Comparison of the electrophilicities of the free and the (tricarbonyl)iron-coordinated tropylium ion

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 10(5)-10(6) 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.