A mechanistic study of the utilization of arachno-diruthenaborane [(Cp*RuCO)2B2H6] as an active alkyne-cyclotrimerization catalyst

Article abstract of DOI:10.1002/chem.201200291

The reaction of nido-[1,2-(Cp*RuH)₂B₃H ₇] (1 a, Cp=I·⁵-C₅Me ₅) with [Mo(CO)₃(CH₃CN)₃] under mild conditions yields the new metallaborane arachno-[(Cp*RuCO) ₂B₂H₆] (2). Compound 2 catalyzes the cyclotrimerization of a variety of internal- and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The reactivities of nido-1 a and arachno-2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne-insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2. The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne-insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum-chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne. Everything changes but Ru: The reactivities of nido-[1,2-(Cp*RuH)₂B₃H₇] and arachno-[(Cp*RuCO)₂B₂H₆] with alkynes show that a change in geometry (nido to arachno) drives a change from alkyne-insertion to catalytic cyclotrimerization, respectively. Copyright

Full text of DOI:10.1002/chem.201200291

DOI: 10.1002/chem.201200291
A Mechanistic Study of the Utilization of arachno-Diruthenaborane
[(Cp*RuCO)2B2H6] as an Active Alkyne-Cyclotrimerization Catalyst
K. Geetharani,[a] Samat Tussupbayev,[b] Julia Borowka,[b] Max C. Holthausen,*[b] and
Sundargopal Ghosh*[a]
Abstract: The reaction of nido-[1,2-
(Cp*RuH)2B3H7] (1a, Cp*=h5-C5Me5)
with [Mo(CO)3ACHTUNGTRENNUNG(CH3CN)3] under mild
arachno drives a change in the reaction
from alkyne-insertion to catalytic cy-
clotrimerization, respectively. Density
functional calculations have been used
to evaluate the reaction pathways of
the cyclotrimerization of alkynes cata-
lyzed by compound 2. The reaction in-
volves the formation of a ruthenacyclic
intermediate and the subsequent
alkyne-insertion step is initiated by
a [2+2] cycloaddition between this in-
termediate and an alkyne. The experi-
mental and quantum-chemical results
also show that the stability of the met-
allacyclic intermediate is strongly de-
pendent on the nature of the substitu-
ents that are present on the alkyne.
conditions yields the new metallabor-
ane arachno-[(Cp*RuCO)2B2H6] (2).
Compound 2 catalyzes the cyclotrime-
rization of a variety of internal- and
terminal alkynes to yield mixtures of
1,3,5- and 1,2,4-substituted benzenes.
The reactivities of nido-1a and arach-
no-2 with alkynes demonstrates that
a change in geometry from nido to
Keywords: boron · cyclotrimeriza-
tion · density functional calcula-
tions · reaction mechanisms · ruthe-
nium
Introduction
The chemistry of metallaboranes, which are a class of
structures that result from the combination of borane cages
and metal clusters, has been studied for some time now.[7,8]
The reactions of boranes with alkynes lead to carboranes[9,10]
that can further incorporate transition-metal fragments,
thereby resulting in the development of metallacarborane
chemistry.[11–13] In 1974, Grimes et al. put forward a pioneer-
ing report on the generation of nido-[1-(h5-C5H5)CoC2B3H7]
from nido-[2-(h5-C5H5)CoB4H8][14] and, subsequently, others
provided some notable accounts of the reactions of metalla-
boranes with alkynes.[15–17]
However, by and large, the lack of efficient preparative
routes to metallaboranes has encumbered the development
of this field. Recent renewed thrust has stemmed from the
availability of convenient, high-yielding routes to metalla-
boranes that replace the older, intricate procedures, which
involved long reaction times under pyrolytic conditions.
With these new practical routes in hand,[18] metallaboranes
that contain Group 4–9 metals have become accessible and
a variety of reactions has subsequently been investigated. To
the best of our knowledge, only a single catalytic process of
metallaboranes has been reported, that is, the catalytic activ-
ity of [1,2-(Cp*Rh)2B3H7] (1b) with alkynes,[19] which has
been a boon for the synthesis of metallaboranes with other
transition-metal centers. We have recently reported the re-
action of nido-[1,2-(Cp*RuH)2B3H7] (1a)[20] with [Mo(CO)3-
The transition-metal-mediated cyclotrimerization of alkynes
represents an elegant preparative route to substituted ben-
zenes. Substantial research efforts have been devoted in the
past to examine the scope of this reaction and to understand
its underlying mechanistic details.[1–3] However, the other-
wise-rich chemistry of p-block elements[4,5] has remained
largely unexplored in this particular context. Our interest to
probe the corresponding catalytic potential of main-group
elements was sparked by the fact that boron in particular
exhibits a rich organometallic chemistry, with a broad range
of useful chemical transformations.[6] In view of the consid-
erable body of knowledge that has been gathered in the
field of metallaborane chemistry, it appears rewarding to ex-
plore the catalytic potential of this class of compounds for
the cyclotrimerization of alkynes.
[a] K. Geetharani, Dr. S. Ghosh
Department of Chemistry
Indian Institute of Technology Madras
Chennai 600036 (India)
Fax : (+91)44-2257-4202
E-mail: sghosh@iitm.ac.in
[b] Dr. S. Tussupbayev, Dr. J. Borowka, Prof. Dr. M. C. Holthausen
Johann Wolfgang Goethe Universitꢀt
AHCTNUTGERGUN(NN CH3CN)3], which led to arachno-[(Cp*RuCO)2B2H6] (2) in
high yield.[21] Herein, we report a detailed study of the reac-
tivity of compound 2 with a variety of terminal- and internal
alkynes, thereby leading to metallaborane-catalyzed cyclotri-
merization reactions. To obtain a detailed mechanistic in-
sight into these reactions, we have evaluated the cyclotrime-
Institut fꢁr Anorganische und Analytische Chemie
Max-von-Laue-Strasse 7, 60438 Frankfurt (Germany)
Fax : (+49)69-798-29417
E-mail: max.holthausen@chemie.uni-frankfurt.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200291.
8482
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Chem. Eur. J. 2012, 18, 8482 – 8489

FULL PAPER
rization reaction that is mediated by the arachno-diruthena-
tetraborane by means of density functional calculations for
1H NMR spectroscopy, and GC analysis. The formation of
new metallaboranes or metallacarboranes was not observed.
The activities and selectivities for the cyclotrimerization of
a variety of terminal and internal alkynes are listed in
Table 1 and Table 2, respectively. The activities are higher
ꢀ
acetylene and HC CCOOEt as a model substrate.
Results and Discussion
Table 1. Cyclotrimerization of terminal alkynes catalyzed by 2.[a]
Cyclotrimerization of alkynes mediated by compound 2: A
few accounts of metallaborane reactivity with alkynes have
been put forward: 1) a facile reaction was observed between
compound 1a and a variety of substituted alkynes,[22,23]
2) the use of [(Cp*Ir)2B4H10][24] yields a range of metallacar-
borane products, and 3) the reaction of compound 1b with
internal- and terminal alkynes[19] leads to catalytic cyclotri-
merization. Owing to the presence of two additional hydride
atoms in the ruthenaborane, compounds 1a and 1b are isoe-
lectronic with very similar molecular structures (Scheme 1).
Entry
R
Cat./substrate
t
T
[8C]
Yield[b]
[%]
Ratio
[h]
1
2
3
4
5
6
7
8
9
Ph
1:60
1:20
1:20
1:20
1:20
1:40
1:24
1:14
1:20
1:20
36
30
72
40
60
24
70
60
40
36
75
75
90
75
75
75
75
75
75
50
35
75
1:3
1:3
–
1:1
–
1:6
–
–
p-CH3C6H4
p-CF3C6H4
p-NH2C6H4
2,4-F2C6H3
CO2Et
n.c.[c]
42
n.c.[c]
85
CH2OH
n.c.[c]
n.c.[c]
n.c.[c]
40
C
N
[d]
C10H7
SiMe3
–
1:3
10
[a] Catalyst to substrate ratio, reaction time, t, reaction temperature, T,
total yield, ratio of the 1,3,5- and 1,2,4-trisubstituted benzenes. [b] Yields
are given based on product formation as determined by 1H NMR spec-
troscopy. [c] n.c.=no cyclotrimerization. [d] 1-Naphthyl.
Table 2. Cyclotrimerization of internal alkynes catalyzed by compound
2.[a]
Entry R1
R2
Cat./substrate
t
T
Yield[b] Ratio[c]
[h] [8C] [%]
1
2
3
4
5
6
Ph
Ph
Ph
Me
SiMe3 1:20
SiMe3 1:20
SiMe3 1:20
1:20
1:20
1:95
40
48
70
48
60
60
75
75
55
75
75
55
51
21
Hb[d]
Me
Me
Ph
SiMe3
C4H9
Tpb[e]
n.c.[f]
20
–
Tmsb[g]
n.c.[f]
n.c.[f]
–
–
[h]
[a] Catalyst to substrate ratio, reaction time, t, reaction temperature, T,
total yield, ratio of the 1,3,5- and 1,2,4-trisubstituted benzenes. [b] Yields
are given based on product formation as determined by 1H NMR spec-
troscopy. [c] A single isomer was isolated. [d] Hb=hexaphenylbenzene.
[e] Tpb=1,2,4-trimethyl-3,5,6-triphenylbenzene. [f] n.c.=no cyclotrimeri-
zation. [g] Tmsb=1,2,4-trimethylsilyl-3,5,6-triphenylbenzene. [h] n-butyl.
Scheme 1. Structures of metallaborane compounds 1a, 1b, 2, and 3 and
the reactivity of compounds 1a, 1b, and 2 towards alkynes (Cp*=h5-
C5Me5).
Against this background, we explored the reactivity of
compound 2 with various internal- and terminal alkynes,
which led to the formation of a mixture of 1,3,5- and 1,2,4-
substituted benzenes through catalyzed cyclotrimerization
reactions. However, we found that compound 2 was catalyti-
cally inactive towards acetylene cyclotrimerization, which
was confirmed by thin-layer chromotography (TLC),
for terminal alkynes than for internal alkynes and the yields
are in the range 20–85% (no attempts were made to opti-
mize the reaction conditions). The ratios of the isomers
1
were determined by H NMR spectroscopy on the reaction
mixture after removing any unreacted alkyne and solvent
or, in the cases where side-reactions were observed, by
1H NMR spectroscopy after thin-layer chromatography. In
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8483

M. C. Holthausen, S. Ghosh et al.
all cases, the 1,2,4-isomer is predominant, except for p-
ꢀ
ꢀ
NH2C6H4C CH. The reaction of compound 2 with MeC
CPh yields 1,2,4-trimethyl-3,5,6-triphenylbenzene as the
1
only isomer that is detectable by H NMR spectroscopy.
We found that electron-withdrawing substituents generally
enhanced the reactivity of an alkyne. For example, with
ꢀ
HC CCO2Et, we observed a high yield of the cyclotrimer-
ized product, which was comparable to the results for rhoda-
ꢀ
borane 1b. The reactions of HC CR (R=p-CF3C6H4 or 2,4-
F2C6H3) gave insoluble materials and no cyclotrimerized
products.
To evaluate the scope of this catalytic process, the reac-
tion was also carried out by using the symmetrical alkyne
ꢀ
PhC CPh, thereby affording the fully substituted arene,
hexaphenylbenzene. Notably, rhodaborane 1b, which cata-
lyzes the cyclotrimerization of a variety of internal alkynes,
does not yield the cyclotrimerization product of this
alkyne.[19] The decreasing activities of compound 2 for RC
CSiMe3 (R=H, Ph, SiMe3, and C4H9) imply the presence of
a significant steric factor in the reaction mechanism.
ꢀ
There is one striking aspect of these observations with re-
spect to the earlier work of Fehlner and co-workers:[19] in
any presumable mechanistic picture, a key requirement for
the metal-catalyzed cyclotrimerization is the coordination of
the alkyne to the metal center, which, in turn, requires the
presence of a vacant metal site or labile ligands. We note
that neither compound 2 nor compound 1b contain vacant
metal sites, yet compound 2 contains two potentially labile
CO ligands. To establish their role, we tested the cyclotrime-
rization of alkynes by using [(Cp*Ru)(Cp*RuPMe2Ph)B2H6]
(3),[25] in which the CO ligands were substituted by a more-
labile PMe2Ph ligand. The reaction led to decomposition at
room temperature and no sign of the cyclotrimerized prod-
uct was observed. Up to this point, the catalytic activity re-
mained puzzling and it is still not clear why the presence of
the Cp*RuH site in compound 1a leads to alkyne insertion,
the presence of the Cp*RuCO site in compound 2 leads to
catalytic cyclotrimerization, and the presence of a Cp*RuP-
Scheme 2. Four pathways that were investigated for the initial interaction
between the alkyne and the catalyst.
the Cp* ligands in compound 2 with Cp ligands). Calcula-
ꢀ ꢁ
tions were performed with HC C COOEt as a representa-
tive example for the cyclotrimerization reaction. Scheme 2
shows four conceivable pathways that we considered for the
initial alkyne interactions with the catalyst. First, a dissocia-
tive path (path A) involves the initial release of one of the
carbonyl ligands to provide a vacant Ru-coordination site
for subsequent alkyne addition. The associative paths
(paths B–D) commence with the coordination of the alkyne
to three different sites in compound 2’. Path B starts with
the attack of the alkyne on one of the boron atoms of the
B2H5 group. Alternatively, addition to one of the metal cen-
ters involves a concomitant change of the coordination
mode of either the B2H5 group (from bridging to terminal,
path C) or of the adjacent Cp ring (change in hapticity from
h5 to h1, path D).
Me2Ph fragment in compound
(Scheme 1).
3 offers no reaction
Thus, we performed a detailed computational study to es-
tablish the mechanism of the cyclotrimerization reaction
that is catalyzed by binuclear ruthenaborane compound 2.
Although numerous theoretical papers have focused on the
cyclotrimerization of alkynes catalyzed by mononuclear
complexes,[26] to the best of our knowledge, only one study
has focused on catalysis by binuclear complexes.[27] This
latter computational study,[27] which was on the tetrameriza-
tion of acetylene by a nickel catalyst, revealed that terminal-
ly coordinated acetylene ligands insert more-readily into
Path A involves the release of CO from compound 2’,
thereby leading to either structures A1 or A2 (Scheme 3).
The calculations revealed that this dissociation is highly en-
dothermic (DRH298 =39.7 and 44.6 kcalmolꢁ1, respectively),
and, therefore, we did not consider this pathway any further.
Along path B, we examined the potential reactivity of the
B2H5 group in compound 2’ with respect to alkyne attack.
The only transition state that we could locate in extensive
potential-energy-surface scans was TS2’–B1, in which the
alkyne attacks the terminal boron atom (Scheme 3). In this
ꢁ
Ni C bonds than bridging acetylene ligands and, thus, the
most-favored dinuclear path strongly resembles the mono-
nuclear pathway.
ꢁ
ꢁ
transition state, one of the Ru B bonds is broken and a B
C bond is formed synchronously to yield intermediate B1
and a boracyclopropene, that is, the alkyne abstracts a BH
Density functional study on cyclotrimerization pathways:
The reaction pathways have been explored by employing
[(CpRuCO)2B2H6] (2’) as a model catalyst (that is, replacing
moiety from compound 2’. However, the calculated energy
¼
barrier of this transformation is prohibitively high (DH
=
298
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An arachno-Diruthenaborane Catalyst
FULL PAPER
sponding Ru atom. The coordination of a second alkyne
molecule onto structure C3 to yield structure C4 proceeds
without a barrier in a highly exothermic step (DRH298
=
ꢁ24.7 kcalmolꢁ1).
¼
The next step, which is both facile (DH 298 =4.9 kcal
molꢁ1) and highly exothermic (DRH298 =ꢁ41.6 kcalmolꢁ1), is
ꢁ
the C C coupling between the vinyl ligand and the coordi-
nated alkyne in structure C4 to produce butadienyl complex
C5 (Scheme 3). A subsequent intramolecular rearrangement
leads to the loss of the p-interaction between the Ru atom
and the butadienyl ligand in structure C5; instead, an agostic
interaction Ru···H···Cd is present in structure C6. The con-
version is slightly endothermic (DRH298 =1.5 kcalmolꢁ1) and
¼
essentially barrierless (DH 298 =2.3 kcalmolꢁ1). Transfer of
the hydrogen atom that is involved in the agostic interaction
Ru···H···Cd in structure C6 onto the B2H5 ligand yields struc-
ture C7 and the subsequent formation of structure C8 is an
Scheme 3. The calculated reaction steps along paths A and B for the cy-
endothermic process (DRH298 =12.1 kcalmolꢁ1) that proceeds
ꢀ ꢁ
clotrimerization reaction of substituted-alkyne HC C COOEt. Enthal-
pies (DH298) are given relative to compound 2’ in kcalmolꢁ1
.
¼
with
a
considerable energy barrier (DH 298 =22.0 kcal
molꢁ1).
Coordination of the third alkyne molecule to compound
C8 leads to alkyne–ruthenacyclopentadiene complex C9
(Scheme 5) in a slightly endothermic step (DRH298 =8.1 kcal
molꢁ1). This transformation occurs with a considerable
37.9 kcalmolꢁ1) and, therefore, this route was also not con-
sidered further.
The results of path C are shown in Scheme 4 and
Scheme 5. The first sequence of reaction steps leads to the
formation of a ruthenacyclopentadiene intermediate (C8).
This sequence starts with the coordination of the alkyne to
compound 2’ to yield structure C1. As noted above, this step
is associated with a change in the coordination mode of the
B2H5 moiety from bridging to terminal. We located an
isomer of compound 2’ with a terminal B2H5 fragment at
Ru1 (2a’, not shown in Scheme 4) that was 28.4 kcalmolꢁ1
higher in energy than compound 2’. The isomerization step
involves the loss of an agostic Ru2···H···B interaction that is
present in compound 2’, whereas the Ru-H-Ru bridging
bond is retained. This process generates a vacant coordina-
tion site at the Ru2 center and, therefore, a subsequent coor-
dination of the alkyne to compound 2a’ is virtually barrier-
less. Overall, the coordination of the first alkyne molecule is
a slightly endothermic step (DRH298 =2.1 kcalmolꢁ1) with
¼
energy barrier (DH 298 =21.0 kcalmolꢁ1) and is accompa-
ꢁ
nied by a substantial increase in the Ru Ru distance.
Subsequent [4+2] cycloaddition via a concerted transition
state (TSC9–C10) leads directly to benzene complex C10. The
¼
C C coupling is facile (DH 298 =8.7 kcalmolꢁ1) and highly
ꢁ
exothermic (DRH298 =ꢁ65.6 kcalmolꢁ1). Finally, elimination
of the h2-coordinated benzene molecule from compound
C10 generates structure C11, which is an isomer of the ini-
tial catalyst (2’). The elimination step is thermodynamically
favorable (DRH298 =ꢁ19.9 kcalmolꢁ1) and proceeds without
a barrier. Intermediate C11 contains two bridging CO li-
gands and the B2H6 ligand is attached to one of the Ru
atoms, whereas the other Ru atom remains coordinatively
unsaturated. Therefore, this atom readily coordinates anoth-
er incoming alkyne molecule to form compound C12, there-
by closing the catalytic cycle through a facile isomerization
a substantial energy barrier of DH 298 =28.8 kcalmolꢁ1.
(DH 298 =1.7 kcalmolꢁ1) into compound C1.
¼
¼
It has been demonstrated previously that the introduction
of alkynes into pentacarbonyl complexes accelerates the
rate of carbonyl substitution owing to the propensity of al-
kynes to act as four-electron donors.[28,29] This result led us
to examine CO elimination from structure C1, but the for-
mation of the resulting complex (C2) is prohibitively endo-
thermic (DRH298 =36.2 kcalmolꢁ1). Instead, the transfer of
the hydrogen atom, which was formerly involved in the
agostic B···H···Ru interaction, from the borane group onto
the coordinated alkyne affords vinyl complex C3
Another possibility for the generation of a vacant coordi-
nation site at the Ru center in compound 2’ is a change in
the hapticity of the Cp ring, which is well-known to play an
important role in ligand-substitution reactions.[30] We exam-
ined this Cp-ring slippage along path D, which was induced
by alkyne coordination to compound 2’ to yield structure
D1 (Scheme 6). The calculations reveal a barrier for this
¼
step (DH 298 =24.2 kcalmolꢁ1) that is 4.6 kcalmolꢁ1 lower
than the initial barrier along path C, but intermediate D1
(DRH298 =13.4 kcalmolꢁ1) is significantly less-stable than
structure C1. Furthermore, it is well-known that the pres-
ence of electron-donating methyl substituents at the Cp* li-
gands render ring-slippage processes energetically less favor-
able.[30a] Therefore, we performed calculations on the real
system with Cp* for both paths C and D. As expected, the
replacement of Cp with Cp* increases the energy barrier to
(Scheme 3). This step is essentially thermoneutral (DRH298
=
ꢁ0.4 kcalmolꢁ1) and occurs with a very low energy barrier
¼
(DH 298 =7.3 kcalmolꢁ1). As a result of the hydrogen trans-
fer, the terminal CO ligand that is adjacent to the coordinat-
ed alkyne in structure C1 becomes bridging in structure C3,
thereby generating a vacant coordination site at the corre-
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8485

M. C. Holthausen, S. Ghosh et al.
ꢀ ꢁ
Scheme 4. Reaction sequence along path C (part 1) of the cyclotrimerization reaction of substituted-alkyne HC C COOEt. Enthalpies (DH298) are given
relative to compound 2’ in kcalmolꢁ1
.
29.1 kcalmolꢁ1 for the initial step along path D. Moreover,
this step becomes even-less thermodynamically favorable
(DRH298 =20.7 kcalmolꢁ1) and the energy barrier of the re-
verse step decreases to only 8.4 kcalmolꢁ1, thus rendering
the formation of intermediate D1 improbable. In contrast,
for path C, the replacement of Cp with Cp* results in a no-
ticeable lowering of the energy barrier to 23.3 kcalmolꢁ1
and an additional stabilization of intermediate C1 by
1.9 kcalmolꢁ1. The presence of Cp* also induces changes in
the coordination sphere of transition state TS2’–C1 and struc-
ture C1 such that the CO ligands become bridging, which
we see as one of the reasons for the additional stabilization.
with the experimental observation that the reaction with
acetylene does not lead to the cyclotrimerization product.
We identified a significant contribution of the dispersion
correction (about 15 kcalmolꢁ1) to the lowering of the rela-
tive enthalpy of the transition steps and intermediates when
ꢁ
the bulkier C2H COOEt molecule was used as a substrate;
without the inclusion of these effects, some of the steps have
prohibitively high energy barriers.
Conclusion
Calculations that were performed with acetylene as a sub-
We have demonstrated the ruthenaborane-catalyzed cyclo-
trimerization reactions of both internal- and terminal al-
kynes under mild conditions with wide functional-group
compatibility. This reaction is a rare example of this type of
¼
strate revealed a prohibitively high energy barrier (DH
=
298
38.6 kcalmolꢁ1) for the initial step along path C, that is, acet-
ylene coordination to compound 2’. This result is in line
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Chem. Eur. J. 2012, 18, 8482 – 8489

An arachno-Diruthenaborane Catalyst
FULL PAPER
ꢀ ꢁ
Scheme 5. Reaction sequence along path C (part 2) of the cyclotrimerization reaction of substituted-alkyne HC C COOEt. Enthalpies (DH298) are given
relative to compound 2’ in kcalmolꢁ1
.
function of the second metal ion is to keep the B2H5 frag-
ment in a suitable stereochemical arrangement, from which
a hydrogen atom can be readily transferred. Hence, the
B2H5 ligand plays an important role as a hydrogen-atom
buffer. For example, a migration of the bridging hydride in
species 2’, together with the change in the coordination
mode of the B2H5 fragment in the initial step of path C,
opens a vacant coordination site that is suitable for accept-
ing the incoming alkyne. Subsequently, a hydrogen transfer
from the B2H6 moiety to the coordinated h2-alkyne molecule
in structure C1 to yield h1-vinyl ligands in structure C2 gen-
erates a vacant space for the incoming second alkyne mole-
cule. Furthermore, this hydrogen atom shifts back to the
B2H5 ligand from the butadienyl ligand in structure C6 to
produce ruthenacyclopentadiene complex C8.
The dissociation of CO ligands is thermodynamically un-
favorable and cyclotrimerization occurs without substitution
of the carbonyl ligands in compound 2’. During the course
of the transformations, the CO ligands change their coordi-
nation mode within the catalyst from terminal to bridging,
thus linking the metal ions together. This finding explains
the loss of catalytic activity that is observed for the
[(Cp*Ru)(Cp*RuPMe2Ph)B2H6] complex, because the
monophosphine PMe2Ph ligand cannot act as a bridging
linker.
Scheme 6. Reaction sequence along path D of the cyclotrimerization re-
ꢀ ꢁ
action of substituted-alkyne HC C COOEt. Enthalpies (DH298
given relative to compound 2’ in kcalmolꢁ1
) are
.
catalytic activity of metallaboranes, which commonly pro-
duce metallacarborane complexes upon reaction with al-
kynes.
The experimental results were complemented and ration-
alized by means of a computational study on the mechanism
of the cyclotrimerization reaction. We considered several
different reaction pathways, the most-favorable of which
starts with alkyne coordination onto compound 2’, along
with a change in the coordination mode of the B2H5 frag-
ment from bridging to terminal, which is preserved along
the whole reaction pathway. Both Ru atoms act cooperative-
ly to promote the cyclotrimerization reaction: all of the
The rate-determining step of this mechanistic scenario is
the initial coordination of the alkyne to the reactant (2’),
¼
thereby yielding intermediate C1 with a barrier of D H298
=
28.8 kcalmolꢁ1. However, we predict a noticeably lower
energy barrier for the real system with Cp* instead of Cp
that is used in our molecular model. The calculations also
demonstrate the influence of substituents on the alkyne
(C2HR, where R=H or COOEt) on the stability of the in-
termediates and transition states: the conversion barriers
are prohibitively high if acetylene is used as a substrate.
ꢁ
alkyne-coordination and the C C-coupling steps take place
within the coordination sphere of one of the Ru atoms. The
Chem. Eur. J. 2012, 18, 8482 – 8489
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8487

M. C. Holthausen, S. Ghosh et al.
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Generous support from the Department of Science and Technology, DST
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Received: January 26, 2012
Published online: June 1, 2012
Chem. Eur. J. 2012, 18, 8482 – 8489
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8489