Diiodoacetylene: compact, strong ditopic halogen bond donor?

CrystEngComm Pub Date: 2012-02-28 DOI: 10.1039/C2CE00029F

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) ?; C–I?O > 170°] or C–I?N [I?N 2.715(3)–2.832(7) ?; 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) ?; 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 Csp–I?N halogen bonds are generally shorter Csp2–I?N halogen bonds.

Graphical abstract: Diiodoacetylene: compact, strong ditopic halogen bond donor
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