Electronic Structure Tuning of Lanthanidocene Photocatalysts for C-F Bond Cleavage

A set of nine new robust, tunable cerium complexes supported by an ansa-bis(cyclopentadienyl) ligand, [Me₂Si(η⁵-Cp^R)₂]CeX (^anCp^R)CeX, are excellent homogeneous visible-light photocatalysts for the monodefluoroalkylation of trifluorotoluene with Mg(CH₂C₆H₅)₂THF₂ (R = Me₄, SiMe₃, X = N(SiMe₃)₂ (N″), X = CH(SiMe₃)₂ (R''), Cl, OC₆H₂^tBu₂-2,6,Me-4 (OAr)). The trends in photocatalytic activity within the series are explained by photophysical spectroscopic analyses. The aryloxide complex [Me₂Si(Cp^SiMe3)₂]CeOAr, which has the highest activity (95% substrate conversion in 27 h), shows the most negative (most reducing) excited-state reduction potential (-2.71 V vs Fc). The precatalyst excited-state lifetimes are also exceptionally long. Detailed photoluminescence, NMR spectroscopic, and kinetic studies on chloride [Me₂Si(Cp^Me4)₂]CeCl suggest that the "ate" complex [{Me₂Si(Cp^Me4)₂}₂Ce^IIIClBn][MgBn] is the active catalyst in the alkylation reaction to form PhCF₂CH₂Ph with high selectivity over PhCF₂H. The reaction rates are up to 30 times higher than previously reported for organometallic rare-earth photocatalysts for these Ce complexes and comparable to established Ir-based photoredox systems.