Five-membered N-heterocyclic beryllium(i) compounds: fluctuating electronic structures with ambiphilic reactivity

The structure, bonding, and reactivity of the five-membered N-heterocyclic beryllium compounds (NHBe), BeN2C2H4 (1) and BeN2(CH3)(2)C2H2 (2) were studied at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The molecular orbital analysis indicates that NHBe is an aromatic 6 pi-electron system with an unoccupied sigma-type sp(n)-hybrid orbital on Be. Energy decomposition analysis combined with natural orbitals for chemical valence has been carried out with Be and L (L = N2C2H4 (1), N-2(CH3)(2)C2H2 (2)) in their different electronic states as fragments at the BP86/TZ2P level of theory. The results indicate that the best bonding representation can be considered as an interaction between Be+ having the 2s(0)2p(x)(1)2p(y)(0)2p(z)(0) electronic configuration and L-. Accordingly, L- forms two donor-acceptor sigma-bonds and one electron sharing pi-bond with Be+. Compounds 1 and 2 show high proton and hydride affinity at beryllium, indicating its ambiphilic reactivity. The protonated structure results from adding a proton on the lone pair of electrons in the doubly excited state. On the other hand, the hydride adduct is formed by donating electrons from the hydride to an unoccupied sigma-type sp(n)-hybrid orbital on Be. These compounds show very high exothermic reaction energy for adduct formation with two electron donor ligands such as cAAC, CO, NHC, and PMe3.