Organometallics
Article
pure silica materials at 523 K for 4 h and in the case of the hybrid
materials at 313 K for 12 h to avoid degradation of the grafted
organometallics. The Brunauer−Emmett−Teller (BET) specific
surface area was calculated from the nitrogen adsorption branch of
the isotherm in the relative pressure range of 0.07−0.15 for the pure
SiO2 materials and the hybrid materials.58 Pore size distributions (dV/
dD) were calculated from the nitrogen desorption branch using the
Barrett−Joyner−Halenda (BJH) method.59
be observed. The mixture was stirred for 15 min, and then the solvent
was removed in vacuo. The resulting crude product was recrystallized
from C6H4F2 to yield 159.8 mg (0.141 mmol, 98%) of [Ce(OiPr)4]3
1
as blocky, yellow crystals. H NMR (C6D6, 400.16 MHz, 299 K): δ
5.25 (sept, 12 H, OCHMe2, J = 6.05 Hz), 1.53 (d, 72 H, OCHMe2, J
= 6.04 Hz) ppm. 13C NMR (C6D6, 100.6 MHz, 298 K): δ 76.60
(OCHMe2), 28.12 (OCHMe2) ppm. DRIFTS: ν 2961 (vs), 2926 (s),
̃
2847 (s), 2612 (m), 1458 (m), 1358 (s), 1327 (s), 1124 (vs), 989 (s),
973 (vs), 954 (s), 836 (s), 809 (m), 528 (s), 482 (s), 444 (s) cm−1.
Anal. Calcd for C36H84Ce3O12 (1129.41 g mol−1): C, 38.29; H, 7.50.
Materials. All silica support materials were synthesized according
55,56
̈
to a modified Stober method,
adapted from Lin et al.82 The
amounts of all starting materials for all MSN-MCM-41500 materials
are given in Table 5. In a typical synthesis, CTAB was dissolved in
Found: C, 38.75; H, 7.45. Evans (C6D6, 400.16 MHz, 299 K): χmol
=
7.72 × 10−4 emu mol−1, χmolT = 0.23 emu K mol−1, μeff = 0.70 μB, Δ =
3.8 Hz, c = 0.0148 mol L−1.
[Ce(OiPr)3]n. Ce[N(SiMe3)2]3 (137.4 mg, 0.221 mmol) was
dissolved in n-hexane (5 mL), and an excess of iPrOH (10 drops,
ca. 100 mg, ca. 1.7 mmol, ca. 8 equiv) was added. The mixture was
briefly shaken, and a discoloration of the yellow solution along with
the formation of a slightly greenish white precipitate could be
observed. Solvents and liberated HN(SiMe3)2 were removed in vacuo
to yield 58.5 mg of [Ce(OiPr)3]n (0.184 mmol, 83%) as a slightly off-
Table 5. Amounts of Starting Materials Used in the
Syntheses of MSN-MCM-41500
a
MSN-MCM-41-a MSN-MCM-41-b MSN-MCM-41-c
CTAB
H2O
2.014/5.526
2.002/5.493
2.007/5.507
960.023/53.28 mol 960.01/53.27 mol 960.07/53.28 mol
NaOH (2 M) 8.0 mL/16.0
TEOS 10.014/48.1
8.0 mL/16.0
9.4/44.8
8.0 mL/16.0
9.34/44.8
white powder. DRIFTS: ν 2959 (s), 2924 (w), 2859 (w), 2817 (w),
̃
2695 (vw), 2609 (vw), 1460 (w), 1375 (w), 1357 (w), 1331 (w),
1158 (s), 1130 (s), 991 (m), 966 (vs), 824 (m), 518 (m), 487 (w),
445 (w) cm−1. Anal. Calcd for C9H21CeO3 (317.38 g mol−1): C,
34.06; H, 6.67. Found: C, 33.66; H, 6.36. 6.36. For comparison, the
routinely observed formation of the oxo cluster Ln5O(OiPr)13 would
correspond to C39H91Ce5O14 (1484.73 g mol−1): Ce, 31.55; H, 6.18.
exemplary grafting procedure, the synthesis of the hybrid material
[Ce(OiPr)4]3@MCM-41500-MSN (H1) is given here. The activated
silica material (MSN-MCM-41500-a, 400.4 mg, ca. 0.44 mmol of
SiOH) was suspended in n-hexane and the organometallic precursor
([Ce(OiPr)4]3, 159.7 mg, 0.141 mmol, 0.42 mmol of Ce), dissolved
in n-hexane, was added. After the mixture was stirred overnight, the
solid hybrid material was separated from the supernatant by
centrifugation and washed with n-hexane three times before residual
a
Listed as g/mmol if not stated otherwise.
demineralized H2O in a polypropylene (PP) bottle (Nalgene/Thermo
Fisher Scientific). After a completely homogeneous solution was
obtained, 2 M NaOH was added, the temperature was adjusted to 338
K, and the solution was stirred for ca. 1 h to achieve thermal
equilibrium. The stirring (triangular-prismatic stir bar, length ca. 4
cm) was adjusted to 500 min−1 before tetraethyl orthosilicate (TEOS)
was added dropwise during 4 min. Upon complete addition, the
solution turned white and turbid. The suspension was stirred for 1:45
h before stirring and heating were stopped, and it was cooled naturally
to ambient temperature. The solid material was separated by
centrifugation (sigma 3-30KS, sigma Laborzentrifugen GmbH) at
21000 min−1 (41415g) and washed three times each with MeOH and
H2O before being dried overnight at 333 K. Template residues were
removed by extraction in MeOH/HCl. To this end, the dried MSN-
MCM-41as‑made samples were suspended in a mixture of MeOH (160
mL) and concentrated HCl (37%, 5 mL) and heated to reflux for 24
h, before the separation and washing process was repeated. After
drying at 333 K overnight again, the resulting MSN-MCM-41extracted
sample were activated at 753 K and <1 × 10−4 mbar for 12 h to yield
solvent was removed in vacuo to yield [Ce(OiPr)4]3@MCM-41500
-
MSN (H1) as a light brown powder. The supernatant was checked for
1
remaining precursor complex by H NMR (or ICP-OES), and the
grafting experiment was deemed successful (consumption of all
accessible Si−OH groups) since there was still precursor present. The
remaining organometallic precursor complex could easily be
reclaimed by filtration and subsequent evaporation of the supernatant.
Anal. Found: Ce, 9.8 (ICP-OES) C, 10.52; H, 2.15. aBET 786 m2 g−1,
MSN-MCM-41500
.
Vpore 0.47 cm3 g−1, dpore 1.8 nm. DRIFTS: ν
̃
2969 (w), 2932 (vw),
MSN-MCM-41500-a. A 1.501 g amount (24.98 mmol, 52%) of
MSN-MCM-41500-a was obtained as a white powder. Anal. Found: C,
0.78; H, 0.44; N, 0.06. aBET 1062 m2 g−1, Vpore 0.72 cm3 g−1, dpore 2.6
2859 (vw), 1462 (vw), 1379 (vw), 1327 (vw), 1238 (m), 1065 (vs),
821 (m), 449 (s) cm−1.
nm. Surface Si−OH groups 1.11 mmol g−1. DRIFTS: ν
̃
3743 (w),
Catalysis. All operations regarding the catalytic MPV reduction
were performed under rigorous exclusion of air and moisture using
Schlenk (Table 4, entries 6 and 7) and glovebox techniques (MBraun
MB150B-G-II). All solvents and liquid chemicals were carefully
predried and degassed using appropriate methods85 and stored over
molecular sieves (3 Å) in a glovebox. To the iPrOH used as a solvent
and hydride source was added ca. 5 mg/mL of n-dodecane as an
internal standard for GC measurements. Solid chemicals that were
packed under an Ar atmosphere were stored in a glovebox and used as
received; others were degassed in vacuo and transferred into a
glovebox under an Ar atmosphere. For a typical catalytic experiment,
the catalyst was dissolved or, in the case of heterogeneous catalysts,
suspended in 5 mL of iPrOH, while the substrate, 4-tBu-cyclo-
hexanone, was dissolved in 4 mL of iPrOH. The substrate solution
was then added with stirring to the catalyst solution or suspension,
and the vessel was washed once with 1 mL of iPrOH which was also
added. GC samples of ca. 0.1 mL each were taken with a syringe at set
time intervals (usually 15 min, 1 h, 2 h, 3 h, and, where required, 24
h), and in the case of heterogeneous catalysts, the solids were
removed by centrifugation and decanting of the solution. The GC
samples were immediately measured. Where temperatures other than
ambient temperature were employed, the reactions were performed
outside the glovebox under Schlenk technique conditions in 25 mL
1238 (m), 1075 (vs), 820 (m), 448 (s) cm−1.
MSN-MCM-41500-b. A 1.640 g amount (27.28 mmol, 61%) of
MSN-MCM-41500-b was obtained as a white powder. Anal. Found: C,
1.14; H, 0.48; N, 0.22. aBET 1134 m2 g−1, Vpore 0.89 cm3 g−1, dpore 2.6
nm. Surface Si−OH groups 1.04 mmol g−1. DRIFTS: ν
̃
3743 (w),
1238 (m), 1058 (vs), 825 (m), 440 (s) cm−1.
MSN-MCM-41500-c. A 1.563 g amount (26.0 mmol, 58%) of
MSN-MCM-41500-c was obtained as a white powder. Anal. Found: C,
0.86; H, 0.41; N, 0.28. aBET 1107 m2 g−1, Vpore 0.77 cm3 g−1, dpore 2.6
nm. Surface Si−OH groups 1.11 mmol g−1. DRIFTS ν
̃
3743 (w),
1237 (m), 1070 (vs), 819 (m), 442 (s) cm−1.
Organometallic Precursors. The syntheses of Ce[N(SiMe3)2]3,
La[N(SiMe3)2]3, Ce[N(SiHMe2)2]4, and [Ce(OCH2tBu)4]3 are
described in detail elsewhere.33,35,83,84
[Ce(OiPr)4]3 (1). Ce[N(SiHMe2)2]4 (291.3 mg, 0.435 mmol) was
dissolved in n-hexane (5 mL), and iPrOH (105.5 mg, 1.755 mmol,
4.03 equiv) was equally mixed with n-hexane (5 mL). The iPrOH
solution was then added dropwise to the Ce[N(SiHMe2)2]4 solution.
Upon complete addition, a color change from deep red to translucent
yellow could be observed. Alternatively, the iPrOH was used in slight
excess (6−8 equiv) and the iPrOH/n-hexane mixture was added to
the Ce[N(SiHMe2)2]4 solution until a color change could no longer
J
Organometallics XXXX, XXX, XXX−XXX