Thorium(IV) and Uranium(IV) trans-calix[2]benzene[2]pyrrolide Alkyl and Alkynyl Complexes: Synthesis, Reactivity and Electronic Structure

The first thorium(IV) and uranium(IV) hydrocarbyl complexes of a trans-calix[2]benzene[2]pyrrolide macrocycle can use ligand noninnocence to enable multiple C–H bond activation reactions at the metal. Both alkyl and alkynyl complexes supported by the L dianion and L–2H tetraanion are reported. The ThIV and UIV monoalkyl-ate complexes [M(L–2H)An(R)] (M = K for R = CH2Ph, M = Li for R = Me, CH2SiMe3), in which the ligand aryl groups are metalated, add C–H bonds of terminal alkynes across the metal and ligand, forming the AnIV-alkynyl complexes [(L)An(C≡CR′)2] (R′ = SiMe3, SiiPr3). This ligand reprotonation from (L–2H)4– to (L)2– is accompanied by a change in coordination mode of the ligand from η5:η1:η5:η1 to η5:η5. Alternatively, the original alkyl group can be retained if the ligand is reprotonated using [Et3NH][BPh4], affording the ThIV cations [(L)Th(R)][BPh4] (R = CH2Ph, N(SiMe3)2). Here, ligand rearrangement to the κ1:η6:κ1:η6 coordination mode occurs. These complexes provide rare examples of bis(arene) actinide sandwich geometry. The two η1-alkynides in [(L)Th(C≡CSiMe3)2] rearrange upon coordination of [Ni0], forming [(L)Th(C≡CSiMe3)2·Ni(PR″3)] (R″ = phenyl, cyclohexyl), featuring the shortest yet reported distance between Th and Ni and giving unprecedented insight into the changes in macrocyclic ligand coordination between κ1:η6:κ1:η6 and η5:η5 coordination modes. A computational study of this conformational change demonstrates the η5:η5 coordination mode to be more stable in the Th/Ni bimetallics (and hypothetical Pt analogues), an observation rationalized by detailed analysis of the Kohn–Sham orbital structure of the κ1:η6:κ1:η6 and η5:η5 conformers. Although remarkably inert to even high pressures of CO2 at room temperature, the bis(alkynyl) complexes [(L)An(C≡CSiMe3)2] completely cleave one CO bond of CO2 when they are heated under 1 bar pressure, resulting in the formation, and elimination from the metal, of a new, CO-inserted, bicyclic, carbonylated macrocycle with complete control over the C–C and C–N bond forming reactions.