Page 7 - Interaction of Multiple Bonded and Unsaturated Heavier Main Group Compounds with Hydrogen, Ammonia, Olefins, and Related Molecules

Reactivity of Multiple Bonded and Main Group Compounds Power

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FIGURE 5. Structure of Ar SnSnAr (C 2 H 4 ) 2 (Ar =C 6 H 3 -2,6(C 6 H 3 -2,6-Pr 2 ) 2 (two molecules per unit cell).
SCHEME 6. Reversible Reactions of Distannynes with Ethylene

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readily dissociate ethylene. A van't Hoff analysis of VT HNMR suggest considerable internal strain energy. It thus seems that
spectra afforded relatively small molar enthalpies of associa- the energy gained in the formation of four SnC σ-bondsisin
tion of 48 and 27 kJ mol 1 for the Ar SnSnAr (C 2 H 4 ) 2 and approximate balance with the loss of two ethylene CC
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3,5-PrAr*SnSnAr*-3,5-Pr 2 (C 2 H 4 ) 2 complexes (Scheme 6). DFT π-bonds plus the increase in strain energy. As a result, the
calculationsonthemodelspeciesPhSnSnPh(C 2 H 4 ) 2 reproduced equilibrium in Scheme 6 is strongly affected by the entropic
the core structural parameters and afforded a more negative TΔS factor so that relatively small physical changes can induce
ΔH assn of133kJmol 1 whichcanberationalizedonthebasis rapid dissociation of the ethylenes. Because of this internal
of the reduced steric crowding in the model complex. The strain, the distannyneethylene complexes need just a small
apparent paradox of weak complexation in the face of slight outside stimulus (e.g., gentle heating or vacuum) to dissociate
elongation of the core bonds lengths can be accounted for, at and it is noteworthy that propene did not display any olefin
least in part, on the basis of their distorted geometries which complex formation due to the slight increase in steric

Vol. 44, No. 8 ’ 2011 ’ 627–637 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 633