Theory and X‑ray Absorption Spectroscopy for Aluminum Coordination Complexes – Al K‑Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR₂, and (BDI)AlX₂ Complexes

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)­Al, (BDI)­AlX₂, and (BDI)­AlR₂ coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal–ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)­AlX₂ systems with more electronegative X^1– ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)­AlH₂ the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core–Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)­AlH₂ are similar relative to the (BDI)­AlX₂ and (BDI)­AlMe₂ compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b₁ symmetry that is localized on Al and orthogonal to the (BDI)­Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)­AlX₂ and (BDI)­AlR₂. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds.