The He I valence photoelectron spectra of the Lewis acid-base adducts, Me3Al(py) and Me2(BHT)Al(py) (BHT = 2,6-di-tert-butyl-4-methylphenoxide, py = pyridine, Me = methyl) have been obtained in order to characterize the electronic structure and bonding in four-coordinate organometallic complexes of aluminum. In the photoelectron spectrum of the BHT ligand coordinated to aluminum in Me2(BHT)Al(py), the phenyl π a2 ionization of the phenoxide ligand (Phπ a2) retains the vibrational structure observed in the free alcohol (BHT-H), but the individual vibrational components are lost in the ionization, which corresponds most closely with the Phπ b1. The loss of vibrational fine structure associated with ionization from the Phπ b1 orbital in the coordinated phenoxide is one evidence that the phenoxide is involved in a π interaction with the Me2Al(py) portion of the molecule. The similarity of the splits of the Phπ a2 and b1 ionizations in Me2(BHT)Al(PMe3) and Me2(BHT)Al(py) shows that the extent of the π interaction of the BHT ligand with the Me2Al(PMe3) and Me2Al(py) portions of the molecules is about the same. The Phπ a2 and b1 ionizations of the BHT ligand experience a destabilizing shift on proceeding from Me2(BHT)Al(py) to Me2(BHT)Al(PMe3). This increase in negative charge potential at the metal center when the trialkyl-phosphine replaces the pyridine follows from the expected better σ-donor ability of the trialkylphosphine and the potentially better π-acceptor ability of the pyridine ligand. The pyridine ionizations show several interesting trends. First, the pyridine π ionizations in Me3Al(py) are destabilized in comparison to the same ionizations of free pyridine, showing that the pyridine experiences a net negative charge potential when it is coordinated to Me3Al. Since the aluminum is in its highest formal oxidation state, the net negative charge potential must involve the Al-Me σ bonds and/or π donation from the methyl C-H σ bonds. Second, on proceeding from Me3Al(py) to Me2(BHT)Al(py), the pyridine π ionizations show the expected stabilization from the more positive potential at the aluminum center that results from replacing a methyl group with the more electronegative alkoxide. However, the coordinated nitrogen lone-pair ionization shows no significant shift. This again traces to overlap interaction with the π-donor orbital of the alkoxide. These results suggest that π-symmetry interactions with ligands from the first row of the periodic table may be a prevalent aspect of the chemistry of four-coordinate aluminum complexes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry