Effect of ancillary ligands in the hapticity of the pyrrolyl ligand in [Ti(pyrrolyl)(NMe2)xCl3−x] (x = 0, 1, 2, 3) complexes

Two complete series of titanium complexes of the type [Ti(pyrrolyl)(NMe2)xCl3−x] (pyrrolyl = 2,3,4,5-tetramethylpyrrolyl (TMP), x = 3 (1), 2 (2), 1 (3), 0 (4); pyrrolyl = 2,5-dimethylpyrrolyl (DMP), x = 3 (5), 2 (6), 1 (7), 0 (8)) were prepared by reaction of equimolar amounts of either [Ti(NMe2)xCl4−x] (x = 4, 3 and 2) with the corresponding pyrrole (1–3 and 5–7) or TiCl4 with Li(pyrrolyl) (4 and 8). A study on the effect on the pyrrolyl coordination mode of the number of strong π-donor amido ligands present in the Ti coordination sphere was carried out by 13C NMR spectroscopy and by X-ray diffraction, when possible. For both pyrrolyl ligands, the η5-coordination to the metal is favoured when there are two or less coordinated amido ligands. However, the presence of a third amido ligand inhibits this type of coordination, shifting the pyrrolyl ring to a σN binding mode (η1), due to an increase of the electron density at the metal centre. For these two complete series of Ti monopyrrolyl complexes we have observed that 13C NMR deshielding parameters (in relation to the parent pyrrole molecules), in the absence of detectable η5–η1 dynamic behaviour, could be used as an assignment criterion for the N-heterocyclic ring type of coordination in Ti pyrrolyl complexes.

An increasing number of NMe2 pπ → dπ donor ligands in the coordination sphere of η5-pyrrolyl titanium complexes enhances the pyrrolyl ring slippage until reaching a σN-coordination. This trend was evaluated by X-ray diffraction and by 13C NMR for two series of compounds [Ti(pyrrolyl)(NMe2)xCl3−x] (pyrrolyl = 2,5-dimethylpyrrolyl and 2,3,4,5-tetramethylpyrrolyl; x = 0−3).Figure optionsDownload as PowerPoint slide