Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis

Abstract

Cationic amidotitanocene complexes [Cp₂ Ti(NPhAr)][B(C₆ F₅ )₄ ] (Cp=η⁵ -C₅ H₅ ; Ar=phenyl (1 a), p-tolyl (1 b), p-anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti-N bond in 1 a-c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a-c undergo photolytic Ti-N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H₃ BNHMe₂ results in fast homolytic Ti-N cleavage to give [Cp₂ Ti(H₃ BNHMe₂ )][B(C₆ F₅ )₄ ] (3). 1 a-c are highly active precatalysts in olefin hydrogenation and silanes/amines cross-dehydrogenative coupling, whilst 3 efficiently catalyzes amine-borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H₂ activation key step was disclosed using the Activation Strain Model (ASM).

Keywords: activation strain model; catalysis; energy decomposition analysis; hydrogen; titanium.