One-coordinate aluminum

25 November 2020 - Chemical Zoo

RAl_Power_2020.PNG In the news this week a one-coordinate Al(I) compound as reported by Philip Power in JACS (Queen et al. DOI). His group has a long standing interest in making one-coordinate compounds based on different elements and aluminum was next on the list. Making a compound like R-Al was a bigger challenge because monovalent aluminum is very reactive. In regular organoaluminum compounds like diisobutylaluminium hydride or triethylaluminium, the metal has 4 neighbors.

The carbon part in the new R-Al compound (an alanediyl) is a terphenyl group with a total of 8 isopropyl groups making it very bulky with aluminum stuck in the middle and shielded from the environment. In 2020 speech you could say aluminum got quarantined or aluminum is wearing a giant facemask. The compound is perfectly stable right up to it's melting point of 238 degrees Celsius. It was found to rapidly react with hydrogen to an aluminum hydride.

Understanding the entire synthetic procedure for compound R-Al required some delving back in time in the literature. Power has been working on sterically crowding ligands since 2000. The quick sketch presented here may contain errors but please do not call the chemistry police on me (again). The key ipso position on the central aryl group is introduced very early on as an iodine substituent. Through lithium it arrives as a trihydridoaluminate salt in the 2020 article.

This salt was found to react with methyl iodide to a aluminum(III) halide. For the removal of iodine atoms, the usual reducing agents were found to fail (hydrogen took over) except for one: sodium metal supported on sodium chloride.