Diverse catalytic activity of the cationic actinide complex [(Et2N)3U][BPh4] in the dimerization and hydrosilylation of terminal alkynes. Characterization of the first f-element alkyne π-complex [(Et2N)2U(CCtBu)(η2-HCC tBu)][BPh4]

Article abstract of DOI:10.1016/S0022-328X(00)00207-2

The cationic actinide complex [(Et₂N)₃U][BPh₄] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis-dimer is also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex [(Et₂N)₂UCCR][BPh₄] with the concomitant reversible elimination of Et₂NH, followed by the formation of the alkyne π-complex [(Et₂N)₂UCCR(RCCH)][BPh₄]. This latter complex (R=^tBu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the CC bond insertion of the terminal alkyne into the actinide-acetylide bond to give the corresponding alkenyl complex with ΔH^?=15.6(3) kcal mol^-1 and ΔS^?=-11.4(6) eu. The following step, protonolysis of the uranium-carbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH₃CCD, allowing the formation of trimers. The unexpected cis-isomer is presumably obtained by the isomerization of the trans-alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et₂N)₃U][BPh₄] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH₃ at room temperature produced a myriad of products among which the cis- and trans-vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)CCH and PhSiH₃ with [(Et₂N)₃U][BPh₄] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium-hydride complex [(Et₂N)₂U-H][BPh₄], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.

Full text of DOI:10.1016/S0022-328X(00)00207-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 604 (2000) 83–98
Diverse catalytic activity of the cationic actinide complex
[(Et2N)3U][BPh4] in the dimerization and hydrosilylation of
terminal alkynes. Characterization of the first f-element alkyne
p-complex [(Et2N)2U(CꢀCtBu)(h2-HCꢀCtBu)][BPh4]
Aswini K. Dash a, Jia Xi Wang a, Jean Claude Berthet b, Michel Ephritikhine b,
Moris S. Eisen a,*
a Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology, Haifa 32000, Israel
b DSM, DRECAM, Ser6ice de Chimie Mole´culaire, CNRS URA 331, CEA Saclay, 91191 Gif sur Y6ette, France
Received 7 March 2000; accepted 12 April 2000
Abstract
The cationic actinide complex [(Et2N)3U][BPh4] is an active catalytic precursor for the selective dimerization of terminal
alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis-dimer is
also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex
[(Et2N)2UCꢀCR][BPh4] with the concomitant reversible elimination of Et2NH, followed by the formation of the alkyne p-complex
[(Et2N)2UCꢀCR(RCꢀCH)][BPh4]. This latter complex (R=tBu) has been characterized spectroscopically. The kinetic rate law is
first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne
concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The
turnover-limiting step is the CꢀC bond insertion of the terminal alkyne into the actinideꢁacetylide bond to give the corresponding
alkenyl complex with DH‡=15.6(3) kcal mol−1 and DS‡= −11.4(6) eu. The following step, protonolysis of the uraniumꢁcarbon
bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH3CꢀCD, allowing the
formation of trimers. The unexpected cis-isomer is presumably obtained by the isomerization of the trans-alkenyl intermediate via
an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic
complex [(Et2N)3U][BPh4] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The
chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction
temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH3 at room temperature produced a myriad of
products among which the cis- and trans-vinylsilanes, the alkene and the silylalkyne are the major components. At higher
temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon
moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)CꢀCH and PhSiH3 with
[(Et2N)3U][BPh4] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the
uranium–hydride complex [(Et2N)2UꢁH][BPh4], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible
mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.
© 2000 Elsevier Science S.A. All rights reserved.
Keywords: Organoactinide; p-Complexes; Alkyne complexes; Dimerization of alkynes; Hydrosilylation; Catalysis
1. Introduction
In recent years, the catalytic aspects of the
organometallic complexes of d0/fn-block have attracted
a major attention and have been in particular the focus
of numerous investigations for the functionalization of
* Corresponding author. Tel.: +972-4-8292680; fax: +972-4-
8233735.
E-mail address: chmoris@techunix.technion.ac.il (M.S. Eisen).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00207-2

84
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
unsaturated organic molecules [1–14]. Metal-mediated
oligomerization of terminal alkynes is currently of con-
siderable interest because it can lead to a variety of
organic enynes and oligoacetylene products [4,14] that
are useful synthetic precursors for the synthesis of
natural products [15] and also for organic conducting
polymers [16]. Enynes are the simplest oligomeriza-
tion products of alkynes and the key steps in their
formation involve the generation of a MꢁCꢀCR carbyl
moiety, insertion of the alkyne to yield the Mꢁ
C(H)ꢂC(R)CꢀCR alkenyl intermediate and protonoly-
sis with additional alkyne to release the dimer and
regenerate the MCꢀCR species. Higher oligomers are
formed by further insertion of alkyne into the
MꢁC(H)ꢂC(R)CꢀCR species. Lately, we have demon-
strated that organoactinides complexes of the type
tures from those of the late transition complexes, have
been reported to catalyze the hydrosilylation reaction
of unsaturated hydrocarbons very effectively [22].
Considerable attention has been paid in recent years
to the versatile and rich chemistry of vinylsilanes, which
are considered as important building blocks in organic
synthesis [23]. The synthesis of vinylsilanes has been
extensively studied and one of the most convenient and
straightforward methods is the hydrosilylation of alky-
nes [23–25]. In general, hydrosilylation of terminal
alkynes produces the three different isomers, cis, trans
and geminal, as a result of both 1,2 (syn and anti ) and
2,1 additions, respectively, as shown in Reaction (1).
The distribution of the products is found to vary con-
siderably with the nature of the catalyst, substrates and
also the specific reaction conditions [23–25].
(1)
A number of mechanisms have been presented for
the hydrosilylation process and one of the most widely
accepted was first proposed by Chalk and Harrod in
1965 for the Pt-catalyzed hydrosilylation of alkenes
[26]. The main feature of this mechanism (Scheme 1(a))
is the insertion of a coordinated alkene into a
metalꢁhydrogen bond, followed by reductive elimina-
tion of the alkyl and silyl ligands. If the intermediate
alkyl complex undergoes reversible b-hydride elimina-
tion and reinsertion with opposite regiochemistry, then
the Chalk–Harrod mechanism provides an explanation
for the olefin isomerization and deuterium scrambling
during the hydrosilylation reactions [18a]. However,
this mechanism is unable to explain the production of
vinylsilanes from the hydrosilylation reaction of alke-
nes. In some cases vinylsilanes are produced more
readily than the hydrosilylation product [27]. To ac-
count for this competing process, a number of different
mechanisms, so called modified Chalk–Harrod mecha-
nisms, have been proposed [28]. In the basic mechanism
(Scheme 1(b)), an alkene inserts into a metalꢁsilicon
bond and the reductive elimination of the resulting
b-silaalkyl and hydride ligands leads to the hydrosilyla-
tion product. A competing b-hydride elimination from
the b-silaalkyl moiety permits the formation of the
vinylsilane.
Cp2*AnMe2 (Cp*=C5Me5; An=U, Th) are active cat-
alysts for the linear oligomerization of terminal alkynes
and the extent of oligomerization was found to be
strongly dependent on the electronic and steric proper-
ties of the alkyne substituents [17]. For example, bulky
alkynes reacted with high regioselectivity towards
dimers and/or trimers whereas for non-bulky alkynes,
the oligomerization yielded dimers to decamers with
total lack of regioselectivity. The addition of primary
amines, for An=Th, allowed the chemoselective for-
mation of dimers whereas for An=U this control was
not achieved [14b].
The metal-catalyzed hydrosilylation reaction, which
is the addition of a SiꢁH bond across a carbonꢁcarbon
multiple bond, is one of the most important reactions in
organosilicon chemistry and has been studied exten-
sively for half a century [18]. The hydrosilylation reac-
tion is used in the industrial production of
organosilicon compounds (adhesives, binders and cou-
pling agents), and in research laboratories as an effi-
cient route to a variety of organosilicon compounds,
silicon-based polymers and new type of dendrimeric
materials [18]. Since the discovery of Speier’s catalyst
(H2PtCl6/iPrOH) in 1957, [19] catalytic asymmetric hy-
drosilylation and new reactions related to hydrosilyla-
tion have been discovered and developed [18–20].
Interestingly, most of the research has been devoted to
late-transition metal complexes [18–21]. More recently,
metallocene complexes of either Group 3, 4, lan-
thanides and actinides, which exhibit distinctive fea-
For organolanthanides and organoyttrium com-
plexes, in the hydrosilylation of alkenes and for
organoactinides, in the hydrosilylation of alkynes, the
processes are proposed to proceed via the Chalk–Har-

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
85
rod mechanism (insertion of an olefin into a metalꢁ
hydride bond) with the classical oxidative addition–re-
ductive elimination steps replaced with s-bond metathe-
sis reactions (Scheme 1(c)) [6,22].
reactivity and selectivity of a well defined cationic ac-
tinide complex [(Et2N)3U][BPh4] as a catalytic precursor
for the selective dimerization of a variety of terminal
alkynes. In addition we have shown with as well as the
spectroscopic characterization of the first uranium–
alkyne p-complex as the key organometallic intermediate
in the catalytic cycles; we present here a thorough kinetic,
thermodynamic and mechanistic study. In addition, we
present a comprehensive study on the reactivity of this
cationic uranium complex in the hydrosilylation of ter-
minal alkynes; This reaction seems to follow the hydride
pathway rather than the silane route, as evidenced by the
lack of dehydrogenative coupling of silanes.
In contrast to the neutral organoactinide complexes,
homogeneous cationic d0/fn actinide complexes have
been used as catalysts for the polymerization of a-olefins
[29], similarly to their isolobal Group 4 complexes. Some
heterogeneous complexes have been used for the rapid
hydrogenation of aromatic molecules and CꢁH activa-
tion of alkanes [30]. Regarding alkyne activations with
cationic Group 4 complexes, Cp2*ZrMe+ selectively
t
dimerizes BuCꢀCH to the head-to-tail dimer but con-
n
verts PrCꢀCH and MeCꢀCH into mixtures of dimers
and trimers [31]. The less bulky Cp2ZrMe+ cation reacts
with these alkynes to form catalytically inactive dinuclear
compounds [32]. Thus, catalytic alkyne oligomerization
is a useful probe of insertion and s-bond metathesis
reactivity of complexes. For cationic actinide complexes,
virtually nothing is known regarding their reactivity with
terminal alkynes [33]. Expanding their rich potential as
homogeneous catalysts, in this publication we report the
2. Experimental
2.1. Materials and methods
All manipulations of air-sensitive materials were per-
formed with the rigorous exclusion of oxygen and
moisture in flamed Schlenk-type glassware on a dual
manifold Schlenk line, or interfaced to a high vacuum
(10−5 torr) line, or in a nitrogen filled ‘Vacuum Atmo-
spheres’ glove box with a medium capacity recirculator
(1–2 ppm O2). Argon and nitrogen were purified by
passage through a MnO oxygen-removal column and a
,
Davison 4A molecular sieve column. Hydrocarbon sol-
vents (toluene-d8, benzene-d6, THF-d8) were distilled
under nitrogen from Na/K alloy. All solvents for vacuum
line manipulations were stored in vacuo over Na/K alloy
in resealable bulbs. Acetylenic compounds and phenylsi-
lane (Aldrich) were dried and stored over activated
,
molecular sieves (4 A), degassed and freshly vacuum-dis-
tilled. Deuterium oxide was purchased from Cambridge
isotopes. Et2NH (Fluka) was dried over Na/K alloy and
,
stored over activated molecular sieve (4 A), degassed and
freshly vacuum-distilled. [(Et2N)3U][BPh4] was prepared
according to the literature [34]. NMR spectra were
recorded on Bruker AM 200 and Bruker AM 400
spectrometers. Chemical shifts for 1H- and 13C-NMR are
referenced to internal solvent resonances and are re-
ported relative to tetramethylsilane. For 29Si-NMR,
Si(TMS)4 was used as internal standard (SiMe3= −7.80
ppm), and the experiments were measured using either
the INEPT or DEPT programs. GC–MS experiments
were conducted with a GCMS (Finnigan Magnum)
spectrometer. The NMR experiments were conducted in
Teflon valve-sealed tubes (J. Young) after vacuum trans-
fer of the liquids in a high vacuum line.
2.2. General procedure for the catalytic oligomerization
of alkynes
Scheme 1. Proposed mechanisms for the hydrosilylation of alkenes:
Chalk–Harrod (1a), modified Chalk–Harrod (1b) and modified
Chalk–Harrod for early-transition metal complexes (1c).
In a typical procedure, the amount of the specific
alkyne was vacuum transferred into a J. Young NMR

86
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
tube containing 18 mg (0.0233 mmol) of
[(Et2N)3U][BPh4] in 0.6 ml of toluene-d8. The sealed
tube was then heated in an oil bath to 110°C for 24 h.
The organic products were vacuum transferred (10−6
mmHg) to another J. Young NMR tube, sealed and
Evaporation of the solvent gave a white solid. The
Schlenk tube was cooled to −85°C and under argon
flush 0.1 ml (5.5 mmol) of D2O was added. After
stirring for 10 min, the mixture was warmed to r.t. and
the evolved gas, MeCꢀCD, was transferred and trapped
into a J. Young NMR tube containing 10 mg (0.013
mmol) of [(Et2N)3U][BPh4] in 0.6 ml of toluene-d8. The
sealed tube was heated in an oil bath at 110°C for 24 h.
The volatiles were vacuum transferred to another J.
Young NMR tube and characterized by NMR methods
to be the gem-D2CꢂC(Me)CꢀCMe [1f] in 92% yield
1
both residue and volatiles were characterized by H-,
13C- and 2D-NMR, GC–MS spectroscopy and by com-
paring with literature known compounds. Spectroscopic
data of compounds 1b, 1c, 1d, 2d, 3d, 4d, 1e, 3e, 1g are
described in references [14] and [35].
1
with 8% of trimers. H-NMR: (200 MHz, toluene-d8): l
2.2.1. Catalytic dimerization of MeCꢀCH
1.83 (s, 3 H, CꢂCCH3), 1.64 (s, 3 H, CꢀCCH3). 13C-
NMR: (50 MHz, toluene-d8): l 119.5 (quintet, J=35
Hz, D2CꢂC), 118.0 (s, D2CꢂC), 84.6 (s, CꢀCMe), 81.2
(s, CꢀCMe), 23.3 (q, J=127.6 Hz, CꢂCCH3), 3.3 (q,
J=131.3 Hz, CꢀCCH3).
As described above, 48 mg (1.2 mmol) of MeCꢀCH
(measured at low temperature, −40°C, where the
alkyne is a liquid) were catalytically dimerized to head-
to-tail dimer H2CꢂC(Me)CꢀCMe (1a) in 94% yield (the
1
remaining 6% was found to be starting material). H-
NMR (200 MHz, toluene-d8), l 5.30 (s, 1 H, HHCꢂC),
5.06 (s, 1 H, HHCꢂC), 1.83 (s, 3 H, CꢂCCH3), 1.65 (s,
3 H, CꢀCCH3). 13C-NMR (50 MHz, toluene-d8), l
136.4 (s, H2CꢂC), 119.7 (t, J=159.5 Hz, H2CꢂC), 84.7
(s, CꢀCMe), 81.5 (s, CꢀCMe), 23.3 (q, J=127.6 Hz,
H2CꢂCCH3), 3.2 (q, J=131.3 Hz, CꢀCCH3).
2.2.7. Catalytic oligomerization of PhCꢀCH
As described above, 0.13 ml (1.162 mmol) of
PhCꢀCH were catalytically oligomerized to a mixture
of gem-dimer (1g) (32%) and trimers (58%).
2.3. Preparation of
n
2.2.2. Catalytic dimerization of BuCꢀCH
[(Et2N)2U(CꢀCtBu)(HCꢀCtBu)][BPh4]
n
As described above, 0.13 ml (1.1 mmol) of BuCꢀCH
were catalytically dimerized into the head-to-tail gemi-
In a typical procedure, 61.6 mg (7.97×10−2 mmol)
of [(Et2N)3U][BPh4] was charged in an NMR tube and
dissolved in 0.5 ml of benzene-d6 which was added by
vacuum transfer through a greaseless vacuum line. The
solution was transferred in the glovebox by a gas-tight
syringe into another NMR tube containing 0.0196 ml
nal dimer H2CꢂC(nBu)CꢀCnBu (1b) in 94% yield.
i
2.2.3. Catalytic dimerization of PrCꢀCH
As described above, 0.13 ml (1.248 mmol) of
iPrCꢀCH were catalytically dimerized into the gem-
H2CꢂC(iPr)CꢀCiPr (1c) in 75% yield.
(15.9×10−2 mmol) of BuCꢀCH in 0.5 ml of benzene-
t
d6. The mixture was stirred at r.t. for 12 h, leading to
the formation of the complex [(Et2N)2U(CꢀC-
tBu)(HCꢀCtBu)][BPh4] and traces of the gem-dimer
H2CꢂC(tBu)(CꢀCtBu). The complex [(Et2N)2U(Cꢀ
CtBu)(HCꢀCtBu)][BPh4] decomposes totally in 24 h.
1H-NMR (200 MHz, benzene-d6): l −2.14 (s, 1H,
HC), 1.24 (s, 9H, C(CH3)3), 1.26 (b, 6H, CH2CH3),
1.28 (b, 6H, CH2CH3), 1.30 (s, 9H, C(CH3)3), 7.19–7.31
(m, 20H, Ph), 25.07 (b, 8H, CH2CH3). 13C{H}-NMR
(200 MHz, benzene-d6): l −19.85 (d, J=250 Hz,
HCꢀC), 15.25 (q, J=119 Hz, Me3CCꢀCH), 24.33 (s,
CMe3), 27.54 (q, J=115 Hz, Me3CCꢀC), 31.26 (q,
J=122 Hz, CH3CH2), 31.76 (q, J=122 Hz, CH3CH2),
44.51 (t, J=144 Hz, CH3CH2), 57.76 (bs,
Me3CꢁCꢀCH), 104.96 (bs, Me3CꢁCꢀCU), 123.06 (bs,
Me3CꢁCꢀCU), 139.4–151.97 (m, Ph). IR (dry
parathone oil) cm−1: 3079, 2967, 2934, 2908, 2875,
2059 (CꢀC str), 2032 (CꢀC str), 1486, 1368, 1275, 1210,
2.2.4. Catalytic oligomerization of TMSCꢀCH
As described above, 0.13 ml (0.903 mmol) of
TMSCꢀCH were catalytically oligomerized to a mixture
of the gem(1d):trans(2d):cis(3d) dimers=43:7:16, re-
spectively, and the head-to-tail-to-head trimer, TM-
SC(H)ꢂC(H)ꢁC(H)ꢂC(TMS)CꢀCTMS (4d) in 33%
yield.
t
2.2.5. Catalytic dimerization of BuCꢀCH
As described above, 0.13 ml (1.036 mmol) of
tBuCꢀCH were catalytically oligomerized to a mixture
of gem-H2CꢂC(tBu)CꢀCtBu (1e) in 74% yield, and the
cis-tBuC(H)ꢂC(H)CꢀCtBu (3e) in 25% yield.
2.2.6. Catalytic oligomerization of MeCꢀCD
An excess of propyne (ꢀ5 ml) was condensed at
−100°C into a Schlenk tube containing 3.13 ml (5.0
t
1157, 1045, 1005, 742, 709 (the IR of the free BuCꢀCH
n
mmol) of a 1.6 M solution of BuLi in hexane. The
alkyne under the same condition is: 3313(CꢁH str),
2978, 2048, 2036, 2820, 2108 (CꢀC str), 1484, 1465,
1370, 1250, 1212, 638.
mixture was stirred for 30 min and then the tempera-
ture was allowed to rise to room temperature (r.t.).

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
87
2.4. Kinetic study of controlled oligomerization
trans-iPrCHꢂCHSiH2Ph
(5c)
(27.5%),
cis-
iPrCHꢂCHSiH2Ph (6c) (10.4%), iPrCꢀCSiH2Ph (7c)
(21.8%), iPrCHꢂCH2 (8c) (28.4%), gem-H2Cꢂ
C(iPr)CꢀCiPr (1c) (9.5%) and Et2NSiH2Ph (9) (2.4%).
(b) According to the general procedure described
above, 100% conversion was obtained by the reaction
In a typical experiment, a NMR sample was pre-
pared as described in the typical NMR scale catalytic
reactions section but maintained at −78°C until ki-
netic measurements were initiated. The sealed tube was
heated in a temperature controlled oil bath and at time
intervals NMR data were acquired using eight scans
per time interval with a long pulse delay to avoid
saturation of the signal. The kinetics were usually mon-
itored by the intensity changes in the substrate reso-
nances and in the product resonances over three or
more half-lives. The substrate concentration (C) was
measured from the area (As) of the 1H-normalized
signal of the solvent (Ab). All the data collected could
convincingly fit (R\0.98) by least-squares to Eq. (1)
where Co (Co=Aso/Abo) is the initial concentration of
substrate, C(As/Ab) is the substrate concentration at
time t.
i
of 0.098 ml of PrCꢀCH (0.96 mmol) and 0.092 ml of
PhSiH3 (0.75 mmol), catalyzed by [(Et2N)3U][BPh4]
(0.013 mmol) in benzene-d6 at 78°C, for 6 h, producing
trans-iPrCHꢂCHSiH2Ph
(5c)
(20.5%),
cis-
iPrCHꢂCHSiH2Ph (6c) (16.9%), iPrCꢀCSiH2Ph (7c)
(26.4%), iPrCHꢂCH2 (8c) (20.5%), gem-H2Cꢂ
C(iPr)CꢀCiPr (1c) (7.7%) and traces of gem-
H2CꢂC(iPr)CHꢂC(iPr)CꢀC(iPr) (1.5%), trans-iPrCHꢂ
CHSi(H)(Ph)CꢀCiPr (1.5%), PrCHꢂC(SiH2Ph)2 (1.9%)
i
and Et2NSiH2Ph (9) (2.4%) as observed from GC–MS
measurements.
(c) According to the general procedure described
above, 100% conversion was obtained by the reaction
i
of 0.078 ml of PrCꢀCH (0.768 mmol) and 0.092 ml of
mt=log(C/C0)
(1)
PhSiH3 (0.75 mmol), catalyzed by [(Et2N)3U][BPh4]
(0.013 mmol) in THF-d8 at 65°C, for 12 h, producing
trans-iPrCHꢂCHSiH2Ph (5c) (16.0%), cis-iPrCHꢂ
The ratio of catalyst to substrate was accurately
measured by calibration with internal FeCp2. Turnover
frequencies (Nt, h−1) were calculated from the least-
squares determined slopes (m) of the resulting plots.
Typical initial alkyne concentrations were in the range
0.052–5.92 M and typical catalyst concentrations were
in the range 9.7–35 mM.
i
CHSiH2Ph (6c) (4.3%), PrCꢀCSiH2Ph (7c) (29.9%),
iPrCHꢂCH2 (8c) (31.0%), gem-H2CꢂC(iPr)CꢀC(iPr) (1c)
(15.5%) and Et2NSiH2Ph (9) (3.2%).
t
2.5.2. Hydrosilylation of BuCꢀCH with PhSiH3
catalyzed by [(Et2N)3U][BPh4]
2.5. General procedure for the catalytic hydrosilylation
of terminal alkynes
(a) According to the general procedure described
above, 100% conversion was obtained by the reaction
of 0.098 ml of BuCꢀCH (0.79 mmol) and 0.098 ml of
PhSiH3 (0.79 mmol), catalyzed by [(Et2N)3U][BPh4]
t
In a typical procedure, the amount of the specific
alkyne and PhSiH3 were vacuum transferred in a high
vacuum line into a J. Young NMR tube containing 10
mg of [(Et2N)3U][BPh4] in 0.6 ml of THF-d8 or ben-
zene-d6. The sealed tube was then heated in an oil bath
or kept at r.t. until 100% conversion of the alkyne was
detected by the disappearance of the acetylenic hydro-
(0.013 mmol) in benzene-d6 at r.t. for 48 h, producing
trans-tBuCHꢂCHSiH2Ph
(5e)
(53.8%),
cis-
tBuCHꢂCHSiH2Ph (6e) (10.5%), BuCꢀCSiH2Ph (7e)
(2.0%), tBuCHꢂCH2 (8e) (12.0%), tBuCHꢂC(SiH2-
Ph)Si(H)(Ph)CꢀCtBu (11e) (15.5%), Et2NSiH2Ph (9)
(3.2%) and trace amounts of gem-H2CꢂC(tBu)Cꢀ
CtBu (1c) (0.8%) and tBuCHꢂCHSi(H)(Ph)CꢀCtBu
(2.2%).
t
1
gen by H-NMR spectroscopy. The organic products
were vacuum transferred (10−6 mm Hg) to another J.
Young NMR tube, sealed and both residue and
1
volatiles were identified by H-, 13C-, 29Si-, 2D-NMR
2.5.2.1. Spectroscopic data for tBuCHꢂC(SiH2-
1
(COSY, CꢁH correlation, SiꢁH correlation, NOESY),
GC–MS spectroscopy and by comparing with literature
known compounds. Spectroscopic data for compounds
5b–e, 6b–e and 7b–e are described in the literature
[22q].
Ph)Si(H)(Ph)CꢀCtBu (11e). H-NMR (200 MHz, ben-
zene-d6): l 7.12–7.68 (m, 10 H, Ph), 7.34 (s, 1 H,
CH-based on 2D CꢁH correlation to the signal at 176.6
ppm), 5.27 (s, 1 H, PhSiH), 5.1 (s, 2 H, PhSiH2), 1.09
(s, 9 H, C(CH3)3), 1.02 (s, 9 H, C(CH3)3). 13C-NMR (50
MHz, benzene-d6): l 176.6 (s, HCtBu), 137.2, 133.1,
131.6 (CꢁHꢁPh), 132.6 (s, ipso CꢁSi ), 77.1 (CꢀC),
42.4 (s, CMe3), 30.8 (s, C(CH3)3), 30.4 (s, C(CH3)3).
The quaternary carbons for the tBu and acetylide
moieties were not detected due to large relaxation
times. 29Si-NMR (79.5 MHz, benzene-d6): l 21.4 (d,
J=210 Hz, PhSiH), 4.0 (t, J=205 Hz, PhSiH2). GC–
MS data: m/z 376 [M+], 375 [M+−H], 361 [M+
i
2.5.1. Hydrosilylation of PrCꢀCH with PhSiH3
catalyzed by [(Et2N)3U][BPh4]
(a) According to the general procedure described
above, 100% conversion was obtained by the reaction
i
of 0.098 ml of PrCꢀCH (0.96 mmol) and 0.092 ml of
PhSiH3 (0.75 mmol), catalyzed by [(Et2N)3U][BPh4]
(0.013 mmol) in benzene-d6 at r.t., for 48 h, producing

88
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
Table 1
Product distribution of the [(Et2N)3U][BPh4] catalyzed oligomerization of terminal alkynes (RCCH)a
R in RCCH
gem-H2CꢂC(R)CCR (1) (%)
cis-H2CꢂC(R)CCR (3) (%)
Trimer (4) (%)
Me (a)
nBu (b)
iPr (c)
94
94
75
43
74
92
32
–
–
–
16
25
–
–
–
–
33
–
8
TMS (d) b
tBu (e)
Me (f) c
Ph (g)
–
58
a The reaction was carried out in toluene-d8 at 110°C.
b The trans-H(TMS)CꢂCHCCTMS (2d; 7%) was also observed.
c The alkyne in this entry is MeCCD.
−CH3], 333 [M+−C3H7], 319 [M+−tBu, 100%), 187
[tBuCꢀCSiHPh+], 159 [tBuCꢀCSiH2Ph+−C2H5], 145
CꢂCHCꢀC(TMS) (4.3%), cis-H(TMS)CꢂCHCꢀC(TMS)
(1.1%) and trans-(TMS)CHꢂCHꢁCHꢂC(TMS)Cꢀ
C(TMS) (12.2%), gem-H2CꢂC(TMS)CHꢂC(TMS)Cꢀ
C(TMS) (5.7%). No reaction was observed at r.t. For
the latter compounds, the stereochemistry was assigned
by comparison of their GC–MS retention times to pure
compounds [14a].
[tBuCꢀCSiH2Ph+−C3H7],
131
[tBuCꢀCSiH2Ph+
−tBu], 105 [PhSi+].
(b) According to the general procedure described
above, 100% conversion was obtained by the reaction
t
of 0.098 ml of BuCꢀCH (0.79 mmol) and 0.098 ml of
PhSiH3 (0.79 mmol), catalyzed by [(Et2N)3U][BPh4]
(0.013 mmol) in benzene-d6 at 78°C for 6 h, producing
trans-tBuCHꢂCHSiH2Ph (5e) (35.8%), cis-tBuCHꢂ
CHSiH2Ph (6e) (15.8%), tBuCHꢂCH2 (8e) (17.4%),
tBuCHꢂC(SiH2Ph)Si(H)(Ph)CꢀCtBu (11e) (21.7%),
Et2NSiH2Ph (9) (4.2%) and trace amounts of
3. Results and discussion
The products from the reactions of the terminal
alkynes RCꢀCH are noted by a, b, c, d, e and g
corresponding to R=Me, Bu, Pr, TMS, Bu and Ph,
t
n
i
t
PhSiH2ꢁSiH2Ph (2.3%), BuCHꢂC(SiH2Ph)2 (0.9%) and
tBuCHꢂCHSi(H)(Ph)CꢀCtBu (1.9%).
respectively.
n
2.5.3. Hydrosilylation of BuCꢀCH with PhSiH3
3.1. Catalytic dimerization of alkynes
catalyzed by [(Et2N)3U][BPh4]
According to the general procedure described above,
100% conversion was obtained by the reaction of 0.117
ml of BuCꢀCH (1.0 mmol) and 0.118 ml of PhSiH3
(0.96 mmol), catalyzed by [(Et2N)3U][BPh4] (0.013
Reaction of [(Et2N)3U][BPh4] [34] with an excess of
n
i
terminal alkyne RCꢀCH (R=Me, Bu, Pr; toluene-d8,
alkyne/[(Et2N)3U][BPh4] ratio 50: 1) resulted in the
chemo- and regio-selective catalytic formation of the
head-to-tail gem-dimers (1a–c) with no formation of
the trans isomers or major oligomers (Reaction 2). For
PhCꢀCH, the reaction was less chemoselective allowing
the formation of trimers (geminal dimer (1g): trimers
ratio=32:58). For TMSCꢀCH, besides the formation
of the geminal head-to-tail dimer (1d), the trans-head-
to-head dimer (2d), and the regioselective head-to-tail-
to-head-trimer (E,E)-1,4,6-tris(trimethylsilyl)-1-3-hexa-
dien-5-yne (4d), the unexpected head-to-head cis dimer
(3d) was also formed (Reaction 3) [35]. Likewise, for
tBuCꢀCH, the geminal dimer (1e) and the unexpected
cis-dimer (3e) were formed (Reaction 4, Table 1) [35].
n
mmol) in benzene-d6, at 78°C for 6 h, producing trans-
nBuCHꢂCHSiH2Ph
(5b)
(25.7%),
cis-nBuCHꢂ
n
CHSiH2Ph (6b) (16.9%) BuCꢀCSiH2Ph (7b) (19.3%),
nBuCHꢂCH2 (8b) (26.9%), gem-H2CꢂC(nBu)CꢀCnBu
(1b) (5.0%) Et2NSiH2Ph (9) (2.7%), nBuCHꢂCH-
(SiHPh)2 (2.9%) and trace amounts of gem-
H2CꢂC(nBu)CHꢂC(nBu)CꢀCnBu (0.5%).
2.5.4. Hydrosilylation of (TMS)CꢀCH with PhSiH3
catalyzed by [(Et2N)3U][BPh4]
According to the general procedure described above,
100% conversion was obtained by the reaction of 0.118
ml of (TMS)CꢀCH (0.834 mmol) and 0.107 ml of
PhSiH3 (0.868 mmol), catalyzed by [(Et2N)3U][BPh4]
(0.013 mmol) in benzene-d6, at 78°C for 24 h, produc-
ing trans-(TMS)CHꢂCHSiH2Ph (5d) (12.2%), cis-
(TMS)CHꢂCHSiH2Ph (6d) (18.5%) (TMS)CꢀCSiH2Ph
(7d) (21.3%), (TMS)CHꢂCH2 (8d) (10.6%), Et2NSiH2Ph
(9) (2.6%) and the oligomerization products, gem-
H2CꢂC(TMS)CꢀC(TMS) (1d) (11.4%) trans-H(TMS)-
(2)

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
89
(3)
(4)
Mechanistically, the presence of relatively low-lying
empty s-bonding orbitals, the relatively polar
metalꢁligand bonds, and the absence of energetically
accessible metal oxidation states for oxidative addition/
reductive elimination processes, [36] would implicate a
‘four-center’ heterolytic transition state in the
metalꢁcarbon bond cleavage. Thus, the reaction of the
metal acetylide with another terminal alkyne will be in
a syn mode, and the s-bond metathesis step of the
alkenyl complex with the terminal alkyne hydrogen
(replacement of the metal center by a hydrogen of the
terminal alkyne) will produce the trans product. Hence,
the formation of dimers 3d and 3e argues for an
isomerization pathway before the products are released
from the metal center.
plexes [Cp*2 AnMe][B(C6F5)4] (An=Th, U), only the
geminal dimer was formed with no trace formation of
either cis or trans dimers [14b]. In addition, in the
oligomerization reaction promoted by Cp*2 AnMe2, the
cis-dimer was not observed [14a].
In the reaction of [(Et2N)3U][BPh4] with terminal
alkynes, one equivalent of the Et2NH amine was re-
leased to the solution, as observed in the NMR, form-
ing presumably the bisamido acetylide cationic complex
[(Et2N)2UꢁCꢀCR][BPh4]. This is a slow equilibrium
process and the addition of different equimolar
amounts of Et2NH to the reaction mixture led to a
lowering of the reaction rate linearly (Fig. 1).
Considering that in the reactions of alkynes the
amount of the free amine formed is stoichiometric with
that of the catalyst, it seems plausible that the free
terminal alkyne is the major protonolytic agent releas-
ing the dimer from the metal–alkenyl complex. To
corroborate this protonolytic hypothesis, a novel strat-
egy was implemented to increase the chemoselectivity
towards a trimer. This was accomplished by providing a
kinetic delay for the presumed fast protonolysis by the
alkyne, to allow trimer formation, through replacement
of the terminal hydrogen at the alkyne with deuterium
(Reaction 5). This strategy indeed enabled us to influ-
ence the chemoselectivity of the oligomerization allow-
ing the formation of the deuterated geminal dimer (1f)
and some deuterated trimer formation (measured by
GC–MS) [37].
It is important to point out that in the oligomeriza-
tion of terminal alkynes promoted by the cationic com-
Fig. 1. Rate of catylic dimerization of nBuCꢀCH by [(Et2N)3U][BPh4].
(—) without the addition of external diethylamine; (- - -) with addi-
tion of an excess of diethyl amine.
(5)

90
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
Fig. 2. Determination of the reaction order in [(E2N)3U][BPh4] concentration for the dimerization of nBuCꢀCH in benzene-d6 at 25°C.
the second process, only at higher concentrations, is the
rate-limiting step toward the dimer formation.
Kinetic measurements on the oligomerization reac-
tion of nBuCꢀCH were studied by in situ 1H-NMR
n
The derived activation parameters for the dimeriza-
spectroscopy. The reaction of an excess of BuCꢀCH
n
tion of BuCꢀCH (Fig. 4) are characterized by a rather
was controlled with constant catalyst concentration un-
til the complete consumption of the substrate. The
disappearance of the CꢀCH 1H resonance (l=2.28)
was normalized. The turnover frequency of the reaction
was calculated from the slope of the kinetic plots of
substrate to catalyst ratio vs. time. When the initial
concentration of the terminal alkyne is held constant
and the concentration of the catalytic precursor is
varied over a ca. fourfold concentration range (9.7–35
mM), a plot of reaction rate versus precatalyst concen-
tration indicates that the reaction is first-order depen-
dent in precatalyst, in analogy with the oligomerization
of terminal alkynes promoted by Cp*2 AnMe2 (Fig. 2)
[14]. When the concentration of the catalyst is main-
tained at a constant and the concentration of the alkyne
is varied over a 10-fold concentration range (0.052–
5.92 M) (Fig. 3), a plot of the reaction rate versus
alkyne concentration shows a two domain behavior. At
low concentrations, an inverse proportionality is ob-
served, indicating that the reaction is in an inverse first
order, and at higher concentrations, the reaction ex-
hibits a zero order in alkyne. An inverse proportionality
in catalytic systems is consistent with an equilibrium
before the rate-determining step. The change from an
inverse rate to a zero rate is compatible with two
equilibrium processes. One of those is routing the com-
plex out of the catalytic cycle (inverse order), whereas
small enthalpy of activation (DH†=15.6(3) kcal
mol−1) and a negative entropy of activation (DS†= −
11.4(6) eu). This DS† parameter suggests an ordered
transition state with considerable bond-making to com-
pensate for bond breaking. Interestingly, the oligomer-
ization of TMSCꢀCH by Cp*2 AnMe2, which proceeds
via a four-centered-transition state, exhibits rather simi-
lar enthalpy of activation (DH†=11.1(3) kcal mol−1
)
although a larger negative entropy of activation (DS†=
−45(5) eu) [14].
A possible mechanism for the dimerization of
nBuCꢀCH is proposed in Scheme 2. The mechanism
consists of a sequence of well-established elementary
reactions such as terminal alkyne insertion into a
metalꢁcarbon s-bond and s-bond metathesis [12]. The
initial step in the catalytic cycle is the alkyne CꢁH
activation by the cationic uranium amide complex and
the formation of the bisamido carbyl complex
[(Et2N)2UꢁCꢀCnBu][BPh4] (A) together with Et2NH
(step 1) [38]. Complex A may either be in equilibrium
with an alkyne forming the p-alkyne acetylide uranium
complex B, which drives the active species out of the
catalytic cycle (inverse rate dependence), or undergo
with an alkyne a head-to-tail insertion into the ura-
niumꢁcarbon s-bond, yielding the substituted uranium
alkenyl complex C (step 2) [39]. This complex under-
goes a s-bond metathesis with an additional alkyne

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
91
(step 3) leading to the corresponding dimer and regen-
erating the active carbyl complex A [40].
DEPT and in the 2D CꢁH correlation experiments to a
carbon at l= −19.85, exhibiting a coupling constant
Complex B (for R=tBu) has been trapped and its
structure determined spectroscopically. The 1H- and
13C-NMR spectra of complex B show sharp lines as
found for other actinide (IV) type of complexes. The
1H-NMR spectrum exhibits one acetylide signal (ꢀCꢁH)
at l= −2.14 which was found to correlate in the
of J=250 Hz. In addition, two Bu group signals were
found in either the 1H-, 13C-NMR, DEPT or CꢁH
correlation spectra. These results clearly indicate that
free alkyne must be coordinated to the metal center. A
confirmation of the formation of an alkyne h2-complex,
as compared to an acetylide complex or to a free alkyne
1
t
Fig. 3. Determination of the reaction order in alkyne concentration for the dimerization of nBuCꢀCH mediated by [(Et2N)3U][BPh4] as the
precatalyst in benzene-d6 at 25°C.
Fig. 4. Erying plot for the dimerization of nBuCꢀCH using [(E2N)3U][BPh4] as the precatalyst in toluene-d8. This line represents the least-squares
fit to the data points.

92
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
Scheme 2. Plausible mechanism for the dimerization of terminal alkynes promoted by [(Et2N)3U][BPh4].
k
−1k2[cat]
has been acquired by FT-IR spectroscopy. The CꢀC
stretching of the free alkyne (2108 cm−1) disappears
giving rise to two signals at low frequencies, as expected
for h2-transition metal complexes, one at 2032 cm−1
similar to acetylide lanthanides, and the second one at
2059 cm−1 [12]. The turnover-limiting step for the
catalytic dimerization was found to be the insertion of
the alkyne into the uranium–carbyl complex A (step 2).
This result implies that the s-bond metathesis between
the cationic complex [(Et2N)3U][BPh4] and the alkyne,
and the protonolysis of C by the alkyne are faster than
the insertion of the alkyne into complex A. In addition,
this result also suggests that trimers are only expected if
a kinetic delay in the protonolysis is induced corrobo-
rating our observations.
6=
(2)
k2k−2
k3[alkyne]
k1+k2−
For sterically demanding alkyne substituents (TMS,
tBu), it seems essential that the protonolysis step rate is
lower than that of the isomerization of the metalla–
alkenyl complex C%, allowing the formation of the
unexpected cis-dimer. This process takes place proba-
bly through a metalla-cyclopropyl cation, alike the
‘envelope isomerization’ mechanism (Reaction 6) [41].
The preference towards the cis-isomer is possibly due to
the b-hydrogen interaction (agostic interaction) to the
metal center. In addition, if a metal–acetylide complex
is formed for C%, the bulkier alkynes will promote the
isomerization process (Reaction 6) to remove the steric
hindrance from around the metal center.
Thus, the derived rate law for the oligomerization of
terminal alkynes promoted by the cationic complex
[(Et2N)3U][BPh4] is given by Eq. (2) (see Appendix A).
(6)

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
93
3.2. Catalytic hydrosilylation reactions of alkynes
Interestingly, no hydrosilylation or dehydrogenative
coupling products were obtained from TMSCꢀCH and
PhSiH3 under similar reaction conditions at r.t. In
addition, none of the oligomerization dimers or trimers
of (TMS)CꢀCH were observed [14].
3.2.1. Reaction scope at room temperature
The hydrosilylation reaction of terminal alkynes pro-
moted by Cp*2 AnMe2 (An=Th, U) have been thor-
oughly investigated, showing that the mechanism
proceeds by the intermediacy of the corresponding hy-
dride complex (Chalk–Harrod mechanism) [22g,26]. A
conceptual question regards the possibility to form a
similar cationic hydride complex, as an intermediate, in
the catalytic hydrosilylation of terminal alkynes pro-
moted by [(Et2N)3U][BPh4]. The r.t. reaction in benzene
of [(Et2N)3U][BPh4] with an excess of terminal alkynes
RCꢀCH (R=iPr, tBu) and PhSiH3 resulted in the
catalytic formation of a myriad of products showing
the large reactivity of this complex. The products ob-
served to account for 100% conversion with respect to
the alkyne were: the geminal dimer 1 trans-vinylsilane
RCHꢂCHSiH2Ph (R=iPr (5c), tBu (5e)), cis-vinylsilane
3.2.2. Reaction scope at high temperature
At high temperature (65–78°C), the chemoselectivity
and regioselectivity of the products formed in the
cationic organouranium-catalyzed hydrosilylation of
terminal alkynes with PhSiH3 were found to be differ-
ent from those obtained at r.t. The reactions were
sensitive to the nature of the substituent on the alkyne
and to the polarity of the solvent.
The hydrosilylation of RCꢀCH (R=nBu, Pr, Bu)
with PhSiH3 catalyzed by [(Et2N)3U][BPh4] at high
temperature (reflux of the solvent, 65–78°C), produced,
in addition to the hydrosilylation products at r.t.
(Reaction 7), the corresponding double hydrosilylated
i
t
t
RCHꢂCHSiH2Ph (R=iPr (6c), Bu (6e)), the dehydro-
compounds:
RCHꢂC(SiH2Ph)2
(R=nBu
(12b),
genative silylalkyne RCꢀCSiH2Ph (R=iPr (7c), tBu
(7e)), the corresponding alkenes RCHꢂCH2 (R=iPr
(8c), tBu (8e)) and the aminosilane Et2NSiH2Ph (9). For
the bulky t-butylacetylene, the tertiary silanes trans-
iPr (12c), Bu (12e)), and trimers. Only for BuCꢀCH,
trace amounts of the dehydrogenative coupling of the
silane PhH2SiꢁSiH2Ph (13) [42] were observed. Since
the same myriad of products was obtained from
the hydrosilylation of terminal alkynes promoted
by Cp2*AnMe2 (An=Th, U) [22q], it seems plau-
sible that the same type of mechanism is operative
here.
t
t
t
tBuCHꢂCHSi(HPh)(CꢀCtBu) (10e), BuCHꢂC(SiH2Ph)-
Si(HPh)(CꢀCtBu) (11e), were also observed (Reaction
7, Table 2). Formation of the tertiary silanes 10e and
11e can be accounted for by the metathesis reactions of
the trans-alkenylsilane (5e) and the double hydrosily-
lated compound 12e [22g] with the metal acetylide
complex A, respectively, as shown in Reactions 8 and 9.
(7)
The formation of an active uranium hydride complex
D is conceivable either by the reaction of the starting
cationic complex with a silane molecule, giving the
corresponding aminosilane compound 9 [43], or/and by
the reaction of the acetylide complex A with silane,
producing the corresponding silaalkyne 7 as described
in Reactions 10 and 11, respectively. The proposed
mechanism that takes into account the formation of all
the products is described in Scheme 3.
(8)
(9)
This mechanism consists of a sequence of well-estab-
lished elementary reactions, such as insertion of a ter-
minal alkyne into a metalꢁhydride s-bond, s-bond

94
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
Table 2
a
Product distribution of the [(Et2N)3U][BPh4] catalyzed hydrosilylation of terminal alkynes (RCꢀCH) with PhSiH3
R in RCꢀCH
Temperature
(°C)
trans-RCHꢂCHSiH2Ph
(5) (%)
cis-RCHꢂCHSiH2Ph
(6) (%)
RCCSiH2Ph
(7) (%)
RCHꢂCH2
(8) (%)
gem-H2CꢂC(R)CꢀCR
(1) (%)
b
nBu (b)
78
20
78
65
78
20
78
26
28
21
16
12
54
36
17
10
17
4
19
11
16
19
22
26
30
21
2
26
28
21
31
11
12
17
5
10
8
16
11
1
iPr (c)
iPr (c) c
iPr (c) d
TMS (d) e
tBu (e) f
tBu (e) g
–
–
a The reaction was carried out in benzene-d6 unless otherwise mentioned. Small amount of 9 (ca 2%) was formed in each reaction.
b Trace amounts of 12b (3%) and gem-trimer (1%) were also observed.
c Traces of trimer 4c, 10c and 12c were also detected from GC/MS spectroscopy.
d The reaction was carried out in THF-d8.
e No reaction was observed at r.t. Alkyne dimers (5%) and trimers (18%) were observed.
f The compound 11e (16%) and traces of 10e (2%) were also observed.
g The compound 11e (22%) and traces of 12e (1%) and 10e (2%) were also observed.
Scheme 3. Plausible mechanism for the room and high temperature hydrosilylation of terminal alkynes promoted by [(Et2N)3U][BPh4]. The
transformation of the starting complex into the acetylide complex [(Et2N)2UꢁCꢀCR][BPh4] (A) was described in Scheme 2, and is omitted here
for clarity.
(10)
(11)
metathesis by a silane, and protonolysis by an acidic
alkyne hydrogen. The precatalyst [(Et2N)3U][BPh4] in
the presence of alkyne is converted to the acetylide
complex A removing one of the amido ligands (Scheme
2). Complex A reacts with PhSiH3 to give the sily-
lalkyne 7 and the actinide hydride D (step 4) (a compet-
itive pathway is the formation of the aminosilane 9).
The hydride complex D may reinsert the silaalkyne in a

A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
95
very regiospecific manner, as already observed in the
thorium and uranium catalyzed hydrosilylation of alky-
nes, forming complex E (step 5) [36]. Complex D will
also react rapidly with the alkyne to produce the
alkenyl uranium complex F (step 6), which is pre-
sumably in rapid equilibrium with complex D. For
example, in the hydrogenation of alkenes by the metal-
locene thorium bis(hydride) complex, this insertion step
was found to be reversible [12]. Complex F will react
with PhSiH3, producing back the organouranium hy-
dride complex D and the trans-hydrosilylated product 5
(step 7). Under the catalytic conditions, complex F may
also react with a second alkyne giving the alkene 8 and
the acetylide complex A (step 8). Complex E may react
with a silane (step 9) yielding complex D and the
double hydrosilylation product 12 or with an alkyne
(step 10) yielding complex A and the cis-isomer 6.
Formation of compounds 9, 10 and 11 has been ex-
plained by the reactions described in Reactions 10, 8
and 9, respectively.
instead of the hydride complex D either from steps 4, 7
or 9 in the catalytic cycle (Scheme 3). In these steps, the
silane is acting as the protonolytic source.
4. Conclusions
These results demonstrated that cationic actinide
complexes are active catalysts for the dimerization of
terminal alkynes by a mechanism that consists of inser-
tion and s-bond metathesis reactions. A delicate bal-
ance between alkyne insertion and alkyne CꢁH s-bond
metathesis determines the dimer: trimer oligomer ratio
and the geminal:cis:trans isomer ratio. The trapped
p-alkyne acetylide complex is, to the best of our knowl-
edge, the first characterized actinide p-alkyne complex.
Cationic uranium complexes were also found to be very
efficient for the catalytic hydrosilylation of terminal
alkynes. All these reactions likely involve uranium
acetylide and uranium hydride species as active inter-
mediates. Further studies are presently under investiga-
tion.
This mechanistic scenario takes into account the
higher yields observed for the alkene compound 8 as
compared with those obtained for the silylalkyne 7. For
i
TMSCꢀCH and PrCꢀCH, at high temperature, the
amount of the hydrosilylated products is larger than
that of the alkenes, indicating that an optional compet-
ing equilibrium route should be operative. This would
involve the transformation of the hydride D back into
the acetylide complex A by reaction with the alkyne
(Reaction 12) allowing the production of more sily-
lalkyne without producing the alkene. The hydride D
could alternatively react with PhSiH3 to give the sily-
lorganometallic compound [(Et2N)2USiH2Ph][BPh4]
(Reaction 13) which would further react with PhSiH3 or
RCꢀCH to give back the hydride D and PhH2SiꢁSiH2-
Ph or PhH2SiCꢀCR, respectively.
Acknowledgements
This research was supported by The Israel Science
Foundation, administered by The Israel Academy of
Sciences and Humanities under contract 69/97-2; by the
Fund for the Promotion of Research at the Technion,
and by Technion V.P.R. fund Loewengart Research
Fund. M.E. and M.S.E. thank the Israel Ministry of
Sciences and the French Ministe`re de Affaires Etran-
ge`res for funding the Arc-en-Ciel/Keshet Project no. 50.
A.K.D. thanks the Technion for
fellowship.
a postdoctoral
(12)
Appendix A. Derivation of the kinetic rate equation
based on the mechanism as presented in Scheme 2
#P/#t=k3[C][RCꢀCH]
(A1)
steady state approximation on [C]
#C/#t:0: −k3[C][RCꢀCH]−k−2[C]
+k2[A][RCꢀCH]
(A2)
(A3)
(A4)
[C]=k2[A][RCꢀCH]/k3[RCꢀCH]+k−2
[C]=k2[A]/k3 [ k3[RCꢀCH]ꢁk−2
(13)
It is noteworthy that only for the hydrosilylation
t
reaction of BuCꢀCH at high temperature (78°C), a
This assumption is made since no scrambling of alkynes
have been observed even at low alkyne concentrations
when the reaction is the fastest.
small amount of the dehydrogenative coupling of
phenylsilane was observed. This result argues for the
formation of a compound with an uranium silicon
bond although not as a major operative intermediate.
The compound [(Et2N)2USiH2Ph][BPh4] can be formed
#P/#t=k2[A][RCꢀCH]
(A5)
steady state approximation on [A]

96
A.K. Dash et al. / Journal of Organometallic Chemistry 604 (2000) 83–98
Stern, T.J. Marks, Organometallics 16 (1997) 4705. (c) C.M.
Haar C.L. Stern, T.J. Marks, Organometallics 15 (1996) 1765.
(d) G.A. Molander, J. Winterfeld, J. Organomet. Chem. 524
(1996) 275. (e) M.A. Giardello, V.P. Conticello, L. Brard, M.R.
Gagne´, T.J. Marks, J. Am. Chem. Soc. 116 (1994) 10241. (f)
G.A. Molander, J.O. Hoberg, J. Am. Chem. Soc. 114 (1992)
3123. (g) G. Jeske, H. Lauke, H. Mauermann, H. Schumann,
T.J. Marks, J. Am. Chem. Soc. 107 (1985) 8111. (h) W.J. Evans,
I. Bloom, W.E. Hunter, J.L. Atwood, J. Am. Chem. Soc. 105
(1983) 1401.
#A/#t:0: −k1[A][RCꢀCH]−k2[A][RCꢀCH]
+k−1[B]+k−2[C]
(A6)
#A/#t:0: −k1[A][RCꢀCH]−k2[A][RCꢀCH]
+k−1[B]
+k−2k2[A][RCꢀCH]/k3[RCꢀCH] +k−2
−1[B]=[A][RCꢀCH]
{k1+k2−k2k−2/k3[RCꢀCH]+k−2
−1[B]
[RCꢀCH] {k1+k2−k2k−2/k3[RCꢀCH]+k−2
k
[4] For examples of catalytic activity of organolanthanides in dimer-
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B.M. Novak, Macromolecules 30 (1997) 3494. (b) W.J. Evans,
P.M. DeCoster, J. Greaves, Macromolecules 28 (1995) 7929. (c)
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Kai, Macromol. Chem. Phys. 197 (1996) 1909. (f) P.-F. Fu, T.J.
Marks, J. Am. Chem. Soc. 117 (1995) 10747. (g) C.J. Schaverien,
Organometallics 13 (1994) 69. (h) H.J. Heeres, J.H. Teuben,
Organometallics 10 (1991) 1980. (i) H.J. Heeres, J. Renkema, M.
Booij, A. Meetsma, J.H. Teuben, Organometallics 7 (1988) 2495
and references therein. (j) G. Jeske, L.E. Schock, P.N. Swepson,
H. Schumann, T.J. Marks, J. Am. Chem. Soc. 107 (1985) 8091.
(k) P.L. Watson, G.W. Parshall, Acc. Chem. Res. 18 (1985) 51.
[5] For examples of catalytic activity of organolanthanides in hy-
droamination, see: (a) V.M. Arredondo, F.E. McDonald, T.J.
Marks, J. Am. Chem. Soc. 120 (1998) 4871. (b) Y. Li, T.J.
Marks, J. Am. Chem. Soc. 118 (1996) 9295. (c) Y. Li, T.J.
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(1994) 10241 and references therein.
}
(A7)
k
1
[A]=
}
(A8)
Substituting for [A] in Eq. (A5)
#P
k−1k2[B]
=
(A9)
#t {k1+k2−k2k−2/k3[RCꢀCH]+k−2
}
as before k3[RCꢀCH]ꢁk−2
#P
k
−1k2[B]
=
(A10)
#t k1+k2−k2k−2/k3[RCꢀCH]
Thus the derived rate law for the oligomerization of
terminal alkynes promoted by cationic [(Et2N)3U]+
[BPh4]− is given by Eq. (A11)
k
−1k2[cat]
6=
(A11)
k2k−2
k3[alkyne]
k1+k2−
[6] For examples of catalytic activity of organolanthanides in hy-
drosilylation see: (a) P.-F. Fu, L. Brard, Y. Li, T.J. Marks, J.
Am. Chem. Soc. 117 (1995) 7157. (b) G.A. Molander, P.J.
Nichols, J. Am. Chem. Soc. 117 (1995) 4415. (c) G.A. Molander,
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Noll, Organometallics 17 (1998) 3754.
[7] For examples of catalytic activity of organolanthanides in hy-
drophosphination, see: M.A. Giardello, W.A. King, S.P. Nolan,
M. Porchia, C. Sishta, T.J. Marks, in: J.A. Martinho Simoes
(Ed.), Energetics of Organometallic Species, Kluwer, Dodrecht,
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[8] For examples of catalytic activity of organolanthanides in hy-
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(Eds.), Applied Homogeneous Catalysis with Organometallic
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[9] For examples of catalytic activity of organolanthanides in ring
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Chem. Soc. 118 (1996) 7900. (b) X. Yang, A.M. Seyam, P.-F.
Fu, T.J. Marks, Macromolecules 27 (1994) 4625.
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i
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[37] The addition of external amine to the reaction mixture contain-
ing MeCꢀCD allows the formation of the geminal dimer [1f]
.
.