ja5b05854_si_001.pdf (1.14 MB)
Download fileTheory 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
journal contribution
posted on 19.08.2015, 00:00 authored by Alison
B. Altman, C. D. Pemmaraju, Clément Camp, John Arnold, Stefan G. Minasian, David Prendergast, David K. Shuh, Tolek TyliszczakPolarized
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 X1– 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.