Synthesis and grafting of CAN-derived tetravalent cerium alkoxide silylamide precursors onto mesoporous silica MCM-41

Article abstract of DOI:10.1039/c3dt33005b

The heteroleptic tetravalent cerium complex [Ce(OiPr) ₃{N(SiMe₃)₂}]₂ was synthesised by treating ceric ammonium nitrate (CAN) sequentially with sodium isopropoxide and lithium bis(trimethylsilyl)amide in THF. The trivalent ate complex [Ce(OiPr)₂{N(SiMe₃)₂]₂}][Li(thf) ₂] was also isolated from these reaction mixtures. A transsilylamination reaction of [Ce(OiPr)₃{N(SiMe₃) ₂}]₂ with tetramethyldisilazane produced a considerable amount of homoleptic Ce[N(SiHMe₂)₂]₄. The polymeric complex [Li₂Ce₂(OiPr)₁₀(1,4-dioxane)] _n was isolated as an additional ligand redistribution product. When tetravalent complexes Ce[N(SiHMe₂)₂]₄, Ce[N(SiMe₃)₂]₃Cl and Cp₃CeCl were allowed to react with samples of periodic mesoporous silica MCM-41, Ce(iv) hybrid materials were produced. All hybrid materials were characterised via N₂ physisorption, elemental analysis and DRIFT spectroscopy.

Full text of DOI:10.1039/c3dt33005b

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Synthesis and grafting of CAN-derived tetravalent
cerium alkoxide silylamide precursors onto
Cite this: Dalton Trans., 2013, 42, 5491
mesoporous silica MCM-41†
a
b
b
a
Alan R. Crozier, Christoph Schädle, Cäcilia Maichle-Mössmer, Karl W. Törnroos
b
and Reiner Anwander*
The heteroleptic tetravalent cerium complex [Ce(OiPr)
ammonium nitrate (CAN) sequentially with sodium isopropoxide and lithium bis(trimethylsilyl)amide in
THF. The trivalent ate complex [Ce(OiPr) {N(SiMe }][Li(thf) ] was also isolated from these reaction mix-
tures. A transsilylamination reaction of [Ce(OiPr) {N(SiMe }] with tetramethyldisilazane produced a
considerable amount of homoleptic Ce[N(SiHMe . The polymeric complex [Li Ce (OiPr)10(1,4-dioxane)]
was isolated as an additional ligand redistribution product. When tetravalent complexes Ce[N(SiHMe
Ce[N(SiMe Cl and Cp CeCl were allowed to react with samples of periodic mesoporous silica MCM-41,
3 3 2 2
{N(SiMe ) }] was synthesised by treating ceric
2
3
)
2
]
2
2
3
3 2 2
)
2
)
2
]
4
2
2
n
Received 14th December 2012,
Accepted 24th January 2013
2 2 4
) ] ,
) ]
3 2 3
3
DOI: 10.1039/c3dt33005b
2
Ce(IV) hybrid materials were produced. All hybrid materials were characterised via N physisorption,
www.rsc.org/dalton
elemental analysis and DRIFT spectroscopy.
suitable as grafting precursors – organometallics, (silyl)-
amides, and alkoxides. Examples of tetravalent alkoxides
Introduction
8
Tetravalent cerium compounds such as ceric ammonium sul- dominate the literature due to the oxophilic nature of the
2
–4,9
phate or nitrate (CAN) are well-known single-electron oxidants cerium metal centre.
While the number of Ce(IV) (silyl)-
1
for many organic transformations. Cerium is a unique amide complexes has increased in the past 15 years, develop-
member of the rare-earth metals in its ability to exist in a high- ments have been hampered by the choice of an oxidant
valent state in solution. The Ce ion is isoelectronic to xenon intractable reaction pathways. Organometallic complexes are
with a closed-shell 4f configuration, yet pathways to generate even fewer, with the oxidation state of cerocene and its deriva-
tetravalent cerium complexes remain challenging. Recent tives still under investigation, the range of complexes are
research areas of tetravalent cerium complexes include limited to N-heterocyclic carbene (NHC)
It is of interest to note that when
lactide), electrochemistry, oxo and peroxo complexes, NHC approaching an (uSiO) Ce (NR ) (x + y = 4) surface species,
6
,10
or
4
+
11
0
1
2
6
,13
5
and η -cyclopenta-
2
10d,14
(
MO)CVD and ALD, polymerisation reactions (ring-opening of dienyl ligands.
3
4
5
IV
x
2 y
6
7
complexes and cationic species. Soluble and reactive such mixed alkoxo(siloxo)/amido ligand sets provide a stabili-
pseudo)organometallic complexes are desirable in the study sing environment for tetravalent cerium (Chart 1).
6
,15
(
of silica-based surface organometallic chemistry. When target-
In this report, the synthesis of the mixed alkoxide/silyl-
{N(SiMe from ceric ammonium
least one readily hydrolysable Ce–X bond, assuming the nitrate is illustrated and its use for an alternative synthesis
ing a Ce(IV) grafted silica, chemical precursors must contain at amide complex [Ce(OiPr)
3
3 2 2 2
) } ]
8
16
surface of a dehydrated silica has a pK value of 5–7. With of the homoleptic silylamide complex Ce[N(SiHMe2)2]4.
a
this in mind, three complex categories appear synthetically Moreover, three Ce(IV) hybrid materials have been produced
through the grafting of tetravalent precursors Ce[N(SiHMe ) ] ,
2
2 4
Ce[N(SiMe ) ] Cl and Cp CeCl onto periodic mesoporous silica
MCM-41 and the surface species are discussed.
3
2 3
3
a
Department of Chemistry, University of Bergen, Allégaten 41, N-5007 Bergen,
Norway
b
Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der
Morgenstelle 18, D-72076 Tübingen, Germany.
E-mail: reiner.anwander@uni-tuebingen.de; Fax: +49 7071 292436
Results and discussion
†
Electronic supplementary information (ESI) available: Experimental data,
powder X-ray diffraction pattern, pore size distribution and transmission elec- Cerium silylamide synthesis
tron microscopy image of parent MCM-41 silica, DRIFT spectra of all materials
in the region 400–4000 cm−1. CCDC 915627, 915628 and 915629. For ESI
and crystallographic data in CIF or other electronic format see DOI:
The choice of ceric ammonium nitrate (CAN) as a starting
material for the generation of soluble Ce(IV) precursors has
10.1039/c3dt33005b
several advantages – (i) the desired oxidation state is already
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Fig. 1 Molecular structure of [Ce(OiPr)
All hydrogen atoms have been omitted for clarity. Selected interatomic distances
Å) and angles (°). Ce1–O1 2.058(5), Ce1–O2 2.325(5), Ce1–O3 2.061(5), Ce1–
3 3 2 2
{N(SiMe ) }] 1 (ellipsoids set to 50%).
(
Chart 1 Mixed silylamide/alkoxide Ce(IV) complexes, structurally authenticated
N1 2.289(6), Ce⋯Ce 3.892(7); O3–Ce–O1 95.7(2), O3–Ce–N1 101.5(2), O2–Ce1–
O2’ 67.9(2), Ce1–N1–Si1 112.9(3), Ce1–N1–Si2 122.3(3).
by X-ray crystallography along with the synthesis protocols (N = N(SiMe
N(SiMe )(C OMe-2); N’’ = N(SiMe )(C iPr -2,6)).
3 2
) ; N’ =
3
6
H
4
3
6
H
3
2
0
.4 ppm (singlet; SiMe
3
), 1.3 ppm (doublet; OCHMe
2
) and
5
.1 ppm (septet; OCHMe
2
), indicating the presence of two
species – one diamagnetic and one paramagnetic (see ESI†).
Redissolving this solid in hexane and cooling to −35 °C
afforded red crystals in 40% yield. An X-ray diffraction experi-
ment was undertaken and the molecular structure of 1 is
shown in Fig. 1.
Complex 1 is a tetravalent dinuclear isopropoxide/hexa-
methyl silylamide complex with the cerium atoms adopting a
geometry intermediate to square pyramidal and trigonal bipyra-
midal. Each cerium atom is bridged by an alkoxide ligand
forming a four-membered ring with cerium–oxygen distances
of 2.369(5) and 2.325(5) Å and angles of 67.9(2) and 112.0(2)°
at the cerium and oxygen atoms, respectively. The terminal
alkoxide Ce–O (avg. 2.059 Å) and bridging alkoxide (avg.
Scheme 1 Reaction pathway to complexes 1–4.
2.347 Å) bond lengths compare well with other Ce(IV)
achieved, (ii) the reactions of CAN with many alkali metal alkoxide complexes, such as mixed-valent [Ce
3
(OtBu)10NO
3
]
2
,9h,14b,17,18
15,19
(
pseudo)organometallics have been described,
(terminal: 2.062(4)–2.069(4) Å; μ : 2.261(4)–2.519(4) Å),
2
however, if organometallic reactants such as NaCp are used 5-coordinate [Ce(OCMe
reduction occurs,
2
iPr)
(iii) the separation of by-products is 2.468(3) Å), 7-coordinate [Ce
uncomplicated as both ammonia and insoluble nitrate salts (terminal: 2.122(4)–2.137(5) Å; μ : 2.328((4) and 2.411(4) Å)
4
] (terminal: avg. 2.085; μ
2
: 2.291(2)–
NMe ) ]
2 2
1
7
9i
(OiPr) (OC NMeC
H
H
2
6
2
4
2
4
9
e
2
1
5
3 2 2 2
are formed, (iv) other than an initial dehydration step, it is and 4-coordinate [Ce{N(SiMe ) } (OtBu) ] (2.052(6) Å). The
possible to use CAN in an ‘off the shelf’ manner unlike many Ce–N distance of 2.289(6) Å was found to be considerably
metal chlorides which require donor solvation. When CAN was longer than in the 5-coordinate complex [Ce{N(CH CH -
2
2
1
0a
reacted with 5 equivalents of NaOiPr in THF a white precipitate NSiMe
and bubbles of gas were observed (Scheme 1). Removing the plexes [Ce{N(SiMe
volatiles via vacuum and performing a toluene extraction gave (X = Cl, Br; avg. 2.218 Å)
2 3
tBu) }I] (2.202 Å) as well as in the 4-coordinate com-
1
5
) } (OtBu) ] (2.260(7) Å), [Ce{N(SiMe ) }
X]
and [Ce(NCy ) ] (Cy = cyclohexyl;
3 2 2 2 3 2 3
1
0b,c
2
4
1
1a
a yellow-orange solution, which after removing the volatiles 2.247 Å),
once more yielded a sticky yellow-orange solid presumably [Ce{p-MeOC
Ce(OiPr) (NO )(thf) ]. Without further characterisation this The trivalent ate complex 2 was isolated as colourless crys-
solid was redissolved in THF and to this one equivalent of LiN tals in low yield from solutions of 1 when left for periods of
SiMe
was added. A further white precipitate and a colour over two weeks at −35 °C. The complex could also be achieved
change from orange-yellow to red were observed. Filtering and by using an excess of LiOiPr in the reaction mixture. The mole-
drying under vacuum gave a red solid. cular structure of 2 is shown in Fig. 2 with both cerium and
of this residue gave sharp lithium metal centres in a distorted tetrahedral geometry. The
however, shorter than in the amidinate complex
10d
H C(NSiMe ) } Cl] (avg. 2.432 Å).
4 3 2 3
6
[
3
3
2
(
3 2
)
1
6 6
H NMR spectroscopy in C D
peaks at 0.5 ppm (singlet; SiMe ), 1.2 ppm (doublet; OCHMe ) cerium atom is four-coordinate with two terminal hexamethyl
3
2
and at 4.8 ppm (septet; OCHMe
2
), and broader peaks at silylamido ligands and two isopropoxy ligands bridging to a
5
492 | Dalton Trans., 2013, 42, 5491–5499
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lithium atom with two coordinated THF molecules. The ate complexes appear uncommon. Only one other complex
central four-membered ring has average Ce–O and Li–O bond [Ce{N(SitBuMe )(2-C H N-6-Me)} (µ-OtBu) Na(tmeda)] was re-
2
5
3
2
2
2
4
lengths of 2.252 and 1.957 Å and bond angles of 78.2 and ported.
9
Other amido-containing ate complexes such
1
1a
3.1°, respectively. The Ce–N bond length (avg. 2.408(3) Å) lies as [Ce[NCy
2
5
]
4
Li(thf)]
and [Ce{(µ-N(H)(CH
2 2 6 4
tBu)} C H -1,2)-
2
well within the range of other relevant trivalent cerium amide Na (OEt )] share similar cerium alkali metal separation
3
2
2
0
complexes [Ce{N(SiMe
(N(SiMe )C(C tBu-4))
considerably longer than in homoleptic 3-coordinate [Ce-
3
)
2
}
4
][Na(thf)
4
(OEt
) }
3 2 2
2
)] (2.440 Å),
[Ce- distances with 2 (Ce⋯Li = 3.089(6), 3.077(5) and 3.442(3) Å,
] (2.453 Å), being respectively).
When a trans(silyl)amination reaction with a slight excess
] (2.323 Å). While 2 of HN(SiHMe was attempted on 1 in hexane, only an oily red
2
1
{
3
6
H
4
2
CH}{N(SiMe
2
2
23
{
N(SiMe
3
)
2
}
3
] (2.320 Å) and [Ce(TMP)
3
2 2
)
can be considered an undesirable by-product with 1, it is not product was obtained after removing the volatiles. The FTIR
without interest as trivalent cerium alkoxide/(silyl)amide data for this oil showed evidence of amido ligand exchange
with the characteristic Si–H peak close to 2000 cm− . Redissol-
1
ving this oil in hexane and adding 2 equivalents of 1,4-dioxane
caused an immediate colourless precipitate to form. After
filtering this precipitate and removing the volatiles from the
filtrate a red solid was isolated. The Schlenk 1,4-dioxane dis-
2
6,27
proportionation/precipitation method
seems to apply also
in the case of Ce(IV), presumably co-producing insoluble poly-
meric alkoxides. The FTIR spectrum for this red solid also
showed a peak at 2000 cm− indicating the retention of the
1
1
tetramethyl silylamido ligand. H NMR spectroscopy showed
a doublet at δ 0.34 and a septet at δ 6.01 with a ratio of 6 : 1
as expected for the N(SiHMe ) ligand. Performing a recrystalli-
2
2
sation on this red solid from a cooled hexane solution
produced red single crystals. An X-ray structure analysis con-
firmed the formation of Ce[N(SiHMe ) ] (3) which was pre-
2
2 4
viously synthesised via an oxidation/redistribution sequence
employing Ce[N(SiHMe ](thf) and chlorinating reagents
such as Ph CCl. When stored for longer times colourless
2
)
2
2
1
6
2 3 2 2 2
Fig. 2 Molecular structure of [Ce(OiPr) {N(SiMe ) } ][Li(thf) ] 2 (ellipsoids set
3
to 30%). All hydrogen atoms have been omitted for clarity. Selected interatomic
2 2 n
crystals of [Li Ce (OiPr)10(1,4-dioxane)] (4) were isolated and
distances (Å) and angles (°). Ce1–N1 2.408(3), Ce1–N2 2.408(3), Ce1–O1
characterised by X-ray crystallography. The solid-state structure
of 4 (Fig. 3) shows a tetravalent polymeric isopropoxide ate
complex consisting of two 6-coordinate anionic cerium entities
2
.253(2), Ce1–O2 2.250(2), Li1–O1 1.962(7), Li1–O2 1.953(8), Li1–O3, 1.962(7),
Li1–O4 1.970(8), Ce1⋯Li1 3.089(6); O1–Ce1–O2 78.22(9), Ce1–N1–Si1 121.0(1),
Ce1–N1–Si2 114.3(1), Ce1–N2–Si3 119.9(1), Ce1–N2–Si4 113.8(1).
Fig. 3 Molecular structure of [Li
2 2 n
Ce (OiPr)10(1,4-dioxane)] (4) (ellipsoids set to 30%). All hydrogen atoms and peripheral methyl groups as well as the disorder
of the dioxane molecule have been omitted for clarity. Selected interatomic distances (Å) and angles (°). Ce–O1 2.440(4), Ce–O2 2.257(4), Ce–O3 2.095(4),
#
1
Ce–O4 2.080(4), Ce–O5 2.234(4), Li–O1 2.098(12), Li–O5 1.899(12), Li–O6 2.066(11), Li⋯Li’’ 5.117(4); Ce–O1–Li 91.0(3), Ce–Li–O6 119.2(4), O1–Ce–O3 90.93(15).
#
1
=
−x + 1/2, −y + 1/2, −z + 1/2.
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= 1040 m g−1, d
2
= 3.3 nm
charge-balanced with two 4-coordinate lithium cations. A dis- has the following characteristics: a
s
p
ordered 1,4-dioxane molecule bridges between two lithium and SiOHpopulation = 2.9 mmol g−
1 47
.
atoms accomplishing
cerium centres adopt a distorted octahedral arrangement Cp
with O–Ce–O isopropoxy bond angles ranging from 71.2(2) to material MCM-41, peach Ce[N(SiHMe ) ] @MCM-41 (5),
a
polymeric chain structure. The
When solutions of Ce(IV) precursors 3, Ce[N(SiMe
3 2 3
) ] Cl and
3
CeCl were added to suspended samples of the parent
2
2 4
1
02.3(2)°. There are three different types of isopropoxy ligands brown Ce[N(SiMe
per cerium: two terminal, two μ -bridging (to lithium), two Cp CeCl@MCM-41 (7) hybrid materials were obtained, respect-
μ3-bridging (to lithium and the other cerium). As a conse- ively (Fig. 4).
3 2 3
) ] Cl@MCM-41 (6) and dark grey
2
3
quence, distinct 4-membered metal–oxygen rings are featured
The DRIFT spectra of the hybrid materials 5 and 6 showed
in this structure. The Ce–O1–Li–O5 ring shows Ce–O and Li–O complete consumption of the surface silanol groups with the
bond lengths of avg. 2.327 and 1.999 Å and bond angles of absence of a peak at 3747 cm− found in the parent material
1
−
1
7
5.44 and 90.9°, respectively. The two terminal Ce–O distances (Fig. 5). The v(C–H) band between 3043 and 2824 cm indi-
average 2.087 Å corresponding to the terminal Ce–OR alkoxy cates the presence of the methyl groups from the silylamido
9
d
groups in [Ce (OiPr) (HOiPr) ] . The 1,4-dioxane molecule ligands. The use of the tetramethyl silylamido ligand and its
2
8
2 n
bridges a Li⋯Li distance of 6.82(2) Å. Only one structurally usefulness as a spectroscopic probe can be seen in the spec-
−
1
authenticated trivalent example of a 1,4-dioxane-bridged trum of material 5. The v(Si–H) peak centred at 2116 cm
3
28
−1
cerium compound is known ([Ce(η -C H ) (1,4-dioxane)] )
differs from its precursor with an increase of 100 cm . The
reaction of the cyclopentadienyl complex Cp CeCl with the
silica support did not lead to complete consumption of
3
5 3
n
2
9,30
and further examples have also been reported for La,
3
3
0,31
30,32
30
31
Sm,
Nd,
Pr, and Y.
−
1
the silanols. Material 7 shows a residual peak at 3747 cm but
Hybrid materials
While CAN-derived hybrid materials (support: silica,
3
3
3
4
35
perfluorinated resin-sulphonic acid (Nafion), alumina or
3
6
charcoal ) have been successfully employed for oxidative
organic transformations, to our knowledge surface organo-
cerium(IV) chemistry (SOCe(IV)C) has not been tackled so far.
There are, however, reports on combining Ce(IV) complexes
3
7
with silica using methods such as incipient wetness, sol–
3
3d
33e
gel
exploited the alkoxide complexes [Ce(OiPr)
for grafting onto periodic mesoporous silica (PMS) MCM-41
and CVD. The most relevant work from Bell et al. has
4
] and [Ce(OtBu) ]
4
3
3f
and SBA-15 in order to synthesise CeO nanoparticles. While
2
nanoparticles were produced, the latter report describes
incomplete grafting with surface SiOH groups remaining after
the first alkoxide treatment and requiring a subsequent graft-
ing step to achieve higher loadings. We anticipated that the
heterogeneously performed silylamide route, previously proved
3
8
39
for rare-earth metals (Ln(III), Ln(II) ), main group (Mg(II),
4
0
41
42
43
Ba(II), Al(III) ) and 3d transition metals (Ti(IV), Fe(II)/(III),
4
4
Fig. 4 Proposed surface species for Ce[N(SiHMe
Ce[N(SiMe Cl (6).
2 2 4
) ] @MCM-41 (5) and
Zn(II) ) might as well apply for Ce(IV) silylamides. Using pre-
3 2 3
) ]
cursors exhibiting higher proligand pK values such as metal
a
silylamides (≈ 25) should ensure complete surface silanol con-
sumption and higher initial loadings. Particularly, the strongly
reducing hybrid materials Sm(II)(NR ) @PMS revealed a stabi-
2
2
3
8
lising effect for ketyl radicals via pore confinement. More-
over, surface attachment via the formation of Ce–O(siloxide)-
bonds (electron-withdrawing effect of the silica matrix) might
not only stabilise the +IV oxidation state, but also counteract
ligand redistribution. Accordingly, this approach was tested by
using complexes 3, Ce[N(SiMe ) ] Cl and Cp CeCl as three can-
3
2 3
3
didates for grafting precursors in the production of Ce(IV)
hybrid silicas.
Periodic mesoporous silica MCM-41 was chosen as the
support material due to its hexagonal (P6mm) pore arrange-
ment with uniform 1-D channels and relatively narrow pore
Fig. 5 IR spectra (DRIFT) of the parent material MCM-41, Ce[N(SiH-
Me
2
)
2
]
4
3 2 3 3
@MCM-41 (5), Ce[N(SiMe ) ] Cl (6) and CeCp Cl@MCM-41 (7) in the
4
5,46
−1
size distribution.
The MCM-41 material used in this study range 1300–4000 cm .
5
494 | Dalton Trans., 2013, 42, 5491–5499
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peak and a new peak appearing at 2152 cm− assigned as
v(Si–H). While the reaction of metal silylamides with a silica
surface and more specifically, the surface silylation side-
reaction is well understood, less so is the reaction of metal
1
Table 1 Analytical data of parent and Ce(IV)-grafted MCM-41 materials
a
b
c
d
d
Sample
a
s
d
p
V
p
C
N
Parent MCM-41
1040
725
472
375
712
3.3
2.7
2.3
2.2
2.7
1.11
0.71
0.44
0.36
0.77
—
5.97
14.13
11.20
13.34
—
—
0.79
0.90
—
HN(SiHMe
Ce[N(SiHMe
3 2 3
Ce[N(SiMe ) ]
2
)
2
2
)
@MCM-41
@MCM-41 (5)
Cl@MCM-41 (6)
3
Cl@MCM-41 (7)
cyclopentadienyl derivatives and the accompanying side-
CeCl can occur in two ways, firstly
via the loss of a [C H ] fragment from the complex forming
]
2 4
48
reactions. Grafting of Cp
3
−
CeCp
5 5
free cyclopentadiene or via the loss of the chloro ligand atom
forming HCl. While the former reaction would involve proto-
samples were pretreated at 250 °C (parent material) and 25 °C (hybrid nolysis at a sterically highly encumbered Ce(IV) centre, the
a
Specific BET surface area [m g−1]. Pore diameter [nm]. Pore
2
b
c
= 0.996 [cm g−1]; all
3
volume determined at the relative pressure P/P
o
materials) in vacuo until the pressure was <10− Torr. Expressed
in wt%.
3
d
latter would preferentially take place in the presence of a base
like triethylamine. Not surprisingly, an attempted chlorine
analysis of hybrid material 7 out rules any surface reaction via
the Ce(IV)–Cl bond. Further, the mild reaction conditions
(
ambient temperature) might also suggest surface interaction
of complexes Cp CeCl via Ce–Cl⋯HOSiu and Cp⋯HOSiu
3
hydrogen bonds. A more detailed investigation of 7-type
materials is currently pursued.
While silylamides generate silylamines of the form
HN(SiRx)2 upon grafting, which can react further with the
surface, the cyclopentadienyl fragments released are unable to
react further with the silica surface. It is noteworthy that steri-
cally encumbered metal silylamide complexes such as 3 do not
necessarily have to react via the metal–nitrogen bond but also
4
3
via peripheral Si–N silazane moieties. This lack of additional
side-reactions may also be responsible for the incomplete con-
sumption of the surface silanols. Furthermore, cerium–silica
Fig. 6 Nitrogen adsorption–desorption isotherms of MCM-41 parent material
(
blue squares), HN(SiHMe
MCM-41 (5, black circles) Ce[N(SiMe
Cp CeCl@MCM-41 (7, orange circles).
2 2 2 2 4
) @MCM-41 (green diamonds) Ce[N(SiHMe ) ] @
3
)
2
]
3
Cl@MCM-41(6, red triangles), and species containing Cp ligands may sterically inhibit access to
another SiOH site adjacent and so force an additional incom-
3
ing precursor to interact with a less crowded SiOH site. This
can also be rationalised upon the addition of HN(SiHMe
2 2
)
also two broad bands between 3600 and 3100 cm− indicating (7a) which is small enough to access such unreacted SiOH
1
significant interaction of the complex Cp
surface.
3
3
CeCl with the silica sites while the larger Cp CeCl is unable to.
The carbon loadings of materials 5 and 6 are high (14 and
1 wt%) compared to that of 7 (13 wt%) (Table 1), when taking
Conclusions
1
into consideration the carbon contents of the Ce(IV) precursors A CAN-based salt metathesis protocol gives access to hetero-
3: 28.71; Ce[N(SiMe ) ] Cl: 32.92; Cp CeCl: 48.58%). The nitro- leptic silylamide/alkoxide complexes as evidenced for
(
3
2 3
3
gen content of 5 is lower than 6 indicating an overall higher [Ce(OiPr) {N(SiMe ) }] . The reactions of tetravalent cerium
3
3 2 2
podality, meaning a comparatively higher amount of species involving the tetramethyl silylamido ligand show a marked
IV
(
uSiO) Ce [N(SiR ) ] (5B > 6B). Furthermore, competitive difference in complexity of ligand redistribution and reduction
2 3 2 2
surface silylation is a well-documented feature of silylamide chemistry. In the case of homoleptic Ce(IV) silylamide com-
3
8a,40,42–44,47
functionalised silica.
reflected in the nitrogen physisorption data as for material saturation with Ce(IV)⋯SiH β-agostic interactions appear to
Ce[N(SiMe Cl@MCM-41 (6) bearing the bulkier silylamido increase the stability compared with the hexamethyl conge-
These differences are also plexes the decrease in symmetry and increase in coordinative
3 2 3
) ]
ligand the decreases in pore volume and pore diameter are ners. The large electron-withdrawing effect of the silica surface
more pronounced than in the case of Ce[N(SiHMe ) ] @ can be seen as a very bulky siloxide ligand and the site-
2
2 4
MCM-41 (5) (Fig. 6). Organocerium(IV) hybrid material isolation of the cerium metal on the surface increases the
Cp CeCl@MCM-41 (7) maintains a higher surface area, pore stability of Ce(IV) complexes that appear unstable in solution.
3
diameter and pore volume as expected for an incompletely Undesirable phenomena such as decomposition, ligand
2
−1
3
−1
grafted material (712 m
respectively).
g
, 2.7 nm and 0.77 cm
g
,
scrambling and bimolecular deactivation are avoided via the
stabilising effect of the silica surface. The absence of side reac-
When an excess of 1,1,4,4-tetramethyldisilylazane was tions associated with incomplete grafting with Cp CeCl@PMS
3
added to material
7 complete SiOH consumption was may be considered advantageous as the material produced is
achieved, affording HN(SiHMe ) @Cp CeCl@MCM-41 (7a, not not as hydrophobic with SiOH species contributing to the
2
2
3
shown). This was evidenced by the disappearance of the SiOH polar nature of the surface.
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3
0 min at ambient temperature, to which a THF slurry
Experimental
General considerations
of NaOiPr (0.748 g, 9.120 mmol) was added. An orange
suspension was formed along with gas bubbles and a white
The synthesis of all cerium-containing compounds and hybrid precipitate. The volatiles were removed via vacuum and a
materials was performed in an argon-filled glovebox (MBraun toluene (15 ml) extraction was performed followed by centrifu-
MB150B-G; <1 ppm O
THF were dried and degassed using Grubbs columns oily solid (presumably Ce(NO
MBraun SPS800, solvent purification system) and stored in a this solid in THF (10 ml) and addition of LiN(SiMe3)2
glovebox. C was obtained from Deutero GmbH, degassed, (0.1281 g, 0.766 mmol) in THF (5 ml) gave a deep red solution
2
, <1 ppm H
2
O). Hexane, toluene and gation. Removing the toluene via vacuum gave a red-orange
3
)(OiPr) (thf) ). Redissolving
3
x
(
6 6
D
dried over sodium metal and filtered. 1,1,3,3-Tetramethyl- with further precipitation of a white solid. Removal of the
disilazane and ceric ammonium nitrate (CAN) were purchased volatiles produced a red/brown oily solid, a component
from ABCR (97%; 98% purity). Sodium isopropoxide was syn- of which could be extracted into hexane producing a deep red
thesised from sodium metal and anhydrous isopropanol solution. Concentration and cooling of the solution led to the
(Sigma-Aldrich, 99.5% purity). Lithium bis(trimethylsilyl)- formation of red crystals suitable for X-ray diffraction
amide was synthesised from n-butyllithium (Sigma 2.5 M in (yield 40%). IR (DRIFT): vmax = 2960 (vs), 2894 (m,sh), 2864
hexanes) and 1,1,1,3,3,3-hexamethyldisilazane. Tetraethyl (m,sh), 2618 (w), 1585 (w,br), 1463 (m), 1454 (m), 1379 (m),
orthosilicate (98% purity) was purchased from Fluka, CTMABr 1360 (m) 1327 (m), 1329 (m), 1247 (s), 1163 (m), 1130 (s),
(
98% purity), 1,4-dioxane (99%) and tetramethylammonium 1048 (m), 1016 (s,sh), 976 (vs), 917 (m,sh), 870 (s,sh), 845 (vs),
hydroxide (25 wt% in H O) were bought from Sigma-Aldrich 770 (m), 746 (m,sh), 664 (m), 599 (m), 533 (m), 467 (m),
and C1
was synthesised according to the literature pro- 444 (m), 418 (m) cm− . H NMR (400 MHz, C D ): δ = 0.53
2
1
1
6-3-1
6 6
4
9
cedure. All chemicals were used as bought except CAN which (s, 18H, SiMe
was heated under vacuum for 5 h at 60 °C then stored in a (septet,
glovebox. MCM-41 was synthesised in bulk using previous C H Ce N O Si : C, 37.71; H, 8.23; N, 2.93. Found: C, 37.91;
3
), 1.22 (d, J = 15 Hz, 18H, OC(H)Me
2
), 4.82
J
=
15 Hz, 3H, OC(Me )H). Anal. Calc. for
2
3
0
78
2
2
6
4
4
5
descriptions (cf., ESI†). As-synthesised MCM-41 was calcined H, 8.86; N, 2.69%.
450 °C, 24 h) in air and dehydrated (270 °C, 10− Torr). Parent
material MCM-41 was characterised by FTIR spectroscopy, was slurried in THF (10 ml), to which NaOiPr (0.449 g,
powder X-ray diffraction, N -physisorption at 77.4 K, trans- 5.472 mmol) was added dropwise. An orange suspension
5
(
2 3 2 2 2
Ce(OiPr) {N(SiMe ) } Li(thf) (2). CAN (1.000 g, 1.824 mmol)
2
mission electron microscopy (see ESI†) and stored in a glove- was formed along with gas bubbles and a white precipitate.
box. The SiOH population of the parent material was Volatiles were removed via vacuum and a toluene (15 ml)
determined by the carbon content of a sample reacted with extraction was performed followed by centrifugation. Remov-
5
0
an excess of 1,1,3,3-tetramethyldisilazane in hexane.
ing the toluene via vacuum gave a red orange oily solid
1
0b
10d
Ce[N(SiMe ) ] Cl
and Cp CeCl
3
were prepared according (presumably Ce(NO ) (OiPr)(thf) ). Redissolving this solid in
3
2 3
3 3
x
3 2
to previous reports. IR spectra were recorded on a NICOLET THF (10 ml) and adding of LiN(SiMe ) (0.1070 g, 0.639 mmol)
6
700 FTIR spectrometer using a DRIFT chamber with KBr/ in THF (5 ml) gave a red solution with further precipitation
1
sample mixtures. H NMR spectra were recorded in C D solu- of a white solid. Removal of volatiles produced a red/
tions on
6
6
1
a Bruker-ADVANCE-DMX400 (5 mm BB, H: brown oily solid, a component of which could be extracted
1
4
00.13 Hz), H shifts are referenced to the internal solvent and into hexane producing a deep red solution. Concentration
reported in parts per million relative to TMS. CHN elemental and cooling of this solution led to the formation of colourless
1
analyses were performed on an Elementar Vario MICRO cube. crystals suitable for X-ray diffraction. H NMR (400 MHz,
N
2
physisorption was carried out on either an ASAP 2010 or
6 6
C D ): δ = −3.86 (s), 0.37 (s), 0.43 (s), 1.24 (d, J = 15 Hz),
ASAP 2020 volumetric adsorption apparatus (Micromeritics) at 1.46 (s), 4.0 (s), 4.86 (septet, J = 15 Hz). Anal. Calc. for
2
7
7.4 K [a
outgassing step of 3 h, 10 Torr and at ambient temperature H, 7.47; N, 4.09%.
before measurement began. The specific surface areas of the Ce[N(SiHMe ) ] (3) and [Li Ce (OiPr) (1,4-dioxane)] (4). A
m
(N
2
, 77 K) = 0.162 nm ). All samples underwent an
26 2 4
C H66CeLiN Si : C, 42.77; H, 9.11; N, 3.84. Found: C, 41.26;
−3
2
2 4
2
2
10
n
5
1
samples were obtained using the BET method. Pore size dis- 10 ml pentane solution of 1 (0.221 g, 0.211 mmol) was
tributions were determined via the Barrett–Joyner–Halenda added to a 2 ml pentane solution containing a slight excess
(
BJH) method using the Kelvin equation to calculate the mean of tetramethyldisilazane (0.035 g, 0.263 mmol) dropwise
5
2
pore diameter.
Powder X-ray diffraction patterns were under stirring. This dark red solution was allowed to stir
obtained using a Bruker D8 ADVANCE instrument in the step/ for 1 h. The volatiles were removed via vacuum leaving a
scan mode (step width = 0.00825; accumulation time = 2 s per red oil (89% yield). Redispersal in a 1,4-dioxane/pentane or
step; range (2θ) = 0.50–10.00°) using monochromatic CuKα1 THF solution caused precipitation, whereupon filtration, con-
(
λ = 1.540562 Å).
centration and cooling to −35 °C produced dark red
crystals suitable for X-ray diffraction, analysed as 3. N.B.
Colourless crystals were also formed with prolonged
1
6
Synthesis of cerium complexes
Ce(OiPr) {N(SiMe }] (1). In
CAN (1.000 g, 1.824 mmol) was stirred in THF (10 ml) for crystallographically.
[
3
3
)
2
2
a
2 2 n
two-step procedure, storage and found to be [Li Ce (OiPr)10(1,4-dioxane)] (4)
5
496 | Dalton Trans., 2013, 42, 5491–5499
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Paper
Table 2 Crystallographic data for 1, 2 and 4
Table 2, and in the CIF file; CCDC reference numbers are
915627 (1), 915628 (2) and 915629 (4).
1
2
4
Formula
C
30
H
78Ce
2
N
2
O
Si
6 4
C
26
H
66CeLiN
2
4
O Si
4
C
34 2 2 12
H78Ce Li O
F
w
803.51
Orthorhombic
730.23
Orthorhombic
973.08
Monoclinic
Acknowledgements
Crystal
system
This research was supported by the German Science
Foundation and the NANOSCIENCE program of the University
of Bergen. The authors acknowledge the efforts of Egil
S. Erichsen for TEM images.
Space group Pbca
P2
1
2
1
2
1
C2/c
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å
Z
15.2304(16)
12.5799(3)
16.2153(3)
19.6833(5)
—
—
—
4015.13(16)
4
1540
18.9892(5)
16.7465(5)
15.0041(4)
—
101.0900(10)
—
4682(2)
4
2000
18.3123(18)
16.960(2)
—
—
—
4730.2(9)
4
1968
3
Notes and references
F(000)
173(2)
1.342
2.035
0.0673
0.1475
1.151
173(2)
1.208
1.280
0.0443
0.0771
1.203
100(2)
1.380
1.968
0.0486
0.1160
1.340
−
3
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b
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o
2 2
= Σ(|F
o
1/2
| − |F
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o
> 2δ(F
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). wR
2
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2
)
2
2
]
4
−1
@MCM-41
(5). Peach-coloured
hybrid
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sh), 2891 (m), 2114 (m,br), 1462 (m), 1246 (s), 1063 (vs), 899
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1
.
(
C, 14.1; H, 2.4; N, 0.8%.
Ce[N(SiMe Cl@MCM-41 (6). Brown-coloured hybrid
3 2 3
) ]
2
−1
material (375 m g ). IR (DRIFT): vmax = 2960 (m,sh), 2924 (m,
sh), 2855 (m), 1459 (w), 1462 (m), 1243 (s), 1070 (vs), 844 (m),
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.
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C, 11.2; H, 2.1; N, 1.7%.
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1
X-ray crystallography and crystal structure determination of
complexes 1, 2 and 4. Crystals were grown by standard
techniques from saturated solutions using hexane at −35 °C.
Suitable crystals for diffraction experiments were selected in a
glovebox and mounted in Paratone-N (Hampton Research) on
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2
T, and 4 on a Bruker AXS APEX II instrument using MoK
α
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3
54
refinement was done using Stoe’s X-Area, SMART and
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55
56
56
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5
7
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Paper
Dalton Transactions
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