Crystal engineering with ambidentate thiocyanate: thiocyanate nitrogen as a halogen bond acceptor in platinum(ii) complexes

Abstract

Halogen bonding (HaB) serves as a directional noncovalent interaction between electrophilic halogen donors and nucleophilic acceptors. This study demonstrates the unique role of thiocyanate nitrogen as a HaB acceptor in the supramolecular assembly of transition metal complexes. Pt(II)–thiocyanate complexes featuring halogen-substituted aryl isocyanides [Pt(ppy)(SCN)(CNAr)] (ppy = 2-phenylpyridinato-C²,N; Ar = C₆H₃-2-I-4-Br 4, C₆H₃-2,4-I₂ 5, C₆H₃-2,4-Br₂ 6) were synthesized and fully characterized (HR ESIMS, IR, NMR, UV-vis, luminescence). Solution studies revealed a dynamic S/N-thiocyanate isomer equilibrium, while crystallization yielded exclusively S-coordinated solvates (4^S·CHCl₃, 5^S·CHCl₃, 6^S·MeNO₂). X-ray diffraction confirmed isomorphic structures stabilized by directional C–X⋯NCS (X = Br, I) HaBs, with the thiocyanate nitrogen acting as the preferred acceptor despite its coordination to platinum. Theoretical analyses (DFT, QTAIM, NCIplot, ELF, NBO) validated the σ-hole-driven interactions, revealing binding energies of 1.61–4.07 kcal mol⁻¹. A Cambridge Structural Database survey underscores the rarity of such N⋯X contacts in metal–thiocyanate systems compared to prevalent S⋯X interactions. These findings establish thiocyanate nitrogen as an underutilized supramolecular synthon for crystal engineering of metal–organic materials.