Synthesis, Structure, and Reactivity of Magnesium Pentalenides

The first magnesium pentalenide complexes have been synthesized via deprotonative metalation of 1,3,4,6-tetraphenyldihydropentalene (Ph₄PnH₂) with magnesium alkyls. Both the nature of the metalating agent and the reaction solvent influenced the structure of the resulting complexes, and an equilibrium between Mg[Ph₄Pn] and [ⁿBuMg]₂[Ph₄Pn] was found to exist and investigated by NMR, XRD, and UV-vis spectroscopic techniques. Studies on the reactivity of Mg[Ph₄Pn] with water, methyl iodide, and trimethylsilylchloride revealed that the [Ph₄Pn]^2- unit undergoes electrophilic addition at 1,5-positions instead of 1,4-positions known for the unsubstituted pentalenide, Pn^2-, highlighting the electronic influence of the four aryl substituents on the pentalenide core. The ratio of syn/anti addition was found to be dependent on the size of the incoming electrophile, with methylation yielding a 60:40 mixture, while silylation yielded exclusively the anti-isomer.