COOPERATIVE EFFECTS IN PI-LIGAND BRIDGED DINUCLEAR COMPLEXES .15. HETERODINUCLEAR FE-CO COMPLEXES FROM MONONUCLEAR ETA(5)-CYCLOOCTATRIENYL IRON COMPLEXES

The reaction of CpFe(1.5-eta-C8H8-8-exo-R) (R = CH(CO2Et)2: 2a; R = H: 2b) with CpCo(C2H4)2 at T less-than-or-equal-to 40-degrees-C yields the diamagnetic heterodinuclear cyclo-C-8 bridged FeCo complexes [{(CpFeXCpCo)}mu-(eta'-C8H8R)] (R = CH(CO2Et)2: 3; R = H: 4a) which are themally labile. For 3 the substituent R = CH(CO2Et)2 is thermally cleaved with increasing temperature yielding the paramagnetic ESR active 35 valence electron (ve) compound [{(CpFe)(CpCO)}mu-Cot] (5). The ESR data of 5 indicate a cobalt localized unpaired electron, 5 is easily oxidized by O2, [Ph3C]BF4 or [FeCP2]PF6 yielding the stable diamagnetic 34 ve cation [{(CpFe)(CpCo)}mu-Cot] + (5+). Crystal structure analysis of 5BF4 establishes a synfacial eta5 : eta5 coordination with a very long Fe-Co distance of about 287(1) pm which is assumed to be too long for a Fe-Co single bond. The cyclic voltammetry study of 5+ reveals two chemically as well as electrochemically reversible redox couples 5/5+, 5+/5(2+) and one quasi-reversible redox couple 5-15. Evidence for the formation of the radical dication 52, containing a Co-centred unpaired electron, can be got from in situ ESR measurements. The thermal lability of 4a leads irreversibly to the isomeric complex 4b at elevated temperature. The activation energy DELTAE(a) of this molecular transformation was estimated by H-1 NMR spectroscopy as 103 kJ/mol. Preliminary crystal structure determinations of the high temperature isomer 4b confirm a synfacial 1,2,6,7-eta:3-5-eta bonding mode for the cyclooctatrienyl ligand. The cyclooctatrienyl ligand of the low temperature isomer 4a is assumed to coordinate via a 1-eta:6,7-eta-bonding mode to the Co centre whereas the coordination mode of the cyclo-C-8 ligand to the Fe centre should occur in 2-5-eta fashion. C(s) molecular symmetry for 4a, which can be deduced from the H-1 NMR spectra, is explained by a low energy twitching motion of 4a.