Tetra(tert-butyl)zirconium: synthesis, characterisation and reaction with carbon dioxide

The reaction of zirconium tetrachloride with t-butyllithium in diethyl ether at −78°C gives the very unstable molecule (t-Bu)4Zr. This product was characterised by 13C and 91Zr NMR spectroscopy and could be partially stabilised as its bis-pyridine adduct. Treatment of (t-Bu)4Zr with carbon dioxide at −78°C followed by acid hydrolysis gives predominantly the ketone, (t-Bu)2CO. In contrast, other R4Zr complexes (R = benzyl or allyl) under the same conditions yield equimolar amounts of the carbinol R3COH and carboxylic acid RCO2H almost quantitatively. The reaction of CO2 with a mixture of R4Zr complexes did not yield any cross-over products suggesting that an intramolecular mechanism is operative. A mechanistic scheme is proposed to account for all these observations.