Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

Abstract

Chromium(II) chloride reacts with lithium pentamethylcyclopentadienide (LiCp*, Cp* = C₅Me₅) and LiE(SiMe₃)₂ (E = P or As) to give the heterocubane chromium phosphide [(η⁵-Cp*Cr)(μ₃-P)]₄ (1) or the chromium arsenide cage [(η⁵-Cp*Cr)₃(μ₃-As)₂] (2), respectively. The crystal and molecular structures of both compounds have been determined by X-ray crystallography. The reactions represent unusual oxidative additions of pnictogen atoms to chromium(II), which in the case of 1 results in the formation of the unusual chromium(IV) formal oxidation state, and in the case of 2 provides access to chromium(III). Compound 1 is also a rare example of a transition metal μ₃-phosphide-ligated cubane. Magnetic susceptibility and magnetization measurements, with support from DFT calculations, indicate S = 2 and S = 9/2 ground states for 1 and 2, respectively, which can be rationalized by considering the electronic structure in terms of chromium–chromium bonding.