Octamethyloctahydrodibenzofluorenyl: Electronic Comparisons between a Sterically Expanded Ligand and Its Cyclopentadienyl Analogues

Incorporating the octamethyloctahydrodibenzofluorenyl (Oct) ligand into metallocene and constrained geometry olefin polymerization catalysts has profound catalytic consequences. The steric influences are undoubtedly important, but it is shown herein that electronics likely also play a crucial role. The electron richness of the Oct− anion was directly measured by competitive deprotonation experiments, which reveal that the pKa of OctH is 3.9 units higher than that of fluorene. The HOMO−LUMO gap decreases by about 0.6 kcal/mol for each additional tertiary alkyl group appended to the metallocene R2C(C5H4)(C13H8)ZrCl2 (R = Me or Ph) in the 2, 3, 6, and 7 positions of the fluorenyl moiety, indicating the ability of these groups to increase the HOMO energy by electron donation. The carbonyl stretching frequencies for η5-OctMn(CO)3 (2009, 1924 cm−1) demonstrated that the Oct ligand is the most electron donating in the series of CpMn(CO)3, Cp*Mn(CO)3, and (C13H9)Mn(CO)3. DFT calculations universally corroborate these experimental findings.