First 17−18−19-Electron Triads of Stable Isostructural Organometallic Complexes. The 17-Electron Complexes [Fe(C5R5)(arene)]2+ (R = H or Me), a Novel Family of Strong Oxidants:  Isolation, Characterization, Electronic Structure, and Redox Properties (original) (raw)

1998, Journal of the American Chemical Society

The 18-electron complexes [M II (C 5 R 5)(arene)] + (M) Fe: R) H or Me, arene) C 6 H 6-n Me n (n) 0-6), C 6 H 5 NMe 2 , or C 6 Me 5 NH 2 ; M) Ru: R) Me, arene) C 6 Me 6) are oxidized to M III complexes between 0.92 and 1.70 V vs [FeCp 2 ] according to a single-electron process that is reversible in SO 2 if at least one of the rings is permethylated. The dinuclear complex [Fe II 2 (fulvalenyl)(C 6 Me 6)][PF 6 ] 2 is oxidized in two one-electron reversible waves in SO 2 separated by 0.38 V to the mixed-valence species trication and to the 34-electron dioxidized tetracation. Stoichiometric oxidation of the yellow complexes [Fe II Cp*(arene)][EX 6 ] (EX 6) PF 6 or SbCl 6) is achieved by using SbCl 5 in CH 2 Cl 2 at 20°C or SbF 5 in SO 2 at-10°C or by Br 2 + [Ag][SbF 6 ] and gives the purple 17-electron complexes [Fe III Cp*(arene)][SbX 6 ] 2 (X) F or Cl) if arene) hexa-, penta-, and 1,2,4,5-tetramethylbenzene. No oxidation is observed for complexes of less methylated arene ligands, which shows that the oxidation power of SbX 5 is limited to 1.0 V Vs [FeCp 2 ] for monocations. The complex [Fe III Cp*(C 6 Me 6)][SbCl 6 ] 2 , 1[SbCl 6 ] 2 , is also obtained by SbCl 5 oxidation of the 19-electron complex [Fe I Cp*(C 6 Me 6)], 1, at-80°C. The 17-electron complexes are characterized by elemental analyses, ESR, Mössbauer, and UV/vis spectra, magnetic susceptibility, cyclic voltammetry, and quantitative singleelectron reduction by ferrocene. The complex 1[SbCl 6 ] 2 is used as a very strong single-electron oxidant to also oxidize [Ru(bpy) 3 ][PF 6 ] 2 to the 17-electron Ru III species and the neutral cluster [FeCp(µ 3-CO)] 4 to its mono-and dications. The complex [Fe II Cp(C 6 Me 6)][PF 6 ] is a redox catalyst for the anodic oxidation of furfural on Pt in SO 2 via the Fe II /Fe III redox system. Density functional theory (DFT) calculations on various 17electron compounds [Fe(C 5 R 5)(C 6 R 6)] +/2+ (R) H, Me) and [FeCp(C 6 H 5 NH 2)] 2+ , as well as on the isoelectronic complexes ferrocenium and [Fe(C 6 H 6) 2 ] 3+ and their 18-electron parents, allowed a detailed comparison of the electronic structure, bonding, UV-visible spectra, and ionization potentials of these species. Although the nature of the HOMO is not always the same within the series of their 18-electron parents, all the computed 17-electron complexes have the same 2 E 2 ground state corresponding to the metallic (a 1) 2 (e 2) 3 electron configuration. Full geometry optimizations lead to the prediction of their molecular structures for the lowest 2 E 2 and 1 A 1 states.