Ring-opening of a furyl group appended to the cyclopentadienyl ligand in rare-earth metal half-sandwich complexes

A series of scandium, lutetium, and yttrium complexes containing a tetramethylcyclopentadienyl ligand with a pendant furyl group, [Ln{eta(5)-C5Me4SiMe2(C4H2RO-2)}(CH2SiMe3)(2)(THF)] (Ln = Sc, Lu, Y; R = H, Me), have been synthesized and structurally characterized. Single-crystal X-ray diffraction studies of the scandium complexes [Sc{eta(5)-C5Me4SiMe2(C4H2RO-2)}(CH2SiMe3)(2)(THF)] (R = H, Me) revealed a pseudo-four-coordinate metal center without additional coordination of the furyl group. The previously elusive lutetium complex [Lu{eta(5)-C5Me4SiMe2(C4H2MeO-2)}(CH2SiMe3)(2)(THF)] could be isolated and exhibited structural features analogous to those of the scandium compound. Variable-temperature H-1 NMR spectroscopic studies of the entire series confirmed the labile nature of both the THF ligands and furyl donor in solution. Rates for the ring-opening reaction, triggered by intramolecular C-H bond activation, were shown to depend primarily on the metal size. The ring-opening reaction resulted in the formation of the corresponding dinuclear yne-enolate complexes [Ln{eta(5):eta(1)-C5Me4SiMe2(CCCHCRO)}(CH2SiMe3)](2) (Ln = Sc, Lu, Y; R = H, Me). Substitution at the 5-furyl position did not markedly influence the rate of the ring-opening reaction but shifted the position of the monomer-dimer equilibrium toward the monomer. Only a large excess of THF allowed the observation of monomeric yne-enolate complexes such as [Y{eta(5):eta(1)-C5Me4SiMe2(CCCHCHO)}(CH2SiMe3)(THF)(2)], which was characterized by X-ray crystallography. Addition of THF increased the stability of the lutetium bis(alkyl) complexes and allowed the formation of the corresponding cationic compounds by treatment with [NEt3H][BPh4] to yield [Lu{eta(5):eta(1)-C5Me4SiMe2(C4H2RO-2)}(CH2SiMe3)(THF)(n)][BPh4] (Ln = Lu; R = H, Me), which resisted ring- opening of the furyl group.