Hypercoordinate β-carbon in Grubbs and Schrock olefin metathesis metallacycles

Abstract

Metallacyclobutane (MCB) intermediates of Grubbs and Schrock olefin metathesis catalysts are well-known for their unusually short single bond-like metal to C_β distance and unusually long C_αC_β distances. From the analysis of structural, bond order, electron density and ¹³C NMR data of a large variety of MCB systems, we show that the C_β of the metallacycle possesses pentacoordinate geometry due to the agostic type interaction of the metal with the C_αC_β bonds. The pentacoordination of C_β to the metal center is characterized by a catastrophe ring critical point (RCP) in the quantum theory of atoms-in-molecule (QTAIM) analysis. Fine tuning of the ligand environment changes the catastrophe point to a fifth bond critical point (BCP) which is clearly brought out in the case of two ruthenium olefin metathesis systems. Several Ru and W agostic MCB complexes exhibiting pentacoordinate C_β as well as their non-agostic isomers have been reported at the BP86/def2-TZVPP level of DFT. The agostic systems showed a significant bond order between metal and C_β (0.17–0.36), single bond-like electron density values at the catastrophe RCP/BCP and a significantly large difference in ¹³C NMR chemical shift values between C_α and C_β atoms.