Chemistry Reference
In-Depth Information
EDT-TTF
Ethylenedithio-tetrathiafulvalene
SOMO
Singly occupied molecular orbital
TEMPO
2,2,6,6-Tetramethyl(piperidin-1-yloxyl)
TSF
Tetraselenafulvalene
TTF
Tetrathiafulvalene
1
Introduction
Crystal engineering, defined by Desiraju as “the understanding of intermole-
cular interactions in the context of crystal packing and the utilization of such
understanding in the design of new solids with desired physical and chem-
ical properties”, has been the subject of several excellent reviews [1-3]. As
a favourite tool to build such supramolecular crystalline systems, directional
intermolecular interactions, such as the prototypical hydrogen bond [4],
have demonstrated their remarkable efficiency, extended since to the weaker
C - H
X hydrogen bonds [5]. However, among these desired physical prop-
erties invoked by Desiraju, a metallic conductivity or a magnetic ordering
are not considered to possibly derive solely from the structural requirements
brought by hydrogen or halogen bonding interactions in molecular solids. In-
deed, molecular conductors are primarily based on
radical molecules
interact-
ing more or less strongly with each other in one-, two- or three-dimensional
structures, allowing for the formation of partially filled conduction bands
and their associated metallic behaviour through the overlap interaction of
the singly occupied molecular orbitals (SOMOs) [6-8]. Similarly, a magnetic
ordering in molecular solids requires a precise distribution and topology of
spin density interactions between radical molecules [9]. However, it was soon
discovered that the band structures of molecular conductors are excessively
sensitive to minute structural modifications [10], and hence that intermole-
cular interactions as weak as C - H
···
X interactions could bring dramatic
effects on the conduction properties [11]. Similarly, magnetic interactions,
much weaker in essence, can possibly be transmitted through such weak
C - H
···
X intermolecular interactions [12, 13].
The combination of both approaches, toward a
crystal engineering of rad-
ical molecules
, has led to numerous conducting or magnetic molecular mate-
rials which exhibit normal hydrogen bond interactions thanks to their func-
tionalization with hydrogen bond donor groups (- OH, N - H). This has been
recently reviewed in the field of molecular conductors [14, 15], while many ex-
amples can also be found in the family of nitroxide radicals [16]. Therefore,
it is not totally surprising that halogen bonding, once identified as a pow-
erful structural tool, would also be considered in the frame of molecular
conductors and magnets. This is the topic of the present review article, which
will be divided into two parts, a shorter one dedicated to halogen bonding
···
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