Cycloheptatrienyl−Cyclopentadienyl−Zirconium Sandwich Complexes: Structure and Bonding

Abstract

The reactions of the 16-electron cycloheptatrienyl−cyclopentadienyl−zirconium sandwich complex [(η7-C7H7)Zr(η5-C5H5)] (1) with tert-butyl isocyanide (tBuNC) and 2,6-dimethylphenyl isocyanide (o-XyNC) have been studied. The isocyanide complexes [(η7-C7H7)(η5-C5H5)Zr(CNR)] (R = tBu, 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 tBuNC has been examined in further detail by dynamic NMR spectroscopy, allowing an enthalpy for the formation of 2a of ΔH° = −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.