Comparison of Ce(iv)/Th(iv)-alkynyl complexes and observation of a trans-influence ligand series for Ce(iv)

Organometallic cerium(iv) complexes have been challenging to isolate and characterize due to the strongly oxidizing nature of the cerium(iv) cation. Herein, we report two cerium(iv) alkynyl complexes, [Ce(TriNOx)(C = C-SiMe₃)] (1-CeTMS) and [Ce(TriNOx)(C C-Ph)] (1-CePh) (TriNOx³⁻ = tris(2-tert-butylhydroxylaminato)benzylamine), that include terminal alkyne moieties. The isostructural thorium analogue [Th(TriNOx)(C C-SiMe₃)] (1-ThTMS) was also synthesized and compared with 1-CeTMS in bond distance, ¹³C-NMR spectra, vibrational spectra and electronic structure. The Ce-C bond distances were 2.501(3) Å for 1-CePh and 2.513(5) Å for 1-CeTMS on the shorter end of the few reported Ce^IV-C single bonds (2.478(3)-2.705(2) Å), possibly indicating significant Ce 5d- and 4f-orbital involvement. ¹³C-NMR spectroscopy was also consistent with Ce-C covalency, with significantly deshielded resonances ranging from 185-213 ppm. Such ¹³C-NMR shifts demonstrate a strong influence from spin-orbit coupling (SOC) effects, corroborated by computational studies. Raman analysis showed ν_{C C} stretching frequencies of 2000 cm⁻¹ (1-CeTMS) and 2052 cm⁻¹ (1-CePh), indicating the cerium(iv)-alkynyl interaction, compared to the parent HC CPh (IR = 2105 cm⁻¹ and Raman = 2104 cm⁻¹). L₃-edge X-ray absorption measurements revealed a predominant Ce(iv) electronic configuration, and magnetic measurements revealed temperature-independent paramagnetism. Electrochemical studies similarly revealed the electron donating ability of the alkynyl ligands, stronger than either fluoride or imido ligands for the Ce(iv)(TriNOx)-framework, with a cerium(iv/iii) reduction potential of E_pc = −1.58 to −1.66 V vs. Fc/Fc⁺. Evidence for a trans-influence has been observed by evaluating a series including previously reported [Ce^IV(TriNOx)X]^+/0 complexes with axial ligands X = THF, I⁻, Br⁻, Cl⁻, F⁻⁻C C-Ph, ⁻C C-SiMe₃⁻NH(3,5-(CF₃)₂-Ar), ⁻OSiPh₃⁻N(M(L))(3,5-(CF₃)₂-Ar) [M(L) = Li(TMEDA), K(DME)₂ or Cs(2,2,2-crypt)]. These data stand in contrast with previous reports of an inverse trans-influence at cerium(iv) and point to differences in involvement of cerium 4f- versus 5d-orbitals in the electronic structures of the complexes.