Pentadienyls vs Cyclopentadienyls and Reversal of Metal−Ligand Bonding Affinity with Metal Oxidation State:  Synthesis, Molecular Structures, and Electronic Structures of High-Valent Zirconium Pentadienyl Complexes

Molecules of the form Cp(6,6-dmch)ZrX₂ (Cp = η⁵-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = η⁵-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp₂ZrX₂ analogues, these Cp(6,6-dmch)ZrX₂ molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr−C distances to the 6,6-dmch pentadienyl ligand are about 0.1 Å longer than the average Zr−C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe₃)₂ (tmch = η⁵-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX₂ complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp₂ZrX₂ analogues, the Cp(6,6-dmch)ZrX₂ molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.