Silica-Supported MnII Sites as Efficient Catalysts for Carbonyl Hydroboration, Hydrosilylation, and Transesterification

Article abstract of DOI:10.1002/chem.201903638

Manganese, the third most abundant transition-metal element after iron and titanium, has recently been demonstrated to be an effective homogeneous catalyst in numerous reactions. Herein, the preparation of silica-supported Mn^II sites is reported using Surface Organometallic Chemistry (SOMC), combined with tailored thermolytic molecular precursors approach based on Mn₂[OSi(OtBu)₃]₄ and Mn{N(SiMe₃)₂}₂?THF. These supported Mn^II sites, free of organic ligands, efficiently catalyze numerous reactions: hydroboration and hydrosilylation of ketones and aldehydes as well as the transesterification of industrially relevant substrates.

Full text of DOI:10.1002/chem.201903638

DOI: 10.1002/chem.201903638
Communication
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Heterogeneous Catalysis |Hot Paper|
Silica-Supported MnII Sites as Efficient Catalysts for Carbonyl
Hydroboration, Hydrosilylation, and Transesterification
Behnaz Ghaffari, Jorge Mendes-Burak, Ka Wing Chan, and Christophe Copꢀret*[a]
lated metal sites at the surface of materials through grafting of
Abstract: Manganese, the third most abundant transition-
metal siloxide precursors (M(OSi(OtBu)3)x) followed by a ther-
metal element after iron and titanium, has recently been
mal treatment that removes all remaining organic ligands.[15]
demonstrated to be an effective homogeneous catalyst in
The resulting surface sites are low-coordinated metal centers
numerous reactions. Herein, the preparation of silica-sup-
with unusual coordination geometry that readily activate CÀH
bonds and a broad range of small molecules participating in
numerous catalytic reactions[16,17] (ethylene polymerization,[16a,b]
olefin metathesis,[16c,d] methane-to-methanol conversion,[16e]
alkane dehydrogenation,[16f,g] hydroamination[16h] to name a
few). We thus reasoned that this SOMC/TMP approach could
be an ideal strategy to generate highly reactive silica-support-
ed MnII sites from bis-tris(tert-butoxy)siloxide (1) and bis-tri-
methylsilyl amide (2) MnII as molecular precursors
(Scheme 1b).
ported MnII sites is reported using Surface Organometallic
Chemistry (SOMC), combined with tailored thermolytic
molecular precursors approach based on Mn2[OSi(OtBu)3]4
and Mn{N(SiMe3)2}2·THF. These supported MnII sites, free of
organic ligands, efficiently catalyze numerous reactions:
hydroboration and hydrosilylation of ketones and alde-
hydes as well as the transesterification of industrially rele-
vant substrates.
Here, we describe their development and demonstrate that
these MnII sites, free of organic ligand, display high catalytic ac-
tivity towards the hydrosilylation and hydroboration of carbon-
yls and transesterification reactions.
Catalysis is one of the pillars of green chemistry and sustain-
able development. One important aspect in developing sus-
tainable processes is the use of nontoxic and earth-abundant
metals. It is thus not surprising that a growing research effort
has been directed at developing catalytic processes with non-
noble metals.[1] Although manganese could fill this role due to
its low toxicity and relatively high abundance (950 mgkgÀ1 in
the Earth’s crust),[2] this element has been rarely used in cataly-
sis[3] because of its often complex coordination chemistry. The
most prominent example of its use in catalysis is for the asym-
metric epoxidation of alkenes.[4–6] Only recently, manganese
has been used in reactions such as hydrogenation,[1,7,8] hydrosi-
lylation,[8,9] and hydroboration[10] (Scheme 1a). Among the first-
row transition metals like Fe,[11a,b] Co,[11c,d] Ni,[11e,f] Mn[8,9]-based
catalysts show the highest activities (turnover frequencies) in
the hydrosilylation of carbonyl moieties. In addition, the Mn-
catalyzed hydroboration and hydrosilylation of inactivated/
nonpolar bonds has recently been demonstrated.[8–10] In all
cases, homogeneous Mn catalysts require the use of specific
and complex ligand scaffolds, even if, for most of these catalyt-
ic reactions, the metal site mostly acts as a Lewis acid to acti-
vate incoming reactants.
In the first step, Mn2[OSi(OtBu)3]4 (1)[18] was prepared as a
white solid in 80% yield from Mn{N(SiMe3)2}2·THF (2).[19] Crystals
of 1 were grown from a saturated solution in toluene at
À408C. The unit cell contains two independent molecules with
very similar bond lengths and angles. The structure of [{Mn-
(OSi(OtBu)3)2}2] is similar to its CrII counterpart[16a] (see Figure S1
in the Supporting Information). Next, SiO2–700 (0.36 mmol ÀOH
per gram) was exposed to a benzene solution of 1 for 3 h
(Scheme 2a). IR Spectroscopy following washing of the materi-
al with benzene and drying reveals the disappearance of isolat-
ed OH sites, whereas new n(CÀH) and d(CÀH) bands corre-
sponding to the organic ligands appear at 2850–2910 and
1367–1606 cmÀ1, respectively (Figure S2). Elemental analysis
(EA) of this material 1@SiO2 indicates a manganese loading of
1.50%wt (0.27 mmol Mn per g), which correspond to a similar
Mn density as the starting density of OH groups in SiO2–700
.
Subsequent thermal treatment under high vacuum (10À5 mbar)
at 4008C for 3 h yields a material 1@SiO2–400 with a similar
amount of Mn (1.65%wt and 0.3 mmol Mn per g). The thermal-
ly treated material is free of organic bands as evidenced by IR,
whereas isolated silanols are regenerated as shown by the
presence of an intense band at 3745 cmÀ1 (Figure S2). Expo-
sure of this material to CO reveals a blue-shifted band of weak
intensity at 2195 cmÀ1 in the IR spectrum, consistent with the
coordination of CO to Lewis acid sites (Figure S4). To further
confirm the Lewis acidic properties of 1@SiO2–400, this material
was exposed to pyridine atmosphere.[20,21,16h] IR bands at 1489
and 1577 cmÀ1 correspond to pyridine coordinated to Lewis
acid sites and features at 1448 and 1606 cmÀ1 demonstrate the
In parallel, Surface Organometallic Chemistry (SOMC)[12,13]
combined with Thermolytic Molecular Precursor (TMP)[14] ap-
proach has emerged as a powerful approach to generate iso-
[a] B. Ghaffari, J. Mendes-Burak, K. W. Chan, Prof. Dr. C. Copꢀret
Department of Chemistry and Applied Biosciences, ETH Zꢁrich
Vladimir-Prelog-Weg 1–5, 8093, Zꢁrich (Switzerland)
E-mail: ccoperet@inorg.chem.ethz.ch
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201903638.
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Scheme 1. a) State-of-the-art and representative examples of homogeneous MnII catalysts used in the hydrosilylation and hydroboration of carbonyls. b) Strat-
egy to generate surface MnII sites free of organic ligand using Surface Organometallic Chemistry (SOMC) combined with a thermolytic molecular precursor
(TMP) approach and associated TMPs 1 and 2.
Scheme 2. Preparation of 1@SiO2–400 and 2@SiO2–400 and proposed structures for the MnII sites free of organic ligands generated from grafting the TMPs a) 1
or b) 2 followed by thermal treatment under vacuum. Note that the presence of dimeric MnII sites cannot be excluded for 1@SiO2–400
.
presence of hydrogen-bonded pyridine.[20,21] Upon treatment
of this pre-exposed material at higher temperatures under
vacuum, the coordinated pyridine was removed, indicating the
weak Lewis acidic sites (Figure S5).
1@SiO2–400 shows substantial differences; one considerably nar-
rower signal with the g=2.01 is observed (Figure S9) which
may suggest that 1@SiO2–400 does not maintain the dimeric
structure upon thermal treatment. The presence of MnII surface
species is further supported by X-ray absorption spectroscopy
(XAS) measured at the Mn K-edge. In addition, although the X-
ray absorption near-edge structure (XANES) spectra of 1 and
1@SiO2 show very similar edge energies and pre-edge features
indicating the oxidation states and geometries of the Mn cen-
ters remain similar upon grafting, the edge position of 1@SiO2–
To further investigate the nature of 1@SiO2–400, EPR measure-
ments were performed. At room temperature, 1 display a wide
spectrum in the 50–650 mT range with g=2.16 for the princi-
ple line (Figure S7, Supporting Information), which is consistent
with previously reported compounds.[22] Upon grafting, the
spectrum shows differences in structure of the bands, and the
g factor of 1@SiO2 is 2.02 (Figure S8). The EPR spectrum of
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is shifted slightly towards higher energy suggesting some
400
changes around the MnII centers (see below).
The formation of MnII sites was also investigated starting
from 2. Contacting 2 with SiO2–700 (0.36 mmol ÀOH per g) for
3 h in benzene yields a solid 2@SiO2 (Scheme 2b). Elemental
analysis of 2@SiO2 indicates a manganese loading of 1.77%wt
(0.32 mmol Mn per g). Upon grafting of 2, IR spectroscopy re-
veals complete consumption of the OH moieties. Moreover,
new bands between 2898 and 2989 cmÀ1 associated with or-
ganic ligands appear. Note that the competing passivation of
the surface (formation of OSiMe3) resulting from the reaction
of (Me3Si)2NH evolved upon protonolysis of the ligand with sur-
face silanol is also evidenced (Figure S3, Supporting Informa-
tion). Subjecting 2@SiO2 to a thermal treatment under high
vacuum (10À5 mbar) at 4008C for 3 hours results in the forma-
tion of 2@SiO2–400. Elemental analysis reveals a loading of 1.8%
(0.33 mmol Mn per g). IR Spectroscopy shows the presence of
remaining Me3SiO group (Figure S10), as previously observed
when trimethylsilyl amide derivatives are used as precur-
sor.[23,24] Although EPR measurements of 2 show a fairly com-
plex pattern (Figures S10 and S11), with an unusual g factor of
2.80 for 2, that is likely due to spin-orbit coupling and symme-
try of molecule,[25] a broad line at g=2.09 is observed for
2@SiO2. The EPR signal of the manganese site free of organic
ligand 2@SiO2–400 (Figure S12) is similar to that of 1@SiO2–400
with a single isotropic resonance and identical g=2.01 (Fig-
ure S13), indicating a similar environment after thermal treat-
ment and suggesting that monomeric MnII sites could be pres-
ent in both materials. The XANES spectra of 2, 2@SiO2, and
2@SiO2–400, share almost identical edge energies and pre-edge
features indicating that similar oxidation states and geometries
are retained throughout (Figure 1b).
Figure 1. XANES spectra of a) 1, 1@SiO2, and 1@SiO2–400 and b) 2, 2@SiO2,
and 2@SiO2–400
.
Table 1. Catalytic evaluation of Mn-catalyzed reduction of benzalde-
hyde.[a]
With 1@SiO2–400 and 2@SiO2–400 in hand, we chose to investi-
gate their catalytic activity in the reduction of benzaldehyde
(A) at room temperature using pinacolborane (HBpin), phenyl-
silane (PhSiH3), triethoxysilane ((EtO)3SiH), and H2 (Table 1). In
the case of hydroboration, full conversion was achieved within
5 minutes at 0.5 mol% catalyst loading, whereas no reaction is
observed in the absence of catalyst or in the presence of the
support itself (silica).
Entry
Catalyst
Reductant
(HE)
Time [min]
Yield
[%][b]
1
2
3
4
5
6
7
8[c]
None
MnO
HBpin
HBpin
HBpin
HBpin
HBpin
PhSiH3
SiH(EtO)3
H2
60
60
5
5
5
10
60
960
0
0
98
>99
>99
>99
97
Zn@SiO2
1@SiO2–400
2@SiO2–400
2@SiO2-400
2@SiO2–400
2@SiO2–400
We also compared the reactivity of 1@SiO2–400 with the pre-
viously reported ZnII analogue.[16h] Both catalysts show similar
reactivity, which is consistent with the ZnII and MnII Lewis
acidic properties. Using 2@SiO2–400, full conversion in the hy-
droboration of the carbonyl moiety was achieved in less than
5 minutes. Given that both 1@SiO2–400 and 2@SiO2–400 demon-
strated similar conversion, we chose to focus on 2@SiO2–400 as
it is more synthetically accessible. The reduction can also be
0
[a] Conditions: benzaldehyde (0.15 mmol), reductant (0.25 mmol), catalyst
(0.5 mol%) and C6D6 (0.25 mL), rt, under inert atmosphere of N2. [b] De-
termined by 1H NMR spectroscopy using tridecane as internal standard
(ISTD). [c] 25 bar of H2, catalyst (2 mol%), and 808C.
achieved with PhSiH3 in less than 10 minutes with 2@SiO2–400
,
whereas triethoxysilane leads to considerably lower reaction
rates, and no reaction is observed when employing H2 as a re-
ductant. Given that pinacolborane shows greater reactivity, we
explored the combination of 2@SiO2–400 and HBpin for the re-
duction of various industrially important and representative al-
dehydes and ketones (Table 2).[26] All reactions were carried out
in a 0.25 mmol scale using 0.5 mol% of 2@SiO2–400 and a 6 h
standard reaction time.[27] The products were analyzed by GC-
1
MS and H NMR spectroscopy.
With cinnamaldehyde, the carbonyl moiety was selectively
reduced (>99%) whereas the olefin moiety remained intact,
providing the cinnamyl alcohol (B) in high yield (92%). The
heteroaromatic furfuryl alcohol, (C), an important intermediate
used in the production of plasticizers and lubricants,[28,29] is
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Once again, 2@SiO2–400 shows greater reactivity than
1@SiO2–400, which may be attributed in part to the passivat-
ed surface being less hydrophilic and therefore better for
desorption of polar reactants/products. No reaction was
observed in the absence of catalyst or with silica alone.
In summary, we have described the synthesis of silica-
supported MnII sites using the SOMC/TMP approach based
on easily accessible molecular compounds bearing inex-
pensive ligands. Due to inherent Lewis acidity of MnII sites,
these heterogeneous supported manganese catalysts are
active towards transesterification under moderate condi-
tions and chemoselective towards hydroboration of ke-
tones and aldehydes under mild reaction conditions. This
report indicates isolated Lewis acid sites based on non-
noble metals like MnII, free of complex stabilizing organic li-
gands, are highly efficient as catalysts for numerous trans-
Table 2. Mn-catalyzed hydroboration of ketones and aldehydes.[a,b]
[a] Conditions: substrate (0.25 mmol), HBpin (0.25 mmol), catalyst (0.5 mol%)
1
and C6D6 (0.25 mL), 6 h, rt, under N2. [b] Yields determined by H NMR spectros-
copy and GC-MS using tridecane as an internal standard.
also obtained in 92% yield from readily available
Table 3. Mn-catalyzed transesterification.[a,b]
furfural. Similarly, simple heptyl alcohols (D and
H) are easily formed from their corresponding
starting materials. Additionally, high yields to-
wards the reduction of carbonyls are observed
using the corresponding ketone to form diphe-
nylmethanol (E), and adding steric bulk does not
affect the hydroboration, as evidenced by the
clean and facile reduction of 2,2-dimethyl-1-phe-
nylpropan-1-one (F). Aliphatic hex-5-en-2-one
also yields the corresponding alcohol in excellent
yield without the reduction of olefin moiety (G).
However, studies towards the hydroboration of
olefins (styrene/1-nonene) have remained unsuc-
cessful.
[a] Conditions: substrate (0.05 mmol), 1-butanol (0.2 mL), 24 h, and 808C, under inert at-
mosphere of N2. [b] Yields determined by GC-MS using tridecane used as an internal stan-
dard.
Lastly, the recyclability of the catalyst was also
evaluated: no loss in activity was observed upon
recycling the catalysts three times, whereas additional cycles
indicated small loss of activity (5% and 10% activity loss was
observed at 4th and 5th cycle, respectively, see the Supporting
Information).
formations. We are currently exploring the use of these materi-
als in a broad range of reactions.
Acknowledgements
The reactivity of 2@SiO2–400 was also compared to a repre-
sentative homogeneous catalyst developed by Trovitch and
co-workers,[9b] using equimolar ratio of benzophenone and
PhSiH3 in presence of 1 mol% [Mn]. Under identical reaction
conditions, 2@SiO2–400 shows full conversion in 10 versus
20 min for the homogeneous catalyst, showing that a support-
ed MnII site having no additional stabilizing organic ligand dis-
plays similar and possibly greater activity as a molecular cata-
lyst.
The authors acknowledge Dr. Keith Searles, Dr. Michael Wçrle,
Dr. Florian Allouche, and Dr. Deven Estes for assistance with X-
ray crystallography as well as Dr. Reinhard Kissner for helpful
discussions. We acknowledge the Paul Scherrer Institute, Villi-
gen, Switzerland for provision of synchrotron radiation beam
time at beamline Super XAS of the SLS and would like to
thank Dr. Olga Safonova for assistance.
We also evaluated the reactivity of 1@SiO2–400 and 2@SiO2–
towards the Lewis acid-catalyzed transesterification reac-
Conflict of interest
400
tion, a very important industrial process used in various appli-
cations in the polymer, coating, fragrance, and perfume indus-
try, as well as in the production of fatty esters derived from
biomass.[30] As representative of industrially valuable esters, re-
actions of methyl methacrylate (I), ethyl cinnamate (J), glycerol
tributyrate (K), and methyl benzoate (L) with 1-butanol were
examined. All four substrates (I–L) undergo the transformation
in high yields using 2% catalyst loading at 808C (Table 3).
The authors declare no conflict of interest.
Keywords: heterogeneous catalysis
·
hydroboration
·
hydrosilylation · manganese · transesterification
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Manuscript received: August 8, 2019
Accepted manuscript online: August 29, 2019
Version of record online: && &&, 0000
Chem. Eur. J. 2019, 25, 1 – 6
www.chemeurj.org
5
ꢁ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Heterogeneous Catalysis
B. Ghaffari, J. Mendes-Burak, K. W. Chan,
C. Copꢀret*
&& – &&
Silica-Supported MnII Sites as Efficient
Catalysts for Carbonyl Hydroboration,
Hydrosilylation, and
Manganese is free and active: Silica-
supported MnII sites were prepared
starting from Mn2[OSi(OtBu)3]4 and
Mn{N(SiMe3)2}2·THF using Surface
gands, efficiently catalyze the hydrobo-
ration and hydrosilylation of ketones
and aldehydes as well as the transesteri-
fication of industrially relevant sub-
strates.
Transesterification
Organometallic Chemistry (SOMC). The
generated MnII sites, free of organic li-
&
&
Chem. Eur. J. 2019, 25, 1 – 6
www.chemeurj.org
6
ꢁ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!