Cationic Thorium Alkyl Complexes of Rigid NON- and NNN-Donor Ligands: π-Arene Coordination as a Persistent Structural Motif

Abstract

Reaction of the neutral dialkyl complex [(XA2)Th(CH2SiMe3)2] {1; XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene} with [CPh3][B(C6F5)4] in benzene or toluene at room temperature resulted in the formation of [(XA2)Th(CH2SiMe3)(η6-arene)][B(C6F5)4] {arene = C6H6 (5) and toluene (5B)}, which were characterized by 1H, 13C, and 2D NMR spectroscopy and by X-ray crystallography (for 5). In close analogy, reaction of [(XA2)Th(CH2Ph)2] (3) with [CPh3][B(C6F5)4] in toluene at room temperature resulted in the formation of 1 equiv of Ph3CCH2R (R = Ph) and precipitation of an insoluble orange-brown oil, which upon layering with hexanes yielded crystals of [(XA2)Th(η2-CH2Ph)(η6-C6H5Me)][B(C6F5)4] (6). NMR investigation of the reactions of 1 and 3 with substoichiometric amounts of [CPh3][B(C6F5)4] provided no evidence for dinuclear monocation formation. By contrast, reaction of [(BDPP)Th(CH2Ph)2] {4; BDPP = 2,6-bis(2,6-diisopropylanilidomethyl)pyridine} with 0.5 equiv of [CPh3][B(C6F5)4] resulted in the precipitation of an insoluble oil containing a mixture of a mononuclear and a dinuclear cation; the dinuclear cation was identified as [(BDPP)Th(η2-CH2Ph)(μ-η1:η6-CH2Ph)Th(η1-CH2Ph)(BDPP)][B(C6F5)4] (7) by X-ray crystallography. Reaction of complex 3 with B(C6F5)3 resulted in the formation of [(XA2)Th(η1-CH2Ph)][η6-PhCH2B(C6F5)3] (9), which was characterized in solution by NMR spectroscopy. Complexes 5, 5B, and 6 are rare examples of arene solvent-separated ion pairs, complex 9 exists as a tight contact ion pair, and dinuclear 7 exhibits a unique benzyl ligand bridging mode. The structures of cations 5−7 and 9 highlight a pronounced tendency for these systems to engage in arene π-coordination.