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{Os5Lu20}I24, the First Extended Cluster Complex of Lutetium with Eight-Coordinate Endohedral Osmium Atoms in Two Different Environments


  • Dedicated to Professor John D. Corbett on the occasion of his 85th birthday


{Os5Lu20}I24 was obtained from a conproportionation reaction of lutetium and lutetium triiodide in the presence of osmium in an arc-welded tantalum container at 850–1200 °C. The crystal structure has been determined by single-crystal X-ray diffraction analysis. Square antiprisms and cubes of lutetium atoms in a 4:1 ratio share common square faces in a chain; they encapsulate osmium atoms and are surrounded by iodide ions. Os–Os distances within the cluster chains (299, 303, and 325 pm) imply only little if no bonding between neighbouring osmium atoms. The cluster complex chains are packed in the fashion of a hexagonal closest packing of rods. There are only van der Waals interactions between these chains as the shortest inter-chain Lu–I distances of 468 pm attest. Electronic structure calculations show mainly Os–Lu and Lu–I bonding, with lesser Lu–Lu and negligible Os–Os contributions. Remarkable is the considerable population of Lu-5d states close to the Fermi level. Susceptibility measurements show complex magnetic behavior with ferromagnetic coupling in the {Os5Lu20} chains and antiferromagnetic coupling between the chains, which results in geometric frustration. As a consequence, the spin system has a cluster-glass-like ground state.