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

Abstract

The reaction of a previously reported phosphine-imine ligand with 9bromofluorene 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 C 6 D 6 . 1 H 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 (Et 2 O 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 η 2−6 interactions with the alkali metals.

Contrasting a series of bidentate amido phosphine oxide, sulfide, or selenide ligands and complexes of dimethyl aluminum and indium

“…In our previous work with the fluorenyl substituted analogue of 1 , every isolated phosphonium salt was in the ( E ) conformation as well, with the N–H chemical shift observed at 9.98 ppm in CDCl 3 . 24 Proton transfer to the imine conformation was calculated to be a lower energy barrier starting at the ( Z )- vs. ( E )-enamine. This is thought to “lock out” the ( E )-enamine, leading to the greater proportion in most instances and the ability for it to be isolated as a crystalline solid ( 2b ).…”