Space Craft-like Octanuclear Co(II)-Silsesquioxane Nanocages: Synthesis, Structure, Magnetic Properties, Solution Behavior, and Catalytic Activity for Hydroboration of Ketones

Article abstract of DOI:10.1021/acs.inorgchem.9b00137

Two novel space craft-like octanuclear Co(II)-silsesquioxane nanocages, {Co₈[(MeSiO₂)₄]₂(dmpz)₈} (SD/Co8a) and {Co₈[(PhSiO₂)₄]₂(dmpz)₈} (SD/Co8b)

Full text of DOI:10.1021/acs.inorgchem.9b00137

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Space Craft-like Octanuclear Co(II)-Silsesquioxane Nanocages:
Synthesis, Structure, Magnetic Properties, Solution Behavior, and
Catalytic Activity for Hydroboration of Ketones
†
#
,‡
§
c,
̌ ́
Wen-Guang Wang,*,†
⊥
Ya-Nan Liu, Hai-Feng Su, Yun-Wu Li,* Qing-Yun Liu, Zvonko Jaglici
Chen-Ho Tung,† and Di Sun*,†,‡
†Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, State
Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, People’s Republic of China
‡
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, and School of Chemistry and
Chemical Engineering, Liaocheng University, Liaocheng 252000, P. R. China
#
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, 361005, People’s Republic of China
⊥
Faculty of Civil and Geodetic Engineering & Institute of Mathematics, Physics and Mechanics University of Ljubljana, Jamova 2,
1
000 Ljubljana, Slovenia
§
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, P. R.
China
*
S Supporting Information
ABSTRACT: Two novel space craft-like octanuclear Co(II)-silsesquioxane nanocages,
{
Co [(MeSiO ) ] (dmpz) } (SD/Co8a) and {Co [(PhSiO ) ] (dmpz) } (SD/Co8b) (SD =
8 2 4 2 8 8 2 4 2 8
SunDi; Hdmpz = 3,5-dimethylpyrazole), have been constructed from two similar multidentate
silsesquioxane ligands assisted with a pyrazole ligand. The Co skeleton consists of eight
8
tetrahedral Co(II) ions arranged in a ring and is further capped by two (MeSiO ) ligands up
2
4
and down. The auxiliary dmpz− ligands seal the ring finally. Electrospray ionization mass
spectrometry revealed SD/Co8a and SD/Co8b are highly stable in CH Cl . Magnetic analysis
2
2
implies that SD/Co8a announces antiferromagnetic interactions between Co(II) ions.
Moreover, both of them display good homogeneous catalytic activity for hydroboration of
ketones in the presence of pinacolborane under mild conditions.
6
−13
INTRODUCTION
rarely studied to date.
Generally, precursor {R Si(OR ) }
1 2 3
■
is actually metastable in alkaline solution and can transform
into various oligomeric [RSiO ] (n = 1, 2, 4, 5, 6, 7, 8, 9, 10,
Thus, it is conceivable that using in situ
hydrolysis of {R Si(OR ) } to generate multidentate cyclic or
acyclic silsesquioxane ligands is a promising strategy to
construct a diverse range of MONCs with interesting
properties. Indeed, as the literature has documented, sporadic
The ongoing research interest in polynuclear metal−organic
nanocages (MONCs) is motivated by their fascinating
1
.5 n
6
−13
1
12) intermediates.
structures and miscellaneous applications, including catalysts,
2
3
1
2 3
sensors, and molecular magnets. Up to now, numerous
MONCs have been springing up and exhibit diverse structures,
which pours new energy into the modern coordination
4
chemistry field. As we know, the rational selection of
7
8
silsesquioxane-based MONCs containing Cu(II), Fe(III),
multidentate ligands of special geometry bearing suitable
coordination sites to “capture” or “ligate” multiple metal ions is
extremely crucial to fabricate MONCs. From small M L
9
10
11
12
Ni(II), Co(II), Cd(II), and bimetal have been isolated.
For example, the {Cu } MONC with charming geometry and
5
2
4
6
12
good catalytic activity has been reported as the highest-
nuclearity silsesquioxane-based Cu(II) MONC until now.
Another hexanuclear Fe(III) MONC incorporating two kinds
octahedra, M L cuboctahedra, M24L rhombicuboctahedra,
1
2
24
48
7a
to the currently largest M L icosidodecahedron reported by
the Fujita group, the special ligand effect in the construction of
MONCs is fully demonstrated.
30 60
5
e−h
of different silsesquioxanes with interesting magnetic proper-
Given this, silsesquioxane
8a
ties was also reported. Normally, cobalt complexes are much
more attractive due to their variable valences (+2 and +3) and
[
RSiO ] with varied oligomeric forms, preorganized multiple
1
.5 n
O coordination sites, high coordination flexibility and reactivity
can be the next-generation macrocyclic ligands used to
construct MONCs with increased stability due to cooperativity
between the binding sites, but such kinds of ligands have been
Received: January 15, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00137
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the flexible coordination geometries (4-coordinated tetrahe-
dron and 6-coordinated octahedron), which endow them
fascinating magnetism and catalytic activity. However, Co(II)-
silsesquioxane cages are relatively rare and develop slowly
among various metal-silsesquioxane MONCs. Currently the
nuclearity Co(II)-silsesquioxane MONCs based on tetrameric
silsesquioxanes.
Crystal Structures of SD/Co8a and SD/Co8b. The SD/
Co8a and SD/Co8b crystallize in the monoclinic P2 /n and
1
orthorhombic Pbcn space groups, respectively (Table S1). The
asymmetric units of SD/Co8a and SD/Co8b consist of a
complete and one-half of the molecules, respectively. They
exhibit similar space craft-like nanocage structures protected by
highest-nuclearity Co(II)-silsesquioxane MONC is a {Co }
6
1
0a
until now.
On the basis of these considerations and our previous work
in transitional metal clusters, we successfully isolated two
14
−
outer silsesquioxanes and auxiliary dmpz ligands (Figure 1a−
highest-nuclearity {Co } cages in the Co(II)-silsesquioxane
8
MONCs family (Scheme 1), namely, {Co [(MeSiO ) ] -
8
2 4 2
a
Scheme 1. Synthetic Routes for SD/Co8a and SD/Co8b
a
The photos are the crystals of SD/Co8a and SD/Co8b taken by a
digital camera under the microscope.
(
dmpz) } (SD/Co8a), and {Co [(PhSiO ) ] (dmpz) } (SD/
8 8 2 4 2 8
Co8b) (Hdmpz = 3,5-dimethylpyrazole). Both of them
present similar space craft-like cage structures containing an
equatorial ring of eight Co atoms which are capped by two
(
RSiO ) silsesquioxanes up and down and then sealed by
2 4
−
eight dmpz ligands near the equator region. The {(RSiO ) }
2
4
ligand exhibits a 4-fold symmetry with four lower-rim O atoms
dangling for reinforcing the internal {Co } core. Their
Figure 1. (a, b) Total structures of SD/Co8a and SD/Co8b; (c, d)
the space craft-like skeleton of octanuclear Co(II)-silsesquioxane
nanocage; (e) the crown-like {Co O } core; (f) the tetrameric
8
structures, solution stabilities, magnetic properties, and the
catalytic activities have been studied in detail.
8
8
[
RSiO ] ligands in SD/Co8a and SD/Co8b. Color code: Co, cyan;
2 4
Si, yellow; O, red; C, gray; N, blue.
RESULTS AND DISCUSSION
■
Syntheses of SD/Co8a and SD/Co8b. The syntheses of
Co(II)-methylsilsesquioxane nanocages need a two-step
fashion. In the first step, in situ hydrolysis of methyltrialkox-
d). Both of them are in an approximate C symmetry. As
shown in Figure 1a,b, both compounds comprise eight Co
atoms, two silsesquioxane ligands, and four dmpz ligands. On
4
−
ysilane {MeSi(OMe) } in the presence of NaOH in alcohol
3
16
can produce a mixture of oligomeric siloxanolate [(MeSi(O)-
the basis of bond-valence sum (BVS) analysis, all Co ions are
in the +2 oxidation state (Table S4 and Table S5). Eight
Co(II) ions are coplanar (Figure 1c) and arranged in an eight-
membered ring connected by eight O atoms to form a
{Co O } crown (Figure 1e). The Co−Co separations are in
ONa) ] intermediates. In the second step, the intermediate
n
siloxanolate [(MeSi(O)ONa) ] was reacted with Co(NO ) ·
n
3 2
−
6
H O and auxiliary dmpz ligand in DMF/MeCN solution
2
under the solvothermal condition. After standing the reaction
solution for 2−3 days at room temperature, blue cubic crystals
of SD/Co8a were formed (yield: 50% based on Co(NO ) ·
8
8
the range of 3.13−3.18 and 3.11−3.18 Å for SD/Co8a and
−
SD/Co8b, respectively. Eight μ -dmpz ligands seal the Co
3
2
2
8
−
6
H O). The octanuclear Co(II)-silsesquioxane structure of
cage near the equator region. Eight dmpz ligands are
2
SD/Co8a was confirmed by single crystal X-ray analysis. By
distributed evenly up and down the plane of Co ring. Eight
8
switching silsesquioxanes from {MeSi(OMe) } to {PhSi-
Co(II) ions exhibit the same four-coordinated tetrahedral
geometry by coordinating two O atoms from two silsesquiox-
3
(
OMe) } a similar structure, SD/Co8b, can also be isolated
3 ,
−
by a similar method (yield: 60% based on Co(NO ) ·6H O).
ane ligands and two N atoms from two dmpz ligands. The
3
2
2
The bulk samples were collected as the crystals (Figure S1).
It is noted that siloxanolates are inclined to transform into
various intermediate [RSiO ] (n = 1, 2, 4, 5, 6, 7, 8, 9, 10,
Co−O, Co−N bond lengths and O−Co−O, N−Co−N, O−
Co−N bond angles (Table S2 and Table S3) are all located in
17
the normal range of Co(II) compounds. Two silsesquioxane
ligands also present the same coordination mode by bearing
1
.5 n
6
−13
1
2)
structures in the first step. However, the tetrameric
silsesquioxane was only observed in di- and tetranuclear Ti(IV)
MONCs previously. In this work, we obtained two highest
four terminal μ -O atoms to bridge eight Co(II) ions to give a
2
15
{Co O } crown (Figure 1e). Two [RSiO ] ligands up and
8
8
2 4
B
DOI: 10.1021/acs.inorgchem.9b00137
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Inorganic Chemistry
Article
down the plane of the Co ring are staggered with the rotation
angle of 45.7 and 45.1° for SD/Co8a and SD/Co8b,
respectively (Figure S2). It is worthy to note that the planar
Magnetic Properties. Magnetic properties were inves-
tigated on SD/Co8a using a Quantum Design MPMS XL-5
magnetometer. Susceptibility has been measured between 2
and 300 K in a constant magnetic field of 1 kOe, and the
magnetization was measured between −50 kOe and 50 kOe at
a constant temperature of 2 K. The data presented in Figure 3
were corrected for the temperature-independent contribution
8
{
Co } core and the cyclic tetrasiloxide ligand are very rare in
8
Co clusters and metallasiloxanes fields, respectively.
Electrospray Ionization Mass Spectrometry (ESI-MS).
Recently, the ESI-MS technique was used more and more in
coordination chemistry to detect the solution behaviors and
19a
of core electrons as obtained from Pascal’s tables.
18
solution assembly mechanism. In order to investigate the
stabilities of SD/Co8a and SD/Co8b in solvents, ESI-MS of
them dissolved in dichloromethane were measured in a
positive-ion mode. As depicted in Figure 2a,b, both SD/
Figure 3. (a) Temperature-dependent susceptibility χ(T) of SD/
Co8a. The red (dashed) and the green (full) line are the best fits with
eqs 1 and 2, respectively. Inset shows the same χ(T) in an expanded
scale around the maximum of the susceptibility. (b) Isothermal
magnetization at 2 K and effective magnetic moment measured in a
magnetic field of 1 kOe (inset).
The susceptibility of SD/Co8a χ = M/H measured in a
magnetic field of 1 kOe (Figure 3a) monotonically increases
from χ = 0.062 emu/mol at 300 K to a local maximum of 0.256
emu/mol at 9.4 K (see inset in Figure 3a). From the room
Figure 2. Positive-ion ESI-MS of SD/Co8a (a) and SD/Co8b (b)
dissolved in dichloromethane. The experimental (black line) and
simulated (red line) isotope distributions of 1a to 1d and 2a to 2b.
temperature susceptibility, an effective magnetic moment μ
eff
9b
1
per cobalt ion can be calculated as μ = 2.828 χT/8,
eff
Co8a and SD/Co8b display four identifiable species with the
charge state of +1 in the ranges of m/z = 1800−1950 and
where the factor 8 in the denominator is due to the 8 cobalt
ions per formula unit. The obtained value μeff(T=300 K) = 4.3 μB
is slightly higher than the theoretically expected value for
Co(II) ions with no orbital contribution (L = 0) which
2
300−2450 for SD/Co8a and SD/Co8b, respectively. The
predominant species (1d and 2b), centered at m/z =
929.9524 and 2351.9749, can be assigned to [SD/Co8a +
19c
1
amounts to 3.87 μB.
However, the measured effective
+
+
dmpz + 2H] (calcd m/z = 1929.9381) and [SD/Co8b + Na]
calcd m/z = 2351.9913), respectively. The parent peaks of
SD/Co8a were observed as 1a and 1b, ascribed to [SD/Co8a
magnetic moment of 4.3 μ is in a range of usually measured
B
(
values for Co(II) ions with a quantum number of total
electronic spin angular momentum S = 3/2 and a small but
nonzero quantum number of total orbital angular momentum
+
+
+
H] (calcd m/z = 1832.8676) and [SD/Co8a + Na] (calcd
19c
m/z = 1855.8685). The details of assigned formulas for 1a−1d
L.
and 2a−2d are listed in Table S6. On the basis of these
The maximum at 9.4 K is a clear indication of an
formulas, we found all labeled peaks have the {Co } skeleton,
antiferromagnetic interaction between the Co(II) ions in a
molecule. The same conclusion can be made observing the
temperature dependence of effective magnetic moment per
8
which means SD/Co8a and SD/Co8b retain their structural
integrity in dichloromethane.
C
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Inorganic Chemistry
Article
Co(II) ion (inset in Figure 3b). It decreases from the already
such a Hamiltonian is highly demanding numerical task, and
enough computer power should be available.
mentioned room temperature value of 4.3 μ down to 0.8 μ at
B
B
2
K.
Below 5 K the susceptibility starts to increase again with
In order to estimate the interaction parameter J and
considering at least phenomenologically also the single ion
effects, χ(T) was analyzed using Rueff’s approach using eq 2:
decreasing temperature. This so-called “Curie tail” can be
attributed to a minute concentration of localized, non-
interacting spin in the sample.
Figure 3b shows an isothermal magnetization M(H)
measured at 2 K. A small value of the magnetization (at 50
E1
E2
A exp −
+ B exp −
(
)
(
)
k T
k T
B
B
χ =
(
2)
T
kOe it is only ∼0.2 μ per Co(II) ion, while in saturation one
B
where A + B is the Curie constant, and E and E represent the
1 2
can expect approximately g·S·μ ≈ 3 μ per Co(II) ion) and
B
B
activation energies corresponding to the single ion effects, and
the antiferromagnetic interaction parameter J. The excellent
agreement between the experimental data for T > 6 K and eq 2
is shown as a full green line in Figure 3a with parameters A + B
linear dependence of M vs H at the beginning are again the
confirmation of the antiferromagnetic ordering of magnetic
moments in a molecule. At the magnetic field H of
approximately 40 kOe, the M(H) curve starts to increase
more rapidly (the increase of a derivative dM/dH can be
observed). At this magnetic field, the interaction energy
between the individual Co(II) magnetic moment and the
external magnetic field becomes comparable with the energy
due to the antiferromagnetic exchange coupling between
Co(II) magnetic moments. The external magnetic field starts
to rotate magnetic moments more efficiently in the direction of
the external magnetic field.
According to the structure of SD/Co8a, a considerable
magnetic interaction can be expected only between eight
intramolecular Co(II) ions. They built a circle, each Co(II)
bridged with two neighbors with the two bridges: Co−O−Co
and Co−N−N-Co. As the distances between all nearest
neighbor Co(II) ions and the bond angles between them are
similar, the same exchange interaction parameter J between
them can be proposed.
=
21.8 emu K/mol (from which follows g = 2.4), E /k = 52 K,
1 B
and E /k = 8.2 K. The obtained g-factor and the single ion
effects are on the order of values found in the literature.
The activation energy E /k , corresponding to the interaction
parameter J = −2E /k = −16 K (−11.1 cm ) is a bit larger as
the one when cobalt magnetic moments were treated as
2
B
19d,e
1
B
−1
1
B
−1
−1
classical vectors (J/k of −11.1 cm versus −7.6 cm ). We
B
can conclude that the maximum in the measured susceptibility
at 9.4 K can be attributed to the intramolecular antiferro-
magnetic interaction between eight neighboring Co(II) ions
making a circle with the interaction parameter on the order of
−1
J/k ≈ −10 cm .
B
Hydroboration of Ketones by SD/Co8a and SD/Co8b.
The catalytic reduction of carbonyl compounds to alcohols is
one of the most important chemical transformations in organic
chemistry, which provides an invaluable tool in fine chemical
20
production and natural product synthesis. Although reduc-
As the first approximation to estimate the interaction
parameter J a circle of eight Co(II) S = 3/2 ions is considered
as an “infinite” chain of classical vectors with interaction
tion of carbonyl compounds has been reported by precious
21
22
23
metals, main group elements, rare earth metals, and alkali
24
metals, catalytic carbonyl hydroboration remains relatively
25
Hamiltonian between two neighbors/int = −2 J ΣS S . In this
1
2
undeveloped by first-row earth-abundant transition-metal,
case, the temperature-dependent susceptibility can be
described with the following eq 1:
and which is currently of great interest. Some advances in
modulating the selectivity or catalytic efficiency of catalysts
have also been made for carbonyl hydroboration, but
synergistic catalysis of hydroboration of unsaturated com-
pounds using polynuclear transition metal clusters as catalysts
has not yet been reported until now. Herein, the modeling
trials were performed using SD/Co8a and SD/Co8b as
homogeneous catalysts to explore the possibility of hydro-
boration of acetophenone with pinacolborane (HBpin).
Initially, we chose acetophenone as the model substrate to
conduct the hydroboration reaction with HBpin in the
presence of octanuclear Co(II)-silsesquioxane nanocages SD/
Co8a and SD/Co8b. Acetophenone can be hydroborated to
afford 2-(phenylethoxy)pinacolborane with 58% yield at a 2
mol % loading of SD/Co8a in n-hexane at 60 °C for 24 h
2
2
Ng μ S(S + 1)
1
+ u
B
χ = 8
3k T
1 − u
(1)
B
Ä
É
Ä
É
Å
Ñ
Å
Ñ
Å
Ñ
Å
Ñ
Å 2JS(S + 1)Ñ
Å
Å
k T
Ñ
Ñ
Å
Ñ
B
u = cothÅ
Ñ − Å
Ñ
Å
Ñ
Å
Ñ
Å
Å
ÇÅÅ
Ñ
Ñ
Å
Ñ
k T
Å 2JS(S + 1)Ñ
B
ÖÑ ÇÅ
ÖÑ
The prefactor 8 is used due to eight cobalt ions in a formula
unit. Other symbols acquire their usual meanings. The spin S
was set to S = 3/2, while the interaction parameter J and g-
factor were left as free parameters during the fitting procedure.
The best correspondence between eq 1 and the measured data
for T > 6 K (below 6 K the prevailing signal is the “Curie tail”
due to uncoupled magnetic ions for which eq 1 is not
(
entry 1, Table 1). Encouraged by this initial result, we then
focused our attention toward the optimization of the reaction
−
1
applicable) was obtained with J/k = −10.9 K (−7.6 cm )
B
conditions (Table 1). Various solvents were tested first (entries
and g = 2.2 (see the red dashed line in Figure 3a). A rather
large difference between the model and the measured data is
mainly because eq 1 does not take into account the single ion
effects, like spin−orbit couplingwe have shown the quantum
number of total orbital angular momentum L is not zeroand
a crystal field effect. For the theoretical description of a system
1
−4, Table 1), the reaction also proceeded very well in polar
solvents such as CDCl and THF, and the latter was found to
be the best solvent, giving almost quantitative yield (entry 4,
Table 1) as inferred from H NMR analysis.
In order to check the influence of catalyst dosage on
catalytic reaction efficiency, the reactions were further carried
out with 1 mol % loading of SD/Co8a, which resulted in a
slightly low yield (entry 5, Table 1). When shortening reaction
time and lowering temperature (entries 6 and 7, Table 1), the
yield of targeted products becomes quite low. Under the same
3
1
of eight interacting cobalt ions the Hamiltonian: / = /
+
int
/
+ /
+ / , where /
takes account of the
SO
CF
Z
int
interaction, / spin−orbit coupling, / crystal field, and
SO
CF
/
is the Zeeman term, should be constructed. The solving of
z
D
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Inorganic Chemistry
Article
a
Table 1. Optimization of the Reaction Conditions
Table 2. Hydroboration of Ketones Catalyzed by SD/Co8a
and SD/Co8b
entry cat. (mol %)
solvent
time (h) temp (°C) yield (%)
b
1
2
3
4
5
6
7
8
9
1
2
2
2
2
1
2
2
2
1
n-hexane
toluene
CDCI3
THF
THF
THF
THF
THF
THF
THF
24
24
24
24
24
12
24
24
24
24
60
60
60
60
60
60
50
60
60
60
58
58
73
99
92
45
19
99
85
4
b
b
b
b
b
b
c
c
d
0
a
Reaction conditions: acetophenone (0.2 mmol), pinacolborane
HBpin) (0.22 mmol), SD/Co8a or SD/Co8b catalysts and 1,3,5-
trimethoxybenzene (0.05 mmol, internal standard) in 0.5 mL of
(
b
c
solvent. SD/Co8a as catalyst. SD/Co8b as catalyst. Yields were
determined by H NMR analysis based on acetophenone. Without
catalyst.
1
d
condition, however, SD/Co8b exhibited excellent catalytic
performance (entry 8, Table 1). Thus, 2 mol % loading of SD/
Co8a or SD/Co8b with a mild reaction temperature of 60 °C
for 24 h in THF was determined as an optimized condition for
the hydroboration of ketones. Control experiments also
indicated that the hydroboration reaction can hardly proceed
in the absence of catalyst under the same condition (entry 10,
Table 1), which is consistent with the results reported in other
2
6
studies.
With the optimized reaction conditions in hand, a variety of
commercially available ketones were subjected to the hydro-
boration reactions to study the substrate scope capabilities, and
the results are presented in Table 2. Both aromatic and alkyl
ketones can be hydroborated by SD/Co8a and SD/Co8b to
boronate esters in moderate to excellent yields under the
optimized conditions (64−99%). The electron-withdrawing
and -donating groups, such as −CF , −Cl, −Br, −NO , and
3
2
−
OCH groups, were tolerated on ketones substrates. In
3
particular, the halogen group was keep intact in this protocol,
which offered the possibility of further functionalization of the
hydroborated products. Note that the ketones substrates with
unsaturated functional groups such as nitro (entries 3 and 6,
Table 2) selectively hydroborated CO bond. When SD/
Co8a was used as catalyst for ortho-substituted substrates
a
Reaction conditions: ketones (0.2 mmol), pinacolborane (HBpin)
(
0.22 mmol), catalysts SD/Co8a or SD/Co8b (2 mol %) in 0.5 mL
b c
of solvent. SD/Co8a as catalyst. SD/Co8b as catalyst. Yields were
determined by H NMR analysis based on ketones.
(
8
entries 2 and 3, Table 2), the yields are diminished (64% and
5%) compared to para-substituted substrates (entries 5 and 6,
1
Table 2), which clearly demonstrated the steric hindrance
effect of substrates. Moreover, a series of compared catalytic
experiments also indicated that the catalytic activities of SD/
Co8b are superior to those of SD/Co8a under same condition,
as seen in entries 2, 3, and 8.
As mentioned before, octanuclear Co(II)-silsesquioxane
nanocages SD/Co8a and SD/Co8b are paramagnetic in
both solution and in the solid state, which precludes us to
monitor the catalytic reaction process by NMR spectroscopy.
To gain insights into the mechanism of this hydroboration
reaction, ESI-MS was used to check the catalytic active species
or intermediates by extracting the mother liquor after reaction
proceeded for 1 h. We set three parallel experiments to
examine the interactions between catalyst and reactants. When
examining the ESI-MS of reaction solution of SD/Co8b and
acetophenone in THF, we did not observed any peaks
corresponding to the catalyst-ketone adducts (Figure S7a).
However, the ESI-MS of reaction solution of SD/Co8b and
HBpin clearly showed the peak of catalyst-borane adducts. By
matching the experimental and simulated isotopic distribu-
tions, we can assign the dominant peak at m/z = 2479.0852 to
+
[(SD/Co8b + HBpin) + Na)] (calcd m/z = 2479.0973,
Figure S7b), which indicates the catalyst SD/Co8b can
combine with one reactant to form active intermediates. We
further examined the reaction solution of SD/Co8b, HBpin,
and acetophenone by ESI-MS, but only SD/Co8b + HBpin
adducts can be detected as well. The ESI-MS results
E
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Article
alternatively justified the stability and robustness of the cage
cores during the catalytic reaction process. On the basis of the
above observations, two possible hydroboration mechanisms
are proposed with different bond cleavage fashions around the
Co center, that is, Co−N or Co−O breaking (Figures 4).
Accession Codes
CCDC 1882225−1882226 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
E-mail: dsun@sdu.edu.cn (D.S.).
E-mail: wwg@sdu.edu.cn (W.G.W.).
E-mail: liyunwu@lcu.edu.cn (Y.W.L.).
■
*
*
*
ORCID
Yun-Wu Li: 0000-0001-7673-7787
Wen-Guang Wang: 0000-0002-4108-7865
Chen-Ho Tung: 0000-0001-9999-9755
Di Sun: 0000-0001-5966-1207
Notes
The authors declare no competing financial interest.
Figure 4. Proposed mechanisms for hydroboration by SD/Co8a and
SD/Co8b.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21822107,
■
2
1571115, 21671172, 21771095), the Slovenian Research
Initially, catalyst-borane adducts formed by a Lewis acid cobalt
center and an adjacent Lewis-basic N or O atoms in catalyst to
Agency (Grant No. P2-0348), bilateral project PR China-
Slovenian (Grant No. BI-CN/17-18-004), the Natural Science
Foundation of Shandong Province (Nos. JQ201803,
ZR2017JL013, and ZR2014BM027), the Qilu Youth Scholar
Funding of Shandong University and the Fundamental
Research Funds of Shandong University (104.205.2.5).
2
7
ligate HBpin. Subsequent catalyst-borane adducts were
dissociated accompanying the transfer of borenium to the
oxygen atom of substrate, which would then be susceptible to
nucleophilic attack of the hydride released from cobalt hydride
and regenerates the catalyst for further reaction. Considering
the stronger Co−O bond strength, the hydroboration route
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