Supported Single-Site Ti(IV) on a Metal-Organic Framework for the Hydroboration of Carbonyl Compounds

Article abstract of DOI:10.1021/acs.organomet.7b00544

A stable and structurally well-defined titanium alkoxide catalyst supported on a metal-organic-framework (MOF) of UiO-67 topology (ANL1-Ti(OⁱPr)₂) was synthesized and fully characterized by a variety of analytical and spectroscopic techniques, including BET, TGA, PXRD, XAS, DRIFT, SEM, and DFT computations. The Ti-functionalized MOF was demonstrated active for the catalytic hydroboration of a wide range of aldehydes and ketones with HBpin as the boron source. Compared to traditional homogeneous and supported hydroboration catalysts, ANL1-Ti(OⁱPr)₂ is completely recyclable and reusable, making it a promising hydroboration catalyst alternative for green and sustainable chemical synthesis. In addition, ANL1-Ti(OⁱPr)₂ catalyst exhibits remarkable hydroboration selectivity toward aldehydes vs ketone in competitive study. DFT calculations suggest that the catalytic hydroboration proceeds via a (1) hydride transfer between the active Ti-hydride species and a carbonyl moiety (rate-determining step) and (2) alkoxide transfer (intramolecular σ-bond metathesis) to generate the borate ester product.

Full text of DOI:10.1021/acs.organomet.7b00544

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX-XXX
Supported Single-Site Ti(IV) on a Metal−Organic Framework for the
Hydroboration of Carbonyl Compounds
Zhiyuan Huang,†,‡ Dong Liu,†,‡ Jeffrey Camacho-Bunquin,‡ Guanghui Zhang,§ Dali Yang,†,‡
Juan M. Lopez-Encarnacion,‡,∥ Yunjie Xu,§ Magali S. Ferrandon,‡ Jens Niklas,‡ Oleg G. Poluektov,‡
́
́
Julius Jellinek,*,‡ Aiwen Lei,*,†,‡ Emilio E. Bunel,*,‡ and Massimiliano Delferro*,‡
†College of Chemistry & Molecular Sciences, Institute of Advanced Studies, Wuhan University, Wuhan 430072, PR China
‡Chemical Sciences & Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
§Department of Biological and Chemical Sciences, Illinois Institute of Technology, Chicago, Illinois 60616, United States
∥Department of Mathematics-Physics, University of Puerto Rico at Cayey, Cayey, Puerto Rico 00736, United States
S
* Supporting Information
ABSTRACT: A stable and structurally well-defined titanium alkoxide
catalyst supported on a metal−organic-framework (MOF) of UiO-67
topology (ANL1-Ti(OiPr)2) was synthesized and fully characterized
by a variety of analytical and spectroscopic techniques, including BET,
TGA, PXRD, XAS, DRIFT, SEM, and DFT computations. The Ti-
functionalized MOF was demonstrated active for the catalytic
hydroboration of a wide range of aldehydes and ketones with
HBpin as the boron source. Compared to traditional homogeneous
and supported hydroboration catalysts, ANL1-Ti(OiPr)2 is com-
pletely recyclable and reusable, making it a promising hydroboration
catalyst alternative for green and sustainable chemical synthesis. In
addition, ANL1-Ti(OiPr)2 catalyst exhibits remarkable hydroboration
selectivity toward aldehydes vs ketone in competitive study. DFT
calculations suggest that the catalytic hydroboration proceeds via a
(1) hydride transfer between the active Ti-hydride species and a carbonyl moiety (rate-determining step) and (2) alkoxide
transfer (intramolecular σ-bond metathesis) to generate the borate ester product.
a σ-bond metathesis mechanism.5c,14 The reaction is commonly
first-order in HBpin and the catalyst and zero-order in the
carbonyl substrate, suggesting that the rate-determining step is σ-
bond metathesis between HBpin and the alkoxide intermediate.
INTRODUCTION
■
The reduction of aldehydes and ketones to functionalized
alcohols is an important tool for the synthesis of natural products
and fine chemicals.1 Because of its importance, various
In addition, other mechanisms have been proposed, including a
stoichiometric and catalytic carbonyl reduction strategies have
Ti(II) η2-carbonyl/Ti(IV) metallacycle scenario15 and a Mg(II)-
been established.2 Most approaches employ stoichiometric
amounts of reducing agents, e.g., LiAlH4 and NaBH4, which, in
general, effect poor selectivity (over-reduction) and functional
catalyzed hydroboration of esters via a zwitterionic alkoxyborate
reaction pathway.5a
Despite the growing scope of homogeneous hydroboration
group tolerance, modest reaction rates, and often require harsh
conditions.3 Several molecular and supported catalysts for
catalysts, these systems are generally plagued with deactivation
processes arising from irreversible ligand redistribution path-
carbonyl hydroboration have been shown effective and selective
for the synthesis of functionalized alcohols.4 Less expensive and
ways. Such catalyst deactivation mechanisms can be prevented
abundant s-, p-, and f-block precatalysts, e.g., Mg,5 Ga,6 Sn,7 and
via kinetic stabilization of the active site using sterically
La8 (Chart 1), have been used for hydroboration of aldehydes
encumbered ligands. However, this approach results in lower
and ketones.9 These systems are proposed to function through
activity since the increased steric constraints renders the active
site inaccessible for substrate binding and activation.
an active metal-hydride that forms in situ from the reaction
Alternatively, stabilization of highly reactive intermediates has
between an H-BR2 species, i.e., HBpin and HBcat, and the
been achieved via surface organometallic chemistry (SOMC).16
precatalyst. Other examples of molecular hydroboration catalysts
include base and precious transition metals such as Ti(OiPr)4,10
Supported organometallic catalysts have garnered significant
Shvo’s catalyst (Ru),11 (IPr)CuOtBu (IPr = N,N′-bis(2,6-
attention in the past 25 years as a result of the ever increasing
diisopropylphenyl)imidazol-2-ylidene),12 and Zn-
13
(iminooxazoline)(OTf)2 (Chart 1). For the systems men-
Received: July 19, 2017
tioned, carbonyl (and pyridine) hydroboration proceeds through
© XXXX American Chemical Society
A
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via direct solvothermal reaction of bpdcOAc (2,2′-bis(acetoxy)-
1,1′-biphenyl-4,4′-dicarboxylic acid) and H2bpdc (1,1′-biphen-
yl-4,4′-dicarboxylic acid) ligands (molar ratio of 1.5:1) with
ZrCl4 in N,N′-dimethylformamide (DMF) and acetic acid
(HOAc, as a modulator) at 120 °C for 24 h. Notably,
solvothermal synthesis from bpdcOAc ligand under the same
conditions failed. The resulting white microcrystalline powder
was washed with DMF and methanol, then dried at 150 °C under
vacuum for 2 h to give ANL1 in ∼83% yield (based on H2bpdc).
1H NMR spectroscopy upon digestion of ANL1 with D2SO4/d6-
DMSO solution confirmed the complete deprotection of the
acetylated bpdcOAc ligand, with a molar ratio between the two
ligands determined to be ∼1:1 (Figure S3). Powder X-ray
diffraction (PXRD) analysis reveals the classic isoreticular nature
of ANL-1 and high crystallinity typical of the UiO-67 topology
(Figure 1). Meanwhile, scanning electron microscopy (SEM)
Chart 1. Examples of Homogenous and Supported
Hydroboration Catalysts
demand for enhanced catalyst efficiencies and amenability to
spectroscopic characterization.17 The latter has drastically
contributed to understanding of structure−property relation-
ships, which is a key tool enabling the rational design of more
efficient catalysts. In this context, Pruski, Sadow, and co-
workers18 have successfully developed well-defined, isolated
amidozirconium(IV) sites on mesoporous silica (Chart 1) for the
catalytic hydroboration of a range of carbonyl compounds.
Recently, Lin and co-workers19 have shown that single-site
magnesium alkyl sites supported on a node of a metal−organic
framework (TPHN-MOF of UiO-69 topology; TPHN = 4,4-
bis(carboxyphenyl)-2-nitro-1,1′biphenyl, Chart 1) are catalyti-
cally active for hydroboration of ketones, aldehydes, and imines.
Our catalyst (vide infra) shows some notable advantages to the
prior works referenced above, including higher activity than the
homogeneous counterpart and the use of commercially available,
nonpyrophoric starting material.
Figure 1. PXRD of UiO-67-bpdc, ANL1, ANL1-Ti(OiPr)2, and ANL1-
Ti(OiPr)2 post-catalysis.
MOFs are 3D, atomically periodic, chemically and thermally
robust porous materials composed of inorganic nodes and
organic linkers.20 In contrast to the irregular surfaces of bulk
inorganic oxides, i.e., SiO2 or Al2O3, MOFs have intrinsically
uniform surface structures, making them an ideal support
platform for supported-but-homogeneous-in-function cata-
lysts.21 Two strategies are commonly employed for incorporating
catalytic active site into MOFs: (1) covalent attachment via
functionalized organic linkers22 or (2) grafting to inorganic node
groups.23 Given the periodicity of MOFs, we consider them a
promising underexplored platform for supporting molecular
catalysts. Here, we report the synthesis and detailed character-
ization of a stable and well-defined titanium alkoxide catalyst
supported on a MOF of UiO-6724 topology (ANL1-Ti(OiPr)2).
It is seen that ANL1-Ti(OiPr)2 is an efficient and recyclable
catalyst for hydroboration of a diverse scope of carbonyl
compounds using HBpin as reductant. Additionally, computa-
tional and mechanistic studies suggest that the catalytic
hydroboration proceeds via (1) hydride transfer between the
active Ti-hydride species and a carbonyl moiety (rate-
determining step) and (2) alkoxide transfer (intramolecular σ-
bond metathesis) to generate the borate ester product.
analysis shows octahedral-shaped, phase-pure products with
homogeneous particle morphology and crystal sizes of ∼1 μm
(Figure S9). Gas adsorption analysis of ANL1 gives a BET
surface area of 1876 m2 g−1 (N2 gas at 77 K), lower than the
typical UiO-67-bpdc (2411 m2 g−1); this is attributable to the
presence of OH groups (Table S1). In addition, N2 adsorption
isotherm confirms that mesoporosity is maintained in ANL1,
with an average pore-size distribution of 10.9 Å (Figures S1 and
S2). Also, thermogravimetric analysis (TGA) shows that ANL1
is stable up to 400 °C (Figure S4). The diffuse reflectance FT-IR
(DRIFTS) spectrum of ANL1 exhibits characteristic stretching
frequencies of the node −OH/−OH2 groups at 3673 cm−1 and
the 2,2-biphenyldiol (BIPHENOL) linker −OH at 3582 cm−1,
respectively (Figure 2). Accessibility of the BIPHENOL
chelating sites in ANL1 to early transition metals was evidenced
by postsynthetic metalation with a molecular Ti(IV) precursor. A
toluene solution of Ti(OiPr)4 (1.05 mmol, 0.128 g) was added to
a toluene slurry of microcrystalline ANL1 (0.30 mmol equiv of
bpdcOH, 0.220 g) at 25 °C for 30 h in a N2 glovebox. The air-
sensitive yellow product ANL1-Ti(OiPr)2 was washed repeat-
edly with toluene to remove the excess, unreacted Ti(OiPr)4, and
the toluene supernatant exchanged with Et2O to facilitate solvent
removal from the functionalized MOF.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of ANL1 and ANL1-
Ti(OiPr)2. ANL1, a MOF of UiO-67 topology, was synthesized
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−OH groups of ANL1 at 3673 cm−1 remain intact after the
metalation reaction (Figure 2, purple line vs green line). Note
that negligible reaction occurs between the Ti(IV) precursor and
the Zr6 nodes in UiO-67-bpdc, as suggested by DRIFT
spectroscopy (Ti-UiO-67-bpdc, Figure 2C−D).26
Results of the PXRD analysis of ANL1-Ti(OiPr)2 confirm
retention of the MOF crystallinity after Ti incorporation (Figure
1). This was further confirmed by SEM−EDX analysis where
retention of the MOF crystallinity was observed (Figure S9).
Notably, although the BET surface area of ANL1-Ti(OiPr)2
slightly decreased to 1554 m2 g−1 after metalation (Table S1), the
N2 adsorption isotherms reveal that mesoporosity is maintained
(Figure S1 and S2).
The XAS pre-edge and edge energies of ANL1-Ti(OiPr)2 (a,
Figure 3) are consistent with the Ti4+ standard TiO2 (Anatase).27
Figure 2. Diffuse reflectance FT-IR (DRIFTS) of ANL1, ANL1-
Ti(OiPr)2, UiO-67-bpdc, and Ti-UiO-67-bpdc. See Figure S8 for full
spectra.
1H NMR scale metalation experiments reveal the complete
consumption of the Ti(OiPr)4 precursor, along with the
generation of a substoichiometric amount (1.75 equiv) of
isopropanol (Figures S6 and S7). This suggests that each ANL1-
based Ti(IV) is stabilized by a dianionic BIPHENOL unit, two
anionic isopropoxy groups.25
X-ray absorption spectroscopy (XAS) reveal hexacoordination
of the Ti(IV) centers (vide infra), suggesting that the Ti(IV)
center is datively bound to neutral donor ligands, consistent with
a precatalyst formula unit of [(BIPHENOL)Ti(OiPr)2(L)2],
where L is a residual isopropanol or Et2O molecule. The presence
of labile donor groups was confirmed through thermogravimetric
analysis (TGA) showing a weight loss of 1.4% (1.2% theoretical)
upon heating ANL1-Ti(OiPr)2 between 110 and 200 °C,
corresponding to the departure of two isopropanol/Et2O
molecules (Figure S5).
The presence of Ti in ANL1-Ti(OiPr)2 (Scheme 1) was
confirmed by ICP−atomic emission spectroscopy. The
elemental composition suggests that on average there are 2.3
Ti atoms for every Zr6 node. This ratio translates to ∼80%
metalation of the BIPHENOL linkers. This degree of metalation
is consistent with results of DRIFTS measurements where
significant attenuation of the O−H vibration at 3583 cm−1, along
with the detection of evident alkyl C−H stretching vibration of
the isopropoxy groups (Figure 2, blue line vs red line), was
observed. Additionally, the DRIFTS spectra suggest that node
Figure 3. (a) X-ray absorption near edge structure (XANES) and (b)
EXAFS spectra of Ti(0) foil (green), TiO2 (Anatase) (blue), and ANL1-
Ti(OiPr)2 (red).
Scheme 1. Synthesis of ANL1 via Direct Solvothermal Method and Metalation with Ti(OiPr)4 to yield ANL1-Ti(OiPr)2
C
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Specifically, the pre-edge energy feature at 4970.8 eV, observed at
a normalized height of 0.21, is attributable to a 6-fold
coordination. The X-ray absorption fine structure (EXAFS)
spectra further confirm the 6-fold coordination number (Figures
S10−S12). Three Ti−O distances were observed (1.79, 1.94, and
2.44 Å, Table 1), indicative of the presence of three different
is isolated by simple filtration in 99% yield after 5 days (Figure
S42).
ANL1-Ti(OiPr)2 was also demonstrated efficient for the
hydroboration of a wide range of carbonyl compounds with
excellent functional group tolerance. Table 4 summarizes the
Table 3. Hydroboration of Acetophenone with HBpin in the
Presence of Different Catalysts
a
Table 1. EXAFS-Based Bonding Parameters for ANL1-
a
Ti(OiPr)2
bonds
CN
bond length (Å)
computed bond length (Å)
Ti−O1
Ti−O2
Ti−O3
2
2
2
1.79
1.94
2.44
1.82
1.98
2.13
b
entry
catalyst
ANL1-Ti(OiPr)2
yield (%)
a
Fitting range: k-range = 3.2−12 Å−1; R-range = 0.95−2.23 Å.
c
1
2
3
4
78 (65)
ANL1
0
donor groups around the Ti(IV) center and consistent with the
results of the 1H NMR-scale metalation experiments (vide supra).
Altogether, these spectroscopic features are consistent with a
precatalyst structure where each isolated Ti(IV) center is
stabilized by a anionic BIPHENOL unit, two anionic
isopropoxide groups, and two datively bound neutral donor
molecules.
Ti(OiPr)4
−
14
0
a
Reaction conditions: acetophenone (0.4 mmol, 48.1 mg); HBpin (0.6
mmol, 76.8 mg), toluene/hexane (1.5/0.5 mL), 25 °C, N2, 24 h.
Yield was determined by H NMR with 1,3,5-trimethoxybenzene as
b
1
c
the internal standard. Isolated yield.
Catalytic Hydroboration of Carbonyl Compounds.
ANL1-Ti(OiPr)2 exhibits excellent reactivity in the hydro-
boration of benzaldehyde with 1.5 equiv of HBpin.28 At 2 mol %
Ti loading, benzaldehyde is quantitatively converted to the
corresponding benzyl alcohol borate ester at room temperature
within 5 h (Table 2, entry 1; isolated yield = 98%). Under the
scope of functionalized aldehydes reduced to the corresponding
Bpin borate esters. ANL1-Ti(OiPr)2 quantitatively converts aryl-
functionalized aldehydes (Table 4) to the corresponding borate
ester products in good to quantitative yields with good functional
group tolerance (phenyl, naphthyl, C6H4CH3, C6H4CF3,
C6H4Br, C6H4F, C6F5, and OMe). Notably, aldehydes with a
heteroaromatic substituent such as pyridyl could be quantita-
tively reduced with complete tolerance of the heteroaryl
functionality (entry 4). Less electrophilic, linear, and cyclic
alkyl-substituted aldehydes are reduced in good yields at slower
rates (entries 5 and 6). In addition, trans-cinnamaldehyde, an
α,β-unsaturated aldehyde, is selectively reduced at the carbonyl
position, leaving the vinylic CC bond intact (entry 7). The
slower rates of hydroboration for n-hexyl- and styryl-substituted
aldehydes (entries 6 and 7) are attributed to active pore blockage
by the bulkier borate ester product. The same is observed in the
case of bulkier ketone substrates (vide infra).
The reactivity of ketones (Table 5) is, in general, slightly lower
compared to aldehydes, likely rates due to the formation of
bulkier borate esters that could block the catalytic pore, and they
require higher catalyst loadings (7.5 mol % Ti) and prolonged
reaction time (24 h). Importantly, under the same conditions
(Table 3), homogeneous Ti(OiPr)4 catalyst converts acetophe-
none with a total yield of only 14%, whereas ANL1-Ti(OiPr)2
gives rise to the corresponding hydroborate product as a racemic
mixture with an NMR yield of 78% (65% isolated yield). Notably,
negligible conversion is observed in the absence of the catalyst,
confirming again the catalytic role of ANL1-Ti(OiPr)2. In
general, ketone hydroboration rates appear to be mainly
influenced by steric effects. Electron-donating and withdrawing
groups (−CH3, −OCH3, C10H7, −CF3, I, and NO2; Table 5,
entries 2−7) do not affect acetophenone hydroboration rates. In
addition, ketones bearing a heteroaromatic substituent such as
pyridyl are converted with remarkably yield. Cyclic and linear
aliphatic ketones such as cyclopentanone and 2-pentanone
generate the corresponding products at comparable yields, 77
and 80%, respectively (entries 9 and 10). Notably, the sterically
encumbered carbonyl group in benzophenone undergoes slower
hydroboration, giving moderate yield of the borate ester
Table 2. Hydroboration of Benzaldehyde with HBpin in the
a
Presence of Different Catalysts
b
entry
catalyst
yield (%)
c
1
2
3
4
5
ANL1-Ti(OiPr)2
>99 (98)
ANL1
20
22
46
1
Ti-UiO-67-bpdc
Ti(OiPr)4
−
a
Reaction conditions: benzaldehyde (0.4 mmol, 42.4 mg), HBpin (0.6
b
mmol, 76.8 mg), toluene/hexane (1.5/0.5 mL), 25 °C, N2, 5 h. Yield
was determined by 1H NMR with 1,3,5-trimethoxybenzene as the
c
internal standard. Isolated yield.
same conditions, less than 1% of the product was generated in the
absence of the catalyst, confirming the catalytic role of ANL1-
Ti(OiPr)2 for this transformation (Table 2, entry 4). Meanwhile,
unfunctionalized ANL1 and Ti-UiO-67-bpdc effected 20 and
22% conversion, respectively (Table 2, entries 2 and 3),
suggesting that the observed quantitative conversion in Entry 1
mainly mediated by the supported Ti(IV) center. Markedly,
when molecular Ti(OiPr)4 was used as the catalyst, 46% yield of
the product was obtained. This result further supports the
advantage of heterogenizing the catalytic sites for this trans-
formation. Furthermore, the ANL1-Ti(OiPr)2-catalyzed hydro-
boration of benzaldehyde can be scaled up (>1 g) successfully
without significant loss in efficiency, and the borate-ester product
D
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Table 4. ANL1-Ti(OiPr)2-Catalyzed Hydroboration of
Aldehydes
Table 5. ANL1-Ti(OiPr)2-Catalyzed Hydroboration of
a
a
Ketones
a
Reaction conditions: 7.5 mol % ANL1-Ti(OiPr)2 (32.3 mg), ketones
a
Reaction conditions: 5 mol % ANL1-Ti(OiPr)2 (8.6 or 21.5 mg),
(0.4 mmol), HBpin (0.6 mmol, 76.8 mg), toluene/hexane (1.5/0.5
b
mL), 25 °C. Yield was determined by 1H NMR with 1,3,5-
aldehydes (0.4 mmol), HBpin (0.6 mmol, 76.8 mg), toluene/hexane
(1.5/0.5 mL), 25 °C. Yield was determined by H NMR with 1,3,5
b
1
trimethoxybenzene as the internal standard (Figures S17−S40).
trimethoxybenzene as the internal standard (Figures S14−S26).
c
d
Using 2% catalyst loading, 5 h. Using 2% catalyst loading, 24 h
Due to the markedly different rates of reduction of aldehydes
and ketones exhibited by ANL1-Ti(OiPr)2, competition experi-
ment was carried out to directly probe hydroboration selectivity.
In the reaction of 4-acetylbenzaldehyde (Scheme 2, Figure S41)
with 1.5 equiv of HBpin at 94% conversion (5 mol % Ti, 5 h), the
aldehyde-only hydroboration product is obtained in 99% yield,
with only 1% of the dihydroborated and ketone-only hydro-
derivative (entry 11), compared to less sterically demanding
acetone (entry 12). Moreover, hydroboration of α,β-unsaturated
ketones proceeds selectively the carbonyl group without
detectable reduction of the CC bond (entries 13 and 14).
E
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acetophenone were chosen to represent, respectively, the
aldehyde and the ketone families studied. Two plausible
mechanistic scenarios for the hydroboration of carbonyl groups
were taken into account and probed by DFT calculations.30
In the first scenario (Figure 4), which mimics more closely the
experimental conditions, both reagents (HBpin and benzalde-
hyde or HBpin and acetophenone) were added simultaneously
to the active titanium(IV) dihydride (TiH2) center, which is
formed in situ from the reaction between the HBpin and the
Scheme 2. Competitive Experiment: Aldehyde/Ketone
Hydroboration Selectivity in 4-Acetylbenzaldehyde
precatalyst.5−9 Note that H NMR-scale reaction of ANL1-
1
Ti(OiPr)2 with HBpin shows the formation of iPrOBpin,
suggesting the formation of the Ti-hydride species (Figure S43).
The full cycle of the catalytic hydroboration in this scenario is
depicted in Figure 4B, whereas the Gibbs free energies
corresponding to all the intermediates and transition states
involved are shown in Figure 4C (1, catalytic center; 2, HBpin;
3a, benzaldehyde; 3b, acetophenone). The intermediacy of metal
hydrides has been previously demonstrated in metal-catalyzed
hydroelementation of carbonyl and N-heterocycle com-
pounds.4,8,31 Our results obtained within the natural bond
orbital (NBO)32 analysis scheme are consistent with a hydridic
active species (1), where each titanium-bound hydrogen has a
charge of −0.3 (see Table S3). The DFT calculations indicate a
barrierless coadsorption of the reactants on 1 to give
intermediate 4, followed by 1,2-addition of a Ti−H moiety
across a CO moiety (intramolecular hydride transfer) to give
monoalkoxy monohydride titanium(IV) intermediate 5. Inter-
mediate 5 then undergoes σ-bond metathesis between the Ti−
O(alkoxy) and B−H moieties to regenerate the TiH2 fragment
and a Ti-bound borate ester (7). Ultimately, barrierless
desorption of the hydroborated product regenerates the active
TiH2 species (1). The overall hydroboration reaction is
exothermic with the hydride transfer from Ti to the carbonyl
moiety as the rate-limiting step. The hydroboration of
acetophenone follows an energy profile, which is similar to that
of benzaldehyde, albeit with an overall upward shift in energy.
Interestingly, this shift is most significant for the transition state
TS1 that defines the rate-limiting step. This finding is consistent
boration products. Such remarkable selectivity for the hydro-
boration of aldehyde versus ketone with HBpin has only been
shown before few times using homogeneous catalysts, such as
La(N(TMS)2)3,8a [Ru(p-cymene)Cl2]2,29 and [{(2,4,6-Me3-
C6H2)NC(Me)}2(Me)(H)]AlH.9b
To test the heterogeneity of ANL1-Ti(OiPr)2, a hot filtration
experiment was carried out after 0.5 h of hydroboration of
benzaldehyde with HBpin (conversion = 27%), after which no
further conversion of the substrate was observed. Furthermore,
neither significant leaching (<0.1 ppm Ti) of titanium nor any
change in Zr/Ti ratio (2.3 Ti/Zr6 node) were observed, as
evidenced by ICP-OES analysis of the filtrate and the MOF after
catalysis. ANL1-Ti(OiPr)2 also displayed remarkable recycla-
bility (vide supra), forming the borate ester derivative in excellent
yields (>99%) and selectivity (100%) over five cycles. Between
each run, the catalyst was recovered and directly used for the next
reaction. The crystallinity was maintained after each cycle,
confirmed by PXRD (Figure 1), showing the robust nature of
ANL1-Ti(OiPr)2 under catalytic conditions and stirring required
for these reaction. SEM analysis of the MOF after hydroboration
(Figure S9) also indicates that crystallites are largely undamaged
and N2 adsorption isotherms reveal that mesoporosity is
maintained (Figure S1). However, BET surface area of ANL1-
Ti(OiPr)2 slightly decreased to 1329 m2 g−1 (Table S1 and
Figure S2), likely due the blocking of some active pores.
DFT Calculations and Reaction Mechanisms. The
fragment of the support with the catalyst that was used in the
calculations is shown in Table S3. Benzaldehyde and
Figure 4. Calculated catalytic pathway for ANL1-Ti(OiPr)2-mediated hydroboration of benzaldehyde (a) and acetophenone (b). (A) Formation of
TiH2 active species. (B) Proposed hydroboration mechanistic summary. (C) Corresponding energetic profile (the zero of energy is defined as the sum of
Gibbs free energies of noninteracting species 1, 2, and 3).
F
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Figure 5. Calculated energetic profile (in term of Gibbs free energies) for the first half of the catalytic cycle along the second scenario for the
hydroboration of benzaldehyde (the zero of energy is defined as the sum of Gibbs free energies of noninteracting species 1, two benzaldehydes, and one
HBpin). “1” denotes the catalyst (the same as in Figure 4).
with experimental observations where hydroboration of
aldehydes is more favored than that of ketones.
of a Ti-alkoxide complex into the BIPHENOL linkers of ANL1.
Compared to traditional homogeneous catalyst, purposeful site
isolation of active Ti centers in ANL1-Ti(OiPr)2 enabled the
stabilization of highly active, robust, and reusable catalyst. This
makes it an attractive hydroboration catalyst for green and
sustainable chemical synthesis. High selectivity for aldehyde
hydroboration over ketones and good functional group tolerance
for many other groups is observed. In addition, DFT calculations
suggest that the catalytic hydroboration proceeds via (1) hydride
transfer between the active Ti-hydride species and a carbonyl
moiety (rate-determining step), and (2) alkoxide transfer
(intramolecular σ-bond metathesis) to generate the borate
ester product.
The second scenario assumes concomitant addition of two
molecules of a carbonyl compound, i.e., benzaldehyde or
acetophenone, to the Ti(IV) dihydride (TiH2) center, followed
by Ti−H insertions into the carbonyls to yield [Ti(O-CR2H)2]
and subsequent σ-bond metathesis with HBpin, which leads to
transfer of H to Ti and formation of a hydroborated carbonyl as a
product of the first half of the full catalytic cycle (Figure 5).
Similarly to the first scenario, the first half of the second scenario
is overall exothermic with the rate-limiting step also being a
hydride transfer, in this case the transfer of the second hydride.
The energy (21.7 kcal/mol) of the transition state for this
transfer (TS2 in Figure 5) is slightly higher than the energy (21.1
kcal/mol) of the transition state for the hydride transfer in the
first scenario (TS1 on Figure 4C). The second half of the full
catalytic cycle of the second scenario, which leads to the second
hydroborated carbonyl product and regeneration of the
titanium(IV) dihydride (TiH2) catalyst, is then completed by
adding the second HBpin reagent and following or closely
mimicking the part of the first scenario after step 5 (Figure 4).
Overall, the two mechanistic scenarios explored here can be
viewed as energetically competitive, with the first scenario being
slightly more facile. In addition, the first scenario, as mentioned
above, more closely corresponds to our experimental conditions.
The 1.5:1 ratio of the concentrations of the HBpin to carbonyl
reagents, together with the higher or comparable binding energy
of HBpin (28.6 kcal/mol) as compared to those of the carbonyls
(24.9 kcal/mol for benzaldehyde and 29.6 kcal/mol for
acetophenone) to the catalytic center, favor initial coadsorption
of a HBpin/carbonyl pair, rather than that of a pair of carbonyls.
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations of air-sensitive
materials were performed with rigorous exclusion of O2 and moisture
in oven-dried Schlenk-type glassware on a dual manifold Schlenk line
and in a N2-filled atmosphere glovebox with a high capacity recirculator
(<1 ppm of O2). Toluene and diethyl ether were sparged with N2, dried
using activated alumina columns according to the method described by
Grubbs,33 transferred into the glovebox, and stored over 4 Å molecular
sieves prior to use. Unless specified, all chemicals and other solvents
were purchased and used as received from Sigma-Aldrich, Acros
Organics, Strem Chemicals, TCI America. 2,2′-Bis(acetoxy)-1,1′-
biphenyl-4,4′-dicarboxylic acid (bpdcOAc) was prepared according to
a previous report.34 1,1′-Biphenyl-4,4′-dicarboxylic acid (H2bpdc) were
purchased from TCI America.
Nuclear magnetic resonance (NMR) spectra (1H and 13C) were
recorded using a Bruker UltraShield 500 MHz spectrometer (1H = 500
MHz, 13C = 125 MHz) and analyzed using MestReNova (v11.0.1,
MestreLab Research S. L., Santiago de Compostela, Spain) software.
1
Chemical shifts for H and 13C spectra were referenced using internal
solvent resonances and reported relative to tetramethylsilane (TMS).
Gas chromatography mass spectrometry (GC-MS) and flame ionization
detection (GC-FID) data were collected using a Thermo Scientific
Trace GC Ultra Gas Chromograph system equipped with a Tri Plus
RSH autosampler, a Thermo Scientific ISQ GC-MS detector, and a FID
CONCLUSIONS
■
A new, structurally well-defined MOF-based catalyst for
hydroboration of carbonyl groups was synthesized by integration
G
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Article
detector. The data was analyzed using the Themo Xcalibur 2.2 SP1.48
software. Elemental analysis (% C, H, Zr, Ti) was conducted by
Galbraith Laboratories, Inc. (Knoxville, TN). Inductively coupled
plasma−atomic emission spectroscopy (ICP−AES) data were collected
on a Thermo Scientific iCAP 600 spectrometer with errors within 5%.
Digestion and Analysis by H NMR. Approximately 3 mg of dried
MOF material was digested with sonication in 500 μL of d6-DMSO with
35 μL of D2SO4 (96−98 wt % in D2O).
maximum of the pre-edge peak. Standard procedures based on Demeter
0.9.24 software package were used to extract the extended X-ray
absorption fine structure (EXAFS) data. The coordination parameters
were obtained by a least-square fit in R-space using k1-, k2-, and k3-
weighted Fourier transformed data.
Electron Paramagnetic Resonance Spectroscopy (EPR). Continu-
ous wave (CW) X-band (9−10 GHz) EPR experiments were carried out
with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker Biospin,
Rheinstetten, Germany), equipped with a TE102 rectangular EPR
resonator (Bruker ER 4102ST). A He gas-flow cryostat (ICE Oxford,
UK) and an intelligent temperature controller (ITC503, Oxford
Instruments, UK) were used for measurements at cryogenic temper-
atures (T = 30 K). Data processing was done using Xepr (Bruker
BioSpin, Rheinstetten) and Matlab 7.11.2 (The MathWorks, Inc.,
Natick) environment.
1
Powder X-ray Diffraction Analysis. Approximately 20−30 mg of
MOF samples were dried under vacuum before PXRD analysis. PXRD
data were collected at ambient temperature on a Bruker Diffractometer
D8 Advance operating with the following parameters: Cu Kα radiation
of 40 mA, 40 kV, Kλ = 0.15418 nm, 2θ scanning range of 2−40°, a scan
step size of 0.02° and a time of 3 s per step. The samples were grinded to
smaller particles and placed on a zero-background silicon holder (MTI
Corp.) for analysis.
Brunauer−Emmett−Teller (BET) Surface Area Analysis. Approx-
imately 50−100 mg of the MOF sample was dried under vacuum line for
2 h at 150 °C. The sample was then transferred to a preweighed sample
tube and degassed at 150 °C using Micromeritics ASAP 2020 Surface
Area and Porosity Analyzer (Micromeritics, Norcross, GA) overnight or
until the outgas rate was <5 mmHg. The sample tube was then cooled to
room temperature and reweighed to obtain a consistent weight. BET
surface area (m2/g) measurements and full isotherm data were collected
at 77 K with N2 on a Micromeritics ASAP 2020 Surface Area and
Porosity Analyzer employing the volumetric technique.
Thermogravimetric Analysis. Approximately 10 mg of the MOF
material was used for TGA measurements. Samples were analyzed under
a stream of N2 using a Mettler Toledo TGA 851 unit, running from
room temperature to 600 °C at a scan rate of 5 °C/min.
Synthesis of UiO-67-bpdcOH/bpdc (ANL1). A Schlenk flask (100
mL) branched with a Teflon adaptor equipped with pressure relief was
charged with bpdcOAc (0.226 g, 0.63 mmol), H2bpdc (0.102 g, 0.42
mmol), and DMF (40 mL). The resulting mixture was sonicated for ∼5
min, followed by the addition of ZrCl4 (0.245 g, 1.05 mmol) and glacial
acetic acid (1.8 mL, 31.5 mmol). The reaction mixture was further
sonicated for ∼5 min, sealed with a greased glass stopper, and incubated
at 120 °C for 24 h. After cooling, the solid was collected by filtration,
washed with DMF (10 mL × 2), MeOH (10 mL × 2), and dried under
vacuum at 150 °C for 2 h (258 mg, white powder, yield: 83% based on
H2bpdc). Anal. Calcd for Zr6O4(OH)4[bpdcOH]3[bpdc]3
(Zr6C84H64O38, Mw = 2227.65): C, 45.25; H, 2.90, Zr, 24.57. Found:
C, 39.70; H, 2.39, Zr, 25.3. Satisfactory C% and H% were not obtained,
likely due to the presence of defects in the MOF. However, digestion
1
and analysis by H NMR and PXRD suggest acceptable purity of the
Diffuse Reflectance Infrared Fourier Transform Measurements.
Infrared (IR) spectra were collected under nitrogen atmosphere in a
glovebox using a Bruker Alpha FTIR spectrometer (Bruker Optics,
Billerica, MA) with a single-reflection diamond ATR setup. Samples
were activated at 150 °C under vacuum for 4 h and mixed with dry KBr
(chromatographically pure) before each measurement (sample content,
ca. 10% in KBr). A sample of solid KBr was utilized as the background.
Spectra were collected at 4 cm−1 resolution over 64 scans. All IR data
were normalized, baseline corrected, and analyzed using OPUS (v7.0,
Bruker Optics, Billerica, MA) software.
Scanning Electron Microscopy. Approximately 2−5 mg of dried
MOF sample was suspended in 3 mL of acetone, and the mixture was
sonicated to make a slurry. Two drops of the slurry was transferred to a
conductive carbon tape on a sample holder disk and air-dried. A
Phenomworld SEM instrument was used for acquiring images using a 10
kV energy source under vacuum.
X-ray Absorption Spectroscopy. XAS experiments were conducted
in the bending magnet beamline of the Materials Research Collaborative
Access Team (MRCAT) at the Advanced Photon Source (APS),
Argonne National Laboratory. XAS data were acquired in transmission
step scan mode with photon energies selected using a water-cooled,
double-crystal Si(111) monochromator. The monochromator was
detuned by approximately 50%, reducing harmonic reflections. The
ionization chambers were optimized for the maximum current with
linear response (∼1010 photons detected/sec) with 10% absorption
(30% N2 and 70% He) in the incident ion chamber and 70% absorption
(90% N2 and 10% Ar) in the transmission detector. A Ti foil spectrum
(edge energy 4966 eV) was acquired simultaneously with each
measurement for energy calibration.
material.
Chemisorption of Ti(OiPr)4 Complex on ANL1. In a glovebox,
ANL1 (220 mg, 0.30 mmol equiv of bpdcOH) was added to a solution
of Ti(OiPr)4 (0.128 g, 0.45 mmol) in anhydrous toluene (4 mL). Upon
addition of Ti(OiPr)4, the MOF immediately turned from white to
yellow. The suspension was allowed to stand for 30 h at room
temperature. The resulting yellow solid of ANL1-Ti(OiPr)2 was soaked
first with anhydrous toluene (2 × 5 mL in 12 h intervals) followed by
anhydrous Et2O (5 mL × 2 in 12 h intervals) to remove all
uncoordinated Ti(OiPr)4. The obtained ANL1-Ti(OiPr)2 was activated
by heating at 150 °C in vacuo for 2 h. The extent of titanium uptake was
estimated through the Ti/Zr ratio based on ICP-OES analysis (atomic
ratio Ti/Zr6 = 2.3). Anal. Calcd for Zr6O4(OH)4[bpdcOTi(i-
OPr)2]3[bpdc]3 (Zr6Ti3C108H118O44, Mw = 2809.64): C, 46.13; H,
4.23; Ti, 5.11, Zr, 19.48. Found: C, 36.33; H, 2.64; Ti, 4.36; Zr, 21.83.
Satisfactory C% and H% were not obtained, likely due to the presence of
defects in the MOF. However, digestion and analysis by 1H NMR and
PXRD suggest acceptable purity of the material.
Catalytic Hydroboration of Carbonyl Compounds. All hydro-
boration experiments were carried out inside a N2-atmosphere glovebox.
In a typical experiment, ANL1-Ti(OiPr)2 (Ti loading: 2 or 5 mol % for
aldehydes and 7.5 mol % for ketones) was added to 20 mL vial charged
with dry toluene (1.5 mL). A solution of the carbonyl compound (0.4
mmol) and HBpin (0.6 mmol, 88 μL) in 0.5 mL of hexane was then
added to the catalyst−toluene suspension. The resulting mixture was
shaken at room temperature under N2 atmosphere until completion
(confirmed by TLC analysis). The solid was then filtered off and washed
with hexane (3 × 2 mL). The solvent fraction was removed in vacuo to
yield the borate ester product. Hydroborated products were identified
based on comparison of 1H NMR data with the literature.3−13
Preparative-Scale Reaction Procedure. In a glovebox, ANL1-
Ti(OiPr)2 (0.215 g, 0.2 mmol) was added to an oven-dried 50 mL
Schlenk flask and suspended in dry toluene (10 mL). Then, a solution of
benzaldehyde (1.06 g, 10 mmol) and HBpin (2.2 mL, 15 mmol) in 37.5
mL of dry toluene and 12.5 mL of hexane were transferred to the flask
using syringe. The flask was shaken at room temperature for 5 days.
Then, the mixture was filtered, and the solid was washed multiple times
with ethyl acetate. The solutions were combined in a preweighed
Schlenk flask and the volatiles were removed in vacuo, yielding 2-
Each sample (TiO2; anatase, and ANL1-Ti(OiPr)2) was mixed with
boron nitride to a weight ratio of about 4% with Ti under inert and
moisture-free atmosphere. The mixture was mixed well with a mortar
and pestle, and 10 mg of the mixture then was pressed into a cylindrical
sample holder consisting of six wells with a radius of 2.0 mm, forming a
self-supporting wafer. The sample holder was placed in a quartz tube (1
in. OD, 10 in. length) sealed with Kapton windows by two Ultra-Torr
fittings and used for transmission mode measurement.
The edge energy of the XANES spectrum was determined from the
inflection point in the edge, i.e., the maximum in the first derivative of
the XANES spectrum. The pre-edge energy was determined from the
H
DOI: 10.1021/acs.organomet.7b00544
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Article
(benzyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (99% yield; Figure
S42).
*E-mail: aiwenlei@whu.edu.cn.
*E-mail: ebunel@anl.gov.
*E-mail: delferro@anl.gov.
Recycling Experiment. A series of catalyst recyclability tests were
carried out with benzaldehyde as test substrate. After each catalyst
testing cycle, the recovered catalyst was dried under vacuum for 1 h. The
dried catalyst was then added to an oven-dried 20 mL vial and suspended
in dry toluene (1.5 mL). A solution of benzaldehyde (0.4 mmol, 41 μL)
and HBpin (0.6 mmol, 88 μL) in 0.5 mL of hexane was then transferred
to the catalyst−toluene suspension. The resultant mixture was shaken at
room temperature for 5 h. The spent catalyst was then filtered off and
washed with hexane (3 × 2 mL). The solvent fraction was removed in
vacuo to yield the borate ester product.
ORCID
Jeffrey Camacho-Bunquin: 0000-0003-2297-3404
Guanghui Zhang: 0000-0002-5854-6909
Jens Niklas: 0000-0002-6462-2680
Julius Jellinek: 0000-0001-8241-3623
Aiwen Lei: 0000-0001-8417-3061
Massimiliano Delferro: 0000-0002-4443-165X
Hot Filtration Test. In a glovebox, ANL1-Ti(OiPr)2 (2 mol %, 8.6
mg) was added to an oven-dried 20 mL vial and suspended in dry
toluene (1.5 mL). Then, a solution of benzaldehyde (0.4 mmol, 41 μL)
and HBpin (0.6 mmol, 88 μL) in 0.5 mL of hexane was transferred to the
vial using syringe. The resultant mixture was shaken at room
temperature under nitrogen atmosphere for 0.5 h. Then, the reaction
mixture was quickly filtered in glovebox. The filtrate was analyzed by 1H
NMR (1,3,5-trimethoxybenzene as the internal standard), shaken at
room temperature under nitrogen atmosphere for 4.5 h, and analyzed by
1H NMR.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences, and Biosciences,
under Contract DE-AC02-06CH11357. Use of the Advanced
Photon Source is supported by the U.S. Department of Energy
(DOE), Office of Science, and Office of Basic Energy Sciences,
under Contract DE-AC02-06CH11357. MRCAT operations are
supported by the Department of Energy and the MRCAT
member institutions. The computations were performed using
the facilities of the National Energy Research Scientific
Computer Center (NERSC) and ANL’s Laboratory Computing
Resource Center (LCRC), both supported by DOE. J.M.L.E. was
supported by DOE Office of Science, Office of Workforce
Development for Teachers and Scientists (WDTS) under the
Visiting Faculty Program (VFP).
Computational Methods. The computations were performed
using the spin-unrestricted formalism of the density functional theory
(DFT) as implemented in the NWChem package.35 We employed the
meta-generalized gradient approximation (meta-GGA) with the M06-L
exchange-correlation functional36 and all-electron double-ζ
(LANL2DZ) basis sets37 for all the elements involved, except Ti. The
latter was characterized by an effective core potential, which
incorporates the 10 inner electrons in the ionic core, in combination
with the LANL2DZ basis set for the 12 valence electrons
(3s23p64s24d2). This computational framework has been shown in
earlier studies to provide adequate accuracy for systems similar to those
studied here.8 Our tests on reactions involving Ti+, C6H6, and C7H6O,
for which experimental data are available and which are species akin to
those forming our supported catalyst, provide further corroboration of
the accuracy of the calculations (see Table S11). The structural
optimizations of all the reactants and products involved in the
elementary reaction steps were performed using gradient-based
techniques with no symmetry constraints imposed. The nudged elastic
band (NEB) technique was employed to identify the transition state
geometries and their energies. The identities of the reactants and the
products connected via a given transition state were confirmed via a
small distortion of the transition state configuration along the
eigenmode corresponding to the imaginary frequency in the positive
and negative directions and optimization of the distorted structures.
Results of ab initio molecular dynamics (AIMD) trajectories run under
the conditions of constant energy or constant temperature were
analyzed to visualize and understand the mechanism of interaction of the
reactants with the ANL1-Ti(H)2 catalyst.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00544.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jellinek@anl.gov.
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DOI: 10.1021/acs.organomet.7b00544
Organometallics XXXX, XXX, XXX−XXX