Comparison of Ce(IV)/Th(IV)-Alkynyl Complexes, and Observation of a Trans-Influence Ligand Series for Ce(IV)

Abstract

of the cerium(IV) cation. Herein, we report the 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, 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 CeIV–C single bonds (2.478(3)-2.705(2) Å), and 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 νC≡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, 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 Epc = −1.58 to −1.66 V vs Fc/Fc+. Evidence for a trans-influence has been observed by evaluating a series including previously reported [CeIV(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.