Reduction of Thorium Tris(amido)arene Complexes: Reversible Double and Single C–C Couplings

The reduction chemistry of thorium complexes is less explored compared to that of their uranium counterparts. Here, we report the synthesis, characterization, and reduction chemistry of two thorium­(IV) complexes, (AdTPBN3)­ThCl (1) and (DtbpTPBN3)­ThCl­(THF) (4) [RTPBN3 = 1,3,5-[2-(RN)­C6H4]3C6H3; R = 1-adamantyl (Ad) or 3,5-di-tert-butylphenyl (Dtbp); THF = tetrahydrofuran], supported by tripodal tris­(amido)­arene ligands with different N-substituents. Reduction of 1 with excessive potassium in n-pentane yielded a double C–C coupling product, [(AdTPBN3)­ThK­(Et2O)2]2 (3), featuring a unique tetraanionic tricyclic core. On the other hand, reduction of 4 with 1 equiv of KC8 in hexanes/1,2-dimethoxyethane (DME) afforded a single C–C coupling product, [(DtbpTPBN3)­Th­(DME)]2 (5), with a dianionic bis­(cyclohexadienyl) core. The solid- and solution-state structures of dinuclear thorium­(IV) complexes 3 and 5 were established by X-ray crystallography and NMR spectroscopy. In addition, reactivity studies show that 3 and 5 can behave as thorium­(II) and thorium­(III) synthons to reduce organic halides. For instance, 3 and 5 are able to reduce 4 and 2 equiv of benzyl chloride, respectively, to regenerate 1 and 4 with concomitant formation of dibenzyl. Reversible C–C couplings under redox conditions provide an alternative approach to exploiting the potential of thorium arene complexes in redox chemistry.