Heterovalent clusters: Ln(4)Se(SePh)(8) (Ln(4) = Sm-4, Yb-4, Sm2Yb2, Nd2Yb2)

The heterovalent (2 Ln(2+), 2 Ln(3+)) tetranuclear (DME)(4)Ln(4)Se(SePh)(8) clusters (Ln = Sm, Yb, Nd-(III)/Yb(II), Sm(III)/Yb(II); DME = dimethoxyeth ane) can be prepared either by the reduction of Se-C bonds with Ln or by th e reaction of Ln(SePh)(2) with elemental Se in DME. The Sm(II) compound is difficult to isolate because it slowly redissolves after precipitation. The Yb-4 compound and Nd2Yb2, compounds are considerably more stable, and can be isolated in higher yield. For the Ln = Sm-4, Yb-4, Sm2Yb2, and Nd2Yb2 cl usters, low-temperature structural characterization reveals a square array of four metal ions with a crystallographically imposed disorder that render s the Ln(II) and Ln(III) sites indistinguishable; the Nd2Yb2 compound also crystallizes in a lattice-solvated unit cell in which there are distinct Ln (II)/Ln(III) metal ions. Connecting the four metals in all structures is a selenido ligand that caps the square array of Ln ions, and pairs of SePh li gands that bridge each of the four edges, with a chelating DME ligand satur ating the primary coordination sphere of the seven-coordinate Ln ions. Yb-L -3 X-ray absorption edge measurements on the disordered Yb-4 cluster reveal a strong bimodal white Line feature that clearly indicates the presence of both Yb(II) and Yb(III) ions. Magnetic susceptibility measurements on the Yb-4 cluster indicate that the material is a static, inhomogeneous mixed va lent material with well-defined Yb(II) and Yb(III) ions and a crossover in the Curie-Weiss behavior at low temperatures is interpreted in terms of cry stal field and antiferromagnetic interactions. These small cluster fragment s represent intermediates in the stepwise formation of larger clusters, i.e ., dimerization of the Ln(4) in a face-to-face arrangement gives the metal core of the now familiar octanuclear Ln(8)E(6)(EPh)(12) clusters.