Molecular titanium nitrides: nucleophiles unleashed

Abstract

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ₂-K(OEt₂)]₂[(PN)₂TiN]₂ (1) (PN⁻ = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pK_a range of the NH group in (PN)₂TiNH to be between 26–36. Complex 1 could be produced by a reductively promoted elimination of N₂ from the azide precursor (PN)₂TiN₃, whereas reductive splitting of N₂ could not be achieved using the complex (PN)₂TiNNTi(PN)₂ (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)^tBu and OCCPh₂ to form NC^tBu and KNCCPh₂, respectively, along with the terminal oxo complex (PN)₂TiO, which was also characterized. A combination of solid state ¹⁵N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)₂TiN], and the discrete salt [K(2,2,2-Kryptofix)][(PN)₂TiN] as well as the origin of the highly downfield ¹⁵N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of ¹⁵N_nitride resonance in the ¹⁵N NMR spectrum was found to be linked to the K⁺ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.