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-Trip(2), Trip = C6H2-2,4,6-Pr-3(i), 1), GaAr# (Ar# = C6H3-2,6(Bu(t)Dipp)(2), Bu(t)Dipp = C6H2-2,6-Pr-2(i)-4-Bu-t, 4), and the dimeric (GaAr')(2) (Ar' = C6H3-2,6-DiPP(2), Dipp = C6H3-2,6-Pr-2(i), 6) were synthesized by the reaction of Gal 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 LIV/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) Angstrom and a trans-bent CGaGaC core array. The 1,2-diiodo- 1,2-diaryldigal lane 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 ICGaGaCl core arrays with Ga-Ga distances near 2.49 Angstrom, 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 I 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*Cl-2 with Na2Fe(CO)(4). Reaction of 1 with 2,3-dimethyl-1,3-butadiene afforded the compound {Ar*GaCH2C(Me)C(Me)CH2}(2) (12) that had a 10-membered 1,5-Ga2C8 ring with no Ga-Ga interaction. Stirring I or 6 with sodium readily gave Na-2{Ar*GaGaAr*} (13) and Na-2(Ar'GaGaAr') (14). The former species 13 had been synthesized previously by reduction of GaAr*Cl-2 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) Angstrom, which is slightly (ca. 0.02 Angstrom) longer than that reported for 13. Reaction of Ga{N(Dipp)C(Me)}(2)CH, 15 (i.e., GaNboolean ANDNDiPP(2)), which is sterically related to 1, 4, and 6, with Fe(CO)(5) yielded Dipp(2)Nboolean ANDNGaFe(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 Gal afforded the new paramagnetic cluster Ga(11)Ar4 (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.