Experimental and theoretical evidence for low-lying excited states in [Cr$_{6}$E$_{8}$(PEt$_{3}$)$_{6}$] (E = S, Se, Te) cluster molecules

Three [Cr$_{6}$E$_{8}$(PEt$_{3}$)$_{6}$] cluster molecules with E = S, Se, and Te have been synthesized by reaction of stoichiometric mixtures of Cr(II) and Cr(III) metal salts with silylated chalcogen reagents E(SiMe$_{3}$)$_{2}$ (E = S, Se, Te) in the presence of L = PEt$_{3}$ = triethylphosphine. For the sulfide- and selenide-bridged clusters two crystallographic forms (trigonal R$\bar{3}$ and triclinic P$\bar{1}$), which differ in the presence of lattice solvent molecules, have been isolated. Structural data, optical spectra and quantum chemical calculations reveal the presence of low-lying excited states in [Cr$_{6}$E$_{8}$(PEt$_{3}$)$_{6}$] (E = S, Se), which would help in rationalizing the non-vanishing magnetic moments at 2 K revealed by DC magnetic measurements and EPR spectroscopy. These findings are partially in contrast to a previous report by Saito and co-workers (S. Kamiguchi, H. Imoto, T. Saito, Inorg. Chem., 1998, 37, 6852–6857.), who postulated an incorporated hydrogen atom as the source of paramagnetism at low temperatures for the trigonal forms of [Cr$_{6}$E$_{8}$(PEt$_{3}$)$_{6}$] (E = S, Se).