Syntheses, Properties and Structures of [{(C5Me5)2Nb}2NiTe4] and [(tBuC5H4Nb)2Ni5Te7(Ph2PCH2PPh2)2]: The Quest for Tetratelluronickelate Clusters

The reaction of [Ni(COD)2] with [Cp*2NbTe2H] (1; Cp* = η-C5Me5) in the presence of Ph2PCH2PPh2 (dppm) in boiling toluene gives black-violet [(Cp*2Nb)2NiTe4] (3). If [Cp′2NbTe2H] (2; Cp′ = tBuC5H4) is used under similar conditions dark-brown [(Cp′Nb)2Ni5Te7(dppm)2] (4) is formed. The structures of 3 and 4 have been determined crystallographically. Complex 3 contains a severely distorted NiTe4 tetrahedron to which two niobocene groups are coordinated. Density functional analysis of the electronic structure of the NiTe4 building block shows that it is best described as an [Ni(η2-Te2)2]2– fragment. The structure of 4 reveals the presence of two Ni5 and Te5 tetragonal pyramids stacked into each other in an opposite way. Six of the Te bridges form two triangular faces, each of which is capped by a Cp′Nb fragment. Additionally, two dppm ligands bridge the long Ni–Ni edges of the Ni4 bases. Oxidation of 3 with [(C5H5)2Fe]BPh4 gives paramagnetic [(Cp*2Nb)2NiTe4]BPh4 (6), whereas oxidation with [(C5H5)2Fe]PF6 proceeds with fluoride transfer to give [Cp*2NbF2]PF6 (5). Electrochemical studies of 3 reveal the existence of the redox couples [3]2+/[3]+/[3]0/[3]–/[3]2–. Density functional calculations carried out on the different oxidation states of 3 reveal the following main features: the persistent presence of Nb–Ni bonds and the retention of Te–Te bonds in cationic and neutral species and their breaking off in reduced forms. Irrespective of the initial site of electrochemical attack, the Te atoms and Cp rings are principally affected in the final products.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)