Dissecting the Diverse Reactivity of β-Diketiminate Aluminum(I) Compound towards Azaarenes: Insight From DFT Calculations

Abstract

Interest in aluminum(I) complexes has surged in recent decades due to the unusual role of electropositive aluminum as donor atoms in ligands. Numerous Al(I) complexes, which were previously considered too unstable, have been isolated. Among these, β-diketiminate aluminum(I) complex, NacNacAl(I), stands out for its unique reactivities including oxidative addition and π-bond activation. However, the understanding of reactions involving NacNacAl(I) has not yet been fully established. This study unveils the mechanisms behind the diverse reactivity of NacNacAl(I) with five structurally similar azaarenes through DFT calculations. Interestingly, computational results indicate that some of the five reactions can proceed via radical processes. A holistic comparison of all results highlights the mechanistic differences between monocyclic and bicyclic azaarenes. In the initial step with NacNacAl(I), monocyclic azaarenes form Al(I)-azaarene adducts, whereas bicyclic azaarenes generate Al(II)-azaarene biradicals. These intermediates are critical for understanding their distinctive reactivity. For monocyclic azaarenes, electronic effects of their substituents on the azaarene adducts result in varying reaction outcomes, while for bicyclic azaarenes, subsequent intermolecular or intramolecular coordination of biradicals leads to different products. This study provides deeper mechanistic insights into reactions associated with NacNacAl(I) complexes, thereby contributing to a more comprehensive understanding of these reactions.

Keywords: aluminum(I) complexes; biradical singlets; density functional calculations; nitrogen heterocycles; reaction mechanisms.

Figure 1

First examples of stable Al(I) complexes.

Scheme 1

Reactions of NacNacAl(I) with 3,5‐lutidine (LUT), 4‐picoline (PIC), 4‐dimethylaminopyridine (DMAP), quinoline (QUI) and phthalazine (PHT).

Figure 2

Energy profile calculated for the reaction of NacNacAl(I) with 3,5‐lutidine. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 3

Energy profiles of other less favorable pathways for the reaction of NacNacAl(I) with 3,5‐lutidine: (a) nucleophilic attack on the C(4) position of a free 3,5‐lutidine molecule by 3,5‐lutidine‐coordinated NacNacAl(I), (b) nucleophilic attack on the C(4) position of the 3,5‐lutidine moiety in the 3,5‐lutidine‐coordinated NacNacAl(I) by uncoordinated NacNacAl(I) and (c) oxidative addition of the C(4)─H bond of 3,5‐lutidine. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 4

Energy profile calculated for the proposed pathway for the reaction of NacNacAl(I) with 4‐picoline yielding 2 molecules of products. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 5

Energy profiles of other less favorable pathways for the reaction of NacNacAl(I) with 4‐picoline: (a) direct deprotonation of a 4‐picoline methyl C─H bond utilizing PIC_1, the 4‐picoline‐coordinated NacNacAl(I) and (b) oxidative addition of a 4‐picoline methyl C─H bond. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 6

Energy profile calculated for the proposed pathway for the reaction of NacNacAl(I) with DMAP yielding two molecules of products. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 7

Energy profile calculated for an alternative pathway for the reaction of NacNacAl(I) with DMAP: deprotonation of the NacNacAl(I)‐DMAP adduct by another NacNacAl(I)‐DMAP adduct yielding 2 molecules of products. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 8

Energy profile calculated for the singlet biradical pathway for the reaction of NacNacAl(I) with quinoline. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 9

Spin Natural Orbitals calculated for the NacNacAl(I)‐quinoline adduct QUI_1.

Figure 10

Relative stability among QUI_P_cis , QUI_2 and QUI_P_trans . Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 11

Energy profiles of other less favorable pathways for the reaction of NacNacAl(I) with quinoline: (a) intramolecular nucleophilic attack on the C(8) position of quinoline by Al(I) center of quinoline‐NacNacAl(I) adduct, (b) [4+1] cycloaddition with quinoline at the N1 and C(4) positions by uncoordinated NacNacAl(I) and (c) [4+1] cycloaddition with quinoline at the C(5) and C(8) positions by uncoordinated NacNacAl(I). Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 12

Energy profile calculated for the singlet biradical pathway for the reaction of NacNacAl(I) with phthalazine. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 13

Spin natural orbitals calculated for the singlet biradical NacNacAl(I)‐phthalazine adduct PHT_1.

Figure 14

Energy profile of the less favorable pathway for the reaction of NacNacAl(I) with phthalazine: nucleophilic addition of NacNacAl(I) to phthalazine. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 15

Relative free energies (kcal/mol) of the singlet biradical forms of the NacNacAl(I)‐dimethylaminopyridine (DMAP) adduct, NacNacAl(I)‐quinoline (QUI) adduct and NacNacAl(I)‐phthalazine (PHT) adduct, with respect to their respective normal singlet states.