Theoretical study of complexes of Ti-doped aluminide cluster Al@Al11Ti with ligands L containing multiple bonds

Abstract

The Al12Ti(π-L) complexes with ligands L = C2H2, C2H4, HCN, N2H2, C6H6, and N2 in the singlet and triplet states have been calculated within the B3LYP approximation of the density functional theory using the 6–31G* basis set. Their calculated structures and properties have been compared with the results of analogous calculations of the titanium porphyrin complexes with the same ligands L. It has been demonstrated that, in both series of compounds, the side-on coordination of the ligands (through the multiple bond, π type) to the titanium atom is accompanied by the weakening and elongation of the C-C, C-N, and N-N bonds by 0.05–0.20A and the long-wavelength shift of the stretching vibration modes νstr(CC), νstr(CN), and νstr(NN) by a few hundreds of cm−1. For the Ti aluminide complexes, these activation effects are much more clearly pronounced than for their Ti porphyrin analogues. The aluminide complexes (except the nitrogenyl one) have the singlet ground state; however, the nearest triplet is close lying to the singlet (within 1–14 kcal/mol). The singlet is characterized by the considerable electron density transfer from the Al12Ti cluster to the ligand L, the displacement of the Ti atom from the aluminum cage to the ligand, and the distortion of the Al12 cage. In the triplet states, the ligand activation depends on the character of spin density delocalization between the ligand, the Ti atom, and the Al12 cage. If the spin density is distributed between the Ti atom and the Al12 cage or if both unpaired electrons are localized on the Ti atom, then the structure, stability, and spectroscopic properties of the “active” Ti-L moiety in these triplets differ only slightly from those in the singlet state. If the spin density is distributed between the Ti atom and the ligand, the singlet-triplet excitation is accompanied by the elongation and weakening of the Ti-ligand bond and the decrease in the ligand activation effect. Complexes with several ligands L coordinated to the Ti atom in the Al12Ti cluster have been calculated. There are some trends in the change in the molecular characteristics of the Ti-ligand bonds in different series of the complexes.