[3]FERROCENOPHANE BRIDGE REVERSAL BARRIERS .3. ENERGIES OF TE2S, TE2SE AND TE3 BRIDGED COMPOUNDS, AND A CNDO/2 INVESTIGATION OF THE MECHANISM OF THE BRIDGE REVERSAL PROCESS

Dynamic NMR studies have shown that the energies of the bridge reversal fluxion in the [3]ferrocenophanes [Fe(C5H4Te)2E] (E = S, Se, Te) are, in terms of DELTA-G double-ended-dagger (298 K) data, 56.3, 55.4 and 51.8 kJ mol-1 respectively. These values, which are a function of total bridge length, are compared with values for the other trichalcogena-[3]ferrocenophanes. Relative magnitudes of torsional barriers about chalcogen-chalcogen bonds calculated from these data, showed the Te-Te torsion energy to be 1.7 kJ mol-1 lower than the Se-Te torsion and 2.2 kJ mol-1 lower than the S-Te torsion. The mechanism of the bridge reversal process was investigated by CNDO/2 calculations on [Fe(C5H4E)2E] (E = S, Se, Te), which showed that a transition state structure involving staggered Cp rings, akin to the half chair conformation of cyclohexane, is considerably more favoured than a structure with eclipsed Cp rings and a planar trichalcogen bridge.