The thiocyanate anion as a polydentate halogen bond acceptor

Abstract

Co-crystallisation of the Et₄N⁺ or n-Bu₄N⁺ salts of the thiocyanate anion with o-, m- and p-diodoperfluorobenzene or the sym-trifluorotriiodobenzene allowed for the isolation of six different salts which were structurally characterized by single crystal X-ray diffraction. Halogen bonding interactions are observed between the neutral iodinated molecules acting as halogen bond donors and the S or N ends of the thiocyanate anion, with a variety of bonding modes (terminal as well as bridging), and characterised by short I⋯(N,S) contacts and linear C–I⋯(N,S) motifs. On the other hand, the I⋯N≡C angles present deviations from linearity (145–170°) while the I⋯S–C angles are close to 90°, revealing a type II halogen bond interaction with the S atom acting as pseudo halogen. The observation of structures where only the S or the N atom are engaged in halogen bonding suggests that the two coordination modes are close in energy, as confirmed by DFT calculations and ELF analysis which give also a rationale for the observed structural preferences. This bidentate nature of the SCN⁻ anion in halogen bonding interactions is reminiscent of its various coordination modes with the Cd²⁺ cation, best understood in terms of the hard–soft acid–base concept.