Titankomplexe mit tetradentaten Liganden für die Aktivierung von CO$_{2}$

  • Titanium complexes with tetradentate ligands for the activation of CO$_{2}$

van Krüchten, Franziska Dorothea; Okuda, Jun (Thesis advisor); Albrecht, Markus (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

The aim of this thesis was to investigate pathways for the synthesis of a dititanoxycarbene stabilized by tetradentate ligand systems. Hereby terminal and cationic bridged formate complexes were synthesized as intermediates which should be deprotonated with bases. The synthesis of the terminal formate complex [Ti(OCHO)(Me$_{3}$Si-N$_{3}$N)] and the cationic bridged formate complexes [{Ti(Me$_{3}$Si-N$_{3}$N)}2($\mu$-OCHO-$\eta$O:$\eta$O‘)]A (A = [B(3,5-Cl$_{2}$C$_{6}$H$_{3}$)$_{4}$] and [B(C6F5)4]) is shown. By adding KHMDS, the terminal formate complex is deprotonated and decarbonylated to the anionic titanium oxo complex K[OTi(Me$_{3}$Si-N$_{3}$N)]. The instability of the amide-silyl-bonds towards bases prevents the deprotonation of the bridged formate complexes to a dititanoxycarbene. CO$_{2}$ inserts into the titanium amide bonds of the cationic complexes to form the carbamate complex [{Ti(Me$_{3}$Si-N$_{2}$ ($\mu$-O$_{2}$CN-SiMe$_{3}$-$\eta$O:$\eta$O‘)N)} $_{2}$]A$_{2}$. Starting from [Ti(CH$_{2}$SiMe$_{3}$)(Xy-N$_{3}$N)], both the coordinatively saturated [Ti(Xy-N$_{3}$N)]A as well as the coordinatively unsaturated cationic complexes [Ti(Xy-N$_{3}$N)(L)]A were prepared. These complexes are not suitable for the polymerization of THF, but THF coordinated to [Ti(Xy-N$_{3}$N)(thf)]A can be selectively ring opened with NEt$_{3}$ to [Ti(OnBuNEt$_{3}$)(Xy-N$_{3}$N)]A and with [HBPh$_{3}$] to [Ti(OnBu)(Xy-N$_{3}$N)]. When the Lewis acidic trityl salts are added to [TiR(Xy-N$_{3}$N)] (R = NMe$_{32}$, CH$_{2}$SiMe$_{3}$), a hydride from the ligand backbone is abstracted and the formation of [TiR(Xy-N$_{2}$(Xy-NCHCH$_{2}$)N)]A is observed. The addition of KH deprotonates [Ti(NMe$_{2}$)(Xy-N$_{2}$(Xy-NCHCH$_{2}$)N)]A to [Ti(NMe$_{2}$)(Xy-N$_{2}$ (Xy-NCHCH)N)]. [TiR(Xy-N$_{3}$N)] (R = Me, CH$_{2}$SiMe$_{3}$) reacts with PhSiH$_{3}$ and H$_{2}$ to the trivalent complex [{Ti(Xy-N$_{2}$($\mu$-N-Xy-$\eta$N)N)} $_{2}$]. A terminal formate complex stabilized by Xy-N$_{3}$N is not accessible via the synthesis routes described in this work.Furthermore, the synthesis of the water- and air-stable terminal formate complex [Ti(OCHO)(O$_{3}$N)] and the cationic bridged formate complexes [{Ti(O$_{3}$N)}$_{2}$ ($\mu$-OCHO-$\eta$O:$\eta$O‘)]A is described. These formate complexes were converted with Brønsted- and non-nucleophilic Lewis-acids. [Ti(OCHO)(O$_{3}$N)] probably reacts with LDA to the anionic titanium oxo complex [Li(thf)x][OTi(O$_{3}$N)] (x = 0 to 2). The cationic bridging formate complexes cannot be converted to dititanoxycarbene, since the dimers dissociate into two species upon addition of base. Moreover, the reactivity of [TiCl(tmtaa)] and the trivalent alkyl complexes [TiR(tmtaa)] (R = Me, CH$_{2}$SiMe$_{3}$) with oxygen and hydrogen was investigated. The alkyl complexes react with traces of oxygen to the titanium μ-oxo complex [{Ti(tmtaa)}$_{2}$O]. Whereas the tetravalent chlorido complex reacts to the cationic titanium μ-oxo complex [(tmtaa)Ti(Cl)-O-Ti(tmtaa)]Cl. The synthesis of a trivalent titanium hydride complex was verified by the formation of the terminal formate complex [Ti(OCHO)(tmtaa)] after the addition of CO$_{2}$. Starting from the terminal formate complex no bridged formate complex was obtained. Moreover, the synthesis of intramolecular, tmtaa-stabilized titanium formate complex was not successful.

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