β-sih-containing tris(silazido) rare-earth complexes as homogeneous and grafted single-site catalyst precursors for hydroamination

Article abstract of DOI:10.1021/acs.organomet.6b00956

A series of homoleptic rare-earth silazido compounds and their silica-grafted derivatives were prepared to compare spectroscopic and catalytic features under homogeneous and interfacial conditions. Trivalent tris-(silazido) compounds Ln{N(SiHMe₂

Full text of DOI:10.1021/acs.organomet.6b00956

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pubs.acs.org/Organometallics
β‑SiH-Containing Tris(silazido) Rare-Earth Complexes as
Homogeneous and Grafted Single-Site Catalyst Precursors for
Hydroamination
Naresh Eedugurala, Zhuoran Wang, KaKing Yan, Kasuni C. Boteju, Umesh Chaudhary,
Takeshi Kobayashi, Arkady Ellern, Igor I. Slowing, Marek Pruski, and Aaron D. Sadow*
Department of Chemistry and U.S. Department of Energy Ames Laboratory, Iowa State University, 1605 Gilman Hall, Ames, Iowa
50011, United States
S
* Supporting Information
ABSTRACT: A series of homoleptic rare-earth silazido
compounds and their silica-grafted derivatives were prepared
to compare spectroscopic and catalytic features under
homogeneous and interfacial conditions. Trivalent tris-
(silazido) compounds Ln{N(SiHMe2)tBu}3 (Ln = Sc (1), Y
(2), Lu (3)) are prepared in high yield by salt metathesis
reactions. Solution-phase and solid-state characterization of 1−
3 by NMR and IR spectroscopy and X-ray diffraction reveals
Ln↼H−Si interactions. These features are retained in solvent-
coordinated 2·Et2O, 2·THF, and 3·THF. The change in
spectroscopic features characterizing the secondary interac-
tions (νSiH, 1JSiH) from the unactivated SiH in the silazane HN(SiHMe2)tBu follows the trend 3 > 2 > 1 ≈ 2·Et2O > 2·THF ≈ 3·
THF. Ligand lability follows the same pattern, with Et2O readily dissociating from 2·Et2O while THF is displaced only during
surface grafting reactions. 1 and 2·THF graft onto mesoporous silica nanoparticles (MSN) to give Ln{N(SiHMe2)tBu}n@MSN
(Ln = Sc (1@MSN), Y (2@MSN)) along with THF and protonated silazido as HN(SiHMe2)tBu and H2NtBu. The surface
species are characterized by multinuclear and multidimensional solid-state (SS) NMR spectroscopic techniques, as well as diffuse
reflectance FTIR, elemental analysis, and reaction stoichiometry. A key 1JSiH SSNMR measurement reveals that the grafted sites
most closely resemble Ln·THF adducts, suggesting that siloxane coordination occurs in grafted compounds. These species
catalyze the hydroamination/bicyclization of aminodialkenes, and both solution-phase and interfacial conditions provide the
bicyclized product with equivalent cis:trans ratios. Similar diastereoselectivities mediated by catalytic sites under the two
conditions suggest similar effective environments.
important starting materials, but their large ionic radii and low
numbers of X-type ligands (either two or three) add to the
challenge of preparing reactive monometallic species.14
INTRODUCTION
■
Complexes containing only one type of ligand (MXn), known
as homoleptic compounds, represent the simplest systems for
characterizing the nature of metal−ligand interactions because
all ligands equivalently contribute electronic and steric effects.
The resulting complexes often have intriguing structural and
spectroscopic features that are associated with secondary
metal−ligand interactions and non-VSEPR geometries.1−5 In
addition, homoleptic compounds in high oxidation states are
often electronically and/or coordinatively unsaturated, giving
highly electrophilic metal centers and geometric distortions to
counterbalance low electron counts. The nature of the M−X
bond is also important to their reactivity; for example, selective
substitution of these groups with ancillary ligands (LX)
provides routes to reactive complexes, including catalysts.
While the rich chemistry of surface-supported organometallic
compounds indicates that alkyl species are desirable,6 work with
grafted early-metal amides suggests their emerging potential in
catalysis.7−13 In rare-earth chemistry, homoleptic organo-
metallic and pseudo-organometallic compounds are particularly
As a result, disilazido groups, such as hexamethyldisilazide
and tetramethyldisilazide, are the primary N-based ligand types
to support monometallic homoleptic rare-earth compounds.
Trivalent Ln{N(SiMe3)2}315−17 and Ln{N(SiHMe2)2}318,19 and
divalent19 compounds are prevalent starting materials for a
range of rare-earth chemistries, including as homogeneous
catalysts20−23 and as precursors for single-site supported rare-
earth catalysts.18,24−28 Such surface-grafted materials catalyze
alkyne dimerization,27 Tishchenko aldehyde dimerization,26,27
hydroamination (the addition of amines and olefins),28 and
polymerization.26,29 However, a downside of disilazido
complexes as catalyst precursors is that HN(SiMe3)2 and
especially HN(SiHMe2)2 can be poor leaving groups due to
their relatively high acidity, with pKa values of 25.7 and 22.6,
Received: December 23, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00956
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respectively.30 Disilazanes are effective silylating agents, and
grafting of Ln{N(SiHMe2)2}3 on silica results in significant
surface silylation. In addition, compounds containing the
smaller N(SiHMe2)2 ligand are often multimetallic (e.g.,
[La{N(SiHMe2)2}3]2).31 Thus, the basicity of the bulkier
silazido ligand N(SiMe3)tBu was invoked for reactions of
Ln{N(SiMe3)tBu}3 (Ln = Y, La) as a precursor to
homogeneous hydroamination catalysts.32 Still, the SiH group
in N(SiHMe2)2 provides a valuable spectroscopic handle for
both NMR and IR analysis,18 stabilization of coordinatively and
electronically unsaturated metal centers through labile secon-
dary interactions,18,30,33,34 and a site for reactivity.35−38
monocyclization reaction of aminodialkenes and amino-
dialkynes to give optically active pyrrolidines.45 In addition,
Marks and co-workers showed in their seminal study that
diastereoselectivity in Cp*2LaCH(SiMe3)2-catalyzed hydro-
aminations of chiral aminoalkenes is also influenced by
concentration.46 Rare-earth compounds and a few zirconium
catalysts give hydroamination/bicyclization products through a
two-step process in which the second cyclization requires
conditions more forcing than those in the first.32,47−51,48−50
Ligand−metal or surface−metal center interactions might
provide control over selectivity in the hydroamination of
aminodialkenes. Experiments are needed to test for surface
effects on (a) monocyclization vs bicyclization of amino-
dialkenes and (b) cis:trans selectivity of the products to
elucidate factors that ultimately control selectivity in such C−N
bond forming reactions. Controlling selectivity in these
reactions has synthetic value in terms of additional trans-
formations of the olefin-substituted heterocycles.52 In addition,
the azabicyclo[2.2.1]heptane product contains motifs found in
natural products and biologically active substances, and the exo
and endo selectivity is also important for their applica-
tions.53−55
Thus, homoleptic monometallic compounds of the type
Ln{N(SiHMe2)tBu}3 may be effective precatalysts and
precursors for single-site heterogeneous catalysts. The present
study describes our efforts to synthesize a series of homoleptic
rare-earth silazido compounds. The NMR, IR, and structural
properties of Ln{N(SiHMe2)tBu}3 and their ethereal solvent
adducts were analyzed to provide molecular models for surface-
bonded species. Such surface-supported analogues are obtained
by grafting on mesoporous silica nanoparticles (MSN).
Characterization of MSN-supported rare-earth silazido materi-
als via IR and solid-state NMR spectroscopy provides a
molecular picture of the surface sites. With this picture and the
spectroscopic comparison between solution-phase homoleptic
vs grafted species in hand, we studied their catalytic reactivity
(activity and selectivity) in hydroamination/cyclization of
aminoalkenes and aminodialkenes.
The silazide N(SiHMe2)tBu incorporates a number of these
desired features: enhanced steric protection, a more basic amide
group, and the SiH moiety. This silazido ligand has been
underutilized as a supporting ligand in homoleptic compounds
in comparison to the disilazido ligands, despite the early
promise of the only trivalent homoleptic Er{N(SiHMe2)
39
tBu}3 and the rich chemistry of Cp2Zr{N(SiHMe2)tBu}X
(X = hydride, halide, alkyl).33,34,40−42 Both of these systems, as
well as the main-group compound [Mg{N(SiHMe2)tBu}2]2,43
exhibit structural and spectroscopic features associated with
multicenter M↼H−Si interactions, including short M···H
distances and small ∠M−N−Si angles in X-ray diffraction
studies, low-energy νSiH bands in infrared spectra, upfield δSiH
1
signals in 1H NMR spectra, and low JSiH values in 29Si and 1H
NMR spectra. The NMR properties, however, have not been
evaluated for Er{N(SiHMe2)tBu}3 because of its paramagnet-
ism, although the solid-state structure and infrared spectra
established that all three SiH groups interact with the rare-earth
center.39 Thus, the N(SiHMe2)tBu ligand could provide useful
precursors for catalysis, such as hydroamination.
Despite the high reactivity of disilazido rare-earth com-
pounds as precatalysts for this process,44 examples of grafted
single-site rare-earth hydroamination catalysts are limited.28
Moreover, those examples suggested that silica-supported
catalysts are diminished in activity in comparison to the
homogeneous analogues. A number of additional challenges
face the development of the catalytic hydroamination reaction,
including functional group tolerance, catalytic efficiency for
intermolecular additions, and control over selectivity.
The selectivity and activity in catalytic conversions of
aminodialkenes could provide a means for examining the effect
of surface and pore environment on hydroamination processes;
because both mono- and bicyclization products are possible,
each product has cis and trans diastereomers (Scheme 1) and
the diastereoselectivity is sensitive to reaction conditions. For
example, we recently reported that substrate concentration
affected the cis:trans ratio in an enantioselective Zr-catalyzed
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization of
Ln{N(SiHMe2)tBu}3 and Ln{N(SiHMe2)tBu}3L. Reactions of
three equiv of [LiN(SiHMe2)tBu] and LnCl3 (Ln = Y, Lu) or
LnCl3THF3 (Ln = Sc, Lu) in THF or Et2O provide
Ln{N(SiHMe2)tBu}3 or Ln{N(SiHMe2)tBu}3L, as outlined in
Scheme 2 (Ln = Sc (1), Y (2, 2·Et2O, 2·THF), Lu (3, 3·
THF)). Compound 1 is isolated as a light yellow sticky solid,
and neither Et2O nor THF is retained in the scandium’s
coordination sphere. The solvent-free Y compound may be
obtained by subliming 2·Et2O or by performing the synthesis
under concentrated conditions (0.46 M). The solvent-free Lu
compound is obtained from the reaction of LuCl3 in Et2O. The
complexes 2·Et2O, 2·THF, and 3·THF are isolated as white
sticky solids from pentane crystallization or precipitation.
Sublimation of the sticky solids of 2·THF and 3·THF affords
analytically pure powders while retaining the THF ligand.
Scheme 1. Cis and Trans Diastereomers Accessible from
Monocyclization and Bicyclization of Aminodialkenes
1
While the H NMR spectrum of 2·THF is not altered by
sublimation, the νSiH region of the infrared spectra is slightly
sharper after this treatment (see the Supporting Information).
The infrared spectra of the series of compounds contained
bands attributable to Si−H stretching modes, ranging from
2019 to 1849 cm−1 (Figure 1 and Table 1; see the Supporting
Information for full IR spectra). Spectra for 1−3 and 2·Et2O
B
DOI: 10.1021/acs.organomet.6b00956
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Scheme 2. Synthesis of Homoleptic Silazido Rare-Earth Compounds
appearing at higher energy than the signal for the ether-
coordinated yttrium species. The signal for the νSiH band of 3
(1849 cm−1) appeared with the lowest energy of the series,
which follows the trend 3 < Er{N(SiHMe2)tBu}3 (1858
cm−1)39 < 2 < 1. In contrast, the νSiH region for nonsublimed 2·
THF contained two peaks at 2019 and 1967 cm−1 at notably
higher energy; once it was sublimed, a signal at 2117 cm−1 was
detected due to sharpening of the broad 2019 cm−1 band. The
signals for the SiH group in the lutetium THF adduct were
observed around 2000 cm−1. For comparison, the SiH
stretching frequencies of the silazane HN(SiHMe2)tBu and
lithium silazido [LiN(SiHMe2)tBu] appeared at higher energy
in comparison to those of the rare-earth silazido compounds.
Room-temperature 1H NMR spectra suggested that the
homoleptic rare-earth species are C3v symmetric on the basis of
three resonances, which were assigned to the SiH, SiMe2, and
3
tBu groups in equivalent silazido ligands. The JHH coupling in
SiHMe2 is small and resolved clearly as doublets (3 Hz) for the
Me in 2, 2·THF, 3, and 3·THF. The 1JSiH values (Table 1) vary
depending on the rare-earth element and the coordinated THF
or Et2O ligands but are generally low and suggest Ln↼H−Si
bonding motifs. The 1JSiH values in HN(SiHMe2)tBu (192 Hz)
and [LiN(SiHMe2)tBu] (168 Hz) are larger than those in the
rare-earth compounds. At low temperature (190 K), the SiMe2
Figure 1. Infrared spectra of HN(SiHMe2)tBu, [LiN(SiHMe2)tBu], 1,
2·Et2O, and 2·THF (before sublimation) corresponding to Si−H
stretching modes. Full spectra are shown in the Supporting
Information.
revealed a single strong band assigned to bridging Ln↼H−Si
groups, with the tricoordinated scandium complex’s peak
1
signals in the H NMR spectra of 1, 2·THF, and 3·THF
Table 1. Spectroscopic Data for Silazido Compounds
compound
δSiH, ppm
1JSiH, Hz
δSi, ppm
δN, ppm
νSiH, cm−1
Sc{N(SiHMe2)tBu}3 (1)
Y{N(SiHMe2)tBu}3 (2)
2·Et2O
2·THF
Lu{N(SiHMe2)tBu}3 (3)
3·THF
4.18
4.26
4.30
4.59
4.42
4.63
4.83
4.87
125
124
126
143
121
137
192
168
−22.9
−24.8
−25.9
−30.5
−20.5
−28.7
−18.8
−23.1
−208
−221
−222
−231
−221
−232
−329
−301
1893
1860
1864
2019, 1967
1849
1988
HN(SiHMe2)tBu
[LiN(SiHMe2)tBu]
2135, 2104
2055
C
DOI: 10.1021/acs.organomet.6b00956
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appeared as two signals of equal intensity, implying low-
temperature C3-symmetric structures. Interestingly, the SiH
chemical shift and coupling constants were identical in spectra
acquired down to 190 K, and this observation suggested that
the fluxional process did not involve disruption of the
1
secondary Ln↼H−Si interactions. In contrast, the H NMR
spectra of 2, 2·Et2O, and 3 merely broadened at 190 K in
toluene-d8.
The 29Si NMR spectra vary from −20.5 to −30.5 ppm
depending on the identity of the rare-earth element, and these
were slightly upfield in comparison to HN(SiHMe2)tBu. A
similar trend was observed in the 29Si NMR spectra of the rare-
earth disilazido compounds Ln{N(SiHMe2)2}3THFn, which are
ca. 10 ppm upfield in comparison to the disilazane HN-
(SiHMe2)2 (−11.1 ppm).18 In addition, 1H−15N HMQC
experiments (at natural abundance) revealed cross-peaks
between N and tBu signals but not with the SiHMe2 group.
The 15N NMR chemical shifts were downfield in comparison to
those of HN(SiHMe2)tBu and [LiN(SiHMe2)tBu] (see Table
1). We also noticed the same trend in the 15N NMR chemical
shifts for Sc{N(SiHMe2)2}3THF (−253 ppm) and Y{N-
(SiMe3)2}3 (−243.1 ppm), which are downfield of those for
HN(SiHMe2)2 (−365 ppm) and HN(SiMe3)2 (−354 ppm).
Likewise, the 15N NMR chemical shifts for Cp2Zr{N(SiHMe2)
tBu}H (−260 ppm) and Cp2Zr{N(SiHMe2)2}H (−292 ppm)
are downfield with respect to those of HN(SiHMe2)tBu (−329
ppm) and HN(SiHMe2)2 (−365.3 ppm).36
Figure 3. Rendered thermal ellipsoid plot (50% probability) of
Y{N(SiHMe2)2}3THF (2·THF). See the Supporting Information for
solid-state structures of 2·Et2O and 3·THF. H atoms bonded to Si
were located objectively in the Fourier difference map and are included
in the rendition; all other H atoms are not included for clarity.
X-ray Crystallography. Single-crystal X-ray diffraction
studies provided solid-state structures of 1 (Figure 2; see the
groups are directed toward the rare-earth center. The methyl
groups in the SiMe2 are inequivalent in these structures, and
1
this is consistent with the low-temperature H NMR spectrum
described above.
The LnN3 cores of 1−3 adopt similar trigonal geometries
distorted by pyramidalization (∑NLnN: Sc, 348.62(9)°; Y,
351.3(2)°; Lu, 349.0(2)° (vs Er, 350.42°)). There are three
short Ln···H and three short Ln···Si distances. Remarkably, the
scandium−silicon distances in 1 (Sc1−Si1, 2.8603(3) Å; Sc1−
Si2, 2.8343(4) Å; Sc1−Si3, 2.8557(4) Å) are similar to the
distance in the scandium silyl compound Cp2ScSi(SiMe3)3THF
(2.863(2) Å)56 and only slightly longer than that in
Cp*2ScSiH2SiPh3 (2.797(1) Å),57 both of which contain
bona fide two-center−two-electron Sc−Si bonds. These short
distances are complemented by the Sc−N−Si angles, which are
much smaller than the 120° expected for a trigonal-planar N
center (98.47(4), 97.98(4), and 97.93(4)°). Taking into
consideration the short distances to N and Si, the N3Si3
atoms form a trigonal prism, with the smaller N3 end-capping
triangle twisted from the triangular face composed of Si3
vertices. The Sc center is 0.41 and 1.02 Å from the N3 and
Si3 planes, respectively. The scandium−hydrogen distances
(Sc1−H1s, 2.26(1) Å; Sc1−H2s, 2.20(2) Å; Sc1−H3s, 2.23(1)
Å), however, are significantly longer than the calculated
distance in ScH3 (1.82 Å).58 For comparison, the homoleptic,
solvent-free tris(amido)scandium compound Sc{N(SiMe3)2}3
is pyramidal in the solid state (∑NScN = 346.5°) but planar in
the gas phase.59 In that compound, the solid-state and gas-
phase Sc−N distances (2.047(2) and 2.02(3) Å, respectively)
are slightly shorter than those in 1 (Sc1−N1, 2.0656(6) Å;
Sc1−N2, 2.063(1) Å; Sc1−N3, 2.071(2) Å). The Sc−N
distances in 1, however, are similar to those in the four-
coordinate THF adduct Sc{N(SiHMe2)2}3THF.18 As expected,
the Ln−N, Ln···Si, and Ln···H distances in 2 and 3 are longer
than those in 1 (see the Supporting Information). The Y−N
distances (2.223(2), 2.223(2), and 2.227(2) Å) are slightly
Figure 2. Rendered thermal ellipsoid plot of Sc{N(SiHMe2)tBu}3 (1)
at 50% probability. See the Supporting Information for isostructural
yttrium (2) and lutetium (3). H atoms bonded to Si were located
objectively in the Fourier difference map and are included in the
rendition; all other H atoms are not included for clarity.
Supporting Information for other structures), 2, 2·Et2O, 2·THF
(Figure 3), 3, and 3·THF for comparison to Er{N(SiHMe2)
tBu}3.39 The molecular structures of 1−3 and the erbium
analogue are similarly pseudo-C3 with the N(SiHMe2)tBu
ligands adopting a propeller-like conformation. All three SiH
groups are located (identified objectively on the difference
Fourier map) on the same face of the LnN3 core, and these
D
DOI: 10.1021/acs.organomet.6b00956
Organometallics XXXX, XXX, XXX−XXX