Three Arsenic Atoms, Three Oxidation States

Compounds with three arsenic atoms are rare to begin with, but Andrey V. Protchenko, Michael F. Lappert, and coworkers of the University of Sussex, in England, have made one that stands out among this small club (Chem. Commun. 2009, 428). “What is particularly interesting about this compound is the three different formal oxidation states of the arsenic atoms all in the same unit,” says chemistry professor Bryan Eichhorn of the University of Maryland, whose group has synthesized other types of catenated arsenic compounds. Lappert’s group made the novel compound by first reacting AsI₃ with potassium β-diiminate; the diiminate ligand (L) is quite bulky: RNCHC(Ph)CHNR, where R is 2,6-diisopropylphenyl and Ph is phenyl. The researchers isolated a crystalline intermediate product, AsI₂L, and then reduced it by reaction with potassium graphite, KC₈, to produce the trinuclear arsenic compound L₂As-As=AsL. On the basis of crystal structure data, Lappert and coworkers conclude that there is one As=As bond and that the formal oxidation states for the arsenic atoms under these conditions are As²⁺, As⁰, and As¹⁺, from left to right, as depicted.