1-Titanacyclobuta-2,3-diene – an elusive four-membered cyclic allene

Abstract

The synthesis and characterisation of a 1-titanacyclobuta-2,3-diene complex, an organometallic analog of elusive 1,2-cyclobutadiene, is presented.

A Ghost Trapped: Realization of the 1‐Titanacyclobuta‐2,3‐diene as the First Four‐Membered Group 4 Metallacycloallene

“…Whereas the title compound has a very large angle of 150(1)° at the allenic C‐atoms, this is with 130–135° significant smaller for the examples from group 6 metals (Mo, W) for which a M‐β‐carbon atom bond is drawn in the papers . This is in contrast to the findings of of Reiß and Beweries for the new Ti complex (Scheme and Scheme ) . With this metallacyclic analogue of the non‐existent 1,2‐cyclobutadiene and similar compounds further studies regarding unknown structural principles and reactivity become possible for the future.…”

“…When using the pentamethylcyclopentadienyl complex Cp* 2 TiCl 2 instead of Cp 2 ZrCl 2 in the reaction with [Li 2 (Me 3 SiC 3 SiMe 3 )] an organic coupling product together with unidentified titanium species were found (Scheme ). The coupling product is obtained in a high yield and could be formed by dimerization of two carbenes in the coordination sphere of titanium …”

“…Further alteration of the cyclopentadienyl scaffold, i.e. introduction of the chelating rac ‐(ebthi) ligand, gave a breakthrough in the reaction of [Li 2 (Me 3 SiC 3 SiMe 3 )] and resulted in the synthesis of rac ‐(ebthi)Ti[η 2 ‐C(SiMe 3 )=C=C(SiMe 3 )‐] (Scheme ), the first mononuclear four ‐membered all‐C ‐metallacyclooallene based on a Group 4 metal that was sought‐after since 2004 . Notably, this compound represents the hitherto smallest compound in the series of small ring strained metallacycles of a Group 4 metal!…”

“…For the four‐ membered titanacycloallene, further investigations and calculations were performed in the new paper of Reiß and Beweries for a better understanding of these unusual metallacycles . Despite the short interatomic distance between titanium and the β‐carbon atom, there is no significant bonding interaction between both atoms and a ring critical point was found near the centre of the TiC 3 ring .…”

“…For the four‐ membered titanacycloallene, further investigations and calculations were performed in the new paper of Reiß and Beweries for a better understanding of these unusual metallacycles . Despite the short interatomic distance between titanium and the β‐carbon atom, there is no significant bonding interaction between both atoms and a ring critical point was found near the centre of the TiC 3 ring . Scheme shows the main resonance structures which best describe the bonding situation in this unusual complex as a singlet biradical, composed of a Ti(III) center which is antiferromagnetically coupled with a monoanionic allene radical, thus allowing for the NMR spectroscopic characterization of this species.…”