Ligand Substituent Effect Observed for Ytterbocene 4‘-Cyano-2,2‘:6‘,2‘ ‘-terpyridine

A new N-heterocyclic complex of ytterbocene (Cp*2YbII, Cp* = C5Me5) has been prepared by the addition of
4‘-cyano-2,2‘:6‘,2‘ ‘-terpyridine (tpyCN) to Cp*2YbII(OEt2) in toluene to give a dark blue species designated as
Cp*2Yb(tpyCN). The effect of the electron-withdrawing group (−C⋮N) on the redox potentials of the charge-transfer
form of this complex [in which an electron is transferred from the f 14 metal center to the lowest unoccupied (π*)
molecular orbital of the tpyCN ligand to give a 4f13−π*1 electronic configuration] has been quantified by cyclic
voltammetry. The tpyCN ligand stabilizes this configuration by 60 mV more than that in the unsubstituted tpy ligand
complex and by 110 mV more than that in the unsubstituted bpy ligand complex. Magnetic susceptibility measurements
corroborate the enhanced stabilization of the 4f13−π*1 configuration by the substituted terpyridyl ligand complex.
Furthermore, the temperature dependence of the magnetic data is most consistent with a thermally induced valence
tautomeric equilibrium between this paramagnetic 4f13−π*1 form that dominates near room temperature and the
diamagnetic 4f14−π*0 form that dominates at low temperature. Differing coordination modes for the tpyCN ligand to
the ytterbocene center have also been confirmed by isolation and X-ray crystallographic characterization of complexes
binding through either the cyano nitrogen of tpyCN or the three terpyridyl nitrogen atoms of tpyCN.