Bent N-Heterocyclic Allenes From a Well-Defined Titanium Vinylidene Complex

Area of Science:

• Organometallic Chemistry
• Organic Synthesis
• Structural Chemistry

Background:

• Allenes usually have linear geometry, but exceptions exist with electron-donating groups.
• Push-push allenes (carbodicarbenes) are known bent ligands, but push-pull allene geometry was unconfirmed.
• Computational studies predicted bent geometries for push-pull allenes, lacking experimental evidence.

Purpose of the Study:

• To experimentally synthesize and characterize push-pull allenes.
• To investigate the geometric impact of electron-donating and withdrawing capping groups on allene structures.
• To explore the reactivity and ligand utility of these novel allenes.

Main Methods:

• Synthesis of push-pull allenes using a titanium vinylidene complex intermediate.
• Isolation and X-ray diffraction analysis of the titanium vinylidene complex.
• Structural characterization of push-pull allenes with fluorenylidene and diarylmethylidene capping groups.

Main Results:

• First experimental confirmation of bent C═C═C units in push-pull allenes, particularly with fluorenylidene groups (α_c-c-c < 140°).
• Diarylmethylidene analogues showed more linear geometries due to less polarization.
• The titanium vinylidene complex was successfully isolated and structurally characterized.
• Geometric differences correlated with distinct reactivity patterns upon thermal activation.

Conclusions:

• Push-pull allenes with N-heterocyclic and fluorenylidene capping groups exhibit significantly bent C═C═C geometries.
• These novel allenes function effectively as carbon-donor ligands for transition metal complexes.
• The study provides crucial experimental validation for predicted allene geometries and expands their synthetic utility.