The Reactions of Organometallic Compounds Involving Silicon. Reactions of Methyldiphenyl-, Triphenylsilyl-sodium, -potassium, -rubidium, and -caesium with 9-Methylfluorene in Tetrahydrofuran

The reactions of triphenylsilyl-sodium(Ph3SiNa), diphenylmethylsilyl-sodium(Ph2MeSiNa), triphenylsilyl-potassium (Ph3SiK), triphenylsilyl-rubidium (Ph3SiRb), and triphenylsilyl-caesium (Ph3SiCs) with 9-methylfluorene in tetrahydrofuran (THF) were studied using the stop-flow technique at low temperatures. The rate constant k (1 mol−1 s−1) depends on the nature of the cation; at −50 °C k is 11.5 for Ph3SiNa, 29.5 for Ph2MeSiNa, 26.8 for Ph3SiK, 35.6 for Ph3SiRb, and 47.9 for Ph3SiCs. It seems that the organosilylalkali–metal compounds (R3SiM) exist in the form of contact ion-pair and therefore the reactivity of an ion-pair is determined by the Coulombic binding energy of the pair. The thermodynamic constants of activation for these reactions were calculated for −50 °C and compared with the corresponding values of triphenylsilyl-lithium Ph3SiLi reaction with 9-methylfluorene in THF. The plots of log k, ΔS≠, and ΔH≠ against 1/(rc + 2) and ΔS≠ against ΔH≠ were drawn to show that Li+ due to its existence as solvent separated ion-pair behaves differently to its Na+, K+, Rb+, and Cs+ counterparts.The electronic spectra of R3SiM, 9-methyl-fluoren-9-yl-sodium, -potassium, -rubidium and -caesium were determined at temperatures varying from 20 to −80 °C. A red shift was observed in spectra of R3SiNa at low temperatures; this was attributed to the increase in dipole moment which occurs during the electronic transition. The spectra of 9-methylfluoren-9-yl-sodium, -potassium, -rubidium, and -caesium were compared with their lithium counterpart to explain the effects of gegenion on the solvation of these species.