New reactivity and synthesis of titanium imide complexes
<p>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). Add...
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| Format: | Thesis |
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2018
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| _version_ | 1797106293269331968 |
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| author | Slusarczyk, M |
| author2 | Weller, A |
| author_facet | Weller, A Slusarczyk, M |
| author_sort | Slusarczyk, M |
| collection | OXFORD |
| description | <p>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.</p> <p><b>Chapter Two</b> describes the dehydrogenative coupling of amine-boranes using cyclopentadienyl-amidinate supported titanium imido catalysts Cp*Ti{MeC(NMe)<sub>2</sub>}(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)<sub>2</sub>}(N<sup>t</sup>Bu) (2.1) featured a turnover frequency of TOF90% = 1600 h-1 (RT, toluene, [DMAB]<sub>0</sub>=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)<sub>2</sub>}(NR) represents the active species. Significantly, the catalysis features two distinct kinetic regimes which can be exploited to isolate Me<sub>2</sub>NH.BH<sub>2</sub>NMe<sub>2</sub>.BH<sub>3</sub> (and it’s deuterium labelled isotopologues) directly and catalytically from DMAB in high yields on the preparative scale.</p> <p><b>Chapter Three</b> describes the reactivity of one of the first known transition metal borylimide complexes, namely Cp*Ti(HPP){NB(NDippCH)<sub>2</sub>} (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)]<sub>2</sub> (3.22) as well as borylimide N(CHTol){B(NDippCH)<sub>2</sub>} (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)<sub>2</sub>}{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)<sub>2</sub>})(OR’).</p> <p><b>Chapter Four</b> describes a novel synthetic protocol to access titanium borylimides, namely the oxidative addition of borylazides N<sub>3</sub>BR to a Ti(II) precursor, namely (Cp''<sub>2</sub>Ti)<sub>2</sub>(μ<sub>2</sub>:η<sup>1</sup>,η<sup>1</sup>-N<sub>2</sub>) (1.16). Three different examples of highly reactive, base-free titanium borylimide complexes of type Cp''<sub>2</sub>Ti=NBR<sub>2</sub> could be prepared this way, specifically Cp''<sub>2</sub>Ti=NB{N(Me)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>} (4.11), Cp''<sub>2</sub>Ti=NB(NDippCH)<sub>2</sub> (4.12) and Cp''<sub>2</sub>Ti=NBMes<sub>2</sub> (4.1). In contrast to the vast majority of borylimide complexes that feature two nitrogen atoms adjacent to the boron centre, Cp''<sub>2</sub>Ti=NBMes<sub>2</sub> (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''<sub>2</sub>Ti=NBMes<sub>2</sub> (4.1) to be more reactive towards small molecule activation than the previously studied Cp*Ti(HPP)(NB(NDippCH)<sub>2</sub>) (1.83). Significantly, dihydrogen is activated rapidly under ambient conditions to form the titanium hydride amide Cp''<sub>2</sub>Ti(H)N(H)BMes<sub>2</sub> (4.21). This complex is characterised by X-ray crystallography, representing the first structurally authenticated example of a titanium hydride amide.</p> |
| first_indexed | 2024-03-07T06:59:41Z |
| format | Thesis |
| id | oxford-uuid:ff4b38ce-0376-446c-b2d1-d2477c5aba25 |
| institution | University of Oxford |
| last_indexed | 2024-03-07T06:59:41Z |
| publishDate | 2018 |
| record_format | dspace |
| spelling | oxford-uuid:ff4b38ce-0376-446c-b2d1-d2477c5aba252022-03-27T13:43:49ZNew reactivity and synthesis of titanium imide complexesThesishttp://purl.org/coar/resource_type/c_db06uuid:ff4b38ce-0376-446c-b2d1-d2477c5aba25ORA Deposit2018Slusarczyk, MWeller, A<p>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.</p> <p><b>Chapter Two</b> describes the dehydrogenative coupling of amine-boranes using cyclopentadienyl-amidinate supported titanium imido catalysts Cp*Ti{MeC(NMe)<sub>2</sub>}(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)<sub>2</sub>}(N<sup>t</sup>Bu) (2.1) featured a turnover frequency of TOF90% = 1600 h-1 (RT, toluene, [DMAB]<sub>0</sub>=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)<sub>2</sub>}(NR) represents the active species. Significantly, the catalysis features two distinct kinetic regimes which can be exploited to isolate Me<sub>2</sub>NH.BH<sub>2</sub>NMe<sub>2</sub>.BH<sub>3</sub> (and it’s deuterium labelled isotopologues) directly and catalytically from DMAB in high yields on the preparative scale.</p> <p><b>Chapter Three</b> describes the reactivity of one of the first known transition metal borylimide complexes, namely Cp*Ti(HPP){NB(NDippCH)<sub>2</sub>} (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)]<sub>2</sub> (3.22) as well as borylimide N(CHTol){B(NDippCH)<sub>2</sub>} (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)<sub>2</sub>}{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)<sub>2</sub>})(OR’).</p> <p><b>Chapter Four</b> describes a novel synthetic protocol to access titanium borylimides, namely the oxidative addition of borylazides N<sub>3</sub>BR to a Ti(II) precursor, namely (Cp''<sub>2</sub>Ti)<sub>2</sub>(μ<sub>2</sub>:η<sup>1</sup>,η<sup>1</sup>-N<sub>2</sub>) (1.16). Three different examples of highly reactive, base-free titanium borylimide complexes of type Cp''<sub>2</sub>Ti=NBR<sub>2</sub> could be prepared this way, specifically Cp''<sub>2</sub>Ti=NB{N(Me)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>} (4.11), Cp''<sub>2</sub>Ti=NB(NDippCH)<sub>2</sub> (4.12) and Cp''<sub>2</sub>Ti=NBMes<sub>2</sub> (4.1). In contrast to the vast majority of borylimide complexes that feature two nitrogen atoms adjacent to the boron centre, Cp''<sub>2</sub>Ti=NBMes<sub>2</sub> (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''<sub>2</sub>Ti=NBMes<sub>2</sub> (4.1) to be more reactive towards small molecule activation than the previously studied Cp*Ti(HPP)(NB(NDippCH)<sub>2</sub>) (1.83). Significantly, dihydrogen is activated rapidly under ambient conditions to form the titanium hydride amide Cp''<sub>2</sub>Ti(H)N(H)BMes<sub>2</sub> (4.21). This complex is characterised by X-ray crystallography, representing the first structurally authenticated example of a titanium hydride amide.</p> |
| spellingShingle | Slusarczyk, M New reactivity and synthesis of titanium imide complexes |
| title | New reactivity and synthesis of titanium imide complexes |
| title_full | New reactivity and synthesis of titanium imide complexes |
| title_fullStr | New reactivity and synthesis of titanium imide complexes |
| title_full_unstemmed | New reactivity and synthesis of titanium imide complexes |
| title_short | New reactivity and synthesis of titanium imide complexes |
| title_sort | new reactivity and synthesis of titanium imide complexes |
| work_keys_str_mv | AT slusarczykm newreactivityandsynthesisoftitaniumimidecomplexes |