ELECTRONIC INTERPLAY BETWEEN 2 IRON CENTERS ACROSS POLYAROMATIC LIGANDS - SYNTHESES, REDOX CHEMISTRY, AND ELECTRONIC-STRUCTURES OF THE ELECTRON-RESERVOIR 36- TO 38-ELECTRON COMPLEXES ECP-ASTERISK)(2)(MU(2)-ETA(12)-POLYAROMATIC)](Q-2) INCLUDING MIXED VALENCES AND BIRADICALS() (Q=0)

A new series of diiron polyaromatic complexes, )Cp)(2)(mu(2)-eta(12)- polyaromatic)](2+)(PF6-)(2) (Cp = eta(5)-C(5)Me(5); polyaromatic = di hydrophenanthrene, fluorene, diphenylmethane, fluorene, phenanthrene, triphenylmethane, pyrene, [2.2]paracyclophane), has been synthesized b y reaction of [FeCp(CO)(2)Br] with Al2Cl6 and Al(2)Cl(3)Me(3), avoidi ng hydrogenation of the polyaromatic ligand. One-electron and two-elec tron reductions have been achieved to investigate the electronic commu nication between the two iron atoms in the 37-electron (37e) mixed-val ence complexes and in the 38e complexes. The Cp (Cp* = eta(5)-C(5)Me( 5)) ligand stabilizes the 37e mixed-valence complexes which were synth esized by comproportionation between the 36e and 38e complexes. Mossba uer spectra show that these complexes are average valence on the Mossb auer time scale. The 38e complexes are thermally stable up to -20 degr ees C and were studied by H-1 NMR, ESR, and Mossbauer spectroscopy for phenanthrene, pyrene, and triphenylene. The absence of H-1 NMR spectr a in the diamagnetic region and the three g values around 2 in ESR are characteristic of 19e Fe-I complexes and indicate that these complexe s do not undergo intramolecular chemical coupling (formation of the ex ocyclic double bond), in contrast to the dihydrophenanthene and cyclop hane complexes, which are shown by H-1 NMR to be diamagnetic. The comp ared Mossbauer parameters of the 36e (FeFeII)-Fe-II, 37e (FeFeII)-Fe-I , and 38e (FeFeI)-Fe-I polyaromatic complexes mentioned above are very similar, indicating that the 37th and 38th electrons are in mainly po lyaromatic-based orbitals. The Mossbauer IS and QS values of the 38e c omplexes, however, are slightly higher than those of the 36e and 37e s eries, consistent with a slightly higher metal contribution. The cycli c voltammograms of the 36e polyaromatic complexes show a redox series of five oxidation states, contrasting with those of dihydrophenanthren e and [2.2]paracyclophane, which give only two waves because of intram olecular chemical coupling at the second reduction stage. Among the fo ur one-electron waves, only the last one is chemically and electrochem ically irreversible, showing the structural rearrangement in the cours e of the fourth reduction. SCF MS-X alpha calculations have been perfo rmed on the diiron pyrene species and were compared to similar calcula tions carried out on the related diphenyl system. The reduction of the 36e diphenyl complex leads to the occupation of its b(u) LUMO, which is significantly cc-bonding on the exocyclic bond, thus inducing chemi cal coupling. In the case of the 36e pyrene complex, the corresponding b(u) vacant orbital is not the LUMO and has a poorer pi-bonding chara cter on the C-C bond susceptible to be involved in the chemical coupli ng. Therefore, this b, level is not involved upon reduction. Instead, the a(u) LUMO and the next a(g) level are successively populated by th e incoming electrons, the latter being more metal-centered than the fo rmer.