New supporting ligands in actinide chemistry: tetramethyltetraazaannulene complexes with thorium and uranium†
We report the synthesis, characterization, and preliminary reactivity of new heteroleptic thorium and uranium complexes supported by the macrocyclic TMTAA ligand (TMTAA = Tetramethyl-tetra-aza-annulene). The dihalide complexes Th(TMTAA)Cl2(THF)2 (1), [UCl2(TMTAA)]2 (2) and U(TMTAA)I2 (3) are further functionalized to the Cp* derivatives ThCp*(TMTAA)Cl (4), UCp*(TMTAA)Cl (5) and UCp*(TMTAA)I (6) (Cp* = pentamethylcyclopentadienide). Compounds 4–6 are also obtained through a one-pot reaction from standard thorium(IV) and uranium(IV) starting materials, Li2TMTAA and KCp*. Complexes 1–6 function as valuable starting materials for salt metathesis chemistry. Treatment of precursors 4 or 5 with trimethylsilylmethyllithium (LiCH2TMS) results in the new actinide TMTAA alkyl complexes ThCp*(TMTAA)(CH2TMS) (7) and UCp*(TMTAA)(CH2TMTS) (8), respectively. The TMTAA-derived alkyl complexes (7 and 8) show unexpected stability and are stable for several weeks at room temperature in solution and in the solid-state. Additionally, double substitution of the halide ligands in 1–3 shows a strong dependence on the nucleophile used. While weaker nucleophiles, such as amides, and more sterically demanding nucleophiles, such as Cp (Cp = cyclopenadienide), favour the formation of bis-TMTAA “sandwich” complexes [An(TMTAA)2] (An = Th (9) and An = U (10)), the use of oxygen-functionalized ligands like the ODipp anion (Dipp = diisopropylphenyl) results in the formation of the doubly substituted species Th(ODipp)2TMTAA (11) and U(ODipp)2TMTAA (12). We also describe the divergent reactivity of the TMTAA ligand towards uranium(III). Unlike the syntheses of actinide(IV) TMTAA complexes, the synthesis of a uranium(III) TMTAA was not successful and only uranium(IV) species could be obtained.