Investigations into the Mechanochemical Synthesis of Sterically Bulky Allyl Complexes

Abstract

Mechanochemical methods of synthesis (specifically grinding and milling) have not yet been widely used by organometallic chemists. This thesis surveys recent developments in the field, outlining differences that can occur when organometallic reactions are conducted in the absence of solvents. Advances in the synthesis of bulky allyl complexes by conventional (solvent-based) methods are also described, as is the direct reactivity of these complexes with gaseous CO. Unsolvated tri(allyl)aluminum, Al(C₃H₅)₃, has never been isolated, although adducts with THF, OPPh₃ and pyridine are known. Attempts to make a base-free derivative with the bulky 1,3-bis(trimethylsilyl)allyl anion (A´) from the reaction of aluminum halides and K[A´] in ethers, or by deprotonating HA´ with AlR₃ (R = Me, Et) were not successful. However, grinding AlX₃ and K[A´] in the solid state produces AlA´₃ in high yield. AlA´₃ reacts immediately with benzophenone in hexanes to produce Al(OCPh₂A´)₃. Grinding AlA´₃ with ScCl₃ yields no observable product, although the same reactions with K[A´] produces the hexane soluble ScA´₃. The reaction of AsX₃ (X = I) and SbX₃ (X = Cl, I) with K[A´], conducted either in solution or by mechanochemical methods (ball milling) leads to the production of the tris(allyl) complexes, [EA´₃]. The corresponding bismuth compound can only be prepared from [AlA´₃]. All [EA´₃] complexes are found in two diastereomeric forms of C₁ and C₃ symmetry. Mechanochemical synthesis increases the C₁:C₃ ratio relative to that produced in solution. In addition, the C₁ form becomes less favored moving down group 15. These findings are supported with the aid of density functional theory (DFT) calculations. Passage of CO at at-mospheric pressure through solutions of MA´₂ (M = Fe, Co, Ni) in hexanes produces the corresponding allyl complexes FeA´₂(CO)₂, CoA´(CO)₃, and NiA´₂(CO), respectively; the latter is the first example of a nickel allyl carbonyl complex. DFT calculations were performed on various conformations of the complexes, which reproduced the observed frequency-lowering effect of the trimethylsilyl groups.