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 H-2 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)Ph-2}(2)-4,2,6) follows a stepwise reaction course. The addition of the first H-2 gives the singly bridged species LGe(mu-H)GeHL, which rearranges with very low activation barriers to the symmetrically hydrogenated compound LHGeGeHL and to the most stable isomer LGeGe(H)(2)L, which is experimentally observed. The addition of the second H-2 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)(2)L -> L(H)(2)GeGe(H)(2)L 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 H-2 to the distannyne L'SnSnL' is the doubly bridged species L'Sn(mu-H)(2)SnL', which has been experimentally observed when bulky groups are employed. Analysis of the H-2-L'EEL' interactions in the transition state for the addition of the first H-2 with the EDA-NOCV method reveals that the HOMO-LUMO and LUMO-HOMO interactions have similar magnitudes.