Experimental and computational studies of R3Al-ER′3 (E = P, As, Sb, Bi; R = Et, t-bu; R′ = SiMe3, i-Pr) donor-acceptor complexes:: Role of the central pnictine and the substituents on the structure and stability of alane adducts

ddDonor-acceptor interactions within analogously substituted Lewis acid-base adducts of the type Et3Al-E(SiMe3)(3) and t-Bu3Al-E(i-Pr)(3) (E = P, As, Sb, Bi) were investigated in the solid state, on the basis of single-crystal X-ray structure analyses, and in solution by temperature-dependent NMR spectroscopy. In addition, density functional theory computations (B3LYP/SDD) were performed to analyze molecular structures and to derive dissociation energies (De) of the R3Al-ER'(3) adducts. The thermodynamic stability of these depends both on the electronic strength of the Lewis acid and base, which is influenced by the central group 13 and 15 elements and their substituents R and R', and on steric interactions between the Lewis acid and base. Such repulsive interactions are decisive for the stability of adducts containing small central elements such as P and large substituents such as i-Pr and t-Bu. Comparisons between central structure parameters (Al-E and Al-C(H) bond distances; C(H)-Al-C(H) bond angles) and D-e values of several adducts show that the Al-E bond length does not necessarily display the thermodynamic stability of an adduct, while the Al-C(H) bond distance and C(H)-Al-C(H) bond angles are useful structural parameters to estimate the stability of an adduct in the solid state.