Prediction and Characterization of Alkaline‐Earth (M=Be, Mg, Ca, Sr, and Ba) Metallacyclopentadienes and Relevant Derivatives

Abstract

AbstractAlkaline‐earth metallacyclopentadienes are an active area of research in synthetic chemistry, notable for their roles as precursors of key cyclopentadiene derivatives. However, experimental characterization is limited due to their challenging synthesis and subsequent isolation. Herein, we report systematic theoretical studies of MC4H4 (M=Be, Mg, Ca, Sr, Ba) rings and experimentally relevant complexes. Benchmarking against CCSD(T) and experiment shows reasonable performance for the B3LYP, BP86, and M06‐2X density functionals in predicting the equilibrium geometries, thermochemistry, and vibrational spectra of these species. NICS indices and extra cyclic resonance energies confirm that the compounds in question possess antiaromatic character. For both the bare MC4H4 rings and larger complexes, the Be‐containing compounds were found to be the most strongly bound species. Such viability is explained via ring strain, electron densities, natural bond orders, and orbital analyses. Given that the second least viable Mg complex has been reported in recent experiments, synthesis and characterization of the other group IIA analogs are anticipated.