Electrophilic d0 Cations of Group 4 Metals (M = Ti, Zr, Hf) Derived from Monopentafulvene Complexes: Direct Formation of Tridentate Cp, O, P -Ligands
Fischer M., Schaper R., Jaugstetter M., Schmidtmann M., Beckhaus R.
The reactions of the monopentafulvene complexes Ti1a and Ti1b with the general formula [CpTi(Cl)(π-η5:σ-η1-C5H4=CR2)] (R = p-tolyl (Ti1a); CR2 = adamantylidene (Ti1b)) with the bidentate P,O-ligand precursor L1, featuring a diphenylphosphine and a hydroxyl functional group, are reported, yielding the corresponding complexes Ti2a and Ti2b in good yields as the result of deprotonation. A chloride/methyl exchange reaction and subsequent reaction with B(C6F5)3 was envisaged to yield the corresponding cationic complexes. Instead, the methylation reactions of Ti2a and Ti2b with methyllithium or methylmagnesium bromide selectively yielded the doubly methylated titanium complexes Ti3a and Ti3b with abstraction of LiCl and the lithium salt of the bidentate P,O-ligand. To avoid this reaction, the P,O-ligand precursor L2 was prepared, featuring a carbonyl group instead of the hydroxyl functional group. This change in the general reaction sequence allowed the preparation of a new family of cationic titanium complexes Ti6a and Ti6b and was transferred to the heavier congeners zirconium (Zr4) and hafnium (Hf4). Every step of the reaction pathway was performed under mild reaction conditions and in good to very good yields. The insertion of the carbonyl group into the M-Cexo bond of the monopentafulvene complexes Ti1a, Ti1b, Zr1, and Hf1, and consequently the formation of a C-C bond, proved to be mandatory for the methylation and subsequent abstraction of the methyl group by B(C6F5)3. In effect, a tridentate Cp,O,P-ligand was directly introduced into the coordination spheres of the respective group 4 metals within the cationic complexes. In all cases the phosphorus shows a persistent interaction between the Lewis acidic metal center and the Lewis basic phosphine moiety, as shown by NMR analyses and in the solid state. Every complex was thoroughly characterized, including several X-ray diffraction analyses of each class of compounds reported here.