Nanoscale Organo-Rare-Earth-Metal Clusters

Abstract

Although rare-earth-metal clusters serve as established model systems for distinct reaction patterns, the formation process of metal clusters is rather complex and often not well understood.

Primarily, the process of formation of high-nuclear lanthanide clusters was scrutinized. The halfsandwich tetramethylaluminate complexes CpRLn(AlMe4)2 (R = Me3Si (Cp'), Me5 (Cp*)) were treated with different halidogenido-transfer reagents (Me3SiI, Me3GeBr, Me3GeCl) to afford multinuclear Ln clusters. By applying modular changes to the synthesis protocol, effects of various parameters on the cluster formation could be revealed. The sterically less demanding Cp' ligand enabled cluster enlargement to the dodecametallic lanthanum clusters [Cp'LaX2]12 (X = I, Br) and characterization by NMR spectroscopy in contrast to the Cp* ligand, yielding the nonanuclear cluster [Cp*LaI2]9. Similarly, the larger iodido and bromido ligands afforded the larger coordination clusters [Cp'LaX2]12 (X = I, Br) compared to the smaller chlorido ligand, where the decametallic cluster [Cp'LaCl2]10 was isolated. Changing the rare-earth-metal center from lanthanum to marginally smaller cerium or praseodymium metal centers revealed a size effect for the metal center by the isolation of structurally diverse clusters, e.g. the hexadecanuclear lanthanide nanowheel [Cp'4Ln4I8]4 (Ln = Ce, Pr). Lastly, establishing a kinetically controlled reaction pathway via a diffusion synthesis protocol enabled the isolation of lanthanide clusters with distinct structural motifs that can be exclusively obtained by this route, such as the desilylated octanuclear lanthanum cluster [(μ-Cp)2Cp'8Ce8I14]. Further mechanistic studies by NMR spectroscopy were conducted on the trinuclear model cluster [Cp*3Y3(μ2-Me)3(μ3-CH2)(μ3-H)(thf)3], featuring a rare mixed methyl/methylidene/hydrido derivative, which gave detailed insight on the process of formation of simple lanthanide clusters.

Additionally, the physicochemical properties of the complexes were investigated. The cerium precursor complexes and clusters both showed interesting photoluminescence behavior, which can be precisely tuned by changing the ligand system. Furthermore, clusters showing singlemolecule magnet (SMM) behavior were targeted by using ferromagnetic lanthanides as metal centers. The obtained tetranuclear dysprosium cluster [Cp*DyI2]4 then displayed singlemolecule magnetism properties, hence establishement of a straightforward synthesis route toward potential SMM clusters was feasible.