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.