Reaction Pathways for Addition of H₂ to Amido-Ditetrylynes R₂N–EE–NR₂ (E = Si, Ge, Sn). A Theoretical Study

Quantum chemical calculations of the reaction profiles for addition of one and two H₂ molecules to amido-substituted ditetrylynes have been carried using density functional theory at the BP86/def2-TZVPP//BP86/def2-TZVPP level of theory for the model systems L′EEL′ and BP86/def2-TZVPP//BP86/def-SVP for the real compounds. The hydrogenation of the digermyne LGeGeL (L = N­(SiMe₃)­Ar*; Ar* = C₆H₂Me­{C­(H)­Ph₂}₂-4,2,6) follows a stepwise reaction course. The addition of the first H₂ gives the singly bridged species LGe­(μ-H)­GeHL, which rearranges with very low activation barriers to the symmetrically hydrogenated compound LHGeGeHL and to the most stable isomer LGeGe­(H)₂L, which is experimentally observed. The addition of the second H₂ proceeds with a higher activation energy under rupture of the Ge–Ge bond, yielding LGeH and LGeH₃ as reaction products. Energy calculations which consider dispersion interactions using Grimme’s D3 term suggest that the latter reaction is thermodynamically unfavorable. The second hydrogenation reaction LGeGe­(H)₂L → L­(H)₂GeGe­(H)₂L possesses an even higher activation barrier than the bond-breaking hydrogenation step. Further calculations which consider solvent effects change the theoretically predicted reaction profile very little. The calculations of the model system L′GeGeL′ (L′ = NMe₂) give a very similar reaction profile. Calculations of the model disilyne and distannyne homologues L′SiSiL′ and L′SnSnL′ suggest that the reactivity of the amido-substituted ditetrylynes always has the order Si > Ge > Sn. The most stable product of the addition of one H₂ to the distannyne L′SnSnL′ is the doubly bridged species L′Sn­(μ-H)₂SnL′, which has been experimentally observed when bulky groups are employed. Analysis of the H₂–L′EEL′ interactions in the transition state for the addition of the first H₂ with the EDA-NOCV method reveals that the HOMO–LUMO and LUMO–HOMO interactions have similar magnitudes.