Convenient Synthesis of Cationic Titanium Complexes with Tridentate Cp, N, P-Ligand Framework: FLP-Like Reactivity at the Ti-N Bond and Unexpected Ligand Hydrogenation Reaction
Fischer M., Barbul D., Schmidtmann M., Beckhaus R.
The convenient synthesis of cationic titanium complexes 4a,b is reported, starting from the titanium monopentafulvene complex 1 (Cp∗Ti(Cl)(π-η5:σ- η1-C5H4=CR2; CR2 = adamantylidene) and the bidentate P,N-ligand precursor compounds L1 and L2 (2-R2PC6H4CN; R = Ph, iPr) with a nitrile functional group each and differently substituted phosphine moieties. In effect, 4a,b feature novel tridentate Cp,N,P-ligand systems, which were introduced directly into the coordination sphere of the titanium in conclusive two-step syntheses. Complexes 4a,b react under mild conditions with acetone and phenylacetylene selectively at the Ti-N bond in a FLP-like manner to give the corresponding complexes 5a,b and 6a,b via protonation of the nitrogen and addition of the anionic enolate and acetylide moieties to the titanium. Addition of dihydrogen to a solution of 4b at room temperature results in an unexpected hydrogenation reaction of the C=N bond of the ancillary Cp,N,P-ligand framework to yield complex 7b. In order to obtain further insight into the ligand design with respect to enabling subsequent reactions, another cationic titanium complex 10 was prepared with a bidentate Cp,N-ligand framework bearing no phosphine functional group, starting from 1 and 4-chlorobenzonitrile. 10 does not react with acetone, phenylacetylene, or dihydrogen, and consequently only 10 could be reisolated, even under harsher reaction conditions, showing that the phosphorus donor side is mandatory for subsequent reactions in a FLP-like manner.