Diiodoacetylene: compact, strong ditopic halogen bond donor

Abstract

Diiodoacetylene, C₂I₂, is the smallest ditopic halogen bond donor other than I₂ or other dihalogens. A convenient synthesis of diiodoacetylene from the common Sonagashira coupling reagent Me₃SiC≡CH, is described. The halogen-bonded adducts of C₂I₂ with dimethylformamide (DMF), pyrazine (pyz) and 1,4-diazabicyclooctane (dabco) have been characterised by X-ray crystallography. All adopt 1D halogen-bonded chains linked via short C–I⋯O [I⋯O 2.834(4)–2.888(4) Å; C–I⋯O > 170°] or C–I⋯N [I⋯N 2.715(3)–2.832(7) Å; C–I⋯N > 175°] interactions. Attempts to synthesise the adduct of C₂I₂ with hexamethylenetetramine (hmta) resulted in isolation and crystallographic characterisation of the adduct of C₂I₄·hmta, indicating decomposition of C₂I₂ to yield C₂I₄ in solution. The adduct comprises two independent C₂I₄ molecules that act, respectively, as tetratopic and ditopic halogen bond donors forming C–I⋯N interactions [I⋯N 2.948(7)–2.999(8) Å; C–I⋯N > 165°], occupying three of the four nitrogen sites on htma. The remaining nitrogen sites engage in N⋯C(π) interactions directed orthogonal to the plane of the ditopic C₂I₄ molecules. Separate surveys of halogen bonds formed by diiodo(poly)alkynes I(C≡C)_nI (n = 1–3) and by C₂I₄ molecules indicate that C–I⋯N halogen bonds are shorter, when normalised for van der Waals radii, and, by inference, stronger than halogen bonds involving other acceptor groups, and demonstrates that C_sp–I⋯N halogen bonds are generally shorter C_sp²–I⋯N halogen bonds.