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

Cationic amidotitanocene complexes [Cp2Ti(NPhAr)][B(C6F5)(4)] (Cp=eta(5)-C5H5; 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 sigma and pi 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 H3BNHMe2 results in fast homolytic Ti-N cleavage to give [Cp2Ti(H3BNHMe2)][B(C6F5)(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 H-2 activation key step was disclosed using the Activation Strain Model (ASM).