Cycloheptatrienyl−Cyclopentadienyl−Zirconium Sandwich Complexes: Structure and Bonding

The reactions of the 16-electron cycloheptatrienyl−cyclopentadienyl−zirconium sandwich complex [(η 7 -C 7 H 7 )Zr(η 5 -C 5 H 5 )] ( 1 ) with tert- butyl isocyanide ( t BuNC) and 2,6-dimethylphenyl isocyanide ( o -XyNC) have been studied. The isocyanide complexes [(η 7 -C 7 H 7 )(η 5 -C 5 H 5 )Zr(CNR)] (R = t Bu, 2a; R = o -Xy, 2b ) can be isolated in quantitative yield as stable orange ( 2a ) or red ( 2b ) crystalline solids. The CN stretching vibrations are observed at 2156 ( 2a ) and 2134 cm - 1 ( 2b ) in KBr, indicating a relatively weak zirconium−isocyanide interaction. Solution NMR studies consequently reveal that the isocyanides quickly exchange on the NMR time scale at room temperature. The equilibrium reaction of 1 with t BuNC has been examined in further detail by dynamic NMR spectroscopy, allowing an enthalpy for the formation of 2a of Δ H ° = −39.6 ± 6.6 kJ mol - 1 to be established. DFT calculations reveal that the weakness of the zirconium−isocyanide bonds in 2a and 2b can be attributed to the strong and appreciably covalent zirconium−cycloheptatrienyl interaction, leading to highly stabilized frontier orbitals and consequently to a diminishing π-electron release capability and a small propensity to efficiently interact with σ-donor/π-acceptor ligands. The X-ray crystal structures of 1 and 2a are also reported.