Gallium monoiodide. Gallium monoiodide is an inorganic gallium compound with the formula GaI or Ga4I4. It is a pale green solid and mixed valent gallium compound, which can contain gallium in the 0, +1, +2, and +3 oxidation states. It is used as a pathway for many gallium-based products. Unlike the gallium(I) halides first crystallographically characterized, gallium monoiodide has a more facile synthesis allowing a synthetic route to many low-valent gallium compounds.

Synthesis
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. 

When the incompletely reacted product was probed by NMR spectroscopy, it showed the presence gallium metal. When probed by 127I NQR, it showed the presence of Ga2I4 and further confirms the [Ga0]2[Ga+][GaI4−] assignment. 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.

Gallium Lewis base adducts

Gallium monoiodide reacts with various monodentate Lewis bases to form Ga(II), Ga(III), or mixed valent compounds, as well as gallium-based dimers and trimers. For example, gallium monoiodide can react with primary, secondary, and tertiary amines, secondary or tertiary phosphines or ethers to form Ga(II)-Ga(II) dimers. Gallium monoiodide can also react with triphenylphosphine (PPh3) to form Ga(III)I3PPh3. It also reacts with the less sterically hindered triethylphosphine (PEt3) to form a Ga(II)-Ga(I)-Ga(II) mixed valent complex with datively coordinated PEt3 ligands. These reactions are believed to be a disproportionation, as gallium metal is produced in these reactions.

Gallium monoiodide reacts with triphenylstibine to produce an SbPh3 fragment datively bonded to a GaPhI2 fragment. 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.

N-heterocyclic carbenes reacts with gallium monoiodide to form a complex with a sterically hindered isopropyl ligand. However, gallium monoiodide reacts with diazabutadienes and subsequent reduction by potassium metal to form Ga analogs of N-heterocyclic carbenes. Other Ga-based carbenes can be produced from gallium monoiodide precursor using Li(NacNac).

Gallium monoiodide reacts with multidentate Lewis bases, such as bipyridine, phenyl-terpyridine, and bis(imino)pyridine ligands to form Ga(III) complexes. 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˚). Despite these similar reactivities and bond characteristics, when gallium monoiodide was reacted with imino-substituted pyridines (RN=C(H)Py), unique reactivity was observed. Reductive coupling of the imino-substituted pyridines formed diamido-digallium(III) complexes. These reactions display the ability of gallium monoiodides to form new C-C bonds.

Gallium heterocycles
Gallium monoiodide can also be used as a precursor to form gallium-based heterocycles. Reactions with diazabutadienes, {RN=C(H)}2, forms monomers or dimers based on the substituents on the diazabutadienes. More sterically hindered substituents such as tert-butyl have resulted in the formation of gallium(II) dimers, whereas reactions with alkyl or aryl substituted diazabutadienes have formed Ga(III) monomers. Gallium monoiodide can be reacted with phenyl-substituted 1,4-diazabuta-1,3-dienes to form a gallium heterocycle with a diazabutadiene monoanion. EPR spectroscopy has revealed that the diazabutadiene fragment is a paramagnetic monoanionic species rather than an ene-diamido dianion or a neutral ligand. Thus, gallium monoiodide undergoes a disproportionation reaction to form a gallium(III) complex with deposition of a gallium metal. Upon further reaction with a 1,4-dilithiated diazabutadiene, this gallium heterocycle forms a new complex with the diazabutadiene monoanion fragment datively bonded to the gallium center and an ene-diamido dianion covalently bonded to the Ga center.

One very important reactivity of this gallium(III) heterocycle is its ability to access gallium analogues of N-heterocyclic carbenes upon reduction with potassium metal. 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].

Cyclopentadienyl complexes
Gallium monoiodide can easily be converted to half-sandwich complexes, (pentamethylcyclopentadienyl)gallium(I) and cyclopentadienylgallium. (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.

Cyclopentadienylgallium, which is less sterically hindered than (pentamethylcyclopentadienyl)gallium(I), can also be accessed using a gallium monoiodide. This ligand can be synthesized with a metathesis reaction of NaCp with gallium monoiodide. 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 both of these two complexes, the (pentamethylcyclopentadienyl)gallium(I) analogues have been synthesized and x-ray crystallography has supported that, as expected, (pentamethylcyclopentadienyl)gallium(I) is a slightly stronger donor than cyclopentadienylgallium.

Like (pentamethylcyclopentadienyl)gallium(I), cyclopentadienylgallium can also coordinate to transition metal complexes such as Cr(CO)5(cyclooctene) or Co2(CO)8 to yield CpGa–Cr(CO)5 or (thf)GaCp{Co(CO)4}2. 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.

Unlike reactivity with Cr(CO)5(cyclooctene), reactivities of (pentamethylcyclopentadienyl)gallium(I) and cyclopentadienylgallium with Co2(CO)8 diverge significantly. Dicobalt octacarbonyl, or Co2(CO)8, exists in various isomeric states. One such isomer contains two bridging CO ligands. When (pentamethylcyclopentadienyl)gallium(I) reacts with Co2(CO)8, two equivalents of CO gas are released, forming (CO)3Co[μ2-(η5-GaCp*)]2-Co(CO)3. This is a derivative of the dicobalt octacarbonyl complex where the bridging CO moieties are replaced by bridging (pentamethylcyclopentadienyl)gallium(I) moieties. 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. Overall, straightforward synthesis of cyclopentadienylgallium from a gallium monoiodide precursor has many merits in expanding the scope of transition metal chemistry with lower valent species.

Gallium clusters

A variety of gallium clusters have also been synthesized from gallium monoiodide. These clusters have often been isolated as salts with bulky silyl or germyl anions, such as [Si(SiMe3)3]−. An example of an isolated gallium cluster is [Ga9{Si(SiMe3)3}6]−, which has a pentagonal bipyramidal polyhedral structure. It is synthesized by reacting gallium monoiodide with Li(thf)3Si(SiMe3)3 in toluene at -78 ˚C. 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. Other clusters that been isolated via similar reaction pathways include [Ga10{Si(SiMe3)3}6], which is a conjuncto-polyhedral cluster, and a closo-silatetragallane anion, which contains three 2-electron-2-center and three 2-electron-3-center bonds. Interestingly, this latter species can only be synthesized when sub-stoichiometric quantities of I2 are utilized to access a "Ga2I3" intermediate species. This is equivalent to reacting liquid gallium metal and iodine to pre-completion, which, as explained above, accesses the [Ga0]2[Ga+]2[Ga2I62-] variant of gallium monoiodide. This highlights the versatility of the gallium monoiodide precursor in accessing a wide range of gallium-based complexes.

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. This molecule is structurally similar to cubane, where the corners are metal and bridging iodine atoms, with one corner removed. This is a particularly unique Co-GaI cluster due to its unusual geometry for transition metal compounds containing heavy group 13 atoms such as gallium. The bond critical points and bond paths, as computed with QTAIM analysis, support that while there are Co-Ga bonds, there are no Ga-Ga bonds.

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

 Gallium(III) iodide
 Persistent carbene

References

Gallium compounds
Iodides