Nature | Vol 577 | 2 January 2020 | 65sheet edges, alternating between nickel centres bound by bipyridine
(site I) and sites facing the tetrahedral cages of the framework (site
II). The third and fourth sites form the sheet interior, comprising six
symmetry-equivalent nickel octahedra (site III) surrounding a central
nickel site (site IV). Nickel site occupancies in the 1 (NiBr 2 )9.9 structure
decrease from 78.4(1)% for site I and 43.2(9)% for site II at the sheet edges
to 39.9(9)% for site III and 23.3(17)% for site IV at the centre (values in
parentheses correspond to standard uncertainties calculated from
the crystallographic refinement). Overall, these occupancies amount
to 52.2(5)% of the expected loading for a Ni 19 Br 38 cluster and suggest
that complete sheets fill 23% of the framework cages and partial sheets
take up 20%, and a combination of mononuclear bipyridine–NiBr 2
complexes and unmetallated linkers probably occupy the remaining
cages. Optimizing the reaction conditions by lowering the concen-
tration of coordinating solvent (as further discussed below) led to a
higher overall Ni occupancy of 80.5(3)% in the structure of 1 (NiBr 2 ) 15.
The average nearest Ni∙∙∙Ni separation (3.723(18) Å) in this structure
closely matches the separation in bulk NiBr 2 (3.723(10) Å), further cor-
roborating the similarity of these sheets to those in the bulk structure^18.
Bulk NiCl 2 adopts the same layered structure type as NiBr 2 , but
exhibits contracted lattice dimensions as a result of having shorter
nickel–halide bonds^19. To probe whether 1 could also stabilize nickel(ii)
chloride sheets, the framework was treated with a solution of Ni(DME)
Cl 2 in diglyme at 120 °C. The structure of the resulting framework
1 (NiCl 2 ) 13 at 100 K (Fig. 2a and Supplementary Fig. 2) confirmed the
formation of analogous nickel(ii) chloride sheets. Notably, the flex-
ibility of the framework allows the bipyridine linkers to conform to the
more compact nickel(ii) chloride lattice. Consistent with its greater
lattice stabilization energy^20 , nickel(ii) chloride affords a higher crystal-
lographic Ni loading (69.9(4)%) compared with nickel(ii) bromide under
similar reaction conditions. Moreover, the nickel site occupancies were
found to be 78.4(7)%, 67.1(7)%, 64.5(7)% and 68.4(16)% for sites I, II, III
and IV, respectively, implying that 65% of the octahedral cages contain
full sheets, whereas only 3% contain partial sheets. These results sug-
gest that nickel(ii) chloride preferentially forms complete clusters.
Unlike the nickel(ii) bromide structure, 1 (NiCl 2 ) 13 features a slightly
expanded average Ni∙∙∙Ni separation of 3.578(17) Å in 1 (NiCl 2 ) 13 com-
pared with 3.483(6) Å in the bulk structure^19 , which probably reflects a
subtle interplay between the stabilization gained from forming an ideal
NiCl 2 lattice and the strain incurred upon contraction of the bipyridine
linkers of the framework around the cluster.
Encouraged by the stabilization of nickel(ii) halide clusters in 1 ,
we pursued the extension of this chemistry to cobalt(ii) chloride and
iron(ii) chloride; however, attempts under similar reaction conditions
resulted in metallation of only the bipyridine sites. Recognizing that
an equilibrium between the metal(ii) halide clusters and solvated
metal species governs sheet assembly, we conducted reactions under
reduced concentrations of coordinating solvent to drive the equilib-
rium towards sheet formation. Specifically, performing the reaction
of single crystals of 1 with either CoCl 2 or FeCl 2 in a 10% (v/v) solvent
mixture of DME and 1,2-difluorobenzene (DFB) at 120 °C facilitates the
growth of cobalt(ii) and iron(ii) chloride sheets in the framework to
yield 1 (CoCl 2 ) 14 (Fig. 2b and Supplementary Fig. 3) and 1 (FeCl 2 ) 17 (Fig. 2c
and Supplementary Fig. 4), respectively. Close inspection of the cobaltbaZr 6 O 4 (OH) 4 (bpydc) 6 (NiBr 2 ) 15Zr 6 O 4 (OH) 4 (bpydc) 6IIIIIIIVFig. 1 | Solid-state structures. a, b, Portions of the structures of 1 (a) and
1 (NiBr 2 ) 15 (b) at 100 K as determined by single-crystal X-ray diffraction. The four
crystallographically distinct Ni(ii) sites in the structure of 1 (NiBr 2 ) 15 are labelled
with Roman numerals at the upper right of each site. Yellow, green, dark red,
red, blue and grey spheres represent Zr, Ni, Br, O, N and C atoms, respectively; H
atoms are omitted for clarity.