Cationic, Neutral, and Anionic Allyl Magnesium Compounds: Unprecedented Ligand Conformations and Reactivity Toward Unsaturated Hydrocarbons

Starting from bis­(allyl)magnesium [Mg­(C3H5)2], a set of cationic, neutral, anionic, and dianionic
allyl magnesium compounds has been isolated and characterized, including
[Mg­(C3H5)­(THF)5]­[B­(C6F5)4] (3), [Mg­(C3H5)2(1,4-dioxane)­(THF)] (2), [KMg­(C3H5)3(THF)] (6), and [MMg­(C3H5)4] (8: M = K2; 9: M = Ca). In solution, the allyl ligands of the
compounds display fluxional behavior, even at low temperatures. Single
crystal X-ray analysis reveals unusual μ2-η1:η3- and unprecedented μ3-η1:η3:η3-coordination
modes in the heterobimetallic compounds 6 and [8·(THF)2]. Density functional theory calculations
confirm that these metal–allyl conformations are energetically
stable. The magnesium compounds have been investigated as initiators
for butadiene polymerization and ethylene oligomerization. The heterobimetallic
compounds display initiation properties, including higher reaction
rates, that are distinctively different from those of the monometallic
species. Reactivity trends depend on the formal charge of the magnesium
compounds (dianionic, higher-order magnesiate > monoanionic, lower-order
magnesiate) and on the nature of the counterion (K+ >
Ca2+).