Structures, Bonding, and Reaction Chemistry of the Neutral Organogallium(I) Compounds (GaAr)_n (n = 1 or 2) (Ar = Terphenyl or Related Ligand):  An Experimental Investigation of Ga−Ga Multiple Bonding

The synthesis, structure, and properties of several new organogallium(I) compounds are reported. The monovalent compounds GaAr* (Ar* = C₆H₃-2,6-Trip₂, Trip = C₆H₂-2,4,6-Prⁱ₃, 1), GaAr^# (Ar^# = C₆H₃-2,6(Bu^tDipp)₂, Bu^tDipp = C₆H₂-2,6-Prⁱ₂-4-Bu^t, 4), and the dimeric (GaAr‘)₂ (Ar‘ = C₆H₃-2,6-Dipp₂, Dipp = C₆H₃-2,6-Prⁱ₂, 6) were synthesized by the reaction of “GaI” with (Et₂O)LiAr*, (Et₂O)LiAr^# (3), or (LiAr‘)₂. Compounds 1 and 4 were isolated as green crystals, whereas 6 was obtained as a brown-red crystalline solid. All three compounds dissolved in hydrocarbon solvents to give green solutions and almost identical UV/visible spectra. Cryoscopy of 1 and 6 showed that they were monomeric in cyclohexane. Crystals of 1 and 4 were unsuitable for X-ray crystal structure determinations, but an X-ray data set for 6 showed that it was weakly dimerized in the solid with a long Ga−Ga bond of 2.6268(7) Å and a trans-bent CGaGaC core array. The 1,2-diiodo-1,2-diaryldigallane compounds {Ga(Ar*)I}₂ (2), {Ga(Ar^#)I}₂ (5), and {Ga(Ar‘)I}₂ (7) were isolated as byproducts of the synthesis of 1, 4, and 6. The crystal structures of 2 and 7 showed that they had planar ICGaGaCI core arrays with Ga−Ga distances near 2.49 Å, consistent with Ga−Ga single bonding. Treatment of 1, 4, and 6 with B(C₆F₅)₃ immediately afforded the 1:1 donor−acceptor complexes ArGa{B(C₆F₅)₃} (Ar = Ar*, 8; Ar^#, 9; Ar‘, 10) that featured almost linear gallium coordination, Ga−B distances near the sum of the covalent radii of gallium and boron, as well as some close Ga···F contacts. Compound 1 also reacted with Fe(CO)₅ under ambient conditions to give Ar*GaFe(CO)₄ (11), which had been previously synthesized by the reaction of GaAr*Cl₂ with Na₂Fe(CO)₄. Reaction of 1 with 2,3-dimethyl-1,3-butadiene afforded the compound (12) that had a 10-membered 1,5-Ga₂C₈ ring with no Ga−Ga interaction. Stirring 1 or 6 with sodium readily gave Na₂{Ar*GaGaAr*} (13) and Na₂(Ar‘GaGaAr‘) (14). The former species 13 had been synthesized previously by reduction of GaAr*Cl₂ with sodium and was described as having a Ga−Ga triple bond because of the short Ga−Ga distance and the electronic relationship between [Ar*GaGaAr*]^2- and the corresponding neutral group 14 alkyne analogues. Compound 14 has a similar structure featuring a trans-bent CGaGaC core, bridged by sodiums which were also coordinated to the flanking aryl rings of the Ar‘ ligands. The Ga−Ga bond length was found to be 2.347(1) Å, which is slightly (ca. 0.02 Å) longer than that reported for 13. Reaction of Ga{N(Dipp)C(Me)}₂CH, 15 (i.e., GaN^∧NDipp₂), which is sterically related to 1, 4, and 6, with Fe(CO)₅ yielded Dipp₂N^∧NGaFe(CO)₄ (16), whose Ga−Fe bond is slightly longer than that observed in 11. Reaction of the less bulky LiAr‘ ‘ (Ar‘ ‘ = C₆H₃-2,6-Mes₂) with “GaI” afforded the new paramagnetic cluster Ga₁₁Ar₄‘ ‘ (17). The ready dissociation of 1, 4, and 6 in solution, the long Ga−Ga distance in 6, and the chemistry of these compounds showed that the Ga−Ga bonds are significantly weaker than single bonds. The reduction of 1 and 6 with sodium to give 13 and 14 supplies two electrons to the di-gallium unit to generate a single bond (in addition to the weak interaction in the neutral precursor) with retention of the trans-bent geometry. It was concluded that the stability of 13 and 14 depends on the matching size of the sodium ion, and the presence of Na−Ga and Na−Ar interactions that stabilize their Na₂Ga₂ core structures.