Methyl or ethyl makes the difference: Synthesis and solid-state structure of 9,10-dialkyl-9,10-dihydro-9,10-digallaanthracenes

The donor-free 9,10-dialkyl-9,10-dihydro-9,10-digallaanthracenes 1 (alkyl: methyl) and 2 (alkyl: ethyl) were prepared by reaction of 1,2-di(chloromercurio)benzene with the corresponding trialkylgallium and isolated as colourless air- and moisture sensitive crystalline compounds. In the solid-state structure of 1, two slightly different monomers 1A and 1B are found, which form a dimer 1A⋯1B held together by “medium” strong gallium arene π-interactions. Further weak π-interactions between 1A and 1A and 1B and 1B constitute a one-dimensional coordination polymer containing strands of the composition [⋯(1A⋯1B)⋯(1B⋯1A)⋯]n. In contrast, compound 2 crystallizes in the form of distinct molecular units without any further intermolecular π-interactions. The molecular units possess D2d symmetry and are built by strong π-interactions between two digallaanthracene monomers. Two symmetrical aryl group bridges between two gallium atoms are observed for the first time in the subunits of 2. By addition of a Lewis-base (THF, Pyridine) to 2, a monomeric planar digallaanthracene framework is restored, as proven by an X-ray crystal structure analysis of 2 · 2Py. The different structures of 1 and 2 are explained on the basis of steric effects.