Experimental and Theoretical Approach to the Understanding of TiCl4 Interacting with Arenes. Isolation of a d0-Metal-Arene Complex and Cyclotrimerization of But-2-yne Promoted by TiCl4

The reaction of TiCl4 with C6Me6 in CH2Cl2 or 1,2-C6H4Cl2 led to a d0-arene complex [(eta6-C6Me6)TiCl3]+[Ti2Cl9]-(2), which was structurally characterized by an X-ray analysis. The structure consists of a three-leg piano-stool metallic fragment (TiCl3)+ eta6 binding the arene moiety. The H-1 NMR-spectrum of the reaction solution indicated the presence of a charge transfer intermediate. The high stability of 2 is evidenced by the TiCl4-promoted stoichiometric cyclotrimerization of but-2-yne. The theoretical calculations on the model compounds [(eta6-C6H6)TiX3]+ (X = H, F, Cl) and [(176-C6H6)TiX3] (X = F, Cl) explain the high stability of the titanium(IV) derivatives, as well as the weaker arene-metal interaction in the titanium(III) derivatives. Also, a strong positive charge was found on the benzene hydrogens, consistent with an electrophilic activation of the benzene ring. Theoretical calculations have been carried out on some possible precursors to 2, like [(eta6-C6H6)TiCl4], [(eta2-C6H6)TiCl4], and [Cl3Ti-Cl...eta6-C6H6]. Crystallographic details: 2 is orthorhombic, space group Pca2(1), with a = 17.263(2) angstrom, b = 8.712(1) angstrom, c = 17.256(2) angstrom, alpha = beta = gamma = 90-degrees, Z = 4, and R = 0.059.