Chemistry Reference
In-Depth Information
substituted tetrathiafulvalene derivatives, which are interesting molecular
conductors [95-106]. On interacting with unfunctionalized monohaloalka-
nes,nelectrondonorsusuallygiveS
N
2 type products rather than halogen-
bonded adducts, as the entrance of electron donors from the back-side of the
C - X covalent bond (nucleophilic attack at carbon) prevails over the front-
side entrance (nucleophilic attack at halogen). Particularly strong electron-
withdrawing residues geminal to the halogen atom of monohaloalkanes boost
the Lewis acidity of the halogen and allow it to give strong XBs. This
is the case, among others [107, 108], for various iodomethyl onium salts
(e.g. Ph
3
P
+
-CH
2
-I anion
-
) wherein both the anion
···
I-C distances and the
anion
I-C angles show the presence of strong XB [109-111]. Similarly,
short Cl
-
···
Cl-C XBs are present in chloromethyl onium salts [112, 113] (e.g.
Me
3
N
+
-CH
2
-Cl Cl
-
[114] and in Ph
3
P
+
-CH
2
-Cl ICl
4
-
[115]).
For steric and electronic reasons, the entrance of n electron donors
from the front-side of the C - X covalent bond is favoured over the back-
side entrance in polyhaloalkanes. The latter compounds thus routinely form
XBs. The interaction can further evolve into chemical reactions [63, 64, 116].
Tri- and tetrabromomethane have been extensively used to form halogen-
bonded adducts [117], and iodoperfluoroalkanes are particularly tailored to
XB-based crystal engineering [118]. The general rule that strong electron-
withdrawing groups geminal or vicinal to the halogen atom boost its XB
donor ability holds also for polyhaloalkanes [119]. The Br
-
···
Br distances
in tribromomethyl-triphenylphosphonium bromide (Ph
3
P
+
-CBr
3
Br
-
)are
shorter than in the tetraphenylphosphonium bromide/tetrabromomethane
adduct (Ph
4
P
+
Br
-
···
/
CBr
4
) [120, 121].
2.4
Structure and Role of the Halocarbon Partner
Increased electron density on the XB acceptor site increases its Lewis ba-
sicity and favours XB formation. In solution, the XB acceptor ability of sp
2
hybridized oxygens increases moving from acetone, to dimethylsulfoxide, to
hexamethylphosphortriamide [67, 68]. Nitrogen is usually a better XB accep-
tor than oxygen (see onwards), but the reverse is true when pyridines and
corresponding
N
-oxides are compared, consistent with the electron densi-
ties on the respective electron donor sites [122]. 1,4-Dicyanobenzene does
not give a co-crystal with 1,4-DITFB, but 1,4-dicyano-tetramethylbenzene
does [123]. Pyrazine and its 2-methyl derivative afford solid and halogen-
bonded co-crystals when interacting with 1,2-diiodotetrafluoroethane, but
this is not the case starting from pyrazinecarbonitrile, even on cool-
ing to - 20
◦
C [124]. Neither the oxygen nor the nitrogen atoms of
ω
-
iodoperfluoroalkylsulfonamides form XBs [125], but when a formal negative
charge is on the sulfonamide group, intermolecular O
···
IXBoccursandgives
rise to infinite chains. Similarly,
ω
-iodoperfluoroalkylsulfonic salts present
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