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

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[triple bond, length as m-dash]C-SiMe₃)] (1-Ce^TMS) and [Ce(TriNOx)(C[triple bond, length as m-dash]C-Ph)] (1-Ce^Ph) (TriNOx^3- = <i>tris</i>(2-<i>tert</i>-butylhydroxylaminato)benzylamine), that include terminal alkyne moieties. The isostructural thorium analogue [Th(TriNOx)(C[triple bond, length as m-dash]C-SiMe₃)] (1-Th^TMS) was also synthesized and compared with 1-Ce^TMS in bond distance, ¹³C-NMR spectra, vibrational spectra and electronic structure. The Ce-C bond distances were 2.501(3) Å for 1-Ce^Ph and 2.513(5) Å for 1-Ce^TMS 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 <i>ν</i> _{C[triple bond, length as m-dash]C} stretching frequencies of 2000 cm^-1 (1-Ce^TMS) and 2052 cm^-1 (1-Ce^Ph), indicating the cerium(iv)-alkynyl interaction, compared to the parent HC[triple bond, length as m-dash]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 <i>E</i> _pc = -1.58 to -1.66 V <i>vs.</i> Fc/Fc⁺. Evidence for a <i>trans</i>-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[triple bond, length as m-dash]C-Ph, ^-C[triple bond, length as m-dash]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 <i>trans</i>-influence at cerium(iv) and point to differences in involvement of cerium 4f- <i>versus</i> 5d-orbitals in the electronic structures of the complexes.