Novel mechanism for interesting C-C coupling and cleavage reactions and control of thermodynamic stability involving [L₂M(μ-CCR)₂ML₂] and [L₂M(μ-RCC-CCR)ML₂] complexes (M = Ti, Zr; L = η ⁵-C₅H₅, Cl, H; R = H, F, CN): a theoretical study

Abstract

Possible reaction mechanisms for C-C coupling and cleavage reactions involving [(η⁵-C₅H₅)M(μ-CCR)₂M(η⁵-C₅H₅)₂] and [(η⁵-C₅H₅)M(μ-RCC-CCR)M(η⁵-C₅H₅)₂] (M = Ti, Zr) complexes are discussed on the basis of theoretical studies. A series of realistic and simplified model complexes, [L₂M(μ-CCR)₂ML₂] 8 and [L₂M(μ-RCC-CCR)ML₂] 9 (M = Ti, Zr; L = η⁵-C₅H₅, Cl, H; R = H, F, CN) were calculated using ab initio MO and density functional methods. The formation of the bisacetylide bridging complex starting from either metal acetylides or butadiyne is explained by a mechanistic scheme involving metallacyclocumulenes (5), binuclear complexed metallacyclocumulenes (6), and the M^+2-M⁺⁴ complexes (7). The latter rather than 6 led to the C-C coupled products 9 via 8. The experimentally observed unusual C-C coupling in the dimeric bis(η⁵-cyclopentadienyl)phenylethynyltitanium complex and the lack of it in the corresponding zirconium complex are influenced by substituents on the ethynyl group. Theoretical studies demonstrate that the electron-withdrawing substituent R = F drives the thermodynamic equilibrium distinctively towards the C-C coupled product, 9 both for M = Ti and Zr. An unusually short Zr-Zr interatomic distance in [(η⁵-C₅H₅)Zr(μ-CCR)₂Zr(η⁵-C₅H₅)₂] compared to Ti-Ti distance in the corresponding Ti complex is observed theoretically and from X-ray crystal structures of related complexes. Substitution of cyclopentadienyls by Cl and even H to obtain computationally viable models does not change the relative energies of the ground and transition states of the binuclear Zr complexes. The Ti complexes are affected somewhat more by these changes but the trends in relative energies between Ti and Zr isomers are retained. Geometric parameters and relative energies obtained at the B3LYP/LANL2DZ level are found to be better than those at the HF level.