HALF-SANDWICH COMPLEXES OF GROUP III (Sc, Y, and La) METALS WITH CYCLOOCTATERAENE

Abstract

1,3,5,7 -- Cyclooctatetraene (COT, C$_8$H$_8$) is one of the most versatile ligands used in organometallic chemistry, with coordination modes of $\eta$$^2$, $\eta$$^4$, $\eta$$^6$, and $\eta$$^{8}$. COT is non-aromatic with a tub conformation; however, its dianion (C$_8$H$_8$) is aromatic and planar. In this work, we have studied group III M-COT (M = Sc, Y, and La) 1:1 complexes using pulsed field ionization - zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory. The ZEKE spectra of these complexes show a strong 0-0 transition and a major metal-ligand stretching progression. The ionization energies are measured to be 42261(5), 40747(5), and 36641(5) cm$^{-1}$; and the M$^+$-COT stretching frequencies are 338, 300, and 278 cm$^{-1}$ for the triad. The ionization energies of the metal complexes decrease down the group as expected from those of the bare metal atoms. The metal-ligand stretching frequencies decrease with increasing the mass of the metal atoms. In coordination with theory, the neutral and ionized complexes are determined to be in C$_{4v}$ point group, with M/M$^+$ in $\eta$$^8$ binding to the planar COT; the observed transition is assigned to $^{1}$A$_1$$\leftarrow$ $^{2}$A$_1$. The conformation change of the COT molecule upon metal coordination is associated with the metal to ligand electron transfer and metal-ligand orbital interaction.