Gallium monoiodide

In 1990, Malcolm Green synthesized gallium monoiodide by the ultrasonication of liquid gallium metal with iodine in toluene yielding a pale green powder referred to as gallium monoiodide. The chemical composition of gallium monoiodide was not determined until the early to mid-2010s despite its simple synthesis. In 2012, the pale green gallium monoiodide was determined to be a combination of gallium metal and gallium(I,III) iodide, having the chemical composition [Ga0]2[Ga+][GaI4−]. However, in 2014, it was found that the incomplete reaction of gallium metal with iodine yielded gallium monoiodide with this chemical composition. Gallium monoiodide synthesized with longer reaction times for complete reaction had a different chemical composition [Ga0]2[Ga+]2[Ga2I62-]. The resultant gallium monoiodide is highly air sensitive, but stable under inert atmosphere conditions for up to a year at -35 ˚C. ==Characterization==

When gallium monoiodide was first produced, it was proposed that gallium monoiodide is a combination of gallium metal, Ga2I3 and Ga2I4 based on the characteristic Raman spectra of these constituents. This hypothesis was confirmed as two variants of gallium monoiodide were determined to have the chemical compositions [Ga0]2[Ga+][GaI4−], simplified as Ga2I4·2Ga, and [Ga0]2[Ga+]2[Ga2I62-], simplified as Ga2I3·Ga. Raman spectroscopy has also confirmed this composition assignment. All of the evidence from other spectroscopic methods, and power x-ray diffraction patterns, validates the assignment of [Ga0]2[Ga+][GaI4−] for the incompletely reacted gallium monoiodide variant. When the completely reacted product was probed by 127I NQR, it showed the presence of Ga2I3. Raman spectroscopy has also confirmed this assignment, as it aligned with those from a Ga4I6 reference. Finally, power x-ray diffraction supports that this gallium monoiodide variant matches that of characteristic Ga2I3, which is different from that of GaI2. [Ga0]2[Ga+][GaI4−] converts to [Ga0]2[Ga+]2[Ga2I62-] over time. ==Reactions and derivatives==

Gallium monoiodide is used as a precursor for a variety of reactions, acting as a lewis acid and a reducing agent. Early-on, gallium monoiodide was shown to produce alkylgallium diiodides via oxidative addition by reacting liquid gallium metal and iodine in the presence of an alkyl iodide. Since then, other organogallium complexes have been synthesized, as well as Lewis base adducts and gallium based clusters. (L = phosphines, ethers, amines). Gallium monoiodide can also react with triphenylphosphine (PPh3) to form Ga(III)I3PPh3. The difference in reactivity between PPh3 and SbPh3, a heavy atom analogue of PPh3, can be attributed to a weaker Sb-C bond, allowing for transfer of a phenyl group from antimony to gallium. This suggests that gallium monoiodide can be used as a reducing agent as well. Crystallographically, the bipyridine derivative has a distorted octahedral geometry, with a Ga–N bond length of 2.063 Å. The phenyl-terpyridine derivative adopts a distorted trigonal bipyramidal geometry where the two equatorial Ga–N bonds (as drawn) are longer than the axial Ga-N bond, with 2.104 Å and 2.007(5) Å, respectively. The average Ga-N bond length (2.071 Å) is similar to that of a neutral GaCl3(terpy) Lewis base adduct (2.086 Å). The bis(imino)pyridine derivative has a distorted square-based pyramidal geometry. Like for the phenyl-terpyridine derivative, the equatorial imino Ga-N bonds (2.203 Å) are longer than the axial pyridyl Ga-N bond (2.014(7) A˚). EPR spectroscopy has revealed that the diazabutadiene fragment is a paramagnetic monoanionic species rather than an ene-diamido dianion or a neutral ligand. Although a gallium analogue of N-heterocyclic carbenes had been synthesized previously, having access to heavier analogues of N-heterocylic carbenes from a synthetically more facile gallium monoiodide route has opened new avenues in coordination chemistry, such as access to new Ga-M bonds. Gallium monoiodide can also be used to access six-membered gallium(I) heterocycles that have parallels to gallium analogues of N-heterocyclic carbenes. These neutral gallium(I) heterocycles can be synthesized by reacting gallium monoiodide and Li[nacnac]. File:Ga 6-member NHC new.png|center|thumb|400x400px|Reaction of gallium monoiodide (slurry) and Li[nacnac] in a dry ice/acetone bath to access a gallium(I) heterocycle. Excess potassium metal can be added to circumvent a Ga(II) derivative of the six-member gallium(I) heterocycle. (Pentamethylcyclopentadienyl)gallium(I) can be easily produced by reacting gallium monoiodide with a potassium salt of the desired ligand under toluene to avoid side products. This cyclopentadienylgallium ligand has been used to access a GaCp2I complex with datively bonded cyclopentadienylgallium. This complex showcases an uncommon donor-acceptor Ga-Ga bond. Cyclopentadienylgallium can also be used to access a Lewis acid B(C6F5)3 complex with a datively bonded cyclopentadienylgallium ligand. For CpGa–Cr(CO)5, the Ga-Cr bond length (239.6 pm) is similar to that for a (pentamethylcyclopentadienyl)gallium(I) analogue (240.5 pm). For this complex, the trans effect is also observed, where the Cr-CO bond trans to the cyclopentadienylgallium ligand is contracted (186 pm) relative to the cis Cr-CO bonds (189.5 pm). While cyclopentadienylgallium can act as a terminal ligand similar to (pentamethylcyclopentadienyl)gallium(I), it was determined that cyclopentadienylgallium analogues react faster than their (pentamethylcyclopentadienyl)gallium(I) counterparts. This can be attributed to the lower steric bulk of cyclopentadienylgallium. On the other hand, cyclopentadienylgallium enables oxidative addition to Co2(CO)8 to form (thf)GaCp{Co(CO)4}2, where gallium has sigma interactions to two Co(CO)4 units. The average Ga–Co bond length is 248.5 pm and gallium is in a formally +3 oxidation state in this new complex. These clusters have often been isolated as salts with bulky silyl or germyl anions, such as [Si(SiMe3)3]−. This reaction has been shown to access a wide array of products, which may be attributed to the wide range of gallium monoiodide compositions that have been subsequently probed. Of these products, [Ga9{Si(SiMe3)3}6]− is especially unique because Ga was found to have a very low average oxidation state (0.56) and also because this cluster has fewer R substituents than polyhedron vertices. Interestingly, this latter species can only be synthesized when sub-stoichiometric quantities of I2 are utilized to access a "Ga2I3" intermediate species. Gallium monoiodide can also form cluster-type compounds with transition metals precursors. One example is the reaction between gallium monoiodide and (2,6-Pmp2C6H3)2Co, (Pmp = C6Me5), which yields a nido-type cluster. Finally, gallium monoiodide has been able to form clusters with heavy gold atoms by acting as a reducing reagent when combined with (pentamethylcyclopentadienyl)gallium(I) and triphenylphosphine-gold complexes(i.e. AuI(PPh3) or AuCl(PPh3)). This cluster contained the first crystallographically confirmed Ga-Au bonds, consisting of a Au3 cluster ligated by Ga ligands. In addition, NBO analysis showed that the charge on the galliums within the (pentamethylcyclopentadienyl)gallium(I) ligands were much higher than the charge on the Au atoms and the charge on the gallium atoms within the GaI2 motifs. This suggests that non-bridging Ga-Au bonds are highly polarized, whereas the μ-bridging Ga-Au bonds are more non-polar covalent in character. ==See also==