Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-15 for Asymmetric Hydrosilylation

Article abstract of DOI:10.1021/acs.organomet.5b00714

The C₂-symmetric bis(oxazoline)s (4S,4S)-R₁-BOX, with R₁ = tBu and Ph, react with ZnEt₂ straightforwardly to the four-coordinate complexes ((4S,4S)-R₁-BOX)ZnEt₂ in high yield. Their immobil

Full text of DOI:10.1021/acs.organomet.5b00714

Article
pubs.acs.org/Organometallics
Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-
15 for Asymmetric Hydrosilylation
Erwan Le Roux,*,† Aimery De Mallmann,‡ Nicolas Merle,‡ Mostafa Taoufik,‡ and Reiner Anwander§
†Kjemisk Institutt, Universitetet i Bergen, Alleg
́
aten 41, N-5007, Bergen, Norway
́
Lyon 1) LCOMS−CPE-Lyon, Universite de Lyon, 43 Boulevard du 11 Novembre
‡ICL, C2P2 UMR 5265 (CNRS-CPE-Universite
1918, F-69616, Villeurbanne, France
́
§Institut fur Anorganische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, D-72076, Tubingen, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The C2-symmetric bis(oxazoline)s (4S,4S)-R1-
BOX, with R1 = tBu and Ph, react with ZnEt2 straightforwardly
to the four-coordinate complexes ((4S,4S)-R1-BOX)ZnEt2 in
high yield. Their immobilization on highly dehydroxylated
(700 °C) mesoporous silica SBA-15 with enlarged pore
diameters (dp = 15.3 nm) led to the well-defined surface
species [(SiO)Zn(Et)((4S,4S)-R1-BOX)]. These ethyl zinc-
terminated materials react readily with one equivalent of
anhydrous methanol and acetic acid to give predominantly and
selectively the corresponding surface ligand-exchanged species
[(SiO)(Zn(R)((4S,4S)-R1-BOX)] (R = OMe, OAc). All
1
hybrid materials were characterized via N2 physisorption, elemental analysis, DRIFT, H/13C MAS NMR, and EXAFS
spectroscopies. The SBA-15 supported heteroleptic bis(oxazoline) zinc ethyl, methoxide, and acetate surface species were
scrutinized as catalysts for the asymmetric hydrosilylation of acetophenone.
bonds.6 In addition, site isolation of the BOX organozinc
INTRODUCTION
■
complexes would not only counteract complicated equilibrium
phenomena present in solution but also the formation of
polynuclear entities which affect the activity and stereochemical
course of the reaction.3b,c For instance, our recent investigation
of the stability of chiral BOX organozinc complexes ((4S)-R1-
BOX)ZnEt (with R1 = tBu, Ph) in solution revealed
uncontrolled reactivity toward protic reagents leading to ligand
redistribution and thus rendering the BOX zinc type complexes
inadequate for AHS.7 Indeed, the steric effect of substituted
BOX ligands in position C4 was not sufficient to prevent the
monomeric compounds ((4S,4S)-R1-BOX)Zn-X (X = OMe,
OAc) from undergoing Schlenk-type equilibria and inevitably
gave homoleptic (4S,4S)-R1-BOX)2Zn and [Zn(X)2]n com-
pounds through the dimeric intermediates [((4S,4S)-R1-
BOX)Zn(μ-X)]2.7
To date, only the groups of Gauvin and Thomas reported a
well-defined monopodal chiral pyBOX Zn(II) alkyl silica
surface species through the SOMC approach showing
improved activity and enantioselectivity in the asymmetric
silylcyanation of benzaldehyde.8 On the basis of these
observations, we envisaged that site isolation of C2-symmetric
BOX Zn-alkyl complexes via the SOMC approach could readily
stabilize and counteract any ligand redistribution. Herein, we
report the synthesis and the characterization of heteroleptic
Chiral bis(oxazoline)s (BOX) and their derivatives with
organozinc complexes are of great interest due to their wide
applicability as reagents and catalysts in fine chemical synthesis
and polymerization.1 Over the past 20 years, several chiral BOX
organozinc complexes have been immobilized on polymers,
dendrimers, and inorganic oxides in order to enhance their
separation and recyclability of expensive chiral ligands.2 Several
predominant routes have been developed for the solution-
immobilization of enantioselective organozinc complexes:
adsorption, electrostatic interaction, encapsulation, covalent
tethering via the ligand (including sequential or convergent
approaches),1i,3 and grafting of the organozinc catalysts onto
inorganic oxides via a covalent Zn−O(support) bond (Surface
OrganoMetallic Chemistry = SOMC).4 Among all of the
approaches toward immobilized chiral organozinc catalysts, the
covalent SOMC approach is the most promising and
straightforward one in comparison to either (i) the weak and
nonspecific interactions (ionic, hydrogen bonding, coordina-
tive, van der Waals, etc.) prone to metal leaching or (ii) the
covalent immobilization by sequential or convergent ap-
proaches which routinely involve far more sophisticated
preparation methods and are less attractive economically.3
Such an SOMC approach could be of interest regarding
asymmetric hydrosilylation (AHS) since chiral organozinc
complexes are probably the most cost-efficient enantioselective
homogeneous catalysts5 and exhibit excellent catalytic proper-
ties for the reduction of prochiral unsaturated CO and CN
Received: August 24, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
BOX zinc(II) ethyl compounds immobilized on SBA-15, which
is a high surface-area periodic mesoporous silica material with
large pores. The controlled reactivity of the obtained hybrid
materials with protic reagents HX (X = OMe, OAc) was
assessed along with their activity in the AHS of acetophenone,
constituting a pioneering example of well-defined heterogen-
ized chiral zinc catalysts on inorganic supports for the AHS
reaction.
SBA-15 mesoporous silica (surface silanols: 1.12 mmolOH g−1;
specific BET surface area: 530 m2 g−1; pore diamter: 15.3 nm;
Table 1) dehydroxylated at 700 °C (namely SBA-15−700), solids
2a and b were obtained. While the DRIFT spectra of both
compounds show the irreversible disappearance of the isolated
silanols’ stretching band ν(O−H) at 3747 cm−1 (Figure 1 and
RESULTS AND DISCUSSION
■
Synthesis of BOX Zinc Alkyl Complexes. The addition
of neutral ((4S,4S)-R1-BOX) with R1 = tBu (a) and Ph (b) to
ZnEt2 in toluene at ambient temperature followed by heating at
80 °C gave the heteroleptic ethyl zinc(II) compounds ((4S,4S)-
R1-BOX)ZnEt2 (with R1 = tBu, (1a); Ph, (1b), Scheme 1) in
Scheme 1. Synthesis of Zinc Alkyl Molecular Precursors 1
Figure 1. DRIFT spectra of (a) SBA-15−700, (b) 1b, and hybrid
materials (c) 2b, (d) 3b, and (e) 4b.
Figures S4−S5), a broad band of very weak intensity remained
at lower wave numbers (3740−3550 cm−1) showing the
presence of a small amount of residual silanols. Concomitantly,
two groups of bands appeared in the 3060−2850, 1665, and
1480−1360 cm−1 regions, assigned to ν(C−H), ν(CN), and
δ(C−H), respectively, for both materials. An extra absorption
frequency appeared at 1609 cm−1 ascribed to the ν(CC)
stretching of the aromatic ring for 2b. The slight shift of the
ν(CN) band from the oxazolinyl ring to higher wavenumbers
(Δν = 6−8 cm−1) indicates a shortening of CN bonds upon
coordination with the electron withdrawing siloxy ligand
89−91% yield. The isolated pure compounds 1a−b were
1
characterized by DRIFT, H/13C{1H} NMR spectroscopies,
and elemental analysis. The infrared data show a slight shift to
lower frequencies of the imine ν(CN) stretching band (Δν =
3−5 cm−1, free ligands 1664 (a) and 1660 cm−1 (b)) indicating
their coordination to the zinc metal.7−10 The H and 13C{1H}
1
NMR spectra of 1a−b show the appearance of characteristic
signals of BOX and ethyl ligands (Figures S1−S3). The most
remarkable trend for both compounds 1a−b is the splitting of
the geminal Zn−CH2CH3 proton signal into two doublets of
quartets, at 0.34/0.33 ppm for 1a and 0.17/0.18 ppm for 1b,
originating from the diastereotopic splitting due to the C2-
symmetric BOX ligand and the coupling with adjacent CH3
protons. Such diastereotopic protons induced by the chiral
ligand conformation have already been observed with
monoanionic chiral aminotroponiminate (ATI) and BOX zinc
complexes: [(S,S)-{iPrATI)2diph}(ZnEt)2] (with diph =
diamino-1,2-diphenylethane) and (R1,R2-BOX)ZnEt (with R1
= (4S)-tBu, R2 = H; R1 = (4S)-Ph, R2 = H; R1 = (4S)-Ph, R2 =
(5S)-Ph).7,10,11
comparable to ((S,S)-iPr-pyBOX)ZnEt2 immobilized onto
8
SiO2−800
.
The structural parameters derived from the isotherms of
parent SBA-15 and hybrid materials are summarized in Table 1.
According to the Brunauer−Deming−Deming−Teller
(BDDT) classification, all isotherms in Figure S6 are of type
IV, which refers to mesoporous materials.12 Furthermore, the
isotherms have two overlapping hysteresis loops between
relative pressures of 0.7 and 1.0, originating from the
overlapping of large pore size in the mesoporous region and
textural or interparticle porosity.13 At lower relative pressure
between 0.7 and 0.9, the hysteresis loops resemble type H1,
suggesting the presence of cylindrical mesopores.12 Besides the
Immobilization of BOX Zinc Alkyl Complexes on SBA-
15. When contacting 1.1 equiv of zinc(II) alkyl compounds
1a−b in hexane at room temperature with extra-large pore
Table 1. Analytical Data, Pore Volume, Surface Area, and Effective Mean Pore Diameter of SBA-15 and Hybrid Materials 2a−b,
3a−b, and 4a−b
a
b
c
d
hybrid material
As (m2 g−1
)
Vp (cm3 g−1
)
dp,ads (nm)
Zn (mmol g−1
)
N (mmol g−1) (N/Zn)
C (mmol g−1) (C/Zn:C/N)
SBA-15−700
530
380
340
210
230
240
240
0.95
0.69
0.65
0.42
0.43
0.58
0.48
15.3
13.0
12.3
13.9
12.6
12.3
11.6
2a
2b
3a
3b
4a
4b
0.62
0.54
1.32(2.1)
1.07(1.9)
1.05
12.75(20.5:9.6)
13.32(24.6:12.1)
9.85(−:9.4)
13.93(−:11.7)
10.91(−:10.3)
13.48(−:11.6)
e
0.60
e
0.58
1.19
e
0.58
1.06
e
0.57
1.16
a
Pretreatment temperature: 500 °C, 8 h, and then 700 °C, 4 h, 10−3 mbar for SBA-15−700; 25 °C, 5 h, 10−3 mbar for 2a−b, 3a−b, and 4a−b.
b
c
d
Specific BET surface area. BJH from desorption branch cumulative pore volume of pores between 1.0 and 30 nm diameter. Pore diameter
e
according to the maximum of the BJH pore size distribution calculated from the adsorption branch. Determined from Nwt %, Cwt %, and C/N ratio.
B
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 2. 13C CP MAS NMR spectra (ns = 35000−40000, d1 = 2 s, P15 = 4 ms, and LB = 50 Hz). Spinning side bands (*) and residual toluene ($)
and hexane (¥) peaks.
mesoporous structures, all hybrid materials exhibit also high
adsorption capacity at the low relative pressure range of 0−0.2
related to a small amount of micropores (dp < 2 nm)
corroborating the presence of residual silanol groups observed
by DRIFT spectroscopy after immobilization of 1a−b. The
analysis of the Barret−Joyner−Halenda (BJH) pore size
distributions from the desorption branch shows reduced pore
diameters for 2a (Δdp = 2.3 nm) and even more for 2b bearing
encumbered Ph substituents on the BOX ligand (Δdp = 3.0
nm) in comparison with the parent SBA-15 materials (Table
1).
Chemical analysis of the solids obtained is in agreement with
a high surface coverage of Zn (0.95−0.98 Zn nm−2) onto SBA-
15−700 and the partial consumption of silanol groups (≈75% of
the silanol groups according to the silanol population, 1.12
mmolOH g−1 with a surface coverage of 1.27 OH nm−2) for
these materials. Nitrogen and carbon elemental analyses (Table
1) give a stoichiometric ratio in the expected range for a
monopodal surface species (Table 1; theoretical values, N/Zn,
2; C/Zn, 19 for 2a and 23 for 2b).
those of the molecular precursor ((S,S)-iPr-pyBOX)ZnEt2
before immobilization onto silica.8 Note that no transfer of
Zn-Et group to the strained siloxane bridges was observed by
solid-state NMR spectroscopy.14
Zn K-edge XAS measurements were performed on 2b in
order to elucidate the structure of the surface complex in more
detail (oxidation state, first and eventually second coordination
sphere of the metal). XANES revealed that the main edge (1s
→ 4p electronic transition) for 2b is at the same position
(9661.5 0.5 eV) as for ZnO (cf., Zn(0) metal foil: 9658.6
eV15) suggesting a Zn(II) surface complex (Figure S9). The
parameters extracted from the fit of the k3.χ(k) EXAFS
spectrum (Table 2 and Figure S10) are consistent with ca. one
oxygen atom at 1.90(1) Å and two nitrogen atoms and ca. one
carbon atom bonded to zinc at 1.99(2) Å (unresolved
contributions, then N and C considered at the same distance).
This is in accordance with the proposed structure for 2b
(Scheme 2), with bonding distances matching those extracted
a
Table 2. EXAFS Parameters for Hybrid Materials 2b and 3b
Mesoporous hybrid materials 2a−b were further charac-
1
terized by solid-state NMR spectroscopy. The H MAS NMR
neighboring
atom of Zn
number of
atoms N
distance from Zn
Debye−Waller
factor σ (Å2)
R (Å)
spectra of 2a−b give overlapping broad signals accounting for
aromatic and aliphatic protons from residual toluene solvent,
tBu and Ph groups (Figure S7). The 13C CP MAS NMR
spectra are more conclusive and show a new set of peaks
originating from the coordination of the electron-withdrawing
siloxy ligand to the molecular chiral precursors 1a−b (Figure 2
and Figure S8). Indeed, the chemical resonances of both C1/C2
from the oxazolinic ring are shifted to the downfield region with
ΔδC1/C2 = 4.5/1.7 ppm for 2a and with ΔδC1/C2 = 2.7/0.7 ppm
for 2b, which is entirely in accordance with subsequent ligand
exchange from a donor to an electron-withdrawing group.
Surprisingly, the α- and β-carbons of the Zn-Et show the
opposite tendency with chemical resonances (13C NMR: −0.9/
11.6 ppm for 2a and −3.8/10.7 ppm for 2b, respectively)
shifted to the upfield region. A similar trend was observed for
the surface species [(SiO)Zn(Et)((S,S)-iPr-pyBOX)] where
both Cα and Cβ chemical shifts were shielded compared to
Hybrid Material 2b
Zn−O (OSi)
Zn--C (CH2Me)
Zn−N (BOX)
Zn−C (CH2Me)
Zn--C (BOX)
Hybrid Material 3b
Zn−O
1.1(3)
1.90(1)
0.0050(19)
0.0094(42)
0.013(5)
b
b
0.9(3)
1.99
2.0
1.0
4.0
1.99(2)
2.99(3)
3.31(3)
0.0089(40)
0.014(6)
2.0(2)
2.0
1.90(1)
2.01(1)
2.94(2)
3.34(2)
0.0048(6)
0.0069(11)
0.0039(18)
0.011(3)
Zn−N (BOX)
Zn--C (OMe)
Zn--C (BOX)
1.0
4.0
a
The errors generated by the EXAFS fitting program “RoundMid-
night” are indicated in parentheses. Δk, [1.3−15.0 Å−1]; ΔR, [0.6−3.3
Å]; S02 = 0.84; ΔE0 = 4.15 1.0 eV (fitted); fit residue ρ = 2.6% (2b)
or 1.7% (3b); quality factor, (Δχ)2/ν = 2.94 (2b) or 3.22 (3b), with ν
b
= 14/25 (2b) or 15/25 (3b). Shell constrained to a parameter above.
C
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
peaks related to Zn−OCOMe at 181.4 and 179.5 ppm for 4a−
b corroborate the surface species structure proposed in Scheme
3. In a control experiment, a solution of complex 1b was treated
Scheme 2. Proposed Surface Species for 2a−b
Scheme 3. Proposed Surface Species after Ligand Exchange
from the crystallographic data of analogous molecular
complexes for Zn−C (1.95−1.99 Å for methyl and ethyl
ligands),7,16 Zn−O, and Zn−N bond lengths (1.89−1.99 Å and
1.94−2.14 Å, respectively).16−18 Moreover, the fit could be
improved by considering two further layers of carbon
backscatterers, one carbon atom at 2.99(3) Å and four carbon
atoms at 3.31(3) Å from the Zn center, which could be,
respectively, attributed to the carbon atom of the methyl in the
ethyl ligand and to the sp2 and sp3 carbon atoms connected to
the N atoms of the BOX ligand. Consequently, the combined
characterizations suggest the formation of monopodal [(
SiO)Zn(Et)((4S,4S)-R1-BOX)] as the major surface species
(Scheme 2).
with 1 equiv of glacial AcOH to evaluate the coordination
behavior of the BOX ligand under acidic conditions during the
1
ligand exchange. The in situ H and 13C NMR spectroscopies
revealed that (i) the intensity of the proton signals assigned to
Zn-Et are decreased with concomitant evolution of ethane, (ii)
the signals corresponding to Zn−O(CO)Me proton and
carbon atoms appeared, and (iii) the characteristic signals of
the BOX and ethyl ligands are shifted to lower field (Figures
S9−10).20 In addition to the new set of chemical resonances,
which greatly differs from that of the free ligand b in toluene
solution (Figure S9), the proton signal integration of the
mixture resulting from complex 1b with AcOH indicates most
likely the formation of ((4S,4S)-Ph-BOX)ZnEt(OAc).20 These
results confirmed a selective ligand exchange (Zn-Et → Zn-
OAc) and the stability of Zn-BOX bonds upon the addition of a
Brønsted-acidic reagent. The completeness of the surface ligand
exchange was further confirmed by the elemental analysis data
determined for 3a−b and 4a−b (Table 1).
In order to further characterize the protonolytic ligand
exchange reactions involving the Zn-Et fragment, Zn K-edge
XAS measurements were also performed on both SBA-15
supported zinc species 3b and 4b (Figure 3 and Figures S11−
13). XANES revealed that the main edge is at the same position
for species 2b, 3b, 4b, and ZnO, suggesting a Zn(II) surface
species. The EXAFS parameters resulting from the fits of the
spectra (Tables 2 and S1) agree with the proposed structures
for 3b and 4b. For 3b, the EXAFS data are consistent with two
oxygen atoms at 1.90 Å and two nitrogen atoms at 2.01 Å
coordinated to the Zn center (Table 2). These distances are in
good agreement with average values obtained from crystallo-
graphic data of similar molecular complexes.16−18 As for surface
species 2b, the fit could be improved by adding two further
layers of carbon backscatterers, one carbon atom at 2.94(2) Å
and four carbon atoms at 3.34(2) Å from the Zn center, which
could be, respectively, attributed to the carbon atom of the
methoxy and to the carbon atoms connected to the N atoms of
the BOX ligand. In the EXAFS fit obtained for 4b, very similar
parameters were obtained, and a third oxygen atom as a non-σ-
bonded one of an acetate could be introduced but not at a
distance lower than 3.09(5) Å and might be assigned to a
weakly coordinated carbonyl group from the carboxylate ligand
in accordance with the νas(COO−) and νs(COO−) carboxylato
stretching band separation observed by DRIFT spectroscopy
(Figure 1 and Scheme S1). In conclusion, the 4- and pseudo 5-
coordinate structures proposed for 3b and 4b, respectively, are
fully compatible with all of the spectroscopic and analytical data
(Scheme 3).
Reactivity of Immobilized BOX Alkyl Zinc Complexes
with Protic Reagents. While Zn-alkyl moieties are known to
react readily with Brønsted acids, the reactivity and the stability
of Zn−O bonds (D°298 ≤ 250 kJ mol−1) at a molecular level are
rather uncertain, and the SiO-Zn bond could possibly be
cleaved off in the presence of Brønsted acids. The ligand
exchange reactions with protic reagents were first monitored by
DRIFT spectroscopy. Upon contacting 2a−b either with 1.1
equiv of anhydrous MeOH or glacial AcOH (pKa = 15.5 and
4.76 in H2O, respectively) in hexane at ambient temperature,
the hybrid materials 3a−b and 4a−b were obtained as colorless
solids. The ν(CN) stretching band is slightly shifted by 1−2
cm−1 to lower frequency with the concomitant appearance of
the asymmetric νas(COO−) and symmetric νs(COO−)
carboxylato stretching frequencies for 4a−b (Figure 1 and
Figures S4−5). The magnitude of separation of about Δexp
=
118
1 cm−1 of these two stretching bands indicates the
monodentate chelation of the carboxylato group to the Zn
center.19 For all hybrid materials, no free Si−OH stretching
bands at 3747 cm−1 were observed upon ligand exchange,
showing that no reaction occurs between SiO-Zn and the
protic reagents under these conditions.
The N2 adsorption−desorption isotherms of hybrid materials
3a−b and 4a−b display the same characteristics as the ones
observed for 2a−b with the pore size increasing by ca. 0.5 nm
for 3a−b and decreasing by ca. 0.7 nm for 4a−b compared to
2a−b (Table 1 and Figure S6). These values are reflecting the
exchange of the ethyl ligand by either a slightly less bulky
methoxy or a sterically more demanding carboxylato ligand.
1
The H MAS NMR spectra of 3a−b display peaks at about
3.4 ppm attributable to the protons of Zn-OCH3, whereas the
1H MAS NMR spectra of 4a−b are inconclusive due to the
overlapping of broad peaks of aliphatic protons (Figure S7).
The 13C CP MAS NMR spectra for both hybrid materials 3a−b
and 4a−b show clearly the complete disappearance of Zn-Et
peaks and a shift of the signal of the C2 atom to the downfield
region (compared to 2a−b) indicating that the ligand
exchanges were effective (Figure 2 and Figure S8).
Furthermore, the 13C peaks attributable to Zn-OMe were
observed at 52.4 and 50.3 ppm for 3a and 3b, respectively, and
D
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 3. k3-Weighted zinc K-edge EXAFS spectra (left) and corresponding Fourier transform (right, modulus and imaginary part) with comparison
to simulated curves for 3b. Solid lines, experimental; dashed lines, spherical wave theory.
Asymmetric Hydrosilylation of Acetophenone. Pre-
liminary catalytic tests in the AHS of the standard substrate
acetophenone were carried out with the heterogenized zinc
complexes 2−4 using relatively inexpensive HSi(OEt)3 as a
silylation reagent for 5 h at 60 °C in acetonitrile-d3 on the
NMR scale and compared to the unsupported zinc complexes
1a−b. The results are summarized in Table 3. Acetophenone
pseudoequatorial position enhancing the complex flexibility and
thus facilitating the substrate approach by its si face (Chart 1).
Chart 1
Table 3. Asymmetric Hydrosilylation of Acetophenone
a
Catalyzed by BOX Zinc Complexes
a
b
c
c
entry
catalyst
yield
29
%
ee %
absol. config.
1
2
3
4
5
6
7
8
9
1a
1b
2a
2b
3a
3b
19
88
14
50
9
R
R
R
R
R
R
R
R
38
32
53
65
86
52
28
13
32
28
30
d
3b
e
3b
f
f
4b
n.d.
n.d.
A similar trend was observed with the supported BOX zinc
complexes 2a−b (Table 3, entries 3−4) showing the same
absolute configuration as the homogeneous analogues but
lower ee values. This can be ascribed to the bulkier silica ligand
diminishing the degree of flexibility of the BOX zinc complexes
upon immobilization.3e Nonetheless, the activity was slightly
enhanced upon immobilization of molecular complexes and
takes most likely its origin by the presence of the siloxy
electron-withdrawing ligand in the Zn coordination sphere,
presenting similar activity for the reported pyBOX Zn(II) silica
surface species for asymmetric silylcyanation.8 Additional
control experiments demonstrated that the reaction of hybrid
material 2b with HSi(OEt)3 releases less than 3% of free BOX
b ligand and that the formation of soluble Zn hydride species is
a
Conditions: 8.7 μmolZn, 0.66 mmol HSi(OEt)3 and 0.26 mmol
b
1
PhC(O)Me in 0.5 mL of acetonitrile-d3. Yield determined by H
NMR spectroscopy based on PhC(O)Me and PhCHMeOSi(OEt)3
c
chemical shifts. ee = enantiomeric excess and absolute configuration
were determined by 1H NMR in chloroform-d using chiral derivatizing
reagents (R)-(−)-α-acetylmandelic acid.21 1st recycling. 2nd
d
e
f
recycling. n.d. = not determined.
was hydrosilylated in moderate yield accomplishing the same
absolute configuration (R) for both homogeneous complexes
1a−b but with markedly improved ee (19% → 88%) for 1b
(Table 3, entries 1−2).
The enantio-directing effect of both BOX ligands is similar to
that found for BOX Zn(II) and Cu(II) complexes, and the
coordination of the incoming substrate interferes with the BOX
substituents in C4 position promoting the favored approach to
the si face.22 Moreover, the poor enantioselectivity observed
with complex 1a can be ascribed to the structural inflexibility
upon coordination of prochiral ketone when the tBu-BOX
ligand is employed. In this case, as already observed with similar
BOX Zn(II) complexes, both tBu substituents in C4 position
are in pseudoaxial position decreasing the flexibility, while the
Ph-BOX ligand has one substituent in pseudoaxial and one in
1
not observable either by H NMR or DRIFT spectroscopies
indicating the absence of any homogeneous AHS reactions.23
While the preliminary characterization by 1H MAS NMR
spectroscopy of the hybrid materials obtained from treatment
of 2b with HSi(OEt)3 was inconclusive due to the broad
chemical resonances of HC4/H2C5 protons from the oxazolinic
ring overlapping with putative Zn−H species (expected range
for Zn−H: 4.2−5.6 ppm),24 DRIFT spectroscopy pointed
toward the presence of Zn−H species with a stretching band
ν(Zn−H) at 1605 cm−1 (Figure S14; terminal/bridging Zn−H
in solution, 1770−1720/1500 cm−1).24 These observations
E
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
deliver initial insights into the formation of BOX Zn−H active
surface species, as depicted in Chart 1.
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed with
rigorous exclusion of air and water, using standard Schlenk, high-
vacuum and glovebox techniques (MB Braun MB200B-G; < 1 ppm of
O2, < 1 ppm of H2O). Hexane and toluene were purified by using
Grubbs columns (MB Braun Solvent Purification System 800).
Chloroform-d, acetonitrile-d3, toluene-d8, and benzene-d6 were
obtained from Sigma-Aldrich, dried over CaH2 or sodium, vacuum
transferred, degassed, and filtered prior to use. 1-(3-(Dimethylamino)-
propyl)-3-ethylcarbodiimide hydrochloride was purchased from Alfa
Aesar and used as received. All other chemicals were purchased from
Sigma-Aldrich and used as received. Hexagonal SBA-15 with enlarged
pores was prepared according to the literature.28 Surfactant-free
material was obtained from the calcinations of the as-made material at
540 °C for 4 h. SBA-15−700 came from the high temperature
dehydroxylation at 500 °C (8 h, heating rate 36 °C h−1) and then at
700 °C (4 h, heating rate 36 °C h−1) of the surfactant-free material
under high vacuum (10−3 mbar).
Previous studies on Zn(II)-based AHS reactions showed that
not only the chiral ligand is important but also the nature of the
coligands can affect significantly the activity and enantioselecti-
vity.6b,i,25 The Zn-Et/OMe ligand exchange shows a rise in
activity for both catalysts 3a and 3b. Such a rise in activity could
conceivably be attributed to the presence of methanol traces,
after ligand exchange, enhancing the reaction rate by cleaving
off the Zn-OR bond via protonolysis. Such an acceleration
effect was observed with secondary diamine/ZnEt2 complexes
in the presence of methanol increasing substantially the activity
and at the same time lowering the ee’s for acetophenone.6b,25 In
our case, the presence of methanol is also lowering the ee’s and
most probably promotes the displacement of the BOX ligands
under these catalytic conditions. This trend is further
corroborated for the heterogeneous Zn(II) catalyst bearing
bidentate carboxylato ligand: the activity for 4b dropped down
to 13% (Table 3, entry 9), and 4a showed no activity in
acetophenone reduction.6f,g,p−r,26 We assume that (i) the
silylated acetate ligand formed competes with acetophenone,
which would lower the activity, and that (ii) the residual
physisorbed AcOH is presumably more effective in the BOX
ligands displacement, thus reducing the formation of
enantioselective active Zn species.
Spectroscopic Methods. The NMR spectra of air and moisture
sensitive compounds were recorded by using J. Young valve NMR
tubes at 25 1 °C on a Bruker-AVANCE-DMX400 spectrometer (5
mm BB, 1H, 400.13 MHz; 13C, 100.62 MHz) and a Bruker-BIOSPIN-
AV500 spectrometer (5 mm BB, 1H, 500.13 MHz; 13C, 125.75 MHz).
1H and 13C shifts are referenced to internal solvent resonances and
reported in parts per million relative to TMS. Solid-state NMR spectra
were recorded on a Bruker DLX-500 spectrometer at 10 kHz,
equipped with a standard 4 mm probe head, and operating at 500.13
1
MHz for H and 125.77 MHz for 13C. The samples were filled into
The recycling of the most active system 3b was performed,
and it could be recovered and reused twice showing constant ee
values (Table 3, entries 6−8) but with substantial loss of
activity. The reduced activity of the recycled hybrid material 3b
might originate from the presence of methanol traces having a
significant influence on the mechanism as demonstrated by
Carpentier’s group.6b,25 Also, pore blockage affecting the
diffusion of the substrate to the active site cannot be excluded.27
Leaching of active Zn species seems to be not responsible for
the loss of activity since the addition of a second batch of
acetophenone to the filtered supernatant did not produce any
significant activity in AHS. The unchanged enantioselectivities
observed during the recycling studies corroborate the displace-
ment of BOX ligands by residual protic reagents, the amount of
which is significantly reduced for the recycled materials
affecting considerably less the ee’s.
zirconia rotors in a glovebox and tightly closed. Compressed air was
used for both bearing and driving the rotors. Chemical shifts are given
1
with respect to adamantane for H and 13C with TMS as an external
reference. Nitrogen adsorption−desorption isotherms were measured
with an ASAP 2020 volumetric adsorption apparatus (Micromeritics)
at 77.4 K for relative pressures from 10−2 to 0.99 (am(N2, 77 K) =
0.162 nm2). The samples were outgassed in the degas port of the
adsorption analyzer. The BET specific surface area was obtained from
the nitrogen adsorption data in the relative pressure range from 0.04 to
0.20.12 The pore size distributions were derived from the adsorption
branches using the BJH method.12 IR spectra were recorded on a
́ ́
Nicolet FT-IR Protege 460 spectrometer with a DRIFT collector
(Spectra-Tech Collector II from Thermo Scientific), and the samples
were prepared in a glovebox and mixed in a KBr powder matrix before
to be loaded in the microcup. Then, the cup is mounted onto a
microcup holder and placed into the DRIFT dome (equipped with
KBr windows). The spectra were averaged over 64 scans; the
resolution was 4 cm−1. Elemental analyses of C, H, and N were
performed on an Elementar Vario EL III.
CONCLUSIONS
■
((S,S)-tBu-BOX)ZnEt2 (1a). In a pressure tube, a solution of (S,S)-
tBu-BOX (200 mg, 0.68 mmol) in toluene (10 mL) was added to a
solution of ZnEt2 (0.68 mL of a 1 M solution in hexane, 0.68 mmol) in
toluene (5 mL). The colorless solution was stirred at 80 °C for 22 h.
The reaction mixture was transferred into a vial, and all volatiles were
removed in vacuo, leading to a white solid of 1a. Yield: 259 mg (91%).
1H NMR (400.13 MHz, benzene-d6, 25 °C): δ 3.8−3.7 (m, 6 H, H2C5
+ HC4); 1.64 (s, 6 H, C1-(CH3)2); 1.43 (t, J = 8.2 Hz, 6H, Zn-
CH2CH3); 0.83 (s, 18 H, C-(CH3)3); 0.34/0.33 (dq, 4H, J = 8.2 Hz,
3.2 Hz, Zn−CH2CH3) ppm. 13C{1H} NMR (100.62 MHz, benzene-
d6, 25 °C): δ 169.1 (C2); 75.8 (C4); 68.9 (C5); 39.5 (C1); 33.9 (C-
(CH3)3); 25.8 (C-(CH3)3); 25.3 (C1-(CH3)2); 12.6 (Zn-CH2CH3);
4.9 (Zn-CH2CH3) ppm. DRIFT (KBr, ν/cm−1, [4000−400]): 2975s,
2957vs, 2904m, 2904s, 2984s, 2880s, 2866s, 1659vs, 1479m, 1450m,
1389m, 1383m, 1354m, 1333w, 1297w, 1249s, 1211w, 1193vw,
1171vw, 1142s, 1118s, 1096vw, 1046w, 1024vw, 1013vw, 981s, 950vw,
923s, 897vw, 883m, 855vw, 789vw, 750vw, 732vw, 677vw, 611w,
551w, 529w, 515m, 449m. Anal. Calcd for C21H40N2O2Zn (417.93):
C, 60.35; H, 9.65; N, 6.70%. Found: C, 60.06; H, 9.37; N, 6.78%.
((S,S)-Ph-BOX)ZnEt2 (1b). Following the procedure described
above, (S,S)-Ph-BOX (268 mg, 0.80 mmol) in toluene and ZnEt2
(0.80 mL of a 1 M solution in hexane, 0.80 mmol) were stirred at 80
Heteroleptic neutral bidentate bis(oxazoline) zinc(II) ethyl
compounds can be straightforwardly immobilized on extra-large
pore SBA-15 giving exclusively the monopodal surface species
[(SiO)Zn(Et)((4S,4S)-R1-BOX)], with R1 = tBu, Ph. The
SBA-15 supported BOX Zn(II) alkyl complexes are readily
accessible to ethyl/HL ligand exchange by preserving
monopodal surface species and without cleaving off or
decomposing the BOX zinc complexes. This simplified
approach illustrates that fine-tuning of such site-isolated zinc
alkyl surface species is available by subsequent ligand exchange
via alkoxy or acetato groups without ligand redistribution (i.e.,
no formation of homoleptic or dimeric compounds). Such
covalently bonded chiral BOX Zn(II) complexes can be
efficient in the AHS of prochiral ketones. Further research is
underway addressing the identification and deactivation of the
active species which could contribute to a mechanistic
understanding of heterogenized chiral zinc(II) complexes for
the AHS as well as extending the scope of the reaction to other
unsaturated substrates.
F
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
°C for 22 h giving 1b as a yellowish solid. Yield: 327 mg (89%). H
NMR (400.13 MHz, benzene-d6, 25 °C): δ 7.2−7.0 (m, 10 H, ArH);
5.03 (dd, 2 H, J = 8.1, 9.8 Hz, HaC5); 4.09 (dd, 2 H, J = 8.1, 9.8 Hz,
HbC5); 3.77 (t, 2 H, J = 8.1 Hz, HC4); 1.71 (s, 6 H, C1-(CH3)2); 1.33
(t, 6 H, J = 8.1 Hz, Zn-CH2CH3); 0.18/0.17 (dq, 4 H, J = 8.1 Hz, 7.4
a suspended solution of 2a (250 mg, 155 μmol) in hexane (5 mL) was
added dropwise a solution of glacial acetic acid (171 μmol, 1.1 equiv)
in hexane (4 mL). The reaction mixture was stirred at ambient
temperature overnight. Then, the solid was washed with hexane several
times and dried overnight under vacuum. 1H MAS NMR (500.13
MHz, 25 °C): δ 3.7 (br, OCO−CH3, H2C5 + HC4); 1.4 (br, C1-
(CH3)); 0.80 (br, C-(CH3)3) ppm. 13C CP MAS NMR (125.76 MHz,
25 °C): δ 181.4 (OCO−CH3); 176.2 (C2); 75.3 (C4); 71.3 (C5); 41.7
(C1); 35.0 (C-(CH3)3); 26.2 (C-(CH3)3 + C1-(CH3)2 + OCO-CH3)
ppm. DRIFT (KBr, ν/cm−1, [4000−1300]): 3650wbr, 2960s, 2913m,
2873m, 1664vs, 1630s, 1609s, 1598vs, 1481m, 1448m, 1404m, 1389m,
1373m, 1325m. Elemental analysis found (%): C 13.10, H 1.90, N
1.48.
1
Hz, Zn−CH2CH3) ppm. H NMR (500.13 MHz, toluene-d8, 25 °C):
δ 7.15−6.90 (m, 10 H, ArH); 4.99 (dd, 2 H, J = 8.6, 10.8 Hz, HaC5);
4.05 (dd, 2 H, J = 8.6, 10.8 Hz, HbC5); 3.72 (t, 2 H, J = 8.1 Hz, HC4);
1.60 (s, 6 H, C1-(CH3)2); 1.19 (t, 6 H, J = 8.1 Hz, Zn-CH2CH3); 0.06/
0.05 (dq, 4 H, J = 8.5 Hz, 7.9 Hz, Zn−CH2CH3) ppm. 13C{1H} NMR
(100.62 MHz, benzene-d6, 25 °C): δ 170.0 (C2); 142.8 (ipso-CAr);
128.9 (ortho-CAr); 127.8 (para-CAr); 127.1 (meta-CAr); 75.1 (C4); 69.9
(C5); 39.6 (C1); 25.1 (C1-(CH3)2); 13.8 (Zn-CH2CH3); 3.0 (Zn-
CH2CH3) ppm. DRIFT (KBr, ν/cm−1, [4000−500]): 3084w, 3061m,
3029m, 2983m, 2964m, 2934m, 2916m, 2898m, 2849m, 1952w,
1883w, 1812w, 1657vs, 1603w, 1494m, 1471m, 1455m, 1386w,
1352w, 1306w, 1274w, 1239m, 1207w, 1143s, 1115s, 1078m, 1041w,
1027m, 981s, 933w, 915m, 885vw, 800w, 761m, 701s, 611w, 540w.
Anal. Calcd for C25H32N2O2Zn (457.91): C, 65.57; H, 7.04; N, 6.12%.
Found: C, 65.21; H, 6.94; N, 6.07%.
Representative Procedure for the Synthesis of [((4S,4S)-tBu-
BOX)ZnEt]@SBA-15−700 (2a). SBA-15−700 (555 mg, 1.12 mmolOH g-
1) was suspended in toluene (10 mL). Then, a solution of 1a (259.8
mg, 622 μmol, 1 equiv) in toluene (5 mL) was added to this
suspension. The mixture was stirred overnight at room temperature.
The solid was washed with toluene several times and subsequently
evaporated overnight. 1H MAS NMR (500.13 MHz, 25 °C): δ 4.2 (br,
H2C5 + HC4); 1.4 (br, C1-(CH3)2 + Zn-CH2CH3); 0.8 (br, C-(CH3)3
+ Zn−CH2CH3) ppm. 13C CP MAS NMR (125.76 MHz, 25 °C): δ
173.6 (C2); 75.6 (C4); 71.3 (C5); 41.2 (C1); 34.9 (C-(CH3)3); 26.2
(C-(CH3)3 + C1-(CH3)2); 11.6 (Zn-CH2CH3); −0.9 (Zn-CH2CH3)
ppm. DRIFT (KBr, ν/cm−1, [4000−1300]): 3654wbr, 2960s, 2909m,
2875m, 2850w, 1665vs, 1481m, 1450vw, 1399w, 1371m. Elemental
analysis found (%): C 15.32, H 3.78, N 1.85, Zn 4.06.
[((4S,4S)-Ph-BOX)ZnOAc]@SBA-15−700 (4b). 1H MAS NMR
(500.13 MHz, 25 °C): δ 7.1 (m, ArH); 1.4 (br, C1-(CH3)2) ppm.
The corresponding peaks of OCO−CH3, H2C5 and HC4 were not
observed under these conditions. 13C CP MAS NMR (125.76 MHz,
25 °C): δ 179.5 (OCO−CH3); 175.1 (C2); 139.3 (ipso-CAr); 128.2
(ortho-/meta-/para-CAr); 76.4 (C4); 68.7 (C5); 40.6 (C1); 22.7 (C1-
(CH3)2 + OCO-CH3) ppm. DRIFT (KBr, ν/cm−1, [4000−1300]):
3654wbr, 3087vw, 3066m, 3033m, 2998m, 2984m, 2967m, 2935m,
2918m, 2876m, 2857w, 1661vs, 1606s, 1569s, 1495m, 1477m, 1451s,
1385m, 1351w, 1325w. Elemental analysis found (%): C 16.19, H 1.28,
N 1.63.
General Procedure for Hydrosilylation. In an argon-filled
glovebox, the Zn catalyst (8.7 μmol), acetophenone (0.26 mmol), 0.5
mL of acetonitrile-d3, and triethoxysilane (0.66 mmol) were added to a
J. Young NMR tube. The mixture was then heated at 60 °C (oil bath).
1
After 5 h, the progress of the reaction mixture was measured by H
NMR spectroscopy by integrating intrinsic signals of the methyl
groups from the educt and the product at 2.56 and 1.46 ppm,
respectively. The mixture was then transferred to a vial with CH2Cl2,
hydrolyzed with aqueous HCl, and extracted with CH2Cl2 (3 × 3 mL).
The organic layer was washed with distilled water (2 × 3 mL), dried
over MgSO4, and concentrated. The enantiomeric excess and absolute
[((4S,4S)-Ph-BOX)ZnEt]@SBA-15−700 (2b). 1H MAS NMR
(500.13 MHz, 25 °C): δ 7.1 (m, ArH); 1.4 (br, C1-(CH3)2); 0.90
(br, Zn-CH2CH3); 0.07 (br, Zn−CH2CH3) ppm. H2C5 and HC4 are
not observable under these conditions. 13C CP MAS NMR (125.76
MHz, 25 °C): δ 172.7 (C2); 139.7 (ipso-CAr); 128.5 (ortho-/meta-/
para-CAr); 75.6 (C4); 68.8 (C5); 40.3 (C1); 24.2 (C1-(CH3)2); 10.7
(Zn-CH2CH3); −3.8 (Zn-CH2CH3) ppm. DRIFT (KBr, ν/cm−1,
[4000−1300]): 3628wbr, 3085w, 3062m, 3031s, 2982m, 2925m,
2885m, 1946w, 1861w, 1805w, 1736vw, 1665vs, 1609m, 1587w,
1498s, 1477m, 1458m, 1380m. Elemental analysis found (%): C 16.00,
H 5.13, N 1.53, Zn 3.52.
Representative Procedure for the Synthesis of [((4S,4S)-tBu-
BOX)ZnOMe]@SBA-15−700 (3a). In an oven-dried scintillation vial,
to a suspended solution of 2a (110 mg, 66 μmol) in hexane (5 mL)
was added dropwise a solution of anhydrous methanol (72.6 μmol, 1.1
equiv) in hexane (4 mL). The reaction mixture was stirred at ambient
temperature overnight. Then, the solid was washed with hexane for
several times and dried overnight under vacuum. 1H MAS NMR
(500.13 MHz, 25 °C): δ 3.4 (br, OCH3 + H2C5 + HC4); 1.3 (br, C1-
(CH3)2); 0.8 (br, C-(CH3)3) ppm. 13C CP MAS NMR (125.76 MHz,
25 °C): δ 174.6 (C2); 75.2 (C4); 71.2 (C5); 52.4 (OCH3); 41.7 (C1);
34.9 (C-(CH3)3); 26.2 (C-(CH3)3 + C1-(CH3)2) ppm. DRIFT (KBr,
ν/cm−1, [4000−1300]): 3652wbr, 2956s, 2913m, 2876m, 2849w,
2821vw, 1663vs, 1481m, 1456vw, 1394w, 1373m. Elemental analysis
found (%): C 11.83, H 1.83, N 1.47.
1
configuration were determined by H NMR spectroscopy from the
reaction with the chiral derivatizing reagents (R)-(−)-α-acetylmandelic
acid and the mixture of (R)- and (S)-1-phenyl-ethanol leading to the
corresponding diasteromeric ester according to the reported
procedure.20
Representative Recovery and Reuse Procedure. All oper-
ations were carried out in a glovebox. After the completed catalytic
reaction (Table 2, entry 6), the solution was evaporated to dryness
under vacuum for 4 h, followed by several consecutive washing−
decantation (≈ 4 × 1 mL of hexane) and drying steps. Then, the
recovered supported Zn catalyst was used for the second run (vide
supra).
EXAFS Determination for 2−4b. X-ray absorption spectra were
acquired at room temperature at the zinc K-edge, with a double crystal
Si(111) monochromator detuned 70% to reduce the higher harmonics
of the beam. The spectra were recorded in the transmission mode
between 9.4 and 11.0 keV (every 0.5 eV in the edge area). The Zn
samples were packaged inside an argon filled glovebox into a double
airtight sample holder equipped with kapton windows. The spectra
analyzed were the results of four such acquisitions, and no evolution
could be observed between the first and last acquisitions. The data
analyses were performed by standard procedures using in particular
the program “Athena”29 and the EXAFS fitting program “RoundMid-
night”30 from the “MAX” package, using spherical waves. The program
[((4S,4S)-Ph-BOX)ZnOMe]@SBA-15−700 (3b). 1H MAS NMR
(500.13 MHz, 25 °C): δ 7.1 (m, ArH); 3.4 (br, H2C5 + HC4); 3.4 (br,
OCH3); 1.2 (br, C1-(CH3)2) ppm. 13C CP MAS NMR (125.76 MHz,
25 °C): δ 173.7 (C2); 140.4 (ipso-CAr); 128.3 (ortho-/meta-/para-CAr);
76.4 (C4); 68.3 (C5); 50.3 (OCH3); 40.2 (C1); 23.2 (C1-(CH3)2) ppm.
DRIFT (KBr, ν/cm−1, [4000−1300]): 3647wbr, 3088vw, 3066m,
3033m, 2984m, 2951m, 2915w, 2847w, 1663vs, 1606w, 1496m,
1476m, 1458m, 1389w, 1373w, 1356w. Elemental analysis found (%):
C 16.73, H 1.78, N 1.67.
FEFF8 was used to calculate theoretical files for phases and amplitudes
based on model clusters of atoms.31 The value of the scale factor, S0
=
2
0.84, was determined from the k2 and k3·χ(k) spectra of a ZnO
reference, diluted in boron nitride and carefully mixed and pressed as a
pellet (four oxygen at 1.95(1) Å in the first coordination sphere of
Zn). The refinements were performed by fitting the structural
parameters Ni, Ri, σi, and the energy shift, ΔE0 (the same for all
shells). The fit residue, ρ (%), was calculated by the following formula:
Representative Procedure for the Synthesis of [((4S,4S)-tBu-
BOX))ZnOAc]@SBA-15700 (4a). In an oven-dried scintillation vial, to
G
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
∑k [k3χexp(k) − k3χ (k)]2
Chav
10−22.
́
ez, D.; Parra-Hake, M.; Aguirre, G. Mini-Rev. Org. Chem. 2010, 7,
cal
ρ =
× 100
∑k [k3χexp(k)]2
(3) (a) Bianchini, C.; Barbaro, P. Top. Catal. 2002, 19, 17−32.
(b) Song, C. E.; Lee, S.-g. Chem. Rev. 2002, 102, 3495−3524.
(c) Fraile, J. M.; García, J. I.; Mayoral, J. A. Chem. Rev. 2009, 109,
360−417. (d) Fraile, J. M.; Garcia, J. I.; Herrerias, C. I.; Mayoral, J. A.;
Pires, E. Chem. Soc. Rev. 2009, 38, 695−706. (e) Trindade, A. F.; Gois,
P. M. P.; Afonso, C. A. M. Chem. Rev. 2009, 109, 418−514.
As recommended by the Standards and Criteria Committee of the
International XAFS Society,32 the quality factor, (Δχ)2/ν, where ν is
the number of degrees of freedom in the signal, was calculated, and its
minimization considered in order to control the number of variable
parameters in the fits.
(f) Zamboulis, A.; Moitra, N.; Moreau, J. J. E.; Cattoen, X.; Wong Chi
̈
Man, M. J. Mater. Chem. 2010, 20, 9322−9338. (g) Dal Santo, V.;
Liguori, F.; Pirovano, C.; Guidotti, M. Molecules 2010, 15, 3829−3856.
(4) (a) Anwander, R. Chem. Mater. 2001, 13, 4419−4438.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00714.
■
S
́
(b) Coperet, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-
M. Angew. Chem., Int. Ed. 2003, 42, 156−181. (c) Basset, J.-M.; Psaro,
R.; Roberto, D.; Ugo, R. Modern Surface Organometallic Chemistry;
Wiley-VCH: Weinheim, Germany, 2009. (d) Liang, Y.; Anwander, R.
Dalton Trans. 2013, 42, 12521−12545 and references therein..
(5) For comprehensive reviews, see: (a) Nishiyama, H.; Itoh, K.
Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley VCH:
New York, 2000, Chapter 2. (b) Nishiyama, H. Comprehensive
Asymmetric Catalysis I−III; Jacobsen, E. N., Pfaltz, A., Yamamoto, H.,
Eds.; Springer-Verlag: NewYork, 1999; Vol. 1, Chapter 6.
(c) Nishiyama, H.; Itoh, K. Transition Metals for Organic Synthesis,
2nd ed.; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany,
2004; Vol. 2, p 182. (d) Carpentier, J. F.; Bette, V. Curr. Org. Chem.
2002, 6, 913−936. (e) Riant, O.; Mostefaï, N.; Courmarcel, J. Synthesis
1H NMR spectra of 1a-b and a-b, 13C{1H} NMR spectra
of 1a-b, DRIFT spectra of 2−4a, nitrogen physisorption
1
isotherms SBA-15−700 and 2−4a/2−4b, H MAS NMR
spectra of 2−4a/2−4b, 13C CP MAS spectra 2a-4a,
XANES spectra of 2−4b, EXAFS spectra of 2b/4b, and
parameters of 4b (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Erwan.LeRoux@uib.no.
■
́
2004, 2943−2958. (f) Díez-Gonzalez, S.; Nolan, S. P. Org. Prep.
Proced. Int. 2007, 39, 523−560. (g) Marciniec, B. Hydrosilylation: A
Comprehensive Review on Recent Advances; Springer: The Netherlands,
2009. (h) Arena, C. G. Mini-Rev. Org. Chem. 2009, 6, 159−167.
(i) Enthaler, S. ACS Catal. 2013, 3, 150−158.
Notes
The authors declare no competing financial interest.
(6) Selected examples of homogeneous chiral zinc complexes for
AHS: (a) Mimoun, H.; de Saint Laumer, J. Y.; Giannini, L.; Scopelliti,
R.; Floriani, C. J. Am. Chem. Soc. 1999, 121, 6158−6158. (b) Bette, V.;
Mortreux, A.; Savoia, D.; Carpentier, J. F. Adv. Synth. Catal. 2005, 347,
289−302. (c) Gerard, S.; Pressel, Y.; Riant, O. Tetrahedron: Asymmetry
́
2005, 16, 1889−1891. (d) Bandini, M.; Melucci, M.; Piccinelli, F.;
Sinisi, R.; Tommasi, S.; Umani-Ronchi, A. Chem. Commun. 2007,
ACKNOWLEDGMENTS
■
This research was supported by the Nanoscience program of
the University of Bergen and the Norwegian Research Council
(NRC-SYNKNØYT, project no. 227702). We gratefully
acknowledge the assistance of the Dr. Herman Emerich at
the Swiss-Norwegian beamlines at the European Synchrotron
Radiation Facility (project no. 01-01-905, Grenoble, France)
during the XAS acquisition and Dr. Thomas Deschner for the
́
4519−4521. (e) Gajewy, J.; Kwit, M.; Gawronski, J. Adv. Synth. Catal.
2009, 351, 1055−1063. (f) Inagaki, T.; Yamada, Y.; Phong, L. T.;
Furuta, A.; Ito, J.-i.; Nishiyama, H. Synlett 2009, 2009, 253−256.
(g) Ozasa, H.; Kondo, K.; Aoyama, T. Chem. Pharm. Bull. 2010, 58,
989−990. (h) Marinos, N. A.; Enthaler, S.; Driess, M. ChemCatChem
2010, 2, 846−853. (i) Gajewy, J.; Gawronski, J.; Kwit, M. Org. Biomol.
Chem. 2011, 9, 3863−3870. (j) Muthupandi, P.; Sekar, G. Tetrahedron
Lett. 2011, 52, 692−695. (k) Muthupandi, P.; Sekar, G. Tetrahedron:
Asymmetry 2011, 22, 512−517. (l) Park, B.-M.; Feng, X.; Yun, J. Bull.
nitrogen physisorption measurements (Institut fur Anorgani-
̈
sche Chemie, Universitat Tubingen, Germany).
̈
̈
REFERENCES
■
(1) (a) Bolm, C. Angew. Chem., Int. Ed. Engl. 1991, 30, 542−543.
(b) Pfaltz, A. Acc. Chem. Res. 1993, 26, 339−345. (c) Ghosh, A. K.;
Mathivanan, P.; Cappiello, J. Tetrahedron: Asymmetry 1998, 9, 1−45.
Korean Chem. Soc. 2011, 32, 2960−2964. (m) Junge, K.; Moller, K.;
̈
Wendt, B.; Das, S.; Gordes, D.; Thurow, K.; Beller, M. Chem. - Asian J.
̈
́
(d) Gomez, M.; Muller, G.; Rocamora, M. Coord. Chem. Rev. 1999,
2012, 7, 314−320. (n) Liu, S.; Peng, J.; Yang, H.; Bai, Y.; Li, J.; Lai, G.
Tetrahedron 2012, 68, 1371−1375. (o) Zaman, T.; Frauenlob, R.;
McCarthy, R.; Walsh, C. M.; Bergin, E. J. Organomet. Chem. 2012, 716,
159−166. (p) Gajewy, J.; Gawronski, J.; Kwit, M. Eur. J. Org. Chem.
2013, 2013, 307−318. (q) Pang, S.; Peng, J.; Li, J.; Bai, Y.; Xiao, W.;
Lai, G. Chirality 2013, 25, 275−280. (r) Lowicki, D.; Bezlada, A.;
Mlynarski, J. Adv. Synth. Catal. 2014, 356, 591−595.
193, 769−835. (e) Jørgensen, K. A.; Johannsen, M.; Yao, S.; Audrain,
H.; Thorhauge, J. Acc. Chem. Res. 1999, 32, 605−613. (f) Johnson, J.
S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325−335. (g) Fache, F.;
Schutz, E.; Tommasino, M. L.; Lemaire, M. Chem. Rev. 2000, 100,
2159−2232. (h) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed.
2004, 43, 6618−6639. (i) Rechavi, D.; Lemaire, M. Chem. Rev. 2002,
102, 3467−3494. (j) Desimoni, G.; Faita, G.; Quadrelli, P. Chem. Rev.
2003, 103, 3119−3154. (k) Nakano, K.; Kosaka, N.; Hiyama, T.;
Nozaki, K. Dalton Trans. 2003, 4039−4050. (l) McManus, H. A.;
Guiry, P. J. Chem. Rev. 2004, 104, 4151−4202. (m) Desimoni, G.;
Faita, G.; Jørgensen, K. A. Chem. Rev. 2006, 106, 3561−3651.
(n) Nishiyama, H. Chem. Soc. Rev. 2007, 36, 1133−1141.
(o) Desimoni, G.; Faita, G.; Jørgensen, K. A. Chem. Rev. 2011, 111,
PR284−PR437.
(7) Le Roux, E.; Merle, N.; Tornroos, K. W. Dalton Trans. 2011, 40,
̈
1768−1777.
(8) Ternel, J.; Delevoye, L.; Agbossou-Niedercorn, F.; Roisnel, T.;
Gauvin, R. M.; Thomas, C. M. Dalton Trans. 2010, 39, 3802−3804.
(9) (a) Niven, L.; Percy, G. C. Transition Met. Chem. 1978, 3, 267−
271. (b) Singh, R. P. Bull. Soc. Chim. Fr. 1997, 134, 765−768. (c) Diaz-
Acosta, I.; Baker, J.; Cordes, W.; Pulay, P. J. Phys. Chem. A 2001, 105,
238−244.
(2) (a) De Vos, D. E.; Vankelecom, I. F. J.; Jacobs, P. A. Chiral
Catalysts Immobilization and Recycling; Wiley-VCH: Weinheim,
Germany, 2000. (b) Shaikh, N. S.; Deshpande, V. H.; Bedekar, A. V.
Tetrahedron Lett. 2002, 43, 5587−5589. (c) Bolm, C.; Hermans, N.;
(10) Merle, N.; Tornroos, K. W.; Jensen, V. R.; Le Roux, E. J.
̈
Organomet. Chem. 2011, 696, 1691−1697.
(11) Meyer, N.; Roesky, P. W. Organometallics 2009, 28, 306−311.
(12) (a) Brunauer, S.; Deming, L.; Deming, W.; Teller, E. J. Am.
Chem. Soc. 1940, 62, 1723−1732. (b) Sing, K. S. W.; Everett, D. H.;
Claβen, A.; Muniz, K. Bioorg. Med. Chem. Lett. 2002, 12, 1795−1798.
̃
́ ́
(d) Somanathan, R.; Flores-Lopez, L. Z.; Montalvo-Gonzalez, R.;
H
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.;
Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603−619. (c) Rouquerol,
J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. M.;
Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl.
Chem. 1994, 66, 1739−1758. (d) Kruk, M.; Jaroniec, M. Chem. Mater.
2001, 13, 3169−3183.
(13) (a) Yu, J.; Yu, J. C.; Leung, M. K. P.; Ho, W.; Cheng, B.; Zhao,
X.; Zhao, J. J. Catal. 2003, 217, 69−78. (b) Yu, J.; Liu, S.; Yu, H. J.
Catal. 2007, 249, 59−66.
(24) Examples of soluble Zn hydride complexes: (a) Mimoun, H. J.
Org. Chem. 1999, 64, 2582−2589. (b) Chandrasekhar, S.; Reddy, M.
V.; Chandraiah, L. Synth. Commun. 1999, 29, 3981−3987.
(c) Mastranzo, V. M.; Quintero, L.; de Parrodi, C. A.; Juaristi, E.;
Walsh, P. J. Tetrahedron 2004, 60, 1781−1789. (d) Das, S.; Addis, D.;
Zhou, S.; Junge, K.; Beller, M. J. Am. Chem. Soc. 2010, 132, 1770−
1771. (e) Wu, X.-F. Chem. - Asian J. 2012, 7, 2502−2509 and refs 5i,
6a, c, i, and p.
(25) Bette, V.; Mortreux, A.; Lehmann, C. W.; Carpentier, J. F. Chem.
Commun. 2003, 332−333.
(14) For selected examples of metal−alkyl transfer to siloxane
bridges, see: (a) Anwander, R.; Palm, C.; Groeger, O.; Engelhardt, G.
Organometallics 1998, 17, 2027−2036. (b) Tao, T.; Maciel, G. E. J. Am.
Chem. Soc. 2000, 122, 3118−3126. (c) Le Roux, E.; Chabanas, M.;
(26) Examples of active [Zn-OAc] in AHS: (a) Das, S.; Addis, D.;
Junge, K.; Beller, M. Chem. - Eur. J. 2011, 17, 12186−12192. (b) Das,
S.; Moller, K. D.; Junge, K.; Beller, M. Chem. - Eur. J. 2011, 17, 7414−
̈
7417.
Baudouin, A.; De Mallmann, A.; Coper
Thivolle-Cazat, J.; Basset, J.-M.; Lukens, W.; Lesage, A.; Emsley, L.;
́
et, C.; Quadrelli, E. A.;
(27) Le Roux, E.; Liang, Y.; Storz, M. P.; Anwander, R. J. Am. Chem.
Soc. 2010, 132, 16368−16371.
Sunley, G. J. J. Am. Chem. Soc. 2004, 126, 13391−13399. (d) Schroder,
̈
(28) Zhang, H.; Sun, J.; Ma, D.; Weinberg, G.; Su, D. S.; Bao, X. J.
Phys. Chem. B 2006, 110, 25908−25915.
F.; Hermes, S.; Parala, H.; Hikov, T.; Muhler, M.; Fischer, R. A. J.
Mater. Chem. 2006, 16, 3565−3574. (e) Le Roux, E.; Michel, O.;
(29) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537−
541.
Sommerfeldt, H.-M.; Liang, Y.; Maichle-Mossmer, C.; Tornroos, K.
̈
̈
W.; Anwander, R. Dalton Trans. 2010, 39, 8552−8559. (f) Pelletier, J.;
Espinas, J.; Vu, N.; Norsic, S.; Baudouin, A.; Delevoye, L.; Trebosc, J.;
Le Roux, E.; Santini, C.; Basset, J.-M.; Gauvin, R. M.; Taoufik, M.
Chem. Commun. 2011, 47, 2979−2981. (g) Liang, Y.; Erichsen, E. S.;
Song, C.; Anwander, R. Eur. J. Inorg. Chem. 2012, 2012, 5741−5751.
(15) Bearden, J. A.; Burr, A. F. Rev. Mod. Phys. 1967, 39, 125−142.
(16) Xiong, Y.; Yao, S.; Driess, M. Dalton Trans. 2010, 39, 9282−
9287.
(30) (a) Michalovicz, A. Logiciels pour la Chimie; S.F.C.: Paris, 1991;
pp 102−103. (b) Michalowicz, A.; Moscovici, J.; Muller-Bouvet, D.;
Provost, K. J. Phys.: Conf. Ser. 2009, 190, 012034.
(31) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys.
Rev. B: Condens. Matter Mater. Phys. 1998, 58, 7565−7576.
(32) Reports of the Standards and Criteria Committee of the
International XAFS Society 2000. http://ixs.iit.edu/subcommittee_
reports/sc.
(17) Reported Zn-OR bond distances for zinc(II) complexes: (a) Su,
K.; Tilley, T. D.; Sailor, M. J. J. Am. Chem. Soc. 1996, 118, 3459−3468.
(b) Gardiner, M. G.; Lawrence, S. M.; Raston, C. L.; Skeltonc, B. W.;
White, A. H. Chem. Commun. 1996, 2491−2492. (c) Bukhaltsev, E.;
Goldberg, I.; Vigalok, A. Organometallics 2004, 23, 4540−4543.
(d) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organometallics 2005, 24,
5732−5736. (e) Jana, S.; Berger, R. J. F.; Frohlich, R.; Pape, T.; Mitzel,
̈
N. W. Inorg. Chem. 2007, 46, 4293−4297. (f) Petrus, R.; Sobota, P.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2012, 68, m275−
m280.
(18) Recent reported Zn-N bond distances for BOX zinc(II)
complexes: (a) Takacs, J. M.; Hrvatin, P. M.; Atkins, J. M.; Reddy, D.
S.; Clark, J. L. New J. Chem. 2005, 29, 263−265. (b) Atkins, J. M.;
Moteki, S. A.; DiMagno, S. G.; Takacs, J. M. Org. Lett. 2006, 8, 2759−
2762. (c) Kang, Y. S.; Son, J.-H.; Hwang, I.-C.; Ahn, K. H. Polyhedron
2006, 25, 3025−3031.
(19) (a) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33,
227−250. (b) Sutton, C. C. R.; da Silva, G.; Franks, G. V. Chem. - Eur.
J. 2015, 21, 6801−6805.
(20) In situ NMR experiment: addition of 1 equiv of HOAc to
complex 1b (43 mg) in toluene-d8 (0.6 mL) at ambient temperature.
1H NMR (500.13 MHz, toluene-d8, 25 °C): δ 7.2−6.9 (m, 10 H,
ArH); 5.00 (br s, 2 H, HaC5); 4.14 (br s, 2 H, HbC5); 3.74 (t, 2 H, J =
8.1 Hz, HC4); 1.85 (s, 3 H, ZnO(CO)CH3); 1.64 (s, 6 H, C1-(CH3)2);
1.34 (br s, 3 H, Zn-CH2CH3); 0.23 (br s, 2 H, Zn-CH2CH3) and 0.77
(s, ethane) ppm. 13C{1H} NMR (125.75 MHz, toluene-d8, 25 °C): δ
179.2 (ZnO(CO)CH3); 170.9 (C2); 141.0 (ipso-CAr); 125.5−124.5
(para /ortho/meta-CAr); 75.6 (C4); 69.6 (C5); 39.6 (C1); 24.9 (C1-
(CH3)2); 23.7 (ZnO(CO)CH3); 13.4 (Zn-CH2CH3); −1.8 (Zn-
CH2CH3) and 6.8 (ethane) ppm.
(21) Faraldos, J. A.; Giner, J.-L.; Smith, D. H.; Wilson, M.;
Ronhovde, K.; Wilson, E.; Clevette, D.; Holmes, A. E.; Rouhier, K. J.
Chem. Educ. 2011, 88, 334−336.
(22) Thorhauge, J.; Roberson, M.; Hazell, R. G.; Jørgensen, K. A.
Chem. - Eur. J. 2002, 8, 1888−1898.
(23) Reaction of hybrid material 2b with HSi(OEt)3: to a suspension
of 2b (200 mg, 0.11 mmolZn) in acetonitrile (6 mL) was added 25 μL
of neat of HSi(OEt)3 (0.13 mmol, 1.2 equiv). The mixture was stirred
for 1 h at 60 °C. After centrifugation, the solid was washed with
acetonitrile (3 × 2 mL), and then, both the supernatant (and
combined washing solutions) and the solid were evaporated overnight.
I
DOI: 10.1021/acs.organomet.5b00714
Organometallics XXXX, XXX, XXX−XXX