Gallium monoiodide (GaI or Ga4I4) is a low-valent gallium species that acts as a reactive intermediate for many gallium-based products. Gallium(I) halides were first crystallographically characterized by Schnöckel and coworkers and have allowed a synthetic route to many low-valent gallium species. However, chemical syntheses that employ “GaI” rather than gallium(I) halide precursors have been increasingly investigated given the ease of synthesis of this reagent. While the synthetic method of Schnöckel and coworkers to synthesize gallium(I) halides require extraordinarily high temperatures, the straightforward preparation of “GaI” at near room temperature has allowed for the exploration of new gallium-based chemistries.
Synthesis
In 1990, Malcolm Green and coworkers synthesized a “GaI” species, whose method of preparation is most widely followed. They found that ultrasonication of liquid gallium metal with iodine in a toluene solvent yields a new pale green powder that they referred to as “GaI”. Although this intermediate was relatively simple to synthesize, the chemical composition of “GaI” was not determined until the early to mid 2010s.
In 2012, the pale green GaI powder was determined to be a combination of various gallium-based species. In 2012, Widdifield and coworkers determined that “GaI” synthesized from the complete reaction of gallium metal with iodine had the chemical composition [Ga0]2[Ga+][GaI4−]. However, in 2014, Malbrecht and coworkers found in a somewhat contradictory study that the incomplete reaction of gallium metal with iodine yielded "GaI" with this chemical composition. “GaI” synthesized with longer reaction times for complete reaction had a different chemical composition [Ga0]2[Ga+]2[Ga2I62-]. Because different variants of "GaI" can access different types of chemical reactivities, these contradictory results support that the chemical composition of "GaI" must be confirmed for each preparation prior to further experimentation.
A schematic of the preparation utilized by Malbrecht and coworkers to access different "GaI" compositions is depicted above. All variants were sonicated for the amount of time listed in toluene at 30 ˚C. The resultant "GaI" powder was found to be highly air sensitive, but stable under inert atmosphere conditions for up to a year at -35 ˚C.
Characterization
The composition of “GaI” has been probed with FT-Raman spectroscopy, powder X-ray diffraction, 69/71Ga solid-state NMR spectroscopy and 127I nuclear quadrupole resonance (NQR). M. Wilkinson and I. J. Worrall, who synthesized a “GaI” species via a different preparation in 1975, determined that “GaI” is likely a combination of gallium(0) metal, Ga2I3 and Ga2I4 based on the characteristic Raman spectra of these constituents. This hypothesis was corroborated as two variants of “GaI” were determined to have the chemical compositions [Ga0]2[Ga+][GaI4−] and [Ga0]2[Ga+]2[Ga2I62-], where [Ga+][GaI4−] can also be expressed as Ga2I4 or GaI2 and [Ga+]2[Ga2I62-] can also be expressed as Ga2I3.
[Ga0]2[Ga+][GaI4−]
When probed by 69/71Ga solid-state NMR spectroscopy, “GaI” with chemical composition [Ga0]2[Ga+][GaI4−] has a highly deshielded signal at δiso = 4488.1(0.3) ppm with no significant gallium quadrupole interaction (QI) tensor. This is representative of a liquid gallium(0) metal rather than solid gallium(0) metal. When probed by 127I NQR, four peaks were observed at 113.69, 132.04, 134.39, and 163.71 MHz. The chemical shifts align with a Ga2I4 reference and further defends the [Ga0]2[Ga+][GaI4−] assignment. Raman spectroscopy has also confirmed this composition assignment with vibrational frequencies identified at 141 (strong), 230 (weak) and 85 (weak) cm−1. While these frequencies match the reference for Ga2I4, they also overlap with a GaI3 reference. However, given the shape of the spectra, evidence from other spectroscopic methods, and power x-ray diffraction patterns, the assignment of [Ga0]2[Ga+][GaI4−] for this “GaI” variant was validated.
[Ga0]2[Ga+]2[Ga2I62-]
When probed by 127I NQR, three peaks were observed at 106.35, 107.83, and 123.54 MHz, which align with the assignment to Ga2I3. Raman spectroscopy has also confirmed this assignment with vibrational frequencies at 124 (strong), 292 (weak) and 188 (weak) cm−1. These frequencies align with those from a Ga4I6 reference. Finally, power x-ray diffraction supports that this “GaI” variant matches that of characteristic Ga2I3, which is different from that of GaI2.
