Properties of Anions, Radicals, and Radical Cations Derived from 9-(Dialkylamino)fluorenes

Article abstract of DOI:10.1021/ja00232a034

Equilibrium acidities for seven 9-(dialkylamino)fluorenes together with their oxidation potentials and those of their conjugate bases have been measured in Me2SO solution.These data are used to estimate (a) the homolytic bond dissociation energies, (BDEs) of the 9-C-H bonds in these molecules, (b) the acidities of the corresponding radical cations, and (c) the relative stabilities of the radicals formed on deprotonation of their radical cations.Comparisons are made with similar estimates of the effects of 9-(1-imidazolyl), 9-(3-methyl-1-pyrazolyl), 2-Me2N, and 2,7-(Me2N)2 groups.The 2-Me2N and 2,7-(Me2N)2 groups decrease the acidity of the fluorene radical cation by 18 and 24 kcal/mol, respectively but have little effect on the BDEs of the 9-C-H bonds.On the other hand, the 9-dialkylamino groups were found to lower the BDEs of the 9-C-H bonds by 5-12.8 kcal/mol relative to that of fluorene (79.5) and to decrease the acidity of the fluorene radical cation by an average of only 6.5 pK_HA.+ units.The latter decreases are the net result of making bont E_ox(HA) more negative (acid weakening) and E_ox(A-) more negative (acid strengthening).By contrast, the 9-imidazolyl group weakens the 9-C-H bond by only 3.3 kcal/mol and increases the acidity of the corresponding radical cation by 3.6 pK_HA.+ units, relative to that of fluorene.The Me3N+ moiety of 9-Me3NFlH+ increases the 9-C-H BDE by 5 kcal/mol.