Electrophilic Attack on the Carbon Terminus of a Niobium-Bound Formaldehyde Ligand

The niobocene formaldehyde complex Cp‘2Nb(Cl)(η2-CH2O) (1, Cp‘ = η5-C5H4SiMe3) reacts with [Ph3C][BF4] to give an alkoxide (2) of formula [Cp‘2Nb(Cl)(OCH2CPh3)][BF4] and with HBF4·OEt2 to give analogue 4 ([Cp‘2Nb(Cl)(OMe)][BF4]); these processes constitute examples of electrophilic attack on the formaldehyde carbon, instead of the more typical addition at oxygen. Compound 2 is hydrolyzed to Cp‘2Nb(Cl)(OBF3) (3), a Lewis acid adduct of the corresponding metal oxide; both 2 and 3 have been characterized by X-ray crystallography.