Elemental Carbon Chain Bridging Two Iron Centers:  Syntheses and Spectroscopic Properties of Donor−Acceptor [Fe]−C₄−[Fe] Complexes Isolated in Two Different Oxidation States. X-ray Crystal Structure of [Cp*(dppe)Fe−C₄−Fe(CO)₂Cp*]

Abstract

The complexes Fe(η 5 -C 5 R 5 )(CO) 2 (C⋮CC⋮CSiMe 3 ) (R = Ph, 2a; R = Me, 2b ) were obtained from the reaction of (η 5 -C 5 Ph 5 )Fe(CO) 2 Br ( 1a ) or (Cp*)Fe(CO) 2 I (Cp* = η 5 -C 5 Me 5, 1b ) with Me 3 SiC⋮CC⋮CLi (1.05 equiv) in THF at −80 °C (70−75%). The butadiynyl complex (η 5 -C 5 Me 5 )Fe(dppe)(C⋮CC⋮CSiMe 3 ) ( 2c ) was prepared upon photolysis of 2b in a toluene/acetonitrile mixture (95/5 v/v) in the presence of dppe (1,2-bis(diphenylphosphino)ethane) (85%). Treatment of complexes 2a, 2b with 1.1 equiv of potassium fluoride in a 50/50 CH 3 OH/THF mixture at 20 °C afforded the terminal butadiyne complex (η 5 -C 5 R 5 )Fe(CO) 2 (C⋮CC⋮CH) (R = Ph, 3a; R = Me, 3b ), isolated in 85 and 95% yields respectively. Complex 3c was obtained by reaction of 2c with 0.2 equiv of Bu n 4 NF in THF (95%). The mononuclear complexes 2a − c and 3a − c were characterized by cyclic voltammetry and IR, 1 H, 13 C, and 31 P NMR, and Mössbauer spectroscopy. The binuclear complexes 5 a,b were obtained (59−69%) in a one-step procedure by treatment of (η 5 -C 5 R 5 )Fe(CO) 2 (C⋮CC⋮CH) ( 3 a,b ) with 1 equiv of Cp*Fe(dppe)Cl ( 4 ) in the presence of KPF 6 and KOBu t in methanol. The lower IR frequencies of the carbonyl groups in 5 a,b relative to the isolated acceptor group in 2 a,b and 3 a,b are indicative of the electronic communication between the metal centers through the −C 4 − spacer. In the complexes 5 a,b, the polarization of the spacer was shown by the 13 C NMR chemical shifts of the carbon of the −C 4 − chain. The Mössbauer spectrum of 5b establishes that the electron density at the two iron atoms is quite different. The structure of 5b has been determined by X-ray diffraction. Cyclic voltammograms of complexes 5 a,b from −0.8 to −1.4 V/SCE displayed one fully reversible wave and a second one almost reversible, showing that the binuclear systems undergo two successive one-electron oxidations at the electrode. Comparison of both redox potentials and current ratio with those of the corresponding monomers 2a − c demonstrates that a strong electronic communication between the metal centers takes place across the all-carbon bridge. Treatment of the neutral complexes with 1 equiv of ferrocenium allowed isolation of the salts [5 a,b ][PF 6 ] in 85−90% yield. Mössbauer spectroscopy showed that both salts were trapped Fe(II)-Fe(III) mixed-valence compounds. The g tensors and couplings with the 31 P nuclei are very close for both Fe(III) low-spin radicals. Intense absorption bands were observed at 810 and 829 nm for [5a][PF 6 ] and [5b][PF 6 ], respectively. A second absorption band was observed in the near-infrared for [5b][PF 6 ] (1600 nm) and ascribed to an ICT band. It allowed determination of the electronic coupling between the electron-poor and the electron-rich iron centers through the all-carbon spacer ( V ab = 0.021 eV).