Diiodoacetylene: compact, strong ditopic halogen bond donor

Diiodoacetylene, C2I2, is the smallest ditopic halogen bond donor other than I-2 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 center dot center dot center dot O [I center dot center dot center dot O 2.834(4)-2.888(4) angstrom; C-I center dot center dot center dot O > 170 degrees] or C-I center dot center dot center dot N [I center dot center dot center dot N 2.715(3)-2.832(7) angstrom; C-I center dot center dot center dot N > 175 degrees] 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 center dot center dot center dot N interactions [I center dot center dot center dot N 2.948(7)-2.999(8) angstrom; C-I center dot center dot center dot N > 165 degrees], occupying three of the four nitrogen sites on htma. The remaining nitrogen sites engage in N center dot center dot center dot C(pi) interactions directed orthogonal to the plane of the ditopic C2I4 molecules. Separate surveys of halogen bonds formed by diiodo(poly)alkynes I(C C)(n)I (n = 1-3) and by C2I4 molecules indicate that C-I center dot center dot center dot 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 center dot center dot center dot N halogen bonds are generally shorter C-sp2-I center dot center dot center dot N halogen bonds.