Density functional theory studies of reaction mechanisms for titanium alkylamide incorporation onto functionalized aromatic self-assembled monolayers

Abstract

We have studied the reaction kinetics of the initial layer deposition of titanium-based alkylamide organometallic precursors on aromatic self-assembled monolayers (SAMs) possessing different terminal groups. Using density functional theory and Møller–Plesset (MP2) perturbation theory, we found the initial reaction rate constant for deposition to decrease in the order: OH > SH > NH₂, similar to results we found previously for alkyl SAMs. Aromatic amine- and hydroxyl-terminated SAMs were kinetically and thermodynamically slightly more favorable than alkyl counterparts, with a very small (2 kcal mol⁻¹) reduction in activation barrier. In contrast, the reactivity of thiol-terminated aromatic ligands was similar to corresponding alkyl SAMs due to the thiol group's small electronegativity. Branching on the surface functional group reduced the kinetic favorability of both aromatic and alkyl amine-terminated SAMs. Fluorine substitution in the aromatic backbone of amine-terminated SAMs is kinetically and thermodynamically much more favorable than corresponding unsubstituted aromatic SAMs and fluoryl-substituted alkyl SAMs. Investigation of the kinetics of several competing side unimolecular decomposition reactions led to the discovery of new kinetically favorable pathways that are competitive with transamination at temperatures as low as 250 K and which lead to a Ti : N ratio of 1 : 3 in agreement with experiments by Dube et al. (A. Dube, A. R. Chadeayne, M. Sharma, P. T. Wolczanski and J. R. Engstrom, J. Am. Chem. Soc., 2005, 127, 14299–14309). Significant enhancement in kinetic favorability was shown to result from the steric hindrance of large groups.