A Study of the Mechanism of Platinum(II)/Tin(II) Dichloride Mediated Hydrogenation of Alkynes and Alkenes Employing Parahydrogen-Induced Polarization

Article abstract of DOI:10.1002/(SICI)1099-0682(199812)1998:12<1915::AID-EJIC1915>3.0.CO;2-W

The mechanism of hydrogenation of alkynes catalyzed by the [(PR₃)₂PtHX]/SnX₂ system (PR₃ = PPh₃, PMePh₂; X = Cl, Br) has been studied by means of parahydrogen-induced polarization of ¹H spectra (PHIP). Dihydride intermediates confirming the stepwise hydrogenation at room temperature were observed when the reaction was run in acetone. The obtained ¹H-PHIP spectra, together with NMR data for related species, are consistent with the formulation of these intermediates as cis-[H₂Pt(PR₃)(SnX ₃)(σ-alkenyl)(acetone)], where the σ-alkenyl ligand originates from an insertion reaction of the alkyne (1-phenyl-1-propyne, 1-phenyl-1-butyne, diphenylacetylene, 3,3-dimethylbutyne). At elevated temperatures, the hydrogenation in acetone proceeds as a cis-synchronous transfer of the two hydrogen atoms of parahydrogen to the substrate molecule. A mechanism for this synchronous hydrogenation is suggested.

Full text of DOI:10.1002/(SICI)1099-0682(199812)1998:12<1915::AID-EJIC1915>3.0.CO;2-W

FULL PAPER
A Study of the Mechanism of Platinum(II)/Tin(II) Dichloride Mediated
Hydrogenation of Alkynes and Alkenes Employing Parahydrogen-Induced
Polarization
Christina Deibelea, Alexei B. Perminb, Valery S. Petrosyanb, and Joachim Bargon*a
Institute of Physical and Theoretical Chemistry, University of Bonna,
Wegelerstrasse 12, D-53115 Bonn, Germany
Fax: (internat.) ϩ49 (0)228/ 739424
E-mail: bargon@uni-bonn.de
Department of Chemistry, M. V. Lomonosov Moscow State Universityb,
Vorobyevy Gory, 119899 Moscow, Russia
Fax: (internat.) ϩ7-095/ 9395546
E-mail: alp@org.chem.msu.su
Received August 18, 1998
Keywords: Hydrogenations / Parahydrogen / Platinum / Alkynes / Alkenes
The mechanism of hydrogenation of alkynes catalyzed by the
[(PR3)2PtHX]/SnX2 system (PR3 = PPh3, PMePh2; X = Cl, Br)
has been studied by means of parahydrogen-induced
polarization of 1H spectra (PHIP). Dihydride intermediates
confirming the stepwise hydrogenation at room temperature
were observed when the reaction was run in acetone. The
obtained 1H-PHIP spectra, together with NMR data for
related species, are consistent with the formulation of these
intermediates as cis-[H2Pt(PR3)(SnX3)(σ-alkenyl)(acetone)],
where the σ-alkenyl ligand originates from an insertion
reaction of the alkyne (1-phenyl-1-propyne, 1-phenyl-1-
butyne, diphenylacetylene, 3,3-dimethylbutyne). At elevated
temperatures, the hydrogenation in acetone proceeds as a
cis-synchronous transfer of the two hydrogen atoms of
parahydrogen to the substrate molecule. A mechanism for
this synchronous hydrogenation is suggested.
talyzed by rhodium[10Ϫ13], iridium[14][15], palladium[16][17]
,
Introduction
and ruthenium[18] complexes have been studied.
Although platinum complexes are well-known catalysts
for hydrogenation[19Ϫ21], hydroformylation[22Ϫ25], and hy-
drosilylation[26] reactions, there have been only a few mech-
anistic studies using the PHIP technique. In a previous
paper, we showed that spin-polarized products arise from
the hydrogenation of alkynes in the presence of Pt0 com-
plexes with either coordinated alkynes or alkenes, as well as
with the usual PtII phosphane hydrido chloro complexes,
The parahydrogen-induced polarization (PHIP) phenom-
enon was first observed accidentally by Bryndza and co-
workers as early as 1983[1]. This NMR technique is now
recognized as a valuable mechanistic probe for investigating
homogeneously catalyzed hydrogenation reactions[2]. The
phenomenon was proposed theoretically in 1986 by Bowers
and Weitekamp[3], and soon thereafter was confirmed ex-
perimentally[4]. Furthermore, it was recognized that PHIP
allows examination of the reaction mechanisms of the sin-
gle-step transfer of the two parahydrogen atoms to the sub-
strate molecule, and sometimes allows identification of the
unstable dihydride intermediates participating in the cata-
lytic cycle[5][6]. Moreover, study of the hydrogenation
activated by tin(II) chloride[27]
.
In principal, the latter fact is contradictory to the gener-
ally accepted mechanism for PtII/SnCl2 mediated hydroge-
nations (Scheme 1).
Since 1965, it has been accepted that the first step of the
mechanism by traditional methods is troublesome because hydrogenation is the reduction of the pre-catalyst, ac-
in many cases the reaction is accompanied by significant companied by the formation of platinum monohydrides
degrees of isomerization of the starting materials and/or 2[28][29]. These monohydrides have been shown to undergo
products[7]. The PHIP technique offers further information, rapid, equilibrium addition reactions to carbonϪcarbon
even on the stereochemistry of the fundamental step of the multiple bonds of the substrate, with the formation of σ-C
addition of dihydrogen to the organometallic catalyst. Quite intermediates 3[30][31]. Furthermore, the third step of the
possibly, in situ PHIP-NMR spectroscopy may provide the asynchronous hydrogenation, namely hydrogenolysis of the
data relating to the structures of active intermediates that PtIIϪC(alkyl) bond in 3, has been shown to proceed under
are otherwise difficult or impossible to obtain.
relatively mild conditions[32]. The appearance of polarized
The physical aspects of the processes leading to PHIP- products during PtII/SnCl2 mediated hydrogenation using
NMR spectra are discussed in a number of papers (e.g., para-H2 prompted us to engage in a more detailed study of
refs.[8][9]). Using this technique, hydrogenation reactions ca- the reaction mechanism using the PHIP technique.
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ1948/98/1212Ϫ1915 $ 17.50ϩ.50/0
1915

C. Deibele, A. B. Permin, V. S. Petrosyan, J. Bargon
FULL PAPER
Scheme 1. Generally assumed mechanism for the PtIIϪtin dichlo- Ϫ90°C and thus the minor discrepancies with these data
ride catalyzed hydrogenation, asynchronous mechanism
are obviously due to the different temperatures used.
Though the dihydrido-dichloro PtIV complexes were pre-
viously shown to arise as a result of an oxidative addition
of HX to PtII monohydrides[36Ϫ38], our attempts to observe
them using the PHIP technique during the reduction of di-
chloride 1 by parahydrogen were unsuccessful. Moreover,
no polarization patterns were found as a consequence of
the parahydrogen action on platinumϪtin monohydrides.
On the other hand, we observed strong, sharp polarization
signals after the addition of an alkyne followed by parahy-
drogen to the tin-activated monohydride acetone solution
(Figure 1). The relatively high field values of the chemical
shift of the observed polarization patterns are consistent
with PtIV species, since published data show a substantial
high-field shift for PtIV dihydrides as compared to anal-
ogous PtII complexes[36][39][40][41]
.
Figure 1. PHIP 1H-NMR spectra of the system trans-
[(PPh3)2PtHCl]/SnCl2/1-phenyl-1-propyne/para-H2 in [D6]acetone;
a) at room temp. (119Sn and 117Sn satellite signals are marked *);
b) the same sample after cooling; c) PHIP 1H-NMR spectrum of
the isolated insertion product 5a in CD2Cl2, at 3 bar of para-H2,
room temp.; d) PHIPϩϩ simulation of the monophosphane dihy-
dride 6; e) PHIPϩϩ simulation of the diphosphane dihydride 3
(values for cis- and trans-coupling constants are taken to be as in
c); f) PHIPϩϩ simulation of the diphosphane dihydride 3 (values
for cis and trans coupling constants are taken as in c))
It is well known that activation of PtII complexes by
tin(II) halides is strongly solvent-dependent. Solvents
of medium solvating ability and polarity, such as acetone,
play quite a significant role in platinum-phosphane chemi-
stry, promoting extensive phosphane redistribution reac-
tions[33Ϫ35]. This dependence can also result in changes in
catalytic activity and selectivity[22], as well as of the reaction
mechanism, therefore necessitating studies in a variety of
solvents.
In this paper we describe the results of PHIP studies of
the PtII/SnII mediated hydrogenation of alkynes and alkenes
in acetone and some other solvating media.
Results and Discussion
PHIP in the System [(PPh3)2PtHCl]/SnCl2/1-Phenyl-1-propyne
Since the generally accepted catalytic cycle involves plati-
num hydride as an active intermediate, we avoided the acti-
vation stage (a, Scheme 1), as well as extensive HCl forma-
tion, by the use of pre-synthesized platinum hydride instead
of the usually employed dichloro complex. The interaction
of platinum monohydride with SnCl2 in acetone proceeds
rapidly, and the resulting clear, orange-yellow solution
shows new signals in its 1H-NMR spectrum attributable
to trans-[(PPh3)2PtH(SnCl3)], 2, {δ(PtH) ϭ Ϫ9.75 [br,
1J(PtH) ϭ 1105 Hz]}, as well as to the products resulting
from a phosphane redistribution reaction {[(PPh3)3PtH]ϩ,
δ(PtH) ϭ Ϫ5.75 [br, d, 1J(PtH) ϭ 154 Hz]; trans-
2
[(PPh3)PtH(SnCl3)2]Ϫ, δ(PtH) ϭ 13.355 [d, J(PH) ϭ 131
The obtained PHIP patterns exhibit features that allow
Hz, 1J(PtH) ϭ 412 Hz]} in accord with previously pub- us to establish the structures of the transient platinum dihy-
lished data[34]. The spectra in ref.[34] were recorded at drides. First of all, two isomers with similar couplings are
1916
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923

Mechanism of Mediated Hydrogenation
FULL PAPER
Table 1. Spectral NMR parameters of intermediate dihydrides observed in the systems [(PPh3)2PtH(SnX3)]/alkyne/[D6]acetone (δ, relative
to [D5]acetone; J, Hz). The letter a corresponds to the major isomer, the letter b to the minor isomer
[a]
Alkyne
X
Isomer δ(H1)
δ(H2)
1J(Pt-H1) 1J(Pt-H2) 2J(P-H1) 2J(P-H2) 2J(119Sn-H1)
2J(Sn-H2) [b]
PhCϵCMe
PhCϵCMe
PhCϵCMe
Cl
Cl
Br
Cl
Br
a
Ϫ10.71 Ϫ11.24
Ϫ10.91 Ϫ11.34
Ϫ10.58 Ϫ11.275
Ϫ10.82 Ϫ11.33
Ϫ11.46 Ϫ10.248
Ϫ11.63 Ϫ10.498
Ϫ10.71 Ϫ11.181
Ϫ10.893 Ϫ11.334
Ϫ11.443 Ϫ10.194
Ϫ11.629 Ϫ10.404
Ϫ10.922 Ϫ11.32
Ϫ11.621 Ϫ10.364
Ϫ10.950 Ϫ11.356
913.7
906.7
869.2
918
920.5
918.4
911.2
907.3
920.7
919.2
568.1
567.8
542.0
567
565.4
566.4
566.1
565.2
563.6
563.8
9.3
9.3
9.4
167.8
168.5
165.4
167.2
166.8
167.7
168.5
169.3
166.3
166.7
169.0
165.9
172.1
1842 (1760)
1840
25.9
b
[c]
[d]
[e]
a
b
a
b
a
9.5
9.6
8.9
10.1
9.7
9.7
9.9
9.4
9.3
1850 (1768)
40.1
33.5
PhCϵCCH2CH3
PhCϵCCH2CH3
1839 (1759)
1837
1853 (1779)
47.3
46.3
b
[f]
Diphenylacetylene
Diphenylacetylene
3,3-Dimethylbutyne
Cl
Br
Cl
[f]
[f]
913.6
940.6
558.3
588.2
1857 (1771)
44.9
2
2
[a] Values in brackets correspond to J(117SnϪH). Ϫ [b] Averaged J(117/119SnϪH). Ϫ [c] Solution in CD2Cl2. Ϫ [d] In the absence of added
acetone. Ϫ [e] 0.15 ml of [D6]acetone was added. Ϫ [f] One isomer.
clearly discernible. This conclusion was confirmed exper- tion of the presence of this ligand was also derived from
imentally upon cooling of the solution under parahydrogen the following considerations: (a) the role of the substrate
(Figure 1, b) which resulted in a non-concerted change in could be either to coordinate to platinum as a π-ligand or
the intensities and widths of the relevant resonances. Each to insert into a PtϪH bond, thereby producing a σ-bonded
isomer exhibits a typical ABX pattern, with large trans- and carbon ligand; (b) the obtained PHIP corresponds to the
small cis-1HϪ31P couplings (Figure 1a, Table 1). The pat- spectrum of a dihydride, and not to a trihydride, estab-
terns for both isomers are accompanied by 195Pt and lishing the insertion suggestion in (a); and (c) the isolated
117/119Sn satellites, permitting an unambiguous assignment insertion product, i.e. the σ-phenylpropenyl platinum com-
of the ligands in the equatorial plane of the coordination plex, shows analogous though more simple polarization
octahedron of 6a. Only one phosphane ligand is incorpor- patterns upon reaction with parahydrogen (Figure 1, c).
ated into the coordination sphere of the transient dihydride, The precise structure of the alkenyl radical in the transient
1
since the spectrum exhibits only one H-31P coupling for dihydride is not clear, but it seems plausible that the exist-
each proton. This was confirmed by PHIP simulations of ence of the two dihydrides is due to an isomerism of the
the spectra of the mono- and diphosphane dihydrides (Fig- alkenyl radical and, therefore, the dihydrides are in fact 2-
ure 1, d, e, f). The assignment of the axial positions is not phenyl-1-methylethenyl and 1-phenylprop-1-enyl com-
so clear-cut, but is supported by the observation that the plexes.
presence of both an alkyne and acetone is essential for the
We were fortunately also able to isolate the product of
development of polarization patterns. We suggest that the insertion of 1-phenyl-1-propyne into the PtϪH bond of the
possible role of acetone lies in promoting the dissociation starting monohydride. Light-yellow crystals of the product
of one phosphane ligand, which, apparently, is followed by were formed on leaving to stand an acetone solution of the
coordination to the platinum center. It has previously been [(PPh3)2PtH(SnCl3)] complex together with the alkyne in
well-documented[34][35] that the reaction of excess tin dihal- the absence of hydrogen, as well as during hydrogenation
ides with platinum hydrides in acetone at room temperature experiments. The 1H-NMR spectrum of this product in
rapidly leads to the formation of mixture of phosphane re- CD2Cl2 clearly shows the signal of the CH3 protons of the
distribution products, which contain monophosphane com- σ-alkenyl ligand with a full set of 195Pt and 117/119Sn satel-
plexes.
lites, and the signal of the proton adjacent to the double
1
Addition of 1-phenyl-1-propyne to an acetone solution bond with partially obscured satellite signals. The H spec-
of 2 leads to the disappearance of the Pt-H signals in the trum is essentially similar to published data for the anal-
hydride region in the thermal spectrum and gives rise to a ogous triethylphosphane complex[42], in spite of the large
broadened singlet at approximately δ ϭ 0.7. This singlet is deviation of the chemical shift value for methyl protons
symmetrically surrounded by broadened signals, the posi- [δ ϭ 2.23, J(PtH) ϭ 56.5 Hz[42]], which is apparently due
3
tions and intensities of which are consistent with the notion to the difference between an acido ligand and the phos-
that they are in part 195Pt and unresolved 119/117Sn satellites. phane. Together with the obtained 13C and 31P data, this
The singlet was thus assigned to the methyl resonance of spectrum allows the unambiguous assignment of the struc-
the σ-alkenyl ligand, resulting from an insertion of the tri- ture of this product as 5a. The doublet splitting of the main
4
ple bond of the substrate into the PtϪH bond (vide infra). methyl resonance (ca. 2 Hz) can be attributed to a J cis-
This signal was invariably present in all cases where polari- coupling with the olefinic proton. Large values of
zation in the PtϪH region was observed, which prompted 4J(119/117Sn-1H) are in accord with a trans-position of the
us to ascribe the key role of a σ-bonded carbon ligand in trichlorostannato ligand relative to the alkenyl group, and
the formation of PHIP-active PtIV dihydrides. The sugges- they match published values for tinϪproton couplings in
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923
1917

C. Deibele, A. B. Permin, V. S. Petrosyan, J. Bargon
FULL PAPER
The polarization spectrum originating from reaction of
parahydrogen with the solution of 5a in dichloromethane
indicates the presence of only one isomer. These patterns
are much less intense than those obtained for catalytic mix-
tures in acetone, which is apparently due to the fact that
acetone is present in the system only in minor amounts as
an impurity or as a solvent of crystallization, as is evident
from the 1H and 13C spectra. The low intensity of the polar-
ization signals can be also attributed to the enhanced sta-
bility of the isolated isomer, which is hydrogenated only to
1
a small extent. The differences between the PHIP H-NMR
spectra obtained for the isolated insertion product and
those obtained from the catalytic system are due to the
change of medium, since addition of 0.15 ml of [D6]acetone
to a CDCl3 solution of 5a, followed by parahydrogen, shifts
the dihydride signals towards coincidence with the position
observed in situ (Table 1).
Catalytic Systems with Other Substrates
Besides 1-phenyl-1-propyne, other substrates can give
transient dihydrides detectable by the PHIP technique.
Thus, we have attempted to hydrogenate alkynes (phenyl-
acetylene, diphenylacetylene, 1-phenyl-1-butyne, 1,4-di-
phenylbutadiyne), and alkenes (norbornadiene, 3,3-dimeth-
ylbutene) under the same conditions as used for 1-phenyl-
other platinum trichlorostannato complexes[43]. It is also 1-propyne. Our observations are summarized qualitatively
noteworthy that addition of several equivalents of DMF re- in Table 2.
sults in the cleavage of the trichlorostannato ligand with
The interaction of the system [(PPh3)2PtHCl]/SnCl2/1-
formation of the corresponding chloro complex, 5b, in ac- phenyl-1-butyne/acetone with parahydrogen proceeds simi-
cord with the reported behavior of σ-alkylplatinum com- larly to the reaction of 1-phenyl-1-propyne (Figure 2). Pola-
plexes[31]
.
rization patterns show clearly resolved 117Sn and 119Sn sat-
Table 2. Qualitative summary of results obtained for the studied systems [(PR3)2PtHX]/SnX2/substrate/acetone
[b]
PR3
X
Substrate
Pt dihydride(s), intensity,
Insertion product
Synchronous/ asynchro-
[a]
[c]
number of isomers
nous hydrogenation
PPh3
Br
Br
Br
Cl
Cl2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Cl
Cl
1-Phenyl-1-propyne
1-Phenyl-1-butyne
Diphenylacetylene
1-Phenyl-1-propyne
1-Phenyl-1-propyne
1-Phenyl-1-butyne
Diphenylacetylene
3,3-Dimetylbutyne
1,4-Diphenylbutadiyne
Phenylacetylene
ϩϩϩ, 2
ϩϩϩ, 3
ϩϩϩ, 1
ϩϩϩ, 2
no
ϩ
ϩϩ
ϩϩϩ, th.
PPh3
n.d.
n.d.
ϩ
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
PPh3
th.
PPh3
ϩϩ, th.
th.
ϩϩ
[d]
PPh3
PPh3
ϩϩϩ, 3
ϩ, 1
ϩϩ, 2
no
PPh3
PPh3
th.
PPh3
ϩϩ
ϩϩϩ, th.
no
PPh3
no
PPh3
3,3-Dimetylbutene
Norbornadiene
no
PPh3
no
n.d.
n.d.
n.d.
ϩ, br.
n.d.
ϩ
th.
PPh3[e]
PPh3[f]
PPh3[g]
PPh3
1-Phenyl-1-propene
1-Phenyl-1-butyne
1-Phenyl-1-propyne
1-Phenyl-1-propyne
1-Phenyl-1-propyne
1-Phenyl-1-propyne
1-Phenyl-1-propyne
no
no
no
no
no
ϩ, th.
ϩ, th.
ϩ, th.
th.
no
PMePh2
PMePh2
PMe2Ph
ϩϩ, 2
ϩ
ϩ
n.d.
ϩ
th.
[a] ؉؉؉, strong polarization signals, with 195Pt and 117/119Sn satellites visible in spectrum; ؉؉, medium polarization, 117/119Sn satellites
are not visible due to low signal/noise ratio; ؉, low polarization, only central part of the patterns is clearly visible. Ϫ [b] ؉, the signal(s)
[c]
attributable to the insertion product are present; n.d., not detected, the signals of the insertion product are presumably obscured. Ϫ
؉؉؉, ؉؉, and ؉, strong, medium, and weak polarization of product signals, respectively; th., continuous rise of the thermal product
signals without polarization. Ϫ [d] The corresponding dichloro complex was used instead of [(PR3)2PtHX]. Ϫ [e] In acetonitrile as solvent;
1 mol equivalent of SnCl2. Ϫ [f] In CDCl3/[D8]THF mixture, 7:1 (v/v). Ϫ [g] In CDCl3.
1918
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923

Mechanism of Mediated Hydrogenation
FULL PAPER
ellites for both isomers, confirming the general assignment temperatures, a reversible formation of the second isomer is
of the transient dihydrides to the structures 6b. Upon heat- observable. Since in this case no changes in the alkenyl li-
ing, extensive isomerization of the alkenyl part of the σ- gand can readily occur without undergoing skeletal re-
ligand occurs, complicating the spectrum and leading to the arrangements, we suggest these two reversible isomers to be
appearance of a third isomer. This isomer could be due to the 3,3-dimethylbutenyl-1-platinum complex as the major
an isomerization of the side-chain of the alkyne into an component, and the corresponding -2-platinum complex as
allenic structure. With 1-phenyl-1-butyne as the substrate, the minor component. Phenylacetylene and 1,4-diphenylbu-
1
the insertion product was not discernible in the H spec- tadiyne gave no detectable dihydrides, but polarized signals
trum owing to the low intensity and complicated patterns of styrene and 1,4-diphenyl-3-en-1-butyne, respectively,
due to its ethyl group, but it was readily observable in the were seen on heating (vide infra).
31P spectrum of the reaction mixture [δP ϭ 17.75,
1J(PtϪP) ϭ 2995 Hz].
PHIP in Related Catalytic Systems
Figure 2. PHIP 1H-NMR spectra of the systems trans-
[(PPh3)2PtHCl]/SnCl2/alkyne/para-H2 ([D6]acetone, room temp.);
a) alkyne ϭ 1-phenyl-1-butyne; b) central lines of the spectrum (a)
in the range δ ϭ Ϫ10 to Ϫ12; c) PHIPϩϩ simulation of the central
part of the spectrum (a); d) alkyne ϭ diphenylacetylene; e) al-
kyne ϭ 3,3-dimethylbutyne
Bromide Systems: The interactions of the systems
[(PPh3)2PtHBr]/SnBr2/alkyne (alkyne ϭ 1-phenyl-1-pro-
pyne, 1-phenyl-1-butyne, diphenylacetylene) in acetone with
parahydrogen also give rise to polarization in the PtϪH
spectral region (Figure 3). The difference in chemical shifts
of the two hydride resonances provides clear evidence for
first-order patterns of an AMX spin system in these cases.
In general, bromide systems give more intense and sharp
signals compared to the corresponding hydrido chloro com-
plexes. The resulting PtIV dihydrides possess an enhanced
thermal stability compared with the chloro analogues, and
are readily observable even upon heating. Furthermore, the
initial patterns are fully restored on cooling once more (al-
1
Figure 3. PHIP H-NMR spectra of the systems [L2PtHBr]/SnBr2/
alkyne/para-H2 in [D6]acetone; a) L ϭ PPh3, alkyne ϭ 1-phenyl-1-
propyne, room temp.; b) L ϭ PPh3, alkyne ϭ 1-phenyl-1-butyne,
room temp., before heating; c) L ϭ PPh3, alkyne ϭ 1-phenyl-1-
butyne, on heating; d) L ϭ PPh3, alkyne ϭ 1-phenyl-1-butyne, at
room temp. after heating; e) L ϭ PPh3, alkyne ϭ diphenylacetylene,
room temp.; f) L ϭ PMePh2, alkyne ϭ 1-phenyl-1-propyne, room
temp.
As expected, the system [(PPh3)2PtHCl]/SnCl2/diphenyl-
acetylene produced simple polarization patterns upon reac-
tion with parahydrogen in acetone (Figure 2, d) due to the
symmetry of the substrate and, therefore, resulted in the
presence of only one dihydride, 6c. The intensity of the sig-
nals was much lower than in the case of the 1-phenyl-1-
propyne system. The reaction with 3,3-dimethylbutyne
(Figure 2, e) also gave a simple polarization spectrum, de-
spite the lack of symmetry of the substrate. This simplicity
is apparently due to the fact that one alkenyl isomer
strongly predominates in this case. The major isomer is sug-
gested to be the 3,3-dimethylbutenyl-1 complex (6d), in view
of the considerable steric requirements of the tert-butyl
group. When the PHIP spectrum is recorded at elevated
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923
1919

C. Deibele, A. B. Permin, V. S. Petrosyan, J. Bargon
FULL PAPER
kyne ϭ 1-phenyl-1-propyne, Figure 3, a). With 1-phenyl-1- diphenylacetylene in the presence of [(PPh3)2PtHCl]/SnCl2.
butyne, the additional third isomer of the dihydride is Nevertheless, in addition to these signals, in some cases rel-
formed only after heating (Figure 3, bϪd). With diphenyl- atively strong polarization patterns are observed on heating
acetylene, the bromide system gives strong and well-re- (Figure 4). In general, hydrogenation of 1-phenyl-1-butyne
solved polarization patterns with a full set of satellite sig- and 1-phenyl-1-propyne gives moderately intense polari-
nals (Figure 3, e).
zation signals of the corresponding cis-alkenes (Figure 4, a,
Variation of the Phosphane Ligands: Generally speaking, b). Phenylacetylene is hydrogenated readily, giving rise to
electron-donating phosphanes show less intense and poorly strongly polarized styrene signals as a result of cis-addition
resolved polarization patterns in the PtϪH region under the of parahydrogen (Figure 4 c). The cis-addition of para-H2
conditions employed. The results obtained for the was confirmed by means of the PHIPϩϩ program simu-
[(PR3)2PtHX]/SnX2/alkyne/acetone systems are summar- lation. Due to the ALTADENA conditions of the hydro-
ized in Tables 2 and 3. The triethylphosphane monohydride genation (i.e. the addition of parahydrogen proceeds in the
system did not give rise to PHIP. The system [(PMe- EarthЈs magnetic field, and the subsequent transfer to the
Ph2)2PtHCl]/SnCl2/1-phenyl-1-propyne/acetone produces strong B0 field for the NMR measurement is adiabatic),
only weak polarization in the PtϪH region (Figure 3, f). extensive polarization transfer to the methyl and phenyl
Table 3. Spectral NMR parameters of intermediate dihydrides observed in systems with other phosphanes, [(PR3)2PtH(SnX3)]/1-phenyl-
1-propyne/[D6]acetone (δ, relative to [D5]acetone; J, Hz). The letter a corresponds to the major isomer, the letter b to the minor isomer.
PR3
X
Isomer
δ(H1)
δ(H2)
1J(Pt-H1)
1J(Pt-H2)
2J(P-H1)
2J(P-H2)
PMePh2
PMePh2
PMe2Ph
Cl
Br
Cl
a
b
a
Ϫ11.17
Ϫ
Ϫ
Ϫ
9.5
Ϫ
Ϫ11.392
Ϫ11.875
Ϫ12.102
Ϫ11.869
Ϫ10.25
Ϫ11.487
951.3
948.6
9.5
563?
Ϫ
9.3
9.2
Ϫ
163.2
164.5
b
[a]
[a] One isomer.
Figure 4. Polarization of product signals during hydrogenation with
para-H2 of alkynes in the presence of [(PPh3)2PtHX]/SnX2 catalytic
systems in [D6]acetone on heating; a) X ϭ Br, alkyne ϭ 1-phenyl-
1-butyne; b) X ϭ Br, alkyne ϭ 1-phenyl-1-propyne; c) X ϭ Cl,
alkyne ϭ phenylacetylene. The absorption signals of the products,
i.e. of the cis-alkenes, are marked *.
The methyl resonance attributable to the insertion product,
analogous to 5, is also observable in this system [δ(CH3) ϭ
3
1.24, J(195PtϪH) ϭ 53.4 Hz]. Due to the low intensity and
broadness of the signals, the exact composition of the inser-
tion product remains unclear.
Using the [(PMePh2)2PtHBr]/SnBr2 system, the hydro-
genation of 1-phenyl-1-propyne proceeds rapidly at room
temperature, mainly via an asynchronous mechanism, i.e.
not pairwise. This is reflected in the low polarization signals
of the hydrogenation product, as opposed to rapidly arising
thermal ones, and furthermore, the polarization in the
PtϪH region is low. These Pt-H PHIP signals become
stronger upon heating. It should be noted, however, that
the signals attributable to the insertion product are clearly
visible even at elevated temperature.
Only weak and poorly-resolved polarization patterns in
the PtIV dihydride region are observed in the course of
hydrogenation of PhCϵCMe in the presence of
[(PMe2Ph)2PtHCl] ϩ SnCl2 in acetone. This hydrogenation
proceeds without polarization of the product signals.
The Relationship between Synchronous (Pairwise) and
Asynchronous Hydrogenation Mechanisms
The observed PtIV dihydrides are assumed to be the inter-
mediates in the stepwise hydrogenation of an alkyne. De-
composition of these dihydrides leads via reductive elimi-
nation to an alkene and a platinum monohydride. Conse-
quently, only thermal signals are observed from the prod-
uct, as is the case during the hydrogenation of
1920
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923

Mechanism of Mediated Hydrogenation
FULL PAPER
groups of the substrate is also observed. The efficient pola- should be favored at higher temperatures, as well as by an
rization transfer under these conditions is due to strong iso- increased electronegativity of the π ligand. This process is
tropic mixing of the proton spin functions in the whole mol- of little significance for alkenes, where asynchronous hydro-
ecule at low magnetic field[44], as was previously shown for genation is observed, giving rise to thermal signals of the
the hydrogenation of the diphenylacetylene-Pt0 complex, products. It is also noteworthy that the PtIV dihydrides (6a)
[(PPh3)2Pt(PhCϵCPh)][27]. Alternatively, for the hydroge- are not usually observable in the spectrum at elevated tem-
nation of 1-phenyl-1-propyne, the polarized signals of the peratures in cases where the polarized signals of the prod-
methyl group of the product can be accounted for by as- ucts appear instead.
suming a 1,3-dihydrogen addition to the intermediate plat-
inacyclobutene, formed by methyl CϪH activation, al-
though the geometry of such an addition should favor the
trans-alkene as a primary product, rather than the cis-iso-
mer, which is in fact obtained. Furthermore, the same effect
is also operative for the methyl group of 1-phenyl-1-butyne,
and seems to have the same origin.
The Role of Structural Factors and Reaction Conditions for the
Appearance of PHIP in the PtII/SnII Catalytic Systems
Solvent Effects: The described polarization patterns of
the PtIV dihydrides were observed only with acetone as the
The mechanism of the synchronous dihydrogen addition solvent. The sole exception was the interaction of isolated
observed in the SnX2-activated systems described here is 5a with parahydrogen in dichloromethane solution, as dis-
suggested to be related to the Pt0-mediated hydrogenation.
cussed above. The synchronous hydrogenation of alkynes
This mechanism could involve the η2-coordination of an proceeded well in acetone, but was hardly reproducible in
alkyne, and the reductive elimination of HX together with benzene or in other non-solvating media. Experiments in
SnX2, followed by oxidative addition of dihydrogen to the acetonitrile, which is a much poorer solvent for SnCl2, did
resulting Pt0 complex with subsequent formation of an al- not lead to any PHIP effects either. Strongly solvating addi-
kene (Scheme 2). The formation of the Pt0 intermediate tives, such as DMF and THF, can easily destroy the cata-
lytic system, facilitating the cleavage of solvated SnX2 mol-
Scheme 2. The proposed mechanism for the synchronous hydroge-
ecules from the active platinum hydride and/or from a σ-
nation of alkynes catalyzed with the [(PR3)2-
carbon intermediate (3 in Scheme 1[31][45]). Since PtIV dihy-
PtHX]ϪSnX2 system in acetone
dride contains only one phosphane ligand, the promotion
of phosphane dissociation by the attached trihalotin ligand
and acetone as the solvent seems to be necessary for the
generation of PHIP-detectable dihydrides. It is noteworthy
that phosphane redistribution proceeds only for platinum
monohydrides with PPh3 and PMePh2[46], and correspond-
ingly, polarized PtIV dihydrides are observable mainly with
these ligands. Furthermore, the processes suggested to be
essential for synchronous hydrogenation, the formation of
cationic intermediates 7 (Scheme 2), as well as their depro-
tonation with formation of Pt0 intermediates 8, are strongly
favored with polar, solvating acetone as the solvent.
Substrate and Halide Ligands: The appearance of polar-
ized PtIV dihydrides depends principally on the hydroge-
nation substrate. The intensity of the polarization pattern
reflects the suggested stability of the insertion product,
while the number of isomers corresponds to the possible
number of isomeric alkenyl radicals. For symmetrical
alkynes, only one isomer is observed; for 1-phenyl-1-pro-
pyne two main insertion products are possible, giving rise
initially to two dihydrides. Accordingly, 1-phenyl-1-butyne,
being able to form more isomers, gives a complicated mix-
ture of polarized dihydrides after heating. Alkenes, es-
pecially internal ones, generally adopt a quite unfavorable
equilibrium position for insertion into PtϪH bond[31]. This
factor is believed to be the reason for the failure to observe
intermediates of type 6 in the course of the hydrogenation
of alkenes such as NBD and 3,3-dimethylbutene. Further-
more, steric bulkiness of an alkyne (e.g. diphenylacetylene)
can prevent extensive formation of the alkenyl complex,
thereby decreasing the intensity of the signals of the PtIV di-
hydrides.
Eur. J. Inorg. Chem. 1998, 1915Ϫ1923
1921

C. Deibele, A. B. Permin, V. S. Petrosyan, J. Bargon
FULL PAPER
[s, CPh, 2J(PtC) ϭ 94.8 Hz], 23.50 [s, CH3, 2J(PtC) ϭ 28.9 Hz].
Tribromostannato hydrides give insertion products with
higher equilibrium constants, and the resulting σ-insertion
products are less fluxional compared to the chloro ana-
logues. This feature is reflected in the higher intensity and
sharpness of the corresponding polarization patterns.
Ϫ
31P NMR: δ ϭ 18.85 [s, 1J(PtP) ϭ 3007, 2J(119SnP) ϭ 240.4,
2J(117SnP) ϭ 230.7 Hz].
Reaction of 5a with Dimethylformamide: To a solution of isolated
5a (46 mg, 43 µmol) in 0.7 ml of CD2Cl2, 10 µl of dimethylformam-
1
3
ide was added. H NMR of 5b: δ ϭ 1.131 [s, J(PtH) ϭ 48.8 Hz].
Ϫ
We thank the Russian Foundation for Basic Research ( RFBR) ,
and the Deutsche Forschungsgemeinschaft ( DFG) for financial sup-
port, and Dr. S. Klages for many helpful discussions.
31P NMR: δ ϭ 24.607 [s, J(PtP) ϭ 3306 Hz].
1
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1923