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

Markéta Suvova, Kieran T. P. O’brien, Joy H. Farnaby, Jason B. Love, Nikolas Kaltsoyannis, Polly L. Arnold

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The first thorium(IV) and uranium(IV) hydrocarbyl complexes of a trans-calix[2]benzene[2]pyrrolide macrocycle can use ligand non-innocence 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 mono-alkyl –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 metallated, add the 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:η5 to η5:η1:η5:η1. 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). Again, 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 the more stable in the Th/Ni bimetallics (and hypothetical Pt analogues), an observation rationalised 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 heated under a 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.
Original languageEnglish
Early online date16 Oct 2017
Publication statusE-pub ahead of print - 16 Oct 2017

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