Syntheses of ruthenium(II) complexes containing polyphosphine ligands and their applications in the homogeneous hydrogenation

Article abstract of DOI:10.1016/S0277-5387(98)00319-2

A series of ruthenium(II) complexes of polyphosphine ligands, RuHCl(CO)(etp) (2), RuHCl(CO)(tdpme) (3) and RuHCl(PP₃) (4) [etp PhP(CH₂CH₂PPh₂)₂; tdpme = CH₃C(CH₂PPh₂)₃; PP₃ = P(CH₂CH₂PPh₂)₃] have been prepared from RuHCl(CO)(AsPh₃)₃ (1) through typical substitution reactions and characterized by spectroscopic (IR and ¹H NMR) methods and elemental analyses. These compounds have been used as catalyst precursors in the homogeneous hydrogenation of cyclohexene, cyclohexanone, propanal and 2-cyclohexen-l-one. These polyphosphine Ru(Il) complexes show enhanced catalytic activities in comparison with monodentate or bidentate phosphine or arsine analogues. The selectivities of compound 3 for substrates have been found to decrease in the order cyclohexanone > propanal > cyclohexene. In the case of hydrogenation of 2-cyclohexen-l-one, cyclohexanol has been produced by way of cyclohexanone. The catalytic activities are directly proportional to the concentration of catalysts. The dependence of catalytic activities upon the hydrogen pressure has been also studied. The Arrhenius activation energy of cyclohexanone for compound 3 is found to be E_a = 97.2 kJ mol^-1 and the activation parameters are ΔH? = 94.0 kJ mol^-1 and ΔS? = -0.72kJ K^-1 mol^-1. 1999 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0277-5387(98)00319-2

\
Polyhedron 07 "0888# 358Ð368
PERGAMON
Syntheses of ruthenium"II# complexes containing polyphosphine
ligands and their applications in the homogeneous hydrogenation
Kie!Muon Sung\ Seong Huh\ Moo!Jin Junꢀ
Department of Chemistry\ Yonsei University\ Seoul 019!638\ South Korea
Received 16 March 0886^ accepted 18 August 0887
Abstract
A series of ruthenium"II# complexes of polyphosphine ligands\ RuHCl"CO#"etp# "1#\ RuHCl"CO#"tdpme# "2# and RuHCl"PP2# "3#
ðetp PhP"CH1CH1PPh1#1^ tdpmeꢁCH2C"CH1PPh1#2^ PP2ꢁP"CH1CH1PPh1#2Ł have been prepared from RuHCl"CO#"AsPh2#2 "0#
through typical substitution reactions and characterized by spectroscopic "IR and 0H NMR# methods and elemental analyses[ These
compounds have been used as catalyst precursors in the homogeneous hydrogenation of cyclohexene\ cyclohexanone\ propanal and
1!cyclohexen!0!one[ These polyphosphine Ru"Il# complexes show enhanced catalytic activities in comparison with monodentate or
bidentate phosphine or arsine analogues[ The selectivities of compound 2 for substrates have been found to decrease in the order
cyclohexanone×propanal×cyclohexene[ In the case of hydrogenation of 1!cyclohexen!0!one\ cyclohexanol has been produced by
way of cyclohexanone[ The catalytic activities are directly proportional to the concentration of catalysts[ The dependence of catalytic
activities upon the hydrogen pressure has been also studied[ The Arrhenius activation energy of cyclohexanone for compound 2 is
found to be Eaꢁ86[1 kJ mol−0 and the activation parameters are DH%ꢁ83[9 kJ mol−0 and DS%ꢁ−9[61 kJ K−0 mol−0[ Þ 0888 Elsevier
Science Ltd[ All rights reserved[
Keywords] Ruthenium"II# complexes^ Polyphosphine ligands^ Homogeneous hydrogenation
0[ Introduction
many other Ru"II# complexes[ Tertiary phosphines are
generally excellent ligating agents to metal ions and\ thus\
are very important constituents of compounds for cataly!
sis[ Polyphosphines have favorable behavior over their
comparable monodentate phosphines and exhibit "i#
excellent bonding activity to metals\ "ii# an increased
basicity "or nucleophilicity# at the metal\ "iii# strong trans
in~uence\ "iv# formation of stable complexes in a variety
of metal oxidation states and "v# detailed structural and
bonding information[ For these facts\ many chemists
have focused on and extensively studied polyphosphineÐ
metal complexes and their reactions ð01Ð03Ł[
According to the results published by our research
group\ chelating diphosphine Ru"II# complexes generally
have better catalytic activity in the homogeneous hydro!
genation of alkenes ð6\04Ł and aldehydes ð05\06Ł[ It has
been reported that Ru"II# complexes of larger chelate
ring sizes have better catalytic activities[ Since the chelate
ring opening is regarded as the rate determining step in
catalytic processes\ complexes having larger chelate rings
should be considered to open the ring faster[ Because
there are no separations of any ligands\ an energetically
favorable system can be obtained using a metal!chelating
ligand in terms of entropy[ Moreover\ since poly!
phosphine complexes usually have two or more chelate
rings\ the chelate e}ect is augmented and the unwanted
A number of ruthenium"II# complexes including
ðRuCl1"PPh2#nŁ "nꢁ2\ 3# ð0\1Ł\ ðRuHCl"PPh2#2Ł ð2Ł and
ðRuH1"PPh2#3Ł ð3Ł have been reported to be e.cient cata!
lyst precursors for the homogeneous hydrogenation of
alkenes\ alkynes and CO1[ They also reduce many other
unsaturated groups such as carbon monoxide\ aldehydes\
ketones and nitrogroups[ Because of their outstanding
reactivity\ ruthenium metal complexes have been widely
used
as
catalyst
precursors[
The
complex
RuHCl"CO#"PPh2#2\ _rst reported by Vaska and Diluzino
in 0850 ð4Ł has been widely used as a catalyst due to its
facility in preparation\ air!stable property and ease of
further substitution reactions[ This neutral ruthenium
complex has shown good catalytic properties in a number
of homogeneous catalytic reactions such as C1C bond
migration ð5Ł\ hydrogenation of ole_ns ð6Ł\ aldehydes\
ketones\ a\b!unsaturated aldehydes ð7Ł\ nitroarenes ð8Ł\
N! and S!heterocycles ð09Ł and hydroformylation of
alkenes ð00Ł[ Owing to these properties\ much interest
have been given to RuHCl"CO#"PPh2#2 to be used as a
catalyst or a starting material for further preparation of
ꢀ Corresponding author[
9166!4276:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[
PII] S9166!4276"87#99220!3

369
K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
isomers which often appear in monodentate or bidentate
octahedral complexes are considered to be diminished[
In view of such advantages\ the polyphosphine metal
complexes were studied as the catalysts for homogeneous
hydrogenation[
1[1[ Preparation of RuHCl"CO#"AsPh2#2 "0#
Complex 0 was prepared similarly to the literature
method ð19\10Ł[
The goals of this research are two!fold] "i# to prepare
polyphosphine Ru"II# complexes containing hydride and
to characterize their structures and "ii# to study how much
the catalytic activities increased in the homogeneous
hydrogenation of several unsaturated organic
compounds[ Cyclohexene\ cyclohexanone\ propanal and
1!cyclohexen!0!one were tested as substrates in hydro!
genations[ Suitable mechanisms for the hydrogenation
catalysed by terdentate phosphine Ru"II# complexes were
studied via calculating activation parameters and per!
forming the hydrogenation reaction in several di}erent
solvents[
1[2[ Preparation of RuHCl"CO#"etp# "1#
Complex 1 was prepared according to the same method
from the previous work of our research group ð11Ł[
1[3[ Preparation of RuHCl"CO#"tdpme# "2#
Complex 0 "9[43 g\ 9[4 mmol# and CH2C"CH1PPh1#2
"tdpme] 9[21 g\ 9[40 mmol# were added to toluene "19 ml#
and re~uxed for 1[4 h[ The pale yellow suspension turned
into a clear yellow solution and\ as the reaction _nished\
the yellow crystalline solid was obtained[ After being
cooled to room temperature\ the solid was _ltered out in
air\ washed with 29 ml of n!heptane and then dried in
vacuo "yield] 9[177 g\ 62)#[ Anal[ calcd for C31H39
OClP2Ru] C\ 52[7^ H\ 4[0[ Found] C\ 51[8^ H 4[5[
1[ Experimental
1[0[ General procedures and instruments
1[4[ Preparation of RuHCl"CO#"PP2# "3#
All synthetic reactions were carried out using the typi!
cal Schlenk method ð07Ł under a pure nitrogen atmo!
sphere[ RuCl2 = xH1O\ AsPh2\ etp\ tdpme and PP2 were
purchased from Aldrich and used without further puri!
_cation[ All other chemicals and solvents were com!
mercial reagent grade and were puri_ed following the
literature ð08Ł[ Cyclohexanone\ cyclohexene\ propanal
and 1!cyclohexene!0!one\ all of which were the substrates
for the homogeneous hydrogenation reactions\ were pur!
i_ed from fractional distillations and hydrogen gas for
those reactions was of 88[888)[
Complex
0 "9[16 g\ 9[14 mmol# and P"CH1CH1
PPh1#2 "PP2] 9[19 g\ 9[2 mmol# were dissolved in toluene
"19 ml# and re~uxed for 1 h[ The light green solution
turned yellow as the reaction completed[ After this solu!
tion was cooled gradually to room temperature\ the small
amount of impurities was removed by _ltration[ This
_ltrate was _ltered again through celite and concentrated
to ca[ half of the original volume under a reduced pres!
sure[ The addition of 29 ml of n!pentane precipitated an
ivory solid which was _ltered out in air\ washed with
29 ml of n!pentane and dried in vacuo "yield] 9[044 g\
63)#[ Anal[ calcd for C32H32OClP2Ru] C\ 50[7^ H\ 4[1[
Found] C\ 50[7^ H 4[2[
FT!IR spectra were recorded on a Nicolet Impact 399
FT!IR spectrophotometer in KBr discs[ 010[36 MHz 20
P
NMR spectra were obtained from a Gemini 299 spec!
trometer using 74) H2PO3 as an external standard[
149 MHz 0H NMR spectra were obtained from a Bruker
1[5[ Homogeneous hydrogenation reactions
0
DPX!149 or 499 MHz H NMR spectra from a Bruker
AMX 499 with chemical shifts expressed in ppm relative
to tetramethylsilane[ Elemental analyses were performed
at the Basic Science Research Center\ Seoul branch[ The
hydrogenation reactions were run at high pressure and
high temperature conditions in a Parr Series 3459 Bench
Top Mini Reactor "099 ml# manufactured by Parr Instru!
ment Company[ The progress of the hydrogen reactions
was traced by analyzing the amount of the reactants
and products with a Hewllet Packard 4789 Series II Gas
Chromatograph equipped with FID and HP!4 "cross!
linked 4) phenylmethyl silicone phase^ 14 m×
9[1 mm×9[00 mm _lm thickness# capillary column[ The
amount of the substrate and the corresponding product
in hydrogenation reactions were known by the internal
standard "n!heptane# method[ The gas chromatograph
was connected to an HP 2283A integrator[
Compounds 0Ð3 were used as catalyst precursors in
the homogeneous hydrogenations of cyclohexanone\
cyclohexene\ propanal and 1!cyclohexen!0!one[ To
compare catalytic activities for the hydrogenation
of cyclohexanone\ complex RuHCl"PPh2#2 and
RuHCl"CO#"PPh2#"dppe# were also used as catalysts[
Unless otherwise noted\ the reactions were run generally
as follows] 1[9×09−1 mmol of catalyst\ 19[9 mmol of sub!
strate and ca[ 9[1 g of n!heptane "internal standard
material# in 59 ml of solvent were introduced to the auto!
clave equipped with a gas inlet valve and a sampling
valve[ Toluene\ benzene\ THF\ DMF and DMSO were
used as solvents in separate experiments[ The system was
purged three times with nitrogen gas of ca[ 79 psi and
then once with hydrogen gas of ca[ 149 psi at room tem!
perature to remove the air in the vessel[ At this time

K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
360
the reactor was heated rapidly to 099>C\ the reaction
temperature[ As soon as the heating was completed and
the temperature was adjusted to 099>C\ the pressure of
hydrogen gas was _tted to the exact reaction pressure\
19 atm "183 psi# and stirring was started[ This point was
regarded as zero time and sampling was performed inter!
mittently\ using a needle attached to the sampling valve[
The temperature and pressure was maintained constantly
throughout the reaction[ The extracted sample was
induced to an aluminum capped vial "1 ml# and
quenched at −19>C quickly to prevent further reactions[
After 4 min\ the amount of the substrate and the product
was determined by gas chromatograph[
complexes[ Substitution reactions giving 1\ 2 and 3 from
the starting material 0 are summarized in Scheme 0[
The linear terdentate phosphine ligand\ etp\ is able to
coordinate to Ru metal center either facially or mer!
idionally to give two geometrical isomers[ The carbonyl
stretching frequency in the IR spectrum of the complex
1 at n"CO# at 0855 cm−0\ which is somewhat di}erent and
shifted to a higher frequency region relative to the n"CO#
of complex 0\ shows that a good p!accepting phosphine
ligand is located trans to CO making the bond between
C and O stronger due to the back donation from Ru to
a phosphorus atom[
The 0H NMR spectral data of 1 in Table 0 indicate the
fact that there have to be two isomers\ 1a and 1b[ The
doublet of triplets at d −5[4 ppm of 1a is ascribed to the
facial coordination mode of etp[ One of the phosphorus
atoms in etp is coordinated to the ruthenium metal trans
to the hydride and the other two are cis to hydride[ The
trans HÐP couplings "normally 89Ð059 Hz# are larger than
the cis ones "09Ð29 Hz# ð13Ł\ and thus one can assign the
structure of 1a from the data in Table 0[ From the two
di}erent cis 0HÐ20P coupling constants "17[1 and 07[4 Hz#\
the chemical environment of the two cis phosphorus
atoms are considered to be inequivalent[ The quartet
signal at d −04[8 ppm suggests that all three phosphorus
atoms in etp ligands coordinate to the metal in cis to the
hydride[ In case of 1a\ there are two possible structures
like 1a!0 and 1a!1[ At this point\ however\ it is impossible
to assign the exact structure of the complex from the IR
and 0H NMR spectro!scopic data alone[
2[ Results and discussion
2[0[ Syntheses and characterizations of polyphosphine
ruthenium"II# complexes
Complex 0 has been thought to have similar stereo!
chemistry to RuHCl"CO#"PPh2#2[ Due to the steric hin!
drance of the bulky triphenylarsine ligands\ a meridional
arrangement rather than a facial one is favored for the
three triphenylarsine ligands[ Carbonyl and chloride
ligands\ the sizes of which are relatively larger than
hydride\ are thought to be located trans to each other to
minimize the steric hindrances between them[ For many
reasons we have chosen complex 0 as the starting material
to prepare complexes 1\ 2 and 3[ Triphenylarsine is more
labile than the phosphine analogue due to its bulkiness
and somewhat weak bonding characters compared with
Complex 2 was synthesized via the same method used
to prepare complex 1[ Due to the characteristic tripodal
type structure of the tdpme ligand\ it must coordinate to
the ruthenium metal facially and there is only one possible
structure in complex 2[ This structure of complex 2 is
0
phosphines ð12Ł[ The metal!hydride resonances in H
NMR spectra of complexes 1\ 2 and 3 are so clear that
identi_cation for the location of their ligands can be made
more de_nite[ In other words\ if the complex
RuHCl"CO#"PPh2#2 is used as a starting complex for fur!
ther substitutions\ any triphenylphosphine ligand which
might not be substituted makes it di.cult to interpretate
the NMR data of the phosphineÐhydride metal
0
elucidated by the IR and H NMR spectroscopic data
shown in Table 0[ The carbonyl stretching frequency
n"CO# at 0869 cm−0 con_rms the structure shown above\
where one of the three phosphorus donor atoms is located
Table 0
Selected 0H NMR and IR data of Ru"II# complexes
b
0H NMR "SiMe3#a
IR "cm−0
#
Complex
d"RuÐH# "ppm#
1J"HÐP# "Hz#
n"CO#
n"RuÐH#
0881
0
−7[6\ sd
−5[4\ dq\ −04[8\ q
−4[7\ dq
0812
0855
0869
1c
2
84[0\ 17[1\ 07[4\ 06[4
82[8\ 08[1\ 04[3
82[1\ 17[2\ 06[2\ 11[9\ 06[9
1902
3c
−5[6\ dq\ −05[4\ dt
0832 "0869e#
aIn CDCl2\ 182 K[
bIn KBr discs[
cThese complexes have structural isomers[
dSꢁsinglet\ dtꢁdoublet of triplets\ dqꢁdoublet of quartets\ qꢁquartet[
eIn CHCl2[

361
K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
Scheme 0[
trans to CO[ The 0H NMR spectrum also shows that the
hydride is cis to two phosphorus atoms and trans to the
other[ Complex 2 has been found to have somewhat
interesting characteristics unlike other polyphosphine
Ru"II# complexes[ The fact that there is only one possible
structure paves the way to giving a very pure product
during the synthetic process[ Upon cooling down the
system from the boiling temperature\ the low solubility
of the microcrystals of complex 2 at room temperature
in toluene makes it easier to separate the product from
slightly excess ligandÐsolvent solution[ Complex 2 is
exceptionally air!stable both in solid state and in solution\
and no decomposition has been observed[ Though the
solubility is low in toluene or benzene\ complex 2 is highly
soluble in chloroform and dichloromethane[ The 0H
NMR spectrum was successfully obtained using the
CDCl2 medium[ Moreover\ complex 2 shows e.cient
catalytic properties in the homogeneous hydrogenation
reactions\ as will be mentioned later[
valent of PP2 based on Ru in boiling toluene\ complex
3 was prepared with three of the four phosphine sites
coordinated to the ruthenium metal center while the other
phosphine arm remains free[ According to the IR spec!
troscopic data of complex 3 shown in Table 0\ n"CO# is
at 0832 cm−0\ which is also shifted to a higher frequency
region[ Therefore\ the carbonyl ligand is thought to be
located trans to a phosphorus donor atom among the
four[ The IR spectrum measured in CHCl2 solution has
shown only single n"CO# band at 0869 cm−0\ therefore we
could not detect any isomers by IR data[ The hydrideÐ
0
phosphorus resonances in H NMR spectrum are very
similar to those of complex 1 except for the integration
ratio[ The ratio is about 0]09 which tells us that 3b is the
major product[ Doublet of triplets at −05[4 ppm shows
that the hydride ligand exists in axial position cis to all
coordinating phosphorus atoms\ which are in an equa!
torial plane[ Doublet of quartets signal at −5[6 ppm rep!
resenting 3a shows similar stereochemical geometry to
1a[ Thus 3a has two possible structural isomers of 3a!0
and 3a!1 [ 20P NMR spectrum of 3 measured in CDCl2
has shown four groups of multiplet in the range of 14Ð
PP2\ a tetradentate chelating phosphine\ acts as a terd!
entate ligand[ By simple substitution of three triphenyl
arsine ligands in RuHCl"CO#"AsPh2#2 with one equi!

K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
362
41 ppm which could not be analyzed unambiguously
because of poor resolution[ Free ligand resonance signals
and RuHCl"CO#"tdpme# "2#[ The comparison of catalytic
activities for monodentate\ bidentate and terdentate
phosphine Ru"II# complexes is clearly displayed[ The rate
constants and relative activities of these complexes are
summarized in Table 1[
Complex 2\ a polyphosphine Ru"II# complex\ converts
cyclohexanone to cyclohexanol about 1[4 times faster
than RuHCl"CO#"PPh2#"dppe#\ where bidentate phos!
phine ligand dppe is coordinated to the Ru"II# metal
center[ It has been previously reported that the catalytic
activities of RuHCl"CO#"PPh2#"dppe#\ RuHCl"CO#
2
have been observed clearly at −01[2 ppm "doublet\ J
ꢁ16[0 Hz# and −05[6 ppm "quintet\ 2Jꢁ14[6 Hz#[
Although we were unable to elucidate the structure of 3
by 20P NMR data\ we may well believe the presence of
isomers of 3 by 0H NMR data[
2[1[ Homogeneous hydrogenations of cyclohexanone\
cyclohexene and propanal
The catalytic properties of polyphosphine Ru"II# com!
plexes 0\ 1\ 2 and 3 were examined in the homogeneous
hydrogenation reactions of cyclohexanone\ cyclohexene
and propanal\ the substrates representing a ketone\ an
alkene and an aldehyde\ respectively[ All the hydro!
genation reactions\ catalysed by these complexes obey
the pseudo!_rst order rate law as described in Fig[ 0[ The
observed rate constants kobs can be obtained from the
equation −d"099−)conv#:dtꢁkobs"099−)conv# and
from the slope of the straight line in the graph of
ln"099−)conv# vs time "min#[
The hydrogenation reactions catalysed by 0\ 1\ 2 and
3 at ambient temperature were too slow for the reaction
processes to be traced and at high temperatures over
199>C\ decomposition of the Ru"II# complexes could be
observed[ At 099>C and 19 atm of hydrogen pressure\ the
turnover numbers are found to reach over 899 within 5Ð
7 h in most hydrogenation reactions which is suitable
to investigate the catalytic activities of the complexes\
although slightly di}erent results are obtained depending
on solvents[
"PPh2#"arphos#
ðarphosꢁPh1AsCH1CH1PPh1Ł
and
RuHCl"CO#"diarsine# ðdiarsineꢁPh1AsCH1CH1AsPh1Ł\
all of which have _ve!membered chelate rings\ are con!
siderably higher than those of non!chelate complexes
RuHCl"CO#"PPh2#2 and RuHCl"CO#"AsPPh2#2 "0# in the
homogeneous hydrogenation of propanal ð6\04\05\06Ł[
Here\ the same trend has been observed in the homo!
geneous hydrogenation of cyclohexanone[
RuHCl"CO#"PPh2#2 converts cyclohexanone to
cyclohexanol slightly faster than complex 0\ with both
complexes having non!chelating monodentate phosphine
or arsine ligands[ The di}erence in catalytic activities is
considered to arise from the electronic and steric di}er!
ences between PPh2 and AsPh2[ The increase in catalytic
activity of RuHCl"CO#"tdpme# "2# is in accordance with
the earlier proposals suggested[ The polyphosphine com!
plexes may contribute de_nitely to increased catalytic
activities due to the increased chelate e}ect and stability[
In order to compare the hydrogenation of ketone with
that of alkene\ cyclohexanone and cyclohexene have been
chosen[ Cyclohexanone and cyclohexene are often used
as basic materials for substrates for homogeneous hydro!
genation\ because they are rather simple hydrocarbons
which a}ord no reduced products containing chiral
centers\ or isomerized forms ð14Ł[
Figure 0 shows the typical pseudo!_rst order hydro!
genation of cyclohexanone in THF catalysed by
RuHCl"CO#
"AsPh2#2
"0#\
RuHCl"CO#"PPh2#2\
RuHCl"CO#"PPh2#"dppe#
ðdppeꢁPh1PCH1CH1PPh1Ł
Fig[ 0[ Pseudo!_rst order plots for the hydrogenation of cyclohexanone "099>C^ 19 atm of H1 pressure^ ðcyclohexanoneŁ:ðcatalystŁꢁ0999 in THF#[

363
K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
Table 1
Rate constants and relative catalytic activities for the hydrogenation of cyclohexanone
a
Complex
092kobs "min−0
#
TONb
Relative activityc
RuHCl"CO#"AsPh2#2 "0#
RuHCl"CO#"PPh2#2
RuHCl"CO#"PPh2#"dppe#
RuHCl"CO#"tdpme# "2#
9[0929[96
9[4329[91
3[6229[90
01[2729[93
14
71
0[9
4[3
36[7
014[9
349
514d
a−d"099−)conv#:dtꢁkobs"099−)conv#[
bTONꢁðProductŁ:ðCatalystŁ for 039 min except for 2[
ckobs:kobs\ for RuHCl"CO#"AsPh2#2[
dFor 79 min[
Reductions of cyclohexene to cyclohexane using
rapidly in propanal "Table 2#[ But the reaction catalysed
by complex has been performed satis!
RuHCl"CO#"etp# "1#\ RuHCl"CO#"tdpme# "2# and
RuHCl"CO#"PP2# "3# were examined under the same con!
ditions "099>C and 19 atm H1 pressure# as the hydro!
genation of cyclohexanone[ Ketones are not reduced with
the classical Wilkinson|s catalyst\ although some cationic
rhodium complexes show some catalytic activities ð15Ł[ It
is usually observed that hydrogenation of ketone requires
more drastic conditions than that of aldehydes or alkenes
ð16Ł[ But there are some cases where diphosphine com!
plexes of Rh and Ru have been used as catalysts for the
asymmetric hydrogenation of functionalized ketones and
diphosphineÐRu complexes have also been excellent
hydrogenation catalysts for the hydrogenation of a!sub!
stituted b!keto esters ð17Ł[
The catalytic activities in the hydrogenation of
cyclohexene increases in the order 1³2³3 as depicted in
Table 2 where the pseudo!_rst order rate constants kobs
are summarized[ In the case of complex 1\ the possible
isomerization during the phosphine dissociationÐassoci!
ation processes in the catalytic cycles seems to in~uence
the reaction rate to be slower than complex 2[ In the
case of complex 3\ however\ a non!chelating dangling
phosphine site in PP2 ligand may accelerate the phosphine
association step\ thereby increasing catalytic activities[
Hydrogenations of propanal with complexes 1 and 3
were unsuccessful because the complexes decomposed
2
factorily and kobs for the hydrogenation reaction is found
to be 4[6029[90 min−0[ When we simply examine the
catalytic activities of RuHCl"CO#"tdpme# "2# in the
hydrogenation of cyclohexene\ cyclohexanone and pro!
panal\ a quite surprising result is obtained[ Complex 2
catalyses in the order cyclohexanone×propanal×
cyclohexene\ in opposition to our expectation "Fig[ 1#[
As mentioned earlier\ it is usually more di.cult to reduce
ketones and aldehydes with Rh catalysts ð14Ł\ cyclohexene
has also been expected to be reduced most easily by these
catalytic systems[ The exact explanation for this trend
has not been well established\ but the nucleophilicity of
the substrates as well as their steric factors might be
considered to in~uence the reaction rates[
2[2[ Homogeneous hydrogenation of 1!cyclohexen!0!one
1!Cyclohexen!0!one\ an a\b!unsaturated ketone\ was
chosen as the substrate in hydrogenation reaction to _nd
out the preferred selectivity between the ole_n group
and the ketone in an a\b!unsaturated ketone[ Figure 2
describes the percent composition changes of reactant
and products depending on time[ Polyphosphine Ru"II#
complexes RuHCl"CO#"etp# "1# and RuHCl"CO#"tdpme#
"2# have been used as catalyst precursors[ Both complexes
complete the reactions within 399 min and the reaction
rate by complex 2 is almost 1 times faster than complex
1 and such a trend is consistent with the case of cyclohex!
ene[ Conversion of 1!cyclohexen!0!one to cyclohexanol
occurs by way of cyclohexanone\ which indicates a higher
selectivity for ole_n group reduction[ Interestingly\ the
rate of the hydrogenation of cyclohexen!0!one to cyclo!
hexanone is faster than that of cyclohexene to cyclo!
hexane\ indicating that the presence of the carbonyl
group in 1!cyclohexen!0!one has a favorable e}ect on the
reduction of the ole_n[ This can be attributed to either
an enhanced coordination strength of the substrate to
the metal or to the carbonyl making the bond more
susceptible to the nucleophilic attack[ The selectivity for
Table 2
Rate constants for the hydrogenation of cyclohexene and propanal with
polyphosphine Ru"II# complexes
a
Complex
092kobs "min−0
#
cyclohexene
propanal
b
RuHCl"CO#"etp# "1#
RuHCl"CO#"tdpme# "2#
RuHCl"CO#"PP2# "3#
1[1929[91
2[5629[90
4[2329[92
4[6029[90
b
a−d"099−)conv#:dtꢁkobs"099−)conv#[
bData non!available due to the decomposition of catalysts[

K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
364
Fig[ 1[ Comparison of catalytic activities of RuHCl"CO#"tdpme# "2# in the homogeneous hydrogenation of cyclohexene\ cyclohexanone and propanal
"099>C^ 19 atm of H1 pressure^ in toluene#[
a C1C bond is higher than that for a C1O bond in both
cases of the complexes 1 and 2[ It has been reported that in
hydrogenation of a\b!unsaturated carbonyl compounds
with monohydride catalysts\ p!oxapropenyl inter!
mediates give rise to the selective hydrogenations for the
ole_n side ð18Ł[ This is an interesting fact that accounts
for the possible pathway from 1!cyclohexen!0!one to
cyclohexanol[
most labile phosphine site would be the one trans to the
hydride arising from the trans e}ect of hydride ð29Ł\ but
the transfer of the hydride to cyclohexanone group is
unfavored stereochemically[ It is reasonable to assume
that the phosphine site of tdpme ligand trans to the
hydride is dissociated a}ording the metalÐcyclohexanone
intermediate B[ If the phosphine trans to hydride is dis!
sociated\ the transfer of hydride should take place before
the addition of hydrogen to metal\ which forms a steri!
cally congested conformation where two non!coor!
dinating phosphorus donors have to be dangling
afterward[ Intermediate C is formed via hydride transfer
followed by the addition of hydrogen molecule to Ru
metal center to give the intermediate D[ There is no direct
evidence that the h1!dihydrogen metal complex inter!
mediate such as D is more stable than the h0!dihydride
intermediate\ but the formation of the product\ cyclohex!
anol\ is certainly completed through another hydride
transfer to the metal!alkoxy intermediate[
During the catalytic process\ there are no ligand dis!
sociations separating the complex into two or more mol!
ecules completely because the polyphosphine ligand\
tdpme\ links the phosphorus donors together[ The
entropy of activation DS% is found to be −9[61 kJ K−0
mol−0 "Fig[ 3#\ a highly negative value\ which strongly
supports the fact that unlike Wilkinson|s catalysis of
cyclohexene "DS%ꢁ¦9[43 kJ K−0 mol−0#\ the transition
state undergoes a highly congested intermediate ð20Ł[ The
entropy of activation DS% and enthalpy of activation DH%
to the transition state have been calculated from the
Eyring equation[ DH% is found to be 83[9 kJ mol−0 and
Arrhenius activation energy Ea "Fig[ 4# in the hydro!
2[3[ Mechanism for the hydrogenation of cyclohexanone
with RuHCl"CO#"tdpme# "2#
The mechanism for the hydrogenation of cyclohex!
anone with RuHCl"CO#"tdpme# "2# has been studied as
a representative case for the homogeneous hydrogenation
catalysed by polyhodphine ruthenium"II# complexes[
Since "i# complex 2 is the most stable compound among
the three polyphosphine Ru"II# complexes in this work\
"ii# there are no other geometrical isomers in the complex
and "iii# the hydrogenation by complex 2 is very suitable
in many ways for the study of catalytic mechanisms[ On
the other hand\ there are few reports about the mech!
anism for ketone reductions using ruthenium complexes
as catalyst[
Based partly on the mechanism for hydrogenation of
aldehydes or ketones by RuHCl"CO#"PPh2#2 ð16Ł\ a mech!
anism is proposed for the hydrogenation of cyclohex!
anone with 2 as shown in Scheme 1[ Complex 2\ which is
an 07!electron species\ should make room for the
incoming substrate\ cyclohexanone\ a}ording one phos!
phine ligating site of Wpme ligand to be dissociated[ So
A is considered to be an active species for the catalytic
cycle and complex 2 acts as a catalyst precursor[ The
genation of cyclohexanone by complex
2 to be

365
K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
Fig[ 2[ Hydrogenation of 1!cyclohexen!0!one with "a# RuHCl"CO#"etp# "1# and "b# RuHCl"CO#"tdpme# "2# "099>C^ 19 atm of H1 pressure^ in toluene#[
86[1 kJ mol−0 from the Arrhenius equation[ Arrhenius
activation energy and the other activation parameters are
obtained from observing kobs at four di}erent tempera!
tures\ 79\ 89\ 099 and 009>C[ The Arrhenius activation
energy has almost the same value as the value of Wil!
kinson|s catalyst in the hydrogenation of cyclohexene
"85[1 kJ mol−0# ð21Ł[ From this fact\ complex 2 can be
known as a very e}ective catalytic system in the hydro!
genation of ketones[
The kinetic equations given in Scheme 2 can be derived
from the proposed mechanism shown in Scheme 1[ There\
we can simply _nd that the rate is linearly proportional
to the concentrations of catalyst as well as hydrogen[
Figure 5 shows the correlation of the rate constants vs
the concentrations of catalyst[ This is in accordance with
the proposed kinetic equation[ However\ the relationship
between the rate and hydrogen pressure in Fig[ 6 does
not exactly follow that equation and shows a large devi!
ation at 39 atm[ If we consider that the actual con!
centration of hydrogen is that of the dissolved hydrogen
gas in solvent\ the hydrogen pressure may not stand for
the concentration of hydrogen gas[
The hydrogenation reactions in di}erent solvents have
been run in order to _nd the solvent e}ect on the catalytic
activities[ Hydrogenations were carried out in DMF\
toluene\ benzene\ DMSO and THF\ all of which are
aprotic solvents having higher boiling points above 59>C
and di}erent polarities[ Unfortunately\ however\ regular

K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
366
Scheme 1[ Proposed mechanism for the hydrogenation of cyclohexanone to cyclohexanol with RuHCl"CO#"Wpme# "2#[
correlations between catalytic activities and solvent
polarities can hardly be found[ The pseudo!_rst order
plots representing catalytic activities of RuHCl
"CO#"tdpme# "2# in the hydrogenation of cyclohexanone
in various solvents are described in Fig[ 7[
steric e}ect of solvent molecules and their binding ability
to the metal center leading to another possible metal!
complex species[
The correlation between solvent polarity and catalytic
activity was pretty unclear[ Table 3 shows that the
polarity of the solvent is not the only factor which in~u!
ences catalytic activity[ The solvent e}ect may be related
not only with the polarity of the solvent but also with the
Acknowlegements
This research was supported by a grant from the Basic
Science Research Institute\ Ministry of Education of
Republic of Korea "Grant No[ BSRI!83!2315#[

367
K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
Scheme 2[
Fig[ 3[ Eyring plot for the pseudo!_rst order hydrogenation reaction of
cyclohexanone using RuHCl"CO#"tdpme# "2# "in toluene^ 19 atm of H1^
ðcyclohexanoneŁ:ðcatŁꢁ0999#[
Fig[ 5[ Rate dependence on the concentration of catalyst in the hydro!
genation of cyclohexanone with RuHCl"CO# "tdpme# "2# "099>C and
19 atm of H1 pressure in toluene#[
Fig[ 4[ Arrhenius plot for the pseudo!_rst order hydrogenation reaction
of cyclohexanone using RuHCl"CO# "tdpme# "2# "in toluene^ 19 atm of
H1^ ðcyclohexanoneŁ:ðcatŁꢁ0999#[
Fig[ 6[ Rate dependence on the hydrogen pressure in the hydrogenation
of cyclohexanone with RuHCl"CO#"tdpme# "2# "099>C in toluene#[

K[!M[ Sung et al[ : Polyhedron 07 "0888# 358Ð368
368
Fig[ 7[ Hydrogenation of cyclohexanone in various solvents[ð099>C\ 19 atm H1 of pressure\ catꢁRuHCl"CO#"tdpme# "2#Ł[
Table 3
ð02Ł Bianchini C\ Perez PJ\ Peruzzini M\ Zanobini F\ Vacca A[ Inorg
Chem 0880^29]168[
Catalytic activities of RuHCl"CO#"tdpme# "2# in di}erent solvents
ð03Ł Bianchini C\ Fabbri D\ Gladiali S\ Meli A\ Pohl W\ Vizza F[
Organometallics 0885^04]3593[
a
Solvent
092kobs "min−0
#
Polarityb
ð04Ł Huh S\ Sung K!M\ Cho Y\ Jun M!J\ Whang D\ Kim K[ Polyhedron
0885^04]0362[
ð05Ł Na K!I Huh S\ Sung K!M\ Jun M!J[ Polyhedron 0885^04]0730[
ð06Ł Han S!H\ Sung K!M\ Huh S\ Jun M!J\ Whang D\ Kim K[ Poly!
hedron 0885^04]2700[
ð07Ł Shriver DF\ Drezdzon MA[ The Manipulations of Air!Sensitive
Compounds John Wiley and Sons\ New York\ 0875[
ð08Ł Perrin DD\ Armarego LF[ Puri_cation of Laboratory Chemicals[
Pergamon\ New York\ 0877[
ð19Ł Ahmad N\ Levinson JJ\ Robinson SD\ Uttely MF[ Inorg Synth
0863^04]37[
DMF
0[5429[96
4[8129[92
6[8829[92
7[8629[94
01[2729[93
25[6
1[3
1[2
35[6
6[5
Toluene
Benzene
DMSO
THF
a−d"099−)conv#:dtꢁkobs"099−)conv#^
ꢁ0999^ 099>C^ 19 atm of H1 pressure[
bDielectric constant[
ðcyclohexanoneŁ:ðcatŁ
ð10Ł Sanchez!Delgado RA\ Lee WY\ Cho YH\ Jun M!J\ Choi SR[
Trans Met Chem 0889^05]130[
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