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Heterovalent Clusters:  Ln4Se(SePh)8 (Ln4 = Sm4, Yb4, Sm2Yb2, Nd2Yb2)

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journal contribution
posted on 1999-11-30, 00:00 authored by Deborah Freedman, Safak Sayan, Thomas J. Emge, Mark Croft, John G. Brennan
The heterovalent (2 Ln2+, 2 Ln3+) tetranuclear (DME)4Ln4Se(SePh)8 clusters (Ln = Sm, Yb, Nd(III)/Yb(II), Sm(III)/Yb(II); DME = dimethoxyethane) can be prepared either by the reduction of Se−C bonds with Ln or by the 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 Yb4 compound and Nd2Yb2 compounds are considerably more stable, and can be isolated in higher yield. For the Ln = Sm4, Yb4, Sm2Yb2, and Nd2Yb2 clusters, low-temperature structural characterization reveals a square array of four metal ions with a crystallographically imposed disorder that renders 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 ligands that bridge each of the four edges, with a chelating DME ligand saturating the primary coordination sphere of the seven-coordinate Ln ions. Yb-L3 X-ray absorption edge measurements on the disordered Yb4 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 Yb4 cluster indicate that the material is a static, inhomogeneous mixed valent 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 crystal field and antiferromagnetic interactions. These small cluster fragments represent intermediates in the stepwise formation of larger clusters, i.e., dimerization of the Ln4 in a face-to-face arrangement gives the metal core of the now familiar octanuclear Ln8E6(EPh)12 clusters.

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