Halogen Bonding in Crystal Engineering - Halogen Bonding: Fundamentals and Applications - page 111

interaction energy (density functional theory, DFT) is 3.29 kcal mol -1 .The
same parameters become 2.851 A and 5.81 kcal mol -1 , respectively, when 1,4-
diiodotetrafluorobenzene (1,4-DITFB) is used [50] (Table 2).
Similarly, the experimentally determined N I distances in the co-crystals

given by ( E )-1,2-bis(4-pyridyl)ethylene (BPE) and hexamethylenetetramine
(HMTA) with 1,4-DIB were longer than with its tetrafluorinated analogue and
the computed N

···

Ienergiesweaker.TheI -

I-C interaction length in the
adduct between a cyclopropenium iodide and iodobenzene is 3.535 A when
one nitro group is on the aromatic ring and becomes 3.431 and 3.328 A when
two and three nitro groups are present, respectively [53].
In keeping with the anisotropic electrostatic potential model for the halo-
gen atoms, halogens show an “amphiphilic” character and can work both as
the electrophilic sites and the nucleophilic sites when involved in short con-
tacts (Fig. 3).
The halogen acts as an electron-deficient site when it gives contacts to-
wards the pole (electrophilic end) [54, 55], while the same halogen can act as
an electron-rich site when it gives contacts towards the equator (nucleophilic
end) (for instance with metal ions, protons [56], or halogens [57]).

··· ···

Table 3 Interaction of XB donors with different acceptors

a d

C - I ∆ C-I I ··· donor distance ∆ I ··· donor

Expected b Observed c

distance

[A] [A] [A] [%] [%]

1,4-DITFB 2.075

1,4-BPY ··· 1,4-DITFB 2.094 0.92 3.53 2.851 19.2

1,4-BPE ··· 1,4-DITFB 2.089 0.67 3.53 2.768 21.6

TMDCN ··· 1,4-DITFB 2.079 0.19 3.53 3.061 13.3

DABCO ··· 1,4-DITFB 2.119, 2.121 2.12, 2.22 3.53 2.739, 2.748 22.4, 22.1

DMAB ··· 1,4-DITFB 2.084 0.43 3.50 3.097 11.5

2,2-BPY ··· 1,4-DITFB 2.086 0.53 3.53 3.111 11.9

TMPDA ··· 1,4-DITFB 2.091 0.77 3.53 2.935 16.8

TMACl ··· 1,4-DITFB 2.096, 2.083 1.01, 0.38 3.65 3.134/3.153 14.0, 13.6

When a given XB donor interacts with different acceptors, the shortening of the D ··· X
interaction ( ∆ D ··· I ) is greater than the lengthening of the C - Xbond( ∆ C-I ). This proves

how the D ··· I distance is more sensitive to XB strength than the C - Idistance.

a

∆ C-I = difference between C - IdistanceintheXB
donor as pure compound and as an halogen bonded complex
b

Percentage change of C - Idistance,

Sum of van der Waals radii of XB donor and acceptor atom

c

From single crystal X-ray analysis

d

∆ I ··· donor = difference between the expected
and the observed distances. TMBDCN 2,3,5,6-tetramethyl-1,4-benzodicarbonitrile, DMAB
4,4 -bis(dimethylamino)benzophenone,

···

Percentage change of I donor distance,

N , N , N , N -tetramethyl- p -phenylenedi-

TMPDA

amine, TMACl tetramethylammonium chloride