Halogen Bonding with Dihalogens and Interhalogens - Halogen Bonding: Fundamentals and Applications - page 68

2
Crystal Structures

2.1
General Observations

Structural results for N-donor Lewis base molecules (B) with dihalogens (X 2 )
and interhalogens (XY) began to appear with regularity in the late 1950s
and 1960s with the extensive work of Hassel [36-42], Stromme [39-42],
Romming [42, 45, 46], McCullough [47-51], and others [52-54]. Since then,
a large number of such complexes have been studied. This interest has
been driven in large part by research in two areas: the unusual electric
properties imparted to materials when doped with dihalogens (particu-
larly iodine) [55], and the study of dihalogen and interhalogen complexes
with potential antithyroid drugs and related derivatives, including dithiole
thiones [56-65], imidazole thiones [19, 66-73] and selones [68, 74-76], thi-
azole thiones [77] and selones [78-80], oxazole thiones [81], and various
thiourea derivatives [77, 82]. Additional activity has involved the fundamen-
tal study of halogen bonding in particular classes of compounds, such as
aromatic nitrogen heterocycles [31, 83-94, 96], thioethers [95-106], phos-
phines [107-118] and arsines [119-124], and phosphine sulfides [125-132]
and selenides [133-136].
A recent study of halogen bond geometry based on 141 homo- and het-
eronuclear dihalogens provides a thorough investigation of the metric data
for halogen bond geometry, and supports the generally recognized similari-
ties between this interaction and hydrogen bonding [137]. In particular, the
authors verify that halogen bonding in the solid state and for a wide as-
sortment of interhalogens (XY) and Lewis bases (B) is essentially identical
to that for gas phase complexes as reported earlier [138-142]. For the com-
plexes studied, the following was found. (1) The mean B X-Y bond angle

is 176 ◦ ,whichis11-38 ◦ more linear than the corresponding O-H

···

Bhydro-
gen bonds. (2) The direction of the halogen bond is very close to the axis
of the nonbonding electron pair for sp 3 -hybridizedN,P,andAs,and sp 2 -
hybridized N bases. (3) For thio- and selenoethers and carbonyl bases, the
halogen bond is quasi-perpendicular to the R 1 S(Se)R 2 plane or to the plane
bisector of the R 1 CR 2 angle. But for thio- or selenoamides, the NCS(Se)

···

··· X

90 ◦ when bulky R groups crowd
theSorSelonepairs.(4)ForR 3 P = Sor = Se bases, the chalcogen lone
pairs of electrons are staggered relative to the substituents on P. (5) The
lengtheningoftheX-Ybondobeysabondordermodel[ n (X-Y)+ n (B

0or180 ◦ to torsion angle changes from ∼ ∼

···

X)
= 1]. (6) The Lewis acid strength (Br 2 < I 2 < IBr < ICl) is independent of the
softness of the base. (7) The length of the B X halogen bond and the elon-

gation of the X-Y bond are significantly related to the donation strength of
the Lewis base.

···