Synthesis of a terminal Ce(iv) oxo complex by photolysis of a Ce(iii) nitrate complex

Reaction of [Ce(NR₂)₃] (R = SiMe₃) with LiNO₃ in THF, in the presence of 2,2,2-cryptand, results in the formation of the Ce(iii) "ate" complex, [Li(2,2,2-cryptand)][Ce(κ²-O₂NO)(NR₂)₃] (1) in 38% yield. Photolysis of 1 at 380 nm affords [Li(2,2,2-cryptand)][Ce(O)(NR₂)₃] (2), in 33% isolated yield after reaction work-up. Complex 2 is the first reported example of a Ce(iv) oxo complex where the oxo ligand is not supported by hydrogen bonding or alkali metal coordination. Also formed during photolysis are [Li(2,2,2-cryptand)]₂[(μ₃-O){Ce(μ-O)(NR₂)₂}₃] (3) and [Li(2,2,2-cryptand)][Ce(OSiMe₃)(NR₂)₃] (4). Their identities were confirmed by X-ray crystallography. Complex 4 can also be prepared via reaction of [Ce(NR₂)₃] with LiOSiMe₃ in THF, in the presence of 2,2,2-cryptand. When synthesized in this fashion, 4 can be isolated in 47% yield. To rationalize the presence of 2, 3, and 4 in the reaction mixture, we propose that photolysis of 1 first generates 2 and NO₂, via homolytic cleavage of the N-O bond in its nitrate co-ligand. Complex 2 then undergoes decomposition via two separate routes: (1) ligand scrambling and oligomerization to form 3; and, (2) abstraction of a trimethylsilyl cation to form a transient Ce(iv) silyloxide, [Ce^IV(OSiMe₃)(NR₂)₃], followed by 1e^- reduction to form 4. Alternatively, complex 4 could form directly via ·SiMe₃ abstraction by 2.