Uncovering a CF3 Effect on X-ray Absorption Energies of [Cu(CF3)4]− and Related Cu Compounds Using Resonant Diffraction Anomalous Fine Structure (DAFS) Measurements

Understanding the electronic structures of high-valent metal complexes aids the advancement of metal-catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF 3 ) 4 ] − ( 1 ), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of 1 by X-ray spectroscopies have led previous authors to contradictory conclusions, motivating the re-examination of its X-ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS). From analysis of DAFS measurements for a series of seven mononuclear Cu complexes including 1 , here it is shown that there is a systematic trifluoromethyl effect on X-ray absorption that blue shifts the resonant Cu K-edge energy by 2-3 eV per CF 3 , completely accounting for observed changes in DAFS profiles between formally Cu(III) complexes like 1 and formally Cu(I) complexes like (Ph 3 P) 3 CuCF 3 ( 3 ). Thus, in agreement with the inverted ligand field model, the data presented herein imply that 1 is best described as containing a Cu(I) ion with d n count approaching 10. The electronic structures of [Cu(CF 3 ) 4 ] − and other Cu-CF 3 complexes are (re-)evaluated using resonant X-ray diffraction methods. A systematic trend was identified by which each additional CF 3 ligand shifts the resonant Cu K-edge energy by 2-3 eV, completely accounting for observed differences between complexes with different formal oxidation states. The recently debated assignment of [Cu(CF 3 ) 4 ] − as containing a Cu(I) ion is, thus, verified.