Please cite this article in press as: Fang et al., Ultra-Small Face-Centered-Cubic Ru Nanoparticles Confined within a Porous Coordination Cage
Face-centered-cubic (fcc) noble-metal (Ru/Rh/Pd) NPs are more active catalytically
than other close-packing structures29 because the more active crystal surfaces are
exposed in an fcc structure.30 Recent research indicates that the rare fcc Ru NPs31
are more efficient than common hcp Ru NPs, although the data available in the
literature only apply to particle sizes > 3.0 nm.32,33 Presumably, this could be
extended to the size range < 3.0 nm. However, obtaining stable noble-metal
NPs, particularly fcc Ru NPs, with an ultra-small diameter (<3.0 nm) and in highly
crystalline form is very difficult but highly desirable. Although theoretical studies
on the catalytic activity of fcc Ru NPs have been presented,34,35 their synthesis is
extremely rare.31,36 As a result, the reactivity of these M NPs has not yet been
explored. It occurred to us that by using highly crystalline MOPs as host to encap-
sulate M NPs, we would most likely obtain highly crystalline M NPs with tunable
structure.
To have high catalytic activity toward designated reactions, a soluble porous mate-
rial that can strongly interact with and encapsulate M NPs is needed. Here, we
propose a suitable candidate of MOP species, namely porous coordination cages
(PCCs), for such a purpose. PCCs often have intrinsic micro- and mesopores and
are usually soluble in common solvents. Furthermore, specific functional groups
can be introduced to the PCCs, intensifying the interaction between the support
(PCC) and the M NPs. Encapsulated M NPs may adopt the same geometry as the
host cavity, stabilizing their unusual close-packing form and exposing more active
crystal surface. In addition, PCCs can prevent agglomeration of the M NPs, making
the active surface more accessible during the catalytic process.
RESULTS AND DISCUSSION
Synthesis and Characterization of PCC-2
PCC-2 was designed to have an octahedral cage structure and was assembled by
panel ligands, vertex ligands, and metal clusters (Figure 1A). It was synthesized by
a solvothermal reaction of vertex ligand (Na4H4V), panel ligand (H3L), and CoCl2
at 80ꢀC in methanol for 12 hr (Scheme S1). It crystallizes in the rhombohedral R3
space group as large, purple blocks (Figures 1B and S2; Table S2). Each PCC-2
cage molecule comprises six tetranuclear cobalt clusters [Co4(m4-OH)] and six vertex
ligands as vertices and eight panel ligands as the faces of an octahedral cage, giving
rise to the anionic host {[Co4(m4-OH)V]6L8}30À. Each cobalt atom coordinates one sul-
fonic oxygen, two phenolate oxygen atoms from V, two carboxylate oxygens from L,
and a m4-OH, leading to an octahedral coordination environment. On the basis of the
˚
crystal structure, the longest inner cavity dimension of PCC-2 is 25.1 A (the distance
between two m4-OH of two opposite Co4 clusters), and the diameter of the sphere in
1Department of Chemistry, Texas A&M
University, College Station, TX 77843-3255, USA
˚
which the octahedral cage is inscribed is 42.2 A. From the packing diagram of the
2Department of Chemistry, Faculty of Science,
Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japan
crystal structure, multiple pores from micro- to meso-range are found in the solid
state (Figure 1C). A pore-size distribution is given on the basis of the crystal structure
˚
of PCC-2 with van der Waals radii; the most populated intermolecular cavity is 14 A in
3Department of Chemistry, Shanghai Jiaotong
Univeristy, 800 Dongchuan Road, Shanghai
200240, China
diameter (Figure S3).
Cationic cages37–40 and neutral giant cages41–43 have been reported previously, but
PCC-2 represents one of the most negatively charged coordination cages44
reported to date. For PCC-2, the charge is derived from the following: coordination
of the Co2+ ions produces a total of 48 positive charges, which are compensated by
the phenolate groups from V (24 negative charges) and the carboxylates from L (24
negative charges). However, there are still an additional 24 negative charges from V,
as well as 6 negative charges from the anionic m4-OH group in the center of the Co4
4Department of Chemistry, Northwestern
University, Evanston, IL 60208-3113, USA
5Department of Materials Science and
Engineering, Texas A&M University, College
Station, TX 77842, USA
6Lead Contact
2
Chem 4, 1–9, March 8, 2018
Fang, Yu
