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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

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posted on 11.02.2003, 00:00 authored by Ned J. Hardman, Robert J. Wright, Andrew D. Phillips, Philip P. Power
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(ButDipp)2, ButDipp = C6H2-2,6-Pri2-4-But, 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., GaNNDipp2), which is sterically related to 1, 4, and 6, with Fe(CO)5 yielded Dipp2NNGaFe(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.