Heavier congeners of CO and CO2 as ligands: from zero-valent germanium ('germylone') to isolable monomeric GeX and GeX2 complexes (X = S, Se, Te)

In contrast to molecular CO and CO2, their heavier mono- and dichalcogenide homologues, EX and EX2 (E = Si, Ge, Sn, Pb; X = O, S, Se, Te), are important support materials (e.g., SiO2) and/or semiconductors (e.g., SiS2) and exist typically as insoluble crystalline or amorphous polymers under normal conditions. Herein, we report the first successful synthesis and characterisation of an extraordinary series of isolable monomeric GeX and GeX2 complexes (X = S, Se, Te), representing novel classes of compounds and heavier congeners of CO and CO2. This could be achieved by solvent-dependent oxidation reactions of the new zero-valent germanium ('germylone')-GaCl3 precursor adduct (bis-NHC)Ge-0 -> GaCl3 1 (bis-NHC = H2C[{NC(H)=C(H)N(Dipp)}C:](2), Dipp = 2,6-iPr(2)C(6)H(3)) with elemental chalcogens, affording the donor-acceptor stabilised monomeric germanium(IV) dichalcogenide (bis-NHC)Ge-IV(=X)=X -> GaCl3 (X = S, 2; X = Se, 3) and germanium(II) monochalcogenide complexes (bis-NHC)Ge-II= X -> GaCl3 (X = Se, 4; X = Te, 5), respectively. Moreover, the reactivity of 4 and 5 towards elemental sulphur, selenium, and tellurium has been investigated. In THF, the germanium(II) monoselenide complex 4 reacts with activated elemental selenium to afford the desired germanium(IV) diselenide complex 3. Unexpectedly, both reactions of 4 and 5 with elemental sulphur, however, lead to the formation of germanium(IV) disulfide complex 2 under liberation of elemental Se and Te as a result of further oxidation of the germanium centre and replacement of the Se and Te atoms by sulphur atoms. All novel compounds 1-5 have been fully characterised, including single-crystal X-ray diffraction analyses, and studied by DFT calculations.