Comparison of the Electrophilicities of the Free and the (Tricarbonyl)iron-Coordinated Tropylium Ion

Abstract

The kinetics of the reactions of the tropylium ion (1) and the (tricarbonyl)iron-coordinated tropylium ion (6) with allylsilanes, allylstannanes, and other uncharged nucleophiles were studied photometrically and conductometrically. The second-order rate constants were independent of the counterions, indicating rate-determining carbon−carbon bond formation. The electrophilicity parameters of the (tricarbonyl)iron complexes of the tropylium ion E(6) = −3.81 ± 0.24 and of the dihydrotropylium ion E(22) = −9.88 indicate that the former is 105−106 times more reactive toward nucleophiles. Comparison with the electrophilicity parameter of the free tropylium ion E(1) = −4.62 ± 0.57 shows that coordination by Fe(CO)3 affects its electrophilic reactivity only slightly. Density-functional calculations are used to rationalize the relative reactivities in terms of thermodynamic effects and frontier orbital interactions. On the basis of the linear free enthalpy relationship log k = s(E + N) one can predict that the free and the Fe(CO)3-coordinated tropylium ion react with nucleophiles (N > −1) at room temperature.