Heterometallic alumo- and gallodisilicates with M(O-Si-O) 2M′ and [M(O-Si-O)2]2M′ Cores (M = Al, Ga; M′ = Ti, Zr, Hf)

Article abstract of DOI:10.1021/ic400156n

The synthesis and stabilization of alumo- and gallodisilicates [HC{C(Me)N(2,6-iPr₂C₆H₃)}₂]M[(μ- O)Si(OH)(OtBu)₂]₂ [M = Al (1), Ga (2)] containing two silicate subunits have been achieved through reactions between 2 equiv of the silanediol (tBuO)₂Si(OH)₂ and the aluminum hydride [HC{C(Me)N(2,6-iPr₂C₆H₃)}₂]AlH ₂ or the gallium amide [HC{C(Me)N(2,6-iPr₂C ₆H₃)}₂]Ga(NHEt)₂, respectively. Compounds 1 and 2 exhibit M(O-SiO₂-OH)₂ moiety and represent the first molecular metallosilicate-based analogues of neighboring silanol groups found in silicate surfaces. The substitution of both SiOH groups led to the formation of bimetallic compounds with 4R topologies, which are regularly found in zeolitic materials. Thus, reactions between group 4 metal amides M′(NEt₂)₄ (M′ = Ti, Zr, Hf) and 1 and 2 resulted in the formation of nine heterometallic silicates (3-11) containing inorganic M(O-Si-O)₂M′ and [M(O-Si-O)₂] ₂M′ cores with 4R and spiro-4R topologies, respectively. The latter have M···M distances of 0.81 nm. NMR studies of the heterometallic derivatives showed a fluxional behavior at room temperature due to a high flexibility of the eight-membered ring.

Full text of DOI:10.1021/ic400156n

Article
pubs.acs.org/IC
Heterometallic Alumo- and Gallodisilicates with M(O−Si−O)2M′ and
[M(O−Si−O)2]2M′ Cores (M = Al, Ga; M′ = Ti, Zr, Hf)
Raul Huerta-Lavorie, Diego Solis-Ibarra, Dana Victoria Baez-Rodríguez, Marisol Reyes-Lezama,
́
́
M. de las Nieves Zavala-Segovia, and Vojtech Jancik*
Centro Conjunto de Investigacion
́
en Química Sustentable UAEM-UNAM, Car. Toluca-Atlacomulco km 14.5, Toluca 50200, Estado
de Mexico, Mexico
́
́
S
* Supporting Information
ABSTRACT: The synthesis and stabilization of alumo- and gallodisilicates [HC{C(Me)N(2,6-iPr2C6H3)}2]M[(μ-O)Si(OH)-
(OtBu)2]2 [M = Al (1), Ga (2)] containing two silicate subunits have been achieved through reactions between 2 equiv of the
silanediol (tBuO)2Si(OH)2 and the aluminum hydride [HC{C(Me)N(2,6-iPr2C6H3)}2]AlH2 or the gallium amide
[HC{C(Me)N(2,6-iPr2C6H3)}2]Ga(NHEt)2, respectively. Compounds 1 and 2 exhibit M(O−SiO2−OH)2 moiety and
represent the first molecular metallosilicate-based analogues of neighboring silanol groups found in silicate surfaces. The
substitution of both SiOH groups led to the formation of bimetallic compounds with 4R topologies, which are regularly found in
zeolitic materials. Thus, reactions between group 4 metal amides M′(NEt2)4 (M′ = Ti, Zr, Hf) and 1 and 2 resulted in the
formation of nine heterometallic silicates (3−11) containing inorganic M(O−Si−O)2M′ and [M(O−Si−O)2]2M′ cores with 4R
and spiro-4R topologies, respectively. The latter have M···M distances of 0.81 nm. NMR studies of the heterometallic derivatives
showed a fluxional behavior at room temperature due to a high flexibility of the eight-membered ring.
and soluble compounds, is a promising alternative for the better
understanding of these materials. Moreover, molecular models
could give valuable insight in the nature of silicate bonding and
could eventually serve as building units for hybrid materials,
particularly with the recent development of new families of
porous materials such as periodic mesoporous organosilicas
(PMOs),3a silicon based metal−organic frameworks
(MOFs),3b−d and, more recently, eni-carbon silicates (ECS-
s).3e,f However, stabilization of simple silicate rings in a
molecular form is difficult due to the ease of formation and
dissociation of Al−O and Si−O bonds and the high
oligomerization tendency of these units.4 Recently, we have
reported on the synthesis of 3R type heterobimetallic alumo-
and gallosilicates with M′−O−Si−O−M−O connectivity based
on [HC{C(Me)N(2,6-iPr2C6H3)}2]M(OH·thf)(μ-O)Si(OH)-
(OtBu)2 (M = Al (a), Ga (b)) precursors with highly reactive
M−OH and Si−OH groups in the same molecule (Figure 1).5
INTRODUCTION
■
One of the most important types of natural inorganic
compounds containing cyclic subunits are silicates and
alumosilicates. These minerals or their synthetic analogues,
belonging to the zeolite class, are commercially used in a wide
variety of applications such as ion exchange,1a catalysis (mainly
in the petrochemical industry),1b,c,g−i separations (most notably
N2 and O2 from air),1d and support for nanoparticles among
others.1e These features are the result of a combination of
inorganic rings with different sizes, formed by 6 (3R), 8 (4R),
12 (6R), 16 (8R), 18 (9R), and 24 (12R) atoms, with the 4R
and 6R rings being the most common.1f The elucidation of the
interaction of these rigid frameworks with the precursors is
difficult because of the heterogeneous nature of the reactions
and the lack of suitable characterization techniques. Therefore,
the development of simple systems with similar connectivity is
a viable route to overcome these obstacles. In fact, just recently
the first heterogeneous 2D-alumosilicate films were proposed as
a model for zeolites.2 Therefore, the use of molecular silicates
that resemble the structure of zeolites, but remain as crystalline
Received: January 21, 2013
Published: May 29, 2013
© 2013 American Chemical Society
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Article
some signals. Therefore, these are not reported. Similarly, 13C and 29Si
NMR spectra are not reported because of very broad signals due to
1
conformational changes (vide infra). Selected H NMR spectra for
compounds 3−11 can be found in the Supporting Information. The IR
spectra were measured on Bruker Tensor 27 or Bruker Alpha.
X-ray Crystallography. Single crystals were mounted either on a
Bruker Smart Apex (Apex 1k detector) or a Bruker APEX DUO
diffractometer equipped with an Apex II CCD detector at 100 K.
Frames were collected using ω scans and integrated with SAINT.16
Multiscan absorption correction (SADABS)16 was applied. The
structures were solved by direct methods (SHELXS)17 and refined
using full-matrix least-squares on F2 with SHELXL-9717 using the
SHELXLE GUI.18 Weighted R factors, Rw, and all goodness-of-fit
indicators, are based on F2. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms of the C−H bonds were placed in
idealized positions, whereas the hydrogen atoms from the OH
moieties were localized from the difference electron density map and
their position was refined with Uiso tied to the parent atom with
distance restraints. The disordered groups and solvent molecules (1 1
× OtBu, 2 × tBu; 2 1 × OtBu, 1 × tBu; 6 1 × iPr; 7 1 × iPr, 2.73 ×
toluene; 8 2.88 × toluene; 9 1 × OtBu, 1.5 toluene + hexane; 10 2.83
× toluene; 11 1 × OtBu, 1.5 hexane + toluene) were refined using
geometry (SADI, SAME, FLAT) and Uij restraints (SIMU, DELU,
ISOR) implemented in SHELXL-97.15 The OH distances in 1 and 2
were restrained using DFIX.
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(OH)-
(OtBu)]2 (1) follow: A solution of (tBuO)2Si(OH)2 (0.54 g, 2.59
mmol) in THF (10 mL) was slowly added to a stirred solution of
[HC{C(Me)N(2,6-iPr2C6H3)}2]AlH2 (0.50 g, 1.12 mmol) in THF
(30 mL) at −108 °C. After reaching room temperature, the reaction
was stirred for 6 h giving a colorless solution. The solvent was
removed and the product crystallized from toluene as transparent
crystals. Yield: 0.79 g (76%). Mp 163 °C (dec). Anal. (%) Calcd for
C45H79AlN2O8Si2 (859.27 g mol−1): C 62.90, H 9.27, N 3.26. Found:
Figure 1. Compounds a and b.
A similar synthetic strategy may be used for the preparation
of simple 4R type heterobimetallic silicate Si−O−M−O−Si−
O−M′−O (M, M′ = different metals) rings, which to the best
of our knowledge are unknown. Until now, only monometallic
compounds with Si3MO4 rings6 and homobimetallic Si2M2O4
ring-containing species have been reported.6a,e,7,8 This is due to
a low degree of control over the association reactions and the
limited access to the M(O−Si−OH)2 precursors necessary for
the formation of the heterobimetallic 4R systems in a stepwise
reactions. To date, only three such compounds have been
reported on the basis of aluminum ([(2,6-iPr2C6H3)N(SiMe2-
iPr)SiO(OH)2]2AlC(SiMe3)3·3THF),9 niobium (tBu2Si-
(O)2[(tBu2Si(OH)O)2Nb]2[C5Me4EtNb(Cl)]μ2-Cl)(μ2-
O)2(μ3-O)),8 and tin ([Me2N(CH2)3]2Sn(OSitBu2OH)2),10
but only the aluminum compound can be prepared in a yield
over 40%. Silicon based systems with such vicinal Si−OH
groups are restricted to partially hydrolyzed silsesquioxanes,11
a
few acyclic Si(O−Si−OH)2 siloxanes,12 and only one example
of a silicate (tBuO)2Si[(μ-O)Si(OtBu)2(OH)]2.13 None of
these have been used for the preparation of bimetallic systems
with the desired connectivity. Herein, we report on a facile
synthesis of mono- and spirocyclic 4R type heterobimetallic
alumo- and gallodisilicates based on the [HC{C(Me)N(2,6-
iPr2C6H3)}2]M[(μ-O)Si(OtBu)2(OH)]2 (M = Al (1), Ga (2))
molecular disilicates.
C 62.97, H 9.28, N 3.28. FT-IR (KBr pellet) ν
̃
/cm−1: 3620, 3505 (w,
1
br, ν O−H). EI-MS: m/z (%) 859(31) [M]+, 41(100) [C3H5]+. H
3
NMR (400 MHz, C6D6, 20 °C): δ = 1.22 (d, 12 H, JH−H = 6.8 Hz,
CH(CH3)2), 1.30 (s, 36 H, C(CH3)3), 1.53 (s, 6 H, CH3), 1.53 (d, 12
3
H, JH−H = 6.8 Hz, CH(CH3)2), 3.04 (s, 2 H, OH), 3.47 (sept, 4 H,
3JH−H = 6.8 Hz, CH(CH3)2), 4.97 (s, 1 H, γ-H), 7.19 ppm (m, 6 H, H
of Ar). 13C{1H} NMR (75.43 MHz, C6D6, 20 °C): δ = 24.1 (CH3),
25.4 (CH(CH3)2), 26.3 (CH(CH3)2), 28.7 (CH(CH3)2), 31.9
(C(CH3)3), 72.2 (C(CH3)3), 99.2 (γ-C), 125.1, 127.6, 141.2, 145.0
(C of Ar), 171.8 ppm (CN). 29Si{1H} NMR (99.33 MHz, C6D6, 20
°C): δ = −97 ppm (O2Si(OtBu)2).
EXPERIMENTAL SECTION
■
General Considerations. All manipulations described below were
performed under a dried nitrogen atmosphere using Schlenk and
glovebox techniques. The solvents were purchased from Aldrich and
dried prior to use with an MBraun Solvent Purification System using
Grubs’ type columns. M′(NR2)4 (M′ = Ti, Zr, R = Me; M′ = Hf, R =
Et) were purchased from Aldrich and used as received, whereas
(tBuO)2Si(OH)2,13 [HC{C(Me)N(2,6-iPr2C6H3)}2]AlH2,14a [HC{C-
(Me)N(2,6-iPr2C6H3)}2]Ga(NHEt)2,14b Ti(NEt2)4,14c and Zr-
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(OH)-
(OtBu)]2 (2) follow: A solution of (tBuO)2Si(OH)2 (0.40 g, 1.80
mmol) in THF (10 mL) was slowly added to a stirred thawing
solution of [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga(NHEt)2 (0.50 g, 0.90
mmol) in THF (30 mL). The reaction mixture was allowed to reach
room temperature and stirred for 2 h giving a transparent solution.
The solvent was removed and the product crystallized from hexane as
colorless crystals. Yield: 0.51 g (65%). Mp 135 °C. Anal. (%) Calcd for
C45H79GaN2O8Si2 (902.01 g mol−1): C 59.92, H 8.83, N 3.11. Found:
14c
(NEt2)4 were prepared according to literature procedures. C6D6
and tol-d8 were distilled from Na/K alloy and degassed before use.
NMR spectroscopic data were recorded either on a Varian
NMRSystem 500 MHz, Bruker Avance III 400 MHz, or Bruker
Avance III 300 MHz spectrometer and referenced to residual protons
of the deuterated solvent. Mass spectrometry for compounds 3−11
was futile due to poor ionization of the molecules (ESI) or because the
molecular mass of the compounds lies outside the detection limits of
the available equipment (EI).15 Therefore, mass spectra of compounds
3−11 are not reported. Elemental analyses (C, H, N) were carried out
either on a VarioMICRO cube analyzer or in Galbraith Laboratories
Inc. (Knoxville, TN). Dried monocrystals were used for these analyses
for all compounds. All samples were prepared in a glovebox and kept
under a nitrogen atmosphere prior to the analysis. The high reactivity
of the compounds, varying content of solvents, and possible formation
of silicon and aluminum carbides during the combustion are
responsible for the observed differences between the theoretical and
C 60.2, H 8.9, N 3.2. FT-IR (ATR) ν
̃
/cm−1: 3690, 3509 (w, br, ν O−
H). EI-MS: m/z (%) 900(5) [M]+, 692(32) [M−(OH)2Si(OtBu)2]+,
1
41(100) [C3H5]+. H NMR (500 MHz, C6D6, 20 °C): δ = 1.21 (d,
6H, 3JH−H = 6.8 Hz, CH(CH3)2), 1.31 (s, 18H, (CH3)3COSi), 1.50 (s,
3
6 H, CH3), 1.55 (d, 6H, JH−H = 6.8 Hz, CH(CH3)2), 3.42 (s, 2H,
3
SiOH), 3.48 (sept, 4H, JH−H = 6.8 Hz, CH(CH3)2), 4.80 (s, 1H, γ-
CH), 7.10−7.25 ppm (m, 6H, H of Ar). 13C{1H} NMR (75.43 MHz,
C6D6, 25 °C): δ = 24.0 (CH3), 25.0 (CH(CH3)2), 25.8 (CH(CH3)2),
28.3 (CH(CH3)2), 31.5 (C(CH3)2), 71.9 (C(CH3)3), 97.0 (γ-CH),
124.6, 127.6, 140.6, 144.6 (C of Ar), 171.3 (CN) ppm. 29Si{1H}
NMR (99.33 MHz, C6D6, 20 °C): δ = −94 ppm (O2Si(OtBu)2).
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2M′(NEt2)2 (3 M′ = Zr; 5 M′ = Hf) follow: A solution of
1 (0.30 g, 0.35 mmol) in toluene (15 mL) was added dropwise to a
stirred solution of Zr(NEt2)4 (0.14 g, 0.38 mmol) for 3 or Hf(NEt2)4
(0.18 g, 0.38 mmol) for 5, respectively, in toluene (30 mL) at −78 °C.
1
experimental values. H NMR spectra of compounds 3−11 at room
1
temperature (25 °C) are too broad to be assigned; thus, H NMR
spectra are reported at 0 or −10 °C. Even at low temperatures, it is not
possible to accurately determine the coupling constants (JH−H) of
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Inorganic Chemistry
Article
The reaction mixture was allowed to reach room temperature and
stirred for additional 12 h resulting in a clear solution. After removing
all volatiles, the crude product was crystallized from toluene at −30 °C.
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2M′(NEt2)2 (4 M′ = Zr; 6 M′ = Hf) follow: A solution of
2 (0.20 g, 0.22 mmol) in THF (10 mL) was added dropwise to a
thawing solution of Zr(NEt2)4 (0.09 g, 0.24 mmol) for 4 or Hf(NEt2)4
(0.10 g, 0.24 mmol) for 6, in THF (10 mL). The reaction mixture was
allowed to reach room temperature and stirred for 2 h resulting in a
clear slightly yellow solution. After removing all volatiles, the crude
product was crystallized from hexane at −30 °C.
resulting in a slightly yellow solution. After removing all volatiles the
crude product was crystallized from toluene at −30 °C.
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2}2M (9 M = Zr, 11 M = Hf) follow: A solution of
Zr(NMe2)4 (0.03 g, 0.11 mmol) for 9 or Hf(NEt2)4 (0.04 g, 0.10
mmol) for 11, in THF (10 mL), was added dropwise to a thawing
solution of [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(OH)-
(OtBu)2]2 (2) (0.20 g, 0.22 mmol) in THF (30 mL). The reaction
mixture was allowed to slowly warm to room temperature and stirred
for additional 4 h, resulting in slightly yellow solutions. After removing
all volatiles, the crude product was crystallized from hexane at −30 °C.
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2}2Ti (7) follow: White crystals. Yield: 0.27 g (66%). Mp
126 °C (dec). Anal. (%) Calcd for C90H154Al2N4O16Si4Ti (1762.38 g
mol−1): C 61.34, H 8.81, N 3.18. Found: C 61.87, H 8.77, N 3.03. 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 1.00 (s, 18 H, C(CH3)3),
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2Zr(NEt2)2 (3) follow: White crystals. Yield: 0.28 g (74%).
Mp 273 − 275 °C (dec). Anal. (%) Calcd for C53H97AlN4O8Si2Zr
(1092.74 g mol−1): C 58.25, H 8.95, N 5.13. Found: C 57.87, H 8.92,
1
N 5.00. H NMR (500 MHz, toluene-d8, 0 °C): δ = 1.07 (s, 18 H,
3
3
3
C(CH3)3), 1.10 (d, 6 H, JH−H = 6.8 Hz, CH(CH3)2), 1.24 (t, 12 H,
1.02 (d, 6 H, JH−H = 6.5 Hz, CH(CH3)2), 1.07 (d, 6 H, JH−H = 6.5
Hz, CH(CH3)2), 1.30 (d, 6 H, 3JH−H = 6.5 Hz, CH(CH3)2), 1.31 (s, 18
H, C(CH3)3), 1.44 (s, 6 H, CH3), 1.48 (s, 18 H, C(CH3)3), 1.48 (s, 6
H, br, CH(CH3)2), 1.52 (d, 6 H, 3JH−H = 7.0 Hz, CH(CH3)2), 1.58 (s,
3
3JH−H = 6.8 NCH2CH3), 1.43 (d, 6 H, JH−H = 6.5 Hz, CH(CH3)2),
1.47 (d, 6 H, 3JH−H = 6.8 Hz, CH(CH3)2), 1.54 (s, 6 H, CH3), 1.58 (s,
18 H, C(CH3)3), 1.62 (d, 6 H, 3JH−H = 6.5 Hz, CH(CH3)2), 3.18 (dq,
3
2
3
6 H, CH3), 1.61 (d, 6 H, JH−H = 7.0 Hz, CH(CH3)2), 1.63 (d, 6 H,
4 H, JH−H = 13.8 Hz, JH−H = 6.8 Hz, NCH2CH3), 3.28 (sept, 2 H,
3JH−H = 7.0 Hz, CH(CH3)2), 1.68 (s, 18 H, C(CH3)3), 1.91 (d, 6 H,
3JH−H = 6.8 Hz, CH(CH3)2), 3.34 (dq, 4 H, 2JH−H = 13.8 Hz, 3JH−H
=
3JH−H = 6.5 Hz, CH(CH3)2), 3.24 (sept, 2 H, JH−H = 6.5 Hz,
3
6.8 Hz, NCH2CH3), 3.84 (sept, 2 H, 3JH−H = 6.5 Hz, CH(CH3)2), 5.05
(s, 1 H, γ-H), 7.12−7.20 ppm (m, 6 H, H of Ar).
CH(CH3)2), 3.40 (sept, 2 H, 3JH−H = 6.5 Hz, CH(CH3)2), 3.55 (sept,
3
3
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2Zr(NEt2)2 (4) follow: White crystals. Yield: 0.11 g (44%).
Mp 236−237 °C. Anal. (%) Calcd for C53H97GaN4O8Si2Zr (1092.74 g
mol−1): C 56.05, H 8.62, N 4.94. Found: C 56.47, H 8.32, N 4.66). 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 0.90 (t, 12 H, 3JH−H = 7.0
Hz, NCH2CH3), 0.97 (s, 18 H, C(CH3)3), 1.05 (d, 6 H, br,
CH(CH3)2), 1.07 (d, 6 H, br, CH(CH3)2), 1.28 (s, 6 H, CH3), 1.44 (s,
18 H, C(CH3)3), 1.52 (d, 6 H, 3JH−H = 7.0 Hz, CH(CH3)2), 1.61 (d, 6
2 H, JH−H = 7.0 Hz, CH(CH3)2), 3.69 (sept, 2 H, JH−H = 7.0 Hz,
CH(CH3)2), 4.93 (s, 2 H, γ-H), 7.14−7.42 ppm (m, 12 H, H of Ar).
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2}2Zr (8) follow: White crystals. Yield: 0.28 g (68%). Mp
121 °C (dec). Anal. (%) Calcd for C90H154Al2N4O16Si4Zr (1805.73 g
mol−1): C 59.86, H 8.60, N 3.10. Found: C 59.51, H 8.70, N 2.88. 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 1.00 (s, 18 H, C(CH3)3),
1.08 (s, 12 H, br, CH(CH3)2), 1.21 (s, 18 H, C(CH3)3), 1.37 (d, 6 H,
3JH−H = 6.5 Hz, CH(CH3)2), 1.48 (s, 6 H, CH3), 1.48 (s, 6 H, br,
CH(CH3)2), 1.53 (s, 12 H, br, CH(CH3)2), 1.53 (s, 18 H, C(CH3)3),
3
H, JH−H = 6.5 Hz, CH(CH3)2), 3.22 (dq, 4 H, br, NCH2CH3), 3.43
(dq, 4 H, br, NCH2CH3), 3.62 (sept, 4 H, br, CH(CH3)2), 4.77 (s, 1
H, γ-H), 7.02−7.40 ppm (m, 6 H, H of Ar).
3
1.59 (s, 6 H, CH3), 1.63 (d, 12 H, JH−H = 6.5 Hz, CH(CH3)2), 1.67
3
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2Hf(NEt2)2 (5) follow: White crystals. Yield: 0.30 g (73%).
Mp 269−271 °C (dec). Anal. (%) Calcd for C53H97AlN4O8Si2Hf
(1180.61 g mol−1): C 53.95, H 8.29, N 4.75. Found: C 53.08, H 8.20,
N 4.52. 1H NMR (500 MHz, toluene-d8, − 10 °C): δ = 1.08 (s, 18 H,
(s, 18 H, C(CH3)3), 1.83 (d, 6 H, JH−H = 6.5 Hz, CH(CH3)2), 3.21
3
(sept, 2 H, JH−H = 6.5 Hz, CH(CH3)2), 3.37 (sept, 2 H, br,
CH(CH3)2), 3.72 (sept, 4 H, br, CH(CH3)2), 5.00 (s, 2 H, γ-H),
7.14−7.31 ppm (m, 12 H, H of Ar).
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2}2Zr (9) follow: White crystals. Yield: 0.14 g (68%). Mp
> 400 °C. Anal. (%) Calcd for C90H154Ga2N4O16Si4Zr (1889.71 g
mol−1): C 57.15, H 8.21, N 2.96. Found: C 55.51, H 7.92, N 2.55. 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 0.99 (s, 18 H, C(CH3)3),
3
C(CH3)3), 1.10 (d, 6 H, JH−H = 7.0 Hz, CH(CH3)32), 1.24 (t, 12 H,
3JH−H = 6.5 Hz, NCH2CH3), 1.43 (d, 6 H, JH−H = 7.0 Hz,
3
CH(CH3)2), 1.47 (d, 6 H, JH−H = 6.5 Hz, CH(CH3)2), 1.53 (s, 6 H,
3
CH3), 1.58 (s, 18 H, C(CH3)3), 1.63 (d, 6 H, JH−H = 6.5 Hz,
3
3
3
CH(CH3)2), 3.26 (br, 4 H, NCH2CH3), 3.26 (sept, 2 H, JH−H = 7.0
1.02 (d, 6 H, JH−H = 6.6 Hz, CH(CH3)2), 1.08 (d, 6 H, JH−H = 6.6
Hz, CH(CH3)2), 1.30 (d, 6 H, br, CH(CH3)2), 1.31 (s, 18 H,
C(CH3)3), 1.44 (s, 6 H, CH3), 1.48 (s, 18 H, C(CH3)3), 1.52 (d, 6 H,
br, CH(CH3)2), 1.58 (s, 6 H, CH3), 1.62 (d, 12 H, br, CH(CH3)2),
1.63 (d, 6 H, br, CH(CH3)2), 1.69 (s, 18 H, C(CH3)3), 1.90 (d, 6 H,
3
Hz, CH(CH3)2), 3.36 (br, 4 H, NCH2CH3), 3.82 (sept, 2 H, JH−H
=
6.5 Hz, CH(CH3)2), 5.04 (s, 1 H, γ-H), 7.13−7.18 ppm (m, 6 H, H of
Ar).
Details for [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2Hf(NEt2)2 (6) follow: White crystals. Yield: 0.28 g (74%).
Mp 273−275 °C (dec). Anal. (%) Calcd for C53H97AlN4O8Si2Hf
(1221.89 g mol−1): C 52.05, H 8.00, N 4.58. Found: C 52.45, H 8.20,
3
3JH−H = 6.8 Hz, CH(CH3)2), 3.24 (sept, 2 H, JH−H = 6.8 Hz,
CH(CH3)2), 3.39 (sept, 2 H, 3JH−H = 6.6 Hz, CH(CH3)2), 3.54 (sept,
3
3
2 H, JH−H = 6.5 Hz, CH(CH3)2), 3.69 (sept, 2 H, JH−H = 6.5 Hz,
CH(CH3)2), 4.93 (s, 2 H, γ-H), 7.18−7.42 ppm (m, 12 H, H of Ar).
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2}2Hf (10) follow: White crystals. Yield: 0.25 g (58%). Mp
132 °C (dec). Anal. (%) Calcd for C90H154Al2N4O16Si4Hf (1893.00 g
mol−1): C 57.10, H 8.20, N 2.96. Found: C 57.11, H 8.34, N 2.83. 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 1.00 (s, 18 H, C(CH3)3),
1
N 4.12. H NMR (500 MHz, toluene-d8, −10 °C): δ = 1.07 (s, 18 H,
3
C(CH3)3), 1.09 (d, 6 H, JH−H = 6.8 Hz, CH(CH3)32), 1.23 (t, 12 H,
3JH−H = 6.8 Hz, NCH2CH3), 1.42 (d, 6 H, JH−H = 6.8 Hz,
3
CH(CH3)2), 1.47 (d, 6 H, JH−H = 6.8 Hz, CH(CH3)2), 1.52 (s, 6 H,
3
CH3), 1.56 (s, 18 H, C(CH3)3), 1.61 (d, 6 H, JH−H = 6.8 Hz,
2
3
CH(CH3)2), 3.17 (dq, 4 H, JH−H = 13.4 Hz, JH−H = 6.8 Hz,
NCH2CH3), 3.27 (sept, 2 H, 3JH−H = 6.8 Hz, CH(CH3)2), 3.33 (dq, 4
H, br, NCH2CH3), 3.83 (sept, 2 H, 3JH−H = 6.8 Hz, CH(CH3)2), 5.03
(s, 1 H, γ-H), 7.02−7.18 ppm (m, 6 H, H of Ar).
3
1.07 (d, 6 H, br, CH(CH3)2), 1.08 (d, 6 H, JH−H = 6.25 Hz,
CH(CH3)2), 1.22 (s, 18 H, C(CH3)3), 1.37 (d, 6 H, 3JH−H = 6.25 Hz,
CH(CH3)2), 1.48 (s, 6 H, CH3), 1.48 (s, 6 H, br, CH(CH3)2), 1.53 (s,
12 H, br, CH(CH3)2), 1.53 (s, 18 H, C(CH3)3), 1.59 (s, 6 H, CH3),
1.63 (d, 12 H, 3JH−H = 7.5 Hz, CH(CH3)2), 1.67 (s, 18 H, C(CH3)3),
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)-
(OtBu)2]2}2M (7 M = Ti, 8 M = Zr, 10 M = Hf) follow: A solution
of Ti(NMe2)4 (0.05 g, 0.23 mmol) for 7, Zr(NMe2)4 (0.06 g, 0.23
mmol) for 8, or Hf(NEt2)4 (0.11 g, 0.23 mmol) for 10, respectively, in
toluene (10 mL) was added dropwise to a stirred solution of
[HC{C(Me)N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(OH)(OtBu)2]2 (1) (0.40
g, 0.46 mmol) in toluene (30 mL) at −78 °C. The reaction mixture
was allowed to slowly warm to room temperature and stirred for 16 h,
3
3
1.82 (d, 6 H, JH−H = 7.5 Hz, CH(CH3)2), 3.21 (sept, 2 H, JH−H
=
3
6.25 Hz, CH(CH3)2), 3.36 (sept, 2 H, JH−H = 6.25 Hz, CH(CH3)2),
3.71 (sept, 4 H, br, CH(CH3)2), 5.01 (s, 2 H, γ-H), 7.14−7.30 ppm
(m, 12 H, H of Ar).
Details for {[HC{C(Me)N(2,6-iPr2C6H3)}2]Ga[(μ-O)Si(μ-O)-
(OtBu)2]2}2Hf (11) follow: White crystals. Yield: 0.15 g (70%). Mp
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1H NMR spectra of 1 and 2 are devoid of the hydride or
NHEt signals, but signals for the (tBuO)2Si (δ 1.30 (1), 1.31
ppm (2)) and for the Si−OH groups (δ 3.04 (1), 3.45 ppm
(2)) are present. The singlets at δ −97 (1), −94 ppm (2) in the
29Si{1H} NMR spectra confirmed the presence of two
equivalent silicate substituents attached to the metal atom.
This clearly shows that the hydrogen bond between the silicate
groups is not strong enough to persist in solution and,
consequently, the free movement of these groups is responsible
for the slow decomposition of 1 and 2 in solution via
condensation of the OH groups. Unfortunately, despite all our
efforts, we have not been able to elucidate the decomposition
pathway of 1 and 2, and thus, the only identified decomposition
product corresponds to the ligand [HC{C(Me)N(2,6-
iPr2C6H3)}2]H.
> 400 °C. Anal. (%) Calcd for C90H154Ga2N4O16Si4Hf (1976.98 g
mol−1): C 54.63, H 7.85, N 2.83. Found: C 54.51, H 7.62, N 2.50. 1H
NMR (500 MHz, toluene-d8, − 10 °C): δ = 1.12 (s, 18 H, C(CH3)3),
1.17 (d, 6 H, br, CH(CH3)2), 1.19 (d, 6 H, br, CH(CH3)2), 1.20 (d, 6
H, br, CH(CH3)2, 1.37 (d, 6 H, br, CH(CH3)2), 1.49 (s, 6 H, CH3),
1.57 (s, 6 H, br, CH(CH3)2), 1.53 (s, 12 H, br, CH(CH3)2), 1.62 (d,
12 H, br, CH(CH3)2), 1.67 (s, 18 H, C(CH3)3), 3.15 (sept, 2 H, br,
CH(CH3)2), 3.27 (sept, 2 H, br, CH(CH3)2), 3.33 (sept, 4 H, br,
CH(CH3)2), 3.76 (sept, 4 H, br, CH(CH3)2), 4.81 (s, 2 H, γ-H),
7.05−7.16 ppm (m, 12 H, H of Ar).
RESULTS AND DISCUSSION
■
The synthesis of 1 and 2 represents a major challenge due the
reduced control over the kinetic and thermodynamic factors
involved in its isolation. Compared to a and b (Figure 1),
where the formation of the SiO2M rings is hampered due the
high steric bulk of the ligand [HC{C(Me)N(2,6-iPr2C6H3)}2]−
and its high ring strain, the substitution of the M−OH group
for a highly reactive M−O−Si−OH substituent reduces this
steric control over the condensation of the terminal −OH
groups and, therefore, over the formation of the six-membered
Si−O−Si−O−M−O ring.19 To suppress this condensation, the
reactions between [HC{C(Me)N(2,6-iPr2C6H3)}2]AlH2 or
[HC{C(Me)N(2,6-iPr2C6H3)}2]Ga(NHEt)2 and 2 equiv of
(tBuO)2Si(OH)2 were performed at low temperature. The slow
addition of the silanediol and relatively short reaction times
resulted in a clean formation of the alumo- and gallodisilicates
[HC{C(Me)N(2,6-iPr2C6H3)}2]M[(μ-O)Si(OH)(OtBu)2]2
(M = Al (1), Ga (2)) in high yields as white crystalline solids
(Scheme 1). The compounds are highly soluble in organic
Single crystals of the disilicates 1 and 2 were grown from
saturated toluene (1) or hexane (2) solutions at −30 °C.
Compounds 1 and 2 are isomorphous and crystallize in the
monoclinic space group P21/n with one molecule of the
disilicate in the asymmetric unit (Figure 2). The solid-state
Scheme 1. Synthesis of Disilicates 1 and 2
Figure 2. Molecular structures of 1 and 2. Carbon ellipsoids and
carbon-bound hydrogen atoms have been omitted for clarity. Thermal
ellipsoids of noncarbon atoms are set at 50% probability level.
structures of both compounds present characteristics features
analogous to those observed in a and b. Compounds 1 and 2
feature a highly distorted tetrahedral geometry for the Al or Ga
atoms, as a result of the reduced bite angle of the ligand
[HC{C(Me)N(2,6-iPr2C6H3)}2]− [N−M−N 98.6(1)° (1),
99.2(1)° (2)].
solvents, display similar spectroscopic characteristics, are
thermally stable up to 135 °C, and can be stored in a
crystalline form for several months under an inert atmosphere.
However, slow decomposition occurs in solution after 24 h, and
it is likely caused by the condensation of the silanol groups with
the concomitant loss of water (vide infra).
The M−O bond distances reflect the size and the acidity of
the metals and are shorter for the aluminum atom in 1
(1.701(2), 1.715(1) Å) than for the gallium atom in 2
(1.782(1), 1.801(1) Å), but analogous to a (1.711(1) Å) and b
(1.800(1) Å), respectively. The SiO4 moieties in compounds 1
and 2 are similar, and all the Si atoms exhibit a higher distorted
tetrahedral geometry as can be seen from the values for the O−
Si−O angles [103.9(3)−113.5(8) ° (1), 102.7(1)−114.5(1) °
(2)]. The Si−O bond lengths in 1 (1.586(2)−1.640(2) Å) and
in 2 (1.582(1)−1.638(1) Å) are identical; however, they feature
large variations. Table 1 contains selected distances and angles
for compounds 1 and 2, whereas the crystallographic details are
summarized in Table 2. The molecular structures also confirm
the formation of an intramolecular hydrogen bond between one
Si−OH group and an oxygen atom from a OtBu group from
the second silicate moiety with a O···O distance of 2.972(2) Å
(1) and 2.954(2) Å (2). These distances are slightly longer
The solid state IR spectra of compounds 1 and 2 show two
̃
different bands for two nonequivalent Si−OH moieties (ν =
3620, 3505 for 1 and 3690, 3509 for 2 cm−1), with the broader
lower energy vibration band, indicating the formation of a
hydrogen bond. The presence of two different OH groups and
one hydrogen bond in the solid state was confirmed by X-ray
diffraction studies and will be discussed later. On the other
hand, NMR studies reveal different behavior in solution.
1
Through H NMR spectroscopy it is possible to observe a
signal pattern for the hydrocarbon backbone of the ligand that
corresponds to a C2v symmetry. This symmetry is characteristic
for two equivalent substituents attached to the M center in the
[HC{C(Me)N(2,6-iPr2C6H3)}2]MX2 systems. Furthermore,
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5b
(μ-O)Si(OH)(OtBu)2 (2.753(2) Å) or in a (Si−OH···O-
(H)Al; 2.762(3) Å) and b (Si−OH···O(H)Ga; 2.705(4) Å)
containing strong intramolecular hydrogen bonds. The hydro-
gen bonds found in 1 and 2 are the weakest in the series of
related silicates (Table S1 in Supporting Information). The
steric bulk around the metal atom in 1 and 2 results in one
nearly linear M−O−Si angle (1 174.7(1)°, 2 (172.4(1)°),
whereas the other is with 136.3(1)° (1) or 129.8(1)° (2)
comparable to those found in a and b where the formation of
the hydrogen bonds caused rather acute M−O−Si angles
[132.8(1)° (a) and 128.9(2)° (b)]. The flexibility of the M−
O−Si angle is well demonstrated by the values observed in the
following related compounds: [HC{C(Me)N(2,6-
iPr2C6H3)}2]Al(SH)(μ-O)Si(OH)(OtBu)2 146.5(1)°,5a [(HC-
{C(Me)N(2,6-iPr2C6H3)}2)Al(SLi)(μ-O)Si(OLi·2thf)-
(OtBu)2]2 147.4(2)°,5a [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga-
(NHEt)(μ-O)Si(OH)(OtBu)2 136.6(1)°, and [HC{C(Me)N-
(2,6-iPr2C6H3)}2]Ga(Cl)(μ-O)Si(OH)(OtBu)2 138.2(1)°.5b It
is noteworthy that the second Si−OH group is not involved in
any hydrogen bond due to its location between hydrocarbon
groups (two iPr and three tBu groups) away from the inorganic
core. This explains the stability of 1 and 2 in the solid state.
Consequently, the reactivities of 1 and 2 were explored with
group 4 metal amides to determine the effect of one extra Si−O
unit in the Si−O−M−O−Si−O−M′−O eight-membered ring
on its flexibility and geometry when compared to the Si−O−
Table 1. Selected Bond Lengths [Å] and Angles [deg] for
Compounds 1 and 2
c
d
1
2
M−O(Si)
M−N
1.701(2)
1.782(1)
1.715(2)
1.890(2)
1.892(2)
1.586(2)
1.607(2)
1.630(2)
1.640(2)
1.616(2)
113.2(1)
98.6(1)
1.801(1)
1.919(2)
1.932(2)
1.582(1)
1.606(1)
1.638(2)
1.628(1)
1.626(3)
113.1(1)
99.2(1)
Si−O(M)
Si−O(H)
a
Si−O(tBu)
O−M−O
N−M−N
b
O−Si−O
105.8(1)−113.3(1)
103.9(3)−113.5(8)
174.7(1)
105.5(1)−114.6(1)
102.7(1)−114.5(9)
172.4(1)
M−O−Si
136.3(1)
129.8(1)
a
b
Average distances. Smallest and widest angles for Si1 or Si2,
c
d
respectively. M = Al. M = Ga.
than the O···O distance in the intermolecular Si−OH···OtBu
interaction in the related silicate [HC{C(Me)N(2,6-
iPr2C6H3)}2]Al(SH)(μ-O)Si(OH)(OtBu)2 (2.891(2) Å).
However, they are significantly longer compared to those
O···O distances in [HC{C(Me)N(2,6-iPr2C6H3)}2]Ga(NHEt)-
5a
Table 2. Crystallographic Information for Compounds 1−11
1
2
3
4
5
6
chemical formula C45H79AlN2O8Si2
C45H79GaN2O8Si2
902.00
C53H97AlN4O8Si2Zr
1092.73
C53H97GaN4O8Si2Zr
1135.47
C53H97AlHfN4O8Si2
1180.00
C53H97GaHfN4O8Si2
1222.74
fw
859.26
space group
a/Å
P21/n (No. 14, variation) P21/n (No. 14, variation)
P21/c (No. 14)
12.4257(4)
13.0671(5)
37.4401(13)
91.881(1)
6075.8(4)
4
P21/c (No. 14)
12.4717(5)
13.0571(5)
37.4659(14)
91.955(1)
6097.6(4)
4
P21/c (No. 14)
12.4329(7)
13.0387(7)
37.4929(19)
91.999(1)
6074.2(6)
4
P21/c (No. 14)
12.4739(4)
13.0290(4)
37.5017(11)
92.158(1)
6090.5(3)
4
14.278(2)
20.082(3)
17.910(2)
103.98(2)
4983.2(12)
4
14.501(2)
19.850(2)
17.756(3)
102.88(2)
4982.4(12)
4
b/Å
c/Å
β/deg
V/Å3
Z
T/K
100(2)
0.710 73
0.138
100(2)
0.710 73
0.649
100(2)
100(2)
100(2)
100(2)
λ/Å
0.710 73
0.285
0.710 73
0.710 73
0.710 73
μ/mm−1
ρcalcd/g cm−3
R1 (I > 2σ(I))
0.702
1.821
2.235
1.145
1.202
1.195
1.237
1.290
1.333
a
0.0529
0.0326
0.0297
0.0296
0.0291
0.0188
b
wR2 (all data)
0.1285
0.0844
0.0721
0.0701
0.0626
0.0427
7·2.73toluene
8·2.88toluene
9·1.12toluene·0.38hexane
10·2.83toluene
11·1.378hexane·0.122toluene
chemical
formula
C109.11H175.84Al2N4O16Si4Ti
C110.16H177.04Al2N4O16Si4Zr
C99.38H172.75Ga2N4O16Si4Zr
C109.81H176.64Al2HfN4O16Si4
C99.12H174.26Ga2HfN4O16Si4
fw
2013.92
2071.06
2022.68
P21/c (No. 14)
13.7916(3)
33.8092(8)
24.2899(6)
101.304(1)
11106.3(5)
4
2153.72
2108.45
space group
a/Å
P21/n (No. 14, variation)
12.7898(13)
46.817(5)
19.4595(19)
90.127(2)
11652(2)
4
P21/n (No. 14, variation)
12.768(2)
46.784(5)
19.654(3)
90.24(3)
11740(3)
4
P21/n (No. 14, variation)
12.777(2)
46.851(5)
19.643(2)
90.32(2)
11758(3)
4
P21/c (No. 14)
13.8092(9)
33.807(2)
24.3049(15)
101.2160(10)
11130.1(12)
4
b/Å
c/Å
β/deg
V/Å3
Z
T/K
100(2)
100(2)
100(2)
100(2)
100(2)
λ/Å
0.710 73
0.190
0.710 73
0.209
0.710 73
0.676
0.710 73
1.002
0.710 73
1.513
μ/mm−1
ρcalcd/g cm−3
R1 (I > 2σ(I))
1.148
1.172
1.210
1.217
1.258
a
0.0530
0.0615
0.0346
0.0415
0.0496
b
wR2 (all data)
0.1298
0.1464
0.0875
0.0935
0.0955
a
b
2
2
R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. wR2 = [Σw(Fo − Fc2)2/Σ(Fo )2]1/2
.
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Scheme 2. Synthesis of Compounds 3−11
Figure 3. VT-500 MHz 1H NMR spectra of compound 3 in toluene-d8, in the 5.2−0.8 ppm region. Peaks are assigned for the spectrum measured at
0 °C.
M−O−M′−O six-membered rings based on a and b.3b,c The
high oxophilicity of group 4 metals, the high reactivity of their
amides, and the relevance of zeolites modified with group 4
metals in catalytic processes were relevant factors for the choice
of these metals.1b,g−i Furthermore, their stable tetrahedral
coordination environment can help to create 4R rings with the
same coordination numbers observed in zeolites. Thus,
reactions between 1 or 0.5 equiv of a group 4 amide
M′(NEt2)4 (M′ = Ti, Zr, Hf) and 1 or 2 were performed.
The equimolar reactions led to the isolation of four crystalline
solids for the heavier group 4 metals with formulas [HC{C-
( M e ) N ( 2 , 6 - i P r 2 C 6 H 3 ) } 2 ] M [ ( μ - O ) S i ( μ - O ) -
(OtBu)2]2M′(NEt2)2 [M′ = Zr, M = Al (3), Ga (4); M′ = Hf,
M = Al (5), Ga (6)] and the desired 4R topology. However, the
reaction between Ti(NEt2)4 and compound 1 always resulted
in the formation of the spiro-cyclic compound {[HC{C(Me)-
N(2,6-iPr2C6H3)}2]Al[(μ-O)Si(μ-O)(OtBu)2]2}2Ti (7). The
titanium derivatives [HC{C(Me)N(2,6-iPr2C6H3)}2]M[(μ-
O)Si(μ-O)(OtBu)2]2Ti(NEt2)2 (M = Al, Ga) could not be
isolated even with a large excess of the amide. Such reactions
resulted mainly in the decomposition of 1 and 2 as confirmed
1
by H NMR analysis. However, the use of 0.5 equiv of amide
toward 1 or 2 led to the clean isolation of 7 and four other
spiro-cyclic derivatives {[HC{C(Me)N(2,6-iPr2C6H3)}2]M[(μ-
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Table 3. Selected Bond Lengths [Å] and Angles [deg] for the Inorganic Rings in Compounds 3−6
b,
d
c
,
d
b,
e
c
,
e
b
,
d
c
,
d
b,
e
c,e
distance (Å)
3
4
5
6
angle (deg)
3
4
5
6
M′−O(Si)
1.970(1)
1.970(1)
1.702(1)
1.721(1)
1.624(1)
1.623(1)
1.601(1)
1.605(1)
2.038(2)
1.902(2)
1.967(1)
1.970(1)
1.781(1)
1.803(1)
1.624(1)
1.625(1)
1.599(1)
1.600(1)
2.038(2)
1.938(2)
1.634(2)
1.949(2)
1.954(2)
1.704(2)
1.721(2)
1.629(2)
1.624(2)
1.600(2)
1.606(2)
2.024(2)
1.902(3)
1.952(1)
1.952(1)
1.782(1)
1.803(1)
1.625(1)
1.627(1)
1.599(1)
1.598(1)
2.027(2)
1.936(2)
1.634(2)
O−Si−O
110.8(1)
112.5(1)
152.3(1)
155.1(1)
151.1(1)
159.7(1)
108.9(1)
103.6(1)
116.1(1)
112.4(1)
113.5(1)
152.0(1)
157.3(1)
145.6(1)
157.6(1)
108.7(1)
103.4(1)
116.8(1)
110.6(1)
112.4(1)
152.2(1)
155.5(1)
150.8(1)
159.8(1)
108.9(1)
103.9(1)
115.9(1)
111.4(1)
113.4(1)
151.7(1)
157.8(1)
145.4(1)
157.9(1)
109.0(1)
103.9(1)
116.6(1)
M−O(Si)
Si−O(M′)
Si−O(M)
M′−O−Si
M−O−Si
O−M−O
N−M−N
O−E−O
a
M′−N
M−N
a
a
Si−O(OtBu)
1.634(1)
b
1.635(2)
a
c
d
e
Average bond lengths. M = Al. M = Ga. M′ = Zr. M′ = Hf.
Table 4. Selected Bond Lengths [Å] and Angles [deg] for the Inorganic Rings in Compounds 7−11
b,
d
b,
e
c
,
e
b,
f
c
,f
b,
d
b,
e
c
,
e
b,
f
c,f
distance (Å)
7
8
9
10
11
angle (deg)
7
8
9
10
11
M′−O(Si)
1.792(1)
1.799(1)
1.802(1)
1.808(1)
1.700(1)
1.708(1)
1.718(1)
1.720(1)
1.635(1)
1.636(1)
1.646(1)
1.647(1)
1.594(1)
1.594(1)
1.599(1)
1.599(1)
1.911(2)
1.626(1)
1.928(2)
1.943(2)
1.943(2)
1.950(2)
1.698(2)
1.706(2)
1.713(2)
1.718(2)
1.625(2)
1.627(2)
1.631(2)
1.636(2)
1.592(2)
1.599(2)
1.600(2)
1.603(2)
1.907(3)
1.938(2)
1.948(2)
1.945(2)
1.952(2)
1.796(2)
1.784(2)
1.805(2)
1.786(2)
1.628(2)
1.638(2)
1.629(2)
1.633(2)
1.595(2)
1.597(2)
1.595(2)
1.595(2)
1.948(2)
1.920(2)
1.933(2)
1.934(2)
1.942(2)
1.701(2)
1.712(2)
1.719(2)
1.720(2)
1.626(2)
1.629(2)
1.633(2)
1.636(2)
1.593(2)
1.594(2)
1.602(2)
1.605(2)
1.908(3)
1.931(3)
1.939(3)
1.940(3)
1.944(3)
1.789(3)
1.791(3)
1.798(3)
1.805(3)
1.626(3)
1.628(3)
1.631(3)
1.636(3)
1.594(3)
1.594(3)
1.595(3)
1.595(3)
1.948(4)
O−M′−O
106.4(1)
106.8(1)
112.4(1)
112.4(1)
109.9(1)
110.8(1)
110.9(1)
111.2(1)
155.4(1)
157.0(1)
158.1(1)
158.7(1)
151.9(1)
155.5(1)
156.7(1)
158.9(1)
104.6(1)
105.2(1)
113.4(1)
113.4(1)
109.7(1)
111.0(1)
111.0(1)
111.0(1)
152.8(1)
155.0(1)
158.0(1)
159.0(1)
151.0(1)
155.8(1)
158.6(1)
161.7(1)
104.7(1)
105.9(1)
112.7(1)
114.8(1)
112.3(1)
111.2(1)
112.2(1)
110.1(1)
156.7(1)
159.1(1)
158.2(1)
156.4(1)
154.4(1)
152.5(1)
153.0(1)
156.2(1)
105.3(1)
106.0(1)
113.3(1)
113.5(1)
109.9(1)
111.0(1)
111.1(1)
111.1(1)
152.8(1)
154.8(1)
157.2(1)
158.6(1)
150.9(1)
155.9(1)
158.5(1)
161.3(1)
105.2(1)
106.4(1)
112.6(1)
114.6(1)
110.1(1)
111.4(1)
112.3(1)
112.4(1)
156.4(1)
156.5(1)
157.7(2)
158.6(2)
152.5(2)
153.4(2)
154.3(2)
156.1(2)
O−M−O
O−Si−O
M−O(Si)
Si−O(M′)
Si−O(M)
M′−O−Si
M−O−Si
a
M−N
a
Si−O(tBu)
1.625(2)
1.630(2)
1.627(2)
1.628(3)
f
a
b
c
d
e
Average bond lengths. M = Al. M = Ga. M′ = Ti. M′ = Zr. M′ = Hf.
O)Si(μ-O)(OtBu)2]2}2M′ [M′ = Zr, M = Al (8), Ga (9); M′ =
Hf, M = Al (10), Ga (11)] with spiro-4R topology (Scheme 2).
Until now, we have not been able to isolate any titanium
derivative of 2. This is probably due to the high acidity of
titanium and weaker Ga−O bonds when compared to the Al−
O bond energy. Compounds 3−11 are highly soluble in organic
solvents including hexane, which makes their isolation rather
difficult and leads to lower yields.
observed. This confirms the high flexibility of the inorganic
rings at low temperature, but also the steric bulk of the ligand L,
which prevents the coalescence at high temperatures (Figures
S1−S12 in Supporting Information).
The cyclic derivatives 3−6 crystallized from toluene in the
monoclinic space group P21/c with one molecule of the
corresponding compound in the asymmetric unit and are
isomorphous. The alumodisilicate spirocyclic derivatives 7, 8,
and 10 are isomorphous and crystallized from toluene in the
monoclinic space group P21/n with one molecule of the
particular compound and 2.73, 2.88, or 2.83 molecules of
toluene, respectively, in the asymmetric unit. Finally, 9 and 11
crystallized from a hexane/toluene mixture in the monoclinic
space group P21/c, with one molecule of the compound and a
mixture of hexane and toluene (together 1.5 molecules) in the
asymmetric unit (Tables 2−4, Figure 4).
1
The absence of signals for the −OH groups in IR and H
NMR spectra of 3−6 together with the presence of signals for
the NEt2 substituents confirmed the deprotonation of the
silanol moieties and the concomitant formation of the M′−O
1
bonds. The H NMR spectra of the heterometallic derivatives
exhibit broad signals at room temperature. This was attributed
to a fluxional behavior caused by the flexible eight-membered
1
rings. In consequence, low temperature H NMR experiments
were performed for all compounds observing decoalescence for
most of the signals in the spectra in the range 0 to −10 °C with
a pattern that corresponds to C2v or D4h symmetries for the
cyclic (3−6) and spiro-cyclic (7−11) derivatives, respectively.
Full variable temperature 1H NMR experiments were
performed for 3 and 7 between −50 (for 3) or −70 (for 7)
and 85 °C, respectively (Figure 3). However, even in this range,
full coalescence or decoalescence of the signals could not be
All compounds exhibit eight-membered inorganic rings with
M(O−Si−O)2M′ connectivity in a boat−chair conformation
for the cyclic derivatives 3−6 and two almost perpendicular
diplanar rings for the spirocyclic compounds 7−11. It is
noteworthy that the size of the spirocyclic inorganic cores is
0.81 nm. The bond lengths in the inorganic rings are similar for
both cyclic and spirocyclic compounds, and the size is related to
the acidity of the metal atom M′(IV) or M(III) present in the
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Figure 4. Molecular structures of 3−11. Carbon ellipsoids and carbon-bound hydrogen atoms have been omitted for clarity. Thermal ellipsoids of
noncarbon atoms are set at 50% probability level.
molecule, where shorter distances for the more acidic atoms are
observed. Thus, M′−O bond lengths for the heavier M′(IV)
zirconium and hafnium compounds are similar (1.931(3) Å ≤
M′−O ≤ 1.970(1) Å, for 3−6, 8−11) but longer than those
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Article
found in the titanium spiro-cycle 7 (1.792(1) Å ≤ Ti−O ≤
1.808(1) Å). Nevertheless, those distances are similar to the
related tetracoordinated group 4 alumo- and gallosilicates
[HC{C(Me)N(2,6-iPr2 C6 H3 )}2 ]M(μ-O)2 Si(μ-O)-
(OtBu)2M′(NR2)2 or [(HC{C(Me)N(2,6-iPr2C6H3)}2)M(μ-
O)2Si(μ-O)(OtBu)2]2M′ (M′ = Zr or Hf, M = Al, 1.961(2) Å
≤ M′−O ≤ 1.987(1) Å; M = Ga, 1.952(3) Å ≤ M′−O ≤
1.989(2) Å and M′ = Ti, M = Al, 1.832(2) Å ≤ Ti−O ≤
1.842(2) Å),5b,c as well as to the tetra- and pentacoordinated
group 4 silicates M′[OSi(OtBu)3]4, M′[OSi(μ-OtBu)(OtBu)2]-
[OSi(OtBu)3]3 (1.945(6) ≤ Zr−O ≤ 2.038(6);20a 1.912(4) ≤
Hf−O ≤ 2.021(4)20a,b Å) and [(tBuO)2Ti{μ-O2Si[OSi-
(OtBu)3]2}]2 (1.812(4) ≤ Ti−O ≤ 1.816(4) Å).7n The M−
O bond distances for the heterobimetallic alumodisilicates 3, 5,
7, 8, 10 (1.700(1) Å ≤ Al−O ≤ 1.721 (2) Å) are shorter than
those found in the gallodisilicates 4, 6, 9, 11 (1.781(1) Å ≤
Ga−O ≤ 1.805(3) Å) and in both cases comparable to those in
the related group 4 alumo- and gallosilicate derivatives
[HC{C(Me)N(2,6-iPr2 C6 H3 )}2 ]M(μ-O)2 Si(μ-O)-
(OtBu)2M′(NR2)2 or [(HC{C(Me)N(2,6-iPr2C6H3)}2)M(μ-
O)2Si(μ-O)(OtBu)2]2M′ (M′ = Ti, Zr or Hf, M = Al, 1.716(2)
Å ≤ Al−O ≤ 1.732(1) Å; M = Ga, 1.800(2) Å ≤ Ga−O ≤
1.817(3) Å).5b,c They are also similar to those found in the
alumosilicates Al[OSi(OtBu)3]3(iPrOH)·1/2[Al(OiPr)3]4,
[(OiPr)2AlOSi(OtBu)3]2, {[HC{C(Me)N(2,6-iPr2C6H3)}2]Al-
(μ-O)[(tBuO)2Si](μ-O)}2, and the silicoalumophosphate
[(tBuO)3SiO]2Al[(μ-O)2P(OtBu)2]2Al(Me)-OSi(OtBu)3
(1.684(6) ≤ Al−O ≤ 1.721(2) Å),7n,21a,b and are in the range
observed in the mineral gismondine (Al−O ∼ 1.74 Å)20a or the
synthetic zeolites ABW (LiMSiO4·H2O, M = Al, Ga; Al−O ∼
1.75 Å, Ga−O ∼ 1.804(4) Å, respectively)22b but shorter than
those in the bridged system{Me2Al[μ-OSi(OtBu)3]2}21c (Al−O
∼ 1.865(4) Å). In all compounds, the geometry of the silicate
moiety remains almost unchanged as is evidenced by similar
Si−O bond lengths in all compounds (1.623(1) Å ≤ Si−O(M′)
≤ 1.647.(1) Å, 1.593(2) Å ≤ Si−O(M) ≤ 1.606(2) Å, for 3 −
11), which are comparable to the average distance in several
zeolite frameworks (1.61 Å).22 The angles in the inorganic rings
are similar for all the compounds although a higher flexibility is
observed in the gallodisilicate derivatives (4, 6, 9, 11) due the
longer Ga−O bonds: M′−O−Si (151.7(1)−159.1(1)°) and the
Ga−O−Si (145.4(1)−157.9(1)°) angles compared to the
respective alumodisilicates (3, 5, 8, 10) M′−O−Si
(152.2(1)−155.5(1)°) and Al−O−Si (150.8(1)−159.8(1)°)
angles. Nevertheless, similar O−M−O (112.4(1)−116.8(1)°),
O−M′−O (104.6(1)−109.0(1)°) and O−Si−O (109.7(1)−
113.5(1)°) angles are found in both systems. The angles
observed in 3−11 are similar to those found in the eight
membered cycles present in the titanosilicate [(tBuO)2Ti{μ-
O2Si[OSi(OtBu)3]2}]2,13 the alumosilicate {[HC{C(Me)N-
(2,4,6-Me3C6H2)}2]Al(μ-O)[(tBuO)2Si](μ-O)}2,7n and the
alumoazasilicates [(2,6-iPr2C6H3)N(SiMe3)Si(OH)O2Al-
(iBu)·thf]2, [(2,6-iPr2-C6H3)N(SiMe2iPr)SiO3Al·thf]4, [(2,6-
Me2-C6H3)N(SiMe3)SiO3Al·thf]4, and [(2,6-Me2-C6H3)N-
CONCLUSION
■
The stabilization of the first metallodisilicates based on
aluminum and gallium with two terminal Si−OH groups was
achieved by kinetic control using a bulky β-diketiminate ligand
and protecting OtBu groups. The compounds have been
successfully used in the preparation of cyclic and spirocyclic
heterobimetallic silicates containing group 4 metals and 4R
rings known from zeolites. The spirocyclic compounds feature
unique inorganic silicate cores of 0.81 nm, and neither of these
compounds have any precedent in the literature. The disilicates
1 and 2 are proposed to be models for vicinal −OH groups on
silica surface.
ASSOCIATED CONTENT
* Supporting Information
Additional crystallographic details (including data in CIF
format) and 1H NMR and IR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: vjancik@unam.mx. Fax: (+) 52-55-5616-2217.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support from DGAPA-UNAM (PAPIIT Grant
IN211910) and CONACyT (Grant 79531) is gratefully
acknowledged. R.H.-L. and D.S.-I. gratefully acknowledge the
Ph.D. fellowships provided by CONACyT (Grants 195791 and
268796, respectively). Alejandra Nunez-Pineda and Lizbeth
Triana Cruz are gratefully acknowledged for their technical
support.
́
̃
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