The He I valence photoelectron spectra of the Lewis acid-base adducts Me3Al(PMe3) and Me2(BHT)Al(PMe3) (BHT-H = 2,6-di-tert-butyl-4-methylphenoI) have been obtained to characterize the electronic structure and bonding in four-coordinate organometallic complexes of aluminum. To aid in the assignment of the spectrum of Me2(BHT)Al(PMe3), the spectrum of the free alcohol, BHT-H, was also obtained. The first and second ionizations of the free BHT-H alcohol show vibrational progressions associated with the symmetric C-C phenyl ring stretching modes, consistent with the b1 and a2 Π ionizations, respectively, of monosubstituted phenyl rings. In the photoelectron spectrum of BHT coordinated to aluminum in Me2(BHT)Al(PMe3), the corresponding phenoxide a2 ionization retains the vibrational structure, but the individual vibrational components are lost in the ionization that corresponds most closely with the b1. The loss of vibrational fine structure associated with ionization from the phenyl Π b1 orbital in the coordinated phenoxide shows that the phenoxide is involved in a Π interaction with the Me2Al(PMe3) portion of the molecule. In addition, the aluminum center in Me2(BHT)Al(PMe3) feels a more negative charge potential than the aluminum center in Me3Al(PMe3), as shown by the Al-P σ ionization occurring at lower binding energy in Me2(BHT)Al(PMe3). This is counter to the σ inductive effects of an alkoxide compared to an alkyl and shows that the BHT is acting as a π electron donor. The change in band shape of the Al-P σ ionization between Me3Al(PMe3) and Me2(BHT)Al(PMe3) indicates that the oxygen p σ orbital of the phenoxide ligand is interacting directly with the Al-P bonding orbital. The relationship between experimental ionization potentials and bond strengths of the Al-P σ bond in Me3Al(PMe3) and Me2(BHT)Al(PMe3) is developed, and the results show that the Al-P σ bond is stronger in Me3Al(PMe3) than in Me2(BHT)Al(PMe3), consistent with σ donation from the phenoxide into the predominantly Al-P σ* orbital.
ASJC Scopus subject areas
- Colloid and Surface Chemistry