Thermodynamic and Kinetic Effects of the Double Coordination of Carbonyl Groups by Bidentate Lewis Acids

Treatment of 1,2,3,4-tetrahexylbenzene or 1,2,4,5-tetrahexylbenzene with Hg(OOCCF 3 ) 2 produces 1,2-phenylenedimercury bis(trifluoroacetate) 7 or its 1,4-isomer 9, which are bidentate Lewis acids. IR studies in CH 2 Cl 2 established that 1,2-isomer 7 forms doubly coordinated 1:1 complexes with amides in which both atoms of mercury interact simultaneously with the carbonyl oxygen atom of the bound amide, whereas 1,4-isomer 9 forms more weakly bound singly coordinated complexes. Variable-temperature IR studies revealed that for the double coordination of diethylformamide (DEF) by 1,2-isomer 7, Δ H ° = −4.5 ± 0.2 kcal mol -1 and Δ S ° = −6.9 ± 1.0 eu, whereas for the single coordination of DEF by 1,4-isomer 9, Δ H ° = −3.4 ± 0.2 kcal mol -1 and Δ S ° = −8.1 ± 1.0 eu. These measurements establish that double coordination is significantly more exothermic than single coordination. In addition, variable-temperature NMR studies showed that Δ G ‡ for rotation around the N−C(O) bond of dimethylpivalamide (normally 12.2 ± 0.1 kcal mol -1 in CDCl 3 ) is increased more by the addition of an equimolar amount of 1,2-isomer 7 (12.9 ± 0.1 kcal mol -1 ) than by the addition of an equimolar amount of 1,4-isomer 9 (12.3 ± 0.1 kcal mol -1 ). This demonstrates that the double coordination of carbonyl compounds by bidentate Lewis acids can have measurable effects on the rates of subsequent chemical transformations.