Meerwein-Ponndorf-Verley-type reduction processes in aluminum and indium isopropoxide complexes of imino-phenolate ligands

Article abstract of DOI:10.1021/om300481q

Unexpected Meerwein-Ponndorf-Verley (MPV) reduction products have been obtained during attempts to prepare aluminum and indium isopropoxide complexes of an imino-phenolate ligand derived from 8-aminoquinoline ({ONN ^qui}^-). Reaction of the imino-phenol proligand {ONN ^qui}H and Al(OiPr)₃ in toluene/THF at 90 °C affords selectively [{ONN^qui}{ON^HN^qui}Al] (1a), which contains a monoanionic{ONN^qui}^- ligand and another dianionic ligand ({ON^HN^qui}^2-) that results from the reduction of the latter imino group into an amido moiety. The same and similar MPV reduction products 1a and [{ONN^qui}{ON^HN ^qui}In] (5a) were obtained from the treatment of {ONN ^qui}AlMe₂ (2a) and dialkylindium compounds {ONN ^qui}InMe₂ (3a) and {ONN^qui}In(CH ₂SiMe₃)₂ (4a), respectively, with isopropyl alcohol in THF at room temperature. Related reactions of Al(OiPr)₃ with imino-phenol(ate) (pro)ligands bearing N-benzyl- and N-methyl(2-pyridinyl) substituents or of iPrOH with {ON^Bn}AlMe₂ (2b) and {ONN^CH2pyr}AlMe₂ (2c) did not lead to MPV reduction products but to the expected {ON^Bn}₂Al(OiPr) (6b) and {ONN^CH2Pyr}₂Al(OiPr) (6c) compounds (and other unidentified products). The solid-state structures of 1a, 3a, 4a, and 5a were determined by X-ray diffraction studies. DFT computations performed on the putative complex {ONN^qui}₂Al(OiPr) ( 6a = Ia) and its isolated analogue {ON^Bn}₂Al(OiPr) (6b) indicated an intramolecular MPV pathway with direct hydrogen transfer. Formation of 1a from the presumed alkoxide intermediate Ia is calculated to be a highly exergonic reaction which features a relatively low activation barrier. On the other hand, formation of the MPV reaction products from 6b appears to be highly unfavorable on both thermodynamic and kinetic grounds.

Full text of DOI:10.1021/om300481q

Article
pubs.acs.org/Organometallics
Meerwein−Ponndorf−Verley-Type Reduction Processes in Aluminum
and Indium Isopropoxide Complexes of Imino-Phenolate Ligands
Mickael Normand, Evgeny Kirillov,* Thierry Roisnel, and Jean-Franco̧ is Carpentier*
Organometallics: Materials and Catalysis, UMR 6226 Institut des Sciences Chimiques de Rennes, CNRS-Universite
35042 Rennes cedex, France
́
de Rennes 1,
S
* Supporting Information
ABSTRACT: Unexpected Meerwein−Ponndorf−Verley (MPV)
reduction products have been obtained during attempts to prepare
aluminum and indium isopropoxide complexes of an imino-
phenolate ligand derived from 8-aminoquinoline ({ONNqui}−).
Reaction of the imino-phenol proligand {ONNqui}H and Al(OiPr)3
in toluene/THF at 90 °C affords selectively [{ONNqui}{ONHNqui}-
Al] (1a), which contains a monoanionic{ONNqui}− ligand and
another dianionic ligand ({ONHNqui}2−) that results from the
reduction of the latter imino group into an amido moiety. The same
and similar MPV reduction products 1a and [{ONNqui}{ONHNqui}In] (5a) were obtained from the treatment of
{ONNqui}AlMe2 (2a) and dialkylindium compounds {ONNqui}InMe2 (3a) and {ONNqui}In(CH2SiMe3)2 (4a), respectively,
with isopropyl alcohol in THF at room temperature. Related reactions of Al(OiPr)3 with imino-phenol(ate) (pro)ligands bearing
N-benzyl- and N-methyl(2-pyridinyl) substituents or of iPrOH with {ONBn}AlMe2 (2b) and {ONNCH2pyr}AlMe2 (2c) did not
lead to MPV reduction products but to the expected {ONBn}2Al(OiPr) (6b) and {ONNCH2Pyr}2Al(OiPr) (6c) compounds (and
other unidentified products). The solid-state structures of 1a, 3a, 4a, and 5a were determined by X-ray diffraction studies. DFT
computations performed on the putative complex {ONNqui}2Al(OiPr) (“6a” = Ia) and its isolated analogue {ONBn}2Al(OiPr)
(6b) indicated an intramolecular MPV pathway with direct hydrogen transfer. Formation of 1a from the presumed alkoxide
intermediate Ia is calculated to be a highly exergonic reaction which features a relatively low activation barrier. On the other
hand, formation of the MPV reaction products from 6b appears to be highly unfavorable on both thermodynamic and kinetic
grounds.
Another class of highly desired group 13−Schiff base
compounds are alkoxide derivatives. This is especially the
case for the very topical ROP of cyclic esters,2 because
alkoxides are ideal initiators for this polymerization process, not
least because they mimic the propagating species well (also an
alkoxy-metal species). However, preparation of group 13
alkoxide compounds is often more delicate than that of the
aforementioned halides and alkyls, where alcoholysis (of
alkyls), salt metathesis (of halides), or protonolysis (from
group 13 tris-alkoxides) routes are employed. Indeed, although
a significant number of aluminum, and to a much lesser extent
gallium and indium, alkoxide complexes incorporating Schiff
base ligands have been prepared,1,2 many reports mention
synthetic troubles in specific cases, with the inability to isolate
pure compounds and/or formation of complicated mixtures of
unidentified compounds.
INTRODUCTION
■
Phenoxy-imine, salen, and related Schiff base complexes of
group 13 elements are important and versatile compounds used
in a large variety of applications, as either materials or catalysts.1
Among the many catalytic applications, essentially of aluminum
compounds, noteworthy examples include ring-opening poly-
merization (ROP) of cyclic esters2 and asymmetric processes
such as aldol-type reactions,3 cyanosilylation of carbonyl
compounds,4 hydrophosphonylation of aldehydes and aldi-
mines,5 etc.
The most frequently prepared compounds of this type are
halide and alkyl complexes. The latter alkyls are usually
prepared by protonolysis of a tris-alkyl group 13 compound
with the Schiff base proligand (e.g., an imino-phenol or imino-
alcohol). Such reactions take place by selective release of the
corresponding alkanes and formation of the desired alkyl−
Schiff base complexes; side reactions are barely ever mentioned
in the case of group 13 elements, which somewhat contrasts
with decomposition processes sometimes encountered with
alkyl−Schiff base zirconium compounds, where migration (1,2-
migratory insertion or radical) of an alkyl group to imine6 or
abstraction of a hydrogen from the methylene backbone in
fluorinated alkoxy-imino ligands7 has been unveiled.
In line with our work devoted to the design of well-defined
aluminum− and indium−Schiff base compounds for their use
as catalysts/initiators in the controlled ROP of cyclic
esters,2o−q,t,u we report herein the attempted preparation of
aluminum and indium isopropoxide complexes of imino-
Received: May 31, 2012
Published: July 23, 2012
© 2012 American Chemical Society
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phenolates. With an imino-phenolate ligand derived from 8-
aminoquinoline ({ONNqui}−, Scheme 1), unexpected com-
Scheme 1
pounds resulting from a Meerwein−Ponndorf−Verley (MPV)
reduction of the imine group have been obtained under a
variety of conditions. The MPV reduction process proved to be
dependent on the nature of the residue installed on the imino
group of the ligand. These results shed light on some of the
possible troubles encountered in the synthesis of group 13−
Schiff base alkoxides; they also evidence that care must be taken
when applying in situ protocols (e.g., in catalytic processes such
as the ROP of cyclic esters) implying alcohols and alkyl group
13 precursors bearing imino ligand frameworks sensitive to
reduction.
Figure 1. Molecular structure of [{ONNqui}{ONHNqui}Al] (1a). All
hydrogen atoms, except those on the imino and amino carbons, and
phenylsilyl groups have been omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) in
[{ONNqui}{ONHNqui}M] (M = Al (1a), In (5a))
1a (M = Al)
5a (M = In)
M−O(1)
M−O(2)
M−N(1_1)
M−N(1_2)
M−N(2_1)
M−N(2_2)
N(1_1)−C
N(2_1)−C
O(1)−M−O(2)
O(1)−M−N(2_1)
N(2_1)−M−N(1_1)
O(2)−M−N(2_2)
N(1_1)−M−N(2_2)
1.842(1)
1.800(1)
2.037(2)
2.110(2)
1.909(2)
2.101(2)
1.323(2)
1.310(2)
96.10(6)
99.16(6)
167.06(7)
173.21(6)
89.50(6)
2.108(2)
2.082(2)
2.210(2)
2.281(2)
2.167(2)
2.312(3)
1.343(4)
1.390(4)
101.46(8)
101.14(9)
169.5(1)
162.54(8)
97.25(9)
RESULTS AND DISCUSSION
■
The reaction between the proligand {ONNqui}H and Al(OiPr)3
does not lead to any of the expected products of the type
{ONNqui}nAl(OiPr)2−n (n = 1, 2). When the latter reagents
were mixed in a 1/1 ratio in toluene solution, for 18 h at 80 °C,
no reaction was observed at all; application of vacuum−argon
cycles at time intervals to remove the potentially formed
isopropyl alcohol did not affect the reaction course. On the
other hand, conducting the reaction in toluene/THF (4/1)
solution, for 48 h at 90 °C, with vacuum−argon cycles at
regular time intervals, led to the unanticipated formation of the
heteroleptic complex [{ONNqui}{ONHNqui}Al] (1a) (Scheme
1). Complex 1a was isolated as a pure compound in 34% and
45% yields when the reaction was carried out, respectively, in
1/1 and 2/1 {ONNqui}H/Al(OiPr)3 ratios.
1
The H and 13C NMR data of 1a confirm the existence of a
single species in THF-d8 and C6D6 solution, with a pristine
imino-phenolate ligand ({ONNqui}−) and a reduced amido-
Compound 1a is soluble in THF-d8 and slightly so in
benzene-d6 and toluene-d8, in which it is stable at 100 °C for at
least 18 h. This compound was characterized in solution by 1H
and 13C NMR spectroscopy and in the solid state by
combustion analysis and a single-crystal X-ray diffraction
study. 1a contains a regular monoanionic imino-phenolate
ligand ({ONNqui}−) and another dianionic ligand (hereafter
abbreviated {ONHNqui}2−) that formally results from the
reduction of the latter imine group into an amido moiety.
The molecular structure of 1a in the solid state features the
aluminum center in a slightly distorted octahedral environment,
hexacoordinated by the {ONNqui}− and {ONHNqui}2− ligands
(Figure 1; see Table 1 for selected bond distances and angles).
The last two ligands each adopt a near-planar coordination
(maximum deviations of the phenyl Caro(Me) from the O−N−
N−Al mean planes: 0.91 and 1.10 Å), forming an almost
perfect (89.9°) orthogonal angle with each other. The Al−
N(amido) bond distance (1.9090(15) Å) is expectedly shorter
than the Al−N(imino) bond distance (2.0371(15) Å), both
distances falling in the usual ranges for similar bonds.2n,8
phenolate ligand ({ONHNqui}2−). In particular, the H NMR
1
spectra in both solvents contain diagnostic signals for
nonequivalent ligands, including two distinct singlets for the
methyl substituents on the phenolate rings, a downfield singlet
integrating for one hydrogen for the NCH group, and an AB
set of doublets (2JH−H = 17 Hz) for the newly formed
NCHH(aryl) moiety (Figure 2 and Figure S6 (Supporting
Information)).
Product 1a is formally the result of an in situ Meerwein−
Ponndorf−Verley (MPV) reduction9 of an ancillary {ONNqui}−
ligand by an active Al−OiPr moiety.10 We have recently
reported a related phenomenon when attempting to coordinate
fluorinated β-diketonate ligands of the type “{hfacac}−” onto Al
centers; in this case, reduction of one carbonyl group took place
to generate the bridging dianionic ligand {4H-hfacac}2−.11
Basically, two limit processes can be considered to account for
such reductions (Scheme 2): (i) the case where the reduction
of the imine group in the ligand (or proligand) occurs at an
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1
Figure 2. H NMR spectrum (500 MHz, THF-d8, 298 K) of [{ONNqui}{ONHNqui}Al] (1a). Asterisks denote residual signals of THF-d8.
early stage of the reaction, that is when the initial precursor
Al(OiPr)3 and/or intermediary species of the type
“[{ONNqui}nAl(OiPr)2−n]” might still be available, and would
proceed in an intermolecular fashion and (ii) the case where
the reduction of the imine function occurs in an intramolecular
fashion, i.e. in the Al coordination sphere from an isopropoxide
moiety of an intermediate of the type [Al{ONNqui}2(OiPr)].
Considering the high selectivity for 1a and its straightfor-
wardness, route ii appears to be the most reasonable (as
confirmed hereafter by a DFT study; vide infra). However, all
our attempts to prepare independently the intermediate
[Al{ONNqui}2(OiPr)] via different routes12 or to detect it in
the course of the reaction of Al(OiPr)3 with {ONNqui}H under
a variety of conditions failed. On the other hand, the synthesis
of dimethylaluminum compounds {ONR}AlMe2 (2a−c) could
be efficiently achieved by alkane elimination (Scheme 3).
The reactivity of {ONNqui}AlMe2 (2a) toward isopropyl
alcohol was thus explored (Scheme 4). No reaction was
observed in toluene solution at 70 °C, but the overall reduction
process took place readily in THF at room temperature, leading
to the isolation of 1a (68% yield based on {ONN} moieties).
Because of the {ONN}/Al ratio of 1 and 2 in the reagent (2a)
Scheme 2. Possible Schematic Routes for the Formation of
the MPV Reduction Product 1a: (i) Intermolecular and (ii)
Intramolecular Processes (R1 = 8-Quinolyl)
Scheme 3. Synthesis of Dialkylaluminum and Dialkylindium Complexes of Imino-Phenolate Ligands
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a
Scheme 4. Reaction of Dimethylaluminum Compound {ONqui}AlMe2 (2a) with Isopropyl Alcohol
a
The scheme in the dashed box shows a possible pathway for the formation of 1a.
and product (1a), respectively, this process must be
accompanied formally by ligand redistribution, and the eventual
formation of equimolar amounts of Al(OiPr)3 and acetone,
possibly following the aforementioned pathway (ii, Scheme 2)
(Scheme 4); however, these coproducts were not searched for
in the reaction mixtures.
yield from the reaction of 4a in toluene at 110 °C as a dark red
solid and characterized by NMR spectroscopy in solution,
combustion analysis, and an X-ray diffraction study (Figure 3).
In order to assess the scope of this reduction process, the
reactivity of indium−{quinolyl-imino-phenolate} analogues of
1a was also explored. The dimethyl {ONNqui}InMe2 (3a) and
bis(trimethylsilylmethyl) {ONNqui}In(CH2SiMe3)2 (4a) deriv-
atives were first prepared in high yields from protonolysis by
the proligand {ONNqui}H of the parent precursors “[InMe3]n”
and In(CH2SiMe3)3, respectively (Scheme 3). The crystal
structures of those compounds are reported in Figures S29 and
S30 (Supporting Information).
While no reaction of 3a with isopropyl alcohol in toluene at
room temperature was observed, reduction of compounds 3a
and 4a into 5athe indium analogue of 1atook place in
toluene at 100 °C (Scheme 5).13 As in the aluminum case
(Scheme 4), the reaction of 3a with isopropyl alcohol
proceeded more easily in THF and was eventually observed
at room temperature. Compound 5a was best isolated in 70%
Figure 3. Molecular structure of [{ONNqui}{ONHNqui}In] (5a). All
hydrogen atoms, except those on the imino and amino carbons, and
phenylsilyl groups have been omitted for clarity.
Scheme 5. Reaction of Dialkylindium Compounds
a
{ONqui}AlMe2 (3a, 4a) with Isopropyl Alcohol
The last study revealed that the gross structural features of 5a
(Table 1) are similar to those of the aluminum analogue 1a,
albeit with a significantly more distorted octahedral geometry,
which is most likely due to the larger ionic radius of indium.14
To further probe the origin and scope of this reduction
process, the reactivity of (pro)ligands having different
substituents at the imino function was evaluated (Scheme 6).
Proligand {ONBn}H, bearing a N-benzyl substituent, reacted
with Al(OiPr)3 (1 equiv) in toluene/THF (4/1) at 100 °C to
afford the corresponding {ONBn}2Al(OiPr) compound (6b) in
30−40% isolated yields. The latter compound was obtained in
higher yield (47%) from the reaction of {ONBn}AlMe2 (1b;
Scheme 3) with iPrOH (5 equiv) in THF at 50 °C for 16 h.
The formation of 1a was never detected in these reactions. The
a
The by-products co-generated in the formation of 5a have been
omitted.
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on two molecular systems having different substitutions at the
imino group, namely from the putative complex {ONNqui}2Al-
(OiPr) (“6a” = Ia) and its isolated analogue {ONBn}2Al(OiPr)
(6b), which bear N-8-quinolyl and N-benzyl groups,
respectively. The overall free energy profiles for the two
molecular systems are illustrated in Scheme 7.
Scheme 6. Reactivity of N-Benzyl- and N-Methylene(2-
pyridyl) Iminophenol(ate) (Pro)ligands and Al Complexes
The calculations were carried out starting from the six-
coordinated Ia and the five-coordinated 6b isopropoxy
precursors, used as free-energy references. Chelated, six-
membered transition states (six-coordinated TSa and five-
coordinated TSb) for the hydride transfer processes were then
located. Interestingly, the energy barrier found for the hydrogen
transfer in the 8-quinolyl-substituted system appeared to be
significantly lower (by 11.8 kcal mol−1) than that in the benzyl-
substituted system. For each molecular system, stable
intermediates IIa and IIb were found. These two species are
the intermediary products of the MPV reaction, each of them
having a coordinated molecule of acetone. Also, six-coordinated
IIa appeared by more stable 23.1 kcal mol−1 than its five-
coordinated congener IIb. Further dissociation of acetone
molecules from IIa and IIb resulted in the formation of the
thermodynamically stable six-coordinated 1a and unstable four-
coordinated IIIb, respectively.
Thus, formation of 1a from the presumed alkoxide
intermediate Ia is a highly exergonic reaction and also features
a relatively low activation barrier. On the other hand, formation
of the MPV reaction products from 6b appears to be highly
unfavorable on both thermodynamic and kinetic grounds. The
results of these computations are consistent with the proposed
intramolecular mechanism and substantiate well other obser-
vations on the influence of R1 imino substituents on the MPV
process.
similar reactions of proligand {ONCH2pyr}H, bearing a methyl-
(2-pyridyl) substituent at the imine group, were less selective. A
complex mixture of products, poorly soluble in aromatic
solvents, was obtained from the reaction with 1 equiv of
Al(OiPr)3 in toluene/THF (4/1) at 100 °C. On the other
hand, the reaction of {ONCH2pyr}AlMe2 (1c; Scheme 3) with
iPrOH (5 equiv) in THF at 60 °C for 16 h gave
{ONCH2pyr}2Al(OiPr) (6c) as the main product (>80% NMR
yield).15
Computational Study of the MPV Reaction. In order to
get a better insight in the operative mechanistic pathway and
the factors that govern the formation of reduction product 1a,
we carried out a theoretical study of the most plausible MPV
route, that is the intramolecular route starting from bis(ligand)-
aluminum isopropoxide intermediates with “direct hydrogen
transfer”16 (route ii, Scheme 2). This computational study was
done at the DFT level using the hybrid ONIOM model (see
details in the Experimental Section). The study was conducted
CONCLUDING REMARKS
The absence of reduction processes in these {ONBn} and
{ONNCH2pyr} bi- and tridentate (pro)ligand frameworks
■
a
Scheme 7. Energy Profiles Computed at the ONIOM(BP86:PM6) Level for the MPV Process
a
Free energies in kcal mol−1 relative to Ia and 6b. R1 = 8-quinolyl (route a) and R1 = benzyl (route b).
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20 mL), and dried under vacuum to give {ONBn}H as a yellow solid
(1.98 g, 43%). H NMR (200 MHz, C6D6, 298 K): δ 1.98 (s, 3H,
suggests that the chemoselectivity in those reactions is not
controlled by the denticity of the ligand but is rather driven by
the electronic nature of the substituent at the imine group.
Apparently, the electron-withdrawing 8-quinolyl substituent on
the N-imino group activates the latter group toward MPV
reduction, which is not the case with electron-donating benzyl
and methyl(2-pyridyl) substituents. The DFT computations
performed on models for the intramolecular route confirm
these observations on the influence of R1 imino substituents
and their probable electronic origin. Another noteworthy point
is the dramatically faster reduction process observed in THF
(operative from room temperature) as compared to that in
toluene (observed in some cases at 100−110 °C). This is in line
with our previous observations in the chemistry of Al−(hfacac)
compounds, where MPV reduction of the fluorinated β-
diketonate ligand was observed only in the presence of THF.11
These observations hint at a strong polarity effect and are
consistent with the fact that most MPV reductions for synthetic
purposes are performed in 2-propanol (used as reductant and
solvent).9,13a
In conclusion, we have shown that the preparation of
aluminum and indium isopropoxide complexes bearing
activated imino-phenolate type ligands can be plagued by
MPV reduction processes of the imine group. Those side
reactions take place in synthetic protocols involving the imino-
phenol proligand and tris(isopropoxide) group 13 precursors.
Less expectedly, we have demonstrated that MPV reduction
can also occur during treatment of aluminum− or indium−
dialkyl imino-phenolate complexes with isopropyl alcohol,
especially when the reaction is performed in a polar solvent
such as THF. This finding is important because the
aforementioned protocol, which consists of treating alkyl
group 13 precursors with an alcohol, is a common practice in
the ROP of cyclic esters.2 Therefore, care must be taken when
applying such protocols to alkyl group 13 precursors bearing
Schiff base ligand frameworks sensitive to reduction.
1
ArCH3), 4.11 (s, 2H, NCH2Ph), 6.73 (m, 1H, Harom), 6.92 (m, 2H,
Harom), 7.03 (m, 3H, Harom), 7.17 (m, 10H, Harom), 7.43 (m, 1H,
Harom), 7.74 (br s, 1H, ArCHN), 7.90 (m, 5H, Harom), 13.80 (s, 1H,
ArOH). 13C{1H} NMR (50 MHz, C6D6, 298 K): δ 20.40 (Ar-CH3),
63.13 (CH2Ph), 117.88, 121.50, 127.27, 127.30, 127.76, 127.69,
128.04, 128.59, 129.20, 129.78, 134.19, 134.82, 135.20, 136.39, 138.01,
142.02, 164.12 (CHN), 165.53 (C−OH). Mp: 143 °C.
{ONNCH2pyr}H. A solution of 4-methyl-6-triphenylsilylsalicylalde-
hyde (1.03 g, 2.60 mmol), 2-(aminomethyl)pyridine (0.337 g, 3.10
mmol), and PTSA (cat.) in benzene (20 mL) was stirred under reflux
with a Dean−Stark apparatus at 110 °C for 48 h. The reaction mixture
was cooled to room temperature, and the volatiles were removed
under vacuum. The resulting solid residue was then recrystallized in
methanol at −30 °C to give {ONNCH2pyr}H as a yellow solid (0.390 g,
31%). 1H NMR (400 MHz, C6D6, 298 K): δ 1.95 (s, 3H, ArCH3), 4.44
(s, 2H, NCH2pyr), 6.49 (m, 1H, Harom), 6.74 (s, 1H, Harom), 6.77−6.79
(m, 1H, Harom), 6.89 (m, 1H, Harom), 7.17 (m, 11H, Harom), 7.43 (br s,
1H, Harom), 7.89 (m, 5H, Harom), 8.35 (m, 1H, CHN), 13.70 (br s,
1H, ArOH). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ 20.01
(ArCH3), 64.72 (NCH2pyr), 118, 12, 121.49, 121.65, 122.04, 129.20,
134.42, 135.13, 136.63, 142.22, 149.14, 158.08, 164.50 (CHN),
166.78 (C−OH). Mp: 127 °C.
[{ONNqui}{ONHNqui}Al] (1a). (a). Synthesis from {ONNqui}AlMe2
(2a) and iPrOH. To a solution of {ONNqui}AlMe2 (2a; 0.300 g, 0.520
mmol) in THF (ca. 15 mL) was added isopropyl alcohol (0.080 mL,
1.04 mmol) by microsyringe. The reaction mixture was stirred at room
temperature for 96 h. Then, the volatiles were removed in vacuo and
the resulting material was washed with n-hexane (ca. 10 mL) and
finally dried under vacuum to give 1a as a dark red solid (0.188 g,
0.177 mmol, 34% based on Al).
(b). Synthesis from {ONNqui}H and Al(OiPr)3. A solution of
{ONNqui}H (0.200 g, 0.340 mmol) and Al(OiPr)3 (0.035 g, 0.170
mmol) in a mixture of toluene and THF (4/1, ca. 10 mL) was stirred
at 90 °C for 48 h. Every ca. 3 h, the atmosphere was quickly evacuated
in vacuo to remove the formed isopropyl alcohol. Then, the volatiles
were removed in vacuo, n-hexane (ca. 10 mL) was vacuum-condensed
in, and the resulting solution was filtered off. The resulting solution
was concentrated under vacuum, and the residue was dried to give 1a
as a dark red solid (0.082 g, 45%). Crystals of 1a suitable for X-ray
diffraction studies were grown from a saturated benzene solution at
room temperature. 1H NMR (500 MHz, C6D6, 298 K): δ 2.01 (s, 3H,
ArCH3), 2.19 (s, 3H, ArCH3), 4.31 (d, 2JH−H = 17 Hz, 1H, ArCHHN),
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under a purified argon atmosphere using standard Schlenk techniques
or in a glovebox. Solvents were distilled from Na/benzophenone
(THF, Et2O) or Na/K alloy (toluene, n-pentane, and n-hexane) under
argon and degassed thoroughly prior to use. Deuterated solvents
(benzene-d6, toluene-d8, THF-d8; >99.5% D, Eurisotop) were vacuum-
transferred from Na/K alloy into storage tubes. 4-Methyl-6-
triphenylsilylsalicylaldehyde17 and proligand {ONNqui}H18 were
prepared using reported procedures. Other starting materials were
purchased from Acros, Strem, and Aldrich and used as received.
NMR spectra of complexes were recorded on Bruker AC-300,
Avance DRX 400, and AM-500 spectrometers in Teflon-valved NMR
tubes at 298 K unless otherwise indicated. 1H and 13C NMR chemical
shifts are reported in ppm vs SiMe4 and were determined by reference
to the residual solvent peaks. Assignment of resonances for
organometallic complexes was made from 2D 1H−1H COSY and
1H−13C HMQC and HMBC NMR experiments. Elemental analyses
(C, H, N) were performed using a Flash EA1112 CHNS Thermo
Electron apparatus and are the average of two independent
determinations.
2
4.69 (d, JH−H = 17 Hz, 1H, ArCHHN), 5.79 (dd, JH−H = 3.5 and 4.7
Hz, 1H, Harom), 5.98 (dd, JH−H = 3.5 and 4.7 Hz, 1H, Harom), 6.36 (d,
3JH−H = 7.8 Hz, 1H, Harom), 6.45 (d, 3JH−H = 7.8 Hz, 1H, Harom), 6.68−
3
7.09 (m, 30H, Harom), 7.47 (d, JH−H = 6.7 Hz, 7H, Harom), 7.70 (d,
3JH−H = 6.7 Hz, 6H, Harom), 7.98 (s, 1H, NCH), 8.22 (dd, JH−H
=
1.4, 3.3 Hz, 1H, Harom). 1H NMR (500 MHz, THF-d8, 298 K): δ 2.10
2
(s, 3H, ArCH3), 2.13 (s, 3H, ArCH3), 3.95 (d, JH−H = 17 Hz, 1H,
2
2
ArCHHN), 4.42 (d, JH−H = 17 Hz, 1H, ArCHHN), 6.22 (d, JH−H
=
8.0 Hz, 1H, Harom), 6.40 (d, JH−H = 8.0 Hz, 1H, Harom), 6.52 (m, 1H,
Harom), 6.67 (t, 3JH−H = 7.5 Hz, 3H, Harom), 6.75 (m, 3H, Harom), 6.86
(m, 3H, Harom), 6.90−6.96 (m, 3H, Harom), 7.12−7.24 (m, 14H,
Harom), 7.31−7.35 (m, 8H, Harom), 7.46 (m, 4H, Harom), 7.51 (d, JH−H
= 7.9 Hz, 1H, Harom), 7.59 (d, JH−H = 8.2 Hz, 1H, Harom), 7.67 (d, JH−H
= 8.2 Hz, 1H, Harom), 7.72 (d, JH−H = 7.7 Hz, 1H, Harom), 8.10 (d, JH−H
= 8.3 Hz, 1H, Harom), 8.27 (d, JH−H = 3.5 Hz, 1H, Harom), 8.62 (s, 1H,
CHN). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 17.55 (CH3),
18.02 (CH3), 49.09 (CH2N), 100.10, 103.80, 112.78, 117.19, 118.14,
120.04, 120.40, 120.57, 121.39, 122.84, 123.41, 124.32, 124.74, 125.49,
129.64, 126.13, 127.29, 127.77, 128.13, 128.57, 130.49, 133.14, 133.52,
133.34, 133.91, 134.77, 135.29, 135.61, 135.74, 136.05, 136.28, 137.26,
138.21, 143.36, 146.17, 148.76, 161.20 (CHN), 162.94 (C-O-Al,
reduced ligand), 169.96 (C-O-Al). Anal. Calcd for C70H55AlN4O2Si2:
C, 78.77; H, 5.19; N, 5.25. Found: C, 78.43; H, 5.27; N, 5.32.
{ONNqui}AlMe2 (2a). A solution of {ONNqui}H (1.00 g, 1.92
mmol) in toluene (20 mL) was added to a solution of AlMe3 (0.96 mL
of a 2.0 M solution in hexane, 1.96 mmol) in toluene (10 mL). The
{ONBn}H. A solution of 4-methyl-6-triphenylsilylsalicylaldehyde
(3.73 g, 9.45 mmol), benzylamine (1.22 g, 11.3 mmol), and formic
acid (98%, 2 drops) in methanol (50 mL) was stirred under reflux at
70 °C for 18 h. The reaction mixture was cooled to room temperature,
and the volatiles were removed under vacuum until the appearance of
a precipitate. The suspension was then stored at 4 °C for 2 h. The
resulting precipitate was filtered off, washed with cold methanol (3 ×
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Organometallics
Article
reaction mixture was stirred at 50 °C for 24 h. Then, the volatiles were
evaporated and the resulting material washed with n-hexane (ca. 10
mL), and dried under vacuum to give 2a as a dark red solid (0.914 g,
82%). 1H NMR (400 MHz, C6D6, 298 K): δ −0.51 (s, 6H, Al(CH3)2),
2.02 (s, 3H, ArCH3), 6.49 (dd, JH−H = 4.0 and 8 Hz, 1H, Harom), 6.69
(d, JH−H = 6.7 Hz, 1H, Harom), 6.77 (s, 1H, Harom), 6.90−7.09 (m, 3H,
Harom), 7.28 (m, 9H, Harom), 7.72 (m, 1H, Harom), 8.03 (m, 7H, Harom
and CHN), 8.32 (s, 1H). 13C{1H} NMR (100 MHz, C6D6, 298 K):
δ −5.20 (Al(CH3)2), 19.93 (ArCH3), 114.88, 118.20, 122.34, 124.18,
124.32, 127.02, 128.54, 129.06, 135.31, 135.64, 136.76, 136.95, 139.38,
142.13, 146.34, 149.91, 165.21 (ArCHN), 173.79 (C-O-Al). Anal.
Calcd for C37H33AlN2OSi: C, 77.05; H, 5.77; N, 4.86. Found: C,
76.95; H, 5.92; N, 4.93.
concentrated under vacuum to give 4a as a bright red solid (0.170 g,
1
0.210 mmol, 44%). H NMR (500 MHz, C6D62, 298 K): δ −0.55 (d,
2JH−H = 12.3 Hz, 2H, CHHSiMe3), −0.37 (d, JH−H = 12.3 Hz, 2H,
CHHSiMe3), −0.13 (s, 18H, Si(CH3)3), 2.02 (s, 3H, ArCH3), 6.64
(dd, JH−H = 4.4 and 8.3 Hz, 1H, Harom), 6.84 (m, 1H, Harom), 6.92 (dd,
JH−H = 3.1 and 7.1 Hz, 1H, Harom), 7.00−7.06 (m, 3H, Harom), 7.25−
7.32 (m, 8H, Harom), 7.37 (dd, JH−H = 1.6 and 8.3 Hz, 1H, Harom), 7.60
(d, JH−H = 2.5 Hz, 1H, Harom), 7.97−7.99 (m, 5H, Harom), 8.22 (s, 1H,
CHN), 8.29 (dd, JH−H = 1.7 and 4.4 Hz, 1H, Harom). 1H NMR (400
2
MHz, THF-d8, 298 K): δ − 0.98 (d, JH−H = 12.2 Hz, 2H,
2
CHHSiMe3), −0.74 (d, JH−H = 12.2 Hz, 2H, CHHSiMe3), −0.49 (s,
18H, Si(CH3)3), 2.09 (s, 3H, ArCH3), 7.12−7.31 (m, 12H, Harom),
7.62 (m, 7H, Harom), 7.68 (m, 1H, Harom), 7.83 (d, JH−H = 8.0 Hz, 1H,
Harom), 7.94 (d, JH−H = 7.6 Hz, 1H, Harom), 8.46 (d, JH−H = 8.2 Hz, 1H,
Harom), 8.69 (br s, 1H, Harom), 8.88 (s, 1H, CHN). 13C{1H} NMR
(125 MHz, C6D6, 298 K): δ −0.10 (CH2SiMe3), 2.09 (Si(CH3)3),
19.90 (ArCH3), 117.18, 118.80, 121.78, 122.65, 124.30, 128.86,
128.95, 129.48, 136.34, 136.89, 138.93, 140.15, 144.39, 146.83, 149.28,
167.28 (CHN), 177.85 (C-O-In). 13C{1H} NMR (100 MHz, THF-
d8, 298 K): δ −2.16 (CH2SiMe3), 0.00 (Si(CH3)3), 17.96 (ArCH3),
116.22, 117.54, 120.56, 121.08, 123.70, 125.76, 126.76, 127.07, 134.91,
135.17, 137.16, 137.80, 142.85, 146.11, 146.92, 166.60 (CHN),
175.85 (C-O-In). Anal. Calcd for C43H49InN2OSi3: C, 63.84; H, 6.11;
N, 3.46. Found: C, 63.65; H, 6.21; N, 3.55. Crystals suitable for X-ray
diffraction studies were grown from a saturated Et2O solution layered
by hexanes at room temperature.
{ONBn}AlMe2 (2b). This compound was prepared as described
above for 2a, starting from a solution of {ONBn}H (200 mg, 0.41
mmol) in toluene (10 mL) and a solution of AlMe3 (0.207 mL of a 2.0
M solution in hexane, 0.41 mmol) in toluene (4 mL). Reaction for 18
h at room temperature and workup gave 2b as a yellow solid (0.151 g,
68%). 1H NMR (300 MHz, C6D6, 298 K): δ −0.74 (s, 6H, Al(CH3)2),
1.88 (s, 3H, ArCH3), 4.02 (s, 2H, CH2Ph), 6.40 (m, 1H, Harom), 6.82
(dd, JH−H = 2.0 and 3.4 Hz, 1H, Harom), 7.01 (m, 3H, Harom), 7.14 (s,
2H, Harom), 7.20−7.27 (m, 8H, Harom), 7.28 (s, 1H, CHN), 7.51 (s,
1H), 7.83 (dd, JH−H = 2.0 and 3.6 Hz, 6H, Harom). 13C{1H} NMR (100
MHz, C6D6, 298 K): δ −10.12 (Al(CH3)2), 19.69 (ArCH3), 59.46
(CH2Ph), 117.88, 125.58, 128.24, 128.74, 128.85, 128.98, 129.12,
134.68, 135.26, 136.57, 136.75, 147.14, 167.21 (C-O-Al), 170.46
(ArCHN). Anal. Calcd for C35H34AlNOSi: C, 77.89; H, 6.35; N,
2.60. Found: C, 77.20; H, 6.08; N, 2.32.
[{ONNqui}{ONHNqui}In] (5a). (a). Synthesis from {ONNqui}InMe2
(3a). A solution of 3a (0.030 g, 0.075 mmol) and iPrOH (7.2 μL,
0.150 mmol) in THF (ca. 5 mL) was stirred at room temperature for
18 h. Volatiles were removed in vacuo, and the solid residue was
washed with n-hexane (ca. 5 mL). The resulting material was dried
under vacuum to give 5a as a dark red solid (0.009 g, 0.0078 mmol,
10%).
{ONNCH2pyr}AlMe2 (2c). A solution of {ONNCH2pyr}H (0.100 g,
0.210 mmol) in toluene (5 mL) was added to a solution of AlMe3
(0.210 mL of a 1.0 M solution in n-heptane, 0.210 mmol) in toluene
(4 mL). The reaction mixture was stirred at room temperature for 18
h. Volatiles were removed in vacuo, and the solid residue was washed
with n-hexane (ca. 10 mL) and then dried under vacuum to give 2c as
a yellow solid (0.080 g, 67%). 1H NMR (400 MHz, THF-d8, 298 K): δ
−1.28 (s, 6H, Al(CH3)2), 2.07 (s, 3H, ArCH3), 5.00 (s, 2H, CH2py),
7.10 (s, 1H, Harom), 7.17 (s, 1H, Harom), 7.24−7.36 (m, 10H, Harom),
7.58−7.60 (m, 7H, Harom), 7.80 (t, JH−H = 7 Hz, 1H, Harom), 8.41 (s,
1H, Harom), 8.46 (s, 1H, ArCHN). 13C{1H} NMR (125 MHz, THF-
d8, 298 K): δ −9.27 (Al(CH3)2), 17.41 (ArCH3), 57.83 (CH2pyr),
116.02, 119.09, 121.06, 121.34, 124.34, 125.18, 126.57, 133.88, 134.65,
135.98, 144.65, 145.10, 152.45, 168.59 (C-O-Al), 170.33 (ArCHN).
Anal. Calcd for C34H33AlN2OSi: C, 75.52; H, 6.15; N, 5.18. Found: C,
75.26; H, 6.35; N, 5.30.
(b). Synthesis from {ONNqui}In(CH2SiMe3)2 (4a). A solution of 4a
(0.0526 g, 0.065 mmol) and iPrOH (20.0 μL, 0.260 mmol) in toluene
(ca. 10 mL) was stirred at 110 °C for 16 h. Volatiles were then
removed in vacuo, and the solid residue was washed with n-hexane (ca.
10 mL). The resulting material was dried under vacuum to give 5a as a
1
dark red solid (0.061 g, 0.053 mmol, 70%). H NMR (500 MHz,
THF-d8, 298 K): δ 2.13 (s, 3H, ArCH3), 2.17 (s, 3H, ArCH3), 4.23 (d,
2JH−H = 15.0 Hz, 1H, CHHN), 4.46 (d, 2JH−H = 15.0 Hz, 1H, CHHN),
2
6.50 (dd, JH−H = 3.0 and 8.0 Hz, 2H, Harom), 6.66 (t, JH−H = 7.5 Hz,
6H, Harom), 6.80 (m, 10H, Harom), 6.38 (m, 4H, Harom), 7.00 (br s, 1H,
Harom), 7.11 (m, 3H, Harom), 7.23 (d, JH−H = 7.0 Hz, 5H, Harom), 7.29
(m, 5H, Harom), 7.40 (d, JH−H = 7.0 Hz, 5H, Harom), 7.63−7.84 (m, 5H,
Harom), 8.16 (br s, 1H, Harom), 8.28 (d, JH−H = 8.0 Hz, 1H, Harom), 8.56
(s, 1H, CHN). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 17.58
(ArCH3), 18.07 (ArCH3), 51.48 (CH2N), 101.19, 103.81, 114.67,
116.62, 118.37, 120.03, 120.89, 121.50, 121.54, 123.20, 124.02, 124.28,
124.78, 125.26, 125.31, 125.35, 125.39, 126.57, 126.57, 126.82, 126.88,
127.58, 127.76, 128.16, 128.62, 131.60, 133.67, 134.01, 134.30, 134.37,
134.67, 135.02, 135.50, 136.34, 136.66, 137.36, 138.82, 138.95, 145.32,
145.81, 147.57, 165.94 (CHN), 168.53 (C-O-In, reduced ligand),
175.01 (C-O-In). Anal. Calcd for C70H55InN4O2Si2: C, 72.78; H, 4.80;
N, 4.85. Found: C, 72.54; H, 4.78; N, 4.89.
{ONNqui}InMe2 (3a). To a stirred suspension of InCl3 (0.100 g,
0.450 mmol) in Et2O (ca. 5 mL) was added a solution of MeLi (0.81
mL of a 1.2 M solution in Et2O, 1.30 mmol). The reaction mixture was
stirred for 30 min at room temperature, and a solution of {ONNqui}H
(0.235 g, 0.450 mmol) in toluene (ca. 5 mL) was added in. The
resulting reaction mixture was stirred at room temperature for 18 h.
Volatiles were removed in vacuo, and the crude product was washed
with n-hexane (ca. 10 mL) to give 3a as a bright red solid (0.140 g,
1
0.112 mmol, 47%). H NMR (500 MHz, C6D6, 298 K): δ −0.10 (s,
6H, In(CH3)2), 2.08 (s, 3H, ArCH3), 6.50 (dd, JH−H = 5.0 and 2.6 Hz,
1H, Harom), 6.75 (dd, JH−H = 1.3 and 7.5 Hz, 1H, Harom), 6.86 (br s,
1H, Harom), 6.95−7.02 (m, 2H, Harom), 7.42−7.31 (m, 10H, Harom),
7.67 (s, 1H, Harom), 7.86 (dd, JH−H = 1.7 and 2.6 Hz, 1H, Harom), 8.04
(m, 7H, Harom and ArCHN). 13C{1H} NMR (125 MHz, C6D6, 298
K): δ −6.85 (In(CH3)2), 20.03 (ArCH3), 117.23, 118.96, 121.82,
122.53, 124.04, 127.97, 128.77, 129.94, 135.31, 136.58, 136.86, 136.94,
138.64, 140.33, 145.12, 146.39, 148.21, 167.38 (CHN), 177.63 (C-
O-In). Anal. Calcd for C37H33InN2OSi: C, 66.87; H, 5.01; N, 4.22.
Found: C, 66.41; H, 5.16; N, 4.17. Crystals suitable for X-ray
diffraction studies were grown from a saturated Et2O solution layered
by hexanes at room temperature.
{ONBn}2Al(OiPr) (6b). (a). Preparation from the Reaction of
{ONBn}H with Al(OiPr)3. A solution of {ONBn}H (0.080 g, 0.165
mmol) and Al(OiPr)3 (0.034 g, 0.165 mmol, 1.0 equiv) in a 4/1 v/v
toluene/THF mixture (10 mL) was stirred at 100 °C for 18 h. Every
ca. 3 h, the atmosphere was quickly evacuated in vacuo to remove the
formed isopropyl alcohol. Then, the volatiles were removed in vacuo,
n-hexane (ca. 10 mL) was vacuum-condensed in, and the resulting
solution was filtered off. The resulting material was then dried in vacuo
to give 6b as a yellow solid (0.052 g, 30% vs Al).
(b). Generation from the Reaction of {ONBn}AlMe2 (2b) with
Isopropyl Alcohol. A solution of {ONBn}AlMe2 (0.080 g, 0.148 mmol)
and iPrOH (57.0 μL, 0.741 mmol, 5.0 equiv) in THF (ca. 5 mL) was
stirred at 50 °C for 18 h. Volatiles were then removed in vacuo, n-
hexane (ca. 10 mL) was vacuum-condensed in, and the resulting
solution was filtered off. The resulting material was then dried in vacuo
{ONNqui}In(CH2SiMe3)2 (4a). A solution of {ONNqui}H (0.249 g,
0.480 mmol) in toluene (8 mL) and In(CH2SiMe3)3 (0.200 g, 0.480
mmol) in Et2O (2 mL) was stirred at 70 °C for 18 h. Then, volatiles
were removed in vacuo, n-hexane (ca. 10 mL) was vacuum-condensed
in, and the resulting solution was filtered off. The filtrate was
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Organometallics
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to give 6b as a yellow solid (0.073 g, 47% vs Al). 1H NMR (300 MHz,
minima.27 The optimized geometrical data for 1a, obtained using the
given theoretical model, were found to be in good agreement with the
X-ray crystallography results (Table S2, Supporting Information). Free
energies (in hartree units) are reported in Table S3 (Supporting
Information).
3
C6D6, 298 K): δ 1.22 (d, JH−H = 5.8 Hz, 3H, CH(CH3)2), 1.31 (d,
3JH−H = 5.9 Hz, 3H, CH(CH3)2), 1.86 (s, 6H, Ar−CH3), 3.70 (d,
2JH−H = 17 Hz, 2H, CH2Ph), 4.17 (m, 1H, CH(CH3)2), 4.77 (d, 2JH−H
= 16.5 Hz, 2H, CH2Ph), 6.30 (s, 2H, Harom), 6.93 (s, 2H, Harom), 6.95
(s, 2H, Harom), 7.05−7.14 (m, 24H, Harom), 7.27 (d, JH−H = 2 Hz, 4H,
Harom), 7.83 (m, 12H, Harom). 1H NMR (500 MHz, THF-d8, 298 K): δ
ASSOCIATED CONTENT
■
3
3
0.91 (d, JH−H = 1.2 Hz, 3H, CH(CH3)2), 0.92 (d, JH−H = 1.05 Hz,
S
* Supporting Information
2
3H, CH(CH3)2), 2.06 (s, 6H, Ar-CH3), 3.40 (d, JH−H = 15 Hz, 2H,
CH2Ph), 3.80 (m, JH−H = 5.9 Hz, 1H, CH(CH3)2), 4.40 (d, JH−H
3
2
CIF and PDB files, figures, and tables giving X-ray crystallo-
graphic data for 1a, 3a, 4a, and 5a, molecular structures of 3a
=
16.4 Hz, 2H, CH2Ph), 6.71−6.76 (m, 6H, Harom), 6.91 (d, JH−H = 2.1
Hz, 2H, Harom), 6.99 (s, 2H, CHN), 7.12−7.18 (m, 26H, Harom),
7.61 (d, JH−H = 6.9 Hz, 10H, Harom). 13C{1H} NMR (125 MHz, THF-
d8, 298 K): δ 19.13 (ArCH3), 27.05 (CH(CH3)2), 27.51 (CH(CH3)2),
56.81 (CH2Ph), 62.28 (CH(CH3)2), 118.97, 123.67, 125.20, 126.80,
127.32, 127.50, 127.85, 129.97, 129.81, 135.03, 135.96, 136.12, 136.21,
137.01, 137.12, 144.48, 166.34 (CHN), 166.36 (CHN).
1
and 4a, representative H and 13C{1H} NMR spectra of the
prepared complexes, Cartesian coordinates for the computed
structures, a comparison of experimental and calculated
geometrical parameters for 1a, and calculated free energy
values for the different reagents, intermediates, transition states,
and products. This material is available free of charge via the
Internet at http://pubs.acs.org.
Reaction of {ONNCH2pyr}AlMe2 (2c) with Isopropyl Alcohol.
Generation of {ONNCH2pyr}2Al(OiPr) (6c). A solution of
{ONNCH2pyr}AlMe2 (0.020 g, 0.037 mmol) and iPrOH (14.0 μL,
0.180 mmol, 5.0 equiv) in THF (ca. 5 mL) was stirred at 60 °C for 18
h. The solution became dark blue. Volatiles were then removed in
vacuo, n-hexane (ca. 10 mL) was vacuum-condensed in, and the
resulting solution was filtered off. The resulting material was then
dried in vacuo to give 6c as a dark blue solid (0.073 g, 47% vs Al). 1H
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jean-francois.carpentier@univ-rennes1.fr (J.-F.C.);
evgueni.kirillov@univ-rennes1.fr (E.K.).
3
NMR (500 MHz, THF-d8, 298 K): δ 0.50 (d, JH−H = 5.9 Hz, 3H,
Notes
3
CH(CH3)2), 0.52 (d, JH−H = 5.9 Hz, 3H, CH(CH3)2), 2.09 (s, 6H,
The authors declare no competing financial interest.
2
ArCH3), 3.98 (m, 1 H, CH(CH3)2), 4.32 (d, JH−H = 13.3 Hz, 1H,
CH2pyr), 5.28 (d, 2JH−H = 13.3 Hz, 1H, CH2pyr), 6.32 (s, 2H, Harom),
6.79 (s, 2H, Harom), 6.91 (t, JH-H = 7.3 Hz, 4H, Harom), 6.96 (t, JH-H
= 7.4 Hz, 10H, Harom), 7.19 (s, 2H, Harom), 7.16−7.29 (m, 10H, Harom),
7.55 (d, JH−H = 6.8 Hz, 8H, Harom), 7.61 (m, 4H, Harom), 8.12 (s, 2H,
ArCHN). 13C{1H} NMR (125 MHz, C6D6, 298 K): δ 17.41
(ArCH3), 25.43 (CH(CH3)2), 25.53 (CH(CH3)2), 59.51 (CH2Ph),
60.96 (CH(CH3)2), 118.55, 125.31, 125.64, 126.60, 126.98, 133.71,
134.25, 134.41, 134.75, 169.32 (CHN).
ACKNOWLEDGMENTS
■
This work was supported by the Ministere de L’Enseignement
̀
Super
́
ieur et de La Recherche (Ph.D. fellowship to M.N.).
REFERENCES
■
(1) For leading general reviews, see: (a) Atwood, D. A.; Harvey, M. J.
Chem. Rev. 2001, 101, 37. (b) Lewinski, J.; Zachara, J.; Justyniak, I.;
Dranka, M. Coord. Chem. Rev. 2005, 249, 1185. (c) Matsumoto, K.;
Saito, B.; Katsuki, T. Chem. Commun. 2007, 3619. (d) Dagorne, S.;
Atwood, D. A. Chem. Rev. 2008, 108, 4037. (e) Bellemin-Laponnaz, S.;
Dagorne, S. Coordination chemistry and applications of salen, salan
and salalen metal complexes. In The Chemistry of Metal Phenolates;
Zabicky, J., Ed.; Wiley: Chichester, U.K., 2012; in press.
Crystal Structure Determination of 1a, 3a, 4a, and 5a.
Diffraction data were collected at 100(2) K using a Bruker APEX CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.710 73 Å). A combination of ω and ϕ scans was carried out to obtain
at least a unique data set. The crystal structures were solved by direct
methods, and the remaining atoms were located from difference
Fourier synthesis followed by full-matrix least-squares refinement
based on F2 (programs SIR97 and SHELXL-97)19 with the aid of the
WINGX program.20 Many hydrogen atoms could be found from the
Fourier difference analysis. Other hydrogen atoms were placed at
calculated positions and forced to ride on the attached atom. The
hydrogen atom contributions were calculated but not refined. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. The locations of the largest peaks in the final difference
Fourier map calculation and the magnitude of the residual electron
densities were of no chemical significance. Crystal data and details of
data collection and structure refinement for the different compounds
are given in Table S1 (Supporting Information). The crystallographic
data (excluding structure factors) are available as Supporting
Information, as CIF files.
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Computational Details. DFT calculations were carried out using
the Gaussian 09 package,21 employing the two-layer hybrid molecular
orbital (MO) method ONIOM,22 combining the density functional
method BP8623 with the semiempirical MO method PM6.24 In the
ONIOM model, the “low level” layer contained exclusively the atoms
from the phenyl groups of the SiPh3 substituents, while all other atoms
were included in the “high level” layer. Carbon, oxygen, nitrogen,
silicon, and hydrogen atoms were described with all-electron 6-
31G(d,p) double-ζ quality basis sets.25 Aluminum was represented by
def2-TZVP, a triple-ζ valence basis set augmented by 2d1f polarization
functions.26 All stationary points were fully characterized via analytical
frequency calculations as either true minima (all positive eigenvalues)
or transition states (one imaginary eigenvalue). The IRC procedure
was used to confirm the nature of each transition state connecting two
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Organometallics
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(10) This MPV process is thought to also generate acetone, but the
latter compound could not be identified, because of the low amounts
generated, its volatility, and possibly its reactivity in the reaction
mixture as well.
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(12) The preparation of [Al{ONNqui}2(OiPr)] was envisioned via
salt metathesis or alcoholysis reactions of MOiPr salts (M = Na, K) or
iPrOH with {ONNquin}2AlX (X = Cl, Me, respectively) intermediates;
however, none of the latter compounds could be isolated in pure form
from the reaction of AlMe2X reagents with 2 equiv of {ONNquin}H
under a variety of conditions.
(13) MPV reductions mediated by indium compounds are far less
common than those with aluminum derivatives. MPV reduction of
aliphatic and aromatic aldehydes in 2-propanol mediated by
“In(OiPr)3” has just been reported: (a) Lee, J.; Ryu, T.; Park, S.;
Lee, P. H. J. Org. Chem. 2012, 77, 4821. For an example with a
heterogeneous Mg−In oxide catalyst, see: (b) Aramendia, M. A.;
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times less active than their aluminum analogues.
(14) Effective ionic radii for six-coordinate metal centers: Al3+, 0.535
Å; In3+, 0.80 Å. Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32,
751.
(15) The minor byproduct formed could not be identified thus far;
however, its NMR data are inconsistent with a reduction product of
the imino functionality and also do not correspond to Al(OiPr)3.
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