Halogen Bonding in Crystal Engineering - Halogen Bonding: Fundamentals and Applications - page 118

partners) of the XB. These features give a substantial contribution to the reli-
ability of XB-based crystal engineering and allow a precise structural control
of the formation of the supramolecular aggregates to be obtained whenever
reliable tectons are used.

3.1
1D Architectures

As discussed above, XBs tend to be linear, namely to be formed along the
C - X bond axis on the XB donor module and along the lone pair axis of
the heteroatom on the XB acceptor module. The angles between these axes
determine the halogen-bonded adducts geometry.
For instance, pyramidal tetramers are formed when a pyramidal and tri-
dentate XB donor interacts with a monodentate XB acceptor (e.g. the ad-
ducts CHI 3 /isoquinoline [32] and CHBr 3 /tris(tetrakis(4-nitrophenyl)me-
thane) [154]). According to the same geometric paradigm, linear trimers are
formed when a linear and bidentate XB donor interacts with a monoden-
tate XB acceptor (e.g. the adducts 1,4-DITFB/Ph 4 P + Cl - ,1,4-DITFB/Ph 4 P + Br -
[155], 1,4-di(2-iodoethinyl)-2,5-dimethylbenzene/bis(tetrathiafulvalenium) +
Cl - [83]) and when a monodentate XB donor interacts with a linear and
bidentate XB acceptor (e.g. the adduct C 6 F 5 I (Me 2 N) 3 S + F - [156]).

When both the donor and the acceptor modules are bidentate, infinite
chain (1D polymers) are formed. The simplest case is when the axes of the
donor and acceptor sites are parallel and coaxial so that linear polymers are
formed. This is the case in the homopolymers formed by bidentate and self-
complementary tectons (e.g. 4-iodopyridine [157], 4-iodobenzonitrile [71,
72], halocyanoacetylenes [70]) and in the co-polymers formed when dihalo-
carbons interact with dinitrogen, or dioxygen, substituted hydrocarbons (e.g.
the systems formed when 1,4-DITFB, or 1,4-DIB, interact with 4,4 -BPY [50],
when 1,4-dinitrobenzene interacts with 1,4-DIB [158-162] 1 , and when 1,4-
DITFB interacts with DABCO [163]) (Fig. 7).
Weaker XBs usually result in less linear geometries and this accounts
for the waved geometry in the copolymer given by 1,4-DITFB with 1,4-
dicyano,2,3,5,6-tetramethylbenzene [123] and for the reduced linearity of
the homopolymer given by 4-bromobenzonitrile compared to 4-iodobenzo-
nitrile [71].
Stepped infinite chain are formed when the binding sites, while remaining
parallel, are no more collinear, but are translated from each other. Examples
are the homopolymer given by 4 -bromo-2 ,3 ,5 ,6 -tetrafluorostilbazole [163]
and the copolymers obtained starting from bidentate XB donors having
collinear sites, and XB acceptors with translated sites (e.g. the adducts 1,4-
DITFB/BPE, 1,4-dibromotetrafluorobenzene (1,4-DBTFB)/BPE [49, 50], 1,4-

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1 M. Bolte, private communication, CCDC structure reference code ISIHUN)