Reaction Mechanism of the Symmetry-Forbidden [2+2] Addition of Ethylene and Acetylene to Amido-Substituted Digermynes and Distannynes Ph2N-EE-NPh2, (E = Ge, Sn): A Theoretical Study

Quantum chemical calculations of reaction mechanisms for the formal [2+2] addition of ethylene and acetylene to the amido-substituted digermyne and distannyne Ph2N-EE-NPh2 (E = Ge, Sn) have been carried out by using density functional theory at the BP86/def2-TZVPP level. The nature and bonding situations were studied with the NBO method and with the charge and energy decomposition analysis EDA-NOCV. The addition of ethylene to Ph2N-EE-NPh2 takes place through an initial [2+1] addition to one metal atom and consecutive rearrangement to four-membered cyclic species, which feature a weak E-E bond. Rotation about the C-C bond with concomitant rupture of the E-E bond leads to the 1,2-disubstituted ethanes, which have terminal E(NPh2) groups. The overall reaction Ph2N-EE-NPh2+C2H4→(Ph2N)E-C2H4-E(NPh2) has very low activation barriers and is slightly exergonic for E = Ge but slightly endergonic for E = Sn. The analysis of the electronic structure shows that there is charge donation of nearly one electron to the ethylene moiety already in the first part of the reaction. The energy partitioning analysis suggests that the HOMO(Ph2N-EE-NPh2)→LUMO(C2H4) interaction has a similar strength as the HOMO(C2H4)→LUMO(Ph2N-EE-NPh2) interaction. The [2+2] addition of acetylene to Ph2N-EE-NPh2 also takes place through an initial [2+1] approach, which eventually leads to 1,2-disubstituted olefins (Ph2N)E-C2H2-E(NPh2). The formation of the energetically lowest lying conformations of cis-(Ph2N)E-C2H2-E(NPh2), which occurs with very low activation barriers, is clearly exergonic for the germanium and the tin compound. The trans-coordinated isomers of (Ph2N)E-C2H2-E(NPh2) are slightly lower in energy than the cis form but they are separated by a substantial energy barrier for the rotation about the C-C bond. The energy decomposition analysis indicates that the initial reaction takes place under formation of electron-sharing bonds between triplet fragments rather than HOMO-LUMO interactions.