Synthesis and Characterization of Zirconium and Iron Complexes Containing Substituted Indenyl Ligands: Evaluation of Steric and Electronic Parameters

Evaluation of the steric and electronic influence of a family of silyl- and alkyl-substituted
indenyl ligands on zirconium and iron centers has been accomplished by a combination of
X-ray diffraction, IR spectroscopy, solution NMR dynamics, and electrochemical measurements. Three tetrasubstituted, bis-indenyl zirconocene dichloride complexes have been
characterized by X-ray diffraction and adopt a gauche ligand conformation such that the
interactions between tertiary substituents on adjacent rings are minimized. Similar solid
state conformations were also observed in two of the corresponding iron compounds.
Evaluation of the electronic environment about each zirconium center was achieved by
measurement of the CO stretching frequencies of the dicarbonyl derivatives. Simple inductive
effects govern the electronic properties of each zirconocene where silyl groups are relatively
electron withdrawing and alkyl groups electron donating. For the most hindered zirconocene
dicarbonyl derivatives, population of three vibrationally distinct rotamers has been detected
by IR spectroscopy. Independent assessment of these stereoelectronic parameters by variable-temperature NMR spectroscopy and electrochemistry with the analogous series of iron
complexes provided the same relative ordering of the indenyl ligands.