Electronic Structure of Ti³⁺–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

Abstract

The interaction of tetrahedrally coordinated Ti³⁺ ions generated in the framework of TiAlPO-5 microporous materials with ^12,13C₂H₄ leads to the formation of side-on η² {Ti³⁺C₂H₄} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C₂H₄ results from the σ overlap of low lying C₂H₄ filled π orbitals with the 3<i>d</i>_<i>z</i>² empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3<i>d</i>_<i>yz</i> orbital of titanium and the empty π* orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π*-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time