Synthesis, Characterization and Real Molecule DFT Calculations for Neutral Organogallium(I) Aryl Dimers and Monomers: Weakness of Gallium-Gallium Bonds in Digallenes and Digallynes

A series of stable aryl gallium(I) terphenyl derivatives was synthesized and characterized spectroscopically, structurally and by density functional calculations. Dimeric structures with trans-bent planar CGaGaC core arrangements were observed for [(GaAr*-4-tBu)(2)] (7, Ar*-4-r-tBu=C6H2-2,6(C6H2-2,4,6-iPr(3))(2)-4-tBu) and [(GaAr*-4-CF3)(2)] (8, Ar*-4-CF3=C6H2-2,6(C6H2-2,4,6-iPr(3))(2)-4-CF3), whereas monomeric structures featuring one coordinate gallium were observed for the more crowded complexes [:GaAr*-3,5-iPr(2)] (10, Ar*-3,5-iPr(2)=C6H-2,6(C6H2-2,4-6-iPr(3))(2)-3,5-iPr(2)) and [GaAr'-3,5-iPr(2)] (11, Ar'-3.5-iPr(2)=C6H-2,6(C6H3-2,6-iPr(2))(2)-3,5-iPr(2)). Complexes 7 and 8 dissociate to monomers in hydrocarbon solution and their electronic spectra closely resemble those of 10 and 11 as well as those of [Ar'GaGaAr'] (Ar'=C6H3-2,6(C6H3-2,6-iPr(3))(2)) and [(GaAr*)(n)] (Ar*=C6H3-2,6(C6H2-2,4,6-iPr(3))(2)). The calculations showed that the binding energies of the compounds are weak, resemble closed-shell interactions and average approximate to 5 kcal mol(-1), as in [Ar*GaGaAr*] with a lowest value of approximate to-2 kcal mol(-1) for monomeric 10 and a highest value approximate to 9 kcal mol(-1) for the least crowded species [Ar'GaGaAr']. The weak bonding in the complexes supports the view that the GaGa bonding in the previously published doubly reduced Na-2[Ar*GaGaAr*] and Na-2[Ar'GaGaAr'] is also weak and is consistent with approximate single bonding.