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

Allenes typically display a linear geometry. A notable exception occurs in allenes bearing two electron-donating N-heterocyclic capping groups, which can display a strongly bent C═C═C unit. These push-push allenes, known as carbodicarbenes, are powerful carbon-donor ligands with broad utility in chemistry. Computational studies have suggested that push-pull allenes, featuring both an electron-donating and withdrawing capping group, can also adopt a bent geometry. However, experimental confirmation has so far been lacking. Herein, we report allenes featuring an N-heterocyclic capping group on one side, and either diarylmethylidene or fluorenylidene capping groups on the other. The synthesis of these allenes was accomplished via a titanium vinylidene complex. The latter could be isolated and analyzed by X-ray diffraction, and it represents the first structurally characterized Ti vinylidene complex. The push-pull allenes with fluorenylidene capping groups revealed markedly bent C═C═C units (α_c-c-c < 140°), whereas the less polarized diarylmethylidene analogues were found to display a more linear geometry. The geometric differences correlate with a divergent reactivity upon thermal activation. Both types of N-heterocyclic allenes can be used as carbon-donor ligands for transition metal complexes.