A synthetic route to a molecular galloxane dihydroxide and its group 4 heterobimetallic compounds

Article abstract of DOI:10.1021/ic4010054

Controlled hydrolysis of ^MeLGaCl₂ (^MeL = HC[(CMe)N(2,4,6-Me₃C₆H₂)]₂ ^-) (1) in the presence of a N-heterocyclic carbene, as a HCl acceptor, led to the unprecedented molecular galloxane dihydroxide [{ ^MeLGa(OH)}₂(μ-O)] (2) in high yield. Compound 2 was used in the assembly of the heterobimetallic galloxanes with group 4 metals [{(^MeLGa)₂(μ-O)}(μ-O)₂{M(NR ₂)₂}] (M = Ti, R = Me (6); M = Zr (7), Hf (8), R = Et).

Full text of DOI:10.1021/ic4010054

Article
pubs.acs.org/IC
A Synthetic Route to a Molecular Galloxane Dihydroxide and Its
Group 4 Heterobimetallic Compounds
Erandi Bernabe-Pablo, Vojtech Jancik,† and Monica Moya-Cabrera*,†
́
́
Centro Conjunto de Investigacion
Estado de Mexico, Mexico
́
en Química Sustentable, UAEM-UNAM, Carr. Toluca-Atlacomulco Km 14.5, 50200 Toluca,
́
S
* Supporting Information
ABSTRACT: Controlled hydrolysis of MeLGaCl2
(
MeL = HC[(CMe)N(2,4,6-
−
Me3C6H2)]2 ) (1) in the presence of a N-heterocyclic carbene, as a HCl acceptor, led
to the unprecedented molecular galloxane dihydroxide [{MeLGa(OH)}2(μ-O)] (2) in
high yield. Compound 2 was used in the assembly of the heterobimetallic galloxanes with
group 4 metals [{(MeLGa)2(μ-O)}(μ-O)2{M(NR2)2}] (M = Ti, R = Me (6); M = Zr (7),
Hf (8), R = Et).
the reactivity patterns observed previously for the alumoxane
INTRODUCTION
■
dihydroxide [{MeLAl(OH)}2(μ-O)].3a
The controlled hydrolysis of compounds with group 13 metals
has been a topic of extensive study. The major interest in this
issue has focused on the aluminum derivatives due to their
potential application as cocatalysts in the polymerization of a
wide range of organic monomers.1 A long-lasting problem of
alumoxanes is the difficulty to stabilize them in low aggregation
and crystalline forms.2 In this regard, our research group has
been interested in the synthesis of functionalized molecular
alumoxanes and their heterobimetallic derivatives.3,4 In contrast
to alumoxanes, structural information concerning soluble
molecular galloxanes is limited to a handful of examples,5−8
none of which contain potentially useful functional groups or
can be isolated in suitable yields.9 Furthermore, the task of
assembling compounds with Ga−O−Ga moieties is also
hampered by the limited availability of appropriate starting
materials for gallium compared to those used for aluminum.
Nonetheless, functionalized galloxanes can be used as starting
materials for the construction of heterobimetallic species. In
fact, heterobimetallic systems containing M−O−M′ frame-
works are particularly important because they bring the metals
into close proximity with each other allowing pronounced
chemical communication between them.10 However, com-
pounds bearing Ga−O−M moieties11−13 remain scarce, while
heterobimetallic galloxanes are virtually unknown. An interest-
ing strategy used for the preparation of molecular gallium
hydroxides was achieved by using a strong nucleophilic reagent,
a N-heterocyclic carbene, as an HCl acceptor for the controlled
hydrolysis of gallium halides.11,14 Consequently, we focused
this strategy on the assembly of functonalized dinuclear gallium
species, particularly molecular galloxane dihydroxides, based on
Herein, we report on the preparation of the unprecedented
molecular galloxane containing two terminal OH groups
[{MeLGa(OH)}2(μ-O)] (MeL = HC[(CMe)N(2,4,6-
−
Me3C6H2)]2 ) (2) obtained from the controlled hydrolysis of
[MeLGaCl2] using 1,3-di-tert-butylimidazol-2-ilydene as a hydro-
gen chloride acceptor,15 along with the synthesis of its group 4
heterobimetallic derivatives [{(LGa)2(μ-O)}(μ-O)2{M-
(NR2)2}] (M = Ti, R = Me (6); M = Zr (7), Hf (8), R = Et).
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under a
dry and oxygen-free atmosphere (N2) using Schlenk-line and glovebox
techniques. The solvents were dried using a MBraun Solvent
Purification System. Commercially available chemicals were purchased
from Sigma-Aldrich and used without further purification. Zr(NEt2)4
was prepared according to the literature procedure.16 C6D6 was dried
with a Na/K alloy and distilled through vacuum transfer (−196 °C)
using a Swagelok system, while a similar procedure was used for
CDCl3 using P2O5. NMR spectra were recorded on a Bruker Avance
1
III 300 MHz, and H chemical shifts were reported with reference to
the residual protons of the deuterated solvent unless otherwise stated.
IR spectra were recorded on a Bruker Alpha FT-IR spectrometer with
an ATR measurement setup (diamond) under inert atmosphere in a
glovebox in the 4000−400 cm−1 range. Mass spectra were obtained on
a Shimadzu GCMS-QP2010 Plus using the electron impact (EI)
ionization technique. Elemental analyses (C, H, N) were performed on
an Elementar MicroVARIO Cube analyzer. Melting points were
measured in sealed glass tubes on a Buchi B-540 melting point
̈
apparatus. Crystallographic data for compounds 2, 3, 5, 7, and 8 were
Received: January 28, 2013
© XXXX American Chemical Society
A
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Article
Table 1. Crystallographic Data for the Structural Analyses of Compounds 2, 3, 5, 7, and 8
2
3
5
7·toluene
8·toluene
chemical formula
formula weight
space group
a, Å
C46H60Ga2N4O3
856.42
C46H58Cl2Ga2N4O
893.30
Fdd2
C25H35GaN2
433.27
C57.5H82Ga2N6O3Zr
1135.95
C57.5H82Ga2N6O3Hf
1223.22
Fdd2
P212121
13.966(2)
17.174(2)
19.935(3)
90
P1
̅
P1
̅
20.1713(7)
48.0892(16)
8.9712(3)
90
20.539(3)
47.199(5)
9.343(2)
90
10.4374(4)
13.5148(5)
21.8467(8)
95.605(1)
90.835(1)
112.228(2)
2834.6(2)
2
10.4382(3)
13.4996(4)
21.7611(7)
95.8000(10)
90.4520(10)
112.2750(10)
2819.48(15)
2
b, Å
c, Å
α, deg
β, deg
90
90
90
γ, deg
V, Å3
90
90
90
8702.3(5)
8
9057(3)
8
4781.5(11)
8
Z
temp, K
100(2)
0.71073
1.282
100(2)
0.71073
1.345
100(2)
100(2)
100(2)
λ, Å
0.71073
1.163
0.71073
0.71073
μ, mm−1
ρcalc/g cm−3
1.171
2.832
1.307
1.310
1.204
1.331
1.441
a
R1 (I > 2σ(I))
0.0153, 0.0415
0.0252, 0.0548
0.0548, 0.1055
0.0684, 0.1100
0.0277, 0.0685
0.0347, 0.0685
0.0162, 0.0404
0.0174, 0.0410
b
wR2 (all data)
0.0155, 0.0416
0.0268, 0.0553
a
b
2
2
R1 = ∑||Fo| − | Fc||/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑(Fo )2]1/2
.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds 2 and 3
a
b
a
b
2
3
2
3
Ga(1)−O(1)
Ga(1)−X
1.794(1)
1.845(1)
1.946(1)
1.936(1)
1.783(1)
2.197(1)
1.933(2)
1.925(2)
Ga(1)−O(1)−Ga(1A)
N(1)−Ga(1)−N(2)
O(1)−Ga(1)−X
121.6(1)
96.7(1)
124.2(1)
98.0(1)
111.6(1)
110.9(1)
Ga(1)−N(1)
Ga(1)−N(2)
114.1(1)
109.1(1)
N(1)−Ga(1)−X
a
b
X = O(2). X = Cl(1).
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Compounds 7 and 8
a
b
a
b
7·toluene
8·toluene
7·toluene
8·toluene
Ga(1)−O(1)
Ga(1)−O(2)
Ga(2)−O(1)
Ga(2)−O(3)
Ga(1)−N(1)
Ga(1)−N(2)
Ga(2)−N(3)
Ga(2)−N(4)
M(1)−O(2)
M(1)−O(3)
1.808(2)
1.825(2)
1.788(2)
1.833(2)
1.942(2)
1.937(2)
1.962(2)
1.959(2)
1.927(2)
1.945(2)
1.808(2)
1.825(2)
1.788(2)
1.833(2)
1.942(2)
1.937(2)
1.962(2)
1.959(2)
1.920(1)
1.935(1)
Ga(1)−O(1)−Ga(2)
N(1)−Ga(1)−N(2)
N(3)−Ga(1)−N(4)
O(1)−Ga(1)−O(2)
O(1)−Ga(2)−O(3)
O(2)−M(1)−O(3)
Ga(1)−O(2)−M(1)
Ga(2)−O(3)−M(1)
N(5)−M(1)−N(6)
126.4(1)
96.3(1)
112.2(1)
96.2(1)
94.9(1)
94.9(1)
111.9(1)
116.1(1)
103.4(1)
133.1(1)
128.5(1)
108.4(1)
111.9(1)
116.0(1)
104.1(1)
132.5(1)
128.3(1)
108.7(1)
a
b
M = Zr. M = Hf.
collected on a Bruker SMART APEX DUO three-circle diffractometer
equipped with an Apex II CCD detector using MoKα (Microfocus
sealed tube with a graphite monochromator). The crystals were coated
with a hydrocarbon oil, picked up with a nylon loop, and immediately
mounted in the cold nitrogen stream (−173 °C) of the diffractometer.
Frames were collected by omega scans, integrated using SAINT
program, and semiempirical absorption correction (SADABS) was
applied.17 The structures were solved by direct methods (SHELXS),
and refined by the full-matrix least-squares on F2 with SHELXL-9718
using the SHELXLE GUI.19 Weighted R factors, Rw, and all goodness
of fit indicators are based on F2. All non-hydrogen atoms were refined
anisotropicaly. Hydrogen atoms were placed in idealized geometrical
positions and refined with Uiso tied to the parent atom with the riding
model, whereas the hydrogen atoms of the OH moieties in 2 were
localized from the difference electron-density map and refined
isotropically. The crystallographic data and refinement details for
compounds 2, 3, 5, 7, and 8 are given in Table 1. Selected interatomic
distances and angles are provided in Tables 2 and 3.
Preparation of [MeLGaCl2] (1). A solution of nBuLi (2.5 M in
hexanes, 2.39 mL, 5.98 mmol) was added dropwise to a diethylether
solution (20 mL) of MeLH (2.00 g, 5.94 mmol) at −79 °C. After 1 h
under stirring at ambient temperature, the reaction mixture was added
to a solution of GaCl3 (1.05 g, 5.9 mmol) in diethylether (20 mL) at
−79 °C, and the reaction mixture was stirred for 2 h at ambient
temperature. The resulting solution was then filtered through Celite,
and all of the volatiles were removed under vacuum and the remaining
yellow solid was washed with hexane (3 × 3 mL). Yield 84% (2.36 g).
Mp 219 °C. 1H NMR (300 MHz, CDCl3, 25 °C): δ 1.82 (s, 6H, CH3),
2.27 (s, 6H, p-Ar−CH3), 2.28 (s, 12H, o-Ar−CH3), 5.21 (s, 1H, γ-
CH), 6.92 (s, 4H, m-Ar−H). 13C NMR (75.58 MHz, CDCl3, 25 °C): δ
18.9 (CH3), 21.0 (p-Ar−CH3), 23.4 (o-Ar−CH3), 96.6 (γ-CH), 129.7
(o-Ar−C), 133.6 (p-Ar−C), 136.7 (m-Ar−C), 138.3 (i-Ar−C), 171.4
ppm (CN). IR: v 1538 cm−1 (CN). MS−EI (70 eV) m/z: 474
̃
[M+]. Anal. Calcd for C23H29Cl2Ga2N2 (474.12): C, 58.27; H, 6.17; N,
5.91. Found: C, 58.19; H, 6.12; N, 5.59.
Preparation of [{MeLGa(OH)}2(μ-O)] (2). 1 (0.5 g, 1.05 mmol), 1,3-
di-tert-butylimidazol-2-ilydene (0.42 g, 2.37 mmol), and toluene (20
B
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Inorganic Chemistry
Article
mL) were placed in a Schlenk flask. A solution of H2O (1.0 M, 1.84
mmol) in THF was slowly added at −79 °C. The reaction mixture was
allowed to warm to ambient temperature and stirred for 4 h; after this
time, the insoluble material was filtered off and the volatiles were
removed under vacuum. The white solid obtained was washed with
cold hexane (2 × 5 mL) and dried under vacuum. Yield 85% (0.38 g).
Mp 279 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ −1.38 (s, 1H, OH),
1.41 (s, 6H, CH3), 1.97, 2.43 (s, 12H, o-Ar−CH3), 2.24 (s, 6H, p-Ar−
CH3), 4.66 (s, 1H, γ-CH), 6.76, 6.85 ppm (s, 4H, m-Ar−H). 13C NMR
(75 MHz, C6D6, 25 °C): δ 18.3 (CH3), 19.0 (p-Ar−CH3), 21.0, 22.4
(o-Ar−CH3), 94.4 (γ-CH3), 129.3, 129.6 (m-Ar−C), 134.4, 134.6 (o-
Ar−C), 135.2 (p-Ar−C), 141.6 (i-Ar−C), 168.5 ppm (CN). IR
−79 °C. The reaction mixture was allowed to warm to ambient
temperature and stirred for 2 h after which the solution was filtered.
All volatiles were removed under vacuum leaving a white residue,
which was treated with cold pentane (2 × 5 mL). After filtration and
drying under vacuum, 3 was obtained as a white powder. Yield 64%
1
(0.18 g). Mp 253 °C. H NMR (300 MHz, C6D6, 25 °C): δ 1.29 (s,
6H, CH3), 1.95, 2.31 (s, 12H, o-Ar−CH3), 2.30 (s, 6H, p-Ar−CH3),
3.13 [s, 6H, N(CH3)2], 4.77 (s, 1H, γ-CH), 6.70, 6.83 ppm (s, 4H, m-
Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ = 18.5 (CH3), 19.1, 22.9
(o-Ar−CH3), 21.1 (p-Ar−CH3), 45.9 [N(CH3)2], 96.9 (γ-CH3), 129.4,
130.1 (m-Ar−C), 133.7, 134.3 (o-Ar−C), 134.4 (p-Ar−C), 142.51 (i-
Ar−C), 169.5 ppm (CN). MS−EI (70 eV) m/z: 990 [M+ − H], 902
[M+ − 2NMe2]. Anal. Calcd for C50H70Ga2N6O3Ti (990.44): C,
60.63; H, 7.12; N, 8.49. Found: C, 60.61; H, 7.07; N, 8.41.
(ATR): v 3623 cm−1 (GaO−H). MS−EI (70 eV) m/z: 855 [M+ − H],
̃
838 [M+ − H2O]. Anal. Calcd for C46H60Ga2N4O3 (856.42): C, 64.51;
H, 7.06; N, 6.54. Found: C, 64.53; H, 6.97; N, 6.52.
Preparation of [{(MeLGa)2(μ-O)}(μ-O)2{Zr(NEt2)2}] (7). Compound
7 was synthesized using the same procedure outlined above for 6
starting from Zr(NEt2)4 (0.13 g, 0.35 mmol) and 2 (0.25 g, 0.29
Preparation of [{MeLGa(Cl)}2(μ-O)] (3). A modification to the
reported procedure for the synthesis of [{LAl(Cl)}2(μ-O)] (L =
1
20
−
mmol). Yield 55% (0.17 g). Mp 217 °C. H NMR (300 MHz, C6D6,
HC[(CMe)N(Me)]2 ) was used. To a solution of 1 (1.0 g 2.1
mmol) in THF (30 mL) was added Ag2O (0.24 g, 1.05 mmol) at
ambient temperature. The reaction mixture was refluxed for 48 h, and
the yellow solution was filtered. The volatiles were removed under
vacuum, and the resulting white solid was rinsed with hexane (2 × 5
25 °C): δ 1.30 [t, 6H, N(CH2CH3)2], 1.31 (s, 6H, CH3), 1.97, 2.32 (s,
12H, o-Ar−CH3), 2.31 (s, 6H, o-Ar−CH3), 3.18 [q, 4H, N-
(CH2CH3)2], 4.76 (s, 1H, γ-CH), 6.73, 6.87 ppm (s, 4H, m-Ar−H).
13C NMR (75 MHz, C6D6, 25 °C): δ 16.6 [N(CH2CH3)2], 18.6
(CH3), 19.1, 23.0 (p-Ar−CH3), 21.2 (p-Ar−CH3), 46.1 [N-
(CH2CH3)2], 97.0 (γ-CH3), 129.4, 130.1 (m-Ar−C), 133.9, 134.2 (o-
Ar−C), 134.3(p-Ar−C), 142.6 (i-Ar−C), 169.4 ppm (CN). MS−EI
(70 eV) m/z: 1017 [M+ − NEt2], 944 [M+ − 2NEt2]. Anal. Calcd for
C54H78Ga2N6O3Zr (1089.91): C, 59.51; H, 7.21; N, 7.71. Found: C,
60.05; H, 7.19; N, 7.63.
1
mL). Yield 77% (0.73 g). Mp 345 °C (dec). H NMR (300 MHz,
CDCl3, 25 °C): δ 1.58 (s, 6H, CH3), 1.67, 2.31 (s, 12H, o-Ar−CH3),
2.15 (s, 6H, p-Ar−CH3), 4.91 (s, 1H, γ-CH), 6.64, 6.88 (br s, 4H, m-
Ar−H). 13C (75.58 MHz, CDCl3, 25 °C): δ 18.1 (CH3), 19.0, 22.9 (o-
Ar−CH3), 20.9 (p-Ar−CH3), 95.8 (γ-CH), 128.7 (m-Ar−CH), 129.3
(m-Ar−CH), 133.5, 134.2 (o-Ar−C), 135.0 (p-Ar−C), 140.2 (p-Ar−
Preparation of [{(MeLGa)2(μ-O)}(μ-O)2{Hf(NEt2)2}] (8). Compound
8 was synthesized using the same procedure outlined above for 6 and
7, starting from Hf(NEt2)4 (0.16 g, 0.35 mmol) and 2 (0.25 g, 0.29
C), 169.0 ppm (CN). IR (ATR): v 1534 cm−1. MS−EI (70 eV) m/
̃
z: 545 [C22H27Cl2Ga2O]+. Anal. Calcd for C46H58Cl2Ga2N4O
(893.33): C, 61.85; H, 6.54; N, 6.27. Found: C, 61.63; H, 6.38; N,
6.16.
1
mmol). Yield 62% (0.21 g). Mp 261 °C. H NMR (300 MHz, C6D6,
Preparation of [MeLGaH2] (4). A 0.7 M solution of AlH3·NMe3 (5.6
mL, 3.9 mmol) in toluene was added to a suspension of 3 (1.0 g, 1.12
mmol) in toluene at −79 °C. The solution was stirred for 12 h and
then filtered. The volatile materials were removed under vacuum, and
the remaining residue was washed with pentane (2 × 3 mL) giving a
25 °C): δ 1.31 [t, 6H, N(CH2CH3)2], 1.32 (s, 6H, CH3), 1.97, 2.32 (s,
12H, o-Ar−CH3), 2.31 (s, 6H, p-Ar−CH3), 3.20 [q, 4H, N-
(CH2CH3)2], 4.77 (s, 1H, γ-CH), 6.73, 6.87 ppm (s, 4H, m-Ar−H).
13C NMR (75 MHz, C6D6, 25 °C): δ 16.7 [N(CH2CH3)2], 18.6
(CH3), 19.2, 23.0 (o-Ar−CH3), 21.2 (p-Ar−CH3), 47.0 [N-
(CH2CH3)2], 97.0 (γ-CH3), 129.4, 130.0 (m-Ar−C), 133.9, 134.2 (o-
Ar−C), 134.3 (p-Ar−C), 142.67 (i-Ar−C), 169.38 ppm (CN). MS−
EI (70 eV) m/z: 1032 [M+ − 2NEt2]. Anal. Calcd for
C54H78Ga2N6O3Hf (1177.17): C, 55.10; H, 6.68; N, 7.14. Found: C,
55.26; H, 6.46; N, 6.97.
1
white solid. Yield 70% (0.63 g). Mp 149 °C (dec). H NMR (300
MHz, C6D6, 25 °C): δ 1.51 (s, 6H, CH3), 2.10 (s, 6H, p-Ar−CH3),
2.30 (s, 12H, o-Ar−CH3), 4.69 (s, 1H, γ-CH), 5.26 (s, 2H, Ga−H),
6.74 (br s, 4H, m-Ar−H). IR (ATR): v 1836, 1876 cm−1 (GaH2). MS−
̃
EI (70 eV) m/z: 403 [M+ − H]. Anal. Calcd for C23H31Ga2N2
(405.23): C, 68.17; H, 7.71; N, 6.91. Found: C, 68.17; H, 7.74; N,
6.90.
RESULTS AND DISCUSSION
■
Preparation of [MeLGaMe2] (5). A solution of GaMe3 (1.0 M in
toluene, 6.0 mL, 6 mmol) was added to a solution of MeLH (2.0 g 6.0
mmol) in toluene (10 mL) at ambient temperature. After the
evolution of methane ceased, the solution was stirred for 2 h. The
resulting yellow solution was filtered, the volatiles were removed under
vacuum, and the residue was washed with pentane (2 × 3 mL) after
which a white solid was obtained. Yield 89% (2.31 g). Mp 97.2 °C. 1H
NMR (300 MHz, C6D6, 25 °C): δ −0.67 (s, 6H, GaCH3), 1.68 (s, 6H,
CH3), 2.20 (s, 6H, p-Ar−CH3), 2.28 (s, 6H, o-Ar−CH3), 4.85 (s, 1H,
γ-CH), 6.89 (s, 4H, m-Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ
−6.90 (GaCH3), 18.5 (CH3), 20.8 (o-Ar−CH3), 22.8 (p-Ar−CH3),
94.0 (γ-CH3), 129.4 (m-Ar−C), 132.9 (o-Ar−C), 134.4 (p-Ar−C),
141.9 (i-Ar−C), 166.9 ppm (CN). MS−EI (70 eV) m/z (%): 417
[M+ − CH3]. Anal. Calcd for C25H35Ga2N2 (432.21): C, 69.30; H,
8.14; N, 6.47. Found: C, 69.21; H, 8.12; N, 6.39.
Treatment of [MeLGaCl2] (1) (MeL = HC[(CMe)N(2,4,6-
−
Me3C6H2)]2 ) with a stoichiometric amount of water in the
presence of 1,3-di-tert-butylimidazol-2-ilydene results in the
formation of 2 in 85% yield (Scheme 1). The 1,3-di-tert-
Scheme 1. Synthesis of the Molecular Galloxanes 2 and 3
Hydrolysis Studies for 4 and 5. A 0.5 M solution of H2O in THF
(6.0 mL, 3.0 mmol) was added to a solution of 4 (or 8) (2.0 mmol) in
toluene (20 mL) at either −30 °C or ambient temperature. The
reactions were stirred at ambient temperature for 48 h or refluxed for
the same amount of time. The solvent was removed under vacuum,
and the white solid was washed (3 × 3 mL) with hexane. The products
1
from these reactions were identified as the starting materials by H
NMR analysis.
Preparation of [{(MeLGa)2(μ-O)}(μ-O)2{Ti(NMe2)2}] (6). A solution
of Ti(NMe2)4 (0.08 g, 0.35 mmol) in toluene (10 mL) was added
dropwise to a solution of 2 (0.25 g, 0.29 mmol) in toluene (10 mL) at
C
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Inorganic Chemistry
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butylimidazolium chloride formed can be easily separated due
to its insolubility in toluene, and thus 2 can be isolated in high
purity. Furthermore, attempts to prepare the intermediate
galloxane dichloride [{MeLGa(Cl)}2(μ-O)] (3) starting from 2
equiv of 1, 1 equiv of H2O, and 2 equiv of the N-heterocyclic
carbene failed leading exclusively to 2. However, an alternative
approach for the preparation of 3 was achieved by treating 1
with Ag2O in THF at ambient temperature (Scheme 1).
Compound 3 was treated with an excess (3 equiv) of
AlH3·NMe3 as an endeavor to prepare the galloxane dihydride
[{MeLGa(H)}2(μ-O)]. Instead, the monometallic dihydride
gallium complex 4 was obtained in 80% yield (Scheme 2).
Similar results, albeit with lower yields, were obtained with a
1:1 molar ratio of 2 and AlH3·NMe3 or with LiAlH4.
corresponding terminal group (Figures 1 and 2). The Ga−
O(H) bond length [1.845(1) Å] in 2 is longer than those in the
Scheme 2. Synthesis of Compound 4
Compounds 1−4 were unambiguously characterized by
means of spectroscopic, spectrometric, and, in the case of 2
and 3, also by X-ray diffraction techniques. Compounds 2 and 4
are highly soluble in common organic solvents (toluene, THF,
CH2Cl2) but insoluble in hexane and pentane, whereas 3 is
soluble in CH2Cl2 and THF. The EI mass spectrum of 2
exhibited a peak at m/z 855 corresponding to the loss of a
hydrogen atom from the molecular ion, while the most intense
peak appeared at m/z 838 due to the fragment [M+ − H2O].
Similarly, a peak at m/z 403 in 4 is due to the loss of a
hydrogen atom from the molecular ion. On the other hand, a
peak at m/z 545 in 3 corresponds to the isotopic pattern of the
bimetallic fragment [C22H27Cl2Ga2O]+. The IR spectrum of 2
Figure 1. Molecular structure of 2; hydrogen atoms (except OH) are
omitted for clarity. Thermal ellipsoids are set at 50% probability level.
displays a sharp band at v 3623 cm−1 due to the stretching
̃
frequency of the hydroxides groups, while the IR spectrum of 4
shows a set of two sharp bands (v 1836 and 1876 cm−1)
̃
corresponding to the symmetric and asymmetric stretching of
the GaH2 group.
1
The H NMR spectra of 2 and 3 exhibit a different pattern
for the ligand backbone as compared to that in 1 and 4,
showing two signals for the protons from the methyl group in
the ortho position and two signals for the aromatic protons in
the meta position. This behavior is consistent with the lower
symmetry observed in the bimetallic compounds 2 and 3, as
compared to that in the monometallic complexes 1 and 4.
Furthermore, 2 shows a single signal at δ −1.38 ppm ascribed
to the OH groups, which is shifted upfield relative to the OH
groups in the alumoxane analogue [{MeLAl(OH)}2(μ-O)] (δ
−0.64 ppm).3a On the other hand, the 1H NMR spectrum of 4
exhibits a broad signal at δ 5.26 ppm corresponding to the
protons from the GaH2 group.
Suitable X-ray single crystals of 2 and 3 were grown at
ambient temperature within several days from a toluene and a
toluene/THF solution, respectively. Compounds 2 and 3
crystallize in the orthorhombic space group Fdd2 with one-half
of the corresponding molecule in the asymmetric unit.
In compounds 2 and 3, the gallium center exhibits a distorted
tetrahedral geometry with coordination to two nitrogen atoms
from the β-diketiminate ligand, one oxygen atom, and to the
Figure 2. Molecular structure of 3; hydrogen atoms are omitted for
clarity. Thermal ellipsoids are set at 50% probability level.
gallium hydroxides [(2,6-Mes2C6H3)2GaOH] (Mes = 2,4,6-
Me3C6H2) [1.783(2) Å],21 [iPrLGa(OH)2] [1.777(1)−1.820(1)
Å],14 and [iPrLGa(Me)OH] [1.831(1) Å].11 In a similar
manner, the Ga−Cl bond in 3 [2.197(1) Å] is more elongated
than those in the monometallic species [{(2,6-Mes2C6H3)Ga-
(Cl)(μ-OH)}2] [2.166(1) and 2.176(1) Å]21 and [{(2,6-
Trip2C6H3)Ga(Cl)(μ-OH)}2] (Trip = 2,6-iPr2C6H3)
[2.146(2) Å].22 The Ga−(μ-O) distances in 2 [1.794(1) Å]
and 3 [1.783(1) Å] are comparable to those in [{[CH-
(SiMe3)2]2Ga}(μ-O)] [1.795(2), 1.787(2) Å],5 while the Ga−
O−Ga angles [121.6(1)° for 2 and 124.6(1)° for 3] are more
acute as compared to those found in [{(CH(SiMe3)2)2Ga}(μ-
O)] [141.4(1)°]5 and [{(Mn(CO)5)ArGa}(μ-O)] (Ar = 2,4,6-
t-Bu3C6H2) [150.2(5)°].6 Furthermore, the terminal groups in
these compounds are in a syn conformation with dihedral
D
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angles defined by the planes X−Ga(1)−O(1) and O(1)−
Ga(1A)−Y corresponding to 50.6° for 2 [X = O(2), Y =
O(2A)] and 56.3° for 3 [X = Cl(1), Y = Cl(1A)].
It is noteworthy that several other reactions were studied as
alternative synthetic routes for 2. In this regard, our primary
interest in the synthesis of [MeLGaMe2] (5) (Figure 3) was the
devoid of GaO−H stretching vibration, thus confirming the
deprotonation of the hydroxyl groups. The 1H NMR spectra of
6−8 show the same pattern for the galloxane backbone as that
in 2 along with the corresponding signals for the NR2 moieties
(δ 3.13 ppm for 6, δ 1.31 and 3.18 ppm for 7, and δ 1.32 and
3.21 for 8). The EI−MS spectra of 6−8 show peaks with the
characteristic isotopic patterns at m/z 990 [M − H]+, 1017 [M
− NEt2]+, and 1032 [M − 2NEt2]+, respectively.
Suitable X-ray single crystals of 7 and 8 were obtained from
their saturated toluene solutions at room temperature within
several days. Compounds 7 and 8 are isomorphous and
crystallize in the triclinic space group P1 with one molecule of
̅
the heterobimetallic compound and one molecule of toluene in
the asymmetric unit (Figures 4 and 5).
Figure 3. Molecular structure of 5 showing one of the two
crystallographically independent molecules; hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50% probability level.
development of readily available precursors for the formation of
2. Experiments were performed on both 4 and 5 in NMR tubes
using 0.06 mmol of the corresponding gallium compound, and
1
the progress of the reactions was monitored by H NMR
spectroscopy. It should be pointed out that controlled
hydrolysis reactions of 4 and 5 were also performed in boiling
toluene at a larger scale. However, in all of these cases, the only
identifiable products were the corresponding starting materials.
Furthermore, attempts to obtain 2 from the controlled
hydrolysis of 1 using amines as HCl scavengers (triethylamine,
pyridine, and 1,8-bis(dimethylamino)naphthalene) were un-
successful leading exclusively to the starting material.
Figure 4. Molecular structure of 7; hydrogen atoms are omitted for
clarity. Thermal ellipsoids are set at 50% probability.
The thermal stability of 2 and the apparent Bronsted acidity
̈
of the GaO−H protons make it an advantageous starting
material for the synthesis of heterometallic systems. Nonethe-
less, attempts to react 2 with the organometallic reagents RLi
In both compounds, the group 4 metal displays a distorted
tetrahedral geometry with coordination to two oxygen atoms
and two nitrogen atoms from the amide groups. The six-
membered inorganic MOGaOGaO rings display planar
arrangements with a mean deviation from the plane of 0.04
and 0.05 Å for 7 and 8, respectively. The M−O bond lengths in
n
t
(R = Bu, Bu, Me), MMe3 (M = Al, Ga, In), and Me2M′Cp2
(M′ = Ti, Zr, Hf) were unsuccessful leading to the isolation of
the starting materials. The limited reactivity of the former
compounds may be due to the inherent strength of their M−C
bonds and thus results in their resistance to facile
substitution.23 However, compound 2 reacts smoothly with
the group 4 amides M(NR2)4 (M = Ti, R = Me; M = Zr, H, R =
Et) in toluene leading to the formation of the heterobimetallic
compounds 6−8 (Scheme 3).
Compounds 6−8 are extremely air and moisture-sensitive
and are highly soluble in common organic solvents including
aliphatic hydrocarbons. The IR spectra of these compounds are
Scheme 3. Preparation of the Heterobimetallic Galloxanes
6−8
Figure 5. Molecular structure of 8; hydrogen atoms are omitted for
clarity. Thermal ellipsoids are set at 50% probability.
E
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7 [1.927(2) and 1.945(2) Å] and 8 [1.920(2) and 1.935(1) Å]
are comparable to those in [{(iPrLGaMe)(Cp2ZrMe)}(μ-O)]
[1.926(1) Å],11 but significantly longer than those in the six-
membered gallosilicate systems [{iPrLGa(μ-O)Si(OtBu)2}(μ-
O)2{M(NEt2)2}] [M = Zr (1.812(1) Å); M = Hf (1.818(3)
Å].12 The Ga−O−M angles [133.1(1)° and 128.5(1)° for 7
and 132.5(1) and 128.3(1)° for 8] are more acute than those
reported for [{(iPrLGaMe)(Cp2ZrMe)}(μ-O)] [146.7(1)°] but
more obtuse than those in [{iPrLGa(μ-O)Si(OtBu)2}(μ-
O)2{M(NEt2)2}] [M = Zr (127.6(1)°; M = Hf (127.1(1)°)]
and in the spirocyclic compounds [{(iPrLGa(μ-O)Si(OtBu)2)-
(μ-O)2}2M] [M = Zr (127.9(1)°, 127.0(1)°); Hf (127.5(1)°,
126.7(1)°)].12 The O−M−O angles [103.4(1)° for 7 and
104.1(1)° for 8] are more obtuse as compared to the
corresponding endocyclic angles in [{iPrLGa(μ-O)Si(OtBu)2}-
(μ-O)2{M(NEt2)2}][M = Zr (101.1(1)°); Hf (102.0(1)°)] and
[{iPrLGa(μ-O)Si(OtBu)2}(μ-O)2]2M [M = Zr (99.5(1)°,
100.0(1)°); Hf (100.6(1)°, 100.5(1)°)].
The Ga−O−M framework present in these compounds
makes them potential candidates for catalytic studies,
particularly for olefin polymerization. Consequently, prelimi-
nary screening of compounds 6 and 8 for their ability to
catalyze ethylene polymerization was undertaken using MAO as
a cocatalyst.24 These compounds were active in the polymer-
ization of ethylene under mild conditions, albeit with low yields.
In summary, a facile method for the preparation of a
molecular galloxane bearing terminal OH groups was achieved.
The structural arrangement exhibited in this molecule allowed
the preparation of multimetallic systems with group 4 metal
amides (6−8). Compounds 6−8 are, to the best of our
knowledge, the first examples of fully characterized hetero-
bimetallic galloxanes. Furthermore, preliminary studies on the
reactivity of these compounds show promising chemical
features. Overall, the synthetic approach developed in this
work provides access to a new class of annular systems with
main group, transition, or lanthanide metals, through oxygen
bridging. The preparation of such multimetallic systems is the
subject of ongoing research.
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Author
*Phone: +52 (722) 276 66 10, ext 7726. Fax: (+52) 55 56 16
22 17. E-mail: monica.moya@unam.mx.
(17) SAINT and SADABS; Bruker AXS Inc.: Madison, WI, 2007.
(18) SHELX: Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64,
112−122.
Present Address
†Academic staff from the Universidad Nacional Auton
́
oma de
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Mexico (UNAM).
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Direccion
́
General de Asuntos
del Personal Academ
IN210710). E.B.-P. thanks the CONACyT for the Ph.D.
fellowship (227105). M. Granados, A. Nunez, L. Triana, and N.
́
ico from the UNAM (PAPIIT Grant
(24) Ethylene polymerizations were carried out in a high vacuum line
(10−5 Torr) using a glass reactor equipped with magnetic stirring. 100
mL of toluene was introduced into the reactor followed by the catalyst
(10 μmol), and the temperature was fixed at 30 °C. MAO (1 mmol)
́
̃
Zavala are acknowledged for their technical assistance.
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was added into the system, and stirring was kept for 20 min for
activation. After this step, the system was closed and set under vacuum.
The polymerization was initiated by starting the flux of ethylene into
the reactor, the system was set at 0.2 mbar, and the polymerization was
carried for 1 h. The ethylene consumption was continuous but low.
The reaction was quenched using 15% acidified methanol, and the
white polyethylene formed was collected by filtration and dried. 0.1
and 0.2 g of polyethylene were collected for 6 and 8, respectively.
G
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