Ligand effects in allyl, cyclopentadienyl, and related complexes of the transition and main group metals

Abstract

Ligand effects can play a substantial role in the stability and reactivity of organometallic complexes. For this research, sterically bulky substituents (e.g., trimethylsilyl and isopropyl groups) were incorporated as part of the ligand frameworks for use in the syntheses of organometallic complexes. Subsequent reactions involving the substituted organometallic species were studied to investigate how the steric bulk of the ligands affects reaction outcomes. The addition of steric bulk can add kinetic stability to the resulting organometallic complexes, as seen in the syntheses of the trimethylsilyl-substituted bis(allyl) complexes of nickel, cobalt, and iron. Chapter one reflects research using the disubstituted trimethylsilyl allyl ligand, [1,3-(SiMe₃)₂C₃H₃]^– (allyl´), to synthesize a bis(allyl´)Ni complex. Unlike the unstable (C₃H₅)₂Ni, which decomposes at 293 K and is pyrophoric in air, [1,3-(SiMe₃)₂C₃H₃]₂Ni is stable above 373 K and only slowly decomposes in air. Eclipsed and staggered conformers are found in solution and in the solid state for [1,3-(SiMe₃)₂C₃H₃]₂Ni. Reactions with Br₂ or I₂ result in dimeric complexes. Reactions with phosphines were also studied, and a novel P–P bond formation reaction with PMe₃ to yield (MeP)₄ was discovered. Chapter two depicts the investigation of carbonyl derivatives of bis(allyl´)M complexes (M = Ni, Co, Fe). Reactions with other small molecules such as phosphines and halogens were run for bis(allyl´)Fe and bis(allyl´)Co. The addition of CO resulted in diamagnetic complexes with terminal CO groups. The iron and cobalt allyl congeners were dissimilar to that of nickel with respect to reactions with halogens. Chapter three describes a series of <i>s</i>-block organometallic complexes that were synthesized using [1,3-(SiMe₃)₂C₃H₃]^–. The bis(allyl´)strontium is isostructural to its calcium analog, whereas a chiral heterometallic Ba₂/K(allyl´)₅ polymer was obtained. A polymeric cesium species was also structurally characterized. These <i>s</i>-block allyl complexes were shown to polymerize methyl methacrylate. Chapters four and five present work involving the use of bulky ligands in organometallic alkaline-earth metal complexes. Group 2 diguaiazulenide complexes have been shown to thermally decompose, releasing guaiazulene and metal products. Although a sterically bulky beryllocene was not isolated using the trimethylsilyated cyclopentadienyl anion (1,2,4-(SiMe₃)₃C₅H₂)^–, DFT calculations suggest that the substituted beryllocene is feasible.