Adduct formation of [(η7-C7H7)Zr(η5-C5H5)] with phosphines and N-heterocyclic carbenes: An experimental and theoretical study

The reaction of [(η7-C7H7)Zr(η5-C5H5)] with two Lewis bases, tetramethylimidazolin-2-ylidene and PMe3, is reported and their stability probed via spectroscopic and theoretical methods. The strongly σ-basic N-heterocyclic carbene forms a stable adduct which has been structurally characterised, whilst the PMe3 ligand coordinates weakly to the metal centre. Variable temperature 31P NMR spectroscopy has been used to determine the activation energy for this process (ΔG‡ = 40.5 ± 1.9 kJ mol−1). DFT calculations have been performed on both complexes and the structures discussed. In addition, the enthalpies for the formation of these compounds have been calculated [ΔH0(Zr–IMe) = −56.3 kJ mol−1; ΔH0(Zr–PMe3) = −2.3 kJ mol−1] and show that the N-heterocyclic carbene forms a thermodynamically much more stable adduct than that with PMe3.