Thorium(IV) and Uranium(IV) trans-Calix[2]benzene[2]pyrrolide Alkyl and Alkynyl Complexes: Synthesis, Reactivity, and Electronic Structure
Suvova, M; O'Brien, KTP; Famaby, JoyH; Love, JB; Kaltsoyannis, N; Arnold, PL
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 CH 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 CH bonds of terminal alkynes across the metal and ligand, forming the An(IV)-alkynyl complexes [(L)An(C equivalent to CR')(2)] (R' = SiMe3, SiPr3). This ligand reprotonation from (L-2H)(4-) to (L)(2-) is accompanied by a change in coordination mode of the ligand from eta(5):eta(6):kappa(1):eta(6) to eta(5):eta(5). 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 kappa(1):eta(6):kappa(1):eta(6) coordination mode occurs. These complexes provide rare examples of bis(arene) actinide sandwich geometry. The two eta(1)-alkynides in [(L)Th(C equivalent to CSiMe3)(2)] rearrange upon coordination of [Ni-0], forming [(L)Th(C equivalent to CSiMe3)(2)center dot 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 kappa(1):eta(6):kappa(1):eta(6) and eta(5):eta(5) coordination modes. A computational study of this conformational change demonstrates the eta(5):eta(5) 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 kappa(1):eta(6):kappa(1):eta(6) and eta(5):eta(5) conformers. Although remarkably inert to even high pressures of CO2 at room temperature, the bis(alkynyl) complexes [(L)An(C equivalent to 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.