Synthesis and structures of the first triiodoarsenate(III) anion, EtAsI3−, and zwitterions, (HPy)2As2I6

Abstract

The first examples of organotriiodoarsenates(III) in the form of zwitterions, (3-HPy)2As2I6 and (4-HPy)2As2I6, have been obtained from aqueous solutions, and their structures have been established by XRD. The dimeric molecules in the crystals are built from two square pyramidal (HPy)AsI4 units joined by 2 bridging I atoms. The crystal packing of both compounds is largely defined by interlayer As⋯I contacts, which fulfill the coordination sphere of the As atoms to 6 in the 4-HPy isomer and result in its characteristic red colour and lower solubility. These intermolecular interactions in the crystal structure can be suppressed by the inclusion of a solvent, showing them to be not very strong. The intramolecular bonding via the two bridging I atoms was proven to be rather weak as well, by the formation of the salt [Et4N][EtAsI3] with a monomeric triiodoarsenate(III) anion from acetonitrile solution. In the closely related compound p-H2NC6H4AsI2·HI, one iodide can be substituted for the OTs− anion to give [p-H3NC6H4AsI2]OTs, showing the iodide belongs to the outer sphere rather than being coordinatively bound to the As atom. The lengths of the trans-I–As–I bonds in the determined structures can be formalized by a semi-empirical modified Pauling equation, which relates them to the bond order, taking into account the antibonding influence of the 4s orbital of the As atom.