Synthesis, properties and structures of the tris(cyclopentadienyl)thorium(III) complexes [Th{η5-C5H3(SiMe2R)2-1,3}3] (R=Me or tBu)

The homoleptic, dark-blue, crystalline (disubstituted-cyclopentadienyl)thorium(III) complexes [Th{η5-C5H3(SiMe2R)2-1,3}3] (R=Me 1 or tBu 3) were obtained in good yield from the appropriate tris(cyclopentadienyl)thorium(IV) chloride by treatment with an excess of sodium–potassium alloy in toluene at 20–35 °C with sonication. Complex 1 was also accessible by a similar reduction of [ThCp″2Cl2] [Cp″=η5-C5H3(SiMe3)2-1,3]; with possibly a transient Th(II) cyclopentadienyl as intermediate. Complex 1 with wet toluene afforded [(ThCp″3)2(μ-O)] (2); it was unreactive with THF, DME, CO or H2, but with Na–K alloy in THF it afforded a green, diamagnetic, readily oxidisable cyclopentadienylthorium complex. Complexes 1 and 3 showed singlet EPR spectra in methylcyclohexane at 298 K, whence it is concluded that they have a 6d1 rather than a 5f1 electronic ground state. Variable temperature EPR spectra, magnetic measurements and optical spectra of 1 are described and discussed. The X-ray structures of crystalline 1 and 3 show that each has approximate D3h symmetry about the Th atom, which can be regarded as bound to the centroid of each of the three cyclopentadienyl rings.