Reaction Mechanism of the Symmetry-Forbidden [2+2] Addition of Ethylene and Acetylene to Amido-Substituted Digermynes and Distannynes Ph₂NNPh₂, (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 Ph₂NNPh₂ (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 Ph₂NNPh₂ 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 bond. Rotation about the C-C bond with concomitant rupture of the E bond leads to the 1,2-disubstituted ethanes, which have terminal E(NPh₂) groups. The overall reaction Ph₂NNPh₂+C₂H₄→(Ph₂N)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(Ph₂NNPh₂)→LUMO(C₂H₄) interaction has a similar strength as the HOMO(C₂H₄)→LUMO(Ph₂NNPh₂) interaction. The [2+2] addition of acetylene to Ph₂NNPh₂ also takes place through an initial [2+1] approach, which eventually leads to 1,2-disubstituted olefins (Ph₂N)C₂H₂-E(NPh₂). The formation of the energetically lowest lying conformations of cis-(Ph₂N)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)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. Really forbidden? DFT calculations were used to study the reaction mechanisms for the formal symmetry-forbidden [2+2] addition of C₂H₄/C₂H₂ to the amido-digermyne/-distannyne Ph₂NNPh₂ (E=Ge, Sn) (see scheme). The reactions occur through an initial [2+1] addition to one metal atom and consecutive rearrangement with concomitant rupture of the E bond. NBO and EDA-NOCV methods are employed to analyze the nature and bonding situations.