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*]

The complexes Fe(eta(5)-C5R5)(CO)(2)(C=CC=CSiMe3) (R = Ph, 2a; R = Me, 2b) were obtained from the reaction of (eta(5)-C5Ph5)Fe(CO)(2)Br (1a) or (Cp*)Fe(CO)(2)I (Cp* = eta(5)-C5Me5, 1b) with Me3SiC=CC=CLi (1.05 equiv) in THF at -80 degrees C (70-75%). The butadiynyl complex (eta(5)-C5Me5)Fe(dppe)(C=CC=CSiMe3) (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 CH3-OH/THF mixture at 20 degrees C afforded the terminal butadiyne complex (eta(5)-C5R5)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 (Bu4NF)-N-n in THF (95%). The mononuclear complexes 2a-c and 3a-c were characterized by cyclic voltammetry and IR, H-1, C-13, and P-31 NMR, and Mossbauer spectroscopy. The binuclear complexes 5a,b were obtained (59-69%) in a one-step procedure by treatment of (eta(5)-C5R5)Fe(CO)(2)(C=CC=CH) (3a,b) with 1 equiv of Cp*Fe(dppe)Cl (4) in the presence of KPF6 and KOBut in methanol. The lower IR frequencies of the carbonyl groups in 5a,b relative to the isolated acceptor group in 2a,b and 3a,b are indicative of the electronic communication between the metal centers through the -C-4- spacer. In the complexes 5a,b, the polarization of the spacer was shown by the C-13 NMR chemical shifts of the carbon of the -C-4- chain. The Mossbauer 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 5a,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 [5a,b] [PF6] in 85-90% yield. Mossbauer spectroscopy showed that both salts were trapped Fe(II)-Fe(III) mixed-valence compounds. The g tensors and couplings with the P-31 nuclei are very close for both Fe(III) low-spin radicals. Intense absorption bands were observed at 810 and 829 nm for [5a][PF6] and [5b][PF6], respectively. A second absorption band was observed in the near-infrared for [5b][PF6] (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).