Transfer hydrogenation reactions catalyzed by free or silica-immobilized [RuCl2(ampy){RN(CH2PPh2)2}] complexes

Article abstract of DOI:10.1002/ejic.200700044

The complexes cis-[RuCl₂(ampy){R¹N(CH ₂PPh₂)₂}] [ampy = (2-aminomethyl)pyridine, R¹ = C₆H₅ or (CH₃CH ₂O)₃Si-(CH₂)₃

Full text of DOI:10.1002/ejic.200700044

FULL PAPER
DOI: 10.1002/ejic.200700044
Transfer Hydrogenation Reactions Catalyzed by Free or Silica-Immobilized
[RuCl2(ampy){RN(CH2PPh2)2}] Complexes
Alessandro Del Zotto,*[a] Carla Greco,[a,b] Walter Baratta,[a] Katia Siega,[a] and
Pierluigi Rigo[a]
Keywords: Ruthenium / Transfer hydrogenation / Heterogeneous catalysis / Silica / Ketones
The complexes cis-[RuCl2(ampy){R1N(CH2PPh2)2}] [ampy =
(2-aminomethyl)pyridine, R1
C6H5 or (CH3CH2O)3Si-
(CH2)3] showed very high catalytic activity in the homogen-
eous transfer hydrogenation of acetophenone (TOF
300000 h–1) with the use of 2-propanol as the hydrogen donor
and in the presence of sodium isopropoxide. The ligand
(CH3CH2O)3Si(CH2)3N(CH2PPh2)2 (ATM) was prepared in
high yield by reacting (3-aminopropyl)triethoxysilane, para-
formaldehyde, and diphenylphosphane in toluene heated at
80 °C. The –N(CH2PPh2)2 function was attached to the sur-
face of three different kinds of silica by means of two alterna-
tive methods. Thus, MA-Si-150 and mesoporous MA-Si-
MCM-41 were prepared by the reaction of the inorganic ma-
alized silica gel with HCHO and PHPh2 in refluxing toluene.
The –(CH2)3N(CH2PPh2)2 functionalized inorganic materials
were used to anchor the RuCl2(ampy) moiety; thus, three dif-
ferent silica-immobilized versions of the cis-[RuCl2(ampy)-
(ATM)] complex were obtained. The silica-anchored com-
plexes were tested in the transfer hydrogenation of aceto-
phenone, which was found to be fast and quantitative; the
effect of the nature of the silica support on the activity of the
catalyst was almost negligible. It was possible to reuse the
catalytic system for a second cycle without a decrease in the
activity, but the efficiency of the catalyst considerably dimin-
ished in successive reuses.
=
Ͼ
trix with ATM, whereas MA-Si-60 was synthesized by reac- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
tion of the commercially available 3-aminopropyl-function-
Germany, 2007)
Recently, we reported the synthesis of a series of phos-
phanyl–RuII complexes bearing the (2-aminomethyl)pyr-
idine (ampy)[5–8] ligand or the related 6-(4Ј-methylphenyl)-
2-pyridylmethylamine ligand,[9,10] and examined their abil-
ity to catalyze the reduction of ketones. Among complexes
with the ampy ligand, exceptionally high catalytic activity
was observed for complexes also bearing diphosphane li-
gands (Figure 1) with the cis isomer showing higher effi-
ciency. TOF values up to 4ϫ105 h–1 and ee values up to
94% with the use of chiral diphosphane ligands were ob-
tained.[7] The analogous species with two triphenylphos-
phane ligands was less active;[7] however, they seemed to be
suitable precursors for the anchoring of the RuCl2(ampy)
moiety to a solid support. The grafting of a complex active
in the homogeneous phase to an inert solid support is find-
Introduction
The catalytic transfer hydrogenation of ketones has re-
cently emerged as a useful and convenient method to pre-
pare secondary alcohols, including chiral compounds.[1]
Several transition-metal complexes have been found to cata-
lyze the reduction of ketones by using 2-propanol as a hy-
drogen donor or the system formic acid/triethylamine. In
particular, significant results were obtained, mainly by
Noyori and co-workers, using ruthenium(II) complexes
with monotosylated diamines[2] or amino alcohols.[3] These
catalysts, as well as those based on rhodium and iridium
with the same type of ligands, have been largely investigated
in recent years by several research groups.[4] Many efforts
have been made to design highly efficient catalysts but, for
large-scale applications, increasing activity and productivity
of the catalytic reactions is still a target of primary impor-
tance as is the ability to separate, recover, and recycle the
catalyst.
[a] Dipartimento di Scienze e Tecnologie Chimiche, Università di
Udine
Via Cotonificio 108, 33100 Udine, Italy
E-mail: alessandro.delzotto@uniud.it
[b] Serichim s.r.l.
P. le Marinotti 1, 33050 Torviscosa, Udine, Italy
Figure 1. RuII–ampy complexes efficient in transfer hydrogenation.
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A. Del Zotto, C. Greco, W. Baratta, K. Siega, P. Rigo
FULL PAPER
Scheme 1.
ing fast growing interest because the use of immobilized
catalysts can provide a significant improvement to over-
come the problems of separation and recycling of the cata-
lyst. Silica is the most common support for the heterogeni-
zation of molecular catalysts[11] owing to its high stability,
inertness, and low cost, all of which render this matrix at-
tractive for medium-scale synthesis and industrial applica-
tions. However, the use of catalysts immobilized on inor-
ganic supports for the transfer hydrogenation of ketones is
still rare.
Results and Discussion
Synthesis and Characterization of the Diphosphanes and
Diphosphanyl-Functionalized Silicas
Following a known synthetic method,[15] the diphos-
phane (3-{bis[(diphenylphosphanyl)methyl]amino}propyl)-
triethoxysilane (ATM) was obtained as a colorless oil by
reacting (3-aminopropyl)triethoxysilane with paraformal-
dehyde and diphenylphosphane in a 1:2.5:2 molar ratio in
toluene heated at 80 °C [Equation (1)]. Unreacted para-
formaldehyde was easily eliminated by filtration after dis-
solving the crude product in ethyl ether. The product, which
was isolated in high yield, showed a single resonance at δ =
With the very active cis-[RuCl2(ampy)(dppp)][7] [dppp =
1,3-bis(diphenylphosphanyl)propane] complex in mind, we
turned our attention to the R1N(CH2PPh2)2 ligand
(Scheme 1), which is topologically analogous to dppp,
easily prepared, and used to anchor the RuCl2(ampy) moi-
ety to a solid support. A suitable choice for the R1 group,
as in the case of (OR2)3Si(CH2)3–, allows the diphosphane
to be conveniently grafted onto a silica support. Alterna-
tively, the –N(CH2PPh2)2 function linked to the inorganic
matrix can be obtained by reacting a suitable amino-func-
tionalized silica with H2CO and PPh2H (Scheme 1).
Similar RuII-functionalized silicas, where the metal is
surrounded by a Cl2N2P2 donor set, were prepared by Ku-
mar and Ghosh starting from supports with ethylenedi-
–28.0 ppm in the 31P{1H} NMR spectrum and a clean H
NMR spectrum, which confirmed its very high purity.
1
(1)
As the morphology of the inorganic support (surface
amino arms.[12] Such catalytic systems were successfully ap- area and porosity) as well as the distance between the cata-
plied to the hydrogenation of carbonyl compounds. lytic sites could influence the activity of the anchored tran-
Furthermore, Reek, van Leeuwen, and co-workers used sil- sition-metal complex,[11] we prepared three different silica-
ica-immobilized RuII complexes for the asymmetric re- immobilized versions of the ATM ligand. The first one was
duction of acetophenone where the metal was anchored to synthesized according to Equation (2) by starting from the
the inert support by chelation to an aminoalcohol moi- commercially available 3-aminopropyl-functionalized silica
ety.[13] Finally, Tu and co-workers investigated the asymmet- gel (mean pore size: 60 Å, surface area: 550 m2 g–1, loading:
ric transfer hydrogenation of different prochiral ketones by 1 mmolg–1), which was heated at reflux in toluene for 48 h
using RuII complexes immobilized on amorphous or meso- together with HCHO and PHPh2. The product, 3-{bis-
porous silicas functionalized through chiral diamine [(diphenylphosphanyl)methyl]amino}propyl-functionalized
arms.[14]
silica gel (MA-Si-60) was obtained as a white cream powder,
In this paper, we present the preparation and characteri- which was characterized by elemental analysis (C, H, N, P),
zation of the RuII complexes [RuCl2(ampy){R1N(CH2- TGA measurements, and CP-MAS 31P NMR spectroscopy.
PPh2)2}] [R1 = C6H5 or (CH2)3Si(OCH2CH3)3] as well as The latter confirmed the presence of the anchored diphos-
of three different –N(CH2PPh2)2 functionalized silicas phane function (broad singlet at δ = –25.3 ppm). Two minor
along with the corresponding RuII derivatives containing broad signals centered at δ = 20.3 and 30.8 ppm, within the
the RuCl2(ampy) moiety. Furthermore, the catalytic appli- range for the P=O resonances, and a shoulder on the high-
cation of both free and immobilized complexes to the trans- field signal were indicative of the formation of small
fer hydrogenation of acetophenone by using 2-propanol as amounts of products resulting from the oxidation of the
the hydrogen source is reported and discussed.
ligand.[16] Furthermore, from the elemental analysis of ni-
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Transfer Hydrogenation Reactions Catalyzed by RuII Complexes
FULL PAPER
trogen and phosphorus, it was possible to determine 0.86 ples showed about 5% weight loss due to water release,
and 0.70 mmolg–1 loading, respectively. The relative P/N whereas the progressive weight decrease above 250 °C was
molar ratio (1.6) is lower than that expected (2.0) for total attributed to organic material release only. Thus, in the
conversion of the –NH2 functions to –N(CH2- range 250–800 °C, MA-Si-60, MA-Si-150, and MA-Si-
PPh2)2. This finding could be reasonably explained in terms MCM-41 lost 24, 15, and 20% of their weight, respectively.
of a material still showing about 20% of the amine function These data, combined with the relative amounts of –(CH2)3-
unchanged. The possibility of formation of HNCH2PPh2 NH2 and –(CH2)3N(CH2PPh2)2 groups, present in each ma-
functions on the inorganic surface seems to be excluded by terial, calculated from the P/N molar ratio, allowed MA-Si-
the NMR spectroscopic data, though the presence of both 60, MA-Si-150, and MA-Si-MCM-41 to be obtained fol-
–NHCH2PPh2 and –N(CH2PPh2)2 groups cannot be ex- lowing the loading 0.64, 0.39, and 0.50 mmolg–1, respec-
cluded as the resonances of the P nuclei could be superim- tively. Such values are lower than those obtained from the
posed. It should be noted that no appreciable variation in elemental analysis of phosphorus.
the loading was observed when the reaction time was in-
creased, whereas a lower value (0.49 mmolg–1, based on P
analysis) was determined when the reaction was stopped
after 24 h. Two further preparations of MA-Si-60 con-
firmed the good reproducibility of the synthesis.
(3)
Synthesis of the Ruthenium(II)–ampy Complexes
(2)
The synthesis of the six-coordinate cis-[RuCl2-
(ampy){R1N(CH2PPh2)2}] [R1 = C6H5 1, R1 = (CH2)3Si-
(OCH2CH3)3 2] (Figure 2) complexes was carried out as de-
scribed previously for analogous compounds starting from
Two other 3-{bis[(diphenylphosphanyl)methyl]amino}-
propyl-functionalized silicas, namely, MA-Si-150 and MA-
[RuCl2(PPh3)3], ampy, and the diphosphane in a 1:1:1 mo-
Si-MCM-41, were conveniently prepared starting from Da-
lar ratio.[7] The complexes precipitated spontaneously as
visil silica with a 150 Å mean pore size and a surface area
sand yellow microcrystals from hot solutions of toluene and
were almost insoluble in common solvents. However, ac-
ceptable quality 31P{1H} NMR spectra were recorded from
saturated CD2Cl2 solutions. The spectra of 1 and 2 ap-
peared as AB spin systems with a coupling constant of
about 45 Hz; the doublets of 1 were centered at δ = 39.7
and 43.2 ppm and those of 2 at δ = 37.8 and 45.9 ppm. As
the NMR spectroscopic parameters agree with those found
for the cis-[RuCl2(ampy)(dppp)][7] complex, we propose for
complexes 1 and 2 the same stereochemistry to that shown
by cis-[RuCl2(ampy)(dppp)], with P-trans-Cl and P-trans-
N(pyridine) arrangements. As a matter of fact, this repre-
sents the common stable geometry shown by several com-
plexes bearing ampy and different diphosphane ligands
with C3 and C4 backbones.[7]
of 300 m2 g–1 and MCM-41 mesoporous silica with a sur-
face area of 1000 m2 g–1, respectively. Both were obtained
by combining ATM and the appropriate matrix in toluene
heated at 80 °C according to Equation (3). As per MA-Si-
60, the two products were characterized by elemental analy-
sis. The calculated loading, based on phosphorus, was
0.46 mmolg–1 for MA-Si-150 and 0.59 mmolg–1 for MA-Si-
MCM-41. As observed for MA-Si-60, the syntheses of MA-
Si-150 and MA-Si-MCM-41 showed very good reproducib-
ility. Compounds MA-Si-60, MA-Si-150, and MA-Si-
MCM-41 were also characterized by TGA measurements,
which were run under an atmosphere of argon in the tem-
perature range 20–800 °C. From 20 to ca. 250 °C, all sam-
Synthesis and Characterization of Silica-Immobilized
Ruthenium(II) Complexes
As depicted in Equation (4), by reacting the appropriate
functionalized silica and the trans,cis-[RuCl2(PPh3)2(ampy)]
complex in boiling 2-propanol, three different RuII-based
immobilized catalysts, namely, Ru-MA-Si-60, Ru-MA-Si-
150, and Ru-MA-Si-MCM-41, were prepared. The catalysts
were thoroughly washing with dichloromethane to elimin-
ate all traces of physisorbed metal–organic species from the
inert support. All materials appeared as yellow to yellow–
orange microcrystalline solids, which were characterized by
Figure 2. RuII–ampy complexes with the ligands PhN(CH2PPh2)2
and ATM.
Eur. J. Inorg. Chem. 2007, 2909–2916
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A. Del Zotto, C. Greco, W. Baratta, K. Siega, P. Rigo
FULL PAPER
(4)
elemental analysis (C, H, N, Ru) and TGA measurements complex 2. Analogous results were obtained by monitoring
in the range 20–800 °C to assess the metal loading. The the trans,cis-[RuCl2(PPh3)2(ampy)]/[C6H5N(CH2PPh2)2]
yield of the reaction (based on ruthenium), was generally system, for which the exclusive formation of 1 was ob-
high (88, 91, and 84% for Ru-MA-Si-60, Ru-MA-Si-150, served. These findings confirm that the in situ prepared
and Ru-MA-Si-MCM-41, respectively). The calculated catalytic systems effectively correspond to isolated com-
loading values were 0.35, 0.29, and 0.33 mmolg–1 of com- plexes 1 and 2, which are obtained in almost insoluble form
plex, in the respective order. To address the reaction to the in refluxing toluene.
formation of the cis dichloro isomer immobilized on silica,
A comparison of the catalytic efficiency of the complexes
the reagents were heated at reflux and allowed to react for bearing R1N(CH2PPh2)2 ligands with that of isostructural
2 h. In the case of Ru-MA-Si-60, a 31P CP/MAS NMR cis-[RuCl2(ampy)(dppp)] shows that the modification of the
spectroscopic investigation was undertaken. The spectrum diphosphane by substitution of the central carbon atom of
showed, in addition to signals due to uncoordinated di- the P–C–C–C–P skeleton with a N–R1 moiety has only a
phosphane and the corresponding oxide, two signals at δ = little influence on the activity of the relative ruthenium
47 and 53 ppm. The rather large line width may be the complex. As a matter of fact, by using complexes 1 and 2,
reason for the lack of observable coupling between the two the catalytic reduction of acetophenone was completed
inequivalent P nuclei. The NMR measurements are in (98%) within 1 min (Table 1, Entries 1 and 2), and the cor-
agreement with the presence of a unique RuII species immo- responding very high TOF values (Ͼ300000 h–1 at 50%
bilized on the inert support.
conversion) were of the same magnitude to that found for
cis-[RuCl2(ampy)(dppp)] (220000 h–1).[7]
Transfer Hydrogenation of Acetophenone in the
Homogeneous Phase
Transfer Hydrogenation of Acetophenone in the
Heterogeneous Phase
The metal-catalyzed transfer hydrogenation of ketones
was accomplished by using 2-propanol as the hydrogen do-
nor heated at reflux [Equation (5)]. It is well-documented
that the presence of a strong base as a cocatalyst enhances
the rate of the reaction.[17] Accordingly, all catalytic tests
were run in the presence of sodium 2-propoxide generated
by dissolution of metallic sodium into the alcohol.
All three silica-immobilized ruthenium complexes were
examined as catalysts in the transfer hydrogenation of ace-
tophenone. Standard reactions were carried out by using
500:1 and 20:1 acetophenone/Ru and (CH3)2CHO–Na+/Ru
molar ratios, respectively, at the reflux temperature of 2-
propanol (82 °C). The results are collected in Table 1. Al-
though, as expected, the immobilized complexes were much
less active than their counterparts in the homogeneous
phase, a quantitative conversion of the substrate into 1-
phenylethanol was observed within 90 min (Table 1, En-
tries 3–5), and the results showed that the nature of the in-
organic support played an almost negligible role on the per-
(5)
As complexes 1 and 2 showed only partial solubility in formance of the catalyst. An analogous behavior was ob-
2-propanol, even when heated at reflux and in catalytic con- served for ruthenium complexes anchored on mesoporous
ditions, we turned our attention to in situ prepared catalytic MCM-41 and MCM-48 and amorphous silicas.[12] The con-
systems. Thus, the catalytic reactions were carried out with centration of the strong base is crucial for the activity of
the use of a 0.1  solution of acetophenone in 2-propanol the supported complex. In fact, a decrease in the (CH3)2-
in the presence of 0.05 mol-% of the cis,cis-[RuCl2(PPh3)2- CHO–Na+/Ru molar ratio from 20:1 to 5:1 resulted in a
(ampy)] complex, 0.06 mol-% of the appropriate R1N- 94% conversion achieved in 8 h by using Ru-MA-Si-MCM-
(CH2PPh2)2 ligand, and 2 mol-% of (CH3)2CHO–Na+ (sub- 41 (Table 1, Entry 6). It should be stressed that the stability
strate/Ru/base = 2000:1:40) under reflux conditions. 31P of the functionalized silicas Ru-MA-Si-60, Ru-MA-Si-150,
NMR spectroscopic monitoring of a dilute CDCl3 solution and Ru-MA-Si-MCM-41, as well as that of the precursor
containing trans,cis-[RuCl2(PPh3)2(ampy)] and the ligand 3-aminopropyl-functionalized silica gel, towards (CH3)2-
ATM in a 1:1 molar ratio showed that on increasing the CHO–Na+ was analyzed. All samples, after treatment with
temperature the PPh3 ligands are completely replaced by a 20-fold excess of base for 2 h at 100 °C showed a 15–20%
the diphosphane ligand and that after prolonged heating reduction in the linked organic moiety (based on nitrogen
the only Ru-containing species detectable in solution was analysis). These results are in agreement with the metal
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Transfer Hydrogenation Reactions Catalyzed by RuII Complexes
FULL PAPER
Table 1. Catalytic transfer hydrogenation of acetophenone with the use of silica-supported complexes at 82 °C.
Entry[a]
Catalyst
S/C[b]
B/C[c]
Yield[d] [%]
Time [min]
[e]
1
2
3
4
5
6
7
7(1)
7(2)
8
8(1)
8(2)
8(3)
9
cis,cis-[RuCl2(PPh3)2(ampy)]/PhN(CH2PPh2)2
2000
2000
500
500
500
500
500
–
–
100
–
–
–
40
40
20
20
20
5
20
–
–
20
–
–
–
20
98
98
97
99
99
94
99
99
12
99
98
98
82
6
1[f]
cis,cis-[RuCl2(PPh3)2(ampy)]/ATM[e]
1[g]
90
Ru-MA-Si-60
Ru-MA-Si-150
Ru-MA-Si-MCM-41
Ru-MA-Si-MCM-41
Ru-MA-Si-150
–
–
90
90
480
90
240
1440
60
Ru-MA-Si-150
–
60
–
–
720
1440
720
Ru-MA-Si-MCM-41[h]
500
[a] The values reported in parenthesis for Entries 7 and 8 indicate the successive reuses of the catalyst. [b] Substrate/catalyst(Ru) molar
ratio. [c] Base/catalyst molar ratio. [d] The yield of 1-phenylethanol was determined by GC. [e] Reaction run in the homogeneous phase,
Ru/ligand, 1:1.2 molar ratio. [f] TOF = 320000 h–1 (turnover frequency = mol of acetophenone converted to 1-phenylethanol per mole
of catalyst per hour at 50% conversion). [g] TOF = 340000 h–1. [h] At 40 °C.
leaching measured after the first use of the catalyst (see be- spect to the metal) indicated that for both samples about
low).
15% of the ruthenium present in the immobilized catalyst
Recycling tests were run on all three silica-immobilized was released into the solution. Not surprisingly, analogous
ruthenium complexes with the use of both the 500:1 and the results concerning the metal leaching were obtained by me-
100:1 substrate/ruthenium molar ratios. As almost identical asuring the ruthenium amount in solution after the first
results were observed with the three different silica sup- catalytic trial. By contrast, an almost negligible metal leach-
ports, only those obtained with the representative Ru-MA- ing (Ͻ2%) was measured in the successive reuse of the cata-
Si-150 catalyst are presented in Table 1. By using a high lyst. Thus, as the activity of the immobilized complex re-
substrate/ruthenium molar ratio, such as 500:1, the activity mained unchanged from the first to the second use, one can
of the recycled catalyst in the second run resulted reduced conclude that the ruthenium released into the solution is
[Table 1, Entries 7 and 7(1)], although complete conversion present in a catalytically inactive form. It should be noted
of acetophenone into 1-phenylethanol was achieved. In the that 31P NMR spectroscopic measurements run on the re-
next reuse, only low amounts of product were obtained, covered solutions did not give any useful information about
which indicates that there is a breakdown in the catalytic the decomposition pattern of the catalyst. The neat decline
system [Table 1, Entry 7(2)]. The measurements were re- of the efficiency of the catalyst in the successive reuses can
peated by using a lower substrate/ruthenium molar ratio be ascribed to a decomposition of the structure of the sup-
(100:1). As can be seen in Table 1 [Entries 8–8(3)], the effi- ported metal complex.
ciency of the catalyst was apparently retained in the first
reuse, but then it drastically decreased; however, in the third
run the substrate underwent complete transformation into
1-phenylethanol. It should be stressed that in each success-
Conclusions
ive catalytic trial, the recovered catalyst was mixed not only
We
have
shown
that
the
cis-[RuCl2(ampy)-
with acetophenone and solvent, but also with a 20-fold ex- {R1N(CH2PPh2)2}] complexes are highly efficient homo-
cess of (CH3)2CHO–Na+ with respect to the amount of ru- geneous catalysts in the transfer hydrogenation of acetophe-
thenium effectively present. It is important to note that the none with 2-propanol and that they may have great poten-
efficiency of the catalyst did not appreciably fade from the tial for a broad application in the reduction of carbonyl
first to the second use. This finding should be compared compounds. We have also demonstrated that the anchoring
with the observed breakdown in the activity of complexes of the RuCl2(ampy) moiety to a silica support can be easily
1 and 2, as well as of all similar derivatives,[7] when reused accomplished when the surface of the inorganic matrix is
as homogeneous catalysts.
prior functionalized with –N(CH2PPh2)2 arms. Such a
As decomposition processes should be reduced by work- modification of the support was done by two alternative
ing at low temperature, the catalytic potential of Ru-MA- pathways, which allowed the syntheses of three different
Si-MCM-41 was verified at 40 °C. In this case, the reaction mesoporous or amorphous silicas and of the relative immo-
proceeded at an extremely low rate as only 6% conversion bilized ruthenium(II) complexes. These systems have shown
was observed after 12 h (Table 1, Entry 9). ICP MS mea- to be catalytically active in the transfer hydrogenation of
surements carried out on both Ru-MA-Si-150 and Ru-MA- acetophenone, which is rapidly and quantitatively trans-
Si-MCM-41 systems treated at 100 °C for 2 h in the pres- formed into 1-phenylethanol. Recovery and recycling of the
ence of a strong excess of (CH3)2CHO–Na+ (20:1 with re- inorganic material still resulted in good catalytic perform-
Eur. J. Inorg. Chem. 2007, 2909–2916
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FULL PAPER
NMR (200 MHz, CDCl3, 295 K): δ = 7.1–7.5 (m, 20 H, phenyl),
3.73 (q, JH,H = 7.0 Hz, 6 H, CH2CH3), 3.55 (d, JP,H = 3.3 Hz, 4 H,
NCH2P), 2.82 (m, 2 H, CH2CH2N), 1.48 (m, 2 H, CH2CH2CH2),
1.16 (t, JH,H = 7.0 Hz, 9 H, CH3), 0.46 (m, 2 H, CH2Si) ppm.
ance but only for the first reuse as decomposition of the
catalyst limited successive reuses.
13C{1H} NMR (50 MHz, CDCl3, 295 K): δ = 138.2 (d, JC,P
=
Experimental Section
12.8 Hz, Ci), 133.0 (d, JC,P = 18.3 Hz, Co), 128.3 (Cp), 128.2 (d,
JC,P = 8.5 Hz, Cm), 59.2 (t, JC,P = 9.2 Hz, CH2CH2N), 58.6 (dd,
JC,P = 5.5, 9.1 Hz, NCH2P), 58.2 (CH2CH3), 19.7 (CH2CH2CH2),
18.3 (CH2CH3), 7.6 (CH2Si) ppm.31P{1H} NMR (81 MHz, CDCl3,
295 K): δ = –28.0 ppm.
General: All reagents were purchased from Aldrich and used with-
out further purification. Commercial reagent grade solvents were
dried according to standard methods and freshly distilled under an
atmosphere of argon before use. All syntheses and manipulations
were carried out under an atmosphere of argon by using either
cis-[RuCl2(ampy){PhN(CH2PPh2)2}] (1): [RuCl2(PPh3)3] (384 mg,
0.40 mmol) and 2-(aminomethyl)pyridine (45 mg, 0.42 mmol) were
mixed in toluene (15 mL), and the slurry was heated at reflux for
2 h. The ligand PhN(CH2PPh2)2 (196 mg, 0.40 mmol) was then
added, and the mixture was heated at reflux for another 20 h. Upon
cooling, a sand yellow powder precipitated, which was isolated by
filtration, washed with ethyl ether, and dried under reduced pres-
sure. Yield: 225 mg (73 %). C38H37Cl2N3P2Ru (769.66): calcd. C
59.30, H 4.85, N 5.46; found C 59.23, H 4.66, N 5.31. 31P{1H}
NMR (81 MHz, CD2Cl2): δ = 39.7 (d, JP,P = 44.7 Hz), 43.2 (d, JP,P
= 44.7 Hz) ppm.
standard Schlenk or glove box techniques. The diphosphane
[15b]
PhN(CH2PPh2)2
and the complexes [RuCl2(PPh3)3],[18]
trans,cis-[RuCl2(PPh3)2(ampy)],[7]
and
cis,cis-[RuCl2(PPh3)2-
(ampy)][7] were synthesized according to literature procedures. The
solution of (CH3)2CHO–Na+ in 2-propanol used in the catalytic
trials was prepared by dissolving 46 mg of freshly cut Na into
20 mL of solvent and kept under an atmosphere of argon in the
dark. The 1H and 31P NMR spectra in solution (at 200.13 and
81.02 MHz, respectively) were recorded with a Bruker AC 200 F
QNP spectrometer. The 1H chemical shifts were referenced to
SiMe4, whereas positive 31P NMR chemical shifts are reported
downfield from 85% H3PO4 as an external standard. The NMR
characterization of solid samples was performed with a Bruker AC
200 spectrometer equipped for solid state analysis. Samples were
spun at 7000 Hz in 7 mm diameter zirconia rotors with Kel-F caps.
31P SPE MAS NMR spectra were obtained at 80.90 MHz by using
a single pulse experiment and high-power proton decoupling, with
120 s relaxing delay. 31P NMR chemical shifts were externally cal-
ibrated with respect to solid triphenylphosphane at δ = –7.2 ppm
and were referenced to 85% H3PO4. The GC analyses of the cata-
lytic mixtures were run with a Fisons GC 8000 Series gas chroma-
tograph equipped with a Supelco PTA-5 column [30 m long,
0.53 mm i.d., coated with a 3.0 µm poly(5% diphenyl-95% dimeth-
ylsiloxane) film] Injector temperature was kept at 250 °C and the
column temperature was programmed from 50 °C to 170 °C with a
gradient of 8 °Cmin–1. The elemental analyses (C, H, N) were car-
ried out at the Microanalytical Laboratory of the Dipartimento di
Scienze e Tecnologie Chimiche, Università di Udine, with a Carlo
Erba 1106 elemental analyzer. Phosphorus and ruthenium quanti-
tative determination were carried out at the Dipartimento di Sci-
enze e Tecnologie Chimiche and the Dipartimento di Scienze
Agrarie e Ambientali, Università di Udine by using a Varian Vista
MPX axial Inductively Coupled Plasma-Optical Emission spec-
trometer (for the determination of P) and a Spectro Analytical In-
struments Spectromass 2000 Type MSDIA10B Inductively Cou-
pled Plasma Mass spectrometer (for the determination of Ru).
Samples (20 mg) were digested on a microwave apparatus MILE-
STONE Mega 1200 by use of 1 mL of 65% HNO3, 0.4 mL of 30%
H2O2 and 0.1 mL of 40% HF with a mineralization program at
650 W for 20 min in Teflon vessels. Thermogravimetric analyses
were performed by using a TA Instrument (Waters) TGA Q500
apparatus. Weight loss was measured in the range 20–800 °C with
a heating rate of 10 °Cmin–1, under an argon atmosphere.
cis-[RuCl2(ampy)(ATM)] (2): As for 1, but with the use of the ATM
ligand instead of PhN(CH2PPh2)2. Sand yellow solid. Yield:
246 mg (68%). C41H53Cl2N3O3P2RuSi (897.90): calcd. C 54.84, H
5.95, N 4.68; found C 54.01, H 5.92, N 4.59. 31P{1H} NMR
(81 MHz, CD2Cl2): δ = 37.8 (d, JP,PЈ = 46.2 Hz), 45.9 (d, JP,PЈ
=
46.2 Hz) ppm.
Preparation of 3-{Bis[(diphenylphosphanyl)methyl]amino}propyl-
Functionalized Silica Gels
MA-Si-60 from Silica with 60 Å Mean Pore Size: A mixture of 3-
aminopropyl-functionalized silica gel (60 Å mean pore size) (1.00 g,
1.0 mmol of amine), paraformaldehyde (90 mg, 3.0 mmol), and di-
phenylphosphane (465 mg, 2.5 mmol) was stirred in toluene
(20 mL) and heated at reflux for 48 h. A cream white product was
recovered by filtration, washed with ethyl ether, and dried under
vacuum. Yield: 1.15 g. Composition found: C 19.90, H 2.30, N
1.21, P 4.32. TGA analysis (20–800 °C, under argon): ∆w = 24%
(water loss excluded).
MA-Si-150 from Silica with 150 Å Mean Pore Size: A mixture of
Davisil silica gel (150 Å mean pore size) (510 mg), which was acti-
vated by heating at 150 °C under vacuum for 15 h, and ATM
(302 mg) was stirred in toluene (10 mL) and heated at 110 °C for
20 h. The pale yellow mixture was cooled, filtered, washed with
toluene, dichloromethane, ethanol, and ethyl ether, and finally
dried under reduced pressure to obtain a cream white solid. Yield:
782 mg. Composition found: C 13.01, H 1.56, N 0.77, P 2.83. TGA
analysis (20–800 °C, under argon): ∆w = 14% (water loss excluded).
MA-Si-MCM-41 from MCM-41 Mesoporous Silica: A mixture of
MCM-41 mesoporous silica gel (440 mg) which was activated by
treatment at 150 °C under vacuum for 2 h, and ATM (310 mg) was
stirred in toluene (10 mL) and heated at 110 °C for 18 h. After cool-
ing, the mixture was filtered, and the white solid was washed with
toluene, dichloromethane, ethanol, and ethyl ether. The white prod-
uct was then dried under reduced pressure. Yield: 714 mg. Compo-
sition found: C 19.87, H 2.21, N 0.96, P 3.64. TGA analysis (20–
800 °C, under argon): ∆w = 20% (water loss excluded).
(3-{Bis[(diphenylphosphanyl)methyl]amino}propyl)triethoxysilane
(ATM): A mixture of (3-aminopropyl)triethoxysilane (0.89 g,
4.0 mmol), paraformaldehyde (0.30 g, 10 mmol) and diphenylphos-
phane (1.49 g, 8.0 mmol) was stirred in toluene (20 mL) and heated
at 80 °C for 1 h with complete dissolution of solid paraformalde-
hyde. The solvent was removed under vacuum. The resulting oil
was treated with ethyl ether (20 mL), the mixture was filtered, and
the product was recovered from the solution as a colorless oil by
Synthesis of Silica-Supported RuII Complexes
RuII Complex Supported on MA-Si-60 Silica (Ru-MA-Si-60): A sus-
pension of silica MA-Si-60 (100 mg) and trans,cis-[RuCl2(PPh3)2-
(ampy)] (32 mg) was heated at reflux in 2-propanol (5 mL) for 2 h.
1
elimination of the solvent under vacuum. Yield: 2.18 g (88%). H
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Eur. J. Inorg. Chem. 2007, 2909–2916

Transfer Hydrogenation Reactions Catalyzed by RuII Complexes
FULL PAPER
After cooling, the mixture was filtered, and the solid was carefully
washed with 2-propanol, dichloromethane (until the discarded sol-
vent resulted colorless), and ethyl ether to give a yellow powder,
which was dried under reduced pressure. Yield: 102 mg. Composi-
tion found: C 21.29, H 2.47, N 1.55, Ru 3.48. TGA analysis (20–
800 °C, under argon): ∆w = 20% (water loss excluded).
of the recovered catalyst to maintain a constant substrate/metal/
base ratio. Analogous procedures were adopted for successive re-
uses of the catalysts.
Acknowledgments
RuII Complex Supported on MA-Si-150 Silica (Ru-MA-Si-150): Ru-
MA-Si-150 was prepared as indicated above for Ru-MA-Si-60 from
silica MA-Si-150 (100 mg) and trans,cis-[RuCl2(PPh3)2(ampy)]
(26 mg). The isolated product is a yellow powder. Yield: 92 mg.
Composition found: C 15.38, H 1.70, N 1.10, Ru 3.22. TGA analy-
sis (20–800 °C, under argon): ∆w = 16% (water loss excluded).
The authors are indebted to Federchimica, Rome, Italy for a fellow-
ship to C. G. We thank Prof. C. de Leitenburg, Dr. F. Tubaro, and
Mr. P. Polese (Dipartimento di Scienze e Tecnologie Chimiche,
University of Udine) for carrying out TGA measurements, ICP-
MS, and elemental analyses, respectively. We also thank Dr. A.
Sassi (Dipartimento di Processi Chimici dellЈIngegneria, University
of Padua) and Dr. M. Contin (Dipartimento di Scienze Agrarie e
Ambientali, University of Udine) for performing solid state NMR
and ICP-OE measurements, respectively.
RuII Complex Supported on MA-Si-MCM-41 Silica (Ru-MA-Si-
MCM-41): Ru-MA-Si-MCM-41 was prepared as indicated above
for Ru-MA-Si-60 from functionalized mesoporous silica MA-Si-
MCM-41 (100 mg) and trans,cis-[RuCl2(PPh3)2(ampy)] (32 mg).
The product is a yellow–orange microcrystalline powder. Yield:
99 mg. Composition found: C 20.95, H 2.18, N 1.34, Ru 3.41. TGA
analysis (20–800 °C, under argon): ∆w = 20% (water loss excluded).
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(CH3)2CHO–Na+ (0.1  in 2-propanol, 2 mL). The mixture was
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mL Schlenk was charged with the silica-anchored ruthenium(II)
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(CH3)2CHONa (0.1  in 2-propanol, 0.2 mL) were added and the
mixture was heated at reflux. Finally, by addition of acetophenone
(0.5 mmol) the catalytic reaction started. Samples for the GC
analysis were prepared as indicated above.
General Procedure for Recycling of Silica-Supported Catalysts: The
mixture recovered after the first catalytic trial was centrifuged, and
the solution was discarded. The solid was washed with dichloro-
methane and 2-propanol and vacuum dried. It was then weighed
and transferred into a 25-mL Schlenk, which was charged under
an atmosphere of argon, with 2-propanol and (CH3)2CHO–Na+
(0.1  in 2-propanol). The mixture was warmed to reflux, and the
catalytic reaction was started by the addition of acetophenone.
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Eur. J. Inorg. Chem. 2007, 2909–2916
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2915

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Received: January 13, 2007
Published Online: May 4, 2007
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Eur. J. Inorg. Chem. 2007, 2909–2916