64 | Nature | Vol 577 | 2 January 2020
Article
Confinement of atomically defined metal
halide sheets in a metal–organic framework
Miguel I. Gonzalez1,6, Ari B. Turkiewicz1,6, Lucy E. Darago^1 , Julia Oktawiec^1 , Karen Bustillo^2 ,
Fernande Grandjean^3 , Gary J. Long^3 & Jeffrey R. Long1,4,5*The size-dependent and shape-dependent characteristics that distinguish nanoscale
materials from bulk solids arise from constraining the dimensionality of an inorganic
structure^1 –^3. As a consequence, many studies have focused on rationally shaping these
materials to influence and enhance their optical, electronic, magnetic and catalytic
properties^4 –^6. Although a select number of stable clusters can typically be synthesized
within the nanoscale regime for a specific composition, isolating clusters of a
predetermined size and shape remains a challenge, especially for those derived from
two-dimensional materials. Here we realize a multidentate coordination environment
in a metal–organic framework to stabilize discrete inorganic clusters within a porous
crystalline support. We show confined growth of atomically defined nickel(ii)
bromide, nickel(ii) chloride, cobalt(ii) chloride and iron(ii) chloride sheets through
the peripheral coordination of six chelating bipyridine linkers. Notably, confinement
within the framework defines the structure and composition of these sheets and
facilitates their precise characterization by crystallography. Each metal(ii) halide
sheet represents a fragment excised from a single layer of the bulk solid structure, and
structures obtained at different precursor loadings enable observation of successive
stages of sheet assembly. Finally, the isolated sheets exhibit magnetic behaviours
distinct from those of the bulk metal halides, including the isolation of
ferromagnetically coupled large-spin ground states through the elimination of long-
range, interlayer magnetic ordering. Overall, these results demonstrate that the pore
environment of a metal–organic framework can be designed to afford precise control
over the size, structure and spatial arrangement of inorganic clusters.Several reports have demonstrated the uniform incorporation of nano-
particles or clusters in metal–organic frameworks through encapsu-
lation of preformed particles or serendipitous self-assembly during
framework synthesis^7 –^10. Constraining cluster formation within frame-
work pores has proven to be more difficult, as the absence of sufficiently
stabilizing interactions in most metal–organic frameworks leads to
nonselective agglomeration and unrestricted growth^7 ,^8. Nonetheless,
frameworks bearing coordinating groups have, in a few cases, been
shown to encourage site-specific nucleation of clusters or nanoparti-
cles^11 –^15. Although these methods afford some control over cluster size
and distribution, correlating the properties of the resulting species to
their atomic structure remains challenging.
We proposed that pre-organization of the coordinating groups
in a metal–organic framework could enable the templated growth
of discrete inorganic clusters. Thus, we selected the framework
Zr 6 O 4 (OH) 4 (bpydc) 6 ( 1 ) (Fig. 1a; where bpydc2− = 2,2′-bipyridine-5,5′-
dicarboxylate), which features roughly 1.3-nm-wide octahedral cages
lined with chelating sites that readily bind a variety of metal sources as
isolated, mononuclear complexes, including metal(ii) halides^12 –^14 ,^16 ,^17.
Notably, metallation of the bipyridine linkers of this framework induces
a single-crystal-to-single-crystal transformation that results in crystal-
lographic ordering of the metal–linker complexes^16 ,^17 , thereby enabling
their structure determination by crystallography. Once metallated, six
bipyridine linkers point towards the centre of each octahedral cavity,
providing nucleation sites and creating a multidentate scaffold for
cluster growth.
Reaction of 1 with Ni(DME)Br 2 (where DME = 1,2-dimethoxyethane) in
bis(2-methoxyethyl) ether (diglyme) at 120 °C afforded 1 (NiBr 2 )9.9, and
characterization of single crystals by X-ray diffraction at 100 K revealed
the growth of isolated nickel(ii) bromide sheets within the octahedral
cages of the framework (Fig. 1b and Supplementary Fig. 1). Coordination
of six bipyridine linkers to edge nickel sites constrains the diameter of
each sheet to about 1.5 nm, with the octahedral cage distorting slightly
to accommodate the sheet dimensions. At full occupancy, each cluster
represents a monolayer of 19 edge-sharing nickel octahedra that closely
resembles a portion of a single layer within the structure of bulk NiBr 218.
Each cluster contains four crystallographically distinct nickel(ii) sites.
Two of these sites correspond to twelve nickel centres that define thehttps://doi.org/10.1038/s41586-019-1776-0
Received: 18 June 2019
Accepted: 26 September 2019
Published online: 18 November 2019
(^1) Department of Chemistry, University of California, Berkeley, CA, USA. (^2) National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
(^3) Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, MO, USA. (^4) Department of Chemical and Biomolecular Engineering, University of
California, Berkeley, CA, USA.^5 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.^6 These authors contributed equally: Miguel I. Gonzalez, Ari B. Turkiewicz.
*e-mail: [email protected]