Chalcogen Rich Lanthanide Clusters from Halide Starting Materials (II): Selenido Compounds

Lanthanides reduce mixtures of I2 and PhSeSePh in THF, and the resultant heteroligand mixture reacts further
with elemental Se in pyridine to give (THF)6Ln4I2(SeSe)4(μ4-Se)·THF (Ln = Tm, Ho, Er, Yb). These selenium rich
clusters contain a square array of Ln(III) ions connected through a single (μ4-Se) ligand. There are two I- ligands
coordinating nonadjacent Ln(III) ions on the side of the cluster opposite the (μ4-Se), and the edges of the square
are bridged by μ2-SeSe groups. The electronic spectrum of the Yb compound contains two absorption maxima that
can tentatively be assigned as Se2- to Yb and SeSe to Yb charge-transfer absorptions, by comparison with the
featureless absorption spectra of the Tm, Ho, and Er derivatives. With a 1/1/1/1 Yb/I/Ph2S2/Se stoichiometry, chalcogen
rich compounds are not obtained, but instead, in Yb chemistry, the selenido cluster (THF)10Yb6Se6I6 can be isolated
in 51% yield. The molecular structure of this compound contains a Yb4Se4 cubane fragment, with an additional
Yb2Se2 layer capping one face of the cube. Each Yb coordinates a terminal I-. This intensely colored compound
also has an absorption maximum in the visible spectrum. Upon thermolysis, the selenium rich compounds give
Ln2Se3 that is free of iodide contamination.