Date of Award
Doctor of Philosophy (PhD)
Dr. Michael J. McGlinchey
The chemistry of crowded organic and organometallic complexes has been explored by the incorporation of bulky substituents into cyclopentadienone and benzene architectures, with the ultimate goal of creating a system that exhibits correlated rotation of peripheral groups.
The inclusion of naphthyl groups into η4-(2,3,4,5-tetraaryl)cyclopentadienone rhodium acetylacetonate derivatives increases the steric hindrance of the propeller blades, as observed by the generation of numerous rotamers at low-temperature. By using the 31P NMR signal of the triphenylephosphine moiety, the barrier to rotation of the naphthyl substituents was determined to be 8.2 kcal mol-1. In hexa-β-naphthylbenzene, the barrier to rotation is approximately 17 kcal mol-1, which is comparable to that of a meta-substituted phenyl group. The presence of numerous proximal/distal naphthyl rotational isomers is evident in the structure of penta-β-naphthylferrocenylbenzene, in which the substituents experience significant steric interplay and offer the potential for the observation of correlated rotation.
The influence of an organometallic unit on the stabilization of positive charge in a sterically crowded cyclopentadienyl cation has also been explored. In the absence of the metal moiety, the cation formed by the protonation of 2,3,4,5-tetraphenylcyclopentadienone (1.26) undergoes extensive rearrangement to give 5-H benzo[a]fluorene-5-one (4.38), whereas 3-ferrocenyl-2,4,5-triphenylcyclopentadienone (1.27) is stabilized by metal participation, as evidenced by the barrier to rotation of the resulting hydroxy-fulvalene cation; these results have been rationalized using DFT calculations.
Furthermore, the potential for control over the barrier to rotation has been investigated for the first time by the examination of indenyl and fluorenyl groups as peripheral substituents. The synthesis and dynamic behaviour of an indenyl-triptycene complex revealed the severe degree of interaction between the blades of the triptycene and the indenyl unit. The attempted incorporation of a metal fragment onto the indene yields instead addition to one of the triptycene blades, thus preventing the possibility of utilizing haptotropic shifts to alter the barrier to triptycene rotation.
Finally, the attempted synthesis of pentaphenylfluorenyl benzene was thwarted by the formation and reaction of fluorenyl radicals. The result was the unanticipated preparation of a Gomberg dimer, as well as substituted naphthacene molecules.
Harrington, Laura E., "The Syntheses, Structures and Dynamic Behaviour of Sterically Crowded Organic and Organometallic Complexes" (2004). Open Access Dissertations and Theses. Paper 782.