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 t he diamagnetic heterodinuclear cyclo-C-8 bridged FeCo complexes [{(CpF eXCpCo)}mu-(eta'-C8H8R)] (R = CH(CO2Et)2: 3; R = H: 4a) which are them ally labile. For 3 the substituent R = CH(CO2Et)2 is thermally cleaved with increasing temperature yielding the paramagnetic ESR active 35 v alence electron (ve) compound [{(CpFe)(CpCO)}mu-Cot] (5). The ESR data of 5 indicate a cobalt localized unpaired electron, 5 is easily oxidi zed 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 lon g Fe-Co distance of about 287(1) pm which is assumed to be too long fo r 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 unpaire d electron, can be got from in situ ESR measurements. The thermal labi lity of 4a leads irreversibly to the isomeric complex 4b at elevated t emperature. The activation energy DELTAE(a) of this molecular transfor mation was estimated by H-1 NMR spectroscopy as 103 kJ/mol. Preliminar y crystal structure determinations of the high temperature isomer 4b c onfirm a synfacial 1,2,6,7-eta:3-5-eta bonding mode for the cyclooctat rienyl ligand. The cyclooctatrienyl ligand of the low temperature isom er 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 th e Fe centre should occur in 2-5-eta fashion. C(s) molecular symmetry f or 4a, which can be deduced from the H-1 NMR spectra, is explained by a low energy twitching motion of 4a.