPTS ([TPPTS]
= 34.8 mmol/L), respectively. Addition of five equivalents of
free TPPTS suppressed the reaction rate to ca. 1/20. Such re-
tardation effect is considered to arise from the competitive co-
ordination of TPPTS to 1b to block the fifth coordination site
at Pt for β-hydrogen elimination12 and/or from preventing the
prerequisite formation of 3-coordinate intermediate which
would be formed by dissociation of one TPPTS ligand.13
However, neither 3-coordinate species [PtEt2(TPPTS)] nor 5-
coordinate species [PtEt2(TPPTS)3] was detected by NMR in
the presence or absence of added TPPTS, indicating that the
complex 1b always keeps its square planar 4-coordinate struc-
ture during the thermolysis. If the competitive association of
TPPTS to 4-coordinate species is responsible for the retarda-
tion effect, a considerable amount of 5-coordinate species
should be detected in the presence of TPPTS, and therefore the
former associative mechanism is excluded. Thus, a reaction
mechanism via rate limiting dissociation of TPPTS giving an
unstable three coordinate T-shape intermediate is proposed, as
previously described by Whitesides10a,b and us10c as shown in
Scheme 2.
The reversibility of the β-hydrogen elimination was also ex-
amined by the deuterium labeling experiment. 1b-d6, cis-
[Pt(CH2CD3)2(TPPTS)2] was prepared and decomposed in wa-
ter at 80 °C. The ethylene liberated in the thermolysis was an-
alyzed by studying the IR spectrum of the gas and was found
to consist of only CH2wCD2, cis- and trans-CHDwCHD, and
CD2wCHD, while no absorptions due to C2H4, CH2wCHD, and
C2D4 were detected. Deuterium scrambling in evolved ethyl-
ene and absence of ethylene-d0, d1, and d4 clearly indicate that
the facile H–D exchange was taking place within one ethyl
ligand and the other ethyl group was not participate this H–D
exchange process. The facile and reversible β-hydrogen elimi-
nation giving a ethyl(ethylene)hydridoplatinum(Ⅱ) intermedi-
ate, followed by re-insertion into the Pt–H bond, is proposed as
a mechanism as shown in Scheme 3. This process must take
place after the prerequisite phosphine dissociation as well as
before the reductive elimination of ethane. Such a dissociative
80 °C
(1)
1b
CH2wCH2 + CH3CH3
94% 90%
H2O
On the other hand, the ratio of ethylene to ethane was ap-
proximately constant during the thermolysis of 1b, indicating
occurrence of clean disproportionation of the ethyl groups.
Figure 1 shows the time-yield curves of ethylene and ethane
for the thermolysis of 1b in H2O. Platinum(0) product may be
formed, as previously reported in the thermolysis of dialkyl-
platinum(Ⅱ) complexes in aprotic solvents.10 Such a clean dis-
proportionation of the ethyl groups also excludes the possible
free radical mechanism, since the ethyl radical is known to fa-
vor coupling rather than disproportionation.11 Thus the con-
certed β-hydrogen elimination giving a ethyl(ethylene)hydri-
doplatinum(Ⅱ) intermediate, followed by reductive elimination
of ethane at Pt, would be the most plausible mechanism. In or-
der to get more mechanistic insight of the β-hydrogen elimina-
tion process, the effect of added TPPTS on the time-course of
thermolysis was examined (Fig. 1).
The thermolysis reactions were found to obey first-order ki-
netics with rate constant kobsd
.
Fig. 1. Time-courses of thermolysis of 1b at 80 °C in water.
Ethylene (ꢀ) and ethane (ꢁ) evolved in the absence of
TPPTS ([1b] = 7.2 mmol/L); ethylene (ꢂ) and ethane (ꢃ)
evolved in the presence of TPPTS ([1b] = 7.0 mmol/L,
[TPPTS] = 34.8 mmol/L).
Scheme 2.

S. Komiya et al.
Bull. Chem. Soc. Jpn., 76, No. 1 (2003) 187
Chemical Innovation Institute (JCⅡ) through the R&D pro-
gram for the Next-Generation Chemical Process Technologies.
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4
5
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6
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Scheme 3.
process is essentially the same as the mechanism established in
the thermolysis of [PtR2(PPh3)2] in non-aqueous organic sol-
vents.10 It is interesting to note that the thermolysis of 1b in
water in the absence of excess phosphine proceeds slightly
faster than that of triphenylphsophine analog [PtEt2(PPh3)2] in
CH2Ph2. The effect of added phosphine ligand is less pro-
nounced in water. This suggests that the phosphine dissocia-
tion of 1b for β-hydrogen elimination in water is slower than
that of [PtEt2(PPh3)2] in CH2CPh2, probably due to the weaker
electron-donating ability of TPPTS than PPh3, and/or that the
rate of β-hydrogen elimination followed by elimination of eth-
ylene and ethane and the rate of association of TPPTS to the 3-
coordinate intermediate are in the same order.13
The present results display the intrinsic thermal stability of
the Pt–C bond in water and the occurrence of a typical organo-
metallic reaction such as β-hydrogen elimination in preference
to hydrolysis even in water, suggesting that transition metal-
mediated organic transformations and catalyses via organome-
tallic intermediates, which are frequently performed in aprotic
solvents, can be extended to the reactions in aqueous medium.
7
B. L. Crawford, Jr., J. E. Lancaster, and R. G. Inskeep, J.
Chem. Phys., 21, 678 (1953).
A. Yamamoto, “Organotransition Metal Chemistry, Funda-
8
mental Concepts and Applications,” Wiley Interscience, New York
(1986), p. 180.
9
R. J. Goodfellow, M. J. Hardy, and B. F. Taylor, J. Chem.
Soc. Dalton Trans., 1973, 2450.
10 a) G. M. Whitesides, Pure Appl. Chem., 53, 287 (1981). b)
T. J. McCarthy, R. G. Nuzzo, and G. M. Whitesides, J. Am. Chem.
Soc., 103, 1676, 3396, 3404 (1981). c) S. Komiya, Y. Morimoto,
A. Yamamoto, and T. Yamamoto, Organometallics, 1, 1528
(1982).
11 Disproportionation-combination rate constant ratio is 0.14
for Et radical and the value is relatively invariable in various sol-
vents: “Free Radicals,” ed by J. K. Kochi, Wiley-Interscience Pub-
lication, New York (1973), Vol 1, p. 11.
12 If the competitive coordination of TPPTS to 1b to block
the fifth coordination site at Pt for β-hydrogen elimination is re-
sponsible for the retardation effect, the following rate equation is
derived by assuming pre-equilibrium of competitive coordination
of TPPTS and β-hydrogen to Pt. In order to obtain the present re-
tardation effect of added TPPTS, a significant amount of [PtR2L3]
should be observed during the thermolysis. However, the complex
1b always keeps its square planar 4-coordinate structure during
This work was supported by New Energy and Industrial
Technology Development Organization (NEDO) and Japan

188 Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Water-Soluble Dialkylplatinum(ꢀ) Complexes
the thermolysis and the equilibrium constants K1 and K2[TPPTS]
estimated by NMR were too small (at least smaller than 0.01) to
show the effect, excluding the associative mechanism (Scheme 4
and Eq. 3).
mediate, from which a facile reversible β-hydrogen elimination
takes place followed by reductive elimination of ethane and libera-
tion of ethylene, gives the following rate equation which is consis-
tent with the experimental data with approximate values of k1 and
k−1/k2 are 5.6 × 10−4 s−1 and 5.7 × 102 L/mol. The observed
weaker magnitude of the retardation effect of added TPPTS sug-
gests the considerably smaller 1/k2K value for 1b than [PtEt2-
(PPh3)2]. This may be due to larger phosphine dissociation con-
stant and/or facile succeeding processes of 1b.
Scheme 4.
Scheme 5.
d
kK1
−
[PtEt2 ]total
=
[PtEt2 ]total
(3)
dt
1 + K1 + K2[TPPTS]
d
k1k2
(4)
[PtEt2(TPPTS)2 ]
−
[PtEt2(TPPTS)2 ] =
13 The steady state approximation of the 3-coordinate inter-
dt
k2 + k−1[TPPTS]