ACS Catalysis
Research Article
active species or affording solution-inaccessible molecular
catalysts via active-site isolation.41−48 The inorganic nodes of
many MOFs feature functional OH groups to mimic metal
oxides/hydroxides. Due to the presence of highly disperse and
uniform OH groups at the nodes, tunable pores, thermal and
chemical stability, and predictable structures, MOFs offer a
unique platform to develop robust single-site nickel-hydride
catalysts.35,36,49,50
Moreover, unlike the traditional supported catalysts, MOF
catalysts combine advantages offered by heterogeneous
catalysts, such as high stability, facile catalyst separation and
recovery, and less leaching, and those provided by their
homogeneous counterparts, such as homogeneity of the active
sites, reproducibility, and selectivity.51−55 Aluminum MOF,
owing to its simple and economical synthesis, high thermal
stability, large pore size, and electrophilic nature of the hydroxy
groups at SBUs, offers superior support to develop highly
electrophilic nickel-hydride catalysts at SBUs. Herein, we
report a highly chemoselective and reusable single-site
nickel(II) hydride catalyst supported by the nodes of a robust
and porous aluminum metal−organic framework (DUT-5) for
hydrogenation of nitro and nitrile compounds to the
corresponding amines and also for hydrogenolysis of aryl
ethers under mild reaction conditions (Figure 1).
(PXRD) pattern to that obtained after the simulation of
aluminum MOF using the structure of MIL-53(Al)-MOF
(Figure 2b).78,79 DUT-5 has one-dimensional chains of μ2-
hydroxide-bridged, octahedrally coordinated Al3+ cations that
are linked via bpdc2− linkers to give a three-dimensional
framework with rhombic channels. The solid-state infrared
(KBr) spectrum of DUT-5 showed the υμ2‑O−H stretching band
at 3700 cm−1 and strong carboxylate stretching bands at 1598
cm−1 (Figure S6, Supporting Information (SI)).49 Nitrogen
adsorption isotherms indicated a highly porous structure with a
Brunauer−Emmett−Teller (BET) surface of 1618 m2/g, a
pore volume of 0.91 cm3/g, and a pore size of 1.2 nm (Figure
2c).78
Nickel-functionalized DUT-5 (DUT-5-NiBr) was synthe-
sized by metalation of the SBUs via deprotonation of μ2-OH
followed by coordination of μ2-O− to a nickel ion. DUT-5 was
first treated with 1.5 equiv of n-BuLi in THF at room
temperature to give an intermediate Al(OLi)(bpdc)35,80 and
then reacted with NiBr2 to afford DUT-5-NiBr as an off-white
solid via a salt-metathesis reaction. Transmission electron
microscopy showed that the DUT-5-NiBr particles have a
platelike morphology and an average diameter of the particles
of 0.9 nm (Figure S3, SI). Both the organic linker and
carboxylate groups remained intact upon lithiation and
1
Primary and secondary amines including anilines are key
intermediates in the synthesis of fine chemicals, pharmaceut-
icals, agrochemicals, and industrial materials.56−58 Among
many methods, earth-abundant metal-catalyzed chemoselective
hydrogenation of nitro or nitrile compounds is one of the most
sustainable and economic methods due to the availability of
starting materials, low price, and high atom efficiency.57−59
Methods for such transformations generally require high
hydrogen pressure and high temperature and also often
necessitate the addition of a base and thus have potential
safety issues.59−68 Despite significant progress with non-noble
metal nanoparticles in recent years,69−72 the development of
single-site well-defined heterogeneous and chemoselective
base-metal catalysts with excellent tolerance of functional
groups under mild reaction conditions is a challenge. Our
single-site DUT-5-supported nickel catalyst is not only highly
active for hydrogenation of nitroarenes and nitriles to give
primary amines under 1 bar H2 in the absence of any base but
also affords symmetric and unsymmetric secondary amines
selectively in similar mild conditions (Figure 1). Our
straightforward, atom-efficient, and environmentally friendly
protocols using the easily affordable MOF catalyst are able to
hydrogenate not only common aromatic and aliphatic nitro
and nitrile substrates but also several industrially important
dinitriles and active pharmaceutical ingredients. Moreover, the
same MOF catalyst is also efficient for selective hydrogenolysis
of ether bonds of aryl ethers as models for lignin under 1 bar
H2 at temperatures as low as 70 °C without the addition of a
base, which is important for the production of value-added
chemicals from biomass.50,73−77
metalation of SBUs as evidenced by the H NMR spectrum
of the digested MOF (Figures S4, SI) and by the retention of
carbonyl stretching frequency of carboxylates in the IR
spectrum (Figure S6, SI). The crystallinity of DUT-5 was
maintained upon metalation as evidenced by similarity in the
PXRD pattern of freshly prepared DUT-5-NiBr to that of
pristine DUT-5 (Figure 2b). Inductively coupled plasma
optical emission spectroscopy (ICP-OES) showed 70% nickel
loading with respect to the μ2-OH moiety. DUT-5-NiBr is
thermally stable up to 450 °C as evaluated by thermogravi-
metric analysis (Figure S5, SI). The estimated BET surface
area and pore size of DUT-5-NiBr are 1252 m2/g and 1.1 nm,
respectively (Figure 2c). The reduction of surface area and
pore size of DUT-5-NiBr as compared to those of pristine
DUT-5 is attributed to the partial filling of the void space of
MOF with the nickel bromide moiety. The coordination
environment of nickel in DUT-5-NiBr was optimized by the
density functional theory (DFT) method (B3LYP) and a basis
set of 6-311G(d,p) using the Gaussian 09 software suite. The
DFT-optimized structure revealed a slightly distorted square-
planar nickel ion, which is ligated with one anionic bridging
oxo (from the deprotonation of the μ2−OH group), two
carboxylate oxygen, and one bromide to afford the [(μ3-
O−)(carboxylate-O)2NiBr] species. The Ni−(μ3-O−) distance
is 1.85 Å, while the Ni−O(carboxylate) distances are 1.91 and
The treatment of DUT-5-NiBr with 1.2 equiv of NaBEt3H
resulted in the formation of DUT-5-NiH as a black solid.
Heating the mixture of DUT-5-NiH and 1 equiv of H2O leads
to the formation of 1.0 equiv of H2 as analyzed by gas
chromatography (GC). The similarity of the PXRD pattern
implies that the crystallinity and the structural integrity of the
MOF are maintained during the bromide/hydride exchange.
X-ray absorption near-edge structure (XANES) analysis
indicated that the oxidation state of the nickel centers in
both DUT-5-NiBr and DUT-5-NiH is +2, as determined by
comparing their energies of the pre-edge peaks to NiBr2 as the
reference compound (Figure 2d). The Ni coordination in the
DFT-optimized structure of DUT-5-NiH converged at a
RESULTS AND DISCUSSION
■
Synthesis and Characterization of DUT-5 and Nickel-
Metalated DUT-5. The solvothermal synthesis between
aluminum trichloride hexahydrate and 4,4′-biphenyldicarbox-
ylic acid in DMF at 120 °C for 48 h afforded DUT-5 (DUT-5)
as a white crystalline solid with the formula of Al(OH)(bpdc)
(Figure 2a).78 The crystallinity and structure of DUT-5 were
established by the similarity of its Powder X-ray diffraction
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ACS Catal. 2021, 11, 3943−3957