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

3 years ago

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

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

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 Th^IV and U^IV 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 An^IV-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 Th^IV 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.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00633

DOI: 10.1021/acs.organomet.7b00633