Physical Sciences ︱ Akiko Hori

Crystal engineering
for solid-state
molecular recognition
Host-guest interactions in
crystalline frameworks canbe exploited to selectivelyand reversibly adsorb
molecular species within acrystalline lattice. This process
is important in a variety oftechnological fields, including
gas separation, catalysis,trapping of pollutants andcarbon capture. Professor
Akiko Hori (Shibaura Instituteof Technology) is developing
chemical approaches for
optimising the building blocksof these solid-state systems.
Perfluorination of these hostmolecules induces opposite
quadrupole moments thattailors their affinities for
specific molecular adsorbates.

M

olecular recognition is a process
through which moleculesinteract to form non-covalently
bonded assemblies, often comprisingmultiple components and exhibiting
complex structures. The binding ofthese complementary molecules arises
from weak interactions, includinghydrogen bonds, metal coordination,π···π and π···metal interactions, van der
Waals forces and electrostatic attraction.
Molecular recognition occurs naturally inseveral biological processes, including
antigene-antibody interactions andDNA-protein binding, and it is at thecore of the mechanism of action of
specific classes of drugs, like antibiotics.
Synthetic systems exhibiting molecular
recognition have also been created,and they have been put forward aspromising materials for the selective
manipulation of small molecules in avariety of technological applications.
Solid-state frameworks with a highly
porous structure have received
particular attention as flexible “smarthosts” for small- to medium-size
molecules, in view of their potential
ability to bind and transport molecules
and bio-antibody models withhigh efficiency and selectivity. Hostframeworks can be designed byexploiting weak interactions between
their building blocks, to form porous
network structures with cavities capableof accommodating guest molecules.
However, controlling the growth ofthe crystal network, and, especially,
the size and structural properties of
the host cavities can be challenging.
Frequently, unexpected transformations
occur in the crystal structure during
the incorporation of guest molecules

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and cavity mismatch can occur inconsequence to the formation ofinterpenetrating structures.
To overcome the limitations of current
materials growth processes, Professor
Hori has been pioneering the synthesis
and use of non-porous materials
with high adsorption selectivity
and efficiency, which are preparedthrough self-assembly of chemicallyfunctionalised monomers. In particular,
she has been able to show that thefluorination of organic ligands can beused as a simple and powerful approachto prepare host materials with theability to bind reversibly or irreversibly
wide classes of molecular species, andto provide new promising solid-state
media for molecular recognition.
COORDINATION COMPLEXESAS BUILDING BLOCKS
Host crystalline structures can becreated through the self-assembly of
discrete molecular species. The selfassembly process is spontaneous andis regulated by the size, shape andchemical properties of these building
blocks. Transition metal coordination
compounds composed of a metal cation
surrounded by an organic ligand withextended aromatic bonds are good
examples of self-assembling building
blocks. These compounds exhibit avariety of structures and properties,
which depend on the nature of thecation and of the coordinated ligand.
The assembly of these monomers toform perfect crystals, however, is notcontrolled simply by the nature of
the cation itself and by the external
conditions under which the selfassembly process is carried out, like theBenzene is non-polar andnegative quadrupolar
molecule.

Single x-ray crystallographic study is a powerful tool for understanding the weakintermolecular interactions between host and guest materials.

Fluorine is a unique substituent that
dramatically changes the intramolecularelectron withdrawing nature andintermolecular interactions ofaromatic materials.
temperature and type of solvent usedduring the crystallization.
Furthermore, the crystallization ofcoordination compounds typically
only yields uniform crystals with a very
regular structure and high structural
rigidity, which makes them unsuitable
for molecular recognition. Polymorphic
materials, which can exist in different
crystal structures, and pseudopolymorphic materials, which adoptdifferent crystal structures dependingon the presence of water or othersolvents, can provide suitable alternativeroutes for the development of synthetic
systems for molecular recognition.

Hexafluorobenzene is
non-polar and positive
quadrupolar molecule.

aromatic groups of ligands without
significantly changing their physicalproperties and molecular structures.
The perfluorinated aromatic rings show
positive quadrupole moments (e.g.
hexafluorobenzene is +3.2x10-39 C m2) togive completely opposite and different
molecular recognition events. Solid-state
samples grown from fully fluorinatedmonomers were found to exhibit muchhigher flexibility than their unfluorinatedcounterparts. This flexibility can beexplained in terms of changes in theelectrostatic interactions between thecations in the coordination complexes(which are enhanced by the presenceof the electron withdrawing fluorineatoms), along with anion···π, CF···H
and arene-perfluroarene (π···π-hole)interactions. Repulsive F···F interactions
also play an important role. Altogether,
these interactions are responsible for theexistence of multiple states with different
crystal structures, which is a crucial
feature of systems exhibiting guest
recognition properties.

Four pentafluorophenyl
rings are attached to givea host molecule.

CONTROL OF CRYSTAL GROWTH
THROUGH FLUORINATION
To favour the formation of pseudopolymorphic crystals with enhancedhost features, Professor Hori andher collaborators have developed asimple chemical approach, in which
the molecular flexibility, electrostatic
potential and local electron density ofthe monomers are modified throughthe addition of fluorine atoms in theirmolecular structure. Fluorine has a large
intra-molecular electron withdrawing
nature, which dramatically affects theintermolecular interactions between

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Professor Akiko Hori
E: ahori@shibaura-it.ac.jp T: +81-48-720-6350 W: https://www.shibaura-it.ac.jp/en/
W: http://www.sic.shibaura-it.ac.jp/~ahori/molecular_assemblies/index.html

Co-crystal structure of benzene,naphthalene, and fully-fluorinated complex.

MOLECULAR RECOGNITION IN
FLUORINATED COMPOUNDSThe research team have synthesised
several flexible host frameworks offluorinated mono-nuclear and di-nuclear
coordination complexes of varioustransition metal cations (including Cu2+,Fe3+, Co2+, Ni2+, Pd2+ and Pt2+) withvery specific molecular recognition
properties. These frameworks show twotypes of guest recognition events: theselective binding of CO2 and the metal
ion-selective adsorption of small gas
molecules, such as benzene, N2, O2, H2,NO and CH4. Crucially, host structures
obtained from the corresponding nonfluorinated coordination complexes showno tendency to guest encapsulation.
QUADRUPOLAR HOST-GUEST
INTERACTIONS
Professor Hori has also studied indetail the structural and electronic

These interactions are responsible forthe existence of multiple states withdifferent crystal structures, which is a
crucial feature of systems exhibiting
guest recognition properties.
properties of hosts obtained fromfluorinated complexes, to devise amodel for the origin of the enhanced
molecular recognition propertiesof these systems. She used x-ray
crystallography measurements tounderstand what features in the localstructure of the adsorption site areresponsible for promoting the selectivebinding of specific guest molecules.
For instance, in the case of fully
fluorinated pentafluorophenyl ligandswith Cu2+, she demonstrated that guest
CO2 adsorption was
only observed for fullyfluorinated complex.

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aromatic molecules (e.g. benzene
derivatives) arrange themselves in
the most suitable position, within
the flexible cavities of the host, toestablish direct π···cation and areneperfluoroarene interactions. In thisspecific case, the planar nature of theligand allows access to a free axial
coordination site, which acts as abinding site for the guest molecule.
Furthermore, the ability of the cation toact as a coordination site is enhancedby the presence of fluorine atoms
on the ligand, which promote thedisplacement of electron density awayfrom Cu2+, effectively strengthening
electrostatic π···cation interactions. Thefluorine atoms also induce on a ligand
a net quadrupole moment, promoting
attractive interactions with guest
molecules of opposite quadrupolemoment. This explains the remarkableability of the systems prepared by Prof
Hori’s team to act as efficient hosts for
non-polar and negative quadrupolar
molecules like CO2 (-1.4x10-39 C m2)
and benzene derivatives (e.g. benzene
is -2.9x10-39 C m2).

Crystal engineering for solidstate molecular recognition
Research Objectives

DetailAkiko Hori’s research is concerned with supramolecularchemistry and coordination compounds, in particular weakintermolecular interactions.

Akiko Hori, Shibaura Institute of Technology 307 Fukasaku,Minuma-ku, Saitama 337-8570, Japan.

References
Hori A. (2012). Chapter 7, Arene-perfluoroarene interactions
in coordination architectures. The importance of piinteractions in crystal engineering. Frontiers in Crystal
Engineering III ((Eds. Tiekink ERT & Zukerman-Schpector J).
John Wiley & Sons, pp. 163-185.
Hori A; Nakajima K; Akimoto Y; Naganuma K; Yuge H. (2014).
Guest-adjusted encapsulation and thermal studies of nonporous mononuclear Cu(II) coordination complexes throughelectrostatic interactions induced by fluorine substitution.
CrystEngComm, 16, 8805-8817.
 ori A; Gonda R; Rzeznicka II. (2017). Enhanced adsorption ofH
small gas molecules in metal (Cu2+, Pd2+, Pt2+) complexes
induced by ligand fluorination. CrystEngComm, 19, 6263-6266.
Gonda R; Rzeznicka II; Kinoshita Y; Uchida S; Hori A.
(2019). Guest encapsulations in non-porous crystals of fully
fluorinated dinuclear metal complexes with the M2O2 core
(M = Fe3+, Co2+, Ni2+). Dalton Trans, 48, 9062-9066.
Ikumura
Y; Habuka Y; Sakai S; Shinohara T; Yuge H; Rzeznicka
II; Hori A. (2020). Enhanced and heteromolecular guest
encapsulation in nonporous crystals of a perfluorinatedtriketonato dinuclear copper complex. Chem Eur J, 26,5051-5060.
Kusakawa T; Goto T; Hori A. (2020). Supramolecularassociation of M2+...π induced by different electrostatic
properties using naphthyl substituted ꞵ-diketonate complexes(metal = Cu, Pd, Pt). CrystEngComm, 22, 3090-3094.

BioAkiko Hori is a Professor in Department of Applied Chemistry,Graduate School of Engineering and Science at Shibaura
Institute of Technology. She received her PhD in 2002 from
Nagoya University, Japan.
Funding
Japan Society for the Promotion of Science (JSPS)
Collaborators
Prof Izabela Irena Rzeznicka (Shibaura Institute of Technology)
Prof Gary James Richards (Shibaura Institute of Technology)
Prof Hidetaka Yuge (Kitasato University)

Personal Response
Fluorination appears to be a very simple and generalapproach to obtain new materials with enhanced
molecular recognition properties. How did you become
interested and develop this approach, and what
technological fields do you think will benefit most fromyour work?
The research of molecular recognition has moved from
relatively strong interactions such as polarity and hydrogenbonding to weaker intermolecular forces. Since carbon
dioxide and aromatic hydrocarbons, which are associatedwith environmental problems, are non-polar molecules,
and both happen to have negative quadrupole moments,
we noticed that the design of molecules with positive
quadrupole moments using fluorine atoms is necessary for
their recognition and separation. The technological fields
of our research will benefit the purification and storageof molecules for chemical products and the separation ofenvironmental pollutions.