Immobilization, Characterization, and Preliminary Reactivity Studies of Halfsandwich Ruthenium Complexes on Silica

Article abstract of DOI:10.1007/s00706-003-0049-6

The complexes [RuCp(CH₃CN)₂(Ph₂PCH ₂CH₂Si(OMe)₃)]PF₆ and [RuCp(CH ₃CN) (Ph₂PCH₂CH₂Si(OMe) ₃)₂]PF₆ were obtained in good yields by treatment of [RuCp(CH₃CN)₃]PF₆ with 1 and 2 equivs of Ph₂PCH₂CH₂Si(OMe)₃. Both free Ph₂PCH₂CH₂Si(OMe)₃ and the two complexes were grafted onto the surface of powdered silica. The surface coverage was determined independently by ³¹P solid state NMR and IR spectroscopy. IR data revealed that for Ph₂PCH₂CH ₂Si(OMe)₃ and the complexes 52, 52, and 18 molecules, respectively, were immobilized per 100 nm² of SiO₂ surface. Similar values were obtained from ³¹P MAS NMR measurements. With the immobilized first complex the catalytic redox isomerization of allyl alcohol to propanal has been studied by means of IR spectroscopy and ¹H NMR spectroscopy showing the quantitative formation of aldehyde. While in the first cycle satisfactory turnover numbers were achieved, the subsequent cycles showed only low conversions to aldehyde presumably due to decomposition of the complex. The immobilized second complex was catalytically inactive.

Full text of DOI:10.1007/s00706-003-0049-6

Monatshefte f€ur Chemie 134, 1167–1175 (2003)
DOI 10.1007/s00706-003-0049-6
Immobilization, Characterization,
and Preliminary Reactivity Studies
of Halfsandwich Ruthenium Complexes
on Silica
Christina M. Standfest-Hauser1, Thomas Lummerstorfer1,
Roland Schmid1, Karl Kirchner1;ꢀ
,
Helmuth Hoffmann1;ꢀ, and Michael Puchberger2
1
Institute of Applied Synthetic Chemistry, Vienna University of Technology,
A-1060 Vienna, Austria
2
Institute of Material Chemistry, Vienna University of Technology, A-1060 Vienna, Austria
Received February 25, 2003; accepted March 24, 2003
Published online August 18, 2003 # Springer-Verlag 2003
Summary. The complexes [RuCp(CH3CN)2(Ph2PCH2CH2Si(OMe)3)]PF6 and [RuCp(CH3CN)
(Ph2PCH2CH2Si(OMe)3)2]PF6 were obtained in good yields by treatment of [RuCp(CH3CN)3]PF6
with 1 and 2 equivs of Ph2PCH2CH2Si(OMe)3. Both free Ph2PCH2CH2Si(OMe)3 and the two
complexes were grafted onto the surface of powdered silica. The surface coverage was deter-
mined independently by 31P solid state NMR and IR spectroscopy. IR data revealed that for
Ph2PCH2CH2Si(OMe)3 and the complexes 52, 52, and 18 molecules, respectively, were immobilized
per 100 nm2 of SiO2 surface. Similar values were obtained from 31P MAS NMR measurements. With
the immobilized first complex the catalytic redox isomerization of allyl alcohol to propanal has been
studied by means of IR spectroscopy and 1H NMR spectroscopy showing the quantitative formation of
aldehyde. While in the first cycle satisfactory turnover numbers were achieved, the subsequent cycles
showed only low conversions to aldehyde presumably due to decomposition of the complex. The
immobilized second complex was catalytically inactive.
Keywords. Ruthenium complexes; Immobilization; Alkylsiloxane monolayers; Surface confined
catalysts.
Introduction
Immobilization of organometallic compounds on solid substrate surfaces offers a
number of interesting and useful applications and has been the subject of numerous
ꢀ
tuwien.ac.at (H. H.)
Corresponding authors. E-mails: kkirch@mail.zserv.tuwien.ac.at (K. K.), hhoffman@email.

1168
C. M. Standfest-Hauser et al.
studies [1]. Apart from the fundamental interest in the structure and properties of
surface-confined metal complexes and their reactivity in this ‘frozen’ state com-
pared to the conventional environment of a homogeneous solution, several tech-
nological applications such as nonlinear optical materials [2], nanolithographic
masks of organic-organometallic block copolymers [3], or covalently bonded, sup-
ported transition metal catalysts [4] have been discovered and have urged com-
bined efforts of classical organometallic research with surface chemistry and
surface analysis.
The ultimate goal of immobilizing a reactant on a solid support is to gain
control over the local concentration, the geometric orientation, and the molecular
structure of a reactive species and to be able to tune its reactivity to specific,
practical demands. One of the most successful strategies to anchor molecules on
surfaces in a controlled and uniform orientation is based on the self-assembly
process [5], where bifunctional, ꢀ,!-disubstituted hydrocarbons X–(CH2)n–Y are
adsorbed from dilute solutions via a surface anchor group –X and form highly
ordered, monomolecular films, whose outer surfaces are exclusively comprised
of the second functionality –Y [6]. The present study is part of an ongoing effort
in our group to exploit these favorable features of self-assembled adsorption for a
controlled immobilization of transition metal complexes on solid supports. We
have previously investigated a model reaction [7] of the cationic 18-electron Ru
complex [RuCp(ꢁ4–C5H4O)(L)]+, tethered to a silica surface via a link ligand
L ¼ Cl3Si–(CH2)16–Y (Y ¼ C5H4N, CN), with several tertiary phosphines and have
found that in this surface-bound configuration, even strong bases react only with
the diene ligand, whereas the metal center, which is the primary target in homo-
geneous solution, appears to be inaccessible on the surface due to the rigid,
densely-packed molecular environment. In the present study, we have chosen a
catalytically active model complex [RuCp(CH3CN)2(PR3)]+which has been shown
to catalyze the redox isomerization of allyl alcohols to aldehydes in homoge-
neous solution with high efficiency and high yields [8]. Both the basicity and the
size of the phosphine ligand PR3 were found to strongly influence the reaction
pattern. Immobilization of this complex on silica via a !-trialkoxysilyl-ligand
Ph2PCH2CH2Si(OMe)3 was therefore expected to have a similar strong effect on
the reaction rates and product distributions. Using a combination of FT-IR and
NMR techniques for solid and liquid samples, we have attempted to a) elucidate
the structure and packing density of the adsorbed catalyst, b) characterize the rate
of the allyl alcohol isomerization, and c) assess the possibility of recycling and
reusing the immobilized catalyst.
Results and Discussion
Synthesis and Immobilization of Ph2PCH2CH2Si(OMe)3
and Phosphine RuCp Complexes
The
complexes
[RuCp(CH3CN)2(Ph2PCH2CH2Si(OMe)3)]PF6
(1)
and
[RuCp(CH3CN)(Ph2PCH2CH2Si(OMe)3)2]PF6 (2) were obtained in 68 and 73%
isolated yields by treatment of [RuCp(CH3CN)3]PF6 with 1 and 2 equivs of
Ph2PCH2CH2Si(OMe)3. Characterization was accomplished by a combination of

Halfsandwich Ruthenium Complexes on Silica
1169
Scheme 1
1H, 13C{1H} NMR, IR spectroscopy, and elemental analyses. The NMR spectra of
these compounds bear no unusual features and it is sufficient to point out that in the
31P{1H} NMR spectrum of 1 and 2 signals at ꢂ ¼ 47.5 and 44.8 ppm are observed.
The ligand Ph2PCH2CH2Si(OMe)3 and complexes 1 and 2 were all grafted onto
the silica surface according to the simplified reaction Scheme 1.
The surface coverage was determined independently by 31P MAS NMR and IR
spectroscopy utilizing internal standards. FT-IR spectroscopy was used to examine
the adsorption of the metal complexes on silica (Fig. 1). Characteristic peaks of
the non-adsorbed compound are at ꢃꢀ¼ 2840 cmꢁ 1 (symmetric C–H stretch of the
Si–OMe groups) and at 2280 and 2271 cmꢁ 1 assignable to ꢃꢀ(CN) of 1 and 2. A
further sharp band occurs at 1438 cmꢁ 1 which may be assigned either to one of the
two ꢃꢀ(C–C) of the phenyl substituents or the ꢂ(C6H5)-vibration of the phosphine
moiety of the complexes [9]. These vibrations can be also detected in the spectra of
the loaded silica providing evidence for the successful anchoring of 1 and 2 on
the support material. The CH-region of the loaded silica shows no absorbance
of the SiOMe groups indicating a high extent of hydrolysis of the methoxy-
groups. Values for the surface coverage of Ph2PCH2CH2Si(OMe)3, complexes 1
and 2 are 52 (8.63ꢂ10ꢁ 4 mmol=m2), 52 (8.63ꢂ10ꢁ 4 mmol=m2), and 18 molecules
(2.99ꢂ10ꢁ 4 mmol=m2) per 100 nm2 of SiO2 surface. Comparable values were
obtained from single pulse 31P solid state NMR spectroscopy revealing that 39,
66, and 13 molecules of Ph2PCH2CH2Si(OMe)3 and complexes 1 and 2 were
immobilized per 100 nm2 of SiO2 surface. The 31P-CP=MAS NMR spectra of free
and immobilized 1 and 2 are shown in Fig. 2. These data correspond well to the
solution chemical shifts (see Exp.).

1170
C. M. Standfest-Hauser et al.
Fig. 1. IR spectra of 1 and 2 in the solid reference state (bottom) and adsorbed on silica (top); the
hatched bands represent SiO2 vibrations
The following conclusions can be drawn from these results: a) The surface
coverage is determined primarily by the size of the phosphine ligand, as indicated
by the fact that the coverages of the isolated ligand Ph2PCH2CH2Si(OMe)3 and
1 are essentially the same. Complex 2, on the other hand, yields a coverage of
only about 30% of complex 1. b) The surface coverage of unsubstituted, long-
chain hydrocarbon films obtained by adsorption of octadecyltrimethoxysilane
CH3(CH2)17Si(OMe)3, for example, is on the order of 500 molecules per
100 nm2 of silica surface [10], which is about one order of magnitude higher than
the phosphine-substituted compounds in this study. Thus, the larger cone angle of
the phosphine group in diphenylalkylphosphines Ph2PR appears to prevent a dense
packing and interlocking of the alkyl groups as in unsubstituted hydrocarbon films.
However, the coverages achieved here are still about a factor of 5–10 higher than
literature results with comparable complexes such as Ph2PCH2CH2Si(OMe)3
Ru(CO)4 [9] or Ph2PCH2CH2Si(OEt)3Rh(CO)Cl [4c].
Redoxisomerization of Allyl Alcohol to Propanal
Catalyzed by Immobilized 1
Recently Kulawiec and Trost have shown [11] that the isomerization of allyl alco-
hols to the corresponding aldehyde is catalyzed by RuCp(PPh3)2Cl in the presence

Halfsandwich Ruthenium Complexes on Silica
1171
Fig. 2. 31P-CP=MAS NMR spectra of 1 and 2 in the solid reference state (bottom) and adsorbed on
silica (top)
of NH4PF6. Since the catalyst is substitutionally inert, high temperatures (typically
100ꢃC in dioxane as the solvent) and high concentrations of both a chloride
scavenger (10 mol%) as well as the catalyst (5 mol%) have been used. We have
improved this process by utilizing the cationic complex [RuCp(PR3)(CH3CN)2]+
(R ¼ Me, Ph, Cy) [8]. Here we investigate the isomerization of allyl alcohol to
propanal catalyzed by immobilized 1 and 2 by means of IR spectroscopy and
1H NMR spectroscopy.
Treatment of immobilized 1 (2 mg, 2.05 mol% of pre-catalyst) with allyl alco-
hol in fluorobenzene at 60ꢃC affords quantitatively propanal as monitored in situ by
IR spectroscopy (Fig. 3). The decreasing OH-peak of the allyl alcohol and the
increasing peak at ꢃꢀ¼ 2714 cmꢁ 1 (one of the two CH-peaks of the CHO-group
of the emerging aldehyde) were periodically monitored by IR spectroscopy. The
ꢃꢀ(CO) stretching frequency of propanal could not be used for monitoring the
isomerization process since this signal was superimposed by the intensive Si–O
stretching frequencies of SiO2 (see Fig. 1, hatched bands). After each reaction
cycle, the reactant solution was removed by syringe. The catalyst was dried and
recovered by purging N2 through the cell for 15 minutes. As shown in Fig. 3, only
the first cycle exhibits satisfactory turnover numbers of allyl alcohol. The second
and third cycles show only low conversions to aldehyde while a decrease of the

1172
C. M. Standfest-Hauser et al.
Fig. 3. Band intensities of ꢃꢀðOHÞ at 3620 cmꢁ 1 (bottom) and of ꢃꢀðCHÞ at 2714 cmꢁ 1 (top) for the
isomerization of allyl alcohol to propanal as a function of reaction time; the labels 1, 2, and 3
represent three consecutive reaction cycles with freshly prepared reactant solution in contact with the
recovered catalyst
OH-band of allyl alcohol is still observed. A side reaction of allyl alcohol with the
substrate SiO2 could be excluded from measurements of pure SiO2 (without cat-
alyst) in contact with allyl alcohol in fluorobenzene, which did not show any
spectral changes with time. The adsorbed catalyst after 3 reaction cycles showed
the same intensity for the ꢃꢀ(C–C)=ꢂ(C6H5)-vibration at ꢃꢀ¼ 1438 cmꢁ 1 as the
initial, freshly adsorbed catalyst. Likewise, the 31P-CP=MAS NMR spectrum of
the recycled catalyst was identical with that of freshly immobilized 1. Thus, leach-
ing of the catalyst appears to be unlikely but cannot be conclusively ruled out. The
ꢃꢀ(CN) peak at ꢃꢀ¼ 2280 cmꢁ 1, however, disappeared after the first reaction cycle
suggesting that the catalyst may be converted into another species, which no longer
catalyzes the reduction of allyl alcohol to propanal, but still converts allyl alcohol
to some unknown product concomitant with loss of the hydroxyl group. The reason
for this behavior, however, remains unclear.
The same reaction has been also performed with immobilized 1 (80 mg,
0.15 mol% of pre-catalyst) in neat allyl alcohol (1 cm3) at 70ꢃC for 24 h. The
formation of propanal was monitored by 1H NMR spectroscopy revealing that after
30 min, 3, 6, and 24 h approximately 2, 18, 46, and 100% of propanal were formed.
There was no evidence for other side products. The catalyst was recovered by
removal of the solvent, drying under vacuum, and washing with n-pentane and
the next catalytic cycle was performed under the same reaction conditions. How-
ever, 1H NMR spectroscopy showed that only 8% of allyl alcohol was converted to
propanal. In subsequent cycles the solid sample exhibited no longer any catalytic
activity.

Halfsandwich Ruthenium Complexes on Silica
1173
Finally, it has to be noted that immobilized 2 was inactive for the redoxisome-
rization of allyl alcohol and no propanal could be detected.
Conclusion
In the present contribution mono- and bisphosphine complexes of the types
[RuCp(CH3CN)2(Ph2PCH2CH2Si(OMe)3)]PF6 (1) and [RuCp(CH3CN)(Ph2PCH2
CH2Si(OMe)3)2]PF6 (2) were prepared and subsequently immobilized on SiO2. This
was achieved under mild conditions with a high surface coverage. The structure and
packing density of the adsorbed complexes could be determined by two independent
methods viz 31P MAS NMR and IR spectroscopy. Immobilized 1 and 2 and were
tested as catalysts for the redoxisomerization of allyl alcohol to propanal. Whereas
with 1 quantitative conversion was achieved in the first run, decomposition of the
catalyst occurred in subsequent reaction cycles with concomitant loss of catalytic
activity. In contrast to immobilized 1, complex 2 was catalytically inactive.
Experimental
General
All manipulations were performed under an inert atmosphere of N2 or Ar by using Schlenk techniques
and a glove box. All chemicals were standard reagent grade and used without further purification. The
solvents were purified according to standard procedures [12]. The deuterated solvents were dried over
˚
4 A molecular sieves. [RuCp(CH3CN)3]PF6 and Ph2PCH2CH2Si(OMe)3 were prepared according
to Refs. [13, 14]. All experiments were carried out with Merck silica (specific surface area
388 ꢄ 2 m2=g). The silica was treated in an UV-cleaning chamber prior to use to remove adsorbed
organic impurities but was not dried at elevated temperatures. The complexes gave satisfactory C
and H elemental analyses.
Liquid-State NMR and IR Spectroscopy
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker Avance-250 spectrometer operat-
ing at 250.13, 62.86, and 101.26 MHz and were referenced to SiMe4 and H3PO4 (85%). Infrared
spectra were recorded on Mattson RS-10000 and Perkin-Elmer 16PC FT-IR spectrometers.
Solid-State NMR Spectroscopy
All 31P NMR spectra were recorded on a Bruker Avance 300 spectrometer (standard bore), equipped
with a 4 mm broad-band MAS probehead and ZrO2 rotors. The rotational speed for all experiments
was 12 kHz. Phosphorous spectra were referenced with respect to 85% H3PO4 (aq) by setting the
31P NMR peak of solid PPh3 to ꢁ 7.0 ppm. 31P-CP=MAS spectra (cross polarization with magic angle
spinning) were measured with a contact time of 3 ms and a relaxation delay of 6 s. Quantitative
measurements were done by 31P single pulse experiments with a relaxation delay time of 120 s.
Synthesis of [RuCp(CH3CN)2(Ph2PCH2CH2Si(OMe)3)]PF6 (1)
A solution of [RuCp(CH3CN)3]PF6 (0.100g, 0.230mmol) and 1 equiv of Ph2PCH2CH2Si(OMe)3
(0.077 g, 0.230 mmol) in 5 cm3 of CH2Cl2 was stirred for 2 h at room temperature. Upon removal
of the solvent under reduced pressure, a yellow compound was obtained which was collected on a glass
frit, washed with petroleum ether, and dried under vacuum. Yield 0.109 g (68%); 1H NMR (acetone-d6,

1174
C. M. Standfest-Hauser et al.
20ꢃC): ꢂ ¼ 7.66–7.40 (m, 10H, PPh2), 4.53 (s, 5H, Cp), 3.53 (s, 9H, Si(OMe)3), 2.70–2.55 (m,
2H, PCH2CH2), 2.38 (d, JPH ¼ 1.42Hz, 6 H, NC–CH3), 0.71–0.53 (m, 2H, PCH2CH2); 13C{1H}
NMR (acetone-d6, 20ꢃC): ꢂ ¼ 132.2 (d, JPC ¼ 40.4Hz, Ph1), 132.9 (d, JPC ¼ 10.1 Hz, Ph2, 6), 130.4
1
(d, JPC ¼ 2.8 Hz, Ph4), 128.9 (d, JPC ¼ 9.7 Hz, Ph3, 5), 128.6 (s, NC–CH3), 22.9 (d, JPC ¼ 22.9Hz,
2
PCH2CH2), 76.5 (d, JCP ¼ 1.84Hz, Cp), 50.4 (d, JPC ¼ 1.8 Hz, Si(OMe)3), 4.00 (d, JPC ¼ 2.76 Hz,
PCH2CH2), 3.2 (d, JPC ¼ 0.9 Hz, NC–CH3); 31P{1H} NMR (acetone-d6, 20ꢃC): ꢂ ¼ 47.5
(PPh2), ꢁ 146.5 (1JPF ¼ 707.9 Hz, PF6); IR (KBr): ꢃꢀ¼ 2280cmꢁ 1 (CN).
Synthesis of [RuCp(CH3CN)(Ph2PCH2CH2Si(OMe)3)2]PF6 (2)
This complex has been obtained analogously to 1 utilizing 0.1 g of [RuCp(CH3CN)3]PF6 (0.230 mmol)
1
and 0.154 g of Ph2PCH2CH2Si(OMe)3 (0.460mmol). Yield 0.173g (73%); H NMR (acetone-d6,
20ꢃC): ꢂ ¼ 7.61–7.21 (m, 20H, PPh2), 4.76 (s, 5H, Cp), 3.47 (s, 18H, Si(OMe)3), 2.53 (s, 3H, NC–
CH3), 2.28–2.09, 1.86–1.76 (m, 4H, PCH2CH2), 0.45–0.26 (m, 4H, PCH2CH2); 13C{1H} NMR
(acetone-d6, 20ꢃC): ꢂ ¼ 137.1 (m, Ph), 134.5 (m, Ph), 132.8 (m, Ph), 130.6 (s, Ph), 130.2 (s, Ph),
129.6 (s, NC–CH3), 128.9 (m, Ph), 82.3 (d, JPC ¼ 0.5 Hz, Cp), 50.4 (s, Si(OMe)3), 21.9 (t,
2
1JPC ¼ 11.3Hz, PCH2CH2), 4.6 (t, JPC ¼ 3.2 Hz, PCH2CH2), 3.8 (s, NC–CH3); 31P{1H} NMR
(acetone-d6, 20ꢃC): ꢂ ¼ 44.8 (PPh2), ꢁ 143.0 (1JPF ¼ 707.7 Hz, PF6); IR (KBr): ꢃꢀ¼ 2271 cmꢁ 1 (CN).
Immobilization of Ph2PCH2CH2Si(OMe)3, 1, and 2
The phosphine ligand Ph2PCH2CH2Si(OMe)3 as well as complexes 1 and 2 were immobilized by
rigorously stirring a suspension of SiO2 and an appropriate amount of ligand or complex (typically
280 mmol of substrate per 250 mg of SiO2) in 20cm3 of toluene under dry N2 for 24h at room
temperature. The solid was separated by centrifuging. Upon removal of the supernatant, the silica
was washed several times with dry CH2Cl2 and again centrifuged. The samples were dried under
vacuum at room temperature for 24 h and characterized by means of 31P-CP=MAS NMR: signals at
ꢁ 14.7, 49.1, and 39.9 ppm for immobilized Ph2PCH2CH2Si(OMe)3, 1, and 2.
The surface coverage was determined by two independent methods. Method A. An IR-calibration
curve was generated by mixing weighted amounts of the respective complex and silica in order to
simulate silica with different surface coverages. IR-spectra were then obtained by mixing these
powders with KBr and pressing them into disks containig 5.6 w=w% of the calibration mixture. The
ꢂ(C6H5)-vibration of the phosphine group of the complex was integrated and the peak area as a
function of surface coverage was determined. The liquid phosphine ligand Ph2PCH2CH2Si(OMe)3
was polymerised in 5 w=w% HCl, filtered off, and dried in vacuo prior to use. Values for the surface
coverage for Ph2PCH2CH2Si(OMe)3, 1, and 2 are 52molecules=100 nm2 (8.63ꢂ10ꢁ 4 mmol=m2),
52molecules=100 nm2 (8.63ꢂ10ꢁ 4 mmol=m2), and 18 molecules=100 nm2 (2.99ꢂ10ꢁ 4 mmol=m2 ).
Method B. The surface coverage was determined by means of 31P MAS NMR spectroscopy
integrating the 31P signals of mixtures of the surface-immobilized compounds together with a weighted
amount of ½NBun4ꢅPF6 for the free ligand and Ph3P ¼ O for complexes 1 and 2 as a standard utilizing
single pulse experiments with a pulse delay of 120s. The integral of the 31P signal of PF6ꢁ counterion in
1 and 2 has been set to unity. Values for the surface coverage for Ph2PCH2CH2Si(OMe)3, 1, and 2 are
39molecules=100 nm2 (6.43ꢂ10ꢁ 4 mmol=m2), 66molecules=100 nm2 (11.00ꢂ10ꢁ 4 mmol=m2), and
13molecules=100 nm2 (2.18ꢂ10ꢁ 4 mmol=m2).
Redoxisomerization of Allyl Alcohol to Propanal Catalyzed by Immobilized 1
Method A. Allyl alcohol was dissolved in fluorobenzene (1v=v% solution) to provide IR-transmittance
of the sample. The reaction was monitored between CaF2-plates separated by a 0.2 mm teflon spacer.
Upon addition of 2 mg of immobilized 1 (corresponds to ca. 2.05 mol% of pre-catalyst) and 180 mm3
of allyl alcohol in fluorobenzene a start spectrum was recorded. The reactant solution was heated to

Halfsandwich Ruthenium Complexes on Silica
1175
60ꢃC and additional spectra were recorded after defined intervals. All spectra were referenced against
neat fluorobenzene. The changes in the IR spectra were used to monitor the conversion of allyl alcohol
to propanal by integration of the decreasing OH-peak of allyl alcohol and the increasing peak at
ꢃꢀ¼ 2714 cmꢁ 1 (one of the two CH-peaks of the CHO-group of the emerging aldehyde). After each
reaction cycle, the reactant solution was removed by syringe. The catalyst was dried and recovered by
purging N2 through the cell for 15 minutes. The cell was refilled with fresh dilute solution of allyl
alcohol and the next catalytic cycle was performed.
Method B. A suspension of 80mg of immobilized 1 (corresponds to ca. 0.15mol% of pre-catalyst)
1
in 1 cm3 of neat allyl alcohol was stirred in a Schlenk tube at 70ꢃC for 24h. After that a H NMR
spectrum was recorded revealing the quantitative formation of propanal. The catalyst was recovered by
removal of the solvent, drying under vacuum, and washing with n-pentane and the next catalytic cycle
1
was performed. H NMR spectroscopy showed that only 8% of allyl alcohol had been converted to
propanal. In subsequent cycles the solid sample exhibited no longer catalytic activity. Under the same
reaction conditions, immobilized 2 was catalytically inactive and no propanal could be detected even
after prolonged reaction times.
Acknowledgements
€
Financial support by the ‘‘Fonds zur Forderung der wissenschaftlichen Forschung’’ is gratefully
acknowledged (Projects No. P14681-CHE and P12769-CHE).
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