Journal of the American Chemical Society
Article
increased stability and a greater range of structures and
compositions. Beyond harnessing the intrinsic chemical and
physical properties of coordination network-forming liquids to
establish new structure−property relationships and realize new
functionalities, expanded access to molten phases of metal−
organic frameworks would also afford new opportunities for
materials processing,8 growth of large single crystals,9 and glass
formation.10
melting transitions. Melting transitions are governed by the
difference in enthalpy (ΔHfus) and entropy (ΔSfus) between
solid and liquid phases, with the transition temperature given
by ΔHfus = TmΔSfus. The promotion of low melting
temperatures thus requires minimizing ΔHfus, maximizing
ΔSfus, or some combination of both.
Minimizing ΔHfus entails balancing the strength of cohesive
interactions within the solid and liquid phases. In most metal−
organic materials, such interactions involve a convolution of
attractive and repulsive forces, including those mediated by
coordination bonds, hydrogen bonds, van der Waal inter-
actions, Coulombic interactions, and steric repulsions. For a
three-dimensional metal−organic network to transition to a
liquid, the network of coordination bonds holding the
compound together must dissociate to at least some degree
to enable the inherent microscopic fluctuations that give rise to
the macroscopic fluidity of liquids. Indeed, simulations have
indicated that tetrahedral Zn centers in Zn(imidazolate)2 (ZIF-
4) transition to an average coordination number of ∼3.3−3.6
in the liquid phase.4a,c Bridging ligands bearing coordination
groups that form relatively weak bonds with metalssuch as
sulfonates, esters, amides, and nitrilesshould facilitate the
partial breaking of coordination bonds during melting and thus
promote a lower enthalpy changeand lower melting
temperaturethan the carboxylate- and azolate-based ligands
that are typically used to form metal−organic frameworks with
high thermal stability.6f,g,i,k In addition, ΔHfus can be further
reduced by minimizing electrostatic attractions through charge
delocalization and the spatial separation of anions and
cationsa strategy that is routinely applied in the design of
ionic liquids.11
The existence of matter in the liquid state requires that
attractive interactions between constituent atoms, ions,
molecules, or other chemical species are sufficiently counter-
acted by repulsive interactionsimparting fluidity while still
maintaining cohesion. In principle, every metal−organic
network has an equilibrium melting temperature, Tm. In
practice, however, the decomposition temperature, Tdec, of
many compounds falls below Tm. Therefore, the primary
challenge in the synthesis of coordination network-forming
liquids is to design systems for which Tm < Tdec.
6c To date, the
limited efforts to explore melting transitions in two- and three-
dimensional metal−organic networks have focused on
increasing Tdec through the use of ligandssuch as
imidazoleswith high thermal stabilities, while efforts to
design compounds with intrinsically lower Tm have received
comparatively little attention.6c,e−g,j Herein, we establish
generalizable thermodynamic strategies to lower the melting
temperature of three-dimensional metal−organic networks
through the synthesis and characterization of a series of
bis(acetamide)-based compounds that feature stable liquid
phases (Figure 1).
Maximizing ΔSfus requires minimizing solid-state entropy
and maximizing liquid-state entropy. In most cases, the entropy
of metal−organic materials can be partitioned into contribu-
tions from configurational, vibrational, and rotational entropy
for solid phases, with an added contribution from translational
entropy for liquid phases.6e,f,12 Although much remains to be
understood about relationships between the structure,
composition, and entropy of metal−organic networks, their
configurational, vibrational, and rotational entropy should, in
principle, be conducive to predictable manipulation through
ligand design. For instance, low-symmetry, high-flexibility
organic bridging ligands will access more conformations in
the liquid phase relative to the solid,13 and consequently,
compounds constructed from such ligands should experience a
greater increase in entropy upon melting. Similarly, extended
structures that better restrict the residual motion of organic
ligands in the solid state should undergo higher entropy
increases upon melting than structures for which ligands have
more rotational and vibrational degrees of freedom in the solid
state.
Toward the goal of minimizing ΔHfus and maximizing ΔSfus,
we targeted extended networks composed of flexible bridging
ligands with weak, neutral donor groups and noncoordinating
anions to provide charge balance (Figure 1). Although there
are limited examples of three-dimensional coordination
networks formed exclusively from neutral bridging ligands,
select polymethylene bis(amides) are known to form
crystalline two- and three-dimensional extended solids with
several first-row transition metals (Table S13),14 a few of
which even have reported visual melting points.14b We
hypothesized that metal−amide bonds would provide an
interaction weak enough to facilitate melting, but strong
Figure 1. Illustation of design principles to promote the formation of
coordination network-forming liquids from extended metal−organic
materials by minimizing the enthalpy of fusion, ΔHfus, and maximizing
the entropy of fusion, ΔSfus, to achieve low melting temperatures, Tm.
Our approach draws from the well-established “crystal
engineering” design rules of metal−organic framework
chemistry to control structure through directional coordination
bonds and from the “anti-crystal engineering” design rules of
ionic liquid synthesis to promote low melting temper-
atures.3,6f,11 Specifically, we targeted organic ligands predis-
posed to (1) bridge multiple metals and (2) provide an
enthalpic and entropic driving force for low-temperature
2802
J. Am. Chem. Soc. 2021, 143, 2801−2811