Alkali Metal Amide Salts of a Bulky Nitrogen Tethered Phosphonium Fluorenide Ligand

The reaction of a previously reported phosphine-imine ligand with 9-bromofluorene afforded a phosphonium bromide (E)-enamine that, upon deprotonation, affords an enamine tethered to a phosphonium fluorenide moiety. Isolation of various crystals of this enamine and subsequent analysis by single-crystal X-ray diffraction showed that both (E)- and (Z)-isomers occur, which is supported by three dynamic isomers in C6D6. 1H NMR spectroscopy revealed that the enamine phosphonium fluorenide is the dominant isomer and is also observed within all four solid-state crystal structures of 3. The solvent-free 3 and 0.5(THF, n-hexane) solvate of 3 occur as the (Z)-enamine, while co-crystallization of either diethyl ether or 3 molecules of THF resulted in the (E)-enamine of 3. The ability for compound 3 to participate in two types of hydrogen bonds between either the methyne or amine protons with THF was observed from the 0.5(THF, n-hexane) and 3(THF) solvates, respectively. Subsequent deprotonation of the secondary amine with lithium-, sodium-, and potassium-containing bases yielded metal amides, forming mostly monomeric structures. Each alkali metal is chelated by one ligand with a coordinating solvent molecule (Et2O or THF), with the only exception being a potassium complex that lacked a coordinating solvent. This potassium complex has this donor vacancy filled by an intermolecular interaction with the backside of an adjacent 6-membered fluorenide ring, forming a polymer with n-hexane within the channels. This crystalline form readily de-solvates upon crushing and prolonged drying in vacuo. The single crystal X-ray structure of these complexes as various solvates with diethyl ether (Li, K), THF (Li, Na), and n-hexane (K) revealed the highly dynamic donor properties of the phosphonium fluorenide undergoing a wide range of η2–6 interactions with the alkali metals.