Interaction of pseudohalide anions, A-  (A-  = N3-, NCO-, NCS-) with electrophilic bromocarbons, R-Br (R-Br = CBr4, CBr3NO2, CBr3CONH2, CBr3H, CBr3F, CBr3CN or C3Br2F6) in solution results in formation of [R-Br, A-] complexes. These associates are characterized by intense absorption bands in the 200 - 350 nm range showing distinct Mulliken correlation with the frontier (HOMO/LUMO) orbitals' separations of the interacting anion and R-Br electrophile. X-ray crystallographic studies established the structural features of the halogen-bonded associates between bromocarbons and polydentate pseudohalide anions. Specifically, in the (Pr4N)NCO�CBr4, (Pr4N)N3�CBr4 and (Pr4N)NCO�CBr3NO2 co-crystals, bromine substituents of the electrophiles are halogen-bonded with the (C=N or N=N) p-bonds of the cyanate or azide anions. Co-crystals of CBr4 with(Pr4N) NCS show two modes (C-Br�S-C and C-Br�N=C) of halogen bonding, while tribromoacetamide molecules form C-Br�S-C halogen bonds and N-H�N=C hydrogen bonds with thiocyanate anions. Principal geometric characteristics of halogen bonding between bromocarbons and zinc-coordinated pseudohalides are similar to those involving separate anions. Structures and energetics of the halogen-bonded complexes resulted from the M06-2X/6-311+G(dp) computations of various R-Br/A- pairs were consistent with the experimental data. These computations revealed that the variations of the intramolecular (C-Br) and intermolecular (Br�A-) bond lengths are correlated with the A- ? R-Br charge transfer determined via Natural Bond Orbital analysis. Also, scrutiny of the structural data indicated that the locations of the intermolecular contacts in these associates are determined primarily by the frontier orbital shapes of the halogen-bonded species. On the whole, spectral and structural data point out that   molecular-orbital (charge-transfer) interactions play a significant role in formation of halogen bonded complexes involving preudohalides and bromocarbons. The basic geometric characteristics of these associates established herein provide a background for the use of halogen bonding with pseudohalides for the molecular recognition of these anions and crystal engineering of their solid-state materials.

Halogen bonding of bromosubstituted electrophiles with pseudohalide  and pseudohalometallate anions

248th National Meeting (2014)

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