Ru Complexes in Catalytic Hydrogenation
Organometallics, Vol. 16, No. 1, 1997 137
Ta ble 5. An a lytica l Da ta
calcd, %
found, %
compd no.
mp, °C (dec)
color
empirical formula
C45H30O3Ru
C
H
C
H
2
3
4
5
6
7
242
160
235
yellow
yellow
yellow
yellow
yellow
beige
75.10
72.55
68.70
4.17
4.18
3.56
75.02
73.10
69.17
4.41
4.61
3.78
C
C
39H26O3Ru
45H28Cl2O3Ru
256
204-5
C
C
45H28Cl2O3Ru
37H28O7Ru
68.70
64.81
3.56
4.11
67.95
64.68
3.43
4.18
The left-hand cycle (Scheme 2) represents the viable
hydrogenation cycle which transforms alkynes to alk-
enes and subsequently to alkanes. The identifiable
complexes 8 and 12 must function as intermediates in
this cycle. Since 8 persists during hydrogenation, it is
reasonable that the rate-limiting step is its interaction
with an alkyne (8a ). Complex 8, is an 18e coordinately
saturated complex; therefore a reasonable proposition
that may account for alkyne complexation is an η5 f η3
rearrangement of the cyclopentadienol system, as de-
scribed by 8a , followed by Ru-H cis addition to the
triple bond to generate 12. This sequence of structural
events may in fact be considered as the known associa-
tive ligand substitution reaction of an 18 electron
complex.9 In essence, ring slippage provides an empty
coordination site. The η3 species (8a ) has been assigned
an arbitrary electronic arrangement of the cyclopenta-
dienol ring. We are unaware of previous mentioning of
such a slippage mechnism of a Cp ring in conjunction
with catalysis.
Our previous notion that 12 can reductively eliminate
an alkene, and generate 9, which may then continue
the cycle (Scheme 2), is apparently incorrect. As stated
previously, 12 is an extremely stable complex. In fact
we have experimentally demonstrated (vide supra) that
it does release a reduced substrate but only in the
presence of dihydrogen (Scheme 1). In order to explain
the interaction of 12, an 18e complex, with dihydrogen,
we must again invoke an associative type mechanism
resulting in 13. Reductive elimination of a reduced
substrate from 13 would generate 8, thus completing
the catalytic hydrogenation cycle.
In practice, an alkane (dimethyl succinate) rather
than alkene (dimethyl maleate) has been isolated. A
completely analogous hydrogenation cycle may be con-
sidered for the hydrogenation of alkenes to alkanes. It
is also plausible that the alkene does not depart from
the metal after the reductive elimination from 13.
An alternative route for the formation of 8a via
oxidative addition of H2 to 10 (Scheme 2) is unlikely,
since it would in fact require thermal loss of dihydrogen
from 8 to generate 9 under H2 pressure. However it can
not be strictly ruled out.
The above analysis (Scheme 2) is based on the
isolation and X-ray structural characterization of two
complexes 6 and 7 which are represented by 11 and 12
(Scheme 2). Admittedly, they have not been isolated
from the same reaction. It appears that complexes of
type 12 are unstable with most alkynes as they could
not be isolated but in the case of DMA. Thus, the Ru-C
bond in the DMA complex 7 (2.096 Å) is stronger than
that in 6 (2.114 Å). Possibly, the two carbomethoxy
groups in 7 provide a lower π* energy level for Ru back-
donation. The infrared spectral data (Table 1) support
this argument as well, in as much as the CO stretching
frequencies of 7 are higher than those of 6 indicative of
better back donation in 7.
Con clu sion s
Hydrogenation of alkynes with complex 1 (or with its
ring-substituted derivatives) as a precatalyst generates
a new complex in which the alkyne substrate is bound,
via two sp2-C atoms, to the Ru atom as well as to the
oxygen atom of the cyclopentadienol ring system. Such
complexes can be prepared independent of the hydro-
genation reaction by a simple thermal reaction of
alkynes and complexes of the general structure 1. They
are extremely stable both thermally and chemically, and
when formed during hydrogenation reaction of alkynes,
they slowly poison the hydrogenation process by virtue
of their catalytic inertness. Their mode of formation is
described in Scheme 2.
Complexes of type 7 (isolated and characterized in the
case of dimethyl acetylenedicarboxylate), which are the
formal addition products of Ru-H to a triple bond, are
the viable hydrogenation intermediates. A complex of
type 8a , with η3-coordination of the cyclopentadienol
ring system, is a logical precursor for 7. Being ther-
mally stable, the mode of decomposition of 7 to products
requires dihydrogen as described in Scheme 2.
Exp er im en ta l Section
Gen er a l P r oced u r e for th e P r ep a r a tion of Com p lexes
1-7. Dimer (0.8 mmol) and an alkyne (1.6 mmol) in toluene
(10 mL) were heated under a nitrogen blanket in a closed
reactor at 140 °C for 24 h. The toluene was evaporated in
vacuum and the residue taken in methylene chloride and
chromatographed on Silica 60. Elution of the column with
methylene chloride-petroleum ether mixture (1:1) gave mi-
crocrystalline yellow solids which were further purified by
crystallization from a methylene chloride-hexane mixture (1:
1).
Hyd r ogen a tion of 7. A solution of complex 7 (27.2 mg) in
THF (10 mL) was heated is a closed SS reactor in a glass sleeve
under dihydrogen (500 psi) at 140 °C for 6 h. Infrared
spectrum of the colorless solution: 2013, 1954, 1743 cm-1. The
first two bands are identical with those of 8, obtained by
hydrogenation of 1 under the above conditions. TLC of the
two solutions gave the same spot for 8. The last band was
found to be identical with that of dimethyl succinate measured
in THF.
Cr ysta l Str u ctu r e An a lyses. The X-ray diffraction mea-
surements were carried out at room temperature (ca. 298K)
on an automated CAD4 diffractometer equipped with
a
graphite monochromator, using Mo KR (λ ) 0.7107 Å) radia-
tion. Intensity data were collected out to 2θ ) 46° by the ω-2θ
scan mode with a constant scan speed of 4 deg/min for 6 and
2 deg/min for 7. Possible deterioration of the analyzed crystals
was tested by detecting periodically the intensities of three
standard reflections from different zones of the reciprocal space
and was found negligible during the experiment. A total of
(9) Basolo, F. Isr. J . Chem. 1986, 27, 233.
Shvo, Youval
