New reactivity and synthesis of titanium imide complexes

This Thesis reports the synthesis and reactivity of titanium imide complexes. The dehydrocoupling of amines with boranes was studied as novel catalytic application of transition metal imides. A particular focus lies on the mechanism of the dehydrocoupling of dimethylamine-borane (DMAB). Additionally, synthetic routes to novel titanium borylimides were explored, as well as their reactivity in the activation of small molecules.
Chapter Two describes the dehydrogenative coupling of amine-boranes using cyclopentadienyl-amidinate supported titanium imido catalysts Cp*Ti{MeC(NMe)2}(NR). The substrate scope of the systems are described, and several differently substituted amine-boranes are shown to be coupled efficiently. For the dehydrocoupling of DMAB, catalyst Cp*Ti{MeC(NMe)2}(NtBu) (2.1) featured a turnover frequency of TOF90% = 1600 h-1 (RT, toluene, [DMAB]0=0.38 M), the highest TOF number for any group 4 system to date. The focus of this Chapter lies on a detailed mechanistic study of the dehydrocoupling process using DMAB as model substrate. The study suggests a mechanism with a constant oxidation state of Ti(IV), and in which Cp*Ti{MeC(NMe)2}(NR) represents the active species. Significantly, the catalysis features two distinct kinetic regimes which can be exploited to isolate Me2NH.BH2NMe2.BH3 (and it’s deuterium labelled isotopologues) directly and catalytically from DMAB in high yields on the preparative scale.
Chapter Three describes the reactivity of one of the first known transition metal borylimide complexes, namely Cp*Ti(HPP){NB(NDippCH)2} (1.83). To begin, the chemistry with a variety of terminal and internal alkynes will be explored. Terminal alkynes exclusively reacted via C-H bond activation since [2+2] cycloaddition is sterically disfavoured. Next, [2+2] cycloadditions followed by cycloreversions with the heterocumulenes CO2, and isocyanates ArNCO, as well as their heavier thio-congeners CS2, and ArNCS will be presented. Finally, the reactivity with a variety of small organic carbonyls will be discussed, including aldehydes, ketones, esters, as well as amides. Tolylaldehyde was found to undergo initial [2+2] cycloaddition followed by cycloreversion to form titanium dioxo complex [Cp*Ti(HPP)(μ-O)]2 (3.22) as well as borylimide N(CHTol){B(NDippCH)2} (3.29). By contrast, enolisable ketones and esters RC(O)CH3 featuring protons in α-position were shown to form titanium enolate species of type Cp*Ti(HPP-H){NB(NDippCH)2}{OC(CH2)R}. In the case of esters, where R = OR’, these titanium enolate species rearranged to release ketenes and form titanium alkoxy/aryloxy species Cp*Ti(HPP)(NH{B(NDippCH)2})(OR’).
Chapter Four describes a novel synthetic protocol to access titanium borylimides, namely the oxidative addition of borylazides N3BR to a Ti(II) precursor, namely (Cp''2Ti)2(μ2:η1,η1-N2) (1.16). Three different examples of highly reactive, base-free titanium borylimide complexes of type Cp''2Ti=NBR2 could be prepared this way, specifically Cp''2Ti=NB{N(Me)2C6H4} (4.11), Cp''2Ti=NB(NDippCH)2 (4.12) and Cp''2Ti=NBMes2 (4.1). In contrast to the vast majority of borylimide complexes that feature two nitrogen atoms adjacent to the boron centre, Cp''2Ti=NBMes2 (4.1) features two aryl groups. The solid state structure and MO DFT calculations suggest only negligible π-interactions between the aryl groups and the boron based p-orbital, thus pointing to significant “Ti=N=B” cumulene character. Preliminary reactivity studies suggest Cp''2Ti=NBMes2 (4.1) to be more reactive towards small molecule activation than the previously studied Cp*Ti(HPP)(NB(NDippCH)2) (1.83). Significantly, dihydrogen is activated rapidly under ambient conditions to form the titanium hydride amide Cp''2Ti(H)N(H)BMes2 (4.21). This complex is characterised by X-ray crystallography, representing the first structurally authenticated example of a titanium hydride amide.

Advisor: Weller, A